RFC3022: Traditional IP Network Address Translator (Traditional NAT)

Download in PDF format Download in text format

Obsoletes:  RFC1631
Related keywords:  (Internet) (ports) (Private) (protocol)

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.


   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.


   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

Srisuresh & Egevang          Informational                      [Page 1]
RFC 3022                    Traditional NAT                 January 2001

   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

Srisuresh & Egevang          Informational                      [Page 2]
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

Srisuresh & Egevang          Informational                      [Page 3]
RFC 3022                    Traditional NAT                 January 2001

   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 [RFC 1918].
   Stub A's NAT is assigned the class C address block,
   and Stub B's NAT is assigned the class C address block  The class C addresses are globally unique no other
   NAT boxes can use them.

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

                      Figure 2: Basic NAT Operation

Srisuresh & Egevang          Informational                      [Page 4]
RFC 3022                    Traditional NAT                 January 2001

   When stub A host wishes to send a packet to stub B host, it uses the globally unique address as
   destination, and sends the packet to its primary router.  The stub
   router has a static route for net so the packet is
   forwarded to the WAN-link.  However, NAT translates the source
   address of the IP header to the globally unique 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, 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  The stub router's WAN interface is
   assigned an IP address by the service provider.

Srisuresh & Egevang          Informational                      [Page 5]
RFC 3022                    Traditional NAT                 January 2001

                                     \ | /
                                   |Service Provider Router|
                                 WAN |
                 Stub A .............|....
         ^{s=,sport=1024, |  v{s=, sport = 23,
         ^ d=,dport=23}   |  v d=, dport = 1024}
                         |Stub Router w/NAPT|
                           |  LAN
        |        ^{s=,sport=3017, |  v{s=, sport=23,
        |        ^ d=,dport=23} |  v d=, dport=3017}
        |                                  |
       +--+      +--+                    +--+
       |--|      |--|                    |--|
      /____\    /____\                  /____\   .....

      Figure 3: Network Address Port Translation (NAPT) Operation

   When stub A host sends a telnet packet to host,
   it uses the globally unique address as destination, and
   sends the packet to it's primary router.  The stub router has a
   static route for the subnet so the packet is forwarded
   to the WAN-link.  However, NAPT translates the tuple of source
   address and source TCP port 3017 in the IP and TCP headers
   into the globally unique and a uniquely assigned TCP
   port, say 1024, before the packet is forwarded.  Packets on the
   return path go through similar address and TCP port translations for
   the target IP address and target TCP port.  Once again, notice that
   this requires no changes to hosts or routers.  The translation is
   completely transparent.

   In this setup, only TCP/UDP sessions are allowed and must originate
   from the local network.  However, there are services such as DNS that
   demand inbound access.  There may be other services for which an
   organization wishes to allow inbound session access.  It is possible
   to statically configure a well known TU port service [RFC 1700] on
   the stub router to be directed to a specific node in the private

Srisuresh & Egevang          Informational                      [Page 6]
RFC 3022                    Traditional NAT                 January 2001

   In addition to TCP/UDP sessions, ICMP messages, with the exception of
   REDIRECT message type may also be monitored by NAPT router.  ICMP
   query type packets are translated similar to that of TCP/UDP packets,
   in that the identifier field in ICMP message header will be uniquely
   mapped to a query identifier of the registered IP address.  The
   identifier field in ICMP query messages is set by Query sender and
   returned unchanged in response message from the Query responder.  So,
   the tuple of (Local IP address, local ICMP query identifier) is
   mapped to a tuple of (registered IP address, assigned ICMP query
   Identifier) by the NAPT router to uniquely identify ICMP queries of
   all types from any of the local hosts. Modifications to ICMP error
   messages are discussed in a later section, as that involves
   modifications to ICMP payload as well as the IP and ICMP headers.

