| Rfc | 5902 |
|---|
| Title | IAB Thoughts on IPv6 Network Address Translation |
|---|
| Author | D. Thaler, L.
Zhang, G. Lebovitz |
|---|
| Date | July 2010 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Internet Architecture Board (IAB) D. Thaler
Request for Comments: 5902 L. Zhang
Category: Informational G. Lebovitz
ISSN: 2070-1721 July 2010
IAB Thoughts on IPv6 Network Address Translation
Abstract
There has been much recent discussion on the topic of whether the
IETF should develop standards for IPv6 Network Address Translators
(NATs). This document articulates the architectural issues raised by
IPv6 NATs, the pros and cons of having IPv6 NATs, and provides the
IAB's thoughts on the current open issues and the solution space.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Architecture Board (IAB)
and represents information that the IAB has deemed valuable to
provide for permanent record. Documents approved for publication by
the IAB are not a candidate for any level of Internet Standard; see
Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5902.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
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
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. What is the problem? . . . . . . . . . . . . . . . . . . . . . 3
2.1. Avoiding Renumbering . . . . . . . . . . . . . . . . . . . 3
2.2. Site Multihoming . . . . . . . . . . . . . . . . . . . . . 4
2.3. Homogenous Edge Network Configurations . . . . . . . . . . 4
2.4. Network Obfuscation . . . . . . . . . . . . . . . . . . . 5
2.4.1. Hiding Hosts . . . . . . . . . . . . . . . . . . . . . 5
2.4.2. Topology Hiding . . . . . . . . . . . . . . . . . . . 8
2.4.3. Summary Regarding NAT as a Tool for Network
Obfuscation . . . . . . . . . . . . . . . . . . . . . 8
2.5. Simple Security . . . . . . . . . . . . . . . . . . . . . 9
2.6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Architectural Considerations of IPv6 NAT . . . . . . . . . . . 9
4. Solution Space . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. IAB Members at the Time of Approval . . . . . . . . . . . . . 13
7. Informative References . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
In the past, the IAB has published a number of documents relating to
Internet transparency and the end-to-end principle, and other IETF
documents have also touched on these issues as well. These documents
articulate the general principles on which the Internet architecture
is based, as well as the core values that the Internet community
seeks to protect going forward. Most recently, RFC 4924 [RFC4924]
reaffirms these principles and provides a review of the various
documents in this area.
Facing imminent IPv4 address space exhaustion, recently there have
been increased efforts in IPv6 deployment. However, since late 2008
there have also been increased discussions about whether the IETF
should standardize network address translation within IPv6. People
who are against standardizing IPv6 NAT argue that there is no
fundamental need for IPv6 NAT, and that as IPv6 continues to roll
out, the Internet should converge towards reinstallation of the end-
to-end reachability that has been a key factor in the Internet's
success. On the other hand, people who are for IPv6 NAT believe that
NAT vendors would provide IPv6 NAT implementations anyway as NAT can
be a solution to a number of problems, and that the IETF should avoid
repeating the same mistake as with IPv4 NAT, where the lack of
protocol standards led to different IPv4 NAT implementations, making
NAT traversal difficult.
An earlier effort, [RFC4864], provides a discussion of the real or
perceived benefits of NAT and suggests alternatives for most of them,
with the intent of showing that NAT is not required to get the
desired benefits. However, it also identifies several gaps remaining
to be filled.
This document provides the IAB's current thoughts on this debate. We
believe that the issue at hand must be viewed from an overall
architectural standpoint in order to fully assess the pros and cons
of IPv6 NAT on the global Internet and its future development.
2. What is the problem?
The discussions on the desire for IPv6 NAT can be summarized as
follows. Network address translation is viewed as a solution to
achieve a number of desired properties for individual networks:
avoiding renumbering, facilitating multihoming, making configurations
homogenous, hiding internal network details, and providing simple
security.
2.1. Avoiding Renumbering
As discussed in [RFC4864], Section 2.5, the ability to change service
providers with minimal operational difficulty is an important
requirement in many networks. However, renumbering is still quite
painful today, as discussed in [RFC5887]. Currently it requires
reconfiguring devices that deal with IP addresses or prefixes,
including DNS servers, DHCP servers, firewalls, IPsec policies, and
potentially many other systems such as intrusion detection systems,
inventory management systems, patch management systems, etc.
