draft-ietf-mpls-label-encaps-01.txt   draft-ietf-mpls-label-encaps-02.txt 
Network Working Group Eric C. Rosen Network Working Group Eric C. Rosen
Internet Draft Yakov Rekhter Internet Draft Yakov Rekhter
Expiration Date: August 1998 Daniel Tappan Expiration Date: January 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
February 1998 July 1998
MPLS Label Stack Encoding MPLS Label Stack Encoding
draft-ietf-mpls-label-encaps-01.txt draft-ietf-mpls-label-encaps-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
"Multi-Protocol Label Switching (MPLS)" [1,2,3] requires a set of "Multi-Protocol Label Switching (MPLS)" [1,2,3] 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
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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 ........................................ 4
2.1 Encoding the Label Stack ............................... 4 2.1 Encoding the Label Stack ............................... 4
2.2 Determining the Network Layer Protocol ................. 7 2.2 Determining the Network Layer Protocol ................. 7
2.3 Processing the Time to Live Field ...................... 8 2.3 Generating ICMP Messages for Labeled IP Packets ........ 8
2.3.1 Definitions ............................................ 8 2.3.1 Tunneling through a Transit Routing Domain ............. 8
2.3.2 Protocol-independent rules ............................. 8 2.3.2 Tunneling Private Addresses through a Public Backbone .. 9
2.3.3 IP-dependent rules ..................................... 8 2.4 Processing the Time to Live Field ...................... 9
2.3.4 Translating Between Different Encapsulations ........... 9 2.4.1 Definitions ............................................ 9
3 Fragmentation and Path MTU Discovery ................... 9 2.4.2 Protocol-independent rules ............................. 9
3.1 Terminology ............................................ 10 2.4.3 IP-dependent rules ..................................... 10
3.2 Maximum Initially Labeled IP Datagram Size ............. 12 2.4.4 Translating Between Different Encapsulations ........... 10
3.3 When are Labeled IP Datagrams Too Big? ................. 13 3 Fragmentation and Path MTU Discovery ................... 11
3.4 Processing Labeled IPv4 Datagrams which are Too Big .... 13 3.1 Terminology ............................................ 12
3.5 Processing Labeled IPv6 Datagrams which are Too Big .... 14 3.2 Maximum Initially Labeled IP Datagram Size ............. 13
3.6 Implications with respect to Path MTU Discovery ........ 15 3.3 When are Labeled IP Datagrams Too Big? ................. 14
3.6.1 Tunneling through a Transit Routing Domain ............. 15 3.4 Processing Labeled IPv4 Datagrams which are Too Big .... 14
3.6.2 Tunneling Private Addresses through a Public Backbone .. 16 3.5 Processing Labeled IPv6 Datagrams which are Too Big .... 15
4 Transporting Labeled Packets over PPP .................. 16 3.6 Implications with respect to Path MTU Discovery ........ 16
4.1 Introduction ........................................... 16 4 Transporting Labeled Packets over PPP .................. 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 . 18 4.4 Label Switching Control Protocol Configuration Options . 19
5 Transporting Labeled Packets over LAN Media ............ 18 5 Transporting Labeled Packets over LAN Media ............ 19
6 Security Considerations ................................ 19 6 Security Considerations ................................ 19
7 Authors' Addresses ..................................... 19 7 Authors' Addresses ..................................... 19
8 References ............................................. 20 8 References ............................................. 20
1. Introduction 1. Introduction
"Multi-Protocol Label Switching (MPLS)" [1,2,3] requires a set of "Multi-Protocol Label Switching (MPLS)" [1,2,3] 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
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1. Bottom of Stack (S) 1. Bottom of Stack (S)
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 2.3. The processing of this field is described in section 2.4.
3. Class of Service (CoS) 3. Class of Service (CoS)
This three-bit field is used to identify a "Class of Service". This three-bit field is used to identify a "Class of Service".
The setting of this field is intended to affect the scheduling The setting of this field is intended to affect the scheduling
and/or discard algorithms which are applied to the packet as it and/or discard algorithms which are applied to the packet as it
is transmitted through the network. is transmitted through the network.
When an unlabeled packet is initially labeled, the value When an unlabeled packet is initially labeled, the value
assigned to the CoS field in the label stack entry is assigned to the CoS field in the label stack entry is
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(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.
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".
