protocol provides reliable, flow-controlled, two-way
transmission of data.
It is a byte-stream protocol used to
uses the standard
Internet address format and, in addition, provides a per-host
Thus, each address is composed
of an Internet address specifying the host and network,
with a specific
port on the host identifying the peer entity.
Sockets utilizing the
protocol are either
Active sockets initiate connections to passive
sockets are created active; to create a
passive socket, the
system call must be used
after binding the socket with the
Only passive sockets may use the
call to accept incoming connections.
Only active sockets may use the
call to initiate connections.
Passive sockets may
their location to match
incoming connection requests from multiple networks.
This technique, termed
allows a single
server to provide service to clients on multiple networks.
To create a socket which listens on all networks, the Internet
must be bound.
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.
supports a number of socket options which can be set with
and tested with
Information about a socket's underlying TCP session may be retrieved
by passing the read-only option
It accepts a single argument: a pointer to an instance of
Vt struct tcp_info .
This API is subject to change; consult the source to determine
which fields are currently filled out by this option.
Fx specific additions include
send window size,
receive window size,
bandwidth-controlled window space.
Under most circumstances,
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
defeats this algorithm.
By default, a sender- and
will negotiate among themselves to determine the maximum segment size
to be used for each connection.
option allows the user to determine the result of this negotiation,
and to reduce it if desired.
usually sends a number of options in each packet, corresponding to
extensions which are provided in this implementation.
The boolean option
is provided to disable
option use on a per-connection basis.
By convention, the
will set the
bit, and begin transmission immediately (if permitted) at the end of
every user call to
When this option is set to a non-zero value,
will delay sending any data at all until either the socket is closed,
or the internal send buffer is filled.
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
Fx router deployment is to enable
based routers to interwork with Cisco equipment at peering points.
Support for this feature conforms to RFC 2385.
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
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
call is the protocol number for
All options are declared in
In netinet/tcp.h .
Options at the
transport level may be used with
Incoming connection requests that are source-routed are noted,
and the reverse source route is used in responding.
protocol implements a number of variables in the
branch of the
Implement the window scaling and timestamp options of RFC 1323
(default is true).
The default value used for the maximum segment size
when no advice to the contrary is received from MSS negotiation.
Log any connection attempts to ports where there is not a socket
The value of 1 limits the logging to
(connection establishment) packets only.
That of 2 results in any
packets to closed ports being logged.
Any value unlisted above disables the logging
(default is 0, i.e., the logging is disabled).
The number of packets allowed to be in-flight during the
slow-start phase on a non-local network.
The number of packets allowed to be in-flight during the
slow-start phase to local machines in the same subnet.
The Maximum Segment Lifetime, in milliseconds, for a packet.
Timeout, in milliseconds, for new, non-established
Amount of time, in milliseconds, that the connection must be idle
before keepalive probes (if enabled) are sent.
The interval, in milliseconds, between keepalive probes sent to remote
(default 8) probes are sent, with no response, the connection is dropped.
is set on all
connections, the kernel will
periodically send a packet to the remote host to verify the connection
is still up.
unreachable messages may abort connections in
Flush packets in the
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.
Delay ACK to try and piggyback it onto a data packet.
Maximum amount of time, in milliseconds, before a delayed ACK is sent.
NewReno Fast Recovery algorithm,
as described in RFC 2582.
Enable Path MTU Discovery.
Size of the
control-block hash table
This may be tuned using the kernel option
or by setting
Number of active process control blocks
Determines whether or not
cookies should be generated for outbound
cookies are a great help during
flood attacks, and are enabled by default.
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
recycling for a few minutes.
rexmit_min , rexmit_slop
Adjust the retransmit timer calculation for
The slop is
typically added to the raw calculation to take into account
occasional variances that the
(smoothed round-trip time)
is unable to accommodate, while the minimum specifies an
While a number of
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
For this reason, we use 200ms of slop and a near-0
minimum, which gives us an effective minimum of 200ms (similar to
bandwidth-delay product limiting.
An attempt will be made to calculate
the bandwidth-delay product for each individual
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
situations, without this option, you may experience high interactive
latencies or packet loss due to the overloading of intermediate routers
Note that bandwidth-delay product limiting only effects
the transmit side of a
Enable debugging for the bandwidth-delay product algorithm.
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.
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
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
reduction of the default inflight code is not sufficient.
If this case occurs, you should first try reducing
and, if that does not
work, reduce both
trying values of
15, 10, or 5 for the latter.
Never use a value less than 5.
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.
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
When this feature is enabled, the
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
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
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
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.
MIB variable controls the maximum number of these structures allocated.
By default, it is initialized to
Suppress creating of compressed TCP TIME_WAIT states for connections in
which both endpoints are local.
connections faster when the socket is marked as
(no user process has the socket open, data received on
the socket cannot be read).
The timeout used here is
Timeout to use for fast recycling of
Defaults to 60 seconds.
A socket operation may fail with one of the following errors returned:
Bq Er EISCONN
when trying to establish a connection on a socket which
already has one;
Bq Er ENOBUFS
when the system runs out of memory for
an internal data structure;
Bq Er ETIMEDOUT
when a connection was dropped
due to excessive retransmissions;
Bq Er ECONNRESET
when the remote peer
forces the connection to be closed;
Bq Er ECONNREFUSED
when the remote
peer actively refuses connection establishment (usually because
no process is listening to the port);
Bq Er EADDRINUSE
when an attempt
is made to create a socket with a port which has already been
Bq Er EADDRNOTAVAIL
when an attempt is made to create a
socket with a network address for which no network interface
Bq Er EAFNOSUPPORT
when an attempt is made to bind or connect a socket to a multicast