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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.



















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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


























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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.






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   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.






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   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



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   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).




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   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.




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   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.




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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.





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   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.








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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.






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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




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      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.







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   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
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
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   might or might not be available; nor does it represent that it has
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

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









Baker & Savola           Best Current Practice                 [Page 16]

=========================================================================



Internet Engineering Task Force (IETF)                         K. Sriram
Request for Comments: 8704                                 D. Montgomery
BCP: 84                                                         USA NIST
Updates: 3704                                                    J. Haas
Category: Best Current Practice                   Juniper Networks, Inc.
ISSN: 2070-1721                                            February 2020


         Enhanced Feasible-Path Unicast Reverse Path Forwarding

Abstract

   This document identifies a need for and proposes improvement of the
   unicast Reverse Path Forwarding (uRPF) techniques (see RFC 3704) for
   detection and mitigation of source address spoofing (see BCP 38).
   Strict uRPF is inflexible about directionality, the loose uRPF is
   oblivious to directionality, and the current feasible-path uRPF
   attempts to strike a balance between the two (see RFC 3704).
   However, as shown in this document, the existing feasible-path uRPF
   still has shortcomings.  This document describes enhanced feasible-
   path uRPF (EFP-uRPF) techniques that are more flexible (in a
   meaningful way) about directionality than the feasible-path uRPF (RFC
   3704).  The proposed EFP-uRPF methods aim to significantly reduce
   false positives regarding invalid detection in source address
   validation (SAV).  Hence, they can potentially alleviate ISPs'
   concerns about the possibility of disrupting service for their
   customers and encourage greater deployment of uRPF techniques.  This
   document updates RFC 3704.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8704.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Requirements Language
   2.  Review of Existing Source Address Validation Techniques
     2.1.  SAV Using Access Control List
     2.2.  SAV Using Strict Unicast Reverse Path Forwarding
     2.3.  SAV Using Feasible-Path Unicast Reverse Path Forwarding
     2.4.  SAV Using Loose Unicast Reverse Path Forwarding
     2.5.  SAV Using VRF Table
   3.  SAV Using Enhanced Feasible-Path uRPF
     3.1.  Description of the Method
       3.1.1.  Algorithm A: Enhanced Feasible-Path uRPF
     3.2.  Operational Recommendations
     3.3.  A Challenging Scenario
     3.4.  Algorithm B: Enhanced Feasible-Path uRPF with Additional
           Flexibility across Customer Cone
     3.5.  Augmenting RPF Lists with ROA and IRR Data
     3.6.  Implementation and Operations Considerations
       3.6.1.  Impact on FIB Memory Size Requirement
       3.6.2.  Coping with BGP's Transient Behavior
     3.7.  Summary of Recommendations
       3.7.1.  Applicability of the EFP-uRPF Method with Algorithm A
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Source address validation (SAV) refers to the detection and
   mitigation of source address (SA) spoofing [RFC2827].  This document
   identifies a need for and proposes improvement of the unicast Reverse
   Path Forwarding (uRPF) techniques [RFC3704] for SAV.  Strict uRPF is
   inflexible about directionality (see [RFC3704] for definitions), the
   loose uRPF is oblivious to directionality, and the current feasible-
   path uRPF attempts to strike a balance between the two [RFC3704].
   However, as shown in this document, the existing feasible-path uRPF
   still has shortcomings.  Even with the feasible-path uRPF, ISPs are
   often apprehensive that they may be dropping customers' data packets
   with legitimate source addresses.

   This document describes enhanced feasible-path uRPF (EFP-uRPF)
   techniques that aim to be more flexible (in a meaningful way) about
   directionality than the feasible-path uRPF.  It is based on the
   principle that if BGP updates for multiple prefixes with the same
   origin AS were received on different interfaces (at border routers),
   then incoming data packets with source addresses in any of those
   prefixes should be accepted on any of those interfaces (presented in
   Section 3).  For some challenging ISP-customer scenarios (see
   Section 3.3), this document also describes a more relaxed version of
   the enhanced feasible-path uRPF technique (presented in Section 3.4).
   Implementation and operations considerations are discussed in
   Section 3.6.

   Throughout this document, the routes under consideration are assumed
   to have been vetted based on prefix filtering [RFC7454] and possibly
   origin validation [RFC6811].

   The EFP-uRPF methods aim to significantly reduce false positives
   regarding invalid detection in SAV.  They are expected to add greater
   operational robustness and efficacy to uRPF while minimizing ISPs'
   concerns about accidental service disruption for their customers.  It
   is expected that this will encourage more deployment of uRPF to help
   realize its Denial of Service (DoS) and Distributed DoS (DDoS)
   prevention benefits network wide.

