Offer/Answer

1
Offer/Answer Multipart Alternative

• Cullen Jennings
• ltfluffy_at_cisco.comgt
• Medhavi Bhatia
• John Oliver
• mbhatia, joliver_at_nextone.com

2
Documents

• draft-jennings-sipping-certs-04
• Cullen Jennings
• Jon Peterson
• draft-bhatia-mmusic-sdp-altcap-01.txt
• Medhavi Bhatia
• John Oliver
• mbhatia, joliver_at_nextone.com

3
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.

4
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.

5
Example

• Content-Type multipart/alternative
  boundaryxxx
• --xxx
• Content-Type application/sdp
• Content-ID lto100_at_spock.example.comgt
• v0
• o- 25678 753849 IN IP4 128.96.41.1
• s
• cIN IP4 128.96.41.1
• t0 0
• maudio 3456 RTP/AVP 0
• mvideo 3458 RTP/AVP 31
• --xxx
• Content-Type application/sdp
• Content-ID lto101_at_spock.example.comgt
• v0
• o- 25678 753849 IN IP4 128.96.41.1

Offer
Answer
Content-Type application/sdp Content-Reference
lto100_at_spock.example.comgt v0 o- 25678 753849 IN
IP4 128.96.41.1 s cIN IP4 128.96.41.1 t0
0 maudio 49000 RTP/AVP 0 mvideo 59000 RTP/AVP
31
6
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.