Network Working Group R. Zhang
Internet-Draft China Telecom
Intended status: Standards Track Z. Cao
Expires: March 1, 2015 H. Deng
China Mobile
R. Pazhyannur
S. Gundavelli
Cisco
L. Xue
Huawei
August 28, 2014
Alternate Tunnel Encapsulation for Data Frames in CAPWAP
draft-ietf-opsawg-capwap-alt-tunnel-02
Abstract
CAPWAP defines a specification to encapsulate a station's data frames
between the Wireless Transmission Point (WTP) and Access Controller
(AC) using CAPWAP. 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 it is desirable to encapsulate date frames to an entity
different from the AC for example to an Access Router (AR). Further,
it may also be desirable to use different tunnel encapsulations to
carry the stations' data frames. This document provides a
specification for this and refers to it as Alternate tunnel
encapsulation. The Alternate tunnel encapsulation allows 1) the WTP
to tunnel non-management data frames to an endpoint different from
the AC and 2) the WTP to tunnel using one of many known encapsulation
types such as IP-IP, IP-GRE, CAPWAP. The WTP may advertise support
for Alternate tunnel encapsulation during the discovery or join
process and AC may select one of the supported Alternate Tunnel
encapsulation types while configuring the WTP.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 1, 2015.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Alternate Tunnel Encapsulation . . . . . . . . . . . . . . . 6
2.1. Description . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Considerations . . . . . . . . . . . . . . . . . . . 8
3.1. Supported Alternate Tunnel Encapsulations . . . . . . . . 8
3.2. Alternate Tunnel Encapsulations Type . . . . . . . . . . 9
3.3. IEEE 802.11 WTP Alternate Tunnel Failure Indication . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Service Providers are deploying very large Wi-Fi deployments (ranging
from hundreds of thousands of APs (referred to as WTPs in CAPWAP
terminology) to millions of APs). These networks are designed to
carry traffic generated from mobile users. The volume in mobile user
traffic is already very large (in the order of petabytes per day) and
expected to continue growing rapidly. As a result, operators are
looking for solutions that can scale to meet the increasing demand.
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One way to meet the scalability requirement is to split the control/
management plane from the data plane. This separation enables the
data plane be scaled independently of the control/management plane.
This document provides a description of a CAPWAP specification change
that enables the separation of data plane from control plane.
CAPWAP ([RFC5415], [RFC5416]) defines a tunnel mode that specifies
the frame tunneling type to be used for 802.11 data frames from
stations associated with the WLAN. The following types are
supported:
o Local Bridging: All user traffic is to be locally bridged.
o 802.3 Tunnel: All user traffic is to be tunneled to the AC in
802.3 format.
o 802.11 Tunnel: All user traffic is to be tunneled to the AC in
802.11 format.
There are two shortcomings with currently specified tunneled modes:
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]). Next, we describe what is driving the
above mentioned two requirements.
Some operators deploying large number of Access Points prefer to
centralize the management and control of Access Points while
distributing the handling of data traffic to increase scaling. This
motivates an architecture as shown in Figure 1 that has the AC in a
centralized location and one or more tunnel gateways (or Access
Routers) that terminate the data tunnels from the various WTPs. This
split architecture has two benefits over an architecture where data
traffic is aggregated at the AC: 1) reduces the scale requirement on
data traffic handling capability of the AC and 2) leads to more
efficient/optimal routing of data traffic.
