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Versions: 00 01                                                         
Multi6                                                    I. van Beijnum
Internet-Draft                                             June 30, 2003
Expires: December 29, 2003

      Provider-Internal Aggregation based on Geography to Support
                          Multihoming in IPv6

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on December 29, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   Current 6bone backbone routing guidelines prohibit traditional
   multihoming in IPv6, because current IPv4-style multihoming doesn't
   scale. This stands in the way of successful adoption of IPv6. The
   solution outlined in this memo proposes aggregating the routing
   information for multihomed destinations inside service provider
   networks based on geography to accomplish scalable multihoming in
   IPv6 using current protocols and implementations. This solution does
   not require network operators to increase the density of
   interconnection; nor does it require significant cooperation or
   simultaneous adoption.

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1. Introduction

   Current IPv4 and IPv6 interdomain routing operational practices
   depend heavily on aggregation in order to reach the necessary
   scalability. Current aggregation is exclusively service provider
   based: ISPs (Internet Service Providers) obtain blocks of address
   space from the Regional Internet Registries (RIRs) and assign their
   customers addresses from these blocks. Then they announce a single
   route for each block to other networks. This aggregation makes it
   possible for millions of

   organizations to be connected to the internet while limiting the
   global routing table to only slightly more than a hundred thousand
   destination prefixes.

   Unfortunately, provider-based aggregation doesn't work for networks
   connected to the internet over more than one connection
   ("multi-homed" networks). In the current IPv4 internet, multihoming
   is typically done by announcing a route for an independent address
   block to two or more ISPs. The address block may actually be part of
   a larger PA (provider aggregatable) block, but it must be visible in
   the global routing table independently from possible aggregates to
   make multihoming work under all circumstances. This makes it
   impossible for many millions of networks to multihome: the global
   routing table would grow beyond what routers can handle.

   There are efforts underway to provide in IPv6 the failover and load
   balancing functionality present in current "IPv4 style" multihoming
   in different ways that wouldn't increase the size the global routing
   table. However, all these new multihoming solutions are still on the
   drawing board and need changes to protocols and implementations. In
   the mean time, the current 6Bone backbone routing guidelines
   [RFC2772] don't allow non-aggregated routes in the IPv6 global
   routing table and thereby make IPv4-style multihoming impossible.

   This memo proposes new operational practices that will allow networks
   to handle a much larger global routing table, so multihoming in IPv6
   can be made possible within a very short time frame. However, it is
   very important to note this isn't a perfect "one size fits all"
   solution that scales to huge numbers of multihomed networks without
   any pain or effort. (See the Limitations section later in this
   document.) But at least this mechanism makes multihoming possible
   almost immediately, without having to wait for protocols and
   implementations to be changed or even for network operators to
   reconfigure their networks. The latter can be done later, and on a
   per-network basis, as the size of the global routing table becomes
   problematic for individual networks. The idea is to make multihoming
   possible now, while providing networks with the means to control the

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   size of the routing table in their routers later as necessary.

   After implementing the necessary filtering mechanisms, growth to
   several million multihomed networks world wide should be possible
   without much trouble. In theory, this mechanism can support many
   hundreds of millions multihomed networks, but this will be hard to
   accomplish in practice, so work on more advanced multihoming
   solutions should continue.


   This mechanism does NOT require networks to announce geographical
   aggregate routes to anyone; those aggregates are only used
   internally. In this respect, the mechanism discussed here is very
   different from earlier geographical aggregation proposals.

   The full routing information for all destinations connected to the
   internet is still present in each network (AS) that doesn't use a
   default route (in other words, is part of the default-free zone).
   It's just that this information is distributed over all the routers
   in the network so each router holds part of the information, rather
   than being replicated as is done today, where each router holds a
   full copy of the information.

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2. How It Works

   To make multihoming (as we know it today) possible, individual routes
   must be present in the global routing table. But in order to fit the
   routing table into a router, there must be aggregation. These
   requirements seem at odds with each other. This is because there is a
   hidden assumption: the full global routing table must be present in
   all routers that are part of the default-free zone. Dropping this
   requirement makes everything much more complex, but it is possible.
   The global routing table can then be split into several parts, where
   individual routers all handle one (or a few) of those parts.

