TCP(7) Linux Programmer's Manual TCP(7)
tcp - TCP protocol.
tcp_socket = socket(PF_INET, SOCK_STREAM, 0);
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)
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 adminis-
trative 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.
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.
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).
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
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.
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.
Enable RFC2883 TCP Duplicate SACK support. It is enabled by default.
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.
Enable TCP Forward Acknowledgement support. It is enabled by default.
How many seconds to wait for a final FIN packet before the socket is forcibly closed. This is strictly a violation of the TCP spec-
ification, but required to prevent denial-of-service (DoS) attacks. The default value in 2.4 kernels is 60, down from 180 in 2.2.
The number of seconds between TCP keep-alive probes. The default value is 75 seconds.
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.
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.
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. Lower-
ing 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.
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 modifed to keep
TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the kernel be recompiled.
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.
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 mem-
ory 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.
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
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 unnec-
essary back off and retransmits provoked by reordering of packets on a connection.
Try to send full-sized packets during retransmit. This is enabled by default.
The number of times TCP will attempt to retransmit a packet on an established connection normally, without the extra effort of get-
ting 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.
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 aproximately between 13 to 30 minutes, depending on the retransmission timeout. The RFC1122 specified
minimum limit of 100 seconds is typically deemed too short.
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.
This is a vector of 3 integers: [min, default, max]. These parameters are used by TCP to regulate receive buffer sizes. TCP dynam-
ically 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.
Enable RFC2018 TCP Selective Acknowledgements. It is enabled by default.
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.
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.
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 over-
loaded or misconfigured conditions. For recommended alternatives see tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_over-
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.
Enable RFC1323 TCP timestamps. This is enabled by default.
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).
Enable RFC1323 TCP window scaling. It is enabled by default. This feature allows the use of a large window (> 64K) on a TCP con-
nection, 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.
This is a vector of 3 integers: [min, default, max]. These parameters are used by TCP to regulate send buffer sizes. TCP dynami-
cally adjusts the size of the send buffer from the default values listed below, in the range of these sysctl variables, depending on
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_RCVBUF 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_win-
dow_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_RCVBUF on a socket. The default value is 128K bytes. It is low-
ered to 64K depending on the memory available in the system.
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).
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.
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.
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.
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.
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.
The time (in seconds) between individual keepalive probes. This option should not be used in code intended to be portable.
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.
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.
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 send-
ing of small packets, which results in poor utilization of the network. This option cannot be used at the same time as the option
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.
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.
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.
These ioctls can be accessed using ioctl(2). The correct syntax is:
error = ioctl(tcp_socket, ioctl_type, &value);
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.
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.
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.
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.
When an error occurs doing a connection setup occurring in a socket write SIGPIPE is only raised when the SO_KEEPALIVE socket option is
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 interoper-
ability with other stacks. It can be changed by the tcp_stdurg sysctl.
EPIPE The other end closed the socket unexpectedly or a read is executed on a shut down socket.
The other end didn't acknowledge retransmitted data after some time.
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.
Not all errors are documented.
IPv6 is not described.
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
The default values and descriptions for the sysctl variables given above are applicable for the 2.4 kernel.
This man page was originally written by Andi Kleen. It was updated for 2.4 by Nivedita Singhvi with input from Alexey Kuznetsov's Documen-
socket(7), socket(2), ip(7), bind(2), listen(2), accept(2), connect(2), sendmsg(2), recvmsg(2), sendfile(2), sysctl(2), getsockopt(2).
RFC793 for the TCP specification.
RFC1122 for the TCP requirements and a description of the Nagle algorithm.
RFC1323 for TCP timestamp and window scaling options.
RFC1644 for a description of TIME_WAIT assasination hazards.
RFC2481 for a description of Explicit Congestion Notification.
RFC2581 for TCP congestion control algorithms.
RFC2018 and RFC2883 for SACK and extensions to SACK.
Linux Man Page 2002-04-20 TCP(7)