NAT Working Group D. Senie
INTERNET-DRAFT Amaranth Networks Inc.
Category: Informational March 2001
Expires in six months
NAT Friendly Application Design Guidelines
draft-ietf-nat-app-guide-05.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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To view the list Internet-Draft Shadow Directories, see
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Copyright Notice
Copyright (C) The Internet Society (1999-2001). All Rights Reserved.
Preface:
While many common Internet applications will operate cleanly in the
presence of Network Address Translators, others suffer from a variety
of problems when crossing these devices. This document discusses
those things that application designers might wish to consider when
designing new protocols. Guidelines are presented herein to help
ensure new protocols and applications will, to the extent possible,
be compatible with NAT.
1. Introduction
Other documents that describe Network Address Translation (NAT)
discuss the Terminology and Considerations [RFC2663] and Protocol
Issues [RFC3022], [RFC3027] or discuss the perceived implications of
NAT [RFC2993]. All of those relate to various issues with the NAT
mechanism and its effects on protocols that already exist. It is the
focus of this document to instruct authors of new protocols what to
think about when designing new protocols such that special handling
is not required at NAT gateway points.
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When a protocol is unable to pass cleanly through a NAT, the use of
an Application Level Gateway (ALG) can still permit operation of the
protocol. Depending on the encoding used in a protocol, the ALG may
be difficult or easy to construct. While adjunct to NAT, the
formulation of protocols that cannot directly operate through NAT
should be considered such that the ALG design may be simple and
automated. ALGs typically operate inside small routers along with the
NAT component. Ideally, the ALG should be simple and not require
excessive computation or state storage.
The same issues in application design that create issues for NAT (and
thus can require ALG support) are also issues for firewalls. An
application designer would do well to keep this in mind, as any
protocol that does require special handling by NAT or firewall
products will be more difficult to deploy than those that require no
special handling.
2. Discussion
Network Address Translation presents a challenge to some existing
applications. It should be possible for developers of new
applications to avoid problems if they understand the issues
involved. This document aims to provide the application designer with
information on what to do, and what to avoid, when building
applications.
The proliferation of NAT, especially in homes and small offices
cannot be dismissed. The emerging technologies for providing high
bandwidth to these types of locations often allow only a single IP
address per location. As such NAT is a clear choice for connecting
more than a single system per location.
Clearly the most common problem associated with NAT implementations
is the passing of addressing data between stations. Where possible,
applications should find alternatives to such schemes. Studying a few
existing protocols will serve to highlight the different approaches
possible.
Two common forms of Traditional NAT exist. With Basic NAT, only the
IP addresses of packets are altered by the NAT implementation. Many
applications will operate correctly with Basic NAT. The other common
form is Network Address Port Translation. With NAPT, both the IP
addresses and the source and destination ports (for TCP and UDP) are
potentially altered by the gateway. As such, applications passing
only port number information will work with Basic NAT, but not with
NAPT.
Application designers should ensure compatibility with NAPT, as this
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form of NAT is the most widely deployed. This is also the form of NAT
that will likely see the greatest penetration in homes and small
offices.
3. Recommendations and Examples
When possible, applications designers should strive to avoid the
necessity of ALG functions in NAT implementations. Since the same
issues apply to firewalls, application designers have added incentive
to consider these recommendations.
Applications that work in such fashion as to not need ALG support
will be much easier to deploy, and have the greatest chance of
operation on a broad spectrum of NAT implementations.
Each of the areas called out below are examples of issues to consider
when building an application. This list is likely not comprehensive,
but does cover a number of important issues and considerations.
3.1 Issues and Recommendations affecting all types of Network
Address Translators
3.1.1. Peer-to-Peer Applications Limitations
Peer to peer applications are problematic in a NAT world. Client-
server applications are more generally workable. Peer-to-peer
applications rely on each peer being reachable as a server for the
other to connect to. With NAPT, there are likely many machines
behind one address. With other types of NAT such as one-to-one
mappings, there is a greater chance of making such applications
work.
