Subject: how to if a frame is RTP

From: Chris James <[email-masked]>
Date: Thu, 10 Jun 1999 12:56:17 -0700 (PDT)

First, appologies for the newbee question.

Given a hex dump of a UDP frame, how is it
possible to know if the next layer is RTP?
There is no assigned port to it.  Is there
something in the frame data that distinguishes
it?

Any help/pointers are appreciated.
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From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 10 Jun 1999 15:57:11 -0400

From the dump of a single packet, there is no way to be certain. RTP is
not on a well known port, nor are there magic cookies anywhere. Certain
fields in the header generally have a fixed value; in particular, the
version number is 2. So, you could tell that a packet is NOT RTP if the
version number weren't 2. With 2 packets, you have a better shot. The
SSRC stays the same, and other fields, like SN and timestamp, increase
by fixed amounts. In this fashion, a stateful process could detect, with
increasing probability of success, that the packet was RTP. But it could
never be 100%.

-Jonathan R.

-- 
Jonathan D. Rosenberg                       Lucent Technologies
Member of Technical Staff                   101 Crawfords Corner Rd.
High Speed Networks Research                Holmdel, NJ 07733
FAX:   (732) 834-5379                       Rm. 4C-526
EMAIL: [email-masked]
URL: http://www.cs.columbia.edu/~jdrosen

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From: Stanley Pinjiang Feng <[email-masked]>
Date: Fri, 11 Jun 1999 11:29:49 -0400

This may be of interest. The following is from the newest version of
RFC1890: RTP Profile for Audio and Vedio Conferences with Minimal Control
section 8:

     Port numbers 5004 and 5005 have been registered for use with this
     profile for those applications that choose to use them as the
     default pair.  Applications that operate multiple profiles may use
     this port pair as an indication to select this profile if they are
     not subject to the constraint of the previous paragraph.

I would very much prefer to have these default UDP ports for RTP.  Again, we
can use this default pair to agree on some other pair of ports so that
mutiple RTP sessions can be carried simultaneously between two IP addresses.

Thank you

Stanley Feng