Non200 response to PRACK Due to rejected SDP

Non-200 response to PRACK (Due to rejected SDP offer or other reasons) Christer Holmberg (christer. holmberg@ericsson. com)

PROBLEM • Currently RFC 3262 more or less assumes that a PRACK will be responded to using the 200 OK response code • SDP offer received in PRACK cannot be rejected using a 4 xx response code – Recommendation that only ”unrejectable” SDP offers are sent in PRACK • One cannot assume that a PRACK will not be rejected, because of SDP offer rejection or some other reason

PREVIOUSLY IN IETF • Agreement that it shall be allowed to send a non-200 response to PRACK – 488 response to reject SDP offer

OPEN ISSUE • Does a non-200 response cease the retransmission of the reliable 18 x response associated with the PRACK? • Issue if non-200 ceases retransmission: – PRACK may be rejected by intermediate, which means that retransmission of reliable 18 x responses from the UAS will continue • Issue if non-200 does not cease retransmission: – Text in RFC 3262 seems to assume that retransmission will always be ceased • Retransmission is ceased when the PRACK is received, before it is known what the response code will be

PROPOSAL • Clarify that a non-200 response does not cease retransmission of the reliable provisional response associated with the PRACK.

Use cases for Problems with SDP Offer in first reliable provisional response Sanjay Sinha Christer Holmberg

PROBLEM RFC 3262 Offer/Answer rule: • For INVITE without SDP offer, first reliable provisional response MUST contain SDP offer.

PROBLEM USE-CASE 1: SIP-H 323 Call Setup with Slowstart • Media capabilities of H. 323 endpoint is not known until Open. Logical. Channel. Ack (OLC Ack) • With Slowstart, OLC Ack is not received until endpoint answers the call • If a SIP message has to be sent in response to Q 931 ALERT/PROGRESS, then there will be no SDP to send in 18 x • The interworking gateway needs to wait till CONNECT and OLC Ack to send SDP in 200 OK

PROBLEM USE CASE 1: SIP to H 323 Slow. Start Call Setup H 323 SIP Q 931 Setup Q 931 ALERT/PROGRESS INVITE (no SDP) Signaling Gateway Q 931 CONNECT 18 X (no sdp or dummy sdp) Capabilities/H 245 PRACK with SDP Capabilities/H 245. Open Logical Channel/ H 245 Acknowledgement H 323 endpoint media information is not available to be sent in 18 x Media Caps Open Logical Channel / H 245 Acknowledgement RTP 200 with SDP

PROBLEM USE CASE 2: 181 Call is being forwarded • INVITE without SDP comes into a SIP UA (or proxy or PBX) for user A, which has call forwarding (any reason) enabled on it’s device • 181 is sent to UAC to indicate that call is being forwarded to user B • 181 can not have sdp since call has not been routed yet to user B • If reliable provisional is required, this violates the rule

RCF 3262 IMPACT Chapter 5 Paragraph 2 Old: “Similarly, if a reliable provisional response is the first reliable message sent back to the UAC, and the INVITE did not contain an offer, one MUST appear in that reliable provisional response. ” New: “Similarly, if a reliable provisional response is the first reliable message sent back to the UAC, and the INVITE did not contain an offer, one MAY appear in that reliable provisional response. ”

Concern with relaxing restriction • Backward compatibility: UAC strictly follows the rule and so fails the call when it does not get offer sdp in first reliable 18 x • Possible follow-up action: Gather data at SIPIt about how many UACs will fail the call when offer is not in first reliable 18 x

THANK YOU!