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INTERNET-DRAFT                                               A. Campbell
<draft-ietf-mobileip-cellularip-00.txt>                         J. Gomez
Expires July 2000                                               C-Y. Wan
                                                                  S. Kim
                                                     Columbia University
                                                              Z. Turanyi
                                                                A. Valko
                                                                Ericsson
                                                            January 2000

                              Cellular IP

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This document specifies a protocol that allows routing IP datagrams
   to a mobile host.  The protocol is intended to provide local mobility
   and handoff support.  It can interwork with Mobile IP [1] to provide
   wide area mobility support.  Four fundamental design principles of
   the protocol are: (1) location information is stored in distributed
   data bases (2) location information referring to a mobile host is
   created and updated by regular IP datagrams originated by the said
   mobile host (3) location information is stored as soft state (4)
   location management for idle mobile hosts is separated from location
   management of hosts that are actively transmitting or receiving data.












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Table of Contents

 1. Introduction                                                       3
     1.1. Applicability . . . . . . . . . . . . . . . . . . . . . .    3
     1.2. New Architectural Entities  . . . . . . . . . . . . . . .    4
     1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . .    4
     1.4. Protocol Overview . . . . . . . . . . . . . . . . . . . .    6
     1.5. Location Management and Routing . . . . . . . . . . . . .    7
 2. Cellular IP Functions                                              8
     2.1. Location Management . . . . . . . . . . . . . . . . . . .    8
     2.2. Routing . . . . . . . . . . . . . . . . . . . . . . . . .    9
     2.3. Handoff . . . . . . . . . . . . . . . . . . . . . . . . .   10
     2.4. Wide Area Mobility  . . . . . . . . . . . . . . . . . . .   10
     2.5. Security  . . . . . . . . . . . . . . . . . . . . . . . .   11
 3. Protocol Details                                                  11
     3.1. Protocol Parameters . . . . . . . . . . . . . . . . . . .   11
     3.2. Beacon Signal Structure . . . . . . . . . . . . . . . . .   12
     3.3. Packet Formats  . . . . . . . . . . . . . . . . . . . . .   12
          3.3.1. Data packet  . . . . . . . . . . . . . . . . . . .   12
          3.3.2. Route-update packet  . . . . . . . . . . . . . . .   12
          3.3.3. Paging-update packet . . . . . . . . . . . . . . .   14
          3.3.4. Paging-teardown packet . . . . . . . . . . . . . .   14
     3.4. Addressing  . . . . . . . . . . . . . . . . . . . . . . .   14
     3.5. Security  . . . . . . . . . . . . . . . . . . . . . . . .   14
     3.6. Cellular IP Routing . . . . . . . . . . . . . . . . . . .   15
          3.6.1. Topology . . . . . . . . . . . . . . . . . . . . .   15
          3.6.2. Uplink Routing . . . . . . . . . . . . . . . . . .   15
          3.6.3. Downlink Routing . . . . . . . . . . . . . . . . .   16
     3.7. Cellular IP Gateway . . . . . . . . . . . . . . . . . . .   17
     3.8. Cellular IP Mobile Host . . . . . . . . . . . . . . . . .   18
 4. Extensions to Cellular IP                                         19
     4.1. Semi-soft Handoff . . . . . . . . . . . . . . . . . . . .   19
     4.2. Multiple Gateway Networks . . . . . . . . . . . . . . . .   20
     4.3. Charging  . . . . . . . . . . . . . . . . . . . . . . . .   20
 5. Security Considerations                                           20
 6. Intellectual Property Right Notice  . . . . . . . . . . . . . .   20
 References . . . . . . . . . . . . . . . . . . . . . . . . . . . .   21
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .   21
 APPENDIX A. Uplink Neighbor Selection  . . . . . . . . . . . . . .   21

0. What's Changed

   The following major improvements have been made to the protocol
   compared to <draft-valko-cellularip-00>:

   - Security support for Cellular IP has been added.
   - Paging Areas have been introduced. As long as an idle mobile host
     is moving inside a Paging Area, it is not necessary to transmit any
     control packets.
   - Semi-soft handoff has been introduced to improve handoff
     performance.
   - Each node maintains only one valid Route Cache mapping and only one
     valid Paging Cache mapping for each mobile host. There is one
     exception to this in the case of semisoft handoff.



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   In addition, the following minor changes have been made:

   - Cache mappings can not be created or modified (but still can be
     refreshed) by data packets.
   - Paging-update packets remove Route Cache entries.
   - An optional paging-teardown packet has been introduced that
     explicitly removes Paging Cache mappings.
   - The Base Station's beacon signal has been extended to include
     Paging Area ID.
   - The example algorithm in Appendix A. has been extended to
     distribute Network ID, Gateway IP address and Paging Area IDs.
   - Control packet format has been changed to ICMP.
   - Control packets must contain timestamp and authentication
     information.
   - Cache mappings now contain timestamp information of the update
     packet that created the mapping.
   - Cache mappings also contain the MAC address of the downlink
     Cellular IP node to allow multiple Cellular IP nodes to reside a
     shared medium.  The notion of Uplink and Downlink I/Fs has been
     replaced by Uplink and Downlink neighbors.

1. Introduction

   Hosts connecting to the Internet via a wireless interface are likely
   to change their point of access frequently.  A mechanism is required
   that ensures that packets addressed to moving hosts are successfully
   delivered with high probability.  A change of access point during
   active data transmission or reception is called a handoff.  During or
   immediately after a handoff, packet losses may occur due to delayed
   propagation of new location information.  These losses should be
   minimized in order to avoid a degradation of service quality as
   handoff become more frequent.

