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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 Manage collection of connection level statistics using the
299 Each dropped segment is taken into account in the TCP protocol statistics.
300 .It Dv TCP_TXTLS_ENABLE
301 Enable in-kernel Transport Layer Security (TLS) for data written to this
304 .Vt struct tls_so_enable
305 argument defines the encryption and authentication algorithms and keys
306 used to encrypt the socket data as well as the maximum TLS record
309 All data written to this socket will be encapsulated in TLS records
310 and subsequently encrypted.
311 By default all data written to this socket is treated as application data.
312 Individual TLS records with a type other than application data
313 (for example, handshake messages),
314 may be transmitted by invoking
316 with a custom TLS record type set in a
317 .Dv TLS_SET_RECORD_TYPE
319 The payload of this control message is a single byte holding the desired
322 Data read from this socket will still be encrypted and must be parsed by
323 a TLS-aware consumer.
325 At present, only a single key may be set on a socket.
326 As such, users of this option must disable rekeying.
327 .It Dv TCP_TXTLS_MODE
328 The integer argument can be used to get or set the current TLS mode of a
330 Setting the mode can only used to toggle between software and NIC TLS after
331 TLS has been initially enabled via the
334 The available modes are:
335 .Bl -tag -width "Dv TCP_TLS_MODE_IFNET"
336 .It Dv TCP_TLS_MODE_NONE
337 In-kernel TLS framing and encryption is not enabled for this socket.
338 .It Dv TCP_TLS_MODE_SW
339 TLS records are encrypted by the kernel prior to placing the data in the
341 Typically this encryption is performed in software.
342 .It Dv TCP_TLS_MODE_IFNET
343 TLS records are encrypted by the network interface card (NIC).
344 .It Dv TCP_TLS_MODE_TOE
345 TLS records are encrypted by the NIC using a TCP offload engine (TOE).
349 The option level for the
351 call is the protocol number for
354 .Xr getprotobyname 3 ,
357 All options are declared in
362 transport level may be used with
366 Incoming connection requests that are source-routed are noted,
367 and the reverse source route is used in responding.
369 The default congestion control algorithm for
373 Other congestion control algorithms can be made available using the
379 protocol implements a number of variables in the
384 .Bl -tag -width ".Va TCPCTL_DO_RFC1323"
385 .It Dv TCPCTL_DO_RFC1323
387 Implement the window scaling and timestamp options of RFC 1323
389 .It Dv TCPCTL_MSSDFLT
391 The default value used for the maximum segment size
393 when no advice to the contrary is received from MSS negotiation.
394 .It Dv TCPCTL_SENDSPACE
399 .It Dv TCPCTL_RECVSPACE
405 Log any connection attempts to ports where there is not a socket
406 accepting connections.
407 The value of 1 limits the logging to
409 (connection establishment) packets only.
410 That of 2 results in any
412 packets to closed ports being logged.
413 Any value unlisted above disables the logging
414 (default is 0, i.e., the logging is disabled).
416 The Maximum Segment Lifetime, in milliseconds, for a packet.
418 Timeout, in milliseconds, for new, non-established
421 The default is 75000 msec.
423 Amount of time, in milliseconds, that the connection must be idle
424 before keepalive probes (if enabled) are sent.
425 The default is 7200000 msec (2 hours).
427 The interval, in milliseconds, between keepalive probes sent to remote
428 machines, when no response is received on a
431 The default is 75000 msec.
433 Number of probes sent, with no response, before a connection
435 The default is 8 packets.
436 .It Va always_keepalive
441 connections, the kernel will
442 periodically send a packet to the remote host to verify the connection
447 unreachable messages may abort connections in
453 reassembly queue if the system is low on mbufs.
455 If enabled, disable sending of RST when a connection is attempted
456 to a port where there is not a socket accepting connections.
460 Delay ACK to try and piggyback it onto a data packet.
462 Maximum amount of time, in milliseconds, before a delayed ACK is sent.
463 .It Va path_mtu_discovery
464 Enable Path MTU Discovery.
468 control-block hash table
470 This may be tuned using the kernel option
473 .Va net.inet.tcp.tcbhashsize
477 Number of active process control blocks
480 Determines whether or not
482 cookies should be generated for outbound
486 cookies are a great help during
488 flood attacks, and are enabled by default.
