Alternate Tunnel Encapsulation for Data Frames in CAPWAP
draft-ietf-opsawg-capwap-alt-tunnel-05

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Replaces draft-zhang-opsawg-capwap-cds
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Network Working Group                                           R. Zhang
Internet-Draft                                             China Telecom
Intended status: Standards Track                                  Z. Cao
Expires: October 28, 2015                                        H. Deng
                                                            China Mobile
                                                           R. Pazhyannur
                                                           S. Gundavelli
                                                                   Cisco
                                                                  L. Xue
                                                                  Huawei
                                                          April 26, 2015

        Alternate Tunnel Encapsulation for Data Frames in CAPWAP
                 draft-ietf-opsawg-capwap-alt-tunnel-05

Abstract

   Control And Provisioning of Wireless Access Points (CAPWAP) defines a
   specification to encapsulate a station's data frames between the
   Wireless Transmission Point (WTP) and Access Controller (AC).
   Specifically, the station's IEEE 802.11 data frames can be either
   locally bridged or tunneled to the AC.  When tunneled, a CAPWAP data
   channel is used for tunneling.  In many deployments encapsulating
   data frames to an entity other than the AC (for example to an Access
   Router (AR)) is desirable.  Further, it may also be desirable to use
   different tunnel encapsulations to carry the stations' data frames.
   This document provides a specification for this and refers to it as
   Alternate tunnel encapsulation.  The Alternate tunnel encapsulation
   allows 1) the WTP to tunnel non-management data frames to an endpoint
   different from the AC and 2) the WTP to tunnel using one of many
   known encapsulation types such as IP-IP, IP-GRE, CAPWAP.  The WTP may
   advertise support for Alternate tunnel encapsulation during the
   discovery or join process and AC may select one of the supported
   Alternate Tunnel encapsulation types while configuring the WTP.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 28, 2015.

Copyright Notice

   Copyright (c) 2015 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
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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
     1.1.  Conventions used in this document . . . . . . . . . . . .   6
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Alternate Tunnel Encapsulation  . . . . . . . . . . . . . . .   7
     2.1.  Description . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Protocol Considerations . . . . . . . . . . . . . . . . . . .   9
     3.1.  Supported Alternate Tunnel Encapsulations . . . . . . . .   9
     3.2.  Alternate Tunnel Encapsulations Type  . . . . . . . . . .  10
     3.3.  IEEE 802.11 WTP Alternate Tunnel Failure Indication . . .  11
     3.4.  CAPWAP based Alternate Tunnel . . . . . . . . . . . . . .  11
     3.5.  PMIPv6 based Alternate Tunnel . . . . . . . . . . . . . .  12
     3.6.  Alternate Tunnel Information Elements . . . . . . . . . .  13
       3.6.1.  Access Router Information Sub-Elements  . . . . . . .  13
       3.6.2.  Tunnel DTLS Policy Sub-Element  . . . . . . . . . . .  15
       3.6.3.  IEEE 802.11 Tagging Mode Policy Sub-Element . . . . .  16
       3.6.4.  CAPWAP Transport Protocol Sub-Element . . . . . . . .  16
       3.6.5.  GRE Key Sub-Element . . . . . . . . . . . . . . . . .  17
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

   Service Providers are deploying very large Wi-Fi deployments (ranging
   from hundreds of thousands of Access Points, APs (referred to as WTPs
   in CAPWAP terminology) to millions of APs.  These networks are
   designed to carry traffic generated from mobile users.  The volume in
   mobile user traffic is already very large and expected to continue
   growing rapidly.  As a result, operators are looking for scalable
   solutions that can meet the increasing demand.  The scalability
   requirement can be met by splitting the control/management plane from
   the data plane.  This enables the data plane to scale independent of
   the control/management plane.  This specification provides a way to
   enable such separation.

   CAPWAP ([RFC5415], [RFC5416]) defines a tunnel mode that describes
   how the WTP handles the data plane (user traffic).  The following
   types are defined:

   o  Local Bridging: All data frames are locally bridged.
   o  802.3 Tunnel: All data frames are tunneled to the AC in 802.3
      format.
   o  802.11 Tunnel: All data frames are tunneled to the AC in 802.11
      format.

   Figure 1 describes a system with Local Bridging.  The AC is in a
   centralized location.  The data plane is locally bridged by the WTPs
   leading to a system with centralized control plane with distributed
   data plane.  This system has two benefits: 1) reduces the scale
   requirement on data traffic handling capability of the AC and 2)
   leads to more efficient/optimal routing of data traffic while
   maintaining centralized control/management.

