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Versions: (draft-melia-mipshop-mstp-solution) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5677

Mipshop WG                                                 T. Melia, Ed.
Internet-Draft                                            Alcatel-Lucent
Intended status: Standards Track                                G. Bajko
Expires: August 1, 2009                                            Nokia
                                                                  S. Das
                                             Telcordia Technologies Inc.
                                                               N. Golmie
                                                                    NIST
                                                              JC. Zuniga
                                        InterDigital Communications, LLC
                                                        January 28, 2009


         IEEE 802.21 Mobility Services Framework Design (MSFD)
                  draft-ietf-mipshop-mstp-solution-12

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 1, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.

Abstract

   This document describes a mobility services framework design (MSFD)
   for the IEEE 802.21 Media Independent Handover (MIH) protocol that
   addresses identified issues associated with the transport of MIH
   messages.  The document also describes mechanisms for mobility
   service (MoS) discovery and transport layer mechanisms for the
   reliable delivery of MIH messages.  This document does not provide
   mechanisms for securing the communication between a mobile node (MN)
   and the mobility service (MoS).  Instead, it is assumed that either
   lower layer (e.g., link layer) security mechanisms, or overall
   system-specific proprietary security solutions, are used.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].






























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Deployment Scenarios . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Scenario S1: Home Network MoS  . . . . . . . . . . . . . .  7
     3.2.  Scenario S2: Visited Network MoS . . . . . . . . . . . . .  8
     3.3.  Scenario S3: Third party MoS . . . . . . . . . . . . . . .  8
     3.4.  Scenario S4: Roaming MoS . . . . . . . . . . . . . . . . .  9
   4.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Architecture . . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  MIHF Identifiers (FQDN, NAI) . . . . . . . . . . . . . . . 12
   5.  MoS Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  MoS Discovery when MN and MoSh are in the home network
           (Scenario S1)  . . . . . . . . . . . . . . . . . . . . . . 13
     5.2.  MoS Discovery when MN and MoSv both are in visited
           network (Scenario S2)  . . . . . . . . . . . . . . . . . . 14
     5.3.  MoS Discovery when MIH services are in a 3rd party
           remote network (Scenario S3) . . . . . . . . . . . . . . . 14
     5.4.  MoS Discovery when the MN is in a visited Network and
           Services are at the Home network . . . . . . . . . . . . . 15
   6.  MIH Transport Options  . . . . . . . . . . . . . . . . . . . . 15
     6.1.  MIH Message size . . . . . . . . . . . . . . . . . . . . . 16
     6.2.  MIH Message rate . . . . . . . . . . . . . . . . . . . . . 17
     6.3.  Retransmission . . . . . . . . . . . . . . . . . . . . . . 17
     6.4.  NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . 18
     6.5.  General guidelines . . . . . . . . . . . . . . . . . . . . 18
   7.  Operation Flows  . . . . . . . . . . . . . . . . . . . . . . . 18
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
     8.1.  Security Considerations for MoS Discovery  . . . . . . . . 21
     8.2.  Security Considerations for MIH Transport  . . . . . . . . 21
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     11.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24














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1.  Introduction

   This document proposes a solution to the issues identified in the
   problem statement document [RFC5164] for the layer 3 transport of
   IEEE 802.21 MIH protocols.

   The MIH Layer 3 transport problem is divided into 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 MSFD reference model and
   MIHF identifiers.  MoS discovery procedures explain how the MN
   discovers MoS in its home network, in a visited network or in a third
   party network.  The remainder of this document describes the MIH
   transport architecture, example message flows for several signaling
   scenarios, and security issues.

   This document does not provide mechanisms for securing the
   communication between a mobile node and the mobility service.
   Instead, it is assumed that either lower layer (e.g., link layer)
   security mechanisms, or overall system-specific proprietary security
   solutions, are used.  The details of such lower layer and/or
   proprietary mechanisms are beyond the scope of this document.  It is
   RECOMMENDED against using this protocol without careful analysis that
   these mechanisms meet the desired requirements, and encourages future
   standardization work in this area.  The IEEE 802.21a Task Group has
   recently started work on MIH security issues that may provide some
   solution in this area.  For further information, please refer to
   Section 8.


