SIP and H.323 Interworking question

Paul E. Jones paulej at PACKETIZER.COM
Thu Sep 14 13:39:36 EDT 2000


I thought the same thing you did, but I was corrected on this issue: the
'm=' line corresponds to the "receive" side only.  It tell you nothing about
what port numbers will be used for sending an RTP stream.

For example, if I send an INVITE message with this line:
  m=audio 53000 RTP/AVP 4

If the called endpoint wishes to send media to the caller, it would do so to
port 53000.  However, if the called endpoint wants to also receive media, it
would return an SDP 'm=' line in a message like 200.  Suppose it was:
  m=audio 26000 RTP/AVP 4

The caller may send media to the called party to port 26000, but there is
nothing in SIP or SDP that says that the caller shall use port 53000 to send
media to the callee's port 26000-- this is a different RTP stream.  So, in
effect, there are two RTP streams:

caller             callee
port               port
 ??/?? -----------> 26000/26001
53000/53001 <------ ?? / ??

The question marks indicate that, until media starts flowing, the other
endpoint does not know what port number will be utilized.


----- Original Message -----
From: "Hemant Agrawal" <hemantag at>
To: <ITU-SG16 at>
Sent: Thursday, September 14, 2000 7:08 AM
Subject: Re: SIP and H.323 Interworking question

> Hi Paul
>           SDP used by SIP contains the connection address in the 'c='
>          while it contains the RTP port information in 'm = ' line.  the
> RTCP port used will be RTP port +1
>                     c=<network type> <address type> <connection address>
>                     m=<media> <port> <transport> <fmt list>
>          The call originator will sends it's RTP port information &
> connection address in the INVITE message, and it will get the other end
> port info & connection address in the response ( e.g.200 OK).
> Thanks  & Regards,
> Hemant Agrawal
> > Folks,
> >
> > I was recently pondering issues related to H.323/SIP interworking--
> > particularly the handling of media streams.
> >
> > If I understand things correctly, the 'm=' line in the SDP used by SIP
> > indicates only the media address to which RTP packets will be sent.  It
> says
> > nothing about the RTP/RTCP port used for sending media.  Those can be
> > another pair of ports, apparently.
> >
> > How, then, can an H.323 IWF send an OLC to an H.323 endpoint?  (Let's
> assume
> > that the IWF does not want to proxy the media-- only handle signaling.)
> >
> > Assume that the IWF has received the INVITE message, containing the RTP
> > address of the reverse channel.  It then goes ahead and establishes the
> call
> > with the H.323 endpoint, starts H.245 and exchanges capability
> information.
> >
> > Suppose the IWF now wishes to open a channel.  Part of the
> > forwardLogicalChannel parameters of the OLC is a required field called
> > "mediaControlChannel".  However, this information is not passed in the
> SDP.
> >
> > Are people assuming, as I had done, that the 'm=' line contained the RTP
> > address for both sending and receiving media?
> >
> > Am I misreading something or is there a real interworking issue here?
> >
> > Thanks,
> > Paul
> >
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > For help on this mail list, send "HELP ITU-SG16" in a message to
> > listserv at
> >
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> For help on this mail list, send "HELP ITU-SG16" in a message to
> listserv at

For help on this mail list, send "HELP ITU-SG16" in a message to
listserv at

More information about the sg16-avd mailing list