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Versions: 00                                                            
Network Working Group                                           T. Li
INTERNET DRAFT                                       Juniper Networks
Expire in six months                                    November 1996

         Internet Service Provider Address Coalitions (ISPACs)

Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

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   Please check the 1id-abstracts.txt listing contained in the
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1. Introduction

   The introduction of CIDR [1, 2] substantially changed the way that
   address allocation was performed in the Internet.  The result is that
   address allocation is provider-based: the address used by a host
   depends on which service provider is used by the host.  One of the
   unfortunate implications is that if a host changes provider, it must
   renumber.  In many cases this is awkward, expensive, and difficult.

   This note describes an organization that may be formed to help reduce
   the need for user renumbering while still providing the benefits of
   CIDR to the remainder of the network.  Such organizations do not
   currently exist, so this note is at best a discussion of the
   foreseeable future and is not a catalog of existing experience.

   These organizations are formed by grouping Internet Service Providers
   (ISPs) into topologically connected sets, each of which shares one or
   more IP address prefixes.  Such an organization is called an ISP
   Address Coalition (ISPAC, pronounced "ice pack").  To simplify the
   discussion, we assume that the ISPAC shares a single prefix.  The
   discussion generalizes to multiple shared prefixes in the obvious

2. Creation of an ISPAC

   An ISPAC can be created from a set of ISPs which are willing to share
   an address prefix and are willing to meet certain technical and
   administrative requirements.  There may also be additional legal
   requirements, such as anti-trust issues, that are beyond the scope of
   this document.

   The primary technical requirement is that all members of the ISPAC be
   willing to cooperate and each treat the shared prefix as a common
   resource.  Further, each member of the ISPAC should perform
   aggregation on the shared prefix address space when advertised
   outside of the ISPAC.  Note that this aggregation may be performed in
   accordance with normal CIDR procedures, so this does not necessarily
   imply that only the aggregated prefix is advertised.  This
   exceptional case is discussed in more detail below.

   Because all members of an ISPAC are advertising the aggregated shared
   prefix, any member may attract traffic for portions of the shared
   prefix that need to be routed to another member.  For this reason, it
   is essential that all members of the ISPAC be able to route to any
   other member of the ISPAC without leaving the ISPAC (i.e., the ISPAC
   is connected).  One way that this requirement could be satisfied is
   for all members of the ISPAC to be present at a common interconnect.

   The primary administrative requirement to form an ISPAC is the
   selection of an addressing authority for the shared prefix.  A
   complete discussion of the issues in the selection of such an
   authority are beyond the scope of this document, but may include
   renumeration for providing for address space administration and
   impartiality of the authority.  Once an addressing authority is
   selected, it is necessary to acquire the shared prefix.  This would
   be done according to the normal address allocation procedures.  The
   address space request from an ISPAC should be regarded as if it came
   from any ISP with the properties of the union of the members.  No
   special privileges are accorded to requests from ISPACs, so normal
   justifications for address space would apply.

3. Operation of an ISPAC

   The day to day operations of an ISPAC has certain special
   considerations which impact all of the members of the ISPAC.  Because
   sites may change providers within the ISPAC, intra-ISPAC routing must
   be carefully managed.  A user should be able to be assured that they
   are the only ones using a component in the shared prefix and that
   their routing reliability is comparable to that provided to users
   within the ISP assigned address prefixes.  To accomplish this, the
   members of the ISPAC must exchange routing information about
   components of the shared prefix.  The exact mechanisms that are used
   to accomplish this are beyond the scope of this document, but there
   are many possibilities, including a common IGP, or via BGP.  Members
   of the ISPAC should agree on a particular set of mechanisms for
   exchanging routing information about the shared prefix, including
   protocols and any administrative filtering for sanity checking.

   Because components of the shared prefix are generally not advertised
   outside of the ISPAC, if the ISPAC itself is internally partitioned,
   it will cause an outage between users of the shared prefix.  To
   prevent this, members of the ISPAC are strongly encouraged to insure
   that the ISPAC is biconnected or triconnected.  One way that this can
   be accomplished is to have multiple interconnects at which all
   members of the ISPAC are present.  These may be private or public
   interconnects.  Alternately, increased connectivity can be provided
   by a number of pairwise interconnects.

