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Versions: 00

Mobility for IP: Performance,                                 R. Hancock
Signaling and Handoff Optimization                           E. Hepworth
Internet-Draft                                                       RMR
Expires: December 21, 2006                                 June 19, 2006


       Supporting Media Independent Handover Protocols with GIST
                 draft-hancock-mipshop-gist-for-mih-00

Status of this Memo

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   This Internet-Draft will expire on December 21, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The MIPSHOP working group is working on common functionality
   necessary for the support of media independent handover (MIH)
   protocols, initially focussing on the architectures being developed
   with the 802.21 working group of the IEEE.  A problem statement draft
   describing the context and scope of the common functionality is
   already in progress, and a second draft has been prepared that
   describes the main protocol design issues that have to be taken into
   consideration.  Although the common functionality (relating to



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   transport and security requirements) is mainly standard, there is
   still a significant element of protocol design to integrate them with
   the MIH service itself, including in particular secure peer discovery
   and capability negotiation.  This draft presents a solution for the
   common protocol functionality based on the use of the General
   Internet Signaling Transport (GIST) protocol, which has been
   developed in the NSIS working group.  GIST can already satisfy the
   bulk of the protocol requirements in its current form, and this draft
   explains how this existing GIST functionality fits into the MIH
   problem space.  The main special requirements for MIH relate to
   signalling node discovery and message routing, and this draft
   outlines three options for supporting these requirements within the
   GIST framework.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Functionality Overview . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Discovery and Routing  . . . . . . . . . . . . . . . . . .  4
     2.2.  Message Transport  . . . . . . . . . . . . . . . . . . . .  5
     2.3.  State Management . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Multiplexing and Extensibility . . . . . . . . . . . . . .  6
     2.5.  Network Layer Interactions . . . . . . . . . . . . . . . .  6
     2.6.  Message Security . . . . . . . . . . . . . . . . . . . . .  7
     2.7.  Overload Management  . . . . . . . . . . . . . . . . . . .  8
     2.8.  Failure Handling . . . . . . . . . . . . . . . . . . . . .  9
   3.  Extended Message Routing Methods . . . . . . . . . . . . . . .  9
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  A Message Routing Method for MIH Services  . . . . . . . . 10
     3.3.  Multi-MIH Service Discovery  . . . . . . . . . . . . . . . 12
   4.  MIH Service Development  . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 16
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21













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1.  Introduction

   The MIPSHOP working group is currently working on common
   functionality necessary for the support of media independent handover
   (MIH) protocols, initially focussing on the architectures being
   developed with the 802.21 working group of the IEEE.  MIH protocols
   are used to exchange information between terminals and networks to
   convey information about radio conditions, available resources and
   other handover requirements in order to allow optimised handover
   decisions to be taken and executed in heterogeneous wireless
   environments (that is, environments where more than one type of radio
   access technology is available or in use at any one time).  Thus,
   although the MIH protocols carry link layer information, the
   protocols themselves are not tightly bound to any specific link
   layer.  Instead, they can be carried at the network layer, both
   within the radio access infrastructure and over the air.

   A problem statement describing the context and scope of the common
   functionality is already in progress [1].  This draft describes a
   decomposition of the overall MIH protocol problem into a common part,
   and a set of specific users or MIH services which is layered over it.
   This structure is consistent with the architecture defined by 802.21
   [2], where the services in question have been identified as the
   Event, Command and Information Services (ES/CS/IS).  The problem
   statement itself outlines a division of functionality between the
   common part and the individual services, with the intention that the
   common part will be generally useful even in non-802.21
   architectures.  The expectation is that the MIPSHOP working group
   will develop a solution for the common part which meets the 802.21
   requirements as expressed in the problem statement draft.  A second
   draft outlining more detailed design considerations for the common
   part has also been prepared [3], which formalises some of the open
   issues the scope of the common functionality, and lists the key
   design choices that have to be made and the criteria that should be
   taken into account in doing so.  Although [3] refers to components of
   possible solutions as examples, it is not a solution proposal in its
   own right.

