





Internet Engineering Task Force (IETF)                        A. Muhanna
Request for Comments: 5845                                     M. Khalil
Category: Standards Track                                       Ericsson
ISSN: 2070-1721                                            S. Gundavelli
                                                                K. Leung
                                                                   Cisco
                                                               June 2010


  Generic Routing Encapsulation (GRE) Key Option for Proxy Mobile IPv6

Abstract

   This specification defines a new mobility option for allowing the
   mobile access gateway and the local mobility anchor to negotiate
   Generic Routing Encapsulation (GRE) encapsulation mode and exchange
   the downlink and uplink GRE keys that are used for marking the
   downlink and uplink traffic that belong to a specific mobility
   session.  In addition, the same mobility option can be used to
   negotiate the GRE encapsulation mode without exchanging the GRE keys.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc5845.

Copyright Notice

   Copyright (c) 2010 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





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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  3
     2.1.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  GRE Encapsulation and Key Exchange . . . . . . . . . . . . . .  4
     3.1.  GRE Encapsulation Overview . . . . . . . . . . . . . . . .  4
     3.2.  GRE Encapsulation Mode Only  . . . . . . . . . . . . . . .  6
     3.3.  GRE Encapsulation and Key Exchange . . . . . . . . . . . .  6
       3.3.1.  Initial GRE Key Exchange . . . . . . . . . . . . . . .  6
       3.3.2.  GRE Key Exchange during Binding Re-Registration  . . .  7
   4.  Mobile Access Gateway Considerations . . . . . . . . . . . . .  8
     4.1.  Extensions to the Conceptual Data Structure  . . . . . . .  8
     4.2.  Operational Summary  . . . . . . . . . . . . . . . . . . .  9
   5.  Local Mobility Anchor Considerations . . . . . . . . . . . . . 10
     5.1.  Extensions to the Binding Cache Entry  . . . . . . . . . . 10
     5.2.  Operational Summary  . . . . . . . . . . . . . . . . . . . 11
   6.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  GRE Key Option . . . . . . . . . . . . . . . . . . . . . . 12
     6.2.  Proxy Binding Update Message Extension . . . . . . . . . . 13
     6.3.  Proxy Binding Acknowledgement Message Extension  . . . . . 14
     6.4.  Status Codes . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Data Packets Processing Considerations . . . . . . . . . . . . 15
     7.1.  Tunneling Format . . . . . . . . . . . . . . . . . . . . . 15
     7.2.  TLV-Header Tunneling Negotiation . . . . . . . . . . . . . 16
     7.3.  Mobile Access Gateway Operation  . . . . . . . . . . . . . 18
       7.3.1.  Sending and Receiving Data Packets . . . . . . . . . . 18
     7.4.  Local Mobility Anchor Operation  . . . . . . . . . . . . . 19
       7.4.1.  Sending and Receiving Data Packets . . . . . . . . . . 20
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     11.2. Informative References . . . . . . . . . . . . . . . . . . 22











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1.  Introduction

   The Proxy Mobile IPv6 specification [RFC5213] and IPv4 Support for
   Proxy Mobile IPv6 [RFC5844] allow the use of IPv6 and IPv4
   encapsulation modes as specified in [RFC2473] and [RFC2003] for the
   tunneled traffic between the local mobility anchor (LMA) and the
   mobile access gateway (MAG).  There are scenarios where these
   encapsulation modes are not sufficient to uniquely identify the
   destination of packets of a specific mobility session.  Thus, there
   is a need for an encapsulation mode with richer semantics.  The
   Generic Routing Encapsulation (GRE) [RFC2784], and the Key extension
   as defined in [RFC2890], has the required semantics to allow such a
   distinction for use in Proxy Mobile IPv6.

   This specification defines the GRE Key option to be used for the
   negotiation of GRE encapsulation mode and exchange of the uplink and
   downlink GRE keys.  The negotiated downlink and uplink GRE keys can
   be used for marking the downlink and uplink traffic for a specific
   mobility session.  In addition, this specification enables the mobile
   access gateway and the local mobility anchor to negotiate the use of
   GRE encapsulation mode without exchanging the GRE keys.

   This specification has no impact on IPv4 or IPv6 mobile nodes.

2.  Conventions and Terminology

2.1.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   specification are to be interpreted as described in RFC 2119
   [RFC2119].

2.2.  Terminology

   All the general mobility-related terminology and abbreviations are to
   be interpreted as defined in the Mobile IPv6 [RFC3775], Proxy Mobile
   IPv6 [RFC5213], and IPv4 Support for Proxy Mobile IPv6 [RFC5844]
   specifications.  The following terms are used in this specification.

   Downlink Traffic

      The traffic in the tunnel between the local mobility anchor and
      the mobile access gateway, heading towards the mobile access
      gateway and tunneled at the local mobility anchor.  This traffic
      is also called forward direction traffic.





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   Uplink Traffic

      The traffic in the tunnel between the mobile access gateway and
      the local mobility anchor, heading towards the local mobility
      anchor and tunneled at the mobile access gateway.  This traffic is
      also called reverse direction traffic.

   Downlink GRE Key

      The GRE key is assigned by the mobile access gateway and used by
      the local mobility anchor to mark the downlink traffic that
      belongs to a specific mobility session as described in this
      specification.

   Uplink GRE Key

      The GRE key is assigned by the local mobility anchor and used by
      the mobile access gateway to mark the uplink traffic that belongs
      to a specific mobility session as described in this specification.


