Mipshop WG                                                 T. Melia, Ed.
Internet-Draft                                                     CISCO
Intended status: Standards Track                                G. Bajko
Expires: October 19, 2008                                          Nokia
                                                                  S. Das
                                             Telcordia Technologies Inc.
                                                               N. Golmie
                                                              JC. Zuniga
                                                          April 17, 2008

                   Mobility Services Framework Design

Status of this Memo

   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 becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on October 19, 2008.


   This document describes a design solution for the IEEE 802.21 Media
   Independent Handover (MIH) protocol that addresses identified issues
   associated with the transport of MIH messages.  The document
   describes mechanisms for mobility service (MoS) discovery and
   transport layer mechanisms for the reliable delivery of MIH messages.

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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Deployment Scenarios . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Scenario S1: Home Network MoS  . . . . . . . . . . . . . .  6
     3.2.  Scenario S2: Visited Network MoS . . . . . . . . . . . . .  6
     3.3.  Scenario S3: Roaming MoS . . . . . . . . . . . . . . . . .  7
     3.4.  Scenario S4: Third party MoS . . . . . . . . . . . . . . .  7
   4.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Architecture . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  MIHF Identifiers (FQDN, NAI) . . . . . . . . . . . . . . . 10
   5.  MoS Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  MoS Discovery when MN and MoSh are in the home network
           (Scenario S1)  . . . . . . . . . . . . . . . . . . . . . . 11
     5.2.  MoS Discovery when MN is in visited network and MoSv
           is also in visited network (Scenario S2) . . . . . . . . . 12
     5.3.  MOS Discovery when the MN is in a visited Network and
           Services are at the Home network (Scenario S3) . . . . . . 13
     5.4.  MoS discovery when MIH services are in a 3rd party
           remote network (scenario S4) . . . . . . . . . . . . . . . 16
   6.  MIH Transport Options  . . . . . . . . . . . . . . . . . . . . 17
     6.1.  MIH Message size . . . . . . . . . . . . . . . . . . . . . 18
     6.2.  MIH Message rate . . . . . . . . . . . . . . . . . . . . . 19
     6.3.  Retransmission . . . . . . . . . . . . . . . . . . . . . . 19
     6.4.  NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . 19
     6.5.  General guidelines . . . . . . . . . . . . . . . . . . . . 20
   7.  Operation Flows  . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     11.2. Informative References . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
   Intellectual Property and Copyright Statements . . . . . . . . . . 28

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1.  Introduction

   This document proposes a solution to the issues identified in the
   problem statement document [I-D.ietf-mipshop-mis-ps] for the layer 3
   transport of IEEE 802.21 MIH protocols.

   The MIH Layer 3 transport problem is divided in two main parts: the
   discovery of a node that supports specific Mobility Services (MoS)
   and the transport of the information between a mobile node (MN) and
   the discovered node.  The discovery process is required for the MN to
   obtain the information needed for MIH protocol communication with a
   peer node.  The information includes the transport address (e.g., the
   IP address) of the peer node and the types of MoS provided by the
   peer node.

   This document lists the major MoS deployment scenarios.  It describes
   the solution architecture, including the MSTP reference model and
   MIHF identifiers.  MoS discovery procedures explain how the MN
   discovers MoS both in its home network or while being connected to a
   remote network.  The remainder of this document describes the MIH
   transport architecture, example message flows for several signaling
   scenarios, and security issues.

2.  Terminology

   The following acronyms and terminology are used in this document:

   MIH  Media Independent Handover: the handover support architecture
      defined by the IEEE 802.21 working group that consists of the MIH
      Function (MIHF), MIH Network Entities and MIH protocol messages.

   MIHF  Media Independent Handover Function: a switching function that
      provides handover services including the Event Service (ES),
      Information Service (IS), and Command Service (CS), through
      service access points (SAPs) defined by the IEEE 802.21 working
      group [IEEE80221].

   MIHF User  an entity that uses the MIH SAPs to access MIHF services,
      and which is responsible for initiating and terminating MIH

   MIHFID  Media Independent Handover Function Identifier: an identifier
      required to uniquely identify the MIHF endpoints for delivering
      mobility services (MoS); it is implemented as either a FQDN or

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   MoS  Mobility Services: those services, as defined in the MIH problem
      statement document [I-D.ietf-mipshop-mis-ps] , which includes the
      MIH IS, CS, and ES services defined by the IEEE 802.21 standard.

   MoSh:  Mobility Services assigned in the mobile node's Home Network

   MoSv:  Mobility Services assigned in the Visited Network, which is
      any network other than the mobile node's home network

   MoS3:  Mobility Services assigned in a 3rd Party Network, which is a
      network that is neither the Home Network nor the current Visited

   MN Mobile Node: an Internet device whose location changes, along with
      its point of connection to the network.

   MSTP  Mobility Services Transport Protocol: a protocol that is used
      to deliver MIH protocol messages from an MIHF to other MIH-aware
      nodes in a network.

   IS Information Service: a MoS that originates at the lower or upper
      layers and sends information to the local or remote upper or lower
      layers.  It can use secure or insecure ports to transport
      information elements (IEs) and information about various
      neighboring nodes.  Its architecture is outside the scope of the
      IEEE 802.21 draft document.

