TCP(4) FreeBSD Kernel Interfaces Manual TCP(4)
NAME
tcp - Internet Transmission Control Protocol
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.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_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 struct
tcp_info.
This API is subject to change; consult the source
to determine which fields are currently filled out
by this option. FreeBSD specific additions include
send window size, receive window size, and
bandwidth-controlled window space.
TCP_CCALGOOPT Set or query congestion control algorithm specific
parameters. See mod_cc(4) for details.
TCP_CONGESTION Select or query the congestion control algorithm
that TCP will use for the connection. See
mod_cc(4) for details.
TCP_FASTOPEN Enable or disable TCP Fast Open (TFO). To use this
option, the kernel must be built with the
TCP_RFC7413 option.
This option can be set on the socket either before
or after the listen(2) is invoked. Clearing this
option on a listen socket after it has been set has
no effect on existing TFO connections or TFO
connections in progress; it only prevents new TFO
connections from being established.
For passively-created sockets, the TCP_FASTOPEN
socket option can be queried to determine whether
the connection was established using TFO. Note
that connections that are established via a TFO
SYN, but that fall back to using a non-TFO SYN|ACK
will have the TCP_FASTOPEN socket option set.
In addition to the facilities defined in RFC7413,
this implementation supports a pre-shared key (PSK)
mode of operation in which the TFO server requires
the client to be in posession of a shared secret in
order for the client to be able to successfully
open TFO connections with the server. This is
useful, for example, in environments where TFO
servers are exposed to both internal and external
clients and only wish to allow TFO connections from
internal clients.
In the PSK mode of operation, the server generates
and sends TFO cookies to requesting clients as
usual. However, when validating cookies received
in TFO SYNs from clients, the server requires the
client-supplied cookie to equal
SipHash24(key=16-byte-psk, msg=cookie-sent-to-client)
Multiple concurrent valid pre-shared keys are
supported so that time-based rolling PSK
invalidation policies can be implemented in the
system. The default number of concurrent pre-
shared keys is 2.
This can be adjusted with the TCP_RFC7413_MAX_PSKS
kernel option.
TCP_FUNCTION_BLK Select or query the set of functions that TCP will
use for this connection. This allows a user to
select an alternate TCP stack. The alternate TCP
stack must already be loaded in the kernel. To
list the available TCP stacks, see
functions_available in the MIB Variables section
further down. To list the default TCP stack, see
functions_default in the MIB Variables section.
TCP_KEEPINIT This setsockopt(2) option accepts a per-socket
timeout argument of u_int in seconds, for new, non-
established TCP connections. For the global
default in milliseconds see keepinit in the MIB
Variables section further down.
TCP_KEEPIDLE This setsockopt(2) option accepts an argument of
u_int for the amount of time, in seconds, that the
connection must be idle before keepalive probes (if
enabled) are sent for the connection of this
socket. If set on a listening socket, the value is
inherited by the newly created socket upon
accept(2). For the global default in milliseconds
see keepidle in the MIB Variables section further
down.
TCP_KEEPINTVL This setsockopt(2) option accepts an argument of
u_int to set the per-socket interval, in seconds,
between keepalive probes sent to a peer. If set on
a listening socket, the value is inherited by the
newly created socket upon accept(2). For the
global default in milliseconds see keepintvl in the
MIB Variables section further down.
TCP_KEEPCNT This setsockopt(2) option accepts an argument of
u_int and allows a per-socket tuning of the number
of probes sent, with no response, before the
connection will be dropped. If set on a listening
socket, the value is inherited by the newly created
socket upon accept(2). For the global default see
the keepcnt in the MIB Variables section further
down.
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. Outgoing traffic is digested; digests on
incoming traffic are verified. When this option is
enabled on a socket, all inbound and outgoing TCP
segments must be signed with MD5 digests.
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.
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 can only be specified on a per-host basis at
this time.
If an SADB entry cannot be found for the
destination, the system does not send any outgoing
segments and drops any inbound segments. However,
during connection negotiation, a non-signed segment
will be accepted if an SADB entry does not exist
between hosts. When a non-signed segment is
accepted, the established connection is not
protected with MD5 digests.
TCP_STATS Manage collection of connection level statistics
using the stats(3) framework.
Each dropped segment is taken into account in the
TCP protocol statistics.
TCP_TXTLS_ENABLE Enable in-kernel Transport Layer Security (TLS) for
data written to this socket. See ktls(4) for more
details.
