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.\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd February 3, 2023
.Dt TCP 4
.Os
.Sh NAME
.Nm tcp
.Nd Internet Transmission Control Protocol
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netinet/in.h
.In netinet/tcp.h
.Ft int
.Fn socket AF_INET SOCK_STREAM 0
.Sh DESCRIPTION
The
.Tn TCP
protocol provides reliable, flow-controlled, two-way
transmission of data.
It is a byte-stream protocol used to
support the
.Dv SOCK_STREAM
abstraction.
.Tn TCP
uses the standard
Internet address format and, in addition, provides a per-host
collection of
.Dq "port addresses" .
Thus, each address is composed
of an Internet address specifying the host and network,
with a specific
.Tn TCP
port on the host identifying the peer entity.
.Pp
Sockets utilizing the
.Tn TCP
protocol are either
.Dq active
or
.Dq passive .
Active sockets initiate connections to passive
sockets.
By default,
.Tn TCP
sockets are created active; to create a
passive socket, the
.Xr listen 2
system call must be used
after binding the socket with the
.Xr bind 2
system call.
Only passive sockets may use the
.Xr accept 2
call to accept incoming connections.
Only active sockets may use the
.Xr connect 2
call to initiate connections.
.Pp
Passive sockets may
.Dq underspecify
their location to match
incoming connection requests from multiple networks.
This technique, termed
.Dq "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
.Dv INADDR_ANY
must be bound.
The
.Tn 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.
.Pp
.Tn TCP
supports a number of socket options which can be set with
.Xr setsockopt 2
and tested with
.Xr getsockopt 2 :
.Bl -tag -width ".Dv TCP_FUNCTION_BLK"
.It Dv TCP_INFO
Information about a socket's underlying TCP session may be retrieved
by passing the read-only option
.Dv TCP_INFO
to
.Xr getsockopt 2 .
It accepts a single argument: a pointer to an instance of
.Vt "struct tcp_info" .
.Pp
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.
.It Dv TCP_CCALGOOPT
Set or query congestion control algorithm specific parameters.
See
.Xr mod_cc 4
for details.
.It Dv TCP_CONGESTION
Select or query the congestion control algorithm that TCP will use for the
connection.
See
.Xr mod_cc 4
for details.
.It Dv TCP_FASTOPEN
Enable or disable TCP Fast Open (TFO).
To use this option, the kernel must be built with the
.Dv TCP_RFC7413
option.
.Pp
This option can be set on the socket either before or after the
.Xr 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.
.Pp
For passively-created sockets, the
.Dv 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
.Tn SYN ,
but that fall back to using a non-TFO
.Tn SYN|ACK
will have the
.Dv TCP_FASTOPEN
socket option set.
.Pp
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.
.Pp
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
.Bd -literal -offset left
SipHash24(key=\fI16-byte-psk\fP, msg=\fIcookie-sent-to-client\fP)
.Ed
.Pp
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.
.Pp
This can be adjusted with the
.Dv TCP_RFC7413_MAX_PSKS
kernel option.
.It Dv 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
.Va functions_available
in the
.Sx MIB (sysctl) Variables
section further down.
To list the default TCP stack, see
.Va functions_default
in the
.Sx MIB (sysctl) Variables
section.
.It Dv TCP_KEEPINIT
This
.Xr setsockopt 2
option accepts a per-socket timeout argument of
.Vt "u_int"
in seconds, for new, non-established
.Tn TCP
connections.
For the global default in milliseconds see
.Va keepinit
in the
.Sx MIB (sysctl) Variables
section further down.
.It Dv TCP_KEEPIDLE
This
.Xr setsockopt 2
option accepts an argument of
.Vt "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
.Xr accept 2 .
For the global default in milliseconds see
.Va keepidle
in the
.Sx MIB (sysctl) Variables
section further down.
.It Dv TCP_KEEPINTVL
This
.Xr setsockopt 2
option accepts an argument of
.Vt "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
.Xr accept 2 .
For the global default in milliseconds see
.Va keepintvl
in the
.Sx MIB (sysctl) Variables
section further down.
