netdev - Re: [PATCH net] net: sctp: fix multihoming retransmission path selection to rfc4960

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Date:	Thu, 20 Feb 2014 13:48:19 +0100
From:	Daniel Borkmann <dborkman@...hat.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH net] net: sctp: fix multihoming retransmission path selection
 to rfc4960

On 02/20/2014 01:25 PM, David Laight wrote:
> From: Daniel Borkmann
>>
>> Problem statement: 1) both paths (primary path1 and alternate
>> path2) are up after the association has been established i.e.,
>> HB packets are normally exchanged, 2) path2 gets inactive after
>> path_max_retrans * max_rto timed out (i.e. path2 is down completely),
>> 3) now, if a transmission times out on the only surviving/active
>> path1 (any ~1sec network service impact could cause this like
>> a channel bonding failover), then the retransmitted packets are
>> sent over the inactive path2; this happens with partial failover
>> and without it.
>>
>> Besides not being optimal in the above scenario, a small failure
>> or timeout in the only existing path has the potential to cause
>> long delays in the retransmission (depending on RTO_MAX) until
>> the still active path is reselected.
>
> The current behaviour doesn't seem very good - real networks tend
> to have non-zero packet loss these days (for all sorts of reasons).
>
> I guess that under moderate traffic flow retransmit requests from
> the remote system recover the data before a timeout actually occurs.
>
> That probably means that a path with a high error rate will continue
> to be used when an alternate path would be much better.
>
> I was wondering whether it is valid (or even reasonable) to send
> the retransmit down multiple paths?  Particularly if they are
> not known to be working.

As far as I can see, the RFC says that we should pick one, and
not broadcast through all paths, besides HB should monitor these
anyway.

Future work, however, could select a retransmission path "more
intelligent" based on further transport path properties, but
that is certainly not net material, plus it seems we would need
additional state logic indicating that a path has been used before
to not exclude other less optimal transports on successive
retransmits.

> Or maybe resend heartbeats in a desperate attempt to find a working
> path?

Yes, that is done through HBs, see 1.5.7 of RFC4960.

> Do you guys know which kernel version(s) have that patch?

git describe 4141ddc02a92
v2.6.26-rc4-210-g4141ddc

> We have a few customers using sctp (for m3ua) and I really ought
> to keep track of the 'good' and 'bad' kernel versions.
>
> 	David
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