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'SBGP1' -- Possible downref: Non-RFC (?) normative reference: ref. 'SBGP2' -- Obsolete informational reference (is this intentional?): RFC 2028 (Obsoleted by RFC 9281) -- Obsolete informational reference (is this intentional?): RFC 2434 (Obsoleted by RFC 5226) Summary: 14 errors (**), 0 flaws (~~), 9 warnings (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT V. Gill 2 draft-ietf-rtgwg-rfc3682bis-00.txt J. Heasley 3 D. Meyer 4 Category Experimental 5 Expires: September 2004 March 2004 7 The Generalized TTL Security Mechanism (GTSM) 8 10 Status of this Document 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 This document is a product of the RTG WG. Comments should be 32 addressed to the authors, or the mailing list at rtgwg@ietf.org. 34 Copyright Notice 36 Copyright (C) The Internet Society (2004). All Rights Reserved. 38 Abstract 40 The use of a packet's TTL (IPv4) or Hop Limit (IPv6) to protect a 41 protocol stack from CPU-utilization based attacks has been proposed 42 in many settings (see for example, RFC 2461). This document 43 generalizes these techniques for use by other protocols such as BGP 44 (RFC 1771), MSDP, Bidirectional Forwarding Detection, and LDP (RFC 45 3036). While the Generalized TTL Security Mechanism (GTSM) is most 46 effective in protecting directly connected protocol peers, it can 47 also provide a lower level of protection to multi-hop sessions. GTSM 48 is not directly applicable to protocols employing flooding mechanisms 49 (e.g., multicast), and use of multi-hop GTSM should be considered on 50 a case-by-case basis. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. Assumptions Underlying GTSM. . . . . . . . . . . . . . . . . . 4 56 2.1. GTSM Negotiation. . . . . . . . . . . . . . . . . . . . . . 4 57 2.2. Assumptions on Attack Sophistication. . . . . . . . . . . . 4 58 3. GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . . 5 59 3.1. Multi-hop Scenarios . . . . . . . . . . . . . . . . . . . . 6 60 3.1.1. Intra-domain Protocol Handling . . . . . . . . . . . . . 6 61 4. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 7 62 5. Security Considerations. . . . . . . . . . . . . . . . . . . . 7 63 5.1. TTL (Hop Limit) Spoofing. . . . . . . . . . . . . . . . . . 7 64 5.2. Tunneled Packets. . . . . . . . . . . . . . . . . . . . . . 8 65 5.2.1. IP in IP . . . . . . . . . . . . . . . . . . . . . . . . 8 66 5.2.2. IP in MPLS . . . . . . . . . . . . . . . . . . . . . . . 9 67 5.3. Multi-Hop Protocol Sessions . . . . . . . . . . . . . . . . 10 68 6. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 11 69 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 70 7.1. Normative References. . . . . . . . . . . . . . . . . . . . 11 71 7.2. Informative References. . . . . . . . . . . . . . . . . . . 12 72 8. Author's Addresses . . . . . . . . . . . . . . . . . . . . . . 13 73 9. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13 74 10. Intellectual Property . . . . . . . . . . . . . . . . . . . . 14 75 11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 14 77 1. Introduction 79 The Generalized TTL Security Mechanism (GTSM) is designed to protect 80 a router's TCP/IP based control plane from CPU-utilization based 81 attacks. In particular, while cryptographic techniques can protect 82 the router-based infrastructure (e.g., BGP [RFC1771]) from a wide 83 variety of attacks, many attacks based on CPU overload can be 84 prevented by the simple mechanism described in this document. Note 85 that the same technique protects against other scarce-resource 86 attacks involving a router's CPU, such as attacks against processor- 87 line card bandwidth. 89 GTSM is based on the fact that the vast majority of protocol peerings 90 are established between routers that are adjacent [PEERING]. Thus 91 most protocol peerings are either directly between connected 92 interfaces or at the worst case, are between loopback and loopback, 93 with static routes to loopbacks. Since TTL spoofing is considered 94 nearly impossible, a mechanism based on an expected TTL value can 95 provide a simple and reasonably robust defense from infrastructure 96 attacks based on forged protocol packets. 98 Finally, the GTSM mechanism is equally applicable to both TTL (IPv4) 99 and Hop Limit (IPv6), and from the perspective of GTSM, TTL and Hop 100 Limit have identical semantics. As a result, in the remainder of this 101 document the term "TTL" is used to refer to both TTL or Hop Limit (as 102 appropriate). 