.It Dv TCP_KEEPCNT
This
.Xr setsockopt 2
option accepts an argument of
.Vt "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
.Xr accept 2 .
For the global default see the
.Va keepcnt
in the
.Sx MIB (sysctl) Variables
section further down.
.It Dv TCP_NODELAY
Under most circumstances,
.Tn 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
.Dv TCP_NODELAY
defeats this algorithm.

.It Dv TCP_MAXSEG
By default, a sender- and
.No receiver- Ns Tn TCP
will negotiate among themselves to determine the maximum segment size
to be used for each connection.
The
.Dv TCP_MAXSEG
option allows the user to determine the result of this negotiation,
and to reduce it if desired.
.It Dv TCP_MAXUNACKTIME
This
.Xr setsockopt 2
option accepts an argument of
.Vt "u_int"
to set the per-socket interval, in seconds, in which the connection must
make progress. Progress is defined by at least 1 byte being acknowledged within
the set time period. If a connection fails to make progress, then the
.Tn TCP
stack will terminate the connection with a reset. Note that the default
value for this is zero which indicates no progress checks should be made.
.It Dv TCP_NOOPT
.Tn TCP
usually sends a number of options in each packet, corresponding to
various
.Tn TCP
extensions which are provided in this implementation.
The boolean option
.Dv TCP_NOOPT
is provided to disable
.Tn TCP
option use on a per-connection basis.
.It Dv TCP_NOPUSH
By convention, the
.No sender- Ns Tn TCP
will set the
.Dq push
bit, and begin transmission immediately (if permitted) at the end of
every user call to
.Xr write 2
or
.Xr writev 2 .
When this option is set to a non-zero value,
.Tn TCP
will delay sending any data at all until either the socket is closed,
or the internal send buffer is filled.
.It Dv 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.
.Pp
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.
.Pp
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
.Xr setkey 8
utility.
This entry can only be specified on a per-host basis at this time.
.Pp
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.
.It Dv TCP_STATS
Manage collection of connection level statistics using the
.Xr stats 3
framework.
.Pp
Each dropped segment is taken into account in the TCP protocol statistics.
.It Dv TCP_TXTLS_ENABLE
Enable in-kernel Transport Layer Security (TLS) for data written to this
socket.
See
.Xr ktls 4
for more details.
.It Dv TCP_TXTLS_MODE
The integer argument can be used to get or set the current TLS transmit mode
of a socket.
See
.Xr ktls 4
for more details.
.It Dv TCP_RXTLS_ENABLE
Enable in-kernel TLS for data read from this socket.
See
.Xr ktls 4
for more details.
.It Dv 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:
.Bl -tag -width "Dv TCP_REUSPORT_LB_NUMA"
.It Dv TCP_REUSPORT_LB_NUMA_NODOM
Remove NUMA filtering for this listen socket.
.It Dv 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.
.El
.It Dv TCP_REMOTE_UDP_ENCAPS_PORT
Set and get the remote UDP encapsulation port.
It can only be set on a closed TCP socket.
.El
.Pp
The option level for the
.Xr setsockopt 2
call is the protocol number for
.Tn TCP ,
available from
.Xr getprotobyname 3 ,
or
.Dv IPPROTO_TCP .
All options are declared in
.In netinet/tcp.h .
.Pp
Options at the
.Tn IP
transport level may be used with
.Tn TCP ;
see
.Xr ip 4 .
Incoming connection requests that are source-routed are noted,
and the reverse source route is used in responding.
.Pp
The default congestion control algorithm for
.Tn TCP
is
.Xr cc_newreno 4 .
Other congestion control algorithms can be made available using the
.Xr mod_cc 4
framework.
.Ss MIB (sysctl) Variables
The
.Tn TCP
protocol implements a number of variables in the
.Va net.inet.tcp
branch of the
.Xr sysctl 3
MIB, which can also be read or modified with
.Xr sysctl 8 .
.Bl -tag -width ".Va v6pmtud_blackhole_mss"
.It Va always_keepalive
Assume that
.Dv SO_KEEPALIVE
is set on all
.Tn TCP
connections, the kernel will
periodically send a packet to the remote host to verify the connection
is still up.
