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: }

Defined variables

tcptimers defined in line 77; used 3 times

Defined macros

TCPTV_MSL defined in line 58; used 4 times
TCPTV_SRTTBASE defined in line 59; used 1 times
TCP_ALPHA defined in line 98; used 1 times
TCP_BETA defined in line 99; used 1 times
TCP_LINGERTIME defined in line 72; used 1 times

Usage of this include

Last modified: 1986-06-05
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