[e2e] Are Packet Trains / Packet Bursts a Problem in TCP?
detlef.bosau at web.de
Mon Sep 25 07:08:04 PDT 2006
Craig Partridge wrote:
> At some point, however, burst sizes (or frequency of bursts), becomes
> a problem. Exactly what size/frequency combinations cause grief has, to
> my knowledge, not been studied very much.
Hm. Is this really the problem? What, if we knew what size/frequency
combinations cause problems? Would we then adapt our protocols? E.g.
turn on / off TCP pacing mechanisms?
Isn´t it a more fundamental question, wether burstiness may cause grief
in a significant number of scenarios, so that it would be useful to
avoid burstiness at all?
However, what is the exact meaning of "burstiness" then? The more papers
I read about pacing, AQM etc., the more I wonder how "Telco-like" the
internet shall become.
In addition: Do we have actual statistics about the length of a TCP
flow? Whenever I read papers on AQM, congestion control etc., I see
always the same stuff. NS2 simulations with impressively huge (up to 10)
or even giant (up to 100) numbers of flows with greedy sources. In some
cases, even the "dynamics" of the network are described with stationary
equations which (hopefuly) hold, when all flows lasted at least one week
up to the beginning of the calculation ;-)
Now, I remember a paper where it was mentioned that 95 % of all TCP
flows consist of less then 20 packets. (Spoken more drastically: Are
unlikely to leave the initial slowstart and achieve congestion avoidance
at all.) In addition, users often initiate TCP flows independently (not
in a statistical sense), however there might be many TCP flows initiated
at the same time. Perhaps, there are some "evening news effects", i.e.
after the evening news on TV, a huge number of people turns on the light
in the living room (causing the utilities to fail ;-)) and afterwords
use their toilet (causing the water supply to fail ;-)).
What I'm curious about is:
- is such a bevhaviour annoying?
- if so: do our AQM / congestion control / pacing mechanisms really
tackle the reasons of burstiness?
- _can_ we influence this behaviour at all? (start the evening news on
at least 20 TV channels, each with a time offset of five minuts... ;-))
Perhaps its a problem of my understanding and someone could give me a clue.
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