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'SBGP1' -- Possible downref: Non-RFC (?) normative reference: ref. 'SBGP2' == Outdated reference: A later version (-02) exists of draft-katz-ward-bfd-00 -- Obsolete informational reference (is this intentional?): RFC 2028 (Obsoleted by RFC 9281)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: 8 errors (**), 0 flaws (~~), 12 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT V. Gill 2 draft-gill-gtsh-04.txt J. Heasley 3 D. Meyer 4 Category Experimental 5 Expires: April 2004 October 2003 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 The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 32 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 33 document are to be interpreted as described in RFC 2119 [RFC 2119]. 35 This document is an individual submission. Comments are solicited and 36 should be addressed to the author(s). 38 Copyright Notice 40 Copyright (C) The Internet Society (2003). All Rights Reserved. 42 Abstract 44 The use of a packet's TTL (IPv4) or Hop Limit (IPv6) to protect a 45 protocol stack from CPU-utilization based attacks has been proposed 46 in many settings (see for example, RFC 2461). This document 47 generalizes these techniques for use by other protocols such as BGP 48 (RFC 1771), MSDP, Bidirectional Forwarding Detection, and LDP (RFC 49 3036). While the Generalized TTL Security Mechanism (GTSM) is most 50 effective in protecting directly connected protocol peers, it can 51 also provide a lower level of protection to multi-hop sessions. GTSM 52 is not directly applicable to protocols employing flooding mechanisms 53 (e.g., multicast), and use of multi-hop GTSM should be considered on 54 a case-by-case basis. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. Assumptions Underlying GTSM. . . . . . . . . . . . . . . . . . 4 60 2.1. GTSM Negotiation. . . . . . . . . . . . . . . . . . . . . . 4 61 2.2. Assumptions on Attack Sophistication. . . . . . . . . . . . 4 62 3. GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . . 5 63 3.1. Multi-hop Scenarios . . . . . . . . . . . . . . . . . . . . 6 64 3.1.1. Intra-domain Protocol Handling . . . . . . . . . . . . . 6 65 4. Intellectual Property. . . . . . . . . . . . . . . . . . . . . 6 66 5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 7 67 6. Security Considerations. . . . . . . . . . . . . . . . . . . . 7 68 6.1. TTL (Hop Limit) Spoofing. . . . . . . . . . . . . . . . . . 7 69 6.2. Tunneled Packets. . . . . . . . . . . . . . . . . . . . . . 8 70 6.2.1. IP in IP . . . . . . . . . . . . . . . . . . . . . . . . 8 71 6.2.2. IP in MPLS . . . . . . . . . . . . . . . . . . . . . . . 9 72 6.3. Multi-Hop Protocol Sessions . . . . . . . . . . . . . . . . 10 73 7. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 11 74 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 75 8.1. Normative References. . . . . . . . . . . . . . . . . . . . 11 76 8.2. Informative References. . . . . . . . . . . . . . . . . . . 12 77 9. Author's Addresses . . . . . . . . . . . . . . . . . . . . . . 13 78 10. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 13 80 1. Introduction 82 The Generalized TTL Security Mechanism (GTSM) is designed to protect 83 a router's TCP/IP based control plane from CPU-utilization based 84 attacks. In particular, while cryptographic techniques can protect 85 the router-based infrastructure (e.g., BGP [RFC1771]) from a wide 86 variety of attacks, many attacks based on CPU overload can be 87 prevented by the simple mechanism described in this document. Note 88 that the same technique protects against other scarce-resource 89 attacks involving a router's CPU, such as attacks against processor- 90 line card bandwidth. 92 GTSM is based on the fact that the vast majority of protocol peerings 93 are established between routers that are adjacent [PEERING]. Thus 94 most protocol peerings are either directly between connected 95 interfaces or at the worst case, are between loopback and loopback, 96 with static routes to loopbacks. Since TTL spoofing is considered 97 nearly impossible, a mechanism based on an expected TTL value can 98 provide a simple and reasonably robust defense from infrastructure 99 attacks based on forged protocol packets. 