wdiff rfc8350v1.txt rfc8350.txt

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) defines is a
specification to encapsulate
protocol for encapsulating a station's data frames between the
Wireless Transmission Point (WTP) and Access Controller (AC).
Specifically, the station's IEEE 802.11 data frames can be either
locally bridged or tunneled to the AC. When tunneled, a CAPWAP data
channel Data
Channel is used for tunneling. In many deployments, 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.

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.

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. 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 . . . . . . . . . . . . 15
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 . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.1. Normative References . . . . . . . . . . . . . . . . . . 25
8.2. Informative References . . . . . . . . . . . . . . . . . 26
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27

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.

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
leading WTPs;
this leads to a system with a centralized control plane with and a
distributed data plane. This system has two benefits: 1) it reduces
the scale requirement on the data traffic handling capability of the
AC, 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).

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.

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 Control and data channel Data Channel between the WTP and AC.
The CAPWAP data channel 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 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
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.

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.

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

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

On detecting a tunnel failure, the WTP SHALL forward data frames to
the AC and discard the frames. In addition, the WTP may dissociate
existing clients and refuse association requests from new clients.
Depending on the implementation and deployment scenario, the AC may
choose to reconfigure the WLAN (on the WTP) to a Local Bridging 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.

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
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 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 [TS29281].

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:

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].)
o IEEE 802.11 Tagging Mode Policy: It is used to specify how the
CAPWAP data channel Data Channel packets are to be tagged for QoS purposes (see
[RFC5416] for more details).

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

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.

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

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. This For any Tunnel-Type,
this information element SHOULD be contained whatever included in the tunnel type is. Alternate Tunnel
Encapsulations Type message element.

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

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

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
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.

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. If In this element, if all ARs share the same tunnel DTLS
policy, in this
element, there will not won't be an AR information Information field and or its specific tunnel
DTLS policy.

5.3. IEEE 802.11 Tagging Mode Policy Element

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

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

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

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tagging Mode Policy Ele. Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |P|Q|D|O|I|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. AR Information .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |P|Q|D|O|I|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. AR Information .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |P|Q|D|O|I|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 15: IEEE 802.11 Tagging Mode Policy Element

Type: 3

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

Reserved: A set of reserved bits for future use.

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

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

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

O: When the 'D' bit is set, the 'O' bit is used by the AC to
communicate to the WTP how 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. Data
Channels. When run over IPv6, the CAPWAP data channel 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].

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

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=4 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. AR Information .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. AR Information .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 16: CAPWAP Transport Protocol Element

Type: 4

Length: 1

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

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

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

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

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

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Key Element Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Key |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. AR Information .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Key |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. AR Information .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 17: GRE Key Element

Type: 5

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

GRE Key: The Key field contains a 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.

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.

Tunnel-Type Type

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 [TS29281] [3GPP TS 29.281]

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].

Type Type

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>.

[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>.

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>.

[TS29281] 3GPP, "General Packet Radio System (GPRS) Tunnelling
Protocol User Plane (GTPv1-U)", 3GPP TS 29.281, V13.1.0,
March 2016.

Contributors

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

Authors' Addresses

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

Email: zhangr@gsta.com

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

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