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 # not yet ready..
 167 #   bool '    IP: ARP support' CONFIG_IP_PNP_ARP
 168 config NET_IPIP
 169         tristate "IP: tunneling"
 170         select INET_TUNNEL
 171         ---help---
 172           Tunneling means encapsulating data of one protocol type within
 173           another protocol and sending it over a channel that understands the
 174           encapsulating protocol. This particular tunneling driver implements
 175           encapsulation of IP within IP, which sounds kind of pointless, but
 176           can be useful if you want to make your (or some other) machine
 177           appear on a different network than it physically is, or to use
 178           mobile-IP facilities (allowing laptops to seamlessly move between
 179           networks without changing their IP addresses).
 180
 181           Saying Y to this option will produce two modules ( = code which can
 182           be inserted in and removed from the running kernel whenever you
 183           want). Most people won't need this and can say N.
 184
 185 config NET_IPGRE_DEMUX
 186         tristate "IP: GRE demultiplexer"
 187         help
 188          This is helper module to demultiplex GRE packets on GRE version field criteria.
 189          Required by ip_gre and pptp modules.
 190
 191 config NET_IPGRE
 192         tristate "IP: GRE tunnels over IP"
 193         depends on (IPV6 || IPV6=n) && NET_IPGRE_DEMUX
 194         help
 195           Tunneling means encapsulating data of one protocol type within
 196           another protocol and sending it over a channel that understands the
 197           encapsulating protocol. This particular tunneling driver implements
 198           GRE (Generic Routing Encapsulation) and at this time allows
 199           encapsulating of IPv4 or IPv6 over existing IPv4 infrastructure.
 200           This driver is useful if the other endpoint is a Cisco router: Cisco
 201           likes GRE much better than the other Linux tunneling driver ("IP
 202           tunneling" above). In addition, GRE allows multicast redistribution
 203           through the tunnel.
 204
 205 config NET_IPGRE_BROADCAST
 206         bool "IP: broadcast GRE over IP"
 207         depends on IP_MULTICAST && NET_IPGRE
 208         help
 209           One application of GRE/IP is to construct a broadcast WAN (Wide Area
 210           Network), which looks like a normal Ethernet LAN (Local Area
 211           Network), but can be distributed all over the Internet. If you want
 212           to do that, say Y here and to "IP multicast routing" below.
 213
 214 config IP_MROUTE
 215         bool "IP: multicast routing"
 216         depends on IP_MULTICAST
 217         help
 218           This is used if you want your machine to act as a router for IP
 219           packets that have several destination addresses. It is needed on the
 220           MBONE, a high bandwidth network on top of the Internet which carries
 221           audio and video broadcasts. In order to do that, you would most
 222           likely run the program mrouted. Information about the multicast
 223           capabilities of the various network cards is contained in
 224           <file:Documentation/networking/multicast.txt>. If you haven't heard
 225           about it, you don't need it.
 226
 227 config IP_MROUTE_MULTIPLE_TABLES
 228         bool "IP: multicast policy routing"
 229         depends on IP_MROUTE && IP_ADVANCED_ROUTER
 230         select FIB_RULES
 231         help
 232           Normally, a multicast router runs a userspace daemon and decides
 233           what to do with a multicast packet based on the source and
 234           destination addresses. If you say Y here, the multicast router
 235           will also be able to take interfaces and packet marks into
 236           account and run multiple instances of userspace daemons
 237           simultaneously, each one handling a single table.
 238
 239           If unsure, say N.
 240
 241 config IP_PIMSM_V1
 242         bool "IP: PIM-SM version 1 support"
 243         depends on IP_MROUTE
 244         help
 245           Kernel side support for Sparse Mode PIM (Protocol Independent
 246           Multicast) version 1. This multicast routing protocol is used widely
 247           because Cisco supports it. You need special software to use it
 248           (pimd-v1). Please see <http://netweb.usc.edu/pim/> for more
 249           information about PIM.
 250
 251           Say Y if you want to use PIM-SM v1. Note that you can say N here if
 252           you just want to use Dense Mode PIM.
 253
 254 config IP_PIMSM_V2
 255         bool "IP: PIM-SM version 2 support"
 256         depends on IP_MROUTE
 257         help
 258           Kernel side support for Sparse Mode PIM version 2. In order to use
 259           this, you need an experimental routing daemon supporting it (pimd or
 260           gated-5). This routing protocol is not used widely, so say N unless
 261           you want to play with it.
