Multimob Working Group                                            H. Liu
Internet-Draft                                                M. McBride
Intended status: Informational                       Huawei Technologies
Expires: January 13, 2013                                  July 12, 2012

         IGMP/MLD Optimizations 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 [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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 13, 2013.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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 . . . . . . . . . . . . . . .  5
   3.  IGMP/MLD Optimization for Wireless and Mobile Networks . . . .  6
     3.1.  Switching Between Unicast and Multicast Queries  . . . . .  6
     3.2.  General Query Supplemented with Unicast Query  . . . . . .  6
     3.3.  Retransmission of Queries  . . . . . . . . . . . . . . . .  7
     3.4.  General Query Suppression  . . . . . . . . . . . . . . . .  7
     3.5.  Tuning Response Delay According to Link Type and Status  .  8
     3.6.  Triggering Reports and Queries Quickly During Handover . .  9
   4.  Applicability and Interoperability Considerations  . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   With the wide deployment of various wireless access techniques and
   the tendency to support video applications on wireless 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 under 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 different wireless link
   types and mobile scenarios, thus should be considered to be enhanced
   to be more applicable in these environments.

   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 and without introducing
   interoperability issues.  These solutions can also be applied in
   wired network when efficiency or reliability is required.

   For generality, this memo does not put limitations on the type of
   wireless techniques running below IGMP or MLD.  They could be
   cellular, WiMAX, WiFi and etc, and are modeled as different abstract
   link models as described in section 2.2.  Even though some of them
   (such as WiFi) have multicast limitations, it is probable that IGMP/
   MLD is enabled on the wireless terminal and multicast is supported
   across the network.  The mobile IP protocol adopted on the core side,
   upstream from the access router, could be PMIP, MIPv4, or MIPv6.

2.  Requirements

2.1.  Characteristics of Wireless and Mobile Multicast

   Several limitations 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

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   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
   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.

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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.  Currently the version in-use
   includes IGMPv2 [RFC2236] and its IPv6 counterpart MLDv1 [RFC2710],
   IGMPv3 [RFC3376] and its IPv6 counterpart MLDv2 [RFC3810], and LW-
   IGMPv3/MLDv2 [RFC5790].  All these versions have basic group
   management capability 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 excluding-source function.  Among these versions, (LW-)
   IGMPv3/MLDv2 has the capability of explicitly tracking each host

   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 in a
   short time interval may have the tendency to deteriorate wireless
   network conditions.  IGMP and MLD should be optimized to reduce the
   probability of packet burst.

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3.  IGMP/MLD Optimization for Wireless and Mobile Networks

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

3.1.  Switching Between Unicast and Multicast Queries

   IGMP/MLD protocol uses multicast Queries whose destinations are
   multicast addresses and also allows use of unicast Query with unicast
   destination to be sent only to one host.  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 [RFC6636].  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 Queries 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 terminal which is in 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 [RFC6636], the switching
   operation mostly applies to General Queries.

3.2.  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 these reports are not generated or
   generated but 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,

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   without affecting the majority of receivers that have already
   responded.  Unicast Queries under this condition could be sent at the
   end of the [Maximum Response Delay] after posting a General Query,
   and be retransmitted for [Last Member Query Count] times, at an
   interval of [Last Member Query Interval].

3.3.  Retransmission of Queries

   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 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.  The
   retransmission can also be applied to Group-Specific or Group-and-
   Source-Specific Query on a router without explicit tracking
   capability, when these Specific Queries cannot collect valid
   response, to prevent missing valid members caused by lost Queries and

   The above compensating Queries could be sent [Last Member Query
   Count] times, at the interval of [Last Member Query Interval], if the
   router cannot get any feedback from the receivers.

3.4.  General Query Suppression

   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 hosts 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 host 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

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   after sending Group-Specific or Group-and-Source-Specific Query.

   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 hosts on a
   link, suppressing it when it is not needed is beneficial for both the
   link efficiency and terminal power saving.

3.5.  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 latter of
   which is determined by the router and is carried in Query messages to
   inform the hosts for calculation of the response delay.  A larger
   value will lessen the burst better but will increase leave latency
   (the time taken to cease the traffic flowing after the last member
   requests the escaping of a channel).

   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, longer Maximum Response Delay is recommended; if the expected
   number of reporters is small and the link condition is good, smaller
   Maximum response Delay should be set.

   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.
   The measures to determine the instant value of Maximum Response Delay

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   are out of this document's scope.

3.6.  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.

   An access router could trigger a Query to a terminal as soon as it
   detects the terminal's attaching on its link.  This could be a
   General Query if the number of the entering terminals is not small
   (e.g when they are simultaneously in a moving train).  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'(3.1) and 'General Query Supplemented with Unicast
   Query'(3.2) require a router to know beforehand the valid members
   connected through an interface, thus require explicit tracking
   capability.  An IGMP/MLD implementation could choose any combination
   of the methods listed from 3.1 to 3.6 to optimize multicast
   communication on a specific wireless or mobile network.

   For example, an explicit-tracking IGMPv3 router, can switch to
   unicast General Queries if the number of members on a link is small
   (3.1), can trigger unicast Query to a previously valid receiver if
   failing to get expected responses from it (3.2), can retransmit a
   General Query if after the previous one cannot collect reports from
   all valid members (3.3), and can stop sending a General Query when
   the last member leaves the group (3.4), and etc.

   For interoperability, it is required 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

   Since the methods only involve the tuning of protocol behavior by
   e.g. retransmission, changing delay parameter, or other compensating
   operations, they do not introduce additional security weaknesses.
   The security consderations described in [RFC2236], [RFC3376],
   [RFC2710] and [RFC3810] can be reused.  And to achieve some security
   level in insecure wireless network, it is possible to take stronger
   security procedures during IGMP/MLD message exchange, which are out
   of the scope of this memo.

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.  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

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              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Version 2 (MLDv2) Protocols", RFC 5790,
              February 2010.

   [RFC6636]  Asaeda, H., Liu, H., and Q. Wu, "Tuning the Behavior of
              the Internet Group Management Protocol (IGMP) and
              Multicast Listener Discovery (MLD) for Routers in Mobile
              and Wireless Networks", RFC 6636, May 2012.

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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