Internet DRAFT - draft-ietf-nat-app-guide


NAT Working Group                                               D. Senie
INTERNET-DRAFT                                    Amaranth Networks Inc.
Category: Informational                                    November 2001
Expires in six months

               NAT Friendly Application Design Guidelines


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
   Task Force (IETF), its areas, and its working groups. Note that other
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Copyright Notice

   Copyright (C) The Internet Society (1999-2001). All Rights Reserved.


   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 implications of NAT
   [RFC2993]. All of those relate to various issues with the NAT
   mechanism, effects on protocols and effects upon general Internet
    It is the focus of this document to provide recommendations to
   authors of new protocols about the effects to consider when designing

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   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

   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

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   potentially altered by the gateway. As such, applications passing
   only port number information will work with Basic NAT, but not with

   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

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      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 TLS 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
      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,
      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,
      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

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      networks where their applications will be deployed are able to
      process multicast traffic if their applications rely on that

    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

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      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

      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

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      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 will also use timers to guess when TCP
      sessions have disappeared as well. 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.

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    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

    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

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      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

    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

      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

      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

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      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

      Requirements for multi-homed NAT are the same as for Basic NAT or
      NAPT, depending on how the multi-homed NAT is implemented and

    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

      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

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      high-performance NAT boxes may be built, protocols that open many
      sessions instead of multiplexing will be slower than those that do

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

   [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.

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   [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, 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


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