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 possession 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 FIB support
TCP sockets are FIB-aware. They inherit the FIB of the
process which created the socket, or that of the
listening socket for sockets created by accept(2). In
particular, the FIB is not inherited from that of the
interface where the initiating SYN packet was received.
When an incoming connection request arrives to a
listening socket, the initial handshake also occurs in
the FIB of the listening socket, not that of the
received packet.
By default, a TCP listening socket can accept
connections originating from any FIB. If the
net.inet.tcp.bind_all_fibs tunable is set to 0, a
listening socket will only accept connections
originating from the FIB's listening socket.
Connection requests from other FIBs will be treated as
though there is no listening socket for the destination
address and port. In this mode, multiple listening
sockets owned by the same user can listen on the same
address and port so long as they belong to different
FIBs, similar to the behavior of the SO_REUSEPORT
socket option. If the tunable is set to 0, all sockets
added to a load-balancing group created with the
SO_REUSEPORT_LB socket option must belong to the same
FIB. MIB (sysctl) Variables section further down. To
list the default TCP stack, see functions_default in
the MIB (sysctl) 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 (sysctl) 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 (sysctl)
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 (sysctl)
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
(sysctl) 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_MAXUNACKTIME This setsockopt(2) option accepts an argument of 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 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.
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
following 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_cubic(4). Other
congestion control algorithms can be made available using the mod_cc(4)
framework.
MIB (sysctl) Variables
The TCP protocol implements a number of variables in the net.inet.tcp
branch of the sysctl(3) MIB, which can also be read or modified with
sysctl(8).
ack_war_timewindow, ack_war_cnt
The challenge ACK throttling algorithm defined in
RFC 5961 limits the number of challenge ACKs sent
per TCP connection to ack_war_cnt during the time
interval specified in milliseconds by
ack_war_timewindow. Setting ack_war_timewindow or
ack_war_cnt to zero disables challenge ACK
throttling.
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.
blackhole If enabled, disable sending of RST when a
connection is attempted to a port where there is
no socket accepting connections. See
blackhole(4).
blackhole_local See blackhole(4).
cc A number of variables for congestion control are
under the net.inet.tcp.cc node. See mod_cc(4).
cc.newreno Variables for NewReno congestion control are under
the net.inet.tcp.cc.newreno node. See
cc_newreno(4).
delacktime Maximum amount of time, in milliseconds, before a
delayed ACK is sent.
delayed_ack Delay ACK to try and piggyback it onto a data
packet or another ACK.
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_tcpdrain Flush packets in the TCP reassembly queue if the
system is low on mbufs.
drop_synfin Drop TCP packets with both SYN and FIN set.
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)
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.
4 Negotiate on incoming connection for
Accurate ECN, ECN, or no ECN. Outgoing
connections will not request ECN.
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.
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.
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 1.
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.
finwait2_timeout Timeout to use for fast recycling of TCP
FIN_WAIT_2 connections (fast_finwait2_recycle).
Defaults to 60 seconds.
functions_available List of available TCP function blocks (TCP
stacks).
functions_default The default TCP function block (TCP stack).
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 hostcache
variables under this node. See hostcache.enable.
hostcache.bucketlimit The maximum number of entries for the same hash.
Defaults to 30.
hostcache.cachelimit Overall entry limit for hostcache. Defaults to
hashsize * bucketlimit.
hostcache.count The current number of entries in the host cache.
hostcache.enable Enable/disable the host cache:
0 Disable the host cache.
1 Enable the host cache. (default)
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.hashsize Size of TCP hostcache hashtable. This number has
to be a power of two, or will be rejected.
Defaults to 512.
hostcache.histo Provide a Histogram of the hostcache hash
utilization.
hostcache.list Provide a complete list of all current entries in
the host cache.
hostcache.prune Time in seconds between pruning expired host cache
entries. Defaults to 300 (5 minutes).
hostcache.purge Expire all entries on next pruning of host cache
entries. Any non-zero setting will be reset to
zero, once the purge 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.purgenow Immediately purge all entries once set to any
value. Setting this to 2 will also reseed the
hash salt.
icmp_may_rst Certain ICMP unreachable messages may abort
connections in SYN-SENT state.
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.
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.
insecure_ack Use criteria defined in RFC793 for validating
SEG.ACK. Default is false.
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.
keepcnt Number of keepalive probes sent, with no response,
before a connection is dropped. The default is 8
packets.
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).
keepinit Timeout, in milliseconds, for new, non-established
TCP connections. The default is 75000 msec (75K
msec, 75 sec).
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 (75K msec, 75 sec).
log_in_vain Log any connection attempts to ports where there
is no socket accepting connections. The value of
1 limits the logging to SYN (connection
establishment) packets only. A value of 2 results
in any TCP packets to closed ports being logged.
