Internet Draft                                     Alper Yegin (Editor)
Document: draft-yegin-dna-l2-hints-00.txt               DoCoMo USA Labs
Expires: April 2004                                        Eric Njedjou
                                                     France Telecom R&D
                                                        Siva Veerepalli
                                                          Qualcomm, Inc
                                                      Nicolas Montavont
                                                            Thomas Noel
                                       LSIIT - University Louis Pasteur


           Link-layer Hints for Detecting Network Attachments


                          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
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Abstract

Certain link-layer technologies are capable of providing various link
status information to the IP module. Indicating the status of the link,
such as connected or disconnected, and the link identifier can help the
IP module make intelligent decisions regarding configuration changes.
It has been identified that such information can be used as hints for
network attachment detection purposes. This draft provides a
non-exhaustive catalogue of such hints from well-known link-layer
technologies. Furthermore, a high-level abstraction is defined to
categorize such hints.








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Table of Contents

   1.0  Introduction.................................................2
   2.0  Terminology..................................................3
   3.0  Link-layer Hints in Various Systems..........................6
   3.1.  GPRS........................................................6
   3.1.1.  Network Reference Model...................................7
   3.1.2.  Link-layer Hints..........................................7
   3.2.  3GPP2......................................................11
   3.2.1.  Network Reference Model..................................12
   3.2.2.  Link-layer Hints.........................................14
   3.3.  WLAN.......................................................15
   3.3.1. Network Reference Model...................................16
   3.3.2.  Link-layer Hints.........................................17
   4.0  Abstraction.................................................18
   4.1.  Link Identifier............................................18
   4.2.  Link-up Hint...............................................19
   4.3.  Link-down Hint.............................................19
   5.0  Security Considerations.....................................20
   6.0  References..................................................20
   7.0  Acknowledgements............................................21
   Appendix A.......................................................21
   Authors' Addresses...............................................22
   Full Copyright Statement.........................................23





1.0  Introduction

   It is not an uncommon occurrence for a host to change its point-of
   attachment to the network. This can happen due to mobile usage
   (e.g., a mobile phone moving among base stations) or nomadic usage
   (e.g., road-warrior case).

   Each time a host changes its point-of attachment, it is possible
   that it will also have to change its IP-layer configurations, such
   as its IP address and default gateway information. In order to make
   these changes, the IP module has to detect the new network
   attachment, realize that the old configuration is no longer valid
   and obtain the new configuration parameters. The network detection
   phase can usually use network-layer indications such as a change in
   the advertised prefixes. But generally reliance on such indications
   does not yield rapid detection, since these indications are not
   readily available upon a link change. It has been identified that
   receiving explicit hints from the link-layer would expedite the
   detection process. The link-layer indicating that the host has
   established a new connection can be used as a hint to further probe


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   the network for a possible configuration change. Such an indication
   cannot be used to positively determine the need for a configuration
   change as it might very well be the case that the host is still
   connected to the same IP subnet despite the link change. For
   example, there might be several IEEE 802.11b access points connected
   to the same access router. Moving among these access points does not
   warrant any IP-layer configuration change. This is why the link-
   layer hints should be used as "advisory-only" unless stated
   otherwise.

   In order to enable an enhanced network attachment detection scheme,
   we need to identify types of link-layer hints that can be
   realistically expected from various access technologies. The
   objective of this draft is to provide a catalogue of existing link-
   layer hints in various architectures. Later, this catalogue is used
   to define an abstraction for these hints. The movement detection
   schemes should be able to incorporate these hints in their abstract
   form for simplicity. Furthermore, this abstraction should enable the
   link-layer developers to align their implementations according to
   the needs of IP. This is not an API design, although the abstraction
   can be used to define one.

   The document limits itself to the minimum set of link-layer hints
   that are necessary for detecting network attachment. These hints are
   considered with hosts in mind, although they may also be available
   on the network side (e.g., on the access router).


2.0  Terminology

   Some of the terminology differs among the discussed architectures.
   An architecture name is provided in parenthesis when a term has
   limited applicability.

   3GPP  Third Generation Partnership Project

   3GPP2 Third Generation Partnership Project 2

   ANID  Access Network Identifier. Identifies the packet switched area
         served by a unique combination of RAN and MSC area. (3GPP2)

   AP    Access Point. An access point is an entity that provides
         bridging between the radio link and the wired network. (WLAN)

   APN   Access Point Name. A parameter of the PDP context, in the form
         of a logical name that is used to select the GGSN or the
         external IP network. (3GPP)

   AT    Access Terminal. Another term used for Mobile Terminal in 3GPP2
         networks. (3GPP2)

   BSC   Base Station Controller. A BSC controls a set of BTS. (3GPP,



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         3GPP2)

   BSS   Base Station System. The system that is made up of BTSs and
         BSCs. (3GPP)

   BSS   Basic Service Set. A BSS is composed of one AP and its attached
         MNs. (WLAN)

   BSSID Basic Service Set Identification. A unique identifier of a BSS.
         In infrastructure mode, it is the MAC address of the AP.
         (WLAN)

   BTS   Base Transceiver Station. Mobile terminalÆs radio attachment
         point to the network. A BTS is responsible for MTs within a
         given radio cell.(3GPP, 3GPP2)

   ESS   Extended Service Set. The set composed of APs and associated
         MNs(BSSs) that share a common distribution system. (WLAN)

   FA    Foreign Agent. A router on a mobile node's visited network
         which provides routing services to the mobile node while
         registered.

