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rfc:rfc2710

Network Working Group S. Deering Request for Comments: 2710 Cisco Systems Category: Standards Track W. Fenner

                                                        AT&T Research
                                                          B. Haberman
                                                                  IBM
                                                         October 1999
            Multicast Listener Discovery (MLD) for IPv6

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

 This document specifies the protocol used by an IPv6 router to
 discover the presence of multicast listeners (that is, nodes wishing
 to receive multicast packets) on its directly attached links, and to
 discover specifically which multicast addresses are of interest to
 those neighboring nodes.  This protocol is referred to as Multicast
 Listener Discovery or MLD.  MLD is derived from version 2 of IPv4's
 Internet Group Management Protocol, IGMPv2.  One important difference
 to note is that MLD uses ICMPv6 (IP Protocol 58) message types,
 rather than IGMP (IP Protocol 2) message types.

1. Definitions

 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 [KEYWORDS].

2. Introduction

 The purpose of Multicast Listener Discovery (MLD) is to enable each
 IPv6 router to discover the presence of multicast listeners (that is,
 nodes wishing to receive multicast packets) on its directly attached
 links, and to discover specifically which multicast addresses are of
 interest to those neighboring nodes.  This information is then

Deering, et al. Standards Track [Page 1] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

 provided to whichever multicast routing protocol is being used by the
 router, in order to ensure that multicast packets are delivered to
 all links where there are interested receivers.
 MLD is an asymmetric protocol, specifying different behaviors for
 multicast listeners and for routers.  For those multicast addresses
 to which a router itself is listening, the router performs both parts
 of the protocol, including responding to its own messages.
 If a router has more than one interface to the same link, it need
 perform the router part of MLD over only one of those interfaces.
 Listeners, on the other hand, must perform the listener part of MLD
 on all interfaces from which an application or upper-layer protocol
 has requested reception of multicast packets.

3. Message Format

 MLD is a sub-protocol of ICMPv6, that is, MLD message types are a
 subset of the set of ICMPv6 messages, and MLD messages are identified
 in IPv6 packets by a preceding Next Header value of 58.  All MLD
 messages described in this document are sent with a link-local IPv6
 Source Address, an IPv6 Hop Limit of 1, and an IPv6 Router Alert
 option [RTR-ALERT] in a Hop-by-Hop Options header.  (The Router Alert
 option is necessary to cause routers to examine MLD messages sent to
 multicast addresses in which the routers themselves have no
 interest.)
 MLD messages have the following format:
  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      |     Code      |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Maximum Response Delay    |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                                                               +
 |                                                               |
 +                       Multicast Address                       +
 |                                                               |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Deering, et al. Standards Track [Page 2] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

3.1. Type

 There are three types of MLD messages:
 Multicast Listener Query (Type = decimal 130)
    There are two subtypes of Multicast Listener Query messages:
  1. General Query, used to learn which multicast addresses have

listeners on an attached link.

  1. Multicast-Address-Specific Query, used to learn if a

particular multicast address has any listeners on an attached

      link.
    These two subtypes are differentiated by the contents of the
    Multicast Address field, as described in section 3.6.
    Multicast Listener Report (Type = decimal 131)
    Multicast Listener Done (Type = decimal 132)
 In the rest of this document, the above messages types are referred
 to simply as "Query", "Report", and "Done".

3.2. Code

 Initialized to zero by the sender; ignored by receivers.

3.3. Checksum

 The standard ICMPv6 checksum, covering the entire MLD message plus a
 "pseudo-header" of IPv6 header fields [ICMPv6,IPv6].

3.4. Maximum Response Delay

 The Maximum Response Delay field is meaningful only in Query
 messages, and specifies the maximum allowed delay before sending a
 responding Report, in units of milliseconds.  In all other messages,
 it is set to zero by the sender and ignored by receivers.
 Varying this value allows the routers to tune the "leave latency"
 (the time between the moment the last node on a link ceases listening
 to a particular multicast address and moment the routing protocol is
 notified that there are no longer any listeners for that address), as
 discussed in section 7.8.  It also allows tuning of the burstiness of
 MLD traffic on a link, as discussed in section 7.3.

Deering, et al. Standards Track [Page 3] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

3.5. Reserved

 Initialized to zero by the sender; ignored by receivers.

