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OPSEC Working Group                                            K. Sriram
Internet-Draft                                             D. Montgomery
Intended status: Best Current Practice                          USA NIST
Expires: October 22, 2018                                        J. Haas
                                                  Juniper Networks, Inc.
                                                          April 20, 2018


         Enhanced Feasible-Path Unicast Reverse Path Filtering
                 draft-ietf-opsec-urpf-improvements-00

Abstract

   This document identifies a need for improvement of the unicast
   Reverse Path Filtering techniques (uRPF) [BCP84] for source address
   validation (SAV) [BCP38].  The 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 [BCP84].  However, as shown in this draft, the existing
   feasible-path uRPF still has short comings.  This document describes
   an enhanced feasible-path uRPF technique, which aims to be more
   flexible (in a meaningful way) about directionality than the
   feasible-path uRPF.  It can potentially alleviate ISPs' concerns
   about the possibility of disrupting service for their customers, and
   encourage greater deployment of uRPF techniques.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 22, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Review of Existing Source Address Validation Techniques . . .   3
     2.1.  SAV using Access Control List . . . . . . . . . . . . . .   4
     2.2.  SAV using Strict Unicast Reverse Path Filtering . . . . .   4
     2.3.  SAV using Feasible-Path Unicast Reverse Path Filtering  .   5
     2.4.  SAV using Loose Unicast Reverse Path Filtering  . . . . .   6
   3.  SAV using Enhanced Feasible-Path uRPF . . . . . . . . . . . .   7
     3.1.  Description of the Method . . . . . . . . . . . . . . . .   7
       3.1.1.  Algorithm A: Enhanced Feasible-Path uRPF  . . . . . .   8
     3.2.  Operational Recommendations . . . . . . . . . . . . . . .   9
     3.3.  A Challenging Scenario  . . . . . . . . . . . . . . . . .   9
     3.4.  Algorithm B: Enhanced Feasible-Path uRPF with Additional
           Flexibility Across Customer Cone  . . . . . . . . . . . .  10
     3.5.  Implementation Considerations . . . . . . . . . . . . . .  11
       3.5.1.  Impact on FIB Memory Size Requirement . . . . . . . .  11
     3.6.  Summary of Recommendations  . . . . . . . . . . . . . . .  12
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This internet draft identifies a need for improvement of the unicast
   Reverse Path Filtering (uRPF) techniques [RFC2827] for source address
   validation (SAV) [RFC3704].  The 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 [RFC3704].  However, as shown in this draft, the existing
   feasible-path uRPF still has short comings.  Even with the feasible-
   path uRPF, ISPs are often apprehensive that they may be dropping
   customers' data packets with legitimate source addresses.





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   This document describes an enhanced feasible-path uRPF technique,
   which aims 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 considerations are discussed in Section 3.5.

   Note: Definition of Reverse Path Filtering (RPF) list: The list of
   permissible source address prefixes for incoming data packets on a
   given interface.

   Note: Throughout this document, the routes in consideration are
   assumed to have been vetted based on prefix filtering [RFC7454] and
   possibly (in the future) origin validation [RFC6811].

   The enhanced feasible-path uRPF methods described here 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 DDoS prevention benefits
   network wide.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Review of Existing Source Address Validation Techniques

   There are various existing techniques for mitigation against DDoS
   attacks with spoofed addresses [RFC2827] [RFC3704].  There are also
   some techniques used for mitigating reflection attacks [RRL]
   [TA14-017A], which are used to amplify the impact in DDoS attacks.
   Employing a combination of these preventive techniques (as
   applicable) in enterprise and ISP border routers, broadband and
   wireless access network, data centers, and DNS servers provides
   reasonably effective protection against DDoS attacks.

   Source address validation (SAV) is performed in network edge devices
   such as border routers, Cable Modem Termination Systems (CMTS),
   Digital Subscriber Line Access Multiplexers (DSLAM), and Packet Data
   Network (PDN) gateways in mobile networks.  Ingress Access Control



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   List (ACL) and unicast Reverse Path Filtering (uRPF) are techniques
   employed for implementing SAV [RFC2827] [RFC3704] [ISOC].

2.1.  SAV using Access Control List

   Ingress/egress Access Control Lists (ACLs) are maintained which list
   acceptable (or alternatively, unacceptable) prefixes for the source
   addresses in the incoming Internet Protocol (IP) packets.  Any packet
   with a source address that does not match the filter 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, and hence operationally difficult or infeasible.

   Typically, the egress ACLs in access aggregation devices (e.g.  CMTS,
   DSLAM) 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 (i.e. belongs to obviously disallowed prefix blocks, RFC
   1918 prefixes, or provider's own prefixes).

2.2.  SAV using Strict Unicast Reverse Path Filtering

   In the strict unicast Reverse Path Filtering (uRPF) method, an
   ingress packet at 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 multi-homed and there is asymmetric
   routing of 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.














