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RFC 5213

 
 
 

Proxy Mobile IPv6

Part 3 of 4, p. 41 to 67
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6.  Mobile Access Gateway Operation

   The Proxy Mobile IPv6 protocol described in this document introduces
   a new functional entity, the mobile access gateway (MAG).  The mobile
   access gateway is the entity that is responsible for detecting the
   mobile node's movements to and from the access link and sending the
   Proxy Binding Update messages to the local mobility anchor.  In
   essence, the mobile access gateway performs mobility management on
   behalf of a mobile node.

   The mobile access gateway is a function that typically runs on an
   access router.  However, implementations MAY choose to split this
   function and run it across multiple systems.  The specifics on how
   that is achieved or the signaling interactions between those
   functional entities are beyond the scope of this document.

   The mobile access gateway has the following key functional roles:

   o  It is responsible for detecting the mobile node's movements on the
      access link and for initiating the mobility signaling with the
      mobile node's local mobility anchor.

   o  Emulation of the mobile node's home link on the access link by
      sending Router Advertisement messages containing the mobile node's
      home network prefix(es), each prefix carried using the Prefix
      Information option [RFC4861].

   o  Responsible for setting up the forwarding for enabling the mobile
      node to configure one or more addresses from its home network
      prefix(es) and use it from the attached access link.

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6.1.  Extensions to Binding Update List Entry Data Structure

   Every mobile access gateway MUST maintain a Binding Update List.
   Each entry in the Binding Update List represents a mobile node's
   mobility binding with its local mobility anchor.  The Binding Update
   List is a conceptual data structure, described in Section 11.1 of
   [RFC3775].

   For supporting this specification, the conceptual Binding Update List
   entry data structure needs be extended with the following additional
   fields.

   o  The identifier of the attached mobile node, MN-Identifier.  This
      identifier is acquired during the mobile node's attachment to the
      access link through mechanisms outside the scope of this document.

   o  The link-layer identifier of the mobile node's connected
      interface.  This can be acquired from the received Router
      Solicitation messages from the mobile node or during the mobile
      node's attachment to the access network.  This is typically a
      link-layer identifier conveyed by the mobile node; however, the
      specific details on how that is conveyed is out of scope for this
      specification.  If this identifier is not available, this variable
      length field MUST be set to two (octets) and MUST be initialized
      to a value of ALL_ZERO.

   o  A list of IPv6 home network prefixes assigned to the mobile node's
      connected interface.  The home network prefix(es) may have been
      statically configured in the mobile node's policy profile, or, may
      have been dynamically allocated by the local mobility anchor.
      Each of these prefix entries will also include the corresponding
      prefix length.

   o  The Link-local address of the mobile access gateway on the access
      link shared with the mobile node.

   o  The IPv6 address of the local mobility anchor serving the attached
      mobile node.  This address is acquired from the mobile node's
      policy profile or from other means.

   o  The interface identifier (if-id) of the point-to-point link
      between the mobile node and the mobile access gateway.  This is
      internal to the mobile access gateway and is used to associate the
      Proxy Mobile IPv6 tunnel to the access link where the mobile node
      is attached.

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   o  The tunnel interface identifier (tunnel-if-id) of the bi-
      directional tunnel between the mobile node's local mobility anchor
      and the mobile access gateway.  This is internal to the mobile
      access gateway.  The tunnel interface identifier is acquired
      during the tunnel creation.

6.2.  Mobile Node's Policy Profile

   A mobile node's policy profile contains the essential operational
   parameters that are required by the network entities for managing the
   mobile node's mobility service.  These policy profiles are stored in
   a local or a remote policy store.  The mobile access gateway and the
   local mobility anchor MUST be able to obtain a mobile node's policy
   profile.  The policy profile MAY also be handed over to a serving
   mobile access gateway as part of a context transfer procedure during
   a handoff or the serving mobile access gateway MAY be able to
   dynamically generate this profile.  The exact details on how this
   achieved is outside the scope of this document.  However, this
   specification requires that a mobile access gateway serving a mobile
   node MUST have access to its policy profile.

   The following are the mandatory fields of the policy profile:

   o  The mobile node's identifier (MN-Identifier)

   o  The IPv6 address of the local mobility anchor (LMAA)

   The following are the optional fields of the policy profile:

   o  The mobile node's IPv6 home network prefix(es) assigned to the
      mobile node's connected interface.  These prefixes have to be
      maintained on a per-interface basis.  There can be multiple unique
      entries for each interface of the mobile node.  The specific
      details on how the network maintains this association between the
      prefix set and the interfaces, specially during the mobility
      session handoff between interfaces, is outside the scope of this
      document.

   o  The mobile node's IPv6 home network Prefix lifetime.  This
      lifetime will be the same for all the hosted prefixes on the link,
      as they all are part of one mobility session.  This value can also
      be the same for all the mobile node's mobility sessions.

   o  Supported address configuration procedures (Stateful, Stateless,
      or both) for the mobile node in the Proxy Mobile IPv6 domain

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6.3.  Supported Access Link Types

   This specification supports only point-to-point access link types,
   and thus, it assumes that the mobile node and the mobile access
   gateway are the only two nodes on the access link.  The link is
   assumed to have multicast capability.

   This protocol may also be used on other link types, as long as the
   link is configured in such a way that it emulates point-to-point
   delivery between the mobile node and the mobile access gateway for
   all the protocol traffic.

   It is also necessary to be able to identify mobile nodes attaching to
   the link.  Requirements relating to this are covered in Section 6.6.

   Finally, while this specification can operate without link-layer
   indications of node attachment and detachment to the link, the
   existence of such indications either on the network or mobile node
   side improves the resulting performance.

6.4.  Supported Address Configuration Modes

   A mobile node in the Proxy Mobile IPv6 domain can configure one or
   more global IPv6 addresses on its interface (using Stateless,
   Stateful address autoconfiguration procedures or manual address
   configuration) from the hosted prefix(es) on that link.  The Router
   Advertisement messages sent on the access link specify the address
   configuration methods permitted on that access link for that mobile
   node.  However, the advertised flags, with respect to the address
   configuration, will be consistent for a mobile node, on any of the
   access links in that Proxy Mobile IPv6 domain.  Typically, these
   configuration settings will be based on the domain-wide policy or
   based on a policy specific to each mobile node.

   When stateless address autoconfiguration is supported on the access
   link, the mobile node can generate one or more IPv6 addresses from
   the hosted prefix(es) by standard IPv6 mechanisms such as Stateless
   Autoconfiguration [RFC4862] or Privacy extensions [RFC4941].

   When stateful address autoconfiguration is supported on the link, the
   mobile node can obtain the address configuration from the DHCP server
   located in the Proxy Mobile IPv6 domain, by standard DHCP mechanisms,
   as specified in [RFC3315].  The obtained address(es) will be from its
   home network prefix(es).  Section 6.11 specifies the details on how
   this configuration can be achieved.

