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Internet Engineering Task Force                                 J. Palet
Internet-Draft                                                   M. Diaz
Expires: July 28, 2005                                       Consulintel
                                                               P. Savola
                                                               CSC/FUNET
                                                        January 24, 2005

         Analysis of IPv6 Tunnel End-point Discovery Mechanisms
                 draft-palet-v6ops-tun-auto-disc-03.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 28, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   To be able to automate setting up IPv6-in-IPv4 tunnels, it is
   important to be able to automatically determine the tunnel end-point
   for the tunneling mechanism.  This memo presents a short analysis and
   conclusions on the different approaches for discovering the IPv6


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   tunnel endpoint on a node.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Manual Configuration . . . . . . . . . . . . . . . . . . .  3
   2.  Applicability of Tunnel Endpoint Discovery . . . . . . . . . .  4
     2.1   Scope of Tunnel Endpoint Discovery . . . . . . . . . . . .  4
     2.2   Assumptions about Network Topologies . . . . . . . . . . .  5
   3.  Analysis of Solutions  . . . . . . . . . . . . . . . . . . . .  5
     3.1   Anycast-based Solutions  . . . . . . . . . . . . . . . . .  5
     3.2   DNS-based Solutions  . . . . . . . . . . . . . . . . . . .  7
       3.2.1   Storing the TEP Information  . . . . . . . . . . . . .  8
       3.2.2   Prefixing the DNS Search Path  . . . . . . . . . . . .  8
       3.2.3   IP-address Query from Reverse DNS  . . . . . . . . . .  9
     3.3   DHCP-based Solutions . . . . . . . . . . . . . . . . . . . 10
     3.4   PPP-based Solutions  . . . . . . . . . . . . . . . . . . . 11
     3.5   SLP-based Solutions  . . . . . . . . . . . . . . . . . . . 11
     3.6   Combined Solutions . . . . . . . . . . . . . . . . . . . . 11
   4.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1   Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2   Informative References . . . . . . . . . . . . . . . . . . 14
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
   A.  More Discussion of Anycast Discovery . . . . . . . . . . . . . 15
   B.  Centralized Broker-based Solutions . . . . . . . . . . . . . . 16
       Intellectual Property and Copyright Statements . . . . . . . . 17











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1.  Introduction

   It is important to make setting up IPv6 connectivity simpler, so that
   IPv6-ignorant novice users can get the benefit of IPv6 transparently,
   without the user even having to know that IPv6 connectivity has been
   obtained.

   While this has become possible with Teredo and 6to4, they do not
   provide well for managed infrastructure, where the addresses come
   from the service provider's prefix.

   This document presents a short analysis and conclusions for different
   options to automatically determine the IPv6-in-IPv4 (with or without
   UDP encapsulation) tunnel end-points to a tunnel server, so that the
   set up of tunnels could be automated.

   Note that the other end-point ("tunnel server") typically also needs
   to have a means to configure the client's end-point, but that is
   assumed to be solved by the tunnel server mechanism [1][7], and
   beyond the scope of this memo.

   Some form of automatic discovery already exists in some already
   specified mechanisms; the generic discovery is out of scope, but we
   contrast the approaches to already-deployed methods when appropriate.
   For example, 6to4 [2] uses global anycast [3] and/or vendor's branch
   of DNS, Teredo [8] uses vendor's branch of DNS, and ISATAP [9] uses
   search-path -prefixed DNS.

   First, we look at manual configuration, and why it is not considered
   sufficient.

1.1  Manual Configuration

   Users typically expect to be able to manually configure the tunnel
   endpoint information, and the implementations obviously should allow
   that.

   Some implementations may also provide some default values (e.g.,
   using a vendor branch of DNS, as described in Section 3.2).  This is
   a non-interoperability issue, and may also be a good idea.

   Often the ISPs also provide CD-ROMs or other material to the
   (non-knowledgeable) users which will automatically reset the network
   connectivity settings to the values used by the ISP.  Such
   configuration could also include the tunnel endpoint if the ISP would
   like to roll it out to every customer.

   This raises the question whether it is strictly required to have an


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   automatic discovery process, rather than wait and rely on the ISPs to
   do a massive roll-out.

