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RFC 6058 - Transient Binding for Proxy Mobile IPv6


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Internet Engineering Task Force (IETF)                   M. Liebsch, Ed.
Request for Comments: 6058                                           NEC
Category: Experimental                                        A. Muhanna
ISSN: 2070-1721                                                 Ericsson
                                                                O. Blume
                                                Alcatel-Lucent Bell Labs
                                                              March 2011

                Transient Binding for Proxy Mobile IPv6

Abstract

   This document specifies a mechanism that enhances Proxy Mobile IPv6
   protocol signaling to support the creation of a transient binding
   cache entry that is used to optimize the performance of dual radio
   handover, as well as single radio handover.  This mechanism is
   applicable to the mobile node's inter-MAG (Mobility Access Gateway)
   handover while using a single interface or different interfaces.  The
   handover problem space using the Proxy Mobile IPv6 base protocol is
   analyzed and the use of transient binding cache entries at the local
   mobility anchor is described.  The specified extension to the Proxy
   Mobile IPv6 protocol ensures optimized forwarding of downlink as well
   as uplink packets between mobile nodes and the network infrastructure
   and avoids superfluous packet forwarding delay or even packet loss.

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 a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6058.

Copyright Notice

   Copyright (c) 2011 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
   (http://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 ....................................................4
   2. Conventions and Terminology .....................................5
      2.1. Conventions Used in This Document ..........................5
      2.2. Terminology and Functional Components ......................5
   3. Analysis of the Problem Space ...................................6
      3.1. Handover Using a Single Interface ..........................6
      3.2. Handover between Interfaces ................................6
           3.2.1. Issues with Downlink Traffic ........................7
           3.2.2. Issues with Uplink Traffic ..........................9
      3.3. Need for a Common Solution ................................10
   4. Use of Transient Binding Cache Entries .........................11
      4.1. General Approach ..........................................11
      4.2. Impact on Binding Management ..............................13
      4.3. Role of the LMA and nMAG in Transient State Control .......14
           4.3.1. Control at the nMAG ................................14
           4.3.2. Control at the LMA .................................15
      4.4. LMA Forwarding State Diagram ..............................15
      4.5. MAG Operation .............................................18
      4.6. LMA Operation .............................................19
           4.6.1. Initiation of a Transient BCE ......................19
           4.6.2. Activation of a Transient BCE ......................20
      4.7. MN Operation ..............................................22
      4.8. Status Values .............................................22
      4.9. Protocol Stability ........................................22
   5. Message Format .................................................24
      5.1. Transient Binding Option ..................................24
   6. IANA Considerations ............................................25
   7. Security Considerations ........................................25
   8. Protocol Configuration Variables ...............................26
   9. Contributors ...................................................26
   10. Acknowledgments ...............................................26
   11. References ....................................................26
      11.1. Normative References .....................................26
      11.2. Informative References ...................................26
   Appendix A.  Example Use Cases for Transient BCE ..................28
     A.1.  Use Case for Single Radio Handover ........................28
     A.2.  Use Case for Dual Radio Handover ..........................30
   Appendix B.  Applicability and Use of Static Configuration at
                the LMA ..............................................33
     B.1.  Early Uplink Traffic from the nMAG ........................33
     B.2.  Late Uplink Traffic from the pMAG .........................33
     B.3.  Late Switching of Downlink Traffic to nMAG ................34

1.  Introduction

   The IETF specified Proxy Mobile IPv6 (PMIPv6) [RFC5213] as a protocol
   for network-based localized mobility management, which takes basic
   operation for registration, tunnel management, and deregistration
   into account.  In order to eliminate the risk of lost packets, this
   document specifies an extension to PMIPv6 that utilizes a new
   mobility option in the Proxy Binding Update (PBU) and the Proxy
   Binding Acknowledgement (PBA) between the new Mobility Access Gateway
   (nMAG) and the Local Mobility Anchor (LMA).

   According to the PMIPv6 base specification, an LMA updates a mobile
   node's (MN's) Binding Cache Entry (BCE) and switches the forwarding
   tunnel after receiving a Proxy Binding Update (PBU) message from the
   mobile node's new MAG (nMAG).  At the same time, the LMA disables the
   forwarding entry towards the mobile node's previous MAG (pMAG).  In
   case of an inter-technology handover, the mobile node's handover
   target interface must be configured according to the Router
   Advertisement being sent by the nMAG.  Address configuration as well
   as possible access-technology-specific radio bearer setup may delay
   the complete set up of the mobile node's new interface before it is
   ready to receive or send data packets.  In case the LMA performs
   operation according to [RFC5213] and forwards packets to the mobile
   node's new interface after the reception of the PBU from the nMAG,
   some packets may get lost or experience major packet delay.  The
   transient BCE extension, as specified in this document, increases
   handover performance (optimized packet loss and forwarding delay)
   experienced by MNs, which have multiple network interfaces
   implemented while handing over from one interface to the other.  The
   transient BCE extension also increases handover performance for
   single radio MNs, which build on available radio layer forwarding
   mechanisms, hence re-use existing active handover techniques.

   Some implementation-specific solutions, such as static configuration
   on the LMA to accept uplink packets from the old MAG in addition to
   accepting packets from the new MAG for a short duration during the
   handover and buffering at the new MAG, can help to address some of
   the issues identified in this document.  Please see Appendix B for
   more details.  A dynamic solution by means of the proposed protocol
   operation helps to optimize the performance for a variety of handover
   situations and different radio characteristics.

   Additionally, this document specifies an advanced binding cache
   management mechanism at the LMA according to well-defined transient
   BCE states.  This mechanism ensures that forwarding states at LMAs
   are inline with the different handover scenarios.  During a transient
   state, a mobile node's BCE refers to two proxy Care-of-Address
   (Proxy-CoA) entries, one from the mobile node's pMAG, another from

   its nMAG.  MAGs can establish settings of a transient binding on the
   LMA by means of signaling.  An LMA can establish or change the
   settings of a transient binding according to events, such as a
   timeout, a change of the radio technology due to a handover, or a
   completed set up of a radio bearer or configuration of an MN's IP
   address.  Such an event may also be triggered by other protocols,
   e.g., Authentication, Authorization, and Accounting (AAA) messages.
   This document specifies advanced binding cache control by means of a
   Transient Binding option, which can be used with PMIPv6 signaling to
   support transient BCEs.  Furthermore, this document specifies
   forwarding characteristics according to the current state of a
   binding to switch the forwarding tunnel at the LMA from the pMAG to
   the nMAG during inter-MAG handover according to the handover
   conditions.  As a result of transient binding support, handover
   performance can considerably be improved to smooth an MN's handover
   without introducing major complexity into the system.

2.  Conventions and Terminology

2.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.2.  Terminology and Functional Components

   o  IF - Interface.  Any network interface, which offers a mobile node
      wireless or wired access to the network infrastructure.  In case a
      mobile node has multiple interfaces implemented, they are numbered
      (IF1, IF2, etc.).

   o  Transient Binding Cache Entry.  A temporary state of the mobile
      node Binding Cache Entry that defines the forwarding
      characteristics of the mobile node forwarding tunnels to the nMAG
      and pMAG.  This transient BCE state is created when the Transient
      Binding option is included in the PBU and PBA as specified in this
      document.  The LMA forwards the mobile node traffic according to
      current transient BCE characteristics as specified in this
      document.  The transient BCE state is transparent to the pMAG.

   o  Active Binding Cache Entry.  A valid mobile node Binding Cache
      Entry according to [RFC5213], which is not in transient state.

3.  Analysis of the Problem Space

   This section summarizes the analysis of the handover problem space
   for inter-technology handover as well as intra-technology handover
   when using the PMIPv6 protocol as in [RFC5213].

3.1.  Handover Using a Single Interface

   In some active handover scenarios, it is necessary to prepare the
   nMAG as the handover target prior to the completion of the link-layer
   handover procedures.  Packets sent by the LMA to the nMAG before the
   completion of the link-layer handover procedure will be lost unless
   they are buffered.

   In some systems, the nMAG will be the recipient of uplink traffic
   prior to the completion of the procedure that would result in the
   PBU/PBA handshake.  These packets cannot be forwarded to the LMA.

   During an intra-technology handover, some of the MN's uplink traffic
   may still be in transit through the pMAG.  Currently, and as per the
   PMIPv6 base protocol [RFC5213], the LMA forwards the MN's uplink
   traffic received from a tunnel only as long as the source IP address
   of the MN's uplink traffic matches the IP address of the mobile
   node's registered Proxy-CoA in the associated BCE.  As a result,
   packets received at the LMA from the MN's pMAG after the LMA has
   already switched the tunnel to point to the nMAG will be dropped.

