tcp.txt

TCP(7)                                                                             Linux Programmer's Manual                                                                            TCP(7)

NAME
       tcp - TCP protocol

SYNOPSIS
       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <netinet/tcp.h>

       tcp_socket = socket(AF_INET, SOCK_STREAM, 0);

DESCRIPTION
       This  is  an  implementation  of the TCP protocol defined in RFC 793, RFC 1122 and RFC 2001 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  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 incoming 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) successfully 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 /proc/sys/net/ipv4/tcp_wmem and /proc/sys/net/ipv4/tcp_rmem files, or on individual sockets by using the SO_SNDBUF and SO_RCVBUF socket  options  with  the  setsock‐
       opt(2) call.

       The  maximum  sizes for socket buffers declared via the SO_SNDBUF and SO_RCVBUF mechanisms are limited by the values in the /proc/sys/net/core/rmem_max and /proc/sys/net/core/wmem_max
       files.  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 buf‐
       fer  as  requested  in the setsockopt(2) call.  TCP uses the extra space for administrative purposes and internal kernel structures, and the /proc file values 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 kernel 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-specified fcntl(2) F_SETOWN operation).  When the SO_OOBINLINE socket option is enabled, urgent data is put into the
       normal data stream (a program can test for its location using the SIOCATMARK ioctl described below), otherwise it can be received only 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 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 supports point-to-point communication only; broadcasting and multicasting are not  sup‐
       ported.

   /proc interfaces
       System-wide  TCP  parameter  settings  can  be  accessed by files in the directory /proc/sys/net/ipv4/.  In addition, most IP /proc interfaces also apply to TCP; see ip(7).  Variables
       described as Boolean take an integer value, with a nonzero value ("true") meaning that the corresponding option is enabled, and a zero value ("false") meaning that the option is  dis‐
       abled.

       tcp_abc (Integer; default: 0; since Linux 2.6.15)
              Control  the  Appropriate  Byte  Count  (ABC), defined in RFC 3465.  ABC is a way of increasing the congestion window (cwnd) more slowly in response to partial acknowledgments.
              Possible values are:

              0  increase cwnd once per acknowledgment (no ABC)

              1  increase cwnd once per acknowledgment of full sized segment

              2  allow increase cwnd by two if acknowledgment is of two segments to compensate for delayed acknowledgments.

       tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
              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 listening 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; since Linux 2.4)
              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 application and kernel.  TCP maintains part of the buffer as the TCP window, this is the size of the receive window adver‐
              tised 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_allowed_congestion_control (String; default: see text; since Linux 2.4.20)
              Show/set the congestion control algorithm choices available to unprivileged processes (see the description of the TCP_CONGESTION socket option).  The items in the list are sep‐
              arated by white space and terminated by a newline character.  The list is a subset of those listed in tcp_available_congestion_control.  The default  value  for  this  list  is
              "reno" plus the default setting of tcp_congestion_control.

       tcp_autocorking (Boolean; default: enabled; since Linux 3.14)
              If this option is enabled, the kernel tries to coalesce small writes (from consecutive write(2) and sendmsg(2) calls) as much as possible, in order to decrease the total number
              of sent packets.  Coalescing is done if at least one prior packet for the flow is waiting in Qdisc queues or device transmit queue.  Applications can  still  use  the  TCP_CORK
              socket option to obtain optimal behavior when they know how/when to uncork their sockets.

       tcp_available_congestion_control (String; read-only; since Linux 2.4.20)
              Show a list of the congestion-control algorithms that are registered.  The items in the list are separated by white space and terminated by a newline character.  This list is a
              limiting set for the list in tcp_allowed_congestion_control.  More congestion-control algorithms may be available as modules, but not loaded.

       tcp_app_win (integer; default: 31; since Linux 2.4)
              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_base_mss (Integer; default: 512; since Linux 2.6.17)
              The initial value of search_low to be used by the packetization layer Path MTU discovery (MTU probing).  If MTU probing is enabled, this is the initial MSS used by the  connec‐
              tion.

       tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
              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 linear RTT fairness as well as good scalability.  Under small congestion windows, binary search increase provides TCP friendliness.

       tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to 2.6.13)
              Set 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; Linux 2.4.27/2.6.6 to 2.6.13)
              Force BIC TCP to more quickly respond to changes in congestion window.  Allows two flows sharing the same connection to converge more rapidly.

       tcp_congestion_control (String; default: see text; since Linux 2.4.13)
              Set  the  default  congestion-control  algorithm to be used for new connections.  The algorithm "reno" is always available, but additional choices may be available depending on
              kernel configuration.  The default value for this file is set as part of kernel configuration.

       tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
              Lower limit, in bytes, of the size of socket reads that will be offloaded to a DMA copy engine, if one is present in the system and the kernel  was  configured  with  the  CON‐
              FIG_NET_DMA option.

       tcp_dsack (Boolean; default: enabled; since Linux 2.4)
              Enable RFC 2883 TCP Duplicate SACK support.

       tcp_ecn (Integer; default: se below; since Linux 2.4)
              Enable RFC 3168 Explicit Congestion Notification.

              This file can have one of the following values:

              0      Disable ECN.  Neither initiate nor accept ECN.  This was the default up to and including Linux 2.6.30.

              1      Enable ECN when requested by incoming connections and also request ECN on outgoing connection attempts.

              2      Enable ECN when requested by incoming connections, but do not request ECN on outgoing connections.  This value is supported, and is the default, since Linux 2.6.31.

              When enabled, connectivity to some destinations could be affected due to older, misbehaving middle boxes along the path, causing connections to be dropped.  However, to facili‐
              tate and encourage deployment with option 1, and to work around such buggy equipment, the tcp_ecn_fallback option has been introduced.

       tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
              Enable RFC 3168, Section 6.1.1.1. fallback.  When enabled, outgoing ECN-setup SYNs that time out within the normal SYN retransmission timeout will be resent with  CWR  and  ECE
              cleared.

       tcp_fack (Boolean; default: enabled; since Linux 2.2)
              Enable TCP Forward Acknowledgement support.

       tcp_fin_timeout (integer; default: 60; since Linux 2.2)
              This  specifies  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.  In Linux 2.2, the default value was 180.

       tcp_frto (integer; default: 0; since Linux 2.4.21/2.6)
              Enable F-RTO, an enhanced recovery algorithm for TCP retransmission timeouts (RTOs).  It is particularly beneficial in wireless environments where packet loss is typically  due
              to random radio interference rather than intermediate router congestion.  See RFC 4138 for more details.

              This file can have one of the following values:

              0  Disabled.

              1  The basic version F-RTO algorithm is enabled.

              2  Enable SACK-enhanced F-RTO if flow uses SACK.  The basic version can be used also when SACK is in use though in that case scenario(s) exists where F-RTO interacts badly with
                 the packet counting of the SACK-enabled TCP flow.

              Before Linux 2.6.22, this parameter was a Boolean value, supporting just values 0 and 1 above.

       tcp_frto_response (integer; default: 0; since Linux 2.6.22)
              When F-RTO has detected that a TCP retransmission timeout was spurious (i.e, the timeout would have been avoided had TCP set a longer retransmission timeout), TCP  has  several
              options concerning what to do next.  Possible values are:

              0  Rate halving based; a smooth and conservative response, results in halved congestion window (cwnd) and slow-start threshold (ssthresh) after one RTT.

              1  Very conservative response; not recommended because even though being valid, it interacts poorly with the rest of Linux TCP; halves cwnd and ssthresh immediately.

