| LSR Working Group | L. Ginsberg |
| Internet-Draft | P. Wells |
| Updates: 5305 6232 (if approved) | Cisco Systems |
| Intended status: Standards Track | T. Li |
| Expires: December 24, 2020 | Arista Networks |
| T. Przygienda | |
| S. Hegde | |
| Juniper Networks, Inc. | |
| June 22, 2020 |
Invalid TLV Handling in IS-IS
draft-ietf-lsr-isis-invalid-tlv-02
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 which 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 insure that interoperability is maximized.
This document updates RFC5305 and RFC6232.
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.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 24, 2020.
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.
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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.
A corollary to ignoring unknown TLVs is having the validation of PDUs be independent from the validation of the TLVs contained in the PDU. PDUs which are valid must be accepted [ISO10589] even if an individual TLV contained within that PDU is invalid in some way (e.g., incorrect syntax, data value out of range, etc.).
The set of TLVs (and sub-TLVs) which 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 RFC 5305 (see Section 3.3) and RFC 6232 (see Section 3.4).
[RFC3563] established the IANA-managed IS-IS TLV Codepoints Registry for recording assigned TLV code points [TLV_CODEPOINTS]. The initial contents of this registry were based on [RFC3359].
The registry includes a set of columns indicating in which PDU types a given TLV is allowed:
IIH - TLV is allowed in Intermediate System to Intermediate System Hello (IIH) PDUs (Point-to-point and LAN)
LSP - TLV is allowed in Link State PDUs (LSP)
SNP - TLV is allowed in Sequence Number PDUs (SNP) (Partial Sequence Number PDUs (PSNP) and Complete Sequence Number PDUS (CSNP))
Purge - TLV is allowed in LSP Purges [RFC6233]
If "Y" is entered in a column it means the TLV is allowed in the corresponding PDU type.
If "N" is entered in a column it means the TLV is not allowed in the corresponding PDU type.
This section describes the correct behavior when a PDU is received which contains a TLV which is specified as disallowed in the TLV Codepoints Registry.
[ISO10589] defines the behavior required when a PDU is received containing a TLV which is "not recognised". It states (see Sections 9.3 - 9.13):
"Any codes in a received PDU that are not
recognised shall be ignored."
This is the model to be followed when a TLV is received which is disallowed. Therefore TLVs in a PDU (other than LSP purges) which are disallowed MUST be ignored and MUST NOT cause the PDU itself to be rejected by the receiving IS.
When purging LSPs, [ISO10589] recommends (but does not require) the body of the LSP (i.e., all TLVs) be removed before generating the purge. LSP purges which have TLVs in the body are accepted though any TLVs which are present are ignored.
When cryptographic authentication [RFC5304] was introduced, this looseness when processing received purges had to be addressed in order to prevent attackers from being able to initiate a purge without having access to the authentication key. [RFC5304] therefore imposed strict requirements on what TLVs were allowed in a purge (authentication only) and specified that:
"ISes MUST NOT accept purges that contain TLVs
other than the authentication TLV".
This behavior was extended by [RFC6232] which introduced the Purge Originator Identification (POI) TLV and [RFC6233] which added the "Purge" column to the TLV Codepoints registry to identify all the TLVs which are allowed in purges.
The behavior specified in [RFC5304] is not backwards compatible with the behavior defined by [ISO10589] and therefore can only be safely enabled when all nodes support cryptographic authentication. Similarly, the extensions defined by [RFC6233] are not compatible with the behavior defined in [RFC5304], therefore can only be safely enabled when all nodes support the extensions.
It is RECOMMENDED that implementations provide controls for the enablement of behaviors that are not backward compatible.
[RFC5305] introduced sub-TLVs, which are TLV tuples advertised within the body of a parent TLV. Registries associated with sub-TLVs are associated with the TLV Codepoints Registry and specify in which TLVs a given sub-TLV is allowed. Section 2 of [RFC5305] is updated by the following sentence:
"As with TLVs, it is required that sub-TLVs which
are disallowed MUST be ignored on receipt.".
The existing sentence in Section 2 of [RFC5305] :
"Unknown sub-TLVs are to be ignored and skipped upon
receipt."
is replaced by:
"Unknown sub-TLVs MUST be ignored and skipped upon
receipt."
An error was introduced by [RFC6232] when specifying in which PDUs the POI TLV is allowed. Section 3 of [RFC6232] stated:
"The POI TLV SHOULD be found in all purges and
MUST NOT be found in LSPs with a non-zero
Remaining Lifetime."
However, the IANA section of the same document stated:
"The additional values for this TLV should be
IIH:n, LSP:y, SNP:n, and Purge:y."
The correct setting for "LSP" is "n". This document updates [RFC6232] by correcting that error.
The correct format of a TLV and its associated sub-TLVs, if applicable, are defined in the document(s) which introduce each codepoint. The definition MUST include what action to take when the format/content of the TLV does not conform to the specification (e.g., "MUST be ignored on receipt"). When making use of the information encoded in a given TLV (or sub-TLV) receiving nodes MUST verify that the TLV conforms to the standard definition. This includes cases where the length of a TLV/sub-TLV is incorrect and/or cases where the value field does not conform to the defined restrictions.
However, the unit of flooding for the IS-IS Update process is an LSP. The presence of a TLV (or sub-TLV) with content which does not conform to the relevant specification MUST NOT cause the LSP itself to be rejected. Failure to follow this requirement will result in inconsistent LSP Databases on different nodes in the network which will compromise the correct operation of the protocol.
LSP Acceptance rules are specified in [ISO10589] . Acceptance rules for LSP purges are extended by [RFC5304] and and are further extended by [RFC6233].
[ISO10589] also specifies the behavior when an LSP is not accepted. This behavior is NOT altered by extensions to the LSP Acceptance rules i.e., regardless of the reason for the rejection of an LSP the Update process on the receiving router takes the same action.
IANA is requested to add this document as a reference for the TLV Codepoints Registry.
IANA is also requested to modify the entry for the Purge Originator Identification TLV in the TLV Codepoints Registry to be:
IIH:n, LSP:n, SNP:n, and Purge:y.
The reference field should be updated to point to this document.
As this document makes no changes to the protocol there are no new security issues introduced.
The clarifications discussed in this document are intended to make it less likely that implementations will incorrectly process received LSPs, thereby also making it less likely that a bad actor could exploit a faulty implementation.
Security concerns for IS-IS are discussed in [ISO10589], [RFC5304], and [RFC5310].
The authors would like to thank Alvaro Retana.
| [RFC3359] | Przygienda, T., "Reserved Type, Length and Value (TLV) Codepoints in Intermediate System to Intermediate System", RFC 3359, DOI 10.17487/RFC3359, August 2002. |