   This works as long as traffic for a certain subset of the destination
   networks present in the global routing table is always sent to a
   router containing that part of the global routing table. The obvious
   way to accomplish this is for each router to announce an aggregate
   covering the part of the global routing table it serves. For
   instance, if a network has four routers and wants to divide routing
   information for the IPv6 global unicast address space over those
   routers, it could have router A handle 2000::/5, router B 2800::/5,
   router C 3000::/5 and router D 3800::/5. So if this network peers
   with another network that announces 2200:abc::/35 and 3ffe:def::/35,
   all routers except router A filter out the first route, and all
   routers except router D filter out the second route. When router C
   then has a packet for 2200:abc:1:2::1, it sends the packet to router
   A (because router A announces the 2000::/5 aggregate) and router A
   delivers the packet to the right peer. Note that this behavior is
   completely hidden from the peer: the aggregates are only used within
   the local network, they are not announced to peers. To avoid
   confusion with regular provider aggregatable routes, the term "pilot
   routes" will be used for this type of private aggregates.

   This practice scales relatively well: by adding more routers, it is
   possible to accommodate a global routing table of arbitrary size.
   (These extra routers must be "border routers" that interconnect with
   other networks.) However, there is a major problem: traffic for
   certain address ranges must always first be transported to the
   location of the router handling this address range. So if two
   end-users in Europe want to communicate, but the address range for
   one of them is handled in North America by the other's ISP, and the
   other's address range is handled by a router in Japan, this traffic
   that has the potential to stay within the region has to circle the
   globe. This "scenic routing" can be avoided by assigning address
   space to multihomers in a geographically aggregatable manner. This
   way, networks can have a range of addresses be handled by a router in
   the region where the addresses are used. However, this is not a
   strict requirement. For instance, a network that only has a presence
   in the US doesn't necessarily have to interconnect with other

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   networks in Europe or Asia. In practice, it will have routers at the
   US East Coast (where many European networks are present) handle the
   European address ranges, and routers at the US West Coast (where many
   Asian networks are present) handle the Asian address ranges.

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3. Operational Details

   First of all: more specific routes from customers are usually not
   filtered. They are announced to peers at all interconnect locations.
   It is up to the network receiving the routes to filter them. Only
   when two networks agree on where to exchange routing information for
   certain geographic aggregates, there may be outbound filtering of
   more specific multihomed routes.

   The aggregation scheme works as follows. The network is divided into
   zones. The exact way in which this is done depends on the particular
   topology of the network, and doesn't have to match the layout of
   other networks. Static pilot routes for all address ranges used
   within the zone are configured on at least two routers (for
   redundancy) in that zone (or as close to the zone as is practical).
   Then both EBGP and IBGP filters are configured per peer. The IBGP
   filters are applied to all sessions with routers in other zones (not
   to sessions with other routers within the zone) and filter out the
   more specific routes falling within the address ranges used in the
   zone. The EBGP filters do the opposite and allow only more specific
   routes for destinations within the region. This makes sure more
   specific multihomed routes are allowed in the routing table within
   the zone, but aren't announced over IBGP to other zones.

3.1 Interconnection

   Since interconnection is not an exact science, there may not be
   adequate interconnection within the zone with some peer networks.
   When this is the rule rather than the exception, this indicates the
   zones are too small. Increasing the zone or merging several zones
   will make sure there is interconnection with most peer networks
   within the zone itself. For the few networks for which
   interconnection within the zone isn't possible, EBGP filters that
   always allow all more specific routes are used. Also, these routes
   are tagged with an internal community that prevents them from being
   filtered in IBGP. As a result, there is no aggregation for these
   peers, but there is still full connectivity. It should be possible to
   limit this de-aggregation to a small number of zones rather than the
   entire network with more sophisticated filtering.

3.2 Zone Partitioning

   It is important that regions are never partitioned, because when this
   happens, packets for certain destinations will loop. The router
   inside the zone will route them outside the zone because of the more
   specifics pointing to the other partition of the zone over a router
   that isn't part of the zone, and the first router outside the zone
   will route the packets back into the region to the closest router

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   announcing the pilot route.

