MULTIMOB Working Group H. Asaeda
Internet-Draft Y. Uchida
Expires: August 27, 2011 Keio University
H. Liu
Q. Wu
Huawei Technologies
February 23, 2011
Tuning the Behavior of IGMP and MLD for Mobile Hosts and Routers
draft-asaeda-multimob-igmp-mld-optimization-05
Abstract
IGMP and MLD are the protocols used by hosts to report their IP
multicast group memberships to neighboring multicast routers. This
document describes the ways of IGMPv3 and MLDv2 protocol optimization
for mobility, and aims to become a guideline for query and other
timers tuning.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Explicit Tracking of Membership Status . . . . . . . . . . . . 5
4. Tuning IGMP/MLD Timers and Values . . . . . . . . . . . . . . 6
4.1. Tuning IGMP/MLD General Query Interval . . . . . . . . . . 6
4.2. Tuning IGMP/MLD Query Response Interval . . . . . . . . . 6
4.3. Tuning Last Member Query Timer (LMQT) and Last
Listener Query Timer (LLQT) . . . . . . . . . . . . . . . 7
4.4. Tuning Startup Query Interval . . . . . . . . . . . . . . 8
4.5. Tuning Robustness Variable . . . . . . . . . . . . . . . . 8
5. Destination Address of Specific Query . . . . . . . . . . . . 9
6. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Unicasting General Query . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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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 [11] that maintain multicast membership
information 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 upon its movement. 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 multicast membership management condition, IGMP
and MLD protocols could be tuned to "ease a mobile host's processing
cost or battery power consumption by IGMP/MLD Query transmission
timing coordination by routers" and "realize fast state convergence
by successive monitoring whether downstream members exist or not".
This document describes the ways of tuning the IGMPv3 and MLDv2
protocol behavior for mobility, including query and other timers
tuning. The selective optimization that provides tangible benefits
to the mobile hosts and routers is given by keeping track of
downstream hosts' membership status and varying IGMP/MLD Query types
and values to tune the number of responses. The proposed behavior
interoperates with the IGMPv3 and MLDv2 protocols.
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2. Terminology
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].
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3. Explicit Tracking of Membership Status
Mobile hosts use IGMP and MLD to request to join or leave multicast
sessions. When the adjacent upstream routers receive the IGMP/MLD
Report messages, they recognize the membership status on the link.
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.
The "explicit tracking function" [9] is the possible approach to
reduce the transmitted number of IGMP/MLD messages and contribute to
mobile communications. It enables the router to keep track of the
membership status of the downstream IGMPv3 or MLDv2 member hosts.
The explicit tracking function reduces the chance of Group-Specific
or Group-and-Source Specific Query transmission. Whenever a router
that does not enable the explicit tracking function receives the
State-Change Report and the router's membership state is changed to
block some source or group, it sends the corresponding Group-Specific
or Group-and-Source Specific Query messages to confirm whether the
Report sender is the last member host or not. However, if a router
enables the explicit tracking function, it does not always need to
ask Current-State Report message transmission to the receiver hosts
since the router recognizes the (potential) last member host when it
receives the State-Change Report. The router can therefore send
IGMP/MLD Group-Specific and Group-and-Source Specific Queries LMQC/
LLQC times (see Section 4.3 for LMQC/LLQC) only when it recognizes
the last member has left from the network. This reduces the
transmitted number of Current-State Report messages.
Enabling the explicit tracking function is advantageous for mobile
multicast, but the function requires additional processing capability
and a possibly large memory for routers to keep all membership
status. Especially when a router needs to maintain a large number of
receiver hosts, this resource requirement may be potentially-
impacted. Therefore, in this document, we propose that adjacent
upstream multicast routers SHOULD enable the explicit tracking
function for IP multicast communications on wireless networks, if
they have enough resources. If operators think that their routers do
not have enough resources, they MAY decide to disable this function
on their routers. Note that whether routers enable the explicit
tracking function or not, they need to maintain downstream membership
status by sending IGMPv3/MLDv2 General Query messages as some IGMPv3/
MLDv2 messages may be lost during transmission.