3.6. Multicast Address

 In a Query message, the Multicast Address field is set to zero when
 sending a General Query, and set to a specific IPv6 multicast address
 when sending a Multicast-Address-Specific Query.
 In a Report or Done message, the Multicast Address field holds a
 specific IPv6 multicast address to which the message sender is
 listening or is ceasing to listen, respectively.

3.7. Other fields

 The length of a received MLD message is computed by taking the IPv6
 Payload Length value and subtracting the length of any IPv6 extension
 headers present between the IPv6 header and the MLD message.  If that
 length is greater than 24 octets, that indicates that there are other
 fields present beyond the fields described above, perhaps belonging
 to a future backwards-compatible version of MLD.  An implementation
 of the version of MLD specified in this document MUST NOT send an MLD
 message longer than 24 octets and MUST ignore anything past the first
 24 octets of a received MLD message.  In all cases, the MLD checksum
 MUST be computed over the entire MLD message, not just the first 24
 octets.

4. Protocol Description

 Note that defaults for timer values are described later in this
 document.  Timer and counter names appear in square brackets.
 Routers use MLD to learn which multicast addresses have listeners on
 each of their attached links.  Each router keeps a list, for each
 attached link, of which multicast addresses have listeners on that
 link, and a timer associated with each of those addresses.  Note that
 the router needs to learn only that listeners for a given multicast
 address are present on a link; it does NOT need to learn the identity
 (e.g., unicast address) of those listeners or even how many listeners
 are present.
 For each attached link, a router selects one of its link-local
 unicast addresses on that link to be used as the IPv6 Source Address
 in all MLD packets it transmits on that link.

Deering, et al. Standards Track [Page 4] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

 For each interface over which the router is operating the MLD
 protocol, the router must configure that interface to listen to all
 link-layer multicast address that can be generated by IPv6
 multicasts.  For example, an Ethernet-attached router must set its
 Ethernet address reception filter to accept all Ethernet multicast
 addresses that start with the hexadecimal value 3333 [IPv6-ETHER]; in
 the case of an Ethernet interface that does not support the filtering
 of such a range of multicast address, it must be configured to accept
 ALL Ethernet multicast addresses, in order to meet the requirements
 of MLD.
 With respect to each of its attached links, a router may assume one
 of two roles: Querier or Non-Querier.  There is normally only one
 Querier per link.  All routers start up as a Querier on each of their
 attached links.  If a router hears a Query message whose IPv6 Source
 Address is numerically less than its own selected address for that
 link, it MUST become a Non-Querier on that link.  If [Other Querier
 Present Interval] passes without receiving, from a particular
 attached link, any Queries from a router with an address less than
 its own, a router resumes the role of Querier on that link.
 A Querier for a link periodically [Query Interval] sends a General
 Query on that link, to solicit reports of all multicast addresses of
 interest on that link.  On startup, a router SHOULD send [Startup
 Query Count] General Queries spaced closely together [Startup Query
 Interval] on all attached links in order to quickly and reliably
 discover the presence of multicast listeners on those links.
 General Queries are sent to the link-scope all-nodes multicast
 address (FF02::1), with a Multicast Address field of 0, and a Maximum
 Response Delay of [Query Response Interval].
 When a node receives a General Query, it sets a delay timer for each
 multicast address to which it is listening on the interface from
 which it received the Query, EXCLUDING the link-scope all-nodes
 address and any multicast addresses of scope 0 (reserved) or 1
 (node-local).  Each timer is set to a different random value, using
 the highest clock granularity available on the node, selected from
 the range [0, Maximum Response Delay] with Maximum Response Delay as
 specified in the Query packet.  If a timer for any address is already
 running, it is reset to the new random value only if the requested
 Maximum Response Delay is less than the remaining value of the
 running timer.  If the Query packet specifies a Maximum Response
 Delay of zero, each timer is effectively set to zero, and the action
 specified below for timer expiration is performed immediately.

