Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State
Internet Engineering Task Force (IETF) J. Tantsura
Request for Comments: 8814 Apstra, Inc.
Category: Standards Track U. Chunduri
ISSN: 2070-1721 Futurewei Technologies
Amazon Web Services
Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol -
This document defines a way for a Border Gateway Protocol - Link
State (BGP-LS) speaker to advertise multiple types of supported
Maximum SID Depths (MSDs) at node and/or link granularity.
Such advertisements allow entities (e.g., centralized controllers) to
determine whether a particular Segment Identifier (SID) stack can be
supported in a given network.
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
Copyright (c) 2020 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
1.1. Conventions Used in This Document
1.1.2. Requirements Language
2. Advertisement of MSD via BGP-LS
3. Node MSD TLV
4. Link MSD TLV
5. IANA Considerations
6. Manageability Considerations
7. Security Considerations
8.1. Normative References
8.2. Informative References
When Segment Routing (SR) [RFC8402] paths are computed by a
centralized controller, it is critical that the controller learns the
Maximum SID Depth (MSD) that can be imposed at each node/link on a
given SR path. This ensures that the Segment Identifier (SID) stack
depth of a computed path doesn't exceed the number of SIDs the node
is capable of imposing.
[RFC8664] defines how to signal MSD in the Path Computation Element
Protocol (PCEP). The OSPF and IS-IS extensions for the signaling of
MSD are defined in [RFC8476] and [RFC8491], respectively.
However, if PCEP is not supported/configured on the head-end of an SR
tunnel or a Binding-SID anchor node, and the controller does not
participate in IGP routing, it has no way of learning the MSD of
nodes and links. BGP-LS [RFC7752] defines a way to expose topology
and associated attributes and capabilities of the nodes in that
topology to a centralized controller.
This document defines extensions to BGP-LS to advertise one or more
types of MSDs at node and/or link granularity. Other types of MSDs
are known to be useful. For example, [OSPF-ELC] and [ISIS-ELC]
define Entropy Readable Label Depth (ERLD), which is used by a head-
end to insert an Entropy Label (EL) at a depth that can be read by
In the future, it is expected that new MSD-Types will be defined to
signal additional capabilities, e.g., ELs, SIDs that can be imposed
through recirculation, or SIDs associated with another data plane
such as IPv6. MSD advertisements may be useful even if SR itself is
not enabled. For example, in a non-SR MPLS network, MSD defines the
maximum label depth.
1.1. Conventions Used in This Document
MSD: Maximum SID Depth - the number of SIDs supported by a node or a
link on a node
PCE: Path Computation Element
PCEP: Path Computation Element Protocol
Show full document text