Opsawg Working Group
Internet Engineering Task Force (IETF)                          R. Zhang
Internet-Draft
Request for Comments: 8350                                 China Telecom
Intended status:
Category: Experimental                                     R. Pazhyannur
Expires: August 2, 2018
ISSN: 2070-1721                                            S. Gundavelli
                                                                   Cisco
                                                                  Z. Cao
                                                                 H. Deng
                                                                   Z. Du
                                                                  Huawei
                                                        January 29,
                                                              April 2018

           Alternate Tunnel Encapsulation for Data Frames in CAPWAP
                 draft-ietf-opsawg-capwap-alt-tunnel-12
      Control and Provisioning of Wireless Access Points (CAPWAP)

Abstract

   Control and Provisioning of Wireless Access Points (CAPWAP) defines is a
   specification to encapsulate
   protocol for encapsulating 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 Data
   Channel is used for tunneling.  In many deployments deployments, encapsulating
   data frames to an entity other than the AC (for example example, to an Access
   Router (AR)) is desirable.  Furthermore, it may also be desirable to
   use different tunnel encapsulation modes between the WTP and the
   Access Router.  This document defines an extension to the CAPWAP
   protocol
   for supporting that supports this capability 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 types, such as IP-IP, IP-GRE, or CAPWAP.  The WTP may
   advertise support for alternate tunnel encapsulation during the
   discovery and join
   process process, and the AC may select one of the
   supported alternate tunnel encapsulation types while configuring the
   WTP.

Status of This Memo

   This Internet-Draft document is submitted in full conformance with the
   provisions of BCP 78 not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and BCP 79.

   Internet-Drafts are working documents
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list  It represents the consensus of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents valid approved by the IESG are candidates for a maximum any level of
   Internet Standard; see Section 2 of six months RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
   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 August 2, 2018.
   https://www.rfc-editor.org/info/rfc8350.

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   Copyright (c) 2018 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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions used Used in this document This Document . . . . . . . . . . . .   7
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
     1.3.  History of the document Document . . . . . . . . . . . . . . . . .   8
   2.  Alternate Tunnel Encapsulation Overview . . . . . . . . . . .   8
   3.  Extensions for CAPWAP Protocol Message Elements Extensions . . . . . . . . .  11
     3.1.  Supported Alternate Tunnel Encapsulations . . . . . . . .  11
     3.2.  Alternate Tunnel Encapsulations Type  . . . . . . . . . .  11
     3.3.  IEEE 802.11 WTP Alternate Tunnel Failure Indication . . .  12
   4.  Alternate Tunnel Types  . . . . . . . . . . . . . . . . . . .  13
     4.1.  CAPWAP based  CAPWAP-Based Alternate Tunnel . . . . . . . . . . . . . .  13
     4.2.  PMIPv6 based  PMIPv6-Based Alternate Tunnel . . . . . . . . . . . . . .  14
     4.3.  GRE based  GRE-Based Alternate Tunnel  . . . . . . . . . . . . . . .  15
   5.  Alternate Tunnel Information Elements . . . . . . . . . . . .  15
     5.1.  Access Router Information Elements  . . . . . . . . . . .  15  16
       5.1.1.  AR IPv4 List Element  . . . . . . . . . . . . . . . .  16
       5.1.2.  AR IPv6 List Element  . . . . . . . . . . . . . . . .  16  17
     5.2.  Tunnel DTLS Policy Element  . . . . . . . . . . . . . . .  17
     5.3.  IEEE 802.11 Tagging Mode Policy Element . . . . . . . . .  19
     5.4.  CAPWAP Transport Protocol Element . . . . . . . . . . . .  20
     5.5.  GRE Key Element . . . . . . . . . . . . . . . . . . . . .  22
     5.6.  IPv6 MTU Element  . . . . . . . . . . . . . . . . . . . .  23
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   8.  Contributors  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
   9.
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     8.2.  Informative References  . . . . . . .  25
     9.1.  Normative References . . . . . . . . . .  26
   Contributors  . . . . . . . .  25
     9.2.  Informative References . . . . . . . . . . . . . . . . .  26 .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   Service Providers are deploying very large Wi-Fi network networks containing
   hundreds of thousands of Access Points (APs), which are referred to
   as Wireless Transmission Points (WTPs) in Control and Provisioning of
   Wireless Access Points (CAPWAP) terminology [RFC5415].  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]) [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  IEEE 802.3 Tunnel: All data frames are tunneled to the Access
      Controller (AC) in IEEE 802.3 format.

   o  IEEE 802.11 Tunnel: All data frames are tunneled to the AC in IEEE
      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 WTPs;
   this leads to a system with a centralized control plane with and a
   distributed data plane.  This system has two benefits: 1) it reduces
   the scale requirement on the data traffic handling capability of the AC
   AC, and 2) it leads to more efficient/optimal routing of data traffic
   while maintaining centralized control/management.

