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'SBGP1' -- Possible downref: Non-RFC (?) normative reference: ref. 'SBGP2' -- Obsolete informational reference (is this intentional?): RFC 2434 (Obsoleted by RFC 5226) Summary: 14 errors (**), 0 flaws (~~), 7 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 INTERNET-DRAFT V. Gill 3 draft-ietf-rtgwg-rfc3682bis-02.txt J. Heasley 4 D. Meyer 5 Category Experimental 6 Expires: October 2004 April 2004 8 The Generalized TTL Security Mechanism (GTSM) 9 11 Status of this Memo 13 This document is an Internet-Draft and is subject to all provisions 14 of Section 10 of RFC2026. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/1id-abstracts.html 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html 32 This document is a product of the RTG WG WG. Comments should be 33 addressed to the authors, or the mailing list at rtgwg@ietf.org. 35 Copyright Notice 37 Copyright (C) The Internet Society (2004). All Rights Reserved. 39 Abstract 41 The use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6) 42 to protect a protocol stack from CPU-utilization based attacks has 43 been proposed in many settings (see for example, RFC 2461). This 44 document generalizes these techniques for use by other protocols such 45 as BGP (RFC 1771), Multicast Source Discovery Protocol (MSDP), 46 Bidirectional Forwarding Detection, and Label Distribution Protocol 47 (LDP) (RFC 3036). While the Generalized TTL Security Mechanism (GTSM) 48 is most effective in protecting directly connected protocol peers, it 49 can also provide a lower level of protection to multi-hop sessions. 50 GTSM is not directly applicable to protocols employing flooding 51 mechanisms (e.g., multicast), and use of multi-hop GTSM should be 52 considered on a case-by-case basis. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Assumptions Underlying GTSM. . . . . . . . . . . . . . . . . . 4 58 2.1. GTSM Negotiation. . . . . . . . . . . . . . . . . . . . . . 4 59 2.2. Assumptions on Attack Sophistication. . . . . . . . . . . . 5 60 3. GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . . 5 61 3.1. Multi-hop Scenarios . . . . . . . . . . . . . . . . . . . . 6 62 3.1.1. Intra-domain Protocol Handling . . . . . . . . . . . . . 6 63 4. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 7 64 5. Security Considerations. . . . . . . . . . . . . . . . . . . . 7 65 5.1. TTL (Hop Limit) Spoofing. . . . . . . . . . . . . . . . . . 7 66 5.2. Tunneled Packets. . . . . . . . . . . . . . . . . . . . . . 8 67 5.2.1. IP in IP . . . . . . . . . . . . . . . . . . . . . . . . 8 68 5.2.2. IP in MPLS . . . . . . . . . . . . . . . . . . . . . . . 9 69 5.3. Multi-Hop Protocol Sessions . . . . . . . . . . . . . . . . 10 70 6. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 11 71 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 72 7.1. Normative References. . . . . . . . . . . . . . . . . . . . 11 73 7.2. Informative References. . . . . . . . . . . . . . . . . . . 12 74 8. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13 75 9. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13 76 10. Intellectual Property . . . . . . . . . . . . . . . . . . . . 14 77 11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 14 79 1. Introduction 81 The Generalized TTL Security Mechanism (GTSM) is designed to protect 82 a router's TCP/IP based control plane from CPU-utilization based 83 attacks. In particular, while cryptographic techniques can protect 84 the router-based infrastructure (e.g., BGP [RFC1771], [RFC1772]) from 85 a wide variety of attacks, many attacks based on CPU overload can be 86 prevented by the simple mechanism described in this document. Note 87 that the same technique protects against other scarce-resource 88 attacks involving a router's CPU, such as attacks against processor- 89 line card bandwidth. 