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Network Working Group F. Baker
Request for Comments: 3704 Cisco Systems
Updates: 2827 P. Savola
BCP: 84 CSC/FUNET
Category: Best Current Practice March 2004
Ingress Filtering for Multihomed Networks
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
BCP 38, RFC 2827, is designed to limit the impact of distributed
denial of service attacks, by denying traffic with spoofed addresses
access to the network, and to help ensure that traffic is traceable
to its correct source network. As a side effect of protecting the
Internet against such attacks, the network implementing the solution
also protects itself from this and other attacks, such as spoofed
management access to networking equipment. There are cases when this
may create problems, e.g., with multihoming. This document describes
the current ingress filtering operational mechanisms, examines
generic issues related to ingress filtering, and delves into the
effects on multihoming in particular. This memo updates RFC 2827.
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RFC 3704 Ingress Filtering for Multihomed Networks March 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Different Ways to Implement Ingress Filtering . . . . . . . . 4
2.1 Ingress Access Lists . . . . . . . . . . . . . . . . . . . 4
2.2 Strict Reverse Path Forwarding . . . . . . . . . . . . . . 5
2.3 Feasible Path Reverse Path Forwarding . . . . . . . . . . 6
2.4 Loose Reverse Path Forwarding . . . . . . . . . . . . . . 6
2.5 Loose Reverse Path Forwarding Ignoring Default Routes . . 7
3. Clarifying the Applicability of Ingress Filtering . . . . . . 8
3.1 Ingress Filtering at Multiple Levels . . . . . . . . . . . 8
3.2 Ingress Filtering to Protect Your Own Infrastructure . . . 8
3.3 Ingress Filtering on Peering Links . . . . . . . . . . . . 9
4. Solutions to Ingress Filtering with Multihoming . . . . . . . 9
4.1 Use Loose RPF When Appropriate . . . . . . . . . . . . . . 10
4.2 Ensure That Each ISP's Ingress Filter Is Complete . . . . 11
4.3 Send Traffic Using a Provider Prefix Only to That Provider 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Conclusions and Future Work . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 14
9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
BCP 38, RFC 2827 [1], is designed to limit the impact of distributed
denial of service attacks, by denying traffic with spoofed addresses
access to the network, and to help ensure that traffic is traceable
to its correct source network. As a side effect of protecting the
Internet against such attacks, the network implementing the solution
also protects itself from this and other attacks, such as spoofed
management access to networking equipment. There are cases when this
may create problems, e.g., with multihoming. This document describes
the current ingress filtering operational mechanisms, examines
generic issues related to ingress filtering and delves into the
effects on multihoming in particular.
RFC 2827 recommends that ISPs police their customers' traffic by
dropping traffic entering their networks that is coming from a source
address not legitimately in use by the customer network. The
filtering includes but is in no way limited to the traffic whose
source address is a so-called "Martian Address" - an address that is
reserved [3], including any address within 0.0.0.0/8, 10.0.0.0/8,
127.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 224.0.0.0/4, or
240.0.0.0/4.
The reasoning behind the ingress filtering procedure is that
Distributed Denial of Service Attacks frequently spoof other systems'
source addresses, placing a random number in the field. In some
attacks, this random number is deterministically within the target
network, simultaneously attacking one or more machines and causing
those machines to attack others with ICMP messages or other traffic;
in this case, the attacked sites can protect themselves by proper
filtering, by verifying that their prefixes are not used in the
source addresses in packets received from the Internet. In other
attacks, the source address is literally a random 32 bit number,
resulting in the source of the attack being difficult to trace. If
the traffic leaving an edge network and entering an ISP can be
limited to traffic it is legitimately sending, attacks can be
somewhat mitigated: traffic with random or improper source addresses
can be suppressed before it does significant damage, and attacks can
be readily traced back to at least their source networks.
This document is aimed at ISP and edge network operators who 1) would
like to learn more of ingress filtering methods in general, or 2) are
already using ingress filtering to some degree but who would like to
expand its use and want to avoid the pitfalls of ingress filtering in
the multihomed/asymmetric scenarios.
