Subject: Re: RTSP

From: Rob Lanphier <[email-masked]>
Date: Thu, 17 Oct 1996 20:05:57 -0700

This is an question Henning posed last week on confctrl about the RTP
compression design found in the RTSP  draft.  As per the request to discuss
the transport aspects of this in rem-conf, I'm cc'ing that group as well.

At 08:16 AM 10/11/96 -0400, "Henning G. Schulzrinne"
<[email-masked]> wrote:
>I also question the gain this provides. The RTP header is 12 bytes, the
>"compressed" version 5 bytes, for a saving of 7 bytes. RealAudio 28.8
>(14.4 doesn't seem to work on Solaris) seems to send 247 bytes per UDP
>packet. This amounts to a saving of an amazing 2.83 % of bandwidth (or
>2.6% if you figure in the IP header). 

Actually, for fixed interval packets, we can get the compressed packet size
down to 3 bytes by computing the timestamp based on the sequence number.

Part of the reason we send such large packets is to avoid being eaten alive
by header size.  The smaller we can get the header, the more headers we can
get away with sending.  That's not to say that anyone would, but its nice to
minimize the penalty should someone need to send smaller packets.  

That aside, even the small savings you compute really helps us out.  Lots of
engineering dollars are spent on schemes to squeeze much less savings out of
an audio codec, so it is incumbant upon us to eek out whatever savings we
can in the protocol.  Also, on really low bitrate codecs the packet size may
get smaller (if calculated by RTP-AV Payload criteria, with a typical packet
interval of 20 ms), and the 9 bytes could well mean a large overhead.

I do want to make clear that we intend to be diehard supporters of
RTP/UDP/IP compression once that is decided upon and products are released
that support it.  We will make every effort to have a smooth but speedy
transition to that once it is possible, because it is in our best interest
to do so (the savings will be enormous).  In the interim, though, we need
incremental improvement, and that's what this provides.

Rob

---
Rob Lanphier               Voice: (206)674-2322         Fax: (206)674-2699
Program Manager-Protocols                         Email: [email-masked]
Progressive Networks-Home of RealAudio       Web: http://www.realaudio.com