Any value not listed above disables the logging
(default is 0, i.e., the logging is disabled).
minmss Minimum TCP Maximum Segment Size; used to prevent
a denial of service attack from an unreasonably
low MSS.
msl The Maximum Segment Lifetime, in milliseconds, for
a packet.
msl_local The Maximum Segment Lifetime, in milliseconds, for
a packet when both endpoints are local. msl_local
is only used if nolocaltimewait, which is
deprecated, is zero.
mssdflt The default value used for the TCP Maximum Segment
Size ("MSS") for IPv4 when no advice to the
contrary is received from MSS negotiation.
newcwv 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.
nolocaltimewait Suppress the creation of TCP TIME_WAIT states for
connections in which both endpoints are local.
The default is 0. nolocaltimewait is deprecated
and will be removed in FreeBSD 16. msl_local can
be used instead.
path_mtu_discovery Enable Path MTU Discovery.
pcbcount Number of active protocol control blocks (read-
only).
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.
persmax Maximum persistence interval, msec.
persmin Minimum persistence interval, msec.
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.
reass.cursegments The current total number of segments present in
all reassembly queues.
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.
reass.maxsegments The maximum limit on the total number of segments
across all reassembly queues. The limit can be
adjusted as a tunable.
recvbuf_auto Enable automatic receive buffer sizing as a
connection progresses.
recvbuf_max Maximum size of automatic receive buffer.
recvspace Initial TCP receive window (buffer size).
retries Maximum number of consecutive timer based
retransmits sent after a data segment is lost
(default and maximum is 12).
rexmit_drop_options Drop TCP options from third and later
retransmitted SYN segments of a connection.
rexmit_initial, rexmit_min, rexmit_slop, rexmit_max
Adjust the retransmit timer calculation for TCP.
A new connection starts with timer set to
rexmit_initial. The rexmit_slop 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. The rexmit_min
and rexmit_max set minimum and maximum timer
values that a connection may have.
rfc1323 Implement the window scaling and timestamp options
of RFC 1323/RFC 7323 (default is 1). Settings:
0 Disable window scaling and timestamp
option.
1 Enable window scaling and timestamp
option.
2 Enable only window scaling.
3 Enable only timestamp option.
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.
rfc6191 Enable RFC 6191 connection recycling, which allows
faster connection recycling in certain
circumstances when the new connection has TCP
timestamps enabled.
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.globalholes Global number of TCP SACK holes currently
allocated.
sack.globalmaxholes Maximum number of SACK holes per system, across
all connections. Defaults to 65536.
sack.lrd Enable Lost Retransmission Detection for SACK-
enabled sessions, enabled 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 or purge of the SACK
scoreboard.
sack.maxholes Maximum number of SACK holes per connection.
Defaults to 128.
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. sack.revised is
deprecated and will be removed in FreeBSD 16.
sack.enable will always follow RFC6675.
sendbuf_auto Enable automatic send buffer sizing.
sendbuf_auto_lowat Modify threshold for auto send buffer growth to
account for SO_SNDLOWAT.
sendbuf_inc Incrementor step size of automatic send buffer.
sendbuf_max Maximum size of automatic send buffer.
sendspace Initial TCP send window (buffer size).
syncache Variables under the net.inet.tcp.syncache node are
documented in syncache(4).
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).)
syncookies_only See syncookies(4).
tcbhashsize Size of the TCP control-block hash table (read-
only). This is tuned using the kernel option
TCBHASHSIZE or by setting net.inet.tcp.tcbhashsize
in the loader(8).
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.
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.
tso Enable TCP Segmentation Offload.
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.
udp_tunneling_port The local UDP encapsulation port. A value of 0
indicates that UDP encapsulation is disabled. The
default is 0.
v6mssdflt The default value used for the TCP Maximum Segment
Size ("MSS") for IPv6 when no advice to the
contrary is received from MSS negotiation.
v6pmtud_blackhole_mss MSS to try for IPv6 if PMTU blackhole detection is
turned on. See pmtud_blackhole_detection.
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), setfib(2), socket(2), stats(3), sysctl(3), blackhole(4),
dtrace_mib(4), inet(4), intro(4), ip(4), ktls(4), mod_cc(4), siftr(4),
syncache(4), tcp_bbr(4), tcp_rack(4), setkey(8), sysctl(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.
A. Ramaiah, R. Stewart, and M. Dalal, Improving TCP's Robustness to Blind
In-Window Attacks, RFC 5961.
F. Gont, Reducing the TIME-WAIT State Using TCP Timestamps, RFC 6191.
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 15.1-STABLE-HBSD June 17, 2026 TCP(4)