   This memo specifies Cellular IP, a protocol that provides mobility
   and handoff support for frequently moving hosts.  It is intended to
   be used on a local level, for instance in a campus or metropolitan
   area network.  Cellular IP can interwork with Mobile IP [1] to
   support wide area mobility, that is, mobility between Cellular IP
   Networks.

1.1. Applicability

   Cellular IP supports local mobility, that is, mobility inside an
   access network.  To provide global mobility support, Mobile IP [1]
   should be used in conjunction with Cellular IP.

   Cellular IP is designed to support frequently migrating, rarely
   moving or static hosts as well.

   Cellular IP assumes that a random access L2 protocol covers the air
   interface.






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1.2. New Architectural Entities

      Cellular IP Node
         A Cellular IP Network consists of interconnected Cellular IP
         nodes.  The role of nodes is twofold.  They route IP packets
         inside the Cellular IP Network and communicate with mobile
         hosts via a wireless interface.  Referring to the latter role,
         a Cellular IP node that has a wireless interface is also called
         a Base Station.

      Cellular IP Base Station
         See Cellular IP node.

      Cellular IP Gateway
         A Cellular IP node that is connected to a regular IP network by
         at least one of its interfaces.

      Cellular IP Mobile Host
         A mobile host that implements the Cellular IP protocol.

1.3. Terminology

      Active Mobile Host
         A mobile host is in active state if it is transmitting or
         receiving IP packets.  (Exact definition is given in Section
         3.8.)

      Cellular IP Network Identifier
         A unique identifier assigned to Cellular IP Networks.

      Control packet
         Paging-update, paging-teardown and route-update packet.

      Data packet
         An IP packet that is not a control packet.

      Downlink
         Directed to a mobile host.

      Downlink neighbor
         All neighbors of a Cellular IP node except its Uplink neighbor
         are referred to as Downlink neighbors.

      Idle Mobile Host
         A mobile host is in idle state if it has not recently
         transmitted or received IP packets.  (Exact definition is given
         in Section 3.8.)

      Internet
         A Cellular IP Network provides access to a regular IP network.
         This IP network in this memo is referred to as "Internet", but
         it can also be a corporate intranet, for example.

      Neighbor



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         One Cellular IP node is said to be the neighbor of another if
         they are connected directly. Neighbors are identified in a
         Cellular IP node by interface and MAC address.

      Paging Area
         A set of Base Stations. Idle mobile hosts crossing cell
         boundaries within a Paging Area do not need to transmit control
         packets to update their position. (Exact definition is given in
         Section 2.1.)

      Paging Cache
         A cache maintained by some Cellular IP nodes, used to route
         packets to mobile hosts.

      Paging-timeout
         Validity time of mappings in Paging Caches.

      Paging-update packet
         A control packet transmitted by Cellular IP mobile hosts in
         order to update Paging Cache.

      Paging-update-time
         Time between consecutive paging-update packets.

      Paging-teardown packet
         A control packet transmitted by Cellular IP mobile hosts in
         order to explicitly disconnect from the Cellular IP Network.

      Route-timeout
         Validity time of mappings in Route Cache.

      Route-update packet
         A control packet transmitted by Cellular IP mobile hosts in
         order to update Route Cache.

      Route-update-time
         Time between consecutive route-update packets.

      Route Cache
         A cache maintained by all Cellular IP nodes, used to route
         packets to mobile hosts.

      Update packet
         Paging-update and route-update packet.

      Uplink
         Originated by a mobile host.

      Uplink neighbor
         The neighbor of a Cellular IP node which is the next hop on the
         shortest path towards the Gateway.

1.4. Protocol Overview




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   The figure shown below presents a schematic view of multiple Cellular
   IP Networks providing access to the Internet.

           ..............................................
           .                                            .
           .      Internet Backbone with Mobile IP      .
           .                                            .
           ..............................................
                /                 |                \
               /                  |                 \
           +--+                 +--+                  +--+
           |GW|                 |GW|                  |GW|
           +--+                 +--+                  +--+
          /                       |                      \
   +-------------+      +--------------------+      +-------------+
   |             |      |                    |      |             |
   | Cellular IP |      |     Cellular IP    |      | Cellular IP |
   |   Network   |      |       Network      |      |   Network   |
   |             |      |  __     __     __  |      |             |
   +-------------+      +-|BS|---|BS|---|BS|-+      +-------------+
                           --     --     --


                           +     ...      +
                          MH             MH

   In what follows, we present an overview of the operation of Cellular
   IP, followed by a figure illustrating the functional entities that
   comprise Cellular IP.

   Base Stations periodically emit beacon signals.  Mobile hosts use
   these beacon signals to locate the nearest Base Station.  A mobile
   host can transmit a packet by relaying it to the nearest Base
   Station.

   All IP packets transmitted by a mobile host are routed from the Base
   Station to the Gateway by hop-by-hop shortest path routing,
   regardless of the destination address.

   Cellular IP nodes maintain Route Cache.  Packets transmitted by the
   mobile host create and update entries in each node's Cache.  An entry
   maps the mobile host's IP address to the neighbor from which the
   packet arrived to the node.

   The chain of cached mappings referring to a single mobile host
   constitutes a reverse path for downlink packets addressed to the same
   mobile host.  As the mobile host migrates, the chain of mappings
   always points to its current location because its uplink packets
   create new and change old mappings.