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   Additionally, other address configuration mechanisms specific to the
   access link between the mobile node and the mobile access gateway may
   also be used for delivering the address configuration to the mobile
   node.  This specification does not modify the behavior of any of the
   standard IPv6 address configuration mechanisms.

6.5.  Access Authentication and Mobile Node Identification

   When a mobile node attaches to an access link connected to the mobile
   access gateway, the deployed access security protocols on that link
   SHOULD ensure that the network-based mobility management service is
   offered only after authenticating and authorizing the mobile node for
   that service.  The exact specifics on how this is achieved or the
   interactions between the mobile access gateway and the access
   security service are outside the scope of this document.  This
   specification goes with the stated assumption of having an
   established trust between the mobile node and the mobile access
   gateway before the protocol operation begins.

6.6.  Acquiring Mobile Node's Identifier

   All the network entities in a Proxy Mobile IPv6 domain MUST be able
   to identify a mobile node, using its MN-Identifier.  This identifier
   MUST be stable and unique across the Proxy Mobile IPv6 domain.  The
   mobility entities in the Proxy Mobile IPv6 domain MUST be able to use
   this identifier in the signaling messages and unambiguously identify
   a given mobile node.  The following are some of the considerations
   related to this MN-Identifier.

   o  The MN-Identifier is typically obtained as part of the access
      authentication or from a notified network attachment event.  In
      cases where the user identifier authenticated during access
      authentication uniquely identifies a mobile node, the MN-
      Identifier MAY be the same as the user identifier.  However, the
      user identifier MUST NOT be used if it identifies a user account
      that can be used from more than one mobile node operating in the
      same Proxy Mobile IPv6 domain.

   o  In some cases, the obtained identifier, as part of the access
      authentication, can be a temporary identifier and further that
      temporary identifier may be different at each re-authentication.
      However, the mobile access gateway MUST be able to use this
      temporary identifier and obtain the mobile node's stable
      identifier from the policy store.  For instance, in AAA-based
      systems, the Remote Authentication Dial-In User Service (RADIUS)
      attribute, Chargeable-User-Identifier [RFC4372] may be used, as
      long as it uniquely identifies a mobile node, and not a user
      account that can be used with multiple mobile nodes.

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   o  In some cases and for privacy reasons, the MN-Identifier that the
      policy store delivers to the mobile access gateway may not be the
      true identifier of the mobile node.  However, the mobility access
      gateway MUST be able to use this identifier in the signaling
      messages exchanged with the local mobility anchor.

   o  The mobile access gateway MUST be able to identify the mobile node
      by its MN-Identifier, and it MUST be able to associate this
      identity to the point-to-point link shared with the mobile node.

6.7.  Home Network Emulation

   One of the key functions of a mobile access gateway is to emulate the
   mobile node's home network on the access link.  It must ensure the
   mobile node does not detect any change with respect to its layer-3
   attachment even after it changes its point of attachment in that
   Proxy Mobile IPv6 domain.

   For emulating the mobile node's home link on the access link, the
   mobile access gateway must be able to send Router Advertisement
   messages advertising the mobile node's home network prefix(es)
   carried using the Prefix Information option(s) [RFC4861] and with
   other address configuration parameters consistent with its home link
   properties.  Typically, these configuration settings will be based on
   the domain-wide policy or based on a policy specific to each mobile
   node.

   Typically, the mobile access gateway learns the mobile node's home
   network prefix(es) details from the received Proxy Binding
   Acknowledgement message, or it may obtain them from the mobile node's
   policy profile.  However, the mobile access gateway SHOULD send the
   Router Advertisements advertising the mobile node's home network
   prefix(es) only after successfully completing the binding
   registration with the mobile node's local mobility anchor.

   When advertising the home network prefix(es) in the Router
   Advertisement messages, the mobile access gateway MAY set the prefix
   lifetime value for the advertised prefix(es) to any chosen value at
   its own discretion.  An implementation MAY choose to tie the prefix
   lifetime to the mobile node's binding lifetime.  The prefix lifetime
   can also be an optional configuration parameter in the mobile node's
   policy profile.

6.8.  Link-local and Global Address Uniqueness

   A mobile node in the Proxy Mobile IPv6 domain, as it moves from one
   mobile access gateway to the other, will continue to detect its home
   network and does not detect a change of layer-3 attachment.  Every

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   time the mobile node attaches to a new link, the event related to the
   interface state change will trigger the mobile node to perform
   Duplicate Address Detection (DAD) operation on the link-local and
   global address(es).  However, if the mobile node is Detecting Network
   Attachment in IPv6 (DNAv6) enabled, as specified in [DNAV6], it may
   not detect the link change due to DNAv6 optimizations and may not
   trigger the duplicate address detection (DAD) procedure for its
   existing addresses, which may potentially lead to address collisions
   after the mobile node's handoff to a new link.

   The issue of address collision is not relevant to the mobile node's
   global address(es).  Since the assigned home network prefix(es) are
   for the mobile node's exclusive usage, no other node shares an
   address (other than Subnet-Router anycast address that is configured
   by the mobile access gateway) from the prefix(es), and so the
   uniqueness for the mobile node's global address is assured on the
   access link.

   The issue of address collision is however relevant to the mobile
   node's link-local addresses since the mobile access gateway and the
   mobile node will have link-local addresses configured from the same
   link-local prefix (FE80::/64).  This leaves a room for link-local
   address collision between the two neighbors (i.e., the mobile node
   and the mobile access gateway) on that access link.  For solving this
   problem, this specification requires that the link-local address that
   the mobile access gateway configures on the point-to-point link
   shared with a given mobile node be generated by the local mobility
   anchor and be stored in the mobile node's Binding Cache entry.  This
   address will not change for the duration of that mobile node's
   mobility session and can be provided to the serving mobile access
   gateway at every mobile node's handoff, as part of the Proxy Mobile
   IPv6 signaling messages.  The specific method by which the local
   mobility anchor generates the link-local address is out of scope for
   this specification.

   It is highly desirable that the access link on the mobile access
   gateway shared with the mobile node be provisioned in such a way that
   before the mobile node completes the DAD operation [RFC4862] on its
   link-local address, the mobile access gateway on that link is aware
   of its own link-local address provided by the local mobility anchor
   that it needs to use on that access link.  This essentially requires
   a successful completion of the Proxy Mobile IPv6 signaling by the
   mobile access gateway before the mobile node completes the DAD
   operation.  This can be achieved by ensuring that link-layer
   attachment does not complete until the Proxy Mobile IPv6 signaling is

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   completed.  Alternatively, network and local mobility anchor capacity
   and signaling retransmission timers can be provisioned in such a way
   that signaling is likely to complete during the default waiting
   period associated with the DAD process.

