1 CMPT 471 Networking II IGMP (IPv4) and MLD (IPv6) © Janice Regan, 2006-2013

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CMPT 471Networking II

IGMP (IPv4) and MLD (IPv6)

© Janice Regan, 2006-2013

© Janice Regan, 2006-2013 2

Group Management Protocols Internet Group Management Protocol (IGMP)

RFC 2236 IGMPv2, RFC 3376 IGMPv3 Multicast Listener Discover Protocol (MLD)

Part of ICMPv6 RFC 2710 MLDv1, RFC 3810 MLDv2

Governs exchange of multicast group information between hosts and multicast routers, and between multicast routers

IGMPv3 and MLDv2 support SFM (source filtered multicast)


IGMPv2 (MLDv1) and IGMPv3 (MLDv2)

Three types of Messages transmitted in IP datagrams Membership Query: sent by router to monitor

group membership Membership Report: sent by hosts to

join a group, leave a group Update source filtering information for the host

(ICMPv3 and MLDv2 only) Reply to a membership query and indicate desire

to remain a member of the group Leave group message (ICMPv2 only)

Encapsulation IGMP IGMP messages are sent in the IP layer ttl for IGMP messages is 1


IP Header IGMP message

Encapsulation MLD

Hop by hop options header contains Router Alert Option to allow routers to receive messages sent to multicast addresses to which the router is not yet listening

MLD messages are sent with a hop count of 1 using link local addresses


IP HeaderNext Header = 0

MLD messageHop by Hop HeaderNext header = 58


IGMP and MLD Group membership

A host can send traffic to any multicast group, group membership is not required.

A host must be a member of a multicast group to receive multicast messages from that group

A host leaves a group by sending and IGMP or MLD message indicating it is leaving the group

A host joins group by sending an IGMP or MLD message indicating it is joining the group

Details of joining and leaving groups (including details of message types) are different for IGMPv2, IGMPv3, MLDv1 and MLDv2


IGMP and MLD Operation Routers periodically issue query message

Sent to all-hosts multicast address ( 224.0.0.1 or FF02::1), includes records for each multicast group that has been joined by at least one host in the net

Hosts that want to stay in groups must read all-hosts query messages and respond with one or more reports To prevent overload caused by many reports from

many members on a LAN each host will be assigned a random delay before it will transmit its response.

Details of how the reports are generated and sent are dependent on the version of IGMP or MLD being used


IGMPv2 and MLDv1 Operation: (1)

Each host receiving the query chooses the multicast groups to which belongs. For each group it is a member of the host builds a response message and

1. Sets a timer, (random time between 0 and maximum response time, the maximum time before a response can be sent )

2. Watches for response messages to the same multicast group sent by other hosts on the network.


IGMPv2 and MLDv1 Operation:(2)

Each host receiving the query chooses the multicast groups to which belongs.

For each group it is a member of the host builds a response message and

3. Drops its own response message and stops its timer, if it sees a response message for the same multicast group from another host

4. Transmits the response when the timer expires, and no other response messages have been seen

IGMPv2 and MLDv1 messages Begin by considering report messages MLDv1

done messages and ICMPv2 leave group messages

Report messages are used to Maintain membership in a group Join a group

MLDv1 done messages and ICMPv2 leave messages are used to Leave a multicast group

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IGMPv2 report messages

© Janice Regan, 2006-2013 11Diagram from RFC 2236

Reply to query: type 0x16 Membership Report sent to group address in response to a membership query,

join message: type 0x16 Membership Report, sent to group address, unsolicited, sent when host wants to join a group

Leave message: type 0x17 sent to the all routers address (224.0.0.20 when host wishes to leave the group, not always sent, must be sent by the last host to respond to a membership solicitation for the group. Group address field hold the multicast address of the group

MLDv1 Multicast listener report

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Type (131 ) Message Checksum

Multicast address of group

...

