netdev - Re: [PATCH v1 net] TCP_USER_TIMEOUT and tcp_keepalive should conform to RFC5482

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Date:   Tue, 08 Aug 2017 02:59:49 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Rao Shoaib <rao.shoaib@...cle.com>
Cc:     davem@...emloft.net, kuznet@....inr.ac.ru, netdev@...r.kernel.org
Subject: Re: [PATCH v1 net] TCP_USER_TIMEOUT and tcp_keepalive should
 conform to RFC5482

On Mon, 2017-08-07 at 11:16 -0700, Rao Shoaib wrote:
> Change from version 0: Rationale behind the change:
> 
> The man page for tcp(7) states
> 
> when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will
> override keepalive to  determine  when to close a connection due to keepalive
> failure.
> 
> This is ambigious at best. user expectation is most likely that the connection
> will be reset after TCP_USER_TIMEOUT milliseconds of inactivity.
> 
> The code however waits for the keepalive to kick-in (default 2hrs) and than
> after one failure resets the conenction. 
> 
> What is the rationale for that ? The same effect can be obtained by simply
> changing the value of tcp_keep_alive_probes.
> 
> Since the TCP_USER_TIMEOUT option was added based on RFC 5482 we need to follow 
> the RFC. Which states
> 
> 4.2 TCP keep-Alives:
>    Some TCP implementations, such as those in BSD systems, use a
>    different abort policy for TCP keep-alives than for user data.  Thus,
>    the TCP keep-alive mechanism might abort a connection that would
>    otherwise have survived the transient period without connectivity.
>    Therefore, if a connection that enables keep-alives is also using the
>    TCP User Timeout Option, then the keep-alive timer MUST be set to a
>    value larger than that of the adopted USER TIMEOUT.
> 
> This patch enforces the MUST and also dis-associates user timeout from keep
> alive.  A man page patch will be submitted separately.
> 
> Signed-off-by: Rao Shoaib <rao.shoaib@...cle.com>
> ---
>  net/ipv4/tcp.c       | 10 ++++++++--
>  net/ipv4/tcp_timer.c |  9 +--------
>  2 files changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 71ce33d..f2af44d 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2628,7 +2628,9 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
>  		break;
>  
>  	case TCP_KEEPIDLE:
> -		if (val < 1 || val > MAX_TCP_KEEPIDLE)
> +		/* Per RFC5482 keepalive_time must be > user_timeout */
> +		if (val < 1 || val > MAX_TCP_KEEPIDLE ||
> +		    ((val * HZ) <= icsk->icsk_user_timeout))
>  			err = -EINVAL;
>  		else {
>  			tp->keepalive_time = val * HZ;
> @@ -2724,8 +2726,12 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
>  	case TCP_USER_TIMEOUT:
>  		/* Cap the max time in ms TCP will retry or probe the window
>  		 * before giving up and aborting (ETIMEDOUT) a connection.
> +		 * Per RFC5482 TCP user timeout must be < keepalive_time.
> +		 * If the default value changes later -- all bets are off.
>  		 */
> -		if (val < 0)
> +		if (val < 0 || (tp->keepalive_time &&
> +				tp->keepalive_time <= msecs_to_jiffies(val)) ||
> +		   net->ipv4.sysctl_tcp_keepalive_time <= msecs_to_jiffies(val))


When TCP_USER_TIMEOUT socket option is attempted, maybe keepalive option
was not used.

Yet your new tests assume it is engaged.

It might break some usages.

>  			err = -EINVAL;
>  		else
>  			icsk->icsk_user_timeout = msecs_to_jiffies(val);
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index c0feeee..d39fe60 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -664,14 +664,7 @@ static void tcp_keepalive_timer (unsigned long data)
>  	elapsed = keepalive_time_elapsed(tp);
>  
>  	if (elapsed >= keepalive_time_when(tp)) {
> -		/* If the TCP_USER_TIMEOUT option is enabled, use that
> -		 * to determine when to timeout instead.
> -		 */
> -		if ((icsk->icsk_user_timeout != 0 &&
> -		    elapsed >= icsk->icsk_user_timeout &&
> -		    icsk->icsk_probes_out > 0) ||
> -		    (icsk->icsk_user_timeout == 0 &&
> -		    icsk->icsk_probes_out >= keepalive_probes(tp))) {
> +		if (icsk->icsk_probes_out >= keepalive_probes(tp)) {
>  			tcp_send_active_reset(sk, GFP_ATOMIC);
>  			tcp_write_err(sk);
>  			goto out;