Network working group Q. Wu
Internet Draft H. Liu
Category: Informational Huawei
Created: October 25, 2010
Expires: April 2011
Proposal for Tuning IGMPv3/MLDv2 Protocol Behavior in Wireless and
Mobile networks
draft-wu-multimob-igmp-mld-tuning-03
Abstract
This document proposes a variety of optimization approaches for
tuning IGMPv3 and MLDv2 protocols. It aims to provide useful
guideline to allow efficient multicast communication in wireless and
mobile networks using the current IGMP/MLD protocols.
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 [RFC2119].
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
Wu,et al Expires April 25, 2011 [Page 1]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on August 15, 2009.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document.
Table of Contents
1. Introduction..................................................3
2. Impact of wireless and mobility on IGMP/MLD...................3
2.1. Comparison analysis between wired and wireless multicast.4
2.2. Link models analysis for wireless multicast..............5
2.3. Requirements of wireless and mobile multicast on IGMP/MLD8
3. Evaluation of IGMP/MLD on wireless and mobile multicast.......9
4. IGMP/MLD tuning optimization for Wireless or Mobile Network..11
4.1. Explicit Tracking and Query Suppression.................11
4.2. Report Suppression for the hosts........................13
4.3. Query Suppression for the routers.......................13
4.4. Minimizing Query Frequency by increasing interval each time
............................................................14
4.5. Switching Between Unicast Query and Multicast Query.....15
4.6. Using General Query with Unicast Query..................16
4.7. Retransmission of General Queries.......................16
4.8. General Query Suppression with no receiver..............17
4.9. Tuning Response Delay according to link type and status.17
4.10. Triggering reports and queries quickly during handover.18
5. Security Considerations......................................19
6. Acknowledgement..............................................19
7. References...................................................19
Wu,et al Expires April 25, 2011 [Page 2]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
7.1. Normative References...................................19
7.2. Informative Referencess................................20
Authors' Addresses............................................21
1. Introduction
Multicasting is more efficient a method of supporting group
communication than unicasting. With the wide deployment of
different wireless networks, multicast communication over wireless
network comes to attract more and more interests from content and
service providers, but still faces great challenges when considering
dynamic group membership and constant update of delivery path due to
node movement, which is highly required in the wireless or mobile
network. On the other hand, unlike wired network, some of wireless
networks often offer limited reliability, consume more power and
cost more transmission overhead, thus in worse case are more prone
to loss and congestion.
Multicast network is generally constructed by IGMP/MLD group
management protocol to track valid receivers and by multicast
routing protocol to build multicast delivery paths. This document
focuses only on IGMP/MLD protocols, which are used by a mobile user
to subscribe a multicast group and are most possibly to be exposed
to wireless link to support terminal mobility. As IGMP and MLD are
designed for fixed users using wired link, they does not work
perfectly for wireless link types. They should be enhanced or tuned
to adapt to wireless and mobile environment to meet the reliability
and efficiency requirements in the scenarios described in
[REQUIRE][RFC 5757].
This memo proposes a variety of optimization approaches for tuning
IGMP/MLD protocols in wireless or mobile communication environment.
It aims to make the minimum tuning on the protocol behavior without
introducing interoperability issues, and to improve the performance
of wireless and mobile multicast networks. These solutions can also
be used in wired network when efficiency and reliability are
required. They are discussed in detail in Section 4.
2. Impact of wireless and mobility on IGMP/MLD
This section analyzes the impact of wireless or mobility on IGMP/MLD
by comparing wireless multicast with wired multicast and comparing
different wireless link models. It then gives the requirements of
Wu,et al Expires April 25, 2011 [Page 3]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
wireless and mobile multicast on IGMP/MLD protocols according to the
analysis.
2.1. Comparison analysis between wired and wireless multicast
Existing multicast support for fixed user can be extended to mobile
users in wireless environments. However applying such support to
wireless multicast is difficult for the following five reasons.
O Limited Bandwidth: In contrast with wired link, wireless link
usually has limited bandwidth. This situation will be made even
worse if wireless link has to carry high volume video multicast
data. Also the bandwidth available in upstream direction and
downstream direction may not be equal.
