RFC relevance of procedure

As per Relevance of the word procedure, we have this rfc below:

Network Working Group E.
Request for Comments: 3032 D.
Category: Standards Track G.
Cisco Systems, Inc
Y.
Juniper
D.
T.
Procket Networks, Inc
A.
TranSwitch
January 2001

MPLS Label Stack

Status of this

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

Copyright

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

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

Rosen, et al. Standards Track [Page 1]

RFC 3032 MPLS Label Stack Encoding January 2001

Table of

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.

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

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RFC 3032 MPLS Label Stack Encoding January 2001

This document specifies the encoding to be used by an LSR in order
transmit labeled packets on PPP data links and on LAN data links
The specified encoding may also be useful for other data links
well

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

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

1.1. Specification of

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

2. The Label

2.1. Encoding the Label

The label stack is represented as a sequence of "label
entries". Each label stack entry is represented by 4 octets.
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 | Exp |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

Figure 1

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RFC 3032 MPLS Label Stack Encoding January 2001

The label stack entries appear AFTER the data link layer headers,
BEFORE any network layer headers. The top of the label stack
earliest in the packet, and the bottom appears latest. The
layer packet immediately follows the label stack entry which has
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
(i.e., for the bottom of the stack), and zero for all
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

This three-bit field is reserved for experimental use

4. Label

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

When a labeled packet is received, the label value at the
of the stack is looked up. As a result of a successful
one learns

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

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

In addition to learning the next hop and the label
operation, one may also learn the outgoing data
encapsulation, and possibly other information which is
in order to properly forward the packet

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RFC 3032 MPLS Label Stack Encoding January 2001

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
stack. It indicates that the label stack must be popped
and the forwarding of the packet must then be based on
IPv4 header

ii. A value of 1 represents the "Router Alert Label".
label value is legal anywhere in the label stack except
the bottom. When a received packet contains this
value at the top of the label stack, it is delivered to
local software module for processing. The
forwarding of the packet is determined by the
beneath it in the stack. However, if the packet
forwarded further, the Router Alert Label should be
back onto the label stack before forwarding. The use
this label is analogous to the use of the "Router
Option" in IP packets [5]. Since this label cannot
at the bottom of the stack, it is not associated with
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
stack. It indicates that the label stack must be popped
and the forwarding of the packet must then be based on
IPv6 header

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

v. Values 4-15 are reserved

2.2. Determining the Network Layer

When the last label is popped from a packet's label stack (
in the stack being emptied), further processing of the packet
based on the packet's network layer header. The LSR which pops
last label off the stack must therefore be able to identify
packet's network layer protocol. However, the label stack does
contain any field which explicitly identifies the network

Rosen, et al. Standards Track [Page 5]

RFC 3032 MPLS Label Stack Encoding January 2001

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

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

Adherence to these conditions does not necessarily
intermediate nodes to identify a packet's network layer protocol
Under ordinary conditions, this is not necessary, but there are
conditions under which it is desirable. For instance, if
intermediate LSR determines that a labeled packet is undeliverable
it may be desirable for that LSR to generate error messages which
specific to the packet's network layer. The only means
intermediate LSR has for identifying the network layer is
of the top label and the network layer header. So if
nodes are to be able to generate protocol-specific error messages
labeled packets, all labels in the stack must meet the
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
data link MTU), and either its network layer protocol cannot
identified, or there are no specified protocol-dependent rules
handling the error condition, then the packet MUST be
discarded

2.3. Generating ICMP Messages for Labeled IP

Section 2.4 and section 3 discuss situations in which it is
to generate ICMP messages for labeled IP packets. In order for
particular LSR to be able to generate an ICMP packet and have
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
labeled packet is an IP packet

2. it must be possible for that LSR to route to the packet's
source address

Rosen, et al. Standards Track [Page 6]

RFC 3032 MPLS Label Stack Encoding January 2001

Condition 1 is discussed in section 2.2. The following
subsections discuss condition 2. However, there will be some
in which condition 2 does not hold at all, and in these cases it
not be possible to generate the ICMP message