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                     Locally Bridged
             +-----+ Data Frames   +----------------+
             | WTP |===============|  Access Router |
             +-----+               +----------------+
                    \\
                     \\  CAPWAP Control Channel   +----------+
                       ++=========================|   AC     |
                      // CAPWAP Data Channel:     |          |
                     //  IEEE 802.11 Mgmt traffic +----------+
                    //
             +-----+               +----------------+
             | WTP |============== |  Access Router |
             +=====+               +----------------+
                    Locally Bridged
                    Data Frames

            Figure 1: Centralized Control with Distributed Data

   The AC handles control of WTPs.  In addition, the AC also handles the
   IEEE 802.11 management traffic to/ from the stations.  There is
   CAPWAP Control and Data Channel between the WTP and the AC.  Note
   that even though there is no user traffic transported between the WTP
   and AC, there is still a CAPWAP Data Channel.  The CAPWAP Data
   channel carries the IEEE 802.11 management traffic (like IEEE 802.11
   Action Frames).

   Figure 2 shows a system where the tunnel mode is configured to tunnel
   data frames between the WTP and the AC either using 802.3 Tunnel or
   802.11 Tunnel configurations.  Operators deploy this configuration
   when they need to tunnel the user traffic.  The tunneling requirement
   may be driven by the need to apply policy at the Access Router or a
   legal requirement to support lawful intercept of user traffic.  This
   requirement could be met in the locally bridged system (Figure 1) if
   the access router implemented the required policy.  However, in many
   deployments the operator managing the WTP is different than the
   operator managing the Access Router.  When the operators are
   different, the policy has to be enforced in a tunnel termination
   point in the WTP operator's network.

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                    +-----+
                    | WTP |
                    +-----+
                        \\
                         \\
                          \\  CAPWAP Control Channel   +----------+
                            ++=========================|   AC     |
                           // CAPWAP Data Channel:     |          |
                          //  IEEE 802.11 Mgmt traffic |          |
                         //   Data Frames              +----------+
                        //
                    +-----+
                    | WTP |
                    +=====+

            Figure 2: Centralized Control and Centralized Data

   The key difference with the locally bridged system is that the data
   frames are tunneled to the AC instead of being locally bridged.
   There are two shortcomings with system in Figure 2.  1) They do not
   allow the WTP to tunnel data frames to an endpoint different from the
   AC and 2) They do not allow the WTP to tunnel data frames using any
   encapsulation other than CAPWAP (as specified in Section 4.4.2 of
   [RFC5415]).

   Figure 3 shows a system where the WTP tunnels data frames to an
   alternate entity different from the AC.  The WTP also uses an
   alternate tunnel encapsulation such as such as L2TP, L2TPv3, IP-in-
   IP, IP/GRE, etc.  This enables 1) independent scaling of data plane
   and 2) leveraging of commonly used tunnel encapsulations such as
   L2TP, GRE, etc

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          Alternate Tunnel to AR (L2TPv3, IP-IP, CAPWAP, etc)
                      _________
         +-----+      (         )              +-----------------+
         | WTP |======+Internet +==============|Access Router(AR)|
         +-----+      (_________}              +-----------------+
               \\      ________  CAPWAP Control
                \\    (        ) Channel                +--------+
                   ++==Internet+========================|   AC   |
                  //  (        )CAPWAP Data Channel:    +--------+
                 //            IEEE 802.11 Mgmt traffic
                //   ---------
         +-----+    (         )                +----------------+
         | WTP |====+Internet +================|  Access Router |
         +=====+    (_________}                +----------------+
          Alternate Tunnel to AR (L2TPv3, IP-IP, CAPWAP, etc)

       Figure 3: Centralized Control with Alternate Tunnel for Data

   The WTP may support widely used encapsulation types such as L2TP,
   L2TPv3, IP-in-IP, IP/GRE, etc.  The WTP advertises the different
   alternate tunnel encapsulation types it can support.  The AC
   configures one of the advertised types.  As shown in the figure there
   is a CAPWAP control and data channel between the WTP and AC.  The
   CAPWAP data channel carries the stations' management traffic as in
   the case of the locally bridged system.  The main reason to maintain
   a CAPWAP data channel is to maintain similarity with the locally
   bridged system.  The WTP maintains three tunnels: CAPWAP Control,
   CAPWAP Data, and another alternate tunnel for the data frame.  The
   data frames are transported by an alternate tunnel between the WTP
   and a tunnel termination point such as an Access Router.  This
   specification describes how the alternate tunnel can be established.
   The specification defines message elements for the WTP to advertise
   support for alternate tunnel encapsulation, the AC to configure
   alternate tunnel encapsulation, and for the WTP to report failure of
   the alternate tunnel.

