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Versions: (draft-djsmith-bgp-flowspec-oid) 00 01         Standards Track
          02 03 04 05 06 07 08 09 10 11 12 13 14                        
Network Working Group                                          J. Uttaro
Internet-Draft                                                      AT&T
Updates: 8955 (if approved)                                   J. Alcaide
Intended status: Standards Track                             C. Filsfils
Expires: October 14, 2021                                       D. Smith
                                                                   Cisco
                                                            P. Mohapatra
                                                        Sproute Networks
                                                          April 12, 2021


        Revised Validation Procedure for BGP Flow Specifications
                   draft-ietf-idr-bgp-flowspec-oid-13

Abstract

   This document describes a modification to the validation procedure
   defined for the dissemination of BGP Flow Specifications.  The
   dissemination of BGP Flow Specifications requires that the originator
   of the Flow Specification matches the originator of the best-match
   unicast route for the destination prefix embedded in the Flow
   Specification.  For an iBGP received route, the originator is
   typically a border router within the same autonomous system.  The
   objective is to allow only BGP speakers within the data forwarding
   path to originate BGP Flow Specifications.  Sometimes it is desirable
   to originate the BGP Flow Specification any place within the
   autonomous system itself, for example, from a centralized BGP route
   controller.  However, the validation procedure will fail in this
   scenario.  The modification proposed herein relaxes the validation
   rule to enable Flow Specifications to be originated within the same
   autonomous system as the BGP speaker performing the validation.
   Additionally, this document revises AS_PATH validation rules so Flow
   Specifications received from an eBGP peer can be validated when such
   peer is a BGP route server.

   This document updates the validation procedure in RFC8955.

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/.





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   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 October 14, 2021.

Copyright Notice

   Copyright (c) 2021 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
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   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.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Revised Validation Procedure  . . . . . . . . . . . . . . . .   6
     4.1.  Revision of Route Feasibility . . . . . . . . . . . . . .   6
     4.2.  Revision of AS_PATH Validation  . . . . . . . . . . . . .   7
   5.  Topology Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  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.







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2.  Introduction

   [RFC8955] defines a BGP NLRI [RFC4271] that can be used to distribute
   traffic Flow Specifications amongst BGP speakers in support of
   traffic filtering.  The primary intention of [RFC8955] is to enable
   downstream autonomous systems to signal traffic filtering policies to
   upstream autonomous systems.  In this way, traffic is filtered closer
   to the source and the upstream autonomous system(s) avoid carrying
   the traffic to the downstream autonomous system only to be discarded.
   [RFC8955] also enables more granular traffic filtering based upon
   upper layer protocol information (e.g., protocol port numbers) as
   opposed to coarse IP destination prefix-based filtering.  Flow
   specification NLRIs received from a BGP peer are subject to validity
   checks before being considered feasible and subsequently installed
   within the respective Adj-RIB-In.

   The validation procedure defined within [RFC8955] requires that the
   originator of the Flow Specification NLRI matches the originator of
   the best-match unicast route for the destination prefix embedded in
   the Flow Specification.  The aim is making sure that only speakers on
   the forwarding path can originate the Flow Specification.  Let's
   consider the particular case where the Flow Specification is
   originated in any location within the same autonomous system than the
   speaker performing the validation (for example by a centralized BGP
   route controller), and the best-match unicast route is originated in
   another autonomous system.  In order for validation to succeed for a
   Flow Specification received from an iBGP peer, it could be possible
   to disseminate such Flow Specification NLRIs directly from the
   specific border router (within the local autonomous system) that is
   advertising the corresponding best-match unicast route to the local
   autonomous system.  This approach would be, however, operationally
   cumbersome in an autonomous system with a large number of border
   routers having complex BGP policies.

   Figure 1 illustrates this principle.  R1 (the upstream router) and RR
   need to validate the Flow Specification whose embedded destination
   prefix has a best-match unicast route (dest-route) originated by
   ASBR2.  ASBR2 could originate the Flow Specification, and it would be
   validated when received by RR and R1.  Sometimes the Flow
   Specification needs to be originated on AS1.  ASBR1 could originate
   it, and Flow Specification would still be validated.  In both cases,
   the Flow Specification is originated by a router in the same
   forwarding path as the dest-route.  For the case where AS1 has
   thousands of ASBRs, it becomes impractical to originate different
   rules on each ASBR in AS1 based on which ASBR each dest- route is
   learned from.  The objective is to advertise all the Flow
   Specifications from the same route-controller.




