TCP(7) - phpMan

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  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    /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 setsockopt(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 buffer as requested in the setsockopt(2)
       call.  TCP uses the extra space for administrative purposes and  inter-
       nal  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 informa-
       tion.
       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 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 Notifi-
       cation, 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 supported.
   /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  corre-
       sponding option is enabled, and a zero value ("false") meaning that the
       option is disabled.
       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 seg-
                 ments 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 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; 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  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_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_CONGES-
              TION socket option).  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_con-
              trol.
       tcp_available_congestion_control   (String;   read-only;   since  Linux
       2.4.20)
              Show a list of the congestion-control algorithms that are regis-
              tered.    This   list   is  a  limiting  set  for  the  list  in
              tcp_allowed_congestion_control.  More  congestion-control  algo-
              rithms 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 connection.
       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 lin-
              ear RTT fairness as well as good scalability.  Under small  con-
              gestion  windows,  binary search increase provides TCP friendli-
              ness.
       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  con-
              verge 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 configu-
              ration.  The default value for this file is set as part of  ker-
              nel 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 CONFIG_NET_DMA option.
       tcp_dsack (Boolean; default: enabled; since Linux 2.4)
              Enable RFC 2883 TCP Duplicate SACK support.
       tcp_ecn (Boolean; default: disabled; since Linux 2.4)
              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
              be dropped.
       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 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 (integer; default: 0; since Linux 2.4.21/2.6)
              Enable F-RTO, an enhanced recovery algorithm for TCP retransmis-
              sion 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 ver-
                 sion 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,  sup-
              porting 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 concern-
              ing 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  cau-
                 tious); 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  giv-
              ing  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 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; 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 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; 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.   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; 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 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; 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-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.
       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  con-
                        sumption.   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 Discov-
              ery.  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 estab-
              lished in the near future can use these to  set  initial  condi-
              tions.   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  imme-
              diately 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, 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
                        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_win-
                        dow_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  conges-
              tion  window after an idle period.  An idle period is defined as
              the current RTO (retransmission timeout).  If disabled, the con-
              gestion 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  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_syn_retries (integer; default: 5; since Linux 2.2)
              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_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  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,
              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 since this causes problems when  work-
              ing with NAT (Network Address Translation).
       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  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-
              less 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 nego-
              tiate  the  use of window scaling with the other end during con-
              nection 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 proto-
                        cols.  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  calcu-
                        lated using the formula
                            max(65536, min(4MB, tcp_mem[1]*PAGE_SIZE/128))
                        (On  Linux  2.4, the default value is 128K bytes, low-
                        ered 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 set-
       sockopt(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)
              Get  or  set  the  congestion-control algorithm for this socket.
              The optval argument is a pointer to a character-string buffer.
              For getsockopt() *optlen specifies the amount of space available
              in  the buffer pointed to by optval, which should be at least 16
              bytes (defined by the kernel-internal constant TCP_CA_NAME_MAX).
              On return, the buffer pointed to by optval is set to a null-ter-
              minated string containing the  name  of  the  congestion-control
              algorithm  for this socket, and *optlen is set to the minimum of
              its original value and TCP_CA_NAME_MAX.  If the value passed  in
              *optlen  is  too  small,  then the string returned in *optval is
              silently truncated, and no terminating null byte is  added.   If
              an  empty  string  is  returned,  then  the  socket is using the
              default congestion-control algorithm,  determined  as  described
              under tcp_congestion_control above.
              For  setsockopt() optlen specifies the length of the congestion-
              control algorithm name contained in the  buffer  pointed  to  by
              optval;  this length need not include any terminating null byte.
              The algorithm "reno" is always permitted; other  algorithms  may
              be  available,  depending  on  kernel  configuration.   Possible
              errors from setsockopt() include: algorithm not  found/available
              (ENOENT);  setting  this  algorithm  requires  the CAP_NET_ADMIN
              capability (EPERM); and failure getting kernel module (EBUSY).
       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 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.
       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 connec-
              tion.   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 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.
       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 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.
       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_WINDOW_CLAMP (since Linux 2.4)
              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.
   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  nor-
       mal  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    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-sup-
       plied  buffer.   Since  Linux 2.4.4, MSG_PEEK 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 (EIN-
              VAL)  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 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.
       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>.   Alter-
              natively,  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  exe-
              cuted 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 algo-
       rithm.
       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 3.53 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                             2013-06-21                            TCP(7)