101 Finally, the GTSM mechanism is equally applicable to both TTL (IPv4) 102 and Hop Limit (IPv6), and from the perspective of GTSM, TTL and Hop 103 Limit have identical semantics. As a result, in the remainder of this 104 document the term "TTL" is used to refer to both TTL or Hop Limit (as 105 appropriate). 107 2. Assumptions Underlying GTSM 109 GTSM is predicated upon the following assumptions: 111 (i). The vast majority of protocol peerings are between adjacent 112 routers [PEERING]. 114 (ii). It is common practice for many service providers to 115 ingress filter (deny) packets that have the provider's 116 loopback addresses as the source IP address. 118 (iii). Use of GTSM is OPTIONAL, and can be configured on a 119 per-peer (group) basis. 121 (iv). The router supports a method of classifying traffic 122 destined for the route processor into interesting/control 123 and not-control queues. 125 (iv). The peer routers both implement GTSM. 127 2.1. GTSM Negotiation 129 This document assumes that GTSM will be manually configured between 130 protocol peers. That is, no automatic GTSM capability negotiation, 131 such as is envisioned by RFC 2842 [RFC2842] is assumed or defined. 133 2.2. Assumptions on Attack Sophistication 135 Throughout this document, we assume that potential attackers have 136 evolved in both sophistication and access to the point that they can 137 send control traffic to a protocol session, and that this traffic 138 appears to be valid control traffic (i.e., has the source/destination 139 of configured peer routers). 141 We also assume that each router in the path between the attacker and 142 the victim protocol speaker decrements TTL properly (clearly, if 143 either the path or the adjacent peer is compromised, then there are 144 worse problems to worry about). 146 Since the vast majority of our peerings are between adjacent routers, 147 we can set the TTL on the protocol packets to 255 (the maximum 148 possible for IP) and then reject any protocol packets that come in 149 from configured peers which do NOT have an inbound TTL of 255. 151 GTSM can be disabled for applications such as route-servers and other 152 large diameter multi-hop peerings. In the event that an the attack 153 comes in from a compromised multi-hop peering, that peering can be 154 shut down (a method to reduce exposure to multi-hop attacks is 155 outlined below). 157 3. GTSM Procedure 159 GTSM SHOULD NOT be enabled by default. The following process 160 describes the per-peer behavior: 162 (i). If GTSM is enabled, an implementation performs the 163 following procedure: 165 (a). For directly connected routers, 167 o Set the outbound TTL for the protocol connection to 168 255. 170 o For each configured protocol peer: 172 Update the receive path Access Control List (ACL) 173 or firewall to only allow protocol packets to pass 174 onto the Route Processor (RP) that have the correct 175 tuple. The TTL must 176 either be 255 (for a directly connected peer), or 177 255-(configured-range-of-acceptable-hops) 178 for a multi-hop peer. We specify a range here to 179 achieve some robustness to changes in topology. Any 180 directly connected check MUST be disabled for such 181 peerings. 183 It is assumed that a receive path ACL is an ACL 184 that is designed to control which packets are 185 allowed to go to the RP. This procedure will only 186 allow protocol packets from adjacent router to pass 187 onto the RP. 189 (b). If the inbound TTL is 255 (for a directly connected 190 peer), or 255-(configured-range-of-acceptable-hops) 191 (for multi-hop peers), the packet is NOT 192 processed. Rather, the packet is placed into a low 193 priority queue, and subsequently logged and/or 194 silently discarded. In this case, an ICMP message 195 MUST NOT be generated. 197 (ii). If GTSM is not enabled, normal protocol behavior is followed. 199 3.1. Multi-hop Scenarios 201 When a multi-hop protocol session is required, we set the expected 202 TTL value to be 255-(configured-range-of-acceptable-hops). This 203 approach provides a qualitatively lower degree of security for the 204 protocol implementing GTSM (i.e., an DoS attack could be 205 theoretically be launched by compromising some box in the path). 206 However, GTSM will still catch the vast majority of observed DDoS 207 attacks against a given protocol. Note that since the number of hops 208 can change rapidly in real network situations, it is considered that 209 GTSM may not be able to handle this scenario adequately and an 210 implementation MAY provide OPTIONAL support. 