Network Working Group E. Rosen

Network Working Group E. Rosen
Request for Comments: 3032 D. Tappan
Category: Standards Track G. Fedorkow
Cisco Systems, Inc.
Y. Rekhter
Juniper Networks
D. Farinacci
T. Li
Procket Networks, Inc.
A. Conta
TranSwitch Corporation
January 2001

MPLS Label Stack Encoding

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

"Multi-Protocol Label Switching (MPLS)" [1] requires a set of
procedures for augmenting network layer packets with "label stacks",
thereby turning them into "labeled packets". Routers which support
MPLS are known as "Label Switching Routers", or "LSRs". In order to
transmit a labeled packet on a particular data link, an LSR must
support an encoding technique which, given a label stack and a
network layer packet, produces a labeled packet. This document
specifies the encoding to be used by an LSR in order to transmit
labeled packets on Point-to-Point Protocol (PPP) data links, on LAN
data links, and possibly on 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 described here MUST be used to
encode the remainder of the label stack. This document also
specifies rules and procedures for processing the various fields of
the label stack encoding.

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RFC 3032 MPLS Label Stack Encoding January 2001

Table of Contents

1 Introduction ........................................... 2
1.1 Specification of Requirements .......................... 3
2 The Label Stack ........................................ 3
2.1 Encoding the Label Stack ............................... 3
2.2 Determining the Network Layer Protocol ................. 5
2.3 Generating ICMP Messages for Labeled IP Packets ........ 6
2.3.1 Tunneling through a Transit Routing Domain ............. 7
2.3.2 Tunneling Private Addresses through a Public Backbone .. 7
2.4 Processing the Time to Live Field ...................... 8
2.4.1 Definitions ............................................ 8
2.4.2 Protocol-independent rules ............................. 8
2.4.3 IP-dependent rules ..................................... 9
2.4.4 Translating Between Different Encapsulations ........... 9
3 Fragmentation and Path MTU Discovery ................... 10
3.1 Terminology ............................................ 11
3.2 Maximum Initially Labeled IP Datagram Size ............. 12
3.3 When are Labeled IP Datagrams Too Big? ................. 13
3.4 Processing Labeled IPv4 Datagrams which are Too Big .... 13
3.5 Processing Labeled IPv6 Datagrams which are Too Big .... 14
3.6 Implications with respect to Path MTU Discovery ........ 15
4 Transporting Labeled Packets over PPP .................. 16
4.1 Introduction ........................................... 16
4.2 A PPP Network Control Protocol for MPLS ................ 17
4.3 Sending Labeled Packets ................................ 18
4.4 Label Switching Control Protocol Configuration Options . 18
5 Transporting Labeled Packets over LAN Media ............ 18
6 IANA Considerations .................................... 19
7 Security Considerations ................................ 19
8 Intellectual Property .................................. 19
9 Authors' Addresses ..................................... 20
10 References ............................................. 22
11 Full Copyright Statement ............................... 23

1. Introduction

"Multi-Protocol Label Switching (MPLS)" [1] requires a set of
procedures for augmenting network layer packets with "label stacks",
thereby turning them into "labeled packets". Routers which support
MPLS are known as "Label Switching Routers", or "LSRs". In order to
transmit a labeled packet on a particular data link, an LSR must
support an encoding technique which, given a label stack and a
network layer packet, produces a labeled packet.

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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.
The specified encoding may also be useful for other data links as
well.

This document also specifies rules and procedures for processing the
various fields of the label stack encoding. Since MPLS is
independent of any particular network layer protocol, the majority of
such procedures are also protocol-independent. A few, however, do
differ for different protocols. In this document, we specify the
protocol-independent procedures, and we specify the protocol-
dependent procedures for IPv4 and IPv6.

LSRs that are implemented on certain switching devices (such as ATM
switches) may use different encoding techniques for encoding the top
one or two entries of the label stack. When the label stack has
additional entries, however, the encoding technique described in this
document MUST be used for the additional label stack entries.

1.1. Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].