   In NAPT setup, where the registered IP address is the same as the IP
   address of the stub router WAN interface, the router has to be sure
   to make distinction between TCP, UDP or ICMP query sessions
   originated from itself versus those originated from the nodes on
   local network.  All inbound sessions (including TCP, UDP and ICMP
   query sessions) are assumed to be directed to the NAT router as the
   end node, unless the target service port is statically mapped to a
   different node in the local network.

   Sessions other than TCP, UDP and ICMP query type are simply not
   permitted from local nodes, serviced by a NAPT router.

3.0. Translation phases of a session.

   The translation phases with traditional NAT are same as described in
   [NAT-TERM].  The following sub-sections identify items that are
   specific to traditional NAT.

3.1. Address binding:

   With Basic NAT, a private address is bound to an external address,
   when the first outgoing session is initiated from the private host.
   Subsequent to that, all other outgoing sessions originating from the
   same private address will use the same address binding for packet

   In the case of NAPT, where many private addresses are mapped to a
   single globally unique address, the binding would be from the tuple
   of (private address, private TU port) to the tuple of (assigned
   address, assigned TU port).  As with Basic NAT, this binding is
   determined when the first outgoing session is initiated by the tuple
   of (private address, private TU port) on the private host.  While not
   a common practice, it is possible to have an application on private
   host establish multiple simultaneous sessions originating from the

Srisuresh & Egevang          Informational                      [Page 7]
RFC 3022                    Traditional NAT                 January 2001

   same tuple of (private address, private TU port).  In such a case, a
   single binding for the tuple of (private address, private TU port)
   may be used for translation of packets pertaining to all sessions
   originating from the same tuple on a host.

3.2. Address lookup and translation:

   After an address binding or (address, TU port) tuple binding in case
   of NAPT is established, a soft state may be maintained for each of
   the connections using the binding.  Packets belonging to the same
   session will be subject to session lookup for translation purposes.
   The exact nature of translation is discussed in the follow-on

3.3. Address unbinding:

   When the last session based on an address or (address, TU port) tuple
   binding is terminated,  the binding itself may be terminated.

4.0. Packet Translations

   Packets pertaining to NAT managed sessions undergo translation in
   either direction.  Individual packet translation issues  are covered
   in detail in the following sub-sections.

4.1. IP, TCP, UDP and ICMP Header Manipulations

   In Basic NAT model, the IP header of every packet must be modified.
   This modification includes IP address (source IP address for outbound
   packets and destination IP address for inbound packets) and the IP

   For TCP ([TCP]) and UDP ([UDP]) sessions, modifications must include
   update of checksum in the TCP/UDP headers.  This is because TCP/UDP
   checksum also covers a pseudo header which contains the source and
   destination IP addresses.  As an exception, UDP headers with 0
   checksum should not be modified.  As for ICMP Query packets ([ICMP]),
   no further changes in ICMP header are required as the checksum in
   ICMP header does not cover IP addresses.

   In NAPT model, modifications to IP header are similar to that of
   Basic NAT.  For TCP/UDP sessions, modifications must be extended to
   include translation of TU port (source TU port for outbound packets
   and destination TU port for inbound packets) in the TCP/UDP header.
   ICMP header in ICMP Query packets must  also be modified to replace
   the query ID and ICMP header checksum.  Private host query ID must be

Srisuresh & Egevang          Informational                      [Page 8]
RFC 3022                    Traditional NAT                 January 2001

   translated into assigned ID on the outbound and the exact reverse on
   the inbound.  ICMP header checksum must be corrected to account for
   Query ID translation.