   Optionally, implementations MAY choose to configure a fixed link-
   local address across all the access links in a Proxy Mobile IPv6
   domain and without a need for carrying this address from the local
   mobility anchor to the mobile access gateway in the Proxy Mobile IPv6
   signaling messages.  The configuration variable
   FixedMAGLinkLocalAddressOnAllAccessLinks determines the enabled mode
   in that Proxy Mobile IPv6 domain.

6.9.  Signaling Considerations

6.9.1.  Binding Registrations

6.9.1.1.  Mobile Node Attachment and Initial Binding Registration

   1.   After detecting a new mobile node on its access link, the mobile
        access gateway MUST identify the mobile node and acquire its MN-
        Identifier.  If it determines that the network-based mobility
        management service needs to be offered to the mobile node, it
        MUST send a Proxy Binding Update message to the local mobility
        anchor.

   2.   The Proxy Binding Update message MUST include the Mobile Node
        Identifier option [RFC4283], carrying the MN-Identifier for
        identifying the mobile node.

   3.   The Home Network Prefix option(s) MUST be present in the Proxy
        Binding Update message.  If the mobile access gateway learns the
        mobile node's home network prefix(es) either from its policy
        store or from other means, the mobile access gateway MAY choose
        to request the local mobility anchor to allocate the specific
        prefix(es) by including a Home Network Prefix option for each of
        those requested prefixes.  The mobile access gateway MAY also
        choose to include just one Home Network Prefix option with the
        prefix value of ALL_ZERO, for requesting the local mobility
        anchor to do the prefix assignment.  However, when including a
        Home Network Prefix option with the prefix value of ALL_ZERO,
        there MUST be only one instance of the Home Network prefix
        option in the request.

   4.   The Handoff Indicator option MUST be present in the Proxy
        Binding Update message.  The Handoff Indicator field in the
        Handoff Indicator option MUST be set to a value indicating the
        handoff hint.

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        *  The Handoff Indicator field MUST be set to a value of 1
           (Attachment over a new interface) if the mobile access
           gateway determines (under the Handoff Indicator
           considerations specified in this section) that the mobile
           node's current attachment to the network over this interface
           is not as a result of a handoff of an existing mobility
           session (over the same interface or through a different
           interface), but as a result of an attachment over a new
           interface.  This essentially serves as a request to the local
           mobility anchor to create a new mobility session and not
           update any existing Binding Cache entry created for the same
           mobile node connected to the Proxy Mobile IPv6 domain through
           a different interface.

        *  The Handoff Indicator field MUST be set to a value of 2
           (Handoff between two different interfaces of the mobile node)
           if the mobile access gateway definitively knows the mobile
           node's current attachment is due to a handoff of an existing
           mobility session between two different interfaces of the
           mobile node.

        *  The Handoff Indicator field MUST be set to a value of 3
           (Handoff between mobile access gateways for the same
           interface) if the mobile access gateway definitively knows
           the mobile node's current attachment is due to a handoff of
           an existing mobility session between two mobile access
           gateways and for the same interface of the mobile node.

        *  The Handoff Indicator field MUST be set to a value of 4
           (Handoff state unknown) if the mobile access gateway cannot
           determine if the mobile node's current attachment is due to a
           handoff of an existing mobility session.

   5.   The mobile access gateway MUST apply the below considerations
        when choosing the value for the Handoff Indicator field.

        *  The mobile access gateway can choose to use the value 2
           (Handoff between two different interfaces of the mobile
           node), only when it knows that the mobile node has, on
           purpose, switched from one interface to another, and the
           previous interface is going to be disabled.  It may know this
           due to a number of factors.  For instance, most cellular
           networks have controlled handovers where the network knows
           that the host is moving from one attachment to another.  In
           this situation, the link-layer mechanism can inform the
           mobility functions that this is indeed a movement, not a new
           attachment.

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        *  Some link layers have link-layer identifiers that can be used
           to distinguish (a) the movement of a particular interface to
           a new attachment from (b) the attachment of a new interface
           from the same host.  Option value 3 (Handoff between mobile
           access gateways for the same interface) is appropriate in
           case (a) and a value of 1 (Attachment over a new interface)
           in case (b).

        *  The mobile access gateway MUST NOT set the option value to 2
           (Handoff between two different interfaces of the mobile node)
           or 3 (Handoff between mobile access gateways for the same
           interface) if it cannot be determined that the mobile node
           can move the address between the interfaces involved in the
           handover or that it is the same interface that has moved.
           Otherwise, Proxy Mobile IPv6-unaware hosts that have multiple
           physical interfaces to the same domain may suffer unexpected
           failures.

        *  Where no support from the link layer exists, the host and the
           network would need to inform each other about the intended
           movement.  The Proxy Mobile IPv6 protocol does not specify
           this and simply requires that knowledge about movements can
           be derived either from the link-layer or from somewhere else.
           The method by which this is accomplished is outside the scope
           of this specification.

   6.   Either the Timestamp option or a valid sequence number
        maintained on a per mobile node's mobility session basis as
        specified in [RFC3775] (if the Sequence-Number-based scheme is
        in use) MUST be present.  This can be determined based on the
        value of the configuration flag TimestampBasedApproachInUse.
        When Timestamp option is added to the message, the mobile access
        gateway SHOULD also set the Sequence Number field to a value of
        a monotonically increasing counter (maintained at each mobile
        access gateway and not to be confused with the per mobile node
        sequence number specified in [RFC3775]).  The local mobility
        anchor will ignore this field when there is a Timestamp option
        present in the request, but will return the same value in the
        Proxy Binding Acknowledgement message.  This will be useful for
        matching the reply to the request message.

   7.   The Mobile Node Link-layer Identifier option carrying the link-
        layer identifier of the currently attached interface MUST be
        present in the Proxy Binding Update message, if the mobile
        access gateway is aware of the same.  If the link-layer
        identifier of the currently attached interface is not known or
        if the identifier value is ALL_ZERO, this option MUST NOT be
        present.

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   8.   The Access Technology Type option MUST be present in the Proxy
        Binding Update message.  The access technology type field in the
        option SHOULD be set to the type of access technology by which
        the mobile node is currently attached to the mobile access
        gateway.

   9.   The Link-local Address option MUST be present in the Proxy
        Binding Update message only if the value of the configuration
        variable FixedMAGLinkLocalAddressOnAllAccessLinks is set to a
        value of ALL_ZERO; otherwise, the Link-local Address option MUST
        NOT be present in the request.  Considerations from Section 6.8
        MUST be applied when using the Link-local Address option.

        *  For querying the local mobility anchor to provide the link-
           local address that it should use on the point-to-point link
           shared with the mobile node, this option MUST be set to
           ALL_ZERO value.  This essentially serves as a request to the
           local mobility anchor to provide the link-local address that
           it can use on the access link shared with the mobile node.