   We do not consider manual configuration sufficient for two main
   reasons (XXX: feel free to send some more..):

   1.  Many deployments are expected to happen at pre-production stage,
       and those service providers would not yet be ready to integrate
       the configuration in their CD-ROM etc.  material.

   2.  The CD-ROM materials have mainly been targeted to the users who
       set up their (IPv4) connectivity which either requires software
       or out-of-band configuration.  There is no reason to require
       out-of-band configuration if the discovery could be done in a
       feasible manner.

2.  Applicability of Tunnel Endpoint Discovery

2.1  Scope of Tunnel Endpoint Discovery

   There are three main areas of applicability for tunnel endpoint
   discovery:

   1.  No discovery: always assume the client pre-configures the
       endpoint information.

   2.  Discovery of the end-point at the "care-of" ISP only; that is,
       discovery is only supported inside the ISP of the network the
       user plugs into.

   3.  Discovery of the endpoint everywhere; contrast to the use of 6to4
       anycast address.  This is very problematic administratively,
       financially and technically, because the IPv6 prefix is not
       provider-independent as with 6to4.

   We decree 3) out of scope for this study; this is too extensive a
   problem to be solved here.  Therefore, we concentrate on 2).

   In addition to automatic discovery, the implementations should
   naturally provide a manual configuration option.  The manual
   configuration could be used to override the automatic discovery
   process or to configure a tunnel server at "home ISP" which the
   discovery would not find if the user is visiting a network where no
   tunnel server exists (compare to configuring MIPv6 Home Agent).

   When there are multiple inputs to which tunnel server to use,
   implementations will have to make a policy decision which one to


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   pick; this is rocket science, and some implementations (e.g.,
   Microsoft 6to4/ISATAP) already something like this.  The main options
   are:

   a.  First try local discovery, if it fails, try manual config,

   b.  Manual config first, then try local discovery, or

   c.  Only local discovery or only manual config.

2.2  Assumptions about Network Topologies

   The following assumptions about network topologies apply to the
   discovery process:

   1.  The CPE device can run either in bridged or routed mode.

   2.  In routed mode, the router typically is doing NAT with private IP
       addresses.  The router is also a DHCPv4 server, so DHCPv4 tunnel
       endpoint option is not relayed to the user.

   3.  The router may also inject its own DNS search string (e.g.,
       "home", "lan") instead relaying the one received from the ISP,
       though how often that happens is unknown.

   4.  The user may also have deployed NAT boxes of his/her own.

3.  Analysis of Solutions

   Several possible solutions to discovering the tunnel end-point can be
   imagined; this section describes them in detail.

3.1  Anycast-based Solutions

   An "anycast" (shared-unicast by some terminology: see [10]) address
   identifies a group of hosts, usually server hosts.  When a client
   sends a datagram to a shared-unicast address, it is delivered to one
   of the shared-unicast servers based on the routing topology and
   metrics.

   There are two possible ways of using "anycast": as a global service
   (where a shared-unicast prefix is the same for everyone, and
   advertised in the Inter-domain routing) or as a local service, where
   the service provider is sharing one of its own addresses on multiple
   nodes for example for load-balancing or redundancy reasons.  As local
   anycast is invisible to the users, it is not further discussed here.


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   A packet to a shared-unicast address may end up being delivered to
   more than one node.  In addition, there is no guarantee that two
   consecutive datagrams sent from the same host towards the same
   shared-unicast address are going to be delivered to the same node.
   However, when the routing topology is stable and metrics are
   well-designed, the packets are regularly delivered to the same nodes.
   Operational issues relating to managemeng of anycast services have
   been described in [4].

   A global anycast address could be leveraged in two fundamentally
   different ways:

   1.  Use the anycast address only for the initial handshake, to
       establish a stable unicast address of the end-point (and possibly
       to perform some initial negotiation, e.g., nonces).  All the
       subsequent packets are sent to the unicast address which is
       included in the payload of the reply.  An example of such use is
       in [5].

   2.  Use the anycast address for all the communications (e.g., as with
       6to4).

   The former approach is much more suitable in this situation as the
   IPv6 address/prefix of the tunnel service depends on the operator of
   the service.  The cost is at least one additional roundtrip.

