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Versions: 00 01 02 03 04 05 06 07 08 09 10 rfc5082                      
Routing WG                                                       V. Gill
Internet-Draft                                                J. Heasley
Obsoletes: 3682 (if approved)                                   D. Meyer
Intended status: Standards Track                               P. Savola
Expires: May 26, 2007                                  November 22, 2006

             The Generalized TTL Security Mechanism (GTSM)

Status of this Memo

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

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   This Internet-Draft will expire on May 26, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2006).


   The use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6)
   to verify whether the packet originated within the same link has been
   used in many recent protocols.  This document generalizes this
   technique.  This document obsoletes RFC 3682.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Assumptions Underlying GTSM  . . . . . . . . . . . . . . . . .  3
     2.1.  GTSM Negotiation . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Assumptions on Attack Sophistication . . . . . . . . . . .  4
   3.  GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
     5.1.  TTL (Hop Limit) Spoofing . . . . . . . . . . . . . . . . .  7
     5.2.  Tunneled Packets . . . . . . . . . . . . . . . . . . . . .  7
       5.2.1.  IP in IP . . . . . . . . . . . . . . . . . . . . . . .  7
       5.2.2.  IP in MPLS . . . . . . . . . . . . . . . . . . . . . .  8
     5.3.  Multi-Hop Protocol Sessions  . . . . . . . . . . . . . . .  9
   6.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  9
     6.1.  Backwards Compatibility  . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.   Multihop GTSM  . . . . . . . . . . . . . . . . . . . 11
   Appendix B.   Changes Since RFC3682  . . . . . . . . . . . . . . . 12
   Appendix C.   Draft Changelog  . . . . . . . . . . . . . . . . . . 12
   Appendix C.1. Changes between -06 and -07  . . . . . . . . . . . . 12
   Appendix C.2. Changes between -05 and -06  . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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1.  Introduction

   The Generalized TTL Security Mechanism (GTSM) is designed to protect
   a router's IP based control plane from CPU-utilization based attacks.
   In particular, while cryptographic techniques can protect the router-
   based infrastructure (e.g., BGP [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 .  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
   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 RFC 2119 [RFC2119].

2.  Assumptions Underlying GTSM

   GTSM is predicated upon the following assumptions:

   1.  The vast majority of protocol peerings are between adjacent

   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.

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   4.  The peer routers both implement GTSM.

   5.  The router supports a method 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 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 implementation-specific.

   The 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
   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

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   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 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
   multi-hop peerings.  In the event that an attack comes in from a
   compromised multi-hop peering, that peering can be shut down.

3.  GTSM Procedure

   If GTSM is not built into the protocol and used as an additional
   feature (e.g., for BGP, LDP, or MSDP), it SHOULD NOT be enabled by

   If GTSM is enabled for a protocol session, the following steps are
   added to the IP packet sending and reception procedures:

      Sending protocol packets:

         The TTL field in all IP packets used for transmission of
         messages associated with GTSM-enabled protocol sessions MUST be
         set to 255.  This also applies to related error handling
         messages such as TCP RSTs or ICMP errors.

         On some architectures, the TTL of control plane originated
         traffic is under some configurations decremented in the
         forwarding plane.  The TTL of GTSM-enabled sessions MUST NOT be

      Receiving protocol packets:

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

         +  Unknown: these are packets that cannot be associated with
            any registered GTSM-enabled session, and hence GTSM cannot
            make any judgement on the level of risk associated with

         +  Trusted: these are packets that have been identified as
            belonging to one of the GTSM-enabled sessions, and their TTL

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            values are within the expected range.

         +  Dangerous: these are packets that have been identified as
            belonging to one of the GTSM-enabled sessions, but their TTL
            values are NOT within the expected range, and hence GTSM
            believes there is a risk that the packets have been spoofed.

         The exact policies applied to packets of different
         classifications are not postulated in this document and are
         expected to be configurable.  Configurability is likely
         necessary particular with the treatment of related messages
         such as ICMP errors and TCP RSTs.  It should be noted that
         fragmentation may restrict the amount of information available
         to the classification.

