man Pages(7):Device and Network Interfaces

tcp(7P)

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | DIAGNOSTICS

NAME

tcp, TCP- Internet Transmission Control Protocol

SYNOPSIS

#include <sys/socket.h>

#include <netinet/in.h>

s = socket(AF_INET, SOCK_STREAM, 0);

t = t_open("/dev/tcp", O_RDWR);

DESCRIPTION

TCP is the virtual circuit protocol of the Internet protocol family. It provides reliable, flow-controlled, in order, two-way transmission of data. It is a byte-stream protocol layered above the Internet Protocol (IP), the Internet protocol family's internetwork datagram delivery protocol.

Programs can access TCP using the socket interface as a SOCK_STREAM socket type, or using the Transport Level Interface (TLI) where it supports the connection-oriented ( T_COTS_ORD ) service type.

TCP uses IP's host-level addressing and adds its own per-host collection of "port addresses." The endpoints of a TCP connection are identified by the combination of an IP address and a TCP port number. Although other protocols, such as the User Datagram Protocol (UDP), may use the same host and port address format, the port space of these protocols is distinct. See inet(7P) for details on the common aspects of addressing in the Internet protocol family.

Sockets utilizing TCP are either "active" or "passive". Active sockets initiate connections to passive sockets. Both types of sockets must have their local IP address and TCP port number bound with the bind(3N) system call after the socket is created. By default, TCP sockets are active. A passive socket is created by calling the listen(3N) system call after binding the socket with bind() . This establishes a queueing parameter for the passive socket. After this, connections to the passive socket can be received with the accept(3N) system call. Active sockets use the connect(3N) call after binding to initiate connections.

By using the special value INADDR_ANY , the local IP address can be left unspecified in the bind() call by either active or passive TCP sockets. This feature is usually used if the local address is either unknown or irrelevant. If left unspecified, the local IP address will be bound at connection time to the address of the network interface used to service the connection.

Once a connection has been established, data can be exchanged using the read(2) and write(2) system calls.

Under most circumstances, TCP sends data when it is presented. When outstanding data has not yet been acknowledged, TCP gathers small amounts of output to be sent in a single packet once an acknowledgement has been 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. To circumvent this problem, TCP provides a socket-level boolean option, TCP_NODELAY. TCP_NODELAY is defined in <netinet/tcp.h> , and is set with setsockopt(3N) and tested with getsockopt(3N) . The option level for the setsockopt() call is the protocol number for TCP, available from getprotobyname(3N) .

Another socket level option, SO_RCVBUF, can be used to control the window that TCP advertises to the peer. IP level options may also be used with TCP. See ip(7P) .

TCP provides an urgent data mechanism, which may be invoked using the out-of-band provisions of send(3N) . The caller may mark one byte as "urgent" with the MSG_OOB flag to send(3N) . This sets an "urgent pointer" pointing to this byte in the TCP stream. The receiver on the other side of the stream is notified of the urgent data by a SIGURG signal. The SIOCATMARK ioctl(2) request returns a value indicating whether the stream is at the urgent mark. Because the system never returns data across the urgent mark in a single read(2) call, it is possible to advance to the urgent data in a simple loop which reads data, testing the socket with the SIOCATMARK ioctl() request, until it reaches the mark.

Incoming connection requests that include an IP source route option are noted, and the reverse source route is used in responding.

A checksum over all data helps TCP implement reliability. Using a window-based flow control mechanism that makes use of positive acknowledgements, sequence numbers, and a retransmission strategy, TCP can usually recover when datagrams are damaged, delayed, duplicated or delivered out of order by the underlying communication medium.

If the local TCP receives no acknowledgements from its peer for a period of time, as would be the case if the remote machine crashed, the connection is closed and an error is returned to the user. If the remote machine reboots or otherwise loses state information about a TCP connection, the connection is aborted and an error is returned to the user.

SunOS supports TCP Extensions for High Performance (RFC 1323) which includes the window scale and time stamp options, and Protection Against Wrap Around Sequence Numbers (PAWS). SunOS also supports Selective Acknowledgment (SACK) capabilities (RFC 2018).

