Linux lhjmq-records 5.15.0-118-generic #128-Ubuntu SMP Fri Jul 5 09:28:59 UTC 2024 x86_64
Your IP : 52.14.7.103
# SPDX-License-Identifier: GPL-2.0-only
#
# IP configuration
#
config IP_MULTICAST
bool "IP: multicasting"
help
This is code for addressing several networked computers at once,
enlarging your kernel by about 2 KB. You need multicasting if you
intend to participate in the MBONE, a high bandwidth network on top
of the Internet which carries audio and video broadcasts. More
information about the MBONE is on the WWW at
<https://www.savetz.com/mbone/>. For most people, it's safe to say N.
config IP_ADVANCED_ROUTER
bool "IP: advanced router"
help
If you intend to run your Linux box mostly as a router, i.e. as a
computer that forwards and redistributes network packets, say Y; you
will then be presented with several options that allow more precise
control about the routing process.
The answer to this question won't directly affect the kernel:
answering N will just cause the configurator to skip all the
questions about advanced routing.
Note that your box can only act as a router if you enable IP
forwarding in your kernel; you can do that by saying Y to "/proc
file system support" and "Sysctl support" below and executing the
line
echo "1" > /proc/sys/net/ipv4/ip_forward
at boot time after the /proc file system has been mounted.
If you turn on IP forwarding, you should consider the rp_filter, which
automatically rejects incoming packets if the routing table entry
for their source address doesn't match the network interface they're
arriving on. This has security advantages because it prevents the
so-called IP spoofing, however it can pose problems if you use
asymmetric routing (packets from you to a host take a different path
than packets from that host to you) or if you operate a non-routing
host which has several IP addresses on different interfaces. To turn
rp_filter on use:
echo 1 > /proc/sys/net/ipv4/conf/<device>/rp_filter
or
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
Note that some distributions enable it in startup scripts.
For details about rp_filter strict and loose mode read
<file:Documentation/networking/ip-sysctl.rst>.
If unsure, say N here.
config IP_FIB_TRIE_STATS
bool "FIB TRIE statistics"
depends on IP_ADVANCED_ROUTER
help
Keep track of statistics on structure of FIB TRIE table.
Useful for testing and measuring TRIE performance.
config IP_MULTIPLE_TABLES
bool "IP: policy routing"
depends on IP_ADVANCED_ROUTER
select FIB_RULES
help
Normally, a router decides what to do with a received packet based
solely on the packet's final destination address. If you say Y here,
the Linux router will also be able to take the packet's source
address into account. Furthermore, the TOS (Type-Of-Service) field
of the packet can be used for routing decisions as well.
If you need more information, see the Linux Advanced
Routing and Traffic Control documentation at
<https://lartc.org/howto/lartc.rpdb.html>
If unsure, say N.
config IP_ROUTE_MULTIPATH
bool "IP: equal cost multipath"
depends on IP_ADVANCED_ROUTER
help
Normally, the routing tables specify a single action to be taken in
a deterministic manner for a given packet. If you say Y here
however, it becomes possible to attach several actions to a packet
pattern, in effect specifying several alternative paths to travel
for those packets. The router considers all these paths to be of
equal "cost" and chooses one of them in a non-deterministic fashion
if a matching packet arrives.
config IP_ROUTE_VERBOSE
bool "IP: verbose route monitoring"
depends on IP_ADVANCED_ROUTER
help
If you say Y here, which is recommended, then the kernel will print
verbose messages regarding the routing, for example warnings about
received packets which look strange and could be evidence of an
attack or a misconfigured system somewhere. The information is
handled by the klogd daemon which is responsible for kernel messages
("man klogd").
config IP_ROUTE_CLASSID
bool
config IP_PNP
bool "IP: kernel level autoconfiguration"
help
This enables automatic configuration of IP addresses of devices and
of the routing table during kernel boot, based on either information
supplied on the kernel command line or by BOOTP or RARP protocols.