 262
 263 config ARPD
 264         bool "IP: ARP daemon support"
 265         ---help---
 266           The kernel maintains an internal cache which maps IP addresses to
 267           hardware addresses on the local network, so that Ethernet/Token Ring/
 268           etc. frames are sent to the proper address on the physical networking
 269           layer. Normally, kernel uses the ARP protocol to resolve these
 270           mappings.
 271
 272           Saying Y here adds support to have an user space daemon to do this
 273           resolution instead. This is useful for implementing an alternate
 274           address resolution protocol (e.g. NHRP on mGRE tunnels) and also for
 275           testing purposes.
 276
 277           If unsure, say N.
 278
 279 config SYN_COOKIES
 280         bool "IP: TCP syncookie support"
 281         ---help---
 282           Normal TCP/IP networking is open to an attack known as "SYN
 283           flooding". This denial-of-service attack prevents legitimate remote
 284           users from being able to connect to your computer during an ongoing
 285           attack and requires very little work from the attacker, who can
 286           operate from anywhere on the Internet.
 287
 288           SYN cookies provide protection against this type of attack. If you
 289           say Y here, the TCP/IP stack will use a cryptographic challenge
 290           protocol known as "SYN cookies" to enable legitimate users to
 291           continue to connect, even when your machine is under attack. There
 292           is no need for the legitimate users to change their TCP/IP software;
 293           SYN cookies work transparently to them. For technical information
 294           about SYN cookies, check out <http://cr.yp.to/syncookies.html>.
 295
 296           If you are SYN flooded, the source address reported by the kernel is
 297           likely to have been forged by the attacker; it is only reported as
 298           an aid in tracing the packets to their actual source and should not
 299           be taken as absolute truth.
 300
 301           SYN cookies may prevent correct error reporting on clients when the
 302           server is really overloaded. If this happens frequently better turn
 303           them off.
 304
 305           If you say Y here, you can disable SYN cookies at run time by
 306           saying Y to "/proc file system support" and
 307           "Sysctl support" below and executing the command
 308
 309           echo 0 > /proc/sys/net/ipv4/tcp_syncookies
 310
 311           after the /proc file system has been mounted.
 312
 313           If unsure, say N.
 314
 315 config INET_AH
 316         tristate "IP: AH transformation"
 317         select XFRM
 318         select CRYPTO
 319         select CRYPTO_HMAC
 320         select CRYPTO_MD5
 321         select CRYPTO_SHA1
 322         ---help---
 323           Support for IPsec AH.
 324
 325           If unsure, say Y.
 326
 327 config INET_ESP
 328         tristate "IP: ESP transformation"
 329         select XFRM
 330         select CRYPTO
 331         select CRYPTO_AUTHENC
 332         select CRYPTO_HMAC
 333         select CRYPTO_MD5
 334         select CRYPTO_CBC
 335         select CRYPTO_SHA1
 336         select CRYPTO_DES
 337         ---help---
 338           Support for IPsec ESP.
 339
 340           If unsure, say Y.
 341
 342 config INET_IPCOMP
 343         tristate "IP: IPComp transformation"
 344         select INET_XFRM_TUNNEL
 345         select XFRM_IPCOMP
 346         ---help---
 347           Support for IP Payload Compression Protocol (IPComp) (RFC3173),
 348           typically needed for IPsec.
 349
 350           If unsure, say Y.
 351
 352 config INET_XFRM_TUNNEL
 353         tristate
 354         select INET_TUNNEL
 355         default n
 356
 357 config INET_TUNNEL
 358         tristate
 359         default n
 360
 361 config INET_XFRM_MODE_TRANSPORT
 362         tristate "IP: IPsec transport mode"
 363         default y
 364         select XFRM
 365         ---help---
 366           Support for IPsec transport mode.
 367
 368           If unsure, say Y.
 369
 370 config INET_XFRM_MODE_TUNNEL
 371         tristate "IP: IPsec tunnel mode"
 372         default y
 373         select XFRM
 374         ---help---
 375           Support for IPsec tunnel mode.
 376
 377           If unsure, say Y.
 378
 379 config INET_XFRM_MODE_BEET
 380         tristate "IP: IPsec BEET mode"
 381         default y
 382         select XFRM
 383         ---help---
 384           Support for IPsec BEET mode.
 385
 386           If unsure, say Y.
 387
 388 config INET_LRO
 389         tristate "Large Receive Offload (ipv4/tcp)"