2. The Label Stack

2.1. Encoding the Label Stack

The label stack is represented as a sequence of "label stack
entries". Each label stack entry is represented by 4 octets. This
is shown in Figure 1.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
| Label | Exp |S| TTL | Stack
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

Label: Label Value, 20 bits
Exp: Experimental Use, 3 bits
S: Bottom of Stack, 1 bit
TTL: Time to Live, 8 bits

Figure 1

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The label stack entries appear AFTER the data link layer headers, but
BEFORE any network layer headers. The top of the label stack appears
earliest in the packet, and the bottom appears latest. The network
layer packet immediately follows the label stack entry which has the
S bit set.

Each label stack entry is broken down into the following fields:

1. Bottom of Stack (S)

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
label stack entries.

2. Time to Live (TTL)

This eight-bit field is used to encode a time-to-live value.
The processing of this field is described in section 2.4.

3. Experimental Use

This three-bit field is reserved for experimental use.

4. Label Value

This 20-bit field carries the actual value of the Label.

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
one learns:

a) the next hop to which the packet is to be forwarded;

b) the operation to be performed on the label stack before
forwarding; this operation may be to replace the top label
stack entry with another, or to pop an entry off the label
stack, or to replace the top label stack entry and then to
push one or more additional entries on the 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.

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There are several reserved label values:

i. A value of 0 represents the "IPv4 Explicit NULL Label".
This label value is only legal at the bottom of the label
stack. It indicates that the label stack must be popped,
and the forwarding of the packet must then be based on the
IPv4 header.

ii. A value of 1 represents the "Router Alert Label". This
label value is legal anywhere in the label stack except at
the bottom. When a received packet contains this label
value at the top of the label stack, it is delivered to a
local software module for processing. The actual
forwarding of the packet is determined by the label
beneath it in the stack. However, if the packet is
forwarded further, the Router Alert Label should be pushed
back onto the label stack before forwarding. The use of
this label is analogous to the use of the "Router Alert
Option" in IP packets [5]. Since this label cannot occur
at the bottom of the stack, it is not associated with a
particular network layer protocol.

iii. A value of 2 represents the "IPv6 Explicit NULL Label".
This label value is only legal at the bottom of the label
stack. It indicates that the label stack must be popped,
and the forwarding of the packet must then be based on the
IPv6 header.

iv. A value of 3 represents the "Implicit NULL Label". This
is a label that an LSR may assign and distribute, but
which never actually appears in the encapsulation. When
an LSR would otherwise replace the label at the top of the
stack with a new label, but the new label is "Implicit
NULL", the LSR will pop the stack instead of doing the
replacement. Although this value may never appear in the
encapsulation, it needs to be specified in the Label
Distribution Protocol, so a value is reserved.

v. Values 4-15 are reserved.

2.2. Determining the Network Layer Protocol

When the last label is popped from a packet's label stack (resulting
in the stack being emptied), further processing of the packet is
based on the packet's network layer header. The LSR which pops the
last label off the stack must therefore be able to identify the
packet's network layer protocol. However, the label stack does not
contain any field which explicitly identifies the network layer

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protocol. This means that the identity of the network layer protocol
must be inferable from the value of the label which is popped from
the bottom of the stack, possibly along with the contents of the
network layer header itself.

Therefore, when the first label is pushed onto a network layer
packet, either the label must be one which is used ONLY for packets
of a particular network layer, or the label must be one which is used
ONLY for a specified set of network layer protocols, where packets of
the specified network layers can be distinguished by inspection of
the network layer header. Furthermore, whenever that label is
replaced by another label value during a packet's transit, the new
value must also be one which meets the same criteria. If these
conditions are not met, the LSR which pops the last label off a
packet will not be able to identify the packet's network layer
protocol.

Adherence to these conditions does not necessarily enable
intermediate nodes to identify a packet's network layer protocol.
Under ordinary conditions, this is not necessary, but there are error
conditions under which it is desirable. For instance, if an
intermediate LSR determines that a labeled packet is undeliverable,
it may be desirable for that LSR to generate error messages which are
specific to the packet's network layer. The only means the
intermediate LSR has for identifying the network layer is inspection
of the top label and the network layer header. So if intermediate
nodes are to be able to generate protocol-specific error messages for
labeled packets, all labels in the stack must meet the criteria
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

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.

