Subject: Re: [dcp] Window counter

From: Sally Floyd <[email-masked]>
Date: Tue, 18 Jun 2002 16:11:32 -0700

Sara -

My apologies for the very late reply.

>As I understand it the purpose of the window counter option
>(CCID 3) is to replace the RTT estimate. Basically the receiver needs to
>know the RTT in order to report the current receive rate and to know
>when to send feedback among other things. My question is how the window
>counter option is to be used for this.

This use of a coarse-grained counter that increments every quarter
RTT in CCID 3 is largely to determine when multiple losses or marks
belong to the same loss event.  This is discussed in Section 8 of
draft-padhye-dcp-ccid3-03.txt, and is discussed briefly in Section
3.2.1 of draft-ietf-tsvwg-tfrc-04.txt.  The TFRC draft states that
the receiver requires either the timestamp along with the sender's
estimate of the RTT, or the coarse-grained counter, to determine
when multiple losses belong to the same loss event.  The DCCP/CCID 3 
draft was changed to use the coarse-grained counter instead of
the timestamp and the sender's estimate of the RTT.

You are right that the receiver also needs an RTT estimate to set
the feedback timer.  With the new version of DCCP/CCID 3, the receiver
will have to use the info in the coarse-grained counter to set the
feedback timer.  We need to add this to the DCCP/CCID 3 draft.

The receiver reports the received rate X_recv in pkts per second,
so this does not require the RTT estimate.

>By the way, is it still recommended that the sender increases the window
>counter by 4 once it receives a report of a congestion event?

The current draft draft-padhye-dcp-ccid3-03.ps recommends the following:

"When the sender receives an acknowledgement
acknowledging a data packet with window counter i, the sender can
increase its window counter, if necessary, so that subsequent data
packets are sent with window counter values of at least i+4."

This is still recommended.

Many thanks, 
- Sally
http://www.icir.org/floyd/

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