1.1.  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]

1.2.  Terminology

   Station (STA): A device that contains an IEEE 802.11 conformant
   medium access control (MAC) and physical layer (PHY) interface to the
   wireless medium (WM).

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   Access Controller (AC): The network entity that provides WTP access
   to the network infrastructure in the data plane, control plane,
   management plane, or a combination therein.

   Wireless Termination Point (WTP), The physical or network entity that
   contains an RF antenna and wireless Physical Layer (PHY) to transmit
   and receive station traffic for wireless access networks.

   CAPWAP Control Channel: A bi-directional flow defined by the AC IP
   Address, WTP IP Address, AC control port, WTP control port, and the
   transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Control
   packets are sent and received.

   CAPWAP Data Channel: A bi-directional flow defined by the AC IP
   Address, WTP IP Address, AC data port, WTP data port, and the
   transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Data
   packets are sent and received.  In certain WTP modes, the CAPWAP Data
   Channel only transports IEEE 802.11 management frames and not the
   data plane (user traffic).

2.  Alternate Tunnel Encapsulation

2.1.  Description

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   +-+-+-+-+-+-+                             +-+-+-+-+-+-+
   |    WTP    |                             |    AC     |
   +-+-+-+-+-+-+                             +-+-+-+-+-+-+
          |Join Request[Supported Alternate Tunnel  |
          |       Encapsulations ]                  |
          |---------------------------------------->|
          |                                         |
          |Join Response                            |
          |<----------------------------------------|
          |                                         |
          |IEEE 802.11 WLAN Config. Request [       |
          | IEEE 802.11 Add WLAN,                   |
          | Alternate Tunnel Encapsulation (        |
          |   Tunnel Type, Tunnel Info Element)     |
          | ]                                       |
          |<----------------------------------------|
          |                                         |
          |                                         |
     +-+-+-+-+-+-+                                  |
     | Setup     |                                  |
     | Alternate |                                  |
     | Tunnel    |                                  |
     +-+-+-+-+-+-+                                  |
          |                                         |
          |IEEE 802.11 WLAN Config. Response        |
          |---------------------------------------->|
          |                                         |
          |                                         |
     +-+-+-+-+-+-+                                  |
     | Tunnel    |                                  |
     | Failure   |                                  |
     +-+-+-+-+-+-+                                  |
          |WTP Alternate Tunnel Failure Indication  |
          |(report failure)                         |
          |---------------------------------------->|
          |                                         |
     +-+-+-+-+-+-+-+                                |
     | Tunnel      |                                |
     | Established |                                |
     +-+-+-+-+-+-+-+                                |
          |WTP Alternate Tunnel Failure Indication  |
          |(report clearing failure)                |
          |---------------------------------------->|
          |                                         |

                    Figure 4: Setup of Alternate Tunnel

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   The above example describes how the alternate tunnel encapsulation
   may be established.  When the WTP joins the AC, it should indicate
   its alternate tunnel encapsulation capability.  The AC determines
   whether an alternate tunnel configuration is required.  If an
   appropriate alternate tunnel type is selected, then the AC provides
   the alternate tunnel encapsulation message element containing the
   tunnel type and a tunnel-specific information element.  (The tunnel-
   specific information element, for example, may contain information
   like the IP address of the tunnel termination point.)  The WTP sets
   up the alternate tunnel using the alternate tunnel encapsulation
   message element.

   On detecting a tunnel failure, WTP shall forward data frames to the
   AC and discard the frames.  In addition, WTP may dissociate existing
   clients and refuse association requests from new clients.  Depending
   on the implementation and deployment scenario, the AC may choose to
   reconfigure the WLAN (on the WTP) to a local bridging mode or to
   tunnel frames to the AC.  When the WTP detects an alternate tunnel
   failure, the WTP informs the AC using a message element, WTP
   Alternate Tunnel Fail Indication (defined in this specification).
   The message element has a status field that indicates whether the
   message denotes reporting a failure or the clearing of the previously
   reported failure.

