1# 2# IP configuration 3# 4config 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/>. For most people, it's safe to say N. 13 14config IP_ADVANCED_ROUTER 15 bool "IP: advanced router" 16 ---help--- 17 If you intend to run your Linux box mostly as a router, i.e. as a 18 computer that forwards and redistributes network packets, say Y; you 19 will then be presented with several options that allow more precise 20 control about the routing process. 21 22 The answer to this question won't directly affect the kernel: 23 answering N will just cause the configurator to skip all the 24 questions about advanced routing. 25 26 Note that your box can only act as a router if you enable IP 27 forwarding in your kernel; you can do that by saying Y to "/proc 28 file system support" and "Sysctl support" below and executing the 29 line 30 31 echo "1" > /proc/sys/net/ipv4/ip_forward 32 33 at boot time after the /proc file system has been mounted. 34 35 If you turn on IP forwarding, you should consider the rp_filter, which 36 automatically rejects incoming packets if the routing table entry 37 for their source address doesn't match the network interface they're 38 arriving on. This has security advantages because it prevents the 39 so-called IP spoofing, however it can pose problems if you use 40 asymmetric routing (packets from you to a host take a different path 41 than packets from that host to you) or if you operate a non-routing 42 host which has several IP addresses on different interfaces. To turn 43 rp_filter on use: 44 45 echo 1 > /proc/sys/net/ipv4/conf/<device>/rp_filter 46 or 47 echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter 48 49 Note that some distributions enable it in startup scripts. 50 For details about rp_filter strict and loose mode read 51 <file:Documentation/networking/ip-sysctl.txt>. 52 53 If unsure, say N here. 54 55config IP_FIB_TRIE_STATS 56 bool "FIB TRIE statistics" 57 depends on IP_ADVANCED_ROUTER 58 ---help--- 59 Keep track of statistics on structure of FIB TRIE table. 60 Useful for testing and measuring TRIE performance. 61 62config IP_MULTIPLE_TABLES 63 bool "IP: policy routing" 64 depends on IP_ADVANCED_ROUTER 65 select FIB_RULES 66 ---help--- 67 Normally, a router decides what to do with a received packet based 68 solely on the packet's final destination address. If you say Y here, 69 the Linux router will also be able to take the packet's source 70 address into account. Furthermore, the TOS (Type-Of-Service) field 71 of the packet can be used for routing decisions as well. 72 73 If you are interested in this, please see the preliminary 74 documentation at <http://www.compendium.com.ar/policy-routing.txt> 75 and <ftp://post.tepkom.ru/pub/vol2/Linux/docs/advanced-routing.tex>. 76 You will need supporting software from 77 <ftp://ftp.tux.org/pub/net/ip-routing/>. 78 79 If unsure, say N. 80 81config IP_ROUTE_MULTIPATH 82 bool "IP: equal cost multipath" 83 depends on IP_ADVANCED_ROUTER 84 help 85 Normally, the routing tables specify a single action to be taken in 86 a deterministic manner for a given packet. If you say Y here 87 however, it becomes possible to attach several actions to a packet 88 pattern, in effect specifying several alternative paths to travel 89 for those packets. The router considers all these paths to be of 90 equal "cost" and chooses one of them in a non-deterministic fashion 91 if a matching packet arrives. 92 93config IP_ROUTE_VERBOSE 94 bool "IP: verbose route monitoring" 95 depends on IP_ADVANCED_ROUTER 96 help 97 If you say Y here, which is recommended, then the kernel will print 98 verbose messages regarding the routing, for example warnings about 99 received packets which look strange and could be evidence of an 100 attack or a misconfigured system somewhere. The information is 101 handled by the klogd daemon which is responsible for kernel messages 102 ("man klogd"). 