Network Working Group V. Rawat
Request for Comments: 3070 ONI Systems, Inc.
Category: Standards Track R. Tio
S. Nanji
Redback Networks, Inc.
R. Verma
Deloitte Consulting
February 2001
Layer Two Tunneling Protocol (L2TP) over Frame Relay
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 Internet Society (2001). All Rights Reserved.
Abstract
Layer Two Tunneling Protocol (L2TP) describes a mechanism to tunnel
Point-to-Point (PPP) sessions. The protocol has been designed to be
independent of the media it runs over. The base specification
describes how it should be implemented to run over the User Datagram
Protocol (UDP) and the Internet Protocol (IP). This document
describes how L2TP is implemented over Frame Relay Permanent Virtual
Circuits (PVCs) and Switched Virtual Circuits (SVCs).
Applicability
This specification is intended for those implementations which desire
to use facilities which are defined for L2TP and applies only to the
use of Frame Relay pont-to-point circuits.
1.0 IntrodUCtion
L2TP [1] defines a general purpose mechanism for tunneling PPP over
various media. By design, it insulates L2TP operation from the
details of the media over which it operates. The base protocol
specification illustrates how L2TP may be used in IP environments.
This document specifies the encapsulation of L2TP over native Frame
Relay and addresses relevant issues.
2.0 Conventions
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 [2].
3.0 Problem Space Overview
In this section we describe in high level terms the scope of the
problem being addressed. Topology:
+------+ +---------------+
PSTN Frame Relay
User-- ----LAC === === LNS --+ LANs
ISDN Cloud
+------+ +---------------+
An L2TP Access Concentrator (LAC) is a device attached to the
switched network fabric (e.g., PSTN or ISDN) or co-located with a PPP
end system capable of handling the L2TP protocol. The LAC need only
implement the media over which L2TP is to operate to pass traffic to
one or more LNS"s. It may tunnel any protocol carried within PPP.
L2TP Network Server (LNS) operates on any platform capable of PPP
termination. The LNS handles the server side of the L2TP protocol.
L2TP is connection-oriented. The LNS and LAC maintain state for each
user that is attached to an LAC. A session is created when an end-
to-end PPP connection is attempted between a user and the LNS. The
datagrams related to a session are sent over the tunnel between the
LAC and LNS. A tunnel is defined by an LNS-LAC pair. The tunnel
carries PPP datagrams between the LAC and the LNS.
L2TP protocol operates at a level above the particular media over
which it is carried. However, some details of its connection to
media are required to permit interoperable implementations. L2TP
over IP/UDP is described in the base L2TP specification [1]. Issues
related to L2TP over Frame Relay are addressed in later sections of
this document.
4.0 Encapsulation and Packet Format
L2TP MUST be able to share a Frame Relay virtual circuit (VC) with
other protocols carried over the same VC. The Frame Relay header
format for data packet needs to be defined to identify the protocol
being carried in the packets. The Frame Relay network may not
understand these formats.
All protocols over this circuit MUST encapsulate their packets within
a Q.922 frame. Additionally, frames must contain information
necessary to identify the protocol carried within the frame relay
Protocol Data Unit (PDU), thus allowing the receiver to properly
process the incoming packet.
The frame format for L2TP MUST be SNAP encapsulation as defined in
RFC1490 [6] and FRF3.1 [3]. SNAP format uses NLPID followed by
Organizationally Unique Identifier and a PID.
NLPID
The single octet identifier provides a mechanism to allow easy
protocol identification. For L2TP NLPID value 0x80 is used which
indicates the presence of SNAP header.
OUI & PID
The three-octet Organizationally Unique Identifier (OUI) 0x00-00-5E
identifies IANA who administers the meaning of the Protocol
Identifier (PID) 0x0007. Together they identify a distinct protocol.
Format of L2TP frames encapsulated in Frame Relay is given in Figure
1.
