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

[<[e-mail]>]

I agree with Alan that one should ask Q12 why they used a specific approach for the modelling of TTL in MPLS.  However, one has to observe that in a co-ps mode transport network based on proper connections then a TTL function is unnecessary....so asking whether we are accurately modelling something that is just plain wrong is a rather strange thing to focus on (and I'll wager this is why it has a strange handling model in G.8110).

Perhaps Stewart's mail is the catalyst to ask the question why we simply don't null the TLL function in MPLS-TP?

Aside=> We can't appeal to an argument that 'it's a profile of MPLS' because MPLS-TP is creating a new layer co-ps mode network technology since it adds new functional behaviours not found in any existing spin of MPLS (eg logically OOB CP, new addressing types, new OAM) in addition to reusing as much of the existing MPLS specification as it can......even inappropriate bits unfortunately....which is the case we have here.

regards, Neil

> -----Original Message-----
> From: [e-mail] 
> [mailto:[e-mail]] On Behalf Of [e-mail]
> Sent: 29 June 2010 08:37
> To: [e-mail]; [e-mail]
> Cc: [e-mail]
> Subject: Re: [mpls-tp] Proposed liaison to ITU-T on G.8110
> 
> Stewart,
> Whilst I don't agree with your proposed liaison I'd like to 
> make a suggestion. Your statement that " We note that 
> according to G.805, the CI is supposed to be delivered 
> end-to-end between MPLS APs without modification or 
> inspection" is incorrect by definition. CI information is 
> transferred between Termination Connection Points (TCPs) and 
> not between AP's. Indeed the point of the source termination 
> function is to add information to adapted information which 
> traverses the AP and thus to generate CI and the 
> corresponding sink processes information.
> 
> This is quite clear from G.805 and is also well described in 
> Sexton and Reid. I would therefore suggest you correct your liaison.
> 
> Regarding the model for TTL, I am sure that Q12 will be happy 
> to explain the reasoning behind the way it was constructed 
> and how the TTL value is decremented on a per hop basis. But 
> perhaps it might be more appropriate to ask for an 
> explanation of how the model works before assuming that it is wrong.
> 
> Best regards
> Alan
> 
> > -----Original Message-----
> > From: [e-mail]
> > [mailto:[e-mail]] On Behalf Of Stewart Bryant
> > Sent: lunedì 28 giugno 2010 19.42
> > To: [e-mail]
> > Subject: [mpls-tp] Proposed liaison to ITU-T on G.8110
> > 
> > We note that according to G.805, the CI is supposed to be delivered 
> > end-to-end between MPLS APs without modification or 
> inspection 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
> > 
> > ===============
> > 
> > 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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> > [e-mail]
> > https://www.ietf.org/mailman/listinfo/mpls-tp
> > 
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