2.3.1. Tunneling through a Transit Routing

Suppose one is using MPLS to "tunnel" through a transit
domain, where the external routes are not leaked into the domain'
interior routers. For example, the interior routers may be
OSPF, and may only know how to reach destinations within that
domain. The domain might contain several Autonomous System
Routers (ASBRs), which talk BGP to each other. However, in
example the routes from BGP are not distributed into OSPF, and
LSRs which are not ASBRs do not run BGP

In this example, only an ASBR will know how to route to the source
some arbitrary packet. If an interior router needs to send an
message to the source of an IP packet, it will not know how to
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
which must leave the domain (such as an ICMP packet) gets sent to
router which has full routing information. The routers with
routing information will label the packets before sending them
through the transit domain, so the use of default routing within
transit domain does not cause any loops

This solution only works for packets which have globally
addresses, and for networks in which all the ASBRs have
routing information. The next subsection describes a solution
works when these conditions do not hold

2.3.2. Tunneling Private Addresses through a Public

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
packet at all. This would be the case, for example, if the
addresses carried in the packet were private (i.e., not
unique) addresses, and MPLS were being used to tunnel those
through a public backbone. Default routing to an ASBR will not
in this environment

In this environment, in order to send an ICMP message to the
of a packet, one can copy the label stack from the original packet
the ICMP message, and then label switch the ICMP message. This

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RFC 3032 MPLS Label Stack Encoding January 2001

cause the message to proceed in the direction of the
packet's destination, rather than its source. Unless the message
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
original packet, at which point the message will be sent in
proper direction

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

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

Note that if a packet's TTL expiration is due to the presence of
routing loop, then if this technique is used, the ICMP message
loop as well. Since an ICMP message is never sent as a result
receiving an ICMP message, and since many implementations
the rate at which ICMP messages can be generated, this is
expected to pose a problem

2.4. Processing the Time to Live

2.4.1.

The "incoming TTL" of a labeled packet is defined to be the value
the TTL field of the top label stack entry when the packet
received

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

a) one less than the incoming TTL
b) zero

2.4.2. Protocol-independent

If the outgoing TTL of a labeled packet is 0, then the labeled
MUST NOT be further forwarded; nor may the label stack be
off and the packet forwarded as an unlabeled packet. The packet'
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
be simply discarded, or it may be passed to the
"ordinary" network layer for error processing (e.g., for
generation of an ICMP error message, see section 2.3).

When a labeled packet is forwarded, the TTL field of the label
entry at the top of the label stack MUST be set to the outgoing
value

Note that the outgoing TTL value is a function solely of the
TTL value, and is independent of whether any labels are pushed
popped before forwarding. There is no significance to the value
the TTL field in any label stack entry which is not at the top of
stack

2.4.3. IP-dependent

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
entry MUST BE set to the value of the IP TTL field. (If the IP
field needs to be decremented, as part of the IP processing, it
assumed that this has already been done.)

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

It is recognized that there may be situations where a
administration prefers to decrement the IPv4 TTL by one as
traverses an MPLS domain, instead of decrementing the IPv4 TTL by
number of LSP hops within the domain

2.4.4. Translating Between Different

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

In this case, the value of the "incoming TTL" is determined by
procedures used for carrying labeled packets on, e.g., LC-
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
link, and may need to send it out, say, an LC-ATM interface.
the incoming packet will be received using the
specified in this document, but the outgoing packet will not be
using the encapsulation specified in this document. In this case
the procedure for carrying the value of the "outgoing TTL"
determined by the procedures used for carrying labeled packets on
e.g., LC-ATM interfaces

3. Fragmentation and Path MTU

Just as it is possible to receive an unlabeled IP datagram which
too large to be transmitted on its output link, it is possible
receive a labeled packet which is too large to be transmitted on
output link

It is also possible that a received packet (labeled or unlabeled
which was originally small enough to be transmitted on that
becomes too large by virtue of having one or more additional
pushed onto its label stack. In label switching, a packet may
in size if additional labels get pushed on. Thus if one receives
labeled packet with a 1500-byte frame payload, and pushes on
additional label, one needs to forward it as frame with a 1504-
payload

This section specifies the rules for processing labeled packets
are "too large". In particular, it provides rules which ensure
hosts implementing Path MTU Discovery [4], and hosts using IPv
[7,8], will be able to generate IP datagrams that do not
fragmentation, even if those datagrams get labeled as they
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
MTUs in use on most data links today are 1500 bytes or more,
probability that such datagrams will need to get fragmented, even
they get labeled, is very small

Some hosts that do not implement Path MTU Discovery [4] will
IP datagrams containing 1500 bytes, as long as the IP Source
Destination addresses are on the same subnet. These datagrams
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
classful network number. This is the one case in which there is
risk of fragmentation when such datagrams get labeled. (Even so

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RFC 3032 MPLS Label Stack Encoding January 2001

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

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

3.1.

