draft-ietf-mpls-label-encaps-02.txt   draft-ietf-mpls-label-encaps-03.txt 
Network Working Group Eric C. Rosen Network Working Group Eric C. Rosen
Internet Draft Yakov Rekhter Internet Draft Yakov Rekhter
Expiration Date: January 1999 Daniel Tappan Expiration Date: March 1999 Daniel Tappan
Dino Farinacci Dino Farinacci
Guy Fedorkow Guy Fedorkow
Cisco Systems, Inc. Cisco Systems, Inc.
Tony Li Tony Li
Juniper Networks, Inc. Juniper Networks, Inc.
Alex Conta Alex Conta
Lucent Technologies Lucent Technologies
July 1998 September 1998
MPLS Label Stack Encoding MPLS Label Stack Encoding
draft-ietf-mpls-label-encaps-02.txt draft-ietf-mpls-label-encaps-03.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
"Multi-Protocol Label Switching (MPLS)" [1,2,3] requires a set of "Multi-Protocol Label Switching (MPLS)" [1,2] requires a set of
procedures for augmenting network layer packets with "label stacks", procedures for augmenting network layer packets with "label stacks",
thereby turning them into "labeled packets". Routers which support thereby turning them into "labeled packets". Routers which support
MPLS are known as "Label Switching Routers", or "LSRs". In order to MPLS are known as "Label Switching Routers", or "LSRs". In order to
transmit a labeled packet on a particular data link, an LSR must transmit a labeled packet on a particular data link, an LSR must
support an encoding technique which, given a label stack and a support an encoding technique which, given a label stack and a
network layer packet, produces a labeled packet. This document network layer packet, produces a labeled packet. This document
specifies the encoding to be used by an LSR in order to transmit specifies the encoding to be used by an LSR in order to transmit
labeled packets on PPP data links, on LAN data links, and possibly on labeled packets on PPP data links, on LAN data links, and possibly on
other data links as well. On some data links, the label at the top other data links as well. On some data links, the label at the top
of the stack may be encoded in a different manner, but the techniques of the stack may be encoded in a different manner, but the techniques
described here MUST be used to encode the remainder of the label described here MUST be used to encode the remainder of the label
stack. This document also specifies rules and procedures for stack. This document also specifies rules and procedures for
processing the various fields of the label stack encoding. processing the various fields of the label stack encoding.
Table of Contents Table of Contents
1 Introduction ........................................... 3 1 Introduction ........................................... 3
1.1 Specification of Requirements .......................... 3 1.1 Specification of Requirements .......................... 3
2 The Label Stack ........................................ 4 2 The Label Stack ........................................ 3
2.1 Encoding the Label Stack ............................... 4 2.1 Encoding the Label Stack ............................... 3
2.2 Determining the Network Layer Protocol ................. 7 2.2 Determining the Network Layer Protocol ................. 6
2.3 Generating ICMP Messages for Labeled IP Packets ........ 8 2.3 Generating ICMP Messages for Labeled IP Packets ........ 7
2.3.1 Tunneling through a Transit Routing Domain ............. 8 2.3.1 Tunneling through a Transit Routing Domain ............. 7
2.3.2 Tunneling Private Addresses through a Public Backbone .. 9 2.3.2 Tunneling Private Addresses through a Public Backbone .. 8
2.4 Processing the Time to Live Field ...................... 9 2.4 Processing the Time to Live Field ...................... 8
2.4.1 Definitions ............................................ 9 2.4.1 Definitions ............................................ 8
2.4.2 Protocol-independent rules ............................. 9 2.4.2 Protocol-independent rules ............................. 9
2.4.3 IP-dependent rules ..................................... 10 2.4.3 IP-dependent rules ..................................... 9
2.4.4 Translating Between Different Encapsulations ........... 10 2.4.4 Translating Between Different Encapsulations ........... 10
3 Fragmentation and Path MTU Discovery ................... 11 3 Fragmentation and Path MTU Discovery ................... 10
3.1 Terminology ............................................ 12 3.1 Terminology ............................................ 11
3.2 Maximum Initially Labeled IP Datagram Size ............. 13 3.2 Maximum Initially Labeled IP Datagram Size ............. 13
3.3 When are Labeled IP Datagrams Too Big? ................. 14 3.3 When are Labeled IP Datagrams Too Big? ................. 14
3.4 Processing Labeled IPv4 Datagrams which are Too Big .... 14 3.4 Processing Labeled IPv4 Datagrams which are Too Big .... 14
3.5 Processing Labeled IPv6 Datagrams which are Too Big .... 15 3.5 Processing Labeled IPv6 Datagrams which are Too Big .... 15
3.6 Implications with respect to Path MTU Discovery ........ 16 3.6 Implications with respect to Path MTU Discovery ........ 16
4 Transporting Labeled Packets over PPP .................. 17 4 Transporting Labeled Packets over PPP .................. 17
4.1 Introduction ........................................... 17 4.1 Introduction ........................................... 17
4.2 A PPP Network Control Protocol for MPLS ................ 17 4.2 A PPP Network Control Protocol for MPLS ................ 17
4.3 Sending Labeled Packets ................................ 18 4.3 Sending Labeled Packets ................................ 18
4.4 Label Switching Control Protocol Configuration Options . 19 4.4 Label Switching Control Protocol Configuration Options . 19
5 Transporting Labeled Packets over LAN Media ............ 19 5 Transporting Labeled Packets over LAN Media ............ 19
6 Security Considerations ................................ 19 6 Security Considerations ................................ 19
7 Authors' Addresses ..................................... 19 7 Authors' Addresses ..................................... 20
8 References ............................................. 20 8 References ............................................. 21
1. Introduction 1. Introduction
"Multi-Protocol Label Switching (MPLS)" [1,2,3] requires a set of "Multi-Protocol Label Switching (MPLS)" [1,2] requires a set of
procedures for augmenting network layer packets with "label stacks", procedures for augmenting network layer packets with "label stacks",
thereby turning them into "labeled packets". Routers which support thereby turning them into "labeled packets". Routers which support
MPLS are known as "Label Switching Routers", or "LSRs". In order to MPLS are known as "Label Switching Routers", or "LSRs". In order to
transmit a labeled packet on a particular data link, an LSR must transmit a labeled packet on a particular data link, an LSR must
support an encoding technique which, given a label stack and a support an encoding technique which, given a label stack and a
network layer packet, produces a labeled packet. network layer packet, produces a labeled packet.
