RFC 3443 (rfc3443) - Page 2 of 10


Time To Live (TTL) Processing in Multi-Protocol Label Switching (MPLS) Networks



Alternative Format: Original Text Document



RFC 3443            TTL Processing in MPLS Networks         January 2003


   In particular, a new mode of operation (referred to as the Pipe
   Model) is introduced to support the practice of configuring MPLS LSPs
   such that packets transiting the LSP see the tunnel as a single hop
   regardless of the number of intermediary label switch routers (LSR).
   The Pipe Model for TTL is currently being used in multiple networks
   and is provided as an option configurable by the network operator by
   several vendors.

   This document formalizes the TTL processing in MPLS networks and ties
   it with the terminology introduced in [MPLS-DS].

2. TTL Processing in MPLS Networks

2.1. Changes to RFC 3032 [MPLS-ENCAPS]

   a) [MPLS-ENCAPS] only covers the Uniform Model and does NOT address
      the Pipe Model or the Short Pipe Model.  This document addresses
      these two models and for completeness will also address the
      Uniform Model.

   b) [MPLS-ENCAPS] does not cover hierarchical LSPs.  This document
      addresses this issue.

   c) [MPLS-ENCAPS] does not define TTL processing in the presence of
      Penultimate Hop Popping (PHP).  This document addresses this
      issue.

2.2. Terminology and Background

   As defined in [MPLS-ENCAPS], MPLS packets use a MPLS shim header that
   indicates the following information about a packet:

   a) MPLS Label (20 bits)
   b) TTL (8 bits)
   c) Bottom of stack (1 bit)
   d) Experimental bits (3 bits)

   The experimental bits were later redefined in [MPLS-DS] to indicate
   the scheduling and shaping behavior that could be associated with an
   MPLS packet.

   [MPLS-DS] also defined two models for MPLS tunnel operation: Pipe and
   Uniform Models.  In the Pipe Model, a MPLS network acts like a
   circuit when MPLS packets traverse the network such that only the LSP
   ingress and egress points are visible to nodes that are outside the
   tunnel.  A Short variation of the Pipe Model is also defined in
   [MPLS-DS] to differentiate between different egress forwarding and
   QoS treatments.  On the other hand, the Uniform Model makes all the



Agarwal & Akyol             Standards Track