.It Va blackhole
If enabled, disable sending of RST when a connection is attempted
to a port where there is no socket accepting connections.
See
.Xr blackhole 4 .
.It Va blackhole_local
See
.Xr blackhole 4 .
.It Va cc
A number of variables for congestion control are under the
.Va net.inet.tcp.cc
node.
See
.Xr mod_cc 4 .
.It Va cc.newreno
Variables for NewReno congestion control are under the
.Va net.inet.tcp.cc.newreno
node.
See
.Xr cc_newreno 4 .
.It Va delacktime
Maximum amount of time, in milliseconds, before a delayed ACK is sent.
.It Va delayed_ack
Delay ACK to try and piggyback it onto a data packet or another ACK.
.It Va do_lrd
Enable Lost Retransmission Detection for SACK-enabled sessions, disabled by
default.
Under severe congestion, a retransmission can be lost which then leads to a
mandatory Retransmission Timeout (RTO), followed by slow-start.
LRD will try to resend the repeatedly lost packet, preventing the time-consuming
RTO and performance reducing slow-start.
.It Va 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).
.It Va 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).
.It Va do_tcpdrain
Flush packets in the
.Tn TCP
reassembly queue if the system is low on mbufs.
.It Va drop_synfin
Drop TCP packets with both SYN and FIN set.
.It Va 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.
.Bl -tag -compact
.It 0
Disable ECN.
.It 1
Allow incoming connections to request ECN.
Outgoing connections will request ECN.
.It 2
Allow incoming connections to request ECN.
Outgoing connections will not request ECN.
(default)
.It 3
Negotiate on incoming connection for Accurate ECN, ECN, or no ECN.
Outgoing connections will request Accurate ECN and fall back to
ECN depending on the capabilities of the server.
.It 4
Negotiate on incoming connection for Accurate ECN, ECN, or no ECN.
Outgoing connections will not request ECN.
.El
.It Va 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.
.It Va fast_finwait2_recycle
Recycle
.Tn TCP
.Dv FIN_WAIT_2
connections faster when the socket is marked as
.Dv SBS_CANTRCVMORE
(no user process has the socket open, data received on
the socket cannot be read).
The timeout used here is
.Va finwait2_timeout .
.It Va fastopen.acceptany
When non-zero, all client-supplied TFO cookies will be considered to be valid.
The default is 0.
.It Va fastopen.autokey
When this and
.Va net.inet.tcp.fastopen.server_enable
are non-zero, a new key will be automatically generated after this specified
seconds.
The default is 120.
.It Va fastopen.ccache_bucket_limit
The maximum number of entries in a client cookie cache bucket.
The default value can be tuned with the
.Dv TCP_FASTOPEN_CCACHE_BUCKET_LIMIT_DEFAULT
kernel option or by setting
.Va net.inet.tcp.fastopen_ccache_bucket_limit
in the
.Xr loader 8 .
.It Va fastopen.ccache_buckets
The number of client cookie cache buckets.
Read-only.
The value can be tuned with the
.Dv TCP_FASTOPEN_CCACHE_BUCKETS_DEFAULT
kernel option or by setting
.Va fastopen.ccache_buckets
in the
.Xr loader 8 .
.It Va fastopen.ccache_list
Print the client cookie cache.
Read-only.
.It Va 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.
.It Va fastopen.keylen
The key length in bytes.
Read-only.
.It Va fastopen.maxkeys
The maximum number of keys supported.
Read-only,
.It Va fastopen.maxpsks
The maximum number of pre-shared keys supported.
Read-only.
.It Va fastopen.numkeys
The current number of keys installed.
Read-only.
.It Va fastopen.numpsks
The current number of pre-shared keys installed.
Read-only.
.It Va 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
.Brq 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
.Dv TCP_FASTOPEN_PATH_DISABLE_TIME_DEFAULT .
.It Va 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.
.It Va 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
.Va 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.
