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 RFC 793,
RFC 1122 and RFC 2001 with the NewReno and SACK extensions. It pro-
vides 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 gener-
ates and checks a per-packet checksum to catch transmission errors.
TCP does not preserve record boundaries.
A newly created 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 incom-
ing connections, first bind(2) the socket to a local address and port
and then call listen(2) to put the socket into the listening state.
After that a new socket for each incoming connection can be accepted
using accept(2). A socket which has had accept(2) or connect(2) suc-
cessfully called on it is fully specified and may transmit data. Data
cannot be transmitted on listening or not yet connected sockets.
Linux supports RFC 1323 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 suc-
ceeding getsockopt(2) call will not return the same size of buffer as
requested in the setsockopt(2) call. TCP uses the extra space 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(2) or connect(2) 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 ker-
nel sends a SIGURG signal to the process or process group that has been
set as the socket "owner" using the SIOCSPGRP or FIOSETOWN ioctls (or
the POSIX.1-2001-specified fcntl(2) F_SETOWN operation). When the
SO_OOBINLINE socket option is enabled, urgent data is put into the nor-
mal data stream (a program can test for its location using the SIOCAT-
MARK ioctl described below), otherwise it can be only received when the
MSG_OOB flag is set for recv(2) or recvmsg(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 zero-copy sendfile(2), Explicit Congestion Notifi-
cation, 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). Variables described as Boolean take an integer
value, with a non-zero value ("true") meaning that the corresponding
option is enabled, and a zero value ("false") meaning that the option
tcp_abort_on_overflow (Boolean; default: disabled)
Enable resetting connections if the listening service is too
slow and unable to keep up and accept them. 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 listen-
ing daemon cannot be tuned to accept connections faster.
Enabling this option can harm the clients of your server.
tcp_adv_win_scale (integer; default: 2)
Count buffering overhead as bytes/2^tcp_adv_win_scale, if
tcp_adv_win_scale is greater than 0; or bytes-
bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less than
or equal to zero.
The socket receive buffer space is shared between the applica-
tion 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 applica-
tion 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 (integer; default: 31)
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.
tcp_bic (Boolean; default: disabled)
Enable BIC TCP congestion control algorithm. BIC-TCP is a
sender-side only change that ensures a linear RTT fairness under
large windows while offering both scalability and bounded TCP-
friendliness. The protocol combines two schemes called additive
increase and binary search increase. When the congestion window
is large, additive increase with a large increment ensures lin-
ear RTT fairness as well as good scalability. Under small con-
gestion windows, binary search increase provides TCP friendli-
tcp_bic_low_window (integer; default: 14)
Sets the threshold window (in packets) where BIC TCP starts to
adjust the congestion window. Below this threshold BIC TCP
behaves the same as the default TCP Reno.
tcp_bic_fast_convergence (Boolean; default: enabled)
Forces BIC TCP to more quickly respond to changes in congestion
window. Allows two flows sharing the same connection to con-
verge more rapidly.
tcp_dsack (Boolean; default: enabled)
Enable RFC 2883 TCP Duplicate SACK support.
tcp_ecn (Boolean; default: disabled)
Enable RFC 2884 Explicit Congestion Notification. When enabled,
connectivity to some destinations could be affected due to
older, misbehaving routers along the path causing connections to
tcp_fack (Boolean; default: enabled)
Enable TCP Forward Acknowledgement support.
tcp_fin_timeout (integer; default: 60)
This specifies how many seconds to wait for a final FIN packet
before the socket is forcibly closed. This is strictly a viola-
tion of the TCP specification, but required to prevent denial-
of-service attacks. In Linux 2.2, the default value was 180.
tcp_frto (Boolean; default: disabled)
Enables F-RTO, an enhanced recovery algorithm for TCP retrans-
mission timeouts. It is particularly beneficial in wireless
environments where packet loss is typically due to random radio
interference rather than intermediate router congestion.
tcp_keepalive_intvl (integer; default: 75)
The number of seconds between TCP keep-alive probes.
