[e2e] TCP Loss Differentiation
David P. Reed
dpreed at reed.com
Mon Feb 9 11:29:21 PST 2009
Fahad Dogar wrote:
> I guess you are restricting yourself to 'well behaved' 802.11
> settings. Multi-hop networks (with outdoor links) and mobility
> scenarios (such as wifi from moving cars) do experience losses even
> with link layer reliability and no loss of connection.
I was (perhaps not very clearly) including multihop and mobility in
"loss of connection" cases. I meant loss of PHY or layer 2
connectivity - not "session connectivity." Those situations do, as you
suggest, drop packets when "link layer reliability" fails - but I would
call the cause of that loss process a "loss of connectivity" however
transient or healable.
My main point was that these loss processes are not characterizable by a
"link loss rate". They are not like Poisson losses at all, which are
statistically a single parameter (called "rate"), memoryless
distribution. They are causal, correlated, memory-full processes. And
more importantly, one end or the other of the relevant link experiences
a directly sensed "loss of connectivity" event.
Thus my point: one SHOULD NOT model practical TCP/IP congestion/flow
control based on an assumption of "links" with "loss rates" as if they
were Poisson loss processes. One should instead focus on modeling loss
processes that come from congestion and from loss of link connectivity
or route changes arising from responses to connectivity loss in the
appropriate ways that reflect practical reality.
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