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Versions: 00 01 02 03 RFC 3988

MPLS                                                            B. Black
Internet-Draft                                           Layer8 Networks
Expires: December 30, 2002                                   K. Kompella
                                                        Juniper Networks
                                                            July 1, 2002


                   MTU Signalling Extensions for LDP
                 draft-ietf-mpls-ldp-mtu-extensions-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 30, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   Proper functioning of RFC 1191 path MTU detection requires that IP
   routers have knowledge of the MTU for each link to which they are
   connected.  As currently specified, LDP does not have the ability to
   signal the MTU for an LSP to ingress LSRs.  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 extensions to the LDP label distribution
   protocol in support of LSP MTU signalling.




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1. Introduction

   As currently specified in [3], 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 [1].  Without knowledge of the MTU for an LSP, edge
   LSRs may transmit packets along that LSP which are, according to [4],
   too big.  Such packets may be silently discarded by LSRs along the
   LSP, effectively preventing communication between certain end hosts.

   The solution proposed in this document enables automatic
   determination of the MTU for an LSP with the addition of a TLV to
   carry MTU information for a 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 which is no worse than could be
   generated with local information on the ingress LSR.


































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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 Signalling Procedure

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

   1.  First, L computes the MTU for each FEC:

       1.  If L is the egress LSR for the FEC, L set the MTU to the MTU
           of the egress interface, unless local policy specifies
           otherwise.

       2.  If L is not the egress LSR for the FEC, L SHOULD set the MTU
           to 0xffff, indicating that it is not the egress LSR and has
           not yet received an MTU other than 0xffff from downstream
           LSRs.  Local policy may dictate the selection of a value
           other than 0xffff, but the default in the absence of such
           policy should be 0xffff.

       3.  If L is not the egress LSR for a FEC, and L receives a
           Mapping for a FEC which includes an MTU TLV with a value
           other than 0xffff, L calculates the MTU according to the
           rules in Section 2.2.  If L receives multiple Mapping
           messages for this FEC, it first chooses between them by some
           policy, then applies the calculation for the chosen Mapping.
           This is the "active Mapping" for this FEC.

       4.  If L receives a Mapping for a FEC without an MTU TLV from a
           directly connected neighbor, L MAY act as if it received an
           MTU TLV with MTU 0xffff, and follow the procedure in Step
           1.2.  Otherwise, L MUST send Mappings for this FEC without an
           MTU TLV.

       5.  If L receives a Mapping for a FEC from a peer to which it is
           not directly connected, it must first find an LSP by which L
           can reach the peer.  (Note that this procedure may be
           recursively applied.)  Once the appropriate LSP has been
           determined, the MTU is calculated according to the rules in
           Section 2.2, using the MTU of the selected LSP as the link
           MTU.

   2.  For each direct LDP neighbor of L to which L decides to send a
       Mapping for a FEC, L attaches an MTU TLV with the MTU that it



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       computed for this FEC.  Mapping messages sent to "remote" LDP
       neighbors need not have an MTU TLV.

   3.  When a new MTU is received for a label mapping from a downstream
       LSR, or the active Mapping for a FEC changes, L returns to Step
       1.  If the newly computed MTU is unchanged, L does not advertise
       new information to its neighbors.

       This behavior is standard for attributes such as path vector and
       hop count, and the same rules apply, as specified in [3].

   4.  In some cases, a node may act as both an LER and an LSR for the
       same LSP.  In these situations, the node will calculate multiple
       MTUs: the MTU advertised to upstream LSRs for labelled traffic
       and the MTU used locally when processing unlabelled traffic.  The
       procedure for calculating each of these MTUs is unchanged from
       the steps above, although the series of steps taken will differ
       depending on which MTU is being calculated.


2.2 Calculating Local MTU

   There is a wide variety of policies which may be used in determining
   the MTU advertised by a node, however there are restrictions which
   MUST be adhered to in order to ensure proper operation of MTU
   signalling and minimization of signalling traffic during topology
   changes.

      If the local policy is based entirely on the egress interface for
      the LSP, the calculated MTU must be less than or equal to the
      egress interface MTU.

