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TCP(4)		       FreeBSD Kernel Interfaces Manual			TCP(4)

     tcp -- Internet Transmission Control Protocol

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>

     socket(AF_INET, SOCK_STREAM, 0);

     The TCP protocol provides reliable, flow-controlled, two-way transmission
     of	data.  It is a byte-stream protocol used to support the	SOCK_STREAM
     abstraction.  TCP uses the	standard Internet address format and, in addi-
     tion, provides a per-host collection of ``port addresses''.  Thus,	each
     address is	composed of an Internet	address	specifying the host and	net-
     work, with	a specific TCP port on the host	identifying the	peer entity.

     Sockets utilizing the TCP protocol	are either ``active'' or ``passive''.
     Active sockets initiate connections to passive sockets.  By default, TCP
     sockets are created active; to create a passive socket, the listen(2)
     system call must be used after binding the	socket with the	bind(2)	system
     call.  Only passive sockets may use the accept(2) call to accept incoming
     connections.  Only	active sockets may use the connect(2) call to initiate

     Passive sockets may ``underspecify'' their	location to match incoming
     connection	requests from multiple networks.  This technique, termed
     ``wildcard	addressing'', allows a single server to	provide	service	to
     clients on	multiple networks.  To create a	socket which listens on	all
     networks, the Internet address INADDR_ANY must be bound.  The TCP port
     may still be specified at this time; if the port is not specified,	the
     system will assign	one.  Once a connection	has been established, the
     socket's address is fixed by the peer entity's location.  The address
     assigned to the socket is the address associated with the network inter-
     face through which	packets	are being transmitted and received.  Normally,
     this address corresponds to the peer entity's network.

     TCP supports a number of socket options which can be set with
     setsockopt(2) and tested with getsockopt(2):

     TCP_INFO	     Information about a socket's underlying TCP session may
		     be	retrieved by passing the read-only option TCP_INFO to
		     getsockopt(2).  It	accepts	a single argument: a pointer
		     to	an instance of struct tcp_info.

		     This API is subject to change; consult the	source to
		     determine which fields are	currently filled out by	this
		     option.  FreeBSD specific additions include send window
		     size, receive window size,	and bandwidth-controlled win-
		     dow space.

     TCP_CONGESTION  Select or query the congestion control algorithm that TCP
		     will use for the connection.  See mod_cc(4) for details.

     TCP_KEEPINIT    This write-only setsockopt(2) option accepts a per-socket
		     timeout argument of u_int in seconds, for new, non-estab-
		     lished TCP	connections.  For the global default in	mil-
		     liseconds see keepinit in the MIB Variables section fur-
		     ther down.

     TCP_KEEPIDLE    This write-only setsockopt(2) option accepts an argument
		     of	u_int for the amount of	time, in seconds, that the
		     connection	must be	idle before keepalive probes (if
		     enabled) are sent for the connection of this socket.  If
		     set on a listening	socket,	the value is inherited by the
		     newly created socket upon accept(2).  For the global
		     default in	milliseconds see keepidle in the MIB Variables
		     section further down.

     TCP_KEEPINTVL   This write-only setsockopt(2) option accepts an argument
		     of	u_int to set the per-socket interval, in seconds,
		     between keepalive probes sent to a	peer.  If set on a
		     listening socket, the value is inherited by the newly
		     created socket upon accept(2).  For the global default in
		     milliseconds see keepintvl	in the MIB Variables section
		     further down.

     TCP_KEEPCNT     This write-only setsockopt(2) option accepts an argument
		     of	u_int and allows a per-socket tuning of	the number of
		     probes sent, with no response, before the connection will
		     be	dropped.  If set on a listening	socket,	the value is
		     inherited by the newly created socket upon	accept(2).
		     For the global default see	the keepcnt in the MIB
		     Variables section further down.

     TCP_NODELAY     Under most	circumstances, TCP sends data when it is pre-
		     sented; when outstanding data has not yet been acknowl-
		     edged, it gathers small amounts of	output to be sent in a
		     single packet once	an acknowledgement is received.	 For a
		     small number of clients, such as window systems that send
		     a stream of mouse events which receive no replies,	this
		     packetization may cause significant delays.  The boolean
		     option TCP_NODELAY	defeats	this algorithm.

     TCP_MAXSEG	     By	default, a sender- and receiver-TCP will negotiate
		     among themselves to determine the maximum segment size to
		     be	used for each connection.  The TCP_MAXSEG option
		     allows the	user to	determine the result of	this negotia-
		     tion, and to reduce it if desired.

     TCP_NOOPT	     TCP usually sends a number	of options in each packet,
		     corresponding to various TCP extensions which are pro-
		     vided in this implementation.  The	boolean	option
		     TCP_NOOPT is provided to disable TCP option use on	a per-
		     connection	basis.

