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Maximum Transmission Unit Signalling Extensions for the Label Distribution Protocol
RFC 3988

Document Type RFC - Experimental (February 2005)
Authors Kireeti Kompella , Benjamin Black
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
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IESG Responsible AD Alex D. Zinin
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RFC 3988
Network Working Group                                           B. Black
Request for Comments: 3988                               Layer8 Networks
Category: Experimental                                       K. Kompella
                                                        Juniper Networks
                                                            January 2005

            Maximum Transmission Unit Signalling Extensions
                  for the Label Distribution Protocol

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   Proper functioning of RFC 1191 path Maximum Transmission Unit (MTU)
   discovery requires that IP routers have knowledge of the MTU for each
   link to which they are connected.  As currently specified, the Label
   Distribution Protocol (LDP) does not have the ability to signal the
   MTU for a Label Switched Path (LSP) to the ingress Label Switching
   Router (LSR).  In the absence of this functionality, the MTU for each
   LSP must be statically configured by network operators or by
   equivalent off-line mechanisms.

   This document specifies experimental extensions to LDP in support of
   LSP MTU discovery.

1.  Introduction

   As currently specified in [2], the LDP protocol for MPLS does not
   support signalling of the MTU for LSPs to ingress LSRs.  This
   functionality is essential to the proper functioning of RFC 1191 path
   MTU detection [3].  Without knowledge of the MTU for an LSP, edge
   LSRs may transmit packets along that LSP which are, according to [4],
   too big.  These packets may be silently discarded by LSRs along the
   LSP, effectively preventing communication between certain end hosts.

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   The solution proposed in this document enables automatic
   determination of the MTU for an LSP by adding a Type-Length-Value
   triplet (TLV) to carry MTU information for a Forwarding Equivalence
   Class (FEC) between adjacent LSRs in LDP Label Mapping messages.
   This information is sufficient for a set of LSRs along the path
   followed by an LSP to discover either the exact MTU for that LSP, or
   an approximation that is no worse than could be generated with local
   information on the ingress LSR.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119 [1].

2.  MTU Signalling

   The signalling procedure described in this document employs the
   addition of a single TLV to LDP Label Mapping messages and a simple
   algorithm for LSP MTU calculation.

2.1.  Definitions

   Link MTU: The MTU of a given link.  This size includes the IP header
   and data (or other payload) and the label stack but does not include
   any lower-layer headers.  A link may be an interface (such as
   Ethernet or Packet-over-SONET), a tunnel (such as GRE or IPsec), or
   an LSP.

   Peer LSRs: For LSR A and FEC F, this is the set of LSRs that sent a
   Label Mapping for FEC F to A.

   Downstream LSRs: For LSR A and FEC F, this is the subset of A's peer
   LSRs for FEC F to which A will forward packets for the FEC.
   Typically, this subset is determined via the routing table.

   Hop MTU: The MTU of an LSP hop between an upstream LSR, A, and a
   downstream LSR, B.  This size includes the IP header and data (or
   other payload) and the part of the label stack that is considered
   payload as far as this LSP goes.  It does not include any lower-level
   headers.  (Note: If there are multiple links between A and B, the Hop
   MTU is the minimum of the Hop MTU of those links used for

   LSP MTU: The MTU of an LSP from a given LSR to the egress(es), over
   each valid (forwarding) path.  This size includes the IP header and
   data (or other payload) and any part of the label stack that was
   received by the ingress LSR before it placed the packet into the LSP

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   (this part of the label stack is considered part of the payload for
   this LSP).  The size does not include any lower-level headers.

2.2.  Example

   Consider LSRs A - F, interconnected as follows:

                     M       P
                   _____ C =====
                  /      |      \
         A ~~~~~ B ===== D ----- E ----- F
             L       N       Q       R

   Say that the link MTU for link L is 9216; for links M, Q, and R,
   4470; and for N and P, is 1500.

   Consider an FEC X for which F is the egress, and say that all LSRs
   advertise X to their neighbors.

   Note that although LDP may be running on the C - D link, it is not
   used for forwarding (e.g., because it has a high metric).  In
   particular, D is an LDP neighbor of C, but D is not one of C's
   downstream LSRs for FEC X.

   E's peers for FEC X are C, D, and F.  Say that E chooses F as its
   downstream LSR for X.  E's Hop MTU for link R is 4466.  If F
   advertised an implicit null label to E, then E MAY set the Hop MTU
   for R to 4470.

