One document matched: draft-asaeda-multimob-igmp-mld-optimization-03.txt
Differences from draft-asaeda-multimob-igmp-mld-optimization-02.txt
MULTIMOB Working Group H. Asaeda
Internet-Draft Y. Uchida
Expires: January 13, 2011 Keio University
July 12, 2010
Tuning the Behavior of IGMP and MLD for Mobile Hosts and Routers
draft-asaeda-multimob-igmp-mld-optimization-03
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.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Explicit Tracking of Membership Status . . . . . . . . . . . . 5
3.1. Advantages . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Considerations . . . . . . . . . . . . . . . . . . . . . . 6
4. IGMP/MLD Query Processing . . . . . . . . . . . . . . . . . . 7
5. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 9
6. Timers, Counters, and Their Default Values . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . . 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 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. Explicit Tracking of Membership Status
3.1. Advantages
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 network
congestion. 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. It enables the
router to keep track of the membership status of the downstream
IGMPv3 or MLDv2 member hosts.
Routers that enable the explicit tracking function can unicast IGMP/
MLD General Query messages. (The possibility of unicast General
Query is described in Section 4.) 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 pay
attention the messages and induces power consumption to the member
hosts. This also allows the upstream router to proceed fast leaves
by tuning [Query Interval] value, because the router can immediately
converge and update the membership information, ideally. (Section 4
details the point and Section 6 show the potential timer value.)
This document therefore recommends to enable the explicit tracking
function on adjacent upstream multicast routers. However, this
function requires router resources and does not completely eliminate
the query-and-report communication between a router and host.
Details are described in the following sections.
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3.2. Considerations
Even with the explicit tracking function, routers need to maintain
downstream membership status by sending IGMPv3/MLDv2 Query messages
because of 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 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.
Regarding the last bullet, the IGMPv3 specification [2] mentions that
an IGMPv3 Report is usually sent with a valid IP source address,
although it 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. The
MLDv2 specification [3] mentions that an MLDv2 Report MUST be sent
with a valid IPv6 link-local source address, although 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 or sent with the unspecified address, without
taking any action, because of the security consideration.
Another concern is that the explicit tracking function requires
additional processing capability and a possibly large memory for
routers to keep all membership status. Especially when a router
needs to maintail a large number of receiver hosts, this resource
requirement may be potentially-impacted. Operators may decide to
disable this function when their routers do not have enough
resources.
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4. 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 still 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., whose default value is 125 seconds [2][3].
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
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). 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 General Query messages do not affect resources of non-
member hosts. And since the router recognizes the (mostly) actual
member hosts, whether IGMP/MLD General Query messages are transmitted
by unicast or multicast, the router can configure longer [Query
Interval] value and send IGMP/MLD Group-Specific and Group-and-Source
Specific Queries when it recognizes the last member has left from the
network. This will reduce the number of both IGMP/MLD General Query
and Current-State Report messages.
Note that longer query interval may increase join latency and leave
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latency when an unsolicited message with State-Change Record is not
reached to the router.
There is another 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 send the same join requests (i.e., unsolicited Report
messages) again, 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 as proposed by [9],
whereas it will require the protocol extension this document does not
focus on.
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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5. 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 multicast routers that deal with mobile hosts
and 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]. This
document does not consider interoperability with older version
protocols.
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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.
This document proposes to inherit the default [Query Interval] value,
125 seconds, in case that a router enables the explicit tracking
function and sends General Query to tracked member hosts by unicast.
Nevertheless, the last-hop router may want to configure longer [Query
Interval] value such as 150 seconds when operators expect the router
will potentially maintains a large number of member hosts such as
1000 hosts on the wireless link.
This document also proposes 180 seconds for the [Query Interval]
value in case that a router enables the explicit tracking function
and sends General Query by multicast. This longer value contributes
to minimizing traffic of Report messages and battery power
consumption for mobile hosts. Multicast General Query is necessary
to update membership information if it is not correctly synchronized
due to missing Reports.
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. Its default value is 10 seconds expressed
by "100" for IGMPv3 [2] and "10000" for [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.
This document proposes 5 seconds (expressed by "50" for IGMPv3 and
"5000" for MLDv2) for the [Query Response Interval] value in case
that a router enables the explicit tracking function and sends
General Query by unicast. This shorter value introduces quick
response, which is valuable for the use of unicast General Query.
Note that the shorter [Query Response Interval] value may cause
sudden increase in a traffic especially when a large number of member
hosts attaches on the network; therefore it SHOULD NOT be 1 or
smaller second.
In case that a router multicasts General Query, the default [Query
Response Interval] value, 10 seconds, can be reasonablly used.
To cover the possibility of unsolicited reports being missed by
multicast routers, unsolicited reports are retransmitted [Robustness
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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". 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. And
whether the explicit tracking function is enabled or not, the
[Robustness Variable] value SHOULD NOT be bigger than "2".
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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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, Thomas C.
Schmidt, 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.
9.2. 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 252-0816
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 252-0816
Japan
Email: uchida@sfc.wide.ad.jp
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