net/ipv4/Kconfig

   1 #
   2 # IP configuration
   3 #
   4 config IP_MULTICAST
   5         bool "IP: multicasting"
   6         help
   7           This is code for addressing several networked computers at once,
   8           enlarging your kernel by about 2 KB. You need multicasting if you
   9           intend to participate in the MBONE, a high bandwidth network on top
  10           of the Internet which carries audio and video broadcasts. More
  11           information about the MBONE is on the WWW at
  12           <http://www.savetz.com/mbone/>. Information about the multicast
  13           capabilities of the various network cards is contained in
  14           <file:Documentation/networking/multicast.txt>. For most people, it's
  15           safe to say N.
  16
  17 config IP_ADVANCED_ROUTER
  18         bool "IP: advanced router"
  19         ---help---
  20           If you intend to run your Linux box mostly as a router, i.e. as a
  21           computer that forwards and redistributes network packets, say Y; you
  22           will then be presented with several options that allow more precise
  23           control about the routing process.
  24
  25           The answer to this question won't directly affect the kernel:
  26           answering N will just cause the configurator to skip all the
  27           questions about advanced routing.
  28
  29           Note that your box can only act as a router if you enable IP
  30           forwarding in your kernel; you can do that by saying Y to "/proc
  31           file system support" and "Sysctl support" below and executing the
  32           line
  33
  34           echo "1" > /proc/sys/net/ipv4/ip_forward
  35
  36           at boot time after the /proc file system has been mounted.
  37
  38           If you turn on IP forwarding, you should consider the rp_filter, which
  39           automatically rejects incoming packets if the routing table entry
  40           for their source address doesn't match the network interface they're
  41           arriving on. This has security advantages because it prevents the
  42           so-called IP spoofing, however it can pose problems if you use
  43           asymmetric routing (packets from you to a host take a different path
  44           than packets from that host to you) or if you operate a non-routing
  45           host which has several IP addresses on different interfaces. To turn
  46           rp_filter on use:
  47
  48           echo 1 > /proc/sys/net/ipv4/conf/<device>/rp_filter
  49            or
  50           echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
  51
  52           Note that some distributions enable it in startup scripts.
  53           For details about rp_filter strict and loose mode read
  54           <file:Documentation/networking/ip-sysctl.txt>.
  55
  56           If unsure, say N here.
  57
  58 config IP_FIB_TRIE_STATS
  59         bool "FIB TRIE statistics"
  60         depends on IP_ADVANCED_ROUTER
  61         ---help---
  62           Keep track of statistics on structure of FIB TRIE table.
  63           Useful for testing and measuring TRIE performance.
  64
  65 config IP_MULTIPLE_TABLES
  66         bool "IP: policy routing"
  67         depends on IP_ADVANCED_ROUTER
  68         select FIB_RULES
  69         ---help---
  70           Normally, a router decides what to do with a received packet based
  71           solely on the packet's final destination address. If you say Y here,
  72           the Linux router will also be able to take the packet's source
  73           address into account. Furthermore, the TOS (Type-Of-Service) field
  74           of the packet can be used for routing decisions as well.
  75
  76           If you are interested in this, please see the preliminary
  77           documentation at <http://www.compendium.com.ar/policy-routing.txt>
  78           and <ftp://post.tepkom.ru/pub/vol2/Linux/docs/advanced-routing.tex>.
  79           You will need supporting software from
  80           <ftp://ftp.tux.org/pub/net/ip-routing/>.
  81
  82           If unsure, say N.
  83
  84 config IP_ROUTE_MULTIPATH
  85         bool "IP: equal cost multipath"
  86         depends on IP_ADVANCED_ROUTER
  87         help
  88           Normally, the routing tables specify a single action to be taken in
  89           a deterministic manner for a given packet. If you say Y here
  90           however, it becomes possible to attach several actions to a packet
  91           pattern, in effect specifying several alternative paths to travel
  92           for those packets. The router considers all these paths to be of
  93           equal "cost" and chooses one of them in a non-deterministic fashion
  94           if a matching packet arrives.
