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_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
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,
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
Fx 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
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.
#include <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
TCPFIN_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
TCPFIN_WAIT_2
connections.
Defaults to 60 seconds.
ERRORS
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
allocated;
Bq Er EADDRNOTAVAIL
when an attempt is made to create a
socket with a network address for which no network interface
exists;
Bq Er EAFNOSUPPORT
when an attempt is made to bind or connect a socket to a multicast
address.
"V. Jacobson"
"R. Braden"
"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
BSD 4.2
The RFC 1323 extensions for window scaling and timestamps were added
in
BSD 4.4
The
TCP_INFO
option was introduced in
Linux 2.6
and is
subject to change
