Multiprotocol Label Switching (MPLS) has defined a mechanism to load-balance traffic flows using Entropy Labels (EL). An ingress Label Switching Router (LSR) cannot insert ELs for packets going into a given Label Switched Path (LSP) unless an egress LSR has indicated via signaling that it has the capability to process ELs, referred to as the Entropy Label Capability (ELC), on that LSP. In addition, it would be useful for ingress LSRs to know each LSR's capability for reading the maximum label stack depth and performing EL-based load-balancing, referred to as Entropy Readable Label Depth (ERLD). This document defines a mechanism to signal these two capabilities using OSPFv2 and OSPFv3, and Border Gateway Protocol - Link State (BGP-LS).

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 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9089.

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[RFC 6790] describes a method to load-balance Multiprotocol Label Switching (MPLS) traffic flows using Entropy Labels (EL). It also introduces the concept of Entropy Label Capability (ELC) and defines the signaling of this capability via MPLS signaling protocols. Recently, mechanisms have been defined to signal labels via link-state Interior Gateway Protocols (IGP) such as OSPFv2 [RFC 8665] and OSPFv3 [RFC 8666]. This document defines a mechanism to signal the ELC using OSPFv2 and OSPFv3. In cases where Segment Routing (SR) is used with the MPLS data plane (e.g., SR-MPLS [RFC 8660]), it would be useful for ingress LSRs to know each intermediate LSR's capability of reading the maximum label stack depth and performing EL-based load-balancing. This capability, referred to as Entropy Readable Label Depth (ERLD) as defined in [RFC 8662], may be used by ingress LSRs to determine the position of the EL label in the stack, and whether it is necessary to insert multiple ELs at different positions in the label stack. This document defines a mechanism to signal the ERLD using OSPFv2 and OSPFv3.

This memo makes use of the terms defined in [RFC 6790] and [RFC 8662]. 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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here. The key word OSPF is used throughout the document to refer to both OSPFv2 and OSPFv3.

Even though ELC is a property of the node, in some cases it is advantageous to associate and advertise the ELC with a prefix. In multi-area networks, routers may not know the identity of the prefix originator in a remote area or may not know the capabilities of such an originator. Similarly, in a multi-domain network, the identity of the prefix originator and its capabilities may not be known to the ingress LSR. If a router has multiple interfaces, the router MUST NOT announce ELC unless all of its interfaces are capable of processing ELs. If the router supports ELs on all of its interfaces, it SHOULD advertise the ELC with every local host prefix it advertises in OSPF.

[RFC 7684] defines the OSPFv2 Extended Prefix TLV to advertise additional attributes associated with a prefix. The OSPFv2 Extended Prefix TLV includes a one-octet Flags field. A new flag in the Flags field is used to signal the ELC for the prefix:

0x20 - E-Flag (ELC Flag):

Set by the advertising router to indicate that the prefix originator is capable of processing ELs.

The ELC signaling MUST be preserved when an OSPF Area Border Router (ABR) distributes information between areas. To do so, an ABR MUST originate an OSPFv2 Extended Prefix Opaque Link State Advertisement (LSA) [RFC 7684] including the received ELC setting. When an OSPF Autonomous System Border Router (ASBR) redistributes a prefix from another instance of OSPF or from some other protocol, it SHOULD preserve the ELC signaling for the prefix if it exists. To do so, an ASBR SHOULD originate an Extended Prefix Opaque LSA [RFC 7684] including the ELC setting of the redistributed prefix. The flooding scope of the Extended Prefix Opaque LSA MUST match the flooding scope of the LSA that an ASBR originates as a result of the redistribution. The exact mechanism used to exchange ELC between protocol instances on an ASBR is outside of the scope of this document.

[RFC 5340] defines the OSPFv3 PrefixOptions field to indicate capabilities associated with a prefix. A new bit in the OSPFv3 PrefixOptions field is used to signal the ELC for the prefix:

0x40 - E-Flag (ELC Flag):

Set by the advertising router to indicate that the prefix originator is capable of processing ELs.

The ELC signaling MUST be preserved when an OSPFv3 Area Border Router (ABR) distributes information between areas. The setting of the ELC Flag in the Inter-Area-Prefix-LSA [RFC 5340] or in the Inter-Area-Prefix TLV [RFC 8362], generated by an ABR, MUST be the same as the value the ELC Flag associated with the prefix in the source area. When an OSPFv3 Autonomous System Border Router (ASBR) redistributes a prefix from another instance of OSPFv3 or from some other protocol, it SHOULD preserve the ELC signaling for the prefix if it exists. The setting of the ELC Flag in the AS-External-LSA, Not-So-Stubby Area LSA (NSSA-LSA) [RFC 5340], or in the External-Prefix TLV [RFC 8362], generated by an ASBR, MUST be the same as the value of the ELC Flag associated with the prefix in the source domain. The exact mechanism used to exchange ELC between protocol instances on the ASBR is outside of the scope of this document.

