Internet Engineering Task Force M. Ford, Ed.
Internet-Draft Internet Society
Intended status: Informational M. Boucadair
Expires: April 18, 2011 France Telecom
A. Durand
Juniper Networks
P. Levis
France Telecom
P. Roberts
Internet Society
October 15, 2010
Issues with IP Address Sharing
draft-ietf-intarea-shared-addressing-issues-02
Abstract
The completion of IPv4 address allocations from IANA and the RIRs is
causing service providers around the world to question how they will
continue providing IPv4 connectivity service to their subscribers
when there are no longer sufficient IPv4 addresses to allocate them
one per subscriber. Several possible solutions to this problem are
now emerging based around the idea of shared IPv4 addressing. These
solutions give rise to a number of issues and this memo identifies
those common to all such address sharing approaches. Solution-
specific discussions are out of scope.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on April 18, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Shared Addressing Solutions . . . . . . . . . . . . . . . . . 4
3. Analysis of Issues as they Relate to First and Third
Parties . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Content Provider Example . . . . . . . . . . . . . . . . . . . 8
5. Port Allocation . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Outgoing Ports . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Incoming Ports . . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Port Negotiation . . . . . . . . . . . . . . . . . . . 11
5.2.2. Connection to a Well-Known Port Number . . . . . . . . 11
6. Impact on Applications . . . . . . . . . . . . . . . . . . . . 12
7. Geo-location and Geo-proximity . . . . . . . . . . . . . . . . 13
8. Tracking Service Usage . . . . . . . . . . . . . . . . . . . . 14
9. ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10. MTU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 15
12. Traceability . . . . . . . . . . . . . . . . . . . . . . . . . 15
13. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
13.1. Abuse Logging and Penalty Boxes . . . . . . . . . . . . . 17
13.2. Authentication . . . . . . . . . . . . . . . . . . . . . . 17
13.3. Spam . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
13.4. Port Randomisation . . . . . . . . . . . . . . . . . . . . 18
13.5. IPsec . . . . . . . . . . . . . . . . . . . . . . . . . . 18
13.6. Policing Forwarding Behaviour . . . . . . . . . . . . . . 18
14. TCP Control Block Sharing . . . . . . . . . . . . . . . . . . 19
15. Reverse DNS . . . . . . . . . . . . . . . . . . . . . . . . . 19
16. Load Balancing . . . . . . . . . . . . . . . . . . . . . . . . 19
17. IPv6 Transition Issues . . . . . . . . . . . . . . . . . . . . 20
18. Introduction of Single Points of Failure . . . . . . . . . . . 20
19. State Maintenance Reduces Battery Life . . . . . . . . . . . . 20
20. Support of Multicast . . . . . . . . . . . . . . . . . . . . . 21
21. Support of Mobile-IP . . . . . . . . . . . . . . . . . . . . . 21
22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
23. Security Considerations . . . . . . . . . . . . . . . . . . . 21
24. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 21
25. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
26. Annex . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
26.1. Classes of Address Sharing Solution . . . . . . . . . . . 22
26.2. Address Space Multiplicative Factor . . . . . . . . . . . 23
27. Informative References . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
Allocations of IPv4 addresses from the Internet Assigned Numbers
Authority (IANA) are currently forecast to be complete during 2011
[IPv4_Report]. Allocations from some Regional Internet Registries
(RIRs) are anticipated to be complete around a year later, although
the exact date will vary from registry to registry. This is causing
service providers around the world to start to question how they will
continue providing IPv4 connectivity service to their subscribers
when there are no longer sufficient IPv4 addresses to allocate them
one per subscriber. Several possible solutions to this problem are
now emerging based around the idea of shared IPv4 addressing. These
solutions give rise to a number of issues and this memo identifies
those common to all such address sharing approaches and collects them
in a single document.
Over the long term, deploying IPv6 is the only way to ease pressure
on the public IPv4 address pool and thereby mitigate the need for
address sharing mechanisms that give rise to the issues identified
herein. In the short term, maintaining growth of IPv4 services in
the presence of IPv4 address depletion will require address sharing.
Address sharing will cause issues for end-users, service providers
and third parties such as law enforcement agencies and content
providers. This memo is intended to highlight and briefly discuss
these issues.
Increased IPv6 deployment should reduce the burden being placed on an
address sharing solution, and should reduce the costs of operating
that solution. Increasing IPv6 deployment should cause a reduction
in the number of concurrent IPv4 sessions per subscriber. If the
percentage of end-to-end IPv6 traffic significantly increases, so
that the volume of IPv4 traffic begins decreasing, then the number of
IPv4 sessions will decrease. The smaller the number of concurrent
IPv4 sessions per subscriber, the higher the number of subscribers
able to share the same IPv4 public address, and consequently, the
lower the number of IPv4 public addresses required. However, this
effect will only occur for subscribers who have both an IPv6 access
and a shared IPv4 access. This motivates a strategy to
systematically bind a shared IPv4 access to an IPv6 access. It is
difficult to foresee to what extent growing IPv6 traffic will reduce
the number of concurrent IPv4 sessions, but in any event, IPv6
deployment and use should reduce the pressure on the available public
IPv4 address pool.
2. Shared Addressing Solutions
In many networks today a subscriber is provided with a single public
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IPv4 address at their home or small business. For instance, in fixed
broadband access, an IPv4 public address is assigned to each CPE
(Customer Premises Equipment). CPEs embed a NAT function which is
responsible for translating private IPv4 addresses ([RFC1918]
addresses) assigned to hosts within the local network, to the public
IPv4 address assigned by the service provider (and vice versa).
Therefore, devices located with the LAN share the single public IPv4
address and they are all associated with a single subscriber account
and a single network operator.