                     Locally Bridged
             +-----+ Data Frames   +----------------+
             | WTP |===============|  Access Router |
             +-----+               +----------------+
                    \\
                     \\  CAPWAP Control Channel   +----------+
                       ++=========================|   AC     |
                      // CAPWAP Data Channel:     |          |
                     //  IEEE 802.11 Mgmt traffic 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 a
   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 either the IEEE 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 AC.  This requirement could be met in the locally bridged system
   (Figure 1) if the Access Router (AR) implemented the required policy.
   However, in many deployments 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.

              +-----+
              | WTP |
              +-----+
                  \\
                    \\  CAPWAP Control Channel   +----------+
                      ++=========================|   AC     |
                     // CAPWAP Data Channel:     |          |
                    //  IEEE 802.11 Mgmt traffic 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 the system in Figure 2. 2: 1) They do it does
   not allow the WTP to tunnel data frames to an endpoint different from
   the AC AC, and 2) They do it does 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 L2TP, Layer 2 Tunneling Protocol
   (L2TP), L2TPv3, IP-in-IP, IP/
   GRE, IP/GRE, etc.  This enables 1) independent
   scaling of data plane and 2) leveraging of commonly used tunnel
   encapsulations such as L2TP, GRE, etc.

          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 Traffic
                //   _________
         +-----+    (         )                +----------------+
         | WTP |====+Internet +================|  Access Router |
         +-----+    (_________)                +----------------+
          Alternate Tunnel to AR (L2TPv3, IP-in-IP, CAPWAP, etc.)

      Figure 3: Centralized Control with an 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 is shown in the figure Figure 3,
   there is a CAPWAP control Control and data channel Data Channel between the WTP and AC.
   The CAPWAP data channel Data Channel carries the stations' management traffic traffic, as
   in the case of the locally bridged system.  The main reason to
   maintain a CAPWAP data channel 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
   frames.  The data frames are transported by an alternate tunnel
   between the WTP and a tunnel termination point 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, for the AC
   to configure alternate tunnel encapsulation, and for the WTP to
   report failure of the alternate tunnel.

   The alternate tunnel encapsulation also supports the third-party WLAN
   service provider scenario (i.e. (i.e., Virtual Network Operator, VNO). Operator (VNO)).
   Under this scenario, the WLAN provider owns the WTP and AC resources, resources
   while the VNOs can rent the WTP resources from the WLAN provider for
   network access.  The AC belonging to the WLAN service provider
   manages the WTPs in the centralized mode.

   As shown in Figure 4, VNO 1&2 1 and VNO 2 don't possess the network
   access
   resources, however resources; however, they provide services by acquiring
   resources from the WLAN provider.  Since a WTP is capable of
   supporting up to 16 Service Set Identifiers (SSIDs), the WLAN
   provider may provide network access service for different providers
   with different SSIDs.  For example, SSID1 is advertised by the WTP
   for VNO1; while VNO 1 while SSID2 is advertised by the WTP for VNO2.  Therefore VNO 2.  Therefore,
   the data traffic from the user can be directly steered to the
   corresponding access router Access Router of the VNO who owns that user.  As is
   shown in Figure 4, AC can notify multiple AR addresses for load
   balancing or redundancy.

                                     +----+
                                     | AC |
                                     +--+-+
                          CAPWAP-CTL    |
                      +-----------------+
                      |   CAPWAP-DATA: IEEE 802.11 Mgmt traffic Traffic
                      |
         WLAN Provider|                            VNO 1
                +-----+   CAPWAP-DATA (SSID1)    +---------------+
         SSID1  | WTP +--------------------------|Access Router 1|
         SSID2  +--+-++                          +---------------+
                   | |
                   | |                             VNO 1
                   | |    GRE-DATA (SSID1)       +---------------+
                   | +---------------------------|Access Router 2|
                   |                             +---------------+
                   |
                   |                               VNO 2
                   |      CAPWAP-DATA (SSID2)    +---------------+
                   +-----------------------------|Access Router 3|
                                                 +---------------+

                Figure 4: Third-party Third-Party WLAN Service Provider

1.1.  Conventions used Used in this document This Document

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

1.2.  Terminology

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

   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.