Deering, et al. Standards Track [Page 5] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

 When a node receives a Multicast-Address-Specific Query, if it is
 listening to the queried Multicast Address on the interface from
 which the Query was received, it sets a delay timer for that address
 to a random value selected from the range [0, Maximum Response
 Delay], as above.  If a timer for the address is already running, it
 is reset to the new random value only if the requested Maximum
 Response Delay is less than the remaining value of the running timer.
 If the Query packet specifies a Maximum Response Delay of zero, the
 timer is effectively set to zero, and the action specified below for
 timer expiration is performed immediately.
 If a node's timer for a particular multicast address on a particular
 interface expires, the node transmits a Report to that address via
 that interface; the address being reported is carried in both the
 IPv6 Destination Address field and the MLD Multicast Address field of
 the Report packet.  The IPv6 Hop Limit of 1 (as well as the presence
 of a link-local IPv6 Source Address) prevent the packet from
 traveling beyond the link to which the reporting interface is
 attached.
 If a node receives another node's Report from an interface for a
 multicast address while it has a timer running for that same address
 on that interface, it stops its timer and does not send a Report for
 that address, thus suppressing duplicate reports on the link.
 When a router receives a Report from a link, if the reported address
 is not already present in the router's list of multicast address
 having listeners on that link, the reported address is added to the
 list, its timer is set to [Multicast Listener Interval], and its
 appearance is made known to the router's multicast routing component.
 If a Report is received for a multicast address that is already
 present in the router's list, the timer for that address is reset to
 [Multicast Listener Interval].  If an address's timer expires, it is
 assumed that there are no longer any listeners for that address
 present on the link, so it is deleted from the list and its
 disappearance is made known to the multicast routing component.
 When a node starts listening to a multicast address on an interface,
 it should immediately transmit an unsolicited Report for that address
 on that interface, in case it is the first listener on the link.  To
 cover the possibility of the initial Report being lost or damaged, it
 is recommended that it be repeated once or twice after short delays
 [Unsolicited Report Interval].  (A simple way to accomplish this is
 to send the initial Report and then act as if a Multicast-Address-
 Specific Query was received for that address, and set a timer
 appropriately).

Deering, et al. Standards Track [Page 6] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

 When a node ceases to listen to a multicast address on an interface,
 it SHOULD send a single Done message to the link-scope all-routers
 multicast address (FF02::2), carrying in its Multicast Address field
 the address to which it is ceasing to listen.  If the node's most
 recent Report message was suppressed by hearing another Report
 message, it MAY send nothing, as it is highly likely that there is
 another listener for that address still present on the same link.  If
 this optimization is implemented, it MUST be able to be turned off
 but SHOULD default to on.
 When a router in Querier state receives a Done message from a link,
 if the Multicast Address identified in the message is present in the
 Querier's list of addresses having listeners on that link, the
 Querier sends [Last Listener Query Count] Multicast-Address-Specific
 Queries, one every [Last Listener Query Interval] to that multicast
 address.  These Multicast-Address-Specific Queries have their Maximum
 Response Delay set to [Last Listener Query Interval].  If no Reports
 for the address are received from the link after the response delay
 of the last query has passed, the routers on the link assume that the
 address no longer has any listeners there; the address is therefore
 deleted from the list and its disappearance is made known to the
 multicast routing component.  This process is continued to its
 resolution (i.e. until a Report is received or the last Multicast-
 Address-Specific Query is sent with no response) despite any
 transition from Querier to Non-Querier on this link.
 Routers in Non-Querier state MUST ignore Done messages.
 When a router in Non-Querier state receives a Multicast-Address-
 Specific Query, if its timer value for the identified multicast
 address is greater than [Last Listener Query Count] times the Maximum
 Response Delay specified in the message, it sets the address's timer
 to that latter value.

5. Node State Transition Diagram

 Node behavior is more formally specified by the state transition
 diagram below.  A node may be in one of three possible states with
 respect to any single IPv6 multicast address on any single interface:
  1. "Non-Listener" state, when the node is not listening to the address

on the interface (i.e., no upper-layer protocol or application has

    requested reception of packets to that multicast address).  This
    is the initial state for all multicast addresses on all
    interfaces; it requires no storage in the node.

Deering, et al. Standards Track [Page 7] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

  1. "Delaying Listener" state, when the node is listening to the

address on the interface and has a report delay timer running for

    that address.
  1. "Idle Listener" state, when the node is listening to the address on

the interface and does not have a report delay timer running for

    that address.
 There are five significant events that can cause MLD state
 transitions:
  1. "start listening" occurs when the node starts listening to the

address on the interface. It may occur only in the Non-Listener

    state.
  1. "stop listening" occurs when the node stops listening to the

address on the interface. It may occur only in the Delaying

    Listener and Idle Listener states.
  1. "query received" occurs when the node receives either a valid

