1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. 3.\" Copyright (c) 2010-2011 The FreeBSD Foundation 4.\" All rights reserved. 5.\" 6.\" Portions of this documentation were written at the Centre for Advanced 7.\" Internet Architectures, Swinburne University of Technology, Melbourne, 8.\" Australia by David Hayes under sponsorship from the FreeBSD Foundation. 9.\" 10.\" Redistribution and use in source and binary forms, with or without 11.\" modification, are permitted provided that the following conditions 12.\" are met: 13.\" 1. Redistributions of source code must retain the above copyright 14.\" notice, this list of conditions and the following disclaimer. 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in the 17.\" documentation and/or other materials provided with the distribution. 18.\" 3. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" From: @(#)tcp.4 8.1 (Berkeley) 6/5/93 35.\" 36.Dd November 17, 2023 37.Dt TCP 4 38.Os 39.Sh NAME 40.Nm tcp 41.Nd Internet Transmission Control Protocol 42.Sh SYNOPSIS 43.In sys/types.h 44.In sys/socket.h 45.In netinet/in.h 46.In netinet/tcp.h 47.Ft int 48.Fn socket AF_INET SOCK_STREAM 0 49.Sh DESCRIPTION 50The 51.Tn TCP 52protocol provides reliable, flow-controlled, two-way 53transmission of data. 54It is a byte-stream protocol used to 55support the 56.Dv SOCK_STREAM 57abstraction. 58.Tn TCP 59uses the standard 60Internet address format and, in addition, provides a per-host 61collection of 62.Dq "port addresses" . 63Thus, each address is composed 64of an Internet address specifying the host and network, 65with a specific 66.Tn TCP 67port on the host identifying the peer entity. 68.Pp 69Sockets utilizing the 70.Tn TCP 71protocol are either 72.Dq active 73or 74.Dq passive . 75Active sockets initiate connections to passive 76sockets. 77By default, 78.Tn TCP 79sockets are created active; to create a 80passive socket, the 81.Xr listen 2 82system call must be used 83after binding the socket with the 84.Xr bind 2 85system call. 86Only passive sockets may use the 87.Xr accept 2 88call to accept incoming connections. 89Only active sockets may use the 90.Xr connect 2 91call to initiate connections. 92.Pp 93Passive sockets may 94.Dq underspecify 95their location to match 96incoming connection requests from multiple networks. 97This technique, termed 98.Dq "wildcard addressing" , 99allows a single 100server to provide service to clients on multiple networks. 101To create a socket which listens on all networks, the Internet 102address 103.Dv INADDR_ANY 104must be bound. 105The 106.Tn TCP 107port may still be specified 108at this time; if the port is not specified, the system will assign one. 109Once a connection has been established, the socket's address is 110fixed by the peer entity's location. 111The address assigned to the 112socket is the address associated with the network interface 113through which packets are being transmitted and received. 114Normally, this address corresponds to the peer entity's network. 115.Pp 116.Tn TCP 117supports a number of socket options which can be set with 118.Xr setsockopt 2 119and tested with 120.Xr getsockopt 2 : 121.Bl -tag -width ".Dv TCP_FUNCTION_BLK" 122.It Dv TCP_INFO 123Information about a socket's underlying TCP session may be retrieved 124by passing the read-only option 125.Dv TCP_INFO 126to 127.Xr getsockopt 2 . 128It accepts a single argument: a pointer to an instance of 129.Vt "struct tcp_info" . 130.Pp 131This API is subject to change; consult the source to determine 132which fields are currently filled out by this option. 133.Fx 134specific additions include 135send window size, 136receive window size, 137and 138bandwidth-controlled window space. 139.It Dv TCP_CCALGOOPT 140Set or query congestion control algorithm specific parameters. 141See 142.Xr mod_cc 4 143for details. 144.It Dv TCP_CONGESTION 145Select or query the congestion control algorithm that TCP will use for the 146connection. 147See 148.Xr mod_cc 4 149for details. 150.It Dv TCP_FASTOPEN 151Enable or disable TCP Fast Open (TFO). 152To use this option, the kernel must be built with the 153.Dv TCP_RFC7413 154option. 155.Pp 156This option can be set on the socket either before or after the 157.Xr listen 2 158is invoked. 159Clearing this option on a listen socket after it has been set has no effect on 160existing TFO connections or TFO connections in progress; it only prevents new 161TFO connections from being established. 