   ES Event Service: a MoS that originates at a remote MIHF or the lower
      layers of protocol stack and sends information to the local MIHF
      or local higher layers.  The purpose of the ES is to report
      changes in link status (e.g.  Link Going Down messages) and
      transmission status.

   CS Command Service: a MoS that sends commands from the remote MIHF or
      local upper layers to the local lower layers of the protocol stack
      to switch links or to get link status.

   FQDN:  Fully-Qualified Domain Name: a complete domain name for a host
      on the Internet, consisting of a host name followed by a domain
      name (e.g. hostname.domain.org)

   NAI  Network Access Identifier: the user ID that a user submits
      during PPP authentication.  For mobile users, the NAI identifies
      the user and helps to route the authentication request message.

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   NAT  Network Address Translator: A device that implements the Network
      Address Translation function described in [RFC3022], in which
      local or private network layer addresses are mapped to valid
      network addresses and port numbers.

   DHCP  Dynamic Host Configuration Protocol: a protocol described in
      [RFC2131] that allows Internet devices to obtain IP addresses,
      subnet masks, default gateway addresses, and other IP
      configuration information from DHCP servers.

   DNS  Domain Name System: a protocol described in [RFC1035] that
      translates domain names to IP addresses.

   AAA  Authentication, Authorization and Accounting: a set of network
      management services that respectively determine the validity of a
      user's ID, determine whether a user is allowed to use network
      resources, and track users' use of network resources.

   Home AAA  AAAh: an AAA server located on the MN's home network.

   Visited AAA  AAAv: an AAA server located a visited network that is
      not the MN's home network.

   MIH ACK  MIH Acknowledgement Message: a MIH signaling message that a
      MIHF sends in response to an MIH message from a sending MIHF, when
      UDP is used as the MSTP.

   PoS  Point of Service, a network-side MIHF instance that exchanges
      MIH messages with a MN-based MIHF.

   NAS  Network Access Server: a server to which a MN initially connects
      when it is trying to gain a connection to a network and which
      determines whether the MN is allowed to connect to the NAS's

   UDP  User Datagram Protocol: a connectionless transport layer
      protocol used to send datagrams between a source and a destination
      at a given port, defined in RFC 768.

   TCP  Transmission Control Protocol: a stream-oriented transport layer
      protocol that provides a reliable delivery service with congestion
      control, defined in RFC 793.

   RTT  Round-Trip Time: an estimation of the time required for a
      segment to travel from a source to a destination and an
      acknowledgement to return to the source that is used by TCP in
      connection with timer expirations to determine when a segment is
      considered lost and should be resent.

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   MTU  Maximum Transmission Unit: the largest size packet that can be
      sent on a network without requiring fragmentation [RFC1191].

   TLS  Transport Layer Security Protocol: an application layer protocol
      that assures privacy and data integrity between two communicating
      network entities [RFC4346].

3.  Deployment Scenarios

   This section describes the various possible deployment scenarios for
   the MN and the MoS.  The relative positioning of MN and MoS affects
   MoS discovery as well as the performance of the MIH signaling

3.1.  Scenario S1: Home Network MoS

   In this scenario, the MN and the services are located in the home
   network.  We refer to this set of services as MoSh as in Figure 1.
   The MoSh can be located at the access network the MN uses to connect
   to the home network, or it can be located elsewhere.

   +--------------+  +====+
   | HOME NETWORK |  |MoSh|
   +--------------+  +====+
   |   MN   |

                     Figure 1: MoS in the Home network

3.2.  Scenario S2: Visited Network MoS

   In this scenario, the MN is in the visited network and mobility
   services are provided by the visited network.  We refer to this as
   MoSv as shown in Figure 2.

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             | HOME NETWORK |
    +====+ +-----------------+
    +====+ +-----------------+
               |   MN   |

                   Figure 2: MoSV in the Visited Network

3.3.  Scenario S3: Roaming MoS

   In this scenario, the MN is located in the visited network and all
   MIH services are provided by the home network, as shown in Figure 3.

    +====+   +--------------+
    |MoSh|   | HOME NETWORK |
    +====+   +--------------+
          | VISITED NETWORK |
               |   MN   |

            Figure 3: MoS provided by the home while in visited

3.4.  Scenario S4: Third party MoS

   In this scenario, the MN is in its home network or in a visited
   network and services are provided by a 3rd party network.  We refer
   to this situation as MoS3 as shown in Figure 4.  (Note that MoS can
   exist both in home and in visited networks).

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                                      | HOME NETWORK |
   +====+    +--------------+         +--------------+
   |MoS3|    | THIRD PARTY  |  <===>        /\
   +====+    +--------------+               ||
                                    | VISITED NETWORK |
                                        |   MN   |

                     Figure 4: MoS form a third party

   Different types of MoS can be provided independently of other types
   and there is no strict relationship between ES, CS and IS, nor is
   there a requirement that the entities that provide these services
   should be co-located.  However, while IS tends to involve a large
   amounts of static information, ES and CS are dynamic services and
   some relationships between them can be expected, e.g., a handover
   command (CS) could be issued upon reception of a link event (ES).
   Hence, while in theory MoS can be implemented in different locations,
   it is expected that ES and CS will be co-located, whereas IS can be
   co-located with ES/CS or located elsewhere.  Therefore, having the
   flexibility at the MSTP to discover different services in different
   locations is an important feature that can be used to optimize
   handover performance.  MoS discovery is discussed in more detail in
   Section 5.