  95
  96 config IP_ROUTE_VERBOSE
  97         bool "IP: verbose route monitoring"
  98         depends on IP_ADVANCED_ROUTER
  99         help
 100           If you say Y here, which is recommended, then the kernel will print
 101           verbose messages regarding the routing, for example warnings about
 102           received packets which look strange and could be evidence of an
 103           attack or a misconfigured system somewhere. The information is
 104           handled by the klogd daemon which is responsible for kernel messages
 105           ("man klogd").
 106
 107 config IP_ROUTE_CLASSID
 108         bool
 109
 110 config IP_PNP
 111         bool "IP: kernel level autoconfiguration"
 112         help
 113           This enables automatic configuration of IP addresses of devices and
 114           of the routing table during kernel boot, based on either information
 115           supplied on the kernel command line or by BOOTP or RARP protocols.
 116           You need to say Y only for diskless machines requiring network
 117           access to boot (in which case you want to say Y to "Root file system
 118           on NFS" as well), because all other machines configure the network
 119           in their startup scripts.
 120
 121 config IP_PNP_DHCP
 122         bool "IP: DHCP support"
 123         depends on IP_PNP
 124         ---help---
 125           If you want your Linux box to mount its whole root file system (the
 126           one containing the directory /) from some other computer over the
 127           net via NFS and you want the IP address of your computer to be
 128           discovered automatically at boot time using the DHCP protocol (a
 129           special protocol designed for doing this job), say Y here. In case
 130           the boot ROM of your network card was designed for booting Linux and
 131           does DHCP itself, providing all necessary information on the kernel
 132           command line, you can say N here.
 133
 134           If unsure, say Y. Note that if you want to use DHCP, a DHCP server
 135           must be operating on your network.  Read
 136           <file:Documentation/filesystems/nfs/nfsroot.txt> for details.
 137
 138 config IP_PNP_BOOTP
 139         bool "IP: BOOTP support"
 140         depends on IP_PNP
 141         ---help---
 142           If you want your Linux box to mount its whole root file system (the
 143           one containing the directory /) from some other computer over the
 144           net via NFS and you want the IP address of your computer to be
 145           discovered automatically at boot time using the BOOTP protocol (a
 146           special protocol designed for doing this job), say Y here. In case
 147           the boot ROM of your network card was designed for booting Linux and
 148           does BOOTP itself, providing all necessary information on the kernel
 149           command line, you can say N here. If unsure, say Y. Note that if you
 150           want to use BOOTP, a BOOTP server must be operating on your network.
 151           Read <file:Documentation/filesystems/nfs/nfsroot.txt> for details.
 152
 153 config IP_PNP_RARP
 154         bool "IP: RARP support"
 155         depends on IP_PNP
 156         help
 157           If you want your Linux box to mount its whole root file system (the
 158           one containing the directory /) from some other computer over the
 159           net via NFS and you want the IP address of your computer to be
 160           discovered automatically at boot time using the RARP protocol (an
 161           older protocol which is being obsoleted by BOOTP and DHCP), say Y
 162           here. Note that if you want to use RARP, a RARP server must be
 163           operating on your network. Read
 164           <file:Documentation/filesystems/nfs/nfsroot.txt> for details.
 165
 166 config NET_IPIP
 167         tristate "IP: tunneling"
 168         select INET_TUNNEL
 169         ---help---
 170           Tunneling means encapsulating data of one protocol type within
 171           another protocol and sending it over a channel that understands the
 172           encapsulating protocol. This particular tunneling driver implements
 173           encapsulation of IP within IP, which sounds kind of pointless, but
 174           can be useful if you want to make your (or some other) machine
 175           appear on a different network than it physically is, or to use
 176           mobile-IP facilities (allowing laptops to seamlessly move between
 177           networks without changing their IP addresses).
 178
 179           Saying Y to this option will produce two modules ( = code which can
 180           be inserted in and removed from the running kernel whenever you
 181           want). Most people won't need this and can say N.
 182
 183 config NET_IPGRE_DEMUX
 184         tristate "IP: GRE demultiplexer"
 185         help
 186          This is helper module to demultiplex GRE packets on GRE version field criteria.
 187          Required by ip_gre and pptp modules.
 188
 189 config NET_IPGRE
 190         tristate "IP: GRE tunnels over IP"
 191         depends on (IPV6 || IPV6=n) && NET_IPGRE_DEMUX
 192         help
 193           Tunneling means encapsulating data of one protocol type within
 194           another protocol and sending it over a channel that understands the
 195           encapsulating protocol. This particular tunneling driver implements
 196           GRE (Generic Routing Encapsulation) and at this time allows
 197           encapsulating of IPv4 or IPv6 over existing IPv4 infrastructure.