   IP packets addressed to a mobile host are routed by the chain of
   cached mappings associated with the said mobile host.

   To prevent its mappings from timing out, a mobile host can



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   periodically transmit control packets.  Control packets are ICMP
   packets with specific authentication payloads.

   Mobile hosts that are not actively transmitting or receiving data but
   want to be reachable for incoming packets, let their Route Cache
   mappings time out but maintain Paging Cache mappings.  IP packets
   addressed to these mobile hosts will be routed by Paging Caches.
   Paging Caches have a longer timeout value than Route Caches and are
   not necessarily maintained in every node.

                           +--------+
                           |host in |
                           |Internet|
                           +--------+
                                |                        Internet
                                |      --------------------------
                           +--------+         Cellular IP Network
                           |Cell. IP|
                           |Gateway |
                           +--------+
                                |
                  -             :
                  |             :
                  |             :\___________ Uplink neighbor
    A network of  |             |                  (=shortest path
                  |        +--------+               toward Gateway)
     Cellular IP  |        |Cellular|
                  |        |IP node |
        nodes     |        +--------+
                  |             | ___________ Downlink neighbors
                  |             :/                 (=all other
                  -             :                   neighbors)
                                :
                                |
                           +--------+
uplink                     |Cellular|
  ^                        |IP node |
  |                        +--------+
  |                      air    |
  |                    interface|
  V                        +--------+
downlink                   | Mobile |
                           |  host  |
                           +--------+

1.5. Location Management and Routing

   Cellular IP uses two parallel cache systems to store the information
   related to the location of mobile hosts.  The two systems basically
   operate in the same way.  This section is intended to clarify why we
   use two distinct caches.

   When a mobile host is in active state, the network must follow its
   movement from Base Station to Base Station to be able to deliver



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   packets without searching for the mobile host.  As a consequence
   active mobile hosts must notify the network about each handoff.  For
   idle mobile hosts exact location tracking is less important, instead
   mimizing communication to save battery is of higher priority.  By
   deploying two caches, the granularity of location tracking can be
   different for idle and active mobile hosts.

   Separating the location tracking for idle and active mobile hosts
   also has a performance benefit.  Supposing there is just one set of
   cache, for each downlink packet the entire cache must be searched to
   find the destination mobile host.  It is expected, however, that only
   a portion of the hosts will be in active state at any given time and
   that most of the packets are destined for active mobile hosts.  Thus
   by separating the caches for active and idle mobile hosts only a
   smaller cache needs to be searched for most of the packets.  This
   results in faster lookups and better scalability [5].

2. Cellular IP Functions

2.1. Location Management

   Cellular IP allows idle mobile hosts to roam large geographic areas
   without the need to transmit location update packets at cell borders.
   The network operator can group cells into Paging Areas, each
   comprising of an arbitrary number of (typically adjacent) cells.
   Each Paging Area has an identifier that is unique in the given
   Cellular IP Network.  Each Base Station transmits its Paging Area
   Identifier in its periodic beacon signals, thus enabling mobile hosts
   to notice when they move into a new Paging Area.

   An idle mobile host that moves into a new Paging Area must transmit a
   paging-update packet.  Paging-update packets are routed from the Base
   Station to the Gateway using hop-by-hop routing.  Selected nodes of
   the Cellular IP network are equipped with Paging Cache. These nodes
   monitor passing paging-update packets and update Paging Cache
   mappings to point toward the new Paging Area.  Paging-update packets
   reach the Gateway and are discarded there to isolate Cellular IP
   specific operations from the Internet.

   When the idle mobile host moves within a Paging Area, it transmits a
   paging-update packet only when the system specific time, paging-
   update-time expires.  Outdated mappings of Paging Caches are cleared
   if no update arrives before paging-timeout expires.

   When an IP packet arrives at a Cellular IP node, addressed to a
   mobile host for which no up-to-date Route Cache mapping is available,
   the Paging Cache is used to route the packet.  This is called
   "implicit paging".  If the node has no Paging Cache, it forwards the
   packet to all Downlink neighbors.  A node that has Paging Cache but
   has no mapping in it for the destination mobile host discards the
   packet.

   On the path from the gateway to the mobile host there may be Cellular
   IP nodes with and without Paging Cache.  After the paging packet



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   leaves the last node which has a Paging Cache it is effectively
   downlink broadcast by all nodes it passes.  The set of cells that are
   reached by the paging packet forms a Paging Area.  The number, size
   and population of Paging Areas in a Cellular IP network are
   determined by the topology of the network and the placement of Paging
   Caches.  Based on the configuration of a Paging Area each base
   station (with Paging Cache configured) could be considered an
   autonomous Paging Area. The other extreme case is when a Cellular IP
   Network has no Paging Cache configured in which case the complete
   network represents a Paging Area where paging devolves to
   broadcasting throughout the network.

   When the mobile host receives the paging packet, it moves to active
   state and creates its Route Cache mappings by sending a route-update
   packet.  Subsequent IP packets addressed to the same host will be
   routed by Route Caches as long as the mobile host keeps the Route
   Caches updated.

2.2. Routing

   Packets transmitted by mobile hosts are routed to the Gateway using
   shortest path hop-by-hop routing.  Cellular IP nodes monitor these
   passing data packets and use them to create and update Route Cache
   mappings.  These map mobile host IP addresses to Downlink neighbors
   of the Cellular IP node.  Packets addressed to the mobile host are
   routed along the reverse path, on a hop-by-hop basis, by these Route
   Cache mappings.