   A Policy Check

      When a local mobility anchor receives an initial, handoff-
      triggered Binding Lifetime Extension, or Binding Lifetime
      Extension Proxy Binding Update for a mobility session, the local
      mobility anchor determines if the GRE encapsulation mode only or
      GRE encapsulation and GRE keys are required based on a policy
      check.  This policy could be a per-MAG-LMA pair, a per-LMA local
      policy, a per-MN policy, or the combination of any of them.

3.  GRE Encapsulation and Key Exchange

   This section describes how GRE encapsulation mode is negotiated and
   the GRE keys are dynamically exchanged using Proxy Mobile IPv6
   protocol [RFC5213] signaling.

3.1.  GRE Encapsulation Overview

   Using the GRE Key option defined in this specification, the mobile
   access gateway and the local mobility anchor can negotiate GRE
   encapsulation mode only or GRE encapsulation mode and exchange the
   GRE keys for marking the downlink and uplink traffic.  In the case
   when GRE encapsulation mode only is negotiated between the mobile
   access gateway and the local mobility anchor, then no GRE keys are
   used.





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   However, once the GRE keys have been exchanged between the mobile
   access gateway and the local mobility anchor as per this
   specification, the mobile access gateway will use the uplink GRE key
   that is assigned by the local mobility anchor in the GRE header of
   the uplink payload packet.  Similarly, the local mobility anchor will
   use the downlink GRE key as negotiated with the mobile access gateway
   in the GRE header of the downlink payload packet.

   The following illustration explains the use of GRE encapsulation mode
   and the GRE keys for supporting the usecase where overlapping IPv4
   private address [RFC1918] allocation is in use.

                                                          +------------+
                                                          | Operator-A |
                                                          |            |
                                                          | 10.x.0.0/16|
                                                          +------------+
                                                                   /
        +------+                                      +------+    /
        |      |      ==========================      |      |   /
 MN-1---|      |    /                            \    |      |  / Key-1
        |  M   |   / ---Flows with GRE Key-1 ---- \   |  L   | / Traffic
 MN-2---|  A   |--|                                |--|  M   |-
        |  G   |   \ ---Flows with GRE Key-2 ---- /   |  A   | \ Key-2
 MN-3---|      |    \                            /    |      |  \Traffic
        |      |      ==========================      |      |   \
 MN-4---|      |       Proxy Mobile IPv6 Tunnel       |      |    \
        +------+                                      +------+     \
                                                                    \
                   Operator-C: Access Network             +------------+
                                                          | Operator-B |
                                                          |            |
                                                          | 10.x.0.0/16|
                                                          +------------+

    Figure 1: GRE Tunneling for IPv4 Private Address Space Overlapping

   Figure 1 illustrates a local mobility anchor providing mobility
   service to mobile nodes that are from different operators and are
   assigned IPv4 addresses from overlapping private address space.  In
   this scenario, the mobile access gateway and the local mobility
   anchor must be able to distinguish flows belonging to different
   operators.

   The mobile nodes MN-1 and MN-2 are visiting from Operator-A, and the
   mobile nodes MN-3 and MN-4 are visiting from Operator-B.  The mobile
   access gateway and the local mobility anchor exchange a specific pair




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   of downlink and uplink GRE keys and save them as part of the mobile
   node's binding to be used for identifying the flows belonging to each
   mobile node.

   The LMA and the MAG will be able to distinguish each mobile node
   flow(s) based on the GRE key present in the GRE header of the
   tunneled payload packet, and route them accordingly.  However, the
   GRE keys, as in this specification, apply to the individual mobility
   binding updated by the Proxy Binding Update but not to all bindings
   that the mobile may have registered following procedures described in
   [RFC5648].

3.2.  GRE Encapsulation Mode Only

   In order for the mobile access gateway to request GRE encapsulation
   mode only without exchanging the GRE keys, the mobile access gateway
   MUST include the GRE Key option but omit the GRE Key Identifier field
   in the Proxy Binding Update.

   If the local mobility anchor supports GRE encapsulation and the
   received Proxy Binding Update contains the GRE Key option but the GRE
   Key Identifier field is omitted, the mobile access gateway is
   requesting GRE encapsulation without exchanging the GRE keys
   dynamically.  If the Proxy Binding Update processing is successful,
   the local mobility anchor sends a successful Proxy Binding
   Acknowledgement message with the GRE Key option but the GRE Key
   Identifier field is omitted.

   When the mobile access gateway and the local mobility anchor
   successfully negotiate the GRE encapsulation mode only, then no GRE
   keys are used.

3.3.  GRE Encapsulation and Key Exchange

   The following subsections describe how the mobile access gateway and
   the local mobility anchor negotiate GRE encapsulation and exchange
   downlink and uplink GRE keys using the Proxy Mobile IPv6 registration
   procedure.

3.3.1.  Initial GRE Key Exchange

   When the mobile access gateway determines, based on, e.g., private
   IPv4 address support [RFC1918], the mobile access gateway local
   policy, or the MAG-LMA peer agreement, that GRE encapsulation is
   needed and GRE keys are required, the mobile access gateway MUST
   include the GRE Key option in the initial Proxy Binding Update





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   message sent to the local mobility anchor.  The mobile access gateway
   MUST include the downlink GRE key in the GRE Key Identifier field of
   the GRE Key option.