4.  Solution Overview

   As mentioned in Section 1, the solution space is being divided into
   two functional domains: discovery and transport.  The following
   assumptions have been made:

   o  The solution is aimed at supporting IEEE 802.21 MIH services,
      namely Information Service (IS), Event Service (ES), and Command
      Service (CS).

   o  If the MIHFID is available, FQDN or NAI's realm is used for
      mobility service discovery.

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   o  The solutions are chosen to cover all possible deployment
      scenarios as described in Section 3.

   o  MoS discovery can be performed during initial network attachment
      or thereafter.

   The MN could know or not know the realm of the MoS to be discovered.
   In any case after the acquisition of the target realm (e.g. via
   Information Server or statically configured) the MN could either be
   pre-configured with the address of the MoS, or this address could be
   obtained through DHCP or DNS.  The dynamic assignment methods are
   described in Section 5.

   The configuration of the MoS could be executed either upon network
   attachment or after successful IP configuration.  The methodology to
   be used depends on the considered deployment scenario.

   Once the MIHF peer has been discovered, MIH information can be
   exchanged between MIH peers over a transport protocol such as UDP or
   TCP.  The usage of transport protocols is described in Section 6.

4.1.  Architecture

   Figure 5 depicts the MSTP reference model and its components within a
   node.  The topmost layer is the MIHF user.  This set of applications
   consists of one or more MIH clients that are responsible for
   operations such as generating query and response, processing Layer 2
   triggers as part of the ES, and initiating and carrying out handover
   operations as part of the CS.  Beneath the MIHF user is the MIHF
   itself.  This function is responsible for MoS discovery, as well as
   creating, maintaining, modifying, and destroying MIH signaling
   associations with other MIHFs located in MIH peer nodes.  Below the
   MIHF are various transport layer protocols as well as address
   discovery functions.

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    |       MIHF User          |
    |           MIHF           |
        ||         ||       ||
    +---------+ +------+ +-----+
    | TCP/UDP | | DHCP | | DNS |
    +---------+ +------+ +-----+

                            Figure 5: MN stack

   The MIHF relies on the services provided by TCP and UDP for
   transporting MIH messages, and relies on DHCP and DNS for peer
   discovery.  In cases where the peer MIHF IP address is not pre-
   configured, the source MIHF needs to discover it either via DHCP or
   DNS or a combination of both as described in Section 5.  Once the
   peer MIHF is discovered, MIHF must exchange messages with its peer
   over either UDP or TCP.  Specific recommendations regarding the
   choice of transport protocols are provided in Section 6.

   The above reference architecture however does not include other
   services such as message fragmentation and security.  Depending upon
   the MIH service type (e.g., ES, CS and IS) the message size can be
   very large.  In the case where the underlying layers do not support
   fragmentation, this may be an issue.  There are no security features
   currently defined as part of the MIH protocol level.  However,
   security can be provided either at the transport or IP layer where it
   is necessary.  Section 8 provides some guidelines and recommendations
   for security.

4.2.  MIHF Identifiers (FQDN, NAI)

   MIHFID is an identifier required to uniquely identify the MIHF end
   points for delivering the mobility services (MoS).  Thus an MIHF
   identifier needs to be unique within a domain where mobility services
   are provided and independent of the configured IP addresse(s).  An
   MIHFID MUST be represented either in the form of an FQDN [RFC2181] or
   NAI [RFC4282].  An MIHFID can be pre-configured or discovered through
   the discovery methods described in Section 5.

5.  MoS Discovery

   The MoS discovery method depends on whether the MN attempts to
   discover an MoS in the home network, in the visited network, or in a

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   3rd party remote network that is neither the home network nor the
   visited network.

   In the case where MoS is provided locally (scenarios S1 and S2) , the
   discovery techniques described in [I-D.bajko-mos-dhcp-options] and
   [I-D.bajko-mos-dns-discovery] are both applicable and either one MAY
   be used to discover the MoS.

   In the case where MoS is provided in the home network while the MN is
   in the visited network (scenario S3), the DNS based discovery
   described in [I-D.bajko-mos-dns-discovery] is applicable, while the
   DHCP based discovery method would require an interaction between the
   DHCP and the AAA infrastructure, similarly to what is specified in
   [I-D.ietf-mip6-bootstrapping-integrated-dhc] .  This latter case
   assumes that MoS assignment is performed during access authentication
   and authorization.

   In the case where MoS is provided by a third party network which is
   different from the current visited network (scenario S4), only the
   DNS based discovery method described in [I-D.bajko-mos-dns-discovery]
   is applicable.

5.1.  MoS Discovery when MN and MoSh are in the home network (Scenario

   To discover an MoS in the home network, the MN SHOULD use the DNS
   based MoS discovery method described in
   [I-D.bajko-mos-dns-discovery].  In order to use that mechanism, the
   MN MUST first find out the domain name of its home network.  Home
   domains are usually pre-configured in the MNs (i.e. subscription is
   tied to a network), thus the MN can simply read its configuration
   data to find out the home domain name (scenario S1).  The DNS query
   option is shown in Figure 6a.  Alternatively, the MN MAY use the DHCP
   options for MoS discovery[I-D.bajko-mos-dhcp-options] as shown
   inFigure 6b.

