draft-ietf-mpls-arch-01.txt   draft-ietf-mpls-arch-02.txt 
Network Working Group Eric C. Rosen Network Working Group Eric C. Rosen
Internet Draft Cisco Systems, Inc. Internet Draft Cisco Systems, Inc.
Expiration Date: September 1998 Expiration Date: January 1999
Arun Viswanathan Arun Viswanathan
Lucent Technologies Lucent Technologies
Ross Callon Ross Callon
IronBridge Networks, Inc. IronBridge Networks, Inc.
March 1998 July 1998
Multiprotocol Label Switching Architecture Multiprotocol Label Switching Architecture
draft-ietf-mpls-arch-01.txt draft-ietf-mpls-arch-02.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
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Abstract Abstract
This internet draft specifies the architecture for multiprotocol This internet draft specifies the architecture for multiprotocol
label switching (MPLS). The architecture is based on other label label switching (MPLS). The architecture is based on other label
switching approaches [2-11] as well as on the MPLS Framework document switching approaches [2-11] as well as on the MPLS Framework document
[1]. [1].
Table of Contents Table of Contents
1 Introduction to MPLS ............................... 4 1 Introduction to MPLS ............................... 4
1.1 Overview ........................................... 4 1.1 Overview ........................................... 4
1.2 Terminology ........................................ 6 1.2 Terminology ........................................ 6
1.3 Acronyms and Abbreviations ......................... 9 1.3 Acronyms and Abbreviations ......................... 9
1.4 Acknowledgments .................................... 10 1.4 Acknowledgments .................................... 10
2 Outline of Approach ................................ 11 2 Outline of Approach ................................ 10
2.1 Labels ............................................. 11 2.1 Labels ............................................. 11
2.2 Upstream and Downstream LSRs ....................... 12 2.2 Upstream and Downstream LSRs ....................... 12
2.3 Labeled Packet ..................................... 12 2.3 Labeled Packet ..................................... 12
2.4 Label Assignment and Distribution; Attributes ...... 12 2.4 Label Assignment and Distribution .................. 12
2.5 Label Distribution Protocol (LDP) .................. 13 2.5 Attributes of a Label Binding ...................... 12
2.6 The Label Stack .................................... 13 2.6 Label Distribution Protocol (LDP) .................. 13
2.7 The Next Hop Label Forwarding Entry (NHLFE) ........ 14 2.7 Downstream vs. Downstream-on-Demand ................ 13
2.8 Incoming Label Map (ILM) ........................... 14 2.8 Label Retention Mode ............................... 13
2.9 Stream-to-NHLFE Map (STN) .......................... 15 2.9 The Label Stack .................................... 14
2.10 Label Swapping ..................................... 15 2.10 The Next Hop Label Forwarding Entry (NHLFE) ........ 14
2.11 Scope and Uniqueness of Labels ..................... 15 2.11 Incoming Label Map (ILM) ........................... 15
2.12 Label Switched Path (LSP), LSP Ingress, LSP Egress . 16 2.12 FEC-to-NHLFE Map (FTN) ............................. 15
2.13 Penultimate Hop Popping ............................ 18 2.13 Label Swapping ..................................... 16
2.14 LSP Next Hop ....................................... 19 2.14 Scope and Uniqueness of Labels ..................... 16
2.15 Route Selection .................................... 20 2.15 Label Switched Path (LSP), LSP Ingress, LSP Egress . 17
2.16 Time-to-Live (TTL) ................................. 21 2.16 Penultimate Hop Popping ............................ 19
2.17 Loop Control ....................................... 22 2.17 LSP Next Hop ....................................... 20
2.17.1 Loop Prevention .................................... 23 2.18 Invalid Incoming Labels ............................ 21
2.17.2 Interworking of Loop Control Options ............... 25 2.19 LSP Control: Ordered versus Independent ............ 21
2.18 Merging and Non-Merging LSRs ....................... 26 2.20 Aggregation ........................................ 22
2.18.1 Stream Merge ....................................... 27 2.21 Route Selection .................................... 24
2.18.2 Non-merging LSRs ................................... 27 2.22 Time-to-Live (TTL) ................................. 25
2.18.3 Labels for Merging and Non-Merging LSRs ............ 28 2.23 Loop Control ....................................... 26
2.18.4 Merge over ATM ..................................... 29 2.23.1 Loop Prevention .................................... 27
2.18.4.1 Methods of Eliminating Cell Interleave ............. 29 2.23.2 Interworking of Loop Control Options ............... 29
2.18.4.2 Interoperation: VC Merge, VP Merge, and Non-Merge .. 29 2.24 Label Encodings .................................... 30
2.19 LSP Control: Egress versus Local ................... 30 2.24.1 MPLS-specific Hardware and/or Software ............. 31
2.20 Granularity ........................................ 32 2.24.2 ATM Switches as LSRs ............................... 31
2.21 Tunnels and Hierarchy .............................. 33 2.24.3 Interoperability among Encoding Techniques ......... 33
2.21.1 Hop-by-Hop Routed Tunnel ........................... 33 2.25 Label Merging ...................................... 33
2.21.2 Explicitly Routed Tunnel ........................... 33 2.25.1 Non-merging LSRs ................................... 34
2.21.3 LSP Tunnels ........................................ 33 2.25.2 Labels for Merging and Non-Merging LSRs ............ 35
2.21.4 Hierarchy: LSP Tunnels within LSPs ................. 34 2.25.3 Merge over ATM ..................................... 36
2.21.5 LDP Peering and Hierarchy .......................... 34 2.25.3.1 Methods of Eliminating Cell Interleave ............. 36
2.22 LDP Transport ...................................... 36 2.25.3.2 Interoperation: VC Merge, VP Merge, and Non-Merge .. 36
2.23 Label Encodings .................................... 36 2.26 Tunnels and Hierarchy .............................. 37
2.23.1 MPLS-specific Hardware and/or Software ............. 36 2.26.1 Hop-by-Hop Routed Tunnel ........................... 38
2.23.2 ATM Switches as LSRs ............................... 37 2.26.2 Explicitly Routed Tunnel ........................... 38
2.23.3 Interoperability among Encoding Techniques ......... 38 2.26.3 LSP Tunnels ........................................ 38
2.24 Multicast .......................................... 39 2.26.4 Hierarchy: LSP Tunnels within LSPs ................. 39
3 Some Applications of MPLS .......................... 39 2.26.5 LDP Peering and Hierarchy .......................... 39
3.1 MPLS and Hop by Hop Routed Traffic ................. 39 2.27 LDP Transport ...................................... 40
3.1.1 Labels for Address Prefixes ........................ 39 2.28 Multicast .......................................... 41
3.1.2 Distributing Labels for Address Prefixes ........... 39 3 Some Applications of MPLS .......................... 41
3.1.2.1 LDP Peers for a Particular Address Prefix .......... 39 3.1 MPLS and Hop by Hop Routed Traffic ................. 41
3.1.2.2 Distributing Labels ................................ 40 3.1.1 Labels for Address Prefixes ........................ 41
3.1.3 Using the Hop by Hop path as the LSP ............... 41 3.1.2 Distributing Labels for Address Prefixes ........... 41
3.1.4 LSP Egress and LSP Proxy Egress .................... 41 3.1.2.1 LDP Peers for a Particular Address Prefix .......... 41
3.1.5 The POP Label ...................................... 42 3.1.2.2 Distributing Labels ................................ 42
3.1.6 Option: Egress-Targeted Label Assignment ........... 43 3.1.3 Using the Hop by Hop path as the LSP ............... 43
3.2 MPLS and Explicitly Routed LSPs .................... 44 3.1.4 LSP Egress and LSP Proxy Egress .................... 43
3.2.1 Explicitly Routed LSP Tunnels: Traffic Engineering . 44 3.1.5 The Implicit NULL Label ............................ 44
3.3 Label Stacks and Implicit Peering .................. 45 3.1.6 Option: Egress-Targeted Label Assignment ........... 45
3.4 MPLS and Multi-Path Routing ........................ 46 3.2 MPLS and Explicitly Routed LSPs .................... 46
3.5 LSP Trees as Multipoint-to-Point Entities .......... 46 3.2.1 Explicitly Routed LSP Tunnels: Traffic Engineering . 46
3.6 LSP Tunneling between BGP Border Routers ........... 47 3.3 Label Stacks and Implicit Peering .................. 47
3.7 Other Uses of Hop-by-Hop Routed LSP Tunnels ........ 49 3.4 MPLS and Multi-Path Routing ........................ 48
3.8 MPLS and Multicast ................................. 49 3.5 LSP Trees as Multipoint-to-Point Entities .......... 48
4 LDP Procedures for Hop-by-Hop Routed Traffic ....... 50 3.6 LSP Tunneling between BGP Border Routers ........... 49
4.1 The Procedures for Advertising and Using labels .... 50 3.7 Other Uses of Hop-by-Hop Routed LSP Tunnels ........ 50
4.1.1 Downstream LSR: Distribution Procedure ............. 50 3.8 MPLS and Multicast ................................. 51
4.1.1.1 PushUnconditional .................................. 51 4 LDP Procedures for Hop-by-Hop Routed Traffic ....... 51
4.1.1.2 PushConditional .................................... 51 4.1 The Procedures for Advertising and Using labels .... 51
4.1.1.3 PulledUnconditional ................................ 52 4.1.1 Downstream LSR: Distribution Procedure ............. 52
4.1.1.4 PulledConditional .................................. 52 4.1.1.1 PushUnconditional .................................. 52
4.1.2 Upstream LSR: Request Procedure .................... 53 4.1.1.2 PushConditional .................................... 53
4.1.2.1 RequestNever ....................................... 53 4.1.1.3 PulledUnconditional ................................ 53
4.1.2.2 RequestWhenNeeded .................................. 53 4.1.1.4 PulledConditional .................................. 54
4.1.2.3 RequestOnRequest ................................... 53 4.1.2 Upstream LSR: Request Procedure .................... 55
4.1.3 Upstream LSR: NotAvailable Procedure ............... 54 4.1.2.1 RequestNever ....................................... 55
4.1.3.1 RequestRetry ....................................... 54 4.1.2.2 RequestWhenNeeded .................................. 55
4.1.3.2 RequestNoRetry ..................................... 54 4.1.2.3 RequestOnRequest ................................... 55
4.1.4 Upstream LSR: Release Procedure .................... 54 4.1.3 Upstream LSR: NotAvailable Procedure ............... 56
4.1.4.1 ReleaseOnChange .................................... 54 4.1.3.1 RequestRetry ....................................... 56
4.1.4.2 NoReleaseOnChange .................................. 54 4.1.3.2 RequestNoRetry ..................................... 56
4.1.5 Upstream LSR: labelUse Procedure ................... 55 4.1.4 Upstream LSR: Release Procedure .................... 56
4.1.5.1 UseImmediate ....................................... 55 4.1.4.1 ReleaseOnChange .................................... 56
4.1.5.2 UseIfLoopFree ...................................... 55 4.1.4.2 NoReleaseOnChange .................................. 57
4.1.5.3 UseIfLoopNotDetected ............................... 55 4.1.5 Upstream LSR: labelUse Procedure ................... 57
4.1.6 Downstream LSR: Withdraw Procedure ................. 56 4.1.5.1 UseImmediate ....................................... 57
4.2 MPLS Schemes: Supported Combinations of Procedures . 56 4.1.5.2 UseIfLoopFree ...................................... 57
4.2.1 TTL-capable LSP Segments ........................... 57 4.1.5.3 UseIfLoopNotDetected ............................... 58
4.2.2 Using ATM Switches as LSRs ......................... 57 4.1.6 Downstream LSR: Withdraw Procedure ................. 58
4.2.2.1 Without Multipoint-to-point Capability ............. 58 4.2 MPLS Schemes: Supported Combinations of Procedures . 59
4.2.2.2 With Multipoint-To-Point Capability ................ 58 4.2.1 TTL-capable LSP Segments ........................... 59
4.2.3 Interoperability Considerations .................... 59 4.2.2 Using ATM Switches as LSRs ......................... 60
4.2.4 How to do Loop Prevention .......................... 60 4.2.2.1 Without Label Merging .............................. 60
4.2.5 How to do Loop Detection ........................... 60 4.2.2.2 With Label Merging ................................. 61
4.2.6 Security Considerations ............................ 60 4.2.3 Interoperability Considerations .................... 62
5 Authors' Addresses ................................. 60 5 Security Considerations ............................ 63
6 References ......................................... 61 6 Authors' Addresses ................................. 63
7 References ......................................... 64
1. Introduction to MPLS 1. Introduction to MPLS
1.1. Overview 1.1. Overview
In connectionless network layer protocols, as a packet travels from In connectionless network layer protocols, as a packet travels from
one router hop to the next, an independent forwarding decision is one router hop to the next, an independent forwarding decision is
made at each hop. Each router runs a network layer routing made at each hop. Each router runs a network layer routing
algorithm. As a packet travels through the network, each router algorithm. As a packet travels through the network, each router
analyzes the packet header. The choice of next hop for a packet is analyzes the packet header. The choice of next hop for a packet is
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algorithm. algorithm.
Packet headers contain considerably more information than is needed Packet headers contain considerably more information than is needed
simply to choose the next hop. Choosing the next hop can therefore be simply to choose the next hop. Choosing the next hop can therefore be
thought of as the composition of two functions. The first function thought of as the composition of two functions. The first function
partitions the entire set of possible packets into a set of partitions the entire set of possible packets into a set of
"Forwarding Equivalence Classes (FECs)". The second maps each FEC to "Forwarding Equivalence Classes (FECs)". The second maps each FEC to
a next hop. Insofar as the forwarding decision is concerned, a next hop. Insofar as the forwarding decision is concerned,
different packets which get mapped into the same FEC are different packets which get mapped into the same FEC are
indistinguishable. All packets which belong to a particular FEC and indistinguishable. All packets which belong to a particular FEC and
which travel from a particular node will follow the same path. Such which travel from a particular node will follow the same path.
a set of packets may be called a "stream".
In conventional IP forwarding, a particular router will typically In conventional IP forwarding, a particular router will typically
consider two packets to be in the same stream if there is some consider two packets to be in the same FEC if there is some address
address prefix X in that router's routing tables such that X is the prefix X in that router's routing tables such that X is the "longest
"longest match" for each packet's destination address. As the packet match" for each packet's destination address. As the packet traverses
traverses the network, each hop in turn reexamines the packet and the network, each hop in turn reexamines the packet and assigns it to
assigns it to a stream. a FEC.
In MPLS, the assignment of a particular packet to a particular stream In MPLS, the assignment of a particular packet to a particular FEC is
is done just once, as the packet enters the network. The stream to done just once, as the packet enters the network. The FEC to which
which the packet is assigned is encoded with a short fixed length the packet is assigned is encoded with a short fixed length value
value known as a "label". When a packet is forwarded to its next known as a "label". When a packet is forwarded to its next hop, the
hop, the label is sent along with it; that is, the packets are label is sent along with it; that is, the packets are "labeled".
"labeled".
At subsequent hops, there is no further analysis of the packet's At subsequent hops, there is no further analysis of the packet's
network layer header. Rather, the label is used as an index into a network layer header. Rather, the label is used as an index into a
table which specifies the next hop, and a new label. The old label table which specifies the next hop, and a new label. The old label
is replaced with the new label, and the packet is forwarded to its is replaced with the new label, and the packet is forwarded to its
next hop. If assignment to a stream is based on a "longest match", next hop. If assignment to a FEC is based on a "longest match", this
this eliminates the need to perform a longest match computation for eliminates the need to perform a longest match computation for each
each packet at each hop; the computation can be performed just once. packet at each hop; the computation can be performed just once.
Some routers analyze a packet's network layer header not merely to Some routers analyze a packet's network layer header not merely to
choose the packet's next hop, but also to determine a packet's choose the packet's next hop, but also to determine a packet's
"precedence" or "class of service", in order to apply different "precedence" or "class of service", in order to apply different
discard thresholds or scheduling disciplines to different packets. discard thresholds or scheduling disciplines to different packets.
MPLS allows the precedence or class of service to be inferred from MPLS allows the precedence or class of service to be inferred from
the label, so that no further header analysis is needed; in some the label, so that no further header analysis is needed; in some
cases MPLS provides a way to explicitly encode a class of service in cases MPLS provides a way to explicitly encode a class of service in
the "label header". the "label header".
The fact that a packet is assigned to a stream just once, rather than The fact that a packet is assigned to a FEC just once, rather than at
at every hop, allows the use of sophisticated forwarding paradigms. every hop, allows the use of sophisticated forwarding paradigms. A
A packet that enters the network at a particular router can be packet that enters the network at a particular router can be labeled
labeled differently than the same packet entering the network at a differently than the same packet entering the network at a different
different router, and as a result forwarding decisions that depend on router, and as a result forwarding decisions that depend on the
the ingress point ("policy routing") can be easily made. In fact, ingress point ("policy routing") can be easily made. In fact, the
the policy used to assign a packet to a stream need not have only the policy used to assign a packet to a FEC need not have only the
network layer header as input; it may use arbitrary information about network layer header as input; it may use arbitrary information about
the packet, and/or arbitrary policy information as input. Since this the packet, and/or arbitrary policy information as input. Since this
decouples forwarding from routing, it allows one to use MPLS to decouples forwarding from routing, it allows one to use MPLS to
support a large variety of routing policies that are difficult or support a large variety of routing policies that are difficult or
impossible to support with just conventional network layer impossible to support with just conventional network layer
forwarding. forwarding.
Similarly, MPLS facilitates the use of explicit routing, without Similarly, MPLS facilitates the use of explicit routing, without
requiring that each IP packet carry the explicit route. Explicit requiring that each IP packet carry the explicit route. Explicit
routes may be useful to support policy routing and traffic routes may be useful to support policy routing and traffic
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A general discussion of issues related to MPLS is presented in "A A general discussion of issues related to MPLS is presented in "A
Framework for Multiprotocol Label Switching" [1]. Framework for Multiprotocol Label Switching" [1].
1.2. Terminology 1.2. Terminology
This section gives a general conceptual overview of the terms used in This section gives a general conceptual overview of the terms used in
this document. Some of these terms are more precisely defined in this document. Some of these terms are more precisely defined in
later sections of the document. later sections of the document.
aggregate stream synonym of "stream"
DLCI a label used in Frame Relay networks to DLCI a label used in Frame Relay networks to
identify frame relay circuits identify frame relay circuits
flow a single instance of an application to flow a single instance of an application to
application flow of data (as in the RSVP application flow of data (as in the RSVP
and IFMP use of the term "flow") and IFMP use of the term "flow")
forwarding equivalence class a group of IP packets which are forwarding equivalence class a group of IP packets which are
forwarded in the same manner (e.g., forwarded in the same manner (e.g.,
over the same path, with the same over the same path, with the same
forwarding treatment) forwarding treatment)
frame merge stream merge, when it is applied to frame merge label merging, when it is applied to
operation over frame based media, so that operation over frame based media, so that
the potential problem of cell interleave the potential problem of cell interleave
is not an issue. is not an issue.
label a short fixed length physically label a short fixed length physically
contiguous identifier which is used to contiguous identifier which is used to
identify a stream, usually of local identify a FEC, usually of local
significance. significance.
label information base the database of information containing label merging the replacement of multiple incoming
label bindings labels for a particular FEC with a single
outgoing label
label swap the basic forwarding operation consisting label swap the basic forwarding operation consisting
of looking up an incoming label to of looking up an incoming label to
determine the outgoing label, determine the outgoing label,
encapsulation, port, and other data encapsulation, port, and other data
handling information. handling information.
label swapping a forwarding paradigm allowing label swapping a forwarding paradigm allowing
streamlined forwarding of data by using streamlined forwarding of data by using
labels to identify streams of data to be labels to identify classes of data
forwarded. packets which are treated
indistinguishably when forwarding.
label switched hop the hop between two MPLS nodes, on which label switched hop the hop between two MPLS nodes, on which
forwarding is done using labels. forwarding is done using labels.
label switched path the path created by the concatenation of label switched path the path created by the concatenation of
one or more label switched hops, allowing one or more label switched hops, allowing
a packet to be forwarded by swapping a packet to be forwarded by swapping
labels from an MPLS node to another MPLS labels from an MPLS node to another MPLS
node. node.
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label stack an ordered set of labels label stack an ordered set of labels
loop survival a method of dealing with loops in which loop survival a method of dealing with loops in which
data may be transmitted over a loop, but data may be transmitted over a loop, but
means are employed to limit the amount of means are employed to limit the amount of
network resources which may be consumed network resources which may be consumed
by the looping data by the looping data
label switched path The path through one or more LSRs at one label switched path The path through one or more LSRs at one
level of the hierarchy followed by a level of the hierarchy followed by a
stream. packets in a particular FEC.
label switching router an MPLS node which is capable of label switching router an MPLS node which is capable of
forwarding native L3 packets forwarding native L3 packets
merge point the node at which multiple streams and merge point a node at which label merging is done
switched paths are combined into a single
stream sent over a single path.
Mlabel abbreviation for MPLS label
MPLS core standards the standards which describe the core MPLS core standards the standards which describe the core
MPLS technology MPLS technology
MPLS domain a contiguous set of nodes which operate MPLS domain a contiguous set of nodes which operate
MPLS routing and forwarding and which are MPLS routing and forwarding and which are
also in one Routing or Administrative also in one Routing or Administrative
Domain Domain
MPLS edge node an MPLS node that connects an MPLS domain MPLS edge node an MPLS node that connects an MPLS domain
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domain. Note that if an LSR has a domain. Note that if an LSR has a
neighboring host which is not running neighboring host which is not running
MPLS, that that LSR is an MPLS edge node. MPLS, that that LSR is an MPLS edge node.
