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

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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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