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In section 2, several different ways to implement ingress filtering
are described and examined in the generic context. In section 3,
some clarifications on the applicability of ingress filtering methods
are made. In section 4, ingress filtering is analyzed in detail from
the multihoming perspective. In section 5, conclusions and potential
future work items are identified.
2. Different Ways to Implement Ingress Filtering
This section serves as an introduction to different operational
techniques used to implement ingress filtering as of writing this
memo. The mechanisms are described and analyzed in general terms,
and multihoming-specific issues are described in Section 4.
There are at least five ways one can implement RFC 2827, with varying
impacts. These include (the names are in relatively common usage):
o Ingress Access Lists
o Strict Reverse Path Forwarding
o Feasible Path Reverse Path Forwarding
o Loose Reverse Path Forwarding
o Loose Reverse Path Forwarding ignoring default routes
Other mechanisms are also possible, and indeed, there are a number of
techniques that might profit from further study, specification,
implementation, and/or deployment; see Section 6. However, these are
out of scope.
2.1. Ingress Access Lists
An Ingress Access List is a filter that checks the source address of
every message received on a network interface against a list of
acceptable prefixes, dropping any packet that does not match the
filter. While this is by no means the only way to implement an
ingress filter, it is the one proposed by RFC 2827 [1], and in some
sense the most deterministic one.
However, Ingress Access Lists are typically maintained manually; for
example, forgetting to have the list updated at the ISPs if the set
of prefixes changes (e.g., as a result of multihoming) might lead to
discarding the packets if they do not pass the ingress filter.
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Naturally, this problem is not limited to Ingress Access Lists -- it
is inherent to Ingress Filtering when the ingress filter is not
complete. However, usually Ingress Access Lists are more difficult
to maintain than the other mechanisms, and having an outdated list
can prevent legitimate access.
2.2. Strict Reverse Path Forwarding
Strict Reverse Path Forwarding (Strict RPF) is a simple way to
implement an ingress filter. It is conceptually identical to using
access lists for ingress filtering, with the exception that the
access list is dynamic. This may also be used to avoid duplicate
configuration (e.g., maintaining both static routes or BGP prefix-
list filters and interface access-lists). The procedure is that the
source address is looked up in the Forwarding Information Base (FIB)
- and if the packet is received on the interface which would be used
to forward the traffic to the source of the packet, it passes the
check.
Strict Reverse Path Forwarding is a very reasonable approach in front
of any kind of edge network; in particular, it is far superior to
Ingress Access Lists when the network edge is advertising multiple
prefixes using BGP. It makes for a simple, cheap, fast, and dynamic
filter.
But Strict Reverse Path Forwarding has some problems of its own.
First, the test is only applicable in places where routing is
symmetrical - where IP datagrams in one direction and responses from
the other deterministically follow the same path. While this is
common at edge network interfaces to their ISP, it is in no sense
common between ISPs, which normally use asymmetrical "hot potato"
routing. Also, if BGP is carrying prefixes and some legitimate
prefixes are not being advertised or not being accepted by the ISP
under its policy, the effect is the same as ingress filtering using
an incomplete access list: some legitimate traffic is filtered for
lack of a route in the filtering router's Forwarding Information
Base.
There are operational techniques, especially with BGP but somewhat
applicable to other routing protocols as well, to make strict RPF
work better in the case of asymmetric or multihomed traffic. The ISP
assigns a better metric which is not propagated outside of the
router, either a vendor-specific "weight" or a protocol distance to
prefer the directly received routes. With BGP and sufficient
machinery in place, setting the preferences could even be automated,
using BGP Communities [2]. That way, the route will always be the
best one in the FIB, even in the scenarios where only the primary
connectivity would be used and typically no packets would pass
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through the interface. This method assumes that there is no strict
RPF filtering between the primary and secondary edge routers; in
particular, when applied to multihoming to different ISPs, this
assumption may fail.
