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Network Working Group T. Melia, Ed. Request for Comments: 5164 Cisco Systems Category: Informational March 2008

           Mobility Services Transport: Problem Statement

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.


 There are ongoing activities in the networking community to develop
 solutions that aid in IP handover mechanisms between heterogeneous
 wired and wireless access systems including, but not limited to, IEEE
 802.21.  Intelligent access selection, taking into account link-layer
 attributes, requires the delivery of a variety of different
 information types to the terminal from different sources within the
 network and vice-versa.  The protocol requirements for this
 signalling have both transport and security issues that must be
 considered.  The signalling must not be constrained to specific link
 types, so there is at least a common component to the signalling
 problem, which is within the scope of the IETF.  This document
 presents a problem statement for this core problem.

Melia, et al. Informational [Page 1] RFC 5164 Mobility Services Transport March 2008

Table of Contents

 1. Introduction ....................................................2
 2. Terminology .....................................................3
    2.1. Requirements Language ......................................3
 3. Definition of Mobility Services .................................4
 4. Deployment Scenarios for MoS ....................................4
    4.1. End-to-End Signalling and Transport over IP ................5
    4.2. End-to-End Signalling and Partial Transport over IP ........5
    4.3. End-to-End Network-to-Network Signalling ...................6
 5. MoS Transport Protocol Splitting ................................7
    5.1. Payload Formats and Extensibility Considerations ...........8
    5.2. Requirements on the Mobility Service Transport Layer .......8
 6. Security Considerations ........................................11
 7. Conclusions ....................................................12
 8. Acknowledgements ...............................................13
 9. References .....................................................13
    9.1. Normative References ......................................13
    9.2. Informative References ....................................13
 Contributors ......................................................14

1. Introduction

 This document provides a problem statement for the exchange of
 information to support handover in heterogeneous link environments
 [1].  This mobility support service allows more sophisticated
 handover operations by making available information about network
 characteristics, neighboring networks and associated characteristics,
 indications that a handover should take place, and suggestions for
 suitable target networks to which to handover.  The mobility support
 services are complementary to IP mobility mechanisms [4], [5], [6],
 [7], [8], [9] to enhance the overall performance and usability
 There are two key attributes to the handover support service problem
 for inter-technology handovers:
 1. The Information: the information elements being exchanged.  The
     messages could be of a different nature, such as information,
     commands to perform an action, or events informing of a change,
     potentially being defined following a common structure.

Melia, et al. Informational [Page 2] RFC 5164 Mobility Services Transport March 2008

 2. The Underlying Transport: the transport mechanism to support
     exchange of the information elements mentioned above.  This
     transport mechanism includes information transport, discovery of
     peers, and the securing of this information over the network.
 The initial requirement for this protocol comes from the need to
 provide a transport for the Media Independent Handover (MIH) protocol
 being defined by IEEE 802.21 [1], which is not bound to any specific
 link layer and can operate over more that one network-layer hop.  The
 solution should be flexible to accommodate evolution in the MIH
 standard, and should also be applicable for other new mobility
 signalling protocols that have similar message patterns and discovery
 and transport requirements.
 The structure of this document is as follows.  Section 3 defines
 Mobility Services.  Section 4 provides a simple model for the
 protocol entities involved in the signalling and their possible
 relationships.  Section 5 describes a decomposition of the signalling
 problem into service-specific parts and a generic transport part.
 Section 5.2 describes more detailed requirements for the transport
 component.  Section 6 provides security considerations.  Section 7
 summarizes the conclusions and open issues.

2. Terminology

 The following abbreviations are used in the document:
    MIH: Media Independent Handover
    MN: Mobile Node
    NN: Network Node, intended to represent some device in the network
    (the location of the node, e.g., in the access network, the home
    network is not specified, and for the moment it is assumed that
    they can reside anywhere).
    EP: Endpoint, intended to represent the terminating endpoints of
    the transport protocol used to support the signalling exchanges
    between nodes.

2.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC 2119 [2].

Melia, et al. Informational [Page 3] RFC 5164 Mobility Services Transport March 2008

3. Definition of Mobility Services

 As mentioned in the Introduction, mobility (handover) support in
 heterogeneous wireless environments requires functional components
 located either in the mobile terminal or in the network to exchange
 information and eventually to make decisions upon this information
 exchange.  For instance, traditional host-based handover solutions
 could be complemented with more sophisticated network-centric
 solutions.  Also, neighborhood discovery, potentially a complex
 operation in heterogeneous wireless scenarios, can result in a
 simpler step if implemented with a unified interface towards the
 access network.
 In this document, the different supporting functions for Media
 Independent Handover (MIH) management are generally referred to as
 Mobility Services (MoS) that have different requirements for the
 transport protocol.  These requirements and associated
 functionalities are the focus of this document.  Speaking 802.21
 terminology, MoS can be regarded as Information Services (IS), Event
 Services (ES), and Command Service (CS).

