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Network Working Group
Request for Comments: 3753
Category: Informational
J. Manner, Ed.
M. Kojo, Ed.
June 2004

Mobility Related Terminology

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.

Copyright Notice

Copyright © The Internet Society (2004).

Abstract

There is a need for common definitions of terminology in the work to be done around IP mobility. This document defines terms for mobility related terminology. The document originated out of work done in the Seamoby Working Group but has broader applicability for terminology used in IETF-wide discourse on technology for mobility and IP networks. Other working groups dealing with mobility may want to take advantage of this terminology.

Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  General Terms . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Mobile Access Networks and Mobile Networks. . . . . . . . . .  10
   4.  Handover Terminology. . . . . . . . . . . . . . . . . . . . .  15
       4.1.  Scope of Handover . . . . . . . . . . . . . . . . . . .  16
       4.2.  Handover Control. . . . . . . . . . . . . . . . . . . .  17
       4.3.  Simultaneous connectivity to Access Routers . . . . . .  19
       4.4.  Performance and Functional Aspects. . . . . . . . . . .  19
       4.5.  Micro Diversity, Macro Diversity, and IP Diversity. . .  21
       4.6.  Paging, and Mobile Node States and Modes. . . . . . . .  22
       4.7.  Context Transfer. . . . . . . . . . . . . . . . . . . .  24
       4.8.  Candidate Access Router Discovery . . . . . . . . . . .  24
       4.9.  Types of Mobility . . . . . . . . . . . . . . . . . . .  25
   5.  Specific Terminology for Mobile Ad-Hoc Networking . . . . . .  26
   6.  Security-related Terminology. . . . . . . . . . . . . . . . .  27
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   8.  Contributors. . . . . . . . . . . . . . . . . . . . . . . . .  28
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  29
   10. Informative References. . . . . . . . . . . . . . . . . . . .  29
   11. Appendix A - Index of Terms . . . . . . . . . . . . . . . . .  31
   12. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  35
   13. Full Copyright Statement. . . . . . . . . . . . . . . . . . .  36

1. Introduction

This document presents terminology to be used for documents and discussions within the Seamoby Working Group. Other mobility related working groups could take advantage of this terminology, in order to create a common terminology for the area of mobility in IP networks.

Some terms and their definitions that are not directly related to the IP world are included for the purpose of harmonizing the terminology. For example, 'Access Point' and 'base station' refer to the same component, from the point of view of IP, but 'Access Router' has a very different meaning. The presented terminology may also, it is hoped, be adequate to cover mobile ad-hoc networks.

The proposed terminology is not meant to assert any new terminology. Rather the authors would welcome discussion on more exact definitions as well as missing or unnecessary terms. This work is a collaborative enterprise between people from many different engineering backgrounds and so already presents a first step in harmonizing the terminology.

The terminology in this document is divided into several sections. First, there is a list of terms for general use and mobile access networks followed by terms related to handovers, and finally some terms used within the MANET and NEMO working groups.

