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For this RFC, original HTML is available from the RFC-Editor: RFC8885



Internet Engineering Task Force (IETF)                     CJ. Bernardos
Request for Comments: 8885                                A. de la Oliva
Category: Experimental                                              UC3M
ISSN: 2070-1721                                                 F. Giust
                                                                 Athonet
                                                              JC. Zúñiga
                                                                  SIGFOX
                                                               A. Mourad
                                                            InterDigital
                                                            October 2020


    Proxy Mobile IPv6 Extensions for Distributed Mobility Management

Abstract

   Distributed Mobility Management solutions allow networks to be set up
   in such a way that traffic is distributed optimally and centrally
   deployed anchors are not relied upon to provide IP mobility support.

   There are many different approaches to address Distributed Mobility
   Management -- for example, extending network-based mobility protocols
   (like Proxy Mobile IPv6) or client-based mobility protocols (like
   Mobile IPv6), among others.  This document follows the former
   approach and proposes a solution based on Proxy Mobile IPv6, in which
   mobility sessions are anchored at the last IP hop router (called the
   mobility anchor and access router).  The mobility anchor and access
   router is an enhanced access router that is also able to operate as a
   local mobility anchor or mobility access gateway on a per-prefix
   basis.  The document focuses on the required extensions to
   effectively support the simultaneous anchoring several flows at
   different distributed gateways.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8885.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Terminology
   3.  PMIPv6 DMM Extensions
     3.1.  Initial Registration
     3.2.  The CMD as PBU/PBA Relay
     3.3.  The CMD as MAAR Locator
     3.4.  The CMD as PBU/PBA Proxy
     3.5.  De-registration
     3.6.  Retransmissions and Rate Limiting
     3.7.  The Distributed Logical Interface (DLIF) Concept
   4.  Message Format
     4.1.  Proxy Binding Update
     4.2.  Proxy Binding Acknowledgement
     4.3.  Anchored Prefix Option
     4.4.  Local Prefix Option
     4.5.  Previous MAAR Option
     4.6.  Serving MAAR Option
     4.7.  DLIF Link-Local Address Option
     4.8.  DLIF Link-Layer Address Option
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Distributed Mobility Management (DMM) paradigm aims at minimizing
   the impact of currently standardized mobility management solutions,
   which are centralized (at least to a considerable extent) [RFC7333].

   The two most relevant examples of current IP mobility solutions are
   Mobile IPv6 [RFC6275] and Proxy Mobile IPv6 (PMIPv6) [RFC5213].
   These solutions offer mobility support at the cost of handling
   operations at a cardinal point (i.e., the mobility anchor) and
   burdening it with data forwarding and control mechanisms for a large
   number of users.  The mobility anchor is the home agent for Mobile
   IPv6 and the local mobility anchor for PMIPv6.  As stated in
   [RFC7333], centralized mobility solutions are prone to several
   problems and limitations: longer (sub-optimal) routing paths,
   scalability problems, signaling overhead (and most likely a longer
   associated handover latency), more complex network deployment, higher
   vulnerability due to the existence of a potential single point of
   failure, and lack of granularity of the mobility management service
   (i.e., mobility is offered on a per-node basis because it is not
   possible to define finer granularity policies, for example, on a per-
   application basis).

   The purpose of DMM is to overcome the limitations of the traditional
   centralized mobility management [RFC7333] [RFC7429]; the main concept
   behind DMM solutions is indeed bringing the mobility anchor closer to
   the mobile node (MN).  Following this idea, the central anchor is
   moved to the edge of the network and is deployed in the default
   gateway of the MN.  That is, the first elements that provide IP
   connectivity to a set of MNs are also the mobility managers for those
   MNs.  In this document, we call these entities Mobility Anchors and
   Access Routers (MAARs).

   This document focuses on network-based DMM; hence, the starting point
   is making PMIPv6 work in a distributed manner [RFC7429].  Mobility is
   handled by the network without the MN's involvement.  But differently
   from PMIPv6, when the MN moves from one access network to another,
   the router anchoring the MN's address may change, hence requiring
   signaling between the anchors to retrieve the MN's previous
   location(s).  Also, a key aspect of network-based DMM is that a
   prefix pool belongs exclusively to each MAAR in the sense that those
   prefixes are assigned by the MAAR to the MNs attached to it and are
   routable at that MAAR.  Prefixes are assigned to MNs attached to a
   MAAR at that time, but remain with those MNs as mobility occurs,
   remaining always routable at that MAAR as well as towards the MN
   itself.

   We consider partially distributed schemes, where only the data plane
   is distributed among access routers similar to mobile access gateways
   (MAGs), whereas the control plane is kept centralized towards a
   cardinal node (used as an information store), which is discharged
   from any route management and MN's data forwarding tasks.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Terminology

   The following terms used in this document are defined in the PMIPv6
   specification [RFC5213]:

      BCE:  Binding Cache Entry

      LMA:  Local Mobility Anchor

      MAG:  Mobile Access Gateway

      MN:  Mobile Node

      P-CoA:  Proxy Care-of Address

      PBA:  Proxy Binding Acknowledgement

      PBU:  Proxy Binding Update

   The following terms used in this document are defined in the Mobile
   IPv6 (MIPv6) specification [RFC6275]:

      CN:  Correspondent Node

   The following terms are used in this document:

   Home Control-Plane Anchor (Home-CPA or H-CPA):
      The Home-CPA function hosts the MN's mobility session.  There can
      be more than one mobility session for an MN, and those sessions
      may be anchored on the same or different Home-CPAs.  The Home-CPA
      will interface with the Home-DPA for managing the forwarding
      state.

   Home Data Plane Anchor (Home-DPA or H-DPA):
      The Home-DPA is the topological anchor for the MN's IP addresses
      and/or prefixes.  The Home-DPA is chosen by the Home-CPA on a
      session basis.  The Home-DPA is in the forwarding path for all the
      MN's IP traffic.

   Access Control Plane Node (Access-CPN or A-CPN):
      The Access-CPN is responsible for interfacing with the MN's Home-
      CPA and the Access-DPN.  The Access-CPN has a protocol interface
      to the Home-CPA.

