MULTIMOB Group H. Asaeda Internet-Draft Keio University Expires: January 7, 2010 July 6, 2009 IGMP and MLD Optimization for Mobile Hosts and Routers draft-asaeda-multimob-igmp-mld-optimization-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. 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. This Internet-Draft will expire on January 7, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of Asaeda Expires January 7, 2010 [Page 1] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Asaeda Expires January 7, 2010 [Page 2] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 Abstract To notify neighboring multicast routers of their IP multicast group memberships, hosts must support IGMP and MLD protocols. This document describes the ways of IGMPv3 and MLDv2 protocol optimization for mobility. The optimization includes a query timer tuning and an explicit membership notification operation. Asaeda Expires January 7, 2010 [Page 3] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 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 RFC 2119 [1]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Optimization . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Tracking of Membership Status . . . . . . . . . . . . . . 7 2.2. IGMP/MLD Query Coordination . . . . . . . . . . . . . . . 7 2.2.1. Unicasting IGMP/MLD General Query . . . . . . . . . . 7 2.2.2. Multicasting IGMP/MLD Group-Specific Query . . . . . . 8 2.3. IGMP/MLD Querier Selection . . . . . . . . . . . . . . . . 9 2.4. Multicast Source Filter . . . . . . . . . . . . . . . . . 9 3. Explicit Membership Notification . . . . . . . . . . . . . . . 11 4. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 13 5. Timers, Counters, and Their Default Values . . . . . . . . . . 14 6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 8. Normative References . . . . . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 Asaeda Expires January 7, 2010 [Page 4] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 1. Introduction The Internet Group Management Protocol (IGMP) [2] for IPv4 and the Multicast Listener Discovery Protocol (MLD) [3] for IPv6 are the standard protocols for hosts to initiate joining or leaving multicast sessions. These protocols must be also supported by multicast routers or IGMP/MLD proxies [7] that serve multicast member hosts on their downstream interfaces. Conceptually, IGMP and MLD work on wireless networks. However, wireless access technologies operate on a shared medium or a point-to-point link with limited frequency and bandwidth. In many wireless regimes, it is desirable to minimize multicast-related signaling to preserve the limited resources of battery powered mobile devices and the constrained transmission capacities of the networks. A mobile host may cause initiation and termination of a multicast service in the new or the previous network. Slow multicast service activation following a join may degrade reception quality. Slow service termination triggered by IGMP/MLD querying or by a rapid departure of the mobile host without leaving the group in the previous network may waste network resources. To create the optimal condition for mobile hosts and routers, it is required to "ease processing cost or battery power consumption by eliminating transmission of a large number of IGMP/MLD messages via flooding" and "realize fast state convergence by successive monitoring whether downstream members exist or not". One of the possible approaches to support these requirements is that multicast routers enable the "explicit tracking function". According to the specification of IGMPv3 [2] and MLDv2 [3], the downstream IGMPv3 or MLDv2 member hosts must send their membership reports to the upstream router upon data reception. This enables the router to keep track of the membership status of the downstream IGMPv3 or MLDv2 member hosts. On the other hand, routers still need to maintain downstream membership status by sending IGMPv3/MLDv2 query messages due to the following reasons. o IGMP/MLD messages are non-reliable and may be lost in the transmission, therefore routers need to confirm the membership by sending query messages. o Routers need additional processing capability and a possibly large memory to keep track of membership status, and therefore the routers usually disable the function for keeping track of membership status. Asaeda Expires January 7, 2010 [Page 5] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 o To preserve compatibility with older versions of IGMP/MLD, routers need to support downstream hosts that are not upgraded to the latest versions of IGMP/MLD and run the report suppression mechanism. This document describes the ways of IGMPv3 and MLDv2 protocol optimization for mobility. The optimization includes a query timer tuning and an explicit membership notification operation. The selective optimization that provides tangible benefits to the mobile hosts and routers is given by "keeping track of downstream