Internet Engineering Task Force
Internet Draft                                     Jordi Palet
Document: draft-palet-v6ops-proto41-nat-03.txt     Cesar Olvera
Category:                                          David Fernandez
Expires: April 2004                                October 2003

                    Forwarding Protocol 41 in NAT Boxes


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
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   Some IPv4-only NAT boxes/routers allow the establishment of IPv6
   tunnels from systems in the private LAN (using private IPv4
   addresses) to routers or tunnel servers in the public Internet.

   As far as we know [2] this is not a common way of using IPv6 tunnels;
   the usual way is to finish the tunnel directly in a device with an
   IPv4 public address.

   This behavior provides a big opportunity to rapidly deploy a huge
   number of IPv6 nodes and networks, without the need of new transition
   mechanism. This option is very important to facilitate the IPv6
   deployment when is not possible to offer native IPv6 or 6to4 [3].

   From this point of view, this mechanism should be considered only as
   a temporary solution until native IPv6 routers, or those that support
   6to4, will become widely available.

   Not all the IPv4-only NAT boxes/routers support this mechanism, but
   this document describes this behavior and consequently provides hints
   that should be applied in the IPv4-only NAT boxes and tunnel brokers
   to facilitate it.

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Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [4].

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Table of Contents

   1. Introduction...................................................4

   2. Rationale of proto-41 forwarding...............................6

   3. Behavior of different NAT types................................7

      3.1 3.1 Traditional (or) Outbound NAT..........................7

      3.2 Bi-directional (or) Two-Way NAT............................8

   4. Applicability..................................................8

   5. NAT design considerations and recommendations..................9

   6. Tunnel broker design considerations...........................10

   7. Security Considerations.......................................10

   8. References....................................................11


   Authors' Addresses...............................................11

   Intellectual Property Statement..................................12

   Full Copyright Statement.........................................12


1. Introduction

   Most of the existing solutions for the transition to IPv6 rely in
   tunnels assuming that the client end-point is an IPv6 capable router.
   However, nowadays the installed base of IPv4-only NAT boxes/routers
   is still quite big, while most of the client operating systems
   already support IPv6.

   The ability of some IPv4-only NAT boxes/routers to establish IPv6
   tunnels from systems inside the private LAN (even using private IPv4
   addresses) to routers or tunnel servers in the public Internet has
   been used for some time. However, it has not been documented so far.

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   The goal of this document is to describe in detail that functionality
   and to show the rationale behind it, as well as to provide some
   recommendations for IPv4-only NAT boxes and tunnel broker
   implementers in order to facilitate its use and deployment.

   The basic scenario of the mechanism presented is shown in the Figure
   below. As can be seen, a Tunnel Client (a host or a router), which is
   connected to Internet through an IPv4-only NAT box using a private
   IPv4 address, establishes an IPv6 tunnel to a Tunnel Server with the
   help of a Tunnel Broker. The mechanism can also be used without a
   tunnel broker, ending the tunnel in an IPv6 router, which is
   configured manually.

                            (    )
                           ( IPv6 )
        +--------+        +--------+
        | Tunnel |________| Tunnel |
        | Broker |        | Server |
        +--------+        +--------+
                  \      /     |
                   \____/      |
                   (    )      |
                  ( IPv4 )     |
                   (____)      | IPv6 Tunnel
                     |         |
                        Public IPv4 |         |
                  +-----+      |
                  | NAT |      |
                  | Box |      |
                  +-----+      |
                     |         |
                       Private IPv4 |         |
                +--------+     |
                | Tunnel |------
                | Client |------> (possible IPv6 or dual stack network)

   Typically, IPv6 routers on the Tunnel Server side support the
   establishment of these tunnels without any additional configuration.
   However, in the case of some clients under certain operating systems,
   the tunnel configuration process or the tunnel broker scripts have to
   be modified to reflect the private/public addressing conversion.

   This fact should be taken into consideration by tunnel broker
   implementations in future versions, in order to properly create the
   script in case the client is located in a private network.

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   This document describes the reasons why this scenario works as it is
   using present NAT implementations. We consider that exploring this
   option is very important to facilitate the IPv6 deployment, as it can
   be used as a temporary fallback solution when neither native IPv6 nor
   6to4 mechanisms are available.

   The document does not discuss how the local private network is
   organized, for example, in case the Tunnel Client is an IPv6 router
   providing IPv6 connectivity to other systems. The behavior in this
   case should be the same as any other IPv6 native network (that is
   using stateless or stateful autoconfiguration, or any other typical
   functionalities like Home Agent, etc).

   Although this mechanism is not usable on all existing IPv4-only NAT
   boxes/routers, the large number of them that already support it gives
   an opportunity to rapidly deploy a huge number of IPv6 nodes and
   networks (in case the node behind the NAT is an IPv6 router) without
   the need of using or designing new transition mechanisms.