              2  Aggressive  response; undoes congestion-control measures that are now known to be unnecessary (ignoring the possibility of a lost retransmission that would require TCP to be
                 more cautious); cwnd and ssthresh are restored to the values prior to timeout.

       tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
              The number of seconds between TCP keep-alive probes.

       tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
              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.

       tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
              The number of seconds a connection needs to be idle before TCP begins sending out keep-alive probes.  Keep-alives are sent only 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_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6)
              If enabled, the TCP stack makes decisions that prefer lower latency as opposed to higher throughput.  It this option is disabled, then higher throughput is preferred.  An exam‐
              ple of an application where this default should be changed would be a Beowulf compute cluster.

       tcp_max_orphans (integer; default: see below; since Linux 2.4)
              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.   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 (integer; default: see below; since Linux 2.2)
              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).

              Prior to Linux 2.6.20, it was recommended that if this needed to be increased above 1024, the size of the SYNACK hash table (TCP_SYNQ_HSIZE) in include/net/tcp.h should be mod‐
              ified to keep

                  TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog

              and the kernel should be recompiled.  In Linux 2.6.20, the fixed sized TCP_SYNQ_HSIZE was removed in favor of dynamic sizing.

       tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
              The  maximum number of sockets in TIME_WAIT state allowed in the system.  This limit exists only to prevent simple denial-of-service 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_moderate_rcvbuf (Boolean; default: enabled; since Linux 2.4.17/2.6.7)
              If enabled, TCP performs receive buffer auto-tuning, attempting to automatically size the buffer (no greater than tcp_rmem[2]) to match the size required by the path  for  full
              throughput.

       tcp_mem (since Linux 2.4)
              This  is  a  vector of 3 integers: [low, pressure, high].  These bounds, measured in units of the system page size, 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 kernel.

       tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
              This parameter controls TCP Packetization-Layer Path MTU Discovery.  The following values may be assigned to the file:

              0  Disabled

              1  Disabled by default, enabled when an ICMP black hole detected

              2  Always enabled, use initial MSS of tcp_base_mss.

       tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
              By default, TCP saves various connection metrics in the route cache when the connection closes, so that connections established in the near future can use these to set  initial
              conditions.   Usually, this increases overall performance, but it may sometimes cause performance degradation.  If tcp_no_metrics_save is enabled, TCP will not cache metrics on
              closing connections.

       tcp_orphan_retries (integer; default: 8; since Linux 2.4)
              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; since Linux 2.4)
              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 connection.

       tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
              Try to send full-sized packets during retransmit.

       tcp_retries1 (integer; default: 3; since Linux 2.2)
              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; since Linux 2.2)
              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; since Linux 2.2)
              Enable  TCP  behavior  conformant  with  RFC 1337.   When  disabled, 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 (since Linux 2.4)
              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 values, depending on memory available in the system.

              min       minimum  size of the receive buffer used by each TCP socket.  The default value is the system page size.  (On Linux 2.4, the default value is 4K, 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.  (On Linux 2.4, this will be 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 is calculated using the formula

                            max(87380, min(4MB, tcp_mem[1]*PAGE_SIZE/128))

                        (On Linux 2.4, the default is 87380*2 bytes, lowered to 87380 in low-memory systems).

       tcp_sack (Boolean; default: enabled; since Linux 2.2)
              Enable RFC 2018 TCP Selective Acknowledgements.

       tcp_slow_start_after_idle (Boolean; default: enabled; since Linux 2.6.18)
              If enabled, provide RFC 2861 behavior and time out the congestion window after an idle period.  An idle period is defined as the current RTO (retransmission timeout).  If  dis‐
              abled, the congestion window will not be timed out after an idle period.

       tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
              If  this  option is enabled, then use the RFC 1122 interpretation of the TCP urgent-pointer field.  According to this interpretation, the urgent pointer points to the last byte
              of urgent data.  If this option is disabled, then use the BSD-compatible interpretation 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_syn_retries (integer; default: 5; since Linux 2.2)
              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_synack_retries (integer; default: 5; since Linux 2.2)
              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.