   C's peers for FEC X are B, D, and E.  Say that C chooses E as its
   downstream LSR for X.  Similarly, A chooses B, B chooses C and D
   (equal cost multi-path), D chooses E, and E chooses F (respectively)
   as downstream LSRs.

   C's Hop MTU to E for FEC X is 1496.  B's Hop MTU to C is 4466 and to
   D is 1496.  A's LSP MTU for FEC X is 1496.  If A has another LSP for
   FEC Y to F (learned via targeted LDP) that rides over the LSP for FEC
   X, the MTU for that LSP would be 1492.

   If B had a targeted LDP session to E (e.g., over an RSVP-TE tunnel T)
   and B received a Mapping for FEC X over the targeted LDP session,
   then E would also be B's peer, and E may be chosen as a downstream
   LSR for B.  In that case, B's LSP MTU for FEC X would then be the
   smaller of {(T's MTU - 4), E's LSP MTU for X}.

   This memo describes how A determines its LSP MTU for FECs X and Y.

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2.3.  Signalling Procedure

   The procedure for signalling the MTU is performed hop-by-hop by each
   LSR L along an LSP for a given FEC, F.  The steps are as follows:

   1. First, L computes its LSP MTU for FEC F:

      A. If L is the egress for F, L sets the LSP MTU for F to 65535.

      B. [OPTIONAL] If L's only downstream LSR is the egress for F
         (i.e., L is a penultimate hop for F) and L receives an implicit
         null label as its Mapping for F, then L can set the Hop MTU for
         its downstream link to the link MTU instead of (link MTU - 4
         octets).  L's LSP MTU for F is the Hop MTU.

      C. Otherwise (L is not the egress LSR), L computes the LSP MTU for
         F as follows:

         a) L determines its downstream LSRs for FEC F.

         b) For each downstream LSR Z, L computes the minimum of the Hop
            MTU to Z and the LSP MTU in the MTU TLV that Z advertised to
            L.  If Z did not include the MTU TLV in its Label Mapping,
            then Z's LSP MTU is set to 65535.

         c) L sets its LSP MTU to the minimum of the MTUs it computed
            for its downstream LSRs.

   2. For each LDP neighbor (direct or targeted) of L to which L decides
      to send a Mapping for FEC F, L attaches an MTU TLV with the LSP
      MTU that it computed for this FEC.  L MAY (because of policy or
      for other reasons) advertise a smaller MTU than it has computed,
      but L MUST NOT advertise a larger MTU.

   3. When a new MTU is received for FEC F from a downstream LSR or the
      set of downstream LSRs for F changes, L returns to step 1.  If the
      newly computed LSP MTU is unchanged, L SHOULD NOT advertise new
      information to its neighbors.  Otherwise, L readvertises its
      Mappings for F to all its peers with an updated MTU TLV.

      This behavior is standard for attributes such as path vector and
      hop count, and the same rules apply, as specified in [2].

      If the LSP MTU decreases, L SHOULD readvertise the new MTU
      immediately; if the LSP MTU increases, L MAY hold down the

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2.4.  MTU TLV

   The MTU TLV encodes information on the maximum transmission unit for
   an LSP, from the advertising LSR to the egress(es) over all valid

   The encoding for the MTU TLV is as follows:

       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
      |1|1|      MTU TLV (0x0601)     |            Length             |
      |              MTU              |


   This is a 16-bit unsigned integer that represents the MTU in octets
   for an LSP or a segment of an LSP.

   Note that the U and F bits are set.  An LSR that doesn't recognize
   the MTU TLV MUST ignore it when it processes the Label Mapping
   message and forward the TLV to its peers.  This may result in the
   incorrect computation of the LSP MTU; however, silently forwarding
   the MTU TLV preserves the maximal amount of information about the LSP

3.  Example of Operation

   Consider the network example in Section 2.2.  For each LSR, Table 1
   describes the links to its downstream LSRs, the Hop MTU for the peer,
   the LSP MTU received from the peer, and the LSR's computed LSP MTU.

   Now consider the same network with the following changes: There is an
   LSP T from B to E, and a targeted LDP session from B to E.  B's peer
   LSRs are A, C, D, and E; B's downstream LSRs are D and E; to reach E,
   B chooses to go over T.  The LSP MTU for LSP T is 1496.  This
   information is depicted in Table 2.