   The structure and basic operation of routing is similar to that of
   location management.  To clarify the duality between the two, we
   summarize the operation of Paging Caches and Route Caches in the
   following table.  For the reasons of separating the two functions,
   see Section 1.7.

   ----------------------------------------------------------------------
                       Paging Caches                  Route Caches
   ----------------------------------------------------------------------
   refreshed by    all uplink packets (data,            data and
                 paging-update, route-update)     route-update packets

   updated by        all update packets           route-update packets
                (paging-update, route-update)

   updated when   moving to a new Paging             moving to a new
                      Area, or after                 cell, or after
                    paging-update-time              route-update-time

   scope          both idle and active MHs         active mobile hosts

   purpose        route downlink packets if           route downlink
                there is no Route Cache entry            packets
   ----------------------------------------------------------------------

   The mobile host may keep receiving data packets without sending data



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   for possibly long durations.  To keep its Route Cache mappings up to
   date and to avoid repeated paging, mobile hosts in active state that
   have no data to send must send periodic route-update packets.  Like
   uplink data packets, route-update packets update Route Caches and
   ensure that the hop-by-hop route from the Gateway to the mobile host
   does not time out.

   In addition, active mobile hosts must transmit a route-update packet
   when they cross cell borders. This is required because the Route
   Cache mappings associated with the new Base Station can only be
   created by authenticated route-update packets. Data packets are not
   required to carry authentication information and hence can refresh,
   but not modify Route Cache mappings.

   For reliability and timeliness, Paging Caches also contain mobile
   hosts that are contained by Route Caches.  For this reason, Paging
   Caches are updated by all uplink update packets and refreshed by all
   uplink packets including data packets as well.

2.3. Handoff

   Handoff is initiated by the mobile host.  As an active host
   approaches a new Base Station, it transmits a route-update packet and
   redirects its packets from the old to the new Base Station.  The
   route-update packet will configure Route Caches along the way from
   the new Base Station to the Gateway.  (The paths leading to the old
   and new Base Stations may overlap.  In nodes where the two paths
   coincide, the route-update packet simply refreshes the old mapping
   and the handoff remains unnoticed.)

   An idle mobile host, moving to a new Base Station, transmits a
   paging-update packet only if the new Base Station is in a new Paging
   Area. During handoffs between Base Stations within the same Paging
   Area idle mobile hosts may remain silent, as paging is performed
   within the entire Paging Area.

2.4. Wide Area Mobility

   Wide area mobility occurs when the mobile host moves between Cellular
   IP Networks.  The mobile host can identify Cellular IP Networks by
   the Cellular IP Network Identifier contained in the Base Stations'
   beacon signals.  The beacon signal also contains the IP address of
   the Gateway.  For security and charging purposes, authentication and
   other user-related information may need to be provided by the mobile
   host, when it first contacts a Cellular IP Network.  This information
   will be inserted in the payload of the first paging-update packet and
   may be repeated in a few subsequent paging-update packets for
   reliability.  Upon receiving the first paging-update packet, the
   Gateway performs admission control that may involve technical and
   charging decisions.  The Gateway's response is sent to the mobile
   host in regular IP packet(s).  If the request was accepted, the
   response may also carry the required setting for protocol parameters.
   After successful authentication to the Cellular IP network the mobile
   host can send a Mobile IP registration message to its home agent,



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   specifying the Gateway's IP address as the care-of-address.
   (Alternatively, the Gateway can register at the Home Agent on behalf
   of the mobile host.)

   The mobile host may leave the service area at any time without prior
   notice.  Mappings associated to the host will be cleared after the
   timeout.  Alternatively, as a performance optimization the host may
   send a paging-teardown packet to clear Cache mappings from both Route
   and Paging Caches.

2.5. Security

   Cellular IP control packets (paging-update, route-update and paging-
   teardown packets) carry mandatory authentication information.  This
   prevents malicious mobile hosts from changing location information
   related to other mobile hosts using a spoofed source address; details
   of the authentication mechanism can be found in Section 3.5.

   Data security issues are not discussed in this document.  We note
   that any further authentication or encryption can be performed in
   addition to control packet authentication built into Cellular IP.

3. Protocol Details

3.1. Protocol Parameters

   The following parameters shall be set by network management.  The
   values listed here are for information only.  Note that in the most
   typical case a mobile host that is in active state will regularly
   transmit data packets and hence route-update packets will need to be
   transmitted at handoffs only.

   -------------------------------------------------------------------
   Name                           Meaning                Typical Value
   -------------------------------------------------------------------
   route-update-time     Maximal inter-arrival time          3 sec
                          of packets updating the
                                Route Cache

   route-timeout             Validity of Route               9 sec
                               Cache mappings

   paging-update-time    Maximal inter-arrival time          3 min
                          of packets updating the
                               Paging Cache

   paging-timeout            Validity of Paging              9 min
                               Cache mappings
   -------------------------------------------------------------------








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3.2. Beacon Signal Structure

   Cellular IP Base Stations must periodically transmit beacon signals
   to allow for mobile hosts to identify an available Base Station.
   Information elements carried by the beacon signal include:

   - Layer2 parameters related to the Base Station;
   - the Cellular IP Network Identifier;
   - the IP address of the Gateway; and
   - the ID of the Paging Area.

   All parameters can be configured by network management. As an
   alternative, in Appendix A we present an example algorithm for
   automatically distributing the Cellular IP Network Identifier, the IP
   address of the Gateway and the Paging Area IDs to Base Stations.