In practice today, renumbering does not seem to be a significant
problem in consumer networks, such as home networks, where addresses
or prefixes are typically obtained through DHCP and are rarely
manually configured in any component. However, in managed networks,
renumbering can be a serious problem.
We also note that many, if not most, large enterprise networks avoid
the renumbering problem by using provider-independent (PI) IP address
blocks. The use of PI addresses is inherent in today's Internet
operations. However, in smaller managed networks that cannot get
provider-independent IP address blocks, renumbering remains a serious
issue. Regional Internet Registries (RIRs) constantly receive
requests for PI address blocks; one main reason that they hesitate in
assigning PI address blocks to all users is the concern about the PI
addresses' impact on the routing system scalability.
2.2. Site Multihoming
Another important requirement in many networks is site multihoming.
A multihomed site essentially requires that its IP prefixes be
present in the global routing table to achieve the desired
reliability in its Internet connectivity as well as load balancing.
In today's practice, multihomed sites with PI addresses announce
their PI prefixes to the global routing system; multihomed sites with
provider-allocated (PA) addresses also announce the PA prefix they
obtained from one service provider to the global routing system
through another service provider, effectively disabling provider-
based prefix aggregation. This practice makes the global routing
table scale linearly with the number of multihomed user networks.
This issue was identified in [RFC4864], Section 6.4. Unfortunately,
no solution except NAT has been deployed today that can insulate the
global routing system from the growing number of multihomed sites,
where a multihomed site simply assigns multiple IPv4 addresses (one
from each of its service providers) to its exit router, which is an
IPv4 NAT box. Using address translation to facilitate multihoming
support has one unique advantage: there is no impact on the routing
system scalability, as the NAT box simply takes one address from each
service provider, and the multihomed site does not inject its own
routes into the system. Intuitively, it also seems straightforward
to roll the same solution into multihoming support in the IPv6
deployment. However, one should keep in mind that this approach
brings all the drawbacks of putting a site behind a NAT box,
including the loss of reachability to the servers behind the NAT box.
It is also important to point out that a multihomed site announcing
its own prefix(es) achieves two important benefits that NAT-based
multihoming support does not provide. First, end-to-end
communications can be preserved in face of connectivity failures of
individual service providers, as long as the site remains connected
through at least one operational service provider. Second,
announcing one's prefixes also gives a multihomed site the ability to
perform traffic engineering and load balancing.
2.3. Homogenous Edge Network Configurations
Service providers supporting residential customers need to minimize
support costs (e.g., help desk calls). Often a key factor in
minimizing support costs is ensuring customers have homogenous
configurations, including the addressing architecture. Today, when
IPv4 NATs are provided by a service provider, all customers get the
same address space on their home networks, and hence the home gateway
always has the same address. From a customer-support perspective,
this perhaps represents the most important property of NAT usage
today.
In IPv6, link-local addresses can be used to ensure that all home
gateways have the same address, and to provide homogenous addresses
to any other devices supported by the service provider. Unlike IPv4,
having a globally unique address does not prevent the use of a
homogenous address within the subnet. It is only in the case of
multi-subnet customers that IPv6 NAT would provide some homogeneity
that wouldn't be provided by link-local addresses. For multi-subnet
customers (e.g., a customer using a wireless access point behind the
service provider router/modem), service providers today might only
discuss problems (for IPv4 or IPv6) from computers connected directly
to the service provider router.
It is currently unknown whether IPv6 link-local addresses provide
sufficient homogeneity to minimize help desk calls. If they do not,
providers might still desire IPv6 NATs in the residential gateways
they provide.
2.4. Network Obfuscation
Most network administrators want to hide the details of the computing
resources, information infrastructure, and communications networks
within their borders. This desire is rooted in the basic security
principle that an organization's assets are for its sole use and all
information about those assets, their operation, and the methods and
tactics of their use are proprietary secrets. Some organizations use
their information and communication technologies as a competitive
advantage in their industries. It is a generally held belief that
measures must be taken to protect those secrets. The first layer of
protection of those secrets is preventing access to the secrets or
knowledge about the secrets whenever possible. It is understandable
why network administrators would want to keep the details about the
hosts on their network, as well as the network infrastructure itself,
private. They believe that NAT helps achieve this goal.