212 3.1.1. Intra-domain Protocol Handling 214 In general, GTSM is not used for intra-domain protocol peers or 215 adjacencies. The special case of iBGP peers can be protected by 216 filtering at the network edge for any packet that has a source 217 address of one of the loopback addresses used for the intra-domain 218 peering. In addition, the current best practice is to further protect 219 such peers or adjacencies with an MD5 signature [RFC2385]. 221 4. Intellectual Property 223 The IETF takes no position regarding the validity or scope of any 224 intellectual property or other rights that might be claimed to 225 pertain to the implementation or use of the technology described in 226 this document or the extent to which any license under such rights 227 might or might not be available; neither does it represent that it 228 has made any effort to identify any such rights. Information on the 229 IETF's procedures with respect to rights in standards-track and 230 standards-related documentation can be found in BCP-11 [RFC2028]. 231 Copies of claims of rights made available for publication and any 232 assurances of licenses to be made available, or the result of an 233 attempt made to obtain a general license or permission for the use of 234 such proprietary rights by implementors or users of this 235 specification can be obtained from the IETF Secretariat. 237 The IETF invites any interested party to bring to its attention any 238 copyrights, patents or patent applications, or other proprietary 239 rights which may cover technology that may be required to practice 240 this standard. Please address the information to the IETF Executive 241 Director. 243 5. Acknowledgments 245 The use of the TTL field to protect BGP originated with many 246 different people, including Paul Traina and Jon Stewart. Ryan 247 McDowell also suggested a similar idea. Steve Bellovin, Jay 248 Borkenhagen, Randy Bush, Vern Paxon, Pekka Savola, and Robert Raszuk 249 also provided useful feedback on earlier versions of this document. 250 David Ward provided insight on the generalization of the original 251 BGP-specific idea. 253 6. Security Considerations 255 GTSM is a simple procedure that protects single hop protocol 256 sessions, except in those cases in which the peer has been 257 compromised. 259 6.1. TTL (Hop Limit) Spoofing 261 The approach described here is based on the observation that a TTL 262 (or Hop Limit) value of 255 is non-trivial to spoof, since as the 263 packet passes through routers towards the destination, the TTL is 264 decremented by one. As a result, when a router receives a packet, it 265 may not be able to determine if the packet's IP address is valid, but 266 it can determine how many router hops away it is (again, assuming 267 none of the routers in the path are compromised in such a way that 268 they would reset the packet's TTL). 270 Note, however, that while engineering a packet's TTL such that it has 271 a particular value when sourced from an arbitrary location is 272 difficult (but not impossible), engineering a TTL value of 255 from 273 non-directly connected locations is not possible (again, assuming 274 none of the directly connected neighbors are compromised, the packet 275 hasn't been tunneled to the decapsulator, and the intervening routers 276 are operating in accordance with RFC 791 [RFC791]). 278 6.2. Tunneled Packets 280 An exception to the observation that a packet with TTL of 255 is 281 difficult to spoof occurs when a protocol packet is tunneled to a 282 decapsulator who then forwards the packet to a directly connected 283 protocol peer. In this case the decapsulator (tunnel endpoint) can 284 either be the penultimate hop, or the last hop itself. A related case 285 arises when the protocol packet is tunneled directly to the protocol 286 peer (the protocol peer is the decapsulator). 288 When the protocol packet is encapsulated in IP, it is possible to 289 spoof the TTL. It may also be impossible to legitimately get the 290 packet to the protocol peer with a TTL of 255, as in the IP in MPLS 291 cases described below. 