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.
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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