162.Pp 163For passively-created sockets, the 164.Dv TCP_FASTOPEN 165socket option can be queried to determine whether the connection was established 166using TFO. 167Note that connections that are established via a TFO 168.Tn SYN , 169but that fall back to using a non-TFO 170.Tn SYN|ACK 171will have the 172.Dv TCP_FASTOPEN 173socket option set. 174.Pp 175In addition to the facilities defined in RFC7413, this implementation supports a 176pre-shared key (PSK) mode of operation in which the TFO server requires the 177client to be in posession of a shared secret in order for the client to be able 178to successfully open TFO connections with the server. 179This is useful, for example, in environments where TFO servers are exposed to 180both internal and external clients and only wish to allow TFO connections from 181internal clients. 182.Pp 183In the PSK mode of operation, the server generates and sends TFO cookies to 184requesting clients as usual. 185However, when validating cookies received in TFO SYNs from clients, the server 186requires the client-supplied cookie to equal 187.Bd -literal -offset left 188SipHash24(key=\fI16-byte-psk\fP, msg=\fIcookie-sent-to-client\fP) 189.Ed 190.Pp 191Multiple concurrent valid pre-shared keys are supported so that time-based 192rolling PSK invalidation policies can be implemented in the system. 193The default number of concurrent pre-shared keys is 2. 194.Pp 195This can be adjusted with the 196.Dv TCP_RFC7413_MAX_PSKS 197kernel option. 198.It Dv TCP_FUNCTION_BLK 199Select or query the set of functions that TCP will use for this connection. 200This allows a user to select an alternate TCP stack. 201The alternate TCP stack must already be loaded in the kernel. 202To list the available TCP stacks, see 203.Va functions_available 204in the 205.Sx MIB (sysctl) Variables 206section further down. 207To list the default TCP stack, see 208.Va functions_default 209in the 210.Sx MIB (sysctl) Variables 211section. 212.It Dv TCP_KEEPINIT 213This 214.Xr setsockopt 2 215option accepts a per-socket timeout argument of 216.Vt "u_int" 217in seconds, for new, non-established 218.Tn TCP 219connections. 220For the global default in milliseconds see 221.Va keepinit 222in the 223.Sx MIB (sysctl) Variables 224section further down. 225.It Dv TCP_KEEPIDLE 226This 227.Xr setsockopt 2 228option accepts an argument of 229.Vt "u_int" 230for the amount of time, in seconds, that the connection must be idle 231before keepalive probes (if enabled) are sent for the connection of this 232socket. 233If set on a listening socket, the value is inherited by the newly created 234socket upon 235.Xr accept 2 . 236For the global default in milliseconds see 237.Va keepidle 238in the 239.Sx MIB (sysctl) Variables 240section further down. 241.It Dv TCP_KEEPINTVL 242This 243.Xr setsockopt 2 244option accepts an argument of 245.Vt "u_int" 246to set the per-socket interval, in seconds, between keepalive probes sent 247to a peer. 248If set on a listening socket, the value is inherited by the newly created 249socket upon 250.Xr accept 2 . 251For the global default in milliseconds see 252.Va keepintvl 253in the 254.Sx MIB (sysctl) Variables 255section further down. 256.It Dv TCP_KEEPCNT 257This 258.Xr setsockopt 2 259option accepts an argument of 260.Vt "u_int" 261and allows a per-socket tuning of the number of probes sent, with no response, 262before the connection will be dropped. 263If set on a listening socket, the value is inherited by the newly created 264socket upon 265.Xr accept 2 . 266For the global default see the 267.Va keepcnt 268in the 269.Sx MIB (sysctl) Variables 270section further down. 271.It Dv TCP_NODELAY 272Under most circumstances, 273.Tn TCP 274sends data when it is presented; 275when outstanding data has not yet been acknowledged, it gathers 276small amounts of output to be sent in a single packet once 277an acknowledgement is received. 278For a small number of clients, such as window systems 279that send a stream of mouse events which receive no replies, 280this packetization may cause significant delays. 281The boolean option 282.Dv TCP_NODELAY 283defeats this algorithm. 284 285.It Dv TCP_MAXSEG 286By default, a sender- and 287.No receiver- Ns Tn TCP 288will negotiate among themselves to determine the maximum segment size 289to be used for each connection. 290The 291.Dv TCP_MAXSEG 292option allows the user to determine the result of this negotiation, 293and to reduce it if desired. 294.It Dv TCP_MAXUNACKTIME 295This 296.Xr setsockopt 2 297option accepts an argument of 298.Vt "u_int" 299to set the per-socket interval, in seconds, in which the connection must 300make progress. Progress is defined by at least 1 byte being acknowledged within 301the set time period. If a connection fails to make progress, then the 302.Tn TCP 303stack will terminate the connection with a reset. Note that the default 304value for this is zero which indicates no progress checks should be made. 305.It Dv TCP_NOOPT 306.Tn TCP 307usually sends a number of options in each packet, corresponding to 308various 309.Tn TCP 310extensions which are provided in this implementation. 