3.2.  Handover between Interfaces

   In client-based mobility protocols, the handover sequence is fully
   controlled by the MN, and the MN updates its binding and associated
   routing information at its mobility anchor after IP connectivity has
   been established on the new link.  On the contrary, PMIPv6 aims to
   relieve the MN from the IP mobility signaling, while the mobile node
   still controls link configuration during a handover.  This introduces
   a problem during an MN's handover between interfaces.  According to
   the PMIPv6 base protocol [RFC5213], the Access Authentication and the
   Proxy Binding Update (PBU) are triggered in the access network by the
   radio attach procedure, transparently for the MN.  In addition, a
   delay for the MN's new interface's address configuration is not
   considered in the handover procedure.  As a consequence, the
   immediate update of the MN's BCE after the PBU from the MN's nMAG has
   been received at the LMA impacts the performance of the MN's downlink
   traffic as well as its uplink traffic.  Performance aspects of
   downlink as well as uplink traffic during a handover between
   interfaces are analyzed in the subsequent subsections.

3.2.1.  Issues with Downlink Traffic

   Delay of availability of an MN's network interface can be caused by
   certain protocol operations that the MN needs to perform to configure
   its new interface, and these operations can take time.  In order to
   complete the address auto-configuration on its new interface, the MN
   needs to send a Router Solicitation and awaits a Router
   Advertisement.  Upon receiving a Router Advertisement from the new
   MAG, the MN can complete its address configuration and may perform
   Duplicate Address Detection (DAD) [RFC4862] on the new interface.
   Only then the MN's new interface is ready to receive packets.

   Address configuration can take more than a second to complete.  If
   the LMA has already switched the mobile node tunnel to point to the
   nMAG and started forwarding data packets for the MN to the nMAG
   during this time, these data packets may get delayed or lost because
   the MN's new interface is not yet ready to receive data.  However,
   delaying the PBU, which is sent from the new MAG to the LMA after the
   MN's new interface has attached to the network, is not possible, as
   the new MAG retrieves configuration data for the MN from the LMA in
   the PBA, such as the MN's Home Network Prefixes (HNPs) and the link-
   local address to be used at the MAG.

   The aforementioned problem is illustrated in Figure 1, which assumes
   that the HNP(s) will be assigned under control of the LMA.  Hence,
   the HNP option in the PBU, which is sent by the new MAG to the LMA,
   is set to ALL_ZERO.  An MN has attached to the network with interface
   (IF) IF1 and receives data on this interface.  When the MN's new
   interface IF2 comes up and is detected by the new MAG, the new MAG
   sends a PBU and receives a PBA from the LMA.  If the LMA decides to
   forward data packets for the MN via the new MAG, the new MAG has to
   buffer these packets until address configuration of the MN's new
   interface has completed and the MN's new interface is ready to
   receive packets.  While setting up IF2, the MN may not reply to
   address resolution signaling [RFC4861], as sent by the new MAG [A].
   If the MAG's buffer overflows or the MN cannot reply to address
   resolution signaling for too long, data packets for the MN are
   dropped and the MN can experience severe packet losses during an
   inter-access handover [B] until IF2 is ready to receive and send data
   [C].

       +------+                 +----+      +----+                 +---+
       |  MN  |                 |pMAG|      |nMAG|                 |LMA|
       +------+                 +----+      +----+                 +---+
       IF2 IF1                    |           |                      |
        |   |                     |           |                      |
        |   |- - - - - - - - - Attach         |                      |
        |   |                     |---------------PBU--------------->|
        |   |                     |<--------------PBA----------------|
        |   |--------RtSol------->|           |                      |
        |   |<-------RtAdv--------|           |                      |
        |  Addr.                  |           |                      |
        |  Conf.                  |           |                      |
        |   |<--------------------|==================data============|--
        |   |                     |           |                      |
        |- - - - - - - - - - - - - - - - - Attach                    |
        |   |                     |           |----------PBU-------->|
        |   |                     |           |<---------PBA---------|
        |   |                     |           |<-====data============|--
    [A]?|<-----------NSol---------------------|<-====data============|--
        |   |                     |      [B] ?|<-====data============|--
        |   |                     |          ?|<-====data============|--
        |-----------RtSol-------------------->|<-====data============|--
        |<----------RtAdv---------------------|            :         |
     Addr.  |                     |           |            :         |
     Conf.  |                     |           |            :         |
        |<-----------NSol---------------------|            :         |
        |------------NAdv------------------->[C]                     |
       !|<------------------------------------|======data============|--
        |   |                     |           |                      |
        |   |                     |           |                      |

                 Figure 1: Issue with dual radio handover

   Another risk for a delay in forwarding data packets from a new MAG to
   the MN's IF2 can be some latency in setting up a particular access
   technology's radio bearer or access-specific security associations
   after the new MAG received the MN's HNP(s) from the LMA via the PBA
   signaling message.

   In case an access network needs the MN's IP address or HNP to set up
   a radio bearer between an MN's IF2 and the network infrastructure,
   the access network might have to wait until the nMAG has received the
   associated information from the LMA in the Proxy Binding
   Acknowledgment.  Delay in forwarding packets from the nMAG to the
   MN's IF2 depends now on the latency in setting up the radio bearer.

   A similar problem can occur in the case in which the setup of a
   required security association between the MN's IF2 and the network
   takes time and such a setup can be performed only after the MN's IP
   address or HNP is available on the nMAG.

   Both scenarios, as depicted above, can be found in [TS23.402], where
   the protocol sequence during a handover between different accesses
   considers a PMIPv6 handshake between the nMAG and the LMA to retrieve
   the MN's HNP(s) before access-specific operations can be completed.

3.2.2.  Issues with Uplink Traffic

   In the case of an inter-technology handover between two interfaces,
   the MN may be able to maintain connectivity on IF1 while it is
   completing address configuration on IF2.  Such a handover mechanism
   is called "make-before-break" and can avoid uplink packet loss in
   client-based Mobile IP.  However, in a PMIPv6 domain, the attachment
   of the MN on IF2 will cause the nMAG to send a PBU to the LMA, which
   will cause the LMA to update the BCE for this mobility session of the
   MN.  According to Section 5.3.5 of the PMIPv6 base specification
   [RFC5213], the LMA may drop all subsequent packets being forwarded by
   the MN's pMAG due to the updated BCE, which refers now to the nMAG as
   a "Proxy-CoA".

   A further issue for uplink packets arises from differences in the
   time of travel between the nMAG and LMA in comparison with the time
   of travel between the pMAG and LMA.  Even if the MN stops sending
   packets on IF1 before the PBU is sent (i.e., before it attaches IF2
   to nMAG), uplink packets from pMAG may arrive at the LMA after the
   LMA has received the PBU from nMAG.  Such a situation can, in
   particular, occur when the MN's previous link has a high delay (e.g.,
   a Global System for Mobile Communications (GSM) link) and is slow
   compared to the handover target link.  This characteristic is
   illustrated in Figure 2.

      +------+              +----+                   +---+
      |  MN  |              |nMAG|                   |LMA|
      +------+              +----+                   +---+
      IF2 IF1                 |                        |
       |   |\                 |                        |BCE exists
       |   |    \             |                        | for pMAG
       |- -|- - - - \- - - - Attach                    |
       |   |           s\     |---------PBU----------->|BCE update
       |   |               l\ |<--------PBA------------| for nMAG
       |   |                   o\                      |
       |   |                  |    w\                  |
       |   |                  |        l\              |
       |   |                  |            i\          |
       |   |                  |               n \      |packet dropped
       |   |                  |                  k --->| as BCE has only
       |   |                  |                        | entry for nMAG
       |   |                  |                        |
       |   |                  |                        |

              Figure 2: Uplink traffic issue with slow links

3.3.  Need for a Common Solution

   To reduce the risk of packet loss, some settings on an MN could be
   chosen appropriately to speed up the process of network interface
   configuration.  Also, tuning some network parameters, such as
   increasing the buffer capacity on MAG components, could improve the
   handover performance.  However, some network characteristics, such as
   access link delay or bearer setup latency, cannot be easily fine
   tuned to suit a particular handover scenario.  Thus, a common
   solution that dynamically controls and enhances this handover
   complexity using a simple extension to the PMIPv6 base protocol is
   preferred.

