Operations Area                                                 F. Baker
Internet-Draft                                             Cisco Systems
Updates: 2827 (if approved)                                    P. Savola
Expires: January 30, 2004                                      CSC/FUNET
                                                                Aug 2003


               Ingress Filtering for Multihomed Networks
                draft-savola-bcp38-multihoming-update-01

Status of this Memo

   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
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on January 30, 2004.

Copyright Notice

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

Abstract

   RFC 2827, BCP 38, is designed to limit the impact of distributed
   denial of service attacks, by denying traffic with spoofed addresses
   access to the network, and to help ensure that traffic is traceable
   to its correct source network. As a side effect of protecting the
   Internet against such attacks, the network implementing the solution
   also protects itself from this and other attacks, such as spoofed
   management access to networking equipment. However, it may cause
   problems of its own when e.g. multihoming. This document describes
   the current ingress filtering operational mechanisms, examines
   generic issues related to ingress filtering and delves into the
   effects on multihoming in particular.  This memo updates RFC 2827.



Baker & Savola          Expires January 30, 2004                [Page 1]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Different Ways to Implement Ingress Filtering  . . . . . . . .  4
   2.1 Ingress Access Lists . . . . . . . . . . . . . . . . . . . . .  4
   2.2 Strict Reverse Path Forwarding . . . . . . . . . . . . . . . .  4
   2.3 Feasible Path Reverse Path Forwarding  . . . . . . . . . . . .  5
   2.4 Loose Reverse Path Forwarding  . . . . . . . . . . . . . . . .  6
   2.5 Loose Reverse Path Forwarding Ignoring Default Routes  . . . .  7
   3.  Clarifying the Applicability of Ingress Filtering  . . . . . .  8
   3.1 Ingress Filtering at Multiple Levels . . . . . . . . . . . . .  8
   3.2 Ingress Filtering to Protect Your Own Infrastructure . . . . .  8
   4.  Solutions to Ingress Filtering with Multihoming  . . . . . . .  9
   4.1 Use Loose RPF When Appropriate . . . . . . . . . . . . . . . .  9
   4.2 Ensure That Each ISP's Ingress Filter Is Complete  . . . . . . 10
   4.3 Send Traffic Using a Provider Prefix Only to That Provider . . 11
   5.  Conclusions and Future Work  . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
       Normative References . . . . . . . . . . . . . . . . . . . . . 16
       Informative References . . . . . . . . . . . . . . . . . . . . 17
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
       Intellectual Property and Copyright Statements . . . . . . . . 18




























Baker & Savola          Expires January 30, 2004                [Page 2]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


1. Introduction

   RFC 2827 [1],  BCP 38, is designed to limit the impact of distributed
   denial of service attacks, by denying traffic with spoofed addresses
   access to the network, and to help ensure that traffic is traceable
   to its correct source network. As a side effect of protecting the
   Internet against such attacks, the network implementing the solution
   also protects itself from this and other attacks, such as spoofed
   management access to networking equipment. However, it may cause
   problems of its own when e.g. multihoming. This document describes
   the current ingress filtering operational mechanisms, examines
   generic issues related to ingress filtering and delves into the
   effects on multihoming in particular.

   RFC 2827 recommends that ISPs police their customers' traffic by
   dropping traffic entering their networks that is coming from a source
   address not legitimately in use by the customer network.  The
   filtering includes but is in no way limited to the traffic whose
   sources address is a so-called "Martian Address" - an address that is
   reserved [2], including any address within 0.0.0.0/8, 10.0.0.0/8,
   127.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 224.0.0.0/4, or
   240.0.0.0/4.