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4. Tuning IGMP/MLD Timers and Values
4.1. Tuning IGMP/MLD General Query Interval
IGMP and MLD are non-reliable protocols; to cover the possibility of
a State-Change Report being missed by one or more multicast routers,
"hosts retransmit the same State-Change Report messages [Robustness
Variable] - 1 more times", at intervals chosen at random from the
range (0, [Unsolicited Report Interval]) [2][3]. Although this
behavior increases the protocol robustness, it does not guarantee
that the State-Change Report is reached to the routers. Therefore,
routers still need to refresh the downstream membership information
by receiving Current-State Report periodically solicited by IGMP/MLD
General Query sent in the [Query Interval] period, in order to be
robust in front of host or link failures and packet loss. It also
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).
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/MLD messages on the network; larger
values cause IGMP/MLD Queries to be sent less often.
This document proposes 150 seconds for the [Query Interval] value by
changing the Querier's Query Interval Code (QQIC) field specified in
the IGMP/MLD Query message, for the case that a router enabling the
explicit tracking function sends General Query and potentially
operates a large number of member hosts such as more than 200 hosts
on the wireless link. This longer interval value contributes to
minimizing traffic of Report messages and battery power consumption
for mobile hosts.
On the other hand, this document also proposes 60 to 90 seconds for
the [Query Interval] value for the case that a router enabling the
explicit tracking function attaches to a wireless link having higher
capacity of the resource. This shorter interval contributes to quick
synchronization of the membership information tracked by the router
but may consume battery power of mobile hosts.
If a router does not enable the explicit tracking function, the
[Query Interval] value would be its default value, 125 seconds.
4.2. Tuning IGMP/MLD Query Response Interval
The [Query Response Interval] is the Max Response Time (or Max
Response Delay) used to calculate the Max Resp Code inserted into the
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periodic General Queries. Its default value is 10 seconds expressed
by "Max Resp Code=100" for IGMPv3 [2] and "Maximum Response
Code=10000" for MLDv2 [3]. By varying the [Query Response Interval],
multicast routers can tune the burstiness of IGMP/MLD messages on the
network; larger values make the traffic less bursty as host responses
are spread out over a larger interval, but will increase join latency
when State-Change Report is missing.
According to our experimental analysis, this document proposes two
tuning scenarios for tuning the [Query Response Interval] value in
different wireless link conditions; one scenario is for a wireless
link with a lower capacity of network resource or a lossy link, and
the other scenario is for a wireless link with enough capacity or
reliable condition for IGMP/MLD message transmission.
Regarding the first scenario, for instance, when a multicast router
attaches to a bursty IEEE 802.11b link, the router configures the
longer [Query Response Interval] value, such as 10 to 20 (sec). This
configuration will reduce congestion of the Current-State Report
messages on a link but may increase join latency and leave latency
when the unsolicited messages (State-Change Record) are lost on the
router.
The second scenario may happen for a multicast router attaching to a
wireless link having higher capacity of the resource or a point-to-
(multi-)point link such as an IEEE 802.16e link, because IGMP/MLD
messages do not seriously affect the link condition. The router can
seek Current-State Report messages with the shorter [Query Response
Interval] value, such as 5 to 10 (sec). This configuration will
contribute to quickly (at some level) discovering non-tracked member
hosts and synchronizing the membership information.
4.3. Tuning Last Member Query Timer (LMQT) and Last Listener Query
Timer (LLQT)
Shortening the Last Member Query Timer (LMQT) for IGMPv3 and the Last
Listener Query Timer (LLQT) for MLDv2 contributes to minimizing leave
latency. LMQT is represented by the Last Member Query Interval
(LMQI), multiplied by the Last Member Query Count (LMQC), and LLQT is
represented by the Last Listener Query Interval (LLQI), multiplied by
the Last Listener Query Count (LLQC).