   After the local mobility anchor successfully processes the initial
   Proxy Binding Update and accepts the GRE encapsulation request and
   the downlink GRE key based on a policy check, the local mobility
   anchor MUST include the GRE Key option with the uplink GRE key in the
   GRE Key Identifier field in a successful Proxy Binding
   Acknowledgement and send it to the mobile access gateway.

3.3.2.  GRE Key Exchange during Binding Re-Registration

   If the local mobility anchor has successfully negotiated and
   exchanged the initial GRE keys with the mobile access gateway for a
   specific mobile node's mobility session, the local mobility anchor
   MUST maintain the same negotiated uplink GRE key for the lifetime of
   that mobility session.  However, for administrative reasons, e.g.,
   local mobility anchor reboot, the local mobility anchor MAY change
   the uplink GRE key for the mobility session.  In that case, some
   packet loss may be experienced.

   If the mobile access gateway has successfully negotiated and
   exchanged the initial GRE keys with the local mobility anchor for a
   specific mobile node's mobility session, the mobile access gateway
   MUST include the GRE Key option with the downlink GRE key in the
   Proxy Binding Update that is used to request a Binding Lifetime
   Extension.  In this case, if the local mobility anchor successfully
   processes the Proxy Binding Update message, the local mobility anchor
   MUST return the same uplink GRE key that was exchanged with the
   mobile access gateway in the last successful Proxy Binding Update for
   the same mobility session in the GRE Key option in a successful Proxy
   Binding Acknowledgement message.

   However, during inter-MAG handoff and if the new mobile access
   gateway determines, based on, e.g., private IPv4 address support, the
   mobile access gateway local policy, the MAG-LMA peer agreement, or an
   indication during the handoff process, that GRE encapsulation and GRE
   keys exchange are required, the new mobile access gateway MUST
   include the GRE Key option with the downlink GRE key in the Proxy
   Binding Update that is used to request an after-handoff Binding
   Lifetime Extension.  In this case, the new mobile access gateway may
   either pick a new downlink GRE key or use the downlink GRE key that
   was used by the previous mobile access gateway for the same binding.
   For the new mobile access gateway to know the downlink GRE key used
   by the previous mobile access gateway, it may require transfer of





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   context from the previous mobile access gateway to the new mobile
   access gateway during a handoff.  Such mechanisms are out of scope
   for this specification.

   If the local mobility anchor successfully processes a handoff-
   triggered Binding Lifetime Extension Proxy Binding Update message
   that contains a GRE Key option with a downlink GRE key included, the
   local mobility anchor MUST return the same uplink GRE key that was
   exchanged with the previous mobile access gateway for the same
   mobility session in the GRE Key option in a successful Proxy Binding
   Acknowledgement.

   If the local mobility anchor receives a handoff-triggered Binding
   Lifetime Extension Proxy Binding Update message without the GRE Key
   option for a Binding Cache entry (BCE) that is using GRE keys and GRE
   encapsulation, the local mobility anchor makes a policy check
   regarding GRE encapsulation and GRE key exchange.  If, according to
   the policy check, GRE encapsulation and GRE key exchange are
   required, the local mobility anchor MUST reject the Proxy Binding
   Update by sending a Proxy Binding Acknowledgement message with the
   Status field set to GRE_KEY_OPTION_REQUIRED as defined in
   Section 6.4.  Otherwise, the local mobility anchor SHOULD accept the
   Proxy Binding Update, and if it is processed successfully, the local
   mobility anchor MUST return a successful Proxy Binding
   Acknowledgement without including the GRE Key option.

4.  Mobile Access Gateway Considerations

4.1.  Extensions to the Conceptual Data Structure

   Every mobile access gateway maintains a Binding Update List (BUL)
   entry for each currently attached mobile node, as explained in
   Section 6.1 of the Proxy Mobile IPv6 specification [RFC5213].  To
   support this specification, the conceptual Binding Update List entry
   data structure must be extended with the following four new
   additional fields.

   o  A flag (GRE-encapsulation-enabled) is used for indicating whether
      GRE encapsulation is enabled for the mobile node's traffic.

   o  The downlink GRE key used in the GRE encapsulation header of the
      tunneled payload packet from the local mobility anchor to the
      mobile access gateway that is destined to the mobile node.  This
      GRE key is generated by the mobile access gateway and communicated
      to the local mobility anchor in the GRE Key option in the Proxy
      Binding Update message.





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   o  The uplink GRE key used in the GRE encapsulation header of the
      tunneled payload packet from the mobile access gateway to the
      local mobility anchor that is originating from the mobile node.
      This GRE key is obtained from the GRE Key Identifier field of the
      GRE Key option present in the received Proxy Binding
      Acknowledgement message sent by the local mobility anchor as
      specified in this document.

   o  A flag indicating whether UDP-based TLV-header format
      (Section 7.2) is enabled for the mobile node's traffic.  This flag
      is TRUE only when UDP tunneling as in [RFC5844] and GRE
      encapsulation as in this specification are both enabled for this
      mobility session.

