The Generalized TTL Security Mechanism (GTSM)
The information below is for an old version of the document that is already published as an RFC.
This is an older version of an Internet-Draft that was ultimately published as RFC 3682.
|Authors||Vijay Gill , John Heasley , David Meyer|
|Last updated||2015-10-14 (Latest revision 2003-10-10)|
|RFC stream||Independent Submission|
|IESG||IESG state||RFC 3682 (Experimental)|
|Responsible AD||Alex D. Zinin|
|Send notices to||(None)|
INTERNET-DRAFT V. Gill draft-gill-gtsh-04.txt J. Heasley D. Meyer Category Experimental Expires: April 2004 October 2003 The Generalized TTL Security Mechanism (GTSM) <draft-gill-gtsh-04.txt> Status of this Document This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC 2119]. This document is an individual submission. Comments are solicited and should be addressed to the author(s). Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Gill, Heasley, and Meyer [Page 1] INTERNET-DRAFT Expires: April 2004 October 2003 Abstract The use of a packet's TTL (IPv4) or Hop Limit (IPv6) to protect a protocol stack from CPU-utilization based attacks has been proposed in many settings (see for example, RFC 2461). This document generalizes these techniques for use by other protocols such as BGP (RFC 1771), MSDP, Bidirectional Forwarding Detection, and LDP (RFC 3036). While the Generalized TTL Security Mechanism (GTSM) is most effective in protecting directly connected protocol peers, it can also provide a lower level of protection to multi-hop sessions. GTSM is not directly applicable to protocols employing flooding mechanisms (e.g., multicast), and use of multi-hop GTSM should be considered on a case-by-case basis. Gill, Heasley, and Meyer [Page 2] INTERNET-DRAFT Expires: April 2004 October 2003 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Assumptions Underlying GTSM. . . . . . . . . . . . . . . . . . 4 2.1. GTSM Negotiation. . . . . . . . . . . . . . . . . . . . . . 4 2.2. Assumptions on Attack Sophistication. . . . . . . . . . . . 4 3. GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Multi-hop Scenarios . . . . . . . . . . . . . . . . . . . . 6 3.1.1. Intra-domain Protocol Handling . . . . . . . . . . . . . 6 4. Intellectual Property. . . . . . . . . . . . . . . . . . . . . 6 5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations. . . . . . . . . . . . . . . . . . . . 7 6.1. TTL (Hop Limit) Spoofing. . . . . . . . . . . . . . . . . . 7 6.2. Tunneled Packets. . . . . . . . . . . . . . . . . . . . . . 8 6.2.1. IP in IP . . . . . . . . . . . . . . . . . . . . . . . . 8 6.2.2. IP in MPLS . . . . . . . . . . . . . . . . . . . . . . . 9 6.3. Multi-Hop Protocol Sessions . . . . . . . . . . . . . . . . 10 7. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 11 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.1. Normative References. . . . . . . . . . . . . . . . . . . . 11 8.2. Informative References. . . . . . . . . . . . . . . . . . . 12 9. Author's Addresses . . . . . . . . . . . . . . . . . . . . . . 13 10. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 13 1. Introduction The Generalized TTL Security Mechanism (GTSM) is designed to protect a router's TCP/IP based control plane from CPU-utilization based attacks. In particular, while cryptographic techniques can protect the router-based infrastructure (e.g., BGP [RFC1771]) from a wide variety of attacks, many attacks based on CPU overload can be prevented by the simple mechanism described in this document. Note that the same technique protects against other scarce-resource attacks involving a router's CPU, such as attacks against processor- line card bandwidth. GTSM is based on the fact that the vast majority of protocol peerings are established between routers that are adjacent [PEERING]. Thus most protocol peerings are either directly between connected interfaces or at the worst case, are between loopback and loopback, with static routes to loopbacks. Since TTL spoofing is considered nearly impossible, a mechanism based on an expected TTL value can provide a simple and reasonably robust defense from infrastructure attacks based on forged protocol packets. Gill, Heasley, and Meyer Section 1. [Page 3] INTERNET-DRAFT Expires: April 2004 October 2003 Finally, the GTSM mechanism is equally applicable to both TTL (IPv4) and Hop Limit (IPv6), and from the perspective of GTSM, TTL and Hop Limit have identical semantics. As a result, in the remainder of this document the term "TTL" is used to refer to both TTL or Hop Limit (as appropriate). 