Network Address Translator (NAT)-Friendly Application Design Guidelines

Network Working Group                                           D. Senie
Request for Comments: 3235                        Amaranth Networks Inc.
Category: Informational                                     January 2002


               Network Address Translator (NAT)-Friendly
                     Application Design Guidelines

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 (2002).  All Rights Reserved.

Abstract

   This document discusses those things that application designers might
   wish to consider when designing new protocols.  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.  Guidelines are presented herein to help
   ensure new protocols and applications will, to the extent possible,
   be compatible with NAT (Network Address Translation).

1. Introduction

   Other documents that describe Network Address Translation (NAT)
   discuss the Terminology and Considerations [RFC2663] and Protocol
   Issues [RFC3022], [RFC3027] or discuss the implications of NAT
   [RFC2993].  All of those relate to various issues with the NAT
   mechanism, effects on protocols and effects upon general Internet
   architecture.

   It is the focus of this document to provide recommendations to
   authors of new protocols about the effects to consider when designing
   new protocols such that special handling is not required at NAT
   gateway points.

   When a protocol is unable to pass cleanly through a NAT, the use of
   an Application Level Gateway (ALG) may still permit operation of the
   protocol.  Depending on the encoding used in a protocol, an ALG may
   be difficult or easy to construct, though in some cases it may not be
   possible at all.  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.

   Many of 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.  In many cases, it should be possible for developers of
   new applications to avoid problems if they understand the issues.
   This document aims to provide the application designer with
   information on what things they can do and what to avoid when trying
   to build applications that are able to function across NAT.

   The proliferation of NAT, especially in homes and small offices
   cannot be dismissed.  The marketing of these technologies to homes
   and small businesses is often focused on a single-computer
   environment, and thus providers only give out a single IP address to
   each user.  NAT has become a popular 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 strive for compatibility with NAPT, as
   this 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.  Not all applications lend themselves to the
   architectural model imposed by NAPT.

3. Recommendations and Examples

   Application designers who work within the constraints of NAT, and who
   do not rely on the presence of ALGs will generally find the easier
   acceptance in user communities where NAT is common.  When designing a
   new application or service, the requirement for an ALG will limit
   deployment until the required additional code is incorporated into
   the many devices which implement NAT.

   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 (i.e.,
   bound to a listening port, and able to accept connections) for the
   other to connect to.  With NAPT, there are likely many machines
   behind one address.  With other types of NAT such as Basic NAT with
   Static Address Assignment (providing 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.  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.  The source IP
   address is NOT used in this case to match incoming packets to UDP
   sessions, allowing any source address using the UDP port number to be
   translated.

   NAT devices which track source and destination IP addresses, in
   addition to port numbers, will not permit third-party packets.  NAT
   is often implemented in conjunction along with stateful-inspection
   firewall functionality.  As such the latter implementation of UDP
   association tracking would be considered more secure.

   NAT/Firewall device implementations could be constructed to have a
   software switch within them, permitting the consumer the ability to
   select whether they want the greater security, or greater ability to
   run peer-to-peer 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 Transport Layer Security (TLS) [RFC2246] as a
   transport mode that will traverse NAT cleanly.  See [RFC2709] for
   additional discussion on combining NAT with Tunnel-mode IPSec
   security on the same device.

3.1.3. Use DNS Names, Not IP Addresses In Payload

   Applications should, where possible, 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 the
   networks where their applications will be deployed 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
   from one session (association between machines, for whatever protocol
   for a period of time) 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 host IP address may not function without an ALG.

   Another consideration is the number of addressing realms.  It is
   entirely possible to have multiple levels of NAT implementations
   between the two end points involved.  As such, one must think about
   the lifetime of such mappings at all such levels.

   Load balancers and other devices may use a single IP address and port
   to map to multiple actual end points.  Many products implement
   variations on this theme, sometimes using NAT, sometimes using other
   technologies.  The lack of guarantee of mapping is important to
   understand, since the mapping to one actual system to another may not
   survive across such intermediate boxes.

   Don't assume systems know their own IP addresses.  A system behind a
   NAT may be reachable via a particular IP address, but that address
   may not be recognized by the system itself.  Consider the case of
   Static, one-to-one mapping using Basic NAT.  A server in this context
   will have an IP address from the private realm, and may not know the
   public address which maps to it.  Similarly, some such systems may
   not know their own DNS names, while others may.  This is largely
   dependent on the configuration of the servers and the network within
   the private realm.