3.3 Example Picture

   The following picture represents an AS with four routers and eight
   peers, divided into two zones that each handle routing for three

          [S]   [T]               [U]   [V]
           |     |                 |     |
         b E   c E     ZONE 1      E c   E c
         a B   b B     REGIONS     B c   B b
         a G   a G     A, B, C     G b   G a
         v P   v P                 P v   P v
           |     |                 |     |
        +--+-----+-+  <abbccc    +-+-----+--+
        |   RTR 1  +---I-B-G-P---+   RTR 2  |
        +--+------++    cbbaaa>  +-+-----+--+
         C |      |                |     | C
         B I    +-|----I-B-G-P-----+     I B
         A |    | | GFE>     <ABC        | A
         v B    | |                      B v
           G ^  | |                    ^ G
           | E  | | CBA>     <EFG      E |
           P F  | +----I-B-G-P-----+   F P
           | G  |                  |   G |
        +--+----+--+  <eefggg    +-+-----+--+
        |   RTR 3  +---I-B-G-P---+   RTR 4  |
        +--+-----+-+    gfffee>  +-+-----+--+
           |     |                 |     |
         ^ E   ^ E                 E ^   E ^
         e B   e B     ZONE 2      B e   B e
         f G   f G     REGIONS     G f   G g
         f P   g P     E, F, G     P g   P g
           |     |                 |     |
          [W]   [X]               [Y]   [Z]

   [S], [T], [U], [V],  Peer EBGP routers
   [W], [X], [Y], [Z]
   RTR 1, RTR 2         Routers in zone 1
   RTR 3, RTR 4         Routers in zone 2
   A, B, C, E, F, G     Pilot (aggregate) routes
   a, b, c, e, f, g     Individual /48 routes for end-user networks
   <, > ^, v           The direction of the routing information flow

                Figure 1: Geographic aggregation example

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4. Migration

   Migration from a regular, non-aggregated setup to full geographical
   aggregation doesn't have to be immediate. The process can be carried
   out is several steps:

   1.  The border router handling most of the traffic to a specific
       geographical destination or aggregate of several destinations is
       promoted to "designated router" for the matching address range.
       The designated router is configured to announce a pilot route
       over IBGP and with filters that don't allow more specifics for
       the destinations covered by the pilot route to be announced over
       IBGP to non-border routers. Now only border routers have the more
       specific routes.

   2.  Border routers are configured with EBGP filters to filter out
       incoming more specific routes covered by pilot routes announced
       by far away designated routers. (For instance, routers in Europe
       are configured to filter out American more specifics for which an
       American router announces a pilot route.) The designated router
       is configured to no longer send these more specifics over IBGP to
       the routers that now filter those same routes on EBGP sessions.
       (For the American routers, their European IBGP neighbors now
       essentially become part of the group "non-border routers".) Now
       each border router only has a subset of all multihomed more
       specifics in its routing table.

   Step 1 can be implemented on individual routers one at a time, and,
   barring configuration mistakes, doesn't pose any risks. There is only
   one pilot route, and only more specific routes announced by the same
   router as the pilot route are suppressed. Since both the new pilot
   route and the now suppressed more specific routes point to the same
   border router, the way packets are routed through the network is
   completely identical and there is no risk of loops. If different a
   router than the designated router has the preferred external route
   for a more specific, this more specific route will be announced as
   before, since only the designated router is configured to filter out
   these more specifics.

   When the designated router is the one holding the best external
   route, non-border routers won't see any more specific routes for this
   destination. The designated router has a filter, and the other border
   routers don't announce the route over IBGP because they aren't the
   ones holding the best route. To aid aggregation, the designated
   router can be configured to increase the IBGP Local Preference
   attribute for the more specifics it acts as designated router for.
   This way, the route over the designated router is always preferred,
   even if another router has a matching more specific with a shorter AS

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   path or better Multi Exit Discriminator metric.

   When the designated router becomes unreachable or loses its external
   routes, there will be automatic de-aggregation: more specific routes
   are announced by other routers.

   Step 2 can also be implemented one router at a time. The new EBGP
   filters should be installed first, after which the designated router
   can be configured to no longer announce more specifics to the border
   routers with the new EBGP filters. If this is done the other way
   around, more specifics will leak over IBGP and there will be
   non-optimal routing. Without step 2, there is no aggregation in
   border routers: they need to hear the designated router announce a
   "better" more specific, or they will start to announce their own over
   IBGP. Introducing step 2 introduces the risk that certain
   destinations become unreachable when there is an outage. For
   instance, when European routers no longer see American more
   specifics, and the European and American parts of the network become
   partitioned, it is no longer possible for the European routers to
   send traffic to American destinations, even if there is peering in
   Europe that would have made this possible before. This step should
   only be taken if the risk of network partitions is negligible.