2. Assumptions Underlying GTSM GTSM is predicated upon the following assumptions: (i). The vast majority of protocol peerings are between adjacent routers [PEERING]. (ii). It is common practice for many service providers to ingress filter (deny) packets that have the provider's loopback addresses as the source IP address. (iii). Use of GTSM is OPTIONAL, and can be configured on a per-peer (group) basis. (iv). The router supports a method of classifying traffic destined for the route processor into interesting/control and not-control queues. (iv). The peer routers both implement GTSM. 2.1. GTSM Negotiation This document assumes that GTSM will be manually configured between protocol peers. That is, no automatic GTSM capability negotiation, such as is envisioned by RFC 2842 [RFC2842] is assumed or defined. 2.2. Assumptions on Attack Sophistication Throughout this document, we assume that potential attackers have evolved in both sophistication and access to the point that they can send control traffic to a protocol session, and that this traffic appears to be valid control traffic (i.e., has the source/destination of configured peer routers). Gill, Heasley, and Meyer Section 2.2. [Page 4] INTERNET-DRAFT Expires: April 2004 October 2003 We also assume that each router in the path between the attacker and the victim protocol speaker decrements TTL properly (clearly, if either the path or the adjacent peer is compromised, then there are worse problems to worry about). Since the vast majority of our peerings are between adjacent routers, we can set the TTL on the protocol packets to 255 (the maximum possible for IP) and then reject any protocol packets that come in from configured peers which do NOT have an inbound TTL of 255. GTSM can be disabled for applications such as route-servers and other large diameter multi-hop peerings. In the event that an the attack comes in from a compromised multi-hop peering, that peering can be shut down (a method to reduce exposure to multi-hop attacks is outlined below). 3. GTSM Procedure GTSM SHOULD NOT be enabled by default. The following process describes the per-peer behavior: (i). If GTSM is enabled, an implementation performs the following procedure: (a). For directly connected routers, o Set the outbound TTL for the protocol connection to 255. o For each configured protocol peer: Update the receive path Access Control List (ACL) or firewall to only allow protocol packets to pass onto the Route Processor (RP) that have the correct <source, destination, TTL> tuple. The TTL must either be 255 (for a directly connected peer), or 255-(configured-range-of-acceptable-hops) for a multi-hop peer. We specify a range here to achieve some robustness to changes in topology. Any directly connected check MUST be disabled for such peerings. It is assumed that a receive path ACL is an ACL that is designed to control which packets are allowed to go to the RP. This procedure will only Gill, Heasley, and Meyer Section 3. [Page 5] INTERNET-DRAFT Expires: April 2004 October 2003 allow protocol packets from adjacent router to pass onto the RP. (b). If the inbound TTL is 255 (for a directly connected peer), or 255-(configured-range-of-acceptable-hops) (for multi-hop peers), the packet is NOT processed. Rather, the packet is placed into a low priority queue, and subsequently logged and/or silently discarded. In this case, an ICMP message MUST NOT be generated. (ii). If GTSM is not enabled, normal protocol behavior is followed. 3.1. Multi-hop Scenarios When a multi-hop protocol session is required, we set the expected TTL value to be 255-(configured-range-of-acceptable-hops). This approach provides a qualitatively lower degree of security for the protocol implementing GTSM (i.e., an DoS attack could be theoretically be launched by compromising some box in the path). However, GTSM will still catch the vast majority of observed DDoS attacks against a given protocol. Note that since the number of hops can change rapidly in real network situations, it is considered that GTSM may not be able to handle this scenario adequately and an implementation MAY provide OPTIONAL support. 3.1.1. Intra-domain Protocol Handling In general, GTSM is not used for intra-domain protocol peers or adjacencies. The special case of iBGP peers can be protected by filtering at the network edge for any packet that has a source address of one of the loopback addresses used for the intra-domain peering. In addition, the current best practice is to further protect such peers or adjacencies with an MD5 signature [RFC2385]. 4. Intellectual Property The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in Gill, Heasley, and Meyer Section 4. [Page 6] INTERNET-DRAFT Expires: April 2004 October 2003 this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11 [RFC2028]. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. 