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Network Working Group T. Hain Request for Comments: 2993 Microsoft Category: Informational November 2000

                 Architectural Implications of NAT

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.


 In light of the growing interest in, and deployment of network
 address translation (NAT) RFC-1631, this paper will discuss some of
 the architectural implications and guidelines for implementations. It
 is assumed the reader is familiar with the address translation
 concepts presented in RFC-1631.

Table of Contents

 1.  Introduction................................................... 2
 2.  Terminology.................................................... 4
 3.  Scope.......................................................... 6
 4.  End-to-End Model............................................... 7
 5.  Advantages of NATs............................................. 8
 6.  Problems with NATs............................................ 10
 7.  Illustrations................................................. 13
  7.1 Single point of failure...................................... 13
  7.2.  ALG complexity............................................. 14
  7.3. TCP state violations........................................ 15
  7.4.  Symmetric state management................................. 16
  7.5.  Need for a globally unique FQDN when advertising public
        services................................................... 18
  7.6.  L2TP tunnels increase frequency of address collisions...... 19
  7.7.  Centralized data collection system report correlation...... 20
 8.  IPv6.......................................................... 21
 9.  Security Considerations....................................... 22
 10.  Deployment Guidelines........................................ 23
 11.  Summary...................................................... 24
 12.  References................................................... 27

Hain Informational [Page 1] RFC 2993 Architectural Implications of NAT November 2000

 13.  Acknowledgments.............................................. 28
 14.  Author's Address............................................. 28
 15.  Full Copyright Statement..................................... 29

1. Introduction

 Published in May 1994, written by K. Egevang and P. Francis, RFC-1631
 [2] defined NAT as one means to ease the growth rate of IPv4 address
 use.  But the authors were worried about the impact of this
 technology.  Several places in the document they pointed out the need
 to experiment and see what applications may be adversely affected by
 NAT's header manipulations, even before there was any significant
 operational experience.  This is further evidenced in a quote from
 the conclusions: 'NAT has several negative characteristics that make
 it inappropriate as a long term solution, and may make it
 inappropriate even as a short term solution.'
 Now, six years later and in spite of the prediction, the use of NATs
 is becoming widespread in the Internet.  Some people are proclaiming
 NAT as both the short and long term solution to some of the
 Internet's address availability issues and questioning the need to
 continue the development of IPv6.  The claim is sometimes made that
 NAT 'just works' with no serious effects except on a few legacy
 applications.  At the same time others see a myriad of difficulties
 caused by the increasing use of NAT.
 The arguments pro & con frequently take on religious tones, with each
 side passionate about its position.
  1. Proponents bring enthusiasm and frequently cite the most popular

applications of Mail & Web services as shining examples of NAT

    transparency.  They will also point out that NAT is the feature
    that finally breaks the semantic overload of the IP address as
    both a locator and the global endpoint identifier (EID).
 -  An opposing view of NAT is that of a malicious technology, a weed
    which is destined to choke out continued Internet development.
    While recognizing there are perceived address shortages, the
    opponents of NAT view it as operationally inadequate at best,
    bordering on a sham as an Internet access solution. Reality lies
    somewhere in between these extreme viewpoints.
 In any case it is clear NAT affects the transparency of end-to-end
 connectivity for transports relying on consistency of the IP header,
 and for protocols which carry that address information in places
 other than the IP header.  Using a patchwork of consistently
 configured application specific gateways (ALG's), endpoints can work
 around some of the operational challenges of NAT.  These operational
 challenges vary based on a number of factors including network and

Hain Informational [Page 2] RFC 2993 Architectural Implications of NAT November 2000

 application topologies and the specific applications in use.  It can
 be relatively easy to deal with the simplest case, with traffic
 between two endpoints running over an intervening network with no
 parallel redundant NAT devices.  But things can quickly get quite
 complicated when there are parallel redundant NAT devices, or where
 there are more distributed and multi-point applications like multi-
 party document sharing.  The complexity of coordinating the updates
 necessary to work around NAT grows geometrically with the number of
 endpoints.  In a large environment, this may require concerted effort
 to simultaneously update all endpoints of a given application or
 The architectural intent of NAT is to divide the Internet into
 independent address administrations, (also see "address realms",
 RFC-2663 [3]) specifically facilitating casual use of private address
 assignments RFC-1918 [4].  As noted by Carpenter, et al RFC-2101 [5],
 once private use addresses were deployed in the network, addresses
 were guaranteed to be ambiguous.  For example, when simple NATs are
 inserted into the network, the process of resolving names to or from
 addresses becomes dependent on where the question was asked.  The
 result of this division is to enforce a client/server architecture
 (vs. peer/peer) where the servers need to exist in the public address
 A significant factor in the success of the Internet is the
 flexibility derived from a few basic tenets.  Foremost is the End-
 to-End principle (discussed further below), which notes that certain
 functions can only be performed in the endpoints, thus they are in
 control of the communication, and the network should be a simple
 datagram service that moves bits between these points.  Restated, the
 endpoint applications are often the only place capable of correctly
 managing the data stream.  Removing this concern from the lower layer
 packet-forwarding devices streamlines the forwarding process,
 contributing to system-wide efficiency.
 Another advantage is that the network does not maintain per
 connection state information.  This allows fast rerouting around
 failures through alternate paths and to better scaling of the overall
 network.  Lack of state also removes any requirement for the network
 nodes to notify each other as endpoint connections are formed or
 dropped.  Furthermore, the endpoints are not, and need not be, aware
 of any network components other than the destination, first hop
 router(s), and an optional name resolution service.  Packet integrity
 is preserved through the network, and transport checksums and any
 address-dependent security functions are valid end-to-end.

Hain Informational [Page 3] RFC 2993 Architectural Implications of NAT November 2000

 NAT devices (particularly the NAPT variety) undermine most of these,
 basic advantages of the end-to-end model, reducing overall
 flexibility, while often increasing operational complexity and
 impeding diagnostic capabilities.  NAT variants such as RSIP [6] have
 recently been proposed to address some of the end-to-end concerns.
 While these proposals may be effective at providing the private node
 with a public address (if ports are available), they do not eliminate
 several issues like network state management, upper layer constraints
 like TCP_TIME_WAIT state, or well-known-port sharing. Their port
 multiplexing variants also have the same DNS limitations as NAPT, and
 each host requires significant stack modifications to enable the
 technology (see below).
 It must be noted that firewalls also break the end-to-end model and
 raise several of the same issues that NAT devises do, while adding a
 few of their own.  But one operational advantage with firewalls is
 that they are generally installed into networks with the explicit
 intent to interfere with traffic flow, so the issues are more likely
 to be understood or at least looked at if mysterious problems arise.
 The same issues with NAT devices can sometimes be overlooked since
 NAT devices are frequently presented as transparent to applications.
 One thing that should be clearly stated up front is, that attempts to
 use a variant of NAT as a simple router replacement may create
 several significant issues that should be addressed before
 deployment.  The goal of this document is to discuss these with the
 intent to raise awareness.

