TCP (7)

SYNOPSIS

#include <sys/socket.h> #include <netinet/in.h> tcp_socket = socket(PF_INET, SOCK_STREAM, 0);

DESCRIPTION

This is an implementation of the TCP protocol defined in RFC793, RFC1122 and RFC2001 with the NewReno and SACK extensions. It provides a reliable, stream oriented, full duplex connection between two sockets on top of ip (7). TCP guarantees that the data arrives in order and retransmits lost packets. It generates and checks a per packet checksum to catch transmission errors. TCP does not preserve record boundaries.

A fresh TCP socket has no remote or local address and is not fully specified. To create an outgoing TCP connection use connect (2) to establish a connection to another TCP socket. To receive new incoming connections bind (2) the socket first to a local address and port and then call listen (2) to put the socket into listening state. After that a new socket for each incoming connection can be accepted using accept (2). A socket which has had accept or connect successfully called on it is fully specified and may transmit data. Data cannot be transmitted on listening or not yet connected sockets.

Linux 2.2 supports the RFC1323 TCP high performance extensions. This includes large TCP windows to support links with high latency or bandwidth. In order to make use of them, the send and receive buffer sizes must be increased. They can be be set globally with the net.core.wmem_default and net.core.rmem_default sysctls, or on individual sockets by using the SO_SNDBUF and SO_RCVBUF socket options. The maximum sizes for socket buffers are limited by the global net.core.rmem_max and net.core.wmem_max sysctls. See socket (7) for more information.

TCP supports urgent data. Urgent data is used to signal the receiver that some important message is part of the data stream and that it should be processed as soon as possible. To send urgent data specify the MSG_OOB option to send (2). When urgent data is received, the kernel sends a SIGURG signal to the reading process or the process or process group that has been set for the socket using the FIOCSPGRP or FIOCSETOWN ioctls. When the SO_OOBINLINE socket option is enabled, urgent data is put into the normal data stream (and can be tested for by the SIOCATMARK ioctl), otherwise it can be only received when the MSG_OOB flag is set for sendmsg (2).

ADDRESS FORMATS

TCP is built on top of IP (see ip (7)). The address formats defined by ip (7) apply to TCP. TCP only supports point-to-point communication; broadcasting and multicasting are not supported.

SYSCTLS

These sysctls can be accessed by the /proc/sys/net/ipv4/* files or with the sysctl (2) interface. In addition, most IP sysctls also apply to TCP; see ip (7).

tcp_window_scaling

Enable RFC1323 TCP window scaling.

tcp_sack

Enable RFC2018 TCP Selective Acknowledgements.

tcp_timestamps

Enable RFC1323 TCP timestamps.

tcp_fin_timeout

How many seconds to wait for a final FIN packet before the socket is forcibly closed. This is strictly a violation of the TCP specification, but required to prevent denial-of-service attacks.

tcp_keepalive_probes

Maximum TCP keep-alive probes to send before giving up. Keep-alives are only sent when the SO_KEEPALIVE socket option is enabled.

tcp_keepalive_time

The number of seconds after no data has been transmitted before a keep-alive will be sent on a connection. The default is 10800 seconds (3 hours).

tcp_max_ka_probes

How many keep-alive probes are sent per slow timer run. To prevent bursts, this value should not be set too high.

tcp_stdurg

Enable the strict RFC793 interpretation of the TCP urgent-pointer field. The default is to use the BSD-compatible interpretation of the urgent-pointer, pointing to the first byte after the urgent data. The RFC793 interpretation is to have it point to the last byte of urgent data. Enabling this option may lead to interoperatibility problems.

tcp_syncookies

Enable TCP syncookies. The kernel must be compiled with CONFIG_SYN_COOKIES . Syncookies protects a socket from overload when too many connection attempts arrive. Client machines may not be able to detect an overloaded machine with a short timeout anymore when syncookies are enabled.

tcp_max_syn_backlog

Length of the per-socket backlog queue. As of Linux 2.2, the backlog specified in listen (2) only specifies the length of the backlog queue of already established sockets. The maximum queue of sockets not yet established (in SYN_RECV state) per listen socket is set by this sysctl. When more connection requests arrive, Linux starts to drop packets. When syncookies are enabled the packets are still answered and this value is effectively ignored.

tcp_retries1

Defines how many times an answer to a TCP connection request is retransmitted before giving up.

tcp_retries2

Defines how many times a TCP packet is retransmitted in established state before giving up.

tcp_syn_retries

Defines how many times to try to send an initial SYN packet to a remote host before giving up and returns an error. Must be below 255. This is only the timeout for outgoing connections; for incoming connections the number of retransmits is defined by tcp_retries1 .

tcp_retrans_collapse

Try to send full-sized packets during retransmit. This is used to work around TCP bugs in some stacks.