INTERNET-DRAFT                                    Vijay Gill
draft-gill-btsh-01.txt                          John Heasley
                                                 David Meyer
Category                                       Informational
Expires: November 2003                              May 2003

                    The BGP TTL Security Hack (BTSH)

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
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Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   The BGP TTL Security Hack (BTSH) is designed to protect the BGP
   [RFC1771] infrastructure from CPU-utilization based attacks. While
   BTSH is most effective in protecting directly connected BGP peers, it
   can also provide a lower level of protection to multi-hop sessions.

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                           Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2. Assumptions Underlying BTSH. . . . . . . . . . . . . . . . . .   3
    2.1. Assumptions on Attack Sophistication. . . . . . . . . . . .   3
   3. BTSH Procedure . . . . . . . . . . . . . . . . . . . . . . . .   4
    3.1. Multi-hop Scenarios . . . . . . . . . . . . . . . . . . . .   5
     3.1.1. iBGP Handling. . . . . . . . . . . . . . . . . . . . . .   5
   4. Intellectual Property. . . . . . . . . . . . . . . . . . . . .   5
   5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .   6
   6. Security Considerations. . . . . . . . . . . . . . . . . . . .   6
   7. IANA Considerations. . . . . . . . . . . . . . . . . . . . . .   6
   8. References . . . . . . . . . . . . . . . . . . . . . . . . . .   6
    8.1. Normative References. . . . . . . . . . . . . . . . . . . .   7
    8.2. Informative References. . . . . . . . . . . . . . . . . . .   7
   9. Author's Addresses . . . . . . . . . . . . . . . . . . . . . .   8
   10. Full Copyright Statement. . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The BGP TTL Security Hack (BTSH) is designed to protect the BGP
   [RFC1771] infrastructure from CPU-utilization based attacks. In
   particular, while cryptographic techniques can protect the routed
   infrastructure from a wide variety of attacks, many attacks based on
   CPU-overload can be prevented by the simple mechanism described in
   this document.

   BTSH is based on the fact that the vast majority of ISP eBGP peerings
   are established between routers that are adjacent [PEERING]. Thus
   most eBGP 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 [BALDWIN2001] 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 BGP packets.

   SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined
   in RFC 2119 [RFC2119].

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2.  Assumptions Underlying BTSH

   BTSH is predicated upon the following assumptions:

    (i).   The vast majority of eBGP 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 BTSH 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 BTSH.

2.1.  Assumptions on Attack Sophistication

   Throughout this document, we assume that attackers have evolved in
   both sophistication and access to the point that they can send
   control traffic to a BGP 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 BGP speaker decrements TTL properly (clearly, if the
   either the path or the adjacent peer is compromised, then there are
   worse problems we have to worry about).

   Since the vast majority of our peerings are between adjacent routers,
   we can set the TTL on the BGP packets to 255 (the maximum possible
   for IP) and then reject any BGP packets that come in from configured
   peers which do NOT have a TTL in the range 255-254. That is, the
   receive TTL is expected to be within a small range of 1 or 2
   (254-255). The actual value depends upon the architecture, but is it
   is expected that the receiver will verify the range.

   BTSH 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

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   shut down (a method to reduce exposure to multi-hop attacks is
   outlined below).

3.  BTSH Procedure

   BTSH SHOULD not be enabled by default. The following process
   described the per-peer behavior:

   (i).   If BTSH is enabled, do the following:

           (a).    For directly connected routers,

                   o Set the TCP TTL for the BGP connection a value
                     in the range 255-254.

                   o For each configured eBGP peer:

                     Update the receive path ACL/firewall to only
                     allow BGP packets to pass onto the Route
                     Processor (RP) that have the correct
                     <source,destination,TTL> tuple. The TTL must
                     either be in the range 255-254 (directly
                     connected peer), or 255-(configured-range-of-hops)
                     for a multi-hop peer. We specify a range here
                     to achieve some robustness to changes in
                     topology. The connected check should be
                     disabled for such non-direct 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 BGP packets from adjacent router to
                     pass onto the RP.