Octet 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 Q.922 Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3 Control 0x03 pad 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5 NLPID 0x80 OUI 0x00
+-+-+-+-+-+-+-+-+ +
7 OUI 0x00-5E
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9 PID 0x0007
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

L2TP packet

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FCS
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 Format for L2TP frames encapsulated in
Frame Relay
5.0 MTU Considerations
FRF.12 [5] is the Frame Relay Fragmentation Implementation Agreement.
If fragmentation is not supported, the two Frame Relay endpoints MUST
support an MTU size of at least 1526 which is based on adding the PPP
Max-Receive-Unit size with the PPP header size with the Max L2TP
Header Size with the Frame Relay header size (PPP header size is the
protocol field size plus HDLC framing bytes, which is required by
L2TP). To avoid packet discards on the Frame Relay interface, the
RECOMMENDED default Frame Relay MTU is 1564 based on a PPP default
MRU of 1500. The means to ensure these MTU settings are left to
implementation.
6.0 QOS Issues
In general, QoS mechanisms can be roughly provided for with
proprietary mechanisms localized within the LAC or LNS. QoS
considerations are beyond the scope of this document.
7.0 Frame Relay and L2TP Interaction
In case of Frame Relay SVCs, connection setup will be triggered when
L2TP tries to create a tunnel. Details of triggering mechanism are
left to implementation. There SHALL NOT be any change in Frame Relay
SVC signaling due to L2TP. The endpoints of the L2TP tunnel MUST be
identified by X.121/E.164 addresses in case of Frame Relay SVC.
These addresses MAY be oBTained as tunnel endpoints for a user as
defined in [4]. In case of PVCs, the Virtual Circuit to carry L2TP
traffic MAY be configured administratively. The endpoints of the
tunnel MUST be identified by DLCI, assigned to the PVC at
configuration time. This DLCI MAY be obtained as tunnel endpoints
for a user as defined in [4].
There SHALL be no framing issues between PPP and Frame Relay. PPP
frames received by LAC from remote user are stripped of CRC, link
framing, and transparency bytes, encapsulated in L2TP, and forwarded
over Frame Relay tunnel.
8.0 Security Considerations
Currently there is no standard specification for Frame Relay security
although the Frame Relay Forum is working on a Frame Relay Privacy
Agreement. In light of this work, the issue of security will be re-
examined at a later date to see if L2TP over Frame Relay specific
protection mechanisms are still required. In the interim, basic
security issues are discussed in the base L2TP specification [1].
9.0 Acknowledgments
Ken Pierce (3Com Corporation) and (Rick Dynarski 3Com Corporation)
contributed to the editing of this document.
10.0 References
[1] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and
B. Palter "Layer Two Tunneling Protocol "L2TP"", RFC2661,
August 1999.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[3] Multiprotocol Encapsulation Implementation Agreement, FRF.3.1 ,
Frame Relay Forum Technical Committee, June 1995.
[4] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M. and
I. Goyret, "RADIUS Attributes for Tunnel Protocol Support", RFC
2868, June 2000.
[5] Frame Relay Fragmentation Implementation Agreement, FRF.12,
Frame Relay Forum Technical Committee, December 1997.
[6] Bradley, T., Brown, C. and A. Malis, "Multiprotocol Interconnect
over Frame Relay", RFC1490, July 1993.
11.0 Authors" Addresses
Vipin Rawat
ONI Systems, Inc.
166 Baypointe Parkway
San Jose CA 95134
EMail: vrawat@oni.com
Rene Tio
Redback Networks, Inc.
300 Holger Way
San Jose, CA 95134
EMail: tor@redback.com
Rohit Verma
Deloitte Consulting
180 N. Stetson Avenue
Chicago Illinois 60601
EMail: rverma@dc.com
Suhail Nanji
Redback Networks, Inc.
300 Holger Way
San Jose, CA 95134
EMail: suhail@redback.com
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