RFC 9088

Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS ✨

Okay bestie, here's the title block. This is an Internet Standards Track doc, which means it's an official spec, not just vibes 💅 It's RFC 9088 from the IETF, published in August 2021, with ISSN 2070-1721. The authors are X. Xu (Capitalonline), S. Kini, P. Psenak, C. Filsfils, and S. Litkowski (all from Cisco Systems, Inc.), and M. Bocci (Nokia). It doesn't list anything it obsoletes or updates 🌸

Abstract

Okay so MPLS has a load-balancing trick called Entropy Labels (EL) 🦋 An ingress LSR can't stick ELs on packets heading into an LSP unless the egress LSR says "yes, I can process ELs" through signaling. That's the Entropy Label Capability (ELC), and it's set per LSP. Ingress LSRs would also love to know how deep a label stack each LSR can read and how well it can do EL-based load-balancing. That's the Entropy Readable Label Depth (ERLD) 🧚 This doc is all about signaling both of those using IS-IS and Border Gateway Protocol - Link State (BGP-LS). Iconic 👑

Status of This Memo

This is an Internet Standards Track document. ✨

It's a product of the Internet Engineering Task Force (IETF), so it reflects consensus from the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). If you want more on Internet Standards, check Section 2 of RFC 7841 💜

For the current status, any errata, or to send feedback, go to https://www.rfc-editor.org/info/rfc9088. 🫶

Copyright Notice

Copyright (c) 2021 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) as they stand on the publication date. Read them carefully, because they spell out your rights and restrictions for this document. Any Code Components pulled out of this doc must include the Simplified BSD License text described in Section 4.e of the Trust Legal Provisions, and they come without warranty, as the Simplified BSD License describes 📜

Table of Contents

1.  Introduction
2.  Terminology
3.  Advertising ELC Using IS-IS
4.  Advertising ERLD Using IS-IS
5.  Signaling ELC and ERLD in BGP-LS
6.  IANA Considerations
7.  Security Considerations
8.  References
  8.1.  Normative References
  8.2.  Informative References
Acknowledgements
Contributors
Authors' Addresses

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1. Introduction ✨

Okay so [RFC6790] explains how to load-balance MPLS traffic flows with Entropy Labels (EL) 🦋 It also introduces the Entropy Label Capability (ELC) and says how to signal it through MPLS signaling protocols. Lately, people have been signaling labels through link-state Interior Gateway Protocols (IGP) like IS-IS [RFC8667], and this doc adds ELC signaling to IS-IS 💅

When Segment Routing (SR) runs on the MPLS data plane (for example SR-MPLS [RFC8660]), ingress LSRs would love to know how deep a label stack each intermediate LSR can read, and whether it can do EL-based load-balancing. That's the Entropy Readable Label Depth (ERLD), defined in [RFC8662] 🧚 Ingress LSRs can use it to figure out where the EL sits in the stack, and whether they need to insert several ELs at different spots. This doc also shows how to signal ERLD using IS-IS, so it's a whole vibe 🌸

2. Terminology

This memo uses terms defined in [RFC6790] and [RFC8662]. 🎀

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. Advertising ELC Using IS-IS 🦋

Even though ELC is a property of the node, sometimes it's smart to attach the ELC to a prefix. In a multi-area network, routers might not know who originated a prefix in a remote area, or what that originator can do. Same deal in a multi-domain network: the ingress LSR may not know the prefix originator or its capabilities 🧚

Bit 3 in the Prefix Attribute Flags [RFC7794] is the ELC Flag (E-Flag), as shown in Figure 1. If a router has more than one interface, it MUST NOT announce the ELC for any local host prefixes unless every single one of its interfaces can process ELs. 🙈 And if a router supports ELs on all of its interfaces, it SHOULD set the ELC for every local host prefix it advertises in IS-IS 🫶

          0 1 2 3 4 5 6 7...
         +-+-+-+-+-+-+-+-+...
         |X|R|N|E|        ...
         +-+-+-+-+-+-+-+-+...