1.1.  Terminology

   The Reverse Path Forwarding (RPF) list is the list of permissible
   source-address prefixes for incoming data packets on a given
   interface.

   Peering relationships considered in this document are provider-to-
   customer (P2C), customer-to-provider (C2P), and peer-to-peer (P2P).
   Here, "provider" refers to a transit provider.  The first two are
   transit relationships.  A peer connected via a P2P link is known as a
   lateral peer (non-transit).

   AS A's customer cone is A plus all the ASes that can be reached from
   A following only P2C links [Luckie].

   A stub AS is an AS that does not have any customers or lateral peers.
   In this document, a single-homed stub AS is one that has a single
   transit provider and a multihomed stub AS is one that has multiple
   (two or more) transit providers.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Review of Existing Source Address Validation Techniques

   There are various existing techniques for the mitigation of DoS/DDoS
   attacks with spoofed addresses [RFC2827] [RFC3704].  SAV is performed
   in network edge devices, such as border routers, Cable Modem
   Termination Systems (CMTS) [RFC4036], and Packet Data Network
   Gateways (PDN-GWs) in mobile networks [Firmin].  Ingress Access
   Control List (ACL) and uRPF are techniques employed for implementing
   SAV [RFC2827] [RFC3704] [ISOC].

2.1.  SAV Using Access Control List

   Ingress/egress ACLs are maintained to list acceptable (or
   alternatively, unacceptable) prefixes for the source addresses in the
   incoming/outgoing Internet Protocol (IP) packets.  Any packet with a
   source address that fails the filtering criteria is dropped.  The
   ACLs for the ingress/egress filters need to be maintained to keep
   them up to date.  Updating the ACLs is an operator-driven manual
   process; hence, it is operationally difficult or infeasible.

   Typically, the egress ACLs in access aggregation devices (e.g., CMTS,
   PDN-GW) permit source addresses only from the address spaces
   (prefixes) that are associated with the interface on which the
   customer network is connected.  Ingress ACLs are typically deployed
   on border routers and drop ingress packets when the source address is
   spoofed (e.g., belongs to obviously disallowed prefix blocks, IANA
   special-purpose prefixes [SPAR-v4][SPAR-v6], provider's own prefixes,
   etc.).

2.2.  SAV Using Strict Unicast Reverse Path Forwarding

   Note: In the figures (scenarios) in this section and the subsequent
   sections, the following terminology is used:

   *  "fails" means drops packets with legitimate source addresses.

   *  "works (but not desirable)" means passes all packets with
      legitimate source addresses but is oblivious to directionality.

   *  "works best" means passes all packets with legitimate source
      addresses with no (or minimal) compromise of directionality.

   *  The notation Pi[ASn ASm ...] denotes a BGP update with prefix Pi
      and an AS_PATH as shown in the square brackets.

   In the strict uRPF method, an ingress packet at a border router is
   accepted only if the Forwarding Information Base (FIB) contains a
   prefix that encompasses the source address and forwarding information
   for that prefix points back to the interface over which the packet
   was received.  In other words, the reverse path for routing to the
   source address (if it were used as a destination address) should use
   the same interface over which the packet was received.  It is well
   known that this method has limitations when networks are multihomed,
   routes are not symmetrically announced to all transit providers, and
   there is asymmetric routing of data packets.  Asymmetric routing
   occurs (see Figure 1) when a customer AS announces one prefix (P1) to
   one transit provider (ISP-a) and a different prefix (P2) to another
   transit provider (ISP-b) but routes data packets with source
   addresses in the second prefix (P2) to the first transit provider
   (ISP-a) or vice versa.  Then, data packets with a source address in
   prefix P2 that are received at AS2 directly from AS1 will get
   dropped.  Further, data packets with a source address in prefix P1
   that originate from AS1 and traverse via AS3 to AS2 will also get
   dropped at AS2.

