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Abstract -- Stratum-Based Aggregation of Routing Information Network and Application Engineering Track
N2: Routing and Addressing

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Stratum-Based Aggregation of Routing Information

Rekhter, Yakov ( yakov@watson.ibm.com)


Growth of the Internet strains the Internet routing systems to its limits. Classless Inter-Domain Routing (CIDR) is presently being adopted as the mechanism to contain the volume of routing information that needs to be maintained throughout the Internet routing system.

Organizationally the Internet can be partitioned into a set of Network Service Providers, and Network Service Subscribers. Providers are interconnected with each other at "exchanges". An exchange may be either bilateral (e.g. two providers interconnected via a point-to-point line), or Network Layer multilateral (a "backbone" IP provider that interconnects several other providers), or Data Link Layer multilateral (e.g. NAPs of the new NSFNET architecture).

Presently aggregation of the inter-domain routing information occurs at the boundary between a subscriber and its provider, and at the boundary between providers. Aggregation at the subscriber's boundary alone is insufficient with respect to the reduction of routing information. Aggregation at the provider's boundary may be sufficient only as long as the total number of providers (Internet-wide) is sufficiently moderate (so that it would be feasible to maintain routes to individual providers Internet-wide), and that as subscribers switch from one provider to another, the subscribers renumber to enable aggregation at their provider's boundaries.

To improve scaling properties of the Internet routing system we suggest to augment the current aggregation boundaries with the strata-based aggregation.

We define a "stratum" as a set of providers P connected to a set of exchanges E, such that connectivity among the providers in P is achieved solely by the providers in P and the set of exchanges in E. A provider connected only to the exchange(s) within a single stratum is called an "intra-stratum" provider. A provider connected to the exchanges in multiple strata is called an "inter-stratum" provider. A given stratum can be connected to other strata by one or more inter-strata providers. An inter-stratum provider that is connected to a given stratum, and provides a uniform transit service for all the intra-stratum providers (and their subscribers) within the stratum is called an "equal access" inter-stratum provider.

An exchange of a stratum acts as a concentration point for routing. Therefore, the set of exchanges of a given stratum presents a natural boundary for another level aggregation of routing information.

To make the most effective use of stratum-based aggregation, it is necessary that a stratum be assigned an appropriate contiguous block of IP addresses. Intra-stratum providers within the stratum would allocate their addresses from this block. Subsequent address allocation to individual subscribers is expected to be consistent with the guidelines specified by CIDR.

Routing information reduction due to the stratum-based address allocation assumes the existence of equal access inter-stratum providers. Such a provider that is (directly) connected to an exchange of a given stratum should be able to aggregate reachability information for all the subscribers whose addresses are taken out of the block assigned to the stratum (which may include subscribers of any intra-stratum provider that is part of the stratum) into a single address prefix (rather than carrying multiple address prefixes, one per provider), thus reducing the volume of routing information that needs to be carried by the Internet routing system.

It is the equal access inter-stratum providers that are expected to gain most out of the stratum-based address allocation. Inter-stratum providers that don't provide equal access are likely to derive little or no benefits from the stratum-based address allocation.

By definition, an equal access inter-stratum provider for a given stratum can not discriminate its transit services with respect to the individual intra-stratum providers (and/or their subscribers) of the stratum. Therefore, an equal access inter-stratum provider connected to an exchange has to have fairly uniform peering agreements with all the intra-stratum providers connected to that exchange. It is not clear how this would impact the ability of providers to operate as equal address providers.

Use of stratum-based address allocation should be carefully balanced against possible long-term instabilities of the exchanges that are part of a stratum. Such instabilities may cause dissolvement of the stratum. Dissolving a stratum may either result in injecting additional routing information, or would require massive renumbering that spawns multiple providers with all of their subscribers.

The above proposal applies both to IPv4 and IPv6.