Multi6 I. van Beijnum
Internet-Draft June 30, 2003
Expires: December 29, 2003
Provider-Internal Aggregation based on Geography to Support
Multihoming in IPv6
draft-van-beijnum-multi6-isp-int-aggr-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on December 29, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
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.
NOTE WELL
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
regions:
[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
circuit.
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
time.
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
topology.
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
implementations.
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
networks.
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
NL
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
progress
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