Internet DRAFT - draft-ietf-multimob-igmp-mld-tuning
draft-ietf-multimob-igmp-mld-tuning
MULTIMOB Working Group H. Asaeda
Internet-Draft Keio University
Intended status: Informational H. Liu
Expires: September 27, 2012 Q. Wu
Huawei
March 26, 2012
Tuning the Behavior of IGMP and MLD for Routers in Mobile and Wireless
Networks
draft-ietf-multimob-igmp-mld-tuning-06
Abstract
IGMP and MLD are the protocols used by hosts and multicast routers to
exchange their IP multicast group memberships with each other. This
document describes the ways of IGMPv3 and MLDv2 protocol optimization
for mobility, and aims to become a guideline for tuning of IGMPv3/
MLDv2 Queries and timer and counter values.
Status of this Memo
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This Internet-Draft will expire on September 27, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Explicit Tracking of Membership Status . . . . . . . . . . . . 4
4. Tuning IGMP/MLD Timers and Values . . . . . . . . . . . . . . 4
4.1. Tuning IGMP/MLD General Query Interval . . . . . . . . . . 5
4.2. Tuning IGMP/MLD Query Response Interval . . . . . . . . . 6
4.3. Tuning Last Member Query Timer (LMQT) and Last
Listener Query Timer (LLQT) . . . . . . . . . . . . . . . 6
4.4. Tuning Startup Query Interval . . . . . . . . . . . . . . 7
4.5. Tuning Robustness Variable . . . . . . . . . . . . . . . . 7
4.6. Tuning Scenarios for Various Mobile IP Networks . . . . . 8
5. Destination Address of Specific Query . . . . . . . . . . . . 9
6. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Unicasting General Query . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
The Internet Group Management Protocol (IGMP) [1] for IPv4 and the
Multicast Listener Discovery Protocol (MLD) [2] for IPv6 are the
standard protocols for hosts to initiate joining or leaving of
multicast sessions. These protocols must be also supported by
multicast routers or IGMP/MLD proxies [5] that maintain multicast
membership information on their downstream interfaces. Conceptually,
IGMP and MLD work on both wireless and mobile networks. However,
wireless access technologies operate on a shared medium or a point-
to-point link with limited spectrum 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. The motion
of a host may cause disruption of a multicast service initiation and
termination in the new or previous network upon its movement. Slow
multicast service activation following a join may incur additional
delay in receiving multicast packets and degrade reception quality.
Slow service termination triggered by a rapid departure of the mobile
host without leaving the group in the previous network may waste
network resources.
When IGMP and MLD are used with mobile IP protocols, the proximity of
network entities should be considered. For example, when bi-
directional tunnel is used with the mobility entities described in
[6][7], the mobile host experiences additional latency, because the
round-trip time using bi-directional tunnel between mobility entities
is larger comparing to the case that a host and an upstream router
attach to a LAN.
This document describes the ways of tuning the IGMPv3 and MLDv2
protocol behavior on multicast router and proxy side for wireless and
mobile networks, including query and related 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. This document does not make any changes to
the IGMPv3 and MLDv2 protocls and only suggests optimal settings for
the configurable parameters of the protocols in mobile and wireless
environments.
2. Terminology
In this document, "router" means both multicast router and IGMP/MLD
proxy.
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3. Explicit Tracking of Membership Status
Mobile hosts use IGMP and MLD to request to join or leave multicast
sessions. When an adjacent upstream router receives the IGMP/MLD
Report messages, it recognizes the membership status on the link. To
update the membership status reliably, the router sends IGMP/MLD
Query messages periodically, and sends Group-Specific and/or Group-
and-Source Specific Queries when a member host reports its leave.
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 "explicit tracking function" [8] is the possible approach to
reduce the transmitted number of IGMP/MLD messages and contribute to
the efficiency of mobile communications. It enables the router to
keep track of the membership status of the downstream IGMPv3 or MLDv2
member hosts. That is, if a router enables the explicit tracking
function, it does not always need to ask Current-State Report
message's transmission from the receiver hosts since the router
implicitly recognizes the (potential) last member host when it
receives the State-Change Report reporting a leave. The router can
therefore send IGMP/MLD Group-Specific and Group-and-Source Specific
Queries LMQC/LLQC times (see Section 4.3 for LMQC/LLQC) only when it
recognizes the last member has left from the network. This reduces
the transmitted number of Current-State Report messages.
Enabling the explicit tracking function is advantageous for mobile
multicast, but the function requires additional processing capability
and possibly a large memory for routers to keep all membership
status. Especially when a router needs to maintain a large number of
receiver hosts, this resource requirement is potentially impacted.