91 GTSM is based on the fact that the vast majority of protocol peerings 92 are established between routers that are adjacent [PEERING]. Thus 93 most protocol peerings are either directly between connected 94 interfaces or at the worst case, are between loopback and loopback, 95 with static routes to loopbacks. Since TTL spoofing is considered 96 nearly impossible, a mechanism based on an expected TTL value can 97 provide a simple and reasonably robust defense from infrastructure 98 attacks based on forged protocol packets. 100 Finally, the GTSM mechanism is equally applicable to both TTL (IPv4) 101 and Hop Limit (IPv6), and from the perspective of GTSM, TTL and Hop 102 Limit have identical semantics. As a result, in the remainder of this 103 document the term "TTL" is used to refer to both TTL or Hop Limit (as 104 appropriate). 106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 108 document are to be interpreted as described in BCP 14, RFC 2119 109 [RFC2119]. 111 2. Assumptions Underlying GTSM 113 GTSM is predicated upon the following assumptions: 115 (i) The vast majority of protocol peerings are between adjacent 116 routers [PEERING]. 118 (ii) It is common practice for many service providers to 119 ingress filter (deny) packets that have the provider's 120 loopback addresses as the source IP address. 122 (iii) Use of GTSM is OPTIONAL, and can be configured on a 123 per-peer (group) basis. 125 (iv) The router supports a method of classifying traffic 126 destined for the route processor into interesting/control 127 and not-control queues. 129 (v) The peer routers both implement GTSM. 131 2.1. GTSM Negotiation 133 This document assumes that GTSM will be manually configured between 134 protocol peers. That is, no automatic GTSM capability negotiation, 135 such as is envisioned by RFC 2842 [RFC2842] is assumed or defined. 137 2.2. Assumptions on Attack Sophistication 139 Throughout this document, we assume that potential attackers have 140 evolved in both sophistication and access to the point that they can 141 send control traffic to a protocol session, and that this traffic 142 appears to be valid control traffic (i.e., has the source/destination 143 of configured peer routers). 145 We also assume that each router in the path between the attacker and 146 the victim protocol speaker decrements TTL properly (clearly, if 147 either the path or the adjacent peer is compromised, then there are 148 worse problems to worry about). 150 Since the vast majority of our peerings are between adjacent routers, 151 we can set the TTL on the protocol packets to 255 (the maximum 152 possible for IP) and then reject any protocol packets that come in 153 from configured peers which do NOT have an inbound TTL of 255. 155 GTSM can be disabled for applications such as route-servers and other 156 large diameter multi-hop peerings. In the event that an the attack 157 comes in from a compromised multi-hop peering, that peering can be 158 shut down (a method to reduce exposure to multi-hop attacks is 159 outlined below). 161 3. GTSM Procedure 163 GTSM SHOULD NOT be enabled by default. The following process 164 describes the per-peer behavior: 166 (i) If GTSM is enabled, an implementation performs the 167 following procedure: 169 (a) For directly connected routers, 171 o Set the outbound TTL for the protocol connection to 172 255. 174 o For each configured protocol peer: 176 Update the receive path Access Control List (ACL) 177 or firewall to only allow protocol packets to pass 178 onto the Route Processor (RP) that have the correct 179 tuple. The TTL must 180 either be 255 (for a directly connected peer), or 181 255-(configured-range-of-acceptable-hops) 182 for a multi-hop peer. We specify a range here to 183 achieve some robustness to changes in topology. Any 184 directly connected check MUST be disabled for such 185 peerings. 