L2TPEXT Working Group G. Bourdon Internet Draft France Telecom R&D Document: draft-ietf-l2tpext-mcast-00.txt July 2001 Category: Experimental L2TP Multicast Extension Status of this Memo This document is an Internet-Draft and is in full conformance with 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) [RFC2661] provides a standard method for tunneling PPP [RFC1661] 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. Negotiation of multicast capability.........................4 3.1. LAC behaviour...............................................4 3.2. LNS behaviour...............................................5 4. L2TP multicast session establishment........................5 4.1. IGMP states in LNS..........................................5 4.2. Considerations about IGMP versions..........................6 4.3. Triggering..................................................7 Bourdon Expires January 2002 [Page 1] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 5. L2TP multicast session opening process......................7 5.1. OCRQ........................................................7 5.2. OCRP........................................................8 5.3. OCCN........................................................8 6. Session maintenance and management..........................8 6.1. Outgoing Sessions List updates..............................9 6.1.1. New Outgoing Sessions AVP...................................9 6.1.2. Withdraw Outgoing Sessions AVP.............................10 6.2. Multicast Packets Priority AVP.............................11 6.2.1. Global configuration.......................................12 6.2.2. Individual configuration...................................12 6.2.3. Priority...................................................13 7. Multicast session teardown.................................13 7.1. Operations.................................................13 7.2. Result Codes...............................................14 8. Traffic merging............................................14 9. IANA Considerations........................................15 10. Security Considerations....................................15 11. References.................................................15 12. Acknowledgments............................................16 13. Author's Addresses.........................................16 Appendix A: IGMPv3 filters considerations..........................16 1. Introduction The deployment of IP multicast services may co-exist 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 to transmit multicast data 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 services, it is assumed that the routers of the IP network that act as LNS may participate 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 have the traffic replicated in the LAC only. This is assumed to spare transmission resources in the network that supports L2TP tunnels. This multicast extension for L2TP is designed so that it does not affect the behavior of L2TP equipment under normal conditions. The need for a solution to carry multicast data once in an L2TP tunnel is crucial for service providers since the edge equipment of the network are aggregating more and more users. This is particularly true for operators who are deploying xDSL (Digital Subscriber Line) and cable infrastructure. Therefore, the Bourdon Expires January 2002 [Page 2] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 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 the 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 L2TP unicast session or unicast session These terms refer to the definition of "Session", as it is described in the terminology section of [RFC2661]. In addition, this document may use this term to designate other connection types than PPP connection. L2TP multicast session or multicast session These terms refer to a connection between the LAC and the LNS. This connection is opened, maintained and closed as it is performed for L2TP unicast sessions, using the same mechanisms described in [RFC2661]. Additional messages and 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 only intended to convey multicast traffic. L2TP session or session These terms are used when there is no need to dissociate multicast from unicast sessions, and thus designate both. M-IGP Designate a Multicast Internet Gateway Protocol. PIM-SM (and SSM extension), PIM-DM, M-OSPF, DVMRP, CBT are M-IGPs. (*, G) Designate a multicast group state, considering the group G and all sources sending to this group G. (S, G) Bourdon Expires January 2002 [Page 3] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 Designate a multicast group state, considering the group G and the source S sending to this group G. (*-{S}, G) Designate a multicast group state, considering the group G and all source addresses sending to the group G, but source address(es) S. 2. Motivation for a session-based solution Multicast data has to be seen as a singular flow which concerns all kinds of protocols carried within 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 users. 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 has to be in charge of the IP multicast service and the related engineering aspects. He must be capable of filtering multicast flows on a multicast source basis, on a multicast group basis, and on a user basis (who access the network using an L2TP session ending in this LNS). - Having an 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 done within the network between the LAC and the LNS. Having the multicast traffic conveyed within an L2TP tunnel ensures a multicast service between the LNS and end-users, without necessity of having a multicast capability in the underlying network. 