MPLS egress node an MPLS edge node in its role in handling MPLS egress node an MPLS edge node in its role in handling
traffic as it leaves an MPLS domain traffic as it leaves an MPLS domain
MPLS ingress node an MPLS edge node in its role in handling MPLS ingress node an MPLS edge node in its role in handling
traffic as it enters an MPLS domain traffic as it enters an MPLS domain
MPLS label a label placed in a short MPLS shim MPLS label a label which is carried in a packet
header used to identify streams header, and which represents the packet's
FEC
MPLS node a node which is running MPLS. An MPLS MPLS node a node which is running MPLS. An MPLS
node will be aware of MPLS control node will be aware of MPLS control
protocols, will operate one or more L3 protocols, will operate one or more L3
routing protocols, and will be capable of routing protocols, and will be capable of
forwarding packets based on labels. An forwarding packets based on labels. An
MPLS node may optionally be also capable MPLS node may optionally be also capable
of forwarding native L3 packets. of forwarding native L3 packets.
MultiProtocol Label Switching an IETF working group and the effort MultiProtocol Label Switching an IETF working group and the effort
associated with the working group associated with the working group
network layer synonymous with layer 3 network layer synonymous with layer 3
stack synonymous with label stack stack synonymous with label stack
stream an aggregate of one or more flows,
treated as one aggregate for the purpose
of forwarding in L2 and/or L3 nodes
(e.g., may be described using a single
label). In many cases a stream may be the
aggregate of a very large number of
flows. Synonymous with "aggregate
stream".
stream merge the merging of several smaller streams
into a larger stream, such that for some
or all of the path the larger stream can
be referred to using a single label.
switched path synonymous with label switched path switched path synonymous with label switched path
virtual circuit a circuit used by a connection-oriented virtual circuit a circuit used by a connection-oriented
layer 2 technology such as ATM or Frame layer 2 technology such as ATM or Frame
Relay, requiring the maintenance of state Relay, requiring the maintenance of state
information in layer 2 switches. information in layer 2 switches.
VC merge stream merge when it is specifically VC merge label merging where the MPLS label is
applied to VCs, specifically so as to carried in the ATM VCI field (or combined
allow multiple VCs to merge into one VPI/VCI field), so as to allow multiple
single VC VCs to merge into one single VC
VP merge stream merge when it is applied to VPs, VP merge label merging where the MPLS label is
specifically so as to allow multiple VPs carried din the ATM VPI field, so as to
to merge into one single VP. In this case allow multiple VPs to be merged into one
the VCIs need to be unique. This allows single VP. In this case two cells would
cells from different sources to be have the same VCI value only if they
originated from the same node. This
allows cells from different sources to be
distinguished via the VCI. distinguished via the VCI.
VPI/VCI a label used in ATM networks to identify VPI/VCI a label used in ATM networks to identify
circuits circuits
1.3. Acronyms and Abbreviations 1.3. Acronyms and Abbreviations
ATM Asynchronous Transfer Mode ATM Asynchronous Transfer Mode
BGP Border Gateway Protocol BGP Border Gateway Protocol
DLCI Data Link Circuit Identifier DLCI Data Link Circuit Identifier
FEC Forwarding Equivalence Class FEC Forwarding Equivalence Class
STN stream to NHLFE Map FTN FEC to NHLFE Map
IGP Interior Gateway Protocol IGP Interior Gateway Protocol
ILM Incoming Label Map ILM Incoming Label Map
IP Internet Protocol IP Internet Protocol
LIB Label Information Base
LDP Label Distribution Protocol LDP Label Distribution Protocol
L2 Layer 2 L2 Layer 2
L3 Layer 3 L3 Layer 3
LSP Label Switched Path LSP Label Switched Path
LSR Label Switching Router LSR Label Switching Router
MPLS MultiProtocol Label Switching MPLS MultiProtocol Label Switching
MPT Multipoint to Point Tree MPT Multipoint to Point Tree
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George Swallow for their inputs and ideas. George Swallow for their inputs and ideas.
2. Outline of Approach 2. Outline of Approach
In this section, we introduce some of the basic concepts of MPLS and In this section, we introduce some of the basic concepts of MPLS and
describe the general approach to be used. describe the general approach to be used.
2.1. Labels 2.1. Labels
A label is a short, fixed length, locally significant identifier A label is a short, fixed length, locally significant identifier
which is used to identify a stream. The label is based on the stream which is used to identify a FEC. The label which is put on a
or Forwarding Equivalence Class that a packet is assigned to. The particular packet represents the Forwarding Equivalence Class to
label does not directly encode the network layer address. The choice which that packet is assigned.
of label depends on the network layer address only to the extent that
the Forwarding Equivalence Class depends on that address.
If Ru and Rd are LSRs, and Ru transmits a packet to Rd, they may Most commonly, packets are assigned to FECS based on their
agree to use label L to represent stream S for packets which are sent destination network layer addresses. However, the label is never an
from Ru to Rd. That is, they can agree to a "mapping" between label encoding of the destination network layer address.
L and stream S for packets moving from Ru to Rd. As a result of such
an agreement, L becomes Ru's "outgoing label" corresponding to stream
S for such packets; L becomes Rd's "incoming label" corresponding to
stream S for such packets.
Note that L does not necessarily correspond to stream S for any If Ru and Rd are LSRs, they may agree that when Ru transmits a packet
packets other than those which are being sent from Ru to Rd. Also, L to Rd, Ru will label with packet with label value L if and only if
is not an inherently meaningful value and does not have any network- the packet is a member of a particular FEC F. That is, they can
wide value; the particular value assigned to L gets its meaning agree to a "binding" between label L and FEC F for packets moving
solely from the agreement between Ru and Rd. from Ru to Rd. As a result of such an agreement, L becomes Ru's
"outgoing label" representing FEC F, and L becomes Rd's "incoming
label" representing FEC F.
Note that L does not necessarily represent FEC F for any packets
other than those which are being sent from Ru to Rd. L is an
arbitrary value whose binding to F is local to Ru and Rd.
When we speak above of packets "being sent" from Ru to to Rd, we do
not imply either that the packet originated at Ru or that its
destination is Rd. Rather, we mean to include packets which are
"transit packets" at one or both of the LSRs.
Sometimes it may be difficult or even impossible for Rd to tell, of Sometimes it may be difficult or even impossible for Rd to tell, of
an arriving packet carrying label L, that the label L was placed in an arriving packet carrying label L, that the label L was placed in
the packet by Ru, rather than by some other LSR. (This will the packet by Ru, rather than by some other LSR. (This will
typically be the case when Ru and Rd are not direct neighbors.) In typically be the case when Ru and Rd are not direct neighbors.) In
such cases, Rd must make sure that the mapping from label to FEC is such cases, Rd must make sure that the binding from label to FEC is
one-to-one. That is, in such cases, Rd must not agree with Ru1 to one-to-one. That is, in such cases, Rd must not agree with Ru1 to
use L for one purpose, while also agreeing with some other LSR Ru2 to bind L to FEC F1, while also agreeing with some other LSR Ru2 to bind
use L for a different purpose. L to a different FEC F2. It is the responsibility of each LSR to
ensure that it can uniquely interpret its incoming labels.
2.2. Upstream and Downstream LSRs 2.2. Upstream and Downstream LSRs
Suppose Ru and Rd have agreed to map label L to stream S, for packets Suppose Ru and Rd have agreed to bind label L to FEC F, for packets
sent from Ru to Rd. Then with respect to this mapping, Ru is the sent from Ru to Rd. Then with respect to this binding, Ru is the
"upstream LSR", and Rd is the "downstream LSR". "upstream LSR", and Rd is the "downstream LSR".
The notion of upstream and downstream relate to agreements between To say that one node is upstream and one is downstream with respect
nodes of the label values to be assigned for packets belonging to a to a given binding means only that a particular label represents a
particular stream that might be traveling from an upstream node to a particular FEC in packets travelling from the upstream node to the
downstream node. This is independent of whether the routing protocol downstream node. This is NOT meant to imply that packets in that FEC
actually will cause any packets to be transmitted in that particular would actually be routed from the upstream node to the downstream
direction. Thus, Rd is the downstream LSR for a particular mapping node.
for label L if it recognizes L-labeled packets from Ru as being in
stream S. This may be true even if routing does not actually forward
packets for stream S between nodes Rd and Ru, or if routing has made
Ru downstream of Rd along the path which is actually used for packets
in stream S.
2.3. Labeled Packet 2.3. Labeled Packet
A "labeled packet" is a packet into which a label has been encoded. A "labeled packet" is a packet into which a label has been encoded.
The encoding can be done by means of an encapsulation which exists In some cases, the label resides in an encapsulation header which
specifically for this purpose, or by placing the label in an exists specifically for this purpose. In other cases, the label may
available location in either of the data link or network layer reside in an existing data link or network layer header, as long as
headers. Of course, the encoding technique must be agreed to by the there is a field which is available for that purpose. The particular
entity which encodes the label and the entity which decodes the encoding technique to be used must be agreed to by both the entity
label. which encodes the label and the entity which decodes the label.
2.4. Label Assignment and Distribution; Attributes 2.4. Label Assignment and Distribution
For unicast traffic in the MPLS architecture, the decision to bind a In the MPLS architecture, the decision to bind a particular label L
particular label L to a particular stream S is made by the LSR which to a particular FEC F is made by the LSR which is DOWNSTREAM with
is downstream with respect to that mapping. The downstream LSR then respect to that binding. The downstream LSR then informs the
informs the upstream LSR of the mapping. Thus labels are upstream LSR of the binding. Thus labels are "downstream-assigned",
"downstream-assigned", and are "distributed upstream". and label bindings are distributed in the "downstream to upstream"
direction.
A particular mapping of label L to stream S, distributed by Rd to Ru, 2.5. Attributes of a Label Binding
A particular binding of label L to FEC F, distributed by Rd to Ru,
may have associated "attributes". If Ru, acting as a downstream LSR, may have associated "attributes". If Ru, acting as a downstream LSR,
also distributes a mapping of a label to stream S, then under certain also distributes a binding of a label to FEC F, then under certain
conditions, it may be required to also distribute the corresponding conditions, it may be required to also distribute the corresponding
attribute that it received from Rd. attribute that it received from Rd.
2.5. Label Distribution Protocol (LDP) 2.6. Label Distribution Protocol (LDP)
A Label Distribution Protocol (LDP) is a set of procedures by which A Label Distribution Protocol (LDP) is a set of procedures by which
one LSR informs another of the label/Stream mappings it has made. one LSR informs another of the label/FEC bindings it has made. Two
Two LSRs which use an LDP to exchange label/Stream mapping LSRs which use an LDP to exchange label/FEC binding information are
information are known as "LDP Peers" with respect to the mapping known as "LDP Peers" with respect to the binding information they
information they exchange; we will speak of there being an "LDP exchange. If two LSRs are LDP Peers, we will speak of there being an
Adjacency" between them. "LDP Adjacency" between them.
(N.B.: two LSRs may be LDP Peers with respect to some set of (N.B.: two LSRs may be LDP Peers with respect to some set of
mappings, but not with respect to some other set of mappings.) bindings, but not with respect to some other set of bindings.)
The LDP also encompasses any negotiations in which two LDP Peers need The LDP also encompasses any negotiations in which two LDP Peers need
to engage in order to learn of each other's MPLS capabilities. to engage in order to learn of each other's MPLS capabilities.
2.6. The Label Stack 2.7. Downstream vs. Downstream-on-Demand
The MPLS architecture allows an LSR to explicitly request, from its
next hop for a particular FEC, a label binding for that FEC. This is
known as "downstream-on-demand" label distribution.
The MPLS architecture also allows an LSR to distribute bindings to
LSRs that have not explicitly requested them. This is known as
"downstream" label distribution.
Both of these label distribution techniques may be used in the same
network at the same time. However, on any given LDP adjacency, the
upstream LSR and the downstream LSR must agree on which technique is
to be used.
2.8. Label Retention Mode
An LSR Ru may receive (or have received) a label binding for a
particular FEC from an LSR Rd, even though Rd is not Ru's next hop
(or is no longer Ru's next hop) for that FEC.
Ru then has the choice of whether to keep track of such bindings, or
whether to discard such bindings. If Ru keeps track of such
bindings, then it may immediately begin using the binding again if Rd
eventually becomes its next hop for the FEC in question. If Ru
discards such bindings, then if Rd later becomes the next hop, the
binding will have to be reacquired.
If an LSR supports "Liberal Label Retention Mode", it maintains the
bindings between a label and a FEC which are received from LSRs which
are not its next hop for that FEC. If an LSR supports "Conservative
Label Retention Mode", it discards such bindings.
Liberal label retention mode allows for quicker adaptation to routing
changes, especially if loop prevention (see section 2.23) is not
being used. Conservative label retention mode though requires an LSR
to maintain many fewer labels.
2.9. The Label Stack
So far, we have spoken as if a labeled packet carries only a single So far, we have spoken as if a labeled packet carries only a single
label. As we shall see, it is useful to have a more general model in label. As we shall see, it is useful to have a more general model in
which a labeled packet carries a number of labels, organized as a which a labeled packet carries a number of labels, organized as a
last-in, first-out stack. We refer to this as a "label stack". last-in, first-out stack. We refer to this as a "label stack".
IN MPLS, EVERY FORWARDING DECISION IS BASED EXCLUSIVELY ON THE LABEL IN MPLS, EVERY FORWARDING DECISION IS BASED EXCLUSIVELY ON THE LABEL
AT THE TOP OF THE STACK. AT THE TOP OF THE STACK.
Although, as we shall see, MPLS supports a hierarchy, the processing Although, as we shall see, MPLS supports a hierarchy, the processing
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An unlabeled packet can be thought of as a packet whose label stack An unlabeled packet can be thought of as a packet whose label stack
is empty (i.e., whose label stack has depth 0). is empty (i.e., whose label stack has depth 0).
If a packet's label stack is of depth m, we refer to the label at the If a packet's label stack is of depth m, we refer to the label at the
bottom of the stack as the level 1 label, to the label above it (if bottom of the stack as the level 1 label, to the label above it (if
such exists) as the level 2 label, and to the label at the top of the such exists) as the level 2 label, and to the label at the top of the
stack as the level m label. stack as the level m label.
The utility of the label stack will become clear when we introduce The utility of the label stack will become clear when we introduce
the notion of LSP Tunnel and the MPLS Hierarchy (sections 2.21.3 and the notion of LSP Tunnel and the MPLS Hierarchy (section 2.26).
2.21.4).
2.7. The Next Hop Label Forwarding Entry (NHLFE) 2.10. The Next Hop Label Forwarding Entry (NHLFE)
The "Next Hop Label Forwarding Entry" (NHLFE) is used when forwarding The "Next Hop Label Forwarding Entry" (NHLFE) is used when forwarding
a labeled packet. It contains the following information: a labeled packet. It contains the following information:
1. the packet's next hop 1. the packet's next hop
2. the data link encapsulation to use when transmitting the packet 2. the data link encapsulation to use when transmitting the packet
3. the way to encode the label stack when transmitting the packet 3. the way to encode the label stack when transmitting the packet
4. the operation to perform on the packet's label stack; this is 4. the operation to perform on the packet's label stack; this is
one of the following operations: one of the following operations:
a) replace the label at the top of the label stack with a a) replace the label at the top of the label stack with a
specified new label specified new label
b) pop the label stack b) pop the label stack
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make another forwarding decision, based on what remains after the make another forwarding decision, based on what remains after the
label stacked is popped. This may still be a labeled packet, or it label stacked is popped. This may still be a labeled packet, or it
may be the native IP packet. may be the native IP packet.
This implies that in some cases the LSR may need to operate on the IP This implies that in some cases the LSR may need to operate on the IP
header in order to forward the packet. header in order to forward the packet.
If the packet's "next hop" is the current LSR, then the label stack If the packet's "next hop" is the current LSR, then the label stack
operation MUST be to "pop the stack". operation MUST be to "pop the stack".
2.8. Incoming Label Map (ILM) 2.11. Incoming Label Map (ILM)
The "Incoming Label Map" (ILM) is a mapping from incoming labels to The "Incoming Label Map" (ILM) is a mapping from incoming labels to
NHLFEs. It is used when forwarding packets that arrive as labeled NHLFEs. It is used when forwarding packets that arrive as labeled
packets. packets.
2.9. Stream-to-NHLFE Map (STN) 2.12. FEC-to-NHLFE Map (FTN)
The "Stream-to-NHLFE" (STN) is a mapping from stream to NHLFEs. It is The "FEC-to-NHLFE" (FTN) is a mapping from FECs to NHLFEs. It is used
used when forwarding packets that arrive unlabeled, but which are to when forwarding packets that arrive unlabeled, but which are to be
be labeled before being forwarded. labeled before being forwarded.
2.10. Label Swapping 2.13. Label Swapping
Label swapping is the use of the following procedures to forward a Label swapping is the use of the following procedures to forward a
packet. packet.
In order to forward a labeled packet, a LSR examines the label at the In order to forward a labeled packet, a LSR examines the label at the
top of the label stack. It uses the ILM to map this label to an top of the label stack. It uses the ILM to map this label to an
NHLFE. Using the information in the NHLFE, it determines where to NHLFE. Using the information in the NHLFE, it determines where to
forward the packet, and performs an operation on the packet's label forward the packet, and performs an operation on the packet's label
stack. It then encodes the new label stack into the packet, and stack. It then encodes the new label stack into the packet, and
forwards the result. forwards the result.
In order to forward an unlabeled packet, a LSR analyzes the network In order to forward an unlabeled packet, a LSR analyzes the network
layer header, to determine the packet's stream. It then uses the STN layer header, to determine the packet's FEC. It then uses the FTN to
to map this to an NHLFE. Using the information in the NHLFE, it map this to an NHLFE. Using the information in the NHLFE, it
determines where to forward the packet, and performs an operation on determines where to forward the packet, and performs an operation on
the packet's label stack. (Popping the label stack would, of course, the packet's label stack. (Popping the label stack would, of course,
be illegal in this case.) It then encodes the new label stack into be illegal in this case.) It then encodes the new label stack into
the packet, and forwards the result. the packet, and forwards the result.
IT IS IMPORTANT TO NOTE THAT WHEN LABEL SWAPPING IS IN USE, THE NEXT IT IS IMPORTANT TO NOTE THAT WHEN LABEL SWAPPING IS IN USE, THE NEXT
HOP IS ALWAYS TAKEN FROM THE NHLFE; THIS MAY IN SOME CASES BE HOP IS ALWAYS TAKEN FROM THE NHLFE; THIS MAY IN SOME CASES BE
DIFFERENT FROM WHAT THE NEXT HOP WOULD BE IF MPLS WERE NOT IN USE. DIFFERENT FROM WHAT THE NEXT HOP WOULD BE IF MPLS WERE NOT IN USE.
2.11. Scope and Uniqueness of Labels 2.14. Scope and Uniqueness of Labels
A given LSR Rd may map label L1 to stream S, and distribute that A given LSR Rd may bind label L1 to FEC F, and distribute that
mapping to LDP peer Ru1. Rd may also map label L2 to stream S, and binding to LDP peer Ru1. Rd may also bind label L2 to FEC F, and
distribute that mapping to LDP peer Ru2. Whether or not L1 == L2 is distribute that binding to LDP peer Ru2. Whether or not L1 == L2 is
not determined by the architecture; this is a local matter. not determined by the architecture; this is a local matter.
A given LSR Rd may map label L to stream S1, and distribute that A given LSR Rd may bind label L to FEC F1, and distribute that
mapping to LDP peer Ru1. Rd may also map label L to stream S2, and binding to LDP peer Ru1. Rd may also bind label L to FEC F2, and
distribute that mapping to LDP peer Ru2. IF (AND ONLY IF) RD CAN distribute that binding to LDP peer Ru2. IF (AND ONLY IF) RD CAN
TELL, WHEN IT RECEIVES A PACKET WHOSE TOP LABEL IS L, WHETHER THE TELL, WHEN IT RECEIVES A PACKET WHOSE TOP LABEL IS L, WHETHER THE
LABEL WAS PUT THERE BY RU1 OR BY RU2, THEN THE ARCHITECTURE DOES NOT LABEL WAS PUT THERE BY RU1 OR BY RU2, THEN THE ARCHITECTURE DOES NOT
REQUIRE THAT S1 == S2. In general, Rd can only tell whether it was REQUIRE THAT F1 == F2.
Ru1 or Ru2 that put the particular label value L at the top of the
label stack if the following conditions hold:
- Ru1 and Ru2 are the only LDP peers to which Rd distributed a In general, Rd can only tell whether it was Ru1 or Ru2 that put the
mapping of label value L, and particular label value L at the top of the label stack if the
following conditions hold:
- Ru1 and Ru2 are the only LDP peers to which Rd distributed a
binding of label value L, and
- Ru1 and Ru2 are each directly connected to Rd via a point-to- - Ru1 and Ru2 are each directly connected to Rd via a point-to-
point interface. point interface.
When these conditions hold, an LSR may use labels that have "per When these conditions hold, an LSR may use labels that have "per
interface" scope, i.e., which are only unique per interface. When interface" scope, i.e., which are only unique per interface. When
these conditions do not hold, the labels must be unique over the LSR these conditions do not hold, the labels must be unique over the LSR
which has assigned them. which has assigned them.
If a particular LSR Rd is attached to a particular LSR Ru over two If a particular LSR Rd is attached to a particular LSR Ru over two
point-to-point interfaces, then Rd may distribute to Rd a mapping of point-to-point interfaces, then Rd may distribute to Rd a binding of
label L to stream S1, as well as a mapping of label L to stream S2, label L to FEC F1, as well as a binding of label L to FEC F2, F1 !=
S1 != S2, if and only if each mapping is valid only for packets which F2, if and only if each binding is valid only for packets which Ru
Ru sends to Rd over a particular one of the interfaces. In all other sends to Rd over a particular one of the interfaces. In all other
cases, Rd MUST NOT distribute to Ru mappings of the same label value cases, Rd MUST NOT distribute to Ru bindings of the same label value
to two different streams. to two different FECs.
This prohibition holds even if the mappings are regarded as being at This prohibition holds even if the bindings are regarded as being at
different "levels of hierarchy". In MPLS, there is no notion of different "levels of hierarchy". In MPLS, there is no notion of
having a different label space for different levels of the hierarchy. having a different label space for different levels of the hierarchy;
when interpreting a label, the level of the label is irrelevant.
