BGP Next-next Hop Nodes
draft-wang-idr-next-next-hop-nodes-02
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Kevin Wang , Jeffrey Haas , Changwang Lin , Jeff Tantsura | ||
| Last updated | 2024-12-02 | ||
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| Intended RFC status | (None) | ||
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draft-wang-idr-next-next-hop-nodes-02
IDR K. Wang
Internet-Draft J. Haas
Intended status: Standards Track Juniper Networks
Expires: 5 June 2025 C. Lin
New H3C Technologies
J. Tantsura
Nviadia
2 December 2024
BGP Next-next Hop Nodes
draft-wang-idr-next-next-hop-nodes-02
Abstract
BGP speakers learn their next hop addresses for NLRI in RFC-4271 in
the NEXT_HOP field and in RFC-4760 in the "Network Address of Next
Hop" field. Under certain circumstances, it might be desirable for a
BGP speaker to know both the next hops and the next-next hops of NLRI
to make optimal forwarding decisions. One such example is global
load balancing (GLB) in a Clos network.
Draft-ietf-idr-entropy-label defines the "Next Hop Dependent
Characteristics Attribute" (NHC) which allows a BGP speaker to signal
the forwarding characteristics associated with a given next hop.
This document defines a new NHC characteristic, the Next-next Hop
Nodes (NNHN) characteristic, which can be used to advertise the next-
next hop nodes associated with a given next hop.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 June 2025.
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Copyright Notice
Copyright (c) 2024 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. BGP Next-next Hop Nodes (NNHN) Characteristic . . . . . . . . 4
2.1. Encoding NNHN . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Sending NNHN . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Receiving NNHN . . . . . . . . . . . . . . . . . . . . . 5
2.4. NNHN Error Handling . . . . . . . . . . . . . . . . . . . 6
3. Operational Considerations . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Alternative Solutions . . . . . . . . . . . . . . . 7
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
BGP speakers learn their next hop addresses for NLRI in [RFC4271] in
the NEXT_HOP field and in [RFC4760] in the "Network Address of Next
Hop" field. Under certain circumstances, it might be desirable for a
BGP speaker to know both the next hops and the next-next hops of NLRI
to make optimal forwarding decisions. One such example is the global
load balancing (GLB) in a Clos network
[I-D.cheng-rtgwg-adaptive-routing-framework].
When a route's ECMP has multiple next hops, packets forwarded using
that ECMP are hashed to the member next hops for load balancing
purposes. If one of the member next hop links is congested due to
uneven hashing, dynamic load balancing (DLB) allows the node to
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adjust the hashing so that the congestion on that link can be
mitigated. When all next hop link(s) are congested, DLB on the local
node will not help to mitigate the congestion. Such nodes will
require help from the previous hop(s) to shift the traffic towards
alternative nodes to mitigate such congestion. This process is
called global load balancing.
In a Clos network, a congested link will affect the load balancing
decisions of the previous layer nodes equally. Because of this, the
previous-previous layer nodes do not need to change their load
balancing decisions towards the previous layer nodes to mitigate this
link congestion. This means we only need to know the link congestion
status of the next-next hops of given BGP route in order to make GLB
decisions. The combined link quality of each next hop and its
corresponding next-next hops can be used as the feedback for DLB.
The purpose of this document is to provide a method for BGP to learn
the next-next hops - or more specifically, the next-next hop nodes.
When a next hop node has more than one next-next hops towards a next-
next hop node, DLB helps to balance the load between the multiple
next-next hops by locally adjusting the volume of traffic hashed over
a given ECMP member link. Thus, only the overall link congestion
between the next hop node and the next-next hop node is important for
GLB.
Note that the mechanism for detecting link congestion and
communicating them to the previous hop nodes is out of the scope of
this document.
This document defines a new NHC characteristic, the Next-next Hop
Nodes (NNHN) characteristic, for the BGP Next Hop Dependent
Characteristics Attribute (NHC) defined in
[I-D.ietf-idr-entropy-label]. A downstream BGP speaker can use the
NNHN to advertise the next-next hop nodes corresponding to the next
hop of an NLRI. This allows the upstream BGP speaker to learn the
next-next hop nodes corresponding to each of its next hop 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.
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2. BGP Next-next Hop Nodes (NNHN) Characteristic
[I-D.ietf-idr-entropy-label] defines NHC as a container for
characteristic TLVs. Next-next Hop Nodes is one such characteristic.
It specifies the next-next hop nodes corresponding to the next hop
field in the NHC.
2.1. Encoding NNHN
The NNHN TLV has the NHC characteristic code 2, as assigned in
Section 5 of [I-D.ietf-idr-entropy-label]. The NHC characteristic
length specifies the remaining number of octets in the NNHN TLV. The
NNHN characteristic format is shown in Figure 1:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Characteristic Code = 2 |Characteristic Length(variable)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next-hop BGP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Next-next-hop BGP IDs (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: NNHN Characteristic TLV Format
Next-hop BGP ID:
32-bit BGP Identifier of the next hop node attaching this NHC
characteristic.
