Mipshop WG                                                 T. Melia, Ed.
Internet-Draft                                            Alcatel-Lucent
Intended status: Standards Track                                G. Bajko
Expires: March 7, 2009                                             Nokia
                                                                  S. Das
                                             Telcordia Technologies Inc.
                                                               N. Golmie
                                                              JC. Zuniga
                                        InterDigital Communications, LLC
                                                       September 3, 2008

               Mobility Services Framework Design (MSFD)

Status of this Memo

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   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

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   reliable delivery of MIH messages.

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: Third party MoS . . . . . . . . . . . . . . .  7
     3.4.  Scenario S4: Roaming MoS . . . . . . . . . . . . . . . . .  7
   4.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Architecture . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  MIHF Identifiers (FQDN, NAI) . . . . . . . . . . . . . . . 10
   5.  MoS Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 11
     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 MIH services are in a 3rd party
           remote network (scenario S3) . . . . . . . . . . . . . . . 12
     5.4.  MoS Discovery when the MN is in a visited Network and
           Services are at the Home network . . . . . . . . . . . . . 13
   6.  MIH Transport Options  . . . . . . . . . . . . . . . . . . . . 13
     6.1.  MIH Message size . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  MIH Message rate . . . . . . . . . . . . . . . . . . . . . 15
     6.3.  Retransmission . . . . . . . . . . . . . . . . . . . . . . 15
     6.4.  NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . 16
     6.5.  General guidelines . . . . . . . . . . . . . . . . . . . . 16
   7.  Operation Flows  . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     11.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 23

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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 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 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.

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 [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, 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 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 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.

   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, consisting of a host name followed by a domain
      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.

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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] and [RFC3315] that allows 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.

   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

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      considered lost and should be resent.

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

   PMTU  Path MTU.

   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

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: 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).

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

                     Figure 3: MoS form 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.

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

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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 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.

   o  If the MN changes its IP address (e.g. upon handover) it MUST
      refresh MIHF peers binding (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

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   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.

    |       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,
   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.

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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
   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

   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

   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 the home domain pre-configured in the MNs
   (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
   inFigure 6b (in some deployments DHCP relay may not be present).

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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.ietf-mipshop-mos-dhcp-options] as shown in Figure 7.

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

                      MoS Discovery using DHCP options

                   Figure 7: Discovery using DHCP option

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

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   specific network or the domain name of a network at a specific
   location (as in Figure 8 part (a), 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

   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
   inFigure 8 part(b).  It should be noted that step b can only be
   performed upon obtaining the domain name of the remote network.

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

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

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

            (b) using DNS Query in the remote network

   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

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   capability discovery and subscription procedures 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 is
   preferred to avoid using MIH ACKs 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 where PMTU is not implemented).  The minimum PMTU depends
   on the IP version used for transmission, and is the lesser of 576
   bytes for IPv4 [RFC1122] and 1280 bytes for IPv6 [RFC2460], although
   applications may reduce these values to guard against the presence of

   It should be noted that MIH layer fragmentation MUST NOT be used
   together with IP layer fragmentation as specified in [IEEE80221].

   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

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   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 PUSH bit can alleviate this problem by triggering
   transmission of a segment less than the SMSS.

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 token bucket parameter settings are as follow:

   o  If MIHF knows the RTT, the rate can be based upon this

   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 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

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   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.  The default
   number of retransmissions is set to 2 and retransmissions are
   controlled by a timer with a default value of 10s.  No backoff
   mechanism is specified.

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.
   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.  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

6.5.  General guidelines

   Since ES and CS messages are small in nature and have tight latency
   requirements, UDP in combination with MIH acknowledgement MAY 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.  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.ietf-mipshop-mos-dns-discovery] to discover
   additional transport protocols supported by the server.

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7.  Operation Flows

   Figure 9 gives an example operation flow between MIHF peers when an
   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.
   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.ietf-mipshop-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
   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|         |          |
    |==========================>|==========>|         |          |
    |                 |         |           |         |          |
    |                 |         |           |         |          |

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    |                 |         |  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   |          |
    |                 |<=============================>|          |
    |                 |         |           |         |          |
    |                 |         |           |         |          |
    |                 |         |           |         |          |
    |                 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)|         |           |         |          |
    |                 |         |           |         |          |

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          Figure 9: 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 [RFC5164]

   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 case where DHCP authentication mechanism
   is not available administrators may need to rely upon 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 check integrity
   protection 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.

   In the case where reliable transport protocol such as TCP is used for
   transport connection between two MIHF peers, TLS [RFC5246] with
   server-side certificates SHOULD be used for server only
   authentication, message confidentiality and data integrity.  Certain
   subscriptions may include client certificates, and in those cases
   servers MAY require the clients to authenticate themselves using
   client-side certificates.  Readers should also follow the
   recommendations in [RFC5246] that provides generic extension
   mechanisms for the TLS protocol suitable for wireless environments.

   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.

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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

              Bajko, G. and S. Das, "Dynamic Host Configuration Protocol
              (DHCPv4 and DHCPv6) Options for Mobility  Server (MoS)
              discovery", draft-ietf-mipshop-mos-dhcp-options-03 (work
              in progress), June 2008.

              Bajko, G., "Locating Mobility Servers using DNS",
              draft-ietf-mipshop-mos-dns-discovery-01 (work in
              progress), May 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.

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

11.2.  Informative References

              Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers",
              draft-ietf-tsvwg-udp-guidelines-10 (work in progress),
              August 2008.

              "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
              Control", RFC 2581, April 1999.

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

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

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   [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.

Authors' Addresses

   Telemaco Melia (editor)
   Route de Villejust
   Nozay  91620

   Email: telemaco.melia@gmail.com

   Gabor Bajko

   Email: Gabor.Bajko@nokia.com

   Subir Das
   Telcordia Technologies Inc.

   Email: subir@research.telcordia.com

   Nada Golmie

   Email: nada.golmie@nist.gov

   Juan Carlos Zuniga
   InterDigital Communications, LLC

   Email: j.c.zuniga@ieee.org

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Full Copyright Statement

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