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Date: Mon, 31 Aug 2009 16:04:11 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: unlisted-recipients:; (no To-header on input)
CC: Damian Lukowski <damian@....rwth-aachen.de>,
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 a écrit :
> 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);
>
Oh well, typical min/max error, sorry :)
>
>> +
>> + 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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