 390         default y
 391         ---help---
 392           Support for Large Receive Offload (ipv4/tcp).
 393
 394           If unsure, say Y.
 395
 396 config INET_DIAG
 397         tristate "INET: socket monitoring interface"
 398         default y
 399         ---help---
 400           Support for INET (TCP, DCCP, etc) socket monitoring interface used by
 401           native Linux tools such as ss. ss is included in iproute2, currently
 402           downloadable at:
 403
 404             http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2
 405
 406           If unsure, say Y.
 407
 408 config INET_TCP_DIAG
 409         depends on INET_DIAG
 410         def_tristate INET_DIAG
 411
 412 config INET_UDP_DIAG
 413         depends on INET_DIAG
 414         def_tristate INET_DIAG && IPV6
 415
 416 menuconfig TCP_CONG_ADVANCED
 417         bool "TCP: advanced congestion control"
 418         ---help---
 419           Support for selection of various TCP congestion control
 420           modules.
 421
 422           Nearly all users can safely say no here, and a safe default
 423           selection will be made (CUBIC with new Reno as a fallback).
 424
 425           If unsure, say N.
 426
 427 if TCP_CONG_ADVANCED
 428
 429 config TCP_CONG_BIC
 430         tristate "Binary Increase Congestion (BIC) control"
 431         default m
 432         ---help---
 433         BIC-TCP is a sender-side only change that ensures a linear RTT
 434         fairness under large windows while offering both scalability and
 435         bounded TCP-friendliness. The protocol combines two schemes
 436         called additive increase and binary search increase. When the
 437         congestion window is large, additive increase with a large
 438         increment ensures linear RTT fairness as well as good
 439         scalability. Under small congestion windows, binary search
 440         increase provides TCP friendliness.
 441         See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/
 442
 443 config TCP_CONG_CUBIC
 444         tristate "CUBIC TCP"
 445         default y
 446         ---help---
 447         This is version 2.0 of BIC-TCP which uses a cubic growth function
 448         among other techniques.
 449         See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
 450
 451 config TCP_CONG_WESTWOOD
 452         tristate "TCP Westwood+"
 453         default m
 454         ---help---
 455         TCP Westwood+ is a sender-side only modification of the TCP Reno
 456         protocol stack that optimizes the performance of TCP congestion
 457         control. It is based on end-to-end bandwidth estimation to set
 458         congestion window and slow start threshold after a congestion
 459         episode. Using this estimation, TCP Westwood+ adaptively sets a
 460         slow start threshold and a congestion window which takes into
 461         account the bandwidth used  at the time congestion is experienced.
 462         TCP Westwood+ significantly increases fairness wrt TCP Reno in
 463         wired networks and throughput over wireless links.
 464
 465 config TCP_CONG_HTCP
 466         tristate "H-TCP"
 467         default m
 468         ---help---
 469         H-TCP is a send-side only modifications of the TCP Reno
 470         protocol stack that optimizes the performance of TCP
 471         congestion control for high speed network links. It uses a
 472         modeswitch to change the alpha and beta parameters of TCP Reno
 473         based on network conditions and in a way so as to be fair with
 474         other Reno and H-TCP flows.
 475
 476 config TCP_CONG_HSTCP
 477         tristate "High Speed TCP"
 478         depends on EXPERIMENTAL
 479         default n
 480         ---help---
 481         Sally Floyd's High Speed TCP (RFC 3649) congestion control.
 482         A modification to TCP's congestion control mechanism for use
 483         with large congestion windows. A table indicates how much to
 484         increase the congestion window by when an ACK is received.
 485         For more detail see http://www.icir.org/floyd/hstcp.html
 486
 487 config TCP_CONG_HYBLA
 488         tristate "TCP-Hybla congestion control algorithm"
 489         depends on EXPERIMENTAL
 490         default n
 491         ---help---
 492         TCP-Hybla is a sender-side only change that eliminates penalization of
 493         long-RTT, large-bandwidth connections, like when satellite legs are
 494         involved, especially when sharing a common bottleneck with normal
 495         terrestrial connections.
 496
 497 config TCP_CONG_VEGAS
 498         tristate "TCP Vegas"
 499         depends on EXPERIMENTAL
 500         default n
 501         ---help---
 502         TCP Vegas is a sender-side only change to TCP that anticipates
 503         the onset of congestion by estimating the bandwidth. TCP Vegas
 504         adjusts the sending rate by modifying the congestion
 505         window. TCP Vegas should provide less packet loss, but it is
 506         not as aggressive as TCP Reno.
 507
 508 config TCP_CONG_SCALABLE
 509         tristate "Scalable TCP"
 510         depends on EXPERIMENTAL
 511         default n
 512         ---help---
 513         Scalable TCP is a sender-side only change to TCP which uses a
 514         MIMD congestion control algorithm which has some nice scaling
 515         properties, though is known to have fairness issues.
