One document matched: draft-asaeda-multimob-igmp-mld-mobility-extensions-00.txt
MULTIMOB Group H. Asaeda
Internet-Draft Keio University
Expires: May 15, 2008 November 12, 2007
IGMP and MLD Extensions for Mobile Hosts and Routers
draft-asaeda-multimob-igmp-mld-mobility-extensions-00
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Abstract
This document describes the IGMP and MLD protocol extensions for
mobile hosts and routers. IGMP and MLD are necessary protocols for
hosts to request join or leave multicast sessions. While the regular
IGMP and MLD protocols support communication between mobile hosts and
routers over wireless networks, this document discusses the
conditions how mobile hosts and routers use IGMP and MLD in their
communication more effectively.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Configurations . . . . . . . . . . . . . . . . . . . . . . . . 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
2.2.3. Values in IGMP/MLD Query . . . . . . . . . . . . . . . 9
2.3. IGMP/MLD Querier Selection . . . . . . . . . . . . . . . . 10
3. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 11
4. Implementations . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Host-Side Implementation . . . . . . . . . . . . . . . . . 13
4.2. Router-Side Implementation . . . . . . . . . . . . . . . . 13
5. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Normative References . . . . . . . . . . . . . . . . . . . 16
7.2. Informative References . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
The Internet Group Management Protocol (IGMP) for IPv4 and the
Multicast Listener Discovery Protocol (MLD) for IPv6 are the
necessary protocols for hosts to request to join or leave multicast
sessions. These protocols must be also supported by the upstream
multicast routers that have the downstream multicast member hosts on
the same LAN. By definition, IGMP and MLD work on wireless networks;
there is not necessary to change these protocol specs for wireless
communication environments. However, when mobile hosts or routers
attached on a wireless link multicast IGMP/MLD messages, the
transmitted IGMP/MLD messages are flooded to all mobile hosts and
routers on the link, where a large amount of flooding data consumes
battery power of each mobile host. In addition, it takes higher
costs for the upstream router to maintain a large number of IGMP/MLD
messages, and in this situation, it takes longer time for the router
to converge the membership state (i.e. existence of downstream member
hosts), where the longer convergence negatively affects leave
latency.
To create the feasible condition to communicate mobile hosts and
routers with IGMP/MLD, 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". The possible approach to fulfill these requirements is
relevant; if the upstream router traces all downstream members by
limiting the number of solicited membership reports (by periodical
IGMP/MLD Query), the number of transmitted IGMP/MLD messages is
effectively reduced, and the upstream router can immediately update
the membership information and proceed the fast leave.
The function that enables to trace all downstream members is
supported by IGMPv3 [2] and MLDv2 [3]. In the previous version
protocols, IGMPv1 [4], IGMPv2 [5], and MLDv1 [6], 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. This
specification effectively reduced a possibility of network congestion
or message flooding, but precluded the function for an upstream
router to track membership status. On the other hand, in IGMPv3 and
MLDv2, the membership report suppression mechanism has been removed,
and therefore all downstream member hosts must send their membership
reports to an upstream router and the router can keep track of
membership status.
If the report suppression mechanism is removed from the host-side
protocols, the upstream router supporting IGMPv3/MLDv2 receives all
membership reports from the downstream hosts. One may deduce that
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the router does not need to periodically send IGMPv3/MLDv2 Query
messages to trace membership status. However, IGMPv3/MLDv2 capable
routers usually configure to send periodical IGMP/MLD Query messages
to seek membership information to the downstream hosts, and disable
the function that keeps track of membership status. One of the
reasons is that IGMP/MLD message is non-reliable and may be lost in
the transmission, and therefore the router would need to confirm the
membership by sending query messages. The other reason is that, for
keeping track of membership status, the router needs additional
processing cost and a possibly large size of the memory to record all
member information. The requirement to keep the compatibility mode
with older version IGMP/MLD is also the reason, because the router
needs to support the downstream hosts that are not upgraded to the
latest versions of IGMP/MLD and run the report suppression mechanism.
There is one more important function in IGMPv3 and MLDv2. IGMPv3 and
MLDv2 provide the ability for hosts to report interest in receiving
packets only from specific source addresses to upstream routers.
With IGMPv3/MLDv2, the mobile host specifies the interesting
multicast and source addresses with INCLUDE filter mode in the 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) [7]. SSM is
advantageous to multicast routing tree establishment and satisfies
current and future needs.
