Network Working Group Rahul Aggarwal
Internet Draft Juniper Networks
Expiration Date: July 2004 W. Mark Townsley
Cisco Systems
Editors
Transport of Ethernet Frames over L2TPv3
draft-ietf-l2tpext-pwe3-ethernet-01.txt
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Abstract
This document describes transport of Ethernet frames over Layer 2
Tunneling Protocol (L2TPv3). This includes the transport of Ethernet
port to port frames as well as the transport of Ethernet VLAN frames.
The mechanism described in this document can be used in the creation
of Pseudo Wires to transport Ethernet frames over an IP network.
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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 RFC-2119 [KEYWORDS].
Contributors
Following is the complete list of contributors to this document.
Rahul Aggarwal
Juniper Networks
Xipeng Xiao
Riverstone Networks
W. Mark Townsley
Stewart Bryant
Cisco Systems
Cheng-Yin Lee
Alcatel
Tissa Senevirathne
Consultant
Mitsuru Higashiyama
Anritsu Corporation
Table of Contents
1 Introduction........................................ 3
1.1 Abbreviations....................................... 3
1.2 Requirements........................................ 3
2 PW Establishment.................................... 3
2.1 LCCE-LCCE Control Connection Establishment.......... 4
2.2 PW Session Establishment............................ 4
2.3 PW Session Monitoring............................... 4
2.3.1 SLI Message......................................... 5
3 Packet Processing................................... 6
3.1 Encapsulation....................................... 6
3.2 Sequencing.......................................... 6
3.3 MTU Handling........................................ 6
4 Security Considerations............................. 7
5 IANA Considerations................................. 7
6 Acknowledgements.................................... 7
7 References.......................................... 7
8 Author Information.................................. 8
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1. Introduction
L2TPv3 can be used as a control protocol and for data encapsulation
to set up Pseudo Wires (PW) for transporting layer 2 Packet Data
Units across an IP network [L2TPv3]. This document describes the
transport of Ethernet frames over L2TPv3 including the PW
establishment and data encapsulation.
1.1. Abbreviations
CE Customer Edge. (Typically also the L2TPv3 Remote System)
LCCE L2TP Control Connection Endpoint (See [L2TPv3])
PE Provider Edge (Typically also the LCCE).
PSN Packet Switched Network
PW Pseudo-Wire
PWE3 Pseudo-Wire Emulation Edge to Edge (Working Group)
NSP Native Service Processing
1.2. Requirements
An Ethernet PW emulates a single Ethernet link between exactly two
endpoints. The following figure depicts the PW termination relative
to the NSP and PSN tunnel within a LCCE [PWE3-ARCH]. The Ethernet
interface may be connected to one or more CEs i.e. one or more L2TPv3
Remote Systems. The LCCE may or may not be a PE.
+---------------------------------------+
| LCCE |
+-+ +-----+ +------+ +------+ +-+
|P| | | |PW ter| | PSN | |P|
Ethernet <==>|h|<=>| NSP |<=>|minati|<=>|Tunnel|<=>|h|<==> PSN
Interface |y| | | |on | | | |y|
+-+ +-----+ +------+ +------+ +-+
| |
+---------------------------------------+
Figure 1: PW termination
The PW termination point receives untagged (also referred to as
'raw') or tagged ethernet frames and delivers them unaltered to the
PW termination point on the remote LCCE. Hence it can provide
untagged or tagged Ethernet link emulation service.
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The "NSP" function includes packet processing needed to translate the
Ethernet packets that arrive at the CE-LCCE interface to/from the
Ethernet packets that are applied to the PW termination point. Such
functions may include stripping, overwriting or adding VLAN tags.
The NSP functionality can be used in conjunction with local
provisioning to provide heterogeneous services where the CE-LCCE
encapsulations at the two ends may be different.
The physical layer between the CE and LCCE, and any adaptation (NSP)
functions between it and the PW termination, are outside of the scope
of PWE3 and are not defined here.
2. PW Establishment
With L2TPv3 as the tunneling protocol, Ethernet PWs are L2TPv3
sessions. A L2TP control connection has to be set up first between
the two LCCEs. Individual PWs can then be established as L2TP
sessions.