   10.  The Proxy Binding Update message MUST be constructed as
        specified in Section 6.9.1.5.

   11.  If there is no existing Binding Update List entry for that
        mobile node, the mobile access gateway MUST create a Binding
        Update List entry for the mobile node upon sending the Proxy
        Binding Update message.

6.9.1.2.  Receiving Proxy Binding Acknowledgement

   On receiving a Proxy Binding Acknowledgement message (format
   specified in Section 8.2) from the local mobility anchor, the mobile
   access gateway MUST process the message as specified below.

   1.   The received Proxy Binding Acknowledgement message (a Binding
        Acknowledgement message with the (P) flag set to value of 1)
        MUST be authenticated as described in Section 4.  When IPsec is
        used for message authentication, the SPI in the IPsec header
        [RFC4306] of the received packet is needed for locating the
        security association, for authenticating the Proxy Binding
        Acknowledgement message.

   2.   The mobile access gateway MUST observe the rules described in
        Section 9.2 of [RFC3775] when processing Mobility Headers in the
        received Proxy Binding Acknowledgement message.

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   3.   The mobile access gateway MUST apply the considerations
        specified in Section 5.5 for processing the Sequence Number
        field and the Timestamp option (if present) in the message.

   4.   The mobile access gateway MUST ignore any checks, specified in
        [RFC3775], related to the presence of a Type 2 Routing header in
        the Proxy Binding Acknowledgement message.

   5.   The mobile access gateway MAY use the mobile node identifier
        present in the Mobile Node Identifier option for matching the
        response to the request messages that it sent recently.
        However, if there is more than one request message in its
        request queue for the same mobile node, the sequence number
        field can be used for identifying the exact message from those
        messages.  There are other ways to achieve this and
        implementations are free to adopt the best approach that suits
        their implementation.  Additionally, if the received Proxy
        Binding Acknowledgement message does not match any of the Proxy
        Binding Update messages that it sent recently, the message MUST
        be ignored.

   6.   If the received Proxy Binding Acknowledgement message has any
        one or more of the following options, Handoff Indicator option,
        Access Technology Type option, Mobile Node Link-layer Identifier
        option, Mobile Node Identifier option, carrying option values
        that are different from the option values present in the
        corresponding request (Proxy Binding Update) message, the
        message MUST be ignored as the local mobility anchor is expected
        to echo back all these listed options and with the same option
        values in the reply message.  In this case, the mobile access
        gateway MUST NOT retransmit the Proxy Binding Update message
        until an administrative action is taken.

   7.   If the received Proxy Binding Acknowledgement message has the
        Status field value set to PROXY_REG_NOT_ENABLED (Proxy
        registration not enabled for the mobile node), the mobile access
        gateway SHOULD NOT send a Proxy Binding Update message again for
        that mobile node until an administrative action is taken.  It
        MUST deny the mobility service to that mobile node.

   8.   If the received Proxy Binding Acknowledgement message has the
        Status field value set to TIMESTAMP_LOWER_THAN_PREV_ACCEPTED
        (Timestamp value lower than previously accepted value), the
        mobile access gateway SHOULD try to register again to reassert
        the mobile node's presence on its access link.  The mobile
        access gateway is not specifically required to synchronize its
        clock upon receiving this error code.

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   9.   If the received Proxy Binding Acknowledgement message has the
        Status field value set to TIMESTAMP_MISMATCH (Invalid timestamp
        value), the mobile access gateway SHOULD try to register again
        only after it has synchronized its clock to a common time source
        that is used by all the mobility entities in that domain for
        their clock synchronization.  The mobile access gateway SHOULD
        NOT synchronize its clock to the local mobility anchor's system
        clock, based on the timestamp present in the received message.

   10.  If the received Proxy Binding Acknowledgement message has the
        Status field value set to NOT_AUTHORIZED_FOR_HOME_NETWORK_PREFIX
        (The mobile node is not authorized for one or more of the
        requesting home network prefixes), the mobile access gateway
        SHOULD NOT request the same prefix(es) again, but MAY request
        the local mobility anchor to do the assignment of prefix(es) by
        including only one Home Network Prefix option with the prefix
        value set to ALL_ZERO.

   11.  If the received Proxy Binding Acknowledgement message has the
        Status field value set to any value greater than or equal to 128
        (i.e., if the binding is rejected), the mobile access gateway
        MUST NOT advertise the mobile node's home network prefix(es) in
        the Router Advertisement messages sent on that access link and
        MUST deny the mobility service to the mobile node by not
        forwarding any packets received from the mobile node using an
        address from the home network prefix(es).  It MAY also tear down
        the point-to-point link shared with the mobile node.

   12.  If the received Proxy Binding Acknowledgement message has the
        Status field value set to 0 (Proxy Binding Update accepted), the
        mobile access gateway MUST establish a bi-directional tunnel to
        the local mobility anchor (if there is no existing bi-
        directional tunnel to that local mobility anchor).
        Considerations from Section 5.6.1 MUST be applied for managing
        the dynamically created bi-directional tunnel.

   13.  The mobile access gateway MUST set up the route for forwarding
        the packets received from the mobile node using address(es) from
        its home network prefix(es) through the bi-directional setup for
        that mobile node.  The created tunnel and the routing state MUST
        result in the forwarding behavior on the mobile access gateway
        as specified in Section 6.10.5.

   14.  The mobile access gateway MUST also update the Binding Update
        List entry to reflect the accepted binding registration values.
        It MUST also advertise the mobile node's home network prefix(es)
        as the hosted on-link prefixes, by including them in the Router
        Advertisement messages that it sends on that access link.

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   15.  If the received Proxy Binding Acknowledgement message has the
        address in the Link-local Address option set to a NON_ZERO
        value, the mobile access gateway SHOULD configure that link-
        local address on that point-to-point link and SHOULD NOT
        configure any other link-local address without performing a DAD
        operation [RFC4862].  This will avoid any potential link-local
        address collisions on that access link.  However, if the link-
        local address generated by the local mobility anchor happens to
        be already in use by the mobile node on that link, the mobile
        access gateway MUST NOT use that address, but SHOULD configure a
        different link-local address.  It SHOULD also upload this link-
        local address to the local mobility anchor by immediately
        sending a Proxy Binding Update message and by including this
        address in the Link-local Address option.

6.9.1.3.  Extending Binding Lifetime

   1.  For extending the lifetime of a currently registered mobile node
       (i.e., after a successful initial binding registration from the
       same mobile access gateway), the mobile access gateway can send a
       Proxy Binding Update message to the local mobility anchor with a
       new lifetime value.  This re-registration message MUST be
       constructed with the same set of options as the initial Proxy
       Binding Update message, under the considerations specified in
       Section 6.9.1.1.  However, the following exceptions apply.