2.12. Label Switched Path (LSP), LSP Ingress, LSP Egress 2.15. Label Switched Path (LSP), LSP Ingress, LSP Egress
A "Label Switched Path (LSP) of level m" for a particular packet P is A "Label Switched Path (LSP) of level m" for a particular packet P is
a sequence of routers, a sequence of routers,
<R1, ..., Rn> <R1, ..., Rn>
with the following properties: with the following properties:
1. R1, the "LSP Ingress", is an LSR which pushes a label onto P's 1. R1, the "LSP Ingress", is an LSR which pushes a label onto P's
label stack, resulting in a label stack of depth m; label stack, resulting in a label stack of depth m;
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3. which ends (at an "LSP Egress") when a forwarding decision is 3. which ends (at an "LSP Egress") when a forwarding decision is
made by label Switching on a level m-k label, where k>0, or made by label Switching on a level m-k label, where k>0, or
when a forwarding decision is made by "ordinary", non-MPLS when a forwarding decision is made by "ordinary", non-MPLS
forwarding procedures. forwarding procedures.
A consequence (or perhaps a presupposition) of this is that whenever A consequence (or perhaps a presupposition) of this is that whenever
an LSR pushes a label onto an already labeled packet, it needs to an LSR pushes a label onto an already labeled packet, it needs to
make sure that the new label corresponds to a FEC whose LSP Egress is make sure that the new label corresponds to a FEC whose LSP Egress is
the LSR that assigned the label which is now second in the stack. the LSR that assigned the label which is now second in the stack.
We will call a sequence of LSRs the "LSP for a particular stream S" We will call a sequence of LSRs the "LSP for a particular FEC F" if
if it is an LSP of level m for a particular packet P when P's level m it is an LSP of level m for a particular packet P when P's level m
label is a label corresponding to stream S. label is a label corresponding to FEC F.
Consider the set of nodes which may be LSP ingress nodes for stream Consider the set of nodes which may be LSP ingress nodes for FEC F.
S. Then there is an LSP for stream S which begins with each of those Then there is an LSP for FEC F which begins with each of those nodes.
nodes. If a number of those LSPs have the same LSP egress, then one If a number of those LSPs have the same LSP egress, then one can
can consider the set of such LSPs to be a tree, whose root is the LSP consider the set of such LSPs to be a tree, whose root is the LSP
egress. (Since data travels along this tree towards the root, this egress. (Since data travels along this tree towards the root, this
may be called a multipoint-to-point tree.) We can thus speak of the may be called a multipoint-to-point tree.) We can thus speak of the
"LSP tree" for a particular stream S. "LSP tree" for a particular FEC F.
2.13. Penultimate Hop Popping 2.16. Penultimate Hop Popping
Note that according to the definitions of section 2.11, if <R1, ..., Note that according to the definitions of section 2.15, if <R1, ...,
Rn> is a level m LSP for packet P, P may be transmitted from R[n-1] Rn> is a level m LSP for packet P, P may be transmitted from R[n-1]
to Rn with a label stack of depth m-1. That is, the label stack may to Rn with a label stack of depth m-1. That is, the label stack may
be popped at the penultimate LSR of the LSP, rather than at the LSP be popped at the penultimate LSR of the LSP, rather than at the LSP
Egress. Egress.
From an architectural perspective, this is perfectly appropriate. From an architectural perspective, this is perfectly appropriate.
The purpose of the level m label is to get the packet to Rn. Once The purpose of the level m label is to get the packet to Rn. Once
R[n-1] has decided to send the packet to Rn, the label no longer has R[n-1] has decided to send the packet to Rn, the label no longer has
any function, and need no longer be carried. any function, and need no longer be carried.
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require the egress to do TWO lookups, either two label lookups or a require the egress to do TWO lookups, either two label lookups or a
label lookup followed by an address lookup. label lookup followed by an address lookup.
If, on the other hand, penultimate hop popping is used, then when the If, on the other hand, penultimate hop popping is used, then when the
penultimate hop looks up the label, it determines: penultimate hop looks up the label, it determines:
- that it is the penultimate hop, and - that it is the penultimate hop, and
- who the next hop is. - who the next hop is.
The penultimate node then pops the stack, and forward the packet The penultimate node then pops the stack, and forwards the packet
based on the information gained by looking up the label that was at based on the information gained by looking up the label that was
the top of the stack. When the LSP egress receives the packet, the previously at the top of the stack. When the LSP egress receives the
label at the top of the stack will be the label which it needs to packet, the label which is now at the top of the stack will be the
look up in order to make its own forwarding decision. Or, if the label which it needs to look up in order to make its own forwarding
packet was only carrying a single label, the LSP egress will simply decision. Or, if the packet was only carrying a single label, the
see the network layer packet, which is just what it needs to see in LSP egress will simply see the network layer packet, which is just
order to make its forwarding decision. what it needs to see in order to make its forwarding decision.
This technique allows the egress to do a single lookup, and also This technique allows the egress to do a single lookup, and also
requires only a single lookup by the penultimate node. requires only a single lookup by the penultimate node.
The creation of the forwarding fastpath in a label switching product The creation of the forwarding "fastpath" in a label switching
may be greatly aided if it is known that only a single lookup is product may be greatly aided if it is known that only a single lookup
every required: is ever required:
- the code may be simplified if it can assume that only a single - the code may be simplified if it can assume that only a single
lookup is ever needed lookup is ever needed
- the code can be based on a "time budget" that assumes that only a - the code can be based on a "time budget" that assumes that only a
single lookup is ever needed. single lookup is ever needed.
In fact, when penultimate hop popping is done, the LSP Egress need In fact, when penultimate hop popping is done, the LSP Egress need
not even be an LSR. not even be an LSR.
However, some hardware switching engines may not be able to pop the However, some hardware switching engines may not be able to pop the
label stack, so this cannot be universally required. There may also label stack, so this cannot be universally required. There may also
be some situations in which penultimate hop popping is not desirable. be some situations in which penultimate hop popping is not desirable.
Therefore the penultimate node pops the label stack only if this is Therefore the penultimate node pops the label stack only if this is
specifically requested by the egress node, or if the next node in the specifically requested by the egress node, OR if the next node in the
LSP does not support MPLS. (If the next node in the LSP does support LSP does not support MPLS. (If the next node in the LSP does support
MPLS, but does not make such a request, the penultimate node has no MPLS, but does not make such a request, the penultimate node has no
way of knowing that it in fact is the penultimate node.) way of knowing that it in fact is the penultimate node.)
An LSR which is capable of popping the label stack at all MUST do An LSR which is capable of popping the label stack at all MUST do
penultimate hop popping when so requested by its downstream LDP peer. penultimate hop popping when so requested by its downstream LDP peer.
Initial LDP negotiations must allow each LSR to determine whether its Initial LDP negotiations MUST allow each LSR to determine whether its
neighboring LSRS are capable of popping the label stack. A LSR will neighboring LSRS are capable of popping the label stack. A LSR MUST
not request an LDP peer to pop the label stack unless it is capable NOT request an LDP peer to pop the label stack unless it is capable
of doing so. of doing so.
It may be asked whether the egress node can always interpret the top It may be asked whether the egress node can always interpret the top
label of a received packet properly if penultimate hop popping is label of a received packet properly if penultimate hop popping is
used. As long as the uniqueness and scoping rules of section 2.11 used. As long as the uniqueness and scoping rules of section 2.14
are obeyed, it is always possible to interpret the top label of a are obeyed, it is always possible to interpret the top label of a
received packet unambiguously. received packet unambiguously.
2.14. LSP Next Hop 2.17. LSP Next Hop
The LSP Next Hop for a particular labeled packet in a particular LSR The LSP Next Hop for a particular labeled packet in a particular LSR
is the LSR which is the next hop, as selected by the NHLFE entry used is the LSR which is the next hop, as selected by the NHLFE entry used
for forwarding that packet. for forwarding that packet.
The LSP Next Hop for a particular stream is the next hop as selected The LSP Next Hop for a particular FEC is the next hop as selected by
by the NHLFE entry indexed by a label which corresponds to that the NHLFE entry indexed by a label which corresponds to that FEC.
stream.
Note that the LSP Next Hop may differ from the next hop which would Note that the LSP Next Hop may differ from the next hop which would
be chosen by the network layer routing algorithm. We will use the be chosen by the network layer routing algorithm. We will use the
term "L3 next hop" when we refer to the latter. term "L3 next hop" when we refer to the latter.
2.15. Route Selection 2.18. Invalid Incoming Labels
What should an LSR do if it receives a labeled packet with a
particular incoming label, but has no binding for that label? It is
tempting to think that the labels can just be removed, and the packet
forwarded as an unlabeled IP packet. However, in some cases, doing
so could cause a loop. If the upstream LSR thinks the label is bound
to an explicit route, and the downstream LSR doesn't think the label
is bound to anything, and if the hop by hop routing of the unlabeled
IP packet brings the packet back to the upstream LSR, then a loop is
formed.
It is also possible that the label was intended to represent a route
which the cannot be inferred the IP header.
Therefore, when a labeled packet is received with an invalid incoming
label, it MUST be discarded, UNLESS it is determined by some means
(not within the scope of the current document) that forwarding it
unlabeled cannot cause any harm.
2.19. LSP Control: Ordered versus Independent
Some FECs correspond to address prefixes which are distributed via a
dynamic routing algorithm. The setup of the LSPs for these FECs can
be done in one of two ways: Independent LSP Control or Ordered LSP
Control.
In Independent LSP Control, each LSR, upon noting that it recognizes
a particular FEC, makes an independent decision to bind a label to
that FEC and to distribute that binding to its LDP peers. This
corresponds to the way that conventional IP datagram routing works;
each node makes an independent decision as to how to treat each
packet, and relies on the routing algorithm to converge rapidly so as
to ensure that each datagram is correctly delivered.
In Ordered LSP Control, an LSR only binds a label to a particular FEC
if it is the egress LSR for that FEC, or if it has already received a
label binding for that FEC from its next hop for that FEC.
If one wants to ensure that traffic in a particular FEC follows a
path with some specified set of properties (e.g., that the traffic
does not traverse any node twice, that a specified amount of
resources are available to the traffic, that the traffic follows an
explicitly specified path, etc.) ordered control must be used. With
independent control, some LSRs may begin label switching a traffic in
the FEC before the LSP is completely set up, and thus some traffic in
the FEC may follow a path which does not have the specified set of
properties. Ordered control also needs to be used if the recognition
of the FEC is a consequence of the setting up of the corresponding
LSP.
Ordered LSP setup may be initiated either by the ingress or the
egress.
Ordered control and independent control are fully interoperable.
However, unless all LSRs in an LSP are using ordered control, the
overall effect on network behavior is largely that of independent
control, since one cannot be sure that an LSP is not used until it is
fully set up.
This architecture allows the choice between independent control and
ordered control to be a local matter. Since the two methods
interwork, a given LSR need support only one or the other. Generally
speaking, the choice of independent versus ordered control does not
appear to have any effect on the LDP mechanisms which need to be
defined.
2.20. Aggregation
One way of partitioning traffic into FECs is to create a separate FEC
for each address prefix which appears in the routing table. However,
within a particular MPLS domain, this may result in a set of FECs
such that all traffic in all those FECs follows the same route. For
example, a set of distinct address prefixes might all have the same
egress node, and label swapping might be used only to get the the
traffic to the egress node. In this case, within the MPLS domain,
the union of those FECs is itself a FEC. This creates a choice:
should a distinct label be bound to each component FEC, or should a
single label be bound to the union, and that label applied to all
traffic in the union?
The procedure of binding a single label to a union of FECs which is
itself a FEC (within some domain), and of applying that label to all
traffic in the union, is known as "aggregation". The MPLS
architecture allows aggregation. Aggregation may reduce the number
of labels which are needed to handle a particular set of packets, and
may also reduce the amount of LDP control traffic needed.
Given a set of FECs which are "aggregatable" into a single FEC, it is
possible to (a) aggregate them into a single FEC, (b) aggregate them
into a set of FECs, or (c) not aggregate them at all. Thus we can
speak of the "granularity" of aggregation, with (a) being the
"coarsest granularity", and (c) being the "finest granularity".
When order control is used, each LSR should adopt, for a given set of
FECs, the granularity used by its next hop for those FECs.
When independent control is used, it is possible that there will be
two adjacent LSRs, Ru and Rd, which aggregate some set of FECs
differently.
If Ru has finer granularity than Rd, this does not cause a problem.
Ru distributes more labels for that set of FECs than Rd does. This
means that when Ru needs to forward labeled packets in those FECs to
Rd, it may need to map n labels into m labels, where n > m. As an
option, Ru may withdraw the set of n labels that it has distributed,
and then distribute a set of m labels, corresponding to Rd's level of
granularity. This is not necessary to ensure correct operation, but
it does result in a reduction of the number of labels distributed by
Ru, and Ru is not gaining any particular advantage by distributing
the larger number of labels. The decision whether to do this or not
is a local matter.
If Ru has coarser granularity than Rd (i.e., Rd has distributed n
labels for the set of FECs, while Ru has distributed m, where n > m),
it has two choices:
- It may adopt Rd's finer level of granularity. This would require
it to withdraw the m labels it has distributed, and distribute n
labels. This is the preferred option.
- It may simply map its m labels into a subset of Rd's n labels, if
it can determine that this will produce the same routing. For
example, suppose that Ru applies a single label to all traffic
that needs to pass through a certain egress LSR, whereas Rd binds
a number of different labels to such traffic, depending on the
individual destination addresses of the packets. If Ru knows the
address of the egress router, and if Rd has bound a label to the
FEC which is identified by that address, then Ru can simply apply
that label.
In any event, every LSR needs to know (by configuration) what
granularity to use for labels that it assigns. Where ordered control
is used, this requires each node to know the granularity only for
FECs which leave the MPLS network at that node. For independent
control, best results may be obtained by ensuring that all LSRs are
consistently configured to know the granularity for each FEC.
However, in many cases this may be done by using a single level of
granularity which applies to all FECs (such as "one label per IP
prefix in the forwarding table", or "one label per egress node").
2.21. Route Selection
Route selection refers to the method used for selecting the LSP for a Route selection refers to the method used for selecting the LSP for a
particular stream. The proposed MPLS protocol architecture supports particular FEC. The proposed MPLS protocol architecture supports two
two options for Route Selection: (1) Hop by hop routing, and (2) options for Route Selection: (1) hop by hop routing, and (2) explicit
Explicit routing. routing.
Hop by hop routing allows each node to independently choose the next Hop by hop routing allows each node to independently choose the next
hop for the path for a stream. This is the normal mode today with hop for each FEC. This is the usual mode today in existing IP
existing datagram IP networks. A hop by hop routed LSP refers to an networks. A "hop by hop routed LSP" is an LSP whose route is selected
LSP whose route is selected using hop by hop routing. using hop by hop routing.
An explicitly routed LSP is an LSP where, at a given LSR, the LSP In an explicitly routed LSP, each LSR does not independently choose
next hop is not chosen by each local node, but rather is chosen by a the next hop; rather, a single LSR, generally the LSP ingress or the
single node (usually the ingress or egress node of the LSP). The LSP egress, specifies several (or all) of the LSRs in the LSP. If a
sequence of LSRs followed by an explicitly routed LSP may be chosen single LSR specifies the entire LSP, the LSP is "strictly" explicitly
by configuration, or may be selected dynamically by a single node routed. If a single LSR specifies only some of the LSP, the LSP is
(for example, the egress node may make use of the topological "loosely" explicitly routed.
The sequence of LSRs followed by an explicitly routed LSP may be
chosen by configuration, or may be selected dynamically by a single
node (for example, the egress node may make use of the topological
information learned from a link state database in order to compute information learned from a link state database in order to compute
the entire path for the tree ending at that egress node). Explicit the entire path for the tree ending at that egress node).
routing may be useful for a number of purposes such as allowing
policy routing and/or facilitating traffic engineering. With MPLS
the explicit route needs to be specified at the time that labels are
assigned, but the explicit route does not have to be specified with
each IP packet. This implies that explicit routing with MPLS is
relatively efficient (when compared with the efficiency of explicit
routing for pure datagrams).
For any one LSP (at any one level of hierarchy), there are two Explicit routing may be useful for a number of purposes such as
possible options: (i) The entire LSP may be hop by hop routed from policy routing or traffic engineering. With MPLS the explicit route
ingress to egress; (ii) The entire LSP may be explicit routed from needs to be specified at the time that labels are assigned, but the
ingress to egress. Intermediate cases do not make sense: In general, explicit route does not have to be specified with each IP packet.
an LSP will be explicit routed specifically because there is a good This makes MPLS explicit routing much more efficient than the
reason to use an alternative to the hop by hop routed path. This alternative of IP source routing.
implies that if some of the nodes along the path follow an explicit
route but some of the nodes make use of hop by hop routing, then
inconsistent routing will result and loops (or severely inefficient
paths) may form.
For this reason, it is important that if an explicit route is When an LSP is explicitly routed (either loosely or strictly), it is
specified for an LSP, then that route must be followed. Note that it essential that packets travelling along the LSP reach its end before
is relatively simple to *follow* an explicit route which is specified they revert to hop by hop routing. Otherwise inconsistent routing
in a LDP setup. We therefore propose that the LDP specification and loops might form.
require that all MPLS nodes implement the ability to follow an
explicit route if this is specified.
It is not necessary for a node to be able to create an explicit It is not necessary for a node to be able to create an explicit
route. However, in order to ensure interoperability it is necessary route. However, in order to ensure interoperability it is necessary
to ensure that either (i) Every node knows how to use hop by hop to ensure that either (i) Every node knows how to use hop by hop
routing; or (ii) Every node knows how to create and follow an routing; or (ii) Every node knows how to create and follow an
explicit route. We propose that due to the common use of hop by hop explicit route. We propose that due to the common use of hop by hop
routing in networks today, it is reasonable to make hop by hop routing in networks today, it is reasonable to make hop by hop
routing the default that all nodes need to be able to use. routing the default that all nodes need to be able to use.
2.16. Time-to-Live (TTL) 2.22. Time-to-Live (TTL)
In conventional IP forwarding, each packet carries a "Time To Live" In conventional IP forwarding, each packet carries a "Time To Live"
(TTL) value in its header. Whenever a packet passes through a (TTL) value in its header. Whenever a packet passes through a
router, its TTL gets decremented by 1; if the TTL reaches 0 before router, its TTL gets decremented by 1; if the TTL reaches 0 before
the packet has reached its destination, the packet gets discarded. the packet has reached its destination, the packet gets discarded.
This provides some level of protection against forwarding loops that This provides some level of protection against forwarding loops that
may exist due to misconfigurations, or due to failure or slow may exist due to misconfigurations, or due to failure or slow
convergence of the routing algorithm. TTL is sometimes used for other convergence of the routing algorithm. TTL is sometimes used for other
functions as well, such as multicast scoping, and supporting the functions as well, such as multicast scoping, and supporting the
"traceroute" command. This implies that there are two TTL-related "traceroute" command. This implies that there are two TTL-related
issues that MPLS needs to deal with: (i) TTL as a way to suppress issues that MPLS needs to deal with: (i) TTL as a way to suppress
loops; (ii) TTL as a way to accomplish other functions, such as loops; (ii) TTL as a way to accomplish other functions, such as
limiting the scope of a packet. limiting the scope of a packet.
When a packet travels along an LSP, it should emerge with the same When a packet travels along an LSP, it SHOULD emerge with the same
TTL value that it would have had if it had traversed the same TTL value that it would have had if it had traversed the same
sequence of routers without having been label switched. If the sequence of routers without having been label switched. If the
packet travels along a hierarchy of LSPs, the total number of LSR- packet travels along a hierarchy of LSPs, the total number of LSR-
hops traversed should be reflected in its TTL value when it emerges hops traversed SHOULD be reflected in its TTL value when it emerges
from the hierarchy of LSPs. from the hierarchy of LSPs.
The way that TTL is handled may vary depending upon whether the MPLS The way that TTL is handled may vary depending upon whether the MPLS
label values are carried in an MPLS-specific "shim" header, or if the label values are carried in an MPLS-specific "shim" header, or if the
MPLS labels are carried in an L2 header such as an ATM header or a MPLS labels are carried in an L2 header, such as an ATM header or a
frame relay header. frame relay header.
If the label values are encoded in a "shim" that sits between the If the label values are encoded in a "shim" that sits between the
data link and network layer headers, then this shim should have a TTL data link and network layer headers, then this shim MUST have a TTL
field that is initially loaded from the network layer header TTL field that SHOULD be initially loaded from the network layer header
field, is decremented at each LSR-hop, and is copied into the network TTL field, SHOULD be decremented at each LSR-hop, and SHOULD be
layer header TTL field when the packet emerges from its LSP. copied into the network layer header TTL field when the packet
emerges from its LSP.
If the label values are encoded in an L2 header (e.g., the VPI/VCI If the label values are encoded in a data link layer header (e.g.,
field in ATM's AAL5 header), and the labeled packets are forwarded by the VPI/VCI field in ATM's AAL5 header), and the labeled packets are
an L2 switch (e.g., an ATM switch). This implies that unless the data forwarded by an L2 switch (e.g., an ATM switch), and the data link
link layer itself has a TTL field (unlike ATM), it will not be layer (like ATM) does not itself have a TTL field, then it will not
possible to decrement a packet's TTL at each LSR-hop. An LSP segment be possible to decrement a packet's TTL at each LSR-hop. An LSP
which consists of a sequence of LSRs that cannot decrement a packet's segment which consists of a sequence of LSRs that cannot decrement a
TTL will be called a "non-TTL LSP segment". packet's TTL will be called a "non-TTL LSP segment".
When a packet emerges from a non-TTL LSP segment, it should however When a packet emerges from a non-TTL LSP segment, it SHOULD however
be given a TTL that reflects the number of LSR-hops it traversed. In be given a TTL that reflects the number of LSR-hops it traversed. In
the unicast case, this can be achieved by propagating a meaningful the unicast case, this can be achieved by propagating a meaningful
LSP length to ingress nodes, enabling the ingress to decrement the LSP length to ingress nodes, enabling the ingress to decrement the
TTL value before forwarding packets into a non-TTL LSP segment. TTL value before forwarding packets into a non-TTL LSP segment.
