TCP

NAME

tcp - TCP protocol

SYNOPSIS

#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.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), for both v4 and v6 versions. 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 supports RFC1323 TCP high performance extensions. These include Protection Against Wrapped Sequence Numbers (PAWS), Window Scaling and Timestamps. Window scaling allows the use of large (> 64K) TCP windows in order to support links with high latency or bandwidth. To make use of them, the send and receive buffer sizes must be increased. They can be set globally with the net.ipv4.tcp_wmem and net.ipv4.tcp_rmem sysctl variables, or on individual sockets by using the SO_SNDBUF and SO_RCVBUF socket options with the setsockopt(2) call.

The maximum sizes for socket buffers declared via the SO_SNDBUF and SO_RCVBUF mechanisms are limited by the global net.core.rmem_max and net.core.wmem_max sysctls. Note that TCP actually allocates twice the size of the buffer requested in the setsockopt(2) call, and so a succeeding getsockopt(2) call will not return the same size of buffer as requested in the setsockopt(2) call. TCP uses this for administrative purposes and internal kernel structures, and the sysctl variables reflect the larger sizes compared to the actual TCP windows. On individual connections, the socket buffer size must be set prior to the listen() or connect() calls in order to have it take effect. 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 SIOCSPGRP or FIOSETOWN 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).

Linux 2.4 introduced a number of changes for improved throughput and scaling, as well as enhanced functionality. Some of these features include support for zerocopy sendfile(2), Explicit Congestion Notification, new management of TIME_WAIT sockets, keep-alive socket options and support for Duplicate SACK extensions.

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 variables 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_abort_on_overflow

Enable resetting connections if the listening service is too slow and unable to keep up and accept them. It is not enabled by default. It means that if overflow occurred due to a burst, the connection will recover. Enable this option _only_ if you are really sure that the listening daemon cannot be tuned to accept connections faster. Enabling this option can harm the clients of your server.

tcp_adv_win_scale

Count buffering overhead as bytes/2^tcp_adv_win_scale (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale), if it is <= 0. The default is 2.

The socket receive buffer space is shared between the application and kernel. TCP maintains part of the buffer as the TCP window, this is the size of the receive window advertised to the other end. The rest of the space is used as the "application" buffer, used to isolate the network from scheduling and application latencies. The tcp_adv_win_scale default value of 2 implies that the space used for the application buffer is one fourth that of the total.

tcp_app_win

This variable defines how many bytes of the TCP window are reserved for buffering overhead.

A maximum of (window/2^tcp_app_win, mss) bytes in the window are reserved for the application buffer. A value of 0 implies that no amount is reserved. The default value is 31.

tcp_dsack

Enable RFC2883 TCP Duplicate SACK support. It is enabled by default.

tcp_ecn

Enable RFC2884 Explicit Congestion Notification. It is not enabled by default. When enabled, connectivity to some destinations could be affected due to older, misbehaving routers along the path causing connections to be dropped.

tcp_fack

Enable TCP Forward Acknowledgement support. It is enabled by default.

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 (DoS) attacks. The default value in 2.4 kernels is 60, down from 180 in 2.2.

tcp_keepalive_intvl

The number of seconds between TCP keep-alive probes. The default value is 75 seconds.

tcp_keepalive_probes

The maximum number of TCP keep-alive probes to send before giving up and killing the connection if no response is obtained from the other end. The default value is 9.

tcp_keepalive_time

The number of seconds a connection needs to be idle before TCP begins sending out keep-alive probes. Keep-alives are only sent when the SO_KEEPALIVE socket option is enabled. The default value is 7200 seconds (2 hours). An idle connection is terminated after approximately an additional 11 minutes (9 probes an interval of 75 seconds apart) when keep-alive is enabled.

Note that underlying connection tracking mechanisms and application timeouts may be much shorter.

tcp_max_orphans

The maximum number of orphaned (not attached to any user file handle) TCP sockets allowed in the system. When this number is exceeded, the orphaned connection is reset and a warning is printed. This limit exists only to prevent simple DoS attacks. Lowering this limit is not recommended. Network conditions might require you to increase the number of orphans allowed, but note that each orphan can eat up to ~64K of unswappable memory. The default initial value is set equal to the kernel parameter NR_FILE. This initial default is adjusted depending on the memory in the system.

tcp_max_syn_backlog

The maximum number of queued connection requests which have still not received an acknowledgement from the connecting client. If this number is exceeded, the kernel will begin dropping requests. The default value of 256 is increased to 1024 when the memory present in the system is adequate or greater (>= 128Mb), and reduced to 128 for those systems with very low memory (<= 32Mb). It is recommended that if this needs to be increased above 1024, TCP_SYNQ_HSIZE in include/net/tcp.h be modified to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the kernel be recompiled.