hosts' membership status", "varying IGMP/MLD Query types and values to tune the number of responses", and "using a source filtering mechanism in a lightweight manner". Aside from a modified protocol semantic, optional "Notification function" for the IGMPv3 and MLDv2 protocols is introduced. The proposed optimization interoperates with the IGMPv3 and MLDv2 protocols. This condition is advantageous to the deployment. Asaeda Expires January 7, 2010 [Page 6] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 2. Optimization 2.1. Tracking of Membership Status Mobile hosts use IGMP and MLD to request to join or leave multicast sessions. When the upstream routers receive the IGMP/MLD reports, they recognize the membership status on the LAN. To update the membership status, the routers send IGMP/MLD Query messages periodically as a soft-state approach does, and the member hosts reply IGMP/MLD report messages upon reception. IGMP/MLD Query is therefore necessary to obtain the up-to-date membership information, but a large number of the reply messages sent from all member hosts may cause network congestion or consume network bandwidth. To escape from the trouble, a membership report suppression mechanism was proposed in the traditional IGMP and MLD [4][5][6]. By the report suppression mechanism, a host would cancel sending a pending membership reports requested by IGMP/MLD Query if a similar report was observed from another member on the network. However, the report suppression mechanism precluded the function for an upstream router to track membership status. In IGMPv3 and MLDv2, it is hence decided that the membership report suppression mechanism has been removed, and all downstream member hosts must send their membership reports to an upstream router. 2.2. IGMP/MLD Query Coordination 2.2.1. Unicasting IGMP/MLD General Query IGMP and MLD are non-reliable protocols; to cover the possibility of a State-Change Report being missed by one or more multicast routers, a host retransmits the same State-Change Report [Robustness Variable] - 1 more times, at intervals chosen at random from the range (0, [Unsolicited Report Interval]) [2][3]. However, this manner does not guarantee that the State-Change Report is reached to the routers. The routers therefore need to refresh the downstream membership information by receiving Current-State Report periodically solicited by IGMP/MLD General Query, in order to be robust in front of host or link failures and packet loss. It supports the situation that mobile hosts turn off or move from the wireless network to other wireless network managed by the different router without any notification (e.g., leave request). A multicast router periodically transmits IGMP/MLD General Query in the [Query Interval] sec. The default value of [Query Interval] is 125 sec. specified in the standard IGMPv3 and MLDv2 specifications [2][3]. Unfortunately, periodical message flooding using the all- hosts multicast address (i.e. 224.0.0.1 or ff02::1) as its IP Asaeda Expires January 7, 2010 [Page 7] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 destination address gives the unwilling situation to the mobile hosts. It consumes the bandwidth of a wireless link and may wake all mobile hosts up by IGMP/MLD General Query. In fact, when mobile hosts are operating in dormant mode and not communicating with others, they should keep sleeping for saving the battery power. In this case, only the hosts that are receiving multicast contents should make the response to the router. A multicast router attached on a wireless link may want to configure longer query interval as the [Multicast Query Interval] value Section 5, in order to reduce the number of IGMP/MLD General Query messages via multicast. Yet, longer query interval will increase join latency when an unsolicit Join message with State-Change Record requesting joining a multicast session is not reached to the router, or it will increase leave latency when an unsolicit Leave message with State-Change Record requesting leaving a session is not reached to the router. IGMPv3 and MLDv2 specifications [2][3] mention that a host MUST accept and process any Query whose IP Destination Address field contains any of the addresses (unicast or multicast) assigned to the interface on which the Query arrives. According to the scenario, a router can unicast the message to tracked member hosts in the [Unicast Query Interval] (described in Section 5), especially when a multicast router has a small number of mobile hosts that are listening different multicast sessions. In this case, the router multicasts IGMP/MLD General Query with longer [Multicast Query Interval] (described in Section 5) to recognize hosts that were not tracked. 