4. Deployment Scenarios for MoS

 The deployment scenarios are outlined in the following sections.
    Note: while MN-to-MN signalling exchanges are theoretically
    possible, these are not currently being considered.
 The following scenarios are discussed for understanding the overall
 problem of transporting MIH protocol.  Although these are all
 possible scenarios and MIH services can be delivered through
 link-layer specific solutions and/or through a "layer 3 or above"
 protocol, this problem statement focuses on the delivery of
 information for Mobility Services only for the latter case.

Melia, et al. Informational [Page 4] RFC 5164 Mobility Services Transport March 2008

4.1. End-to-End Signalling and Transport over IP

 In this case, the end-to-end signalling used to exchange the handover
 information elements (the Information Exchange) runs end-to-end
 between MN and NN.  The underlying transport is also end-to-end.
    +------+                              +------+
    |  MN  |                              |  NN  |
    | (EP) |                              | (EP) |
    +------+                              +------+
                 Information Exchange
       <          Transport over IP           >
    Figure 1: End-to-End Signalling and Transport

4.2. End-to-End Signalling and Partial Transport over IP

 As before, the Information Exchange runs end-to-end between the MN
 and the second NN.  However, in this scenario, some transport means
 other than IP are used from the MN to the first NN, and the transport
 over IP is used only between NNs.  This is analogous to the use of
 EAP end-to-end between Supplicant and Authentication Server, with an
 upper-layer multihop protocol, such as Remote Authentication Dial-In
 User Service (RADIUS), used as a backhaul transport protocol between
 an Access Point and the Authentication Server.
    +------+           +------+           +------+
    |  MN  |           |  NN  |           |  NN  |
    |      |           | (EP) |           | (EP) |
    +------+           +------+           +------+
                 Information Exchange
         (Transport over  /------------------\
        <--------------->< Transport over IP  >
             e.g. L2)     \------------------/
          Figure 2: Partial Transport

Melia, et al. Informational [Page 5] RFC 5164 Mobility Services Transport March 2008

4.3. End-to-End Network-to-Network Signalling

 In this case, NN to NN signalling is envisioned.  Such a model should
 allow different network components to gather information from each
 other.  This is useful for instance in conditions where network
 components need to make decisions and instruct mobile terminals of
 operations to be executed.
    +------+          +------+
    |  NN  |          |  NN  |
    | (EP) |          | (EP) |
    +------+          +------+
       Information Exchange
      <    Transport     >
    Figure 3: Information Exchange between Different NNs
 Network nodes exchange information about the status of connected

Melia, et al. Informational [Page 6] RFC 5164 Mobility Services Transport March 2008

5. MoS Transport Protocol Splitting

 Figure 4 shows a model where the Information Exchanges are
 implemented by a signalling protocol specific to a particular
 mobility service, and these are relayed over a generic transport
 layer (the Mobility Service Transport Layer).
                      +----------------+          ^
                      |Mobility Support|          |
                      |   Service 2    |          |
   +----------------+ |                |          | Mobility Service
   |Mobility Support| +----------------+          |    Signaling
   |    Service 1   |    +----------------+       |      Layer
   |                |    |Mobility Support|       |
   +----------------+    |   Service 3    |       |
                         |                |       |
                         +----------------+       V
    +---------------------------------------+     ^ Mobility Service
    |  Mobility Service Transport Protocol  |     |    Transport
    +---------------------------------------+     V      Layer
    |                   IP                  |
        Figure 4: Handover Services over IP
 The Mobility Service Transport Layer provides certain functionality
 (outlined in Section 5.2) to the higher-layer mobility support
 services in order to support the exchange of information between
 communicating Mobility Service functions.  The transport layer
 effectively provides a container capability to mobility support
 services, as well as any required transport and security operations
 required to provide communication, without regard to the protocol
 semantics and data carried in the specific Mobility Services.
 The Mobility Support Services themselves may also define certain
 protocol exchanges to support the exchange of service-specific
 information elements.  It is likely that the responsibility for
 defining the contents and significance of the information elements is
 the responsibility of standards bodies other than the IETF.  Example
 Mobility Services include the Information Services, Event Services,
 and Command Services.

