Network Working Group                                       P. Srisuresh
Request for Comments: 2663                                   M. Holdrege
Category: Informational                              Lucent Technologies
                                                             August 1999

    IP Network Address Translator (NAT) Terminology and Considerations

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 (1999).  All Rights Reserved.


   The motivation behind this document is to provide clarity to the
   terms used in conjunction with Network Address Translators.  The term
   "Network Address Translator" means different things in different
   contexts. The intent of this document is to define the various
   flavors of NAT and standardize the meaning of terms used.

   The authors listed are editors for this document and owe the content
   to contributions from members of the working group. Large chunks of
   the document titled, "IP Network Address Translator (NAT)" were
   extracted almost as is, to form the initial basis for this document.
   The editors would like to thank the authors Pyda Srisuresh and Kjeld
   Egevang for the same. The editors would like to thank Praveen
   Akkiraju for his contributions in describing NAT deployment
   scenarios. The editors would also like to thank the IESG members
   Scott Bradner, Vern Paxson and Thomas Narten for their detailed
   review of the document and adding clarity to the text.


   Network Address Translation is a method by which IP addresses are
   mapped from one realm to another, in an attempt to provide
   transparent routing to hosts. Traditionally, NAT devices are used to
   connect an isolated address realm with private unregistered addresses
   to an external realm with globally unique registered addresses. This
   document attempts to describe the operation of NAT devices and the
   associated considerations in general, and to define the terminology
   used to identify various flavors of NAT.

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1. Introduction and Overview

   The need for IP Address translation arises when a network's internal
   IP addresses cannot be used outside the network either because they
   are invalid for use outside, or because the internal addressing must
   be kept private from the external network.

   Address translation allows (in many cases, except as noted in
   sections 8 and 9) hosts in a private network to transparently
   communicate with destinations on an external network and vice versa.
   There are a variety of flavors of NAT and terms to match them. This
   document attempts to define the terminology used and to identify
   various flavors of NAT. The document also attempts to describe other
   considerations applicable to NAT devices in general.

   Note, however, this document is not intended to describe the
   operations of individual NAT variations or the applicability of NAT

   NAT devices attempt to provide a transparent routing solution to end
   hosts trying to communicate from disparate address realms. This is
   achieved by modifying end node addresses en-route and maintaining
   state for these updates so that datagrams pertaining to a session are
   routed to the right end-node in either realm. This solution only
   works when the applications do not use the IP addresses as part of
   the protocol itself. For example, identifying endpoints using DNS
   names rather than addresses makes applications less dependent of the
   actual addresses that NAT chooses and avoids the need to also
   translate payload contents when NAT changes an IP address.

   The NAT function cannot by itself support all applications
   transparently and often must co-exist with application level gateways
   (ALGs) for this reason. People looking to deploy NAT based solutions
   need to determine their application requirements first and assess the
   NAT extensions (i.e., ALGs) necessary to provide application
   transparency for their environment.

   IPsec techniques which are intended to preserve the Endpoint
   addresses of an IP packet will not work with NAT enroute for most
   applications in practice. Techniques such as AH and ESP protect the
   contents of the IP headers (including the source and destination
   addresses) from modification. Yet, NAT's fundamental role is to alter
   the addresses in the IP header of a packet.

2. Terminology and concepts used

   Terms most frequently used in the context of NAT are defined here for

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2.1. Address realm or realm

   An address realm is a network domain in which the network addresses
   are uniquely assigned to entities such that datagrams can be routed
   to them. Routing protocols used within the network domain are
   responsible for finding routes to entities given their network
   addresses. Note that this document is limited to describing NAT in
   IPv4 environment and does not address the use of NAT in other types
   of environment. (e.g. IPv6 environments)

2.2. Transparent routing

   The term "transparent routing" is used throughout the document to
   identify the routing functionality that a NAT device provides.  This
   is different from the routing functionality provided by a traditional
   router device in that a traditional router routes packets within a
   single address realm.

   Transparent routing refers to routing a datagram between disparate
   address realms, by modifying address contents in the IP header to be
   valid in the address realm into which the datagram is routed.
   Section 3.2 has a detailed description of transparent routing.

2.3. Session flow vs. Packet flow

   Connection or session flows are different from packet flows.  A
   session flow  indicates the direction in which the session was
   initiated with reference to a network interface. Packet flow is the
   direction in which the packet has traveled with reference to a
   network interface. Take for example, an outbound telnet session.  The
   telnet session consists of packet flows in both inbound and outbound
   directions. Outbound telnet packets carry terminal keystrokes and
   inbound telnet packets carry screen displays from the telnet server.

   For purposes of discussion in this document, a session is defined as
   the set of traffic that is managed as a unit for translation.
   TCP/UDP sessions are uniquely identified by the tuple of (source IP
   address, source TCP/UDP port, target IP address, target TCP/UDP
   port). ICMP query sessions are identified by the tuple of (source IP
   address, ICMP query ID, target IP address). All other sessions are
   characterized by the tuple of (source IP address, target IP address,
   IP protocol).

   Address translations performed by NAT are session based and would
   include translation of incoming as well as outgoing packets belonging
   to that session. Session direction is identified by the direction of
   the first packet of that session (see sec 2.5).

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   Note, there is no guarantee that the idea of a session, determined as
   above by NAT, will coincide with the application's idea of a session.
   An application might view a bundle of sessions (as viewed by NAT) as
   a single session and might not even view its communication with its
   peers as a session. Not all applications are guaranteed to work
   across realms, even with an ALG (defined below in section 2.9)

2.4. TU ports, Server ports, Client ports

   For the reminder of this document, we will refer TCP/UDP ports
   associated with an IP address simply as "TU ports".