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.







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   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
      signaling.

   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
      NAI.

   MoS  Mobility Services: those services, as defined in the MIH problem
      statement document [RFC5164] , 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.

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

   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 of the protocol stack and sends information to the local or
      remote upper or lower layers of the protocol stack.  The purpose
      of IS is to exchange information elements (IEs) relating to
      various neighboring network information.

   ES Event Service: a MoS that originates at a remote MIHF or the lower
      layers of the local 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
      various lower layer events.






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   CS Command Service: MoS that sends commands from the remote MIHF or
      local upper layers to the remote or 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, showing (in reverse order) the full delegation
      path from the DNS root and top level domain down to the host name
      (e.g. myexample.example.org).

   NAI  Network Access Identifier: the user ID that a user submits
      during network access authentication [RFC4282].  For mobile users,
      the NAI identifies the user and helps to route the authentication
      request message.

   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 routable
      (outside the NAT domain) network addresses and port numbers.

   DHCP  Dynamic Host Configuration Protocol: protocols described in
      [RFC2131] and [RFC3315] that allow Internet devices to obtain
      respectively IPv4 and IPv6 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 in 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.







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   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
      network.

   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.

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

   PMTU  Path MTU: the largest size of an IP packet that can be sent on
      an end-to-end network path without requiring IP fragmentation.

   TLS  Transport Layer Security Protocol: an application layer protocol
      that primarily assures privacy and data integrity between two
      communicating network entities [RFC5246].

   SMSS  Sender Maximum Segment Size: size of the largest segment that
      the sender can transmit as per [RFC2581].


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
   service.  This document addresses the scenarios listed in [RFC5164]
   and specifies transport options to carry the MIH protocol over IP.

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.



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   +--------------+  +====+
   | 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.


             +--------------+
             | HOME NETWORK |
             +--------------+
                   /\
                   ||
                   \/
    +====+ +-----------------+
    |MoSv| | VISITED NETWORK |
    +====+ +-----------------+
                   /\
                   ||
                   \/
               +--------+
               |   MN   |
               +--------+

                   Figure 2: MoSv in the visited network

3.3.  Scenario S3: 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 3.  (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 3: MoS from a third party

3.4.  Scenario S4: 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 4.


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

            Figure 4: MoS provided by the home while in visited

   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
   amount of static information, ES and CS are dynamic services and some
   relationships between them can be expected, e.g., a handover command



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   (CS) could be issued upon reception of a link event (ES).  This
   document does not make any assumption on the location of the MoS
   (although there might be some preferred configurations), and aims at
   flexible MSFD to discover different services in different locations
   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 primarily 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.

   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 at any time thereafter.

   The MN may know the realm of the MoS to be discovered.  The MN may
   also be pre-configured with the address of the MoS to be used.  In
   case the MN does not know what realm/MoS to query, dynamic assignment
   methods are described in Section 5.

   The discovery of the MoS (and the related configuration at MIHF
   level) is required to bind two MIHF peers (e.g.  MN and MoS) with
   their respective IP addresses.  Discovery MUST be executed in the
   following conditions:

   o  Bootstrapping: upon successful layer 2 network attachment the MN
      MAY be required to use DHCP for address configuration.  These
      procedures can carry the required information for MoS
      configuration in specific DHCP options.

   o  If the MN does not receive MoS information during network
      attachment and the MN does not have a pre-configured MoS, it MUST
      run a discovery procedure upon initial IP address configuration.






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   o  If the MN changes its IP address (e.g. upon handover) it MUST
      refresh MIHF peer bindings (i.e., MIHF registration process).  In
      case the MoS used is not suitable anymore (e.g. too large RTT
      experienced) the MN MAY need to perform a new discovery procedure.

   o  If the MN is a multi-homed device and it communicates with the
      same MoS via different IP addresses it MAY run discovery
      procedures if one of the IP addresses changes.