3.3. Packet Formats

3.3.1. Data packet

   Cellular IP forwards regular IP packets without modification,
   segmentation, encapsulation or tunneling.

3.3.2. Route-update packet

   A route-update packet is an ICMP packet where

   - the source address is the IP address of the sending mobile host;
   - the destination address is the Gateway; and
   - the type is Cellular IP Control Packet and the code is Route-update.

   The payload of the route-update packet carries authentication and
   control information in the following format:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                   Timestamp (64 bits long)                    |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  CU |S| AType | Auth. Length  |             CU                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                Authentication (variable length)               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |             Control information (variable length)             |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Timestamp       Contains a timestamp used to determine the order
                      in which update packets are sent.  The timestamp



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                      field is formatted as specified by the Network
                      Time Protocol [2].  The low-order 32 bits of the
                      NTP format represent fractional seconds, and those
                      bits which are not available from a time source
                      should be generated from a good source of
                      randomness.  Mobile hosts must ensure that the 64
                      bit value of timestamps is strictly increasing in
                      consecutive control packets.

      CU              Currently Unused. Must be set to 0.

      S flag          Set to 1 to indicate semi-soft handoff. Default
                      value is 0.  Any Cellular IP node that does not
                      support semi-soft handoffs may ignore this bit.
                      (See Section 4.1.)

      AType           Denotes the authentication method used. The
                      default authentication method is described in [4].
                      All authentication methods must utilize the
                      timestamp field.

      Auth. Length    Denotes the length of the authentication
                      information in bytes.

      Authentication  Contains the authentication information.

   Alternatively the Authentication Header [3] could also be used for
   authenticating control packets. This issue is for further study.

   Control information is encoded in the following Type-Length-Value
   format:

     0                   1                   2
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-   -+-+-+-+-+-+-+-+-
    |     Type      |    Length     |    Data ...       |     Type ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-   -+-+-+-+-+-+-+-+-

      Type     Indicates the particular type of control information.

      Length   Indicates the length (in bytes) of the following data
               field within.  The length does not include the Type and
               Length bytes.

      Data     This field may be zero or more bytes in length.  The
               meaning, format and length of the data field is
               determined by the Type and Length fields.

   Currently the following type of control information is defined
   (details are for further study):

      Registration request
         Used when a mobile host enters the Cellular IP Network.




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3.3.3. Paging-update packet

   A paging-update packet is an ICMP packet where

   - the source address is the IP address of the sending mobile host;
   - the destination address is the Gateway; and
   - the type is Cellular IP Control Packet and the code is Paging-update.

   The payload of the paging-update packet carries authentication and
   control information in the same format as the route-update packet.
   The S flag must be 0 for paging-update packets.

3.3.4. Paging-teardown packet

   A paging-teardown packet is a ICMP packet where

   - the source address is the IP address of the sending mobile host;
   - the destination address is the Gateway; and
   - the type is Cellular IP Control Packet and the code is Paging-teardown.

   The payload of the paging-teardown packet carries authentication and
   control information in the same format as the route-update packet.
   The S flag must be 0 for paging-teardown packets.

3.4. Addressing

   Cellular IP requires no address space allocation beyond what is
   present in IP.  Mobile hosts are identified by their home IP
   addresses.

3.5. Security

   Each Cellular IP Network has a secret network key of arbitrary length
   known to all Cellular IP nodes.  The network key is kept secret from
   mobile hosts and other nodes outside the Cellular IP Network,
   however.  Upon initial registration the Gateway must authenticate and
   possibly authorize the mobile host.  This initial authentication and
   authorization can be based on any known symmetric or asymmetric
   method.  After authentication the Gateway concatenates the key of the
   network and the IP address of the mobile host and calculates the PID
   of the mobile host by an MD5 Hash similarly as in [4]:

   PID := MD5(network key, IP address of MH)

   Then it acquires the public key of the mobile host from a trusted
   party, encrypts the PID and sends it to the mobile host.  This way
   the mobile host and the Cellular IP network have a shared secret.
   The PID remains the same during handoff and can be easily computed by
   each Base Station.

   The PID can be used to authenticate (and optionally to encrypt) IP
   packets over the air interface.  Authentication is performed by
   creating a short hash from the (PID, timestamp, packet content)
   triple that is placed into the transmitted packets.  The validity of



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   each packet can be easily checked by any Base Station even
   immediately after a handoff and without prior communication with the
   mobile host or with the old Base Station.


   In addition to authenticating control packets, PID can optionally
   also be used to provide security for data packets transmitted over
   the wireless link.  To this avail, any known shared secret based
   security mechanism can be used where PID serve as the shared secret.

3.6. Cellular IP Routing

   Cellular IP nodes need only to implement the algorithm described in
   this section.  They do not need regular IP routing capability.  This
   section describes the routing algorithm in Cellular IP nodes other
   than the Gateway.  The extra functions required only in the Cellular
   IP Gateway are described in Section 3.7.

3.6.1 Topology

   In uplink direction (toward the Gateway), packets are routed in the
   Cellular IP Network on a hop-by-hop basis.  The neighbor to which a
   node will forward a packet toward the Gateway is referred to as the
   node's Uplink neighbor.  The Uplink neighbor at each node may be
   designated by network management.  Alternatively, a simplified
   shortest path algorithm can select Uplink neighbors  instead of
   manual configuration.  (A regular shortest path algorithm is also
   applicable but is more complex than required since it determines
   routes to all nodes in the network.)  A simple algorithm that
   configures Uplink neighbors and automatically reconfigures them if
   necessary after a topology change is described in Appendix A.

