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.\" $FreeBSD$ 36.\" 37.Dd July 20, 2022 38.Dt TCP 4 39.Os 40.Sh NAME 41.Nm tcp 42.Nd Internet Transmission Control Protocol 43.Sh SYNOPSIS 44.In sys/types.h 45.In sys/socket.h 46.In netinet/in.h 47.In netinet/tcp.h 48.Ft int 49.Fn socket AF_INET SOCK_STREAM 0 50.Sh DESCRIPTION 51The 52.Tn TCP 53protocol provides reliable, flow-controlled, two-way 54transmission of data. 55It is a byte-stream protocol used to 56support the 57.Dv SOCK_STREAM 58abstraction. 59.Tn TCP 60uses the standard 61Internet address format and, in addition, provides a per-host 62collection of 63.Dq "port addresses" . 64Thus, each address is composed 65of an Internet address specifying the host and network, 66with a specific 67.Tn TCP 68port on the host identifying the peer entity. 69.Pp 70Sockets utilizing the 71.Tn TCP 72protocol are either 73.Dq active 74or 75.Dq passive . 76Active sockets initiate connections to passive 77sockets. 78By default, 79.Tn TCP 80sockets are created active; to create a 81passive socket, the 82.Xr listen 2 83system call must be used 84after binding the socket with the 85.Xr bind 2 86system call. 87Only passive sockets may use the 88.Xr accept 2 89call to accept incoming connections. 90Only active sockets may use the 91.Xr connect 2 92call to initiate connections. 93.Pp 94Passive sockets may 95.Dq underspecify 96their location to match 97incoming connection requests from multiple networks. 98This technique, termed 99.Dq "wildcard addressing" , 100allows a single 101server to provide service to clients on multiple networks. 102To create a socket which listens on all networks, the Internet 103address 104.Dv INADDR_ANY 105must be bound. 106The 107.Tn TCP 108port may still be specified 109at this time; if the port is not specified, the system will assign one. 110Once a connection has been established, the socket's address is 111fixed by the peer entity's location. 112The address assigned to the 113socket is the address associated with the network interface 114through which packets are being transmitted and received. 115Normally, this address corresponds to the peer entity's network. 116.Pp 117.Tn TCP 118supports a number of socket options which can be set with 119.Xr setsockopt 2 120and tested with 121.Xr getsockopt 2 : 122.Bl -tag -width ".Dv TCP_FUNCTION_BLK" 123.It Dv TCP_INFO 124Information about a socket's underlying TCP session may be retrieved 125by passing the read-only option 126.Dv TCP_INFO 127to 128.Xr getsockopt 2 . 129It accepts a single argument: a pointer to an instance of 130.Vt "struct tcp_info" . 131.Pp 132This API is subject to change; consult the source to determine 133which fields are currently filled out by this option. 134.Fx 135specific additions include 136send window size, 137receive window size, 138and 139bandwidth-controlled window space. 140.It Dv TCP_CCALGOOPT 141Set or query congestion control algorithm specific parameters. 142See 143.Xr mod_cc 4 144for details. 145.It Dv TCP_CONGESTION 146Select or query the congestion control algorithm that TCP will use for the 147connection. 148See 149.Xr mod_cc 4 150for details. 151.It Dv TCP_FASTOPEN 152Enable or disable TCP Fast Open (TFO). 153To use this option, the kernel must be built with the 154.Dv TCP_RFC7413 155option. 156.Pp 157This option can be set on the socket either before or after the 158.Xr listen 2 159is invoked. 160Clearing this option on a listen socket after it has been set has no effect on 161existing TFO connections or TFO connections in progress; it only prevents new 162TFO connections from being established. 163.Pp 164For passively-created sockets, the 165.Dv TCP_FASTOPEN 166socket option can be queried to determine whether the connection was established 167using TFO. 168Note that connections that are established via a TFO 169.Tn SYN , 170but that fall back to using a non-TFO 171.Tn SYN|ACK 172will have the 173.Dv TCP_FASTOPEN 174socket option set. 175.Pp 176In addition to the facilities defined in RFC7413, this implementation supports a 177pre-shared key (PSK) mode of operation in which the TFO server requires the 178client to be in posession of a shared secret in order for the client to be able 179to successfully open TFO connections with the server. 