   Access Router (AR): A specialized router usually residing at the edge
   or boundary of a network.  This router ensures the connectivity of
   its network with external networks, a wide area network network, or the
   Internet.

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

   CAPWAP Control Channel: A bi-directional bidirectional 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 bidirectional 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).

1.3.  History of the document Document

   This document was started to accommodate Service Providers' need of a
   more flexible deployment mode with alternative tunnels [RFC7494].
   Experiments and tests have been done for this alt-tunnel alternate tunnel
   network infrastructure.  However important, the deployment of
   relevant technology is yet to complete. be completed.  This experimental Experimental
   document is intended to serve as an archival record for any future
   work as to on the operational and deployment requirements.

2.  Alternate Tunnel Encapsulation Overview
           +-+-+-+-+-+-+                             +-+-+-+-+-+-+
           |    WTP    |                             |    AC     |
           +-+-+-+-+-+-+                             +-+-+-+-+-+-+
                 |Join Request [ Supported Alternate       |
                 |       Tunnel Encapsulations ]           |
                 |---------------------------------------->|
                 |                                         |
                 |Join Response                            |
                 |<----------------------------------------|
                 |                                         |
                 |IEEE 802.11 WLAN Configuration Request [ |
                 | IEEE 802.11 Add WLAN,                   |
                 | Alternate Tunnel Encapsulation (        |
                 |   Tunnel Type, Tunnel Info Element)     |
                 | ]                                       |
                 |<----------------------------------------|
                 |                                         |
                 |                                         |
            +-+-+-+-+-+-+                                  |
            | Setup     |                                  |
            | Alternate |                                  |
            | Tunnel    |                                  |
            +-+-+-+-+-+-+                                  |
                 |IEEE 802.11 WLAN Configuration Response  |
                 |[ Alternate Tunnel Encapsulation (       |
                 |   Tunnel Type, Tunnel Info Element) ]   |
                 |---------------------------------------->|
                 |                                         |
                 |                                         |
            +-+-+-+-+-+-+                                  |
            | Tunnel    |                                  |
            | Failure   |                                  |
            +-+-+-+-+-+-+                                  |
                 |WTP Alternate Tunnel Failure Indication  |
                 |(report failure
                 |(Report Failure (AR address(es))) Address(es)))        |
                 |---------------------------------------->|
                 |                                         |
         +-+-+-+-+-+-+-+                                   |
         | Tunnel      |                                   |
         | Established |                                   |
         +-+-+-+-+-+-+-+                                   |
                 |WTP Alternate Tunnel Failure Indication  |
                 |(report clearing failure)
                 |(Report Clearing Failure)                |
                 |---------------------------------------->|
                 |                                         |

                  Figure 5: Setup of an Alternate Tunnel
   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 Alternate Tunnel Encapsulations Type message element containing
   the tunnel type and a tunnel-specific information element.  The tunnel-
   specific
   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 Alternate Tunnel
   Encapsulations Type message element.

   Since an AC can configure a WTP with more than one AR available for
   the WTP to establish the data tunnel(s) for user traffic, it may be
   useful for the WTP to communicate the selected AR.  To enable this,
   the IEEE 802.11 WLAN Configuration Response may carry the alternate
   tunnel encapsulation Alternate
   Tunnel Encapsulations Type message element containing the AR list
   element corresponding to the selected AR as shown in Figure 5.

   On detecting a tunnel failure, the WTP SHALL forward data frames to
   the AC and discard the frames.  In addition, the 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 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, IEEE
   802.11 WTP Alternate Tunnel Fail Failure Indication (defined in this specification).
   Section 3.3).  It MAY be carried in the WTP Event Request message message,
   which is defined in [RFC5415].

   The WTP also needs to notify the AC of which AR(s) are unavailable.
   Particularly, in the VNO scenario, the AC of the WLAN service
   provider needs to maintain the association of the AR addresses of the
   VNOs and SSIDs, SSIDs and provide this information to the WTP for the
   purpose of load balancing or master-slave mode.

   The message element has a status Status field that indicates whether the
   message denotes is reporting a failure or the clearing of the previously reported
   failure.

