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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
306 .It Dv TCP_TXTLS_MODE
307 The integer argument can be used to get or set the current TLS transmit mode
312 .It Dv TCP_RXTLS_ENABLE
313 Enable in-kernel TLS for data read from this socket.
317 .It Dv TCP_REUSPORT_LB_NUMA
318 Changes NUMA affinity filtering for an established TCP listen
320 This option takes a single integer argument which specifies
321 the NUMA domain to filter on for this listen socket.
322 The argument can also have the follwing special values:
323 .Bl -tag -width "Dv TCP_REUSPORT_LB_NUMA"
324 .It Dv TCP_REUSPORT_LB_NUMA_NODOM
325 Remove NUMA filtering for this listen socket.
326 .It Dv TCP_REUSPORT_LB_NUMA_CURDOM
327 Filter traffic associated with the domain where the calling thread is
329 This is typically used after a process or thread inherits a listen
330 socket from its parent, and sets its CPU affinity to a particular core.
334 The option level for the
336 call is the protocol number for
339 .Xr getprotobyname 3 ,
342 All options are declared in
347 transport level may be used with
351 Incoming connection requests that are source-routed are noted,
352 and the reverse source route is used in responding.
354 The default congestion control algorithm for
358 Other congestion control algorithms can be made available using the
364 protocol implements a number of variables in the
369 .Bl -tag -width ".Va TCPCTL_DO_RFC1323"
370 .It Dv TCPCTL_DO_RFC1323
372 Implement the window scaling and timestamp options of RFC 1323/RFC 7323
374 .It Va tolerate_missing_ts
375 Tolerate the missing of timestamps (RFC 1323/RFC 7323) for
377 segments belonging to
379 connections for which support of
381 timestamps has been negotiated.
382 (default is 0, i.e., the missing of timestamps is not tolerated).
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 New Congestion Window Validation mechanism as described in RFC 7661.
543 This gently reduces the congestion window during periods, where TCP is
544 application limited and the network bandwidth is not utilized completely.
545 That prevents self-inflicted packet losses once the application starts to
546 transmit data at a higher speed.
548 Perform SACK loss recovery using the Proportional Rate Reduction (PRR) algorithm
549 described in RFC6937.
550 This improves the effectiveness of retransmissions particular in environments
551 with ACK thinning or burst loss events, as chances to run out of the ACK clock
552 are reduced, preventing lengthy and performance reducing RTO based loss recovery
554 .It Va do_prr_conservative
555 While doing Proportional Rate Reduction, remain strictly in a packet conserving
556 mode, sending only one new packet for each ACK received.
557 Helpful when a misconfigured token bucket traffic policer causes persistent
558 high losses leading to RTO, but reduces PRR effectiveness in more common settings
561 Calculate the bytes in flight using the algorithm described in RFC 6675, and
562 is also an improvement when Proportional Rate Reduction is enabled.
563 Also enables two other mechanisms from RFC6675.
564 Rescue Retransmission helps timely loss recovery, when the trailing segments
565 of a transmission are lost, while no additional data is ready to be sent.
566 In case a partial ACK without a SACK block is received during SACK loss
567 recovery, the trailing segment is immediately resent, rather than waiting
568 for a Retransmission timeout.
569 SACK loss recovery is also engaged, once two segments plus one byte are
570 SACKed - even if no traditional duplicate ACKs were seen.
572 Enable the Limited Transmit algorithm as described in RFC 3042.
573 It helps avoid timeouts on lossy links and also when the congestion window
574 is small, as happens on short transfers.
576 Enable support for RFC 3390, which allows for a variable-sized
577 starting congestion window on new connections, depending on the
578 maximum segment size.
579 This helps throughput in general, but
580 particularly affects short transfers and high-bandwidth large
581 propagation-delay connections.
583 Enable support for RFC 2018, TCP Selective Acknowledgment option,
584 which allows the receiver to inform the sender about all successfully
585 arrived segments, allowing the sender to retransmit the missing segments
588 Maximum number of SACK holes per connection.
590 .It Va sack.globalmaxholes
591 Maximum number of SACK holes per system, across all connections.
594 When a TCP connection enters the
596 state, its associated socket structure is freed, since it is of
597 negligible size and use, and a new structure is allocated to contain a
598 minimal amount of information necessary for sustaining a connection in
599 this state, called the compressed TCP TIME_WAIT state.
600 Since this structure is smaller than a socket structure, it can save
601 a significant amount of system memory.