2.4.1. Hiding Hosts
As a specific measure of network obfuscation, network administrators
wish to keep secret any and all information about the computer
systems residing within their network boundaries. Such computer
systems include workstations, laptops, servers, function-specific
end-points (e.g., printers, scanners, IP telephones, point-of-sale
machines, building door access-control devices), and such. They want
to prevent an external entity from counting the number of hosts on
the network. They also want to prevent host fingerprinting, i.e.,
gaining information about the constitution, contents, or function of
a host. For example, they want to hide the role of a host, as
whether it is a user workstation, a finance server, a source code
build server, or a printer. A second element of host-fingerprinting
prevention is to hide details that could aid an attacker in
compromising the host. Such details might include the type of
operating system, its version number, any patches it may or may not
have, the make and model of the device hardware, any application
software packages loaded, those version numbers and patches, and so
on. With such information about hosts, an attacker can launch a more
focused, targeted attack. Operators want to stop both host counting
and host fingerprinting.
Where host counting is a concern, it is worth pointing out some of
the challenges in preventing it. [Bellovin] showed how one can
successfully count the number of hosts behind a certain type of
simple NAT box. More complex NAT deployments, e.g., ones employing
Network Address Port Translators (NAPTs) with a pool of public
addresses that are randomly bound to internal hosts dynamically upon
receipt of any new connection, and do so without persistency across
connections from the same host are more successful in preventing host
counting. However, the more complex the NAT deployment, the less
likely that complex connection types like the Session Initiation
Protocol (SIP) [RFC3261] and the Stream Control Transmission Protocol
(SCTP) [RFC4960] will be able to successfully traverse the NAT. This
observation follows the age-old axiom for networked computer systems:
for every unit of security you gain, you give up a unit of
convenience, and for every unit of convenience you hope to gain, you
must give up a unit of security.
If fields such as fragment ID, TCP initial sequence number, or
ephemeral port number are chosen in a predictable fashion (e.g.,
sequentially), then an attacker may correlate packets or connections
coming from the same host.
To prevent counting hosts by counting addresses, one might be tempted
to use a separate IP address for each transport-layer connection.
Such an approach introduces other architectural problems, however.
Within the host's subnet, various devices including switches,
routers, and even the host's own hardware interface often have a
limited amount of state available before causing communication that
uses a large number of addresses to suffer significant performance
problems. In addition, if an attacker can somehow determine an
average number of connections per host, the attacker can still
estimate the number of hosts based on the number of connections
observed. Hence, such an approach can adversely affect legitimate
communication at all times, simply to raise the bar for an attacker.
Where host fingerprinting is concerned, even a complex NAT cannot
prevent fingerprinting completely. The way that different hosts
respond to different requests and sequences of events will indicate
consistently the type of a host that it is, its OS, version number,
and sometimes applications installed, etc. Products exist that do
this for network administrators as a service, as part of a
vulnerability assessment.
These scanning tools initiate connections of various types across a
range of possible IP addresses reachable through that network. They
observe what returns, and then send follow-up messages accordingly
until they "fingerprint" the host thoroughly. When run as part of a
network assessment process, these tools are normally run from the
inside of the network, behind the NAT. If such a tool is set outside
a network boundary (as part of an external vulnerability assessment
or penetration test) along the path of packets, and is passively
observing and recording connection exchanges, over time it can
fingerprint hosts only if it has a means of determining which
externally viewed connections are originating from the same internal
host. If the NATing is simple and static, and each host's internal
address is always mapped to the same external address and vice versa,
the tool has 100% success fingerprinting the host. With the internal
hosts mapped to their external IP addresses and fingerprinted, the
attacker can launch targeted attacks into those hosts, or reliably
attempt to hijack those hosts' connections. If the NAT uses a single
external IP, or a pool of dynamically assigned IP addresses for each
host, but does so in a deterministic and predictable way, then the
operation of fingerprinting is more complex, but quite achievable.