4.2.  Operational Summary

   o  If the mobile access gateway determines that GRE encapsulation
      mode only is required, the mobile access gateway MUST include the
      GRE Key option but omit the GRE Key Identifier field in the Proxy
      Binding Update message that is sent to the local mobility anchor.

   o  If the mobile access gateway determines that GRE encapsulation and
      GRE keys are required, the mobile access gateway MUST include the
      GRE Key option with the downlink GRE key in the GRE Key Identifier
      field in the Proxy Binding Update message that is sent to the
      local mobility anchor.

   o  After receiving a successful Proxy Binding Acknowledgement message
      with the GRE Key option with the GRE Key Identifier field omitted,
      the mobile access gateway MUST update the mobile node's Binding
      Update List entry described in Section 4.1 by only setting the
      GRE-encapsulation-enabled flag.

   o  After receiving a successful Proxy Binding Acknowledgement message
      with the GRE Key option and the uplink GRE key included in the GRE
      Key Identifier field, the mobile access gateway MUST update the
      related fields in the mobile node's Binding Update List entry
      described in Section 4.1.  Additionally, the mobile access gateway
      MUST use the assigned uplink GRE Key for tunneling all the traffic
      that belongs to this mobile node BUL entry and that originated
      from the mobile node before forwarding the tunneled traffic to the
      local mobility anchor.

   o  If the mobile access gateway includes the GRE Key option in the
      Proxy Binding Update for a specific mobile node and the local
      mobility anchor accepts the Proxy Binding Update by sending a
      Proxy Binding Acknowledgement with a success status code (less
      than 128) other than GRE_KEY_OPTION_NOT_REQUIRED, but without the



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      GRE Key option, then the mobile access gateway MUST consider that
      the local mobility anchor does not support the GRE Key option as
      per this specification.  The mobile access gateway SHOULD NOT
      include the GRE Key option in any subsequent Proxy Binding Update
      message that is sent to that local mobility anchor.

   o  If the mobile access gateway sent a Proxy Binding Update message
      without the GRE Key option, but the received Proxy Binding
      Acknowledgement has the status code GRE_KEY_OPTION_REQUIRED,
      indicating that GRE encapsulation and GRE keys are required, the
      mobile access gateway SHOULD resend the Proxy Binding Update
      message with the GRE Key option.  If the mobile access gateway
      does not support the GRE Key option, it MAY log the event and
      possibly raise an alarm to indicate a possible misconfiguration.

   o  If the mobile access gateway sent a Proxy Binding Update message
      with the GRE Key option and the downlink GRE key included and
      received a successful Proxy Binding Acknowledgement message with
      the status code GRE_KEY_OPTION_NOT_REQUIRED, the mobile access
      gateway MUST consider that GRE encapsulation and GRE keys are not
      required for this specific mobility session.  The mobile access
      gateway follows procedures in the Proxy Mobile IPv6 specification
      [RFC5213] for the handling of uplink and downlink traffic and MUST
      NOT include the GRE Key option in any subsequent Proxy Binding
      Update message that is sent to the local mobility anchor for this
      mobility session.

   o  If the mobile access gateway has successfully negotiated GRE
      encapsulation and exchanged the GRE keys with the local mobility
      anchor for a specific mobility session, the mobile access gateway
      SHOULD NOT include the GRE Key option in the de-registration Proxy
      Binding Update.

   o  On receiving a packet from the tunnel with the GRE header, the
      mobile access gateway MUST use the GRE key present in the GRE
      extension header as an additional identifier to determine to which
      mobility session this packet belongs.  The GRE header is removed
      before further processing takes place.

5.  Local Mobility Anchor Considerations

5.1.  Extensions to the Binding Cache Entry

   When the local mobility anchor and the mobile access gateway
   successfully negotiate GRE encapsulation and exchange downlink and
   uplink GRE keys, the local mobility anchor MUST maintain the downlink
   and uplink GRE keys as part of the mobile node's BCE.  This requires
   the BCE described in Section 5.1 of the Proxy Mobile IPv6



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   specification [RFC5213] to be extended.  To support this
   specification, the BCE must be extended with the following four
   additional fields.

   o  A flag indicating whether GRE encapsulation is enabled for the
      mobile node's traffic flows.

   o  The downlink GRE key, assigned by the mobile access gateway and
      used in the GRE encapsulation header of the tunneled payload
      packet from the local mobility anchor to the mobile access
      gateway.

   o  The uplink GRE key, assigned by the local mobility anchor and used
      in the GRE encapsulation header of the tunneled payload packet
      from the mobile access gateway to the local mobility anchor.

   o  A flag indicating whether UDP-based TLV-header format
      (Section 7.2) is enabled for the mobile node's traffic.  This flag
      is TRUE only when UDP tunneling as in [RFC5844] and GRE
      encapsulation as in this specification are both enabled for this
      mobility session.

5.2.  Operational Summary

   o  If the local mobility anchor successfully processes a Proxy
      Binding Update message with the GRE Key option, but the GRE Key
      Identifier field is omitted for initial GRE key exchange, the
      local mobility anchor MUST include the GRE Key option but omit the
      GRE Key Identifier field when responding with a successful Proxy
      Binding Acknowledgement message.

   o  If the local mobility anchor successfully processes a Proxy
      Binding Update message with the GRE Key option and the downlink
      GRE key included in the GRE Key Identifier field for initial GRE
      key exchange as in Section 3.3.1, the local mobility anchor MUST
      include the GRE Key option with the uplink GRE key included in the
      GRE Key Identifier field when responding with a successful Proxy
      Binding Acknowledgement message.

   o  If the GRE tunneling is negotiated and the downlink and uplink GRE
      keys have been exchanged between the mobile access gateway and the
      local mobility anchor for a specific mobility session, the local
      mobility anchor MUST use the negotiated downlink GRE key in the
      GRE header of every packet that is destined to the mobile node of
      this specific mobility session over the GRE tunnel to the mobile
      access gateway.





