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 Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 Bourdon Expires September 2003 [Page 1] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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 Bourdon Expires September 2003 [Page 2] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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) Bourdon Expires September 2003 [Page 3] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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 Bourdon Expires September 2003 [Page 4] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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 Bourdon Expires September 2003 [Page 5] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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: Bourdon Expires September 2003 [Page 6] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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: Bourdon Expires September 2003 [Page 7] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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: Bourdon Expires September 2003 [Page 8] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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: Bourdon Expires September 2003 [Page 9] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Bourdon Expires September 2003 [Page 10] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 | ... | 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 Bourdon Expires September 2003 [Page 11] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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 Bourdon Expires September 2003 [Page 12] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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. Bourdon Expires September 2003 [Page 13] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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; Bourdon Expires September 2003 [Page 14] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 - 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. Bourdon Expires September 2003 [Page 15] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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- Bourdon Expires September 2003 [Page 16] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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 Bourdon Expires September 2003 [Page 17] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 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 Bourdon Expires September 2003 [Page 18] Internet Draft draft-ietf-l2tpext-mcast-03.txt March 2003 Phone: +33 1 4529-4645 Email: gilles.bourdon@francetelecom.com Full Copyright Statement "Copyright (C) The Internet Society (2003). All Rights Reserved. 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