2.2.2. Multicasting IGMP/MLD Group-Specific Query In the standard protocols [2][3], an IGMP/MLD Group-Specific Query is sent to verify there are no hosts that desire reception of the specified group or to rebuild the desired reception state for a particular group. The Group-Specific Query is sent when a router receives State-Change Records indicating a host is leaving a group, and never in response to Current-State Records. A Group-Specific Query builds and refreshes group membership state of hosts on attached networks. Since a Group-Specific Query specifies the corresponding multicast address (not the all-hosts multicast address) as its IP destination address, dormant mode hosts that do not join any multicast session are not woken up by these specific Queries, and only active group member hosts that have been receiving multicast contents with the specified address reply IGMP/MLD reports. However, sending many Group-Specific Queries for all corresponding Asaeda Expires January 7, 2010 [Page 8] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 groups may increase the total number of transmitted IGMP/MLD messages. [TODO: Therefore, it is necessary to know the condition in which a Group-Specific Query is used for maintaining the group membership state of all hosts on a link, instead of a General Query.] The [Multicast Group-Query Interval] is the interval between Group- Specific Queries sent by the querier, i.e., the router that sends the Group-Specific Query. [TODO: Define [Multicast Group-Query Interval]. We currently think this value is same of the default [Query Interval] value the regular IGMP and MLD define [2][3].] 2.3. IGMP/MLD Querier Selection [TODO: To consider the case that multiple multicast routers exist in a single wireless link, do we need to propose a new IGMP/MLD querier selection mechanism and the corresponding timer values or intervals? The Querier's Query Interval Code (QQIC) field that specifies the [Query Interval] used by the querier may be tuned. The actual interval, called the Querier's Query Interval (QQI), is derived from QQIC. Multicast routers that are not the current querier adopt the QQI value from the most recently received Query as their own [Query Interval] value.] 2.4. Multicast Source Filter IGMPv3 and MLDv2 provide the ability for hosts to report source- specific subscriptions. With IGMPv3/MLDv2, a mobile host can specify a channel of interest, using multicast group and source addresses with INCLUDE filter mode in its join request. Upon reception, the upstream router establishes the shortest path tree toward the source without coordinating a shared tree. This function is called the source filtering function and required to support Source-Specific Multicast (SSM) [8]. IGMPv3 and MLDv2 support another operation with EXCLUDE filter mode. When a mobile host specifies multicast and source addresses with EXCLUDE filter mode in the join request, an upstream router forwards the multicast packets sent from all sources *except* the specified sources. However, practical applications do not use EXCLUDE mode to block sources very often, because a user or application usually wants to specify desired source addresses, not undesired source addresses. In addition, this scheme leads an implementation cost to mobile hosts and complex procedures to maintain coexisting situation of the interesting source address lists with INCLUDE filter mode or non- interesting source address lists with EXCLUDE filter mode. Asaeda Expires January 7, 2010 [Page 9] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 Furthermore, specifying non-interesting source addresses with EXCLUDE filter mode reduces the advantage of scalable routing tree coordination produced by SSM. An upstream router needs to maintain a shared tree (e.g., RPT in PIM-SM) whenever the router receives join request with EXCLUDE filter mode from the downstream hosts. This increases the tree maintenance cost to the multicast routers on the routing paths. While the mobile multicast communication does not prohibit a traditional (*,G) join request (which is a join request with EXCLUDE filter mode without specifying any source address), all other join requests with EXCLUDE filter mode should be eliminated from the mobile multicast communication. Recently, Lightweight-IGMPv3 (LW-IGMPv3) and Lightweight-MLDv2 (LW- MLDv2) [9] are proposed in the IETF MBONED working group. These protocols are the simplified versions of IGMPv3 and MLDv2, and eliminate an EXCLUDE filter mode operation. Not only are LW-IGMPv3 and LW-MLDv2 fully compatible with the full version of these protocols (i.e., the standard IGMPv3 and MLDv2), but also the protocol operations made by hosts and routers are simplified in the lightweight manner, and complicated operations are effectively reduced. LW-IGMPv3 and LW-MLDv2 give the opportunity to grow SSM use. In the lightweight protocols, EXCLUDE mode on the host part is preserved only for EXCLUDE (*,G) join/leave, which denotes a non- source-specific group report (known as the traditional (*,G) join/ leave or Any-Source Multicast (ASM)) and is equivalent to the group membership join/leave triggered by IGMPv2/IGMPv1/MLDv1. This document hence recommend to adopt LW-IGMPv3 and