      If the local policy is based on a group of egress interfaces, the
      calculated MTU MUST be less than or equal to the MTU of the egress
      interface with the largest MTU in the group minus any label
      overhead, but SHOULD be less than or equal to the MTU of the
      egress interface with the smallest MTU in the group minus any
      label overhead.

      If the local LSR is the ingress LER for the FEC in question, any
      label overhead introduced must be subtracted from the calculated
      MTU to determine the actual path MTU.  For example, if 2 labels
      are pushed onto the stack, the LSR MUST subtract 8 bytes from the
      MTU value it has calculated based on local link MTUs and MTU
      values received from downstream LDP neighbors.

      Under no circumstances must the advertised MTU exceed the received
      MTU.



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2.3 MTU TLV

   The MTU TLV encodes information on the maximum transmission unit for
   an LSP, either for the entire path or only for a segment of the path.

   The encoding for the MTU TLV is:

       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|0|      MTU TLV (0x0XXX)     |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              MTU              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   MTU

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
































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3. Example of Operation

   The figure and below describes a simple LSR topology.  Ri and Re are
   the ingress and egress LSRs for LSP P1.  Rx and Re are the ingress
   and egress LSRs for LSP P2.  From Rx to Re, LSP P1 is encapsulated in
   LSP P2.  Ry is an intermediate LSR which does not act as ingress or
   egress for any LSPs.  L1 through L3 are links connecting the LSRs.
   Le is the egress link.


                                                                 MTU
                                                       Media    w/ P2
        +--+      +--+      +--+      +--+       Link   MTU    overhead
      --|Ri|--L1--|Rx|--L2--|Ry|--L3--|Re|--Le   ----  ------  --------
        +--+      +--+      +--+      +--+        L1    9216     9216
          |         |                  ^^         L2    4470     4466
          |         |                  ||         L3    9216     9212
          |         +---P2-------------+|         Le    9216     9216
          |                             |
          +-------------P1--------------+


   Figure 1.  Sample LSR Topology

   The following four time steps illustrate the calculation of the MTU
   for P1.  Let FEC F represent traffic mapped to LSP P1.

   At t[0]:

   1) Re sets the MTU for F to 9216 (the MTU of the egress interface)
   and sends a Mapping message for F to Ry.

   2) Ri, Rx, and Ry have not received Mappings for F.

   At t[1]:

   1) Ry receives a Mapping for F from Re with an MTU of 9216.  Ry
   compares 9216 to 9216 (Ry does not push a label onto the stack for
   either P1 or P2), and sends a mapping message for F with an MTU of
   9216 to Rx.

   2) Ri and Rx have not received Mappings for F.

   At t[2]:

   1) Rx receives a Mapping for F from Ry with an MTU of 9216.  Rx
   compares 9212 (9216 - 4) to 4466, and sends a Mapping message for F
   with an MTU of 4466 to Ri.



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   2) Ri has not received Mappings for F.

   At t[3]:

   1) Ri receives a Mapping for F from Rx with an MTU of 4462.  Ri
   compares 4466 to 9216, and sets the MTU for P1 to 4462 (4466 minus
   the overhead of 1 label pushed onto the stack).












































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4. Protocol Interaction

4.1 Interaction With LSRs Which 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 which do not support MTU signalling MUST accept these messages,
   but MAY ignore them (see Section 2.1).

4.2 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 using CR-LDP, as specified in [2].

   Note that LDP/CR-LDP LSPs may tunnel through other LSPs signalled
   using LDP, CR-LDP or RSVP-TE [5]; the mechanism suggested here
   applies in all these cases.

































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5. 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 this sort of attack is not new to LDP.














































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6. Acknowledgments

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














































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References (Normative)

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

   [2]  Jamoussi, J., "Constraint-Based LSP Setup Using LDP", July 2000.

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

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

   [5]  Awduche, D., Berger, L. and D. Gan, "RSVP-TE: Extensions to RSVP
        for LSP Tunnels", February 2001.


Authors' Addresses

   Benjamin Black
   Layer8 Networks

   EMail: ben@layer8.net


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

   EMail: kireeti@juniper.net


















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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Acknowledgement

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



















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