2. Terminology

 Recognizing that many of these terms are defined in detail in RFC
 2663 [3], the following are summaries as used in this document.
 NAT - Network Address Translation in simple form is a method by which
 IP addresses are mapped from one address administration to another.
 The NAT function is unaware of the applications traversing it, as it
 only looks at the IP headers.
 ALG - Application Layer Gateway: inserted between application peers
 to simulate a direct connection when some intervening protocol or
 device prevents direct access.  It terminates the transport protocol,
 and may modify the data stream before forwarding.
 NAT/ALG - combines ALG functions with simple NAT.  Generally more
 useful than pure NAT, because it embeds components for specific
 applications that would not work through a pure NAT.

Hain Informational [Page 4] RFC 2993 Architectural Implications of NAT November 2000

 DNS/ALG -  a special case of the NAT/ALG, where an ALG for the DNS
 service interacts with the NAT component to modify the contents of a
 DNS response.
 Firewall - access control point that may be a special case of an ALG,
 or packet filter.
 Proxy - A relay service designed into a protocol, rather than
 arbitrarily inserted.  Unlike an ALG, the application on at least one
 end must be aware of the proxy.
 Static NAT - provides stable one-to-one mapping between address
 Dynamic NAT - provides dynamic mapping between address spaces
 normally used with a relatively large number of addresses on one side
 (private space) to a few addresses on the other (public space).
 NAPT - Network Address Port Translation accomplishes translation by
 multiplexing transport level identifiers of multiple addresses from
 one side, simultaneously into the transport identifiers of a single
 address on the other.  See 4.1.2 of RFC-2663.  This permits multiple
 endpoints to share and appear as a single IP address.
 RSIP - Realm Specific IP allows endpoints to acquire and use the
 public address and port number at the source.  It includes mechanisms
 for the private node to request multiple resources at once.  RSIP
 clients must be aware of the address administration boundaries, which
 specific administrative area its peer resides in for each
 application, and the topology for reaching the peer.  To complete a
 connection, the private node client requests one or more addresses
 and/or ports from the appropriate RSIP server, then initiates a
 connection via that RSIP server using the acquired public resources.
 Hosts must be updated with specific RSIP software to support the
 tunneling functions.
 VPN - For purposes of this document, Virtual Private Networks
 technically treat an IP infrastructure as a multiplexing substrate,
 allowing the endpoints to build virtual transit pathways, over which
 they run another instance of IP.  Frequently the 2nd instance of IP
 uses a different set of IP addresses.
 AH - IP Authentication Header, RFC-2401 [7], which provides data
 integrity, data origin authentication, and an optional anti-replay

Hain Informational [Page 5] RFC 2993 Architectural Implications of NAT November 2000

 ESP - Encapsulating Security Payload protocol, RFC 2401, may provide
 data confidentiality (encryption), and limited traffic flow
 confidentiality.  It also may provide data integrity, data origin
 authentication, and an anti-replay service.
 Address administration - coordinator of an address pool assigned to a
 collection of routers and end systems.
 Addressing realm -  a collection of routers and end systems
 exchanging locally unique location knowledge.  (Further defined in
 RFC-2663 NAT Terminology.)  NAT is used a means to distribute address
 allocation authority and provide a mechanism to map addresses from
 one address administration into those of another administration.

3. Scope

 In discussing the architectural impact of NATs on the Internet, the
 first task is defining the scope of the Internet.  The most basic
 definition is; a concatenation of networks built using IETF defined
 technologies.  This simple description does not distinguish between
 the public network known as the Internet, and the private networks
 built using the same technologies (including those connected via
 NAT).  Rekhter, et al in RFC-1918 defined hosts as public when they
 need network layer access outside the enterprise, using a globally
 unambiguous address.  Those that need limited or no access are
 defined as private.  Another way to view this is in terms of the
 transparency of the connection between any given node and the rest of
 the Internet.
 The ultimate resolution of public or private is found in the intent
 of the network in question.  Generally, networks that do not intend
 to be part of the greater Internet will use some screening technology
 to insert a barrier.  Historically barrier devices between the public
 and private networks were known as Firewalls or Application Gateways,
 and were managed to allow approved traffic while blocking everything
 else.  Increasingly, part of the screening technology is a NAT, which
 manages the network locator between the public and private-use
 address spaces, and then, using ALGs adds support for protocols that
 are incompatible with NAT.  (Use of NAT within a private network is
 possible, and is only addressed here in the context that some
 component of the private network is used as a common transit service
 between the NAT attached stubs.)
 RFC-1631 limited the scope of NAT discussions to stub appendages of a
 public Internet, that is, networks with a single connection to the
 rest of the Internet.  The use of NAT in situations in which a
 network has multiple connections to the rest of the Internet is
 significantly more complex than when there is only a single

Hain Informational [Page 6] RFC 2993 Architectural Implications of NAT November 2000

 connection since the NATs have to be coordinated to ensure that they
 have a consistent understanding of address mapping for each
 individual device.

4. End-to-End Model

 The concept of the End-to-End model is reviewed by Carpenter in
 Internet Transparency [8].  One of the key points is "state should be
 maintained only in the endpoints, in such a way that the state can
 only be destroyed when the endpoint itself breaks"; this is termed
 "fate-sharing".  The goal behind fate-sharing is to ensure
 robustness.  As networks grow in size, likelihood of component
 failures affecting a connection becomes increasingly frequent. If
 failures lead to loss of communication, because key state is lost,
 then the network becomes increasingly brittle, and its utility
 degrades.  However, if an endpoint itself fails, then there is no
 hope of subsequent communication anyway.  Therefore the End-to-End
 model argues that as much as possible, only the endpoints should hold
 critical state.
 For NATs, this aspect of the End-to-End model translates into the NAT
 becoming a critical infrastructure element:  if it fails, all
 communication through it fails, and, unless great care is taken to
 assure consistent, stable storage of its state, even when it recovers
 the communication that was passing through it will still fail
 (because the NAT no longer translates it using the same mappings).
 Note that this latter type of failure is more severe than the failure
 of a router; when a router recovers, any communication that it had
 been forwarding previous can continue to be successfully forwarded
 through it.
 There are other important facets to the End-to-End model:
  1. when state is held in the interior of the network, then traffic

dependent on that state cannot be routed around failures unless

    somehow the state is replicated to the fail-over points, which can
    be very difficult to do in a consistent yet efficient and timely
 -  a key principle for scaling networks to large size is to push
    state-holding out to the edges of the network.  If state is held
    by elements in the core of the network, then as the network grows
    the amount of state the elements must holds likewise grows.  The
    capacities of the elements can become severe chokepoints and the
    number of connections affected by a failure also grows.
 -  if security state must be held inside the network (see the
    discussion below), then the possible trust models the network can
    support become restricted.

