Re: [Idr] AD Review of draft-ietf-idr-bgp-flowspec-oid-12

"Juan Alcaide (jalcaide)" <jalcaide@cisco.com> Wed, 24 February 2021 12:52 UTC

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From: "Juan Alcaide (jalcaide)" <jalcaide@cisco.com>
To: Alvaro Retana <aretana.ietf@gmail.com>, Susan Hares <shares@ndzh.com>, "draft-ietf-idr-bgp-flowspec-oid@ietf.org" <draft-ietf-idr-bgp-flowspec-oid@ietf.org>
CC: "idr-chairs@ietf.org" <idr-chairs@ietf.org>, IDR List <idr@ietf.org>
Thread-Topic: [Idr] AD Review of draft-ietf-idr-bgp-flowspec-oid-12
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Date: Wed, 24 Feb 2021 12:52:24 +0000
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Subject: Re: [Idr] AD Review of draft-ietf-idr-bgp-flowspec-oid-12
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We assumed it was understood, but it's always better to have a new set of eyes 😉



We did some rewording below. Tried to simplify the abstract and expand some explanation in introduction (simpler than in Motivation section)



Let us know if there is need for more rewording.







Network Working Group                                          J. Uttaro
Internet-Draft                                                      AT&T
Updates: 8955 (if approved)                                   J. Alcaide
Intended status: Standards Track                             C. Filsfils
Expires: August 28, 2021                                        D. Smith
                                                                   Cisco
                                                            P. Mohapatra
                                                        Sproute Networks
                                                       February 24, 2020


        Revised Validation Procedure for BGP Flow Specifications
                   draft-ietf-idr-bgp-flowspec-oid-12

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, typically an autonomous system border router.  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 from a
   centralized BGP route controller.

   This document updates 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
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   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."




Uttaro, et al.           Expires August 28, 2021                [Page 1]

Internet-Draft    Revised Flowspec Validation Procedure    February 2020


   This Internet-Draft will expire on August 28, 2021.

Copyright Notice

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Table of Contents

   1.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Revised Validation Procedure  . . . . . . . . . . . . . . . .   6
     4.1.  Revision of Route Feasibility . . . . . . . . . . . . . .   6
     4.2.  Revision of AS_PATH Validation  . . . . . . . . . . . . .   7
   5.  Other RFC8955 Considerations  . . . . . . . . . . . . . . . .   8
   6.  Topology Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  10
   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.

2.  Introduction

   [RFC8955] defined a new BGP [RFC4271] capability 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



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   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.  This allows only BGP speakers within the
   data forwarding path (such as autonomous system border routers) to
   originate BGP Flow Specification NLRIs.  Though it is possible to
   disseminate such Flow Specification NLRIs directly from border
   routers, it may be operationally cumbersome in an autonomous system
   with a large number of border routers having complex BGP policies.

   Figure 1 illustrates this principle.  R1 needs 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 then the validation procedure
   would pass.  Sometimes the Flow Specification needs to be originated
   on AS1.  ASBR1 could originate it, and validation procedure would
   still pass.  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.

           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




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   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 ASN1 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 ingress border routers
   of ASN (see RTR-A and RTR-B of ASN1 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 ingress border routers select paths from different egress
   border routers as best path based upon IGP distance.  For example, in
   figure 2:

      RTR-A chooses RTR-D's path as best

      RTR-B chooses RTR-E as the best path
























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                     / - - - - - - - - - - - - -  -
                     |           ASN1              |
                       +-------+        +-------+
                     | |       |        |       |  |
                       | RTR-A |        | RTR-B |
                     | |       |        |       |  |
                       +-------+        +-------+
                     |       \           /         |
                        iBGP  \         / iBGP
                     |         \       /           |
                               +-------+
                     |         |       |           |
                               | RTR-C |
                     |         |  RC   |           |
                               +-------+
                     |           /   \             |
                                /     \
                     |   iBGP  /       \ iBGP      |
                       +-------+        +-------+
                     | | RTR-D |        | RTR-E |  |
                       |       |        |       |
                     | |       |        |       |  |
                       +-------+        +-------+
                     |     |                 |     |
                      - - -|- - - - - - - - -|- - -/
                           | eBGP       eBGP |
                      - - -|- - - - - - - - -|- - -/
                     |     |                 |     |
                       +-------+        +-------+
                     | |       |        |       |  |
                       | RTR-F |        | RTR-G |
                     | |       |        |       |  |
                       +-------+        +-------+
                     |           ASN2              |
                     / - - - - - - - - - - - - -  -

                                 Figure 2

   It is highly desirable that the mechanisms exist to protect each ASN
   independently from network security attacks using the BGP Flow
   Specification NLRI for intra-domain purposes only.  Network operators
   often deploy a dedicated Security Operations Center (SOC) within
   their ASN to monitor and detect such security attacks.  To mitigate
   attacks within a domain (AS or group of ASes), operators require the
   ability to originate intra-domain 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




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

   To originate a Flow Specification NLRI, a central BGP route
   controller must set itself as the originator in the Flow
   Specification NLRI.  This is necessary given the route controller is
   originating the Flow Specification rather than reflecting it, and to
   avoid the complexity of having to determine the egress border router
   whose path was chosen as the best in each of the ingress border
   routers.  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 does not
          contain AS_SET and/or AS_SEQUENCE segments.

