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Versions: 00 01 02 rfc3424                                              
Network Working Group                                          L. Daigle
Internet-Draft                                                    Editor
Expires: July 10, 2002                       Internet Architecture Board
                                                                     IAB
                                                         January 9, 2002


     IAB Considerations for UNilateral Self-Address Fixing (UNSAF)
                 draft-iab-unsaf-considerations-00.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
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on July 10, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   In order to alleviate the fundamental problem with current NA[P]T
   middleboxes altering the relationship between the apparent location
   and true identity of endpoints, various proposals have been made for
   "UNilateral Self-Address Fixing (UNSAF)" processes.  These are
   processes 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.

   This document outlines the reasons for which these proposals can be



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   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 predicted some years ago, the fundamental problem with current
   NA[P]T middleboxes is that they alter the relationship between the
   apparent location and true identity of endpoints.  For some purposes,
   endpoints need to know their own apparent location, or fix a public
   address from which they will receive packets.  "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.  Use of these workarounds MUST be
   considered transitional in IETF protocols; a better architectural
   solution is being sought.  The explicit intention is to deprecate any
   such workarounds when sound technical approaches are available.

2. Architectural Considerations for UNSAF Systems

   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 UNSAF partner is with respect to the source;
      how does a client find an appropriate server to reflect its
      address?

   o  specifically because it is impossible to tell where "outside" or
      "public" is, an address can only be determined relative to one
      specific point in the network.  If the UNSAF partner that
      reflected a client's address is in a different NAT-masked 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.

   o  absent "middlebox communication (midcom)" there is no usable way
      to let incoming communications make their way through a firewall
      under proper supervision:  that is, respecting the firewall
      policies and as opposed to circumventing security mechanisms.




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   o  the proposed "ping"-like services create stateful conditions;
      there is no guarantee that the state will remain consistent for
      the duration of the communication.

   o  since the reflecting service is not integrated with the middlebox,
      it does not really know what the middlebox thinks it is doing and
      can only guess in an attempt to use past behavior as a predictor
      of future behavior.

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

   Work-arounds 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).

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

   o  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; this is why  "short term
      fixes usually aren't".

   o  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.

   o  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.

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





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3. Security Considerations

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


Authors' Addresses

   Leslie Daigle
   Editor


   Internet Architecture Board
   IAB

   EMail: iab@iab.org

Appendix A. IAB Members at the time of this writing

      Harald Alvestrand

      Ran Atkinson

      Rob Austein

      Fred Baker

      Brian Carpenter

      Steve Bellovin

      Jon Crowcroft

      Leslie Daigle

      Steve Deering

      Sally Floyd

      Geoff Huston

      John Klensin

      Henning Schulzrinne



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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Acknowledgement

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



















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