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