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. In fact, this operation gives the complexity in the host-
side procedure. If any application running on a host requests an
EXCLUDE filter mode operation, the host sets the interface state to
EXCLUDE mode for the requested multicast address, and the source
address list of the interface record is the intersection of the
source address lists requested by all applications in EXCLUDE mode,
minus the source addresses that appear in any application in INCLUDE
mode. The state transition that maintains the interface record is
complex, and the implementation cost will be relatively high for
mobile hosts.
Furthermore, specifying non-interesting source addresses with EXCLUDE
filter mode reduces the advantage of scalable routing tree
establishment in an SSM manner, because an upstream router needs to
refresh and re-generate some or all of the corresponding routing tree
including the RPT whenever the router receives join request with
EXCLUDE filter mode from the downstream hosts. This manner increases
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the tree maintenance cost to not only the upstream multicast routers
but other routers existed 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.
This document describes the IGMP and MLD protocol extensions for
mobile hosts and routers, and discusses the conditions how mobile
hosts and routers use IGMP and MLD in their communication over
wireless networks effectively. The selective solutions that provide
tangible benefits to the mobile hosts and routers are given by
"keeping track of membership status by eliminating a report
suppression mechanism", "varying IGMP/MLD Query types and values to
tune the number of responses", and "using a source filtering
mechanism in a lightweight manner". The proposed solutions do not
require changing the IGMP and MLD protocols. This condition is
advantageous to the deployment.
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2. Configurations
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.
By eliminating membership report suppression, an IGMPv3 or MLDv2
capable upstream router could trace all downstream members and track
per-host membership status on the interface. This 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 LAN 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.
2.2. IGMP/MLD Query Coordination
2.2.1. Unicasting IGMP/MLD General Query
IGMP and MLD are asymmetric and non-reliable protocols; multicast
routers still need to solicited membership reports by periodical
IGMP/MLD Query, in order to be robust in front of host or link
failures and packet loss. Moreover, it happens that mobile hosts may
turn off or move from the wireless network to other wireless network
managed by the different router without any notification (i.e. leave
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report). Therefore, even though multicast routers keep track of the
interests of downstream member hosts attached on the same LAN, IGMP/
MLD Query must be sent periodically.
However, periodical message flooding using the all-hosts multicast
address (i.e. 224.0.0.1 or ff02::1) as its IP destination address
gives the unwilling situation to the mobile hosts. When the mobile
hosts are operating in a dormant mode and not communicating with
others, they should not be woken up by IGMP/MLD General Query and
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.
IGMPv3 and MLDv2 specifications [2][3] say that a host MUST accept
and procss 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, it unicasts the
message to tracked member hosts in the [Unicast Query Interval]. It
is happened especially when a multicast router has a small number of
mobile hosts that are listening different multicast sessions. In
this situation, the router multicasts IGMP/MLD General Query with
longer [Query Interval] (described in Section 2.2.3).
[TODO: Define [Unicast Query Interval] value. The value could be
same of the default [Query Interval]?]
2.2.2. Multicasting IGMP/MLD Group-Specific Query
In the standard protocols [2][3], 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. Group-Specific Queries are sent when a router
receives a State-Change record indicating a host is leaving a group.
In a dormant mode operation, IGMP/MLD Group-Specific Query can be
also used to build and refresh the group membership state of hosts on
attached networks. When more than one mobile host join the multicast
contents whose multicast address is same, IGMP/MLD Group-Specific
Query can be sent to maintain the group membership state of mobile
hosts on attached networks, instead of IGMP/MLD General Query. Since
IGMP/MLD 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 the IGMP/MLD Group-Specific Query and only active
group member hosts that have been receiving multicast contents would
reply IGMP/MLD reports. This manner contributes to reducing the
number of transmitted IGMP/MLD messages.
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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. This value is same of the default [Query
Interval] value the regular IGMP and MLD define [2][3].
[TODO: Define [Multicast Group-Query Interval].]
2.2.3. Values in IGMP/MLD Query
A multicast router operating in a 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 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 [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 value the regular IGMP and MLD 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
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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
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.]
2.3. IGMP/MLD Querier Selection
[TODO: Is there any condition or assumption in which multiple
multicast routers exist in a single wireless link? If there is the
case, do we need to consider IGMP/MLD querier selection mechanism and
the corresponding timer values or intervals? The Querier's Query
Interval Code (QQIC) field 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.]
