RFC4719: Transport of Ethernet Frames over Layer 2 Tunneling Protocol Version 3 (L2TPv3)
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Related keywords:
(port-to-port)
(vlan)
Network Working Group R. Aggarwal, Ed.
Request for Comments: 4719 Juniper Networks
Category: Standards Track M. Townsley, Ed.
M. Dos Santos, Ed.
Cisco Systems
November 2006
Transport of Ethernet Frames over
Layer 2 Tunneling Protocol Version 3 (L2TPv3)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2006).
Abstract
This document describes the transport of Ethernet frames over the
Layer 2 Tunneling Protocol, Version 3 (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 Pseudowires to transport Ethernet frames
over an IP network.
Aggarwal, et al. Standards Track [Page 1]
RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
Table of Contents
1. Introduction ....................................................2
1.1. Specification of Requirements ..............................2
1.2. Abbreviations ..............................................3
1.3. L2TPv3 Control Message Types ...............................3
1.4. Requirements ...............................................3
2. PW Establishment ................................................4
2.1. LCCE-LCCE Control Connection Establishment .................4
2.2. PW Session Establishment ...................................4
2.3. PW Session Monitoring ......................................6
3. Packet Processing ...............................................7
3.1. Encapsulation .............................................7
3.2. Sequencing ................................................7
3.3. MTU Handling ..............................................7
4. Applicability Statement .........................................8
5. Congestion Control .............................................10
6. Security Considerations ........................................10
7. IANA Considerations ............................................11
8. Contributors ...................................................11
9. Acknowledgements ...............................................11
10. References ....................................................12
10.1. Normative References .....................................12
10.2. Informative References ...................................12
1. Introduction
The Layer 2 Tunneling Protocol, Version 3 (L2TPv3) can be used as a
control protocol and for data encapsulation to set up Pseudowires
(PWs) 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.
The term "Ethernet" in this document is used with the intention to
include all such protocols that are reasonably similar in their
packet format to IEEE 802.3 [802.3], including variants or extensions
that may or may not necessarily be sanctioned by the IEEE (including
such frames as jumbo frames, etc.). The term "VLAN" in this document
is used with the intention to include all virtual LAN tagging
protocols such as IEEE 802.1Q [802.1Q], 802.1ad [802.1ad], etc.
1.1. 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].
Aggarwal, et al. Standards Track [Page 2]
RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
1.2. Abbreviations
AC Attachment Circuit (see [RFC3985])
CE Customer Edge (Typically also the L2TPv3 Remote System)
LCCE L2TP Control Connection Endpoint (see [RFC3931])
NSP Native Service Processing (see [RFC3985])
PE Provider Edge (Typically also the LCCE) (see [RFC3985])
PSN Packet Switched Network (see [RFC3985])
PW Pseudowire (see [RFC3985])
PWE3 Pseudowire Emulation Edge to Edge (Working Group)
1.3. L2TPv3 Control Message Types
Relevant L2TPv3 control message types (see [RFC3931]) are listed for
reference.
SCCRQ L2TPv3 Start-Control-Connection-Request control message
SCCRP L2TPv3 Start-Control-Connection-Reply control message
SCCCN L2TPv3 Start-Control-Connection-Connected control message
StopCCN L2TPv3 Stop-Control-Connection-Notification control message
ICRQ L2TPv3 Incoming-Call-Request control message
ICRP L2TPv3 Incoming-Call-Reply control message
ICCN L2TPv3 Incoming-Call-Connected control message
OCRQ L2TPv3 Outgoing-Call-Request control message
OCRP L2TPv3 Outgoing-Call-Reply control message
OCCN L2TPv3 Outgoing-Call-Connected control message
CDN L2TPv3 Call-Disconnect-Notify control message
SLI L2TPv3 Set-Link-Info control message
1.4. 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 an 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.
+---------------------------------------+
| LCCE |
+-+ +-----+ +------+ +------+ +-+
|P| | | |PW ter| | PSN | |P|
Ethernet <==>|h|<=>| NSP |<=>|minati|<=>|Tunnel|<=>|h|<==> PSN
Interface |y| | | |on | | | |y|
+-+ +-----+ +------+ +------+ +-+
| |
+---------------------------------------+
Figure 1: PW termination
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RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
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 frames that arrive at the CE-LCCE interface to/from the
Ethernet frames 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 an L2TP
Control Connection first as described in [RFC3931]. Hence, an
Ethernet PW Type must be included in the Pseudowire 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
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
via the standard Incoming Call three-way handshake described in
Section 3.4.1 of [RFC3931].
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RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
Note that an L2TP Outgoing Call is essentially a method of
controlling the originating point of a Switched Virtual Circuit
(SVC), allowing it to be established from any reachable L2TP-enabled
device able to perform outgoing calls. The Outgoing Call model and
its corresponding OCRQ, OCRP, and OCCN control messages are mainly
used within the dial arena with L2TPv2 today and has not been found
applicable for PW applications yet.
