$NetBSD: tcp.4,v 1.3 1994/11/30 16:22:35 jtc Exp $
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@(#)tcp.4 8.1 (Berkeley) 6/5/93
.Dd $Mdocdate: August 11 2022 $ .Dt TCP 4 .Os .Sh NAME .Nm tcp .Nd Internet Transmission Control Protocol .Sh SYNOPSIS n sys/socket.h n netinet/in.h n netinet/tcp.h .Ft int .Fn socket AF_INET SOCK_STREAM 0 .Ft int .Fn socket AF_INET6 SOCK_STREAM 0 .Sh DESCRIPTION The .Tn TCP protocol provides a reliable, flow-controlled, two-way transmission of data. It is a byte-stream protocol used to support the .Dv SOCK_STREAM abstraction. TCP uses the standard Internet address format and, in addition, provides a per-host collection of .Dq port addresses . Thus, each address is composed of an Internet address specifying the host and network, with a specific .Tn TCP port on the host identifying the peer entity.
p Sockets utilizing the TCP protocol are either .Dq active or .Dq passive . Active sockets initiate connections to passive sockets. By default .Tn TCP sockets are created active; to create a passive socket the .Xr listen 2 system call must be used after binding the socket with the .Xr bind 2 system call. Only passive sockets may use the .Xr accept 2 call to accept incoming connections. Only active sockets may use the .Xr connect 2 call to initiate connections.
p Passive sockets may .Dq underspecify their location to match incoming connection requests from multiple networks. This technique, termed .Dq wildcard addressing , allows a single server to provide service to clients on multiple networks. To create a socket which listens on all networks, the Internet address .Dv INADDR_ANY must be bound. The .Tn TCP port may still be specified at this time; if the port is not specified the system will assign one. Once a connection has been established, the socket's address is fixed by the peer entity's location. The address assigned to the socket is the address associated with the network interface through which packets are being transmitted and received. Normally this address corresponds to the peer entity's network.
p .Tn TCP supports several socket options which are set with .Xr setsockopt 2 and tested with .Xr getsockopt 2 . l -ohang t Cd TCP_INFO Retrieve information about a socket's underlying TCP session. .Dv TCP_INFO is only used with .Fn getsockopt . The argument is a pointer to an instance of .Vt "struct tcp_info" (from n netinet/tcp.h ) . t Cd TCP_NODELAY Under most circumstances, .Tn TCP sends data when it is presented; when outstanding data has not yet been acknowledged, it gathers small amounts of output to be sent in a single packet once an acknowledgement is received. For a small number of clients, such as window systems that send a stream of mouse events which receive no replies, this packetization may cause significant delays. Therefore, .Tn TCP provides a boolean option, .Dv TCP_NODELAY (from n netinet/tcp.h ) , to defeat this algorithm. t Cd TCP_NOPUSH By convention, the .Tn TCP sender will set the .Dq push bit and begin transmission immediately (if permitted) at the end of every user call to .Xr write 2 or .Xr writev 2 . When this option is set to a non-zero value, .Tn TCP will delay sending any data at all until either the socket is closed, the internal send buffer is filled, or this option is set to a zero value. t Cd TCP_MAXSEG Set the maximum segment size for this connection. The maximum segment size can only be lowered. t Cd TCP_SACK_ENABLE Use selective acknowledgements for this connection. See .Xr options 4 . t Cd TCP_MD5SIG Use TCP MD5 signatures per RFC 2385. This requires .Em Security Associations to be set up, which can be done using .Xr ipsecctl 8 . When a listening socket has .Em TCP_MD5SIG set, it accepts connections with MD5 signatures only from sources for which a .Em Security Association is set up. Connections without MD5 signatures are only accepted from sources for which no .Em Security Association is set up. The connected socket only has .Em TCP_MD5SIG set if the connection is protected with MD5 signatures. .El
p The option level for the .Xr setsockopt 2 call is the protocol number for .Tn TCP , available from .Xr getprotobyname 3 .
p Options at the .Tn IP transport level may be used with .Tn TCP ; see .Xr ip 4 or .Xr ip6 4 . Incoming connection requests that are source-routed are noted, and the reverse source route is used in responding. .Sh DIAGNOSTICS A socket operation may fail with one of the following errors returned: l -tag -width [EADDRNOTAVAIL] t Bq Er EISCONN when trying to establish a connection on a socket which already has one; t Bq Er ENOBUFS when the system runs out of memory for an internal data structure; t Bq Er ETIMEDOUT when a connection was dropped due to excessive retransmissions; t Bq Er ECONNRESET when the remote peer forces the connection to be closed; t Bq Er ECONNREFUSED when the remote peer actively refuses connection establishment (usually because no process is listening to the port); t Bq Er EADDRINUSE when an attempt is made to create a socket with a port which has already been allocated; t Bq Er EADDRNOTAVAIL when an attempt is made to create a socket with a network address for which no network interface exists. .El .Sh SEE ALSO .Xr tcpbench 1 , .Xr getsockopt 2 , .Xr socket 2 , .Xr inet 4 , .Xr inet6 4 , .Xr ip 4 , .Xr ip6 4 , .Xr netintro 4 , .Xr ipsecctl 8 , .Xr tcpdrop 8 .Sh HISTORY The .Nm protocol stack appeared in x 4.2 .