For a UA, the rport just declares that "I can receive UDP on the same port I transmitted". A UA not supoprting rport could, in theory, be transmitting on one UDP port but be listening on another. So sending back to the port you received on would not work - you have to send back to the port in the sent-by. Having said that the asymetric usage of UDP SIP signalling is very rare - I saw it in UA many years ago. Most (today I would say (I hope) 100% of) UAs will send using a UDP source port which matches their UDP listening port. And using rport declares this "symmetric" behaviour. Of course a UA won't directly use the rport information - it just allows upstream proxies to know that they can send UDP responses back to the source. In fact, since the UDP symmetry is so common these some SIP proxies (or B2B UAs) add an rport header even if one wasn't included. So some people are already doing what you suggested (at the very small risk that the request-sending-UA doesn't have UDP SIP signalling symmetry) Regards, Attila
________________________________ From: [EMAIL PROTECTED] on behalf of Iñaki Baz Castillo Sent: Sun 28/09/2008 00:14 To: [email protected] Subject: [Sip-implementors] Is really "rport" needed? Hi, RFC 3261 says (like assuming NAT doesn't exist) that when a UDP request arrives to an UAS, replies must be sent to the Via "received" address (real source IP) and the port indicated in "sent-by". In the real world full of NAT's it needs the "rport" complement. Hi, imagine a SIP UDP UAS that, instead of implementing RFC 3581 ("rport"), just replies *always* using the real incoming source UDP address:port. Could it generate any issue? Note that in TCP replies must be sent using the incoming connection (this involves real source IP and real source port), but in UDP it just involves real source IP, that IMHO makes no sense (why just the IP?). Other way to ask the same would be: does really exist any case in which replying to real source IP but not to the real source port (this is, port in "sent-by") is useful? Thanks. -- Iñaki Baz Castillo _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