2. General Terms

   Bandwidth
      The total width of the frequency band available to or used by a
      communications channel.  Usually measured in Hertz (Hz).  The
      bandwidth of a channel limits the available channel capacity.
   Bandwidth utilization
      The actual rate of information transfer achieved over a link,
      expressed as a percentage of the theoretical maximum channel
      capacity on that link, according to Shannon's Law.
   Beacon
      A control message broadcast by a node (especially, a base station)
      informing all the other nodes in its neighborhood of the
      continuing presence of the broadcasting node, possibly along with
      additional status or configuration information.
   Binding Update (BU)
      A message indicating a mobile node's current mobility binding, and
      in particular its care-of address.
   Care-of-Address (CoA)
      An IP address associated with a mobile node while visiting a
      foreign link; the subnet prefix of this IP address is a foreign
      subnet prefix.  A packet addressed to the mobile node which
      arrives at the mobile node's home network when the mobile node is
      away from home and has registered a Care-of Address will be
      forwarded to that address by the Home Agent in the home network.
   Channel
      A subdivision of the physical medium allowing possibly shared
      independent uses of the medium.  Channels may be made available by
      subdividing the medium into distinct time slots, or distinct
      spectral bands, or decorrelated coding sequences.
   Channel access protocol
      A protocol for mediating access to, and possibly allocation of,
      the various channels available within the physical communications
      medium.  Nodes participating in the channel access protocol agree
      to communicate only when they have uncontested access to one of
      the channels, so that there will be no interference.
   Channel capacity
      The total capacity of a link to carry information (typically bits)
      per unit time.  With a given bandwidth, the theoretical maximum
      channel capacity is given by Shannon's Law.  The actual channel
      capacity of a channel is determined by the channel bandwidth, the
      coding system used, and the signal to noise ratio.
   Control message
      Information passed between two or more network nodes for
      maintaining protocol state, which may be unrelated to any specific
      application.
   Distance vector
      A characteristic of some routing protocols in which, for each
      desired destination, a node maintains information about the
      distance to that destination, and a vector (next hop) towards that
      destination.
   Fairness
      A property of channel access protocols whereby a medium is made
      fairly available to all eligible nodes on the link.  Fairness does
      not strictly imply equality, especially in cases where nodes are
      given link access according to unequal priority or classification.
   Flooding
      The process of delivering data or control messages to every node
      within the network under consideration.
   Foreign subnet prefix
      A bit string that consists of some number of initial bits of an IP
      address which identifies a node's foreign link within the Internet
      topology.
   Forwarding node
      A node which performs the function of forwarding datagrams from
      one of its neighbors to another.
   Home Address (HoA)
      An IP address assigned to a mobile node, used as the permanent
      address of the mobile node.  This address is within the mobile
      node's home link.  Standard IP routing mechanisms will deliver
      packets destined for a mobile node's home address to its home link
      [9].
   Home Agent (HA)
      A router on a mobile node's home link with which the mobile node
      has registered its current care-of address.  While the mobile node
      is away from home, the home agent intercepts packets on the home
      link destined to the mobile node's home address, encapsulates
      them, and tunnels them to the mobile node's registered care-of
      address.
   Home subnet prefix
      A bit string that consists of some number of initial bits of an IP
      address which identifies a node's home link within the Internet
      topology (i.e., the IP subnet prefix corresponding to the mobile
      node's home address, as defined in [9]).
   Interface
      A node's point of attachment to a link.
   IP access address
      An IP address (often dynamically allocated) which a node uses to
      designate its current point of attachment to the local network.
      The IP access address is typically to be distinguished from the
      mobile node's home address; in fact, while visiting a foreign
      network the IP access address may be considered unsuitable for use
      as an end-point address by any but the most short-lived
      applications.  Instead, the IP access address is typically used as
      the care-of address of the node.
   Link
      A communication facility or physical medium that can sustain data
      communications between multiple network nodes, such as an Ethernet
      (simple or bridged).  A link is the layer immediately below IP.
      In a layered network stack model, the Link Layer (Layer 2) is
      normally below the Network (IP) Layer (Layer 3), and above the
      Physical Layer (Layer 1).
   Asymmetric link
      A link with transmission characteristics which are different
      depending upon the relative position or design characteristics of
      the transmitter and the receiver of data on the link.  For
      instance, the range of one transmitter may be much higher than the
      range of another transmitter on the same medium.
   Link establishment
      The process of establishing a link between the mobile node and the
      local network.  This may involve allocating a channel, or other
      local wireless resources, possibly including a minimum level of
      service or bandwidth.
   Link-layer trigger (L2 Trigger)
      Information from the link layer that informs the network layer of
      the detailed events involved in handover sequencing at the link
      layer.  L2 triggers are not specific to any particular link layer,
      but rather represent generalizations of link layer information
      available from a wide variety of link layer protocols [4].
   Link state
      A characterization of some routing protocols in which every node
      within the network is expected to maintain information about every
      link within the network topology.
   Link-level acknowledgment
      A protocol strategy, typically employed over wireless media,
      requiring neighbors to acknowledge receipt of packets (typically
      unicast only) from the transmitter.  Such strategies aim to avoid
      packet loss or delay resulting from lack of, or unwanted
      characteristics of, higher level protocols.  Link-layer
      acknowledgments are often used as part of Automatic Repeat-Request
      (ARQ) algorithms for increasing link reliability.
   Local broadcast
      The delivery of data to every node within range of the
      transmitter.
   Loop-free
      A property of routing protocols whereby the path taken by a data
      packet from source to destination never traverses through the same
      intermediate node twice before arrival at the destination.
   Medium Access Protocol (MAC)
      A protocol for mediating access to, and possibly allocation of,
      the physical communications medium.  Nodes participating in the
      medium access protocol can communicate only when they have
      uncontested access to the medium, so that there will be no
      interference.  When the physical medium is a radio channel, the
      MAC is the same as the Channel Access Protocol.
   Mobile network prefix
      A bit string that consists of some number of initial bits of an IP
      address which identifies the entire mobile network within the
      Internet topology.  All nodes in a mobile network necessarily have
      an address containing this prefix.
   Mobility factor
      The relative frequency of node movement, compared to the frequency
      of application initiation.
   Multipoint relay (MPR)
      A node which is selected by its one-hop neighbor to re-transmit
      all broadcast messages that it receives.  The message must be new
      and the time-to-live field of the message must be greater than
      one.  Multipoint relaying is a technique to reduce the number of
      redundant re-transmissions while diffusing a broadcast message in
      the network.
   Neighbor
      A "neighbor" is any other node to which data may be propagated
      directly over the communications medium without relying on the
      assistance of any other forwarding node.
   Neighborhood
      All the nodes which can receive data on the same link from one
      node whenever it transmits data.
   Next hop
      A neighbor which has been selected to forward packets along the
      way to a particular destination.
   Payload
      The actual data within a packet, not including network protocol
      headers which were not inserted by an application.  Note that
      payloads are different between layers:  application data is the
      payload of TCP, which are the payload of IP, which three are the
      payload of link layer protocols etc.  Thus, it is important to
      identify the scope when talking about payloads.
   Prefix
      A bit string that consists of some number of initial bits of an
      address.
   Routing table
      The table where forwarding nodes keep information (including next
      hop) for various destinations.
   Route entry
      An entry for a specific destination (unicast or multicast) in the
      routing table.
   Route establishment
      The process of determining a route between a source and a
      destination.
   Route activation
      The process of putting a route into use after it has been
      determined.
   Routing proxy
      A node that routes packets by overlays, e.g., by tunneling,
      between communicating partners.  The Home Agent and Foreign Agent
      are examples of routing proxies, in that they receive packets
      destined for the mobile node and tunnel them to the current
      address of the mobile node.
   Shannon's Law
      A statement defining the theoretical maximum rate at which error-
      free digits can be transmitted over a bandwidth-limited channel in
      the presence of noise.  No practical error correction coding
      system exists that can closely approach the theoretical
      performance limit given by Shannon's law.
   Signal strength
      The detectable power of the signal carrying the data bits, as seen
      by the receiver of the signal.
   Source route
      A source route from node A to node B is an ordered list of IP
      addresses, starting with the IP address of node A and ending with
      the IP address of the node B.  Between A and B, the source route
      includes an ordered list of intermediate hops between A and B, as
      well as the interface index of the interface through which the
      packet should be transmitted to reach the next hop.  The list of
      intermediate hops might not include all visited nodes, some hops
      might be omitted for a reason or another.
   Spatial re-use
      Simultaneous use of channels with identical or close physical
      characteristics, but located spatially far enough apart to avoid
      interference (i.e., co-channel interference)
   System-wide broadcast
      Same as flooding, but used in contrast to local broadcast.
   Subnet
      A subnet is a logical group of connected network nodes.  In IP
      networks, nodes in a subnet share a common network mask (in IPV4)
      or a network prefix (in IPv6).
   Topology (Network Topology)
      The interconnection structure of a network: which nodes are
      directly connected to each other, and through which links they are
      connected.  Some simple topologies have been given names, such as
      for instance 'bus topology', 'mesh topology', 'ring topology',
      'star topology' and 'tree topology'.
   Triggered update
      A solicited route update transmitted by a router along a path to a
      destination.

