The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_timer.h

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    1 /*
    2  * Copyright (c) 1982, 1986, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  *
    5  * Redistribution and use in source and binary forms, with or without
    6  * modification, are permitted provided that the following conditions
    7  * are met:
    8  * 1. Redistributions of source code must retain the above copyright
    9  *    notice, this list of conditions and the following disclaimer.
   10  * 2. Redistributions in binary form must reproduce the above copyright
   11  *    notice, this list of conditions and the following disclaimer in the
   12  *    documentation and/or other materials provided with the distribution.
   13  * 3. All advertising materials mentioning features or use of this software
   14  *    must display the following acknowledgement:
   15  *      This product includes software developed by the University of
   16  *      California, Berkeley and its contributors.
   17  * 4. Neither the name of the University nor the names of its contributors
   18  *    may be used to endorse or promote products derived from this software
   19  *    without specific prior written permission.
   20  *
   21  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   24  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   31  * SUCH DAMAGE.
   32  *
   33  *      @(#)tcp_timer.h 8.1 (Berkeley) 6/10/93
   34  * $FreeBSD: releng/5.0/sys/netinet/tcp_timer.h 101975 2002-08-16 09:07:59Z alfred $
   35  */
   36 
   37 #ifndef _NETINET_TCP_TIMER_H_
   38 #define _NETINET_TCP_TIMER_H_
   39 
   40 /*
   41  * The TCPT_REXMT timer is used to force retransmissions.
   42  * The TCP has the TCPT_REXMT timer set whenever segments
   43  * have been sent for which ACKs are expected but not yet
   44  * received.  If an ACK is received which advances tp->snd_una,
   45  * then the retransmit timer is cleared (if there are no more
   46  * outstanding segments) or reset to the base value (if there
   47  * are more ACKs expected).  Whenever the retransmit timer goes off,
   48  * we retransmit one unacknowledged segment, and do a backoff
   49  * on the retransmit timer.
   50  *
   51  * The TCPT_PERSIST timer is used to keep window size information
   52  * flowing even if the window goes shut.  If all previous transmissions
   53  * have been acknowledged (so that there are no retransmissions in progress),
   54  * and the window is too small to bother sending anything, then we start
   55  * the TCPT_PERSIST timer.  When it expires, if the window is nonzero,
   56  * we go to transmit state.  Otherwise, at intervals send a single byte
   57  * into the peer's window to force him to update our window information.
   58  * We do this at most as often as TCPT_PERSMIN time intervals,
   59  * but no more frequently than the current estimate of round-trip
   60  * packet time.  The TCPT_PERSIST timer is cleared whenever we receive
   61  * a window update from the peer.
   62  *
   63  * The TCPT_KEEP timer is used to keep connections alive.  If an
   64  * connection is idle (no segments received) for TCPTV_KEEP_INIT amount of time,
   65  * but not yet established, then we drop the connection.  Once the connection
   66  * is established, if the connection is idle for TCPTV_KEEP_IDLE time
   67  * (and keepalives have been enabled on the socket), we begin to probe
   68  * the connection.  We force the peer to send us a segment by sending:
   69  *      <SEQ=SND.UNA-1><ACK=RCV.NXT><CTL=ACK>
   70  * This segment is (deliberately) outside the window, and should elicit
   71  * an ack segment in response from the peer.  If, despite the TCPT_KEEP
   72  * initiated segments we cannot elicit a response from a peer in TCPT_MAXIDLE
   73  * amount of time probing, then we drop the connection.
   74  */
   75 
   76 /*
   77  * Time constants.
   78  */
   79 #define TCPTV_MSL       ( 30*hz)                /* max seg lifetime (hah!) */
   80 #define TCPTV_SRTTBASE  0                       /* base roundtrip time;
   81                                                    if 0, no idea yet */
   82 #define TCPTV_RTOBASE   (  3*hz)                /* assumed RTO if no info */
   83 #define TCPTV_SRTTDFLT  (  3*hz)                /* assumed RTT if no info */
   84 
   85 #define TCPTV_PERSMIN   (  5*hz)                /* retransmit persistence */
   86 #define TCPTV_PERSMAX   ( 60*hz)                /* maximum persist interval */
   87 
   88 #define TCPTV_KEEP_INIT ( 75*hz)                /* initial connect keepalive */
   89 #define TCPTV_KEEP_IDLE (120*60*hz)             /* dflt time before probing */
   90 #define TCPTV_KEEPINTVL ( 75*hz)                /* default probe interval */
   91 #define TCPTV_KEEPCNT   8                       /* max probes before drop */
   92 
   93 /*
   94  * Minimum retransmit timer is 3 ticks, for algorithmic stability.
