Architectural Implications of NAT
draft-iab-nat-implications-09

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IAB                                                             T. Hain 
Internet Draft                                                Microsoft 
Document: draft-iab-nat-implications-09.txt                 August 2000 
Category: Informational                                                 
 
                   Architectural Implications of NAT 
 
Status of this Memo 
    
   This document is an Internet-Draft and is in full conformance with 
   all provisions of Section 10 of RFC 2026 [1].  
    
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   "This memo provides information for the Internet community. This 
   memo does not specify an Internet standard of any kind. Distribution 
   of this memo is unlimited." 
    
Abstract 
    
   In light of the growing interest in, and deployment of network 
   address translation (NAT) RFC-1631, this paper will discuss some of 
   the architectural implications and guidelines for implementations. 
   It is assumed the reader is familiar with the address translation 
   concepts presented in RFC-1631. 
    
    

  
Hain             Informational - Expires January 2001                1 
 

                  Architectural Implications of NAT       August 2000 
 
    
Table of Contents 
 Abstract............................................................1 
 1.  Introduction....................................................3 
 2.  Terminology.....................................................5 
 3.  Scope...........................................................7 
 4.  End-to-End Model................................................7 
 5.  Advantages of NATs..............................................9 
 6.  Problems with NATs.............................................11 
 7.  Illustrations..................................................13 
  7.1 Single point of failure.......................................13 
  7.2.  ALG complexity..............................................14 
  7.3. TCP state violations.........................................15 
  7.4.  Symmetric state management..................................16 
  7.5.  Need for a globally unique FQDN when advertising public 
  services..........................................................17 
  7.6.  L2TP tunnels increase frequency of address collisions.......18 
  7.7.  Centralized data collection system report correlation.......19 
 8.  IPv6...........................................................19 
 9.  Security Considerations........................................21 
 10.  Deployment Guidelines.........................................23 
 11.  Summary.......................................................24 
 12.  References....................................................25 
 13.  Acknowledgments...............................................27 
 14.  Author's Addresses............................................27 

  
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                  Architectural Implications of NAT       August 2000 
 
1.  Introduction 
    
   Published in May 1994, written by K. Egevang and P. Francis, RFC-
   1631 [2] defined NAT as one means to ease the growth rate of IPv4 
   address use. But the authors were worried about the impact of this 
   technology. Several places in the document they pointed out the need 
   to experiment and see what applications may be adversely affected by 
   NAT's header manipulations, even before there was any significant 
   operational experience. This is further evidenced in a quote from 
   the conclusions: 'NAT has several negative characteristics that make 
   it inappropriate as a long term solution, and may make it 
   inappropriate even as a short term solution.' 
    
   Now, six years later and in spite of the prediction, the use of NATs 
   is becoming widespread in the Internet.  Some people are proclaiming 
   NAT as both the short and long term solution to some of the 
   Internet's address availability issues and questioning the need to 
   continue the development of IPv6.  The claim is sometimes made that 
   NAT 'just works' with no serious effects except on a few legacy 
   applications. At the same time others see a myriad of difficulties 
   caused by the increasing use of NAT.