   This document specifies transient BCEs as an extension to the PMIPv6
   protocol.  Set up and configuration of a transient BCE can be
   performed by means of extended PMIPv6 signaling messages between the
   MAG and the LMA component using a new Transient Binding mobility
   option.  The transient BCE mechanism supports three clearly
   distinguished sequences of transient states to suit various handover
   scenarios and to improve handover performance for both inter- and
   intra-technology handover.  As a result of using transient BCEs,
   excessive packet buffering at the nMAG during the MN's handover
   process is not necessary and packet losses and major jitter can be
   avoided.

4.  Use of Transient Binding Cache Entries

4.1.  General Approach

   The use of transient BCE during an MN's handover (HO) enables greater
   control on the forwarding of uplink (Ul) and downlink (Dl) traffic to
   harmonize handover performance characteristics with the capabilities
   of the handover source and target access networks.  Updating of an
   MN's BCE at an LMA is split into different phases before and after
   the radio setup and IP configuration being associated with the MN's
   handover from a pMAG to an nMAG.

   The use of a transient BCE during an MN's handover splits into an
   initiation phase and a phase turning the transient BCE into an active
   BCE.  Figure 3 illustrates the procedure to enter and leave a
   transient BCE during an MN's handover.  As a result of the MN's
   attachment at the nMAG, the first PBU from the MN's nMAG can turn the
   MN's BCE at the LMA and the nMAG into transient state by including a
   Transient Binding option (Section 5.1).  The LMA enters the nMAG as a
   further forwarding entry to the MN's BCE without deleting the
   existing forwarding entry and marks the BCE state as 'transient'.
   Alternatively, in case the nMAG does not include a Transient Binding
   option, the LMA can make the decision to use a transient BCE during
   an MN's handover and notify the nMAG about this decision by adding a
   Transient Binding option in the PBA.  After receiving the PBA, the
   nMAG enters the MN's data, such as the assigned HNP(s), into its
   Binding Update List (BUL) and marks the MN's binding with the LMA as
   'transient', which serves as an indication to the nMAG that the
   transient BCE needs to be turned into an active BCE.

   During the transient state, the LMA accepts uplink packets from both
   MAGs, the pMAG and the nMAG, for forwarding.  To benefit from the
   still available downlink path from pMAG to MN, the LMA forwards
   downlink packets towards the pMAG until the transient BCE is turned
   into an active BCE.  Such a downlink forwarding characteristic is
   denoted as "late path switch" (L).  During a dual radio handover, an
   MN can receive downlink packets via its previous interface; during a
   single radio handover, the late path switch supports re-using
   available forwarding mechanisms in the radio access network.
   Appendix A describes both use cases.

   Decisions about the classification of an MN's BCE as transient during
   a handover can be made either by the nMAG or the LMA.  Detailed
   mechanisms showing how an nMAG or an LMA finds out to use a transient
   BCE procedure are out of scope of this document.

   A transient BCE can be turned into an active BCE by different means,
   such as a timeout at the LMA, a PBU from the nMAG, which has no
   Transient Binding option included, or a deregistration PBU from the
   pMAG.  As soon as the MN's BCE has been initiated to turn into an
   active BCE, the LMA switches the forwarding path for downlink packets
   from the pMAG to the nMAG.

    +-----+            +----+    +----+                +-----+
    | MN  |            |pMAG|    |nMAG|                | LMA |
    +-----+            +----+    +----+                +-----+
       |                  |         |                     |[pMAG serves
       |                  |         |                     | MN as
       |                  |         |                     | Proxy-CoA]
       |                  |         |                     |
       |<-----------------|===============data tunnel=====|--->data
       |                  |         |                     |
   [Handoff               |         |                     |
     Start]               |         |                     |
       |                  |         |                     |
      e|-----------------------[MN Attach]                |
      x|                  |         |                     |
      e|                  |         |---PBU(transient)--->|[Add nMAG to
      c|                  |         |                     | MN's BCE,
      u|                  |         |<--PBA(transient)----| enter trans-
      t|                  |         |                     | ient state]
      i|                  |         |                     |
      o|<-----Dl+Ul-------|===============data tunnel=====|--->data
      n|--------Ul------------------|=====data tunnel=====|--->data
       |                  |         |                     |
    [Handoff/             |         |                     |
   Configuration          |         |                     |
    Completed]            |   [HO Complete]               |
       |                  |         |--------PBU--------->|[Activate
       |                  |         |                     | MN's BCE,
       |                  |         |<-------PBA ---------| update for-
       |                  |         |                     | warding path
       |                  |         |                     | to nMAG]
       |                  |         |                     |
       |<---------------------------|=====data tunnel=====|--->data
       |                  |         |                     |

     Figure 3: General mechanism and forwarding characteristics during
                        handover with transient BCE

   This specification considers an optional state when turning the
   transient BCE into an active BCE of a transient BCE with a late path
   switch, which keeps the pMAG for some more time as the forwarding

   entry in the transient BCE, solely to ensure forwarding of delayed
   uplink packets from the pMAG.  This optional activation state has a
   lifetime associated, and termination does not need any signaling.

   Whether or not to enter this optional activation state is decided by
   the LMA.  The LMA may take information about the access technology
   associated with the MN's pMAG and nMAG from the MN's BCE to decide if
   using the activation state is beneficial, e.g., since a slow link is
   associated with the pMAG and uplink packets from the pMAG may arrive
   delayed at the LMA.

   The Transient Binding option allows configuration of the transient
   BCE late path switch and signaling of associated settings.  Signaling
   of the Transient Binding option and the LMA's decision whether or not
   to use an optional activation state defines the sequence through the
   clearly defined transient BCE states, as illustrated and described in
   Section 4.4.  Section 4.2 describes the required extension to an
   LMA's binding cache to support transient BCE operation.  Section 4.3
   provides a concise overview about the possible roles of the nMAG and
   the LMA to control a transient BCE handover sequence.  Details about
   the Transient Binding option and its use are described in Sections
   4.5 and 4.6.

4.2.  Impact on Binding Management

   The use of a transient BCE requires temporary maintenance of two
   forwarding entries in the MN's BCE at the LMA, one referring to the
   MN's pMAG and the other referring to its nMAG.  Forwarding entries
   are represented according to [RFC5213] and comprise the interface
   identifier of the associated tunnel interface towards each MAG, as
   well as the associated access technology information.

   Each forwarding entry is assigned a forwarding rule to admit and
   control forwarding of uplink and downlink traffic to and from the
   associated MAG.  Hence, according to this specification, a forwarding
   entry can have either a rule that allows only forwarding of uplink
   traffic from the associated MAG, or a rule that allows bidirectional
   forwarding from and to the associated MAG.  At any time, only one of
   the two forwarding entries can have a bi-directional forwarding rule.
   The interface identifier and access technology type info can be taken
   from the PBU received at the LMA and linked to each forwarding entry
   accordingly.

   MAGs should maintain the status of an MN's binding and the lifetime
   associated with a transient BCE at the LMA in their binding update
   list.  This is particularly important if the new MAG needs to
   explicitly turn a binding into an active BCE after the associated
   MN's new interface has proven to be ready to handle IP traffic.

4.3.  Role of the LMA and nMAG in Transient State Control

   This section provides an overview about the nMAG's and the LMA's
   possibility to control a transient BCE.  Please refer to the Protocol
   Operations sections for a detailed protocol description (Sections 4.5
   and 4.6).

4.3.1.  Control at the nMAG

   Initiate a late path switch -  Since the nMAG needs to have knowledge
      about the nature of a handover to set the Handoff Indicator (HI)
      option in the PBU and whether or not the handover implies a change
      in the used radio interface or technology, the nMAG is a suitable
      entity to make the decision to delay the downlink path switch in a
      controlled manner by means of a transient BCE.  The nMAG can make
      the decision to initiate a transient BCE handover for an MN only
      when it knows that the MN supports a delayed downlink path switch
      (Section 4.7) according to this specification.  It may know this
      due to a number of factors.  For instance, during dual radio
      handover, 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 and that the MN has sufficient control on its
      interfaces to support a transient BCE handover.  Where no support
      from the link layer exists and no such indication can be provided
      to the nMAG by the network, the nMAG MUST assume that the host is
      incapable of this mode of operation and employ standard behavior
      as specified in [RFC5213].  In other words, the nMAG initiates a
      regular [RFC5213] handover.