104 The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 105 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 106 document are to be interpreted as described in RFC 2119 [RFC2119]. 108 2. Assumptions Underlying GTSM 110 GTSM is predicated upon the following assumptions: 112 (i). The vast majority of protocol peerings are between adjacent 113 routers [PEERING]. 115 (ii). It is common practice for many service providers to 116 ingress filter (deny) packets that have the provider's 117 loopback addresses as the source IP address. 119 (iii). Use of GTSM is OPTIONAL, and can be configured on a 120 per-peer (group) basis. 122 (iv). The router supports a method of classifying traffic 123 destined for the route processor into interesting/control 124 and not-control queues. 126 (iv). The peer routers both implement GTSM. 128 2.1. GTSM Negotiation 130 This document assumes that GTSM will be manually configured between 131 protocol peers. That is, no automatic GTSM capability negotiation, 132 such as is envisioned by RFC 2842 [RFC2842] is assumed or defined. 134 2.2. Assumptions on Attack Sophistication 136 Throughout this document, we assume that potential attackers have 137 evolved in both sophistication and access to the point that they can 138 send control traffic to a protocol session, and that this traffic 139 appears to be valid control traffic (i.e., has the source/destination 140 of configured peer routers). 142 We also assume that each router in the path between the attacker and 143 the victim protocol speaker decrements TTL properly (clearly, if 144 either the path or the adjacent peer is compromised, then there are 145 worse problems to worry about). 147 Since the vast majority of our peerings are between adjacent routers, 148 we can set the TTL on the protocol packets to 255 (the maximum 149 possible for IP) and then reject any protocol packets that come in 150 from configured peers which do NOT have an inbound TTL of 255. 152 GTSM can be disabled for applications such as route-servers and other 153 large diameter multi-hop peerings. In the event that an the attack 154 comes in from a compromised multi-hop peering, that peering can be 155 shut down (a method to reduce exposure to multi-hop attacks is 156 outlined below). 158 3. GTSM Procedure 160 GTSM SHOULD NOT be enabled by default. The following process 161 describes the per-peer behavior: 163 (i). If GTSM is enabled, an implementation performs the 164 following procedure: 166 (a). For directly connected routers, 168 o Set the outbound TTL for the protocol connection to 169 255. 171 o For each configured protocol peer: 173 Update the receive path Access Control List (ACL) 174 or firewall to only allow protocol packets to pass 175 onto the Route Processor (RP) that have the correct 176 tuple. The TTL must 177 either be 255 (for a directly connected peer), or 178 255-(configured-range-of-acceptable-hops) 179 for a multi-hop peer. We specify a range here to 180 achieve some robustness to changes in topology. Any 181 directly connected check MUST be disabled for such 182 peerings. 184 It is assumed that a receive path ACL is an ACL 185 that is designed to control which packets are 186 allowed to go to the RP. This procedure will only 187 allow protocol packets from an adjacent router to 188 pass onto the RP. 190 (b). If the inbound TTL is less than 255 (for a directly 191 connected peer), or 255-(configured-range-of-acceptable-hops) 192 (for multi-hop peers), the packet is NOT 193 processed. Rather, the packet is placed into a low 194 priority queue, and subsequently logged and/or 195 silently discarded. In this case, an ICMP message 196 MUST NOT be generated. 198 (ii). If GTSM is not enabled, normal protocol behavior is followed. 200 3.1. Multi-hop Scenarios 202 When a multi-hop protocol session is required, we set the expected 203 TTL value to be 255-(configured-range-of-acceptable-hops). This 204 approach provides a qualitatively lower degree of security for the 205 protocol implementing GTSM (i.e., an DoS attack could be 206 theoretically be launched by compromising some box in the path). 207 However, GTSM will still catch the vast majority of observed DDoS 208 attacks against a given protocol. Note that since the number of hops 209 can change rapidly in real network situations, it is considered that 210 GTSM may not be able to handle this scenario adequately and an 211 implementation MAY provide OPTIONAL support. 