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   o  If the received Proxy Binding Update message does not contain the
      GRE Key option, and if the local mobility anchor based on a policy
      check determines that GRE encapsulation and GRE keys are required,
      e.g., overlapping IPv4 private addressing is in use, a local
      mobility anchor local policy, or LMA-MAG peer agreement, the local
      mobility anchor MUST reject the request and send a Proxy Binding
      Acknowledgement message to the mobile access gateway with the
      status code GRE_KEY_OPTION_REQUIRED as defined in Section 6.4.
      This indicates that GRE encapsulation and GRE keys are required.

   o  If, after receiving and successfully processing a Proxy Binding
      Update message with the GRE Key option, the local mobility anchor
      determines, based on a policy check, that GRE encapsulation and
      GRE keys are not required for this specific binding, e.g., private
      IPv4 addressing is not in use, the local mobility anchor SHOULD
      send a successful Proxy Binding Acknowledgement message to the
      mobile access gateway with the status code
      GRE_KEY_OPTION_NOT_REQUIRED.  In this case, the local mobility
      anchor MUST NOT include the GRE Key option in the Proxy Binding
      Acknowledgement.

   o  If the local mobility anchor successfully processes a de-
      registration Proxy Binding Update message, the local mobility
      anchor follows the same de-registration process as described in
      the Proxy Mobile IPv6 specification [RFC5213] to clean the Binding
      Cache entry and all associated resources including the downlink
      and uplink GRE keys.

   o  On receiving a packet from the tunnel with the GRE header, the
      local mobility anchor MUST use the GRE key in the GRE extension
      header as an additional identifier to determine to which mobility
      session this packet belongs.  The GRE header is removed before
      further processing takes place.

6.  Message Formats

   This section defines an extension to the Mobile IPv6 protocol
   [RFC3775] messages.  The use of the GRE Key option for supporting GRE
   tunneling and GRE key exchange for Proxy Mobile IPv6 is defined in
   this specification.

6.1.  GRE Key Option

   A new mobility option, the GRE Key option, is defined for use in the
   Proxy Binding Update and Proxy Binding Acknowledgement messages
   exchanged between the mobile access gateway and the local mobility
   anchor.  This option can be used for negotiating GRE encapsulation
   mode only or GRE encapsulation and exchanging the downlink and uplink



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   GRE keys.  These GRE keys can be used by the peers in all GRE
   encapsulated payload packets for marking that specific mobile node's
   data traffic.

   The alignment requirement for this option is 4n.

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      GRE Key Identifier                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 2: GRE Key Option

   Type

      33

   Length

      An 8-bit unsigned integer indicating the length in octets of the
      option, excluding the Type and Length fields.  If the Length field
      is set to 2, it indicates that the GRE Key Identifier field is not
      being carried in the option.  If the Length field is set to a
      value of 6, it means that either the downlink or the uplink GRE
      key is carried.

   Reserved

      These fields are unused.  They MUST be initialized to zero by the
      sender and MUST be ignored by the receiver.

   GRE Key Identifier

      A 32-bit field containing the downlink or the uplink GRE key.
      This field is present in the GRE Key option only if the GRE keys
      are being exchanged using the Proxy Binding Update and Proxy
      Binding Acknowledgement messages.

6.2.  Proxy Binding Update Message Extension

   This specification extends the Proxy Binding Update message as
   defined in [RFC5213] with the new TLV-header format (T) flag.  The
   new (T) flag is described below and shown as part of the Proxy
   Binding Update message as in Figure 3.




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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                      |          Sequence #           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |A|H|L|K|M|R|P|F|T|  Reserved   |           Lifetime            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: Proxy Binding Update Message

   TLV-header format (T)

      When set, this flag indicates that the mobile access gateway
      requests the use of the TLV header for encapsulating IPv6 or IPv4
      packets in IPv4.  The TLV-header format is described in
      Section 7.2.  None of the other fields or flags in the Proxy
      Binding Update are modified by this specification.

6.3.  Proxy Binding Acknowledgement Message Extension

   This specification extends the Proxy Binding Acknowledgement message
   as defined in [RFC5213] with the new TLV-header format (T) flag.  The
   new (T) flag is described below and shown as part of the Proxy
   Binding Acknowledgement message as in Figure 4.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                      |    Status     |K|R|P|T|   Res |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Sequence #          |           Lifetime            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 4: Proxy Binding Acknowledgement Message

   TLV-header format (T)

      When set, this flag indicates that the sender of the Proxy Binding
      Acknowledgement, the LMA, supports tunneling IPv6-or-IPv4 in IPv4
      using TLV-header format.  None of the other fields or flags in the
      Proxy Binding Acknowledgement are modified by this specification.

6.4.  Status Codes

   The following status code values are defined for use in the Binding
   Acknowledgement message when using Proxy Mobile IPv6.





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   GRE_KEY_OPTION_NOT_REQUIRED (2)

      When the local mobility anchor receives a Proxy Binding Update
      with the GRE Key option, and based on a policy check it determines
      that GRE encapsulation is not required for this specific mobility
      session, it uses this code to indicate to the mobile access
      gateway that the Proxy Binding Update has been processed
      successfully but GRE encapsulation and GRE keys are not required.