O Large packets Loss: In contrast with wired multicast, wireless
multicast has packet loss that range between 1% and 30%, based on
the links types and conditions. And when packets have to travel
between home and access networks e.g. through tunnel, the packets
are prone to be lost if the distance between the two networks is
long.
O Frequent Membership change: In fixed multicast, membership change
only happens when a user leave or joins a group while in the
mobile multicast, membership changes may also occur when a user
changes its location.
O Prone to performance degradation: Due to possible unwanted
interaction of protocols across layers and user movement, the
wireless network may be overwhelmed with more excessive traffic
than wired network. In worse case, this may lead to network
performance degrading and network connection complete loss.
O Increased Leave Latency: Unlike fixed multicast, the leave latency
in the mobile multicast will be increased due to user movement.
And if the traffic has to be transmitted between access network
and the home network, or if the handshake is required between
these two networks, the Leave Latency will be increased further
more.
Figure 1 shows the details for the difference between wired/fixed
multicast and wireless/mobile multicast.
Wu,et al Expires April 25, 2011 [Page 4]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
+--------------+---------------------+----------------------------+
| | Wired or fixed | Wireless/mobile |
| Issues | Multicast | multicast |
+--------------+---------------------+----------------------------+
| | | Limited and variable |
| Bandwidth | Plentiful | possibly asymmetric |
+--------------+---------------------+----------------------------+
| | | |
| Loss of | Infrequent(<1%) | Frequent and variable |
| Packets | | (1%-30% based on links) |
+--------------+---------------------+----------------------------+
| | | |
| Membership | Only when a user | Also when a user moves |
| Changes | leaves and joins | to another location |
| | a group | |
+--------------+---------------------+----------------------------+
| | | More complex due to |
| | Possible use of a | wireless links and user |
| Reliability | transport-layer | mobility; possible unwanted|
| | protocol(such as the| interaction of protocols |
| | Multicast File | at transport and link |
| | Transfer Protocol) | layers |
+--------------+---------------------+----------------------------+
| | | Increased due to |
|Leave Latency | not changed by | user movement |
| | user movement | and lost packet |
-------------------------------------+----------------------------
Figure 1. Comparison between wired/fixed multicast
and wireless/mobile multicast
2.2. Link models analysis for wireless multicast
There are various types of wireless links, each with different
feature and performance. In this document, we according to the
transmission mode categorize the wireless link type into three
typical link models:
O Point To Point (PTP) link model
O Point To Multipoint (PTMP) link model
O Broadcast link model
PTP link model is the model with one dedicated link that connects
exactly two communication facilities. For multicast transmission,
each PTP link has only one receiver and the bandwidth is dedicated
Wu,et al Expires April 25, 2011 [Page 5]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
for each receiver. Also one unique prefix or set of unique prefixes
will be assigned to each receiver. Such link model can be
accomplished by running PPP on the link or having separate VLAN for
each receiver.
PTMP link model is the model with multipoint link which consists of
a series of receivers and one centralized transmitter. Unlike P2P
link model, PTMP provide downlink common channels and dedicated
uplink channel for each user. Bandwidth and prefix in this model
are shared by all the receivers on the same link. Therefore
Duplicate Address Detection (DAD) should be performed to check
whether the assigned address is used by other receivers.
Broadcast link model is the model with the link connecting two or
more nodes and supporting broadcast transmission. Such link model is
quite similar to fixed Ethernet link model and its link resource is
shared in both uplink and downlink directions. The bandwidth and
prefix are shared by all the receivers and DAD is required to avoid
address collision.
Figure 2 shows the details for the difference between different
wireless link models.
