Multimob Working Group                                            H. Liu
Internet-Draft                                                M. McBride
Intended status: Informational                       Huawei Technologies
Expires: September 3, 2012                                 March 2, 2012

         IGMP/MLD Optimization in Wireless and Mobile Networks


   This document proposes a variety of optimization approaches for IGMP
   and MLD in wireless and mobile networks.  It aims to provide useful
   guideline to allow efficient multicast communication in these
   networks using IGMP or MLD protocols.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on September 3, 2012.

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   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Characteristics of Wireless and mobile Multicast . . . . .  3
     2.2.  Wireless Link Model  . . . . . . . . . . . . . . . . . . .  4
     2.3.  Requirements on IGMP and MLD . . . . . . . . . . . . . . .  4
   3.  IGMP/MLD optimization for Wireless and Mobile Network  . . . .  5
     3.1.  Minimizing Query Frequency by increasing intervals . . . .  6
     3.2.  Switching Between Unicast and Multicast Queries  . . . . .  7
     3.3.  General Query Supplemented with Unicast Query  . . . . . .  7
     3.4.  Retransmission of General Query  . . . . . . . . . . . . .  8
     3.5.  General Query Supplemented with Unicast Query  . . . . . .  8
     3.6.  Tuning Response Delay according to link type and status  .  9
     3.7.  Triggering Reports and Queries quickly during handover . . 10
   4.  Applicability and interoperability considerations  . . . . . . 10
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   With the wide deployment of various wireless access techniques and
   the tendency to support video applications on these networks,
   wireless and mobile multicast come to attract more and more interests
   from content and service providers, but still face great challenges
   when considering dynamic group membership management and constant
   update of delivery path introduced by node movement, and high
   probability of loss and congestion due to limited reliability and
   capacity of wireless links.

   Multicast network is generally constructed by IGMP and MLD group
   management protocol (respectively for IPv4 and IPv6 networks) to
   track valid receivers and by multicast routing protocol to build
   multicast delivery paths.  This document focuses only on IGMP and
   MLD, which are used by a host to subscribe a multicast group and are
   most possibly to be exposed to wireless link to support terminal
   mobility.  As IGMP and MLD were designed for fixed users using wired
   link, they do not necessarily work well for all kinds of wireless
   link types and mobile scenarios, thus should be enhanced to be
   adapted to these environment.

   This memo proposes a variety of optimizations for IGMP and MLD in
   wireless and mobile networks to improve network performance, with
   minimum changes on the protocol behavior while without introducing
   interoperability issues.  These solutions can also be applied in
   wired network when efficiency or reliability is required.

2.  Requirements

2.1.  Characteristics of Wireless and mobile Multicast

   Several aspects should be considered when supporting IP multicast in
   wireless and mobile networks, including:

   O Limited link bandwidth: wireless link usually has limited
   bandwidth, and the situation will be made even worse if high volume
   video multicast data has to be carried.  Also the bandwidth available
   in the upstream and downstream directions may be asymmetrical.

   O High loss rate: wireless link usually has packet loss ranging from
   1% to 30% according to different links types and conditions.  Also
   when packets have to travel between home and access networks (e.g.
   through tunnel), they are prone to loss if the two networks are
   distant from each other.

   O Frequent membership change: in fixed multicast, membership change

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   only happens when a user leaves or joins a group, while in mobile
   scenario membership may also change when a user changes its location.

   O Prone to performance degradation: the possible increased
   interaction of protocols across layers for mobility management, and
   the limitation of link capacity, may lead to network performance
   degradation and even to complete connection loss.

   O Increased Leave Latency: the leave latency in mobile multicast
   might be increased due to user movement, especially if the traffic
   has to be transmitted between access and home networks, or if there
   is a handshake between networks.

2.2.  Wireless Link Model

   Wireless links can be categorized by their different transmission
   modes into three typical models: point-to-point (PTP), point-to-
   multipoint (PTMP), and broadcast link models.

   In PTP model, one link is dedicated for two communication facilities.
   For multicast transmission, each PTP link normally has only one
   receiver and the bandwidth is dedicated for that receiver.  Such link
   model may be implemented by running PPP on the link or having
   separate VLAN assignment for each receiver.  In mobile network,
   tunnel between entities of home and foreign networks should be
   recognized as a PTP link.

   PTMP is the model for multipoint transmission wherein there is one
   centralized transmitter and multiple distributed receivers.  PTMP
   provides common downlink channels for all receivers and dedicated
   uplink channel for each receiver.  Bandwidth downstream is shared by
   all receivers on the same link.

   Broadcast link can connect two or more nodes and supports broadcast
   transmission.  It is quite similar to fixed Ethernet link model and
   its link resource is shared in both uplink and downlink directions.

