OSPF Prefix Originator Extensions
draft-ietf-lsr-ospf-prefix-originator-06

Abstract

This document defines OSPF extensions to include information associated with the node originating a prefix along with the prefix advertisement.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Protocol Extensions
• 2.1. Prefix Source Router-ID Sub-TLV
• 2.2. Prefix Originator Sub-TLV
• 3. Elements of Procedure
• 4. Security Considerations
• 5. IANA Considerations
• 6. Acknowledgement
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Inter-Area Topology Retrieval Process
• Appendix B. Special Considerations on Inter-Area Topology Retrieval
• Authors' Addresses

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 identification of the originating router for a prefix in OSPF varies by the type of the prefix and is currently not always possible. For intra-area prefixes, the originating router is identified by the advertising Router ID field of the area-scoped LSA used for those prefix advertisements. However, for the inter-area prefixes advertised by the Area Border Router (ABR), the advertising Router ID 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 this 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 ID 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 conveyed 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. The IPv4/IPv6 Router Address as defined in [RFC3630] and [RFC5329] for OSPFv2 and OSPFv3 respectively provide an address to reach that router.

The primary use case for the extensions proposed in this document is to be able to identify the originator of the 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 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].

Applications related to use of the prefix originating node information for topology reconstruction process on a controller and the associated limitations are described in Appendix A and Appendix B.

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 capitals, as shown here.

2. Protocol Extensions

This document defines the Prefix Source Router-ID and the Prefix Originator Sub-TLVs for inclusion of the Router ID and a reachable address information for the router originating the prefix as a prefix attribute.

2.1. Prefix Source Router-ID Sub-TLV

For OSPFv2, the Prefix Source Router-ID Sub-TLV is an optional Sub-TLV of the OSPFv2 Extended Prefix TLV [RFC7684]. For OSPFv3, the Prefix Source Router-ID Sub-TLV is an optional Sub-TLV of the Intra-Area-Prefix TLV, Inter-Area-Prefix TLV, and External-Prefix TLV [RFC8362] when originating either an IPv4 [RFC5838] or an IPv6 prefix advertisement.

The Prefix Source Router-ID Sub-TLV has the following format:

  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                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
        Figure 1: Prefix Source Router-ID Sub-TLV Format
 
 Where:  

• Type: 4 for OSPFv2 and 27 for OSPFv3
• Length: 4
• OSPF Router ID : the OSPF Router ID of the OSPF router that originated the prefix advertisement in the OSPF domain.

A prefix advertisement MAY include more than one Prefix Source Router-ID sub-TLV, one corresponding to each of the Equal-Cost Multi-Path (ECMP) nodes that originated the given prefix.

A received Prefix Source Router-ID Sub-TLV with OSPF Router ID set to 0 MUST be considered invalid and ignored. Additionally, reception of such Sub-TLV SHOULD be logged as an error (subject to rate-limiting).

2.2. Prefix Originator Sub-TLV

For OSPFv2, the Prefix Originator Sub-TLV is an optional Sub-TLV of the OSPFv2 Extended Prefix TLV [RFC7684]. For OSPFv3, the Prefix Originator Sub-TLV is an optional Sub-TLV of the Intra-Area-Prefix TLV, Inter-Area-Prefix TLV, and External-Prefix TLV [RFC8362] when originating either an IPv4 [RFC5838] or an IPv6 prefix advertisement.

The Prefix Originator Sub-TLV has the following format:

  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 octects)                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
        Figure 2: Prefix Originator Sub-TLV Format
 
 Where:  

• Type: TBD1 for OSPFv2 and TBD2 for OSPFv3
• Length: 4 or 16
• Router Address: A reachable IPv4 or IPv6 router address for the router that originated the IPv4 or IPv6 prefix advertisement. Such an address would be semantically equivalent to what may be advertised in the OSPFv2 Router Address TLV [RFC3630] or in the OSPFv3 Router IPv6 Address TLV [RFC5329].

A prefix advertisement MAY include more than one Prefix Originator sub-TLV, one corresponding to each of the Equal-Cost Multi-Path (ECMP) nodes that originated the given prefix.

A received Prefix Originator Sub-TLV that has an invalid length (not 4 or 16) or a Reachable Address containing an invalid IPv4 or IPv6 address (dependent on address family of the associated prefix) MUST be considered invalid and ignored. Additionally, reception of such Sub-TLV SHOULD be logged as an error (subject to rate-limiting).

[RFC7794] provides similar functionality for the Intermediate System to Intermediate System (IS-IS) protocol.

3. Elements of Procedure

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

The OSPF Router ID of the Prefix Source 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 that value is set in the Router Address field of the Prefix Originator Sub-TLV. When the orignating node is not advertising such an address, implementations MAY support mechanisms to determine a reachable address belonging to the originating node to set in the Router Address field. Such mechanisms are outside the scope of this document.

Implementations MAY support the selection of specific prefixes for which the originating node information needs to be included with their prefix advertisements.

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 Router-ID and Prefix Originator Sub-TLVs present in the inter-area prefix advertisement originated into the backbone area by an ABR for another non-backbone area. The ABR performs its prefix calculation to determine the set of nodes that contribute to the best prefix reachability. It MUST use the prefix originator information only from this set of nodes. The ABR MUST NOT include the Prefix Source Router-ID or the Prefix Originator Sub-TLVs when it is unable to determine the information of the best originating node.

