From: David Ahern <dsahern@kernel.org>
To: "Nabil S. Alramli" <nalramli@fastly.com>,
sbhogavilli@fastly.com, davem@davemloft.net, edumazet@google.com,
kuba@kernel.org, pabeni@redhat.com, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org
Cc: srao@fastly.com, dev@nalramli.com
Subject: Re: [net] ipv4: Fix broken PMTUD when using L4 multipath hash
Date: Fri, 13 Oct 2023 11:19:58 -0500 [thread overview]
Message-ID: <e18c52e8-116e-f258-7f2c-030a80e88343@kernel.org> (raw)
In-Reply-To: <20231012234025.4025-1-nalramli@fastly.com>
On 10/12/23 5:40 PM, Nabil S. Alramli wrote:
> From: Suresh Bhogavilli <sbhogavilli@fastly.com>
>
> On a node with multiple network interfaces, if we enable layer 4 hash
> policy with net.ipv4.fib_multipath_hash_policy=1, path MTU discovery is
> broken and TCP connection does not make progress unless the incoming
> ICMP Fragmentation Needed (type 3, code 4) message is received on the
> egress interface of selected nexthop of the socket.
known problem.
>
> This is because build_sk_flow_key() does not provide the sport and dport
> from the socket when calling flowi4_init_output(). This appears to be a
> copy/paste error of build_skb_flow_key() -> __build_flow_key() ->
> flowi4_init_output() call used for packet forwarding where an skb is
> present, is passed later to fib_multipath_hash() call, and can scrape
> out both sport and dport from the skb if L4 hash policy is in use.
are you sure?
As I recall the problem is that the ICMP can be received on a different
path. When it is processed, the exception is added to the ingress device
of the ICMP and not the device the original packet egressed. I have
scripts that somewhat reliably reproduced the problem; I started working
on a fix and got distracted.
next prev parent reply other threads:[~2023-10-13 16:20 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20231012005721.2742-2-nalramli@fastly.com>
2023-10-12 23:40 ` [net] ipv4: Fix broken PMTUD when using L4 multipath hash Nabil S. Alramli
2023-10-13 16:19 ` David Ahern [this message]
2023-10-16 18:51 ` Nabil S. Alramli
2024-02-09 17:11 ` Suresh Bhogavilli
2024-02-09 22:27 ` David Ahern
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