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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 At present, only a single transmit key may be set on a socket.
323 As such, users of this option must disable rekeying.
324 .It Dv TCP_TXTLS_MODE
325 The integer argument can be used to get or set the current TLS transmit mode
327 Setting the mode can only used to toggle between software and NIC TLS after
328 TLS has been initially enabled via the
331 The available modes are:
332 .Bl -tag -width "Dv TCP_TLS_MODE_IFNET"
333 .It Dv TCP_TLS_MODE_NONE
334 In-kernel TLS framing and encryption is not enabled for this socket.
335 .It Dv TCP_TLS_MODE_SW
336 TLS records are encrypted by the kernel prior to placing the data in the
338 Typically this encryption is performed in software.
339 .It Dv TCP_TLS_MODE_IFNET
340 TLS records are encrypted by the network interface card (NIC).
341 .It Dv TCP_TLS_MODE_TOE
342 TLS records are encrypted by the NIC using a TCP offload engine (TOE).
344 .It Dv TCP_RXTLS_ENABLE
345 Enable in-kernel TLS for data read from this socket.
347 .Vt struct tls_so_enable
348 argument defines the encryption and authentication algorithms and keys
349 used to decrypt the socket data.
351 Each received TLS record must be read from the socket using
353 Each received TLS record will contain a
355 control message along with the decrypted payload.
356 The control message contains a
357 .Vt struct tls_get_record
358 which includes fields from the TLS record header.
359 If a corrupted TLS record is received,
364 At present, only a single receive key may be set on a socket.
365 As such, users of this option must disable rekeying.
366 .It Dv TCP_RXTLS_MODE
367 The integer argument can be used to get the current TLS receive mode
369 The available modes are the same as for
373 The option level for the
375 call is the protocol number for
378 .Xr getprotobyname 3 ,
381 All options are declared in
386 transport level may be used with
390 Incoming connection requests that are source-routed are noted,
391 and the reverse source route is used in responding.
393 The default congestion control algorithm for
397 Other congestion control algorithms can be made available using the
403 protocol implements a number of variables in the
408 .Bl -tag -width ".Va TCPCTL_DO_RFC1323"
409 .It Dv TCPCTL_DO_RFC1323
411 Implement the window scaling and timestamp options of RFC 1323
413 .It Dv TCPCTL_MSSDFLT
415 The default value used for the maximum segment size
417 when no advice to the contrary is received from MSS negotiation.
418 .It Dv TCPCTL_SENDSPACE
423 .It Dv TCPCTL_RECVSPACE
429 Log any connection attempts to ports where there is not a socket
430 accepting connections.
431 The value of 1 limits the logging to
433 (connection establishment) packets only.
434 That of 2 results in any
436 packets to closed ports being logged.
437 Any value unlisted above disables the logging
438 (default is 0, i.e., the logging is disabled).
440 The Maximum Segment Lifetime, in milliseconds, for a packet.
442 Timeout, in milliseconds, for new, non-established
445 The default is 75000 msec.
447 Amount of time, in milliseconds, that the connection must be idle
448 before keepalive probes (if enabled) are sent.
449 The default is 7200000 msec (2 hours).
451 The interval, in milliseconds, between keepalive probes sent to remote
452 machines, when no response is received on a
455 The default is 75000 msec.
457 Number of probes sent, with no response, before a connection
459 The default is 8 packets.
460 .It Va always_keepalive
465 connections, the kernel will
466 periodically send a packet to the remote host to verify the connection
471 unreachable messages may abort connections in
477 reassembly queue if the system is low on mbufs.
479 If enabled, disable sending of RST when a connection is attempted
480 to a port where there is not a socket accepting connections.
484 Delay ACK to try and piggyback it onto a data packet.
486 Maximum amount of time, in milliseconds, before a delayed ACK is sent.
487 .It Va path_mtu_discovery
488 Enable Path MTU Discovery.
492 control-block hash table
494 This may be tuned using the kernel option
497 .Va net.inet.tcp.tcbhashsize
501 Number of active process control blocks
504 Determines whether or not
506 cookies should be generated for outbound
510 cookies are a great help during
512 flood attacks, and are enabled by default.