Therefore, in this document it is recommended that adjacent upstream
multicast routers enable the explicit tracking function for IP
multicast communications on wireless and mobile networks, if they
have enough resources. If operators think that their routers do not
have enough resources, they may disable this function on their
routers. Note that whether routers enable the explicit tracking
function or not, they need to maintain downstream membership status
by sending IGMPv3/MLDv2 General Query messages as some IGMPv3/MLDv2
messages may be lost during transmission.
4. Tuning IGMP/MLD Timers and Values
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4.1. Tuning IGMP/MLD General Query Interval
IGMP and MLD are non-reliable protocols; to cover the possibility of
a State-Change Report being missed by one or more multicast routers,
"hosts retransmit the same State-Change Report messages [Robustness
Variable] - 1 more times", at intervals chosen at random from the
range (0, [Unsolicited Report Interval]) [1][2]. Although this
behavior increases the protocol robustness, it does not guarantee
that the State-Change Report reaches the routers. Therefore, routers
still need to refresh the downstream membership information by
receiving Current-State Report periodically solicited by IGMP/MLD
General Query sent in the [Query Interval] period, in order to
enhance robustness of the host in case of link failures and packet
loss. It also supports the situation that mobile hosts turn off or
move from a network to other network managed by a different router
without any notification (e.g., leave request).
The [Query Interval] is the interval between General Queries sent by
the regular IGMPv3/MLDv2 querier, and the default value is 125
seconds [1][2]. By varying the [Query Interval], multicast routers
can tune the number of IGMP/MLD messages on the network; larger
values cause IGMP/MLD Queries to be sent less often.
This document proposes 150 seconds for the [Query Interval] value by
changing the Querier's Query Interval Code (QQIC) field specified in
the IGMP/MLD Query message, for the case that a router enabling the
explicit tracking function potentially operates a large number of
member hosts such as more than 200 hosts on the wireless link. This
longer interval value contributes to minimizing traffic of Report
messages and battery power consumption for mobile hosts.
On the other hand, this document also proposes 60 to 90 seconds for
the [Query Interval] value for the case that a router enabling the
explicit tracking function attaches to a wireless link with higher
capacity. This shorter interval contributes to quick synchronization
of the membership information tracked by the router but may consume
battery power of mobile hosts.
If a router does not enable the explicit tracking function, the
[Query Interval] value would be its default value, 125 seconds.
In situations where Mobile IPv6 [7] is used, when the home agent
implements multicast router functionality and multicast data packets
are tunneled to and from the home agent, the home agent may want to
slow down Query periodicity. It happens, for example, when the home
agent detects network congestion. In this case, the home agent
starts forwarding queries with the default [Query Interval] value and
increases the value in a gradual manner.
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4.2. Tuning IGMP/MLD Query Response Interval
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 "Max Resp Code=100" for IGMPv3 [1] and "Maximum Response
Code=10000" for MLDv2 [2]. 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's
responses are spread out over a larger interval, but will increase
join latency when State-Change Report (i.e., join request) is
missing.
According to our experimental analysis, this document proposes two
tuning scenarios for tuning the [Query Response Interval] value in
different wireless link conditions; one scenario is for a wireless
link with a lower capacity of network resource or a lossy link, and
the other scenario is for a wireless link with enough capacity or
reliable condition for IGMP/MLD message transmission.
Regarding the first scenario, for instance, when a multicast router
attaches to a bursty IEEE 802.11b link, the router configures the
longer [Query Response Interval] value, such as 10 to 20 (sec). This
configuration will reduce congestion of the Current-State Report
messages on a link but may increase join latency and leave latency
when the unsolicited messages (State-Change Record) are lost on the
router. Note that as defined in Section 4.1.1 of [1], in IGMPv3, the
Max Resp Code larger than 128 represents the exponential values, and
changes the granularity. For example, if one wants to set the Max
Response Time to 20.0 seconds, the Max Resp Code should be expressed
with "0b10001001", which is divided into "mant=0b1001" and
"exp=0b000".
The second scenario may happen for a multicast router attaching to a
wireless link having higher capacity of the resource or a point-to-
(multi-)point link such as an IEEE 802.16e link, because IGMP/MLD
messages do not seriously affect the link condition. The router can
seek Current-State Report messages with the shorter [Query Response
Interval] value, such as 5 to 10 (sec). This configuration will
contribute to quickly (at some level) discovering non-tracked member
hosts and synchronizing the membership information.
4.3. Tuning Last Member Query Timer (LMQT) and Last Listener Query
Timer (LLQT)
Shortening the Last Member Query Timer (LMQT) for IGMPv3 and the Last
Listener Query Timer (LLQT) for MLDv2 contributes to minimizing leave
latency. LMQT is represented by the Last Member Query Interval
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(LMQI), multiplied by the Last Member Query Count (LMQC), and LLQT is
represented by the Last Listener Query Interval (LLQI), multiplied by
the Last Listener Query Count (LLQC).