      The nMAG is also a suitable entity to estimate a maximum delay
      until the new connection can be used, as it knows about its
      locally connected radio network characteristics.  Hence, the nMAG
      can set the maximum lifetime to delimit the transient BCE
      softstate at the LMA.  The LMA may still override the proposed
      lifetime and notify the nMAG about the new lifetime in the
      Transient Binding option included in the PBA.

   Activation of a transient BCE to perform a downlink path switch -
      During a transient BCE handover, the nMAG may get an indication
      that the MN's radio link can be used and the MN has completed the
      setup of the IP address to send and receive data packets via the
      new link.  In this case, the nMAG can initiate turning a transient
      BCE into an active BCE before the expiration of the associated
      maximum transient BCE lifetime.  To do that, the nMAG sends a PBU
      message without the Transient Binding option to the LMA.  This
      results in a downlink path switch to the nMAG.

4.3.2.  Control at the LMA

   Initiate a late path switch -  If the LMA has received a PBU without
      a Transient Binding option included, the LMA can take a decision
      to use a transient BCE to optimize the handover performance.  The
      LMA indicates its selected settings for the late path switch (L)
      and the associated maximum lifetime in the Transient Binding
      option, which is included in the PBA and sent to the nMAG.

   Decision to use an optional activation state -  The LMA is a suitable
      entity to decide about the use of an optional activation state, as
      the LMA has the knowledge about the MN's previous and new access
      technology.  Hence, the LMA can make this decision to use an
      activation state to temporarily keep alive the forwarding of
      uplink packets from both MAGs, the pMAG, and the nMAG, even though
      the downlink path has been switched to the nMAG already.  One
      reason to enter such an activation state may be a slow link
      between the pMAG and the LMA as described in Section 3.2.2.

4.4.  LMA Forwarding State Diagram

   The current specification of transient BCEs covers three clearly
   defined transient BCE states at the LMA, which can be used during an
   MN's handover.  Each state implies a dedicated characteristic
   regarding forwarding entries, in which forwarding rules for uplink
   traffic are maintained separately from downlink traffic.  This
   section explains how the forwarding state sequentially changes during
   the optimized handoff.  To suit different handover scenarios,
   different sequences through the forwarding states can be entered.
   Figure 4 depicts the possible cases, their sequence of forwarding
   states, and the triggers for the transitions.  Two example use cases
   are described in detail in Appendix A to illustrate which sequence
   through the forwarding states suits a particular handover.

   According to this specification, each BCE has a state associated,
   which can be either 'Active' or any of the specified transient states
   'Transient-L', 'Transient-LA', or 'Transient-A'.  In the case that a
   BCE is in 'Active' state, the information in a BCE and associated
   forwarding conforms to [RFC5213].

   Any of the transient states imply that the transient BCE has two
   forwarding entries, which are denoted as pMAG and nMAG in the
   forwarding state diagram.  The diagram includes information about the
   forwarding rule along with each forwarding entry.  This rule
   indicates whether a forwarding entry is meant to perform forwarding
   only for Uplink (Ul) traffic or to perform bi-directional forwarding
   for Uplink (Ul) and Downlink (Dl) traffic.

   State transitions can be triggered as a result of processing a
   received PBU or by a local timeout event on the LMA.  In the
   forwarding state chart below, the presence of a Transient Binding
   option in a PBU is indicated by 'Topt' as an argument to a PBU or
   PBA, respectively.  As a further argument to a PBU message, the
   source of the message is indicated, which can be either the MN's nMAG
   or pMAG.  A PBA is always sent by the LMA and addressed to the
   originator of the associated PBU.

   A handover with transient BCE is either triggered when the nMAG sends
   a PBU with a Transient Binding option or when the LMA decides to
   answer a normal PBU with a PBA after including a Transient Binding
   option.  Figure 4 illustrates the possible transitions between an
   active BCE and a transient BCE from the LMA's point of view.  It also
   shows the direct transition between two active BCE states during an
   MN's handover according to [RFC5213], bypassing any transient states.

   The diagram refers to two timeout events.  TIMEOUT_1 is set according
   to the Lifetime value in a Transient Binding option (see Section 5
   for the format of the Transient Binding option), whereas TIMEOUT_2 is
   set to ACTIVATIONDELAY (see Section 8 for the default value).

   The first sequence of a transient BCE handover is followed when the
   LMA decides not to use the optional activation state and is going
   through Transient-L state, in which the LMA continues forwarding
   downlink packets to the pMAG, whereas uplink packets are accepted and
   forwarded from both, the pMAG and the nMAG.  On reception of a PBU
   without a Transient Binding option from the nMAG, a TIMEOUT_1 event,
   or the reception of a deregistration PBU from the pMAG, the
   forwarding entry of the pMAG is removed from the MN's BCE, and the
   BCE state changes to active.

   If the LMA decides to use the activation state, the second sequence
   is used.  In this case, the BCE state turns into Transient-LA.
   Forwarding characteristics in the Transient-LA state are the same as
   for the Transient-L state, but the Transient-LA state follows a
   Transient-A state when the LMA receives a PBU from the nMAG without a
   Transient Binding option included or a TIMEOUT_1 event occurs.  In
   the Transient-A state, the LMA performs a downlink forwarding path
   switch from the pMAG to the nMAG, whereas uplink packets are still
   accepted and forwarded from both, the pMAG and the nMAG.  The
   Transient-A state is terminated by a TIMEOUT_2 event, the forwarding
   entry of the pMAG is removed from the MN's BCE, and the BCE state
   turns to active.  If the LMA receives a deregistration PBU from the
   pMAG while the associated MN's BCE is in Transient-LA state, the
   uplink forwarding rule of the pMAG is no longer valid and the
   transition through Transient-A state is skipped.  In such a case, the
   BCE turns into active state immediately.

                                +----------------+              Before
          PBU(nMAG) & PBA(LMA)  |    Active      |              Handover
        +-----------------------|                |              --------
        |                       |  pMAG [Dl,Ul]  |                   .
        |                       *----------------*                   .
        |                               |                            .
        |                               |                            V
        |               PBU(nMAG, Topt) | PBU(nMAG) & PBA(LMA, Topt) .
        |                               |                            .
        |                               |                            .
        |                               V                      Handover
        |                           __________                 Procedure
        |                          /   LMA    \                      .
        |               _________ /  selects   \ _________           .
        |            No|          \ activation /          |Yes       .
        |              |           \_state_?__/           |          .
        |              |                                  |          V
        |              V                                  V          .
        |       +--------------+                  +--------------+   .
        |       | Transient-L  |                  | Transient-LA |   .
        |       |              |                  |              |   .
        |       | pMAG [Dl,Ul] |          +-------| pMAG [Dl,Ul] |   .
        |       | nMAG [Ul]    |          |       | nMAG [Ul]    |   .
        |       +--------------+          |       +--------------+   .
        |              |                  |               |
        |              |       PBU(pMAG,  |     PBU(nMAG) | TIMEOUT_1
        |              |       lifetime=0)|               |          .
        |              |                  |               V          .
        |              |                  |       +--------------+   .
        |              |                  |       | Transient-A  |   .
        |    PBU(nMAG) | TIMEOUT_1        |       |              |   .
        |              |                  |       | nMAG [Dl,Ul] |   .
        |              |PBU(pMAG,         |       | pMAG [Ul]    |   .
        |              | lifetime=0)      |       +--------------+   .
        |              |                  |               |
        |              |                  |   PBU(pMAG,   | TIMEOUT_2
        |              |                  |    lifetime=0)|          .
        |              |                  |               |          V
        |              |                  |               |    -------
        |              |                  |               |    Handover
        |              |                  |               V    Complete
        |              |                  |        +--------------+
        |              |                  +------->|    Active    |
        |              +-------------------------->|              |
        +----------------------------------------->| nMAG [Dl,Ul] |
                                                   +--------------+

     Figure 4: Possible transient forwarding states during a handover

4.5.  MAG Operation

   In case of a handover, the MN's nMAG may decide to control the MN's
   handover at the LMA to perform a late path switch according to the
   transient BCE procedure.  In such a case, the nMAG includes the
   Transient Binding option in the PBU and sets the L-flag to 1 to
   indicate a late path switch.  Furthermore, the nMAG MUST set the
   Lifetime field of the Transient Binding option to a value larger than
   0 to propose a maximum lifetime of the transient BCE and to delimit
   the delay of switching the downlink path to the nMAG.  The chosen
   lifetime value for the Transient Binding option SHOULD be smaller
   than the chosen lifetime value for the PBU registration.  Other
   fields and options of the PBU are used according to [RFC5213].