           (c).    If the TTL is not in the range 255-254 (or
                   255-(configured-range-of-hops) for multi-hop
                   peers), punt into low priority queue, log, or
                   silently discard.

   (ii).   If BTSH is not enabled for a particular peering, normal
           RFC 1772 [RFC1772] protocol behavior is followed.

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3.1.  Multi-hop Scenarios

   When a multi-hop BGP session is required, we set the expected TTL
   value to be 255-(configured-range-of-acceptable-of-hops). While this
   approach provides a qualitatively lower degree of security for BGP
   (i.e., an DoS attack could be theoretically be launched by
   compromising some box in the path). However, BTSH will still catch
   the vast majority of observed DDoS attacks against eBGP.

3.1.1.  iBGP Handling

   BTSH is not used for iBGP peer groups. Current best practice is to
   protect peers (both eBGP and iBGP) with an MD5 signature [RFC2385].
   Such sessions can be further protected by filtering (deny) at the
   network edge for any packet that has a source address of one of the
   loopbacks addresses used for iBGP peering.

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
   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.  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

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5.  Acknowledgments

   The BTSH concept originated with many different people, including
   Paul Traina and Jon Stewart. Ryan McDowell also suggested a similar
   idea. Steve Bellovin, Jay Borkenhagen and Randy Bush also provided
   useful feedback on early versions of this document.

6.  Security Considerations

   BTSH is a simple procedure that protects single hop BGP sessions,
   except in those cases where the directly connected peer has been
   compromised. While the method is less effective for multi-hop BGP
   sessions, it still closes the window on several forms of attack.

   Protection of the BGP infrastructure beyond this method will likely
   require cryptographic machinery such as is envisioned by Secure BGP
   (S-BGP) [SBGP1,SBGP2], and/or other extensions. For example, consider
   the class of attacks based on forged SYN packets directed to port
   179/tcp on a large core infrastructure routers. In this case, the
   routers respond with SYN/ACKs (or ICMP messages) towards the victim,
   resulting in flooding of the victim's link being flooded with SYN/ACK
   or ICMP traffic. Preventing such attacks will likely require that BGP
   speakers send SYN/ACKs only to configured neighbors, and they never
   send ICMP messages related to these events.

   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 some robustness to different forms of attack.

7.  IANA Considerations

   This document creates a no new requirements on IANA namespaces

8.  References

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8.1.  Normative References

   [RFC1771]       "A Border Gateway Protocol (BGP-4)", Y. Rekhter,
                   T. Li, Editors, RFC 1771, March, 1995

   [RFC1772]       "Application of the Border Gateway Protocol in
                   the Internet", Y. Rekhter, P. Gross, RFC 1772,
                   March, 1995

   [RFC2385]       "Protection of BGP Sessions via the TCP MD5
                   Signature Option", A. Heffernan, RFC 2385,
                   August, 1998.

   [SBGP1]         "Secure Border Gateway Protocol (Secure-BGP)",
                   Stephen Kent and Charles Lynn and Karen Seo,
                   IEEE Journal on Selected Areas in Communications,
                   volume 18, number 4, April, 2000.

   [SBGP2]         "Secure Border Gateway Protocol (S-BGP) -- Real
                   World Performance and Deployment Issues", Stephen
                   Kent and Charles Lynn and Joanne Mikkelson and
                   Karen Seo, Proceedings of the IEEE Network and
                   Distributed System Security Symposium, February,

8.2.  Informative References


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

   [RFC2119]       "Key words for use in RFCs to Indicate
                   Requirement Levels", S. Bradner, RFC 2119, March,
   [RFC2434]       Narten, T., and H. Alvestrand, "Guidelines for
                   Writing an IANA Considerations Section in
                   RFCs", RFC 2434/BCP 0026, October, 1998.

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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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