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Internet Engineering Task Force                                P. Savola
Internet-Draft                                                 CSC/FUNET
Expires: September 30, 2004                                  J. Soininen
                                                                Apr 2004

         Evaluation of v6ops Tunneling Scenarios and Mechanisms

Status of this Memo

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Copyright Notice

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


   This memo analyses the v6ops scenarios/analysis work (Unmanaged,
   3GPP, ISP and Enterprise) for their requirements for tunneling
   solutions, and analyses the proposed mechanisms on how they might fit
   in these requirements, and discusses possibilities for choosing

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Selection Process  . . . . . . . . . . . . . . . . . . . .  3
   3.  Scenarios  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1   3GPP Networks  . . . . . . . . . . . . . . . . . . . . . .  3
     3.2   Unmanaged Networks . . . . . . . . . . . . . . . . . . . .  4
     3.3   Enterprise Scenarios . . . . . . . . . . . . . . . . . . .  6
     3.4   ISP Scenarios  . . . . . . . . . . . . . . . . . . . . . .  7
     3.5   Additional Scenarios: IP Mobility  . . . . . . . . . . . .  7
   4.  Scenarios and Mechanisms Evaluation  . . . . . . . . . . . . .  8
     4.1   Scenarios Evaluation . . . . . . . . . . . . . . . . . . .  8
     4.2   Mechanisms Evaluation  . . . . . . . . . . . . . . . . . .  9
     4.3   Evaluation Summary . . . . . . . . . . . . . . . . . . . . 10
     4.4   Features of the Mechanisms . . . . . . . . . . . . . . . . 11
   5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.1   Normative References . . . . . . . . . . . . . . . . . . . . 13
   8.2   Informative References . . . . . . . . . . . . . . . . . . . 14
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
       Intellectual Property and Copyright Statements . . . . . . . . 16

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1.  Introduction

   This memo analyzes the v6ops scenarios/analysis work (Unmanaged [1],
   3GPP [2], ISP [3], Enterprise [4]) at a bit more length, summarizes
   the exact requirements for tunneling in the scenarios, and analyzes
   how different mechanisms would fit into those specific tunneling

   The mechanisms analyzed in this document are Teredo [5], ISATAP [6],
   STEP [7], and TSP [8] The latter two are examples of the tunnel
   server/broker concept [9]. Already-specified, and in many cases
   applicable tunneling mechanisms are 6to4 [10] and configured
   tunneling [11].  Some others include so-called "6over4" [12] and
   Layer 2 Tunneling Protocol (L2TP) [13].

2.  The Selection Process

   It is strongly desirable to be able to recommend as few mechanisms as
   reasonably possible.  This is an important tradeoff to consider.
   However, it is likely that one has to give up some features to
   achieve that goal.

   However, it is recognized that there are implementations out there:
   therefore, one can submit I-Ds specifying currently implemented (but
   only that; no additions) mechanisms to RFC-editor as invidial
   contributions for Informational or Experimental RFC.  These documents
   will include a clear IESG Note and/or an applicability statement that
   these are not recommended mechanisms.  The goal of this process is to
   improve interoperability of the implementations that already exist,
   and some have deemed fit to implement.  XXX: anything about the
   timing when these can be submitted to the RFC-editor?

3.  Scenarios

   This section described the specific cases identified inside the
   scenarios where a form of tunneling is desirable.

3.1  3GPP Networks

   There are two closely related cases:

   1.  Providing IPv6 connectivity to User Equipment (UE) when it roams
       to an another operator's network, and the other operator does not
       support IPv6 PDP contexts.

   2.  Providing IPv6 connectivity to UEs when the "home" 3GPP operator
       has not deployed even minimal IPv6 PDP context support yet in its
       network -- but would like to enable IPv6 connectivity through a

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       transition mechanism which can be transparently overlayed on the
       existing IPv4 3GPP network.

   Note that the current document strongly recommends at least initial
   IPv6 PDP context support: this only requires support from HLR, SGSN
   and one GGSN.

