netdev - Re: [net-next v2 02/13] tcp: tcp_v[46]_conn_request: fix snt

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Date:	Mon, 30 Jun 2014 14:50:22 +0300
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	Netdev <netdev@...r.kernel.org>, Daniel Lee <longinus00@...il.com>,
	Yuchung Cheng <ycheng@...gle.com>, Jerry Chu <hkchu@...gle.com>
Subject: Re: [net-next v2 02/13] tcp: tcp_v[46]_conn_request: fix snt_synack initialization

On Sat, Jun 28, 2014 at 5:16 AM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Wed, Jun 25, 2014 at 10:09 AM, Octavian Purdila
> <octavian.purdila@...el.com> wrote:
>> Commit 016818d07 (tcp: TCP Fast Open Server - take SYNACK RTT after
>> completing 3WHS) changes the code to only take a snt_synack timestamp
>> when a SYNACK transmit or retransmit succeeds. This behaviour is later
>> broken by commit 843f4a55e (tcp: use tcp_v4_send_synack on first
>> SYN-ACK), as snt_synack is now updated even if tcp_v4_send_synack
>> fails.
>>
>> Also, commit 3a19ce0ee (tcp: IPv6 support for fastopen server) misses
>> the required IPv6 updates for 016818d07.
>>
>> This patch makes sure that snt_synack is updated only when the SYNACK
>> trasnmit/retransmit succeeds, for both IPv4 and IPv6.
>>
>> Cc: Cardwell <ncardwell@...gle.com>
>> Cc: Daniel Lee <longinus00@...il.com>
>> Cc: Yuchung Cheng <ycheng@...gle.com>
>>
>> Signed-off-by: Octavian Purdila <octavian.purdila@...el.com>
>
>> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
>> index 08ae3da..a962455 100644
>> --- a/net/ipv6/tcp_ipv6.c
>> +++ b/net/ipv6/tcp_ipv6.c
>> @@ -497,6 +497,8 @@ static int tcp_v6_send_synack(struct sock *sk, struct dst_entry *dst,
>>                 skb_set_queue_mapping(skb, queue_mapping);
>>                 err = ip6_xmit(sk, skb, fl6, np->opt, np->tclass);
>>                 err = net_xmit_eval(err);
>> +               if (!tcp_rsk(req)->snt_synack && !err)
>> +                       tcp_rsk(req)->snt_synack = tcp_time_stamp;
>>         }
>
> Octavian, thanks for finding this issue!
>

Hi Neal,

> I think we should resolve the inconsistency you have discovered by
> converging on the approach in Yuchung's 843f4a55e (tcp: use
> tcp_v4_send_synack on first SYN-ACK), which restores the original
> behavior of snt_synack from the commit where it was added by Jerry:
> 9ad7c049f0f79 ("tcp: RFC2988bis + taking RTT sample from 3WHS for the
> passive open side") in v3.1. In that commit tcp_v4_conn_request() and
> tcp_v6_conn_request() unconditionally store the time at which the
> server received the first client SYN in snt_synack:
>
>   tcp_rsk(req)->snt_synack = tcp_time_stamp;
>
> At Google we have found that behavior very useful because it allows
> extracting useful performance metrics summarizing how long it took to
> establish a passive TCP connection.

In that case perhaps it is better to add a new field (or rename the
existing one if it is not needed anymore) to store the syn arrival
time? I think it is confusing to store the syn arrival time in the
"synack sent time" field.
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