Entropy Label IS-IS: Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS
How IS-IS routers announce whether they can handle entropy labels and how deep into a label stack they can read.
Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 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>.
@misc{rfc9088,
series = {Request for Comments},
number = 9088,
howpublished = {RFC 9088},
publisher = {RFC Editor},
doi = {10.17487/RFC9088},
url = {https://www.rfc-editor.org/info/rfc9088},
author = {X. Xu and S. Kini and P. Psenak and C. Filsfils and S. Litkowski and M. Bocci},
title = {{Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS}},
pagetotal = 7,
year = 2021,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) X. Xu
Request for Comments: 9088 Capitalonline
Category: Standards Track S. Kini
ISSN: 2070-1721
P. Psenak
C. Filsfils
S. Litkowski
Cisco Systems, Inc.
M. Bocci
Nokia
August 2021
Signaling Entropy Label Capability and Entropy Readable Label Depth
Using IS-IS
Abstract
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
IS-IS and Border Gateway Protocol - Link State (BGP-LS).
Status of This Memo
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/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) 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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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
1. Introduction
[RFC6790] 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 IS-IS [RFC8667]. This
document defines a mechanism to signal the ELC using IS-IS.
In cases where Segment Routing (SR) is used with the MPLS data plane
(e.g., SR-MPLS [RFC8660]), 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
[RFC8662], may be used by ingress LSRs to determine the position of
the EL label in the stack, and whether it's necessary to insert
multiple ELs at different positions in the label stack. This
document defines a mechanism to signal the ERLD using IS-IS.
2. Terminology
This memo makes use of the 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, in some cases it is
advantageous to associate and advertise the ELC with a prefix. In a
multi-area network, routers may not know the identity of the prefix
originator in a remote area or may not know the capabilities of such
originator. Similarly, in a multi-domain network, the identity of
the prefix originator and its capabilities may not be known to the
ingress LSR.
Bit 3 in the Prefix Attribute Flags [RFC7794] is used as the ELC Flag
(E-Flag), as shown in Figure 1. If a router has multiple interfaces,
the router MUST NOT announce the ELC for any local host prefixes
unless all of its interfaces are capable of processing ELs. 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) - Set for local host prefix of the originating
node if it supports ELC on all interfaces.
The ELC signaling MUST be preserved when a router propagates a prefix
between IS-IS levels [RFC5302].
When redistributing a prefix between two IS-IS protocol instances or
redistributing from another protocol to an IS-IS protocol instance, a
router SHOULD preserve the ELC signaling for that prefix if it
exists. The exact mechanism used to exchange ELC between protocol
instances running on an Autonomous System Border Router is outside of
the scope of this document.
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. An MSD-Type code 2 has been
assigned by IANA for ERLD-MSD. The MSD-Value field is set to the
ERLD in the range between 0 to 255. The scope of the advertisement
depends on the application. 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 its
interfaces.
The absence of ERLD-MSD advertisements indicates only that the
advertising node does not support advertisement of this capability.
The considerations for advertising the ERLD are specified in
[RFC8662].
If the ERLD-MSD type is received in the Link MSD sub-TLV, it MUST be
ignored.
5. Signaling ELC and ERLD in BGP-LS
The IS-IS extensions defined in this document can be advertised via
BGP-LS (distribution of Link-State and TE information using BGP)
[RFC7752] using existing BGP-LS TLVs.
The ELC is advertised using the Prefix Attribute Flags TLV as defined
in [RFC9085].
The ERLD-MSD is advertised using the Node MSD TLV as defined in
[RFC8814].
6. IANA Considerations
IANA has completed the following actions for this document:
* Bit 3 in the "Bit Values for Prefix Attribute Flags Sub-TLV"
registry has been assigned to the ELC Flag. IANA has updated the
registry to reflect the name used in this document: ELC Flag
(E-Flag).
* Type 2 in the "IGP MSD-Types" registry has been assigned for the
ERLD-MSD. IANA has updated the registry to reflect the name used
in this document: ERLD-MSD.
7. Security Considerations
This document specifies the ability to advertise additional node
capabilities using IS-IS and BGP-LS. As such, the security
considerations as described in [RFC7752], [RFC7794], [RFC7981],
[RFC8491], [RFC8662], [RFC8814], and [RFC9085] 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 MPLS traffic being discarded on the egress node.
Incorrectly setting the ERLD value may lead to poor or no load-
balancing of the MPLS traffic.
8. References
8.1. 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>.
[RFC5302] Li, T., Smit, H., and T. Przygienda, "Domain-Wide Prefix
Distribution with Two-Level IS-IS", RFC 5302,
DOI 10.17487/RFC5302, October 2008,
<https://www.rfc-editor.org/info/rfc5302>.
[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>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., 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>.
[RFC7794] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
March 2016, <https://www.rfc-editor.org/info/rfc7794>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[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>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
[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>.