The following are the signaling elements needed for the Ethernet PW
establishment:
a) Pseudowire Type: The type of a Pseudowire can be either "Ethernet
port" or "Ethernet VLAN". Each LCCE signals its Pseudowire type
in the Pseudowire Type AVP [RFC3931]. The assigned values for
"Ethernet port" and "Ethernet VLAN" Pseudowire types are captured
in the "IANA Considerations" of this document. The Pseudowire
Type AVP MUST be present in the ICRQ.
b) Pseudowire ID: Each PW is associated with a Pseudowire ID. The
two LCCEs of a PW have the same Pseudowire ID for it. The Remote
End Identifier AVP [RFC3931] is used to convey the Pseudowire 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 that 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. For ICRQ and ICRP, the Circuit Status AVP MUST indicate
that the circuit status is for a new circuit (refer to N bit in
Section 2.3.3). An implementation MAY send an ICRQ or ICRP before
an Ethernet interface is ACTIVE, as long as the Circuit Status AVP
(refer to A bit in Section 2.3.3) in the ICRQ or ICRP reflects the
correct status of the Ethernet port or Ethernet VLAN link. A
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. For ICCN and SLI (refer to Section 2.3.2), the
Circuit Status AVP MUST indicate that the circuit status is for an
existing circuit (refer to N bit in Section 2.3.3) and reflect the
current status of the link (refer to A bit in Section 2.3.3).
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RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
2.3. PW Session Monitoring
2.3.1. Control Connection Keep-alive
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.
2.3.2. 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 Ethernet interface state changes without bringing down
the L2TP session. Note that change in the Ethernet interface state
will trigger an 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 that establish and tear down 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).
2.3.3. Use of Circuit Status AVP for Ethernet
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 indicates whether the circuit status is for a new (1)
Ethernet circuit or an existing (0) Ethernet circuit.
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RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
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 frame check
sequence (FCS), is encapsulated in L2TPv3 and is sent as a single
packet by the ingress LCCE. This is done regardless of whether or
not a VLAN tag is present in the Ethernet frame. 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 mode, 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 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.
The maximum length of the Ethernet frame carried as the PW payload is
irrelevant as far as the PW is concerned. If anything, that value
would only be relevant when quantifying the faithfulness of the
emulation.
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 IP packet resulting from
the encapsulation is M + N bytes longer than the Ethernet frame
without the preamble or FCS. Here M is the length of the IP header
along with associated options and extension headers, and the value of
N depends on the following fields:
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RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
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;
N = 16-28, for L2TPv3 data messages over UDP;
(N does not include the IP header).
Fragmentation in the PSN can occur when using Ethernet over L2TP,
unless proper configuration and management of MTU sizes are in place
between the Customer Edge (CE) router and Provider Edge (PE) router,
and across the PSN. This is not specific only to Ethernet over
L2TPv3, and the base L2TPv3 specification [RFC3931] provides general
recommendations with respect to fragmentation and reassembly in
Section 4.1.4. "PWE3 Fragmentation and Reassembly" [RFC4623]
expounds on this topic, including a fragmentation and reassembly
mechanism within L2TP itself in the event that no other option is
available. Implementations MUST follow these guidelines with respect
to fragmentation and reassembly.
4. Applicability Statement
The Ethernet PW emulation allows a service provider to offer a
"port-to-port"-based Ethernet service across an IP Packet Switched
Network (PSN), while the Ethernet VLAN PW emulation allows an "VLAN-
to-VLAN"-based Ethernet 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 port ACs, and Ethernet VLAN PW
connects two Ethernet VLAN ACs, which both support 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 VLAN tag in the frame. The egress LCCE
receives the Ethernet frame from the PW and regenerates the
preamble and 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].
Aggarwal, et al. Standards Track [Page 8]
RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
o While architecturally [RFC3985] outside the scope of the L2TPv3 PW
itself, if VLAN tags are present, the NSP may rewrite VLAN tags on
ingress or egress from the PW (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 the SLI message (see Sections
2.3.2 and 2.3.3). Loss of connectivity between LCCEs can be
detected by the L2TPv3 keep-alive mechanism (see Section 2.3.1 of
this document and Section 4.4 of [RFC3931]). The LCCE can convey
these indications back to its attached Remote System.
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 of this document 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 (QoS) features of the
LCCEs and the underlying PSN. For example, for Ethernet 802.1Q
[802.1Q] VLAN transport, the ingress LCCE MAY consider the user
priority field (i.e., 802.1p) of the VLAN tag for traffic
classification and QoS treatments, such as determining the
Differentiated Services (DS) field [RFC2474] of the encapsulating
IP header. Similarly, the egress LCCE MAY consider the DS field
of the encapsulating IP header when rewriting the user priority
field of the VLAN tag or queuing the Ethernet frame before
forwarding the frame to the Remote System. The mapping between
the user priority field and the IP header DS field as well as the
Quality-of-Service model deployed are application specific and are
outside the scope of this document.
