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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)
, 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 modifed 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 aproximately 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_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_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_RCVBUF
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.
See Also
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.
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