MULTIMOB Group H. Asaeda
Internet-Draft Keio University
Expires: January 15, 2009 T. C. Schmidt
HAW Hamburg
July 14, 2008
IGMP and MLD Extensions for Mobile Hosts and Routers
draft-asaeda-multimob-igmp-mld-mobility-extensions-01
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Abstract
This document describes 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. Aside from a modified protocol semantic, an optional
"Listener Hold" function for the IGMP and MLD protocol is introduced,
which requires an extended signaling.
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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
3.1. IGMP/MLD Hold and Release Operations . . . . . . . . . . . 11
3.1.1. Action of Mobile Host and Router for IGMP/MLD Hold . . 11
3.1.2. Action of Mobile Host and Router for IGMP/MLD
Release . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.3. IGMP/MLD Hold Message Format . . . . . . . . . . . . . 13
3.1.4. IGMP/MLD Hold Timer . . . . . . . . . . . . . . . . . 15
3.1.5. Interoperability of IGMP/MLD Hold and Release . . . . 15
3.2. Lightweight-IGMP/MLD . . . . . . . . . . . . . . . . . . . 15
4. Implementations . . . . . . . . . . . . . . . . . . . . . . . 17
4.1. Host-Side Implementation . . . . . . . . . . . . . . . . . 17
4.2. Router-Side Implementation . . . . . . . . . . . . . . . . 17
5. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . . . 24
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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 the upstream
multicast routers that serve multicast member hosts or IGMP/MLD proxy
[7] on their downstream interfaces. Conceptually, IGMP and MLD work
on wireless networks. However, wireless access technologies operate
on a shared medium 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
(e.g., [12]). Additionally, 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 feasible condition for mobile hosts and routers
communicating 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". One possible approach to support these requirements is
explicit tracking. Upstream router traces all downstream members,
thereby 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 explicit tracking function 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
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membership reports from the downstream hosts. One may deduce that
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.
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. 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
coordination produced by SSM. An upstream router needs to maintain a
shared tree (e.g., RPT in PIM-SM) 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
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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 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 (e.g., leave
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request). 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 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, a router may
want to unicast the message to tracked member hosts in the [Unicast
Query Interval], 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], an 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. The Group-Specific Query is sent when a router
receives State-Change Records indicating a host is leaving a group,
and never in response to Current-State Records.
In a dormant mode operation, a Group-Specific Query can be 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, a Group-Specific Query can be sent
to maintain the group membership state of mobile hosts on attached
networks. Since a 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 these specific Queries and only
active group member hosts that have been receiving multicast contents
would reply IGMP/MLD reports.
However, sending many Group-Specific Queries for all corresponding
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groups may increase the total number of transmitted IGMP/MLD
messages. [TODO: Therefore, it is necessary to know the condition in
which a Group-Specific Query is used for maintaining the group
membership state of all hosts on a link, instead of a General Query.]
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. [TODO: Define [Multicast Group-Query
Interval]. We currently think this value is same of the default
[Query Interval] value the regular IGMP and MLD define [2][3].]
2.2.3. Values in IGMP/MLD Query
A multicast router operating in 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
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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
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
3.1. IGMP/MLD Hold and Release Operations
A mobile host sends a join/leave request for particular multicast
session to its upstream router (i.e., a proxy router or an adjacent
router), and the router will maintain IGMP/MLD state of the host and
serve multicast data to the host. According to a fast handover
scenario (e.g., using FMIPv6 [10]), a mobile host wants to accelerate
multicast service termination in the previous network before handoff
and immediately rejoin the session after the movement. As the
router's behavior, when the router remains part of the multicast
delivery tree, it keeps the session active without leaving from the
session, while the data transmission is paused until the host
completes the movement and resumed after the movement.
This document describes an IGMP/MLD Hold operation that enables to
keep the membership state for fast packet forwarding, and an IGMP/MLD
Release operation that ressumes forwarding the multicast session
after the host movement. Originally, an idea of MLD Hold was
proposed in [13].
