Invalid TLV Handling in IS-IS
RFC 8918
Internet Engineering Task Force (IETF) L. Ginsberg
Request for Comments: 8918 P. Wells
Updates: 5305, 6232 Cisco Systems
Category: Standards Track T. Li
ISSN: 2070-1721 Arista Networks
T. Przygienda
S. Hegde
Juniper Networks, Inc.
September 2020
Invalid TLV Handling in IS-IS
Abstract
The key to the extensibility of the Intermediate System to
Intermediate System (IS-IS) protocol has been the handling of
unsupported and/or invalid Type-Length-Value (TLV) tuples. Although
there are explicit statements in existing specifications, deployment
experience has shown that there are inconsistencies in the behavior
when a TLV that is disallowed in a particular Protocol Data Unit
(PDU) is received.
This document discusses such cases and makes the correct behavior
explicit in order to ensure that interoperability is maximized.
This document updates RFCs 5305 and 6232.
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/rfc8918.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction
1.1. Requirements Language
2. TLV Codepoints Registry
3. TLV Acceptance in PDUs
3.1. Handling of Disallowed TLVs in Received PDUs Other Than LSP
Purges
3.2. Special Handling of Disallowed TLVs in Received LSP Purges
3.3. Applicability to Sub-TLVs
3.4. Correction to POI "TLV Codepoints Registry" Entry
4. TLV Validation and LSP Acceptance
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The Intermediate System to Intermediate System (IS-IS) protocol
[ISO10589] utilizes Type-Length-Value (TLV) encoding for all content
in the body of Protocol Data Units (PDUs). New extensions to the
protocol are supported by defining new TLVs. In order to allow
protocol extensions to be deployed in a backwards compatible way, an
implementation is required to ignore TLVs that it does not
understand. This behavior is also applied to sub-TLVs [RFC5305],
which are contained within TLVs.
Also essential to the correct operation of the protocol is having the
validation of PDUs be independent from the validation of the TLVs
contained in the PDU. PDUs that are valid must be accepted
[ISO10589] even if an individual TLV contained within that PDU is not
understood or is invalid in some way (e.g., incorrect syntax, data
value out of range, etc.).
The set of TLVs (and sub-TLVs) that are allowed in each PDU type is
documented in the "TLV Codepoints Registry" established by [RFC3563]
and updated by [RFC6233] and [RFC7356].
This document is intended to clarify some aspects of existing
specifications and, thereby, reduce the occurrence of non-conformant
behavior seen in real-world deployments. Although behaviors
specified in existing protocol specifications are not changed, the
clarifications contained in this document serve as updates to
[RFC5305] (see Section 3.3) and [RFC6232] (see Section 3.4).
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
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