Re: [RFC 0/2] Account for duplicate ACKs with invalid SACK-blocks

[Christoph Paasch <christoph.paasch@uclouvain.be>]

On 19/08/13 - 14:22:46, Corey Hickey wrote:
> On 2013-08-19 12:29, Christoph Paasch wrote:
> > There exist sequence-number rewriting middleboxes, who do not modify the 
> > sequence-number in the SACK-blocks.
> > Duplicate acknowledgments with these (invalid) SACK-blocks will not be
> > accounted in sacked_out, and thus no fast-retransmit will trigger.
> > So, the only way to recover from a packet-loss is through an RTO,
> > effectively killing the performance of TCP in this case.
> > 
> > Performance-results can be seen here:
> > http://tools.ietf.org/agenda/87/slides/slides-87-tcpm-11.pdf
> > 
> > Another solution might be to simply disable SACK, as soon as an invalid
> > SACK-block has been received (as suggested by Phil Oester). This however
> > might be too aggressive.
> > 
> > Christoph Paasch (2):
> >   Use acked_out for reno-style ack acounting instead of sacked_out
> >   Account acked_out in sack, if the sack is invalid
> > 
> >  include/linux/tcp.h      |   1 +
> >  include/net/tcp.h        |  17 +++++---
> >  net/ipv4/tcp_input.c     | 103 ++++++++++++++++++++++++++++++++---------------
> >  net/ipv4/tcp_minisocks.c |   1 +
> >  net/ipv4/tcp_output.c    |   6 +--
> >  net/ipv4/tcp_timer.c     |   2 +-
> >  6 files changed, 89 insertions(+), 41 deletions(-)
> 
> I tested this patchset, and, unfortunately, the behavior seems to be the
> same.

Yes, because the problem is still on conntrack's side too.

As soon as conntrack will let pass the invalid SACK-blocks, you will
experience bad performance because every packet-loss can only be recovered
by an RTO.
My patchset tries to fix this last issue.


Cheers,
Christoph

> 
> I'm still planning on working with the Cisco device, when I can get some
> of the network admin's time, but that won't happen immediately.
> 
> Thanks,
> Corey