This document specifies the encoding to be used by an LSR in order to This document specifies the encoding to be used by an LSR in order to
transmit labeled packets on PPP data links and on LAN data links. transmit labeled packets on PPP data links and on LAN data links.
The specified encoding may also be useful for other data links as The specified encoding may also be useful for other data links as
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dependent procedures for IPv4 and IPv6. dependent procedures for IPv4 and IPv6.
LSRs that are implemented on certain switching devices (such as ATM LSRs that are implemented on certain switching devices (such as ATM
switches) may use different encoding techniques for encoding the top switches) may use different encoding techniques for encoding the top
one or two entries of the label stack. When the label stack has one or two entries of the label stack. When the label stack has
additional entries, however, the encoding technique described in this additional entries, however, the encoding technique described in this
document MUST be used for the additional label stack entries. document MUST be used for the additional label stack entries.
1.1. Specification of Requirements 1.1. Specification of Requirements
In this document, several words are used to signify the requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
of the specification. These words are often capitalized. "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
MUST
This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT
This phrase means that the definition is an absolute prohibition
of the specification.
SHOULD
This word, or the adjective "recommended", means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications must be understood and carefully
weighed before choosing a different course.
MAY
This word, or the adjective "optional", means that this item is
one of an allowed set of alternatives. An implementation which
does not include this option MUST be prepared to interoperate
with another implementation which does include the option.
2. The Label Stack 2. The Label Stack
2.1. Encoding the Label Stack 2.1. Encoding the Label Stack
The label stack is represented as a sequence of "label stack The label stack is represented as a sequence of "label stack
entries". Each label stack entry is represented by 4 octets. This entries". Each label stack entry is represented by 4 octets. This
is shown in Figure 1. is shown in Figure 1.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
| Label | CoS |S| TTL | Stack | Label | Exp |S| TTL | Stack
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry
Label: Label Value, 20 bits Label: Label Value, 20 bits
CoS: Class of Service, 3 bits Exp: Experimental Use, 3 bits
S: Bottom of Stack, 1 bit S: Bottom of Stack, 1 bit
TTL: Time to Live, 8 bits TTL: Time to Live, 8 bits
Figure 1 Figure 1
The label stack entries appear AFTER the data link layer headers, but The label stack entries appear AFTER the data link layer headers, but
BEFORE any network layer headers. The top of the label stack appears BEFORE any network layer headers. The top of the label stack appears
earliest in the packet, and the bottom appears latest. The network earliest in the packet, and the bottom appears latest. The network
layer packet immediately follows the label stack entry which has the layer packet immediately follows the label stack entry which has the
S bit set. S bit set.
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This bit is set to one for the last entry in the label stack This bit is set to one for the last entry in the label stack
(i.e., for the bottom of the stack), and zero for all other (i.e., for the bottom of the stack), and zero for all other
label stack entries. label stack entries.
2. Time to Live (TTL) 2. Time to Live (TTL)
This eight-bit field is used to encode a time-to-live value. This eight-bit field is used to encode a time-to-live value.
The processing of this field is described in section 2.4. The processing of this field is described in section 2.4.
3. Class of Service (CoS) 3. Experimental Use
This three-bit field is used to identify a "Class of Service".
The setting of this field is intended to affect the scheduling
and/or discard algorithms which are applied to the packet as it
is transmitted through the network.
When an unlabeled packet is initially labeled, the value
assigned to the CoS field in the label stack entry is
determined by policy. Some possible policies are:
- the CoS value is a function of the IP ToS value
- the CoS value is a function of the packet's input interface
- the CoS value is a function of the "flow type"
Of course, many other policies are also possible.
When an additional label is pushed onto the stack of a packet
that is already labeled:
- in general, the value of the CoS field in the new top stack
entry should be equal to the value of the CoS field of the
old top stack entry;
- however, in some cases, most likely at boundaries between This three-bit field is reserved for experimental use.
network service providers, the value of the CoS field in
the new top stack entry may be determined by policy.
4. Label Value 4. Label Value
This 20-bit field carries the actual value of the Label. This 20-bit field carries the actual value of the Label.
When a labeled packet is received, the label value at the top When a labeled packet is received, the label value at the top
of the stack is looked up. As a result of a successful lookup of the stack is looked up. As a result of a successful lookup
one learns: one learns:
(a) information needed to forward the packet, such as the (a) the next hop to which the packet is to be forwarded;
next hop and the outgoing data link encapsulation;
however, the precise queue to put the packet on, or
information as to how to schedule the packet, may be a
function of both the label value AND the CoS field
value;
(b) the operation to be performed on the label stack before (b) the operation to be performed on the label stack before
forwarding; this operation may be to replace the top forwarding; this operation may be to replace the top
label stack entry with another, or to pop an entry off label stack entry with another, or to pop an entry off
the label stack, or to replace the top label stack entry the label stack, or to replace the top label stack entry
and then to push one or more additional entries on the and then to push one or more additional entries on the
label stack. label stack.
In addition to learning the next hop and the label stack
operation, one may also learn the outgoing data link
encapsulation, and possibly other information which is needed
in order to properly forward the packet.