   For the case where AC is unreachable but the tunnel end point is
   still reachable, the WTP behavior is up to the implementation.  For
   example, the WTP could either choose to tear down the alternate
   tunnel or let the existing user's traffic continue to be tunneled.

3.  Protocol Considerations

3.1.  Supported Alternate Tunnel Encapsulations

   This message element is sent by a WTP to communicate its capability
   to support alternate tunnel encapsulations.  The message element
   contains the following fields:

    0               1               2               3
    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Num_Tunnels   | Tunnel-Type 1 |  Tunnel-Type [2..N]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 5: Supported Alternate Tunnel Encapsulations

      Type: <IANA-1> for Supported Alternate Tunnel Encapsulations
      Length: The length in bytes is 1 + Num_Tunnels

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      Num_Tunnels: This refers to number of tunnel types present in the
      message element.  At least one tunnel type must be present.
      Tunnel-Type: This is identified by value defined in Section 3.2

3.2.  Alternate Tunnel Encapsulations Type

   This message element is sent by the AC.  This message element allows
   the AC to select the alternate tunnel encapsulation.  This message
   element may be provided along with the IEEE 802.11 Add WLAN message
   element.  When the message element is present the following fields of
   the IEEE 802.11 Add WLAN element shall be set as follows: MAC mode is
   set to 0 (Local MAC) and Tunnel Mode is set to 0 (Local Bridging).
   The message element contains the following fields

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Tunnel-Type            |  Info Element Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Info Element
     +-+-+-+-+-+-+-+-+-+

              Figure 6: Alternate Tunnel Encapsulations Type

      Type: <IANA-2> for Alternate Tunnel Encapsulation Type
      Length: > 4
      Tunnel-Type: The tunnel type is specified by a 2 byte value.  This
      specification defines the values from zero (0) to five (5) as
      given below.  The remaining values are reserved for future use.

         0: CAPWAP.  This refers to a CAPWAP data channel described in
         [RFC5415][RFC5416].
         1: L2TP.  This refers to tunnel encapsulation described in
         [RFC2661].
         2: L2TPv3.  This refers to tunnel encapsulation described in
         [RFC3931].
         3: IP-in-IP.  This refers to tunnel encapsulation described in
         [RFC2003].
         4: PMIPv6.  This refers to the tunneling encapsulation
         described in [RFC5213]
         5: GRE-IPv4.  This refers to GRE encapsulation with IPv4 as the
         delivery protocol as described in RFC2874.
         6: GRE-IPv6.  This refers to GRE encapsulation with IPv6 as the
         delivery protocol as described in RFC2874.
      Info Element: This field contains tunnel specific configuration
      parameters to enable the WTP to setup the alternate tunnel.  This
      specification provides details for this elements for CAPWAP and

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      PMIPv6.  We anticipate that message elements for the other
      protocols (like L2TPv3, etc) will be defined in other
      specifications in the future

3.3.  IEEE 802.11 WTP Alternate Tunnel Failure Indication

   The Alternate Tunnel Failure Indication message element is sent by
   the WTP to inform the AC about the status of the Alternate Tunnel.
   The message element contains the following fields

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Radio ID  |  WLAN ID      |    Status     |   Reserved      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 7: IEEE 802.11 WTP Alternate Tunnel Failure Indication

      Type: <IANA-3> for IEEE 802.11 WTP Alternate Tunnel Failure
      Indication
      Length: == 4
      Radio ID: The Radio Identifier, whose value is between one (1) and
      31, typically refers to some interface index on the WTP.
      WLAN ID: An 8-bit value specifying the WLAN Identifier.  The value
      MUST be between one (1) and 16.
      Status: An 8-bit boolean indicating whether the radio failure is
      being reported or cleared.  A value of zero is used to clear the
      event, while a value of one is used to report the event.

3.4.  CAPWAP based Alternate Tunnel

   If the CAPWAP encapsulation is selected by the AC and configured by
   the AC to the WTP, the Info Element field defined in Section 3.2
   should contain the following information:

   o  Access Router Information: IPv4 address or IPv6 address or Fully
      Qualified Domain Name (FQDN), of the Access Router for the
      alternate tunnel.
   o  Tunnel DTLS Policy: The CAPWAP protocol allows optional protection
      of data packets using DTLS.  Use of data packet protection on a
      WTP is not mandatory but determined by the associated AC policy
      (This is consistent with the WTP behavior described in [RFC5415]).
   o  IEEE 802.11 Tagging Mode Policy: It is used to specify how the
      CAPWAP data channel packet are to be tagged for QoS purposes (see
      [RFC5416] for more details).