   Access Data Plane Node (Access-DPN or A-DPN):
      The Access-DPN function is hosted on the first-hop router where
      the MN is attached.  This function is not hosted on a Layer 2 (L2)
      bridging device such as an eNode(B) or Access Point.

   The following terms are defined and used in this document:

   MAAR (Mobility Anchor and Access Router):
      First-hop router where the MNs attach.  It also plays the role of
      mobility manager for the IPv6 prefixes it anchors, running the
      functionalities of PMIP's MAG and LMA.  Depending on the prefix,
      it plays the role of Access-DPN, Home-DPA, and Access-CPN.

   CMD (Central Mobility Database):
      The node that stores the BCEs allocated for the MNs in the
      mobility domain.  It plays the role of Home-CPA.

   P-MAAR (Previous MAAR):
      When an MN moves to a new point of attachment, a new MAAR might be
      allocated as its anchor point for future IPv6 prefixes.  The MAAR
      that served the MN prior to new attachment becomes the P-MAAR.  It
      is still the anchor point for the IPv6 prefixes it had allocated
      to the MN in the past and serves as the Home-DPA for flows using
      these prefixes.  There might be several P-MAARs serving an MN in
      cases when the MN is frequently switching points of attachment
      while maintaining long-lasting flows.

   S-MAAR (Serving MAAR):
      The MAAR to which the MN is currently attached.  Depending on the
      prefix, it plays the role of Access-DPN, Home-DPA, and Access-CPN.

   Anchoring MAAR:
      A MAAR anchoring an IPv6 prefix used by an MN.

   DLIF (Distributed Logical Interface):
      It is a logical interface at the IP stack of the MAAR.  For each
      active prefix used by the MN, the S-MAAR has a DLIF configured
      (associated with each MAAR still anchoring flows).  In this way,
      an S-MAAR exposes itself towards each MN as multiple routers, one
      as itself and one per P-MAAR.

3.  PMIPv6 DMM Extensions

   The solution consists of decoupling the entities that participate in
   the data and the control planes: the data plane becomes distributed
   and managed by the MAARs near the edge of the network, while the
   control plane, besides those on the MAARs, relies on a central entity
   called the Central Mobility Database (CMD).  In the proposed
   architecture, the hierarchy present in PMIPv6 between LMA and MAG is
   preserved but with the following substantial variations:

   *  The LMA is discharged from the data forwarding role; only the
      Binding Cache and its management operations are maintained.
      Hence, the LMA is renamed as "CMD", which is therefore a Home-CPA.
      Also, the CMD is able to send and parse both PBU and PBA messages.

   *  The MAG is enriched with the LMA functionalities, hence the name
      Mobility Anchor and Access Router (MAAR).  It maintains a local
      Binding Cache for the MNs that are attached to it, and it is able
      to send and parse PBU and PBA messages.

   *  The Binding Cache will be extended to include information
      regarding P-MAARs where the MN was anchored and still retains
      active data sessions.

   *  Each MAAR has a unique set of global prefixes (which are
      configurable) that can be allocated by the MAAR to the MNs but
      must be exclusive to that MAAR, i.e., no other MAAR can allocate
      the same prefixes.

   The MAARs leverage the CMD to access and update information related
   to the MNs, which is stored as mobility sessions; hence, a
   centralized node maintains a global view of the network status.  The
   CMD is queried whenever an MN is detected joining/leaving the
   mobility domain.  It might be a fresh attachment, a detachment, or a
   handover, but as MAARs are not aware of past information related to a
   mobility session, they contact the CMD to retrieve the data of
   interest and eventually take the appropriate action.  The procedure
   adopted for the query and the message exchange sequence might vary to
   optimize the update latency and/or the signaling overhead.  Here, one
   method for the initial registration and three different approaches
   for updating the mobility sessions using PBUs and PBAs are presented.
   Each approach assigns a different role to the CMD:

   *  The CMD is a PBU/PBA relay;

   *  The CMD is only a MAAR locator;

   *  The CMD is a PBU/PBA proxy.

   The solution described in this document allows per-prefix anchoring
   decisions -- for example, to support the anchoring of some flows at a
   central Home-DPA (like a traditional LMA) or to enable an application
   to switch to the locally anchored prefix to gain route optimization,
   as indicated in [RFC8563].  This type of per-prefix treatment would
   potentially require additional extensions to the MAARs and signaling
   between the MAARs and the MNs to convey the per-flow anchor
   preference (central, distributed), which are not covered in this
   document.

   Note that an MN may move across different MAARs, which might result
   in several P-MAARs existing at a given moment of time, each of them
   anchoring a different prefix used by the MN.

3.1.  Initial Registration

   Initial registration is performed when an MN attaches to a network
   for the first time (rather than attaching to a new network after
   moving from a previous one).

   In this description (shown in Figure 1), it is assumed that:

   1.  The MN is attaching to MAAR1.

   2.  The MN is authorized to attach to the network.

   Upon MN attachment, the following operations take place:

   1.  MAAR1 assigns a global IPv6 prefix from its own prefix pool to
       the MN (Pref1).  It also stores this prefix (Pref1) in the
       locally allocated temporary BCE.

   2.  MAAR1 sends a PBU [RFC5213] with Pref1 and the MN's MN-ID to the
       CMD.

   3.  Since this is an initial registration, the CMD stores a BCE
       containing the MN-ID, Pref1, and MAAR1's address (as a Proxy-CoA)
       as the primary fields.

   4.  The CMD replies with a PBA with the usual options defined in
       PMIPv6 [RFC5213], meaning that the MN's registration is fresh and
       no past status is available.

   5.  MAAR1 stores the BCE described in (1) and unicasts a Router
       Advertisement (RA) to the MN with Pref1.

   6.  The MN uses Pref1 to configure an IPv6 address (IP1) (e.g., with
       stateless address autoconfiguration (SLAAC)).

   Note that:

   1.  Alternative IPv6 autoconfiguration mechanisms can also be used,
       though this document describes the SLAAC-based one.