   For most TCP/IP hosts, TU port range 0-1023 is used by servers
   listening for incoming connections. Clients trying to initiate a
   connection typically select a source TU port in the range of 1024-
   65535. However, this convention is not universal and not always
   followed. Some client stations initiate connections using a source TU
   port number in the range of 0-1023, and there are servers listening
   on TU port numbers in the range of 1024-65535.

   A list of assigned TU port services may be found in RFC 1700 [Ref 2].

2.5. Start of session for TCP, UDP and others

   The first packet of every TCP session tries to establish a session
   and contains connection startup information. The first packet of a
   TCP session may be recognized by the presence of SYN bit and absence
   of ACK bit in the TCP flags. All TCP packets, with the exception of
   the first packet, must have the ACK bit set.

   However, there is no deterministic way of recognizing the start of a
   UDP based session or any non-TCP session. A heuristic approach would
   be to assume the first packet with hitherto non-existent session
   parameters (as defined in section 2.3) as constituting the start of
   new session.

2.6. End of session for TCP, UDP and others

   The end of a TCP session is detected when FIN is acknowledged by both
   halves of the session or when either half receives a segment with the
   RST bit in TCP flags field. However, because it is impossible for a
   NAT device to know whether the packets it sees will actually be
   delivered to the destination (they may be dropped between the NAT
   device and the destination), the NAT device cannot safely assume that
   the segments containing FINs or SYNs will be the last packets of the
   session (i.e., there could be retransmissions).  Consequently, a
   session can be assumed to have been terminated only after a period of

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   4 minutes subsequent to this detection. The need for this extended
   wait period is described in RFC 793 [Ref 7], which suggests a TIME-
   WAIT duration of 2 * MSL (Maximum Segment Lifetime) or 4 minutes.

   Note that it is also possible for a TCP connection to terminate
   without the NAT device becoming aware of the event (e.g., in the case
   where one or both peers reboot). Consequently, garbage collection is
   necessary on NAT devices to clean up unused state about TCP sessions
   that no longer exist. However, it is not possible in the general case
   to distinguish between connections that have been idle for an
   extended period of time from those that no longer exist.  In the case
   of UDP-based sessions, there is no single way to determine when a
   session ends, since UDP-based protocols are application specific.

   Many heuristic approaches are used to terminate sessions. You can
   make the assumption that TCP sessions that have not been used for
   say, 24 hours, and non-TCP sessions that have not been used for a
   couple of minutes, are terminated. Often this assumption works, but
   sometimes it doesn't. These idle period session timeouts vary a great
   deal both from application to application and for different sessions
   of the same application. Consequently, session timeouts must be
   configurable. Even so, there is no guarantee that a satisfactory
   value can be found. Further, as stated in section 2.3, there is no
   guarantee that NAT's view of session termination will coincide with
   that of the application.

   Another way to handle session terminations is to timestamp entries
   and keep them as long as possible and retire the longest idle session
   when it becomes necessary.

2.7. Public/Global/External network

   A Global or Public Network is an address realm with unique network
   addresses assigned by Internet Assigned Numbers Authority (IANA) or
   an equivalent address registry. This network is also referred as
   External network during NAT discussions.

2.8. Private/Local network

   A private network is an address realm independent of external network
   addresses. Private network may also be referred alternately as Local
   Network. Transparent routing between hosts in private realm and
   external realm is facilitated by a NAT router.

   RFC 1918 [Ref 1] has recommendations on address space allocation for
   private networks. Internet Assigned Numbers Authority (IANA) has
   three blocks of IP address space, namely 10/8, 172.16/12, and
   192.168/16 set aside for private internets. In pre-CIDR notation, the

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   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 coordination with IANA or any other
   Internet registry such as APNIC, RIPE and ARIN.  The address space
   can thus be used privately by many independent organizations at the
   same time. However, if those independent organizations later decide
   they wish to communicate with each other or the public Internet, they
   will either have to renumber their networks or enable NAT on their
   border routers.

2.9. Application Level gateway (ALG)

   Not all applications lend themselves easily to translation by NAT
   devices; especially those that include IP addresses and TCP/UDP ports
   in the payload. Application Level Gateways (ALGs) are application
   specific translation agents that allow an application on a host in
   one address realm to connect to its counterpart running on a host in
   different realm transparently. An ALG may interact with NAT to set up
   state, use NAT state information, modify application specific payload
   and perform whatever else is necessary to get the application running
   across disparate address realms.

   ALGs may not always utilize NAT state information. They may glean
   application payload and simply notify NAT to add additional state
   information in some cases. ALGs are similar to Proxies, in that, both
   ALGs and proxies facilitate Application specific communication
   between clients and servers. Proxies use a special protocol to
   communicate with proxy clients and relay client data to servers and
   vice versa. Unlike Proxies, ALGs do not use a special protocol to
   communicate with application clients and do not require changes to
   application clients.

3. What is NAT?

   Network Address Translation is a method by which IP addresses are
   mapped from one address realm to another, providing transparent
   routing to end hosts. There are many variations of address
   translation that lend themselves to different applications.  However,
   all flavors of NAT devices should share the following

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          a) Transparent Address assignment.
          b) Transparent routing through address translation.
             (routing here refers to forwarding packets, and not
             exchanging routing information)
          c) ICMP error packet payload translation.

   Below is a diagram illustrating a scenario in which NAT is enabled on
   a stub domain border router, connected to the Internet through a
   regional router made available by a service provider.

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

        Figure 1: A typical NAT operation scenario

3.1. Transparent Address Assignment

   NAT binds addresses in private network with addresses in global
   network and vice versa to provide transparent routing for the
   datagrams traversing between address realms. The binding in some
   cases may extend to transport level identifiers (such as TCP/UDP
   ports). Address binding is done at the start of a session. The
   following sub-sections describe two types of address assignments.

3.1.1. Static Address assignment

   In the case of static address assignment, there is one-to-one address
   mapping for hosts between a private network address and an external
   network address for the lifetime of NAT operation.  Static address
   assignment ensures that NAT does not have to administer address
   management with session flows.

