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rfc:rfc8350

Internet Engineering Task Force (IETF) R. Zhang Request for Comments: 8350 China Telecom Category: Experimental R. Pazhyannur ISSN: 2070-1721 S. Gundavelli

                                                                 Cisco
                                                                Z. Cao
                                                               H. Deng
                                                                 Z. Du
                                                                Huawei
                                                            April 2018
         Alternate Tunnel Encapsulation for Data Frames in
    Control and Provisioning of Wireless Access Points (CAPWAP)

Abstract

 Control and Provisioning of Wireless Access Points (CAPWAP) is a
 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 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 an extension to the CAPWAP
 protocol 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, such as IP-IP, IP-GRE, or CAPWAP.  The WTP may
 advertise support for alternate tunnel encapsulation during the
 discovery and join process, and the AC may select one of the
 supported alternate tunnel encapsulation types while configuring the
 WTP.

Zhang, et al. Experimental [Page 1] RFC 8350 Alternate Tunnel April 2018

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are candidates for any level of
 Internet Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8350.

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.

Zhang, et al. Experimental [Page 2] RFC 8350 Alternate Tunnel April 2018

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 . . . . . . . . . . .   9
 3.  Extensions for CAPWAP Protocol Message Elements . . . . . . .  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 . . . . . . . . . . . .  16
   5.1.  Access Router Information Elements  . . . . . . . . . . .  16
     5.1.1.  AR IPv4 List Element  . . . . . . . . . . . . . . . .  16
     5.1.2.  AR IPv6 List Element  . . . . . . . . . . . . . . . .  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 . . . . . . . . . . . . . . . . . . . . .  24
 7.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
 8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
   8.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
   8.2.  Informative References  . . . . . . . . . . . . . . . . .  27
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1. Introduction

 Service Providers are deploying very large Wi-Fi 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.

Zhang, et al. Experimental [Page 3] RFC 8350 Alternate Tunnel April 2018

 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  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;
 this leads to a system with a centralized control plane 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, 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 +----------+
                  //
           +-----+               +----------------+
           | 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).

Zhang, et al. Experimental [Page 4] RFC 8350 Alternate Tunnel April 2018

 Figure 2 shows a system where the tunnel mode is configured to tunnel
 data frames between the WTP and the AC 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, 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 |          |
                 //   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) it does
 not allow the WTP to tunnel data frames to an endpoint different from
 the AC, and 2) 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 Layer 2 Tunneling Protocol
 (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.

Zhang, et al. Experimental [Page 5] RFC 8350 Alternate Tunnel April 2018

        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 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 Figure 3,
 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 and VNO 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

Zhang, et al. Experimental [Page 6] RFC 8350 Alternate Tunnel April 2018

 with different SSIDs.  For example, SSID1 is advertised by the WTP
 for VNO 1 while SSID2 is advertised by the WTP for VNO 2.  Therefore,
 the data traffic from the user can be directly steered to the
 corresponding 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
                    |
       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", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 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 (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.

Zhang, et al. Experimental [Page 7] RFC 8350 Alternate Tunnel April 2018

 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 a Radio Frequency (RF) antenna and wireless Physical layer
 (PHY) to transmit and receive station traffic for wireless access
 networks.
 CAPWAP Control Channel: A 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 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

 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 alternate tunnel
 network infrastructure.  However important, the deployment of
 relevant technology is yet to be completed.  This Experimental
 document is intended to serve as an archival record for any future
 work on the operational and deployment requirements.

Zhang, et al. Experimental [Page 8] RFC 8350 Alternate Tunnel April 2018

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 (AR Address(es)))        |
               |---------------------------------------->|
               |                                         |
       +-+-+-+-+-+-+-+                                   |
       | Tunnel      |                                   |
       | Established |                                   |
       +-+-+-+-+-+-+-+                                   |
               |WTP Alternate Tunnel Failure Indication  |
               |(Report Clearing Failure)                |
               |---------------------------------------->|
               |                                         |
                Figure 5: Setup of an Alternate Tunnel

Zhang, et al. Experimental [Page 9] RFC 8350 Alternate Tunnel April 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 Encapsulations Type 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
 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 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 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 Failure Indication (defined in
 Section 3.3).  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 is reporting a failure or clearing the previously reported
 failure.
 For the case where an AC is unreachable but the tunnel endpoint is
 still reachable, the WTP behavior is up to the implementation.  For
 example, the WTP could choose to either tear down the alternate
 tunnel or let the existing user's traffic continue to be tunneled.

Zhang, et al. Experimental [Page 10] RFC 8350 Alternate Tunnel April 2018

3. Extensions for CAPWAP Protocol Message Elements

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: 54 for Supported Alternate Tunnel Encapsulations Type
 o  Length: The length in bytes; two bytes for each Alternative
    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, as is shown
    in Figure 6.

3.2. Alternate Tunnel Encapsulations Type

 This message element can be sent by the AC, allows the AC to select
 the alternate tunnel 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), 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

Zhang, et al. Experimental [Page 11] RFC 8350 Alternate Tunnel April 2018

 o  Type: 55 for Alternate Tunnel Encapsulations Type
 o  Length: > 4
 o  Tunnel-Type: The Tunnel-Type is specified by a 2-byte value.  This
    specification defines the values from 0 to 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 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 of [RFC5845].
  • 5: GRE. This refers to GRE tunnel encapsulation as described

in [RFC2784].

