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32 .\" From: @(#)tcp.4 8.1 (Berkeley) 6/5/93
33 .\" $FreeBSD: src/share/man/man4/tcp.4,v 1.11.2.14 2002/12/29 16:35:38 schweikh Exp $
34 .\" $DragonFly: src/share/man/man4/tcp.4,v 1.2 2003/06/17 04:36:59 dillon Exp $
41 .Nd Internet Transmission Control Protocol
47 .Fn socket AF_INET SOCK_STREAM 0
51 protocol provides reliable, flow-controlled, two-way
52 transmission of data. It is a byte-stream protocol used to
55 abstraction. TCP uses the standard
56 Internet address format and, in addition, provides a per-host
59 Thus, each address is composed
60 of an Internet address specifying the host and network, with
63 port on the host identifying the peer entity.
65 Sockets utilizing the tcp protocol are either
69 Active sockets initiate connections to passive
72 sockets are created active; to create a
75 system call must be used
76 after binding the socket with the
79 passive sockets may use the
81 call to accept incoming connections. Only active sockets may
84 call to initiate connections.
86 also supports a more datagram-like mode, called Transaction
93 their location to match
94 incoming connection requests from multiple networks. This
96 .Dq wildcard addressing ,
98 server to provide service to clients on multiple networks.
99 To create a socket which listens on all networks, the Internet
104 port may still be specified
105 at this time; if the port is not specified the system will assign one.
106 Once a connection has been established the socket's address is
107 fixed by the peer entity's location. The address assigned the
108 socket is the address associated with the network interface
109 through which packets are being transmitted and received. Normally
110 this address corresponds to the peer entity's network.
113 supports a number of socket options which can be set with
117 .Bl -tag -width TCP_NODELAYx
119 Under most circumstances,
121 sends data when it is presented;
122 when outstanding data has not yet been acknowledged, it gathers
123 small amounts of output to be sent in a single packet once
124 an acknowledgement is received.
125 For a small number of clients, such as window systems
126 that send a stream of mouse events which receive no replies,
127 this packetization may cause significant delays.
130 defeats this algorithm.
132 By default, a sender\- and receiver-TCP
133 will negotiate among themselves to determine the maximum segment size
134 to be used for each connection. The
136 option allows the user to determine the result of this negotiation,
137 and to reduce it if desired.
140 usually sends a number of options in each packet, corresponding to
143 extensions which are provided in this implementation. The boolean
146 is provided to disable
148 option use on a per-connection basis.
150 By convention, the sender-TCP
153 bit and begin transmission immediately (if permitted) at the end of
160 option is provided to allow servers to easily make use of Transaction
163 When the option is set to a non-zero value,
165 will delay sending any data at all until either the socket is closed,
166 or the internal send buffer is filled.
169 The option level for the
171 call is the protocol number for
174 .Xr getprotobyname 3 ,
177 All options are declared in
178 .Aq Pa netinet/tcp.h .
182 transport level may be used with
186 Incoming connection requests that are source-routed are noted,
187 and the reverse source route is used in responding.
191 protocol implements a number of variables in the
196 .Bl -tag -width TCPCTL_DO_RFC1644
197 .It Dv TCPCTL_DO_RFC1323
199 Implement the window scaling and timestamp options of RFC 1323
201 .It Dv TCPCTL_DO_RFC1644
203 Implement Transaction
205 as described in RFC 1644.
206 .It Dv TCPCTL_MSSDFLT
208 The default value used for the maximum segment size
210 when no advice to the contrary is received from MSS negotiation.
211 .It Dv TCPCTL_SENDSPACE
213 Maximum TCP send window.
214 .It Dv TCPCTL_RECVSPACE
216 Maximum TCP receive window.
218 Log any connection attempts to ports where there is not a socket
219 accepting connections.
220 The value of 1 limits the logging to SYN (connection establishment)
222 That of 2 results in any TCP packets to closed ports being logged.
223 Any value unlisted above disables the logging
224 (default is 0, i.e., the logging is disabled).
225 .It tcp.slowstart_flightsize
226 The number of packets allowed to be in-flight during the
228 slow-start phase on a non-local network.
229 .It tcp.local_slowstart_flightsize
230 The number of packets allowed to be in-flight during the
232 slow-start phase to local machines in the same subnet.
234 The Maximum Segment Lifetime for a packet.
236 Timeout for new, non-established TCP connections.
238 Amount of time the connection should be idle before keepalive
239 probes (if enabled) are sent.
241 The interval between keepalive probes sent to remote machines.
244 (default 8) probes are sent, with no response, the connection is dropped.
245 .It tcp.always_keepalive
250 connections, the kernel will
251 periodically send a packet to the remote host to verify the connection
256 unreachable messages may abort connections in
262 reassembly queue if the system is low on mbufs.
264 If enabled, disable sending of RST when a connection is attempted
265 to a port where there is not a socket accepting connections.