   2.  There MUST be a Home Network Prefix option for each of the
       assigned home network prefixes assigned for that mobility session
       and with the prefix value in the option set to that respective
       prefix value.

   3.  The Handoff Indicator field in the Handoff Indicator option MUST
       be set to a value of 5 (Handoff state not changed - Re-
       Registration).

6.9.1.4.  Mobile Node Detachment and Binding De-Registration

   1.  If at any point the mobile access gateway detects that the mobile
       node has moved away from its access link, or if it decides to
       terminate the mobile node's mobility session, it SHOULD send a
       Proxy Binding Update message to the local mobility anchor with
       the lifetime value set to zero.  This de-registration message
       MUST be constructed with the same set of options as the initial
       Proxy Binding Update message, under the considerations specified
       in Section 6.9.1.1.  However, the following exceptions apply.

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   2.  There MUST be a Home Network Prefix option for each of the
       assigned home network prefixes assigned for that mobility session
       and with the prefix value in the option set to the respective
       prefix value.

   3.  The Handoff Indicator field in the Handoff Indicator option MUST
       be set to a value of 4 (Handoff state unknown).

   Either upon receipt of a Proxy Binding Acknowledgement message from
   the local mobility anchor with the Status field set to 0 (Proxy
   Binding Update Accepted), or after INITIAL_BINDACK_TIMEOUT [RFC3775]
   timeout waiting for the reply, the mobile access gateway MUST do the
   following:

   1.  It MUST remove the Binding Update List entry for the mobile node
       from its Binding Update List.

   2.  It MUST remove the created routing state for tunneling the mobile
       node's traffic.

   3.  If there is a dynamically created tunnel to the mobile node's
       local mobility anchor and if there are not other mobile nodes for
       which the tunnel is being used, then the tunnel MUST be deleted.

   4.  It MUST tear down the point-to-point link shared with the mobile
       node.  This action will force the mobile node to remove any IPv6
       address configuration on the interface connected to this point-
       to-point link.

6.9.1.5.  Constructing the Proxy Binding Update Message

   o  The mobile access gateway, when sending the Proxy Binding Update
      message to the local mobility anchor, MUST construct the message
      as specified below.

          IPv6 header (src=Proxy-CoA, dst=LMAA)
            Mobility header
               - BU /* P & A flags MUST be set to value 1 */
              Mobility Options
               - Mobile Node Identifier option            (mandatory)
               - Home Network Prefix option(s)            (mandatory)
               - Handoff Indicator option                 (mandatory)
               - Access Technology Type option            (mandatory)
               - Timestamp option                         (optional)
               - Mobile Node Link-layer Identifier option (optional)
               - Link-local Address option                (optional)

                Figure 12: Proxy Binding Update Message Format

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   o  The Source Address field in the IPv6 header of the message MUST be
      set to the global address configured on the egress interface of
      the mobile access gateway.  When there is no Alternate Care-of
      Address option present in the request, this address will be
      considered as the Proxy-CoA for this Proxy Binding Update message.
      However, when there is an Alternate Care-of Address option present
      in the request, this address will be not be considered as the
      Proxy-CoA, but the address in the Alternate Care-of Address option
      will be considered as the Proxy-CoA.

   o  The Destination Address field in the IPv6 header of the message
      MUST be set to the local mobility anchor address.

   o  The Mobile Node Identifier option [RFC4283] MUST be present.

   o  At least one Home Network Prefix option MUST be present.

   o  The Handoff Indicator option MUST be present.

   o  The Access Technology Type option MUST be present.

   o  The Timestamp option MAY be present.

   o  The Mobile Node Link-layer Identifier option MAY be present.

   o  The Link-local Address option MAY be present.

   o  If IPsec is used for protecting the signaling messages, the
      message MUST be protected, using the security association existing
      between the local mobility anchor and the mobile access gateway.

   o  Unlike in Mobile IPv6 [RFC3775], the Home Address option [RFC3775]
      MUST NOT be present in the IPv6 Destination Options extension
      header of the Proxy Binding Update message.

6.9.2.  Router Solicitation Messages

   A mobile node may send a Router Solicitation message on the access
   link shared with the mobile access gateway.  The Router Solicitation
   message that the mobile node sends is as specified in [RFC4861].  The
   mobile access gateway, on receiving the Router Solicitation message
   or before sending a Router Advertisement message, MUST apply the
   following considerations.

   1.  The mobile access gateway, on receiving the Router Solicitation
       message, SHOULD send a Router Advertisement message containing
       the mobile node's home network prefix(es) as the on-link
       prefix(es).  However, before sending the Router Advertisement

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       message containing the mobile node's home network prefix(es), it
       SHOULD complete the binding registration process with the mobile
       node's local mobility anchor.

   2.  If the local mobility anchor rejects the Proxy Binding Update
       message, or, if the mobile access gateway failed to complete the
       binding registration process for whatever reason, the mobile
       access gateway MUST NOT advertise the mobile node's home network
       prefix(es) in the Router Advertisement messages that it sends on
       the access link.  However, it MAY choose to advertise a local
       visited network prefix to enable the mobile node for regular IPv6
       access.

   3.  The mobile access gateway SHOULD add the MTU option, as specified
       in [RFC4861], to the Router Advertisement messages that it sends
       on the access link.  This will ensure the mobile node on the link
       uses the advertised MTU value.  The MTU value SHOULD reflect the
       tunnel MTU for the bi-directional tunnel between the mobile
       access gateway and the local mobility anchor.  Considerations
       from Section 6.9.5 SHOULD be applied for determining the tunnel
       MTU value.

6.9.3.  Default-Router

   In Proxy Mobile IPv6, the mobile access gateway is the IPv6 default-
   router for the mobile node on the access link.  However, as the
   mobile node moves from one access link to another, the serving mobile
   access gateway on those respective links will send the Router
   Advertisement messages.  If these Router Advertisements are sent
   using a different link-local address or a different link-layer
   address, the mobile node will always detect a new default-router
   after every handoff.  For solving this problem, this specification
   requires all the mobile access gateways in the Proxy Mobile IPv6
   domain to use the same link-local and link-layer address on any of
   the access links wherever the mobile node attaches.  These addresses
   can be fixed addresses across the entire Proxy Mobile IPv6 domain,
   and all the mobile access gateways can use these globally fixed
   address on any of the point-to-point links.  The configuration
   variables FixedMAGLinkLocalAddressOnAllAccessLinks and
   FixedMAGLinkLayerAddressOnAllAccessLinks SHOULD be used for this
   purpose.  Additionally, this specification allows the local mobility
   anchor to generate the link-local address and provide it to the
   mobile access gateway as part of the signaling messages.

