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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 Each dropped segment is taken into account in the TCP protocol statistics.
296 .It Dv TCP_TXTLS_ENABLE
297 Enable in-kernel Transport Layer Security (TLS) for data written to this
300 .Vt struct tls_so_enable
301 argument defines the encryption and authentication algorithms and keys
302 used to encrypt the socket data as well as the maximum TLS record
305 All data written to this socket will be encapsulated in TLS records
306 and subsequently encrypted.
307 By default all data written to this socket is treated as application data.
308 Individual TLS records with a type other than application data
309 (for example, handshake messages),
310 may be transmitted by invoking
312 with a custom TLS record type set in a
313 .Dv TLS_SET_RECORD_TYPE
315 The payload of this control message is a single byte holding the desired
318 Data read from this socket will still be encrypted and must be parsed by
319 a TLS-aware consumer.
321 At present, only a single key may be set on a socket.
322 As such, users of this option must disable rekeying.
323 .It Dv TCP_TXTLS_MODE
324 The integer argument can be used to get or set the current TLS mode of a
326 Setting the mode can only used to toggle between software and NIC TLS after
327 TLS has been initially enabled via the
330 The available modes are:
331 .Bl -tag -width "Dv TCP_TLS_MODE_IFNET"
332 .It Dv TCP_TLS_MODE_NONE
333 In-kernel TLS framing and encryption is not enabled for this socket.
334 .It Dv TCP_TLS_MODE_SW
335 TLS records are encrypted by the kernel prior to placing the data in the
337 Typically this encryption is performed in software.
338 .It Dv TCP_TLS_MODE_IFNET
339 TLS records are encrypted by the network interface card (NIC).
343 The option level for the
345 call is the protocol number for
348 .Xr getprotobyname 3 ,
351 All options are declared in
356 transport level may be used with
360 Incoming connection requests that are source-routed are noted,
361 and the reverse source route is used in responding.
363 The default congestion control algorithm for
367 Other congestion control algorithms can be made available using the
373 protocol implements a number of variables in the
378 .Bl -tag -width ".Va TCPCTL_DO_RFC1323"
379 .It Dv TCPCTL_DO_RFC1323
381 Implement the window scaling and timestamp options of RFC 1323
383 .It Dv TCPCTL_MSSDFLT
385 The default value used for the maximum segment size
387 when no advice to the contrary is received from MSS negotiation.
388 .It Dv TCPCTL_SENDSPACE
393 .It Dv TCPCTL_RECVSPACE
399 Log any connection attempts to ports where there is not a socket
400 accepting connections.
401 The value of 1 limits the logging to
403 (connection establishment) packets only.
404 That of 2 results in any
406 packets to closed ports being logged.
407 Any value unlisted above disables the logging
408 (default is 0, i.e., the logging is disabled).
410 The Maximum Segment Lifetime, in milliseconds, for a packet.
412 Timeout, in milliseconds, for new, non-established
415 The default is 75000 msec.
417 Amount of time, in milliseconds, that the connection must be idle
418 before keepalive probes (if enabled) are sent.
419 The default is 7200000 msec (2 hours).
421 The interval, in milliseconds, between keepalive probes sent to remote
422 machines, when no response is received on a
425 The default is 75000 msec.
427 Number of probes sent, with no response, before a connection
429 The default is 8 packets.
430 .It Va always_keepalive
435 connections, the kernel will
436 periodically send a packet to the remote host to verify the connection
441 unreachable messages may abort connections in
447 reassembly queue if the system is low on mbufs.
449 If enabled, disable sending of RST when a connection is attempted
450 to a port where there is not a socket accepting connections.
454 Delay ACK to try and piggyback it onto a data packet.
456 Maximum amount of time, in milliseconds, before a delayed ACK is sent.
457 .It Va path_mtu_discovery
458 Enable Path MTU Discovery.
462 control-block hash table
464 This may be tuned using the kernel option
467 .Va net.inet.tcp.tcbhashsize
471 Number of active process control blocks
474 Determines whether or not
476 cookies should be generated for outbound
480 cookies are a great help during
482 flood attacks, and are enabled by default.
485 .It Va isn_reseed_interval
486 The interval (in seconds) specifying how often the secret data used in
487 RFC 1948 initial sequence number calculations should be reseeded.
488 By default, this variable is set to zero, indicating that
489 no reseeding will occur.
490 Reseeding should not be necessary, and will break
492 recycling for a few minutes.
493 .It Va reass.cursegments
494 The current total number of segments present in all reassembly queues.
495 .It Va reass.maxsegments
496 The maximum limit on the total number of segments across all reassembly
498 The limit can be adjusted as a tunable.
499 .It Va reass.maxqueuelen
500 The maximum number of segments allowed in each reassembly queue.
501 By default, the system chooses a limit based on each TCP connection's
502 receive buffer size and maximum segment size (MSS).
503 The actual limit applied to a session's reassembly queue will be the lower of
504 the system-calculated automatic limit and the user-specified
505 .Va reass.maxqueuelen
507 .It Va rexmit_initial , rexmit_min , rexmit_slop
508 Adjust the retransmit timer calculation for
511 typically added to the raw calculation to take into account
512 occasional variances that the
514 (smoothed round-trip time)
515 is unable to accommodate, while the minimum specifies an
520 second minimum, these RFCs tend to focus on streaming behavior,
521 and fail to deal with the fact that a 1 second minimum has severe
522 detrimental effects over lossy interactive connections, such
523 as a 802.11b wireless link, and over very fast but lossy
524 connections for those cases not covered by the fast retransmit
526 For this reason, we use 200ms of slop and a near-0
527 minimum, which gives us an effective minimum of 200ms (similar to
529 The initial value is used before an RTT measurement has been performed.
530 .It Va initcwnd_segments
531 Enable the ability to specify initial congestion window in number of segments.
532 The default value is 10 as suggested by RFC 6928.
533 Changing the value on fly would not affect connections using congestion window
536 This regulates the burst of packets allowed to be sent in the first RTT.
537 The value should be relative to the link capacity.
538 Start with small values for lower-capacity links.
539 Large bursts can cause buffer overruns and packet drops if routers have small
540 buffers or the link is experiencing congestion.