Turn on the window scale option in one of the following ways:

An application can set SO_SNDBUF or SO_RCVBUF size in the setsockopt() option to be larger than 64K. This must be done before the program calls listen () or connect() , because the window scale option is negotiated when the connection is established. Once the connection has been made, it is too late to increase the send or receive window beyond the default TCP limit of 64K.
For all applications, use ndd(1M) to modify the configuration parameter tcp_wscale_always . If tcp_wscale_always is set to 1 , the window scale option will always be set when connecting to a remote system. If tcp_wscale_always is 0, the window scale option will be set only if the user has requested a send or receive window larger than 64K. The default value of tcp_wscale_always is 0 .
Regardless of the value of tcp_wscale_always , the window scale option will always be included in a connect acknowledgement if the connecting system has used the option.

Turn on SACK capabilities in the following way:

Use ndd to modify the configuration parameter tcp_sack_permitted . If tcp_sack_permitted is set to 0 , TCP will not accept SACK or send out SACK information. If tcp_sack_permitted is set to 1 , TCP will not initiate a connection with SACK permitted option in the SYN segment, but will respond with SACK permitted option in the SYN|ACK segment if an incoming connection request has the SACK permitted option. This means that TCP will only accept SACK information if the other side of the connection also accepts SACK information. If tcp_sack_permitted is set to 2 , it will both initiate and accept connections with SACK information. The default for tcp_sack_permitted is 1 .

Turn on the time stamp option in the following way:

Use ndd to modify the configuration parameter tcp_tstamp_always . If tcp_tstamp_always is 1 , the time stamp option will always be set when connecting to a remote machine. If tcp_tstamp_always is 0 , the time stamp option will not be set when connecting to a remote system. The default for tcp_tstamp_always is 0 .

Regardless of the value of tcp_tstamp_always , the time stamp option will always be included in a connect acknowledgement (and all succeeding packets) if the connecting system has used the time stamp option.

Use the following procedure to turn on the time stamp option only when the window scale option is in effect:

Use ndd to modify the configuration parameter tcp_tstamp_if_wscale . Setting tcp_tstamp_if_wscale to 1 will cause the time stamp option to be set when connecting to a remote system, if the window scale option has been set. If tcp_tstamp_if_wscale is 0 , the time stamp option will not be set when connecting to a remote system. The default for tcp_tstamp_if_wscale is 0 .

Protection Against Wrap Around Sequence Numbers (PAWS) is always used when the time stamp option is set.

SunOS also supports multiple methods of generating initial sequence numbers. One of these methods is the improved technique suggested in RFC 1948. We HIGHLY recommended that you set sequence number generation parameters to be as close to boot time as possible. This prevents sequence number problems on connections that use the same connection-ID as ones that used a different sequence number generation. The /etc/init.d/inetinit script contains commands which configure initial sequence number generation. The script reads the value contained in the configuration file /etc/default/inetinit to determine which method to use.

The /etc/default/inetinit file is an unstable interface, and may change in future releases.

DIAGNOSTICS

A socket operation may fail if:

EISCONN: A connect() operation was attempted on a socket on which a connect() operation had already been performed.
ETIMEDOUT: A connection was dropped due to excessive retransmissions.
ECONNRESET: The remote peer forced the connection to be closed (usually because the remote machine has lost state information about the connection due to a crash).
ECONNREFUSED: The remote peer actively refused connection establishment (usually because no process is listening to the port).
EADDRINUSE: A bind() operation was attempted on a socket with a network address/port pair that has already been bound to another socket.
EADDRNOTAVAIL: A bind() operation was attempted on a socket with a network address for which no network interface exists.
EACCES: A bind() operation was attempted with a "reserved" port number and the effective user ID of the process was not the privileged user.
ENOBUFS: The system ran out of memory for internal data structures.

SunOS 5.7 Last Revised 19 Dec 1997

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | DIAGNOSTICS