 198           This driver is useful if the other endpoint is a Cisco router: Cisco
 199           likes GRE much better than the other Linux tunneling driver ("IP
 200           tunneling" above). In addition, GRE allows multicast redistribution
 201           through the tunnel.
 202
 203 config NET_IPGRE_BROADCAST
 204         bool "IP: broadcast GRE over IP"
 205         depends on IP_MULTICAST && NET_IPGRE
 206         help
 207           One application of GRE/IP is to construct a broadcast WAN (Wide Area
 208           Network), which looks like a normal Ethernet LAN (Local Area
 209           Network), but can be distributed all over the Internet. If you want
 210           to do that, say Y here and to "IP multicast routing" below.
 211
 212 config IP_MROUTE
 213         bool "IP: multicast routing"
 214         depends on IP_MULTICAST
 215         help
 216           This is used if you want your machine to act as a router for IP
 217           packets that have several destination addresses. It is needed on the
 218           MBONE, a high bandwidth network on top of the Internet which carries
 219           audio and video broadcasts. In order to do that, you would most
 220           likely run the program mrouted. Information about the multicast
 221           capabilities of the various network cards is contained in
 222           <file:Documentation/networking/multicast.txt>. If you haven't heard
 223           about it, you don't need it.
 224
 225 config IP_MROUTE_MULTIPLE_TABLES
 226         bool "IP: multicast policy routing"
 227         depends on IP_MROUTE && IP_ADVANCED_ROUTER
 228         select FIB_RULES
 229         help
 230           Normally, a multicast router runs a userspace daemon and decides
 231           what to do with a multicast packet based on the source and
 232           destination addresses. If you say Y here, the multicast router
 233           will also be able to take interfaces and packet marks into
 234           account and run multiple instances of userspace daemons
 235           simultaneously, each one handling a single table.
 236
 237           If unsure, say N.
 238
 239 config IP_PIMSM_V1
 240         bool "IP: PIM-SM version 1 support"
 241         depends on IP_MROUTE
 242         help
 243           Kernel side support for Sparse Mode PIM (Protocol Independent
 244           Multicast) version 1. This multicast routing protocol is used widely
 245           because Cisco supports it. You need special software to use it
 246           (pimd-v1). Please see <http://netweb.usc.edu/pim/> for more
 247           information about PIM.
 248
 249           Say Y if you want to use PIM-SM v1. Note that you can say N here if
 250           you just want to use Dense Mode PIM.
 251
 252 config IP_PIMSM_V2
 253         bool "IP: PIM-SM version 2 support"
 254         depends on IP_MROUTE
 255         help
 256           Kernel side support for Sparse Mode PIM version 2. In order to use
 257           this, you need an experimental routing daemon supporting it (pimd or
 258           gated-5). This routing protocol is not used widely, so say N unless
 259           you want to play with it.
 260
 261 config ARPD
 262         bool "IP: ARP daemon support"
 263         ---help---
 264           The kernel maintains an internal cache which maps IP addresses to
 265           hardware addresses on the local network, so that Ethernet
 266           frames are sent to the proper address on the physical networking
 267           layer. Normally, kernel uses the ARP protocol to resolve these
 268           mappings.
 269
 270           Saying Y here adds support to have an user space daemon to do this
 271           resolution instead. This is useful for implementing an alternate
 272           address resolution protocol (e.g. NHRP on mGRE tunnels) and also for
 273           testing purposes.
 274
 275           If unsure, say N.
 276
 277 config SYN_COOKIES
 278         bool "IP: TCP syncookie support"
 279         ---help---
 280           Normal TCP/IP networking is open to an attack known as "SYN
 281           flooding". This denial-of-service attack prevents legitimate remote
 282           users from being able to connect to your computer during an ongoing
 283           attack and requires very little work from the attacker, who can
 284           operate from anywhere on the Internet.
