Internet Engineering Task Force                         A.Durand
INTERNET-DRAFT                             SUN Microsystems,inc.
June, 24, 2004                                         F. Parent
Expires December 23, 2004                                 Hexago

                  Requirements for assisted tunneling

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

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   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document defines requirements for a tunnel set-up protocol that
   could be used by an ISP to jumpstart its IPv6 offering to its
   customers by providing them IPv6 connectivity without having to
   upgrade its access network.

1. Goal and Scope of the Document

   The v6ops working group has worked on requirements and scenarios for
   IPv6 deployment by soliciting input from network operators. This work
   has identified a need for an "assisted tunneling" mechanism. For
   example, an ISP starting its IPv6 offering to its customers without
   upgrading its access network to support IPv6 could use a "tunnel
   brokering solution" [ISP, section 5.1.] a la [3053]. What has been
   identified as missing from that RFC is a tunnel set-up protocol.

   In an ISP network, getting IPv6 connectivity to the customers
   involves upgrading the access network to support IPv6, which can take
   a long time and/or be costly.  A tunneled infrastructure can be used
   as a low cost migration path [ISP, section 5.1].

   With such an infrastructure, the ISP can connect its customers to its
   IPv6 network using its production IPv6 address space, thus
   facilitating migration towards native IPv6 deployment. The IPv6
   deployment roadmap for connecting customers becomes:

      - assisted tunneling infrastructure to early adopters,
      - native IPv6 to customers where economically justified,
      - native IPv6 to all customers.

   Note that, as the addressing space used during the transition to
   native remains the same the customer routing, filtering, accounting
   [ISP, section 5.] stay the same, and there is no need to maintain any
   kind of relay.

   "Assisted tunneling" is used in this document to described a
   transition mechanism where the parameters to configure a bi-
   directional tunnel between an end-node (or leaf network) and a router
   in the core of an ISP are exchanged through a tunnel set-up protocol.
   Although this exchange can be automated, this remains different from
   transition mechanisms like 6to4 that is fully automatic.  Teredo and
   ISATAP, on the other hand, practically require receiving information
   about the available prefix or address from the server/router,
   comparable to the most simple form of assisted tunneling.

   In particular, the 'registered' mode defined in section 4 enables
   explicit access control to the tunneled IPv6 connectivity, where the
   other transion mechanisms have to rely on other kinds of control
   (e.g., access control based on the IPv4 address).

   This document analyze the requirements for such a tunnel set-up
   protocol. The v6ops WG scenario and evaluation documents for
   deploying IPv6 within common network environments are used as input
   to this document.

2. Applicability

   Assisted tunneling is applicable in different IPv6 transition
   scenarios. The focus of this document is to define the requirements
   to apply this mechanism in the IPv4 ISP context making the following

      - ISP is offering IPv6 connectivity to its customers initially
      using controlled tunneling infrastructure [ISP, 5.1 Steps in
      Transitioning Customer Connection Networks]

      - The customer configuration may be diverse, and not necessarily
      predictable by the ISP. The protocol must be able to adapt to the
      following cases, for example by choosing the most optimal tunnel
      encapsulation depending on the presence of a NAT.
         - a single node,
         - a leaf network,
         - using a globally routable IPv4 address,
         - behind a NAT (customer or ISP owned),
         - using dynamic IPv4 address (internally or externally to the

   There are actually two cases where the IPv4 address of the customer
   tunnel end point can be dynamic, and both must be supported:

      - The device used as tunnel end point is using a dynamic IPv4
      address provided by the ISP.
      - The device used as tunnel end point is located behind a customer
      owned NAT box that is also acting as a local DHCP server. In that
      case, the device IPv4 address may change after a reboot.

   Althought the main focus of this document is the ISP scenario,
   assisted tunneling is applicable in all the other scenarios:
   unmanaged, enterprise and 3GPP.

   In unmanaged networks, assisted tunneling is applicable in the case A
   (a gateway which does not provide IPv6 at all) [UNMANAGED, section 3]
   and C (a dual-stack gateway connected to an IPv4-only ISP)
   [UNMANAGED, section 5].

   In 3GPP networks, assisted tunneling can be used in the context of
   dual stack UE connecting to IPv6 nodes through a 3GPP network that
   only supports IPv4 PDP contexts [3GPP, 3.1].

3. Requirements for Simplicity

   The tunnel set-up protocol must be simple to implement and easy to
   deploy. In particular, it should not depend on any complex, yet to be
   designed, protocols or infrastructure pieces.

   This protocol is a transition mechanism, thus does not need to be
   perfect. As a matter of fact, making it perfect would be counter
   productive, at it would first delay its definition, then make its
   deployment more cumbersome and, last but not least, diminish the
   incentives to deploy native IPv6.