LW-MLDv2 to mobile hosts and routers, or eliminate EXCLUDE filter mode operation from mobile hosts if IGMPv3 and MLDv2 are adopted to the hosts. Asaeda Expires January 7, 2010 [Page 10] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 3. Explicit Membership Notification This document proposes an IGMP/MLD Notification operation, in which a mobile host *periodically* sends Current-State Record messages expressing which multicast sessions the host is joining, even the host is not requested to report the membership information by its upstream router (i.e., no reception of General Query message). The IGMP/MLD Notification operation enables the longer [Multicast Query Interval] value for IGMP/MLD General Query than the default [Query Interval]. If mobile hosts support the IGMP/MLD Notification operation, a multicast router can obtain downstream membership information without periodical and spontaneous membership solicitation by IGMP/MLD General Query. The router only needs to refresh downstream membership information by solicit IGMP/MLD General Query to the hosts that do not support the IGMP/MLD Notification operation or leave from network without sending any message to the router. If timers are tuned by dynamic nature of membership, the IGMP/MLD Notification operation reduces the number of IGMP/MLD General Query periodically sent by a router and the total number of IGMP/MLD messages. Since a router only needs to refresh downstream membership information by solicit General Query to hosts that do not support the Notification operation, both [Unicast Query Interval] and [Multicast Query Interval] can be set to longer values. This mechanism may conserve battery power of dormant mode hosts, as dormant mode hosts do not pay attention to the General Query messages at short intervals. The IGMP/MLD Notification operation also contributes to fast handover, because a host receiving data immediately sends unsolicited reports without waiting for IGMP/MLD General Query at the new network. The [Notification Interval] value (described in Section 5) is the interval of Current-State Records periodically sent by a member host that joins at least one multicast session. Since a mobile host periodically unicasts Current-State Record in [Notification Interval] that is shorter than the regular General Query interval (i.e. [Query Interval] value) and [Multicast Query Interval] and [Unicast Query Interval], even if a router tracking membership status misses State- Change report that requests a leave operation, the router can operate a leave procedure faster than the regular case. When mobile hosts receive IGMP/MLD General Query, they reset their [Notification Interval] timer value and restart it. When a multicast router works with the Notification operation, it Asaeda Expires January 7, 2010 [Page 11] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 must maintain the following information for each multicast session to recognize receiver host's membership status; 1 Receiver address - indicates an address of a receiver host sending the Current-State Report. 2 Last membership report - indicates the time that the router receives the last Current-State Report. 3 Filter mode - indicates either INCLUDE or EXCLUDE as defined in [2][3]. 4 Source addresses and multicast address - indicates the address pair that the receiver joins. Asaeda Expires January 7, 2010 [Page 12] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 4. Interoperability This document assumes multicast routers that deal with mobile hosts MUST be IGMPv3/MLDv2 capable (regardless whether the protocols are the full or lightweight version). Therefore all interoperability conditions are inherited from [2][3][9], and this document does not need to consider interoperability with older version protocols. An IGMP/MLD Notification operation is a simple optimization for mobile hosts to spontaneously send IGMP/MLD Current-State Report to their upstream multicast routers. Since a multicast router solicits downstream membership information by IGMP/MLD General Query, non- upgraded mobile hosts can coexist in the network. However, join and leave latency for non-upgraded mobile hosts may become longer due to the longer [Query Interval] timer configuration for multicast routers. Note that the IGMP/MLD Notification operation does not require any modification to multicast routers. Asaeda Expires January 7, 2010 [Page 13] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 5. Timers, Counters, and Their Default Values A multicast router operating in dormant mode keeps track of the membership status and checks the membership status by transmitting unicast IGMP/MLD General Query or multicast IGMP/MLD Group-Specific Query. Cooperating with these scenarios, the message interval between IGMP/MLD General Queries is set to longer than the default [Query Interval] value. The [Query Interval] is the interval between General Queries sent by the regular IGMPv3/MLDv2 querier, and the default value is 