Subject: RE: [SIP] Re: SDP issues

From: Christian Huitema <[email-masked]>
Date: Mon, 24 Apr 2000 10:43:34 -0700

> Whatever you might wish the SDP spec would say, it's fairly clear on
> this issue: if there's no s= line, you should treat the SDP as
> corrupted and act accordingly.  If you send SDP without the s= line,
> you are definitely not compliant with RFC 2327 and shouldn't expect
> your message to be accepted.

What we are seeing here is a conflict between SAP and SIP. SDP was
originally designed for the multicast session directory programs (sd, sdr)
that use a session id to correlate advertizements from multiple possible
announcers. If you do that, then you need a very complete and autonomous
session description, including session-id and timing parameters.

SDP is then used in SIP (and also MGCP and Megaco) as a way to describe and
negotiate the characteristics of a "private session." If the beginning and
end of the sessions are bracketed by the INVITE and BYE requests, if the
sessions are identified by a SIP "Call-id," then the SDP level session
identifiers and timing indicators are somewhat redundant.

What we probably need is a profile of some sort, that qualifies what is
mandatory and when. For example, I believe that a fully formed "s=" field
MUST be present when announcements of multicast sessions are multicast in
SAP. The same requirement would logically apply when invitations to the same
sessions are sent via SIP. But the requirement is not all that logical for
invitations to ephemeral point to point sessions -- the robustness principle
would dictate that inviters SHOULD fill up a session field, but that
invitees should not reject invitations to point to point sessions when the
"s=" field is absent.

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