Internet DRAFT - draft-ietf-idr-next-hop-capability
draft-ietf-idr-next-hop-capability
Network Working Group B. Decraene
Internet-Draft Orange
Updates: 6790 (if approved) K. Kompella
Intended status: Standards Track Juniper Networks, Inc.
Expires: 10 December 2022 W. Henderickx
Nokia
8 June 2022
BGP Next-Hop dependent capabilities
draft-ietf-idr-next-hop-capability-08
Abstract
RFC 5492 advertises the capabilities of the BGP peer. When the BGP
peer is not the same as the BGP Next-Hop, it is useful to also be
able to advertise the capability of the BGP Next-Hop, in particular
to advertise forwarding plane features. This document defines a
mechanism to advertise such BGP Next Hop dependent Capabilities.
This document defines a new BGP non-transitive attribute to carry
Next-Hop Capabilities. This attribute is guaranteed to be deleted or
updated when the BGP Next Hop is changed, in order to reflect the
capabilities of the new BGP Next-Hop.
This document also defines a Next-Hop capability to advertise the
ability to process the MPLS Entropy Label as an egress LSR for all
NLRI advertised in the BGP UPDATE. It updates RFC 6790 with regard
to this BGP signaling.
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 10 December 2022.
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Copyright Notice
Copyright (c) 2022 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 Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. BGP Next-Hop dependent Capabilities Attribute . . . . . . . . 3
3.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Attribute Operation . . . . . . . . . . . . . . . . . . . 4
3.3. Interpreting received Capability . . . . . . . . . . . . 5
3.4. Attribute Error Handling . . . . . . . . . . . . . . . . 5
3.5. Network operation considerations . . . . . . . . . . . . 6
4. Entropy Label Next-Hop dependent Capability . . . . . . . . . 6
4.1. Readable Label Depth . . . . . . . . . . . . . . . . . . 7
4.2. Entropy Label Next-Hop Capability error handling . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5.1. Next-Hop Capabilities Attribute . . . . . . . . . . . . . 9
5.2. Next-Hop Capability registry . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Changes / Author Notes . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
[RFC5492] advertises the capabilities of the BGP peer. When the BGP
peer is not the same as the BGP Next-Hop, it is useful to also be
able to advertise the capability of the BGP Next-Hop, in particular
to advertise forwarding plane features. This document defines a
mechanism to advertise such BGP Next Hop Capabilities.
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This document defines a new BGP non-transitive attribute to carry
Next-Hop Capabilities. This attribute is guaranteed to be deleted or
updated when the BGP Next Hop is changed, in order to reflect the
capabilities of the new BGP Next-Hop. Hence it allows advertising
capabilities which are dependent of the BGP Next-Hop.
This attribute advertises the capabilities of the BGP Next-Hop for
the NLRI advertised in the same BGP update. A BGP Next-Hop may
advertise different capabilities for different set of NLRI.
This document also defines a first application to advertise the
capability to handle the MPLS Entropy Label defined in [RFC6790].
Note that RFC 6790 had originally defined a BGP attribute for this
but it has been latter deprecated in [RFC7447].
2. 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.
3. BGP Next-Hop dependent Capabilities Attribute
3.1. Encoding
The BGP Next-Hop dependent Capabilities Attribute is an optional,
non-transitive BGP Attribute, of value TBD1. The attribute consists
of a set of Next-Hop Capabilities.
The inclusion of a Next-Hop Capability "X" in a BGP UPDATE message,
indicates that the BGP Next-Hop, encoded in either the NEXT_HOP
attribute defined in [RFC4271] or the Network Address of Next Hop
field of the MP_REACH_NLRI attribute defined in [RFC4760], supports
the capability "X" for the NLRI advertised in this BGP UPDATE.
This document does not make a distinction between these two Next-Hop
fields and uses the term 'BGP Next-Hop' to refer to whichever one is
used in a given BGP UPDATE message.
A Next-Hop Capability is a triple (Capability Code, Capability
Length, Capability Value) aka a TLV:
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+------------------------------+
| Capability Code (2 octets) |
+------------------------------+
| Capability Length (2 octets) |
+------------------------------+
| Capability Value (variable) |
~ ~
+------------------------------+
Figure 1: BGP Next-Hop Capability
Capability Code: a two-octets unsigned binary integer which indicates
the type of "Next-Hop Capability" advertised and unambiguously
identifies an individual capability.
Capability Length: a two-octets unsigned binary integer which
indicates the length, in octets, of the Capability Value field. A
length of 0 indicates that no Capability Value field is present.