General Query message, or a valid Multicast-Address-Specific Query

    message.  To be valid, the Query message MUST come from a link-
    local IPv6 Source Address, be at least 24 octets long, and have a
    correct MLD checksum.  The Multicast Address field in the MLD
    message must contain either zero (a General Query) or a valid
    multicast address (a Multicast- Address-Specific Query).  A
    General Query applies to all multicast addresses on the interface
    from which the Query is received.  A Multicast-Address-Specific
    Query applies to a single multicast address on the interface from
    which the Query is received.  Queries are ignored for addresses in
    the Non-Listener state.
  1. "report received" occurs when the node receives a valid MLD Report

message. To be valid, the Report message MUST come from a link-

    local IPv6 Source Address, be at least 24 octets long, and have a
    correct MLD checksum.  A Report applies only to the address
    identified in the Multicast Address field of the Report, on the
    interface from which the Report is received.  It is ignored in the
    Non-Listener or Idle Listener state.
  1. "timer expired" occurs when the report delay timer for the address

on the interface expires. It may occur only in the Delaying

    Listener state.

Deering, et al. Standards Track [Page 8] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

 All other events, such as receiving invalid MLD messages or MLD
 message types other than Query or Report, are ignored in all states.
 There are seven possible actions that may be taken in response to the
 above events:
  1. "send report" for the address on the interface. The Report message

is sent to the address being reported.

  1. "send done" for the address on the interface. If the flag saying

we were the last node to report is cleared, this action MAY be

    skipped.  The Done message is sent to the link-scope all-routers
    address (FF02::2).
  1. "set flag" that we were the last node to send a report for this

address.

  1. "clear flag" since we were not the last node to send a report for

this address.

  1. "start timer" for the address on the interface, using a delay value

chosen uniformly from the interval [0, Maximum Response Delay],

    where Maximum Response Delay is specified in the Query.  If this
    is an unsolicited Report, the timer is set to a delay value chosen
    uniformly from the interval [0, [Unsolicited Report Interval] ].
  1. "reset timer" for the address on the interface to a new value,

using a delay value chosen uniformly from the interval [0, Maximum

    Response Delay], as described in "start timer".
  1. "stop timer" for the address on the interface.
 In all of the following state transition diagrams, each state
 transition arc is labeled with the event that causes the transition,
 and, in parentheses, any actions taken during the transition.  Note
 that the transition is always triggered by the event; even if the
 action is conditional, the transition still occurs.

Deering, et al. Standards Track [Page 9] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

                           ________________
                          |                |
                          |                |
                          |                |
                          |                |
                --------->|  Non-Listener  |<---------
               |          |                |          |
               |          |                |          |
               |          |                |          |
               |          |________________|          |
               |                   |                  |
               | stop listening    | start listening  | stop listening
               | (stop timer,      |(send report,     | (send done if
               |  send done if     | set flag,        |  flag set)
               |  flag set)        | start timer)     |
       ________|________           |          ________|________
      |                 |<---------          |                 |
      |                 |                    |                 |
      |                 |<-------------------|                 |
      |                 |   query received   |                 |
      |     Delaying    |    (start timer)   |      Idle       |
 ---->|     Listener    |------------------->|     Listener    |
|     |                 |   report received  |                 |
|     |                 |    (stop timer,    |                 |
|     |                 |     clear flag)    |                 |
|     |_________________|------------------->|_________________|
| query received    |        timer expired
| (reset timer if   |        (send report,
|  Max Resp Delay   |         set flag)
|  < current timer) |
 -------------------
 The link-scope all-nodes address (FF02::1) is handled as a special
 case.  The node starts in Idle Listener state for that address on
 every interface, never transitions to another state, and never sends
 a Report or Done for that address.
 MLD messages are never sent for multicast addresses whose scope is 0
 (reserved) or 1 (node-local).
 MLD messages ARE sent for multicast addresses whose scope is 2
 (link-local), including Solicited-Node multicast addresses [ADDR-
 ARCH], except for the link-scope, all-nodes address (FF02::1).

Deering, et al. Standards Track [Page 10] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

6. Router State Transition Diagram

 Router behavior is more formally specified by the state transition
 diagrams below.
 A router may be in one of two possible states with respect to any
 single attached link:
  1. "Querier", when this router is designated to transmit MLD Queries

on this link.

  1. "Non-Querier", when there is another router designated to transmit

MLD Queries on this link.

