Proposed TLA and NLA Assignment Rule
RFC 2450

Document Type RFC - Informational (December 1998; No errata)
Author Bob Hinden 
Last updated 2013-03-02
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Network Working Group                                      R. Hinden
Request for Comments: 2450                                     Nokia
Category: Informational                                December 1998

                 Proposed TLA and NLA Assignment Rules

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

1.0 Introduction

   This document proposes rules for Top-Level Aggregation Identifiers
   (TLA ID) and Next-Level Aggregation Identifiers (NLA ID) as defined
   in [AGGR].  These proposed rules apply to registries allocating TLA
   ID's and to organizations receiving TLA ID's.

   This proposal is intended as input from the IPng working group to the
   IANA and Registries.  It is not intended for any official IETF
   status.  Its content represents the result of extensive discussion
   between the IPng working group, IANA, and Registries.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].

2.0 Scope

   The proposed TLA and NLA assignment rules described in this document
   are intended for the first two years of IPv6 TLA address assignments.
   As routing technology evolves and we gain additional experience with
   allocating IPv6 addresses the procedures proposed in this document
   may change.

Hinden                       Informational                      [Page 1]
RFC 2450         Proposed TLA and NLA Assignment Rules     December 1998

3.0 IPv6 Aggregatable Global Unicast Address Format

   This document proposes assignment rules for the TLA ID and NLA ID
   fields in the IPv6 Aggregatable Global Unicast Address Format.  This
   address format is designed to support both the current provider-based
   aggregation and a new type of exchange-based aggregation.  The
   combination will allow efficient routing aggregation for sites that
   connect directly to providers and for sites that connect to
   exchanges.  Sites will have the choice to connect to either type of
   aggregation entity.

   While this address format is designed to support exchange-based
   aggregation (in addition to current provider-based aggregation) it is
   not dependent on exchanges for its overall route aggregation
   properties.  It will provide efficient route aggregation with only
   provider-based aggregation.

   The aggregatable global unicast address format as defined in [AGGR]
   is as follows:

      | 3|  13 | 8 |   24   |   16   |          64 bits               |
      |FP| TLA |RES|  NLA   |  SLA   |         Interface ID           |
      |  | ID  |   |  ID    |  ID    |                                |

      <--Public Topology--->   Site
                                      <------Interface Identifier----->


      FP           Format Prefix (001)
      TLA ID       Top-Level Aggregation Identifier
      RES          Reserved for future use
      NLA ID       Next-Level Aggregation Identifier
      SLA ID       Site-Level Aggregation Identifier
      INTERFACE ID Interface Identifier

4.0 Technical Motivation

   The design choices for the size of the fields in the aggregatable
   address format were based on the need to meet a number of technical
   requirements that are described in [AGGR].  An extract of the
   technical requirements from [AGGR] is as follows:

Hinden                       Informational                      [Page 2]
RFC 2450         Proposed TLA and NLA Assignment Rules     December 1998

      The size of the Top-Level Aggregation Identifier is 13 bits.  This
      allows for 8,192 TLA ID's.  This size was chosen to insure that
      the default-free routing table in top level routers in the
      Internet is kept within the limits, with a reasonable margin, of
      the current routing technology.  The margin is important because
      default-free routers will also carry a significant number of
      longer (i.e., more-specific) prefixes for optimizing paths
      internal to a TLA and between TLAs.

      The important issue is not only the size of the default-free
      routing table, but the complexity of the topology that determines
      the number of copies of the default-free routes that a router must
      examine while computing a forwarding table.  In current practice
      with IPv4, it is common to see a prefix announced fifteen times
      via different paths.  The complexity of Internet topology is very
      likely to increase in the future.  It is important that IPv6
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