5. Acknowledgments The use of the TTL field to protect BGP originated with many different people, including Paul Traina and Jon Stewart. Ryan McDowell also suggested a similar idea. Steve Bellovin, Jay Borkenhagen, Randy Bush, Vern Paxon, Pekka Savola, and Robert Raszuk also provided useful feedback on earlier versions of this document. David Ward provided insight on the generalization of the original BGP-specific idea. 6. Security Considerations GTSM is a simple procedure that protects single hop protocol sessions, except in those cases in which the peer has been compromised. 6.1. TTL (Hop Limit) Spoofing The approach described here is based on the observation that a TTL (or Hop Limit) value of 255 is non-trivial to spoof, since as the Gill, Heasley, and Meyer Section 6.1. [Page 7] INTERNET-DRAFT Expires: April 2004 October 2003 packet passes through routers towards the destination, the TTL is decremented by one. As a result, when a router receives a packet, it may not be able to determine if the packet's IP address is valid, but it can determine how many router hops away it is (again, assuming none of the routers in the path are compromised in such a way that they would reset the packet's TTL). Note, however, that while engineering a packet's TTL such that it has a particular value when sourced from an arbitrary location is difficult (but not impossible), engineering a TTL value of 255 from non-directly connected locations is not possible (again, assuming none of the directly connected neighbors are compromised, the packet hasn't been tunneled to the decapsulator, and the intervening routers are operating in accordance with RFC 791 [RFC791]). 6.2. Tunneled Packets An exception to the observation that a packet with TTL of 255 is difficult to spoof occurs when a protocol packet is tunneled to a decapsulator who then forwards the packet to a directly connected protocol peer. In this case the decapsulator (tunnel endpoint) can either be the penultimate hop, or the last hop itself. A related case arises when the protocol packet is tunneled directly to the protocol peer (the protocol peer is the decapsulator). When the protocol packet is encapsulated in IP, it is possible to spoof the TTL. It may also be impossible to legitimately get the packet to the protocol peer with a TTL of 255, as in the IP in MPLS cases described below. Finally, note that the security of any tunneling technique depends heavily on authentication at the tunnel endpoints, as well as how the tunneled packets are protected in flight. Such mechanisms are, however, beyond the scope of this memo. 6.2.1. IP in IP Protocol packets may be tunneled over IP directly to a protocol peer, or to a decapsulator (tunnel endpoint) that then forwards the packet to a directly connected protocol peer (e.g., in IP-in-IP [RFC2003], GRE [RFC2784], or various forms of IPv6-in-IPv4 [RFC2893]). These cases are depicted below. Gill, Heasley, and Meyer Section 6.2.1. [Page 8] INTERNET-DRAFT Expires: April 2004 October 2003 Peer router ---------- Tunnel endpoint router and peer TTL=255 [tunnel] [TTL=255 at ingress] [TTL=255 at egress] Peer router ---------- Tunnel endpoint router ----- On-link peer TTL=255 [tunnel] [TTL=255 at ingress] [TTL=254 at ingress] [TTL=254 at egress] In the first case, in which the encapsulated packet is tunneled directly to the protocol peer, the encapsulated packet's TTL can be set arbitrary value. In the second case, in which the encapsulated packet is tunneled to a decapsulator (tunnel endpoint) which then forwards it to a directly connected protocol peer, RFC 2003 specifies the following behavior: When encapsulating a datagram, the TTL in the inner IP header is decremented by one if the tunneling is being done as part of forwarding the datagram; otherwise, the inner header TTL is not changed during encapsulation. If the resulting TTL in the inner IP header is 0, the datagram is discarded and an ICMP Time Exceeded message SHOULD be returned to the sender. An encapsulator MUST NOT encapsulate a datagram with TTL = 0. Hence the inner IP packet header's TTL, as seen by the decapsulator, can be set to an arbitrary value (in particular, 255). As a result, it may not be possible to deliver the protocol packet to the peer with a TTL of 255. 6.2.2. IP in MPLS Protocol packets may also be tunneled over MPLS to a protocol peer which either the penultimate hop (when the penultimate hop popping (PHP) is employed [RFC3032]), or one hop beyond the penultimate hop. These cases are depicted below. Peer router ---------- Penultimate Hop (PH) and peer TTL=255 [tunnel] [TTL=255 at ingress] [TTL<=254 at egress] Peer router ---------- Penultimate Hop -------- On-link peer TTL=255 [tunnel] [TTL=255 at ingress] [TTL <=254 at ingress] [TTL<=254 at egress] Gill, Heasley, and Meyer Section 6.2.2. [Page 9] INTERNET-DRAFT Expires: April 2004 October 2003 TTL handling for these cases is described in RFC 3032. RFC 3032 states that when the IP packet is first labeled: ... the TTL field of the label stack