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RFC 3424 - IAB Considerations for UNilateral Self-Address Fixing


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Network Working Group                                     L. Daigle, Ed.
Request for Comments: 3424                   Internet Architecture Board
Category: Informational                                              IAB
                                                           November 2002

     IAB Considerations for UNilateral Self-Address Fixing (UNSAF)
                   Across Network Address Translation

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

   As a result of the nature of Network Address Translation (NAT)
   Middleboxes, communicating endpoints that are separated by one or
   more NATs do not know how to refer to themselves using addresses that
   are valid in the addressing realms of their (current and future)
   peers.  Various proposals have been made for "UNilateral Self-Address
   Fixing (UNSAF)" processes.  These are processes whereby some
   originating endpoint attempts to determine or fix the address (and
   port) by which it is known to another endpoint - e.g. to be able to
   use address data in the protocol exchange, or to advertise a public
   address from which it will receive connections.

   This document outlines the reasons for which these proposals can be
   considered at best as short term fixes to specific problems and the
   specific issues to be carefully evaluated before creating an UNSAF
   proposal.

1. Introduction

   As a result of the nature of Network Address (and port) Translation
   (NAT) Middleboxes, communicating endpoints that are separated by one
   or more NATs do not know how to refer to themselves using addresses
   that are valid in the addressing realms of their (current and future)
   peers - the address translation is locked within the NAT box.  For
   some purposes, endpoints need to know the addresses (and/or ports) by
   which they are known to their peers.  There are two cases: 1) when
   the client initiates communication, starting the communication has
   the side effect of creating an address binding in the NAT device and

   allocating an address in the realm that is external to the NAT box;
   and 2) a server will be accepting connections from outside, but
   because it does not initiate communication, no NAT binding is
   created.  In such cases, a mechanism is needed to fix such a binding
   before communication can take place.

   "UNilateral Self-Address Fixing (UNSAF)" is a process whereby some
   originating process attempts to determine or fix the address (and
   port) by which it is known - e.g. to be able to use address data in
   the protocol exchange, or to advertise a public address from which it
   will receive connections.

   There are only heuristics and workarounds to attempt to achieve this
   effect; there is no 100% solution.  Since NATs may also dynamically
   reclaim or readjust translations, "keep-alive" and periodic re-
   polling may be required.  Use of these workarounds MUST be considered
   transitional in IETF protocols, and a better architectural solution
   is being sought.  The explicit intention is to deprecate any such
   workarounds when sound technical approaches are available.

2. Architectural issues affecting UNSAF Systems

   Generally speaking, the proposed workarounds are for cases where a
   standard protocol communication is to take place between two
   endpoints,  but in order for this to occur, a separate step of
   determining (or fixing) the perceived address of an endpoint in the
   other endpoint's addressing realm is required.  Proposals require
   that an endpoint seeking to "fix" its address contact a participating
   service (in a different address realm) to determine (reflect) its
   address.  Thus, there is an "UNSAF client" partnering with some form
   of "UNSAF service" that may or may not be associated with the target
   endpoint of the actual desired communication session.  Throughout
   this memo, the terms "UNSAF server" and "UNSAF service" should be
   understood to generically refer to whatever process is participating
   in the UNSAF address determination for the originating process (the
   UNSAF client).

   Any users of these workarounds should be aware that specific
   technical issues that impede the creation of a general solution
   include:

   o  there *is* no unique "outside" to a NAT - it may be impossible to
      tell where the target endpoint is with respect to the initiator;
      how does an UNSAF client find an appropriate UNSAF server to
      reflect its address?  (See Appendix C).

   o  specifically because it is impossible to tell where address realms
      are bounded ("inside" or "outside", "private" or "public", or
      several "private" realms routing traffic), an address can only be
      determined relative to one specific point in the network.  If the
      UNSAF service that reflected an UNSAF client's address is in a
      different NAT-masqueraded subnet from some other service X that
      the client wishes to use, there is _no_ guarantee that the
      client's "perceived" address from the UNSAF partner would be the
      same as the address viewed from the perspective of X.  (See
      Appendix C).

