Routing WG                                                       V. Gill
Internet-Draft                                                J. Heasley
                                                    D. Meyer
Category                                   Proposed Standard
Obsoletes:                                          RFC 3682 (if approved)                                   D. Meyer
Intended status: Standards Track                               P. Savola
Expires: March 1, 2007                                   August 28, 2006

             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.

   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 becomes
   aware will be disclosed, in accordance with RFC 3668. Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

   Copyright (C) The Internet Society (2005). All Rights Reserved. (2006).


   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 verify whether the packet originated within the same link has been proposed
   used in many settings (see for example, RFC 2461). recent protocols.  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
   technique.  This document obsoletes RFC 3682.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Assumptions Underlying GTSM. GTSM  . . . . . . . . . . . . . . . . .   4  3
     2.1.  GTSM Negotiation. Negotiation . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Assumptions on Attack Sophistication. Sophistication . . . . . . . . . . .   5  4
   3.  GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . .  5
    3.1. Multi-hop Scenarios
   4.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.1. Intra-domain Protocol Handling
   5.  Security Considerations  . . . . . . . . . . . . .   7
   4. Acknowledgments. . . . . . .  6
     5.1.  TTL (Hop Limit) Spoofing . . . . . . . . . . . . . . . . .  7
   5. Security Considerations.
     5.2.  Tunneled Packets . . . . . . . . . . . . . . . . . . .   7
    5.1. TTL (Hop Limit) Spoofing. . .  7
       5.2.1.  IP in IP . . . . . . . . . . . . . . .   8
    5.2. Tunneled Packets. . . . . . . . .  8
       5.2.2.  IP in MPLS . . . . . . . . . . . . .   8
     5.2.1. IP in IP . . . . . . . . .  8
     5.3.  Multi-Hop Protocol Sessions  . . . . . . . . . . . . . . .  9
     5.2.2. IP in MPLS
   6.  Applicability Statement  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . .  10
    5.3. Multi-Hop Protocol Sessions . . . . . . . . . . . . . . . .  11
   6. Applicability Statement. . 10
   8.  Changelog  . . . . . . . . . . . . . . . . . . .  12
   7. IANA Considerations. . . . . . . . 10
     8.1.  Changes between -05 and -06  . . . . . . . . . . . . . . .  12
   8. 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  12
    8.1. 11
     9.1.  Normative References. References . . . . . . . . . . . . . . . . . . .  12
    8.2. 11
     9.2.  Informative References. References . . . . . . . . . . . . . . . . . .  14
   9. 11
   Appendix A.  Multihop GTSM . . . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  14

1. . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

1.  Introduction

   The Generalized TTL Security Mechanism (GTSM) is designed to protect
   a router's TCP/IP IP based control plane from CPU-utilization based attacks.
   In particular, while cryptographic techniques can protect the router-based router-
   based infrastructure (e.g., BGP [RFC1771], [RFC1772]) [RFC4271], [RFC4272]) 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-
   wire on-
   the-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 appropriate).

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119 [RFC2119].

2.  Assumptions Underlying GTSM

   GTSM is predicated upon the following assumptions:


   1.  The vast majority of protocol peerings are between adjacent
       routers [PEERING].


   2.  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.


   3.  Use of GTSM is OPTIONAL, and can be configured on a per-peer
       (group) basis.


   4.  The peer routers both implement GTSM.

   5.  The router supports a method of classifying traffic
          destined for to use separate resource pools
       (e.g., queues, processing quotas) for differently classified

   Note that this document does not prescribe further restrictions that
   a router may apply to the route processor into interesting/control packets not matching the GTSM filtering
   rules, such as dropping packets that do not match any configured
   protocol session and rate-limiting the rest.  This document also does
   not suggest the actual means of resource separation, as those are
   hardware and not-control queues.

    (v) implementation-specific.