Capability Value: a variable-length field. It is interpreted
according to the value of the Capability Code.
BGP speakers SHOULD NOT include more than one instance of a Next-Hop
capability with the same Capability Code, Capability Length, and
Capability Value. Note, however, that processing of multiple
instances of such capability does not require special handling, as
additional instances do not change the meaning of the announced
capability; thus, a BGP speaker MUST be prepared to accept such
multiple instances.
BGP speakers MAY include more than one instance of a capability (as
identified by the Capability Code) with non-zero Capability Length
field, but with different Capability Value and either the same or
different Capability Length. Processing of these capability
instances is specific to the Capability Code and MUST be described in
the document introducing the new capability.
3.2. Attribute Operation
The BGP Next-Hop dependent Capabilities attribute being non-
transitive, as per [RFC4271], a BGP speaker which does not understand
it will quietly ignore it and not pass it along to other BGP peers.
A BGP speaker that understands the BGP Next-Hop dependent
Capabilities Attribute and does not change the BGP Next-Hop, SHOULD
NOT change the BGP Next-Hop dependent Capabilities Attribute and
SHOULD pass the attribute unchanged along to other BGP peers.
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A BGP speaker that understands the BGP Next-Hop dependent
Capabilities Attribute and changes the BGP Next-Hop, MUST remove or
update the received BGP Next-Hop dependent Capabilities Attribute
before propagating the BGP UPDATE to other BGP peers. If the
capability is not removed, it MUST be updated to only advertise the
capabilities of the new BGP Next-Hop for these NLRIs. An
implementation MAY allow, by configuration, to not advertise some of
the capabilities of a BGP Next-Hop. If a received capability is
unknown, it can't be updated hence unknown capabilities MUST be
removed when the BGP Next-Hop is changed.
The BGP Next-Hop Capability Code MUST reflect the capability of the
router indicated in the BGP Next-Hop, for the NLRI advertised in the
BGP UPDATE. If a BGP speaker sets the BGP Next-Hop to an address of
a different router, it MUST NOT advertise a BGP Next-Hop Capability
not supported by this router for these NLRI.
3.3. Interpreting received Capability
A BGP speaker receiving a BGP Next-Hop Capability Code that it
supports behave as defined in the document defining this Capability
Code. A BGP speaker receiving a BGP Next-Hop Capability Code that it
does not support MUST ignore this BGP Next-Hop Capability Code. In
particular, this MUST NOT be handled as an error. In both cases, the
BGP speaker MUST examine the remaining BGP Next-Hop Capability
Code(s) that may be present in the BGP Next-Hop Capabilities
Attribute.
The presence of a Next-Hop Capability SHOULD NOT influence route
selection or route preference, unless tunneling is used to reach the
BGP Next-Hop or the selected route has been learnt from EBGP (i.e.
the Next-Hop is in a different AS). Indeed, it is in general
impossible for a node to know that all BGP routers of the Autonomous
System (AS) will understand a given Next-Hop Capability; and having
different routers, within an AS, use a different preference for a
route, may result in forwarding loops if tunnelling is not used to
reach the BGP Next-Hop.
3.4. Attribute Error Handling
A BGP Next-Hop dependent Capabilities Attribute is considered
malformed if the length of the Attribute is not equal to the sum of
all (BGP Next-Hop dependent Capability Length +4) of the capabilities
carried in this attribute. Note that "4" is the length of the fields
"Type" and "Length" of each BGP Next Hop dependent Capability, as the
capability length only account for the length of the Value field.
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A BGP UPDATE message with a malformed BGP Next-Hop dependent
Capabilities Attribute SHALL be handled using the approach of
"attribute discard" defined in [RFC7606].
Unknown Next-Hop Capabilities Codes MUST NOT be considered as an
error.
A document that specifies a new Next-Hop Capability SHOULD provide
specifics regarding what constitutes an error for that Next-Hop
Capability.
If a Next-Hop dependent Capability is malformed, this Capability MUST
be ignored and removed. Others Next-Hop Capabilities MUST be
processed as usual.
3.5. Network operation considerations
In the corner case where multiple nodes use the same IP address as
their BGP Next-Hop, aka anycast nodes as described in [RFC4786], a
BGP speaker MUST NOT advertise a given Next-Hop Capability unless all
nodes sharing this same IP address support this Next-Hop Capability.