Type 131: response to query send to the multicast address in response to query Type 131: join message: send to the multicast address, unsolicitedType 132: done (leave) send to all routers multicast address (FF02::2), may not be

sent if node did not reply to the last periodic query

Diagram from RFC 2710

IGMPv2 and MLD1 queries Sent by multicast router General query (to 224.0.0.1, or :: )

Which groups have members on attached network Group address field and number of sources are 0

Group-specific query (to group address) Does group have members on an attached network Group address field contains IP address of the

multicast group being queried Send to verify if there are any remaining hosts on

the network after the router receives leave msg

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IGMPv2 query messages

© Janice Regan, 2006-2013 14Diagram from RFC 2236

Checksum for all IGMPv2 messages is the ones complement of the ones complement sum of all 16 bit words in the IGMPv2 messageMembership query: type 0x11. General query, used to learn which groups have members on the network attached to the router. Group address is 0.0.0.0. message sent to all hosts (224.0.0.1)Group specific query: used to learn if a particular group still has members on the network attached to the router. Group address is the group being queried Sent to group being queried.

MLDv1 query

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Type (130 )

Multicast address of group

...

Analogous to ICMPv3 queryIPv6 all hosts address is FF02::1:

Diagram from RFC 2710

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Joining a group To join group, host sends either

IGMP membership report message or MLD multicast listener report

The multicast address of the multicast group being joined is in both the multicast address field of the IGMPv2 or MLDv1

message the Destination Address field in the encapsulating IP

header. Router will receive report message and add the group if

there are no current members of the group, or add the host to the group if there are current members.

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Leaving a group An IGMPv2 host leave a multicast group by sending

leave group message to all-routers static multicast address (224.0.0.2). The multicast address is the leave message is the multicast address of the group being left.

An MLDv1 host leaves a multicast group by sending a multicast listener done message to the all routers multicast address (FF02::2). The multicast address in the done message is the multicast address of the group being left

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Leaving a group If the process that joined the multicast group is

terminated a leave message is also sent When the multicast router for the network receives a

leave message it determines if there are any remaining group members using group-specific query message (more about query messages later)

If there are any other hosts who are member of the multicast group the host just left they will send reports to the router and the router will know to keep the multicast group alive.

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IGMPv2, MLDv1: Keeping Lists Valid Router does not need to know every host

in a group Needs to know at least one group member

still active Each host in group sets timer with random

delay Host that hears another claim membership

cancels own report If timer expires, host sends report Only one member of each group reports to

router

20© Janice Regan, 2006-2013

State machine for an entry in host's multicast route table

Comer 2000: fig 17.4


IGMP states: 1 Consider a router with at least one host on a connected network

that is a member of a particular multicast group Every 125 seconds the router will generate a query and send it

to the multicast group.. When the host receives the query it moves from member state

to delaying member state, it builds its report and sets its random delay timer (random in a specified interval which defaults to 10 sec)

If it hears another report before it is due to send its own it will drop the report without sending an moves back to idle member state.

When its timer expires it sends its report and moves back to idle member state.


IGMP states: 2 Consider a router with a connected network. No host on

that connected network is a member of the multicast group. Then consider that one host on the connected network wishes to join the multicast group The host will be in the nonmember state. The host builds a membership report packet. The host starts a timer and sends the membership report packet The host moves into the delaying member state. When the timer expires the host resends the member report

packet and moves into the member state. Sending the membership report twice (each time in a best effort

UDP packet) provides a measure of security that the membership report will reach the server

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IGMPv3 and MLDv2 Operation Each host receiving the query chooses the multicast

groups it belongs to Each host builds a response message, containing

information (one record per multicast group) for all multicast groups it belongs to

Each host sets a timer for a random time (0-maximum response delay), and transmits the report when that timer expires (sent to 224.0.0.22 or FF02::16)

Responses to multiple queries may in some cases be sent as a single response. A responses to a previous query will be dropped if a response to a newer query for the same group is being scheduled


IGMPv3 and MLDv2 messages Begin by considering report messages Report messages are used to

Maintain membership in a group Join a group Leave a group To filter group membership accepting or

rejecting traffic from certain members in the group

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IGMPv3 Membership Report

Stallings 2003:

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IGMPv3 Message types Type Number Message Name

0x11 Membership Query

0x22 V 3 Membership Report 0x16 V2 Membership

Report

0x17 V2 Leave Group

Unrecognized message types are silently ignored

MLDv2 Multicast listener report

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Type (143 ) Reserved

Reserved

Checksum

Number of multicast address records

Multicast address record of groupCan be many such records one for each group

...

...

