From: "Vikram Chhibber" <[EMAIL PROTECTED]>
I think I have missed something here. Why do we need to maintain codec
numbers relationship with the previous offer/answer (Between Alice and Bob)?
My understanding is that offer/answer model does not have any state
dependency with previous one except for "o" line and maintaining media
description ordering.
I will say preserving m line ordering is definitely the
reason. Codec number for dynamic payload can change within same
session.
RFC 3264 section 8.3.2:
8.3.2 Changing the Set of Media Formats
The list of media formats used in the session MAY be changed. To do
this, the offerer creates a new media description, with the list of
media formats in the "m=" line different from the corresponding media
stream in the previous SDP. This list MAY include new formats, and
> MAY remove formats present from the previous SDP. However, in the
> case of RTP, the mapping from a particular dynamic payload type
> number to a particular codec within that media stream MUST NOT change
> for the duration of a session. For example, if A generates an offer
with G.711 assigned to dynamic payload type number 46, payload type
number 46 MUST refer to G.711 from that point forward in any offers
or answers for that media stream within the session. However, it is
acceptable for multiple payload type numbers to be mapped to the same
codec, so that an updated offer could also use payload type number 72
for G.711.
The mappings need to remain fixed for the duration of the session
because of the loose synchronization between signaling exchanges
of SDP and the media stream.
Dale
_______________________________________________
Sip-implementors mailing list
[email protected]
https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
