rfc2036.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                          G. Huston
Request for Comments: 2036                              Telstra Internet
Category: Informational                                     October 1996


          Observations on the use of Components of the Class A
                   Address Space within the Internet

Status of this Memo

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

Abstract

   This document is a commentary on the recommendation that IANA
   commence allocation of the presently unallocated components of the
   Class A address space to registries, for deployment within the
   Internet as class-less address blocks.

   The document examines the implications for service providers and end
   clients within this environment. The document notes the major
   conclusion that widespread adoption of class-less routing protocols
   is required, within a relatively rapid timeframe for this
   recommendation to be effective.

Introduction

   The Address Lifetime Expectancy (ALE) Working Group of the IETF has
   recorded the allocation of Internet addresses from the unallocated
   address pool. ALE has noted that the existing practice of drawing
   addresses from the Class C space (192/3 address prefix) will result
   in near to medium term exhaustion of this section of the unallocated
   address pool. The largest remaining pool is in the Class A space,
   where some 25% of Internet addresses (the upper half of the Class A
   space) remain, to date, unallocated.

   This document is a commentary on the potential recommendation that
   the Internet Assigned Numbers Authority (IANA), through delegated
   registries, commence allocation of the presently unallocated
   components of the Class A  address space to registries, for
   deployment within the Internet through the mechanism of allocation of
   class-less address prefixes.

   The deployment of class-less address prefixes from the Class A space
   within the Internet will require some changes to the routing
   structure within Internet component network domains. The motivation



Huston                       Informational                      [Page 1]

RFC 2036        Components of the Class A Address Space     October 1996


   for, and nature of, such changes as they effect network domains and
   network service providers are outlined in this document.

Current Practice with Address Allocations

   To date the allocation of class-less network prefixed address blocks
   has followed a conservative practice of using address allocations
   which are compatible superblocks of Class C addresses, while the
   allocation of addresses within the space of Class A and Class B
   networks has continued to be aligned with the class-based prefix
   structure.

   Within this address allocation environment for non-transit network
   domains there is accordingly the option to continue to use address
   deployment strategies which involve fixed subnet address structures
   within contiguous areas, and use Class-full interior routing
   protocols. In the situation where variable length subnet masks or
   disconnected subnets are deployed within the network domain's routing
   structure, interior routing protocols which use subnet-based routing
   of Class-full networks can still be successfully deployed and the end
   network has the option of using an explicit or implicit sink subnet
   default route. Where such non-transit network domains are connected
   to the Internet infrastructure the boundary exchange between the
   non-transit network and the network service provider (this term is
   used as a synonym for a transit network domain, which provides a
   traffic transit service to other non-transit and peer transit network
   domains) is either a class-full advertisement of routes, or an
   aggregated address advertisement where the aggregate is a superblock
   of the deployed component class-full networks. At the boundary points
   of the non-transit network it is a requirement that the non-transit
   network's subnet default route (if used explicitly) not be directed
   to the network service provider's domain, to avoid a routing loop at
   the domain boundary point.

   For network service providers the interior routing protocol can use
   either aggregated routing or explicit class-full routing within this
   environment. At the network service provider's boundary peering
   points the strongly recommended practice is to advertise aggregated
   routes to transit peers, which in turn may be further aggregated
   across the Internet, within the parameters of permissible policies.











Huston                       Informational                      [Page 2]

RFC 2036        Components of the Class A Address Space     October 1996


Implications of Address Allocation from the Class A space

Network Service Providers Must Use Class-less Routing

   For network service providers within the deployed Internet the
   implications from this recommendation to deploy prefixes from the
   Class A address space add more pressure to the requirement to
   uniformly deploy class-less routing protocols. While this is already
   a mandatory requirement for any domain which operates without a
   default  route (ie. the provider carries full Internet routing and
   effectively  calculates default), other providers currently can use
   an imported default route and operate within a class-full routing
   configuration. This mode of operation is sub-optimal, in so far as
   the task of aggregating routes falls on peer network service
   providers performing proxy aggregation of contiguous class-full
   address blocks.