Hain Informational [Page 7] RFC 2993 Architectural Implications of NAT November 2000

 A network for which endpoints need not trust network service
 providers has a great deal more security flexibility than one which
 does.  (Picture, for example, a business traveler connecting from
 their hotel room back to their home office: should they have to trust
 the hotel's networking staff with their security keys?, or the staff
 of the ISP that supplies the hotel with its networking service?  How
 about when the traveler connects over a wireless connection at an
 Related to this, RFC-2101 notes:
   Since IP Security authentication headers assume that the addresses
   in the network header are preserved end-to-end, it is not clear
   how one could support IP Security-based authentication between a
   pair of hosts communicating through either an ALG or a NAT.
 In addition, there are distributed applications that assume that IP
 addresses are globally scoped, globally routable, and all hosts and
 applications have the same view of those addresses.  Indeed, a
 standard technique for such applications to manage their additional
 control and data connections is for one host to send to another the
 address and port that the second host should connect to.  NATs break
 these applications.  Similarly, there are other applications that
 assume that all upper layer ports from a given IP address map to the
 same endpoint, and port multiplexing technologies like NAPT and RSIP
 break these.  For example, a web server may desire to associate a
 connection to port 80 with one to port 443, but due to the possible
 presence of a NATPT, the same IP address no longer ensures the same
 Limiting such applications is not a minor issue: much of the success
 of the Internet today is due to the ease with which new applications
 can run on endpoints without first requiring upgrades to
 infrastructure elements.  If new applications must have the NATs
 upgraded in order to achieve widespread deployment, then rapid
 deployment is hindered, and the pace of innovation slowed.

5. Advantages of NATs

 A quick look at the popularity of NAT as a technology shows that it
 tackles several real world problems when used at the border of a stub
  1. By masking the address changes that take place, from either dial-

access or provider changes, minimizes impact on the local network

    by avoiding renumbering.
 -  Globally routable addresses can be reused for intermittent access
    customers.  This pushes the demand for addresses towards the
    number of active nodes rather than the total number of nodes.

Hain Informational [Page 8] RFC 2993 Architectural Implications of NAT November 2000

  1. There is a potential that ISP provided and managed NATs would

lower support burden since there could be a consistent, simple

    device with a known configuration at the customer end of an access
 -  Breaking the Internet into a collection of address authorities
    limits the need for continual justification of allocations allows
    network managers to avoid the use of more advanced routing
    techniques such as variable length subnets.
 -  Changes in the hosts may not be necessary for applications that
    don't rely on the integrity of the packet header, or carry IP
    addresses in the payload.
 -  Like packet filtering Firewalls, NAPT, & RSIP block inbound
    connections to all ports until they are administratively mapped.
 Taken together these explain some of the strong motivations for
 moving quickly with NAT deployment.  Traditional NAT [2] provides a
 relatively simple function that is easily understood.
 Removing hosts that are not currently active lowers address demands
 on the public Internet.  In cases where providers would otherwise end
 up with address allocations that could not be aggregated, this
 improves the load on the routing system as well as lengthens the
 lifetime of the IPv4 address space.  While reclaiming idle addresses
 is a natural byproduct of the existing dynamic allocation, dial-
 access devices, in the dedicated connection case this service could
 be provided through a NAT.  In the case of a NAPT, the aggregation
 potential is even greater as multiple end systems share a single
 public address.
 By reducing the potential customer connection options and minimizing
 the support matrix, it is possible that ISP provided NATs would lower
 support costs.
 Part of the motivation for NAT is to avoid the high cost of
 renumbering inherent in the current IPv4 Internet.  Guidelines for
 the assignment of IPv4 addresses RFC-2050 [9] mean that ISP customers
 are currently required to renumber their networks if they want to
 switch to a new ISP.  Using a NAT (or a firewall with NAT functions)
 means that only the Internet facing IP addresses must be changed and
 internal network nodes do not need to be reconfigured. Localizing
 address administration to the NAT minimizes renumbering costs, and
 simultaneously provides for a much larger local pool of addresses
 than is available under current allocation guidelines. (The registry
 guidelines are intended to prolong the lifetime of the IPv4 address
 space and manage routing table growth, until IPv6 is ready or new
 routing technology reduces the pressure on the routing table.  This
 is accomplished by managing allocations to match actual demand and to
 enforce hierarchical addressing.  An unfortunate byproduct of the

Hain Informational [Page 9] RFC 2993 Architectural Implications of NAT November 2000

 current guidelines is that they may end up hampering growth in areas
 where it is difficult to sort out real need from potential hoarding.)
 NAT is effective at masking provider switching or other requirements
 to change addresses, thus mitigates some of the growth issues.
 NAT deployments have been raising the awareness of protocol designers
 who are interested in ensuring that their protocols work end-to-end.
 Breaking the semantic overload of the IP address will force
 applications to find a more appropriate mechanism for endpoint
 identification and discourage carrying the locator in the data
 stream.  Since this will not work for legacy applications, RFC-1631
 discusses how to look into the packet and make NAT transparent to the
 application (i.e.: create an application gateway).  This may not be
 possible for all applications (such as IP based authentication in
 SNMP), and even with application gateways in the path it may be
 necessary to modify each end host to be aware when there are
 intermediaries modifying the data.
 Another popular practice is hiding a collection of hosts that provide
 a combined service behind a single IP address (i.e.: web host load
 sharing).  In many implementations this is architecturally a NAT,
 since the addresses are mapped to the real destination on the fly.
 When packet header integrity is not an issue, this type of virtual
 host requires no modifications to the remote applications since the
 end client is unaware of the mapping activity.  While the virtual
 host has the CPU performance characteristics of the total set of
 machines, the processing and I/O capabilities of the NAT/ALG device
 bound the overall performance as it funnels the packets back and

6. Problems with NATs

  1. NATs break the flexible end-to-end model of the Internet.
  2. NATs create a single point where fates are shared, in the device

maintaining connection state and dynamic mapping information.