4. Protocol Requirements

   Assisted tunneling can be provided in two different modes. "Non-
   registered" mode is a simple mode with no user registration,
   essentially deployed in a controlled and "authenticated" environment
   where the service is made available to all the IPv4 customers..  The
   "registered" mode is aimed at production deployment which requires
   the user to be authenticated. This mode can be used to offer the
   tunneling service to roaming users or restrict the service to
   specific (authenticated) users. The tunnel set-up protocol must
   support both modes, however ISP deploying it may choose to only
   support one mode of operation.

   Assisted tunneling in the non-registered mode is defined for simple
   "plug & play" scenarios. In this mode, the tunnel establishment is
   triggered through the execution of a simple program, without any pre-
   configuration or pre-registration required from the end-user.

   The tunnel established is "anonymous" in a sense as the end-user does
   not register explicitly to the server. An ISP using the protocol in
   this mode may be offering a free service and doesn't wish to require
   any form of registration. This free service can also be used to offer
   trial IPv6 service limited to the ISP customers by relying on IPv4
   access control.

   The registered mode offers some extra features documented in latter
   sections. This mode is most valuable in a provider network where
   deployment of IPv6 is done in a more controlled manner or when the
   provider cannot rely on IPv4 related authentication (e.g. roaming
   customers). In such networks, ease of debugging, traceability,
   filtering and so on are important features.

4.1. Address and Prefix Delegation

   Assignment of an IPv6 address (/128) to the end-node must be
   supported in both modes.

   In non-registered mode, the IPv6 address is "transient" and may
   change, but the protocol SHOULD offer a mechanism to provide IPv6
   address stability in this mode (e.g. cookie mechanism). The
   implementation of this mechanism MUST allow this feature to be turned
   off.  In registered mode, the protocol MUST be able to support
   servers willing to offer stable IPv6 prefixes to the authenticated

   The registered mode MUST support delegation of a single address or a
   whole prefix. The length of the IPv6 prefix delegated MUST be
   configurable on the server. The non-registered mode MAY support
   prefix delegation.

   See section 5.9 for DNS considerations.

4.2 Registration

   The registration of credentials is external to the protocol. A
   service provider SHOULD be able to use its existing authentication
   database. If necessary, an implementation may provide hooks to
   facilitate integration with the ISP management infrastructure (e.g.
   RADIUS for AAA, billing).

   The protocol MAY send information about registration procedure when a
   non-registered client requests registered mode (ex: URL to provider
   registration web page).

4.3. Authentication

   The authentication mechanism supported should be compatible with
   standardized methods that are generally deployed.  In order to assure
   interoperability, at least one common authentication method MUST be
   supported. Other authentication MAY be supported and should be
   negotiated between the client and server (e.g., SASL [2222]).

4.4. Service Discovery

   In order to offer "plug & play", the implementation SHOULD allow a
   mechanism to discover the address of the server that will provide the
   tunnel connectivity. This discovery should be automatic within an ISP

4.5. NAT Traversal

   Tunneling through IPv4 NAT MUST be supported. The protocol should
   detect if an IPv4 NAT is in the path during the set-up phase (section
   5.3). If a NAT is present, an extra level of encapsulation is
   necessary to tunnel IPv6 across the NAT. If no NAT is detected,
   IPv6-over-IPv4 tunneling (IP protocol 41) is enough.

4.6. Firewall Traversal

   Even if no NAT is in the tunnel path, there may be a firewall which
   prohibits IP protocol 41. In such case, the tunnel encapsulation
   selection based on NAT detection (section 5.3) will select a tunnel
   that will not work.
   The implementation MUST allow a user to explicitly specify the
   desired tunnel encapsulation, regardless of the NAT detection

4.7. Accounting

   The assisted tunneling should include tools for managing and
   monitoring the provided service. Such information can be used to plan
   service capacity (traffic load) or billing information.

   Some useful accounting data are (not exhaustive list):
      - Tunnel counters (traffic in/out)
      - User utilization (tunnel uptime)
      - System logging (authentication failures, resource exhaustion,

   The interface used to provide such information can be through SNMP or
   an AAA protocol (e.g., RADIUS accounting).

4.8. Security Threat Analysis

   The non registered mode does not require explicit authentication of
   the user. Access control may be performed using e.g., IPv4 address.
   The mode essentially offer the IPv6 service to any of its IPv4
   customers.  Deployment of the service with this mode must be done
   carefully. In particular, security considerations must be taken into

   Non registered service should be limited to the provider network,
   where access of the service can be controlled based on the IPv4
   address of the users (filtering).  If specific filtering is not in
   place in the ISP core network, anyone on the Internet could start
   using its tunneling infrastructure to get free IPv6 connectivity,
   transforming effectively the ISP into a IPv6 transit provider.

