Network Working Group G. Bourdon
Internet Draft France Telecom R&D
Document: draft-ietf-l2tpext-mcast-03.txt March 2003
Category: Experimental
L2TP Multicast Extension
<draft-ietf-l2tpext-mcast-03.txt>
Status of this Memo
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of Section 10 of RFC2026.
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Abstract
The Layer Two Tunneling Protocol (L2TP) provides a standard method
for tunneling PPP packets. This document describes an extension to
L2TP, in order to have an efficient use of L2TP tunnels within the
context of deploying multicast services whose data will have to be
conveyed by such tunnels.
Table of Contents
1. Introduction................................................2
1.1. Conventions used in this document...........................3
1.2. Terminology.................................................3
2. Motivation for a session-based solution.....................4
3. Control Connection establishment............................4
3.1. Negotiation phase...........................................4
3.2. Multicast Capability AVP (SCCRQ, SCCRP).....................4
4. L2TP multicast session establishment decision...............5
4.1. IGMP states in LNS..........................................5
4.2. Triggering..................................................6
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5. L2TP multicast session opening process......................6
5.1. Multicast-Session-Request (MSRQ)............................7
5.2. Multicast-Session-Response (MSRP)...........................8
5.3. Multicast-Session-Established (MSE).........................8
6. Session maintenance and management..........................9
6.1. Multicast-Session-Information (MSI).........................9
6.2. Outgoing Sessions List updates.............................10
6.2.1. New Outgoing Sessions AVP (MSI)............................10
6.2.2. New Outgoing Sessions Acknowledgement AVP (MSI)............11
6.2.3. Withdraw Outgoing Sessions AVP (MSI).......................12
6.3. Multicast Packets Priority AVP (MSI).......................12
6.3.1. Global configuration.......................................14
6.3.2. Individual configuration...................................14
6.3.3. Priority...................................................14
7. Multicast session teardown.................................14
7.1. Operations.................................................15
7.2. Multicast-Session-End-Notify (MSEN)........................15
7.3. Result Codes...............................................16
8. Traffic merging............................................16
9. IANA Considerations........................................17
10. Security Considerations....................................17
11. References.................................................18
12. Acknowledgments............................................18
13. Author's Addresses.........................................18
1. Introduction
The deployment of IP multicast-based services may have to deal with
L2TP tunnel engineering. From this perspective, the forwarding of
multicast data within L2TP sessions may impact the throughput of L2TP
tunnels. This proposal aims to reduce this impact by applying
replication mechanism of multicast traffic only when necessary.
The solution described herein provides a mechanism for transmitting
multicast data only once for all the L2TP sessions that have been
established in a tunnel, each multicast group having a dedicated L2TP
session.
Within the context of deploying IP multicast-based services, it is
assumed that the routers of the IP network that embed a L2TP Network
Server (LNS) capability may be involved in the forwarding of
multicast data, towards users who access the network through an L2TP
tunnel. Then the LNS is in charge of replicating the multicast data
for a multicast group G for each L2TP session that is used by a
receiver who has actually subscribed to group G. The solution
described here gives the ability for a LNS to send multicast data
once and make the L2TP Access Concentrator (LAC) perform the traffic
replication. By doing so, it is expected to spare transmission
resources in the core network that supports L2TP tunnels. This
multicast extension to L2TP is designed so that it does not affect
the behavior of L2TP equipment under normal conditions. A solution to
carry multicast data once in a L2TP tunnel is interesting for service
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providers since edge devices are aggregating more and more users.
This is particularly true for operators who are deploying xDSL
(Digital Subscriber Line) 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 towards 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
their framing scheme is.
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]. (Also: L2TP unicast session)
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. (Also: L2TP multicast session)
Session
This term is used when there is no need to dissociate multicast from
unicast sessions, and thus designates both. (Also: L2TP session)
M-IGP
Designates a Multicast Interior Gateway Protocol.
(*, G)
Designates a multicast group state, considering the group G and all
sources sending to this group G.
(S, G)
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Designates a multicast group state, considering the group G and the
source S sending to this group G.
2. Motivation for a session-based solution
Multicast data have to be seen as a singular flow that potentially
concerns all L2TP sessions already existing in a tunnel. It means
that a given L2TP session can be dedicated for the forwarding of a
unique multicast flow that is addressed to multiple receivers. A
session carrying IP multicast data is independent from the framing
scheme and is therefore compatible with any new framing scheme that
may be supported by the L2TP protocol.
