RFC4045 - Extensions to Support Efficient Carrying of Multicast Traffic in Layer-2 Tunneling Protocol (L2TP)

A solution whereby multicast data is carried only once in a L2TP
tunnel is of interest to service providers, as edge devices are
aggregating more and more users. This is particularly true for
operators who are deploying xDSL (Digital Subscriber Line) services
and cable infrastructures. Therefore, L2TP tunnels that may be
supported by the network will have to carry multiple redundant
multicast data more often. The solution described in this document
applies to downstream traffic exclusively; i.e., data coming from the
LNS toward end-users connected to the LAC. This downstream multicast
traffic is not framed by the LNS but by the LAC, thus ensuring
compatibility for all users in a common tunnel, whatever the framing
scheme.

1.1. Conventions Used in This Document

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].

1.2. Terminology

Unicast session

This term refers to the definition of "Session" as it is described
in the terminology section of [RFC2661].

Multicast session

This term refers to a connection between the LAC and the LNS.
Additional Control Messages and Attribute-Value-Pairs (AVPs) are
defined in this document to open and maintain this connection for
the particular purpose of multicast traffic transportation. This
connection between the LAC and the LNS is intended to convey
multicast traffic only.

Session

This term is used when there is no need to dissociate multicast
from unicast sessions, and thus it designates both.

M-IGP

Designates a Multicast Interior Gateway Protocol.

Multicast flow

Designates datagrams sent to a group from a set of sources for
which multicast reception is desired.

GMP

Group Management Protocol, such as:

- IGMPv1 ([RFC1112])
- IGMPv2 ([RFC2236])
- MLD ([RFC2710], [RFC3590])

SFGMP

Source Filtering Group Management Protocol, such as:

- IGMPv3 ([RFC3376])
- MLDv2 ([RFC3810])

2. Motivation for a Session-Based Solution

Multicast data have to be seen as a singular flow that may be
conveyed into all the L2TP sessions that have been established in a

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