   A node's neighbors other than the Uplink neighbor are called Downlink
   neighbors.

3.6.2 Uplink Routing

   A packet arriving at a node from one of its Downlink neighbors is
   assumed to be coming from a mobile host.  The packet is first used to
   update the node's Route and Paging Caches and is then forwarded to
   the node's Uplink neighbor.

   To update the Caches, the node reads the packet type, port number and
   the source IP address.  Paging-update packets update the Paging Cache
   only.  Route-update packets update both Route and Paging Caches.
   Data packets only refresh the soft state of both caches, but do not
   change it.  Both types of caches consist of

      { IP-address, interface, MAC address, expiration time, timestamp }

   5-tuples, called mappings.  The IP address is the address of the
   mobile host the mapping corresponds to.  The interface and the MAC
   address denote the Downlink neighbor toward the mobile host.  The
   timestamp field contains the timestamp of the control packet that has



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   established the mapping.

   When a data packet arrives from a Downlink neighbor, the Route Cache
   entry of the source IP address is searched first.  If the data packet
   is coming from the same neighbor as indicated by the cache entry then
   it is sent from the direction where the mobile host was last seen.
   In that case the mapping is only refreshed: the expiration time is
   set to the current time + route-timeout. If the node has Paging
   Cache, then the expiration time of the mapping in the Paging Cache is
   set to current time + paging-timeout as well. Then the packet is
   forwarded to the uplink.

   If the data packet arrived from a different neighbor than that is in
   its mapping or no mapping exists for the IP address, then the packet
   is dropped.

   When an update packet arrives from a Downlink neighbor then the
   authentication is first validated.  Packets with invalid
   authentication must be dropped and the event should be logged as a
   potential tampering attempt.  For valid packets the node creates the
   following 5-tuple:

      { the newly arrived packet's source IP address,
        the interface through which it arrived,
        the source MAC address of the arrived packet,
        current time + route-timeout,
        the timestamp in the arrived update packet }

   This mapping is used to update Route Cache, if the incoming packet is
   a route-update packet.  If a valid mapping for the source IP address
   already exists, then it is replaced by the new 5-tuple, if the
   timestamp is newer, otherwise the packet is dropped.  If no mapping
   exists for the source IP address then the mapping is added to the
   Route Cache. The Paging Cache is updated in the same way, but using
   paging-timeout instead of route-timeout.  If the node has no Paging
   Cache then only the Route Cache is updated.  If the incoming packet
   is a paging-update, then only the Paging Cache is updated (if any).

   If the packet is a paging-teardown packet and the authentication
   information is valid, then mappings of the mobile host with timestamp
   earlier than the timestamp of the packet are removed from both the
   Route and the Paging Cache.

   After cache modifications the control packet is forwarded to the
   Uplink neighbor.

3.6.3 Downlink Routing

   A packet arriving to a Cellular IP node from the Uplink neighbor is
   assumed to be addressed to a mobile host.  The node first checks if
   the destination IP address has a valid mapping in the Route Cache.
   If such a mapping exists, the packet is forwarded to the Downlink
   neighbor found in the mapping.




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   If the Route Cache contains no mapping for the destination IP address
   and the node has no Paging Cache, then the packet is broadcast on all
   interfaces of the node except the interface of the Uplink neighbor.

   If the node has Paging Cache and there is a mapping for the
   destination IP address, then the packet is forwarded to the neighbor
   found in that mapping.

   If the node has Paging Cache, but there is no mapping for the
   destination IP address, then the packet is dropped.

3.7. Cellular IP Gateway

   The following figure is a schematic view of a Cellular IP Gateway.
   The Gateway can logically be divided into three building blocks: a
   regular Cellular IP node, a Gateway Packet Filter and a Gateway
   Controller.

                               IP network
                               ===================
                                         |
          +------------------------------|--------+
          |                              |        |
          | +----------+          +-------------+ |
          | | Gateway  |__________|   Gateway   | |
          | |Controller|          |Packet Filter| |
          | +----------+          +-------------+ |
          |                              |\_______|___Uplink neighbor
          |                              |        |
          |                       +-------------+ |
          |    Cellular IP        | Cellular IP | |
          |      Gateway          |    node     | |
          |                       +-------------+ |
          |                         |    |    |   |
          +-------------------------|----|----|---+
                                                 <----Downlink neighbors

   Uplink packets update the Route and/or Paging Caches in the Cellular
   IP node block and are forwarded towards the Gateway filter.  The
   Gateway filter reads the destination IP address.  If this is the
   Gateway's address, the packet is forwarded to the Gateway controller.
   Most of these packets are control packets with empty control
   information field and are immediately dropped.  If the packet carries
   control information, for instance a registration request, it is
   interpreted and processed by the Gateway controller.

   If the destination address is not the Gateway's, the packet is
   forwarded to the Internet.  (This means that a packet sent from a
   mobile host to another mobile host in the same Cellular IP Network
   goes through the destination Home Agent.  However, this is not the
   case if route optimization is used.  To operate efficiently even
   without Mobile IP route optimization, the Gateway Packet Filter can
   also check if the destination address of an uplink packet has a valid
   mapping in any of the Gateway's caches.  If a mapping is found, the



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   packet is "turned back" and is treated as a downlink packet.)