 285
 286           SYN cookies provide protection against this type of attack. If you
 287           say Y here, the TCP/IP stack will use a cryptographic challenge
 288           protocol known as "SYN cookies" to enable legitimate users to
 289           continue to connect, even when your machine is under attack. There
 290           is no need for the legitimate users to change their TCP/IP software;
 291           SYN cookies work transparently to them. For technical information
 292           about SYN cookies, check out <http://cr.yp.to/syncookies.html>.
 293
 294           If you are SYN flooded, the source address reported by the kernel is
 295           likely to have been forged by the attacker; it is only reported as
 296           an aid in tracing the packets to their actual source and should not
 297           be taken as absolute truth.
 298
 299           SYN cookies may prevent correct error reporting on clients when the
 300           server is really overloaded. If this happens frequently better turn
 301           them off.
 302
 303           If you say Y here, you can disable SYN cookies at run time by
 304           saying Y to "/proc file system support" and
 305           "Sysctl support" below and executing the command
 306
 307           echo 0 > /proc/sys/net/ipv4/tcp_syncookies
 308
 309           after the /proc file system has been mounted.
 310
 311           If unsure, say N.
 312
 313 config NET_IPVTI
 314         tristate "Virtual (secure) IP: tunneling"
 315         select INET_TUNNEL
 316         depends on INET_XFRM_MODE_TUNNEL
 317         ---help---
 318           Tunneling means encapsulating data of one protocol type within
 319           another protocol and sending it over a channel that understands the
 320           encapsulating protocol. This can be used with xfrm mode tunnel to give
 321           the notion of a secure tunnel for IPSEC and then use routing protocol
 322           on top.
 323
 324 config INET_AH
 325         tristate "IP: AH transformation"
 326         select XFRM_ALGO
 327         select CRYPTO
 328         select CRYPTO_HMAC
 329         select CRYPTO_MD5
 330         select CRYPTO_SHA1
 331         ---help---
 332           Support for IPsec AH.
 333
 334           If unsure, say Y.
 335
 336 config INET_ESP
 337         tristate "IP: ESP transformation"
 338         select XFRM_ALGO
 339         select CRYPTO
 340         select CRYPTO_AUTHENC
 341         select CRYPTO_HMAC
 342         select CRYPTO_MD5
 343         select CRYPTO_CBC
 344         select CRYPTO_SHA1
 345         select CRYPTO_DES
 346         ---help---
 347           Support for IPsec ESP.
 348
 349           If unsure, say Y.
 350
 351 config INET_IPCOMP
 352         tristate "IP: IPComp transformation"
 353         select INET_XFRM_TUNNEL
 354         select XFRM_IPCOMP
 355         ---help---
 356           Support for IP Payload Compression Protocol (IPComp) (RFC3173),
 357           typically needed for IPsec.
 358
 359           If unsure, say Y.
 360
 361 config INET_XFRM_TUNNEL
 362         tristate
 363         select INET_TUNNEL
 364         default n
 365
 366 config INET_TUNNEL
 367         tristate
 368         default n
 369
 370 config INET_XFRM_MODE_TRANSPORT
 371         tristate "IP: IPsec transport mode"
 372         default y
 373         select XFRM
 374         ---help---
 375           Support for IPsec transport mode.
 376
 377           If unsure, say Y.
 378
 379 config INET_XFRM_MODE_TUNNEL
 380         tristate "IP: IPsec tunnel mode"
 381         default y
 382         select XFRM
 383         ---help---
 384           Support for IPsec tunnel mode.
 385
 386           If unsure, say Y.
 387
 388 config INET_XFRM_MODE_BEET
 389         tristate "IP: IPsec BEET mode"
 390         default y
 391         select XFRM
 392         ---help---
 393           Support for IPsec BEET mode.
 394
 395           If unsure, say Y.
 396
 397 config INET_LRO
 398         tristate "Large Receive Offload (ipv4/tcp)"
 399         default y
 400         ---help---
 401           Support for Large Receive Offload (ipv4/tcp).
 402
 403           If unsure, say Y.
 404
 405 config INET_DIAG
 406         tristate "INET: socket monitoring interface"
 407         default y
 408         ---help---
 409           Support for INET (TCP, DCCP, etc) socket monitoring interface used by
 410           native Linux tools such as ss. ss is included in iproute2, currently
 411           downloadable at:
 412
 413             http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2
 414
 415           If unsure, say Y.
 416
 417 config INET_TCP_DIAG
 418         depends on INET_DIAG
 419         def_tristate INET_DIAG
 420
 421 config INET_UDP_DIAG
 422         tristate "UDP: socket monitoring interface"
 423         depends on INET_DIAG && (IPV6 || IPV6=n)
 424         default n
 425         ---help---
 426           Support for UDP socket monitoring interface used by the ss tool.
 427           If unsure, say Y.
 428
 429 menuconfig TCP_CONG_ADVANCED
 430         bool "TCP: advanced congestion control"
 431         ---help---
 432           Support for selection of various TCP congestion control
 433           modules.
