Subject: AS parameter in SDP

From: David Lide <[email-masked]>
Date: Tue, 6 Nov 2001 15:38:03 -0500

There seems to be some confusion on the exact definition of the AS parameter
in SDP.  Should this
value reflect just the RTP payload requirements (e.g. 64 kbps for PCM voice)
or include the RTP header, IP, UDP header, security overhead, etc. 

thanks,
dave

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From: Flemming Andreasen <[email-masked]>
Date: Tue, 20 Nov 2001 11:00:56 -0500

I have asked this question myself before, but never gotten an official answer. I
have had discussions with a few people in private, and the opinion there has
been that it should include RTP header overhead. The reason for at least
including RTP header overhead is, that if it doesn't somehow account for the
varying header overhead that different packetization periods will lead to, then
it's unclear what benefit the parameter provides. Letting it apply to any lower
layers could be considered a layer violation.

Colin:    Assuming nobody disagrees with the above, can we get this clarified in
the SDP-new draft ?

Thanks

        Flemming

--
Flemming Andreasen
Cisco Systems

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From: Colin Perkins <[email-masked]>
Date: Tue, 20 Nov 2001 11:49:05 -0500

I always assumed this was the RTP session bandwidth (section 6.2 of the RTP
specification), and hence included RTP/UDP/IP header overheads. 

Colin

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From: Flemming Andreasen <[email-masked]>
Date: Tue, 20 Nov 2001 12:13:35 -0500

> specification), and hence included RTP/UDP/IP header overheads.

While this is simple for well-known fixed size overhead as e.g. 20+8 bytes for
IP+UDP, it's not clear to me that an application would have access to other types of
overhead. For example, consider the use of IPSec or IPv6. Do we really want the app
to be concerned about such things ?

-- Flemming

--
Flemming Andreasen
Cisco Systems

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From: Colin Perkins <[email-masked]>
Date: Tue, 20 Nov 2001 22:48:07 -0500

--> Flemming Andreasen writes:
>While this is simple for well-known fixed size overhead as e.g. 20+8 bytes
>for IP+UDP, it's not clear to me that an application would have access to
>other types of overhead. For example, consider the use of IPSec or IPv6.
>Do we really want the app to be concerned about such things ?

If you're using the bandwidth information for resource reservation or
capacity planning, you pretty much need to include the header sizes. 
If not, then it's advisory and the application can make a reasonable
guess and hope (nothing should break too badly if the values are not
quite right...).

Colin

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From: Flemming Andreasen <[email-masked]>
Date: Wed, 21 Nov 2001 13:44:02 -0500

> capacity planning, you pretty much need to include the header sizes.

Sure (however lower layer compression may actually make this inaccurate as well).

> guess and hope (nothing should break too badly if the values are not

All right - network layer and up it is then. As long as we get this clarified in
the SDP-new spec I'm happy.

-- Flemming

--
Flemming Andreasen
Cisco Systems