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   o  CAPWAP Transport Protocol: The CAPWAP protocol supports both UDP
      and UDP-Lite (see RFC3828).  When run over IPv4, UDP is used for
      the CAPWAP data channels.  When run over IPv6, the CAPWAP data
      channel may use either UDP or UDP-lite.

   The message element structure for CAPWAP encapsulation is shown in
   Figure 8:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Tunnel-Type=0             |   Info Element Length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .              Access Router Information Sub-Element            .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .              Tunnel DTLS Policy Sub-Element                   .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .             IEEE 802.11 Tagging Mode Policy Sub-Element       .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .             CAPWAP Transport Protocol Sub-Element             .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 8: Alternate Tunnel Encapsulation - CAPWAP

3.5.  PMIPv6 based Alternate Tunnel

   Proxy Mobile IPv6 (PMIPv6) (defined in [RFC5213]) can also be used
   for alternate tunnel encapsulation between the WTP and the AR.  In
   this scenario, a WTP acts as the Mobile Access Gateway (MAG) function
   that manages the mobility-related signaling for a station that is
   attached to the WTP IEEE 802.11 radio access.  The Local Mobility
   Anchor (LMA) function is at the AR.  If PMIPv6 encapsulation is
   selected by the AC and configured by the AC to a WTP, the Info
   Element field defined in Section 3.2 should contain the following
   information:

   o  Access Router (acts as LMA) Information: IPv6 address or Fully
      Qualified Domain Name (FQDN) for the alternate tunnel endpoint.

   The message element structure for PMIPv6 encapsulation is shown in
   Figure 9:

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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Tunnel-Type=4             |   Info Element Length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .             Access Router (LMA) Information Sub-element       .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 9: Alternate Tunnel Encapsulation - PMIPv6

3.6.  Alternate Tunnel Information Elements

   This section defines the various sub-elements described in
   Section 3.4 and Section 3.5

3.6.1.  Access Router Information Sub-Elements

   The Access Router Information Sub-Elements allow the AC to notify a
   WTP of which AR(s) are available for establishing a data tunnel.  The
   AR information may be IPv4 address, IPv6 address, or AR domain name.
   If a WTP obtains the correct AR FQDN, the Name-to-IP address mapping
   is handled in the WTP (see RFC2782).

   The following are the Access Router Information Sub-Elements defined
   in this specification.  The AC can use one of them to notify the
   destination information of the data tunnel to the WTP.  The Sub-
   Elements containing the AR IPv4 address MUST NOT be used if an IPv6
   data channel such as PMIPv6 or GREv6 is used.

3.6.1.1.  AR IPv4 List Sub-Element

   This Sub-Element (see Figure 10) is used by the AC to configure a WTP
   with the AR IPv4 address available for the WTP to establish the data
   tunnel for user traffic.

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AR IPv4 Sub-Element Type     |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                     AR IPv4 Address-1                         .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                     AR IPv4 Address-2                         .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                     AR IPv4 Address-N                         .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 10: AR IPv4 List Sub-Element

   Length: This refers to the total length in octets of the sub-element
   excluding the Type and Length fields.

   AR IPv4 Address: IPv4 address of the AR.  At least one IPv4 address
   shall be present.  Multiple addresses may be provided for load
   balancing or redundancy.

3.6.1.2.  AR IPv6 List Sub-Element

   This Sub-Element (see Figure 11) is used by the AC to configure a WTP
   with the AR IPv6 address available for the WTP to establish the data
   tunnel for user traffic.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AR IPv6 Sub-Element Type    |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                     AR IPv6 Address-1                         .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                     AR IPv6 Address-2                         .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                     AR IPv6 Address-N                         .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 11: AR IPv6 List Sub-Element

   Length: This refers to the total length in octets of the sub-element
   excluding the Type and Length fields.

   AR IPv6 Address: IPv6 address of the AR.  At least one IPv6 address
   shall be present.  Multiple addresses may be provided for load
   balancing or redundancy.

