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rfc:rfc3022

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

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 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

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

 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.

Srisuresh & Egevang Informational [Page 5] 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|
                       +------------------+
                         |
                         |  LAN
   --------------------------------------------
      |        ^{s=10.0.0.10,sport=3017, |  v{s=138.76.29.7, sport=23,
      |        ^ d=138.76.29.7,dport=23} |  v d=10.0.0.10, dport=3017}
      |                                  |
     +--+      +--+                    +--+
     |--|      |--|                    |--|
    /____\    /____\                  /____\
   10.0.0.1  10.0.0.2   .....        10.0.0.10
    Figure 3: Network Address Port Translation (NAPT) Operation
 When stub A host 10.0.0.10 sends a telnet packet to host 138.76.29.7,
 it uses the globally unique address 138.76.29.7 as destination, and
 sends the packet to it's primary router.  The stub router has a
 static route for the subnet 138.76.0.0/16 so the packet is forwarded
 to the WAN-link.  However, NAPT translates the tuple of source
 address 10.0.0.10 and source TCP port 3017 in the IP and TCP headers
 into the globally unique 138.76.28.4 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
 network.

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
 translation.
 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
 section.

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
 checksum.
 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=chksum[0]*256+chksum[1];
   x=~x & 0xFFFF;
   while (olen)
   {
       old=optr[0]*256+optr[1]; optr+=2;
       x-=old & 0xffff;
       if (x<=0) { x--; x&=0xffff; }
       olen-=2;
   }
   while (nlen)
   {
       new=nptr[0]*256+nptr[1]; nptr+=2;
       x+=new & 0xffff;
       if (x & 0x10000) { x++; x&=0xffff; }
       nlen-=2;
   }
   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
 implementations.
 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 10.0.0.0/8, 172.16.0.0/12,
 and 192.168.0.0/16 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.

References

 [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
            1981.

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

 [ICMP]     Postel, J., "INTERNET CONTROL MESSAGE (ICMP)
            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
 U.S.A.
 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

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 or assist in its implementation may be prepared, copied, published
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 The limited permissions granted above are perpetual and will not be
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Acknowledgement

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

Srisuresh & Egevang Informational [Page 16]

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