Melia, et al. Informational [Page 7] RFC 5164 Mobility Services Transport March 2008

5.1. Payload Formats and Extensibility Considerations

 The format of the Mobility Service Transport Protocol (MSTP) is as
    |Mobility Service|           Opaque Payload               |
    |Transport Header|     (Mobility Support Service)         |
                 Figure 5: Protocol Structure
 This figure shows the case for an MIH message that is smaller than
 the MTU of the path to the destination.  A larger payload may require
 the transport protocol to transparently fragment and reassemble the
 MIH message.
 The opaque payload encompasses the Mobility Support Service (MSTP)
 information that is to be transported.  The definition of the
 Mobility Service Transport Header is something that is best addressed
 within the IETF.  MSTP does not inspect the payload, and any required
 information will be provided by the MSTP users.

5.2. Requirements on the Mobility Service Transport Layer

 The following section outlines some of the general transport
 requirements that should be supported by the Mobility Service
 Transport Protocol.  Analysis has suggested that at least the
 following need to be taken into account:
 Discovery:  MNs need the ability to either discover nodes that
    support certain services or discover services provided by a
    certain node.  The service discovery can be dealt with using
    messages as defined in [1].  This section refers to node-discovery
    in either scenario.  There are no assumptions about the location
    of these Mobility Service nodes within the network.  Therefore,
    the discovery mechanism needs to operate across administrative
    boundaries.  Issues such as speed of discovery, protection against
    spoofing, when discovery needs to take place, and the length of
    time over which the discovery information may remain valid; all
    need to be considered.  Approaches include:
  • Hard coding information into the MN, indicating either the IP

address of the NN, or information about the NN that can be

       resolved onto an IP address.  The configuration information
       could be managed dynamically, but assumes that the NN is
       independent of the access network to which the MN is currently

Melia, et al. Informational [Page 8] RFC 5164 Mobility Services Transport March 2008

  • Pushing information to the MN, where the information is

delivered to the MN as part of other configuration operations,

       for example, via DHCP or Router Discovery exchange.  The
       benefit of this approach is that no additional exchanges with
       the network would be required, but the limitations associated
       with modifying these protocols may limit applicability of the
  • MN dynamically requesting information about a node, which may

require both MN and NN support for a particular service

       discovery mechanism.  This may require additional support by
       the access network (e.g., multicast or anycast) even when it
       may not be supporting the service directly itself.
    Numerous directory and configuration services already exist, and
    reuse of these mechanisms may be appropriate.  There is an open
    question about whether multiple methods of discovery would be
    needed, and whether NNs would also need to discover other NNs.
    The definition of a service also needs to be determined, including
    the granularity of the description.  For example, IEEE 802.21
    specifies three different types of Mobility Services (Information
    Services, Command Services, and Event Services) that can be
    located in different portions of the network.  An MN could
    therefore run a discovery procedure of any service running in the
    (home or visited) network or could run a discovery procedure for a
    specific service.
 Information from a trusted source:  The MN uses the Mobility Service
    information to make decisions about what steps to take next.  It
    is essential that there is some way to ensure that the information
    received is from a trustworthy source.  This requirement should
    reuse trust relationships that have already been established in
    the network, for example, on the relationships established by the
    Authentication, Authorization, and Accounting (AAA) infrastructure
    after a mutual authentication, or on the certificate
    infrastructure required to support SEND [10].  Section 6 provides
    a more complete analysis.
 Security association management:  A common security association
    negotiation method, independent of any specific MSTP user, should
    be implemented between the endpoints of the MSTP.  The solution
    must also work in the case of MN mobility.
 Secure delivery:  The Mobility Service information must be delivered
    securely (integrity and confidentiality) between trusted peers,
    where the transport may pass though untrusted intermediate nodes
    and networks.  The Mobility Service information should also be
    protected against replay attacks and denial-of-service attacks.

