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rfc:rfc5902

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.

Thaler, et al. Informational [Page 1] RFC 5902 IPv6 NAT Considerations July 2010

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.

Thaler, et al. Informational [Page 2] RFC 5902 IPv6 NAT Considerations July 2010

 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.

Thaler, et al. Informational [Page 3] RFC 5902 IPv6 NAT Considerations July 2010

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

Thaler, et al. Informational [Page 4] RFC 5902 IPv6 NAT Considerations July 2010

 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.,

Thaler, et al. Informational [Page 5] RFC 5902 IPv6 NAT Considerations July 2010

 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.

Thaler, et al. Informational [Page 6] RFC 5902 IPv6 NAT Considerations July 2010

 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

Thaler, et al. Informational [Page 7] RFC 5902 IPv6 NAT Considerations July 2010

 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;

Thaler, et al. Informational [Page 8] RFC 5902 IPv6 NAT Considerations July 2010

 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.

Thaler, et al. Informational [Page 9] RFC 5902 IPv6 NAT Considerations July 2010

 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.

Thaler, et al. Informational [Page 10] RFC 5902 IPv6 NAT Considerations July 2010

 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

Thaler, et al. Informational [Page 11] RFC 5902 IPv6 NAT Considerations July 2010

     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

Thaler, et al. Informational [Page 12] RFC 5902 IPv6 NAT Considerations July 2010

 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

Thaler, et al. Informational [Page 13] RFC 5902 IPv6 NAT Considerations July 2010

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.

Thaler, et al. Informational [Page 14] RFC 5902 IPv6 NAT Considerations July 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

Thaler, et al. Informational [Page 15]

/data/webs/external/dokuwiki/data/pages/rfc/rfc5902.txt · Last modified: 2010/07/16 23:26 (external edit)