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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.

Abstract

 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.

Baker & Savola Best Current Practice [Page 1] 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

Baker & Savola Best Current Practice [Page 2] 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 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 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.

Baker & Savola Best Current Practice [Page 3] 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.

Baker & Savola Best Current Practice [Page 4] 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
 check.
 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.  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

Baker & Savola Best Current Practice [Page 5] 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
 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 (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).

Baker & Savola Best Current Practice [Page 6] 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.

Baker & Savola Best Current Practice [Page 7] 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
 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 Best Current Practice [Page 8] 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
    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:
 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.

Baker & Savola Best Current Practice [Page 9] 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
     4.3.
 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.

Baker & Savola Best Current Practice [Page 10] 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
 RIR.
 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.

Baker & Savola Best Current Practice [Page 11] 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

Baker & Savola Best Current Practice [Page 12] 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
 appropriate.

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.

Baker & Savola Best Current Practice [Page 13] 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
 implement.
 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.
 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.

Baker & Savola Best Current Practice [Page 14] 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
 US
 EMail: fred@cisco.com
 Pekka Savola
 CSC/FUNET
 Espoo
 Finland
 EMail: psavola@funet.fi

Baker & Savola Best Current Practice [Page 15] RFC 3704 Ingress Filtering for Multihomed Networks March 2004

10. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  This document is subject
 to the rights, licenses and restrictions contained in BCP 78 and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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Baker & Savola Best Current Practice [Page 16]

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