       tcp_syncookies (Boolean; since Linux 2.2)
              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, and tcp_abort_on_overflow.

       tcp_timestamps (Boolean; default: enabled; since Linux 2.2)
              Enable RFC 1323 TCP timestamps.

       tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
              This  parameter controls what percentage of the congestion window can be consumed by a single TCP Segmentation Offload (TSO) frame.  The setting of this parameter is a tradeoff
              between burstiness and building larger TSO frames.

       tcp_tw_recycle (Boolean; default: disabled; since Linux 2.4)
              Enable fast recycling of TIME_WAIT sockets.  Enabling this option is not recommended for devices communicating with the general Internet or using NAT (Network Address  Transla‐
              tion).  Since some NAT gateways pass through IP timestamp values, one IP can appear to have non-increasing timestamps.  See RFC 1323 (PAWS), RFC 6191.

       tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
              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_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 2.6.13)
              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 Reno.

       tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
              Enable TCP Westwood+ congestion control algorithm.  TCP Westwood+ is a sender-side-only modification of the TCP Reno protocol stack that optimizes the performance of  TCP  con‐
              gestion control.  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 West‐
              wood+ adaptively sets a slow start threshold and a congestion window which takes into account the bandwidth used at the time congestion is experienced.  TCP Westwood+  signifi‐
              cantly increases fairness with respect to TCP Reno in wired networks and throughput over wireless links.

       tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
              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 negotiate the use of window scaling with the other end during connection setup.

       tcp_wmem (since Linux 2.4)
              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 values, depending on memory available.

              min       Minimum size of the send buffer used by each TCP socket.  The default value is the system page size.  (On Linux 2.4, 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  /proc/sys/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 /proc/sys/net/ipv4/tcp_window_scaling must be set to a nonzero value (default).

              max       The maximum size of the send buffer used by each TCP socket.  This value does not override the value in /proc/sys/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 calculated using the formula

                            max(65536, min(4MB, tcp_mem[1]*PAGE_SIZE/128))

                        (On Linux 2.4, the default value is 128K bytes, lowered 64K depending on low-memory systems.)

       tcp_workaround_signed_windows (Boolean; default: disabled; since Linux 2.6.26)
              If  enabled,  assume  that  no  receipt of a window-scaling option means that the remote TCP is broken and treats the window as a signed quantity.  If disabled, assume that the
              remote TCP is not broken even if we do not receive a window scaling option from it.

   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 IPPROTO_TCP.  Unless otherwise  noted,  optval
       is a pointer to an int.  In addition, most IPPROTO_IP socket options are valid on TCP sockets.  For more information see ip(7).

       TCP_CONGESTION (since Linux 2.6.13)
              The  argument for this option is a string.  This option allows the caller to set the TCP congestion control algorithm to be used, on a per-socket basis.  Unprivileged processes
              are restricted to choosing one of the algorithms in tcp_allowed_congestion_control (described above).  Privileged processes (CAP_NET_ADMIN) can choose from any of the available
              congestion-control algorithms (see the description of tcp_available_congestion_control above).

       TCP_CORK (since Linux 2.2)
              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.  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.

       TCP_DEFER_ACCEPT (since Linux 2.4)
              Allow 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 (since Linux 2.4)
              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 (since Linux 2.4)
              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 (since Linux 2.4)
              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 (since Linux 2.4)
              The time (in seconds) between individual keepalive probes.  This option should not be used in code intended to be portable.

       TCP_LINGER2 (since Linux 2.4)
              The  lifetime  of orphaned FIN_WAIT2 state sockets.  This option can be used to override the system-wide setting in the file /proc/sys/net/ipv4/tcp_fin_timeout for 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.  In Linux 2.2 and earlier, and in Linux 2.6.28 and later, 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
              is overridden by TCP_CORK; however, setting this option forces an explicit flush of pending output, even if TCP_CORK is currently set.