187 It is assumed that a receive path ACL is an ACL 188 that is designed to control which packets are 189 allowed to go to the RP. This procedure will only 190 allow protocol packets from adjacent router to pass 191 onto the RP. 193 (b) If the inbound TTL is less than 255 for a directly 194 connected peer, or less than 195 255-(configured-range-of-acceptable-hops) for a 196 multi-hop peer, the packet is NOT processed. Rather, 197 the packet is placed into a low priority queue, and 198 subsequently logged and/or silently discarded. In 199 this case, an ICMP message MUST NOT be generated. 201 (ii) If GTSM is not enabled, normal protocol behavior is followed. 203 3.1. Multi-hop Scenarios 205 When a multi-hop protocol session is required, we set the expected 206 TTL value to be 255-(configured-range-of-acceptable-hops). This 207 approach provides a qualitatively lower degree of security for the 208 protocol implementing GTSM (i.e., a DoS attack could theoretically be 209 launched by compromising some box in the path). However, GTSM will 210 still catch the vast majority of observed DDoS attacks against a 211 given protocol. Note that since the number of hops can change rapidly 212 in real network situations, it is considered that GTSM may not be 213 able to handle this scenario adequately and an implementation MAY 214 provide OPTIONAL support. 216 3.1.1. Intra-domain Protocol Handling 218 In general, GTSM is not used for intra-domain protocol peers or 219 adjacencies. The special case of iBGP peers can be protected by 220 filtering at the network edge for any packet that has a source 221 address of one of the loopback addresses used for the intra-domain 222 peering. In addition, the current best practice is to further protect 223 such peers or adjacencies with an MD5 signature [RFC2385]. 225 4. Acknowledgments 227 The use of the TTL field to protect BGP originated with many 228 different people, including Paul Traina and Jon Stewart. Ryan 229 McDowell also suggested a similar idea. Steve Bellovin, Jay 230 Borkenhagen, Randy Bush, Alfred Hoenes, Vern Paxon, Pekka Savola, and 231 Robert Raszuk also provided useful feedback on earlier versions of 232 this document. David Ward provided insight on the generalization of 233 the original BGP-specific idea. 235 5. Security Considerations 237 GTSM is a simple procedure that protects single hop protocol 238 sessions, except in those cases in which the peer has been 239 compromised. 241 5.1. TTL (Hop Limit) Spoofing 243 The approach described here is based on the observation that a TTL 244 (or Hop Limit) value of 255 is non-trivial to spoof, since as the 245 packet passes through routers towards the destination, the TTL is 246 decremented by one. As a result, when a router receives a packet, it 247 may not be able to determine if the packet's IP address is valid, but 248 it can determine how many router hops away it is (again, assuming 249 none of the routers in the path are compromised in such a way that 250 they would reset the packet's TTL). 252 Note, however, that while engineering a packet's TTL such that it has 253 a particular value when sourced from an arbitrary location is 254 difficult (but not impossible), engineering a TTL value of 255 from 255 non-directly connected locations is not possible (again, assuming 256 none of the directly connected neighbors are compromised, the packet 257 hasn't been tunneled to the decapsulator, and the intervening routers 258 are operating in accordance with RFC 791 [RFC791]). 260 5.2. Tunneled Packets 262 An exception to the observation that a packet with TTL of 255 is 263 difficult to spoof occurs when a protocol packet is tunneled to a 264 decapsulator who then forwards the packet to a directly connected 265 protocol peer. In this case the decapsulator (tunnel endpoint) can 266 either be the penultimate hop, or the last hop itself. A related case 267 arises when the protocol packet is tunneled directly to the protocol 268 peer (the protocol peer is the decapsulator). 