2.3.  Requirements on IGMP and MLD

   IGMP and MLD are usually run between mobile or wireless terminals and
   their first-hop access routers (i.e.home or foreign routers) to
   subscribe an IP multicast channel, and sometimes run between home
   router and access router (in PMIP case [RFC6224]) to proxy IGMP/MLD
   in mobile network the summarized IGMP/MLD reports from the latter to
   the former.  Currently the version in-use includes IGMPv2 [RFC2236]
   and its IPv6 counterpart MLDv1 [RFC2710], IGMPv3 [RFC3376] and its
   IPv6 counterpart MLDv2 [RFC3810], and lightweight IGMPv3/MLDv2
   [RFC5790].  All these versions have basic group management capability

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   required by a multicast subscription.  The differences lie in that
   IGMPv2 and MLDv1 can only join and leave a non-source specific group,
   while IGMPv3 and MLDv2 can select including and excluding specific
   sources for their join and leave operation, and LW-IGMPv3/MLDv2
   simplifies IGMPv3/MLDv2 procedures by discarding useless excluding-
   source function.  Among these versions, (LW-) IGMPv3/MLDv2 has the
   capability of explicit track each host member.

   From the illustration given in section 2.1 and 2.2, it is desirable
   for IGMP and MLD to have the following characteristics when used in
   wireless and mobile networks:

   o Adaptive to link conditions: wireless network has various link
   types, each with different bandwidth and performance features.  IGMP
   or MLD should be able to be adaptive to different link model and link
   conditions to optimize its protocol operation.

   o Minimal group Join/Leave latency: because mobility and handover may
   cause a user to join and leave a multicast group frequently, fast
   join and leave by the user helps to accelerate service activation and
   to release unnecessary resources quickly to optimize resource

   o Robust to packet loss: the unreliable packet transmission due to
   instable wireless link conditions and limited bandwidth, or long
   distance transmission in mobile network put more strict robustness
   requirement on delivery of IGMP and MLD protocol messages.

   o Reducing packet exchange: wireless link resources are usually more
   limited, precious, and congested compared to their wired counterpart.
   This requires packet exchange be minimized without degrading protocol

   o Packet burst avoidance: large number of packets generated within a
   short time interval may have the tendency to deteriorate wireless
   network conditions.  IGMP and MLD should be optimized if their
   protocol message generation has the potential of introducing packet

3.  IGMP/MLD optimization for Wireless and Mobile Network

   This section introduces several optimization methods for IGMP and MLD
   in wireless or mobile environment.  The aim is to meet the
   requirements described in section 2.3.  These measures could be
   applied between host and access routers in mobile or wireless
   network, and in mobile PMIPv6 [RFC6224] case, could also be used
   between access and home routers.  It should be noted that because an

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   enhancement in one direction might result in weakening effect in
   another, balances should be taken cautiously to realize overall
   performance elevation.

3.1.  Minimizing Query Frequency by increasing intervals

   In IGMP and MLD, Group-Specific Query and Group-and-Source-Specific
   Query are triggered on reception of a Leave message, and are sent for
   [Last Member Query Count] times with fixed [Last Member Query
   Interval], to learn whether there are valid members from an attached
   link.  If a network is undergoing congestion, multiple transmissions
   of the Queries as above may further deteriorate the conditions.  To
   avoid congestion or make remedy during congestion, these Queries can
   be slowed down when a router cannot collect successfully expected

   The slowing down process of the Queries could be arranged in a
   prolonged time interval as described in [ADAPTIVE].  The slowdown
   process should be: a router after sending a Query, if acquires the
   expected responses from the receivers, refreshes its state database
   and optionally stop the querying retransmission process, or if after
   a time interval fails to get the expected report responses, resends a
   Query with an increased (e.g. double) interval.  This process can be
   repeated, each time the retransmission is arranged in a progressively
   prolonged time interval, till the router receives the expected
   responses, or determines the receiver is unreachable and stops the
   sending of the Query.

   Query slowdown can be applied in the cases as listed in the following

   O When Group-Specific Query and Group-and-Source-Specific Query are
   used to track other members but cannot get any response.

   O When all group members leave a group or move out of scope, the
   General Query sent by the router cannot solicit any response on a
   network, as illustrated in section 3.4.

   O When General Query is retransmitted due to possible loss or
   congestion deducing from no responses from valid members in the
   database, on the primes that explicit tracking is used.

   O When General Query is retransmitted by a router on startup but no
   response can be acquired.

   O When unicast Query is sent to query a particular valid receiver,
   from whom no response could be collected, as described in section 3.2
   and 3.3.  This requires explicit tracking to be enabled.