   GRE_TUNNELING_BUT_TLV_HEADER_NOT_SUPPORTED (3)

      If the local mobility anchor receives a Proxy Binding Update with
      the GRE Key option and TLV-header format (T) flag set, the local
      mobility anchor uses this code to indicate to the mobile access
      gateway that GRE encapsulation has been successfully negotiated
      but TLV-header format is NOT supported.

   GRE_KEY_OPTION_REQUIRED (163)

      When the local mobility anchor receives a Proxy Binding Update
      without the GRE Key option while based on a policy check, the
      local mobility anchor determines that GRE encapsulation is
      required for this specific mobility session and uses this code to
      reject the Proxy Binding Update and indicate to the mobile access
      gateway that GRE encapsulation and GRE keys are required.

7.  Data Packets Processing Considerations

   This section describes how the local mobility anchor and mobile
   access gateway encapsulate and decapsulate data packets when GRE
   encapsulation and GRE keys are used for tunneling the mobile node's
   data traffic between these two mobile nodes.

7.1.  Tunneling Format

   When GRE encapsulation is used, the mobile access gateway is allowed
   to use various tunneling formats depending on the mobile access
   gateway location and the network capabilities between the mobile
   access gateway and the local mobility anchor.  Using GRE
   encapsulation, as described in [RFC2784] and [RFC2890], the mobile
   access gateway can tunnel the IPv6-or-IPv4 payload packet in IPv6 or
   in IPv4 following the rules in [RFC5213] and [RFC5844].

   If UDP-based tunneling is used in conjunction with GRE encapsulation
   between the mobile access gateway and the local mobility anchor, the
   TLV-header UDP tunneling format as shown in Figure 5 MUST be used.





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         [IPv4 Header]

         [UDP Header]

         [TLV Header]

         [GRE Header]

         [Payload - IPv6 or IPv4 Header]

         Upper Layer protocols

         Figure 5: TLV-Header UDP-Based Encapsulation Header Order

   When a UDP-based tunneling format is used between the mobile access
   gateway and the local mobility anchor, the use of the TLV header is
   negotiated during the Proxy Binding Update/Acknowledgement exchange
   as described in Sections 7.3 and 7.4.  If the TLV-header format is
   agreed upon between the mobile access gateway and local mobility
   anchor, the local mobility anchor expects the TLV header to follow
   the UDP header as shown in Figure 5.  The TLV header contains the
   Type field, the following payload packet header type, and its length.
   The Type field in the TLV header is always set to a value of 0 to
   enhance the processing of the received packet by ensuring that the
   receiver can differentiate whether what came after the UDP header is
   a TLV-header Type field or an IP version field of an IP header.
   Hence, the TLV header can carry traffic other than IP as indicated in
   the Next Header field.  The distinction between IP and TLV
   encapsulation is needed, because the Proxy Binding Update (IP packet)
   and the data packets (GRE packets) can be sent over the same UDP
   tunnel.

   When processing a UDP packet with the TLV-header format, if the
   receiving node found that the payload packet length as calculated
   from the UDP header length field is different than its length as
   calculated from the TLV-header length field, the receiving node MUST
   discard the received IP packet.

7.2.  TLV-Header Tunneling Negotiation

   The mobile access gateway negotiates the format for tunneling payload
   traffic during the Proxy Mobile IPv6 registration procedure.  If the
   mobile access gateway is required to use the TLV-header UDP
   encapsulation format, the mobile access gateway MUST set the TLV-
   header format (T) flag in the Proxy Binding Update message sent to
   the local mobility anchor.  If the local mobility anchor supports the
   TLV-header UDP tunneling format, the local mobility anchor SHOULD set
   the TLV-header format (T) flag in the Proxy Binding Acknowledgement.



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   Otherwise, the TLV-header format (T) flag is cleared.  The setting of
   the TLV-header Format (T) flag in the Proxy Binding Acknowledgement
   indicates to the mobile access gateway that it MUST use the TLV-
   header UDP encapsulation format for all packets tunneled to the local
   mobility anchor for the entire duration the mobile node is attached
   to the mobile access gateway.  The TLV-header UDP tunneling format
   SHOULD NOT change during a Binding Lifetime Extension Proxy Binding
   Update (re-registration) from the same mobile access gateway.

   Any Proxy Binding Update message triggered by a handoff (Section
   5.3.4 of [RFC5213]) may renegotiate the tunneling format.  Therefore,
   in order to avoid interoperability issues, the local mobility anchor
   MUST NOT set the TLV-header format (T) flag unless it was set in the
   Proxy Binding Update received from the mobile access gateway.

   The TLV-header format is as shown below in Figure 6.

       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  |  Res. |  Next Header  |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 6: TLV-Header Format

   Type

      This field is always 0 (zero) and distinguishes the TLV header
      from the IPv4 and IPv6 headers.

   Res.

      These fields are Reserved and unused.  They MUST be initialized to
      zero by the sender and MUST be ignored by the receiver.

   Next Header

      An 8-bit unsigned integer that indicates the protocol number of
      the payload header following this TLV header.  It is set to the
      protocol number as assigned by IANA in the "Assigned Internet
      Protocol Numbers" registry.  For example, if an IPv6 header
      follows, it should be '41'; if a GRE header follows, it should be
      '47'.

   Length

      A 16-bit unsigned integer indicating the length in octets of the
      payload following this header, excluding the TLV header itself.