You need to say Y only for diskless machines requiring network
access to boot (in which case you want to say Y to "Root file system
on NFS" as well), because all other machines configure the network
in their startup scripts.
config IP_PNP_DHCP
bool "IP: DHCP support"
depends on IP_PNP
help
If you want your Linux box to mount its whole root file system (the
one containing the directory /) from some other computer over the
net via NFS and you want the IP address of your computer to be
discovered automatically at boot time using the DHCP protocol (a
special protocol designed for doing this job), say Y here. In case
the boot ROM of your network card was designed for booting Linux and
does DHCP itself, providing all necessary information on the kernel
command line, you can say N here.
If unsure, say Y. Note that if you want to use DHCP, a DHCP server
must be operating on your network. Read
<file:Documentation/admin-guide/nfs/nfsroot.rst> for details.
config IP_PNP_BOOTP
bool "IP: BOOTP support"
depends on IP_PNP
help
If you want your Linux box to mount its whole root file system (the
one containing the directory /) from some other computer over the
net via NFS and you want the IP address of your computer to be
discovered automatically at boot time using the BOOTP protocol (a
special protocol designed for doing this job), say Y here. In case
the boot ROM of your network card was designed for booting Linux and
does BOOTP itself, providing all necessary information on the kernel
command line, you can say N here. If unsure, say Y. Note that if you
want to use BOOTP, a BOOTP server must be operating on your network.
Read <file:Documentation/admin-guide/nfs/nfsroot.rst> for details.
config IP_PNP_RARP
bool "IP: RARP support"
depends on IP_PNP
help
If you want your Linux box to mount its whole root file system (the
one containing the directory /) from some other computer over the
net via NFS and you want the IP address of your computer to be
discovered automatically at boot time using the RARP protocol (an
older protocol which is being obsoleted by BOOTP and DHCP), say Y
here. Note that if you want to use RARP, a RARP server must be
operating on your network. Read
<file:Documentation/admin-guide/nfs/nfsroot.rst> for details.
config NET_IPIP
tristate "IP: tunneling"
select INET_TUNNEL
select NET_IP_TUNNEL
help
Tunneling means encapsulating data of one protocol type within
another protocol and sending it over a channel that understands the
encapsulating protocol. This particular tunneling driver implements
encapsulation of IP within IP, which sounds kind of pointless, but
can be useful if you want to make your (or some other) machine
appear on a different network than it physically is, or to use
mobile-IP facilities (allowing laptops to seamlessly move between
networks without changing their IP addresses).
Saying Y to this option will produce two modules ( = code which can
be inserted in and removed from the running kernel whenever you
want). Most people won't need this and can say N.
config NET_IPGRE_DEMUX
tristate "IP: GRE demultiplexer"
help
This is helper module to demultiplex GRE packets on GRE version field criteria.
Required by ip_gre and pptp modules.
config NET_IP_TUNNEL
tristate
select DST_CACHE
select GRO_CELLS
default n
config NET_IPGRE
tristate "IP: GRE tunnels over IP"
depends on (IPV6 || IPV6=n) && NET_IPGRE_DEMUX
select NET_IP_TUNNEL
help
Tunneling means encapsulating data of one protocol type within
another protocol and sending it over a channel that understands the
encapsulating protocol. This particular tunneling driver implements
GRE (Generic Routing Encapsulation) and at this time allows
encapsulating of IPv4 or IPv6 over existing IPv4 infrastructure.
This driver is useful if the other endpoint is a Cisco router: Cisco
likes GRE much better than the other Linux tunneling driver ("IP
tunneling" above). In addition, GRE allows multicast redistribution
through the tunnel.
config NET_IPGRE_BROADCAST
bool "IP: broadcast GRE over IP"
depends on IP_MULTICAST && NET_IPGRE
help
One application of GRE/IP is to construct a broadcast WAN (Wide Area
Network), which looks like a normal Ethernet LAN (Local Area
Network), but can be distributed all over the Internet. If you want
to do that, say Y here and to "IP multicast routing" below.
config IP_MROUTE_COMMON
bool
depends on IP_MROUTE || IPV6_MROUTE
config IP_MROUTE
bool "IP: multicast routing"
depends on IP_MULTICAST
select IP_MROUTE_COMMON
help
This is used if you want your machine to act as a router for IP
packets that have several destination addresses. It is needed on the
MBONE, a high bandwidth network on top of the Internet which carries
audio and video broadcasts. In order to do that, you would most
likely run the program mrouted. If you haven't heard about it, you
don't need it.
config IP_MROUTE_MULTIPLE_TABLES
bool "IP: multicast policy routing"
depends on IP_MROUTE && IP_ADVANCED_ROUTER
select FIB_RULES
help
Normally, a multicast router runs a userspace daemon and decides
what to do with a multicast packet based on the source and
destination addresses. If you say Y here, the multicast router
will also be able to take interfaces and packet marks into
account and run multiple instances of userspace daemons
simultaneously, each one handling a single table.