There are several reserved label values: There are several reserved label values:
i. A value of 0 represents the "IPv4 Explicit NULL Label". i. A value of 0 represents the "IPv4 Explicit NULL Label".
When this label value is the sole label stack entry, it This label value is only legal when it is the sole
indicates that the label stack must be popped, and the label stack entry. It indicates that the label stack
forwarding of the packet must then be based on the IPv4 must be popped, and the forwarding of the packet must
header. When this label value is not the sole label then be based on the IPv4 header.
stack entry, it indicates that the label stack must be
popped, and the forwarding of the packet must then be
based on the label value which then rises to the top of
the stack.
ii. A value of 1 represents the "Router Alert Label". This ii. A value of 1 represents the "Router Alert Label". This
label value is legal anywhere in the label stack except label value is legal anywhere in the label stack except
at the bottom. When a received packet contains this at the bottom. When a received packet contains this
label value at the top of the label stack, it is label value at the top of the label stack, it is
delivered to a local software module for processing. delivered to a local software module for processing.
The actual forwarding of the packet is determined by The actual forwarding of the packet is determined by
the label beneath it in the stack. However, if the the label beneath it in the stack. However, if the
packet is forwarded further, the Router Alert Label packet is forwarded further, the Router Alert Label
should be pushed back onto the label stack before should be pushed back onto the label stack before
forwarding. The use of this label is analogous to the forwarding. The use of this label is analogous to the
use of the "Router Alert Option" in IP packets [7]. use of the "Router Alert Option" in IP packets [6].
Since this label cannot occur at the bottom of the Since this label cannot occur at the bottom of the
stack, it is not associated with a particular network stack, it is not associated with a particular network
layer protocol. layer protocol.
iii. A value of 2 represents the "IPv6 Explicit NULL Label". iii. A value of 2 represents the "IPv6 Explicit NULL Label".
This label value is only legal when it is the sole This label value is only legal when it is the sole
label stack entry. It indicates that the label stack label stack entry. It indicates that the label stack
must be popped, and the forwarding of the packet must must be popped, and the forwarding of the packet must
then be based on the IPv6 header. then be based on the IPv6 header.
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conditions under which it is desirable. For instance, if an conditions under which it is desirable. For instance, if an
intermediate LSR determines that a labeled packet is undeliverable, intermediate LSR determines that a labeled packet is undeliverable,
it may be desirable for that LSR to generate error messages which are it may be desirable for that LSR to generate error messages which are
specific to the packet's network layer. The only means the specific to the packet's network layer. The only means the
intermediate LSR has for identifying the network layer is inspection intermediate LSR has for identifying the network layer is inspection
of the top label and the network layer header. So if intermediate of the top label and the network layer header. So if intermediate
nodes are to be able to generate protocol-specific error messages for nodes are to be able to generate protocol-specific error messages for
labeled packets, all labels in the stack must meet the criteria labeled packets, all labels in the stack must meet the criteria
specified above for labels which appear at the bottom of the stack. specified above for labels which appear at the bottom of the stack.
If a packet cannot be forwarded for some reason (e.g., it exceeds the
data link MTU), and either its network layer protocol cannot be
identified, or there are no specified protocol-dependent rules for
handling the error condition, then the packet MUST be silently
discarded.
2.3. Generating ICMP Messages for Labeled IP Packets 2.3. Generating ICMP Messages for Labeled IP Packets
Section 2.4 and section 3 discuss situations in which it is desirable Section 2.4 and section 3 discuss situations in which it is desirable
to generate ICMP messages for labeled IP packets. In order for a to generate ICMP messages for labeled IP packets. In order for a
particular LSR to be able to generate an ICMP packet and have that particular LSR to be able to generate an ICMP packet and have that
packet sent to the source of the IP packet, two conditions must hold: packet sent to the source of the IP packet, two conditions must hold:
1. it must be possible for that LSR to determine that a particular 1. it must be possible for that LSR to determine that a particular
labeled packet is an IP packet; labeled packet is an IP packet;
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(b) zero. (b) zero.
2.4.2. Protocol-independent rules 2.4.2. Protocol-independent rules
If the outgoing TTL of a labeled packet is 0, then the labeled packet If the outgoing TTL of a labeled packet is 0, then the labeled packet
MUST NOT be further forwarded; nor may the label stack be stripped MUST NOT be further forwarded; nor may the label stack be stripped
off and the packet forwarded as an unlabeled packet. The packet's off and the packet forwarded as an unlabeled packet. The packet's
lifetime in the network is considered to have expired. lifetime in the network is considered to have expired.
Depending on the label value in the label stack entry, the packet MAY Depending on the label value in the label stack entry, the packet MAY
be simply discarded, or it may be passed to "ordinary" network layer be simply discarded, or it may be passed to the appropriate
for error processing (e.g., for the generation of an ICMP error "ordinary" network layer for error processing (e.g., for the
message, see section 2.3). generation of an ICMP error message, see section 2.3).
When a labeled packet is forwarded, the TTL field of the label stack When a labeled packet is forwarded, the TTL field of the label stack
entry at the top of the label stack must be set to the outgoing TTL entry at the top of the label stack MUST be set to the outgoing TTL
value. value.
Note that the outgoing TTL value is a function solely of the incoming Note that the outgoing TTL value is a function solely of the incoming
TTL value, and is independent of whether any labels are pushed or TTL value, and is independent of whether any labels are pushed or
popped before forwarding. There is no significance to the value of popped before forwarding. There is no significance to the value of
the TTL field in any label stack entry which is not at the top of the the TTL field in any label stack entry which is not at the top of the
stack. stack.
2.4.3. IP-dependent rules 2.4.3. IP-dependent rules
We define the "IP TTL" field to be the value of the IPv4 TTL field, We define the "IP TTL" field to be the value of the IPv4 TTL field,
or the value of the IPv6 Hop Limit field, whichever is applicable. or the value of the IPv6 Hop Limit field, whichever is applicable.