 434
 435           Nearly all users can safely say no here, and a safe default
 436           selection will be made (CUBIC with new Reno as a fallback).
 437
 438           If unsure, say N.
 439
 440 if TCP_CONG_ADVANCED
 441
 442 config TCP_CONG_BIC
 443         tristate "Binary Increase Congestion (BIC) control"
 444         default m
 445         ---help---
 446         BIC-TCP is a sender-side only change that ensures a linear RTT
 447         fairness under large windows while offering both scalability and
 448         bounded TCP-friendliness. The protocol combines two schemes
 449         called additive increase and binary search increase. When the
 450         congestion window is large, additive increase with a large
 451         increment ensures linear RTT fairness as well as good
 452         scalability. Under small congestion windows, binary search
 453         increase provides TCP friendliness.
 454         See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/
 455
 456 config TCP_CONG_CUBIC
 457         tristate "CUBIC TCP"
 458         default y
 459         ---help---
 460         This is version 2.0 of BIC-TCP which uses a cubic growth function
 461         among other techniques.
 462         See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
 463
 464 config TCP_CONG_WESTWOOD
 465         tristate "TCP Westwood+"
 466         default m
 467         ---help---
 468         TCP Westwood+ is a sender-side only modification of the TCP Reno
 469         protocol stack that optimizes the performance of TCP congestion
 470         control. It is based on end-to-end bandwidth estimation to set
 471         congestion window and slow start threshold after a congestion
 472         episode. Using this estimation, TCP Westwood+ adaptively sets a
 473         slow start threshold and a congestion window which takes into
 474         account the bandwidth used  at the time congestion is experienced.
 475         TCP Westwood+ significantly increases fairness wrt TCP Reno in
 476         wired networks and throughput over wireless links.
 477
 478 config TCP_CONG_HTCP
 479         tristate "H-TCP"
 480         default m
 481         ---help---
 482         H-TCP is a send-side only modifications of the TCP Reno
 483         protocol stack that optimizes the performance of TCP
 484         congestion control for high speed network links. It uses a
 485         modeswitch to change the alpha and beta parameters of TCP Reno
 486         based on network conditions and in a way so as to be fair with
 487         other Reno and H-TCP flows.
 488
 489 config TCP_CONG_HSTCP
 490         tristate "High Speed TCP"
 491         default n
 492         ---help---
 493         Sally Floyd's High Speed TCP (RFC 3649) congestion control.
 494         A modification to TCP's congestion control mechanism for use
 495         with large congestion windows. A table indicates how much to
 496         increase the congestion window by when an ACK is received.
 497         For more detail see http://www.icir.org/floyd/hstcp.html
 498
 499 config TCP_CONG_HYBLA
 500         tristate "TCP-Hybla congestion control algorithm"
 501         default n
 502         ---help---
 503         TCP-Hybla is a sender-side only change that eliminates penalization of
 504         long-RTT, large-bandwidth connections, like when satellite legs are
 505         involved, especially when sharing a common bottleneck with normal
 506         terrestrial connections.
 507
 508 config TCP_CONG_VEGAS
 509         tristate "TCP Vegas"
 510         default n
 511         ---help---
 512         TCP Vegas is a sender-side only change to TCP that anticipates
 513         the onset of congestion by estimating the bandwidth. TCP Vegas
 514         adjusts the sending rate by modifying the congestion
 515         window. TCP Vegas should provide less packet loss, but it is
 516         not as aggressive as TCP Reno.
 517
 518 config TCP_CONG_SCALABLE
 519         tristate "Scalable TCP"
 520         default n
 521         ---help---
 522         Scalable TCP is a sender-side only change to TCP which uses a
 523         MIMD congestion control algorithm which has some nice scaling
 524         properties, though is known to have fairness issues.