              +------------+ ---- P1[AS2 AS1] ---> +------------+
              | AS2(ISP-a) | <----P2[AS3 AS1] ---- | AS3(ISP-b) |
              +------------+                       +------------+
                       /\                             /\
                        \                             /
                         \                           /
                          \                         /
                    P1[AS1]\                       /P2[AS1]
                            \                     /
                           +-----------------------+
                           |      AS1(customer)    |
                           +-----------------------+
                             P1, P2 (prefixes originated)

             Consider data packets received at AS2
             (1) from AS1 with a source address (SA) in P2, or
             (2) from AS3 that originated from AS1 with a SA in P1:
                       * Strict uRPF fails
                       * Feasible-path uRPF fails
                       * Loose uRPF works (but not desirable)
                       * Enhanced feasible-path uRPF works best

     Figure 1: Scenario 1 for Illustration of Efficacy of uRPF Schemes

2.3.  SAV Using Feasible-Path Unicast Reverse Path Forwarding

   The feasible-path uRPF technique helps partially overcome the problem
   identified with the strict uRPF in the multihoming case.  The
   feasible-path uRPF is similar to the strict uRPF, but in addition to
   inserting the best-path prefix, additional prefixes from alternative
   announced routes are also included in the RPF list.  This method
   relies on either (a) announcements for the same prefixes (albeit some
   may be prepended to effect lower preference) propagating to all
   transit providers performing feasible-path uRPF checks or (b)
   announcement of an aggregate less-specific prefix to all transit
   providers while announcing more-specific prefixes (covered by the
   less-specific prefix) to different transit providers as needed for
   traffic engineering.

   As an example, in the multihoming scenario (see Scenario 2 in
   Figure 2), if the customer AS announces routes for both prefixes (P1,
   P2) to both transit providers (with suitable prepends if needed for
   traffic engineering), then the feasible-path uRPF method works.  It
   should be mentioned that the feasible-path uRPF works in this
   scenario only if customer routes are preferred at AS2 and AS3 over a
   shorter non-customer route.  However, the feasible-path uRPF method
   has limitations as well.  One form of limitation naturally occurs
   when the recommendation (a) or (b) mentioned above regarding
   propagation of prefixes is not followed.

   Another form of limitation can be described as follows.  In Scenario
   2 (described here, illustrated in Figure 2), it is possible that the
   second transit provider (ISP-b or AS3) does not propagate the
   prepended route for prefix P1 to the first transit provider (ISP-a or
   AS2).  This is because AS3's decision policy permits giving priority
   to a shorter route to prefix P1 via a lateral peer (AS2) over a
   longer route learned directly from the customer (AS1).  In such a
   scenario, AS3 would not send any route announcement for prefix P1 to
   AS2 (over the P2P link).  Then, a data packet with a source address
   in prefix P1 that originates from AS1 and traverses via AS3 to AS2
   will get dropped at AS2.


             +------------+  routes for P1, P2   +------------+
             | AS2(ISP-a) |<-------------------->| AS3(ISP-b) |
             +------------+        (P2P)         +------------+
                       /\                            /\
                        \                            /
                  P1[AS1]\                          /P2[AS1]
                          \                        /
            P2[AS1 AS1 AS1]\                      /P1[AS1 AS1 AS1]
                            \                    /
                           +-----------------------+
                           |      AS1(customer)    |
                           +-----------------------+
                             P1, P2 (prefixes originated)

           Consider data packets received at AS2 via AS3
           that originated from AS1 and have a source address in P1:
           * Feasible-path uRPF works (if the customer route to P1
             is preferred at AS3 over the shorter path)
           * Feasible-path uRPF fails (if the shorter path to P1
             is preferred at AS3 over the customer route)
           * Loose uRPF works (but not desirable)
           * Enhanced feasible-path uRPF works best

     Figure 2: Scenario 2 for Illustration of Efficacy of uRPF Schemes

2.4.  SAV Using Loose Unicast Reverse Path Forwarding

   In the loose uRPF method, an ingress packet at the border router is
   accepted only if the FIB has one or more prefixes that encompass the
   source address.  That is, a packet is dropped if no route exists in
   the FIB for the source address.  Loose uRPF sacrifices
   directionality.  It only drops packets if the source address is
   unreachable in the current FIB (e.g., IANA special-purpose prefixes
   [SPAR-v4][SPAR-v6], unallocated, allocated but currently not routed).

2.5.  SAV Using VRF Table

   The Virtual Routing and Forwarding (VRF) technology [RFC4364]
   [Juniper] allows a router to maintain multiple routing table
   instances separate from the global Routing Information Base (RIB).
   External BGP (eBGP) peering sessions send specific routes to be
   stored in a dedicated VRF table.  The uRPF process queries the VRF
   table (instead of the FIB) for source address validation.  A VRF
   table can be dedicated per eBGP peer and used for uRPF for only that
   peer, resulting in strict mode operation.  For implementing loose
   uRPF on an interface, the corresponding VRF table would be global,
   i.e., contains the same routes as in the FIB.

