[e2e] TCP Loss Differentiation
Jon.Crowcroft at cl.cam.ac.uk
Mon Feb 23 15:01:56 PST 2009
um - i am not talking about whether ECN works right in the presence of
lost packets - i am talking about what you need to differentiate
non-congestive loss. you need to be somewhat more ingeneious -
if each router were to record in every packet in a flow, all the
packets it had seen, the order it had seen them, and the routers own
address, then when any packet arrives, you'd have a cmplete history of
packets predeceessors and successors and gaps, and where gaps are
caused, and so a receiver can disambiguate on a _per packet_ base
when a loss was not congestive...
In missive <3D86E9E6-8FB4-458C-9DC2-8883F8805161 at cisco.com>, Fred Baker typed:
>>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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