   In case the nMAG does not include a Transient Binding option but the
   LMA decides to perform a handover according to the transient BCE
   procedure, the nMAG may receive a Transient Binding option along with
   the PBA from the LMA as a result of the PBU it sent to the LMA.

   In case the nMAG receives a PBA with a Transient Binding option
   having the L-flag set to 1, it SHOULD link the information about the
   transient BCE sequence and the associated transient BCE lifetime with
   the MN's entry in the BUL.  Since the L-flag of the Transient Binding
   option is set to 1 to indicate a late path switch, the nMAG MAY turn
   an MN's transient BCE into an active BCE before the expiration of the
   transient BCE lifetime (TIMEOUT_1), e.g., when the MN's nMAG detects
   or gets informed that address configuration and radio bearer setup
   has been completed.  To initiate turning a transient BCE into an
   active BCE, the nMAG sends a PBU to the LMA without including the
   Transient Binding option.  All fields of the PBU are set according to
   the procedure for the binding lifetime extension described in Section
   5.3.3 of [RFC5213].  In case the lifetime of a transient BCE expires
   or the LMA approves turning a transient BCE into an active BCE as a
   result of a PBU sent by the nMAG, the nMAG MUST delete all
   information associated with the transient BCE from the MN's BUL
   entry.

   In case the nMAG includes a Transient Binding option into the PBU,
   only one instance of the Transient Binding option per PBU is allowed.

   A MAG, which serves the MN current Proxy-CoA while the LMA already
   has an active or transient binding for the MN pointing to this MAG,
   SHALL NOT include a Transient Binding option in any subsequent PBU to
   create or update a transient BCE for the MN's current registration
   with this MAG.

4.6.  LMA Operation

4.6.1.  Initiation of a Transient BCE

   In case the LMA receives a handover PBU from an MN's nMAG that does
   not include a Transient Binding option and the associated MN's BCE is
   active and not in transient state, the LMA MAY take the decision to
   use a transient BCE and inform the nMAG about the transient BCE
   characteristics by including a Transient Binding option in the PBA.
   In such a case, the LMA should know about the nMAG's capability to
   support the Transient Binding option.  The configuration of the MN's
   transient BCE is performed according to the description in this
   section and the selected transient state.  Otherwise, the LMA
   processes the PBU according to the PMIPv6 protocol [RFC5213] and
   performs a normal update of the MN's BCE.

   In case the PBU from the nMAG has a Transient Binding option
   included, the LMA must enter the sequence of transient BCE states
   according to its decision whether or not to use an optional
   activation state.  In case the LMA decides not to use an activation
   state, it configures the MN's transient BCE and the forwarding rules
   according to Transient-L state.  As a result, the LMA performs a late
   path switch and forwards downlink packets for the MN towards the MN's
   pMAG, whereas uplink packets being forwarded from both Proxy-CoAs,
   the MN's pMAG, as well as from its nMAG, will be routed by the LMA.

   In case the PBU from the nMAG has a Transient Binding option included
   and the LMA decides to use an optional activation state, the LMA
   configures the MN's transient BCE and the forwarding rules according
   to Transient-LA state.  As a result, the LMA performs a late path
   switch and forwards downlink packets for the MN towards the MN's
   pMAG, whereas uplink packets being forwarded from both Proxy-CoAs,
   the MN's pMAG, as well as from its nMAG, will be routed by the LMA.
   In addition, the LMA marks the transient BCE to enter a temporary
   activation phase in Transient-A state after the LMA received an
   indication to turn a transient BCE into an active BCE.

   The LMA sets the lifetime of the transient BCE according to the
   lifetime indicated by the nMAG in the Transient Binding option's
   lifetime field or may decide to reduce the lifetime according to its
   policy.  If the lifetime value in the Transient Binding option
   exceeds the lifetime value associated with the PBU message, the LMA
   MUST reduce the lifetime of the transient BCE to a value smaller than
   the registration lifetime value in the PBU message.  In the case of a
   successful transient BCE registration, the LMA sends a PBA with a
   Transient Binding option back to the nMAG.  The L-flag of the

   Transient Binding option MUST be set to 1 in this version of the
   specification.  The lifetime field is set to the value finally chosen
   by the LMA.

   In any case where the LMA finds the L-flag of the received Transient
   Binding option set to 1, but the lifetime field of the Transient
   Binding option is set to 0, the LMA MUST ignore the Transient Binding
   option and process the PBU according to [RFC5213].  After the PBU has
   been processed successfully, the LMA sends back a PBA with the status
   field set to PBU_ACCEPTED_TB_IGNORED_SETTINGSMISMATCH.

   In case the LMA receives a Transient Binding option with the L-flag
   set to 0, this version of the specification mandates the LMA to
   ignore the Transient Binding option and process the PBU according to
   [RFC5213].  After the PBU has been processed successfully, the LMA
   sends back a PBA with the status field set to
   PBU_ACCEPTED_TB_IGNORED_SETTINGSMISMATCH.

   In case the LMA receives a PBU with a Transient Binding option
   included from a MAG that serves already as Proxy-CoA to the
   associated MN in an active or transient BCE, the LMA MUST ignore the
   Transient Binding option and process the PBU according to [RFC5213].
   After the PBU has been processed successfully, the LMA sends back a
   PBA with the status field set to
   PBU_ACCEPTED_TB_IGNORED_SETTINGSMISMATCH.  In case the MN's BCE was
   in transient state before receiving such PBU from the MAG, the LMA
   SHALL interpret this PBU as indication to turn a transient BCE into
   an active BCE and proceed with leaving the Transient-L or
   Transient-LA state, respectively.

   In any case where the LMA includes a Transient Binding option in the
   PBA, only one instance of the Transient Binding option per PBA is
   allowed.

4.6.2.  Activation of a Transient BCE

   When the LMA receives a PBU from the MN's nMAG that has no Transient
   Binding option included but the MN's BCE is in a transient state or
   the LMA receives a local event trigger due to expiration of the MN's
   transient BCE, the LMA should check whether the forwarding rules for
   the associated MN are set to route the MN's downlink traffic to the
   MN's pMAG.  If the forwarding entry for downlink packets refers to
   the MN's pMAG, the LMA must update the forwarding information to
   forward downlink packets towards the MN's nMAG.  After the forwarding
   path has been switched, the LMA must update the MN's BCE accordingly.

   If the transient BCE indicates that the LMA must consider an
   activation state Transient-A after leaving a transient BCE has been
   initiated, the LMA must keep both forwarding entries for the pMAG and
   the nMAG for uplink packets and perform forwarding of packets it
   receives from both Proxy-CoAs.  If no activation phase is indicated,
   the LMA sets the state of the MN's BCE to active and deletes any
   forwarding entry referring to the MN's pMAG.  The LMA must delete any
   scheduled timeout event for the MN that is associated with a
   transient BCE.

   When the LMA receives a deregistration PBU from the MN's pMAG, which
   has the registration lifetime set to 0 and the MN's BCE is in
   transient state, the LMA must update the forwarding rules for the MN
   and switch the downlink traffic path from the pMAG to the nMAG.
   Furthermore, the LMA sets the state of the MN's BCE to active and
   removes any forwarding entry towards the pMAG from the MN's BCE,
   irrespective of whether or not the transient BCE was configured to
   enter an activation state of Transient-A.

   When the LMA receives a local event trigger due to the expiration of
   a timer that has been set to ACTIVATIONDELAY and scheduled to
   terminate the activation state of an MN's transient BCE, the LMA sets
   the state of the MN's BCE to active and removes any forwarding entry
   towards the pMAG from the MN's BCE.

   When the LMA receives a PBU for binding lifetime extension from the
   MN's pMAG while the MN's BCE is in transient state, the LMA must
   approve the lifetime extension to pMAG according to [RFC5213] and
   proceed with the transient BCE handover towards nMAG according to
   this specification.

   When the LMA receives a PBU from pMAG or a (n+1)MAG, which indicates
   a handover, e.g., according to the indications specified in
   [RFC5213], while the MN's BCE is in any of the specified transient
   states, the LMA MUST terminate the transient state and perform a
   handover to pMAG or (n+1)MAG, respectively, according to [RFC5213].
   After the PBU has been processed successfully, the LMA sends back a
   PBA to the MAG that sent the PBU.  If the PBU included a Transient
   Binding option, the LMA must ignore the Transient Binding option and
   set the status code of the PBA to
   PBU_ACCEPTED_TB_IGNORED_SETTINGSMISMATCH.