 The following three events can cause the router to change states:
  1. "query timer expired" occurs when the timer set for query

transmission expires. This event is significant only when in the

    Querier state.
  1. "query received from a router with a lower IP address" occurs when

a valid MLD Query is received from a router on the same link with

    a lower IPv6 Source Address. To be valid, the Query message MUST
    come from a link-local IPv6 Source Address, be at least 24 octets
    long, and have a correct MLD checksum.
  1. "other querier present timer expired" occurs when the timer set to

note the presence of another querier with a lower IP address on

    the link expires.  This event is significant only when in the
    Non-Querier state.
 There are three actions that may be taken in response to the above
 events:
  1. "start general query timer" for the attached link to [Query

Interval].

  1. "start other querier present timer" for the attached link to [Other

Querier Present Interval].

  1. "send general query" on the attached link. The General Query is

sent to the link-scope all-nodes address (FF02::1), and has a

    Maximum Response Delay of [Query Response Interval].

Deering, et al. Standards Track [Page 11] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

  1. ——————————-

_| gen. query timer | ——— | | expired | | Initial |—————→| | (send general query, | ——— (send gen. q., | | start gen. q. timer) | start initial gen. q. | |←——————— timer) | Querier | | | —–| |←– | | | | | || | query received from a | | other querier router with a lower | | present timer IP address | | expired (start other querier | | (send gen. query, present timer) | | | | start gen. q. timer) | | | | | | | | —→| Non |—- | Querier | | | | | —→| |—- | || | | query received from a | | router with a lower IP | | address | | (start other querier | | present timer) | ————————— A router starts in the Initial state on all attached links, and immediately transitions to Querier state. In addition, to keep track of which multicast addresses have listeners, a router may be in one of three possible states with respect to any single IPv6 multicast address on any single attached link: - "No Listeners Present" state, when there are no nodes on the link that have sent a Report for this multicast address. This is the initial state for all multicast addresses on the router; it requires no storage in the router. - "Listeners Present" state, when there is a node on the link that has sent a Report for this multicast address. Deering, et al. Standards Track [Page 12] RFC 2710 Multicast Listener Discovery for IPv6 October 1999 - "Checking Listeners" state, when the router has received a Done message but has not yet heard a Report for the identified address. There are five significant events that can cause router state transitions: - "report received" occurs when the router receives a Report for the address from the link. To be valid, the Report message MUST come from a link-local IPv6 Source Address, be at least 24 octets long, and have a correct MLD checksum. - "done received" occurs when the router receives a Done message for the address from the link. To be valid, the Done message MUST come from a link-local IPv6 Source Address, be at least 24 octets long, and have a correct MLD checksum. This event is significant only in the "Listerners Present" state and when the router is a Querier. - "multicast-address-specific query received" occurs when a router receives a Multicast-Address-Specific Query for the address from the link. To be valid, the Query message MUST come from a link- local IPv6 Source Address, be at least 24 octets long, and have a correct MLD checksum. This event is significant only in the "Listeners Present" state and when the router is a Non-Querier. - "timer expired" occurs when the timer set for a multicast address expires. This event is significant only in the "Listeners Present" or "Checking Listeners" state. - "retransmit timer expired" occurs when the timer set to retransmit a Multicast-Address-Specific Query expires. This event is significant only in the "Checking Listeners" state. There are seven possible actions that may be taken in response to the above events: - "start timer" for the address on the link - also resets the timer to its initial value [Multicast Listener Interval] if the timer is currently running. - "start timer*" for the address on the link - this alternate action sets the timer to the minimum of its current value and either [Last Listener Query Interval] * [Last Listener Query Count] if this router is a Querier, or the Maximum Response Delay in the Query message * [Last Listener Query Count] if this router is a non-Querier. Deering, et al. Standards Track [Page 13] RFC 2710 Multicast Listener Discovery for IPv6 October 1999 - "start retransmit timer" for the address on the link [Last Listener Query Interval]. - "clear retransmit timer" for the address on the link. - "send multicast-address-specific query" for the address on the link. The Multicast-Address-Specific Query is sent to the address being queried, and has a Maximum Response Delay of [Last Listener Query Interval]. - "notify routing +" internally notify the multicast routing protocol that there are listeners to this address on this link. - "notify routing -" internally notify the multicast routing protocol that there are no longer any listeners to this address on this link. The following state diagrams apply per group per link. There are two diagrams; one for routers in Querier state and one for routers in Non-Querier state. The transition between Querier and Non-Querier state on a link is handled specially. All groups on that link in "No Listeners Present" or "Listeners Present" states switch state transition diagrams when the Querier/Non-Querier state transition occurs. However, any groups in "Checking Listeners" state continue with the same state transition diagram until the "Checking Listeners" state is exited. E.g. a router that starts as a Querier, receives a Done message for a group and then receives a Query from a router with a lower address (causing a transition to the Non-Querier state) continues to send multicast-address-specific queries for the group in question until it either receives a Report or its timer expires, at which time it starts performing the actions of a Non-Querier for this group. Deering, et al. Standards Track [Page 14] RFC 2710 Multicast Listener Discovery for IPv6 October 1999 The state transition diagram for a router in Querier state follows: | | | |timer expired timer expired| |(notify routing -, (notify routing -)| No Listeners |clear rxmt tmr) ——→| Present |←——– | | | | | | | | | || | ————— | | | | rexmt timer | | report received| | | expired | | (notify routing +,| | | (send m-a-s | | start timer)| | | query, | | | |_| st rxmt |

