2 * util/rtt.c - UDP round trip time estimator for resend timeouts.
4 * Copyright (c) 2007, NLnet Labs. All rights reserved.
6 * This software is open source.
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39 * This file contains a data type and functions to help estimate good
40 * round trip times for UDP resend timeout values.
45 /** calculate RTO from rtt information */
47 calc_rto(const struct rtt_info* rtt)
49 /* From Stevens, Unix Network Programming, Vol1, 3rd ed., p.598 */
50 int rto = rtt->srtt + 4*rtt->rttvar;
51 if(rto < RTT_MIN_TIMEOUT)
52 rto = RTT_MIN_TIMEOUT;
53 if(rto > RTT_MAX_TIMEOUT)
54 rto = RTT_MAX_TIMEOUT;
59 rtt_init(struct rtt_info* rtt)
63 rtt->rto = calc_rto(rtt);
64 /* default value from the book is 0 + 4*0.75 = 3 seconds */
65 /* first RTO is 0 + 4*0.094 = 0.376 seconds */
69 rtt_timeout(const struct rtt_info* rtt)
75 rtt_unclamped(const struct rtt_info* rtt)
77 if(calc_rto(rtt) != rtt->rto) {
78 /* timeout fallback has happened */
81 /* return unclamped value */
82 return rtt->srtt + 4*rtt->rttvar;
86 rtt_update(struct rtt_info* rtt, int ms)
88 int delta = ms - rtt->srtt;
89 rtt->srtt += delta / 8; /* g = 1/8 */
91 delta = -delta; /* |delta| */
92 rtt->rttvar += (delta - rtt->rttvar) / 4; /* h = 1/4 */
93 rtt->rto = calc_rto(rtt);
97 rtt_lost(struct rtt_info* rtt, int orig)
99 /* exponential backoff */
101 /* if a query succeeded and put down the rto meanwhile, ignore this */
105 /* the original rto is doubled, not the current one to make sure
106 * that the values in the cache are not increased by lots of
107 * queries simultaneously as they time out at the same time */
109 if(rtt->rto <= orig) {
111 if(rtt->rto > RTT_MAX_TIMEOUT)
112 rtt->rto = RTT_MAX_TIMEOUT;
116 int rtt_notimeout(const struct rtt_info* rtt)
118 return calc_rto(rtt);