  1. NATs complicate the use of multi-homing by a site in order to

increase the reliability of their Internet connectivity. (While

    single routers are a point of fate sharing, the lack of state in a
    router makes creating redundancy trivial.  Indeed, this is on of
    the reasons why the Internet protocol suite developed using a
    connectionless datagram service as its network layer.)
 -  NATs inhibit implementation of security at the IP level.
 -  NATs enable casual use of private addresses.  These uncoordinated
    addresses are subject to collisions when companies using these
    addresses merge or want to directly interconnect using VPNs.
 -  NATs facilitate concatenating existing private name spaces with
    the public DNS.

Hain Informational [Page 10] RFC 2993 Architectural Implications of NAT November 2000

  1. Port versions (NAPT and RSIP) increase operational complexity when

publicly published services reside on the private side.

  1. NATs complicated or may even invalidate the authentication

mechanism of SNMPv3.

  1. Products may embed a NAT function without identifying it as such.
 By design, NATs impose limitations on flexibility.  As such, extended
 thought about the introduced complications is called for.  This is
 especially true for products where the NAT function is a hidden
 service, such as load balancing routers that re-write the IP address
 to other public addresses.  Since the addresses may be all in
 publicly administered space these are rarely recognized as NATs, but
 they break the integrity of the end-to-end model just the same.
 NATs place constraints on the deployment of applications that carry
 IP addresses (or address derivatives) in the data stream, and they
 operate on the assumption that each session is independent. However,
 there are applications such as FTP and H.323 that use one or more
 control sessions to set the characteristics of the follow-on sessions
 in their control session payload.  Other examples include SNMP MIBs
 for configuration, and COPS policy messages.  Applications or
 protocols like these assume end-to-end integrity of addresses and
 will fail when traversing a NAT.  (TCP was specifically designed to
 take advantage of, and reuse, the IP address in combination with its
 ports for use as a transport address.) To fix how NATs break such
 applications, an Application Level Gateway needs to exist within or
 alongside each NAT.  An additional gateway service is necessary for
 each application that may imbed an address in the data stream.  The
 NAT may also need to assemble fragmented datagrams to enable
 translation of the application stream, and then adjust TCP sequence
 numbers, prior to forwarding.
 As noted earlier, NATs break the basic tenet of the Internet that the
 endpoints are in control of the communication.  The original design
 put state control in the endpoints so there would be no other
 inherent points of failure.  Moving the state from the endpoints to
 specific nodes in the network reduces flexibility, while it increases
 the impact of a single point failure.  See further discussion in
 Illustration 1 below.
 In addition, NATs are not transparent to all applications, and
 managing simultaneous updates to a large array of ALGs may exceed the
 cost of acquiring additional globally routable addresses.  See
 further discussion in Illustration 2 below.
 While RSIP addresses the transparency and ALG issues, for the
 specific case of an individual private host needing public access,
 there is still a node with state required to maintain the connection.

Hain Informational [Page 11] RFC 2993 Architectural Implications of NAT November 2000

 Dynamic NAT and RSIP will eventually violate higher layer assumptions
 about address/port number reuse as defined in RFC-793 [10] RFC-1323
 [11].  The TCP state, TCP_TIME_WAIT, is specifically designed to
 prevent replay of packets between the 4-tuple of IP and port for a
 given IP address pair.  Since the TCP state machine of a node is
 unaware of any previous use of RSIP, its attempt to connect to the
 same remote service that its neighbor just released (which is still
 in TCP_TIME_WAIT) may fail, or with a larger sequence number may open
 the prior connection directly from TCP_TIME_WAIT state, at the loss
 of the protection afforded by the TCP_TIME_WAIT state (further
 discussion in 2.6 of RFC-2663 [3]).
 For address translators (which do not translate ports) to comply with
 the TCP_TIME_WAIT requirements, they must refrain from assigning the
 same address to a different host until a period of 2*MSL has elapsed
 since the last use of the address, where MSL is the Maximum Segment
 Lifetime defined in RFC-793 as two minutes.  For address-and-port
 translators to comply with this requirement, they similarly must
 refrain from assigning the same host/port pair until 2*MSL has
 elapsed since the end of its first use.  While these requirements are
 simple to state, they can place a great deal of pressure on the NAT,
 because they temporarily reduce the pool of available addresses and
 ports.  Consequently, it will be tempting or NAT implementers to
 ignore or shorten the TCP_TIME_WAIT requirements, at the cost of some
 of TCP's strong reliability.  Note that in the case where the strong
 reliability is in fact compromised by the appearance of an old
 packet, the failure can manifest itself as the receiver accepting
 incorrect data.  See further discussion in Illustration 3 below.
 It is sometimes argued that NATs simply function to facilitate
 "routing realms", where each domain is responsible for finding
 addresses within its boundaries.  Such a viewpoint clouds the
 limitations created by NAT with the better-understood need for
 routing management.  Compartmentalization of routing information is
 correctly a function of routing protocols and their scope of
 application.  NAT is simply a means to distribute address allocation
 authority and provide a mechanism to map addresses from one address
 realm into those of another realm.
 In particular, it is sometimes erroneously believed that NATs serve
 to provide routing isolation.  In fact, if someone were to define an
 OSPF ALG it would actually be possible to route across a NAT
 boundary.  Rather than NAT providing the boundary, it is the
 experienced operators who know how to limit network topology that
 serve to avoid leaking addresses across a NAT.  This is an
 operational necessity given the potential for leaked addresses to
 introduce inconsistencies into the public infrastructure.

Hain Informational [Page 12] RFC 2993 Architectural Implications of NAT November 2000

 One of the greatest concerns from the explosion of NATs is the impact
 on the fledgling efforts at deploying network layer end-to-end IP
 security.  One fundamental issue for IPSec is that with both AH and
 ESP, the authentication check covers the TCP/UDP checksum (which in
 turn covers the IP address).  When a NAT changes the IP address, the
 checksum calculation will fail, and therefore authentication is
 guaranteed to fail.  Attempting to use the NAT as a security boundary
 fails when requirement is end-to-end network layer encryption, since
 only the endpoints have access to the keys.  See further discussion
 in Illustration 4 below.
 Finally, while the port multiplexing variants of NAT (popular because
 they allow Internet access through a single address) work modestly
 well for connecting private hosts to public services, they create
 management problems for applications connecting from public toward
 private.  The concept of a well-known port is undermined because only
 one private side system can be mapped through the single public-side
 port number.  This will affect home networks, when applications like
 multi-player Internet games can only be played on one system at a
 time.  It will also affect small businesses when only one system at a
 time can be operated on the standard port to provide web services.
 These may sound like only medium-grade restrictions for the present,
 but as a basic property of the Internet, not to change years into the
 future, it is highly undesirable.  The issue is that the public
 toward private usage requires administrative mapping for each target
 prior to connection.  If the ISP chooses to provide a standardized
 version of these to lower configuration options, they may find the
 support costs of managing the ALGs will exceed the cost of additional
 address space.  See further discussion in Illustration 6 below.

