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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 1.4 2005/03/06 05:12:42 hsu 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.msl 226The Maximum Segment Lifetime for a packet. 227.It tcp.keepinit 228Timeout for new, non-established TCP connections. 229.It tcp.keepidle 230Amount of time the connection should be idle before keepalive 231probes (if enabled) are sent. 232.It tcp.keepintvl 233The interval between keepalive probes sent to remote machines. 234After 235.Dv TCPTV_KEEPCNT 236(default 8) probes are sent, with no response, the connection is dropped. 237.It tcp.always_keepalive 238Assume that 239.Dv SO_KEEPALIVE 240is set on all 241.Tn TCP 242connections, the kernel will 243periodically send a packet to the remote host to verify the connection 244is still up. 245.It tcp.icmp_may_rst 246Certain 247.Tn ICMP 248unreachable messages may abort connections in 249.Tn SYN-SENT 250state. 251.It tcp.do_tcpdrain 252Flush packets in the 253.Tn TCP 254reassembly queue if the system is low on mbufs. 255.It tcp.blackhole 256If enabled, disable sending of RST when a connection is attempted 257to a port where there is not a socket accepting connections. 258See 259.Xr blackhole 4 . 260.It tcp.delayed_ack 261Delay ACK to try and piggyback it onto a data packet. 262.It tcp.delacktime 263Maximum amount of time before a delayed ACK is sent. 264.It tcp.newreno 265Enable TCP NewReno Fast Recovery algorithm, 266as described in RFC 2582. 267.It tcp.path_mtu_discovery 268Enables Path MTU Discovery. PMTU Discovery is helpful for avoiding 269IP fragmentation when tranferring lots of data to the same client. 270For web servers, where most of the connections are short and to 271different clients, PMTU Discovery actually hurts performance due 272to unnecessary retransmissions. Turn this on only if most of your 273TCP connections are long transfers or are repeatedly to the same 274set of clients. 275.It tcp.tcbhashsize 276Size of the 277.Tn TCP 278control-block hashtable 279(read-only). 280This may be tuned using the kernel option 281.Dv TCBHASHSIZE 282or by setting 283.Va net.inet.tcp.tcbhashsize 284in the 285.Xr loader 8 . 286.It tcp.pcbcount 287Number of active process control blocks 288(read-only). 289.It tcp.syncookies 290Determines whether or not syn cookies should be generated for 291outbound syn-ack packets. Syn cookies are a great help during 292syn flood attacks, and are enabled by default. 293.It tcp.isn_reseed_interval 294The interval (in seconds) specifying how often the secret data used in 295RFC 1948 initial sequence number calculations should be reseeded. 296By default, this variable is set to zero, indicating that 297no reseeding will occur. 298Reseeding should not be necessary, and will break 299.Dv TIME_WAIT 300recycling for a few minutes. 301.It tcp.inet.tcp.rexmit_{min,slop} 302Adjust the retransmit timer calculation for TCP. The slop is 303typically added to the raw calculation to take into account 304occasional variances that the SRTT (smoothed round trip time) 305is unable to accomodate, while the minimum specifies an 306absolute minimum. While a number of TCP RFCs suggest a 1 307second minimum these RFCs tend to focus on streaming behavior 308and fail to deal with the fact that a 1 second minimum has severe 309detrimental effects over lossy interactive connections, such 310as a 802.11b wireless link, and over very fast but lossy 311connections for those cases not covered by the fast retransmit 312code. For this reason we suggest changing the slop to 200ms and 313setting the minimum to something out of the way, like 20ms, 314which gives you an effective minimum of 200ms (similar to Linux). 315.It tcp.inflight_enable 316Enable 317.Tn TCP 318bandwidth delay product limiting. An attempt will be made to calculate 319the bandwidth delay product for each individual TCP connection and limit 320the amount of inflight data being transmitted to avoid building up 321unnecessary packets in the network. This option is recommended if you 322are serving a lot of data over connections with high bandwidth-delay 323products, such as modems, GigE links, and fast long-haul WANs, and/or 324you have configured your machine to accomodate large TCP windows. In such 325situations, without this option, you may experience high interactive 326latencies or packet loss due to the overloading of intermediate routers 327and switches. Note that bandwidth delay product limiting only effects 328the transmit side of a TCP connection. 329.It tcp.inflight_debug 330Enable debugging for the bandwidth delay product algorithm. This may 331default to on (1) so if you enable the algorithm you should probably also 332disable debugging by setting this variable to 0. 333.It tcp.inflight_min 334This puts an lower bound on the bandwidth delay product window, in bytes. 335A value of 1024 is typically used for debugging. 6000-16000 is more typical 336in a production installation. Setting this value too low may result in 337slow ramp-up times for bursty connections. Setting this value too high 338effectively disables the algorithm. 339.It tcp.inflight_max 340This puts an upper bound on the bandwidth delay product window, in bytes. 341This value should not generally be modified but may be used to set a 342global per-connection limit on queued data, potentially allowing you to 343intentionally set a less then optimum limit to smooth data flow over a 344network while still being able to specify huge internal TCP buffers. 345.It tcp.inflight_stab 346The bandwidth delay product algorithm requires a slightly larger window 347then it otherwise calculates for stability. This parameter determines the 348extra window in maximal packets / 10. The default value of 20 represents 3492 maximal packets. Reducing this value is not recommended but you may 350come across a situation with very slow links where the ping time 351reduction of the default inflight code is not sufficient. If this case 352occurs you should first try reducing tcp.inflight_min and, if that does not 353work, reduce both tcp.inflight_min and tcp.inflight_stab, trying values of 35415, 10, or 5 for the latter. Never use a value less then 5. Reducing 355tcp.inflight_stab can lead to upwards of a 20% underutilization of the link 356as well as reducing the algorithm's ability to adapt to changing 357situations and should only be done as a last resort. 358.El 359.Sh ERRORS 360A socket operation may fail with one of the following errors returned: 361.Bl -tag -width Er 362.It Bq Er EISCONN 363when trying to establish a connection on a socket which 364already has one; 365.It Bq Er ENOBUFS 366when the system runs out of memory for 367an internal data structure; 368.It Bq Er ETIMEDOUT 369when a connection was dropped 370due to excessive retransmissions; 371.It Bq Er ECONNRESET 372when the remote peer 373forces the connection to be closed; 374.It Bq Er ECONNREFUSED 375when the remote 376peer actively refuses connection establishment (usually because 377no process is listening to the port); 378.It Bq Er EADDRINUSE 379when an attempt 380is made to create a socket with a port which has already been 381allocated; 382.It Bq Er EADDRNOTAVAIL 383when an attempt is made to create a 384socket with a network address for which no network interface 385exists. 386.It Bq Er EAFNOSUPPORT 387when an attempt is made to bind or connect a socket to a multicast 388address. 389.El 390.Sh SEE ALSO 391.Xr getsockopt 2 , 392.Xr socket 2 , 393.Xr sysctl 3 , 394.Xr blackhole 4 , 395.Xr inet 4 , 396.Xr intro 4 , 397.Xr ip 4 , 398.Xr ttcp 4 399.Rs 400.%A V. Jacobson 401.%A R. Braden 402.%A D. Borman 403.%T "TCP Extensions for High Performance" 404.%O RFC 1323 405.Re 406.Rs 407.%A R. Braden 408.%T "T/TCP \- TCP Extensions for Transactions" 409.%O RFC 1644 410.Re 411.Sh HISTORY 412The 413.Nm 414protocol appeared in 415.Bx 4.2 . 416The RFC 1323 extensions for window scaling and timestamps were added 417in 418.Bx 4.4 . 419