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               +----+         |Domain |
               | MN |-------->|Name   |
               +----+         |Server |

                             (a) using DNS Query

                            +-----+      +------+
               +----+       |     |      |DHCP  |
               | MN |<----->| DHCP|<---->|Server|
               +----+       |Relay|      |      |
                            +-----+      +------+

                             (b)  Using DHCP Option

    Figure 6: MOS Discovery (a) Using DNS query, (b) using DHCP option

5.2.  MoS Discovery when MN is in visited network and MoSv is also in
      visited network (Scenario S2)

   To discover an MoS in the visited network, the MN SHOULD attempt to
   use the DHCP options for MoS discovery [I-D.bajko-mos-dhcp-options]
   as shown in Figure 7a.  If the DHCP method fails, the MN SHOULD
   attempt to use the DNS based MoS discovery method described in
   [I-D.bajko-mos-dns-discovery] as shown in Figure 7b.  In order to use
   that, the MN MUST first learn the domain name of the local network.
   There are a number of ways how the domain name of a network can be

   DHCP --  In order to find out the domain name of the local network,
      the MN SHOULD use the dhcpv4 option 15 for learning the domain
      name [RFC1533].  A similar solution is available for dhcpv6
      [I-D.ietf-dhc-dhcpv6-opt-dnsdomain] .

   Reverse dns query --  When DHCP does not provide the required domain
      name, the MN MAY use reverse DNS (DNS PTR record) to find the
      domain name associated with the IP address it is using in the
      visited network.  Note, that when a NAT device exists between the
      MN and the visited network, the MN will first need to find out the
      external IP address of the NAT device.  Some possible methods for
      determining the NAT's external IP address are STUN [RFC3849] or
      UPnP [UPnP_IDG_DCP].  Once the MN has determined the external IP
      address of the NAT device, it MUST use that address in the reverse

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      DNS query.

                         +-----+      +------+
            +----+       |     |      |DHCP  |
            | MN |<----->| DHCP|<---->|Server|
            +----+       |Relay|      |      |
                         +-----+      +------+

                   (a) MoS Discovery using DHCP options

            +----+         |Domain |
            | MN |-------->|Name   |
            +----+         |Server |

                    (b) Reverse DNS Query (starting from the IP address)

         Figure 7: Discovery (a) using DHCP option, (b) Using DNS

   It should be noted, that the usage of DHCP options to discover an MoS
   in this particular scenario is recommended because of its simplicity
   over the DNS based discovery method: the DNS discovery method
   requires the MN to learn the domain name of the local network first,
   possibly using DHCP, and then perform the DNS query.  The usage of
   the DHCP based discovery method does not require any additional

   When the discovery of an MoS at the visited network, using the FQDN
   returned in the reverse DNS query, is not successful, the MN MAY
   attempt to remove portions from the left side of the FQDN and attempt
   discovery again.  The process MAY be repeated iteratively until a
   successful discovery.

5.3.  MOS Discovery when the MN is in a visited Network and Services are
      at the Home network (Scenario S3)

   To discover an MoS in the visited network when MIH services are
   provided by the home network, both the DNS based discovery method
   described in [I-D.bajko-mos-dns-discovery] and the DHCP based
   discovery method described in [I-D.bajko-mos-dhcp-options] are

   To discover the MoS at home while in a visited network using DNS, the

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   MN SHOULD use the procedures described in Section 5.1

   Alternatively, the MN MAY also use the DHCP based discovery method.
   Using the DHCP based discovery may be required in deployments where
   the usage of MoS located in the home network is enforced and included
   in the subscription profile.  Similar to
   [I-D.ietf-mip6-bootstrapping-integrated-dhc] in this integrated
   scenario the mobile node is required to perform network access
   authentication before it can obtain the MoS information.  This allows
   for MoS discovery at the time of access authentication and
   authorization.  Also, the mechanism defined in this document requires
   the NAS to support MIH specific AAA attributes and a collocated DHCP
   relay agent.  In order to provide the mobile node with information
   about the assigned MoS, the AAAh conveys the assigned MoS's
   information to the NAS via AAA protocol as defined in
   [I-D.ietf-dime-mip6-integrated] and described in

   In these deployment scenarios the AAAh sends the MoS address at home
   to the AAAv during the network access authentication.  The relation
   beween functional components supporting such procedure is shown in
   Figure 8.

   The mobile node executes the network access authentication procedure
   (e.g., IEEE 802.11i/802.1X) and it interacts with the NAS.  The NAS
   is in the visited and it interacts with AAAh via AAAv to authenticate
   the mobile node.  In the process of authorizing the mobile node, the
   AAAh verifies in the AAA profile that the mobile node is allowed to
   use MoS services.  The AAAh assigns the MoS in the home and returns
   this information to the NAS.  The NAS may keep the received
   information for a configurable duration or it may keep the
   information for as long as the MN is connected to the NAS.

   The mobile node sends a DHCPv6 Information Request message [RFC3315]
   to the All_DHCP_Relay_Agents_and_Servers multicast address.  In this
   message the mobile node (DHCP client) MUST include the Option Code
   for MoS Identifier Option [I-D.bajko-mos-dhcp-options] in the
   OPTION_ORO.  The mobile node MUST also include the OPTION_CLIENTID to
   identify itself to the DHCP server.

