Enhanced Feasible-Path Unicast Reverse Path Filtering
draft-sriram-opsec-urpf-improvements-01

Opsec Working Group                                            K. Sriram
Internet-Draft                                                      NIST
Intended status: Best Current Practice                     D. Montgomery
Expires: November 4, 2017                                        US NIST
                                                             May 3, 2017

         Enhanced Feasible-Path Unicast Reverse Path Filtering
                draft-sriram-opsec-urpf-improvements-01

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 proposes
   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 expected to alleviate ISPs' concerns about
   the possibility of disrupting service for their customers, and
   encourage greater deployment of uRPF.

Status of This Memo

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   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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 . . . . . . . . . . . . . .   3
     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.  Proposed New Technique: SAV using Enhanced Feasible-Path uRPF   6
     3.1.  Description of the Method . . . . . . . . . . . . . . . .   6
     3.2.  Operational Recommendations . . . . . . . . . . . . . . .   8
     3.3.  Customer Cone Consideration . . . . . . . . . . . . . . .   9
     3.4.  Implementation Consideration  . . . . . . . . . . . . . .   9
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This internet draft identifies a need for improvement of the unicast
   Reverse Path Filtering techniques (uRPF) [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 denying
   customers' data packets with legitimate source addresses.  This
   document proposes 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 an edge router), then data
   packets with source addresses in any of those prefixes are allowed to
   be received on any of those interfaces.  This technique is expected

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   to add greater operational logic and efficacy to uRPF, and alleviate
   ISPs' concerns about the possibility of disrupting service for their
   customers.  It should encourage greater deployment of uRPF to 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 deterrence against DDoS
   attacks with spoofed addresses [RFC2827] [RFC3704].  There are also
   some techniques used for prevention of reflection-amplification
   attacks [RRL] [TA14-017A], which are used in achieving greater impact
   in DDoS attacks.  Employing a combination of these preventive
   techniques in enterprise and ISP border routers, DNS servers,
   broadband and wireless access networks, and data centers provides the
   necessary protections 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
   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.  Hence, this method may be
   operationally difficult or infeasible in dynamic environments such as
   when a customer network is multihomed, has address space allocations
   from multiple ISPs, or dynamically varies its BGP announcements (i.e.
   routing) for traffic engineering purposes.

   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

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   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 on an interface at the border router is accepted only
   if the Forwarding Information Base (FIB) contains a prefix that
   encompasses the source address and packet forwarding for that prefix
   points to said interface.  In other words, the best path for routing
   to that source address (if it were used as a destination address)
   should point to said interface.  It is well known that this method
   has limitations when a network or autonomous system is 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.

              +------------+ ---- 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.

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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 the difference
   is that instead of inserting one best route in the FIB (or an
   equivalent RPF table), alternative routes are also added there.  This
   method relies on announcements for the same prefixes (albeit some may
   be prepended to effect lower preference) propagating to all the
   routers performing feasible-path uRPF check.  So 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 route is preferred at AS2 and AS3
   over the shorter path.

             +------------+  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 preferred
             at AS3 over shorter path)
           * Feasible-path uRPF fails (if shorter path 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.

   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 heeded.  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

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   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.  In most cases, this method is
   not useful for prevention of address spoofing.  It only drops packets
   if the spoofed address is non-routable (e.g.  RFC 1918, unallocated,
   allocated but currently not routed).

3.  Proposed New Technique: SAV using Enhanced Feasible-Path uRPF

3.1.  Description of the Method

   Enhanced feasible-path uRPF adds greater operational logic and
   efficacy to existing uRPF methods discussed in Section 2.  It can be
   best explained with an example.  Let us say, a border router of ISP-A
   has in its Adj-RIB-in the set of prefixes {Q1, Q2, Q3} each of which
   has AS-x as its origin and AS-x belongs 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.  All these prefixes passed route filtering and/or
   origin validation (i.e. the origin AS-x is deemed legitimate).  In
   this example scenario, the enhanced feasible-path uRPF method allows
   source addresses to belong in {Q1, Q2, Q3} on any of the three
   specific interfaces in question (customer, peer, provider) on which
   the three routes were learned.