491 .It Va isn_reseed_interval
492 The interval (in seconds) specifying how often the secret data used in
493 RFC 1948 initial sequence number calculations should be reseeded.
494 By default, this variable is set to zero, indicating that
495 no reseeding will occur.
496 Reseeding should not be necessary, and will break
498 recycling for a few minutes.
499 .It Va reass.cursegments
500 The current total number of segments present in all reassembly queues.
501 .It Va reass.maxsegments
502 The maximum limit on the total number of segments across all reassembly
504 The limit can be adjusted as a tunable.
505 .It Va reass.maxqueuelen
506 The maximum number of segments allowed in each reassembly queue.
507 By default, the system chooses a limit based on each TCP connection's
508 receive buffer size and maximum segment size (MSS).
509 The actual limit applied to a session's reassembly queue will be the lower of
510 the system-calculated automatic limit and the user-specified
511 .Va reass.maxqueuelen
513 .It Va rexmit_initial , rexmit_min , rexmit_slop
514 Adjust the retransmit timer calculation for
517 typically added to the raw calculation to take into account
518 occasional variances that the
520 (smoothed round-trip time)
521 is unable to accommodate, while the minimum specifies an
526 second minimum, these RFCs tend to focus on streaming behavior,
527 and fail to deal with the fact that a 1 second minimum has severe
528 detrimental effects over lossy interactive connections, such
529 as a 802.11b wireless link, and over very fast but lossy
530 connections for those cases not covered by the fast retransmit
532 For this reason, we use 200ms of slop and a near-0
533 minimum, which gives us an effective minimum of 200ms (similar to
535 The initial value is used before an RTT measurement has been performed.
536 .It Va initcwnd_segments
537 Enable the ability to specify initial congestion window in number of segments.
538 The default value is 10 as suggested by RFC 6928.
539 Changing the value on fly would not affect connections using congestion window
542 This regulates the burst of packets allowed to be sent in the first RTT.
543 The value should be relative to the link capacity.
544 Start with small values for lower-capacity links.
545 Large bursts can cause buffer overruns and packet drops if routers have small
546 buffers or the link is experiencing congestion.
548 Enable the New Congestion Window Validation mechanism as described in RFC 7661.
549 This gently reduces the congestion window during periods, where TCP is
550 application limited and the network bandwidth is not utilized completely.
551 That prevents self-inflicted packet losses once the application starts to
552 transmit data at a higher speed.
554 Calculate the bytes in flight using the algorithm described in RFC 6675, and
555 is also a prerequisite to enable Proportional Rate Reduction.
557 Enable the Limited Transmit algorithm as described in RFC 3042.
558 It helps avoid timeouts on lossy links and also when the congestion window
559 is small, as happens on short transfers.
561 Enable support for RFC 3390, which allows for a variable-sized
562 starting congestion window on new connections, depending on the
563 maximum segment size.
564 This helps throughput in general, but
565 particularly affects short transfers and high-bandwidth large
566 propagation-delay connections.
568 Enable support for RFC 2018, TCP Selective Acknowledgment option,
569 which allows the receiver to inform the sender about all successfully
570 arrived segments, allowing the sender to retransmit the missing segments
573 Maximum number of SACK holes per connection.
575 .It Va sack.globalmaxholes
576 Maximum number of SACK holes per system, across all connections.
579 When a TCP connection enters the
581 state, its associated socket structure is freed, since it is of
582 negligible size and use, and a new structure is allocated to contain a
583 minimal amount of information necessary for sustaining a connection in
584 this state, called the compressed TCP TIME_WAIT state.
585 Since this structure is smaller than a socket structure, it can save
586 a significant amount of system memory.
588 .Va net.inet.tcp.maxtcptw
589 MIB variable controls the maximum number of these structures allocated.
590 By default, it is initialized to
591 .Va kern.ipc.maxsockets
593 .It Va nolocaltimewait
594 Suppress creating of compressed TCP TIME_WAIT states for connections in
595 which both endpoints are local.