   The case where there is no IPv6 connectivity at all in the 3GPP
   network is considered out of scope (but a solution can be achieved
   using one of the Unmanaged connectivity mechanisms if appropriate).

   In this scenario, it is assumed that the users are typically using
   private IPv4 addresses, but there is no NAT in the path between the
   different users in the same 3GPP network.

   There are a couple of differences with the two cases described above:
   in the first case, the operator has deployed IPv6 PDP context
   support, as recommended, but some other ISP has not: waiting for all
   the operators to deploy IPv6 PDP context support prior to launching
   the service would be unacceptable -- so there has to be a solution to

   Also, note that the tunnel from the foreign 3GPP network is
   terminated at the local 3GPP operator's network, inducing delay and a
   bottle-neck; "direct connectivity" optimization is not much different
   whether the tunnel would be terminated at a tunnel server, or
   communicating directly.  In the second case, one could argue that the
   deployment should only be temporary, with at least minimal IPv6 PDP
   context support being the goal.

   In any case, the critical question in this specific scenario is, how
   desirable is it to have direct tunneling between the UEs in the same
   3GPP network -- instead of a slightly longer "leg" through a tunnel
   server?  Clearly, this would be desirable -- but not a strict
   requirement.  Also, to re-iterate the (strongly) recommended
   deployment scenario in the 3GPP networks is the native (even if
   minimal) IPv6 PDP context support; accepting non-direct connectivity
   could be an acceptable tradeoff in the early adopter phase, also
   encouraging to move to proper IPv6 support.

3.2  Unmanaged Networks

   The unmanaged scenarios seem to include two cases, with a subcase,
   totalling 3 different cases; these are derived from both unmanaged
   and ISP scenario documents.

   1.  When the user's direct ISP does not offer any IPv6 service at
       all, and connectivity must be obtained automatically.

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       1.  When the connectivity must be obtained with as little
           infrastructure as possible, without any signups or contracts,

       2.  When the connectivity can be obtained from a third party,
           through a sign-up, contract, etc. -- for higher
           manageability, more control, increased security benefits, or
           for other reasons. (Whether such "3rd party connectivity" is
           feasible or good enough is a separate question.)

   2.  When the user's direct ISP would like to offer IPv6 service, but
       would require tunneling for some reason (e.g., access router,
       access link, or the gateway in the unmanaged network is incapable
       of IPv6).  In this case the options are basically tunneling from
       the gateway, a separate IPv6 gateway or tunneling from the

   It should be noted that a solution to problem 1.2) would solve
   problem 2) as well, but a solution to problem 2) would not
   necessarily be adequate for solving 1.2).

   NAT traversal must be supported.  Dynamic IPv4 addresses must be
   supported -- but the solution does not necessarily have to be better
   than dynamic IPv4 addresses are today; e.g., a dynamic IPv6 address
   would be acceptable as well.

   When the gateway has been upgraded to support IPv6 (but access router
   or link has not, resulting in tunneling from the gateway, NAT
   traversal doesn't need to be used as often.  That is, often the
   gateway has a public IPv4 address on its Internet-side interface, so
   a mechanism like 6to4 can be used on the gateway to provide
   "native-like" IPv6 support to the unmanaged network.  However, there
   are a number of cases where this is not sufficient -- for example,
   when the gateway is connecting to a privately-addressed access
   network shared by multiple ISPs.  In such case, the gateway may be
   unable to obtain IPv6 connectivity.

   It seems obvious that direct tunneling between users is required at
   least when there is no ISP support -- to minimize the latency
   increase, and to decrease the bandwidth aggregation.  However, it is
   not a strict requirement with hosts in the same ISP: in such a case,
   the slight increase in latency and concentration of traffic is
   probably manageable. It should be re-iterated that tunneling is not
   meant to be a permanent solution, and accepting that the connectivity
   may not be fully optimal should be acceptable.

   One should note that direct connectivity that traverses NATs, as is
   requirement here, is a very difficult thing to do right; essentially,

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   this is what Teredo is doing.  On the other hand, if the ISP is
   offering service which does not provide direct connectivity between
   hosts, the use of another mechanism (such as 6to4 or Teredo) "on the
   side" could perhaps provide at least partial direct connectivity.

