[AVTCORE] Max SSRC declaration: Objection to retrofitting rules

Harald Alvestrand <harald@alvestrand.no> Wed, 26 October 2011 17:19 UTC

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Date: Wed, 26 Oct 2011 10:19:06 -0700
From: Harald Alvestrand <harald@alvestrand.no>
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Subject: [AVTCORE] Max SSRC declaration: Objection to retrofitting rules
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Hello,
I have read through draft-westerlund-avtcore-max-ssrc, and generally I 
find it a fine specification, which allows us to be explicit about an 
aspect of RTP usage that was not previously explicit.

However, I do have one strong objection to two specific paragraph:

    When max-send-ssrc or max-recv-ssrc are not included in the SDP, it
    MUST be interpreted as equivalent to a limit of one, unless sendonly
    or recvonly attributes are specified, in which case the limit is
    implicitly zero for the corresponding unused direction.
.....
    In case an answer fails to include any of the limitation attributes,
    the agent SHOULD be interpreted as capable of supporting only a
    single stream in the direction for which attributes are missing.

If I read this correctly:

  - There is no way to tell from SDP whether or not the sender or 
recipient intends to conform
    to the max-ssrc draft: The extension itself is not signalled.
  - Therefore, it seems that the intent of the paragraph is to change 
the rules about how to interpret
    SDP messages that do NOT contain any hint that their sender is 
conforming to max-ssrc
  - Therefore, the draft, if taken literally as written, would 
retroactively declare all usage
    of multiple SSRCs in a single RTP session to be noncompliant with 
its SDP declaration.

That seems to me to be an entirely unreasonable thing to do, given that 
implementations are using multiple SSRCs on single RTP sessions every 
minute of every day.

I would not object to a section that instead said:

    When max-send-ssrc or max-recv-ssrc are not included in the SDP, no
    information about the supported number of SSRCs is available, unless 
sendonly
    or recvonly attributes are specified, in which case the limit is
    implicitly zero for the corresponding unused direction.

    If a specific deployment is protected by gateways that modify SDP, 
and have
   knowledge of a specific limit active within that deployment, those 
gateways
   MAY choose to add max-*-ssrc attributes to the SDP exchanged with the
   outside world, to indicate the limited capacity of those devices.
.....
    In case an answer fails to include any of the limitation attributes,
    local heuristics SHOULD be applied to produce a reasonable guess at the
    capacity of the responding agent.

Declaring a class of deployments non-conformant by IETF fiat is not, in 
my opinion, a reasonable option.
Encouraging the deployers of limited-function devices to be explicit 
about those limitations is a reasonable option.

                        Harald