The network operator operating those anycast nodes is responsible for
enforcing that an anycast node does not advertise a BGP Next-Hop
capability not supported by all nodes advertising this anycast
address. This can be performed by using anycast nodes sharing the
same capabilities or by filtering the BGP Next-Hop Capabilities which
are not shared by all anycast nodes.
For security considerations, a network operator may want to filter
the BGP Next-Hop capabilities advertised from or to external
Autonomous Systems on a per capability, capability type or attribute
basis.
4. Entropy Label Next-Hop dependent Capability
The Entropy Label Next-Hop Capability has type code 1 and a length of
0 or 1 octet.
The inclusion of the "Entropy Label" Next-Hop Capability indicates
that the BGP Next-Hop can be sent packets, for all routes indicated
in the NRLI, with a MPLS entropy label (ELI, EL) added immediately
after the label stack advertised with the NLRI.
On the receiving side, suppose BGP speaker S has determined that
packet P is to be forwarded according to BGP route R, where R is a
route of one of the labeled address families. And suppose that L is
the label stack embedded in the NLRI of route R. Then to forward
packet P according to route R, S either replaces P's top label with
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L, or else pushes L onto the MPLS label stack. If the EL-Capability
is advertised in the BGP UPDATE advertising this route R, S knows
that it may safely place the ELI and an EL on the label stack
immediately beneath L.
A BGP speaker S that sends an UPDATE with the BGP Next-Hop "NH" MAY
include the Entropy Label Next-Hop Capability only if the NLRI are
labelled and for all the NLRI in the BGP UPDATE, either of the
following is true:
* Egress case: NH is the egress of the LSP advertised with the NLRI
and its capable of handling the ELI during the lookup of the MPLS
top label.
* Transit LSR case: NH is a transit LSR for the LSP advertised with
the NLRI (i.e. NH swaps one of the label advertised in the NLRI)
and next downstream BGP Next-Hop(s) has(have) advertised the
Entropy Label Next-Hop Capability (or a similar capability
signalled by protocol P if the route is redistributed, by NH, from
protocol P into BGP).
4.1. Readable Label Depth
When stacked LSPs are used and a LSR nests LSP(s) inside this BGP
signalled LSP, its useful for the ingress LSRs to know how many
labels the BGP Next-Hop and its downstream LSR(s) may read when load-
balancing based on the Entropy Label. In other words, how many
labels the ingress LER may push, before pushing an entropy label that
will be seen by the BGP Next-Hop and its downstream LSR(s).
This maximum number of labels is called the Readable Label Depth
(RLD) of the LSP(s). It is related, yet different, to the RLD of an
node which is defined in [RFC8662]
The RLD of the LSP(s) advertised in the NLRI, may be advertised in
the first octet of value field of the Entropy Label Next-Hop
Capability. This value field is optional. If present, the value
field is a one-octet unsigned binary integer which indicates the
maximum Readable Label Depth (RLD) of the LSP(s) advertised in the
NLRI. In other words, this is the maximum number of MPLS labels that
may be pushed by the ingress, before pushing the ELI, EL labels,
where the BGP Next-Hop and its downstream LSR(s) are capable of
performing load-balancing based on the entropy label.
S SHOULD advertise a RLD of:
* If S is the egress of the LSP(s) advertised in the NLRI: its own
local RLD;
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* If S is propagating in BGP a route received in BGP: the minimum
of:
- its own node RLD;
- the RLD of the LSP from itself to the BGP NEXT_HOP of its
received route minus (Number of Labels in the received NLRI -
Number of Labels in the sent NLRI);
- 0 if a RLD is not present in its received routes or the RLD in
the received BGP route minus (Number of Labels in the received
NLRI - Number of Labels in the sent NLRI).
- Note that the first term represents the limitation of the new
BGP NEXT_HOP (S), the second term the contribution of the
LSR(s) between the new BGP NEXT_HOP (S) and the old (received)
BGP NEXT_HOP (S'), the third term represent the contribution
from the old BGP NEXT_HOP (S') toward the egress.
* If S is propagating in BGP a route received in protocol X: the
minimum of:
- its own node RLD;
- the minimum of thg RLD(s) in the received protocol X to reach
the NLRI(s).
255 is a reserved value.
Note that the local RLD is meant as a node value. If a router has
multiple line cards with different capabilities, the router SHOULD
advertise the smallest one. However, a router MAY choose to only
consider the line cards that may be used by the BGP routers receiving
the ELC. e.g. if the ELC is advertised over an EBGP session with peer
A, a router MAY consider only the line cards connected to peer A.