269 Delay ACK to try and piggyback it onto a data packet.
271 Maximum amount of time before a delayed ACK is sent.
273 Enable TCP NewReno Fast Recovery algorithm,
274 as described in RFC 2582.
275 .It tcp.path_mtu_discovery
276 Enable Path MTU Discovery
280 control-block hashtable
282 This may be tuned using the kernel option
285 .Va net.inet.tcp.tcbhashsize
289 Number of active process control blocks
292 Determines whether or not syn cookies should be generated for
293 outbound syn-ack packets. Syn cookies are a great help during
294 syn flood attacks, and are enabled by default.
295 .It tcp.isn_reseed_interval
296 The interval (in seconds) specifying how often the secret data used in
297 RFC 1948 initial sequence number calculations should be reseeded.
298 By default, this variable is set to zero, indicating that
299 no reseeding will occur.
300 Reseeding should not be necessary, and will break
302 recycling for a few minutes.
303 .It tcp.inet.tcp.rexmit_{min,slop}
304 Adjust the retransmit timer calculation for TCP. The slop is
305 typically added to the raw calculation to take into account
306 occasional variances that the SRTT (smoothed round trip time)
307 is unable to accomodate, while the minimum specifies an
308 absolute minimum. While a number of TCP RFCs suggest a 1
309 second minimum these RFCs tend to focus on streaming behavior
310 and fail to deal with the fact that a 1 second minimum has severe
311 detrimental effects over lossy interactive connections, such
312 as a 802.11b wireless link, and over very fast but lossy
313 connections for those cases not covered by the fast retransmit
314 code. For this reason we suggest changing the slop to 200ms and
315 setting the minimum to something out of the way, like 20ms,
316 which gives you an effective minimum of 200ms (similar to Linux).
317 .It tcp.inflight_enable
320 bandwidth delay product limiting. An attempt will be made to calculate
321 the bandwidth delay product for each individual TCP connection and limit
322 the amount of inflight data being transmitted to avoid building up
323 unnecessary packets in the network. This option is recommended if you
324 are serving a lot of data over connections with high bandwidth-delay
325 products, such as modems, GigE links, and fast long-haul WANs, and/or
326 you have configured your machine to accomodate large TCP windows. In such
327 situations, without this option, you may experience high interactive
328 latencies or packet loss due to the overloading of intermediate routers
329 and switches. Note that bandwidth delay product limiting only effects
330 the transmit side of a TCP connection.
331 .It tcp.inflight_debug
332 Enable debugging for the bandwidth delay product algorithm. This may
333 default to on (1) so if you enable the algorithm you should probably also
334 disable debugging by setting this variable to 0.
336 This puts an lower bound on the bandwidth delay product window, in bytes.
337 A value of 1024 is typically used for debugging. 6000-16000 is more typical
338 in a production installation. Setting this value too low may result in
339 slow ramp-up times for bursty connections. Setting this value too high
340 effectively disables the algorithm.
342 This puts an upper bound on the bandwidth delay product window, in bytes.
343 This value should not generally be modified but may be used to set a
344 global per-connection limit on queued data, potentially allowing you to
345 intentionally set a less then optimum limit to smooth data flow over a
346 network while still being able to specify huge internal TCP buffers.
347 .It tcp.inflight_stab
348 The bandwidth delay product algorithm requires a slightly larger window
349 then it otherwise calculates for stability. This parameter determines the
350 extra window in maximal packets / 10. The default value of 20 represents
351 2 maximal packets. Reducing this value is not recommended but you may
352 come across a situation with very slow links where the ping time
353 reduction of the default inflight code is not sufficient. If this case
354 occurs you should first try reducing tcp.inflight_min and, if that does not
355 work, reduce both tcp.inflight_min and tcp.inflight_stab, trying values of
356 15, 10, or 5 for the latter. Never use a value less then 5. Reducing
357 tcp.inflight_stab can lead to upwards of a 20% underutilization of the link
358 as well as reducing the algorithm's ability to adapt to changing
359 situations and should only be done as a last resort.
362 A socket operation may fail with one of the following errors returned:
365 when trying to establish a connection on a socket which
368 when the system runs out of memory for
369 an internal data structure;
371 when a connection was dropped
372 due to excessive retransmissions;
375 forces the connection to be closed;
376 .It Bq Er ECONNREFUSED
378 peer actively refuses connection establishment (usually because
379 no process is listening to the port);
382 is made to create a socket with a port which has already been
384 .It Bq Er EADDRNOTAVAIL
385 when an attempt is made to create a
386 socket with a network address for which no network interface
388 .It Bq Er EAFNOSUPPORT
389 when an attempt is made to bind or connect a socket to a multicast
405 .%T "TCP Extensions for High Performance"
410 .%T "T/TCP \- TCP Extensions for Transactions"
418 The RFC 1323 extensions for window scaling and timestamps were added