    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

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    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.

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    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.

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       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

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       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
       is multiply homed within network service providers.

       There are also additional constraints placed on the non-transit
       network domain where the network has exterior connections to other
       peer networks. Even in the case where the network domain uses a
       class-less interior routing protocol, there is the additional
       consideration that this requirement for use of a class-less routing
       domain is transitive to other connected network domains. An second
       network domain, externally connected to the class-less domain routing
       part of the Class A space, will interpret the boundary reachability
       advertisement as a complete Class A network advertisement, if using
       class-full routing. Even if both network domains are connected to the
       same network provider the provider's default routing  advertisement
       default to the class-full domain will be overridden by the assumed
       class A advertisement through the domain-to-domain connection,
       leading to unintended traffic diversion. The diversion occurs in this
       case as the traffic directed to parts of the Class A network which
       are not deployed within the first domain will transit the first
       domain before entering the network service provider's domain.

       It is also possible to have configurations with unintended routing
       holes. An example of such a configuration is two stub clients of
       different network service providers, both using class-less interior
       routing (X and Y), both directly connected to a third network domain

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       (Z), which uses class-full interior routing, which is configured as a
       transit between X and Y. X's advertisement of a component of a Class
       A to Z will be assumed by Z to be a complete Class A network, and as
       such will be advertised to Y, overriding Y's default route received
       from the network service provider. Y will pass all Class A addressed
       traffic to Z, who will in turn pass it to X. As X is configured as a
       non-transit stub network X must discard all non-locally addressed
       traffic.

       Thus reasonable operational practice would be to ensure that if a
       network domain deploys a component of the Class A address space, the
       network domain is configured to use class-less interior routing
       protocols, and the network has a single exterior connection to a
       class-less network provider domain, with the boundary configured as a
       class-less routing exchange. Multiply homed network domains do infer
       a common requirement of class-less routing exchanges and interior
       class-less routing protocols across all peer connected network
       domains.

       It is possible to propose that multi homed network domains should
       probably not get subnets of a class A for these reasons, although
       with an increasing diversity of network service providers instances
       of multi-homed network domains may become more prevalent, and the
       requirement to transition to an interior class-less routing structure
       as a consequence of moving to a multi-homed configuration may not be
       explicitly apparent to all network domains.

    Potential Guidelines for Allocation of an Address Prefix from the Class
       A Address Space

       To summarise the possible guidelines for allocation from the Class A
       space, such addresses should only be assigned to network domains
       which:

        - have no exterior connection (in which case the domain can use
          either class-full or class-less interior routing protocols without
          further implication),

        or

        - are a component of a private internet domain which uses class-full
          routing exchanges and no other part of the same Class A is
          assigned into the domain (this is probably an unlikely scenario
          given a probable direction to use the Class A space as the major
          resource for the unallocated pool of addresses for allocation),

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        or

        - have a single default exterior connection to a class-less routing
          domain, use class-full routing  protocols and explicitly direct a
          subnet default route to the exterior connection,

        or

        - use class-less interior routing protocols and connect only to
          other network domains which also use class-less interior routing
          protocols.

       It is a reasonable objective to nominate a transition objective to
       the final configuration (uniform use of class-less routing domains
       within the Internet) which would enable deployment of components of
       the Class A space uniformly across the Internet.

    Related Potential Activities

       Given the pressures on the remaining Class C address space in the
       unallocated address pool, it is noted that there would be widespread
       deployment of components of the remaining Class A space in class-less
       allocation guidelines. There is a consequent requirement for
       widespread deployment of class-less interior routing protocols in
       order to ensure continued correct operation of the routed Internet.
       This is a more significant transition than that deployed to date with
       the network service providers' deployment of Class-less Inter-Domain
       Routing (CIDR) protocols, in that there is a necessary transition to
       deploy Class-less Interior Routing Protocols (CIRP) within a large
       number of network domains which are currently configured with class-
       full routing.

       However this would appear to be a necessary task if we wish to
       continue to utilise a pool of globally unique Internet addresses to
       allocate to new systems and networks, but one requiring significant
       effort considering the space of the routing transition required to
       make this work.

       There are a number of directed activities which can assist in this
       transition:

        - The network registries commence initial class-less allocation from
          the unallocated Class A space to those entities who either:

          o  operate a CIRP environment, and either have no external
             connectivity, or are singly homed to a network service provider
             using a CIDR environment, with no other exterior connections,

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          or

          o  operate a class-full routing protocol, and either have no
             external connectivity, or are singly homed to a network service
             provider using a CIDR environment, with no other exterior
             connections, and are willing to point the subnet default route
             towards the network service provider.

        - In deploying the Class A space there is a requirement within the
          vendors' product sets to allow explicit configuration of whether
          the router operates in a class-less or class-full mode, with
          correct behaviour of the default route in each case. Class-full
          mode of operation must also allow explicit configuration of
          subnet default behaviour as to whether to follow the default
          route, or to operate a subnet default sink.

        - There is a similar, but longer term, activity within the host
          configuration environment to support a mode of address
          configuration which uses a local network prefix and host address,
          possibly in addition to the current configuration mode of class-
          full network, subnet and host address

        - Internet Service Providers also must support full class-less
          configurations in both interior routing configurations and
          interdomain peering routing exchanges, and provide support to
          client network domains operating a class-less boundary routing
          exchange configuration and be able to undertake proxy aggregation
          as permitted.

    Security Considerations

       Correct configuration of the routing environment of the Internet is
       essential to the secure operation of the Internet.

       The potential use of the Class A space raises no additional
       considerations in this area.

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    References

       [CIDR]
            Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless Inter-
            Domain Routing (CIDR): an Address Assignment and Aggregation
            Strategy", RFC 1519, BARRnet, cisco, MERIT, OARnet, September
            1993.

    Author's Address

          Geoff Huston
          Telstra Internet
          Locked Bag 5744
          Canberra  ACT  2601
          Australia

          phone: +61 6 208 1908
          email: gih@telstra.net

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