The ERLD is advertised in a Node Maximum SID Depth (MSD) TLV [RFC 8476] using the ERLD-MSD type defined in [RFC 9088]. If a router has multiple interfaces with different capabilities of reading the maximum label stack depth, the router MUST advertise the smallest value found across all of its interfaces. The absence of ERLD-MSD advertisements indicates only that the advertising node does not support advertisement of this capability. When the ERLD-MSD type is received in the OSPFv2 or OSPFv3 Link MSD sub-TLV [RFC 8476], it MUST be ignored. The considerations for advertising the ERLD are specified in [RFC 8662].

The OSPF extensions defined in this document can be advertised via BGP-LS (distribution of Link-State and TE information using BGP) [RFC 7752] using existing BGP-LS TLVs. The ELC is advertised using the Prefix Attribute Flags TLV as defined in [RFC 9085]. The ERLD-MSD is advertised using the Node MSD TLV as defined in [RFC 8814].

IANA has completed the following actions for this document:

• Flag 0x20 in the "OSPFv2 Extended Prefix TLV Flags" registry has been allocated to the E-Flag (ELC Flag).
• Bit 0x40 in the "OSPFv3 Prefix Options (8 bits)" registry has been allocated to the E-Flag (ELC Flag).

This document specifies the ability to advertise additional node capabilities using OSPF and BGP-LS. As such, the security considerations as described in [RFC 5340], [RFC 7684], [RFC 7752], [RFC 7770], [RFC 8476], [RFC 8662], [RFC 8814], and [RFC 9085] are applicable to this document. Incorrectly setting the E-Flag during origination, propagation, or redistribution may lead to poor or no load-balancing of the MPLS traffic or to the MPLS traffic being discarded on the egress node. Incorrectly setting of the ERLD value may lead to poor or no load-balancing of the MPLS traffic.

[RFC2119]

S. Bradner, "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>.

[RFC5340]

R. Coltun, D. Ferguson, J. Moy, and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.

[RFC6790]

K. Kompella, J. Drake, S. Amante, W. Henderickx, 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>.

[RFC7684]

P. Psenak, H. Gredler, R. Shakir, W. Henderickx, J. Tantsura, and A. Lindem, "OSPFv2 Prefix/Link Attribute Advertisement", RFC 7684, DOI 10.17487/RFC7684, November 2015,
<https://www.rfc-editor.org/info/rfc7684>.

[RFC7752]

H. Gredler, J. Medved, S. Previdi, A. Farrel, and S. Ray, "North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP", RFC 7752, DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.

[RFC7770]

A. Lindem, N. Shen, JP. Vasseur, R. Aggarwal, and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, February 2016,
<https://www.rfc-editor.org/info/rfc7770>.

[RFC8174]

B. Leiba, "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>.

[RFC8362]

A. Lindem, A. Roy, D. Goethals, V. Reddy Vallem, and F. Baker, "OSPFv3 Link State Advertisement (LSA) Extensibility", RFC 8362, DOI 10.17487/RFC8362, April 2018,
<https://www.rfc-editor.org/info/rfc8362>.

[RFC8476]

J. Tantsura, U. Chunduri, S. Aldrin, and P. Psenak, "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/info/rfc8476>.

[RFC8662]

S. Kini, K. Kompella, S. Sivabalan, S. Litkowski, R. Shakir, 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>.

[RFC8814]

J. Tantsura, U. Chunduri, K. Talaulikar, G. Mirsky, and N. Triantafillis, "Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State", RFC 8814, DOI 10.17487/RFC8814, August 2020,
<https://www.rfc-editor.org/info/rfc8814>.

[RFC9085]

S Previdi, K Talaulikar, C Filsfils, H Gredler, and M Chen, "Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment Routing", RFC 9085, DOI 10.17487/RFC9085, August 2021,
<https://www.rfc-editor.org/info/rfc9085>.

[RFC9088]

X Xu, S Kini, P Psenak, C Filsfils, S Litkowski, and M Bocci, "Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS", RFC 9088, DOI 10.17487/RFC9088, August 2021,
<https://www.rfc-editor.org/info/rfc9088>.

[RFC8660]

A. Bashandy, C. Filsfils, S. Previdi, B. Decraene, S. Litkowski, and R. Shakir, "Segment Routing with the MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.

[RFC8665]

P. Psenak, S. Previdi, C. Filsfils, H. Gredler, R. Shakir, W. Henderickx, and J. Tantsura, "OSPF Extensions for Segment Routing", RFC 8665, DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.

[RFC8666]

P. Psenak, and S. Previdi, "OSPFv3 Extensions for Segment Routing", RFC 8666, DOI 10.17487/RFC8666, December 2019,
<https://www.rfc-editor.org/info/rfc8666>.

The authors would like to thank Yimin Shen, George Swallow, Acee Lindem, Les Ginsberg, Ketan Talaulikar, Jeff Tantsura , Bruno Decraene, and Carlos Pignataro for their valuable comments.

The following people contributed to the content of this document and should be considered coauthors:

Gunter Van de Velde (editor)

Wim Henderickx

Keyur Patel

Xiaohu Xu

Sriganesh Kini

Peter Psenak

Clarence Filsfils

Stephane Litkowski

Matthew Bocci