   The reasoning behind the ingress filtering procedure is that
   Distributed Denial of Service Attacks frequently spoof other systems'
   source addresses, placing a random number in the field. In some
   attacks, this random number is deterministically within the target
   network, simultaneously attacking one or more machines and causing
   those machines to attack others with ICMP messages or other traffic;
   in this case, the attacked sites can protect themselves by proper
   filtering, by verifying that their prefix are not used in the source
   addresses in packets received from the Internet. In other attacks,
   the source address is literally a random 32 bit number, resulting in
   the source of the attack being difficult to trace. If the traffic
   leaving an edge network and entering an ISP can be limited to traffic
   it is legitimately sending, attacks can be somewhat mitigated:
   traffic with random or improper source addresses can be suppressed
   before it does significant damage, and attacks can be readily traced
   back to at least their source networks.

   In section 2, several different ways to implement ingress filtering
   are described and examined in the  generic context.  In section 3,
   some clarifications on the applicability of ingress filtering methods
   are made.  In section 4, ingress filtering is analyzed in detail from
   the multihoming perspective.  In section 5, conclusions and potential
   future work items are identified.





Baker & Savola          Expires January 30, 2004                [Page 3]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


2. Different Ways to Implement Ingress Filtering

   This section serves as an introduction to different operational
   techniques used to implement ingress filtering as of writing this
   memo. The mechanisms are described and analyzed in general terms, and
   multihoming-specific issues are described in Section 4.

   There are at least five ways one can implement RFC 2827, with varying
   impacts. These include:

   o  Ingress Access Lists

   o  Strict Reverse Path Forwarding

   o  Feasible Path Reverse Path Forwarding

   o  Loose Reverse Path Forwarding

   o  Loose Reverse Path Forwarding ignoring default routes

   Other mechanisms are also possible, and indeed, there are a number of
   techniques that might profit from further study, specification,
   implementation, and/or deployment; see Section 5.  However, these are
   out of scope.

2.1 Ingress Access Lists

   An Ingress Access List is a filter that checks the source address of
   every message received from a network against a list of acceptable
   prefixes, dropping any packet that does not match the filter. While
   this is by no means the only way to implement an ingress filter, it
   is the one proposed by BCP 38 [1], and in some sense the most
   deterministic one.

   Ingress Access Lists have some problems, however, when the list of
   prefixes is incomplete. For example, if a multihomed edge network is
   using provider-based addressing using two or more prefixes, one can
   expect each of its providers to accept traffic using their provided
   prefix, but they may not accept traffic from other prefixes. If they
   do not, then any traffic offered them sourced by a machine using the
   other prefix will find itself black-holed - that machine will only be
   able to access systems within its edge network.

2.2 Strict Reverse Path Forwarding

   Strict Reverse Path Forwarding (Strict RPF) is a simple way to
   implement an ingress filter. It is conceptually identical to using
   access lists for ingress filtering, with the exception that the



Baker & Savola          Expires January 30, 2004                [Page 4]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


   access list is dynamic. This may also be used to avoid duplicate
   configuration (e.g. maintaining both static routes or BGP prefix-list
   filters and interface access-lists). The procedure is that the source
   address is looked up in the Forwarding Information Base (FIB) - the
   same route table used in destination address routing. If the previous
   hop, or the previous hop interface, is one of the routing neighbors
   that traffic responding to the datagram might be forwarded to, the
   message is deemed to have come from a reasonable direction.

   Strict Reverse Path Forwarding is a very reasonable approach in front
   of any kind of edge network; in particular, it is far superior to
   Ingress Access Lists when the network edge is advertising multiple
   prefixes using BGP. It makes for a simple, cheap, fast, and dynamic
   filter.

   But Strict Reverse Path Forwarding has some problems of its own.
   First, the test is only applicable in places where routing is
   symmetrical - where IP datagrams in one direction and responses from
   the other deterministically follow the same path. While this is
   common at edge network interfaces to their ISP, it is in no sense
   common between ISPs, which normally use asymmetrical "hot potato"
   routing. Also, if BGP is carrying prefixes and some legitimate
   prefixes are not being advertised or not being accepted by the ISP
   under its policy, the effect is the same as ingress filtering using
   an incomplete access list: some legitimate traffic is filtered for
   lack of a route in the filtering router's Forwarding Information
   Base.