This label value is only legal when it is the sole When this label value is the sole label stack entry, it
label stack entry. It indicates that the label stack indicates that the label stack must be popped, and the
must be popped, and the forwarding of the packet must forwarding of the packet must then be based on the IPv4
then be based on the IPv4 header. header. When this label value is not the sole label
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
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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.
2.3. Processing the Time to Live Field 2.3. Generating ICMP Messages for Labeled IP Packets
2.3.1. Definitions 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
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:
1. it must be possible for that LSR to determine that a particular
labeled packet is an IP packet;
2. it must be possible for that LSR to route to the packet's IP
source address.
Condition 1 is discussed in section 2.2. The following two
subsections discuss condition 2. However, there will be some cases
in which condition 2 does not hold at all, and in these cases it will
not be possible to generate the ICMP message.
2.3.1. Tunneling through a Transit Routing Domain
Suppose one is using MPLS to "tunnel" through a transit routing
domain, where the external routes are not leaked into the domain's
interior routers. For example, the interior routers may be running
OSPF, and may only know how to reach destinations within that OSPF
domain. The domain might contain several Autonomous System Border
Routers (ASBRs), which talk BGP to each other. However, in this
example the routes from BGP are not distributed into OSPF, and the
LSRs which are not ASBRs do not run BGP.
In this example, only an ASBR will know how to route to the source of
some arbitrary packet. If an interior router needs to send an ICMP
message to the source of an IP packet, it will not know how to route
the ICMP message.
One solution is to have one or more of the ASBRs inject "default"
into the IGP. (N.B.: this does NOT require that there be a "default"
carried by BGP.) This would then ensure that any unlabeled packet
which must leave the domain (such as an ICMP packet) gets sent to a
router which has full routing information. The routers with full
routing information will label the packets before sending them back
through the transit domain, so the use of default routing within the
transit domain does not cause any loops.
This solution only works for packets which have globally unique
addresses, and for networks in which all the ASBRs have complete
routing information. The next subsection describes a solution which
works when these conditions do not hold.
2.3.2. Tunneling Private Addresses through a Public Backbone
In some cases where MPLS is used to tunnel through a routing domain,
it may not be possible to route to the source address of a fragmented
packet at all. This would be the case, for example, if the IP
addresses carried in the packet were private (i.e., not globally
unique) addresses, and MPLS were being used to tunnel those packets
through a public backbone. Default routing to an ASBR will not work
in this environment.
In this environment, in order to send an ICMP message to the source
of a packet, one can copy the label stack from the original packet to
the ICMP message, and then label switch the ICMP message. This will
cause the message to proceed in the direction of the original
packet's destination, rather than its source. Unless the message is
label switched all the way to the destination host, it will end up,
unlabeled, in a router which does know how to route to the source of
original packet, at which point the message will be sent in the
proper direction.
2.4. Processing the Time to Live Field
2.4.1. Definitions
The "incoming TTL" of a labeled packet is defined to be the value of The "incoming TTL" of a labeled packet is defined to be the value of
the TTL field of the top label stack entry when the packet is the TTL field of the top label stack entry when the packet is
received. received.
The "outgoing TTL" of a labeled packet is defined to be the larger The "outgoing TTL" of a labeled packet is defined to be the larger
of: of:
(a) one less than the incoming TTL, (a) one less than the incoming TTL,
(b) zero. (b) zero.
2.3.2. 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; the packet's lifetime in the network MUST NOT be further forwarded; nor may the label stack be stripped
is considered to have expired. off and the packet forwarded as an unlabeled packet. The packet's
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 silently discarded, or the packet MAY have its label stack be simply discarded, or it may be passed to "ordinary" network layer
stripped off, and passed as an unlabeled packet to the ordinary for error processing (e.g., for the generation of an ICMP error
processing for network layer packets which have exceeded their message, see section 2.3).
maximum lifetime in the network. However, even if the label stack is
stripped, the packet MUST NOT be further forwarded.
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.
2.3.3. 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 MUST BE replaced with the outgoing TTL
value, as defined above. In IPv4 this also requires modification of value, as defined above. In IPv4 this also requires modification of
the IP header checksum. the IP header checksum.
2.3.4. 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 [11], 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
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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 RFC 1191 Path MTU Discovery, and hosts using IPv6,
will be able to generate IP datagrams that do not need fragmentation, will be able to generate IP datagrams that do not need fragmentation,
even if they get labeled as the traverse the network. even if they get labeled as the traverse the network.