     TCP_NOPUSH	     By	convention, the	sender-TCP will	set the	``push'' bit,
		     and begin transmission immediately	(if permitted) at the
		     end of every user call to write(2)	or writev(2).  When
		     this option is set	to a non-zero value, TCP will delay
		     sending any data at all until either the socket is
		     closed, or	the internal send buffer is filled.

     TCP_MD5SIG	     This option enables the use of MD5	digests	(also known as
		     TCP-MD5) on writes	to the specified socket.  Outgoing
		     traffic is	digested; digests on incoming traffic are ver-
		     ified if the net.inet.tcp.signature_verify_input sysctl
		     is	nonzero.  The current default behavior for the system
		     is	to respond to a	system advertising this	option with
		     TCP-MD5; this may change.

		     One common	use for	this in	a FreeBSD router deployment is
		     to	enable based routers to	interwork with Cisco equipment
		     at	peering	points.	 Support for this feature conforms to
		     RFC 2385.	Only IPv4 (AF_INET) sessions are supported.

		     In	order for this option to function correctly, it	is
		     necessary for the administrator to	add a tcp-md5 key
		     entry to the system's security associations database
		     (SADB) using the setkey(8)	utility.  This entry must have
		     an	SPI of 0x1000 and can therefore	only be	specified on a
		     per-host basis at this time.

		     If	an SADB	entry cannot be	found for the destination, the
		     outgoing traffic will have	an invalid digest option
		     prepended,	and the	following error	message	will be	visi-
		     ble on the	system console:	tcp_signature_compute: SADB
		     lookup failed for %d.%d.%d.%d.

     The option	level for the setsockopt(2) call is the	protocol number	for
     TCP, available from getprotobyname(3), or IPPROTO_TCP.  All options are
     declared in <netinet/tcp.h>.

     Options at	the IP transport level may be used with	TCP; see ip(4).
     Incoming connection requests that are source-routed are noted, and	the
     reverse source route is used in responding.

     The default congestion control algorithm for TCP is cc_newreno(4).	 Other
     congestion	control	algorithms can be made available using the mod_cc(4)

   MIB Variables
     The TCP protocol implements a number of variables in the net.inet.tcp
     branch of the sysctl(3) MIB.

     TCPCTL_DO_RFC1323	(rfc1323) Implement the	window scaling and timestamp
			options	of RFC 1323 (default is	true).

     TCPCTL_MSSDFLT	(mssdflt) The default value used for the maximum seg-
			ment size (``MSS'') when no advice to the contrary is
			received from MSS negotiation.

     TCPCTL_SENDSPACE	(sendspace) Maximum TCP	send window.

     TCPCTL_RECVSPACE	(recvspace) Maximum TCP	receive	window.

     log_in_vain	Log any	connection attempts to ports where there is
			not a socket accepting connections.  The value of 1
			limits the logging to SYN (connection establishment)
			packets	only.  That of 2 results in any	TCP packets to
			closed ports being logged.  Any	value unlisted above
			disables the logging (default is 0, i.e., the logging
			is disabled).

     msl		The Maximum Segment Lifetime, in milliseconds, for a

     keepinit		Timeout, in milliseconds, for new, non-established TCP
			connections.  The default is 75000 msec.

     keepidle		Amount of time,	in milliseconds, that the connection
			must be	idle before keepalive probes (if enabled) are
			sent.  The default is 7200000 msec (2 hours).

     keepintvl		The interval, in milliseconds, between keepalive
			probes sent to remote machines,	when no	response is
			received on a keepidle probe.  The default is 75000

     keepcnt		Number of probes sent, with no response, before	a con-
			nection	is dropped.  The default is 8 packets.

     always_keepalive	Assume that SO_KEEPALIVE is set	on all TCP connec-
			tions, the kernel will periodically send a packet to
			the remote host	to verify the connection is still up.

     icmp_may_rst	Certain	ICMP unreachable messages may abort connec-
			tions in SYN-SENT state.

     do_tcpdrain	Flush packets in the TCP reassembly queue if the sys-
			tem is low on mbufs.

     blackhole		If enabled, disable sending of RST when	a connection
			is attempted to	a port where there is not a socket
			accepting connections.	See blackhole(4).

     delayed_ack	Delay ACK to try and piggyback it onto a data packet.

     delacktime		Maximum	amount of time,	in milliseconds, before	a
			delayed	ACK is sent.

			Enable Path MTU	Discovery.

     tcbhashsize	Size of	the TCP	control-block hash table (read-only).
			This may be tuned using	the kernel option TCBHASHSIZE
			or by setting net.inet.tcp.tcbhashsize in the

     pcbcount		Number of active process control blocks	(read-only).

     syncookies		Determines whether or not SYN cookies should be	gener-
			ated for outbound SYN-ACK packets.  SYN	cookies	are a
			great help during SYN flood attacks, and are enabled
			by default.  (See syncookies(4).)

