Summary

  • RFC 2374 named TLA, NLA and SLA fields inside an IPv6 address, but explicitly assumed that routers would forward by longest-prefix match on arbitrary bit boundaries without understanding those names.
  • Its hierarchy was a bounded plan for allocation and aggregation, not a self-executing map of provider authority, route acceptance, exchange portability or delivered service.

The diagram looked like a chain of command

RFC 2374 gave the first 64 bits of its aggregatable global unicast format a careful vocabulary. A three-bit format prefix introduced a 13-bit Top-Level Aggregation identifier, eight reserved bits, a 24-bit Next-Level Aggregation identifier and a 16-bit Site-Level Aggregation identifier. The remaining 64 bits identified an interface.

On paper, the structure resembled an institutional map. TLA and NLA described public topology. SLA described the site's internal topology. A TLA holder could divide its NLA space among lower providers and sites; those recipients could divide again. The address appeared to carry the route from the top of the public system to a local interface.

But the specification refused that interpretation at the forwarding boundary. It assumed that the Internet routing system used longest-prefix match on arbitrary bit boundaries and had no knowledge of the address's internal structure. The names were for assignment and allocation. Apart from the distinction between unicast and multicast, the router was not instructed to treat the bit fields as semantic commands.

A field boundary was not a route

This distinction separated the plan from the running network. A registry or upstream could delegate a prefix at a boundary suggested by the format. An operator could announce that prefix, a neighbour could accept it under policy, and a router could install a forwarding entry. Those were different acts.

The address could not show whether an allocation was current, whether custody had changed, whether a more-specific route existed, whether policy rejected an announcement or whether traffic followed the anticipated provider. A router could carry a route whose length cut through a named field. Conversely, a beautifully structured address could remain unreachable.

The useful receipt was therefore not “this address contains an NLA.” It was a chain: the allocation rule in force, the actual delegation record, the announced prefix, the accepting policy, the installed forwarding entry, the observed path and the reachable service.

The 8,192 ceiling was a planning envelope

The 13-bit TLA field allowed 8,192 top-level identifiers. RFC 2374 chose that scale to keep default-free routing tables within the limits of contemporary technology, while leaving margin for longer routes used to optimise paths within and between top-level regions. It also worried about computation: a prefix might arrive by many paths, so route count alone understated the work.

Eight reserved bits provided an escape. A later decision could expand TLA or NLA into that field. Another format prefix could also carry the same architecture. The deployed router was not expected to infer when that future had arrived. A new allocation and standards decision had to change the shared interpretation.

This was a minimum initial specification in numerical form: define a workable top-level envelope, retain contingency space and avoid claiming that the first hierarchy exhausted the future.

Delegation traded flexibility for aggregation

Each TLA holder received 24 NLA bits and could arrange them into its own hierarchy. The next holder could subdivide again. RFC 2374 recommended slow-start allocation and recognised the central trade-off: deeper hierarchy supported aggregation and smaller routing tables; flatter assignment made allocation and attachment easier but exposed more routes.

Within a site, the 16 SLA bits supported 65,535 subnet values, and the site decided their internal structure. A very large organisation could obtain another site identifier. The public allocator did not need to design the site's subnet plan, and the site did not gain authority over the public route merely because it arranged its SLA bits well.

The format distributed decisions. It did not eliminate them.

Portability was promised beyond the scope line

The most ambitious part of RFC 2374 was exchange-based aggregation. An organisation attached through an exchange could, the document said, become independent of a long-haul provider, change that provider without renumbering and multihome without taking a prefix from each carrier.

The very next boundary mattered: the mechanisms for provider selection and portability were not discussed. The address format reserved a place in an allocation topology. It did not define exchange governance, route-selection policy, authentication, failover, commercial eligibility or the operational procedure that would preserve service while a provider changed.

RFC 2450 later proposed evidence and registry duties for top-level recipients: native service within three months, a transit track record verified through routing observations, public assignment records, utilisation reports and possible revocation. Yet that memo was Informational input for the first two years, not permanent IETF law. It explicitly expected procedures to evolve with experience.

The experiment changed the vocabulary

The 6bone used pseudo-TLA and pseudo-NLA formats to prototype the architecture, and initial sub-TLA assignments put real records behind part of the plan. Those facts establish an experiment and an allocation history. They do not prove that every promised exchange mechanism or hierarchical relationship became durable.

In 2003, RFC 3587 made RFC 2374's TLA/NLA structure Historic. It replaced the prescribed public hierarchy with a general global routing prefix, retained the site's subnet function under the simpler name “subnet ID,” and warned implementations not to assume that 2000::/3 would always be the special global-unicast space.

The change did not abolish aggregation or longest-prefix routing. It removed one mandated story about how public topology should be read from fixed address fields. The running network had always followed prefixes; the standard finally made that separation harder to overlook.

The evidence survived the field names

Lu Heng's running-code principle places authority in advertisements, accepted policy, forwarding entries and observed paths rather than in the symbolic dignity of TLA or NLA. Minimum initial specification explains why an early field plan could be useful without deserving permanence. Reality layers supply the audit rule: address syntax, allocation, registry custody, route propagation, path selection and delivered service are adjacent but non-interchangeable facts.

RFC 2374 tried to make aggregation administrable by giving the address a vocabulary. Its lasting lesson came from the sentence that limited that vocabulary. The router did not read the hierarchy. It read the route.