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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" From: @(#)tcp.4 8.1 (Berkeley) 6/5/93 33.\" $FreeBSD: src/share/man/man4/tcp.4,v 1.11.2.14 2002/12/29 16:35:38 schweikh Exp $ 34.\" $DragonFly: src/share/man/man4/tcp.4,v 1.2 2003/06/17 04:36:59 dillon Exp $ 35.\" 36.Dd February 14, 1995 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.Ft int 47.Fn socket AF_INET SOCK_STREAM 0 48.Sh DESCRIPTION 49The 50.Tn TCP 51protocol provides reliable, flow-controlled, two-way 52transmission of data. It is a byte-stream protocol used to 53support the 54.Dv SOCK_STREAM 55abstraction. TCP uses the standard 56Internet address format and, in addition, provides a per-host 57collection of 58.Dq port addresses . 59Thus, each address is composed 60of an Internet address specifying the host and network, with 61a specific 62.Tn TCP 63port on the host identifying the peer entity. 64.Pp 65Sockets utilizing the tcp protocol are either 66.Dq active 67or 68.Dq passive . 69Active sockets initiate connections to passive 70sockets. By default 71.Tn TCP 72sockets are created active; to create a 73passive socket the 74.Xr listen 2 75system call must be used 76after binding the socket with the 77.Xr bind 2 78system call. Only 79passive sockets may use the 80.Xr accept 2 81call to accept incoming connections. Only active sockets may 82use the 83.Xr connect 2 84call to initiate connections. 85.Tn TCP 86also supports a more datagram-like mode, called Transaction 87.Tn TCP , 88which is described in 89.Xr ttcp 4 . 90.Pp 91Passive sockets may 92.Dq underspecify 93their location to match 94incoming connection requests from multiple networks. This 95technique, termed 96.Dq wildcard addressing , 97allows a single 98server to provide service to clients on multiple networks. 99To create a socket which listens on all networks, the Internet 100address 101.Dv INADDR_ANY 102must be bound. The 103.Tn TCP 104port may still be specified 105at this time; if the port is not specified the system will assign one. 106Once a connection has been established the socket's address is 107fixed by the peer entity's location. The address assigned the 108socket is the address associated with the network interface 109through which packets are being transmitted and received. Normally 110this address corresponds to the peer entity's network. 111.Pp 112.Tn TCP 113supports a number of socket options which can be set with 114.Xr setsockopt 2 115and tested with 116.Xr getsockopt 2 : 117.Bl -tag -width TCP_NODELAYx 118.It Dv TCP_NODELAY 119Under most circumstances, 120.Tn TCP 121sends data when it is presented; 122when outstanding data has not yet been acknowledged, it gathers 123small amounts of output to be sent in a single packet once 124an acknowledgement is received. 125For a small number of clients, such as window systems 126that send a stream of mouse events which receive no replies, 127this packetization may cause significant delays. 128The boolean option 129.Dv TCP_NODELAY 130defeats this algorithm. 131.It Dv TCP_MAXSEG 132By default, a sender\- and receiver-TCP 133will negotiate among themselves to determine the maximum segment size 134to be used for each connection. The 135.Dv TCP_MAXSEG 136option allows the user to determine the result of this negotiation, 137and to reduce it if desired. 138.It Dv TCP_NOOPT 139.Tn TCP 140usually sends a number of options in each packet, corresponding to 141various 142.Tn TCP 143extensions which are provided in this implementation. The boolean 144option 145.Dv TCP_NOOPT 146is provided to disable 147.Tn TCP 148option use on a per-connection basis. 149.It Dv TCP_NOPUSH 150By convention, the sender-TCP 151will set the 152.Dq push 153bit and begin transmission immediately (if permitted) at the end of 154every user call to 155.Xr write 2 156or 157.Xr writev 2 . 158The 159.Dv TCP_NOPUSH 160option is provided to allow servers to easily make use of Transaction 161TCP (see 162.Xr ttcp 4 ) . 163When the option is set to a non-zero value, 164.Tn TCP 165will delay sending any data at all until either the socket is closed, 166or the internal send buffer is filled. 167.El 168.Pp 169The option level for the 170.Xr setsockopt 2 171call is the protocol number for 172.Tn TCP , 173available from 174.Xr getprotobyname 3 , 175or 176.Dv IPPROTO_TCP . 177All options are declared in 178.Aq Pa netinet/tcp.h . 179.Pp 180Options at the 181.Tn IP 182transport level may be used with 183.Tn TCP ; 184see 185.Xr ip 4 . 