Date:	Tue, 01 Sep 2009 13:49:59 +0200
From:	Damian Lukowski <damian@....rwth-aachen.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 3/3] Revert Backoff [v3]: Calculate TCP's connection close
 threshold as a time value.

Eric Dumazet schrieb:
> Damian Lukowski a écrit :
>> RFC 1122 specifies two threshold values R1 and R2 for connection timeouts,
>> which may represent a number of allowed retransmissions or a timeout value.
>> Currently linux uses sysctl_tcp_retries{1,2} to specify the thresholds
>> in number of allowed retransmissions.
>>
>> For any desired threshold R2 (by means of time) one can specify tcp_retries2
>> (by means of number of retransmissions) such that TCP will not time out
>> earlier than R2. This is the case, because the RTO schedule follows a fixed
>> pattern, namely exponential backoff.
>>
>> However, the RTO behaviour is not predictable any more if RTO backoffs can be
>> reverted, as it is the case in the draft
>> "Make TCP more Robust to Long Connectivity Disruptions"
>> (http://tools.ietf.org/html/draft-zimmermann-tcp-lcd).
>>
>> In the worst case TCP would time out a connection after 3.2 seconds, if the
>> initial RTO equaled MIN_RTO and each backoff has been reverted.
>>
>> This patch introduces a function retransmits_timed_out(N),
>> which calculates the timeout of a TCP connection, assuming an initial
>> RTO of MIN_RTO and N unsuccessful, exponentially backed-off retransmissions.
>>
>> Whenever timeout decisions are made by comparing the retransmission counter
>> to some value N, this function can be used, instead.
>>
>> The meaning of tcp_retries2 will be changed, as many more RTO retransmissions
>> can occur than the value indicates. However, it yields a timeout which is
>> similar to the one of an unpatched, exponentially backing off TCP in the same
>> scenario. As no application could rely on an RTO greater than MIN_RTO, there
>> should be no risk of a regression.
>>
>> Signed-off-by: Damian Lukowski <damian@....rwth-aachen.de>
>> ---
>>  include/net/tcp.h    |   18 ++++++++++++++++++
>>  net/ipv4/tcp_timer.c |   11 +++++++----
>>  2 files changed, 25 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/net/tcp.h b/include/net/tcp.h
>> index c35b329..17d1a88 100644
>> --- a/include/net/tcp.h
>> +++ b/include/net/tcp.h
>> @@ -1247,6 +1247,24 @@ static inline struct sk_buff *tcp_write_queue_prev(struct sock *sk, struct sk_bu
>>  #define tcp_for_write_queue_from_safe(skb, tmp, sk)			\
>>  	skb_queue_walk_from_safe(&(sk)->sk_write_queue, skb, tmp)
>>  
>> +static inline bool retransmits_timed_out(const struct sock *sk,
>> +					 unsigned int boundary)
>> +{
>> +	int limit, K;
>> +	if (!inet_csk(sk)->icsk_retransmits)
>> +		return false;
>> +
>> +	K = ilog2(TCP_RTO_MAX/TCP_RTO_MIN);
>> +
>> +	if (boundary <= K)
>> +		limit = ((2 << boundary) - 1) * TCP_RTO_MIN;
>> +	else
>> +		limit = ((2 << K) - 1) * TCP_RTO_MIN +
>> +			(boundary - K) * TCP_RTO_MAX;
> 
> Doing this computation might allow us to respect RFC 1122 here :
>  
> "The value of R2 SHOULD correspond to at least 100 seconds."
> 
> adding a third parameter to retransmits_timed_out(), min_limit,
> being 100*HZ if sysctl_tcp_retries2 was used...
> 
> limit = min(min_limit, limit);

Hi.
Hm, with this restriction, we would make it a MUST instead of a SHOULD.
The current approach does also allow retries2 values, which can yield
lower timeouts than 100 seconds.
I could implement the min_timeout, but in my opinion, the 100 seconds
shouldn't be enforced. We could make a patch later, which introduces a
lower limit to the sysctl, so the user gets feedback, if he tries to adjust
the limit below the recommended 100 seconds, or something like that.

Maybe the others would like to comment on this?

Regards
 Damian

>> +
>> +	return (tcp_time_stamp - tcp_sk(sk)->retrans_stamp) >= limit;
>> +}
>> +
>>  static inline struct sk_buff *tcp_send_head(struct sock *sk)
>>  {
>>  	return sk->sk_send_head;
>> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
>> index a3ba494..2972d7b 100644
>> --- a/net/ipv4/tcp_timer.c
>> +++ b/net/ipv4/tcp_timer.c
>> @@ -137,13 +137,14 @@ static int tcp_write_timeout(struct sock *sk)
>>  {
>>  	struct inet_connection_sock *icsk = inet_csk(sk);
>>  	int retry_until;
>> +	bool do_reset;
>>  
>>  	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
>>  		if (icsk->icsk_retransmits)
>>  			dst_negative_advice(&sk->sk_dst_cache);
>>  		retry_until = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries;
>>  	} else {
>> -		if (icsk->icsk_retransmits >= sysctl_tcp_retries1) {
>> +		if (retransmits_timed_out(sk, sysctl_tcp_retries1)) {
>>  			/* Black hole detection */
>>  			tcp_mtu_probing(icsk, sk);
>>  
>> @@ -155,13 +156,15 @@ static int tcp_write_timeout(struct sock *sk)
>>  			const int alive = (icsk->icsk_rto < TCP_RTO_MAX);
>>  
>>  			retry_until = tcp_orphan_retries(sk, alive);
>> +			do_reset = alive ||
>> +				   !retransmits_timed_out(sk, retry_until);
>>  
>> -			if (tcp_out_of_resources(sk, alive || icsk->icsk_retransmits < retry_until))
>> +			if (tcp_out_of_resources(sk, do_reset))
>>  				return 1;
>>  		}
>>  	}
>>  
>> -	if (icsk->icsk_retransmits >= retry_until) {
>> +	if (retransmits_timed_out(sk, retry_until)) {
>>  		/* Has it gone just too far? */
>>  		tcp_write_err(sk);
>>  		return 1;
>> @@ -385,7 +388,7 @@ void tcp_retransmit_timer(struct sock *sk)
>>  out_reset_timer:
>>  	icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
>>  	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
>> -	if (icsk->icsk_retransmits > sysctl_tcp_retries1)
>> +	if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1))
>>  		__sk_dst_reset(sk);
>>  
>>  out:;

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