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   Query retransmission with incremental interval enables the router to
   reduce the total query-response times and consequently the packet
   count.  The variable time interval and the termination condition of
   retransmission should be configurable and could be set according to
   actual network condition, which is out the scope of this document.

3.2.  Switching Between Unicast and Multicast Queries

   IGMP/MLD protocols use multicast Queries whose destination addresses
   are multicast addresses and also allow use of unicast Query with
   unicast destination to be sent only for one destination.  Unicast
   Query has the advantage of not affecting other hosts on the same
   link, and is desirable for wireless communication because a mobile
   terminal often has limited battery power.  But if the number of valid
   receivers is large, using unicast Query for each receiver is
   inefficient because large number of Unicast Queries have to be
   generated, in which situation normal multicast Query will be a good
   choice because only one General Query is needed.  If the number of
   receivers to be queried is small, unicast Query is advantageous over
   the multicast one.

   More flexibly, the router can choose to switch between unicast and
   multicast Query according to the practical network conditions.  For
   example, if the receiver number is small, the router could send
   unicast Queries respectively to each receiver, without arousing other
   non-member host which is in the dormant state.  When the receiver
   number reaches a predefined level, the router could change to use
   multicast Queries.  To have the knowledge of the number of the valid
   receivers, a router is required to enable explicit tracking, and
   because Group-Specific Query and Group-and-Source-Specific Query are
   usually not used under explicit tracking, the switching only applies
   to General Queries.

3.3.  General Query Supplemented with Unicast Query

   Unicast Query also can be used in assistance to General Query to
   improve the robustness of solicited reports when General Query fails
   to collect all of its valid members.  It requires the explicit
   tracking to be enabled and can be used when a router after sending a
   periodical General Query collects successfully most of the valid
   members' responses while losing some of which are still valid in its
   database.  This may be because the non-respondent ones silently leave
   the network without any notification, or because their reports are
   lost for some unknown reasons.  The router could choose to unicast a
   Query respectively to each non-respondent valid receiver to check
   whether they are still alive for the multicast reception, without
   affecting the majority of receivers that have already responded.
   Unicast Queries under this condition could be sent at the end of the

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   [Maximum Response Delay] after posting a General Query, and be
   retransmitted for [Last Member Query Count] times, at a constant
   interval, or at incremental interval as described in section 3.1.

3.4.  Retransmission of General Query

   In IGMP and MLD, apart from the continuously periodical transmission,
   General Query is also transmitted during a router's startup.  It is
   transmitted for [Startup Query Count] times by [Startup Query
   Interval].  There are some other cases where retransmission of
   General Query is beneficial which are not covered by current IGMP and
   MLD protocols as shown as following.

   For example, a router which keeps track of all its active receivers,
   if after sending a General Query, fails to get any response from the
   receivers which are still valid in its membership database.  This may
   be because all these valid receivers have left the group silently or
   moved out of range, or all the responses of the receivers happen to
   be lost, or the sent Query does not arrive at the other side of the
   link to the receivers.  The router could compensate this situation by
   retransmitting the General Query to solicit its active members.

   This compensating General Query could be sent several times, if the
   router cannot get any feedback from the receivers which are valid in
   the database.  The repetition of the transmission could be in fixed
   interval, or in prolonged interval as described in section 3.1.  The
   method can also be applied to router without explicit tracking
   enabled, in which case General Query is retransmitted if no valid
   response can be collected for a group or source-specific group which
   previously has valid reception state.

3.5.  General Query Supplemented with Unicast Query

   In IGMP and MLD, General Query is sent periodically and continuously
   without any limitation.  It helps soliciting the state of current
   valid member but has to be processed by all terminals on the link,
   whether they are valid multicast receivers or not.  When there is no
   receiver, the transmission of the General Query is a waste of
   resources for both the terminals and the router.

   An IGMP/MLD router could suppress its transmission of General Query
   if it knows there is no valid multicast receiver on an interface,
   e.g. in the following cases:

   O When the last member reports its leave for a group.  This could be
   judged by an explicit tracking router checking its membership
   database, or by a non-explicit-tracking router getting no response
   after sending Group-Specific Query or Group-and-Source-Specific Query

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   O When the only member on a PTP link reports its leaving

   O When a router after retransmitting General Queries on startup fails
   to get any response

   O When a router previously has valid members but fails to get any
   response after several rounds of General Queries.

   In these cases the router could make the decision that no member is
   on the interface and totally stop its transmission of periodical
   General Queries.  If afterwards there is any valid member joins a
   group, the router could resume the original cycle of general
   Querying.  Because General Query has influences on all terminals on a
   link, suppressing it when it is not needed is beneficial for both the
   link efficiency and terminal power saving.