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5. Limitations

   Since this scheme depends on geography for aggregation, it only works
   well for organizations that connect to the internet in locations that
   are close together. An organization with a network spanning multiple
   countries and connecting to the internet in all those countries isn't
   geographically aggregatable, and neither is an organization
   connecting to ISPs very far way, for instance by means of a satellite

   These types of organizations must choose address space falling within
   a geographic area that doesn't (fully) fit if they elect to use the
   type of address space this aggregation scheme uses. This choice will
   have consequences on routing efficiency, and when the infrastructure
   changes, the organization may need to adopt a new address range to
   minimize the routing efficiencies created by the change.

   Although the notion of geographic aggregation has been discussed many
   times within the IETF over the past eight years or so, this approach
   is generally believed to be flawed since the topology of the networks
   that make up the internet and the interconnection between those
   networks doesn't align well with geography. This is indeed a problem,
   but it doesn't automatically make geographical aggregation useless,
   it only makes it less effective. Since network topology is under
   constant revision, and as networks get faster, the main disadvantage
   of "scenic routing" (the increased speed of light delay) becomes more
   acute as bandwidth increases, it is certainly not unthinkable for the
   topology of the internet to align itself more with geography over

   Additionally, while the past decade or so the trend among high speed
   IP backbones was to run IP as directly over the physical
   infrastructure as possible, today this trend seems to be reversing
   with the adoption of MPLS and switched optical services. This allows
   two routers to communicate directly at the IP level without the need
   for a direct physical connection, making it possible for two routers
   in the same aggregation area that don't share a physical connection
   to exchange packets without the need for routers outside the
   aggregation area to know routing information for the area. Instead, a
   direct virtual lower layer connection is used so the traffic can pass
   through areas where the routing information isn't known, enabling
   aggregation to become largely independent from the physical network

   The same can be achieved through tunneling.

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6. Route Visibility for Customers

   In order to be able to do traffic engineering for outbound traffic,
   multihomed customers need to receive a consistent view of the global
   routing table from all their ISPs. If the aggregation levels of
   different ISPs used by a multihomed customer don't match, because of
   the longest match first rule, most of the traffic will flow over the
   ISP doing the least aggregation. To avoid this, ISPs are strongly
   encouraged to provide their customers with a full, unaggregated view
   of the global routing table. If an ISP aggregates internally, such a
   view could be obtained by the customer by having an EBGP (multihop if
   necessary) session with one or more route servers, in addition to the
   regular EBGP session to the next hop router.

   ISPs should also provide their customers with pilot routes at all
   aggregation levels, even if the ISPs themselves don't (yet)
   aggregate. This makes it possible for customers to filter out more
   specifics and still maintain a consistent view of the global routing
   table. If an ISP can't do this immediately (adding a large number of
   pilot routes is a lot of work) the ISP should establish a time frame
   for implementing the necessary pilot routes and communicate this to
   existing and potential customers. A reasonable time frame would be
   six months to implement continent/country/province/state level pilot
   routes for the whole world, a year to implement metropolitan area
   pilot routes for the regions the ISP is active, and 18 months to
   implement world wide metropolitan area pilot routes, starting from
   the moment a geographically aggregatable address allocation mechanism
   is implemented.

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7. Traffic Flow

   Larger ISP and ISP-like networks that interconnect with other
   networks in more than one location must have a policy on how to
   select the interconnect location used for traffic to those other
   networks. At present, the most widely adopted policy is "early exit"
   or "hot potato": packets are routed to the closest interconnect
   location where the other network is present and delivered to the
   destination network there. As a result, packets travel most of the
   way over the destination network. If both networks use the early exit
   policy, traffic in one direction will travel most of the way over one
   network, and traffic in the other direction most of the way over the
   other network, so the policy is "fair" as long as the traffic volumes
   are fairly equal in both directions. This policy is implemented by
   not changing the default behavior for the most widely available BGP

   Since the aggregation scheme described in this document requires
   traffic to be transported to a location where more specific routing
   information is known, and this location is presumably close to the
   destination of the packet, adoption of this scheme leads to a "late
   exit" routing policy for multihomed traffic. Assuming early exit is
   still used for single homed traffic, there are four possible
   permutations for the traffic flow between any two hosts:

   1.  Hosts A and B both single homed: both early exit = "fair"

   2.  Host A single homed, host B multihomed: traffic is exchanged
       close to host B = host A's network does most of the work

   3.  Host A multihomed, host B single homed: traffic is exchanged
       close to host A = host B's network does most of the work

   4.  Hosts A and B both multihomed: both late exit = "fair"

   Since networks can control the level of late exit routing by
   (selectively) de-aggregating and many interconnection (peering)
   agreements call for equal traffic volumes in both directions, the
   potential for changes in the flow of traffic should not adversely
   affect existing networks.