542 Enable the Limited Transmit algorithm as described in RFC 3042.
543 It helps avoid timeouts on lossy links and also when the congestion window
544 is small, as happens on short transfers.
546 Enable support for RFC 3390, which allows for a variable-sized
547 starting congestion window on new connections, depending on the
548 maximum segment size.
549 This helps throughput in general, but
550 particularly affects short transfers and high-bandwidth large
551 propagation-delay connections.
553 Enable support for RFC 2018, TCP Selective Acknowledgment option,
554 which allows the receiver to inform the sender about all successfully
555 arrived segments, allowing the sender to retransmit the missing segments
558 Maximum number of SACK holes per connection.
560 .It Va sack.globalmaxholes
561 Maximum number of SACK holes per system, across all connections.
564 When a TCP connection enters the
566 state, its associated socket structure is freed, since it is of
567 negligible size and use, and a new structure is allocated to contain a
568 minimal amount of information necessary for sustaining a connection in
569 this state, called the compressed TCP TIME_WAIT state.
570 Since this structure is smaller than a socket structure, it can save
571 a significant amount of system memory.
573 .Va net.inet.tcp.maxtcptw
574 MIB variable controls the maximum number of these structures allocated.
575 By default, it is initialized to
576 .Va kern.ipc.maxsockets
578 .It Va nolocaltimewait
579 Suppress creating of compressed TCP TIME_WAIT states for connections in
580 which both endpoints are local.
581 .It Va fast_finwait2_recycle
585 connections faster when the socket is marked as
587 (no user process has the socket open, data received on
588 the socket cannot be read).
589 The timeout used here is
590 .Va finwait2_timeout .
591 .It Va finwait2_timeout
592 Timeout to use for fast recycling of
596 Defaults to 60 seconds.
598 Enable support for TCP Explicit Congestion Notification (ECN).
599 ECN allows a TCP sender to reduce the transmission rate in order to
606 Allow incoming connections to request ECN.
607 Outgoing connections will request ECN.
609 Allow incoming connections to request ECN.
610 Outgoing connections will not request ECN.
612 .It Va ecn.maxretries
613 Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
615 This is needed to help with connection establishment
616 when a broken firewall is in the network path.
617 .It Va pmtud_blackhole_detection
618 Turn on automatic path MTU blackhole detection.
619 In case of retransmits OS will
620 lower the MSS to check if it's MTU problem.
621 If current MSS is greater than
622 configured value to try, it will be set to configured value, otherwise,
623 MSS will be set to default values
624 .Po Va net.inet.tcp.mssdflt
626 .Va net.inet.tcp.v6mssdflt
628 .It Va pmtud_blackhole_mss
629 MSS to try for IPv4 if PMTU blackhole detection is turned on.
630 .It Va v6pmtud_blackhole_mss
631 MSS to try for IPv6 if PMTU blackhole detection is turned on.
632 .It Va pmtud_blackhole_activated
633 Number of times configured values were used in an attempt to downshift.
634 .It Va pmtud_blackhole_activated_min_mss
635 Number of times default MSS was used in an attempt to downshift.
636 .It Va pmtud_blackhole_failed
637 Number of connections for which retransmits continued even after MSS
639 .It Va functions_available
640 List of available TCP function blocks (TCP stacks).
641 .It Va functions_default
642 The default TCP function block (TCP stack).
643 .It Va functions_inherit_listen_socket_stack
644 Determines whether to inherit listen socket's tcp stack or use the current
645 system default tcp stack, as defined by
646 .Va functions_default .
649 Use criteria defined in RFC793 instead of RFC5961 for accepting RST segments.
652 Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments.
654 .It Va ts_offset_per_conn
655 When initializing the TCP timestamps, use a per connection offset instead of a
656 per host pair offset.
657 Default is to use per connection offsets as recommended in RFC 7323.
660 A socket operation may fail with one of the following errors returned:
663 when trying to establish a connection on a socket which
665 .It Bo Er ENOBUFS Bc or Bo Er ENOMEM Bc
666 when the system runs out of memory for
667 an internal data structure;
669 when a connection was dropped
670 due to excessive retransmissions;
673 forces the connection to be closed;
674 .It Bq Er ECONNREFUSED
676 peer actively refuses connection establishment (usually because
677 no process is listening to the port);
680 is made to create a socket with a port which has already been
682 .It Bq Er EADDRNOTAVAIL
683 when an attempt is made to create a
684 socket with a network address for which no network interface
686 .It Bq Er EAFNOSUPPORT
687 when an attempt is made to bind or connect a socket to a multicast
690 when trying to change TCP function blocks at an invalid point in the session;
692 when trying to use a TCP function block that is not available;
711 .%T "TCP Extensions for High Performance"
716 .%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
720 .%A "K. Ramakrishnan"
723 .%T "The Addition of Explicit Congestion Notification (ECN) to IP"
731 The RFC 1323 extensions for window scaling and timestamps were added
736 option was introduced in
739 .Em subject to change .
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