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

Detlef Bosau detlef.bosau at web.de
Fri Sep 29 13:21:30 PDT 2006

Sireen Malik wrote:
> Heavy-tailed sized files cause correlated arrivals of packets. Large 
> variance in file sizes causes correlations on large time scales. This 
> is why this is called long range memory, or long range dependence (LRD). 
I haven´t read all the relevant papers yet, but hasn´t been there some
rumour that these LRD need at least a minimum buffer size on the
routers? In other words: If we take care not to configure buffers too
large, would this cure the problem?

>> Not quite. This discussion ignores ACK clocking. Ideally, TCP should 
>> not be bursty. Ideally (Freds remaks on TDM like behaviour) TCP 
>> packets and ACK packets shall form an evenly spaced packet train 
>> which wraps around downstream and upstream.
>> At least, that´s the idea behind the congavoid paper and following ones.
> It has been argued that RTTs have a heavy-tailed distribution!

Really? I don´t believe this for practical cases. If RTT would have a
heavy-tailed distribution it must be able to take arbitrary long values.
In other words: This argument assumes and holds with infinite router
queues. Due to some lack of silicon in the universe, router queues are
necessariley finite ;-)

In addition: Sometimes I read of "burst intervals" with several hours.
So, there should be some autoregressive proces on a line which
oscillates with a period of several hours. Doesn´t that assume that
there exist e.g. TCP connections which exist long enough to produce such
a behaviour?

In other words: Are this results based on realistic system models? Are
the system models used here "result oriented"? Or "reality oriented"? ;-)

If we consider TCP and ACK packets as "energy" and consider how this
energy is distributed in queues and links, so all this osciallating
behaviour is a result of redistribution of energy, as in any oscillating
symstem, right?
Now when, say (I don´t know! I would appreciate actual statistics here!)
one TCP flow in 10000 lasts longer than half an hour and so can exhibit
a self similar behaviour,  does this flow contain enough enegery to
cause a problem to the whole system? Or couldn´t we simply ignore it?


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