Wu,et al Expires April 25, 2011 [Page 6]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
+---------------+-----------------+---------------+---------------+
| Features | PTP | PTMP | Broadcast |
| | link model | link model | link model |
+---------------+-----------------+---------------+---------------|
| | | Common | |
| Shared link/ |Dedicated uplink | downlink | |
| Dedicated link|and downlink | channels and |common downlink|
| |channels for each| dedicated | Channel for |
| |user | uplink |each user |
| | | channels for | |
| | | each | |
| | | user | |
+---------------+-----------------+---------------+---------------|
| | | Prefix shared | Prefix shared |
| Shared Prefix | Per Prefix for | by all | by all |
| /Dedicated | each receiver | receivers | receivers |
| Prefix | No need DAD |DAD is required|DAD is required|
+---------------+-----------------+---------------+---------------|
| | | | |
|Shared Service | | | |
| Support | Not Support | Support | Support |
| | | | |
+---------------+-----------------+---------------+---------------|
| | Only one node | Link Layer | Broadcast |
| | On the link | Multicast | Support |
| | Forward | Support | at L2 |
| link layer | multicast | using | using switch |
| Broadcast | packets to | Backend | |
| Multicast | the only | (e.g.,AR) | IGMP/MLD |
| Support | receiver | IGMP/MLD | Snooping |
| | on the | Snooping | at switch |
| | link | at AR | |
+---------------+-----------------+---------------+---------------|
| | | | |
| | | | Ethernet |
| Ethernet | Not support | Not support | Support By |
| link Support | | | Implementing |
| | | | Bridge |
| | | | |
+---------------+-----------------+---------------+---------------+
Figure 2. Wireless Link Models Analysis
Wu,et al Expires April 25, 2011 [Page 7]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
2.3. Requirements of wireless and mobile multicast on IGMP/MLD
Due to the characteristics of wireless and mobile multicast
described in the section 2.1 and 2.2, it is desirable for IGMP and
MLD to have the following characteristics when used in wireless and
mobile networks [REQUIRE]:
o Adaptive to different link characteristics: IGMP and MLD are
originally designed for wired multicast and some of their processing
is not applicable to wireless multicast for its asymmetrical link,
limited bandwidth, larger packet loss rate, increased leave latency,
and etc. Also Wireless network has various link types, each of them
has different bandwidth and performance. These require IGMP/MLD
protocol behavior should be tuned to adapt to different link model
and link conditions.
o Minimal Join and Leave Latency: Fast join and leave of a
subscriber helps to improve the user's experience during channel
join and channel zapping. Fast leave also facilitates releasing of
unused network resources quickly. Besides, mobility and handover
may cause a user to join and leave a multicast group frequently,
which also require fast join and leave to accelerate service
activation and to optimize resource usages.
o Robustness to packet loss: Wireless link has the characteristic
that packet transmission is unreliable due to instable link
conditions and limited bandwidth. For mobile IP network, packets
sometimes have to travel between home network and foreign network
and have the possibility of being lost due to long distance
transmission. These network scenarios have more strict robustness
requirement on delivery of IGMP and MLD protocol messages.
o Minimum packet transmission: Wireless link resources are usually
more precious and limited compared to their wired counterpart, and
are prone to be congested when carrying high volume multicast stream.
Minimizing packet exchange without degrading general protocol
performance should also be emphasized to improve efficiency and make
good use of network capacity and processing capability.
o Avoiding packet burst: Large number of packets generated within a
short time interval may have the tendency to deteriorate wireless
network conditions. IGMP and MLD when using in wireless and mobile
networks should be optimized if their protocol message generation
has the potential of introducing packet burst.
Wu,et al Expires April 25, 2011 [Page 8]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
According to these requirements, in the following parts of the
document, current versions of IGMP/MLD protocols are evaluated
whether their various protocol aspects are applicable to wireless
and mobile multicast communications. They will be optimized to meet
these requirements without new features introduced on the wire or
link, without new message type defined, and without interoperability
issues introduced, which is referred to as "tuning" of IGMP/MLD
protocols.
3. Evaluation of IGMP/MLD on wireless and mobile multicast
This section analyzes the applicability of IGMP and MLD to wireless
communication in the following aspects:
O General evaluation of different versions: IGMPv2 [RFC2236] and
MLDv1 [RFC2710] only support ASM communication mode. They do not
support SSM subscription and explicit tracking. IGMPv3 [RFC3376]
and MLDv2 [RFC3810] and their lightweight version LW-IGMPv3/LW-
MLDv2 [RFC5760] support all the features of ASM/SSM communication
modes and explicit tracking. Because SSM is more efficient and
secure than ASM for IPTV application, and explicit tracking
enables faster channel zapping and better manageability capability,
IGMPv3/MLDv2 and LW-IGMPv3/MLDv2 are more promising to be deployed
widely than IGMPv2 and MLDv1.