A number of proposals currently under consideration in the IETF rely
upon the mechanism of multiplexing multiple subscribers' connections
over a smaller number of shared IPv4 addresses. This is a
significant change. These proposals include Carrier Grade NAT (CGN.
a.k.a., LSN for Large Scale NAT) [I-D.nishitani-cgn], Dual-Stack-Lite
[I-D.ietf-softwire-dual-stack-lite], NAT64
[I-D.ietf-behave-v6v4-xlate-stateful] [I-D.ietf-behave-v6v4-xlate] ,
Address+Port (A+P) proposals [I-D.ymbk-aplusp],
[I-D.boucadair-port-range] and SAM [I-D.despres-sam]. Section 26
provides a classification of these different types of solutions and
discusses some of the design considerations to be borne in mind when
deploying large-scale address sharing. Whether we're talking about
Dual-Stack-Lite, A+P, NAT64 or CGN isn't especially important - it's
the view from the outside that matters, and given that, most of the
issues identified below apply regardless of the specific address
sharing scenario in question. Issues specific to A+P proposals are
addressed in [I-D.thaler-port-restricted-ip-issues].
In these new proposals, a single public IPv4 address would be shared
by multiple homes or small businesses (i.e., multiple subscribers) so
the operational paradigm described above would no longer apply. In
this document we refer to this new paradigm as large-scale address
sharing. All these proposals extend the address space by adding port
information, they differ in the way they manage the port value.
Security issues associated with NAT have long been documented (see
[RFC2663] and [RFC2993] ). However, sharing IPv4 addresses across
multiple subscribers by any means, either moving the NAT
functionality from the home gateway to the core of the service
provider network, or restricting the port choice in the subscriber's
NAT, creates additional issues for subscribers, content providers and
network operators. Many of these issues are created today by public
wi-fi hotspot deployments. As such large-scale address sharing
solutions become more widespread in the face of IPv4 address
depletion, these issues will become both more severe and more
commonly felt. NAT issues in the past typically only applied to a
single legal entity; as large-scale address sharing becomes more
prevalent these issues will increasingly span across multiple legal
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entities simultaneously.
All large-scale address sharing proposals share technical and
operational issues and these are addressed in the sections that
follow. These issues are common to any service-provider NAT,
enterprise NAT, and also non-NAT solutions that share individual IPv4
addresses across multiple subscribers. This document is intended to
bring all of these issues together in one place.
3. Analysis of Issues as they Relate to First and Third Parties
In this section we present an analysis of whether the issues
identified in the remainder of this document are applicable to the
organisation deploying the shared addressing mechanism (and by
extension their subscribers) and/or whether these issues impact third
parties (e.g., content providers, law enforcement agencies, etc.).
In this analysis, issues that affect end-users are deemed to affect
1st parties, as end-users can be expected to complain to their
service provider when problems arise. Where issues can expect to be
foreseen and addressed by the party deploying the shared addressing
solution, they are not attributed.
In Figure 1 we have also tried to indicate (with 'xx') where issues
are newly created in addition to what could be expected from the
introduction of a traditional NAT device. Issues marked with a
single 'x' are already present today in the case of typical CPE NAT,
however they can be expected to be more severe and widespread in the
case of large-scale address sharing.
+------------------------------------------------+--------+---------+
| Issue | 1st | 3rd |
| | party | parties |
+------------------------------------------------+--------+---------+
| Overly restrictive allocations of outgoing | x | |
| ports will impact performance for end users | | |
| | | |
| Incoming port negotiation mechanisms may fail | xx | |
| | | |
| Incoming connections to Well-Known Ports will | x | |
| not work | | |
| | | |
| Some applications will fail to operate | x | x |
| | | |
| TCP control block sharing will be affected | x | x |
| | | |
+------------------------------------------------+--------+---------+
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+------------------------------------------------+--------+---------+
| Issue | 1st | 3rd |
| | party | parties |
+------------------------------------------------+--------+---------+
| Reverse DNS will be affected | x | x |
| | | |
| Inbound ICMP will fail in many cases | x | x |
| | | |
| Amplification of security issues | xx | xx |
| | | |
| Fragmentation will require special handling | x | |
| | | |
| Single points of failure and increased | x | |
| network instability | | |
| | | |
| Port randomisation will be affected | x | |
| | | |
| Service usage monitoring and abuse logging | xx | xx |
| will be impacted for all elements in the chain | | |
| between service provider and content provider | | |
| | | |
| Penalty boxes will no longer work | xx | xx |
| | | |
| Spam blacklisting will be affected | xx | xx |
| | | |
| Geo-location services will be impacted | xx | xx |
| | | |
| Geo-proximity mechanisms will be impacted | xx | xx |
| | | |
| Load balancing algorithms may be impacted | | xx |
| | | |
| Authentication mechanisms may be impacted | | x |
| | | |
| Traceability of network usage and abusage will | | xx |
| be affected | | |
| | | |
| IPv6 transition mechanisms will be affected | xx | |
| | | |
| Frequent keep-alives reduce battery life | x | |
| | | |
+------------------------------------------------+--------+---------+
Figure 1: Shared addressing issues for first and third parties
As can be seen from Figure 1, deployment of large-scale address
sharing will create almost as many problems for third parties as it
does for the service provider deploying the address sharing
mechanism. Several of these issues are specific to the introduction
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of large-scale address sharing as well. All of these issues are
discussed in further detail below.