   However, both of these approaches (a link-local address generated by
   the local mobility anchor or when using a globally fixed link-local
   address) have implications on the deployment of SEcure Neighbor
   Discovery (SEND) [RFC3971].  In SEND, routers have certificates and

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   public key pairs, and their Router Advertisements are signed with the
   private keys of these key pairs.  When a number of different routers
   use the same addresses, the routers either all have to be able to
   construct these signatures for the same key pair, or the used key
   pair and the router's cryptographic identity must change after a
   movement.  Both approaches are problematic.  Sharing of private key
   information across multiple nodes in a PMIP6 domain is poor design
   from a security perspective.  And changing even the cryptographic
   identity of the router goes against the general idea of the Proxy
   Mobile IPv6 being as invisible to the hosts as possible.

   There is, however, ongoing work in the IETF to revise the SEND
   specifications.  It is suggested that these revisions also address
   the above problem.  Other revisions are needed to deal with other
   problematic cases (such as Neighbor Discovery proxies) before wide-
   spread deployment of SEND.

6.9.4.  Retransmissions and Rate Limiting

   The mobile access gateway is responsible for retransmissions and rate
   limiting the Proxy Binding Update messages that it sends to the local
   mobility anchor.  The Retransmission and the Rate Limiting rules are
   as specified in [RFC3775].  However, the following considerations
   MUST be applied.

   1.  When the mobile access gateway sends a Proxy Binding Update
       message, it should use the constant, INITIAL_BINDACK_TIMEOUT
       [RFC3775], for configuring the retransmission timer, as specified
       in Section 11.8 [RFC3775].  However, the mobile access gateway is
       not required to use a longer retransmission interval of
       InitialBindackTimeoutFirstReg, as specified in [RFC3775], for the
       initial Proxy Binding Update message.

   2.  If the mobile access gateway fails to receive a valid matching
       response for a registration or re-registration message within the
       retransmission interval, it SHOULD retransmit the message until a
       response is received.  However, the mobile access gateway MUST
       ensure the mobile node is still attached to the connected link
       before retransmitting the message.

   3.  As specified in Section 11.8 of [RFC3775], the mobile access
       gateway MUST use an exponential back-off process in which the
       timeout period is doubled upon each retransmission, until either
       the node receives a response or the timeout period reaches the
       value MAX_BINDACK_TIMEOUT [RFC3775].  The mobile access gateway
       MAY continue to send these messages at this slower rate
       indefinitely.

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   4.  If the Timestamp-based scheme is in use, the retransmitted Proxy
       Binding Update messages MUST use the latest timestamp.  If the
       Sequence Number scheme is in use, the retransmitted Proxy Binding
       Update messages MUST use a Sequence Number value greater than
       that was used for the previous transmission of this Proxy Binding
       Update message, just as specified in [RFC3775].

6.9.5.  Path MTU Discovery

   It is important that mobile node, mobile access gateway, and local
   mobility anchor have a correct understanding of MTUs.  When the
   mobile node uses the correct MTU, it can send packets that do not
   exceed the local link MTU and do not cause the tunneled packets from
   the mobile access gateway to be fragmented.  This is important both
   from the perspective of efficiency, as well as preventing hard-to-
   diagnose MTU problems.  The following are some of the considerations
   related to Path MTU discovery.

   o  The local mobility anchor and mobile access gateway MAY use the
      Path MTU discovery mechanisms, as specified in [RFC1981] or in
      [RFC4821], for determining the Path MTU (PMTU) for the (LMA-MAG)
      paths.  The specific discovery mechanism to be used in a given
      deployment can be configurable.

   o  The mobility entities MUST implement and SHOULD support ICMP-based
      Path MTU discovery mechanism, as specified in [RFC1981].  However,
      this mechanism may not work correctly if the Proxy Mobile IPv6
      network does not deliver or process ICMP Packet Too Big messages.

   o  The mobility entities MAY implement Packetization Layer Path MTU
      discovery mechanisms, as specified in [RFC4821], and use any
      application traffic as a payload for the PMTU discovery.  Neither
      the Proxy Mobile IPv6 protocol or the tunnel between the mobile
      access gateway and local mobility agent can easily be used for
      this purpose.  However, implementations SHOULD support at least
      the use of an explicit ICMP Echo Request/Response for this
      purpose.

   o  The mobility entities MAY choose to perform Path MTU discovery for
      all the (LMA-MAG) paths at the boot time and may repeat this
      operation periodically to ensure the Path MTU values have not
      changed for those paths.  If the dynamic PMTU discovery mechanisms
      fail to determine the Path MTU, an administratively configured
      default value MUST be used.

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   o  The IPv6 tunnel MTU for an established tunnel between the local
      mobility anchor and the mobile access gateway MUST be computed
      based on the determined Path MTU value for that specific path and
      the computation should be as specified in Section 6.7 of
      [RFC2473].

   o  The mobile access gateway SHOULD use the determined tunnel Path
      MTU value (for the tunnel established with the mobile node's local
      mobility anchor) as the MTU value in the MTU option that it sends
      in the Router Advertisements on the access link shared with the
      mobile node.  But, if the MTU value of the access link shared with
      the mobile node is lower than the determined Path MTU value, then
      the MTU of the access link MUST be used in the MTU option.

   o  If the mobile access gateway detects a change in the MTU value for
      any of the paths (LMA-MAG) and at any point of time, the
      corresponding tunnel MTU value MUST be updated to reflect the
      change in Path MTU value.  The adjusted tunnel MTU value (lower of
      the Path MTU and the access link MTU) SHOULD be notified to the
      impacted mobile nodes by sending additional Router Advertisement
      messages.  Additionally, the adjusted tunnel MTU value MUST be
      used in all the subsequent Router Advertisement messages as well.

6.10.  Routing Considerations

   This section describes how the mobile access gateway handles the
   traffic to/from the mobile node that is attached to one of its access
   interfaces.

                 Proxy-CoA                   LMAA
                    |                          |
    +--+          +---+                      +---+          +--+
    |MN|----------|MAG|======================|LMA|----------|CN|
    +--+          +---+                      +---+          +--+
                            IPv6 Tunnel

                    Figure 13: Proxy Mobile IPv6 Tunnel

6.10.1.  Transport Network

   As per this specification, the transport network between the local
   mobility anchor and the mobile access gateway is an IPv6 network.
   The document [IPV4-PMIP6] specifies the required extensions for
   negotiating IPv4 transport and the corresponding encapsulation mode.

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6.10.2.  Tunneling and Encapsulation Modes

   An IPv6 address that a mobile node uses from its home network
   prefix(es) is topologically anchored at the local mobility anchor.
   For a mobile node to use this address from an access network attached
   to a mobile access gateway, proper tunneling techniques have to be in
   place.  Tunneling hides the network topology and allows the mobile
   node's IPv6 datagram to be encapsulated as a payload of another IPv6
   packet and to be routed between the local mobility anchor and the
   mobile access gateway.  The Mobile IPv6 base specification [RFC3775]
   defines the use of IPv6-over-IPv6 tunneling [RFC2473] between the
   home agent and the mobile node, and this specification extends the
   use of the same tunneling mechanism for use between the local
   mobility anchor and the mobile access gateway.