   While the common functionality (relating to transport and security
   requirements) is mainly standard, there is still a significant
   element of protocol design to integrate them with the MIH service
   itself, in particular secure peer discovery and capability
   negotiation.  This draft presents an outline solution for the common
   protocol functionality, based on the use of the General Internet
   Signaling Transport (GIST) protocol [4].  GIST has been developed in
   the NSIS working group to provide common protocol support for a
   diverse set of signalling applications, such as resource reservation
   [5] and middlebox control [6].  Within the overall NSIS protocol



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   suite, GIST has responsibility for the discovery (implicit or
   explicit) of the signalling peers that should take part in a
   transaction, and the actual transfer of the signalling messages
   between them (including security and transport issues).  In this
   sense, it is already aligned at a high level with the requirements
   for the MIH support, especially so far as message transfer
   functionality is concerned.  For discovery, the initial use cases for
   GIST have followed the 'path-coupled' signalling paradigm, where the
   signalling peers are assumed to lie on the path taken by a flow in
   the network; however, GIST has the concept of modular message routing
   methods to implement alternative discovery paradigms, and this
   flexibility can be used to develop a simple extension of GIST for the
   MIH problem space if this turns out to be necessary.

   This draft is structured as follows.  Section 2 provides an overview
   of how GIST can be used to support the various functional
   requirements for the MIH support; in most cases, there is a single
   natural approach and only a brief discussion is required.  The case
   of routing and discovery is more complex; this is discussed in more
   detail in Section 3, which outlines three possible design options and
   their advantages and disadvantages.  Section 4 describes the process
   of implementing an MIH service within such a protocol framework.
   Security considerations are discussed in Section 5, and finally
   Section 6 summarises the open issues and provides conclusions and
   recommendations.


2.  Functionality Overview

   This section follows the structure of section 3 of [3] in addressing
   each design consideration in turn.

2.1.  Discovery and Routing

   Discovery and routing are the most MIH-specific parts of the
   signalling problem.  The considerations of [3] imply that this
   functionality should be integrated as part of the GIST layer at least
   to some extent, for example because it interacts very strongly with
   the re-use of standard security and transport protocols which are the
   basis of GIST functionality.  The current GIST specification includes
   one routing method that could be used to allow the MIH services to
   specify the signalling peer address directly, and this could be used
   as a fallback method.  However, it is likely that a GIST extension to
   support a different type of message routing will provide better
   performance.  This issue is therefore discussed in more detail in
   Section 3.





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2.2.  Message Transport

   All the standard message transport requirements can be supported by
   GIST, by virtue of it being able to negotiate the use of standard
   transport protocols.  The baseline GIST specification supports a
   minimal unreliable mode (using UDP), and reliable, congestion
   controlled and flow controlled transport over TCP, which is also the
   only mode to allow large messages.  As an implementation and policy
   issue, GIST is allowed to create multiple TCP connections to avoid
   head of line blocking or to support differential transmission
   priorities.  Between two peers, messages may be transported reliably,
   unreliably, or any mixture of the two.  Note that the MIH services
   would not directly select between these various possibilities; they
   just indicate the transmission requirements for a particular message
   and rely on GIST to transport it accordingly.

   It is not forseen that additional transport protocols would be needed
   for the support of MIH services.  Services that intermittently
   required a large volume of unreliably transported real-time data
   (e.g. for measurement reporting) could benefit from the use of DCCP,
   although careful service design and implementation would be needed to
   avoid saturating the radio links with reporting data.  A GIST
   extension to use SCTP has already been proposed [7], which would
   allow a single transport connection to support multiple priority
   streams as well as mixing reliable and unreliable data, which would
   lead to efficiency savings; however, support for SCTP in mobile nodes
   is not currently widespread.

2.3.  State Management

   Conceptually, GIST provides a stateless service to the upper layers
   (MIH services).  It is up to the MIH service to define its own
   transaction structure; whether or not transactions nest or overlap or
   how a transaction structure is even indicated at all is left open.
   Also, there is no hard binding or dependency between state at the
   GIST level and state at the MIH service level: they can be managed
   largely independently.  For example, the opening or closing of a TCP
   connection within GIST has no significance to an MIH service -
   transport connection state can be managed separately.  In particular,
   if the MIH service needs to refresh state periodically at a peer or
   tear it down, this is done with explicit messages at the service
   level rather than implicitly by configuration operations on GIST.

