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Appendix A. Multihomed Hosts

This appendix discusses considerations for hosts with multiple interfaces or hosts connected to multiple networks (multihomed hosts) when using Neighbor Discovery.

A.1. Overview​

A multihomed host is a host with:

  • Multiple physical interfaces (e.g., Ethernet and Wi-Fi)
  • Multiple logical interfaces on a single physical interface
  • Connections to multiple networks or subnets

Multihoming introduces complexities in Neighbor Discovery, particularly in:

  • Source address selection
  • Router selection
  • Prefix management
  • Default route selection

A.2. Source Address Selection​

When a multihomed host sends packets, it must select an appropriate source address. The selection affects:

Reachability: The selected source address must be reachable from the destination, considering routing policies and ingress filtering.

Performance: Different interfaces may have different characteristics (bandwidth, latency, cost).

Policy: Administrative policies may dictate which interface to use for certain destinations.

RFC 6724 defines the default source address selection algorithm, which considers factors such as:

  • Scope matching
  • Preferred addresses vs. deprecated addresses
  • Home addresses vs. care-of addresses (Mobile IPv6)
  • Label matching and policy tables

A.3. Router Discovery on Multiple Interfaces​

A multihomed host receives Router Advertisements on multiple interfaces. Key considerations include:

Per-Interface Router Lists: The host SHOULD maintain separate Default Router Lists for each interface, as routers on one interface may not be reachable from another interface.

Default Router Selection: When selecting a default router, the host must consider:

  • Which interface the router is reachable through
  • Router preferences (if advertised via RFC 4191)
  • Interface preferences based on local policy

Prefix Information: Prefixes learned via Router Advertisements are associated with the interface on which they were received. A host SHOULD NOT use an address from one interface to send packets via a different interface unless explicitly configured to do so.

For a multihomed host, on-link determination becomes more complex:

  • An address may be on-link on one interface but not on another
  • The host must track which prefixes are on-link on which interfaces
  • When sending packets, the host must select both the source address and the outgoing interface consistently

A.5. Redirect Processing​

Redirect messages are specific to the interface on which they are received. A multihomed host:

  • MUST associate Redirect information with the interface on which the Redirect was received
  • MUST NOT apply Redirect information from one interface to routing decisions for another interface

A.6. Neighbor Unreachability Detection​

Neighbor Unreachability Detection is performed independently on each interface. A neighbor that is unreachable on one interface may still be reachable on another interface (if multihomed itself).

A.7. Multiple Addresses per Interface​

A single interface may have multiple addresses:

  • Link-local address (required)
  • One or more global addresses from different prefixes
  • Privacy addresses (RFC 4941)
  • Temporary addresses

Each address has its own Preferred and Valid Lifetimes, which must be tracked independently.

A.8. Interaction with Mobile IPv6​

Mobile IPv6 adds additional complexity for multihomed hosts:

  • Home address vs. care-of address selection
  • Multiple care-of addresses when multiple interfaces are available
  • Dynamic home agent discovery across interfaces

Refer to RFC 3775 and related documents for details on Mobile IPv6 interactions.

A.9. Implementation Recommendations​

For multihomed hosts, implementations SHOULD:

  1. Maintain Per-Interface State: Keep separate Neighbor Caches, Destination Caches, Prefix Lists, and Default Router Lists for each interface.

  2. Proper Source Address Selection: Implement RFC 6724 source address selection algorithm.

  3. Interface Selection: When multiple interfaces can reach a destination, select based on:

    • Longest matching prefix
    • Router preferences
    • Local policy (administrator configuration)
    • Interface characteristics

  4. Avoid Cross-Interface Assumptions: Do not assume that information learned on one interface applies to another.

  5. Handle Interface State Changes: Properly handle interface enable/disable events, updating routing and address information accordingly.

  6. Monitor All Interfaces: Process Router Advertisements on all active interfaces.

A.10. Common Pitfalls​

Pitfall 1: Using Wrong Source Address Using a source address from interface A when sending via interface B can result in:

  • Ingress filtering dropping packets
  • Return traffic going to the wrong interface
  • Broken connections

Pitfall 2: Cross-Interface Redirects Applying Redirect information from one interface to routing decisions on another interface.

Pitfall 3: Single Default Router List Maintaining a single Default Router List for all interfaces can result in trying to reach routers on the wrong interface.

Pitfall 4: Ignoring Interface Preferences Not considering interface-specific characteristics (bandwidth, cost, reliability) when selecting routes.

A.11. Future Work​

Areas for potential future work related to multihomed hosts include:

  • Better algorithms for interface selection
  • Mechanisms for sharing reachability information between interfaces
  • Enhanced router preferences that consider multiple paths
  • Integration with traffic engineering and quality of service mechanisms
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