   On most operating systems, a tunnel is implemented as a virtual
   point-to-point interface.  The source and the destination address of
   the two endpoints of this virtual interface along with the
   encapsulation mode are specified for this virtual interface.  Any
   packet that is routed over this interface gets encapsulated with the
   outer header as specified for that point-to-point tunnel interface.

   For creating a point-to-point tunnel to any local mobility anchor,
   the mobile access gateway may implement a tunnel interface with the
   Source Address field set to a global address on its egress interface
   (Proxy-CoA) and the destination address field set to the global
   address of the local mobility anchor (LMAA).

   The following is the supported packet encapsulation mode that can be
   used by the mobile access gateway and the local mobility anchor for
   routing mobile node's IPv6 datagrams.

   o  IPv6-In-IPv6 - IPv6 datagram encapsulated in an IPv6 packet
      [RFC2473].

   The companion document [IPV4-PMIP6] specifies other encapsulation
   modes for supporting IPv4 transport.

   o  IPv6-In-IPv4 - IPv6 datagram encapsulation in an IPv4 packet.  The
      details on how this mode is negotiated are specified in
      [IPV4-PMIP6].

   o  IPv6-In-IPv4-UDP - IPv6 datagram encapsulation in an IPv4 UDP
      packet.  This mode is specified in [IPV4-PMIP6].

   o  IPv6-In-IPv4-UDP-TLV - IPv6 datagram encapsulation in an IPv4 UDP
      packet with a TLV header.  This mode is specified in [IPV4-PMIP6].

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6.10.3.  Local Routing

   If there is data traffic between a visiting mobile node and a
   correspondent node that is locally attached to an access link
   connected to the mobile access gateway, the mobile access gateway MAY
   optimize on the delivery efforts by locally routing the packets and
   by not reverse tunneling them to the mobile node's local mobility
   anchor.  The flag EnableMAGLocalRouting MAY be used for controlling
   this behavior.  However, in some systems, this may have an
   implication on the mobile node's accounting and policy enforcement as
   the local mobility anchor is not in the path for that traffic and it
   will not be able to apply any traffic policies or do any accounting
   for those flows.

   This decision of path optimization SHOULD be based on the policy
   configured on the mobile access gateway, but enforced by the mobile
   node's local mobility anchor.  The specific details on how this is
   achieved are beyond of the scope of this document.

6.10.4.  Tunnel Management

   All the considerations mentioned in Section 5.6.1 for the tunnel
   management on the local mobility anchor apply for the mobile access
   gateway as well.

6.10.5.  Forwarding Rules

   Forwarding Packets Sent to the Mobile Node's Home Network:

   o  On receiving a packet from the bi-directional tunnel established
      with the mobile node's local mobility anchor, the mobile access
      gateway MUST use the destination address of the inner packet for
      forwarding it on the interface where the destination network
      prefix is hosted.  The mobile access gateway MUST remove the outer
      header before forwarding the packet.  Considerations from
      [RFC2473] MUST be applied for IPv6 decapsulation.  If the mobile
      access gateway cannot find the connected interface for that
      destination address, it MUST silently drop the packet.  For
      reporting an error in such a scenario, in the form of an ICMP
      control message, the considerations from [RFC2473] MUST be
      applied.

   o  On receiving a packet from a correspondent node that is connected
      to the mobile access gateway as a regular IPv6 host (see Section
      6.14) destined to a mobile node that is also locally attached, the
      mobile access gateway MUST check the flag EnableMAGLocalRouting to
      determine if the packet can be delivered directly to the mobile
      node.  If the mobile access gateway is not allowed to route the

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      packet directly, it MUST route the packet towards the local
      mobility anchor where the destination address is topologically
      anchored, else it can route the packet directly to the mobile
      node.

   Forwarding Packets Sent by the Mobile Node:

   o  On receiving a packet from a mobile node connected to its access
      link, the mobile access gateway MUST ensure that there is an
      established binding for that mobile node with its local mobility
      anchor before forwarding the packet directly to the destination or
      before tunneling the packet to the mobile node's local mobility
      anchor.

   o  On receiving a packet from a mobile node connected to its access
      link for a destination that is locally connected, the mobile
      access gateway MUST check the flag EnableMAGLocalRouting, to
      ensure the mobile access gateway is allowed to route the packet
      directly to the destination.  If the mobile access gateway is not
      allowed to route the packet directly, it MUST route the packet
      through the bi-directional tunnel established between itself and
      the mobile node's local mobility anchor.  Otherwise, it MUST route
      the packet directly to the destination.

   o  On receiving a packet from a mobile node connected to its access
      link, to a destination that is not directly connected, the packet
      MUST be forwarded to the local mobility anchor through the bi-
      directional tunnel established between itself and the mobile
      node's local mobility anchor.  However, the packets that are sent
      with the link-local source address MUST NOT be forwarded.

   o  The format of the tunneled packet is shown below.  Considerations
      from [RFC2473] MUST be applied for IPv6 encapsulation.  However,
      when using IPv4 transport, the format of the tunneled packet is as
      described in [IPV4-PMIP6].

        IPv6 header (src= Proxy-CoA, dst= LMAA  /* Tunnel Header */
           IPv6 header (src= MN-HoA, dst= CN )  /* Packet Header */
              Upper layer protocols             /* Packet Content*/

                  Figure 14: Tunneled Packet from MAG to LMA

   o  The format of the tunneled packet is shown below, when payload
      protection using IPsec is enabled for the mobile node's data
      traffic.  However, when using IPv4 transport, the format of the
      packet is as described in [IPV4-PMIP6].

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        IPv6 header (src= Proxy-CoA, dst= LMAA     /* Tunnel Header */
           ESP Header in tunnel mode               /* ESP Header */
              IPv6 header (src= MN-HoA, dst= CN )  /* Packet Header */
                 Upper layer protocols             /* Packet Content*/

      Figure 15: Tunneled Packet from MAG to LMA with Payload Protection

6.11.  Supporting DHCP-Based Address Configuration on the Access Link

   This section explains how Stateful Address Configuration using DHCP
   support can be enabled in a Proxy Mobile IPv6 domain.  It also
   identifies the required configuration in DHCP and mobility
   infrastructures for supporting this address configuration mode and
   also identifies the protocol interactions between these two systems.

   o  For supporting Stateful Address Configuration using DHCP, the DHCP
      relay agent [RFC3315] service MUST be supported on all the mobile
      access gateways in the Proxy Mobile IPv6 domain.  Further, as
      specified in Section 20 of [RFC3315], the DHCP relay agent should
      be configured to use a list of destination addresses, which MAY
      include unicast addresses, the All_DHCP_Servers multicast address,
      or other addresses as required in a given deployment.

   o  The DHCP infrastructure needs to be configured to assign addresses
      from each of the prefixes assigned to a link in that Proxy Mobile
      IPv6 domain.  The DHCP relay agent indicates the link to which the
      mobile node is attached by including an IPv6 address from any of
      the prefixes assigned to that link in the link-address field of
      the Relay Forward message.  Therefore, for each link in the Mobile
      IPv6 domain, the DHCP infrastructure will:

      *  be configured with a list of all of the prefixes associated
         with that link;

      *  identify the link to which the mobile node is attached by
         looking up the prefix for the link-address field in the Relay
         Forward message in the list of prefixes associated with each
         link;

      *  assign to the host an address from each prefix associated with
         the link to which the mobile node is attached.