180This is useful, for example, in environments where TFO servers are exposed to 181both internal and external clients and only wish to allow TFO connections from 182internal clients. 183.Pp 184In the PSK mode of operation, the server generates and sends TFO cookies to 185requesting clients as usual. 186However, when validating cookies received in TFO SYNs from clients, the server 187requires the client-supplied cookie to equal 188.Bd -literal -offset left 189SipHash24(key=\fI16-byte-psk\fP, msg=\fIcookie-sent-to-client\fP) 190.Ed 191.Pp 192Multiple concurrent valid pre-shared keys are supported so that time-based 193rolling PSK invalidation policies can be implemented in the system. 194The default number of concurrent pre-shared keys is 2. 195.Pp 196This can be adjusted with the 197.Dv TCP_RFC7413_MAX_PSKS 198kernel option. 199.It Dv TCP_FUNCTION_BLK 200Select or query the set of functions that TCP will use for this connection. 201This allows a user to select an alternate TCP stack. 202The alternate TCP stack must already be loaded in the kernel. 203To list the available TCP stacks, see 204.Va functions_available 205in the 206.Sx MIB Variables 207section further down. 208To list the default TCP stack, see 209.Va functions_default 210in the 211.Sx MIB Variables 212section. 213.It Dv TCP_KEEPINIT 214This 215.Xr setsockopt 2 216option accepts a per-socket timeout argument of 217.Vt "u_int" 218in seconds, for new, non-established 219.Tn TCP 220connections. 221For the global default in milliseconds see 222.Va keepinit 223in the 224.Sx MIB Variables 225section further down. 226.It Dv TCP_KEEPIDLE 227This 228.Xr setsockopt 2 229option accepts an argument of 230.Vt "u_int" 231for the amount of time, in seconds, that the connection must be idle 232before keepalive probes (if enabled) are sent for the connection of this 233socket. 234If set on a listening socket, the value is inherited by the newly created 235socket upon 236.Xr accept 2 . 237For the global default in milliseconds see 238.Va keepidle 239in the 240.Sx MIB Variables 241section further down. 242.It Dv TCP_KEEPINTVL 243This 244.Xr setsockopt 2 245option accepts an argument of 246.Vt "u_int" 247to set the per-socket interval, in seconds, between keepalive probes sent 248to a peer. 249If set on a listening socket, the value is inherited by the newly created 250socket upon 251.Xr accept 2 . 252For the global default in milliseconds see 253.Va keepintvl 254in the 255.Sx MIB Variables 256section further down. 257.It Dv TCP_KEEPCNT 258This 259.Xr setsockopt 2 260option accepts an argument of 261.Vt "u_int" 262and allows a per-socket tuning of the number of probes sent, with no response, 263before the connection will be dropped. 264If set on a listening socket, the value is inherited by the newly created 265socket upon 266.Xr accept 2 . 267For the global default see the 268.Va keepcnt 269in the 270.Sx MIB Variables 271section further down. 272.It Dv TCP_NODELAY 273Under most circumstances, 274.Tn TCP 275sends data when it is presented; 276when outstanding data has not yet been acknowledged, it gathers 277small amounts of output to be sent in a single packet once 278an acknowledgement is received. 279For a small number of clients, such as window systems 280that send a stream of mouse events which receive no replies, 281this packetization may cause significant delays. 282The boolean option 283.Dv TCP_NODELAY 284defeats this algorithm. 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_NOOPT 295.Tn TCP 296usually sends a number of options in each packet, corresponding to 297various 298.Tn TCP 299extensions which are provided in this implementation. 300The boolean option 301.Dv TCP_NOOPT 302is provided to disable 303.Tn TCP 304option use on a per-connection basis. 305.It Dv TCP_NOPUSH 306By convention, the 307.No sender- Ns Tn TCP 308will set the 309.Dq push 310bit, and begin transmission immediately (if permitted) at the end of 311every user call to 312.Xr write 2 313or 314.Xr writev 2 . 315When this option is set to a non-zero value, 316.Tn TCP 317will delay sending any data at all until either the socket is closed, 318or the internal send buffer is filled. 