   The Relay Agent intercepts the Information-request from the mobile
   node and forwards it to the DHCP server.  The Relay Agent also
   includes the received MoS information from the AAAh in the IPv6 Relay
   Agent MoS Option [I-D.bajko-mos-dhcp-options].  If a NAS
   implementation does not store the received information as long as the
   MN's session remains in the visited network, and if the MN delays
   sending DHCP request, the NAS/DHCP relay does not include the IPv6
   Relay Agent MoS Option in the Relay Forward message.

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   The DHCP server identifies the client by looking at the DUID for the
   client in the OPTION_CLIENTID.  The DHCP server determines that the
   MoS is allocated by the AAAh by looking at the IPv6 Relay Agent Sub-
   Option in the IPv6 Relay Agent MoS Option.  The DHCP server extracts
   the allocated MoS information from the IPv6 Relay Agent Sub-Option
   and includes it in the MoS Information Option
   [I-D.bajko-mos-dhcp-options] in the Reply Message.  If the requested
   information is not available in the DHCP server, it follows the
   behavior described in [RFC3315].

   The Relay Agent relays the Reply Message from the DHCP server to the
   mobile node.  At this point, the mobile node has the MoS information
   that it requested.

   In should be noted, that using the DHCP Options and procedures
   defined in [I-D.bajko-mos-dhcp-options] the MN can not specify the
   network (local or home) where it wants the MoS address from.  Whether
   the MN receives an MoS address from local or home network will depend
   on the actual network deployment (scenario S2 or S3) and operator
   policy.  In an integrated scenario, where the network access
   authentication is performed by the home network the MoS information
   will be always sent to the AAAv, then stored in the relay agent and
   ultimately sent to the MN if the MN asks for it, using the procedures
   defined in [I-D.bajko-mos-dhcp-options].

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                           Visited             |          Home
                           +-------+           |        +-------+
                           |       |           |        |       |
                           |AAAV   |-----------|--------|AAAH   |
                           |       |           |        |       |
                           |       |           |        |       |
                           +-------+           |        +-------+
                               |               |
                               |               |
                               |               |
                               |               |
                               |               |       +--------+
                               |               |       |        |
                               |               |       |  MoSh  |
                           +-----+    +------+ |       +--------+
               +----+      |     |    |DHCP  | |
               | MN |------| NAS/|----|Server| |
               +----+      | DHCP|    |      | |
                           |Relay|    |      | |
                           +-----+    +------+ |

          AAAv -- Visited AAA
          AAAH -- Home AAA
          NAS  -- Network Access Server

   Figure 8: MOS Discovery using Network Access Authentication and DHCP

   This section assumes the use of IPv6 and DHCPv6 based mechanisms to
   discover MoS services in home while the MN is in visited network.  If
   similar functionalities are desired for IPv4 additional DHCPv4
   extensions would be required.  Since use cases requiring these
   extensions were not identified at the time of writing this document,
   they were excluded from the scope of the document.

5.4.  MoS discovery when MIH services are in a 3rd party remote network
      (scenario S4)

   To discover an MoS in a remote network other than home network, the
   MN MUST use the DNS based MoS discovery method described in
   [I-D.bajko-mos-dns-discovery].  The MN MUST first learn the domain
   name of the network containing the MoS it is searching for.  If the
   MN does not yet know the domain name of the network, learning it may
   require more than one operation, as DHCP based discovery can not be

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   used and pre-configuration is not a feasible solution in case of an
   arbitrary remote network.  The MN MAY attempt to first discover an
   MoS in either the local or home network (as in Figure 9 part (a)) and
   query that MoS to find out the domain name of a specific network or
   the domain name of a network at a specific location (as in Figure 9
   part (b)).  Alternatively, the MN MAY query an MoS previously known
   to learn the domain name of the desired network (e.g., via an IS
   Query).  Finally, the MN MUST use DNS queries to find MoS in the
   remote network as inFigure 9 part(c).  It should be noted that step c
   can only be performed upon obtaining the domain name of the remote

                    +----+         |DHCP   |
                    | MN |-------->|       |
                    +----+         |Server |

           (a) Discover MoS in local network with DHCP
                +----+         |            |
                |    |         |Information |
                | MN |-------->| Server     |
                |    |         |(previously |
                +----+         |discovered) |

      (b) Using IS query to find the FQDN on the remote network

                  +----+         |Domain |
                  | MN |-------->|Name   |
                  +----+         |Server |

            (c) using DNS Query in the remote network

     Figure 9: MOS Discovery using (a) DHCP Options, (b) IS Query to a
                      known IS Server, (c) DNS Query

6.  MIH Transport Options

   Once the Mobility Services have been discovered, MIH peers MAY
   exchange information over TCP, UDP or any other transport supported
   by both the server and client, as described in

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   [I-D.rahman-mipshop-mih-transport].  The client MAY use the DNS
   discovery mechanism to discover which transport protocols are
   supported by the server in addition to TCP and UDP that are
   recommended in this document.  While either protocol can provide the
   basic transport functionality required, there are performance trade-
   offs and unique characteristics associated with each that need to be
   considered in the context of the MIH services for different network
   loss and congestion conditions.  The objectives of this section are
   to discuss these trade-offs for different MIH settings such as the
   MIH message size and rate, and the retransmission parameters.  In
   addition, factors such as NAT traversal are also discussed.  Given
   the reliability requirements for the MIH transport, it is assumed in
   this discussion that the MIH ACK mechanism is to be used in
   conjunction with UDP, while it is preferred to avoid using MIH ACKs
   with TCP since TCP includes acknowledgement and retransmission

