OSPF Multi-Instance Extensions
draft-acee-ospf-multi-instance-02.txt

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Abstract

OSPFv3 includes a mechanism for supporting multiple instances on the same link. OSPFv2 could benefit from such a mechanism in order to support multiple routing domains on the same subnet. The OSPFv2 instance ID is reserved for support of separate OSPFv2 protocol instances. This is different from OSPFv3 where it could be used for other purposes such as putting the same link in multiple areas. OSPFv2 supports this capability using a separate subnet or the OSPF multi-area adjacency capability.

Table of Contents

1.  Introduction
    1.1.  Requirements notation
2.  OSPFv2 Instance Packet Encoding
3.  OSPF Interface Instance ID
    3.1.  Sending and Receiving OSPF packets
4.  State Sharing Optimizations between OSPF instances
5.  Backward Compatibility and Deployment Considerations
6.  Security Considerations
7.  IANA Considerations
8.  Normative References
Appendix A.  Acknowledgments
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

OSPFv3 [OSPFV3] (Coltun, R., Ferguson, D., Moy, J., and A. Lindem, “OSPF for IPv6,” July 2008.) includes a mechanism for supporting multiple instances on the same link. OSPFv2 [OSPFV2] (Moy, J., “OSPF Version 2,” April 1998.) could benefit from such a mechanism in order to support multiple routing domains on the same subnet. The OSPFv2 instance ID is reserved for support of separate OSPFv2 protocol instances. This is different from OSPFv3 where it could be used for other purposes such as putting the same link in multiple areas. OSPFv2 supports this capability using a separate subnet or the OSPF multi-area adjacency capability [MULTI‑AREA] (Mirtorabi, S., Psenak, P., Lindem, A., and A. Oswal, “OSPF Multi-Area Adjacency,” May 2008.).

1.1.  Requirements notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC‑KEYWORDS] (Bradner, S., “Key words for use in RFC's to Indicate Requirement Levels,” March 1997.).

2.  OSPFv2 Instance Packet Encoding

OSPFv2 currently doesn't offer a mechanism to differentiate packets for different instances sent and received on the same interface. In support of this capability, this document introduces a modified packet header format when the Authentication Type field is split into an instance ID and type.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Version #   |     Type      |         Packet length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Router ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Area ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum             |  Instance ID  |  AuType       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Authentication                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Authentication                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version #

The OSPFv2 version number - 2

Type

The OSPFv2 packet type as specified [OSPFV2] (Moy, J., “OSPF Version 2,” April 1998.).

Packet length

The length of the OSPF protocol packet in bytes. This length includes the standard OSPF header.

Router ID

The Router ID of the packet's source.

Area ID

A 32-bit number identifying the area corresponding the packet as specified in [OSPFV2] (Moy, J., “OSPF Version 2,” April 1998.).

Checksum

OSPFv2 standard checksum calculation as specified in specified in [OSPFV2] (Moy, J., “OSPF Version 2,” April 1998.).

Instance ID

Enables multiple instances of OSPF to be run over a single link. Each protocol instance would be assigned a separate Instance ID; the Instance ID has local subnet significance only. Received packets with an Instance ID not equal to one of the configured OSPF Instance IDs on the receiving interface MUST be discarded.

AuType

OSPFv2 authentication type as specified in specified in [OSPFV2] (Moy, J., “OSPF Version 2,” April 1998.).

Authentication

A 64-bit field for Authentication type dependent authentication data.

3.  OSPF Interface Instance ID

OSPF [OSPFV2] (Moy, J., “OSPF Version 2,” April 1998.) describes the conceptual interface data structure in section 9. The OSPF Interface ID will be added to this structure. The Interface Instance ID will default to 0. Its setting to a non-zero value may be accomplished through configuration or implied by some usage beyond the scope of this document.

3.1.  Sending and Receiving OSPF packets

When sending OSPF packets, if the interface instance ID has a non-zero value, it will be set in the OSPF packet header. When receiving OSPF packets, the OSPFv2 Header Instance ID will be used to aid in demultiplexing the packet and routing it to the correct OSPFv2 instance. Received packets with an Instance ID not equal to one of the configured OSPF Instance IDs on the receiving interface MUST be discarded.

4.  State Sharing Optimizations between OSPF instances

This is beyond the scope of this draft and is an area for further study.

5.  Backward Compatibility and Deployment Considerations

When there are OSPF routers that support this capability on the same broadcast capable link as those that do not, packets with non-zero Instance IDs will be received by those legacy routers. Since the authentication type will be unknown to them they will not process the packet. This is exactly what is desired.

Previously, a concern was that some implementations will log every single authentication type mismatch. However, discussions with implementers have led us to the conclusion that this is not as current a problem as we'd first thought and it will be even less of a problem by the time the mechanism in this draft is standardized, implemented, and deployed. Hence, the controversial mechanisms to avoid legacy routers receiving multicast OSPF packets with a non-zero instance ID have been removed.

6.  Security Considerations

The enhancement described herein doesn't add any additional security considerations to OSPFv2. Security considerations for OSPFv2 are described in [OSPFV2] (Moy, J., “OSPF Version 2,” April 1998.).

Given that only three OSPFv2 authentication types have been standardized, it seems reasonable to reduce the OSPF packet header field to 8 bits.

7.  IANA Considerations

A new registry will be added for OSPF Instance IDs. The allocation is TBD.

Dependent on the approach, two new multicast addresses from the IPv4 Multicast Addresses registry would need to be allocated.

Dependent on the approach, a new protocol ID may need to be allocated from the Protocol Numbers registry.

8. Normative References

Appendix A.  Acknowledgments

The RFC text was produced using Marshall Rose's xml2rfc tool.

Thanks to Paul Wells for commenting on the backward compatibility issues.

Authors' Addresses

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