When an IP packet is first labeled, the TTL field of the label stack When an IP packet is first labeled, the TTL field of the label stack
entry MUST BE set to the value of the IP TTL field. (If the IP TTL entry MUST BE set to the value of the IP TTL field. (If the IP TTL
field needs to be decremented, as part of the IP processing, it is field needs to be decremented, as part of the IP processing, it is
assumed that this has already been done.) assumed that this has already been done.)
When a label is popped, and the resulting label stack is empty, then When a label is popped, and the resulting label stack is empty, then
the value of the IP TTL field MUST BE replaced with the outgoing TTL the value of the IP TTL field SHOULD BE replaced with the outgoing
value, as defined above. In IPv4 this also requires modification of TTL value, as defined above. In IPv4 this also requires modification
the IP header checksum. of the IP header checksum.
It is recognized that there may be situations where a network
administration prefers to decrement the IPv4 TTL by one as it
traverses an MPLS domain, instead of decrementing the IPv4 TTL by the
number of LSP hops within the domain.
2.4.4. Translating Between Different Encapsulations 2.4.4. Translating Between Different Encapsulations
Sometimes an LSR may receive a labeled packet over, say, a label Sometimes an LSR may receive a labeled packet over, say, a label
switching controlled ATM (LC-ATM) interface [11], and may need to switching controlled ATM (LC-ATM) interface [10], and may need to
send it out over a PPP or LAN link. Then the incoming packet will send it out over a PPP or LAN link. Then the incoming packet will
not be received using the encapsulation specified in this document, not be received using the encapsulation specified in this document,
but the outgoing packet will be sent using the encapsulation but the outgoing packet will be sent using the encapsulation
specified in this document. specified in this document.
In this case, the value of the "incoming TTL" is determined by the In this case, the value of the "incoming TTL" is determined by the
procedures used for carrying labeled packets on, e.g., LC-ATM procedures used for carrying labeled packets on, e.g., LC-ATM
interfaces. TTL processing then proceeds as described above. interfaces. TTL processing then proceeds as described above.
Sometimes an LSR may receive a labeled packet over a PPP or a LAN Sometimes an LSR may receive a labeled packet over a PPP or a LAN
link, and may need to send it out, say, an LC-ATM interface. Then link, and may need to send it out, say, an LC-ATM interface. Then
the incoming packet will be received using the encapsulation the incoming packet will be received using the encapsulation
specified in this document, but the outgoing packet will not be send specified in this document, but the outgoing packet will not be sent
using the encapsulation specified in this document. In this case, using the encapsulation specified in this document. In this case,
the procedure for carrying the value of the "outgoing TTL" is the procedure for carrying the value of the "outgoing TTL" is
determined by the procedures used for carrying labeled packets on, determined by the procedures used for carrying labeled packets on,
e.g., LC-ATM interfaces. e.g., LC-ATM interfaces.
3. Fragmentation and Path MTU Discovery 3. Fragmentation and Path MTU Discovery
Just as it is possible to receive an unlabeled IP datagram which is Just as it is possible to receive an unlabeled IP datagram which is
too large to be transmitted on its output link, it is possible to too large to be transmitted on its output link, it is possible to
receive a labeled packet which is too large to be transmitted on its receive a labeled packet which is too large to be transmitted on its
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which was originally small enough to be transmitted on that link which was originally small enough to be transmitted on that link
becomes too large by virtue of having one or more additional labels becomes too large by virtue of having one or more additional labels
pushed onto its label stack. In label switching, a packet may grow pushed onto its label stack. In label switching, a packet may grow
in size if additional labels get pushed on. Thus if one receives a in size if additional labels get pushed on. Thus if one receives a
labeled packet with a 1500-byte frame payload, and pushes on an labeled packet with a 1500-byte frame payload, and pushes on an
additional label, one needs to forward it as frame with a 1504-byte additional label, one needs to forward it as frame with a 1504-byte
payload. payload.
This section specifies the rules for processing labeled packets which This section specifies the rules for processing labeled packets which
are "too large". In particular, it provides rules which ensure that are "too large". In particular, it provides rules which ensure that
hosts implementing RFC 1191 Path MTU Discovery, and hosts using IPv6, hosts implementing Path MTU Discovery [5], and hosts using IPv6
will be able to generate IP datagrams that do not need fragmentation, [8,9], will be able to generate IP datagrams that do not need
even if they get labeled as the traverse the network. fragmentation, even if those datagrams get labeled as they traverse
the network.
In general, IPv4 hosts which do not implement RFC 1191 Path MTU In general, IPv4 hosts which do not implement Path MTU Discovery [5]
Discovery send IP datagrams which contain no more than 576 bytes. send IP datagrams which contain no more than 576 bytes. Since the
Since the MTUs in use on most data links today are 1500 bytes or MTUs in use on most data links today are 1500 bytes or more, the
more, the probability that such datagrams will need to get probability that such datagrams will need to get fragmented, even if
fragmented, even if they get labeled, is very small. they get labeled, is very small.
Some hosts that do not implement RFC 1191 Path MTU Discovery will Some hosts that do not implement Path MTU Discovery [5] will generate
generate IP datagrams containing 1500 bytes, as long as the IP Source IP datagrams containing 1500 bytes, as long as the IP Source and
and Destination addresses are on the same subnet. These datagrams Destination addresses are on the same subnet. These datagrams will
will not pass through routers, and hence will not get fragmented. not pass through routers, and hence will not get fragmented.
Unfortunately, some hosts will generate IP datagrams containing 1500 Unfortunately, some hosts will generate IP datagrams containing 1500
bytes, as long the IP Source and Destination addresses do not have bytes, as long the IP Source and Destination addresses do not have
the same classful network number. This is the one case in which the same classful network number. This is the one case in which
there is any risk of fragmentation when such datagrams get labeled. there is any risk of fragmentation when such datagrams get labeled.