   The peer routers both implement GTSM. possibility of DoS attack prevention, however, is based on the
   assumption that packet classification and separation of their paths
   is done before they go through a scarce resource in the system.  In
   practice, this means that, the closer GTSM processing is done to the
   line-rate hardware, the more resistant the system is to the DoS

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]
   3392 [RFC3392] 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
   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, BGP, LDP, or LDP), MSDP), it SHOULD NOT be enabled by
   default. Each session protected with

   If GTSM is associated with enabled for a
   variable TrustRadius that denotes the distance from the node
   performing the GTSM check to the trusted sources of protocol packets.

    (i)   If GTSM is enabled, an implementation performs session, the following procedure:

          (a)  For directly connected routers,

               o Set steps are
   added to the outbound IP packet sending and reception procedures:

      Sending protocol packets:

         The TTL field in all IP packets used for the transmission of
         messages associated with GTSM-enabled protocol connection sessions MUST be
         set to 255.

               o For each configured protocol peer:

                 Update the receive path Access Control List (ACL)  This also related error handling messages such as
         TCP RSTs or firewall to only allow protocol packets to pass
                 onto the Route Processor (RP) that have ICMP errors.

         On some architectures, 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 of control plane originated
         traffic is under some robustness to changes in topology. Any
                 directly connected (i.e., such as may be used configurations decremented in a
                 BGP implementation to determine whether a peer is
                 directly connected) check the
         forwarding plane.  The TTL of GTSM-enabled sessions MUST NOT be disabled for
                 such peerings.

                 It is assumed that a receive path ACL is an ACL
                 that is designed

      Receiving protocol packets:

         The GTSM packet identification step associates each received
         packet addressed to the router's control which packets plane with one of the
         following three trustworthiness categories:

         +  Unknown: these are
                 allowed to go to packets that cannot be associated with
            any registered GTSM-enabled session, and hence GTSM cannot
            make any judgement on the RP.  This procedure will only
                 allow protocol level of risk associated with

         +  Trusted: these are packets from adjacent router that have been identified as
            belonging to pass
                 onto one of the RP.

           (b)   Otherwise, a GTSM-enabled sessions, and their 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,
            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 are within the expected
   TTL value range.

         +  Dangerous: these are packets that have been identified as
            belonging to be 255 - TrustRadius. This approach provides a
   qualitatively lower degree one of security for the protocol implementing
   GTSM (i.e., a DoS attack could theoretically be launched by
   compromising some box in GTSM-enabled sessions, but their TTL
            values are NOT within the path).  However, expected range, and hence GTSM will still catch
   the vast majority of observed DDoS attacks (launched from outside the
   network) against
            believes there is a given protocol. Note risk that since the number packets have been spoofed.

         The exact policies applied to packets of hops
   can change rapidly in real network situations, it is considered that
   GTSM may different
         classifications are not be able to handle postulated in this scenario adequately document 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 are
         expected to be protected by
   filtering at configurable.  Configurability is likely
         necessary particular with the network edge for any packet that has a source
   address of one treatment of related messages
         such as ICMP errors and TCP RSTs.  It should be noted that
         fragmentation may restrict the loopback addresses used for amount of information available
         to the intra-domain
   peering. In addition, classification.

         However, by default, the current best practice is to further protect
   such peers or adjacencies implementations:

         +  SHOULD ensure that packets classified as Dangerous do not
            compete for resources with an MD5 signature [RFC2385]. packets classified as Trusted or

         +  MUST NOT drop (as part of GTSM processing) packets
            classified as Trusted or Unknown.

         +  MAY drop packets classified as Dangerous.

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 generalization of the
   original BGP-specific idea.  Alex Zinin and Alia Atlas provided
   significant amount of feedback for the newer versions 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.

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]). [RFC0791]).

5.2.  Tunneled Packets

   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.

