TCP(4) BSD Kernel Interfaces Manual TCP(4)NAMEtcp — 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 addi‐
tion, provides a per-host collection of “port addresses”. Thus, each
address is composed of an Internet address specifying the host and net‐
work, 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) sys‐
tem 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 con‐
nection 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 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 pre‐
sented; when outstanding data has not yet been acknowl‐
edged, 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 pack‐
etization 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, cor‐
responding to various TCP extensions which are provided in
this implementation. The boolean option TCP_NOOPT is pro‐
vided 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 the
TCP_NOPUSH 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_SIGNATURE_ENABLE
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 advertis‐
ing this option with TCP-MD5; this may change.
One common use for this in a DragonFlyBSD router deployment
is to enable based routers to interwork with Cisco equip‐
ment 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 nec‐
essary 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: tcpsignature_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 branch
of the sysctl(3) MIB.
TCPCTL_DO_RFC1323 (tcp.rfc1323) Implement the window scaling and time‐
stamp options of RFC 1323 (default true).
TCPCTL_MSSDFLT (tcp.mssdflt) The default value used for the maximum
segment size (“MSS”) when no advice to the contrary is
received from MSS negotiation.
TCPCTL_SENDSPACE (tcp.sendspace) Maximum TCP send window.
TCPCTL_RECVSPACE (tcp.recvspace) Maximum TCP receive window.
tcp.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).
tcp.msl The Maximum Segment Lifetime for a packet.
tcp.keepinit Timeout for new, non-established TCP connections.
tcp.keepidle Amount of time the connection should be idle before
keepalive probes (if enabled) are sent.
tcp.keepintvl The interval between keepalive probes sent to remote
machines. After TCPTV_KEEPCNT (default 8) probes are
sent, with no response, the connection is dropped.
tcp.always_keepalive
Assume that SO_KEEPALIVE is set on all TCP connec‐
tions, the kernel will periodically send a packet to
the remote host to verify the connection is still up.
tcp.icmp_may_rst Certain ICMP unreachable messages may abort connec‐
tions in SYN-SENT state.
tcp.do_tcpdrain Flush packets in the TCP reassembly queue if the sys‐
tem is low on mbufs.
tcp.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).
tcp.delayed_ack Delay ACK to try and piggyback it onto a data packet.
tcp.delacktime Maximum amount of time before a delayed ACK is sent.
tcp.newreno Enable TCP NewReno Fast Recovery algorithm, as
described in RFC 2582.
tcp.path_mtu_discovery
Enables Path MTU Discovery. PMTU Discovery is helpful
for avoiding IP fragmentation when tranferring lots of
data to the same client. For web servers, where most
of the connections are short and to different clients,
PMTU Discovery actually hurts performance due to
unnecessary retransmissions. Turn this on only if
most of your TCP connections are long transfers or are
repeatedly to the same set of clients.
tcp.tcbhashsize Size of the TCP control-block hashtable (read-only).
This may be tuned using the kernel option TCBHASHSIZE
or by setting net.inet.tcp.tcbhashsize in the
loader(8).
tcp.pcbcount Number of active process control blocks (read-only).
tcp.syncookies Determines whether or not syn cookies should be gener‐
ated for outbound syn-ack packets. Syn cookies are a
great help during syn flood attacks, and are enabled
by default.
tcp.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.
tcp.inet.tcp.rexmit_{min,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 con‐
nections, 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 suggest changing the slop to 200ms and setting the
minimum to something out of the way, like 20ms, which
gives you an effective minimum of 200ms (similar to
Linux).
tcp.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 prod‐
uct limiting only affects the transmit side of a TCP
connection.
tcp.inflight_debug
Enable debugging for the bandwidth delay product algo‐
rithm. This may default to on (1) so if you enable
the algorithm you should probably also disable debug‐
ging by setting this variable to 0.
tcp.inflight_min This puts an lower bound on the bandwidth delay prod‐
uct 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 connec‐
tions. Setting this value too high effectively dis‐
ables the algorithm.
tcp.inflight_max This puts an upper bound on the bandwidth delay prod‐
uct window, in bytes. This value should not generally
be modified but may be used to set a global per-con‐
nection 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.
tcp.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 time reduction of
the default inflight code is not sufficient. If this
case occurs you should first try reducing
tcp.inflight_min and, if that does not work, reduce
both tcp.inflight_min and tcp.inflight_stab, trying
values of 15, 10, or 5 for the latter. Never use a
value less than 5. Reducing tcp.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.
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 net‐
work address for which no network interface exists.
[EAFNOSUPPORT] when an attempt is made to bind or connect a socket to
a multicast address.
SEE ALSOgetsockopt(2), socket(2), sysctl(3), blackhole(4), inet(4), intro(4),
ip(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.
BSD February 14, 1995 BSD