293 Finally, note that the security of any tunneling technique depends 294 heavily on authentication at the tunnel endpoints, as well as how the 295 tunneled packets are protected in flight. Such mechanisms are, 296 however, beyond the scope of this memo. 298 6.2.1. IP in IP 300 Protocol packets may be tunneled over IP directly to a protocol peer, 301 or to a decapsulator (tunnel endpoint) that then forwards the packet 302 to a directly connected protocol peer (e.g., in IP-in-IP [RFC2003], 303 GRE [RFC2784], or various forms of IPv6-in-IPv4 [RFC2893]). These 304 cases are depicted below. 306 Peer router ---------- Tunnel endpoint router and peer 307 TTL=255 [tunnel] [TTL=255 at ingress] 308 [TTL=255 at egress] 310 Peer router ---------- Tunnel endpoint router ----- On-link peer 311 TTL=255 [tunnel] [TTL=255 at ingress] [TTL=254 at ingress] 312 [TTL=254 at egress] 314 In the first case, in which the encapsulated packet is tunneled 315 directly to the protocol peer, the encapsulated packet's TTL can be 316 set arbitrary value. In the second case, in which the encapsulated 317 packet is tunneled to a decapsulator (tunnel endpoint) which then 318 forwards it to a directly connected protocol peer, RFC 2003 specifies 319 the following behavior: 321 When encapsulating a datagram, the TTL in the inner IP 322 header is decremented by one if the tunneling is being 323 done as part of forwarding the datagram; otherwise, the 324 inner header TTL is not changed during encapsulation. If 325 the resulting TTL in the inner IP header is 0, the 326 datagram is discarded and an ICMP Time Exceeded message 327 SHOULD be returned to the sender. An encapsulator MUST 328 NOT encapsulate a datagram with TTL = 0. 330 Hence the inner IP packet header's TTL, as seen by the decapsulator, 331 can be set to an arbitrary value (in particular, 255). As a result, 332 it may not be possible to deliver the protocol packet to the peer 333 with a TTL of 255. 335 6.2.2. IP in MPLS 337 Protocol packets may also be tunneled over MPLS to a protocol peer 338 which either the penultimate hop (when the penultimate hop popping 339 (PHP) is employed [RFC3032]), or one hop beyond the penultimate hop. 340 These cases are depicted below. 342 Peer router ---------- Penultimate Hop (PH) and peer 343 TTL=255 [tunnel] [TTL=255 at ingress] 344 [TTL<=254 at egress] 346 Peer router ---------- Penultimate Hop -------- On-link peer 347 TTL=255 [tunnel] [TTL=255 at ingress] [TTL <=254 at ingress] 348 [TTL<=254 at egress] 350 TTL handling for these cases is described in RFC 3032. RFC 3032 351 states that when the IP packet is first labeled: 353 ... the TTL field of the label stack entry MUST BE set to the 354 value of the IP TTL field. (If the IP TTL field needs to be 355 decremented, as part of the IP processing, it is assumed that 356 this has already been done.) 358 When the label is popped: 360 When a label is popped, and the resulting label stack is empty, 361 then the value of the IP TTL field SHOULD BE replaced with the 362 outgoing TTL value, as defined above. In IPv4 this also 363 requires modification of the IP header checksum. 365 where the definition of "outgoing TTL" is: 367 The "incoming TTL" of a labeled packet is defined to be the 368 value of the TTL field of the top label stack entry when the 369 packet is received. 371 The "outgoing TTL" of a labeled packet is defined to be the larger of: 373 a) one less than the incoming TTL, 374 b) zero. 376 In either of these cases, the minimum value by which the TTL could be 377 decremented would be one (the network operator prefers to hide its 378 infrastructure by decrementing the TTL by the minimum number of LSP 379 hops, one, rather than decrementing the TTL as it traverses its MPLS 380 domain). As a result, the maximum TTL value at egress from the MPLS 381 cloud is 254 (255-1), and as a result the check described in section 382 3 will fail. 384 6.3. Multi-Hop Protocol Sessions 386 While the GTSM method is less effective for multi-hop protocol 387 sessions, it does close the window on several forms of attack. 388 However, in the multi-hop scenario GTSM is an OPTIONAL extension. 389 Protection of the protocol infrastructure beyond what is provided by 390 the GTSM method will likely require cryptographic machinery such as 391 is envisioned by Secure BGP (S-BGP) [SBGP1,SBGP2], and/or other 392 extensions. Finally, note that in the multi-hop case described above, 393 we specify a range of acceptable TTLs in order to achieve some 394 robustness to topology changes. This robustness to topological change 395 comes at the cost of the loss some robustness to different forms of 396 attack. 