   An exception to the observation that a packet with TTL of 255 is
   difficult to spoof occurs may occur 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 and
   the penultimate hop, or tunnel is not integrity-protected (i.e., the last hop itself. A related case
   arises when lower layer is

   When the protocol packet is tunneled directly to the protocol peer
   (the protocol peer is the decapsulator). decapsulator), the GTSM provides no added
   protection as the security depends entirely on the integrity of the

   When the protocol packet is encapsulated in IP, it is possible tunneled to
   spoof the TTL. It may also be impossible to legitimately get penultimate hop which
   then forwards the packet to the protocol peer with a directly connected protocol peer, TTL of 255,
   is decremented as described below except in the IP in MPLS some myriad Bump-in-the-
   Wire (BITW) cases [BITW].

   In IP-in-MPLS cases described below.

   Finally, note that below, the security of any tunneling technique depends
   heavily on authentication TTL is always decremented by
   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. least one.

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]). [RFC4213]).  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 could possibly offer a slightly more limited security
   properties also when used with 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 sessions (see
   Appendix A), we do not specify GTSM for multi-hop scenarios due to different
   simplicity, lack of attack. deployment and implementation.

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

   Experimentation on GTSM's applicability and security properties is
   needed in which multi-hop scenarios.  The multi-hop scenarios where GTSM
   might be applicable is expected to have the following
   characteristics: the topology between peers is fairly static and well
   known, and in which the intervening network (between the peers) is

7.  IANA Considerations

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

8.  References  Changelog

   NOTE to the RFC-editor: please remove this section before

8.1.  Changes between -05 and -06

   o  Clarify the assumptions wrt. resource separation and protection
      based on comments from Alex Zinin.

   o  Rewrite the GTSM procedure based on text from Alex Zinin.

   o  Reduce TrustRadius and multi-hop scenarios to a mention in an

   o  Describe TCP-RST, ICMP error and "related messages" handling.

   o  Update the tunneling security considerations text.

   o  Editorial updates (e.g., shortening the abstract).

9.  References

9.1.  Normative References


   [RFC0791]  Postel, J., "Internet Protocol Specification", Protocol", 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 within 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

   [RFC2461]  Narten, T., Nordmark, E. E., and W. Simpson, "Neighbor Discover
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "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. E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T. T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC3667]       Bradner, S., "IETF Rights in Contributions",
                   BCP 78,

   [RFC3392]  Chandra, R. and J. Scudder, "Capabilities Advertisement
              with BGP-4", RFC 3667, February, 2004.

   [RFC3668]       Bradner, S., "Intellectual Property Rights in
                   IETF Technology", BCP 79, 3392, November 2002.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 3668, February,

   [SBGP1]         Kent, S., C. Lynn, 4213, October 2005.

   [RFC4271]  Rekhter, Y., Li, T., and K. Seo, "Secure S. Hares, "A Border Gateway
              Protocol (Secure-BGP)", IEEE Journal on
                   Selected Areas in Communications, volume 18,
                   number 4, April 2000.

   [SBGP2]         Kent, 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4272]  Murphy, 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 "BGP Security Symposium, February, 2000.

8.2. Vulnerabilities Analysis",
              RFC 4272, January 2006.

9.2.  Informative References

   [BFD]           Katz, D.

   [BITW]     "Thread: 'IP-in-IP, TTL decrementing when forwarding and D. Ward, "Bidirectional Forwarding
                   Detection", draft-ietf-bfd-base-02.txt, Work in
              BITW' on int-area list, Message-ID:
              June 2006, <

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

   [RFC2434]       Narten, T.,

Appendix A.  Multihop GTSM

   NOTE: This is a non-normative part of the specification.

   The main applicability of GTSM is for directly connected peers.  GTSM
   could be used for non-directly connected sessions as well, where the
   recipient would check that the TTL is within "TrustRadius" (e.g., 1)
   of 255 instead of 255.  As such deployment is expected to have a more
   limited applicability and H. Alvestrand, "Guidelines for
                   Writing an IANA Considerations Section different security implications, it is not
   specified 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. this document.

Authors' Addresses

   Vijay Gill


   John Heasley


   David Meyer


   Pekka Savola


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