While LMQI and LLQI are changeable, it is reasonable to use the
default values (i.e., 1 second) for LMQI and LLQI in a wireless
network. LMQC and LLQC, whose default value is the [Robustness
Variable] value, are also tunable. Therefore, LMQC and LLQC can be
set to "1" for routers enabling the explicit tracking function, and
then LMQT and LLQT are set to 1 second. However, setting LMQC and
LLQC to 1 increases the risk of missing the last member; LMQC and
LLQC ought to be set to 1 only when network operators think that
their wireless link is stable enough.
On the other hand, if network operators think that their wireless
link is lossy (e.g., due to a large number of attached hosts or
limited resources), they can set LMQC and LLQC to "2" for their
routers enabling the explicit tracking function. Although bigger
LMQC and LLQC values may cause longer leave latency, the risk of
missing the last member will be reduced.
4.4. Tuning Startup Query Interval
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, a shortened value such as 1 second would help
contribute to shortening handover delay for mobile hosts in, e.g.,
the base solution with PMIPv6 [9]. 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
need to properly configure this value.
4.5. Tuning Robustness Variable
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 [1][2]. The QRV (Querier's Robustness Variable) field
in IGMP/MLD Query contains the [Robustness Variable] value used by
the querier. The default [Robustness Variable] value defined in
IGMPv3 [1] and MLDv2 [2] is "2".
This document proposes "2" for the [Robustness Variable] value for
mobility, when a router attaches to a wireless link having lower
capacity of the resource or a large number of hosts. For a router
that attaches to a wireless link having higher capacity or is known
to be reliable, it is not required to retransmit the same State-
Change Report message; hence the router sets the [Robustness
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Variable] to "1".
4.6. Tuning Scenarios for Various Mobile IP Networks
In mobile IP networks, IGMP and MLD are used either with three
deployment scenarios; (1) running directly between host and access
router on a wireless network, (2) running between host and home
router through a tunnel link, and (3) running between home router and
foreign router through a tunnel link.
When a receiver host connects directly through a wireless link to a
foreign access router or a home router, the tuning of the IGMP/MLD
protocol parameters should be the same as suggested in the previous
sections. The example of this scenario occurs when in Proxy Mobile
IPv6 (PMIPv6) [6], the mobile access gateway, whose role is similar
to a foreign router, acts as a multicast router or proxy.
The second scenario occurs when bi-directional tunnel established
between host and home router is used to exchange IGMP/MLD messages
such as in [7][10]. There are difficulties in tuning the parameters
in this situation, because the tunnel link condition is diverse and
changeable. When a host is far away from the home router, the
transmission delay between the two entities may be longer and the
packet delivery may be more unreliable. Thus the effects of the
IGMP/MLD message transmission through a tunnel link ought to be
considered during the parameter setting. For example, the [Query
Interval] and [Query Response Interval] could be set shorter to
compensate the transmission delay, and the [Robustness Variable]
could be increased for possible packet loss.
The third scenario occurs in [9], in which the mobile access gateway
(i.e., foreign router) acts as the IGMP/MLD Proxy [5] in PMIPv6 [6].
Through the bi-directional tunnel established with the local mobility
anchor (i.e., home router), the mobile access gateway sends summary
reports of its downstream member hosts to the local mobility anchor.
Apart from the distance factor that influences the parameter setting,
the [Query Response Interval] on the local mobility anchor could be
set to a smaller value because the number of the mobile access
gateways is much smaller compared to that of the host and the chances
of packet burst is low for the same reason. And the power
consumption due to a lower query interval is not an issue for the
moble access gateways because the mobile access gateways are usually
not battery-powered.
Ideally, the IGMP/MLD querier router adjusts its parameter setting
according to the actual mobile IP network conditions to benefit
service performance and resource utilization. It would be desirable
that a home router determines aforementioned timers and values
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according to the delay between the initiating IGMP/MLD Query and the
responding IGMP/MLD Report, while describing such mechanism
dynamically adjusting these timers and values is out of scope of this
document.
5. Destination Address of Specific Query
IGMP/MLD Group-Specific and Group-and-Source Specific Queries defined
in [1][2] 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.
Group-specific queries are sent with an IP destination address equal
to the multicast address of interest, as defined in [1][2]. Using
the multicast group of interest in the specific query is preferred in
this environment because hosts that do not join the multicast session
do not pay attention to these specific Queries, and only active
member hosts that have been receiving multicast contents with the
specified address reply to IGMP/MLD reports.
6. Interoperability
IGMPv3 [1] and MLDv2 [2] 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 is required to support
Source-Specific Multicast (SSM) [3].
