rfc3022.txt

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Network Working Group                                       P. Srisuresh
Request for Comments: 3022                              Jasmine Networks
Obsoletes: 1631                                               K. Egevang
Category: Informational                                Intel Corporation
                                                            January 2001


      Traditional IP Network Address Translator (Traditional NAT)

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 (2001).  All Rights Reserved.

Preface

   The NAT operation described in this document extends address
   translation introduced in RFC 1631 and includes a new type of network
   address and TCP/UDP port translation.  In addition, this document
   corrects the Checksum adjustment algorithm published in RFC 1631 and
   attempts to discuss NAT operation and limitations in detail.

Abstract

   Basic Network Address Translation or Basic NAT is a method by which
   IP addresses are mapped from one group to another, transparent to end
   users.  Network Address Port Translation, or NAPT is a method by
   which many network addresses and their TCP/UDP (Transmission Control
   Protocol/User Datagram Protocol) ports are translated into a single
   network address and its TCP/UDP ports.  Together, these two
   operations, referred to as traditional NAT, provide a mechanism to
   connect a realm with private addresses to an external realm with
   globally unique registered addresses.

1. Introduction

   The need for IP Address translation arises when a network's internal
   IP addresses cannot be used outside the network either for privacy
   reasons or because they are invalid for use outside the network.

   Network topology outside a local domain can change in many ways.
   Customers may change providers, company backbones may be reorganized,
   or providers may merge or split.  Whenever external topology changes



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   with time, address assignment for nodes within the local domain must
   also change to reflect the external changes.  Changes of this type
   can be hidden from users within the domain by centralizing changes to
   a single address translation router.

   Basic Address translation would (in many cases, except as noted in
   [NAT-TERM] and section 6 of this document) allow hosts in a private
   network to transparently access the external network and enable
   access to selective local hosts from the outside.  Organizations with
   a network setup predominantly for internal use, with a need for
   occasional external access are good candidates for this scheme.

   Many Small Office, Home Office (SOHO) users and telecommuting
   employees have multiple Network nodes in their office, running
   TCP/UDP applications, but have a single IP address assigned to their
   remote access router by their service provider to access remote
   networks.  This ever increasing community of remote access users
   would be benefited by NAPT, which would permit multiple nodes in a
   local network to simultaneously access remote networks using the
   single IP address assigned to their router.

   There are limitations to using the translation method.  It is
   mandatory that all requests and responses pertaining to a session be
   routed via the same NAT router.  One way to ascertain this would be
   to have NAT based on a border router that is unique to a stub domain,
   where all IP packets are either originated from the domain or
   destined to the domain.  There are other ways to ensure this with
   multiple NAT devices.  For example, a private domain could have two
   distinct exit points to different providers and the session flow from
   the hosts in a private network could traverse through whichever NAT
   device has the best metric for an external host.  When one of the NAT
   routers fail, the other could route traffic for all the connections.
   There is however a caveat with this approach, in that, rerouted flows
   could fail at the time of switchover to the new NAT router.  A way to
   overcome this potential problem is that the routers share the same
   NAT configuration and exchange state information to ensure a fail-
   safe backup for each other.

   Address translation is application independent and often accompanied
   by application specific gateways (ALGs) to perform payload monitoring
   and alterations.  FTP is the most popular ALG resident on NAT
   devices.  Applications requiring ALG intervention must not have their
   payload encoded, as doing that would effectively disables the ALG,
   unless the ALG has the key to decrypt the payload.

   This solution has the disadvantage of taking away the end-to-end
   significance of an IP address, and making up for it with increased
   state in the network.  As a result, end-to-end IP network level



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RFC 3022                    Traditional NAT                 January 2001


   security assured by IPSec cannot be assumed to end hosts, with a NAT
   device enroute.  The advantage of this approach however is that it
   can be installed without changes to hosts or routers.

   Definition of terms such as "Address Realm", "Transparent Routing",
   "TU Ports", "ALG" and others, used throughout the document, may be
   found in [NAT-TERM].

2. Overview of traditional NAT

   The Address Translation operation presented in this document is
   referred to as "Traditional NAT".  There are other variations of NAT
   that will not be explored in this document.  Traditional NAT would
   allow hosts within a private network to transparently access hosts in
   the external network, in most cases.  In a traditional NAT, sessions
   are uni-directional, outbound from the private network.  Sessions in
   the opposite direction may be allowed on an exceptional basis using
   static address maps for pre-selected hosts.  Basic NAT and NAPT are
   two variations of traditional NAT, in that translation in Basic NAT
   is limited to IP addresses alone, whereas translation in NAPT is
   extended to include IP address and Transport identifier (such as
   TCP/UDP port or ICMP query ID).

   Unless mentioned otherwise, Address Translation or NAT throughout
   this document will pertain to traditional NAT, namely Basic NAT as
   well as NAPT.  Only the stub border routers as described in figure 1
   below may be configured to perform address translation.