(Even so, fragmentation is not likely unless the packet must traverse (Even so, fragmentation is not likely unless the packet must traverse
an ethernet of some sort between the time it first gets labeled and an ethernet of some sort between the time it first gets labeled and
the time it gets unlabeled.) the time it gets unlabeled.)
This document specifies procedures which allow one to configure the This document specifies procedures which allow one to configure the
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With respect to a particular data link, we can use the following With respect to a particular data link, we can use the following
terms: terms:
- Frame Payload: - Frame Payload:
The contents of a data link frame, excluding any data link layer The contents of a data link frame, excluding any data link layer
headers or trailers (e.g., MAC headers, LLC headers, 802.1Q headers or trailers (e.g., MAC headers, LLC headers, 802.1Q
headers, PPP header, frame check sequences, etc.). headers, PPP header, frame check sequences, etc.).
When a frame is carrying an an unlabeled IP datagram, the Frame When a frame is carrying an unlabeled IP datagram, the Frame
Payload is just the IP datagram itself. When a frame is carrying Payload is just the IP datagram itself. When a frame is carrying
a labeled IP datagram, the Frame Payload consists of the label a labeled IP datagram, the Frame Payload consists of the label
stack entries and the IP datagram. stack entries and the IP datagram.
- Conventional Maximum Frame Payload Size: - Conventional Maximum Frame Payload Size:
The maximum Frame Payload size allowed by data link standards. The maximum Frame Payload size allowed by data link standards.
For example, the Conventional Maximum Frame Payload Size for For example, the Conventional Maximum Frame Payload Size for
ethernet is 1500 bytes. ethernet is 1500 bytes.
- True Maximum Frame Payload Size: - True Maximum Frame Payload Size:
The maximum size frame payload which can be sent and received The maximum size frame payload which can be sent and received
properly by the interface hardware attached to the data link. properly by the interface hardware attached to the data link.
On ethernet and 802.3 networks, it is believed that the True On ethernet and 802.3 networks, it is believed that the True
Maximum Frame Payload Size is 4-8 bytes larger than the Maximum Frame Payload Size is 4-8 bytes larger than the
Conventional Maximum Frame Payload Size (as long neither an Conventional Maximum Frame Payload Size (as long as neither an
802.1Q header nor an 802.1p header is present, and as long as 802.1Q header nor an 802.1p header is present, and as long as
neither can be added by a switch or bridge while a packet is in neither can be added by a switch or bridge while a packet is in
transit to its next hop). For example, it is believed that most transit to its next hop). For example, it is believed that most
ethernet equipment could correctly send and receive packets ethernet equipment could correctly send and receive packets
carrying a payload of 1504 or perhaps even 1508 bytes, at least, carrying a payload of 1504 or perhaps even 1508 bytes, at least,
as long as the ethernet header does not have an 802.1Q or 802.1p as long as the ethernet header does not have an 802.1Q or 802.1p
field. field.
On PPP links, the True Maximum Frame Payload Size may be On PPP links, the True Maximum Frame Payload Size may be
virtually unbounded. virtually unbounded.
- Effective Maximum Frame Payload Size for Labeled Packets: - Effective Maximum Frame Payload Size for Labeled Packets:
This is either be the Conventional Maximum Frame Payload Size or This is either the Conventional Maximum Frame Payload Size or the
the True Maximum Frame Payload Size, depending on the True Maximum Frame Payload Size, depending on the capabilities of
capabilities of the equipment on the data link and the size of the equipment on the data link and the size of the data link
the ethernet header being used. header being used.
- Initially Labeled IP Datagram - Initially Labeled IP Datagram:
Suppose that an unlabeled IP datagram is received at a particular Suppose that an unlabeled IP datagram is received at a particular
LSR, and that the the LSR pushes on a label before forwarding the LSR, and that the the LSR pushes on a label before forwarding the
datagram. Such a datagram will be called an Initially Labeled IP datagram. Such a datagram will be called an Initially Labeled IP
Datagram at that LSR. Datagram at that LSR.
- Previously Labeled IP Datagram - Previously Labeled IP Datagram:
An IP datagram which had already been labeled before it was An IP datagram which had already been labeled before it was
received by a particular LSR. received by a particular LSR.
3.2. Maximum Initially Labeled IP Datagram Size 3.2. Maximum Initially Labeled IP Datagram Size
Every LSR which is capable of Every LSR which is capable of
(a) receiving an unlabeled IP datagram, (a) receiving an unlabeled IP datagram,
(b) adding a label stack to the datagram, and (b) adding a label stack to the datagram, and
(c) forwarding the resulting labeled packet, (c) forwarding the resulting labeled packet,
MUST support a configuration parameter known as the "Maximum IP SHOULD support a configuration parameter known as the "Maximum
Datagram Size for Labeling", which can be set to a non-negative Initially Labeled IP Datagram Size", which can be set to a non-
value. negative value.
If this configuration parameter is set to zero, it has no effect. If this configuration parameter is set to zero, it has no effect.
If it is set to a positive value, it is used in the following way. If it is set to a positive value, it is used in the following way.
If: If:
(a) an unlabeled IP datagram is received, and (a) an unlabeled IP datagram is received, and
(b) that datagram does not have the DF bit set in its IP header, (b) that datagram does not have the DF bit set in its IP header,
and and
(c) that datagram needs to be labeled before being forwarded, and (c) that datagram needs to be labeled before being forwarded, and
(d) the size of the datagram (before labeling) exceeds the value (d) the size of the datagram (before labeling) exceeds the value
of the parameter, of the parameter,
then then
(a) the datagram must be broken into fragments, each of whose size (a) the datagram must be broken into fragments, each of whose size
is no greater than the value of the parameter, and is no greater than the value of the parameter, and
(b) each fragment must be labeled and then forwarded. (b) each fragment must be labeled and then forwarded.