   2.  IP1 is routable at MAAR1 in the sense that it is on the path of
       packets addressed to the MN.

   3.  MAAR1 acts as a plain router for packets destined to the MN as no
       encapsulation or special handling takes place.

   In the diagram shown in Figure 1 (and subsequent diagrams), the flow
   of packets is presented using '*'.

     +-----+      +---+                +--+
     |MAAR1|      |CMD|                |CN|
     +-----+      +---+                +*-+
        |           |                   *
       MN           |                   *     +---+
     attach.        |               *****    _|CMD|_
   detection        |         flow1 *       / +-+-+ \
        |           |               *      /    |    \
    local BCE       |               *     /     |     \
    allocation      |               *    /      |      \
        |--- PBU -->|           +---*-+-'    +--+--+    `+-----+
        |          BCE          |   * |      |     |     |     |
        |        creation       |MAAR1+------+MAAR2+-----+MAAR3|
        |<-- PBA ---|           |   * |      |     |     |     |
    local BCE       |           +---*-+      +-----+     +-----+
    finalized       |               *
        |           |         Pref1 *
        |           |              +*-+
        |           |              |MN|
        |           |              +--+

     Operations sequence                  Packet flow

                 Figure 1: First Attachment to the Network

   Note that the registration process does not change regardless of the
   CMD's modes (relay, locator, or proxy) described in the following
   sections.  The procedure is depicted in Figure 1.

3.2.  The CMD as PBU/PBA Relay

   Upon MN mobility, if the CMD behaves as a PBU/PBA relay, the
   following operations take place:

   1.  When the MN moves from its current point of attachment and
       attaches to MAAR2 (now the S-MAAR), MAAR2 reserves an IPv6 prefix
       (Pref2), stores a temporary BCE, and sends a PBU to the CMD for
       registration.

   2.  Upon PBU reception and BC lookup, the CMD retrieves an already
       existing entry for the MN and binds the MN-ID to its former
       location; thus, the CMD forwards the PBU to the MAAR indicated as
       Proxy-CoA (MAAR1) and includes a new mobility option to
       communicate the S-MAAR's global address to MAAR1 (defined as the
       Serving MAAR option in Section 4.6).  The CMD updates the P-CoA
       field in the BCE related to the MN with the S-MAAR's address.

   3.  Upon PBU reception, MAAR1 can install a tunnel on its side
       towards MAAR2 and the related routes for Pref1.  Then MAAR1
       replies to the CMD with a PBA (including the option mentioned
       before) to ensure that the new location has successfully changed.
       The PBA contains the prefix anchored at MAAR1 in the Home Network
       Prefix option.

   4.  The CMD, after receiving the PBA, updates the BCE and populates
       an instance of the P-MAAR list.  The P-MAAR list is an additional
       field on the BCE that contains an element for each P-MAAR
       involved in the MN's mobility session.  The list element contains
       the P-MAAR's global address and the prefix it has delegated.
       Also, the CMD sends a PBA to the new S-MAAR, which contains the
       previous Proxy-CoA and the prefix anchored to it embedded into a
       new mobility option called the Previous MAAR option (defined in
       Section 4.5).  Then, upon PBA arrival, a bidirectional tunnel can
       be established between the two MAARs, and new routes are set
       appropriately to recover the IP flow(s) carrying Pref1.

   5.  Now, packets destined for Pref1 are first received by MAAR1,
       encapsulated into the tunnel, and forwarded to MAAR2, which
       finally delivers them to their destination.  In the uplink, when
       the MN transmits packets using Pref1 as a source address, they
       are sent to MAAR2 (as it is the MN's new default gateway) and
       then tunneled to MAAR1, which routes them towards the next hop to
       the destination.  Conversely, packets carrying Pref2 are routed
       by MAAR2 without any special packet handling both for the uplink
       and downlink.

   +-----+      +---+      +-----+           +--+            +--+
   |MAAR1|      |CMD|      |MAAR2|           |CN|            |CN|
   +-----+      +---+      +-----+           +*-+            +*-+
      |           |           |               *               *
      |           |          MN               *     +---+     *
      |           |        attach.        *****    _|CMD|_    *
      |           |          det.   flow1 *       / +-+-+ \   *flow2
      |           |<-- PBU ---|           *      /    |    \  *
      |          BCE          |           *     /     | *******
      |        check+         |           *    /      | *    \
      |        update         |       +---*-+-'    +--+-*+    `+-----+
      |<-- PBU*---|           |       |   * |      |    *|     |     |
   route          |           |       |MAAR1|______|MAAR2+-----+MAAR3|
   update         |           |       |   **(______)**  *|     |     |
      |--- PBA*-->|           |       +-----+      +-*--*+     +-----+
      |         BCE           |                      *  *
      |        update         |                Pref1 *  *Pref2
      |           |--- PBA*-->|                     +*--*+
      |           |         route         ---move-->|*MN*|
      |           |         update                  +----+

         Operations sequence                  Data Packet flow
   PBU/PBA messages with * contain
        a new mobility option

             Figure 2: Scenario after a Handover, CMD as Relay

   For MN's next movements, the process is repeated, but the number of
   P-MAARs involved increases (according to the number of prefixes that
   the MN wishes to maintain).  Indeed, once the CMD receives the first
   PBU from the new S-MAAR, it forwards copies of the PBU to all the
   P-MAARs indicated in the BCE, namely the P-MAAR registered as the
   current P-CoA (i.e., the MAAR prior to handover) plus the ones in the
   P-MAAR list.  Those P-MAARs reply with a PBA to the CMD, which
   aggregates all of the PBAs into one PBA to notify the S-MAAR, which
   finally can establish the tunnels with the P-MAARs.