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 IP Addresses Specific To A Realm

   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 placing addressing
   information in payloads all together.

   In addition to avoiding addresses and port numbers within packet
   payloads, it is important to avoid assumptions of (address, port)
   tuples are unique beyond the scope of the present session.  Load
   balancing devices implementing NAT may, for example, map subsequent
   sessions to other systems in the private realm.

3.2.2 Avoid Session Bundles

   Independent sessions, such as used by POP or SMTP, are preferred to
   protocols that attempt to manage a bundle of related sessions, such
   as FTP.  The term "session" here is used to refer to any association
   between end systems, and may be using any transport protocol or
   combination of protocols (UDP, TCP, SCTP).

   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 across the
   network.  However, multiplexing traffic over a single connection can
   create problems as well.

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. Choice of Transport Protocol

   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.

   NAT implementations also use timers to guess when TCP sessions have
   disappeared.  While TCP sessions should disappear only after FIN
   packets are exchanged, it is possible that such packets may never
   come, for example if both end stations die.  As such, the NAT
   implementation must use a timer for cleaning up its resources.

   NAT implementers in many cases provide several timeouts, one for live
   TCP sessions, one for TCP sessions on which a FIN has been seen, and
   one for UDP sessions.  It is best when such flexibility is provided,
   but some implementations appear to apply a single timer to all
   traffic.

   Protocols other than TCP and UDP can work 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
   will be problematic unless or until such protocols are programmed
   into the implementations.

   It's important to note that NAPT 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 stations in
   the private realm pick matching fragmentation identifiers, and talk
   to the same remote host, it may be impossible to determine which
   fragments belong to which session.  A clever NAPT implementation
   could track fragmentation identifiers and map those into a unique
   space, though it is not clear how many do so.

   Ideally, applications should limit packet size, use Path MTU
   Discovery or both.  Unfortunately, at least some firewall/NAT devices
   block Path MTU Discovery, apparently believing all ICMP packets are
   evil.

   Some implementations of NAT may implement fragment reassembly prior
   to Forwarding, however many do not.  Application designers are
   advised 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 is not to say that this form
   of NAT is better or worse than NAPT.  Application designers may think
   they could just specify users must use Basic NAT, and many
   application issues would go away.  This is unrealistic, however, as
   many users have no real alternative to NAPT due to the way their
   providers sell service.

   Many of the issues raised earlier still apply to Basic NAT, 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.

   This comes back to the same recommendation made for NAPT, that being
   to use a single connection whenever possible.

   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.  As stated earlier, load
   balancers and similar platforms may well map the same IP address and
   port number to a completely different system.  Thus it is problematic
   to assume an address or port number which is valid in the realm on
   one side of a NAT is valid on the other side.

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 application function as if there were no
   special processing involved at all.

   RSIP uses a view of the world in which there are only two realms, the
   private and public.  This isn't always the case.  Situations with
   multiple levels of NAT implementations are growing.  For example,
   some ISPs are handing out [RFC1918] addresses to their dialup users,
   rather than obtaining real addresses.  Any user of such an ISP who
   also uses a NAT implementation will see two levels of NAT, and the
   advantages of RSIP will have been wasted.

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

   [RFC1579]   Bellovin, S., "Firewall Friendly FTP", RFC 1579, February
               1994.

   [RFC2246]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
               RFC 2246, January 1999.

   [RFC2993]   Hain, T., "Architectural Implications of NAT", RFC 2993,
               November 2000.

   [RFC3027]   Holdrege, M. and P. Srisuresh, "Protocol Complications
               with the IP Network Address Translator (NAT)", RFC 3027,
               January 2001.

   [RFC2663]   Srisuresh, P. and M. Holdrege, "IP Network Address
               Translator (NAT) Terminology and Considerations", RFC
               2663, August 1999.

   [RFC2709]   Srisuresh, P., "Security Model with Tunnel-mode IPsec for
               NAT Domains", RFC 2709, October 1999.

   [RFC3102]   Borella, M., Lo, J., Grabelsky, D. and G. Montenegro,
               "Realm Specific IP: Framework", RFC 3102, October 2001.

   [RFC3022]   Srisuresh, P. and K. Egevang, "Traditional IP Network
               Address Translator (Traditional NAT)", RFC 3022, January
               2001.

   [RFC2694]   Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A.
               Heffernan, "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, and Keith Moore for his extensive comments.

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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