Re: [rtcweb] SDP Offer/Answer draft-jennings-rtcweb-signaling

Iñaki Baz Castillo <> Sat, 15 October 2011 09:46 UTC

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Date: Sat, 15 Oct 2011 11:46:26 +0200
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From: Iñaki Baz Castillo <>
To: Cullen Jennings <>
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Cc: Jonathan Rosenberg <>,,
Subject: Re: [rtcweb] SDP Offer/Answer draft-jennings-rtcweb-signaling
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2011/10/15 Iñaki Baz Castillo <>:
> -----------------------------
> 5.3.3.  OK
> The OK message is used by the receiver of an ANSWER message to
> indicate that it has received the ANSWER message. It has no contents
> itself and is merely used to stop the retransmissions of the ANSWER
> -----------------------------

Hi, some other possible issues with the above ROAP OK message:

ROAP OK message fits well with the ACK for 2XX in SIP. Let's assume
that alice (pure SIP UA) and bob (RTCweb client) can intercommunicate
via a RTCweb server implementing SIP over WebSocket (so JS in both
browser is speaking real SIP rather than a custom protocol).

NOTE: When alice sends a SIP INVITE to bob it arrives unchanged to
bob. bob extracts the SDP and converts it into a ROAP OFFER. So I
write it as "INVITE + ROAP OFFER" but it just means a common SIP
INVITE with an SDP.

case 1)  SDP OFFER in the INVITE

  alice (SIP client)          bob (RTCweb client)

  --------> INVITE + ROAP OFFER (F1)

      200 OK + ROAP ANSWER (F2) <---------

      (200 is lost in the network or server)

      200 OK + ROAP ANSWER (F2) <---------
      200 OK + ROAP ANSWER (F2) <---------

  ---------> ACK (F3)

ISSUE: How would bob receive a ROAP OK message?

Possible response: bob should generate it by itself upon receipt of
the SIP ACK and signal it to the RTCweb stack in his browser. Am I

case 2)  SDP OFFER in the 200 OK

  alice (SIP client)          bob (RTCweb client)

  --------> INVITE (no SDP)

         200 OK + ROAP OFFER (F2) <---------

      (200 is lost in the network or server)

         200 OK + ROAP OFFER (F2) <---------
         200 OK + ROAP OFFER (F2) <---------

  ---------> ACK + ROAP ANSWER (F3)

ISSUE: In this case there is no SIP message to confirm the receipt of
the ACK (which carries the SDP answer) so bob has no chance to send
the ROAP OK message over the wire. Would it break the signaling?

Possible response: there is no need at all in sending the ROAP OK
message (if alice is a pure SIP client it does not need it, and if
alice is a RTCweb client it should autogenerate it).

ISSUE: Now imagine that alice is also a WEBrtc client. In this case
(Case 2) when alice sends the ACK + ROAP ANSWER she will not receive a
new SIP message (which could be mapped to ROAP OK) so, should alice
signal the ROAP OK message to his RTCweb stack once the ACK + ROAP
ANSWER is sent?

Possible response: yes.

Thanks a lot.

Iñaki Baz Castillo