If unsure, say N.
config IP_PIMSM_V1
bool "IP: PIM-SM version 1 support"
depends on IP_MROUTE
help
Kernel side support for Sparse Mode PIM (Protocol Independent
Multicast) version 1. This multicast routing protocol is used widely
because Cisco supports it. You need special software to use it
(pimd-v1). Please see <http://netweb.usc.edu/pim/> for more
information about PIM.
Say Y if you want to use PIM-SM v1. Note that you can say N here if
you just want to use Dense Mode PIM.
config IP_PIMSM_V2
bool "IP: PIM-SM version 2 support"
depends on IP_MROUTE
help
Kernel side support for Sparse Mode PIM version 2. In order to use
this, you need an experimental routing daemon supporting it (pimd or
gated-5). This routing protocol is not used widely, so say N unless
you want to play with it.
config SYN_COOKIES
bool "IP: TCP syncookie support"
help
Normal TCP/IP networking is open to an attack known as "SYN
flooding". This denial-of-service attack prevents legitimate remote
users from being able to connect to your computer during an ongoing
attack and requires very little work from the attacker, who can
operate from anywhere on the Internet.
SYN cookies provide protection against this type of attack. If you
say Y here, the TCP/IP stack will use a cryptographic challenge
protocol known as "SYN cookies" to enable legitimate users to
continue to connect, even when your machine is under attack. There
is no need for the legitimate users to change their TCP/IP software;
SYN cookies work transparently to them. For technical information
about SYN cookies, check out <https://cr.yp.to/syncookies.html>.
If you are SYN flooded, the source address reported by the kernel is
likely to have been forged by the attacker; it is only reported as
an aid in tracing the packets to their actual source and should not
be taken as absolute truth.
SYN cookies may prevent correct error reporting on clients when the
server is really overloaded. If this happens frequently better turn
them off.
If you say Y here, you can disable SYN cookies at run time by
saying Y to "/proc file system support" and
"Sysctl support" below and executing the command
echo 0 > /proc/sys/net/ipv4/tcp_syncookies
after the /proc file system has been mounted.
If unsure, say N.
config NET_IPVTI
tristate "Virtual (secure) IP: tunneling"
depends on IPV6 || IPV6=n
select INET_TUNNEL
select NET_IP_TUNNEL
select XFRM
help
Tunneling means encapsulating data of one protocol type within
another protocol and sending it over a channel that understands the
encapsulating protocol. This can be used with xfrm mode tunnel to give
the notion of a secure tunnel for IPSEC and then use routing protocol
on top.
config NET_UDP_TUNNEL
tristate
select NET_IP_TUNNEL
default n
config NET_FOU
tristate "IP: Foo (IP protocols) over UDP"
select XFRM
select NET_UDP_TUNNEL
help
Foo over UDP allows any IP protocol to be directly encapsulated
over UDP include tunnels (IPIP, GRE, SIT). By encapsulating in UDP
network mechanisms and optimizations for UDP (such as ECMP
and RSS) can be leveraged to provide better service.
config NET_FOU_IP_TUNNELS
bool "IP: FOU encapsulation of IP tunnels"
depends on NET_IPIP || NET_IPGRE || IPV6_SIT
select NET_FOU
help
Allow configuration of FOU or GUE encapsulation for IP tunnels.
When this option is enabled IP tunnels can be configured to use
FOU or GUE encapsulation.
config INET_AH
tristate "IP: AH transformation"
select XFRM_AH
help
Support for IPsec AH (Authentication Header).
AH can be used with various authentication algorithms. Besides
enabling AH support itself, this option enables the generic
implementations of the algorithms that RFC 8221 lists as MUST be
implemented. If you need any other algorithms, you'll need to enable
them in the crypto API. You should also enable accelerated
implementations of any needed algorithms when available.
