Network Working Group
Request for Comments: 3682
Category: Experimental

V. Gill
J. Heasley
D. Meyer
February 2004

The Generalized TTL Security Mechanism (GTSM)

Status of this Memo

This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.

Copyright Notice

Abstract

The use of a packet's Time to Live (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), Multicast Source Discovery Protocol (MSDP), Bidirectional Forwarding Detection, and Label Distribution Protocol (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.

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Assumptions Underlying GTSM. . . . . . . . . . . . . . . . . .  2
       2.1.  GTSM Negotiation . . . . . . . . . . . . . . . . . . . .  3
       2.2.  Assumptions on Attack Sophistication . . . . . . . . . .  3
   3.  GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . .  3
       3.1.  Multi-hop Scenarios. . . . . . . . . . . . . . . . . . .  4
             3.1.1.  Intra-domain Protocol Handling . . . . . . . . .  5
   4.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Security Considerations. . . . . . . . . . . . . . . . . . . .  5
       5.1.  TTL (Hop Limit) Spoofing . . . . . . . . . . . . . . . .  5
       5.2.  Tunneled Packets . . . . . . . . . . . . . . . . . . . .  6
             5.2.1.  IP in IP . . . . . . . . . . . . . . . . . . . .  6
   
             5.2.2.  IP in MPLS . . . . . . . . . . . . . . . . . . .  7
       5.3.  Multi-Hop Protocol Sessions. . . . . . . . . . . . . . .  8
   6.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
       7.1.  Normative References . . . . . . . . . . . . . . . . . .  8
       7.2.  Informative References . . . . . . . . . . . . . . . . .  9
   8.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
   9.  Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11

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], [RFC1772]) 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.

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).

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 BCP 14, RFC 2119 [RFC2119].

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).

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,

Set the outbound TTL for the protocol connection to 255.

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 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., a DoS attack could 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. 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.

5. Security Considerations

GTSM is a simple procedure that protects single hop protocol sessions, except in those cases in which the peer has been compromised.

5.1. 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 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]).

5.2. 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.

5.2.1. 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.

    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.

5.2.2. 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]

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.

5.3. 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 robustness to topology changes. This robustness to topological change comes at the cost of the loss of some robustness to different forms of attack.

6. IANA Considerations

This document creates no new requirements on IANA namespaces [RFC2434].

7. References

7.1. Normative References

   [RFC791]   Postel, J., "Internet Protocol Specification", STD 5, 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.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.
   
   [RFC2461]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor
              Discover for IP Version 6 (IPv6)", RFC 2461, December
              1998.
   
   [RFC2784]  Farinacci, D., "Generic Routing Encapsulation (GRE)", RFC
              2784, March 2000.
   
   [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.
   
   [RFC3032]  Rosen, E. Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T. and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.
   
   [RFC3036]  Andersson, L., Doolan, P., Feldman, N., Fredette, A. and
              B. Thomas, "LDP Specification", RFC 3036, January 2001.
   
   [RFC3392]  Chandra, R. and J. Scudder, "Capabilities Advertisement
              with BGP-4", RFC 3392, November 2002.
   
   [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.

7.2. Informative References

   [BFD]      Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", Work in Progress, June 2003.
   
   [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", BCP 11, RFC 2028, October
              1996.
   
   [RFC2434]  Narten, T., and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.
   
   [RFC3618]  Meyer, D. and W. Fenner, Eds., "The Multicast Source
              Discovery Protocol (MSDP)", RFC 3618, October 2003.

8. Authors' Addresses

   Vijay Gill

EMail:

          vijay@umbc.edu
   
   John Heasley

EMail:

          heas@shrubbery.net
   
   David Meyer
   
   EMail: dmm@1-4-5.net

9. Full Copyright Statement

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Acknowledgement

Funding for the RFC Editor function is currently provided by the Internet Society.