TitleIngress Filtering for Multihomed Networks
AuthorF. Baker, P. Savola
DateMarch 2004
Format:TXT, HTML
Updated byRFC8704

Network Working Group                                           F. Baker
Request for Comments: 3704                                 Cisco Systems
Updates: 2827                                                  P. Savola
BCP: 84                                                        CSC/FUNET
Category: Best Current Practice                               March 2004

               Ingress Filtering for Multihomed Networks

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   BCP 38, RFC 2827, 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.  There are cases when this
   may create problems, e.g., with 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.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

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 . . . . . . . . . . . . . .  5
       2.3 Feasible Path Reverse Path Forwarding  . . . . . . . . . .  6
       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
       3.3 Ingress Filtering on Peering Links . . . . . . . . . . . .  9
   4.  Solutions to Ingress Filtering with Multihoming  . . . . . . .  9
       4.1 Use Loose RPF When Appropriate . . . . . . . . . . . . . . 10
       4.2 Ensure That Each ISP's Ingress Filter Is Complete  . . . . 11
       4.3 Send Traffic Using a Provider Prefix Only to That Provider 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  Conclusions and Future Work  . . . . . . . . . . . . . . . . . 13
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       8.1.  Normative References . . . . . . . . . . . . . . . . . . 14
       8.2.  Informative References . . . . . . . . . . . . . . . . . 14
   9.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
   10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

1.  Introduction

   BCP 38, RFC 2827 [1], 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.  There are cases when this
   may create problems, e.g., with 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
   source address is a so-called "Martian Address" - an address that is
   reserved [3], including any address within,,,,,, or

   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 prefixes 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.

   This document is aimed at ISP and edge network operators who 1) would
   like to learn more of ingress filtering methods in general, or 2) are
   already using ingress filtering to some degree but who would like to
   expand its use and want to avoid the pitfalls of ingress filtering in
   the multihomed/asymmetric scenarios.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

   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.

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 (the names are in relatively common usage):

   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 6.  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 on a network interface 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 RFC 2827 [1], and in some
   sense the most deterministic one.

   However, Ingress Access Lists are typically maintained manually; for
   example, forgetting to have the list updated at the ISPs if the set
   of prefixes changes (e.g., as a result of multihoming) might lead to
   discarding the packets if they do not pass the ingress filter.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

   Naturally, this problem is not limited to Ingress Access Lists -- it
   is inherent to Ingress Filtering when the ingress filter is not
   complete.  However, usually Ingress Access Lists are more difficult
   to maintain than the other mechanisms, and having an outdated list
   can prevent legitimate access.

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
   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)
   - and if the packet is received on the interface which would be used
   to forward the traffic to the source of the packet, it passes the

   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

   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

   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.  With BGP and sufficient
   machinery in place, setting the preferences could even be automated,
   using BGP Communities [2].  That way, the route will always be the
   best one in the FIB, even in the scenarios where only the primary
   connectivity would be used and typically no packets would pass

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

   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 instead of just inserting one
   best route there, the alternative paths (if any) have been added as
   well, and are valid for consideration.  The list is populated using
   routing-protocol 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 routers performing Feasible RPF checking.  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.

   As a general guideline, if an advertisement is filtered, the packets
   will be filtered as well.

   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

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 (even a default route, if applicable), not where the route
   points to.  Practically, this could be considered as a "route
   presence check" ("loose RPF is a misnomer in a sense because there is
   no "reverse path" check in the first place).

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

   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
   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 of 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, i.e., an "explicit route presence check".
   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 only to
   catch traffic with bogus source addresses, with an extensive (or even
   full) list of explicit routes to cover legitimate traffic.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

   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.

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
   defense 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 there -- or will get used, at some
   point in the future; 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

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

   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.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

3.3.  Ingress Filtering on Peering Links

   Ingress filtering on peering links, whether by ISPs or by end-sites,
   is not really that much different from the more typical "downstream"
   or "upstream" ingress filtering.

   However, it's important to note that with mixed upstream/downstream
   and peering links, the different links may have different properties
   (e.g., relating to contracts, trust, viability of the ingress
   filtering mechanisms, etc.).  In the most typical case, just using an
   ingress filtering mechanism towards a peer (e.g., Strict RPF) works
   just fine as long as the routing between the peers is kept reasonably
   symmetric.  It might even be considered useful to be able to filter
   out source addresses coming from an upstream link which should have
   come over a peering link (implying something like Strict RPF is used
   towards the upstream) -- but this is a more complex topic and
   considered out of scope; see Section 6.

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

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

   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

   1.  Do not multihome.

   2.  Do not use ingress filters.

   3.  Accept that service will be incomplete.

   4.  On some interfaces, weaken ingress filtering by using an
       appropriate form of loose RPF check, as described in Section 4.1.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

   5.  Ensure, by BGP or by contract, that each ISP's ingress filter is
       complete, as described in Section 4.2.

   6.  Ensure that edge networks only deliver traffic to their ISPs that
       will in fact pass the ingress filter, as described in Section

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

   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
   filtering may be difficult to deploy using a mechanism such as 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 using 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, except as a
   way to measure whether "martian" or other unrouted addresses are
   being used.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

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

   There are a number of techniques which make it easier to ensure the
   ISP's ingress filter is complete.  Feasible RPF and Strict RPF with
   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.

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.  Alternatively, a set of
   tunnels could be configured instead of multiple connections to the
   same ISP [4][5].  This way the edge routers are configured to first
   inspect the source address of a packet destined to an ISP and shunt
   it into the appropriate tunnel or interface toward the ISP.

   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.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

5.  Security Considerations

   Ingress filtering is typically performed to ensure that traffic
   arriving on one network interface legitimately comes from a computer
   residing on a network reachable through that interface.

   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, a simple 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

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

      propagation of routing information to work; the implications of
      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 different ingress filtering
   mechanisms, 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

6.  Conclusions and Future Work

   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.

   To summarize:

   o  Ingress filtering should always be done between the ISP and a
      single-homed edge network.

   o  Ingress filtering with Feasible RPF or similar Strict RPF
      techniques could almost always be applied between the ISP and
      multi-homed edge networks as well.

   o  Both the ISPs and edge networks should verify that their own
      addresses are not being used in source addresses in the packets
      coming from outside their network.

   o  Some form of ingress filtering is also reasonable between ISPs,
      especially if the number of prefixes is low.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

   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

   One can identify multiple areas where additional work would be

   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.

   o  Consider the more complex case where a network has connectivity
      with different properties (e.g., peers and upstreams), and wants
      to ensure that traffic sourced with a peer's address should not be
      accepted from the upstream.

7.  Acknowledgements

   Rob Austein, Barry Greene, Christoph Reichert, Daniel Senie, Pedro
   Roque, and Iljitsch van Beijnum reviewed this document and helped in
   improving it.  Thomas Narten, Ted Hardie, and Russ Housley provided
   good feedback which boosted the document in its final stages.

8.  References

8.1.  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.

8.2.  Informative References

   [2]  Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities
        Attribute", RFC 1997, August 1996.

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

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

   [4]  Bates, T. and Y. Rekhter, "Scalable Support for Multi-homed
        Multi-provider Connectivity", RFC 2260, January 1998.

   [5]  Hagino, J. and H. Snyder, "IPv6 Multihoming Support at Site Exit
        Routers", RFC 3178, October 2001.

9.  Authors' Addresses

   Fred Baker
   Cisco Systems
   Santa Barbara, CA  93117

   EMail: fred@cisco.com

   Pekka Savola

   EMail: psavola@funet.fi

RFC 3704       Ingress Filtering for Multihomed Networks      March 2004

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