Next-next-hop BGP IDs:
One or more 32-bit BGP Identifiers, each representing a next-next
hop node used by the next hop node for ECMP forwarding for the
NLRI in the BGP Update.
2.2. Sending NNHN
All procedures from Section 2.2 of [I-D.ietf-idr-entropy-label]
apply.
When a BGP speaker S has a BGP route R it wishes to advertise with
next hop self to its peer, it MAY choose to originate an NNHN
characteristic. The "Next-hop BGP ID" field MUST be set to the BGP
Identifier this BGP speaker uses with the peer.
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For all the ECMP paths of route R which are used for forwarding, the
BGP Identifiers of those BGP peers MUST be encoded as the "Next-next-
hop BGP IDs". When more than one paths are from the same BGP peer,
the characteristic MUST have only one BGP Identifier of that peer.
When there are more than one "Next-next-hop BGP IDs" in the
characteristic, they MUST be encoded in the numerically ascending
order treating the BGP Identifier as a network byte order encoded
32-bit unsigned integer.
An NNHN with no "Next-next-hop BGP IDs" MUST NOT be sent.
When a BGP speaker S has a BGP route R it wishes to advertise with
next hop self to its peer, it MUST NOT forward the NNHN
characteristic received from downstream peers. It either originates
its own NNHN characteristic as described above or does not send one.
When a BGP speaker S has a BGP route R it wishes to advertise with
the next hop that has not been set to self, it MUST NOT originate an
NNHN characteristic. However, if a NNHN characteristic has been
received for route R and passed the NHC validation as defined in
[I-D.ietf-idr-entropy-label], the NNHN characteristic SHOULD be
forwarded.
A BGP speaker MUST NOT include more than one instance of NNHN in an
NHC.
2.3. Receiving NNHN
All procedures from Section 2.3 of [I-D.ietf-idr-entropy-label]
apply.
When a BGP speaker wishes to enforce hop-by-hop eBGP propagation of
the NNHN, if the received NNHN characteristic's Next-hop BGP
Identifier does not match the BGP Identifier of the BGP speaker the
UPDATE was received from, it MUST be ignored and discarded.
The receiver of the NNHN characteristic MUST be able to handle any
order of the "Next-next-hop BGP IDs".
Duplicate BGP Identifiers in the "Next-next-hop BGP IDs" MUST be
silently ignored.
The details for the use of the NNHN characteristic for global load
balancing is out of the scope of this document.
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2.4. NNHN Error Handling
The NNHN characteristic length MUST be at least 8 and MUST be
divisible by 4, otherwise it is malformed. Malformed NNHN
characteristics MUST be discarded and SHOULD be logged.
If more than one instance of NNHN is included in an NHC, instances
beyond the first MUST be discarded and SHOULD be logged.
3. Operational Considerations
Since BGP Identifiers are used to identify the next-next hop nodes,
we need to make sure they are unique across the network where NNHN
characteristic is sent.
4. IANA Considerations
NHC Characteristic Code 2, has been assigned in Section 5 of
[I-D.ietf-idr-entropy-label], for the NNHN characteristic defined in
this document.
5. Security Considerations
Insertion of a syntactically valid but bogus NNHN characteristic by
an attacker could potentially make the forwarding behavior of the
route non-optimal.
6. References
6.1. 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>.
[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>.
[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>.
[I-D.ietf-idr-entropy-label]
Decraene, B., Scudder, J., Kompella, K., Satya, M. R.,
Wen, B., Wang, K., and S. Krier, "BGP Next Hop Dependent
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Characteristics Attribute", Work in Progress, Internet-
Draft, draft-ietf-idr-entropy-label-16, 26 September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
entropy-label-16>.
6.2. Informative References
[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>.
[I-D.cheng-rtgwg-adaptive-routing-framework]
Cheng, W., Lin, C., Wang, K., Ye, J., Zhuang, R., and P.
Huo, "Adaptive Routing Framework", Work in Progress,
Internet-Draft, draft-cheng-rtgwg-adaptive-routing-
framework-03, 20 October 2024,
<https://datatracker.ietf.org/doc/html/draft-cheng-rtgwg-
adaptive-routing-framework-03>.
Appendix A. Alternative Solutions
An alternative way to carry next-next hops is via a separate path
attribute. We evaluated both approaches and choose the NNHN
characteristic approach for several reasons:
* Next-next hops depend on next hops, this makes it naturally fit
into the existing NHC attribute.
* The next hop carried in the existing NHC attribute can help to
validate that the next-next hop nodes are indeed for the next hop
of the NLRI.
* Carrying next-next hop nodes via a seperate path attribute will
cost an additional attribute code, which is supposed to be
allocated for more generally used attributes.
Acknowledgements
TBD.
Contributors
TBD.
Authors' Addresses
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Kevin Wang
Juniper Networks
10 Technology Park Dr
Westford, MA 01886
United States of America
Email: kfwang@juniper.net
Jeff Haas
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
United States of America
Email: jhaas@juniper.net
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Jeff Tantsura
Nviadia
United States of America
Email: jefftant.ietf@gmail.com
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