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

3.  Extensions for CAPWAP Protocol Message Elements Extensions

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 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 1            |      Tunnel-Type 2            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            ...                |      Tunnel-Type N            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 6: Supported Alternate Tunnel Encapsulations

   o  Type: <IANA-1> 54 for Supported Alternate Tunnel Encapsulations Type

   o  Length: The length in bytes, bytes; two bytes for each Alternative tunnel
      type
      Tunnel-Type that is included

   o  Tunnel-Type: This is identified by the value defined in
      Section 3.2.  There may be one or more Tunnel-Types Tunnel-Types, as shows is shown
      in Figure 6.

3.2.  Alternate Tunnel Encapsulations Type

   This message element can be sent by the AC.  This message element AC, allows the AC to select
   the alternate tunnel encapsulation.  This
   message element encapsulation, and 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) MAC), and Tunnel
   Mode is set to 0 (Local Bridging).  Besides, the message element can
   also be sent by the WTP to communicate the selected AR(s).

   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 7: Alternate Tunnel Encapsulations Type
   o  Type: <IANA-2> 55 for Alternate Tunnel Encapsulation Encapsulations Type

   o  Length: > 4

   o  Tunnel-Type: The tunnel type Tunnel-Type is specified by a 2 byte 2-byte value.  This
      specification defines the values from zero (0) 0 to six (6) 6 as given below.  The
      remaining values are reserved for future use.

      *  0: CAPWAP.  This refers to a CAPWAP data channel Data Channel described in
         [RFC5415] and [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-UDP.  This refers to the UDP encapsulation mode for
         Proxy Mobile IPv6 (PMIPv6) described in [RFC5844].  This
         encapsulation mode is the basic encapsulation mode and does not
         include the TLV header specified in section 7.2, Section 7.2 of [RFC5845].

      *  5: GRE.  This refers to GRE tunnel encapsulation as described
         in [RFC2784].

      *  6: GTPv1-U.  This refers to GTPv1 user plane the GPRS Tunnelling Protocol (GTP)
         User Plane mode as described in [TS29281].

   o  Info Element: This field contains tunnel specific tunnel-specific configuration
      parameters to enable the WTP to setup set up the alternate tunnel.  This
      specification provides details for this elements element for CAPWAP,
      PMIPv6, and GRE.  This specification reserves the tunnel type
      values for the key tunnel types and defines the most common
      message elements.  It is anticipated that message elements for the
      other protocols (like L2TPv3, etc.) L2TPv3) will be defined in other
      specifications in the future.

3.3.  IEEE 802.11 WTP Alternate Tunnel Failure Indication

   The WTP MAY include the Alternate Tunnel Failure Indication message
   in a WTP Event Request message to inform the AC about the status of
   the Alternate Tunnel. alternate tunnel.  For the case where the WTP establishes data
   tunnels with multiple ARs (e.g., under a VNO scenario), the WTP needs
   to notify the AC of which AR(s) are unavailable.  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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      WLAN ID  |     Status    |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .              Access Router Information Element                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 8: IEEE 802.11 WTP Alternate Tunnel Failure Indication

   o  Type: <IANA-3> 1062 for IEEE 802.11 WTP Alternate Tunnel Failure Indication

   o  Length: > 4

   o  WLAN ID: An 8-bit value specifying the WLAN Identifier.  The value
      MUST be between one (1) 1 and 16.

   o  Status: An 8-bit boolean indicating whether the radio failure is
      being reported or cleared.  A value of zero 0 is used to clear the
      event, while a value of one 1 is used to report the event.

   o  Reserved: MUST be set to a value of 0 and MUST be ignored by the
      receiver.

   o  Access Router Information Element: The IPv4 address or IPv6 address of the
      Access Router that terminates the alternate tunnel.  The Access
      Router Information Elements allow the WTP to notify the AC of
      which AR(s) are unavailable.

4.  Alternate Tunnel Types

4.1.  CAPWAP based  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: The IPv4 address or IPv6 address 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 is determined by the associated AC policy
      policy.  (This is consistent with the WTP behavior described in [RFC5415]).
      [RFC5415].)
   o  IEEE 802.11 Tagging Mode Policy: It is used to specify how the
      CAPWAP data channel packet Data Channel packets are to be tagged for QoS purposes (see
      [RFC5416] for more details).

   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. Data Channels.  When run over IPv6, the CAPWAP data
      channel Data
      Channel may use either UDP or UDP-lite. UDP-Lite.