311The boolean option 312.Dv TCP_NOOPT 313is provided to disable 314.Tn TCP 315option use on a per-connection basis. 316.It Dv TCP_NOPUSH 317By convention, the 318.No sender- Ns Tn TCP 319will set the 320.Dq push 321bit, and begin transmission immediately (if permitted) at the end of 322every user call to 323.Xr write 2 324or 325.Xr writev 2 . 326When this option is set to a non-zero value, 327.Tn TCP 328will delay sending any data at all until either the socket is closed, 329or the internal send buffer is filled. 330.It Dv TCP_MD5SIG 331This option enables the use of MD5 digests (also known as TCP-MD5) 332on writes to the specified socket. 333Outgoing traffic is digested; 334digests on incoming traffic are verified. 335When this option is enabled on a socket, all inbound and outgoing 336TCP segments must be signed with MD5 digests. 337.Pp 338One common use for this in a 339.Fx 340router deployment is to enable 341based routers to interwork with Cisco equipment at peering points. 342Support for this feature conforms to RFC 2385. 343.Pp 344In order for this option to function correctly, it is necessary for the 345administrator to add a tcp-md5 key entry to the system's security 346associations database (SADB) using the 347.Xr setkey 8 348utility. 349This entry can only be specified on a per-host basis at this time. 350.Pp 351If an SADB entry cannot be found for the destination, 352the system does not send any outgoing segments and drops any inbound segments. 353However, during connection negotiation, a non-signed segment will be accepted if 354an SADB entry does not exist between hosts. 355When a non-signed segment is accepted, the established connection is not 356protected with MD5 digests. 357.It Dv TCP_STATS 358Manage collection of connection level statistics using the 359.Xr stats 3 360framework. 361.Pp 362Each dropped segment is taken into account in the TCP protocol statistics. 363.It Dv TCP_TXTLS_ENABLE 364Enable in-kernel Transport Layer Security (TLS) for data written to this 365socket. 366See 367.Xr ktls 4 368for more details. 369.It Dv TCP_TXTLS_MODE 370The integer argument can be used to get or set the current TLS transmit mode 371of a socket. 372See 373.Xr ktls 4 374for more details. 375.It Dv TCP_RXTLS_ENABLE 376Enable in-kernel TLS for data read from this socket. 377See 378.Xr ktls 4 379for more details. 380.It Dv TCP_REUSPORT_LB_NUMA 381Changes NUMA affinity filtering for an established TCP listen 382socket. 383This option takes a single integer argument which specifies 384the NUMA domain to filter on for this listen socket. 385The argument can also have the follwing special values: 386.Bl -tag -width "Dv TCP_REUSPORT_LB_NUMA" 387.It Dv TCP_REUSPORT_LB_NUMA_NODOM 388Remove NUMA filtering for this listen socket. 389.It Dv TCP_REUSPORT_LB_NUMA_CURDOM 390Filter traffic associated with the domain where the calling thread is 391currently executing. 392This is typically used after a process or thread inherits a listen 393socket from its parent, and sets its CPU affinity to a particular core. 394.El 395.It Dv TCP_REMOTE_UDP_ENCAPS_PORT 396Set and get the remote UDP encapsulation port. 397It can only be set on a closed TCP socket. 398.El 399.Pp 400The option level for the 401.Xr setsockopt 2 402call is the protocol number for 403.Tn TCP , 404available from 405.Xr getprotobyname 3 , 406or 407.Dv IPPROTO_TCP . 408All options are declared in 409.In netinet/tcp.h . 410.Pp 411Options at the 412.Tn IP 413transport level may be used with 414.Tn TCP ; 415see 416.Xr ip 4 . 417Incoming connection requests that are source-routed are noted, 418and the reverse source route is used in responding. 419.Pp 420The default congestion control algorithm for 421.Tn TCP 422is 423.Xr cc_newreno 4 . 424Other congestion control algorithms can be made available using the 425.Xr mod_cc 4 426framework. 427.Ss MIB (sysctl) Variables 428The 429.Tn TCP 430protocol implements a number of variables in the 431.Va net.inet.tcp 432branch of the 433.Xr sysctl 3 434MIB, which can also be read or modified with 435.Xr sysctl 8 . 436.Bl -tag -width ".Va v6pmtud_blackhole_mss" 437.It Va always_keepalive 438Assume that 439.Dv SO_KEEPALIVE 440is set on all 441.Tn TCP 442connections, the kernel will 443periodically send a packet to the remote host to verify the connection 444is still up. 445.It Va blackhole 446If enabled, disable sending of RST when a connection is attempted 447to a port where there is no socket accepting connections. 