O Robustness: IGMP/MLD actively sends unsolicited Report or Leave
message to join or leave a group, and solicited Report to respond
to Queries. Unsolicited Report and Leave messages are more
important for ensuring satisfactory user experience and should be
guaranteed to improve service performance. Current IGMP and MLD
provide the reliability for these messages by non responsive
retransmission, which is not adequate from both the robustness and
efficiency aspects when they are used on unreliable wireless link
or have to be exchanged over the tunnel between home network and
access network separated by long distance [ROBUST][ACK]. For
IGMPv3/MLDv2, because unsolicited report and leave messages will
not be suppressed by report from other host, it is possible to
adopt acknowledgement-retransmission to improve reliability and
reduce superfluous packet transmission [IGMP-ACK].
Besides, for IGMPv3/MLDv2, because the router could by explicit
tracking establishes membership database recording each valid
receiver, it is possible to deduce the possible loss of some
protocol messages according to the feedback after their
transmission, and to take some remedies (e.g. by retransmission)
Wu,et al Expires April 25, 2011 [Page 9]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
to enable more reliable transmission of these messages in bad
conditions.
O Efficiency: IGMPv2 and MLDv1 use host suppression to suppress
duplicated membership reports on the link. In IGMPv3 and MLDv2,
because host suppression is not adopted, the report count will be
numerous if the number of valid receivers on the network is large.
IGMPv3 and MLDv2 should be optimized to try to minimize
unnecessary packet transmission to compensate this drawback. As an
example, because an IGMPv3/MLDv2 router has record of each user in
its state database by explicit tracking, it is possible to
eliminate the need for query timeouts when receiving leave
messages and to improve the efficiency by reducing both the
unnecessary Queries and reports generated on a network.
And as described in [REQUIRE] and [RFC5757], the default timer
values and counter values specified in IGMP and MLD were not
designed for the mobility context. This may result in a slow
reaction following a client join or leave, in possible packet loss
under worse conditions, or in overburdening the wireless link by
excessive packets exchange than necessary. These issues can be
addressed by tuning these parameters for the expected packet loss on
a link to optimize service performance and resource usage.
The comparison between IGMPv2/MLDv1 and IGMPv3/MLDv2 is illustrated
in figure 3. In summary, it is desirable to choose IGMPv3/MLDv2 or
LW-IGMPv3/MLDv2 as the group management protocol for wireless or
mobile multicast. They should be optimized to adapt to wireless and
mobile networks to meet the efficiency and reliability requirement
for these networks. These optimizations range from the tuning of the
parameters (e.g. the Query Interval and other variables), to the
tuning of protocol behavior without introducing interoperability
issues. Considering an enhancement in one direction might introduce
side effects in another one, balances should be taken carefully to
avoid defects and improve protocol performance as a whole.
Wu,et al Expires April 25, 2011 [Page 10]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
+---------------------+----------------------+-------------------+
| Issues | IGMPv2/MLDv1 | IGMPv3/MLDv2 |
+---------------------+----------------------+-------------------+
|Default Timer and | Not designed for | Not designed for|
|Robustness Variable | Mobility context | Mobility context|
| | Need to be tuned | Need to be tuned|
+---------------------+----------------------+-------------------+
| | | |
| Explicit Tracking | Not Support | Support |
| | | |
+---------------------+----------------------+-------------------+
| ASM and SSM | Only Support ASM | |
| Subscription | Subscription | Both Support |
+---------------------+----------------------+-------------------+
| | | |
| Explicit Join | | |
| and Leave | Support | Support |
| | | |
+---------------------+----------------------+-------------------+
| | | |
|Host Suppression | Support | Not Support |
+---------------------+----------------------+-------------------+
Figure 3. Comparison between IGMPv2/MLDv1 and IGMPv3/MLDv2
4. IGMP/MLD tuning optimization for Wireless or Mobile Network
As mentioned in section 2, IGMPv3/MLDv2 or LW-IGMPv3/MLDv2 is
recommended to be used as the basis for optimization of IGMP/MLD to
adapt to wireless and mobile networks. In this section, taking
these characteristics requirement into account, we will discuss
several optimization approaches for tuning of IGMPv3 and MLDv2 in
wireless environment. The optimizations try to minimize the packet
transmission for both the Reports and Queries, and at the meanwhile
take the factor of improving reliability into account, with minimum
cost. Different link types are also considered for the tuning
behavior.
