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TCP(4) |
FreeBSD Kernel Interfaces Manual |
TCP(4) |
tcp —
Internet Transmission Control Protocol
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.
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.
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;
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.
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.
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