3. Negotiation of multicast capability Multicast extension capability is negotiated by LAC and LNS during the tunnel establishment phase. 3.1. LAC behaviour The procedure used is described in [SVCTYPE], with the following particularities: - The Service Capabilities List AVP includes a Service Type with a value = X corresponding to the L2TP multicast capability (Note: value X to be assigned as defined in [SVCTYPE]). Bourdon Expires January 2002 [Page 4] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 - The L2TP multicast capability value is sent in a Service Capabilities List AVP only from a LAC to an LNS. It SHOULD NOT be sent from the LNS to the LAC. As described in [SVCTYPE], the basic tunnel establishment procedures defined in [RFC2661] remain unchanged. 3.2. LNS behaviour Upon reception by an LNS of a Service Capabilities List AVP embedding a service type corresponding to the L2TP multicast capability, an LNS may have three distinct behaviours: 1) The LNS doesn't implement service types as defined in [SVCTYPE]: the Service Capabilities List AVP is ignored, and the LNS will not initiate any L2TP multicast action. 2) The LNS implements service types as defined in [SVCTYPE], but doesn't activate or recognize the L2TP multicast capability parameter: the LNS will not initiate any L2TP multicast action. 3) The LNS implements service types as defined in [SVCTYPE], and supports the L2TP multicast capability parameter: the LNS is granted to send L2TP specific commands for multicast towards the LAC. The L2TP multicast capability applies exclusively to the tunnel for which the parameter has been received during tunnel establishment phase. 4. L2TP multicast session establishment 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 IGMP (Internet Group Management Protocol, v1, v2 or v3) and acts as a IGMP querier. The router MUST also be Designated Router for the M-IGP running in the network, or be IGMP proxy (as defined in [PROXY]). 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 which is defined either by a (*, G) or by an (S, G) state. 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 such a list. Using this list, the LNS can build for each subscribed group within a tunnel a list of the associated L2TP sessions. This "Outgoing Sessions List" (OSL) gives the ability to identify which L2TP sessions are conveying a common multicast group, Bourdon Expires January 2002 [Page 5] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 and this for each 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 8. There is one OSL maintained per L2TP multicast session within an L2TP tunnel. The LAC does not have any IGMP activity. IGMP processing is only 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 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. Considerations about IGMP versions [IGMPv3] provides source filtering and thus gives the ability for a receiver to report interest in receiving packets from specific source addresses or from all but specific source addresses. The complexity of combinations which can be achieved with IGMPv3 reports to set a filter makes difficult the establishment of an optimal L2TP multicast session. The L2TP Multicast Extension model gives the opportunity for developers to have their own way to handle IGMPv3 filters to establish L2TP multicast sessions: the decision process of L2TP multicast session creation, the OSL set up and multicast traffic filtering capability based on the source address are hosted by the LNS. Thus developers have all degrees of liberty to set up their own multicast L2TP session creation rules. Here is a non-exhaustive list of rules that might be applied to handle IGMPv3 filters: a- Establishment of an L2TP multicast session based on the IGMPv3 filter. b- Establishment of multiple (S, G) multicast sessions for "INCLUDE" IGMPv3 filters, and filter-based multicast sessions for "EXCLUDE" IGMPv3 filters (which would be (*, G) multicast sessions with packet filtering performed by the LNS based on the source addresses which have been explicitly excluded in the IGMPv3 report). c- Establishment of multiple (S, G) multicast sessions, resulting from the decomposition of IGMPv3 filters for the group G. Each session creation would be triggered by reception by the LNS of a multicast packet for group G coming from source S and matching subscriptions recorded for several unicast sessions. IGMPv1/2 subscriptions would be translated in multiple (S, G) sessions, except if there are only IGMPv1/2 reports. d- Establishment of (Si, G) and (*-{Si}, G) L2TP multicast sessions to match all IGMPv3 filters existing for the users in the considered tunnel. Individual filters are then composed by an appropriate set of L2TP multicast sessions forwarding rules (i.e. determined by OSL entries). Bourdon Expires January 2002 [Page 6] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 The solutions briefly mentioned above are more detailed in Appendix A. Usage of IGMPv1 ([RFC1112]) or IGMPv2 ([RFC2236]) does not require such a particular attention: there can be only one L2TP multicast session for a group within a tunnel, and there is no filtering features required in the LNS equipment. For simplicity purposes, this document refers to L2TP multicast sessions as sessions conveying all the traffic for a given multicast group. Of course, this has to be moderated by what the IGMPv3 filter might be. Triggering rules as well as session maintenance described in this document are operation functions separated from optimisation policies introduced by IGMPv3. 