   For inter-ISPAC routing, each member of the ISPAC must advertise the
   shared prefix.  Normally, this will be done with BGP, so the shared
   prefix may appear to have multiple source autonomous systems.  For
   this reason, advertising the existence and membership of the ISPAC
   and the particular prefix to the administrators of other domains is
   useful to help eliminate false alarms in interdomain sanity checking

   By default, simple advertisement of the shared prefix will cause all
   traffic to enter the ISPAC at the nearest member.  This will only be
   optimal for sites which have their optimal connection to that
   neighbor.  In many cases, it would be preferable to determine optimal
   entry points for components of the shared prefix.  At the same time,
   it is essential to not advertise all components to the remainder of
   the network, as this would defeat the aggregation of the prefix.  To
   preserve aggregation and provide more optimal routing, there is a
   useful technique which can be borrowed from normal interdomain
   routing.  In this application, this technique a member of the ISPAC
   may advertise some of the components of the shared prefix, but the
   member must insure that the scope of these advertisements is
   carefully restricted.  The restriction of scope is usually
   accomplished by administrative filtering and careful coordination the
   receiver of the advertisements.

   Because any advertisement of the shared prefix by a member of the
   ISPAC may attract traffic, it may be the case that a member may end
   up providing transit service to other members of the ISPAC.  It is up
   to the members of the ISPAC to determine their own policy regarding
   this transit traffic.  It may be that such traffic is equally
   distributed, in which case responsibilities are shared equally.
   Alternately, due to customer dissimilarities, the traffic loads may
   be highly skewed.  In this case, the shared prefix may only be
   advertised to external providers at interconnects where all members
   and external providers are present.  In this case, it is likely that
   members would not advertise the shared prefix across their private
   interconnects.  This will affect the quality of the routing for sites
   within the shared prefix.

   Note that nothing in this discussion precludes a provider from
   joining multiple ISPACs.  Further, these ISPACs may share members in
   any fashion.  ISPACs may be disjoint, may overlap, may be tangential
   or may intersect freely.

4. Benefits of an ISPAC

            The primary benefit of an ISPAC is to reduce the requirement
   on renumbering when a site changes provider.  This benefit only
   occurs when the site changes providers within the ISPAC.  As a
   result, a site gains the most benefit from the ISPAC when there are
   many providers within the ISPAC.  Further, a site may have
   substantial costs if changing to a provider which is distant.  Thus,
   a site derives the most benefit if there are multiple providers
   within the same geographic area.  Note that this is most effective if
   the different members of the ISPAC are closely comparable
   competitors.  The site will consider reliability, bandwidth and
   service when selecting an alternate provider, in addition to
   considering membership in the ISPAC.

   Note that the end site itself is not a member of the ISPAC and has no
   obligation whatsoever to change to another provider within the ISPAC.

   Another benefit of an ISPAC is for sites that wish to be multi-homed
   through multiple providers.  If such sites are numbered from the
   shared prefix and are connected through multiple providers that are
   all members of the ISPAC, then no extra routing information needs to
   be injected into global routing.  This is a significant benefit
   because even with CIDR, the scalability of global routing can fall
   victim due to the number of prefixes injected due to multi-homed

   Another benefit that aids in the aggregation of routing information
   is the ability of the ISPAC to take multiple smaller providers and
   advertise only the shared prefixes.  Without the ISPAC, each ISP
   would be forced to inject their own routes.  The additional
   aggregation resulting from membership in the ISPAC should greatly
   improve the level of aggregation that can happen across small
   providers, both by allowing the ISPAC a larger block of addresses and
   by automatically aggregating the ISPAC shared prefixes at the border
   of the ISPAC.  An ISPAC also allows a smaller provider to receive
   significant amounts of address space which is not allocated through
   an upstream provider.  This is advantageous as the smaller provider
   may wish to be associated with one or more larger providers.  An
   association with any single larger provider might result in inferior
   routing, and an inability to leave that large provider without
   renumbering.  With an ISPAC, such a member would instead be dependent
   on the continued existence of the ISPAC.

5. Drawbacks of an ISPAC

   Along with these many benefits, there are also a number of drawbacks
   to membership in an ISPAC.  The primary difficulty stems from the
   inability to perform aggregation on the shared prefix within the
   ISPAC.  As noted above, this implies that members of the ISPAC must
   be willing to devote a certain amount of resources to supporting
   ISPAC routing.  Also as mentioned above, the quality of routing for a
   shared prefix may suffer as compared to a private prefix due to the
   aggregation at the ISPAC boundary.  Due to this aggregation, traffic
   will tend to enter the nearest border of the ISPAC.  Without
   aggregation, traffic would seek the closest entry to the ISP.  In
   cases in which these two entries differ, the routing may be

   Another extremely important point about ISPACs is that they do not
   eliminate the need to renumber.  If a site chooses to leave the
   ISPAC, they will have to renumber, just as if they left a non-ISPAC
   provider.  As with the normal renumbering case, certain
   accommodations may be made to ease the transition to the sites new
   prefix.  These circumstances and operations are part of normal CIDR
   prefix management and are not detailed here.