319.It Dv TCP_MD5SIG 320This option enables the use of MD5 digests (also known as TCP-MD5) 321on writes to the specified socket. 322Outgoing traffic is digested; 323digests on incoming traffic are verified. 324When this option is enabled on a socket, all inbound and outgoing 325TCP segments must be signed with MD5 digests. 326.Pp 327One common use for this in a 328.Fx 329router deployment is to enable 330based routers to interwork with Cisco equipment at peering points. 331Support for this feature conforms to RFC 2385. 332.Pp 333In order for this option to function correctly, it is necessary for the 334administrator to add a tcp-md5 key entry to the system's security 335associations database (SADB) using the 336.Xr setkey 8 337utility. 338This entry can only be specified on a per-host basis at this time. 339.Pp 340If an SADB entry cannot be found for the destination, 341the system does not send any outgoing segments and drops any inbound segments. 342However, during connection negotiation, a non-signed segment will be accepted if 343an SADB entry does not exist between hosts. 344When a non-signed segment is accepted, the established connection is not 345protected with MD5 digests. 346.It Dv TCP_STATS 347Manage collection of connection level statistics using the 348.Xr stats 3 349framework. 350.Pp 351Each dropped segment is taken into account in the TCP protocol statistics. 352.It Dv TCP_TXTLS_ENABLE 353Enable in-kernel Transport Layer Security (TLS) for data written to this 354socket. 355See 356.Xr ktls 4 357for more details. 358.It Dv TCP_TXTLS_MODE 359The integer argument can be used to get or set the current TLS transmit mode 360of a socket. 361See 362.Xr ktls 4 363for more details. 364.It Dv TCP_RXTLS_ENABLE 365Enable in-kernel TLS for data read from this socket. 366See 367.Xr ktls 4 368for more details. 369.It Dv TCP_REUSPORT_LB_NUMA 370Changes NUMA affinity filtering for an established TCP listen 371socket. 372This option takes a single integer argument which specifies 373the NUMA domain to filter on for this listen socket. 374The argument can also have the follwing special values: 375.Bl -tag -width "Dv TCP_REUSPORT_LB_NUMA" 376.It Dv TCP_REUSPORT_LB_NUMA_NODOM 377Remove NUMA filtering for this listen socket. 378.It Dv TCP_REUSPORT_LB_NUMA_CURDOM 379Filter traffic associated with the domain where the calling thread is 380currently executing. 381This is typically used after a process or thread inherits a listen 382socket from its parent, and sets its CPU affinity to a particular core. 383.El 384.It Dv TCP_REMOTE_UDP_ENCAPS_PORT 385Set and get the remote UDP encapsulation port. 386It can only be set on a closed TCP socket. 387.El 388.Pp 389The option level for the 390.Xr setsockopt 2 391call is the protocol number for 392.Tn TCP , 393available from 394.Xr getprotobyname 3 , 395or 396.Dv IPPROTO_TCP . 397All options are declared in 398.In netinet/tcp.h . 399.Pp 400Options at the 401.Tn IP 402transport level may be used with 403.Tn TCP ; 404see 405.Xr ip 4 . 406Incoming connection requests that are source-routed are noted, 407and the reverse source route is used in responding. 408.Pp 409The default congestion control algorithm for 410.Tn TCP 411is 412.Xr cc_newreno 4 . 413Other congestion control algorithms can be made available using the 414.Xr mod_cc 4 415framework. 416.Ss MIB Variables 417The 418.Tn TCP 419protocol implements a number of variables in the 420.Va net.inet.tcp 421branch of the 422.Xr sysctl 3 423MIB. 424.Bl -tag -width ".Va v6pmtud_blackhole_mss" 425.It Va always_keepalive 426Assume that 427.Dv SO_KEEPALIVE 428is set on all 429.Tn TCP 430connections, the kernel will 431periodically send a packet to the remote host to verify the connection 432is still up. 433.It Va blackhole 434If enabled, disable sending of RST when a connection is attempted 435to a port where there is not a socket accepting connections. 436See 437.Xr blackhole 4 . 438.It Va delacktime 439Maximum amount of time, in milliseconds, before a delayed ACK is sent. 440.It Va delayed_ack 441Delay ACK to try and piggyback it onto a data packet. 442.It Va do_lrd 443Enable Lost Retransmission Detection for SACK-enabled sessions, disabled by 444default. 