3. Mobile Access Networks and Mobile Networks

In order to support host mobility a set of nodes towards the network edge may need to have specific functions. Such a set of nodes forms a mobile access network that may or may not be part of the global Internet. Figure 1 presents two examples of such access network topologies. The figure depicts a reference architecture which illustrates an IP network with components defined in this section.

We intend to define the concept of the Access Network (AN) which may also support enhanced mobility. It is possible that to support routing and QoS for mobile nodes, existing routing protocols (e.g., Open Shortest Path First (OSPF) [14]) may not be appropriate to maintain forwarding information for these mobile nodes as they change their points of attachment to the Access Network. These new functions are implemented in routers with additional capabilities. We can distinguish three types of Access Network components: Access Routers (AR) which handle the last hop to the mobile, typically over a wireless link; Access Network Gateways (ANG) which form the boundary on the fixed network side and shield the fixed network from the specialized routing protocols; and (optionally) other internal Access Network Routers which may also be needed in some cases to support the functions. The Access Network consists of the equipment needed to support this specialized routing, i.e., AR or ANG. AR and ANG may be the same physical nodes.

In addition, we present a few basic terms on mobile networks, that is, mobile network, mobile router (MR), and mobile network node (MNN). More terminology for discussing mobile networks can be found in [13]. A more thorough discussion of mobile networks can be found in the working group documents of the NEMO Working Group.

Note: this reference architecture is not well suited for people dealing with Mobile Ad-hoc Networks (MANET).

                                                                   |
                                                                   |
                     ---        ------                    -------  |
        ---  | <-->  | | -------| AR | -------------------|     |  |
        | |--[]      ---        /------          \       /| ANG |--|
        ---            AP      /                  \     / |     |  |
         MH                   /                    \   /  -------  |
   (with wireless     ___    /                    -------          |
        device)       | |----                     | ANR |          |
                      ---                         -------          |
                       AP                          /   \           |
                                                  /     \ -------  |
                     ---       ------            /       \|     |  |
                     | |-------| AR |---------------------| ANG |--|
                     ---       ------                     |     |  |
                      AP                                  -------  |
                                                                   |
                          Access Network (AN) 1                    |
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  -|
                          Access Network (AN) 2                    |
                                                                   |
                                                                   |
                     ---        ------                    -------  |
       ---  | <-->   | | -------| AR | -------------------|     |  |
       | |--[]       ---       /------                   /| ANG |--|
       ---            AP      /                         / |     |  |
        MH                   /                         /  -------  |
   (with wireless    ___    /                         /            |
       device)       | |----                         /             |
                     ---                            /              |
                      AP                           /               |
                                                  /                |
       |            ---       ------         -------               |
   --- |       | <->| |-------| AR |---------| ANR |               |
   | |-|      []    ---  \    ------         -------               |
   --- |  -----|     AP   \                  /                     |
   MNN |--i MR e           \                /                      |
       |  ------    ---     \ ------       /                       |
   --- | (with      | |-------| AR |-------                        |
   | |-| wireless   ---       ------                               |
   --- |  device)    AP                                            |
   MNN                                 'i': MR ingress interface   |
                                       'e': MR egress interface    |
                                                                   |
                Figure 1: Reference Network Architecture
   Mobile Node (MN)
      An IP node capable of changing its point of attachment to the
      network.  A Mobile Node may either be a Mobile Host (no forwarding
      functionality) or a Mobile Router (forwarding functionality).
   Mobile Host (MH)
      A mobile node that is an end host and not a router.  A Mobile Host
      is capable of sending and receiving packets, that is, being a
      source or destination of traffic, but not a forwarder of it.
   Fixed Node (FN)
      A node, either a host or a router, unable to change its point of
      attachment to the network and its IP address without breaking open
      sessions.
   Mobile network
      An entire network, moving as a unit, which dynamically changes its
      point of attachment to the Internet and thus its reachability in
      the topology.  The mobile network is composed of one or more IP-
      subnets and is connected to the global Internet via one or more
      Mobile Routers (MR).  The internal configuration of the mobile
      network is assumed to be relatively stable with respect to the MR.
   Mobile Router (MR)
      A router capable of changing its point of attachment to the
      network, moving from one link to another link.  The MR is capable
      of forwarding packets between two or more interfaces, and possibly
      running a dynamic routing protocol modifying the state by which it
      does packet forwarding.
      A MR acting as a gateway between an entire mobile network and the
      rest of the Internet has one or more egress interface(s)  and one
      or more ingress interface(s).  Packets forwarded upstream to the
      rest of the Internet are transmitted through one of the MR's
      egress interface; packets forwarded downstream to the mobile
      network are transmitted through one of the MR's ingress interface.
   Ingress interface
      The interface of a MR attached to a link inside the mobile
      network.
   Egress interface
      The interface of a MR attached to the home link if the MR is at
      home, or attached to a foreign link if the MR is in a foreign
      network.
   Mobile Network Node (MNN)
      Any node (host or router) located within a mobile network, either
      permanently or temporarily.  A Mobile Network Node may either be a
      mobile node or a fixed node.
   Access Link (AL)
      A last-hop link between a Mobile Node and an Access Point.  That
      is, a facility or medium over which an Access Point and the Mobile
      Node can communicate at the link layer, i.e., the layer
      immediately below IP.
   Access Point (AP)
      An Access Point is a layer 2 device which is connected to one or
      more Access Routers and offers the wireless link connection to the
      Mobile Node.  Access Points are sometimes called base stations or
      access point transceivers.  An Access Point may be a separate
      entity or co-located with an Access Router.
   Radio Cell
      The geographical area within which an Access Point provides radio
      coverage, i.e., where radio communication between a Mobile Node
      and the specific Access Point is possible.
   Access Network Router (ANR)
      An IP router in the Access Network.  An Access Network Router may
      include Access Network specific functionalities, for example,
      related to mobility and/or QoS.  This is to distinguish between
      ordinary routers and routers that have Access Network-related
      special functionality.
   Access Router (AR)
      An Access Network Router residing on the edge of an Access Network
      and connected to one or more Access Points.  The Access Points may
      be of different technology.  An Access Router offers IP
      connectivity to Mobile Nodes, acting as a default router to the
      Mobile Nodes it is currently serving.  The Access Router may
      include intelligence beyond a simple forwarding service offered by
      ordinary IP routers.
   Access Network Gateway (ANG)
      An Access Network Router that separates an Access Network from
      other IP networks, much in the same way as an ordinary gateway
      router.  The Access Network Gateway looks to the other IP networks
      like a standard IP router.  In a small network, an ANG may also
      offer the services of an AR, namely offer the IP connectivity to
      the mobile nodes.
   Access Network (AN)
      An IP network which includes one or more Access Network Routers.
   Administrative Domain (AD)
      A collection of networks under the same administrative control and
      grouped together for administrative purposes [5].
   Serving Access Router (SAR)
      The Access Router currently offering the connectivity to the MN.
      This is usually the point of departure for the MN as it makes its
      way towards a new Access Router (at which time the Serving Access
      Router takes the role of the Previous Access Router).  There may
      be several Serving Access Routers serving the Mobile Node at the
      same time.
   New Access Router (NAR)
      The Access Router that offers connectivity to the Mobile Node
      after a handover.
   Previous Access Router (PAR)
      An Access Router that offered connectivity to the Mobile Node
      prior to a handover.  This is the Serving Access Router that will
      cease or has ceased to offer connectivity to the Mobile Node.
      Often also called Old Access Router (OAR).
   Candidate Access Router (CAR)
      An Access Router to which the Mobile Node may do a handoff.  See
      Section 4.8.

