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Versions: (draft-sriram-opsec-urpf-improvements) 00 01 02 03 04

OPSEC Working Group                                            K. Sriram
Internet-Draft                                             D. Montgomery
Updates: RFC3704 (if approved)                                  USA NIST
Intended status: Best Current Practice                           J. Haas
Expires: January 9, 2020                          Juniper Networks, Inc.
                                                            July 8, 2019


         Enhanced Feasible-Path Unicast Reverse Path Filtering
                 draft-ietf-opsec-urpf-improvements-03

Abstract

   This document identifies a need for improvement of the unicast
   Reverse Path Filtering techniques (uRPF) (see BCP 84) for detection
   and mitigation of source address spoofing (see BCP 38).  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 (see BCP 84).  However, as shown in
   this draft, the existing feasible-path uRPF still has shortcomings.
   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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 January 9, 2020.








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Copyright Notice

   Copyright (c) 2019 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Review of Existing Source Address Validation Techniques . . .   4
     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  . . . . .   7
     2.5.  SAV using VRF Table . . . . . . . . . . . . . . . . . . .   7
   3.  SAV using Enhanced Feasible-Path uRPF . . . . . . . . . . . .   7
     3.1.  Description of the Method . . . . . . . . . . . . . . . .   7
       3.1.1.  Algorithm A: Enhanced Feasible-Path uRPF  . . . . . .   9
     3.2.  Operational Recommendations . . . . . . . . . . . . . . .  10
     3.3.  A Challenging Scenario  . . . . . . . . . . . . . . . . .  10
     3.4.  Algorithm B: Enhanced Feasible-Path uRPF with Additional
           Flexibility Across Customer Cone  . . . . . . . . . . . .  11
     3.5.  Augmenting RPF Lists with ROA and IRR Data  . . . . . . .  12
     3.6.  Implementation and Operations Considerations  . . . . . .  12
       3.6.1.  Impact on FIB Memory Size Requirement . . . . . . . .  12
       3.6.2.  Coping with BGP's Transient Behavior  . . . . . . . .  14
     3.7.  Summary of Recommendations  . . . . . . . . . . . . . . .  14
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18







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1.  Introduction

   Source Address Validation (SAV) refers to the detection and
   mitigation of source address spoofing [RFC2827].  This document
   identifies a need for improvement of the unicast Reverse Path
   Filtering (uRPF) techniques [RFC3704] for SAV.  The 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 draft, 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 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 and operations considerations are discussed in
   Section 3.6.

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

   Throughout this document, the routes under 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].




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2.  Review of Existing Source Address Validation Techniques

   There are various existing techniques for mitigation against DDoS
   attacks with spoofed addresses [RFC2827] [RFC3704].  Source address
   validation (SAV) is performed in network edge devices such as border
   routers, Cable Modem Termination Systems (CMTS) [RFC4036], and Packet
   Data Network (PDN) gateways in mobile networks [Firmin].  Ingress
   Access Control 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/outgoing 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 (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 Filtering

   Note: In the figures (scenarios) that follow, 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.
   Further, 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 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



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   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, 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.

              +------------+ ---- 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 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 Filtering

   The feasible-path uRPF technique 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 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



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   less specific prefix) to different transit providers as needed for
   traffic engineering.  As an example, in the multi-homing scenario
   (see 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 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 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 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 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 not desirable)
           * Enhanced Feasible-path uRPF works best

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




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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.  It only drops packets if the
   spoofed 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 allows a router
   to maintain multiple routing table instances, separate from the
   global Routing Information Base (RIB) [Juniper][RFC4364].  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

   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 avoids 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, 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.  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.



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   Thus, the enhanced feasible-path uRPF (EFP-uRPF) method gathers
   feasible paths for customer interfaces in a more precise way (as
   compared to 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 be extended to design the RPF lists for
   lateral peer interfaces also.  That is, the EFP-uRPF method can be
   applied (and loose uRPF avoided) on lateral peer interfaces.  That
   will help avoid compromise of directionality for lateral peer
   interfaces (which is inevitable with loose uRPF; see Section 2.4).

   Looking back at Scenarios 1 and 2 (Figure 1 and Figure 2), the
   enhanced feasible-path uRPF (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 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 lateral 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 transit 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).

   The above algorithm can be extended to apply EFP-uRPF method to
   lateral peer interfaces also.  However, it is left up to the operator
   to decide whether they should apply 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 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.  (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:

   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.

3.3.  A Challenging Scenario

   It should be observed that in the absence of ASes adhering to 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 (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].)










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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)

          Consider that data packets (sourced from AS1)
          may be received at AS4 with 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-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}.



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   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 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 still the
   proposed EFP-uRPF method with Algorithm B is a much better choice
   (for the scenario under consideration) than applying the loose uRPF
   method which is oblivious to directionality.

   So, applying EFP-uRPF method with Algorithm B is recommended on
   customer interfaces for the challenging scenarios such as those
   described in Section 3.3.  Further, it is recommended that loose uRPF
   method for SAV should be applied on lateral peer and transit provider
   interfaces.

3.5.  Augmenting RPF Lists with ROA and IRR Data

   It is worth emphasizing that an indirect part of the proposal in the
   draft 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.  Prefixes from registered ROAs and IRR route
   objects that include ASes in an ISP's customer cone SHOULD be used to
   augment the appropriate RPF lists.  (Note: The ASes in a customer
   cone are obtained in Step 3 of Algorithm B in Section 3.4.)  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



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   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).  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., 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 Algorithm B in Section 3.4 is used).
   (Note: Most ISP customer cone scenarios would not require Algorithm
   B, but instead be served best by Algorithm A (see Section 3.1.1)
   which requires much less FIB memory.  This is because Algorithm B is
   applicable for the less common scenarios such as Scenario 4 in
   Figure 4 while Algorithm A is applicable for the more common
   scenarios such as Scenario 3 in Figure 3.)

   The following table shows the measured customer cone sizes for
   various types of ISPs [sriram-ripe63]:

   +---------------------------------+---------------------------------+
   | 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.




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   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.  What is meant here by relevant ISP scenarios is that only
   smaller ISPs (and possibly some mid-size 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 re-announced 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 pre-determined 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 and not multi-homed, the AS under consideration SHOULD
       perform ACL-based source address validation (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:

       *  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 on customer interfaces.

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



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       *  In general, loose uRPF method for SAV SHOULD be applied on
          lateral peer and transit provider interfaces.  However, for
          lateral peer interfaces, in some cases an operator MAY apply
          EFP-uRPF method with Algorithm A if they deem it suitable.

   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 appropriate RPF lists.

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, transit 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 Sandy Murphy, 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.

7.  References

7.1.  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>.






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   [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>.

7.2.  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>.

   [Cisco2]   "Cisco Nexus 7000 Series NX-OS Unicast Routing
              Configuration Guide, Release 5.x (Chapter 15: Managing the
              Unicast RIB and FIB)", Cisco Configuration Guides , March
              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_NewChange.html>.

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

   [ISOC]     Vixie (Ed.), P., "Addressing the challenge of IP
              spoofing", ISOC report , September 2015,
              <https://www.internetsociety.org/resources/doc/2015/
              addressing-the-challenge-of-ip-spoofing/>.

   [Juniper]  "Creating Unique VPN Routes Using VRF Tables", Juniper
              Networks TechLibrary , March 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>.








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

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

   [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>.

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






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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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