103 104config IP_ROUTE_CLASSID 105 bool 106 107config IP_PNP 108 bool "IP: kernel level autoconfiguration" 109 help 110 This enables automatic configuration of IP addresses of devices and 111 of the routing table during kernel boot, based on either information 112 supplied on the kernel command line or by BOOTP or RARP protocols. 113 You need to say Y only for diskless machines requiring network 114 access to boot (in which case you want to say Y to "Root file system 115 on NFS" as well), because all other machines configure the network 116 in their startup scripts. 117 118config IP_PNP_DHCP 119 bool "IP: DHCP support" 120 depends on IP_PNP 121 ---help--- 122 If you want your Linux box to mount its whole root file system (the 123 one containing the directory /) from some other computer over the 124 net via NFS and you want the IP address of your computer to be 125 discovered automatically at boot time using the DHCP protocol (a 126 special protocol designed for doing this job), say Y here. In case 127 the boot ROM of your network card was designed for booting Linux and 128 does DHCP itself, providing all necessary information on the kernel 129 command line, you can say N here. 130 131 If unsure, say Y. Note that if you want to use DHCP, a DHCP server 132 must be operating on your network. Read 133 <file:Documentation/filesystems/nfs/nfsroot.txt> for details. 134 135config IP_PNP_BOOTP 136 bool "IP: BOOTP support" 137 depends on IP_PNP 138 ---help--- 139 If you want your Linux box to mount its whole root file system (the 140 one containing the directory /) from some other computer over the 141 net via NFS and you want the IP address of your computer to be 142 discovered automatically at boot time using the BOOTP protocol (a 143 special protocol designed for doing this job), say Y here. In case 144 the boot ROM of your network card was designed for booting Linux and 145 does BOOTP itself, providing all necessary information on the kernel 146 command line, you can say N here. If unsure, say Y. Note that if you 147 want to use BOOTP, a BOOTP server must be operating on your network. 148 Read <file:Documentation/filesystems/nfs/nfsroot.txt> for details. 149 150config IP_PNP_RARP 151 bool "IP: RARP support" 152 depends on IP_PNP 153 help 154 If you want your Linux box to mount its whole root file system (the 155 one containing the directory /) from some other computer over the 156 net via NFS and you want the IP address of your computer to be 157 discovered automatically at boot time using the RARP protocol (an 158 older protocol which is being obsoleted by BOOTP and DHCP), say Y 159 here. Note that if you want to use RARP, a RARP server must be 160 operating on your network. Read 161 <file:Documentation/filesystems/nfs/nfsroot.txt> for details. 162 163config NET_IPIP 164 tristate "IP: tunneling" 165 select INET_TUNNEL 166 select NET_IP_TUNNEL 167 ---help--- 168 Tunneling means encapsulating data of one protocol type within 169 another protocol and sending it over a channel that understands the 170 encapsulating protocol. This particular tunneling driver implements 171 encapsulation of IP within IP, which sounds kind of pointless, but 172 can be useful if you want to make your (or some other) machine 173 appear on a different network than it physically is, or to use 174 mobile-IP facilities (allowing laptops to seamlessly move between 175 networks without changing their IP addresses). 176 177 Saying Y to this option will produce two modules ( = code which can 178 be inserted in and removed from the running kernel whenever you 179 want). Most people won't need this and can say N. 180 181config NET_IPGRE_DEMUX 182 tristate "IP: GRE demultiplexer" 183 help 184 This is helper module to demultiplex GRE packets on GRE version field criteria. 185 Required by ip_gre and pptp modules. 186 187config NET_IP_TUNNEL 188 tristate 189 default n 190 191config NET_IPGRE 192 tristate "IP: GRE tunnels over IP" 193 depends on (IPV6 || IPV6=n) && NET_IPGRE_DEMUX 194 select NET_IP_TUNNEL 195 help 196 Tunneling means encapsulating data of one protocol type within 197 another protocol and sending it over a channel that understands the 198 encapsulating protocol. This particular tunneling driver implements 199 GRE (Generic Routing Encapsulation) and at this time allows 200 encapsulating of IPv4 or IPv6 over existing IPv4 infrastructure. 