   GGSN  Gateway GPRS Support Node. A router between the GPRS core
         network and an external IP network. (3GPP)

   GMM   GPRS Mobility Management. Sub-link-layer protocol between the
         MT and the SGSN for handling MT movement. (3GPP)

   GPRS  General Packet Radio Service. Packet-switched data
         transmission service on top of the GSM network. (3GPP)

   GTP   GPRS Tunneling Protocol. A protocol for encapsulating user data
         traffic between the SGSN and the GGSN. (3GPP)

   HA    A router on a mobile node's home network which tunnels
         datagrams for delivery to the mobile node when it is away from
         home, and maintains current location information for the mobile
         node.

   IMSI  International Mobile Subscriber Identity. A 12-digit number
         that uniquely identifies a GPRS subscriber smart card. (3GPP)

   Link  An association between a host and a link-layer access device
         (e.g., IEEE 802.11b access point) for the purpose of IP
         connectivity service. In an abstract form, a link is identified
         by its end-points. An additional token may be needed to handle
         cases when multiple link instances exist between the same
         end-points. The actual link identifiers are access technology
         and architecture specific.

   LLC   Logical Link Control. Data link protocol between the MT and



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         SGSN. (3GPP)

   MN    Mobile Node. A host or router that changes its point of
         attachment from one network or subnet to another.

   MN    Mobile Node. The conjunction of a Mobile Terminal, a SIM card
         and Terminal Equipment. (3GPP)

   MT    Mobile Terminal. For example, a mobile phone handset or a
         PCMCIA card. (3GPP)

   MS    Mobile Station. For example, a mobile phone or a combination of
         mobile terminal (e.g., a phone) and terminal equipment (e.g., a
         laptop). (3GPP2)

   MUX   Multiplex Layer. A link layer protocol used to multiplex
         signaling and RLP protocols. (3GPP2)

   NSAPI Network Layer Service Access Point Identifier. It is used to
         identify a PDP context between MT and SGSN on top of the
         Logical Link Control layer. It is set by the MT. (3GPP)

   P-TMSI
         Packet TMSI. A temporary IMSI allocated by the GPRS network to
         the MT upon IMSI attach procedure.(3GPP)

   PCF   Packet Control Function (3GPP2)

   PDP Address
         Address of a MN for a given PDP context. (3GPP)

   PDP Context
         Soft state maintained between the Mobile Terminal, the SGSN and
         the GGSN for guaranteeing a negotiated quality of service for
         the IP flows exchanged between the GPRS Mobile Terminal and an
         external Packet Data Network such as Internet. (3GPP)

   PDSN  Packet Data Serving Node. The default gateway router for MNs in
         3GPP2 networks. (3GPP2)

   PLMN  Public Land Mobile Network. A GPRS Network operated on a
         national territory. (3GPP)

   PPP   Point-to-Point Protocol

   RA    Routing Area. Set of adjacent cells. A given number of RAs are
         under the control of one SGSN. (3GPP)

   RAN   Radio Access Network. (3GPP, 3GPP2)





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   RLP   Radio Link Protocol. A link-layer protocol used to improve the
         physical-layer frame error rate over the air. (3GPP2)

   R-P   RAN-to-PDSN interface. Also known as the A10/A11 interface.
         (3GPP2)

   SGSN  Serving GPRS Support Node. A router directly connected to the
         GPRS Radio Sub-System that handles the mobility of terminals
         attached to the RAs under its authority. The SGSN is also the
         Radio Sub-System interface to the GPRS IP core network. It
         could be considered as an equivalent to the IEEE 802.11 access
         point. (3GPP)

   SM    Session Management. Sub-link-layer protocol between the MT and
         the GGSN that handles the activation/deactivation of a PDP
         Context. (3GPP)

   SSID  Service Set Identifier. Identifier of an ESS. (WLAN)

   TE    Terminal Equipment. A user's laptop for example. TE can connect
         to the network via MT. (3GPP)

   TI    Transaction Identifier. This is the association between an
         NSAPI and an identifier corresponding to an operation performed
         on the associated PDP context. For example a "Modify PDP
         Context Request" will be identified by a Transaction
         Identifier. (3GPP)

   TLLI  Temporary Logical Link Identity. It is used by the SGSN to
         identify a particular Mobile Terminal at the logical link
         control layer. (3GPP)


3.0  Link-layer Hints in Various Systems

   This section provides an overview of various architectures and
   discusses associated link-layer hints.


3.1. GPRS

   Multi-interface terminals are changing the face of wireless IP
   connectivity and GPRS [GPRS] is being one of the most pervasive
   types of radio link for enabling multi-technology access to the
   Internet.

   GPRS is an enhancement to the GSM data transmission architecture and
   capabilities, designed to allow for packet switching in user data
   transmission within the GPRS network as well as for higher quality
   of service for the IP traffic of Mobile Terminals with external
   Packets Data Networks (PDN) such as the Internet or corporate LANs.



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   The GPRS architecture consists of a Radio Access Network and a
   packet domain Core Network.

   - The GPRS Radio Access Network is composed of Mobile Terminals, a
   Base Station Subsystem (BSS) and Serving GPRS Support Nodes (SGSN).
   The BSS is made up of radio cells called Base Transceiver Stations
   (BTS) served under the control of Base Station Controllers (BSC).
   So-called Routing Areas are formed by the subdivision of BSCs. Each
   SGSN in the GPRS architecture controls a set of RAs;

   - An IP Core Network that acts as the transport backbone of user
   datagrams between SGSNs and Gateway GPRS Support Nodes (GGSN). The
   GGSN ensures the GPRS IP core network connectivity with external
   networks, such as Internet or Local Area Networks.