4.2. Checksum Adjustment

   NAT modifications are per packet based and can be very compute
   intensive, as they involve one or more checksum modifications in
   addition to simple field translations.  Luckily, we have an algorithm
   below, which makes checksum adjustment to IP, TCP, UDP and ICMP
   headers very simple and efficient.  Since all these headers use a
   one's complement sum, it is sufficient to calculate the arithmetic
   difference between the before-translation and after-translation
   addresses and add this to the checksum.  The algorithm below is
   applicable only for even offsets (i.e., optr below must be at an even
   offset from start of header) and even lengths (i.e., olen and nlen
   below must be even).  Sample code (in C) for this is as follows.

   void checksumadjust(unsigned char *chksum, unsigned char *optr,
   int olen, unsigned char *nptr, int nlen)
   /* assuming: unsigned char is 8 bits, long is 32 bits.
     - chksum points to the chksum in the packet
     - optr points to the old data in the packet
     - nptr points to the new data in the packet
     long x, old, new;
     x=~x & 0xFFFF;
     while (olen)
         old=optr[0]*256+optr[1]; optr+=2;
         x-=old & 0xffff;
         if (x<=0) { x--; x&=0xffff; }
     while (nlen)
         new=nptr[0]*256+nptr[1]; nptr+=2;
         x+=new & 0xffff;
         if (x & 0x10000) { x++; x&=0xffff; }
     x=~x & 0xFFFF;
     chksum[0]=x/256; chksum[1]=x & 0xff;

Srisuresh & Egevang          Informational                      [Page 9]
RFC 3022                    Traditional NAT                 January 2001

4.3. ICMP error packet modifications

   Changes to ICMP error message ([ICMP]) will include changes to IP and
   ICMP headers on the outer layer as well as changes to headers of the
   packet embedded within the ICMP-error message payload.

   In order for NAT to be transparent to end-host, the IP address of the
   IP header embedded within the payload of ICMP-Error message must be
   modified, the checksum field of the embedded IP header must be
   modified, and lastly, the ICMP header checksum must also be modified
   to reflect changes to payload.

   In a NAPT setup, if the IP message embedded within ICMP happens to be
   a TCP, UDP or ICMP Query packet, you will also need to modify the
   appropriate TU port number within the TCP/UDP header or the Query
   Identifier field in the ICMP Query header.

   Lastly, the IP header of the ICMP packet must also be modified.

4.4. FTP support

   One of the most popular applications, "FTP" ([FTP]) would require an
   ALG to monitor the control session payload to determine the ensuing
   data session parameters.  FTP ALG is an integral part of most NAT

   The FTP ALG would require a special table to correct the TCP sequence
   and acknowledge numbers with source port FTP or destination port FTP.
   The table entries should have source address, destination address,
   source port, destination port, delta for sequence numbers and a
   timestamp.  New entries are created only when FTP PORT commands or
   PASV responses are seen.  The sequence number delta may be increased
   or decreased for every FTP PORT command or PASV response.  Sequence
   numbers are incremented on the outbound and acknowledge numbers are
   decremented on the inbound by this delta.

   FTP payload translations are limited to private addresses and their
   assigned external addresses (encoded as individual octets in ASCII)
   for Basic NAT.  For NAPT setup, however, the translations must be
   extended to include the TCP port octets (in ASCII) following the
   address octets.

4.5 DNS support

   Considering that sessions in a traditional NAT are predominantly
   outbound from a private domain, DNS ALG may be obviated from use in
   conjunction with traditional NAT as follows.  DNS server(s) internal
   to the private domain maintain mapping of names to IP addresses for

Srisuresh & Egevang          Informational                     [Page 10]
RFC 3022                    Traditional NAT                 January 2001

   internal hosts and possibly some external hosts.  External DNS
   servers maintain name mapping for external hosts alone and not for
   any of the internal hosts.  If the private network does not have an
   internal DNS server, all DNS requests may be directed to external DNS
   server to find address mapping for the external hosts.

4.6. IP option handling

   An IP datagram with any of the IP options Record Route, Strict Source
   Route or Loose Source Route would involve recording or using IP
   addresses of intermediate routers.  A NAT intermediate router may
   choose not to support these options or leave the addresses
   untranslated while processing the options.  The result of leaving the
   addresses untranslated would be that private addresses along the
   source route are exposed end to end.  This should not jeopardize the
   traversal path of the packet, per se, as each router is supposed to
   look at the next hop router only.