   o  absent "middlebox communication (midcom)", there is no usable way
      to let incoming communications make their way through a middlebox
      (NAT, firewall) under proper supervision.  By circumventing the
      NAT, UNSAF mechanisms may also (inadvertently) circumvent security
      mechanisms.  The particular danger is that internal machines are
      unwittingly exposed to all the malicious communications from the
      external side that the firewall is intended to block.  This is
      particularly unacceptable if the UNSAF process is running on one
      machine which is acting on behalf of several.

   o  proposed workarounds include the use of "ping"-like address
      discovery requests sent from the UNSAF client (initiator) to the
      UNSAF server (listener), to which the listener responds with the
      transport address of the initiator - in the address realm of the
      listener.  However, with connection-less transports, e.g. UDP,
      IPsec ESP, etc., an UNSAF process must take care to react to
      changes in NAT bindings for a given application flow, since it may
      change unpredictably.

   o  if the UNSAF client uses periodic retries to refresh/reevaluate
      the address translation state, both the UNSAF client and the UNSAF
      server are required to maintain information about the presumed
      state of the communication in order to manage the address
      illusion.

   o  since the UNSAF server is not integrated with the middlebox, it
      can only operate on the assumption that past behavior is a
      predictor of future behavior.  It has no special knowledge of the
      address translation heuristic or affecting factors.

   o  the communication exchange is made more "brittle" by the
      introduction of other servers (UNSAF servers) that need to be
      reachable in order for the communication to succeed - more boxes
      that are "fate sharing" in the communication.

   Workarounds may mitigate some of these problems through tight scoping
   of applicability and specific fixes.  For example:

   o  rather than finding the address from "the" outside of the NAT, the
      applicability of the approach may be limited to finding the
      "self-address" from a specific service, for use exclusively with
      that service.

   o  limiting the scope to outbound requests for service (or service
      initiation) in order to prevent unacceptable security exposures.

3. Practical Issues

   From observations of deployed networks, it is clear that different
   NAT box implementations vary widely in terms of how they handle
   different traffic and addressing cases.

   Some of the specific types of observed behaviors have included:

   o  NATs may drop fragments in either direction: without complete
      TCP/UDP headers, the NAT may not make the address translation
      mapping, simply dropping the packet.

   o  Shipping NATs often contain Application Layer Gateways (ALGs)
      which attempt to be context-sensitive, depending on the source or
      destination port number.  The behavior of the ALGs can be hard to
      anticipate and these behaviors have not always been documented.

   o  Most NAT implementations with ALGs that attempt to translate TCP
      application protocols do not perform their functions correctly
      when the substrings they must translate span across multiple TCP
      segments; some of them are also known to fail on flows that use
      TCP option headers, e.g. timestamps.

   o  NAT implementations differ markedly in their handling of packets.
      Quite a few only really work reliably with TCP packets, not UDP.
      Of the ones that do make any attempt to handle UDP packets, the
      timers aging out flows can vary widely making it challenging to
      predict behavior.

   o  Variation in address and port assignments can be quite frequent -
      on NATs, port numbers always change, and change unpredictably;
      there may be multiple NATs in parallel for load-sharing, making IP
      address variations quite likely as well.

4. Architectural Considerations

   By distinguishing these approaches as short term fixes, the IAB
   believes the following considerations must be explicitly addressed in
   any proposal:

   1.  Precise definition of a specific, limited-scope problem that is
       to be solved with the UNSAF proposal.   A short term fix should
       not be generalized to solve other problems.  Such generalizations
       lead to the the prolonged dependence on and usage of the supposed
       short term fix -- meaning that it is no longer accurate to call
       it "short term".

   2.  Description of an exit strategy/transition plan.  The better
       short term fixes are the ones that will naturally see less and
       less use as the appropriate technology is deployed.

