[e2e] MSS and cwnd increase

Ian McDonald ian.mcdonald at jandi.co.nz
Sat Jul 1 15:54:34 PDT 2006

On 7/2/06, Salman Abdul Baset <salman at cs.columbia.edu> wrote:
> I am wondering how the Linux 2.6 updates its congestion window when a CBR
> stream (packet size << MSS) is sent over TCP.
> In general, if I am sending 100 packets every second with a packet size
> of one byte and assuming each packet is ACKed, then increasing window by
> one every RTT (during congestion avoidance) will artifically inflate the
> window. Thus, the cwnd calculation should not assume that the last packet
> sent contained MSS bytes of data.
> The rfc and the code and seem to imply that cwnd increase is MSS-based. I
> haven't confirmed either way (still testing it) and will like to know:
> 1) What is the RFC suggested way?
> 2) How Linux 2.6 is implementing it?
Linux has changed to use RFC 3465 Appropriate Byte Counting (ABC) for
exactly this sort of reason. This can be turned off or on but is on by

Ian McDonald
Web: http://wand.net.nz/~iam4
Blog: http://imcdnzl.blogspot.com
WAND Network Research Group
Department of Computer Science
University of Waikato
New Zealand

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