Alex,
A classic use case is 3PCC: A device in the middle wants to broker a
call between two other endpoints. A priori it doesn't know the
capabilities of either of those endpoints, and doesn't want to put
itself in the media path as a transcoder. So it asks one end to provide
an offer so it can offer that to the other end.
Thanks,
Paul
On 4/23/19 1:48 PM, Alex Balashov wrote:
Hi,
Trying to fill a gaping hole in my knowledge:
What is the actual purpose of late SDP offers (no SDP in initial INVITE,
SDP offer in 2xx reply, SDP answer in end-to-end ACK)?
RFC 3261 mentions them, of course, but I’ve only ever seen them used in
Cisco (CCM and IOS voice gateway) land.
I understand that this puts some control in the hands of the caller - it
gives the caller the flexibility to respond based on the callee's SDP
offer more 'flexibly', since it doesn't have to tip its hand about what
it wants first.
But from what I understand, an SDP stanza is, in principle, a statement
about what / how each endpoint wants to receive, not send. Right? I am
aware that there are some cases where, as a matter of convention more so
than standardisation, some inferences about sending intentions are
permitted on the basis of an SDP advertisement -- such as the 183 early
media case.
Still, in principle, SDP is about what I want to receive and how I want
to receive it, I thought. And in principle, any session can involve
wildly asymmetric and non-isomorphic media stream characteristics, i.e.
two different codecs, packetisation durations, etc. on the respective
legs.
If so, what purpose does it serve for the caller to not have to tip its
hand preemptively about what codecs it is willing to accept, for
example?
Does it mirror some PSTN interoperability need? A lot of the discussion
around it seems to be in the context of third-party call control (3pcc),
but the exact connection is unilluminated, and in any case, that's not a
concept I understand particularly well.
Much technical discussion exists online about what it does and why it
needs to be supported: it allows the caller to respond flexibly based on
the callee’s offer. But I can't find a word about why one might actually
want to do that, what sort of scenario it is meant to support, or
otherwise anything about the underlying philosophical motivation.
Any insight is appreciated!
-- Alex
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