RFC2993 - Architectural Implications of NAT

Network Working Group T. Hain

Request for Comments: 2993 Microsoft

Category: Informational November 2000

Architectural Implications of NAT

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Abstract

In light of the growing interest in, and deployment of network

address translation (NAT) RFC-1631, this paper will discuss some of

the architectural implications and guidelines for implementations. It

is assumed the reader is familiar with the address translation

concepts presented in RFC-1631.

Table of Contents

1. IntrodUCtion................................................... 2

2. Terminology.................................................... 4

3. Scope.......................................................... 6

4. End-to-End Model............................................... 7

5. Advantages of NATs............................................. 8

6. Problems with NATs............................................ 10

7. Illustrations................................................. 13

7.1 Single point of failure...................................... 13

7.2. ALG complexity............................................. 14

7.3. TCP state violations........................................ 15

7.4. Symmetric state management................................. 16

7.5. Need for a globally unique FQDN when advertising public

services................................................... 18

7.6. L2TP tunnels increase frequency of address collisions...... 19

7.7. Centralized data collection system report correlation...... 20

8. IPv6.......................................................... 21

9. Security Considerations....................................... 22

10. Deployment Guidelines........................................ 23

11. Summary...................................................... 24

12. References................................................... 27

13. Acknowledgments.............................................. 28

14. Author's Address............................................. 28

15. Full Copyright Statement..................................... 29

1. Introduction

Published in May 1994, written by K. Egevang and P. Francis, RFC-1631

[2] defined NAT as one means to ease the growth rate of IPv4 address

use. But the authors were worried about the impact of this

technology. Several places in the document they pointed out the need

to eXPeriment and see what applications may be adversely affected by

NAT's header manipulations, even before there was any significant

operational experience. This is further evidenced in a quote from

the conclusions: 'NAT has several negative characteristics that make

it inappropriate as a long term solution, and may make it

inappropriate even as a short term solution.'

Now, six years later and in spite of the prediction, the use of NATs

is becoming widespread in the Internet. Some people are proclaiming

NAT as both the short and long term solution to some of the

Internet's address availability issues and questioning the need to

continue the development of IPv6. The claim is sometimes made that

NAT 'just works' with no serious effects except on a few legacy

applications. At the same time others see a myriad of difficulties

caused by the increasing use of NAT.

The arguments pro & con frequently take on religious tones, with each

side passionate about its position.

- 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

endpoints. In a large environment, this may require concerted effort

to simultaneously update all endpoints of a given application or

service.

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

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 - 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

spaces.

Dynamic NAT - provides dynamic mapping between address spaces

normally used with a relatively large number of addresses on one side

(private space) to a few addresses on the other (public space).

NAPT - Network Address Port Translation accomplishes translation by

multiplexing transport level identifiers of multiple addresses from

one side, simultaneously into the transport identifiers of a single

address on the other. See 4.1.2 of RFC-2663. This permits multiple

endpoints to share and appear as a single IP address.

RSIP - Realm Specific IP allows endpoints to acquire and use the

public address and port number at the source. It includes mechanisms

for the private node to request multiple resources at once. RSIP

clients must be aware of the address administration boundaries, which

specific administrative area its peer resides in for each

application, and the topology for reaching the peer. To complete a

connection, the private node client requests one or more addresses

and/or ports from the appropriate RSIP server, then initiates a

connection via that RSIP server using the acquired public resources.

Hosts must be updated with specific RSIP software to support the

tunneling functions.

VPN - For purposes of this document, Virtual Private Networks

technically treat an IP infrastructure as a multiplexing substrate,

allowing the endpoints to build virtual transit pathways, over which

they run another instance of IP. Frequently the 2nd instance of IP

uses a different set of IP addresses.

AH - IP Authentication Header, RFC-2401 [7], which provides data

integrity, data origin authentication, and an optional anti-replay

service.

ESP - Encapsulating Security Payload protocol, RFC2401, may provide

data confidentiality (encryption), and limited traffic flow

confidentiality. It also may provide data integrity, data origin

authentication, and an anti-replay service.

Address administration - coordinator of an address pool assigned to a

collection of routers and end systems.

Addressing realm - a collection of routers and end systems

exchanging locally unique location knowledge. (Further defined in

RFC-2663 NAT Terminology.) NAT is used a means to distribute address

allocation authority and provide a mechanism to map addresses from

one address administration into those of another administration.

3. Scope

In discussing the architectural impact of NATs on the Internet, the

first task is defining the scope of the Internet. The most basic

definition is; a concatenation of networks built using IETF defined

technologies. This simple description does not distinguish between

the public network known as the Internet, and the private networks

built using the same technologies (including those connected via

NAT). Rekhter, et al in RFC-1918 defined hosts as public when they

need network layer access outside the enterprise, using a globally

unambiguous address. Those that need limited or no access are

defined as private. Another way to view this is in terms of the

transparency of the connection between any given node and the rest of

the Internet.