   Packets arriving from the Internet (using Mobile IP) to mobile hosts
   in the Cellular IP Network are decapsulated and forwarded to the
   Cellular IP node block.  Arriving packets not using Mobile IP are
   assumed to be sent to mobile hosts of which this Cellular IP Network
   is the home network.  If no foregin registration shows that the
   mobile host is away, these packets are forwarded to the Cellular IP
   node block unchanged.

   The Gateway's Cellular IP node block treats these packets as
   determined by the Cellular IP Routing algorithm (Section 3.5)
   according to the mappings in Route and Paging Cache.  It is optional
   whether Cellular IP Nodes have Paging Cache configured or not.
   However, it is recommended that at least the Gateway's Cellular IP
   node has Route Cache configured.  This ensures that packets addressed
   to hosts currently not connected to the Cellular IP Network do not
   enter the network and do not load it in vain but are immediately
   discarded in the Gateway when neither Route, nor Paging Cache mapping
   is found for the destination address.  (It may be advantageous to
   also generate an ICMP message in this case and send it back to the
   packet's source address.)

3.8. Cellular IP Mobile Host

   While connected to a Cellular IP Network, a mobile host must be in
   one of two states: 'active' or 'idle'.  The host moves from idle to
   active state when it receives or wishes to send a data packet.
   Active state is maintained as long as the host is transmitting or
   receiving data packets.  When the host has not received or
   transmitted any data packets for some time (the value of this timer
   may be implementation-specific) then it returns to idle state.

   When the host moves from idle to active state, it must transmit a
   route-update packet.  At the same time, a timer is initiated from a
   value equal to route-update-time.  If the timer expires without any
   data packet being transmitted from the host, again a route-update
   packet is transmitted and the timer is re-initiated.  Any IP packet
   transmitted before the timer expires, resets the timer to route-
   update-time.  This ensures that while the mobile host is in active
   state, the largest interval between two transmitted packets is never
   longer than route-update-time.  The mechanism also ensures that if
   data packets are transmitted with sufficient frequency, no route-
   update packets will be generated, which will probably be typical.

   If the host is in active state, it must immediately transmit a
   route-update packet whenever it connects to a new base station.  This
   typically happens at migration, but is also the case after a wireless
   channel black-out or when the host enters the Cellular IP Network.  A
   packet transmitted this way also resets the route-update packet
   timer.

   In idle state, the mobile host must transmit paging-update packets
   periodically, at intervals of paging-update-time.  In addition, the



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   host must transmit a paging-update packet when it connects to a new
   Base Station which has a different Paging Area ID from the previous
   Base Station.  (When connecting to a Base Station that belongs to the
   same Paging Area as the previous one, the host need not transmit
   paging-update packet.)  Similarly to the route-update packet timer,
   the paging-update timer is reset if a data packet is transmitted.

   The mobile host must ensure that the 64 bit value of timestamps is
   strictly increasing in consecutive control packets.

4. Extensions to Cellular IP

4.1. Semi-soft Handoff

   When a mobile host switches to a new Base Station it sends a route-
   update packet to make the chain of cache bindings to point to the new
   Base Station.  Packets that are traveling on the old path will be
   delivered to the old Base Station and will be lost.  Although this
   loss may be small it can potentially degrade TCP throughput.  This
   kind of handoff, when the mobile switches all at once to the new Base
   Station is called "hard" handoff. For performance details of hard
   handoff in a Cellular IP network see [5].

   To improve the performance of loss sensitive applications, another
   type of handoff may be introduced, called "semi-soft" handoff.
   During semi-soft handoff a mobile host may be in contact with either
   of the old and new Base Stations and receive packets from them.
   Packets intended to the mobile host are sent to both Base Stations,
   so when the mobile host eventually moves to the new location it can
   continue to receive packets without interruption.

   To initiate semi-soft handoff, the moving mobile host transmits a
   route-update packet to the new Base Station and continues to listen
   to the old one.  The S flag is set in this route-update packet to
   indicate semi-soft handoff.  Semi-soft route-update packets create
   new mappings in the Route and Paging Cache similarly to regular
   route-update packets.  When the semi-soft route-update packet reaches
   the cross-over node where the old and new path meet (note that the
   cross-over node already has a mapping for the mobile host), the new
   mapping is added to the cache instead of replacing the old one.
   Packets sent to the mobile host are transmitted to both Downlink
   neighbors.  When the mobile host eventually makes the move then the
   packets will already be underway to the new Base Station and the
   handoff can be performed with minimal packet loss.  After migration
   the mobile host sends a route-update packet to the new Base Station
   with the S bit cleared.  This route-update packet will remove all
   mappings in Route Cache except for the ones pointing to the new Base
   Station.  The semi-soft handoff is then complete.

   If the path to the new Base Station is longer than to the old Base
   Station or it takes non negligible time to switch to the new Base
   Station, then some packets may not reach the mobile host.  To
   overcome the problem, packets sent to the new Base Station can be
   delayed during the semi-soft handoff.  This way a few packets may be



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   delivered twice to the mobile host, but in many cases this results in
   better performance than a few packets lost.  Introduction of packet
   delay can be best performed in the Cellular IP node that has multiple
   mappings for the mobile host as a result of a semi-soft route-update
   packet.  Packets that belong to flows that require low delay but can
   tolerate occasional losses should not be delayed. For performance
   details of semi-soft handoff in a Cellular IP network see [5].

