From: Mat Martineau <mathew.j.martineau@linux.intel.com>
To: Paolo Abeni <pabeni@redhat.com>
Cc: mptcp@lists.linux.dev
Subject: Re: [PATCH v2 mptcp-net 2/2] mptcp: do not reset MP_CAPABLE subflow on mapping errors
Date: Tue, 18 May 2021 10:28:27 -0700 (PDT) [thread overview]
Message-ID: <7b2f1ab8-68e-adb8-aaff-8a8cd080529@linux.intel.com> (raw)
In-Reply-To: <d816c5c2b4bcb0d69effdb3b8fc4fd09fa2a9003.camel@redhat.com>
On Tue, 18 May 2021, Paolo Abeni wrote:
> On Mon, 2021-05-17 at 17:16 -0700, Mat Martineau wrote:
>> On Mon, 17 May 2021, Paolo Abeni wrote:
>>
>>> When some mapping related errors occours we close the main
>>> MPC subflow with a RST. We should instead fallback gracefully
>>> to TCP, and do the reset only for MPJ subflows.
>>>
>>> Fixes: d22f4988ffec ("mptcp: process MP_CAPABLE data option")
>>> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/192
>>> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
>>> ---
>>> net/mptcp/subflow.c | 39 +++++++++++++++++++--------------------
>>> 1 file changed, 19 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
>>> index 278986585088..9befe9fe7bca 100644
>>> --- a/net/mptcp/subflow.c
>>> +++ b/net/mptcp/subflow.c
>>> @@ -1110,10 +1110,9 @@ static bool subflow_check_data_avail(struct sock *ssk)
>>>
>>> status = get_mapping_status(ssk, msk);
>>> trace_subflow_check_data_avail(status, skb_peek(&ssk->sk_receive_queue));
>>> - if (unlikely(status == MAPPING_INVALID)) {
>>> - ssk->sk_err = EBADMSG;
>>> - goto fatal;
>>> - }
>>> + if (unlikely(status == MAPPING_INVALID))
>>> + goto fallback;
>>> +
>>
>> There are a bunch of other ways to get MAPPING_INVALID during the life of
>> a connection, including when there are multiple subflows active and
>> fallback is not a valid option. Can the new fallback condition be more
>> targeted to this "out of order / packet loss at connection time" issue so
>> truly fatal MAPPING_INVALID cases still reset the connection?
>
> What we should do if we get a MAPPING_INVALID and we have a single
> (MPC) subflow? I could not find any specific reference in the RFC. I
> think it's roughly the same as 'no mapping' at all: if we can fallback
> we do fallback, otherwise we reset.
Looking at section 3.7, there aren't many cases where fallback is an
option during operation (after the initial data has been successfully
acked in each direction).
"If a subflow breaks during operation ... then once this is detected ...
the subflow SHOULD be treated as broken and closed with a RST, since no
data can be delivered to the application layer and no fallback signal
can be reliably sent."
In general, it looks like a bad mapping should reset the *subflow* (rather
than the whole MPTCP connection) - what I said above was unclear about
what exactly should be reset.
It seems like the only way to fall back the whole connection later in its
life involves checksum failure. In that case the RFC describes the
conditions where an "infinite mapping" can be used for fallback - but
that's different from the beginning-of-connection fallback code we have
today.
>
> Note that with this patch, under the 'fallback' label, checks if a
> reset is needed or not. The current patch does a reset only if the
> subflow is an MP_JOIN one, but that condition could be additionally
> extended to 'msk has multiple subflows' - even if the latter looks like
> a net-next patch.
>
> WDYT?
As long as the subflow is reset when the mapping failed, that's fine. It
doesn't seem like there are conditions during operation (long after
connection time) where it's correct to do fallback without an infinite
mapping.
--
Mat Martineau
Intel
prev parent reply other threads:[~2021-05-18 17:28 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-17 16:56 [PATCH v2 mptcp-net 1/2] mptcp: always parse mptcp options for MPC reqsk Paolo Abeni
2021-05-17 16:56 ` [PATCH v2 mptcp-net 2/2] mptcp: do not reset MP_CAPABLE subflow on mapping errors Paolo Abeni
2021-05-17 17:22 ` [PATCH v2 mptcp-net 3/2] mptcp: update selftest for fallback due to OoO Paolo Abeni
2021-05-18 0:16 ` [PATCH v2 mptcp-net 2/2] mptcp: do not reset MP_CAPABLE subflow on mapping errors Mat Martineau
2021-05-18 14:17 ` Paolo Abeni
2021-05-18 17:28 ` Mat Martineau [this message]
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