         However, by default, the implementations:

         +  SHOULD ensure that packets classified as Dangerous do not
            compete for resources with 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, 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.  Alex Zinin, Alia Atlas, and John Scudder
   provided significant amount of feedback for the newer versions of the

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 per router.  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 [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 may occur when a protocol packet is tunneled and
   the tunnel is not integrity-protected (i.e., the lower layer is

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

   When the protocol packet is tunneled to the penultimate hop which
   then forwards the packet to a directly connected protocol peer, TTL
   is decremented as described below except in some myriad Bump-in-the-
   Wire (BITW) cases [BITW].

   In IP-in-MPLS cases described below, the TTL is always decremented by
   at 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

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   to a directly connected protocol peer (e.g., in IP-in-IP [RFC2003],
   GRE [RFC2784], or various forms of IPv6-in-IPv4 [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 to an 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), however as the
   decapsulator forwards the protocol packet to the peer, TTL will be

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 the final 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 GTSM could possibly offer some small though difficult to
   quantify degree of protection when used with multi-hop protocol
   sessions (see Appendix A), we do not specify GTSM for multi-hop
   scenarios due to simplicity, lack of deployment and implementation.

6.  Applicability Statement

   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

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   limited to those cases in which protocol peers are directly

   Experimentation on GTSM's applicability and security properties is
   needed in 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

6.1.  Backwards Compatibility

   RFC 3682 did not specify how to handle "related messages" such as TCP
   RSTs or ICMP errors.  This specification mandates setting and
   verifying TTL=255 of those as well as the main protocol packets.

   Setting TTL=255 in related messages does not cause issues for RFC
   3682 implementations.

   Requiring TTL=255 in related messages may have impact with RFC 3682
   implementations, depending on which default TTL the implementation
   uses for originated packets; some implementations are known to use
   255, while 64 or other values are also used.  Related messages from
   the latter category of RFC 3682 implementations would be discarded.
   This is not believed to be a significant problem because protocols do
   not depend on related messages (e.g., typically having a protocol
   exchange for closing the session instead of doing a TCP-RST), and
   indeed the delivery of related messages is not reliable.  As such,
   related messages typically provide an optimization to shorten a
   protocol keepalive timeout.  Regardless of these issues, given that
   related messages provide a significant attack vector to e.g., reset
   protocol sessions, making this further restriction seems sensible.

7.  IANA Considerations

   This document requires no action from IANA.

8.  References

8.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              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.

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

   [RFC3392]  Chandra, R. and J. Scudder, "Capabilities Advertisement
              with BGP-4", RFC 3392, November 2002.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, January 2006.

8.2.  Informative References

   [BITW]     "Thread: 'IP-in-IP, TTL decrementing when forwarding and
              BITW' on int-area list, Message-ID:
              June 2006, <http://www1.ietf.org/mail-archive/web/

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 different security implications, it is not
   specified in this document.

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Appendix B.  Changes Since RFC3682

   o  New text on GTSM applicability and use in new and existing

   o  Explicitly require that related messages (e.g., TCP RSTs, ICMP
      errors) must also be sent and checked to have TTL=255.  See
      Section 6.1 for discussion on backwards compatibility.

   o  Clarifications relating to security with tunneling.

   o  A significant number of editorial improvements and clarifications.

Appendix C.  Draft Changelog

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

Appendix C.1.  Changes between -06 and -07

   o  Be more reserved about multi-hop security properties in section
      'Multi-Hop Protocol Sessions'.

   o  Clarify IP-in-IP tunnel decapsulation/forwarding as decrementing

   o  Add text on related messages backwards compatibility.

   o  Editorial updates.

Appendix C.2.  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).

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Authors' Addresses

   Vijay Gill

   Email: vijay@umbc.edu

   John Heasley

   Email: heas@shrubbery.net

   David Meyer

   Email: dmm@1-4-5.net

   Pekka Savola

   Email: psavola@funet.fi

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