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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>
     #include <netinet/tcp.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 ad-
     dress is composed of an Internet address specifying the host and network,
     with a specific TCP port on the host identifying the peer entity.

     Sockets utilizing the TCP protocol	are either "active" or "passive".  Ac-
     tive 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 con-
     nection 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 as-
     signed to the socket is the address associated with the network interface
     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 pa-
		       rameters.  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 de-

     TCP_FASTOPEN      Enable or disable TCP Fast Open (TFO).  To use this op-
		       tion, the kernel	must be	built with the TCP_RFC7413 op-

		       This option can be set on the socket either before or
		       after the listen(2) is invoked.	Clearing this option
		       on a listen socket after	it has been set	has no effect
		       on existing TFO connections or TFO connections in
		       progress; it only prevents new TFO connections from be-
		       ing established.

		       For passively-created sockets, the TCP_FASTOPEN socket
		       option can be queried to	determine whether the connec-
		       tion was	established using TFO.	Note that connections
		       that are	established via	a TFO SYN, but that fall back
		       to using	a non-TFO SYN|ACK will have the	TCP_FASTOPEN
		       socket option set.

		       In addition to the facilities defined in	RFC7413, this
		       implementation supports a pre-shared key	(PSK) mode of
		       operation in which the TFO server requires the client
		       to be in	posession of a shared secret in	order for the
		       client to be able to successfully open TFO connections
		       with the	server.	 This is useful, for example, in envi-
		       ronments	where TFO servers are exposed to both internal
		       and external clients and	only wish to allow TFO connec-
		       tions from internal clients.

		       In the PSK mode of operation, the server	generates and
		       sends TFO cookies to requesting clients as usual.  How-
		       ever, when validating cookies received in TFO SYNs from
		       clients,	the server requires the	client-supplied	cookie
		       to equal

		       SipHash24(key=16-byte-psk, msg=cookie-sent-to-client)

		       Multiple	concurrent valid pre-shared keys are supported
		       so that time-based rolling PSK invalidation policies
		       can be implemented in the system.  The default number
		       of concurrent pre-shared	keys is	2.

		       This can	be adjusted with the TCP_RFC7413_MAX_PSKS ker-
		       nel option.

     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 al-
		       ready 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

     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

     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 ac-
		       knowledged, it gathers small amounts of output to be
		       sent in a single	packet once an acknowledgement is re-
		       ceived.	For a small number of clients, such as window
		       systems that send a stream of mouse events which	re-
		       ceive no	replies, this packetization may	cause signifi-
		       cant 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.  When this	option is enabled on a socket,
		       all inbound and outgoing	TCP segments must be signed
		       with MD5	digests.

		       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.

		       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 can
		       only be specified on a per-host basis at	this time.

		       If an SADB entry	cannot be found	for the	destination,
		       the system does not send	any outgoing segments and
		       drops any inbound segments.  However, during connection
		       negotiation, a non-signed segment will be accepted if
		       an SADB entry does not exist between hosts.  When a
		       non-signed segment is accepted, the established connec-
		       tion is not protected with MD5 digests.

     TCP_STATS	       Manage collection of connection level statistics	using
		       the stats(3) framework.

		       Each dropped segment is taken into account in the TCP
		       protocol	statistics.

     TCP_TXTLS_ENABLE  Enable in-kernel	Transport Layer	Security (TLS) for
		       data written to this socket.  See ktls(4) for more de-

     TCP_TXTLS_MODE    The integer argument can	be used	to get or set the cur-
		       rent TLS	transmit mode of a socket.  See	ktls(4)	for
		       more details.

     TCP_RXTLS_ENABLE  Enable in-kernel	TLS for	data read from this socket.
		       See ktls(4) for more details.

		       Changes NUMA affinity filtering for an established TCP
		       listen socket.  This option takes a single integer ar-
		       gument which specifies the NUMA domain to filter	on for
		       this listen socket.  The	argument can also have the
		       follwing	special	values:

						Remove NUMA filtering for this
						listen socket.

						Filter traffic associated with
						the domain where the calling
						thread is currently executing.
						This is	typically used after a
						process	or thread inherits a
						listen socket from its parent,
						and sets its CPU affinity to a
						particular core.

		       Set and get the remote UDP encapsulation	port.  It can
		       only be set on a	closed TCP socket.

     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).	 In-
     coming connection requests	that are source-routed are noted, and the re-
     verse 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/RFC	7323 (default is true).

			Tolerate the missing of	timestamps (RFC	1323/RFC 7323)
			for TCP	segments belonging to TCP connections for
			which support of TCP timestamps	has been negotiated.
			As of June 2021, several TCP stacks are	known to vio-
			late RFC 7323, including modern	widely deployed	ones.
			Therefore the default is 1, i.e., the missing of time-
			stamps is tolerated.