3.  SAV Using Enhanced Feasible-Path uRPF

3.1.  Description of the Method

   The enhanced feasible-path uRPF (EFP-uRPF) method adds greater
   operational robustness and efficacy to existing uRPF methods
   discussed in Section 2.  That is because it avoids dropping
   legitimate data packets and compromising directionality.  The method
   is based on the principle that if BGP updates for multiple prefixes
   with the same origin AS were received on different interfaces (at
   border routers), then incoming data packets with source addresses in
   any of those prefixes should be accepted on any of those interfaces.
   The EFP-uRPF method can be best explained with an example, as
   follows:

   Let us say, in its Adj-RIBs-In [RFC4271], a border router of ISP-A
   has the set of prefixes {Q1, Q2, Q3}, each of which has AS-x as its
   origin and AS-x is in ISP-A's customer cone.  In this set, the border
   router received the route for prefix Q1 over a customer-facing
   interface while it learned the routes for prefixes Q2 and Q3 from a
   lateral peer and an upstream transit provider, respectively.  In this
   example scenario, the enhanced feasible-path uRPF method requires Q1,
   Q2, and Q3 be included in the RPF list for the customer interface
   under consideration.

   Thus, the EFP-uRPF method gathers feasible paths for customer
   interfaces in a more precise way (as compared to the feasible-path
   uRPF) so that all legitimate packets are accepted while the
   directionality property is not compromised.

   The above-described EFP-uRPF method is recommended to be applied on
   customer interfaces.  It can also be extended to create the RPF lists
   for lateral peer interfaces.  That is, the EFP-uRPF method can be
   applied (and loose uRPF avoided) on lateral peer interfaces.  That
   will help to avoid compromising directionality for lateral peer
   interfaces (which is inevitable with loose uRPF; see Section 2.4).

   Looking back at Scenarios 1 and 2 (Figures 1 and 2), the EFP-uRPF
   method works better than the other uRPF methods.  Scenario 3
   (Figure 3) further illustrates the enhanced feasible-path uRPF method
   with a more concrete example.  In this scenario, the focus is on
   operation of the EFP-uRPF at ISP4 (AS4).  ISP4 learns a route for
   prefix P1 via a C2P interface from customer ISP2 (AS2).  This route
   for P1 has origin AS1.  ISP4 also learns a route for P2 via another
   C2P interface from customer ISP3 (AS3).  Additionally, AS4 learns a
   route for P3 via a lateral P2P interface from ISP5 (AS5).  Routes for
   all three prefixes have the same origin AS (i.e., AS1).  Using the
   enhanced feasible-path uRPF scheme and given the commonality of the
   origin AS across the routes for P1, P2, and P3, AS4 includes all of
   these prefixes in the RPF list for the customer interfaces (from AS2
   and AS3).

                    +----------+   P3[AS5 AS1]  +------------+
                    | AS4(ISP4)|<---------------|  AS5(ISP5) |
                    +----------+      (P2P)     +------------+
                        /\   /\                        /\
                        /     \                        /
            P1[AS2 AS1]/       \P2[AS3 AS1]           /
                 (C2P)/         \(C2P)               /
                     /           \                  /
              +----------+    +----------+         /
              | AS2(ISP2)|    | AS3(ISP3)|        /
              +----------+    +----------+       /
                       /\           /\          /
                        \           /          /
                  P1[AS1]\         /P2[AS1]   /P3[AS1]
                     (C2P)\       /(C2P)     /(C2P)
                           \     /          /
                        +----------------+ /
                        |  AS1(customer) |/
                        +----------------+
                             P1, P2, P3 (prefixes originated)

            Consider that data packets (sourced from AS1)
            may be received at AS4 with a source address
            in P1, P2, or P3 via any of the neighbors (AS2, AS3, AS5):
            * Feasible-path uRPF fails
            * Loose uRPF works (but not desirable)
            * Enhanced feasible-path uRPF works best

     Figure 3: Scenario 3 for Illustration of Efficacy of uRPF Schemes

3.1.1.  Algorithm A: Enhanced Feasible-Path uRPF

   The underlying algorithm in the solution method described above
   (Section 3.1) can be specified as follows (to be implemented in a
   transit AS):

   1.  Create the set of unique origin ASes considering only the routes
       in the Adj-RIBs-In of customer interfaces.  Call it Set A = {AS1,
       AS2, ..., ASn}.

   2.  Considering all routes in Adj-RIBs-In for all interfaces
       (customer, lateral peer, and transit provider), form the set of
       unique prefixes that have a common origin AS1.  Call it Set X1.

   3.  Include Set X1 in the RPF list on all customer interfaces on
       which one or more of the prefixes in Set X1 were received.