   Thus, enhanced feasible-path uRPF defines feasible paths in a more
   generalized but precise way (as compared to feasible-path uRPF).  In
   the above example, routes for prefixes Q2 and Q3 were not received on
   a customer facing interface at the border router, yet data packets
   with source addresses in Q2 or Q3 are accepted by the router if they
   come in on the same customer interface on which the route for prefix
   Q1 was received (based on these prefix routes having the same origin
   AS).

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   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 for those scenarios.  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 an alternate route for P2 via a
   peer-to-peer (p2p) interface from ISP5 (AS5).  Both routes for P2
   have the same origin AS (i.e.  AS1) as does the route for P1.
   Applying the principle of enhanced feasible-path uRPF, given the
   commonality of the origin AS across the above-mentioned routes for P1
   and P2, AS4 permits the SA in data packets to belong in P1 or P2 on
   any of the three interfaces (from AS2, AS3, and AS5).

                    +----------+   P2[AS5 AS1]  +------------+
                    | AS4(ISP4)|<---------------|  AS5(ISP5) |
                    +----------+      (p2p)     +------------+
                        /\   /\                        /\
                        /     \                        /
            P1[AS2 AS1]/       \P2[AS3 AS1]           /
                 (C2P)/         \(C2P)               /
                     /           \                  /
              +----------+    +----------+         /
              | AS2(ISP2)|    | AS3(ISP3)|        /
              +----------+    +----------+       /
                       /\           /\          /
                        \           /          /
                  P1[AS1]\         /P2[AS1]   /P2[AS1]
                     (C2P)\       /(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 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.

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   Based on the above, it can be possibly rationalized that the proposed
   enhanced feasible-path uRPF method would help alleviate ISP concerns
   about possible service disruption for their customers and encourage
   greater adoption of uRPF.

3.2.  Operational Recommendations

   The following operational recommendations if followed will make
   robust the desired operation of the enhanced feasible-path uRPF
   proposed here.

   For multi-homed stub AS:

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

   For non-stub AS:

   o  A non-stub AS SHOULD announce at least one of its origination
      prefixes to each transit provider AS.

   o  Additionally, from the routes it has learned from customers, a
      non-stub AS SHOULD announce at least one route for each unique
      {prefix, origin AS} pair to each transit provider AS.

   (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 desired.)

   Also, it should be observed that in the absence of ASes adhering the
   above recommendations, the following type of example scenarios may be
   constructed which pose 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.  But this can be clearly avoided if the
   above recommendations for stub and non-stub ASes are followed.  In
   this example, this would mean that the NO_EXPORT is avoided and
   instead AS prepending is used (to depref routes) on the AS1-AS2
   peering session.

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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.

3.3.  Customer Cone Consideration

   An additional degree of flexibility that can be incorporated in the
   enhanced feasible-path uRPF can be described as follows.  Let I =
   {I1, I2, ..., In} represent the set of all directly-connected
   customer interfaces at customer-facing edge routers in a transit
   provider's AS.  Let P = {P1, P2, ..., Pm} represent the set of all
   prefixes for which routes have been received over the interfaces in
   set I.  Then, over all interfaces in the set I, the edge router
   SHOULD permit ingress data packets with SA in any of the prefixes in
   the set P.

3.4.  Implementation Consideration

   The existing RPF checks in edge routers take advantage of existing
   line card implementations to perform the RPF functions.  For
   implementation of the proposed technique, the general necessary
   feature would be to extend the line cards to take arbitrary RPF lists
   that are not necessarily tied to the existing FIB contents.  For
   example, in the proposed method, 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).

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4.  Security Considerations

   This document offers a technique to improve the security features of
   uRPF.  The proposed technique does not warrant any additional
   security considerations.

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 Jeff Haas, Job Snijders, Marco
   Marzetti, Marco d'Itri, Nick Hilliard, Gert Doering, Barry Greene,
   and Joel Jaeggli for comments and suggestions.

7.  Informative References

   [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,
              <http://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, <http://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, <http://www.rfc-editor.org/info/rfc3704>.

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

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

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Authors' Addresses

   Kotikalapudi Sriram
   NIST
   100 Bureau Drive
   Gaithersburg  MD 20899
   USA

   Email: ksriram@nist.gov

   Doug Montgomery
   US NIST
   100 Bureau Drive
   Gaithersburg  MD 20899
   USA

   Email: dougm@nist.gov

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