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Versions: 00 01 02 03 RFC 2663

NAT Working Group                                           P. Srisuresh
INTERNET-DRAFT                                       Lucent Technologies
Category: Informational                                    Matt Holdrege
Expire in six months                               Ascend Communications
                                                               June 1999

   IP Network Address Translator (NAT) Terminology and Considerations
             <draft-ietf-nat-terminology-03.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF),
   its areas, and its working groups.  Note that other groups may
   also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-
   Drafts as reference material or to cite them other than as
   "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Preface

   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 draft, 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

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   review of the document and adding clarity to the text.

Abstract

   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.

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 devices.

   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

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   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 reference.

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
   a IPv4 environment and does not address the use of NAT in any 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

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   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).

   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) enroute.

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.

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   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 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.

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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 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,

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   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
   characteristics.

          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.

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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
   implementations.

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.

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   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 updated.

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.

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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 network.

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

   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.

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

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   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-NAT'.

   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
   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

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   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 200.200.200.0/24 address
   space which is officially assigned to another site in the public
   internet. Host_A (200.200.200.1) in Private space seeks to connect
   to Host_X (200.200.200.100) 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 :

       Private to Public : 200.200.200.0/24 -> 138.76.28.0/24
       Public to Private : 200.200.200.0/24 -> 172.16.1.0/24

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

       a) Within private network

          DA: 172.16.1.100      SA: 200.200.200.1

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       b) After twice-NAT translation

         DA: 200.200.200.100    SA: 138.76.28.1

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

       a) Within Public network

          DA: 138.76.28.1       SA: 200.200.200.100

       b) After twice-NAT translation, in private network

          SA: 200.200.200.1     DA: 172.16.1.100

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.

   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.

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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 realm.

   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 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).

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   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
   ------               -----------              ------

   <Outer IP header, with
   src=Addr-A, Dest=Addr-X>,
   embedding
   <End-to-end packet, with
   src=Addr-k, Dest=Addr-X>
   ----------------------------->

                        <Outer IP header, with
                        src=Addr-k, Dest=Addr-X>,
                        embedding
                        <End-to-end packet, with
                        src=Addr-k,  Dest=Addr-X>
                        --------------------------->

                             .
                             .
                             .

                                              <Outer IP header, with
                                              src=Addr-X, Dest=Addr-k>,
                                              embedding
                                              <End-to-end packet, with
                                              src=Addr-X, Dest=Addr-k>
                                     <---------------------------------

                        <Outer IP header, with
                        src=Addr-X, Dest=Addr-A>,
                        embedding <End-to-end packet,
                        with src=Addr-X, Dest=Addr-k>
              <--------------------------------------

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   Second method, using a tunnel within private realm:
   ==================================================

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

   <Outer IP header, with
   src=Addr-A, Dest=Addr-Np>,
   embedding
   <End-to-end packet, with
   src=Addr-k, Dest=Addr-X>
   ----------------------------->

                        <End-to-end packet, with
                        src=Addr-k, Dest=Addr-X>
                        ------------------------------->

                             .
                             .
                             .

                                             <End-to-end packet, with
                                             src=Addr-X, Dest=Addr-k>
                                    <--------------------------------

                        <Outer IP header, with
                        src=Addr-Np, Dest=Addr-A>,
                        embedding <End-to-end packet,
                        with src=Addr-X, Dest=Addr-k>
                  <----------------------------------

   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-IP".

   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.

Srisuresh & Holdrege                                           [Page 18]

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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 characteristics.

   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.

5.1. 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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Internet Draft     NAT Terminology and Considerations          June 1999

   "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 sessions.

   "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

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Internet Draft     NAT Terminology and Considerations          June 1999

   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.

   First method, using NAPT router enroute to translate:
   ====================================================

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

   <Outer TCP/UDP packet, with
   src=Addr-A, Src Port=T-Na,
   Dest=Addr-X>,
   embedding
   <End-to-end packet, with
   src=Addr-Nx, Src Port=T-Nx, Dest=Addr-X>
   ----------------------------->

                        <Outer TCP/UDP packet, with
                        src=Addr-Nx, Src Port=T-Nxa,
                        Dest=Addr-X>,
                        embedding
                        <End-to-end packet, with
                        src=Addr-Nx, Src Port=T-Nx, Dest=Addr-X>
                        --------------------------------------->

                             .
                             .
                             .

                                             <Outer TCP/UDP packet with
                                             src=Addr-X, Dest=Addr-Nx,
                                             Dest Port=T-Nxa>,
                                             embedding
                                             <End-to-end packet, with
                                             src=Addr-X, Dest=Addr-Nx,
                                             Dest Port=T-Nx>
                                     <----------------------------------

                        <Outer TCP/UDP packet, with
                        src=Addr-X, Dest=Addr-A,
                        Dest Port=T-Na>,
                        embedding
                        <End-to-end packet, with
                        src=Addr-X, Dest=Addr-Nx,
                        Dest Port=T-Nx>
              <-----------------------------------

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   Second method, using a tunnel within private realm:
   ==================================================

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

   <Outer IP header, with
   src=Addr-A, Dest=Addr-Np>,
   embedding
   <End-to-end packet, with
   src=Addr-Nx, Src Port=T-Nx,
   Dest=Addr-X>
   ----------------------------->

                        <End-to-end packet, with
                        src=Addr-Nx, Src Port=T-Nx,
                        Dest=Addr-X>
                        -------------------------------->

                             .
                             .
                             .

                                             <End-to-end packet, with
                                             src=Addr-X, Dest=Addr-Nx,
                                             Dest Port=T-Nx>
                                   <----------------------------------

                        <Outer IP header, with
                        src=Addr-Np, Dest=Addr-A>,
                        embedding
                        <End-to-end packet, with
                        src=Addr-X, Dest=Addr-Nx,
                        Dest Port=T-Nx>
                <----------------------------------

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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Internet Draft     NAT Terminology and Considerations          June 1999

   translation.

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 network.

   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 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 DNS, each DNS RRset is signed with a key from within the
   zone. Moreover, the authenticity of a specific key is verified by

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   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 forwarding.

   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
   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 problem.

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

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Internet Draft     NAT Terminology and Considerations          June 1999

   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 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 routers.

   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.

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Internet Draft     NAT Terminology and Considerations          June 1999

   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.

      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.

REFERENCES

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

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

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

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

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

   [6] J. Postel, J. Reynolds, "FILE TRANSFER PROTOCOL(FTP)", RFC 959

   [7] "TRANSMISSION CONTROL PROTOCOL (TCP) SPECIFICATION",  RFC 793

   [8] J. Postel, "INTERNET CONTROL MESSAGE PROTOCOL SPECIFICATION",
       RFC 792

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

   [10] J. Mogul, J. Postel, "Internet Standard Subnetting Procedure",
        RFC 950

   [11] Brian carpenter, Jon Crowcroft, Yakov Rekhter, "IPv4 Address

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Internet Draft     NAT Terminology and Considerations          June 1999

        Behavior Today", RFC 2101

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

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

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

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

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

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

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

Authors' Addresses

   Pyda Srisuresh
   Lucent technologies
   4464 Willow Road
   Pleasanton, CA 94588-8519
   U.S.A.

   Voice: (925) 737-2153
   Fax:   (925) 737-2110
   EMail: suresh@ra.lucent.com

   Matt Holdrege
   Ascend Communications, Inc.
   One Ascend Plaza
   1701 Harbor Bay Parkway
   Alameda, CA 94502

   Voice: (510) 769-6001
   Fax:   (510) 814-2300
   EMail: matt@ascend.com

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