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Re: [MMUSIC] 4566bis-36: Ignoring/rejecting session description that contains unknown letter

Christer Holmberg <christer.holmberg@ericsson.com> Fri, 26 July 2019 17:09 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: [MMUSIC] 4566bis-36: Ignoring/rejecting session description that contains unknown letter
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Date: Fri, 26 Jul 2019 17:09:38 +0000
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Subject: Re: [MMUSIC] 4566bis-36: Ignoring/rejecting session description that contains unknown letter
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Hi,

>> Due to my involvement in a large number of other drafts, I haven’t 
>> been able to follow the 4566bis work lately.
>> 
>> I do have a question.
>> 
>> The text says:
>> 
>>     “The set of type letters is deliberately small and not intended to be 
>>     extensible -- an SDP parser MUST completely ignore or reject any
>>     session description that contains a type letter that it does not
>>     understand.”
>> 
>> Since the syntax does not allow new letters to begin with, why can’t 
>> an SDP parser simply discard an unknown letter if received? Why does 
>> the parser have to ignore/reject the whole session description?
>
> This has always been part of the description of SDP. It would be odd to change it now. AFAIK there has never been another letter added to the syntax.

True.

So, what does "ignore the session description" actually mean? Discard it, and move on as it if hadn't been received? Perhaps it would then be better to always reject the session description, as there is a syntax error (as the syntax does not allow new letters).

> (Related to the point that the version has never been changed, and there is no mechanism for changing it in a backward compatible way.)

I don't think it would cause any backward compatible issues, as the receiver would only discard the letter and then process the session description.

Regards,

Christer

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