TCP_TXTLS_MODE The integer argument can be used to get or set the
current TLS transmit mode of a socket. See ktls(4)
for more details.
TCP_RXTLS_ENABLE Enable in-kernel TLS for data read from this
socket. See ktls(4) for more details.
TCP_REUSPORT_LB_NUMA Changes NUMA affinity filtering for an established
TCP listen socket. This option takes a single
integer argument which specifies the NUMA domain to
filter on for this listen socket. The argument can
also have the follwing special values:
TCP_REUSPORT_LB_NUMA_NODOM
Remove NUMA filtering for
this listen socket.
TCP_REUSPORT_LB_NUMA_CURDOM
Filter traffic associated
with the domain where the
calling thread is
currently executing. This
is typically used after a
process or thread inherits
a listen socket from its
parent, and sets its CPU
affinity to a particular
core.
TCP_REMOTE_UDP_ENCAPS_PORT
Set and get the remote UDP encapsulation port. It
can only be set on a closed TCP socket.
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.
The default congestion control algorithm for TCP is cc_newreno(4). Other
congestion control algorithms can be made available using the mod_cc(4)
framework.
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/RFC 7323 (default is
true).
tolerate_missing_ts Tolerate the missing of timestamps (RFC 1323/RFC
7323) for TCP segments belonging to TCP
connections for which support of TCP timestamps
has been negotiated. As of June 2021, several TCP
stacks are known to violate RFC 7323, including
modern widely deployed ones. Therefore the
default is 1, i.e., the missing of timestamps is
tolerated.
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).
msl The Maximum Segment Lifetime, in milliseconds, for
a packet.
keepinit Timeout, in milliseconds, for new, non-established
TCP connections. The default is 75000 msec.
keepidle Amount of time, in milliseconds, that the
connection must be idle before keepalive probes
(if enabled) are sent. The default is 7200000
msec (2 hours).
keepintvl The interval, in milliseconds, between keepalive
probes sent to remote machines, when no response
is received on a keepidle probe. The default is
75000 msec.
keepcnt Number of probes sent, with no response, before a
connection is dropped. The default is 8 packets.
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.
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.
reass.cursegments The current total number of segments present in
all reassembly queues.
reass.maxsegments The maximum limit on the total number of segments
across all reassembly queues. The limit can be
adjusted as a tunable.
reass.maxqueuelen The maximum number of segments allowed in each
reassembly queue. By default, the system chooses
a limit based on each TCP connection's receive
buffer size and maximum segment size (MSS). The
actual limit applied to a session's reassembly
queue will be the lower of the system-calculated
automatic limit and the user-specified
reass.maxqueuelen limit.
rexmit_initial, 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). The initial value is
used before an RTT measurement has been performed.
initcwnd_segments Enable the ability to specify initial congestion
window in number of segments. The default value
is 10 as suggested by RFC 6928. Changing the
value on fly would not affect connections using
congestion window from the hostcache. Caution:
This regulates the burst of packets allowed to be
sent in the first RTT. The value should be
relative to the link capacity. Start with small
values for lower-capacity links. Large bursts can
cause buffer overruns and packet drops if routers
have small buffers or the link is experiencing
congestion.
newcwd Enable the New Congestion Window Validation
mechanism as described in RFC 7661. This gently
reduces the congestion window during periods,
where TCP is application limited and the network
bandwidth is not utilized completely. That
prevents self-inflicted packet losses once the
application starts to transmit data at a higher
speed.
do_prr Perform SACK loss recovery using the Proportional
Rate Reduction (PRR) algorithm described in
RFC6937. This improves the effectiveness of
retransmissions particular in environments with
ACK thinning or burst loss events, as chances to
run out of the ACK clock are reduced, preventing
lengthy and performance reducing RTO based loss
recovery (default is true).
do_prr_conservative While doing Proportional Rate Reduction, remain
strictly in a packet conserving mode, sending only
one new packet for each ACK received. Helpful
when a misconfigured token bucket traffic policer
causes persistent high losses leading to RTO, but
reduces PRR effectiveness in more common settings
(default is false).
rfc6675_pipe Calculate the bytes in flight using the algorithm
described in RFC 6675, and is also an improvement
when Proportional Rate Reduction is enabled. Also
enables two other mechanisms from RFC6675. Rescue
Retransmission helps timely loss recovery, when
the trailing segments of a transmission are lost,
while no additional data is ready to be sent. In
case a partial ACK without a SACK block is
received during SACK loss recovery, the trailing
segment is immediately resent, rather than waiting
for a Retransmission timeout. SACK loss recovery
is also engaged, once two segments plus one byte
are SACKed - even if no traditional duplicate ACKs
were seen.