7. Illustrations

7.1 Single point of failure

 A characteristic of stateful devices like NATs is the creation of a
 single point of failure.  Attempts to avoid this by establishing
 redundant NATs, creates a new set of problems related to timely
 communication of the state, and routing related failures.  This
 encompasses several issues such as update frequency, performance
 impact of frequent updates, reliability of the state update
 transaction, a-priori knowledge of all nodes needing this state
 information, and notification to end nodes of alternatives.  (This
 notification could be accomplished with a routing protocol, which
 might require modifications to the hosts so they will listen.)

Hain Informational [Page 13] RFC 2993 Architectural Implications of NAT November 2000

  1. ——- ——–

| Host A |—–| Host B |

  1. ——- | ——–
    1. —————-

| |

  1. —– ——

| AD 1 | | AD 2 |

  1. —– ——

\ /

  1. ——-


  1. ———
  1. ——-

Illustration 1

 In the traditional case where Access Device (AD) 1 & 2 are routers,
 the single point of failure is the end Host, and the only effort
 needed to maintain the connections through a router or link failure
 is a simple routing update from the surviving router.  In the case
 where the ADs are a NAT variant there will be connection state
 maintained in the active path that would need to be shared with
 alternative NATs.  When the Hosts have open connections through
 either NAT, and it fails, the application connections will drop
 unless the state had been previously moved to the surviving NAT. The
 hosts will still need to acquire a routing redirect.  In the case of
 RSIP, the public side address pool would also need to be shared
 between the ADs to allow movement.  This sharing creates another
 real-time operational complexity to prevent conflicting assignments
 at connection setup.  NAT as a technology creates a point fate
 sharing outside the endpoints, in direct contradiction to the
 original Internet design goals.

7.2. ALG complexity

 In the following example of a proposed corporate network, each
 NAT/ALG was to be one or more devices at each physical location, and
 there were to be multiple physical locations per diagramed
 connection.  The logistics of simply updating software on this scale
 is cumbersome, even when all the devices are the same manufacturer
 and model.  While this would also be true with routers, it would be
 unnecessary for all devices to run a consistent version for an
 application to work across an arbitrary path.

Hain Informational [Page 14] RFC 2993 Architectural Implications of NAT November 2000

  1. —————————————

| Corporate Network |

             | Asia |------| Americas |------| Europe |
              ------        ----------        --------
                 |                |                |
             --------         --------         --------
            |NAT/ALGs|       |NAT/ALGs|       |NAT/ALGs|
             --------         --------         --------
                 |                |                |
             |                Internet                |
                 |                |                |
             --------         --------         --------
            |NAT/ALGs|       |NAT/ALGs|       |NAT/ALGs|
             --------         --------         --------
                 |                |                |
     ------------------     --------------     ----------------
     Home Telecommuters     Branch Offices     Partner Networks
     ------------------     --------------     ----------------
  1. ——-

Illustration 2

7.3. TCP state violations

 The full range of upper layer architectural assumptions that are
 broken by NAT technologies may not be well understood without a very
 large-scale deployment, because it sometimes requires the diversity
 that comes with large-scale use to uncover unusual failure modes. The
 following example illustrates an instance of the problem discussed
 above in section 6.

Hain Informational [Page 15] RFC 2993 Architectural Implications of NAT November 2000

  1. ——- ——–

| Host A |—–| Host B |

  1. ——- | ——–
    1. ——-


  1. ——-


  1. ——-


  1. ——-


  1. ——–

| Web |

                            |  Server |
  1. ——-

Illustration 3

 Host A completes its transaction and closes the web service on TCP
 port 80, and the RSIP releases the public side address used for Host
 A.  Host B attempts to open a connection to the same web service, and
 the NAT assigns then next free public side address which is the same
 one A just released.  The source port selection rules on Host B
 happen to lead it to the same choice that A used.  The connect
 request from Host B is rejected because the web server, conforming to
 the TCP specifications, has that 4-tuple in TIME WAIT for 4 minutes.
 By the time a call from Host B gets through to the helpdesk
 complaining about no access, the requested retry will work, so the
 issue is marked as resolved, when it in fact is an on-going, but
 intermittent, problem.

7.4. Symmetric state management

 Operational management of networks incorporating stateful packet
 modifying device is considerably easier if inbound and outbound
 packets traverse the same path.  (Otherwise it's a headache to keep
 state for the two directions synchronized.)  While easy to say, even
 with careful planning it can be difficult to manage using a
 connectionless protocol like IP.  The problem of creating redundant
 connections is ensuring that routes advertised to the private side
 reach the end nodes and map to the same device as the public side
 route advertisements.  This state needs to persist throughout the
 lifetime of sessions traversing the NAT, in spite of frequent or
 simultaneous internal and external topology churn.  Consider the
 following case where the -X- links are broken, or flapping.

Hain Informational [Page 16] RFC 2993 Architectural Implications of NAT November 2000

  1. ——- ——–

| Host A | | Host B |

                       |   Foo  |     |   Bar  |
                        --------       --------
                            |             |
                          ----          ----
                          ----          ----
                            |            |
                           ----         ----
                          |NAT1|       |NAT2|
                           ----         ----
                             |          |
                           |Rtr         Rtr|
                           | /  Internet \ |     ---
                            --------------       ---
                             |          |
                             |          |
                        --------       --------
                       | Host C |     | Host D |
                        --------       --------
  1. ——-

Illustration 4

 To preserve a consistent view of routing, the best path to the
 Internet for Routers 1 & 2 is via NAT1, while the Internet is told
 the path to the address pool managed by the NATs is best found
 through NAT1.  When the path X1 breaks, Router 2 would attempt to
 switch to NAT2, but the external return path would still be through
 NAT1.  This is because the NAT1 device is advertising availability of
 a pool of addresses.  Directly connected routers in this same
 situation would advertise the specific routes that existed after the
 loss.  In this case, redundancy was useless.
 Consider the case that the path between Router 1 & 2 is up, and some
 remote link in the network X2 is down.  It is also assumed that DNS
 returns addresses for both NATs when queried for Hosts A or B.  When
 Host D tries to contact Host B, the request goes through NAT2, but
 due to the internal routing, the reply is through NAT1.  Since the
 state information for this connection is in NAT2, NAT1 will provide a
 new mapping.  Even if the remote path is restored, the connection
 will not be made because the requests are to the public IP of NAT2,
 while the replies are from the public IP of NAT1.