4.3. 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 received by 2 receivers (which means that 2 L2TP unicast sessions are concerned). Reception by the LNS of actual 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 L2TP multicast session is performed by the LNS as described in [RFC2661]. However, since the decision is made by the LNS, the multicast session opening always starts with an OCRQ (Outgoing Call ReQuest) message. If required, the Random Vector AVP has to be used as defined in [RFC2661]. 5.1. OCRQ The LNS opens a L2TP multicast session by sending an OCRQ to the LAC. Due to the particular status of a multicast session, the multicast OCRQ message has some restrictions compared to the rules defined in [RFC2661] for regular L2TP sessions (e.g. no real outgoing call has to be performed by the LAC, although this is an Outgoing Call ReQuest message). The Service Type AVP corresponding to L2TP multicast capability MUST be placed as defined in [SVCTYPE], and may be preceded by a Random Vector AVP if the Service Type AVP has to be hidden. The Service Type AVP gives the ability for the LAC to adapt its behavior for standard OCRQ and for multicast OCRQ. Bourdon Expires January 2002 [Page 7] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 The following AVPs MUST be present in multicast OCRQ : Message Type Service Type Assigned Session ID The following AVP MAY be present in multicast OCRQ: Maximum BPS The Maximum BPS value is set up by the LNS administrator. However, this value should be chosen in accordance with the line capabilities of final users. The Maximum BPS value SHOULD NOT be higher than the highest speed connection for all final users within the L2TP tunnel. 5.2. OCRP OCRP (Outgoing Call ResPonse) is sent by the LAC to the LNS in response to the OCRQ message previously sent. There is no Service Type AVP in OCRP. The following AVPs MUST be present in multicast OCRP: Message Type Assigned Session ID Since Physical Channel ID AVP makes no sense in this context, it SHOULD NOT be present in OCRP, even though its presence is defined as optional in [RFC2661]. If present, the Physical Channel ID AVP MUST be ignored in this context. 5.3. OCCN OCCN is sent by the LAC to the LNS, giving the start for the latter to send necessary multicast information (Section 6) for the group using the newly created L2TP session. There is no Service Type AVP in OCCN. The following AVP MUST be present in multicast OCCN: Message Type The following AVP MAY be present in multicast OCCN: Sequencing Required The sequencing will occur only from the LNS to the LAC since multicast session is only used for downstream purposes. 6. Session maintenance and management Bourdon Expires January 2002 [Page 8] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 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 session, as well as a related optional priority parameter defined in Section 6.2. To achieve this, a new control message type is defined: Multicast Session Information (MSI). The associated Message Type AVP is encoded with the values: Vendor ID = to be defined Attribute Type = 0 Attribute Value = XY (Note: XY to be assigned by IANA) The M-bit MUST be set to 0, the H-bit MUST be set to 0. The MSI control message is always sent by the LNS towards the LAC, and carries additional AVPs to keep the OSL synchronised between the LNS and the LAC, with an optional priority parameter for multicast traffic versus unicast traffic. MSI can also be used to carry future additional features. Each MSI message is specific to a particular multicast session. Therefore, the control message MUST use the associated multicast session number previously assigned by the LAC, except for the case mentioned in 6.2.2. Random Vector AVP can be used with AVPs carried within a MSI control packet according to the procedure specified in [RFC2661]. 6.1. 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 an L2TP tunnel. To update the LAC OSL, the LNS send a New Outgoing Sessions AVP for additional(s) session(s) or send 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 Session List is identified by the Tunnel ID and the multicast Session ID from which the updating AVP is received. 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. 6.1.1. New Outgoing Sessions AVP Bourdon Expires January 2002 [Page 9] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 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 Attribute = ABC (Note: ABC 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ABC | Session ID 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | 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 and sent immediately 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 MUST be ready to forward the multicast traffic towards the indicated sessions as soon as the MSI control message is acknowledged by the LAC. An LNS is allowed to send multicast traffic within the L2TP multicast session and stop sending multicast traffic for the related group within L2TP unicast sessions mentioned in the AVP only when it receives a reception acknowledgement from the LAC about the MSI message carrying these session IDs. It has to be noted that from this point, the multicast traffic for this group SHOULD NOT be transported within each L2TP unicast session, and this to avoid duplicate multicast packets. The multicast traffic can use L2TP unicast sessions again (as these sessions are mentioned in this AVP) when the L2TP multicast session goes down. The M-bit MUST be set to 0, the AVP MAY be hidden (H-bit set to 0 or 1). 