* Copyright (c) 1982 Regents of the University of California.
* All rights reserved. The Berkeley software License Agreement
* specifies the terms and conditions for redistribution.
* @(#)tcp_timer.h 6.5 (Berkeley) %G%
* Definitions of the TCP timers. These timers are counted
* down PR_SLOWHZ times a second.
#define TCPT_REXMT 0 /* retransmit */
#define TCPT_PERSIST 1 /* retransmit persistance */
#define TCPT_KEEP 2 /* keep alive */
#define TCPT_2MSL 3 /* 2*msl quiet time timer */
* The TCPT_REXMT timer is used to force retransmissions.
* The TCP has the TCPT_REXMT timer set whenever segments
* have been sent for which ACKs are expected but not yet
* received. If an ACK is received which advances tp->snd_una,
* then the retransmit timer is cleared (if there are no more
* outstanding segments) or reset to the base value (if there
* are more ACKs expected). Whenever the retransmit timer goes off,
* we retransmit one unacknowledged segment, and do a backoff
* on the retransmit timer.
* The TCPT_PERSIST timer is used to keep window size information
* flowing even if the window goes shut. If all previous transmissions
* have been acknowledged (so that there are no retransmissions in progress),
* and the window is too small to bother sending anything, then we start
* the TCPT_PERSIST timer. When it expires, if the window is nonzero,
* we go to transmit state. Otherwise, at intervals send a single byte
* into the peer's window to force him to update our window information.
* We do this at most as often as TCPT_PERSMIN time intervals,
* but no more frequently than the current estimate of round-trip
* packet time. The TCPT_PERSIST timer is cleared whenever we receive
* a window update from the peer.
* The TCPT_KEEP timer is used to keep connections alive. If an
* connection is idle (no segments received) for TCPTV_KEEP amount of time,
* but not yet established, then we drop the connection. If the connection
* is established, then we force the peer to send us a segment by sending:
* <SEQ=SND.UNA-1><ACK=RCV.NXT><CTL=ACK>
* This segment is (deliberately) outside the window, and should elicit
* an ack segment in response from the peer. If, despite the TCPT_KEEP
* initiated segments we cannot elicit a response from a peer in TCPT_MAXIDLE
* amount of time, then we drop the connection.
#define TCP_TTL 15 /* time to live for TCP segs */
#define TCPTV_MSL ( 15*PR_SLOWHZ) /* max seg lifetime */
#define TCPTV_SRTTBASE 0 /* base roundtrip time;
#define TCPTV_SRTTDFLT ( 5*PR_SLOWHZ) /* assumed RTT if no info */
#define TCPTV_KEEP ( 45*PR_SLOWHZ) /* keep alive - 45 secs */
#define TCPTV_PERSMIN ( 5*PR_SLOWHZ) /* retransmit persistance */
#define TCPTV_MAXIDLE ( 8*TCPTV_KEEP) /* maximum allowable idle
#define TCPTV_MIN ( 1*PR_SLOWHZ) /* minimum allowable value */
#define TCPTV_MAX ( 30*PR_SLOWHZ) /* maximum allowable value */
#define TCP_LINGERTIME 120 /* linger at most 2 minutes */
#define TCP_MAXRXTSHIFT 12 /* maximum retransmits */
{ "REXMT", "PERSIST", "KEEP", "2MSL" };
* Retransmission smoothing constants.
* Smoothed round trip time is updated by
* tp->t_srtt = (tcp_alpha * tp->t_srtt) + ((1 - tcp_alpha) * tp->t_rtt)
* each time a new value of tp->t_rtt is available. The initial
* retransmit timeout is then based on
* tp->t_timer[TCPT_REXMT] = tcp_beta * tp->t_srtt;
* limited, however to be at least TCPTV_MIN and at most TCPTV_MAX.
float tcp_alpha
, tcp_beta
;
* Initial values of tcp_alpha and tcp_beta.
* These are conservative: averaging over a long
* period of time, and allowing for large individual deviations from
* Force a time value to be in a certain range.
#define TCPT_RANGESET(tv, value, tvmin, tvmax) { \
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