6.1.  MIH Message size

   Although the MIH message size varies widely from about 30 bytes (for
   a broadcast capability discovery request) to around 65000 bytes (for
   an IS MIH_Get_Information response primitive), a typical MIH message
   size for the ES/CS service ranges between 50 to 100 bytes
   [IEEE80221].  Thus, considering the effects of the MIH message size
   on the performance of the transport protocol brings us to discussing
   two main issues, related to fragmentation of long messages in the
   context of UDP and the concatenation of short messages in the context
   of TCP.  Since transporting long MIH messages may require
   fragmentation that is not available in UDP, if MIH is using UDP a
   limit MUST be set on the size of the MIH message, unless
   fragmentation functionality is added to the MIH layer or IP layer
   fragmentation is used instead.  In this latter case, the loss of an
   IP fragment leads to the retransmission of an entire MIH message,
   which in turn leads to poor end-to-end delay performance in addition
   to wasted bandwidth.  Additional recommendations in
   [I-D.ietf-tsvwg-udp-guidelines] apply for limiting the size of the
   MIH message when using UDP and assuming IP layer fragmentation.  In
   terms of dealing with short messages, TCP has the capability to
   concatenate very short messages in order to reduce the overall
   bandwidth overhead.  However, this reduced overhead comes at the cost
   of additional delay to complete an MIH transaction, which may not be
   acceptable for CS and ES services.  Note also that TCP is a stream
   oriented protocol and measures data flow in terms of bytes, not
   messages.  Thus it is possible to split messages across multiple TCP
   segments if they are long enough.  Even short messages can be split
   across two segments.  This can also cause unacceptable delays,
   especially if the link quality is severely degraded as is likely to
   happen when the MN is exiting a wireless access coverage area.

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6.2.  MIH Message rate

   The frequency of MIH messages varies according to the MIH service
   type.  It is expected that CS/ES message arrive at a rate of one in
   hundreds of milliseconds in order to capture quick changes in the
   environment and/ or process handover commands.  On the other hand, IS
   messages are exchanged mainly every time a new network is visited
   which may be in order of hours or days.  Therefore a burst of either
   short CS/ES messages or long IS message exchanges (in the case where
   multiple MIH nodes request information) may lead to network
   congestion.  While the built-in rate-limiting controls available in
   TCP may be well suited for dealing with these congestion conditions,
   this may result in large transmission delays that may be unacceptable
   for the timely delivery of ES/CS messages.  On the other hand, if UDP
   is used, a rate-limiting effect similar to the one obtained with TCP
   may be obtained by adequately adjusting the parameters of a token
   bucket regulator as defined in the MIH specifications [IEEE80221].
   Recommendations for tocken bucket parameter settings are specific to
   the scenario considered.

6.3.  Retransmission

   For TCP, the retransmission timeout is adjusted according to the
   measured RTT.  However due to the exponential backoff mechanism, the
   delay associated with retransmission timeouts may increase
   significantly with increased packet loss.

   If UDP is being used to carry MIH messages, MIH SHOULD use MIH ACKs.
   An MIH message is retransmitted if its corresponding MIH ACK is not
   received by the generating node within a timeout interval set by the
   MIHF.  This approach does not include an exponential backoff and
   therefore tends to degrade more gracefully than TCP when the packet
   loss rate becomes large, in the sense that the expected delay does
   not increase exponentially.  The number of retransmissions is
   limited, which reduces head-of-line blocking of other MIH messages,
   but this can cause important ES/CS messages to be lost.

6.4.  NAT Traversal

   There are no known issues for NAT traversal when using TCP.  The
   default connection timeout of 24 hours is considered adequate for MIH
   transport purposes.  However, issues with NAT traversal using UDP are
   documented in [I-D.ietf-tsvwg-udp-guidelines].  Communication
   failures are experienced when middleboxes destroy the per-flow state
   associated with an application session during periods when the
   application does not exchange any UDP traffic.  Hence, communication
   between the MN and the MoS SHOULD be able to gracefully handle such
   failures and implement mechanisms to re-establish their UDP sessions.

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   In addition and in order to avoid such failures, MIH messages MAY be
   sent periodically, similarly to keep-alive messages, to attempt to
   refresh middlebox state (e.g.  ES reports could be used for this
   purpose).  As [RFC4787] requires a minimum state timeout of two
   minutes or more, MIH messages using UDP as transport SHOULD be sent
   once every two minutes.

6.5.  General guidelines

   Since ES and CS messages are small in nature and have tight latency
   requirements, UDP in combination with MIH acknowledgement SHOULD be
   used for transporting ES and CS messages.  On the other hand, IS
   messages are more resilient in terms of latency constraints and some
   long IS messages could exceed the MTU of the path to the destination.
   Therefore, TCP SHOULD be used for transporting IS messages.  For both
   UDP and TCP cases, if a port number is not explicitly assigned (e.g.
   by the DNS SRV), MIH messages sent over UDP, TCP or other supported
   transport MUST use the default port number defined for that
   particular transport.

   MoS server MUST support both UDP and TCP for MIH transport and the MN
   MUST support TCP.  Additionally, the server and MN MAY support
   additional transport mechanisms.  The MN MAY use the procedures
   defined in [I-D.bajko-mos-dns-discovery] to discover additional
   transport protocols supported by the server.