If this configuration parameter is set to a value of 1488, for For example, if this configuration parameter is set to a value of
example, then any unlabeled IP datagram containing more than 1488 1488, then any unlabeled IP datagram containing more than 1488 bytes
bytes will be fragmented before being labeled. Each fragment will be will be fragmented before being labeled. Each fragment will be
capable of being carried on a 1500-byte data link, without further capable of being carried on a 1500-byte data link, without further
fragmentation, even if as many as three labels are pushed onto its fragmentation, even if as many as three labels are pushed onto its
label stack. label stack.
In other words, setting this parameter to a non-zero value allows one In other words, setting this parameter to a non-zero value allows one
to eliminate all fragmentation of Previously Labeled IP Datagrams, to eliminate all fragmentation of Previously Labeled IP Datagrams,
but it may cause some unnecessary fragmentation of Initially Labeled but it may cause some unnecessary fragmentation of Initially Labeled
IP Datagrams. IP Datagrams.
Note that the parameter has no effect on IP Datagrams that have the Note that the setting of this parameter does not affect the
DF bit set, which means that it has no effect on Path MTU Discovery. processing of IP datagrams that have the DF bit set; hence the result
of Path MTU discovery is unaffected by the setting of this parameter.
3.3. When are Labeled IP Datagrams Too Big? 3.3. When are Labeled IP Datagrams Too Big?
A labeled IP datagram whose size exceeds the Conventional Maximum A labeled IP datagram whose size exceeds the Conventional Maximum
Frame Payload Size of the data link over which it is to be forwarded Frame Payload Size of the data link over which it is to be forwarded
MAY be considered to be "too big". MAY be considered to be "too big".
A labeled IP datagram whose size exceeds the True Maximum Frame A labeled IP datagram whose size exceeds the True Maximum Frame
Payload Size of the data link over which it is to be forwarded MUST Payload Size of the data link over which it is to be forwarded MUST
be considered to be "too big". be considered to be "too big".
A labeled IP datagram which is not "too big" MUST be transmitted A labeled IP datagram which is not "too big" MUST be transmitted
without fragmentation. without fragmentation.
3.4. Processing Labeled IPv4 Datagrams which are Too Big 3.4. Processing Labeled IPv4 Datagrams which are Too Big
If a labeled IPv4 datagram is "too big", and the DF bit is not set in If a labeled IPv4 datagram is "too big", and the DF bit is not set in
its IP header, then the LSR MAY discard the datagram. its IP header, then the LSR MAY silently discard the datagram.
Note that discarding such datagrams is a sensible procedure only if Note that discarding such datagrams is a sensible procedure only if
the "Maximum Initially Labeled IP Datagram Size" is set to a non-zero the "Maximum Initially Labeled IP Datagram Size" is set to a non-zero
value in every LSR in the network which is capable of adding a label value in every LSR in the network which is capable of adding a label
stack to an unlabeled IP datagram. stack to an unlabeled IP datagram.
If the LSR chooses not to discard a labeled IPv4 datagram which is If the LSR chooses not to discard a labeled IPv4 datagram which is
too big, or if the DF bit is set in that datagram, then it MUST too big, or if the DF bit is set in that datagram, then it MUST
execute the following algorithm: execute the following algorithm:
skipping to change at page 15, line 10 skipping to change at page 15, line 4
a. convert it into fragments, each of which MUST be at least a. convert it into fragments, each of which MUST be at least
N bytes less than the Effective Maximum Frame Payload N bytes less than the Effective Maximum Frame Payload
Size. Size.
b. Prepend each fragment with the same label header that b. Prepend each fragment with the same label header that
would have been on the original datagram had would have been on the original datagram had
fragmentation not been necessary. fragmentation not been necessary.
c. Forward the fragments c. Forward the fragments
4. If the IP datagram has the "Don't Fragment" bit set in its IP 4. If the IP datagram has the "Don't Fragment" bit set in its IP
header: header:
a. the datagram MUST NOT be forwarded a. the datagram MUST NOT be forwarded
b. Create an ICMP Destination Unreachable Message: b. Create an ICMP Destination Unreachable Message:
i. set its Code field (RFC 792) to "Fragmentation i. set its Code field [4] to "Fragmentation Required
Required and DF Set", and DF Set",
ii. set its Next-Hop MTU field (RFC 1191) to the ii. set its Next-Hop MTU field [5] to the difference
difference between the Effective Maximum Frame between the Effective Maximum Frame Payload Size
Payload Size and the value of N and the value of N
c. If possible, transmit the ICMP Destination Unreachable c. If possible, transmit the ICMP Destination Unreachable
Message to the source of the of the discarded datagram. Message to the source of the of the discarded datagram.
3.5. Processing Labeled IPv6 Datagrams which are Too Big 3.5. Processing Labeled IPv6 Datagrams which are Too Big
To process a labeled IPv6 datagram which is too big, an LSR MUST To process a labeled IPv6 datagram which is too big, an LSR MUST
execute the following algorithm: execute the following algorithm:
1. Strip off the label stack entries to obtain the IP datagram. 1. Strip off the label stack entries to obtain the IP datagram.
skipping to change at page 16, line 24 skipping to change at page 16, line 18
c. Forward the fragments. c. Forward the fragments.
Reassembly of the fragments will be done at the destination Reassembly of the fragments will be done at the destination
host. host.
3.6. Implications with respect to Path MTU Discovery 3.6. Implications with respect to Path MTU Discovery
The procedures described above for handling datagrams which have the The procedures described above for handling datagrams which have the
DF bit set, but which are "too large", have an impact on the Path MTU DF bit set, but which are "too large", have an impact on the Path MTU
Discovery procedures of RFC 1191. Hosts which implement these Discovery procedures of RFC 1191 [5]. Hosts which implement these
procedures will discover an MTU which is small enough to allow n procedures will discover an MTU which is small enough to allow n
labels to be pushed on the datagrams, without need for fragmentation, labels to be pushed on the datagrams, without need for fragmentation,
where n is the number of labels that actually get pushed on along the where n is the number of labels that actually get pushed on along the
path currently in use. path currently in use.
