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Versions: (draft-ymbk-grow-wkc-behavior) 00 01 02 03 04 05 06 07 08

Network Working Group                                     J. Borkenhagen
Internet-Draft                                                      AT&T
Intended status: Best Current Practice                           R. Bush
Expires: November 17, 2019                     Internet Initiative Japan
                                                               R. Bonica
                                                        Juniper Networks
                                                            S. Bayraktar
                                                           Cisco Systems
                                                            May 16, 2019


                  Well-Known Community Policy Behavior
                    draft-ietf-grow-wkc-behavior-04

Abstract

   Popular BGP implementations manipulate Well-known Communities
   differently from one another.  This results in difficulties for
   operators.  Network operators are encouraged to deploy consistent
   community handling across their networks, taking the inconsistent
   behaviors from the various BGP implementations they operate into
   consideration.  This document recommends specific action items to
   limit future inconsistency, namely BGP implementors are expected to
   not create any further inconsistencies from this point forward.

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 November 17, 2019.

Copyright Notice

   Copyright (c) 2019 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.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Manipulation of Communities by Policy . . . . . . . . . . . .   3
   4.  Community Manipulation Policy Differences . . . . . . . . . .   3
   5.  Documentation of Vendor Implementations . . . . . . . . . . .   4
     5.1.  Note on an Inconsistency  . . . . . . . . . . . . . . . .   5
   6.  Note for Those Writing RFCs for New Community-Like Attributes   5
   7.  Action Items  . . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     11.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The BGP Communities Attribute was specified in [RFC1997] which
   introduced the concept of Well-Known Communities.  In hindsight,
   [RFC1997] did not prescribe as fully as it should have how Well-Known
   Communities may be manipulated by policies applied by operators.
   Currently, implementations differ in this regard, and these
   differences can result in inconsistent behaviors that operators find
   difficult to identify and resolve.

   This document describes the current behavioral differences in order
   to assist operators in generating consistent community-manipulation
   policies in a multi-vendor environment, and to prevent the
   introduction of additional divergence in implementations.

   This document recommends specific action items to limit future
   inconsistency.






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2.  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.

3.  Manipulation of Communities by Policy

   [RFC1997] says:

   "A BGP speaker receiving a route with the COMMUNITIES path attribute
   may modify this attribute according to the local policy."

   One basic operational need is to add or remove one or more
   communities to the received set.  The focus of this document is
   another common operational need, to replace all communities with a
   new set.  To simplify this second case, most BGP policy
   implementations provide syntax to "set" community that operators use
   to mean "remove any/all communities present on the route, and apply
   this set of communities instead."

   Some operators prefer to write explicit policy to delete unwanted
   communities rather than using "set;" i.e. using a "delete community
   *:*" and then "add community x:y ..." configuration statements in an
   attempt to replace all received communities.  The same community
   manipulation policy differences described in the following section
   exist in both "set" and "delete community *:*" syntax.  For
   simplicity, the remainder of this document refers only to the "set"
   behaviors, which we refer to collectively as each implementation's
   '"set" directive.'

4.  Community Manipulation Policy Differences

   Vendor implementations differ in the treatment of certain Well-Known
   communities when modified using the syntax to "set" the community.
   Some replace all communities including the Well-Known ones with the
   new set, while others replace all non-Well-Known Communities but do
   not modify any Well-Known Communities that are present.

   These differences result in what would appear to be identical policy
   configurations having very different results on different platforms.








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5.  Documentation of Vendor Implementations

   In this section we document the syntax and observed behavior of the
   "set" directive in several popular BGP implementations.

   In Juniper Networks' Junos OS, "community set" removes all received
   communities, Well-Known or otherwise.