   From the point of view prevailing in detecting network attachment,
   the GPRS access network will be only seen as providing layer 1-2
   reachability even if it is able to provide IP connectivity alone.


3.1.1.  Network Reference Model

   Most of the hints described in this document come from messages
   exchanged on top of the Logical Link Control protocol (LLC) running
   between the Mobile Terminal and the SGSN. The messages are part of
   the GPRS Mobility Management (GMM) and Session Management (SM)
   protocols and ensure functionalities such as GPRS attach, detach,
   PDP Context activation and deactivation, Routing Area update.

                |
           +----|    <-------------GMM/SM-------------->    +-----+
           |    |    <--------------LLC---------------->    |     |
           |    |                                           |     |
           |    |                    \ /                    |     |
           | MT |                  +-----+                  |SGSN |
           |    |  Radio interface |     |<---------------->|     |
           |    |<----Protocols--->| BSS |                  |     |
           +----+  (RLC, MAC, L1)  +-----+                  +-----+

        Figure 1. Signaling protocol stack between MT and SGSN


3.1.2.  Link-layer Hints

   In GPRS networks, only network attachment/detachment and subsequent
   PDP context changing events will directly impact the IP
   configurations, hence should be used as link-layer hints by IP.
   Other events such as routing area and cell change do not directly
   imply potential configuration change. More details on those
   secondary types of events can be found in Appendix A.





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   A GPRS attach is made to the SGSN. The procedure is attempted
   whenever a GPRS-enabled Mobile Terminal is being switched on. The
   attachment can also take place at any time while the MT is switched
   on, for example following a detach forced by the network. The MT
   provides its identity during the attach request to the network in
   the form of a so-called Packet Temporary Mobile Subscriber Identity
   (P-TMSI). If the MT has no valid P-TMSI, it provides its IMSI.

   Before the MT becomes GPRS attached, it scans for available GPRS
   networks, as well as acquires the identities of their cells in the
   covered area. It is also possible for the MT to obtain the radio
   capabilities of these cells.

   When a MT has performed the GPRS attach, it becomes in READY state.
   In this state, the MT is reachable (using the logical link layer -
   LLC) by the GPRS Radio Access Point called the SGSN. Otherwise, its
   state is said to be IDLE. During the IDLE state, no IP level
   communication is possible with an external network, such as
   Internet. The SGSN identifies the logical link with the MT by the
   Temporary Logical Link Identifier (TLLI) it derives from the P-TMSI
   that was assigned to the MT. It has to be noted that the MT or SGSN
   may initiate a detach procedure (Mobile or Network Initiated
   Detach). The MT returns from READY to IDLE STATE upon detachment.

   The MT is actually considered GPRS attached when it has received an
   "Attach Accept" message from the SGSN. This can be a hint to the
   network-layer of the Mobile Node that a GPRS Network has been found
   and that the GPRS interface of the MN is attached to it.

   The MT is considered detached from the GPRS Network when it has
   received/sent a "Detach Accept message" from/to the SGSN. This is an
   indication that the link-layer connectivity is being lost.

   The "Detach Accept" message is also preceded by a "Detach Request"
   message from the side initiating the detachment procedure. This
   message is a hint that a detachment from the GPRS network is about
   to take place. The network-layer could then anticipate the loss of
   connectivity.

   The "Attach Accept" message comes along with an update of the Mobile
   Terminal Mobility Management context held at the GMM/MM level. This
   message contains:

   - The Packet Temporary Mobile Station Identifier (P-TMSI). The P-
   TMSI is a temporary IMSI allocated by the GPRS network upon attach
   (if no P-TMSI was already present). It is used for subscriber
   location hiding purpose in substitution to the IMSI.
   - The current Cell Identity (CI)
   - The current Routing Area Identity (RAI) which identifies the
   serving SGSN
   - The ciphering algorithm, key (Kc) and sequence number (CKSN)




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   Once the GPRS MT is attached, the attached network information can
   be sent to it via the "MM information" message that contains:

   - The network name known as Public Land Mobile Network ID in 3GPP
   terminology
   - Network registration type (Home or Roaming)

   A MN that wants to establish IP-level connections through the GPRS
   MT should first request the GPRS network to settle the necessary
   soft state mechanism (GPRS tunneling protocol) between its serving
   SGSN and the GGSN corresponding to the APN specified in the PDP
   Context parameters. Only after this tunneling mechanism takes place
   can the MN's IP packets be forwarded to/from its remote IP peers.
   The process by which this is made possible is designated as a PDP
   Context Request.

   The aim of this function is also to provide IP-level configuration
   on top of the GPRS link-layer attachment, in order for the MN to get
   IP reachability with external networks, such as Internet. The
   establishment of a PDP context is partially based on link-layer
   characteristics negotiated between the MT and the GPRS network (SGSN
   and GGSN). These characteristics include the QoS profile that will
   be guaranteed by the SGSN and GGSN (e.g., maximum delay, link
   reliability, peak and mean throughputs). When the MT requests a PDP
   context, it selects a Network Service Access Point Identifier
   (NSAPI) that it sends to the SGSN with the request. The NSAPI is
   sent (as part of the PDP Context request message) on top the Logical
   Link Control layer identified for that MT by the TLLI. In this way,
   the SGSN is able to uniquely identify the PDP context.