Note that discussing the procedure of context transfer (CT) for each
mobility protocol (e.g., FMIPv6, PMIPv6 [11]) and the message format
for CT will be defined in other documents. And also, discussing how
a mobile host or a router detects the movement and triggers to send
an IGMP/MLD Hold message is depend on the mobility protocol, and
hence out of scope of this document.
3.1.1. Action of Mobile Host and Router for IGMP/MLD Hold
An IGMP/MLD Hold operation is used to notify an upstream multicast
router (e.g., HA in case of bidirectional tunneling, adjacent router
in case of remote subscription) to stop forwarding data of the
specified multicast sessions if there is no member joining the same
sessions, but maintain the multicast membership on the router's
interface the message was received.
When a mobile host starts moving from a network to a new network, the
host or the gateway that detects the movement sends an IGMP/MLD Hold
message (mentioned in Section 3.1.3) to its upstream router to make a
fast handover for the specified multicast sessions.
When the multicast router receives an IGMP/MLD Hold message from a
mobile host, it records the host's address and specified group and
source address records and filter-mode, and stops the corresponding
group or source timer until the IGMP/MLD Hold Timer (described in
Section 3.1.4) is expired. The router does not send any Group-
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Specific or Group-and-Source Specific Query upon reception of an
IGMP/MLD Hold message.
After the router recognizes the IGMP/MLD Hold message, the router
decides whether it stops forwarding the data or not. The decision is
dependent on whether the router has been keeping track of membership
status. If the router has been tracking all the members, it can
recognize whether the message sender host is the last member joining
the notified session on the interface or not. If the router has not
been tracking, the router waits for receiving the next Current-State
Report and recognizes whether the host is the last member joining the
notified session on the interface or not. And if the message sender
host is the last member, the router stops forwarding data by
disabling the outgoing interface for that session, whereas it keeps
the host's membership state. If the host is not the last member of
the session, the router does not stop forwarding the data even with
reception of this message.
3.1.2. Action of Mobile Host and Router for IGMP/MLD Release
An IGMP/MLD Release operation is used to restart forwarding data of
the specified multicast sessions that are kept with the membership
state at a router, or used to cancel the hold request previously
notified by an IGMP/MLD Hold message.
When a mobile host moves to a new network, the host or the gateway
that detects the movement operates IGMP/MLD Release by sending a
standard IGMP/MLD Current-State Report message as mentioned in
Section 3.1.3) to its upstream router to release the hold request
previously sent.
When a multicast router receives an IGMP/MLD Current-State Report
message, the router examines the message sender and an IGMP/MLD Hold
Timer (Section 3.1.4). If the router has tracked the host's
membership status, it compares the holding Multicast Address Records
and notified Multicast Address Records specified in the IGMP/MLD
Current-State Report message. Regarding the corresponding Multicast
Address Records, the router will delete the host's membership state
if it has, and restart the given group or source timer and forwarding
the data again if the IGMP/MLD Hold Timer is not expired.
If either a multicast router has not tracked the corresponding
membership status given by the host that sent the IGMP/MLD Current-
State Report message, or if the IGMP/MLD Hold Timer has expired, then
the router does not take any action.
If a router that did not recognize an IGMP/MLD Hold message (e.g.,
due to packet loss or some troubles in its transmission) receives an
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IGMP/MLD Current-State Report message, it will accept the message as
a regular Current-State Report IGMP/MLD message as specified in
Section 3.1.5.
3.1.3. IGMP/MLD Hold Message Format
The following figure is the Report message format defined in IGMPv3
[2] and MLDv2 [3]. Type field is either IGMP Version 3 Membership
Report (0x22) or Version 2 Multicast Listener Report (decimal 143);
it is not necessary to change IGMP/MLD Report message format to
support IGMP/MLD Hold messages.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Nr of Mcast Address Records (M)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address Record [1] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . .
. . .
