IAB                                                             T. Hain
Internet Draft                                                Microsoft
Document: draft-iab-nat-implications-09.txt                 August 2000
Category: Informational

                   Architectural Implications of NAT

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026 [1].

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


   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.

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Table of Contents
 1.  Introduction....................................................3
 2.  Terminology.....................................................5
 3.  Scope...........................................................7
 4.  End-to-End Model................................................7
 5.  Advantages of NATs..............................................9
 6.  Problems with NATs.............................................11
 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
  7.6.  L2TP tunnels increase frequency of address collisions.......18
  7.7.  Centralized data collection system report correlation.......19
 8.  IPv6...........................................................19
 9.  Security Considerations........................................21
 10.  Deployment Guidelines.........................................23
 11.  Summary.......................................................24
 12.  References....................................................25
 13.  Acknowledgments...............................................27
 14.  Author's Addresses............................................27

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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.
   -  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
   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

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   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 realm.

   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.

   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

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   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.

   DNS/ALG û 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

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   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

   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 û 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.

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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
   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.

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   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:
   - 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.
   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 airport?)
   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

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   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 domain.

   - 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.
   - 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.

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   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 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 (ie: create an application gateway). This may not be
   possible for all applications (such as IP based authentication in
   SNMPv3), 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 (ie: 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 forth.

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6.  Problems with NATs

   - NATs break the flexible end-to-end model of the Internet.
   - NATs create a single point where fates are shared, in the device
     maintaining connection state and dynamic mapping information.
   - 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.
   - Port versions (NAPT and RSIP) increase operational complexity when
     publicly published services reside on the private side.
   - NATs invalidate the authentication mechanism of SNMPv3.
   - 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

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   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.  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

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   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.

   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.)

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                        --------       --------
                       | Host A |-----| Host B |
                        --------   |   --------
                             |            |
                          ------        ------
                         | AD 1 |      | AD 2 |
                          ------        ------
                              \         /

                             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.

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               |           Corporate Network            |
               | Asia |------| Americas |------| Europe |
                ------        ----------        --------
                   |                |                |
               --------         --------         --------
              |NAT/ALGs|       |NAT/ALGs|       |NAT/ALGs|
               --------         --------         --------
                   |                |                |
               |                Internet                |
                   |                |                |
               --------         --------         --------
              |NAT/ALGs|       |NAT/ALGs|       |NAT/ALGs|
               --------         --------         --------
                   |                |                |
       ------------------     --------------     ----------------
       Home Telecommuters     Branch Offices     Partner Networks
       ------------------     --------------     ----------------

                             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.

                        --------       --------
                       | Host A |-----| Host B |
                        --------   |   --------
                              |   Web   |
                              |  Server |

                             Illustration 3

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   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.

                          --------       --------
                         | Host A |     | Host B |
                         |   Foo  |     |   Bar  |
                          --------       --------
                              |             |
                            ----          ----
                            ----          ----
                              |            |
                             ----         ----
                            |NAT1|       |NAT2|
                             ----         ----
                               |          |
                             |Rtr         Rtr|
                             | /  Internet \ |     ---
                              --------------       ---
                               |          |
                               |          |
                          --------       --------
                         | Host C |     | Host D |
                          --------       --------

                             Illustration 4

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   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.

   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.

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                          --------       --------
                         | Host A |     | Host B |
                         |   Foo  |-----|   Bar  |
                          --------   |   --------   ---
                                    ---             ---
                                 --------      ---
                                 --------      ---
                                    ---             ---
                          --------   |   --------   ---
                         | Host C |-----| Host D |
                         |   Foo  |     |   Bar  |
                          --------       --------

                             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
   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.

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   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.

              ---------------                     ----------------
             |  |--------L2TP-------| Assigned by A  |
             |    Host A     |   ---       ---   |    Host B      |
             |   |--|NAT|-----|NAT|--|   |
              ---------------    ---       ---    ----------------

                             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 NAT.

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.

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   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

   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.

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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;
   - Use of AH is not possible via NAT as the hash protects the IP
     address in the header.
   - Authenticated certificates may contain the IP address as part of
     the subject name for authentication purposes.
   - Encrypted Quick Mode structures may contain IP addresses and ports
     for policy verifications.
   - The Revised Mode of public key encryption includes the peer
     identity in the encrypted payload.
   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.

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   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.

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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.
   - 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.
   - 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 applications.
   - 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.

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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.
   - NATs are a 'fact of life', and will proliferate as an enhancement
     that sustains the existing IPv4 infrastructure.
   - 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

   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

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   class of applications, which creates the need for complex work-
   around scenarios.

   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.

12.  References

   1  RFC-2026 Bradner, S., " The Internet Standards Process --
      Revision 3", BCP 9, RFC 2026, October 1996.

   2  RFC-1631 Egevang, K., Francis, P., "The IP Network Address
      Translator", RFC 1631, May 1994

   3  RFC-2663, Srisuresh & Holdrege, "NAT Terminology and
      Considerations", RFC 2663 August 1999

   4  RFC-1918, Rekhter, et al, "Address Allocation for Private
      Internets", RFC 1918 February 1996

   5  RFC-2101, Carpenter, et al, "IPv4 Address Behavior Today", RFC
      2101, February 1997

   6  draft-ietf-nat-rsip-protocol-06.txt, M. Borella, D. Grabelsky,
      J.lo, K. Tuniguchi, "Realm Specific IP: Protocol Specification",
      March 2000

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                  Architectural Implications of NAT       August 2000

   7  RFC-2401, Kent & Atkinson, "Security Architecture for IP",
      November 1998

   8  RFC-2775, Carpenter, "Internet Transparency", February 2000

   9  RFC-2050, Hubbard, et. Al., "Internet Registry IP Allocation
      Guidelines", November 1996

   10  RFC-793, J. Postel, "Transmission Control Protocol", RFC 793,
      September 1981

   11  RFC-1185, V. Jacobson, R. Braden, L. Zhang, "TCP Extension for
      High-Speed Paths", RFC 1185, October 1990

   12  RFC-1123, R. Braden, "Requirements for Internet Hosts", RFC
      1123, October 1989

   13  draft-ietf-ngtrans-6to4-06.txt, B. Carpenter, K. Moore,
      "Connection of IPv6 Domains via IPv4 Clouds without Explicit
      Tunnels", June 2000

   14  RFC-1752, Bradner & Mankin, "Recommendation for IPng", January

   15  RFC-2694, Srisuresh, et al., "DNS extensions to NAT", September

   16  RFC-2246, T. Dierks, C. Allen, "The TLS Protocol", January 1999

   17  http://home.netscape.com/eng/ssl3/ssl-toc.html   March 1996

   18  draft-ietf-secsh-architecture-02.txt, T. Ylonen, et al, "SSH
      Protocol Architecture", August 1998

   19  RFC-2385, A. Heffernan, "Protection of BGP Sessions via the TCP
      MD5 Signature Option", RFC 2385, August 1998

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13.  Acknowledgments
   Valuable contributions to this draft 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 Addresses
   Tony Hain
   One Microsoft Way            Phone:  1-425-703-6619
   Redmond, Wa. USA             Email:  tonyhain@microsoft.com

Hain             Informational - Expires January 2001               27