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Manual Pages  — TCP


tcp – Internet Transmission Control Protocol



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



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 addition, provides a per-host collection of "port addresses". Thus, each address 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". 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 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 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_FASTOPEN Enable or disable TCP Fast Open (TFO). To use this option, the kernel must be built with the TCP_RFC7413 option.

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 being established.

For passively-created sockets, the TCP_FASTOPEN socket option can be queried to determine whether the connection 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 environments where TFO servers are exposed to both internal and external clients and only wish to allow TFO connections from internal clients.

In the PSK mode of operation, the server generates and sends TFO cookies to requesting clients as usual. However, 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 kernel option.

  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 milliseconds 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 section 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 milliseconds 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 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, corresponding to various TCP extensions which are provided 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. Outgoing 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.

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

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.
  ( 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 violate RFC 7323, including modern widely deployed ones. Therefore the default is 1, i.e., the missing of timestamps is tolerated.
TCPCTL_MSSDFLT ( mssdflt) The default value used for the maximum segment 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 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).
  The Maximum Segment Lifetime, in milliseconds, for a packet.
  Timeout, in milliseconds, for new, non-established TCP connections. The default is 75000 msec.
  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).
  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.
  Number of probes sent, with no response, before a connection is dropped. The default is 8 packets.
  Assume that SO_KEEPALIVE is set on all TCP connections, the kernel will periodically send a packet to the remote host to verify the connection is still up.
  Certain ICMP unreachable messages may abort connections in SYN-SENT state.
  Flush packets in the TCP reassembly queue if the system is low on mbufs.
  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).
  Delay ACK to try and piggyback it onto a data packet.
  Maximum amount of time, in milliseconds, before a delayed ACK is sent.
  Enable Path MTU Discovery.
  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).
  Number of active process control blocks (read-only).
  Determines whether or not SYN cookies should be generated 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 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.
  The current total number of segments present in all reassembly queues.
  The maximum limit on the total number of segments across all reassembly queues. The limit can be adjusted as a tunable.
  The maximum number of segments allowed in each reassembly 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 connections, 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.
  Enable the ability to specify initial congestion window 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.
  Calculate the bytes in flight using the algorithm described in RFC 6675, and is also a prerequisite to enable Proportional Rate Reduction.
  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.
  Enable support for RFC 3390, which allows for a variable-sized starting congestion window on new connections, depending on the maximum segment size. This helps throughput in general, but particularly affects short transfers and high-bandwidth large propagation-delay connections.
  Enable support for RFC 2018, TCP Selective Acknowledgment option, which allows the receiver to inform the sender about all successfully arrived segments, allowing the sender to retransmit the missing segments only.
  Maximum number of SACK holes per connection. Defaults to 128.
  Maximum number of SACK holes per system, across all connections. Defaults to 65536.
  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 allocated to contain a minimal amount of information necessary 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 maximum number of these structures allocated. By default, it is initialized to kern.ipc.maxsockets / 5.
  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 cannot 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.
  Enable support for TCP Explicit Congestion Notification (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 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 default 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 IPv4.
3 Enable path MTU blackhole detection only for IPv6.
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.
fastopen.acceptany 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.
fastopen.ccache_bucket_limit The maximum number of entries in a client cookie cache bucket. The default value can be tuned with the TCP_FASTOPEN_CCACHE_BUCKET_LIMIT_DEFAULT kernel option or by setting net.inet.tcp.fastopen_ccache_bucket_limit in the loader(8).
fastopen.ccache_buckets The number of client cookie cache buckets. Read-only. The value can be tuned with the TCP_FASTOPEN_CCACHE_BUCKETS_DEFAULT kernel option or by setting fastopen.ccache_buckets in the loader(8).
fastopen.ccache_list Print the client cookie cache. Read-only.
fastopen.client_enable When zero, no new active (i.e., client) TFO connections 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 established. 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. Read-only.
fastopen.numkeys The current number of keys installed. Read-only.
fastopen.numpsks The current number of pre-shared keys installed. Read-only.
fastopen.path_disable_time When a failure occurs while trying to create a new active (i.e., client) TFO connection, new active connections 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 disable time for a given path to be reduced if the corresponding client cookie cache entry is reused due to resource pressure before the disable period has elapsed. The default is TCP_FASTOPEN_PATH_DISABLE_TIME_DEFAULT.
fastopen.psk_enable 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 removed. The default is 0.
fastopen.server_enable When zero, no new passive (i.e., server) TFO connections 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 enabled, if fastopen.autokey is non-zero and there are no keys installed, a new key will be generated immediately. 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.
functions_available List of available TCP function blocks (TCP stacks).
functions_default The default TCP function block (TCP stack).
functions_inherit_listen_socket_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.
ts_offset_per_conn When initializing the TCP timestamps, use a per connection offset instead of a per host pair offset. Default is to use per connection offsets as recommended in RFC 7323.


A socket operation may fail with one of the following errors returned:
  when trying to establish a connection on a socket which already has one;
  when the system runs out of memory for an internal data structure;
  when a connection was dropped due to excessive retransmissions;
  when the remote peer forces the connection to be closed;
  when the remote peer actively refuses connection establishment (usually because no process is listening to the port);
  when an attempt is made to create a socket with a port which has already been allocated;
  when an attempt is made to create a socket with a network address for which no network interface exists;
  when an attempt is made to bind or connect a socket to a multicast address.
  when trying to change TCP function blocks at an invalid point in the session;
  when trying to use a TCP function block that is not available;


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

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

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

A. Heffernan, RFC 2385, Protection of BGP Sessions via the TCP MD5 Signature Option,

K. Ramakrishnan, S. Floyd, D. Black, RFC 3168, The Addition of Explicit Congestion Notification (ECN) to IP,


The TCP protocol appeared in BSD 4.2 . The RFC 1323 extensions for window scaling and timestamps were added in BSD 4.4 . The TCP_INFO option was introduced in Linux 2.6 and is subject to change.

TCP (4) June 27, 2021

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