[mpls-tp] Proposed liaison to ITU-T on G.8110

[Stewart Bryant <[e-mail]>]

Several people have pointed out a discrepancy in the model for MPLS as 
documented in G.8110. Since this formal model plays a major role in the  
ITU-T MPLS-TP G.8110.1 specification, the error should to be corrected 
before publication.

I therefore propose that we send the following liaison to the ITU-T.

- Stewart

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To: ITU-T WP3/15
From: IETF

Dear Dr. Trowbridge,

We note that G.8110 is referenced as a normative reference from the 
draft text of the revision of G.8110.1. We also note that G.8110 is
now five years old, and has received no contributions for update over 
that period. G.8110 has been described as "not covering all of MPLS and 
certainly not what has happened in the last five years."

We believe that G.8110.1 should document MPLS-TP accurately. It is 
important, therefore, that where the model for MPLS-TP differs from that 
described in G.8110, the correct model be developed and documented in 
G.8110.1.

We would like to draw your attention in specifically to Section 6.2.2 of 
G.8110 (and, in particular, Figures 1 and 2) that says that the 
Time-To-Live (TTL) field of an MPLS header is part of the Characteristic 
Information (CI) of an MPLS_CI traffic unit. We note that according to 
G.805, the CI is supposed to be delivered end-to-end between MPLS APs 
without modification or inspection. But the function of a TTL in an 
MPLS-TP network is to be decremented at each hop along the path, and to 
be inspected at each hop and tested against zero. Thus, in the model for 
MPLS-TP, the TTL should not form part of the CI.

We request that G.8110.1 be updated to include this revision to the 
model. This might most easily be achieved by augmenting the references 
to G.8110 with updated figures based on those in G.8110 along with 
appropriate text explaining the differences in the model such that 
G.8110.1 correctly captures the model for MPLS-TP.

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