4.7.  MN Operation

   For a single radio handover, this specification does not require any
   additional functionality on the mobile node, when compared to
   [RFC5213].

   During dual radio handover, the MN benefits most from the transient
   BCE extension to PMIPv6 when it is able to keep communication on the
   previous interface while it is setting up its handover target
   interface with the configuration context that has been received as a
   result of the new interface's attachment to the nMAG.  Various
   techniques enable support for such an operation, e.g., the use of a
   virtual interface on top of physical radio interfaces [NETEXT] or
   implementation-specific extensions to the MN's protocol stack.
   Details about how to enable such make-before-break support on the MN
   are out of scope of this document.

4.8.  Status Values

   This section specifies the following PBA status value (6) for
   transient binding cache entry support.  This status value is smaller
   than 128 and has been added to the set of status values specified in
   [RFC5213].

   PBU_ACCEPTED_TB_IGNORED_SETTINGSMISMATCH:  6

      The LMA has processed and accepted the PBU, but the attached
      Transient Binding option has been ignored.

4.9.  Protocol Stability

   The specification and use of transient BCEs ensures that correct
   PMIPv6 operation according to [RFC5213] will not be broken in any
   case.  Such cases include loss of signaling information and
   incompatibility between an nMAG and an LMA in case one or the other
   side does not support the transient BCE option.  The following list
   summarizes such cases and describes how the PMIPv6 protocol operation
   resolves incompatibility or loss of a signaling message.

   LMA does not support transient BCEs:  In case the nMAG sends a PBU
      with a Transient Binding option included to an LMA but the LMA
      does not support transient BCEs, the LMA ignores the unknown
      option [RFC3775] and processes the PBU according to [RFC5213].
      Since the nMAG receives a PBA that has no Transient Binding option
      included, it does not set any transient binding information in the
      MN's BUL entry and operates according to [RFC5213].

   nMAG does not support transient BCEs:  In case the LMA makes the
      decision to perform a handover according to any of the specified
      transient BCE sequences and includes a Transient Binding option in
      the PBA, the receiving nMAG ignores the unknown option [RFC3775]
      and processes the PBA according to [RFC5213].  As the LMA does not
      get any further indication or feedback about the incompatibility
      at the nMAG, the LMA enters the selected transient state, which
      will be terminated at the latest time after (TIMEOUT_1 +
      ACTIVATIONDELAY) seconds.  During this period, the nMAG performs
      according to the PMIPv6 specification [RFC5213], whereas the LMA
      will accept all uplink packets for the MN, from the pMAG, as well
      as from the nMAG according to the transient BCE specification.  It
      is transparent to the nMAG if the LMA forwards downlink packets to
      the pMAG during the transient BCE phase; thus, no protocol
      conflict occurs due to the different states on the nMAG and the
      LMA.

   Loss of Transient Binding option:  As the Transient Binding option is
      included in the PBU and PBA, recovery from signaling packet loss
      is according to the PMIPv6 protocol operation and associated
      re-transmission mechanisms [RFC5213].

   Missing PBU to turn a transient BCE into an active BCE:  According to
      this specification, a lifetime for TIMEOUT_1 is signaled in the
      Transient Binding option, and turning a transient BCE into an
      active BCE is initiated at the latest time after the timer
      TIMEOUT_1 has elapsed.  In case PBU signaling is lost or the nMAG
      fails to initiate turning a transient BCE into an active BCE, the
      transient state of the MN's BCE will be terminated after
      expiration of the set lifetime, i.e., stable operation of the
      PMIPv6 protocol [RFC5213] has reliably recovered.

   Lost connection with pMAG during late path switch:  In case an MN
      loses connectivity to its pMAG during a transient BCE phase with
      late path switch and the nMAG fails to initiate turning a
      transient BCE into an active BCE to perform the path switch to the
      nMAG, in a worst-case scenario, downlink packets are lost until
      the chosen TIMEOUT_1 expires.  After TIMEOUT_1 seconds, the
      protocol operation has been recovered successfully.  However, this
      case is very unlikely for two reasons: If the connectivity to the
      pMAG is lost, the pMAG will send a deregistration PBU for the MN
      to the LMA, which results in turning the transient BCE into an
      active BCE and in a path switch.  Furthermore, the nMAG will
      initiate turning the transient BCE into an active BCE as soon as
      the setup of the data link between the MN and the nMAG has been
      completed (Section 4.4).  Note that this case, in particular,
      affects downlink packets, whereas uplink packets can be sent

      through the new connection after a broken link to the pMAG has
      been detected.

   Binding lifetime extension from pMAG while MN's BCE is transient:  As
      the binding lifetime of the pMAG and the nMAG is not correlated,
      pMAG may send a PBU for binding lifetime extension to the MN's LMA
      while the MN's BCE is in transient state.  In such a case, the LMA
      will approve the binding lifetime extension to pMAG according to
      [RFC5213] and proceed with the transient BCE handover towards nMAG
      according to this specification.

   The specification of the transient BCE extension maintains stable
   operation of PMIPv6 in case the MN performs very frequent handover,
   e.g., movement while the MN's handover between the pMAG and the nMAG
   is still in progress.  Such corner cases are summarized in the
   following list.

   Handover to (n+1)MAG during transient BCE:  In case the MN's BCE is
      transient due to a handover from the pMAG to nMAG and during the
      transient BCE, the MN performs a further handover to a MAG that is
      different from pMAG and nMAG, say to (n+1)MAG, the LMA terminates
      the transient BCE and performs a handover to (n+1)MAG according to
      [RFC5213].

   Handover back to pMAG during transient BCE (ping pong):  In case the
      MN's BCE is transient due to a handover from the pMAG to nMAG and
      the MN moves back from nMAG to pMAG during the transient BCE, the
      LMA terminates the transient BCE and performs a handover to pMAG
      according to [RFC5213].

5.  Message Format

5.1.  Transient Binding Option

   This section describes the format of the Transient Binding option,
   which can be included in a Proxy Binding Update message and a Proxy
   Binding Acknowledge message.  The use of this Mobility Header option
   is optional.

   The Transient Binding option can be included in a PBU message, which
   is sent by an MN's nMAG as a result of a handover.  In such a case,
   the nMAG controls the transient BCE on the LMA.  Alternatively, the
   LMA may attach the Transient Binding option in a PBA for two reasons.
   Either it replies to a received PBU with an attached Transient
   Binding option to approve or correct the transient BCE lifetime, or
   it notifies the nMAG about its decision to enter a transient BCE
   without having received a Transient Binding option from the nMAG in
   the associated PBU beforehand.

   The Transient Binding option has no alignment requirement.  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    |L|   Lifetime    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type: Identifies the Transient Binding option (43).

   Length: 8-bit unsigned integer indicating the length of the option in
   octets, excluding the Type and the Length fields.  This field MUST be
   set to 2.

   L-Flag: Indicates that the LMA applies late path switch according to
   the transient BCE state.  If the L-flag is set to 1, the LMA
   continues to forward downlink packets towards the pMAG.  Different
   setting of the L-Flag may be for future use.

   Lifetime: Maximum lifetime of a Transient-L state in multiple of 100
   ms.

6.  IANA Considerations

   This specification adds a new Mobility Header option, the Transient
   Binding option.  The Transient Binding option is described in
   Section 5.1.  The Type value (43) for this option has been registered
   in the Mobility Options registry, the numbering space allocated for
   the other mobility options, as defined in [RFC3775].

   This specification also adds one status code value to the Proxy
   Binding Acknowledge message, the
   PBU_ACCEPTED_TB_IGNORED_SETTINGSMISMATCH status code (6).  The
   PBU_ACCEPTED_TB_IGNORED_SETTINGSMISMATCH status code is described in
   Section 4.8.  Its value has been assigned from the Status Codes sub-
   registry as defined in [RFC3775] and has a value smaller than 128.

7.  Security Considerations

   Signaling between MAGs and LMAs as well as information carried by PBU
   and PBA messages is protected and authenticated according to the
   mechanisms described in [RFC5213].  No new security considerations
   are introduced in addition to those in [RFC5213].  Thus, the security
   considerations described throughout [RFC5213] apply here as well.

   In case the MAGs or LMAs make use of a further protocol interface to
   an external component, such as for support of transient BCE control,
   the associated protocol must be protected and information must be
   authenticated.

8.  Protocol Configuration Variables

   LMA values:

   o  'ACTIVATIONDELAY': This value is set by default to 2000 ms and can
      be administratively adjusted.

