Non-200 response to PRACK (Due to rejected SDP offer or other reasons)

1
Non-200 response to PRACK(Due to rejected SDP
offer or other reasons)

• Christer Holmberg
• (christer.holmberg_at_ericsson.com)

2
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 4xx 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

3
PREVIOUSLY IN IETF

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

4
OPEN ISSUE

• Does a non-200 response cease the retransmission
  of the reliable 18x response associated with the
  PRACK?
• Issue if non-200 ceases retransmission
• PRACK may be rejected by intermediate, which
  means that retransmission of reliable 18x
  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

5
PROPOSAL

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

6
Use cases for Problems with SDP Offer in first
reliable provisional response

• Sanjay Sinha
• Christer Holmberg

7
PROBLEM

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

8
PROBLEM USE-CASE 1SIP-H323 Call Setup with
Slowstart

• Media capabilities of H.323 endpoint is not known
  until OpenLogicalChannelAck (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
  Q931 ALERT/PROGRESS, then there will be no SDP to
  send in 18x
• The interworking gateway needs to wait till
  CONNECT and OLC Ack to send SDP in 200 OK

9
(No Transcript)
10
PROBLEM USE CASE 2181 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 its 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 provisional is required,
  this violates the rule

11
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.

12
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 18x
• Possible follow-up action Gather data at SIPIt
  about how many UACs will fail the call when offer
  is not in first reliable 18x

13

• THANK YOU!