| blob | bd290160484263ba1507b8ae5b05580430fa9e56 |
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/>. For most people, it's safe to say N.
14 config 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.
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.
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
31 echo "1" > /proc/sys/net/ipv4/ip_forward
33 at boot time after the /proc file system has been mounted.
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:
45 echo 1 > /proc/sys/net/ipv4/conf/<device>/rp_filter
46 or
47 echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
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>.
53 If unsure, say N here.
55 config 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.
62 config 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.
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/>.
79 If unsure, say N.
81 config 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.
93 config 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").
104 config IP_ROUTE_CLASSID
105 bool
107 config 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.
118 config 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.
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.
135 config 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.
150 config 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.
163 config 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).
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.
181 config 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.
187 config NET_IP_TUNNEL
188 tristate
189 default n
191 config 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.
206 config 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.
215 config 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.
226 config 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.
238 If unsure, say N.
240 config 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.
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.
253 config 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.
262 config 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.
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>.
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.
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.
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
292 echo 0 > /proc/sys/net/ipv4/tcp_syncookies
294 after the /proc file system has been mounted.
296 If unsure, say N.
298 config 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.
310 config NET_UDP_TUNNEL
311 tristate
312 select NET_IP_TUNNEL
313 default n
315 config 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.
325 config NET_FOU_IP_TUNNELS
326 bool "IP: FOU encapsulation of IP tunnels"
327 depends on NET_IPIP || NET_IPGRE || IPV6_SIT
328 select NET_FOU
329 ---help---
330 Allow configuration of FOU or GUE encapsulation for IP tunnels.
331 When this option is enabled IP tunnels can be configured to use
332 FOU or GUE encapsulation.
334 config GENEVE
335 tristate "Generic Network Virtualization Encapsulation (Geneve)"
336 depends on INET
337 select NET_UDP_TUNNEL
338 ---help---
339 This allows one to create Geneve virtual interfaces that provide
340 Layer 2 Networks over Layer 3 Networks. Geneve is often used
341 to tunnel virtual network infrastructure in virtualized environments.
342 For more information see:
343 http://tools.ietf.org/html/draft-gross-geneve-01
345 To compile this driver as a module, choose M here: the module
348 config INET_AH
349 tristate "IP: AH transformation"
350 select XFRM_ALGO
351 select CRYPTO
352 select CRYPTO_HMAC
353 select CRYPTO_MD5
354 select CRYPTO_SHA1
355 ---help---
356 Support for IPsec AH.
358 If unsure, say Y.
360 config INET_ESP
361 tristate "IP: ESP transformation"
362 select XFRM_ALGO
363 select CRYPTO
364 select CRYPTO_AUTHENC
365 select CRYPTO_HMAC
366 select CRYPTO_MD5
367 select CRYPTO_CBC
368 select CRYPTO_SHA1
369 select CRYPTO_DES
370 ---help---
371 Support for IPsec ESP.
373 If unsure, say Y.
375 config INET_IPCOMP
376 tristate "IP: IPComp transformation"
377 select INET_XFRM_TUNNEL
378 select XFRM_IPCOMP
379 ---help---
380 Support for IP Payload Compression Protocol (IPComp) (RFC3173),
381 typically needed for IPsec.
383 If unsure, say Y.
385 config INET_XFRM_TUNNEL
386 tristate
387 select INET_TUNNEL
388 default n
390 config INET_TUNNEL
391 tristate
392 default n
394 config INET_XFRM_MODE_TRANSPORT
395 tristate "IP: IPsec transport mode"
396 default y
397 select XFRM
398 ---help---
399 Support for IPsec transport mode.
401 If unsure, say Y.
403 config INET_XFRM_MODE_TUNNEL
404 tristate "IP: IPsec tunnel mode"
405 default y
406 select XFRM
407 ---help---
408 Support for IPsec tunnel mode.
410 If unsure, say Y.
412 config INET_XFRM_MODE_BEET
413 tristate "IP: IPsec BEET mode"
414 default y
415 select XFRM
416 ---help---
417 Support for IPsec BEET mode.
419 If unsure, say Y.
421 config INET_LRO
422 tristate "Large Receive Offload (ipv4/tcp)"
423 default y
424 ---help---
425 Support for Large Receive Offload (ipv4/tcp).
427 If unsure, say Y.
429 config INET_DIAG
430 tristate "INET: socket monitoring interface"
431 default y
432 ---help---
433 Support for INET (TCP, DCCP, etc) socket monitoring interface used by
434 native Linux tools such as ss. ss is included in iproute2, currently
435 downloadable at:
437 http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2
439 If unsure, say Y.
441 config INET_TCP_DIAG
442 depends on INET_DIAG
443 def_tristate INET_DIAG
445 config INET_UDP_DIAG
446 tristate "UDP: socket monitoring interface"
447 depends on INET_DIAG && (IPV6 || IPV6=n)
448 default n
449 ---help---
450 Support for UDP socket monitoring interface used by the ss tool.
451 If unsure, say Y.
453 menuconfig TCP_CONG_ADVANCED
454 bool "TCP: advanced congestion control"
455 ---help---
456 Support for selection of various TCP congestion control
457 modules.
459 Nearly all users can safely say no here, and a safe default
460 selection will be made (CUBIC with new Reno as a fallback).
462 If unsure, say N.
464 if TCP_CONG_ADVANCED
466 config TCP_CONG_BIC
467 tristate "Binary Increase Congestion (BIC) control"
468 default m
469 ---help---
470 BIC-TCP is a sender-side only change that ensures a linear RTT
471 fairness under large windows while offering both scalability and
472 bounded TCP-friendliness. The protocol combines two schemes
473 called additive increase and binary search increase. When the
474 congestion window is large, additive increase with a large
475 increment ensures linear RTT fairness as well as good
476 scalability. Under small congestion windows, binary search
477 increase provides TCP friendliness.
478 See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/
480 config TCP_CONG_CUBIC
481 tristate "CUBIC TCP"
482 default y
483 ---help---
484 This is version 2.0 of BIC-TCP which uses a cubic growth function
485 among other techniques.
486 See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
488 config TCP_CONG_WESTWOOD
489 tristate "TCP Westwood+"
490 default m
491 ---help---
492 TCP Westwood+ is a sender-side only modification of the TCP Reno
493 protocol stack that optimizes the performance of TCP congestion
494 control. It is based on end-to-end bandwidth estimation to set
495 congestion window and slow start threshold after a congestion
496 episode. Using this estimation, TCP Westwood+ adaptively sets a
497 slow start threshold and a congestion window which takes into
498 account the bandwidth used at the time congestion is experienced.
499 TCP Westwood+ significantly increases fairness wrt TCP Reno in
500 wired networks and throughput over wireless links.
502 config TCP_CONG_HTCP
503 tristate "H-TCP"
504 default m
505 ---help---
506 H-TCP is a send-side only modifications of the TCP Reno
507 protocol stack that optimizes the performance of TCP
508 congestion control for high speed network links. It uses a
509 modeswitch to change the alpha and beta parameters of TCP Reno
510 based on network conditions and in a way so as to be fair with
511 other Reno and H-TCP flows.
513 config TCP_CONG_HSTCP
514 tristate "High Speed TCP"
515 default n
516 ---help---
517 Sally Floyd's High Speed TCP (RFC 3649) congestion control.
518 A modification to TCP's congestion control mechanism for use
519 with large congestion windows. A table indicates how much to
520 increase the congestion window by when an ACK is received.
521 For more detail see http://www.icir.org/floyd/hstcp.html
523 config TCP_CONG_HYBLA
524 tristate "TCP-Hybla congestion control algorithm"
525 default n
526 ---help---
527 TCP-Hybla is a sender-side only change that eliminates penalization of
528 long-RTT, large-bandwidth connections, like when satellite legs are
529 involved, especially when sharing a common bottleneck with normal
530 terrestrial connections.
532 config TCP_CONG_VEGAS
533 tristate "TCP Vegas"
534 default n
535 ---help---
536 TCP Vegas is a sender-side only change to TCP that anticipates
537 the onset of congestion by estimating the bandwidth. TCP Vegas
538 adjusts the sending rate by modifying the congestion
539 window. TCP Vegas should provide less packet loss, but it is
540 not as aggressive as TCP Reno.
542 config TCP_CONG_SCALABLE
543 tristate "Scalable TCP"
544 default n
545 ---help---
546 Scalable TCP is a sender-side only change to TCP which uses a
547 MIMD congestion control algorithm which has some nice scaling
548 properties, though is known to have fairness issues.
549 See http://www.deneholme.net/tom/scalable/
551 config TCP_CONG_LP
552 tristate "TCP Low Priority"
553 default n
554 ---help---
555 TCP Low Priority (TCP-LP), a distributed algorithm whose goal is
556 to utilize only the excess network bandwidth as compared to the
557 ``fair share`` of bandwidth as targeted by TCP.