Using a single L2TP session per multicast group G to carry multicast
data is motivated by the following arguments:
- The administrator of the LNS is presumably in charge of the IP
multicast-based services and the related engineering aspects. As
such, he must be capable of filtering multicast flows on a multicast
source basis, on a multicast group basis, and on a user basis (users
who access the network using a L2TP session that terminates in this
LNS).
- Having a L2TP session dedicated for a multicast group gives the
ability to have distinct policies for each group. For instance, it is
possible to allow more bandwidth for some groups, or change the
priority treatment for multicast packets against unicast packets.
- It is not always acceptable nor possible to have multicast
forwarding performed within the network between the LAC and the LNS.
Having the multicast traffic conveyed within a L2TP tunnel ensures a
multicast service between the LNS and end-users, without necessity of
having a multicast capability in the underlying network.
3. Control Connection establishment
3.1. Negotiation phase
The multicast extension capability is negotiated between the LAC and
the LNS during the control connection establishment phase. However,
establishment procedures defined in [RFC2661] remain unchanged. A LAC
indicates its multicast extension capability by using a new AVP, the
"Multicast Capability" AVP. There is no explicit acknowledgement from
the LNS during the control connection establishment phase. Instead,
the LNS is granted to use multicast extension messages to open and
maintain multicast session(s).
3.2. Multicast Capability AVP (SCCRQ, SCCRP)
In order to inform the LNS that a LAC has the ability to handle
multicast sessions, the LAC sends a Multicast Capability AVP during
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the control connection establishment phase.
This AVP is sent either in a SCCRQ or a SCCRP control message by the
LAC towards the LNS.
Upon receipt of the Multicast Capability AVP, a LNS may adopt two
distinct behaviors:
1) The LNS does not implement the L2TP multicast extension: the
Multicast Capability AVP is ignored, and the LNS will not initiate
any L2TP-based multicast action.
2) The LNS implements L2TP multicast extensions, and therefore
supports the Multicast Capability AVP: the LNS is granted to send
L2TP specific commands for conveying multicast traffic towards the
LAC.
The multicast capability exclusively refers to the tunnel for which
the AVP has been received during control connection establishment
phase. It SHOULD be possible for a LNS administrator to shut down
L2TP multicast extension features towards one or a set of LAC(s). In
this case, the LNS behavior is similar to 1).
The AVP has the following format:
Vendor ID = to be defined (0 once TBA1 assigned by IANA)
Attribute = TBA1 (16 bits) (Note: to be assigned by IANA)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The M-bit MUST be set to 0, the AVP MAY be hidden (H-bit set to 0 or
1).
The length of this AVP is 6 octets.
4. L2TP multicast session establishment decision
4.1. IGMP states in LNS
The LNS MUST always be at the origin of the creation of a multicast
L2TP session dedicated for the forwarding of IP multicast datagrams
destined to a multicast group.
The router that embeds the LNS feature MUST support IGMPv1
([RFC1112]) or IGMPv2 ([RFC2236]) or IGMPv3 ([RFC3376]) and acts as
an IGMP Querier for every logical interface represented by a L2TP
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session. However, the L2TP Multicast Extension is not designed to
take advantage of IGMPv3 source filtering, and is restricted to
multicast group operations.
As a multicast router, the equipment that embeds the LNS function
will be involved in the state maintenance related to the multicast
groups for which receivers have subscribed to, i.e. the maintenance
of an OIL (Outgoing Interface List) for every multicast group G
defined by (*, G) and (S, G) states. The OIL for a given multicast
group G will be partly composed by logical interfaces. All or some of
these logical interfaces will correspond to L2TP unicast sessions in
this context.
Implementing IGMP requires the LNS-capable equipment to create and
maintain OILs. Using these tables, the LNS can build for each
subscribed group within a tunnel a list of the associated L2TP
sessions: the Outgoing Sessions List (OSL). An OSL gives the ability
to identify which L2TP sessions connect users interested in receiving
the traffic corresponding to a given multicast group, and this for
each L2TP tunnel. There is one OSL maintained per L2TP multicast
session (i.e. per multicast group) within an L2TP tunnel. Whenever
the OSL gets empty, the LNS MUST stop sending multicast traffic over
the L2TP multicast session. Then the L2TP multicast session MUST be
torn down as described in Section 7 of this document.