   4.  Repeat Steps 2 and 3 for each of the remaining ASes in Set A
       (i.e., for ASi, where i = 2, ..., n).

   The above algorithm can also be extended to apply the EFP-uRPF method
   to lateral peer interfaces.  However, it is left up to the operator
   to decide whether they should apply the EFP-uRPF or loose uRPF method
   on lateral peer interfaces.  The loose uRPF method is recommended to
   be applied on transit provider interfaces.

3.2.  Operational Recommendations

   The following operational recommendations will make the operation of
   the enhanced feasible-path uRPF robust:

   For multihomed stub AS:

   *  A multihomed stub AS should announce at least one of the prefixes
      it originates to each of its transit provider ASes.  (It is
      understood that a single-homed stub AS would announce all prefixes
      it originates to its sole transit provider AS.)

   For non-stub AS:

   *  A non-stub AS should also announce at least one of the prefixes it
      originates to each of its transit provider ASes.

   *  Additionally, from the routes it has learned from customers, a
      non-stub AS SHOULD announce at least one route per origin AS to
      each of its transit provider ASes.

3.3.  A Challenging Scenario

   It should be observed that in the absence of ASes adhering to above
   recommendations, the following example scenario, which poses a
   challenge for the enhanced feasible-path uRPF (as well as for
   traditional feasible-path uRPF), may be constructed.  In the scenario
   illustrated in Figure 4, since routes for neither P1 nor P2 are
   propagated on the AS2-AS4 interface (due to the presence of NO_EXPORT
   Community), the enhanced feasible-path uRPF at AS4 will reject data
   packets received on that interface with source addresses in P1 or P2.
   (For a little more complex example scenario, see slide #10 in
   [Sriram-URPF].)

                    +----------+
                    | AS4(ISP4)|
                    +----------+
                        /\   /\
                        /     \  P1[AS3 AS1]
         P1 and P2 not /       \ P2[AS3 AS1]
           propagated /         \ (C2P)
             (C2P)   /           \
              +----------+    +----------+
              | AS2(ISP2)|    | AS3(ISP3)|
              +----------+    +----------+
                       /\           /\
                        \           / P1[AS1]
       P1[AS1] NO_EXPORT \         / P2[AS1]
       P2[AS1] NO_EXPORT  \       / (C2P)
                    (C2P)  \     /
                        +----------------+
                        |  AS1(customer) |
                        +----------------+
                             P1, P2 (prefixes originated)

          Consider that data packets (sourced from AS1)
          may be received at AS4 with a source address
          in P1 or P2 via AS2:
          * Feasible-path uRPF fails
          * Loose uRPF works (but not desirable)
          * Enhanced feasible-path uRPF with Algorithm A fails
          * Enhanced feasible-path uRPF with Algorithm B works best

              Figure 4: Illustration of a Challenging Scenario

3.4.  Algorithm B: Enhanced Feasible-Path uRPF with Additional
      Flexibility across Customer Cone

   Adding further flexibility to the enhanced feasible-path uRPF method
   can help address the potential limitation identified above using the
   scenario in Figure 4 (Section 3.3).  In the following, "route" refers
   to a route currently existing in the Adj-RIBs-In.  Including the
   additional degree of flexibility, the modified algorithm called
   Algorithm B (implemented in a transit AS) can be described as
   follows:

   1.  Create the set of all directly connected customer interfaces.
       Call it Set I = {I1, I2, ..., Ik}.

   2.  Create the set of all unique prefixes for which routes exist in
       Adj-RIBs-In for the interfaces in Set I.  Call it Set P = {P1,
       P2, ..., Pm}.

   3.  Create the set of all unique origin ASes seen in the routes that
       exist in Adj-RIBs-In for the interfaces in Set I.  Call it Set A
       = {AS1, AS2, ..., ASn}.

   4.  Create the set of all unique prefixes for which routes exist in
       Adj-RIBs-In of all lateral peer and transit provider interfaces
       such that each of the routes has its origin AS belonging in Set
       A.  Call it Set Q = {Q1, Q2, ..., Qj}.

   5.  Then, Set Z = Union(P,Q) is the RPF list that is applied for
       every customer interface in Set I.

   When Algorithm B (which is more flexible than Algorithm A) is
   employed on customer interfaces, the type of limitation identified in
   Figure 4 (Section 3.3) is overcome and the method works.  The
   directionality property is minimally compromised, but the proposed
   EFP-uRPF method with Algorithm B is still a much better choice (for
   the scenario under consideration) than applying the loose uRPF
   method, which is oblivious to directionality.