4. Content Provider Example
Taking a content provider as an example of a third-party, and
focusing on the issues that are created specifically by the presence
of large-scale address sharing, we identify the following issues as
being of particular concern:
o Degraded geolocation for targeted advertising and licensed content
restrictions (see Section 7).
o Additional latency due to indirect routing and degraded
geoproximity (see Section 7).
o Exposure to new amplification attacks (see Section 13).
o Service usage monitoring is made more complicated (see Section 8).
o Incoming port negotiation mechanisms may fail (see Section 5.2.1).
o IP blocking for abuse/spam will cause collateral damage (see
Section 13).
o Load balancing algorithms may be impacted (see Section 16).
o Traceability of network usage and abuse will be impacted (see
Section 12).
5. Port Allocation
When we talk about port numbers we need to make a distinction between
outgoing connections and incoming connections. For outgoing
connections, the actual source port number used is usually
irrelevant. (While this is true today, in a port-range solution it
is necessary for the source port to be within the allocated range).
But for incoming connections, the specific port numbers allocated to
subscribers matter because they are part of external referrals (used
by third parties to contact services run by the subscribers).
The total number of subscribers able to share a single IPv4 address
will depend upon assumptions about the average number of ports
required per active subscriber, and the average number of
simultaneously active subscribers. It is important to realise that
the TCP design makes it undesirable for clients to re-use ports while
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they remain in the TIME-WAIT state (typically 4 minutes after the
connection has concluded). TIME-WAIT state removes the hazard of old
duplicates for "fast" or "long" connections, in which clock-driven
Initial Sequence Number selection is unable to prevent overlap of the
old and new sequence spaces. The TIME-WAIT delay allows all old
duplicate segments time enough to die in the Internet before the
connection is reopened [RFC1337]. Therefore ports in this state must
be included in calculations concerning port usage per subscriber.
Most of the time the source port selected by a client application
will be translated (unless there is direct knowledge of a port-range
restriction in the client's stack), either by a NAT in the
subscriber's device, or by a CPE NAT, or by a CPE NAT and a CGN.
IANA has classified the whole port space into three categories (as
defined in http://www.iana.org/assignments/port-numbers):
o The Well Known Ports are those from 0 through 1023.
o The Registered Ports are those from 1024 through 49151.
o The Dynamic and/or Private Ports are those from 49152 through
65535.
[RFC4787] notes that current NATs have different policies with regard
to this classification; some NATs restrict their translations to the
use of dynamic ports, some also include registered ports, some
preserve the port if it is in the well-known range. [RFC4787] makes
it clear that the use of port space (1024-65535) is safe: "mapping a
source port to a source port that is already registered is unlikely
to have any bad effects". Therefore, for all address sharing
solutions, there is no reason to only consider a subset of the port
space (1024-65535) for outgoing source ports.
5.1. Outgoing Ports
According to measurements the average number of outgoing ports
consumed per active subscriber is much, much smaller than the maximum
number of ports a subscriber can use at any given time. However, the
distribution is heavy-tailed, so there are typically a small number
of subscribers who use a very high number of ports [CGN_Viability].
This means that an algorithm that dynamically allocates outgoing port
numbers from a central pool will typically allow more subscribers to
share a single IPv4 address than algorithms that statically divide
the resource by pre-allocating a fixed number of ports to each
subscriber. Similarly, such an algorithm should be more able to
accommodate subscribers wishing to use a relatively high number of
ports.
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It is important to note here that the desire to dynamically allocate
outgoing port numbers will make a service provider's job of
maintaining records of subscriber port number allocations
considerably more onerous (see Section 12). The number of records
per subscriber will increase from 1 in a scheme where ports are
statically allocated, to a much larger number equivalent to the total
number of outgoing ports consumed by that subscriber during the time
period for which detailed logs must be kept.
A potential problem with dynamic allocation occurs when one of the
subscriber devices behind such a port-shared IPv4 address becomes
infected with a worm, which then quickly sets about opening many
outbound connections in order to propagate itself. Such an infection
could rapidly exhaust the shared resource of the single IPv4 address
for all connected subscribers. It is therefore necessary to impose
limits on the total number of ports available to an individual
subscriber to ensure that the shared resource (the IPv4 address)
remains available in some capacity to all the subscribers using it.
However, static schemes for ports assignment may introduce security
issues [I-D.ietf-tsvwg-port-randomization] when small contiguous port
ranges are statically assigned to subscribers.
Session failure due to NAT state overflow or timeout (when the NAT
discards session state because it's run out of resource) can be
experienced when the configured quota per user is reached or if the
NAT is out of recourses.
5.2. Incoming Ports
It is desirable to ensure that incoming ports remain stable over
time. This is challenging as the network doesn't know anything in
particular about the applications that it is supporting and therefore
has no real notion of how long an application/service session is
still ongoing and therefore requiring port stability.
Early measurements [CGN_Viability] also seem to indicate that, on
average, only very few ports are used by subscribers for incoming
connections. However, a majority of subscribers accept at least one
inbound connection.
This means that it is not necessary to pre-allocate a large number of
incoming ports to each subscriber. It is possible to either pre-
allocate a small number of ports for incoming connections or do port
allocation on demand when the application wishing to receive a
connection is initiated. The bulk of incoming ports can be reserved
as a centralized resource shared by all subscribers using a given
public IPv4 address.
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5.2.1. Port Negotiation
In current deployments, one important and widely used feature of many
CPE devices is the ability to open incoming ports (port forwarding)
either manually, or with a protocol such as UPnP IGD. If a CGN is
present, the port must also be open in the CGN. The situation may be
alleviated somewhat if the CGN architecture is composed of only one
NAT level (no NAT in the CPE) as for DS-Lite, although a service
provider operating this solution will still be required to offer some
means for configuring incoming ports by their subscribers. This may
be either via a UPnP or NAT-PMP relay over a tunnelled direct
connection between the CPE and CGN, or a web interface to configure
the incoming port mapping on the CGN. Note, that such an interface
effectively makes public what was previously a private management
interface and this raises security concerns that must be addressed.