201 This driver is useful if the other endpoint is a Cisco router: Cisco 202 likes GRE much better than the other Linux tunneling driver ("IP 203 tunneling" above). In addition, GRE allows multicast redistribution 204 through the tunnel. 205 206config NET_IPGRE_BROADCAST 207 bool "IP: broadcast GRE over IP" 208 depends on IP_MULTICAST && NET_IPGRE 209 help 210 One application of GRE/IP is to construct a broadcast WAN (Wide Area 211 Network), which looks like a normal Ethernet LAN (Local Area 212 Network), but can be distributed all over the Internet. If you want 213 to do that, say Y here and to "IP multicast routing" below. 214 215config IP_MROUTE 216 bool "IP: multicast routing" 217 depends on IP_MULTICAST 218 help 219 This is used if you want your machine to act as a router for IP 220 packets that have several destination addresses. It is needed on the 221 MBONE, a high bandwidth network on top of the Internet which carries 222 audio and video broadcasts. In order to do that, you would most 223 likely run the program mrouted. If you haven't heard about it, you 224 don't need it. 225 226config IP_MROUTE_MULTIPLE_TABLES 227 bool "IP: multicast policy routing" 228 depends on IP_MROUTE && IP_ADVANCED_ROUTER 229 select FIB_RULES 230 help 231 Normally, a multicast router runs a userspace daemon and decides 232 what to do with a multicast packet based on the source and 233 destination addresses. If you say Y here, the multicast router 234 will also be able to take interfaces and packet marks into 235 account and run multiple instances of userspace daemons 236 simultaneously, each one handling a single table. 237 238 If unsure, say N. 239 240config IP_PIMSM_V1 241 bool "IP: PIM-SM version 1 support" 242 depends on IP_MROUTE 243 help 244 Kernel side support for Sparse Mode PIM (Protocol Independent 245 Multicast) version 1. This multicast routing protocol is used widely 246 because Cisco supports it. You need special software to use it 247 (pimd-v1). Please see <http://netweb.usc.edu/pim/> for more 248 information about PIM. 249 250 Say Y if you want to use PIM-SM v1. Note that you can say N here if 251 you just want to use Dense Mode PIM. 252 253config IP_PIMSM_V2 254 bool "IP: PIM-SM version 2 support" 255 depends on IP_MROUTE 256 help 257 Kernel side support for Sparse Mode PIM version 2. In order to use 258 this, you need an experimental routing daemon supporting it (pimd or 259 gated-5). This routing protocol is not used widely, so say N unless 260 you want to play with it. 261 262config SYN_COOKIES 263 bool "IP: TCP syncookie support" 264 ---help--- 265 Normal TCP/IP networking is open to an attack known as "SYN 266 flooding". This denial-of-service attack prevents legitimate remote 267 users from being able to connect to your computer during an ongoing 268 attack and requires very little work from the attacker, who can 269 operate from anywhere on the Internet. 270 271 SYN cookies provide protection against this type of attack. If you 272 say Y here, the TCP/IP stack will use a cryptographic challenge 273 protocol known as "SYN cookies" to enable legitimate users to 274 continue to connect, even when your machine is under attack. There 275 is no need for the legitimate users to change their TCP/IP software; 276 SYN cookies work transparently to them. For technical information 277 about SYN cookies, check out <http://cr.yp.to/syncookies.html>. 278 279 If you are SYN flooded, the source address reported by the kernel is 280 likely to have been forged by the attacker; it is only reported as 281 an aid in tracing the packets to their actual source and should not 282 be taken as absolute truth. 