   So, applying the EFP-uRPF method with Algorithm B is recommended on
   customer interfaces for the challenging scenarios, such as those
   described in Section 3.3.

3.5.  Augmenting RPF Lists with ROA and IRR Data

   It is worth emphasizing that an indirect part of the proposal in this
   document is that RPF filters may be augmented from secondary sources.
   Hence, the construction of RPF lists using a method proposed in this
   document (Algorithm A or B) can be augmented with data from Route
   Origin Authorization (ROA) [RFC6482], as well as Internet Routing
   Registry (IRR) data.  Special care should be exercised when using IRR
   data because it is not always accurate or trusted.  In the EFP-uRPF
   method with Algorithm A (see Section 3.1.1), if a ROA includes prefix
   Pi and ASj, then augment the RPF list of each customer interface on
   which at least one route with origin ASj was received with prefix Pi.
   In the EFP-uRPF method with Algorithm B, if ASj belongs in Set A (see
   Step #3 Section 3.4) and if a ROA includes prefix Pi and ASj, then
   augment the RPF list Z in Step 5 of Algorithm B with prefix Pi.
   Similar procedures can be followed with reliable IRR data as well.
   This will help make the RPF lists more robust about source addresses
   that may be legitimately used by customers of the ISP.

3.6.  Implementation and Operations Considerations

3.6.1.  Impact on FIB Memory Size Requirement

   The existing RPF checks in edge routers take advantage of existing
   line card implementations to perform the RPF functions.  For
   implementation of the enhanced feasible-path uRPF, the general
   necessary feature would be to extend the line cards to take arbitrary
   RPF lists that are not necessarily the same as the existing FIB
   contents.  In the algorithms (Sections 3.1.1 and 3.4) described here,
   the RPF lists are constructed by applying a set of rules to all
   received BGP routes (not just those selected as best path and
   installed in the FIB).  The concept of uRPF querying an RPF list
   (instead of the FIB) is similar to uRPF querying a VRF table (see
   Section 2.5).

   The techniques described in this document require that there should
   be additional memory (i.e., ternary content-addressable memory
   (TCAM)) available to store the RPF lists in line cards.  For an ISP's
   AS, the RPF list size for each line card will roughly equal the total
   number of originated prefixes from ASes in its customer cone
   (assuming Algorithm B in Section 3.4 is used).  (Note: EFP-uRPF with
   Algorithm A (see Section 3.1.1) requires much less memory than EFP-
   uRPF with Algorithm B.)

   The following table shows the measured customer cone sizes in number
   of prefixes originated (from all ASes in the customer cone) for
   various types of ISPs [Sriram-RIPE63]:

          +------------+---------------------------------------+
          | Type of    | Measured Customer Cone Size in #      |
          | ISP        | Prefixes (in turn this is an estimate |
          |            | for RPF list size on the line card)   |
          +============+=======================================+
          | Very Large | 32393                                 |
          | Global ISP |                                       |
          | #1         |                                       |
          +------------+---------------------------------------+
          | Very Large | 29528                                 |
          | Global ISP |                                       |
          | #2         |                                       |
          +------------+---------------------------------------+
          | Large      | 20038                                 |
          | Global ISP |                                       |
          +------------+---------------------------------------+
          | Mid-size   | 8661                                  |
          | Global ISP |                                       |
          +------------+---------------------------------------+
          | Regional   | 1101                                  |
          | ISP (in    |                                       |
          | Asia)      |                                       |
          +------------+---------------------------------------+

              Table 1: Customer Cone Sizes (# Prefixes) for
                          Various Types of ISPs

   For some super large global ISPs that are at the core of the
   Internet, the customer cone size (# prefixes) can be as high as a few
   hundred thousand [CAIDA], but uRPF is most effective when deployed at
   ASes at the edges of the Internet where the customer cone sizes are
   smaller, as shown in Table 1.

   A very large global ISP's router line card is likely to have a FIB
   size large enough to accommodate 2 million routes [Cisco1].
   Similarly, the line cards in routers corresponding to a large global
   ISP, a midsize global ISP, and a regional ISP are likely to have FIB
   sizes large enough to accommodate about 1 million, 0.5 million, and
   100k routes, respectively [Cisco2].  Comparing these FIB size numbers
   with the corresponding RPF list size numbers in Table 1, it can be
   surmised that the conservatively estimated RPF list size is only a
   small fraction of the anticipated FIB memory size under relevant ISP
   scenarios.  What is meant here by relevant ISP scenarios is that only
   smaller ISPs (and possibly some midsize and regional ISPs) are
   expected to implement the proposed EFP-uRPF method since it is most
   effective closer to the edges of the Internet.