The LAC does not have any IGMP activity; IGMP processing is performed
by the LNS. The LAC is a layer-2 equipment, and is not supposed to
track IGMP messages between users and the LNS in this context.
In order for the LAC to forward the multicast traffic received
through the L2TP multicast session to end-users, the LNS sends to the
LAC the OSL for the related multicast session (see Section 6).
4.2. Triggering
The rules to be enforced by the LNS so as to decide when to open a
dedicated L2TP multicast session for a multicast group SHOULD be
configurable by the LNS administrator. This would typically happen
whenever a number of MULTICAST_SESSION_THRESHOLD receivers/sessions
is reached. This threshold value SHOULD be valued at 2 by default, if
we consider that it is worth opening a dedicated L2TP multicast
session for a multicast group subscribed by two receivers (which
means that two L2TP unicast sessions are concerned).
Reception by the LNS of multicast traffic requested by end-users can
also be taken into account to decide if the associated L2TP multicast
session has to be opened.
5. L2TP multicast session opening process
The opening of an L2TP multicast session is initiated by the LNS. A
three-message exchange is utilized to set up the session. Following
is a typical sequence of events:
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LAC LNS
--- ---
(multicast session
triggering)
<- MSRQ
MSRP ->
(Ready to
replicate)
MSE ->
<- ZLB ACK
ZLB ACK is sent if there are no further messages waiting in 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 mentioned in
section 4.2. It is the first in a three-message exchange used for
establishing a multicast session within a L2TP tunnel.
A LNS MUST NOT send a MSRQ control message if the remote LAC did not
open the L2TP tunnel with the Multicast Capability AVP. The LAC MUST
close the session if it receives a MSRQ control message, while the
L2TP tunnel was not opened with a Multicast Capability AVP.
The following AVPs MUST be present in MSRQ:
Message Type
Assigned Session ID
The following AVP MAY be present in MSRQ:
Random Vector
Maximum BPS
The Maximum BPS value is set by the LNS administrator. However, this
value should be chosen in accordance with the line capabilities of
the end-users. The Maximum BPS value SHOULD NOT be higher than the
highest speed connection for all end-users within the L2TP tunnel.
The associated Message Type AVP is encoded with the values:
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Vendor ID = to be defined (0 once TBA2 assigned by IANA)
Attribute Type = 0
Attribute Value = TBA2 (16 bits) (Note: to be assigned by IANA)
The M-bit MUST be set to 0, the H-bit MUST be set to 0.
5.2. Multicast-Session-Response (MSRP)
Multicast-Session-Response (MSRP) is a control message sent by the
LAC to the LNS in response to a received MSRQ message. It is the
second in a three-message exchange used for establishing a multicast
session within a L2TP tunnel.
MSRP is used to indicate that the MSRQ was successful and the LAC
will attempt to reserve appropriate resources to perform multicast
replication for unicast sessions managed in the pertaining control
connection.
The following AVPs MUST be present in MSRP:
Message Type
Assigned Session ID
The following AVP MAY be present in MSRP:
Random Vector
The associated Message Type AVP is encoded with the values:
Vendor ID = to be defined (0 once TBA3 assigned by IANA)
Attribute Type = 0
Attribute Value = TBA3 (16 bits) (Note: to be assigned by IANA)
The M-bit MUST be set to 0, the H-bit MUST be set to 0.
5.3. Multicast-Session-Established (MSE)
Multicast-Session-Established (MSE) is a control message sent by the
LAC to the LNS to indicate that the LAC is ready to receive necessary
multicast information (Section 6) for the group using the newly
created multicast session. It is the third message in the three-
message sequence used for establishing a multicast session within a
L2TP tunnel.
The following AVP MUST be present in MSE:
Message Type
The following AVP MAY be present in MSE:
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Sequencing Required
Sequencing will occur only from the LNS to the LAC since a
multicast session is used for downstream purposes only.
