NAME
tcp
—
Internet Transmission Control
Protocol
SYNOPSIS
#include
<sys/socket.h>
#include <netinet/in.h>
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 theSOCK_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 acknowledgment 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 unsigned int 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. TheTCP_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_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. TheTCP_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_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, theTCP_KEEPINIT
option value is inherited from the listening socket. This option takes an unsigned int value, with a value greater than 0. TCP_INFO
- Information about a socket's underlying TCP session may be retrieved by
passing the read-only option
TPC_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:
- RFC 1323 extensions for high performance
- Send/receive buffer sizes
- Default maximum segment size (MSS)
- SYN cache parameters
- 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
- RFC 3390: Increased initial window size
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), inet(4), inet6(4), intro(4), ip(4), ip6(4), sysctl(7)
Transmission Control Protocol, RFC, 793, September 1981.
Requirements for Internet Hosts -- Communication Layers, RFC, 1122, October 1989.
HISTORY
The tcp
protocol stack appeared in
4.2BSD.