   A PDP context Activation procedure can also be initiated by the GGSN
   (Network-requested PDP Context Activation) but this alternative is
   not likely to happen so often.

   The network may also decide to modify an existing PDP Context with a
   given MN at any time. Such a modification might be prompted by the
   MN's serving SGSN when it estimates that the negotiated QoS profile
   can no longer be respected. For instance, the GPRS Network might
   temporarily not be able to guarantee the contracted delay, in which
   case it would force the related PDP context parameter to be
   renegotiated. Note that, a MT can decide not to accept such an
   update of its PDP context, in which case it should start a PDP
   context deactivation procedure. Furthermore, a PDP context may be
   deleted at any time at the request of the MT or the network. After a
   PDP context is deleted, the MT returns to simply attached state
   (READY STATE). Finally, a Mobile Terminal can hold several PDP
   contexts, each corresponding to a different NSAPI.







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               +--------------+
               | PDP Context1 |                        +-------+
               |   NSAPI 1    |                        |       |
               | ------------ |       +------+         |       |
               |   GPRS MT    +-------+ TLLI +---------| SGSN  |
               | ------------ |       +------+         |       |
               | PDP Context2 |                        |       |
               |   NSAPI 2    |                        +-------+
               +--------------+

            Figure 2. NSAPI and TLLI (link identifier).


   A PDP context is considered activated on the MT side as soon as an
   "Activate PDP Context Accept" message has been received from the
   GGSN. The reception of this message can be considered as a hint that
   the GPRS network will be providing a certain link-layer quality-of
   service for which parameters (e.g., delay, reliability, throughput)
   are included with the messages described below.

   When the network is about to modify a PDP Context, it informs the MT
   by sending a "Modify PDP Context Request" message. This can also be
   an indication at the MN's network-layer that the link-layer
   characteristics on the GPRS attachment are about to change. The MN
   could then be able to anticipate such a change, which would likely
   be a drop or an increase of service quality. The "Modify PDP Context
   Accept" message confirms the modification and is a hint that the
   initially negotiated PDP context characteristics are no longer
   valid.

   A "Deactivate PDP Context Request" message is sent by the MN or
   received from the SGSN depending on which side has initiated the
   deactivation procedure. The transmission or reception of this
   message can serve as a hint that the IP configuration of the MN's
   GPRS interface or one of its IP configuration (in case multiple PDP
   Contexts are present on the MT), is about to be deleted. This could
   help the MN anticipate the coming loss of IP attachment. A
   "Deactivate PDP Context Accept" sent or received by the MT is a
   confirmation that the PDP context is being deleted.

   The "Activate PDP Context Accept" message comes along with a
   modification of the GMM context that contains the following
   information:

   - The TI (transaction identifier) associated to the procedure of
   activating a PDP context. It consists of the NSAPI generated by the
   MT for that PDP context and an operation identifier,
   - The IP address for that PDP context,
   - The QoS Profile negotiated with the network,
   - The Radio Priority level for data transmission.





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   The "Modify PDP Context Accept" comes along with the following
   information:
   -The TI associated to the procedure,
   -The new QoS profile negotiated with the network,
   -The radio priority level for data transmission.

   The "Deactivate PDP Context Accept" message comes along with the TI
   associated to the procedure.


3.2. 3GPP2

   3GPP2 (cdma2000) packet data services provide mobile users wide area
   high-speed access to packet switched networks. 3GPP2 consists of
   multiple radio access technologies, namely 1x EV, 1x EV-DO and 1x
   EV-DV, where the order shows the evolution of technology in the
   industry. 1x Evolution Data Only (1x EV-DO) and 1x Evolution Data-
   Voice (1x EV-DV) are enhanced air interface technologies that are
   optimized for higher data rates.

   The aforementioned 3GPP2 technologies share a common core network
   infrastructure which enables easy transition to enhanced air
   interface technologies. 3GPP2 networks use the Point-to-Point
   Protocol (PPP) as the link-layer protocol between the mobile node
   and the network access server. Hence, link-layer mechanisms are
   pretty consistent across all air interface technologies. Unless
   specifically called out, all link-layer mechanisms specified in this
   document apply to all 3GPP2 air interface technologies.



























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3.2.1.  Network Reference Model

   Some of the major components of the 3GPP2 packet network
   architecture (see Figure 3) consist of:

   - Mobile Node (also known as Mobile Station or Access Terminal in
   3GPP2), which allows mobile access to packet-switched networks over
   a wireless connection,
   - Radio Access Network, which consists of the Base Station
   Transceivers (BTS), Base Station Controllers (BSC), and the Packet
   Control Function (PCF),
   - Network Access Server known as the Packet Data Switching Node
   (PDSN). The PDSN also serves as the Foreign Agent (FA), in the case
   of Mobile IP service.



                      +-------------------------+
                      |           RAN           |
     +====+           |  +=====+       +=====+  |       +======+
     |    |           |  | BSC/|       |     |  |       |      |
     | MN |-----------|  | BTS |-------| PCF |--|-------| PDSN |
     |    |           |  |     | A8/A9 |     |  |A10/A11|      |
     +====+           |  +=====+       +=====+  |       +======+
                      |                         |
                      +-------------------------+


                Figure 3. Packet Network Reference Model


   Figure 4 shows the hierarchical relationship between the RAN,
   PDSN/FA and HA. The control and bearer interfaces between the BSC
   and PCF are known as the A9 and A8 interface respectively, while the
   control and bearer interfaces between PCF and PDSN are known as the
   A11 and A10 interfaces respectively. Note that, the A11/A10
   interface is also known as the R-P interface (for RAN-PDSN
   interface). The A9 and A11 interfaces are used to establish A8 and
   A10 connections. The A8 and A10 connections are used to tunnel link
   layer data (PPP frames) between the BSC and PDSN.