5. Miscellaneous issues

5.1. Partitioning of Local and Global Addresses

   For NAT to operate as described in this document, it is necessary to
   partition the IP address space into two parts - the private addresses
   used internal to stub domain, and the globally unique addresses.  Any
   given address must either be a private address or a global address.
   There is no overlap.

   The problem with overlap is the following.  Say a host in stub A
   wished to send packets to a host in stub B, but the global addresses
   of stub B overlapped the private addressees of stub A. In this case,
   the routers in stub A would not be able to distinguish the global
   address of stub B from its own private addresses.

5.2. Private address space recommendation

   [RFC 1918] has recommendations on address space allocation for
   private networks.  Internet Assigned Numbers Authority (IANA) has
   three blocks of IP address space, namely,,
   and for private internets.  In pre-CIDR notation, the
   first block is nothing but a single class A network number, while the
   second block is a set of 16 contiguous class B networks, and the
   third block is a set of 256 contiguous class C networks.

   An organization that decides to use IP addresses in the address space
   defined above can do so without any coordination with IANA or an
   Internet registry.  The address space can thus be used privately by

Srisuresh & Egevang          Informational                     [Page 11]
RFC 3022                    Traditional NAT                 January 2001

   many independent organizations at the same time, with NAT operation
   enabled on their border routers.

5.3. Routing Across NAT

   The router running NAT should not advertise the private networks to
   the backbone.  Only the networks with global addresses may be known
   outside the stub.  However, global information that NAT receives from
   the stub border router can be advertised in the stub the usual way.

   Typically, the NAT stub router will have a static route configured to
   forward all external traffic to service provider router over WAN
   link, and the service provider router will have a static route
   configured to forward NAT packets (i.e., those whose destination IP
   address fall within the range of NAT managed global address list) to
   NAT router over WAN link.

5.4. Switch-over from Basic NAT to NAPT

   In Basic NAT setup, when private network nodes outnumber global
   addresses available for mapping (say, a class B private network
   mapped to a class C global address block), external network access to
   some of the local nodes is abruptly cut off after the last global
   address from the address list is used up.  This is very inconvenient
   and constraining.  Such an incident can be safely avoided by
   optionally allowing the Basic NAT router to switch over to NAPT setup
   for the last global address in the address list.  Doing this will
   ensure that hosts on private network will have continued,
   uninterrupted access to the external nodes and services for most
   applications.  Note, however, it could be confusing if some of the
   applications that used to work with Basic NAT suddenly break due to
   the switch-over to NAPT.

6.0. NAT limitations

   [NAT-TERM] covers the limitations of all flavors of NAT, broadly
   speaking.  The following sub-sections identify limitations specific
   to traditional NAT.

6.1. Privacy and Security

   Traditional NAT can be viewed as providing a privacy mechanism as
   sessions are uni-directional from private hosts and the actual
   addresses of the private hosts are not visible to external hosts.

   The same characteristic that enhances privacy potentially makes
   debugging problems (including security violations) more difficult. If
   a host in private network is abusing the Internet in some way (such

Srisuresh & Egevang          Informational                     [Page 12]
RFC 3022                    Traditional NAT                 January 2001

   as trying to attack another machine or even sending large amounts of
   spam) it is more difficult to track the actual source of trouble
   because the IP address of the host is hidden in a NAT router.

6.2. ARP responses to NAT mapped global addresses on a LAN interface

   NAT must be enabled only on border routers of a stub domain.  The
   examples provided in the document to illustrate Basic NAT and NAPT
   have maintained a WAN link for connection to external router (i.e.,
   service provider router) from NAT router.  However, if the WAN link
   were to be replaced by a LAN connection and if part or all of the
   global address space used for NAT mapping belongs to the same IP
   subnet as the LAN segment, the NAT router would be expected to
   provide ARP support for the address range that belongs to the same
   subnet.  Responding to ARP requests for the NAT mapped global
   addresses with its own MAC address is a must in such a situation with
   Basic NAT setup.  If the NAT router did not respond to these
   requests, there is no other node in the network that has ownership to
   these addresses and hence will go unresponded.