      This DHCP infrastructure configuration requirement is identical to
      other IPv6 networks; other than receiving DHCP messages from a
      mobile node through different relay agents (MAGs) over time, the
      DHCP infrastructure will be unaware of the mobile node's
      capability with respect to mobility support.

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   o  The local mobility anchor needs to have the same awareness with
      respect to the links along with the associated prefixes in a Proxy
      Mobile IPv6 domain.  When a local mobility anchor assigns
      prefix(es) to a mobile node, it MUST assign all the prefixes
      associated with a given link and all of those assigned prefixes
      will remain as the home network prefixes for that mobile node
      throughout the life of that mobility session.  The serving mobile
      access gateway that hosts these prefixes is physically connected
      to that link and can function as the DHCP relay agent.  This
      common understanding between DHCP and mobility entities about all
      the links in the domain along with the associated prefixes
      provides the required coordination for allowing mobility entities
      to perform prefix assignment dynamically to a mobile node and
      still allow the DHCP infrastructure to perform address assignment
      for that mobile node only from its home network prefixes.

   o  When a mobile node sends a DHCP request message, the DHCP relay
      agent function on the mobile access gateway will set the link-
      address field in the DHCP message to an address in the mobile
      node's home network prefix (any one of the mobile node's home
      network prefixes assigned to that mobile node's attached
      interface).  The mobile access gateway can generate an
      autoconfiguration address from one of the mobile node's home
      network prefixes [RFC4862] and can use this address link-address
      option, so as to provide a hint to the DHCP Server for the link
      identification.  The DHCP server, on receiving the request from
      the mobile node, will allocate addresses from all the prefixes
      associated with that link (identified using the link-address field
      of the request).

   o  Once the mobile node obtains address(es), moves to a different
      link, and sends a DHCP request (at any time) for extending the
      DHCP lease, the DHCP relay agent on the new link will set the
      link-address field in the DHCP Relay Forward message to one of the
      mobile node's home network prefixes.  The DHCP server will
      identify the client from the Client-DUID option and will identify
      the link from the link-address option present in the request and
      will allocate the same address(es) as before.

   o  For correct operation of the model of network-based mobility
      management in which the host does not participate in any mobility
      management, the mobile node MUST always be assigned an identical
      set of IPv6 addresses regardless of the access link to which the
      mobile node is attached.  For example, the mobile access gateways

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      in the Proxy Mobile IPv6 domain should be configured so that DHCP
      messages from a mobile node will always be handled by the same
      DHCP server or by a server from the same group of coordinated DHCP
      servers serving that domain.  DHCP-based address configuration is
      not recommended for deployments in which the local mobility anchor
      and the mobile access gateway are located in different
      administrative domains.

6.12.  Home Network Prefix Renumbering

   If the mobile node's home network prefix(es) gets renumbered or
   becomes invalid during the middle of a mobility session, the mobile
   access gateway MUST withdraw the prefix(es) by sending a Router
   Advertisement message on the access link with zero prefix lifetime
   for the prefix(es) that is being renumbered.  Also, the local
   mobility anchor and the mobile access gateway MUST delete the created
   routing state for the renumbered prefix(es).  However, the specific
   details on how the local mobility anchor notifies the mobile access
   gateway about the mobile node's home network prefix(es) renumbering
   are outside the scope of this document.

6.13.  Mobile Node Detachment Detection and Resource Cleanup

   Before sending a Proxy Binding Update message to the local mobility
   anchor for extending the lifetime of a currently existing binding of
   a mobile node, the mobile access gateway MUST make sure the mobile
   node is still attached to the connected link by using some reliable
   method.  If the mobile access gateway cannot predictably detect the
   presence of the mobile node on the connected link, it MUST NOT
   attempt to extend the registration lifetime of the mobile node.
   Further, in such a scenario, the mobile access gateway SHOULD
   terminate the binding of the mobile node by sending a Proxy Binding
   Update message to the mobile node's local mobility anchor with
   lifetime value set to 0.  It MUST also remove any local state such as
   the Binding Update List entry created for that mobile node.

   The specific detection mechanism of the loss of a visiting mobile
   node on the connected link is specific to the access link between the
   mobile node and the mobile access gateway and is outside the scope of
   this document.  Typically, there are various link-layer-specific
   events specific to each access technology that the mobile access
   gateway can depend on for detecting the node loss.  In general, the
   mobile access gateway can depend on one or more of the following
   methods for the detection presence of the mobile node on the
   connected link:

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   o  Link-layer event specific to the access technology

   o  Session termination event on point-to-point link types

   o  IPv6 Neighbor Unreachability Detection event from IPv6 stack

   o  Notification event from the local mobility anchor

6.14.  Allowing Network Access to Other IPv6 Nodes

   In some Proxy Mobile IPv6 deployments, network operators may
   provision the mobile access gateway to offer network-based mobility
   management service only to some visiting mobile nodes and enable just
   regular IP access to some other nodes.  This requires the network to
   have control on when to enable network-based mobility management
   service to a mobile node and when to enable regular IPv6 access.
   This specification does not disallow such configuration.

   Upon detecting a mobile node on its access link and after policy
   considerations, the mobile access gateway MUST determine if network-
   based mobility management service should be offered to that mobile
   node.  If the mobile node is entitled to network-based mobility
   management service, then the mobile access gateway must ensure the
   mobile node does not detect any change with respect to its layer-3
   attachment, as explained in various sections of this specification.

   If the mobile node is not entitled to the network-based mobility
   management service, as determined from the policy considerations, the
   mobile access gateway MAY choose to offer regular IPv6 access to the
   mobile node, and in such a scenario, the normal IPv6 considerations
   apply.  If IPv6 access is enabled, the mobile node SHOULD be able to
   obtain IPv6 address(es) using the normal IPv6 address configuration
   procedures.  The obtained address(es) must be from a local visitor
   network prefix(es).  This essentially ensures that the mobile access
   gateway functions as a normal access router to a mobile node attached
   to its access link and without impacting its host-based mobility
   protocol operation.



(page 67 continued on part 4)

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