For port-range solutions, port forwarding capabilities may still be
present at the CPE, with the limitation that the open incoming port
must be within the allocated port-range (for instance it is not
possible to open port 5002 for incoming connections if port 5002 is
not within the allocated port-range).
5.2.1.1. Universal Plug and Play (UPnP)
Using the UPnP semantic, an application asks "I want to use port
number X, is that ok?" and the answer is yes or no. If the answer is
no, the application will typically try the next port in sequence,
until it either finds one that works or gives up after a limited
number of attempts. UPnP has, currently, no way to redirect the
application to use another port number. UPnP IGD 2.0, currently
being defined, should improve this and allow for allocation of any
available port.
5.2.1.2. NAT Port Mapping Protocol (NAT-PMP)
NAT-PMP already has a better semantic here, enabling the NAT to
redirect the application to an available port number.
5.2.2. Connection to a Well-Known Port Number
Once an IPv4 address sharing mechanism is in place, connections to
well-known port numbers will not work in the general case. Any
application that is not port-agile cannot be expected to work. Some
workaround (e.g., redirects to a port-specific URI) could be deployed
given sufficient incentives. There exist several proposals for
'application service location' protocols which would provide a means
of addressing this problem, but historically these proposals have not
gained much deployment traction.
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For example, the use of DNS SRV records [RFC2782] provides a
potential solution for subscribers wishing to host services in the
presence of a shared-addressing scheme. SRV records make it possible
to specify a port value related to a service, thereby making services
accessible on ports other than the Well-Known ports. It is worth
noting that this mechanism is not applicable to HTTP.
6. Impact on Applications
Address sharing solutions will have an impact on the following types
of applications:
o Applications that establish inbound communications - these
applications will have to ensure that ports selected for inbound
communications are either within the allocated range (for port-
range solutions) or are forwarded appropriately by the CGN (for
CGN-based solutions). See Section 5.2 for more discussion of
this;
o Applications that carry address and/or port information in their
payload - where translation of port and/or address information is
performed at the IP and transport layers by the address sharing
solution, an ALG will also be required to ensure application layer
data is appropriately modified;
o Applications that use fixed ports (e.g., well-known ports) - see
Section 5.2.2 for more discussion of this;
o Applications that do not use any port (e.g., ICMP) - where address
sharing solutions map subscribers to (private) IP addresses on a
one-to-one basis this will not be an issue, otherwise such
applications will require special handling - see Section 9 for
more discussion of this;
o Applications that assume the uniqueness of source addresses (e.g.,
IP address as identifier) - such applications will fail to operate
correctly in the presence of multiple, discrete, simultaneous
connections from the same source IP address;
o Applications that explicitly prohibit concurrent connections from
the same address - such applications will fail when multiple
subscribers sharing an IP address attempt to use them
simultaneously.
o Applications that do not use TCP or UDP for transport - All IP
address sharing mechanisms proposed to date are limited to TCP,
UDP, and ICMP, thereby preventing end users from fully utilizing
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the Internet (e.g., SCTP, DCCP, RSVP, protocol 41 (IPv6-over-
IPv4), protocol 50 (IPsec ESP)).
Applications already frequently implement mechanisms in order to
circumvent the presence of NATs (typically CPE NATs):
o Application Layer Gateways (ALGs): Many CPE devices today embed
ALGs that allow applications to behave correctly despite the
presence of NAT on the CPE. When the NAT belongs to the
subscriber, the subscriber has flexibility to tailor the device to
his or her needs. For CGNs, subscribers will be dependent on the
set of ALGs that their service provider makes available. A
service provider-based NAT may, or may not, support NAT-traversal
for IKE [RFC3947] for example. For port-range solutions, ALGs
will require modification to deal with the port-range restriction,
but will otherwise have the same capabilities as today.
o NAT Traversal Techniques: There are several commonly-deployed
mechanisms that support operating servers behind a NAT by
forwarding specific TCP or UDP ports to specific internal hosts
([UPnP-IGD], [I-D.cheshire-nat-pmp], and manual configuration).
All of these mechanisms assume the NAT's WAN address is a
publicly-routable IP address, and fail to work normally when that
assumption is wrong. There have been attempts to avoid that
problem by automatically disabling the NAT function and bridging
traffic instead upon assignment of a private IP address to the WAN
interface (as is required for [Windows-Logo] certification).
Bridging (rather than NATting) has other side effects (DHCP
requests are served by an upstream DHCP server which can increase
complexity of in-home networking).
7. Geo-location and Geo-proximity
IP addresses are frequently used to indicate, with some level of
granularity and some level of confidence, where a host is physically
located. Geo-location services are used by content providers to
allow them to conform with regional content licensing restrictions,
to target advertising at specific geographic areas, or to provide
customised content. Geo-location services are also necessary for
emergency services provision. In some deployment contexts (e.g.,
centralised CGN), shared addressing will reduce the level of
confidence and level of location granularity that IP-based geo-
location services can provide. Viewed from the content provider, a
subscriber behind a CGN geolocates to wherever the prefix of the CGN
appears to be; very often that will be in a different city, and
sometimes in a different country, than the subscriber. Other forms
of geo-location will still work as usual.
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A slightly different use of an IP address is to calculate the
proximity of a connecting host to a particular service delivery
point. This use of IP address information impacts the efficient
delivery of content to an end-user. If a CGN is introduced in
communications and it is far from an end-user connected to it,
application performance may be degraded insofar as IP-based geo-
proximity is a factor.