2.3. Feasible Path Reverse Path Forwarding
Feasible Path Reverse Path Forwarding (Feasible RPF) is an extension
of Strict RPF. The source address is still looked up in the FIB (or
an equivalent, RPF-specific table) but instead of just inserting one
best route there, the alternative paths (if any) have been added as
well, and are valid for consideration. The list is populated using
routing-protocol specific methods, for example by including all or N
(where N is less than all) feasible BGP paths in the Routing
Information Base (RIB). Sometimes this method has been implemented
as part of a Strict RPF implementation.
In the case of asymmetric routing and/or multihoming at the edge of
the network, this approach provides a way to relatively easily
address the biggest problems of Strict RPF.
It is critical to understand the context in which Feasible RPF
operates. The mechanism relies on consistent route advertisements
(i.e., the same prefix(es), through all the paths) propagating to all
the routers performing Feasible RPF checking. For example, this may
not hold e.g., in the case where a secondary ISP does not propagate
the BGP advertisement to the primary ISP e.g., due to route-maps or
other routing policies not being up-to-date. The failure modes are
typically similar to "operationally enhanced Strict RPF", as
described above.
As a general guideline, if an advertisement is filtered, the packets
will be filtered as well.
In consequence, properly defined, Feasible RPF is a very powerful
tool in certain kinds of asymmetric routing scenarios, but it is
important to understand its operational role and applicability
better.
2.4. Loose Reverse Path Forwarding
Loose Reverse Path Forwarding (Loose RPF) is algorithmically similar
to strict RPF, but differs in that it checks only for the existence
of a route (even a default route, if applicable), not where the route
points to. Practically, this could be considered as a "route
presence check" ("loose RPF is a misnomer in a sense because there is
no "reverse path" check in the first place).
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The questionable benefit of Loose RPF is found in asymmetric routing
situations: a packet is dropped if there is no route at all, such as
to "Martian addresses" or addresses that are not currently routed,
but is not dropped if a route exists.
Loose Reverse Path Forwarding has problems, however. Since it
sacrifices directionality, it loses the ability to limit an edge
network's traffic to traffic legitimately sourced from that network,
in most cases, rendering the mechanism useless as an ingress
filtering mechanism.
Also, many ISPs use default routes for various purposes such as
collecting illegitimate traffic at so-called "Honey Pot" systems or
discarding any traffic they do not have a "real" route to, and
smaller ISPs may well purchase transit capabilities and use a default
route from a larger provider. At least some implementations of Loose
RPF check where the default route points to. If the route points to
the interface where Loose RPF is enabled, any packet is allowed from
that interface; if it points nowhere or to some other interface, the
packets with bogus source addresses will be discarded at the Loose
RPF interface even in the presence of a default route. If such
fine-grained checking is not implemented, presence of a default route
nullifies the effect of Loose RPF completely.
One case where Loose RPF might fit well could be an ISP filtering
packets from its upstream providers, to get rid of packets with
"Martian" or other non-routed addresses.
If other approaches are unsuitable, loose RPF could be used as a form
of contract verification: the other network is presumably certifying
that it has provided appropriate ingress filtering rules, so the
network doing the filtering need only verify the fact and react if
any packets which would show a breach in the contract are detected.
Of course, this mechanism would only show if the source addresses
used are "martian" or other unrouted addresses -- not if they are
from someone else's address space.
2.5. Loose Reverse Path Forwarding Ignoring Default Routes
The fifth implementation technique may be characterized as Loose RPF
ignoring default routes, i.e., an "explicit route presence check".
In this approach, the router looks up the source address in the route
table, and preserves the packet if a route is found. However, in the
lookup, default routes are excluded. Therefore, the technique is
mostly usable in scenarios where default routes are used only to
catch traffic with bogus source addresses, with an extensive (or even
full) list of explicit routes to cover legitimate traffic.
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Like Loose RPF, this is useful in places where asymmetric routing is
found, such as on inter-ISP links. However, like Loose RPF, since it
sacrifices directionality, it loses the ability to limit an edge
network's traffic to traffic legitimately sourced from that network.
