Alternate Tunnel Encapsulation for Data Frames in CAPWAP
draft-ietf-opsawg-capwap-alt-tunnel-12

Document Type Active Internet-Draft (opsawg WG)
Last updated 2018-02-08 (latest revision 2018-01-29)
Replaces draft-zhang-opsawg-capwap-cds
Stream IETF
Intended RFC status Experimental
Formats plain text xml pdf html bibtex
Reviews
Stream WG state Submitted to IESG for Publication
Document shepherd Tianran Zhou
Shepherd write-up Show (last changed 2017-10-17)
IESG IESG state RFC Ed Queue
Consensus Boilerplate Yes
Telechat date
Responsible AD Warren Kumari
Send notices to warren@kumari.net, Tianran Zhou <zhoutianran@huawei.com>
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
RFC Editor RFC Editor state EDIT
Opsawg Working Group                                            R. Zhang
Internet-Draft                                             China Telecom
Intended status: Experimental                              R. Pazhyannur
Expires: August 2, 2018                                    S. Gundavelli
                                                                   Cisco
                                                                  Z. Cao
                                                                 H. Deng
                                                                   Z. Du
                                                                  Huawei
                                                        January 29, 2018

        Alternate Tunnel Encapsulation for Data Frames in CAPWAP
                 draft-ietf-opsawg-capwap-alt-tunnel-12

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.  Furthermore, it may also be desirable to
   use different tunnel encapsulation modes between the WTP and the
   Access Router.  This document defines extension to CAPWAP protocol
   for supporting 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 such as IP-IP, IP-GRE, CAPWAP.  The WTP may advertise support
   for alternate tunnel encapsulation during the discovery and 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 https://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

Zhang, et al.            Expires August 2, 2018                 [Page 1]
Internet-Draft              Alternate Tunnel                January 2018

   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.

Copyright Notice

   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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions used in this document . . . . . . . . . . . .   7
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
     1.3.  History of the document . . . . . . . . . . . . . . . . .   8
   2.  Alternate Tunnel Encapsulation Overview . . . . . . . . . . .   8
   3.  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 Alternate Tunnel . . . . . . . . . . . . . .  13
     4.2.  PMIPv6 based Alternate Tunnel . . . . . . . . . . . . . .  14
     4.3.  GRE based Alternate Tunnel  . . . . . . . . . . . . . . .  15
   5.  Alternate Tunnel Information Elements . . . . . . . . . . . .  15
     5.1.  Access Router Information Elements  . . . . . . . . . . .  15
       5.1.1.  AR IPv4 List Element  . . . . . . . . . . . . . . . .  16
       5.1.2.  AR IPv6 List Element  . . . . . . . . . . . . . . . .  16
     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  . . . . . . . . . . . . . . . . . . . . . . . .  25
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25

Zhang, et al.            Expires August 2, 2018                 [Page 2]
Internet-Draft              Alternate Tunnel                January 2018

     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   Service Providers are deploying very large Wi-Fi network 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]) 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 Access
      Controller (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.

Zhang, et al.            Expires August 2, 2018                 [Page 3]
Internet-Draft              Alternate Tunnel                January 2018

                     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 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 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.

Zhang, et al.            Expires August 2, 2018                 [Page 4]
Internet-Draft              Alternate Tunnel                January 2018

              +-----+
              | 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 the 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 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.

          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-in-IP, CAPWAP, etc.)

       Figure 3: Centralized Control with Alternate Tunnel for Data

Zhang, et al.            Expires August 2, 2018                 [Page 5]
Internet-Draft              Alternate Tunnel                January 2018

   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 frames.  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, 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.  Virtual Network Operator, VNO).
   Under this scenario, the WLAN provider owns the WTP and AC 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 don't possess the network access
   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 SSID2 is
   advertised by the WTP for VNO2.  Therefore the data traffic from the
   user can be directly steered to the corresponding access router of
   the VNO who owns that user.  As shown in Figure 4, AC can notify
   multiple AR addresses for load balancing or redundancy.

Zhang, et al.            Expires August 2, 2018                 [Page 6]
Internet-Draft              Alternate Tunnel                January 2018

                                     +----+
                                     | AC |
                                     +--+-+
                          CAPWAP-CTL    |
                      +-----------------+
                      |   CAPWAP-DATA: IEEE 802.11 Mgmt 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 WLAN Service Provider

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).

