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_NODELAY
TCP_NODELAY
(from
<
netinet/tcp.h
>,
to defeat this algorithm.
TCP_MAXSEG
TCP_MAXSEG
option allows the user to determine the result of this negotiation,
and to reduce it if desired.
TCP_MD5SIG
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_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
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
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
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:
EISCONN
]
ENOBUFS
]
ETIMEDOUT
]
ECONNRESET
]
ECONNREFUSED
]
EADDRINUSE
]
EADDRNOTAVAIL
]