| TCP(4) | Device Drivers Manual | TCP(4) | 
tcp —
#include <sys/socket.h>
#include <netinet/in.h>
int
  
  socket(AF_INET,
    SOCK_STREAM,
    0);
int
  
  socket(AF_INET6,
    SOCK_STREAM,
    0);
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 using 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(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.
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_NODELAYTCP_NODELAY (from
      <netinet/tcp.h>, to defeat
      this algorithm.TCP_MAXSEGTCP_MAXSEG option allows the user to determine the
      result of this negotiation, and to reduce it if desired.TCP_MD5SIGOne common use for this in a NetBSD router deployment is to enable based routers to interwork with Cisco equipment at peering points. Support for this feature conforms to RFC 2385. Only IPv4 (AF_INET) sessions are supported.
In order for this option to function correctly, it is necessary for the administrator to add a tcp-md5 key entry to the system's security associations database (SADB) using the setkey(8) utility. This entry must have an SPI of 0x1000 and can therefore only be specified on a per-host basis at this time.
If an SADB entry cannot be found for the destination, the outgoing traffic will have an invalid digest option prepended, and the following error message will be visible on the system console: tcp_signature_compute: SADB lookup failed for %d.%d.%d.%d.
TCP_KEEPIDLETCP_KEEPIDLE option can be used to affect this
      value for a given socket, and specifies the number of seconds of idle time
      between keepalive probes. This option takes an unsigned
      int value, with a value greater than 0.TCP_KEEPINTVLSO_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 retransmits the
      probe after some amount of time. The default value for this retransmit
      interval is 150 seconds. The TCP_KEEPINTVL option
      can be used to affect this value for a given socket, and specifies the
      number of seconds to wait before retransmitting a keepalive probe. This
      option takes an unsigned int value, with a value
      greater than 0.TCP_KEEPCNTSO_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 retransmits the
      probe a certain number of times before a connection is considered to be
      broken. The default value for this keepalive probe retransmit 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 unsigned int
      value, with a value greater than 0.TCP_KEEPINITTCP_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
      connections, the TCP_KEEPINIT option value is
      inherited from the listening socket. This option takes an
      unsigned int value, with a value greater than
    0.TCP_INFOTPC_INFO to
      getsockopt(2). It
      accepts a single argument: a pointer to an instance of
      struct tcp_info.
    This API is subject to change; consult the source to determine which fields are currently filled out by this option. NetBSD specific additions include send window size, receive window size, and bandwidth-controlled window space.
The option level for the setsockopt(2) call is the protocol number for TCP, available from getprotobyname(3).
In the historical BSD TCP implementation,
    if the TCP_NODELAY option was set on a passive
    socket, the sockets returned by
    accept(2) erroneously did not
    have the TCP_NODELAY option set; the behavior was
    corrected to inherit TCP_NODELAY in
    NetBSD 1.6.
Options at the IP network level may be used with TCP; see ip(4) or ip6(4). Incoming connection requests that are source-routed are noted, and the reverse source route is used in responding.
There are many adjustable parameters that control various aspects of the NetBSD TCP behavior; these parameters are documented in sysctl(7), and they include:
EISCONN]ENOBUFS]ETIMEDOUT]ECONNRESET]ECONNREFUSED]EADDRINUSE]EADDRNOTAVAIL]Transmission Control Protocol, RFC, 793, September 1981.
Requirements for Internet Hosts -- Communication Layers, RFC, 1122, October 1989.
tcp protocol stack appeared in
  4.2BSD.
| February 14, 2015 | NetBSD 10.1 |