   There are operational techniques, especially with BGP but somewhat
   applicable to other routing protocols as well, to make strict RPF
   work better in the case of asymmetric or multihomed traffic. The ISP
   assigns a better metric which is not propagated outside of the
   router, either a vendor-specific "weight" or a protocol distance to
   prefer the directly received routes.  That way, the interface will
   always be in the FIB, even in the scenarios where only the primary
   connectivity would be used and typically no packets would pass
   through the interface.  This method assumes that there is no strict
   RPF filtering between the primary and secondary edge routers; in
   particular, when applied to multihoming to different ISPs, this
   assumption may fail.

2.3 Feasible Path Reverse Path Forwarding

   Feasible Path Reverse Path Forwarding (Feasible RPF) is an extension
   of Strict RPF.  The source address is still looked up in the FIB (or
   an equivalent, RPF-specific table) but the data also includes the
   list of alternative paths (if any), i.e., a list of interfaces, not
   just one interface.  The list is populated using routing-protocol



Baker & Savola          Expires January 30, 2004                [Page 5]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


   specific methods, for example by including all or N (where N is less
   than all) feasible BGP paths in the Routing Information Base (RIB).
   Sometimes this method has been implemented as part of a Strict RPF
   implementation.

   In the case of asymmetric routing and/or multihoming at the edge of
   the network, this approach provides a way to relatively easily
   address the biggest problems of Strict RPF.

   It is critical to understand the context in which Feasible RPF
   operates.  The mechanism relies on consistent route advertisements
   (i.e., the same prefix(es), through all the paths) propagating to all
   the destinations.  For example, this may not hold e.g. in the case
   where a secondary ISP does not propagate the BGP advertisement to the
   primary ISP e.g., due to route-maps or other routing policies not
   being up-to-date.  The failure modes are typically similar to
   "operationally enhanced Strict RPF", as described above.

   In consequence, properly defined, Feasible RPF is a very powerful
   tool in certain kinds of asymmetric routing scenarios, but it is
   important to understand its operational role and applicability
   better.

2.4 Loose Reverse Path Forwarding

   Loose Reverse Path Forwarding (Loose RPF) is algorithmically similar
   to strict RPF, but differs in that it checks only for the existence
   of a route, not where the route points to.

   The questionable benefit of Loose RPF is found in asymmetric routing
   situations: a packet is dropped if there is no route at all, such as
   to "Martian addresses" or addresses that are not currently routed,
   but is not dropped if a route exists.

   Loose Reverse Path Forwarding has problems, however. Since it
   sacrifices directionality, it loses the ability to limit an edge
   network's traffic to traffic legitimately sourced from that network,
   in most cases, rendering the mechanism useless as an ingress
   filtering mechanism.

   Also, many ISPs use default routes for various purposes such as
   collecting illegitimate traffic at so-called "Honey Pot" systems or
   discarding any traffic they do not have a "real" route to ,and
   smaller ISPs may well purchase transit capabilities and use a default
   route from a larger provider.  At least some implementations of Loose
   RPF check where the default route points to.  If the route points to
   the interface where Loose RPF is enabled, any packet is allowed from
   that interface; if it points nowhere or to some other interface, the



Baker & Savola          Expires January 30, 2004                [Page 6]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


   packets with bogus source addresses will be discarded at the Loose
   RPF interface even in the presence of a default route.  If such
   fine-grained checking is not implemented, presence of a default route
   nullifies the effect Loose RPF completely.

   One case where Loose RPF might fit well could be an ISP filtering
   packets from its upstream providers, to get rid of packets with
   "Martian" or other non-routed addresses.

   If other approaches are unsuitable, loose RPF could be used as a form
   of contract verification: the other network is presumably certifying
   that it has provided appropriate ingress filtering rules, so the
   network doing the filtering need only verify the fact and react if
   any packets which would show a breach in the contract are detected.
   Of course, this mechanism would only show if the source addresses
   used are "martian" or other unrouted addresses -- not if they are
   from someone else's address space.

