> Prior to RFC 3264, presence of port=0 in the SDP was also used as an
> indication to put the call on hold.
Port 0 was/is a refusal/removal of a media stream instead of a hold.
RFC 2543 section B.1:
If the callee wants to neither send nor receive a stream offered by
the caller, the callee sets the port number of that stream to zero in
its media description.
There currently is no other way than port zero for the
callee to refuse a bidirectional stream offered by the
caller. Both caller and callee need to be aware what media
tools are to be started.
RFC 2543 section B.5:
If a party in a call wants to put the other party "on hold", i.e.,
request that it temporarily stops sending one or more media streams,
a party re-invites the other by sending an INVITE request with a
modified session description. The session description is the same as
in the original invitation (or response), but the "c" destination
addresses for the media streams to be put on hold are set to zero
(0.0.0.0).
> As there is no specific option tag to indicate the support
> of RFC 3264, how should a party receiving port=0 interpret it?
It is refusal, removal, or place holder.
> More specifically, should the receiving party delete the resources
> associated with this stream or just mark this as an inactive stream?
> As per RFC 3264, on receipt of port=0 for a media stream all resources
> associated with this media stream can be released. The slot occupied by
> this stream can be re-used later to add a new media stream.
You found the answer.
> Can sending a valid port in a previously port zeroed stream
> reactivate that stream or is it equivalent to a new offer
> altogether?
An offer is always an offer. I'm not sure that I fully understand the subtle
distinction you making between the two choices since "yes" appears to be the
answer to both questions.
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