   If IPv4 address spoofing is possible within the access network, a
   number of DoS attack can happen. For example, customer A can
   impersonate someone else's IPv4 address during the set-up phase and
   redirect a tunnel to that IP address. A then can, for example, start
   a high bandwidth multimedia flow across that tunnel and saturate its
   victim's up-link.

   But even with filtering in place, the non registered mode is
   vulnerable to denial of service attacks: a customer A behind a NAT
   can use a large number of (private) IPv4 addresses and request
   multiple tunnels.That would quickly saturate the ISP tunnel server
   infrastructure. In order to minimize such attacks, IPv4 return
   routability checks could help blocking many DoS attack. Also, the
   server implementation should offer a way to throttle and limit the
   number of tunnel established to the same (non-private) IPv4 address.

   In registered mode, the protocol must make sure that the tunnel is
   established to the legitimate and authenticated destination, again to
   avoid having someone establish a tunnel to a potential victim. IPv4
   return routability checks could help block possible DoS attacks.

5. General Requirements

5.1 Scalability

   The tunnel set-up protocol must be scalable. Typically, this protocol
   should be deployable in broadband ISP.

5.2 Service discovery requirements

   The discovery part of the tunnel set-up protocol should be as
   automatic as possible.

   The discovery mechanism must be able to scale for large ISP who cover
   different geographical areas and/or have a large number of customers.

   Customers may very well try to use this tunnel set-up protocol even
   if their ISP is not offering the service. In this case, without any
   previous action taken by their ISP, the discovery part of the tunnel
   set-up protocol must be able to abort immediately and display the
   customers a message explaining that no service is available.

5.3 NAT Considerations

   The assisted tunnel established by the protocol to be designed must
   work with the existing infrastructure, in particular it must be
   compatible with the various customer premise equipments available
   today.  This means that, in particular, the tunnels must be able to
   traverse one or many NAT boxes of different kinds. There is no
   requirement for any particular NAT traversal technology. However, as
   NAT traversal usually requires an extra layer of encapsulation, the
   tunnel set-up protocol SHOULD be able to detect automatically the
   presence of one or more NAT boxes in the path.
   The implementation MUST provide an option to to turn on extra
   encapsulation manually.  In order to assure interoperability, at
   least one common tunnel encapsulation type MUST be supported.

5.4 Keep-alive

   When a tunnel has to cross a NAT box, the mapping established by the
   NAT must be preserved as long as the tunnel is in use. This is
   usually achieved by sending keep alive messages across the tunnel.
   Also, the same keep alive messages can enable the ISP tunnel end
   point to perform garbage collection of its resources when tunnels are
   not in use anymore. To enable those two functionalities, the tunnel
   set-up protocol must include the transmission of keep-alive messages.
   A client MAY choose not to send those messages (for example on ISDN
   type links), but should then expect that the tunnel may be
   disconnected at any time by the ISP and be prepared to restart the
   set-up phase.

5.5 Latency in Set-up Phases

   In certain type of networks, keeping tunnels active all the time is
   not possible (e.g., limited number of tunnels on server) or simply
   too expensive (e.g., wireless environments where periodic
   transmission is expensive). In those environments, the protocol must
   be able to set-up tunnels on demand of the IPv6 applications
   requiring external connectivity.

   The tunnel set-up protocol must then have a low enough latency to
   enable quasi-instant configuration.  Latency is usually a function of
   the number of packet exchanges required, so minimizing this parameter
   is important.

5.6 Security

   The tunnel set-up protocol must not introduce any new vulnerability
   to the network.

5.7 Traceability

   In production environment, traceability is an important
   consideration.  The tunnel set-up protocol must be instrumentable to
   enable the collection of usage data that can be used, for example,
   for capacity planning.

5.8 Phase Out

   This assisted tunneling mode is only a transition mechanism to enable
   ISP to jump start IPv6 service without requiring an immediate global
   upgrade of access networks and instead enabling a progressive roll
   out.  Once IPv6 is available natively in the access network
   connecting a customer, there is no reason to keep using tunnels. So
   the implementation SHOULD have a provision to enable the ISP to
   signal the user to use native IPv6 instead.

5.9 Extensibility

   The protocol MUST be extensible to support tunnel encapsulation other
   than IPv6 over IPv4 and IPv6 over transport over IPv4. In particular,
   encapsulation of IPv4 over IPv6 or IPv6 over IPv6 could be defined.

5.10 DNS considerations

   It should be possible to have the server side of the protocol
   automatically register the assigned IPv6 address in the DNS system
   (AAAA and PTR records) using the ISP name space. Nothing specific is
   required in the protocol to support this.  The details can be
   implementation specific.