   GIST does have a concept of signalling sessions; however, a
   signalling session is simply a group of signalling messages with a
   common session identifier (SID).  It is left to the signalling
   application to generate SIDs and assign them to individual messages
   or groups of messages.  The main significance of SID selection is



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   that messages for a single session that are delivered reliably will
   be delivered in order.  (In addition, GIST maintains a separate copy
   of the routing state for each signalling session that it is aware of,
   although this is to handle certain security considerations rather
   than in the expectation that routing state will be SID dependent.)
   In the MIH context, it is likely that only a small number of sessions
   (e.g. 1) will need to be created by an MIH service while it is
   operating for any single mobile node.

   GIST will automatically maintain its internal state to ensure that
   MIH messages can continue to be transported.  MIH services can
   provide timing information on how long it will require such state to
   be kept for.  GIST can provide hints to the MIH service that the
   state of peer node may have changed (see Section 2.8), but it is up
   to the MIH service itself to verify the exact situation and take
   appropriate recovery actions.

2.4.  Multiplexing and Extensibility

   GIST supports multiple signalling applications.  Messages for
   different signalling applications are transported independently, and
   a single identifier distinguishes which application in a node should
   receive the message.  The identifier is the NSIS Signalling Layer
   Protocol Identifier (NSLPID), which is a two byte field managed by
   IANA (see section 9 of [4]).

   MIH services could use a single NSLPID, with further demultiplexors
   defined inside the signalling application payload, or a set of
   NSLPIDs could be allocated.  Because the routing of a message depends
   on the NSLPID, the choice has implications for the design of the
   routing and discovery functionality, see Section 3 below.  The
   minimal approach would be to request a new NSLPID for each MIH
   service, but more sophisticated options are possible.  General
   guidelines on the extensibility of the NSIS protocol suite as a whole
   are contained in [8].

2.5.  Network Layer Interactions

   The signalling messages themselves have to pass through the network
   infrastructure, and thus are subject to the same problems with
   middlebox (NAT and firewall) traversal as all other types of data
   traffic.  The signalling also has to operate in both IPv4 and IPv6
   environments and also possibly in transition scenarios.

   Most of these issues have to be handled in the GIST layer.  Since
   this uses standard transport encapsulations (currently UDP and TCP)
   for all its data, NAT traversal should not be a problem.  (The
   transport connections are initiated from the terminal, which in most



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   cases will be on the 'private' side of any NAT, which is the same
   scenario as typical host-initiated client-server interactions.)  The
   UDP exchanges use a specific well-known port which firewalls would
   have to be configured to pass.  The transport protocol negotiation
   allows agile port numbers to be used for the TCP connections, so this
   would either require GIST-awareness in the firewall to open the
   correct ports, or pinning the port to be offered in the GIST layers
   in the signalling nodes themselves.  GIST is IP version neutral
   because it uses transport protocols which can operate equally over
   IPv4 or IPv6.

   Special problems arise if the signalling application data, i.e. the
   messages defined for the MIH services, contain embedded IP addresses
   for third party nodes (i.e. other than the addresses of the
   signalling nodes themselves).  If this is the case, these payloads
   may need to be translated in an application specific way as the
   message passes through a NAT.  Whether this is actually necessary
   depends on a more detailed analysis of the MIH services themselves;
   if it is, it is potentially a source of considerable complexity
   because it requires the signalling to be aware of the address realm
   topology over a potentially wide area.  It would be ideal if it was
   possible to express any IP addressing information at the MIH service
   level in terms of a single addressing realm (for example, the
   addressing as seen by the terminals themselves).  The current 802.21
   [2] draft does include payloads with such data, and further analysis
   would be needed to understand whether these need to be carried
   explicitly as addresses or simply as references to nodes in the
   signalling exchange.

2.6.  Message Security

   GIST is able to negotiate the use of channel security protocols to
   protect the messages between adjacent signalling peers.  (Here, the
   term 'adjacent' means two nodes supporting the same signalling
   application, without any other node for that signalling application
   between them.  There could be one or several IP hops between the
   peers.)  In principle, any channel security protocol can be
   negotiated; in the initial GIST specification, the single mandatory-
   to-implement mechanism is TLS over TCP, with TLS authentication using
   certificates.  Further security options could be added within the
   GIST framework with relatively simple extensions, such as:

   o  TLS over other transports (DTLS over UDP, TLS over SCTP)

   o  Other TLS authentication mechanisms

   o  IPsec using IKE or KINK for authentication and key exchange




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   The main motivation for defining additional security options would be
   to re-use existing security (authentication) infrastructures with
   existing deployed sets of credentials.  It would be natural to carry
   out such extensions to GIST itself, rather than building something
   MIH specific into the signalling application level.  In particular,
   GIST already includes a protocol negotiation capability which can
   support the addition of further options, and designing such a
   negotiation which is secure against bidding-down and denial of
   service attacks is not trivial.

