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Architectural Implications of NAT

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2993.
Author Tony L. Hain
Last updated 2013-03-02 (Latest revision 2000-08-10)
RFC stream Internet Architecture Board (IAB)
Intended RFC status Informational
Stream IAB state (None)
Consensus boilerplate Unknown
IAB shepherd (None)
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].  
   Internet-Drafts are working documents of the Internet Engineering 
   Task Force (IETF), its areas, and its working groups. Note that 
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of 
   six months and may be updated, replaced, or obsoleted by other 
   documents at any time. It is inappropriate to use Internet- Drafts 
   as reference material or to cite them other than as "work in 
   The list of current Internet-Drafts can be accessed at  
   The list of Internet-Draft Shadow Directories can be accessed at 
   "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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
                        --------       -------- 
                       | 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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                  Architectural Implications of NAT       August 2000 
               |           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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
                          --------       -------- 
                         | 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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
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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                  Architectural Implications of NAT       August 2000 
   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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                  Architectural Implications of NAT       August 2000 
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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                  Architectural Implications of NAT       August 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.  
   - 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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                  Architectural Implications of NAT       August 2000 
   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   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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                  Architectural Implications of NAT       August 2000 
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: 

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