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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>

     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) sys-
     tem 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.  TCP also supports a more datagram-like mode,	called Trans-
     action TCP, which is described in ttcp(4).

     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 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_NODELAY  Under	most circumstances, TCP	sends data when	it is pre-
		  sented; 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 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 negotiation, and to reduce
		  it if	desired.

     TCP_NOOPT	  TCP usually sends a number of	options	in each	packet,	corre-
		  sponding to various TCP extensions which are provided	in
		  this implementation.	The boolean option TCP_NOOPT is	pro-
		  vided	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).  The TCP_NOPUSH
		  option is provided to	allow servers to easily	make use of
		  Transaction TCP (see ttcp(4)).  When the 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.  In the current
		  release, only	outgoing traffic is digested; digests on
		  incoming traffic are not verified.  The current default
		  behavior for the system is to	respond	to a system advertis-
		  ing 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	neces-
		  sary 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 visible
		  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.

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

     TCPCTL_DO_RFC1323	(tcp.rfc1323) Implement	the window scaling and time-
			stamp options of RFC 1323 (default true).

     TCPCTL_DO_RFC1644	(tcp.rfc1644) Implement	Transaction TCP, as described
			in RFC 1644.

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

     TCPCTL_SENDSPACE	(tcp.sendspace)	Maximum	TCP send window.

     TCPCTL_RECVSPACE	(tcp.recvspace)	Maximum	TCP receive window.

     tcp.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).

     tcp.slowstart_flightsize
			The number of packets allowed to be in-flight during
			the TCP	slow-start phase on a non-local	network.

     tcp.local_slowstart_flightsize
			The number of packets allowed to be in-flight during
			the TCP	slow-start phase to local machines in the same
			subnet.

     tcp.msl		The Maximum Segment Lifetime for a packet.

     tcp.keepinit	Timeout	for new, non-established TCP connections.

     tcp.keepidle	Amount of time the connection should be	idle before
			keepalive probes (if enabled) are sent.

     tcp.keepintvl	The interval between keepalive probes sent to remote
			machines.  After TCPTV_KEEPCNT (default	8) probes are
			sent, with no response,	the connection is dropped.

     tcp.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.

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

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

     tcp.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).

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

     tcp.delacktime	Maximum	amount of time before a	delayed	ACK is sent.

     tcp.newreno	Enable TCP NewReno Fast	Recovery algorithm, as
			described in RFC 2582.

     tcp.path_mtu_discovery
			Enable Path MTU	Discovery

     tcp.tcbhashsize	Size of	the TCP	control-block hashtable	(read-only).
			This may be tuned using	the kernel option TCBHASHSIZE
			or by setting net.inet.tcp.tcbhashsize in the
			loader(8).

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

     tcp.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.

     tcp.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.

     tcp.inet.tcp.rexmit_{min,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	accomodate,
			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 suggest changing the	slop to	200ms and setting the
			minimum	to something out of the	way, like 20ms,	which
			gives you an effective minimum of 200ms	(similar to
			Linux).

     tcp.inflight_enable
			Enable TCP bandwidth delay product limiting.  An
			attempt	will be	made to	calculate the bandwidth	delay
			product	for each individual TCP	connection and limit
			the amount of inflight data being transmitted to avoid
			building up unnecessary	packets	in the network.	 This
			option is recommended if you are serving a lot of data
			over connections with high bandwidth-delay products,
			such as	modems,	GigE links, and	fast long-haul WANs,
			and/or you have	configured your	machine	to accomodate
			large TCP windows.  In such situations,	without	this
			option,	you may	experience high	interactive latencies
			or packet loss due to the overloading of intermediate
			routers	and switches.  Note that bandwidth delay prod-
			uct limiting only effects the transmit side of a TCP
			connection.

     tcp.inflight_debug
			Enable debugging for the bandwidth delay product algo-
			rithm.	This may default to on (1) so if you enable
			the algorithm you should probably also disable debug-
			ging by	setting	this variable to 0.

     tcp.inflight_min	This puts an lower bound on the	bandwidth delay	prod-
			uct window, in bytes.  A value of 1024 is typically
			used for debugging.  6000-16000	is more	typical	in a
			production installation.  Setting this value too low
			may result in slow ramp-up times for bursty connec-
			tions.	Setting	this value too high effectively	dis-
			ables the algorithm.

     tcp.inflight_max	This puts an upper bound on the	bandwidth delay	prod-
			uct window, in bytes.  This value should not generally
			be modified but	may be used to set a global per-con-
			nection	limit on queued	data, potentially allowing you
			to intentionally set a less then optimum limit to
			smooth data flow over a	network	while still being able
			to specify huge	internal TCP buffers.

     tcp.inflight_stab	The bandwidth delay product algorithm requires a
			slightly larger	window then it otherwise calculates
			for stability.	This parameter determines the extra
			window in maximal packets / 10.	 The default value of
			20 represents 2	maximal	packets.  Reducing this	value
			is not recommended but you may come across a situation
			with very slow links where the ping time reduction of
			the default inflight code is not sufficient.  If this
			case occurs you	should first try reducing
			tcp.inflight_min and, if that does not work, reduce
			both tcp.inflight_min and tcp.inflight_stab, trying
			values of 15, 10, or 5 for the latter.	Never use a
			value less then	5.  Reducing tcp.inflight_stab can
			lead to	upwards	of a 20% underutilization of the link
			as well	as reducing the	algorithm's ability to adapt
			to changing situations and should only be done as a
			last resort.

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.

SEE ALSO
     getsockopt(2), socket(2), sysctl(3), blackhole(4),	inet(4), intro(4),
     ip(4), ttcp(4)

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

     R.	Braden,	T/TCP -	TCP Extensions for Transactions, RFC 1644.

HISTORY
     The tcp protocol appeared in 4.2BSD.  The RFC 1323	extensions for window
     scaling and timestamps were added in 4.4BSD.

FreeBSD	9.3		       February	14, 1995		   FreeBSD 9.3

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

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