netdev - Re: [net-next v2] tcp: add TCP_INFO status for failed client TFO

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Date:   Fri, 25 Oct 2019 19:25:53 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     jbaron@...mai.com
Cc:     edumazet@...gle.com, netdev@...r.kernel.org, ncardwell@...gle.com,
        cpaasch@...le.com, ycheng@...gle.com
Subject: Re: [net-next v2] tcp: add TCP_INFO status for failed client TFO

From: Jason Baron <jbaron@...mai.com>
Date: Wed, 23 Oct 2019 11:09:26 -0400

> The TCPI_OPT_SYN_DATA bit as part of tcpi_options currently reports whether
> or not data-in-SYN was ack'd on both the client and server side. We'd like
> to gather more information on the client-side in the failure case in order
> to indicate the reason for the failure. This can be useful for not only
> debugging TFO, but also for creating TFO socket policies. For example, if
> a middle box removes the TFO option or drops a data-in-SYN, we can
> can detect this case, and turn off TFO for these connections saving the
> extra retransmits.
> 
> The newly added tcpi_fastopen_client_fail status is 2 bits and has the
> following 4 states:
> 
> 1) TFO_STATUS_UNSPEC
> 
> Catch-all state which includes when TFO is disabled via black hole
> detection, which is indicated via LINUX_MIB_TCPFASTOPENBLACKHOLE.
> 
> 2) TFO_COOKIE_UNAVAILABLE
> 
> If TFO_CLIENT_NO_COOKIE mode is off, this state indicates that no cookie
> is available in the cache.
> 
> 3) TFO_DATA_NOT_ACKED
> 
> Data was sent with SYN, we received a SYN/ACK but it did not cover the data
> portion. Cookie is not accepted by server because the cookie may be invalid
> or the server may be overloaded.
> 
> 4) TFO_SYN_RETRANSMITTED
> 
> Data was sent with SYN, we received a SYN/ACK which did not cover the data
> after at least 1 additional SYN was sent (without data). It may be the case
> that a middle-box is dropping data-in-SYN packets. Thus, it would be more
> efficient to not use TFO on this connection to avoid extra retransmits
> during connection establishment.
> 
> These new fields do not cover all the cases where TFO may fail, but other
> failures, such as SYN/ACK + data being dropped, will result in the
> connection not becoming established. And a connection blackhole after
> session establishment shows up as a stalled connection.
> 
> Signed-off-by: Jason Baron <jbaron@...mai.com>

Applied, thanks Jason.