Questions about Internet Packet Losses

Vern Paxson vern at
Tue Jan 14 07:15:39 UTC 1997

> So you'll note that there are very clear spikes at 552 and 576 total packet
> size.  Further, note that the curve falls off _extremely_ rapidly.  From
> that, I argue that case (2) _cannot_ be the case because any type of random
> distribution would be much smoother.

(2) is a random distribution plus some spikes, plus some modality due to
TCP's that can only generate packets of <= N bytes for different popular
MSS's.  So I think it still works (but will save more detailed analysis
till later ....).

> 2) What OS is using a 512 MSS?

>From a bunch of (~ 20,000) traces in my study, looks like

        Irix 4.0/5.3, BSDI 1.1/2.0, OSF/1 3.0/3.2, and
	often but not always SunOS

use either 512 or 536, or sometimes 500, 524, or 548 (BSDI).  Never
anything higher.  This doesn't mean they *can't* use anything higher,
just that they never found an opportunity to do so.

512 is far and away the most common MSS in the traces.  (All the TCP's
in the traces are BSD variants except for Solaris and Linux.)

> 256?

My best guess is SLIP links.

> 4) 41 bytes is pretty obviously interactive traffic.  Is the intuition
>    correct?

That's my guess too.

> What's so special about 44, 52, 48 and 56?

Ack/SYN/FIN's with options.

>    What do people do
>    with 4, 8, 12 and 16 bytes of data?  And why not any of the odd values?

Because option sizes are rounded up to multiples of 4 bytes.


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