MULTIMOB Working Group H. Asaeda
Internet-Draft Keio University
Expires: April 29, 2010 October 26, 2009
IGMP and MLD Optimization for Mobile Hosts and Routers
draft-asaeda-multimob-igmp-mld-optimization-01
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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. The optimization includes a query timer tuning and an
explicit membership notification operation.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Optimization . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Tracking of Membership Status . . . . . . . . . . . . . . 6
3.2. IGMP/MLD Query Processing . . . . . . . . . . . . . . . . 7
3.3. IGMP/MLD Report Processing . . . . . . . . . . . . . . . . 9
3.4. Multicast Source Filter . . . . . . . . . . . . . . . . . 9
4. Explicit Membership Notification . . . . . . . . . . . . . . . 11
5. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 13
6. Timers, Counters, and Their Default Values . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative 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".
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.
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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. Optimization
3.1. 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. 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 cancels
sending a pending membership report requested by IGMP/MLD Query if it
observes the report that includes the same membership information on
the network. However, the report suppression mechanism precluded the
function for an upstream router to track membership status. In
IGMPv3 [2] and MLDv2 [3], hence the membership report suppression
mechanism has been removed, and all downstream member hosts must send
their membership reports to an upstream router.
The "explicit tracking function" is the possible approach to create
the optimal condition for mobile communications. This enables the
router to keep track of the membership status of the downstream
IGMPv3 or MLDv2 member hosts.
The explicit tracking function reduces the number of solicited
membership reports by periodical IGMP/MLD Query, and finally the
total number of transmitted IGMP/MLD messages can be drastically
reduced. This is beneficial especially to mobile hosts that do not
have enough battery power, since flooding IGMP/MLD messages on a
wireless link makes all multicast members give significant attention
and induces power consumption to the member hosts. This also allows
the upstream router to proceed fast leaves, because the router can
immediately converge and update the membership information, ideally.
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.
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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.
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.
o It is impossible to identify mobile hosts when hosts have the same
IPv6 link-local address in some mobile routing environment such as
PMIPv6 [11].
This document recommends to enable the explicit tracking function at
multicast routers if their resources are enough to handle downstream
membership information and all hosts membership report messages do
not affect wireless communications. While this document defines the
new [Multicast Query Interval] value (described in Section 6), the
standard [Query Interval] value and [Multicast Query Interval] value
can be tuned to be longer when the explicit tracking function is
enabled at adjacent upstream multicast routers.
3.2. IGMP/MLD Query Processing
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. 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. Unfortunately, flooding periodical message whose destination
address is the all-hosts/all-nodes multicast address consumes bettery
power of mobile hosts. Only the active hosts that have been
receiving multicast contents should respond the Query message.
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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 [Query
Interval], if the router keeps track of membership information
(Section 3.1). Unicasting IGMP/MLD General Query is effective
especially when a small number of mobile hosts attaches to a network.
If a multicast router attached on a wireless link enables an explicit
tracking function and unicasts IGMP/MLD General Query for each member
host, the router may configure longer [Multicast Query Interval]
value, which is newly defined in this document and described in
Section 6, in order to reduce the number of IGMP/MLD General Query
messages via multicast.
If either a multicast router does not track the member hosts or a
large number of mobile hosts attaches the network, the router
multicasts IGMP/MLD General Query with shorter [Multicast Query
Interval].
Note that it is necessary to multicast IGMP/MLD General Query even if
a router keeps track of member hosts, in order to be robust from lost
IGMP/MLD messages sent from downstream hosts. In addition, longer
query interval will increase join latency when an unsolicited 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 unsolicited Leave message with State-Change Record
requesting leaving a session is not reached to the router.
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 corresponding 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 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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3.3. IGMP/MLD Report Processing
An IGMPv3 Report is sent with a valid IP source address, and an MLDv2
Report MUST be sent with a valid IPv6 link-local source address.
Note that the IGMPv3 specification [2] permits that a host uses the
0.0.0.0 source address, as it happens that the host has not yet
acquired an IP address, and routers MUST accept a report with a
source address of 0.0.0.0. On the other hand, the MLDv2
specification [3] describes that an MLDv2 Report can be sent with the
unspecified address (::), if the sending interface has not acquired a
valid link-local address yet. However, routers MUST silently discard
a message that is not sent with a valid link-local address, without
taking any action. Thus, an MLDv2 Report sent with the unspecified
address is also discarded by the router, because of the security
consideration.
In summary, routers permit that multiple mobile hosts simultaneously
use the same IPv4 address, including the 0.0.0.0 source address, for
an IGMPv3 Report, or simultaneously use the same IPv6 link-local
address, but not the unspecified address, for an MLDv2 Report. When
routers receive IGMPv3/MLDv2 Reports with duplicate source addresses
or the 0.0.0.0 or the unspecified address, they should disable the
explicit tracking function (described in Section 3.1) even if it has
been enabled.
3.4. 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
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interesting source address lists with INCLUDE filter mode or non-
interesting source address lists with EXCLUDE filter mode.
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 [10]) 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 recommends to adopt LW-IGMPv3 and LW-MLDv2 to mobile
hosts and routers and eliminate EXCLUDE filter mode operation from
mobile hosts.
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4. 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 solicited 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 solicited 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 sleeping hosts, as these
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 6) 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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5. 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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6. Timers, Counters, and Their Default Values
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.
The Querier's Query Interval Code (QQIC) field specifies the [Query
Interval] in the IGMP/MLD query message, and will be tuned by the
querier. 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. The [Multicast Query
Interval] value is calculated from the [Query Interval] value
[TODO: We will provide the appropriate [Query Interval] and
[Multicast Query Interval] values that would fit in the mobile
communication environment based on some experimental results.
Tentatively, this document currently defines the default [Query
Interval] value is 90 sec. and the default [Multicast Query Interval]
value is 180 sec.]
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/MLD 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.]
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. 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".
This document proposes that the [Robustness Variable] value SHOULD
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NOT be bigger than "2" especially when the [Query Response Interval]
is set smaller than its default value.
The default [Notification Interval] value is 60 sec. The
[Notification Interval] value MUST be shorter than both [Query
Interval] and [Multicast Query Interval] values.
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7. Security Considerations
TBD.
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8. Acknowledgements
Marshall Eubanks, Gorry Fairhurst, Behcet Sarikaya, Thomas C.
Schmidt, 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] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
August 1989.
[5] Fenner, W., "Internet Group Management Protocol, Version 2",
RFC 2236, 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-mldv2-06.txt (work in
progress), October 2009.
9.2. Informative References
[10] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[11] Gundavelli, S, Ed., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
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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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