Network Working Group Rahul Aggarwal
Internet Draft Juniper Networks
Expiration Date: April 2006 W. Mark Townsley
Maria A. Dos Santos
Cisco Systems
Editors
October 2005
Transport of Ethernet Frames over L2TPv3
draft-ietf-l2tpext-pwe3-ethernet-05.txt
Status of this Memo
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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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Contributors
Following is the complete list of contributors to this document.
Rahul Aggarwal
Juniper Networks
Xipeng Xiao
Riverstone Networks
W. Mark Townsley
Stewart Bryant
Maria Alice Dos Santos
Cisco Systems
Cheng-Yin Lee
Alcatel
Tissa Senevirathne
Consultant
Mitsuru Higashiyama
Anritsu Corporation
Table of Contents
Status of this Memo.......................................... 1
1. Introduction.............................................. 3
1.1 Abbreviations......................................... 3
1.2 Requirements.......................................... 3
2. PW Establishment.......................................... 4
2.1 LCCE-LCCE Control Connection Establishment............ 4
2.2 PW Session Establishment.............................. 5
2.3 PW Session Monitoring................................. 5
3. Packet Processing......................................... 7
3.1 Encapsulation......................................... 7
3.2 Sequencing............................................ 7
3.3 MTU Handling.......................................... 7
4. Applicability Statement................................... 8
5. Security Considerations................................... 9
6. IANA Considerations....................................... 9
7. Acknowledgements.......................................... 10
8. References................................................ 10
8.1 Normative References.................................. 10
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8.2 Informative References................................ 10
9. Author Information........................................ 10
Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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. 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 [RFC3931]. 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 [RFC3931])
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 [RFC3985]. The Ethernet
interface may be connected to one or more Remote Systems (an L2TPv3
Remote System is referred to as Customer Edge (CE) in this and
associated PWE3 documents). The LCCE may or may not be a PE.
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+---------------------------------------+
| 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.
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. An 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 [RFC3931]. Hence an Ethernet
PW type must be included in the Pseudo Wire Capabilities List as
defined in [RFC3931]. 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. For connecting an Ethernet port to
another Ethernet port, the PW Type MUST be "Ethernet port"; for
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connecting an Ethernet VLAN to another Ethernet VLAN, the PW Type
MUST be "Ethernet VLAN".
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 [RFC3931]. 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 Pseudo Wire
type in the Pseudowire Type AVP [RFC3931]. The assigned values for
"Ethernet port" and "Ethernet VLAN" Pseudo Wire types are captured in
the "IANA Considerations" of this document. The Pseudowire Type AVP
MUST be present in the ICRQ.
b) Pseudo Wire ID: Each PW is associated with a Pseudo Wire ID. The
two LCCEs of a PW have the same Pseudo Wire ID for it. The Remote End
Identifier AVP [RFC3931] is used to convey the Pseudo Wire ID. The
Remote 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 a Remote End Identifier of four
octets known to both LCCEs either by manual configuration or some
other means. Additional Remote End Identifier formats which MAY be
supported are outside the scope of this document.
c) The Circuit Status AVP [RFC3931] MUST be included in ICRQ and ICRP
to indicate the circuit status of the Ethernet port or Ethernet VLAN.
The N (New) bit of the Circuit Status AVP in ICRQ and ICRP MUST be
set to 1 indicating that the status is for a new circuit while the A
(Active) bit is set to 0 (INACTIVE) or 1 (ACTIVE) to reflect the
circuit operational status. Subsequent circuit status change of the
Ethernet port or Ethernet VLAN MUST be conveyed in the Circuit Status
AVP in ICCN or SLI control messages. In ICCN and SLI, the Circuit
Status AVP N bit MUST be set to 0 indicating that the status is for
an existing circuit. The A bit should be set to 0 or 1 to reflect
the circuit operational status at that point in time.
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 set of PWs associated with a Control Connection.
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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 [RFC3931]. 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 [RFC3931]. 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 |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Value is a 16 bit mask with the two least significant bits
defined and the remaining bits reserved for future use. Reserved bits
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 the circuit status
indication is for a new Ethernet circuit, zero (0) otherwise.
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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 an 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 mechanisms described in [RFC3931] 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 following fields:
L2TP Session Header:
Flags, Ver, Res - 4 octets (L2TPv3 over UDP only)
Session ID - 4 octets
Cookie Size - 0, 4 or 8 octets
L2-Specific Sublayer - 0 or 4 octets (i.e., using sequencing)
Hence the range for N in octets is:
N = 4-16, for L2TPv3 data messages over IP;
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N = 16-28, for L2TPv3 data messages over UDP;
(N does not include the IP header).