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           R1(AS1) --- RR(AS1) --- ASBR1(AS1) --- ASBR2(AS2)
                        |
                route-controller(AS1)

                                 Figure 1

   This document describes a modification to the [RFC8955] validation
   procedure allowing Flow Specification NLRIs to be originated from a
   centralized BGP route controller within the local autonomous system
   that is not in the data forwarding path.  While the proposed
   modification cannot be used for inter-domain coordination of traffic
   filtering, it greatly simplifies distribution of intra-domain traffic
   filtering policies within an autonomous system which has a large
   number of border routers having complex BGP policies.  By relaxing
   the validation procedure for iBGP, the proposed modification allows
   Flow Specifications to be distributed in a standard and scalable
   manner throughout an autonomous system.

3.  Motivation

   Step (b) of the validation procedure in [RFC8955], section 6 is
   defined with the underlying assumption that the Flow Specification
   NLRI traverses the same path, in the inter-domain and intra-domain
   route distribution graph, as that of the longest-match unicast route
   for the destination prefix embedded in the Flow Specification.

   In the case of inter-domain traffic filtering, the Flow Specification
   originator at the egress border routers of an AS (e.g.  RTR-D and
   RTR-E of AS1 in Figure 2) matches the eBGP neighbor that advertised
   the longest match destination prefix (see RTR-F and RTR-G
   respectively in Figure 2).

   Similarly, at the upstream routers of an AS (see RTR-A and RTR-B of
   AS1 in Figure 2), the Flow Specification originator matches the
   egress iBGP border routers that had advertised the unicast route for
   the best-match destination prefix (see RTR-D and RTR-E respectively
   in Figure 2).  This is true even when upstream routers select paths
   from different egress border routers as best route based upon IGP
   distance.  For example, in Figure 2:

      RTR-A chooses RTR-D as the best route

      RTR-B chooses RTR-E as the best route








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                     / - - - - - - - - - - - - -  -
                     |            AS1              |
                       +-------+        +-------+
                     | |       |        |       |  |
                       | RTR-A |        | RTR-B |
                     | |       |        |       |  |
                       +-------+        +-------+
                     |       \           /         |
                        iBGP  \         / iBGP
                     |         \       /           |
                               +-------+
                     |         |       |           |
                               | RTR-C |
                     |         |  RC   |           |
                               +-------+
                     |           /   \             |
                                /     \
                     |   iBGP  /       \ iBGP      |
                       +-------+        +-------+
                     | | RTR-D |        | RTR-E |  |
                       |       |        |       |
                     | |       |        |       |  |
                       +-------+        +-------+
                     |     |                 |     |
                      - - -|- - - - - - - - -|- - -/
                           | eBGP       eBGP |
                      - - -|- - - - - - - - -|- - -/
                     |     |                 |     |
                       +-------+        +-------+
                     | |       |        |       |  |
                       | RTR-F |        | RTR-G |
                     | |       |        |       |  |
                       +-------+        +-------+
                     |            AS2              |
                     / - - - - - - - - - - - - -  -

                                 Figure 2

   It is highly desirable that mechanisms exist to protect each AS
   independently from network security attacks using the BGP Flow
   Specification NLRI for intra-AS purposes only.  Network operators
   often deploy a dedicated Security Operations Center (SOC) within
   their AS to monitor and detect such security attacks.  To mitigate
   attacks within an AS, operators require the ability to originate
   intra-AS Flow Specification NLRIs from a central BGP route controller
   that is not within the data forwarding plane.  In this way, operators
   can direct border routers within their AS with specific attack




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   mitigation actions (drop the traffic, forward to a clean-pipe center,
   etc.).

   In addition, an operator MAY extend the requirements above for a
   group of ASes via policy.  This is described in section (b.2.3) of
   the validation procedure.

   A central BGP route controller that originates a Flow Specification
   NLRI should be able to avoid the complexity of having to determine
   the egress border router whose path was chosen as the best for each
   of its neighbors.  When a central BGP route controller originates a
   Flow Specification NLRI, the rest of the speakers within the AS will
   see the BGP controller as the originator of the Flow Specification in
   terms of the validation procedure rules.  Thus, it is necessary to
   modify step (b) of the [RFC8955] validation procedure such that an
   iBGP peer that is not within the data forwarding plane may originate
   Flow Specification NLRIs.

4.  Revised Validation Procedure

4.1.  Revision of Route Feasibility

   Step (b) of the validation procedure specified in [RFC8955], section
   6 is redefined as follows:

   b) One of the following conditions MUST hold true:

      1.  The originator of the Flow Specification matches the
          originator of the best-match unicast route for the destination
          prefix embedded in the Flow Specification (this is the unicast
          route with the longest possible prefix length covering the
          destination prefix embedded in the Flow Specification).

      2.  The AS_PATH attribute of the Flow Specification is empty or
          contains only AS_CONFED_SEQUENCE and/or AS_CONFED_SET segments
          [RFC5065].

          1.  This condition SHOULD be enabled by default (it may be
              disabled by explicit configuration as described on the
              next list item (b.2.1))..  an empty AS_PATH.