   95  * TCPT_RANGESET() will add another TCPTV_CPU_VAR to deal with
   96  * the expected worst-case processing variances by the kernels
   97  * representing the end points.  Such variances do not always show
   98  * up in the srtt because the timestamp is often calculated at
   99  * the interface rather then at the TCP layer.  This value is
  100  * typically 50ms.  However, it is also possible that delayed
  101  * acks (typically 100ms) could create issues so we set the slop
  102  * to 200ms to try to cover it.  Note that, properly speaking,
  103  * delayed-acks should not create a major issue for interactive
  104  * environments which 'P'ush the last segment, at least as 
  105  * long as implementations do the required 'at least one ack
  106  * for every two packets' for the non-interactive streaming case.
  107  * (maybe the RTO calculation should use 2*RTT instead of RTT
  108  * to handle the ack-every-other-packet case).
  109  *
  110  * The prior minimum of 1*hz (1 second) badly breaks throughput on any
  111  * networks faster then a modem that has minor (e.g. 1%) packet loss.
  112  */
  113 #define TCPTV_MIN       ( 3 )                   /* minimum allowable value */
  114 #define TCPTV_CPU_VAR   ( hz/5 )                /* cpu variance allowed (200ms) */
  115 #define TCPTV_REXMTMAX  ( 64*hz)                /* max allowable REXMT value */
  116 
  117 #define TCPTV_TWTRUNC   8                       /* RTO factor to truncate TW */
  118 
  119 #define TCP_LINGERTIME  120                     /* linger at most 2 minutes */
  120 
  121 #define TCP_MAXRXTSHIFT 12                      /* maximum retransmits */
  122 
  123 #define TCPTV_DELACK    (hz / PR_FASTHZ / 2)    /* 100ms timeout */
  124 
  125 #ifdef  TCPTIMERS
  126 static const char *tcptimers[] =
  127     { "REXMT", "PERSIST", "KEEP", "2MSL" };
  128 #endif
  129 
  130 /*
  131  * Force a time value to be in a certain range.
  132  */
  133 #define TCPT_RANGESET(tv, value, tvmin, tvmax) do { \
  134         (tv) = (value) + tcp_rexmit_slop; \
  135         if ((u_long)(tv) < (u_long)(tvmin)) \
  136                 (tv) = (tvmin); \
  137         else if ((u_long)(tv) > (u_long)(tvmax)) \
  138                 (tv) = (tvmax); \
  139 } while(0)
  140 
  141 #ifdef _KERNEL
  142 extern int tcp_keepinit;                /* time to establish connection */
  143 extern int tcp_keepidle;                /* time before keepalive probes begin */
  144 extern int tcp_keepintvl;               /* time between keepalive probes */
  145 extern int tcp_maxidle;                 /* time to drop after starting probes */
  146 extern int tcp_delacktime;              /* time before sending a delayed ACK */
  147 extern int tcp_maxpersistidle;
  148 extern int tcp_rexmit_min;
  149 extern int tcp_rexmit_slop;
  150 extern int tcp_msl;
  151 extern int tcp_ttl;                     /* time to live for TCP segs */
  152 extern int tcp_backoff[];
  153 
  154 void    tcp_timer_2msl(void *xtp);
  155 void    tcp_timer_keep(void *xtp);
  156 void    tcp_timer_persist(void *xtp);
  157 void    tcp_timer_rexmt(void *xtp);
  158 void    tcp_timer_delack(void *xtp);
  159 
  160 #endif /* _KERNEL */
  161 
  162 #endif /* !_NETINET_TCP_TIMER_H_ */

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