The associated Message Type AVP is encoded with the values:
Vendor ID = to be defined (0 once TBA4 assigned by IANA)
Attribute Type = 0
Attribute Value = TBA4 (16 bits) (Note: to be assigned by IANA)
The M-bit MUST be set to 0, the H-bit MUST be set to 0.
6. Session maintenance and management
Once the multicast session is established, the LAC has to be informed
of the L2TP unicast sessions interested in getting the traffic from
the newly-created multicast session, as well as a related optional
priority parameter defined in Section 6.3. To achieve this, a new
control message type is defined: Multicast-Session-Information (MSI).
6.1. Multicast-Session-Information (MSI)
Multicast-Session-Information (MSI) control messages carry AVPs to
keep the OSL synchronised between the LNS and the LAC and to set
optional priority parameter for multicast traffic versus unicast
traffic. MSI may be extended to update the multicast session with
additional parameters as needed.
Each MSI message is specific to a particular multicast session.
Therefore, the control message MUST use the assigned session ID
associated to the multicast session (assigned by the LAC), except for
the case mentioned in 6.3.2.
The associated Message Type AVP is encoded with the values:
Vendor ID = to be defined (0 once TBA5 assigned by IANA)
Attribute Type = 0
Attribute Value = TBA5 (16 bits) (Note: to be assigned by IANA)
The M-bit MUST be set to 0, the H-bit MUST be set to 0.
The following AVPs MUST be present in MSI:
Message Type
The following AVP MAY be present in MSI:
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Random Vector
New Outgoing Sessions
New Outgoing Sessions Acknowledgement
Withdraw Outgoing Sessions
Multicast Packets Priority
New Outgoing Sessions, New Outgoing Sessions Acknowledgement,
Withdraw Outgoing Sessions and Multicast Packets Priority are new
AVPs defined in sections 6.2 and 6.3.
6.2. Outgoing Sessions List updates
Whenever a change occurs in the Outgoing Sessions List, the LNS MUST
inform the LAC of that change. The OSL is built upon subscription
reports recorded by the IGMP process running in the LNS (Section
4.1).
The LAC maintains an OSL as a per-group local table transmitted by
the LNS. As for the LNS, the LAC has to maintain an OSL for each L2TP
multicast session within a L2TP tunnel. To update the LAC OSL, the
LNS sends a New Outgoing Sessions AVP for additional(s) session(s) or
sends a Withdraw Outgoing Sessions AVP to remove session(s). All
sessions mentioned in these AVPs MUST be added or removed by the LAC
from the pertaining OSL. The Outgoing Sessions List is identified by
the tunnel ID and the multicast session ID the updating AVP is
referring to.
To update the OSL, the following AVPs are used:
Additional session(s): New Outgoing Sessions AVP
Session(s) removal: Withdraw Outgoing Sessions AVP
These new AVPs MUST be sent in a MSI message.
6.2.1. New Outgoing Sessions AVP (MSI)
The New Outgoing Sessions AVP can only be carried within a MSI
message type. This AVP piggybacks every Session ID to which the
multicast traffic has to be forwarded.
The AVP has the following format:
Vendor ID = to be defined (0 once TBA6 assigned by IANA)
Attribute = TBA6 (16 bits) (Note: to be assigned by IANA)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA6 | Session ID 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ... | Session ID N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There can be from 1 to N Session IDs present in the New Outgoing
Sessions AVP. This AVP must be placed in a MSI message and sent after
the establishment of the multicast session to indicate the LAC what
are the initial outgoing sessions, and at any time when one or more
outgoing sessions appear during the multicast session lifetime. Upon
reception of this AVP, the LAC sends a New Outgoing Sessions
Acknowledgment AVP to the LNS to notify that the LAC is ready to
replicate the multicast traffic towards the indicated sessions.
Usage of this AVP is incremental: only new outgoing sessions have to
be listed in the AVP.
The M-bit MUST be set to 1, the AVP MAY be hidden (H-bit set to 0 or
1).
6.2.2. New Outgoing Sessions Acknowledgement AVP (MSI)
The New Outgoing Sessions Acknowledgement AVP can only be carried
within a MSI message type. This AVP informs the LNS that the LAC is
ready to replicate traffic for every Session ID listed within.