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

- Frame Payload

The contents of a data link frame, excluding any data
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
Payload is just the IP datagram itself. When a frame
carrying a labeled IP datagram, the Frame Payload consists
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
ethernet is 1500 bytes

- True Maximum Frame Payload Size

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

On ethernet and 802.3 networks, it is believed that the
Maximum Frame Payload Size is 4-8 bytes larger than
Conventional Maximum Frame Payload Size (as long as neither
802.1Q header nor an 802.1p header is present, and as long
neither can be added by a switch or bridge while a packet is
transit to its next hop). For example, it is believed
most ethernet equipment could correctly send and
packets carrying a payload of 1504 or perhaps even 1508 bytes
at least, as long as the ethernet header does not have
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
virtually unbounded

- Effective Maximum Frame Payload Size for Labeled Packets

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

- Initially Labeled IP Datagram

Suppose that an unlabeled IP datagram is received at
particular LSR, and that the the LSR pushes on a label
forwarding the datagram. Such a datagram will be called
Initially Labeled IP Datagram at that LSR

- Previously Labeled IP Datagram

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

3.2. Maximum Initially Labeled IP Datagram

Every LSR which is capable

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

SHOULD support a configuration parameter known as the "
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,
b) that datagram does not have the DF bit set in its IP header

c) that datagram needs to be labeled before being forwarded,
d) the size of the datagram (before labeling) exceeds the value
the parameter

a) the datagram must be broken into fragments, each of whose
is no greater than the value of the parameter,

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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
1488, then any unlabeled IP datagram containing more than 1488
will be fragmented before being labeled. Each fragment will
capable of being carried on a 1500-byte data link, without
fragmentation, even if as many as three labels are pushed onto
label stack

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

Note that the setting of this parameter does not affect
processing of IP datagrams that have the DF bit set; hence the
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
Frame Payload Size of the data link over which it is to be
MAY be considered to be "too big".

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

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

3.4. Processing Labeled IPv4 Datagrams which are Too

If a labeled IPv4 datagram is "too big", and the DF bit is not set
its IP header, then the LSR MAY silently discard the datagram

Note that discarding such datagrams is a sensible procedure only
the "Maximum Initially Labeled IP Datagram Size" is set to a non-
value in every LSR in the network which is capable of adding a
stack to an unlabeled IP datagram

If the LSR chooses not to discard a labeled IPv4 datagram which
too big, or if the DF bit is set in that datagram, then it
execute the following algorithm

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

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RFC 3032 MPLS Label Stack Encoding January 2001

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

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

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

b. Prepend each fragment with the same label header that
have been on the original datagram had fragmentation
been necessary

c. Forward the

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

a. the datagram MUST NOT be

b. Create an ICMP Destination Unreachable Message

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

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

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

3.5. Processing Labeled IPv6 Datagrams which are Too

To process a labeled IPv6 datagram which is too big, an LSR
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
the number of label stack entries).

3. If the IP datagram contains more than 1280 bytes (not
the label stack entries), or if it does not contain a
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-
MTU field to the difference between the Effective
Frame Payload Size and the value of

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

c. discard the labeled IPv6 datagram

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

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

b. Prepend each fragment with the same label header that
have been on the original datagram had fragmentation
been necessary

c. Forward the fragments

Reassembly of the fragments will be done at the
host

3.6. Implications with respect to Path MTU

The procedures described above for handling datagrams which have
DF bit set, but which are "too large", have an impact on the Path
Discovery procedures of RFC 1191 [4]. Hosts which implement
procedures will discover an MTU which is small enough to allow
labels to be pushed on the datagrams, without need for fragmentation
where n is the number of labels that actually get pushed on along
path currently in use

In other words, datagrams from hosts that use Path MTU Discovery
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,
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
where a labeled IP Datagram's fragmentation needs to occur, it
possible to cause an ICMP Destination Unreachable message to
routed to the packet's source address. See section 2.3.