283 284 SYN cookies may prevent correct error reporting on clients when the 285 server is really overloaded. If this happens frequently better turn 286 them off. 287 288 If you say Y here, you can disable SYN cookies at run time by 289 saying Y to "/proc file system support" and 290 "Sysctl support" below and executing the command 291 292 echo 0 > /proc/sys/net/ipv4/tcp_syncookies 293 294 after the /proc file system has been mounted. 295 296 If unsure, say N. 297 298config NET_IPVTI 299 tristate "Virtual (secure) IP: tunneling" 300 select INET_TUNNEL 301 select NET_IP_TUNNEL 302 depends on INET_XFRM_MODE_TUNNEL 303 ---help--- 304 Tunneling means encapsulating data of one protocol type within 305 another protocol and sending it over a channel that understands the 306 encapsulating protocol. This can be used with xfrm mode tunnel to give 307 the notion of a secure tunnel for IPSEC and then use routing protocol 308 on top. 309 310config NET_UDP_TUNNEL 311 tristate 312 select NET_IP_TUNNEL 313 default n 314 315config NET_FOU 316 tristate "IP: Foo (IP protocols) over UDP" 317 select XFRM 318 select NET_UDP_TUNNEL 319 ---help--- 320 Foo over UDP allows any IP protocol to be directly encapsulated 321 over UDP include tunnels (IPIP, GRE, SIT). By encapsulating in UDP 322 network mechanisms and optimizations for UDP (such as ECMP 323 and RSS) can be leveraged to provide better service. 324 325config GENEVE 326 tristate "Generic Network Virtualization Encapsulation (Geneve)" 327 depends on INET 328 select NET_UDP_TUNNEL 329 ---help--- 330 This allows one to create Geneve virtual interfaces that provide 331 Layer 2 Networks over Layer 3 Networks. Geneve is often used 332 to tunnel virtual network infrastructure in virtualized environments. 333 For more information see: 334 http://tools.ietf.org/html/draft-gross-geneve-01 335 336 To compile this driver as a module, choose M here: the module 337 338 339config INET_AH 340 tristate "IP: AH transformation" 341 select XFRM_ALGO 342 select CRYPTO 343 select CRYPTO_HMAC 344 select CRYPTO_MD5 345 select CRYPTO_SHA1 346 ---help--- 347 Support for IPsec AH. 348 349 If unsure, say Y. 350 351config INET_ESP 352 tristate "IP: ESP transformation" 353 select XFRM_ALGO 354 select CRYPTO 355 select CRYPTO_AUTHENC 356 select CRYPTO_HMAC 357 select CRYPTO_MD5 358 select CRYPTO_CBC 359 select CRYPTO_SHA1 360 select CRYPTO_DES 361 ---help--- 362 Support for IPsec ESP. 363 364 If unsure, say Y. 365 366config INET_IPCOMP 367 tristate "IP: IPComp transformation" 368 select INET_XFRM_TUNNEL 369 select XFRM_IPCOMP 370 ---help--- 371 Support for IP Payload Compression Protocol (IPComp) (RFC3173), 372 typically needed for IPsec. 373 374 If unsure, say Y. 375 376config INET_XFRM_TUNNEL 377 tristate 378 select INET_TUNNEL 379 default n 380 381config INET_TUNNEL 382 tristate 383 default n 384 385config INET_XFRM_MODE_TRANSPORT 386 tristate "IP: IPsec transport mode" 387 default y 388 select XFRM 389 ---help--- 390 Support for IPsec transport mode. 391 392 If unsure, say Y. 393 394config INET_XFRM_MODE_TUNNEL 395 tristate "IP: IPsec tunnel mode" 396 default y 397 select XFRM 398 ---help--- 399 Support for IPsec tunnel mode. 400 401 If unsure, say Y. 402 403config INET_XFRM_MODE_BEET 404 tristate "IP: IPsec BEET mode" 405 default y 406 select XFRM 407 ---help--- 408 Support for IPsec BEET mode. 409 410 If unsure, say Y. 411 412config INET_LRO 413 tristate "Large Receive Offload (ipv4/tcp)" 414 default y 415 ---help--- 416 Support for Large Receive Offload (ipv4/tcp). 417 418 If unsure, say Y. 419 420config INET_DIAG 421 tristate "INET: socket monitoring interface" 422 default y 423 ---help--- 424 Support for INET (TCP, DCCP, etc) socket monitoring interface used by 425 native Linux tools such as ss. ss is included in iproute2, currently 426 downloadable at: 427 428 http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2 429 430 If unsure, say Y. 431 432config INET_TCP_DIAG 433 depends on INET_DIAG 434 def_tristate INET_DIAG 435 436config INET_UDP_DIAG 437 tristate "UDP: socket monitoring interface" 438 depends on INET_DIAG && (IPV6 || IPV6=n) 439 default n 440 ---help--- 441 Support for UDP socket monitoring interface used by the ss tool. 442 If unsure, say Y. 443 444menuconfig TCP_CONG_ADVANCED 445 bool "TCP: advanced congestion control" 446 ---help--- 447 Support for selection of various TCP congestion control 448 modules. 