From 6fa12c85031485dff38ce550c24f10da23b0adaa Mon Sep 17 00:00:00 2001 From: Damian Lukowski Date: Wed, 26 Aug 2009 00:16:34 +0000 Subject: Revert Backoff [v3]: Calculate TCP's connection close threshold as a time value. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Acked-by: Ilpo Järvinen Signed-off-by: David S. Miller --- include/net/tcp.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'include/net/tcp.h') diff --git a/include/net/tcp.h b/include/net/tcp.h index 54f212ce8aaf..e5319495f15e 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1252,6 +1252,24 @@ 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; + + 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; -- cgit v1.2.3