   The failure modes of the initial handshake anycast are described in
   Appendix A.

   The advantages of the anycast approach are:

   o  Works well also in the presence of NATs and does not require any
      other components like DNS or DHCP.

   o  The routing stability and leaks are not a major concern if the
      anycast address is only used for initial discovery.  In other
      words, the worst that could happen is that if the initial
      discovery does not work correctly at the moment, and the user is
      either cannot get any service or directed to a tunnel server which
      does not offer any service.

   The drawbacks are:

   o  Setting up an internal anycast route advertisement (e.g., in an
      enterprise) is likely a little bit more difficult than adding a
      name in the DNS or configuring DHCP.

   o  The use of non-local prefixes may require changes in firewall IP


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      prefix, access lists, etc.

   o  The failure modes are a bit more complex than, e.g., just looking
      up a domain name, as one will have to send 1-2 packets to the
      anycast address to see if a working tunnel server is found or not.

   In summary, from the automatic methods, a global anycast based
   solution, using the address only for initial handshake, seems like a
   very promising approach, but has routing operations issues which need
   to be considered.

3.2  DNS-based Solutions

   As DNS is globally deployed and easy to use, it could provide a means
   for discovering the end-point address, either based on the forward or
   reverse tree.

   There are roughly four kinds of different approaches:

   1.  "(forward) global name": the systems look up a globally unique
       name, like www.tunnel-server.net which would point to the global
       anycast address.  This is not considered further as this does not
       solve any problem in itself, because the clients could have been
       configured with IP address instead.

   2.  "(forward) vendor branch": the operating system vendors may
       provide a DNS record which is looked up (contrast to
       "6to4.windows.microsoft.com."), giving the vendor some control
       over already deployed systems.  This could in practice only be
       used to configure the global anycast address, because the authors
       don't expect the vendors would provide a tunnel server to all the
       customers all over the world.  Therefore this approach is not
       considered further.

   3.  "(forward) prefixing the search path" [6]: one could look up a
       service-specific special string, like "_tunnel-server", appended
       by the DNS search path, e.g., "isp.example.com", resulting in a
       query of "_tunnel-server.isp.example.com".

   4.  "(reverse) querying the IP addresses for TEP information": for
       example looking up a special record for the assigned IP address.

   It is also a question where to store the information; the main
   suggestions have been at least A/CNAME, SRV, NAPTR and new type of
   records.  We first discuss these options.

   Approaches 3) and 4) are a bit more complicated and have different
   tradeoffs, so they are elaborated after looking at how to store the


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   information.

3.2.1  Storing the TEP Information

   Forward-tree global name and service branch would obviously use
   A/CNAME records.

   Forward-tree search path prefixing could use A/CNAME records.
   Architecturally a bit "cleaner" approach would be to use SRV records,
   which also provide a bit more fine-grained means for load
   distribution.  NAPTR provide even more extended load distribution,
   but it is not clear what the benefit would be.  A new RR could also
   be defined, but there seems to be no particular reason to do so, and
   a lot of drawbacks in the process.

   Reverse-tree IP address lookup would likely have to define a new
   record type.

   The main benefit of using A/CNAME records would be the applications
   can use simple getaddrinfo() lookups, instead of having to write
   their own or use a non-standardized DNS record lookup functions
   (e.g., getrrsetbyname).

3.2.2  Prefixing the DNS Search Path

   Prefixing the search path bears a bit more analysis; we discussed how
   to store the information above, and now look at where to store the
   records (i.e., the prefix to use, and what to do with the conflicts).

   This approach makes two assumptions; there are cases of both when
   these do not hold:

   1.  There is a decent mapping between the DNS hierarchy and the
       routing topology.

   2.  The information propagated to the end nodes in the "DNS search
       path" is relevant to figure out the domain name of the whoever is
       providing the tunnel service.

   The main advantages of the solution are:

   o  The discovery process is simple, and is already used for example
      by ISATAP (but without NATs in the middle).

   o  Adding the service is very simple, as the ISP is only required to
      add one address in their DNS zone.