The AVP has the following format:
Vendor ID = to be defined (0 once TBA7 assigned by IANA)
Attribute = TBA7 (16 bits) (Note: to be assigned by IANA)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA7 | Session ID 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | Session ID N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This AVP must be placed in a MSI message and sent by the LAC towards
the LNS to acknowledge reception of a New Outgoing Sessions list
received in a New Outgoing Sessions AVP from the LNS.
A LNS is allowed to send multicast traffic within the L2TP multicast
session as soon as a New Outgoing Sessions Acknowledgement AVP is
received for the related L2TP multicast session.
A LNS is allowed to stop sending multicast traffic for the related
group within L2TP unicast sessions only when it receives a MSI
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message with the New Outgoing Session Acknowledgement AVP, and only
for the unicast Session IDs mentioned in the AVP. The multicast
traffic can use L2TP unicast sessions again when the L2TP multicast
session goes down. From this standpoint, multicast traffic related to
this group SHOULD NOT be conveyed within the L2TP unicast sessions
mentioned in the AVP to avoid duplicate multicast packets.
There can be from 1 to N Session IDs present in the New Outgoing
Sessions Acknowledgement AVP. Session IDs mentioned in this AVP that
have not been listed in a previous New Outgoing Sessions AVP should
be ignored. Non-acknowledged Session IDs MAY be listed in future New
Outgoing Sessions AVPs, but multicast traffic MUST be sent to logical
interfaces associated to these Session IDs as long as these Session
IDs are not acknowledged for replication by the LAC.
The M-bit MUST be set to 1, the AVP MAY be hidden (H-bit set to 0 or
1).
6.2.3. Withdraw Outgoing Sessions AVP (MSI)
The Withdraw Outgoing Sessions AVP is sent whenever there is one or
more withdrawn subscriptions for the related multicast group
(designated by the session ID on which the MSI is sent).
The LAC can stop forwarding multicast traffic to users mentioned in
the AVP for the related group as soon as it receives the MSI message
embedding this Withdraw Target Session AVP.
The AVP has the following format:
Vendor ID = to be defined (0 once TBA8 assigned by IANA)
Attribute = TBA8 (16 bits) (Note: to be assigned by the IANA)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA8 | Session ID 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | Session ID N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There can be from 1 to N Session IDs present in the Withdraw Outgoing
Sessions AVP. The M-bit MUST be set to 1, the AVP MAY be hidden (H-
bit set to 0 or 1).
6.3. Multicast Packets Priority AVP (MSI)
The Multicast Packets Priority AVP is an optional AVP intended to
provide the LAC with an indication on how to process multicast
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traffic against unicast traffic. Even though the LAC behavior is
partially described here, the nature of the traffic (layer-2 frames
for unicast traffic and pure IP packets for multicast traffic) is not
a criteria for enforcing a traffic prioritisation policy. Traffic
processing for the provisioning of a uniformly-framed traffic for the
final user is described is section 8.
Three different behaviors can be adopted:
1) Best effort: the traffic is forwarded from the LAC to the end-user
in the order it comes from the LNS, whatever the type of traffic.
2) Unicast traffic priority: traffic coming down the L2TP unicast
session has priority over traffic coming down the L2TP multicast
session.
3) Multicast traffic priority: traffic coming down the L2TP multicast
session has priority over traffic coming down the L2TP unicast
session.
The priority is encoded as a 16-bit quantity, which can take the
values:
0: Best effort (default)
1: Unicast traffic priority
2: Multicast traffic priority
The AVP has the following format:
Vendor ID = to be defined (0 once TBA9 assigned by IANA)
Attribute = TBA9 (16 bits) (Note: to be assigned by the IANA)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA9 | Priority Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It is important to note that the multicast traffic rate can reach up
to Maximum BPS (as indicated in MSRQ). This rate can exceed the
maximum rate allowed for a particular final user. This means that
even with a priority value = 0, the final user might receive
multicast traffic only: unicast packets might be dropped because the
multicast flow overwhelms the LAC forwarding buffer.
The default Priority Value is 0. The M-bit MUST be set to 0, the AVP
MAY be hidden (H-bit set to 0 or 1).
There are two ways of using this AVP: global configuration and
individual configuration.
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6.3.1. Global configuration
The Multicast Priority Packet AVP is sent for all L2TP unicast
sessions concerned by a specific multicast group represented by a
L2TP multicast session.
