[e2e] TCP Loss Differentiation

Fred Baker fred at cisco.com
Mon Feb 23 12:34:46 PST 2009

On Feb 22, 2009, at 4:33 AM, Jon Crowcroft wrote:

> thought experiment - what if a packet that is ECN marked gets lost?

The structure of ECN is such that it really shouldn't matter any more  
than dropping a TCP Ack matters, or not dropping some data segment and  
dropping a different one instead. Yes, there is an effect. The ECN  
loss, or not dropping one data segment and instead dropping a  
different one in Reno/etc, means that *that* packet doesn't trigger a  
cwnd change. If there is one ECN mark/Reno packet loss there is likely  
to be another as the same conditions hold for some period of time, so  
the session that *other* packet is in might adjust cwnd. If there  
isn't an ECN mark on some other packet (if the duration of the  
congestion event was really so short that no other packet was marked)  
one might be justified in asking what the fuss is about. Dropping a  
TCP Ack means that you will skip a small burst (2-3 packets sent in  
response to the Ack) and send one or two larger bursts a little later.

There is an isolated effect, but I don't think it is a huge one.

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