[RFC8814] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
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] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
H., 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>.
8.2. Informative References
[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 would like to thank 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.
Contributors
The following people contributed to the content of this document and
should be considered as coauthors:
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
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
AI ELI5
On this page
§1Why Signal Entropy Labels in IS-IS
RFC 6790 describes how to load-balance MPLS traffic flows using Entropy Labels (EL). It also introduces the Entropy Label Capability (ELC) and defines how to signal it with MPLS signaling protocols. An ingress Label Switching Router (LSR) cannot insert ELs into packets for a given Label Switched Path unless the egress LSR has indicated that it can process them.
Labels can now also be signaled through link-state IGPs such as IS-IS (RFC 8667). This document therefore defines how to signal the ELC using IS-IS.
When Segment Routing is used with the MPLS data plane (SR-MPLS, RFC 8660), ingress LSRs also benefit from knowing each router's Entropy Readable Label Depth (ERLD), defined in RFC 8662. It helps them find where the EL belongs in the stack, and whether several ELs at different positions are needed.
Routers can spread traffic over many paths if packets carry a random tag. Before using the tag, the sender needs to know the receiver understands it, so this document defines a way to announce that.
§3Advertising ELC: The E-Flag
ELC is really a property of a node, but the RFC explains that it is often handy to attach it to a prefix. In a multi-area or multi-domain network, an ingress router may not know who originated a remote prefix or what that originator can do.
The signal is bit 3 of the Prefix Attribute Flags (RFC 7794), used as the ELC Flag, or E-Flag. It is set on a local host prefix of the originating node when the node supports ELC on all interfaces.
- A router with several interfaces
MUST NOTannounce ELC for local host prefixes unless all of its interfaces can process ELs. - If all interfaces can, it
SHOULDset ELC on every local host prefix it advertises in IS-IS. - ELC signaling
MUSTbe preserved when a router propagates a prefix between IS-IS levels (RFC 5302). - When redistributing a prefix between IS-IS instances, or from another protocol into IS-IS, a router
SHOULDpreserve the ELC signaling if it exists. How ELC is exchanged between protocol instances on an Autonomous System Border Router is out of scope.
A router raises a small flag saying it understands the random tags, but only if every one of its doors can handle them. The flag must stay raised as the news passes along.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
X | X | 1 bits | Bit 0, as defined in RFC 7794. | |
R | R | 1 bits | Bit 1, as defined in RFC 7794. | |
N | N | 1 bits | Bit 2, as defined in RFC 7794. | |
E | E | 1 bits | bit 3 | ELC Flag (bit 3): set when the originating node supports ELC on all interfaces. |
REST | Other bits | 4 bits | Remaining flag bits, not shown in the figure. |
§4Advertising ERLD: The ERLD-MSD
To advertise a router's ERLD, the RFC defines a new MSD-Type (RFC 8491) called ERLD-MSD. IANA assigned MSD-Type code 2 to it. The MSD-Value field carries the ERLD, in the range 0 to 255, and the scope of the advertisement depends on the application.
If a router has several interfaces with different abilities to read the label stack, it MUST advertise the smallest value found across all of them.
The absence of an ERLD-MSD advertisement only means that the node does not support advertising this capability. It says nothing about the actual depth. If ERLD-MSD shows up in the Link MSD sub-TLV, it MUST be ignored. Other considerations for advertising ERLD are in RFC 8662.
The router also tells others how many labels deep it can peek into a stack. If its doors differ, it reports the smallest number so nobody is let down.
§5Carrying the Signals in BGP-LS
The IS-IS extensions can be advertised through BGP-LS (RFC 7752), which distributes link-state and traffic engineering information using BGP, by reusing existing BGP-LS TLVs. No new BGP-LS TLVs are defined.
The same news can be passed to far-away systems using an existing message system, with no new kind of message needed.
§6IANA Registry Entries
IANA completed two actions. Bit 3 in the "Bit Values for Prefix Attribute Flags Sub-TLV" registry was assigned to the ELC Flag, and the registry was updated to use the name ELC Flag (E-Flag). Type 2 in the "IGP MSD-Types" registry was assigned to the ERLD-MSD, and the registry was updated to use the name ERLD-MSD.
The official list of numbers now records which flag and which type number mean these two announcements.
§7Security Considerations
The document adds node capabilities to IS-IS and BGP-LS, so the security considerations of RFC 7752, RFC 7794, RFC 7981, RFC 8491, RFC 8662, RFC 8814 and RFC 9085 apply.
Incorrectly setting the E-Flag during origination, propagation or redistribution may lead to poor or no load-balancing of MPLS traffic, or to MPLS traffic being discarded on the egress node. Incorrectly setting the ERLD value may lead to poor or no load-balancing.
If a router raises the flag by mistake, packets may be spread badly or thrown away. A wrong depth number also hurts the spreading.