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3.6.1.3.  AR FQDN List Sub-Element

   This Sub-Element (see Figure 12) is used by the AC to configure a WTP
   with AR FQDN available to establish the data tunnel for user traffic.
   Based on the FQDN, a WTP can acquire the AR IP address via DNS.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AR FQDN Sub-Element Type      |     Sub-element Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Length        |                     AR FQDN-1                 .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Length        |                     AR FQDN-2                 .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Length        |                     AR FQDN-N                 .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 12: AR FQDN List Sub-Element

   Sub-element Length: This refers to the total length in octets of the
   sub-element excluding the Type and sub-element Length fields.

   Length: The length of each AR FQDN.

   AR FQDN: An array of variable-length string containing AR FQDN.  This
   can be used to satisfy load-balance and reliability requirements.

3.6.2.  Tunnel DTLS Policy Sub-Element

   The AC distributes its DTLS usage policy for the CAPWAP data tunnel
   between a WTP and the AR.  There are multiple supported options,
   represented by the bit field below as defined in AC Descriptor
   message elements.  The WTP MUST abide by one of the options for
   tunneling user traffic with AR.  The Tunnel DTLS Policy Sub-Element
   obey the definition in [RFC5415].  If there are more than one ARs
   information provided by the AC for reliability reasons, the same
   Tunnel DTLS Policy (see Figure 13) is generally applied for all
   tunnels associated with the ARs.  Otherwise, Tunnel DTLS Policy MUST
   be bonding together with each of the ARs, then WTP will enforce the
   independent tunnel DTLS policy for each tunnel with a specific AR.

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Tunnel DTLS Sub-element Type   |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Reserved                       |A|D|C|R|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                       AR Information (optional)               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 13: Tunnel DTLS Policy Sub-Element

   Reserved: A set of reserved bits for future use.  All implementations
   complying with this protocol MUST set to zero any bits that are
   reserved in the version of the protocol supported by that
   implementation.  Receivers MUST ignore all bits not defined for the
   version of the protocol they support.

   A: If A bit is set, there is an AR information associated with the
   DTLS policy.  There may be an array of pairs binding DTLS policy
   information and AR information contained in the Tunnel DTLS Policy
   Sub-Element.  Otherwise, the same Tunnel DTLS Policy (see Figure 13)
   is generally applied for all tunnels associated with the ARs
   configured by the AC.

   D: DTLS-Enabled Data Channel Supported (see [RFC5415]).

   C: Clear Text Data Channel Supported (see [RFC5415]).

   R: A reserved bit for future use abide (see [RFC5415]).

3.6.3.  IEEE 802.11 Tagging Mode Policy Sub-Element

   In 802.11 networks, IEEE 802.11 Tagging Mode Policy Sub-Element is
   used to specify how the WTP apply the QoS tagging policy when
   receiving the packets from stations on a particular radio.  When the
   WTP sends out the packet to data channel to the AR(s), the packets
   have to be tagged for QoS purposes (see [RFC5416]).

   The IEEE 802.11 Tagging Mode Policy abides the IEEE 802.11 WTP
   Quality of Service defined in Section 6.22 of [RFC5416].

3.6.4.  CAPWAP Transport Protocol Sub-Element

   The CAPWAP data tunnel supports both UDP and UDP-Lite (see RFC3828).
   When run over IPv4, UDP is used for the CAPWAP data channels.  When
   run over IPv6, the CAPWAP data channel may use either UDP or UDP-

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   lite.  The AC specifies and configure the WTP for which transport
   protocol is to be used for the CAPWAP data tunnel.

   The CAPWAP Transport Protocol Sub-Element abides the definition in
   Section 4.6.14 of [RFC5415].

     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=51                 |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Transport               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   CAPWAP Transport Protocol Sub-Element

   Type: 51 for CAPWAP Transport Protocol [RFC5415].

   Length: 1

   Transport: The transport to use for the CAPWAP Data channel.  The
   following enumerated values are supported:

   1 - UDP-Lite: The UDP-Lite transport protocol is to be used for the
   CAPWAP Data channel.  Note that this option MUST NOT be used if the
   CAPWAP Control channel is being used over IPv4 and AR address is IPv4
   contained in the AR Information Sub-Element.

   2 - UDP: The UDP transport protocol is to be used for the CAPWAP Data
   channel.

3.6.5.  GRE Key Sub-Element

   If a WTP receives the GRE Key Sub-Element in the Alternate Tunnel
   Encapsulation message element for GREv4 or GREv6 selection, the WTP
   must insert the GRE Key to the encapsulation packet (see [RFC2890]).
   An AR acting as decapsulating tunnel endpoint identifies packets
   belonging to a traffic flow based on the Key value.