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

      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 Element                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .              Tunnel DTLS Policy Element                       .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .             IEEE 802.11 Tagging Mode Policy Element           .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .             CAPWAP Transport Protocol Element                 .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 9: Alternate Tunnel Encapsulation - CAPWAP

4.2.  PMIPv6 based  PMIPv6-Based Alternate Tunnel

   Proxy Mobile IPv6 (PMIPv6)

   A user plane based on PMIPv6 (defined in [RFC5213]) based user plane can also be used
   as an 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 UDP 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 (acting as LMA) Information: IPv4 or IPv6 address
      for the alternate tunnel endpoint.

   The message element structure for PMIPv6 encapsulation is shown in
   Figure 10:

      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 Information Element             .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 10: Alternate Tunnel Encapsulation - PMIPv6

4.3.  GRE based  GRE-Based Alternate Tunnel

   A user plane based on Generic Routing Encapsulation (defined in
   [RFC2784]) mode based user
   plane can also be used as an alternate tunnel encapsulation
   between the WTP and the AR.  In this scenario, a WTP and the access router Access
   Router represent the two end points endpoints of the GRE tunnel.  If GRE 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 Information: The IPv4 or IPv6 address for the
      alternate tunnel endpoint.

   o  GRE Key Information: The Key field is intended to be used for
      identifying an individual traffic flow within a tunnel [RFC2890].

   The message element structure for GRE encapsulation is shown in Figure 11:

      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=5             |   Info Element Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .              Access Router Information Element                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                    GRE Key Element                            .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 11: Alternate Tunnel Encapsulation - GRE

5.  Alternate Tunnel Information Elements

   This section defines the various elements described in Section Sections 4.1,
   Section
   4.2, and Section 4.3.

   These information elements can only be included in the Alternate
   Tunnel Encapsulations Type message element, element and the IEEE 802.11 WTP
   Alternate Tunnel Failure Indication message element as their sub-
   elements.

5.1.  Access Router Information Elements

   The Access Router Information Elements allow the AC to notify a WTP
   of which AR(s) are available for establishing a data tunnel.  The AR
   information may be an IPv4 address, or IPv6 address.This address.  For any Tunnel-Type,
   this information element SHOULD be contained whatever included in the tunnel type is. Alternate Tunnel
   Encapsulations Type message element.

   If the Alternate Tunnel Encapsulations Type message element is sent
   by the WTP to communicate the selected AR(s), this Access Router
   Information Element SHOULD be contained. included in it.

   The following are the Access Router Information Elements defined in
   this specification.  The AC can use one of them to notify the WTP
   about the destination information of the data tunnel to the WTP. tunnel.  The Elements
   containing the AR IPv4 address MUST NOT be used if an IPv6 data
   channel with Data
   Channel with IPv6 transport is used.

5.1.1.  AR IPv4 List Element

   This Element element (see Figure 12) 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.

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

                      Figure 12: AR IPv4 List Element

   Type: 0

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

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

5.1.2.  AR IPv6 List Element

   This Element element (see Figure 13) 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 Element Type        |          Length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                     AR IPv6 Address-1                         .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                     AR IPv6 Address-2                         .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                     AR IPv6 Address-N                         .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 13: AR IPv6 List Element

   Type: 1

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

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

5.2.  Tunnel DTLS Policy Element

   The AC distributes its DTLS Datagram Transport Layer Security (DTLS) usage
   policy for the CAPWAP data tunnel between a WTP and the AR.  There
   are multiple supported options, which are represented by the bit field
   fields 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 Element obeys the definition in [RFC5415].
   If, for reliability reasons, the AC has provided more than one AR
   address in the Access Router Information Element, the same Tunnel
   DTLS Policy (the last one in Figure 14) is generally applied for all
   tunnels associated with those ARs.  Otherwise, Tunnel DTLS Policy
   MUST be bonded together with each of the Access Router Information
   Elements, and the WTP will enforce the independent tunnel DTLS policy
   for each tunnel with a specific AR.

      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 Policy Element Type|        Length                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Reserved                         |D|C|R|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Reserved                         |D|C|R|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                         ......                                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Reserved                         |D|C|R|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 14: Tunnel DTLS Policy Element

   Type: 2

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

   Reserved: A set of reserved bits for future use.  All implementations
   complying with this protocol MUST set to zero 0 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.

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

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

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

   AR Information: This means Access Router Information Element.  In
   this context, each address in AR information Information MUST be one of
   previously specified AR addresses.