In this case, the AVP is sent in a L2TP MSI control message for the
corresponding multicast session ID (Session ID = L2TP session for the
corresponding multicast group). The priority value applies to all
L2TP unicast sessions to which the multicast group designated by the
L2TP multicast session is intended, as soon as this AVP is received.
6.3.2. Individual configuration
The Multicast Priority Packet AVP is sent for a specific L2TP unicast
session concerned by adopting a specific behavior for both unicast
and multicast traffic. In this case, the AVP is sent in a L2TP MSI
control message for the L2TP unicast session (Session ID = L2TP
session for the concerned user). The priority value applies to the
individual session only, and doesn't affect other individual
sessions. It is important to note that in this case, all multicast
groups carried in L2TP multicast sessions are treated by the LAC the
same way for the concerned user.
This is the only case where a MSI control message can be sent for a
L2TP unicast session.
6.3.3. Priority
It is the responsibility of the network administrator to decide which
behavior to adopt between global or individual configuration, if the
AVP is sent twice (one for a multicast group and one for an
individual user). By default, only the individual configurations
SHOULD be taken into consideration in that case.
Support of the Multicast Packets Priority AVP is optional and SHOULD
be configurable by the LAC administrator if available.
7. Multicast session teardown
A L2TP multicast session should be torn down whenever there are no
longer users interested in. More generally, we can consider that a
multicast session becomes useless as soon as the related OSL has less
than a predefined number of entries, this number being defined by a
threshold.
Multicast session flapping may occur when the number of OSL entries
is oscillating around the threshold, if the same value is used to
trigger the creation or the deletion of an L2TP multicast session.
To avoid this behavior, two methods can be used:
- The threshold value used to determine if the L2TP multicast
session has to be torn down is lower than the
MULTICAST_SESSION_THRESHOLD value;
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- The MULTICAST_SESSION_THRESHOLD value is used to determine if
the L2TP multicast session has to be torn down. A multicast session
SHOULD be killed after a period of MULTICAST_SESSION_HOLDTIME seconds
if the corresponding OSL maintains less than
MULTICAST_SESSION_THRESHOLD entries. The MULTICAST_SESSION_HOLDTIME
value is 10 seconds by default, and SHOULD be configurable either by
the LAC or the LNS administrator.
The multicast session can be torn down for multiple reasons,
including specific criteria not described here (can be vendor-
specific).
A multicast session teardown can be initiated either by the LAC or
the LNS. However, multicast session teardown MUST be initiated by the
LNS if the termination decision is motivated by the number of users
interested in receiving the traffic corresponding to a multicast
group.
7.1. Operations
The effective termination of a multicast session is initiated with a
new Multicast-Session-End-Notify (MSEN) control message, sent either
by the LAC or by the LNS.
Following is an example of control messages exchange to terminate a
multicast session:
LAC or LNS LAC or LNS
---------- ----------
(multicast session
termination)
<- MSEN
(Clean up)
ZLB ACK ->
(Clean up)
7.2. Multicast-Session-End-Notify (MSEN)
The Multicast-Session-End-Notify (MSEN) is an L2TP control message
sent by either the LAC or the LNS to request termination of a
specific multicast session within the tunnel. Its purpose is to
inform the peer of the termination and the reason why the termination
occurred. The peer MUST clean up any resources, and does not send
back any indication of success or failure.
As defined in [RFC2661], termination of a control connection will
terminate all sessions managed within, including multicast sessions
if any.
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The MSEN message carries a Result Code AVP with an optional Error
Code.
The following AVPs MUST be present in a MSEN message:
Message Type
Result Code
Assigned Session ID
The associated Message Type AVP is encoded with the values:
Vendor ID = to be defined (0 once TBA10 assigned by IANA)
Attribute Type = 0
Attribute Value = TBA10 (16 bits) (Note: to be assigned by IANA)
The M-bit MUST be set to 0, the H-bit MUST be set to 0.
7.3. Result Codes
The following values are the defined result codes for MSEN control
messages:
TBA11 (16 bits) - Session terminated for the reason indicated in
the error code
TBA12 (16 bits) - No multicast traffic for the group
TBA13 (16 bits) - No more receivers
(Note: TBA11, TBA12 and TBA13 to be defined by the IANA)
o The code TBA11 refers to General Error Codes maintained by the
IANA for L2TP.
o The code TBA12 may be used when the LAC detects that no traffic
is coming down the multicast session, or when the LNS doesn't receive
multicast traffic for the related group during a certain period of
time.
o The code TBA13 may be used by the LAC or the LNS when the OSL is
empty.