Hain Informational [Page 17] RFC 2993 Architectural Implications of NAT November 2000

 In a third case, both Host A & B want to contact Host D, when the
 remote link X2 in the Internet breaks.  As long as the path X1 is
 down, Host B is able to connect, but Host A is cut off.  Without a
 thorough understanding of the remote topology (unlikely since
 Internet providers tend to consider that sensitive proprietary
 information), the administrator of Hosts A & B would have no clue why
 one worked and the other didn't.  As far as he can tell the redundant
 paths through the NATs are up but only one connection works.  Again,
 this is due to lack of visibility to the topology that is inherent
 when a stateful device is advertising availability to a pool rather
 than the actual connected networks.
 In any network topology, individual router or link failures may
 present problems with insufficient redundancy, but the state
 maintenance requirements of NAT present an additional burden that is
 not as easily understood or resolved.

7.5. Need for a globally unique FQDN when advertising public services

 The primary feature of NATs is the 'simple' ability to connect
 private networks to the public Internet.  When the private network
 exists prior to installing the NAT, it is unlikely and unnecessary
 that its name resolver would use a registered domain.  As noted in
 RFC 1123 [12] DNS queries may be resolved via local multicast.
 Connecting the NAT device, and reconfiguring it's resolver to proxy
 for all external requests allows access to the public network by
 hosts on the private network.  Configuring the public DNS for the set
 of private hosts that need inbound connections would require a
 registered domain (either private, or from the connecting ISP) and a
 unique name.  At this point the partitioned name space is
 concatenated and hosts would have different names based on inside vs.
 outside queries.

Hain Informational [Page 18] RFC 2993 Architectural Implications of NAT November 2000

  1. ——- ——–

| Host A | | Host B |

                       |   Foo  |-----|   Bar  |
                        --------   |   --------   ---
                                  ---             ---
                               --------      ---
                               --------      ---
                                  ---             ---
                        --------   |   --------   ---
                       | Host C |-----| Host D |
                       |   Foo  |     |   Bar  |
                        --------       --------
  1. ——-

Illustration 5

 Everything in this simple example will work until an application
 embeds a name.  For example, a Web service running on Host D might
 present embedded URL's of the form http://D/bar.html, which would
 work from Host C, but would thoroughly confuse Host A.  If the
 embedded name resolved to the public address, Host A would be happy,
 but Host C would be looking for some remote machine.  Using the
 public FQDN resolution to establishing a connection from Host C to D,
 the NAT would have to look at the destination rather than simply
 forwarding the packet out to the router.  (Normal operating mode for
 a NAT is translate & forward out the other interface, while routers
 do not send packets back on the same interface they came from.)  The
 NAT did not create the name space fragmentation, but it facilitates
 attempts to merge networks with independent name administrations.

7.6. L2TP tunnels increase frequency of address collisions

 The recent mass growth of the Internet has been driven by support of
 low cost publication via the web.  The next big push appears to be
 support of Virtual Private Networks (VPNs) frequently accomplished
 using L2TP.  Technically VPN tunnels treat an IP infrastructure as a
 multiplexing substrate allowing the endpoints to build what appear to
 be clear pathways from end-to-end.  These tunnels redefine network
 visibility and increase the likelihood of address collision when

Hain Informational [Page 19] RFC 2993 Architectural Implications of NAT November 2000

 traversing multiple NATs.  Address management in the private space
 behind NATs will become a significant burden, as there is no central
 body capable of, or willing to do it.  The lower burden for the ISP
 is actually a transfer of burden to the local level, because
 administration of addresses and names becomes both distributed and
 more complicated.
 As noted in RFC-1918, the merging of private address spaces can cause
 an overlap in address use, creating a problem.  L2TP tunnels will
 increase the likelihood and frequency of that merging through the
 simplicity of their establishment.  There are several configurations
 of address overlap which will cause failure, but in the simple
 example shown below the private use address of Host B matches the
 private use address of the VPN pool used by Host A for inbound
 connections.  When Host B tries to establish the VPN interface, Host
 A will assign it an address from its pool for inbound connections,
 and identify the gateway for Host B to use.  In the example, Host B
 will not be able to distinguish the remote VPN gateway address of
 Host A from its own private address on the physical interface, thus
 the connection will fail.  Since private use addresses are by
 definition not publicly coordinated, as the complexity of the VPN
 mesh increases so does the likelihood that there will be a collision
 that cannot be resolved.
  1. ————– —————-

| |——–L2TP——-| Assigned by A |

           |    Host A     |   ---       ---   |    Host B      |
           |   |--|NAT|-----|NAT|--|   |
            ---------------    ---       ---    ----------------
  1. ——-

Illustration 6

7.7. Centralized data collection system report correlation

 It has been reported that NAT introduces additional challenges when
 intrusion detection systems attempt to correlate reports between
 sensors inside and outside the NAT.  While the details of individual
 systems are beyond the scope of this document, it is clear that a
 centralized system with monitoring agents on both sides of the NAT
 would also need access to the current NAT mappings to get this right.
 It would also be critical that the resulting data be indexed properly
 if there were agents behind multiple NATs using the same address
 range for the private side.
 This also applies to management data collected via SNMP.  Any time
 the data stream carries an IP address; the central collector or ALG
 will need to manipulate the data based on the current mappings in the

Hain Informational [Page 20] RFC 2993 Architectural Implications of NAT November 2000


8. IPv6

 It has been argued that IPv6 is no longer necessary because NATs
 relieve the address space constraints and allow the Internet to
 continue growing.  The reality is they point out the need for IPv6
 more clearly than ever.  People are trying to connect multiple
 machines through a single access line to their ISP and have been
 willing to give up some functionality to get that at minimum cost.
 Frequently the reason for cost increases is the perceived scarcity
 (therefore increased value) of IPv4 addresses, which would be
 eliminated through deployment of IPv6.  This crisis mentality is
 creating a market for a solution to a problem already solved with
 greater flexibility by IPv6.
 If NAT had never been defined, the motivation to resolve the
 dwindling IPv4 address space would be a much greater.  Given that
 NATs are enabling untold new hosts to attach to the Internet daily,
 it is difficult to ascertain the actual impact to the lifetime of
 IPv4, but NAT has certainly extended it.  It is also difficult to
 determine the extent of delay NAT is causing for IPv6, both by
 relieving the pressure, and by redirecting the intellectual cycles
 away from the longer-term solution.
 But at the same time NAT functionality may be a critical facilitator
 in the deployment of IPv6.  There are already 100 million or more
 computers running IPv4 on data networks.  Some of these networks are
 connected to and thus part of the Internet and some are on private
 isolated networks.  It is inconceivable that we could have a "flag
 day" and convert all of the existing IPv4 nodes to IPv6 at the same
 time.  There will be a very long period of coexistence while both
 IPv4 and IPv6 are being used in the Internet and in private networks.
 The original IPv6 transition plan relied heavily on having new IPv6
 nodes also be able to run IPv4 - a "dual stack" approach. When the
 dual stack node looks up another node in the DNS it will get back a
 IPv4 or an IPv6 address in response.  If the response is an IPv4
 address then the node uses IPv4 to contact the other node. And if the
 response is an IPv6 address then IPv6 can be used to make the
 contact.  Turning the NAT into a 6to4 [13]router enables widespread
 deployment of IPv6 while providing an IPv4 path if IPv6 is
 unavailable.  While this maintains the current set of issues for IPv4
 connections, it reestablishes the end-to-end principle for IPv6

