BGP Entropy Label Capability, Version 3
draft-ietf-idr-entropy-label-00
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| Last updated | 2022-09-29 | ||
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draft-ietf-idr-entropy-label-00
Internet Engineering Task Force J. G. Scudder, Ed.
Internet-Draft K. Kompella
Intended status: Standards Track Juniper Networks
Expires: 2 April 2023 S. Mohanty
Cisco Systems
J. Uttaro
AT&T
B. Wen
Comcast
29 September 2022
BGP Entropy Label Capability, Version 3
draft-ietf-idr-entropy-label-00
Abstract
This specification defines the Entropy Label Capability Attribute
version 3 (ELCv3), a BGP attribute that can be used to inform an LSP
ingress router about an LSP egress router's ability to process
entropy labels. This version of the attribute corrects a
specification error in the first version, and an improper code point
reuse in the second.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 April 2023.
Copyright Notice
Copyright (c) 2022 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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Entropy Label Capability Path Attribute, Version 3 . . . . . 3
2.1. Sending the ELCv3 . . . . . . . . . . . . . . . . . . . . 4
2.2. Receiving the ELCv3 . . . . . . . . . . . . . . . . . . . 5
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . 6
5.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Other Means of Signaling EL Capability . . . . . . . 7
A.1. Backward Compatibility with ELCv2 . . . . . . . . . . . . 8
Appendix B. Contributors . . . . . . . . . . . . . . . . . . . . 8
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
[RFC6790] defines the Entropy Label Capability attribute (ELC), an
optional, transitive BGP path attribute. For correct operation, it
is necessary that any intermediate node modifying the next hop of a
route must remove the ELC unless the node so doing is able to process
entropy labels. Sadly, these requirements cannot be fulfilled with
the ELC as specified, because it is an optional, transitive
attribute: by definition, a node that does not support the ELC will
propagate the attribute. But such a node might be exactly the one
that we desire to remove it. For this reason, [RFC7447] deprecated
the attribute.
Roughly concurrently with the development and advancement of RFC
7447, Juniper Networks began shipping routing code that implements
what is documented in [I-D.scudder-bgp-entropy-label] and dubbed
Entropy Label Capability version 2 (ELCv2). That implementation uses
the code point that was assigned by RFC 6790 and deprecated by RFC
7447. At time of writing, the functionality is in use in operational
networks.
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The present specification is based on ELCv2 but moves to a new,
previously unallocated, code point.
A related solution to the problem of signaling entropy label
capability is [I-D.ietf-idr-next-hop-capability]. That specification
is based on an optional, non-transitive path attribute. In contrast,
ELCv3 (and ELCv2) is based on an optional, transitive path attribute.
This expands the deployment options available -- in many cases (for
example, route reflectors) it's fine that an intermediate node does
propagate an ELCv3 even if it doesn't itself have the ability to
process entropy labels.
In order to prevent use of the signaled information beyond the
intended perimeter (the problem that led to the deprecation of ELC,
and which is inherently solved by
[I-D.ietf-idr-next-hop-capability]'s use of a non-transitive
attribute), in this specification we take the approach of carrying a
copy of the next hop information in the ELCv3. This allows the node
processing it to know if it can rely on the information carried
therein, while still allowing it to be propagated by all intermediate
nodes.
1.1. 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.
2. Entropy Label Capability Path Attribute, Version 3
The Entropy Label Capability Path Attribute, Version 3 (ELCv3) is an
optional, transitive BGP attribute with type code TBD1. The ELCv3
has as its data a network layer address, representing the next hop of
the route the ELCv3 accompanies. The ELCv3 signals a useful
optimization, so it is desirable to make it transitive; the next hop
data is to ensure correctness if it traverses BGP speakers that do
not understand the ELCv3.
The Attribute Data field of the ELCv3 path attribute is encoded as
shown below:
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+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Length of Next Hop Network Address (1 octet) |
+---------------------------------------------------------+
| Network Address of Next Hop (variable) |
+---------------------------------------------------------+
The meanings of the fields are as given in Section 3 of [RFC4760].
When BGP [RFC4271] is used for distributing labeled Network Layer
Reachability Information (NLRI) as described in, for example,
[RFC8277], the route may include the ELCv3 as part of the Path
Attributes. The inclusion of this attribute with a route indicates
that the egress of the associated Label Switched Path (LSP) can
process entropy labels as an egress Label Switched Router (LSR) for
that route -- see Section 4.2 of [RFC6790]. Below, we refer to this
for brevity as being "EL-capable."
2.1. Sending the ELCv3
When a BGP speaker S has a route R it wishes to advertise with next
hop N to its peer, it MUST NOT include the ELCv3 attribute except if
it knows that the egress of the associated LSP L is EL-capable.
Specifically, this will be true if S:
* Is itself the egress, and knows itself to be EL-capable, or
* Is re-advertising a BGP route it received with a valid ELCv3
attribute, and is not changing the value of N, or
* Is re-advertising a BGP route it received with a valid ELCv3
attribute, and is changing the value of N, and knows (for example,
through configuration) that the router represented by N is either
the LSP egress and is EL-capable, or that it will process the
outer label(s) without processing the entropy label below, as with
a transit LSR, or
* Is redistributing a route learned from another protocol, and that
other protocol conveyed the knowledge that the egress of L was EL-
capable (for example, this might be known through the LDP ELC TLV,
Section 5.1 of [RFC6790]).
In any event, when sending an ELCv3, S MUST set the data portion of
the ELCv3 to be equal to N, using the encoding given in Section 2.
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The ELCv3 MAY be advertised with routes that are labeled, such as
those using SAFI 4 [RFC8277]. It MUST NOT be advertised with
unlabeled routes.