.It Va fastopen.setkey
Install a new key by writing
.Va net.inet.tcp.fastopen.keylen
bytes to this sysctl.
.It Va fastopen.setpsk
Install a new pre-shared key by writing
.Va net.inet.tcp.fastopen.keylen
bytes to this sysctl.
.It Va finwait2_timeout
Timeout to use for fast recycling of
.Tn TCP
.Dv FIN_WAIT_2
connections
.Pq Va fast_finwait2_recycle .
Defaults to 60 seconds.
.It Va functions_available
List of available TCP function blocks (TCP stacks).
.It Va functions_default
The default TCP function block (TCP stack).
.It Va 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
.Va functions_default .
Default is true.
.It Va hostcache
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.
There are a number of
.Va hostcache
variables under this node.
See
.Va hostcache.enable .
.It Va hostcache.bucketlimit
The maximum number of entries for the same hash.
Defaults to 30.
.It Va hostcache.cachelimit
Overall entry limit for hostcache.
Defaults to
.Va hashsize
*
.Va bucketlimit .
.It Va hostcache.count
The current number of entries in the host cache.
.It Va hostcache.enable
Enable/disable the host cache:
.Bl -tag -compact
.It 0
Disable the host cache.
.It 1
Enable the host cache. (default)
.El
.It Va hostcache.expire
Time in seconds, how long a entry should be kept in the
host cache since last accessed.
Defaults to 3600 (1 hour).
.It Va hostcache.hashsize
Size of TCP hostcache hashtable.
This number has to be a power of two, or will be rejected.
Defaults to 512.
.It Va hostcache.histo
Provide a Histogram of the hostcache hash utilization.
.It Va hostcache.list
Provide a complete list of all current entries in the host
cache.
.It Va hostcache.prune
Time in seconds between pruning expired host cache entries.
Defaults to 300 (5 minutes).
.It Va hostcache.purge
Expire all entires on next pruning of host cache entries.
Any non-zero setting will be reset to zero, once the purge
is running.
.Bl -tag -compact
.It 0
Do not purge all entries when pruning the host cache (default).
.It 1
Purge all entries when doing the next pruning.
.It 2
Purge all entries and also reseed the hash salt.
.El
.It Va hostcache.purgenow
Immediately purge all entries once set to any value.
Setting this to 2 will also reseed the hash salt.
.It Va icmp_may_rst
Certain
.Tn ICMP
unreachable messages may abort connections in
.Tn SYN-SENT
state.
.It Va 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 the 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.
.It Va insecure_rst
Use criteria defined in RFC793 instead of RFC5961 for accepting RST segments.
Default is false.
.It Va insecure_syn
Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments.
Default is false.
.It Va 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
.Dv TIME_WAIT
recycling for a few minutes.
.It Va keepcnt
Number of keepalive probes sent, with no response, before a connection
is dropped.
The default is 8 packets.
.It Va keepidle
Amount of time, in milliseconds, that the connection must be idle
before sending keepalive probes (if enabled).
The default is 7200000 msec (7.2M msec, 2 hours).
.It Va keepinit
Timeout, in milliseconds, for new, non-established
.Tn TCP
connections.
The default is 75000 msec (75K msec, 75 sec).
.It Va keepintvl
The interval, in milliseconds, between keepalive probes sent to remote
machines, when no response is received on a
.Va keepidle
probe.
The default is 75000 msec (75K msec, 75 sec).
.It Va log_in_vain
Log any connection attempts to ports where there is no socket
accepting connections.
The value of 1 limits the logging to
.Tn SYN
(connection establishment) packets only.
A value of 2 results in any
.Tn TCP
packets to closed ports being logged.
Any value not listed above disables the logging
(default is 0, i.e., the logging is disabled).
.It Va minmss
Minimum TCP Maximum Segment Size; used to prevent a denial of service attack
from an unreasonably low MSS.
.It Va msl
The Maximum Segment Lifetime, in milliseconds, for a packet.
.It Va mssdflt
The default value used for the TCP Maximum Segment Size
.Pq Dq MSS
for IPv4 when no advice to the contrary is received from MSS negotiation.
.It Va 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.