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              +------------+ ---- 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 source address in P2, or
             (2) from AS3 that originated from AS1
                 with source address in P1:
                       * Strict uRPF fails
                       * Feasible-path uRPF fails
                       * Loose uRPF works (but ineffective in IPv4)
                       * 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 Filtering

   The feasible-path uRPF helps partially overcome the problem
   identified with the strict uRPF in the multi-homing 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 table.  This method
   relies on announcements for the same prefixes (albeit some may be
   prepended to effect lower preference) propagating to all routers
   performing feasible-path uRPF checks.  Therefore, in the multi-homing
   scenario, 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 (see
   Figure 2).  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.










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             +------------+  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 source address in P1:
           * Feasible-path uRPF works (if customer route to P1
             is preferred at AS3 over shorter path)
           * Feasible-path uRPF fails (if shorter path to P1
             is preferred at AS3 over customer route)
           * Loose uRPF works (but ineffective in IPv4)
           * Enhanced Feasible-path uRPF works best

    Figure 2: Scenario 2 for illustration of efficacy of uRPF schemes.

   However, the feasible-path uRPF method has limitations as well.  One
   form of limitation naturally occurs when the recommendation of
   propagating the same prefixes to all routers is not followed.
   Another form of limitation can be described as follows.  In Scenario
   2 (described above, 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 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.  Then a
   data packet with source address in prefix P1 that originates from AS1
   and traverses via AS3 to AS2 will get dropped at AS2.

2.4.  SAV using Loose Unicast Reverse Path Filtering

   In the loose unicast Reverse Path Filtering (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.  This method is not effective
   for prevention of address spoofing since there is little unrouted
   address space in IPv4.  It only drops packets if the spoofed address



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   is unreachable in the current FIB (e.g.  RFC 1918, unallocated,
   allocated but currently not routed).

3.  SAV using Enhanced Feasible-Path uRPF

3.1.  Description of the Method

   Enhanced feasible-path uRPF adds greater operational robustness and
   efficacy to existing uRPF methods discussed in Section 2.  The
   technique 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.  It can be best explained with an example as
   follows:

   Let us say, a border router of ISP-A has in its Adj-RIB-Ins
   [RFC4271].  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.  Further, the
   border router received a route for prefix Q1 over a customer facing
   interface, while it learned 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 in
   consideration.  Loose uRPF (see Section 2.4) is recommended to be
   applied to the peer and provider interfaces in consideration.

   Thus, enhanced feasible-path uRPF defines feasible paths for customer
   interfaces in a more generalized but precise way (as compared to
   feasible-path uRPF).

   Looking back at Scenarios 1 and 2 (Figure 1 and Figure 2), the
   enhanced feasible-path uRPF provides comparable or better performance
   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
   feasible-path uRPF at ISP4 (AS4).  ISP4 learns a route for prefix P1
   via a customer-to-provider (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 peer-to-peer (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, given the
   commonality of the origin AS across the routes for P1, P2 and P3, AS4
   includes all of these prefixes to the RPF list for the customer
   interfaces (from AS2 and AS3).





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                    +----------+   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 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 can
   be specified as follows (to be implemented in a transit AS):

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

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

   3.  Include set X1 in Reverse Path Filter (RPF) list on all customer
       interfaces on which one or more of the prefixes in set X1 were
       received.





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

3.2.  Operational Recommendations

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

   For multi-homed stub AS:

   o  A multi-homed stub AS SHOULD announce at least one of the prefixes
      it originates to each of its transit provider ASes.

   For non-stub AS:

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

   o  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.

   (Note: It is worth noting that in the above recommendations if "at
   least one" is replaced with "all", then even traditional feasible-
   path uRPF will work as effectively.)

3.3.  A Challenging Scenario

   It should be observed that in the absence of ASes adhering the above
   recommendations, the following example scenario may be constructed
   which poses a challenge for the enhanced feasible-path uRPF (as well
   as for traditional feasible-path uRPF).  In the scenario illustrated
   in Figure 4, since routes for neither P1 nor P2 are propagated on the
   AS2-AS4 interface, the enhanced feasible-path uRPF at AS4 will reject
   data packets received on that interface with source addresses in P1
   or P2.















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                    +----------+
                    | 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)

             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-RIB-in.  Including the
   additional degree of flexibility, the modified algorithm (implemented
   in a transit AS) can be described as follows (we call this Algorithm
   B):

   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-RIB-Ins 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-RIB-Ins 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-RIB-Ins of all lateral peer and provider interfaces such that




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       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) represents the RPF list for every
       customer interface in Set I.

   6.  Apply loose uRPF method for SAV on all peer and provider
       interfaces.

   When Algorithm B (which is more flexible than Algorithm A) is
   employed, the type of limitation identified in Figure 4 (Section 3.3)
   goes away.

3.5.  Implementation Considerations

   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 (Section 3.1.1 and Section 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 FIB).