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3. Protocol Extensions
IGMPv3 and MLDv2 enable all member hosts to send membership reports
to the upstream routers. Not only this function, IGMPv3 and MLDv2
support a source filtering function. An IGMPv3 or MLDv2 capable host
can tell its upstream router which group it would like to join by
specifying which sources it does (or does not) intend to receive
multicast traffic from. IGMPv3 and MLDv2 add the capability for a
multicast router to also learn which sources are (and are not) of
interest to neighboring hosts, for packets sent to any particular
multicast address. This source filtering function is required to
invoke Source-Specific Multicast (SSM) [7].
IGMPv3 and MLDv2 introduce antithetic filter modes, INCLUDE and
EXCLUDE filter modes, to expand the source filtering function. If a
host wants to receive from specific sources, it sends an IGMPv3 or
MLDv2 report with the filter mode set to INCLUDE. If the host does
not want to receive from some sources, it sends a report with the
filter mode set to EXCLUDE. A source list for the given sources
shall be included in the report message. INCLUDE and EXCLUDE filter
modes are also defined in a multicast router to process the IGMPv3 or
MLDv2 reports. When a multicast router receives the report messages
from its downstream hosts, it forwards the corresponding multicast
traffic by managing requested group and source addresses.
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.
Recently, Lightweight-IGMPv3 (LW-IGMPv3) and Lightweight-MLDv2 (LW-
MLDv2) [8] 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) and is equivalent to the group membership join/leave triggered
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by IGMPv2/IGMPv1/MLDv1.
The aim of LW-IGMPv3 and LW-MLDv2 is not only for contributing to the
simpler implementation or reducing the memory size on a host.
Another advantage is that it reduces the processing cost on upstream
routers by eliminating the EXCLUDE filter mode operations. If both
INCLUDE and EXCLUDE filter mode operations are supported in the
networks, the routers need to maintain all source addresses joined
from their downstream hosts. Even if a Shortest-Path Tree (SPT) is
well coordinated, the routers need to refresh (and re-generate) some
or all of the corresponding routing paths including the Rendezvous-
Point Tree (RPT) whenever the downstream host requests EXLUDE filter
mode join. LW-IGMPv3 and LW-MLDv2 preclude the unwilling situation.
Since there is no side-effect, this document 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 hosts.
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4. Implementations
4.1. Host-Side Implementation
Mobile hosts should implement IGMPv3 or LW-IGMPv3 for IPv4 multicast
and MLDv2 or LW-MLDv2 for IPv6 multicast. All of these protocols
eliminate a membership report suppression mechanism, and make hosts
work with the function multicast routers use to trace downstream
member hosts. These protocols also support SSM. According to the
protocol requirement aforementioned, however, this document
recommends to implement LW-IGMPv3 for IPv4 and LW-MLDv2 for IPv6 [8]
rather than the full version protocols.
4.2. Router-Side Implementation
To keep track of multicast membership status and cooperate with SSM
capable mobile hosts, multicast routers must implement IGMPv3/
LW-IGMPv3 or MLDv2/LW-MLDv2. The protocol requirement aforementioned
does not require modification of the IGMPv3/LW-IGMPv3 and MLDv2/
LW-MLDv2 protocol specifications. This condition is advantageous to
the deployment. However, regarding the router-side implementation,
the function to trace downstream members requires the hardware
requirement that would cost the router additional hardware resources,
especially CPU and memory resources.
[TODO: This document assumes that multicast routers are not tiny and
non-powerful systems nor battery or power sensitive. Our assumption
is correct?]
As well as the host-side implementation, the elimination of the
EXCLUDE filter mode will greatly simplify the router behavior, e.g.
the action on reception of reports and the setting of the timers.
This document therefore recommends to implement LW-IGMPv3 for IPv4
and LW-MLDv2 for IPv6 rather than their full version protocols. The
detailed operation being simplified is described in [8].
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5. Interoperability
TBD.
[TODO: We believe it would be currently feasible to assume the
routers who take care of mobile hosts MUST be IGMPv3/MLDv2 capable
(regardless whether the protocols are the full version or not). What
we should understand is whether there is the case that mobile hosts
may not be IGMPv3/MLDv2 capable or not.]
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6. Security Considerations
TBD.
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7. References
7.1. 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 2373, July 1997.
[6] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[7] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
RFC 4607, August 2006.
7.2. Informative References
[8] Liu, H., Cao, W., and H. Asaeda, "Lightweight IGMPv3 and MLDv2
Protocols",
draft-ietf-mboned-lightweight-igmpv3-mldv2-01.txt (work in
progress), June 2007.
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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
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