270 When the protocol packet is encapsulated in IP, it is possible to 271 spoof the TTL. It may also be impossible to legitimately get the 272 packet to the protocol peer with a TTL of 255, as in the IP in MPLS 273 cases described below. 275 Finally, note that the security of any tunneling technique depends 276 heavily on authentication at the tunnel endpoints, as well as how the 277 tunneled packets are protected in flight. Such mechanisms are, 278 however, beyond the scope of this memo. 280 5.2.1. IP in IP 282 Protocol packets may be tunneled over IP directly to a protocol peer, 283 or to a decapsulator (tunnel endpoint) that then forwards the packet 284 to a directly connected protocol peer (e.g., in IP-in-IP [RFC2003], 285 GRE [RFC2784], or various forms of IPv6-in-IPv4 [RFC2893]). These 286 cases are depicted below. 288 Peer router ---------- Tunnel endpoint router and peer 289 TTL=255 [tunnel] [TTL=255 at ingress] 290 [TTL=255 at egress] 292 Peer router ---------- Tunnel endpoint router ----- On-link peer 293 TTL=255 [tunnel] [TTL=255 at ingress] [TTL=254 at ingress] 294 [TTL=254 at egress] 296 In the first case, in which the encapsulated packet is tunneled 297 directly to the protocol peer, the encapsulated packet's TTL can be 298 set arbitrary value. In the second case, in which the encapsulated 299 packet is tunneled to a decapsulator (tunnel endpoint) which then 300 forwards it to a directly connected protocol peer, RFC 2003 specifies 301 the following behavior: 303 When encapsulating a datagram, the TTL in the inner IP 304 header is decremented by one if the tunneling is being 305 done as part of forwarding the datagram; otherwise, the 306 inner header TTL is not changed during encapsulation. If 307 the resulting TTL in the inner IP header is 0, the 308 datagram is discarded and an ICMP Time Exceeded message 309 SHOULD be returned to the sender. An encapsulator MUST 310 NOT encapsulate a datagram with TTL = 0. 312 Hence the inner IP packet header's TTL, as seen by the decapsulator, 313 can be set to an arbitrary value (in particular, 255). As a result, 314 it may not be possible to deliver the protocol packet to the peer 315 with a TTL of 255. 317 5.2.2. IP in MPLS 319 Protocol packets may also be tunneled over MPLS to a protocol peer 320 which either the penultimate hop (when the penultimate hop popping 321 (PHP) is employed [RFC3032]), or one hop beyond the penultimate hop. 322 These cases are depicted below. 324 Peer router ---------- Penultimate Hop (PH) and peer 325 TTL=255 [tunnel] [TTL=255 at ingress] 326 [TTL<=254 at egress] 328 Peer router ---------- Penultimate Hop -------- On-link peer 329 TTL=255 [tunnel] [TTL=255 at ingress] [TTL <=254 at ingress] 330 [TTL<=254 at egress] 332 TTL handling for these cases is described in RFC 3032. RFC 3032 333 states that when the IP packet is first labeled: 335 ... the TTL field of the label stack entry MUST BE set to the 336 value of the IP TTL field. (If the IP TTL field needs to be 337 decremented, as part of the IP processing, it is assumed that 338 this has already been done.) 340 When the label is popped: 342 When a label is popped, and the resulting label stack is empty, 343 then the value of the IP TTL field SHOULD BE replaced with the 344 outgoing TTL value, as defined above. In IPv4 this also 345 requires modification of the IP header checksum. 347 where the definition of "outgoing TTL" is: 349 The "incoming TTL" of a labeled packet is defined to be the 350 value of the TTL field of the top label stack entry when the 351 packet is received. 