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

Joe Touch touch at ISI.EDU
Tue Jan 9 12:31:34 PST 2007

Agarwal, Anil wrote:
> =20
> Joe Touch wrote -
>>>> FYI,Internet MSS's are usually in the 500-byte range in general. A
>>>> 5KB file would take 10 packets and be over by the 4th round.
>>> Um, the Internet MSS is usually 1460 bytes, except where it is hacked=

>>> to between 1300 and 1400 bytes to avoid issues with broken Path MTU
>>> Detection in the presence of links with an MTU slightly smaller than
>>> 1500 (mostly ADSL links).
>>> Packets around 500 bytes have become quite rare on the Internet today=
>> http://netweb.usc.edu/~rsinha/pkt-sizes/
>> http://tracer.csl.sony.co.jp/mawi/samplepoint-C/2005/200510250900.html=

>> 'better connected' sites show larger packet sizes (show in the USC
>> traces), but that smaller packets are still used, and that the average=

>> size depends on the protocol (CSL traces).
> Even though smaller packet sizes are observed on the net,
> depending on protocol and application, that does not imply
> that the MSS or path MTU is small. Some applications simply send small
> amounts of data, at a time (telnet, http GETs, etc).
> I suspect, MSS is of the order of 1300-1460 bytes,
> even in these traces.

If that's the case, and such MSSs are indeed predominant throughout the
Internet, not just in well-connected universities talking to the world,
then that begs why the IETF is bothering with updates to path MTU to
avoid black-holing.

One possibility is that black-holing is prevalent, and that sites
accessible only with smaller MTUs whose ICMP 'too big' error messages
are not received are being ignored from these traces.

Anyway, it's probably appropriate to consider both 500 and 1500-byte
MTUs in these calculations. The real question is how much a connection
is sped up by using a larger arithmetic increase factor (2x vs 1.5x),
and how much that matters depends on the size of the transfer, the BW,
the RTT, and the server load. Packet size is part of that equation, but
ultimately not all that critical anyway.


Joe Touch
Sr. Network Engineer, USAF TSAT Space Segment

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