  • 6: GTPv1-U. This refers to the GPRS Tunnelling Protocol (GTP)

User Plane mode as described in [TS.3GPP.29.281].

 o  Info Element: This field contains tunnel-specific configuration
    parameters to enable the WTP to set up the alternate tunnel.  This
    specification provides details for this 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) 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 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:

Zhang, et al. Experimental [Page 12] RFC 8350 Alternate Tunnel April 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      WLAN ID  |     Status    |         Reserved              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .              Access Router Information Element                .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Figure 8: IEEE 802.11 WTP Alternate Tunnel Failure Indication
 o  Type: 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 1 and 16.
 o  Status: An 8-bit boolean indicating whether the radio failure is
    being reported or cleared.  A value of 0 is used to clear the
    event, while a value of 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 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: The IPv4 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.  (This is consistent with the WTP behavior described in
    [RFC5415].)

Zhang, et al. Experimental [Page 13] RFC 8350 Alternate Tunnel April 2018

 o  IEEE 802.11 Tagging Mode Policy: It is used to specify how the
    CAPWAP 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.  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:
    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

 A user plane based on PMIPv6 (defined in [RFC5213]) 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.

Zhang, et al. Experimental [Page 14] RFC 8350 Alternate Tunnel April 2018

 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 Alternate Tunnel

 A user plane based on Generic Routing Encapsulation (defined in
 [RFC2784]) can also be used as an alternate tunnel encapsulation
 between the WTP and the AR.  In this scenario, a WTP and the Access
 Router represent the two endpoints of the GRE tunnel.  If GRE 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 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

Zhang, et al. Experimental [Page 15] RFC 8350 Alternate Tunnel April 2018

5. Alternate Tunnel Information Elements

 This section defines the various elements described in Sections 4.1,
 4.2, and 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 an IPv4 or IPv6 address.  For any Tunnel-Type,
 this information element SHOULD be included in the 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 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.  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

Zhang, et al. Experimental [Page 16] RFC 8350 Alternate Tunnel April 2018

 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.
    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 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
 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

Zhang, et al. Experimental [Page 17] RFC 8350 Alternate Tunnel April 2018

 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 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 MUST be one of
 previously specified AR addresses.

Zhang, et al. Experimental [Page 18] RFC 8350 Alternate Tunnel April 2018

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

Zhang, et al. Experimental [Page 19] RFC 8350 Alternate Tunnel April 2018

 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 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.
 In Figure 15, the last element that has no AR information is the
 default IEEE 802.11 Tagging Mode Policy, which 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 an 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
 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].

Zhang, et al. Experimental [Page 20] RFC 8350 Alternate Tunnel April 2018

 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.  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 if the
    AR address contained in the AR Information Element is an IPv4
    address.
    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 the
 previously specified AR addresses.

Zhang, et al. Experimental [Page 21] RFC 8350 Alternate Tunnel April 2018

 In Figure 16, the last element that has no AR information is the
 default CAPWAP Transport Protocol, which 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 an AR 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
 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 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.

Zhang, et al. Experimental [Page 22] RFC 8350 Alternate Tunnel April 2018

 GRE Key: The Key field contains a 4-octet number 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 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 [ARCH-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 2-octet number indicating the
 minimum IPv6 MTU in the tunnel.

Zhang, et al. Experimental [Page 23] RFC 8350 Alternate Tunnel April 2018

 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

 Per this document, IANA has registered the following values in the
 existing "CAPWAP Message Element Type" registry, defined in
 [RFC5415].
 o  54: Supported Alternate Tunnel Encapsulations Type as defined in
    Section 3.1.
 o  55: Alternate Tunnel Encapsulations Type as defined in
    Section 3.2.
 o  1062: IEEE 802.11 WTP Alternate Tunnel Failure Indication as
    defined in Section 3.3.
 Per this 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 0 through 6 and
 can be found in Section 3.2.  Future allocations of values in this
 namespace are to be assigned by IANA using the "Specification
 Required" policy [RFC8126].  The registry format is given below.
      Description           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             [TS.3GPP.29.281]

Zhang, et al. Experimental [Page 24] RFC 8350 Alternate Tunnel April 2018

 Per this document, IANA has created a registry called "Alternate
 Tunnel 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 with the IEEE 802.11 WTP
 Alternate Tunnel Failure Indication message element as its sub-
 elements.  These information elements contain a Type field.  The
 namespace for the field is 16 bits (0-65535).  This specification
 defines values 0 through 6 in Section 5.  This namespace is managed
 by IANA, and assignments require an Expert 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.

8. References

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>.

Zhang, et al. Experimental [Page 25] RFC 8350 Alternate Tunnel April 2018

 [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>.

Zhang, et al. Experimental [Page 26] RFC 8350 Alternate Tunnel April 2018

8.2. Informative References

 [ARCH-TUNNELS]
            Touch, J. and M. Townsley, "IP Tunnels in the Internet
            Architecture", Work in 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>.
 [TS.3GPP.29.281]
            3GPP, "General Packet Radio System (GPRS) Tunnelling
            Protocol User Plane (GTPv1-U)", 3GPP TS 29.281, V13.1.0,
            March 2016.

Zhang, et al. Experimental [Page 27] RFC 8350 Alternate Tunnel April 2018

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
 United States of America
 Email: rpazhyan@cisco.com
 Sri Gundavelli
 Cisco
 170 West Tasman Drive
 San Jose, CA 95134
 United States of America
 Email: sgundave@cisco.com
 Zhen Cao
 Huawei
 Xinxi Rd. 3
 Beijing  100085
 China
 Email: zhencao.ietf@gmail.com

Zhang, et al. Experimental [Page 28] RFC 8350 Alternate Tunnel April 2018

 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. Experimental [Page 29]

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