1 /* tcp_timer.h 6.2 85/04/17 */ 2 3 /* 4 * Definitions of the TCP timers. These timers are counted 5 * down PR_SLOWHZ times a second. 6 */ 7 #define TCPT_NTIMERS 4 8 9 #define TCPT_REXMT 0 /* retransmit */ 10 #define TCPT_PERSIST 1 /* retransmit persistance */ 11 #define TCPT_KEEP 2 /* keep alive */ 12 #define TCPT_2MSL 3 /* 2*msl quiet time timer */ 13 14 /* 15 * The TCPT_REXMT timer is used to force retransmissions. 16 * The TCP has the TCPT_REXMT timer set whenever segments 17 * have been sent for which ACKs are expected but not yet 18 * received. If an ACK is received which advances tp->snd_una, 19 * then the retransmit timer is cleared (if there are no more 20 * outstanding segments) or reset to the base value (if there 21 * are more ACKs expected). Whenever the retransmit timer goes off, 22 * we retransmit all unacknowledged segments, and do an exponential 23 * backoff on the retransmit timer. 24 * 25 * The TCPT_PERSIST timer is used to keep window size information 26 * flowing even if the window goes shut. If all previous transmissions 27 * have been acknowledged (so that there are no retransmissions in progress), 28 * and the window is shut, then we start the TCPT_PERSIST timer, and at 29 * intervals send a single byte into the peers window to force him to update 30 * our window information. We do this at most as often as TCPT_PERSMIN 31 * time intervals, but no more frequently than the current estimate of 32 * round-trip packet time. The TCPT_PERSIST timer is cleared whenever 33 * we receive a window update from the peer. 34 * 35 * The TCPT_KEEP timer is used to keep connections alive. If an 36 * connection is idle (no segments received) for TCPTV_KEEP amount of time, 37 * but not yet established, then we drop the connection. If the connection 38 * is established, then we force the peer to send us a segment by sending: 39 * <SEQ=SND.UNA-1><ACK=RCV.NXT><CTL=ACK> 40 * This segment is (deliberately) outside the window, and should elicit 41 * an ack segment in response from the peer. If, despite the TCPT_KEEP 42 * initiated segments we cannot elicit a response from a peer in TCPT_MAXIDLE 43 * amount of time, then we drop the connection. 44 */ 45 46 #define TCP_TTL 15 /* time to live for TCP segs */ 47 /* 48 * Time constants. 49 */ 50 #define TCPTV_MSL ( 30*PR_SLOWHZ) /* max seg lifetime */ 51 #define TCPTV_SRTTBASE 0 /* base roundtrip time; 52 if 0, no idea yet */ 53 #define TCPTV_KEEP ( 45*PR_SLOWHZ) /* keep alive - 45 secs */ 54 #define TCPTV_PERSMIN ( 5*PR_SLOWHZ) /* retransmit persistance */ 55 56 #define TCPTV_MAXIDLE ( 8*TCPTV_KEEP) /* maximum allowable idle 57 time before drop conn */ 58 59 #define TCPTV_MIN ( 1*PR_SLOWHZ) /* minimum allowable value */ 60 #define TCPTV_MAX ( 30*PR_SLOWHZ) /* maximum allowable value */ 61 62 #define TCP_LINGERTIME 120 /* linger at most 2 minutes */ 63 64 #define TCP_MAXRXTSHIFT 10 /* maximum retransmits */ 65 66 #ifdef TCPTIMERS 67 char *tcptimers[] = 68 { "REXMT", "PERSIST", "KEEP", "2MSL" }; 69 #endif 70 71 /* 72 * Retransmission smoothing constants. 73 * Smoothed round trip time is updated by 74 * tp->t_srtt = (tcp_alpha * tp->t_srtt) + ((1 - tcp_alpha) * tp->t_rtt) 75 * each time a new value of tp->t_rtt is available. The initial 76 * retransmit timeout is then based on 77 * tp->t_timer[TCPT_REXMT] = tcp_beta * tp->t_srtt; 78 * limited, however to be at least TCPTV_REXMTLO and at most TCPTV_REXMTHI. 79 */ 80 float tcp_alpha, tcp_beta; 81 82 /* 83 * Initial values of tcp_alpha and tcp_beta. 84 * These are conservative: averaging over a long 85 * period of time, and allowing for large individual deviations from 86 * tp->t_srtt. 87 */ 88 #define TCP_ALPHA 0.9 89 #define TCP_BETA 2.0 90 91 /* 92 * Force a time value to be in a certain range. 93 */ 94 #define TCPT_RANGESET(tv, value, tvmin, tvmax) { \ 95 (tv) = (value); \ 96 if ((tv) < (tvmin)) \ 97 (tv) = (tvmin); \ 98 if ((tv) > (tvmax)) \ 99 (tv) = (tvmax); \ 100 } 101