We note that due to the nature of BGP optional transitive path
attributes, any BGP speaker that does not implement this
specification will propagate the ELCv3, the requirements of this
section notwithstanding. However, such a speaker will not update the
data part of the ELCv3.
2.2. Receiving the ELCv3
When a BGP speaker receives an unlabeled route that includes the
ELCv3, it MUST discard the ELCv3.
When a BGP speaker receives a labeled route that includes the ELCv3,
it MUST compare the address given in the ELCv3's data portion to the
next hop of the route. If the two are equal, the egress of the LSP
supports entropy labels, which implies that the receiving BGP
speaker, if acting as ingress, MAY insert an entropy label below the
advertised label, as per Section 4.2 of [RFC6790]. If the two are
not equal, either some intermediate router that does not implement
this specification modified the next hop, or some router on the path
had an incorrect implementation. In either case, the action taken is
the same: the ELCv3 MUST be discarded. The Partial bit MAY be
inspected -- if it is equal to zero, then the mismatch must have been
caused by an incorrect implementation, and the error MAY be logged.
When a BGP speaker receives a route that includes an ELCv3 whose
Attribute Length is less than 4, whose Attribute Length is not
greater than or equal to 4 plus the value encoded in the Length of
Next Hop Network Address carried in the Attribute Data, or whose
Attribute Data is otherwise inconsistent with the encoding specified
in Section 2, it MUST discard the ELCv3.
If an ELCv3 includes data beyond the Network Address of Next Hop
field, such data MUST be disregarded. If the ELCv3 is propagated,
the unknown data MUST be included with it.
3. IANA Considerations
IANA is requested to make a new allocation in the BGP Path Attributes
registry:
* Value = TBD1
* Code = ELCv3
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* Reference = (this document)
4. Security Considerations
Insertion of an ELCv3 by an attacker could cause forwarding to fail.
Deletion of an ELCv3 by an attacker could cause one path in the
network to be overutilized and another to be underutilized. However,
we note that an attacker able to accomplish either of these (below,
an "on-path attacker") could equally insert or remove any other BGP
path attribute or message. The former attack described above denies
service for a given route, which can be accomplished by an on-path
attacker in any number of ways even absent ELCv3. The latter attack
defeats an optimization but nothing more; it seems dubious that an
attacker would go to the trouble of doing so rather than launching
some more damaging attack.
The Attribute Data portion of the ELCv3 contains the next hop the
attribute's originator included when sending it. This will typically
be an IP address of the router in question. This may be an
infrastructure address the network operator does not intend to
announce beyond the border of its Autonomous System, and it may even
be considered in some weak sense, confidential information. Although
the desired operation of the protocol is for the attribute's
propagation scope to be limited to the network operator's own
Autonomous System, it will not always be so -- indeed, that is the
reason this specification had to be written. So, sometimes this
information could leak beyond its intended scope. (Note that it will
only propagate as far as the first router that does support this
specification, at which point it will be discarded per Section 2.2.)
5. References
5.1. Normative References
[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>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
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[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
[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>.
5.2. Informative References
[I-D.ietf-idr-next-hop-capability]
Decraene, B., Kompella, K., and W. Henderickx, "BGP Next-
Hop dependent capabilities", Work in Progress, Internet-
Draft, draft-ietf-idr-next-hop-capability-08, 8 June 2022,
<https://www.ietf.org/archive/id/draft-ietf-idr-next-hop-
capability-08.txt>.
[I-D.scudder-bgp-entropy-label]
Scudder, J. G. and K. Kompella, "BGP Entropy Label
Capability, Version 2", Work in Progress, Internet-Draft,
draft-scudder-bgp-entropy-label-00, 28 April 2022,
<https://www.ietf.org/archive/id/draft-scudder-bgp-
entropy-label-00.txt>.
[RFC7447] 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>.
[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.
Appendix A. Other Means of Signaling EL Capability
A router that supports this specification could also have other means
to know that an egress is EL-capable, for example it could support
ELCv2 [I-D.scudder-bgp-entropy-label], or it could know through
configuration. If a router learns through any means that an egress
is EL-capable, it MAY treat the egress as EL-capable. For example,
reception of a valid ELCv2 would be sufficient (even if a valid ELCv3
is not received), and similarly reception of a valid ELCv3 would be
sufficient (even if a valid ELCv2 is not received). The details of
which methods are accepted for signaling EL capability are beyond the
scope of this specification, but SHOULD be configurable by the user.
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A.1. Backward Compatibility with ELCv2
As was noted in Section 1, there are networks in which ELCv2
(documented in [I-D.scudder-bgp-entropy-label]) is already in use.
Any node that sends the ELCv2 format may also include an ELCv3 per
Section 2.1, so that both formats are sent. The exact set of formats
to send SHOULD be user-configurable.
As discussed above, a route received with either a valid ELCv2 or
ELCv3 may be considered EL-capable.
Appendix B. Contributors
Serge Krier
Cisco Systems
Email: sekrier@cisco.com
Kevin Wang
Juniper Networks
Email: kfwang@juniper.net
Appendix C. Acknowledgements
Thanks to Swadesh Agrawal, Alia Atlas, Bruno Decraene, Martin
Djernaes, John Drake, Adrian Farrell, Keyur Patel, Toby Rees, and
Ravi Singh, for their discussion of this issue.
Authors' Addresses
John G. Scudder (editor)
Juniper Networks
Email: jgs@juniper.net
Kireeti Kompella
Juniper Networks
Email: kireeti@juniper.net
Satya Mohanty
Cisco Systems
Email: satyamoh@cisco.com
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James Uttaro
AT&T
Email: ju1738@att.com
Bin Wen
Comcast
Email: Bin_Wen@comcast.com
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