Overview

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

int
socket(AF_INET, SOCK_STREAM, 0);

TCP (Internet Transmission Control 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 system call must be used after binding the socket with the bind system call. Only passive sockets may use the accept call to accept incoming connections. Only active sockets may use the connect 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 and tested with getsockopt:

• 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_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. 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 advertising 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 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 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 getsockopt call is the protocol number for TCP, available from getprotobyname(), or IPPROTO_TCP. All options are declared in <netinet/tcp.h>.

Options at the IP transport level may be used with TCP; see IP. 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 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 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_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 connections, 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 connections in SYN-SENT state.
• do_tcpdrain
  Flush packets in the TCP reassembly queue if the system 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 generated for outbound SYN-ACK packets. SYN cookies are a great help during SYN flood attacks, and are enabledby default - (see syncookies).
• 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 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).
• 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 accommodate 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 product limiting only effects the transmit side of a TCP connection.
• inflight.debug
  Enable debugging for the bandwidth-delay product algorithm.
• 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 production 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 product window, in bytes. This value should not generally be modified, but may be used to set a global per-connection 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 situation with very slow links where the ping(8) time reduction of the default inflight code is not sufficient. 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 changing 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 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.
  
  

  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 connection has been idle and starts sending again.

• sack.enable
  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.
• 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 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.
• 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 cannot 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.

Status

A socket operation may fail with one of the following errors returned:

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.

Requirements

See Also

getsockopt, socket, sysctl, blackhole, inet, IP, syncache, syncookies

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.