TCP(4) MidnightBSD 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 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_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 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 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 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 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.initburst

Control the number of SACK retransmissions done upon initiation of SACK recovery.

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 network 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 .

HISTORY

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

MidnightBSD 0.3 August 25, 2005 MidnightBSD 0.3