RFC 8350
Alternate Tunnel Encapsulation for Data Frames in Control and Provisioning of Wireless Access Points (CAPWAP)
Pages: 29
Experimental
Experimental
Part 1 of 2 – Pages 1 to 15
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.
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.
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 . . . . . . . . . . . . . . . . . . . . . . . 281. 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; 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).
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 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
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 Provider1.1. 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 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.
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 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:
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 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.
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 - PMIPv64.3. 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
(next page on part 2)