In other words, datagrams from hosts that use Path MTU Discovery will In other words, datagrams from hosts that use Path MTU Discovery will
never need to be fragmented due to the need to put on a label header, never need to be fragmented due to the need to put on a label header,
or to add new labels to an existing label header. (Also, datagrams or to add new labels to an existing label header. (Also, datagrams
from hosts that use Path MTU Discovery generally have the DF bit set, from hosts that use Path MTU Discovery generally have the DF bit set,
and so will never get fragmented anyway.) and so will never get fragmented anyway.)
Note that Path MTU Discovery will only work properly if, at the point Note that Path MTU Discovery will only work properly if, at the point
where a labeled IP Datagram's fragmentation needs to occur, it is where a labeled IP Datagram's fragmentation needs to occur, it is
possible to cause an ICMP Destination Unreachable message to be possible to cause an ICMP Destination Unreachable message to be
routed to the packet's source address. See section 2.3. routed to the packet's source address. See section 2.3.
If it is not possible to forward an ICMP message from within an MPLS If it is not possible to forward an ICMP message from within an MPLS
"tunnel" to a packet's source address, then the LSR at the "tunnel" to a packet's source address, but the network configuration
transmitting end of the tunnel MUST be able to determine the MTU of makes it possible for the LSR at the transmitting end of the tunnel
the tunnel as a whole. It SHOULD do this by sending packets through to receive packets that must go through the tunnel, but are too large
the tunnel to the tunnel's receiving endpoint, and performing Path to pass through the tunnel unfragmented, then:
MTU Discovery with those packets. Then any time the transmitting
endpoint of the tunnel needs to send a packet into the tunnel, and - The LSR at the transmitting end of the tunnel MUST be able to
that packet has the DF bit set, and it exceeds the tunnel MTU, the determine the MTU of the tunnel as a whole. It MAY do this by
transmitting endpoint of the tunnel MUST send the ICMP Destination sending packets through the tunnel to the tunnel's receiving
Unreachable message to the source, with code "Fragmentation Required endpoint, and performing Path MTU Discovery with those packets.
and DF Set", and the Next-Hop MTU Field set as described above.
- Any time the transmitting endpoint of the tunnel needs to send a
packet into the tunnel, and that packet has the DF bit set, and
it exceeds the tunnel MTU, the transmitting endpoint of the
tunnel MUST send the ICMP Destination Unreachable message to the
source, with code "Fragmentation Required and DF Set", and the
Next-Hop MTU Field set as described above.
4. Transporting Labeled Packets over PPP 4. Transporting Labeled Packets over PPP
The Point-to-Point Protocol (PPP) [8] provides a standard method for The Point-to-Point Protocol (PPP) [7] provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP transporting multi-protocol datagrams over point-to-point links. PPP
defines an extensible Link Control Protocol, and proposes a family of defines an extensible Link Control Protocol, and proposes a family of
Network Control Protocols for establishing and configuring different Network Control Protocols for establishing and configuring different
network-layer protocols. network-layer protocols.
This section defines the Network Control Protocol for establishing This section defines the Network Control Protocol for establishing
and configuring label Switching over PPP. and configuring label Switching over PPP.
4.1. Introduction 4.1. Introduction
skipping to change at page 18, line 4 skipping to change at page 18, line 4
4.2. A PPP Network Control Protocol for MPLS 4.2. A PPP Network Control Protocol for MPLS
The MPLS Control Protocol (MPLSCP) is responsible for enabling and The MPLS Control Protocol (MPLSCP) is responsible for enabling and
disabling the use of label switching on a PPP link. It uses the same disabling the use of label switching on a PPP link. It uses the same
packet exchange mechanism as the Link Control Protocol (LCP). MPLSCP packet exchange mechanism as the Link Control Protocol (LCP). MPLSCP
packets may not be exchanged until PPP has reached the Network-Layer packets may not be exchanged until PPP has reached the Network-Layer
Protocol phase. MPLSCP packets received before this phase is reached Protocol phase. MPLSCP packets received before this phase is reached
should be silently discarded. should be silently discarded.
The MPLS Control Protocol is exactly the same as the Link Control The MPLS Control Protocol is exactly the same as the Link Control
Protocol [8] with the following exceptions: Protocol [7] with the following exceptions:
1. Frame Modifications 1. Frame Modifications
The packet may utilize any modifications to the basic frame The packet may utilize any modifications to the basic frame
format which have been negotiated during the Link Establishment format which have been negotiated during the Link Establishment
phase. phase.
2. Data Link Layer Protocol Field 2. Data Link Layer Protocol Field
Exactly one MPLSCP packet is encapsulated in the PPP Exactly one MPLSCP packet is encapsulated in the PPP
skipping to change at page 19, line 10 skipping to change at page 19, line 10
field, where the PPP Protocol field indicates either type hex 0281 field, where the PPP Protocol field indicates either type hex 0281
(MPLS Unicast) or type hex 0283 (MPLS Multicast). The maximum length (MPLS Unicast) or type hex 0283 (MPLS Multicast). The maximum length
of a labeled packet transmitted over a PPP link is the same as the of a labeled packet transmitted over a PPP link is the same as the
maximum length of the Information field of a PPP encapsulated packet. maximum length of the Information field of a PPP encapsulated packet.
The format of the Information field itself is as defined in section The format of the Information field itself is as defined in section
2. 2.