213 3.1.1. Intra-domain Protocol Handling 215 In general, GTSM is not used for intra-domain protocol peers or 216 adjacencies. The special case of iBGP peers can be protected by 217 filtering at the network edge for any packet that has a source 218 address of one of the loopback addresses used for the intra-domain 219 peering. In addition, the current best practice is to further protect 220 such peers or adjacencies with an MD5 signature [RFC2385]. 222 4. Acknowledgments 224 The use of the TTL field to protect BGP originated with many 225 different people, including Paul Traina and Jon Stewart. Ryan 226 McDowell also suggested a similar idea. Steve Bellovin, Jay 227 Borkenhagen, Randy Bush, Alfred Hoenes, Vern Paxon, Pekka Savola, and 228 Robert Raszuk also provided useful feedback on earlier versions of 229 this document. David Ward provided insight on the generalization of 230 the original BGP-specific idea. 232 5. Security Considerations 234 GTSM is a simple procedure that protects single hop protocol 235 sessions, except in those cases in which the peer has been 236 compromised. 238 5.1. TTL (Hop Limit) Spoofing 240 The approach described here is based on the observation that a TTL 241 (or Hop Limit) value of 255 is non-trivial to spoof, since as the 242 packet passes through routers towards the destination, the TTL is 243 decremented by one. As a result, when a router receives a packet, it 244 may not be able to determine if the packet's IP address is valid, but 245 it can determine how many router hops away it is (again, assuming 246 none of the routers in the path are compromised in such a way that 247 they would reset the packet's TTL). 249 Note, however, that while engineering a packet's TTL such that it has 250 a particular value when sourced from an arbitrary location is 251 difficult (but not impossible), engineering a TTL value of 255 from 252 non-directly connected locations is not possible (again, assuming 253 none of the directly connected neighbors are compromised, the packet 254 hasn't been tunneled to the decapsulator, and the intervening routers 255 are operating in accordance with RFC 791 [RFC791]). 257 5.2. Tunneled Packets 259 An exception to the observation that a packet with TTL of 255 is 260 difficult to spoof occurs when a protocol packet is tunneled to a 261 decapsulator who then forwards the packet to a directly connected 262 protocol peer. In this case the decapsulator (tunnel endpoint) can 263 either be the penultimate hop, or the last hop itself. A related case 264 arises when the protocol packet is tunneled directly to the protocol 265 peer (the protocol peer is the decapsulator). 267 When the protocol packet is encapsulated in IP, it is possible to 268 spoof the TTL. It may also be impossible to legitimately get the 269 packet to the protocol peer with a TTL of 255, as in the IP in MPLS 270 cases described below. 272 Finally, note that the security of any tunneling technique depends 273 heavily on authentication at the tunnel endpoints, as well as how the 274 tunneled packets are protected in flight. Such mechanisms are, 275 however, beyond the scope of this memo. 277 5.2.1. IP in IP 279 Protocol packets may be tunneled over IP directly to a protocol peer, 280 or to a decapsulator (tunnel endpoint) that then forwards the packet 281 to a directly connected protocol peer (e.g., in IP-in-IP [RFC2003], 282 GRE [RFC2784], or various forms of IPv6-in-IPv4 [RFC2893]). These 283 cases are depicted below. 285 Peer router ---------- Tunnel endpoint router and peer 286 TTL=255 [tunnel] [TTL=255 at ingress] 287 [TTL=255 at egress] 289 Peer router ---------- Tunnel endpoint router ----- On-link peer 290 TTL=255 [tunnel] [TTL=255 at ingress] [TTL=254 at ingress] 291 [TTL=254 at egress] 293 In the first case, in which the encapsulated packet is tunneled 294 directly to the protocol peer, the encapsulated packet's TTL can be 295 set arbitrary value. In the second case, in which the encapsulated 296 packet is tunneled to a decapsulator (tunnel endpoint) which then 297 forwards it to a directly connected protocol peer, RFC 2003 specifies 298 the following behavior: 300 When encapsulating a datagram, the TTL in the inner IP 301 header is decremented by one if the tunneling is being 302 done as part of forwarding the datagram; otherwise, the 303 inner header TTL is not changed during encapsulation. If 304 the resulting TTL in the inner IP header is 0, the 305 datagram is discarded and an ICMP Time Exceeded message 306 SHOULD be returned to the sender. An encapsulator MUST 307 NOT encapsulate a datagram with TTL = 0. 