2.5 Loose Reverse Path Forwarding Ignoring Default Routes

   The fifth implementation technique may be characterized as Loose RPF
   ignoring default routes. In this approach, the router looks up the
   source address in the route table, and preserves the packet if a
   route is found. However, in the lookup, default routes are excluded.
   Therefore, the technique is mostly usable in scenarios where default
   routes are used in addition to an extensive (or even full) list of
   more specific routes.

   Like Loose RPF, this is useful in places where asymmetric routing is
   found, such as on inter-ISP links. However, like Loose RPF, since it
   sacrifices directionality, it loses the ability to limit an edge
   network's traffic to traffic legitimately sourced from that network.



















Baker & Savola          Expires January 30, 2004                [Page 7]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


3. Clarifying the Applicability of Ingress Filtering

   What may not be readily apparent is that ingress filtering is not
   applied only at the "last-mile" interface between the ISP and the end
   user.  It's perfectly fine, and recommended, to also perform ingress
   filtering at the edges of ISPs where appropriate, at the routers
   connecting LANs to an enterprise network, etc. -- this increases the
   defence in depth.

3.1 Ingress Filtering at Multiple Levels

   Because of wider deployment of ingress filtering, the issue is
   recursive.  Ingress filtering has to work everywhere where it's used,
   not just between the first two parties.  That is, if a user
   negotiates a special ingress filtering arrangement with his ISP, he
   should also ensure (or make sure the ISP ensures) that the same
   arrangements also apply to the ISP's upstream and peering links, if
   ingress filtering is being used -- or will get used, at some point in
   the future -- there; similarly with the upstream ISPs and peers.

   In consequence, manual models which do not automatically propagate
   the information to every party where the packets would go and where
   ingress filtering might be applied have only limited generic
   usefulness.

3.2 Ingress Filtering to Protect Your Own Infrastructure

   Another feature stemming from wider deployment of ingress filtering
   may not be readily apparent.  The routers and other ISP
   infrastructure are vulnerable to several kinds of attacks.  The
   threat is typically mitigated by restricting who can access these
   systems.

   However, unless ingress filtering (or at least, a limited subset of
   it) has been deployed at every border (towards the customers, peers
   and upstreams) -- blocking the use of your own addresses as source
   addresses -- the attackers may be able to circumvent the protections
   of the infrastructure gear.

   Therefore, by deploying ingress filtering, one does not just help the
   Internet as a whole, but protects against several classes of threats
   to your own infrastructure as well.









Baker & Savola          Expires January 30, 2004                [Page 8]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


4. Solutions to Ingress Filtering with Multihoming

   First, one must ask why a site multihomes; for example, the edge
   network might:

   o  use two ISPs for backing up the Internet connectivity to ensure
      robustness,

   o  use whichever ISP is offering the fastest TCP service at the
      moment,

   o  need several points of access to the Internet in places where no
      one ISP offers service, or

   o  be changing ISPs (and therefore multihoming only temporarily).

   One can imagine a number of approaches to working around the
   limitations of ingress filters for multihomed networks. Options
   include:

   o  Do not multihome.

   o  Do not use ingress filters.

   o  Accept that service will be incomplete.

   o  On some interfaces, weaken ingress filtering by using an
      appropriate form of loose RPF check.

   o  Ensure, by BGP or by contract, that each ISP's ingress filter is
      complete.

   o  Ensure that edge networks only deliver traffic to their ISPs that
      will in fact pass the ingress filter.

   The first three of these are obviously mentioned for completeness;
   they are not and cannot be viable positions. However, the final three
   are indeed viable.

   The fourth and the fifth must be ensured in the upstream ISPs as
   well, as described in Section 3.1.

   Next, we now look at the viable ways for dealing with the
   side-effects of ingress filters.

4.1 Use Loose RPF When Appropriate

   Where asymmetric routing is preferred or is unavoidable, ingress



Baker & Savola          Expires January 30, 2004                [Page 9]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


   filtering may be difficult to deploy using a mechanism like plain
   strict RPF which requires the paths to be symmetrical. In many cases,
   using operational methods or feasible RPF may ensure the ingress
   filter is complete, like described below.  Failing that, the only
   real options are to not perform ingress filtering, use a manual
   access-list (possibly in addition to some other mechanisms), or to
   use some form of Loose RPF check.