entry MUST BE set to the value of the IP TTL field. (If the IP TTL field needs to be decremented, as part of the IP processing, it is assumed that this has already been done.) When the label is popped: When a label is popped, and the resulting label stack is empty, then the value of the IP TTL field SHOULD BE replaced with the outgoing TTL value, as defined above. In IPv4 this also requires modification of the IP header checksum. where the definition of "outgoing TTL" is: The "incoming TTL" of a labeled packet is defined to be the value of the TTL field of the top label stack entry when the packet is received. The "outgoing TTL" of a labeled packet is defined to be the larger of: a) one less than the incoming TTL, b) zero. In either of these cases, the minimum value by which the TTL could be decremented would be one (the network operator prefers to hide its infrastructure by decrementing the TTL by the minimum number of LSP hops, one, rather than decrementing the TTL as it traverses its MPLS domain). As a result, the maximum TTL value at egress from the MPLS cloud is 254 (255-1), and as a result the check described in section 3 will fail. 6.3. Multi-Hop Protocol Sessions While the GTSM method is less effective for multi-hop protocol sessions, it does close the window on several forms of attack. However, in the multi-hop scenario GTSM is an OPTIONAL extension. Protection of the protocol infrastructure beyond what is provided by the GTSM method will likely require cryptographic machinery such as is envisioned by Secure BGP (S-BGP) [SBGP1,SBGP2], and/or other extensions. Finally, note that in the multi-hop case described above, we specify a range of acceptable TTLs in order to achieve some Gill, Heasley, and Meyer Section 6.3. [Page 10] INTERNET-DRAFT Expires: April 2004 October 2003 robustness to topology changes. This robustness to topological change comes at the cost of the loss some robustness to different forms of attack. 7. IANA Considerations This document creates a no new requirements on IANA namespaces [RFC2434]. 8. References 8.1. Normative References [RFC791] Postel, J., "INTERNET PROTOCOL PROTOCOL SPECIFICATION", RFC 791, September, 1981. [RFC1771] Rekhter, Y., and T. Li (Editors), "A Border Gateway Protocol (BGP-4)", RFC 1771, March, 1995. [RFC1772] Rekhter, Y., and P. Gross, "Application of the Border Gateway Protocol in the Internet", RFC 1772, March, 1995. [RFC2003] Perkins, C., "IP Encapsulation with IP", RFC 2003, October, 1996. [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August, 1998. [RFC2461] Narten, T., E. Nordmark, and W. Simson, "Neighbor Discover for IP Version 6 (IPv6)", RFC 2461, December, 1998. [RFC2784] Farinacci, D., "Generic Routing Encapsulation (GRE)", RFC 2784, March, 2000. Gill, Heasley, and Meyer Section 8.1. [Page 11] INTERNET-DRAFT Expires: April 2004 October 2003 [RFC2842] Chandra, R. and J. Scudder, "Capabilities Advertisement with BGP-4", RFC 2842, May, 2000. [RFC2893] Gilligan, R., and E. Nordmark, "Transition Mechanisms for IPv6 Hosts and Routers", RFC 2893, August, 2000. [RFC3036] Andersson, L., et. al., "LDP Specification", RFC 3036, January, 2001. January, 2001. [RFC3032] Rosen, E., et. al., "MPLS Label Stack Encoding", RFC 3032, [SBGP1] Kent, S., C. Lynn, and K. Seo, "Secure Border Gateway Protocol (Secure-BGP)", IEEE Journal on Selected Areas in Communications, volume 18, number 4, April, 2000. [SBGP2] Kent, S., C. Lynn, J. Mikkelson, and K. Seo, "Secure Border Gateway Protocol (S-BGP) -- Real World Performance and Deployment Issues", Proceedings of the IEEE Network and Distributed System Security Symposium, February, 2000. 8.2. Informative References [BFD] Katz, D. and D. Ward, "Bidirectional Forwarding Detection", draft-katz-ward-bfd-00.txt, June, 2003. Work in progress. [MSDP] Meyer, D., and W. Fenner (Editors), "The Multicast Source Discovery Protocol (MSDP)", draft-ietf-msdp-spec-20.txt, May 2003. Work in progress. [PEERING] Empirical data gathered from the Sprint and AOL backbones, October, 2002. [RFC2028] Hovey, R. and S. Bradner, "The Organizations Involved in the IETF Standards Process", RFC 2028/BCP 11, October, 1996. Gill, Heasley, and Meyer Section 8.2. [Page 12] INTERNET-DRAFT Expires: April 2004 October 2003 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March, 1997. [RFC2434] Narten, T., and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434/BCP 0026, October, 1998. 9. Author's Addresses Vijay Gill Email: email@example.com John Heasley Email: firstname.lastname@example.org David Meyer Email: email@example.com 10. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING Gill, Heasley, and Meyer Section 10. [Page 13] INTERNET-DRAFT Expires: April 2004 October 2003 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Gill, Heasley, and Meyer Section 10. [Page 14]