Deprecation of BGP Entropy Label Capability Attribute
Retires the BGP Entropy Label Capability attribute, which could not work as an optional transitive attribute.
Scudder, J. and K. Kompella, "Deprecation of BGP Entropy Label Capability Attribute", RFC 7447, DOI 10.17487/RFC7447, February 2015, <https://www.rfc-editor.org/info/rfc7447>.
@misc{rfc7447,
series = {Request for Comments},
number = 7447,
howpublished = {RFC 7447},
publisher = {RFC Editor},
doi = {10.17487/RFC7447},
url = {https://www.rfc-editor.org/info/rfc7447},
author = {J. Scudder and K. Kompella},
title = {{Deprecation of BGP Entropy Label Capability Attribute}},
pagetotal = 4,
year = 2015,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Scudder
Request for Comments: 7447 K. Kompella
Updates: 6790 Juniper Networks
Category: Standards Track February 2015
ISSN: 2070-1721
Deprecation of BGP Entropy Label Capability Attribute
Abstract
The BGP Entropy Label Capability attribute is defined in RFC 6790.
Regrettably, it has a bug: although RFC 6790 mandates that routers
incapable of processing Entropy Labels must remove the attribute,
fulfillment of this requirement cannot be guaranteed in practice.
This specification deprecates the attribute. A forthcoming document
will propose a replacement.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7447.
Copyright Notice
Copyright (c) 2015 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
(http://trustee.ietf.org/license-info) in effect on the date of
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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
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Deprecation of ELCA . . . . . . . . . . . . . . . . . . . . . 2
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 3
5.1. Normative References . . . . . . . . . . . . . . . . . . 3
5.2. Informative References . . . . . . . . . . . . . . . . . 3
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 4
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
[RFC6790] defines the Entropy Label Capability attribute (ELCA), an
optional, transitive BGP path attribute. For correct operation, an
intermediate node modifying the next hop of a route must remove the
ELCA unless the node doing so is able to process entropy labels.
Sadly, this requirement cannot be fulfilled with the ELCA as
specified, because it is an optional, transitive attribute. By
definition, a node that does not support the ELCA will propagate the
attribute (this is a general property of optional, transitive
attributes; see [RFC4271]). But such an ELCA-oblivious node is
likely to be incapable of processing entropy labels and is exactly
the node that we desire to remove the attribute!
This specification updates RFC 6790 by deprecating the version of
ELCA defined in Section 5.2 of that document. A forthcoming document
will propose a replacement. All other sections of RFC 6790 are
unchanged.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Deprecation of ELCA
This document deprecates the ELCA path attribute. This means that
implementations MUST NOT generate the attribute. If received, the
attribute MUST be treated as any other unrecognized optional,
transitive attribute as per [RFC4271], until and unless the code
point is reused by some new specification. (To the authors' best
knowledge, there are no implementations of ELCA at the time of
writing.)
3. IANA Considerations
For the reasons given in Section 1, IANA has marked attribute 28 "BGP
Entropy Label Capability Attribute" in the "BGP Path Attributes"
registry as "deprecated" and has added a reference to this RFC.
4. Security Considerations
ELCA, as defined in Section 5.2 of [RFC6790], has in common with
other optional, transitive path attributes the property that it will
be "tunneled" through intervening routers that don't implement the
relevant specification. Unfortunately, as discussed elsewhere in
this document, implementations of ELCA that receive such "tunneled"
attributes could -- sometimes improperly -- rely on them. The
consequence of doing so could be a black hole in the forwarding path
for the affected routes. Whether or not this is a new security issue
is somewhat debatable, since an attacker would have to be part of the
control-plane path for the route in question in order for the
attacker to exploit the issue. Under those circumstances, an
attacker already has a panoply of mischief-making tools available, as
discussed in [RFC4272].
In any case, this document renders any real or imagined security
issues with ELCA moot, by deprecating it.
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, November 2012,
<http://www.rfc-editor.org/info/rfc6790>.
5.2. Informative References
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
2006, <http://www.rfc-editor.org/info/rfc4271>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC
4272, January 2006,
<http://www.rfc-editor.org/info/rfc4272>.
Acknowledgements
Thanks to Alia Atlas, Bruno Decraene, Martin Djernaes, John Drake,
Adrian Farrel, Keyur Patel, Ravi Singh, and Kevin Wang for their
discussion of this issue.
Authors' Addresses
John G. Scudder
Juniper Networks
EMail: jgs@juniper.net
Kireeti Kompella
Juniper Networks
EMail: kireeti@juniper.net
AI ELI5
On this page
§1Introduction
RFC 6790 defines the Entropy Label Capability attribute (ELCA), an optional, transitive BGP path attribute. A node that changes the next hop of a route must remove the ELCA unless it can process entropy labels.
That requirement cannot be met. By definition a node that does not support an optional, transitive attribute propagates it (RFC 4271), and such a node is likely unable to process entropy labels, which makes it exactly the node that should remove it.
The rule asked the very routers that do not understand the note to be the ones to throw it away. They never could.
§2Deprecation of ELCA
Implementations MUST NOT generate the attribute. If it is received, it MUST be treated like any other unrecognized optional, transitive attribute, until and unless the code point is reused by a new specification. To the authors' best knowledge there were no implementations of ELCA at the time of writing.
Only the version defined in Section 5.2 of RFC 6790 is deprecated; all other sections are unchanged, and a forthcoming document was to propose a replacement.
Stop writing the note. If you see one, just pass it along like any other mystery note.
§3IANA Considerations
IANA marked attribute 28, "BGP Entropy Label Capability Attribute", in the "BGP Path Attributes" registry as deprecated and added a reference to this RFC.
The official list now says this item is retired.
§4Security Considerations
Like other optional, transitive attributes, ELCA is tunneled through routers that do not implement it. Implementations that receive such an attribute could improperly rely on it, which could cause a black hole in the forwarding path. Whether this is a new security issue is debatable, since an attacker would need to be on the control-plane path. Deprecating ELCA makes the issue moot.
A misleading note could send traffic nowhere. Retiring the note removes the problem.