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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 if the
276 .Va net.inet.tcp.signature_verify_input
278 The current default behavior for the system is to respond to a system
279 advertising this option with TCP-MD5; this may change.
281 One common use for this in a
283 router deployment is to enable
284 based routers to interwork with Cisco equipment at peering points.
285 Support for this feature conforms to RFC 2385.
288 sessions are supported.
290 In order for this option to function correctly, it is necessary for the
291 administrator to add a tcp-md5 key entry to the system's security
292 associations database (SADB) using the
295 This entry must have an SPI of 0x1000 and can therefore only be specified
296 on a per-host basis at this time.
298 If an SADB entry cannot be found for the destination, the outgoing traffic
299 will have an invalid digest option prepended, and the following error message
300 will be visible on the system console:
301 .Em "tcp_signature_compute: SADB lookup failed for %d.%d.%d.%d" .
304 The option level for the
306 call is the protocol number for
309 .Xr getprotobyname 3 ,
312 All options are declared in
317 transport level may be used with
321 Incoming connection requests that are source-routed are noted,
322 and the reverse source route is used in responding.
324 The default congestion control algorithm for
328 Other congestion control algorithms can be made available using the
334 protocol implements a number of variables in the
339 .Bl -tag -width ".Va TCPCTL_DO_RFC1323"
340 .It Dv TCPCTL_DO_RFC1323
342 Implement the window scaling and timestamp options of RFC 1323
344 .It Dv TCPCTL_MSSDFLT
346 The default value used for the maximum segment size
348 when no advice to the contrary is received from MSS negotiation.
349 .It Dv TCPCTL_SENDSPACE
354 .It Dv TCPCTL_RECVSPACE
360 Log any connection attempts to ports where there is not a socket
361 accepting connections.
362 The value of 1 limits the logging to
364 (connection establishment) packets only.
365 That of 2 results in any
367 packets to closed ports being logged.
368 Any value unlisted above disables the logging
369 (default is 0, i.e., the logging is disabled).
371 The Maximum Segment Lifetime, in milliseconds, for a packet.
373 Timeout, in milliseconds, for new, non-established
376 The default is 75000 msec.
378 Amount of time, in milliseconds, that the connection must be idle
379 before keepalive probes (if enabled) are sent.
380 The default is 7200000 msec (2 hours).
382 The interval, in milliseconds, between keepalive probes sent to remote
383 machines, when no response is received on a
386 The default is 75000 msec.
388 Number of probes sent, with no response, before a connection
390 The default is 8 packets.
391 .It Va always_keepalive
396 connections, the kernel will
397 periodically send a packet to the remote host to verify the connection
402 unreachable messages may abort connections in
408 reassembly queue if the system is low on mbufs.
410 If enabled, disable sending of RST when a connection is attempted
411 to a port where there is not a socket accepting connections.
415 Delay ACK to try and piggyback it onto a data packet.
417 Maximum amount of time, in milliseconds, before a delayed ACK is sent.
418 .It Va path_mtu_discovery
419 Enable Path MTU Discovery.
423 control-block hash table
425 This may be tuned using the kernel option
428 .Va net.inet.tcp.tcbhashsize
432 Number of active process control blocks
435 Determines whether or not
437 cookies should be generated for outbound
441 cookies are a great help during
443 flood attacks, and are enabled by default.
446 .It Va isn_reseed_interval
447 The interval (in seconds) specifying how often the secret data used in
448 RFC 1948 initial sequence number calculations should be reseeded.
449 By default, this variable is set to zero, indicating that
450 no reseeding will occur.
451 Reseeding should not be necessary, and will break
453 recycling for a few minutes.
454 .It Va rexmit_min , rexmit_slop
455 Adjust the retransmit timer calculation for
458 typically added to the raw calculation to take into account
459 occasional variances that the
461 (smoothed round-trip time)
462 is unable to accommodate, while the minimum specifies an
467 second minimum, these RFCs tend to focus on streaming behavior,
468 and fail to deal with the fact that a 1 second minimum has severe
469 detrimental effects over lossy interactive connections, such
470 as a 802.11b wireless link, and over very fast but lossy
471 connections for those cases not covered by the fast retransmit
473 For this reason, we use 200ms of slop and a near-0
474 minimum, which gives us an effective minimum of 200ms (similar to
476 .It Va initcwnd_segments
477 Enable the ability to specify initial congestion window in number of segments.
478 The default value is 10 as suggested by RFC 6928.
479 Changing the value on fly would not affect connections using congestion window
482 This regulates the burst of packets allowed to be sent in the first RTT.
483 The value should be relative to the link capacity.
484 Start with small values for lower-capacity links.
485 Large bursts can cause buffer overruns and packet drops if routers have small
486 buffers or the link is experiencing congestion.