   However, the main drawbacks of this approach are:


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   o  Some routers and middleboxes may not propagate the search path,
      but to insert their own, and this approach does not work in that
      situation.  It is not clear how widespread this is.  (In case the
      NAT would insert a new search path, e.g., "lan", they also should
      have be authorative for the zone, otherwise the root servers get
      bombarded by lookups.)

   o  In some cases in global enterprises, forward DNS may not map as
      well to the physical topology as IP addresses through reverse DNS
      would.  (NB: then ISATAP would have the same issue as well.)

   In summary, it is questionable how well prefixing the search path
   works under these circumstances, and in particular how common
   propagating (or not) the search path is.

3.2.3  IP-address Query from Reverse DNS

   The node might also try to find out its tunnel endpoint information
   by querying its own IP address in the reverse DNS for a certain DNS
   record type.

   The name needs to be stored somewhere.  The options are basically
   queries like (for IP address 192.0.2.1):

   1.  "QNAME=_tunnel.1.2.0.192.in-addr.arpa.  QTYPE=A"

   2.  "QNAME=1.2.0.192.in-addr.arpa.  QTYPE=TEP"

   In the first case, an arbitrary subname would have to be defined; one
   could query these for A, PTR, or some other records directly.

   In the second case, where the PTR records for the name might already
   exist one should probably use a new record type, though something
   like NAPTR has been used in private experiments.

   Advantages:

   o  IP address maps very well to the topology.

   o  Querying a new record type is an architecturally relatively clean
      approach.

   Drawbacks:

   o  Does not work well with NAT; would require that the ISPs
      prepopulate all the private address space records.

   o  Major management problems.  Wildcards cannot be used because they


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      would be in the middle of the QNAME or would match other records
      (such as PTR) because wildcards are QTYPE-agnostic.

   o  Reverse DNS is not necessarily managed by those who would like to
      configure this service.

   o  A new record type number would be available (for test deployments,
      interoperability etc.) only after an RFC has been published.

   In some cases, the reverse records are generated by scripts which
   could be modified to also add these records.  However, the presence
   of such scripts and the ability to modify them cannot be assumed.

   In short, storing information in the reverse DNS does not look like a
   good approach either.

3.3  DHCP-based Solutions

   In most situations, the users receive the IPv4 information from an
   IPv4 DHCP server.  Consequently, one of the parameters to be provided
   by the DHCP server could be the tunnel end-point address, e.g., as
   described in [11].

   This approach has several drawbacks:

   o  It requires standardizing new parameters/options on this protocol
      and also upgrading the DHCP client/server implementations to
      support this feature.

   o  It will not work if DHCP client is not used, e.g.,  in many
      dial-up scenarios, where only PPP is used; DHCP is not used in
      some (advanced) xDSL setups which use static routing.  Also, some
      managed networks do not use DHCP.  Still, in many cases, DHCP is
      used between a customer and the ISP.

   o  If a router is providing local DHCP information (e.g., an ADSL
      router), the tunnel end-point information would have to be
      automatically "proxied" to the "local DHCP", or manually
      configured on the router to propagate to the hosts in the case
      that the router is not activating the tunnel itself.

   o  It requires manual configuration/update of the ISP's DHCP servers
      when there are changes to the tunnel end-points, similar to
      updating DNS, NTP, etc., server information.

   In short, DHCP-based solutions seem unacceptable because a NAT/router
   does not automatically pass the information to the nodes in an
   "opaque" manner.


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3.4  PPP-based Solutions

   In the case of PPP-like connections, specific PPP parameters could be
   passed to the clients, as part of the AAA signaling process.

   This solution has the same drawbacks as DHCP-based solution.
   Further, there has been resistance to making extensions to PPP (e.g.,
   passing IPv6 prefix options), so it is an open question whether this
   information could be passed as a standardized PPP option at all.

3.5  SLP-based Solutions

   The Service Location Protocol [12] provides a framework for the
   discovery and selection of network services.  Tunnel-End-Point for
   IPv6 tunnels could be defined as a network service which will have
   also assigned a specific service name.

