Hi Actually if we look at the intent of ACK in case of 2xx response things become similar. the ACK for 2xx resonse may carry SDP and it will un-necessarily congest the network if it is sent thru all the proxies which are handling the messaging statefully. Ack is usually exchanged directly purpose of ACK is also to tell the end UAS that the originator has received 200 message (which has SDP) so that conversation can begin. so this signfies the need of ACK.
in case of non-2xx final response, intent of ACK just to ackknowledge the previous node that i have received the non-2xx response. and if ACK is not received retransmit the non-2xx final response. if ACK is not sent then there is no way to tell the pevious node that final response is received. This is not needed in case of 2xx response becos, if 2xx is lost in between, UAC will not send ACK to the UAS and UAS will re-transmit 2xx response again. ACK for 2xx is exchanged directly (normally) between UAC and UAS so it does not congest the network. now if for non-2xx we leave the responsiblity of delivery to UAC (the way it makes sure for 2xx), then even if non-2xx is lost at some intermediate node, UAS will send non-2xx again and will traverse thru all the nodes which is not neded. so by using hop-by-hop reliablity mechanism, some network traffic is saved. UAS must know if 2xx is received at UAC or not because its acknolwedegement of SDP exchanged , else processing at UAS stops (convesation would not beging) for non-2xx response UAS does not care. it does not stop any processing for UAS so in essence: for non-2xx final response (for invite) reliability is ensured hop-by-hope and for 2xx final response (for invite) reliablity is ensured end to end. cheers v. From: "VikramB" <[EMAIL PROTECTED]> To: <[email protected]> CC: [email protected] Subject: [Sip-implementors] sip transcation query Date: Tue, 10 Jan 2006 09:20:06 +0530 Hello All, In rfc 3261 section 6 Definition SIP Transaction : .... ..... If the request is INVITE and the final response is a non-2xx, the transaction also includes an ACK to the response. The ACK for a 2xx response to an INVITE request is a separate transaction. My query is why ACK is considered as a part of same transaction when in case of non-2xx response.? regards VIkram _______________________________________________ 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