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RFC 3032 MPLS Label Stack Encoding January 2001

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

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

- Any time the transmitting endpoint of the tunnel needs to
a packet into the tunnel, and that packet has the DF bit set
and it exceeds the tunnel MTU, the transmitting endpoint of
tunnel MUST send the ICMP Destination Unreachable message
the source, with code "Fragmentation Required and DF Set",
the Next-Hop MTU Field set as described above

4. Transporting Labeled Packets over

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

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

4.1.

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
configuring different network-layer protocols

In order to establish communications over a point-to-point link,
end of the PPP link must first send LCP packets to configure and
the data link. After the link has been established and
facilities have been negotiated as needed by the LCP, PPP must
"MPLS Control Protocol" packets to enable the transmission of
packets. Once the "MPLS Control Protocol" has reached the
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
or MPLS Control Protocol packets close the link down, or until
external event occurs (an inactivity timer expires or
administrator intervention).

4.2. A PPP Network Control Protocol for

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

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

1. Frame

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

2. Data Link Layer Protocol

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

3. Code

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
as unrecognized and should result in Code-Rejects

4.

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

Rosen, et al. Standards Track [Page 17]

RFC 3032 MPLS Label Stack Encoding January 2001

5. Configuration Option

None

4.3. Sending Labeled

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

Exactly one labeled packet is encapsulated in the PPP
field, where the PPP Protocol field indicates either type hex 0281
(MPLS Unicast) or type hex 0283 (MPLS Multicast). The maximum
of a labeled packet transmitted over a PPP link is the same as
maximum length of the Information field of a PPP encapsulated packet

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

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

4.4. Label Switching Control Protocol Configuration

There are no configuration options

5. Transporting Labeled Packets over LAN

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 802.1
headers that may exist

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

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

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

Rosen, et al. Standards Track [Page 18]

RFC 3032 MPLS Label Stack Encoding January 2001

6. IANA

Label values 0-15 inclusive have special meaning, as specified
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

The MPLS encapsulation that is specified herein does not raise
security issues that are not already present in either the
architecture [1] or in the architecture of the network layer
contained within the encapsulation

There are two security considerations inherited from the
architecture which may be pointed out here

- Some routers may implement security procedures which depend
the network layer header being in a fixed place relative to
data link layer header. These procedures will not work
the MPLS encapsulation is used, because that encapsulation
of a variable size

- An MPLS label has its meaning by virtue of an agreement
the LSR that puts the label in the label stack (the "
writer"), and the LSR that interprets that label (the "
reader"). However, the label stack does not provide any
of determining who the label writer was for any
label. If labeled packets are accepted from untrusted sources
the result may be that packets are routed in an
manner

8. Intellectual

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

Rosen, et al. Standards Track [Page 19]

RFC 3032 MPLS Label Stack Encoding January 2001

9. Authors'

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

EMail: erosen@cisco.

Dan
Cisco Systems, Inc
250 Apollo
Chelmsford, MA, 01824

EMail: tappan@cisco.

Yakov
Juniper
1194 N. Mathilda
Sunnyvale, CA 94089

EMail: yakov@juniper.

Guy
Cisco Systems, Inc
250 Apollo
Chelmsford, MA, 01824

EMail: fedorkow@cisco.

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

EMail: dino@procket.

Rosen, et al. Standards Track [Page 20]

RFC 3032 MPLS Label Stack Encoding January 2001

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

EMail: tli@procket.

Alex
TranSwitch
3 Enterprise
Shelton, CT, 06484

EMail: aconta@txc.

Rosen, et al. Standards Track [Page 21]

RFC 3032 MPLS Label Stack Encoding January 2001

10.

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

[2] Bradner, S., "Key words for use in RFCs to Indicate
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
(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
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.

Rosen, et al. Standards Track [Page 22]

RFC 3032 MPLS Label Stack Encoding January 2001

11. Full Copyright

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

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

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

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

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

Rosen, et al. Standards Track [Page 23]

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just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.

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