TCP(4)

NAME
tcp - Internet Transmission Control Protocol

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET, SOCK_STREAM, 0);

DESCRIPTION
The TCP protocol provides reliable, flow-controlled, two-way transmission of data. It is a byte-stream protocol used to support the SOCK_STREAM abstraction. TCP uses the standard Internet address format and, in addition, provides a per-host collection of “port addresses”. Thus, each address is composed of an Internet address specifying the host and network, with a specific TCP port on the host identifying the peer entity.

Sockets utilizing the tcp protocol are either “active” or “passive”. Active sockets initiate connections to passive sockets. By default TCP sockets are created active; to create a passive socket the listen(2) system call must be used after binding the socket with the bind(2) system call. Only passive sockets may use the accept(2) call to accept incoming connections. Only active sockets may use the connect(2) call to initiate connections. TCP also supports a more datagram-like mode, called Transaction TCP, which is described in ttcp(4).

Passive sockets may “underspecify” their location to match incoming connection requests from multiple networks. This technique, termed “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 INADDR_ANY must be bound. The 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 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.

TCP supports a number of socket options which can be set with setsockopt(2) and tested with getsockopt(2):

TCP_NODELAY
    Under most circumstances, 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. The boolean option TCP_NODELAY defeats this algorithm.

TCP_MAXSEG
    By default, a sender- and receiver-TCP will negotiate among themselves to determine the maximum segment size to be used for each connection. The TCP_MAXSEG option allows the user to determine the result of this negotiation, and to reduce it if desired.

TCP_NOOPT
    TCP usually sends a number of options in each packet, corresponding to various TCP extensions which are provided in this implementation. The boolean option TCP_NOOPT is provided to disable TCP option use on a per-connection basis.

TCP_NOPUSH
    By convention, the sender-TCP will set the “push” bit and begin transmission immediately (if permitted) at the end of every user call to write(2) or writev(2). The TCP_NOPUSH option is provided to allow servers to easily make use of Transaction TCP (see ttcp(4) ). When the option is set to a non-zero value, TCP will delay sending any data at all until either the socket is closed, or the internal send buffer is filled.

The option level for the setsockopt(2) call is the protocol number for TCP, available from getprotobyname(3), or IPPROTO_TCP. All options are declared in <netinet/tcp.h>.

Options at the IP transport level may be used with TCP; see ip(4). Incoming connection requests that are source-routed are noted, and the reverse source route is used in responding.

MIB VARIABLES
The tcp protocol implements three variables in the net.inet branch of the sysctl(3) MIB.

TCPCTL_DO_RFC1323
    (tcp.rfc1323) Implement the window scaling and timestamp options of RFC 1323 (default true).

TCPCTL_DO_RFC1644
    (tcp.rfc1644) Implement Transaction TCP, as described in RFC 1644.

TCPCTL_MSSDFLT
    (tcp.mssdflt) The default value used for the maximum segment size (“MSS”) when no advice to the contrary is received from MSS negotiation.

DIAGNOSTICS
A socket operation may fail with one of the following errors returned:

[EISCONN]
    when trying to establish a connection on a socket which already has one;

[ENOBUFS]
    when the system runs out of memory for an internal data structure;

[ETIMEDOUT]
    when a connection was dropped due to excessive retransmissions;

[ECONNRESET]
    when the remote peer forces the connection to be closed;

[ECONNREFUSED]
    when the remote peer actively refuses connection establishment (usually because no process is listening to the port);

[EADDRINUSE]
    when an attempt is made to create a socket with a port which has already been allocated;

[EADDRNOTAVAIL]
    when an attempt is made to create a socket with a network address for which no network interface exists.

[EAFNOSUPPORT]
    when an attempt is made to bind or connect a socket to a multicast address.

SEE ALSO
getsockopt(2), socket(2), sysctl(3), inet(4), intro(4), ip(4), ttcp(4) V. Jacobson, R. Braden, and D. Borman, TCP Extensions for High Performance, RFC 1323. R. Braden, T/TCP - TCP Extensions for Transactions, RFC 1644.

HISTORY
The tcp protocol appeared in 4.2BSD. The RFC 1323 extensions for window scaling and timestamps were added in 4.4BSD.