One document matched: draft-iab-nat-implications-02.txt
Differences from draft-iab-nat-implications-01.txt
IAB T. Hain, Microsoft
Internet Draft
Document: draft-iab-nat-implications-02.txt October 1998
Architectural Implications of NAT
Status of this Memo
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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].
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC-2119].
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Introduction
In discussing the architectural impact of NATs [RFC-1631] 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. An approach
resolving this would be including the resources of Names or
Addresses administered through IANA or its delegates. While this is
more accurate, it still includes many private networks that have
coordinated their names or addresses with the public Internet.
Rekhter, et al [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 the transparency of
the connection between any given node and the rest of the Internet.
True transparency could be stated as; an unambiguous locator known
by a node and identifiable by any other node participating in the
public Internet, with no restrictions on packet delivery.
The ultimate resolution of public or private is found in the intent
of the network in question. Generally networks that use coordinated
names and addresses, but 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
the screening technology is becoming a simple NAT, which manages the
network locator between the public and private use address spaces.
As noted by Carpenter, et al [RFC-2101], once private use addresses
[RFC-1918] were deployed in the network, addresses were guaranteed
to be ambiguous. At the same time when NATs were attached to the
network, the process of resolving names to or from addresses gained
a dependency on where the question was asked; thus both names and
addresses became globally ambiguous. As private use addresses are by
definition not part of the public infrastructure, and an unambiguous
locator is required within a routing realm, NATs are clearly left
sitting at the boundary of the Internet. Here they become another
screening technology for connecting private networks.
In one view, NAT is the feature which finally breaks the semantic
overload of the IP address as both a locator and the end point
identifier (EID). Another view of NAT is that of 'necessary evil',
where there is a real concern that the technology is the weed which
is destined to choke out continued development. In either case,
there is no direct impact on the public Internet, since NATs sit at
the boundary. This leaves the discussion focused on the impact on
end-to-end communications between hosts that use the public Internet
as a transport medium.
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A significant factor in the success of the Internet is the
flexibility derived from a few basic tenets. First and foremost is
the End-to-End principle, which assumes the end points are in
control of the communication and the network simply moves bits
between these points. Restated, the data stream delivered by the
transport protocol of the end points is of no concern to the lower
layer packet routing devices and therefore may contain anything the
end point applications consider appropriate. Another is that the
network does not maintain per connection state information to allow
fast rerouting around failures through parallel paths. Lack of state
also removes any requirement for the network nodes to notify each
other as connections are formed or dropped and enables
connectionless transports. Furthermore, the end points are not, and
need not be, aware of any network components other than the first
hop router(s), name resolution service, and destination. Packet
integrity is preserved through the network, and transport checksums
are valid end to end. NATs (particularly the port multiplexing
variety) break most of these, reducing overall flexibility,
increasing operational complexity, and impeding diagnostic
capabilities.
Terminology
Locator - the address within a packet directing its delivery within
a routing realm.
Routing realm - unambiguous address pool used by a contiguous
collection of routers and end systems.
End point identifier (EID) - used by one end system to identify the
other end of a communication. Uniqueness is required only within the
context of the originating end. Using the Domain Name System as a
common database requires uniqueness throughout the Internet.
NAT - segregates realms of routing information by connecting between
and rewriting packet headers as necessary. A NAT does not interpret
packet contents in any way.
Application Gateway - segregates realms of transport information by
terminating an application data stream then reconstructing packet
contents as necessary before forwarding to the next destination.
Firewall - blocks unauthorized end-to-end connections. Often used
within a routing realm, firewalls adhere to forwarding rules but do
not modify packet headers or contents.
VPN - Virtual Private Network which technically treats an IP
infrastructure as a multiplexing substrate allowing the end points
to build virtual circuits to run another instance of IP over.
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Utility of NATs
A quick look at the popularity of NAT technology shows that it
addresses several real world problems.
- Masking the address changes that take place, from either dial-
access or provider changes, improves stability in the local
network.
- Globally routable addresses can be reused for intermittent access
customers. This lowers the demand and utilization of addresses to
the number of active nodes rather than the total number of nodes.