3.6.2.  Coping with BGP's Transient Behavior

   BGP routing announcements can exhibit transient behavior.  Routes may
   be withdrawn temporarily and then reannounced due to transient
   conditions, such as BGP session reset or link failure recovery.  To
   cope with this, hysteresis should be introduced in the maintenance of
   the RPF lists.  Deleting entries from the RPF lists SHOULD be delayed
   by a predetermined amount (the value based on operational experience)
   when responding to route withdrawals.  This should help suppress the
   effects due to the transients in BGP.

3.7.  Summary of Recommendations

   Depending on the scenario, an ISP or enterprise AS operator should
   follow one of the following recommendations concerning uRPF/SAV:

   1.  For directly connected networks, i.e., subnets directly connected
       to the AS, the AS under consideration SHOULD perform ACL-based
       SAV.

   2.  For a directly connected single-homed stub AS (customer), the AS
       under consideration SHOULD perform SAV based on the strict uRPF
       method.

   3.  For all other scenarios:

       *  The EFP-uRPF method with Algorithm B (see Section 3.4) SHOULD
          be applied on customer interfaces.

       *  The loose uRPF method SHOULD be applied on lateral peer and
          transit provider interfaces.

   It is also recommended that prefixes from registered ROAs and IRR
   route objects that include ASes in an ISP's customer cone SHOULD be
   used to augment the pertaining RPF lists (see Section 3.5 for
   details).

3.7.1.  Applicability of the EFP-uRPF Method with Algorithm A

   The EFP-uRPF method with Algorithm A is not mentioned in the above
   set of recommendations.  It is an alternative to EFP-uRPF with
   Algorithm B and can be used in limited circumstances.  The EFP-uRPF
   method with Algorithm A is expected to work fine if an ISP deploying
   it has only multihomed stub customers.  It is trivially equivalent to
   strict uRPF if an ISP deploys it for a single-homed stub customer.
   More generally, it is also expected to work fine when there is
   absence of limitations, such as those described in Section 3.3.
   However, caution is required for use of EFP-uRPF with Algorithm A
   because even if the limitations are not expected at the time of
   deployment, the vulnerability to change in conditions exists.  It may
   be difficult for an ISP to know or track the extent of use of
   NO_EXPORT (see Section 3.3) on routes within its customer cone.  If
   an ISP decides to use EFP-uRPF with Algorithm A, it should make its
   direct customers aware of the operational recommendations in
   Section 3.2.  This means that the ISP notifies direct customers that
   at least one prefix originated by each AS in the direct customer's
   customer cone must propagate to the ISP.

   On a lateral peer interface, an ISP may choose to apply the EFP-uRPF
   method with Algorithm A (with appropriate modification of the
   algorithm).  This is because stricter forms of uRPF (than the loose
   uRPF) may be considered applicable by some ISPs on interfaces with
   lateral peers.

4.  Security Considerations

   The security considerations in BCP 38 [RFC2827] and RFC 3704
   [RFC3704] apply for this document as well.  In addition, if
   considering using the EFP-uRPF method with Algorithm A, an ISP or AS
   operator should be aware of the applicability considerations and
   potential vulnerabilities discussed in Section 3.7.1.

   In augmenting RPF lists with ROA (and possibly reliable IRR)
   information (see Section 3.5), a trade-off is made in favor of
   reducing false positives (regarding invalid detection in SAV) at the
   expense of another slight risk.  The other risk being that a
   malicious actor at another AS in the neighborhood within the customer
   cone might take advantage (of the augmented prefix) to some extent.
   This risk also exists even with normal announced prefixes (i.e.,
   without ROA augmentation) for any uRPF method other than the strict
   uRPF.  However, the risk is mitigated if the transit provider of the
   other AS in question is performing SAV.

   Though not within the scope of this document, security hardening of
   routers and other supporting systems (e.g., Resource PKI (RPKI) and
   ROA management systems) against compromise is extremely important.
   The compromise of those systems can affect the operation and
   performance of the SAV methods described in this document.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <https://www.rfc-editor.org/info/rfc3704>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

6.2.  Informative References

   [CAIDA]    CAIDA, "Information for AS 174 (COGENT-174)", October
              2019, <https://spoofer.caida.org/as.php?asn=174>.

   [Cisco1]   Cisco, "Internet Routing Table Growth Causes %ROUTING-FIB-
              4-RSRC_LOW Message on Trident-Based Line Cards", January
              2014, <https://www.cisco.com/c/en/us/support/docs/routers/
              asr-9000-series-aggregation-services-routers/116999-
              problem-line-card-00.html>.