3.1.2. Dynamic Address assignment

   In this case, external addresses are assigned to private network
   hosts or vice versa, dynamically based on usage requirements and
   session flow determined heuristically by NAT. When the last session
   using an address binding is terminated, NAT would free the binding so
   that the global address could be recycled for later use. The exact
   nature of address assignment is specific to individual NAT

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3.2. Transparent routing

   A NAT router sits at the border between two address realms and
   translates addresses in IP headers so that when the packet leaves one
   realm and enters another, it can be routed properly. Because NAT
   devices have connections to multiple address realms, they must be
   careful to not improperly propagate information (e.g., via routing
   protocols) about networks from one address realm into another, where
   such an advertisement would be deemed unacceptable.

   There are three phases to Address translation, as follows. Together
   these phases result in creation, maintenance and termination of state
   for sessions passing through NAT devices.

3.2.1. Address binding

   Address binding is the phase in which a local node IP address is
   associated with an external address or vice versa, for purposes of
   translation. Address binding is fixed with static address assignments
   and is dynamic at session startup time with dynamic address
   assignments. Once the binding between two addresses is in place, all
   subsequent sessions originating from or to this host will use the
   same binding for session based packet translation.

   New address bindings are made at the start of a new session, if such
   an address binding didn't already exist. Once a local address is
   bound to an external address, all subsequent sessions originating
   from the same local address or directed to the same local address
   will use the same binding.

   The start of each new session will result in the creation of a state
   to facilitate translation of datagrams pertaining to the session.
   There can be many simultaneous sessions originating from the same
   host, based on a single address binding.

3.2.2. Address lookup and translation

   Once a state is established for a session, all packets belonging to
   the session will be subject to address lookup (and transport
   identifier lookup, in some cases) and translation.

   Address or transport identifier translation for a datagram will
   result in the datagram forwarding from the origin address realm to
   the destination address realm with network addresses appropriately

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3.2.3. Address unbinding

   Address unbinding is the phase in which a private address is no
   longer associated with a global address for purposes of translation.
   NAT will perform address unbinding when it believes that the last
   session using an address binding has terminated.  Refer section 2.6
   for some heuristic ways to handle session terminations.

3.3. ICMP error packet translation

   All ICMP error messages (with the exception of Redirect message type)
   will need to be modified, when passed through NAT. The ICMP error
   message types needing NAT modification would include Destination-
   Unreachable, Source-Quench, Time-Exceeded and Parameter-Problem.  NAT
   should not attempt to modify a Redirect message type.

   Changes to ICMP error message will include changes to the original IP
   packet (or portions thereof) embedded in the payload of the ICMP
   error message. In order for NAT to be completely transparent to end
   hosts, the IP address of the IP header embedded in the payload of the
   ICMP packet must be modified, the checksum field of the same IP
   header must correspondingly be modified, and the accompanying
   transport header. The ICMP header checksum must also be modified to
   reflect changes made to the IP and transport headers in the payload.
   Furthermore, the normal IP header must also be modified.

4.0. Various flavors of NAT

   There are many variations of address translation that lend themselves
   to different applications. NAT flavors listed in the following sub-
   sections are by no means exhaustive, but they do capture the
   significant differences that abound.

   The following diagram will be used as a base model to illustrate NAT
   flavors. Host-A, with address Addr-A is located in a private realm,
   represented by the network N-Pri. N-Pri is isolated from external
   network through a NAT router. Host-X, with address Addr-X is located
   in an external realm, represented by the network N-Ext.  NAT router
   with two interfaces, each attached to one of the realms provides
   transparent routing between the two realms. The interface to the
   external realm is assigned an address of Addr-Nx and the interface to
   private realm is assigned an address of Addr-Np.  Further, it may be
   understood that addresses Addr-A and Addr-Np correspond to N-Pri
   network and the addresses Addr-X and Addr-Nx correspond to N-Ext

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                                 (                )
                                (   External       )    +--+
                               (  Address Realm     )-- |__|
                                (     (N-Ext)      )   /____\
                                 (________________)    Host-X
                                        |              (Addr-X)
                           |              |
                           |  NAT router  |
                           |              |
                    (                )
        +--+       (     Private      )
        |__|------(    Address Realm   )
       /____\      (     (N-pri)      )
       Host-A       (________________)

             Figure 2: A base model to illustrate NAT terms.

4.1. Traditional NAT (or) Outbound NAT

   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. This is in contrast with Bi-directional NAT, which
   permits sessions in both inbound and outbound directions. A detailed
   description of Bi-directional NAT may be found in section 4.2.

   The following is a description of the properties of realms supported
   by traditional NAT. IP addresses of hosts in external network are
   unique and valid in external as well as private networks. However,
   the addresses of hosts in private network are unique only within the
   private network and may not be valid in the external network. In
   other words, NAT would not advertise private networks to the external
   realm. But, networks from the external realm may be advertised within
   the private network.  The addresses used within private network must
   not overlap with the external addresses. Any given address must
   either be a private address or an external address; not both.

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   A traditional NAT router in figure 2 would allow Host-A to initiate
   sessions to Host-X, but not the other way around. Also, N-Ext is
   routable from within N-Pri, whereas N-Pri may not be routable from

   Traditional NAT is primarily used by sites using private addresses
   that wish to allow outbound sessions from their site.

   There are two variations to traditional NAT, namely Basic NAT and
   NAPT (Network Address Port Translation). These are discussed in the
   following sub-sections.

4.1.1. Basic NAT

   With Basic NAT, a block of external addresses are set aside for
   translating addresses of hosts in a private domain as they originate
   sessions to the external domain. For packets outbound from the
   private network, the source IP address and related fields such as IP,
   TCP, UDP and ICMP header checksums are translated. For inbound
   packets, the destination IP address and the checksums as listed above
   are translated.