   The

   In Figure 14, the last element having that has no AR Information in Figure 14 is the
   default tunnel DTLS policy, and which provides options for any address
   not previously mentioned.  Therefore, the AR information Information field here
   is optional.  If  In this element, if all ARs share the same tunnel DTLS
   policy, in this
   element, there will not won't be an AR information Information field and or its specific tunnel
   DTLS policy.

5.3.  IEEE 802.11 Tagging Mode Policy Element

   In IEEE 802.11 networks, the IEEE 802.11 Tagging Mode Policy Element
   is used to specify how the WTP applies 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 by the IEEE 802.11 WTP
   Quality of Service defined in Section 6.22 of [RFC5416].

   If, for reliability reasons, the AC has provided more than one AR
   address in the Access Router Information Element, the same IEEE
   802.11 Tagging Mode Policy (the last one in Figure 15) is generally
   applied for all tunnels associated with those ARs.  Otherwise, IEEE
   802.11 Tagging Mode Policy MUST be bonded together with each of the
   Access Router Information Elements, and the WTP will enforce the
   independent IEEE 802.11 Tagging Mode Policy for each tunnel with a
   specific AR.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Tagging Mode Policy Ele. Type |        Length                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Reserved                     |P|Q|D|O|I|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Reserved                     |P|Q|D|O|I|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                         ......                                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Reserved                     |P|Q|D|O|I|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 15: IEEE 802.11 Tagging Mode Policy Element

   Type: 3

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

   Reserved: A set of reserved bits for future use.

   P: When set, the WTP is to employ the IEEE 802.1p QoS mechanism (see
   [RFC5416]).

   Q: When the 'P' bit is set, the 'Q' bit is used by the AC to
   communicate to the WTP how IEEE 802.1p QoS is to be enforced (see
   [RFC5416]).

   D: When set, the WTP is to employ the DSCP QoS mechanism (see
   [RFC5416]).

   O: When the 'D' bit is set, the 'O' bit is used by the AC to
   communicate to the WTP how DSCP Differentiated Services Code Point (DSCP)
   QoS is to be enforced on the outer (tunneled) header (see [RFC5416]).

   I: When the 'D' bit is set, the 'I' bit is used by the AC to
   communicate to the WTP how DSCP QoS is to be enforced on the
   station's packet (inner) header (see [RFC5416]).

   AR Information: This means Access Router Information Element.  In
   this context, each address in AR information MUST be one of the
   previously specified AR addresses.

   The

   In Figure 15, the last element having that has no AR Information in Figure 15 information is the
   default IEEE 802.11 Tagging Mode Policy, and which provides options for
   any address not previously mentioned.  Therefore, the AR information Information
   field here is optional.  If all ARs share the same IEEE 802.11
   Tagging Mode Policy, in this element, there will not be an AR information
   Information field and its specific IEEE 802.11 Tagging Mode Policy.

5.4.  CAPWAP Transport Protocol 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. Data
   Channels.  When run over IPv6, the CAPWAP data channel Data Channel may use either
   UDP or UDP-lite. UDP-Lite.  The AC specifies and configures the WTP for which
   the transport protocol is to be used for the CAPWAP data tunnel.

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

   If, for reliability reasons, the AC has provided more than one AR
   address in the Access Router Information Element, the same CAPWAP
   Transport Protocol (the last one in Figure 16) is generally applied
   for all tunnels associated with those ARs.  Otherwise, CAPWAP
   Transport Protocol MUST be bonded together with each of the Access
   Router Information Elements, and the WTP will enforce the independent
   CAPWAP Transport Protocol for each tunnel with a specific AR.

      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=4                  |        Length                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Transport               |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Transport               |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                          ......                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Transport               |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 16: CAPWAP Transport Protocol Element

   Type: 4

   Length: 1

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

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

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

   AR Information: This means Access Router Information Element.  In
   this context, each address in AR information MUST be one of the
   previously specified AR addresses.

   The

   In Figure 16, the last element having that has no AR Information in Figure 16 information is the
   default CAPWAP Transport Protocol, and which provides options for any
   address not previously mentioned.  Therefore, the AR information Information
   field here is optional.  If all ARs share the same CAPWAP Transport
   Protocol, in this element, there will not be an AR information Information field
   and its specific CAPWAP Transport Protocol.

5.5.  GRE Key Element

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

   The GRE Key Element field contains a four octet 4-octet number defined in
   [RFC2890].

   If, for reliability reasons, the AC has provided more than one AR
   address in the Access Router Information Element, a GRE Key Element
   MAY be bonded together with each of the Access Router Information
   Elements, and the WTP will enforce the independent GRE Key for each
   tunnel with a specific AR.