2.1. LCCE-LCCE Control Connection Establishment
The two LCCEs that wish to set up Ethernet PWs MUST establish a L2TP
control connection first as described in [L2TPv3]. Hence Ethernet PW
type must be included in the Pseudo Wire Capabilities list. The type
of PW can be either "Ethernet port" or "Ethernet VLAN". This
indicates that the control connection can support the establishment
of Ethernet PWs. Note that there are two Ethernet PW types required.
This can be used for connecting Ethernet port to another Ethernet
port; Ethernet VLAN to another Ethernet VLAN; or for providing
heterogeneous connectivity using NSP processing.
2.2. PW Session Establishment
The provisioning of an Ethernet port or Ethernet VLAN and its
association with a PW triggers the establishment of an L2TP session
as described in [L2TPv3]. The following are the signaling elements
needed for the PW establishment:
a) Pseudo Wire Type: The type of a Pseudo wire can be either
"Ethernet port" or "Ethernet VLAN". Each LCCE signals its PW type in
a PW AVP [L2TPv3]. The Pseudo Wire type for Ethernet port is TBD1 and
the Pseudo Wire type for Ethernet VLAN is TBD2.
b) PW ID: Each PW is associated with a PW ID akin to the VC-ID in
[PWE3-CRTL]. The two LCCEs of a PW have the same PW ID for it. The
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End Identifier AVP in L2TPv3 is used as the PW ID. The End Identifier
AVP MUST be present in the ICRQ in order for the remote LCCE to
determine the PW to associate the L2TP session with. An
implementation MUST support an End Identifier of four octets known to
both LCCEs either by manual configuration or some other means.
Additional End Identifier formats which MAY be supported are outside
the scope of this document.
2.3. PW Session Monitoring
The working status of a PW is reflected by the state of the L2TPv3
session. If the corresponding L2TPv3 session is down, the PW
associated with it MUST be shut down. The control connection keep-
alive mechanism of L2TPv3 can serve as a link status monitoring
mechanism for the PW.
2.3.1. SLI Message
In addition to the control connection keep-alive mechanism of L2TPv3,
Ethernet PW over L2TP makes use of the Set Link Info (SLI) control
message defined in [L2TPv3]. The SLI message is used to signal
Ethernet link status notifications between LCCEs. This can be useful
to indicate the Ethernet interface state change without bringing down
the L2TP session. Note that change in the Ethernet interface state
will trigger a SLI message for each PW associated with that Ethernet
interface. This may be one Ethernet Port PW or more than one
Ethernet VLAN PW. The SLI message MUST be sent any time there is a
status change of any values identified in the Circuit Status AVP. The
only exception to this is the initial ICRQ, ICRP and CDN messages
which establish and teardown the L2TP session itself. The SLI
message may be sent from either LCCE at any time after the first ICRQ
is sent (and perhaps before an ICRP is received, requiring the peer
to perform a reverse Session ID lookup).
Ethernet PW reports Circuit Status with the Circuit Status AVP
defined in [L2TPv3]. For reference, this AVP is shown below:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |A|N|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Value is a 16 bit mask with the two least significant bits
defined and the remaining bits reserved for future use. Reserved bits
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MUST be set to 0 when sending, and ignored upon receipt.
The A (Active) bit indicates whether the Ethernet interface is ACTIVE
(1) or INACTIVE (0).
The N (New) bit SHOULD be set to one (1) if this is the first time
this interface has transitioned to ACTIVE, zero (0) otherwise.
3. Packet Processing
3.1. Encapsulation
The encapsulation described in this section refers to the
functionality performed by the PW termination point depicted in
figure 1, unless otherwise indicated.
The entire Ethernet frame without the preamble or FCS is
encapsulated in L2TPv3 and is sent as a single packet by the ingress
LCCE. This is done regardless of whether an 802.1Q tag is present in
the Ethernet frame or not. For Ethernet port to port mode the remote
LCCE simply decapsulates the L2TP payload and sends it out on the
appropriate interface without modifying the Ethernet header. For
Ethernet VLAN to VLAN, the remote LCCE MAY rewrite the VLAN tag. As
described in section 1, the VLAN tag modification is a NSP function.
The Ethernet PW over L2TP is homogeneous with respect to packet
encapsulation i.e. both the ends of the PW are either untagged or
tagged. The Ethernet PW can still be used to provide heterogeneous
services using NSP functionality at the ingress and/or egress LCCE.