488 Enable the Limited Transmit algorithm as described in RFC 3042.
489 It helps avoid timeouts on lossy links and also when the congestion window
490 is small, as happens on short transfers.
492 Enable support for RFC 3390, which allows for a variable-sized
493 starting congestion window on new connections, depending on the
494 maximum segment size.
495 This helps throughput in general, but
496 particularly affects short transfers and high-bandwidth large
497 propagation-delay connections.
499 Enable support for RFC 2018, TCP Selective Acknowledgment option,
500 which allows the receiver to inform the sender about all successfully
501 arrived segments, allowing the sender to retransmit the missing segments
504 Maximum number of SACK holes per connection.
506 .It Va sack.globalmaxholes
507 Maximum number of SACK holes per system, across all connections.
510 When a TCP connection enters the
512 state, its associated socket structure is freed, since it is of
513 negligible size and use, and a new structure is allocated to contain a
514 minimal amount of information necessary for sustaining a connection in
515 this state, called the compressed TCP TIME_WAIT state.
516 Since this structure is smaller than a socket structure, it can save
517 a significant amount of system memory.
519 .Va net.inet.tcp.maxtcptw
520 MIB variable controls the maximum number of these structures allocated.
521 By default, it is initialized to
522 .Va kern.ipc.maxsockets
524 .It Va nolocaltimewait
525 Suppress creating of compressed TCP TIME_WAIT states for connections in
526 which both endpoints are local.
527 .It Va fast_finwait2_recycle
531 connections faster when the socket is marked as
533 (no user process has the socket open, data received on
534 the socket cannot be read).
535 The timeout used here is
536 .Va finwait2_timeout .
537 .It Va finwait2_timeout
538 Timeout to use for fast recycling of
542 Defaults to 60 seconds.
544 Enable support for TCP Explicit Congestion Notification (ECN).
545 ECN allows a TCP sender to reduce the transmission rate in order to
552 Allow incoming connections to request ECN.
553 Outgoing connections will request ECN.
555 Allow incoming connections to request ECN.
556 Outgoing connections will not request ECN.
558 .It Va ecn.maxretries
559 Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
561 This is needed to help with connection establishment
562 when a broken firewall is in the network path.
563 .It Va pmtud_blackhole_detection
564 Turn on automatic path MTU blackhole detection.
565 In case of retransmits OS will
566 lower the MSS to check if it's MTU problem.
567 If current MSS is greater than
568 configured value to try, it will be set to configured value, otherwise,
569 MSS will be set to default values
570 .Po Va net.inet.tcp.mssdflt
572 .Va net.inet.tcp.v6mssdflt
574 .It Va pmtud_blackhole_mss
575 MSS to try for IPv4 if PMTU blackhole detection is turned on.
576 .It Va v6pmtud_blackhole_mss
577 MSS to try for IPv6 if PMTU blackhole detection is turned on.
578 .It Va pmtud_blackhole_activated
579 Number of times configured values were used in an attempt to downshift.
580 .It Va pmtud_blackhole_activated_min_mss
581 Number of times default MSS was used in an attempt to downshift.
582 .It Va pmtud_blackhole_failed
583 Number of connections for which retransmits continued even after MSS
585 .It Va functions_available
586 List of available TCP function blocks (TCP stacks).
587 .It Va functions_default
588 The default TCP function block (TCP stack).
591 A socket operation may fail with one of the following errors returned:
594 when trying to establish a connection on a socket which
597 when the system runs out of memory for
598 an internal data structure;
600 when a connection was dropped
601 due to excessive retransmissions;
604 forces the connection to be closed;
605 .It Bq Er ECONNREFUSED
607 peer actively refuses connection establishment (usually because
608 no process is listening to the port);
611 is made to create a socket with a port which has already been
613 .It Bq Er EADDRNOTAVAIL
614 when an attempt is made to create a
615 socket with a network address for which no network interface
617 .It Bq Er EAFNOSUPPORT
618 when an attempt is made to bind or connect a socket to a multicast
621 when trying to change TCP function blocks at an invalid point in the session;
623 when trying to use a TCP function block that is not available;
641 .%T "TCP Extensions for High Performance"
646 .%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
650 .%A "K. Ramakrishnan"
653 .%T "The Addition of Explicit Congestion Notification (ECN) to IP"
661 The RFC 1323 extensions for window scaling and timestamps were added
666 option was introduced in
669 .Em subject to change .
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