8. Traffic merging
Both unicast and multicast traffics have to be merged by the LAC in
order to provide properly framed data to the end-user. Multicast
packets are framed by the LAC and transmitted towards the proper end-
user. Methods to achieve this function are not described here, since
it is mostly an implementation specific issue.
All frames conveyed from the LAC to the end-users have to follow the
framing scheme applied for the considered peer to which the traffic
is destined (e.g. the LAC is always aware of the PPP link parameters,
as described in [RFC2661], Section 6.14). It has to be noted that
using L2TP Multicast Extension features is not appropriate for end-
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users who have negotiated a sequenced layer-2 connection with the
LNS: while inserting PPP-encapsulated multicast packets in a session,
the LAC cannot modify PPP sequencing performed by the LNS for each
PPP session.
9. IANA Considerations
This document defines:
- 5 new Message Type (Attribute Type 0) Values:
o Multicast-Session-Request (MSRQ) : TBA2
o Multicast-Session-Response (MSRP) : TBA3
o Multicast-Session-Establishment (MSE) : TBA4
o Multicast-Session-Information (MSI) : TBA5
o Multicast-Session-End-Notify (MSEN) : TBA10
- 5 new Control Message Attribute Value Pairs:
o Multicast Capability : TBA1
o New Outgoing Sessions : TBA6
o New Outgoing Sessions Acknowledgement : TBA7
o Withdraw Outgoing Sessions : TBA8
o Multicast Packets Priority : TBA9
- 3 Result Codes for the MSEN message:
o Session terminated for the reason indicated in the
error code : TBA11
o No multicast traffic for the group : TBA12
o No more receivers : TBA13
IANA will assign, register and maintain values for these new
attributes ([RFC3438]).
10. Security Considerations
The extension described in this document does not introduce any
additional security issues as far as the activation of the L2TP
protocol is concerned.
Injecting appropriate control packets in the tunnel towards a LAC to
modify Outgoing Session List and flood end-users with unwanted
multicast traffic is only possible if the control connection is
hacked. As for any reception of illegitimate L2TP control message:
- If the spoofed control message embeds consistent sequence
numbers, next messages will appear out of synch bringing the control
connection to terminate.
- If sequence numbers are inconsistent with current control
connection states, the spoofed control message will be queued or
discarded, as described in [RFC2661] section 5.8.
The activation of the L2TP multicast capability on a LAC could make
the equipment more sensitive to Denial of Service attacks if the
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control connection or the related LNS is hacked. The LAC might also
be sensitive to the burden generated by the additional replication
work.
As mentioned in [RFC2661] section 9.2, securing L2TP requires that
the underlying transport makes encryption, integrity and
authentication services available for all L2TP traffic, including
L2TP multicast traffic (control and data).
11. References
[RFC1112] S. Deering, "Host Extensions for IP Multicasting",
RFC 1112, August 1989.
[RFC1661] W. Simpson, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2236] W. Fenner, "Internet Group Management Protocol, Version
2", RFC 2236, November 1997.
[RFC2661] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn,
B. Palter, "Layer 2 Tunneling Protocol "L2TP" ",
RFC2661, August 1999.
[RFC3376] B. Cain et. al., "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
[RFC3438] W. Townsley, "Layer Two Tunneling Protocol (L2TP)
Internet Assigned Numbers Authority (IANA)
Considerations Update", RFC 3438, December 2002.
12. Acknowledgments
Thanks to Christian Jacquenet for all the corrections done on this
document and his precious advice, Pierre Levis for his contribution
about IGMP, Francis Houllier for PPP considerations and Xavier Vinet
for his input about thresholds. Many thanks to W. Mark Townsley for
his highly valuable input on protocol definition.
13. Author's Addresses
Gilles Bourdon
France Telecom R&D
38-40, rue du General Leclerc
92794 Issy les Moulineaux Cedex 9 - FRANCE
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Phone: +33 1 4529-4645
Email: gilles.bourdon@francetelecom.com
Full Copyright Statement
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