515 .It Va isn_reseed_interval
516 The interval (in seconds) specifying how often the secret data used in
517 RFC 1948 initial sequence number calculations should be reseeded.
518 By default, this variable is set to zero, indicating that
519 no reseeding will occur.
520 Reseeding should not be necessary, and will break
522 recycling for a few minutes.
523 .It Va reass.cursegments
524 The current total number of segments present in all reassembly queues.
525 .It Va reass.maxsegments
526 The maximum limit on the total number of segments across all reassembly
528 The limit can be adjusted as a tunable.
529 .It Va reass.maxqueuelen
530 The maximum number of segments allowed in each reassembly queue.
531 By default, the system chooses a limit based on each TCP connection's
532 receive buffer size and maximum segment size (MSS).
533 The actual limit applied to a session's reassembly queue will be the lower of
534 the system-calculated automatic limit and the user-specified
535 .Va reass.maxqueuelen
537 .It Va rexmit_initial , rexmit_min , rexmit_slop
538 Adjust the retransmit timer calculation for
541 typically added to the raw calculation to take into account
542 occasional variances that the
544 (smoothed round-trip time)
545 is unable to accommodate, while the minimum specifies an
550 second minimum, these RFCs tend to focus on streaming behavior,
551 and fail to deal with the fact that a 1 second minimum has severe
552 detrimental effects over lossy interactive connections, such
553 as a 802.11b wireless link, and over very fast but lossy
554 connections for those cases not covered by the fast retransmit
556 For this reason, we use 200ms of slop and a near-0
557 minimum, which gives us an effective minimum of 200ms (similar to
559 The initial value is used before an RTT measurement has been performed.
560 .It Va initcwnd_segments
561 Enable the ability to specify initial congestion window in number of segments.
562 The default value is 10 as suggested by RFC 6928.
563 Changing the value on fly would not affect connections using congestion window
566 This regulates the burst of packets allowed to be sent in the first RTT.
567 The value should be relative to the link capacity.
568 Start with small values for lower-capacity links.
569 Large bursts can cause buffer overruns and packet drops if routers have small
570 buffers or the link is experiencing congestion.
572 Enable the New Congestion Window Validation mechanism as described in RFC 7661.
573 This gently reduces the congestion window during periods, where TCP is
574 application limited and the network bandwidth is not utilized completely.
575 That prevents self-inflicted packet losses once the application starts to
576 transmit data at a higher speed.
578 Calculate the bytes in flight using the algorithm described in RFC 6675, and
579 is also a prerequisite to enable Proportional Rate Reduction.
581 Enable the Limited Transmit algorithm as described in RFC 3042.
582 It helps avoid timeouts on lossy links and also when the congestion window
583 is small, as happens on short transfers.
585 Enable support for RFC 3390, which allows for a variable-sized
586 starting congestion window on new connections, depending on the
587 maximum segment size.
588 This helps throughput in general, but
589 particularly affects short transfers and high-bandwidth large
590 propagation-delay connections.
592 Enable support for RFC 2018, TCP Selective Acknowledgment option,
593 which allows the receiver to inform the sender about all successfully
594 arrived segments, allowing the sender to retransmit the missing segments
597 Maximum number of SACK holes per connection.
599 .It Va sack.globalmaxholes
600 Maximum number of SACK holes per system, across all connections.
603 When a TCP connection enters the
605 state, its associated socket structure is freed, since it is of
606 negligible size and use, and a new structure is allocated to contain a
607 minimal amount of information necessary for sustaining a connection in
608 this state, called the compressed TCP TIME_WAIT state.
609 Since this structure is smaller than a socket structure, it can save
610 a significant amount of system memory.
612 .Va net.inet.tcp.maxtcptw
613 MIB variable controls the maximum number of these structures allocated.
614 By default, it is initialized to
615 .Va kern.ipc.maxsockets
617 .It Va nolocaltimewait
618 Suppress creating of compressed TCP TIME_WAIT states for connections in
619 which both endpoints are local.
