Paul, Why can’t it do that with an offer in the initial invite?
— Sent from mobile, with due apologies for brevity and errors. > On Apr 23, 2019, at 5:10 PM, Paul Kyzivat <pkyzi...@alum.mit.edu> wrote: > > 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 > > _______________________________________________ > Sip-implementors mailing list > Sip-implementors@lists.cs.columbia.edu > https://lists.cs.columbia.edu/mailman/listinfo/sip-implementors _______________________________________________ Sip-implementors mailing list Sip-implementors@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