Some implementations of NAT can be made to function for UDP-based
peer-to-peer applications such as interactive gaming. This
capability is dependent on the methodology used to implement the
UDP sessions in the NAT device. If the NAT device tracks the tuple
(private address, private port, public port) then it is possible
for an outbound UDP packet to establish a channel by which
incoming traffic can flow from a source other than that originally
contacted by the system. NAT devices which track source and
destination IP addresses, in addition to port numbers, will not
permit third-party packets. The latter implementation is generally
more secure.
NAT device implementations could be constructed to have a switch
within them, permitting the consumer the ability to select whether
they want greater security, or greater ability to run peer-to-peer
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applications.
3.1.2. Applications Requiring End-to-End IPSec Will Fail
Use of IPSec for end-to-end security will not function in the
presence of a NAT implementation. Application designers may want
to explore the use of TLS as a mechanism that will traverse NAT
cleanly. See [RFC2709] for additional discussion on combining NAT
with Tunnel-mode IPSec security on the same device.
Some vendors have implemented a pass-through capabilty which
permits IPSec tunnel mode from a client in the private realm to
communicate with an endpoint in the public realm.
3.1.3. Use DNS Names, Not IP Addresses In Payload
Applications should take care to use fully qualified domain names,
rather than IP addresses when referring to IP endpoints. When
endpoints are across a NAT gateway, private addresses must not be
allowed to leak to the other endpoint. An example of where this
can happen today is with the HTTP and HTML protocols. It is
possible for web pages to be specified with numeric IP addresses,
rather than with names, for example http://192.168.1.10/index.html
could be used as a URL, but would likely create a problem if this
address is on a server located behind a NAT gateway. Users outside
the gateway would not be able to reach the address 192.168.1.10,
and so would not see the page.
Further exacerbating the problem is the possibility of duplicate
addresses between realms. If a server offers a link with a private
address space IP address embedded within it, such as 192.168.1.10,
the page referenced may resolve to a system on the local network
the browser is on, but would be a completely different server. The
resulting confusion to end-users would be significant. Sessions
involving multiple NAT implementations would be exceptionally
vulnerable to address reuse issues of this sort.
3.1.4. Multicast Considerations
Not all NAT devices implement multicast routing protocols.
Application designers should verify whether the devices in their
networks are able to process multicast traffic if their
applications rely on that capability.
3.1.5. Retention Of Address Mapping
With the exception of statically configured NAT bindings,
applications should not assume address mapping will be maintained
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from one session to another. An example of this is RSVP, which
forms one connection to reserve the resources, then the actual
session for which resources were reserved is started. The sessions
do not necessarily overlap. There is no guarantee that the NAT
implementation will keep the binding association. As such,
applications that rely on subsequent sessions being mapped to the
same destination system may not function without an ALG.
3.2 Recommendations for NAPT
As many of the issues specifically address NAPT issues, this
section will group these issues. NAPT is the most common form of
NAT in actual deployment in routers, especially in smaller offices
and home offices.
3.2.1 Realm Specific IP Addresses
Avoid the use of IP address and port number information within the
payload of packets. While in some cases ALGs will permit such
protocols to function, this presupposes every NAT device can be
updated in a timely fashion to support a new protocol. Since this
is unlikely application writers are urged to avoid addressing
information in payloads all together.
3.2.2 Avoid Session Bundles
Independent sessions, such as used by HTTP, are preferred to
protocols that attempt to manage a bundle of related sessions,
such as FTP.
In the FTP protocol, port information is passed over one TCP
connection and is used to construct a second TCP connection for
passing the actual data. Use of a separate connection to transfer
the file data makes determination of file end quite simple,
however other schemes could be envisioned which could use a single
connection.
The HTTP protocol, for example, uses a header and content length
approach to passing data. In this model, all data is transferred
over the single TCP connection, with the header portion indicating
the length of the data to follow. HTTP has evolved to allow
multiple objects to be passed on a single connection (thereby
cutting the connection establishment overhead). Clearly a new file
transfer function could be built that would perform most of the
functions of FTP without the need for additional TCP connections.