If unsure, say Y.
config INET_ESP
tristate "IP: ESP transformation"
select XFRM_ESP
help
Support for IPsec ESP (Encapsulating Security Payload).
ESP can be used with various encryption and authentication algorithms.
Besides enabling ESP support itself, this option enables the generic
implementations of the algorithms that RFC 8221 lists as MUST be
implemented. If you need any other algorithms, you'll need to enable
them in the crypto API. You should also enable accelerated
implementations of any needed algorithms when available.
If unsure, say Y.
config INET_ESP_OFFLOAD
tristate "IP: ESP transformation offload"
depends on INET_ESP
select XFRM_OFFLOAD
default n
help
Support for ESP transformation offload. This makes sense
only if this system really does IPsec and want to do it
with high throughput. A typical desktop system does not
need it, even if it does IPsec.
If unsure, say N.
config INET_ESPINTCP
bool "IP: ESP in TCP encapsulation (RFC 8229)"
depends on XFRM && INET_ESP
select STREAM_PARSER
select NET_SOCK_MSG
select XFRM_ESPINTCP
help
Support for RFC 8229 encapsulation of ESP and IKE over
TCP/IPv4 sockets.
If unsure, say N.
config INET_IPCOMP
tristate "IP: IPComp transformation"
select INET_XFRM_TUNNEL
select XFRM_IPCOMP
help
Support for IP Payload Compression Protocol (IPComp) (RFC3173),
typically needed for IPsec.
If unsure, say Y.
config INET_TABLE_PERTURB_ORDER
int "INET: Source port perturbation table size (as power of 2)" if EXPERT
default 16
help
Source port perturbation table size (as power of 2) for
RFC 6056 3.3.4. Algorithm 4: Double-Hash Port Selection Algorithm.
The default is almost always what you want.
Only change this if you know what you are doing.
config INET_XFRM_TUNNEL
tristate
select INET_TUNNEL
default n
config INET_TUNNEL
tristate
default n
config INET_DIAG
tristate "INET: socket monitoring interface"
default y
help
Support for INET (TCP, DCCP, etc) socket monitoring interface used by
native Linux tools such as ss. ss is included in iproute2, currently
downloadable at:
http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2
If unsure, say Y.
config INET_TCP_DIAG
depends on INET_DIAG
def_tristate INET_DIAG
config INET_UDP_DIAG
tristate "UDP: socket monitoring interface"
depends on INET_DIAG && (IPV6 || IPV6=n)
default n
help
Support for UDP socket monitoring interface used by the ss tool.
If unsure, say Y.
config INET_RAW_DIAG
tristate "RAW: socket monitoring interface"
depends on INET_DIAG && (IPV6 || IPV6=n)
default n
help
Support for RAW socket monitoring interface used by the ss tool.
If unsure, say Y.
config INET_DIAG_DESTROY
bool "INET: allow privileged process to administratively close sockets"
depends on INET_DIAG
default n
help
Provides a SOCK_DESTROY operation that allows privileged processes
(e.g., a connection manager or a network administration tool such as
ss) to close sockets opened by other processes. Closing a socket in
this way interrupts any blocking read/write/connect operations on
the socket and causes future socket calls to behave as if the socket
had been disconnected.
If unsure, say N.
menuconfig TCP_CONG_ADVANCED
bool "TCP: advanced congestion control"
help
Support for selection of various TCP congestion control
modules.
Nearly all users can safely say no here, and a safe default
selection will be made (CUBIC with new Reno as a fallback).
If unsure, say N.
if TCP_CONG_ADVANCED
config TCP_CONG_BIC
tristate "Binary Increase Congestion (BIC) control"
default m
help
BIC-TCP is a sender-side only change that ensures a linear RTT
fairness under large windows while offering both scalability and
bounded TCP-friendliness. The protocol combines two schemes
called additive increase and binary search increase. When the
congestion window is large, additive increase with a large
increment ensures linear RTT fairness as well as good
scalability. Under small congestion windows, binary search
increase provides TCP friendliness.
See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/
config TCP_CONG_CUBIC
tristate "CUBIC TCP"
default y
help
This is version 2.0 of BIC-TCP which uses a cubic growth function
among other techniques.