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7.3.  Mobile Access Gateway Operation

   When sending a Proxy Binding Update message over an IPv4 transport
   network, the mobile access gateway follows the procedures specified
   in [RFC5844] for using IPv4-UDP encapsulation mode.  However, when
   using GRE header in conjunction with IPv4-UDP encapsulation mode is
   required, the mobile access gateway MUST set the TLV-header format
   (T) flag in the Proxy Binding Update and follow this specification
   for GRE encapsulation negotiation.  If the received Proxy Binding
   Acknowledgement is successful and the TLV-header format (T) flag is
   set and the GRE Key option included, the mobile access gateway MUST
   update the mobile node's Binding Update List entry described in
   Section 4.1 by setting the UDP-based TLV-header format (T) flag.  In
   this case, the mobile access gateway MUST use the TLV-header UDP-
   based encapsulation format as shown in Figure 5.

   If the mobile access gateway receives a Proxy Binding Acknowledgement
   with the status GRE_TUNNELING_BUT_TLV_HEADER_NOT_SUPPORTED in
   response to a Proxy Binding Update with the GRE Key option and the
   (T) flag set, the mobile access gateway MUST use GRE encapsulation
   without UDP encapsulation.  If the mobile access gateway is allowed
   to use GRE encapsulation without UDP tunneling, the mobile access
   gateway MUST update the mobile node's Binding Update List entry
   described in Section 4.1 by setting the GRE-encapsulation-enabled
   flag and the uplink and downlink GRE key fields.  In this case, the
   mobile access gateway MUST set the UDP-based TLV-header format (T)
   flag to FALSE.  A Proxy Binding Acknowledgement message with the
   status code GRE_TUNNELING_BUT_TLV_HEADER_NOT_SUPPORTED has the (T)
   flag cleared.  Alternatively, the mobile access gateway may resend
   the Proxy Binding Update to negotiate different tunneling options,
   e.g., using UDP-based tunneling without GRE encapsulation if possible
   or de-register the mobile node mobility session.

7.3.1.  Sending and Receiving Data Packets

   When the mobile access gateway is located in an IPv6-enabled or IPv4-
   enabled network, it may be required to use GRE encapsulation for
   tunneling IPv6 or IPv4 data packets to the local mobility anchor.  In
   this case, and if the mobile access gateway has successfully
   negotiated GRE encapsulation mode only or GRE encapsulation and GRE
   keys as described in this specification, the mobile access gateway
   encapsulates or decapsulates IPv6-or-IPv4 payload packets following
   the rules described in [RFC5213] and [RFC5844] while ensuring that
   the GRE header is present as shown in Figure 7.







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         [IPv6 or IPv4 Header]

         [GRE Header]

         [Payload - IPv6 or IPv4 Header]

         Upper Layer protocols

         Figure 7: IPv6-or-IPv4 over IPv4 Using GRE Encapsulation

   On the other hand, if the mobile access gateway is located in an
   IPv4-only network where NAT has been detected on the path between the
   mobile access gateway and the local mobility anchor and successfully
   negotiated GRE encapsulation and the TLV-header format, the mobile
   access gateway MUST use UDP TLV-header tunneling format when sending
   an IPv6-or-IPv4 payload packet to the local mobility anchor according
   to the format described in Figure 5.  The source and the destination
   of the IPv4 outer header are mobile node IPv4 Proxy Care-of Address,
   IPv4-Proxy-CoA, and the IPv4 local mobility anchor address, IPv4-
   LMAA, respectively.  In addition, the source and the destination IP
   addresses of the IPv6-or-IPv4 payload data packet are the mobile
   node's IPv6-or-IPv4 home address, IPv6/IPv4-MN-HoA, and the IPv6-or-
   IPv4 corresponding node's address, IPv6/IPv4-CN-Addr, respectively.

7.4.  Local Mobility Anchor Operation

   When the local mobility anchor receives a Proxy Binding Update
   encapsulated in UDP and containing the IPv4 Home Address Request
   option ([RFC5844]), it needs to follow all the steps in [RFC5213] and
   [RFC5844].  In addition, if the TLV-header format (T) flag is set in
   the Proxy Binding Update, the local mobility anchor needs to
   determine whether it can accept the TLV-header UDP-based
   encapsulation format.  If it does, it SHOULD set the TLV-header
   format (T) flag in the Proxy Binding Acknowledgement.  Otherwise, the
   local mobility anchor MUST NOT set the TLV-header format (T) flag in
   the Proxy Binding Acknowledgement.

   If the local mobility anchor (LMA) receives a Proxy Binding Update
   with the GRE Key option and TLV-header format (T) flag set and, based
   on a policy check, the LMA determines that GRE encapsulation is
   required and the LMA supports TLV-header tunneling and the LMA sent a
   successful Proxy Binding Acknowledgement with the TLV-header format
   (T) flag set, the LMA MUST update the mobile node's Binding Cache
   entry described in Section 5.1 by setting the GRE-encapsulation-
   enabled flag and update the uplink and downlink GRE key fields.  In
   addition, the LMA MUST set the UDP-based TLV-header format flag.





