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Versions: 00 01 02                                                      
Inter-Domain Routing                                             J. Haas
Internet-Draft                                          Juniper Networks
Updates: 8955 (if approved)                                 9 April 2021
Intended status: Standards Track
Expires: 11 October 2021

                      BGP Flowspec Capability Bits


   BGP Flowspec (RFC 8955) provides the ability to filter traffic using
   various matching components.  The NLRI format currently defined does
   not permit incremental deployment of new BGP Flowspec components.
   This draft defines a new BGP Capability to permit incremental
   deployment of such new Flowspec component types.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14 [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 October 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  BGP Flowspec Capability Bits  . . . . . . . . . . . . . . . .   3
   3.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Propagation of Known Components and Mismatch with Local
           Filtering Capabilities  . . . . . . . . . . . . . . . . .   5
   5.  BGP Flowspec Implications for Filtered NLRI . . . . . . . . .   5
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Appendix A.  Encoding of the Bit-String . . . . . . . . . . . . .   7
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   BGP Flowspec [RFC8955] provides a mechanism to distribute traffic
   flow spcifications into BGP.  One general purpose of these flow
   specifications is for the distribution of firewall rules to receiving
   routers, particularly for mitigating distributed denial of service
   (DDoS) attacks.  The flow specification rules are encoded as BGP NLRI

   The matching components of a flow specification NLRI is a serialized
   set of optional components.  The components are documented in
   [RFC8955], Section 3.  [RFC8956] defines IPv6-specific components.
   The full set of Flowspec component types is maintained in an IANA
   registry located at the IANA Flow Spec Component Types registry

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   Unknown Flowspec component types require treatment as a malformed
   NLRI ([RFC8955], Section 4.2).  This is due to the lack of a
   mandatory length element for the components in the NLRI.  Without
   such a length, it is not possible to determine how to properly decode
   unknown components in the Flowspec NLRI.

   There has been active interest in the IDR Working Group to extend BGP
   Flowspec for additional purposes.  However, with this difficulty in
   being able to handle unknown components, those new features are
   unable to be deployed in a BGP Flowspec domain in an incremental
   fashion.  Either a carefully managed "flag day" deployment is
   required to avoid disrupting existing sessions, or the Flowspec
   domain is carefully managed such that devices with incompatible sets
   of known/unknown components are carefully separated in a "ships in
   the night" scenario.  Both options are fragile and operationally

   Some initial discussion has begun for a version 2 of Flowspec in
   [I-D.hares-idr-flowspec-v2].  That document may eventually address
   this incremental deployment issue, along with a number of other

   This document proposes to address the issues of incremental
   deployment of new BGP Flowspec component types via a new BGP
   Capability [RFC5492], the BGP Flowpec Capability Bits.

2.  BGP Flowspec Capability Bits

   BGP Flowspec component types are one octet in length with values in
   the range from 0..255.  The BGP Flowspec Capability Bits encode a
   bit-string where each supported component type has its respective bit
   set when the BGP Speaker is willing to receive BGP Flowspec NLRI that
   contain that component type.

   The BGP Flowspec Capability Bits Capability is encoded as follows:

   *  Capability Code of (TBD).

   *  Capability Length of 1..32.

   *  Capability Value contains a bit-string where a bit is set if the
      underlying BGP Flowspec component is willing to be accepted by BGP
      Speaker advertising this capability.

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               Example encoding for Capability Value:

                 0                   1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

      Bit 0 set to 0, bits 1..13 set to 1 showing support for all
      capabilities for IPv6 Flowspec, bits 14 and 15 are set to 0.

3.  Operation

   BGP Flowspec Capability Bits not advertised in the encoded bit-string
   are treated as if they were sent with a value of zero for that bit.

   The Capability Length reflects the number of octets it takes to
   encode the BGP Flowspec Capability Bits.  While the total number of
   octets required to represent the entire range of component types is
   only 32 octets, implementations SHOULD limit the number of octets
   transmitted to those required to encode the final one-bit.  Space in
   BGP Capabilities may be limited in some implementations depending on
   the number of capabilities to be sent.  (See
   [I-D.ietf-idr-ext-opt-param] for discussion on a feature to address
   this point.)

   Bit-values 0 and 255 SHOULD be set to zero as they are RESERVED.

   The BGP Flowspec Capability Bits Capability SHOULD be sent by a BGP
   Speaker utilizing any AFI/SAFI using BGP Flowspec encoding as defined
   in [RFC8955], or [RFC8956].

   The BGP Flowspec Capability Bits Capability MUST be sent by a BGP
   Speaker utilizing BGP Flowspec encoding with a component type not
   defined in those documents previously mentioned.  (I.e. component
   types not in the range 1..13.)

   A BGP Speaker that has received the BGP Flowspec Capability Bits
   Capability MUST NOT originate or propagate a BGP Flowspec encoded
   NLRI that contains a component types that is not present in the
   received bit-string.