If the NAT uses dynamically assigned addresses, with short-term
persistency, but no externally learnable determinism, then the
problem gets harder for the attacker. The observer may be able to
fingerprint a host during the lifetime of a particular IP address
mapping, and across connections, but once that IP mapping is
terminated, the observer doesn't immediately know which new mapping
will be that same host. After much observation and correlation, the
attacker could sometimes determine if an observed new connection in
flight is from a familiar host. With that information, and a good
set of man-in-the-middle attack tools, the attacker could attempt to
compromise the host by hijacking a new connection of adequately long
duration. If temporal persistency is not deployed on the NAT, then
this tactic becomes almost impossible. As the difficulty and cost of
the attack increases, the number of attackers attempting to employ it
decreases. And certainly the attacker would not be able to initiate
a connection toward a host for which the attacker does not know the
current IP address binding. So, the attacker is limited to hijacking
observed connections thought to be from a familiar host, or to
blindly initiating attacks on connections in flight. This is why
network administrators appreciate complex NATs' ability to deter host
counting and fingerprinting, but such deterrence comes at a cost of
host reachability.
2.4.2. Topology Hiding
It is perceived that a network operator may want to hide the details
of the network topology, the size of the network, the identities of
the internal routers, and the interconnection among the routers.
This desire has been discussed in [RFC4864], Sections 4.4 and 6.2.
However, the success of topology hiding is dependent upon the
complexity, dynamism, and pervasiveness of bindings the NAT employs
(all of which were described above). The more complex, the more the
topology will be hidden, but the less likely that complex connection
types will successfully traverse the NAT barrier. Thus, the trade-
off is reachability across applications.
Even if one can hide the actual addresses of internal hosts through
address translation, this does not necessarily prove sufficient to
hide internal topology. It may be possible to infer some aspects of
topological information from passively observing packets. For
example, based on packet timing, delay measurements, the Hop Limit
field, or other fields in the packet header, one could infer the
relative distance between multiple hosts. Once an observed session
is believed to match a previously fingerprinted host, that host's
distance from the NAT device may be learned, but not its exact
location or particular internal subnet.
Host fingerprinting is required in order to do a thorough distance
mapping. An attacker might then use message contents to lump certain
types of devices into logical clusters, and take educated guesses at
attacks. This is not, however, a thorough mapping. Some NATs change
the TTL hop counts, much like an application-layer proxy would, while
others don't; this is an administrative setting on more advanced
NATs. The simpler and more static the NAT, the more possible this
is. The more complex and dynamic and non-persistent the NAT
bindings, the more difficult.
2.4.3. Summary Regarding NAT as a Tool for Network Obfuscation
The degree of obfuscation a NAT can achieve will be a function of its
complexity as measured by:
o The use of one-to-many NAPT mappings;
o The randomness over time of the mappings from internal to external
IP addresses, i.e., non-deterministic mappings from an outsider's
perspective;
o The lack of persistence of mappings, i.e., the shortness of
mapping lifetimes and not using the same mapping repeatedly;
o The use of re-writing in IP header fields such as TTL.
However, deployers be warned: as obfuscation increases, host
reachability decreases. Mechanisms such as STUN [RFC5389] and Teredo
[RFC4380] fail with the more complex NAT mechanisms.
2.5. Simple Security
It is commonly perceived that a NAT box provides one level of
protection because external hosts cannot directly initiate
communication with hosts behind a NAT. However, one should not
confuse NAT boxes with firewalls. As discussed in [RFC4864], Section
2.2, the act of translation does not provide security in itself. The
stateful filtering function can provide the same level of protection
without requiring a translation function. For further discussion,
see [RFC4864], Section 4.2.
2.6. Discussion
At present, the primary benefits one may receive from deploying NAT
appear to be avoiding renumbering, facilitating multihoming without
impacting routing scalability, and making edge consumer network
configurations homogenous.
Network obfuscation (host hiding, both counting and fingerprinting
prevention, and topology hiding) may well be achieved with more
complex NATs, but at the cost of losing some reachability and
application success. Again, when it comes to security, this is often
the case: to gain security one must give up some measure of
convenience.
3. Architectural Considerations of IPv6 NAT
First, it is important to distinguish between the effects of a NAT
box vs. the effects of a firewall. A firewall is intended to prevent
unwanted traffic [RFC4948] without impacting wanted traffic, whereas
a NAT box also interferes with wanted traffic. In the remainder of
this section, the term "reachability" is used with respect to wanted
traffic.