445Under severe congestion, a retransmission can be lost which then leads to a 446mandatory Retransmission Timeout (RTO), followed by slow-start. 447LRD will try to resend the repeatedly lost packet, preventing the time-consuming 448RTO and performance reducing slow-start. 449.It Va do_prr 450Perform SACK loss recovery using the Proportional Rate Reduction (PRR) algorithm 451described in RFC6937. 452This improves the effectiveness of retransmissions particular in environments 453with ACK thinning or burst loss events, as chances to run out of the ACK clock 454are reduced, preventing lengthy and performance reducing RTO based loss recovery 455(default is true). 456.It Va do_prr_conservative 457While doing Proportional Rate Reduction, remain strictly in a packet conserving 458mode, sending only one new packet for each ACK received. 459Helpful when a misconfigured token bucket traffic policer causes persistent 460high losses leading to RTO, but reduces PRR effectiveness in more common settings 461(default is false). 462.It Va do_tcpdrain 463Flush packets in the 464.Tn TCP 465reassembly queue if the system is low on mbufs. 466.It Va ecn.enable 467Enable support for TCP Explicit Congestion Notification (ECN). 468ECN allows a TCP sender to reduce the transmission rate in order to 469avoid packet drops. 470.Bl -tag -compact 471.It 0 472Disable ECN. 473.It 1 474Allow incoming connections to request ECN. 475Outgoing connections will request ECN. 476.It 2 477Allow incoming connections to request ECN. 478Outgoing connections will not request ECN. 479(default) 480.El 481.It Va ecn.maxretries 482Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a 483specific connection. 484This is needed to help with connection establishment 485when a broken firewall is in the network path. 486.It Va fast_finwait2_recycle 487Recycle 488.Tn TCP 489.Dv FIN_WAIT_2 490connections faster when the socket is marked as 491.Dv SBS_CANTRCVMORE 492(no user process has the socket open, data received on 493the socket cannot be read). 494The timeout used here is 495.Va finwait2_timeout . 496.It Va fastopen.acceptany 497When non-zero, all client-supplied TFO cookies will be considered to be valid. 498The default is 0. 499.It Va fastopen.autokey 500When this and 501.Va net.inet.tcp.fastopen.server_enable 502are non-zero, a new key will be automatically generated after this specified 503seconds. 504The default is 120. 505.It Va fastopen.ccache_bucket_limit 506The maximum number of entries in a client cookie cache bucket. 507The default value can be tuned with the 508.Dv TCP_FASTOPEN_CCACHE_BUCKET_LIMIT_DEFAULT 509kernel option or by setting 510.Va net.inet.tcp.fastopen_ccache_bucket_limit 511in the 512.Xr loader 8 . 513.It Va fastopen.ccache_buckets 514The number of client cookie cache buckets. 515Read-only. 516The value can be tuned with the 517.Dv TCP_FASTOPEN_CCACHE_BUCKETS_DEFAULT 518kernel option or by setting 519.Va fastopen.ccache_buckets 520in the 521.Xr loader 8 . 522.It Va fastopen.ccache_list 523Print the client cookie cache. 524Read-only. 525.It Va fastopen.client_enable 526When zero, no new active (i.e., client) TFO connections can be created. 527On the transition from enabled to disabled, the client cookie cache is cleared 528and disabled. 529The transition from enabled to disabled does not affect any active TFO 530connections in progress; it only prevents new ones from being established. 531The default is 0. 532.It Va fastopen.keylen 533The key length in bytes. 534Read-only. 535.It Va fastopen.maxkeys 536The maximum number of keys supported. 537Read-only, 538.It Va fastopen.maxpsks 539The maximum number of pre-shared keys supported. 540Read-only. 541.It Va fastopen.numkeys 542The current number of keys installed. 543Read-only. 544.It Va fastopen.numpsks 545The current number of pre-shared keys installed. 546Read-only. 547.It Va fastopen.path_disable_time 548When a failure occurs while trying to create a new active (i.e., client) TFO 549connection, new active connections on the same path, as determined by the tuple 550.Brq client_ip, server_ip, server_port , 551will be forced to be non-TFO for this many seconds. 