   It should be noted that this design separates the mobility management
   at the prefix granularity, and it can be tuned in order to erase old
   mobility sessions when not required, while the MN is reachable
   through the latest prefix acquired.  Moreover, the latency associated
   with the mobility update is bound to the PBA sent by the furthest
   P-MAAR, in terms of RTT, that takes the longest time to reach the
   CMD.  The drawback can be mitigated by introducing a timeout at the
   CMD, by which, after its expiration, all the PBAs so far collected
   are transmitted, and the remaining are sent later upon their arrival.
   Note that, in this case, the S-MAAR might receive multiple PBAs from
   the CMD in response to a PBU.  The CMD SHOULD follow the
   retransmissions and rate-limiting considerations described in
   Section 3.6, especially when aggregating and relaying PBAs.

   When there are multiple P-MAARs, e.g., k MAARs, a single PBU received
   by the CMD triggers k outgoing packets from a single incoming packet.
   This may lead to packet bursts originating from the CMD, albeit to
   different targets.  Pacing mechanisms MUST be introduced to avoid
   bursts on the outgoing link.

3.3.  The CMD as MAAR Locator

   The handover latency experienced in the approach shown before can be
   reduced if the P-MAARs are allowed to directly signal their
   information to the new S-MAAR.  This procedure reflects what was
   described in Section 3.2 up to the moment the P-MAAR receives the PBU
   with the Serving MAAR option.  At that point, a P-MAAR is aware of
   the new MN's location (because of the S-MAAR's address in the Serving
   MAAR option), and, besides sending a PBA to the CMD, it also sends a
   PBA to the S-MAAR, including the prefix it is anchoring.  This latter
   PBA does not need to include new options, as the prefix is embedded
   in the Home Network Prefix (HNP) option and the P-MAAR's address is
   taken from the message's source address.  The CMD is released from
   forwarding the PBA to the S-MAAR as the latter receives a copy
   directly from the P-MAAR with the necessary information to build the
   tunnels and set the appropriate routes.  Figure 3 illustrates the new
   message sequence.  The data forwarding is unaltered.

   +-----+      +---+      +-----+           +--+            +--+
   |MAAR1|      |CMD|      |MAAR2|           |CN|            |CN|
   +-----+      +---+      +-----+           +*-+            +*-+
      |           |           |               *               *
      |           |          MN               *     +---+     *
      |           |        attach.        *****    _|CMD|_    *
      |           |          det.   flow1 *       / +-+-+ \   *flow2
      |           |<-- PBU ---|           *      /    |    \  *
      |          BCE          |           *     /     | *******
      |        check+         |           *    /      | *    \
      |        update         |       +---*-+-'    +--+-*+    `+-----+
      |<-- PBU*---|           |       |   * |      |    *|     |     |
   route          |           |       |MAAR1|______|MAAR2+-----+MAAR3|
   update         |           |       |   **(______)**  *|     |     |
      |--------- PBA -------->|       +-----+      +-*--*+     +-----+
      |--- PBA*-->|         route                    *  *
      |          BCE        update             Pref1 *  *Pref2
      |         update        |                     +*--*+
      |           |           |           ---move-->|*MN*|
      |           |           |                     +----+

          Operations sequence                  Data Packet flow
   PBU/PBA messages with * contain
        a new mobility option

            Figure 3: Scenario after a Handover, CMD as Locator

3.4.  The CMD as PBU/PBA Proxy

   A further enhancement of previous solutions can be achieved when the
   CMD sends the PBA to the new S-MAAR before notifying the P-MAARs of
   the location change.  Indeed, when the CMD receives the PBU for the
   new registration, it is already in possession of all the information
   that the new S-MAAR requires to set up the tunnels and the routes.
   Thus, the PBA is sent to the S-MAAR immediately after a PBU is
   received, including the Previous MAAR option in this case.  In
   parallel, a PBU is sent by the CMD to the P-MAARs containing the
   Serving MAAR option to notify them about the new MN's location so
   that they receive the information to establish the tunnels and routes
   on their side.  When P-MAARs complete the update, they send a PBA to
   the CMD to indicate that the operation has concluded and the
   information is updated in all network nodes.  This procedure is
   obtained from the first procedure rearranging the order of the
   messages, but the parameters communicated are the same.  This scheme
   is depicted in Figure 4, where, again, the data forwarding is kept
   untouched.

   +-----+      +---+      +-----+           +--+            +--+
   |MAAR1|      |CMD|      |MAAR2|           |CN|            |CN|
   +-----+      +---+      +-----+           +*-+            +*-+
      |           |           |               *               *
      |           |          MN               *     +---+     *
      |           |        attach.        *****    _|CMD|_    *
      |           |          det.   flow1 *       / +-+-+ \   *flow2
      |           |<-- PBU ---|           *      /    |    \  *
      |          BCE          |           *     /     | *******
      |        check+         |           *    /      | *    \
      |        update         |       +---*-+-'    +--+-*+    `+-----+
      |<-- PBU*---x--- PBA*-->|       |   * |      |    *|     |     |
   route          |         route     |MAAR1|______|MAAR2+-----+MAAR3|
   update         |         update    |   **(______)**  *|     |     |
      |--- PBA*-->|           |       +-----+      +-*--*+     +-----+
      |          BCE          |                      *  *
      |         update        |                Pref1 *  *Pref2
      |           |           |                     +*--*+
      |           |           |           ---move-->|*MN*|
      |           |           |                     +----+

          Operations sequence                 Data Packet flow
   PBU/PBA messages with * contain
        a new mobility option

             Figure 4: Scenario after a Handover, CMD as Proxy

3.5.  De-registration

   The de-registration mechanism devised for PMIPv6 cannot be used as is
   in this solution because each MAAR handles an independent mobility
   session (i.e., a single prefix or a set of prefixes) for a given MN,
   whereas the aggregated session is stored at the CMD.  Indeed, if a
   P-MAAR initiates a de-registration procedure because the MN is no
   longer present on the MAAR's access link, it removes the routing
   state for the prefix(es), that would be deleted by the CMD as well,
   hence defeating any prefix continuity attempt.  The simplest approach
   to overcome this limitation is to deny a P-MAAR to de-register a
   prefix, that is, allowing only an S-MAAR to de-register the whole MN
   session.  This can be achieved by first removing any L2 detachment
   event so that de-registration is triggered only when the binding
   lifetime expires, hence providing a guard interval for the MN to
   connect to a new MAAR.  Then, a change in the MAAR operations is
   required, and at this stage, two possible solutions can be deployed:

   *  A P-MAAR stops the BCE timer upon receiving a PBU from the CMD
      containing a "Serving MAAR" option.  In this way, only the S-MAAR
      is allowed to de-register the mobility session, arguing that the
      MN definitely left the domain.