4. Handover Terminology

These terms refer to different perspectives and approaches to supporting different aspects of mobility. Distinctions can be made according to the scope, range overlap, performance characteristics, diversity characteristics, state transitions, mobility types, and control modes of handover techniques.
   Roaming
      An operator-based term involving formal agreements between
      operators that allows a mobile to get connectivity from a foreign
      network.  Roaming (a particular aspect of user mobility) includes,
      for example, the functionality by which users can communicate
      their identity to the local AN so that inter-AN agreements can be
      activated and service and applications in the MN's home network
      can be made available to the user locally.
   Handover
      The process by which an active MN (in the Active State, see
      section 4.6) changes its point of attachment to the network, or
      when such a change is attempted.  The access network may provide
      features to minimize the interruption to sessions in progress.
      Also called handoff.
      There are different types of handover classified according to
      different aspects involved in the handover.  Some of this
      terminology follows the description in [4].

4.1. Scope of Handover

   Layer 2 handover
      A handover where the MN changes APs (or some other aspect of the
      radio channel) connected to the same AR's interface.  This type of
      handover is transparent to the routing at the IP layer (or it
      appears simply as a link layer reconfiguration without any
      mobility implications).
   Intra-AR handover
      A handover which changes the AR's network interface to the mobile.
      That is, the Serving AR remains the same but routing changes
      internal to the AR take place.
   Intra-AN handover
      A handover where the MN changes ARs inside the same AN.  Such a
      handover is not necessarily visible outside the AN.  In case the
      ANG serving the MN changes, this handover is seen outside the AN
      due to a change in the routing paths.  Note that the ANG may
      change for only some of the MN's data flows.
   Inter-AN handover
      A handover where the MN moves to a new AN.  This requires support
      for macro mobility.  Note that this would have to involve the
      assignment of a new IP access address (e.g., a new care-of
      address) to the MN.
   Intra-technology handover
      A handover between equipment of the same technology.
   Inter-technology handover
      A handover between equipment of different technologies.
   Horizontal handover
      This involves MNs moving between access points of the same type
      (in terms of coverage, data rate and mobility), such as, UMTS to
      UMTS, or WLAN to WLAN.
   Vertical handover
      This involves MNs moving between access points of different type,
      such as, UMTS to WLAN.

Note that the difference between a horizontal and vertical handover is vague. For example, a handover from an AP with 802.11b WLAN link to an AP with 802.11g WLAN link may be considered as either a vertical or a horizontal handover, depending on an individual's point of view.

Note also that the IP layer sees network interfaces and IP addresses, rather than specific technologies used by those interfaces. Thus, horizontal and vertical handovers may or may not be noticed at the IP layer. Usually a handover can be noticed if the IP address assigned to the interface changes, the network interface itself changes (which can also change the IP address), or there is a link outage, for example, when the mobile node moves out of coverage for a while. For example, in a GPRS network a horizontal handover happens usually unnoticed by the IP layer. Similarly, a WLAN horizontal handover may be noticed if the IP address of the interface changes. On the other hand, vertical handovers often change the network interface and are, therefore, noticed on the IP layer. Still, some specific network cards may be able to switch between access technologies (e.g., GPRS to UMTS) without changing the network interface. Moreover, either of the two handovers may or may not result in changing the AR. For example, an AR could control WLAN and Bluetooth access points, and the mobile node could do horizontal and vertical handovers under the same AR without changing its IP address or even the network interface.

