June 19, 2007 TCP 4

### NAME

tcp - Internet Transmission Control Protocol

### SYNOPSIS

```int` socket(```AF_INET``, SOCK_STREAM``, 0`)
```int` socket(```AF_INET6``, SOCK_STREAM``, 0`)

### DESCRIPTION

The TCP 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 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_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. Therefore, TCP provides a boolean option, ```TCP_NODELAY` (from <```netinet/tcp.h`>, to defeat 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_MD5SIG`
  This option enables the use of MD5 digests (also known as TCP-MD5) on writes to the specified socket. In the current release, only outgoing traffic is digested; digests on incoming traffic are not verified. The current default behavior for the system is to respond to a system advertising this option with TCP-MD5; this may change.

  One 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_KEEPIDLE`
  TCP probes a connection that has been idle for some amount of time. The default value for this idle period is 4 hours. The ```TCP_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 value, with a value greater than 0.

```TCP_KEEPINTVL`
  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 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 value, with a value greater than 0.

```TCP_KEEPCNT`
  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 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 value, with a value greater than 0.

```TCP_KEEPINIT`
  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 150 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 connections, the ```TCP_KEEPINIT` option value is inherited from the listening socket. This option takes an value, with a value greater than 0.

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 NetBSD1.6.

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:

- RFC 1323 extensions for high performance
- Send/receive buffer sizes
- Default maximum segment size (MSS)
- SYN cache parameters
- Initial window size
- Hughes/Touch/Heidemann Congestion Window Monitoring algorithm
- Keepalive parameters
- newReno algorithm for congestion control
- Logging of connection refusals
- RST packet rate limits
- SACK (Selective Acknowledgment)
- ECN (Explicit Congestion Notification)
- Congestion window increase methods; the traditional packet counting or RFC 3465 Appropriate Byte Counting

### 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.

### SEE ALSO

getsockopt(2), socket(2), sysctl(7), inet(4), inet6(4), intro(4), ip(4), ip6(4)

### HISTORY

The tcp protocol stack appeared in 4.2BSD.