   The operation of the channel security mechanism chosen can be made
   largely invisible to the MIH services.  For any specific message they
   simply have to indicate to GIST that security protection is required.
   The authenticated identifier of the peer can be checked before the
   message is sent, and this is also available at the receiver.  The
   initial negotiation process (what to offer, what to accept and what
   to demand) is policy driven, and it may be appropriate for MIH
   services to require a particular security policy to ensure that
   appropriate security associations are actually created.

2.7.  Overload Management

   The basic requirement for overload management within GIST is to avoid
   causing network layer congestion in the Internet (between signalling
   nodes).  This is achieved mainly by using congestion-controlled
   transport protocols for the bulk of signalling data transfer, coupled
   with rate control for the unreliable message transfer and exponential
   backoff for the initial discovery transactions.  These also serve to
   handle processing overload within the GIST layer itself.

   GIST simplifies the process of writing overload-controlled signalling
   applications by providing a flow controlled transport service between
   them.  The flow control runs directly between the signalling
   application nodes; if the receiving node is not able to accept
   signalling data fast enough from GIST, the sending node will be flow
   controlled.  Overload adaptation mechanisms are left entirely to the
   application: possibilities include discarding messages at the
   receiver after minimal processing, discarding messages at the sender
   when overload is detected, or adapting transaction timers (e.g.
   refresh rates) at the sender.

   As well as overload caused by excessive volumes of (legitimate)
   signalling application data, there is a related problem of overload
   caused by malicious injection of signalling application data as part
   of a denial of service attack.  GIST includes a two-stage defence
   against this type of attack.  Firstly, the initial discovery process
   incorporates a cookie exchange which forces the initiator of the
   discovery process to create local state before a signalling



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   relationship can be created.  Once this has been done, if channel
   security is used this is an efficient mechanism to reject messages
   from non-authenticated peers.

2.8.  Failure Handling

   As part of the discovery process, GIST is responsible for detecting
   when there is no signalling support for a particular service in the
   network.  Detection would result from failure to receive a response
   to a query after some number of retries with an exponential timer
   backoff; the number of retries can be configured as part of local
   GIST policy, and has to be set as a tradeoff between resilience and
   timeliness.  Once a signalling node has been discovered, GIST does
   not perform application liveness monitoring itself, but it is able to
   provide hints to the application that an error condition may have
   occurred if indications from the network or transport layer suggest
   this.


3.  Extended Message Routing Methods

3.1.  Overview

   A key aspect of the operation of any signalling protocol is how the
   peers in the message exchange are defined and discovered.  Distinct
   from typical application protocols, the correct peer will not be
   defined in terms of some global identifier (e.g. a specific DNS name
   or IP address), but will rather be a function of two variables:

   o  The type of signalling to be done (be it for resource reservation,
      middlebox control, MIH services, or whatever)

   o  The network context within which the signalling takes place

   For example, for the MIH Event Service, the destination for a message
   from a terminal can be abstractly stated in some form like "the ES
   server for the network to which I am currently attached".  In the
   NSIS protocol suite, the responsibility for routing messages is
   placed on GIST, which takes the abstract destination specification
   and turns it into message delivery towards a concrete node.  This may
   be done either by transmitting the message directly with a special
   encapsulation so it is intercepted at the correct node without prior
   communication setup; or, by using the first technique to discover the
   peer node explicitly and then send messages point to point.  A
   related element of functionality is that GIST is responsible for
   monitoring the local network state and determine when the peer node
   has or might have changed.  (This is generically referred to as route
   change handling, because a change in the local topology is the



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   commonest cause for such a peer change; however, the same
   functionality handles other situations, such as changes in the
   capability of a signalling node itself.)  Note that discovery is the
   responsibility of the initiator of the signalling exchange, but once
   it has taken place, further transactions can be started by either
   peer (i.e. the association is symmetric); in the following we assume
   that the initiator in MIH scenarios is always the terminal.