8. Tracking Service Usage
As large-scale address sharing becomes more commonplace, monitoring
the number of unique users of a service will become more complex than
simply counting the number of connections from unique IP addresses.
While this is a somewhat inexact methodology today due to the
widespread use of CPE NAT, it will become a much less useful approach
in the presence of widespread large-scale address sharing solutions.
In general, all elements that monitor usage or abusage in the chain
between a service provider that has deployed address sharing and a
content provider will need to be upgraded to take account of the port
value in addition to IP addresses.
9. ICMP
ICMP does not carry any port information and is consequently
problematic for address sharing mechanisms. Sourcing ICMP from hosts
behind an address sharing solution does not pose problems, although
responses to outgoing ICMP will require special handling, such as
making use of the ICMP identifier value to route the response
appropriately.
For inbound ICMP there are two cases. The first case is that of ICMP
sourced from outside the network of the address sharing solution
provider. Several applications make use of this (e.g., P2P
applications) and measurements derived by such applications in the
presence of an address sharing solution will be erroneous. The
second case is that of ICMP sourced from within the network of the
address sharing solution provider (e.g., for network management and
diagnostic purposes). In this case ICMP can be routed normally for
CGN-based solutions owing to the presence of locally unique private
IP addresses for each CPE device. For port-range solutions, ICMP
will not be routable without special handling, e.g., placing a port
number in the ICMP identifier field, and having port-range routers
make routing decisions based upon that field.
Considerations related to ICMP messages handling in NAT-based
environments are specified in [RFC5508].
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10. MTU
In applications where the end hosts are attempting to use path MTU
Discovery [RFC1191] to optimize transmitted packet size with
underlying network MTU, shared addressing has a number of items which
must be considered. As covered in Section 9, ICMP "Packet Too Big"
messages must be properly translated through the address sharing
solution in both directions. However, even when this is done
correctly, MTU can be a concern. Many end hosts cache PMTUd
information for a certain period of time. If the MTU behind the
address sharing solution is inconsistent, the public end host may
have the incorrect MTU value cached. This may cause it to send
packets that are too large, causing them to be dropped if the DF
(Don't Fragment) bit is set, or causing them to be fragmented by the
network, increasing load and overhead. Because the host eventually
will reduce MTU to the lowest common value for all hosts behind a
given public address, it may also send packets that are below optimal
size for the specific connection, increasing overhead and reducing
throughput.
This issue also generates a potential attack vector, that a
malevolent user could send an ICMP "Packet Too Big" (Type 3, Code 4)
message indicating a Next-Hop MTU of anything down to 68 octets.
This value will be cached by the off-net server for all subscribers
sharing the address of the malevolent user. This could lead to a DoS
against both the remote server and the large-scale NAT device itself
(as they will both have to handle many more packets per second).
11. Fragmentation
When a packet is fragmented, transport-layer port information (either
UDP or TCP) is only present in the first fragment. Subsequent
fragments will not carry the port information and so will require
special handling.
12. Traceability
In many jurisdictions, service providers are legally obliged to
provide the identity of a subscriber upon request to the appropriate
authorities. Such legal requests have traditionally included the
source IPv4 address and date (and usually the time), which is
sufficient information when subscribers are assigned IPv4 addresses
for a long duration.
However, where one public IPv4 address is shared between several
subscribers, the IPv4 no longer uniquely identifies a subscriber.
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There are two solutions to this problem:
o The first solution is for servers to additionally log the source
port of incoming connections and for the legal request to include
the source port. The legal request should include the
information: [Source IP address, Source Port, Timestamp] (and
possibly other information). Accurate time-keeping (e.g., use of
NTP or SNTP) is vital because port assignments are dynamic. A
densely populated CGN could mean even very small amounts of clock
skew between a third party's server and the CGN operator will
result in ambiguity about which customer was using a specific port
at a given time.
o The second solution considers it is unrealistic to expect all
servers to log the source port number of incoming connections. To
deal with this, service providers using IPv4 address sharing may
need to log IP destination addresses.
Destination logging is imperfect if multiple subscribers are
accessing the same (popular) server at nearly the same time, it can
be impossible to disambiguate which subscriber accessed the server,
especially with protocols that involve several connections (e.g.,
HTTP). Thus, logging the destination address on the NAT is inferior
to logging the source port at the server.
If neither solution is used (that is, the server is not logging
source port numbers and the NAT is not logging destination IP
addresses), the service provider cannot trace the offending activity
to a specific subscriber. In this circumstance, the service provider
would need to disclose the identity of all subscribers who had active
sessions on the NAT during the time period in question. This may be
a large number of subscribers.
Address sharing solutions must record and store all mappings
(typically during 6-12 months, depending on the local jurisdiction)
that they create. If we consider one mapping per session, a service
provider should record and retain traces of all sessions created by
all subscribers during one year (if the legal storage duration is one
year). This may be challenging due to the volume of data requiring
storage, the volume of data to repeatedly transfer to the storage
location, and the volume of data to search in response to a query.
Address sharing solutions may mitigate these issues to some extent by
pre-allocating groups of ports. Then only the allocation of the
group needs to be recorded, and not the creation of every session
binding within that group. There are trade-offs to be made between
the sizes of these port groups, the ratio of public addresses to
subscribers, whether or not these groups timeout, the impact on
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logging requirements and port randomisation security
[I-D.ietf-tsvwg-port-randomization].
13. Security
Before noting some specific security-related issues caused by large-
scale address sharing, it is perhaps worth noting that, in general,
address sharing creates a vector for attack amplification in numerous
ways. See Section 9 for one example.
13.1. Abuse Logging and Penalty Boxes
When an abuse is reported today, it is usually done in the form: IPv4
address X has done something bad at time T0. This is not enough
information to uniquely identify the subscriber responsible for the
abuse when that IPv4 address is shared by more than one subscriber.