3.5.1.  Impact on FIB Memory Size Requirement

   The techniques described here require that there should be FIB memory
   (i.e., 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 and
   conservatively equal the total number of prefixes in its customer
   cone (assuming the algorithm in Section 3.4 is used).  (Note: Most
   ISP customer cone scenarios would not require the algorithm in
   Section 3.4, but instead be served best by the algorithm in
   Section 3.1.1, which requires much less FIB memory.)  The following
   table shows the measured customer cone sizes for various types of
   ISPs [sriram-ripe63]:














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   +---------------------------------+---------------------------------+
   | Type of ISP                     | Measured Customer Cone Size in  |
   |                                 | # Prefixes (in turn this is an  |
   |                                 | estimate for RPF list size on   |
   |                                 | line card)                      |
   +---------------------------------+---------------------------------+
   | Very Large Global ISP           | 32392                           |
   | ------------------------------- | ------------------------------- |
   | Very Large Global ISP           | 29528                           |
   | ------------------------------- | ------------------------------- |
   | Large Global ISP                | 20038                           |
   | ------------------------------- | ------------------------------- |
   | Mid-size Global ISP             | 8661                            |
   | ------------------------------- | ------------------------------- |
   | Regional ISP (in Asia)          | 1101                            |
   +---------------------------------+---------------------------------+

   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 to 6 million routes [cisco1].
   Similarly, the line cards in routers corresponding to a large global
   ISP, a mid-size 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.

3.6.  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 and not multi-homed, the AS in consideration SHOULD
       perform ACL-based SAV.

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



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   3.  For all other scenarios:

       *  If the scenario does not involve complexity such as NO_EXPORT
          of routes (see Section 3.3, Figure 4), then the enhanced
          feasible-path uRPF method in Algorithm A (see Section 3.1.1)
          SHOULD be applied.

       *  Else, if the scenario involves the aforementioned complexity,
          then the enhanced feasible-path uRPF method in Algorithm B
          (see Section 3.4) SHOULD be applied.

4.  Security Considerations

   The security considerations in BCP 38 [RFC2827] and BCP 84 [RFC3704]
   apply for this document as well.  In addition, AS operator should
   apply the uRPF method that performs best (i.e., with zero or
   insignificant possibility of dropping legitimate data packets) for
   the type of peer (customer, provider, etc.) and the nature of
   customer cone scenario that apply (see Section 3.1.1 and
   Section 3.4).

5.  IANA Considerations

   This document does not request new capabilities or attributes.  It
   does not create any new IANA registries.

6.  Acknowledgements

   The authors would like to thank Job Snijders, Marco Marzetti, Marco
   d'Itri, Nick Hilliard, Gert Doering, Igor Gashinsky, Barry Greene,
   and Joel Jaeggli for comments and suggestions.

7.  Informative References

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

   [cisco1]   "Internet Routing Table Growth Causes ROUTING-FIB-
              4-RSRC_LOW Message on Trident-Based Line Cards", Cisco
              Trouble-shooting Tech-notes , January 2014,
              <https://www.cisco.com/c/en/us/support/docs/routers/asr-
              9000-series-aggregation-services-routers/116999-problem-
              line-card-00.html>.








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   [cisco2]   "Cisco Nexus 7000 Series NX-OS Unicast Routing
              Configuration Guide, Release 5.x (Chapter: Managing the
              Unicast RIB and FIB)", Cisco Configuration Guides , June
              2018, <https://www.cisco.com/c/en/us/td/docs/switches/data
              center/sw/5_x/nx-
              os/unicast/configuration/guide/l3_cli_nxos/
              l3_manage-routes.html#22859>.

   [ISOC]     Vixie (Ed.), P., "Addressing the challenge of IP
              spoofing", ISOC report , September 2015,
              <https://www.us-cert.gov/ncas/alerts/TA14-017A>.

   [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>.

   [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>.

   [RRL]      "Response Rate Limiting in the Domain Name System",
              Redbarn blog , <http://www.redbarn.org/dns/ratelimits>.









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   [sriram-ripe63]
              Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on
              a Router", Presented at RIPE-63; also at IETF-83 SIDR WG
              Meeting, March 2012,
              <http://www.ietf.org/proceedings/83/slides/
              slides-83-sidr-7.pdf>.

   [TA14-017A]
              "UDP-Based Amplification Attacks", US-CERT alert
              TA14-017A , January 2014,
              <https://www.us-cert.gov/ncas/alerts/TA14-017A>.

Authors' Addresses

   Kotikalapudi Sriram
   USA National Institute of Standards and Technology
   100 Bureau Drive
   Gaithersburg  MD 20899
   USA

   Email: ksriram@nist.gov


   Doug Montgomery
   USA National Institute of Standards and Technology
   100 Bureau Drive
   Gaithersburg  MD 20899
   USA

   Email: dougm@nist.gov


   Jeffrey Haas
   Juniper Networks, Inc.
   1133 Innovation Way
   Sunnyvale  CA 94089
   USA

   Email: jhaas@juniper.net












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