4.2. Multiple Gateway Networks

   Cellular IP requires that a mobile host be using exactly one Gateway
   at a time.  This requirement comes from the fact that the Gateway
   serves as the mobile host's Foreign Agent and it relays its packets
   both up and downlink.  It is also required to make uplink routing
   unambiguous.  The Cellular IP Network can have multiple Gateways as
   long as a single host still uses just one Gateway at any time.  (The
   host can change Gateway, involving a Mobile IP location updating.)
   In a Network with multiple Gateways, nodes must be able to determine
   which Gateway a given mobile host is using.  Assignment of Gateways
   can, for instance, be based on geographical partitioning of the
   network, or on partitioning the mobile hosts' address space.  This
   issue is for further study.

4.3. Charging

   Cellular IP Network providers can charge Cellular IP Mobile users for
   connectivity or for transmitted data or both.  Charging information
   is best collected in the Gateway.  The Gateway receives all control
   packets and can determine the time a mobile host was connected to the
   network.  It can also measure through traffic in both directions.

5. Security Considerations

   A Cellular IP Network is a single administrative domain.  It is
   connected to the Internet through a Gateway that may eventually also
   serve as a firewall.  Hence security issues only need to be
   considered at the wireless interface.

   The security of a Cellular IP system will be determined by the
   wireless link.  Security issues relating to wireless links are not
   specific to Cellular IP, and are out of the scope of Cellular IP,
   even though they must be dealt with in practical Cellular IP
   implementations.

   A security problem specific to Cellular IP is the security of the
   control packets, which can be solved by the authentication mechanism
   described in Section 3.5.

6. Intellectual Property Right Notice

   This is to affirm that Telefonaktiebolaget LM Ericsson and its
   subsidiaries, in accordance with corporate policy, will offer patent
   licensing for submissions rightfully made by its employees which are
   adopted or recommended as a standard by your organization as follows:



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   If part(s) of a submission by Ericsson employees is (are) included in
   a standard and Ericsson has patents and/or patent application(s) that
   are essential to implementation of such included part(s) in said
   standard, Ericsson is prepared to grant - on the basis of reciprocity
   (grantback) - a license on such included part(s) on reasonable, non-
   discriminatory terms and conditions.

   Ericsson has filed patent applications that might possibly become
   essential to the implementation of this contribution.

References

   [1] "IP Mobility Support," C. Perkins, ed., IETF RFC 2002, October
       1996.

   [2] "Network Time Protocol (Version 3): Specification, Implementation
       and Analysis," D. Mills, IETF RFC 1305, March 1992.

   [3] "IP Authentication Header," R. Atkinson, IETF RFC 1826, August
       1995.

   [4] "IP Authentication using Keyed MD5," P. Metzger, W. Simpson, IETF
       RFC 1828, August 1995.

   [5] "Cellular IP Performance," A. T. Campbell, J. Gomez, S. Kim, Z.
       Turanyi, A. Valko, C-Y Wan, Work in Progress, <draft-gomez-
       cellularip-performance-00>, October 1999.

Authors' Addresses

   Andrew T. Campbell, Javier Gomez, Sanghyo Kim, Chieh-Yih Wan
   Department of Electrical Engineering, Columbia University
   Rm. 801 Schapiro Research Building
   530 W. 120th Street, New York, N.Y. 10027
   phone: (212) 854 3109
   fax  : (212) 316 9068
   email: {campbell,javierg,shkim2,wan}@comet.columbia.edu

   Zoltan R. Turanyi, Andras G. Valko
   Ericsson Traffic Analysis and Network Performance Laboratory
   H-1300 Bp.3.P.O.Box 197, Hungary
   phone: +36 1 437 7774
   fax  : +36 1 437 7219
   email: {zoltan.turanyi,andras.valko}@eth.ericsson.se

Appendix A. Uplink Neighbor Selection

   This algorithm selects the Uplink neighbor of all nodes of a Cellular
   IP Network and reconfigures them if necessary after a change of
   topology.  An Uplink neighbor is identified by the interface through
   which it is accessible from the node and its corresponding MAC
   address.  The algorithm also distributes the Cellular IP Network
   Identifier, the IP address of the Gateway and the Paging Area IDs to
   the Base Stations.



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   The Gateway periodically creates a control packet called a "Gateway
   broadcast packet".  The Gateway broadcast packet contains

   - the Cellular IP Network Identifier;
   - the IP address of the Gateway;
   - a sequence number increased each time by the Gateway; and
   - a Paging Area ID field initially set to the ID of the Gateway.

   The Gateway broadcasts the packet on all of its interfaces except
   those connected to the Internet.  A Cellular IP node receiving a
   Gateway broadcast packet follows the steps below.

   1) It drops the packet if the sequence number is lower or equal to
      the sequence number of one of the previously received Gateway
      broadcast packets.  In this case no further processing is needed.
   2) It stores the sequence number of the Gateway broadcast packet for
      later comparison.
   3) It stores the Cellular IP Network Identifier and the IP address of
      the Gateway.
   3) It stores the interface through which the packet arrived together
      with source MAC address of the packet (if any) to identify the
      Uplink neighbor.  All other interface/MAC address combinations
      will denote Downlink neighbors.
   4) If the node has a Paging Cache, it overwrites the value of the
      Paging Area ID field in the packet by its own ID.
   5) The value of the (possibly overwritten) Paging Area ID field is
      stored as the Paging Area ID of the node.  This value will be used
      in beacon signals if the node is a Base Station.
   6) It stores the Cellular IP Network Identifier and the IP address of
      the Gateway.  These values will be used in beacon signals if the
      node is a Base Station.
   7) After a short random delay, the node broadcasts the packet through
      all of its interfaces, except the air interface(s) and the
      interface of the Uplink neighbor.























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