3. Clarifying the Applicability of Ingress Filtering
What may not be readily apparent is that ingress filtering is not
applied only at the "last-mile" interface between the ISP and the end
user. It's perfectly fine, and recommended, to also perform ingress
filtering at the edges of ISPs where appropriate, at the routers
connecting LANs to an enterprise network, etc. -- this increases the
defense in depth.
3.1. Ingress Filtering at Multiple Levels
Because of wider deployment of ingress filtering, the issue is
recursive. Ingress filtering has to work everywhere where it's used,
not just between the first two parties. That is, if a user
negotiates a special ingress filtering arrangement with his ISP, he
should also ensure (or make sure the ISP ensures) that the same
arrangements also apply to the ISP's upstream and peering links, if
ingress filtering is being used there -- or will get used, at some
point in the future; similarly with the upstream ISPs and peers.
In consequence, manual models which do not automatically propagate
the information to every party where the packets would go and where
ingress filtering might be applied have only limited generic
usefulness.
3.2. Ingress Filtering to Protect Your Own Infrastructure
Another feature stemming from wider deployment of ingress filtering
may not be readily apparent. The routers and other ISP
infrastructure are vulnerable to several kinds of attacks. The
threat is typically mitigated by restricting who can access these
systems.
However, unless ingress filtering (or at least, a limited subset of
it) has been deployed at every border (towards the customers, peers
and upstreams) -- blocking the use of your own addresses as source
addresses -- the attackers may be able to circumvent the protections
of the infrastructure gear.
Therefore, by deploying ingress filtering, one does not just help the
Internet as a whole, but protects against several classes of threats
to your own infrastructure as well.
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3.3. Ingress Filtering on Peering Links
Ingress filtering on peering links, whether by ISPs or by end-sites,
is not really that much different from the more typical "downstream"
or "upstream" ingress filtering.
However, it's important to note that with mixed upstream/downstream
and peering links, the different links may have different properties
(e.g., relating to contracts, trust, viability of the ingress
filtering mechanisms, etc.). In the most typical case, just using an
ingress filtering mechanism towards a peer (e.g., Strict RPF) works
just fine as long as the routing between the peers is kept reasonably
symmetric. It might even be considered useful to be able to filter
out source addresses coming from an upstream link which should have
come over a peering link (implying something like Strict RPF is used
towards the upstream) -- but this is a more complex topic and
considered out of scope; see Section 6.
4. Solutions to Ingress Filtering with Multihoming
First, one must ask why a site multihomes; for example, the edge
network might:
o use two ISPs for backing up the Internet connectivity to ensure
robustness,
o use whichever ISP is offering the fastest TCP service at the
moment,
o need several points of access to the Internet in places where no
one ISP offers service, or
o be changing ISPs (and therefore multihoming only temporarily).
One can imagine a number of approaches to working around the
limitations of ingress filters for multihomed networks. Options
include:
1. Do not multihome.
2. Do not use ingress filters.
3. Accept that service will be incomplete.
4. On some interfaces, weaken ingress filtering by using an
appropriate form of loose RPF check, as described in Section 4.1.
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5. Ensure, by BGP or by contract, that each ISP's ingress filter is
complete, as described in Section 4.2.
6. Ensure that edge networks only deliver traffic to their ISPs that
will in fact pass the ingress filter, as described in Section
4.3.
The first three of these are obviously mentioned for completeness;
they are not and cannot be viable positions; the final three are
considered below.
The fourth and the fifth must be ensured in the upstream ISPs as
well, as described in Section 3.1.
Next, we now look at the viable ways for dealing with the side-
effects of ingress filters.
4.1. Use Loose RPF When Appropriate
Where asymmetric routing is preferred or is unavoidable, ingress
filtering may be difficult to deploy using a mechanism such as strict
RPF which requires the paths to be symmetrical. In many cases, using
operational methods or feasible RPF may ensure the ingress filter is
complete, like described below. Failing that, the only real options
are to not perform ingress filtering, use a manual access-list
(possibly in addition to some other mechanisms), or to using some
form of Loose RPF check.