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8. Geographical Address Allocation

   This section establishes an address allocation framework for
   Geographically Aggregatable Provider Independent (GAPI) IPv6
   addresses for the purpose of multihoming. A /16 is divided in a
   hierarchical manner over geographical entities such as parts of
   continents, countries, states, provinces and metropolitan areas, with
   each receiving one or more /32 allocations from which end-user
   assignments can be made. The number of /32 allocations for a
   geographical entity depends on the current population.

   Note that this section was previously a separate draft.

   The geographical aggregation scheme splits the global routing domain
   into a number of smaller regional ones, where flat routing happens in
   each region. Ideally, outside the region only aggregates are visible.
   For simplicity and to allow efficient implementation, the framework
   presented here requires "areas" where flat routing takes place to
   have a fixed size: a /32 holding up to 65536 (2^16) fixed sized
   end-user /48 assignments. The maximum number of these /32 areas is
   also 65536. Areas are grouped in CIDR-like fashion if a geographic
   region has a population that warrants allocating more than a single /
   32. The highest level of aggregation is the subcontinent or "zone"
   level. There are 13 entities at this level, in order of population:

   1.   China

   2.   Continental Asia

   3.   India

   4.   Northern Africa

   5.   Asian Islands

   6.   Western Europe

   7.   North America

   8.   South America

   9.   Eastern Europe

   10.  Middle East

   11.  Southern Africa

   12.  Central America

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   13.  Oceania

   The next level is the country level. Every country is assigned a
   range of /32 blocks, depending on population. Countries that are
   medium-sized or larger may be subdivided according to existing
   administrative boundaries, such as by state or province. The
   allocation size per state or province must match the population
   relative to the country and other states or provinces. The lowest
   level of aggregation is the metropolitan level. Cities of sufficient
   size are allocated one or more "metro areas". Assignments to
   end-users in, or very close to, a city are drawn from one of the
   metro area /32s allocated to the city. Addresses for end-users in
   small cities or rural areas are drawn from one of the /32 areas
   allocated to the country (if not subdivided), state or province (a
   country/state/province or "CSP" area).

8.1 Allocation policy

   The goals of the allocation policy are:

   1.  Be completely neutral, fair and unbiased, in order to minimize
       the potential for political complications

   2.  Good aggregation at all levels

   3.  Reasonable flexibility

   4.  Ease of implementation

8.2 Country Allocations

   Each independent country is allocated at least one /32 area. The
   allocation size depends on the country's population figure for the
   year 2001. This is divided by the number D1, which equals 131072. The
   result of the division is rounded up to the next power of two.

   This is the number of /32s constituting the country's allocation.

8.3 Zone Allocations

   The subdivision of the globe in 13 zones is relatively arbitrary.
   However, this division fits current and expected future Regional
   Internet Regions well, and limits the population per zone somewhat
   over a strict by-continent subdivision. Zone allocations are chosen
   such that they are large enough to hold the country allocations for
   all countries located within the geographic bounds of the zone. If
   for any of the zones that encompass more than a single country, the

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   number of /32s not allocated to countries is less than 25%, the zone
   allocation size is doubled.

8.4 Subdivision of Large Countries

   When a country has an allocation of 32 or more /32s, this address
   space may be distributed over the country by allocating blocks of /
   32s to existing sub-entities such as provinces or states. The exact
   geographic bounds of these sub-entities must be clear to the general
   public and not subject to any controversy. The size of each
   allocation is determined by dividing the population of the sub-entity
   by the number D2, which is twice D1.

   At least 40% of the country allocation must remain unallocated. If
   necessary, a higher value than D2 may be used as a divisor in this
   country to reach this objective. The average number of /32 areas per
   state or province must be at least 4.

8.5 Metro Allocations

   All cities with a population of at least D2 within the city limits
   are allocated a block of /32s. The population for small cities or
   municipalities that do not qualify for an address block of their own
   is added to the closest city that qualifies, if there is one within
   40 kilometers. (Distance measured center to center.) The size of the
   address block for a city and its surroundings is determined by taking
   the total populace, dividing it by D2 and rounding down to the next
   power of two.