596 .It Va fast_finwait2_recycle
600 connections faster when the socket is marked as
602 (no user process has the socket open, data received on
603 the socket cannot be read).
604 The timeout used here is
605 .Va finwait2_timeout .
606 .It Va finwait2_timeout
607 Timeout to use for fast recycling of
611 Defaults to 60 seconds.
613 Enable support for TCP Explicit Congestion Notification (ECN).
614 ECN allows a TCP sender to reduce the transmission rate in order to
621 Allow incoming connections to request ECN.
622 Outgoing connections will request ECN.
624 Allow incoming connections to request ECN.
625 Outgoing connections will not request ECN.
627 .It Va ecn.maxretries
628 Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
630 This is needed to help with connection establishment
631 when a broken firewall is in the network path.
632 .It Va pmtud_blackhole_detection
633 Enable automatic path MTU blackhole detection.
634 In case of retransmits of MSS sized segments,
635 the OS will lower the MSS to check if it's an MTU problem.
636 If the current MSS is greater than the configured value to try
637 .Po Va net.inet.tcp.pmtud_blackhole_mss
639 .Va net.inet.tcp.v6pmtud_blackhole_mss
641 it will be set to this value, otherwise,
642 the MSS will be set to the default values
643 .Po Va net.inet.tcp.mssdflt
645 .Va net.inet.tcp.v6mssdflt
650 Disable path MTU blackhole detection.
652 Enable path MTU blackhole detection for IPv4 and IPv6.
654 Enable path MTU blackhole detection only for IPv4.
656 Enable path MTU blackhole detection only for IPv6.
658 .It Va pmtud_blackhole_mss
659 MSS to try for IPv4 if PMTU blackhole detection is turned on.
660 .It Va v6pmtud_blackhole_mss
661 MSS to try for IPv6 if PMTU blackhole detection is turned on.
662 .It Va functions_available
663 List of available TCP function blocks (TCP stacks).
664 .It Va functions_default
665 The default TCP function block (TCP stack).
666 .It Va functions_inherit_listen_socket_stack
667 Determines whether to inherit listen socket's tcp stack or use the current
668 system default tcp stack, as defined by
669 .Va functions_default .
672 Use criteria defined in RFC793 instead of RFC5961 for accepting RST segments.
675 Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments.
677 .It Va ts_offset_per_conn
678 When initializing the TCP timestamps, use a per connection offset instead of a
679 per host pair offset.
680 Default is to use per connection offsets as recommended in RFC 7323.
681 .It Va perconn_stats_enable
682 Controls the default collection of statistics for all connections using the
685 0 disables, 1 enables, 2 enables random sampling across log id connection
686 groups with all connections in a group receiving the same setting.
687 .It Va perconn_stats_sample_rates
688 A CSV list of template_spec=percent key-value pairs which controls the per
689 template sampling rates when
694 A socket operation may fail with one of the following errors returned:
697 when trying to establish a connection on a socket which
699 .It Bo Er ENOBUFS Bc or Bo Er ENOMEM Bc
700 when the system runs out of memory for
701 an internal data structure;
703 when a connection was dropped
704 due to excessive retransmissions;
707 forces the connection to be closed;
708 .It Bq Er ECONNREFUSED
710 peer actively refuses connection establishment (usually because
711 no process is listening to the port);
714 is made to create a socket with a port which has already been
716 .It Bq Er EADDRNOTAVAIL
717 when an attempt is made to create a
718 socket with a network address for which no network interface
720 .It Bq Er EAFNOSUPPORT
721 when an attempt is made to bind or connect a socket to a multicast
724 when trying to change TCP function blocks at an invalid point in the session;
726 when trying to use a TCP function block that is not available;
746 .%T "TCP Extensions for High Performance"
751 .%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
755 .%A "K. Ramakrishnan"
758 .%T "The Addition of Explicit Congestion Notification (ECN) to IP"
766 The RFC 1323 extensions for window scaling and timestamps were added
771 option was introduced in
774 .Em subject to change .
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