   A Basic NAT router in figure 2 may be configured to translate N-Pri
   into a block of external addresses, say Addr-i through Addr-n,
   selected from the external network N-Ext.

4.1.2. Network Address Port Translation (NAPT)

   NAPT extends the notion of translation one step further by also
   translating transport identifier (e.g., TCP and UDP port numbers,
   ICMP query identifiers). This allows the transport identifiers of a
   number of private hosts to be multiplexed into the transport
   identifiers of a single external address. NAPT allows a set of hosts
   to share a single external address. Note that NAPT can be combined
   with Basic NAT so that a pool of external addresses are used in
   conjunction with port translation.

   For packets outbound from the private network, NAPT would translate
   the source IP address, source transport identifier and related fields
   such as IP, TCP, UDP and ICMP header checksums. Transport identifier
   can be one of TCP/UDP port or ICMP query ID. For inbound packets, the
   destination IP address, destination transport identifier and the IP
   and transport header checksums are translated.

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   A NAPT router in figure 2 may be configured to translate sessions
   originated from N-Pri into a single external address, say Addr-i.

   Very often, the external interface address Addr-Nx of NAPT router is
   used as the address to map N-Pri to.

4.2. Bi-directional NAT (or) Two-Way NAT

   With a Bi-directional NAT, sessions can be initiated from hosts in
   the public network as well as the private network. Private network
   addresses are bound to globally unique addresses, statically or
   dynamically as connections are established in either direction.  The
   name space (i.e., their Fully Qualified Domain Names) between hosts
   in private and external networks is assumed to be end-to-end unique.
   Hosts in external realm access private realm hosts by using DNS for
   address resolution. A DNS-ALG must be employed in conjunction with
   Bi-Directional NAT to facilitate name to address mapping.
   Specifically, the DNS-ALG must be capable of translating private
   realm addresses in DNS Queries and responses into their external
   realm address bindings, and vice versa, as DNS packets traverse
   between private and external realms.

   The address space requirements outlined for traditional NAT routers
   are applicable here as well.

   A Bi-directional NAT router in figure 2 would allow Host-A to
   initiate sessions to Host-X, and Host-X to initiate sessions to
   Host-A. Just as with traditional NAT, N-Ext is routable from within
   N-Pri, but N-Pri may not be routable from N-Ext.

4.3. Twice NAT

   Twice NAT is a variation of NAT in that both the source and
   destination addresses are modified by NAT as a datagram crosses
   address realms. This is in contrast to Traditional-NAT and Bi-
   Directional NAT, where only one of the addresses (either source or
   destination) is translated. Note, there is no such term as 'Once-

   Twice NAT is necessary when private and external realms have address
   collisions. The most common case where this would happen is when a
   site had (improperly) numbered its internal nodes using public
   addresses that have been assigned to another organization.
   Alternatively, a site may have changed from one provider to another,
   but chosen to keep (internally) the addresses it had been assigned by
   the first provider. That provider might then later reassign those
   addresses to someone else. The key issue in such cases is that the
   address of the host in the external realm may have been assigned the

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   same address as a host within the local site. If that address were to
   appear in a packet, it would be forwarded to the internal node rather
   than through the NAT device to the external realm. Twice-NAT attempts
   to bridge these realms by translating both source and destination
   address of an IP packet, as the packet transitions realms.

   Twice-NAT works as follows. When Host-A wishes to initiate a session
   to Host-X, it issues a DNS query for Host-X. A DNS-ALG intercepts the
   DNS query, and in the response returned to Host-A the DNS-ALG
   replaces the address for Host-X with one that is properly routable in
   the local site (say Host-XPRIME). Host A then initiates communication
   with Host-XPRIME. When the packets traverse the NAT device, the
   source IP address is translated (as in the case of traditional NAT)
   and the destination address is translated to Host-X. A similar
   translation is performed on return packets coming from Host-X.

   The following is a description of the properties of realms supported
   by Twice-NAT. Network address of hosts in external network are unique
   in external networks, but not within private network.  Likewise, the
   network address of hosts in private network are unique only within
   the private network. In other words, the address space used in
   private network to locate hosts in private and public networks is
   unrelated to the address space used in public network to locate hosts
   in private and public networks.  Twice NAT would not be allowed to
   advertise local networks to the external network or vice versa.

   A Twice NAT router in figure 2 would allow Host-A to initiate
   sessions to Host-X, and Host-X to initiate sessions to Host-A.
   However, N-Ext (or a subset of N-Ext) is not routable from within N-
   Pri, and N-Pri is not routable from N-Ext.

   Twice NAT is typically used when address space used in a Private
   network overlaps with addresses used in the Public space.  For
   example, say a private site uses the address space
   which is officially assigned to another site in the public internet.
   Host_A ( in Private space seeks to connect to Host_X
   ( in Public space. In order to make this connection
   work, Host_X's address is mapped to a different address for Host_A
   and vice versa. The twice NAT located at the Private site border may
   be configured as follows:

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       Private to Public : ->
       Public to Private : ->

       Datagram flow  : Host_A(Private) ->  Host_X(Public)

       a) Within private network

          DA:      SA:

       b) After twice-NAT translation

         DA:    SA:

       Datagram flow Host_X (Public) -> Host_A (Private)

       a) Within Public network

          DA:       SA:

       b) After twice-NAT translation, in private network

          SA:     DA:

4.4. Multihomed NAT

   There are limitations to using NAT. For example, requests and
   responses pertaining to a session must be routed via the same NAT
   router, as a NAT router maintains state information for sessions
   established through it. For this reason, it is often suggested that
   NAT routers be operated on a border router unique to a stub domain,
   where all IP packets are either originated from the domain or
   destined to the domain. However, such a configuration would turn a
   NAT router into a single point of failure.

   In order for a private network to ensure that connectivity with
   external networks is retained even as one of the NAT links fail, it
   is often desirable to multihome the private network to same or
   multiple service providers with multiple connections from the private
   domain, be it from same or different NAT boxes.

