1 /* tcp_timer.h 4.7 82/02/03 */ 2 3 /* 4 * Definitions of the TCP timers. These timers are counted 5 * down PR_SLOWHZ times a second. 6 */ 7 #define TCPT_NTIMERS 5 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 #ifdef TCPTRUEOOB 14 #define TCPT_OOBREXMT 4 /* out-of-band rexmt */ 15 #endif 16 17 /* 18 * The TCPT_REXMT timer is used to force retransmissions. 19 * The TCP has the TCPT_REXMT timer set whenever segments 20 * have been sent for which ACKs are expected but not yet 21 * received. If an ACK is received which advances tp->snd_una, 22 * then the retransmit timer is cleared (if there are no more 23 * outstanding segments) or reset to the base value (if there 24 * are more ACKs expected). Whenever the retransmit timer goes off, 25 * we retransmit all unacknowledged segments, and do an exponential 26 * backoff on the retransmit timer. 27 * 28 * The TCPT_PERSIST timer is used to keep window size information 29 * flowing even if the window goes shut. If an output is attempted when there 30 * is data ready to transmit, but nothing gets sent because the window 31 * is shut, then we start the TCPT_PERSIST timer, and at intervals 32 * send a single byte into the peers window to force him to update 33 * our window information. We do this at most as often as TCPT_PERSMIN 34 * time intervals, but no more frequently than the current estimate of 35 * round-trip packet time. The TCPT_PERSIST timer is cleared whenever 36 * we receive a window update from the peer. 37 * 38 * The TCPT_KEEP timer is used to keep connections alive. If an 39 * connection is idle (no segments received) for TCPTV_KEEP amount of time, 40 * but not yet established, then we drop the connection. If the connection 41 * is established, then we force the peer to send us a segment by sending: 42 * <SEQ=SND.UNA-1><ACK=RCV.NXT><CTL=ACK> 43 * This segment is (deliberately) outside the window, and should elicit 44 * an ack segment in response from the peer. If, despite the TCPT_KEEP 45 * initiated segments we cannot elicit a response from a peer in TCPT_MAXIDLE 46 * amount of time, then we drop the connection. 47 * 48 * The OOBREXMT timer is to force retransmissions of out-of-band indications. 49 * Because out-of-band data is considered critical, it does not exponential 50 * backoff, but runs at a multiple of smoothed round trip time until acked. 51 */ 52 53 #define TCP_TTL 15 /* time to live for TCP segs */ 54 /* 55 * Time constants. 56 */ 57 #define TCPTV_MSL ( 30*PR_SLOWHZ) /* max seg lifetime */ 58 #define TCPTV_SRTTBASE ( 1*PR_SLOWHZ) /* base roundtrip time */ 59 #define TCPTV_KEEP ( 60*PR_SLOWHZ) /* keep alive - 1 min */ 60 #define TCPTV_PERSMIN ( 5*PR_SLOWHZ) /* retransmit persistance */ 61 62 #define TCPTV_MAXIDLE ( 4*TCPTV_KEEP) /* maximum allowable idle 63 time before drop conn */ 64 65 #define TCPTV_MIN ( 1*PR_SLOWHZ) /* minimum allowable value */ 66 #define TCPTV_MAX ( 30*PR_SLOWHZ) /* maximum allowable value */ 67 68 #define TCP_LINGERTIME 120 /* linger at most 2 minutes */ 69 70 #define TCP_MAXRXTSHIFT 10 /* maximum retransmits */ 71 72 #ifdef TCPTIMERS 73 char *tcptimers[] = 74 { "REXMT", "PERSIST", "KEEP", "2MSL", "OOBREXMT" }; 75 #endif 76 77 /* 78 * Retransmission smoothing constants. 79 * Smoothed round trip time is updated by 80 * tp->t_srtt = (tcp_alpha * tp->t_srtt) + ((1 - tcp_alpha) * tp->t_rtt) 81 * each time a new value of tp->t_rtt is available. The initial 82 * retransmit timeout is then based on 83 * tp->t_timer[TCPT_REXMT] = tcp_beta * tp->t_srtt; 84 * limited, however to be at least TCPTV_REXMTLO and at most TCPTV_REXMTHI. 85 */ 86 float tcp_alpha, tcp_beta; 87 88 /* 89 * Initial values of tcp_alpha and tcp_beta. 90 * These are conservative: averaging over a long 91 * period of time, and allowing for large individual deviations from 92 * tp->t_srtt. 93 */ 94 #define TCP_ALPHA 0.9 95 #define TCP_BETA 2.0 96 97 /* 98 * Force a time value to be in a certain range. 99 */ 100 #define TCPT_RANGESET(tv, value, tvmin, tvmax) { \ 101 (tv) = (value); \ 102 if ((tv) < (tvmin)) \ 103 (tv) = (tvmin); \ 104 if ((tv) > (tvmax)) \ 105 (tv) = (tvmax); \ 106 } 107