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TCP(4)			 BSD 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)
     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 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
     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 deter‐
		  mine 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_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).  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.  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.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).

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

     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.

     msl		The Maximum Segment Lifetime, in milliseconds, for a
			packet.

     keepinit		Timeout, in milliseconds, for new, non-established TCP
			connections.

     keepidle		Amount of time, in milliseconds, that the connection
			must be idle before keepalive probes (if enabled) are
			sent.

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

     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.

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

     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).

     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 prod‐
			ucts, such as modems, GigE links, and fast long-haul
			WANs, and/or you have configured your machine to
			accommodate large TCP windows.	In such situations,
			without this option, you may experience high interac‐
			tive latencies or packet loss due to the overloading
			of intermediate routers and switches.  Note that band‐
			width-delay product limiting only effects the transmit
			side of a TCP connection.

     inflight.debug	Enable debugging for the bandwidth-delay product algo‐
			rithm.

     inflight.min	This puts a lower bound on the bandwidth-delay product
			window, in bytes.  A value of 1024 is typically used
			for debugging.	6000-16000 is more typical in a pro‐
			duction installation.  Setting this value too low may
			result in slow ramp-up times for bursty connections.
			Setting this value too high effectively disables the
			algorithm.

     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 than optimum limit, to
			smooth data flow over a network while still being able
			to specify huge internal TCP buffers.

     inflight.stab	The bandwidth-delay product algorithm requires a
			slightly larger window than 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 situa‐
			tion with very slow links where the ping(8) time
			reduction of the default inflight code is not suffi‐
			cient.	If this case occurs, you should first try
			reducing inflight.min and, if that does not work,
			reduce both inflight.min and inflight.stab, trying
			values of 15, 10, or 5 for the latter.	Never use a
			value less than 5.  Reducing inflight.stab can lead to
			upwards of a 20% underutilization of the link as well
			as reducing the algorithm's ability to adapt to chang‐
			ing situations and should only be done as a last
			resort.

     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.

			When this feature is enabled, the slowstart_flightsize
			and local_slowstart_flightsize settings are not
			observed for new connection slow starts, but they are
			still used for slow starts that occur when the connec‐
			tion has been idle and starts sending again.

     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.

     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.

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), 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.

BSD				August 16, 2008				   BSD
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