4.2. Handover Control

A handover must be one of the following two types (a):
      Mobile-initiated handover
         The MN is the one that makes the initial decision to initiate
         the handover.
      Network-initiated handover
         The network makes the initial decision to initiate the
         handover.
A handover is also one of the following two types (b):
      Mobile-controlled handover
         The MN has the primary control over the handover process.
      Network-controlled handover
         The network has the primary control over the handover process.

A handover decision usually involves some sort of measurements about when and where to handover to. Therefore, a handover is also either of these three types (c):

      Mobile-assisted handover
         Information and measurement from the MN are used by the AR to
         decide on the execution of a handover.
      Network-assisted handover
         A handover where the AN collects information that can be used
         by the MN in a handover decision.
      Unassisted handover
         A handover where no assistance is provided by the MN or the AR
         to each other.

Note that it is possible that the MN and the AR both do measurements and decide on the handover.

A handover is also one of the following two types (d):

      Push handover
         A handover either initiated by the PAR, or where the MN
         initiates a handover via the PAR.
      Pull handover
         A handover either initiated by the NAR, or where the MN
         initiates a handover via the NAR.
The handover is also either proactive or reactive (e):
      Planned handover
         A proactive (expected) handover where some signaling can be
         done in advance of the MN getting connected to the new AR,
         e.g., building a temporary tunnel from the previous AR to the
         new AR.
      Unplanned handover
         A reactive (unexpected) handover where no signaling is done in
         advance of the MN's move from the previous AR to the new AR.

The five handover types (a-e) are mostly independent, and every handover should be classifiable according to each of these types.

4.3. Simultaneous connectivity to Access Routers

   Make-before-break (MBB)
      During a MBB handover the MN makes the new connection before the
      old one is broken.  Thus, the MN can communicate simultaneously
      with the old and new AR during the handover.  This should not be
      confused with "soft handover" which relies on macro diversity,
      described in Section 4.5.
   Break-before-make (BBM)
      During a BBM handover the MN breaks the old connection before the
      new connection is made.  Thus, the MN cannot communicate
      simultaneously with the old and the new AR.

4.4. Performance and Functional Aspects

   Handover latency
      Handover latency is the difference between the time a MN is last
      able to send and/or receive an IP packet by way of the PAR, and
      the time the MN is able to send and/or receive an IP packet
      through the NAR.  Adapted from [4].
   Smooth handover
      A handover that aims primarily to minimize packet loss, with no
      explicit concern for additional delays in packet forwarding.
   Fast handover
      A handover that aims primarily to minimize handover latency, with
      no explicit interest in packet loss.
   Seamless handover
      A handover in which there is no change in service capability,
      security, or quality.  In practice, some degradation in service is
      to be expected.  The definition of a seamless handover in the
      practical case should be that other protocols, applications, or
      end users do not detect any change in service capability, security
      or quality, which would have a bearing on their (normal)
      operation.  As a consequence, what would be a seamless handover
      for one less demanding application might not be seamless for
      another more demanding application.  See [7] for more discussion
      on the topic.
   Throughput
      The amount of data from a source to a destination processed by the
      protocol for which throughput is to be measured, for instance, IP,
      TCP, or the MAC protocol.  The throughput differs between protocol
      layers.
   Goodput
      The total bandwidth used, less the volume of control messages,
      protocol overhead from the data packets, and packets dropped due
      to CRC errors.
   Pathloss
      A reduction in signal strength caused by traversing the physical
      medium constituting the link.
   Hidden-terminal problem
      The problem whereby a transmitting node can fail in its attempt to
      transmit data because of destructive interference which is only
      detectable at the receiving node, not the transmitting node.
   Exposed terminal problem
      The problem whereby a transmitting node A prevents another node B
      from transmitting, although node B could have safely transmitted
      to anyone else but the transmitting node A.

4.5. Micro Diversity, Macro Diversity, and IP Diversity

Certain air interfaces (e.g., the Universal Mobile Telephone System (UMTS) Terrestrial Radio Access Network (UTRAN) running in Frequency Division Duplex (FDD) mode) require or at least support macro diversity combining. Essentially, this refers to the fact that a single MN is able to send and receive over two independent radio channels ('diversity branches') at the same time; the information received over different branches is compared and that from the better branch passed to the upper layers. This can be used both to improve overall performance, and to provide a seamless type of handover at layer 2, since a new branch can be added before the old is deleted. See also [6].

It is necessary to differentiate between combining/diversity that occurs at the physical and radio link layers, where the relevant unit of data is the radio frame, and that which occurs at layer 3, the network layer, where what is considered is the IP packet itself.

In the following definitions micro- and macro diversity refer to protocol layers below the network layer, and IP diversity refers to the network layer.

   Micro diversity
      For example, two antennas on the same transmitter send the same
      signal to a receiver over a slightly different path to overcome
      fading.
   Macro diversity
      Duplicating or combining actions taking place over multiple APs,
      possibly attached to different ARs.  This may require support from
      the network layer to move the radio frames between the base
      stations and a central combining point.
   IP diversity
      Refers to the process of duplicating IP packets and sending them
      to the receiver through more than one point of attachment.  This
      is semantically allowed by IP because it does not guarantee packet
      uniqueness, and higher level protocols are assumed to eliminate
      duplicates whenever that is important for the application.

4.6. Paging, and Mobile Node States and Modes

Mobile systems may employ the use of MN states in order to operate more efficiently without degrading the performance of the system. The term 'mode' is also common and means the same as 'state'.

A MN is always in one of the following three states:

   Active state
      When the AN knows the MN's SAR and the MN can send and receive IP
      packets.  The access link may not be active, but the radio layer
      is able to establish one without assistance from the network
      layer.  The MN has an IP address assigned.
   Dormant state
      A state in which the mobile restricts its ability to receive
      normal IP traffic by reducing its monitoring of radio channels.
      The AN knows the MN's Paging Area, but the MN has no SAR and so
      packets cannot be delivered to the MN without the AN initiating
      paging.  Often also called Idle state.
      Time-slotted dormant mode
         A dormant mode implementation in which the mobile alternates
         between periods of not listening for any radio traffic and
         listening for traffic.  Time-slotted dormant mode
         implementations are typically synchronized with the network so
         the network can deliver paging messages to the mobile during
         listening periods.
   Inactive state
      the MN is in neither the Active nor Dormant State.  The MN is no
      longer listening for any packets, not even periodically, and not
      sending packets.  The MN may be in a powered off state, it may
      have shut down all interfaces to drastically conserve power, or it
      may be out of range of a radio access point.  The MN does not
      necessarily have an IP access address from the AN.

