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The Generalized TTL Security Mechanism (GTSM)

The information below is for an old version of the document that is already published as an RFC.
Document Type
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)
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IESG IESG state RFC 3682 (Experimental)
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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)

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-

   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

   The list of Internet-Draft Shadow Directories can be accessed at

   The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.

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   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.

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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. . . . . . . . . . . .   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.

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   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

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).

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   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

               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
                 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

                 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

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                 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

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   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

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

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

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   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.

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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.

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]

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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.

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

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

7.  IANA Considerations

   This document creates a no new requirements on IANA namespaces

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,

   [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,

   [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.

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   [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

   [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.

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   [RFC2119]       Bradner, S., "Key words for use in RFCs to
                   Indicate Requirement Levels", RFC 2119, March,

   [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

   John Heasley

   David Meyer

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

   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

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