   The combination of functions need to perform message routing
   (including peer discovery and route change handling) in GIST is
   referred to as a 'Message Routing Method' (MRM).  An MRM defines a
   specific set of data items on which the message is routed, the
   Message Routing Information (MRI).  Two MRMs are defined in the base
   GIST protocol; it needs to be decided whether to base MIH support on
   one of these, or whether a new MRM should be defined.  The current
   MRMs are:

   Flow-coupled: The signalling message is defined to pass along the
      same set of nodes as the packets of a specific flow; the flow is
      defined by a header N-tuple, which makes up the MRI itself.  This
      MRM is not applicable in the MIH context, because the signalling
      messages are not associated with particular flows.  (This MRM is
      primarily aimed at resource reservation signalling applications.)

   Loose-end: The signalling message is sent in the direction of a
      particular destination identified by its IP address, but is
      expected to be intercepted at a network boundary on the way.

   It would be possible to use the loose-end MRM for MIH support,
   thereby essentially pushing the discovery responsibility into the
   signalling application (MIH service).  The signalling application
   could use any of a number of IP layer discovery techniques to
   determine the destination IP address for a particular MIH service,
   such as DHCP [9][10], SLP [11], mDNS [12], or even link-specific
   techniques, and then use that address directly as the signalling
   destination in the loose-end MRM.  Although such techniques are not
   favoured (for reasons discussed more extensively in [3]), this
   approach can always be used as a fall-back if required.

3.2.  A Message Routing Method for MIH Services

   An alternative approach is to create a new MRM, where the message
   routing information includes precisely the identifiers that are
   needed to select between the possible peers to take part in a
   signalling exchange.  Defining the set of identifiers requires
   further analysis of the set of MIH services, but could include:





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   o  The MIH service identifier (if this is not already provided by the
      NSLPID itself).

   o  Some information about the terminal, such as the realm id of the
      home network (for example if the information service is proxied
      back to nodes in the home network).

   o  Some information about the terminal type or capability.

   Note that the information should be strictly limited to that which is
   necessary for message routing; any other MIH specific information can
   be carried in the service payload itself.

   The terminal sends signalling messages with this routing information
   towards its first-hop IP access router (AR).  Here we are assuming
   that IP connectivity is already available; the case where IP
   connectivity is not available is already handled by lower layer
   protocols in the 802.21 model [2], and would be out of scope for
   other MIH work in the IETF.  An operator deploying MIH services in a
   wireless network is required to configure the ARs so they either
   support the MIH services directly, or know how to forward signalling
   messages to nodes that do.  There are at least two options here:

   1.  The AR is able to parse the GIST message including the MRI, and
       consult local routing information to forward the message on to
       the actual MIH serving node.  The routing information has to be
       installed by the network operator, which could typically be done
       using network management or network autoconfiguration techniques
       (as discussed above) - the actual method is not visible to the
       terminal.  The end result will be that the terminal will peer
       directly with the correct signalling node.  The same technique
       can be used to forward the signalling exchange upwards through a
       chain of proxies.  This mode of operation is conceptually similar
       to the forwarding of AAA messages in AAA protocols such as
       RADIUS[13] or Diameter [14], where the analogue of the message
       routing information is the realm part of the NAI.  A further
       refinement would be to define a default lookup mechanism in the
       AR which could be overridden by specific manual configuration.

   2.  A second technique which requires less GIST-specific processing
       in the AR is to configure the AR to intercept the discovery
       messages at the IP/UDP level and forward them directly to a
       dedicated node which performs the MIH-specific routing.  The
       format of the GIST encapsulation of discovery messages means that
       this is possible on most standard routers (in an implementation
       specific way).  Similar techniques are described for the purpose
       of carrying out a type of off-path signalling in [15].




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   The benefit of this approach compared to putting the discovery
   responsibility into the MIH service is that the terminal requirements
   are minimised and that considerable freedom is left for the internal
   configuration of the network.  The terminal can issue signalling
   messages as soon as IP connectivity is available without waiting for
   a separate discovery process (e.g.  DHCP or SLP) to complete; this is
   particularly valuable in the post-handoff stage where the terminal
   could verify whether the MIH serving nodes have changed within a
   single round trip.  There is also no need to select a single
   discovery protocol, which would be mandatory-to-implement for all
   terminals and network environments, including those in which no such
   protocols are automatically expected to be used (e.g.  IPv6 with
   stateless autoconfiguration).  Indeed, it may be possible to
   generalise this method to the case where direct layer 2 transport is
   being used before full IP connectivity is available at all.  If
   discovery is handled within the GIST layer in this way, GIST can also
   take responsibility for tracking the correctness of the routing state
   by periodically re-probing the network to find if conditions have
   changed.  A further advantage is that the denial of service
   properties of the signalling connection setup can be analysed in an
   integrated way, rather than being a property of the discovery
   protocol selected by the MIH service, the transport protocol it uses,
   and the interaction between them.