tcp_keepalive_probes (integer; default: 9)
The maximum number of TCP keep-alive probes to send before giv-
ing up and killing the connection if no response is obtained
from the other end.
tcp_keepalive_time (integer; default: 7200)
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 termi-
nated 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 applica-
tion timeouts may be much shorter.
tcp_low_latency (Boolean; default: disabled)
If enabled, the TCP stack makes decisions that prefer lower
latency as opposed to higher throughput. It this option is dis-
abled, then higher throughput is preferred. An example of an
application where this default should be changed would be a
Beowulf compute cluster.
tcp_max_orphans (integer; default: see below)
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 denial-of-
service attacks. Lowering this limit is not recommended. Net-
work 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 (integer; default: see below)
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 drop-
ping 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 mem-
ory (<= 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 (integer; default: see below)
The maximum number of sockets in TIME_WAIT state allowed in the
system. This limit exists only to prevent simple denial-of-ser-
vice 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.
(TCP can only use low memory for this, which is limited to
around 900 megabytes on 32-bit systems. 64-bit systems do not
suffer this limitation.)
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
tcp_orphan_retries (integer; default: 8)
The maximum number of attempts made to probe the other end of a
connection which has been closed by our end.
tcp_reordering (integer; default: 3)
The maximum a packet can be reordered in a TCP packet stream
without TCP assuming packet loss and going into slow start. 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
tcp_retrans_collapse (Boolean; default: enabled)
Try to send full-sized packets during retransmit.
tcp_retries1 (integer; default: 3)
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 (integer; default: 15)
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
RFC 1122 specified minimum limit of 100 seconds is typically
deemed too short.
tcp_rfc1337 (Boolean; default: disabled)
Enable TCP behavior conformant with RFC 1337. When disabled, if
a RST is received in TIME_WAIT state, we close the socket imme-
diately 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 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, alloca-
tions 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 buf-
fer 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 (Boolean; default: enabled)
Enable RFC 2018 TCP Selective Acknowledgements.
tcp_stdurg (Boolean; default: disabled)
If this option is enabled, then use the RFC 1122 interpretation
of the TCP urgent-pointer field. According to this interpreta-
tion, the urgent pointer points to the last byte of urgent data.
If this option is disabled, then use the BSD-compatible inter-
pretation of the urgent pointer: the urgent pointer points to
the first byte after the urgent data. Enabling this option may
lead to interoperability problems.
tcp_synack_retries (integer; default: 5)
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.
Enable TCP syncookies. The kernel must be compiled with CON-
FIG_SYN_COOKIES. Send out syncookies when the syn backlog queue
of a socket overflows. The syncookies feature attempts to pro-
tect 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 proto-
col, and conflicts with other areas of TCP such as TCP exten-
sions. 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 recom-
mended alternatives see tcp_max_syn_backlog, tcp_synack_retries,
tcp_syn_retries (integer; default: 5)
The maximum number of times initial SYNs for an active TCP con-
nection 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 (Boolean; default: enabled)
Enable RFC 1323 TCP timestamps.
tcp_tw_recycle (Boolean; default: disabled)
Enable fast recycling of TIME_WAIT sockets. Enabling this
option is not recommended since this causes problems when work-
ing with NAT (Network Address Translation).
tcp_tw_reuse (Boolean; default: disabled)
Allow to reuse TIME_WAIT sockets for new connections when it is
safe from protocol viewpoint. It should not be changed without
advice/request of technical experts.
