RE: TCP limitations (was Re: ISCSI: Urgent Flag requirement violates TCP.)

["Douglas Otis" <dotis@sanlight.net>]

Glen,

Should there be a problem with out of sequence packets due to link
saturations with alternative routes found or misconfigurations, the use of
SACK will reduce the load on a link that may already be overloaded.  I
understand performance of an individual connection will not be significantly
better as a result.  I was trying to indicate a relative advantage in
reducing the link demands.

Doug

> Vern Paxson quoted someone who's mail I had already deleted:
>
> > > Your assumptions about reordering is not accurate for WAN.
> Should there be
> > > more than one long-haul fiber in use, common in metro areas,
> the disparate
> > > physical routes may induce out of sequence events.  Is such a
> case, SACK
> > > reduces the negative effect.
>
> The reordering events should only occur on protection or
> route changes.  Of course, some ISPs can't manage to configure
> stable routes :-)
>
> It was recognised very early that per-packet load sharing across
> parallel links leads to packet reordering, so MAC address hashing
> for switches or IP address hashing for routers has long been standard
> practice and the default configuration setting.  Packets in the same
> flow are then sent through the same forwarding path.
>
> > SACK as currently deployed does not actually help all that much with
> > reordering.  The receiver will still generate duplicate acks and the
> > sender will misinterpret them as indicating packet loss, leading to
> > a congestion response of halving the sending rate.
>
> Our upstream provider misconfigured their links to use per-packet
> load sharing to the US, believing that maximising link utilitisation
> improved the end-to-end performance.
>
> After we got them to change back to the manufacturer's default setting
> we got a 70% decrease in FTP times for large files between our performance
> monitoring systems (Linux 2.2, which has SACK).
>
> --
>  Glen Turner                                 Network Engineer
>  (08) 8303 3936      Australian Academic and Research Network
>  glen.turner@aarnet.edu.au          http://www.aarnet.edu.au/
> --
>  The revolution will not be televised, it will be digitised
>