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.
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 TCP FIN_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 TCP
FIN_WAIT_2 connections. Defaults to 60 seconds.
ecn.enable Enable support for TCP Explicit Congestion
Notification (ECN). ECN allows a TCP sender to
reduce the transmission rate in order to avoid
packet drops.
0 Disable ECN.
1 Allow incoming connections to request ECN.
Outgoing connections will request ECN.
2 Allow incoming connections to request ECN.
Outgoing connections will not request ECN.
(default)
ecn.maxretries Number of retries (SYN or SYN/ACK retransmits)
before disabling ECN on a specific connection.
This is needed to help with connection
establishment when a broken firewall is in the
network path.
pmtud_blackhole_detection
Enable automatic path MTU blackhole detection. In
case of retransmits of MSS sized segments, the OS
will lower the MSS to check if it's an MTU
problem. If the current MSS is greater than the
configured value to try
(net.inet.tcp.pmtud_blackhole_mss and
net.inet.tcp.v6pmtud_blackhole_mss), it will be
set to this value, otherwise, the MSS will be set
to the default values (net.inet.tcp.mssdflt and
net.inet.tcp.v6mssdflt). Settings:
0 Disable path MTU blackhole detection.
1 Enable path MTU blackhole detection for
IPv4 and IPv6.
2 Enable path MTU blackhole detection only
for IPv4.
3 Enable path MTU blackhole detection only
for IPv6.
pmtud_blackhole_mss MSS to try for IPv4 if PMTU blackhole detection is
turned on.
v6pmtud_blackhole_mss MSS to try for IPv6 if PMTU blackhole detection is
turned on.
fastopen.acceptany When non-zero, all client-supplied TFO cookies
will be considered to be valid. The default is 0.
fastopen.autokey When this and net.inet.tcp.fastopen.server_enable
are non-zero, a new key will be automatically
generated after this specified seconds. The
default is 120.
fastopen.ccache_bucket_limit
The maximum number of entries in a client cookie
cache bucket. The default value can be tuned with
the TCP_FASTOPEN_CCACHE_BUCKET_LIMIT_DEFAULT
kernel option or by setting
net.inet.tcp.fastopen_ccache_bucket_limit in the
loader(8).
fastopen.ccache_buckets
The number of client cookie cache buckets. Read-
only. The value can be tuned with the
TCP_FASTOPEN_CCACHE_BUCKETS_DEFAULT kernel option
or by setting fastopen.ccache_buckets in the
loader(8).
fastopen.ccache_list Print the client cookie cache. Read-only.
fastopen.client_enable
When zero, no new active (i.e., client) TFO
connections can be created. On the transition
from enabled to disabled, the client cookie cache
is cleared and disabled. The transition from
enabled to disabled does not affect any active TFO
connections in progress; it only prevents new ones
from being established. The default is 0.
fastopen.keylen The key length in bytes. Read-only.
fastopen.maxkeys The maximum number of keys supported. Read-only,
fastopen.maxpsks The maximum number of pre-shared keys supported.
Read-only.
fastopen.numkeys The current number of keys installed. Read-only.
fastopen.numpsks The current number of pre-shared keys installed.
Read-only.
fastopen.path_disable_time
When a failure occurs while trying to create a new
active (i.e., client) TFO connection, new active
connections on the same path, as determined by the
tuple {client_ip, server_ip, server_port}, will be
forced to be non-TFO for this many seconds. Note
that the path disable mechanism relies on state
stored in client cookie cache entries, so it is
possible for the disable time for a given path to
be reduced if the corresponding client cookie
cache entry is reused due to resource pressure
before the disable period has elapsed. The
default is TCP_FASTOPEN_PATH_DISABLE_TIME_DEFAULT.
fastopen.psk_enable When non-zero, pre-shared key (PSK) mode is
enabled for all TFO servers. On the transition
from enabled to disabled, all installed pre-shared
keys are removed. The default is 0.
fastopen.server_enable
When zero, no new passive (i.e., server) TFO
connections can be created. On the transition
from enabled to disabled, all installed keys and
pre-shared keys are removed. On the transition
from disabled to enabled, if fastopen.autokey is
non-zero and there are no keys installed, a new
key will be generated immediately. The transition
from enabled to disabled does not affect any
passive TFO connections in progress; it only
prevents new ones from being established. The
default is 0.
fastopen.setkey Install a new key by writing
net.inet.tcp.fastopen.keylen bytes to this sysctl.
fastopen.setpsk Install a new pre-shared key by writing
net.inet.tcp.fastopen.keylen bytes to this sysctl.
hostcache.enable The TCP host cache is used to cache connection
details and metrics to improve future performance
of connections between the same hosts. At the
completion of a TCP connection, a host will cache
information for the connection for some defined
period of time.