7.  Operation Flows

   Figure 10 gives an example operation flow between MIHF peers when an
   MIH user requests an IS service.  Scenario 1 is in effect, i.e. the
   MoS and the MN are both in the MN's home network.  Thus DHCP is used
   for MoS discovery and TCP is used for establishing a transport
   connection to carry the IS messages.  When MoS is not pre-configured,
   the MIH user needs to discover the IP address of MoS to communicate
   with the remote MIHF.  Therefore the MIH user sends a discovery
   request message to the local MIHF as defined in [IEEE80221].

   In this example (one could draw similar mechanisms with DHCPv6), we
   assume that MoS discovery is performed before a transport connection
   is established with the remote MIHF, and the DHCP client process is
   invoked via some internal APIs.  DHCP Client sends DHCP INFORM
   message according to standard DHCP and with the MoS option as defined
   in [I-D.bajko-mos-dhcp-options].  DHCP server replies via DHCP ACK
   message with the IP address of the MoS.  The MoS address is then
   passed to the MIHF locally via some internal APIs.  MIHF generates
   the discovery response message and passes it on to the corresponding
   MIH user.  The MIH user generates an IS query addressed to the remote

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   MoS.  MIHF invokes the underlying TCP client which establishes a
   transport connection with the remote peer.  Once the transport
   connection is established, MIHF sends the IS query via MIH protocol
   REQUEST message.  The message and query arrive at the destination
   MIHF and MIH user respectively.  The MoS MIH user responds to the
   corresponding IS query and the MoS MIHF sends the IS response via MIH
   protocol RESPONSE message.  The message arrives at the source MIHF
   which passes the IS response on to the corresponding MIH user.

                MN                                             MoS
|====================================|    |======|   |===================|
 + ---------+                                                 + ---------+
 | MIH USER |       +------+  +------+    +------+   +------+ | MIH USER |
 | +------+ |       | TCP  |  |DHCP  |    |DHCP  |   | TCP  | | +------+ |
 | | MIHF | |       |Client|  |Client|    |Server|   |Server| | | MIHF | |
 +----------+       +------+  +------+    +------+   +------+ +----------+
     |                 |         |           |          |          |
     |MIH Discovery    |         |           |          |          |
     |Request          |         |           |          |          |
     |(MIH User-> MIHF)|         |           |          |          |
     |======>          |         |           |          |          |
     |                 |         |           |          |          |
     |Invoke DHCP Client         |           |          |          |
     |(Internal process with MoS)|DHCP INFORM|          |          |
     |==========================>|==========>|          |          |
     |                 |         |           |          |          |
     |                 |         |           |          |          |
     |                 |         |  DHCP ACK |          |          |
     |                 |         |<==========|          |          |
     |    Inform MoS address     |           |          |          |
     |<==========================|           |          |          |
     |    (internal process)     |           |          |          |
     |                           |           |          |          |
     |Discovery        |         |           |          |          |
     |Response         |         |           |          |          |
     |<======          |         |           |          |          |
     |(MIH User<- MIHF)|         |           |          |          |
     |                 |         |           |          |          |
     |IS Query         |         |           |          |          |
     |=======>         |         |           |          |          |
     |(MIH User-> MIHF)|         |           |          |          |
     |                 |         |           |          |          |
     |Invoke TCP Client|         |           |          |          |
     |================>|         |           |          |          |
     |(Internal process|         |           |          |          |
     |with MOS)        |         |           |          |          |
     |                 |         |           |          |          |
     |                 |  TCP connection established    |          |

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     |                 |<==============================>|          |
     |                 |         |           |          |          |
     |                 |         |           |          |          |
     |                 |         |           |          |          |
     |                 IS  QUERY REQUEST (via MIH protocol)        |
     |                 |         |           |          |          |
     |                 |         |           |          |          |
     |                 |         |           |          |          |
     |                 |         |           |          |  IS QUERY|
     |                 |         |           |          |   REQUEST|
     |                 |         |           |          |=========>|
     |                 |         |           |    (MIHF-> MIH User)|
     |                 |         |           |          |          |
     |                 |         |           |          |     QUERY|
     |                 |         |           |          |  RESPONSE|
     |                 |         |           |          |   <======|
     |                 |         |           |  |(MIHF <-MIH User) |
     |                 |         |           |          |          |
     |                 | IS QUERY RESPONSE (via MIH protoco        |
     |                 |         |           |          |          |
     |    IS           |         |           |          |          |
     |RESPONSE         |         |           |          |          |
     |<========        |         |           |          |          |
     |(MIH User <-MIHF)|         |           |          |          |
     |                 |         |           |          |          |

          Figure 10: Example Flow of Operation Involving MIH User

8.  Security Considerations

   There are a number of security issues that need to be taken into
   account during node discovery and information exchange via a
   transport connection [I-D.ietf-mipshop-mis-ps]

   In the case where DHCP is used for node discovery and authentication
   of the source and content of DHCP messages is required, network
   administrators SHOULD use DHCP authentication option described in
   [RFC3118], where available or rely upon link layer security.  This
   will also protect the DHCP server against denial of service attacks
   to.  [RFC3118] provides mechanisms for both entity authentication and
   message authentication.