   A BGP Speaker that has received a BGP Flowspec related AFI/SAFI
   without this Capability MUST treat the absence as equivalent to
   having received the Capability Bits covered by the base specification
   for its defining RFC, [RFC8955] or [RFC8956].

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4.  Propagation of Known Components and Mismatch with Local Filtering

   There may be circumstances where a BGP Speaker is capable of parsing
   Flowspec components that it is not capable of implementing as
   filters.  Section 4.2 of [RFC8955] specifies that:

      "All combinations of components within a single Flow Specification
      are allowed.  However, some combinations cannot match any packets
      (e.g., "ICMP Type AND Port" will never match any packets) and thus
      SHOULD NOT be propagated by BGP."

   This document updates that text to:

      "All combinations of components within a single Flow Specification
      are allowed.  However, some combinations cannot match any packets
      (e.g., "ICMP Type AND Port" will never match any packets) and thus
      SHOULD NOT be propagated by BGP.

      "When BGP Flowspec component types are understood and the operator
      determines that deployment-wide filtering intent would not be
      comporomised by propagating Flowepec routes that cannot match any
      packets, it SHOULD propagate the route in BGP.  This permits NLRI
      with known components to be propagated to downstream BGP Speakers
      in the deployment."

5.  BGP Flowspec Implications for Filtered NLRI

   BGP Flowspec NLRI encode match operations for traffic filtering
   rules.  Filtering is an ordered operation.  Since the current
   encoding of the NLRI does not supply explicit filtering order, the
   protocol imposes a forwarding order based on the contents of the

   When a BGP Flowspec NLRI is not propagated due to filtering by this
   feature, or by user policy, there is the potential that the network-
   wide filtering intent may be compromised by the missing rules.  The
   exact impact of this on filtering will depend on the relative
   independence of the full set of BGP Flowspec routes in the BGP
   Flowspec routing domain.

   Operators must exercise care when deploying BGP Flowspec features
   with new component types to understand the propagation of such routes
   in their deployment, and the impact that filtering may have on the
   routes they wish to originate.

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6.  Error Handling

   If a BGP Speaker implementing this document has transmitted BGP
   Flowspec Capability Bits to its peer and receives a BGP Flowspec NLRI
   with an unacceptable component (not in its bit-string), it MAY
   terminate the BGP session by sending a NOTIFICATION message.

7.  Acknowledgements

   Thanks to Aseem Choudhary, Jakob Heitz, Christoph Loibl, Robert
   Raszuk for their comments on this proposal.

8.  Security Considerations

   All of the Security Considerations for [RFC8955] and [RFC8956] still

   Additionally, the BGP Flowspec Capability Bits may cause implicit
   filtering of some BGP Flowspec NLRI in a Flowspec domain.  Depending
   on the relative independence of the traffic matched by the BGP
   Flowspec rules in the ordering required by their specifications, such
   filtered NLRI may result in impact to the desired domain-wide
   filtering behaviors.

9.  IANA Considerations

   IANA is requested to assign a new BGP Capability to the Capability
   Codes registry from the First Come, First Served pool.  The Reference
   for the registration is this document.  The Change Controller is

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <https://www.rfc-editor.org/info/rfc5492>.

   [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>.

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   [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,

   [RFC8956]  Loibl, C., Ed., Raszuk, R., Ed., and S. Hares, Ed.,
              "Dissemination of Flow Specification Rules for IPv6",
              RFC 8956, DOI 10.17487/RFC8956, December 2020,

10.2.  Informative References

              Hares, S., "BGP Flow Specification Version 2", Work in
              Progress, Internet-Draft, draft-hares-idr-flowspec-v2-00,
              25 June 2016, <http://www.ietf.org/internet-drafts/draft-

              Chen, E. and J. Scudder, "Extended Optional Parameters
              Length for BGP OPEN Message", Work in Progress, Internet-
              Draft, draft-ietf-idr-ext-opt-param-09, 21 August 2020,

   [RFC2578]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Structure of Management Information
              Version 2 (SMIv2)", STD 58, RFC 2578,
              DOI 10.17487/RFC2578, April 1999,

   [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,

Appendix A.  Encoding of the Bit-String

   IETF has a mixed history in terms of how bit numbering is described.
   The format as used in this document, where the left-most bit sent on
   the wire is bit zero, is consistent with IETF PDU diagrams and also
   the SNMP BITS construct [RFC2578], Section 7.1.4.

   That said, the author is aware of how annoying the code for that
   construct can be.

Appendix B.  Open Issues

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   *  Are there circumstances where advertising capability bits for BGP
      Flowspec NLRI that need to vary on a per AFI-SAFI basis?
      Currently, the IANA registry is a single name space for all
      supported and proposed BGP address families.  As an example, the
      Flowspec for NVO3 feature has components that are defined that do
      not have incremental deployment issues due to being well formed
      with a length field.  However, since it still includes existing
      Flowspec filtering for the outer and inner IP headers, the issues
      addressed by this proposal still apply.

Author's Address

   Jeffrey Haas
   Juniper Networks

   Email: jhaas@juniper.net

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