3.3.  Multi-MIH Service Discovery

   The above approaches to signalling node discovery require a query-
   response transaction for each different MIH signalling node that the
   terminal wishes to communicate with.  Because each MIH service could
   conceptually be implemented on a different node, GIST requires a
   query-response exchange for each MIH service.  This would be the
   behaviour if the approach of using a single NSLPID for each MIH
   service was used, as discussed in Section 2.4.  However, one
   requirement that has been discussed during various stages of the
   802.21 developments is that it would be desirable to be able to carry
   out multiple discovery operations in a single message exchange.

   One solution to this conflict is to reject the requirement.  It is in
   any case a performance optimisation rather than a fundamental goal.
   Whether this is acceptable depends on whether there are many MIH
   services with strong time constraints on the discovery phase.  If
   there are several such services, then there are two ways to aggregate
   them under a single NSLPID, which is an absolute requirement to be
   able to discover multiple service endpoints with a single GIST query.
   (Please note that familiarity with the GIST design [4] is a
   prerequisite for understanding the content of this section.)





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   1.  To carry the required set of service identifiers as piggybacked
       NSLP data in the query message.  The first receiver of the Query
       message can respond on behalf of the services that it supports,
       and forward a modified Query to another node or nodes for the
       remainder.  Although this is conceptually possible, it would be
       very hard to fit into a standard GIST implementation, because the
       GIST stack in the initiating terminal would see multiple
       responses to a single Query, with no information visible at the
       GIST layer to distinguish between them.  Also, for subsequent
       messages, there is no information visible at the GIST layer about
       which service is being invoked, so there would be no method to
       distinguish which peer to send the message to.  Therefore, this
       method is not favoured.

   2.  To carry the required set of service identifiers as part of the
       message routing information (for example, as a capability
       bitmap).  Again, the first receiver generates a response for the
       subset of services that it supports, and re-forwards the Query
       with a reduced set of service identifiers.  Each response creates
       a separate item of GIST routing state for a specific subset of
       supported services.  Subsequent messages would indicate a
       particular service identifier and be matched against the routing
       state for it.

   The second technique is conceptually feasible and does fit into the
   GIST layer model relatively naturally.  It would require some
   analysis of the GIST state machine behaviour to ensure that necessary
   modifications can be confined to the MRM specification.  (The current
   definition of the routing state machine assumes that there is a
   single response to any given Query.  One simple way to handle the
   extension is to require the initiating node to clone the state
   machine when it receives a partial response, which mimics the same
   behaviour as would occur if multiple Queries had been sent in the
   first place.)  Analysis would also be needed of the NAT traversal
   behaviour; however, it is already a requirement that this has to be
   able to allow multiple responses for the same query (because of
   retransmissions).

   It requires further analysis to decide whether the additional
   complexity of such a solution is merited by the performance
   improvement that is gained.  Note that similar problems arise with
   any type of discovery mechanism, where there are potentially multiple
   (and an unknown number) of responses to a single message.


4.  MIH Service Development

   On the assumption that GIST is used to provide the common MIH support



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   functions, the process of formalising the way that an MIH service
   would use it is relatively simple.  If the MIH service is already
   defined as a sequence of message exchanges, the formalisation mainly
   consists of a description of how the MIH service requires GIST to be
   configured and how it invokes GIST to transport each message in turn.

   1.  NSLPIDs for the MIH service need to be chosen; there may be one
       or several (see Section 2.4).  Typically any single message would
       be associated with a single specific NSLPID unless there is some
       type of aggregation concept, which seems not to be applicable
       here.