186Incoming connection requests that are source-routed are noted, 187and the reverse source route is used in responding. 188.Sh MIB VARIABLES 189The 190.Nm 191protocol implements a number of variables in the 192.Li net.inet 193branch of the 194.Xr sysctl 3 195MIB. 196.Bl -tag -width TCPCTL_DO_RFC1644 197.It Dv TCPCTL_DO_RFC1323 198.Pq tcp.rfc1323 199Implement the window scaling and timestamp options of RFC 1323 200(default true). 201.It Dv TCPCTL_DO_RFC1644 202.Pq tcp.rfc1644 203Implement Transaction 204.Tn TCP , 205as described in RFC 1644. 206.It Dv TCPCTL_MSSDFLT 207.Pq tcp.mssdflt 208The default value used for the maximum segment size 209.Pq Dq MSS 210when no advice to the contrary is received from MSS negotiation. 211.It Dv TCPCTL_SENDSPACE 212.Pq tcp.sendspace 213Maximum TCP send window. 214.It Dv TCPCTL_RECVSPACE 215.Pq tcp.recvspace 216Maximum TCP receive window. 217.It tcp.log_in_vain 218Log any connection attempts to ports where there is not a socket 219accepting connections. 220The value of 1 limits the logging to SYN (connection establishment) 221packets only. 222That of 2 results in any TCP packets to closed ports being logged. 223Any value unlisted above disables the logging 224(default is 0, i.e., the logging is disabled). 225.It tcp.slowstart_flightsize 226The number of packets allowed to be in-flight during the 227.Tn TCP 228slow-start phase on a non-local network. 229.It tcp.local_slowstart_flightsize 230The number of packets allowed to be in-flight during the 231.Tn TCP 232slow-start phase to local machines in the same subnet. 233.It tcp.msl 234The Maximum Segment Lifetime for a packet. 235.It tcp.keepinit 236Timeout for new, non-established TCP connections. 237.It tcp.keepidle 238Amount of time the connection should be idle before keepalive 239probes (if enabled) are sent. 240.It tcp.keepintvl 241The interval between keepalive probes sent to remote machines. 242After 243.Dv TCPTV_KEEPCNT 244(default 8) probes are sent, with no response, the connection is dropped. 245.It tcp.always_keepalive 246Assume that 247.Dv SO_KEEPALIVE 248is set on all 249.Tn TCP 250connections, the kernel will 251periodically send a packet to the remote host to verify the connection 252is still up. 253.It tcp.icmp_may_rst 254Certain 255.Tn ICMP 256unreachable messages may abort connections in 257.Tn SYN-SENT 258state. 259.It tcp.do_tcpdrain 260Flush packets in the 261.Tn TCP 262reassembly queue if the system is low on mbufs. 263.It tcp.blackhole 264If enabled, disable sending of RST when a connection is attempted 265to a port where there is not a socket accepting connections. 266See 267.Xr blackhole 4 . 268.It tcp.delayed_ack 269Delay ACK to try and piggyback it onto a data packet. 270.It tcp.delacktime 271Maximum amount of time before a delayed ACK is sent. 272.It tcp.newreno 273Enable TCP NewReno Fast Recovery algorithm, 274as described in RFC 2582. 275.It tcp.path_mtu_discovery 276Enable Path MTU Discovery 277.It tcp.tcbhashsize 278Size of the 279.Tn TCP 280control-block hashtable 281(read-only). 282This may be tuned using the kernel option 283.Dv TCBHASHSIZE 284or by setting 285.Va net.inet.tcp.tcbhashsize 286in the 287.Xr loader 8 . 288.It tcp.pcbcount 289Number of active process control blocks 290(read-only). 291.It tcp.syncookies 292Determines whether or not syn cookies should be generated for 293outbound syn-ack packets. Syn cookies are a great help during 294syn flood attacks, and are enabled by default. 295.It tcp.isn_reseed_interval 296The interval (in seconds) specifying how often the secret data used in 297RFC 1948 initial sequence number calculations should be reseeded. 298By default, this variable is set to zero, indicating that 299no reseeding will occur. 300Reseeding should not be necessary, and will break 301.Dv TIME_WAIT 302recycling for a few minutes. 303.It tcp.inet.tcp.rexmit_{min,slop} 304Adjust the retransmit timer calculation for TCP. The slop is 305typically added to the raw calculation to take into account 306occasional variances that the SRTT (smoothed round trip time) 307is unable to accomodate, while the minimum specifies an 308absolute minimum. While a number of TCP RFCs suggest a 1 309second minimum these RFCs tend to focus on streaming behavior 310and fail to deal with the fact that a 1 second minimum has severe 311detrimental effects over lossy interactive connections, such 312as a 802.11b wireless link, and over very fast but lossy 313connections for those cases not covered by the fast retransmit 314code. For this reason we suggest changing the slop to 200ms and 315setting the minimum to something out of the way, like 20ms, 316which gives you an effective minimum of 200ms (similar to Linux). 