[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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