Note that two codepoints are defined for labeled packets; one for Note that two codepoints are defined for labeled packets; one for
multicast and one for unicast. Once the MPLSCP has reached the multicast and one for unicast. Once the MPLSCP has reached the
Opened state, both label Switched multicasts and label Switched Opened state, both label switched multicasts and label switched
unicasts can be sent over the PPP link. unicasts can be sent over the PPP link.
4.4. Label Switching Control Protocol Configuration Options 4.4. Label Switching Control Protocol Configuration Options
There are no configuration options. There are no configuration options.
5. Transporting Labeled Packets over LAN Media 5. Transporting Labeled Packets over LAN Media
Exactly one labeled packet is carried in each frame. Exactly one labeled packet is carried in each frame.
skipping to change at page 19, line 32 skipping to change at page 19, line 32
and follow any data link layer headers, including, e.g., any 802.1Q and follow any data link layer headers, including, e.g., any 802.1Q
headers that may exist. headers that may exist.
The ethertype value 8847 hex is used to indicate that a frame is The ethertype value 8847 hex is used to indicate that a frame is
carrying an MPLS unicast packet. carrying an MPLS unicast packet.
The ethertype value 8848 hex is used to indicate that a frame is The ethertype value 8848 hex is used to indicate that a frame is
carrying an MPLS multicast packet. carrying an MPLS multicast packet.
These ethertype values can be used with either the ethernet These ethertype values can be used with either the ethernet
encapsulation or the 802.3 SNAP/SAP encapsulation to carry labeled encapsulation or the 802.3 LLC/SNAP encapsulation to carry labeled
packets. packets.
6. Security Considerations 6. Security Considerations
Security considerations are not discussed in this document. The MPLS encapsulation that is specified herein does not raise any
security issues that are not already present in either the MPLS
architecture [1] or in the architecture of the network layer protocol
contained within the encapsulation.
There are two security considerations inherited from the MPLS
architecture which may be pointed out here:
- Some routers may implement security procedures which depend on
the network layer header being in a fixed place relative to the
data link layer header. These procedures will not work when the
MPLS encapsulation is used, because that encapsulation is of a
variable size.
- An MPLS label has its meaning by virtue of an agreement between
the LSR that puts the label in the label stack (the "label
writer") , and the LSR that interprets that label (the "label
reader"). However, the label stack does not provide any means of
determining who the label writer was for any particular label.
If labeled packets are accepted from untrusted sources, the
result may be that packets are routed in an illegitimate manner.
7. Authors' Addresses 7. 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
Dan Tappan Dan Tappan
skipping to change at page 20, line 32 skipping to change at page 21, line 4
Cisco Systems, Inc. Cisco Systems, Inc.
250 Apollo Drive 250 Apollo Drive
Chelmsford, MA, 01824 Chelmsford, MA, 01824
E-mail: fedorkow@cisco.com E-mail: fedorkow@cisco.com
Tony Li Tony Li
Juniper Networks Juniper Networks
385 Ravendale Dr. 385 Ravendale Dr.
Mountain View, CA, 94043 Mountain View, CA, 94043
E-mail: tli@juniper.net E-mail: tli@juniper.net
Alex Conta Alex Conta
Lucent Technologies Lucent Technologies
300 Baker Avenue 300 Baker Avenue
Concord, MA, 01742 Concord, MA, 01742
E-mail: aconta@lucent.com E-mail: aconta@lucent.com
8. References 8. References
[1], "Multiprotocol Label Switching Architecture", 7/98, draft-ietf- [1] Rosen, E., Viswanathan, A., and Callon, R., "Multiprotocol Label
mpls-arch-02.txt, Rosen, Viswanathan, Callon Switching Architecture", Work in Progress, July, 1998
[2] "A Framework for Multiprotocol Label Switching", 11/97, draft- [2] Callon, R., Doolan, P., Feldman, N., Fredette, A., Swallow, G.,
ietf-mpls-framework-02.txt, Callon, Doolan, Feldman, Fredette, Viswanathan, A., "A Framework for Multiprotocol Label Switching",
Swallow, Viswanathan Work in Progress, November 1997.
[3] "Tag Switching Architecture - Overview", 7/97, draft-rekhter- [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
tagswitch-arch-01.txt, Rekhter, Davie, Katz, Rosen, Swallow Levels", RFC 2119, BCP 14, March 1997.
[4] "Internet Protocol", RFC 791, 9/81, Postel [4] Postel, J., "Internet Control Message Protocol", RFC 792,
[5] "Internet Control Message Protocol", RFC 792, 9/81, Postel September 1981.
[6] "Path MTU Discovery", RFC 1191, 11/90, Mogul & Deering [5] Mogul, J. and Deering S., "Path MTU Discovery", RFC 1191,
November 1990.
[7] "IP Router Alert Option", RFC 2113, 2/97, Katz [6] Katz, D., "IP Router Alert Option", RFC 2113, February 1997.
[8] "The Point-to-Point Protocol (PPP)", RFC 1661, 7/94, Simpson [7] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", RFC
1661, STD 51, July 1994.
[9] "Internet Control Message Protocol (ICMPv6) for the Internet [8] Conta, A. and Deering, S., "Internet Control Message Protocol
Protocol Version 6 (IPv6) Specification", RFC 1885, 12/95, Conta, (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification",
Deering RFC 1885, December 1995.
[10] "Path MTU Discovery for IP version 6", [RFC-1981] McCann, J., S. [9] McCann, J., Deering, S. and Mogul, J., "Path MTU Discovery for IP
Deering, J. Mogul version 6", RFC 1981, August 1996.
[11] "Use of Label Switching with ATM", draft-davie-mpls-atm-01.txt, [10] Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y., Rosen, E.
7/98, Davie, Lawrence, McCloghrie, Rekhter, Rosen, Swallow, Doolan and Swallow G., "Use of Label Switching with ATM", Work in Progress,
July 1998.
 End of changes. 

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