   Failing to provide any ingress filter at all essentially trusts the
   downstream network to behave itself, which is not the wisest course
   of action.  However, especially in the case of very large networks of
   even hundreds or thousands of prefixes, maintaining manual
   access-lists may be too much to ask.

   The use of Loose RPF does not seem like a good choice between the
   edge network and the ISP, since it loses the directionality of the
   test. This argues in favor of either using a complete filter in the
   upstream network or ensuring in the downstream network that packets
   the upstream network will reject will never reach it.

   Therefore, the use of Loose RPF cannot be recommended in this
   scenario.

4.2 Ensure That Each ISP's Ingress Filter Is Complete

   For the edge network, if multihoming is being used for robustness or
   to change routing from time to time depending on measured ISP
   behavior, the simplest approach will be to ensure that its ISPs in
   fact carry its addresses in routing. This will often require the edge
   network to use provider-independent prefixes and exchange routes with
   its ISPs with BGP, to ensure that its prefix is carried upstream to
   the major transit ISPs. Of necessity, this implies that the edge
   network will be of a size and technical competence to qualify for a
   separate address assignment and an autonomous system number from its
   RIR.

   There are a number of techniques which make it easier to ensure the
   ISP's ingress filter is complete. Feasible RPF and operational
   techniques both work quite well for multihomed or asymmetric
   scenarios between the ISP and an edge network.

   When a routing protocol is not being used, but rather the customer
   information is generated from databases such as Radius, TACACS, or
   Diameter, the ingress filtering can be the most easily ensured and
   kept up-to-date with Strict RPF or Ingress Access Lists generated
   automatically from such databases.





Baker & Savola          Expires January 30, 2004               [Page 10]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


4.3 Send Traffic Using a Provider Prefix Only to That Provider

   For smaller edge networks that use provider-based addressing and
   whose ISPs implement ingress filters (which they should do), the
   third option is to route traffic being sourced from a given
   provider's address space to that provider.

   This is not a complicated procedure, but requires careful planning
   and configuration. For robustness, the edge network may choose to
   connect to each of its ISPs through two or more different Points of
   Presence (POPs), so that if one POP or line experiences an outage,
   another link to the same ISP can be used. Also, each router that
   connects to an ISP needs to be configured with a line or a tunnel to
   every other ISP-facing router, and the router configured to first
   inspect the source address of a packet destined to an ISP and shunt
   it into the appropriate tunnel or ISP interface.

   If such a scenario is applied exhaustively, so that an exit router is
   chosen in the edge network for every prefix the network uses, traffic
   originating from any other prefix can be summarily discarded instead
   of sending it to an ISP.






























Baker & Savola          Expires January 30, 2004               [Page 11]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


5. Conclusions and Future Work

   Ingress filtering is typically performed to ensure that traffic
   arriving in one network legitimately comes from a computer in the
   other network.

   The closer to the actual source ingress filtering is performed, the
   more effective it is. One could wish that the first hop router would
   ensure that traffic being sourced from its neighboring end system was
   correctly addressed; a router further away can only ensure that it is
   possible that there is such a system within the indicated prefix.
   Therefore, ingress filtering should be done at multiple levels, with
   different level of granularity.

   It bears to keep in mind that while one goal of ingress filtering is
   to make attacks traceable, it is impossible to know whether the
   particular attacker "somewhere in the Internet" is being ingress
   filtered or not.  Therefore, one can only guess whether the source
   addresses have been spoofed or not: in any case, getting a possible
   lead -- e.g. to contact a potential source to ask whether they're
   observing an attack or not -- is still valuable, and more so when the
   ingress filtering gets more and more widely deployed.

   In consequence, every administrative domain should try to ensure a
   sufficient level of ingress filtering on its borders.