   SLP has a number of drawbacks:

   o  SLP is not really widely implemented or deployed.

   o  It requires multicast infrastructure or the additional deployment
      of Directory Agents (DA) for Service Agent (SA) discovery.

   o  If DA is deployed and the network has not multicast support, some
      way for discovery the DA is required.  DHCP could be used [12] but
      this has the same issues why a DHCP-based approach is not
      sufficient (in Section 3.3).

   o  It requires the implementation of a User Agent (UA) on the
      client's host.  This is neither always possible nor feasible.

   o  It can not offer any kind of load-balancing if more than one TEP
      is deployed.

   In short, SLP seems to come awfully short in matching the
   requirements for the solution.

3.6  Combined Solutions

   Many solutions can be combined with each other, but because the
   clients and servers must have a minimum mandatory-to-implement
   mechanism, it is better if only one externally visible mechanism can
   be used.

   Manual configuration override option is of course a good addition to
   any discovery mechanism.


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   Anycasting a locally determined TEP address (e.g., through DNS or
   DHCP) is a useful technique for load balancing purposes.  This is
   invisible to the clients.

   As has been experienced with 6to4, it might be possible to use a
   vendor's DNS branch to configure a global anycast address, but this
   the former requires no interoperability, so it's an implementation's
   internal matter.

4.  Conclusions

   Manual configuration should always be provided, but the question is
   whether the configuration distributed by ISPs (e.g., in CD-ROMs etc)
   is sufficient.

   Global anycast for initial discovery looks like a promising solution,
   but has routing operations issues which need to be considered.

   Both DNS forward and reverse based solutions suffer from various
   problems when a NAT/router is present.

   Centralized brokers are a non-starter.

   DHCP and PPP do not seem to be usable due to restrictions on
   environment where they work.  SLP is not sufficiently deployed,
   implemented or otherwise feasible.

   The following table summarizes the pros/cons of different approaches
   presented in this memo.

   +--------------------+----------------------+-----------------------+
   |                    | Pros                 | Cons                  |
   +--------------------+----------------------+-----------------------+
   | Anycast            | ***                  | *                     |
   | Centralized Broker | -                    | ***                   |
   | Forward DNS w/     | **                   | **                    |
   | Prefixing          |                      |                       |
   | Reverse DNS        | **                   | ***                   |
   | DHCP               | *                    | ***                   |
   | PPP                | -                    | ***                   |
   | SLP                | -                    | ***                   |
   +--------------------+----------------------+-----------------------+

   Qualification of pros/cons:

      -   : None

      *   : Few


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      **  : Medium

      *** : High

5.  Security Considerations

   If the tunnel end-point discovery is done in an insecure fashion, so
   that an attacker could influence the discovery process, the attacker
   could be able to hijack all the IPv6 communications.  This must be
   kept in mind when analyzing the different discovery solutions, and
   spelled-out explicitly in the requirements, if the threats are to be
   mitigated in tunneling mechanisms somehow (e.g., using a return
   routability procedures).

   In particular, the potential weaknesses of DNS bear some
   consideration.

6.  IANA Considerations

   This document requests no action for IANA.

   [[note to RFC-editor: this section can be removed upon publication.]]

7.  Acknowledgements

   This memo was written as a consequence of experience using IPv6 when
   traveling, number of talks during IETF meetings and specially the
   work with the unmanaged, ISP and enterprise v6ops design teams.

   The authors would also like to acknowledge inputs from Carl Williams,
   Brian Carpenter, Jeroen Massar, Alain Durand, Tim Chown, and Florent
   Parent.  This work has been partially funded by the European
   Commission under the Euro6IX project.

8.  References

8.1  Normative References

   [1]  Parent, F., "Goals for Registered Assisted Tunneling",
        Internet-Draft draft-ietf-v6ops-assisted-tunneling-requirements-01
        , October 2004.

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

   [3]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
        RFC 3068, June 2001.


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   [4]  Lindqvist, K. and J. Abley, "Operation of Anycast Services",
        Internet-Draft draft-kurtis-anycast-bcp-00, October 2004.

   [5]  Thaler, D. and L. Vicisano, "IPv4 Automatic Multicast Without
        Explicit Tunnels (AMT)",
        Internet-Draft draft-ietf-mboned-auto-multicast-03, October
        2004.

   [6]  Faltstrom, P. and R. Austein, "Design Choices When Expanding
        DNS", Internet-Draft draft-iab-dns-choices-00, October 2004.