448See 449.Xr blackhole 4 . 450.It Va blackhole_local 451See 452.Xr blackhole 4 . 453.It Va cc 454A number of variables for congestion control are under the 455.Va net.inet.tcp.cc 456node. 457See 458.Xr mod_cc 4 . 459.It Va cc.newreno 460Variables for NewReno congestion control are under the 461.Va net.inet.tcp.cc.newreno 462node. 463See 464.Xr cc_newreno 4 . 465.It Va delacktime 466Maximum amount of time, in milliseconds, before a delayed ACK is sent. 467.It Va delayed_ack 468Delay ACK to try and piggyback it onto a data packet or another ACK. 469.It Va do_lrd 470Enable Lost Retransmission Detection for SACK-enabled sessions, disabled by 471default. 472Under severe congestion, a retransmission can be lost which then leads to a 473mandatory Retransmission Timeout (RTO), followed by slow-start. 474LRD will try to resend the repeatedly lost packet, preventing the time-consuming 475RTO and performance reducing slow-start. 476.It Va do_prr 477Perform SACK loss recovery using the Proportional Rate Reduction (PRR) algorithm 478described in RFC6937. 479This improves the effectiveness of retransmissions particular in environments 480with ACK thinning or burst loss events, as chances to run out of the ACK clock 481are reduced, preventing lengthy and performance reducing RTO based loss recovery 482(default is true). 483.It Va do_tcpdrain 484Flush packets in the 485.Tn TCP 486reassembly queue if the system is low on mbufs. 487.It Va drop_synfin 488Drop TCP packets with both SYN and FIN set. 489.It Va ecn.enable 490Enable support for TCP Explicit Congestion Notification (ECN). 491ECN allows a TCP sender to reduce the transmission rate in order to 492avoid packet drops. 493.Bl -tag -compact 494.It 0 495Disable ECN. 496.It 1 497Allow incoming connections to request ECN. 498Outgoing connections will request ECN. 499.It 2 500Allow incoming connections to request ECN. 501Outgoing connections will not request ECN. 502(default) 503.It 3 504Negotiate on incoming connection for Accurate ECN, ECN, or no ECN. 505Outgoing connections will request Accurate ECN and fall back to 506ECN depending on the capabilities of the server. 507.It 4 508Negotiate on incoming connection for Accurate ECN, ECN, or no ECN. 509Outgoing connections will not request ECN. 510.El 511.It Va ecn.maxretries 512Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a 513specific connection. 514This is needed to help with connection establishment 515when a broken firewall is in the network path. 516.It Va fast_finwait2_recycle 517Recycle 518.Tn TCP 519.Dv FIN_WAIT_2 520connections faster when the socket is marked as 521.Dv SBS_CANTRCVMORE 522(no user process has the socket open, data received on 523the socket cannot be read). 524The timeout used here is 525.Va finwait2_timeout . 526.It Va fastopen.acceptany 527When non-zero, all client-supplied TFO cookies will be considered to be valid. 528The default is 0. 529.It Va fastopen.autokey 530When this and 531.Va net.inet.tcp.fastopen.server_enable 532are non-zero, a new key will be automatically generated after this specified 533seconds. 534The default is 120. 535.It Va fastopen.ccache_bucket_limit 536The maximum number of entries in a client cookie cache bucket. 537The default value can be tuned with the 538.Dv TCP_FASTOPEN_CCACHE_BUCKET_LIMIT_DEFAULT 539kernel option or by setting 540.Va net.inet.tcp.fastopen_ccache_bucket_limit 541in the 542.Xr loader 8 . 543.It Va fastopen.ccache_buckets 544The number of client cookie cache buckets. 545Read-only. 546The value can be tuned with the 547.Dv TCP_FASTOPEN_CCACHE_BUCKETS_DEFAULT 548kernel option or by setting 549.Va fastopen.ccache_buckets 550in the 551.Xr loader 8 . 552.It Va fastopen.ccache_list 553Print the client cookie cache. 554Read-only. 555.It Va fastopen.client_enable 556When zero, no new active (i.e., client) TFO connections can be created. 557On the transition from enabled to disabled, the client cookie cache is cleared 558and disabled. 559The transition from enabled to disabled does not affect any active TFO 560connections in progress; it only prevents new ones from being established. 561The default is 0. 562.It Va fastopen.keylen 563The key length in bytes. 564Read-only. 565.It Va fastopen.maxkeys 566The maximum number of keys supported. 567Read-only, 568.It Va fastopen.maxpsks 569The maximum number of pre-shared keys supported. 570Read-only. 571.It Va fastopen.numkeys 572The current number of keys installed. 