One document matched: draft-ietf-l2tpext-fr-01.txt-11331.txt
Differences from 01.txt-00.txt
Network Working Group Vipin Rawat
INTERNET DRAFT ONI Systems, Inc.
Category: Standards Track Rene Tio
Title: draft-ietf-l2tpext-fr-01.txt Suhail Nanji
Date: November 2000 Redback Networks, Inc.
Rohit Verma
Deloitte Consulting
Layer Two Tunneling Protocol (L2TP) over Frame Relay
<draft-ietf-l2tpext-fr-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full
conformance with all provisions of Section 10 of RFC2026.
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Abstract
Layer Two Tunneling Protocol describes a mechanism to
tunnel 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 UDP and IP. This document describes how the Layer
Two Tunneling Protocol is implemented over Frame Relay
PVCs and 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
cicuits.
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 draft 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 RFC 2119 [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-
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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 draft.
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 RFC 1490 [6] and FRF3.1 [3] . SNAP format uses
NLPID followed by Organizationally Unique Identifier and a
PID.
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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
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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
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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] M. Townsley et al., "Layer Two Tunneling Protocol
'L2TP'", RFC 2661, August 1999.
[2] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[3] Multiprotocol Encapsulation Implementation Agreement,
FRF.3.1 , Frame Relay Forum Technical Committee, June 1995
[4] G. Zorn et al., "RADIUS Attributes for Tunnel
Protocol Support", RFC 2868, June 2000.
[5] Frame Relay Fragmentation Implemenation Agreement,
FRF.12, Frame Relay Forum Technical Committee, December
1997
[6] T. Bradley, C. Brown, A. Malis, "Multiprotocol
Interconnect over Frame Relay", RFC 1490, July 1993
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11.0 Author's Addresses
Vipin Rawat
ONI Systems, Inc.
166 Baypointe Parkway
San Jose CA 95134
vrawat@oni.com
Rene Tio
Redback Networks, Inc.
1195 Borregas Avenue
Sunnyvale, CA 94089
tor@redback.com
Rohit Verma
Deloitte Consulting
180 N. Stetson Avenue
Chicago Illinois 60601
rverma@dc.com
Suhail Nanji
Redback Networks, Inc.
350 Holger Way
San Jose, CA 95134
suhail@redback.com
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