   Similarly, if an ISP leaves an ISPAC, then any of the ISP's customers
   which are numbered out of the shared prefix will have to make a
   difficult decision.  If they choose to remain with the ISP or change
   to a ISP not in the ISPAC, then they will have to renumber.
   Alternately, they can change to another provider within the ISPAC.
   In both this and the above case, the end user ends up renumbering.
   It is important to note these cases as the primary benefit of an
   ISPAC is to decrease, but not eliminate, the need for site

   If an ISPAC is composed of ISPs of differing sizes, there may be some
   difficulty with inequities of responsibilities to the ISPAC.  To see
   this clearly, consider a case where a very large provider and a very
   small provider form an ISPAC.  In this case, the small provider may
   have to provide transit service for the very large provider's
   traffic.  Yet in return, the small provider may receive little
   transit traffic through the very large provider.  Similarly, the very
   large provider may consume a much larger percentage of the shared

6. Generalization

   So far, we have described the simplest and smallest forms of an
   ISPAC.  These consist of a small number of providers aligned around a
   small number of interconnects.  In this section we consider some of
   the situations which can occur with larger ISPACs.  Note that these
   examples are used solely to illustrate the scalability of the concept
   and are not a recommendation for particular ways of using ISPACs.

   An obvious opportunity to use ISPACs is in metropolitan areas,
   grouping providers around population centers.  If the metropolitan
   area has a sufficiently large population, it probably also has a
   significant amount of traffic, one or more interconnects, and a
   number of ISPs.  An ISPAC in this situation bears a superficial
   resemblance to geographic addressing.  However, there are significant
   differences.  Unlike geographic addressing, participation in the
   ISPAC is wholly voluntary.  This maximizes the choices that the site
   has, and so this should be considered a significant advantage.
   Further, this allows competition between ISPACs, which is vital in
   ensuring that the ISPAC provides quality service to its sites.
   Finally, the ISPAC would easily allow private interconnects from
   members to non-members, which may or may not advertise the shared
   prefix.  Scaling the number of interconnects is key to insuring high
   connectivity in the topology, for distributing traffic, and for
   optimizing routing.

   ISPACs may be further generalized along topological or geographic
   boundaries, although neither of these directions is required.  For
   example, if there are several providers which are at a number of
   major interconnects, they may wish to form an ISPAC, even though the
   interconnects may be widely separated.  Such an ISPAC should be more
   robust since it would not partition without the failure of several
   interconnects.  With this size of ISPAC, it is also possible for a
   company to place both its headquarters and its remote offices within
   the ISPAC shared prefix.  This is beneficial in that the company does
   not inject multiple prefixes into global routing.

   If we continue this generalization to its logical extreme, we end up
   with a global ISPAC which all providers belong to.  The shared prefix
   is similar to the provider independent address space that exists
   today.  However, there is one subtle difference in that the address
   space is independently managed under the direction of the ISPAC.

7. Conclusions

   To summarize, Internet Service Provider Address Coalitions (ISPACs)
   are ad hoc organizations which can be formed by groups of ISPs.
   There are significant technical and administrative difficulties to be
   overcome in forming and maintaining these organizations.  However,
   the benefits that result from these organizations can be significant
   both to end users and providers alike.  At the same time, if executed
   correctly, ISPACs are not detrimental to the scalability of Internet
   routing, and may, in some cases, aid that scalability.

   The primary benefit of an ISPAC is a reduction in the need for end
   sites to renumber.  This need is not eliminated with the application
   of ISPACs, but can be significantly reduced, depending on the exact
   deployment of ISPACs.  The benefits to the ISP include the ability to
   offer a desired service to interested sites, the ability to utilize
   address space that is independent of larger providers, and the
   ability to decrease the number of prefixes that it advertises.

7. Security Considerations

   Security issues are not discussed in this document.

8. Acknowledgments

   Much of this memo is the result of ongoing fruitful discussions with
   Yakov Rekhter and the work done on stratum based aggregation.  Joel
   Halpern (NewBridge) and Bill Manning (ISI) were kind enough to review
   and comment on a previous version of this paper.

9. Author's addresses

   Tony Li
   Juniper Networks, Inc.
   3260 Jay St.
   Santa Clara, CA 95051
   Phone:+1 408 327 1906
   Fax:  +1 408 327 1910
   email: tli@juniper.net

10. References

   [1] Fuller, V., Li, T., Yu, J., and Varadhan, K., `Supernetting: an
   Address Assignment and Aggregation Strategy', RFC1519, September 1993

   [2] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
   with CIDR", RFC1518, September 1993