552Note that the path disable mechanism relies on state stored in client cookie 553cache entries, so it is possible for the disable time for a given path to be 554reduced if the corresponding client cookie cache entry is reused due to resource 555pressure before the disable period has elapsed. 556The default is 557.Dv TCP_FASTOPEN_PATH_DISABLE_TIME_DEFAULT . 558.It Va fastopen.psk_enable 559When non-zero, pre-shared key (PSK) mode is enabled for all TFO servers. 560On the transition from enabled to disabled, all installed pre-shared keys are 561removed. 562The default is 0. 563.It Va fastopen.server_enable 564When zero, no new passive (i.e., server) TFO connections can be created. 565On the transition from enabled to disabled, all installed keys and pre-shared 566keys are removed. 567On the transition from disabled to enabled, if 568.Va fastopen.autokey 569is non-zero and there are no keys installed, a new key will be generated 570immediately. 571The transition from enabled to disabled does not affect any passive TFO 572connections in progress; it only prevents new ones from being established. 573The default is 0. 574.It Va fastopen.setkey 575Install a new key by writing 576.Va net.inet.tcp.fastopen.keylen 577bytes to this sysctl. 578.It Va fastopen.setpsk 579Install a new pre-shared key by writing 580.Va net.inet.tcp.fastopen.keylen 581bytes to this sysctl. 582.It Va finwait2_timeout 583Timeout to use for fast recycling of 584.Tn TCP 585.Dv FIN_WAIT_2 586connections 587.Pq Va fast_finwait2_recycle . 588Defaults to 60 seconds. 589.It Va functions_available 590List of available TCP function blocks (TCP stacks). 591.It Va functions_default 592The default TCP function block (TCP stack). 593.It Va functions_inherit_listen_socket_stack 594Determines whether to inherit listen socket's TCP stack or use the current 595system default TCP stack, as defined by 596.Va functions_default . 597Default is true. 598.It Va hostcache 599The TCP host cache is used to cache connection details and metrics to 600improve future performance of connections between the same hosts. 601At the completion of a TCP connection, a host will cache information 602for the connection for some defined period of time. 603There are a number of 604.Va hostcache 605variables under this node. 606See 607.Va hostcache.enable . 608.It Va hostcache.bucketlimit 609The maximum number of entries for the same hash. 610Defaults to 30. 611.It Va hostcache.cachelimit 612Overall entry limit for hostcache. 613Defaults to 614.Va hashsize 615* 616.Va bucketlimit . 617.It Va hostcache.count 618The current number of entries in the host cache. 619.It Va hostcache.enable 620Enable/disable the host cache: 621.Bl -tag -compact 622.It 0 623Disable the host cache. 624.It 1 625Enable the host cache. (default) 626.El 627.It Va hostcache.expire 628Time in seconds, how long a entry should be kept in the 629host cache since last accessed. 630Defaults to 3600 (1 hour). 631.It Va hostcache.hashsize 632Size of TCP hostcache hashtable. 633This number has to be a power of two, or will be rejected. 634Defaults to 512. 635.It Va hostcache.histo 636Provide a Histogram of the hostcache hash utilization. 637.It Va hostcache.list 638Provide a complete list of all current entries in the host 639cache. 640.It Va hostcache.prune 641Time in seconds between pruning expired host cache entries. 642Defaults to 300 (5 minutes). 643.It Va hostcache.purge 644Expire all entires on next pruning of host cache entries. 645Any non-zero setting will be reset to zero, once the purge 646is running. 647.Bl -tag -compact 648.It 0 649Do not purge all entries when pruning the host cache (default). 650.It 1 651Purge all entries when doing the next pruning. 652.It 2 653Purge all entries and also reseed the hash salt. 654.El 655.It Va hostcache.purgenow 656Immediately purge all entries once set to any value. 657Setting this to 2 will also reseed the hash salt. 658.It Va icmp_may_rst 659Certain 660.Tn ICMP 661unreachable messages may abort connections in 662.Tn SYN-SENT 663state. 664.It Va initcwnd_segments 665Enable the ability to specify initial congestion window in number of segments. 666The default value is 10 as suggested by RFC 6928. 667Changing the value on the fly would not affect connections 668using congestion window from the hostcache. 