   For example, a private network could have links to two different
   providers and the sessions from private hosts could flow through the
   NAT router with the best metric for a destination. When one of NAT
   routers fail, the other could route traffic for all connections.

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   Multiple NAT boxes or multiple links on the same NAT box, sharing the
   same NAT configuration can provide fail-safe backup for each other.
   In such a case, it is necessary for backup NAT device to exchange
   state information so that a backup NAT can take on session load
   transparently when the primary NAT fails. NAT backup becomes simpler,
   when configuration is based on static maps.

5.0. Realm Specific IP (RSIP)

   "Realm Specific IP" (RSIP) is used to characterize the functionality
   of a realm-aware host in a private realm, which assumes realm-
   specific IP address to communicate with hosts in private or external

   A "Realm Specific IP Client" (RSIP client) is a host in a private
   network that adopts an address in an external realm when connecting
   to hosts in that realm to pursue end-to-end communication. Packets
   generated by hosts on either end in such a setup would be based on
   addresses that are end-to-end unique in the external realm and do not
   require translation by an intermediary process.

   A "Realm Specific IP Server" (RSIP server) is a node resident on both
   private and external realms, that can facilitate routing of external
   realm packets within a private realm. These packets may either have
   been originated by an RSIP client or directed to an RSIP-client.
   RSIP-Server may also be the same node that assigns external realm
   addresses to RSIP-Clients.

   There are two variations to RSIP, namely Realm-specific Address IP
   (RSA-IP) and Realm-Specific Address and Port IP (RSAP-IP). These
   variations are discussed in the following sub-sections.

5.1. Realm Specific Address IP (RSA-IP)

   A Realm Specific Address IP (RSA-IP) client adopts an IP address from
   the external address space when connecting to a host in external
   realm. Once an RSA-IP client assumes an external address, no other
   host in private or external domain can assume the same address, until
   that address is released by the RSA-IP client.

   The following is a discussion of routing alternatives that may be
   pursued for the end-to-end RSA-IP packets within private realm.  One
   approach would be to tunnel the packet to the destination. The outer
   header can be translated by NAT as normal without affecting the
   addresses used in the internal header. Another approach would be to
   set up a bi-directional tunnel between the RSA-IP Client and the
   border router straddling the two address realms. Packets to and from
   the client would be tunneled, but packets would be forwarded as

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   normal between the border router and the remote destination. Note,
   the tunnel from the client TO the border router may not be necessary.
   You might be able to just forward the packet directly. This should
   work so long as your internal network isn't filtering packets based
   on source addresses (which in this case would be external addresses).

   As an example, Host-A in figure 2 above, could assume an address
   Addr-k from the external realm and act as RSA-IP-Client to allow
   end-to-end sessions between Addr-k and Addr-X. Traversal of end-to-
   end packets within private realm may be illustrated as follows:

   First method, using NAT router enroute to translate:

   Host-A               NAT router               Host-X
   ------               -----------              ------






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   Second method, using a tunnel within private realm:

   Host-A               NAT router               Host-X
   ------               -----------              ------






   There may be other approaches to pursue.

   An RSA-IP-Client has the following characteristics. The collective
   set of operations performed by an RSA-IP-Client may be termed "RSA-

   1. Aware of the realm to which its peer nodes belong.

   2. Assumes an address from external realm when communicating with
      hosts in that realm. Such an address may be assigned statically
      or obtained dynamically (through a yet-to-be-defined protocol)
      from a node capable of assigning addresses from external realm.
      RSA-IP-Server could be the node coordinating external realm
      address assignment.

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   3. Route packets to external hosts using an approach amenable to
      RSA-IP-Server. In all cases, RSA-IP-Client will likely need
      to act as a tunnel end-point, capable of encapsulating
      end-to-end packets while forwarding and decapsulating in the
      return path.

   "Realm Specific Address IP Server" (RSA-IP server) is a node resident
   on both private and external realms, that facilitates routing of
   external realm packets specific to RSA-IP clients inside a private
   realm. An RSA-IP-Server may be described as having the following

   1. May be configured to assign addresses from external realm to
      RSA-IP-Clients, either statically or dynamically.

   2. Must be a router resident on both the private and external
      address realms.

   3. Must be able to provide a mechanism to route external realm
      packets within private realm. Of the two approaches described,
      the first approach requires RSA-IP-Server to be a NAT router
      providing transparent routing for the outer header. This
      approach requires the external peer to be a tunnel end-point.

      With the second approach, an RSA-IP-Server could be any router
      (including a NAT router) that can be a tunnel end-point with
      RSA-IP-Clients.  It would detunnel end-to-end packets outbound
      from RSA-IP-Clients and forward to external hosts. On the
      return path, it would locate RSA-IP-Client tunnel, based on the
      destination address of the end-to-end packet and encapsulate the
      packet in a tunnel to forward to RSA-IP-Client.

   RSA-IP-Clients may pursue any of the IPsec techniques, namely
   transport or tunnel mode Authentication and confidentiality using AH
   and ESP headers on the embedded packets. Any of the tunneling
   techniques may be adapted for encapsulation between RSA-IP-Client and
   RSA-IP-Server or between RSA-IP-Client and external host.  For
   example, IPsec tunnel mode encapsulation is a valid type of
   encapsulation that ensures IPsec authentication and confidentiality
   for the embedded end-to-end packets.

5.2 Realm Specific Address and port IP (RSAP-IP)

   Realm Specific Address and port IP (RSAP-IP) is a variation of RSIP
   in that multiple private hosts use a single external address,
   multiplexing on transport IDentifiers (i.e., TCP/UDP port numbers and
   ICMP Query IDs).

