INTERNET-DRAFT V. Gill
draft-gill-gtsh-03.txt J. Heasley
D. Meyer
Category Experimental
Expires: April 2004 October 2003
The Generalized TTL Security Mechanism (GTSM)
<draft-gill-gtsh-03.txt>
Status of this Document
This document is an Internet-Draft and is in full conformance with
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Abstract
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
appropriate).
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
255.
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
255-(configured-range-of-acceptable-hops)
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
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
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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
Director.
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
compromised.
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.
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 [RFC2839]). 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.
Finally, 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.
6.3. 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
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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 some robustness to different forms of
attack.
7. IANA Considerations
This document creates a no new requirements on IANA namespaces
[RFC2434].
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,
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.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via
the TCP MD5 Signature Option", RFC 2385, August,
1998.
[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
progress.
[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,
1997.
[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
Email: vijay@umbc.edu
John Heasley
Email: heas@shrubbery.net
David Meyer
Email: dmm@1-4-5.net
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
English.
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
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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