   This scenario is unlikely with NAPT setup except when the single
   address used in NAPT mapping is not the interface address of the NAT
   router (as in the case of a switch-over from Basic NAT to NAPT
   explained in 5.4 above, for example).

   Using an address range from a directly connected subnet for NAT
   address mapping would obviate static route configuration on the
   service provider router.

   It is the opinion of the authors that a LAN link to a service
   provider router is not very common.  However, vendors may be
   interested to optionally support proxy ARP just in case.

6.3. Translation of outbound TCP/UDP fragmented packets in NAPT setup

   Translation of outbound TCP/UDP fragments (i.e., those originating
   from private hosts) in NAPT setup are doomed to fail.  The reason is
   as follows.  Only the first fragment contains the TCP/UDP header that
   would be necessary to associate the packet to a session for
   translation purposes.  Subsequent fragments do not contain TCP/UDP
   port information, but simply carry the same fragmentation identifier
   specified in the first fragment.  Say, two private hosts originated
   fragmented TCP/UDP packets to the same destination host.  And, they
   happened to use the same fragmentation identifier.  When the target
   host receives the two unrelated datagrams, carrying same
   fragmentation id, and from the same assigned host address, it is
   unable to determine which of the two sessions the datagrams belong
   to.  Consequently, both sessions will be corrupted.

Srisuresh & Egevang          Informational                     [Page 13]
RFC 3022                    Traditional NAT                 January 2001

7.0. Current Implementations

   Many commercial implementations are available in the industry that
   adhere to the NAT description provided in this document.  Linux
   public domain software contains NAT under the name of "IP
   masquerade".  FreeBSD public domain software has NAPT implementation
   running as a daemon.  Note however that Linux source is covered under
   the GNU license and  FreeBSD software is covered under the UC
   Berkeley license.

   Both Linux and FreeBSD software are free, so you can buy CD-ROMs for
   these for little more than the cost of distribution.  They are also
   available on-line from a lot of FTP sites with the latest patches.

8.0. Security Considerations

   The security considerations described in [NAT-TERM] for all
   variations of NATs are applicable to traditional NAT.


   [NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations", RFC
              2663, August 1999.

   [RFC 1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC 1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
              1700, October 1994.

   [RFC 1122] Braden, R., "Requirements for Internet Hosts --
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC 1123] Braden, R., "Requirements for Internet Hosts --
              Application and Support", STD 3, RFC 1123, October 1989.

   [RFC 1812] Baker, F., "Requirements for IP Version 4 Routers",  RFC
              1812, June 1995.

   [FTP]      Postel, J. and J. Reynolds, "FILE TRANSFER PROTOCOL
              (FTP)", STD 9, RFC 959, October 1985.

   [TCP]      Defense Advanced Research Projects Agency Information
              Processing Techniques Office, "TRANSMISSION CONTROL
              PROTOCOL (TCP) SPECIFICATION", STD 7, RFC 793, September

Srisuresh & Egevang          Informational                     [Page 14]
RFC 3022                    Traditional NAT                 January 2001

              SPECIFICATION", STD 5, RFC 792, September 1981.

   [UDP]      Postel, J., "User Datagram Protocol (UDP)", STD 6, RFC
              768, August 1980.

   [RFC 2101] Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
              Behaviour Today", RFC 2101, February 1997.

Authors' Addresses

   Pyda Srisuresh
   Jasmine Networks, Inc.
   3061 Zanker Road, Suite B
   San Jose, CA 95134

   Phone: (408) 895-5032
   EMail: srisuresh@yahoo.com

   Kjeld Borch Egevang
   Intel Denmark ApS

   Phone: +45 44886556
   Fax:   +45 44886051
   EMail: kjeld.egevang@intel.com
   http:  //www.freeyellow.com/members/kbe

Srisuresh & Egevang          Informational                     [Page 15]
RFC 3022                    Traditional NAT                 January 2001

Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


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

Srisuresh & Egevang          Informational                     [Page 16]