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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

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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.

This technique can be very useful if the ICMP message is a "Time
Exceeded" message or a "Destination Unreachable because fragmentation
needed and DF set" message.

When copying the label stack from the original packet to the ICMP
message, the label values must be copied exactly, but the TTL values
in the label stack should be set to the TTL value that is placed in
the IP header of the ICMP message. This TTL value should be long
enough to allow the circuitous route that the ICMP message will need
to follow.

Note that if a packet's TTL expiration is due to the presence of a
routing loop, then if this technique is used, the ICMP message may
loop as well. Since an ICMP message is never sent as a result of
receiving an ICMP message, and since many implementations throttle
the rate at which ICMP messages can be generated, this is not
expected to pose a problem.

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 TTL field of the top label stack entry when the packet is
received.

The "outgoing TTL" of a labeled packet is defined to be the larger
of:

a) one less than the incoming TTL,
b) zero.

2.4.2. Protocol-independent rules

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
off and the packet forwarded as an unlabeled packet. The packet's
lifetime in the network is considered to have expired.

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RFC 3032 MPLS Label Stack Encoding January 2001

Depending on the label value in the label stack entry, the packet MAY
be simply discarded, or it may be passed to the appropriate
"ordinary" network layer for error processing (e.g., for the
generation of an ICMP error message, see section 2.3).

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
value.

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
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
stack.

2.4.3. IP-dependent rules

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.

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
field needs to be decremented, as part of the IP processing, it is
assumed that this has already been done.)

When a label is popped, and the resulting label stack is empty, then
the value of the IP TTL field SHOULD BE replaced with the outgoing
TTL value, as defined above. In IPv4 this also requires modification
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

Sometimes an LSR may receive a labeled packet over, e.g., a label
switching controlled ATM (LC-ATM) interface [9], and may need to send
it out over a PPP or LAN link. Then the incoming packet will not be
received using the encapsulation specified in this document, but the
outgoing packet will be sent using the encapsulation specified in
this document.

In this case, the value of the "incoming TTL" is determined by the
procedures used for carrying labeled packets on, e.g., LC-ATM
interfaces. TTL processing then proceeds as described above.

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RFC 3032 MPLS Label Stack Encoding January 2001

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
the incoming packet will be received using the encapsulation
specified in this document, but the outgoing packet will not be sent
using the encapsulation specified in this document. In this case,
the procedure for carrying the value of the "outgoing TTL" is
determined by the procedures used for carrying labeled packets on,
e.g., LC-ATM interfaces.

3. Fragmentation and Path MTU Discovery

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
receive a labeled packet which is too large to be transmitted on its
output link.

It is also possible that a received packet (labeled or unlabeled)
which was originally small enough to be transmitted on that link
becomes too large by virtue of having one or more additional labels
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
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
payload.

This section specifies the rules for processing labeled packets which
are "too large". In particular, it provides rules which ensure that
hosts implementing Path MTU Discovery [4], and hosts using IPv6
[7,8], will be able to generate IP datagrams that do not need
fragmentation, even if those datagrams get labeled as they traverse
the network.

In general, IPv4 hosts which do not implement Path MTU Discovery [4]
send IP datagrams which contain no more than 576 bytes. Since the
MTUs in use on most data links today are 1500 bytes or more, the
probability that such datagrams will need to get fragmented, even if
they get labeled, is very small.

Some hosts that do not implement Path MTU Discovery [4] will generate
IP datagrams containing 1500 bytes, as long as the IP Source and
Destination addresses are on the same subnet. These datagrams will
not pass through routers, and hence will not get fragmented.

Unfortunately, some hosts will generate IP datagrams containing 1500
bytes, as long the IP Source and Destination addresses have the same
classful network number. This is the one case in which there is any
risk of fragmentation when such datagrams get labeled. (Even so,

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fragmentation is not likely unless the packet must traverse an
ethernet of some sort between the time it first gets labeled and the
time it gets unlabeled.)