   If stable prefix delegation is done, it is expected that the DNS
   delegation of the associated reverse DNS zone will be also stable and
   thus can be performed out of band, so there is no requirement to
   perform this delegation at the tunnel set-up time.

6. Compatibility with other Transition Mechanisms

6.1 TSP

   The tunnel set-up protocol is not required to be compatible with TSP
   [TSP] or any particular implementation of the tunnel broker model
   [3053]. Although, a great deal of experience can be drawn from the
   operation of tunnel brokers currently using the TSP protocol.


   There is a large number of Teredo clients already deployed, the
   tunnel set-up protocol should explore the avenue of providing a
   compatibility mode with Teredo, at least in the 'simple' mode
   described in section 4. However, it may turn out that supporting a
   compatibility mode with Teredo either requires to change the Teredo
   specifications and/or implement Teredo on the tunnel server side. In
   that case, it might be simpler to say that the compatibility mode
   should be manage on the client side instead of the server side, that
   is leave it up to the client to use either one of them.


   Similar considerations as Teredo, section 6.2, applies to Isatap.
   However, as Isatap can not work accross NAT, it is of much less
   interest in the framework of this document.

7. Security Considerations

   The establishment of a tunnel can be compared to Mobile IP
   technology, where traffic can be redirected to go from one place to
   another one. So similar threats exists. In particular, when a
   customer is asking for the set-up of a tunnel ending at IP address X,
   the ISP should check:
      - the customer is allowed to set-up this tunnel, i.e. he "owns"
      the IPv6 prefix.
      - the customer is allowed to terminate the tunnel where he said he
      would, i.e. he "owns" the IPv4 tunnel endpoint.

   The first check is simply an authentication issue. The second may be
   more complex, but can be omitted if strict ingress filtering is in
   place in the access network, i.e. the customer is effectively
   prevented from sending packet with an IPv4 source address he does not

   See section 4.4 for specific security consideration in the non-
   authenticated mode.

8. Acknowlegements

9. Author Addresses

   Alain Durand
   SUN Microsystems, Inc
   17 Network circle UMPK17-202
   Menlo Park, CA, 94025

   Florent Parent
   2875 boul. Laurier, bureau 300
   Sainte-Foy, QC  G1V 2M2

10. Normative References

   [2119]  Bradner, S., "Key Words for Use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.

   [3053] A. Durand, P. Fasano, I. Guardini, D. Lento.,
          "IPv6 Tunnel Broker", January 2001.

   [ISP]  Lind, M., "Scenarios and Analysis for Introducing IPv6
          into ISP Networks",
          draft-ietf-v6ops-isp-scenarios-analysis-01 (work in
          progress), February 2004.

   [UNMANAGED] Huitema, C., "Evaluation of Transition Mechanisms for
          Unmanaged Networks", draft-ietf-v6ops-unmaneval-01 (work
          in progress), February 2004.

   [3GPP] J. Wiljakka, "Analysis on IPv6 Transition in 3GPP Networks",
          draft-ietf-v6ops-3gpp-analysis-09 (work in progress), March 2004.

11. Informative References

   [2831] Leach, P. and C. Newman, "Using Digest Authentication as a SASL
          Mechanism", RFC 2831, May 2000.

   [2222] Myers, J., "Simple Authentication and Security Layer (SASL)",
          RFC2222, October 1997.

   [ISATAP] Templin, F., Gleeson, T., Talwar, M. and D. Thaler,
          "Intra-Site Automatic Tunnel Addressing Protocol
          (ISATAP)", draft-ietf-ngtrans-isatap-21 (work in
          progress), April 2004.

   [TEREDO] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
          NATs", draft-huitema-v6ops-teredo-01 (work in progress),
          February 2004.

   [TSP]  Blanchet, M., "IPv6 Tunnel Broker with the Tunnel Setup
          Protocol(TSP)", draft-blanchet-v6ops-tunnelbroker-tsp-00
          (work in progress), February 2004.

Appendix 1. Summary of the Protocol Requirements

   The non registered mode:
      - MUST be supported by the protocol to be designed
      - RECOMMENDED to be implemented on client and servers/brokers
      - implementation SHOULD turn it on be default
      - implementation MUST allow it to be turned off
      - MUST support single address assignment
      - SHOULD support stable IPv6 address
      - MAY support prefix delegation

   The registered mode:
      - MUST be supported by the protocol
      - RECOMMENDED to be implemented on clients and servers/brokers
      - implementation SHOULD turn it off by default
      - implementation MUST allow it to be turned on
      - MUST support single address assignment
      - MUST support stable IPv6 address
      - MUST support prefix delegation, SHOULD be able to turn off
   The registered part of the protocol SHOULD not be too different from
   the non registered one, so as to minimize code difference between the
   two modes

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