Melia, et al. Informational [Page 9] RFC 5164 Mobility Services Transport March 2008

 Low latency:  Some of the Mobility Services generate time-sensitive
    information.  Therefore, there is a need to deliver the
    information over quite short timescales, and the required lifetime
    of a connection might be quite short-lived.  As an example, the
    frequency of messages defined in [1] varies according to the MIH
    service type.  It is expected that Events and Commands messages
    arrive at an interval of hundreds of milliseconds in order to
    capture quick changes in the environment and/or process handover
    commands.  On the other hand, Information Service messages are
    mainly exchanged each time a new network is visited that may be in
    the order of hours or days.  For reliable delivery, short-lived
    connections could be set up as needed, although there is a
    connection setup latency associated with this approach.
    Alternatively, a long-lived connection could be used, but this
    requires advanced warning of being needed and some way to maintain
    the state associated with the connection.  It also assumes that
    the relationships between devices supporting the mobility service
    are fairly stable.  Another alternative is connectionless
    operation, but this has interactions with other requirements, such
    as reliable delivery.
 Reliability:  Reliable delivery for some of the Mobility Services may
    be essential, but it is difficult to trade this off against the
    low latency requirement.  It is also quite difficult to design a
    robust, high-performance mechanism that can operate in
    heterogeneous environments, especially one where the link
    characteristics can vary quite dramatically.  There are two main
    approaches that could be adopted:
    1. Assume the transport cannot be guaranteed to support reliable
       delivery.  In this case, the Mobility Support Service itself
       will have to provide a reliability mechanism (at the MIH level)
       to allow communicating endpoints to acknowledge receipt of
    2. Assume the underlying transport will provide reliable delivery.
       There is no need in this case to provide reliability at the MIH
    Guidelines provided in [3] are being considered while writing this
 Congestion Control:  A Mobility Service may wish to transfer small or
    large amounts of data, placing different requirements for
    congestion control in the transport.  As an example, the MIH
    message [1] size varies widely from about 30 bytes (for a
    broadcast capability discovery request) to be normally less than
    64 KB, but may be greater than 64KB (for an IS MIH_Get_Information

Melia, et al. Informational [Page 10] RFC 5164 Mobility Services Transport March 2008

    response primitive).  A typical MIH message size for the Events
    and Commands Services service ranges between 50 to 100 bytes.  The
    solution should consider different congestion control mechanisms
    depending on the amount of data generated by the application (MIH)
    as suggested in [3].
 Fragmentation and reassembly:  ES and CS messages are small in
    nature, are sent frequently, and may wish trade reliability in
    order to satisfy the tight latency requirements.  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.  Depending on the choice of the transport
    protocol, different fragmentation and reassembly strategies are
 Multihoming:  For some Information Services exchanged with the MN,
    there is a possibility that the request and response messages
    could be carried over two different links.  For example, a
    handover command request is on the current link while the response
    could be delivered on the new link.  It is expected that the
    transport protocol is capable of receiving information via
    multiple links.  It is also expected that the MSTP user combines
    information belonging to the same session/transaction.  When
    mobility is applied, the underlying IP mobility mechanism should
    provide session continuity when required.
 IPv4 and IPv6 support:  The MSTP must support both IPv4 and IPv6
    including NAT traversal for IPv4 networks and firewall
    pass-through for IPv4 and IPv6 networks.

6. Security Considerations

 Network-supported Mobility Services aim at improving decision making
 and management of dynamically connected hosts.
 Information Services may not require authorization of the client, but
 both Event and Command Services may authenticate message sources,
 particularly if they are mobile.  Network-side service entities will
 typically need to provide proof of authority to serve visiting
 devices.  Where signalling or radio operations can result from
 received messages, significant disruption may result from processing
 bogus or modified messages.  The effect of processing bogus messages
 depends largely upon the content of the message payload, which is
 handled by the handover services application.  Regardless of the
 variation in effect, message delivery mechanisms need to provide
 protection against tampering, spoofing, and replay attacks.

Melia, et al. Informational [Page 11] RFC 5164 Mobility Services Transport March 2008

 Sensitive and identifying information about a mobile device may be
 exchanged during handover-service message exchange.  Since handover
 decisions are to be made based upon message exchanges, it may be
 possible to trace a user's movement between cells, or predict future
 movements, by inspecting handover service messages.  In order to
 prevent such tracking, message confidentiality and message integrity
 should be available.  This is particularly important because many
 mobile devices are associated with only one user, since divulging of
 such information may violate the user's privacy.  Additionally,
 identifying information may be exchanged during security association
 construction.  As this information may be used to trace users across
 cell boundaries, identity protection should be available, if
 possible, when establishing source addresses (SAs).
 In addition, the user should not have to disclose its identity to the
 network (anymore than it needed to during authentication) in order to
 access the Mobility Support Services.  For example, if the local
 network is just aware that an anonymous user with a subscription to
 "" is accessing the network, the user should not have to
 divulge their true identity in order to access the Mobility Support
 Services available locally.
 Finally, the NNs themselves will potentially be subject to
 denial-of-service attacks from MNs, and these problems will be
 exacerbated if operation of the Mobility Service protocols imposes a
 heavy computational load on the NNs.  The overall design has to
 consider at what stage (e.g., discovery, transport layer
 establishment, and service-specific protocol exchange) denial-of-
 service prevention or mitigation should be built in.