Law enforcement authorities may be particularly impacted because of
this. This particular issue can be fixed by logging port numbers,
although this will increase logging data storage requirements.
A number of services on the network today log the IPv4 source
addresses used in connection attempts to protect themselves from
certain attacks. For example, if a server sees too many requests
from the same IPv4 address in a short period of time, it may decide
to put that address in a penalty box for a certain time during which
requests are denied, or it may require completion of a CAPTCHA for
future requests. If an IPv4 address is shared by multiple
subscribers, this would have unintended consequences in a couple of
ways. First it may become the natural behavior to see many login
attempts from the same address because it is now shared across a
potentially large number of subscribers. Second and more likely is
that one user who fails a number of login attempts may block out
other users who have not made any previous attempts but who will now
fail on their first attempt. In the presence of widespread large-
scale address sharing, penalty box solutions to service abuse simply
will not work.
13.2. Authentication
Simple address-based identification mechanisms that are used to
populate access control lists will fail when an IP address is no
longer sufficient to identify a particular subscriber. Including
port numbers in access control list definitions may be possible at
the cost of extra complexity, and may also require the service
provider to make static port assignments, which conflicts with the
requirement for dynamic assignments discussed in Section 5.1 .
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13.3. Spam
Another case of identifying abusers has to do with spam blacklisting.
When a spammer is behind a CGN or using a port-shared address,
blacklisting of their IP address will result in all other subscribers
sharing that address having their ability to source SMTP packets
restricted to some extent.
13.4. Port Randomisation
A blind attack that can be performed against TCP relies on the
attacker's ability to guess the 5-tuple (Protocol, Source Address,
Destination Address, Source Port, Destination Port) that identifies
the transport protocol instance to be attacked.
[I-D.ietf-tsvwg-port-randomization] describes a number of methods for
the random selection of the source port number, such that the ability
of an attacker to correctly guess the 5-tuple is reduced. With
shared IPv4 addresses, the port selection space is reduced.
Preserving port randomisation is important and may be more or less
difficult depending on the address sharing solution and the size of
the port space that is being manipulated. Allocation of non-
contiguous port ranges could help to mitigate this issue.
It should be noted that guessing the port information may not be
sufficient to carry out a successful blind attack. The exact TCP
Sequence Number (SN) should also be known. A TCP segment is
processed only if all previous segments have been received, except
for some Reset Segment implementations which immediately process the
Reset as long as it is within the Window. If SN is randomly chosen
it will be difficult to guess it (SN is 32 bits long); port
randomisation is one protection among others against blind attacks.
13.5. IPsec
The impact of large-scale IP address sharing for IPsec operation
should be evaluated and assessed. [RFC3947] proposes a solution to
solve issues documented in [RFC3715] . The applicability of
[RFC3947] in the context of shared IP address solutions should be
evaluated. In particular, service providers may wish to ensure that
CGN deployments do not inadvertently block NAT traversal for security
protocols such as IKE (refer to [I-D.gont-behave-nat-security] for
more information).
13.6. Policing Forwarding Behaviour
[RFC2827] motivates and discusses a simple, effective, and
straightforward method for using ingress traffic filtering to
prohibit Denial-of-Service (DoS) attacks which use forged IP
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addresses. Following this recommendation, service providers
operating shared-addressing mechanisms should ensure that source
addresses, or source ports in the case of port-range schemes, are set
correctly in outgoing packets from their subscribers or they should
drop the packets.
If some form of IPv6 ingress filtering is deployed in the broadband
network and DS-Lite service is restricted to those subscribers, then
tunnels terminating at the CGN and coming from registered subscriber
IPv6 addresses cannot be spoofed. Thus a simple access control list
on the tunnel transport source address is all that is required to
accept traffic on the southbound interface of a CGN.
14. TCP Control Block Sharing
[RFC2140] defines a performance optimisation for TCP based on sharing
state between TCP control blocks that pertain to connections to the
same host, as opposed to maintaining state for each discrete
connection. This optimisation assumes that an address says something
about the properties of the path between two hosts, which is clearly
not the case if the address in question is shared by multiple hosts
at different physical network locations. While CPE NAT today causes
problems for sharing TCP control block state across multiple
connections to a given IP address, large-scale address sharing will
make these issues more severe and more widespread.
15. Reverse DNS
Many service providers populate forward and reverse DNS zones for the
public IPv4 addresses that they allocate to their subscribers. In
the case where public addresses are shared across multiple
subscribers, such strings are, by definition, no longer sufficient to
identify an individual subscriber without additional information.
16. Load Balancing
Algorithms used to balance traffic load for popular destinations may
be affected by the introduction of address sharing. Where balancing
is achieved by deterministically routing traffic from specific source
IP addresses to specific servers, sudden imbalances in load may be
experienced as address sharing is enabled for some of those source IP
addresses. This will require re-evaluation of the algorithms used in
the load-balancing design. In general, as the scale of address
sharing grows, load-balancing designs will need to be re-evaluated
and any assumptions about average load per source IP address
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revisited.
17. IPv6 Transition Issues
IPv4 address sharing solutions may interfere with existing IPv4 to
IPv6 transition mechanisms, which were not designed with IPv4
shortage considerations in mind. With port-range solutions for
instance, incoming 6to4 packets should be able to find their way from
a 6to4 relay to the appropriate 6to4 CPE router, despite the lack of
direct port range information (UDP/TCP initial source port did not
pass through the CPE port range translation process). One solution
would be for a 6to4 IPv6 address to embed not only an IPv4 address
but also a port range value.
