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

The LNS MUST send an OSL update to the LAC to reflect L2TP session
list changes, whenever appropriate. If the LNS is configured to
create one replication context per (group, source), L2TP multicast
sessions will be opened in addition to the existing one, depending on
the number of sources for the group.

If new L2TP multicast sessions have to be opened, the LNS SHOULD wait
until these multicast sessions are established before updating the
OSL of the original multicast session. To limit the duration of
temporary packet loss or duplicates to receivers, the LNS has to
minimize delay between OSL updates messages sent to the LAC.

4.4. Multicast Traffic Sent from Group Members

The present document proposes a solution to enhance the forwarding of
downstream multicast traffic exclusively; i.e., data coming from the
LNS toward end-users connected to the LAC. If a group member that
uses an L2TP session is also a multicast source for traffic conveyed
in a multicast session, datagrams may be sent back to the source. To
prevent this behavior, two options can be used in the LNS:

1) Disable the multicast packets' forwarding capability, for those
multicast datagrams sent by users connected to the network by
means of an L2TP tunnel. Protocols using well-known multicast
addresses MUST NOT be impacted.

2) Exclude from the OSL the L2TP session used by a group member
that sends packets matching the replication context of this
OSL. Therefore, the corresponding multicast flow is sent by
the LNS over the user L2TP unicast session, using standard
multicast forwarding rules.

5. L2TP Multicast Session Opening Process

The opening of an L2TP multicast session is initiated by the LNS. A
three-message exchange is used to set up the session. The following
is a typical sequence of events:

LAC LNS
--- ---
(multicast session
triggering)

<- MSRQ
MSRP ->

(Ready to
replicate)

MSE ->
<- ZLB ACK

The ZLB ACK is sent if there are no further messages waiting in the
queue for that peer.

5.1. Multicast-Session-Request (MSRQ)

Multicast-Session-Request (MSRQ) is a control message sent by the LNS
to the LAC to indicate that a multicast session can be created. The
LNS initiates this message according to the rules in Section 4.3. It

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