   |                 |<------------       |                 | tmr)   |
   |                 |                    |                 |<-------
   |                 | report received    |                 |
   |                 | (start timer,      |                 |
   |                 |  clear rxmt tmr)   |                 |
   |    Listeners    |<-------------------|    Checking     |
   |     Present     | done received      |    Listeners    |
   |                 | (start timer*,     |                 |
   |                 |  start rxmt timer, |                 |
   |                 |  send m-a-s query) |                 |

—>| |——————→| |

_ _
report received
(start timer)

—————–

Deering, et al. Standards Track [Page 15] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

 The state transition diagram for a router in Non-Querier state is
 similar, but non-Queriers do not send any messages and are only
 driven by message reception.
                            ________________
                           |                |
                           |                |
              timer expired|                |timer expired
         (notify routing -)|  No Listeners  |(notify routing -)
                 --------->|    Present     |<---------
                |          |                |          |
                |          |                |          |
                |          |                |          |
                |          |________________|          |
                |                   |                  |
                |                   |report received   |
                |                   |(notify routing +,|
                |                   | start timer)     |
        ________|________           |          ________|________
       |                 |<---------          |                 |
       |                 |  report received   |                 |
       |                 |  (start timer)     |                 |
       |    Listeners    |<-------------------|     Checking    |
       |     Present     | m-a-s query rec'd  |    Listeners    |
       |                 | (start timer*)     |                 |
  ---->|                 |------------------->|                 |
 |     |_________________|                    |_________________|
 | report received |
 | (start timer)   |
  -----------------

7. List of timers and default values

 Most of these timers are configurable.  If non-default settings are
 used, they MUST be consistent among all routers on a single link.
 Note that parentheses are used to group expressions to make the
 algebra clear.

7.1. Robustness Variable

 The Robustness Variable allows tuning for the expected packet loss on
 a link.  If a link is expected to be lossy, the Robustness Variable
 may be increased.  MLD is robust to (Robustness Variable - 1) packet
 losses.  The Robustness Variable MUST NOT be zero, and SHOULD NOT be
 one.  Default: 2

Deering, et al. Standards Track [Page 16] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

7.2. Query Interval

 The Query Interval is the interval between General Queries sent by
 the Querier.  Default: 125 seconds.
 By varying the [Query Interval], an administrator may tune the number
 of MLD messages on the link; larger values cause MLD Queries to be
 sent less often.

7.3. Query Response Interval

 The Maximum Response Delay inserted into the periodic General
 Queries.  Default: 10000 (10 seconds)
 By varying the [Query Response Interval], an administrator may tune
 the burstiness of MLD messages on the link; larger values make the
 traffic less bursty, as node responses are spread out over a larger
 interval.  The number of seconds represented by the [Query Response
 Interval] must be less than the [Query Interval].

7.4. Multicast Listener Interval

 The Multicast Listener Interval is the amount of time that must pass
 before a router decides there are no more listeners for an address on
 a link.  This value MUST be ((the Robustness Variable) times (the
 Query Interval)) plus (one Query Response Interval).

7.5. Other Querier Present Interval

 The Other Querier Present Interval is the length of time that must
 pass before a router decides that there is no longer another router
 which should be the querier on a link.  This value MUST be ((the
 Robustness Variable) times (the Query Interval)) plus (one half of
 one Query Response Interval).

7.6. Startup Query Interval

 The Startup Query Interval is the interval between General Queries
 sent by a Querier on startup.  Default: 1/4 the Query Interval.