6.1.2. Withdraw Outgoing Sessions AVP 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). A latency timer may be configurable in the LNS in order to group multiple withdrawals in a single message. A recommended value for this timer would be 0, which gives the best reactivity. Bourdon Expires January 2002 [Page 10] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 Of course, in a very active multicast context this timer should be changed accordingly, in order to avoid multiplicity of Withdraw Outgoing Sessions AVP tranmissions. The LAC can stop forwarding multicast traffic to the users mentioned in the AVP for the related group as soon as it receives the MSI message embedding this Withdraw Target Session AVP. The format of the AVP follows the format adopted for New Outgoing Sessions AVP. The AVP has the following format: Vendor ID = to be defined Attribute = DEF (Note: DEF 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DEF | Session ID 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Session ID N | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ There can be from 1 to N Session ID present in the Withdraw Outgoing Sessions AVP. The M-bit MUST be set to 0, the AVP MAY be hidden (H- bit set to 0 or 1). 6.2. Multicast Packets Priority AVP The Multicast Packets Priority AVP is intended to provide the LAC with indication on how to process multicast against unicast traffic. Even though the LAC behavior is partially described here, the nature of the traffic (layer-2 frames for regular sessions and pure IP for multicast sessions) is not a criteria for priority decisions. Traffic processing to provide a uniformly framed traffic for the final user is described is section 8. Three different behaviors can exist: 1) Best effort: the traffic is forwarded from the LAC to the final user in the order it comes from the LNS, no depending on the type of traffic. If the LAC forwarding interface buffer is full, packets coming from unicast session or multicast session will be dropped with equal chances, depending on their order of arrival. 2) Standard traffic priority: traffic coming down the L2TP unicast session has priority over traffic coming down the L2TP multicast session. Bourdon Expires January 2002 [Page 11] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 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: Standard traffic priority 2: Multicast traffic priority The AVP has the following format: Vendor ID = to be defined Attribute = GHI (Note: GHI 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | GHI | Priority Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ It is important to note that the multicast traffic rate can reach up to Maximum BPS (as indicated in OCRQ). 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 almost only multicast traffic: unicast packets might be dropped because of the multicast flow overwhelming 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. 6.2.1. Global configuration The Multicast Priority Packet AVP is sent for all L2TP unicast sessions concerned by a specific multicast group represented by an L2TP multicast session. In this case, the AVP is sent in a L2TP MSI control message for the related multicast session ID (Session ID = L2TP session for the related 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. 6.2.2. Individual configuration The Multicast Priority Packet AVP is sent for a specific L2TP unicast session concerned by applying specific behavior on unicast and Bourdon Expires January 2002 [Page 12] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 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. This is the only case when a MSI control message can be sent for a L2TP unicast session. 6.2.3. Priority It is an administrator prerogative to decide which behavior has to be applied 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 configuration SHOULD be taken into consideration in that case. 7. Multicast session teardown A L2TP multicast session should be torn down when 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 represented 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: 1) The threshold value used to determine if the L2TP multicast session has to be torn down is lower than the MULTICAST_SESSION_THRESHOLD value; 2) 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 by default, and SHOULD be configurable either by the LAC or LNS administrator. The multicast session can be torn down for multiple reasons, which may include specific criteria not described here (can be vendor- specific). A multicast session teardown can be initiated either by the LAC or the LNS. 7.1. Operations Bourdon Expires January 2002 [Page 13] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 The effective termination of a multicast session is initiated with a CDN control message, sent either by the LAC or by the LNS. The CDN message carries a Result Code AVP with an optional Error Code. The following AVPs MUST be present in multicast CDN: Message Type Result Code Assigned Session ID The following AVP SHOULD NOT be present in multicast CDN (because Q.931 Cause Codes make no sense in a multicast session context): Cause Code However, the Cause Code AVP MUST be ignored if received in this context. 