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                                   +======+
                                   |      |
                                   |  HA  |
                                   |      |
                                   +======+
                                       |
                                       |
                        +--------------+---------------+
                        |              |               |
                    +======+        +======+       +======+
                    |      |        |      |       |      |
                    | PDSN |        | PDSN |       | PDSN |
                    |      |        |      |       |      |
                    +======+        +======+       +======+
                      / \              / \            / \
     A10/A11---------/---\------------/---\----------/---\---------
                    /     \          /     \        /     \
                   /       \        /       \      /       \
             +======+  +======+ +======+     \    /         \
             |      |  |      | |      |    +======+      +======+
             |  PCF |  |  PCF | |  PCF |    |      |      |      |
             |      |  |      | |      |    |  PCF |      |  PCF |
             +======+  +======+ +======+    |      |      |      |
                |         / \       |       +======+      +======+
      A8/A9 ----|--------/---\------|----------|-------------|-----
                |       /     \     |          |             |
             +====+ +====+ +====+ +====+     +====+        +====+
             |    | |    | |    | |    |     |    |        |    |
             |BSC | |BSC | |BSC | |BSC |     |BSC |        |BSC |
             |    | |    | |    | |    |     |    |        |    |
             +====+ +====+ +====+ +====+     +====+        +====+


            +====+
            |    |
            | MS |  ------->
            |    |
            +====+

       Figure 4. Hierarchical relationship between RAN, PDSN and HA


   A PCF controls one or more BSCs. The area served by each PCF is
   identified by the Access Network Identifier (ANID). This is referred
   to as the SUBNET ID in the 1x EV-DO system. Any given BSC is
   associated with one and only one PCF. The combination of BSC and PCF
   is also known as the RAN. Each PCF can communicate with one or more
   PDSNs. However, for a given mobile user, the PCF typically
   establishes a connection with a specific PDSN.






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   Link-layer-related (e.g., handover) information is provided by the
   RAN to the MS via 3GPP2 overhead signaling messages broadcast over
   the air interface.

   A number of other important components of the architecture that
   enable call setup (such as the MSC, HLR, AC and/or AAA servers) are
   left out for the sake of simplicity. None of these components have a
   direct impact on the discussion of link-layer hints.


3.2.2.  Link-layer Hints

   While a PPP connection is in ESTABLISHED state at the MN and PDSN,
   the packet data service state at the MN can be in ACTIVE or DORMANT
   state. In the ACTIVE state, all the bearers between the MN and the
   PDSN are in the established state. In the DORMANT state, the radio
   link bearer and the A8 connection are torn down to conserve radio
   resources. However, the A10 bearer still remains connected, and the
   PPP state is maintained both at the MN and PDSN.

   MN transitions from DORMANT to ACTIVE state when the MN has some
   data to send or the network (PDSN) has data to send to the MN. When
   a MN in ACTIVE packet data service state hands off from one RAN to
   another, it results in an ANID change. An ANID change may or may not
   result in a change in the MN point of attachment to the network
   (i.e., PDSN).

   If the ANID changes, but no change in the network attachment point,
   a new A10 connection between the new PCF and serving PDSN is
   established. If the ANID change results in a change in network
   attachment point (i.e., PDSN), the new PDSN initiates a new PPP
   connection setup with the MN, resulting in an update of the network
   configuration information such as IP address and DNS server address
   on the mobile node. In the case of Mobile IP, PPP resynchronization
   is followed by Mobile IP registration to update the FA (PDSN)
   address in the Mobile IP binding at the HA.

   Hence, a PPP resynchronization from the PDSN could be viewed as a
   link-layer event that updates network configuration information in
   the MN and further provides an indication to the MN that Mobile IP
   registration is required to update the binding in the HA with the
   new FA address.

   On the other hand, when a DORMANT mobile moves, the RAN is not aware
   of the presence of the mobile in its area (as the radio link is not
   in established state). The RAN relies on the MN to inform it of the
   MNÆs presence. The ANID for the RAN, which is broadcast on the
   overhead channel, is used as a link-layer hint by the MN. When a
   dormant MN moves and the ANID changes, the MN registers with the RAN
   to initiate a new A10 connection between the new RAN and PDSN. If
   the ANID change also results in a change in the network attachment
   point, not only is a new A10 connection established, but also a new



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   PPP connection is established between the new PDSN and MN. The RAN
   transitions the MN from DORMANT to ACTIVE state in order to
   resynchronize the PPP connection. This results in an update in the
   network layer configuration information such as IP address and DNS
   server address in the MN. In the case of Mobile IP, PPP
   resynchronization is followed by Mobile IP registration to update
   the FA (PDSN) address in the Mobile IP binding at the HA.

   As described above, a lower-layer hint (ANID change) allows a MN to
   discover a potential change in the network point of attachment. From
   IP's perspective, changes in the PPP link status provide hints about
   the network attachment change.


