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Versions: 00 01 02 03                                                   
Routing Area WG                                                P. Savola
Internet-Draft                                                 CSC/FUNET
Intended status: Informational                             June 13, 2006
Expires: December 15, 2006

            Backbone Infrastructure Attacks and Protections

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
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Copyright Notice

   Copyright (C) The Internet Society (2006).


   A number of countermeasures for attacks against service provider
   backbone network infrastructure have been specified or proposed, each
   of them usually targeting a subset of the problem space.  There has
   never been a more generic analysis of the actual problems, and which
   countermeasures are even necessary (and where).  This document tries
   to provide that higher-level view.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Abbreviations  . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Threat Model . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Attack Vectors . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Lower-layer Attacks  . . . . . . . . . . . . . . . . . . .  5
     2.2.  Generic DoS on the Router  . . . . . . . . . . . . . . . .  5
     2.3.  Generic DoS on a Link  . . . . . . . . . . . . . . . . . .  5
     2.4.  Cryptographic Exhaustion Attacks . . . . . . . . . . . . .  6
     2.5.  Unauthorized Neighbor or Routing Attacks . . . . . . . . .  6
     2.6.  TCP RST Attacks  . . . . . . . . . . . . . . . . . . . . .  7
     2.7.  ICMP Attacks . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Typical Countermeasures  . . . . . . . . . . . . . . . . . . .  7
     3.1.  Address Filtering  . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Route Filtering  . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  GTSM . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  TCP-MD5 and Other Custom Authentication  . . . . . . . . .  9
     3.5.  IPsec and IKE  . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Protocol Analysis  . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  IS-IS  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.3.  BFD  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.4.  BGP  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.5.  Multicast Protocols (PIM, MSDP)  . . . . . . . . . . . . . 11
   5.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 16

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1.  Introduction

   A number of countermeasures for attacks against service provider
   backbone network infrastructure have been specified or proposed, each
   of them usually targeting a subset of the problem space.  There has
   never been a more generic analysis of the actual problems, and which
   countermeasures are even necessary (and where).  This document tries
   to provide that higher-level view.

   The scope of this document are backbone infrastructures and the
   critical protocols that are required to function for legitimate
   traffic to be correctly forwarded through the network.  As such,
   other important services or applications required by infrastructure
   elements such as RADIUS, NTP, remote access, syslog, SNMP, and DNS
   are out of scope.  All such components should be adequately protected
   through appropriate measures, the most important of which are proper
   address and route filtering and restricting authorized access.

   Additionally, the network might run additional routing protocols that
   are not described in this memo, such as (G)MPLS, RSVP-TE or LDP.

1.1.  Abbreviations

   We exclude the common abbreviations such as TCP, ICMP and DNS.

   BGP     Border Gateway Protocol
   BFD     Bidirectional Forwarding Detection
   DoS     Denial of Service
   DSCP    DiffServ Code Point
   GTSM    Generalized TTL Security Mechanism
   IGP     Interior Gateway Protocol
   IKE     Internet Key Exchange
   IRR     Internet Routing Registry
   IS-IS   Integrated System - Integrated System (routing protocol)
   LDP     Label Distribution Protocol
   (G)MPLS (Generalized) Multi-Protocol Label Switching
   MSDP    Multicast Source Discovery Protocol
   NTP     Network Time Protocol
   OSPF    Open Shortest Path First
   PIM     Protocol Independent Multicast
   RADIUS  Remote Authentication Dial-In User Service
   RSVP-TE Resource Reservation Protocol - Traffic Engineering
   SNMP    Simple Network Management Protocol

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1.2.  Assumptions

   This document assumes that the service provider is doing at least
   some form of address filtering at its border devices, i.e., by
   ensuring that only the infrastructure nodes can use infrastructure
   source IP addresses to talk to the other nodes in the infrastructure.
   So, for example, if a router sees an IP packet coming from a source
   address assigned to another router in the backbone, it can be sure
   the packet has been originated inside the backbone (assuming the
   physical security or nodes in the backbone have not been subverted).

   This requirement can be satisfied by applying ingress filtering at
   all the ISP borders [RFC2827][RFC3704], but just filtering
   infrastructure source IP addresses from the outside is also
   sufficient.  Some may even implement this by blocking access to the
   infrastructure destination addresses at the border, but this document
   doesn't describe this approach as that has a number of other issues.