0 Disable the host cache.
1 Enable the host cache. (default)
hostcache.purgenow Immediately purge all entries once set to any
value. Setting this to 2 will also reseed the
hash salt.
hostcache.purge Expire all entires on next pruning of host cache
entries. Any non-zero setting will be reset to
zero, once the pruge is running.
0 Do not purge all entries when pruning the
host cache. (default)
1 Purge all entries when doing the next
pruning.
2 Purge all entries, and also reseed the
hash salt.
hostcache.prune Time in seconds between pruning expired host cache
entries. Defaults to 300 (5 minutes).
hostcache.expire Time in seconds, how long a entry should be kept
in the host cache since last accessed. Defaults
to 3600 (1 hour).
hostcache.count The current number of entries in the host cache.
hostcache.bucketlimit The maximum number of entries for the same hash.
Defaults to 30.
hostcache.hashsize Size of TCP hostcache hashtable. This number has
to be a power of two, or will be rejected.
Defaults to 512.
hostcache.cachelimit Overall entry limit for hostcache. Defaults to
hashsize * bucketlimit.
hostcache.histo Provide a Histogram of the hostcache hash
utilization.
hostcache.list Provide a complete list of all current entries in
the host cache.
functions_available List of available TCP function blocks (TCP
stacks).
functions_default The default TCP function block (TCP stack).
functions_inherit_listen_socket_stack
Determines whether to inherit listen socket's tcp
stack or use the current system default tcp stack,
as defined by functions_default. Default is true.
insecure_rst Use criteria defined in RFC793 instead of RFC5961
for accepting RST segments. Default is false.
insecure_syn Use criteria defined in RFC793 instead of RFC5961
for accepting SYN segments. Default is false.
ts_offset_per_conn When initializing the TCP timestamps, use a per
connection offset instead of a per host pair
offset. Default is to use per connection offsets
as recommended in RFC 7323.
perconn_stats_enable Controls the default collection of statistics for
all connections using the stats(3) framework. 0
disables, 1 enables, 2 enables random sampling
across log id connection groups with all
connections in a group receiving the same setting.
perconn_stats_sample_rates
A CSV list of template_spec=percent key-value
pairs which controls the per template sampling
rates when stats(3) sampling is enabled.
udp_tunneling_port The local UDP encapsulation port. A value of 0
indicates that UDP encapsulation is disabled. The
default is 0.
udp_tunneling_overhead
The overhead taken into account when using UDP
encapsulation. Since MSS clamping by middleboxes
will most likely not work, values larger than 8
(the size of the UDP header) are also supported.
Supported values are between 8 and 1024. The
default is 8.
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] or [ENOMEM]
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.
[EINVAL] when trying to change TCP function blocks at an
invalid point in the session;
[ENOENT] when trying to use a TCP function block that is not
available;
SEE ALSO
getsockopt(2), socket(2), stats(3), sysctl(3), blackhole(4), inet(4),
intro(4), ip(4), ktls(4), mod_cc(4), siftr(4), syncache(4), tcp_bbr(4),
setkey(8), tcp_functions(9)
V. Jacobson, B. Braden, and D. Borman, TCP Extensions for High
Performance, RFC 1323.
D. Borman, B. Braden, V. Jacobson, and R. Scheffenegger, TCP Extensions
for High Performance, RFC 7323.
A. Heffernan, Protection of BGP Sessions via the TCP MD5 Signature
Option, RFC 2385.
K. Ramakrishnan, S. Floyd, and D. Black, The Addition of Explicit
Congestion Notification (ECN) to IP, RFC 3168.
HISTORY
The TCP protocol appeared in 4.2BSD. The RFC 1323 extensions for window
scaling and timestamps were added in 4.4BSD. The TCP_INFO option was
introduced in Linux 2.6 and is subject to change.
FreeBSD 13.1-RELEASE-p6 January 8, 2022 FreeBSD 13.1-RELEASE-p6
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