   2.  The invocation of GIST to transfer a messase is modelled in terms
       of an abstract API (section B of [4]).  The originator invokes
       SendMessage() and the receiver processes it with RecvMessage().
       The actual message payload is defined by the MIH specification
       and is opaque to GIST.  Further parameters influence the message
       transfer process, and the MIH service needs to define how these
       will be used:

       A.  The MIH service needs to decide how messages are to be
           assigned to signalling sessions (see Section 2.3).

       B.  The MIH service needs to provide message routing information
           (see the discussion above in Section 3).  This includes the
           option to require the message to be routed explicitly to the
           same peer that took part in a previous exchange (this can be
           used, for example, for explicit state teardown).

       C.  The MIH service can specify additional security and transport
           attributes (whether channel security is required and whether
           the message needs to be reliably delivered).

       D.  The MIH service may also provide hints about the message
           priority, timeouts on how long delivery should be attempted,
           and how to be notified about error conditions relating to the
           message, although all of these only affect local processing
           within the node rather than end to end behaviour.

   3.  If a new MRM is needed to support the MIH routing and discovery
       requirements, this also needs to be specified as a GIST extension
       in parallel to the MIH specification itself (see Section 3
       above).  Part of this includes the definition of how to detect
       and react to route change events, which can also result in
       notifications over the API between GIST and the MIH services.

   4.  If additional authentication methods or security protocols need
       to be defined (compared to the baseline of TLS + certificates) to



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       ease MIH deployment in particular environments, these should also
       be defined as GIST extensions.  In addition, the criterial for
       security policies for the use of GIST for MIH will need to be
       developed, and a security analysis of the combined (MIH service +
       GIST) solution.  These issues are discussed in the security
       considerations section below, Section 5.


5.  Security Considerations

   This document proposes to address the MIH requirements within a
   layered protocol model, within which some security functionality is
   provided in a generic way by a lower layer (GIST).

   It is important to note that not all the MIH security requirements
   can be met by GIST, nor can the applicability of GIST security for
   MIH be analysed in isolation from other parts of the MIH solution.
   In particular, whether the end result is secure depends on how GIST
   is used (what features are used and how they are invoked).  In
   addition, GIST is completely neutral to certain security
   requirements, specifically in the area of authorisation, and these
   must be handled by the correct implementation and configuration of
   the MIH services that run over it.  A discussion of general threats
   to signalling (with a partial focus on QoS and on-path routing) can
   be found in [16]; most of these issues can be extended to the MIH
   problem space.

   Therefore, the approach taken here is to describe the security
   functionality provided by GIST, and indicate how it could be used as
   part of a more complete security solution.  The main functions are as
   follows (details are given in Section 2.6):

   Message protection: GIST can negotiate the use of a standard channel
      security mechanism (initially TLS) to provide confidentiality and
      integrity protection for messages passing between adjacent
      signalling nodes.  It cannot provide security for messages over
      greater scope than one signalling hop, nor can it assert whether
      the message originator has the right to request whatever actions
      the message implies (i.e. authorisation).  These issues must be
      addressed within the MIH service itself.

   Peer authentication: GIST uses the authentication mechanism within
      the channel security protocol to provide per-peer authentication.
      The default in the current version is limited to TLS
      authentication based on certificates; the authenticated peer
      identities are visible to the MIH service and may be used as part
      of an authorisation decision.  It may be desirable to extend the
      set of authentication mechanisms to improve the applicability and



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      ease of deployment in the MIH problem space.

   Security protocol selection: GIST provides a handshake which can
      negotiate one option from a set of security protocols, in a way
      which is secure against bidding-down attacks and denial-of-service
      attacks.  (This is distinct from any option negotiation within the
      possible security protocols themselves.)  Normally the details of
      this negotiation would be invisible to MIH services; however,
      there may be a need for specific security policies to ensure that
      the level of protection negotiated is appropriate.

   Denial of service mitigation: GIST includes mechanisms to mitigate
      denial of service (flooding) attacks within the GIST layer, and
      can also negotiate channel protection to enable the rejection of
      signalling messages from unauthenticated peers.  An MIH service
      using GIST in this way only needs to care about flooding or other
      DoS attacks launched by authenticated peers.

   An MIH service designed to run over GIST needs to specify the
   required security policy that should be imposed at the GIST level,
   and also needs to define how authorisation is defined and denial of
   service attacks are prevented at the service level.  These latter
   problems are clearly closely related.


6.  Conclusions

   This draft has presented an outline solution for supporting common
   MIH signalling functions using GIST.  Most of the required
   functionality is already present in GIST; some options for GIST
   extensions were also identified.