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   "RSAP-IP-Client" may be defined similar to RSA-IP-Client with the
   variation that RSAP-IP-Client assumes a tuple of (external address,
   transport Identifier) when connecting to hosts in external realm to
   pursue end-to-end communication. As such, communication with external
   nodes for an RSAP-IP-Client may be limited to TCP, UDP and ICMP

   "RSAP-IP-Server" is similar to RSA-IP-Server in that it facilitates
   routing of external realm packets specific to RSAP-IP clients inside
   a private realm. Typically, an RSAP-IP-Server would also be the one
   to assign transport tuples to RSAP-IP-Clients.

   A NAPT router enroute could serve as RSAP-IP-Server, when the outer
   encapsulation is TCP/UDP based and is addressed between the RSAP-IP-
   Client and external peer. This approach requires the external peer to
   be  the end-point of TCP/UDP based tunnel. Using this approach,
   RSAP-IP-Clients may pursue any of the IPsec techniques, namely
   transport or tunnel mode authentication and confidentiality using AH
   and ESP headers on the embedded packets.  Note however, IPsec tunnel
   mode is not a valid type of encapsulation, as a NAPT router cannot
   provide routing transparency to AH and ESP protocols.

   Alternately, packets may be tunneled between RSAP-IP-Client and
   RSAP-IP-Server such that RSAP-IP-Server would detunnel packets
   outbound from RSAP-IP-Clients and forward to external hosts. On the
   return path, RSAP-IP-Server  would locate RSAP-IP-Client tunnel,
   based on the tuple of (destination address, transport Identifier) and
   encapsulate the original packet within a tunnel to forward to RSAP-
   IP-Client. With this approach, there is no limitation on the
   tunneling technique employed between RSAP-IP-Client and RSAP-IP-
   Server. However, there are limitations to applying IPsec based
   security on end-to-end packets.  Transport mode based authentication
   and integrity may be attained.  But, confidentiality cannot be
   permitted because RSAP-IP-Server must be able to examine the
   destination transport Identifier in order to identify the RSAP-IP-
   tunnel to forward inbound packets to. For this reason, only the
   transport mode TCP, UDP and ICMP packets protected by AH and ESP-
   authentication can traverse a RSAP-IP-Server using this approach.

   As an example, say Host-A in figure 2 above, obtains a tuple of
   (Addr-Nx, TCP port T-Nx) from NAPT router to act as RSAP-IP-Client to
   initiate end-to-end TCP sessions with Host-X.  Traversal of end-to-
   end packets within private realm may be illustrated as follows. In
   the first method, outer layer of the outgoing packet from Host-A uses
   (private address Addr-A, source port T-Na) as source tuple to
   communicate with Host-X. NAPT router enroute translates this tuple
   into (Addr-Nx, Port T-Nxa). This translation is independent of RSAP-
   IP-Client tuple parameters used in the embedded packet.

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   First method, using NAPT router enroute to translate:

   Host-A               NAPT router              Host-X
   ------               -----------              ------






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   Second method, using a tunnel within private realm:

   Host-A               NAPT router              Host-X
   ------               -----------              ------






6.0. Private Networks and Tunnels

   Let us consider the case where your private network is connected to
   the external world via tunnels. In such a case, tunnel encapsulated
   traffic may or may not contain translated packets depending upon the
   characteristics of address realms a tunnel is bridging.

   The following subsections discuss two scenarios where tunnels are
   used (a) in conjunction with Address translation, and (b) without

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6.1. Tunneling translated packets

   All variations of  address translations discussed in the previous
   section can be applicable to direct connected links as well as
   tunnels and virtual private networks (VPNs).

   For example, a private network connected to a business partner
   through a VPN could employ traditional NAT to communicate with the
   partner. Likewise, it is possible to employ twice NAT, if the
   partner's address space overlapped with the private network.  There
   could be a NAT device on one end of the tunnel or on both ends of the
   tunnel. In all cases, traffic across the VPN can be encrypted for
   security purposes. Security here refers to security for traffic
   across VPNs alone. End-to-end security requires trusting NAT devices
   within private network.

6.2. Backbone partitioned private Networks

   There are many instances where a private network (such as a corporate
   network) is spread over different locations and use public backbone
   for communications between those locations. In such cases, it is not
   desirable to do address translation, both because large numbers of
   hosts may want to communicate across the backbone, thus requiring
   large address tables, and because there will be more applications
   that depend on configured addresses, as opposed to going to a name
   server. We call such a private network a backbone-partitioned private

   Backbone-partitioned stubs should behave as though they were a non-
   partitioned stub. That is, the routers in all partitions should
   maintain routes to the local address spaces of all partitions. Of
   course, the (public) backbones do not maintain routes to any local
   addresses. Therefore, the border routers must tunnel (using VPNs)
   through the backbones using encapsulation.  To do this, each NAT box
   will set aside a global address for tunneling.

   When a NAT box x in stub partition X wishes to deliver a packet to
   stub partition Y, it will encapsulate the packet in an IP header with
   destination address set to the global address of NAT box y that has
   been reserved for encapsulation. When NAT box y receives a packet
   with that destination address, it decapsulates the IP header and
   routes the packet internally.  Note, there is no address translation
   in the process; merely transfer of private network packets over an
   external network tunnel backbone.

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7.0. NAT operational characteristics

   NAT devices are application unaware in that the translations are
   limited to IP/TCP/UDP/ICMP headers and ICMP error messages only.  NAT
   devices do not change the payload of the packets, as payloads tend to
   be application specific.

   NAT devices (without the inclusion of ALGs) do not examine or modify
   transport payload. For this reason, NAT devices are transparent to
   applications in many cases. There are two areas, however, where NAT
   devices often cause difficulties: 1) when an application payload
   includes an IP address, and 2) when end-to-end security is needed.
   Note, this is not a comprehensive list.

   Application layer security techniques that do not make use of or
   depend on IP addresses will work correctly in the presence of NAT
   (e.g., TLS,  SSL and ssh). In contrast, transport layer techniques
   such as IPSec transport mode or the TCP MD5 Signature Option RFC 2385
   [Ref 17] do not.