- There is a potential that NATs would lower an ISP's support burden
since there could be a consistent, simple device with a known
configuration at the customer end of the access interface.
- Breaking the Internet into a collection of routing realms would
limit the scope of routing knowledge, and thereby the workload on
the routers within each realm.
- For applications which don't care about the integrity of the
packet header, there are no changes necessary in the hosts.
Taken together these are strong motivations for moving quickly with
NAT deployment.
Removing hosts that are not currently active lowers address demands
of the public Internet, which improves the load on the routing
system as well as lengthens the lifetime of the IPv4 address space.
While this 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 port multiplexing
NAT, the aggregation potential is even greater as multiple end
systems share a single public address.
Compartmentalizing routing knowledge and distributing the overhead
by breaking a network into a collection of routing realms might
improve its stability. It could also alleviate some of the pressure
on the routing infrastructure causing ISP's to enforce artificial
boundaries on how much detail they are willing to accept in routing
updates. Since the details of adjacent routing realms could be
completely masked, the level of aggregation possible would dwarf all
prior efforts. The number of entries in the routing table would be
reduced to the number of external attachments (albeit at the expense
of increasing the NAT mapping table at each attachment point).
Determination of the proper exit point is left as an exercise for
the reader.
NAT deployments should raise 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 end point
identification and discourage carrying the locator in the data
stream. Since this will not work for legacy applications, RFC-1631
discusses how to look into the packet and make NAT transparent to
the application (ie: create an application gateway).
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Another popular practice is hiding a collection of hosts behind a
single IP address. 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 overall performance is bounded by the
processing and I/O capabilities of the NAT device as it funnels the
packets back and forth.
Repercussion of NATs
As noted earlier, NATs break the basic tenet of the Internet that
the end points are in control of the communication. The greatest
concern from the explosion of NATs is the impact on the fledgling
efforts at deploying IP security. For lack of another globally
unique EID, the traditional use of the IP address was assumed. This
combination of required global uniqueness of the address, and
assured ambiguity by NAT leaves the IPsec effort with a severely
restricted working set.
In a statement about the use of IPv4 today, RFC-2101 details
architectural issues and notes:
"... it has been considered more useful to deliver the packet,
then worry about how to identify the end points, than to provide
identity in a packet that cannot be delivered."
This argument presumes that delivering the packet has an inherent
value, even if the end points can't be identified. In a self-
fulfillment of that prophecy, the applications developed to date are
structured to assume packets will be delivered and identity is only
assured in controlled environments. In many ways, this fundamental
impediment to basic trust has been the stalling factor in deploying
security across the Internet. In another note from RFC-2101:
"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 (ed:
Application Level Gateway) or a NAT."
A feature 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. 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
appropriate path for each transaction.
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It has been observed that operational management of networks
incorporating stateful packet modifying device is considerably
easier if inbound and outbound packets traverse the same path. While
easy to say, it is difficult to manage even with careful planning.
The problem 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. In many cases this borders on the impossible,
given the internal and external topology churn.
Another major drawback of NAT technology is the process of resolving
addresses from names, or names from addresses. When the public DNS
is required to resolve a given host name on both sides of a NAT
there is no obviously correct answer.
In the example below it is not clear what answer DNS should return
for Host D. Returning the local address will assure global
invisibility, while returning the global address will prevent local
access from Host C. If DNS were to return both, the results would be
unpredictable. By knowing which side the request came from the DNS
server could provide the correct answer, but significant development
would be required to add the capability to DNS for source specific
responses. (note: since Host A has no access to the DNS service it
is required to maintain a local table, but the others may be
expecting DNS to provide the appropriate resolution.)
In the case where Hosts C & D share an address (either time shared
or port multiplexed), there is no way Host B could know which it was
connecting to. DNS would return a public side address for either,
then it is up to the NAT to decide where the packets are eventually
directed. Since Host B cannot rely on the fully qualified domain
name to uniquely identify a specific host, the name space is
fragmented, resulting in pockets of validity.