          2.  This condition MAY be disabled by explicit configuration
              on a BGP speaker.  A possible case would be if we know for
              a fact that only the right egress border routers (i.e.
              those that are also egress border routers for the best
              routes) are originating a Flow Specification NLRI.





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          3.  As an extension to this rule, a given non-empty AS_PATHs
              (or AS_PATHS containing only AS_CONFED_SEQUENCE and/or
              AS_CONFED_SET segments), MAY be validated by policy.  A
              possible case would be if the AS_SEQUENCE and AS_SET
              contained only ASes that are known to belong to our own
              administrative domain.

   Explanation:

      In this context, a local domain includes the local AS or the local
      confederation [RFC5065].  Receiving either an empty AS_PATH or one
      with only AS_CONFED_SEQUENCE and/or AS_CONFED_SET segments
      indicates that the Flow Specification was originated inside the
      local domain.

      With the above modification to the [RFC8955] validation procedure,
      a BGP peer within the local domain that is not within the data
      forwarding path can originate a Flow Specification.

      Disabling the new condition above (b.2.2) may be a good practice
      when the operator knows with certainty that a Flow Specification
      will not be originated inside the local domain.

      Also, policy may be useful to validate a specific set of non-empty
      AS_PATHs (b.2.3).  For example, it could validate a Flow
      Specification whose AS_PATH contains only an AS_SEQUENCE with ASes
      that are all known to belong to the same administrative domain.

4.2.  Revision of AS_PATH Validation

   [RFC8955] states:

   BGP implementations MUST also enforce that the AS_PATH attribute of a
   route received via the External Border Gateway Protocol (eBGP)
   contains the neighboring AS in the left-most position of the AS_PATH
   attribute.

   This rule prevents the exchange of BGP Flow Specification NLRIs at
   Internet exchanges with BGP route servers [RFC7947].  Therefore, this
   document also redefines the [RFC8955] AS_PATH validation procedure
   referenced above as follows:

   BGP Flow Specification implementations MUST enforce that the AS in
   the left-most position of the AS_PATH attribute of a Flow
   Specification route received via the External Border Gateway Protocol
   (eBGP) matches the AS in the left-most position of the AS_PATH
   attribute of the best-match unicast route for the destination prefix
   embedded in the Flow Specification NLRI.



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   Explanation:

      For clarity, the AS in the left-most position of the AS_PATH means
      the AS that was last added to the AS_SEQUENCE.

      This proposed modification enables the exchange of BGP Flow
      Specification NLRIs at Internet exchanges with BGP route servers
      while at the same time, for security reasons, prevents an eBGP
      peer from advertising an inter-domain Flow Specification for a
      destination prefix that it does not provide reachability
      information for.

      Comparing only the last ASes added is sufficient for eBGP learned
      Flow Specification NLRIs.  Requiring a full AS_PATH match would
      limit origination of inter-domain Flow Specifications to the
      origin AS of the best-match unicast route for the destination
      prefix embedded in the Flow Specification only.  As such, a full
      AS_PATH validity check may prevent transit ASes from originating
      inter-domain Flow Specifications, which is not desirable.

      Note, however, that not checking the full AS_PATH allows any rogue
      or misconfigured AS the ability to originate undesired Flow
      Specifications.  This is a security BGP threat, but out of the
      scope of this document.

      Redefinition of this AS_PATH validation rule for a Flow
      Specification does not mean that the original rule in [RFC8955]
      cannot be enforced as well.  Its enforcement remains optional per
      [RFC4271] section 6.3.  That is, a BGP speaker can enforce the
      first AS in the AS_PATH to be the same as the neighbor AS for any
      address-family route (including a Flow Specification).

      Using the new rule to validate a Flow Specification route received
      from an External Border Gateway Protocol (eBGP) peer belonging to
      the same local domain (in the case of a confederation) is out of
      the scope of this document.  Note that although it's possible, its
      utility is dubious.  Although it is conceivable that an router in
      the same local domain (both iBGP and eBGP within the same local
      domain) could send a rogue update, only eBGP (outside the local
      domain) risk is considered within this document (in the same
      spirit of the mentioned beforehand AS_PATH validation in
      [RFC4271]).

5.  Topology Considerations

   [RFC8955] indicates that the originator may refer to the originator
   path attribute (ORIGINATOR_ID) or (if the attribute is not present)
   the transport address of the peer from which the BGP speaker received



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   the update.  If the latter applies, a network should be designed so
   it has a congruent topology amongst ipv4 unicast routes and Flow
   Specification routes.  By congruent topology, it is understood that
   for the two equivalent routes (i.e. the Flow Specification route and
   its best-match unicast route) are learned from the same peer accross
   the AS.  That would likely not be true, for instance, if some peers
   only negotiated one type of address-family or if each address-family
   had a different set of policies.