7.2. Result Codes In order to make the Result Code AVP useful, the following values are defined as additional codes to those listed in [RFC2661], Section 4.4.2: xx - No multicast traffic for the group yy - No more receivers (Note: these are temporary numbers, may be changed later) o The code xx 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 yy may be used by the LAC or the LNS when the OSL is empty. As defined in [RFC2661], termination of a tunnel will terminate all sessions carried within, including multicast sessions if any. The Error Code field can be used within a CDN message, however no additional codes are defined here (can be defined later). 8. Traffic merging Both unicast and multicast traffic 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 a vendor implementation issue. All frames conveyed from the LAC to end-users have to follow the framing scheme applied for the considered peer to which the traffic Bourdon Expires January 2002 [Page 14] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 is intended (e.g. the LAC is always aware of PPP link parameters, as described in [RFC2661], Section 6.14). 9. IANA Considerations This document defines: - 1 Service Type value to be used in the Service Capabilities List AVP (see [SVCTYPE]). - 1 message type (MSI) - 3 AVPs (New Outgoing Sessions, Withdraw Outgoing Sessions, Multicast Packets Priority) - 2 result codes (No multicast traffic for the group, No more receivers) IANA will assign, register and maintain values for these new attributes. 10. Security Considerations This proposal does not introduce any additional issues as far as the activation of the L2TP protocol is concerned. However, activation of the L2TP multicast capability on a LAC could make the equipment more sensitive to Denial of Service attacks if the tunnel or the related LNS is hacked. By injecting appropriate control packets in the tunnel towards a LAC, final users could be flooded by unwanted multicast traffic. The LAC might also be sensitive to the burden generated by the additional replication work. 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. [IGMPv3] B. Cain, S. Deering, W. Fenner, I. Kouvelas, A. Thyagarajan, "Internet Group Management Protocol, Version 3", , Work in Progress, March 2001. Bourdon Expires January 2002 [Page 15] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 [L2TPBIS] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, B. Palter, "Layer 2 Tunneling Protocol "L2TP" ", , Work in Progress, November 2000. [PROXY] W. Fenner, "IGMP-based Multicast Forwarding ("IGMP proxying")", , Work in Progress, April 2001. [SVCTYPE] D. McPherson, S. Nanji, "L2TP Service Type", , Work in Progress, April 2001. 12. Acknowledgments Thanks to Christian Jacquenet for all the corrections done on this document and his precious advice, Pierre Levis for his contribution about IGMPv3 optimisation, Francis Houllier for PPP considerations and Xavier Vinet for his input about thresholds. 13. Author's Addresses Gilles Bourdon France Telecom R&D 38-40, rue du General Leclerc 92794 Issy les Moulineaux Cedex 9 - FRANCE Phone: +33 1 4529-4645 Email: gilles.bourdon@francetelecom.com Appendix A: IGMPv3 filters considerations Section 4.2 mentions some ways to handle IGMPv3 subscription reports. This appendix intends to describe more precisely what might be the decision process to create L2TP multicast session when the LNS has to deal with IGMPv3. Conceptually, there is no obligation for the LNS to proceed in a particular way more than another: the LNS decides what multicast sessions to open and how to forward them to appropriate users, and the LAC follows. Therefore, there is no risk of incompatibility between LAC and LNS implementations. Although, to remain consistent with what L2TP Multicast Extension intends to do, the multicast session opening process on the LNS MUST be respectful of the following precepts: a- End-users MUST NOT be impacted by usage of L2TP Multicast Extension procedures between the LAC and the LNS; b- Usage of L2TP Multicast Extension is done for bandwidth optimisation between the LAC and the LNS. A solution which logically Bourdon Expires January 2002 [Page 16] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 might break this rule MUST NOT be implemented, since it might impact negatively the L2TP service provided. IGMPv3 filters are applied to IGMP capable equipment interfaces getting reports from IGMPv3 capable end-users. Depending on the filter, the Designated Router (DR) initiate appropriate actions using the M-IGP. IGMPv3 gives the ability for users to subscribe to a group with either: - an explicit list of source addresses from which the user wants to get multicast traffic (INCLUDE mode); or - an explicit list of source addresses from which the user does not want to get multicast traffic (EXCLUDE mode); Of course, usual (*, G) reports are still possible with IGMPv3. Depending of the IGMPv3 reports received by a DR, the filter for an interface can be a complex combination of traffic to forward or to discard. The problem could have been simple for the L2TP case since each L2TP unicast session may be considered as a logical interface. However, an L2TP multicast session may aggregate all IGMP states for L2TP unicast session composing a L2TP tunnel. IGMPv3 filtering capabilities raise the problem on how to satisfy users who want to get