3.6.  Tuning Response Delay according to link type and status

   IGMP and MLD use delayed response to spread unsolicited Reports from
   different hosts to reduce possibility of packet burst.  This is
   implemented by a host responding to a Query in a specific time
   randomly chosen between 0 and [Maximum Response Delay].  The value of
   [Maximum Response Delay] is determined by the router and is carried
   in Query messages to inform the hosts for the calculation.  A larger
   value will lessen the burst better but will increase leave latency
   (the time between the last listener request escaping a channel and
   the traffic actually ceases flowing).

   In order to avoid message burst and reduce leave latency, the
   Response Delay may be dynamically calculated based on the expected
   number of responders, and link type and status, as shown in the

   O If the expected number of reporters is large and link condition is
   bad, the system administrator MUST choose the longer Maximum Response
   Delay; if the expected number of reporters is small and the link
   condition is good, smaller Maximum response Delay should be set.  In
   this way, the IGMP/MLD packet burst can be reduced.

   o If the link type is PTP, the Maximum Response Delay can be chosen
   smaller, whereas if the link is PTMP or broadcast medium, the Maximum
   Response Delay can be configured larger.

   The Maximum Response Delay could be configured by the administrator
   as mentioned above, or be calculated automatically by a software tool
   implemented according to experiential model for different link modes.
   How to determine the instant value of Maximum Response Delay is out
   of this document's scope.

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3.7.  Triggering Reports and Queries quickly during handover

   When a mobile terminal is moving from one network to another, if it
   is receiving multicast content, its new access network should try to
   deliver the content to the receiver without disruption or performance
   deterioration.  In order to implement smooth handover between
   networks, the terminal's membership should be acquired as quickly as
   possible by the new access network.

   The access router could trigger a Query to the terminal as soon as it
   detects a new terminal on its link.  This could be a General Query if
   the number of the entering terminals is not small.  Or this Query
   could also be a unicast Query for this incoming terminal to prevent
   unnecessary action of other terminals in the switching area.

   For the terminal, it could send a report immediately if it is
   currently in the multicast reception state, when it begins to connect
   the new network.  This helps establishing more quickly the membership
   state and enable faster multicast stream injection, because with the
   active report the router does not need to wait for the query period
   to acquire the terminal's newest state.

4.  Applicability and interoperability considerations

   Among the optimizations listed above, 'Switching between unicast and
   multicast Queries'(3.2) and 'General Query Supplemented with Unicast
   Query'(3.3) require a router to know beforehand the valid members
   connected through an interface, thus require explicit tracking
   capability.  "Minimizing Query Frequency by increasing intervals"
   (3.1) is only meaningful when for the same network condition the
   retransmission count for a fixed interval is not small (more than the
   default value of 2).  An IGMP/MLD implementation could choose any
   combination of the methods listed from 3.1 to 3.7 to optimize
   multicast communication on a specific wireless or mobile network.

   For example, an explicit-tracking IGMPv3 router, can switch to
   unicast General Query if the number of members on a link is small
   (3.3), can trigger unicast Query to a previously valid receiver if
   failing to get expected responses from it (3.3), can retransmit a
   General Query if after the previous one cannot collect reports from
   valid members (3.4), and can stop sending a General Query when the
   last member leaves the group (3.5), and etc.

   For interoperability, it is suggested if multiple multicast routers
   are connected to the same network for redundancy, each router are
   configured with the same optimization policy to synchronize the
   membership states among the routers..

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5.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

6.  Security Considerations

   They will be described in the later version of this draft.

7.  Acknowledgements

   The authors would like to thank Qin Wu, Stig Venaas, Gorry Fairhurst,
   Thomas C. Schmidt, Marshall Eubanks, Suresh Krishnan, J.William
   Atwood, WeeSan Lee, Imed Romdhani, Hitoshi Asaeda, Liu Yisong and Wei
   Yong for their valuable comments and suggestions on this document.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, November 1997.

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              October 1999.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC5790]  Liu, H., Cao, W., and H. Asaeda, "Lightweight Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Version 2 (MLDv2) Protocols", RFC 5790,
              February 2010.

   [RFC6224]  Schmidt, T., Waehlisch, M., and S. Krishnan, "Base

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              Deployment for Multicast Listener Support in Proxy Mobile
              IPv6 (PMIPv6) Domains", RFC 6224, April 2011.

8.2.  Informative References

              Romdhani, I., Bettahar, H., and A. Bouabdallah, "Adaptive
              Multicast Membership Management for Mobile Multicast
              Receivers", IEEE , 2000.

Authors' Addresses

   Hui Liu
   Huawei Technologies
   Building Q14, No.156, Beiqing Rd.
   Beijing  100095


   Mike McBride
   Huawei Technologies
   2330 Central Expressway
   Santa Clara  CA 95050


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