3.3. WLAN

   WLANs are the wireless extension of the Local Area Networks. A WLAN
   offers MNs short range network access at high rate. The maximum
   coverage area of a node is usually from few meters indoors to more
   than one hundred meter outdoor. The raw bandwidth varies between
   1Mbps to 54Mbps depending on the norm used and the configuration of
   the equipment.

   The IEEE 802.11 series are specified by IEEE since 1997 and the
   currently available standards are IEEE 802.11b [802.11b] and IEEE
   802.11g [802.11g] operating in the 2.4GHz band, and IEEE 802.11a
   [802.11a] operating in the 5GHz band. The specification defines both
   the MAC-layer and the physical-layer (e.g., modulation techniques
   and radio propagation). The MAC level is the same for all these
   technologies.

   Two operating modes are available in the IEEE 802.11 series. In
   ad-hoc mode, each equipment in range may directly communicate with
   each other, i.e. without any infrastructure or intermediate hop. In
   the infrastructure mode, all link-layer frames are transmitted to an
   access point (AP) which then forwards them to the final receiver.

   In this section MN refers to a IEEE 802.11 station without the AP
   functionality.
















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3.3.1. Network Reference Model


   In the infrastructure mode, the network connectivity is offered to
   MN (IEEE 802.11 station) through an AP. An AP is a bridge between
   the wireless domain and the wired domain. The coverage area of an AP
   defines a cell. A BSS (Basic Service Set) is composed of one AP and
   at least one attached MN. A common IEEE 802.11 infrastructure is
   shown in Figure 5.


    Access Router-----Internet-----Access Router
       |                               |
       |    LAN                   LAN  |
      -+----+----            -+--------+------+-
            |                 |               |
            |      ~~~~~~~~~~~AP2~~~~~~~~~    |
     ~~~~~~~AP1~~~~~~~~                ~~~~~~~AP3~~~~~~~~
    ~             ~    ~              ~   ~              ~
    ~             ~    ~              ~   ~              ~
    ~       MN    ~    ~              ~   ~              ~
    ~             ~    ~              ~   ~    MN        ~
    ~             ~ MN ~              ~   ~              ~
     ~~~~~~~~~~~~~~~~~~                ~~~~~~~~~~~~~~~~~~
     BSS of AP1   ~                       ~    BSS of AP3
                  ~          MN           ~
                  ~                       ~
                   ~~~~~~~~~~~~~~~~~~~~~~~
                         BSS of AP2

        Figure 5: Architecture of IEEE 802.11 access.


   When several APs are connected together through a DS (Distribution
   System), the set of all cells is called ESS for Extended Service
   Set. The structure of the DS is not defined in the IEEE 802.11
   standard and any technology can be used as DS medium. The document
   IEEE 802.11f [IAPP] proposes the Inter-AP protocol (IAPP) to be used
   between APs of the same ESS. Some information on attached MNs may be
   exchanged in an ESS in order to enable context transfers and enhance
   MN's roaming.

   When a MN moves out of the coverage area of its current AP, it may
   attach to a new AP. The new AP can be connected to the same access
   network as well as it can be connected to a different access
   network, this makes no difference at the link layer. However, if the
   new AP is connected to the same subnet as the old AP, the MN can
   continue its IP communication through the new AP without any
   configuration change at the network-layer. But if the new AP is
   connected to a different subnet, the MN needs to configure a new IP
   address valid for the new subnet and use some additional mechanism




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   to maintain its ongoing communication sessions, such as Mobile IP
   [MIPv4, MIPv6].

   A MN must be associated with an AP in order to send and receive data
   frames. At any given time, a MN can be associated with only one
   AP on each IEEE 802.11 interface. When a MN moves between two APs,
   it has to switch into promiscuous mode to discover and initiate a
   connection with a new AP. A MN cannot send IP packets during the
   establishment of a connection with an AP.

   Being associated implies that the MN has established a relationship
   with the AP. Three different steps are required for the MN to be
   associated with an AP: first the MN evaluates the potential APs in
   its range. In active mode, the MN scans its default channel to
   identify the available APs (exchange of Probe Request and Probe
   Response). If the MN does not receive any response from AP (e.g., no
   APs are operating in this channel), the MN switches to the next
   channel and continues the scanning. In passive mode, the MN only
   listens to beacons sent by AP to discover the potential APs.

   Once the MN discovers its target AP and its parameters, an
   authentication phase begins (exchange of Authentication
   Request/Response).

   When a MN succeeds the authentication process, it can associate with
   the AP (exchange of Association Request/Response). The MN sends its
   different link-layer parameters and the AP may accept to include the
   MN in the BSS. A MN may also issue a Re-association Request when the
   new AP belongs to the same ESS as the old AP of the MN. The
   Re-association message contains the MAC address of the old AP of the
   MN, allowing the new AP to inform old AP that the MN will now be
   associated with it. It can be noted that even if the two APs belong
   the same ESS, they can be on different IP subnets. No assumption is
   made on the location of APs in IEEE 802.11 series.


3.3.2.  Link-layer Hints

   The roaming of MNs between APs is managed by the link-layer protocol
   and is known as link-layer handover. As long as the MN moves between
   Aps in the same access network, the IP layer is not involved in the
   movement management. However, when the MN handovers to a new AP in
   another IP subnet, the MN needs to perform operations to maintain
   its existing communications [MIPv4, MIPv6]. Therefore, even if a
   link-layer handover occurs at the link layer, it doesn't necessarily
   imply a network-layer handover.