tcp_max_tw_buckets

The maximum number of sockets in TIME_WAIT state allowed in the system. This limit exists only to prevent simple DoS attacks. The default value of NR_FILE*2 is adjusted depending on the memory in the system. If this number is exceeded, the socket is closed and a warning is printed.

tcp_mem

This is a vector of 3 integers: [low, pressure, high]. These bounds are used by TCP to track its memory usage. The defaults are calculated at boot time from the amount of available memory.

low - TCP doesn't regulate its memory allocation when the number of pages it has allocated globally is below this number.

pressure - when the amount of memory allocated by TCP exceeds this number of pages, TCP moderates its memory consumption. This memory pressure state is exited once the number of pages allocated falls below the low mark.

high - the maximum number of pages, globally, that TCP will allocate. This value overrides any other limits imposed by the kernel.

tcp_orphan_retries

The maximum number of attempts made to probe the other end of a connection which has been closed by our end. The default value is 8.

tcp_reordering

The maximum a packet can be reordered in a TCP packet stream without TCP assuming packet loss and going into slow start. The default is 3. It is not advisable to change this number. This is a packet reordering detection metric designed to minimize unnecessary back off and retransmits provoked by reordering of packets on a connection.

tcp_retrans_collapse

Try to send full-sized packets during retransmit. This is enabled by default.

tcp_retries1

The number of times TCP will attempt to retransmit a packet on an established connection normally, without the extra effort of getting the network layers involved. Once we exceed this number of retransmits, we first have the network layer update the route if possible before each new retransmit. The default is the RFC specified minimum of 3.

tcp_retries2

The maximum number of times a TCP packet is retransmitted in established state before giving up. The default value is 15, which corresponds to a duration of approximately between 13 to 30 minutes, depending on the retransmission timeout. The RFC1122 specified minimum limit of 100 seconds is typically deemed too short.

tcp_rfc1337

Enable TCP behaviour conformant with RFC 1337. This is not enabled by default. When not enabled, if a RST is received in TIME_WAIT state, we close the socket immediately without waiting for the end of the TIME_WAIT period.

tcp_rmem

This is a vector of 3 integers: [min, default, max]. These parameters are used by TCP to regulate receive buffer sizes. TCP dynamically adjusts the size of the receive buffer from the defaults listed below, in the range of these sysctl variables, depending on memory available in the system.

min - minimum size of the receive buffer used by each TCP socket. The default value is 4K, and is lowered to PAGE_SIZE bytes in low memory systems. This value is used to ensure that in memory pressure mode, allocations below this size will still succeed. This is not used to bound the size of the receive buffer declared using SO_RCVBUF on a socket.

default - the default size of the receive buffer for a TCP socket. This value overwrites the initial default buffer size from the generic global net.core.rmem_default defined for all protocols. The default value is 87380 bytes, and is lowered to 43689 in low memory systems. If larger receive buffer sizes are desired, this value should be increased (to affect all sockets). To employ large TCP windows, the net.ipv4.tcp_window_scaling must be enabled (default).

max - the maximum size of the receive buffer used by each TCP socket. This value does not override the global net.core.rmem_max. This is not used to limit the size of the receive buffer declared using SO_RCVBUF on a socket. The default value of 87380*2 bytes is lowered to 87380 in low memory systems.

tcp_sack

Enable RFC2018 TCP Selective Acknowledgements. It is enabled by default.

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_synack_retries

The maximum number of times a SYN/ACK segment for a passive TCP connection will be retransmitted. This number should not be higher than 255. The default value is 5.

tcp_syncookies

Enable TCP syncookies. The kernel must be compiled with CONFIG_SYN_COOKIES. Send out syncookies when the syn backlog queue of a socket overflows. The syncookies feature attempts to protect a socket from a SYN flood attack. This should be used as a last resort, if at all. This is a violation of the TCP protocol, and conflicts with other areas of TCP such as TCP extensions. It can cause problems for clients and relays. It is not recommended as a tuning mechanism for heavily loaded servers to help with overloaded or misconfigured conditions. For recommended alternatives see tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

tcp_syn_retries

The maximum number of times initial SYNs for an active TCP connection attempt will be retransmitted. This value should not be higher than 255. The default value is 5, which corresponds to approximately 180 seconds.

tcp_timestamps

Enable RFC1323 TCP timestamps. This is enabled by default.

tcp_tw_recycle

Enable fast recycling of TIME-WAIT sockets. It is not enabled by default. Enabling this option is not recommended since this causes problems when working with NAT (Network Address Translation).

tcp_window_scaling

Enable RFC1323 TCP window scaling. It is enabled by default. This feature allows the use of a large window (> 64K) on a TCP connection, should the other end support it. Normally, the 16 bit window length field in the TCP header limits the window size to less than 64K bytes. If larger windows are desired, applications can increase the size of their socket buffers and the window scaling option will be employed. If tcp_window_scaling is disabled, TCP will not negotiate the use of window scaling with the other end during connection setup.