603 .Va net.inet.tcp.maxtcptw
604 MIB variable controls the maximum number of these structures allocated.
605 By default, it is initialized to
606 .Va kern.ipc.maxsockets
608 .It Va nolocaltimewait
609 Suppress creating of compressed TCP TIME_WAIT states for connections in
610 which both endpoints are local.
611 .It Va fast_finwait2_recycle
615 connections faster when the socket is marked as
617 (no user process has the socket open, data received on
618 the socket cannot be read).
619 The timeout used here is
620 .Va finwait2_timeout .
621 .It Va finwait2_timeout
622 Timeout to use for fast recycling of
626 Defaults to 60 seconds.
628 Enable support for TCP Explicit Congestion Notification (ECN).
629 ECN allows a TCP sender to reduce the transmission rate in order to
636 Allow incoming connections to request ECN.
637 Outgoing connections will request ECN.
639 Allow incoming connections to request ECN.
640 Outgoing connections will not request ECN.
642 .It Va ecn.maxretries
643 Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
645 This is needed to help with connection establishment
646 when a broken firewall is in the network path.
647 .It Va pmtud_blackhole_detection
648 Enable automatic path MTU blackhole detection.
649 In case of retransmits of MSS sized segments,
650 the OS will lower the MSS to check if it's an MTU problem.
651 If the current MSS is greater than the configured value to try
652 .Po Va net.inet.tcp.pmtud_blackhole_mss
654 .Va net.inet.tcp.v6pmtud_blackhole_mss
656 it will be set to this value, otherwise,
657 the MSS will be set to the default values
658 .Po Va net.inet.tcp.mssdflt
660 .Va net.inet.tcp.v6mssdflt
665 Disable path MTU blackhole detection.
667 Enable path MTU blackhole detection for IPv4 and IPv6.
669 Enable path MTU blackhole detection only for IPv4.
671 Enable path MTU blackhole detection only for IPv6.
673 .It Va pmtud_blackhole_mss
674 MSS to try for IPv4 if PMTU blackhole detection is turned on.
675 .It Va v6pmtud_blackhole_mss
676 MSS to try for IPv6 if PMTU blackhole detection is turned on.
677 .It Va functions_available
678 List of available TCP function blocks (TCP stacks).
679 .It Va functions_default
680 The default TCP function block (TCP stack).
681 .It Va functions_inherit_listen_socket_stack
682 Determines whether to inherit listen socket's tcp stack or use the current
683 system default tcp stack, as defined by
684 .Va functions_default .
687 Use criteria defined in RFC793 instead of RFC5961 for accepting RST segments.
690 Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments.
692 .It Va ts_offset_per_conn
693 When initializing the TCP timestamps, use a per connection offset instead of a
694 per host pair offset.
695 Default is to use per connection offsets as recommended in RFC 7323.
696 .It Va perconn_stats_enable
697 Controls the default collection of statistics for all connections using the
700 0 disables, 1 enables, 2 enables random sampling across log id connection
701 groups with all connections in a group receiving the same setting.
702 .It Va perconn_stats_sample_rates
703 A CSV list of template_spec=percent key-value pairs which controls the per
704 template sampling rates when
709 A socket operation may fail with one of the following errors returned:
712 when trying to establish a connection on a socket which
714 .It Bo Er ENOBUFS Bc or Bo Er ENOMEM Bc
715 when the system runs out of memory for
716 an internal data structure;
718 when a connection was dropped
719 due to excessive retransmissions;
722 forces the connection to be closed;
723 .It Bq Er ECONNREFUSED
725 peer actively refuses connection establishment (usually because
726 no process is listening to the port);
729 is made to create a socket with a port which has already been
731 .It Bq Er EADDRNOTAVAIL
732 when an attempt is made to create a
733 socket with a network address for which no network interface
735 .It Bq Er EAFNOSUPPORT
736 when an attempt is made to bind or connect a socket to a multicast
739 when trying to change TCP function blocks at an invalid point in the session;
741 when trying to use a TCP function block that is not available;
763 .%T "TCP Extensions for High Performance"
770 .%A "R. Scheffenegger"
771 .%T "TCP Extensions for High Performance"
776 .%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
780 .%A "K. Ramakrishnan"
783 .%T "The Addition of Explicit Congestion Notification (ECN) to IP"
791 The RFC 1323 extensions for window scaling and timestamps were added
796 option was introduced in
799 .Em subject to change .