573Read-only. 574.It Va fastopen.numpsks 575The current number of pre-shared keys installed. 576Read-only. 577.It Va fastopen.path_disable_time 578When a failure occurs while trying to create a new active (i.e., client) TFO 579connection, new active connections on the same path, as determined by the tuple 580.Brq client_ip, server_ip, server_port , 581will be forced to be non-TFO for this many seconds. 582Note that the path disable mechanism relies on state stored in client cookie 583cache entries, so it is possible for the disable time for a given path to be 584reduced if the corresponding client cookie cache entry is reused due to resource 585pressure before the disable period has elapsed. 586The default is 587.Dv TCP_FASTOPEN_PATH_DISABLE_TIME_DEFAULT . 588.It Va fastopen.psk_enable 589When non-zero, pre-shared key (PSK) mode is enabled for all TFO servers. 590On the transition from enabled to disabled, all installed pre-shared keys are 591removed. 592The default is 0. 593.It Va fastopen.server_enable 594When zero, no new passive (i.e., server) TFO connections can be created. 595On the transition from enabled to disabled, all installed keys and pre-shared 596keys are removed. 597On the transition from disabled to enabled, if 598.Va fastopen.autokey 599is non-zero and there are no keys installed, a new key will be generated 600immediately. 601The transition from enabled to disabled does not affect any passive TFO 602connections in progress; it only prevents new ones from being established. 603The default is 0. 604.It Va fastopen.setkey 605Install a new key by writing 606.Va net.inet.tcp.fastopen.keylen 607bytes to this sysctl. 608.It Va fastopen.setpsk 609Install a new pre-shared key by writing 610.Va net.inet.tcp.fastopen.keylen 611bytes to this sysctl. 612.It Va finwait2_timeout 613Timeout to use for fast recycling of 614.Tn TCP 615.Dv FIN_WAIT_2 616connections 617.Pq Va fast_finwait2_recycle . 618Defaults to 60 seconds. 619.It Va functions_available 620List of available TCP function blocks (TCP stacks). 621.It Va functions_default 622The default TCP function block (TCP stack). 623.It Va functions_inherit_listen_socket_stack 624Determines whether to inherit listen socket's TCP stack or use the current 625system default TCP stack, as defined by 626.Va functions_default . 627Default is true. 628.It Va hostcache 629The TCP host cache is used to cache connection details and metrics to 630improve future performance of connections between the same hosts. 631At the completion of a TCP connection, a host will cache information 632for the connection for some defined period of time. 633There are a number of 634.Va hostcache 635variables under this node. 636See 637.Va hostcache.enable . 638.It Va hostcache.bucketlimit 639The maximum number of entries for the same hash. 640Defaults to 30. 641.It Va hostcache.cachelimit 642Overall entry limit for hostcache. 643Defaults to 644.Va hashsize 645* 646.Va bucketlimit . 647.It Va hostcache.count 648The current number of entries in the host cache. 649.It Va hostcache.enable 650Enable/disable the host cache: 651.Bl -tag -compact 652.It 0 653Disable the host cache. 654.It 1 655Enable the host cache. (default) 656.El 657.It Va hostcache.expire 658Time in seconds, how long a entry should be kept in the 659host cache since last accessed. 660Defaults to 3600 (1 hour). 661.It Va hostcache.hashsize 662Size of TCP hostcache hashtable. 663This number has to be a power of two, or will be rejected. 664Defaults to 512. 665.It Va hostcache.histo 666Provide a Histogram of the hostcache hash utilization. 667.It Va hostcache.list 668Provide a complete list of all current entries in the host 669cache. 670.It Va hostcache.prune 671Time in seconds between pruning expired host cache entries. 672Defaults to 300 (5 minutes). 673.It Va hostcache.purge 674Expire all entires on next pruning of host cache entries. 675Any non-zero setting will be reset to zero, once the purge 676is running. 677.Bl -tag -compact 678.It 0 679Do not purge all entries when pruning the host cache (default). 680.It 1 681Purge all entries when doing the next pruning. 682.It 2 683Purge all entries and also reseed the hash salt. 684.El 685.It Va hostcache.purgenow 686Immediately purge all entries once set to any value. 687Setting this to 2 will also reseed the hash salt. 688.It Va icmp_may_rst 689Certain 690.Tn ICMP 691unreachable messages may abort connections in 692.Tn SYN-SENT 693state. 694.It Va initcwnd_segments 695Enable the ability to specify initial congestion window in number of segments. 696The default value is 10 as suggested by RFC 6928. 