3.3  Enterprise Scenarios

   This scenario/analysis work is still incomplete, so it is not fully
   addressed in this memo.

   The only scenario which is obvious at the moment is when the
   enterprise wishes to deploy IPv6 without changing the gear to be
   dual-stack, without injecting IPv6 into existing VLANs [14], or
   without adding additional IPv6 routers in the VLANs.  In particular,
   this may be the case when the IPv6 deployment is "sparse" -- because
   if it was sufficiently "dense", it would make sense to expand the
   infrastructure in one of the several ways.  It would be nice if the
   tunneling between the nodes is direct from node-to-node, but this is
   not a strict requirement.

   There are at least three subcases of this scenario:

   1.  When the enterprise does not NAT between different parts of the
       internal network.  This case is useful to separate from case 2
       merely because it is generally the common case, where simplicity
       is the most highly valued.

   2.  When NATs exist within the enterprise network, e.g., to connect
       branch offices to the corporate network. Hence, some form of NAT
       traversal must be supported.

   3.  When the enterprise internally uses multicast applications/
       protocols that they want to transition to IPv6.  Here the
       enterprise has already deployed intra-domain IPv4 multicast to
       support this stage. As with case 1, there are no internal NATs in
       this case since IPv4 multicast itself does not cross NATs.

   Case 3 is special because the number of enterprises with all-reaching
   IPv4 multicast deployment is low.  While IPv4 multicast support is
   typically more commonplace than (even unicast) IPv6 support, the
   enterprises are typically in a better position than those which would
   not have IPv4 multicast at all.  Therefore, native IPv6 multicast
   support is a possibility, while using tunneling mechanism(s) which
   support IPv6 multicast by tunneling is a simple short-term deployment

   The need for direct connectivity in all cases is strong due to two

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   o  The high end-to-end bandwidth requirements for enterprise
      applications, e.g., many clients accessing various servers, such
      that bottlenecks occur if all traffic must be funneled through one
      or a few tunnel endpoints.  However, note that heavy usage is
      quite a strong argument for native (or hierarchical/distributed)
      IPv6 deployment.

   o  The use of collaborative applications, possibly including
      high-bandwidth streams such as audio/video.

   Finally, most enterprise networks currently lack IPv6 expertise, and
   have a great need to keep costs low.  Hence simplicity and low
   infrastructure costs are also required.  (This is not really specific
   to enterprises, though.)

3.4  ISP Scenarios

   ISPs have two possible requirements for tunneling: either inside
   their own infrastructure (e.g., through a non-upgraded core network)
   or to their peers or upstreams, or towards customers.

   Internal tunneling requirements can be satisfied with configured
   tunneling or the use of [15] as described in [3] so there is no need
   to discuss that in this memo. Similarly, tunneling requirements
   towards peers or upstreams are satisfied by configured tunneling

   As for tunneling towards customers, ISPs do not have specific
   scenarios which need to be addressed which haven't been already
   mentioned: some ISPs want to provide IPv6 connectivity, possibly over
   a tunnel, to their unmanaged or enterprise customers, or ISPs.
   However, these requirements have already been discussed; only two
   particular considerations should be explicitly mentioned:

   1.  Mechanisms used must be very simple if we want them to be adopted
       by many ISPs,

   2.  Very few ISPs want to be providing service, for free at least, to
       the users which are not their customers.  Therefore being able to
       identify the user as your own customer is very important.
       (Whether this is done by explicit user authentication or basing
       on IP-address -based knowledge of whether the user is your
       customer is a detail.)

3.5  Additional Scenarios: IP Mobility

   Recently, there some concerns have been raised about additional

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   scenarios work, which might be partially worked under v6ops WG. One
   such is work on the scenarios where IP nodes are not stationary,
   i.e., are expected to change IPv4 or IPv6 address relatively rapidly.
   In many cases this implies that either MIPv4 or MIPv6 is being run to
   ensure a relatively stable IP address being available, and to make
   the changes in IP addresses as transparent as possible.