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Locally Bridged
+-----+ DATA +----------------+
| WTP |==========| Access Router |
+-----+ +----------------+
\\
\\ CAPWAP +--------+
++======================+ AC |
// +--------+
//
+-----+// DATA +----------------+
| WTP |===========| Access Router |
+=====+ +----------------+
Locally Bridged
Figure 1: Centralized Control with Distributed Data
The above system (shown in Figure 1) could be achieved by setting the
tunnel mode to Local bridging. In such a case the AC would handle
control of WTPs as well as handle the management traffic to/from the
stations. There is CAPWAP Control and Data Channel between the WTP
and the AC. The CAPWAP Data channel carries the IEEE 802.11
management traffic (like IEEE 802.11 Action Frames). The station's
data frames are locally bridged, i.e., not carried over the CAPWAP
data channel. The station's data frames are handled by the Access
Router. However, in many deployments the operator managing the WTPs/
AC may be different from the operator providing the internet
connectivity to the WTPs. Further, the WTP operator may want (or be
required by legal/regulatory requirements) to tunnel the traffic back
to an Access Router in its network as shown in Figure 2. The
tunneling requirement may be driven by the need to apply policy at
the Access Router or a legal requirement to support lawful intercept
of user traffic. What this means is that local bridging does not
meet their requirements. Their requirements are met either by having
the WTP tunnel the station's traffic to the AC or the WTP support an
alternate tunnel, i.e., a tunnel to an alternate entity different
from the AC. This is the motivation for Alternate Tunnel
encapsulation support where the data tunnels from the WTP are
terminated at an AR (and more specifically at an end point different
from the AC).
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Tunnel to AR _________
+-----+ ( ) +-----------------+
| WTP |======+Internet +==============|Access Router(AR)|
+-----+ (_________} +-----------------+
\\ ________
\\ ( ) CAPWAP +--------+
++==Internet+===============| AC |
// ( ) +--------+
// ________
+-----+// ( ) +----------------+
| WTP |====+Internet +================| Access Router |
+=====+ (_________} +----------------+
Tunnel to AR
Figure 2: Centralized Control with Distributed Data
In the case where the WTP is tunneling data frames to an AR (and not
the AC), the choice of tunnel encapsulation need not be restricted
only to CAPWAP (as described in Section 4.4.2 of [RFC5415]). In
fact, the WTP may additionally support other widely used
encapsulation types such as L2TP, L2TPv3, IP-in-IP, IP/GRE, etc. The
WTP may advertise the different alternate tunnel encapsulation types
supported and the AC can select one of the supported encapsulation
types. As shown in the figure there is still a CAPWAP control and
data channel between the WTP and AC wherein the CAPWAP data channel
carries the stations' management traffic. Thus the WTP will maintain
three tunnels: CAPWAP Control, CAPWAP Data, and another (alternate)
tunnel to the AR. The main reason to maintain a CAPWAP data channel
is to minimize the changes on the WTP and AC required to transport
stations' management frames (like EAP, IEEE 802.11 Action Frames).
These management frames are transported over the CAPWAP data channel
as they are done for case when the WTP's tunnel mode is configured as
the local bridging. In this specification we describe how the WTP
can be configured with this alternate tunnel.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]
1.2. Terminology
Station (STA): A device that contains an IEEE 802.11 conformant
medium access control (MAC) and physical layer (PHY) interface to the
wireless medium (WM).
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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.
Wireless Termination Point (WTP), The physical or network entity that
contains an RF antenna and wireless Physical Layer (PHY) to transmit
and receive station traffic for wireless access networks.
CAPWAP Control Channel: A bi-directional flow defined by the AC IP
Address, WTP IP Address, AC control port, WTP control port, and the
transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Control
packets are sent and received.
CAPWAP Data Channel: A bi-directional flow defined by the AC IP
Address, WTP IP Address, AC data port, WTP data port, and the
transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Data
packets are sent and received.