The ultimate resolution of public or private is found in the intent

of the network in question. Generally, networks that do not intend

to be part of the greater Internet will use some screening technology

to insert a barrier. Historically barrier devices between the public

and private networks were known as Firewalls or Application Gateways,

and were managed to allow approved traffic while blocking everything

else. Increasingly, part of the screening technology is a NAT, which

manages the network locator between the public and private-use

address spaces, and then, using ALGs adds support for protocols that

are incompatible with NAT. (Use of NAT within a private network is

possible, and is only addressed here in the context that some

component of the private network is used as a common transit service

between the NAT attached stubs.)

RFC-1631 limited the scope of NAT discussions to stub appendages of a

public Internet, that is, networks with a single connection to the

rest of the Internet. The use of NAT in situations in which a

network has multiple connections to the rest of the Internet is

significantly more complex than when there is only a single

connection since the NATs have to be coordinated to ensure that they

have a consistent understanding of address mapping for each

individual device.

4. End-to-End Model

The concept of the End-to-End model is reviewed by Carpenter in

Internet Transparency [8]. One of the key points is "state should be

maintained only in the endpoints, in such a way that the state can

only be destroyed when the endpoint itself breaks"; this is termed

"fate-sharing". The goal behind fate-sharing is to ensure

robustness. As networks grow in size, likelihood of component

failures affecting a connection becomes increasingly frequent. If

failures lead to loss of communication, because key state is lost,

then the network becomes increasingly brittle, and its utility

degrades. However, if an endpoint itself fails, then there is no

hope of subsequent communication anyway. Therefore the End-to-End

model argues that as much as possible, only the endpoints should hold

critical state.

For NATs, this ASPect of the End-to-End model translates into the NAT

becoming a critical infrastructure element: if it fails, all

communication through it fails, and, unless great care is taken to

assure consistent, stable storage of its state, even when it recovers

the communication that was passing through it will still fail

(because the NAT no longer translates it using the same mappings).

Note that this latter type of failure is more severe than the failure

of a router; when a router recovers, any communication that it had

been forwarding previous can continue to be successfully forwarded

through it.

There are other important facets to the End-to-End model:

- 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

fashion.

- 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

presence of a NATPT, the same IP address no longer ensures the same

host.

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

interface.

- Breaking the Internet into a collection of address authorities

limits the need for continual justification of allocations allows

network managers to avoid the use of more advanced routing

techniques such as variable length subnets.

- Changes in the hosts may not be necessary for applications that

don't rely on the integrity of the packet header, or carry IP

addresses in the payload.

- Like packet filtering Firewalls, NAPT, & RSIP block inbound

connections to all ports until they are administratively mapped.

Taken together these explain some of the strong motivations for

moving quickly with NAT deployment. Traditional NAT [2] provides a

relatively simple function that is easily understood.

Removing hosts that are not currently active lowers address demands

on the public Internet. In cases where providers would otherwise end

up with address allocations that could not be aggregated, this

improves the load on the routing system as well as lengthens the

lifetime of the IPv4 address space. While reclaiming idle addresses

is a natural byproduct of the existing dynamic allocation, dial-

access devices, in the dedicated connection case this service could

be provided through a NAT. In the case of a NAPT, the aggregation

potential is even greater as multiple end systems share a single

public address.

By reducing the potential customer connection options and minimizing

the support matrix, it is possible that ISP provided NATs would lower

support costs.

Part of the motivation for NAT is to avoid the high cost of

renumbering inherent in the current IPv4 Internet. Guidelines for

the assignment of IPv4 addresses RFC-2050 [9] mean that ISP customers

are currently required to renumber their networks if they want to

switch to a new ISP. Using a NAT (or a firewall with NAT functions)

means that only the Internet facing IP addresses must be changed and

internal network nodes do not need to be reconfigured. Localizing

address administration to the NAT minimizes renumbering costs, and

simultaneously provides for a much larger local pool of addresses

than is available under current allocation guidelines. (The registry

guidelines are intended to prolong the lifetime of the IPv4 address

space and manage routing table growth, until IPv6 is ready or new

routing technology reduces the pressure on the routing table. This

is accomplished by managing allocations to match actual demand and to

enforce hierarchical addressing. An unfortunate byproduct of the

current guidelines is that they may end up hampering growth in areas

where it is difficult to sort out real need from potential hoarding.)

NAT is effective at masking provider switching or other requirements

to change addresses, thus mitigates some of the growth issues.