   [Cisco2]   Cisco, "Cisco Nexus 7000 Series NX-OS Unicast Routing
              Configuration Guide, Release 5.x (Chapter 15: 'Managing
              the Unicast RIB and FIB')", March 2018,
              <https://www.cisco.com/c/en/us/td/docs/switches/
              datacenter/sw/5_x/nx-
              os/unicast/configuration/guide/l3_cli_nxos/
              l3_NewChange.html>.

   [Firmin]   Firmin, F., "The Evolved Packet Core",
              <https://www.3gpp.org/technologies/keywords-acronyms/100-
              the-evolved-packet-core>.

   [ISOC]     Internet Society, "Addressing the challenge of IP
              spoofing", September 2015,
              <https://www.internetsociety.org/resources/doc/2015/
              addressing-the-challenge-of-ip-spoofing/>.

   [Juniper]  Juniper Networks, "Creating Unique VPN Routes Using VRF
              Tables", May 2019,
              <https://www.juniper.net/documentation/en_US/junos/topics/
              topic-map/l3-vpns-routes-vrf-tables.html#id-understanding-
              virtual-routing-and-forwarding-tables>.

   [Luckie]   Luckie, M., Huffaker, B., Dhamdhere, A., Giotsas, V., and
              kc. claffy, "AS Relationships, customer cones, and
              validation", In Proceedings of the 2013 Internet
              Measurement Conference, DOI 10.1145/2504730.2504735,
              October 2013,
              <https://dl.acm.org/doi/10.1145/2504730.2504735>.

   [RFC4036]  Sawyer, W., "Management Information Base for Data Over
              Cable Service Interface Specification (DOCSIS) Cable Modem
              Termination Systems for Subscriber Management", RFC 4036,
              DOI 10.17487/RFC4036, April 2005,
              <https://www.rfc-editor.org/info/rfc4036>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
              Origin Authorizations (ROAs)", RFC 6482,
              DOI 10.17487/RFC6482, February 2012,
              <https://www.rfc-editor.org/info/rfc6482>.

   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811,
              DOI 10.17487/RFC6811, January 2013,
              <https://www.rfc-editor.org/info/rfc6811>.

   [RFC7454]  Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
              and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
              February 2015, <https://www.rfc-editor.org/info/rfc7454>.

   [SPAR-v4]  IANA, "IANA IPv4 Special-Purpose Address Registry",
              <https://www.iana.org/assignments/iana-ipv4-special-
              registry/>.

   [SPAR-v6]  IANA, "IANA IPv6 Special-Purpose Address Registry",
              <https://www.iana.org/assignments/iana-ipv6-special-
              registry/>.

   [Sriram-RIPE63]
              Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on
              a Router", Presented at RIPE 63 and at the SIDR WG meeting
              at IETF 83, March 2012,
              <http://www.ietf.org/proceedings/83/slides/slides-83-sidr-
              7.pdf>.

   [Sriram-URPF]
              Sriram, K., Montgomery, D., and J. Haas, "Enhanced
              Feasible-Path Unicast Reverse Path Filtering", Presented
              at the OPSEC WG meeting at IETF 101, March 2018,
              <https://datatracker.ietf.org/meeting/101/materials/
              slides-101-opsec-draft-sriram-opsec-urpf-improvements-00>.

Acknowledgements

   The authors would like to thank Sandy Murphy, Alvaro Retana, Job
   Snijders, Marco Marzetti, Marco d'Itri, Nick Hilliard, Gert Doering,
   Fred Baker, Igor Gashinsky, Igor Lubashev, Andrei Robachevsky, Barry
   Greene, Amir Herzberg, Ruediger Volk, Jared Mauch, Oliver Borchert,
   Mehmet Adalier, and Joel Jaeggli for comments and suggestions.  The
   comments and suggestions received from the IESG reviewers are also
   much appreciated.

Authors' Addresses

   Kotikalapudi Sriram
   USA National Institute of Standards and Technology
   100 Bureau Drive
   Gaithersburg, MD 20899
   United States of America

   Email: ksriram@nist.gov


   Doug Montgomery
   USA National Institute of Standards and Technology
   100 Bureau Drive
   Gaithersburg, MD 20899
   United States of America

   Email: dougm@nist.gov


   Jeffrey Haas
   Juniper Networks, Inc.
   1133 Innovation Way
   Sunnyvale, CA 94089
   United States of America

   Email: jhaas@juniper.net


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