While LMQI and LLQI are changeable, it is reasonable to use the
default values (i.e., 1 second) for LMQI and LLQI in a wireless
network. LMQC and LLQC, whose default value is the [Robustness
Variable] value, are also tunable. Therefore, LMQC and LLQC MAY be
set to "1" for routers enabling the explicit tracking function, and
then LMQT and LLQT are set to 1 second. However, setting LMQC and
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LLQC to 1 increases the risk of missing the last member; LMQC and
LLQC SHOULD be set to 1 only when network operators think that their
wireless link is stable enough.
On the other hand, if network operators think that their wireless
link is lossy (e.g., due to a large number of attached hosts or
limited resources), they MAY set LMQC and LLQC to "2" for their
routers enabling the explicit tracking function. Although bigger
LMQC and LLQC values may cause longer leave latency, the risk of
missing the last member will be reduced.
4.4. Tuning Startup Query Interval
The [Startup Query Interval] is the interval between General Queries
sent by a Querier on startup. The default value is 1/4 of [Query
Interval]; however, this document recommends the use of its shortened
value such as 1 second since the shorter value would contribute to
smooth handover for mobile hosts using, e.g., PMIPv6 [12]. Note that
the [Startup Query Interval] is a static value and cannot be changed
by any external signal. Therefore operators who maintain routers and
wireless links must properly configure this value.
4.5. Tuning Robustness Variable
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
defined range [2][3]. The QRV (Querier's Robustness Variable) field
in IGMP/MLD Query contains the [Robustness Variable] value used by
the querier. The default [Robustness Variable] value defined in
IGMPv3 [2] and MLDv2 [3] is "2".
This document proposes "2" for the [Robustness Variable] value for
mobility, when a router attaches to a wireless link having lower
capacity of the resource or a large number of hosts. For a router
that attaches to a wireless link having higher capacity of the
resource or reliable condition, it is not required to retransmit the
same State-Change Report message; hence the router sets the
[Robustness Variable] to "1". Note that whether the explicit
tracking function is enabled or not, the [Robustness Variable] value
SHOULD NOT be bigger than "2".
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5. Destination Address of Specific Query
IGMP/MLD Group-Specific and Group-and-Source Specific Queries defined
in [2][3] are sent to verify whether there are hosts that desire
reception of the specified group or a set of sources or to rebuild
the desired reception state for a particular group or a set of
sources. These specific Queries build and refresh multicast
membership state of hosts on an attached network. These specific
Queries should be sent to each desired hosts with specific multicast
address (not the all-hosts/all-nodes multicast address) as their IP
destination addresses, because hosts that do not join the multicast
session do not pay attention to these specific Queries, and only
active member hosts that have been receiving multicast contents with
the specified address reply IGMP/MLD reports.
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6. Interoperability
IGMPv3 [2] and MLDv2 [3] 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 in its join request. Upon its reception, the upstream
router that supports IGMPv3/MLDv2 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].
Recently, the Lightweight-IGMPv3 (LW-IGMPv3) and Lightweight-MLDv2
(LW-MLDv2) [4] protocols have been proposed in the IETF. These
protocols provide protocol simplicity for mobile hosts and routers,
as they eliminate a complex state machine from the full versions of
IGMPv3 and MLDv2, and promote the opportunity to implement SSM in
mobile communications.
This document assumes that both multicast routers and mobile hosts
MUST be IGMPv3/MLDv2 capable, regardless whether the protocols are
the full or lightweight version. And this document does not consider
interoperability with older version protocols. The main reason not
being interoperate with older IGMP/MLD protocols is that the explicit
tracking function does not work properly with older IGMP/MLD
protocols.
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7. Security Considerations
This document neither provides new functions or modifies the standard
functions defined in [2][3][4]. Therefore there is no additional
security consideration provided.
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8. Acknowledgements
Marshall Eubanks, Gorry Fairhurst, Behcet Sarikaya, Stig Venaas,
Jinwei Xia, and others provided many constructive and insightful
comments.
