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rfc:rfc3032

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.

Rosen, et al. Standards Track [Page 1] 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.

Rosen, et al. Standards Track [Page 2] RFC 3032 MPLS Label Stack Encoding January 2001

 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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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

Rosen, et al. Standards Track [Page 3] RFC 3032 MPLS Label Stack Encoding January 2001

 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.

Rosen, et al. Standards Track [Page 4] 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 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

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 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.

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 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

Rosen, et al. Standards Track [Page 7] RFC 3032 MPLS Label Stack Encoding January 2001

 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.

Rosen, et al. Standards Track [Page 8] 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.

Rosen, et al. Standards Track [Page 9] 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,

Rosen, et al. Standards Track [Page 10] RFC 3032 MPLS Label Stack Encoding January 2001

 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:
  1. 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.
  1. 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.
  1. 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.

Rosen, et al. Standards Track [Page 11] RFC 3032 MPLS Label Stack Encoding January 2001

       On PPP links, the True Maximum Frame Payload Size may be
       virtually unbounded.
  1. 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.
  1. 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.
  1. 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

Rosen, et al. Standards Track [Page 12] 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.

Rosen, et al. Standards Track [Page 13] RFC 3032 MPLS Label Stack Encoding January 2001

    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:

Rosen, et al. Standards Track [Page 14] 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.

Rosen, et al. Standards Track [Page 15] RFC 3032 MPLS Label Stack Encoding January 2001

 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:
  1. 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.
  1. 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.

Rosen, et al. Standards Track [Page 16] 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.

Rosen, et al. Standards Track [Page 17] 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.

Rosen, et al. Standards Track [Page 18] 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:
  1. 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.
  1. 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.

Rosen, et al. Standards Track [Page 19] RFC 3032 MPLS Label Stack Encoding January 2001

9. Authors' Addresses

 Eric C. Rosen
 Cisco Systems, Inc.
 250 Apollo Drive
 Chelmsford, MA, 01824
 EMail: erosen@cisco.com
 Dan Tappan
 Cisco Systems, Inc.
 250 Apollo Drive
 Chelmsford, MA, 01824
 EMail: tappan@cisco.com
 Yakov Rekhter
 Juniper Networks
 1194 N. Mathilda Avenue
 Sunnyvale, CA 94089
 EMail: yakov@juniper.net
 Guy Fedorkow
 Cisco Systems, Inc.
 250 Apollo Drive
 Chelmsford, MA, 01824
 EMail: fedorkow@cisco.com
 Dino Farinacci
 Procket Networks, Inc.
 3910 Freedom Circle, Ste. 102A
 Santa Clara, CA 95054
 EMail: dino@procket.com

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: tli@procket.com
 Alex Conta
 TranSwitch Corporation
 3 Enterprise Drive
 Shelton, CT, 06484
 EMail: aconta@txc.com

Rosen, et al. Standards Track [Page 21] 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.

Rosen, et al. Standards Track [Page 22] 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]

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