4.1. Explicit Tracking and Query Suppression
In IGMPv2/MLDv1, the member reports are suppressed if the same
report has already been sent by another host in the network which is
also referred to as host suppression. As described in the A.2 of
[RFC3810], the suppression of multicast listener reports has been
removed in MLDv2 due to the following reasons:
Wu,et al Expires April 25, 2011 [Page 11]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
O Routers may want to track per-host multicast listener status on an
interface. This enables the router to track each individual host
that is joined to a particular group or channel and allow minimal
leave latencies when a host leaves a multicast group or channel.
o Multicast Listener Report suppression does not work well on
bridged LANs. Many bridges and Layer2/Layer3 switches that
implement MLD snooping do not forward MLD messages across LAN
segments in order to prevent multicast listener report suppression.
o By eliminating multicast listener report suppression, hosts have
fewer messages to process; this leads to a simpler state machine
implementation.
o In MLDv2, a single multicast listener report now bundles multiple
multicast address records to decrease the number of packets sent.
In comparison, the previous version of MLD required that each
multicast address be reported in a separate message.
Without host suppression, it is possible to enable explicit tracking
on a router by which the local replication can be used by the router
to inspect incoming join and leave requests, record or refresh the
membership state for each host on the interface, and take
appropriate action to each received report. In the meanwhile, the
router builds a table to track which channel being forwarded to each
port. If the channel being requested to view is already being
received at the router, it can replicate the stream and forward to
this new requester which ensure good response time.
By using the tracking table mentioned above, the router has the
capability to learn if a particular multicast address has any
members on an attached link or if any of the sources from the
specified list for the particular multicast address has any members
on an attached link or not. Such capability makes Group specific
Query or Source-and-Group Specific Queries, which are sent to query
other members when a member leaves, unnecessary to be used because
the router has already known who are active on the interface using
explicit tracking. Therefore it is desirable that these two Queries
are eliminated when explicit tracking is used. But General
periodical Query by a router to solicit current state reports to
refresh existing membership state database should still be used to
prevent incorrectness of the database due to the possible loss of
explicit join and leave message in some cases.
The main benefits of using explicit tracking without Group specific
Query or Source-and-Group Specific Queries are that it provides:
Wu,et al Expires April 25, 2011 [Page 12]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
O minimizing packet number and packet burst: Elimination of Group and
Source-Group specific Queries when a member leaves a group will
reduce the number of transmitted Group Specific Queries. And
finally the total number of Reports in response to Group Specific
Queries can be drastically reduced.
O Minimal leave latencies: an IGMPv3/MLDv2 router configured with
explicit tracking can immediately stop forwarding traffic if the
last host to request to receive traffic from the router indicates
its leave from the group.
O Faster channel changing: The channel change time of the receiver
application depends on the leave latency, that is to say, single
host can not receive the new multicast stream before forwarding of
the old stream has stopped.
O Reducing Power consumption: Due to elimination of the suppression
of membership reports, the host does not need to spend processing
power to hear and determine if the same report has already been
sent by another host in the network, which is beneficial to mobile
hosts that do not have enough battery power.
4.2. Report Suppression for the hosts
The large number of Reports and bad link condition may result in
packets burst. This packet burst can be mitigated by having the
router aggregate the responses (membership reports) from multiple
clients. The router can intercept IGMP/MLD reports coming from hosts,
and forwards a summarized version to the upstream router only when
necessary. Typically this means that the router will forward IGMP/MLD
membership reports as follows:
- Unsolicited membership reports (channel change requests) are
forwarded only when the first subscriber joins a multicast group, or
the last subscriber leaves a multicast group. This tells the upstream
router to begin or stop sending this channel to this router.
- Solicited membership reports (sent in response to a query) are
forwarded once per multicast group. The router may also aggregate
multiple responses together into a single membership report.