Sent to all MLDv2 capable routers (multicast address FF01::16

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membership report fields Reserved fields are filled with zeros

The IGMPv3 checksum the ones complement of the ones complement sum of all 16 bit words in the IP payload (the membership message)

The MLDv2 checksum also includes a pseudo-header in the calculation.

Group records each contain information about one multicast group. This information included the list of hosts to include or exclude

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IGMP or MLD Group Record

Stallings 2003:

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Other group record fields Auxiliary data length should be 0, there is

no auxiliary data defined for IGMP3 or MLDv2, this field is available for future expansion of the protocols

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IGMPv3 and MLDv2 group records The number of group records included in the

membership report. When joining a single group there will be one group

record When joining multiple groups there will be one group

record for each group When responding to a query one response will be

built. It will include one group record for each multicast groups.

Each group record will include a multicast address, a record type (see next slide) and may include a list of hosts to include / exclude

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Record type Value Name

1 MODE_IS_INCLUDE 2 MODE_IS_EXCLUDE indicates the interface filter mode for the specified multicast addresses.

3 CHANGE_TO_INCLUDE_MODE.4 CHANGE_TO_EXCLUDE_MODE indicates that the interface has changed filter mode5 ALLOW_NEW_SOURCES6 BLOCK_OLD_SOURCESthis Group Record contains a list of the sources that the system wishes to add or no longer wishes to hear from,

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IGMPv3 and MLDv2 Operation A member host will send a report packet containing

updates to its filtering information for a group as soon as the filtering information changes. This report package may modify the list of included or excluded hosts, or change from include mode to/from exclude mode.

This report can be a MLDv2 multicast listener report or IGMPv3 report containing “state change” records “filter mode change” records that allow changes from

include mode to exclude mode “source list change” records that add new hosts to or

remove existing hosts from the list of excluded or included hosts


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Joining a group: IGMPv3, MLDv2 A member host will send a report packet

containing updates to its filtering information for a group as soon as the filtering information changes.

To join a group, a host sends a report message Message is sent to multicast group address Message contains multicast group address

of group to join The host filtering information indicates

EXCLUDE mode, and specifies no host list.

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Leaving a group: IGMPv3, MLDv2 A member host will send a report packet

containing updates to its filtering information for a group as soon as the filtering information changes.

To leave a group, a host sends a report message Message is sent to multicast group address Message contains multicast group address of

group to join The host filtering information indicates

INCLUDE mode, and specifies no host list.

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ICMPv3, MLDv2: Keeping Lists Valid Routers periodically issue general query message in a

datagram with all-systems multicast address (224.0.0.1 or FF02::1) Hosts that wish to remain in groups must read

datagrams with this all-hosts address Every host on the network that belongs to a least

one of the queried multicast groups responds by preparing a report message The report message contains one group record

for every multicast group the host is a member of The host sends the prepared report (after waiting a

randomized time)

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Membership Query: IGMPv3 + MLDv2

Stallings 2003:

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Membership Query Fields (1) Max Response Time

Max time before sending report in units of 1/10 second. (>=128 indicates floating point, see RFC)

The IGMPv3 checksum the ones complement of the ones complement sum of all 16 bit words in the IP payload (the membership message)

The MLDv2 checksum also includes a pseudo-header in the calculation.

S Flag 1 indicates that receiving routers should suppress

normal timer updates done on hearing query

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Membership Query Fields (2) QRV (querier's robustness variable)

Routers adopt value from most recently received query A default or statically configured value used if most recently

received QRV is 0 RV dictates number of retransmissions to assure report not

missed (maximum 7) QQIC (querier's querier interval code)

Timer for sending multiple queries Routers not current querier adopt most recently received QI Unless QI was zero, when default QI value used

If you want to know what these are for and understand the details of how these work see RFC’s

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IGMPv3, MLDv2 Membership Queries Sent by multicast router General query (to 224.0.0.1, or :: )

Which groups have members on attached network Group address field and number of sources are 0

Group-specific query (to group address) Does group have members on an attached network Group address field contains IP address of the multicast group

being queried, number of sources is 0

Group-and-source specific query (to group address) Do attached devices want packets sent to specified multicast

address (response to leave by BLOCKing all sources) From any of specified list of sources

Documents

1 CMPT 471 Networking II IGMP (IPv4) and MLD (IPv6) © Janice Regan, 2006-2013