[e2e] end2end-interest Digest, Vol 27, Issue 4

Arjuna Sathiaseelan arjuna.sathiaseelan at gmail.com
Thu May 11 13:00:53 PDT 2006

Ok so how I see this is in 2 scenarios:

Say the no of packets ur sending i.e 10 in ur case could be either a set of
10 packets in the middle of a connection or the last set of packets in a
connection. If its in the middle of a connection and if the last 4 packets
have been lost as u say, the previous 6 packets would have triggered the
sender to send few more packets (depending on whether its in slow start or
congestion avoidance), thus the flight size would not be 4 as u claim - but
would be more than that. If its the latter case where the set of packets
were last in the connection, then if these 4 packets were lost - then they
would be recovered only by a retransmission timeout.


> Message: 2
> Date: Thu, 11 May 2006 17:03:12 +0200
> From: "Daniel Minder" <daniel.minder at informatik.uni-stuttgart.de>
> Subject: [e2e] Question on ssthresh setting in RFC 2581
> To: <end2end-interest at postel.org>
> Message-ID: <001701c6750c$08c16ff0$10d24581 at pcvs7>
> Content-Type: text/plain;       charset="us-ascii"
> Hi,
> can anybody explain why equation (3) in RFC 2581 is
> ssthresh = max (FlightSize / 2, 2*SMSS)
> and why this has changed from RFC 2001 where min(rwnd, cwnd)/2 was used.
> In some postings, I found that FlightSize is usually equal to min(rwnd,
> cwnd) - but not always. According to the RFC, flightsize is "the amount of
> data that has been sent but not yet acknowledged".
> Let's assume that 10 packets have been sent. If all 10 get lost,
> flightsize
> is 10 and ssthresh will be set to 5. But if only the last 4 get lost (and
> no
> more packets are to be sent), flightsize is 4 and ssthresh will be set to
> 2.
> This seems counterproductive to me since the network is propably more
> congested in the first case than in the second. Therefore, ssthresh should
> be set to a lower value if more packets get lost, i.e. when flightsize is
> larger. For example:
> ssthresh = max( (min(rwnd, cwnd) - FlightSize) / 2, 2 * SMSS)
> Or did I miss the point?
> TIA,
> Daniel
> --
> Regards,
> Arjuna
> Postdoctoral Researcher
> Engineering Research Lab,
> Department of Engineering,
> University of Aberdeen
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