353 The "outgoing TTL" of a labeled packet is defined to be the larger of: 355 a) one less than the incoming TTL, 356 b) zero. 358 In either of these cases, the minimum value by which the TTL could be 359 decremented would be one (the network operator prefers to hide its 360 infrastructure by decrementing the TTL by the minimum number of LSP 361 hops, one, rather than decrementing the TTL as it traverses its MPLS 362 domain). As a result, the maximum TTL value at egress from the MPLS 363 cloud is 254 (255-1), and as a result the check described in section 364 3 will fail. 366 5.3. Multi-Hop Protocol Sessions 368 While the GTSM method is less effective for multi-hop protocol 369 sessions, it does close the window on several forms of attack. 370 However, in the multi-hop scenario GTSM is an OPTIONAL extension. 371 Protection of the protocol infrastructure beyond what is provided by 372 the GTSM method will likely require cryptographic machinery such as 373 is envisioned by Secure BGP (S-BGP) [SBGP1,SBGP2], and/or other 374 extensions. Finally, note that in the multi-hop case described 375 above, we specify a range of acceptable TTLs in order to achieve some 376 robustness to topology changes. This robustness to topological 377 change comes at the cost of the loss of some robustness to different 378 forms of attack. 380 6. IANA Considerations 382 This document creates no new requirements on IANA namespaces 383 [RFC2434]. 385 7. References 387 7.1. Normative References 389 [RFC791] Postel, J., "Internet Protocol Specification", 390 STD 5, RFC 791, September 1981. 392 [RFC1771] Rekhter, Y. and T. Li (Editors), "A Border 393 Gateway Protocol (BGP-4)", RFC 1771, March 1995. 395 [RFC1772] Rekhter, Y. and P. Gross, "Application of the 396 Border Gateway Protocol in the Internet", RFC 397 1772, March 1995. 399 [RFC2003] Perkins, C., "IP Encapsulation with IP", RFC 400 2003, October 1996. 402 [RFC2119] Bradner, S., "Key words for use in RFCs to 403 Indicate Requirement Levels", BCP 14, RFC 2119, 404 March 1997. 406 [RFC2385] Heffernan, A., "Protection of BGP Sessions via 407 the TCP MD5 Signature Option", RFC 2385, August 408 1998. 410 [RFC2461] Narten, T., Nordmark, E. and W. Simpson, 411 "Neighbor Discover for IP Version 6 (IPv6)", RFC 412 2461, December 1998. 414 [RFC2784] Farinacci, D., "Generic Routing Encapsulation 415 (GRE)", RFC 2784, March 2000. 417 [RFC2842] Chandra, R. and J. Scudder, "Capabilities 418 Advertisement with BGP-4", RFC 2842, May 2000. 420 [RFC2893] Gilligan, R. and E. Nordmark, "Transition 421 Mechanisms for IPv6 Hosts and Routers", RFC 2893, 422 August 2000. 424 [RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, 425 A. and B. Thomas, "LDP Specification", RFC 3036, 426 January 2001. 428 [RFC3032] Rosen, E. Tappan, D., Fedorkow, G., Rekhter, Y., 429 Farinacci, D., Li, T. and A. Conta, "MPLS Label 430 Stack Encoding", RFC 3032, January 2001. 432 [SBGP1] Kent, S., C. Lynn, and K. Seo, "Secure Border 433 Gateway Protocol (Secure-BGP)", IEEE Journal on 434 Selected Areas in Communications, volume 18, 435 number 4, April 2000. 437 [SBGP2] Kent, S., C. Lynn, J. Mikkelson, and K. Seo, 438 "Secure Border Gateway Protocol (S-BGP) -- Real 439 World Performance and Deployment Issues", 440 Proceedings of the IEEE Network and Distributed 441 System Security Symposium, February, 2000. 443 7.2. Informative References 445 [BFD] Katz, D. and D. Ward, "Bidirectional Forwarding 446 Detection", draft-katz-ward-bfd-02.txt, Work in 447 Progress. 449 [PEERING] Empirical data gathered from the Sprint and AOL 450 backbones, October, 2002. 452 [RFC2028] Hovey, R. and S. Bradner, "The Organizations 453 Involved in the IETF Standards Process", BCP 11, 454 RFC 2028, October 1996. 456 [RFC2434] Narten, T., and H. Alvestrand, "Guidelines for 457 Writing an IANA Considerations Section in RFCs", 458 BCP 26, RFC 2434, October 1998. 460 [RFC3618] Meyer, D. and W. Fenner, Eds., "The Multicast 461 Source Discovery Protocol (MSDP)", RFC 3618, 462 October 2003. 464 8. Authors' Addresses 466 Vijay Gill 467 EMail: vijay@umbc.edu 469 John Heasley 470 EMail: heas@shrubbery.net 472 David Meyer 473 EMail: dmm@1-4-5.net 475 9. Full Copyright Statement 477 Copyright (C) The Internet Society (2004). 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