This document specifies procedures which allow one to configure the
network so that large datagrams from hosts which do not implement
Path MTU Discovery get fragmented just once, when they are first
labeled. These procedures make it possible (assuming suitable
configuration) to avoid any need to fragment packets which have
already been labeled.

3.1. Terminology

With respect to a particular data link, we can use the following
terms:

- Frame Payload:

The contents of a data link frame, excluding any data link
layer headers or trailers (e.g., MAC headers, LLC headers,
802.1Q headers, PPP header, frame check sequences, etc.).

When a frame is carrying an unlabeled IP datagram, the Frame
Payload is just the IP datagram itself. When a frame is
carrying a labeled IP datagram, the Frame Payload consists of
the label stack entries and the IP datagram.

- Conventional Maximum Frame Payload Size:

The maximum Frame Payload size allowed by data link standards.
For example, the Conventional Maximum Frame Payload Size for
ethernet is 1500 bytes.

- True Maximum Frame Payload Size:

The maximum size frame payload which can be sent and received
properly by the interface hardware attached to the data link.

On ethernet and 802.3 networks, it is believed that the True
Maximum Frame Payload Size is 4-8 bytes larger than the
Conventional Maximum Frame Payload Size (as long as neither an
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
transit to its next hop). For example, it is believed that
most ethernet equipment could correctly send and receive
packets 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 field.

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RFC 3032 MPLS Label Stack Encoding January 2001

On PPP links, the True Maximum Frame Payload Size may be
virtually unbounded.

- Effective Maximum Frame Payload Size for Labeled Packets:

This is either the Conventional Maximum Frame Payload Size or
the True Maximum Frame Payload Size, depending on the
capabilities of the equipment on the data link and the size of
the data link header being used.

- Initially Labeled IP Datagram:

Suppose that an unlabeled IP datagram is received at a
particular 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 at that LSR.

- Previously Labeled IP Datagram:

An IP datagram which had already been labeled before it was
received by a particular LSR.

3.2. Maximum Initially Labeled IP Datagram Size

Every LSR which is capable of

a) receiving an unlabeled IP datagram,
b) adding a label stack to the datagram, and
c) forwarding the resulting labeled packet,

SHOULD support a configuration parameter known as the "Maximum
Initially Labeled IP Datagram Size", which can be set to a non-
negative value.

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:

a) an unlabeled IP datagram is received, and
b) that datagram does not have the DF bit set in its IP header,
and
c) that datagram needs to be labeled before being forwarded, and
d) the size of the datagram (before labeling) exceeds the value of
the parameter,
then
a) the datagram must be broken into fragments, each of whose size
is no greater than the value of the parameter, and

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RFC 3032 MPLS Label Stack Encoding January 2001

b) each fragment must be labeled and then forwarded.

For example, if this configuration parameter is set to a value of
1488, then any unlabeled IP datagram containing more than 1488 bytes
will be fragmented before being labeled. Each fragment will be
capable of being carried on a 1500-byte data link, without further
fragmentation, even if as many as three labels are pushed onto its
label stack.

In other words, setting this parameter to a non-zero value allows one
to eliminate all fragmentation of Previously Labeled IP Datagrams,
but it may cause some unnecessary fragmentation of Initially Labeled
IP Datagrams.

Note that the setting of this parameter does not affect the
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?

A labeled IP datagram whose size exceeds the Conventional Maximum
Frame Payload Size of the data link over which it is to be forwarded
MAY be considered to be "too big".

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
be considered to be "too big".

A labeled IP datagram which is not "too big" MUST be transmitted
without fragmentation.

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
its IP header, then the LSR MAY silently discard the datagram.

Note that discarding such datagrams is a sensible procedure only if
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
stack to an unlabeled IP datagram.

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
execute the following algorithm:

1. Strip off the label stack entries to obtain the IP datagram.

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2. Let N be the number of bytes in the label stack (i.e, 4 times
the number of label stack entries).

