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

NAME
     tcp -- Internet Transmission Control Protocol

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

     int
     socket(AF_INET, SOCK_STREAM, 0);

DESCRIPTION
     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
     connections.

     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
		       window space.

     TCP_CCALGOOPT     Set or query congestion control algorithm specific
		       parameters.  See	mod_cc(4) for details.

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

     TCP_FUNCTION_BLK  Select or query the set of functions that TCP will use
		       for this	connection.  This allows a user	to select an
		       alternate TCP stack.  The alternate TCP stack must
		       already be loaded in the	kernel.	 To list the available
		       TCP stacks, see functions_available in the MIB
		       Variables section further down.	To list	the default
		       TCP stack, see functions_default	in the MIB Variables
		       section.

     TCP_KEEPINIT      This setsockopt(2) option accepts a per-socket timeout
		       argument	of u_int in seconds, for new, non-established
		       TCP connections.	 For the global	default	in millisec-
		       onds see	keepinit in the	MIB Variables section further
		       down.

     TCP_KEEPIDLE      This 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 sec-
		       tion further down.

     TCP_KEEPINTVL     This 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 mil-
		       liseconds see keepintvl in the MIB Variables section
		       further down.

     TCP_KEEPCNT       This 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
		       presented; when outstanding data	has not	yet been
		       acknowledged, 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 win-
		       dow systems that	send a stream of mouse events which
		       receive no replies, this	packetization may cause	sig-
		       nificant	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 negoti-
		       ation, 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.  Outgo-
		       ing traffic is digested;	digests	on incoming traffic
		       are verified 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 speci-
		       fied 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 vis-
		       ible 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)
     framework.

   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
			packet.

     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
			msec.

     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.

     path_mtu_discovery
			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
			loader(8).

     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).)

     isn_reseed_interval
			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).

     initcwnd_segments	Enable the ability to specify initial congestion win-
			dow in number of segments.  The	default	value is 10 as
			suggested by RFC 6928.	Changing the value on fly
			would not affect connections using congestion window
			from the hostcache.  Caution: This regulates the burst
			of packets allowed to be sent in the first RTT.	 The
			value should be	relative to the	link capacity.	Start
			with small values for lower-capacity links.  Large
			bursts can cause buffer	overruns and packet drops if
			routers	have small buffers or the link is experiencing
			congestion.

     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
			only.

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

     sack.globalmaxholes
			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.

     fast_finwait2_recycle
			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.

     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.
			Settings:
			0	      Disable ECN.
			1	      Allow incoming connections to request
				      ECN.  Outgoing connections will request
				      ECN.
			2	      Allow incoming connections to request
				      ECN.  Outgoing connections will not
				      request ECN.

     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.

     pmtud_blackhole_detection
			Turn on	automatic path MTU blackhole detection.	 In
			case of	retransmits OS will lower the MSS to check if
			it's MTU problem.  If current MSS is greater than con-
			figured	value to try, it will be set to	configured
			value, otherwise, MSS will be set to default values
			(net.inet.tcp.mssdflt and net.inet.tcp.v6mssdflt).

     pmtud_blackhole_mss
			MSS to try for IPv4 if PMTU blackhole detection	is
			turned on.

     v6pmtud_blackhole_mss
			MSS to try for IPv6 if PMTU blackhole detection	is
			turned on.

     pmtud_blackhole_activated
			Number of times	configured values were used in an
			attempt	to downshift.

     pmtud_blackhole_activated_min_mss
			Number of times	default	MSS was	used in	an attempt to
			downshift.

     pmtud_blackhole_failed
			Number of connections for which	retransmits continued
			even after MSS downshift.

     functions_available
			List of	available TCP function blocks (TCP stacks).

     functions_default	The default TCP	function block (TCP stack).

ERRORS
     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
			retransmissions;

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

     [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.

     [EINVAL]		when trying to change TCP function blocks at an
			invalid	point in the session;

     [ENOENT]		when trying to use a TCP function block	that is	not
			available;

SEE ALSO
     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.

HISTORY
     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			 June 28, 2016			  FreeBSD 11.0

NAME | SYNOPSIS | DESCRIPTION | ERRORS | SEE ALSO | HISTORY

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