1 .\" Copyright (c) 1983, 1991, 1993
2 .\" The Regents of the University of California.
3 .\" Copyright (c) 2010-2011 The FreeBSD Foundation
4 .\" All rights reserved.
6 .\" Portions of this documentation were written at the Centre for Advanced
7 .\" Internet Architectures, Swinburne University of Technology, Melbourne,
8 .\" Australia by David Hayes under sponsorship from the FreeBSD Foundation.
10 .\" Redistribution and use in source and binary forms, with or without
11 .\" modification, are permitted provided that the following conditions
13 .\" 1. Redistributions of source code must retain the above copyright
14 .\" notice, this list of conditions and the following disclaimer.
15 .\" 2. Redistributions in binary form must reproduce the above copyright
16 .\" notice, this list of conditions and the following disclaimer in the
17 .\" documentation and/or other materials provided with the distribution.
18 .\" 3. Neither the name of the University nor the names of its contributors
19 .\" may be used to endorse or promote products derived from this software
20 .\" without specific prior written permission.
22 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
23 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
24 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
25 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
26 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
27 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
28 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
29 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
30 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
31 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34 .\" From: @(#)tcp.4 8.1 (Berkeley) 6/5/93
42 .Nd Internet Transmission Control Protocol
49 .Fn socket AF_INET SOCK_STREAM 0
53 protocol provides reliable, flow-controlled, two-way
55 It is a byte-stream protocol used to
61 Internet address format and, in addition, provides a per-host
63 .Dq "port addresses" .
64 Thus, each address is composed
65 of an Internet address specifying the host and network,
68 port on the host identifying the peer entity.
76 Active sockets initiate connections to passive
80 sockets are created active; to create a
83 system call must be used
84 after binding the socket with the
87 Only passive sockets may use the
89 call to accept incoming connections.
90 Only active sockets may use the
92 call to initiate connections.
96 their location to match
97 incoming connection requests from multiple networks.
98 This technique, termed
99 .Dq "wildcard addressing" ,
101 server to provide service to clients on multiple networks.
102 To create a socket which listens on all networks, the Internet
108 port may still be specified
109 at this time; if the port is not specified, the system will assign one.
110 Once a connection has been established, the socket's address is
111 fixed by the peer entity's location.
112 The address assigned to the
113 socket is the address associated with the network interface
114 through which packets are being transmitted and received.
115 Normally, this address corresponds to the peer entity's network.
118 supports a number of socket options which can be set with
122 .Bl -tag -width ".Dv TCP_FUNCTION_BLK"
124 Information about a socket's underlying TCP session may be retrieved
125 by passing the read-only option
129 It accepts a single argument: a pointer to an instance of
130 .Vt "struct tcp_info" .
132 This API is subject to change; consult the source to determine
133 which fields are currently filled out by this option.
135 specific additions include
139 bandwidth-controlled window space.
141 Set or query congestion control algorithm specific parameters.
145 .It Dv TCP_CONGESTION
146 Select or query the congestion control algorithm that TCP will use for the
151 .It Dv TCP_FUNCTION_BLK
152 Select or query the set of functions that TCP will use for this connection.
153 This allows a user to select an alternate TCP stack.
154 The alternate TCP stack must already be loaded in the kernel.
155 To list the available TCP stacks, see
156 .Va functions_available
159 section further down.
160 To list the default TCP stack, see
161 .Va functions_default
168 option accepts a per-socket timeout argument of
170 in seconds, for new, non-established
173 For the global default in milliseconds see
177 section further down.
181 option accepts an argument of
183 for the amount of time, in seconds, that the connection must be idle
184 before keepalive probes (if enabled) are sent for the connection of this
186 If set on a listening socket, the value is inherited by the newly created
189 For the global default in milliseconds see
193 section further down.
197 option accepts an argument of
199 to set the per-socket interval, in seconds, between keepalive probes sent
201 If set on a listening socket, the value is inherited by the newly created
204 For the global default in milliseconds see
208 section further down.
212 option accepts an argument of
214 and allows a per-socket tuning of the number of probes sent, with no response,
215 before the connection will be dropped.
216 If set on a listening socket, the value is inherited by the newly created
219 For the global default see the
223 section further down.
225 Under most circumstances,
227 sends data when it is presented;
228 when outstanding data has not yet been acknowledged, it gathers
229 small amounts of output to be sent in a single packet once
230 an acknowledgement is received.
231 For a small number of clients, such as window systems
232 that send a stream of mouse events which receive no replies,
233 this packetization may cause significant delays.
236 defeats this algorithm.
238 By default, a sender- and
239 .No receiver- Ns Tn TCP
240 will negotiate among themselves to determine the maximum segment size
241 to be used for each connection.
244 option allows the user to determine the result of this negotiation,
245 and to reduce it if desired.
248 usually sends a number of options in each packet, corresponding to
251 extensions which are provided in this implementation.
254 is provided to disable
256 option use on a per-connection basis.
