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