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Annotation of 43BSD/sys/netinet/tcp_timer.h, revision 1.1.1.1

1.1       root        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 7.1 (Berkeley) 6/5/86
                      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              /* time to live for TCP segs */
                     55: /*
                     56:  * Time constants.
                     57:  */
                     58: #define        TCPTV_MSL       ( 15*PR_SLOWHZ)         /* max seg lifetime */
                     59: #define        TCPTV_SRTTBASE  0                       /* base roundtrip time;
                     60:                                                   if 0, no idea yet */
                     61: #define        TCPTV_SRTTDFLT  (  3*PR_SLOWHZ)         /* assumed RTT if no info */
                     62: 
                     63: #define        TCPTV_KEEP      ( 45*PR_SLOWHZ)         /* keep alive - 45 secs */
                     64: #define        TCPTV_PERSMIN   (  5*PR_SLOWHZ)         /* retransmit persistance */
                     65: 
                     66: #define        TCPTV_MAXIDLE   (  8*TCPTV_KEEP)        /* maximum allowable idle
                     67:                                                   time before drop conn */
                     68: 
                     69: #define        TCPTV_MIN       (  1*PR_SLOWHZ)         /* minimum allowable value */
                     70: #define        TCPTV_MAX       ( 30*PR_SLOWHZ)         /* maximum allowable value */
                     71: 
                     72: #define        TCP_LINGERTIME  120                     /* linger at most 2 minutes */
                     73: 
                     74: #define        TCP_MAXRXTSHIFT 12                      /* maximum retransmits */
                     75: 
                     76: #ifdef TCPTIMERS
                     77: char *tcptimers[] =
                     78:     { "REXMT", "PERSIST", "KEEP", "2MSL" };
                     79: #endif
                     80: 
                     81: /*
                     82:  * Retransmission smoothing constants.
                     83:  * Smoothed round trip time is updated by
                     84:  *    tp->t_srtt = (tcp_alpha * tp->t_srtt) + ((1 - tcp_alpha) * tp->t_rtt)
                     85:  * each time a new value of tp->t_rtt is available.  The initial
                     86:  * retransmit timeout is then based on
                     87:  *    tp->t_timer[TCPT_REXMT] = tcp_beta * tp->t_srtt;
                     88:  * limited, however to be at least TCPTV_MIN and at most TCPTV_MAX.
                     89:  */
                     90: float  tcp_alpha, tcp_beta;
                     91: 
                     92: /*
                     93:  * Initial values of tcp_alpha and tcp_beta.
                     94:  * These are conservative: averaging over a long
                     95:  * period of time, and allowing for large individual deviations from
                     96:  * tp->t_srtt.
                     97:  */
                     98: #define        TCP_ALPHA       0.9
                     99: #define        TCP_BETA        2.0
                    100: 
                    101: /*
                    102:  * Force a time value to be in a certain range.
                    103:  */
                    104: #define        TCPT_RANGESET(tv, value, tvmin, tvmax) { \
                    105:        (tv) = (value); \
                    106:        if ((tv) < (tvmin)) \
                    107:                (tv) = (tvmin); \
                    108:        if ((tv) > (tvmax)) \
                    109:                (tv) = (tvmax); \
                    110: }

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