   The scenario presented has been tested with several IPv4-only NAT
   boxes that have successfully established IPv6 tunnels between tunnel
   clients in a private network and tunnel servers in the public
   Internet. In these test, we have used three well-known Tunnel Broker
   implementations (BT, Freenet6 and TILAB) as well as manually
   configured tunnels with routers from several manufacturers.

2. Rationale of proto-41 forwarding

   As described in RFC 2663 [5]:

   "Address translations performed by NAT are session based and would
   include translation of incoming as well as outgoing packets belonging
   to that session ... 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)."

   Basically, what the NAT router does in the scenario presented in this
   document is a network address translation for protocol identifier 41
   (the one used for IPv6 over IPv4 tunnels). The router considers each
   tuple of the form [source IP address, target IP address, IP protocol
   (41)] a different session, and typically creates a new proto 41 entry
   in its table whenever an IPv6 over IPv4 packet flows from the private
   network to the Internet (as it does, for example, for TCP

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3. Behavior of different NAT types

   As mentioned before, some NAT routers do not support protocol 41
   forwarding. They are usually limited to do network address
   translation for common protocols like TCP, UDP and ICMP. This type of
   NAT routers will not be considered in this document, although some
   recommendations for them will be given in section 5.

   RFC 2663 [5] distinguishes several types of NAT routers. This
   document focuses on how proto 41 forwarding works over the two most
   common types: Traditional NAT and NAPT.

3.1 3.1 Traditional (or) Outbound NAT

   In traditional NAT routers sessions are unidirectional. This means
   that, as IPv6 tunnels are treated as any other NAT dynamic session,
   the tunnel entries are only added to the table whenever an IPv6
   packet is sent from the private network to the public Internet, but
   not with packets flowing in the opposite direction (i.e., coming from
   the external tunnel endpoint).

   Usually, an inactivity timer is started when the NAT entry is
   created, so that the session (and consequently the tunnel) is deleted
   if no packets are sent during the inactivity period (a few minutes
   typically). In case the tunnel entry is deleted due to inactivity, it
   will be created again whenever a new packet is sent from the private

   RFC 2663 distinguish between two types of traditional NAT routers:
   Basic NAT and NAPT. Basic NATs do the address translation by means of
   a one-to-one association between private and public addresses, so
   entries on the NAT table have the form [private address, public

   In the case of NAPT, which is the most widely used at present, each
   public address can be shared among several private systems, by using
   different transport ports for each one. Entries in NAT table have the
   form [source IP address, source TCP/UDP port, target IP address,
   target TCP/UDP port]. Both types can be combined in the same NAT

   Support for protocol 41 forwarding in Traditional NAT routers
   basically means that they should be prepared to manage sessions of
   the form [source IP address, destination IP address, protocol ID] for
   protocol 41.

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3.2 Bi-directional (or) Two-Way NAT

   As stated in [5], “with a Bidirectional 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”.

   RFC 2663 mentions the use of a DNS-ALG algorithm in order to allow
   public hosts to communicate with private ones. Basically, whenever a
   DNS query is made for a private host name, the DNS-ALG in conjunction
   with Bi-directional NAT answers the query using an available public
   address and sets the corresponding NAT table entry.

   However, this mechanism does not fit proto-41 forwarding
   requirements, as no names are normally involved when setting up

   What it is needed in our case is just the basic mechanism included in
   bi-directional NAT to statically associate one of the private
   addresses with a public address, only for protocol 41.

   In this way, all ingoing IPv6 over IPv4 traffic will be forwarded to
   the designated internal system and the tunnel will work in a complete
   bidirectional way, even when no outgoing traffic is generated. IN
   that case, the inactivity timer will probably not be needed, as the
   entry on NAT table for outgoing traffic could also be statically
   configured (by means of a configuration file, http interface, CLI,

4. Applicability

   In the case of Basic NAT and NAPT, IPv6 tunnels can only be initiated
   by inside-to-outside sessions. So in this case, outside-to-inside
   sessions only work whenever a previous inside-to-outside session has
   created the proto-41 entry in the NAT table and the inactivity
   timeout has not been reached.

   This fact is only a problem when IPv6 servers or services inside the
   private network are needed to be accessible from outside. If the
   traffic is client initiated, the session will be created normally as
   soon as the first packet is sent, allowing IPv6 communication.

   The only way to maintain the session permanently is to constantly
   send traffic, for example, with a periodic ping from the Tunnel
   Client, a router solicitation message, or other means. Alternatively,
   some simple keep-alive protocol could be integrated inside tunnel
   broker implementations in order to maintain the tunnel.

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   In the case of Bi-directional NATs, there are means to support also
   incoming sessions, even when no outgoing traffic haa been generated.
   However, they require some type of pre-configuration in the IPv4-only
   NAT box.

   To facilitate that, a default configuration could be defined. For
   example, in the case of simple NAT routers used in most SOHO
   accesses, the default configuration could include a pre-defined
   private network address for the LAN interface and a pre-defined
   private address for the host where all the proto-41 traffic is

   In summary, the application of proto-41 forwarding procedure allows
   in both cases the operation of private IPv6 networks connected by
   means of non-IPv6 aware NAT boxes to tunnel brokers or manually
   configured tunnels.

