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Annotation of 42BSD/sys/netinet/tcp_timer.h, revision 1.1

1.1     ! root        1: /*     tcp_timer.h     6.1     83/07/29        */
        !             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  (  1*PR_SLOWHZ)         /* base roundtrip time */
        !            52: #define        TCPTV_KEEP      ( 45*PR_SLOWHZ)         /* keep alive - 45 secs */
        !            53: #define        TCPTV_PERSMIN   (  5*PR_SLOWHZ)         /* retransmit persistance */
        !            54: 
        !            55: #define        TCPTV_MAXIDLE   (  8*TCPTV_KEEP)        /* maximum allowable idle
        !            56:                                                   time before drop conn */
        !            57: 
        !            58: #define        TCPTV_MIN       (  1*PR_SLOWHZ)         /* minimum allowable value */
        !            59: #define        TCPTV_MAX       ( 30*PR_SLOWHZ)         /* maximum allowable value */
        !            60: 
        !            61: #define        TCP_LINGERTIME  120                     /* linger at most 2 minutes */
        !            62: 
        !            63: #define        TCP_MAXRXTSHIFT 10                      /* maximum retransmits */
        !            64: 
        !            65: #ifdef TCPTIMERS
        !            66: char *tcptimers[] =
        !            67:     { "REXMT", "PERSIST", "KEEP", "2MSL" };
        !            68: #endif
        !            69: 
        !            70: /*
        !            71:  * Retransmission smoothing constants.
        !            72:  * Smoothed round trip time is updated by
        !            73:  *    tp->t_srtt = (tcp_alpha * tp->t_srtt) + ((1 - tcp_alpha) * tp->t_rtt)
        !            74:  * each time a new value of tp->t_rtt is available.  The initial
        !            75:  * retransmit timeout is then based on
        !            76:  *    tp->t_timer[TCPT_REXMT] = tcp_beta * tp->t_srtt;
        !            77:  * limited, however to be at least TCPTV_REXMTLO and at most TCPTV_REXMTHI.
        !            78:  */
        !            79: float  tcp_alpha, tcp_beta;
        !            80: 
        !            81: /*
        !            82:  * Initial values of tcp_alpha and tcp_beta.
        !            83:  * These are conservative: averaging over a long
        !            84:  * period of time, and allowing for large individual deviations from
        !            85:  * tp->t_srtt.
        !            86:  */
        !            87: #define        TCP_ALPHA       0.9
        !            88: #define        TCP_BETA        2.0
        !            89: 
        !            90: /*
        !            91:  * Force a time value to be in a certain range.
        !            92:  */
        !            93: #define        TCPT_RANGESET(tv, value, tvmin, tvmax) { \
        !            94:        (tv) = (value); \
        !            95:        if ((tv) < (tvmin)) \
        !            96:                (tv) = (tvmin); \
        !            97:        if ((tv) > (tvmax)) \
        !            98:                (tv) = (tvmax); \
        !            99: }

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