[e2e] Are Packet Trains / Packet Bursts a Problem in TCP?

Detlef Bosau 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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