4.3. Query Suppression for the routers
The large number of Queries and bad link condition may result in
packets burst. This packet burst can be mitigated by having the
downstream router stop forwarding IGMP/MLD Queries packets sent to
Wu,et al Expires April 25, 2011 [Page 13]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
the hosts and respond with report as proxy to the upstream router.
Typically this means that the router will:
- Never send a specific query to any client, and
- Send general queries only to those clients receiving at least one
multicast group
4.4. Minimizing Query Frequency by increasing interval each time
In IGMPv3/MLDv2, Group Specific Queries and Source and Group
specific Queries are sent for [Last Member Query Count] times with
short fixed [Last Member Query Interval], to learn whether there are
valid members from an attached link. If the network is undergoing
congestion, the multiple transmissions of the queries may further
deteriorate the bad conditions. To eliminate the bad effects for
this, these Queries can be slowed down when a router can not collect
successfully expected members' report responses in the mean while it
detects the network congestion is going to happen. The slowing down
process of the Queries could be arranged in a prolonged time
interval as described in [ADAPTIVE].
The slow down behavior is: a router after sending a Query, if
acquires the expected responses from the receivers, refreshes its
state database and stop the querying retransmission process, or if
after a time interval fails to get the expected report responses,
resends a Query with an increased (e.g. double) interval. This
process can be repeated, for each time the retransmission is
arranged in a prolonged time interval, till the router receives the
expected responses, or determines the receiver is unreachable and
then stops the sending of the Query ultimately. The router can make
judgment on not getting expected response from the Queries in the
following cases:
O When Group Specific Query and Source and Group Specific Queries
are used to track other numbers, the router can not collect any
response from the link.
O When all group members leave the group or move out of scope, the
General Query sent by the router can not solicit any responses
from the link, as mentioned in section 4.9.
O When General Query is retransmitted due to possible loss deducing
from no responses from valid members in the database.
Wu,et al Expires April 25, 2011 [Page 14]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
O When General Query is retransmitted by a router on startup
[RFC3376][RFC3810], it gets no membership response from the
interface.
O When unicast Query is sent to solicit a particular receiver, if
the router can not get responses from the receiver, as described
in section 4.5 and 4.6.
In the above cases, if the router fails to get expected response
from the network, and if the link condition is bad or in congestion,
the router could retransmit the Queries in increased interval. This
query retransmission with incremental interval enables the router to
reduce the total packet retransmission times in the same time period
comparing with retransmission for multiple times with fixed interval,
and at the mean time gain some degree of reliability. The variable
time interval and the termination condition should be configurable
and could be set according to actual network condition, which is out
the scope of this document.
4.5. Switching Between Unicast Query and Multicast Query
IGMP/MLD protocols define the use of multicast Queries whose
destination addresses are multicast addresses and also allow use of
unicast Queries with unicast destination. The unicast Query is sent
only for one destination and has the advantages of not affecting
other host on the same link. This is especially desirable for
wireless communication because the mobile terminal often has limited
battery power. But if the number of valid receivers is large, using
unicast Query instead of multicast Query will introduce large number
of Queries because each Query will be generated for each member,
which will not be an efficient use of link resources. In this case
the normal multicast Query will be a good choice because only one
Query needs to be sent. On the other hand of the number of receivers
to be queried is small, the unicast Query is advantageous over
multicast one.
The router can choose to switch between unicast and multicast Query
according to the practical network conditions. For example, if the
receiver number is small, the router could send unicast Queries
respectively to each receiver to solicit their membership states,
without arousing other host which is in the dormant state. When the
receiver number reaches a predefined level, the router could change
to use multicast Queries. The router could make the switching
flexibly according to practical conditions to improve the efficiency.
Wu,et al Expires April 25, 2011 [Page 15]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
4.6. Using General Query with Unicast Query
Unicast Query also can be used in addition to General Query to
improve the robustness of solicited reports when General Query fails
to collect its valid members. It requires the explicit tracking to
be enabled on the router. Its basic behavior is: a router after
sending a periodical Query collects successfully all the members'
report responses except for one or two which are currently still
valid in its database. This may be because the non-respondent ones
silently leave the network without any notification, or because
their reports are lost due to some unknown reason. The router in
this case could choose to unicast a Query respectively to each non-
respondent receiver to check whether they are still alive for the
multicast reception, without affecting the majority of receivers
that have already responded. Unicast Queries under this condition
could be sent for [Last Member Query Count] times, following the
same rule of [3376] or [3810], or could be resent in incremental
interval, as described in section 4.4.
