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/rfc9084.¶

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

Prefix attributes are advertised in OSPFv2 [RFC2328] using the Extended Prefix Opaque Link State Advertisement (LSA) [RFC7684] and in OSPFv3 [RFC5340] using the various Extended Prefix LSA types [RFC8362].¶

The procedures for identification of the originating router for a prefix in OSPF vary by the type of the prefix and, currently, it is not always possible to produce an accurate result. For intra-area prefixes, the originating router is identified by the Advertising Router field of the area-scoped LSA used for those prefix advertisements. However, for inter-area prefixes advertised by an Area Border Router (ABR), the Advertising Router field of their area-scoped LSAs is set to the ABR itself and the information about the router originating the prefix advertisement is lost in the process of prefix propagation across areas. For Autonomous System (AS) external prefixes, the originating router may be considered as the Autonomous System Border Router (ASBR) and is identified by the Advertising Router field of the AS-scoped LSA used. However, the actual originating router for the prefix may be a remote router outside the OSPF domain. Similarly, when an ABR performs translation of Not-So-Stubby Area (NSSA) [RFC3101] LSAs to AS-external LSAs, the information associated with the NSSA ASBR (or the router outside the OSPF domain) is not propagated across the OSPF domain.¶

While typically the originator of information in OSPF is identified by its OSPF Router ID, it does not necessarily represent a reachable address for the router since the OSPF Router ID is a 32-bit number that is unique in the OSPF domain. For OSPFv2, a common practice is to use one of the IPv4 addresses of the node (e.g., a loopback interface) as the OSPF Router ID. However, this cannot always be assumed and this approach does not apply to IPv6 addresses with OSPFv3. The IPv4/IPv6 Router Address, as respectively defined in [RFC3630] and [RFC5329] for OSPFv2 and OSPFv3, provides an address to reach the advertising router.¶

The primary use case for the extensions proposed in this document is to be able to identify the originator of a prefix in the network. In cases where multiple prefixes are advertised by a given router, it is also useful to be able to associate all these prefixes with a single router even when prefixes are advertised outside of the area in which they originated. It also helps to determine when the same prefix is being originated by multiple routers across areas.¶

This document proposes extensions to the OSPF protocol for the inclusion of information associated with the router originating the prefix along with the prefix advertisement. These extensions do not change the core OSPF route computation functionality. They provide useful information for topology analysis and traffic engineering, especially on a controller when this information is advertised as an attribute of the prefixes via mechanisms such as Border Gateway Protocol - Link State (BGP-LS) [RFC7752] [RFC9085].¶

2. Protocol Extensions

This document defines the Prefix Source OSPF Router-ID and the Prefix Source Router Address Sub-TLVs. They are used, respectively, to include the Router ID of, and a reachable address of, the router that originates the prefix as a prefix attribute.¶

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               Type            |              Length           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        OSPF Router ID                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               Type            |              Length           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |              Router Address (4 or 16 octets)                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3. Elements of Procedure

This section describes the procedure for the advertisement of the Prefix Source OSPF Router-ID and Prefix Source Router Address Sub-TLVs along with the prefix advertisement.¶

The OSPF Router ID of the Prefix Source OSPF Router-ID is set to the OSPF Router ID of the node originating the prefix in the OSPF domain.¶

If the originating node is advertising an OSPFv2 Router Address TLV [RFC3630] or an OSPFv3 Router IPv6 Address TLV [RFC5329], then the same address MUST be used in the Router Address field of the Prefix Source Router Address Sub-TLV. When the originating node is not advertising such an address, implementations can select a unique and reachable local address (for example, advertised with the N-Flag set [RFC7684] or N-bit set [RFC8362]) on the originating node to advertise in the Router Address field.¶

When an ABR generates inter-area prefix advertisements into its non-backbone areas corresponding to an inter-area prefix advertisement from the backbone area, the only way to determine the originating node information is based on the Prefix Source OSPF Router-ID and Prefix Source Router Address Sub-TLVs present in the inter-area prefix advertisement originated into the backbone area by an ABR from another non-backbone area. The ABR performs its prefix calculation to determine the set of nodes that contribute to ECMP paths for the prefix. It MUST only use the prefix originator information from this set of nodes. The ABR MUST NOT include the Prefix Source OSPF Router-ID or the Prefix Source Router Address Sub-TLVs when it is unable to determine the information for the originating nodes contributing ECMP paths.¶

Implementations may support the propagation of the originating node information along with a redistributed prefix into the OSPF domain from another routing domain. The details of such mechanisms are outside the scope of this document. Such implementations may also provide control on whether the Router Address in the Prefix Source Router Address Sub-TLV is set as the ASBR node address or as the address of the actual node outside the OSPF domain that owns the prefix.¶

When translating NSSA prefix advertisements [RFC3101] to AS external prefix advertisements, the NSSA ABR follows the same procedures as an ABR generating inter-area prefix advertisements for the propagation of the originating node information.¶

4. Security Considerations

Since this document extends the OSPFv2 Extended Prefix LSA, the security considerations for [RFC7684] are applicable. Similarly, since this document extends the OSPFv3 E-Intra-Area-Prefix-LSA, E-Inter-Area-Prefix-LSA, E-AS-External-LSA, and E-NSSA-LSA, the security considerations for [RFC8362] are applicable. The new sub-TLVs introduced in this document are optional and do not affect the OSPF route computation and therefore do not affect the security aspects of OSPF protocol operations.¶

A rogue node that can inject prefix advertisements may use the extensions introduced in this document to advertise bogus prefix source information.¶

5. Operational Considerations

Consideration should be given to the operational impact of the increase in the size of the OSPF Link-State Database as a result of the protocol extensions in this document. Based on deployment design and requirements, a subset of prefixes may be identified for which originating node information is required to be included in prefix advertisements.¶

The propagation of prefix source node information for prefix advertisements advertised across an OSPF area or domain boundaries will expose information outside of an area or domain where it would normally be hidden or abstracted by the base OSPF protocol. Based on deployment design and requirements, the propagation of node information across area or domain boundaries may be limited to a subset of prefixes in the ABRs or ASBRs, respectively.¶

The identification of the node that is originating a specific prefix in the network may aid in the debugging of issues related to prefix reachability within an OSPF network.¶

6. IANA Considerations

Per this document, IANA has allocated the following codepoints from the "OSPFv2 Extended Prefix TLV Sub-TLVs" registry under the "Open Shortest Path First v2 (OSPFv2) Parameters" registry.¶

Per this document, IANA has allocated the following codepoints from the "OSPFv3 Extended-LSA Sub-TLVs" registry under the "Open Shortest Path First v3 (OSPFv3) Parameters" registry.¶

Acknowledgement

Many thanks to Les Ginsberg for his suggestions on this document. Also, thanks to Jeff Tantsura, Rob Shakir, Gunter Van de Velde, Goethals Dirk, Smita Selot, Shaofu Peng, John E. Drake, and Baalajee S. for their review and valuable comments. The authors would also like to thank Alvaro Retana for his detailed review and suggestions for the improvement of this document.¶

Authors' Addresses