       TCP_QUICKACK (since Linux 2.4.4)
              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  opera‐
              tion.   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 (since Linux 2.4)
              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_USER_TIMEOUT (since Linux 2.6.37)
              This  option  takes  an unsigned int as an argument.  When the value is greater than 0, it specifies the maximum amount of time in milliseconds that transmitted data may remain
              unacknowledged before TCP will forcibly close the corresponding connection and return ETIMEDOUT to the application.  If the option value is specified as 0, TCP will to use  the
              system default.

              Increasing  user  timeouts allows a TCP connection to survive extended periods without end-to-end connectivity.  Decreasing user timeouts allows applications to "fail fast", if
              so desired.  Otherwise, failure may take up to 20 minutes with the current system defaults in a normal WAN environment.

              This option can be set during any state of a TCP connection, but is effective only during the synchronized states of a connection (ESTABLISHED, FIN-WAIT-1,  FIN-WAIT-2,  CLOSE-
              WAIT,  CLOSING,  and LAST-ACK).  Moreover, when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will override keepalive to determine when to close a connec‐
              tion due to keepalive failure.

              The option has no effect on when TCP retransmits a packet, nor when a keepalive probe is sent.

              This option, like many others, will be inherited by the socket returned by accept(2), if it was set on the listening socket.

              Further details on the user timeout feature can be found in RFC 793 and RFC 5482 ("TCP User Timeout Option").

       TCP_WINDOW_CLAMP (since Linux 2.4)
              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.

   Sockets API
       TCP provides limited support for out-of-band data, in the form of (a single byte of) 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 interoperability with other stacks.  It can be
       changed via /proc/sys/net/ipv4/tcp_stdurg.

       It is possible to peek at out-of-band data using the recv(2) MSG_PEEK flag.

       Since version 2.4, Linux supports the use of MSG_TRUNC in the flags argument of recv(2) (and recvmsg(2)).  This flag causes the received bytes of data to  be  discarded,  rather  than
       passed back in a caller-supplied buffer.  Since Linux 2.4.4, MSG_TRUNC also has this effect when used in conjunction with MSG_OOB to receive out-of-band data.

   Ioctls
       The following ioctl(2) calls return information in value.  The correct syntax is:

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

       ioctl_type is one of the following:

       SIOCINQ
              Returns  the  amount  of  queued  unread  data  in  the receive buffer.  The socket must not be in LISTEN state, otherwise an error (EINVAL) is returned.  SIOCINQ is defined in
              <linux/sockios.h>.  Alternatively, you can use the synonymous FIONREAD, defined in <sys/ioctl.h>.

       SIOCATMARK
              Returns true (i.e., value is nonzero) 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  SIOCATMARK  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 (requesting any number of bytes) as long as
              SIOCATMARK returns false.

       SIOCOUTQ
              Returns the amount of unsent data in the socket send queue.  The socket must not be in LISTEN  state,  otherwise  an  error  (EINVAL)  is  returned.   SIOCOUTQ  is  defined  in
              <linux/sockios.h>.  Alternatively, you can use the synonymous TIOCOUTQ, defined in <sys/ioctl.h>.

   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 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.

ERRORS
       EAFNOTSUPPORT
              Passed socket address type in sin_family was not AF_INET.

       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.

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

VERSIONS
       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, and per-connection keepalive socket options were introduced in 2.3.

BUGS
       Not all errors are documented.
       IPv6 is not described.

SEE ALSO
       accept(2), bind(2), connect(2), getsockopt(2), listen(2), recvmsg(2), sendfile(2), sendmsg(2), socket(2), ip(7), socket(7)

       RFC 793 for the TCP specification.
       RFC 1122 for the TCP requirements and a description of the Nagle algorithm.
       RFC 1323 for TCP timestamp and window scaling options.
       RFC 1337 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.

COLOPHON
       This page is part of release 4.04 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                                                                                     2015-12-05                                                                                    TCP(7)