3pcc Update and Issues

1
3pcc Update and Issues

• Jonathan Rosenberg
• dynamicsoft

2
Initial Call Flow from -00
A Controller B

INV no SDP

lt------------------


200 SDP A
-----------------gt
INV SDP A
-----------------gt


200 SDP B

lt-----------------


ACK
ACK SDP B -----------------gt
lt------------------



RTP
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

• Big problem
• Retransmissions of 200 from A to controller while
  we wait for B

3
Call flow from -01
A Controller B

INV held SDP

lt------------------


200 SDP A
-----------------gt
INV SDP A
ACK -----------------gt
lt-----------------

200 SDP B

lt-----------------


ACK
INV SDP B -----------------gt
lt------------------

200 OK SDP A
------------------gt
ACK

lt------------------
RTP
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxx

• Fixes timeout problem completely
• Each transaction independent
• Big problems though
• Relies on A returning same SDP in re-INVITE
• Otherwise, infinite re-INVITE loops!
• Also assumes controller knows media

4
Latest call flow
A Controller B
INV no SDP time t 0
lt------------------
200 SDP
A1
-----------------gt
ACK SDP
held lt------------------

INV no SDP
-----------------gt

200 SDP B
lt----------------- INV SDP B'
lt------------------

200 SDP A2
-----------------gt

ACK SDP A2' ACK
-----------------gt lt------------------

RTP
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

• Combines best of all call flows
• No timeouts, since reinvite will generally be
  answered fast
• No infinite recursion
• No media assumption
• But
• Complex
• Still requires SDP manipulation for multiple
  streams and differing codec bindings

5
SIP Issues

• Requires that INVITE on hold not generate hold as
  response
• Proposal is to document this in RFC
• Whole reason we chose 0.0.0.0 is that normal SDP
  processing would give hold
• Only special case processing would generate hold
  in response!

• Requires that re-INVITE w/ no SDP mean you tell
  me what SDP to use
• Response is probably same SDP as before, need not
  be
• Agreed to this at bof
• Will document in bis

6
Recommendations

• Call flow 1 still useful in some cases!
• When you know that 2nd party is automata that
  answers right away
• Many examples
• Conference servers
• Media servers
• Messaging servers
• Still simplest and best if timeout not problem

• Call flow 3 most general, and recommended when
  you dont know if 1 will work

7
Issues that remain

• QoS preconditions makes my head hurt
• A lot more thinking needed here
• Allowing ringing to one user
• No way for either user to hear ringback when they
  pick up
• One solution is to reverse direction of one call
  leg
• REFER back to yourself!

• Anything else?
• Early media?
• Most important thing are SIP changes/clarification
  s needed for this service