Subscribers allocated with private addresses will not be able to
utilise 6to4 to access IPv6, but may be able to utilise Teredo.
Shared addresses should be drawn from space designated as such
[RFC1918]. Otherwise their use will break the widely implemented
assumption that public IPv4 addresses are globally unique addresses
and hence break many protocols and applications, e.g., 6to4
[RFC3056]. Some routers enable 6to4 on their WAN link. 6to4 requires
a publicly-routable IPv4 address. Enabling 6to4 when the apparently
public IPv4 WAN address is in fact behind a NAT creates a
disconnected IPv6 island. Issues created by sharing public address
space across multiple hosts are specifically addressed in
[I-D.thaler-port-restricted-ip-issues].
18. Introduction of Single Points of Failure
In common with all deployments of new network functionality, the
introduction of new nodes or functions to handle the multiplexing of
multiple subscribers across shared IPv4 addresses could create single
points of failure in the network. Any IP address sharing solution
should consider the opportunity to add redundancy features in order
to alleviate the impact on the robustness of the offered IP
connectivity service. The ability of the solution to allow hot
swapping from one machine to another should be considered. This is
especially important where the address sharing solution in question
requires the creation of per-flow state in the network.
19. State Maintenance Reduces Battery Life
In order for hosts to maintain network state in the presence of NAT,
keep-alive messages have to be sent at frequent intervals. For
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battery-powered devices, sending these keep-alive messages can result
in significantly reduced battery performance than would otherwise be
the case [Mobile_Energy_Consumption].
20. Support of Multicast
[RFC5135] specifies requirements for a NAT that supports Any Source
IP Multicast or Source-Specific IP Multicast. Port-range routers
that form part of port-range solutions will need to support similar
requirements if multicast support is required.
21. Support of Mobile-IP
IP address sharing within the context of Mobile-IP deployments (in
the home network and/or in the visited network), will require Home
Agents and/or Foreign Agents to be updated so as to take into account
the relevant port information. There may also be issues raised when
an additional layer of encapsulation is required thereby causing, or
increasing the need for, fragmentation and reassembly.
Issues for Mobile-IP in the presence of NAT are discussed in
[I-D.haddad-mext-nat64-mobility-harmful]
22. IANA Considerations
This memo includes no request to IANA.
23. Security Considerations
This memo does not define any protocol and therefore creates no new
security issues. Section 13 discusses some of the security and
identity-related implications of IP address sharing.
24. Contributors
This document is based on sources co-authored by J.L. Grimault and A.
Villefranque of France Telecom.
25. Acknowledgements
This memo was partly inspired by conversations that took place as
part of Internet Society (ISOC) hosted roundtable events for
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operators and content providers deploying IPv6. Participants in
those discussions included John Brzozowski, Leslie Daigle, Tom
Klieber, Yiu Lee, Kurtis Lindqvist, Wes George, Lorenzo Colliti, Erik
Kline, Igor Gashinsky, Jason Fesler, Rick Reed, Adam Bechtel, Larry
Campbell, Tom Coffeen, David Temkin, Pete Gelbman, Mark Winter, Will
Charnock, Martin Levy, Greg Wood and Christian Jacquenet.
The authors are also grateful to Christian Jacquenet, Iain Calder,
Joel Halpern, Brian Carpenter, Gregory Lebovitz, Bob Briscoe, Marcelo
Bagnulo, Dan Wing and Wes George for their helpful comments and
suggestions for improving the document.
This memo was created using the xml2rfc tool.
26. Annex
26.1. Classes of Address Sharing Solution
IP address sharing solutions fall into two classes. Either a
service-provider operated NAT function is introduced and subscribers
are allocated addresses from [RFC1918] space, or public IPv4
addresses are shared across multiple subscribers by restricting the
range of ports available to each subscriber. These classes of
solution are described in a bit more detail below.
o CGN-based solutions: These solutions propose the introduction of a
NAPT function in the service provider's network, denoted also as
Carrier Grade NAT (CGN), or Large Scale NAT (LSN)
[I-D.nishitani-cgn] , or Provider NAT. The CGN is responsible for
translating private addresses to publicly routable addresses.
Private addresses are assigned to subscribers, a pool of public
addresses is assigned to the CGN, and the number of public
addresses is smaller than the number of subscribers. A public
IPv4 address in the CGN pool is shared by several subscribers at
the same time. Solutions making use of a service provider-based
NAT include [I-D.shirasaki-nat444] (two layers of NAT) and
[I-D.ietf-softwire-dual-stack-lite] (a single layer of NAT).
o Port-range solutions: These solutions avoid the presence of a CGN
function. A single public IPv4 address is assigned to several
subscribers at the same time. A restricted port range is also
assigned to each subscriber so that two subscribers with the same
IPv4 address have two different port ranges that do not overlap.
These solutions are called A+P (Address+Port) [I-D.ymbk-aplusp] ,
or Port Range [I-D.boucadair-port-range] , or SAM (Stateless
Address Mapping) [I-D.despres-sam] .
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26.2. Address Space Multiplicative Factor
The purpose of sharing public IPv4 addresses is to increase the
addressing space. A key parameter is the factor by which service
providers want or need to multiply their IPv4 public address space;
and the consequence is the number of subscribers sharing the same
public IPv4 address. We refer to this parameter as the address space
multiplicative factor, the inverse is called the compression ratio.
The multiplicative factor can only be applied to the subset of
subscribers that are eligible for a shared address. The reasons a
subscriber cannot have a shared address can be:
o It would not be compatible with the service they are currently
subscribed to (for example: business subscriber).
o Subscriber CPE is not compatible with the address sharing solution
selected by the service provider (for example it does not handle
port restriction for port-range solutions or it does not allow
IPv4 in IPv6 encapsulation for the DS-Lite solution), and its
replacement is not easy.