-------- --- --- --------
| Host A |--|NAT|------|NAT|--| Host B |
-------- --- --- --------
\ \
--- -------- ---
|NAT|---|Internet|----|DNS|
--- -------- ---
|
-----------------
| |
-------- --------
| Host C | | Host D |
-------- --------
Even if forward mappings are working, implementations that require
an unambiguous reverse mapping from the in-addr.arpa tree will fail
(diagnostic tools come to mind).
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Discussions about an arbitrary mesh of NAT connections will
ultimately exaggerate the issue of name space integration with the
routing infrastructure and show that the only resolution to
appropriately answer name queries in a NAT environment is to locate
the DNS service within each NAT. This brings the additional
complexity of knowing which NAT to look to for remote resolutions.
Since most NATs are engineered to be auto configuring turnkey
devices, and DNS has not been known for its auto configuring
properties, this is not a particularly viable approach.
One proposal to deal with locating the DNS service in each NAT is
the DNS ALG (1). Rather than running the full DNS server in the NAT,
it provides a mapping service by intercepting DNS messages and
modifying the contents appropriately.
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). Technically VPNs treat
an IP infrastructure as a multiplexing substrate allowing the end
points to build what appear to be clear pathways from end to end.
VPNs redefine network visibility and increase the likelihood of
address collision when traversing NATs. Address management in the
hidden 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, which creates a problem. VPNs 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, 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 VPN address of Host A from its own
address so the connection will fail. Since private use addresses are
by definition not coordinated, as the complexity of the VPN mesh
increases so does the likelihood that there will be a collision
which cannot be resolved.
--------------- ----------------
| 10.10.10.10 |--------VPN--------| Assigned by A |
| Host A | --- --- | Host B |
| 10.1.1.1 |--|NAT|-----|NAT|--| 10.10.10.10 |
--------------- --- --- ----------------
----------
1 draft-ietf-nat-dns-alg-00.txt (work in progress 7/98)
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The primary feature of NATs is the ability to simply 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. Connecting the NAT
device, and reconfiguring the 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 name space is partitioned as 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 |
-------- --------
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://bar/*.html, which would
work from Host C, but would thoroughly confuse Host A. If the
embedded name matched the public DNS, Host A would be happy, but
Host C would not. To establish a connection from Host C, the NAT
would have to look at the destination rather than simply forwarding
the packet to a router (which would not send it back on the same
interface it came from).
NATs place constraints on the deployment of applications that carry
IP addresses in the data stream. Applications or protocols that
assume end to end integrity of the address will fail when traversing
a NAT. The resolution to this is to provide an Application Level
Gateway within or alongside each NAT. An additional gateway service
is necessary for each application that may imbed an address. Even
this approach will fail when requirement is end to end encryption
since only the end points have access to the keys.
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Finally, while the port multiplexing NATs (popular because they
allow Internet access through a single address, thus lowering cost)
work modestly well for private toward public connections, they
create management problems for applications connecting from public
toward private. Since only one private side system can be mapped
through the single public side port number, applications like multi-
player Internet games can only be played on one system at a time,
and X-windows services relying on port 6000 need to be manually
mapped to each target prior to connection.
IPv6 Considerations
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.
Beyond all of the above issues, the existence of NATs will
complicate the integration of IPv6 in the Internet because the name
space and end point addresses are not consistent and globally
unique. While multiple addresses are less of a concern to an IPv6
node, the disjoint name space will certainly make management
interesting. If IPv6 nodes are willing to continue in private
networks behind a NAT, they will only need a link local address and
all of the issues become the same as IPv4. If the intent is to move
into a public space as a feature of moving to IPv6, address and name
administration will require explicit effort.
Security Considerations
NATs break most implementation modes of IPsec, and therefore may
stall further deployment of enhanced security across the Internet.
It is difficult to identify all the combinations of header orderings
and options that are possible using NATs, VPNs, and IPsec. It is
even more difficult to clearly state which of those are applicable,
or workable in any given context. For example, use of AH is not
possible via NAT as the hash protects the IP address in the header.
In some cases, 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. While 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 attempts to retrofit IPsec over and existing
NAT infrastructure can be problematic. With all of the restrictions
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placed on deployment flexibility, NATs present the greatest single
threat to security integration being deployed in the Internet today.
Security mechanisms that do not protect or rely on IP addresses as
identifiers, such as TLS (2), SSL (3), or SSH (4) 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.