4.7. Retransmission of General Queries
In IGMPv3 and MLDv2, apart from the continuously periodical
transmission, General Query is also transmitted during a router's
startup. It will be transmitted for [Startup Query Count] times with
[Startup Query Interval], to improve reliability of General Query
during startup. There are some other cases where retransmission of
General Query is beneficial which are not covered by current
IGMPv3/MLDv2 protocols as shown in the following.
For example, a router which keeps track of all its active receivers,
if after sending a General Query, may fail to get any response from
the receivers which are still valid in its membership database. This
may be because all the valid receivers leaves the groups or moves out
of the range of the link at the moment, or because all the responses
of the receivers are lost, or because the sent Query does not arrive
at the other side of the link. If current database indicates the
number of the valid receiver is not small, the router could choose to
compensate this situation by retransmitting the General Query to
solicit its active members.
This compensating General Query could be sent several times, if the
router can not get any feedback from the receivers which are previous
in the database. The repetition of the transmission could in fixed
Wu,et al Expires April 25, 2011 [Page 16]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
interval such as [Last Member Query Interval], or could in prolonged
interval if the link condition is not good.
4.8. General Query Suppression with no receiver
In IGMPv3 and MLDv2, General Query is multicast sent periodically
and continuously without any limitations. It helps solicit the
state of current valid member but has influence on all terminals,
whether they are valid multicast receivers or not. When there is no
receiver on the link, the transmission of the General Query is a
waste of resources for both terminals and the router.
The IGMPv3/MLDv2 router could suppress its transmission of General
Query if there is no valid multicast receiver on the link, e.g. in
the following cases:
O If the last member reports its leave for a group. This could be
judged by an explicit tracking router checking its membership
database, or by a non explicit tracking router sending Group and
Source Group Specific Queries;
O If the only member on a PTP link reports its leaving;
O If the router after retransmission of General Queries on startup
fails to get any response from any member;
O If the router previously has valid members but fails to get any
response from any member after several rounds of General Queries
or Unicast Queries;
In these cases the router could make a decision that no member is on
this link and totally stop its transmission of periodical General
Queries. If afterwards there is valid multicast receiver joins a
group, the router could resume the original cycle of transmission of
General Queries. Because General Query has influences on all the
terminals on the link, suppressing it when it is not needed is
beneficial for both the link efficiency and terminal power saving.
4.9. Tuning Response Delay according to link type and status
IGMPv3 and MLDv2 use delayed response mechanism to spread Report
messages from different hosts over a longer interval which can
greatly reduce possibility of packet burstiness. This is implemented
by the host responding to a Query in a specific time randomly chosen
between 0 and [Maximum Response Delay]. The value of [Maximum
Response Delay] parameter is determined by the router and is carried
Wu,et al Expires April 25, 2011 [Page 17]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
in Query messages to inform the valid hosts to make the selection.
A long delay will lessen the burstiness but will increase leave
latency (the time between when the last listener stops listening to
a source or multicast address and when the traffic stops flowing).
In order to avoid burstiness of MLD messages and reduce leave
latency, explicit tracking with Group Specific Query eliminated is
recommended to be used first to reduce leave latency. Then the
Response Delay may be dynamically calculated based on the expected
number of Reporters for each Query and link type and link status.
O If the expected number of Reporters is large and link condition is
bad, the system administrator MUST choose the longer Maximum
Response Delay; if the expected number of Reporters is small and
the link condition is good, the administrator may choose the
smaller Maximum response Delay. In this case, the IGMP/MLD packet
burstiness can be reduced.
o Another case is if the link type is PTP which means the resource
is dedicated for one receiver on each link, then the Maximum
Response Delay can be chosen smaller, if the link type is shared
medium link or P2MP, then the Maximum Response Delay can be
configured larger.
The Maximum Response Delay can be configured by the administrator as
mentioned above, or be calculated automatically by software tool
implemented according to experiential model on different link modes.