   Upon reception of the Association (or Re-association) response
   message from the AP indicating that the association is accepted, the
   MN is associated to the AP. It can then transmit and receive data
   packets through this AP. This association is valid as long as the MN




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   does not receive a Deauthentication message or a Deassociation
   message from its AP, or the MN moves to a new AP.

   So the reception of a (Re-)Association Response that completes a
   successful association can be taken as a hint that the point of
   attachment to the network of the MN may have changed. There is no
   mechanism at the link-layer that allows the MN to know if it has
   also changed of IP network.

   When the MN receives a Deauthentication message or a Disassociation
   message, it means that the MN is no longer authenticated or
   associated with the AP which sends the message. So this message may
   be a hint to indicate that IP packets can not be sent through this
   interface until the reception of a subsequent Association Response.
   If the MN wants to associate with the AP which sent the message, it
   needs to re-authenticate itself.

   The link identifier used by the MN is the BSSID (Basic Service Set
   Identification). The BSSID is the MAC address of the AP and must
   uniquely identify a BSS. However, several SSIDs can be configured on
   a single AP, to enable different VLANs for example. So a MN can
   switch between two SSIDs and change its network-layer configuration
   while connected to the same AP. Therefore, in the context of DNA,
   use of (BSSID, SSID) tuple is suggested as the link identifier.
   BSSID and SSID information is included in Association Request and
   Authentication Request sent by MN, and in Probe Response sent by the
   AP.


4.0  Abstraction

   Successful usage of link-layer hints by IP depends on the
   availability of a common framework. Abstracting useful link-layer
   hints is essential for providing interoperability between the
   various link-layers and IP. This document limits the scope of hints
   to those that are relevant to network-layer configuration changes.

4.1. Link Identifier

   An identifier should used to address a link on the host side. Any
   given link-layer hint should be accompanied with the associated link
   identifier.

   In GPRS networks, the relevant link-layer events provide a TI
   (Transaction Identifier) that includes NSAPI (Network Layer Service
   Access Point Identifier). NSAPI can be used as the link identifier
   as it can uniquely identify the associated PDP context.

   3GPP2 networks use PPP for the link-layer. Ideally a unique
   identifier associated with the PPP instance should be the link
   identifier. But the architecture does not provide such a handle. The
   closest it provides is ANID (Access Network Identifier), which is



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   associated with a PCF. A change in ANID (PCF) may or may not result
   in a change in PPP link status. When ANID is used as the link
   identifier, its change should not be interpreted as a link change
   from IP perspective. [TBD: the loose relation between ANID and PPP
   link is an issue.]

   IEEE 802.11 networks can rely on using the (BSSID, SSID) tuple as
   the link identifier.


4.2. Link-up Hint

   This hint is asynchronously provided to IP when a new link instance
   is created. This hint may be generated even when the host does not
   change its physical point-of attachment but creates a new link
   instance with the current link-layer access device.

   Network-layer may interpret this hint as a sign of possible
   configuration change. It may react to link-up hint by reconfirming
   its current configuration (e.g.: sending a router solicitation in
   the case of stateless IPv6 address auto-configuration). The detailed
   use of link-up hint for detecting network attachment is outside the
   scope of this draft.

   Creation of a new PDP context can be used to generate a link-up hint
   in GPRS networks. Similarly, a new PPP link establishment can lead
   to a hint in 3GPP2 networks. Note that, these are in reality more
   than simple "hints". They usually lead to network-layer
   configuration changes some of which take place during the link
   establishment (e.g., IP address assignment during PPP link
   establishment). [TBD: another term might be needed for these types]

   In IEEE 802.11 networks, association and re-association events
   indicate that a new link is brought up.


4.3. Link-down Hint

   This hint is asynchronously provided to IP when an existing link
   instance is removed.

   Network-layer may interpret this hint as a sign of possible
   configuration change. This hint might be followed by a link-up hint
   in the case of a handover. The detailed use of link-down hint for
   detecting network attachment is outside the scope of this draft.

   The deactivation of PDP context in GPRS networks can be used to
   generate the link-down hint. Bringing down a PPP connection in 3GPP2
   would have the same effect. De-authentication and disassociation
   events in IEEE 802.11 networks can lead to a link-down hint being
   sent to IP.




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5.0  Security Considerations

   The link-layer hints are advisory only. They SHOULD be used as
   indications of possible network-layer configuration change, not an
   absolute change. When used in this context, potential security
   threats from their use is limited but not necessarily completely
   eliminated. A faked link-layer hint can still be used to launch a
   denial-of service attack on the host and the associated network.
   Secure generation and delivery of these hints MUST be ensured. This
   is a subject for lower-layer designs and therefore it is outside the
   scope of this document.


6.0  References

   [3GPP2/TIA] "IS-835 - cdma2000 Wireless IP Network Standard"

   [3GPP2/TIA] "IS-2001 û Interoperability Specification (IOS) for
   cdma2000 Access Network Interfaces"

   [GPRS-AT] "Digital cellular telecommunications system (Phase 2+); AT
   command set for GPRS Mobile Equipment (ME), (GSM 07.07 version 7.8.0
   Release 98).

   [GPRS] "Digital cellular telecommunications system (Phase 2+);
   General Packet Radio Service (GPRS) Service description; Stage 2",
   (3GPP TS 03.60 version 7.9.0 Release 98).

   [GPRS-LINK]"Digital cellular telecommunications system (Phase 2+);
   Radio subsystem link control", (GSM 03.05 version 7.0.0 Release 98).