Sometimes it can be determined, upon ingress to a non-TTL LSP Sometimes it can be determined, upon ingress to a non-TTL LSP
segment, that a particular packet's TTL will expire before the packet segment, that a particular packet's TTL will expire before the packet
reaches the egress of that non-TTL LSP segment. In this case, the LSR reaches the egress of that non-TTL LSP segment. In this case, the LSR
at the ingress to the non-TTL LSP segment must not label switch the at the ingress to the non-TTL LSP segment must not label switch the
packet. This means that special procedures must be developed to packet. This means that special procedures must be developed to
support traceroute functionality, for example, traceroute packets may support traceroute functionality, for example, traceroute packets may
be forwarded using conventional hop by hop forwarding. be forwarded using conventional hop by hop forwarding.
2.17. Loop Control 2.23. Loop Control
On a non-TTL LSP segment, by definition, TTL cannot be used to On a non-TTL LSP segment, by definition, TTL cannot be used to
protect against forwarding loops. The importance of loop control may protect against forwarding loops. The importance of loop control may
depend on the particular hardware being used to provide the LSR depend on the particular hardware being used to provide the LSR
functions along the non-TTL LSP segment. functions along the non-TTL LSP segment.
Suppose, for instance, that ATM switching hardware is being used to Suppose, for instance, that ATM switching hardware is being used to
provide MPLS switching functions, with the label being carried in the provide MPLS switching functions, with the label being carried in the
VPI/VCI field. Since ATM switching hardware cannot decrement TTL, VPI/VCI field. Since ATM switching hardware cannot decrement TTL,
there is no protection against loops. If the ATM hardware is capable there is no protection against loops. If the ATM hardware is capable
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The MPLS architecture will therefore provide a technique for ensuring The MPLS architecture will therefore provide a technique for ensuring
that looping LSP segments can be detected, and a technique for that looping LSP segments can be detected, and a technique for
ensuring that looping LSP segments are never created. ensuring that looping LSP segments are never created.
All LSRs will be required to support a common technique for loop All LSRs will be required to support a common technique for loop
detection. Support for the loop prevention technique is optional, detection. Support for the loop prevention technique is optional,
though it is recommended in ATM-LSRs that have no other way to though it is recommended in ATM-LSRs that have no other way to
protect themselves against the effects of looping data packets. Use protect themselves against the effects of looping data packets. Use
of the loop prevention technique, when supported, is optional. of the loop prevention technique, when supported, is optional.
2.17.1. Loop Prevention The loop prevention technique presupposes the use of Ordered LSP
Control. The loop detection technique, on the other hand, works with
either Independent or Ordered LSP Control.
2.23.1. Loop Prevention
NOTE: The loop prevention technique described here is being NOTE: The loop prevention technique described here is being
reconsidered, and may be changed. reconsidered, and may be changed.
LSR's maintain for each of their LSP's an LSR id list. This list is a LSR's maintain for each of their LSP's an LSR id list. This list is a
list of all the LSR's downstream from this LSR on a given LSP. The list of all the LSR's downstream from this LSR on a given LSP. The
LSR id list is used to prevent the formation of switched path loops. LSR id list is used to prevent the formation of switched path loops.
The LSR ID list is propagated upstream from a node to its neighbor The LSR ID list is propagated upstream from a node to its neighbor
nodes. The LSR ID list is used to prevent loops as follows: nodes. The LSR ID list is used to prevent loops as follows:
When a node, R, detects a change in the next hop for a given stream, When a node, R, detects a change in the next hop for a given FEC, it
it asks its new next hop for a label and the associated LSR ID list asks its new next hop for a label and the associated LSR ID list for
for that stream. that FEC.
The new next hop responds with a label for the stream and an The new next hop responds with a label for the FEC and an associated
associated LSR id list. LSR id list.
R looks in the LSR id list. If R determines that it, R, is in the R looks in the LSR id list. If R determines that it, R, is in the
list then we have a route loop. In this case, we do nothing and the list then we have a route loop. In this case, we do nothing and the
old LSP will continue to be used until the route protocols break the old LSP will continue to be used until the route protocols break the
loop. The means by which the old LSP is replaced by a new LSP after loop. The means by which the old LSP is replaced by a new LSP after
the route protocols breathe loop is described below. the route protocols breathe loop is described below.
If R is not in the LSR id list, R will start a "diffusion" If R is not in the LSR id list, R will start a "diffusion"
computation [12]. The purpose of the diffusion computation is to computation [12]. The purpose of the diffusion computation is to
prune the tree upstream of R so that we remove all LSR's from the prune the tree upstream of R so that we remove all LSR's from the
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The diffusion computation works as follows: The diffusion computation works as follows:
R adds its LSR id to the list and sends a query message to each of R adds its LSR id to the list and sends a query message to each of
its "upstream" neighbors (i.e. to each of its neighbors that is not its "upstream" neighbors (i.e. to each of its neighbors that is not
the new "downstream" next hop). the new "downstream" next hop).
A node S that receives such a query will process the query as A node S that receives such a query will process the query as
follows: follows:
- If node R is not node S's next hop for the given stream, node S - If node R is not node S's next hop for the given FEC, node S will
will respond to node R will an "OK" message meaning that as far respond to node R will an "OK" message meaning that as far as
as node S is concerned it is safe for node R to switch over to node S is concerned it is safe for node R to switch over to the
the new LSP. new LSP.
- If node R is node S's next hop for the stream, node S will check - If node R is node S's next hop for the FEC, node S will check to
to see if it, node S, is in the LSR id list that it received from see if it, node S, is in the LSR id list that it received from
node R. If it is, we have a route loop and S will respond with a node R. If it is, we have a route loop and S will respond with a
"LOOP" message. R will unsplice the connection to S pruning S "LOOP" message. R will unsplice the connection to S pruning S
from the tree. The mechanism by which S will get a new LSP for from the tree. The mechanism by which S will get a new LSP for
the stream after the route protocols break the loop is described the FEC after the route protocols break the loop is described
below. below.
- If node S is not in the LSR id list, S will add its LSR id to the - If node S is not in the LSR id list, S will add its LSR id to the
LSR id list and send a new query message further upstream. The LSR id list and send a new query message further upstream. The
diffusion computation will continue to propagate upstream along diffusion computation will continue to propagate upstream along
each of the paths in the tree upstream of S until either a loop each of the paths in the tree upstream of S until either a loop
is detected, in which case the node is pruned as described above is detected, in which case the node is pruned as described above
or we get to a point where a node gets a response ("OK" or or we get to a point where a node gets a response ("OK" or
"LOOP") from each of its neighbors perhaps because none of those "LOOP") from each of its neighbors perhaps because none of those
neighbors considers the node in question to be its downstream neighbors considers the node in question to be its downstream
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- Note that when a node is pruned from the tree, the switched path - Note that when a node is pruned from the tree, the switched path
upstream of that node remains "connected". This is important upstream of that node remains "connected". This is important
since it allows the switched path to get "reconnected" to a since it allows the switched path to get "reconnected" to a
downstream switched path after a route change with a minimal downstream switched path after a route change with a minimal
amount of unsplicing and resplicing once the appropriate amount of unsplicing and resplicing once the appropriate
diffusion computation(s) have taken place. diffusion computation(s) have taken place.
The LSR Id list can also be used to provide a "loop detection" The LSR Id list can also be used to provide a "loop detection"
capability. To use it in this manner, an LSR which sees that it is capability. To use it in this manner, an LSR which sees that it is
already in the LSR Id list for a particular stream will immediately already in the LSR Id list for a particular FEC will immediately
unsplice itself from the switched path for that stream, and will NOT unsplice itself from the switched path for that FEC, and will NOT
pass the LSR Id list further upstream. The LSR can rejoin a switched pass the LSR Id list further upstream. The LSR can rejoin a switched
path for the stream when it changes its next hop for that stream, or path for the FEC when it changes its next hop for that FEC, or when
when it receives a new LSR Id list from its current next hop, in it receives a new LSR Id list from its current next hop, in which it
which it is not contained. The diffusion computation would be is not contained. The diffusion computation would be omitted.
omitted.
2.17.2. Interworking of Loop Control Options 2.23.2. Interworking of Loop Control Options
The MPLS protocol architecture allows some nodes to be using loop The MPLS protocol architecture allows some nodes to be using loop
prevention, while some other nodes are not (i.e., the choice of prevention, while some other nodes are not (i.e., the choice of
whether or not to use loop prevention may be a local decision). When whether or not to use loop prevention may be a local decision). When
this mix is used, it is not possible for a loop to form which this mix is used, it is not possible for a loop to form which
includes only nodes which do loop prevention. However, it is possible includes only nodes which do loop prevention. However, it is possible
for loops to form which contain a combination of some nodes which do for loops to form which contain a combination of some nodes which do
loop prevention, and some nodes which do not. loop prevention, and some nodes which do not.
There are at least four identified cases in which it makes sense to There are at least four identified cases in which it makes sense to
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interoperability, where one vendor implements loop prevention but interoperability, where one vendor implements loop prevention but
another vendor does not; (iii) Where there is a mixed ATM and another vendor does not; (iii) Where there is a mixed ATM and
datagram media network, and where loop prevention is desired over the datagram media network, and where loop prevention is desired over the
ATM portions of the network but not over the datagram portions; (iv) ATM portions of the network but not over the datagram portions; (iv)
where some of the ATM switches can do fair access to the buffer pool where some of the ATM switches can do fair access to the buffer pool
on a per-VC basis, and some cannot, and loop prevention is desired on a per-VC basis, and some cannot, and loop prevention is desired
over the ATM portions of the network which cannot. over the ATM portions of the network which cannot.
Note that interworking is straightforward. If an LSR is not doing Note that interworking is straightforward. If an LSR is not doing
loop prevention, and it receives from a downstream LSR a label loop prevention, and it receives from a downstream LSR a label
mapping which contains loop prevention information, it (a) accepts binding which contains loop prevention information, it (a) accepts
the label mapping, (b) does NOT pass the loop prevention information the label binding, (b) does NOT pass the loop prevention information
upstream, and (c) informs the downstream neighbor that the path is upstream, and (c) informs the downstream neighbor that the path is
loop-free. loop-free.
Similarly, if an LSR R which is doing loop prevention receives from a Similarly, if an LSR R which is doing loop prevention receives from a
downstream LSR a label mapping which does not contain any loop downstream LSR a label binding which does not contain any loop
prevention information, then R passes the label mapping upstream with prevention information, then R passes the label binding upstream with
loop prevention information included as if R were the egress for the loop prevention information included as if R were the egress for the
specified stream. specified FEC.
Optionally, a node is permitted to implement the ability of either Optionally, a node is permitted to implement the ability of either
doing or not doing loop prevention as options, and is permitted to doing or not doing loop prevention as options, and is permitted to
choose which to use for any one particular LSP based on the choose which to use for any one particular LSP based on the
information obtained from downstream nodes. When the label mapping information obtained from downstream nodes. When the label binding
arrives from downstream, then the node may choose whether to use loop arrives from downstream, then the node may choose whether to use loop
prevention so as to continue to use the same approach as was used in prevention so as to continue to use the same approach as was used in
the information passed to it. Note that regardless of whether loop the information passed to it. Note that regardless of whether loop
prevention is used the egress nodes (for any particular LSP) always prevention is used the egress nodes (for any particular LSP) always
initiates exchange of label mapping information without waiting for initiates exchange of label binding information without waiting for
other nodes to act. other nodes to act.
2.18. Merging and Non-Merging LSRs 2.24. Label Encodings
Merge allows multiple upstream LSPs to be merged into a single In order to transmit a label stack along with the packet whose label
downstream LSP. When implemented by multiple nodes, this results in stack it is, it is necessary to define a concrete encoding of the
the traffic going to a particular egress nodes, based on one label stack. The architecture supports several different encoding
particular stream, to follow a multipoint to point tree (MPT), with techniques; the choice of encoding technique depends on the
the MPT rooted at the egress node and associated with the stream. particular kind of device being used to forward labeled packets.
This can have a significant effect on reducing the number of labels
that need to be maintained by any one particular node.
If merge was not used at all it would be necessary for each node to 2.24.1. MPLS-specific Hardware and/or Software
provide the upstream neighbors with a label for each stream for each
upstream node which may be forwarding traffic over the link. This
implies that the number of labels needed might not in general be
known a priori. However, the use of merge allows a single label to be
used per stream, therefore allowing label assignment to be done in a
common way without regard for the number of upstream nodes which will
be using the downstream LSP.
The proposed MPLS protocol architecture supports LSP merge, while If one is using MPLS-specific hardware and/or software to forward
allowing nodes which do not support LSP merge. This leads to the labeled packets, the most obvious way to encode the label stack is to
issue of ensuring correct interoperation between nodes which define a new protocol to be used as a "shim" between the data link
implement merge and those which do not. The issue is somewhat layer and network layer headers. This shim would really be just an
different in the case of datagram media versus the case of ATM. The encapsulation of the network layer packet; it would be "protocol-
different media types will therefore be discussed separately. independent" such that it could be used to encapsulate any network
layer. Hence we will refer to it as the "generic MPLS
encapsulation".
2.18.1. Stream Merge The generic MPLS encapsulation would in turn be encapsulated in a
data link layer protocol.
Let us say that an LSR is capable of Stream Merge if it can receive The generic MPLS encapsulation should contain the following fields:
1. the label stack,
2. a Time-to-Live (TTL) field
3. a Class of Service (CoS) field
The TTL field permits MPLS to provide a TTL function similar to what
is provided by IP.
The CoS field permits LSRs to apply various scheduling packet
disciplines to labeled packets, without requiring separate labels for
separate disciplines.
2.24.2. ATM Switches as LSRs
It will be noted that MPLS forwarding procedures are similar to those
of legacy "label swapping" switches such as ATM switches. ATM
switches use the input port and the incoming VPI/VCI value as the
index into a "cross-connect" table, from which they obtain an output
port and an outgoing VPI/VCI value. Therefore if one or more labels
can be encoded directly into the fields which are accessed by these
legacy switches, then the legacy switches can, with suitable software
upgrades, be used as LSRs. We will refer to such devices as "ATM-
LSRs".
There are three obvious ways to encode labels in the ATM cell header
(presuming the use of AAL5):
1. SVC Encoding
Use the VPI/VCI field to encode the label which is at the top
of the label stack. This technique can be used in any network.
With this encoding technique, each LSP is realized as an ATM
SVC, and the LDP becomes the ATM "signaling" protocol. With
this encoding technique, the ATM-LSRs cannot perform "push" or
"pop" operations on the label stack.
2. SVP Encoding
Use the VPI field to encode the label which is at the top of
the label stack, and the VCI field to encode the second label
on the stack, if one is present. This technique some advantages
over the previous one, in that it permits the use of ATM "VP-
switching". That is, the LSPs are realized as ATM SVPs, with
LDP serving as the ATM signaling protocol.
However, this technique cannot always be used. If the network
includes an ATM Virtual Path through a non-MPLS ATM network,
then the VPI field is not necessarily available for use by
MPLS.
When this encoding technique is used, the ATM-LSR at the egress
of the VP effectively does a "pop" operation.
3. SVP Multipoint Encoding
Use the VPI field to encode the label which is at the top of
the label stack, use part of the VCI field to encode the second
label on the stack, if one is present, and use the remainder of
the VCI field to identify the LSP ingress. If this technique
is used, conventional ATM VP-switching capabilities can be used
to provide multipoint-to-point VPs. Cells from different
packets will then carry different VCI values. As we shall see
in section 2.25, this enables us to do label merging, without
running into any cell interleaving problems, on ATM switches
which can provide multipoint-to-point VPs, but which do not
have the VC merge capability.
This technique depends on the existence of a capability for
assigning small unique values to each ATM switch.
If there are more labels on the stack than can be encoded in the ATM
header, the ATM encodings must be combined with the generic
encapsulation.
2.24.3. Interoperability among Encoding Techniques
If <R1, R2, R3> is a segment of a LSP, it is possible that R1 will
use one encoding of the label stack when transmitting packet P to R2,
but R2 will use a different encoding when transmitting a packet P to
R3. In general, the MPLS architecture supports LSPs with different
label stack encodings used on different hops. Therefore, when we
discuss the procedures for processing a labeled packet, we speak in
abstract terms of operating on the packet's label stack. When a
labeled packet is received, the LSR must decode it to determine the
current value of the label stack, then must operate on the label
stack to determine the new value of the stack, and then encode the
new value appropriately before transmitting the labeled packet to its
next hop.
Unfortunately, ATM switches have no capability for translating from
one encoding technique to another. The MPLS architecture therefore
requires that whenever it is possible for two ATM switches to be
successive LSRs along a level m LSP for some packet, that those two
ATM switches use the same encoding technique.
Naturally there will be MPLS networks which contain a combination of
ATM switches operating as LSRs, and other LSRs which operate using an
MPLS shim header. In such networks there may be some LSRs which have
ATM interfaces as well as "MPLS Shim" interfaces. This is one example
of an LSR with different label stack encodings on different hops.
Such an LSR may swap off an ATM encoded label stack on an incoming
interface and replace it with an MPLS shim header encoded label stack
on the outgoing interface.
2.25. Label Merging
Suppose that an LSR has bound multiple incoming labels to a
particular FEC. When forwarding packets in that FEC, one would like
to have a single outgoing label which is applied to all such packets.
The fact that two different packets in the FEC arrived with different
incoming labels is irrelevant; one would like to forward them with
the same outgoing label. The capability to do so is known as "label
merging".
Let us say that an LSR is capable of label merging if it can receive
two packets from different incoming interfaces, and/or with different two packets from different incoming interfaces, and/or with different
labels, and send both packets out the same outgoing interface with labels, and send both packets out the same outgoing interface with
the same label. This in effect takes two incoming streams and merges the same label. Once the packets are transmitted, the information
them into one. Once the packets are transmitted, the information that that they arrived from different interfaces and/or with different
they arrived from different interfaces and/or with different incoming incoming labels is lost.
labels is lost.
Let us say that an LSR is not capable of Stream Merge if, for any two Let us say that an LSR is not capable of label merging if, for any
packets which arrive from different interfaces, or with different two packets which arrive from different interfaces, or with different
labels, the packets must either be transmitted out different labels, the packets must either be transmitted out different
interfaces, or must have different labels. interfaces, or must have different labels.
An LSR which is capable of Stream Merge (a "Merging LSR") needs to Label merging would be a requirement of the MPLS architecture, if not
maintain only one outgoing label for each FEC. AN LSR which is not for the fact that ATM-LSRs using the SVC or SVP Encodings cannot
capable of Stream Merge (a "Non-merging LSR") may need to maintain as perform label merging. This is discussed in more detail in the next
many as N outgoing labels per FEC, where N is the number of LSRs in section.
the network. Hence by supporting Stream Merge, an LSR can reduce its
number of outgoing labels by a factor of O(N). Since each label in
use requires the dedication of some amount of resources, this can be
a significant savings.
2.18.2. Non-merging LSRs If a particular LSR cannot perform label merging, then if two packets
in the same FEC arrive with different incoming labels, they must be
forwarded with different outgoing labels. With label merging, the
number of outgoing labels per FEC need only be 1; without label
merging, the number of outgoing labels per FEC could be as large as
the number of nodes in the network.
With label merging, the number of incoming labels per FEC that a
particular LSR needs is never be larger than the number of LDP
adjacencies. Without label merging, the number of incoming labels
per FEC that a particular LSR needs is as large as the number of
upstream nodes which forward traffic in the FEC to the LSR in
question. In fact, it is difficult for an LSR to even determine how
many such incoming labels it must support for a particular FEC.
The MPLS architecture accommodates both merging and non-merging LSRs,
but allows for the fact that there may be LSRs which do not support
label merging. This leads to the issue of ensuring correct
interoperation between merging LSRs and non-merging LSRs. The issue
is somewhat different in the case of datagram media versus the case
of ATM. The different media types will therefore be discussed
separately.
2.25.1. Non-merging LSRs
The MPLS forwarding procedures is very similar to the forwarding The MPLS forwarding procedures is very similar to the forwarding
procedures used by such technologies as ATM and Frame Relay. That is, procedures used by such technologies as ATM and Frame Relay. That is,
a unit of data arrives, a label (VPI/VCI or DLCI) is looked up in a a unit of data arrives, a label (VPI/VCI or DLCI) is looked up in a
"cross-connect table", on the basis of that lookup an output port is "cross-connect table", on the basis of that lookup an output port is
chosen, and the label value is rewritten. In fact, it is possible to chosen, and the label value is rewritten. In fact, it is possible to
use such technologies for MPLS forwarding; LDP can be used as the use such technologies for MPLS forwarding; LDP can be used as the
"signalling protocol" for setting up the cross-connect tables. "signalling protocol" for setting up the cross-connect tables.
Unfortunately, these technologies do not necessarily support the Unfortunately, these technologies do not necessarily support the
Stream Merge capability. In ATM, if one attempts to perform Stream label merging capability. In ATM, if one attempts to perform label
Merge, the result may be the interleaving of cells from various merging, the result may be the interleaving of cells from various
packets. If cells from different packets get interleaved, it is packets. If cells from different packets get interleaved, it is
impossible to reassemble the packets. Some Frame Relay switches use impossible to reassemble the packets. Some Frame Relay switches use
cell switching on their backplanes. These switches may also be cell switching on their backplanes. These switches may also be
incapable of supporting Stream Merge, for the same reason -- cells of incapable of supporting label merging, for the same reason -- cells
different packets may get interleaved, and there is then no way to of different packets may get interleaved, and there is then no way to
reassemble the packets. reassemble the packets.