317.It tcp.inflight_enable 318Enable 319.Tn TCP 320bandwidth delay product limiting. An attempt will be made to calculate 321the bandwidth delay product for each individual TCP connection and limit 322the amount of inflight data being transmitted to avoid building up 323unnecessary packets in the network. This option is recommended if you 324are serving a lot of data over connections with high bandwidth-delay 325products, such as modems, GigE links, and fast long-haul WANs, and/or 326you have configured your machine to accomodate large TCP windows. In such 327situations, without this option, you may experience high interactive 328latencies or packet loss due to the overloading of intermediate routers 329and switches. Note that bandwidth delay product limiting only effects 330the transmit side of a TCP connection. 331.It tcp.inflight_debug 332Enable debugging for the bandwidth delay product algorithm. This may 333default to on (1) so if you enable the algorithm you should probably also 334disable debugging by setting this variable to 0. 335.It tcp.inflight_min 336This puts an lower bound on the bandwidth delay product window, in bytes. 337A value of 1024 is typically used for debugging. 6000-16000 is more typical 338in a production installation. Setting this value too low may result in 339slow ramp-up times for bursty connections. Setting this value too high 340effectively disables the algorithm. 341.It tcp.inflight_max 342This puts an upper bound on the bandwidth delay product window, in bytes. 343This value should not generally be modified but may be used to set a 344global per-connection limit on queued data, potentially allowing you to 345intentionally set a less then optimum limit to smooth data flow over a 346network while still being able to specify huge internal TCP buffers. 347.It tcp.inflight_stab 348The bandwidth delay product algorithm requires a slightly larger window 349then it otherwise calculates for stability. This parameter determines the 350extra window in maximal packets / 10. The default value of 20 represents 3512 maximal packets. Reducing this value is not recommended but you may 352come across a situation with very slow links where the ping time 353reduction of the default inflight code is not sufficient. If this case 354occurs you should first try reducing tcp.inflight_min and, if that does not 355work, reduce both tcp.inflight_min and tcp.inflight_stab, trying values of 35615, 10, or 5 for the latter. Never use a value less then 5. Reducing 357tcp.inflight_stab can lead to upwards of a 20% underutilization of the link 358as well as reducing the algorithm's ability to adapt to changing 359situations and should only be done as a last resort. 360.El 361.Sh ERRORS 362A socket operation may fail with one of the following errors returned: 363.Bl -tag -width Er 364.It Bq Er EISCONN 365when trying to establish a connection on a socket which 366already has one; 367.It Bq Er ENOBUFS 368when the system runs out of memory for 369an internal data structure; 370.It Bq Er ETIMEDOUT 371when a connection was dropped 372due to excessive retransmissions; 373.It Bq Er ECONNRESET 374when the remote peer 375forces the connection to be closed; 376.It Bq Er ECONNREFUSED 377when the remote 378peer actively refuses connection establishment (usually because 379no process is listening to the port); 380.It Bq Er EADDRINUSE 381when an attempt 382is made to create a socket with a port which has already been 383allocated; 384.It Bq Er EADDRNOTAVAIL 385when an attempt is made to create a 386socket with a network address for which no network interface 387exists. 388.It Bq Er EAFNOSUPPORT 389when an attempt is made to bind or connect a socket to a multicast 390address. 391.El 392.Sh SEE ALSO 393.Xr getsockopt 2 , 394.Xr socket 2 , 395.Xr sysctl 3 , 396.Xr blackhole 4 , 397.Xr inet 4 , 398.Xr intro 4 , 399.Xr ip 4 , 400.Xr ttcp 4 401.Rs 402.%A V. Jacobson 403.%A R. Braden 404.%A D. Borman 405.%T "TCP Extensions for High Performance" 406.%O RFC 1323 407.Re 408.Rs 409.%A R. Braden 410.%T "T/TCP \- TCP Extensions for Transactions" 411.%O RFC 1644 412.Re 413.Sh HISTORY 414The 415.Nm 416protocol appeared in 417.Bx 4.2 . 418The RFC 1323 extensions for window scaling and timestamps were added 419in 420.Bx 4.4 . 421