Subject: [AVT] RTP: "fast-path" check & app header questions

From: "Spiros Spirou" <[email-masked]>
Date: Mon, 23 Dec 2002 08:33:00 +0100

Hi all,

On page 62 of RFC 1889 a "fast-path" check of a part of the RTP fixed
header is suggested as follows:

	"A strong "fast-path" check is possible since with high probability
   	the first four octets in the header of a newly received RTP data
   	packet will be just the same as that of the previous packet from the
   	same SSRC except that the sequence number will have increased by one."

This suggestion is bequeathed to page 79 of
draft-ietf-avt-rtp-new-11. However, if a mixer that identifies
sources in the CSRC list is present in the session, the CC field of
the fixed header will most likely vary, especially when the mixer
performs silence suppression. In this case, a "fast-path" check seems
no longer possible. Does that sound correct?

Another suggestion of RFC 1889 as well as of
draft-ietf-avt-rtp-new-11 (pages 14 and 16 respectively) is about
application specific fields in the fixed header:

 	"If a particular class of applications needs additional
	functionality independent of payload format, the profile under
	which those applications operate SHOULD define additional
	fixed fields to follow immediately after the SSRC field of the
	existing fixed header.  Those applications will be able to
	quickly and directly access the additional fields while
	profile-independent monitors or recorders can still process
	the RTP packets by interpreting only the first twelve octets."

Again, if a mixer is present and thus a potentially non-empty CSRC
list, it seems that the profile-independent monitors or recorders
will not be able to access this list implied by the non-zero CC field
in the first twelve octets, since they will look for it where the
application fields are now located. Furthermore, it is my perception
that the aforementioned extract raises ambiguity as to whether the
RTP fixed header includes the CSRC list, as clearly implied in
section 5.1, or not. What do you think?

Regards,  

Spiros Spirou MSc
Embedded Systems Engineer

New Technologies Department
Technical Division
Intracom S.A.
Markopoulo Avenue
190 02 Peania
Athens, Hellas
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e-mail: [email-masked]
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