Guidelines
Given that NAT devices are being deployed at a fairly rapid pace,
some guidelines are in order. Most of these amount to 'think before
you leap', then think again.
- Determine the mechanism for name resolution, and ensure the
appropriate answer is given for each routing realm. Embedding the
DNS server, or its ALG in the NAT device will be more manageable
than trying to synchronize independent DNS systems across realms.
- 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.
- 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. Avoid port NATs if these apply.
- If there are encrypted payloads, the contents cannot be modified
unless the NAT is a security end point acting as a gateway between
security realms. This precludes end to end confidentiality, as the
NAT becomes the security end point.
- When using VPNs over NATs, identify a clearinghouse for addresses
to avoid collisions.
- Assure that applications that will be used both internally and
externally either avoid embedding names, or use globally unique
ones.
----------
2 draft-ietf-tls-protocol-05.txt (work in progress 11/97)
3 http://home.netscape.com/eng/ssl3/ssl-toc.html March 1996
4 draft-ietf-secsh-architecture-02.txt
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Summary
Wherever they are located topologically, NATs break a long-standing
architectural principle that applications can trust packets to move
between end points without modification beyond ttl or tos. 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 the overall flexibility and
manageability are lowered while support costs go up dealing with the
problems introduced.
NATs are a 'fact of life', and will proliferate as an enhancement
that sustains the existing IPv4 infrastructure. At the same time,
they require strong applicability statements, clearly stating what
works and what does not.
NATs are a 'necessary evil' as well, and by fragmenting the Name
Space, NATs 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. As such, NATs represent the single greatest threat
to a secure Internet.
An overview of the pluses and minuses:
NAT utility NAT repercussions
-------------------------------- --------------------------------
Masks Global Address Changes Mandates Multiple Name Spaces
Routing realms distribute overhead Requires source specific DNS reply
Lowers Address Utilization Allows end-to-end address conflicts
Lowers ISP support burden Increases local support burden
Breaks end-to-end association Breaks end-to-end association
Transparent to end systems Unique development for each app
Load sharing as virtual host Performance impact with scale
Delays need for IPv4 replacement Complicates 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 short sighted 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. At their best, NATs are a diversion from forward motion,
but they do enable wider participation at the present state. At
their worst, they break an association that arguably should never
have been made to begin with.
There have also been many questions about the probability of VPNs
being established which would raise some of the concerns listed
above. While it is hard to predict the future, one way to avoid ALGs
for each application is to establish a VPN over the NATs. This
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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.
The IAB wants to remind everyone to focus on the goal, which is
continued evolution of the Internet, and recognize continued
development of IP (in all current and future versions) is the path.
It has been noted that the success of the Internet is based on the
'living' characteristic of IP. As in life, when growth, evolution,
and forward progress stops, decay overtakes and destroys. History
has shown that protocols that were 'complete and finished' as
presented, have had very short lifetimes while those still 'a work
in progress' manage to survive and continue moving ahead. All
parties need to understand the significant role they are playing in
pursuing the goal, and that none can get there without all the
others.
References
[RFC 1631], Egevang, K., Francis, P., "The IP Network Address
Translator", RFC 1631, May 1994
[RFC 1918], Rekhter, et al, "Address Allocation for Private
Internets", RFC 1918 February 1996
[RFC 2101], Carpenter, et al, "IPv4 Address Behavior Today", RFC
2101, February 1997
[RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997
draft-ietf-nat-dns-alg-00.txt, P. Srisuresh, et al, .DNS extensions
to Network Address Translators., July 1998
draft-ietf-tls-protocol-05.txt, T. Dierks, C. Allen, "The TLS
Protocol", November 1997
draft-ietf-secsh-architecture-02.txt, T. Ylonen, et al, "SSH
Protocol Architecture", August 1998
Acknowledgments
Valuable contributions to this draft came from the IAB, Yakov
Rekhter(cisco) and Eliot Lear (cisco).
Author's Addresses
Tony Hain
Microsoft
One Microsoft Way Phone: 1-425-703-6619
Redmond, Wa. USA Email: tonyhain@microsoft.com
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