259 .No sender- Ns Tn TCP
262 bit, and begin transmission immediately (if permitted) at the end of
267 When this option is set to a non-zero value,
269 will delay sending any data at all until either the socket is closed,
270 or the internal send buffer is filled.
272 This option enables the use of MD5 digests (also known as TCP-MD5)
273 on writes to the specified socket.
274 Outgoing traffic is digested;
275 digests on incoming traffic are verified.
276 When this option is enabled on a socket, all inbound and outgoing
277 TCP segments must be signed with MD5 digests.
279 One common use for this in a
281 router deployment is to enable
282 based routers to interwork with Cisco equipment at peering points.
283 Support for this feature conforms to RFC 2385.
285 In order for this option to function correctly, it is necessary for the
286 administrator to add a tcp-md5 key entry to the system's security
287 associations database (SADB) using the
290 This entry can only be specified on a per-host basis at this time.
292 If an SADB entry cannot be found for the destination,
293 the system does not send any outgoing segments and drops any inbound segments.
295 Each dropped segment is taken into account in the TCP protocol statistics.
298 The option level for the
300 call is the protocol number for
303 .Xr getprotobyname 3 ,
306 All options are declared in
311 transport level may be used with
315 Incoming connection requests that are source-routed are noted,
316 and the reverse source route is used in responding.
318 The default congestion control algorithm for
322 Other congestion control algorithms can be made available using the
328 protocol implements a number of variables in the
333 .Bl -tag -width ".Va TCPCTL_DO_RFC1323"
334 .It Dv TCPCTL_DO_RFC1323
336 Implement the window scaling and timestamp options of RFC 1323
338 .It Dv TCPCTL_MSSDFLT
340 The default value used for the maximum segment size
342 when no advice to the contrary is received from MSS negotiation.
343 .It Dv TCPCTL_SENDSPACE
348 .It Dv TCPCTL_RECVSPACE
354 Log any connection attempts to ports where there is not a socket
355 accepting connections.
356 The value of 1 limits the logging to
358 (connection establishment) packets only.
359 That of 2 results in any
361 packets to closed ports being logged.
362 Any value unlisted above disables the logging
363 (default is 0, i.e., the logging is disabled).
365 The Maximum Segment Lifetime, in milliseconds, for a packet.
367 Timeout, in milliseconds, for new, non-established
370 The default is 75000 msec.
372 Amount of time, in milliseconds, that the connection must be idle
373 before keepalive probes (if enabled) are sent.
374 The default is 7200000 msec (2 hours).
376 The interval, in milliseconds, between keepalive probes sent to remote
377 machines, when no response is received on a
380 The default is 75000 msec.
382 Number of probes sent, with no response, before a connection
384 The default is 8 packets.
385 .It Va always_keepalive
390 connections, the kernel will
391 periodically send a packet to the remote host to verify the connection
396 unreachable messages may abort connections in
402 reassembly queue if the system is low on mbufs.
404 If enabled, disable sending of RST when a connection is attempted
405 to a port where there is not a socket accepting connections.
409 Delay ACK to try and piggyback it onto a data packet.
411 Maximum amount of time, in milliseconds, before a delayed ACK is sent.
412 .It Va path_mtu_discovery
413 Enable Path MTU Discovery.
417 control-block hash table
419 This may be tuned using the kernel option
422 .Va net.inet.tcp.tcbhashsize
426 Number of active process control blocks
429 Determines whether or not
431 cookies should be generated for outbound
435 cookies are a great help during
437 flood attacks, and are enabled by default.
440 .It Va isn_reseed_interval
441 The interval (in seconds) specifying how often the secret data used in
442 RFC 1948 initial sequence number calculations should be reseeded.
443 By default, this variable is set to zero, indicating that
444 no reseeding will occur.
445 Reseeding should not be necessary, and will break
447 recycling for a few minutes.
448 .It Va reass.cursegments
449 The current total number of segments present in all reassembly queues.
450 .It Va reass.maxsegments
451 The maximum limit on the total number of segments across all reassembly
453 The limit can be adjusted as a tunable.
454 .It Va reass.maxqueuelen
455 The maximum number of segments allowed in each reassembly queue.
456 By default, the system chooses a limit based on each TCP connection's
457 receive buffer size and maximum segment size (MSS).
458 The actual limit applied to a session's reassembly queue will be the lower of
459 the system-calculated automatic limit and the user-specified
460 .Va reass.maxqueuelen
462 .It Va rexmit_min , rexmit_slop
463 Adjust the retransmit timer calculation for
466 typically added to the raw calculation to take into account
467 occasional variances that the
469 (smoothed round-trip time)
470 is unable to accommodate, while the minimum specifies an
475 second minimum, these RFCs tend to focus on streaming behavior,
476 and fail to deal with the fact that a 1 second minimum has severe
477 detrimental effects over lossy interactive connections, such
478 as a 802.11b wireless link, and over very fast but lossy
479 connections for those cases not covered by the fast retransmit
481 For this reason, we use 200ms of slop and a near-0
482 minimum, which gives us an effective minimum of 200ms (similar to
484 .It Va initcwnd_segments
485 Enable the ability to specify initial congestion window in number of segments.
486 The default value is 10 as suggested by RFC 6928.
487 Changing the value on fly would not affect connections using congestion window
490 This regulates the burst of packets allowed to be sent in the first RTT.
491 The value should be relative to the link capacity.
492 Start with small values for lower-capacity links.
493 Large bursts can cause buffer overruns and packet drops if routers have small
494 buffers or the link is experiencing congestion.
