1 /* 2 * Copyright (c) 1982, 1986 Regents of the University of California. 3 * All rights reserved. The Berkeley software License Agreement 4 * specifies the terms and conditions for redistribution. 5 * 6 * @(#)tcp_timer.h 7.2 (Berkeley) 06/04/87 7 */ 8 9 /* 10 * Definitions of the TCP timers. These timers are counted 11 * down PR_SLOWHZ times a second. 12 */ 13 #define TCPT_NTIMERS 4 14 15 #define TCPT_REXMT 0 /* retransmit */ 16 #define TCPT_PERSIST 1 /* retransmit persistance */ 17 #define TCPT_KEEP 2 /* keep alive */ 18 #define TCPT_2MSL 3 /* 2*msl quiet time timer */ 19 20 /* 21 * The TCPT_REXMT timer is used to force retransmissions. 22 * The TCP has the TCPT_REXMT timer set whenever segments 23 * have been sent for which ACKs are expected but not yet 24 * received. If an ACK is received which advances tp->snd_una, 25 * then the retransmit timer is cleared (if there are no more 26 * outstanding segments) or reset to the base value (if there 27 * are more ACKs expected). Whenever the retransmit timer goes off, 28 * we retransmit one unacknowledged segment, and do a backoff 29 * on the retransmit timer. 30 * 31 * The TCPT_PERSIST timer is used to keep window size information 32 * flowing even if the window goes shut. If all previous transmissions 33 * have been acknowledged (so that there are no retransmissions in progress), 34 * and the window is too small to bother sending anything, then we start 35 * the TCPT_PERSIST timer. When it expires, if the window is nonzero, 36 * we go to transmit state. Otherwise, at intervals send a single byte 37 * into the peer's window to force him to update our window information. 38 * We do this at most as often as TCPT_PERSMIN time intervals, 39 * but no more frequently than the current estimate of round-trip 40 * packet time. The TCPT_PERSIST timer is cleared whenever we receive 41 * a window update from the peer. 42 * 43 * The TCPT_KEEP timer is used to keep connections alive. If an 44 * connection is idle (no segments received) for TCPTV_KEEP amount of time, 45 * but not yet established, then we drop the connection. If the connection 46 * is established, then we force the peer to send us a segment by sending: 47 * <SEQ=SND.UNA-1><ACK=RCV.NXT><CTL=ACK> 48 * This segment is (deliberately) outside the window, and should elicit 49 * an ack segment in response from the peer. If, despite the TCPT_KEEP 50 * initiated segments we cannot elicit a response from a peer in TCPT_MAXIDLE 51 * amount of time, then we drop the connection. 52 */ 53 54 #define TCP_TTL 30 /* default time to live for TCP segs */ 55 int tcp_ttl; /* time to live for TCP segs */ 56 /* 57 * Time constants. 58 */ 59 #define TCPTV_MSL ( 15*PR_SLOWHZ) /* max seg lifetime */ 60 #define TCPTV_SRTTBASE 0 /* base roundtrip time; 61 if 0, no idea yet */ 62 #define TCPTV_SRTTDFLT ( 3*PR_SLOWHZ) /* assumed RTT if no info */ 63 64 #define TCPTV_KEEP ( 45*PR_SLOWHZ) /* keep alive - 45 secs */ 65 #define TCPTV_PERSMIN ( 5*PR_SLOWHZ) /* retransmit persistance */ 66 67 #define TCPTV_MAXIDLE ( 8*TCPTV_KEEP) /* maximum allowable idle 68 time before drop conn */ 69 70 #define TCPTV_MIN ( 1*PR_SLOWHZ) /* minimum allowable value */ 71 #define TCPTV_MAX ( 30*PR_SLOWHZ) /* maximum allowable value */ 72 73 #define TCP_LINGERTIME 120 /* linger at most 2 minutes */ 74 75 #define TCP_MAXRXTSHIFT 12 /* maximum retransmits */ 76 77 #ifdef TCPTIMERS 78 char *tcptimers[] = 79 { "REXMT", "PERSIST", "KEEP", "2MSL" }; 80 #endif 81 82 /* 83 * Retransmission smoothing constants. 84 * Smoothed round trip time is updated by 85 * tp->t_srtt = (tcp_alpha * tp->t_srtt) + ((1 - tcp_alpha) * tp->t_rtt) 86 * each time a new value of tp->t_rtt is available. The initial 87 * retransmit timeout is then based on 88 * tp->t_timer[TCPT_REXMT] = tcp_beta * tp->t_srtt; 89 * limited, however to be at least TCPTV_MIN and at most TCPTV_MAX. 90 */ 91 float tcp_alpha, tcp_beta; 92 93 /* 94 * Initial values of tcp_alpha and tcp_beta. 95 * These are conservative: averaging over a long 96 * period of time, and allowing for large individual deviations from 97 * tp->t_srtt. 98 */ 99 #define TCP_ALPHA 0.9 100 #define TCP_BETA 2.0 101 102 /* 103 * Force a time value to be in a certain range. 104 */ 105 #define TCPT_RANGESET(tv, value, tvmin, tvmax) { \ 106 (tv) = (value); \ 107 if ((tv) < (tvmin)) \ 108 (tv) = (tvmin); \ 109 if ((tv) > (tvmax)) \ 110 (tv) = (tvmax); \ 111 } 112