   *  P-MAARs can, upon BCE expiry, send de-registration messages to the
      CMD, which, instead of acknowledging the message with a 0
      lifetime, sends back a PBA with a non-zero lifetime, hence
      renewing the session if the MN is still connected to the domain.

3.6.  Retransmissions and Rate Limiting

   The node sending PBUs (the CMD or S-MAAR) SHOULD make use of the
   timeout to also deal with missing PBAs (to retransmit PBUs).  The
   INITIAL_BINDACK_TIMEOUT [RFC6275] SHOULD be used for configuring the
   retransmission timer.  The retransmissions by the node MUST use an
   exponential backoff 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 [RFC6275].
   The node MAY continue to send these messages at this slower rate
   indefinitely.  The node MUST NOT send PBU messages to a particular
   node more than MAX_UPDATE_RATE times within a second [RFC6275].

3.7.  The Distributed Logical Interface (DLIF) Concept

   One of the main challenges of a network-based DMM solution is how to
   allow a MN to simultaneously send/receive traffic that is anchored at
   different MAARs and how to influence the MN's selection process of
   its source IPv6 address for a new flow without requiring special
   support from the MN's IP stack.  This document defines the DLIF,
   which is a software construct in the MAAR that can easily hide the
   change of associated anchors from the MN.

     +---------------------------------------------------+
    (                      Operator's                     )
    (                         core                        )
     +---------------------------------------------------+
               |                               |
       +---------------+     tunnel    +---------------+
       |   IP  stack   |===============|   IP  stack   |
       +---------------+               +-------+-------+
       |    mn1mar1    |--+ (DLIFs) +--|mn1mar1|mn1mar2|--+
       +---------------+  |         |  +-------+-------+  |
       | phy interface |  |         |  | phy interface |  |
       +---------------+  |         |  +---------------+  |
             MAAR1       (o)       (o)       MAAR2       (o)
                                      x                 x
                                        x             x
                           prefA::/64     x         x   prefB::/64
                         (AdvPrefLft=0)     x     x
                                              (o)
                                               |
                                            +-----+
                                prefA::MN1  | MN1 |  prefB::MN1
                               (deprecated) +-----+

         Figure 5: DLIF: Exposing Multiple Routers (One per P-MAAR)

   The basic idea of the DLIF concept is the following: each S-MAAR
   exposes itself to a given MN as multiple routers, one per P-MAAR
   associated with the MN.  Let's consider the example shown in
   Figure 5: MN1 initially attaches to MAAR1, configuring an IPv6
   address (prefA::MN1) from a prefix locally anchored at MAAR1
   (prefA::/64).  At this stage, MAAR1 plays the role of both anchoring
   and serving MAAR and also behaves as a plain IPv6 access router.
   MAAR1 creates a DLIF to communicate (through a point-to-point link)
   with MN1, exposing itself as a (logical) router with specific MAC and
   IPv6 addresses (e.g., prefA::MAAR1/64 and fe80::MAAR1/64) using the
   DLIF mn1mar1.  As explained below, these addresses represent the
   "logical" identity of MAAR1 for MN1 and will "follow" the MN while
   roaming within the domain (note that the place where all this
   information is maintained and updated is out of scope of this
   document; potential examples are to keep it on the home subscriber
   server -- HSS -- or the user's profile).

   If MN1 moves and attaches to a different MAAR of the domain (MAAR2 in
   the example of Figure 5), this MAAR will create a new logical
   interface (mn1mar2) to expose itself to MN1, providing it with a
   locally anchored prefix (prefB::/64).  In this case, since the MN1
   has another active IPv6 address anchored at MAAR1, MAAR2 also needs
   to create an additional logical interface configured to resemble the
   one used by MAAR1 to communicate with MN1.  In this example, MAAR1 is
   the only P-MAAR (MAAR2 is the same as S-MAAR), so only the logical
   interface mn1mar1 is created.  However, the same process would be
   repeated if more P-MAARs were involved.  In order to keep the prefix
   anchored at MAAR1 reachable, a tunnel between MAAR1 and MAAR2 is
   established and the routing is modified accordingly.  The PBU/PBA
   signaling is used to set up the bidirectional tunnel between MAAR1
   and MAAR2, and it might also be used to convey the information about
   the prefix(es) anchored at MAAR1 and the addresses of the associated
   DLIF (i.e., mn1mar1) to MAAR2.

   +------------------------------------------+ +----------------------+
   |                  MAAR1                   | |         MAAR2        |
   |+----------------------------------------+| |+--------------------+|
   ||+------------------++------------------+|| ||+------------------+||
   |||+-------++-------+||+-------++-------+||| |||+-------++-------+|||
   ||||mn3mar1||mn3mar2||||mn2mar1||mn2mar2|||| ||||mn1mar1||mn1mar2||||
   |||| LMAC1 || LMAC2 |||| LMAC3 || LMAC4 |||| |||| LMAC5 || LMAC6 ||||
   |||+-------++-------+||+-------++-------+||| |||+-------++-------+|||
   |||    LIFs of MN3   ||    LIFs of MN2   ||| |||   LIFs of MN1    |||
   ||+------------------++------------------+|| ||+------------------+||
   ||              MAC1   (phy if MAAR1)     || || MAC2 (phy if MAAR2)||
   |+----------------------------------------+| |+--------------------+|
   +------------------------------------------+ +----------------------+
                       x        x                            x
                      x          x                          x
                    (o)          (o)                      (o)
                     |            |                        |
                  +--+--+      +--+--+                  +--+--+
                  | MN3 |      | MN2 |                  | MN1 |
                  +-----+      +-----+                  +-----+

              Figure 6: Distributed Logical Interface Concept

   Figure 6 shows the logical interface concept in more detail.  The
   figure shows two MAARs and three MNs.  MAAR1 is currently serving MN2
   and MN3, while MAAR2 is serving MN1.  Note that an S-MAAR always
   plays the role of anchoring MAAR for the attached (served) MNs.  Each
   MAAR has one single physical wireless interface as depicted in this
   example.