tcp_wmem

This is a vector of 3 integers: [min, default, max]. These parameters are used by TCP to regulate send buffer sizes. TCP dynamically adjusts the size of the send buffer from the default values listed below, in the range of these sysctl variables, depending on memory available.

min - minimum size of the send buffer used by each TCP socket. The default value is 4K bytes. This value is used to ensure that in memory pressure mode, allocations below this size will still succeed. This is not used to bound the size of the send buffer declared using SO_SNDBUF on a socket.

default - the default size of the send buffer for a TCP socket. This value overwrites the initial default buffer size from the generic global net.core.wmem_default defined for all protocols. The default value is 16K bytes. If larger send buffer sizes are desired, this value should be increased (to affect all sockets). To employ large TCP windows, the sysctl variable net.ipv4.tcp_window_scaling must be enabled (default).

max - the maximum size of the send buffer used by each TCP socket. This value does not override the global net.core.wmem_max. This is not used to limit the size of the send buffer declared using SO_SNDBUF on a socket. The default value is 128K bytes. It is lowered to 64K depending on the memory available in the system.

SOCKET OPTIONS

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

TCP_CORK: If set, 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. This option should not be used in code intended to be portable.
TCP_DEFER_ACCEPT: Allows a listener to be awakened only when data arrives on the socket. Takes an integer value (seconds), this can bound the maximum number of attempts TCP will make to complete the connection. This option should not be used in code intended to be portable.
TCP_INFO: Used to collect information about this socket. The kernel returns a struct tcp_info as defined in the file /usr/include/linux/tcp.h. This option should not be used in code intended to be portable.
TCP_KEEPCNT: The maximum number of keepalive probes TCP should send before dropping the connection. This option should not be used in code intended to be portable.
TCP_KEEPIDLE: The time (in seconds) the connection needs to remain idle before TCP starts sending keepalive probes, if the socket option SO_KEEPALIVE has been set on this socket. This option should not be used in code intended to be portable.
TCP_KEEPINTVL: The time (in seconds) between individual keepalive probes. This option should not be used in code intended to be portable.
TCP_LINGER2: The lifetime of orphaned FIN_WAIT2 state sockets. This option can be used to override the system wide sysctl tcp_fin_timeout on this socket. This is not to be confused with the socket(7) level option SO_LINGER. This option should not be used in code intended to be portable.
TCP_MAXSEG: 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 (eventual) interface MTU have no effect. TCP will also impose its minimum and maximum bounds over the value provided.
TCP_NODELAY: If set, disable the Nagle algorithm. This means that segments are always sent as soon as possible, even if there is only a small amount of data. When not set, data is buffered until there is a sufficient amount to send out, thereby avoiding the frequent sending of small packets, which results in poor utilization of the network. This option cannot be used at the same time as the option TCP_CORK.
TCP_QUICKACK: Enable quickack mode if set or disable quickack mode if cleared. In quickack mode, acks are sent immediately, rather than delayed if needed in accordance to normal TCP operation. This flag is not permanent, it only enables a switch to or from quickack mode. Subsequent operation of the TCP protocol will once again enter/leave quickack mode depending on internal protocol processing and factors such as delayed ack timeouts occurring and data transfer. This option should not be used in code intended to be portable.
TCP_SYNCNT: Set the number of SYN retransmits that TCP should send before aborting the attempt to connect. It cannot exceed 255. This option should not be used in code intended to be portable.
TCP_WINDOW_CLAMP: Bound the size of the advertised window to this value. The kernel imposes a minimum size of SOCK_MIN_RCVBUF/2. This option should not be used in code intended to be portable.

IOCTLS

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

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

SIOCINQ: Returns the amount of queued unread data in the receive buffer. Argument is a pointer to an integer. The socket must not be in LISTEN state, otherwise an error (EINVAL) is returned.
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.
SIOCOUTQ: Returns the amount of unsent data in the socket send queue in the passed integer value pointer. The socket must not be in LISTEN state, otherwise an error (EINVAL) is returned.

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 occurring 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.

VERSIONS

Support for Explicit Congestion Notification, zerocopy sendfile, reordering support and some SACK extensions (DSACK) were introduced in 2.4. Support for forward acknowledgement (FACK), TIME_WAIT recycling, per connection keepalive socket options and sysctls were introduced in 2.3.

The default values and descriptions for the sysctl variables given above are applicable for the 2.4 kernel.

AUTHORS

This man page was originally written by Andi Kleen. It was updated for 2.4 by Nivedita Singhvi with input from Alexey Kuznetsov's Documentation/networking/ip-sysctls.txt document.

TCP