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   If the LMA receives a Proxy Binding Update with the GRE Key option
   and TLV-header format (T) flag set and, based on a policy check, the
   LMA determines that GRE encapsulation is required BUT the LMA does
   NOT support TLV-header tunneling and if the Proxy Binding Update has
   been successfully processed, the LMA MUST send a successful Proxy
   Binding Acknowledgement with the status code
   GRE_TUNNELING_BUT_TLV_HEADER_NOT_SUPPORTED.  This way, the LMA
   indicates to the mobile access gateway that GRE encapsulation has
   been successfully negotiated BUT TLV-header UDP-based tunneling
   format is not supported.  In this case, the LMA MUST update the
   mobile node's BCE described in Section 5.1 by setting the GRE
   encapsulation enabled flag and update the uplink and downlink GRE key
   fields.  In this case, the LMA MUST set the UDP-based TLV-header
   format flag to FALSE.

   If the local mobility anchor and the mobile access gateway have
   successfully negotiated the TLV-header UDP-based tunneling format and
   GRE encapsulation for a specific mobility session, the local mobility
   anchor processes data packets as described in the following
   subsection.

7.4.1.  Sending and Receiving Data Packets

   The local mobility anchor may use GRE encapsulation for tunneling an
   IPv6 or IPv4 data packet to the mobile access gateway.  If the local
   mobility anchor has successfully negotiated GRE encapsulation with
   the mobile access gateway for a specific mobility session, the local
   mobility anchor encapsulates and decapsulates IPv6-or-IPv4 payload
   data packets following the rules described in [RFC5213] and [RFC5844]
   while ensuring that the GRE header is present as shown in Figure 7.

   In the case when TLV-tunneling format and GRE encapsulation for a
   specific mobility session have been successfully negotiated between
   the local mobility anchor and the mobile access gateway, the local
   mobility anchor follows the TLV-header UDP-based tunneling format and
   header order as shown in Figure 5 to encapsulate IPv4 or IPv6 payload
   packets in IPv4 before sending the IPv4 packet to the mobile access
   gateway.  In this case, the source and the destination of the IPv4
   outer header are IPv4-LMAA and IPv4-Proxy-CoA, respectively.  In
   addition, the source and the destination IP addresses of the IPv6-or-
   IPv4 payload data packet are IPv6/IPv4-CN-Addr and IPv6/IPv4-MN-HoA,
   respectively.  On the other hand, the local mobility anchor ensures
   the same TLV-header UDP-based tunneling format and header order when
   it decapsulates received IPv4 packets from the mobile access gateway
   for the same mobility session.






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8.  IANA Considerations

   This specification defines a new mobility option, the GRE Key option,
   described in Section 6.1.  This option is carried in the Mobility
   Header.  The type value for this option has been assigned from the
   same numbering space as allocated for the other mobility options
   defined in the Mobile IPv6 specification [RFC3775].

   This specification also defines three new Binding Acknowledgement
   status codes as described in Section 6.4 and IANA has allocated the
   numeric values as specified in Section 6.4 from the "Status Codes"
   registry of the Mobility IPv6 Parameters.

9.  Security Considerations

   The GRE Key option, defined in this specification, when carried in
   Proxy Binding Update and Proxy Binding Acknowledgement messages,
   reveals the group affiliation of a mobile node identified by its
   Network Access Identifier (NAI) or an IP address.  It may help an
   attacker in targeting flows belonging to a specific group.  This
   vulnerability can be prevented, by enabling confidentiality
   protection on the Proxy Binding Update and Proxy Binding
   Acknowledgement messages where the presence of the NAI and GRE Key
   options establish a mobile node's relation to a specific group.  This
   vulnerability can also be avoided by enabling confidentiality
   protection on all the tunneled data packets between the mobile access
   gateway and the local mobility anchor, for hiding all the markings.

   In Proxy Mobile IPv6 [RFC5213], the use of IPsec [RFC4301] for
   protecting a mobile node's data traffic is optional.  Additionally,
   Proxy Mobile IPv6 recommends the use of Encapsulating Security
   Payload (ESP) [RFC4303] in tunnel mode when using ESP for protecting
   the mobile node's data traffic.  However, when GRE encapsulation is
   used, both IPsec tunnel mode and transport mode can be used to
   protect the GRE header.  The IPsec traffic selectors will contain the
   protocol number for GRE, and there is currently no mechanism to use
   the GRE key as a traffic selector.

10.  Acknowledgements

   The authors would like to thank Alessio Casati, Barney Barnowski,
   Mark Grayson, and Parviz Yegani for their input on the need for this
   option.  The authors would like to thank Charlie Perkins, Curtis
   Provost, Irfan Ali, Jouni Korhonen, Julien Laganier, Kuntal
   Chowdhury, Suresh Krishnan, and Vijay Devarapalli for their review
   and comments.





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11.  References

11.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

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

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              March 2000.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, September 2000.

   [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.

   [RFC5844]  Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
              Mobile IPv6", RFC 5844, May 2010.

11.2.  Informative References

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC5648]  Wakikawa, R., Devarapalli, V., Tsirtsis, G., Ernst, T.,
              and K. Nagami, "Multiple Care-of Addresses Registration",
              RFC 5648, October 2009.







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Authors' Addresses

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

   EMail: ahmad.muhanna@ericsson.com


   Mohamed Khalil
   Ericsson, Inc.
   6300 Legacy Dr.
   Plano, TX  75024
   USA

   EMail: Mohamed.khalil@ericsson.com


   Sri Gundavelli
   Cisco
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: sgundave@cisco.com


   Kent Leung
   Cisco
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: kleung@cisco.com















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