   In deploying components of the Class A the use of proxy aggregation
   is no longer sufficient. Where a domain sees a default route and a
   subnet of a Class A route the routing structure, in a class-full
   configuration, may not necessarily follow the default route to reach
   other parts of the Class A network not covered by the advertised
   Class A subnet route.

   Accordingly for Network Service Providers operating within the
   Internet domain the deployment of components of the Class A space
   entails a requirement to deploy class-less routing protocols, even in
   the presence of a default route. It is noted that this absolute
   requirement is not the case at present.

Consideration of Non-Transit Network Configurations

   For disconnected network environments, where the network domain is
   operated with no links to any peer networking domain, such networks
   can continue to use class-full interior routing protocols with subnet
   support. Allocation of addresses using prefix blocks from the Class A
   space within such environments is possible without adding any
   additional routing or address deployment restrictions on the network
   domain.












Huston                       Informational                      [Page 3]

RFC 2036        Components of the Class A Address Space     October 1996


   For non-transit network domains which are connected to one or more
   peer network domains the situation does involve consideration of
   additional factors. The observation which is made in the context of
   this consideration is that there are at present relatively few non-
   transit networks operating a fully class-less interior routing
   protocol, as there has been no absolute requirement for this
   functionality when using single class-full network addresses, or when
   using block prefixed address allocations which are clusters of class-
   full network addresses.

   For non-transit network domains which support external peer
   connections to a network service provider, deployment of a component
   of the Class A space would be supportable using a fully class-less
   interior routing protocol.

   In this case there is an additional constraint placed on the external
   connection such that the non-transit domain either agrees that the
   network service will undertake proxy aggregation of the advertised
   class-less address components, or the network domain is configured to
   advertise to the provider an aggregate route. In both cases the
   aggregate route must be either the allocated address block, or a
   fully contained sub-block. Advertising aggregatable address blocks
   without proxy aggregation permission, or advertising multiple sub-
   blocks of the registry allocated address block is considered overly
   deleterious to the provider's internetworking environment due to
   considerations of consequent growth in routing table size.

   If the externally connected non-transit network domain uses class-
   full interior routing protocols then deployment of Class A address
   space prefixes implies that the domain must configure the Class A
   subnet default route along the same path as the default route to the
   network service provider (which is noted to be the exact opposite of
   the necessary routing configuration for those address prefixes which
   are either aligned to class-full address boundaries or are super
   blocks of such class-full address blocks). The network service
   provider may also receive leaked explicit subnet reachability
   information in such a routing configuration, potentially placing the
   responsibility for advertising the correct aggregate address block
   with the network service provider as a case of proxied aggregation.

   Within this configuration model, even when explicit subnet default
   routing is deployed, there is the risk of unintentional traffic
   leakage and routing loops. If the network service provider is
   undertaking proxy aggregation using the registry allocated address
   block then traffic originating within the non-transit domain which is
   (mis)directed to non-deployed components of the address block will
   loop at the interface between the network domain and the provider. If
   the network service provider is configured to explicitly route only



Huston                       Informational                      [Page 4]

RFC 2036        Components of the Class A Address Space     October 1996


   those address components which are also explicitly routed within the
   non-transit domain, such (mis)directed traffic will be passed through
   the internetworking environment along the default route until a
   default-less routing point is encountered, where it can then be
   discarded. The outcome of this consideration is that the non-transit
   network domain should explicitly configure sink subnet routes for all
   non-deployed components of the allocated address block, and
   conservative operational practice would be to configure the proxy
   aggregation undertaken by the network service provider to aggregate
   according to the registry allocated address block.

   There is an additional constraint placed on the non-transit network
   domain using class-full interior routing protocols, such that the
   domain has no other exterior peer connections to other network
   domains which deploy class-full routing interior routing protocols.

   There is the further constraint placed on the of use of interior
   class-full routing protocols within a non-transit network domain. In
   the case where the non-transit network domain has multiple exterior
   connections to Network Service Providers (ie the network domain is
   multiply homed within a number of network providers) there is the
   possibility that each provider may wish to announce components of the
   same Class A parent. Accordingly the network domain must use a class-
   less interior routing protocol in the case where the network domain