          1.  This condition SHOULD be enabled by default.  This default
              behavior should validate an empty AS_PATH.

          2.  This condition MAY be disabled by configuration on a BGP
              speaker.

          3.  As an exception to this rule, a given AS_PATH with AS_SET
              and/or AS_SEQUENCE segments MAY be validated by policy.

   Explanation:

      In this context, an empty AS_PATH means that it does not have
      AS_SET and/or AS_SEQUENCE segments, and local domain means the
      local AS [RFC4271] or the local confederation of ASes (in the case
      that the local AS belongs to a confederation of ASes [RFC5065]).
      Thus, receiving a Flow Specification with an empty AS_PATH




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      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 there is not a Flow
      Specification 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:

   o  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.  Therefore, this document
   also redefines the [RFC8955] AS_PATH validation procedure referenced
   above as follows:

   o  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.

   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.



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      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.

      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, we 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 received from
      an Internal Border Gateway Protocol (iBGP) peer is out of the
      scope of this document.  Note that in most scenarios such
      validation would be redundant.

      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 that the local AS belongs to a
      confederation of ASes) is out of the scope of this document.  Note
      that although it's possible, its utility is dubious.

5.  Other RFC8955 Considerations

   This section clarifies some of the terminology and rules referenced
   in [RFC8955].  Namely:

   o  In the context of this document and [RFC8955], AS_PATH attribute
      is defined as the reconstructed AS path information (by combining
      AS_PATH and AS4_PATH attributes, if the BGP speaker is a NEW
      speaker and receives the route from an OLD speaker), according to
      section 4.2.3 of [RFC6793].

   o  Support for two-octet AS only implementations is out of the scope
      of this document (i.e. it's assumed that the BGP speaker supports
      [RFC6793]).

6.  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 we received the update.
   If the latter applies, a network should be designed so it has a
   congruent topology.



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   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 following scenarios without the second condition
      (b.2) being added to the validation procedure:

      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.  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 in
          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 examples above, if Flow Specifications are originated in
      the same local domain, AS_PATH will not contain AS_SET and/or
      AS_SEQUENCE segments.  When the second condition (b.2) in the
      validation procedure is used, the validation procedure will pass.
      Thus, non-congruent topologies are supported if the Flow
      Specification is originated in the same local domain.

      Even when the second condition (b.2) is used in the validation
      procedure, a Flow Specification originated in a different local
      domain needs a congruent topology.  AS_SEQUENCE is not empty and
      the first condition (b.1) in the validation procedure needs to be
      evaluated.  Because transport addresses for Flow Specification and
      unicast routes are different, the validation procedure will fail.
      This is true both across domains and within domains.  Consider
      both cases:

      *  Consider the first example.  If the Flow Specification route is
         originated in another AS, the validation procedure will fail
         because the topology is non-congruent within the domain.





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      *  Consider the second example and modify it so AS X and AS Y
         belong to different local domains (no confederation of ASes
         exists).  The validation procedure will fail because the
         topology is non-congruent across domains.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   No new security issues are introduced by relaxing the validation
   procedure for IBGP learned Flow Specifications.  With this proposal,
   the security characteristics of BGP Flow Specifications remain
   equivalent to the existing security properties of BGP unicast
   routing.

   BGP updates learned from iBGP peers are trusted so the Traffic Flow
   Specifications contained in BGP updates are 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.  This proposal
   continues to enforce the validation Procedure for eBGP learned
   Traffic Flow Specifications, as per [RFC8955] rules.  In this way,
   the security properties of [RFC8955] are maintained such that an EBGP
   peer cannot cause a denial-of-service attack by advertising an inter-
   domain Flow Specification for a destination prefix that it does not
   provide reachability information for.

9.  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.

10.  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>.





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   [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>.

   [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<mailto:ju1738@att.com>


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

   Email: jalcaide@cisco.com<mailto:jalcaide@cisco.com>


   Clarence Filsfils
   Cisco

   Email: cf@cisco.com<mailto:cf@cisco.com>








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

   Email: djsmith@cisco.com<mailto:djsmith@cisco.com>


   Pradosh Mohapatra
   Sproute Networks

   Email: mpradosh@yahoo.com<mailto:mpradosh@yahoo.com>






































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