   With the additional second condition (b.2) in the validation
   procedure, non-congruent topologies are supported within the local
   domain if the Flow Specification is originated within the local
   domain.

   Explanation:

      Consider the validation procedure preceding this document.  The
      second condition (b.2) does not exist.  The two following
      scenarios have a non-congruent topology:

      1.  Consider a topology with two BGP speakers with two peering
          sessions between them, one for unicast and one for Flow
          Specification.  This is a non-congruent topology.  Let's
          assume that the ORIGINATOR_ID attribute was not received (e.g.
          a route reflector receiving routes from its clients).  In this
          case, the Flow Specification validation procedure will fail
          because of the first condition (b.1).

      2.  Consider a topology with a BGP speaker within a confederation
          of ASes, inside local AS X.  The ORIGINATOR_ID attribute is
          not advertised within the local domain.  Let's assume the Flow
          Specification route is received from peer A and the best-match
          unicast route is received from peer B.  Both peers belong to
          local AS Y.  Both AS X and AS Y belong to the same local
          domain.  The Flow Specification validation procedure will also
          fail because of the first condition (b.1).

      In the scenarios above, if Flow Specifications are originated in
      the same local domain, the AS_PATH will be empty or contain just
      AS_CONFED_SEQUENCE and/or AS_CONFED_SET segments.  Condition (b.2)
      evaluates to true.  Therefore, using the second condition (b.2),
      as defined by this document, guarantees that the overall
      validation procedure will pass.  Thus, non-congruent topologies
      are supported if the Flow Specification is originated in the same
      local domain.

      Flow Specification originated in a different local domain needs a
      congruent topology.  The reason is that the second condition (b.2)



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      evaluates to false and only the first condition (b.1) is
      evaluated.

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   This document updates the route feasibility validation procedures for
   Flow Specifications learned from iBGP peers and through route
   servers.  This change is in line with the procedures in [rfc8955] and
   thus maintain security characteristics equivalent to the existing
   security properties of BGP unicast routing.

   The security considerations discussed in [RFC8955] apply to this
   specification as well.

   The original AS_PATH validation rule ([RFC4271] section 6.3) becomes
   hereby optional (section 4.2).  If that original rule is actually not
   enforced it may introduce some security risks.  A peer (or a client
   of a route server peer) in AS X could advertise a rogue Flow
   Specification route whose first AS in AS_PATH was Y (assume Y is the
   first AS in the AS_PATH of the best-match unicast route).  This risk
   is impossible to prevent if the Flow Specification route is received
   from a route server peer.  If that peer is known for a fact not to be
   a route server, that optional rule SHOULD be enforced for Flow
   Specification routes.

   BGP updates learned from iBGP peers are considered trusted, so the
   Traffic Flow Specifications contained in BGP updates are also
   considered trusted.  Therefore it is not required to validate that
   the originator of an intra-domain Traffic Flow Specification matches
   the originator of the best-match unicast route for the flow
   destination prefix.  Note that this trustworthy consideration is not
   absolute and the new possibility than an iBGP speaker could send a
   rouge Flow Specification is introduced.

   The changes in Section 4.1 don't affect the validation procedures for
   eBGP-learned routes.

8.  Acknowledgements

   The authors would like to thank Han Nguyen for his direction on this
   work as well as Waqas Alam, Keyur Patel, Robert Raszuk, Eric Rosen
   and Shyam Sethuram for their review comments.





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9.  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>.

   [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>.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065,
              DOI 10.17487/RFC5065, August 2007,
              <https://www.rfc-editor.org/info/rfc5065>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <https://www.rfc-editor.org/info/rfc6793>.

   [RFC7947]  Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker,
              "Internet Exchange BGP Route Server", RFC 7947,
              DOI 10.17487/RFC7947, September 2016,
              <https://www.rfc-editor.org/info/rfc7947>.

   [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>.

   [RFC8955]  Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
              Bacher, "Dissemination of Flow Specification Rules",
              RFC 8955, DOI 10.17487/RFC8955, December 2020,
              <https://www.rfc-editor.org/info/rfc8955>.

Authors' Addresses

   James Uttaro
   AT&T
   200 S. Laurel Ave
   Middletown, NJ  07748
   USA

   Email: ju1738@att.com






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   Juan Alcaide
   Cisco
   7100 Kit Creek Road
   Research Triangle Park, NC  27709
   USA

   Email: jalcaide@cisco.com


   Clarence Filsfils
   Cisco

   Email: cf@cisco.com


   David Smith
   Cisco
   111 Wood Ave South
   Iselin, NJ  08830
   USA

   Email: djsmith@cisco.com


   Pradosh Mohapatra
   Sproute Networks

   Email: mpradosh@yahoo.com























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