 525         See http://www.deneholme.net/tom/scalable/
 526
 527 config TCP_CONG_LP
 528         tristate "TCP Low Priority"
 529         default n
 530         ---help---
 531         TCP Low Priority (TCP-LP), a distributed algorithm whose goal is
 532         to utilize only the excess network bandwidth as compared to the
 533         ``fair share`` of bandwidth as targeted by TCP.
 534         See http://www-ece.rice.edu/networks/TCP-LP/
 535
 536 config TCP_CONG_VENO
 537         tristate "TCP Veno"
 538         default n
 539         ---help---
 540         TCP Veno is a sender-side only enhancement of TCP to obtain better
 541         throughput over wireless networks. TCP Veno makes use of state
 542         distinguishing to circumvent the difficult judgment of the packet loss
 543         type. TCP Veno cuts down less congestion window in response to random
 544         loss packets.
 545         See <http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1177186>
 546
 547 config TCP_CONG_YEAH
 548         tristate "YeAH TCP"
 549         select TCP_CONG_VEGAS
 550         default n
 551         ---help---
 552         YeAH-TCP is a sender-side high-speed enabled TCP congestion control
 553         algorithm, which uses a mixed loss/delay approach to compute the
 554         congestion window. It's design goals target high efficiency,
 555         internal, RTT and Reno fairness, resilience to link loss while
 556         keeping network elements load as low as possible.
 557
 558         For further details look here:
 559           http://wil.cs.caltech.edu/pfldnet2007/paper/YeAH_TCP.pdf
 560
 561 config TCP_CONG_ILLINOIS
 562         tristate "TCP Illinois"
 563         default n
 564         ---help---
 565         TCP-Illinois is a sender-side modification of TCP Reno for
 566         high speed long delay links. It uses round-trip-time to
 567         adjust the alpha and beta parameters to achieve a higher average
 568         throughput and maintain fairness.
 569
 570         For further details see:
 571           http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
 572
 573 choice
 574         prompt "Default TCP congestion control"
 575         default DEFAULT_CUBIC
 576         help
 577           Select the TCP congestion control that will be used by default
 578           for all connections.
 579
 580         config DEFAULT_BIC
 581                 bool "Bic" if TCP_CONG_BIC=y
 582
 583         config DEFAULT_CUBIC
 584                 bool "Cubic" if TCP_CONG_CUBIC=y
 585
 586         config DEFAULT_HTCP
 587                 bool "Htcp" if TCP_CONG_HTCP=y
 588
 589         config DEFAULT_HYBLA
 590                 bool "Hybla" if TCP_CONG_HYBLA=y
 591
 592         config DEFAULT_VEGAS
 593                 bool "Vegas" if TCP_CONG_VEGAS=y
 594
 595         config DEFAULT_VENO
 596                 bool "Veno" if TCP_CONG_VENO=y
 597
 598         config DEFAULT_WESTWOOD
 599                 bool "Westwood" if TCP_CONG_WESTWOOD=y
 600
 601         config DEFAULT_RENO
 602                 bool "Reno"
 603
 604 endchoice
 605
 606 endif
 607
 608 config TCP_CONG_CUBIC
 609         tristate
 610         depends on !TCP_CONG_ADVANCED
 611         default y
 612
 613 config DEFAULT_TCP_CONG
 614         string
 615         default "bic" if DEFAULT_BIC
 616         default "cubic" if DEFAULT_CUBIC
 617         default "htcp" if DEFAULT_HTCP
 618         default "hybla" if DEFAULT_HYBLA
 619         default "vegas" if DEFAULT_VEGAS
 620         default "westwood" if DEFAULT_WESTWOOD
 621         default "veno" if DEFAULT_VENO
 622         default "reno" if DEFAULT_RENO
 623         default "cubic"
 624
 625 config TCP_MD5SIG
 626         bool "TCP: MD5 Signature Option support (RFC2385)"
 627         select CRYPTO
 628         select CRYPTO_MD5
 629         ---help---
 630           RFC2385 specifies a method of giving MD5 protection to TCP sessions.
 631           Its main (only?) use is to protect BGP sessions between core routers
 632           on the Internet.
 633
 634           If unsure, say N.