449 450 Nearly all users can safely say no here, and a safe default 451 selection will be made (CUBIC with new Reno as a fallback). 452 453 If unsure, say N. 454 455if TCP_CONG_ADVANCED 456 457config TCP_CONG_BIC 458 tristate "Binary Increase Congestion (BIC) control" 459 default m 460 ---help--- 461 BIC-TCP is a sender-side only change that ensures a linear RTT 462 fairness under large windows while offering both scalability and 463 bounded TCP-friendliness. The protocol combines two schemes 464 called additive increase and binary search increase. When the 465 congestion window is large, additive increase with a large 466 increment ensures linear RTT fairness as well as good 467 scalability. Under small congestion windows, binary search 468 increase provides TCP friendliness. 469 See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/ 470 471config TCP_CONG_CUBIC 472 tristate "CUBIC TCP" 473 default y 474 ---help--- 475 This is version 2.0 of BIC-TCP which uses a cubic growth function 476 among other techniques. 477 See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf 478 479config TCP_CONG_WESTWOOD 480 tristate "TCP Westwood+" 481 default m 482 ---help--- 483 TCP Westwood+ is a sender-side only modification of the TCP Reno 484 protocol stack that optimizes the performance of TCP congestion 485 control. It is based on end-to-end bandwidth estimation to set 486 congestion window and slow start threshold after a congestion 487 episode. Using this estimation, TCP Westwood+ adaptively sets a 488 slow start threshold and a congestion window which takes into 489 account the bandwidth used at the time congestion is experienced. 490 TCP Westwood+ significantly increases fairness wrt TCP Reno in 491 wired networks and throughput over wireless links. 492 493config TCP_CONG_HTCP 494 tristate "H-TCP" 495 default m 496 ---help--- 497 H-TCP is a send-side only modifications of the TCP Reno 498 protocol stack that optimizes the performance of TCP 499 congestion control for high speed network links. It uses a 500 modeswitch to change the alpha and beta parameters of TCP Reno 501 based on network conditions and in a way so as to be fair with 502 other Reno and H-TCP flows. 503 504config TCP_CONG_HSTCP 505 tristate "High Speed TCP" 506 default n 507 ---help--- 508 Sally Floyd's High Speed TCP (RFC 3649) congestion control. 509 A modification to TCP's congestion control mechanism for use 510 with large congestion windows. A table indicates how much to 511 increase the congestion window by when an ACK is received. 512 For more detail see http://www.icir.org/floyd/hstcp.html 513 514config TCP_CONG_HYBLA 515 tristate "TCP-Hybla congestion control algorithm" 516 default n 517 ---help--- 518 TCP-Hybla is a sender-side only change that eliminates penalization of 519 long-RTT, large-bandwidth connections, like when satellite legs are 520 involved, especially when sharing a common bottleneck with normal 521 terrestrial connections. 522 523config TCP_CONG_VEGAS 524 tristate "TCP Vegas" 525 default n 526 ---help--- 527 TCP Vegas is a sender-side only change to TCP that anticipates 528 the onset of congestion by estimating the bandwidth. TCP Vegas 529 adjusts the sending rate by modifying the congestion 530 window. TCP Vegas should provide less packet loss, but it is 531 not as aggressive as TCP Reno. 532 533config TCP_CONG_SCALABLE 534 tristate "Scalable TCP" 535 default n 536 ---help--- 537 Scalable TCP is a sender-side only change to TCP which uses a 538 MIMD congestion control algorithm which has some nice scaling 539 properties, though is known to have fairness issues. 