Failing to provide any ingress filter at all essentially trusts the
downstream network to behave itself, which is not the wisest course
of action. However, especially in the case of very large networks of
even hundreds or thousands of prefixes, maintaining manual access-
lists may be too much to ask.
The use of Loose RPF does not seem like a good choice between the
edge network and the ISP, since it loses the directionality of the
test. This argues in favor of either using a complete filter in the
upstream network or ensuring in the downstream network that packets
the upstream network will reject will never reach it.
Therefore, the use of Loose RPF cannot be recommended, except as a
way to measure whether "martian" or other unrouted addresses are
being used.
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4.2. Ensure That Each ISP's Ingress Filter Is Complete
For the edge network, if multihoming is being used for robustness or
to change routing from time to time depending on measured ISP
behavior, the simplest approach will be to ensure that its ISPs in
fact carry its addresses in routing. This will often require the
edge network to use provider-independent prefixes and exchange routes
with its ISPs with BGP, to ensure that its prefix is carried upstream
to the major transit ISPs. Of necessity, this implies that the edge
network will be of a size and technical competence to qualify for a
separate address assignment and an autonomous system number from its
RIR.
There are a number of techniques which make it easier to ensure the
ISP's ingress filter is complete. Feasible RPF and Strict RPF with
operational techniques both work quite well for multihomed or
asymmetric scenarios between the ISP and an edge network.
When a routing protocol is not being used, but rather the customer
information is generated from databases such as Radius, TACACS, or
Diameter, the ingress filtering can be the most easily ensured and
kept up-to-date with Strict RPF or Ingress Access Lists generated
automatically from such databases.
4.3. Send Traffic Using a Provider Prefix Only to That Provider
For smaller edge networks that use provider-based addressing and
whose ISPs implement ingress filters (which they should do), the
third option is to route traffic being sourced from a given
provider's address space to that provider.
This is not a complicated procedure, but requires careful planning
and configuration. For robustness, the edge network may choose to
connect to each of its ISPs through two or more different Points of
Presence (POPs), so that if one POP or line experiences an outage,
another link to the same ISP can be used. Alternatively, a set of
tunnels could be configured instead of multiple connections to the
same ISP [4][5]. This way the edge routers are configured to first
inspect the source address of a packet destined to an ISP and shunt
it into the appropriate tunnel or interface toward the ISP.
If such a scenario is applied exhaustively, so that an exit router is
chosen in the edge network for every prefix the network uses, traffic
originating from any other prefix can be summarily discarded instead
of sending it to an ISP.
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5. Security Considerations
Ingress filtering is typically performed to ensure that traffic
arriving on one network interface legitimately comes from a computer
residing on a network reachable through that interface.
The closer to the actual source ingress filtering is performed, the
more effective it is. One could wish that the first hop router would
ensure that traffic being sourced from its neighboring end system was
correctly addressed; a router further away can only ensure that it is
possible that there is such a system within the indicated prefix.
Therefore, ingress filtering should be done at multiple levels, with
different level of granularity.
It bears to keep in mind that while one goal of ingress filtering is
to make attacks traceable, it is impossible to know whether the
particular attacker "somewhere in the Internet" is being ingress
filtered or not. Therefore, one can only guess whether the source
addresses have been spoofed or not: in any case, getting a possible
lead -- e.g., to contact a potential source to ask whether they're
observing an attack or not -- is still valuable, and more so when the
ingress filtering gets more and more widely deployed.
In consequence, every administrative domain should try to ensure a
sufficient level of ingress filtering on its borders.