697Changing the value on the fly would not affect connections 698using congestion window from the hostcache. 699Caution: 700This regulates the burst of packets allowed to be sent in the first RTT. 701The value should be relative to the link capacity. 702Start with small values for lower-capacity links. 703Large bursts can cause buffer overruns and packet drops if routers have small 704buffers or the link is experiencing congestion. 705.It Va insecure_rst 706Use criteria defined in RFC793 instead of RFC5961 for accepting RST segments. 707Default is false. 708.It Va insecure_syn 709Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments. 710Default is false. 711.It Va isn_reseed_interval 712The interval (in seconds) specifying how often the secret data used in 713RFC 1948 initial sequence number calculations should be reseeded. 714By default, this variable is set to zero, indicating that 715no reseeding will occur. 716Reseeding should not be necessary, and will break 717.Dv TIME_WAIT 718recycling for a few minutes. 719.It Va keepcnt 720Number of keepalive probes sent, with no response, before a connection 721is dropped. 722The default is 8 packets. 723.It Va keepidle 724Amount of time, in milliseconds, that the connection must be idle 725before sending keepalive probes (if enabled). 726The default is 7200000 msec (7.2M msec, 2 hours). 727.It Va keepinit 728Timeout, in milliseconds, for new, non-established 729.Tn TCP 730connections. 731The default is 75000 msec (75K msec, 75 sec). 732.It Va keepintvl 733The interval, in milliseconds, between keepalive probes sent to remote 734machines, when no response is received on a 735.Va keepidle 736probe. 737The default is 75000 msec (75K msec, 75 sec). 738.It Va log_in_vain 739Log any connection attempts to ports where there is no socket 740accepting connections. 741The value of 1 limits the logging to 742.Tn SYN 743(connection establishment) packets only. 744A value of 2 results in any 745.Tn TCP 746packets to closed ports being logged. 747Any value not listed above disables the logging 748(default is 0, i.e., the logging is disabled). 749.It Va minmss 750Minimum TCP Maximum Segment Size; used to prevent a denial of service attack 751from an unreasonably low MSS. 752.It Va msl 753The Maximum Segment Lifetime, in milliseconds, for a packet. 754.It Va mssdflt 755The default value used for the TCP Maximum Segment Size 756.Pq Dq MSS 757for IPv4 when no advice to the contrary is received from MSS negotiation. 758.It Va newcwd 759Enable the New Congestion Window Validation mechanism as described in RFC 7661. 760This gently reduces the congestion window during periods, where TCP is 761application limited and the network bandwidth is not utilized completely. 762That prevents self-inflicted packet losses once the application starts to 763transmit data at a higher speed. 764.It Va nolocaltimewait 765Suppress creation of TCP 766.Dv TIME_WAIT 767states for connections in 768which both endpoints are local. 769.It Va path_mtu_discovery 770Enable Path MTU Discovery. 771.It Va pcbcount 772Number of active protocol control blocks 773(read-only). 774.It Va perconn_stats_enable 775Controls the default collection of statistics for all connections using the 776.Xr stats 3 777framework. 7780 disables, 1 enables, 2 enables random sampling across log id connection 779groups with all connections in a group receiving the same setting. 780.It Va perconn_stats_sample_rates 781A CSV list of template_spec=percent key-value pairs which controls the per 782template sampling rates when 783.Xr stats 3 784sampling is enabled. 785.It Va persmax 786Maximum persistence interval, msec. 787.It Va persmin 788Minimum persistence interval, msec. 789.It Va pmtud_blackhole_detection 790Enable automatic path MTU blackhole detection. 791In case of retransmits of MSS sized segments, 792the OS will lower the MSS to check if it's an MTU problem. 793If the current MSS is greater than the configured value to try 794.Po Va net.inet.tcp.pmtud_blackhole_mss 795and 796.Va net.inet.tcp.v6pmtud_blackhole_mss 797.Pc , 798it will be set to this value, otherwise, 799the MSS will be set to the default values 800.Po Va net.inet.tcp.mssdflt 801and 802.Va net.inet.tcp.v6mssdflt 803.Pc . 804Settings: 805.Bl -tag -compact 806.It 0 807Disable path MTU blackhole detection. 808.It 1 809Enable path MTU blackhole detection for IPv4 and IPv6. 810.It 2 811Enable path MTU blackhole detection only for IPv4. 812.It 3 813Enable path MTU blackhole detection only for IPv6. 814.El 815.It Va pmtud_blackhole_mss 816MSS to try for IPv4 if PMTU blackhole detection is turned on. 817.It Va reass.cursegments 818The current total number of segments present in all reassembly queues. 