As the router arrives at a value appropriate for current link type
and conditions, it will encode the value in Query messages to inform
the host to make the response. The determination of the instant
Maximum Response Delay value is out of this document's scope.
4.10. Triggering reports and queries quickly during handover
As a mobile terminal is moving from one network to another, if it is
a multicast receiver from a group, its new access network should try
to deliver the content to the receiver without disruption or
performance deterioration. For the smooth switching between
networks, the terminal's membership should be acquired as quickly as
possible by the new access network.
For the access router, it could trigger a Query to the terminal as
soon as it detects a new terminal on its link. This could be a
General Query if the router does not know whether or not the
terminal is a valid receiver or if the number of the entering
terminals is not small. Or this Query could also be a unicast Query
Wu,et al Expires April 25, 2011 [Page 18]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
for only a small quantity of terminals to prevent unnecessary action
of other terminals in the switching area.
For the terminal, it could trigger a report if it is currently in
the multicast reception state. This helps establish more quickly
the membership states and enable faster multicast stream injection
because active report from the host does not requires the router to
wait for the query-response round in the passive reporting cases.
5. Security Considerations
They will be described in the later version of this draft.
6. Acknowledgement
The authors would like to thank Stig,Venaas, Gorry Fairhurst, Thomas
C. Schmidt, Marshall Eubanks, Suresh Krishnan, J.William Atwood,
WeeSan Lee, Imed Romdhani, Hitoshi Asaeda, Liu Yisong and Wei Yong
for their valuable comments and suggestions on this document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to indicate
requirement levels", RFC 2119, March 1997.
[RFC1112] Deering, S. "Host Extensions for IP Multicasting", RFC1112,
August 1989.
[RFC2236] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2236, November 1997.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October 1999.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version 3", RFC
3376, October 2002.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2(MLDv2) for IPv6", RFC 3810, June 2004.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
Wu,et al Expires April 25, 2011 [Page 19]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
[RFC5790] Liu, H., Cao, W., and H. Asaeda, "Lightweight IGMPv3 and
MLDv2 Protocols", RFC5790, February 2010.
7.2. Informative Referencess
[REQUIRE] H. Liu, Q. Wu, H. Asaeda and TM. Eubanks, "Mobile and
Wireless Multicast Requirements on IGMP/MLD Protocols", draft-liu-
multimob-igmp-mld-mobility-req-03.txt, March 2010.
[ROBUST] A. Sen Mazumder, "Facilitating Robust Multicast Group
Management", NOSSDAV'05, June 13-14, 2005, Stevenson, Washington,
USA.
[ACK] Nikaein, N. and Bonnet, C. "Wireless multicasting in an IP
environment" In Proceedings of the 5th International Workshop on
Mobile Multimedia Communication MoMuc'98 (Berlin, Germany, Oct. 12-
14). IEEE Computer Society Press, 1998.
[IGMP-ACK] H. Liu, Q, Wu, "Reliable IGMP and MLD Protocols in
Wireless Environment", draft-liu-multimob-reliable-igmp-mld-00.txt,
February 2010.
[ADAPTIVE] I. Romdhani, J. Munoz, H. Bettahar, and A. Bouabdallah,
"Adaptive Multicast Membership Management for Mobile Multicast
Receivers", IEEE, 2006.
[RFC5757] Schmidt, T., Waehlisch, M., and G. Fairhurst, "Multicast
Mobility in Mobile IP Version 6 (MIPv6): Problem Statement and Brief
Survey", RFC 5757, February 2010.
Wu,et al Expires April 25, 2011 [Page 20]
Internet-Draft Tuning IGMPv3/MLDv2 Protocol Behavior October 2010
Authors' Addresses
Qin Wu
Huawei Technologies Co., Ltd.
Site B, Floor 12, Huihong Mansion,No.91 Baixia Rd.
Nanjing, Jiangsu 21001
China
Phone: +86-25-84565892
EMail: sunseawq@huawei.com
Hui Liu
Huawei Technologies Co., Ltd.
Huawei Bld., No.3 Xinxi Rd.
Shang-Di Information Industry Base
Hai-Dian Distinct, Beijing 100085
China
EMail: Liuhui47967@huawei.com
Wu,et al Expires April 25, 2011 [Page 21]