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         LSR  |  Link  |  Hop MTU  |  Recvd MTU  |  LSP MTU
          F   |    -   |    65535  |      -      |    65535
          E   |    R   |     4466  |  F:  65535  |     4466
          D   |    Q   |     4466  |  E:   4466  |     4466
          C   |    P   |     1496  |  E:   4466  |     1496
          B   |    M   |     4466  |  C:   1496  |
              |    N   |     1496  |  D:   4466  |     1496
          A   |    L   |     9212  |  B:   1496  |     1496
                              Table 1

         LSR  |  Link  |  Hop MTU  |  Recvd MTU  |  LSP MTU
          F   |    -   |    65535  |      -      |    65535
          E   |    R   |     4466  |  F:  65535  |     4466
          D   |    Q   |     4466  |  E:   4466  |     4466
          C   |    P   |     1496  |  E:   4466  |     1496
          B   |    T   |     1492  |  E:   4466  |
              |    N   |     1496  |  D:   4466  |     1492
          A   |    L   |     9212  |  B:   1492  |     1492
                              Table 2

4.  Using the LSP MTU

   An ingress LSR that forwards an IP packet into an LSP whose MTU it
   knows MUST either fragment the IP packet to the LSP's MTU (if the
   Don't Fragment bit is clear) or drop the packet and respond with an
   ICMP Destination Unreachable message to the source of the packet,
   with the Code indicating "fragmentation needed and DF set", and the
   Next-Hop MTU set to the LSP MTU.  In other words, the LSR behaves as
   RFC 1191 says, except that it treats the LSP as the next hop

   If the payload for the LSP is not an IP packet, the LSR MUST forward
   the packet if it fits (size <= LSP MTU) and SHOULD drop it if it

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5.  Protocol Interaction

5.1.  Interaction with LSRs that Do Not Support MTU Signalling

   Changes in MTU for sections of an LSP may cause intermediate LSRs to
   generate unsolicited label Mapping messages to advertise the new MTU.
   LSRs that do not support MTU signalling will, due to message and TLV
   processing mechanisms specified in RFC3036 [2], accept the messages
   carrying the MTU TLV but will ignore the TLV and forward the TLV to
   the upstream nodes (see Section 2.4).

5.2.  Interaction with CR-LDP and RSVP-TE

   The MTU TLV can be used to discover the Path MTU of both LDP LSPs and
   CR-LDP LSPs.  This proposal is not impacted in the presence of LSPs
   created with CR-LDP, as specified in [5].

   Note that LDP/CR-LDP LSPs may tunnel through other LSPs signalled
   using LDP, CR-LDP, or RSVP-TE [6]; the mechanism suggested here
   applies in all of these cases, essentially by treating the tunnel
   LSPs as links.

6.  References

6.1.  Normative References

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

   [2]  Andersson, L., Doolan, P., Feldman, N., Fredette, A., and B.
        Thomas, "LDP Specification", RFC 3036, January 2001.

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

   [4]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D.,
        Li, T., and A. Conta, "MPLS Label Stack Encoding", RFC 3032,
        January 2001.

   [6]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G.
        Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC
        3209, December 2001.

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6.2.  Informative References

   [5]  Jamoussi, B., Andersson, L., Callon, R., Dantu, R., Wu, L.,
        Doolan, P., Worster, T., Feldman, N., Fredette, A., Girish, M.,
        Gray, E., Heinanen, J., Kilty, T., and A. Malis, "Constraint-
        Based LSP Setup using LDP", RFC 3212, January 2002.

7.  Security Considerations

   This mechanism does not introduce any new weaknesses in LDP.  It is
   possible to spoof TCP packets belonging to an LDP session to
   manipulate the LSP MTU, but LDP has mechanisms to thwart these types
   of attacks.  See Section 5 of [2] for more information on security
   aspects of LDP.

8.  IANA Considerations

   IANA has allocated 0x0601 as a new LDP TLV Type, defined in Section
   2.4.  See:

9.  Acknowledgements

   We would like to thank Andre Fredette for a number of detailed
   comments on earlier versions of the signalling mechanism.  Eric Gray,
   Giles Heron, and Mark Duffy have contributed numerous useful

Authors' Addresses

   Benjamin Black
   Layer8 Networks


   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089


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