        \ | /                 .                                /
   +---------------+  WAN     .           +-----------------+/
   |Regional Router|----------------------|Stub Router w/NAT|---
   +---------------+          .           +-----------------+\
                              .                      |         \
                              .                      |  LAN
                              .               ---------------
                        Stub border

            Figure 1: Traditional NAT Configuration

2.1 Overview of Basic NAT

   Basic NAT operation is as follows.  A stub domain with a set of
   private network addresses could be enabled to communicate with
   external network by dynamically mapping the set of private addresses
   to a set of globally valid network addresses.  If the number of local
   nodes are less than or equal to addresses in the global set, each
   local address is guaranteed a global address to map to.  Otherwise,
   nodes allowed to have simultaneous access to external network are



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   limited by the number of addresses in global set.  Individual local
   addresses may be statically mapped to specific global addresses to
   ensure guaranteed access to the outside or to allow access to the
   local host from external hosts via a fixed public address.  Multiple
   simultaneous sessions may be initiated from a local node, using the
   same address mapping.

   Addresses inside a stub domain are local to that domain and not valid
   outside the domain.  Thus, addresses inside a stub domain can be
   reused by any other stub domain.  For instance, a single Class A
   address could be used by many stub domains.  At each exit point
   between a stub domain and backbone, NAT is installed.  If there is
   more than one exit point it is of great importance that each NAT has
   the same translation table.

   For instance, in the example of figure 2, both stubs A and B
   internally use class A private address block 10.0.0.0/8 [RFC 1918].
   Stub A's NAT is assigned the class C address block 198.76.29.0/24,
   and Stub B's NAT is assigned the class C address block
   198.76.28.0/24.  The class C addresses are globally unique no other
   NAT boxes can use them.

                                    \ | /
                                  +---------------+
                                  |Regional Router|
                                  +---------------+
                                WAN |           | WAN
                                    |           |
                Stub A .............|....   ....|............ Stub B
                                    |           |
                  {s=198.76.29.7,^  |           |  v{s=198.76.29.7,
                   d=198.76.28.4}^  |           |  v d=198.76.28.4}
                    +-----------------+       +-----------------+
                    |Stub Router w/NAT|       |Stub Router w/NAT|
                    +-----------------+       +-----------------+
                          |                         |
                          |  LAN               LAN  |
                    -------------             -------------
                              |                 |
            {s=10.33.96.5, ^  |                 |  v{s=198.76.29.7,
             d=198.76.28.4}^ +--+             +--+ v d=10.81.13.22}
                             |--|             |--|
                            /____\           /____\
                          10.33.96.5       10.81.13.22

                      Figure 2: Basic NAT Operation





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   When stub A host 10.33.96.5 wishes to send a packet to stub B host
   10.81.13.22, it uses the globally unique address 198.76.28.4 as
   destination, and sends the packet to its primary router.  The stub
   router has a static route for net 198.76.0.0 so the packet is
   forwarded to the WAN-link.  However, NAT translates the source
   address 10.33.96.5 of the IP header to the globally unique
   198.76.29.7 before the packet is forwarded.  Likewise, IP packets on
   the return path go through similar address translations.

   Notice that this requires no changes to hosts or routers.  For
   instance, as far as the stub A host is concerned, 198.76.28.4 is the
   address used by the host in stub B.  The address translations are
   transparent to end hosts in most cases.  Of course, this is just a
   simple example.  There are numerous issues to be explored.

2.2. Overview of NAPT

   Say, an organization has a private IP network and a WAN link to a
   service provider.  The private network's stub router is assigned a
   globally valid address on the WAN link and the remaining nodes in the
   organization have IP addresses that have only local significance.  In
   such a case, nodes on the private network could be allowed
   simultaneous access to the external network, using the single
   registered IP address with the aid of NAPT.  NAPT would allow mapping
   of tuples of the type (local IP addresses, local TU port number) to
   tuples of the type (registered IP address, assigned TU port number).

   This model fits the requirements of most Small Office Home Office
   (SOHO) groups to access external network using a single service
   provider assigned IP address.  This model could be extended to allow
   inbound access by statically mapping a local node per each service TU
   port of the registered IP address.

   In the example of figure 3 below, stub A internally uses class A
   address block 10.0.0.0/8.  The stub router's WAN interface is
   assigned an IP address 138.76.28.4 by the service provider.















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RFC 3022                    Traditional NAT                 January 2001


                                     \ | /
                                   +-----------------------+
                                   |Service Provider Router|
                                   +-----------------------+
                                 WAN |
                                     |
                 Stub A .............|....
                                     |
         ^{s=138.76.28.4,sport=1024, |  v{s=138.76.29.7, sport = 23,
         ^ d=138.76.29.7,dport=23}   |  v d=138.76.28.4, dport = 1024}
                         +------------------+
                         |Stub Router w/NAPT|
                         +------------------+
                           |