   This is not a formal scenario as such, but in these events it would
   be extremely desirable to have minimal amount of signalling when the
   attachment point (whether a care-of or home address) changes -- to
   ensure seamless IP mobility.

4.  Scenarios and Mechanisms Evaluation

4.1  Scenarios Evaluation

                 NAT-T Direct  ISP Secure Simple   Low   Mcast Gateway
   Unman 1.1      *       *     N    -      -       -      -      -
   Unman 1.2      *       N     *   -/*     -       -      -      -
   Unman 2        *       -     *    -      *       -      -      -
   3GPP 1         N      -/*    *    -      *       *      -      N
   3GPP 2         N      -/*    *    -      *       *      -      N
   Enterprise 1   N      -/*    *   -/*     *       -      -     -/N
   Enterprise 2   *      -/*    *    -      *       -      -     -/N
   Enterprise 3   N      -/*    *    -      *       -      *     -/N

   Legend: *  = MUST; -  = Nice to have; N  = No

   NAT traversal specifies whether NAT traversal is a requirement.

   Direct tunneling states whether direct tunneling between the nodes
   using the same mechanism is a requirement.

   "ISP" indicates whether the scenario assumes that the ISP provides
   IPv6 support.  That is, whether the mechanism must be able to operate
   (at least to a degree) without explicit support from an ISP which is
   identifying the user.

   "Secure" is an overly simplistic term to evaluate whether the
   scenario requires some specific amount of security.  Here, security
   implies security issues which may not be trivially fixable, whether
   the operational mode or in the mechanism, such that it is difficult
   to secure, whether it creates new significant threats to either IPv4
   or IPv6 infrastructures, etc.

   Simple is a term which refers to the simplicity of the protocol or
   the operational model in which it operates; simple protocols and

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   specifications are easy to understand, evaluate, implement, and
   deploy, and thus preferable to more complex ones.

   "Low overhead" refers to both minimal encapsulation -- as few added
   bytes in the messages or signalling as possible -- and signalling
   latency (e.g., the number of messages/round-trips to set-up, change,
   or tear down a tunnel).

   Multicast states whether IPv6 multicast should be supported.

   Gateway refers to whether the scenario also requires that an upgraded
   gateway, to provide IPv6 connectivity to the local link(s), must be

4.2  Mechanisms Evaluation

   One should note that we are not evaluating the specific version of
   the specification, but rather the mechanism in a more generic sense
   ("which features could this mechanism easily be made to work with?").

          NAT-T Direct ISP Secure Simple  Low   Mcast Gateway Impl. Depl
   Teredo  Y      Y     N     Y      N     N     N      N      R     Y
   ISATAP  N      Y@    Y    N/R     R     Y     N      R      Y     R?
   TSP     Y      N     Y?    Y      R     N     Y#     Y      R     R?
   STEP    Y      N     Y     Y     Y/R    Y     Y#     R      N     N
   L2TP    Y      N     Y     Y      N     N     Y#     R?     Y     Y
   6to4    N      Y$    N     N      Y     Y     N      Y      Y     Y
   6over4  N      Y@    Y    N/R     Y     Y     Y      R      R     N

     @: intra-site, when no NATs are in the path
     #: in a non-optimal fashion
     $: only between public IPv4 addresses

   Legend: Y  = Yes; R  = Relatively good; N  = No

   Notes: 6to4 does not work behind a NAT, so it is not applicable in
   3GPP scenarios, and practically also not applicable in Enterprise
   scenarios.  6over4 requires IPv4 multicast infrastructure, so it is
   practically only applicable in Enterprise scenario 3.

   NAT traversal states whether the mechanism is able to perform full
   NAT traversal.

   Direct tunneling states whether the mechanism is able to provide
   direct tunneling between the nodes using the same mechanism.

   "ISP" indicates whether ISP(s) must explicitly, identifying the user,

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   support the mechanism in order for the mechanism to operate (at least
   to a degree).

