netdev - Re: [PATCH v2 net] tcp: add proper TS val into RST packets

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Message-Id: <20150923.142508.1802022553404500843.davem@davemloft.net>
Date:	Wed, 23 Sep 2015 14:25:08 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	ycheng@...gle.com, ncardwell@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net] tcp: add proper TS val into RST packets

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 23 Sep 2015 14:00:21 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> RST packets sent on behalf of TCP connections with TS option (RFC 7323
> TCP timestamps) have incorrect TS val (set to 0), but correct TS ecr.
> 
> A > B: Flags [S], seq 0, win 65535, options [mss 1000,nop,nop,TS val 100
> ecr 0], length 0
> B > A: Flags [S.], seq 2444755794, ack 1, win 28960, options [mss
> 1460,nop,nop,TS val 7264344 ecr 100], length 0
> A > B: Flags [.], ack 1, win 65535, options [nop,nop,TS val 110 ecr
> 7264344], length 0
> 
> B > A: Flags [R.], seq 1, ack 1, win 28960, options [nop,nop,TS val 0
> ecr 110], length 0
> 
> We need to call skb_mstamp_get() to get proper TS val,
> derived from skb->skb_mstamp
> 
> Note that RFC 1323 was advocating to not send TS option in RST segment,
> but RFC 7323 recommends the opposite :
> 
>   Once TSopt has been successfully negotiated, that is both <SYN> and
>   <SYN,ACK> contain TSopt, the TSopt MUST be sent in every non-<RST>
>   segment for the duration of the connection, and SHOULD be sent in an
>   <RST> segment (see Section 5.2 for details)
> 
> Note this RFC recommends to send TS val = 0, but we believe it is
> premature : We do not know if all TCP stacks are properly
> handling the receive side :
> 
>    When an <RST> segment is
>    received, it MUST NOT be subjected to the PAWS check by verifying an
>    acceptable value in SEG.TSval, and information from the Timestamps
>    option MUST NOT be used to update connection state information.
>    SEG.TSecr MAY be used to provide stricter <RST> acceptance checks.
> 
> 
> In 5 years, if/when all TCP stack are RFC 7323 ready, we might consider
> to decide to send TS val = 0, if it buys something.
> 
> Fixes: 7faee5c0d514 ("tcp: remove TCP_SKB_CB(skb)->when")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied and queued up for -stable, thanks Eric.
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