496 Enable the Limited Transmit algorithm as described in RFC 3042.
497 It helps avoid timeouts on lossy links and also when the congestion window
498 is small, as happens on short transfers.
500 Enable support for RFC 3390, which allows for a variable-sized
501 starting congestion window on new connections, depending on the
502 maximum segment size.
503 This helps throughput in general, but
504 particularly affects short transfers and high-bandwidth large
505 propagation-delay connections.
507 Enable support for RFC 2018, TCP Selective Acknowledgment option,
508 which allows the receiver to inform the sender about all successfully
509 arrived segments, allowing the sender to retransmit the missing segments
512 Maximum number of SACK holes per connection.
514 .It Va sack.globalmaxholes
515 Maximum number of SACK holes per system, across all connections.
518 When a TCP connection enters the
520 state, its associated socket structure is freed, since it is of
521 negligible size and use, and a new structure is allocated to contain a
522 minimal amount of information necessary for sustaining a connection in
523 this state, called the compressed TCP TIME_WAIT state.
524 Since this structure is smaller than a socket structure, it can save
525 a significant amount of system memory.
527 .Va net.inet.tcp.maxtcptw
528 MIB variable controls the maximum number of these structures allocated.
529 By default, it is initialized to
530 .Va kern.ipc.maxsockets
532 .It Va nolocaltimewait
533 Suppress creating of compressed TCP TIME_WAIT states for connections in
534 which both endpoints are local.
535 .It Va fast_finwait2_recycle
539 connections faster when the socket is marked as
541 (no user process has the socket open, data received on
542 the socket cannot be read).
543 The timeout used here is
544 .Va finwait2_timeout .
545 .It Va finwait2_timeout
546 Timeout to use for fast recycling of
550 Defaults to 60 seconds.
552 Enable support for TCP Explicit Congestion Notification (ECN).
553 ECN allows a TCP sender to reduce the transmission rate in order to
560 Allow incoming connections to request ECN.
561 Outgoing connections will request ECN.
563 Allow incoming connections to request ECN.
564 Outgoing connections will not request ECN.
566 .It Va ecn.maxretries
567 Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
569 This is needed to help with connection establishment
570 when a broken firewall is in the network path.
571 .It Va pmtud_blackhole_detection
572 Turn on automatic path MTU blackhole detection.
573 In case of retransmits OS will
574 lower the MSS to check if it's MTU problem.
575 If current MSS is greater than
576 configured value to try, it will be set to configured value, otherwise,
577 MSS will be set to default values
578 .Po Va net.inet.tcp.mssdflt
580 .Va net.inet.tcp.v6mssdflt
582 .It Va pmtud_blackhole_mss
583 MSS to try for IPv4 if PMTU blackhole detection is turned on.
584 .It Va v6pmtud_blackhole_mss
585 MSS to try for IPv6 if PMTU blackhole detection is turned on.
586 .It Va pmtud_blackhole_activated
587 Number of times configured values were used in an attempt to downshift.
588 .It Va pmtud_blackhole_activated_min_mss
589 Number of times default MSS was used in an attempt to downshift.
590 .It Va pmtud_blackhole_failed
591 Number of connections for which retransmits continued even after MSS
593 .It Va functions_available
594 List of available TCP function blocks (TCP stacks).
595 .It Va functions_default
596 The default TCP function block (TCP stack).
598 Use criteria defined in RFC793 instead of RFC5961 for accepting RST segments.
601 Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments.
605 A socket operation may fail with one of the following errors returned:
608 when trying to establish a connection on a socket which
611 when the system runs out of memory for
612 an internal data structure;
614 when a connection was dropped
615 due to excessive retransmissions;
618 forces the connection to be closed;
619 .It Bq Er ECONNREFUSED
621 peer actively refuses connection establishment (usually because
622 no process is listening to the port);
625 is made to create a socket with a port which has already been
627 .It Bq Er EADDRNOTAVAIL
628 when an attempt is made to create a
629 socket with a network address for which no network interface
631 .It Bq Er EAFNOSUPPORT
632 when an attempt is made to bind or connect a socket to a multicast
635 when trying to change TCP function blocks at an invalid point in the session;
637 when trying to use a TCP function block that is not available;
655 .%T "TCP Extensions for High Performance"
660 .%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
664 .%A "K. Ramakrishnan"
667 .%T "The Addition of Explicit Congestion Notification (ECN) to IP"
675 The RFC 1323 extensions for window scaling and timestamps were added
680 option was introduced in
683 .Em subject to change .
projects / FreeBSD / FreeBSD.git / blob