   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 or the
   Internet.

   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.

Zhang, et al.            Expires August 2, 2018                 [Page 7]
Internet-Draft              Alternate Tunnel                January 2018

   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).

1.3.  History of the 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 network
   infrastructure.  However important, the deployment of relevant
   technology is yet to complete.  This experimental document is
   intended to serve as an archival record for any future work as to the
   operational and deployment requirements.

2.  Alternate Tunnel Encapsulation Overview

Zhang, et al.            Expires August 2, 2018                 [Page 8]
Internet-Draft              Alternate Tunnel                January 2018

           +-+-+-+-+-+-+                             +-+-+-+-+-+-+
           |    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 (AR address(es)))        |
                 |---------------------------------------->|
                 |                                         |
         +-+-+-+-+-+-+-+                                   |
         | Tunnel      |                                   |
         | Established |                                   |
         +-+-+-+-+-+-+-+                                   |
                 |WTP Alternate Tunnel Failure Indication  |
                 |(report clearing failure)                |
                 |---------------------------------------->|
                 |                                         |

                    Figure 5: Setup of Alternate Tunnel

Zhang, et al.            Expires August 2, 2018                 [Page 9]
Internet-Draft              Alternate Tunnel                January 2018

   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.

   Since 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 message element containing the AR list element
   corresponding to the selected AR as shown in Figure 5.

   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).  It
   MAY be carried in the WTP Event Request 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, and provide this information to the WTP for the
   purpose of load balancing or master-slave mode.

   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.

Zhang, et al.            Expires August 2, 2018                [Page 10]
Internet-Draft              Alternate Tunnel                January 2018

3.  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> for Supported Alternate Tunnel Encapsulations
   o  Length: The length in bytes, two bytes for each Alternative tunnel
      type that is included
   o  Tunnel-Type: This is identified by value defined in Section 3.2.
      There may be one or more Tunnel-Types as shows in Figure 6.

3.2.  Alternate Tunnel Encapsulations Type

   This message element can be 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).  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> for Alternate Tunnel Encapsulation Type

Zhang, et al.            Expires August 2, 2018                [Page 11]
Internet-Draft              Alternate Tunnel                January 2018

   o  Length: > 4
   o  Tunnel-Type: The tunnel type is specified by a 2 byte value.  This
      specification defines the values from zero (0) to six (6) as given
      below.  The remaining values are reserved for future use.

      *  0: CAPWAP.  This refers to a CAPWAP 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
         described in [RFC5844].  This encapsulation mode is the basic
         encapsulation mode and does not include the TLV header
         specified in 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 mode as described
         in [TS29281].
   o  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,
      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.) 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.  For the case where WTP establishes data
   tunnels with multiple ARs (e.g., under 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

Zhang, et al.            Expires August 2, 2018                [Page 12]
Internet-Draft              Alternate Tunnel                January 2018

   o  Type: <IANA-3> 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) and 16.
   o  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.
   o  Reserved: MUST be set to a value of 0 and MUST be ignored by the
      receiver.
   o  Access Router Information Element: 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 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 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).
   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 9:

Zhang, et al.            Expires August 2, 2018                [Page 13]
Internet-Draft              Alternate Tunnel                January 2018

      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 Alternate Tunnel

   Proxy Mobile IPv6 (PMIPv6) (defined in [RFC5213]) based user plane
   can also be used as 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

Zhang, et al.            Expires August 2, 2018                [Page 14]
Internet-Draft              Alternate Tunnel                January 2018

4.3.  GRE based Alternate Tunnel

   Generic Routing Encapsulation (defined in [RFC2784]) mode based user
   plane can also be used as alternate tunnel encapsulation between the
   WTP and the AR.  In this scenario, a WTP and the access router
   represent the two end points 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: 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 4.1,
   Section 4.2, and Section 4.3.

   These information elements can only be included in the Alternate
   Tunnel Encapsulations Type message 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 IPv4 address, or IPv6 address.This information
   element SHOULD be contained whatever the tunnel type is.

Zhang, et al.            Expires August 2, 2018                [Page 15]
Internet-Draft              Alternate Tunnel                January 2018

   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.