Note: in fact, as well as the MN being in one of these three states, the AN also stores which state it believes the MN is in. Normally these are consistent; the definitions above assume so.

Here are some additional definitions for paging, taking into account the above state definitions.

   Paging
      A procedure initiated by the Access Network to move a Dormant MN
      into the Active State.  As a result of paging, the MN establishes
      a SAR and the IP routes are set up.
   Location updating
      A procedure initiated by the MN, by which it informs the AN that
      it has moved into a new paging area.
   Paging area
      A part of the Access Network, typically containing a number of
      ARs/APs, which corresponds to some geographical area.  The AN
      keeps and updates a list of all the Dormant MNs present in the
      area.  If the MN is within the radio coverage of the area it will
      be able to receive paging messages sent within that Paging Area.
   Paging area registrations
      Signaling from a dormant mode mobile node to the network, by which
      it establishes its presence in a new paging area.  Paging Area
      Registrations thus enable the network to maintain a rough idea of
      where the mobile is located.
   Paging channel
      A radio channel dedicated to signaling dormant mode mobiles for
      paging purposes.  By current practice, the paging channel carries
      only control traffic necessary for the radio link, although some
      paging protocols have provision for carrying arbitrary traffic
      (and thus could potentially be used to carry IP).
   Traffic channel
      The radio channel on which IP traffic to an active mobile is
      typically sent.  This channel is used by a mobile that is actively
      sending and receiving IP traffic, and is not continuously active
      in a dormant mode mobile.  For some radio link protocols, this may
      be the only channel available.

4.7. Context Transfer

   Context
      The information on the current state of a routing-related service
      required to re-establish the routing-related service on a new
      subnet without having to perform the entire protocol exchange with
      the MN from scratch.
   Feature context
      The collection of information representing the context for a given
      feature.  The full context associated with a MN is the collection
      of one or more feature contexts.
   Context transfer
      The movement of context from one router or other network entity to
      another as a means of re-establishing routing-related services on
      a new subnet or collection of subnets.
   Routing-related service
      A modification to the default routing treatment of packets to and
      from the MN.  Initially establishing routing-related services
      usually requires a protocol exchange with the MN.  An example of a
      routing-related service is header compression.  The service may
      also be indirectly related to routing, for example, security.
      Security may not affect the forwarding decision of all
      intermediate routers, but a packet may be dropped if it fails a
      security check (can't be encrypted, authentication failed, etc.).
      Dropping the packet is basically a routing decision.

4.8. Candidate Access Router Discovery

   Capability of an AR
      A characteristic of the service offered by an AR that may be of
      interest to an MN when the AR is being considered as a handoff
      candidate.
   Candidate AR (CAR)
      An AR to which MN has a choice of performing IP-level handoff.
      This means that MN has the right radio interface to connect to an
      AP that is served by this AR, as well as the coverage of this AR
      overlaps with that of the AR to which MN is currently attached.
   Target AR (TAR)
      An AR with which the procedures for the MN's IP-level handoff are
      initiated.  TAR is selected after running a TAR Selection
      Algorithm that takes into account the capabilities of CARs,
      preferences of MN and any local policies.

4.9. Types of Mobility

We can differentiate between host and network mobility, and various types of network mobility. Terminology related more to applications such as the Session Initiation Protocol, such as personal mobility, is out of scope for this document.
      Host mobility support
         Refers to the function of allowing a mobile node to change its
         point of attachment to the network, without interrupting IP
         packet delivery to/from that node.  There may be different sub-
         functions depending on what the current level of service is
         being provided; in particular, support for host mobility
         usually implies active and dormant modes of operation,
         depending on whether the node has any current sessions or not.
         Access Network procedures are required to keep track of the
         current point of attachment of all the MNs or establish it at
         will.  Accurate location and routing procedures are required in
         order to maintain the integrity of the communication.  Host
         mobility is often called 'terminal mobility'.
      Network mobility support
         Refers to the function of allowing an entire network to change
         its point of attachment to the Internet, and, thus, its
         reachability in the topology, without interrupting IP packet
         delivery to/from that mobile network.

Two subcategories of mobility can be identified within both host mobility and network mobility:

      Global mobility
         Same as Macro mobility.
      Local mobility
         Same as Micro mobility.
      Macro mobility
         Mobility over a large area.  This includes mobility support and
         associated address registration procedures that are needed when
         a MN moves between IP domains.  Inter-AN handovers typically
         involve macro-mobility protocols.  Mobile-IP can be seen as a
         means to provide macro mobility.
      Micro mobility
         Mobility over a small area.  Usually this means mobility within
         an IP domain with an emphasis on support for active mode using
         handover, although it may include idle mode procedures also.
         Micro-mobility protocols exploit the locality of movement by
         confining movement related changes and signaling to the access
         network.
      Local mobility management
         Local mobility management (LMM) is a generic term for protocols
         dealing with IP mobility management confined within the access
         network.  LMM messages are not routed outside the access
         network, although a handover may trigger Mobile IP messages to
         be sent to correspondent nodes and home agents.