     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 ac-
			cepting	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 de-
			layed 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 se-
			cret 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.

     reass.cursegments	The current total number of segments present in	all
			reassembly queues.

     reass.maxsegments	The maximum limit on the total number of segments
			across all reassembly queues.  The limit can be	ad-
			justed as a tunable.

     reass.maxqueuelen	The maximum number of segments allowed in each re-
			assembly queue.	 By default, the system	chooses	a
			limit based on each TCP	connection's receive buffer
			size and maximum segment size (MSS).  The actual limit
			applied	to a session's reassembly queue	will be	the
			lower of the system-calculated automatic limit and the
			user-specified reass.maxqueuelen limit.

     rexmit_initial, 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).
			The initial value is used before an RTT	measurement
			has been performed.

     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

     newcwd		Enable the New Congestion Window Validation mechanism
			as described in	RFC 7661.  This	gently reduces the
			congestion window during periods, where	TCP is appli-
			cation limited and the network bandwidth is not	uti-
			lized completely.  That	prevents self-inflicted	packet
			losses once the	application starts to transmit data at
			a higher speed.

     do_prr		Perform	SACK loss recovery using the Proportional Rate
			Reduction (PRR)	algorithm described in RFC6937.	 This
			improves the effectiveness of retransmissions particu-
			lar in environments with ACK thinning or burst loss
			events,	as chances to run out of the ACK clock are re-
			duced, preventing lengthy and performance reducing RTO
			based loss recovery (default is	true).

			While doing Proportional Rate Reduction, remain
			strictly in a packet conserving	mode, sending only one
			new packet for each ACK	received.  Helpful when	a mis-
			configured token bucket	traffic	policer	causes persis-
			tent high losses leading to RTO, but reduces PRR ef-
			fectiveness in more common settings (default is

     rfc6675_pipe	Calculate the bytes in flight using the	algorithm de-
			scribed	in RFC 6675, and is also an improvement	when
			Proportional Rate Reduction is enabled.	 Also enables
			two other mechanisms from RFC6675.  Rescue Retransmis-
			sion helps timely loss recovery, when the trailing
			segments of a transmission are lost, while no addi-
			tional data is ready to	be sent.  In case a partial
			ACK without a SACK block is received during SACK loss
			recovery, the trailing segment is immediately resent,
			rather than waiting for	a Retransmission timeout.
			SACK loss recovery is also engaged, once two segments
			plus one byte are SACKed - even	if no traditional du-
			plicate	ACKs were seen.

     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.
			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.

			Enable automatic path MTU blackhole detection.	In
			case of	retransmits of MSS sized segments, the OS will
			lower the MSS to check if it's an MTU problem.	If the
			current	MSS is greater than the	configured value to
			try (net.inet.tcp.pmtud_blackhole_mss and
			net.inet.tcp.v6pmtud_blackhole_mss), it	will be	set to
			this value, otherwise, the MSS will be set to the de-
			fault values (net.inet.tcp.mssdflt and
			net.inet.tcp.v6mssdflt).  Settings:
			0	Disable	path MTU blackhole detection.
			1	Enable path MTU	blackhole detection for	IPv4
				and IPv6.
			2	Enable path MTU	blackhole detection only for
			3	Enable path MTU	blackhole detection only for

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

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

			When non-zero, all client-supplied TFO cookies will be
			considered to be valid.	 The default is	0.

     fastopen.autokey	When this and net.inet.tcp.fastopen.server_enable are
			non-zero, a new	key will be automatically generated
			after this specified seconds.  The default is 120.

			The maximum number of entries in a client cookie cache
			bucket.	 The default value can be tuned	with the
			or by setting
			net.inet.tcp.fastopen_ccache_bucket_limit in the

			The number of client cookie cache buckets.  Read-only.
			The value can be tuned with the
			by setting fastopen.ccache_buckets in the loader(8).

			Print the client cookie	cache.	Read-only.

			When zero, no new active (i.e.,	client)	TFO connec-
			tions can be created.  On the transition from enabled
			to disabled, the client	cookie cache is	cleared	and
			disabled.  The transition from enabled to disabled
			does not affect	any active TFO connections in
			progress; it only prevents new ones from being estab-
			lished.	 The default is	0.

     fastopen.keylen	The key	length in bytes.  Read-only.

     fastopen.maxkeys	The maximum number of keys supported.  Read-only,

     fastopen.maxpsks	The maximum number of pre-shared keys supported.

     fastopen.numkeys	The current number of keys installed.  Read-only.

     fastopen.numpsks	The current number of pre-shared keys installed.