Different service providers may have very different needs. A long-
lived service provider, whose number of subscribers is rather stable,
may have an existing address pool that will only need a small
extension to cope with the next few years, assuming that this address
pool can be re-purposed for an address sharing solution (small
multiplicative factor, less than 10). A new entrant or a new line of
business will need a much bigger multiplicative factor (e.g., 1000).
A mobile operator may see its addressing needs grow dramatically as
the IP-enabled mobile handset market grows.
When the multiplicative factor is large, the average number of ports
per subscriber is small. Given the large measured disparity between
average and peak port consumption [CGN_Viability] , this will create
service problems in the event that ports are allocated statically.
In this case, it is essential for port allocation to map to need as
closely as possible, and to avoid allocating ports for longer than
necessary. Therefore, the larger the multiplicative factor, the more
dynamic the port assignment has to be.
27. Informative References
[CGN_Viability]
Alcock, S., "Research into the Viability of Service-
Provider NAT", 2008, <http://www.wand.net.nz/~salcock/
someisp/flow_counting/result_page.html>.
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[I-D.boucadair-port-range]
Boucadair, M., Levis, P., Bajko, G., and T. Savolainen,
"IPv4 Connectivity Access in the Context of IPv4 Address
Exhaustion: Port Range based IP Architecture",
draft-boucadair-port-range-02 (work in progress),
July 2009.
[I-D.cheshire-nat-pmp]
Cheshire, S., "NAT Port Mapping Protocol (NAT-PMP)",
draft-cheshire-nat-pmp-03 (work in progress), April 2008.
[I-D.despres-sam]
Despres, R., "Scalable Multihoming across IPv6 Local-
Address Routing Zones Global-Prefix/Local-Address
Stateless Address Mapping (SAM)", draft-despres-sam-03
(work in progress), July 2009.
[I-D.gont-behave-nat-security]
Gont, F. and P. Srisuresh, "Security implications of
Network Address Translators (NATs)",
draft-gont-behave-nat-security-03 (work in progress),
October 2009.
[I-D.haddad-mext-nat64-mobility-harmful]
Haddad, W. and C. Perkins, "A Note on NAT64 Interaction
with Mobile IPv6",
draft-haddad-mext-nat64-mobility-harmful-01 (work in
progress), April 2010.
[I-D.ietf-behave-v6v4-xlate]
Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", draft-ietf-behave-v6v4-xlate-23 (work in
progress), September 2010.
[I-D.ietf-behave-v6v4-xlate-stateful]
Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-12 (work in
progress), July 2010.
[I-D.ietf-softwire-dual-stack-lite]
Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", draft-ietf-softwire-dual-stack-lite-06 (work
in progress), August 2010.
[I-D.ietf-tsvwg-port-randomization]
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Larsen, M. and F. Gont, "Transport Protocol Port
Randomization Recommendations",
draft-ietf-tsvwg-port-randomization-09 (work in progress),
August 2010.
[I-D.nishitani-cgn]
Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida,
"Common requirements for IP address sharing schemes",
draft-nishitani-cgn-05 (work in progress), July 2010.
[I-D.shirasaki-nat444]
Yamagata, I., Shirasaki, Y., Nakagawa, A., Yamaguchi, J.,
and H. Ashida, "NAT444", draft-shirasaki-nat444-02 (work
in progress), July 2010.
[I-D.thaler-port-restricted-ip-issues]
Thaler, D., "Issues With Port-Restricted IP Addresses",
draft-thaler-port-restricted-ip-issues-00 (work in
progress), February 2010.
[I-D.ymbk-aplusp]
Bush, R., "The A+P Approach to the IPv4 Address Shortage",
draft-ymbk-aplusp-05 (work in progress), October 2009.
[IPv4_Report]
Huston, G., "IPv4 Address Report", 2009,
<http://www.potaroo.net/tools/ipv4/index.html>.
[Mobile_Energy_Consumption]
Haverinen, H., Siren, J., and P. Eronen, "Energy
Consumption of Always-On Applications in WCDMA Networks",
2007, <http://research.nokia.com/node/5597>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1337] Braden, B., "TIME-WAIT Assassination Hazards in TCP",
RFC 1337, May 1992.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140,
April 1997.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
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RFC 2663, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2827] 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.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3715] Aboba, B. and W. Dixon, "IPsec-Network Address Translation
(NAT) Compatibility Requirements", RFC 3715, March 2004.
[RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947,
January 2005.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC5135] Wing, D. and T. Eckert, "IP Multicast Requirements for a
Network Address Translator (NAT) and a Network Address
Port Translator (NAPT)", BCP 135, RFC 5135, February 2008.
[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
April 2009.
[UPnP-IGD]
UPnP Forum, "Universal Plug and Play (UPnP) Internet
Gateway Device (IGD)", November 2001,
<http://www.upnp.org/standardizeddcps/igd.asp>.
[Windows-Logo]
Microsoft, "Windows Logo Program Device Requirements",
2006, <http://www.microsoft.com/whdc/winlogo/
hwrequirements/default.mspx>.
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Authors' Addresses
Mat Ford (editor)
Internet Society
Geneva
Switzerland
Email: ford@isoc.org
Mohamed Boucadair
France Telecom
Rennes 35000
France
Email: mohamed.boucadair@orange-ftgroup.com
Alain Durand
Juniper Networks
Email: adurand@juniper.net
Pierre Levis
France Telecom
42 rue des Coutures
BP 6243
Caen Cedex 4 14066
France
Email: pierre.levis@orange-ftgroup.com
Phil Roberts
Internet Society
Reston, VA
USA
Email: roberts@isoc.org
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