We propose to support two solutions to this problem. First, MPLS will We propose to support two solutions to this problem. First, MPLS will
contain procedures which allow the use of non-merging LSRs. Second, contain procedures which allow the use of non-merging LSRs. Second,
MPLS will support procedures which allow certain ATM switches to MPLS will support procedures which allow certain ATM switches to
function as merging LSRs. function as merging LSRs.
Since MPLS supports both merging and non-merging LSRs, MPLS also Since MPLS supports both merging and non-merging LSRs, MPLS also
contains procedures to ensure correct interoperation between them. contains procedures to ensure correct interoperation between them.
2.18.3. Labels for Merging and Non-Merging LSRs 2.25.2. Labels for Merging and Non-Merging LSRs
An upstream LSR which supports Stream Merge needs to be sent only one An upstream LSR which supports label merging needs to be sent only
label per FEC. An upstream neighbor which does not support Stream one label per FEC. An upstream neighbor which does not support label
Merge needs to be sent multiple labels per FEC. However, there is no merging needs to be sent multiple labels per FEC. However, there is
way of knowing a priori how many labels it needs. This will depend on no way of knowing a priori how many labels it needs. This will depend
how many LSRs are upstream of it with respect to the FEC in question. on how many LSRs are upstream of it with respect to the FEC in
question.
In the MPLS architecture, if a particular upstream neighbor does not In the MPLS architecture, if a particular upstream neighbor does not
support Stream Merge, it is not sent any labels for a particular FEC support label merging, it is not sent any labels for a particular FEC
unless it explicitly asks for a label for that FEC. The upstream unless it explicitly asks for a label for that FEC. The upstream
neighbor may make multiple such requests, and is given a new label neighbor may make multiple such requests, and is given a new label
each time. When a downstream neighbor receives such a request from each time. When a downstream neighbor receives such a request from
upstream, and the downstream neighbor does not itself support Stream upstream, and the downstream neighbor does not itself support label
Merge, then it must in turn ask its downstream neighbor for another merging, then it must in turn ask its downstream neighbor for another
label for the FEC in question. label for the FEC in question.
It is possible that there may be some nodes which support merge, but It is possible that there may be some nodes which support label
have a limited number of upstream streams which may be merged into a merging, but can only merge a limited number of incoming labels into
single downstream streams. Suppose for example that due to some a single outgoing label. Suppose for example that due to some
hardware limitation a node is capable of merging four upstream LSPs hardware limitation a node is capable of merging four incoming labels
into a single downstream LSP. Suppose however, that this particular into a single outgoing label. Suppose however, that this particular
node has six upstream LSPs arriving at it for a particular stream. In node has six incoming labels arriving at it for a particular FEC. In
this case, this node may merge these into two downstream LSPs this case, this node may merge these into two outgoing labels.
(corresponding to two labels that need to be obtained from the
downstream neighbor). In this case, the normal operation of the LDP
implies that the downstream neighbor will supply this node with a
single label for the stream. This node can then ask its downstream
neighbor for one additional label for the stream, implying that the
node will thereby obtain the required two labels.
The interaction between explicit routing and merge is FFS. Whether label merging is applicable to explicitly routed LSPs is for
further study.
2.18.4. Merge over ATM 2.25.3. Merge over ATM
2.18.4.1. Methods of Eliminating Cell Interleave 2.25.3.1. Methods of Eliminating Cell Interleave
There are several methods that can be used to eliminate the cell There are several methods that can be used to eliminate the cell
interleaving problem in ATM, thereby allowing ATM switches to support interleaving problem in ATM, thereby allowing ATM switches to support
stream merge: : stream merge: :
1. VP merge 1. VP merge, using the SVP Multipoint Encoding
When VP merge is used, multiple virtual paths are merged into a When VP merge is used, multiple virtual paths are merged into a
virtual path, but packets from different sources are virtual path, but packets from different sources are
distinguished by using different VCs within the VP. distinguished by using different VCs within the VP.
2. VC merge 2. VC merge
When VC merge is used, switches are required to buffer cells When VC merge is used, switches are required to buffer cells
from one packet until the entire packet is received (this may from one packet until the entire packet is received (this may
be determined by looking for the AAL5 end of frame indicator). be determined by looking for the AAL5 end of frame indicator).
VP merge has the advantage that it is compatible with a higher VP merge has the advantage that it is compatible with a higher
percentage of existing ATM switch implementations. This makes it more percentage of existing ATM switch implementations. This makes it more
likely that VP merge can be used in existing networks. Unlike VC likely that VP merge can be used in existing networks. Unlike VC
merge, VP merge does not incur any delays at the merge points and merge, VP merge does not incur any delays at the merge points and
also does not impose any buffer requirements. However, it has the also does not impose any buffer requirements. However, it has the
disadvantage that it requires coordination of the VCI space within disadvantage that it requires coordination of the VCI space within
each VP. There are a number of ways that this can be accomplished. each VP. There are a number of ways that this can be accomplished.
Selection of one or more methods is FFS. Selection of one or more methods is for further study.
This tradeoff between compatibility with existing equipment versus This tradeoff between compatibility with existing equipment versus
protocol complexity and scalability implies that it is desirable for protocol complexity and scalability implies that it is desirable for
the MPLS protocol to support both VP merge and VC merge. In order to the MPLS protocol to support both VP merge and VC merge. In order to
do so each ATM switch participating in MPLS needs to know whether its do so each ATM switch participating in MPLS needs to know whether its
immediate ATM neighbors perform VP merge, VC merge, or no merge. immediate ATM neighbors perform VP merge, VC merge, or no merge.
2.18.4.2. Interoperation: VC Merge, VP Merge, and Non-Merge 2.25.3.2. Interoperation: VC Merge, VP Merge, and Non-Merge
The interoperation of the various forms of merging over ATM is most The interoperation of the various forms of merging over ATM is most
easily described by first describing the interoperation of VC merge easily described by first describing the interoperation of VC merge
with non-merge. with non-merge.
In the case where VC merge and non-merge nodes are interconnected the In the case where VC merge and non-merge nodes are interconnected the
forwarding of cells is based in all cases on a VC (i.e., the forwarding of cells is based in all cases on a VC (i.e., the
concatenation of the VPI and VCI). For each node, if an upstream concatenation of the VPI and VCI). For each node, if an upstream
neighbor is doing VC merge then that upstream neighbor requires only neighbor is doing VC merge then that upstream neighbor requires only
a single VPI/VCI for a particular stream (this is analogous to the a single VPI/VCI for a particular stream (this is analogous to the
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of VCs (identified by a set of VCIs which are significant within a of VCs (identified by a set of VCIs which are significant within a
VP). VP merge nodes would therefore request one VP, with a contained VP). VP merge nodes would therefore request one VP, with a contained
VCI for traffic that it originates (if appropriate) plus a VCI for VCI for traffic that it originates (if appropriate) plus a VCI for
each VC requested from above (regardless of whether or not the VC is each VC requested from above (regardless of whether or not the VC is
part of a containing VP). VC merge node would request only a single part of a containing VP). VC merge node would request only a single
VPI/VCI (since they can merge all upstream traffic into a single VC). VPI/VCI (since they can merge all upstream traffic into a single VC).
Non-merge nodes would pass on any requests that they get from above, Non-merge nodes would pass on any requests that they get from above,
plus request a VPI/VCI for traffic that they originate (if plus request a VPI/VCI for traffic that they originate (if
appropriate). appropriate).
2.19. LSP Control: Egress versus Local 2.26. Tunnels and Hierarchy
There is a choice to be made regarding whether the initial setup of
LSPs will be initiated by the egress node, or locally by each
individual node.
When LSP control is done locally, then each node may at any time pass
label bindings to its neighbors for each FEC recognized by that node.
In the normal case that the neighboring nodes recognize the same
FECs, then nodes may map incoming labels to outgoing labels as part
of the normal label swapping forwarding method.
When LSP control is done by the egress, then initially only the
egress node passes label bindings to its neighbors corresponding to
any FECs which leave the MPLS network at that egress node. Other
nodes wait until they get a label from downstream for a particular
FEC before passing a corresponding label for the same FEC to upstream
nodes.
With local control, since each LSR is (at least initially)
independently assigning labels to FECs, it is possible that different
LSRs may make inconsistent decisions. For example, an upstream LSR
may make a coarse decision (map multiple IP address prefixes to a
single label) while its downstream neighbor makes a finer grain
decision (map each individual IP address prefix to a separate label).
With downstream label assignment this can be corrected by having LSRs
withdraw labels that it has assigned which are inconsistent with
downstream labels, and replace them with new consistent label
assignments.
Even with egress control it is possible that the choice of egress
node may change, or the egress may (based on a change in
configuration) change its mind in terms of the granularity which is
to be used. This implies the same mechanism will be necessary to
allow changes in granularity to bubble up to upstream nodes. The
choice of egress or local control may therefore effect the frequency
with which this mechanism is used, but will not effect the need for a
mechanism to achieve consistency of label granularity. Generally
speaking, the choice of local versus egress control does not appear
to have any effect on the LDP mechanisms which need to be defined.
Egress control and local control can interwork in a very
straightforward manner (although when both methods exist in the
network, the overall behavior of the network is largely that of local
control). With either approach, (assuming downstream label
assignment) the egress node will initially assign labels for
particular FECs and will pass these labels to its neighbors. With
either approach these label assignments will bubble upstream, with
the upstream nodes choosing labels that are consistent with the
labels that they receive from downstream. The difference between the
two approaches is therefore primarily an issue of what each node does
prior to obtaining a label assignment for a particular FEC from
downstream nodes: Does it wait, or does it assign a preliminary label
under the expectation that it will (probably) be correct?
Regardless of which method is used (local control or egress control)
each node needs to know (possibly by configuration) what granularity
to use for labels that it assigns. Where egress control is used, this
requires each node to know the granularity only for streams which
leave the MPLS network at that node. For local control, in order to
avoid the need to withdraw inconsistent labels, each node in the
network would need to be configured consistently to know the
granularity for each stream. However, in many cases this may be done
by using a single level of granularity which applies to all streams
(such as "one label per IP prefix in the forwarding table").
This architecture allows the choice between local control and egress
control to be a local matter. Since the two methods interwork, a
given LSR need support only one or the other.
2.20. Granularity
When forwarding by label swapping, a stream of packets following a
stream arriving from upstream may be mapped into an equal or coarser
grain stream. However, a coarse grain stream (for example, containing
packets destined for a short IP address prefix covering many subnets)
cannot be mapped directly into a finer grain stream (for example,
containing packets destined for a longer IP address prefix covering a
single subnet). This implies that there needs to be some mechanism
for ensuring consistency between the granularity of LSPs in an MPLS
network.
The method used for ensuring compatibility of granularity may depend
upon the method used for LSP control.
When LSP control is local, it is possible that a node may pass a
coarse grain label to its upstream neighbor(s), and subsequently
receive a finer grain label from its downstream neighbor. In this
case the node has two options: (i) It may forward the corresponding
packets using normal IP datagram forwarding (i.e., by examination of
the IP header); (ii) It may withdraw the label mappings that it has
passed to its upstream neighbors, and replace these with finer grain
label mappings.
When LSP control is egress based, the label setup originates from the
egress node and passes upstream. It is therefore straightforward with
this approach to maintain equally-grained mappings along the route.
2.21. Tunnels and Hierarchy
Sometimes a router Ru takes explicit action to cause a particular Sometimes a router Ru takes explicit action to cause a particular
packet to be delivered to another router Rd, even though Ru and Rd packet to be delivered to another router Rd, even though Ru and Rd
are not consecutive routers on the Hop-by-hop path for that packet, are not consecutive routers on the Hop-by-hop path for that packet,
and Rd is not the packet's ultimate destination. For example, this and Rd is not the packet's ultimate destination. For example, this
may be done by encapsulating the packet inside a network layer packet may be done by encapsulating the packet inside a network layer packet
whose destination address is the address of Rd itself. This creates a whose destination address is the address of Rd itself. This creates a
"tunnel" from Ru to Rd. We refer to any packet so handled as a "tunnel" from Ru to Rd. We refer to any packet so handled as a
"Tunneled Packet". "Tunneled Packet".
2.21.1. Hop-by-Hop Routed Tunnel 2.26.1. Hop-by-Hop Routed Tunnel
If a Tunneled Packet follows the Hop-by-hop path from Ru to Rd, we If a Tunneled Packet follows the Hop-by-hop path from Ru to Rd, we
say that it is in an "Hop-by-Hop Routed Tunnel" whose "transmit say that it is in an "Hop-by-Hop Routed Tunnel" whose "transmit
endpoint" is Ru and whose "receive endpoint" is Rd. endpoint" is Ru and whose "receive endpoint" is Rd.
2.21.2. Explicitly Routed Tunnel 2.26.2. Explicitly Routed Tunnel
If a Tunneled Packet travels from Ru to Rd over a path other than the If a Tunneled Packet travels from Ru to Rd over a path other than the
Hop-by-hop path, we say that it is in an "Explicitly Routed Tunnel" Hop-by-hop path, we say that it is in an "Explicitly Routed Tunnel"
whose "transmit endpoint" is Ru and whose "receive endpoint" is Rd. whose "transmit endpoint" is Ru and whose "receive endpoint" is Rd.
For example, we might send a packet through an Explicitly Routed For example, we might send a packet through an Explicitly Routed
Tunnel by encapsulating it in a packet which is source routed. Tunnel by encapsulating it in a packet which is source routed.
2.21.3. LSP Tunnels 2.26.3. LSP Tunnels
It is possible to implement a tunnel as a LSP, and use label It is possible to implement a tunnel as a LSP, and use label
switching rather than network layer encapsulation to cause the packet switching rather than network layer encapsulation to cause the packet
to travel through the tunnel. The tunnel would be a LSP <R1, ..., to travel through the tunnel. The tunnel would be a LSP <R1, ...,
Rn>, where R1 is the transmit endpoint of the tunnel, and Rn is the Rn>, where R1 is the transmit endpoint of the tunnel, and Rn is the
receive endpoint of the tunnel. This is called a "LSP Tunnel". receive endpoint of the tunnel. This is called a "LSP Tunnel".
The set of packets which are to be sent though the LSP tunnel becomes The set of packets which are to be sent though the LSP tunnel
a stream, and each LSR in the tunnel must assign a label to that constitutes a FEC, and each LSR in the tunnel must assign a label to
stream (i.e., must assign a label to the tunnel). The criteria for that FEC (i.e., must assign a label to the tunnel). The criteria for
assigning a particular packet to an LSP tunnel is a local matter at assigning a particular packet to an LSP tunnel is a local matter at
the tunnel's transmit endpoint. To put a packet into an LSP tunnel, the tunnel's transmit endpoint. To put a packet into an LSP tunnel,
the transmit endpoint pushes a label for the tunnel onto the label the transmit endpoint pushes a label for the tunnel onto the label
stack and sends the labeled packet to the next hop in the tunnel. stack and sends the labeled packet to the next hop in the tunnel.
If it is not necessary for the tunnel's receive endpoint to be able If it is not necessary for the tunnel's receive endpoint to be able
to determine which packets it receives through the tunnel, as to determine which packets it receives through the tunnel, as
discussed earlier, the label stack may be popped at the penultimate discussed earlier, the label stack may be popped at the penultimate
LSR in the tunnel. LSR in the tunnel.
A "Hop-by-Hop Routed LSP Tunnel" is a Tunnel that is implemented as A "Hop-by-Hop Routed LSP Tunnel" is a Tunnel that is implemented as
an hop-by-hop routed LSP between the transmit endpoint and the an hop-by-hop routed LSP between the transmit endpoint and the
receive endpoint. receive endpoint.
An "Explicitly Routed LSP Tunnel" is a LSP Tunnel that is also an An "Explicitly Routed LSP Tunnel" is a LSP Tunnel that is also an
Explicitly Routed LSP. Explicitly Routed LSP.
2.21.4. Hierarchy: LSP Tunnels within LSPs 2.26.4. Hierarchy: LSP Tunnels within LSPs
Consider a LSP <R1, R2, R3, R4>. Let us suppose that R1 receives Consider a LSP <R1, R2, R3, R4>. Let us suppose that R1 receives
unlabeled packet P, and pushes on its label stack the label to cause unlabeled packet P, and pushes on its label stack the label to cause
it to follow this path, and that this is in fact the Hop-by-hop path. it to follow this path, and that this is in fact the Hop-by-hop path.
However, let us further suppose that R2 and R3 are not directly However, let us further suppose that R2 and R3 are not directly
connected, but are "neighbors" by virtue of being the endpoints of an connected, but are "neighbors" by virtue of being the endpoints of an
LSP tunnel. So the actual sequence of LSRs traversed by P is <R1, R2, LSP tunnel. So the actual sequence of LSRs traversed by P is <R1, R2,
R21, R22, R23, R3, R4>. R21, R22, R23, R3, R4>.
When P travels from R1 to R2, it will have a label stack of depth 1. When P travels from R1 to R2, it will have a label stack of depth 1.
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to R3. Then it pushes on a new label. This level 2 label has a value to R3. Then it pushes on a new label. This level 2 label has a value
which is meaningful to R21. Switching is done on the level 2 label by which is meaningful to R21. Switching is done on the level 2 label by
R21, R22, R23. R23, which is the penultimate hop in the R2-R3 tunnel, R21, R22, R23. R23, which is the penultimate hop in the R2-R3 tunnel,
pops the label stack before forwarding the packet to R3. When R3 sees pops the label stack before forwarding the packet to R3. When R3 sees
packet P, P has only a level 1 label, having now exited the tunnel. packet P, P has only a level 1 label, having now exited the tunnel.
Since R3 is the penultimate hop in P's level 1 LSP, it pops the label Since R3 is the penultimate hop in P's level 1 LSP, it pops the label
stack, and R4 receives P unlabeled. stack, and R4 receives P unlabeled.
The label stack mechanism allows LSP tunneling to nest to any depth. The label stack mechanism allows LSP tunneling to nest to any depth.
2.21.5. LDP Peering and Hierarchy 2.26.5. LDP Peering and Hierarchy
Suppose that packet P travels along a Level 1 LSP <R1, R2, R3, R4>, Suppose that packet P travels along a Level 1 LSP <R1, R2, R3, R4>,
and when going from R2 to R3 travels along a Level 2 LSP <R2, R21, and when going from R2 to R3 travels along a Level 2 LSP <R2, R21,
R22, R3>. From the perspective of the Level 2 LSP, R2's LDP peer is R22, R3>. From the perspective of the Level 2 LSP, R2's LDP peer is
R21. From the perspective of the Level 1 LSP, R2's LDP peers are R1 R21. From the perspective of the Level 1 LSP, R2's LDP peers are R1
and R3. One can have LDP peers at each layer of hierarchy. We will and R3. One can have LDP peers at each layer of hierarchy. We will
see in sections 3.6 and 3.7 some ways to make use of this hierarchy. see in sections 3.6 and 3.7 some ways to make use of this hierarchy.
Note that in this example, R2 and R21 must be IGP neighbors, but R2 Note that in this example, R2 and R21 must be IGP neighbors, but R2
and R3 need not be. and R3 need not be.
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One performs label Distribution with one's Local LDP Peers by opening One performs label Distribution with one's Local LDP Peers by opening
LDP connections to them. One can perform label Distribution with LDP connections to them. One can perform label Distribution with
one's Remote LDP Peers in one of two ways: one's Remote LDP Peers in one of two ways:
1. Explicit Peering 1. Explicit Peering
In explicit peering, one sets up LDP connections between Remote In explicit peering, one sets up LDP connections between Remote
LDP Peers, exactly as one would do for Local LDP Peers. This LDP Peers, exactly as one would do for Local LDP Peers. This
technique is most useful when the number of Remote LDP Peers is technique is most useful when the number of Remote LDP Peers is
small, or the number of higher level label mappings is large, small, or the number of higher level label bindings is large,
or the Remote LDP Peers are in distinct routing areas or or the Remote LDP Peers are in distinct routing areas or
domains. Of course, one needs to know which labels to domains. Of course, one needs to know which labels to
distribute to which peers; this is addressed in section 3.1.2. distribute to which peers; this is addressed in section 3.1.2.
Examples of the use of explicit peering is found in sections Examples of the use of explicit peering is found in sections
3.2.1 and 3.6. 3.2.1 and 3.6.
2. Implicit Peering 2. Implicit Peering
In Implicit Peering, one does not have LDP connections to one's In Implicit Peering, one does not have LDP connections to one's
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Peers is large. Implicit peering does not require a n-square Peers is large. Implicit peering does not require a n-square
peering mesh to distribute labels to the remote LDP peers peering mesh to distribute labels to the remote LDP peers
because the information is piggybacked through the local LDP because the information is piggybacked through the local LDP
peering. However, implicit peering requires the intermediate peering. However, implicit peering requires the intermediate
nodes to store information that they might not be directly nodes to store information that they might not be directly
interested in. interested in.
An example of the use of implicit peering is found in section An example of the use of implicit peering is found in section
3.3. 3.3.
2.22. LDP Transport 2.27. LDP Transport
LDP is used between nodes in an MPLS network to establish and LDP is used between nodes in an MPLS network to establish and
maintain the label mappings. In order for LDP to operate correctly, maintain the label bindings. In order for LDP to operate correctly,
LDP information needs to be transmitted reliably, and the LDP LDP information needs to be transmitted reliably, and the LDP
messages pertaining to a particular FEC need to be transmitted in messages pertaining to a particular FEC need to be transmitted in
sequence. Flow control is also required, as is the capability to sequence. Flow control is also required, as is the capability to
carry multiple LDP messages in a single datagram. carry multiple LDP messages in a single datagram.
These goals will be met by using TCP as the underlying transport for These goals will be met by using TCP as the underlying transport for
LDP. LDP.
(The use of multicast techniques to distribute label mappings is (The use of multicast techniques to distribute label bindings is for
FFS.) further study.)
2.23. Label Encodings
In order to transmit a label stack along with the packet whose label
stack it is, it is necessary to define a concrete encoding of the
label stack. The architecture supports several different encoding
techniques; the choice of encoding technique depends on the
particular kind of device being used to forward labeled packets.