Recently, the Lightweight-IGMPv3 (LW-IGMPv3) and Lightweight-MLDv2
(LW-MLDv2) [4] protocols have been defined as the proposed standard
protocols 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 that both multicast routers and mobile hosts
are IGMPv3/MLDv2 capable, regardless whether the protocols are the
full or lightweight version. And this document does not consider
interoperability with older version protocols. One of the reasons
not being interoperable with older IGMP/MLD protocols is that the
explicit tracking function does not work properly with older IGMP/MLD
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protocols because of a report suppression mechanism; a host would not
send a pending IGMP/MLD report if a similar report was sent by
another listener on the link.
7. IANA Considerations
This document has no actions for IANA.
8. Security Considerations
This document neither provides new functions or modifies the standard
functions defined in [1][2][4]. Therefore there is no additional
security consideration provided.
9. Acknowledgements
Luis M. Contreras, Marshall Eubanks, Gorry Fairhurst, Dirk von Hugo,
Imed Romdhani, Behcet Sarikaya, Stig Venaas, Jinwei Xia, and others
provided many constructive and insightful comments.
10. References
10.1. Normative References
[1] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version 3",
RFC 3376, October 2002.
[2] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2
(MLDv2) for IPv6", RFC 3810, June 2004.
[3] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
RFC 4607, August 2006.
[4] Liu, H., Cao, W., and H. Asaeda, "Lightweight Internet Group
Management Protocol Version 3 (IGMPv3) and Multicast Listener
Discovery Version 2 (MLDv2) Protocols", RFC 5790,
February 2010.
10.2. Informative References
[5] Fenner, B., He, H., Haberman, B., and H. Sandick, "Internet
Group Management Protocol (IGMP) / Multicast Listener Discovery
(MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")",
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RFC 4605, August 2006.
[6] Gundavelli, S, Ed., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
[7] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 6275, July 2011.
[8] Asaeda, H. and N. Leymann, "IGMP/MLD-Based Explicit Membership
Tracking Function for Multicast Routers",
draft-ietf-pim-explicit-tracking-00.txt (work in progress),
October 2011.
[9] Schmidt, T., Waehlisch, M., and S. Krishnan, "Base Deployment
for Multicast Listener Support in Proxy Mobile IPv6 (PMIPv6)
Domains", RFC 6224, April 2011.
[10] Perkins, Ed., C., "IP Mobility Support for IPv4, Revised",
RFC 5944, November 2010.
Appendix A. Unicasting General Query
This appendix describes the possible IGMP and MLD protocol extensions
for further optimization in mobile and wireless environments. It
might be beneficial to include the following consideration when the
new version or modification of IGMP and MLD protocols are considered
in the future.
IGMPv3 and MLDv2 specifications [1][2] 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. 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. On the other hand, according to [1][2], a router may
be able to unicast General Query to the tracked member hosts in
[Query Interval], if the router keeps track of membership information
(Section 3).
Unicasting IGMP/MLD General Query would reduce the drain on battery
power of mobile hosts as only the active hosts that have been
receiving multicast contents respond the unicast IGMP/MLD General
Query messages and non-active hosts do not need to pay attention to
the IGMP/MLD Query messages. This also allows the upstream router to
proceed fast leaves (or shorten leave latency) by setting LMQC/LLQC
smaller, because the router can immediately converge and update the
membership information, ideally.
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However, there is a 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 retransmit the same join requests (i.e., unsolicited
Report messages), they lose 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. However, using two
different timers for General Queries would require the protocol
extension this document does not focus on. If a router does not
distinguish the multicast and unicast General Query Intervals, the
router should only use and enable multicast General Query.
Also, unicasting General Query does not remove multicasting General
Query. Multicast General Query is necessary to update membership
information if it is not correctly synchronized due to missing
Reports. Therefore, enabling unicast General Query should not be
used for the implementation that does not allow to configure
different query interval timers as [Query Interval] and [Unicast
Query Interval]. If a router does not distinguish these multicast
and unicast General Query Intervals, the router should only use and
enable multicast General Query.
Authors' Addresses
Hitoshi Asaeda
Keio University
Graduate School of Media and Governance
5322 Endo
Fujisawa, Kanagawa 252-0882
Japan
Email: asaeda@wide.ad.jp
URI: http://www.sfc.wide.ad.jp/~asaeda/
Hui Liu
Huawei Technologies Co., Ltd.
Huawei Bld., No.3 Xinxi Rd.
Shang-Di Information Industry Base
Hai-Dian Distinct, Beijing 100085
China
Email: helen.liu@huawei.com
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Qin Wu
Huawei Technologies Co., Ltd.
Site B, Floor 12F, Huihong Mansion
No.91 Baixia Rd.
Nanjing, Jiangsu 21001
China
Email: bill.wu@huawei.com
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