.It Va nolocaltimewait
Suppress creation of TCP
.Dv TIME_WAIT
states for connections in
which both endpoints are local.
.It Va path_mtu_discovery
Enable Path MTU Discovery.
.It Va pcbcount
Number of active process control blocks
(read-only).
.It Va perconn_stats_enable
Controls the default collection of statistics for all connections using the
.Xr 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.
.It Va perconn_stats_sample_rates
A CSV list of template_spec=percent key-value pairs which controls the per
template sampling rates when
.Xr stats 3
sampling is enabled.
.It Va persmax
Maximum persistence interval, msec.
.It Va persmin
Minimum persistence interval, msec.
.It Va 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
.Po Va net.inet.tcp.pmtud_blackhole_mss
and
.Va net.inet.tcp.v6pmtud_blackhole_mss
.Pc ,
it will be set to this value, otherwise,
the MSS will be set to the default values
.Po Va net.inet.tcp.mssdflt
and
.Va net.inet.tcp.v6mssdflt
.Pc .
Settings:
.Bl -tag -compact
.It 0
Disable path MTU blackhole detection.
.It 1
Enable path MTU blackhole detection for IPv4 and IPv6.
.It 2
Enable path MTU blackhole detection only for IPv4.
.It 3
Enable path MTU blackhole detection only for IPv6.
.El
.It Va pmtud_blackhole_mss
MSS to try for IPv4 if PMTU blackhole detection is turned on.
.It Va reass.cursegments
The current total number of segments present in all reassembly queues.
.It Va 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
.Va reass.maxqueuelen
limit.
.It Va reass.maxsegments
The maximum limit on the total number of segments across all reassembly
queues.
The limit can be adjusted as a tunable.
.It Va recvbuf_auto
Enable automatic receive buffer sizing as a connection progresses.
.It Va recvbuf_max
Maximum size of automatic receive buffer.
.It Va recvspace
Initial
.Tn TCP
receive window (buffer size).
.It Va rexmit_drop_options
Drop TCP options from third and later retransmitted SYN segments
of a connection.
.It Va rexmit_initial , rexmit_min , rexmit_slop
Adjust the retransmit timer calculation for
.Tn TCP .
The slop is
typically added to the raw calculation to take into account
occasional variances that the
.Tn SRTT
(smoothed round-trip time)
is unable to accommodate, while the minimum specifies an
absolute minimum.
While a number of
.Tn 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
.Tn Linux ) .
The initial value is used before an RTT measurement has been performed.
.It Va rfc1323
Implement the window scaling and timestamp options of RFC 1323/RFC 7323
(default is 1).
Settings:
.Bl -tag -compact
.It 0
Disable window scaling and timestamp option.
.It 1
Enable window scaling and timestamp option.
.It 2
Enable only window scaling.
.It 3
Enable only timestamp option.
.El
.It Va 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.
.It Va 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.
.It Va rfc6675_pipe
Deprecated and superseded by
.Va sack.revised
.It Va 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.
.It Va sack.globalholes
Global number of TCP SACK holes currently allocated.
.It Va sack.globalmaxholes
Maximum number of SACK holes per system, across all connections.
Defaults to 65536.
.It Va sack.maxholes
Maximum number of SACK holes per connection.
Defaults to 128.
.It Va sack.revised
Enables three updated mechanisms from RFC6675 (default is true).
Calculate the bytes in flight using the algorithm described in RFC 6675, and
is also an improvement when Proportional Rate Reduction is enabled.
Next, 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.
Finally, SACK loss recovery is also engaged, once two segments plus one byte are
SACKed - even if no traditional duplicate ACKs were observed.
.It Va sendbuf_auto
Enable automatic send buffer sizing.
.It Va sendbuf_auto_lowat
Modify threshold for auto send buffer growth to account for
.Dv SO_SNDLOWAT .
.It Va sendbuf_inc
Incrementor step size of automatic send buffer.
.It Va sendbuf_max
Maximum size of automatic send buffer.
.It Va sendspace
Initial
.Tn TCP
send window (buffer size).
