On 6/22/2011 12:40 PM, Nauman Sulaiman wrote: > Hi > > Some UAs do not support PRACK or UPDATE and still work fine with various > Proxies, PBX, Softswitches etc. Are there any scenarios or well known > Proxies,Softswitches that require mandatory PRACK and UPDATE support for a UA > to be interoperable? Or maybe some features will not be available if support > for these methods is not there. > > Just trying to understand where PRACK and UPDATE are used.
Its been a *really* long time since 3262 and 3311 were published. There is no reason for UAs not to support PRACK and UPDATE. There are devices/systems with constraints that may make it necessary to to multiple (typically 2) offer/answer exchanges before establishing a call. The use case that initially motivated PRACK was QoS, but that is not the only one. For instance there are some UAsthat can only support one codec at a time. Such a UA will offer multiple codecs because it doesn't know which ones the answerer can support. But then if the answerer answers with multiple codecs, there is need of another o/a cycle to select one from among those common to offer and answer. (However this is often handled without a second o/a, using "wishful thinking" to assume that only the first of the codecs in the answer will actually be used. I guess if others are then used it just doesn't work.) Thanks, Paul > Thanks > > _______________________________________________ > Sip-implementors mailing list > Sip-implementors@lists.cs.columbia.edu > https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors > _______________________________________________ Sip-implementors mailing list Sip-implementors@lists.cs.columbia.edu https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors