tcp -- Internet Transmission Control Protocol
socket(AF_INET, SOCK_STREAM, 0);
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. TCP also supports a more datagram-like mode, called Transaction
TCP, which is described in ttcp(4).
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). The TCP_NOPUSH
option is provided to allow servers to easily make use of
Transaction TCP (see ttcp(4)). 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
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 [Toc] [Back]
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_DO_RFC1644 (rfc1644) Implement Transaction TCP, as described in
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
The number of packets allowed to be in-flight during
the TCP slow-start phase on a non-local network.
The number of packets allowed to be in-flight during
the TCP slow-start phase to local machines in the same
msl The Maximum Segment Lifetime, in milliseconds, for a
keepinit Timeout, in milliseconds, for new, non-established TCP
keepidle Amount of time, in milliseconds, that the connection
must be idle before keepalive probes (if enabled) are
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.
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
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).)
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.
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 accomodate,
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 accomodate
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.
This may default to on (1), so if you enable
the algorithm, you should probably also disable debugging
by setting this variable to 0.
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
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
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. This is a standards track RFC and
is off by default.
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 propagationdelay
connections. This is a standards track RFC and
support for it is off by default.
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.
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
[ETIMEDOUT] when a connection was dropped due to excessive
[ECONNRESET] when the remote peer forces the connection to be
[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.
getsockopt(2), socket(2), sysctl(3), blackhole(4), inet(4), intro(4),
ip(4), syncache(4), ttcp(4)
V. Jacobson, R. Braden, and D. Borman, TCP Extensions for High
Performance, RFC 1323.
R. Braden, T/TCP - TCP Extensions for Transactions, RFC 1644.
The TCP protocol appeared in 4.2BSD. The RFC 1323 extensions for window
scaling and timestamps were added in 4.4BSD.
FreeBSD 5.2.1 March 13, 2003 FreeBSD 5.2.1 [ Back ]