3. If the IP datagram does NOT have the "Don't Fragment" bit set
in its IP header:

a. convert it into fragments, each of which MUST be at least N
bytes less than the Effective Maximum Frame Payload Size.

b. Prepend each fragment with the same label header that would
have been on the original datagram had fragmentation not
been necessary.

c. Forward the fragments

4. If the IP datagram has the "Don't Fragment" bit set in its IP
header:

a. the datagram MUST NOT be forwarded

b. Create an ICMP Destination Unreachable Message:

i. set its Code field [3] to "Fragmentation Required and DF
Set",

ii. set its Next-Hop MTU field [4] to the difference between
the Effective Maximum Frame Payload Size and the value
of N

c. If possible, transmit the ICMP Destination Unreachable
Message to the source of the of the discarded datagram.

3.5. Processing Labeled IPv6 Datagrams which are Too Big

To process a labeled IPv6 datagram which is too big, an LSR MUST
execute the following algorithm:

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
the number of label stack entries).

3. If the IP datagram contains more than 1280 bytes (not counting
the label stack entries), or if it does not contain a fragment
header, then:

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RFC 3032 MPLS Label Stack Encoding January 2001

a. Create an ICMP Packet Too Big Message, and set its Next-Hop
MTU field to the difference between the Effective Maximum
Frame Payload Size and the value of N

b. If possible, transmit the ICMP Packet Too Big Message to the
source of the datagram.

c. discard the labeled IPv6 datagram.

4. If the IP datagram is not larger than 1280 octets, and it
contains a fragment header, then

a. Convert it into fragments, each of which MUST be at least N
bytes less than the Effective Maximum Frame Payload Size.

b. Prepend each fragment with the same label header that would
have been on the original datagram had fragmentation not
been necessary.

c. Forward the fragments.

Reassembly of the fragments will be done at the destination
host.

3.6. Implications with respect to Path MTU Discovery

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
Discovery procedures of RFC 1191 [4]. Hosts which implement these
procedures will discover an MTU which is small enough to allow n
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
path currently in use.

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,
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,
and so will never get fragmented anyway.)

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
possible to cause an ICMP Destination Unreachable message to be
routed to the packet's source address. See section 2.3.

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If it is not possible to forward an ICMP message from within an MPLS
"tunnel" to a packet's source address, but the network configuration
makes it possible for the LSR at the transmitting end of the tunnel
to receive packets that must go through the tunnel, but are too large
to pass through the tunnel unfragmented, then:

- The LSR at the transmitting end of the tunnel MUST be able to
determine the MTU of the tunnel as a whole. It MAY do this by
sending packets through the tunnel to the tunnel's receiving
endpoint, and performing Path MTU Discovery with those packets.

- 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

The Point-to-Point Protocol (PPP) [6] provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
defines an extensible Link Control Protocol, and proposes a family of
Network Control Protocols for establishing and configuring different
network-layer protocols.

This section defines the Network Control Protocol for establishing
and configuring label Switching over PPP.

4.1. Introduction

PPP has three main components:

1. A method for encapsulating multi-protocol datagrams.

2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.

3. A family of Network Control Protocols for establishing and
configuring different network-layer protocols.

In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure and test
the data link. After the link has been established and optional
facilities have been negotiated as needed by the LCP, PPP must send
"MPLS Control Protocol" packets to enable the transmission of labeled
packets. Once the "MPLS Control Protocol" has reached the Opened
state, labeled packets can be sent over the link.

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RFC 3032 MPLS Label Stack Encoding January 2001

The link will remain configured for communications until explicit LCP
or MPLS Control Protocol packets close the link down, or until some
external event occurs (an inactivity timer expires or network
administrator intervention).

4.2. A PPP Network Control Protocol for MPLS

The MPLS Control Protocol (MPLSCP) is responsible for enabling and
disabling the use of label switching on a PPP link. It uses the same
packet exchange mechanism as the Link Control Protocol (LCP). MPLSCP
packets may not be exchanged until PPP has reached the Network-Layer
Protocol phase. MPLSCP packets received before this phase is reached
should be silently discarded.