   3.  Discussion of specific issues that may render systems more
       "brittle".  For example, approaches that involve using data at
       multiple network layers create more dependencies, increase
       debugging challenges, and make it harder to transition.

   4.  Identify requirements for longer term, sound technical solutions;
       contribute to the process of finding the right longer term
       solution.

   5.  Discussion of the impact of the noted practical issues with
       existing deployed NATs and experience reports.

5. Security Considerations

   As a general class of workarounds, UNSAF proposals may introduce
   security holes because, in the absence of "middlebox communication
   (midcom)", there is no feasible way to let incoming communications
   make their way through a firewall under proper supervision:
   respecting the firewall policies as opposed to circumventing security
   mechanisms.

Appendix A. IAB Members at the time of this writing:

   Harald Alvestrand
   Ran Atkinson
   Rob Austein
   Fred Baker
   Leslie Daigle
   Steve Deering
   Sally Floyd
   Ted Hardie
   Geoff Huston
   Charlie Kaufman
   James Kempf
   Eric Rescorla
   Mike St. Johns

Appendix B. Acknowledgements

   This document has benefited greatly from detailed comments and
   suggestions from Thomas Narten, Bernard Aboba, Keith Moore, and James
   Woodyatt.

   This document was originally drafted when the following people were
   part of the IAB: Steve Bellovin, Brian Carpenter, Jon Crowcroft, John
   Klensin and Henning Schulzrinne; it has benefited considerably from
   their contributions and review.

Appendix C. Example NAT Configuration Scenario

C.1 Generic NATed Network Configuration

   Here is one sample scenario wherein it is difficult to describe a
   single "outside" to a given address realm (bridged by NAPTs).  This
   sort of configuration might arise in an enterprise environment where
   different divisions have their own subnets (each using the same
   private address space); the divisions are connected so that they can
   pass traffic on each others' networks, but to access the global
   Internet, each uses a different NAPT/firewall:

                                    +---------+
                                    | Box C   | (192.168.4.5)
                                    +---+-----+
                                        |
       ---------------------------------+-------
                                        |
                                        | 192.168.3.0/24
                                   +----+----+
                                   | NAT 2   |
                                   +----+----+
                                        | 10.1.0.0/32
                                        |
         -----+-------------------------+------------+----
              |                                      |
              |                                 +----+----+
              |                                 | Box B   | (10.1.1.100)
              |                                 +---------+
              |
         +----+----+
         | NAPT 1  | (10.1.2.27)
         +----+----+
              | 10.1.0.0/32
              |
          ----+-----+--
                    |
                    |
               +----+----+
               | Box A   | (10.1.1.100)
               +---------+

   From the perspective of Box B, Box A's address is (some port on)
   10.1.2.27.  From the perspective of Box C, however, Box A's address
   is some address in the space 192.168.3.0/24.

C.2 Real World Home Network Example

   James Woodyatt provided the following scenario, based on current
   examples of home networking products:

   o  the customer has existing Internet service from some broadband
      service provider, using e.g. a DSL line connected to an appliance
      that integrates a DSL modem with a NAT router/firewall.

   o  these devices are sometimes packaged with automated provisioning
      firmware, so the customer may view them as part of what their ISP
      provides them.

   o  later, the customer wants to use a host with only a wireless LAN
      interface, so they install a wireless access point that ships in
      its default configuration with NAT and a DHCP server enabled.

   o  after this, the customer has a wired LAN in one private address
      realm and a wireless LAN in another private address realm.

   Furthermore, most customers probably have no idea what the phrase
   "address realm" means and shouldn't have to learn it.  All they often
   know is that the printer server is inaccessible to the wireless
   laptop computer.  (Why?  Because the discovery protocol uses UDP
   multicast with TTL=1, but that's okay because any response would just
   be dropped by the NAT anyway, because there's no ALG.)

Authors' Addresses

   Leslie Daigle
   Editor

   Internet Architecture Board
   IAB
   EMail: iab@iab.org

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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