RFC 2993 - Architectural Implications of NAT
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Network Working Group T. Hain
Request for Comments: 2993 Microsoft
Category: Informational November 2000
Architectural Implications of NAT
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
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, RFC 2401, may provide
data confidentiality (encryption), and limited traffic flow
confidentiality. It also may provide data integrity, data origin
authentication, and an anti-replay service.
Address administration - coordinator of an address pool assigned to a
collection of routers and end systems.
Addressing realm - a collection of routers and end systems
exchanging locally unique location knowledge. (Further defined in
RFC-2663 NAT Terminology.) NAT is used a means to distribute address
allocation authority and provide a mechanism to map addresses from
one address administration into those of another administration.
3. Scope
In discussing the architectural impact of NATs on the Internet, the
first task is defining the scope of the Internet. The most basic
definition is; a concatenation of networks built using IETF defined
technologies. This simple description does not distinguish between
the public network known as the Internet, and the private networks
built using the same technologies (including those connected via
NAT). Rekhter, et al in RFC-1918 defined hosts as public when they
need network layer access outside the enterprise, using a globally
unambiguous address. Those that need limited or no access are
defined as private. Another way to view this is in terms of the
transparency of the connection between any given node and the rest of
the Internet.
The ultimate resolution of public or private is found in the intent
of the network in question. Generally, networks that do not intend
to be part of the greater Internet will use some screening technology
to insert a barrier. Historically barrier devices between the public
and private networks were known as Firewalls or Application Gateways,
and were managed to allow approved traffic while blocking everything
else. Increasingly, part of the screening technology is a NAT, which
manages the network locator between the public and private-use
address spaces, and then, using ALGs adds support for protocols that
are incompatible with NAT. (Use of NAT within a private network is
possible, and is only addressed here in the context that some
component of the private network is used as a common transit service
between the NAT attached stubs.)
RFC-1631 limited the scope of NAT discussions to stub appendages of a
public Internet, that is, networks with a single connection to the
rest of the Internet. The use of NAT in situations in which a
network has multiple connections to the rest of the Internet is
significantly more complex than when there is only a single
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
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.
-------- --------
| 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 [RFC 1752], the document that recommended to the IETF
that IPv6 be developed and recommended that a set of working groups
be established to work on a number of specific problems. Header
translation (i.e, NAT) was one of those problems.
Of course, NATs in an IPv6 to IPv4 translation environment encounter
all of the same problems that NATs encounter in a pure IPv4 and the
environment and cautions in this document apply to both situations.
9. Security Considerations
NAT (particularly NAPT) actually has the potential to lower overall
security because it creates the illusion of a security barrier, but
does so without the managed intent of a firewall. Appropriate
security mechanisms are implemented in the end host, without reliance
on assumptions about routing hacks, firewall filters, or missing NAT
translations, which may change over time to enable a service to a
neighboring host. In general, defined security barriers assume that
any threats are external, leading to practices that make internal
breaches much easier.
IPsec RFC-2401 [7] defines a set of mechanisms to support packet-
level authentication and encryption for use in IP networks. While
this may be less efficient than application-level security but in the
words of RFC-1752 [14] "support for basic packet-level authentication
will provide for the adoption of a much needed, widespread, security
infrastructure throughout the Internet."
NATs break IPsec's authentication and encryption technologies because
these technologies depend on an end-to-end consistency of the IP
addresses in the IP headers, and therefore may stall further
deployment of enhanced security across the Internet. NATs raise a
number of specific issues with IPsec. For example;
- 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, RFC 2026, October 1996.
2 Egevang, K. and P. Francis, "The IP Network Address Translator",
RFC 1631, May 1994.
3 Srisuresh, P. and M. Holdrege, "NAT Terminology and
Considerations", RFC 2663, August 1999.
4 Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.
Lear, "Address Allocation for Private Internets", BCP 5, RFC
1918, February 1996.
5 Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
Behavior Today", RFC 2101, February 1997.
6 M. Borella, D. Grabelsky, J., K. Tuniguchi, "Realm Specific IP:
Protocol Specification", Work in Progress, March 2000.
7 Kent, S. and R. Atkinson, "Security Architecture for IP", RFC
2401, November 1998.
8 Carpenter, B., "Internet Transparency", RFC 2775, February 2000.
9 Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D. and J.
Postel, "Internet Registry IP Allocation Guidelines", BCP 12, RFC
2050, November 1996.
10 Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
11 Jacobson, V., Braden, R. and L. Zhang, "TCP Extension for High-
Speed Paths", RFC 1185, October 1990.
12 Braden, R., "Requirements for Internet Hosts", STD 3, RFC 1123,
October 1989.
13 Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4
Clouds without Explicit Tunnels", Work in Progress.
14 Bradner, S. and A. Mankin, "Recommendation for IPng", RFC 1752,
January 1995.
15 Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A. Heffernan, "DNS
extensions to NAT", RFC 2694, September 1999.
16 Dierks, T. and C. Allen, "The TLS Protocol", RFC 2246, 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", RFC 2385, August 1998.
13. Acknowledgments
Valuable contributions to this document came from the IAB, Vern
Paxson (lbl), Scott Bradner (harvard), Keith Moore (utk), Thomas
Narten (ibm), Yakov Rekhter (cisco), Pyda Srisuresh, Matt Holdrege
(lucent), and Eliot Lear (cisco).
14. Author's Address
Tony Hain
Microsoft
One Microsoft Way
Redmond, Wa. USA
Phone: 1-425-703-6619
EMail: tonyhain@microsoft.com
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