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address Record [M] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Multicast Address Record shown in the following figure defines
new Record Types that express an IGMP/MLD Hold message. For an IGMP/
MLD Release operation, the standard IGMP/MLD Current-State Report
message is used.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Record Type | Aux Data Len | Number of Sources (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Source Address [1] .
. .
| |
+- -+
. . .
. . .
. . .
+- -+
| |
. .
. Source Address [N] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Auxiliary Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Value Name and Meaning
----- ----------------
7 HOLD_INCLUDE - indicates that the interface has a filter mode
of INCLUDE for the specified multicast address. The Source
Address [i] fields in this Multicast Address Record contain
the interface's source list for the specified multicast
address. A HOLD_INCLUDE Record is never sent with an empty
source list.
8 HOLD_EXCLUDE - indicates that the interface has a filter mode
of EXCLUDE for the specified multicast address. The Source
Address [i] fields in this Multicast Address Record contain
the interface's source list for the specified multicast
address, if it is non-empty. When a mobile host works with
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LW-IGMPv3/LW-MLDv2 (Section 3.2), the Multicast Address Record
does not contain the Source Address [i] field with
HOLD_EXCLUDE type value.
3.1.4. IGMP/MLD Hold Timer
A router upon receiving an IGMP/MLD Hold message will identify the
corresponding multicast sessions for which the message was received,
and once it accepts the request will wait for IGMP/MLD Hold Timer
period for the sessions for allowing the router on the new link to
resume the sessions. However, if it does not receive any IGMP/MLD
Current-State Record message for an IGMP/MLD Release operation within
that given amount of time, it will delete the hold membership state
given by the IGMP/MLD Hold message and proceed as it receives IGMP
Leave or MLD Done request.
[TODO: We will provide the appropriate [IGMP/MLD Hold Timer] value
that would fit in the mobile communication environment based on some
experimental results. Does it depend on mobile protocols?]
3.1.5. Interoperability of IGMP/MLD Hold and Release
If a multicast router does not support an IGMP/MLD Hold operation, it
will ignore the IGMP/MLD Hold message. In this case, an IGMP/MLD
Current-State Report message sent by a mobile host that previously
requested an IGMP/MLD Hold operation to stop forwarding data is dealt
with as a new join request for the specified multicast sessions (when
the corresponding group or source timer is expired) or simply updates
the corresponding group and/or source timer (when these timers are
not expired and the membership status has been still active).
3.2. Lightweight-IGMP/MLD
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) [8].
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
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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) [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) and is equivalent to the group membership join/leave triggered
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 [9]
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. 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: We assume that multicast routers are not tiny and non-powerful
systems nor battery or power sensitive.]
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 [9].
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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. Acknowledgements
Dave Thaler, Matthias Waehlisch, 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] 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] 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.
8.2. Informative References
[9] Liu, H., Cao, W., and H. Asaeda, "Lightweight IGMPv3 and MLDv2
Protocols",
draft-ietf-mboned-lightweight-igmpv3-mldv2-03.txt (work in
progress), June 2008.
[10] Koodli, Ed., R., "Fast Handovers for Mobile IPv6", RFC 4068,
July 2005.
[11] Leung, K., Devarapalli, V., Chowdhury, K., and B. Patil, "Proxy
Mobile IPv6", draft-ietf-netlmm-proxymip6-16.txt (work in
progress), May 2008.
[12] Schmidt, T., Waehlisch, M., and G. Fairhurst, "Multicast
Mobility in MIPv6: Problem Statement and Brief Survey",
draft-irtf-mobopts-mmcastv6-ps-04.txt (work in progress),
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July 2008.
[13] Jelger, C. and T. Noel, "Multicast for Mobile Hosts in IP
Networks: Progress and Challenges", IEEE Wireless
Comm. pp.58-64, October 2002.
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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
Thomas C. Schmidt
HAW Hamburg
Dept. Informatik
Berliner Tor 7
Hamburg, D-20099
Germany
Email: Schmidt@informatik.haw-hamburg.de
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