See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
config TCP_CONG_WESTWOOD
tristate "TCP Westwood+"
default m
help
TCP Westwood+ is a sender-side only modification of the TCP Reno
protocol stack that optimizes the performance of TCP congestion
control. It is based on end-to-end bandwidth estimation to set
congestion window and slow start threshold after a congestion
episode. Using this estimation, TCP Westwood+ adaptively sets a
slow start threshold and a congestion window which takes into
account the bandwidth used at the time congestion is experienced.
TCP Westwood+ significantly increases fairness wrt TCP Reno in
wired networks and throughput over wireless links.
config TCP_CONG_HTCP
tristate "H-TCP"
default m
help
H-TCP is a send-side only modifications of the TCP Reno
protocol stack that optimizes the performance of TCP
congestion control for high speed network links. It uses a
modeswitch to change the alpha and beta parameters of TCP Reno
based on network conditions and in a way so as to be fair with
other Reno and H-TCP flows.
config TCP_CONG_HSTCP
tristate "High Speed TCP"
default n
help
Sally Floyd's High Speed TCP (RFC 3649) congestion control.
A modification to TCP's congestion control mechanism for use
with large congestion windows. A table indicates how much to
increase the congestion window by when an ACK is received.
For more detail see https://www.icir.org/floyd/hstcp.html
config TCP_CONG_HYBLA
tristate "TCP-Hybla congestion control algorithm"
default n
help
TCP-Hybla is a sender-side only change that eliminates penalization of
long-RTT, large-bandwidth connections, like when satellite legs are
involved, especially when sharing a common bottleneck with normal
terrestrial connections.
config TCP_CONG_VEGAS
tristate "TCP Vegas"
default n
help
TCP Vegas is a sender-side only change to TCP that anticipates
the onset of congestion by estimating the bandwidth. TCP Vegas
adjusts the sending rate by modifying the congestion
window. TCP Vegas should provide less packet loss, but it is
not as aggressive as TCP Reno.
config TCP_CONG_NV
tristate "TCP NV"
default n
help
TCP NV is a follow up to TCP Vegas. It has been modified to deal with
10G networks, measurement noise introduced by LRO, GRO and interrupt
coalescence. In addition, it will decrease its cwnd multiplicatively
instead of linearly.
Note that in general congestion avoidance (cwnd decreased when # packets
queued grows) cannot coexist with congestion control (cwnd decreased only
when there is packet loss) due to fairness issues. One scenario when they
can coexist safely is when the CA flows have RTTs << CC flows RTTs.
For further details see http://www.brakmo.org/networking/tcp-nv/
config TCP_CONG_SCALABLE
tristate "Scalable TCP"
default n
help
Scalable TCP is a sender-side only change to TCP which uses a
MIMD congestion control algorithm which has some nice scaling
properties, though is known to have fairness issues.
See http://www.deneholme.net/tom/scalable/
config TCP_CONG_LP
tristate "TCP Low Priority"
default n
help
TCP Low Priority (TCP-LP), a distributed algorithm whose goal is
to utilize only the excess network bandwidth as compared to the
``fair share`` of bandwidth as targeted by TCP.
See http://www-ece.rice.edu/networks/TCP-LP/
config TCP_CONG_VENO
tristate "TCP Veno"
default n
help
TCP Veno is a sender-side only enhancement of TCP to obtain better
throughput over wireless networks. TCP Veno makes use of state
distinguishing to circumvent the difficult judgment of the packet loss
type. TCP Veno cuts down less congestion window in response to random
loss packets.
See <http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1177186>
config TCP_CONG_YEAH
tristate "YeAH TCP"
select TCP_CONG_VEGAS
default n
help
YeAH-TCP is a sender-side high-speed enabled TCP congestion control
algorithm, which uses a mixed loss/delay approach to compute the
congestion window. It's design goals target high efficiency,
internal, RTT and Reno fairness, resilience to link loss while
keeping network elements load as low as possible.
For further details look here:
http://wil.cs.caltech.edu/pfldnet2007/paper/YeAH_TCP.pdf
config TCP_CONG_ILLINOIS
tristate "TCP Illinois"
default n
help
TCP-Illinois is a sender-side modification of TCP Reno for
high speed long delay links. It uses round-trip-time to
adjust the alpha and beta parameters to achieve a higher average
throughput and maintain fairness.