In general, hosts which do not implement RFC 1191 Path MTU Discovery In general, IPv4 hosts which do not implement RFC 1191 Path MTU
send IP datagrams which contain no more than 576 bytes. Since the Discovery send IP datagrams which contain no more than 576 bytes.
MTUs in use on most data links today are 1500 bytes or more, the Since the MTUs in use on most data links today are 1500 bytes or
probability that such datagrams will need to get fragmented, even if more, the probability that such datagrams will need to get
they get labeled, is very small. fragmented, even if they get labeled, is very small.
Some hosts that do not implement RFC 1191 Path MTU Discovery will Some hosts that do not implement RFC 1191 Path MTU Discovery will
generate IP datagrams containing 1500 bytes, as long as the IP Source generate IP datagrams containing 1500 bytes, as long as the IP Source
and Destination addresses are on the same subnet. These datagrams and Destination addresses are on the same subnet. These datagrams
will not pass through routers, and hence will not get fragmented. will 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.
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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.
2. Let N be the number of bytes in the label stack (i.e, 4 times 2. Let N be the number of bytes in the label stack (i.e, 4 times
the number of label stack entries). the number of label stack entries).
3. If the IP datagram contains more than 576 bytes (not counting 3. If the IP datagram contains more than 1280 bytes (not counting
the label stack entries), then: the label stack entries), then:
a. Create an ICMP Packet Too Big Message, and set its Next- a. Create an ICMP Packet Too Big Message, and set its Next-
Hop MTU field to the difference between the Effective Hop MTU field to the difference between the Effective
Maximum Frame Payload Size and the value of N Maximum Frame Payload Size and the value of N
b. If possible, transmit the ICMP Packet Too Big Message to b. If possible, transmit the ICMP Packet Too Big Message to
the source of the datagram. the source of the datagram.
c. discard the labeled IPv6 datagram. c. discard the labeled IPv6 datagram.
4. If the IP datagram is not larger than 576 octets, then 4. If the IP datagram is not larger than 1280 octets, then
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.
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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.)
However, note that Path MTU Discovery will only work properly if, at Note that Path MTU Discovery will only work properly if, at the point
the point where a labeled IP Datagram's fragmentation needs to occur, where a labeled IP Datagram's fragmentation needs to occur, it is
it is possible to route to the packet's source address. If this is possible to cause an ICMP Destination Unreachable message to be
not possible, then the ICMP Destination Unreachable message cannot be routed to the packet's source address. See section 2.3.
sent to the source.
3.6.1. Tunneling through a Transit Routing Domain
Suppose one is using MPLS to "tunnel" through a transit routing
domain, where the external routes are not leaked into the domain's
interior routers. If a packet needs fragmentation at some router
within the domain, and the packet's DF bit is set, it is necessary to
be able to originate an ICMP message at that router and have it
routed correctly to the source of the fragmented packet. If the
packet's source address is an external address, this poses a problem.
Therefore, in order for Path MTU Discovery to work, any routing
domain in which external routes are not leaked into the interior
routers MUST have a default route which causes all packets carrying
external destination addresses to be sent to a border router. For
example, one of the border routers may inject "default" into the IGP.
3.6.2. Tunneling Private Addresses through a Public Backbone
In other cases where MPLS is used to tunnel through a routing domain,
it may not be possible to route to the source address of a fragmented
packet at all. This would be the case, for example, if the IP
addresses carried in the packet were private addresses, and MPLS were
being used to tunnel those packets through a public backbone.
In such cases, the LSR at the transmitting end of the tunnel MUST be If it is not possible to forward an ICMP message from within an MPLS
able to determine the MTU of the tunnel as a whole. It SHOULD do "tunnel" to a packet's source address, then the LSR at the
this by sending packets through the tunnel to the tunnel's receiving transmitting end of the tunnel MUST be able to determine the MTU of
endpoint, and performing Path MTU Discovery with those packets. Then the tunnel as a whole. It SHOULD do this by sending packets through
any time the transmitting endpoint of the tunnel needs to send a the tunnel to the tunnel's receiving endpoint, and performing Path
packet into the tunnel, and that packet has the DF bit set, and it MTU Discovery with those packets. Then any time the transmitting
exceeds the tunnel MTU, the transmitting endpoint of the tunnel MUST endpoint of the tunnel needs to send a packet into the tunnel, and
send the ICMP Destination Unreachable message to the source, with that packet has the DF bit set, and it exceeds the tunnel MTU, the
code "Fragmentation Required and DF Set", and the Next-Hop MTU Field transmitting endpoint of the tunnel MUST send the ICMP Destination
set as described above. 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) [8] 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
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None. None.
