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rfc:rfc6058

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.

Liebsch, et al. Experimental [Page 1] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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.

Liebsch, et al. Experimental [Page 2] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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

Liebsch, et al. Experimental [Page 3] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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

Liebsch, et al. Experimental [Page 4] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 5] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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.

Liebsch, et al. Experimental [Page 6] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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].

Liebsch, et al. Experimental [Page 7] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

     +------+                 +----+      +----+                 +---+
     |  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.

Liebsch, et al. Experimental [Page 8] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 9] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

    +------+              +----+                   +---+
    |  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.

Liebsch, et al. Experimental [Page 10] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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.

Liebsch, et al. Experimental [Page 11] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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

Liebsch, et al. Experimental [Page 12] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 13] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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.

Liebsch, et al. Experimental [Page 14] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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.

Liebsch, et al. Experimental [Page 15] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 16] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

                              +----------------+              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

Liebsch, et al. Experimental [Page 17] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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.

Liebsch, et al. Experimental [Page 18] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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

Liebsch, et al. Experimental [Page 19] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 20] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 21] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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].

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 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

Liebsch, et al. Experimental [Page 23] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

    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.

Liebsch, et al. Experimental [Page 24] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 25] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 26] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 [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>.

Liebsch, et al. Experimental [Page 27] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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.

Liebsch, et al. Experimental [Page 28] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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.

Liebsch, et al. Experimental [Page 29] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

        +-----+      +----+       +----+                    +-----+
        | 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

Liebsch, et al. Experimental [Page 30] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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].

Liebsch, et al. Experimental [Page 31] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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

Liebsch, et al. Experimental [Page 32] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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.

Liebsch, et al. Experimental [Page 33] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

 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].

Liebsch, et al. Experimental [Page 34] RFC 6058 Transient Binding for Proxy Mobile IPv6 March 2011

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

Liebsch, et al. Experimental [Page 35]

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