   Security properties and applicability of different ingress filtering
   types differ a lot.

   o  Ingress access lists require typically manual maintenance, but are
      the most bulletproof when done properly; typically, ingress access
      lists are best fit between the edge and the ISP when the
      configuration is not too dynamic if strict RPF is not an option,
      between ISPs if the number of used prefixes is low, or as an
      additional layer of protection.

   o  Strict RPF check is a very easy and sure way to implement ingress
      filtering.  It is typically fit between the edge network and the
      ISP.  In many cases, plain strict RPF can be augmented by
      operational procedures in the case of asymmetric traffic patterns,
      or the feasible RPF technique to also account for other
      alternative paths.

   o  Feasible Path RPF check is an extension of Strict RPF.  It is
      suitable in all the scenarios where Strict RPF is, but multihomed
      or asymmetric scenarios in particular.  However, one must remember
      that Feasible RPF assumes the consistent origination and
      propagation of routing information to work; the implications of



Baker & Savola          Expires January 30, 2004               [Page 12]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


      this must be understood especially if a prefix advertisement
      passes through third parties.

   o  Loose RPF primarily filters out unrouted prefixes such as Martian
      addresses. It can be applied in the upstream interfaces to reduce
      the size of DoS attacks with unrouted source addresses.  In the
      downstream interfaces it can only be used as a contract
      verification, that the other network has performed at least some
      ingress filtering.

   When weighing the tradeoffs of ingress filtering, the security
   properties of a more relaxed approach should be carefully considered
   before applying it.  Especially when applied by an ISP towards an
   edge network, there don't seem to be many reasons why a stricter form
   of ingress filtering would not be appropriate.

   One can identify multiple areas where additional work would be
   useful:

   o  Specify the mechanisms in more detail: there is some variance
      between implementations e.g., on whether traffic to multicast
      destination addresses will always pass the Strict RPF filter or
      not.  By formally specifying the mechanisms the implementations
      might get harmonized.

   o  Study and specify Routing Information Base (RIB) -based RPF
      mechanisms, e.g., Feasible Path RPF, in more detail.  In
      particular, consider under which assumptions these mechanisms work
      as intended and where they don't.

   o  Write a more generic note on the ingress filtering mechanisms than
      this memo, after the taxonomy and the details or the mechanisms
      (points above) have been fleshed out.


















Baker & Savola          Expires January 30, 2004               [Page 13]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


6. Security Considerations

   This memo describes ingress filtering techniques in general and the
   options for multihomed networks in particular.

   It is important for ISPs to implement ingress filtering to prevent
   spoofed addresses being used, both to curtail DoS attacks and to make
   them more traceable, and to protect their own infrastructure.  This
   memo describes mechanisms that could be used to achieve that effect,
   and the tradeoffs of those mechanisms.

   This memo will lower the bar for the adoption of ingress filtering
   especially in the scenarios like asymmetric/multihomed networks where
   the general belief has been that ingress filtering is difficult to
   implement.




































Baker & Savola          Expires January 30, 2004               [Page 14]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


7. Acknowledgements

   Rob Austein, Barry Greene, Christoph Reichert, Daniel Senie, and
   Pedro Roque reviewed this document and helped in improving it.















































Baker & Savola          Expires January 30, 2004               [Page 15]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


Normative References

   [1]  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.














































Baker & Savola          Expires January 30, 2004               [Page 16]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


Informative References

   [2]  IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.


Authors' Addresses

   Fred Baker
   Cisco Systems

   Santa Barbara, CA  93117
   US

   EMail: fred@cisco.com


   Pekka Savola
   CSC/FUNET

   Espoo
   Finland

   EMail: psavola@funet.fi




























Baker & Savola          Expires January 30, 2004               [Page 17]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


Intellectual Property Statement

   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
   Director.


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 assignees.

   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
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION



Baker & Savola          Expires January 30, 2004               [Page 18]


Internet-Draft    Ingress Filtering for Multihomed Networks     Aug 2003


   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































Baker & Savola          Expires January 30, 2004               [Page 19]