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5. Scope Zones

5. Scope Zones​

A scope zone, or simply a zone, is a connected region of topology of a given scope. For example, the set of links connected by routers within a particular (multicast) site, and the interfaces attached to those links, comprise a single zone of multicast site-local scope.

Note that a zone is a particular instance of a topological region (e.g., Alice's site or Bob's site), whereas a scope is the size of a topological region (e.g., a site or a link).

The zone to which a particular non-global address pertains is not encoded in the address itself but determined by context, such as the interface from which it is sent or received. Thus, addresses of a given (non-global) scope may be re-used in different zones of that scope. For example, two different physical links may each contain a node with the link-local address fe80::1.

Zones of the different scopes are instantiated as follows:

  • Each interface on a node comprises a single zone of interface-local scope (for multicast only).

  • Each link and the interfaces attached to that link comprise a single zone of link-local scope (for both unicast and multicast).

  • There is a single zone of global scope (for both unicast and multicast) comprising all the links and interfaces in the Internet.

  • The boundaries of zones of a scope other than interface-local, link-local, and global must be defined and configured by network administrators.

Zone boundaries are relatively static features, not changing in response to short-term changes in topology. Thus, the requirement that the topology within a zone be "connected" is intended to include links and interfaces that may only be occasionally connected. For example, a residential node or network that obtains Internet access by dial-up to an employer's (multicast) site may be treated as part of the employer's (multicast) site-local zone even when the dial-up link is disconnected. Similarly, a failure of a router, interface, or link that causes a zone to become partitioned does not split that zone into multiple zones. Rather, the different partitions are still considered to belong to the same zone.

Zones have the following additional properties:

  • Zone boundaries cut through nodes, not links. (Note that the global zone has no boundary, and the boundary of an interface-local zone encloses just a single interface.)

  • Zones of the same scope cannot overlap; i.e., they can have no links or interfaces in common.

  • A zone of a given scope (less than global) falls completely within zones of larger scope. That is, a smaller scope zone cannot include more topology than would any larger scope zone with which it shares any links or interfaces.

  • Each zone is required to be "convex" from a routing perspective; i.e., packets sent from one interface to any other in the same zone are never routed outside the zone. Note, however, that if a zone contains a tunneled link (e.g., an IPv6-over-IPv6 tunnel link [8]), a lower layer network of the tunnel can be located outside the zone without breaking the convexity property.

Each interface belongs to exactly one zone of each possible scope. Note that this means that an interface belongs to a scope zone regardless of what kind of unicast address the interface has or of which multicast groups the node joins on the interface.

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