   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 and
   packing of the MIH messages does not require extra framing since the
   MIH protocol defined in [IEEE80221] already contains a length field.

4.1.  Architecture

   Figure 5 depicts the MSFD 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           |
    +--------------------------+
        ||         ||       ||
        ||      +------+ +-----+
        ||      | DHCP | | DNS |
        ||      +------+ +-----+
        ||         ||       ||
    +--------------------------+
    |         TCP/UDP          |
    +--------------------------+

                            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 as described in Section 5.  Once the peer MIHF is discovered, the
   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.

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



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   visited network.  In the case the MN has already a MoS address pre-
   configured it is not necessary to run the discovery procedure.  If
   the MN does not have pre-configured MoS the following procedure
   applies.

   In the case where MoS is provided locally (scenarios S1 and S2) , the
   discovery techniques described in [I-D.ietf-mipshop-mos-dhcp-options]
   and [I-D.ietf-mipshop-mos-dns-discovery] are both applicable as
   described in Section 5.1 and Section 5.2.

   In the case where MoS is provided in the home network while the MN is
   in the visited network (scenario S4), the DNS based discovery
   described in [I-D.ietf-mipshop-mos-dns-discovery] is applicable.

   In the case where MoS is provided by a third party network which is
   different from the current visited network (scenario S3), only the
   DNS based discovery method described in
   [I-D.ietf-mipshop-mos-dns-discovery] is applicable.

   It should be noted that authorization of a MN to use a specific MoS
   server is neither in scope of this document nor is currently
   specified in IEEE80221.  We further assume all devices can access
   discovered MoS.  In case future deployments will implement
   authorization policies the mobile nodes should fall back to other
   learned MoS if authorization is denied.

5.1.  MoS Discovery when MN and MoSh are in the home network (Scenario
      S1)

   To discover an MoS in the home network, the MN SHOULD use the DNS
   based MoS discovery method described in
   [I-D.ietf-mipshop-mos-dns-discovery].  In order to use that
   mechanism, the MN MUST have its home domain pre-configured (i.e.,
   subscription is tied to a network).  The DNS query option is shown in
   Figure 6a.  Alternatively, the MN MAY use the DHCP options for MoS
   discovery [I-D.ietf-mipshop-mos-dhcp-options] as shown in Figure 6b
   (in some deployments, a DHCP relay may not be present).














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    (a)                       +-------+
               +----+         |Domain |
               | MN |-------->|Name   |
               +----+         |Server |
             MN@example.org   +-------+

    (b)
                            +-----+      +------+
               +----+       |     |      |DHCP  |
               | MN |<----->| DHCP|<---->|Server|
               +----+       |Relay|      |      |
                            +-----+      +------+

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

5.2.  MoS Discovery when MN and MoSv both are 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.ietf-mipshop-mos-dhcp-options] as shown in Figure 7.

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

                Figure 7: MoS Discovery using DHCP options

5.3.  MoS Discovery when MIH services are in a 3rd party remote network
      (Scenario S3)

   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.ietf-mipshop-mos-dns-discovery].  The MN MUST first learn the
   domain name of the network containing the MoS it is searching for.
   The MN can query its current 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 8a, IEEE 802.21 defines information elements
   such as OPERATOR ID and SERVICE PROVIDER ID which can be a domain
   name.  An IS query can provide this information, see [IEEE80221]).

   Alternatively, the MN MAY query a MoS previously known to learn the
   domain name of the desired network .  Finally, the MN MUST use DNS
   based discovery mechanisms to find MoS in the remote network as in
   Figure 8b.  It should be noted that step b can only be performed upon
   obtaining the domain name of the remote network.