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9. References
9.1. 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] Liu, H., Cao, W., and H. Asaeda, "Lightweight IGMPv3 and MLDv2
Protocols", RFC 5790, February 2010.
[5] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
August 1989.
[6] Fenner, W., "Internet Group Management Protocol, Version 2",
RFC 2236, July 1997.
[7] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[8] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
RFC 4607, August 2006.
[9] Asaeda, H. and Y. Uchida, "IGMP/MLD-Based Explicit Membership
Tracking Function for Multicast Routers",
draft-asaeda-mboned-explicit-tracking-01.txt (work in
progress), October 2010.
9.2. Informative References
[10] Asaeda, H. and T. Schmidt, "IGMP and MLD Protocol Extensions
for Mobility",
draft-asaeda-multimob-igmp-mld-mobility-extensions-04.txt (work
in progress), March 2010.
[11] 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.
[12] Gundavelli, S, Ed., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
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Appendix A. Unicasting General Query
IGMPv3 and MLDv2 specifications [2][3] describe 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. In general, the all-hosts
multicast address (224.0.0.1) or link-scope all-nodes multicast
address (FF02::1) is used as the IP destination address of IGMP/MLD
General Query. On the other hand, according to [2][3], a router MAY
be able to unicast General Query to tracked member hosts in [Query
Interval], if the router keeps track of membership information
(Section 3).
Unicasting IGMP/MLD General Query would reduce the drain on battery
power of mobile hosts as only the active hosts that have been
receiving multicast contents respond the unicast IGMP/MLD General
Query messages and non-active hosts do not need to pay attention to
the IGMP/MLD messages. This also allows the upstream router to
proceed fast leaves (or shorten leave latency) by setting LMQC/LLQC
smaller, because the router can immediately converge and update the
membership information, ideally.
However, there is a concern in unicast General Query. If a multicast
router sends General Query "only" by unicast, it cannot discover
potential member hosts whose join requests were lost. Since the
hosts do not retransmit the same join requests (i.e., unsolicited
Report messages), they loose the chance to join the channels unless
the upstream router asks the membership information by sending
General Query by multicast. It will be solved by using both unicast
and multicast General Queries and configuring the [Query Interval]
timer value for multicast General Query and the [Unicast Query
Interval] timer value for unicast General Query. However, using two
different timers for General Queries would require the protocol
extension this document does not focus on. If a router does not
distinguish the multicast and unicast General Query Intervals, the
router should only use and enable multicast General Query.
Also, unicasting General Query does not remove multicasting General
Query. Multicast General Query is necessary to update membership
information if it is not correctly synchronized due to missing
Reports. Therefore, enabling unicast General Query SHOULD NOT be
used for the implementation that does not allow to configure
different query interval timers as [Query Interval] and [Unicast
Query Interval] (See [10] for the detail). If a router does not
distinguish these multicast and unicast General Query Intervals, the
router SHOULD only use and enable multicast General Query.
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Authors' Addresses
Hitoshi Asaeda
Keio University
Graduate School of Media and Governance
5322 Endo
Fujisawa, Kanagawa 252-0882
Japan
Email: asaeda@wide.ad.jp
URI: http://www.sfc.wide.ad.jp/~asaeda/
Yogo Uchida
Keio University
Graduate School of Media and Governance
5322 Endo
Fujisawa, Kanagawa 252-0882
Japan
Email: uchida@sfc.wide.ad.jp
Hui Liu
Huawei Technologies Co., Ltd.
Huawei Bld., No.3 Xinxi Rd.
Shang-Di Information Industry Base
Hai-Dian Distinct, Beijing 100085
China
Email: helen.liu@huawei.com
Qin Wu
Huawei Technologies Co., Ltd.
Site B, Floor 12F, Huihong Mansion
No.91 Baixia Rd.
Nanjing, Jiangsu 21001
China
Email: Sunseawq@huawei.com
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