netdev - Re: [PATCH V4 net 0/2] ipv6: fix flowlabel issue for reset packet

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Date:   Wed, 8 Nov 2017 09:44:51 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     David Miller <davem@...emloft.net>
Cc:     Martin Lau <kafai@...com>, Shaohua Li <shli@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH V4 net 0/2] ipv6: fix flowlabel issue for reset packet

On Fri, Aug 18, 2017 at 3:27 PM, David Miller <davem@...emloft.net> wrote:
> From: Martin KaFai Lau <kafai@...com>
> Date: Fri, 18 Aug 2017 13:51:36 -0700
>
>> It seems like that middle box specifically drops TCP_RST if it
>> does not know anything about this flow.  Since the flowlabel of the TCP_RST
>> (sent in TW state) is always different, it always lands to a different middle
>> box.  All of these TCP_RST cannot be delivered.
>
> This really is illegal behavior.  The flow label is not a flow _KEY_
> by any definition whatsoever.
>
> Flow labels are an optimization, not a determinant for flow matching
> particularly for proper TCP state processing.
>
> I'd rather you invest all of this energy getting that vendor to fix
> their kit.
>
We're now seeing several router vendors recommending people to not use
flow labels for ECMP hashing. This is precisely because when a flow
label changes, network devices that maintain state (firewalls, NAT,
load balancers) can't deal with packets being rerouted so connections
are dropped. Unfortunately, the need for packets of a flow to always
follow the same path has become an implicit requirement that I think
we need follow at least as the default behavior.

Martin: is there any change you could resurrect these patches? In
order to solve the general problem of making routing consistent, I
believe we want to keep sk_tx_hash consistent for the connection from
which a consistent flow label can be derived. To avoid the overhead of
a hash field in sk_common, maybe we could initially set a connection
hash to a five-tuple hash for a flow instead of a random value? So in
TW state the consistent hash can be computed on the fly.

Tom