Offer/Answer &

Multipart Alternative

Cullen Jennings

<fluffy@cisco.com>

&

Medhavi Bhatia

John Oliver

{mbhatia, joliver}@nextone.com

• draft-jennings-sipping-certs-04– Cullen Jennings– Jon Peterson

• draft-bhatia-mmusic-sdp-altcap-01.txt– Medhavi Bhatia– John Oliver– {mbhatia, joliver}@nextone.com

Documents

Problem

• Example that started it:– Alice offers two SDP bodies, one for RTP, one for

SRTP. How does Bob indicate which offer was selected?

– Side note: Can not try SRTP then try RTP. If voicemail does SRTP, will never get a RTP UA.

• General problem:– If Alice sends a multipart/alternative, how does

Bob indicate which alternative was used.

Solution

• Inclusion of Content-ID in each multipart entity.• Definition of a new MIME header:

– Content-Reference• Allows answerer to indicate which MIME body in the

offer is being responded to.

ExampleContent-Type: multipart/alternative;

boundary=’xxx’

--xxxContent-Type: application/sdpContent-ID: <o100@spock.example.com>

v=0o=- 25678 753849 IN IP4 128.96.41.1s= c=IN IP4 128.96.41.1t=0 0m=audio 3456 RTP/AVP 0m=video 3458 RTP/AVP 31

--xxxContent-Type: application/sdpContent-ID: <o101@spock.example.com>

v=0o=- 25678 753849 IN IP4 128.96.41.1s= c=IN IP4 128.96.41.1t=0 0m=audio 3456 RTP/AVP 18m=video 3458 RTP/AVP 34

--xxx--

Content-Type: application/sdpContent-Reference:

<o100@spock.example.com>

v=0o=- 25678 753849 IN IP4 128.96.41.1s= c=IN IP4 128.96.41.1t=0 0m=audio 49000 RTP/AVP 0m=video 59000 RTP/AVP 31

Content-Type: application/sdpContent-Reference:

<o100@spock.example.com>

v=0o=- 25678 753849 IN IP4 128.96.41.1s= c=IN IP4 128.96.41.1t=0 0m=audio 49000 RTP/AVP 0m=video 59000 RTP/AVP 31

Offer

Answer

Require Header

• SIP Require header needed?

• Can implicitly deal with this by saying if a UAS supports multipart/alternative, then it needs to add the header.

• Recommendation: Make it explicit and add an option tag.