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[Megaco] ignoring SDP info; RE: [Foip] COM 16 - C 194 (V.34 fallback in T.38)
Just a side note to 1:
Right, the semantic of ignore for SDP (RFC 2337, 3264, 4566) is one of
the reasons of T.38 interop issues because it undermines the requirement
of unambiguous T.38 configuration settings between both T.38 endpoints.
Further: there's a difference between
a) SIP/SDP-controlled T.38 endpoints and
b) H.248/SDP-controlled T.38 endpoints.
[note: the notion 'H.248/SDP' indicates H.248 text encoding, which uses
SDP in H.248 LD/RD as media description]
Semantic of ignore is allowed in case (a) for SIP, but not allowed for
the master-slave control relationship in H.248. The H.248 MG is not
allowed to ignore unknown SDP, he may react by an error code.
That is one of the motivations of our contribution C-320, which would
solve that particular problem for (a) (the problem does not exist for b,
and also not for H.323-controlled T.38 endpoints).
-Albrecht
> -----Original Message-----
> From: foip-bounces at sipforum.org
> [mailto:foip-bounces at sipforum.org] On Behalf Of Chen, Ximing
> M (Michael)
> Sent: Freitag, 23. Oktober 2009 05:08
> To: Hiroshi Tamura; Schulz, Edward D (Ed); t09sg16q14 at lists.itu.int
> Cc: FoIP at sipforum.org
> Subject: Re: [Foip] COM 16 - C 194 (V.34 fallback in T.38)
>
> Hi, Hiroshi,
>
> Thanks for your comments.
>
> 1. I would expect that the existing GWs will simply ignore
> the fields that they do not understand. This should not be a
> problem for SIP/SDP as some new fields were added in Annex D
> recently (T38VendorInfo, for example).
>
> 2. In the case of G3 only at one GW and V.34 only at the
> other, GWs will have to stay in the VBD/V.152 mode and not to
> switch to a T.38 mode. May be we should state this explicitly
> in the recommendation.
>
> 3. I will looking into the ASN.1 syntax for H.225.0. Perhaps
> the first step is to add T38ModemType in Annex D (instead of
> Annex B) for the SIP/SDP setup?
>
> 4. This document/contribution was originally a working
> document for the SIP Forum's FoIP Task Group
> (http://www.sipforum.org/content/view/310/252/) which has
> recently published a Problem Statement on T.38
> (http://www.sipforum.org/component/option,com_docman/task,cat_
> view/gid,70/Itemid,261/). The purpose of the FoIP task group
> is to investigate various issues (interoperability,
> performance, etc) associated with T.38 in SIP networks. The
> task group is currently converting the Problem Statement into
> an IETF document.
>
> Regards,
>
> Michael
> ________________________________________
> From: Hiroshi Tamura [tamura at cs.ricoh.co.jp]
> Sent: Tuesday, October 20, 2009 3:32 AM
> To: Chen, Ximing M (Michael); Schulz, Edward D (Ed);
> t09sg16q14 at lists.itu.int
> Cc: tamura at cs.ricoh.co.jp
> Subject: COM 16 - C 194 (V.34 fallback in T.38)
>
> Michael, Ed and All,
>
> As I cannot attend the meeting next week, let me comment here.
>
> 1 Combination of New emitting GW and existing receiving GW
> Receiving GW does not understand the new field "T38ModemType".
> Is it guaranteed that it just ignores the field?
> Did you fully consider the compatibility item with the
> current implementation?
>
> 2 G3 only at one GW and V.34 only at the other GW What
> happens in this case? It implicitly says "prefer" type.
>
> 3 Annex B
> As it is H.323 call-setup, you should consider the ASN.1
> syntax to add H.225.0.
> I do not remember the exact recomendation.
>
> 4 Recommendation to Industry
> How do you proceed?
>
> Best regards,
> --
> Hiroshi Tamura
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