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

 
Document Type RFC - Experimental (February 2005; No errata)
Last updated 2013-03-02
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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).

Abstract

   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.

Black & Kompella              Experimental                      [Page 1]
RFC 3988           MTU Signalling Extensions for LDP        January 2005

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   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
   forwarding.)

   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

Black & Kompella              Experimental                      [Page 2]
RFC 3988           MTU Signalling Extensions for LDP        January 2005

   (this part of the label stack is considered part of the payload for
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