   The most usual scope of application of the proto-41 forwarding
   procedure described in this document seems to be SOHO and home
   environments, but it is not only limited to those scenarios.

5. NAT design considerations and recommendations

   This document has been written following a survey with users/vendors
   of different IPv4-only NAT boxes, and the conclusion is that most of
   the manufacturers support protocol-41 forwarding (78% in our survey).
   Nevertheless not all support a bidirectional mode (over 22% of the
   surveyed models do not support it).

   NAT boxes should tend to support native IPv6. If this is not
   feasible, 6to4 should be the second option, and as a last resort,
   proto-41 forwarding.

   6to4 and Proto-41 forwarding can coexist in the same NAT box. In that
   case, an IPv6 over IPv4 packet received, will be forwarded to the
   private LAN only if the IPv6 destination does not belong to the local
   6to4 /48 prefix. Otherwise it will be decapsulated in the NAT box,
   following 6to4 procedures. This fact avoids the problems created by
   mobile users when they visit a network that uses 6to4, in the case
   they have some automatic proto-41 setup.

   New firmware/software versions of the NAT implementations should
   ensure the support of protocol-41 forwarding, as a temporary
   solution, while they are not supporting native IPv6 or 6to4.

   Proto-41 make sense only in IPv4-only routers, but nevertheless, when
   these routers are upgraded to support, for example, 6to4, for

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   compatibility reasons (with existing network configurations), it
   could be still considered to maintain the support proto-41.

   In addition, considering that the code changes needed to support a
   full bidirectional NAT will be minimum, this option should also be
   considered, at least as a configurable option, in an easy way by the
   user (very simple http interface).

   Proto-41 adds an inexpensive feature to existing IPv4-only NAT boxes,
   facilitating the gradual transition to IPv6, while preserving the
   users investment in the existing IPv4 network.

6. Tunnel broker design considerations

   New releases of tunnel brokers should provide means to cope with the
   scenario defined in this document. They should automatically detect
   it or, at least, they should allow the user to specify manually that
   a NAT router is present between the tunnel client and server.

   According to that the tunnel broker must properly create the script
   or configuration file that will setup the client tunnel endpoint. In
   that case they should have requested the public addresses (can be
   automatically detected) and the local interface ID or name of the
   tunnel client.

7. Security Considerations

   It is important to note that IPv6 applications sending traffic over
   the tunnels described here do not suffer the restrictions that apply
   to NAT traversal scenarios, because NAT is made to IPv4 packets that
   transport IPv6 ones, not to IPv6 packets.

   Besides, the protection derived from the unidirectional nature of NAT
   disappears for IPv6. Therefore, some security mechanism (network or
   personal firewalls) could be necessary to protect IPv6 systems in the
   private network.

   A possible security problem is the one related to the DoS Attack than
   can be created if a host in the local network, behind the NAT sends
   IPv6 packets (using protocol 41) to the tunnel endpoint, simulating
   to be the original "owner" of the tunnel. The behavior of the IPv4-
   only NAT box will define the success or failure of this attack. In
   any case, it seems not reasonable that this happens in small networks
   (SOHO and home environments), where the attacker can be easily

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   This considerations are generic to transition mechanisms, as
   described in [6].

8. References

   1  S. Bradner, "The Internet Standards Process -- Revision 3", BCP 9,
      RFC 2026, October 1996.

   2  J. Palet, C. Olvera, D. Fernandez, "IPv6 Tunnels through Routers
      with NAT", Euro6IX Project,
      consulintel_wp4_ipv6_tunnels_nat_v1_6.pdf, April 2003.

   3  B. Carpenter, K. Moore, ”Connection of IPv6 Domains via IPv4
      Clouds”, RFC 3056, February 2001.

   4  S. Bradner, "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

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

   6  P. Savola, ”IPv6 Transition/Co-existence Security Considerations”,
      draft-savola-v6ops-security-overview-00, June 2003 (work in


   The authors would also like to acknowledge the inputs from Tim Chown,
   Miguel Angel Diaz, Alain Durand, Jun-ichiro "itojun" Hagino, Keith
   Moore, Mariana Nikolova, Rute C. Sofia and the European Commission
   support in the co-funding of the Euro6IX project, where this work is
   being developed.

Authors' Addresses

   Jordi Palet Martinez
   San Jose Artesano, 1
   28108 - Alcobendas (Madrid - Spain)
   Phone: +34 91 151 81 99
   Fax:   +34 91 151 81 98

   Cesar Olvera Morales

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   San Jose Artesano, 1
   28108 - Alcobendas (Madrid - Spain)
   Phone: +34 91 151 81 99
   Fax:   +34 91 151 81 98

   David Fernandez
   Technical University of Madrid (UPM)
   Ciudad Universitaria s/n
   28040 – Madrid (Spain)
   Phone: +34 91 549 57 00
   Fax:   +34 91 336 73 33

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