The discussions on IPv6 NAT often refer to the wide deployment of
IPv4 NAT, where people have both identified tangible benefits and
gained operational experience. However, the discussions so far seem
mostly focused on the potential benefits that IPv6 NAT may, or may
not, bring. Little attention has been paid to the bigger picture, as
we elaborate below.
When considering the benefits that IPv6 NAT may bring to a site that
deploys it, we must not overlook a bigger question: if one site
deploys IPv6 NAT, what is the potential impact it brings to the rest
of the Internet that does not do IPv6 NAT? By "the rest of the
Internet", we mean the Internet community that develops, deploys, and
uses end-to-end applications and protocols and hence is affected by
any loss of transparency (see [RFC2993] and [RFC4924] for further
discussion). This important question does not seem to have been
addressed, or addressed adequately.
We believe that the discussions on IPv6 NAT should be put in the
context of the overall Internet architecture. The foremost question
is not how many benefits one may derive from using IPv6 NAT, but more
fundamentally, whether a significant portion of parties on the
Internet are willing to deploy IPv6 NAT, and hence whether we want to
make IP address translation a permanent building block in the
Internet architecture.
One may argue that the answers to the above questions depend on
whether we can find adequate solutions to the renumbering, site
multihoming, and edge network configuration problems, and whether the
solutions provide transparency or not. If transparency is not
provided, making NAT a permanent building block in the Internet would
represent a fundamental architectural change.
It is desirable that IPv6 users and applications be able to reach
each other directly without having to worry about address translation
boxes between the two ends. IPv6 application developers in general
should be able to program based on the assumption of end-to-end
reachability (of wanted traffic), without having to address the issue
of traversing NAT boxes. For example, referrals and multi-party
conversations are straightforward with end-to-end addressing, but
vastly complicated in the presence of address translation.
Similarly, network administrators should be able to run their
networks without the added complexity of NATs, which can bring not
only the cost of additional boxes, but also increased difficulties in
network monitoring and problem debugging.
Given the diversity of the Internet user populations and the
diversity in today's operational practice, it is conceivable that
some parties may have a strong desire to deploy IPv6 NAT, and the
Internet should accommodate different views that lead to different
practices (i.e., some using IPv6 NAT, others not).
If we accept the view that some, but not all, parties want IPv6 NAT,
then the real debate should not be on what benefits IPv6 NAT may
bring to the parties who deploy it. It is undeniable that network
address translation can bring certain benefits to its users.
However, the real challenge we should address is how to design IPv6
NAT in such a way that it can hide its impact within some localized
scope. If IPv6 NAT design can achieve this goal, then the Internet
as a whole can strive for (reinstalling) the end-to-end reachability
model.
4. Solution Space
From an end-to-end perspective, the solution space for renumbering
and multihoming can be broadly divided into three classes:
1. Endpoints get a stable, globally reachable address: In this class
of solutions, end sites use provider-independent addressing and
hence endpoints are unaffected by changing service providers.
For this to be a complete solution, provider-independent
addressing must be available to all managed networks (i.e., all
networks that use manual configuration of addresses or prefixes
in any type of system). However, in today's practice, assigning
provider-independent addresses to all networks, including small
ones, raises concerns with the scalability of the global routing
system. This is an area of ongoing research and experimentation.
In practice, network administrators have also been developing
short-term approaches to resolve today's gap between the
continued routing table growth and limitations in existing router
capacity [NANOG].
2. Endpoints get a stable but non-globally-routable address on
physical interfaces but a dynamic, globally routable address
inside a tunnel: In this class of solutions, hosts use locally-
scoped (and hence provider-independent) addresses for
communication within the site using their physical interfaces.
As a result, managed systems such as routers, DHCP servers, etc.,
all see stable addresses. Tunneling from the host to some
infrastructure device is then used to communicate externally.
Tunneling provides the host with globally routable addresses that
may change, but address changes are constrained to systems that
operate over or beyond the tunnel, including DNS servers and
applications. These systems, however, are the ones that often
can already deal with changes today using mechanisms such as DNS
dynamic update. However, if endpoints and the tunnel
infrastructure devices are owned by different organizations, then
solutions are harder to incrementally deploy due to the incentive
and coordination issues involved.