NAT deployments have been raising the awareness of protocol designers

who are interested in ensuring that their protocols work end-to-end.

Breaking the semantic overload of the IP address will force

applications to find a more appropriate mechanism for endpoint

identification and discourage carrying the locator in the data

stream. Since this will not work for legacy applications, RFC-1631

discusses how to look into the packet and make NAT transparent to the

application (i.e.: create an application gateway). This may not be

possible for all applications (such as IP based authentication in

SNMP), and even with application gateways in the path it may be

necessary to modify each end host to be aware when there are

intermediaries modifying the data.

Another popular practice is hiding a collection of hosts that provide

a combined service behind a single IP address (i.e.: web host load

sharing). In many implementations this is architecturally a NAT,

since the addresses are mapped to the real destination on the fly.

When packet header integrity is not an issue, this type of virtual

host requires no modifications to the remote applications since the

end client is unaware of the mapping activity. While the virtual

host has the CPU performance characteristics of the total set of

machines, the processing and I/O capabilities of the NAT/ALG device

bound the overall performance as it funnels the packets back and

forth.

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 complicated or may even 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 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

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.)

-------- --------

Host A ----- Host B

-------- --------

-----------------

------ ------

AD 1 AD 2

------ ------

\ /

--------

/Internet ----------

--------

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.

----------------------------------------

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

-------- --------

--------

NAT/RSIP

--------

Internet

--------

---------

Web

Server

---------

--------

Illustration 3

Host A completes its transaction and closes the web service on TCP

port 80, and the RSIP releases the public side address used for Host

A. Host B attempts to open a connection to the same web service, and

the NAT assigns then next free public side address which is the same

one A just released. The source port selection rules on Host B

happen to lead it to the same choice that A used. The connect

request from Host B is rejected because the web server, conforming to

the TCP specifications, has that 4-tuple in TIME WAIT for 4 minutes.

By the time a call from Host B gets through to the helpdesk

complaining about no access, the requested retry will work, so the

issue is marked as resolved, when it in fact is an on-going, but

intermittent, problem.

7.4. Symmetric state management

Operational management of networks incorporating stateful packet

modifying device is considerably easier if inbound and outbound

packets traverse the same path. (Otherwise it's a headache to keep

state for the two directions synchronized.) While easy to say, even

with careful planning it can be difficult to manage using a

connectionless protocol like IP. The problem of creating redundant

connections is ensuring that routes advertised to the private side

reach the end nodes and map to the same device as the public side

route advertisements. This state needs to persist throughout the

lifetime of sessions traversing the NAT, in spite of frequent or

simultaneous internal and external topology churn. Consider the

following case where the -X- links are broken, or flapping.

-------- --------

Host A Host B

Foo Bar

-------- --------

---- ----

Rtr1---X1---Rtr2

---- ----

NAT1 NAT2

---- ----

--------------

Rtr Rtr

/ Internet \ ---

Rtr----X2---Rtr----DNS

-------------- ---

-------- --------

Host C Host D

-------- --------

--------

Illustration 4

To preserve a consistent view of routing, the best path to the

Internet for Routers 1 & 2 is via NAT1, while the Internet is told

the path to the address pool managed by the NATs is best found

through NAT1. When the path X1 breaks, Router 2 would attempt to

switch to NAT2, but the external return path would still be through

NAT1. This is because the NAT1 device is advertising availability of

a pool of addresses. Directly connected routers in this same

situation would advertise the specific routes that existed after the

loss. In this case, redundancy was useless.

Consider the case that the path between Router 1 & 2 is up, and some

remote link in the network X2 is down. It is also assumed that DNS

returns addresses for both NATs when queried for Hosts A or B. When

Host D tries to contact Host B, the request goes through NAT2, but

due to the internal routing, the reply is through NAT1. Since the

state information for this connection is in NAT2, NAT1 will provide a

new mapping. Even if the remote path is restored, the connection

will not be made because the requests are to the public IP of NAT2,

while the replies are from the public IP of NAT1.

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

RFC1123 [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.

-------- --------

Host A Host B

Foo ----- Bar

-------- -------- ---

-------------DNS

--- ---

NAT

---

-------- ---

Internet----DNS

-------- ---

---

NAT

--- ---

-------------DNS

-------- -------- ---

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.

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.

--------------- ----------------

10.10.10.10 --------L2TP------- Assigned by A

Host A --- --- Host B

10.1.1.1 --NAT-----NAT-- 10.10.10.10

--------------- --- --- ----------------

--------

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.

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

connections.

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 [RFC1752], the document that recommended to the IETF

that IPv6 be developed and recommended that a set of working groups

be established to work on a number of specific problems. Header

translation (i.e, NAT) was one of those problems.

