

Network Working Group                                      S. E. Deering
Request for Comments: 988                            Stanford University
                                                               July 1986

                  Host Extensions for IP Multicasting


1.  STATUS OF THIS MEMO

   This memo specifies the extensions required of a host implementation
   of the Internet Protocol (IP) to support internetwork multicasting.
   This specification supersedes that given in RFC-966, and constitutes
   a proposed protocol standard for IP multicasting in the
   ARPA-Internet.  The reader is directed to RFC-966 for a discussion of
   the motivation and rationale behind the multicasting extension
   specified here.  Distribution of this memo is unlimited.

2.  INTRODUCTION

   IP multicasting is defined as the transmission of an IP datagram to a
   "host group", a set of zero or more hosts identified by a single IP
   destination address.  A multicast datagram is delivered to all
   members of its destination host group with the same "best-efforts"
   reliability as regular unicast IP datagrams, i.e. the datagram is not
   guaranteed to arrive at all members of the destination group or in
   the same order relative to other datagrams.

   The membership of a host group is dynamic; that is, hosts may join
   and leave groups at any time.  There is no restriction on the
   location or number of members in a host group, but membership in a
   group may be restricted to only those hosts possessing a private
   access key.  A host may be a member of more than one group at a time.
   A host need not be a member of a group to send datagrams to it.

   A host group may be permanent or transient.  A permanent group has a
   well-known, administratively assigned IP address.  It is the address,
   not the membership of the group, that is permanent; at any time a
   permanent group may have any number of members, even zero.  A
   transient group, on the other hand, is assigned an address
   dynamically when the group is created, at the request of a host.  A
   transient group ceases to exist, and its address becomes eligible for
   reassignment, when its membership drops to zero.

   The creation of transient groups and the maintenance of group
   membership information is the responsibility of "multicast agents",
   entities that reside in internet gateways or other special-purpose
   hosts.  There is at least one multicast agent directly attached to
   every IP network or subnetwork that supports IP multicasting.  A host
   requests the creation of new groups, and joins or leaves existing
   groups, by exchanging messages with a neighboring agent.



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RFC 988                                                        July 1986
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   Multicast agents are also responsible for internetwork delivery of
   multicast IP datagrams.  When sending a multicast IP datagram, a host
   transmits it to a local network multicast address which identifies
   all neighboring members of the destination host group.  If the group
   has members on other networks, a multicast agent becomes an
   additional recipient of the local multicast and relays the datagram
   to agents on each of those other networks, via the internet gateway
   system.  Finally, the agents on the other networks each transmit the
   datagram as a local multicast to their own neighboring members of the
   destination group.

   This memo specifies the extensions required of a host IP
   implementation to support IP multicasting, where a "host" is any
   internet host or gateway other than those serving as multicast
   agents.  The algorithms and protocols used within and between
   multicast agents are transparent to non-agent hosts and will be
   specified in a separate document.  This memo also does not specify
   how local network multicasting is accomplished for all types of
   network, although it does specify the required service interface to
   an arbitrary local network and gives an Ethernet specification as an
   example.  Specifications for other types of network will be the
   subject of future memos.

3.  LEVELS OF CONFORMANCE

   There are three levels of conformance to this specification:

   Level 0: no support for IP multicasting.

      There is, at this time, no requirement that all IP implementations
      support IP multicasting.  Level 0 hosts will, in general, be
      unaffected by multicast activity.  The only exception arises on
      some types of local network, where the presence of level 1 or 2
      hosts may cause misdelivery of multicast IP datagrams to level 0
      hosts.  Such datagrams can easily be identified by the presence of
      a class D IP address in their destination address field; they
      should be quietly discarded by hosts that do not support IP
      multicasting.  Class D addresses are defined in section 4 of this
      memo.










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   Level 1: support for sending but not receiving multicast IP
   datagrams.

      Level 1 allows a host to partake of some multicast-based services,
      such as resource location or status reporting, but it does not
      allow a host to create or join any host groups.  An IP
      implementation may be upgraded from level 0 to level 1 very easily
      and with little new code.  Only sections 4, 5, and 6 of this memo
      are applicable to level 1 implementations.

   Level 2: full support for IP multicasting.

      Level 2 allows a host to create, join and leave host groups, as
      well as send IP datagrams to host groups.  It requires
      implementation of the Internet Group Management Protocol (IGMP)
      and extension of the IP and local network service interfaces
      within the host.  All of the following sections of this memo are
      applicable to level 2 implementations.

4.  HOST GROUP ADDRESSES

   Host groups are identified by class D IP addresses, i.e. those with
   "1110" as their high-order four bits.  The remaining 28 bits are
   unstructured as far as hosts are concerned.  The addresses of
   well-known, permanent groups are to be published in "Assigned
   Numbers". Class E IP addresses, i.e. those with "1111" as their
   high-order four bits, are reserved for future addressing modes.

   Appendix II contains some background discussion of several issues
   related to host group addresses.



