398 7. IANA Considerations 400 This document creates a no new requirements on IANA namespaces 401 [RFC2434]. 403 8. References 405 8.1. Normative References 407 [RFC791] Postel, J., "INTERNET PROTOCOL PROTOCOL 408 SPECIFICATION", RFC 791, September, 1981. 410 [RFC1771] Rekhter, Y., and T. Li (Editors), "A Border 411 Gateway Protocol (BGP-4)", RFC 1771, March, 412 1995. 414 [RFC1772] Rekhter, Y., and P. Gross, "Application of the 415 Border Gateway Protocol in the Internet", RFC 416 1772, March, 1995. 418 [RFC2003] Perkins, C., "IP Encapsulation with IP", RFC 419 2003, October, 1996. 421 [RFC2385] Heffernan, A., "Protection of BGP Sessions via 422 the TCP MD5 Signature Option", RFC 2385, August, 423 1998. 425 [RFC2461] Narten, T., E. Nordmark, and W. Simson, "Neighbor 426 Discover for IP Version 6 (IPv6)", RFC 2461, 427 December, 1998. 429 [RFC2784] Farinacci, D., "Generic Routing Encapsulation 430 (GRE)", RFC 2784, March, 2000. 432 [RFC2842] Chandra, R. and J. Scudder, "Capabilities 433 Advertisement with BGP-4", RFC 2842, May, 2000. 435 [RFC2893] Gilligan, R., and E. Nordmark, "Transition 436 Mechanisms for IPv6 Hosts and Routers", RFC 2893, 437 August, 2000. 439 [RFC3036] Andersson, L., et. al., "LDP Specification", RFC 440 3036, January, 2001. January, 2001. 442 [RFC3032] Rosen, E., et. al., "MPLS Label Stack Encoding", 443 RFC 3032, 445 [SBGP1] Kent, S., C. Lynn, and K. Seo, "Secure Border 446 Gateway Protocol (Secure-BGP)", IEEE Journal on 447 Selected Areas in Communications, volume 18, 448 number 4, April, 2000. 450 [SBGP2] Kent, S., C. Lynn, J. Mikkelson, and K. Seo, 451 "Secure Border Gateway Protocol (S-BGP) -- Real 452 World Performance and Deployment Issues", 453 Proceedings of the IEEE Network and Distributed 454 System Security Symposium, February, 2000. 456 8.2. Informative References 458 [BFD] Katz, D. and D. Ward, "Bidirectional Forwarding 459 Detection", draft-katz-ward-bfd-00.txt, June, 460 2003. Work in progress. 462 [MSDP] Meyer, D., and W. Fenner (Editors), "The Multicast 463 Source Discovery Protocol (MSDP)", 464 draft-ietf-msdp-spec-20.txt, May 2003. Work in 465 progress. 467 [PEERING] Empirical data gathered from the Sprint and AOL 468 backbones, October, 2002. 470 [RFC2028] Hovey, R. and S. Bradner, "The Organizations 471 Involved in the IETF Standards Process", RFC 472 2028/BCP 11, October, 1996. 474 [RFC2119] Bradner, S., "Key words for use in RFCs to 475 Indicate Requirement Levels", RFC 2119, March, 476 1997. 478 [RFC2434] Narten, T., and H. Alvestrand, "Guidelines for 479 Writing an IANA Considerations Section in 480 RFCs", RFC 2434/BCP 0026, October, 1998. 482 9. Author's Addresses 484 Vijay Gill 485 Email: vijay@umbc.edu 487 John Heasley 488 Email: heas@shrubbery.net 490 David Meyer 491 Email: dmm@1-4-5.net 493 10. Full Copyright Statement 495 Copyright (C) The Internet Society (2003). All Rights Reserved. 497 This document and translations of it may be copied and furnished to 498 others, and derivative works that comment on or otherwise explain it 499 or assist in its implementation may be prepared, copied, published 500 and distributed, in whole or in part, without restriction of any 501 kind, provided that the above copyright notice and this paragraph are 502 included on all such copies and derivative works. However, this 503 document itself may not be modified in any way, such as by removing 504 the copyright notice or references to the Internet Society or other 505 Internet organizations, except as needed for the purpose of 506 developing Internet standards in which case the procedures for 507 copyrights defined in the Internet Standards process must be 508 followed, or as required to translate it into languages other than 509 English. 511 The limited permissions granted above are perpetual and will not be 512 revoked by the Internet Society or its successors or assigns. 514 This document and the information contained herein is provided on an 515 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 516 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 517 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 518 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 519 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.