      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 Element Type          |        Length                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         GRE Key                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         GRE Key                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                         ......                                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 17: GRE Key Element

   Type: 5

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

   GRE Key: The Key field contains a four octet 4-octet number which that is inserted by
   the WTP according to [RFC2890].

   AR Information: This means Access Router Information Element.  In
   this context, it SHOULD be restricted to a single address, address and MUST be
   the address of one of previously specified AR addresses.

   Any address not explicitly mentioned here does not have a GRE key.

5.6.  IPv6 MTU Element

   If AC has chosen a tunneling mechanism based on IPv6, it SHOULD
   support the minimum IPv6 MTU requirements [RFC8200].  This issue is
   described in [I-D.ietf-intarea-tunnels]. [ARCH-TUNNELS].  AC SHOULD inform the WTP about the IPv6
   MTU information in the "Tunnel Tunnel Info Element" Element field.

   If, for reliability reasons, the AC has provided more than one AR
   address in the Access Router Information Element, an IPv6 MTU Element
   MAY be bonded together with each of the Access Router Information
   Elements, and the WTP will enforce the independent IPv6 MTU for each
   tunnel with a specific AR.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     IPv6 MTU Element Type     |          Length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Minimum IPv6 MTU        |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Minimum IPv6 MTU        |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                       AR Information                          .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         ......                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 18: IPv6 MTU Element

   Type: 6

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

   Minimum IPv6 MTU: The field contains a two octet 2-octet number indicate indicating the
   minimum IPv6 MTU in the tunnel.

   AR Information: This means Access Router Information Element.  In
   this context, each address in AR information MUST be one of
   previously specified AR addresses.

6.  IANA Considerations

   This document requires the following

   Per this document, 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 has registered the following values in the
   existing CAPWAP "CAPWAP Message Element Type 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  54: Supported Alternate Tunnel Encapsulations Type message element as defined in
      Section 3.2.  This element
      needs to be registered in the existing CAPWAP Message Element 3.1.

   o  55: Alternate Tunnel Encapsulations Type
      registry, as defined in [RFC5415].  The Type value for this element
      needs to be between 1 and 1023.
      Section 3.2.

   o  <IANA-3>.  This specification defines the  1062: IEEE 802.11 WTP Alternate Tunnel Failure Indication message element as
      defined 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

   Per this element needs to be between 1024 and 2047.
   o  Alternate Tunnel-Types Registry: document, IANA has created a registry called "Alternate
   Tunnel-Types" under "CAPWAP Parameters".  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) values 0 through six (6) 6 and
   can be found in Section 3.2.  Future allocations of values in this name space
   namespace are to be assigned by IANA using the "Specification
   Required" policy.  IANA needs to
      create a registry called CAPWAP Alternate Tunnel-Types. policy [RFC8126].  The registry format is given below.

        Tunnel-Type           Type

        Description           Value         Reference
        CAPWAP                0            [RFC5415],[RFC5416]             [RFC5415] [RFC5416]
        L2TP                  1             [RFC2661]
        L2TPv3                2             [RFC3931]
        IP-IP                 3             [RFC2003]
        PMIPv6-UDP            4             [RFC5844]
        GRE                   5             [RFC2784]
        GTPv1-U               6             [3GPP TS 29.281]

   o  Alternate

   Per this document, IANA has created a registry called "Alternate
   Tunnel Sub-elements Registry: Sub-elements" under "CAPWAP Parameters".  This specification
   defines the Alternate Tunnel Sub-elements.  Currently, these
   information elements can only be included in the Alternate Tunnel
   Encapsulations Type message element, and element with the IEEE 802.11 WTP
   Alternate Tunnel Failure Indication message element as their its sub-
   elements.  These information elements contains contain a Type field.  The
   namespace for the field is 16 bits (0-65535).  This specification
   defines values, zero (0) values 0 through six (6) 6 in Section 5.  This namespace is managed
   by IANA IANA, and assignments require an Expert
      Review.

        Type                                     Type Review [RFC8126].

        Description                              Value
        AR IPv4 List                             0
        AR IPv6 List                             1
        Tunnel DTLS Policy                       2
        IEEE 802.11 Tagging Mode Policy          3
        CAPWAP Transport Protocol                4
        GRE Key                                  5
        IPv6 MTU                                 6

7.  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 to the control plane compared to
   [RFC5415] and [RFC5416].  In the data plane, if the encapsulation
   type selected itself is not secured, it is suggested to protect the
   tunnel by using known secure methods, such as IPSec. IPsec.