2. Alternate Tunnel Encapsulation
2.1. Description
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+-+-+-+-+-+-+ +-+-+-+-+-+-+
| WTP | | AC |
+-+-+-+-+-+-+ +-+-+-+-+-+-+
|Join Request[Supported Alternate Tunnel |
| Encapsulations ] |
|---------------------------------------->|
| |
|Join Response |
|<----------------------------------------|
| |
|IEEE 802.11 WLAN Config. Request [ |
| IEEE 802.11 Add WLAN, |
| Alternate Tunnel Encapsulation ( |
| Tunnel Type, Tunnel Info Element) |
| ] |
|<----------------------------------------|
| |
| |
+-+-+-+-+-+-+ |
| Setup | |
| Alternate | |
| Tunnel | |
+-+-+-+-+-+-+ |
| |
|IEEE 802.11 WLAN Config. Response |
|---------------------------------------->|
| |
| |
+-+-+-+-+-+-+ |
| Tunnel | |
| Failure | |
+-+-+-+-+-+-+ |
|WTP Alternate Tunnel Failure Indication |
|(report failure) |
|---------------------------------------->|
| |
+-+-+-+-+-+-+-+ |
| Tunnel | |
| Established | |
+-+-+-+-+-+-+-+ |
|WTP Alternate Tunnel Failure Indication |
|(report clearing failure) |
|---------------------------------------->|
| |
Figure 3: Setup of Alternate Tunnel
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The above example describes how the alternate tunnel encapsulation
may be established. When the WTP joins the AC, it should indicate
its alternate tunnel encapsulation capability. The AC determines
whether an alternate tunnel configuration is required. If an
appropriate alternate tunnel type is selected, then the AC provides
the alternate tunnel encapsulation message element containing the
tunnel type and a tunnel-specific information element. (The tunnel-
specific information element, for example, may contain information
like the IP address of the tunnel termination point.) The WTP sets
up the alternate tunnel using the alternate tunnel encapsulation
message element.
When the WTP detects an alternate tunnel failure, the WTP informs the
AC using a message element (defined in this specification), WTP
Alternate Tunnel Fail Indication. The message element has a status
field that indicates whether the message denotes reporting a failure
or the clearing of the previously reported failure.
For the case where AC is unreachable but the tunnel end point is
still reachable, the WTP behavior is up to the implementation. For
example, the WTP could either choose to tear down the tunnel or let
the existing user's traffic continue to be tunneled.
3. Protocol Considerations
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 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
+=+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num_Tunnels | Tunnel-Type 1 | Tunnel-Type [2..N]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Supported Alternate Tunnel Encapsulations
o Type: <IANA-1> for Supported Alternate Tunnel Encapsulations
o Length: The length in bytes is 1 + Num_Tunnels
o Num_Tunnels: This refers to number of tunnel types present in the
message element. At least one tunnel type must be present.
o Tunnel-Type: This is identified by value defined in Section 3.2
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3.2. Alternate Tunnel Encapsulations Type
This message element is sent by the AC. This message element allows
the AC to select the alternate tunnel encapsulation. This message
element may be provided along with the IEEE 802.11 Add WLAN message
element. When the message element is present the following fields of
the IEEE 802.11 Add WLAN element shall be set as follows: MAC mode is
set to 0 (Local MAC) and Tunnel Mode is set to 0 (Local Bridging).
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 5: Alternate Tunnel Encapsulations Type
o Type: <IANA-2> for Alternate Tunnel Encapsulation Type
o Length: > 4
o Tunnel-Type: The tunnel type is specified by a 2 byte value. This
specification defines the values from zero (0) to five (5) as
given below. The remaining values are reserved for future use.
* 0: CAPWAP. This refers to a CAPWAP data channel described in
[RFC5415][RFC5416]. Additional description in
[I-D.xue-opsawg-capwap-alt-tunnel-information].
* 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. This refers to the tunneling encapsulation
described in [RFC5213]
* 5: GRE-IPv4. This refers to GRE encapsulation with IPv4 as the
delivery protocol as described in [RFC2784]
* 6: GRE-IPv6. This refers to GRE encapsulation with IPv6 as the
delivery protocol as described in [RFC2784]
o Info Element: This field contains tunnel specific configuration
parameters to enable the WTP to setup the alternate tunnel. For
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example if the tunnel type is CAPWAP then this field may contain
the following (non-exhaustive) list of parameters
* Access Router IPv4 address
* Access Router IPv6 address
* Tunnel DTLS Policy
* IEEE 802.11 Tagging Policy
This specification only defines a generic container for such
message elements. We anticipate that these message elements (for
the different protocols) will be defined in separate documents,
potentially one for each tunneling protocols. See
[I-D.xue-opsawg-capwap-alt-tunnel-information] for example of such
a specification.