Hain Informational [Page 21] RFC 2993 Architectural Implications of NAT November 2000

 An alternative methodology would be to translate the packets between
 IPv6 and IPv4 at the boarders between IPv4 supporting networks and
 IPv6 supporting networks.  The need for this functionality was
 recognized in [RFC 1752], the document that recommended to the IETF
 that IPv6 be developed and recommended that a set of working groups
 be established to work on a number of specific problems.  Header
 translation (i.e, NAT) was one of those problems.
 Of course, NATs in an IPv6 to IPv4 translation environment encounter
 all of the same problems that NATs encounter in a pure IPv4 and the
 environment and cautions in this document apply to both situations.

9. Security Considerations

 NAT (particularly NAPT) actually has the potential to lower overall
 security because it creates the illusion of a security barrier, but
 does so without the managed intent of a firewall.  Appropriate
 security mechanisms are implemented in the end host, without reliance
 on assumptions about routing hacks, firewall filters, or missing NAT
 translations, which may change over time to enable a service to a
 neighboring host.  In general, defined security barriers assume that
 any threats are external, leading to practices that make internal
 breaches much easier.
 IPsec RFC-2401 [7] defines a set of mechanisms to support packet-
 level authentication and encryption for use in IP networks.  While
 this may be less efficient than application-level security but in the
 words of RFC-1752 [14] "support for basic packet-level authentication
 will provide for the adoption of a much needed, widespread, security
 infrastructure throughout the Internet."
 NATs break IPsec's authentication and encryption technologies because
 these technologies depend on an end-to-end consistency of the IP
 addresses in the IP headers, and therefore may stall further
 deployment of enhanced security across the Internet.  NATs raise a
 number of specific issues with IPsec.  For example;
  1. Use of AH is not possible via NAT as the hash protects the IP

address in the header.

  1. Authenticated certificates may contain the IP address as part of

the subject name for authentication purposes.

  1. Encrypted Quick Mode structures may contain IP addresses and ports

for policy verifications.

  1. The Revised Mode of public key encryption includes the peer

identity in the encrypted payload.

Hain Informational [Page 22] RFC 2993 Architectural Implications of NAT November 2000

 It may be possible to engineer and work around NATs for IPsec on a
 case-by-case basis, but at the cost of restricting the trust model,
 as discussed in section 4 above.  With all of the restrictions placed
 on deployment flexibility, NATs present a significant obstacle to
 security integration being deployed in the Internet today.
 As noted in the RFC-2694 [15], the DNS/ALG cannot support secure DNS
 name servers in the private domain.  Zone transfers between DNSsec
 servers will be rejected when necessary modifications are attempted.
 It is also the case that DNS/ALG will break any modified, signed
 responses.  This would be the case for all public side queries of
 private nodes, when the DNS server is on the private side.  It would
 also be true for any private side queries for private nodes, when the
 DNS server is on the public side.  Digitally signed records could be
 modified by the DNS/ALG if it had access to the source authentication
 key.  DNSsec has been specifically designed to avoid distribution of
 this key, to maintain source authenticity.  So NATs that use DNS/ALG
 to repair the namespace resolutions will either; break the security
 when modifying the record, or will require access to all source keys
 to requested resolutions.
 Security mechanisms that do not protect or rely on IP addresses as
 identifiers, such as TLS [16], SSL [17], or SSH [18] may operate in
 environments containing NATs.  For applications that can establish
 and make use of this type of transport connection, NATs do not create
 any additional complications.  These technologies may not provide
 sufficient protection for all applications as the header is exposed,
 allowing subversive acts like TCP resets.  RFC-2385 [19] discusses
 the issues in more detail.
 Arguments that NATs may operate in a secure mode preclude true End-
 to-End security, as the NAT becomes the security endpoint.
 Operationally the NAT must be managed as part of the security domain,
 and in this mode the packets on the unsecured side of the NAT are
 fully exposed.

10. Deployment Guidelines

 Given that NAT devices are being deployed at a fairly rapid pace,
 some guidelines are in order.  Most of these cautionary in nature and
 are designed to make sure that the reader fully understands the
 implications of the use of NATs in their environment.
  1. Determine the mechanism for name resolution, and ensure the

appropriate answer is given for each address administration.

   Embedding the DNS server, or a DNS ALG in the NAT device will
   likely be more manageable than trying to synchronize independent
   DNS systems across administrations.

Hain Informational [Page 23] RFC 2993 Architectural Implications of NAT November 2000

  1. Is the NAT configured for static one to one mappings, or will it

dynamically manage them? If dynamic, make sure the TTL of the DNS

   responses is set to 0, and that the clients pay attention to the
   don't cache notification.
 - Will there be a single NAT device, or parallel with multiple paths?
   If single, consider the impact of a device failure.  If multiple,
   consider how routing on both sides will insure the packets flow
   through the same box over the connection lifetime of the
 - Examine the applications that will need to traverse the NAT and
   verify their immunity to address changes.  If necessary provide an
   appropriate ALG or establish a VPN to isolate the application from
   the NAT.
 - Determine need for public toward private connections, variability
   of destinations on the private side, and potential for simultaneous
   use of public side port numbers.  NAPTs increase administration if
   these apply.
 - Determine if the applications traversing the NAPT or RSIP expect
   all ports from the public IP address to be the same endpoint.
   Administrative controls to prevent simultaneous access from
   multiple private hosts will be required if this is the case.
 - If there are encrypted payloads, the contents cannot be modified
   unless the NAT is a security endpoint, acting as a gateway between
   security realms.  This precludes end-to-end confidentiality, as the
   path between the NAT and endpoint is exposed.
 - Determine the path for name resolutions.  If hosts on the private
   side of a NAPT or RSIP server need visibility to each other, a
   private side DNS server may be required.
 - If the environment uses secure DNS records, the DNS/ALG will
   require access to the source authentication keys for all records to
   be translated.
 - When using VPNs over NATs, identify a clearinghouse for the private
   side addresses to avoid collisions.
 - Assure that applications used both internally and externally avoid
   embedding names, or use globally unique ones.
 - When using RSIP, recognize the scope is limited to individual
   private network connecting to the public Internet.  If other NATs
   are in the path (including web-server load-balancing devices), the
   advantage of RSIP (end-to-end address/port pair use) is lost.
 - For RSIP, determine the probability of TCP_Time_Wait collisions
   when subsequent private side hosts attempt to contact a recently
   disconnected public side service.

