[e2e] Question on ssthresh setting in RFC 2581
detlef.bosau at web.de
Mon May 15 08:47:40 PDT 2006
Mark Allman wrote:
>>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".
>One hopes that FlightSize == cwnd. However, we introduced the
>FlightSize notion in RFC2581 because this is not always the case. Some
>stacks let cwnd be changed regardless of how much of the window is
^^^^^^^^^ changed or unchanged? ;
Are there actually stacks who change, i.e. decrease, cwnd when cwnd is
not fully used?
>actually used. So, as an example say cwnd grows to 50 segments, but
>only 25 segments per RTT are being injected. In this case, upon a loss
I can pretty well imagine this scenario for persistent non FTP
connections, e.g. HTTP or IMAP.
>>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. But doesn´t flightsize mean
the number of packets being actually on the fly?
So, flightsize should be 10, regardingless of the number of lost packets?
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