8.6 Reserved for Future Use

   The first 1/64th of each allocation at the country/state/province
   level is reserved for future special uses and must not be allocated
   to lower aggregation levels and not be assigned to end-users.

8.7 Subsequent Allocations

   Whenever allocated address space gets close to running out, the IANA,
   Regional Internet Registry or other organization managing (part of)
   the address space should draw new allocations from the next higher
   level. New blocks of address space may be allocated in a way that is
   different from what is outlined here, if analysis of the coordinates
   for current assignments warrants this.

8.8 End-user Address Assignments

   Per country or state/province, only one /32 block is used initially.
   A new block is used when the first is exhausted, and so on. The /32s

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   allocated to a metropolitan area may be put into use concurrently, if
   there is a reason to do so.

   When requesting IPv6 GAPI addresses, an organization should provide
   justification for the use of GAPI space, and information that makes
   it possible to assign addresses from the right geographic area, in
   addition to the information required by current assignment policies.
   Multihoming is justification for the use of GAPI space. Geographic
   information should consist of the longitude and latitude of the
   primary location where the addresses will be used. This information
   should be accurate within 2 kilometers, as long as any inaccuracies
   don't make the organization appear to be at the other side of an
   administrative border or natural barrier (such as a river).
   Preferably, the requesting organization should also include the
   longitude and latitude of the ISP locations they connect to. However,
   this information may be omitted.

   The minimum assignment size is always /48. Future multihoming
   solutions may not support the longest match first rule.

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9. IANA Considerations

   If this scheme is adopted, the number of networks requiring an
   Autonomous System number will rise beyond what can be accommodated
   using the current 16-bit AS number space. There is a draft proposing
   the use of 32-bit AS numbers [32bitAS]. Since having a universally
   recognized AS number is less important for a multihomed "leaf"
   network than for a transit network, it is recommended that the 32-bit
   AS number capability be implemented as soon as possible. All
   multihomed networks requesting an AS number that are capable of using
   a 32-bit AS number should be assigned an AS number higher than 65535,
   so 16-bit compatible AS numbers remain available for transit

   The IANA is requested to allocate /16 worth of IPv6 address space for
   GAPI, and the Regional Internet Registries are asked to further
   assign this address space to end-user organizations. The Regional
   Internet Registries should take the requester's geographic location
   into consideration when assigning address space.

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10. Security Considerations

   Having addresses that are closely tied to an organization's location
   may be undesirable in certain situations. Organizations requesting
   address space should consider the consequences of using GAPI address
   space, and are encouraged to use provider aggregatable address space
   if and when they want to avoid disclosing their location.

   Some organizations may be uncomfortable with providing very accurate
   longitude and latitude information when requesting address space.
   They may introduce a 2 kilometer inaccuracy to avoid exact
   pinpointing, as described in section 6. In addition, the Regional
   Internet Registry or other organization responsible for assigning
   address space must not make location information public.
   Specifically, this information should not appear as a result of whois
   queries. Registries are encouraged to provide aggregated location
   information for policy development purposes, but only as long as this
   information is anonymized and can't be tied to a single organization
   or small group of organizations.

   This aggregation scheme doesn't propose any changes to protocols or
   implementations, so it doesn't introduce any new protocol or
   implementation risks. However, there is one problem: since routing
   information is removed from large parts of the network, it is no
   longer possible to use the routing table to do ingress filtering
   [RFC2267] using the "unicast RPF" feature implemented by several
   router vendors. The alternative, having statically configured filter
   lists, doesn't scale. This leaves networks implementing this
   aggregation scheme with no protection against incoming packets with
   falsified source addresses, so it is highly recommended that network
   operators make sure they don't generate or accept from customers
   packets with falsified source addresses and that vendors implement
   mechanisms to trace back the source of these falsified packets.

Author's Address

   Iljitsch van Beijnum
   Karel Roosstraat 95
   2571 BG  The Hague

   EMail: iljitsch@muada.com

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Appendix A. References

   [RFC2267] RFC 2267, "Network Ingress Filtering: Defeating Denial of
   Service Attacks which employ IP Source Address Spoofing"

   [RFC2772] RFC 2772, "6Bone Backbone Routing Guidelines"

   [32bitAS] "BGP support for four-octet AS number space", work in

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   Funding for the RFC Editor function is currently provided by the
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