5. Specific Terminology for Mobile Ad-Hoc Networking

   Cluster
      A group of nodes located within close physical proximity,
      typically all within range of one another, which can be grouped
      together for the purpose of limiting the production and
      propagation of routing information.
   Cluster head
      A cluster head is a node (often elected in the cluster formation
      process) that has complete knowledge about group membership and
      link state information in the cluster.  Each cluster should have
      one and only one cluster head.
   Cluster member
      All nodes within a cluster except the cluster head are called
      members of that cluster.
   Convergence
      The process of approaching a state of equilibrium in which all
      nodes in the network agree on a consistent collection of state
      about the topology of the network, and in which no further control
      messages are needed to establish the consistency of the network
      topology.
   Convergence time
      The time which is required for a network to reach convergence
      after an event (typically, the movement of a mobile node) which
      changes the network topology.
   Laydown
      The relative physical location of the nodes within the ad hoc
      network.
   Pathloss matrix
      A matrix of coefficients describing the pathloss between any two
      nodes in an ad hoc network.  When the links are asymmetric, the
      matrix is also asymmetric.
   Scenario
      The tuple <laydown, pathloss matrix, mobility factor, traffic>
      characterizing a class of ad hoc networks.

6. Security-related Terminology

This section includes terminology commonly used around mobile and wireless networking. Only a mobility-related subset of the entire security terminology is presented.
      Authorization-enabling extension
         An authentication which makes a (registration) message
         acceptable to the ultimate recipient of the registration
         message.  An authorization-enabling extension must contain an
         SPI (see below) [10].
      Mobility security association
         A collection of security contexts, between a pair of nodes,
         which may be applied to mobility-related protocol messages
         exchanged between them.  In Mobile IP, each context indicates
         an authentication algorithm and mode, a secret (a shared key,
         or appropriate public/private key pair), and a style of replay
         protection in use.  Mobility security associations may be
         stored separately from the node's IPsec Security Policy
         Database (SPD) [10].
      Registration key
         A key used in the Mobility Security Association between a
         mobile node and a foreign agent.  A registration key is
         typically only used once or a very few times, and only for the
         purposes of verifying a small volume of Authentication data
         [12].
      Security context
         A security context between two nodes defines the manner in
         which two nodes choose to mutually authenticate each other, and
         indicates an authentication algorithm and mode.
      Security Parameter Index (SPI)
         An index identifying a security context between a pair of
         routers among the contexts available in the mobility security
         association.

The Mobile IPv6 specification includes more security terminology related to MIPv6 bindings [9]. Terminology about the MIP challenge/response mechanism can be found in [11].

7. Security Considerations

This document presents only terminology. There are no security issues in this document.

8. Contributors

This document was initially based on the work of Tapio Suihko, Phil Eardley, Dave Wisely, Robert Hancock, Nikos Georganopoulos, Markku Kojo, and Jukka Manner.

Charles Perkins has provided input terminology related to ad-hoc networks.

Thierry Ernst has provided the terminology for discussing mobile networks. Henrik Levkowetz did a final check of the definitions in revision -05 and suggested a number of changes.

9. Acknowledgments

This work has been partially performed in the framework of the IST project IST-2000-28584 MIND, which is partly funded by the European Union. Some of the authors would like to acknowledge the help of their colleagues in preparing this document.

Randy Presuhn did a very thorough and helpful review of the -02 version of the terminology.

Some definitions of terminology have been adapted from [1], [2], [3], [4], [7], [8], [9] and [10].

10. Informative References

   [1]  Blair, D., Tweedly, A., Thomas, M., Trostle, J. and M. Ramalho,
        "Realtime Mobile IPv6 Framework", Work in Progress.
   [2]  Calhoun, P., Montenegro, G. and C. Perkins, "Mobile IP
        Regionalized Tunnel Management", Work in Progress.
   [3]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
        Specification", RFC 2460, December 1998.
   [4]   Koodli, R., Ed., "Fast Handovers for Mobile IPv6", Work in
        Progress.
   [5]  Yavatkar, R., Pendarakis, D. and R. Guerin, "A Framework for
        Policy-based Admission Control", RFC 2753, January 2000.
   [6]  Kempf, J., McCann, P. and P. Roberts, "IP Mobility and the CDMA
        Radio Access Network:  Applicability Statement for Soft
        Handoff", Work in Progress.
   [7]  Kempf, J., Ed., "Problem Description:  Reasons For Performing
        Context Transfers Between Nodes in an IP Access Network", RFC
        3374, September 2002.
   [8]  Trossen, D., Krishnamurthi, G., Chaskar, H. and J. Kempf,
        "Issues in candidate access router discovery for seamless IP-
        level handoffs", Work in Progress.
   [9]  Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
        IPv6", RFC 3775, June 2004.
   [10] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
        August 2002.
   [11] Perkins, C., Calhoun, P. and J. Bharatia, "Mobile IPv4
        Challenge/Response Extensions (revised)", Work in Progress.
   [12] Perkins, C. and P. Calhoun, "AAA Registration Keys for Mobile
        IP", Work in Progress.
   [13] Ernst, T. and H. Lach, "Network Mobility Support Terminology",
        Work in Progress.

