--- name: "tcp" section: "4" platform: "macos" package: "xnu" version: "12377.121.6" release: "macOS 26.5" platforms: ["linux", "freebsd", "openbsd", "macos"] url: "https://new.carta.tech/tcp/4" --- # tcp(4) Internet Transmission Control Protocol ## NAME **tcp** – Internet Transmission Control Protocol ## SYNOPSIS **#include \** **#include \** **#include \** *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)](/listen/2.md) system call must be used after binding the socket with the [bind(2)](/bind/2.md) system call. Only passive sockets may use the [accept(2)](/accept/2.md) call to accept incoming connections. Only active sockets may use the [connect(2)](/connect/2.md) or [connectx(2)](/connectx/2.md) call to initiate connections. 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 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. TCP supports a number of socket options which can be set with [setsockopt(2)](/setsockopt/2.md) and tested with [getsockopt(2)](/getsockopt/2.md): - 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)](/write/2.md) or [writev(2)](/writev/2.md). When this 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. - TCP\_KEEPALIVE The TCP\_KEEPALIVE options enable to specify the amount of time, in seconds, that the connection must be idle before keepalive probes (if enabled) are sent. The default value is specified by the MIB variable *net.inet.tcp.keepidle*. - TCP\_CONNECTIONTIMEOUT The TCP\_CONNECTIONTIMEOUT option allows to specify the timeout, in seconds, for new, non established TCP connections. This option can be useful for both active and passive TCP connections. The default value is specified by the MIB variable *net.inet.tcp.keepinit*. - TCP\_KEEPINTVL When keepalive probes are enabled, this option will set the amount of time in seconds between successive keepalives sent to probe an unresponsive peer. - TCP\_KEEPCNT When keepalive probes are enabled, this option will set the number of times a keepalive probe should be repeated if the peer is not responding. After this many probes, the connection will be closed. - TCP\_SENDMOREACKS When a stream of TCP data packets are received, OS X uses an algorithm to reduce the number of acknowledgements by generating a TCP acknowledgement for 8 data packets instead of acknowledging every other data packet. When this socket option is enabled, the connection will always send a TCP acknowledgement for every other data packet. - TCP\_ENABLE\_ECN Using Explicit Congestion Notification (ECN) on TCP allows bi-directional end-to-end notification of congestion without dropping packets. Conventionally TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a mark in the IP header instead of dropping a packet in order to signal impending congestion. The TCP receiver of the packet echoes congestion indication to the TCP sender, which reduces it's transmission rate as if it detected a dropped packet. This will avoid unnecessary retransmissions and will improve latency by saving the time required for recovering a lost packet. - TCP\_NOTSENT\_LOWAT The send socket buffer of a TCP sender has unsent and unacknowledged data. This option allows a TCP sender to control the amount of unsent data kept in the send socket buffer. The value of the option should be the maximum amount of unsent data in bytes. Kevent, poll and select will generate a write notification when the unsent data falls below the amount given by this option. This will allow an application to generate just-in-time fresh updates for real-time communication. - TCP\_FASTOPEN The TCP listener can set this option to use TCP Fast Open feature. After setting this option, an [accept(2)](/accept/2.md) may return a socket that is in SYN\_RECEIVED state but is readable and writable. - TCP\_CONNECTION\_INFO This socket option can be used to obtain TCP connection level statistics. The "struct tcp\_connection\_info" defined in \ is copied to the user buffer. The option level for the [setsockopt(2)](/setsockopt/2.md) call is the protocol number for TCP, available from [getprotobyname(3)](/getprotobyname/3.md), 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)](/ip/4.md). Incoming connection requests that are source-routed are noted, and the reverse source route is used in responding. ### Non-blocking connect When a TCP socket is set non-blocking, and the connection cannot be established immediately, [connect(2)](/connect/2.md) or [connectx(2)](/connectx/2.md) returns with the error EINPROGRESS, and the connection is established asynchronously. When the asynchronous connection completes successfully, [select(2)](/select/2.md) or [poll(2)](/poll/2.md) or [kqueue(2)](/kqueue/2.md) will indicate the file descriptor is ready for writing. If the connection encounters an error, the file descriptor is marked ready for both reading and writing, and the pending error can be retrieved via the socket option SO\_ERROR. Note that even if the socket is non-blocking, it is possible for the connection to be established immediately. In that case [connect(2)](/connect/2.md) or [connectx(2)](/connectx/2.md) does not return with EINPROGRESS. ## 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; - \[EINPROGRESS\] returned by [connect(2)](/connect/2.md) or [connectx(2)](/connectx/2.md) when the socket is set nonblocking, and the connection cannot be immediately established; - \[EALREADY\] returned by [connect(2)](/connect/2.md) or [connectx(2)](/connectx/2.md) when connection request is already in progress for the specified socket. - \[ENODATA\] returned by [recv(2)](/recv/2.md) or [send(2)](/send/2.md) in case a connection is experiencing a data-stall (probably due to a middlebox issue). It is advised that the current connection gets closed by the application and a new attempt is being made. ## SEE ALSO [connect(2)](/connect/2.md), [connectx(2)](/connectx/2.md), [getsockopt(2)](/getsockopt/2.md), [kqueue(2)](/kqueue/2.md), [poll(2)](/poll/2.md), [select(2)](/select/2.md), [socket(2)](/socket/2.md), [sysctl(3)](/sysctl/3.md), [inet(4)](/inet/4.md), [inet6(4)](/inet6/4.md), [ip(4)](/ip/4.md), [ip6(4)](/ip6/4.md), [netintro(4)](/netintro/4.md), [setkey(8)](/setkey/8.md) ## HISTORY The TCP protocol appeared in 4.2BSD. The socket option TCP\_CONNECTIONTIMEOUT first appeared in Mac OS X 10.6.