   "Secure" is an overly simplistic term to evaluate whether the
   mechanism has obvious security issues which may not be trivially
   fixable, whether the operational mode or in the mechanism, such that
   it is difficult to secure, whether it creates new significant threats
   to either IPv4 or IPv6 infrastructures, etc.

   Simple is a term which refers to the simplicity of the protocol or
   the operational model in which it operates; simple protocols and
   specifications are easy to understand, evaluate, implement, and
   deploy, and thus preferable to more complex ones.

   "Low overhead" refers to both minimal encapsulation -- as few added
   bytes in the messages or signalling as possible -- and signalling
   latency (e.g., the number of messages/round-trips to set-up, change,
   or tear down a tunnel).

   Multicast states whether the mechanism supports IPv6 multicast.
   Bidirectional tunnels support it but do not leverage the underlying
   link-layer for packet duplication, as e.g., so-called 6over4 does.

   "Gateway" refers to whether the mechanism can be used on a router to
   provide "native-like" IPv6 connectivity to its link(s), or whether
   the mechanism can only be used at each individual host.

   Implemented refers how widely the mechanism has been implemented
   (e.g., no implementations, one implementation, multiple interoperable
   implementations), and how large part of the mechanism has been

   "Widely deployed" refers to how widely the mechanism has been
   deployed: is it in use as deployed by multiple vendors, or in many
   operational scenarios?

4.3  Evaluation Summary

   By combining the two matrices, we obtain the following:

   o  Unmanaged scenario 1.1) requires Teredo.

   o  Unmanaged 1.2) requires STEP, TSP or L2TP.

   o  Unmanaged 2) requires STEP or TSP.

   o  3GPP 1) can be filled by STEP or ISATAP; if a higher level of
      security is required, ISATAP may not be applicable.

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   o  3GPP 2) can be filled by STEP or ISATAP; only ISATAP if direct
      tunneling is a MUST requirement.

   o  Enterprise 1) requires STEP, TSP, or ISATAP.

   o  Enterprise 2) requires STEP or TSP.

   o  Enterprise 3) requires STEP, TSP, or 6over4.

   This evaluation indicates that Teredo is needed, no matter what.
   STEP can satisfy all the other requirements, and TSP almost all. So,
   the minimum number of recommended "new" mechanisms appears to be 2:
   Teredo and something else.

   6to4 can (continue to) be used when the gateway has been upgraded in
   all the unmanaged scenarios; this does not work in every case,

   On the other hand, if direct connectivity was a strict requirement,
   the minimum number of "new" mechanisms would have to be 3, as Teredo
   and ISATAP would not be sufficient on their own.

   Actually, it would be preferable in some sense to combine TSP and
   STEP as they are best applicable in quite similar scenarios.  This
   might allow one to combine the best features of both proposals.  If
   an auto-discovery feature would be added to that mechanism, we would
   have a very powerful mechanism which would apply well in almost all

4.4  Features of the Mechanisms

   If we would assume that we'd prefer to recommend two solutions, in
   addition to 6to4 and configured tunneling, which are already there,
   we would have to judge, based on the scenario requirements, three
   critical features:

   1.  NAT traversal

   2.  Direct connectivity inside a site/ISP/enterprise

   3.  Operation with third party ISPs

   In several scenarios, it's impossible to have both 1) and 2) without
   a relatively complex solution.  Similarly, there is often a conflict
   with 2) and 3).

   Therefore, we must be able to make a decision which features/
   requirements we are willing to give up in which specific scenarios,

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   or whether we have to specify more mechanism to satisfy all the

   For example, by picking Teredo and {TSP or STEP}, we would have to
   give up direct connectivity inside the 3GPP, Enterprise and the
   unmanaged scenarios where the ISP is offering service -- except where
   it would automatically come along where Teredo (or 6to4) service is
   active in any case.

