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Versions: (RFC 3682) 00 01 02 03 04 05 06 07 08 09 10 RFC 5082

INTERNET-DRAFT                                       V. Gill
draft-ietf-rtgwg-rfc3682bis-04.txt                J. Heasley
                                                    D. Meyer
Category                                   Proposed Standard
Obsoletes:                                          RFC 3682
Expires: March 2005                           September 2004

             The Generalized TTL Security Mechanism (GTSM)

Status of this Memo

   Status of this Memo

   This document is an Internet-Draft and is subject to all
   provisions of section 3 of RFC 3667 [RFC3667]. By submitting
   this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is
   aware have been or will be disclosed, and any of which he or
   she become aware will be disclosed, in accordance with RFC
   3668 [RFC3668].

   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
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   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

   The list of Internet-Draft Shadow Directories can be accessed
   at http://www.ietf.org/shadow.html.

   This document is a product of the RTGWG WG. Comments should be

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   addressed to the authors, or the mailing list at rtgwg@ietf.org.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   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.  This document obsoletes RFC

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Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2. Assumptions Underlying GTSM. . . . . . . . . . . . . . . . . .   4
    2.1. GTSM Negotiation. . . . . . . . . . . . . . . . . . . . . .   4
    2.2. Assumptions on Attack Sophistication. . . . . . . . . . . .   5
   3. GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . .   5
    3.1. Multi-hop Scenarios . . . . . . . . . . . . . . . . . . . .   6
     3.1.1. Intra-domain Protocol Handling . . . . . . . . . . . . .   7
   4. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .   7
   5. Security Considerations. . . . . . . . . . . . . . . . . . . .   7
    5.1. TTL (Hop Limit) Spoofing. . . . . . . . . . . . . . . . . .   8
    5.2. Tunneled Packets. . . . . . . . . . . . . . . . . . . . . .   8
     5.2.1. IP in IP . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.2. IP in MPLS . . . . . . . . . . . . . . . . . . . . . . .  10
    5.3. Multi-Hop Protocol Sessions . . . . . . . . . . . . . . . .  11
   6. Applicability Statement. . . . . . . . . . . . . . . . . . . .  12
   7. IANA Considerations. . . . . . . . . . . . . . . . . . . . . .  12
   8. References . . . . . . . . . . . . . . . . . . . . . . . . . .  12
    8.1. Normative References. . . . . . . . . . . . . . . . . . . .  12
    8.2. Informative References. . . . . . . . . . . . . . . . . . .  14
   9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  14

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 from outside the network.
   Note, however, that GTSM is not a substitute for authentication
   mechanisms. In particular, it does not secure against insider on-the-

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   wire attacks, such as packet spoofing or replay.

   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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

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.

    (v)   The peer routers both implement GTSM.

2.1.  GTSM Negotiation

   This document assumes that, when used with existing protocols, 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.

   If a new protocol is designed with built-in GTSM support, then it is
   recommended that procedures are always used for sending and

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   validating received protocol packets (GTSM is always on, see for
   example [RFC2461]). If, however, dynamic negotiation of GTSM support
   is necessary, protocol messages used for such negotiation MUST be
   authenticated using other security mechanisms to prevent DoS attacks.

   Also note that this specification does not offer a generic GTSM
   capability negotiation mechanism, so messages of the protocol
   augmented with the GTSM behavior will need to be used if dynamic
   negotiation is deemed necessary.

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

   If GTSM is not built into the protocol and used as an additional
   feature (e.g., for BGPv4, or LDP), it SHOULD NOT be enabled by
   default. Each session protected with GTSM is associated with a
   variable TrustRadius that denotes the distance from the node
   performing the GTSM check to the trusted sources of protocol packets.

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    (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

               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, srcPort, destination, destPort, TTL>
                 tuple. The TTL must either be 255 (for a directly
                 connected peer), or 255 - TrustRadius for a
                 multi-hop peer. We specify a range here to achieve
                 some robustness to changes in topology. Any
                 directly connected (i.e., such as may be used in a
                 BGP implementation to determine whether a peer is
                 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)   Otherwise, a TTL value in a received packet is
                 considered valid if it is not less than
                 (255 - TrustRadius).

           In summary, if TrustRadius is set to zero for a particular
           session, only packets from directly connected neighbors
           (TTL=255) will be considered valid. As a result,
           TrustRadius values greater than 0 will allow packets from
           more remote nodes to be accepted.

    (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 - TrustRadius. This approach provides a

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   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 (launched from outside the
   network) 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 SHOULD 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, Alfred Hoenes, Vern Paxon, Pekka Savola,
   Robert Raszuk and Alex Zinin 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. In particular, it does not protect against the wide
   range of on-the-wire attacks; protection from these attacks requires
   more rigorous security mechanisms.

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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.

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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.

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    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]

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   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

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   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.  Applicability Statement

   As described above, GTSM is only applicable to environments with
   inherently limited topologies (and is most effective in those cases
   where protocol peers are directly connected). In particular, its
   application should be limited to those cases in which protocol peers
   are either directly connected, or in which the topology between peers
   is fairly static and well known, and in which the intervening network
   (between the peers) is trusted.

7.  IANA Considerations

   This document creates no new requirements on IANA namespaces

8.  References

8.1.  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.

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   [RFC2385]       Heffernan, A., "Protection of BGP Sessions via
                   the TCP MD5 Signature Option", RFC 2385, August

   [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.

   [RFC3036]       Andersson, L., Doolan, P., Feldman, N., Fredette,
                   A. and B. Thomas, "LDP Specification", RFC 3036,
                   January 2001.

   [RFC3032]       Rosen, E. Tappan, D., Fedorkow, G., Rekhter, Y.,
                   Farinacci, D., Li, T. and A. Conta, "MPLS Label
                   Stack Encoding", RFC 3032, January 2001.

   [RFC3667]       Bradner, S., "IETF Rights in Contributions",
                   BCP 78, RFC 3667, February, 2004.

   [RFC3668]       Bradner, S., "Intellectual Property Rights in
                   IETF Technology", BCP 79, RFC 3668, February,

   [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.

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8.2.  Informative References

   [BFD]           Katz, D. and D. Ward, "Bidirectional Forwarding
                   Detection", draft-ietf-bfd-base-00.txt, Work in

   [PEERING]       Empirical data gathered from the Sprint and AOL
                   backbones, October, 2002.

   [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.

9.  Authors' Addresses

   Vijay Gill
   EMail: vijay@umbc.edu

   John Heasley
   EMail: heas@shrubbery.net

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

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Intellectual Property Statement

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Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


   Funding for the RFC Editor function is currently provided by the
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