540 See http://www.deneholme.net/tom/scalable/ 541 542config TCP_CONG_LP 543 tristate "TCP Low Priority" 544 default n 545 ---help--- 546 TCP Low Priority (TCP-LP), a distributed algorithm whose goal is 547 to utilize only the excess network bandwidth as compared to the 548 ``fair share`` of bandwidth as targeted by TCP. 549 See http://www-ece.rice.edu/networks/TCP-LP/ 550 551config TCP_CONG_VENO 552 tristate "TCP Veno" 553 default n 554 ---help--- 555 TCP Veno is a sender-side only enhancement of TCP to obtain better 556 throughput over wireless networks. TCP Veno makes use of state 557 distinguishing to circumvent the difficult judgment of the packet loss 558 type. TCP Veno cuts down less congestion window in response to random 559 loss packets. 560 See <http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1177186> 561 562config TCP_CONG_YEAH 563 tristate "YeAH TCP" 564 select TCP_CONG_VEGAS 565 default n 566 ---help--- 567 YeAH-TCP is a sender-side high-speed enabled TCP congestion control 568 algorithm, which uses a mixed loss/delay approach to compute the 569 congestion window. It's design goals target high efficiency, 570 internal, RTT and Reno fairness, resilience to link loss while 571 keeping network elements load as low as possible. 572 573 For further details look here: 574 http://wil.cs.caltech.edu/pfldnet2007/paper/YeAH_TCP.pdf 575 576config TCP_CONG_ILLINOIS 577 tristate "TCP Illinois" 578 default n 579 ---help--- 580 TCP-Illinois is a sender-side modification of TCP Reno for 581 high speed long delay links. It uses round-trip-time to 582 adjust the alpha and beta parameters to achieve a higher average 583 throughput and maintain fairness. 584 585 For further details see: 586 http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html 587 588config TCP_CONG_DCTCP 589 tristate "DataCenter TCP (DCTCP)" 590 default n 591 ---help--- 592 DCTCP leverages Explicit Congestion Notification (ECN) in the network to 593 provide multi-bit feedback to the end hosts. It is designed to provide: 594 595 - High burst tolerance (incast due to partition/aggregate), 596 - Low latency (short flows, queries), 597 - High throughput (continuous data updates, large file transfers) with 598 commodity, shallow-buffered switches. 599 600 All switches in the data center network running DCTCP must support 601 ECN marking and be configured for marking when reaching defined switch 602 buffer thresholds. The default ECN marking threshold heuristic for 603 DCTCP on switches is 20 packets (30KB) at 1Gbps, and 65 packets 604 (~100KB) at 10Gbps, but might need further careful tweaking. 605 606 For further details see: 607 http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf 608 609choice 610 prompt "Default TCP congestion control" 611 default DEFAULT_CUBIC 612 help 613 Select the TCP congestion control that will be used by default 614 for all connections. 615 616 config DEFAULT_BIC 617 bool "Bic" if TCP_CONG_BIC=y 618 619 config DEFAULT_CUBIC 620 bool "Cubic" if TCP_CONG_CUBIC=y 621 622 config DEFAULT_HTCP 623 bool "Htcp" if TCP_CONG_HTCP=y 624 625 config DEFAULT_HYBLA 626 bool "Hybla" if TCP_CONG_HYBLA=y 627 628 config DEFAULT_VEGAS 629 bool "Vegas" if TCP_CONG_VEGAS=y 630 631 config DEFAULT_VENO 632 bool "Veno" if TCP_CONG_VENO=y 633 634 config DEFAULT_WESTWOOD 635 bool "Westwood" if TCP_CONG_WESTWOOD=y 636 637 config DEFAULT_DCTCP 638 bool "DCTCP" if TCP_CONG_DCTCP=y 639 640 config DEFAULT_RENO 641 bool "Reno" 642endchoice 643 644endif 645 646config TCP_CONG_CUBIC 647 tristate 648 depends on !TCP_CONG_ADVANCED 649 default y 650 651config DEFAULT_TCP_CONG 652 string 653 default "bic" if DEFAULT_BIC 654 default "cubic" if DEFAULT_CUBIC 655 default "htcp" if DEFAULT_HTCP 656 default "hybla" if DEFAULT_HYBLA 657 default "vegas" if DEFAULT_VEGAS 658 default "westwood" if DEFAULT_WESTWOOD 659 default "veno" if DEFAULT_VENO 660 default "reno" if DEFAULT_RENO 661 default "dctcp" if DEFAULT_DCTCP 662 default "cubic" 663 664config TCP_MD5SIG 665 bool "TCP: MD5 Signature Option support (RFC2385)" 666 select CRYPTO 667 select CRYPTO_MD5 668 ---help--- 669 RFC2385 specifies a method of giving MD5 protection to TCP sessions. 670 Its main (only?) use is to protect BGP sessions between core routers 671 on the Internet. 672 673 If unsure, say N. 674