   On the other hand, ISATAP is unable to properly perform NAT
   traversal, and it is not designed to securely interact with third
   party ISPs.  By picking Teredo and ISATAP, we would not have a
   solution to operate with 3rd party ISPs, ISPs which do not properly
   secure their borders, or in the cases where NAT traversal is required
   and Teredo is seen as too cumbersome a choice.  However, Teredo does
   provide a means to interoperate with a specifically crafted,
   simplistic Tunnel Server implementation; if we assume that it would
   be acceptable to recommend implementation and deployment of Teredo to
   be able to use a tunnel server service, implementing a stub tunnel
   server could provide a means to achieve support in the 3rd party ISP

   Direct connectivity inside (a vague definition of) a "site" is
   sometimes seen as attractive, e.g., to be done with ISATAP.  There
   are two possible arguments: sparse and dense deployment.  In the
   "sparse" deployment case, the requirements of the site can be met
   with an (auto-discovered) tunnel server solution.  In the "dense"
   deployment case, when direct connectivity could be desirable because
   the tunnel servers would get overloaded or the bandwidth could be
   wasted, there is a strong reason to go for a dual-stack (whether
   partial, e.g., with hierarchical tunnel servers or full deployment)
   solution instead. Therefore, it seems that when other choices are
   feasible, "intra-site" direct connectivity is not a progressive way

   As Teredo is the only solution available for scenario 1.1), it seems
   that it cannot be left out.

5.  Conclusions

   There seems to be clear need for Teredo.  There is also clear desire
   to keep using 6to4 in the cases where it's applicable.

   There seems to be clear need for a tunnel server protocol which is
   able to traverse NATs and work with dynamic IPv4 addresses.  This
   tunnel server should be able to automatically discover the server
   address if the service is provided by the ISP.

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   Direct connectivity is desirable but difficult to provide in a few
   specific scenarios when considering the other trade-offs.  However,
   global direct connectivity can be obtained with 6to4 and Teredo;
   local direct connectivity, inside 3GPP, ISP or enterprise is
   something that one could be able to live without.

   (Further TBD.)

6.  Security Considerations

   This memo analyses the tunneling scenario requirements and mechanisms
   trying to address these requirements.  As such, it does not have
   significant security considerations.  When considering which
   mechanism(s) to adopt, the security properties of the mechanisms vary
   considerably -- and this has to be taken in consideration in the
   evaluation and selection. However, mechanism-specific considerations
   are to be addressed in the respective documents.

7.  Acknowledgements

   This memo was written from scratch at IETF59, and it has been
   improved based on feedback received both prior, during, and after the
   session by numerous people.

   Alain Durand, Alain Baudot, Jeroen Massar, and Hesham Soliman sent
   substantive feedback, helping in improving this memo. Dave Thaler
   contributed the majority of enterprise scenarios, and provided other
   feedback as well.

8.  References

8.1  Normative References

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

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

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

   [4]  Bound, J., "IPv6 Enterprise Network Scenarios",
        draft-ietf-v6ops-ent-scenarios-01 (work in progress), February

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

   [6]  Templin, F., Gleeson, T., Talwar, M. and D. Thaler, "Intra-Site
        Automatic Tunnel Addressing Protocol (ISATAP)",
        draft-ietf-ngtrans-isatap-20 (work in progress), February 2004.

   [7]  Savola, P., "Simple IPv6-in-IPv4 Tunnel Establishment Procedure
        (STEP)", draft-savola-v6ops-conftun-setup-02 (work in progress),
        January 2004.

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

8.2  Informative References

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

   [10]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
         IPv4 Clouds", RFC 3056, February 2001.

   [11]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
         IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-02 (work in
         progress), February 2004.

   [12]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
         Domains without Explicit Tunnels", RFC 2529, March 1999.

   [13]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and
         B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661,
         August 1999.

   [14]  Chown, T., "Use of VLANs for IPv4-IPv6 Coexistence in
         Enterprise Networks", draft-chown-v6ops-vlan-usage-00 (work in
         progress), October 2003.

   [15]  Clercq, J., "Connecting IPv6 Islands across IPv4 Clouds with
         BGP", draft-ooms-v6ops-bgp-tunnel-02 (work in progress), March

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Authors' Addresses

   Pekka Savola


   EMail: psavola@funet.fi

   Jonne Soininen

   EMail: jonne.soininen@nokia.com

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