Of course, NATs in an IPv6 to IPv4 translation environment encounter

all of the same problems that NATs encounter in a pure IPv4 and the

environment and cautions in this document apply to both situations.

9. Security Considerations

NAT (particularly NAPT) actually has the potential to lower overall

security because it creates the illusion of a security barrier, but

does so without the managed intent of a firewall. Appropriate

security mechanisms are implemented in the end host, without reliance

on assumptions about routing hacks, firewall filters, or missing NAT

translations, which may change over time to enable a service to a

neighboring host. In general, defined security barriers assume that

any threats are external, leading to practices that make internal

breaches much easier.

IPsec RFC-2401 [7] defines a set of mechanisms to support packet-

level authentication and encryption for use in IP networks. While

this may be less efficient than application-level security but in the

Words of RFC-1752 [14] "support for basic packet-level authentication

will provide for the adoption of a much needed, widespread, security

infrastructure throughout the Internet."

NATs break IPsec's authentication and encryption technologies because

these technologies depend on an end-to-end consistency of the IP

addresses in the IP headers, and therefore may stall further

deployment of enhanced security across the Internet. NATs raise a

number of specific issues with IPsec. For example;

- 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.

Security mechanisms that do not protect or rely on IP addresses as

identifiers, such as TLS [16], SSL [17], or SSH [18] may operate in

environments containing NATs. For applications that can establish

and make use of this type of transport connection, NATs do not create

any additional complications. These technologies may not provide

sufficient protection for all applications as the header is exposed,

allowing subversive acts like TCP resets. RFC-2385 [19] discusses

the issues in more detail.

Arguments that NATs may operate in a secure mode preclude true End-

to-End security, as the NAT becomes the security endpoint.

Operationally the NAT must be managed as part of the security domain,

and in this mode the packets on the unsecured side of the NAT are

fully exposed.

10. Deployment Guidelines

Given that NAT devices are being deployed at a fairly rapid pace,

some guidelines are in order. Most of these cautionary in nature and

are designed to make sure that the reader fully understands the

implications of the use of NATs in their environment.

- 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.

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

conflicts

Lowers ISP support burden Increases local support burden and

complexity

Transparent to end systems in some Unique development for each app

cases

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

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 Bradner, S., " The Internet Standards Process -- Revision 3", BCP

9, RFC2026, October 1996.

2 Egevang, K. and P. Francis, "The IP Network Address Translator",

RFC1631, May 1994.

3 Srisuresh, P. and M. Holdrege, "NAT Terminology and

Considerations", RFC2663, August 1999.

4 Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.

Lear, "Address Allocation for Private Internets", BCP 5, RFC

1918, February 1996.

5 Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address

Behavior Today", RFC2101, February 1997.

6 M. Borella, D. Grabelsky, J., K. Tuniguchi, "Realm Specific IP:

Protocol Specification", Work in Progress, March 2000.

7 Kent, S. and R. Atkinson, "Security Architecture for IP", RFC

2401, November 1998.

8 Carpenter, B., "Internet Transparency", RFC2775, February 2000.

9 Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D. and J.

Postel, "Internet Registry IP Allocation Guidelines", BCP 12, RFC

2050, November 1996.

10 Postel, J., "Transmission Control Protocol", STD 7, RFC793,

September 1981.

11 Jacobson, V., Braden, R. and L. Zhang, "TCP Extension for High-

Speed Paths", RFC1185, October 1990.

12 Braden, R., "Requirements for Internet Hosts", STD 3, RFC1123,

October 1989.

13 Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4

Clouds without Explicit Tunnels", Work in Progress.

14 Bradner, S. and A. Mankin, "Recommendation for IPng", RFC1752,

January 1995.

15 Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A. Heffernan, "DNS

extensions to NAT", RFC2694, September 1999.

16 Dierks, T. and C. Allen, "The TLS Protocol", RFC2246, January

1999.

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

18 T. Ylonen, et al., "SSH Protocol Architecture", Work in Progress,

August 1998.

19 Heffernan, A., "Protection of BGP Sessions via the TCP MD5

Signature Option", RFC2385, August 1998.

13. Acknowledgments

Valuable contributions to this document came from the IAB, Vern

Paxson (lbl), Scott Bradner (harvard), Keith Moore (utk), Thomas

Narten (ibm), Yakov Rekhter (cisco), Pyda Srisuresh, Matt Holdrege

(lucent), and Eliot Lear (cisco).

14. Author's Address

Tony Hain

Microsoft

One Microsoft Way

Redmond, Wa. USA

Phone: 1-425-703-6619

EMail: tonyhain@microsoft.com

Full Copyright Statement

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.