9.  Contributors

   Many thanks to Jun Awano, Suresh Krishnan, Long Le, Kent Leung,
   Basavaraj Patil, and Rolf Sigle for contributing to this document.

10.  Acknowledgments

   The authors would like to thank Telemaco Melia, Vijay Devarapalli,
   Rajeev Koodli, Ryuji Wakikawa, and Pierrick Seite for their valuable
   comments to improve this specification.

11.  References

11.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3775]   Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
               in IPv6", RFC 3775, June 2004.

   [RFC5213]   Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury,
               K., and B. Patil, "Proxy Mobile IPv6", RFC 5213,
               August 2008.

11.2.  Informative References

   [NETEXT]    Melia, T., Ed. and S. Gundavelli, Ed., "Logical Interface
               Support for multi-mode IP Hosts", Work in Progress,
               October 2010.

   [RFC4861]   Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
               "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
               September 2007.

   [RFC4862]   Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
               Address Autoconfiguration", RFC 4862, September 2007.

   [TS23.401]  "General Packet Radio Service (GPRS) enhancements for
               Evolved Universal Terrestrial Radio Access Network
               (E-UTRAN) access", <http://www.3gpp.org>.

   [TS23.402]  "Architecture enhancements for non-3GPP accesses (Release
               9)", <http://www.3gpp.org>.

Appendix A.  Example Use Cases for Transient BCE

A.1.  Use Case for Single Radio Handover

   In some systems, such as the 3GPP Evolved Packet Core, PMIPv6 is
   supported for providing network-based mobility between the Serving
   Gateway (i.e., MAG) and the Packet Data Network Gateway (i.e., LMA)
   and handover mechanisms are implemented in the access network to
   optimize handover for single radio mobile nodes [TS23.401].

   In such a system, a well structured inter-MAG handover procedure has
   been developed and effectively used.  In order to switch the data
   tunnel path between the LMA and the pMAG in a systematic way that
   reduces packet loss and delay, this inter-MAG handover sets up the
   uplink data path from the mobile node through the nMAG and to the LMA
   first.  As soon as the uplink data path is set up, the mobile node is
   able to forward uplink data packets through the nMAG to the LMA.

   Since the downlink data path between the LMA and the nMAG is not set
   up at the same time as the uplink data path, the LMA must continue to
   forward downlink data packets to the pMAG.  Additionally, this system
   utilizes a layer 2 forwarding mechanism from the previous Access
   Network (pAN) to the new Access Network (nAN), which enables the
   delivery of the downlink data packets to the mobile node location
   while being attached to the nMAG.

   In order for the LMA to be able to forward the mobile node uplink
   data packets to the Internet, the transient BCE mechanism is used at
   the nMAG to send a PBU with the Transient Binding option to allow the
   LMA to create a transient BCE for the mobile node with uplink
   forwarding capabilities while maintaining uplink and downlink
   forwarding capabilities for the Proxy-CoA that is hosted at the pMAG.

   During the lifetime of the transient BCE, the LMA continues to accept
   uplink traffic from both previous and new MAG while forwarding
   downlink traffic to the pMAG only.  While the MN is able to receive
   downlink traffic via the pMAG, the mechanism used in the pMAG's
   access network to forward downlink traffic to the current location of
   the mobile node in the nMAG's access network during an intra-
   technology handover is out of scope of this description.

   When the nMAG receives an indication that the inter-MAG handover
   process has completed, the nMAG sends another PBU without including a
   Transient Binding option to update the mobile node's transient BCE to
   a regular PMIPv6 BCE with bi-directional capabilities.  This
   mechanism is used by the LMA as an indication to switch the tunnel to
   point to the nMAG, which results in a smoother handover for the MN.

   An example of using a transient BCE for intra-technology handover is
   illustrated in Figure 5.  When the nMAG receives the indication that
   the MN is moving from the pMAG's access network to the nMAG's area,
   the nMAG sends a PBU on behalf of the MN to the MN's LMA.  In this
   PBU, the nMAG includes the MN-ID, the HNP, and the interface ID as
   per PMIPv6 base protocol [RFC5213].

   Furthermore, the nMAG indicates an intra-technology handover by means
   of the HI option and includes the Transient Binding option to
   indicate to the LMA that this registration should result in a
   transient BCE with a late downlink path switch.  The nMAG sets the
   value of the transient BCE lifetime to a value that is dependent on
   the deployment and operator specific [D].

   After the nMAG receives an indication that the MN has completed the
   handover process and the data path is ready to move the tunnel
   completely from the pMAG to the nMAG, the nMAG SHOULD send a PBU to
   allow the LMA to turn the MN's transient BCE into a regular BCE and
   to switch the data path completely to be delivered through the new
   Proxy-CoA.  In this case, the nMAG sends a PBU with the MN-ID,
   Interface ID, and HNP and at the same time indicates an intra-
   technology handover by means of the HI option.  In this PBU, the nMAG
   MUST NOT include the Transient Binding option, as shown in Figure 5
   [E].

   In the event that the nMAG receives downlink traffic destined to the
   MN from the LMA after sending a PBU with the Transient Binding option
   included, the nMAG MUST deliver the downlink traffic to the MN.  In
   this case, the nMAG SHOULD send a PBU to ensure that the transient
   BCE has been turned into an active BCE.

          +-----+      +----+       +----+                    +-----+
          | MN  |      |pMAG|       |nMAG|                    | LMA |
          +-----+      +----+       +----+                    +-----+
             |            |            |  bi-directional         |
             |            |<<<<<<<<======================>>>>>>>>|<-->
             |            |            |                         |
             |            |            |                         |
       [Handoff Event]    |            |                         |
             |      [MN HO Event]      |                         |
             |            |     [HO Event Acquire]               |
             |            |            |                         |
      [LL Attach to       |            |                         |
           nMAG]          |            |-----PBU(transient)----->|
             |            |            |                        [D]
             |            |            |<-----PBA(transient)-----|
             |            |            |                         |
             |            |          bi-directional              |
             |      |<--->|<<<<<<<<======================>>>>>>>>|<-->
             |     pAN    |            |                         |
             |      |----------->|     |                         |
             |            |     nAN    |                         |
             |<------------------|     |uplink only              |
             |------------------>|---->|>>>>>>===========>>>>>>>>|--->
             |            |            |                         |
             |            |      [HO Complete]                   |
             |            |            |----------PBU----------->|
             |            |            |                        [E]
             |            |            |<---------PBA -----------|
             |            |`           |                         |
             |            |            |<<<<<<<<=========>>>>>>>>|<-->
             |            |            |                         |

     Figure 5: Transient BCE support for an intra-technology handover

A.2.  Use Case for Dual Radio Handover

   During an inter-technology handover, the LMA shall, on the one hand,
   be able to accept uplink packets of the MN as soon as the MN has
   finalized address configuration at the new IF2 and may start using
   the new interface for data traffic, i.e., the PBU for the uplink
   shall be done before the radio setup procedure is finalized.  But, to
   allow the MN to keep sending its data traffic on IF1 during the
   handover, uplink packets with the previously existing binding on IF1
   shall still be accepted by the LMA until the MN detaches from pMAG
   with IF1 and the pMAG has deregistered the MN's attachment at the LMA
   by means of sending a PBU with lifetime 0.  This is of particular
   importance as sending the registration PBU from the nMAG is
   transparent to the mobile node, i.e., the MN does not know when the

   PBU has been sent.  On the other hand, switching the downlink path
   from the pMAG to the nMAG shall be performed at the LMA only after
   completion of the IP configuration at the MN's IF2 and after a
   complete setup of the access link between the MN and the nMAG.  How
   long this takes depends on some interface-specific settings on the MN
   as well as on the duration of the target system's radio layer
   protocols, which is transparent to the LMA but may be known to MAGs.

   Similar to the use case for single radio handover, a transient BCE
   can be utilized for MNs with dual radio capability.  Such MNs are
   still able to send and receive data on the previous interface during
   the address configuration on the new interface.  Forwarding between
   the nMAG and pMAG is not required, but the case in which the LMA
   immediately starts forwarding downlink data packets to the nMAG has
   to be avoided.  This is enabled by a PBU that has the Transient
   Binding option included, so that it is not necessary that MN and LMA
   synchronize the point in time for switching interfaces and turning a
   transient BCE into an active BCE.