			The interval (in seconds) specifying how often the
			secret data used in RFC	1948 initial sequence number
			calculations should be reseeded.  By default, this
			variable is set	to zero, indicating that no reseeding
			will occur.  Reseeding should not be necessary,	and
			will break TIME_WAIT recycling for a few minutes.

     rexmit_min, rexmit_slop
			Adjust the retransmit timer calculation	for TCP.  The
			slop is	typically added	to the raw calculation to take
			into account occasional	variances that the SRTT
			(smoothed round-trip time) is unable to	accommodate,
			while the minimum specifies an absolute	minimum.
			While a	number of TCP RFCs suggest a 1 second minimum,
			these RFCs tend	to focus on streaming behavior,	and
			fail to	deal with the fact that	a 1 second minimum has
			severe detrimental effects over	lossy interactive con-
			nections, such as a 802.11b wireless link, and over
			very fast but lossy connections	for those cases	not
			covered	by the fast retransmit code.  For this reason,
			we use 200ms of	slop and a near-0 minimum, which gives
			us an effective	minimum	of 200ms (similar to Linux).

     rfc3042		Enable the Limited Transmit algorithm as described in
			RFC 3042.  It helps avoid timeouts on lossy links and
			also when the congestion window	is small, as happens
			on short transfers.

     rfc3390		Enable support for RFC 3390, which allows for a	vari-
			able-sized starting congestion window on new connec-
			tions, depending on the	maximum	segment	size.  This
			helps throughput in general, but particularly affects
			short transfers	and high-bandwidth large propagation-
			delay connections.

     sack.enable	Enable support for RFC 2018, TCP Selective Acknowledg-
			ment option, which allows the receiver to inform the
			sender about all successfully arrived segments,	allow-
			ing the	sender to retransmit the missing segments

     sack.maxholes	Maximum	number of SACK holes per connection.  Defaults
			to 128.

			Maximum	number of SACK holes per system, across	all
			connections.  Defaults to 65536.

     maxtcptw		When a TCP connection enters the TIME_WAIT state, its
			associated socket structure is freed, since it is of
			negligible size	and use, and a new structure is	allo-
			cated to contain a minimal amount of information nec-
			essary for sustaining a	connection in this state,
			called the compressed TCP TIME_WAIT state.  Since this
			structure is smaller than a socket structure, it can
			save a significant amount of system memory.  The
			net.inet.tcp.maxtcptw MIB variable controls the	maxi-
			mum number of these structures allocated.  By default,
			it is initialized to kern.ipc.maxsockets / 5.

     nolocaltimewait	Suppress creating of compressed	TCP TIME_WAIT states
			for connections	in which both endpoints	are local.

			Recycle	TCP FIN_WAIT_2 connections faster when the
			socket is marked as SBS_CANTRCVMORE (no	user process
			has the	socket open, data received on the socket can-
			not be read).  The timeout used	here is

     finwait2_timeout	Timeout	to use for fast	recycling of TCP FIN_WAIT_2
			connections.  Defaults to 60 seconds.

     ecn.enable		Enable support for TCP Explicit	Congestion Notifica-
			tion (ECN).  ECN allows	a TCP sender to	reduce the
			transmission rate in order to avoid packet drops.

     ecn.maxretries	Number of retries (SYN or SYN/ACK retransmits) before
			disabling ECN on a specific connection.	This is	needed
			to help	with connection	establishment when a broken
			firewall is in the network path.

     A socket operation	may fail with one of the following errors returned:

     [EISCONN]		when trying to establish a connection on a socket
			which already has one;

     [ENOBUFS]		when the system	runs out of memory for an internal
			data structure;

     [ETIMEDOUT]	when a connection was dropped due to excessive

     [ECONNRESET]	when the remote	peer forces the	connection to be

     [ECONNREFUSED]	when the remote	peer actively refuses connection
			establishment (usually because no process is listening
			to the port);

     [EADDRINUSE]	when an	attempt	is made	to create a socket with	a port
			which has already been allocated;

     [EADDRNOTAVAIL]	when an	attempt	is made	to create a socket with	a net-
			work address for which no network interface exists;

     [EAFNOSUPPORT]	when an	attempt	is made	to bind	or connect a socket to
			a multicast address.

     getsockopt(2), socket(2), sysctl(3), blackhole(4),	inet(4), intro(4),
     ip(4), mod_cc(4), siftr(4), syncache(4), setkey(8)

     V.	Jacobson, R. Braden, and D. Borman, TCP	Extensions for High
     Performance, RFC 1323.

     A.	Heffernan, Protection of BGP Sessions via the TCP MD5 Signature
     Option, RFC 2385.

     K.	Ramakrishnan, S. Floyd,	and D. Black, The Addition of Explicit
     Congestion	Notification (ECN) to IP, RFC 3168.

     The TCP protocol appeared in 4.2BSD.  The RFC 1323	extensions for window
     scaling and timestamps were added in 4.4BSD.  The TCP_INFO	option was
     introduced	in Linux 2.6 and is subject to change.

FreeBSD	11.0			 March 7, 2012			  FreeBSD 11.0


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