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     (a)
                               +------------+
                +----+         |            |
                |    |         |Information |
                | MN |-------->| Server     |
                |    |         |(previously |
                +----+         |discovered) |
                               +------------+

      (b)
                                 +-------+
                  +----+         |Domain |
                  | MN |-------->|Name   |
                  +----+         |Server |
               MN@example.org    +-------+

   Figure 8: MOS Discovery using (a) IS Query to a known IS Server, (b)
                                 DNS Query

5.4.  MoS Discovery when the MN is in a visited Network and Services are
      at the Home network

   To discover an MoS in the visited network when MIH services are
   provided by the home network, the DNS based discovery method
   described in [I-D.ietf-mipshop-mos-dns-discovery] is applicable.  To
   discover the MoS at home while in a visited network using DNS, the MN
   SHOULD use the procedures described in Section 5.1.


6.  MIH Transport Options

   Once the Mobility Services have been discovered, MIH peers run a
   capability discovery and subscription procedure as specified in
   [IEEE80221].  MIH peers MAY exchange information over TCP, UDP or any
   other transport supported by both the server and the client.  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 MUST NOT be



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   used with TCP since TCP includes acknowledgement and retransmission
   functionality.

6.1.  MIH Message size

   Although the MIH message size varies widely from about 30 bytes (for
   a 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 based on the path MTU to destination (or
   the Minimum MTU where PMTU is not implemented).  The Minimum MTU
   depends on the IP version used for transmission, and is the lesser of
   the first hop MTU, and 576 or 1280 bytes for IPv4 [RFC1122] or for
   IPv6 [RFC2460], respectively, although applications may reduce these
   values to guard against the presence of tunnels.

   According to[IEEE80221] when MIH message is sent using an L3 or
   higher layer transport, L3 takes care of any fragmentation issue and
   the MIH protocol does not handle fragmentation in such cases.  Thus,
   MIH layer fragmentation MUST NOT be used together with IP layer
   framentation and MUST not be used when MIH packets are carried over
   TCP.

   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
   [RFC5405] 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.  The use of the TCP_NODELAY
   option can alleviate this problem by triggering transmission of a
   segment less than the SMSS.  (It should be noted that [RFC4960]



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   addresses both of these problems, but discussion of SCTP is omitted
   here, as it is generally not used for the mobility services discussed
   in this document.).

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
   SHOULD be obtained by adequately adjusting the parameters of a token
   bucket regulator as defined in the MIH specifications [IEEE80221].
   Recommendations for token bucket parameter settings are as follow:

   o  If the MIHF knows the RTT (e.g., based on the request/response MIH
      protocol exchange between two MIH peers), the rate can be based
      upon this as specified in [IEEE80221].

   o  If not, then on average it SHOULD NOT send more than one UDP
      message every 3 seconds.

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 MUST 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.  The maximum number of retransmissions is configurable and the
   value of the retransmission timer is computed according to the
   algorithm defined in [RFC2988].  The default maximum number of
   retransmissions is set to 2 and the initial retransmission timer
   (TMO) is set to 3s when RTT is not known.  The maximum TMO is set to
   30s.





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6.4.  NAT Traversal

   There are no known issues for NAT traversal when using TCP.  The
   default connection timeout of 2 hours 4 minutes [RFC5382] (assuming a
   2 hours TCP keep-alive) is considered adequate for MIH transport
   purposes.  However, issues with NAT traversal using UDP are
   documented in [RFC5405].  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.  In addition and in order to avoid
   such failures, MIH messages MAY be sent periodically, similarly to
   keep-alive messages, in an attempt to refresh middlebox state.  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.  Re-registration or Event indication messages as defined in
   [IEEE80221] MAY be used for this purpose.

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.
   TCP SHOULD be used to transport 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 in
   Section 9 for that particular transport.

   A 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.ietf-mipshop-mos-dns-discovery] to discover
   additional transport protocols supported by the server (e.g.  SCTP).


7.  Operation Flows

   Figure 9 gives an example operation flow between MIHF peers when a
   MIH user requests an IS service and both the MN and the MoS are in
   the MN's home network.  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.



