[e2e] Are we doing sliding window in the Internet?

Detlef Bosau detlef.bosau at web.de
Sun Dec 31 11:15:44 PST 2006

Happy New Year, Miss Sophy My Dear!

(Although this sketch is in Englisch, it is hardly known outside Germay 
to my knowledge.)

I wonder whether we´re really doing sliding window in TCP connections 
all the time or whether a number of connections have congestion windows 
of only one segment, i.e. behave like stop´n wait in reality.

When I assume an  Ethernet like MTU, i.e. 1500 byte = 12000 bit, and 10 
ms RTT the throughput is roughly 12000 bit / 10 ms = 1.2 Mbps.

 From this I would expect that in quite a few cases a TCP connection 
will have a congestion window of 1 MSS or even less.

In addition, some weeks ago I read a paper, I don´t remember were, that 
we should reconsider and perhaps resize our MTUs to larger values for 
networks with large bandwidth. The rationale was simply as follows: The 
MTU size is always a tradeoff between overhead and jitter. From Ethernet 
we know that we can accept a maximum packet duration of 12000 bit / (10 
Mbps) = 1.2 ms  and the resultig jitter. For Gigabit Ethernet
a maximum packet duration of 1.2 ms would result in a MTU size of 1500 
kbyte = 1.5 Mbyte.

If so, we would see "stop´n wait like" connections much more frequently 
than today.

Is this view correct?

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