4.3. Sending Labeled Packets 4.3. Sending Labeled Packets
Before any labeled packets may be communicated, PPP must reach the Before any labeled packets may be communicated, PPP must reach the
Network-Layer Protocol phase, and the MPLS Control Protocol must Network-Layer Protocol phase, and the MPLS Control Protocol must
reach the Opened state. reach the Opened state.
Exactly one labeled packet is encapsulated in the PPP Information Exactly one labeled packet is encapsulated in the PPP Information
field, where the PPP Protocol field indicates either type hex 8281 field, where the PPP Protocol field indicates either type hex 0281
(MPLS Unicast) or type hex 8283 (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.
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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], "A Proposed Architecture for MPLS", 7/97, draft-ietf-mpls-arch- [1], "Multiprotocol Label Switching Architecture", 7/98, draft-ietf-
00.txt, Rosen, Viswanathan, Callon mpls-arch-02.txt, Rosen, Viswanathan, Callon
[2] "A Framework for Multiprotocol Label Switching", 11/97, draft- [2] "A Framework for Multiprotocol Label Switching", 11/97, draft-
ietf-mpls-framework-02.txt, Callon, Doolan, Feldman, Fredette, ietf-mpls-framework-02.txt, Callon, Doolan, Feldman, Fredette,
Swallow, Viswanathan Swallow, Viswanathan
[3] "Tag Switching Architecture - Overview", 7/97, draft-rekhter- [3] "Tag Switching Architecture - Overview", 7/97, draft-rekhter-
tagswitch-arch-01.txt, Rekhter, Davie, Katz, Rosen, Swallow tagswitch-arch-01.txt, Rekhter, Davie, Katz, Rosen, Swallow
[4] "Internet Protocol", RFC 791, 9/81, Postel [4] "Internet Protocol", RFC 791, 9/81, Postel
[5] "Internet Control Message Protocol", RFC 792, 9/81, Postel [5] "Internet Control Message Protocol", RFC 792, 9/81, Postel
[6] "Path MTU Discovery", RFC 1191, 11/90, Mogul & Deering [6] "Path MTU Discovery", RFC 1191, 11/90, Mogul & Deering
[7] "IP Router Alert Option", RFC 2113, 2/97, Katz [7] "IP Router Alert Option", RFC 2113, 2/97, Katz
[8] "The Point-to-Point Protocol (PPP)", RFC 1661, 7/94, Simpson [8] "The Point-to-Point Protocol (PPP)", RFC 1661, 7/94, Simpson
[9] "Internet Control Message Protocol (ICMPv6) for the Internet [9] "Internet Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 1885, 12/95, Conta, Protocol Version 6 (IPv6) Specification", RFC 1885, 12/95, Conta,
skipping to change at page 20, line 39 skipping to change at page 21, line 19
[8] "The Point-to-Point Protocol (PPP)", RFC 1661, 7/94, Simpson [8] "The Point-to-Point Protocol (PPP)", RFC 1661, 7/94, Simpson
[9] "Internet Control Message Protocol (ICMPv6) for the Internet [9] "Internet Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 1885, 12/95, Conta, Protocol Version 6 (IPv6) Specification", RFC 1885, 12/95, Conta,
Deering Deering
[10] "Path MTU Discovery for IP version 6", [RFC-1981] McCann, J., S. [10] "Path MTU Discovery for IP version 6", [RFC-1981] McCann, J., S.
Deering, J. Mogul Deering, J. Mogul
[11] "Use of Label Switching with ATM", draft-davie-mpls-atm-00.txt, [11] "Use of Label Switching with ATM", draft-davie-mpls-atm-01.txt,
Davie, Lawrence, McCloghrie, Rekhter, Rosen, Swallow, Doolan 7/98, Davie, Lawrence, McCloghrie, Rekhter, Rosen, Swallow, Doolan
 End of changes. 

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