   [IAPP] IEEE Std. 802.11f/D3, Draft supplement to IEEE Std 802.11,
   1999 Edition, "Recommanded Practice for Multi-Vendor Access Point
   Interoperability via an Inter-Access Point Protocol Across
   Distribution Systems Supporting IEEE 802.11 Operation", January
   2002.

   [802.11b] IEEE Std 802 Part 11, "Information technology -
   Telecomunications anbd information exchange between systems - Local
   and metropolitan area networks - Specific requirements - Part 11:
   Wireless Lan Medium Access Control (MAC) And Physical Layer (PHY)
   Specifications", August 1999.

   [802.11a] IEEE Std 802.11a-1999, supplement to IEEE Std 802.11-1999,
   "Part 11: Wireless MAN Medium Access Control (MAC) and Physical
   Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ
   band, September 1999.

   [802.11g] IEEE Std 802.11g-2003, Amendment to IEEE Std 802.11, 1999
   edition, "Part 11: Wireless "Part 11: Wireless MAN Medium Access
   Control (MAC) and Physical Layer (PHY) specifications. Amendemnt 4:



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   Further Higher Data Rate Extension in the 2.4 GHz Band, June 2003.

   [MIPv4] Perkins, C., "IP Mobility Support for IPv4", RFC3344, August
   2002.

   [MIPv6] Perkins, C. and J. Arko, "Mobility Support in IPv6", I-D
   draft-ietf-mobileip-ipv6-24.txt, June 2003.


7.0  Acknowledgements

   The authors would like to acknowledge Sanjeev Athalye (Qualcomm) and
   Muhammad Mukarram bin Tariq (DoCoMo USA Labs) for their useful
   comments and suggestions.


Appendix A

   This section describes the additional events and the associated
   information with the GPRS networks. Unlike the PDP context changes,
   the following events do not directly imply potential IP
   configuration change.


A.1. Routing Area and Cell Change

   The GPRS Radio Sub-System is organized in sets of Routing Areas
   (RAs), each set managed by a unique SGSN. The RAs are in turn
   divided into cells.

   A GPRS MT detects that it has entered a new cell by comparing the
   cell's identity just received with the cell identity stored in the
   MT's Mobility Management context. A cell update procedure with the
   network then takes place between the MT and the SGSN.

   If the new cell is inside a new RA, the MT detects it by
   periodically comparing the RA identifier stored in its MM context
   with that just received from the new cell and initiates a RA update
   procedure with the SGSN. If the SGSN receiving the RA update request
   realizes that the old RA is not handled by itself, then it knows
   that an inter-SGSN RA update is required. Necessary updates are
   performed in the GPRS Network Sub-System:

   - The new SGSN starts a handover procedure whereby it requests and
   receives the MM and PDP contexts from the old SGSN of the MT, before
   packet tunneling can start to the GGSN.
   - The MT location is updated in the network.


A.1.1. Hints





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   The MT initiates the RA update procedure by sending a "Routing Area
   Update Request" to the new SGSN. This is potentially a hint to the
   IP layer advertising an imminent change of SGSN (GPRS Access Point).

   The network confirms that it has updated the RA (SGSN) by sending a
   "Routing Area Update Accept" to the MT. The MN can utilize this
   message as a hint the MT's SGSN has changed following the handover
   procedure.

   The accept message comes along with an update of the MM context with
   new information as described below:

   - New P-TMSI
   - New Cell Identity
   - New RA Identity
   - New ciphering algorithm, key and sequence number

A.2. Sub-link-layer Hints

   Some of the information, such as the signal quality (e.g., channel's
   Bit Error Rate) and signal level, are not link-layer information but
   rather GPRS Radio Link Control (RLC) parameters. Nonetheless, their
   knowledge at the network-layer might be useful to assess the
   pertinence of deciding to attach in the case where their values are
   below the limits which are deemed necessary or required for the
   attachment.

   The RLC parameters corresponding to the two identified hints are:

   - RXLEV, the received signal strength
   - RXQUAL, the received signal quality


Authors' Addresses

   Alper E. Yegin
   DoCoMo Communications Laboratories USA, Inc.
   181 Metro Drive, Suite 300         Phone: +1 408 451 4743
   San Jose, CA 95110                   Fax: +1 408 451 1090
   USA                                email: alper@docomolabs-usa.com

   Eric Njedjou
   France Telecom R & D
   4, Rue du Clos Courtel BP 91226    Phone: +33 299124202
   35512 Cesson-S‰vign‰            email: eric.njedjou@france-
   telecom.com
   France

   Siva Veerepalli
   Qualcomm, Incorporated.
   5775 Morehouse Drive              Phone : +1 858 658 4628
   San Diego, CA 92131                 Fax : +1 734 661 1812



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   USA                               email : sivav@qualcomm.com

   Nicolas Montavont
   LSIIT - University Louis Pasteur   Phone: (33) 390244587
   Pole API, bureau C430         email: montavont@dpt-info.u-strasbg.fr
   Boulevard Sebastien Brant       URI: www-r2.u-strasbg.fr/~montavont
   Illkirch  67400
   France

   Thomas Noel
   LSIIT - University Louis Pasteur   Phone: (33) 390244592
   Pole API, bureau C428              email: noel@dpt-info.u-strasbg.fr
   Boulevard Sebastien Brant            URI: www-r2.u-strasbg.fr/~noel/
   Illkirch  67400
   France


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