   The following are the Access Router Information 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 Elements
   containing the AR IPv4 address MUST NOT be used if an IPv6 data
   channel with IPv6 transport is used.

5.1.1.  AR IPv4 List Element

   This 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
   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 (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.

Zhang, et al.            Expires August 2, 2018                [Page 16]
Internet-Draft              Alternate Tunnel                January 2018

      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 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 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.

Zhang, et al.            Expires August 2, 2018                [Page 17]
Internet-Draft              Alternate Tunnel                January 2018

      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 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 MUST be one of
   previously specified AR addresses.

   The last element having no AR Information in Figure 14 is the default
   tunnel DTLS policy, and provides options for any address not
   previously mentioned.  Therefore, the AR information field here is
   optional.  If all ARs share the same tunnel DTLS policy, in this
   element, there will not be AR information field and its specific
   tunnel DTLS policy.

Zhang, et al.            Expires August 2, 2018                [Page 18]
Internet-Draft              Alternate Tunnel                January 2018

5.3.  IEEE 802.11 Tagging Mode Policy Element

   In 802.11 networks, 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 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.

Zhang, et al.            Expires August 2, 2018                [Page 19]
Internet-Draft              Alternate Tunnel                January 2018

   P: When set, the WTP is to employ the 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 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 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
   previously specified AR addresses.

   The last element having no AR Information in Figure 15 is the default
   IEEE 802.11 Tagging Mode Policy, and provides options for any address
   not previously mentioned.  Therefore, the AR information field here
   is optional.  If all ARs share the same IEEE 802.11 Tagging Mode
   Policy, in this element, there will not be AR 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.  When run over IPv6, the CAPWAP data channel may use either
   UDP or UDP-lite.  The AC specifies and configures the WTP for which
   transport protocol is to be used for the CAPWAP data tunnel.

   The CAPWAP Transport Protocol Element abides 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.

Zhang, et al.            Expires August 2, 2018                [Page 20]
Internet-Draft              Alternate Tunnel                January 2018

      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.  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 Element.

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

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

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

Zhang, et al.            Expires August 2, 2018                [Page 21]
Internet-Draft              Alternate Tunnel                January 2018

5.5.  GRE Key Element

   If a WTP receives the GRE Key Element in the Alternate Tunnel
   Encapsulation message element for GRE 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 Element field contains a four 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 number which 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, and MUST
   be the address of one of previously specified AR addresses.

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

Zhang, et al.            Expires August 2, 2018                [Page 22]
Internet-Draft              Alternate Tunnel                January 2018

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].  AC SHOULD inform the WTP
   about the IPv6 MTU information in the "Tunnel Info 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 number indicate 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 IANA considerations.

Zhang, et al.            Expires August 2, 2018                [Page 23]
Internet-Draft              Alternate Tunnel                January 2018

   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  Alternate Tunnel-Types Registry: 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.

        Tunnel-Type           Type Value   Reference
        CAPWAP                0            [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 Tunnel Sub-elements Registry: 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 the IEEE 802.11 WTP
      Alternate Tunnel Failure Indication message element as their sub-
      elements.  These information elements contains a Type field.  The
      namespace for the field is 16 bits (0-65535).  This specification
      defines values, zero (0) through six (6) in Section 5.  This
      namespace is managed by IANA and assignments require an Expert
      Review.

Zhang, et al.            Expires August 2, 2018                [Page 24]
Internet-Draft              Alternate Tunnel                January 2018

        Type                                     Type 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.

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.  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>.

Zhang, et al.            Expires August 2, 2018                [Page 25]
Internet-Draft              Alternate Tunnel                January 2018

   [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>.

   [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.  Informative References

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-08 (work in
              progress), 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>.

Zhang, et al.            Expires August 2, 2018                [Page 26]
Internet-Draft              Alternate Tunnel                January 2018

   [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
              Packet Radio System (GPRS) Tunnelling Protocol User Plane
              (GTPv1-U)", 2016.

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

   Email: rpazhyan@cisco.com

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

   Email: sgundave@cisco.com

Zhang, et al.            Expires August 2, 2018                [Page 27]
Internet-Draft              Alternate Tunnel                January 2018

   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

Zhang, et al.            Expires August 2, 2018                [Page 28]