The MTU and fragmentation implications resulting from this are
discussed in section 4.1.4 of [RFC3931].
4. Applicability Statement
The Ethernet PW emulation allows a service provider to offer a "port
to port" Ethernet based service across an IP packet switched network
(PSN) while the Ethernet VLAN PW emulation allows an "Ethernet VLAN
to VLAN" based service across an IP packet switched network (PSN).
The Ethernet or Ethernet VLAN PW emulation has the following
characteristics in relationship to the respective native service:
o Ethernet PW connects two Ethernet ACs while Ethernet VLAN PW
connects two Ethernet VLAN ACs, supporting bi-directional
transport of variable length Ethernet frames. The ingress LCCE
strips the preamble and FCS from the Ethernet frame and transports
the frame in its entirety across the PW. This is done regardless
of the presence of the 802.1Q tag in the frame. The egress LCCE
receives the Ethernet frame from the PW and regenerates the
preamble or FCS before forwarding the frame to the attached Remote
System (See Section 3.1). Since FCS is not being transported
across either Ethernet or Ethernet VLAN PWs, payload integrity
transparency may be lost. To achieve payload integrity
transparency on Ethernet or Ethernet VLAN PWs using L2TP over IP
or L2TP over UDP/IP, the L2TPv3 session can utilize IPSec as
specified in Section 4.1.3 of [RFC3931].
o For Ethernet VLAN PW, VLAN tag rewrite can be achieved by NSP at
the egress LCCE which is outside the scope of this document (See
Section 3.1).
o The Ethernet or Ethernet VLAN PW only supports homogeneous Ethernet
frame type across the PW; both ends of the PW must be either tagged
or untagged. Heterogeneous frame type support achieved with NSP
functionality is outside the scope of this document (See Section
3.1).
o Ethernet port or Ethernet VLAN status notification is provided
using the Circuit Status AVP in SLI message (See Section 2.3.1).
Loss of connectivity between LCCEs can be detected by the L2TPv3
keepalive mechanism (see Section 2.3.1 in [RFC3931]). The LCCE
can convey these indications back to its attached Remote System.
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o The maximum frame size that can be supported is limited by the PSN
MTU minus the L2TPv3 header size, unless fragmentation and
reassembly is used (see Section 3.3 and Section 4.1.4 of
[RFC3931]).
o The packet switched network may reorder, duplicate, or silently
drop packets. Sequencing may be enabled in the Ethernet or
Ethernet VLAN PW for some or all packets to detect lost,
duplicate, or out-of-order packets on a per-session basis
(see Section 3.2).
o The faithfulness of an Ethernet or Ethernet VLAN PW may be
increased by leveraging Quality of Service features of the LCCEs
and the underlying PSN. For example for Ethernet VLAN transport,
the ingress LCCE MAY consider the user priority field of the VLAN
tag for traffic classification and QoS treatments, such as
determining the TOS octet (for example, according to DSCP) of
the encapsulating IP header. Similarly, the egress LCCE MAY
consider the TOS octet of the encapsulating IP header when
rewriting the user priority field of the VLAN tag or queueing the
Ethernet frame before forwarding the frame to the Remote System.
The mapping between the 802.1Q COS and the IP header Qos field and
the Quality of Service model deployed are application specific and
are outside the scope of this document.
5. Security Considerations
Ethernet over L2TPv3 is subject to all of the general security
considerations outlined in [RFC3931].
6. IANA Considerations
The signaling mechanisms defined in this document rely upon the
allocation of following Ethernet Pseudowire Types (see Pseudo Wire
Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3
Pseudowire Types in 10.6 of [RFC3931]) by the IANA (number space
created as part of publication of [RFC3931]):
Pseudowire Types
----------------
0x0004 Ethernet VLAN Pseudowire Type
0x0005 Ethernet Pseudowire Type
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7. 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.
Thanks to Carlos Pignataro for providing a thorough review and
helpful input.
8. References
8.1 Normative References
[RFC3931] J. Lau, M. Townsley, I. Goyret, "Layer Two Tunneling
Protocol (Version 3)", RFC3931, March 2005.
8.2 Informative References
[RFC3985] S. Bryant, P. Pate, "Pseudo Wire Emulation Edge-to-Edge
(PWE3) Architecture", RFC3985, March 2005
9. 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
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e-mail: mark@townsley.net
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
Maria Alice Dos Santos
Cisco Systems
170 W Tasman Dr
San Jose, CA 95134
e-mail: mariados@cisco.com
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