[14] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

11. Appendix A - Index of Terms

   AD ............................................................. 14
   AL ............................................................. 13
   AN ............................................................. 14
   ANG ............................................................ 14
   ANR ............................................................ 13
   AP ............................................................. 13
   AR ............................................................. 14
   Access Link .................................................... 13
   Access Network ................................................. 14
   Access Network Gateway ......................................... 14
   Access Network Router .......................................... 13
   Access Point ................................................... 13
   Access Router .................................................. 14
   Active state ................................................... 22
   Administrative Domain .......................................... 14
   Asymmetric link ................................................. 5
   Authorization-enabling extension ............................... 27
   BBM ............................................................ 19
   BU .............................................................. 3
   Bandwidth ....................................................... 2
   Bandwidth utilization ........................................... 2
   Beacon .......................................................... 3
   Binding Update .................................................. 3
   Break-before-make .............................................. 19
   CAR ............................................................ 15
   CAR ............................................................ 24
   Candidate AR ................................................... 24
   Candidate Access Router ........................................ 15
   Capability of an AR ............................................ 24
   Care-of-Address ................................................. 3
   Channel ......................................................... 3
   Channel access protocol ......................................... 3
   Channel capacity ................................................ 3
   Cluster ........................................................ 26
   Cluster head ................................................... 26
   Cluster member ................................................. 26
   CoA ............................................................. 3
   Context ........................................................ 24
   Context transfer ............................................... 24
   Control message ................................................. 4
   Convergence .................................................... 27
   Convergence time ............................................... 27
   Distance vector ................................................. 4
   Dormant state .................................................. 22
   Egress interface ............................................... 13
   Exposed terminal problem ....................................... 20
   FN ............................................................. 12
   Fairness ........................................................ 4
   Fast handover .................................................. 20
   Feature context ................................................ 24
   Fixed Node ..................................................... 12
   Flooding ........................................................ 4
   Foreign subnet prefix ........................................... 4
   Forwarding node ................................................. 4
   Global mobility ................................................ 25
   Goodput ........................................................ 20
   HA .............................................................. 5
   Handoff ........................................................ 15
   Handover ....................................................... 15
   Handover latency ............................................... 19
   Hidden-terminal problem ........................................ 20
   HoA ............................................................. 4
   Home Address .................................................... 4
   Home Agent ...................................................... 5
   Home subnet prefix .............................................. 5
   Horizontal Handover ............................................ 16
   Host mobility support .......................................... 25
   IP access address ............................................... 5
   IP diversity ................................................... 21
   Inactive state ................................................. 22
   Ingress interface .............................................. 12
   Inter-AN handover .............................................. 16
   Inter-technology handover ...................................... 16
   Interface ....................................................... 5
   Intra-AN handover .............................................. 16
   Intra-AR handover .............................................. 16
   Intra-technology handover ...................................... 16
   L2 Trigger ...................................................... 6
   Laydown ........................................................ 27
   Layer 2 handover ............................................... 16
   Link ............................................................ 5
   Link establishment .............................................. 6
   Link state ...................................................... 6
   Link-layer trigger .............................................. 6
   Link-level acknowledgment ....................................... 6
   Local broadcast ................................................. 6
   Local mobility ................................................. 25
   Local mobility management ...................................... 26
   Location updating .............................................. 23
   Loop-free ....................................................... 6
   MAC ............................................................. 7
   MBB ............................................................ 19
   MH ............................................................. 12
   MN ............................................................. 12
   MNN ............................................................ 13
   MPR ............................................................. 7
   MR ............................................................. 12
   Macro diversity ................................................ 21
   Macro mobility ................................................. 26
   Make-before-break .............................................. 19
   Medium Access Protocol .......................................... 7
   Micro diversity ................................................ 21
   Micro mobility ................................................. 26
   Mobile Host .................................................... 12
   Mobile Network Node ............................................ 13
   Mobile Node .................................................... 12
   Mobile Router .................................................. 12
   Mobile network ................................................. 12
   Mobile network prefix ........................................... 7
   Mobile-assisted handover ....................................... 18
   Mobile-controlled handover ..................................... 18
   Mobile-initiated handover ...................................... 17
   Mobility factor ................................................. 7
   Mobility security association .................................. 27
   Multipoint relay ................................................ 7
   NAR ............................................................ 14
   Neighbor ........................................................ 7
   Neighborhood .................................................... 7
   Network mobility support ....................................... 25
   Network-assisted handover ...................................... 18
   Network-controlled handover .................................... 18
   Network-initiated handover ..................................... 17
   New Access Router .............................................. 14
   Next hop ........................................................ 7
   PAR ............................................................ 15
   Paging ......................................................... 23
   Paging area .................................................... 23
   Paging area registrations ...................................... 23
   Paging channel ................................................. 23
   Pathloss ....................................................... 20
   Pathloss matrix ................................................ 27
   Payload ......................................................... 8
   Planned handover ............................................... 19
   Prefix .......................................................... 8
   Previous Access Router ......................................... 15
   Pull handover .................................................. 18
   Push handover .................................................. 18
   Radio Cell ..................................................... 13
   Registration key ............................................... 28
   Roaming ........................................................ 15
   Route activation ................................................ 8
   Route entry ..................................................... 8
   Route establishment ............................................. 8
   Routing table ................................................... 8
   Routing proxy ................................................... 8
   Routing-related service ........................................ 24
   SAR ............................................................ 14
   SPI ............................................................ 28
   Scenario ....................................................... 27
   Seamless handover .............................................. 19
   Security Parameter Index ....................................... 28
   Security context ............................................... 28
   Serving Access Router .......................................... 14
   Shannon's Law ................................................... 9
   Signal strength ................................................. 9
   Smooth handover ................................................ 19
   Source route .................................................... 9
   Spatial re-use .................................................. 9
   Subnet .......................................................... 9
   System-wide broadcast ........................................... 9
   TAR ............................................................ 25
   Target AR ...................................................... 25
   Throughput ..................................................... 20
   Time-slotted dormant mode ...................................... 22
   Topology ........................................................ 9
   Traffic channel ................................................ 23
   Triggered update ................................................10
   Unassisted handover ............................................ 18
   Unplanned handover ............................................. 19
   Vertical handover .............................................. 17

12. Authors' Addresses

   Jukka Manner
   Department of Computer Science
   University of Helsinki
   P.O. Box 26 (Teollisuuskatu 23)
   FIN-00014 HELSINKI
   Finland

Phone: +358-9-191-44210 Fax: +358-9-191-44441 EMail: jmanner@cs.helsinki.fi

   Markku Kojo
   Department of Computer Science
   University of Helsinki
   P.O. Box 26 (Teollisuuskatu 23)
   FIN-00014 HELSINKI
   Finland

Phone: +358-9-191-44179 Fax: +358-9-191-44441 EMail: kojo@cs.helsinki.fi

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