Subject: [GROW] reviews of draft-ietf-grow-wkc-behavior-04

From: [email-masked]
Date: Wed, 29 May 2019 05:41:02 -0700

thanks for reviews.

-05 is circulating among authors and should be out today ot tomorrow.
here is what we have so far.  comments solicited.

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"Mirja Kühlewind via Datatracker" <[email-masked]> wrote:

> I think this document should update RFC1997.

ok

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Adrian Farrel via Datatracker wrote:

we have most of your comments in; but did not muck with the 2119bis
etc.; preferring to support full employment for the rfc editor. :)

and we leave it to the iseg to have fun deciding bcp, standard, info,
whatever.  above our pay grade.

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Carlos Pignataro via Datatracker wrote:
> 
> 7.  Security Considerations
> 
>    Surprising defaults and/or undocumented behaviors are not good for
>    security.  This document attempts to remedy that.
> 
> Indeed, but this is not an exclusivity of a security consideration. In fact,
> default values and behaviors are covered in
> https://tools.ietf.org/html/rfc5706#appendix-A.

do not understand what change you are suggesting

> Should then the IANA registry be updated to point to this RFC?
> Otherwise someone looking at
> https://www.iana.org/assignments/bgp-well-known-communities/
> 
> Will only see:
>   BGP Well-known Communities
>   Reference
>   [RFC1997]

hmmm.  ok, worth a note in the iana considerations and we'll see what
they say.

> In this Reference, there's a few improvements needed:
> 1. The Author should have "IANA" org
> 2. The Title should be "Border Gateway Protocol (BGP) Well-known Communities"
> 3. The URI should be
> "<https://www.iana.org/assignments/bgp-well-known-communities/>" 4. Should a
> descriptive anchor be "IANA-WKC" instead of "IANA-WKS"?

thanks.  url refs are a pain.

---

Network Working Group J. Borkenhagen
Internet-Draft AT&T
Updates: 1997 (if approved) R. Bush
Intended status: Standards Track R. Bush
Expires: September 11, 2019 Internet Initiative Japan
Expires: November 30, 2019 R. Bonica
Juniper Networks
S. Bayraktar
Cisco Systems
March 10,
May 29, 2019

Well-Known Community Policy Behavior
draft-ietf-grow-wkc-behavior-03
draft-ietf-grow-wkc-behavior-05

Abstract

Well-Known BGP Communities are manipulated inconsistently by differently across various
current implementations. 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.
Also,
This document recommends specific actions to limit future
inconsistency, namely BGP implementors are expected to should not create any further
inconsistencies from this point forward.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
be interpreted as described in RFC 2119 [RFC2119] only when they
appear in all upper case. They may also appear in lower or mixed
case as English words, without normative meaning.

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 September 11, November 30, 2019.

Copyright Notice

Copyright (c) 2019 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
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. Manipulation of Communities by Policy . . . . . . . . . . . . 3
3. Community Manipulation Policy Differences . . . . . . . . . . 3
4. Documentation of Vendor Implementations . . . . . . . . . . . 3
4.1. Note on an Inconsistency . . . . . . . . . . . . . . . . 4
5. Note for Those Writing RFCs for New Community-Like Attributes 5
6. Action Items . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 6
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
10. Normative 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 actions to limit future
inconsistency, namely BGP implementors MUST NOT create further
inconsistencies from this point forward.

2. 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.'

3. 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.

4. Documentation of Vendor Implementations

In this section we document the syntax and observed behavior of the
"set" directive in several popular BGP implementations. implementations to illustrate
the severity of the problem operators face.

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 only by
explicitly enumerating one at a time
by explicit policy; time, 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
communities and sets X.

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

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

4.1. Note on an Inconsistency

The IANA publishes a list of Well-Known Communities [IANA-WKS]. [IANA-WKC].

IOS XR's set of well-known Well-Known communities that "set community" will not
overwrite diverges from the IANA's list. Quite a few well-known Well-Known
communities from IANA's list do not receive special treatment in IOS
XR, and at least one specific community on IOS XR's special treatment
list (internet == 0:0) is not really on IANA's list -- it's taken
from the "Reserved" range [0x00000000-0x0000FFFF].

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

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

Care should be taken when establishing new [RFC1997]-like attributes
(large communities, wide communities, etc) etc.), RFCs MUST make clear how
the new community attribute is to to avoid be handled, avoiding repeating this
mistake.
inconsistencies.

6. Action Items

Network operators are encouraged to limit their use of the "set"
directive (within reason), to improve 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.

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.

7. Security Considerations

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

8. IANA Considerations

This document has no IANA Considerations other than Considerations; though the IANA may wish to be aware that
any future Well-Known Communities will be subject
refer to the policy
treatment described here. this document, if/when published, in [IANA-WKC].

9. Acknowledgments

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

10. Normative References

[IANA-WKS]
"IANA

[IANA-WKC]
IANA, "Border Gateway Protocol (BGP) Well-Known
Communities",
<https://www.iana.org/assignments/bgp-well-known-
communities/bgp-well-known-communities.xhtml>. <https://www.iana.org/assignments/
bgp-well-known-communities>.

[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<http://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,
<http://www.rfc-editor.org/info/rfc2119>.

Authors' Addresses

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

Email: [email-masked]

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

Email: [email-masked]
Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, VA 20171
US

Email: [email-masked]

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

Email: [email-masked]

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