   The GRE Key Sub-Element field contains a four octet number defined in
   [RFC2890].

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | GRE Key Sub-element Type      |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                GRE Key                                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            GRE Key Sub-Element

   GRE Key: The Key field contains a four octet number which is inserted
   by the WTP according to [RFC2890].

4.  IANA Considerations

   This document requires the following IANA considerations.

   o  <IANA-1>.  This specification defines the Supported Alternate
      Tunnel Encapsulations Type message element in Section 3.1.  This
      elements needs to be registered in the existing CAPWAP Message
      Element Type registry, defined in [RFC5415].  The Type value for
      this element needs to be between 1 and 1023 (see Section 15.7 in
      [RFC5415]).
   o  <IANA-2>.  This specification defines the Alternate Tunnel
      Encapsulations Type message element in Section 3.2.  This element
      needs to be registered in the existing CAPWAP Message Element Type
      registry, defined in [RFC5415].  The Type value for this element
      needs to be between 1 and 1023.
   o  <IANA-3>.  This specification defines the IEEE 802.11 WTP
      Alternate Tunnel Failure Indication message element in
      Section 3.3.  This element needs to be registered in the existing
      CAPWAP Message Element Type registry, defined in [RFC5415].  The
      Type value for this element needs to be between 1024 and 2047.
   o  Tunnel-Type: This specification defines the Alternate Tunnel
      Encapsulations Type message element.  This element contains a
      field Tunnel-Type.  The namespace for the field is 16 bits
      (0-65535)).  This specification defines values, zero (0) through
      six (6) and can be found in Section 3.2.  Future allocations of
      values in this name space are to be assigned by IANA using the
      "Specification Required" policy.  IANA needs to create a registry
      called CAPWAP Alternate Tunnel-Types.  The registry format is
      given below.

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        Tunnel-Type           Type Value   Reference
        CAPWAP                0            [RFC5415],[RFC5416]
        L2TP                  1            [RFC2661]
        L2TPv3                2            [RFC3931]
        IP-IP                 3            [RFC2003]
        PMIPv6                4            [RFC5213]
        GRE-IPv4              5            [RFC2784]
        GRE-IPv6              6            [RFC2784]

5.  Security Considerations

   This document introduces three new CAPWAP WTP message elements.
   These elements are transported within CAPWAP Control messages as the
   existing message elements.  Therefore, this document does not
   introduce any new security risks compared to [RFC5415] and [RFC5416].
   In CAPWAP, security for CAPWAP Data Channel is optional and security
   policy is determined by AC.  Similarly, the AC determines the
   security for the Alternate Tunnel between WTP and Alternate Tunnel
   Encapsulation Gateway.  The security considerations described in
   [RFC5415] and [RFC5416] apply here as well.

6.  Contributors

   This document stems from the joint work of Hong Liu, Yifan Chen,
   Chunju Shao from China Mobile Research.

7.  References

7.1.  Normative References

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

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, August 1999.

   [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.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.

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   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

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

   [RFC5415]  Calhoun, P., Montemurro, M., and D. Stanley, "Control And
              Provisioning of Wireless Access Points (CAPWAP) Protocol
              Specification", RFC 5415, March 2009.

   [RFC5416]  Calhoun, P., Montemurro, M., and D. Stanley, "Control and
              Provisioning of Wireless Access Points (CAPWAP) Protocol
              Binding for IEEE 802.11", RFC 5416, March 2009.

7.2.  Informative References

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

Authors' Addresses

   Rong Zhang
   China Telecom
   No.109 Zhongshandadao avenue
   Guangzhou  510630
   China

   Email: zhangr@gsta.com

   Zhen Cao
   China Mobile
   Xuanwumenxi Ave. No. 32
   Beijing  100871
   China

   Phone: +86-10-52686688
   Email: zehn.cao@gmail.com, caozhen@chinamobile.com

   Hui Deng
   China Mobile
   No.32 Xuanwumen West Street
   Beijing  100053
   China

   Email: denghui@chinamobile.com

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   Rajesh S. Pazhyannur
   Cisco
   170 West Tasman Drive
   San Jose, CA 95134
   USA

   Email: rpazhyan@cisco.com

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

   Email: sgundave@cisco.com

   Li Xue
   Huawei
   No.156 Beiqing Rd. Z-park, HaiDian District
   Beijing
   China

   Email: xueli@huawei.com

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