Enhanced Feasible-Path Unicast Reverse Path Forwarding
RFC 8704



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
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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