   In Cisco Systems' IOS XR, "set community" removes all received
   communities except for the following:

            +-------------+-----------------------------------+
            | Numeric     | Common Name                       |
            +-------------+-----------------------------------+
            | 0:0         | internet                          |
            | 65535:0     | graceful-shutdown                 |
            | 65535:1     | accept-own rfc7611                |
            | 65535:65281 | NO_EXPORT                         |
            | 65535:65282 | NO_ADVERTISE                      |
            | 65535:65283 | NO_EXPORT_SUBCONFED (or local-AS) |
            +-------------+-----------------------------------+

                  Communities not removed by Cisco IOS XR

                                  Table 1

   IOS XR does allow Well-Known communities to be removed by explicitly
   enumerating each one, not in the aggregate; for example, "delete
   community accept-own".  Operators are advised to consult IOS XR
   documentation and/or Cisco Systems support for full details.

   On Extreme networks' Brocade NetIron: "set community X" removes all
   received communities and sets X.

   In Huawei's VRP product, "community set" removes all received
   communities, well-Known or otherwise.

   In OpenBSD's OpenBGPD, "set community" does not remove any
   communities, Well-Known or otherwise.

   Nokia's SR OS has several directives that operate on communities.
   Its "set" directive is called using the "replace" keyword, replacing
   all received communities, Well-Known or otherwise, with the specified
   communities.







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5.1.  Note on an Inconsistency

   In IOS XR, "set community" will not overwrite some well-known
   communities.  However, it will overwrite other well-known
   communities.  Conversely, In IOS XR, "set community" will not
   overwrite some communities that are not well-known (e.g., (internet
   == 0:0).

   This merely notes an inconsistency.  It is not a plea to 'protect'
   the entire IANA list from "set community."

6.  Note for Those Writing RFCs for New Community-Like Attributes

   When establishing new [RFC1997]-like attributes (large communities,
   wide communities, etc.), RFC authors should state how the new
   community attribute is to be handled.

7.  Action Items

   Network operators are encouraged to limit their use of the "set"
   directive (within reason), to improve the readability of their
   configurations and hopefully to achieve behavioral consistency across
   platforms.

   Unfortunately, it would be operationally disruptive for vendors to
   change their current implementations.

   Vendors SHOULD clearly document the behavior of "set" directive in
   their implementations.

   Vendors MUST ensure that their implementations' "set" directive
   treatment of any specific community does not change if/when that
   community becomes a new Well-Known Community through future
   standardization.  For most implementations, this means that the "set"
   directive MUST continue to remove the community; for those
   implementations where the "set" directive removes no communities,
   that behavior MUST continue.

   Given the implementation inconsistencies described in this document,
   network operators are urged never to rely on any implicit
   understanding of a neighbor ASN's BGP community handling.  I.e.,
   before announcing prefixes with NO_EXPORT or any other community to a
   neighbor ASN, the operator should confirm with that neighbor how the
   community will be treated.







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8.  Security Considerations

   Surprising defaults and/or undocumented behaviors are not good for
   security.  This document attempts to remedy that.

   Also see Appendix A of [RFC5706].

9.  IANA Considerations

   This document has no IANA Considerations.

10.  Acknowledgments

   The authors thank Martijn Schmidt, Qin Wu for the Huawei data point,
   Greg Hankins, Job Snijders, David Farmer, John Heasley, and Jakob
   Heitz.

11.  References

11.1.  Normative References

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
              <https://www.rfc-editor.org/info/rfc1997>.

   [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>.

   [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>.

11.2.  Informative References

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions",
              RFC 5706, DOI 10.17487/RFC5706, November 2009,
              <https://www.rfc-editor.org/info/rfc5706>.

Authors' Addresses









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   Jay Borkenhagen
   AT&T
   200 Laurel Avenue South
   Middletown, NJ  07748
   United States of America

   Email: jayb@att.com


   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, WA  98110
   United States of America

   Email: randy@psg.com


   Ron Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, VA  20171
   US

   Email: rbonica@juniper.net


   Serpil Bayraktar
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   United States of America

   Email: serpil@cisco.com

















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