309 Hence the inner IP packet header's TTL, as seen by the decapsulator, 310 can be set to an arbitrary value (in particular, 255). As a result, 311 it may not be possible to deliver the protocol packet to the peer 312 with a TTL of 255. 314 5.2.2. IP in MPLS 316 Protocol packets may also be tunneled over MPLS to a protocol peer 317 which either the penultimate hop (when the penultimate hop popping 318 (PHP) is employed [RFC3032]), or one hop beyond the penultimate hop. 319 These cases are depicted below. 321 Peer router ---------- Penultimate Hop (PH) and peer 322 TTL=255 [tunnel] [TTL=255 at ingress] 323 [TTL<=254 at egress] 325 Peer router ---------- Penultimate Hop -------- On-link peer 326 TTL=255 [tunnel] [TTL=255 at ingress] [TTL <=254 at ingress] 327 [TTL<=254 at egress] 329 TTL handling for these cases is described in RFC 3032. RFC 3032 330 states that when the IP packet is first labeled: 332 ... the TTL field of the label stack entry MUST BE set to the 333 value of the IP TTL field. (If the IP TTL field needs to be 334 decremented, as part of the IP processing, it is assumed that 335 this has already been done.) 337 When the label is popped: 339 When a label is popped, and the resulting label stack is empty, 340 then the value of the IP TTL field SHOULD BE replaced with the 341 outgoing TTL value, as defined above. In IPv4 this also 342 requires modification of the IP header checksum. 344 where the definition of "outgoing TTL" is: 346 The "incoming TTL" of a labeled packet is defined to be the 347 value of the TTL field of the top label stack entry when the 348 packet is received. 350 The "outgoing TTL" of a labeled packet is defined to be the larger of: 352 a) one less than the incoming TTL, 353 b) zero. 355 In either of these cases, the minimum value by which the TTL could be 356 decremented would be one (the network operator prefers to hide its 357 infrastructure by decrementing the TTL by the minimum number of LSP 358 hops, one, rather than decrementing the TTL as it traverses its MPLS 359 domain). As a result, the maximum TTL value at egress from the MPLS 360 cloud is 254 (255-1), and as a result the check described in section 361 3 will fail. 363 5.3. Multi-Hop Protocol Sessions 365 While the GTSM method is less effective for multi-hop protocol 366 sessions, it does close the window on several forms of attack. 367 However, in the multi-hop scenario GTSM is an OPTIONAL extension. 368 Protection of the protocol infrastructure beyond what is provided by 369 the GTSM method will likely require cryptographic machinery such as 370 is envisioned by Secure BGP (S-BGP) [SBGP1,SBGP2], and/or other 371 extensions. Finally, note that in the multi-hop case described above, 372 we specify a range of acceptable TTLs in order to achieve some 373 robustness to topology changes. This robustness to topological change 374 comes at the cost of the loss some robustness to different forms of 375 attack. 377 6. IANA Considerations 379 This document creates a no new requirements on IANA namespaces 380 [RFC2434]. 382 7. References 384 7.1. Normative References 386 [RFC791] Postel, J., "INTERNET PROTOCOL PROTOCOL 387 SPECIFICATION", RFC 791, September, 1981. 389 [RFC1771] Rekhter, Y., and T. Li (Editors), "A Border 390 Gateway Protocol (BGP-4)", RFC 1771, March, 391 1995. 393 [RFC1772] Rekhter, Y., and P. Gross, "Application of the 394 Border Gateway Protocol in the Internet", RFC 395 1772, March, 1995. 397 [RFC2003] Perkins, C., "IP Encapsulation with IP", RFC 398 2003, October, 1996. 400 [RFC2119] Bradner, S., "Key words for use in RFCs to 401 Indicate Requirement Levels", RFC 2119, March, 402 1997. 404 [RFC2385] Heffernan, A., "Protection of BGP Sessions via 405 the TCP MD5 Signature Option", RFC 2385, August, 406 1998. 408 [RFC2461] Narten, T., E. Nordmark, and W. Simson, "Neighbor 409 Discovery for IP Version 6 (IPv6)", RFC 2461, 410 December, 1998. 412 [RFC2784] Farinacci, D., "Generic Routing Encapsulation 413 (GRE)", RFC 2784, March, 2000. 415 [RFC2842] Chandra, R. and J. Scudder, "Capabilities 416 Advertisement with BGP-4", RFC 2842, May, 2000. 418 [RFC2893] Gilligan, R., and E. Nordmark, "Transition 419 Mechanisms for IPv6 Hosts and Routers", RFC 2893, 420 August, 2000. 