MULTIMOB Working Group H. Asaeda
Internet-Draft Keio University
Expires: September 9, 2010 S. Venaas
Cisco
March 8, 2010
Tuning the Behavior of IGMP and MLD for Mobile Hosts and Routers
draft-asaeda-multimob-igmp-mld-optimization-02
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, mainly for a query timer tuning.
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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 . . . . . . . . . . . . . . . . 8
3.4. Source-Specific Multicast Support . . . . . . . . . . . . 9
4. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 11
5. Timers, Counters, and Their Default Values . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
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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 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 tuning the IGMPv3 and MLDv2
protocol behavior for mobility, including a 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". A source
filtering mechanism in a lightweight manner is also described for
enabling Source-Specific Multicast. 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. 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. The traditional IGMP and MLD [5][6][7] provide a
membership report suppression mechanism to escape from the trouble.
The report suppression mechanism enables a host to cancel 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 precludes the
function for an upstream router to track membership status. Hence
the membership report suppression mechanism has been removed from
IGMPv3 [2] and MLDv2 [3], 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
unspecified address (::) or the same IPv6 link-local address in
some mobile routing environment.
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. When the explicit tracking
function is enabled at adjacent upstream multicast routers, the
standard [Query Interval] can be tuned to be a longer value as
described in Section 5.
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.
IGMPv3 and MLDv2 specifications [2][3] describe that a host MUST
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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 General Query to tracked member hosts in [Query
Interval] (or [Unicast Query Interval] newly defined in [9]), if the
router keeps track of membership information (Section 3.1).
Unicasting IGMP/MLD General Query would be effective especially when
a wireless link is heavily loaded.
If a multicast router attached to a wireless link enables an explicit
tracking function and unicasts IGMP/MLD General Query for each member
host, the router may configure longer [Query Interval] value, in
order to reduce the number of IGMP/MLD General Query messages via
multicast. If a multicast router does not track the member hosts,
the router multicasts IGMP/MLD General Query with shorter [Query
Interval].
Note that longer query interval will increase join latency or
increase leave latency when an unsolicited message with State-Change
Record 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.
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
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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 all-zero or the unspecified address, they should disable the
explicit tracking function (described in Section 3.1) even if it has
been enabled.
3.4. Source-Specific Multicast Support
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.
Specifying non-interesting source addresses with EXCLUDE filter mode
also reduces the advantage of scalable routing tree coordination,
because 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.
It may be beneficial to implement Lightweight-IGMPv3 (LW-IGMPv3) and
Lightweight-MLDv2 (LW-MLDv2) [4] for mobile hosts and routers, in
order to eliminate an EXCLUDE filter mode operation and promote the
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opportunity to simply use SSM in mobile communications.
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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][4], and this document does not
need to consider interoperability with older version protocols.
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5. 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 [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.
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 NOT be bigger than "2" especially when the
[Query Response Interval] is set smaller than its default value.
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
shortended 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.
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6. Security Considerations
TBD.
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7. Acknowledgements
Marshall Eubanks, Gorry Fairhurst, Behcet Sarikaya, Thomas C.
Schmidt, Jinwei Xia, and others provided many constructive and
insightful comments.
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8. References
8.1. 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.
8.2. Informative References
[9] 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.
[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] 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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Authors' Addresses
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/
Stig Venaas
Cisco
USA
Email: stig@venaas.com
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