Subject: receiver-based RTTs

From: "Timur Friedman" <[email-masked]>
Date: Fri, 11 Aug 2000 12:42:21 -0400

Are special reports needed to allow receivers to caculate RTTs?  Or can the
existing RTCP mechanism be adapted?  This issue came up in Pittsburgh while
discussing the RTCP Reporting Extensions draft.  I will argue for the
special reports that are in the draft.

RTCP allows senders to calculate RTTs.  Receivers echo back a sender's
timestamp along with an indication of how long they waited before the echo
(the RR DLSR field).  However, only a sender knows when it receives the
echo, and so only senders know RTTs.  This is fine for sender-based
congestion control, but does not help receiver-based congestion control.

Dorgham Sisalem and Adam Wolisz have designed MLDA, a receiver-based
congestion control mechanism for RTP.  The receivers need to know RTTs
between themselves and the senders.  We have suggested special reporting
blocks to support this -- essentially a receiver timestamp, and an echo with
delay information from the sender -- as part of the RTCP Reporting
Extensions draft.

In Pittsburgh, it was suggested that a simpler way of achieving the same
effect would be to have all receivers make themselves into senders, by each
sending a small occasional packet.  This would save us from adding
additional mechanism.

It seemed to me at the time as a possible alternative.  But I have discussed
this idea with Dorgham, and he has convinced me that the additional
mechanism is needed.  As I understand his arguments:

(1) If all receivers become senders, then all receivers will by default send
reports on all other receivers.  This is considerable unneeded overhead.
They only need to send reports on the actual data senders.

(2) If all receivers become senders, then this by default does away with the
recommended 1/4 of RTCP bandwidth reserved to data senders.

(3) The data packets sent by the receivers, even if small and occasional, do
consume bandwidth.

Based on this, I see real value in keeping the special reports in the draft
as it moves towards Experimental RFC.

- Timur Friedman

References:

RTCP Reporting Extensions draft
<http://www.ietf.org/internet-drafts/draft-friedman-avt-rtcp-report-extns-01
.txt>

MLDA
D. Sisalem and A. Wolisz, "MLDA: A TCP-friendly Congestion Control Framework
for Heterogeneous Multicast Environments", Eighth International Workshop on
Quality of Service (IWQoS 2000), 5-7 June 2000, Pittsburgh.
<ftp://ftp.fokus.gmd.de/pub/step/papers/Sisa0002:MLDA.ps.gz>