   As discussed before, each MN always "sees" multiple logical routers
   -- one per anchoring MAAR -- independently of its currently S-MAAR.
   From the point of view of the MN, these MAARs are portrayed as
   different routers, although the MN is physically attached to a single
   interface.  This is achieved by the S-MAAR configuring different
   logical interfaces.  MN1 is currently attached to MAAR2 (i.e., MAAR2
   is its S-MAAR) and, therefore, it has configured an IPv6 address from
   MAAR2's pool (e.g., prefB::/64).  MAAR2 has set up a logical
   interface (mn1mar2) on top of its wireless physical interface (phy if
   MAAR2), which is used to serve MN1.  This interface has a logical MAC
   address (LMAC6) that is different from the hardware MAC address
   (MAC2) of the physical interface of MAAR2.  Over the mn1mar2
   interface, MAAR2 advertises its locally anchored prefix prefB::/64.
   Before attaching to MAAR2, MN1 was attached to MAAR1 and configured a
   locally anchored address at that MAAR, which is still being used by
   MN1 in active communications.  MN1 keeps "seeing" an interface
   connecting to MAAR1 as if it were directly connected to the two
   MAARs.  This is achieved by the S-MAAR (MAAR2) configuring an
   additional DLIF, mn1mar1, which behaves as the logical interface
   configured by MAAR1 when MN1 was attached to it.  This means that
   both the MAC and IPv6 addresses configured on this logical interface
   remain the same regardless of the physical MAAR that is serving the
   MN.  The information required by an S-MAAR to properly configure this
   logical interfaces can be obtained in different ways: as part of the
   information conveyed in the PBA, from an external database (e.g., the
   HSS) or by other means.  As shown in the figure, each MAAR may have
   several logical interfaces associated with each attached MN and
   always has at least one (since an S-MAAR is also an anchoring MAAR
   for the attached MN).

   In order to enforce the use of the prefix locally anchored at the
   S-MAAR, the RAs sent over those logical interfaces playing the role
   of anchoring MAARs (different from the serving one) include a zero
   preferred prefix lifetime (and a non-zero valid prefix lifetime, so
   the prefix remains valid while being deprecated).  The goal is to
   deprecate the prefixes delegated by these MAARs (so that they will no
   longer be serving the MN).  Note that ongoing communications may keep
   on using those addresses even if they are deprecated, so this only
   affects the establishment of new sessions.

   The DLIF concept also enables the following use case: suppose that
   access to a local IP network is provided by a given MAAR (e.g., MAAR1
   in the example shown in Figure 5) and that the resources available at
   that network cannot be reached from outside the local network (e.g.,
   cannot be accessed by an MN attached to MAAR2).  This is similar to
   the local IP access scenario considered by 3GPP, where a local
   gateway node is selected for sessions requiring access to services
   provided locally (instead of going through a central gateway).  The
   goal is to allow an MN to be able to roam while still being able to
   have connectivity to this local IP network.  The solution adopted to
   support this case makes use of more specific routes, as discussed in
   RFC 4191 [RFC4191], when the MN moves to a MAAR different from the
   one providing access to the local IP network (MAAR1 in the example).
   These routes are advertised through the DLIF where the MAAR is
   providing access to the local network (MAAR1 in this example).  In
   this way, if MN1 moves from MAAR1 to MAAR2, any active session that
   MN1 may have with a node on the local network connected to MAAR1 will
   survive via the tunnel between MAAR1 and MAAR2.  Also, any potential
   future connection attempt to the local network will be supported even
   though MN1 is no longer attached to MAAR1, so long as a source
   address configured from MAAR1 is selected for new connections (see
   [RFC6724], rule 5.5).

4.  Message Format

   This section defines extensions to the PMIPv6 [RFC5213] protocol
   messages.

4.1.  Proxy Binding Update

   A new flag (D) is included in the PBU to indicate that the PBU is
   coming from a MAAR or a CMD and not from a MAG.  The rest of the PBU
   format remains the same as defined in [RFC5213].

   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |            Sequence #         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |A|H|L|K|M|R|P|F|T|B|S|D| Rsrvd |            Lifetime           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Mobility Options                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   DMM Flag (D)
      The D flag is set to indicate to the receiver of the message that
      the PBU is from a MAAR or a CMD.  When an LMA that does not
      support the extensions described in this document receives a
      message with the D flag set, the PBU in that case MUST NOT be
      processed by the LMA, and an error MUST be returned.

   Mobility Options
      Variable-length field of such length that the complete Mobility
      Header is an integer that is a multiple of 8 octets long.  This
      field contains zero or more TLV-encoded mobility options.  The
      encoding and format of the defined options are described in
      Section 6.2 of [RFC6275].  The receiving node MUST ignore and skip
      any options that it does not understand.

4.2.  Proxy Binding Acknowledgement

   A new flag (D) is included in the PBA to indicate that the sender
   supports operating as a MAAR or CMD.  The rest of the PBA format
   remains the same as defined in [RFC5213].

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |   Status      |K|R|P|T|B|S|D| |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Sequence #            |           Lifetime            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Mobility Options                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   DMM Flag (D)
      The D flag is set to indicate that the sender of the message
      supports operating as a MAAR or CMD.  When a MAG that does not
      support the extensions described in this document receives a
      message with the D flag set, it MUST ignore the message, and an
      error MUST be returned.

   Mobility Options
      Variable-length field of such length that the complete Mobility
      Header is an integer multiple of 8 octets long.  This field
      contains zero or more TLV-encoded mobility options.  The encoding
      and format of the defined options are described in Section 6.2 of
      [RFC6275].  The MAAR MUST ignore and skip any options that it does
      not understand.