   Current operational practices are described in
   [I-D.ietf-opsec-current-practices]; while almost all ISPs are capable
   of employing data plane filtering at the edges, at least one major
   ISP is known not do be able to due to legacy hardware limitations.
   Various filtering capabilities have been discussed at more length in

   If this requirement cannot be satisfied, other approaches are
   warranted.  For example, [I-D.zinin-rtg-dos] suggested an alternative
   (and in any case, provides good analysis); IPsec-protecting all the
   control traffic may be an option if "all bets are off".

1.3.  Threat Model

   In the context of this document, threats are assumed to come from
   external sources, either from customers or other networks.  The
   typical attacks are either meant to cause some form of denial of
   service or to hijack or gain access to a service, such as:

   o  DoS attacks directed at infrastructure (e.g., TCP RSTs, ICMP

   o  DoS attacks directed at someone else but cause harm (e.g., too
      much traffic exceeds forwarding or control processor capacity), or

   o  Hijacking attacks (e.g., unauthorized routing advertisement,
      access control attempt with a spoofed address).

   Other possible attack vectors include inside attacks (e.g.,
   compromised personnel or inadequately protected infrastructure

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   device) and lower-layer attacks as described in Section 2.1.  While
   the insider attack could be devastating, it can be mitigated by
   access controls and automatic configuration audits.  As such the
   first order priority problems typically come from external sources.

2.  Attack Vectors

   This Section describes the most obvious attack vectors.

2.1.  Lower-layer Attacks

   If an attacker has access to a (physical) link, it can obviously
   cause downtime for the link.  In many cases the downtime is not a
   critical threat, as it can be quickly noticed, traffic rerouted, and
   the problem fixed.  Some ISPs are more concerned about other forms of
   attacks: insertion of eavesdropping or man-in-the-middle devices.
   Fortunately, installing such would require downtime, and insertion
   could be noticeable, e.g., as an unscheduled issue gets fixed on its

   However, a lower-layer attack is not specific to routing protocols.
   An attacker could just violate integrity or confidentiality of
   regular packets, instead of tampering with routing.  As such, if a
   lower-layer attack is deemed a concern, full protection for all the
   traffic should be provided and therefore this threat is not addressed
   in this document.

2.2.  Generic DoS on the Router

   A typical attack is to overload a router using various techniques,
   e.g., by sending traffic exceeding the router's forwarding capacity,
   sending special transit packets that go through a "slow-path"
   processing (such functions may also come with problems of their own
   [BLOCKED]), or by sending some packets directed at the router itself.

   Many of these techniques can be mitigated using implementation-
   specific rate-limiting mechanisms, so they are not addressed further
   in this memo.  However, protocol designers should be advised to avoid
   any designs that require noticing and processing any special packets
   from the transit traffic (e.g., messages marked with router alert

2.3.  Generic DoS on a Link

   Overloading the capacity of a link is often more difficult to prevent
   than a router DoS.  Traffic is typically not automatically rerouted
   and even if it did, doing so could make the issue worse unless there

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   is ample spare capacity.

   Mitigation methods include monitoring the usage status of links,
   prioritizing or deprioritizing certain kinds of traffic using DSCPs,
   or devising some form of rate-limiters.

2.4.  Cryptographic Exhaustion Attacks

   A special form of DoS are attacks which target a protocol that uses
   cryptographic mechanisms, for example TCP-MD5 or IPsec.  The attacker
   sends valid protocol messages with cryptographic signatures or other
   properties to the router, which is forced to perform cryptographic
   validation of the message.  If the cryptographic operations are
   computationally expensive, the attack might succeed easier than with
   other generic DoS mechanisms.  Cryptographic protocols employing
   primitives such as stateless cookies, puzzles or return routability
   are typically more resistant to this kind of attacks.

   Some implementation-specific mitigation techniques (rate-limiting
   etc.) have been deployed, but as this affects the choice of a
   countermeasure due to protocol design.

2.5.  Unauthorized Neighbor or Routing Attacks

   Unauthorized nodes can obtain a routing protocol adjacency on links
   where an IGP has been enabled by misconfiguration, or where
   authentication is not used.  This may result in many different kinds
   of attacks, for example traffic redirection

   At least in theory, while it may not be possible to establish an
   adjacency from outside the link, it may be possible to inject packets
   as if the adjacency had been established (e.g., OSPF in Section 3.1.2
   of [I-D.ietf-rpsec-ospf-vuln]).

   Protocols such as BGP and MSDP that process routing information from
   untrusted, external sources may also be attacked, for example by an
   unauthorized advertisement of a prefix.

