From 8f61059a96c2a29c1cc5a39dfe23d06ef5b4b065 Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Mon, 30 Jun 2014 13:52:08 +0200 Subject: net: sctp: improve timer slack calculation for transport HBs RFC4960, section 8.3 says: On an idle destination address that is allowed to heartbeat, it is recommended that a HEARTBEAT chunk is sent once per RTO of that destination address plus the protocol parameter 'HB.interval', with jittering of +/- 50% of the RTO value, and exponential backoff of the RTO if the previous HEARTBEAT is unanswered. Currently, we calculate jitter via sctp_jitter() function first, and then add its result to the current RTO for the new timeout: TMO = RTO + (RAND() % RTO) - (RTO / 2) `------------------------^-=> sctp_jitter() Instead, we can just simplify all this by directly calculating: TMO = (RTO / 2) + (RAND() % RTO) With the help of prandom_u32_max(), we don't need to open code our own global PRNG, but can instead just make use of the per CPU implementation of prandom with better quality numbers. Also, we can now spare us the conditional for divide by zero check since no div or mod operation needs to be used. Note that prandom_u32_max() won't emit the same result as a mod operation, but we really don't care here as we only want to have a random number scaled into RTO interval. Note, exponential RTO backoff is handeled elsewhere, namely in sctp_do_8_2_transport_strike(). Signed-off-by: Daniel Borkmann Signed-off-by: David S. Miller --- include/net/sctp/sctp.h | 21 --------------------- 1 file changed, 21 deletions(-) (limited to 'include/net/sctp/sctp.h') diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h index 8e4de46..c2035c9 100644 --- a/include/net/sctp/sctp.h +++ b/include/net/sctp/sctp.h @@ -388,27 +388,6 @@ static inline int sctp_list_single_entry(struct list_head *head) return (head->next != head) && (head->next == head->prev); } -/* Generate a random jitter in the range of -50% ~ +50% of input RTO. */ -static inline __s32 sctp_jitter(__u32 rto) -{ - static __u32 sctp_rand; - __s32 ret; - - /* Avoid divide by zero. */ - if (!rto) - rto = 1; - - sctp_rand += jiffies; - sctp_rand ^= (sctp_rand << 12); - sctp_rand ^= (sctp_rand >> 20); - - /* Choose random number from 0 to rto, then move to -50% ~ +50% - * of rto. - */ - ret = sctp_rand % rto - (rto >> 1); - return ret; -} - /* Break down data chunks at this point. */ static inline int sctp_frag_point(const struct sctp_association *asoc, int pmtu) { -- cgit v1.1