The goal is to keep to single connections where possible. This
keeps us from needing to pass addressing information of any sort
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across the network.
3.2.3. Session Bundles Originate From Same End
Origination of connections is an important consideration. Where
possible, the client should originate all connections. The FTP
protocol is the most obvious example, where by default the server
opens the data connection to a port on the client (the client
having specified the port number via a PORT command over the
control TCP session).
As pointed out in [RFC1579], the use of the passive open option in
FTP (PASV) remedies this situation as the client is responsible
for opening the connection in this case. With client-opened
connections, the standard functions of NAPT will process the
request as it would any other simple TCP connection, and so an ALG
is not required.
In cases where session bundles are unavoidable, each session in
the bundle should originate from the same end station.
3.2.4. TCP preferred over UDP
NAPT gateways must track which sessions are alive, and flush old
sessions. TCP has clear advantages in this area, since there are
specific beginning and end of session indicators in the packets
(SYN and FIN packets). While UDP works for some types of
applications with NAT, there can be issues when that data is
infrequent. Since there is no clean way to know when an end
station has finished using a UDP session, NAT implementations use
timeouts to guess when a UDP session completes. If an application
doesn't send data for a long period of time, the NAT translation
may time out.
Protocols other than TCP and UDP are workable with Traditional NAT
in many cases, provided they are not carrying addressing
information. For NAPT implementations Use of any protocols other
than TCP and UDP should be discouraged.
It's important to note that NAT deployments are based on the
assumption of a client-server application model, with the clients
in the private realm.
3.2.5. IP Fragmentation
Applications should attempt to avoid fragmentation when packets
pass over NAPT devices. While not always practical or possible,
there are failures that can occur with NAPT. Specifically, if two
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stations in the private realm pick matching fragmentation
identifiers, and talk to the same remote host, it will be
impossible to determine which fragments belong to which session.
Ideally, applications should limit packet size, use Path MTU
Discovery or both.
Some implementations of NAT may implement fragment reassembly
prior to Forwarding, however many do not. Application designers
are advised to examine the devices they plan to deploy, or to
design assuming the devices do not Reassemble fragments.
3.3 Issues and recommendations for Basic NAT
If only Basic NAT implementations are involved, not NAPT, then
many of the issues above do not apply. This form of NAT is not a
panacea. In this case many of the issues still remain, and many
protocols will not function correctly without assistance.
3.3.1. Use IP and TCP/UDP Headers Alone
Applications that use only the information in the IP and TCP or
UDP headers for communication (in other words, do not pass any
additional addressing information in the payload of the packets),
are clearly easier to support in a NAT environment. Where
possible, applications designers should try to limit themselves in
this area.
The X windowing system, for example, uses fixed port numbers to
address X servers. With X, the server (display) is addressed via
ports 6000 through 6000 + n. These map to hostname:0 through
hostname:n server displays. Since only the address and port are
used, the NAT administrator could map these ports to one or more
private addresses, yielding a functioning solution.
The X example, in the case of NAPT, requires configuration of the
NAT implementation. This results in the ability for no more than
one station inside the NAT gateway to use such a protocol. This
approach to the problem is thus OK for NAT but not recommended for
NAPT environments.
3.3.2. Avoid Addressing In Payload
As with NAPT, transporting IP address and/or port number
information in the payload is likely to cause trouble. The IP
address and/or port information valid in the realm on one side of
a NAT may well be invalid or even refer to different resources on
the other side of a NAT gateway.
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3.4 Bi-directional NAT
Bi-directional NAT makes use of DNS mapping of names to point
sessions originating outside the private realm to servers in the
private realm. Through use of a DNS-ALG [RFC2694], lookups are
performed to find the proper host and packets are sent to that
host.
Requirements for applications are the same as for Basic NAT.
Addresses are mapped one-to-one to servers. Unlike Traditional NAT
devices, Bi-directional NAT devices (in conjunction with DNS-ALG)
are amenable to peer-to-peer applications.
3.5 Twice NAT
Twice NAT is address translation where both source and destination
IP addresses are modified due to addressing conflicts between two
private realms. Two bi-directional NAT boxes connected together
would essentially perform the same task, though a common address
space that is not otherwise used by either private realm would be
required.