Security properties and applicability of different ingress filtering
types differ a lot.
o Ingress Access Lists require typically manual maintenance, but are
the most bulletproof when done properly; typically, ingress access
lists are best fit between the edge and the ISP when the
configuration is not too dynamic if strict RPF is not an option,
between ISPs if the number of used prefixes is low, or as an
additional layer of protection.
o Strict RPF check is a very easy and sure way to implement ingress
filtering. It is typically fit between the edge network and the
ISP. In many cases, a simple strict RPF can be augmented by
operational procedures in the case of asymmetric traffic patterns,
or the feasible RPF technique to also account for other
alternative paths.
o Feasible Path RPF check is an extension of Strict RPF. It is
suitable in all the scenarios where Strict RPF is, but multihomed
or asymmetric scenarios in particular. However, one must remember
that Feasible RPF assumes the consistent origination and
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propagation of routing information to work; the implications of
this must be understood especially if a prefix advertisement
passes through third parties.
o Loose RPF primarily filters out unrouted prefixes such as Martian
addresses. It can be applied in the upstream interfaces to reduce
the size of DoS attacks with unrouted source addresses. In the
downstream interfaces it can only be used as a contract
verification, that the other network has performed at least some
ingress filtering.
When weighing the tradeoffs of different ingress filtering
mechanisms, the security properties of a more relaxed approach should
be carefully considered before applying it. Especially when applied
by an ISP towards an edge network, there don't seem to be many
reasons why a stricter form of ingress filtering would not be
appropriate.
6. Conclusions and Future Work
This memo describes ingress filtering techniques in general and the
options for multihomed networks in particular.
It is important for ISPs to implement ingress filtering to prevent
spoofed addresses being used, both to curtail DoS attacks and to make
them more traceable, and to protect their own infrastructure. This
memo describes mechanisms that could be used to achieve that effect,
and the tradeoffs of those mechanisms.
To summarize:
o Ingress filtering should always be done between the ISP and a
single-homed edge network.
o Ingress filtering with Feasible RPF or similar Strict RPF
techniques could almost always be applied between the ISP and
multi-homed edge networks as well.
o Both the ISPs and edge networks should verify that their own
addresses are not being used in source addresses in the packets
coming from outside their network.
o Some form of ingress filtering is also reasonable between ISPs,
especially if the number of prefixes is low.
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This memo will lower the bar for the adoption of ingress filtering
especially in the scenarios like asymmetric/multihomed networks where
the general belief has been that ingress filtering is difficult to
implement.
One can identify multiple areas where additional work would be
useful:
o Specify the mechanisms in more detail: there is some variance
between implementations e.g., on whether traffic to multicast
destination addresses will always pass the Strict RPF filter or
not. By formally specifying the mechanisms the implementations
might get harmonized.
o Study and specify Routing Information Base (RIB) -based RPF
mechanisms, e.g., Feasible Path RPF, in more detail. In
particular, consider under which assumptions these mechanisms work
as intended and where they don't.
o Write a more generic note on the ingress filtering mechanisms than
this memo, after the taxonomy and the details or the mechanisms
(points above) have been fleshed out.
o Consider the more complex case where a network has connectivity
with different properties (e.g., peers and upstreams), and wants
to ensure that traffic sourced with a peer's address should not be
accepted from the upstream.
7. Acknowledgements
Rob Austein, Barry Greene, Christoph Reichert, Daniel Senie, Pedro
Roque, and Iljitsch van Beijnum reviewed this document and helped in
improving it. Thomas Narten, Ted Hardie, and Russ Housley provided
good feedback which boosted the document in its final stages.
8. References
8.1. Normative References
[1] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC 2827, May 2000.
8.2. Informative References
[2] Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities
Attribute", RFC 1997, August 1996.
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[3] IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.
[4] Bates, T. and Y. Rekhter, "Scalable Support for Multi-homed
Multi-provider Connectivity", RFC 2260, January 1998.
[5] Hagino, J. and H. Snyder, "IPv6 Multihoming Support at Site Exit
Routers", RFC 3178, October 2001.
9. Authors' Addresses
Fred Baker
Cisco Systems
Santa Barbara, CA 93117
US
EMail: fred@cisco.com
Pekka Savola
CSC/FUNET
Espoo
Finland
EMail: psavola@funet.fi
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10. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Baker & Savola Best Current Practice [Page 16]