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RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
5. Congestion Control
As explained in [RFC3985], the PSN carrying the PW may be subject to
congestion, with congestion characteristics depending on PSN type,
network architecture, configuration, and loading. During congestion,
the PSN may exhibit packet loss that will impact the service carried
by the Ethernet or Ethernet VLAN PW. In addition, since Ethernet or
Ethernet VLAN PWs carry a variety of services across the PSN,
including but not restricted to TCP/IP, they may or may not behave in
a TCP-friendly manner prescribed by [RFC2914] and thus consume more
than their fair share.
Whenever possible, Ethernet or Ethernet VLAN PWs should be run over
traffic-engineered PSNs providing bandwidth allocation and admission
control mechanisms. IntServ-enabled domains providing the Guaranteed
Service (GS) or DiffServ-enabled domains using EF (expedited
forwarding) are examples of traffic-engineered PSNs. Such PSNs will
minimize loss and delay while providing some degree of isolation of
the Ethernet or Ethernet VLAN PW's effects from neighboring streams.
LCCEs SHOULD monitor for congestion (by using explicit congestion
notification or by measuring packet loss) in order to ensure that the
service using the Ethernet or Ethernet VLAN PW may be maintained.
When severe congestion is detected (for example, when enabling
sequencing and detecting that the packet loss is higher than a
threshold), the Ethernet or Ethernet VLAN PW SHOULD be halted by
tearing down the L2TP session via a CDN message. The PW may be
restarted by manual intervention or by automatic means after an
appropriate waiting time. Note that the thresholds and time periods
for shutdown and possible automatic recovery need to be carefully
configured. This is necessary to avoid loss of service due to
temporary congestion and to prevent oscillation between the congested
and halted states.
This specification offers no congestion control and is not TCP
friendly [TFRC]. Future works for PW congestion control (being
studied by the PWE3 Working Group) will provide congestion control
for all PW types including Ethernet and Ethernet VLAN PWs.
6. Security Considerations
Ethernet over L2TPv3 is subject to all of the general security
considerations outlined in [RFC3931].
Aggarwal, et al. Standards Track [Page 10]
RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
7. IANA Considerations
The signaling mechanisms defined in this document rely upon the
following Ethernet Pseudowire Types (see Pseudowire Capabilities List
as defined in 5.4.3 of [RFC3931] and L2TPv3 Pseudowire Types in 10.6
of [RFC3931]), which were allocated by the IANA (number space created
as part of publication of [RFC3931]):
Pseudowire Types
----------------
0x0004 Ethernet VLAN Pseudowire Type
0x0005 Ethernet Pseudowire Type
8. Contributors
The 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
9. Acknowledgements
This RFC evolved from the document, "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 "Ethernet Service for Layer Two Tunneling Protocol", for
writing the first Ethernet over L2TP document.
Thanks to Carlos Pignataro for providing a thorough review and
helpful input.
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RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
10. References
10.1. Normative References
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4623] Malis, A. and M. Townsley, "Pseudowire Emulation Edge-to-
Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
August 2006.
10.2. Informative References
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December
1998.
[802.3] IEEE, "IEEE std 802.3 -2005/Cor 1-2006 IEEE Standard for
Information Technology - Telecommuincations and
Information Exchange Between Systems - Local and
Metropolitan Area Networks", IEEE Std 802.3-2005/Cor
1-2006 (Corrigendum to IEEE Std 802.3-2005)
[802.1Q] IEEE, "IEEE standard for local and metropolitan area
networks virtual bridged local area networks", IEEE Std
802.1Q-2005 (Incorporates IEEE Std 802.1Q1998, IEEE Std
802.1u-2001, IEEE Std 802.1v-2001, and IEEE Std 802.1s-
2002)
[802.1ad] IEEE, "IEEE Std 802.1ad - 2005 IEEE Standard for Local and
metropolitan area networks - virtual Bridged Local Area
Networks, Amendment 4: Provider Bridges", IEEE Std
802.1ad-2005 (Amendment to IEEE Std 8021Q-2005)
[TFRC] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", RFC
3448, January 2003.
Aggarwal, et al. Standards Track [Page 12]
RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
Author Information
Rahul Aggarwal
Juniper Networks
1194 North Mathilda Avenue
Sunnyvale, CA 94089
EMail: rahul@juniper.net
W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
EMail: mark@townsley.net
Maria Alice Dos Santos
Cisco Systems
170 W Tasman Dr
San Jose, CA 95134
EMail: mariados@cisco.com
Aggarwal, et al. Standards Track [Page 13]
RFC 4719 Transport of Ethernet Frames over L2TPv3 November 2006
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Aggarwal, et al. Standards Track [Page 14]