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NAME    [Toc]    [Back]

       tcp - TCP protocol.

SYNOPSIS    [Toc]    [Back]

       #include <sys/socket.h>
       #include <netinet/in.h>
       tcp_socket = socket(PF_INET, SOCK_STREAM, 0);

DESCRIPTION    [Toc]    [Back]

       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).  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  2.2  supports the RFC1323 TCP high performance extensions.  This
       includes large TCP windows to support links with high latency or  bandwidth.
  In order to make use of them, the send and receive buffer sizes
       must  be  increased.   They  can  be   be   set	 globally   with   the
       net.core.wmem_default and net.core.rmem_default sysctls, or on individual
 sockets by using the SO_SNDBUF and SO_RCVBUF socket	options.   The
       maximum	 sizes	 for   socket	buffers  are  limited  by  the	global
       net.core.rmem_max and net.core.wmem_max sysctls. See socket(7) for more

       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  FIOCSPGRP  or
       FIOCSETOWN  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).

ADDRESS FORMATS    [Toc]    [Back]

       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    [Toc]    [Back]

       These sysctls can be accessed by the /proc/sys/net/ipv4/* files or with
       the  sysctl(2)  interface.  In  addition, most IP sysctls also apply to
       TCP; see ip(7).

	      Enable RFC1323 TCP window scaling.

	      Enable RFC2018 TCP Selective Acknowledgements.

	      Enable RFC1323 TCP timestamps.

	      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

	      Maximum  TCP  keep-alive	probes to send before giving up. Keepalives
 are only sent when  the  SO_KEEPALIVE  socket  option  is

	      The  number of seconds after no data has been transmitted before
	      a keep-alive will be sent on a connection.  The default is 10800
	      seconds (3 hours).

	      How  many keep-alive probes are sent per slow timer run. To prevent
 bursts, this value should not be set too high.

	      Enable the strict  RFC793  interpretation  of  the  TCP  urgentpointer
  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.

	      Enable  TCP  syncookies.	The  kernel must be compiled with CON-
	      FIG_SYN_COOKIES.	Syncookies protects  a	socket	from  overload
	      when  too  many  connection attempts arrive. Client machines may
	      not be able to detect an overloaded machine with a short timeout
	      anymore when syncookies are enabled.

	      Length  of  the  per-socket  backlog queue. As of Linux 2.2, the
	      backlog specified in listen(2) only specifies the length of  the
	      backlog queue of already established sockets.  The maximum queue
	      of sockets not yet established (in SYN_RECV  state)  per	listen
	      socket  is  set  by  this  sysctl. When more connection requests
	      arrive, Linux  starts  to  drop  packets.  When  syncookies  are
	      enabled  the packets are still answered and this value is effectively

	      Defines how many times an answer to a TCP connection request  is
	      retransmitted before giving up.

	      Defines  how  many times a TCP packet is retransmitted in established
 state before giving up.

	      Defines how many times to try to send an initial SYN packet to a
	      remote host before giving up and returns an error. Must be below
	      255.  This is only the timeout  for  outgoing  connections;  for
	      incoming	connections  the  number  of retransmits is defined by

	      Try to send full-sized packets during retransmit. This  is  used
	      to work around TCP bugs in some stacks.

SOCKET OPTIONS    [Toc]    [Back]

       To  set	or get a TCP socket option, call getsockopt(2) to read or set-
       sockopt(2) to write the option with the socket family argument  set  to
       SOL_TCP.   In  addition,  most  SOL_IP  socket options are valid on TCP
       sockets. For more information see ip(7).

       TCP_NODELAY    [Toc]    [Back]
	      Turn the Nagle algorithm off. This means that packets are always
	      sent  as	soon  as possible and no unnecessary delays are introduced,
 at the cost of more packets in the  network.  Expects  an
	      integer boolean flag.

       TCP_MAXSEG    [Toc]    [Back]
	      Set  or  receive 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  interface  MTU  are
	      ignored and have no effect.

       TCP_CORK    [Toc]    [Back]
	      If  enabled  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

IOCTLS    [Toc]    [Back]

       These ioctls can be accessed using ioctl(2).  The correct syntax is:

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

	      Returns  the amount of queued unread data in the receive buffer.
	      Argument is a pointer to an integer.

       SIOCATMARK    [Toc]    [Back]
	      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.

       TIOCOUTQ    [Toc]    [Back]
	      Returns the amount of unsent data in the socket  send  queue  in
	      the passed integer value pointer. Unfortunately, the implementation
 of this ioctl is buggy in all known versions of  Linux  and
	      instead  returns	the  free space (effectively buffer size minus
	      bytes used including metadata) in the send queue. This  will  be
	      fixed  in  future  Linux	versions.  If you use TIOCOUTQ, please
	      include a runtime test for both behaviors for  correct  function
	      on future releases and other Unixes.

ERROR HANDLING    [Toc]    [Back]

       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    [Toc]    [Back]

       When  an  error	occurs	doing  a connection setup occuring in a socket
       write SIGPIPE is only raised when the  SO_KEEPALIVE  socket  option  is

       TCP  has  no  real  out-of-band data; it has urgent data. In Linux this
       means if the other end sends newer out-of-band data  the  older	urgent
       data is inserted as normal data into the stream (even when SO_OOBINLINE
       is not set). This differs from BSD based stacks.

       Linux uses the BSD compatible  interpretation  of  the  urgent  pointer
       field by default. This violates RFC1122, but is required for interoperability
 with other stacks. It can be changed by the tcp_stdurg  sysctl.

ERRORS    [Toc]    [Back]

       EPIPE  The  other  end closed the socket unexpectedly or a read is executed
 on a shut down socket.

       ETIMEDOUT    [Toc]    [Back]
	      The other end didn't acknowledge retransmitted data  after  some

       EAFNOTSUPPORT    [Toc]    [Back]
	      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    [Toc]    [Back]

       Not all errors are documented.

       IPv6 is not described.

       Transparent proxy options are not described.

VERSIONS    [Toc]    [Back]

       The sysctls are new in Linux 2.2.  IP_RECVERR is a new feature in Linux
       2.2.  TCP_CORK is new in 2.2.

SEE ALSO    [Toc]    [Back]

       socket(7), socket(2), ip(7), sendmsg(2), recvmsg(2)
       RFC793 for the TCP specification.
       RFC1122	for  the TCP requirements and a description of the Nagle algorithm.

       RFC2581 for some TCP algorithms.

Linux Man Page			  1999-04-25				TCP(7)
[ Back ]
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