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   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.  The DHCP Client sends a DHCP INFORM
   message according to standard DHCP and with the MoS option as defined
   in [I-D.ietf-mipshop-mos-dhcp-options].  The DHCP server replies via
   a DHCP ACK message with the IP address of the MoS.  The MoS address
   is then passed to the MIHF locally via some internal APIs.  The 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 MoS.  The MIHF invokes the underlying TCP client which
   establishes a transport connection with the remote peer.  Once the
   transport connection is established, the MIHF sends the IS query via
   a 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 a 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             |         |           |         |          |
    |                 |         |           |         |          |
    |Invoke DHCP Client         |           |         |          |
    |(Internal process with MoS)|DHCP INFORM|         |          |
    |==========================>|==========>|         |          |
    |                 |         |           |         |          |
    |                 |         |  DHCP ACK |         |          |
    |                 |         |<==========|         |          |
    |    Inform MoS address     |           |         |          |
    |<==========================|           |         |          |
    |    (internal process)     |           |         |          |
    |                 |         |           |         |          |
  MIH Discovery       |         |           |         |          |
  Response            |         |           |         |          |
    |                 |         |           |         |          |
  IS Query            |         |           |         |          |



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  MIH User-> MIHF     |         |           |         |          |
    |                 |         |           |         |          |
    |Invoke TCP Client|         |           |         |          |
    |================>|         |           |         |          |
  Internal process    |         |           |         |          |
    |                 |  TCP connection established   |          |
    |                 |<=============================>|          |
    |                 |         |           |         |          |
    |                 IS  QUERY REQUEST (via MIH protocol)       |
    |===========================================================>|
    |                 |         |           |         |          |
    |                 |         |           |         |  IS QUERY|
    |                 |         |           |         |   REQUEST|
    |                 |         |           |    MIHF-> MIH User |
    |                 |         |           |         |          |
    |                 |         |           |         |     QUERY|
    |                 |         |           |         |  RESPONSE|
    |                 |         |           |   MIHF <-MIH User  |
    |                 |         |           |         |          |
    |                 | IS QUERY RESPONSE (via MIH protocol)     |
    |<===========================================================|
    |                 |         |           |         |          |
   IS RESPONSE        |         |           |         |          |
   MIH User <-MIHF    |         |           |         |          |
    |                 |         |           |         |          |

          Figure 9: Example Flow of Operation Involving MIH User


8.  Security Considerations

   There are two components to the security considerations: MoS
   Discovery and MIH Transport.  For MoS Discovery, DHCP and DNS
   recommendations are hereby provided per IETF guidelines.  For MIH
   Transport, we describe the security threats and expect that the
   system deployment will have means to mitigate such threats when
   sensitive information is being exchanged between the mobile node and
   MoS.  Since IEEE 802.21 base specification does not provide MIH
   protocol level security, it is assumed that either lower layer
   security (e.g., link layer), or overall system specific (e.g.
   proprietary) security solutions are available.  The present document
   does not provide any guidelines in this regard.  It is should be
   stressed that the IEEE 802.21a Task Group has recently started work
   on MIH security issues that may provide some solution in this area.
   Finally authorization of a MN to use a specific MoS, as stated in
   Section 5, is neither in scope of this document nor is currently
   specified in [IEEE80221].




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8.1.  Security Considerations for MoS Discovery

   There are a number of security issues that need to be taken into
   account during node discovery.  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 the DHCP
   authentication option described in [RFC3118], where available, or
   rely upon link layer security.  [RFC3118] provides mechanisms for
   both entity authentication and message authentication.  In case where
   the DHCP authentication mechanism is not available administrators may
   need to rely upon the underlying link layer security.  In such cases
   the link between DHCP client and layer-2 termination point may be
   protected but the DHCP message source and its messages can not be
   authenticated or the integrity of the latter checked unless there
   exits a security binding between link layer and DHCP layer.

   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
   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.