.It Va syncache
Variables under the
.Va net.inet.tcp.syncache
node are documented in
.Xr syncache 4 .
.It Va syncookies
Determines whether or not
.Tn SYN
cookies should be generated for outbound
.Tn SYN-ACK
packets.
.Tn SYN
cookies are a great help during
.Tn SYN
flood attacks, and are enabled by default.
(See
.Xr syncookies 4 . )
.It Va syncookies_only
See
.Xr syncookies 4 .
.It Va tcbhashsize
Size of the
.Tn TCP
control-block hash table
(read-only).
This is tuned using the kernel option
.Dv TCBHASHSIZE
or by setting
.Va net.inet.tcp.tcbhashsize
in the
.Xr loader 8 .
.It Va tolerate_missing_ts
Tolerate the missing of timestamps (RFC 1323/RFC 7323) for
.Tn TCP
segments belonging to
.Tn TCP
connections for which support of
.Tn 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.
.It Va 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.
.It Va tso
Enable TCP Segmentation Offload.
.It Va 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.
.It Va udp_tunneling_port
The local UDP encapsulation port.
A value of 0 indicates that UDP encapsulation is disabled.
The default is 0.
.It Va v6mssdflt
The default value used for the TCP Maximum Segment Size
.Pq Dq MSS
for IPv6 when no advice to the contrary is received from MSS negotiation.
.It Va v6pmtud_blackhole_mss
MSS to try for IPv6 if PMTU blackhole detection is turned on.
See
.Va pmtud_blackhole_detection .
.El
.Sh ERRORS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width Er
.It Bq Er EISCONN
when trying to establish a connection on a socket which
already has one;
.It Bo Er ENOBUFS Bc or Bo Er ENOMEM Bc
when the system runs out of memory for
an internal data structure;
.It Bq Er ETIMEDOUT
when a connection was dropped
due to excessive retransmissions;
.It Bq Er ECONNRESET
when the remote peer
forces the connection to be closed;
.It Bq Er ECONNREFUSED
when the remote
peer actively refuses connection establishment (usually because
no process is listening to the port);
.It Bq Er EADDRINUSE
when an attempt
is made to create a socket with a port which has already been
allocated;
.It Bq Er EADDRNOTAVAIL
when an attempt is made to create a
socket with a network address for which no network interface
exists;
.It Bq Er EAFNOSUPPORT
when an attempt is made to bind or connect a socket to a multicast
address.
.It Bq Er EINVAL
when trying to change TCP function blocks at an invalid point in the session;
.It Bq Er ENOENT
when trying to use a TCP function block that is not available;
.El
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr socket 2 ,
.Xr stats 3 ,
.Xr sysctl 3 ,
.Xr blackhole 4 ,
.Xr inet 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr ktls 4 ,
.Xr mod_cc 4 ,
.Xr siftr 4 ,
.Xr syncache 4 ,
.Xr tcp_bbr 4 ,
.Xr tcp_rack 4 ,
.Xr setkey 8 ,
.Xr sysctl 8 ,
.Xr tcp_functions 9
.Rs
.%A "V. Jacobson"
.%A "B. Braden"
.%A "D. Borman"
.%T "TCP Extensions for High Performance"
.%O "RFC 1323"
.Re
.Rs
.%A "D. Borman"
.%A "B. Braden"
.%A "V. Jacobson"
.%A "R. Scheffenegger"
.%T "TCP Extensions for High Performance"
.%O "RFC 7323"
.Re
.Rs
.%A "A. Heffernan"
.%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
.%O "RFC 2385"
.Re
.Rs
.%A "K. Ramakrishnan"
.%A "S. Floyd"
.%A "D. Black"
.%T "The Addition of Explicit Congestion Notification (ECN) to IP"
.%O "RFC 3168"
.Re
.Sh HISTORY
The
.Tn TCP
protocol appeared in
.Bx 4.2 .
The RFC 1323 extensions for window scaling and timestamps were added
in
.Bx 4.4 .
The
.Dv TCP_INFO
option was introduced in
.Tn Linux 2.6
and is
.Em subject to change .