tcp_window_scaling (Boolean; default: enabled)
Enable RFC 1323 TCP window scaling. 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 nego-
tiate the use of window scaling with the other end during con-
tcp_vegas_cong_avoid (Boolean; default: disabled)
Enable TCP Vegas congestion avoidance algorithm. TCP Vegas is a
sender-side only change to TCP that anticipates the onset of
congestion by estimating the bandwidth. TCP Vegas adjusts the
sending rate by modifying the congestion window. TCP Vegas
should provide less packet loss, but it is not as aggressive as
tcp_westwood (Boolean; default: disabled)
Enable TCP Westwood+ congestion control algorithm. TCP West-
wood+ is a sender-side only modification of the TCP Reno proto-
col stack that optimizes the performance of TCP congestion con-
trol. It is based on end-to-end bandwidth estimation to set
congestion window and slow start threshold after a congestion
episode. Using this estimation, TCP Westwood+ adaptively sets a
slow start threshold and a congestion window which takes into
account the bandwidth used at the time congestion is experi-
enced. TCP Westwood+ significantly increases fairness with
respect to TCP Reno in wired networks and throughput over wire-
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 mem-
ory available in the system.
To set or get a TCP socket option, call getsockopt(2) to read or set-
sockopt(2) to write the option with the option level argument set to
IPPROTO_TCP. In addition, most IPPROTO_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 use-
ful for prepending headers before calling sendfile(2), or for
throughput optimization. As currently implemented, there is a
200 millisecond ceiling on the time for which output is corked
by TCP_CORK. If this ceiling is reached, then queued data is
automatically transmitted. This option can be combined with
TCP_NODELAY only since Linux 2.5.71. 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 connec-
tion. This option should not be used in code intended to be
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
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 uti-
lization of the network. This option is overridden by TCP_CORK;
however, setting this option forces an explicit flush of pending
output, even if TCP_CORK is currently set.
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 pro-
cessing 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 ker-
nel imposes a minimum size of SOCK_MIN_RCVBUF/2. This option
should not be used in code intended to be portable.
These following ioctl(2) calls return information in value. The cor-
rect syntax is:
error = ioctl(tcp_socket, ioctl_type, &&value);
ioctl_type is one of the following:
Returns the amount of queued unread data in the receive buffer.
The socket must not be in LISTEN state, otherwise an error (EIN-
VAL) is returned.
Returns true (i.e., value is non-zero) if the inbound data
stream is at the urgent mark.
If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
true, then the next read from the socket will return the urgent
data. If the SO_OOBINLINE socket option is not set, and SIOCAT-
MARK returns true, then the next read from the socket will
return the bytes following the urgent data (to actually read the
urgent data requires the recv(MSG_OOB) flag).
Note that a read never reads across the urgent mark. If an
application is informed of the presence of urgent data via
select(2) (using the exceptfds argument) or through delivery of
a SIGURG signal, then it can advance up to the mark using a loop
which repeatedly tests SIOCATMARK and performs a read (request-
ing any number of bytes) as long as SIOCATMARK returns false.
Returns the amount of unsent data in the socket send queue. The
socket must not be in LISTEN state, otherwise an error (EINVAL)
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 IPPROTO_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.
Passed socket address type in sin_family was not AF_INET.
EPIPE The other end closed the socket unexpectedly or a read is exe-
cuted on a shut down socket.
The other end didn't acknowledge retransmitted data after some
Any errors defined for ip(7) or the generic socket layer may also be
returned for TCP.
Support for Explicit Congestion Notification, zero-copy sendfile(2),
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
The default values and descriptions for the sysctl variables given
above are applicable for the 2.4 kernel.
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 RFC 1122, but is required for interop-
erability with other stacks. It can be changed by the tcp_stdurg
Not all errors are documented.
IPv6 is not described.
accept(2), bind(2), connect(2), getsockopt(2), listen(2), recvmsg(2),
sendfile(2), sendmsg(2), socket(2), sysctl(2), ip(7), socket(7)
RFC 793 for the TCP specification.
RFC 1122 for the TCP requirements and a description of the Nagle algo-
RFC 1323 for TCP timestamp and window scaling options.
RFC 1644 for a description of TIME_WAIT assassination hazards.
RFC 3168 for a description of Explicit Congestion Notification.
RFC 2581 for TCP congestion control algorithms.
RFC 2018 and RFC 2883 for SACK and extensions to SACK.
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2007-11-25 TCP(7)