8.2  Informative References

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

   [8]   Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs",
         Internet-Draft draft-huitema-v6ops-teredo-04, January 2005.

   [9]   Templin, F., Gleeson, T., Talwar, M. and D. Thaler, "Intra-Site
         Automatic Tunnel Addressing Protocol (ISATAP)",
         Internet-Draft draft-ietf-ngtrans-isatap-23, December 2004.

   [10]  Hagino, J. and K. Ettican, "An analysis of IPv6 anycast",
         Internet-Draft draft-ietf-ipngwg-ipv6-anycast-analysis-02, June
         2003.

   [11]  Kim, P. and S. Park, "DHCP Option for Configuring IPv6-in-IPv4
         Tunnels", Internet-Draft draft-daniel-dhc-ipv6in4-opt-05,
         October 2004.

   [12]  Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
         Location Protocol, Version 2", RFC 2608, June 1999.

   [13]  Brisco, T., "DNS Support for Load Balancing", RFC 1794, April
         1995.

   [14]  Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
         IPv6", RFC 3775, June 2004.

   [15]  Durand, A. and S. Yamamoto, "Service Discovery using NAPTR
         records in DNS",
         Internet-Draft draft-yamamoto-naptr-service-discovery-00,
         October 2004.




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Authors' Addresses

   Jordi Palet Martinez
   Consulintel
   San Jose Artesano, 1
   Alcobendas - Madrid
   E-28108 - Spain

   Phone: +34 91 151 81 99
   Fax:   +34 91 151 81 98
   Email: jordi.palet@consulintel.es

   Miguel Angel Diaz Fernandez
   Consulintel
   San Jose Artesano, 1
   Alcobendas - Madrid
   E-28108 - Spain

   Phone: +34 91 151 81 99
   Fax:   +34 91 151 81 98
   Email: miguelangel.diaz@consulintel.es

   Pekka Savola
   CSC/FUNET
   Espoo
   Finland

   Email: psavola@funet.fi

Appendix A.  More Discussion of Anycast Discovery

   We do a little bit of more extensive analysis of anycast-based
   solution here to get a better understanding of its operational
   properties.

   At this point, we only discuss the different failure modes of initial
   handshake anycast, and see that these can be solved with robust
   specification and implementation.

   a.  A new server starts advertising the address but is refusing to
       serve the users: established tunnels continue to work, new
       tunnels cannot be established; tracerouting to the server
       identifies where the culprit is.

   b.  The current anycast server goes down: tunnels established to its
       unicast address go down after the event is detected.  New tunnels


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       use the next anycast server (if available) or no server at all
       (in which case the tunnels may get re-created when the server
       comes back up).

   c.  There is no server in the local ISP, and the anycast prefix has
       been filtered out or does not exist globally: the initial
       discovery follows the default route, and either gets discarded
       (and the discovery process notices this after a timeout) or a
       router returns a network/host unreachable ICMP error message.

   d.  There is no server locally, but someone else is advertising it
       here: if the service works, that's OK (though if the service
       works but is bad quality or 100's of milliseconds away is
       undesirable; robust implementations may check the RTT).  If the
       server fails to serve, that's also OK -- the discovery process
       has to be robust against this.

Appendix B.  Centralized Broker-based Solutions

   This solution is described in the appendix because it does not
   actually solve the discovery problem, but has been proposed on the
   assumption that it would.

   Inside a single administrative domain, it would also be possible to
   deploy a centralized server or a "broker" knowing the status of all
   the associated end-points.  Furthermore, it could redirect the users
   to the correct end-points.  This mechanism would still need another
   complementary approach to actually discover the centralized broker.

   This approach is highly assumptive of the tunneling set-up mechanism,
   and likely requires the implementation of lengthy redirection or
   negotiation features which do not work well through a NAT.

   Applying a centralized model over multiple administrative domains,
   e.g., having a single server for the whole Internet, would be
   administratively and management-wise unfeasible.

   A global centralized broker is completely unfeasible.  The only
   benefit of a local broker is the ability to perform more fine-grained
   load balancing or policing, and does not solve the actual problem,
   because the same methods need to be applied to discover the
   centralized broker.




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