[asterisk-dev] why no sequence numbers with video frames over IAX ?

[Russell Bryant]

Luigi Rizzo wrote:
> sorry if it is a stupid question, but i was trying to tunnel
> video through asterisk between SIP and IAX:
> 
> 	[A]----(sip)---[asterisk]---(iax)----[B]
> 
> (A is a windows softphone, B is asterisk with chan_oss video extensions)
> and noticed that video ast_frames reaching B seem to have all the
> same sequence number, f->seqno = 0

The reason is because IAX media frames (meta video frames in this case) do not
carry a sequence number.  They do, however, carry a timestamp.  That is what
must be used to handle reordering or other operations in a jitterbuffer.

The timestamp information should be available in the Asterisk frames, f->ts.


http://tools.ietf.org/id/draft-guy-iax-03.txt

Page 54 begins the section that covers the format for a meta frame, in which
video is sent.

                     1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |F|         Meta Indicator      |V|      Source Call Number     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |?|          time-stamp         |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                         Data                  |
   :                                                               :
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

-- 
Russell Bryant
Senior Software Engineer
Open Source Team Lead
Digium, Inc.