7.7. Startup Query Count

 The Startup Query Count is the number of Queries sent out on startup,
 separated by the Startup Query Interval.  Default: the Robustness
 Variable.

Deering, et al. Standards Track [Page 17] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

7.8. Last Listener Query Interval

 The Last Listener Query Interval is the Maximum Response Delay
 inserted into Multicast-Address-Specific Queries sent in response to
 Done messages, and is also the amount of time between Multicast-
 Address-Specific Query messages.  Default: 1000 (1 second)
 This value may be tuned to modify the "leave latency" of the link.  A
 reduced value results in reduced time to detect the departure of the
 last listener for an address.

7.9. Last Listener Query Count

 The Last Listener Query Count is the number of Multicast-Address-
 Specific Queries sent before the router assumes there are no
 remaining listeners for an address on a link.  Default: the
 Robustness Variable.

7.10. Unsolicited Report Interval

 The Unsolicited Report Interval is the time between repetitions of a
 node's initial report of interest in a multicast address.  Default:
 10 seconds.

8. Message Destinations

 This information is provided elsewhere in the document, but is
 summarized here for convenience.

Message Type IPv6 Destination Address ———— ———————— General Query link-scope all-nodes (FF02::1) Multicast-Address-Specific Query the multicast address being queried Report the multicast address being reported Done link-scope all-routers (FF02::2)

9. Security Considerations

 We consider the ramifications of a forged message of each type.  Note
 that the requirement that nodes verify that the IPv6 Source Address
 of all received MLD messages is a link-local address defends them
 from acting on forged MLD messages originated off-link, so we discuss
 only the effects of on-link forgery.

Deering, et al. Standards Track [Page 18] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

 Query message:
      A forged Query message from a machine with a lower IP address
      than the current Querier will cause Querier duties to be
      assigned to the forger.  If the forger then sends no more Query
      messages, other routers' Other Querier Present timer will time
      out and one will resume the role of Querier.  During this time,
      if the forger ignores Done messages, traffic might flow to
      addresses with no listeners for up to [Multicast Listener
      Interval].
      A forged Query message sent to an address with listeners will
      cause one or more nodes that are listeners to that address to
      send a Report.  This causes a small amount of extra traffic on
      the link, but causes no protocol problems.
 Report message:
      A forged Report message may cause routers to think there are
      listeners for an address present on a link when there are not.
      However, since listening to a multicast address is generally an
      unprivileged operation, a local user may trivially gain the same
      result without forging any messages.
 Done message:
      A forged Done message will cause the Querier to send out
      Multicast-Address-Specific Queries for the address in question.
      This causes extra processing on each router and on each of the
      address's listeners, and extra packets on the link, but cannot
      cause loss of desired traffic.

10. Acknowledgments

 MLD was derived from IGMPv2 [IGMPv2], which was designed by Rosen
 Sharma and Steve Deering and documented by Bill Fenner.

Deering, et al. Standards Track [Page 19] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

11. References

 [ADDR-ARCH]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.
 [ICMPv6]     Conta, A. and S. Deering, "Internet Control Message
              Protocol (ICMPv6) for the Internet Protocol Version 6
              (IPv6) Specification", RFC 2463, December 1998.
 [IGMPv2]     Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, November 1997.
 [IPv6]       Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.
 [IPv6-ETHER] Crawford, M., "Transmission of IPv6 Packets over
              Ethernet Networks", RFC 2464, December, 1998.
 [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RTR-ALERT]  Partridge, C. and A. Jackson, "IPv6 Router Alert
              Option", RFC 2711, October 1999.
 [STD-PROC]   Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

Deering, et al. Standards Track [Page 20] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

12. Authors' Addresses

 Stephen E. Deering
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134-1706
 USA
 Phone: +1 408 527 8213
 EMail: deering@cisco.com
 William C. Fenner
 AT&T Research
 75 Willow Road
 Menlo Park, CA 94025
 USA
 Phone: +1 650 867 6073
 EMail: fenner@research.att.com
 Brian Haberman
 IBM Corporation
 800 Park Office Drive
 Research Triangle Park, NC  27709
 USA
 Phone: +1 919 254 2673
 EMail: haberman@raleigh.ibm.com

Deering, et al. Standards Track [Page 21] RFC 2710 Multicast Listener Discovery for IPv6 October 1999

13. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Deering, et al. Standards Track [Page 22]

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