   Special care needs to be made to ensure that unauthorized neighbors
   are prevented (e.g., by regular configuration audits and OSPF
   protocol filtering at borders).  On the other hand, routing attack
   threats from valid neighbors can be minimized via appropriate route

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2.6.  TCP RST Attacks

   TCP sessions can be closed by attackers that can send a TCP RST
   packet with guessed spoofed endpoint identifiers and a sufficiently
   close sequence number.  The attacks and defenses have been described
   at length in [I-D.ietf-tcpm-tcp-antispoof].  One particular approach
   is modifying the TCP state machine [I-D.ietf-tcpm-tcpsecure].

2.7.  ICMP Attacks

   A slightly newer attack is employing ICMP by sending an ICMP type
   that indicates a hard error condition.  ICMP errors must be
   propagated to applications, and most applications heed the errors as
   they should by closing a connection or session.  ICMP attacks and
   defenses against TCP have been extensively described in

   It is also possible to execute ICMP attacks against other protocols
   such as UDP or IPsec, but the impact and whether/how these protocols
   demultiplex received errors have not been extensively studied.  IPsec
   is protected by ICMP attacks through a lot of assumptions (e.g., that
   only ICMP errors from the end-point are accepted) or manual

3.  Typical Countermeasures

   This Section describes some of the most common countermeasures
   applied today.  This just introduces the techniques; the afforded
   protection is analyzed in Section 4 in the context of each protocol.

3.1.  Address Filtering

   As described in the first section, this document assumes that the
   internal infrastructure is secure from spoofed messages that purport
   to come from inside the infrastructure.  More fine-grained, router-
   specific filters are sometimes deployed as well.

   It is possible to hide the infrastructure by using private or non-
   advertised addressing, but this has numerous drawbacks such as
   breaking address filtering and traceroute, not protecting from the
   ISP's customers that use a default route, etc. so this document
   doesn't recommand doing so.

3.2.  Route Filtering

   Similar principles as used in address filtering can be used to
   mitigate routing attacks.  Specifically, reject any equal or more

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   specific incoming routing advertisements to the ISP's address space
   unless explicitly authorized.  Further, monitor the filtered prefixes
   and use public services (such as RIPE's MyASN [MYASN]) to monitor the
   correctness of advertisements globally.

   In addition, especially in regions where the operational practice is
   to keep Internet Routing Registry (IRR) in sync, it may be possible
   to restrict the prefixes accepted from a peer or a customer to an
   automatically generated list.  In any case, many operators define a
   maximum prefix limit per peer (which typically resets the session if
   exceeded) to prevent misconfiguration or overload attacks.

   As with address filtering, such routing advertisements might still be
   processed by other networks, but at least these steps prevent
   hijacking inside the ISP's own network and allow monitoring of most
   unauthorized attempts.

3.3.  GTSM

   GTSM [I-D.ietf-rtgwg-rfc3682bis] is a technique where the sender of a
   packet sets the TTL/Hop Count to 255 and the receiver verifies it's
   still 255 (or some other preconfigured value).  GTSM can be used to
   protect from off-link attacks (especially spoofing).  This applies
   when GTSM-enabled control traffic is inside a single link: any
   packets coming from outside the link can summarily be discarded as
   they have a TTL/Hop Count smaller than 255.

   The open issue at the moment is how GTSM handles TCP RSTs.  I.e.,
   should it require that RSTs for a GTSM-enabled session should be sent
   with TTL=255 and verified to come with TTL=255 (or a configured
   value)?  Do implementations already do this?  Is there a sensible
   transition plan or need to make a change if any?  Note that this has
   only limited impact on GTSM's security as other TCP RST mitigation
   techniques still apply.

   NOTE IN DRAFT: the following paragraph should be removed in a future
   revision, to be placed to the GTSMbis draft.

   We suggest that the GTSM spec is amended so that TCP RSTs relating to
   a GTSM-enabled protocol port MUST be sent with TTL=255.  (Note that
   this will require a kernel modification, and a means to specify to
   the kernel which ports relate to GTSM.).  The recipient's behaviour
   SHOULD be configurable, and it is RECOMMENDED that the default be to
   discard messages where TTL is not 255 (or 255-TrustRadius).

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3.4.  TCP-MD5 and Other Custom Authentication

   At least BGP, MSDP, and LDP are able to use the TCP-MD5 signature
   option to verify the authenticity of control packets.  TCP-MD5 uses
   manually configured static keys, so changing them typically resets
   the protocol session, so the solution is sub-optimal in cases where
   the security procedures require (e.g., after an employee leaves the
   organization) that the keys must be easily and often changeable.

