[e2e] Question on ssthresh setting in RFC 2581

Detlef Bosau detlef.bosau at web.de
Mon May 15 11:41:32 PDT 2006

Mark Allman wrote:

>There may be some stacks that do CWV and decrease cwnd when it is not
Do someone happen to know stacks which _do_ reduce / not increase CWND 
in case of "underload situations" and which policies are in use for that?

>>>>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.
>>Perhaps, I should have a look at the rfc.
O.k., so I did:

>   FLIGHT SIZE:  The amount of data that has been sent but not yet
>      acknowledged.
So: Daniel, when 10 Packets are sent and not yet acknowledged, 
flightsize is 10. Right?
When do we know, if, that 10 or 4 packets have been lost and why is 
flightsize 10 in the first case and 4 in the second?

Do you talk about a timeout situation, where 10 packets are left 
unacknowledged when RTO occurs and 4 in the second?

Does the RFC define how "flightsize" is determined, particularly what 
happens in case of a timeout?

IIRC in case of timeout, CWND is set to 1 (2) segments to have the 
sender do "stop´n wait" for one round thus having the pipe cleaned.
In that case, flightsitze would be zero afterwards.


Detlef Bosau
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