For further details see:
http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
config TCP_CONG_DCTCP
tristate "DataCenter TCP (DCTCP)"
default n
help
DCTCP leverages Explicit Congestion Notification (ECN) in the network to
provide multi-bit feedback to the end hosts. It is designed to provide:
- High burst tolerance (incast due to partition/aggregate),
- Low latency (short flows, queries),
- High throughput (continuous data updates, large file transfers) with
commodity, shallow-buffered switches.
All switches in the data center network running DCTCP must support
ECN marking and be configured for marking when reaching defined switch
buffer thresholds. The default ECN marking threshold heuristic for
DCTCP on switches is 20 packets (30KB) at 1Gbps, and 65 packets
(~100KB) at 10Gbps, but might need further careful tweaking.
For further details see:
http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
config TCP_CONG_CDG
tristate "CAIA Delay-Gradient (CDG)"
default n
help
CAIA Delay-Gradient (CDG) is a TCP congestion control that modifies
the TCP sender in order to:
o Use the delay gradient as a congestion signal.
o Back off with an average probability that is independent of the RTT.
o Coexist with flows that use loss-based congestion control.
o Tolerate packet loss unrelated to congestion.
For further details see:
D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
delay gradients." In Networking 2011. Preprint: http://goo.gl/No3vdg
config TCP_CONG_BBR
tristate "BBR TCP"
default n
help
BBR (Bottleneck Bandwidth and RTT) TCP congestion control aims to
maximize network utilization and minimize queues. It builds an explicit
model of the bottleneck delivery rate and path round-trip propagation
delay. It tolerates packet loss and delay unrelated to congestion. It
can operate over LAN, WAN, cellular, wifi, or cable modem links. It can
coexist with flows that use loss-based congestion control, and can
operate with shallow buffers, deep buffers, bufferbloat, policers, or
AQM schemes that do not provide a delay signal. It requires the fq
("Fair Queue") pacing packet scheduler.
choice
prompt "Default TCP congestion control"
default DEFAULT_CUBIC
help
Select the TCP congestion control that will be used by default
for all connections.
config DEFAULT_BIC
bool "Bic" if TCP_CONG_BIC=y
config DEFAULT_CUBIC
bool "Cubic" if TCP_CONG_CUBIC=y
config DEFAULT_HTCP
bool "Htcp" if TCP_CONG_HTCP=y
config DEFAULT_HYBLA
bool "Hybla" if TCP_CONG_HYBLA=y
config DEFAULT_VEGAS
bool "Vegas" if TCP_CONG_VEGAS=y
config DEFAULT_VENO
bool "Veno" if TCP_CONG_VENO=y
config DEFAULT_WESTWOOD
bool "Westwood" if TCP_CONG_WESTWOOD=y
config DEFAULT_DCTCP
bool "DCTCP" if TCP_CONG_DCTCP=y
config DEFAULT_CDG
bool "CDG" if TCP_CONG_CDG=y
config DEFAULT_BBR
bool "BBR" if TCP_CONG_BBR=y
config DEFAULT_RENO
bool "Reno"
endchoice
endif
config TCP_CONG_CUBIC
tristate
depends on !TCP_CONG_ADVANCED
default y
config DEFAULT_TCP_CONG
string
default "bic" if DEFAULT_BIC
default "cubic" if DEFAULT_CUBIC
default "htcp" if DEFAULT_HTCP
default "hybla" if DEFAULT_HYBLA
default "vegas" if DEFAULT_VEGAS
default "westwood" if DEFAULT_WESTWOOD
default "veno" if DEFAULT_VENO
default "reno" if DEFAULT_RENO
default "dctcp" if DEFAULT_DCTCP
default "cdg" if DEFAULT_CDG
default "bbr" if DEFAULT_BBR
default "cubic"
config TCP_MD5SIG
bool "TCP: MD5 Signature Option support (RFC2385)"
select CRYPTO
select CRYPTO_MD5
help
RFC2385 specifies a method of giving MD5 protection to TCP sessions.
Its main (only?) use is to protect BGP sessions between core routers
on the Internet.
If unsure, say N.
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