125 seconds [2][3]. By varying the [Query Interval], multicast routers can tune the number of IGMP messages on the network; larger values cause IGMP Queries to be sent less often. [TODO: We will provide the appropriate [Multicast Query Interval] value that would fit in the mobile communication environment based on some experimental results. In our current sense, this value should be larger than the default [Query Interval] value the regular IGMPv3 and MLDv2 define.] The Query Response Interval is the Max Response Time (or Max Response Delay) used to calculate the Max Resp Code inserted into the periodic General Queries, and the default value is 10 seconds [2][3]. By varying the [Query Response Interval], multicast routers can tune the burstiness of IGMP messages on the network; larger values make the traffic less bursty, as host responses are spread out over a larger interval. [TODO: We will provide the appropriate [Query Response Interval] value that would fit in the mobile communication environment based on some experimental results. In our current sense, this value should be less than the default value the regular IGMP and MLD define, because, while the larger Query Interval does not reduce the number of transmitted IGMP/MLD messages, it may cause slow leave latency.] Mobile hosts may receive a variety of Queries on different interfaces and of different kinds (e.g., General Queries, Group-Specific Queries, and Group-and-Source-Specific Queries), each of which may require its own delayed response. [TODO: The timer management for each queries may or should be independent. E.g. the timer value for General Query should be longer than the one of other queries. We will investigate this issue.] To cover the possibility of unsolicited reports being missed by multicast routers, unsolicited reports are retransmitted [Robustness Variable] - 1 more times, at intervals chosen at random from the Asaeda Expires January 7, 2010 [Page 14] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 defined range [2][3]. The QRV (Querier's Robustness Variable) field in IGMP/MLD Query contains the [Robustness Variable] value used by the querier. Routers adopt the QRV value from the most recently received Query as their own [Robustness Variable] value, whose range SHOULD be set between "1" to "7". While the default [Robustness Variable] value defined in IGMPv3 [2] and MLDv2 [3] is "2", the [Robustness Variable] value announced by the querier MUST NOT be "0" and SHOULD NOT be "1". [TODO: We will propose the robustness values that would be adjusted according to the number of receivers. In our current sense, this value SHOULD NOT be bigger than "2" especially when the [Query Response Interval] is set to less than its default value.] The default [Unicast Query Interval] value is 90 sec. The default [Multicast Query Interval] value is 180 sec. The default [Notification Interval] value is 60 sec. The [Notification Interval] value MUST be shorter than [Multicast Query Interval] and [Unicast Query Interval]. Asaeda Expires January 7, 2010 [Page 15] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 6. Security Considerations TBD. Asaeda Expires January 7, 2010 [Page 16] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 7. Acknowledgements Marshall Eubanks, Gorry Fairhurst, Thomas C. Schmidt, Jinwei Xia, and others provided many constructive and insightful comments. Asaeda Expires January 7, 2010 [Page 17] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 8. Normative References [1] Bradner, S., "Key words for use in RFCs to indicate requirement levels", RFC 2119, March 1997. [2] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. Thyagarajan, "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. [3] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June 2004. [4] Deering, S., "Host Extensions for IP Multicasting", RFC 1112, August 1989. [5] Fenner, W., "Internet Group Management Protocol, Version 2", RFC 2373, July 1997. [6] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener Discovery (MLD) for IPv6", RFC 2710, October 1999. [7] Fenner, B., He, H., Haberman, B., and H. Sandick, "Internet Group Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")", RFC 4605, August 2006. [8] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP", RFC 4607, August 2006. [9] Liu, H., Cao, W., and H. Asaeda, "Lightweight IGMPv3 and MLDv2 Protocols", draft-ietf-mboned-lightweight-igmpv3-mldv2-05.txt (work in progress), May 2009. [10] Asaeda, H. and T. Schmidt, "IGMP and MLD Hold and Release Extensions for Mobility", draft-asaeda-multimob-igmp-mld-mobility-extensions-03.txt (work in progress), July 2009. Asaeda Expires January 7, 2010 [Page 18] Internet-Draft IGMP and MLD Optimization for Mobility July 2009 Author's Address Hitoshi Asaeda Keio University Graduate School of Media and Governance 5322 Endo Fujisawa, Kanagawa 252-8520 Japan Email: asaeda@wide.ad.jp URI: http://www.sfc.wide.ad.jp/~asaeda/ Asaeda Expires January 7, 2010 [Page 19]