3.3. IEEE 802.11 WTP Alternate Tunnel Failure Indication
The Alternate Tunnel Encapsulation message element is sent by the WTP
to inform the AC about the status of the Alternate Tunnel. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Radio ID | WLAN ID | Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: IEEE 802.11 WTP Alternate Tunnel Failure Indication
o Type: <IANA-3> for IEEE 802.11 WTP Alternate Tunnel Failure
Indication
o Length: == 4
o Radio ID: The Radio Identifier, whose value is between one (1) and
31, typically refers to some interface index on the WTP.
o WLAN ID: An 8-bit value specifying the WLAN Identifier. The value
MUST be between one (1) and 16.
o Status: An 8-bit boolean indicating whether the radio failure is
being reported or cleared. A value of zero is used to clear the
event, while a value of one is used to report the event.
4. IANA Considerations
This document requires the following IANA considerations.
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o <IANA-1>. This specification defines the Supported Alternate
Tunnel Encapsulations Type message element in Section 3.1. This
elements needs to be registered in the existing CAPWAP Message
Element Type registry, defined in [RFC5415]. The Type value for
this element needs to be between 1 and 1023 (see Section 15.7 in
[RFC5415]).
o <IANA-2>. This specification defines the Alternate Tunnel
Encapsulations Type message element in Section 3.2. This element
needs to be registered in the existing CAPWAP Message Element Type
registry, defined in [RFC5415]. The Type value for this element
needs to be between 1 and 1023.
o <IANA-3>. This specification defines the IEEE 802.11 WTP
Alternate Tunnel Failure Indication message element in
Section 3.3. This element needs to be registered in the existing
CAPWAP Message Element Type registry, defined in [RFC5415]. The
Type value for this element needs to be between 1024 and 2047.
o Tunnel-Type: This specification defines the Alternate Tunnel
Encapsulations Type message element. This element contains a
field Tunnel-Type. The namespace for the field is 16 bits
(0-65535)). This specification defines values, zero (0) through
six (6) and can be found in Section 3.2. The remaining values
(6-65535) are controlled and maintained by IANA and require an
Expert Review. IANA needs to create a Tunnel-Type registry whose
format is given below.
Tunnel-Type Type Value Reference
CAPWAP 0
L2TP 1
L2TPv3 2
IP-IP 3
PMIPv6 4
GRE-IPv4 5
GRE-IPv6 6
5. 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 compared to [RFC5415] and [RFC5416].
The security considerations described in [RFC5415] and [RFC5416]
apply here as well.
6. Contributors
This document stems from the joint work of Hong Liu, Yifan Chen,
Chunju Shao from China Mobile Research.
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7. References
7.1. Normative References
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
[RFC5415] Calhoun, P., Montemurro, M., and D. Stanley, "Control And
Provisioning of Wireless Access Points (CAPWAP) Protocol
Specification", RFC 5415, March 2009.
[RFC5416] Calhoun, P., Montemurro, M., and D. Stanley, "Control and
Provisioning of Wireless Access Points (CAPWAP) Protocol
Binding for IEEE 802.11", RFC 5416, March 2009.
7.2. Informative References
[I-D.xue-opsawg-capwap-alt-tunnel-information]
Liu, D., Zhang, R., Xue, L., Kaippallimalil, J.,
Pazhyannur, R., and S. Gundavelli, "Specification
Alternate Tunnel Information for Data Frames in WLAN",
draft-xue-opsawg-capwap-alt-tunnel-information-00 (work in
progress), July 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Authors' Addresses
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Rong Zhang
China Telecom
No.109 Zhongshandadao avenue
Guangzhou 510630
China
Email: zhangr@gsta.com
Zhen Cao
China Mobile
Xuanwumenxi Ave. No. 32
Beijing 100871
China
Phone: +86-10-52686688
Email: zehn.cao@gmail.com, caozhen@chinamobile.com
Hui Deng
China Mobile
No.32 Xuanwumen West Street
Beijing 100053
China
Email: denghui@chinamobile.com
Rajesh S. Pazhyannur
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Email: rpazhyan@cisco.com
Sri Gundavelli
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Email: sgundave@cisco.com
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Li Xue
Huawei
No.156 Beiqing Rd. Z-park, HaiDian District
Beijing
China
Email: xueli@huawei.com
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