819.It Va reass.maxqueuelen 820The maximum number of segments allowed in each reassembly queue. 821By default, the system chooses a limit based on each TCP connection's 822receive buffer size and maximum segment size (MSS). 823The actual limit applied to a session's reassembly queue will be the lower of 824the system-calculated automatic limit and the user-specified 825.Va reass.maxqueuelen 826limit. 827.It Va reass.maxsegments 828The maximum limit on the total number of segments across all reassembly 829queues. 830The limit can be adjusted as a tunable. 831.It Va recvbuf_auto 832Enable automatic receive buffer sizing as a connection progresses. 833.It Va recvbuf_max 834Maximum size of automatic receive buffer. 835.It Va recvspace 836Initial 837.Tn TCP 838receive window (buffer size). 839.It Va retries 840Maximum number of consecutive timer based retransmits sent after a data 841segment is lost (default and maximum is 12). 842.It Va rexmit_drop_options 843Drop TCP options from third and later retransmitted SYN segments 844of a connection. 845.It Va rexmit_initial , rexmit_min , rexmit_slop 846Adjust the retransmit timer calculation for 847.Tn TCP . 848The slop is 849typically added to the raw calculation to take into account 850occasional variances that the 851.Tn SRTT 852(smoothed round-trip time) 853is unable to accommodate, while the minimum specifies an 854absolute minimum. 855While a number of 856.Tn TCP 857RFCs suggest a 1 858second minimum, these RFCs tend to focus on streaming behavior, 859and fail to deal with the fact that a 1 second minimum has severe 860detrimental effects over lossy interactive connections, such 861as a 802.11b wireless link, and over very fast but lossy 862connections for those cases not covered by the fast retransmit 863code. 864For this reason, we use 200ms of slop and a near-0 865minimum, which gives us an effective minimum of 200ms (similar to 866.Tn Linux ) . 867The initial value is used before an RTT measurement has been performed. 868.It Va rfc1323 869Implement the window scaling and timestamp options of RFC 1323/RFC 7323 870(default is 1). 871Settings: 872.Bl -tag -compact 873.It 0 874Disable window scaling and timestamp option. 875.It 1 876Enable window scaling and timestamp option. 877.It 2 878Enable only window scaling. 879.It 3 880Enable only timestamp option. 881.El 882.It Va rfc3042 883Enable the Limited Transmit algorithm as described in RFC 3042. 884It helps avoid timeouts on lossy links and also when the congestion window 885is small, as happens on short transfers. 886.It Va rfc3390 887Enable support for RFC 3390, which allows for a variable-sized 888starting congestion window on new connections, depending on the 889maximum segment size. 890This helps throughput in general, but 891particularly affects short transfers and high-bandwidth large 892propagation-delay connections. 893.It Va rfc6675_pipe 894Deprecated and superseded by 895.Va sack.revised 896.It Va sack.enable 897Enable support for RFC 2018, TCP Selective Acknowledgment option, 898which allows the receiver to inform the sender about all successfully 899arrived segments, allowing the sender to retransmit the missing segments 900only. 901.It Va sack.globalholes 902Global number of TCP SACK holes currently allocated. 903.It Va sack.globalmaxholes 904Maximum number of SACK holes per system, across all connections. 905Defaults to 65536. 906.It Va sack.maxholes 907Maximum number of SACK holes per connection. 908Defaults to 128. 909.It Va sack.revised 910Enables three updated mechanisms from RFC6675 (default is true). 911Calculate the bytes in flight using the algorithm described in RFC 6675, and 912is also an improvement when Proportional Rate Reduction is enabled. 913Next, Rescue Retransmission helps timely loss recovery, when the trailing segments 914of a transmission are lost, while no additional data is ready to be sent. 915In case a partial ACK without a SACK block is received during SACK loss 916recovery, the trailing segment is immediately resent, rather than waiting 917for a Retransmission timeout. 918Finally, SACK loss recovery is also engaged, once two segments plus one byte are 919SACKed - even if no traditional duplicate ACKs were observed. 920.It Va sendbuf_auto 921Enable automatic send buffer sizing. 922.It Va sendbuf_auto_lowat 923Modify threshold for auto send buffer growth to account for 924.Dv SO_SNDLOWAT . 925.It Va sendbuf_inc 926Incrementor step size of automatic send buffer. 927.It Va sendbuf_max 928Maximum size of automatic send buffer. 929.It Va sendspace 930Initial 931.Tn TCP 932send window (buffer size). 