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rfc:rfc2036

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
 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

Huston Informational [Page 5] RFC 2036 Components of the Class A Address Space October 1996

 (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:
  1. have no exterior connection (in which case the domain can use

either class-full or class-less interior routing protocols without

    further implication),
  or
  1. 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),

Huston Informational [Page 6] RFC 2036 Components of the Class A Address Space October 1996

  or
  1. 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
  1. 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:
  1. 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,

Huston Informational [Page 7] RFC 2036 Components of the Class A Address Space October 1996

    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.
  1. 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.
  1. 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
  1. 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.

Huston Informational [Page 8] RFC 2036 Components of the Class A Address Space October 1996

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

Huston Informational [Page 9]

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