netdev - Re: [PATCH net] sctp: fix the transports round robin issue when init is retransmitted

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Date:	Thu, 10 Mar 2016 10:48:45 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Xin Long <lucien.xin@...il.com>,
	network dev <netdev@...r.kernel.org>,
	linux-sctp@...r.kernel.org
CC:	davem@...emloft.net,
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	Vlad Yasevich <vyasevich@...il.com>
Subject: Re: [PATCH net] sctp: fix the transports round robin issue when init
 is retransmitted

On 03/10/2016 08:31 AM, Xin Long wrote:
> prior to this patch, at the beginning if we have two paths in one assoc,
> they may have the same params other than the last_time_heard, it will try
> the paths like this:
>
> 1st cycle
>    try trans1 fail.
>    then trans2 is selected.(cause it's last_time_heard is after trans1).
>
> 2nd cycle:
>    try  trans2 fail
>    then trans2 is selected.(cause it's last_time_heard is after trans1).
>
> 3rd cycle:
>    try  trans2 fail
>    then trans2 is selected.(cause it's last_time_heard is after trans1).
>
> ....
>
> trans1 will never have change to be selected, which is not what we expect.
> we should keeping round robin all the paths if they are just added at the
> beginning.
>
> So at first every tranport's last_time_heard should be initialized 0, so
> that we ensure they have the same value at the beginning, only by this,
> all the transports could get equal chance to be selected.
>
> Then for sctp_trans_elect_best, it should return the trans_next one when
> *trans == *trans_next, so that we can try next if it fails,  but now it
> always return trans. so we can fix it by exchanging these two params when
> we calls sctp_trans_elect_tie().
>
> Fixes: 4c47af4d5eb2 ('net: sctp: rework multihoming retransmission path selection to rfc4960')
> Signed-off-by: Xin Long <lucien.xin@...il.com>

Seems okay to me. I presume these paths were either all in PF or INACTIVE
state, if not even HBs get through that would actually raise some scores,
so that a good path will be selected again in such case.

Acked-by: Daniel Borkmann <daniel@...earbox.net>