Subject: [mobile-ip] newest text for MIPv6 movement detection

From: "Ed Remmell" <[email-masked]>
Date: Fri, 9 May 2003 15:09:59 -0700

Jari & Erik -

Here is the new MIPv6 movement detection section, incorporating the latest
feedback from the list:

Definitions added to Section 3.2:

    L2 handover - process by which the mobile node changes from one
    link-layer connection to another. For example, a change of wireless
    access point is an L2 handover.

    L3 handover - subsequent to an L2 handover, a mobile node detects
    a change in an on-link subnet prefix that would require a
    change in primary care-of address. For example, a change of access
    router subsequent to a change of wireless access point typically
    results in an L3 handover.

11.5.1 Movement Detection

    The primary goal of movement detection is to detect L3 handovers.
    This section does not attempt to specify a fast movement detection
    algorithm which will function optimally for all types of
    applications, link-layers and deployment scenarios; instead, it
    describes a generic method that uses the facilities of IPv6 Neighbor
    Discovery, including Router Discovery and Neighbor Unreachability
    Detection. At the time of this writing, this method is considered
    well enough understood to recommend for standardization, however it
    is expected that future versions of this specification or other
    specifications may contain updated versions of the movement
    detection algorithm that have better performance.

    Generic move detection uses Neighbor Unreachability Detection to
    detect when the default router is no longer bi-directionally
    reachable, in which case the mobile node must discover a new default
    router (usually on a new link). However, this detection only occurs
    when the mobile node has packets to send, and in the absence of
    frequent Router Advertisements or indications from the link-layer,
    the mobile node might become unaware of an L3 handover that
    occurred. Therefore, the mobile node should supplement this method
    with other information whenever it is available to the mobile node
    (e.g., from lower protocol layers).

    When the mobile node detects an L3 handover, it performs Duplicate
    Address Detection [13] on its link-local address, Router Discovery
    to select a new default router, and Prefix Discovery with that
    router to form new care-of address(es) as described in Section
    11.5.2. It then registers its new primary care-of address with its
    home agent as described in Section 11.7.1. After updating its
    home registration, the mobile node then updates associated mobility
    bindings in correspondent nodes that it is performing route
    optimization with as specified in Section 11.7.2.

    Due to the temporary packet flow disruption and signaling overhead
    involved in updating mobility bindings, the mobile node should avoid
    performing a L3 handover until it is strictly necessary.
    Specifically, if the MN detects that the currently selected default
    router on the old link is still bi-directionally reachable, it
    should generally continue to use that router rather than switch away
    from it to use a new default router.

    Mobile nodes can use the information in received Router
    Advertisements to detect L3 handovers. In doing so the mobile node
    needs to consider the following issues:

    o  There might be multiple routers on the same link, thus hearing a
       new router does not necessarily constitute an L3 handover.

    o  When there are multiple routers on the same link they might
       advertise different prefixes.  Thus even hearing a new router
       with a new prefix might not be a reliable indication of an L3
       handover.

    o  The link-local addresses of routers are not globally unique,
       hence after completing an L3 handover the mobile node might
       continue to receive Router Advertisements with the same
       link-local source address.  This might be common if routers use
       the same link-local address on multiple interfaces.  This issue
       can be avoided when routers use the Router Address (R) bit, since
       that provides a global address of the router.

    In addition, the mobile node should consider the following events
    as indications that an L3 handover may have occurred:

    o  With some types of networks, notification that an L2 handover has
       occurred might be obtained from lower layer protocols or device
       driver software within the mobile node.  However, for some
       wireless technologies and deployment scenarios, an L2 handover
       may not always require an L3 handover, for example, if both the
       old link and the new link share the same access router and thus
       the same on-link prefixes. Therefore, link-layer indications do
       not guarantee that an L3 handover has occurred.

    o  If Router Advertisements that the mobile node receives include
       an Advertisement Interval option, the mobile node may use its
       Advertisement Interval field as an indication of the frequency
       with which it should expect to continue to receive future
       Advertisements from that router.  This field specifies the
       minimum rate (the maximum amount of time between successive
       Advertisements) that the mobile node should expect.  If this
       amount of time elapses without the mobile node receiving any
       Advertisement from this router, the mobile node can be sure that
       at least one Advertisement sent by the router has been lost.
       The mobile node can then implement its own policy to determine
       how many lost Advertisements from its current default router
       constitute a L3 handover indication.

    o  Neighbor Unreachability Detection determines that the default
       router is no longer reachable.

    Upon receiving such indications, the mobile needs to send Router
    Solicitations to discover routers and prefixes on the new link.

Thanks.
- Ed Remmell
Elmic Systems USA, Inc.

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