933.It Va syncache 934Variables under the 935.Va net.inet.tcp.syncache 936node are documented in 937.Xr syncache 4 . 938.It Va syncookies 939Determines whether or not 940.Tn SYN 941cookies should be generated for outbound 942.Tn SYN-ACK 943packets. 944.Tn SYN 945cookies are a great help during 946.Tn SYN 947flood attacks, and are enabled by default. 948(See 949.Xr syncookies 4 . ) 950.It Va syncookies_only 951See 952.Xr syncookies 4 . 953.It Va tcbhashsize 954Size of the 955.Tn TCP 956control-block hash table 957(read-only). 958This is tuned using the kernel option 959.Dv TCBHASHSIZE 960or by setting 961.Va net.inet.tcp.tcbhashsize 962in the 963.Xr loader 8 . 964.It Va tolerate_missing_ts 965Tolerate the missing of timestamps (RFC 1323/RFC 7323) for 966.Tn TCP 967segments belonging to 968.Tn TCP 969connections for which support of 970.Tn TCP 971timestamps has been negotiated. 972As of June 2021, several TCP stacks are known to violate RFC 7323, including 973modern widely deployed ones. 974Therefore the default is 1, i.e., the missing of timestamps is tolerated. 975.It Va ts_offset_per_conn 976When initializing the TCP timestamps, use a per connection offset instead of a 977per host pair offset. 978Default is to use per connection offsets as recommended in RFC 7323. 979.It Va tso 980Enable TCP Segmentation Offload. 981.It Va udp_tunneling_overhead 982The overhead taken into account when using UDP encapsulation. 983Since MSS clamping by middleboxes will most likely not work, values larger than 9848 (the size of the UDP header) are also supported. 985Supported values are between 8 and 1024. 986The default is 8. 987.It Va udp_tunneling_port 988The local UDP encapsulation port. 989A value of 0 indicates that UDP encapsulation is disabled. 990The default is 0. 991.It Va v6mssdflt 992The default value used for the TCP Maximum Segment Size 993.Pq Dq MSS 994for IPv6 when no advice to the contrary is received from MSS negotiation. 995.It Va v6pmtud_blackhole_mss 996MSS to try for IPv6 if PMTU blackhole detection is turned on. 997See 998.Va pmtud_blackhole_detection . 999.El 1000.Sh ERRORS 1001A socket operation may fail with one of the following errors returned: 1002.Bl -tag -width Er 1003.It Bq Er EISCONN 1004when trying to establish a connection on a socket which 1005already has one; 1006.It Bo Er ENOBUFS Bc or Bo Er ENOMEM Bc 1007when the system runs out of memory for 1008an internal data structure; 1009.It Bq Er ETIMEDOUT 1010when a connection was dropped 1011due to excessive retransmissions; 1012.It Bq Er ECONNRESET 1013when the remote peer 1014forces the connection to be closed; 1015.It Bq Er ECONNREFUSED 1016when the remote 1017peer actively refuses connection establishment (usually because 1018no process is listening to the port); 1019.It Bq Er EADDRINUSE 1020when an attempt 1021is made to create a socket with a port which has already been 1022allocated; 1023.It Bq Er EADDRNOTAVAIL 1024when an attempt is made to create a 1025socket with a network address for which no network interface 1026exists; 1027.It Bq Er EAFNOSUPPORT 1028when an attempt is made to bind or connect a socket to a multicast 1029address. 1030.It Bq Er EINVAL 1031when trying to change TCP function blocks at an invalid point in the session; 1032.It Bq Er ENOENT 1033when trying to use a TCP function block that is not available; 1034.El 1035.Sh SEE ALSO 1036.Xr getsockopt 2 , 1037.Xr socket 2 , 1038.Xr stats 3 , 1039.Xr sysctl 3 , 1040.Xr blackhole 4 , 1041.Xr inet 4 , 1042.Xr intro 4 , 1043.Xr ip 4 , 1044.Xr ktls 4 , 1045.Xr mod_cc 4 , 1046.Xr siftr 4 , 1047.Xr syncache 4 , 1048.Xr tcp_bbr 4 , 1049.Xr tcp_rack 4 , 1050.Xr setkey 8 , 1051.Xr sysctl 8 , 1052.Xr tcp_functions 9 1053.Rs 1054.%A "V. Jacobson" 1055.%A "B. Braden" 1056.%A "D. Borman" 1057.%T "TCP Extensions for High Performance" 1058.%O "RFC 1323" 1059.Re 1060.Rs 1061.%A "D. Borman" 1062.%A "B. Braden" 1063.%A "V. Jacobson" 1064.%A "R. Scheffenegger" 1065.%T "TCP Extensions for High Performance" 1066.%O "RFC 7323" 1067.Re 1068.Rs 1069.%A "A. Heffernan" 1070.%T "Protection of BGP Sessions via the TCP MD5 Signature Option" 1071.%O "RFC 2385" 1072.Re 1073.Rs 1074.%A "K. Ramakrishnan" 1075.%A "S. Floyd" 1076.%A "D. Black" 1077.%T "The Addition of Explicit Congestion Notification (ECN) to IP" 1078.%O "RFC 3168" 1079.Re 1080.Sh HISTORY 1081The 1082.Tn TCP 1083protocol appeared in 1084.Bx 4.2 . 1085The RFC 1323 extensions for window scaling and timestamps were added 1086in 1087.Bx 4.4 . 1088The 1089.Dv TCP_INFO 1090option was introduced in 1091.Tn Linux 2.6 1092and is 1093.Em subject to change . 1094