4.3.  Anchored Prefix Option

   A new Anchored Prefix option is defined for use with the PBU and PBA
   messages exchanged between MAARs and CMDs.  Therefore, this option
   can only appear if the D bit is set in a PBU/PBA.  This option is
   used for exchanging the MN's prefix anchored at the anchoring MAAR.
   There can be multiple Anchored Prefix options present in the message.

   The Anchored Prefix option has an alignment requirement of 8n+4.  Its
   format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |   Length      |   Reserved    | Prefix Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                        Anchored Prefix                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      65

   Length
      8-bit unsigned integer indicating the length of the option in
      octets, excluding the type and length fields.  This field MUST be
      set to 18.

   Reserved
      This field is unused at the time of publication.  The value MUST
      be initialized to 0 by the sender and MUST be ignored by the
      receiver.

   Prefix Length
      8-bit unsigned integer indicating the prefix length in bits of the
      IPv6 prefix contained in the option.

   Anchored Prefix
      A 16-octet field containing the MN's IPv6 Anchored Prefix.  Only
      the first Prefix Length bits are valid for the Anchored Prefix
      option.  The rest of the bits MUST be ignored.

4.4.  Local Prefix Option

   A new Local Prefix option is defined for use with the PBU and PBA
   messages exchanged between MAARs or between a MAAR and a CMD.
   Therefore, this option can only appear if the D bit is set in a PBU/
   PBA.  This option is used for exchanging a prefix of a local network
   that is only reachable via the anchoring MAAR.  There can be multiple
   Local Prefix options present in the message.

   The Local Prefix option has an alignment requirement of 8n+4.  Its
   format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |   Length      |   Reserved    | Prefix Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                         Local Prefix                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      66

   Length
      8-bit unsigned integer indicating the length of the option in
      octets, excluding the type and length fields.  This field MUST be
      set to 18.

   Reserved
      This field is unused at the time of publication.  The value MUST
      be initialized to 0 by the sender and MUST be ignored by the
      receiver.

   Prefix Length
      8-bit unsigned integer indicating the prefix length in bits of the
      IPv6 prefix contained in the option.

   Local Prefix
      A 16-octet field containing the IPv6 Local Prefix.  Only the first
      Prefix Length bits are valid for the IPv6 Local Prefix.  The rest
      of the bits MUST be ignored.

4.5.  Previous MAAR Option

   This new option is defined for use with the PBA messages exchanged by
   the CMD to a MAAR.  This option is used to notify the S-MAAR about
   the P-MAAR's global address and the prefix anchored to it.  There can
   be multiple Previous MAAR options present in the message.

   The Previous MAAR option has an alignment requirement of 8n+4.  Its
   format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |   Reserved    | Prefix Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                     Previous MAAR                             +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                    Home Network Prefix                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      67

   Length
      8-bit unsigned integer indicating the length of the option in
      octets, excluding the type and length fields.  This field MUST be
      set to 34.

   Reserved
      This field is unused at the time of publication.  The value MUST
      be initialized to 0 by the sender and MUST be ignored by the
      receiver.

   Prefix Length
      8-bit unsigned integer indicating the prefix length in bits of the
      IPv6 prefix contained in the option.

   Previous MAAR
      A 16-octet field containing the P-MAAR's IPv6 global address.

   Home Network Prefix
      A 16-octet field containing the MN's IPv6 Home Network Prefix.
      Only the first Prefix Length bits are valid for the MN's IPv6 Home
      Network Prefix.  The rest of the bits MUST be ignored.

4.6.  Serving MAAR Option

   This new option is defined for use with the PBU message exchanged
   between the CMD and a P-MAAR.  This option is used to notify the
   P-MAAR about the current S-MAAR's global address.  Its format is as
   follows:

   The Serving MAAR option has an alignment requirement of 8n+6.  Its
   format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |      Type     |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                     S-MAAR's Address                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      68

   Length
      8-bit unsigned integer indicating the length of the option in
      octets, excluding the type and length fields.  This field MUST be
      set to 16.

   Serving MAAR
      A 16-octet field containing the S-MAAR's IPv6 global address.

4.7.  DLIF Link-Local Address Option

   A new DLIF Link-Local Address option is defined for use with the PBA
   message exchanged between MAARs and between a MAAR and a CMD.  This
   option is used for exchanging the link-local address of the DLIF to
   be configured on the S-MAAR so it resembles the DLIF configured on
   the P-MAAR.

   The DLIF Link-Local Address option has an alignment requirement of
   8n+6.  Its format is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |   Type        |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                  DLIF Link-Local Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      69

   Length
      8-bit unsigned integer indicating the length of the option in
      octets, excluding the type and length fields.  This field MUST be
      set to 16.

   DLIF Link-Local Address
      A 16-octet field containing the link-local address of the logical
      interface.

4.8.  DLIF Link-Layer Address Option

   A new DLIF Link-Layer Address option is defined for use with the PBA
   message exchanged between MAARs and between a MAAR and a CMD.  This
   option is used for exchanging the link-layer address of the DLIF to
   be configured on the S-MAAR so it resembles the DLIF configured on
   the P-MAAR.

   The format of the DLIF Link-Layer Address option is shown below.
   Based on the size of the address, the option MUST be aligned
   appropriately, as per the mobility option alignment requirements
   specified in [RFC6275].

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type        |    Length     |          Reserved             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    DLIF Link-Layer Address                    +
   .                              ...                              .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      70

   Length
      8-bit unsigned integer indicating the length of the option in
      octets, excluding the type and length fields.

   Reserved
      This field is unused at the time of publication.  The value MUST
      be initialized to 0 by the sender and MUST be ignored by the
      receiver.

   DLIF Link-Layer Address
      A variable length field containing the link-layer address of the
      logical interface to be configured on the S-MAAR.

      The content and format of this field (including octet and bit
      ordering) is as specified in Section 4.6 of [RFC4861] for carrying
      link-layer addresses.  On certain access links where the link-
      layer address is not used or cannot be determined, this option
      cannot be used.