Implementations MAY provide control on ABRs to selectively disable the propagation of the originating node information across area boundaries.

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 Originator Sub-TLV is set as the ABSR node address or as the address of the actual node outside the OSPF domain that owns the prefix.

When translating the NSSA prefix advertisements [RFC3101] to the 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.

5. IANA Considerations

This document requests IANA for the allocation of the codepoint from the "OSPFv2 Extended Prefix TLV Sub-TLVs" registry under the "Open Shortest Path First v2 (OSPFv2) Parameters" registry.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code  |               Description           |    IANA Allocation    |
| Point |                                     |        Status         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   4   |  Prefix Source Router-ID Sub-TLV    | early allocation done |
|  TBD1 |    Prefix Originator Sub-TLV        |       pending         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 3:  Code Points in OSPFv2 Extended Prefix TLV Sub-TLVs

This document requests IANA for the allocation of the codepoint from the "OSPFv3 Extended Prefix TLV Sub-TLVs" registry under the "Open Shortest Path First v3 (OSPFv3) Parameters" registry.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code  |               Description           |    IANA Allocation    |
| Point |                                     |        Status         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  27   |   Prefix Source Router-ID Sub-TLV   | early allocation done |
| TBD2  |    Prefix Originator Sub-TLV        |       pending         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 4:  Code Points in OSPFv3 Extended-LSA Sub-TLVs

6. Acknowledgement

Many thanks to Les Ginsberg for his suggestions on this draft. Also thanks to Jeff Tantsura, Rob Shakir, Gunter Van De Velde, Goethals Dirk, Smita Selot, Shaofu Peng, and John E Drake for their valuable comments.

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Inter-Area Topology Retrieval Process

When an IP SDN Controller receives BGP-LS [RFC7752] information, it should compare the prefix Network Layer Reachability Information (NLRI) that is included in the BGP-LS NLRI. When it encounters the same prefix but with different source router ID, it should extract the corresponding area-ID, rebuild the link between these two source routers in the non-backbone area. Below is one example that based on the Figure 5 which illustrates a topology where OSPF is running in multiple areas.

                     +-----------------+
                     |IP SDN Controller|
                     +--------+--------+
                              |
                              | BGP-LS
                              |
 +---------------------+------+--------+-----+--------------+
 | +--+        +--+   ++-+   ++-+    +-++   + -+        +--+|
 | |S1+--------+S2+---+R1+---|R0+----+R2+---+T1+--------+T2||
 | +-++   N1   +-++   ++-+   +--+    +-++   ++++   N2   +-++|
 |   |           |     |               |     ||           | |
 |   |           |     |               |     ||           | |
 | +-++        +-++   ++-+           +-++   ++++        +-++|
 | |S4+--------+S3+---+R3+-----------+R4+---+T3+--------+T4||
 | +--+        +--+   ++-+           +-++   ++-+        +--+|
 |                     |               |                    |
 |                     |               |                    |
 |         Area 1      |     Area 0    |      Area 2        |
 +---------------------+---------------+--------------------+

       Figure 5: OSPF Inter-Area Prefix Originator Scenario

R0-R4 are routers in the backbone area, S1-S4 are internal routers in area 1, and T1-T4 are internal routers in area 2. R1 and R3 are ABRs between area 0 and area 1. R2 and R4 are ABRs between area 0 and area 2. N1 is the network between router S1 and S2 and N2 is the network between router T1 and T2. Ls1 is the loopback address of Node S1 and Lt1 is the loopback address of Node T1.

Assuming we want to rebuild the connection between router S1 and router S2 located in area 1:

1. Normally, router S1 will advertise prefix N1 within its router-LSA.
2. When this router-LSA reaches the ABR router R1, it will convert it into summary-LSA, add the Source Router-ID Sub-TLV and the Prefix Originator Sub-TLV, as described in Section 3.
3. R1 then floods this extension summary-LSA to R0, which is using the BGP-LS protocol with IP SDN Controller. The controller then knows the prefix for N1 is from S1.
4. Router S2 will perform a similar process, and the controller will also learn that prefix N1 is also from S2.
5. Then it can reconstruct the link between S1 and S2, using the prefix N1. The topology within Area 1 can then be reconstructed accordingly.

Iterating the above process continuously, the IP SDN controller can retrieve a detailed topology that spans multiple areas.

Appendix B. Special Considerations on Inter-Area Topology Retrieval

The above topology retrieval process can be applied in the case where each point-to-point or multi-access link connecting routers is assigned a unique prefix. However, there are some situations where this heuristic cannot be applied. Specifically, the cases where the link is unnumbered or the prefix corresponding to the link is an anycast prefix.

The Appendix A heuristic to rebuild the topology can normally be used if all links are numbered. For anycast prefixes, if it corresponds to the loopback interface and has a host prefix length, i.e., 32 for IPv4 prefixes and 128 for IPv6 prefixes, Appendix A can also applied since these anycast prefixes are not required to reconstruct the topology.

Authors' Addresses