   Most of the functionality of GIST is obtained by the integration of
   existing security and transport protocols, and conceptually it would
   be possible to design MIH services so they just re-use these
   protocols directly.  Compared to this, the advantages of a GIST-based
   approach include:

   o  integrated negotiation of which security and transport protocols
      to use, including protocol options and port numbers (no need to
      fix on a particular choice);

   o  negotiation which is resistant to denial of service and bidding
      down attacks;

   o  peer discovery and protocol negotiation take place in a single
      round trip, minimising latency;




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   o  ability to support a mixture of transport and security attributes
      under a common API;

   o  optimised discovery of multiple signalling endpoints in a single
      message exchange (with some GIST extensions).

   Further development of a GIST-based solution depends on some more
   detailed analysis of the MIH problem space.  Issues which have been
   noted in this draft include:

   o  whether additional security protocol support will ease deployment
      in typical MIH environments;

   o  what are the more detailed scenarios for signalling endpoint
      location (with implications for how much sophistication is needed
      in the discovery process); and

   o  how to fit the MIH service structure into the overall NSIS
      protocol suite framework, for example as one or several signalling
      applications.


7.  Informative References

   [1]   Hepworth, E., "Media Independent Handovers: Problem Statement",
         draft-hepworth-mipshop-mih-problem-statement-01 (work in
         progress), March 2006.

   [2]   IEEE 802.16, "Draft IEEE Standard for Local and Metropolitan
         Area Networks: Media Independent Handover Services",
         March 2006.

   [3]   Hepworth, E., "Design Considerations for MIH Transport",
         draft-hepworth-mipshop-mih-design-considerations-00 (work in
         progress), March 2006.

   [4]   Schulzrinne, H. and R. Hancock, "GIST: General Internet
         Signaling Transport", draft-ietf-nsis-ntlp-09 (work in
         progress), February 2006.

   [5]   Manner, J., "NSLP for Quality-of-Service Signaling",
         draft-ietf-nsis-qos-nslp-10 (work in progress), March 2006.

   [6]   Stiemerling, M., "NAT/Firewall NSIS Signaling Layer Protocol
         (NSLP)", draft-ietf-nsis-nslp-natfw-11 (work in progress),
         April 2006.

   [7]   Fu, X., "General Internet Signaling Transport (GIST) over



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         SCTP", draft-fu-nsis-ntlp-sctp-01 (work in progress),
         February 2006.

   [8]   Loughney, J., "NSIS Extensibility Model",
         draft-loughney-nsis-ext-02 (work in progress), March 2006.

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

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

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

   [12]  Cheshire, S. and M. Krochmal, "Multicast DNS",
         draft-cheshire-dnsext-multicastdns-05 (work in progress),
         July 2005.

   [13]  Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
         Authentication Dial In User Service (RADIUS)", RFC 2865,
         June 2000.

   [14]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko,
         "Diameter Base Protocol", RFC 3588, September 2003.

   [15]  Hancock, R., "A Problem Statement for Partly-Decoupled
         Signalling in NSIS", draft-hancock-nsis-pds-problem-03 (work in
         progress), March 2006.

   [16]  Tschofenig, H. and D. Kroeselberg, "Security Threats for Next
         Steps in Signaling (NSIS)", RFC 4081, June 2005.


Appendix A.  Acknowledgements

   The authors would like to thank the participants in the NSIS and
   MIPSHOP working groups in the IETF and 802.21 in the IEEE for many
   discussions on signalling protocols in general, and MIH services in
   particular, which have informed the content of this draft.

   Robert Hancock and Eleanor Hepworth are partly funded by Ambient
   Networks, a research project supported by the European Commission
   under its Sixth Framework Program.  The views and conclusions
   contained herein are those of the authors and should not be
   interpreted as necessarily representing the official policies or
   endorsements, either expressed or implied, of the Ambient Networks



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   project or the European Commission.


















































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Authors' Addresses

   Robert Hancock
   Roke Manor Research
   Old Salisbury Lane
   Romsey, Hampshire  SO51 0ZN
   UK

   Email: robert.hancock@roke.co.uk


   Eleanor Hepworth
   Roke Manor Research
   Old Salisbury Lane
   Romsey, Hampshire  SO51 0ZN
   UK

   Email: eleanor.hepworth@roke.co.uk

































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