   Using TCP-MD5 and other similar authentication mechanisms (e.g., for
   IGPs or BFD) also opens an attack vector for cryptographic exhaustion
   attacks unless implementations have appropriate mechanisms to
   throttle or otherwise manage heavy cryptographic operations.

3.5.  IPsec and IKE

   IPsec and IKE have been proposed as a more comprehensive
   countermeasure, but these protocols also require a lot of heavyweight
   protocol machinery, lots of configuration, and cryptographic
   processing.  Vendors have also expressed difficulty in applying IPsec
   to control traffic protection.

4.  Protocol Analysis

   This Section briefly discusses the protocol-specific attack
   properties below.

   ICMP attacks apply to all the IP protocols at least to some degree.
   There is no reasonable way to appropriately protect from these
   attacks by operative methods: the vendors should implement
   countermeasures described in [I-D.ietf-tcpm-icmp-attacks] to mitigate
   these attacks.

4.1.  OSPF

   OSPF attacks have already been analyzed [I-D.ietf-rpsec-ospf-vuln].
   In this context the most important of them are: (1) preventing
   misconfiguration and unauthorized neighbors, and (2) ensuring off-
   path directed attacks as described in Section 3.1.2 of

   The former requires configuration change procedures and regular
   audits of OSPF configuration, and disabling OSPF adjacencies on
   customer-facing links, or adding authentication when there are
   multiple routers.  The latter requires using OSPF authentication,
   dropping all OSPF traffic at all the borders, or moving to another,
   less vulnerable protocol (e.g., IS-IS).

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   OSPF is also used to some degree with provider-provisioned VPNs by
   the customers.  In such scenarios, strict route filtering needs to be
   applied to ensure only the valid prefixes are accepted.

4.2.  IS-IS

   Routing IP with IS-IS has gained popularity in the backbone networks
   lately.  As IS-IS does not use IP, it is sufficient to prevent
   misconfiguration and unauthorized neighbors.  Thus most of the
   attacks and countermeasures of OSPF apply to IS-IS as well:
   configuration change procedures and regular configuration audits and
   disabling IS-IS adjacencies on customer-facing links, or adding
   authentication when there are multiple routers.

4.3.  BFD

   Bidirectional Forwarding Detection (BFD) detects faults in the
   forwarding path between two endpoints.  As a generic mechanism, it
   can be applied to a number of protocols (e.g., OSPF, IS-IS, BGP,
   MPLS, or static routes).

   When BFD is in use for a single-hop scenario, it uses GTSM to protect
   from off-link attackers.  Authentication can also be used for example
   on untrusted links.

4.4.  BGP

   Internal BGP sessions run between loopback addresses.  There is no
   need to run TCP-MD5 for outsider protection as address filtering will
   avoid TCP RST attacks.

   External BGP sessions may run multi-hop between loopback addresses or
   single-hop between interface addresses.  The latter case is much more
   common and easier to protect and applying GTSM provides first-order
   resistance to off-link attackers.

   In any case, assuming address filtering, the session can only be
   reset by the peer, or by attacks from the direction of the peer's
   network (e.g., through lack of peer's border filtering).  One can
   therefore question the necessity of further protection as the peer
   can only shoot itself in the foot by killing the BGP session or
   allowing the BGP session be killed through negligence.

   If the link is not trusted (e.g., in some large Ethernet-based
   Internet Exchange points), it may also be desirable to ensure that
   peers are not able to reset others' sessions, so a mechanism like
   TCP-MD5 may be appropriate.  One should note that the security
   requirements are not necessarily very high as the attacker should

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   already be easily traceable on a single link, and thus re-keying may
   not be worth the trouble.

   As BGP processes data heard from external sources, the routing data
   can be modified in numerous ways, e.g., to create arbitrarily complex
   advertisements using path attributes to crash naive BGP
   implementations.  These and many other BGP attacks are described in
   [RFC4272].  Techniques described in Section 3.2 can mitigate the
   attack vectors to some degree, but a more comprehensive solution to
   securing routing data is needed.

4.5.  Multicast Protocols (PIM, MSDP)

   Multicast routing is typically achieved by PIM-SM
   [I-D.ietf-mboned-routingarch].  MSDP is used for IPv4 source
   discovery.  Multicast routing protocol threats have been analyzed
   separately in [I-D.ietf-mboned-mroutesec] (backbone perspective) and
   [I-D.savola-pim-lasthop-threats] (last-hop perspective).

