From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH 3/3] Revert Backoff [v3]: Calculate TCP's connection close threshold as a time value. Date: Mon, 31 Aug 2009 16:04:11 +0200 Message-ID: <4A9BD85B.9070708@gmail.com> References: <4A950B82.1070807@tvk.rwth-aachen.de> <4A9BD7AB.7060207@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Damian Lukowski , Netdev To: unlisted-recipients:; (no To-header on input) Return-path: Received: from gw1.cosmosbay.com ([212.99.114.194]:49778 "EHLO gw1.cosmosbay.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751302AbZHaOEK (ORCPT ); Mon, 31 Aug 2009 10:04:10 -0400 In-Reply-To: <4A9BD7AB.7060207@gmail.com> Sender: netdev-owner@vger.kernel.org List-ID: Eric Dumazet a =E9crit : > Damian Lukowski a =E9crit : >> RFC 1122 specifies two threshold values R1 and R2 for connection tim= eouts, >> which may represent a number of allowed retransmissions or a timeout= value. >> Currently linux uses sysctl_tcp_retries{1,2} to specify the threshol= ds >> 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 backof= fs 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 initia= l >> RTO of MIN_RTO and N unsuccessful, exponentially backed-off retransm= issions. >> >> 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 retran= smissions >> can occur than the value indicates. However, it yields a timeout whi= ch 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_RT= O, there >> should be no risk of a regression. >> >> Signed-off-by: Damian Lukowski >> --- >> 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) >> =20 >> +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 =3D ilog2(TCP_RTO_MAX/TCP_RTO_MIN); >> + >> + if (boundary <=3D K) >> + limit =3D ((2 << boundary) - 1) * TCP_RTO_MIN; >> + else >> + limit =3D ((2 << K) - 1) * TCP_RTO_MIN + >> + (boundary - K) * TCP_RTO_MAX; >=20 > Doing this computation might allow us to respect RFC 1122 here : > =20 > "The value of R2 SHOULD correspond to at least 100 seconds." >=20 > adding a third parameter to retransmits_timed_out(), min_limit, > being 100*HZ if sysctl_tcp_retries2 was used... >=20 > limit =3D min(min_limit, limit); >=20 Oh well, typical min/max error, sorry :) >=20 >> + >> + return (tcp_time_stamp - tcp_sk(sk)->retrans_stamp) >=3D 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 =3D inet_csk(sk); >> int retry_until; >> + bool do_reset; >> =20 >> if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) { >> if (icsk->icsk_retransmits) >> dst_negative_advice(&sk->sk_dst_cache); >> retry_until =3D icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries= ; >> } else { >> - if (icsk->icsk_retransmits >=3D sysctl_tcp_retries1) { >> + if (retransmits_timed_out(sk, sysctl_tcp_retries1)) { >> /* Black hole detection */ >> tcp_mtu_probing(icsk, sk); >> =20 >> @@ -155,13 +156,15 @@ static int tcp_write_timeout(struct sock *sk) >> const int alive =3D (icsk->icsk_rto < TCP_RTO_MAX); >> =20 >> retry_until =3D tcp_orphan_retries(sk, alive); >> + do_reset =3D alive || >> + !retransmits_timed_out(sk, retry_until); >> =20 >> - if (tcp_out_of_resources(sk, alive || icsk->icsk_retransmits < r= etry_until)) >> + if (tcp_out_of_resources(sk, do_reset)) >> return 1; >> } >> } >> =20 >> - if (icsk->icsk_retransmits >=3D 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 =3D min(icsk->icsk_rto << 1, TCP_RTO_MAX); >> inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, T= CP_RTO_MAX); >> - if (icsk->icsk_retransmits > sysctl_tcp_retries1) >> + if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1)) >> __sk_dst_reset(sk); >> =20 >> out:; >=20 > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >=20 >=20