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tcp(7) Miscellaneous Information Manual tcp(7) NAME tcp - Internet Transmission Control Protocol (TCP) SYNOPSIS #include <sys/socket.h> #include <netinet/in.h> s = socket(AF_INET, SOCK_STREAM, 0); DESCRIPTION The TCP protocol provides reliable, flow-controlled, two-way transmis- sion of data. It is a byte-stream protocol used to support the SOCK_STREAM abstraction. The TCP protocol 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 spec- ifying the host and network, with a specific TCP port on the host iden- tifying the peer entity. Sockets utilizing the TCP 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 function must be used after binding the socket with the bind function. Only passive sockets may use the accept function to accept incoming connections. Only active sockets may use the connect function to initiate connec- tions. Passive sockets may underspecify their location to match incoming con- nection requests from multiple networks. This technique, termed wild- card addressing, allows a single server to provide service to clients on multiple networks. To create a socket which listens on all net- works, 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 sys- tem 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 in- terface through which packets are being transmitted and received. Nor- mally this address corresponds to the peer entity's network. TCP supports the following socket options: 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 gathering of output can cause signifi- cant delays. Therefore, TCP provides a Boolean option, TCP_NODELAY (from the netinet/tcp.h header file), to defeat this algorithm. This option takes an int value. The maximum size of a TCP segment is based on the network MTU for destinations on local networks or on a default MTU of 576 bytes for destinations on nonlocal networks. The default behavior can be altered by setting the TCP_MAXSEG option to an integer value from 1 to 65,535. However, TCP will not use a maximum segment size smaller than 32 or larger than the local network MTU. Setting the TCP_MAXSEG option to a value of zero results in default behavior. The TCP_MAXSEG option can only be set prior to calling listen or connect on the socket. For passive connections, the TCP_MAXSEG option value is inherited from the listening socket. This option takes an int value, with a range of 0 to 65535. When the SO_KEEPALIVE option is enabled, TCP probes a connection that has been idle for some amount of time. The default value for this idle period is 2 hours. The TCP_KEEPIDLE option can be used to affect this value for a given socket, and speci- fies the number of seconds of idle time between keepalive probes. This option takes an int value, with a range of 1 to N (where N is tcp_keepidle divided by PR_SLOWHZ). For more information on tcp_keepi- dle see the information on the inet subsystem configuration attributes in the System Configuration and Tuning guide. PR_SLOWHZ is defined in the <sys/protosw.h> header file. When the SO_KEEPALIVE option is en- abled, TCP probes a connection that has been idle for some amount of time. If the remote system does not respond to a keepalive probe, TCP retransmits the probe after some amount of time. The default value for this retransmit interval is 75 seconds. The TCP_KEEPINTVL option can be used to affect this value for a given socket, and specifies the num- ber of seconds to wait before retransmitting a keepalive probe. This option takes an int value, with a range of 1 to N (where N is tcp_keep- intvl divided by PR_SLOWHZ). For more information on tcp_keepintvl see the information on the inet subsystem configuration attributes in the System Configuration and Tuning guide. PR_SLOWHZ is defined in the <sys/protosw.h> header file. When the SO_KEEPALIVE option is enabled, TCP probes a connection that has been idle for some amount of time. If the remote system does not respond to a keepalive probe, TCP retrans- mits the probe a certain number of times before a connection is consid- ered to be broken. The default value for this keepalive probe retrans- mit limit is 8. The TCP_KEEPCNT option can be used to affect this value for a given socket, and specifies the maximum number of keepalive probes to be sent. This option takes an int value, with a range of 1 to tcp_keepcnt. For more information on tcp_keepcnt see the informa- tion on the inet subsystem configuration attributes in the System Con- figuration and Tuning guide. If a TCP connection cannot be established within some amount of time, TCP will time out the connect attempt. The default value for this initial connection establishment timeout is 75 seconds. The TCP_KEEPINIT option can be used to affect this initial timeout period for a given socket, and specifies the number of seconds to wait before the connect attempt is timed out. For passive connec- tions, the TCP_KEEPINIT option value is inherited from the listening socket. This option takes an int value, with a range of 1 to N (where N is tcp_keepinit divided by PR_SLOWHZ). For more information on tcp_keepinit see the information on the inet subsystem configuration attributes in the System Configuration and Tuning guide. PR_SLOWHZ is defined in the <sys/protosw.h> header file. These options can be re- trieved with the getsockopt function and set with the setsockopt func- tion. The option level for the setsockopt and getsockopt functions is the protocol number for TCP, available from the getprotobyname function. Options at the IP transport level may be used with TCP; see ip(7). In- coming connection requests that are source-routed are noted, and the reverse source route is used in responding. ERRORS If a socket operation fails, errno may be set to one of the following values: The option_len parameter provided to the setsockopt or getsock- opt function is less than sizeof(int) or the option_value parameter provided to the setsockopt function is outside the valid range for the option. The socket to be connected already has a connection. The sys- tem ran out of memory for an internal data structure. Setting the TCP_MAXSEG option is not supported after connect or listen. A connec- tion was dropped due to excessive retransmissions. The remote peer forced the connection to be closed. The remote peer actively refuses connection establishment (usually because no process is listening to the port). An attempt is made to create a socket with a port which has already been allocated. An attempt is made to create a socket with a network address for which no network interface exists. RELATED INFORMATION Functions: getsockopt(2), socket(2) Network Information: netintro(7), inet(7), ip(7) delim off tcp(7)