7. Conclusions

 This document outlined a broad problem statement for the signalling
 of information elements across a network to support Mobility
 Services.  In order to enable this type of signalling service, a need
 for a generic transport solution with certain transport and security
 properties was outlined.  Whilst the motivation for considering this
 problem has come from work within IEEE 802.21, a desirable goal is to
 ensure that solutions to this problem are applicable to a wider range
 of Mobility Services.
 It would be valuable to establish realistic performance goals for the
 solution to this common problem (i.e., transport and security
 aspects) using experience from previous IETF work in this area and
 knowledge about feasible deployment scenarios.  This information
 could then be used as an input to other standards bodies in assisting
 them to design Mobility Services with feasible performance

Melia, et al. Informational [Page 12] RFC 5164 Mobility Services Transport March 2008

 Much of the functionality required for this problem is available from
 existing IETF protocols or combination thereof.  This document takes
 no position on whether an existing protocol can be adapted for the
 solution or whether new protocol development is required.  In either
 case, we believe that the appropriate skills for development of
 protocols in this area lie in the IETF.

8. Acknowledgements

 Thanks to Subir Das, Juan Carlos Zuniga, Robert Hancock, and
 Yoshihiro Ohba for their input.  Thanks to the IEEE 802.21 chair,
 Vivek Gupta, for coordinating the work and supporting the IETF
 liaison.  Thanks to all IEEE 802.21 WG folks who contributed to this
 document indirectly.

9. References

9.1. Normative References

 [1]    "Draft IEEE Standard for Local and Metropolitan Area Networks:
        Media Independent Handover Services", IEEE LAN/MAN Draft IEEE
        P802.21/D07.00, July 2007.
 [2]    Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2. Informative References

 [3]    Eggert, L. and G. Fairhurst, "UDP Usage Guidelines for
        Application Designers", Work in Progress.
 [4]    3GPP, "3GPP system architecture evolution (SAE): Report on
        technical options and conclusions", 3GPP TR 23.882 0.10.1,
        February 2006.
 [5]    Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
        August 2002.
 [6]    Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
        IPv6", RFC 3775, June 2004.
 [7]    Moskowitz, R. and P. Nikander, "Host Identity Protocol (HIP)
        Architecture", RFC 4423, May 2006.
 [8]    Eronen, P., "IKEv2 Mobility and Multihoming Protocol
        (MOBIKE)", RFC 4555, June 2006.

Melia, et al. Informational [Page 13] RFC 5164 Mobility Services Transport March 2008

 [9]    Koodli, R., Ed., "Fast Handovers for Mobile IPv6", RFC 4068,
        July 2005.
 [10]   Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, "SEcure
        Neighbor Discovery (SEND)", RFC 3971, March 2005.

Contributors' Addresses

 Eleanor Hepworth
 Siemens Roke Manor Research
 Roke Manor
 Romsey,   SO51 5RE
 Srivinas Sreemanthula
 Nokia Research Center
 6000 Connection Dr.
 Irving,   TX 75028
 Yoshihiro Ohba
 Toshiba America Research, Inc.
 1 Telcordia Drive
 Piscateway  NJ 08854
 Vivek Gupta
 Intel Corporation
 2111 NE 25th Avenue
 Hillsboro, OR  97124
 Phone: +1 503 712 1754

Melia, et al. Informational [Page 14] RFC 5164 Mobility Services Transport March 2008

 Jouni Korhonen
 TeliaSonera Corporation.
 P.O.Box 970
 FIN-00051 Sonera
 Phone: +358 40 534 4455
 Rui L.A. Aguiar
 Instituto de Telecomunicacoes Universidade de Aveiro
 Aveiro  3810
 Phone: +351 234 377900
 Sam(Zhongqi) Xia
 Huawei Technologies Co., Ltd
 HuaWei Bld., No.3 Xinxi Rd. Shang-Di Information Industry Base
 Hai-Dian District Beijing, P.R. China
 Phone: +86-10-82836136

Authors' Addresses

 Telemaco Melia, Editor
 Cisco Systems International Sarl
 Avenue des Uttins 5
 1180 Rolle
 Switzerland (FR)
 Phone: +41 21 822718

Melia, et al. Informational [Page 15] RFC 5164 Mobility Services Transport March 2008

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Melia, et al. Informational [Page 16]

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