RFC 3070 (rfc3070) - Page 2 of 7
Layer Two Tunneling Protocol (L2TP) over Frame Relay
Alternative Format: Original Text Document
RFC 3070 L2TP over Frame Relay February 2001
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-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.
Rawat, et al. Standards Track