669Caution: 670This regulates the burst of packets allowed to be sent in the first RTT. 671The value should be relative to the link capacity. 672Start with small values for lower-capacity links. 673Large bursts can cause buffer overruns and packet drops if routers have small 674buffers or the link is experiencing congestion. 675.It Va insecure_rst 676Use criteria defined in RFC793 instead of RFC5961 for accepting RST segments. 677Default is false. 678.It Va insecure_syn 679Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments. 680Default is false. 681.It Va isn_reseed_interval 682The interval (in seconds) specifying how often the secret data used in 683RFC 1948 initial sequence number calculations should be reseeded. 684By default, this variable is set to zero, indicating that 685no reseeding will occur. 686Reseeding should not be necessary, and will break 687.Dv TIME_WAIT 688recycling for a few minutes. 689.It Va keepcnt 690Number of keepalive probes sent, with no response, before a connection 691is dropped. 692The default is 8 packets. 693.It Va keepidle 694Amount of time, in milliseconds, that the connection must be idle 695before sending keepalive probes (if enabled). 696The default is 7200000 msec (7.2M msec, 2 hours). 697.It Va keepinit 698Timeout, in milliseconds, for new, non-established 699.Tn TCP 700connections. 701The default is 75000 msec (75K msec, 75 sec). 702.It Va keepintvl 703The interval, in milliseconds, between keepalive probes sent to remote 704machines, when no response is received on a 705.Va keepidle 706probe. 707The default is 75000 msec (75K msec, 75 sec). 708.It Va log_in_vain 709Log any connection attempts to ports where there is not a socket 710accepting connections. 711The value of 1 limits the logging to 712.Tn SYN 713(connection establishment) packets only. 714A value of 2 results in any 715.Tn TCP 716packets to closed ports being logged. 717Any value not listed above disables the logging 718(default is 0, i.e., the logging is disabled). 719.It Va maxtcptw 720When a TCP connection enters the 721.Dv TIME_WAIT 722state, its associated socket structure is freed, since it is of 723negligible size and use, and a new structure is allocated to contain a 724minimal amount of information necessary for sustaining a connection in 725this state, called the compressed TCP 726.Dv TIME_WAIT 727state. 728Since this structure is smaller than a socket structure, it can save 729a significant amount of system memory. 730The 731.Va net.inet.tcp.maxtcptw 732MIB variable controls the maximum number of these structures allocated. 733By default, it is initialized to 734.Va kern.ipc.maxsockets 735/ 5. 736.It Va msl 737The Maximum Segment Lifetime, in milliseconds, for a packet. 738.It Va mssdflt 739The default value used for the maximum segment size 740.Pq Dq MSS 741when no advice to the contrary is received from MSS negotiation. 742.It Va newcwd 743Enable the New Congestion Window Validation mechanism as described in RFC 7661. 744This gently reduces the congestion window during periods, where TCP is 745application limited and the network bandwidth is not utilized completely. 746That prevents self-inflicted packet losses once the application starts to 747transmit data at a higher speed. 748.It Va nolocaltimewait 749Suppress creation of compressed TCP 750.Dv TIME_WAIT 751states for connections in 752which both endpoints are local. 753.It Va path_mtu_discovery 754Enable Path MTU Discovery. 755.It Va pcbcount 756Number of active process control blocks 757(read-only). 758.It Va perconn_stats_enable 759Controls the default collection of statistics for all connections using the 760.Xr stats 3 761framework. 7620 disables, 1 enables, 2 enables random sampling across log id connection 763groups with all connections in a group receiving the same setting. 764.It Va perconn_stats_sample_rates 765A CSV list of template_spec=percent key-value pairs which controls the per 766template sampling rates when 767.Xr stats 3 768sampling is enabled. 769.It Va pmtud_blackhole_detection 770Enable automatic path MTU blackhole detection. 771In case of retransmits of MSS sized segments, 772the OS will lower the MSS to check if it's an MTU problem. 