5.  IANA Considerations

   This document defines six new mobility options: Anchored Prefix,
   Local Prefix, Previous MAAR, Serving MAAR, DLIF Link-Local Address,
   and DLIF Link-Layer Address.  IANA has assigned Type values for these
   options from the same numbering space as allocated for the other
   mobility options in the "Mobility Options" registry defined in
   http://www.iana.org/assignments/mobility-parameters.

   This document reserves a new flag (D) with a value of 0x0010 in the
   "Binding Update Flags" registry and a new flag (D) with a value of
   0x02 in the "Binding Acknowledgment Flags" of the "Mobile IPv6
   parameters" registry (http://www.iana.org/assignments/mobility-
   parameters).

6.  Security Considerations

   The protocol extensions defined in this document share the same
   security concerns of PMIPv6 [RFC5213].  It is recommended that the
   signaling messages, PBU and PBA, exchanged between the MAARs be
   protected using IPsec, specifically by using the established security
   association between them.  This essentially eliminates the threats
   related to the impersonation of a MAAR.

   When the CMD acts as a PBU/PBA relay, the CMD may act as a relay of a
   single PBU to multiple P-MAARs.  In situations with many fast
   handovers (e.g., with vehicular networks), multiple previous (e.g.,
   k) MAARs may exist.  In this situation, the CMD creates k outgoing
   packets from a single incoming packet.  This bears a certain
   amplification risk.  The CMD MUST use a pacing approach in the
   outgoing queue to cap the output traffic (i.e., the rate of PBUs
   sent) to limit this amplification risk.

   When the CMD acts as a MAAR locator, mobility signaling (PBAs) is
   exchanged between P-MAARs and the current S-MAAR.  Hence, security
   associations are REQUIRED to exist between the involved MAARs (in
   addition to the ones needed with the CMD).

   Since de-registration is performed by timeout, measures SHOULD be
   implemented to minimize the risks associated with continued resource
   consumption (DoS attacks), e.g., imposing a limit on the number of
   P-MAARs associated with a given MN.

   The CMD and the participating MAARs MUST be trusted parties
   authorized perform all operations relevant to their role.

   There are some privacy considerations to consider.  While the
   involved parties trust each other, the signaling involves disclosing
   information about the previous locations visited by each MN, as well
   as the active prefixes they are using at a given point of time.
   Therefore, mechanisms MUST be in place to ensure that MAARs and CMDs
   do not disclose this information to other parties or use it for other
   ends than providing the distributed mobility support specified in
   this document.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <https://www.rfc-editor.org/info/rfc4191>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,
              <https://www.rfc-editor.org/info/rfc5213>.

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <https://www.rfc-editor.org/info/rfc6275>.

   [RFC7333]  Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J.
              Korhonen, "Requirements for Distributed Mobility
              Management", RFC 7333, DOI 10.17487/RFC7333, August 2014,
              <https://www.rfc-editor.org/info/rfc7333>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [DISTRIBUTED-ANCHORING]
              Bernardos, C. and J. Zuniga, "PMIPv6-based distributed
              anchoring", Work in Progress, Internet-Draft, draft-
              bernardos-dmm-distributed-anchoring-09, 29 May 2017,
              <https://tools.ietf.org/html/draft-bernardos-dmm-
              distributed-anchoring-09>.

   [DMM-PMIP] Bernardos, C., Oliva, A., and F. Giust, "A PMIPv6-based
              solution for Distributed Mobility Management", Work in
              Progress, Internet-Draft, draft-bernardos-dmm-pmip-09, 8
              September 2017,
              <https://tools.ietf.org/html/draft-bernardos-dmm-pmip-09>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC7429]  Liu, D., Ed., Zuniga, JC., Ed., Seite, P., Chan, H., and
              CJ. Bernardos, "Distributed Mobility Management: Current
              Practices and Gap Analysis", RFC 7429,
              DOI 10.17487/RFC7429, January 2015,
              <https://www.rfc-editor.org/info/rfc7429>.

   [RFC8563]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
              Ed., "Bidirectional Forwarding Detection (BFD) Multipoint
              Active Tails", RFC 8563, DOI 10.17487/RFC8563, April 2019,
              <https://www.rfc-editor.org/info/rfc8563>.

Acknowledgements

   The authors would like to thank Dirk von Hugo, John Kaippallimalil,
   Ines Robles, Joerg Ott, Carlos Pignataro, Vincent Roca, Mirja
   Kühlewind, Éric Vyncke, Adam Roach, Benjamin Kaduk, and Roman Danyliw
   for the comments on this document.  The authors would also like to
   thank Marco Liebsch, Dirk von Hugo, Alex Petrescu, Daniel Corujo,
   Akbar Rahman, Danny Moses, Xinpeng Wei, and Satoru Matsushima for
   their comments and discussion on the documents
   [DISTRIBUTED-ANCHORING] and [DMM-PMIP], on which the present document
   is based.

   The authors would also like to thank Lyle Bertz and Danny Moses for
   their in-depth review of this document and their very valuable
   comments and suggestions.

Authors' Addresses

   Carlos J. Bernardos
   Universidad Carlos III de Madrid
   Av. Universidad, 30
   28911 Leganes Madrid
   Spain

   Phone: +34 91624 6236
   Email: cjbc@it.uc3m.es
   URI:   http://www.it.uc3m.es/cjbc/


   Antonio de la Oliva
   Universidad Carlos III de Madrid
   Av. Universidad, 30
   28911 Leganes Madrid
   Spain

   Phone: +34 91624 8803
   Email: aoliva@it.uc3m.es
   URI:   http://www.it.uc3m.es/aoliva/


   Fabio Giust
   Athonet S.r.l.
   via Ca' del Luogo 6/8
   36050 Bolzano Vicentino (VI)
   Italy

   Email: fabio.giust.research@gmail.com


   Juan Carlos Zúñiga
   SIGFOX
   425 rue Jean Rostand
   31670 Labege
   France

   Email: j.c.zuniga@ieee.org
   URI:   http://www.sigfox.com/


   Alain Mourad
   InterDigital Europe

   Email: Alain.Mourad@InterDigital.com
   URI:   http://www.InterDigital.com/


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