2.23.1. MPLS-specific Hardware and/or Software
If one is using MPLS-specific hardware and/or software to forward
labeled packets, the most obvious way to encode the label stack is to
define a new protocol to be used as a "shim" between the data link
layer and network layer headers. This shim would really be just an
encapsulation of the network layer packet; it would be "protocol-
independent" such that it could be used to encapsulate any network
layer. Hence we will refer to it as the "generic MPLS
encapsulation".
The generic MPLS encapsulation would in turn be encapsulated in a
data link layer protocol.
The generic MPLS encapsulation should contain the following fields:
1. the label stack,
2. a Time-to-Live (TTL) field
3. a Class of Service (CoS) field
The TTL field permits MPLS to provide a TTL function similar to what
is provided by IP.
The CoS field permits LSRs to apply various scheduling packet
disciplines to labeled packets, without requiring separate labels for
separate disciplines.
2.23.2. ATM Switches as LSRs
It will be noted that MPLS forwarding procedures are similar to those
of legacy "label swapping" switches such as ATM switches. ATM
switches use the input port and the incoming VPI/VCI value as the
index into a "cross-connect" table, from which they obtain an output
port and an outgoing VPI/VCI value. Therefore if one or more labels
can be encoded directly into the fields which are accessed by these
legacy switches, then the legacy switches can, with suitable software
upgrades, be used as LSRs. We will refer to such devices as "ATM-
LSRs".
There are three obvious ways to encode labels in the ATM cell header
(presuming the use of AAL5):
1. SVC Encoding
Use the VPI/VCI field to encode the label which is at the top
of the label stack. This technique can be used in any network.
With this encoding technique, each LSP is realized as an ATM
SVC, and the LDP becomes the ATM "signaling" protocol. With
this encoding technique, the ATM-LSRs cannot perform "push" or
"pop" operations on the label stack.
2. SVP Encoding
Use the VPI field to encode the label which is at the top of
the label stack, and the VCI field to encode the second label
on the stack, if one is present. This technique some advantages
over the previous one, in that it permits the use of ATM "VP-
switching". That is, the LSPs are realized as ATM SVPs, with
LDP serving as the ATM signaling protocol.
However, this technique cannot always be used. If the network
includes an ATM Virtual Path through a non-MPLS ATM network,
then the VPI field is not necessarily available for use by
MPLS.
When this encoding technique is used, the ATM-LSR at the egress
of the VP effectively does a "pop" operation.
3. SVP Multipoint Encoding
Use the VPI field to encode the label which is at the top of
the label stack, use part of the VCI field to encode the second
label on the stack, if one is present, and use the remainder of
the VCI field to identify the LSP ingress. If this technique
is used, conventional ATM VP-switching capabilities can be used
to provide multipoint-to-point VPs. Cells from different
packets will then carry different VCI values, so multipoint-
to-point VPs can be provided without any cell interleaving
problems.
This technique depends on the existence of a capability for
assigning small unique values to each ATM switch.
If there are more labels on the stack than can be encoded in the ATM
header, the ATM encodings must be combined with the generic
encapsulation. This does presuppose that it be possible to tell,
when reassembling the ATM cells into packets, whether the generic
encapsulation is also present.
2.23.3. Interoperability among Encoding Techniques
If <R1, R2, R3> is a segment of a LSP, it is possible that R1 will
use one encoding of the label stack when transmitting packet P to R2,
but R2 will use a different encoding when transmitting a packet P to
R3. In general, the MPLS architecture supports LSPs with different
label stack encodings used on different hops. Therefore, when we
discuss the procedures for processing a labeled packet, we speak in
abstract terms of operating on the packet's label stack. When a
labeled packet is received, the LSR must decode it to determine the
current value of the label stack, then must operate on the label
stack to determine the new value of the stack, and then encode the
new value appropriately before transmitting the labeled packet to its
next hop.
Unfortunately, ATM switches have no capability for translating from
one encoding technique to another. The MPLS architecture therefore
requires that whenever it is possible for two ATM switches to be
successive LSRs along a level m LSP for some packet, that those two
ATM switches use the same encoding technique.
Naturally there will be MPLS networks which contain a combination of
ATM switches operating as LSRs, and other LSRs which operate using an
MPLS shim header. In such networks there may be some LSRs which have
ATM interfaces as well as "MPLS Shim" interfaces. This is one example
of an LSR with different label stack encodings on different hops.
Such an LSR may swap off an ATM encoded label stack on an incoming
interface and replace it with an MPLS shim header encoded label stack
on the outgoing interface.
2.24. Multicast 2.28. Multicast
This section is for further study This section is for further study
3. Some Applications of MPLS 3. Some Applications of MPLS
3.1. MPLS and Hop by Hop Routed Traffic 3.1. MPLS and Hop by Hop Routed Traffic
One use of MPLS is to simplify the process of forwarding packets One use of MPLS is to simplify the process of forwarding packets
using hop by hop routing. using hop by hop routing.
3.1.1. Labels for Address Prefixes 3.1.1. Labels for Address Prefixes
In general, router R determines the next hop for packet P by finding In general, router R determines the next hop for packet P by finding
the address prefix X in its routing table which is the longest match the address prefix X in its routing table which is the longest match
for P's destination address. That is, the packets in a given stream for P's destination address. That is, the packets in a given FEC are
are just those packets which match a given address prefix in R's just those packets which match a given address prefix in R's routing
routing table. In this case, a stream can be identified with an table. In this case, a FEC can be identified with an address prefix.
address prefix.
If packet P must traverse a sequence of routers, and at each router If packet P must traverse a sequence of routers, and at each router
in the sequence P matches the same address prefix, MPLS simplifies in the sequence P matches the same address prefix, MPLS simplifies
the forwarding process by enabling all routers but the first to avoid the forwarding process by enabling all routers but the first to avoid
executing the best match algorithm; they need only look up the label. executing the best match algorithm; they need only look up the label.
3.1.2. Distributing Labels for Address Prefixes 3.1.2. Distributing Labels for Address Prefixes
3.1.2.1. LDP Peers for a Particular Address Prefix 3.1.2.1. LDP Peers for a Particular Address Prefix
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3.1.2.2. Distributing Labels 3.1.2.2. Distributing Labels
In order to use MPLS for the forwarding of normally routed traffic, In order to use MPLS for the forwarding of normally routed traffic,
each LSR MUST: each LSR MUST:
1. bind one or more labels to each address prefix that appears in 1. bind one or more labels to each address prefix that appears in
its routing table; its routing table;
2. for each such address prefix X, use an LDP to distribute the 2. for each such address prefix X, use an LDP to distribute the
mapping of a label to X to each of its LDP Peers for X. binding of a label to X to each of its LDP Peers for X.
There is also one circumstance in which an LSR must distribute a There is also one circumstance in which an LSR must distribute a
label mapping for an address prefix, even if it is not the LSR which label binding for an address prefix, even if it is not the LSR which
bound that label to that address prefix: bound that label to that address prefix:
3. If R1 uses BGP to distribute a route to X, naming some other 3. If R1 uses BGP to distribute a route to X, naming some other
LSR R2 as the BGP Next Hop to X, and if R1 knows that R2 has LSR R2 as the BGP Next Hop to X, and if R1 knows that R2 has
assigned label L to X, then R1 must distribute the mapping assigned label L to X, then R1 must distribute the binding
between T and X to any BGP peer to which it distributes that between T and X to any BGP peer to which it distributes that
route. route.
These rules ensure that labels corresponding to address prefixes These rules ensure that labels corresponding to address prefixes
which correspond to BGP routes are distributed to IGP neighbors if which correspond to BGP routes are distributed to IGP neighbors if
and only if the BGP routes are distributed into the IGP. Otherwise, and only if the BGP routes are distributed into the IGP. Otherwise,
the labels bound to BGP routes are distributed only to the other BGP the labels bound to BGP routes are distributed only to the other BGP
speakers. speakers.
These rules are intended to indicate which label mappings must be These rules are intended only to indicate which label bindings must
distributed by a given LSR to which other LSRs, NOT to indicate the be distributed by a given LSR to which other LSRs.
conditions under which the distribution is to be made. That is
discussed in section 2.19.
3.1.3. Using the Hop by Hop path as the LSP 3.1.3. Using the Hop by Hop path as the LSP
If the hop-by-hop path that packet P needs to follow is <R1, ..., If the hop-by-hop path that packet P needs to follow is <R1, ...,
Rn>, then <R1, ..., Rn> can be an LSP as long as: Rn>, then <R1, ..., Rn> can be an LSP as long as:
1. there is a single address prefix X, such that, for all i, 1. there is a single address prefix X, such that, for all i,
1<=i<n, X is the longest match in Ri's routing table for P's 1<=i<n, X is the longest match in Ri's routing table for P's
destination address; destination address;
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the packet's destination address. At that point, the LSP must end and the packet's destination address. At that point, the LSP must end and
the best match algorithm must be performed again. the best match algorithm must be performed again.
Suppose, for example, that packet P, with destination address Suppose, for example, that packet P, with destination address
10.2.153.178 needs to go from R1 to R2 to R3. Suppose also that R2 10.2.153.178 needs to go from R1 to R2 to R3. Suppose also that R2
advertises address prefix 10.2/16 to R1, but R3 advertises advertises address prefix 10.2/16 to R1, but R3 advertises
10.2.153/22, 10.2.154/22, and 10.2/16 to R2. That is, R2 is 10.2.153/22, 10.2.154/22, and 10.2/16 to R2. That is, R2 is
advertising an "aggregated route" to R1. In this situation, packet P advertising an "aggregated route" to R1. In this situation, packet P
can be label Switched until it reaches R2, but since R2 has performed can be label Switched until it reaches R2, but since R2 has performed
route aggregation, it must execute the best match algorithm to find route aggregation, it must execute the best match algorithm to find
P's stream. P's FEC.
3.1.4. LSP Egress and LSP Proxy Egress 3.1.4. LSP Egress and LSP Proxy Egress
An LSR R is considered to be an "LSP Egress" LSR for address prefix X An LSR R is considered to be an "LSP Egress" LSR for address prefix X
if and only if one of the following conditions holds: if and only if one of the following conditions holds:
1. R1 has an address Y, such that X is the address prefix in R1's 1. R1 has an address Y, such that X is the address prefix in R1's
routing table which is the longest match for Y, or routing table which is the longest match for Y, or
2. R contains in its routing tables one or more address prefixes Y 2. R contains in its routing tables one or more address prefixes Y
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2. R contains in its routing tables one or more address prefixes Y 2. R contains in its routing tables one or more address prefixes Y
such that X is a proper initial substring of Y, but R's "LSP such that X is a proper initial substring of Y, but R's "LSP
previous hops" for X do not contain any such address prefixes previous hops" for X do not contain any such address prefixes
Y; that is, R2 is a "deaggregation point" for address prefix X. Y; that is, R2 is a "deaggregation point" for address prefix X.
An LSR R1 is considered to be an "LSP Proxy Egress" LSR for address An LSR R1 is considered to be an "LSP Proxy Egress" LSR for address
prefix X if and only if: prefix X if and only if:
1. R1's next hop for X is R2 R1 and R2 are not LDP Peers with 1. R1's next hop for X is R2 R1 and R2 are not LDP Peers with
respect to X (perhaps because R2 does not support MPLS), or respect to X (perhaps because R2 does not support MPLS), or
2. R1 has been configured to act as an LSP Proxy Egress for X 2. R1 has been configured to act as an LSP Proxy Egress for X
The definition of LSP allows for the LSP Egress to be a node which The definition of LSP allows for the LSP Egress to be a node which
does not support MPLS; in this case the penultimate node in the LSP does not support MPLS; in this case the penultimate node in the LSP
is the Proxy Egress. is the Proxy Egress.
3.1.5. The POP Label 3.1.5. The Implicit NULL Label
The POP label is a label with special semantics which an LSR can bind The Implicit NULL label is a label with special semantics which an
to an address prefix. If LSR Ru, by consulting its ILM, sees that LSR can bind to an address prefix. If LSR Ru, by consulting its ILM,
labeled packet P must be forwarded next to Rd, but that Rd has sees that labeled packet P must be forwarded next to Rd, but that Rd
distributed a mapping of the POP label to the corresponding address has distributed a binding of Implicit NULL to the corresponding
prefix, then instead of replacing the value of the label on top of address prefix, then instead of replacing the value of the label on
the label stack, Ru pops the label stack, and then forwards the top of the label stack, Ru pops the label stack, and then forwards
resulting packet to Rd. the resulting packet to Rd.
LSR Rd distributes a mapping between the POP label and an address LSR Rd distributes a binding between Implicit NULL and an address
prefix X to LSR Ru if and only if: prefix X to LSR Ru if and only if:
1. the rules of Section 3.1.2 indicate that Rd distributes to Ru a 1. the rules of Section 3.1.2 indicate that Rd distributes to Ru a
label mapping for X, and label binding for X, and
2. when the LDP connection between Ru and Rd was opened, Ru 2. when the LDP connection between Ru and Rd was opened, Ru
indicated that it could support the POP label, and indicated that it could support the Implicit NULL label (i.e.,
that it can pop the label stack), and
3. Rd is an LSP Egress (not proxy egress) for X. 3. Rd is an LSP Egress (not proxy egress) for X.
This causes the penultimate LSR on a LSP to pop the label stack. This This causes the penultimate LSR on a LSP to pop the label stack. This
is quite appropriate; if the LSP Egress is an MPLS Egress for X, then is quite appropriate; if the LSP Egress is an MPLS Egress for X, then
if the penultimate LSR does not pop the label stack, the LSP Egress if the penultimate LSR does not pop the label stack, the LSP Egress
will need to look up the label, pop the label stack, and then look up will need to look up the label, pop the label stack, and then look up
the next label (or look up the L3 address, if no more labels are the next label (or look up the L3 address, if no more labels are
present). By having the penultimate LSR pop the label stack, the LSP present). By having the penultimate LSR pop the label stack, the LSP
Egress is saved the work of having to look up two labels in order to Egress is saved the work of having to look up two labels in order to
make its forwarding decision. make its forwarding decision.
However, if the penultimate LSR is an ATM switch, it may not have the However, if the penultimate LSR is an ATM switch, it may not have the
capability to pop the label stack. Hence a POP label mapping may be capability to pop the label stack. Hence a binding of Implicit NULL
distributed only to LSRs which can support that function. may be distributed only to LSRs which can support that function.
If the penultimate LSR in an LSP for address prefix X is an LSP Proxy If the penultimate LSR in an LSP for address prefix X is an LSP Proxy
Egress, it acts just as if the LSP Egress had distributed the POP Egress, it acts just as if the LSP Egress had distributed a binding
label for X. of Implicit NULL for X.
3.1.6. Option: Egress-Targeted Label Assignment 3.1.6. Option: Egress-Targeted Label Assignment
There are situations in which an LSP Ingress, Ri, knows that packets There are situations in which an LSP Ingress, Ri, knows that packets
of several different streams must all follow the same LSP, of several different FECs must all follow the same LSP, terminating
terminating at, say, LSP Egress Re. In this case, proper routing can at, say, LSP Egress Re. In this case, proper routing can be achieved
be achieved by using a single label can be used for all such streams; by using a single label can be used for all such FECs; it is not
it is not necessary to have a distinct label for each stream. If necessary to have a distinct label for each FEC. If (and only if)
(and only if) the following conditions hold: the following conditions hold:
1. the address of LSR Re is itself in the routing table as a "host 1. the address of LSR Re is itself in the routing table as a "host
route", and route", and
2. there is some way for Ri to determine that Re is the LSP egress 2. there is some way for Ri to determine that Re is the LSP egress
for all packets in a particular set of streams for all packets in a particular set of FECs
Then Ri may bind a single label to all FECS in the set. This is Then Ri may bind a single label to all FECS in the set. This is
known as "Egress-Targeted Label Assignment." known as "Egress-Targeted Label Assignment."
How can LSR Ri determine that an LSR Re is the LSP Egress for all How can LSR Ri determine that an LSR Re is the LSP Egress for all
packets in a particular stream? There are a couple of possible ways: packets in a particular FEC? There are a couple of possible ways:
- If the network is running a link state routing algorithm, and all - If the network is running a link state routing algorithm, and all
nodes in the area support MPLS, then the routing algorithm nodes in the area support MPLS, then the routing algorithm
provides Ri with enough information to determine the routers provides Ri with enough information to determine the routers
through which packets in that stream must leave the routing through which packets in that FEC must leave the routing domain
domain or area. or area.
- It is possible to use LDP to pass information about which address - It is possible to use LDP to pass information about which address
prefixes are "attached" to which egress LSRs. This method has prefixes are "attached" to which egress LSRs. This method has
the advantage of not depending on the presence of link state the advantage of not depending on the presence of link state
routing. routing.
If egress-targeted label assignment is used, the number of labels If egress-targeted label assignment is used, the number of labels
that need to be supported throughout the network may be greatly that need to be supported throughout the network may be greatly
reduced. This may be significant if one is using legacy switching reduced. This may be significant if one is using legacy switching
hardware to do MPLS, and the switching hardware can support only a hardware to do MPLS, and the switching hardware can support only a
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3. A means of ensuring that packets sent into the Tunnel will not 3. A means of ensuring that packets sent into the Tunnel will not
loop from the receive endpoint back to the transmit endpoint. loop from the receive endpoint back to the transmit endpoint.
If the transmit endpoint of the tunnel wishes to put a labeled packet If the transmit endpoint of the tunnel wishes to put a labeled packet
into the tunnel, it must first replace the label value at the top of into the tunnel, it must first replace the label value at the top of
the stack with a label value that was distributed to it by the the stack with a label value that was distributed to it by the
tunnel's receive endpoint. Then it must push on the label which tunnel's receive endpoint. Then it must push on the label which
corresponds to the tunnel itself, as distributed to it by the next corresponds to the tunnel itself, as distributed to it by the next
hop along the tunnel. To allow this, the tunnel endpoints should be hop along the tunnel. To allow this, the tunnel endpoints should be
explicit LDP peers. The label mappings they need to exchange are of explicit LDP peers. The label bindings they need to exchange are of
no interest to the LSRs along the tunnel. no interest to the LSRs along the tunnel.
3.3. Label Stacks and Implicit Peering 3.3. Label Stacks and Implicit Peering
Suppose a particular LSR Re is an LSP proxy egress for 10 address Suppose a particular LSR Re is an LSP proxy egress for 10 address
prefixes, and it reaches each address prefix through a distinct prefixes, and it reaches each address prefix through a distinct
interface. interface.
One could assign a single label to all 10 address prefixes. Then Re One could assign a single label to all 10 address prefixes. Then Re
is an LSP egress for all 10 address prefixes. This ensures that is an LSP egress for all 10 address prefixes. This ensures that
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Alternatively, one could assign a distinct label to each interface. Alternatively, one could assign a distinct label to each interface.
Then Re is an LSP proxy egress for the 10 address prefixes. This Then Re is an LSP proxy egress for the 10 address prefixes. This
eliminates the need for Re to look up the network layer addresses in eliminates the need for Re to look up the network layer addresses in
order to forward the packets. However, it can result in the use of a order to forward the packets. However, it can result in the use of a
large number of labels. large number of labels.
An alternative would be to bind all 10 address prefixes to the same An alternative would be to bind all 10 address prefixes to the same
level 1 label (which is also bound to the address of the LSR itself), level 1 label (which is also bound to the address of the LSR itself),
and then to bind each address prefix to a distinct level 2 label. The and then to bind each address prefix to a distinct level 2 label. The
level 2 label would be treated as an attribute of the level 1 label level 2 label would be treated as an attribute of the level 1 label
mapping, which we call the "Stack Attribute". We impose the binding, which we call the "Stack Attribute". We impose the
following rules: following rules:
- When LSR Ru initially labels an untagged packet, if the longest - When LSR Ru initially labels an untagged packet, if the longest
match for the packet's destination address is X, and R's LSP next match for the packet's destination address is X, and R's LSP next
hop for X is Rd, and Rd has distributed to R1 a mapping of label hop for X is Rd, and Rd has distributed to R1 a binding of label
L1 X, along with a stack attribute of L2, then L1 X, along with a stack attribute of L2, then
1. Ru must push L2 and then L1 onto the packet's label stack, 1. Ru must push L2 and then L1 onto the packet's label stack,
and then forward the packet to Rd; and then forward the packet to Rd;
2. When Ru distributes label mappings for X to its LDP peers, 2. When Ru distributes label bindings for X to its LDP peers,
it must include L2 as the stack attribute. it must include L2 as the stack attribute.
3. Whenever the stack attribute changes (possibly as a result 3. Whenever the stack attribute changes (possibly as a result
of a change in Ru's LSP next hop for X), Ru must distribute of a change in Ru's LSP next hop for X), Ru must distribute
the new stack attribute. the new stack attribute.
Note that although the label value bound to X may be different at Note that although the label value bound to X may be different at
each hop along the LSP, the stack attribute value is passed each hop along the LSP, the stack attribute value is passed
unchanged, and is set by the LSP proxy egress. unchanged, and is set by the LSP proxy egress.
Thus the LSP proxy egress for X becomes an "implicit peer" with each Thus the LSP proxy egress for X becomes an "implicit peer" with each
other LSR in the routing area or domain. In this case, explicit other LSR in the routing area or domain. In this case, explicit
peering would be too unwieldy, because the number of peers would peering would be too unwieldy, because the number of peers would
become too large. become too large.
3.4. MPLS and Multi-Path Routing 3.4. MPLS and Multi-Path Routing
If an LSR supports multiple routes for a particular stream, then it If an LSR supports multiple routes for a particular stream, then it
may assign multiple labels to the stream, one for each route. Thus may assign multiple labels to the stream, one for each route. Thus
the reception of a second label mapping from a particular neighbor the reception of a second label binding from a particular neighbor
for a particular address prefix should be taken as meaning that for a particular address prefix should be taken as meaning that
either label can be used to represent that address prefix. either label can be used to represent that address prefix.
If multiple label mappings for a particular address prefix are If multiple label bindings for a particular address prefix are
specified, they may have distinct attributes. specified, they may have distinct attributes.
