TCP(7)

NAME
tcp - TCP protocol.

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.

SOCKET OPTIONS
To set or get a TCP socket option, call getsockopt(2) to read or setsockopt(2) to write the option with the socket family argument set to SOL_TCP. In addition, most SOL_IP socket options are valid on TCP sockets. For more information see ip(7).

TCP_NODELAY
    Turn the Nagle algorithm off. This means that packets are always sent as soon as possible and no unnecessary delays are introduced, at the cost of more packets in the network. Expects an integer boolean flag.

TCP_MAXSEG
    Set or receive the maximum segment size for outgoing TCP packets. If this option is set before connection establishment, it also changes the MSS value announced to the other end in the initial packet. Values greater than the interface MTU are ignored and have no effect.

TCP_CORK
    If enabled don't send out partial frames. All queued partial frames are sent when the option is cleared again. This is useful for prepending headers before calling sendfile(2), or for throughput optimization. This option cannot be combined with TCP_NODELAY.

IOCTLS
These ioctls can be accessed using ioctl(2). The correct syntax is:

    int value;
    error = ioctl(tcp_socket, ioctl_type, &value);

FIONREAD or TIOCINQ
    Returns the amount of queued unread data in the receive buffer. Argument is a pointer to an integer.

SIOCATMARK
    Returns true when the all urgent data has been already received by the user program. This is used together with SO_OOBINLINE. Argument is an pointer to an integer for the test result.

TIOCOUTQ
    Returns the amount of unsent data in the socket send queue in the passed integer value pointer. Unfortunately, the implementation of this ioctl is buggy in all known versions of Linux and instead returns the free space (effectively buffer size minus bytes used including metadata) in the send queue. This will be fixed in future Linux versions. If you use TIOCOUTQ, please include a runtime test for both behaviors for correct function on future releases and other Unixes.

ERROR HANDLING
When a network error occurs, TCP tries to resend the packet. If it doesn't succeed after some time, either ETIMEDOUT or the last received error on this connection is reported.

Some applications require a quicker error notification. This can be enabled with the SOL_IP level IP_RECVERR socket option. When this option is enabled, all incoming errors are immediately passed to the user program. Use this option with care - it makes TCP less tolerant to routing changes and other normal network conditions.

NOTES
When an error occurs doing a connection setup occuring in a socket write SIGPIPE is only raised when the SO_KEEPALIVE socket option is set.

TCP has no real out-of-band data; it has urgent data. In Linux this means if the other end sends newer out-of-band data the older urgent data is inserted as normal data into the stream (even when SO_OOBINLINE is not set). This differs from BSD based stacks.

Linux uses the BSD compatible interpretation of the urgent pointer field by default. This violates RFC1122, but is required for interoperability with other stacks. It can be changed by the tcp_stdurg sysctl.

ERRORS

EPIPE
    The other end closed the socket unexpectedly or a read is executed on a shut down socket.

ETIMEDOUT
    The other end didn't acknowledge retransmitted data after some time.

EAFNOTSUPPORT
    Passed socket address type in sin_family was not AF_INET.

Any errors defined for ip(7) or the generic socket layer may also be returned for TCP.

BUGS
Not all errors are documented.

IPv6 is not described.

Transparent proxy options are not described.

VERSIONS
The sysctls are new in Linux 2.2. IP_RECVERR is a new feature in Linux 2.2. TCP_CORK is new in 2.2.

SEE ALSO
socket(7), socket(2), ip(7), sendmsg(2), recvmsg(2)
RFC793 for the TCP specification.
RFC1122 for the TCP requirements and a description of the Nagle algorithm.
RFC2581 for some TCP algorithms.