773If the current MSS is greater than the configured value to try 774.Po Va net.inet.tcp.pmtud_blackhole_mss 775and 776.Va net.inet.tcp.v6pmtud_blackhole_mss 777.Pc , 778it will be set to this value, otherwise, 779the MSS will be set to the default values 780.Po Va net.inet.tcp.mssdflt 781and 782.Va net.inet.tcp.v6mssdflt 783.Pc . 784Settings: 785.Bl -tag -compact 786.It 0 787Disable path MTU blackhole detection. 788.It 1 789Enable path MTU blackhole detection for IPv4 and IPv6. 790.It 2 791Enable path MTU blackhole detection only for IPv4. 792.It 3 793Enable path MTU blackhole detection only for IPv6. 794.El 795.It Va pmtud_blackhole_mss 796MSS to try for IPv4 if PMTU blackhole detection is turned on. 797.It Va reass.cursegments 798The current total number of segments present in all reassembly queues. 799.It Va reass.maxqueuelen 800The maximum number of segments allowed in each reassembly queue. 801By default, the system chooses a limit based on each TCP connection's 802receive buffer size and maximum segment size (MSS). 803The actual limit applied to a session's reassembly queue will be the lower of 804the system-calculated automatic limit and the user-specified 805.Va reass.maxqueuelen 806limit. 807.It Va reass.maxsegments 808The maximum limit on the total number of segments across all reassembly 809queues. 810The limit can be adjusted as a tunable. 811.It Va recvspace 812Maximum 813.Tn TCP 814receive window. 815.It Va rexmit_initial , rexmit_min , rexmit_slop 816Adjust the retransmit timer calculation for 817.Tn TCP . 818The slop is 819typically added to the raw calculation to take into account 820occasional variances that the 821.Tn SRTT 822(smoothed round-trip time) 823is unable to accommodate, while the minimum specifies an 824absolute minimum. 825While a number of 826.Tn TCP 827RFCs suggest a 1 828second minimum, these RFCs tend to focus on streaming behavior, 829and fail to deal with the fact that a 1 second minimum has severe 830detrimental effects over lossy interactive connections, such 831as a 802.11b wireless link, and over very fast but lossy 832connections for those cases not covered by the fast retransmit 833code. 834For this reason, we use 200ms of slop and a near-0 835minimum, which gives us an effective minimum of 200ms (similar to 836.Tn Linux ) . 837The initial value is used before an RTT measurement has been performed. 838.It Va rfc1323 839Implement the window scaling and timestamp options of RFC 1323/RFC 7323 840(default is true). 841.It Va rfc3042 842Enable the Limited Transmit algorithm as described in RFC 3042. 843It helps avoid timeouts on lossy links and also when the congestion window 844is small, as happens on short transfers. 845.It Va rfc3390 846Enable support for RFC 3390, which allows for a variable-sized 847starting congestion window on new connections, depending on the 848maximum segment size. 849This helps throughput in general, but 850particularly affects short transfers and high-bandwidth large 851propagation-delay connections. 852.It Va rfc6675_pipe 853Deprecated and superseded by 854.Va sack.revised 855.It Va sack.enable 856Enable support for RFC 2018, TCP Selective Acknowledgment option, 857which allows the receiver to inform the sender about all successfully 858arrived segments, allowing the sender to retransmit the missing segments 859only. 860.It Va sack.globalmaxholes 861Maximum number of SACK holes per system, across all connections. 862Defaults to 65536. 863.It Va sack.maxholes 864Maximum number of SACK holes per connection. 865Defaults to 128. 866.It Va sack.revised 867Enables three updated mechanisms from RFC6675 (default is true). 868Calculate the bytes in flight using the algorithm described in RFC 6675, and 869is also an improvement when Proportional Rate Reduction is enabled. 870Next, Rescue Retransmission helps timely loss recovery, when the trailing segments 871of a transmission are lost, while no additional data is ready to be sent. 872In case a partial ACK without a SACK block is received during SACK loss 873recovery, the trailing segment is immediately resent, rather than waiting 874for a Retransmission timeout. 875Finally, SACK loss recovery is also engaged, once two segments plus one byte are 876SACKed - even if no traditional duplicate ACKs were observed. 