3.5. LSP Trees as Multipoint-to-Point Entities 3.5. LSP Trees as Multipoint-to-Point Entities
Consider the case of packets P1 and P2, each of which has a Consider the case of packets P1 and P2, each of which has a
destination address whose longest match, throughout a particular destination address whose longest match, throughout a particular
routing domain, is address prefix X. Suppose that the Hop-by-hop routing domain, is address prefix X. Suppose that the Hop-by-hop
path for P1 is <R1, R2, R3>, and the Hop-by-hop path for P2 is <R4, path for P1 is <R1, R2, R3>, and the Hop-by-hop path for P2 is <R4,
R2, R3>. Let's suppose that R3 binds label L3 to X, and distributes R2, R3>. Let's suppose that R3 binds label L3 to X, and distributes
this mapping to R2. R2 binds label L2 to X, and distributes this this binding to R2. R2 binds label L2 to X, and distributes this
mapping to both R1 and R4. When R2 receives packet P1, its incoming binding to both R1 and R4. When R2 receives packet P1, its incoming
label will be L2. R2 will overwrite L2 with L3, and send P1 to R3. label will be L2. R2 will overwrite L2 with L3, and send P1 to R3.
When R2 receives packet P2, its incoming label will also be L2. R2 When R2 receives packet P2, its incoming label will also be L2. R2
again overwrites L2 with L3, and send P2 on to R3. again overwrites L2 with L3, and send P2 on to R3.
Note then that when P1 and P2 are traveling from R2 to R3, they carry Note then that when P1 and P2 are traveling from R2 to R3, they carry
the same label, and as far as MPLS is concerned, they cannot be the same label, and as far as MPLS is concerned, they cannot be
distinguished. Thus instead of talking about two distinct LSPs, <R1, distinguished. Thus instead of talking about two distinct LSPs, <R1,
R2, R3> and <R4, R2, R3>, we might talk of a single "Multipoint-to- R2, R3> and <R4, R2, R3>, we might talk of a single "Multipoint-to-
Point LSP Tree", which we might denote as <{R1, R4}, R2, R3>. Point LSP Tree", which we might denote as <{R1, R4}, R2, R3>.
This creates a difficulty when we attempt to use conventional ATM This creates a difficulty when we attempt to use conventional ATM
switches as LSRs. Since conventional ATM switches do not support switches as LSRs. Since conventional ATM switches do not support
multipoint-to-point connections, there must be procedures to ensure multipoint-to-point connections, there must be procedures to ensure
that each LSP is realized as a point-to-point VC. However, if ATM that each LSP is realized as a point-to-point VC. However, if ATM
switches which do support multipoint-to-point VCs are in use, then switches which do support multipoint-to-point VCs are in use, then
the LSPs can be most efficiently realized as multipoint-to-point VCs. the LSPs can be most efficiently realized as multipoint-to-point VCs.
Alternatively, if the SVP Multipoint Encoding (section 2.23) can be Alternatively, if the SVP Multipoint Encoding (section 2.24.2) can be
used, the LSPs can be realized as multipoint-to-point SVPs. used, the LSPs can be realized as multipoint-to-point SVPs.
3.6. LSP Tunneling between BGP Border Routers 3.6. LSP Tunneling between BGP Border Routers
Consider the case of an Autonomous System, A, which carries transit Consider the case of an Autonomous System, A, which carries transit
traffic between other Autonomous Systems. Autonomous System A will traffic between other Autonomous Systems. Autonomous System A will
have a number of BGP Border Routers, and a mesh of BGP connections have a number of BGP Border Routers, and a mesh of BGP connections
among them, over which BGP routes are distributed. In many such among them, over which BGP routes are distributed. In many such
cases, it is desirable to avoid distributing the BGP routes to cases, it is desirable to avoid distributing the BGP routes to
routers which are not BGP Border Routers. If this can be avoided, routers which are not BGP Border Routers. If this can be avoided,
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2. The IGP for the Autonomous System maintains a host route for 2. The IGP for the Autonomous System maintains a host route for
each BGP Border Router. Each interior router distributes its each BGP Border Router. Each interior router distributes its
labels for these host routes to each of its IGP neighbors. labels for these host routes to each of its IGP neighbors.
3. Suppose that: 3. Suppose that:
a) BGP Border Router B1 receives an unlabeled packet P, a) BGP Border Router B1 receives an unlabeled packet P,
b) address prefix X in B1's routing table is the longest b) address prefix X in B1's routing table is the longest
match for the destination address of P, match for the destination address of P,
c) the route to X is a BGP route, c) the route to X is a BGP route,
d) the BGP Next Hop for X is B2, d) the BGP Next Hop for X is B2,
e) B2 has bound label L1 to X, and has distributed this e) B2 has bound label L1 to X, and has distributed this
mapping to B1, binding to B1,
f) the IGP next hop for the address of B2 is I1, f) the IGP next hop for the address of B2 is I1,
g) the address of B2 is in B1's and I1's IGP routing tables g) the address of B2 is in B1's and I1's IGP routing tables
as a host route, and as a host route, and
h) I1 has bound label L2 to the address of B2, and h) I1 has bound label L2 to the address of B2, and
distributed this mapping to B1. distributed this binding to B1.
Then before sending packet P to I1, B1 must create a label Then before sending packet P to I1, B1 must create a label
stack for P, then push on label L1, and then push on label L2. stack for P, then push on label L1, and then push on label L2.
4. Suppose that BGP Border Router B1 receives a labeled Packet P, 4. Suppose that BGP Border Router B1 receives a labeled Packet P,
where the label on the top of the label stack corresponds to an where the label on the top of the label stack corresponds to an
address prefix, X, to which the route is a BGP route, and that address prefix, X, to which the route is a BGP route, and that
conditions 3b, 3c, 3d, and 3e all hold. Then before sending conditions 3b, 3c, 3d, and 3e all hold. Then before sending
packet P to I1, B1 must replace the label at the top of the packet P to I1, B1 must replace the label at the top of the
label stack with L1, and then push on label L2. label stack with L1, and then push on label L2.
With these procedures, a given packet P follows a level 1 LSP all of With these procedures, a given packet P follows a level 1 LSP all of
whose members are BGP Border Routers, and between each pair of BGP whose members are BGP Border Routers, and between each pair of BGP
Border Routers in the level 1 LSP, it follows a level 2 LSP. Border Routers in the level 1 LSP, it follows a level 2 LSP.
These procedures effectively create a Hop-by-Hop Routed LSP Tunnel These procedures effectively create a Hop-by-Hop Routed LSP Tunnel
between the BGP Border Routers. between the BGP Border Routers.
Since the BGP border routers are exchanging label mappings for Since the BGP border routers are exchanging label bindings for
address prefixes that are not even known to the IGP routing, the BGP address prefixes that are not even known to the IGP routing, the BGP
routers should become explicit LDP peers with each other. routers should become explicit LDP peers with each other.
3.7. Other Uses of Hop-by-Hop Routed LSP Tunnels 3.7. Other Uses of Hop-by-Hop Routed LSP Tunnels
The use of Hop-by-Hop Routed LSP Tunnels is not restricted to tunnels The use of Hop-by-Hop Routed LSP Tunnels is not restricted to tunnels
between BGP Next Hops. Any situation in which one might otherwise between BGP Next Hops. Any situation in which one might otherwise
have used an encapsulation tunnel is one in which it is appropriate have used an encapsulation tunnel is one in which it is appropriate
to use a Hop-by-Hop Routed LSP Tunnel. Instead of encapsulating the to use a Hop-by-Hop Routed LSP Tunnel. Instead of encapsulating the
packet with a new header whose destination address is the address of packet with a new header whose destination address is the address of
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prefix which is the longest match for the address of the tunnel's prefix which is the longest match for the address of the tunnel's
receive endpoint is pushed on the packet's label stack. The packet receive endpoint is pushed on the packet's label stack. The packet
which is sent into the tunnel may or may not already be labeled. which is sent into the tunnel may or may not already be labeled.
If the transmit endpoint of the tunnel wishes to put a labeled packet If the transmit endpoint of the tunnel wishes to put a labeled packet
into the tunnel, it must first replace the label value at the top of into the tunnel, it must first replace the label value at the top of
the stack with a label value that was distributed to it by the the stack with a label value that was distributed to it by the
tunnel's receive endpoint. Then it must push on the label which tunnel's receive endpoint. Then it must push on the label which
corresponds to the tunnel itself, as distributed to it by the next corresponds to the tunnel itself, as distributed to it by the next
hop along the tunnel. To allow this, the tunnel endpoints should be hop along the tunnel. To allow this, the tunnel endpoints should be
explicit LDP peers. The label mappings they need to exchange are of explicit LDP peers. The label bindings they need to exchange are of
no interest to the LSRs along the tunnel. no interest to the LSRs along the tunnel.
3.8. MPLS and Multicast 3.8. MPLS and Multicast
Multicast routing proceeds by constructing multicast trees. The tree Multicast routing proceeds by constructing multicast trees. The tree
along which a particular multicast packet must get forwarded depends along which a particular multicast packet must get forwarded depends
in general on the packet's source address and its destination in general on the packet's source address and its destination
address. Whenever a particular LSR is a node in a particular address. Whenever a particular LSR is a node in a particular
multicast tree, it binds a label to that tree. It then distributes multicast tree, it binds a label to that tree. It then distributes
that mapping to its parent on the multicast tree. (If the node in that binding to its parent on the multicast tree. (If the node in
question is on a LAN, and has siblings on that LAN, it must also question is on a LAN, and has siblings on that LAN, it must also
distribute the mapping to its siblings. This allows the parent to distribute the binding to its siblings. This allows the parent to
use a single label value when multicasting to all children on the use a single label value when multicasting to all children on the
LAN.) LAN.)
When a multicast labeled packet arrives, the NHLFE corresponding to When a multicast labeled packet arrives, the NHLFE corresponding to
the label indicates the set of output interfaces for that packet, as the label indicates the set of output interfaces for that packet, as
well as the outgoing label. If the same label encoding technique is well as the outgoing label. If the same label encoding technique is
used on all the outgoing interfaces, the very same packet can be sent used on all the outgoing interfaces, the very same packet can be sent
to all the children. to all the children.
4. LDP Procedures for Hop-by-Hop Routed Traffic 4. LDP Procedures for Hop-by-Hop Routed Traffic
4.1. The Procedures for Advertising and Using labels 4.1. The Procedures for Advertising and Using labels
In this section, we consider only label mappings that are used for In this section, we consider only label bindings that are used for
traffic to be label switched along its hop-by-hop routed path. In traffic to be label switched along its hop-by-hop routed path. In
these cases, the label in question will correspond to an address these cases, the label in question will correspond to an address
prefix in the routing table. prefix in the routing table.
There are a number of different procedures that may be used to There are a number of different procedures that may be used to
distribute label mappings. One such procedure is executed by the distribute label bindings. One such procedure is executed by the
downstream LSR, and the others by the upstream LSR. downstream LSR, and the others by the upstream LSR.
The downstream LSR must perform: The downstream LSR must perform:
- The Distribution Procedure, and - The Distribution Procedure, and
- the Withdrawal Procedure. - the Withdrawal Procedure.
The upstream LSR must perform: The upstream LSR must perform:
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However, the MPLS architecture does not support all possible However, the MPLS architecture does not support all possible
combinations of all possible variants. The set of supported combinations of all possible variants. The set of supported
combinations will be described in section 4.2, where the combinations will be described in section 4.2, where the
interoperability between different combinations will also be interoperability between different combinations will also be
discussed. discussed.
4.1.1. Downstream LSR: Distribution Procedure 4.1.1. Downstream LSR: Distribution Procedure
The Distribution Procedure is used by a downstream LSR to determine The Distribution Procedure is used by a downstream LSR to determine
when it should distribute a label mapping for a particular address when it should distribute a label binding for a particular address
prefix to its LDP peers. The architecture supports four different prefix to its LDP peers. The architecture supports four different
distribution procedures. distribution procedures.
Irrespective of the particular procedure that is used, if a label Irrespective of the particular procedure that is used, if a label
mapping for a particular address prefix has been distributed by a binding for a particular address prefix has been distributed by a
downstream LSR Rd to an upstream LSR Ru, and if at any time the downstream LSR Rd to an upstream LSR Ru, and if at any time the
attributes (as defined above) of that mapping change, then Rd must attributes (as defined above) of that binding change, then Rd must
inform Ru of the new attributes. inform Ru of the new attributes.
If an LSR is maintaining multiple routes to a particular address If an LSR is maintaining multiple routes to a particular address
prefix, it is a local matter as to whether that LSR maps multiple prefix, it is a local matter as to whether that LSR binds multiple
labels to the address prefix (one per route), and hence distributes labels to the address prefix (one per route), and hence distributes
multiple mappings. multiple bindings.
4.1.1.1. PushUnconditional 4.1.1.1. PushUnconditional
Let Rd be an LSR. Suppose that: Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table 1. X is an address prefix in Rd's routing table
2. Ru is an LDP Peer of Rd with respect to X 2. Ru is an LDP Peer of Rd with respect to X
Whenever these conditions hold, Rd must map a label to X and Whenever these conditions hold, Rd must bind a label to X and
distribute that mapping to Ru. It is the responsibility of Rd to distribute that binding to Ru. It is the responsibility of Rd to
keep track of the mappings which it has distributed to Ru, and to keep track of the bindings which it has distributed to Ru, and to
make sure that Ru always has these mappings. make sure that Ru always has these bindings.
This procedure would be used by LSRs which are performing downstream
label assignment in the Independent LSP Control Mode.
4.1.1.2. PushConditional 4.1.1.2. PushConditional
Let Rd be an LSR. Suppose that: Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table 1. X is an address prefix in Rd's routing table
2. Ru is an LDP Peer of Rd with respect to X 2. Ru is an LDP Peer of Rd with respect to X
3. Rd is either an LSP Egress or an LSP Proxy Egress for X, or 3. Rd is either an LSP Egress or an LSP Proxy Egress for X, or
Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and
Rn has bound a label to X and distributed that mapping to Rd. Rn has bound a label to X and distributed that binding to Rd.
Then as soon as these conditions all hold, Rd should map a label to X Then as soon as these conditions all hold, Rd should bind a label to
and distribute that mapping to Ru. X and distribute that binding to Ru.
Whereas PushUnconditional causes the distribution of label mappings Whereas PushUnconditional causes the distribution of label bindings
for all address prefixes in the routing table, PushConditional causes for all address prefixes in the routing table, PushConditional causes
the distribution of label mappings only for those address prefixes the distribution of label bindings only for those address prefixes
for which one has received label mappings from one's LSP next hop, or for which one has received label bindings from one's LSP next hop, or
for which one does not have an MPLS-capable L3 next hop. for which one does not have an MPLS-capable L3 next hop.
This procedure would be used by LSRs which are performing downstream
label assignment in the Ordered LSP Control Mode.
4.1.1.3. PulledUnconditional 4.1.1.3. PulledUnconditional
Let Rd be an LSR. Suppose that: Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table 1. X is an address prefix in Rd's routing table
2. Ru is a label distribution peer of Rd with respect to X 2. Ru is a label distribution peer of Rd with respect to X
3. Ru has explicitly requested that Rd bind a label to X and
distribute the binding to Ru
3. Ru has explicitly requested that Rd map a label to X and Then Rd should bind a label to X and distribute that binding to Ru.
distribute the mapping to Ru
Then Rd should map a label to X and distribute that mapping to Ru.
Note that if X is not in Rd's routing table, or if Rd is not an LDP Note that if X is not in Rd's routing table, or if Rd is not an LDP
peer of Ru with respect to X, then Rd must inform Ru that it cannot peer of Ru with respect to X, then Rd must inform Ru that it cannot
provide a mapping at this time. provide a binding at this time.
If Rd has already distributed a mapping for address prefix X to Ru, If Rd has already distributed a binding for address prefix X to Ru,
and it receives a new request from Ru for a mapping for address and it receives a new request from Ru for a binding for address
prefix X, it will map a second label, and distribute the new mapping prefix X, it will bind a second label, and distribute the new binding
to Ru. The first label mapping remains in effect. to Ru. The first label binding remains in effect.
This procedure would be used by LSRs performing downstream-on-demand
label distribution using the Independent LSP Control Mode.
4.1.1.4. PulledConditional 4.1.1.4. PulledConditional
Let Rd be an LSR. Suppose that: Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table 1. X is an address prefix in Rd's routing table
2. Ru is a label distribution peer of Rd with respect to X 2. Ru is a label distribution peer of Rd with respect to X
3. Ru has explicitly requested that Rd map a label to X and 3. Ru has explicitly requested that Rd bind a label to X and
distribute the mapping to Ru distribute the binding to Ru
4. Rd is either an LSP Egress or an LSP Proxy Egress for X, or 4. Rd is either an LSP Egress or an LSP Proxy Egress for X, or
Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and
Rn has bound a label to X and distributed that mapping to Rd, Rn has bound a label to X and distributed that binding to Rd
or
Then as soon as these conditions all hold, Rd should map a label to X Then as soon as these conditions all hold, Rd should bind a label to
and distribute that mapping to Ru. Note that if X is not in Rd's X and distribute that binding to Ru. Note that if X is not in Rd's
routing table, or if Rd is not a label distribution peer of Ru with routing table, or if Rd is not a label distribution peer of Ru with
respect to X, then Rd must inform Ru that it cannot provide a mapping respect to X, then Rd must inform Ru that it cannot provide a binding
at this time. at this time.
However, if the only condition that fails to hold is that Rn has not However, if the only condition that fails to hold is that Rn has not
yet provided a label to Rd, then Rd must defer any response to Ru yet provided a label to Rd, then Rd must defer any response to Ru
until such time as it has receiving a mapping from Rn. until such time as it has receiving a binding from Rn.
If Rd has distributed a label mapping for address prefix X to Ru, and If Rd has distributed a label binding for address prefix X to Ru, and
at some later time, any attribute of the label mapping changes, then at some later time, any attribute of the label binding changes, then
Rd must redistribute the label mapping to Ru, with the new attribute. Rd must redistribute the label binding to Ru, with the new attribute.
It must do this even though Ru does not issue a new Request. It must do this even though Ru does not issue a new Request.
This procedure would be used by LSRs that are performing downstream-
on-demand label allocation in the Ordered LSP Control Mode.
In section 4.2, we will discuss how to choose the particular In section 4.2, we will discuss how to choose the particular
procedure to be used at any given time, and how to ensure procedure to be used at any given time, and how to ensure
interoperability among LSRs that choose different procedures. interoperability among LSRs that choose different procedures.
4.1.2. Upstream LSR: Request Procedure 4.1.2. Upstream LSR: Request Procedure
The Request Procedure is used by the upstream LSR for an address The Request Procedure is used by the upstream LSR for an address
prefix to determine when to explicitly request that the downstream prefix to determine when to explicitly request that the downstream
LSR map a label to that prefix and distribute the mapping. There are LSR bind a label to that prefix and distribute the binding. There
three possible procedures that can be used. are three possible procedures that can be used.
4.1.2.1. RequestNever 4.1.2.1. RequestNever
Never make a request. This is useful if the downstream LSR uses the Never make a request. This is useful if the downstream LSR uses the
PushConditional procedure or the PushUnconditional procedure, but is PushConditional procedure or the PushUnconditional procedure, but is
not useful if the downstream LSR uses the PulledUnconditional not useful if the downstream LSR uses the PulledUnconditional
procedure or the the Pulledconditional procedures. procedure or the the PulledConditional procedures.
This procedure would be used by an LSR when downstream label
distribution and Liberal Label Retention Mode are being used.
4.1.2.2. RequestWhenNeeded 4.1.2.2. RequestWhenNeeded
Make a request whenever the L3 next hop to the address prefix Make a request whenever the L3 next hop to the address prefix
changes, and one doesn't already have a label mapping from that next changes, and one doesn't already have a label binding from that next
hop for the given address prefix. hop for the given address prefix.
This procedure would be used by an LSR whenever Conservative Label
Retention Mode is being used.
4.1.2.3. RequestOnRequest 4.1.2.3. RequestOnRequest
Issue a request whenever a request is received, in addition to Issue a request whenever a request is received, in addition to
issuing a request when needed (as described in section 4.1.2.2). If issuing a request when needed (as described in section 4.1.2.2). If
Rd receives such a request from Ru, for an address prefix for which Rd receives such a request from Ru, for an address prefix for which
Rd has already distributed Ru a label, Rd shall assign a new Rd has already distributed Ru a label, Rd shall assign a new
(distinct) label, map it to X, and distribute that mapping. (Whether (distinct) label, bind it to X, and distribute that binding.
Rd can distribute this mapping to Ru immediately or not depends on (Whether Rd can distribute this binding to Ru immediately or not
the Distribution Procedure being used.) depends on the Distribution Procedure being used.)
This procedure is useful when the LSRs are implemented on This procedure would be used by an LSR which doing downstream-on-
conventional ATM switching hardware. demand label distribution, but is not doing label merging, e.g., an
ATM-LSR which is not capable of VC merge.
4.1.3. Upstream LSR: NotAvailable Procedure 4.1.3. Upstream LSR: NotAvailable Procedure
If Ru and Rd are respectively upstream and downstream label If Ru and Rd are respectively upstream and downstream label
distribution peers for address prefix X, and Rd is Ru's L3 next hop distribution peers for address prefix X, and Rd is Ru's L3 next hop
for X, and Ru requests a mapping for X from Rd, but Rd replies that for X, and Ru requests a binding for X from Rd, but Rd replies that
it cannot provide a mapping at this time, then the NotAvailable it cannot provide a binding at this time, then the NotAvailable
procedure determines how Ru responds. There are two possible procedure determines how Ru responds. There are two possible
procedures governing Ru's behavior: procedures governing Ru's behavior:
4.1.3.1. RequestRetry 4.1.3.1. RequestRetry
Ru should issue the request again at a later time. That is, the Ru should issue the request again at a later time. That is, the
requester is responsible for trying again later to obtain the needed requester is responsible for trying again later to obtain the needed
mapping. binding. This procedure would be used when downstream-on-demand
label distribution is used.