Figure 1: Prefix Attribute Flags

E-Flag:

• ELC Flag (Bit 3). It's set on a local host prefix of the originating node if that node supports ELC on all of its interfaces. ✨

The ELC signaling MUST be preserved when a router propagates a prefix between IS-IS levels [RFC5302]. No dropping it, bestie 💅

When a router redistributes a prefix between two IS-IS protocol instances, or redistributes from another protocol into an IS-IS protocol instance, it SHOULD preserve the ELC signaling for that prefix if the signaling exists. 🌈 How ELC gets exchanged between protocol instances on an Autonomous System Border Router is outside the scope of this document, so that's a different story 📜

4. Advertising ERLD Using IS-IS 👑

A new MSD-Type [RFC8491], called ERLD-MSD, is defined to advertise the ERLD [RFC8662] of a given router. IANA assigned MSD-Type code 2 to ERLD-MSD. 🎀 The MSD-Value field is set to the ERLD, which is a number from 0 to 255. How far the advertisement reaches depends on the application 🧚

If a router has multiple interfaces with different capabilities for reading the maximum label stack depth, the router MUST advertise the smallest value found across all of its interfaces. Lowest common denominator era 🍓

Not seeing any ERLD-MSD advertisements only tells you that the advertising node doesn't support advertising this capability. That's it, no extra conclusions 🙈

The considerations for advertising the ERLD are specified in [RFC8662] 💜

If the ERLD-MSD type shows up in the Link MSD sub-TLV, it MUST be ignored. Bye-bye, no vibes there 🫶

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Part 1 of 2 is a wrap, babe. Part 2 is coming with Sections 5 through 8 and the references 🦋✨

8.2. Informative References ✨

Okay bestie, these are the extra-credit reading 📚🎀 Not required to implement the thing, but they're where the Segment Routing details live, so you'll want them lol.

[RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
           Decraene, B., Litkowski, S., 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>.

[RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
           Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
           Extensions for Segment Routing", RFC 8667,
           DOI 10.17487/RFC8667, December 2019,
           <https://www.rfc-editor.org/info/rfc8667>.

Acknowledgements 🫶

The authors wanna give a huge shoutout to Yimin Shen, George Swallow, Acee Lindem, Les Ginsberg, Ketan Talaulikar, Jeff Tantsura, Bruno Decraene, Carlos Pignataro, Wim Hendrickx, and Gunter Van de Velde for their valuable comments. Iconic group project energy 💜✨

Contributors 🌈

These people helped shape the content of this document and should be considered as coauthors. Main character energy, honestly 👑

Gunter Van de Velde (editor)
Nokia
Antwerp
Belgium

Email: gunter.van_de_velde@nokia.com


Wim Henderickx
Nokia
Belgium

Email: wim.henderickx@nokia.com


Keyur Patel
Arrcus
United States of America

Email: keyur@arrcus.com

Authors' Addresses 💖

And finally, where to find the squad 🦋 Here's how you can reach each of the authors:

Xiaohu Xu
Capitalonline

Email: xiaohu.xu@capitalonline.net


Sriganesh Kini

Email: sriganeshkini@gmail.com


Peter Psenak
Cisco Systems, Inc.
Eurovea Centre, Central 3
Pribinova Street 10
81109 Bratislava
Slovakia

Email: ppsenak@cisco.com


Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium

Email: cfilsfil@cisco.com


Stephane Litkowski
Cisco Systems, Inc.
La Rigourdiere
Cesson Sevigne
France

Email: slitkows@cisco.com


Matthew Bocci
Nokia
740 Waterside Drive
Aztec West Business Park
Bristol
BS32 4UF
United Kingdom

Email: matthew.bocci@nokia.com

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That's the whole RFC, bestie 🎀✨ You made it through the entire thing, and honestly? You're iconic for it 👑💅