The MPLS Control Protocol is exactly the same as the Link Control
Protocol [6] with the following exceptions:

1. Frame Modifications

The packet may utilize any modifications to the basic frame
format which have been negotiated during the Link Establishment
phase.

2. Data Link Layer Protocol Field

Exactly one MPLSCP packet is encapsulated in the PPP
Information field, where the PPP Protocol field indicates type
hex 8281 (MPLS).

3. Code field

Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
Ack and Code-Reject) are used. Other Codes should be treated
as unrecognized and should result in Code-Rejects.

4. Timeouts

MPLSCP packets may not be exchanged until PPP has reached the
Network-Layer Protocol phase. An implementation should be
prepared to wait for Authentication and Link Quality
Determination to finish before timing out waiting for a
Configure-Ack or other response. It is suggested that an
implementation give up only after user intervention or a
configurable amount of time.

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RFC 3032 MPLS Label Stack Encoding January 2001

5. Configuration Option Types

None.

4.3. Sending Labeled Packets

Before any labeled packets may be communicated, PPP must reach the
Network-Layer Protocol phase, and the MPLS Control Protocol must
reach the Opened state.

Exactly one labeled packet is encapsulated in the PPP Information
field, where the PPP Protocol field indicates either type hex 0281
(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
maximum length of the Information field of a PPP encapsulated packet.

The format of the Information field itself is as defined in section
2.

Note that two codepoints are defined for labeled packets; one for
multicast and one for unicast. Once the MPLSCP has reached the
Opened state, both label switched multicasts and label switched
unicasts can be sent over the PPP link.

4.4. Label Switching Control Protocol Configuration Options

There are no configuration options.

5. Transporting Labeled Packets over LAN Media

Exactly one labeled packet is carried in each frame.

The label stack entries immediately precede the network layer header,
and follow any data link layer headers, including, e.g., any 802.1Q
headers that may exist.

The ethertype value 8847 hex is used to indicate that a frame is
carrying an MPLS unicast packet.

The ethertype value 8848 hex is used to indicate that a frame is
carrying an MPLS multicast packet.

These ethertype values can be used with either the ethernet
encapsulation or the 802.3 LLC/SNAP encapsulation to carry labeled
packets. The procedure for choosing which of these two
encapsulations to use is beyond the scope of this document.

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RFC 3032 MPLS Label Stack Encoding January 2001

6. IANA Considerations

Label values 0-15 inclusive have special meaning, as specified in
this document, or as further assigned by IANA.

In this document, label values 0-3 are specified in section 2.1.

Label values 4-15 may be assigned by IANA, based on IETF Consensus.

7. Security Considerations

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.

8. Intellectual Property

The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.

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RFC 3032 MPLS Label Stack Encoding January 2001

9. Authors' Addresses

Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824

EMail: [email protected]

Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824

EMail: [email protected]

Yakov Rekhter
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089

EMail: [email protected]

Guy Fedorkow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824

EMail: [email protected]

Dino Farinacci
Procket Networks, Inc.
3910 Freedom Circle, Ste. 102A
Santa Clara, CA 95054

EMail: [email protected]

Rosen, et al. Standards Track [Page 20]

RFC 3032 MPLS Label Stack Encoding January 2001

Tony Li
Procket Networks, Inc.
3910 Freedom Circle, Ste. 102A
Santa Clara, CA 95054

EMail: [email protected]

Alex Conta
TranSwitch Corporation
3 Enterprise Drive
Shelton, CT, 06484

EMail: [email protected]

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RFC 3032 MPLS Label Stack Encoding January 2001

10. References

[1] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.

[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[3] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
September 1981.

[4] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.

[5] Katz, D., "IP Router Alert Option", RFC 2113, February 1997.

[6] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.

[7] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 1885, December 1995.

[8] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP
version 6", RFC 1981, August 1996.

[9] Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y., Rosen, E.
and G. Swallow, "MPLS Using LDP and ATM VC Switching", RFC 3035,
January 2001.

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RFC 3032 MPLS Label Stack Encoding January 2001

11. Full Copyright Statement

Copyright (C) The Internet Society (2001). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Rosen, et al. Standards Track [Page 23]