   In IPSec transport mode, both AH and ESP have an integrity check
   covering the entire payload. When the payload is TCP or UDP, the
   TCP/UDP checksum is covered by the integrity check. When a NAT device
   modifies an address the checksum is no longer valid with respect to
   the new address. Normally, NAT also updates the checksum, but this is
   ineffective when AH and ESP are used.  Consequently, receivers will
   discard a packet either because it fails the IPSec integrity check
   (if the NAT device updates the checksum), or because the checksum is
   invalid (if the NAT device leaves the checksum unmodified).

   Note that IPsec tunnel mode ESP is permissible so long as the
   embedded packet contents are unaffected by the outer IP header
   translation. Although this technique does not work in traditional NAT
   deployments (i.e., where hosts are unaware that NATs are present),
   the technique is applicable to Realm-Specific IP as described in
   Section 5.0.

   Note also that end-to-end ESP based transport mode authentication and
   confidentiality are permissible for packets such as ICMP, whose IP
   payload content is unaffected by the outer IP header translation.

   NAT devices also break fundamental assumptions by public key
   distribution infrastructures such as Secure DNS RFC 2535 [Ref 18] and
   X.509 certificates with signed public keys. In the case of Secure

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   DNS, each DNS RRset is signed with a key from within the zone.
   Moreover, the authenticity of a specific key is verified by following
   a chain of trust that goes all the way to the DNS root.  When a DNS-
   ALG modifies addresses (e.g., as in the case of Twice-NAT),
   verification of signatures fails.

   It may be of interest to note that IKE (Session key negotiation
   protocol) is a UDP based session layer protocol and is not protected
   by network based IPsec security. Only a portion of the individual
   payloads within IKE are protected. As a result, IKE sessions are
   permissible across NAT, so long as IKE payload does not contain
   addresses and/or transport IDs specific to one realm and not the
   other. Given that IKE is used to setup IPSec associations, and there
   are at present no known ways of making IPSec work through a NAT
   function, it is a future work item to take advantage of IKE through a
   NAT box.

   One of the most popular internet applications "FTP" would not work
   with the definition of NAT as described. The following sub-section is
   devoted to describing how FTP is supported on NAT devices.  FTP ALG
   is an integral part of most NAT implementations. Some vendors may
   choose to include additional ALGs to custom support other
   applications on the NAT device.

7.1. FTP support

   "PORT" command and "PASV" response in FTP control session payload
   identify the IP address and TCP port that must be used for the data
   session it supports. The arguments to the PORT command and PASV
   response are an IP address and a TCP port in ASCII. An FTP ALG is
   required to monitor and update the FTP control session payload so
   that information contained in the payload is relevant to end nodes.
   The ALG must also update NAT with appropriate data session tuples and
   session orientation so that NAT could set up state information for
   the FTP data sessions.

   Because the address and TCP port are encoded in ASCII, this may
   result in a change in the size of packet.  For instance,
   10,18,177,42,64,87 is 18 ASCII characters, whereas
   193,45,228,137,64,87 is 20 ASCII characters. If the new size is same
   as the previous, only the TCP checksum needs adjustment as a result
   of change of data. If the new size is less than or greater than the
   previous, TCP sequence numbers must also be changed to reflect the
   change in length of FTP control data portion.  A special table may be
   used by the ALG to correct the TCP sequence and acknowledge numbers.
   The sequence number and acknowledgement correction will need to be
   performed on all future packet of the connection.

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8.0. NAT limitations

8.1. Applications with IP-address Content

   Not All applications lend themselves easily to address translation by
   NAT devices. Especially, the applications that carry IP address (and
   TU port, in case of NAPT) inside the payload. Application Level
   Gateways, or ALGs must be used to perform translations on packets
   pertaining to such applications. ALGs may optionally utilize address
   (and TU port) assignments made by NAT and perform translations
   specific to the application. The combination of NAT functionality and
   ALGs will not provide end-to-end security assured by IPsec.  However,
   tunnel mode IPsec can be accomplished with NAT router serving as
   tunnel end point.

   SNMP is one such application with address content in payload. NAT
   routers would not translate IP addresses within SNMP payloads. It is
   not uncommon for an SNMP specific ALG to reside on a NAT router to
   perform SNMP MIB translations proprietary to the private network.

8.2. Applications with inter-dependent control and data sessions

   NAT devices operate on the assumption that each session is
   independent.  Session characteristics like session orientation,
   source and destination IP addresses, session protocol, and source and
   destination transport level identifiers are determined independently
   at the start of each new session.

   However, there are applications such as H.323 that use one or more
   control sessions to set the characteristics of the follow-on sessions
   in their control session payload. Such applications require use of
   application specific ALGs that can interpret and translate the
   payload, if necessary. Payload interpretation would help NAT be
   prepared for the follow-on data sessions.

8.3. Debugging Considerations

   NAT increases the probability of mis-addressing. For example, same
   local address may be bound to different global address at different
   times and vice versa. As a result, any traffic flow study based
   purely on global addresses and TU ports could be confused and might
   misinterpret the results.

   If a host is abusing the Internet in some way (such as trying to
   attack another machine or even sending large amounts of junk mail or
   something) it is more difficult to pinpoint the source of the trouble
   because the IP address of the host is hidden in a NAT router.