Requirements for applications to work in the Twice NAT environment
are the same as for Basic NAT. Addresses are mapped one to one.
3.6 Multi-homed NAT
Multi-homed NAT is the use of multiple NAT implementations to
provide redundancy. The multiple implementations share
configuration information so that sessions might continue in the
event of a fail-over. Unless the multiple implementations share
the same external addresses, sessions will have to restart
regardless.
Requirements for multi-homed NAT are the same as for Basic NAT or
NAPT, depending on how the multi-homed NAT is implemented and
configured.
3.7 Realm Specific IP (RSIP)
Realm Specific IP is described in [RFC2663] and defined in [RSIP]
and related documents. Clients within a private realm using RSIP
are aware of the delineation between private and public, and
access a server to allocate address (and optionally port)
information for use in conversing with hosts in the public realm.
By doing this, clients create packets that need not be altered by
the RSIP server on their way to the remote host. This technique
can permit IPSec to function, and potentially makes any
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application function as if there were no special processing
involved at all.
3.8 Performance Implications of Address Translation Implementations
Resource utilization on the NAT gateway should be considered. An
application that opens and closes many TCP connections, for
example, will use up more resources on the NAT router than an
application performing all transfers over a single TCP connection.
HTTP 1.0 opened a connection for each object on a web page,
whereas HTTP 1.1 permits the TCP session to be held open for
additional objects that may need to be transferred. Clearly the
latter imposes a lower overhead on the NAT gateway, as it is only
maintaining state on a single connection instead of multiple
connections.
New session establishment will typically remain a software
function even in implementations where the packet-by-packet
translation work is handled by hardware forwarding engines. While
high-performance NAT boxes may be built, protocols that open many
sessions instead of multiplexing will be slower than those that do
not.
Applications with different types of data, such as interactive
conferencing, require separate streams for the different types of
data. In such cases the protocol needs of each stream must be
optimized. While the goal of multiplexing over a single session is
preferred, clearly there are cases where this is impractical.
The latency of NAT translation overhead is implementation
dependent. On a per-packet basis, for established sessions only
the source or destination IP address is replaced, the source or
destination port (for NAPT) and the checksums for IP, and TCP or
UDP are recalculated. The functionality can be efficiently
implemented in hardware or software.
4. Security Considerations
Network Address Translators have implications for IPSec, as noted
above. When application developers are considering whether their
applications function with NAT implementations, care should be given
to selection of security methodology. Transport Layer Security (TLS)
[RFC2246] operates across translation boundaries. End-to-end IPSec
will prove problematic in many cases.
5. References
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[RFC1579] S. Bellovin, "Firewall Friendly FTP," RFC 1579, February
1994.
[RFC2246] T. Dierks, C. Allen, "The TLS Protocol Version 1.0," RFC
2246, January 1999.
[RFC2993] T. Hain, "Architectural Implications of NAT," RFC 2993,
November 2000.
[RFC3027] M. Holdrege, P. Srisuresh, "Protocol Complications with the
IP Network Address Translator (NAT)," RFC 3027, January 2001.
[RFC2663] P. Srisuresh, M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations," RFC 2663, August 1999.
[RFC2709] P. Srisuresh, "Security Model with Tunnel-mode IPsec for
NAT Domains," RFC 2709, October 1999.
[RSIP] M. Borella, et. al., "Realm Specific IP: Framework," <draft-
ietf-nat-rsip-framework-05.txt> - Work In Progress.
[RFC3022] P. Srisuresh, K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)," RFC 3022, January 2001.
[RFC2694] P. Srisuresh, et. al., "DNS extensions to Network Address
Translators (DNS_ALG)," RFC 2694, September 1999.
6. Acknowledgements
I'd like to thank Pyda Srisuresh for his invaluable input and
feedback.
7. Author's Address
Daniel Senie
Amaranth Networks Inc.
324 Still River Road
Bolton, MA 01740
Phone: (978) 779-6813
EMail: dts@senie.com
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