8.2.  Security Considerations for MIH Transport

   The communication between an MN and an MoS is exposed to a number of
   security threads:

   o  MoS Identity spoofing.  A fake MoS could provide the MNs with
      bogus data and force them to select the wrong network or to make a
      wrong handover decision.

   o  Tampering.  Tampering with the information provided by an MoS may
      result in the MN making wrong network selection or handover
      decisions

   o  Replay attack.  Since MoSs as defined in [IEEE80221] support a
      'PUSH model', they can send bulk of data to the MNs whenever the
      MoSs think that the data is relevant for the MN.  An attacker may
      intercept the data sent my the MoSs to the MNs and replay it at a
      later time, causing the MNs to make network selection or handover
      decisions which are not valid at that point in time.

   o  Eavesdropping.  By snooping the communication between an MN and an
      MoS, an attacker may be able to trace a user's movement between
      networks or cells, or predict future movements, by inspecting
      handover service messages.



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   There are many deployment specific system security solutions
   available and can be used to countermeasure the above mentioned
   threats.  For example, for the MoSh and MoSv scenarios (including
   roaming scenarios), link layer security may be sufficient to protect
   the communication between MN and MoS.  This is a typical mobile
   operator environment where link layer security provides
   authentication, data confidentiality and integrity.  In other
   scenarios, such as the third party MoS, link layer security solutions
   may not be sufficient to protect the communication path between the
   MN and the MoS.  The communication channel between MN and MoS needs
   to be secured by other means.

   The present document does not provide any specific guidelines about
   the way these security solutions should be deployed.  However, if in
   future the IEEE 802.21 Working Group amends the specification with
   MIH protocol level security or recommends the deployment scenarios,
   IETF may revisit the security considerations and recommend specific
   transport layer security as appropriate.


9.  IANA Considerations

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

    Keyword    Decimal                    Description
    --------   ---------------            ------------
    ieee-mih   TBD_BY_IANA/tcp            MIH Services
    ieee-mih   TBD_BY_IANA/udp            MIH Services


10.  Acknowledgements

   The authors would like to thank Yoshihiro Ohba, David Griffith, Kevin
   Noll, Vijay Devarapalli, Patrick Stupar and Sam Xia for their
   valuable comments, reviews and fruitful discussions.


11.  References

11.1.  Normative References

   [I-D.ietf-mipshop-mos-dhcp-options]
              Bajko, G. and S. Das, "Dynamic Host Configuration Protocol
              (DHCPv4 and DHCPv6) Options for IEEE  802.21 Mobility
              Server (MoS) discovery",
              draft-ietf-mipshop-mos-dhcp-options-10 (work in progress),
              January 2009.




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   [I-D.ietf-mipshop-mos-dns-discovery]
              Bajko, G., "Locating IEEE 802.21 Mobility Servers using
              DNS", draft-ietf-mipshop-mos-dns-discovery-04 (work in
              progress), October 2008.

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

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

   [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.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

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

11.2.  Informative References

   [IEEE80221]
              "Draft IEEE Standard for Local and Metropolitan Area
              Networks: Media Independent Handover Services", IEEE LAN/
              MAN Draft  IEEE P802.21/D13.00, August 2008.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion



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              Control", RFC 2581, April 1999.

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

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [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.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5164]  Melia, T., "Mobility Services Transport: Problem
              Statement", RFC 5164, March 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405,
              November 2008.


Authors' Addresses

   Telemaco Melia (editor)
   Alcatel-Lucent
   Route de Villejust
   Nozay  91620
   France

   Email: telemaco.melia@alcatel-lucent.com








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   Gabor Bajko
   Nokia

   Email: Gabor.Bajko@nokia.com


   Subir Das
   Telcordia Technologies Inc.

   Email: subir@research.telcordia.com


   Nada Golmie
   NIST

   Email: nada.golmie@nist.gov


   Juan Carlos Zuniga
   InterDigital Communications, LLC

   Email: j.c.zuniga@ieee.org





























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