877.It Va sendspace 878Maximum 879.Tn TCP 880send window. 881.It Va syncookies 882Determines whether or not 883.Tn SYN 884cookies should be generated for outbound 885.Tn SYN-ACK 886packets. 887.Tn SYN 888cookies are a great help during 889.Tn SYN 890flood attacks, and are enabled by default. 891(See 892.Xr syncookies 4 . ) 893.It Va tcbhashsize 894Size of the 895.Tn TCP 896control-block hash table 897(read-only). 898This is tuned using the kernel option 899.Dv TCBHASHSIZE 900or by setting 901.Va net.inet.tcp.tcbhashsize 902in the 903.Xr loader 8 . 904.It Va tolerate_missing_ts 905Tolerate the missing of timestamps (RFC 1323/RFC 7323) for 906.Tn TCP 907segments belonging to 908.Tn TCP 909connections for which support of 910.Tn TCP 911timestamps has been negotiated. 912As of June 2021, several TCP stacks are known to violate RFC 7323, including 913modern widely deployed ones. 914Therefore the default is 1, i.e., the missing of timestamps is tolerated. 915.It Va ts_offset_per_conn 916When initializing the TCP timestamps, use a per connection offset instead of a 917per host pair offset. 918Default is to use per connection offsets as recommended in RFC 7323. 919.It Va udp_tunneling_overhead 920The overhead taken into account when using UDP encapsulation. 921Since MSS clamping by middleboxes will most likely not work, values larger than 9228 (the size of the UDP header) are also supported. 923Supported values are between 8 and 1024. 924The default is 8. 925.It Va udp_tunneling_port 926The local UDP encapsulation port. 927A value of 0 indicates that UDP encapsulation is disabled. 928The default is 0. 929.It Va v6pmtud_blackhole_mss 930MSS to try for IPv6 if PMTU blackhole detection is turned on. 931See 932.Va pmtud_blackhole_detection . 933.El 934.Sh ERRORS 935A socket operation may fail with one of the following errors returned: 936.Bl -tag -width Er 937.It Bq Er EISCONN 938when trying to establish a connection on a socket which 939already has one; 940.It Bo Er ENOBUFS Bc or Bo Er ENOMEM Bc 941when the system runs out of memory for 942an internal data structure; 943.It Bq Er ETIMEDOUT 944when a connection was dropped 945due to excessive retransmissions; 946.It Bq Er ECONNRESET 947when the remote peer 948forces the connection to be closed; 949.It Bq Er ECONNREFUSED 950when the remote 951peer actively refuses connection establishment (usually because 952no process is listening to the port); 953.It Bq Er EADDRINUSE 954when an attempt 955is made to create a socket with a port which has already been 956allocated; 957.It Bq Er EADDRNOTAVAIL 958when an attempt is made to create a 959socket with a network address for which no network interface 960exists; 961.It Bq Er EAFNOSUPPORT 962when an attempt is made to bind or connect a socket to a multicast 963address. 964.It Bq Er EINVAL 965when trying to change TCP function blocks at an invalid point in the session; 966.It Bq Er ENOENT 967when trying to use a TCP function block that is not available; 968.El 969.Sh SEE ALSO 970.Xr getsockopt 2 , 971.Xr socket 2 , 972.Xr stats 3 , 973.Xr sysctl 3 , 974.Xr blackhole 4 , 975.Xr inet 4 , 976.Xr intro 4 , 977.Xr ip 4 , 978.Xr ktls 4 , 979.Xr mod_cc 4 , 980.Xr siftr 4 , 981.Xr syncache 4 , 982.Xr tcp_bbr 4 , 983.Xr setkey 8 , 984.Xr tcp_functions 9 985.Rs 986.%A "V. Jacobson" 987.%A "B. Braden" 988.%A "D. Borman" 989.%T "TCP Extensions for High Performance" 990.%O "RFC 1323" 991.Re 992.Rs 993.%A "D. Borman" 994.%A "B. Braden" 995.%A "V. Jacobson" 996.%A "R. Scheffenegger" 997.%T "TCP Extensions for High Performance" 998.%O "RFC 7323" 999.Re 1000.Rs 1001.%A "A. Heffernan" 1002.%T "Protection of BGP Sessions via the TCP MD5 Signature Option" 1003.%O "RFC 2385" 1004.Re 1005.Rs 1006.%A "K. Ramakrishnan" 1007.%A "S. Floyd" 1008.%A "D. Black" 1009.%T "The Addition of Explicit Congestion Notification (ECN) to IP" 1010.%O "RFC 3168" 1011.Re 1012.Sh HISTORY 1013The 1014.Tn TCP 1015protocol appeared in 1016.Bx 4.2 . 1017The RFC 1323 extensions for window scaling and timestamps were added 1018in 1019.Bx 4.4 . 1020The 1021.Dv TCP_INFO 1022option was introduced in 1023.Tn Linux 2.6 1024and is 1025.Em subject to change . 1026