4.1.3.2. RequestNoRetry 4.1.3.2. RequestNoRetry
Ru should never reissue the request, instead assuming that Rd will Ru should never reissue the request, instead assuming that Rd will
provide the mapping automatically when it is available. This is provide the binding automatically when it is available. This is
useful if Rd uses the PushUnconditional procedure or the useful if Rd uses the PushUnconditional procedure or the
PushConditional procedure. PushConditional procedure, i.e., if downstream label distribution is
used.
4.1.4. Upstream LSR: Release Procedure 4.1.4. Upstream LSR: Release Procedure
Suppose that Rd is an LSR which has bound a label to address prefix Suppose that Rd is an LSR which has bound a label to address prefix
X, and has distributed that mapping to LSR Ru. If Rd does not happen X, and has distributed that binding to LSR Ru. If Rd does not happen
to be Ru's L3 next hop for address prefix X, or has ceased to be Ru's to be Ru's L3 next hop for address prefix X, or has ceased to be Ru's
L3 next hop for address prefix X, then Rd will not be using the L3 next hop for address prefix X, then Rd will not be using the
label. The Release Procedure determines how Ru acts in this case. label. The Release Procedure determines how Ru acts in this case.
There are two possible procedures governing Ru's behavior: There are two possible procedures governing Ru's behavior:
4.1.4.1. ReleaseOnChange 4.1.4.1. ReleaseOnChange
Ru should release the mapping, and inform Rd that it has done so. Ru should release the binding, and inform Rd that it has done so.
This procedure would be used to implement Conservative Label
Retention Mode.
4.1.4.2. NoReleaseOnChange 4.1.4.2. NoReleaseOnChange
Ru should maintain the mapping, so that it can use it again Ru should maintain the binding, so that it can use it again
immediately if Rd later becomes Ru's L3 next hop for X. immediately if Rd later becomes Ru's L3 next hop for X. This
procedure would be used to implement Liberal Label Retention Mode.
4.1.5. Upstream LSR: labelUse Procedure 4.1.5. Upstream LSR: labelUse Procedure
Suppose Ru is an LSR which has received label mapping L for address Suppose Ru is an LSR which has received label binding L for address
prefix X from LSR Rd, and Ru is upstream of Rd with respect to X, and prefix X from LSR Rd, and Ru is upstream of Rd with respect to X, and
in fact Rd is Ru's L3 next hop for X. in fact Rd is Ru's L3 next hop for X.
Ru will make use of the mapping if Rd is Ru's L3 next hop for X. If, Ru will make use of the binding if Rd is Ru's L3 next hop for X. If,
at the time the mapping is received by Ru, Rd is NOT Ru's L3 next hop at the time the binding is received by Ru, Rd is NOT Ru's L3 next hop
for X, Ru does not make any use of the mapping at that time. Ru may for X, Ru does not make any use of the binding at that time. Ru may
however start using the mapping at some later time, if Rd becomes however start using the binding at some later time, if Rd becomes
Ru's L3 next hop for X. Ru's L3 next hop for X.
The labelUse Procedure determines just how Ru makes use of Rd's The labelUse Procedure determines just how Ru makes use of Rd's
mapping. binding.
There are three procedures which Ru may use: There are three procedures which Ru may use:
4.1.5.1. UseImmediate 4.1.5.1. UseImmediate
Ru may put the mapping into use immediately. At any time when Ru has Ru may put the binding into use immediately. At any time when Ru has
a mapping for X from Rd, and Rd is Ru's L3 next hop for X, Rd will a binding for X from Rd, and Rd is Ru's L3 next hop for X, Rd will
also be Ru's LSP next hop for X. also be Ru's LSP next hop for X. This procedure is used when neither
loop prevention nor loop detection are in use.
4.1.5.2. UseIfLoopFree 4.1.5.2. UseIfLoopFree
Ru will use the mapping only if it determines that by doing so, it Ru will use the binding only if it determines that by doing so, it
will not cause a forwarding loop. will not cause a forwarding loop.
If Ru has a mapping for X from Rd, and Rd is (or becomes) Ru's L3 If Ru has a binding for X from Rd, and Rd is (or becomes) Ru's L3
next hop for X, but Rd is NOT Ru's current LSP next hop for X, Ru next hop for X, but Rd is NOT Ru's current LSP next hop for X, Ru
does NOT immediately make Rd its LSP next hop. Rather, it initiates does NOT immediately make Rd its LSP next hop. Rather, it initiates
a loop prevention algorithm. If, upon the completion of this a loop prevention algorithm. If, upon the completion of this
algorithm, Rd is still the L3 next hop for X, Ru will make Rd the LSP algorithm, Rd is still the L3 next hop for X, Ru will make Rd the LSP
next hop for X, and use L as the outgoing label. next hop for X, and use L as the outgoing label.
This procedure is used when loop prevention is in use.
The loop prevention algorithm to be used is still under The loop prevention algorithm to be used is still under
consideration. consideration.
4.1.5.3. UseIfLoopNotDetected 4.1.5.3. UseIfLoopNotDetected
This procedure is the same as UseImmediate, unless Ru has detected a This procedure is the same as UseImmediate, unless Ru has detected a
loop in the LSP. If a loop has been detected, Ru will discard loop in the LSP. If a loop has been detected, Ru will discard
packets that would otherwise have been labeled with L and sent to Rd. packets that would otherwise have been labeled with L and sent to Rd.
This procedure is used when loop detection, but not loop prevention,
is in use.
This will continue until the next hop for X changes, or until the This will continue until the next hop for X changes, or until the
loop is no longer detected. loop is no longer detected.
4.1.6. Downstream LSR: Withdraw Procedure 4.1.6. Downstream LSR: Withdraw Procedure
In this case, there is only a single procedure. In this case, there is only a single procedure.
When LSR Rd decides to break the mapping between label L and address When LSR Rd decides to break the binding between label L and address
prefix X, then this unmapping must be distributed to all LSRs to prefix X, then this unbinding must be distributed to all LSRs to
which the mapping was distributed. which the binding was distributed.
It is desirable, though not required, that the unmapping of L from X It is desirable, though not required, that the unbinding of L from X
be distributed by Rd to a LSR Ru before Rd distributes to Ru any new be distributed by Rd to a LSR Ru before Rd distributes to Ru any new
mapping of L to any other address prefix Y, where X != Y. If Ru binding of L to any other address prefix Y, where X != Y. If Ru
learns of the new mapping of L to Y before it learns of the unmapping learns of the new binding of L to Y before it learns of the unbinding
of L from X, and if packets matching both X and Y are forwarded by Ru of L from X, and if packets matching both X and Y are forwarded by Ru
to Rd, then for a period of time, Ru will label both packets matching to Rd, then for a period of time, Ru will label both packets matching
X and packets matching Y with label L. X and packets matching Y with label L.
The distribution and withdrawal of label mappings is done via a label The distribution and withdrawal of label bindings is done via a label
distribution protocol, or LDP. LDP is a two-party protocol. If LSR R1 distribution protocol, or LDP. LDP is a two-party protocol. If LSR R1
has received label mappings from LSR R2 via an instance of an LDP, has received label bindings from LSR R2 via an instance of an LDP,
and that instance of that protocol is closed by either end (whether and that instance of that protocol is closed by either end (whether
as a result of failure or as a matter of normal operation), then all as a result of failure or as a matter of normal operation), then all
mappings learned over that instance of the protocol must be bindings learned over that instance of the protocol must be
considered to have been withdrawn. considered to have been withdrawn.
As long as the relevant LDP connection remains open, label mappings As long as the relevant LDP connection remains open, label bindings
that are withdrawn must always be withdrawn explicitly. If a second that are withdrawn must always be withdrawn explicitly. If a second
label is bound to an address prefix, the result is not to implicitly label is bound to an address prefix, the result is not to implicitly
withdraw the first label, but to map both labels; this is needed to withdraw the first label, but to bind both labels; this is needed to
support multi-path routing. If a second address prefix is bound to a support multi-path routing. If a second address prefix is bound to a
label, the result is not to implicitly withdraw the mapping of that label, the result is not to implicitly withdraw the binding of that
label to the first address prefix, but to use that label for both label to the first address prefix, but to use that label for both
address prefixes. address prefixes.
4.2. MPLS Schemes: Supported Combinations of Procedures 4.2. MPLS Schemes: Supported Combinations of Procedures
Consider two LSRs, Ru and Rd, which are label distribution peers with Consider two LSRs, Ru and Rd, which are label distribution peers with
respect to some set of address prefixes, where Ru is the upstream respect to some set of address prefixes, where Ru is the upstream
peer and Rd is the downstream peer. peer and Rd is the downstream peer.
The MPLS scheme which governs the interaction of Ru and Rd can be The MPLS scheme which governs the interaction of Ru and Rd can be
skipping to change at page 57, line 18 skipping to change at page 59, line 30
Only the MPLS schemes which are specified below are supported by the Only the MPLS schemes which are specified below are supported by the
MPLS Architecture. Other schemes may be added in the future, if a MPLS Architecture. Other schemes may be added in the future, if a
need for them is shown. need for them is shown.
4.2.1. TTL-capable LSP Segments 4.2.1. TTL-capable LSP Segments
If Ru and Rd are MPLS peers, and both are capable of decrementing a If Ru and Rd are MPLS peers, and both are capable of decrementing a
TTL field in the MPLS header, then the MPLS scheme in use between Ru TTL field in the MPLS header, then the MPLS scheme in use between Ru
and Rd must be one of the following: and Rd must be one of the following:
<PushUnconditional, RequestNever, N/A, NoReleaseOnChange, 1. <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
UseImmediate> UseImmediate>
<PushConditional, RequestWhenNeeded, RequestNoRetry, *, *> This is downstream label distribution with independent control,
liberal label retention mode, and no loop detection.
The former, roughly speaking, is "local control with downstream label 2. <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
assignment". The latter is an egress control scheme. UseIfLoopNotDetected>
This is downstream label distribution with independent control,
liberal label retention, and loop detection.
3. <PushConditional, RequestWhenNeeded, RequestNoRetry,
ReleaseOnChange, *>
This is downstream label distribution with ordered control and
conservative label retention mode. Loop prevention and loop
detection are optional.
4. <PushConditional, RequestNever, N/A, NoReleaseOnChange, *>
This is downstream label distribution with ordered control and
liberal label retention mode. Loop prevention and loop
detection are optional.
4.2.2. Using ATM Switches as LSRs 4.2.2. Using ATM Switches as LSRs
The procedures for using ATM switches as LSRs depends on whether the The procedures for using ATM switches as LSRs depends on whether the
ATM switches can realize LSP trees as multipoint-to-point VCs or VPs. ATM switches can realize LSP trees as multipoint-to-point VCs or VPs.
Most ATM switches existing today do NOT have a multipoint-to-point Most ATM switches existing today do NOT have a multipoint-to-point
VC-switching capability. Their cross-connect tables could easily be VC-switching capability. Their cross-connect tables could easily be
programmed to move cells from multiple incoming VCs to a single programmed to move cells from multiple incoming VCs to a single
outgoing VC, but the result would be that cells from different outgoing VC, but the result would be that cells from different
skipping to change at page 58, line 5 skipping to change at page 60, line 32
Some ATM switches do support a multipoint-to-point VC-switching Some ATM switches do support a multipoint-to-point VC-switching
capability. These switches will queue up all the incoming cells from capability. These switches will queue up all the incoming cells from
an incoming VC until a packet boundary is reached. Then they will an incoming VC until a packet boundary is reached. Then they will
transmit the entire sequence of cells on the outgoing VC, without transmit the entire sequence of cells on the outgoing VC, without
allowing cells from any other packet to be interleaved. allowing cells from any other packet to be interleaved.
Many ATM switches do support a multipoint-to-point VP-switching Many ATM switches do support a multipoint-to-point VP-switching
capability, which can be used if the Multipoint SVP label encoding is capability, which can be used if the Multipoint SVP label encoding is
used. used.
4.2.2.1. Without Multipoint-to-point Capability 4.2.2.1. Without Label Merging
Suppose that R1, R2, R3, and R4 are ATM switches which do not support Suppose that R1, R2, R3, and R4 are ATM switches which do not support
multipoint-to-point capability, but are being used as LSRs. Suppose label merging, but are being used as LSRs. Suppose further that the
further that the L3 hop-by-hop path for address prefix X is <R1, R2, L3 hop-by-hop path for address prefix X is <R1, R2, R3, R4>, and that
R3, R4>, and that packets destined for X can enter the network at any packets destined for X can enter the network at any of these LSRs.
of these LSRs. Since there is no multipoint-to-point capability, the Since there is no multipoint-to-point capability, the LSPs must be
LSPs must be realized as point-to-point VCs, which means that there realized as point-to-point VCs, which means that there needs to be
needs to be three such VCs for address prefix X: <R1, R2, R3, R4>, three such VCs for address prefix X: <R1, R2, R3, R4>, <R2, R3, R4>,
<R2, R3, R4>, and <R3, R4>. and <R3, R4>.
Therefore, if R1 and R2 are MPLS peers, and either is an LSR which is Therefore, if R1 and R2 are MPLS peers, and either is an LSR which is
implemented using conventional ATM switching hardware (i.e., no cell implemented using conventional ATM switching hardware (i.e., no cell
interleave suppression), the MPLS scheme in use between R1 and R2 interleave suppression), the MPLS scheme in use between R1 and R2
must be one of the following: must be one of the following:
<PulledUnconditional, RequestOnRequest, RequestRetry, 1. <PulledUnconditional, RequestOnRequest, RequestRetry,
ReleaseOnChange, UseImmediate> ReleaseOnChange, UseImmediate>
<PulledConditional, RequestOnRequest, RequestNoRetry, This is downstream-on-demand label distribution with
independent control and conservative label retention mode,
without loop prevention or detection.
2. <PulledUnconditional, RequestOnRequest, RequestRetry,
ReleaseOnChange, UseIfLoopNotDetected>
This is downstream-on-demand label distribution with
independent control and conservative label retention mode, with
loop detection.
3. <PulledConditional, RequestOnRequest, RequestNoRetry,
ReleaseOnChange, *> ReleaseOnChange, *>
The use of the RequestOnRequest procedure will cause R4 to distribute This is downstream-on-demand label distribution with ordered
three labels for X to R3; R3 will distribute 2 labels for X to R2, control (initiated by the ingress), conservative label
and R2 will distribute one label for X to R1. retention mode, and optional loop detection or loop prevention.
The first of these procedures is the "optimistic downstream-on- The use of the RequestOnRequest procedure will cause R4 to
demand" variant of local control. The second is the "conservative distribute three labels for X to R3; R3 will distribute 2
downstream-on-demand" variant of local control. labels for X to R2, and R2 will distribute one label for X to
R1.
An egress control scheme which works in the absence of multipoint- 4.2.2.2. With Label Merging
to-point capability is for further study.
4.2.2.2. With Multipoint-To-Point Capability If R1 and R2 are MPLS peers, at least one of which is an ATM-LSR
which supports label merging, then the MPLS scheme in use between R1
and R2 must be one of the following:
If R1 and R2 are MPLS peers, and either of them is an LSR which is 1. <PulledConditional, RequestOnRequest, RequestNoRetry,
implemented using ATM switching hardware with cell interleave ReleaseOnChange, *>
suppression, and neither is an LSR which is implemented using ATM
switching hardware that does not have cell interleave suppression, This is downstream-on-demand label distribution with
then the MPLS scheme in use between R1 and R2 must be one of the
following;
<PushConditional, RequestWhenNeeded, RequestNoRetry, *, *> <PushConditional, RequestWhenNeeded, RequestNoRetry, *, *>
<PushUnconditional, RequestNever, N/A, NoReleaseOnChange, <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
UseImmediate> UseImmediate>
<PulledConditional, RequestOnRequest, RequestNoRetry,
ReleaseOnChange, *>
The first of these is an egress control scheme. The second is is the The first of these is an ordered control scheme. The second is
"downstream" variant of local control. The third is the is the "downstream" variant of independent control. The third
"conservative downstream-on-demand" variant of local control. is the "conservative downstream-on-demand" variant of
independent control.
4.2.3. Interoperability Considerations 4.2.3. Interoperability Considerations
It is easy to see that certain quintuples do NOT yield viable MPLS It is easy to see that certain quintuples do NOT yield viable MPLS
schemes. For example: schemes. For example:
- <PulledUnconditional, RequestNever, *, *, *> - <PulledUnconditional, RequestNever, *, *, *>
<PulledConditional, RequestNever, *, *, *> <PulledConditional, RequestNever, *, *, *>
In these MPLS schemes, the downstream LSR Rd distributes label In these MPLS schemes, the downstream LSR Rd distributes label
mappings to upstream LSR Ru only upon request from Ru, but Ru bindings to upstream LSR Ru only upon request from Ru, but Ru
never makes any such requests. Obviously, these schemes are not never makes any such requests. Obviously, these schemes are not
viable, since they will not result in the proper distribution of viable, since they will not result in the proper distribution of
label mappings. label bindings.
- <*, RequestNever, *, *, ReleaseOnChange> - <*, RequestNever, *, *, ReleaseOnChange>
In these MPLS schemes, Rd releases mappings when it isn't using In these MPLS schemes, Rd releases bindings when it isn't using
them, but it never asks for them again, even if it later has a them, but it never asks for them again, even if it later has a
need for them. These schemes thus do not ensure that label need for them. These schemes thus do not ensure that label
mappings get properly distributed. bindings get properly distributed.
In this section, we specify rules to prevent a pair of LDP peers from In this section, we specify rules to prevent a pair of LDP peers from
adopting procedures which lead to infeasible MPLS Schemes. These adopting procedures which lead to infeasible MPLS Schemes. These
rules require the exchange of information between LDP peers during rules require the exchange of information between LDP peers during
the initialization of the LDP connection between them. the initialization of the LDP connection between them.
1. Each must state whether it is an ATM switch, and if so, whether 1. Each must state whether it is an ATM-LSR, and if so, whether it
it has cell interleave suppression. has cell interleave suppression (i.e., VC merging).
2. If Rd is an ATM switch without cell interleave suppression, it 2. If Rd is an ATM switch without cell interleave suppression, it
must state whether it intends to use the PulledUnconditional must state whether it intends to use the PulledUnconditional
procedure or the Pulledconditional procedure. If the former, procedure or the Pulledconditional procedure. If the former,
Ru MUST use the RequestRetry procedure; if the latter, Ru MUST Ru MUST use the RequestRetry procedure; if the latter, Ru MUST
use the RequestNoRetry procedure. use the RequestNoRetry procedure.
3. If Ru is an ATM switch without cell interleave suppression, it 3. If Ru is an ATM switch without cell interleave suppression, it
must state whether it intends to use the RequestRetry or the must state whether it intends to use the RequestRetry or the
RequestNoRetry procedure. If Rd is an ATM switch without cell RequestNoRetry procedure. If Rd is an ATM switch without cell
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RequestWhenNeeded and RequestNoRetry, or else RequestNever and RequestWhenNeeded and RequestNoRetry, or else RequestNever and
NoReleaseOnChange, respectively. NoReleaseOnChange, respectively.
5. If Ru is an ATM switch with cell interleave suppression, it 5. If Ru is an ATM switch with cell interleave suppression, it
must specify whether it prefers to use RequestWhenNeeded and must specify whether it prefers to use RequestWhenNeeded and
RequestNoRetry, or else RequestNever and NoReleaseOnChange. If RequestNoRetry, or else RequestNever and NoReleaseOnChange. If
Rd is NOT an ATM switch with cell interleave suppression, it Rd is NOT an ATM switch with cell interleave suppression, it
must then use either PushConditional or PushUnconditional, must then use either PushConditional or PushUnconditional,
respectively. respectively.
4.2.4. How to do Loop Prevention 5. Security Considerations
TBD
4.2.5. How to do Loop Detection
TBD.
4.2.6. Security Considerations
Security considerations are not discussed in this version of this Security considerations are not discussed in this version of this
draft. draft.
5. Authors' Addresses 6. Authors' Addresses
Eric C. Rosen Eric C. Rosen
Cisco Systems, Inc. Cisco Systems, Inc.
250 Apollo Drive 250 Apollo Drive
Chelmsford, MA, 01824 Chelmsford, MA, 01824
E-mail: erosen@cisco.com E-mail: erosen@cisco.com
Arun Viswanathan Arun Viswanathan
Lucent Technologies Lucent Technologies
101 Crawford Corner Rd., #4D-537 101 Crawford Corner Rd., #4D-537
skipping to change at page 61, line 13 skipping to change at page 64, line 5
732-332-5163 732-332-5163
E-mail: arunv@dnrc.bell-labs.com E-mail: arunv@dnrc.bell-labs.com
Ross Callon Ross Callon
IronBridge Networks IronBridge Networks
55 Hayden Avenue, 55 Hayden Avenue,
Lexington, MA 02173 Lexington, MA 02173
+1-781-402-8017 +1-781-402-8017
E-mail: rcallon@ironbridgenetworks.com E-mail: rcallon@ironbridgenetworks.com
6. References 7. References
[1] "A Framework for Multiprotocol Label Switching", R.Callon, [1] "A Framework for Multiprotocol Label Switching", R.Callon,
P.Doolan, N.Feldman, A.Fredette, G.Swallow, and A.Viswanathan, work P.Doolan, N.Feldman, A.Fredette, G.Swallow, and A.Viswanathan, work
in progress, Internet Draft <draft-ietf-mpls-framework-02.txt>, in progress, Internet Draft <draft-ietf-mpls-framework-02.txt>,
November 1997. November 1997.
[2] "ARIS: Aggregate Route-Based IP Switching", A. Viswanathan, N. [2] "ARIS: Aggregate Route-Based IP Switching", A. Viswanathan, N.
Feldman, R. Boivie, R. Woundy, work in progress, Internet Draft Feldman, R. Boivie, R. Woundy, work in progress, Internet Draft
<draft-viswanathan-aris-overview-00.txt>, March 1997. <draft-viswanathan-aris-overview-00.txt>, March 1997.
 End of changes. 

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