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
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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.
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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
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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.
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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.
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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
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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
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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.
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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.
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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
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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.
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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.
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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
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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].
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6. Security Considerations
TBD.
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7. Acknowledgements
Marshall Eubanks, Gorry Fairhurst, Thomas C. Schmidt, Jinwei Xia, and
others provided many constructive and insightful comments.
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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.
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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/
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