The definition of such NSP functionality is outside the scope of this
document.
3.2. Sequencing
Data packet sequencing may be enabled for Ethernet PWs. The
sequencing mechanims described in [L2TPv3] MUST be used for signaling
sequencing support.
3.3. MTU Handling
With L2TPv3 as the tunneling protocol, the packet resulted from the
encapsulation is N bytes longer than Ethernet frame without the
preamble or FCS. The value of N depends on the size of the L2TPv3
cookie, which maybe 0, 4 or 8 bytes. Hence the range for N is:
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N=4-12, L2TPv3 data messages are over IP;
N=12-20, L2TPv3 data messages are over UDP;
(N does not include the IP header).
The fragmentation implications resulting from this are discussed in
[PWE3-FRAG]. The mechanisms outlined in [PWE3-FRAG] SHOULD be
followed with regards to handling fragmentation on an Ethernet PW
over L2TPv3.
4. Security Considerations
Ethernet over L2TPv3 is subject to all of the general security
considerations outlined in [L2TPv3].
5. IANA Considerations
The following value needs assignment by IANA (number space should be
created as part of publication of [L2TPv3]):
TBD1 - Ethernet Pseudowire Type (see Pseudo Wire Capabilities List as
defined in 5.4.3 of [L2TPv3]).
TBD2 - Ethernet VLAN Pseudowire Type (see Pseudo Wire Capabilities
List as defined in 5.4.3 of [L2TPv3]).
6. Acknowledgements
This draft evolves from the draft, "Ethernet Pseudo Wire Emulation
Edge-to-Edge". We would like to thank its authors, T.So, X.Xiao, L.
Anderson, C. Flores, N. Tingle, S. Khandekar, D. Zelig and G. Heron
for their contribution. We would also like to thank S. Nanji, the
author of the draft, "Ethernet Service for Layer Two Tunneling
Protocol", for writing the first ethernet over L2TP draft.
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7. References
[L2TPv3] J. Lau, M. Townsley, A. Valencia, G. Zorn, I. Goyret,
G. Pall, A. Rubens, B. Palter, Layer Two Tunneling
Protocol a.k.a. "L2TPv3," work in progress,
draft-ietf-l2tpext-l2tp-base-03.txt
[L2TP-IANA] Townsley, M., "L2TP IANA Considerations Update",
Internet Draft,
draft-ietf-l2tpext-rfc2661-iana-00.txt
[PWE3-CRTL] L. Martini., et al., "Transport of Layer 2 Frames Over
MPLS", draft-ietf-pwe3-control-protocol-00.txt
[PWE3-FRAG] A. Malis, W. M. Townsley, "PWE3 Fragmentation and
Reassembly", draft-malis-pwe3-fragmentation-00.txt
[PWE3-LAYER] S. Bryant, L. Wood, M. Townsley, D. McPherson, "Protocol
Layering in PWE3", draft-ietf-pwe3-protocol-layer-00.txt
[PWE3-REQ] X. Xiao, D. McPherson, P. Pate, C. White, K. Kompella,
V. Gill, T. D. Nadeau, "Requirements for Pseudo-Wire
Emulation Edge-to-Edge",
draft-ietf-pwe3-requirements-03.txt
8. Author Information
Rahul Aggarwal
Juniper Networks
1194 North Mathilda Avenue
Sunnyvale, CA 94089
e-mail: rahul@juniper.net
XiPeng Xiao
Riverstone Networks
5200 Great America Parkway
Santa Clara, CA 95054
Email: xxiao@riverstonenet.com
W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
e-mail: mark@townsley.net
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Stewart Bryant
Cisco Systems,
4, The Square,
Stockley Park,
Uxbridge UB11 1BL,
United Kingdom.
e-mail: stbryant@cisco.com
Cheng-Yin Lee
Alcatel
600 March Rd, Ottawa
Ontario, Canada K2K 2E6
e-mail: Cheng-Yin.Lee@alcatel.com
Tissa Senevirathne
Consultant
1567 Belleville Way
Sunnywale CA 94087
e-mail: tsenevir@hotmail.com
Mitsuru Higashiyama
Anritsu Corporation
1800 Onna, Atsugi-shi, Kanagawa-prf., 243-8555 Japan
e-mail: Mitsuru.Higashiyama@yy.anritsu.co.jp
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