3. Endpoints get a stable address that gets translated in the
network: In this class of solutions, end sites use non-globally-
routable addresses within the site, and translate them to
globally routable addresses somewhere in the network. In
general, this causes the loss of end-to-end transparency, which
is the subject of [RFC4924] and the documents it surveys. If the
translation is reversible, and the translation is indeed reversed
by the time it reaches the other end of communication, then end-
to-end transparency can be provided. However, if the two
translators involved are owned by different organizations, then
solutions are harder to incrementally deploy due to the incentive
and coordination issues involved.
Concerning routing scalability, although there is no immediate
danger, routing scalability has been a longtime concern in
operational communities, and an effective and deployable solution
must be found. We observe that the question at hand is not about
whether some parties can run NAT, but rather, whether the Internet as
a whole would be willing to rely on NAT to curtail the routing
scalability problem, and whether we have investigated all the
potential impacts of doing so to understand its cost on the overall
architecture. If effective solutions can be deployed in time to
allow assigning provider-independent IPv6 addresses to all user
communities, the Internet can avoid the complexity and fragility and
other unforeseen problems introduced by NAT.
4.1. Discussion
As [RFC4924] states:
A network that does not filter or transform the data that it
carries may be said to be "transparent" or "oblivious" to the
content of packets. Networks that provide oblivious transport
enable the deployment of new services without requiring changes to
the core. It is this flexibility that is perhaps both the
Internet's most essential characteristic as well as one of the
most important contributors to its success.
We believe that providing end-to-end transparency, as defined above,
is key to the success of the Internet. While some fields of traffic
(e.g., Hop Limit) are defined to be mutable, transparency requires
that fields not defined as such arrive un-transformed. Currently,
the source and destination addresses are defined as immutable fields,
and are used as such by many protocols and applications.
Each of the three classes of solution can be defined in a way that
preserves end-to-end transparency.
While we do not consider IPv6 NATs to be desirable, we understand
that some deployment of them is likely unless workable solutions to
avoiding renumbering, facilitating multihoming without adversely
impacting routing scalability, and homogeneity are generally
recognized as useful and appropriate.
As such, we strongly encourage the community to consider end-to-end
transparency as a requirement when proposing any solution, whether it
be based on tunneling or translation or some other technique.
Solutions can then be compared based on other aspects such as
scalability and ease of deployment.
5. Security Considerations
Section 2 discusses potential privacy concerns as part of the Host
Counting and Topology Hiding problems.
6. IAB Members at the Time of Approval
Marcelo Bagnulo
Gonzalo Camarillo
Stuart Cheshire
Vijay Gill
Russ Housley
John Klensin
Olaf Kolkman
Gregory Lebovitz
Andrew Malis
Danny McPherson
David Oran
Jon Peterson
Dave Thaler
7. Informative References
[Bellovin] Bellovin, S., "A Technique for Counting NATted Hosts",
Proc. Second Internet Measurement Workshop,
November 2002,
<http://www.cs.columbia.edu/~smb/papers/fnat.pdf>.
[NANOG] "Extending the Life of Layer 3 Switches in a 256k+ Route
World", NANOG 44, October 2008, <http://www.nanog.org/
meetings/nanog44/presentations/Monday/
Roisman_lightning.pdf>.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4864] Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
E. Klein, "Local Network Protection for IPv6", RFC 4864,
May 2007.
[RFC4924] Aboba, B. and E. Davies, "Reflections on Internet
Transparency", RFC 4924, July 2007.
[RFC4948] Andersson, L., Davies, E., and L. Zhang, "Report from the
IAB workshop on Unwanted Traffic March 9-10, 2006",
RFC 4948, August 2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
Still Needs Work", RFC 5887, May 2010.
Authors' Addresses
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
Lixia Zhang
UCLA Computer Science Department
3713 Boelter Hall
Los Angeles, CA 90095
USA
Phone: +1 310 825 2695
EMail: lixia@cs.ucla.edu
Gregory Lebovitz
Juniper Networks, Inc.
1194 North Mathilda Ave.
Sunnyvale, CA 94089
USA
EMail: gregory.ietf@gmail.com
Internet Architecture Board
EMail: iab@iab.org