   When the handover is finalized, the nMAG sends a second PBU without
   including the Transient Binding option and the LMA turns the MN's BCE
   into an active BCE.  This PBU may overtake packets-on-the-fly from MN
   to LMA via pMAG (e.g., if the previous interface was of type GSM or
   Universal Mobile Telecommunications System (UMTS) with up to 150
   milliseconds of uplink delay).  The LMA has to drop all these packets
   from the pMAG due to the characteristics of the MN's active BCE.
   This can be avoided by entering another transient BCE state
   (Transient-A) during the activation phase and is characteristic for
   this use case.  Whether or not to enter a Transient-A state is
   decided by the LMA.

   The use of a transient BCE for an inter-technology handover is
   exemplarily illustrated in Figure 6.  The MN attaches to the PMIPv6
   network with IF1 according to the procedure described in [RFC5213].
   The MN starts receiving data packets on IF1.  When the MN activates
   IF2 to prepare an inter-technology handover, the nMAG receives an
   attach indication and sends the PBU to the LMA to update the MN's
   point of attachment and to retrieve configuration information for the
   MN (e.g., HNP).  The LMA is able to identify an inter-technology
   handover by means of processing the HI option coming along with the
   PBU sent by the nMAG.  As in this example, the nMAG includes the
   Transient Binding option in the PBU to control the transient BCE at
   the LMA, the LMA updates the MN's BCE according to the transient BCE
   specification described in this document and marks the state of the
   BCE as 'transient' [F].

   As a result of the transient BCE, the LMA keeps using the previous
   forwarding information towards the pMAG binding as forwarding
   information until the transient BCE gets turned into active.  The LMA
   acknowledges the PBU by means of sending a PBA to the nMAG.  The nMAG
   now has relevant information available, such as the MN's HNP, to set
   up a radio bearer and send a Router Advertisement to the MN.  While
   the MN's BCE at the LMA has a transient characteristic, the LMA
   forwards uplink packets from the MN's pMAG as well as from its nMAG.
   The nMAG may recognize when the MN's IF2 is able to send and receive
   data packets and sends a new PBU to the LMA without including the
   Transient Binding option to initiate turning the MN's transient BCE
   into an active BCE [G].  As a result of successfully turning the MN's
   transient BCE into an active BCE, downlink packets will be forwarded
   towards the MN's IF2 via the nMAG [H].

    +------+                 +----+      +----+                 +---+
    |  MN  |                 |pMAG|      |nMAG|                 |LMA|
    +------+                 +----+      +----+                 +---+
    IF2 IF1                    |           |                      |
     |   |                     |           |                      |
     |   |- - - - - - - - - Attach         |                      |
     |   |                     |---------------PBU--------------->|
     |   |                     |<--------------PBA----------------|
     |   |--------RtSol------->|           |                      |
     |   |<-------RtAdv--------|           |                      |
     |  Addr.                  |           |                      |
     |  Conf.                  |           |                      |
     |   |<------------------->|==================data============|<--->
     |   |                     |           |                      |
     |- - - - - - - - - - - - - - - - - Attach                    |
     |   |                     |           |----PBU(transient)--->|
     |   |                     |           |<---PBA(transient)---[F]
     |------RAT Configuration--------------|                      |
     |   |<--------------------|==================data============|<---
     |-------RtSol-(optional)------------->|                      |
     |<-----------RtAdv--------------------|                      |
   Addr. |                     |           |                      |
   Conf  |                     |           |                      |
     |------------NSol-------------------->|---------PBU-------->[G]
     |   |                     |           |<--------PBA----------|
     |<------------------------------------|========data=========[H]<-->
     |   |                     |           |                      |
     |   |                     |           |                      |
     |   |                     |           |                      |

           Figure 6: Late path switch with PMIPv6 transient BCEs

Appendix B.  Applicability and Use of Static Configuration at the LMA

   During the working group discussion of the functionality introduced
   by this document, it was mentioned that some current Home Agents are
   already handling some features and functionality introduced in this
   document via some static configuration.  This Appendix captures the
   analysis that describes which functionality can be handled securely
   using a static configuration and which can not.  In these cases where
   static configuration can be used, this section documents the possible
   disadvantages versus using the procedures captured in this document.

B.1.  Early Uplink Traffic from the nMAG

   This use case is related to the handoff scenario when the access
   network establishes the uplink tunnel to the LMA before the downlink
   portion is done.  Consequently, when the mobile node is attached to
   the nMAG and in the case of active handoff, the UE will start sending
   uplink traffic to the LMA through the nMAG.

   Since the LMA has a proxy BCE for this mobile node that points to the
   Proxy-CoA that is hosted at the pMAG, the LMA has a routing entry for
   the MN HNP that points to the pMAG-LMA tunnel.  Any uplink packet
   coming from the nMAG will be dropped by the LMA.

   Allowing the LMA to forward the received uplink traffic from the nMAG
   to the Internet while the MN BCE points to the Proxy-CoA hosted at
   the pMAG is a violation of all mobility protocols that require a
   secure signaling exchange between the nMAG and the LMA before
   forwarding such traffic to the Internet.  Otherwise, the LMA will be
   modifying the mobile node's routing entry based on an unsecured data
   traffic packet coming from the nMAG.

   Therefore, this case cannot be addressed by any statically configured
   information on the LMA.  On the contrary, a secure signaling using
   Transient Binding option as detailed in this document is required to
   create a transient state for the mobile node BCE at the LMA.  This
   transient state will allow a temporary routing entry of the mobile
   node to point to the nMAG Proxy-CoA.

B.2.  Late Uplink Traffic from the pMAG

   This case is a very common case where the mobile node is handing over
   to another MAG while there is still some uplink traffic in flight
   coming from the pMAG.  In this case, the LMA has the MN BCE points to
   the mobile node location before the handoff, i.e., pMAG Proxy-CoA.
   Then the LMA receives a PBU from the nMAG over a secure signaling
   tunnel, e.g., IPsec tunnel, which indicates some type of handoff as
   per the value in the handoff indicator mobility option.

   If the PBU received from the nMAG was sent using the secure tunnel
   and successfully processed by the LMA, the LMA according to [RFC5213]
   switches the IP-in-IP tunnel to point to the nMAG Proxy-CoA.
   However, as the LMA is fully aware of the mobile node movement via
   secure signaling from the nMAG and the content of the PBU, which, in
   particular, contains the Handoff Indicator mobility option, the LMA
   can process some intelligence to allow the mobile node's late
   in-flight uplink traffic coming over the pMAG-LMA tunnel to proceed
   to the Internet.

   In order to handle all handoff circumstances, the activation
   mechanism as described in this document is preferable over a
   statically configured timer, and it would dynamically help in ending
   the late forwarding from the pMAG based on a protected signaling from
   the pMAG.

B.3.  Late Switching of Downlink Traffic to nMAG

   One main use case of transient bindings is the late switching of
   downlink traffic routing at the LMA.  This allows IP mobility
   protocol signaling between nMAG and LMA to be performed decoupled
   from the setup of the new link-layer connectivity, e.g., for
   performing a handover to an interface with time-consuming link setup
   procedures or for a make-before-break handover between interfaces.

   LMA behavior according to [RFC5213] does not allow for late path
   switching.  The LMA, according to [RFC5213], can only act upon the
   Handover Indicator and has no information on the time of completion
   of link layer setup.  Even if an LMA implementation would be
   configured to delay the path switching by a fixed time, which would
   violate [RFC5213], this would not lead to smooth handover performance
   but would even add latency to the handover.  Only additional
   signaling as provided by this document provides the information that
   late switching is applicable and enables a synchronization of the
   handover sequence, i.e., the switching is adapted both to the
   finalization of the link between mobile terminal and nMAG and to the
   release of the link between mobile terminal and pMAG, whatever comes
   first.  Stable handover performance is achieved using protected
   PMIPv6 signaling as per [RFC5213].

Authors' Addresses

   Marco Liebsch (editor)
   NEC Laboratories Europe
   NEC Europe Ltd.
   Kurfuersten-Anlage 36
   69115 Heidelberg,
   Germany

   Phone: +49 6221 4342146
   EMail: marco.liebsch@neclab.eu

   Ahmad Muhanna
   Ericsson
   2201 Lakeside Blvd.
   Richardson, TX  75082,
   USA

   Phone: +1 (972) 583-2769
   EMail: ahmad.muhanna@ericsson.com

   Oliver Blume
   Alcatel-Lucent Deutschland AG
   Bell Labs
   Lorenzstr. 10
   70435 Stuttgart,
   Germany

   Phone: +49 711 821-47177
   EMail: oliver.blume@alcatel-lucent.com

 

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