multicast traffic for a group G coming from specific sources S, and users who want to get traffic for the same group G, but not from S. This means that the L2TP multicast session can not be seen only as a group G forwarding issue. The following sections give some idea of what is possible, each solution having its own advantages and drawbacks. A.1. Multicast sessions based on IGMPv3 filters This is the easiest solution to understand, and probably to implement. Each logical interface has its own IGMPv3 filter for downstream traffic on the LNS. If the MULTICAST_SESSION_THRESHOLD is reached with a set of matching IGMPv3 filters for logical interfaces belonging to the same L2TP tunnel, then the LNS creates an L2TP multicast session to convey the corresponding traffic. The issue with this solution is that it is likely to have the same multicast traffic conveyed several times in distinct sessions. Example: User A and B subscribe for sources S1 and S2 for group G1; User C and D subscribe for sources S1 and S3 for group G1. Then two multicast sessions would be created for ({S1, S2}, G1) and for ({S1, S3}, G1). (S1, G1) would be conveyed twice, but the bandwidth usage balance is still in favour of the L2TP Multicast Extension solution. A.2. Filter-based sessions and source-based sessions Bourdon Expires January 2002 [Page 17] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 To solve the problem raised in A.1, it is possible to make specific multicast sessions for (S, G), if the filter-mode is INCLUDE. If the filter-mode is EXCLUDE, the process would be the same as described in A.1. Example 1: Users A and B subscribe for sources S1 and S2 for group G1; Users C and D subscribe for sources S1 and S3 for group G1. Then three multicast sessions would be created for (S1, G1) (users A, B, C and D), for (S2, G1) (users A and B) and for (S3, G1) (users C and D). This creates more multicast sessions, but is efficient in term of bandwidth usage. Example 2: Users A and B subscribe to G1 but no traffic from source addresses S1 and S2 Users C and D subscribe to G1 but no traffic from source addresses S1 and S3 Then two multicast sessions would be created for (*-{S1, S2}, G1) and for (*-{S1, S3}, G1). Most of the (*, G1) traffic would be conveyed twice. A.3. Source-based sessions To solve the problem raised in A.2 example 2, a solution would be to create only (S, G) multicast sessions, even with an EXCLUDE filter- mode. INCLUDE filters would be handled as described in A.2. For any EXCLUDE filter-mode applied to a group G: upon reception of (S, G) traffic, the LNS creates an L2TP multicast session except if S is explicitly excluded by all the IGMPv3 filters for the tunnel. If all end-users for a tunnel are only reporting (*, G) interest, there would be no creation of (S, G) multicast sessions. The main risk with this solution is the creation of n multicast sessions if group G has n senders. The LNS may choose to trigger (S, G) multicast sessions creation only upon reception of the related traffic in INCLUDE mode: this can prevent from creating a multicast session if there is no related traffic. A.4. Optimal multicast sessions In order to have a reasonable number of multicast sessions created and an optimal bandwidth usage between the LAC and the LNS, a optimal set of (*-{Si}, G) and (Si, G) sessions can be achieved. - (*, G) interests would be satisfied by sending all (Si, G) sessions and the (*-{Si}, G) session to the interested users; - (*-{Sj}, G) interests would be satisfied by sending the right combination of "outgoing sessions" to the LAC for (Si, G) sessions and for the (*-{Si}, G) session; - (Si, G) interests would be naturally satisfied. Bourdon Expires January 2002 [Page 18] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 Although the optimal set of (*-{Si}, G) and (Si, G) sessions is calculable, it is unlikely to keep it a long time: user subscriptions would often change the existing states, probably leading to a perpetual instable set of multicast sessions. However, this instable behavior can be limited by privileging OSL changes rather than multicast session changes, as demonstrated by the next example. Session changes to obtain an optimal number of multicast sessions opened can be done an a periodic basis. This period time should be long enough to prevent multicast session instability. Example: Users A and B subscribe for sources S1 and S2 for group G1; Users C and D subscribe to G1 but no traffic from source addresses S1 and S3. Then three multicast sessions would be created (S1, G1) (users A and B), (S2, G1) (users A, B, C and D) and (*-{S1, S2, S3}, G1) (users C and D). If a user E report interest for (*, G1), a fourth multicast session (S3, G1) would be created. If users C and D report interest for (*, G1) afterwards, then there are two possibilities: 1- Delete the (S3, G1) session and change (*-{S1, S2, S3}, G1) for (*-{S1, S2}, G1), and add C and D to (S1, G1) OSL. 2- Keep the sessions, and just change the OSL for (S1, G1) and (S3, G1) to add users C and D. Even though solution 1 is optimal in term of opened multicast sessions, stability considerations give the advantage to solution 2. However, both solution are optimal in term of bandwidth usage. Full Copyright Statement "Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. Bourdon Expires January 2002 [Page 19] Internet Draft draft-ietf-l2tpext-mcast-00.txt July 2001 The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Bourdon Expires January 2002 [Page 20]