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8.4. Translation of fragmented FTP control packets

   Translation of fragmented FTP control packets is tricky when the
   packets contain "PORT" command or response to "PASV" command.
   Clearly, this is a pathological case. NAT router would need to
   assemble the fragments together first and then translate prior to

   Yet another case would be when each character of packets containing
   "PORT" command or response to "PASV" is sent in a separate datagram,
   unfragmented. In this case, NAT would simply have to let the packets
   through, without translating the TCP payload. Of course, the
   application will fail if the payload needed to be altered. The
   application could still work in a few cases, where the payload
   contents can be valid in both realms, without modifications enroute.
   For example, FTP originated from a private host would still work
   while traversing a traditional NAT or bi-directional NAT device, so
   long as the FTP control session employed PASV command to establish
   data sessions. The reason being that the address and port number
   specified by FTP server in the PASV response (sent as multiple
   unfragmented packets) is valid to the private host, as is. The NAT
   device will simply view the ensuing data session (also originating
   from private host) as an independent TCP session.

8.5. Compute intensive

   NAT is compute intensive even with the help of a clever checksum
   adjustment algorithm, as each data packet is subject to NAT lookup
   and modifications.  As a result, router forwarding throughput could
   be slowed considerably. However, so long as the processing capacity
   of the NAT device exceeds line processing rate, this should not be a

9.0. Security Considerations

   Many people view traditional NAT router as a one-way (session)
   traffic filter, restricting sessions from external hosts into their
   machines. In addition, when address assignment in NAT router is done
   dynamically, that makes it harder for an attacker to point to any
   specific host in the NAT domain. NAT routers may be used in
   conjunction with firewalls to filter unwanted traffic.

   If NAT devices and ALGs are not in a trusted boundary, that is a
   major security problem, as ALGs could snoop end user traffic payload.
   Session level payload could be encrypted end to end, so long as the
   payload does not contain IP addresses and/or transport identifiers
   that are valid in only one of the realms. With the exception of RSIP,
   end-to-end IP network level security assured by current IPsec

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   techniques is not attainable with NAT devices in between. One of the
   ends must be a NAT box. Refer section 7.0 for a discussion on why
   end-to-end IPsec security cannot be assured with NAT devices along
   the route.

   The combination of NAT functionality, ALGs and firewalls will provide
   a transparent working environment for a private networking domain.
   With the exception of RSIP, end-to-end network security assured by
   IPsec cannot be attained for end-hosts within the private network
   (Refer section 5.0 for RSIP operation). In all other cases, if you
   want to use end-to-end IPsec, there cannot be a NAT device in the
   path. If we make the assumption that NAT devices are part of a
   trusted boundary, tunnel mode IPsec can be accomplished with NAT
   router (or a combination of NAT, ALGs and firewall) serving as tunnel
   end point.

   NAT devices, when combined with ALGs, can ensure that the datagrams
   injected into Internet have no private addresses in headers or
   payload. Applications that do not meet these requirements may be
   dropped using firewall filters. For this reason, it is not uncommon
   to find NAT, ALG and firewall functions co-exist to provide security
   at the borders of a private network. NAT gateways can be used as
   tunnel end points to provide secure VPN transport of packet data
   across an external network domain.

   Below are some additional security considerations associated with NAT

   1. UDP sessions are inherently unsafe. Responses to a datagram
      could come from an address different from the target address
      used by sender ([Ref 4]). As a result, an incoming UDP packet
      might match the outbound session of a traditional NAT router
      only in part (the destination address and UDP port number of
      the packet match, but the source address and port number may
      not). In such a case, there is a potential security compromise
      for the NAT device in permitting inbound packets with partial
      match. This UDP security issue is also inherent to firewalls.

      Traditional NAT implementations that do not track datagrams on
      a per-session basis but lump states of multiple UDP sessions
      using the same address binding into a single unified session
      could compromise the security even further. This is because,
      the granularity of packet matching would be further limited to
      just the destination address of the inbound UDP packets.

   2. Multicast sessions (UDP based) are another source for security
      weakness for traditional-NAT routers. Once again, firewalls face
      the same security dilemma as the NAT routers.

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      Say, a host on private network initiated a multicast session.
      Datagram sent by the private host could trigger responses in the
      reverse direction from multiple external hosts. Traditional-NAT
      implementations that use a single state to track a multicast
      session cannot determine for certain if the incoming UDP packet
      is in response to an existing multicast session or the start of
      new UDP session initiated by an attacker.

   3. NAT devices can be a target for attacks.

      Since NAT devices are Internet hosts they can be the target of a
      number of different attacks, such as SYN flood and ping flood
      attacks. NAT devices should employ the same sort of protection
      techniques as Internet-based servers do.


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

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

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

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

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

   [6]  Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", STD
        9, RFC 959, October 1985.

   [7]  Postel, J., "Transmission Control Protocol (TCP) Specification",
        STD 7,  RFC 793, September 1981.

   [8]  Postel, J., "Internet Control Message Protocol Specification"
        STD 5, RFC 792, September 1981.

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

   [10] Mogul, J. and J. Postel, "Internet Standard Subnetting
        Procedure", STD 5, RFC 950, August 1985.

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RFC 2663           NAT Terminology and Considerations        August 1999

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

   [12] Kent, S. and  R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [13] Kent, S. and  R. Atkinson, "IP Encapsulating Security Payload
        (ESP)", RFC 2406, November 1998.

   [14] Kent, S. and  R. Atkinson, "IP Authentication Header", RFC 2402,
        November 1998.

   [15] Harkins, D. and  D. Carrel, "The Internet Key Exchange (IKE)",
        RFC 2409, November 1998.

   [16] Piper, D., "The Internet IP Security Domain of Interpretation
        for ISAKMP", RFC 2407, November 1998.

   [17] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
        Signature Option", RFC 2385, August 1998.

   [18] Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

Authors' Addresses

   Pyda Srisuresh
   Lucent Technologies
   4464 Willow Road
   Pleasanton, CA 94588-8519

   Phone: (925) 737-2153
   Fax:   (925) 737-2110

   Matt Holdrege
   Lucent Technologies
   1701 Harbor Bay Parkway
   Alameda, CA 94502

   Phone: (510) 769-6001

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Full Copyright Statement

   Copyright (C) The Internet Society (1999).  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
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   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.

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