   In the case where DNS is used for discovering MoS, fake DNS requests
   and responses may cause DoS and the inability of the MN to perform a
   proper handover, respectively.  Where networks are exposed to such

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   DoS, it is RECOMMENDED that DNS service providers use the Domain Name
   System Security Extensions (DNSSEC) as described in [RFC4033].
   Readers may also refer to [RFC4641] to consider the aspects of DNSSEC
   Operational Practices.

   In the case where reliable transport protocol such as TCP is used for
   transport connection between two MIHF peers, TLS [RFC4346] SHOULD be
   used for message confidentiality and data integrity.  In particular,
   TLS is designed for client/server applications and to prevent
   eavesdropping, tampering, or message forgery.  Readers should also
   follow the recommendations in [RFC4366] that provides generic
   extension mechanisms for the TLS protocol suitable for wireless

   In the case where unreliable transport protocol such as UDP is used
   for transport connection between two MIHF peers, DTLS [RFC4347]
   SHOULD be used for message confidentiality and data integrity.  The
   DTLS protocol is based on the Transport Layer Security (TLS) protocol
   and provides equivalent security guarantees.

   Alternatively, generic IP layer security, such as IPSec [RFC2401] MAY
   be used where neither transport layer security for a specific
   transport is available nor server only authentication is required.

9.  IANA Considerations

   This document registers the following TCP and UDP port(s) with IANA:

 Keyword         Decimal   Description
 -------        -------    -----------
 ieee-mih-IS    XXX/tcp    Media Independent Handover Information Services
 ieee-mih-IS    XXX/udp    Media Independent Handover Information Services
 ieee-mih-ES    XXX/tcp    Media Independent Handover Event Services
 ieee-mih-ES    XXX/udp    Media Independent Handover Event Services
 ieee-mih-CS    XXX/tcp    Media Independent Handover Command Services
 ieee-mih-CS    XXX/udp    Media Independent Handover Command Services

10.  Acknowledgements

   The authors would like to thank Patrick Stupar and Sam Xia for their
   valuable comments and fruitful discussions.

11.  References

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11.1.  Normative References

              Bajko, G., "Dynamic Host Configuration Protocol (DHCPv4
              and DHCPv6) Options for Mobility  Servers (MoS)",
              draft-bajko-mos-dhcp-options-00 (work in progress),
              August 2007.

              Bajko, G., "Locating Mobility Servers",
              draft-bajko-mos-dns-discovery-00 (work in progress),
              August 2007.

              Yan, R., "Domain Suffix Option for DHCPv6",
              draft-ietf-dhc-dhcpv6-opt-dnsdomain-04 (work in progress),
              June 2007.

              Korhonen, J., Bournelle, J., Tschofenig, H., Perkins, C.,
              and K. Chowdhury, "Diameter Mobile IPv6: Support for
              Network Access Server to Diameter Server  Interaction",
              draft-ietf-dime-mip6-integrated-08 (work in progress),
              February 2008.

              Chowdhury, K. and A. Yegin, "MIP6-bootstrapping for the
              Integrated Scenario",
              draft-ietf-mip6-bootstrapping-integrated-dhc-05 (work in
              progress), July 2007.

              Jang, H., Yegin, A., Chowdhury, K., and J. Choi, "DHCP
              Options for Home Information Discovery in MIPv6",
              draft-ietf-mip6-hiopt-14 (work in progress), April 2008.

              Melia, T., Hepworth, E., Sreemanthula, S., Ohba, Y.,
              Gupta, V., Korhonen, J., and Z. Xia, "Mobility Services
              Transport: Problem Statement",
              draft-ietf-mipshop-mis-ps-05 (work in progress),
              November 2007.

              Eggert, L. and G. Fairhurst, "UDP Usage Guidelines for
              Application Designers", draft-ietf-tsvwg-udp-guidelines-06
              (work in progress), April 2008.

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              Korhonen, J. and T. Melia, "Diameter extensions for MoS
              discovery", draft-stupar-dime-mos-options-00 (work in
              progress), February 2008.

              "Draft IEEE Standard for Local and Metropolitan Area
              Networks: Media Independent Handover Servicesinnn", IEEE
              LAN/MAN Draft  IEEE P802.21/D07.00, July 2007.

   [RFC1533]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 1533, October 1993.

   [RFC1536]  Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
              Miller, "Common DNS Implementation Errors and Suggested
              Fixes", RFC 1536, October 1993.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3849]  Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
              Reserved for Documentation", RFC 3849, July 2004.

   [RFC4004]  Calhoun, P., Johansson, T., Perkins, C., Hiller, T., and
              P. McCann, "Diameter Mobile IPv4 Application", RFC 4004,
              August 2005.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",

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              RFC 4033, March 2005.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, April 2006.

   [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
              RFC 4641, September 2006.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

11.2.  Informative References

              Rahman, A., "Transport of Media Independent Handover
              Messages Over IP", draft-rahman-mipshop-mih-transport-03
              (work in progress), July 2007.

Authors' Addresses

   Telemaco Melia (editor)

   Email: tmelia@cisco.com

   Gabor Bajko

   Email: Gabor.Bajko@nokia.com

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   Subir Das
   Telcordia Technologies Inc.

   Email: subir@research.telcordia.com

   Nada Golmie

   Email: nada.golmie@nist.gov

   Juan Carlos Zuniga

   Email: j.c.zuniga@ieee.org

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Full Copyright Statement

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