Re: packet reordering at exchange points

[Richard A Steenbergen <ras () e-gerbil net>]

On Mon, Apr 08, 2002 at 02:18:52PM -0700, Paul Vixie wrote:
> > packet reordering at MAE East was extremely common a few years ago. Does
> > anyone have information whether this is still happening?
>
> more to the point, does anybody still care about packet reordering at
> exchange points?  we (paix) go through significant effort to prevent it,
> and interswitch trunking with round robin would be a lot easier.  are
> we chasing an urban legend here, or would reordering still cause pain?

Setup a freebsd system with a dummynet pipe, do a probability match on 50% 
of the packets and send them through a pipe with a few more bytes of 
queueing and 1ms more delay than the rest. Then test the performance of 
TCP across that link.

There is a good paper on the subject that was published by ACM in
Janurary: http://citeseer.nj.nec.com/450712.html

So just how common is packet reordering today? Well I did a quick peak at
a few machines which I don't have any reason to believe are out of the
ordinary, and they all pretty much come out about the same:

        32896155 packets received
                9961197 acks (for 2309956346 bytes)
                96322 duplicate acks
                0 acks for unsent data
                17328137 packets (2667939981 bytes) received in-sequence
                10755 completely duplicate packets (1803069 bytes)
                19 old duplicate packets
                375 packets with some dup. data (38297 bytes duped)
                53862 out-of-order packets (75435307 bytes)

0.3% of non-ACK packets by packet were received out of order, or 2.8% by 
bytes.

-- 
Richard A Steenbergen <ras () e-gerbil net>       http://www.e-gerbil.net/ras
PGP Key ID: 0x138EA177  (67 29 D7 BC E8 18 3E DA  B2 46 B3 D8 14 36 FE B6)