Juniper Networks

jgs@juniper.net

Juniper Networks

kireeti@juniper.net

General Internet Engineering Task Force BGP MPLS 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.

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 ). 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.

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.

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 , 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.)

For the reasons given in , 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.

ELCA, as defined in Section 5.2 of , 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 . In any case, this document renders any real or imagined security issues with ELCA moot, by deprecating it.

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.