620 .It Va fast_finwait2_recycle
624 connections faster when the socket is marked as
626 (no user process has the socket open, data received on
627 the socket cannot be read).
628 The timeout used here is
629 .Va finwait2_timeout .
630 .It Va finwait2_timeout
631 Timeout to use for fast recycling of
635 Defaults to 60 seconds.
637 Enable support for TCP Explicit Congestion Notification (ECN).
638 ECN allows a TCP sender to reduce the transmission rate in order to
645 Allow incoming connections to request ECN.
646 Outgoing connections will request ECN.
648 Allow incoming connections to request ECN.
649 Outgoing connections will not request ECN.
651 .It Va ecn.maxretries
652 Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
654 This is needed to help with connection establishment
655 when a broken firewall is in the network path.
656 .It Va pmtud_blackhole_detection
657 Enable automatic path MTU blackhole detection.
658 In case of retransmits of MSS sized segments,
659 the OS will lower the MSS to check if it's an MTU problem.
660 If the current MSS is greater than the configured value to try
661 .Po Va net.inet.tcp.pmtud_blackhole_mss
663 .Va net.inet.tcp.v6pmtud_blackhole_mss
665 it will be set to this value, otherwise,
666 the MSS will be set to the default values
667 .Po Va net.inet.tcp.mssdflt
669 .Va net.inet.tcp.v6mssdflt
674 Disable path MTU blackhole detection.
676 Enable path MTU blackhole detection for IPv4 and IPv6.
678 Enable path MTU blackhole detection only for IPv4.
680 Enable path MTU blackhole detection only for IPv6.
682 .It Va pmtud_blackhole_mss
683 MSS to try for IPv4 if PMTU blackhole detection is turned on.
684 .It Va v6pmtud_blackhole_mss
685 MSS to try for IPv6 if PMTU blackhole detection is turned on.
686 .It Va functions_available
687 List of available TCP function blocks (TCP stacks).
688 .It Va functions_default
689 The default TCP function block (TCP stack).
690 .It Va functions_inherit_listen_socket_stack
691 Determines whether to inherit listen socket's tcp stack or use the current
692 system default tcp stack, as defined by
693 .Va functions_default .
696 Use criteria defined in RFC793 instead of RFC5961 for accepting RST segments.
699 Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments.
701 .It Va ts_offset_per_conn
702 When initializing the TCP timestamps, use a per connection offset instead of a
703 per host pair offset.
704 Default is to use per connection offsets as recommended in RFC 7323.
705 .It Va perconn_stats_enable
706 Controls the default collection of statistics for all connections using the
709 0 disables, 1 enables, 2 enables random sampling across log id connection
710 groups with all connections in a group receiving the same setting.
711 .It Va perconn_stats_sample_rates
712 A CSV list of template_spec=percent key-value pairs which controls the per
713 template sampling rates when
718 A socket operation may fail with one of the following errors returned:
721 when trying to establish a connection on a socket which
723 .It Bo Er ENOBUFS Bc or Bo Er ENOMEM Bc
724 when the system runs out of memory for
725 an internal data structure;
727 when a connection was dropped
728 due to excessive retransmissions;
731 forces the connection to be closed;
732 .It Bq Er ECONNREFUSED
734 peer actively refuses connection establishment (usually because
735 no process is listening to the port);
738 is made to create a socket with a port which has already been
740 .It Bq Er EADDRNOTAVAIL
741 when an attempt is made to create a
742 socket with a network address for which no network interface
744 .It Bq Er EAFNOSUPPORT
745 when an attempt is made to bind or connect a socket to a multicast
748 when trying to change TCP function blocks at an invalid point in the session;
750 when trying to use a TCP function block that is not available;
770 .%T "TCP Extensions for High Performance"
775 .%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
779 .%A "K. Ramakrishnan"
782 .%T "The Addition of Explicit Congestion Notification (ECN) to IP"
790 The RFC 1323 extensions for window scaling and timestamps were added
795 option was introduced in
798 .Em subject to change .
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