			When a failure occurs while trying to create a new ac-
			tive (i.e., client) TFO	connection, new	active connec-
			tions on the same path,	as determined by the tuple
			{client_ip, server_ip, server_port}, will be forced to
			be non-TFO for this many seconds.  Note	that the path
			disable	mechanism relies on state stored in client
			cookie cache entries, so it is possible	for the	dis-
			able time for a	given path to be reduced if the	corre-
			sponding client	cookie cache entry is reused due to
			resource pressure before the disable period has
			elapsed.  The default is

			When non-zero, pre-shared key (PSK) mode is enabled
			for all	TFO servers.  On the transition	from enabled
			to disabled, all installed pre-shared keys are re-
			moved.	The default is 0.

			When zero, no new passive (i.e., server) TFO connec-
			tions can be created.  On the transition from enabled
			to disabled, all installed keys	and pre-shared keys
			are removed.  On the transition	from disabled to en-
			abled, if fastopen.autokey is non-zero and there are
			no keys	installed, a new key will be generated immedi-
			ately.	The transition from enabled to disabled	does
			not affect any passive TFO connections in progress; it
			only prevents new ones from being established.	The
			default	is 0.

     fastopen.setkey	Install	a new key by writing
			net.inet.tcp.fastopen.keylen bytes to this sysctl.

     fastopen.setpsk	Install	a new pre-shared key by	writing
			net.inet.tcp.fastopen.keylen bytes to this sysctl.

     hostcache.enable	The TCP	host cache is used to cache connection details
			and metrics to improve future performance of connec-
			tions between the same hosts.  At the completion of a
			TCP connection,	a host will cache information for the
			connection for some defined period of time.
			0	Disable	the host cache.
			1	Enable the host	cache. (default)

			Immediately purge all entries once set to any value.
			Setting	this to	2 will also reseed the hash salt.

     hostcache.purge	Expire all entires on next pruning of host cache en-
			tries.	Any non-zero setting will be reset to zero,
			once the pruge is running.
			0	Do not purge all entries when pruning the host
				cache. (default)
			1	Purge all entries when doing the next pruning.
			2	Purge all entries, and also reseed the hash

     hostcache.prune	Time in	seconds	between	pruning	expired	host cache en-
			tries.	Defaults to 300	(5 minutes).

     hostcache.expire	Time in	seconds, how long a entry should be kept in
			the host cache since last accessed.  Defaults to 3600
			(1 hour).

     hostcache.count	The current number of entries in the host cache.

			The maximum number of entries for the same hash.  De-
			faults to 30.

			Size of	TCP hostcache hashtable.  This number has to
			be a power of two, or will be rejected.	 Defaults to

			Overall	entry limit for	hostcache.  Defaults to	hash-
			size * bucketlimit.

     hostcache.histo	Provide	a Histogram of the hostcache hash utilization.

     hostcache.list	Provide	a complete list	of all current entries in the
			host cache.

			List of	available TCP function blocks (TCP stacks).

     functions_default	The default TCP	function block (TCP stack).

			Determines whether to inherit listen socket's tcp
			stack or use the current system	default	tcp stack, as
			defined	by functions_default.  Default is true.

     insecure_rst	Use criteria defined in	RFC793 instead of RFC5961 for
			accepting RST segments.	 Default is false.

     insecure_syn	Use criteria defined in	RFC793 instead of RFC5961 for
			accepting SYN segments.	 Default is false.

			When initializing the TCP timestamps, use a per	con-
			nection	offset instead of a per	host pair offset.  De-
			fault is to use	per connection offsets as recommended
			in RFC 7323.

			Controls the default collection	of statistics for all
			connections using the stats(3) framework.  0 disables,
			1 enables, 2 enables random sampling across log	id
			connection groups with all connections in a group re-
			ceiving	the same setting.

			A CSV list of template_spec=percent key-value pairs
			which controls the per template	sampling rates when
			stats(3) sampling is enabled.

			The local UDP encapsulation port.  A value of 0	indi-
			cates that UDP encapsulation is	disabled.  The default
			is 0.

			The overhead taken into	account	when using UDP encap-
			sulation.  Since MSS clamping by middleboxes will most
			likely not work, values	larger than 8 (the size	of the
			UDP header) are	also supported.	 Supported values are
			between	8 and 1024.  The default is 8.

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

     [ETIMEDOUT]	when a connection was dropped due to excessive re-

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

     [ECONNREFUSED]	when the remote	peer actively refuses connection es-
			tablishment (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	in-
			valid point in the session;

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

     getsockopt(2), socket(2), stats(3), sysctl(3), blackhole(4), inet(4),
     intro(4), ip(4), ktls(4), mod_cc(4), siftr(4), syncache(4), tcp_bbr(4),
     tcp_rack(4), setkey(8), tcp_functions(9)

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

     D.	Borman,	B. Braden, V. Jacobson,	and R. Scheffenegger, TCP Extensions
     for High Performance, RFC 7323.

     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 in-
     troduced in Linux 2.6 and is subject to change.

FreeBSD	13.0		       November	7, 2022			  FreeBSD 13.0


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