   In summary, most of the multicast threats pertain to overloading
   control processors via too much state.  Implementation-specific rate-
   limiters can help in mitigating the risk.  If resetting MSDP sessions
   is a concern, TCP-MD5 option similar to BGP can be used.  Address
   filtering can be applied in particular in PIM Unicast-Register
   message decapsulation; other messages use multicast and already
   employ reverse path forwarding checks.

5.  Summary

   IGPs require a great deal of care to ensure that they are not enabled
   on links where they shouldn't be.  Preventing external OSPF attacks
   also requires OSPF authentication everywhere or filtering OSPF
   packets at the edges.

   ICMP attacks are able to cause a great deal of harm to almost all the
   protocols, including IPsec, and there is little to do to mitigate the
   risk except to implement enhanced ICMP payload verification/
   processing techniques.  More study of the impact on connectionless
   protocols and IPsec should be conducted.

   With border address filtering in place, internal sessions are
   reasonably safe.  With additional GTSM protection, external private
   interconnection links are also reasonably safe, as the session can
   only be reset by the neighbor or due to lack of filtering, someone
   through the neighbor's network.  TCP-MD5 protection is most
   appropriate for Internet Exchange points with multiple neighbors or
   multihop eBGP sessions, but it's worth remembering that the security

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   requirements for the solution are not very high as the attackers have
   very strict topological restrictions.

   IPsec and IKE are obviously an option for heavy-weight protection,
   but impractical (yet) due to configuration complexity and processing
   overhead.  Simplifications in configuration, implementation, and
   cryptographic hardware offloading might help the situation for the
   cases where the use of heavier protection (e.g., possibly Internet
   Exchange points) could be warranted.

6.  IANA Considerations

   This memo makes no request to IANA.

7.  Acknowledgements

   George Jones suggested improvements to the initial version of this
   draft.  Further feedback was received from Sean P. Turner, Seo Boon
   NG, Warren Kumari, Hank Nussbacher, and Jonathan Trostle.

8.  Security Considerations

   This document does not define a protocol but rather describes and
   analyzes the security properties and countermeasures in existing
   service provider backbone network infrastructures.

   The most important issues that should be noted are its security

   o  The main concern is an external attack (from customers or some
      other network); lower-layer attacks are not considered a
      particular concern for routing protocols.

   o  We require at least certain degree of address filtering at
      borders, or else all bets are off.

   o  Generic DoS attacks against routers can be mitigated using
      implementation-specific measures.

   There are a number of activities network operators have to do in
   order to protect the network (e.g., filtering OSPF packets at the
   edges or auditing IGP configurations).  There are also lessons to be
   learned for protocol designers (e.g., OSPF external attacks, ICMP
   attacks against non-TCP, use of GTSM).  Many of the issues listed
   also depend on vendors to implement effective, vendor-specific rate-

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   limiting techniques.

9.  References

9.1.  Normative References

              Savola, P., Lehtonen, R., and D. Meyer, "PIM-SM Multicast
              Routing Security Issues and Enhancements",
              draft-ietf-mboned-mroutesec-04 (work in progress),
              October 2004.

              Kaeo, M., "Operational Security Current Practices",
              draft-ietf-opsec-current-practices-03 (work in progress),
              May 2006.

              Jones, E. and O. Moigne, "OSPF Security Vulnerabilities
              Analysis", draft-ietf-rpsec-ospf-vuln-01 (work in
              progress), December 2004.

              Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", draft-ietf-rpsec-routing-threats-07
              (work in progress), October 2004.

              Gill, V., "The Generalized TTL Security Mechanism (GTSM)",
              draft-ietf-rtgwg-rfc3682bis-05 (work in progress),
              April 2005.

              Gont, F., "ICMP attacks against TCP",
              draft-ietf-tcpm-icmp-attacks-00 (work in progress),
              February 2006.

              Touch, J., "Defending TCP Against Spoofing Attacks",
              draft-ietf-tcpm-tcp-antispoof-04 (work in progress),
              May 2006.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed

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              Networks", BCP 84, RFC 3704, March 2004.

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

9.2.  Informative References

   [BLOCKED]  Cisco Systems, "Cisco Security Advisory: Cisco IOS
              Interface Blocked by IPv4 Packets", 2004, <http://

              Savola, P., "Overview of the Internet Multicast Routing
              Architecture", draft-ietf-mboned-routingarch-03 (work in
              progress), March 2006.

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   [MYASN]    RIPE NCC, "MyASn System",

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Internet-Draft          Attacks Against Backbone               June 2006

Author's Address

   Pekka Savola

   Email: psavola@funet.fi

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Internet-Draft          Attacks Against Backbone               June 2006

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