422 [RFC3036] Andersson, L., et. al., "LDP Specification", RFC 423 3036, January, 2001. January, 2001. 425 [RFC3032] Rosen, E., et. al., "MPLS Label Stack Encoding", 426 RFC 3032, 428 [SBGP1] Kent, S., C. Lynn, and K. Seo, "Secure Border 429 Gateway Protocol (Secure-BGP)", IEEE Journal on 430 Selected Areas in Communications, volume 18, 431 number 4, April, 2000. 433 [SBGP2] Kent, S., C. Lynn, J. Mikkelson, and K. Seo, 434 "Secure Border Gateway Protocol (S-BGP) -- Real 435 World Performance and Deployment Issues", 436 Proceedings of the IEEE Network and Distributed 437 System Security Symposium, February, 2000. 439 7.2. Informative References 441 [BFD] Katz, D. and D. Ward, "Bidirectional Forwarding 442 Detection", draft-katz-ward-bfd-02.txt. Work in 443 progress. 445 [PEERING] Empirical data gathered from the Sprint and AOL 446 backbones, October, 2002. 448 [RFC2028] Hovey, R. and S. Bradner, "The Organizations 449 Involved in the IETF Standards Process", RFC 450 2028/BCP 11, October, 1996. 452 [RFC2434] Narten, T., and H. Alvestrand, "Guidelines for 453 Writing an IANA Considerations Section in 454 RFCs", RFC 2434/BCP 0026, October, 1998. 456 [RFC3618] Meyer, D., and W. Fenner (Editors), "The Multicast 457 Source Discovery Protocol (MSDP)", RFC 3618, 458 October, 2003. 460 8. Author's Addresses 462 Vijay Gill 463 Email: vijay@umbc.edu 465 John Heasley 466 Email: heas@shrubbery.net 468 David Meyer 469 Email: dmm@1-4-5.net 471 9. Full Copyright Statement 473 Copyright (C) The Internet Society (2004). This document is subject 474 to the rights, licenses and restrictions contained in BCP 78 and 475 except as set forth therein, the authors retain all their rights. 477 This document and translations of it may be copied and furnished to 478 others, and derivative works that comment on or otherwise explain it 479 or assist in its implementation may be prepared, copied, published 480 and distributed, in whole or in part, without restriction of any 481 kind, provided that the above copyright notice and this paragraph are 482 included on all such copies and derivative works. However, this 483 document itself may not be modified in any way, such as by removing 484 the copyright notice or references to the Internet Society or other 485 Internet organizations, except as needed for the purpose of 486 developing Internet standards in which case the procedures for 487 copyrights defined in the Internet Standards process must be 488 followed, or as required to translate it into languages other than 489 English. 491 The limited permissions granted above are perpetual and will not be 492 revoked by the Internet Society or its successors or assigns. 494 This document and the information contained herein is provided on an 495 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 496 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 497 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 498 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 499 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 501 10. Intellectual Property 503 The IETF takes no position regarding the validity or scope of any 504 Intellectual Property Rights or other rights that might be claimed to 505 pertain to the implementation or use of the technology described in 506 this document or the extent to which any license under such rights 507 might or might not be available; nor does it represent that it has 508 made any independent effort to identify any such rights. Information 509 on the procedures with respect to rights in RFC documents can be 510 found in BCP 78 and BCP 79. 512 Copies of IPR disclosures made to the IETF Secretariat and any 513 assurances of licenses to be made available, or the result of an 514 attempt made to obtain a general license or permission for the use of 515 such proprietary rights by implementers or users of this 516 specification can be obtained from the IETF on-line IPR repository at 517 http://www.ietf.org/ipr. 519 The IETF invites any interested party to bring to its attention any 520 copyrights, patents or patent applications, or other proprietary 521 rights that may cover technology that may be required to implement 522 this standard. Please address the information to the IETF at ietf- 523 ipr@ietf.org. 525 11. Acknowledgement 527 Funding for the RFC Editor function is currently provided by the 528 Internet Society.