 516         See http://www.deneholme.net/tom/scalable/
 517
 518 config TCP_CONG_LP
 519         tristate "TCP Low Priority"
 520         depends on EXPERIMENTAL
 521         default n
 522         ---help---
 523         TCP Low Priority (TCP-LP), a distributed algorithm whose goal is
 524         to utilize only the excess network bandwidth as compared to the
 525         ``fair share`` of bandwidth as targeted by TCP.
 526         See http://www-ece.rice.edu/networks/TCP-LP/
 527
 528 config TCP_CONG_VENO
 529         tristate "TCP Veno"
 530         depends on EXPERIMENTAL
 531         default n
 532         ---help---
 533         TCP Veno is a sender-side only enhancement of TCP to obtain better
 534         throughput over wireless networks. TCP Veno makes use of state
 535         distinguishing to circumvent the difficult judgment of the packet loss
 536         type. TCP Veno cuts down less congestion window in response to random
 537         loss packets.
 538         See <http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1177186>
 539
 540 config TCP_CONG_YEAH
 541         tristate "YeAH TCP"
 542         depends on EXPERIMENTAL
 543         select TCP_CONG_VEGAS
 544         default n
 545         ---help---
 546         YeAH-TCP is a sender-side high-speed enabled TCP congestion control
 547         algorithm, which uses a mixed loss/delay approach to compute the
 548         congestion window. It's design goals target high efficiency,
 549         internal, RTT and Reno fairness, resilience to link loss while
 550         keeping network elements load as low as possible.
 551
 552         For further details look here:
 553           http://wil.cs.caltech.edu/pfldnet2007/paper/YeAH_TCP.pdf
 554
 555 config TCP_CONG_ILLINOIS
 556         tristate "TCP Illinois"
 557         depends on EXPERIMENTAL
 558         default n
 559         ---help---
 560         TCP-Illinois is a sender-side modification of TCP Reno for
 561         high speed long delay links. It uses round-trip-time to
 562         adjust the alpha and beta parameters to achieve a higher average
 563         throughput and maintain fairness.
 564
 565         For further details see:
 566           http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
 567
 568 choice
 569         prompt "Default TCP congestion control"
 570         default DEFAULT_CUBIC
 571         help
 572           Select the TCP congestion control that will be used by default
 573           for all connections.
 574
 575         config DEFAULT_BIC
 576                 bool "Bic" if TCP_CONG_BIC=y
 577
 578         config DEFAULT_CUBIC
 579                 bool "Cubic" if TCP_CONG_CUBIC=y
 580
 581         config DEFAULT_HTCP
 582                 bool "Htcp" if TCP_CONG_HTCP=y
 583
 584         config DEFAULT_HYBLA
 585                 bool "Hybla" if TCP_CONG_HYBLA=y
 586
 587         config DEFAULT_VEGAS
 588                 bool "Vegas" if TCP_CONG_VEGAS=y
 589
 590         config DEFAULT_VENO
 591                 bool "Veno" if TCP_CONG_VENO=y
 592
 593         config DEFAULT_WESTWOOD
 594                 bool "Westwood" if TCP_CONG_WESTWOOD=y
 595
 596         config DEFAULT_RENO
 597                 bool "Reno"
 598
 599 endchoice
 600
 601 endif
 602
 603 config TCP_CONG_CUBIC
 604         tristate
 605         depends on !TCP_CONG_ADVANCED
 606         default y
 607
 608 config DEFAULT_TCP_CONG
 609         string
 610         default "bic" if DEFAULT_BIC
 611         default "cubic" if DEFAULT_CUBIC
 612         default "htcp" if DEFAULT_HTCP
 613         default "hybla" if DEFAULT_HYBLA
 614         default "vegas" if DEFAULT_VEGAS
 615         default "westwood" if DEFAULT_WESTWOOD
 616         default "veno" if DEFAULT_VENO
 617         default "reno" if DEFAULT_RENO
 618         default "cubic"
 619
 620 config TCP_MD5SIG
 621         bool "TCP: MD5 Signature Option support (RFC2385) (EXPERIMENTAL)"
 622         depends on EXPERIMENTAL
 623         select CRYPTO
 624         select CRYPTO_MD5
 625         ---help---
 626           RFC2385 specifies a method of giving MD5 protection to TCP sessions.
 627           Its main (only?) use is to protect BGP sessions between core routers
 628           on the Internet.
 629
 630           If unsure, say N.