Subject: RTP = application layer protocol?

From: Jason Coombs <[email-masked]>
Date: Wed, 30 Apr 1997 12:21:51 -0700

Greetings.

I've been doing some reading through RFCs and other
documentation concerning Real-time Transport Protocol
(RTP) and I've discovered a few contradicting remarks
concerning where RTP fits in the OSI model.

Is RTP an application layer, presentation layer, session
layer, or transport layer protocol?

RFC 1889 indicates that RTP provides some of the transport
layer in cooperation with UDP in an IP environment but that
RTP essentially sits on top of UDP and can use some other
transport layer instead if necessary.

It seems like RTP doesn't fit nicely into any one OSI layer
and I would appreciate comments from the members of this
list concerning this question.

Regards,

Jason Coombs
Scientist, SCIENCE.ORG
http://www.science.org/jasonc/

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From: Carsten Bormann <[email-masked]>
Date: Wed, 30 Apr 1997 22:05:06 +0200 (MET DST)

> list concerning this question.

The OSI model was invented some 15 years before RTP was standardized.
The world has become more complex (and simpler, too).

There is no known benefit to mankind from generating, stating, or
discussing answers to this question.

(Of course, it would be fun for those of us who like a bit of
nostalgia now and then...  On a mailing list of its own.)

In the usual trade rag misunderstandings you will likely read that RTP
is a session layer protocol, because
1) it's used above transport protocols like UDP
2) it's not really an application on its own
3) it's about RTP sessions :-)

Gruesse, Carsten

PS.: When I had known we were going to do something like RTP when I
wrote papers about the presentation layer around 1981, I would
certainly have made it a part of our concept of the presentation layer...

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From: Jon Crowcroft <[email-masked]>
Date: Thu, 01 May 1997 08:29:47 +0100

RTP is part of the ALF/ILP philosphophy of protocol design

the problem yo udescribe is actually that the OSI model doesn't model
_ANY_ protocol stack once you get up close and personal with it....
(at least not realistically or usefuly)

RTP+UDP is roughly session (if you include rtcp)+transport with a very small (payload type)
presention context....if you really really insist on pushing the model
too far!!

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