558 See http://www-ece.rice.edu/networks/TCP-LP/
560 config TCP_CONG_VENO
561 tristate "TCP Veno"
562 default n
563 ---help---
564 TCP Veno is a sender-side only enhancement of TCP to obtain better
565 throughput over wireless networks. TCP Veno makes use of state
566 distinguishing to circumvent the difficult judgment of the packet loss
567 type. TCP Veno cuts down less congestion window in response to random
568 loss packets.
569 See <http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1177186>
571 config TCP_CONG_YEAH
572 tristate "YeAH TCP"
573 select TCP_CONG_VEGAS
574 default n
575 ---help---
576 YeAH-TCP is a sender-side high-speed enabled TCP congestion control
577 algorithm, which uses a mixed loss/delay approach to compute the
578 congestion window. It's design goals target high efficiency,
579 internal, RTT and Reno fairness, resilience to link loss while
580 keeping network elements load as low as possible.
582 For further details look here:
583 http://wil.cs.caltech.edu/pfldnet2007/paper/YeAH_TCP.pdf
585 config TCP_CONG_ILLINOIS
586 tristate "TCP Illinois"
587 default n
588 ---help---
589 TCP-Illinois is a sender-side modification of TCP Reno for
590 high speed long delay links. It uses round-trip-time to
591 adjust the alpha and beta parameters to achieve a higher average
592 throughput and maintain fairness.
594 For further details see:
595 http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
597 config TCP_CONG_DCTCP
598 tristate "DataCenter TCP (DCTCP)"
599 default n
600 ---help---
601 DCTCP leverages Explicit Congestion Notification (ECN) in the network to
602 provide multi-bit feedback to the end hosts. It is designed to provide:
604 - High burst tolerance (incast due to partition/aggregate),
605 - Low latency (short flows, queries),
606 - High throughput (continuous data updates, large file transfers) with
607 commodity, shallow-buffered switches.
609 All switches in the data center network running DCTCP must support
610 ECN marking and be configured for marking when reaching defined switch
611 buffer thresholds. The default ECN marking threshold heuristic for
612 DCTCP on switches is 20 packets (30KB) at 1Gbps, and 65 packets
613 (~100KB) at 10Gbps, but might need further careful tweaking.
615 For further details see:
616 http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
618 choice
619 prompt "Default TCP congestion control"
620 default DEFAULT_CUBIC
621 help
622 Select the TCP congestion control that will be used by default
623 for all connections.
625 config DEFAULT_BIC
626 bool "Bic" if TCP_CONG_BIC=y
628 config DEFAULT_CUBIC
629 bool "Cubic" if TCP_CONG_CUBIC=y
631 config DEFAULT_HTCP
632 bool "Htcp" if TCP_CONG_HTCP=y
634 config DEFAULT_HYBLA
635 bool "Hybla" if TCP_CONG_HYBLA=y
637 config DEFAULT_VEGAS
638 bool "Vegas" if TCP_CONG_VEGAS=y
640 config DEFAULT_VENO
641 bool "Veno" if TCP_CONG_VENO=y
643 config DEFAULT_WESTWOOD
644 bool "Westwood" if TCP_CONG_WESTWOOD=y
646 config DEFAULT_DCTCP
647 bool "DCTCP" if TCP_CONG_DCTCP=y
649 config DEFAULT_RENO
650 bool "Reno"
651 endchoice
653 endif
655 config TCP_CONG_CUBIC
656 tristate
657 depends on !TCP_CONG_ADVANCED
658 default y
660 config DEFAULT_TCP_CONG
661 string
662 default "bic" if DEFAULT_BIC
663 default "cubic" if DEFAULT_CUBIC
664 default "htcp" if DEFAULT_HTCP
665 default "hybla" if DEFAULT_HYBLA
666 default "vegas" if DEFAULT_VEGAS
667 default "westwood" if DEFAULT_WESTWOOD
668 default "veno" if DEFAULT_VENO
669 default "reno" if DEFAULT_RENO
670 default "dctcp" if DEFAULT_DCTCP
671 default "cubic"
673 config TCP_MD5SIG
674 bool "TCP: MD5 Signature Option support (RFC2385)"
675 select CRYPTO
676 select CRYPTO_MD5
677 ---help---
678 RFC2385 specifies a method of giving MD5 protection to TCP sessions.
679 Its main (only?) use is to protect BGP sessions between core routers
680 on the Internet.
682 If unsure, say N.