8.  Contributors

   The authors would like to thank Andreas Schultz, Hong Liu, Yifan
   Chen, Chunju Shao, Li Xue, Jianjie You, Jin Li, Joe Touch, Alexey
   Melnikov, Kathleen Moriarty, Mirja Kuehlewind, Catherine Meadows, and
   Paul Kyzivat for their valuable comments.

9.  References

9.1.

8.1.  Normative References

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              DOI 10.17487/RFC2003, October 1996,
              <https://www.rfc-editor.org/info/rfc2003>.

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

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, DOI 10.17487/RFC2661, August 1999,
              <https://www.rfc-editor.org/info/rfc2661>.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,
              <https://www.rfc-editor.org/info/rfc2784>.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <https://www.rfc-editor.org/info/rfc2890>.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
              and G. Fairhurst, Ed., "The Lightweight User Datagram
              Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
              2004, <https://www.rfc-editor.org/info/rfc3828>.

   [RFC3931]  Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
              "Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
              RFC 3931, DOI 10.17487/RFC3931, March 2005,
              <https://www.rfc-editor.org/info/rfc3931>.

   [RFC5415]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control And Provisioning of Wireless Access Points
              (CAPWAP) Protocol Specification", RFC 5415,
              DOI 10.17487/RFC5415, March 2009,
              <https://www.rfc-editor.org/info/rfc5415>.

   [RFC5416]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control and Provisioning of Wireless Access Points
              (CAPWAP) Protocol Binding for IEEE 802.11", RFC 5416,
              DOI 10.17487/RFC5416, March 2009,
              <https://www.rfc-editor.org/info/rfc5416>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

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

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

9.2.

8.2.  Informative References

   [I-D.ietf-intarea-tunnels]

   [ARCH-TUNNELS]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-08 (work Work in
              progress), Progress, draft-ietf-intarea-
              tunnels-08, January 2018.

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

   [RFC5844]  Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
              Mobile IPv6", RFC 5844, DOI 10.17487/RFC5844, May 2010,
              <https://www.rfc-editor.org/info/rfc5844>.

   [RFC5845]  Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung,
              "Generic Routing Encapsulation (GRE) Key Option for Proxy
              Mobile IPv6", RFC 5845, DOI 10.17487/RFC5845, June 2010,
              <https://www.rfc-editor.org/info/rfc5845>.

   [RFC7494]  Shao, C., Deng, H., Pazhyannur, R., Bari, F., Zhang, R.,
              and S. Matsushima, "IEEE 802.11 Medium Access Control
              (MAC) Profile for Control and Provisioning of Wireless
              Access Points (CAPWAP)", RFC 7494, DOI 10.17487/RFC7494,
              April 2015, <https://www.rfc-editor.org/info/rfc7494>.

   [TS29281]  "3rd Generation Partnership Project; Technical
              Specification Group Core Network and Terminals; General  3GPP, "General Packet Radio System (GPRS) Tunnelling
              Protocol User Plane (GTPv1-U)", 3GPP TS 29.281, V13.1.0,
              March 2016.

Contributors

   The authors would like to thank Andreas Schultz, Hong Liu, Yifan
   Chen, Chunju Shao, Li Xue, Jianjie You, Jin Li, Joe Touch, Alexey
   Melnikov, Kathleen Moriarty, Mirja Kuehlewind, Catherine Meadows, and
   Paul Kyzivat for their valuable comments.

Authors' Addresses

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

   Email: zhangr@gsta.com

   Rajesh S. Pazhyannur
   Cisco
   170 West Tasman Drive
   San Jose, CA 95134
   USA
   United States of America

   Email: rpazhyan@cisco.com

   Sri Gundavelli
   Cisco
   170 West Tasman Drive
   San Jose, CA 95134
   USA
   United States of America

   Email: sgundave@cisco.com
   Zhen Cao
   Huawei
   Xinxi Rd. 3
   Beijing  100085
   China

   Email: zhencao.ietf@gmail.com

   Hui Deng
   Huawei
   Xinxi Rd. 3
   Beijing 100085
   China

   Email: denghui02@gmail.com

   Zongpeng Du
   Huawei
   No.156 Beiqing Rd. Z-park, HaiDian District
   Beijing  100095
   China

   Email: duzongpeng@huawei.com