Hain Informational [Page 24] RFC 2993 Architectural Implications of NAT November 2000

11. Summary

 Over the 6-year period since RFC-1631, the experience base has grown,
 further exposing concerns raised by the original authors.  NAT breaks
 a fundamental assumption of the Internet design; the endpoints are in
 control.  Another design principle, 'keep-it-simple' is being
 overlooked as more features are added to the network to work around
 the complications created by NATs.  In the end, overall flexibility
 and manageability are lowered, and support costs go up to deal with
 the problems introduced.
 Evangelists, for and against the technology, present their cases as
 righteous while downplaying any rebuttals.
  1. NATs are a 'fact of life', and will proliferate as an enhancement

that sustains the existing IPv4 infrastructure.

  1. NATs are a 'necessary evil' and create an administrative burden

that is not easily resolved. More significantly, they inhibit the

   roll out of IPsec, which will in turn slow growth of applications
   that require a secure infrastructure.
 In either case, NATs require strong applicability statements, clearly
 declaring what works and what does not.
 An overview of the pluses and minuses:
 NAT advantages                      NAT disadvantages
 --------------------------------    --------------------------------
 Masks global address changes        Breaks end-to-end model
 Eases renumbering when providers    Facilitates concatenation of
 change                              multiple name spaces
                                     Breaks IPsec
                                     Stateful points of failure
 Address administrations avoid       Requires source specific DNS reply
 justifications to registries        or DNS/ALG
                                     DNS/ALG breaks DNSsec replies
 Lowers address utilization          Enables end-to-end address
 Lowers ISP support burden           Increases local support burden and
 Transparent to end systems in some  Unique development for each app
 Load sharing as virtual host        Performance limitations with scale
 Delays need for IPv4 replacement    May complicate integration of IPv6

Hain Informational [Page 25] RFC 2993 Architectural Implications of NAT November 2000

 There have been many discussions lately about the value of continuing
 with IPv6 development when the market place is widely deploying IPv4
 NATs.  A shortsighted view would miss the point that both have a
 role, because NATs address some real-world issues today, while IPv6
 is targeted at solving fundamental problems, as well as moving
 forward.  It should be recognized that there will be a long co-
 existence as applications and services develop for IPv6, while the
 lifetime of the existing IPv4 systems will likely be measured in
 decades.  NATs are a diversion from forward motion, but they do
 enable wider participation at the present state.  They also break a
 class of applications, which creates the need for complex work-around
 Efforts to enhance general security in the Internet include IPsec and
 DNSsec.  These technologies provide a variety of services to both
 authenticate and protect information during transit.  By breaking
 these technologies, NAT and the DNS/ALG work-around, hinder
 deployment of enhanced security throughout the Internet.
 There have also been many questions about the probability of VPNs
 being established that might raise some of the listed concerns. While
 it is hard to predict the future, one way to avoid ALGs for each
 application is to establish a L2TP over the NATs. This restricts the
 NAT visibility to the headers of the tunnel packets, and removes its
 effects from all applications. While this solves the ALG issues, it
 raises the likelihood that there will be address collisions as
 arbitrary connections are established between uncoordinated address
 spaces. It also creates a side concern about how an application
 establishes the necessary tunnel.
 The original IP architecture is powerful because it provides a
 general mechanism on which other things (yet unimagined) may be
 built. While it is possible to build a house of cards, time and
 experience have lead to building standards with more structural
 integrity. IPv6 is the long-term solution that retains end-to-end
 transparency as a principle. NAT is a technological diversion to
 sustain the lifetime of IPv4.

Hain Informational [Page 26] RFC 2993 Architectural Implications of NAT November 2000

12. References

 1   Bradner, S., " The Internet Standards Process -- Revision 3", BCP
     9, RFC 2026, October 1996.
 2   Egevang, K. and P. Francis, "The IP Network Address Translator",
     RFC 1631, May 1994.
 3   Srisuresh, P. and M. Holdrege, "NAT Terminology and
     Considerations", RFC 2663, August 1999.
 4   Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.
     Lear, "Address Allocation for Private Internets", BCP 5, RFC
     1918, February 1996.
 5   Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
     Behavior Today", RFC 2101, February 1997.
 6   M. Borella, D. Grabelsky, J., K. Tuniguchi, "Realm Specific IP:
     Protocol Specification", Work in Progress, March 2000.
 7   Kent, S. and R. Atkinson, "Security Architecture for IP", RFC
     2401, November 1998.
 8   Carpenter, B., "Internet Transparency", RFC 2775, February 2000.
 9   Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D. and J.
     Postel, "Internet Registry IP Allocation Guidelines", BCP 12, RFC
     2050, November 1996.
 10  Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
     September 1981.
 11  Jacobson, V., Braden, R. and L. Zhang, "TCP Extension for High-
     Speed Paths", RFC 1185, October 1990.
 12  Braden, R., "Requirements for Internet Hosts", STD 3, RFC 1123,
     October 1989.
 13  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4
     Clouds without Explicit Tunnels", Work in Progress.
 14  Bradner, S. and A. Mankin, "Recommendation for IPng", RFC 1752,
     January 1995.
 15  Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A. Heffernan, "DNS
     extensions to NAT", RFC 2694, September 1999.

Hain Informational [Page 27] RFC 2993 Architectural Implications of NAT November 2000

 16  Dierks, T. and C. Allen, "The TLS Protocol", RFC 2246, January
 17, March 1996.
 18  T. Ylonen, et al., "SSH Protocol Architecture", Work in Progress,
     August 1998.
 19  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
     Signature Option", RFC 2385, August 1998.

13. Acknowledgments

 Valuable contributions to this document came from the IAB, Vern
 Paxson (lbl), Scott Bradner (harvard), Keith Moore (utk), Thomas
 Narten (ibm), Yakov Rekhter (cisco), Pyda Srisuresh, Matt Holdrege
 (lucent), and Eliot Lear (cisco).

14. Author's Address

 Tony Hain
 One Microsoft Way
 Redmond, Wa. USA
 Phone:  1-425-703-6619

Hain Informational [Page 28] RFC 2993 Architectural Implications of NAT November 2000

Full Copyright Statement

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 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
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 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
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 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Hain Informational [Page 29]

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