Advertisement of the RLD is optional. When used, changes in IGP
routing may trigger BGP re-advertisement and hence will increase BGP
churn. If the RLD is decreased, it SHOULD be readvertised
immediatly. If the RLD is increased readvertisement MAY be delayed.
We note however that labelled BGP routes are typically not advertised
outside of an administrative domain hence the churn would be limited
to this administrative domain.
4.2. Entropy Label Next-Hop Capability error handling
If the Entropy Label Next-Hop Capability is present more than once,
it MUST be considered as received once with a length of 0.
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If the Entropy Label Next-Hop Capability is received with a length
other than 0 or 1, it is not considered malformed, and its semantics
are exactly the same as if it had a length of 1. In other words,
additional octets MUST be ignored. This allows for the graceful
addition of future extensions.
5. IANA Considerations
5.1. Next-Hop Capabilities Attribute
IANA is requested to allocate a new Path Attribute, called "Next-Hop
Capabilities", type Code TBD1, from the "BGP Path Attributes"
registry.
5.2. Next-Hop Capability registry
The IANA is requested to create and maintain a registry entitled "BGP
Next-Hop Capabilities".
The registration policies [RFC8126] for this registry are:
0 Reserved
1-63 IETF Review
64-65534 First Come First Served
65535 Reserved
IANA is requested to make the following initial assignments:
Registry Name: Next-Hop Capability.
Value Meaning Reference
---------- ---------------------------------------- ---------
0 Reserved (not to be allocated) This document
1 Entropy Label This document
2-65534 Unassigned
65535 Reserved for future registry extension This document
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6. Security Considerations
This document does not introduce new security vulnerabilities in BGP.
Specifically, an operator who is relying on the information carried
in BGP must have a transitive trust relationship back to the source
of the information. Specifying the mechanism(s) to provide such a
relationship is beyond the scope of this document. Please refer to
the Security Considerations section of [RFC4271] for security
mechanisms applicable to BGP.
As this attribute is removed when the BGP Next-Hop is changed, the
source of the information is the router which IP address is indicated
in the BGP Next-Hop. Such Next-Hop is typically either within the AS
when a BGP Next-Hop Self policy is configured, or in the neighboring
AS with which an interconnection and an EBGP has been established.
If this neighboring AS is not trusted with regards to information
carried in the BGP attribute, or carried in a specific capability,
this attribute or specific capability should be removed when
received. Note that in some cases, this Next-Hop may advertise
information based on information it has received from its own
downstream BGP Next-Hop, hence the transitive trust relationship. If
the underlying transport between both ASes is not trusted, BGP
transport should be protected for integrity and authentification.
The advertisement of BGP Next-Hop capabilities to EBGP peers may
disclose, to the peer AS, some capabilities of the BGP node and may
help fingerprinting its hardware model and software version. This
may be mitigated by filtering the capability advertised to EBGP
peers.
Security of the Entropy Label capability advertisement is unchanged
compared to [RFC6790] which originally defined this signaling.
7. Acknowledgement
The Entropy Label Next-Hop Capability defined in this document is
based on the ELC BGP attribute defined in section 5.2 of [RFC6790].
The authors wish to thank John Scudder for the discussions on this
topic and Eric Rosen for his in-depth review of this document.
The authors wish to thank Jie Dong and Robert Raszuk for their review
and comments.
8. References
8.1. Normative References
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[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>.
[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>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
8.2. Informative References
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <https://www.rfc-editor.org/info/rfc4786>.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.
[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>.
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[RFC8662] Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
Shakir, R., and J. Tantsura, "Entropy Label for Source
Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
DOI 10.17487/RFC8662, December 2019,
<https://www.rfc-editor.org/info/rfc8662>.
Appendix A. Changes / Author Notes
[RFC Editor: Please remove this section before publication]
Changes -01:
* Capability code and length encoded over 2 octets (from one). IANA
registry is now mainly FCFS.
* Addition of section "Network operation considerations", in
particular to discuss anycast nodes.
* Enhanced Security consideration (capability advertised to external
ASes).
* Editorial changes and typo corrections.
Changes -02: No change. Refresh only.
Changes -03: Addition of the optional Readable Label Depth.
Changes -04: Update to security section, following discussion on the
IDR mailing list.
Changes -05 to -08: No change. Refresh only.
Authors' Addresses
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
Kireeti Kompella
Juniper Networks, Inc.
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
United States of America
Email: kireeti.kompella@gmail.com
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Wim Henderickx
Nokia
Copernicuslaan 50
95134 Antwerp 2018
Belgium
Email: wim.henderickx@nokia.com
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