[e2e] A simple scenario. (Basically the reason for the sliding window thread ; -))

Detlef Bosau detlef.bosau at web.de
Sun Jan 21 15:02:37 PST 2007

David Borman wrote:
> There are real-world scenarios where the insertion of a splitter into 
> a TCP path does make a lot of sense.  The cases I am familiar with all 
> are necessitated by a severe mismatch in MTU, buffering and 
> performance, the splitter is in the only path by which the packets can 
> travel, and it is sitting at the crossover between the two disparate 
> paths.  In the specific case that I dealt with, the splitter's main 
> purpose was to change the TCP MSS option, send larger window sizes, 
> and buffer/repackage data.

In addition to the MTU issue you mention let me point to
Joseph Ishac, Mark Allman. / On the Performance of TCP Spoofing in 
Satellite Networks <http://www.icir.org/mallman/papers/milcom01.pdf>/. 
IEEE Milcom. October 2001.

The issue here is the extremely large round trip time in satellite 
networks which causes TCP to need a quite long time to achieve 
sufficient throughput.

In fact, the RTT described by Mark Allman aren´t even that bad. I´m 
still working on the issue of opportunistic scheduling in mobile networks.
I just found a technical report on that issue:
TCP Performance in Wireless Systems with Opportunistic Scheduling, R. 
Srinivasan and J. S. Baras, 
TR 2002-48, *Year:* 2002 
*Advisor:* John Baras 

As far as I see, this is really excellent work. In the example at the 
beginning at the paper opportunistic scheduling introduces a delay 
jitter of up to 1 second into the flow.

I currently simulate  networks with a physcical bandwidth of 10 Mbps and 
an average throughput of 100 kbps at the link layer due to 
retransmissions and accept a delay jitter of up to 1 second. When a 
equalize the delay spikes by buffering to an extent that the TCP fully 
exploits the average throughput at the link layer the round trip time as 
perceived by the sender can reach up to 10 seconds. It´s simply the 
question in wich kind of application 10 seconds RTT will be accepted.

Particularly for mobile networks with no splitting or spoofing I clearly 
expect the quite strict alternative that a network will either exhibit 
acceptable round trip times or acceptable throughput. Of course delay 
spikes themselves will be annoying in interactive appliccations. But one 
second delay may be acceptable for a user whereas the same user would 
simply abolish an application / connection with 10 seconds round trip time.


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