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RFC 988                                                        July 1986
Host Extensions for IP Multicasting


5.  MODEL OF A HOST IP IMPLEMENTATION

   The multicast extensions to a host IP implementation are specified in
   terms of the layered model illustrated below.  In this model, ICMP
   and (for level 2 hosts) IGMP are considered to be implemented within
   the IP module, and the mapping of IP addresses to local network
   addresses is considered to be the responsibility of local network
   modules.  This model is for expository purposes only, and should not
   be construed as constraining an actual implementation.

      |                                                          |    
      |              Upper-Layer Protocol Modules                |    
      |__________________________________________________________|    
                                    
   --------------------- IP Service Interface ----------------------- 
       __________________________________________________________     
      |                            |              |              |    
      |                            |     ICMP     |     IGMP     |    
      |             IP             |______________|______________|    
      |           Module                                         |    
      |                                                          |    
      |__________________________________________________________|    
                                    
   ---------------- Local Network Service Interface ----------------- 
       __________________________________________________________     
      |                            |                             |    
      |           Local            | IP-to-local address mapping |    
      |          Network           |         (e.g. ARP)          |    
      |          Modules           |_____________________________|    
      |      (e.g. Ethernet)                                     |    
      |                                                          |    

   To support level 2 IP multicasting, a host IP implementation must
   provide three new services:  (1) sending multicast IP datagrams, (2)
   receiving multicast IP datagrams, and (3) managing group membership.
   Only the first service need be provided in level 1 hosts.  Each of
   these services is described in a separate section, below.  For each
   service, extensions are specified for the IP service interface, the
   IP module, the local network service interface, and an Ethernet local
   network module.  Extensions to local network modules other than
   Ethernet are mentioned briefly, but are not specified in detail.








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RFC 988                                                        July 1986
Host Extensions for IP Multicasting


6.  SENDING MULTICAST IP DATAGRAMS

   6.1. Extensions to the IP Service Interface

      No change to the IP service interface is required to support the
      sending of multicast IP datagrams.  An upper-layer protocol module
      merely specifies an IP host group destination, rather than an
      individual IP destination, when it invokes the existing "Send IP"
      operation.

   6.2. Extensions to the IP Module

      To support the sending of multicast IP datagrams, the IP module
      must be extended to recognize IP host group addresses when routing
      outgoing datagrams.  Most IP implementations include the following
      logic:

         if IP-destination is on the same local network,
            send datagram locally to IP-destination
         else
            send datagram locally to GatewayTo(IP-destination)

      To allow multicast transmissions, the routing logic must be
      changed to:

         if IP-destination is on the same local network
         or IP-destination is a host group,
            send datagram locally to IP-destination
         else
            send datagram locally to GatewayTo(IP-destination)

      If the sending host is itself a member of the destination group, a
      copy of the outgoing datagram must be looped-back for local
      delivery if and only if loopback was requested when the host
      joined the group (see section 8.1).  (This issue does not arise in
      level 1 implementations.)

      On hosts attached to more than one network, each multicast IP
      datagram must be transmitted via one network interface only,
      leaving it to the multicast agents to effect delivery to any other
      required networks.

      A host group address should not be placed in the source address
      field of an outgoing IP datagram.  A host group address may be
      used in a source routing option as the last element only.

      It should be noted that a small IP time-to-live (TTL) value can


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RFC 988                                                        July 1986
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      prevent delivery to some members of a destination group.  Thus, a
      large TTL value should be used to reach all members.  Conversely,
      a small TTL value can be used to advantage to reach only "nearby"
      members of a widely-dispersed group.  In clusters of low-delay
      local area networks, the TTL field acts as a hop limit; thus, one
      can perform expanding-ring searches by starting with a TTL of 1
      and incrementing on each retransmission, up to some limit defined
      by the diameter of the cluster.

   6.3. Extensions to the Local Network Service Interface

      No change to the local network service interface is required to
      support the sending of multicast IP datagrams.  The IP module
      merely specifies an IP host group destination, rather than an
      individual IP destination, when it invokes the existing "Send
      Local" operation.

   6.4. Extensions to an Ethernet Local Network Module

      The Ethernet directly supports the sending of local multicast
      packets by allowing multicast addresses in the destination field
      of Ethernet packets.  All that is needed to support the sending of
      multicast IP datagrams is a procedure for mapping IP host group
      addresses to Ethernet multicast addresses.

      An IP host group address is mapped to an Ethernet multicast
      address by placing the low-order 28-bits of the IP address into
      the low-order 28 bits of an Ethernet address.  The high-order 20
      bits of the Ethernet address are set to a well-known value, to be
      published in "Assigned Numbers".

      [At time of publication of this memo, a block of Ethernet
      multicast addresses with 28 unspecified bits had not yet been
      obtained from the allocating authority.  If such a block of
      addresses cannot be obtained, an alternative mapping scheme will
      be specified.]

   6.5. Extensions to Local Network Modules other than Ethernet

      Other networks that directly support multicasting, such as rings
      or buses conforming to the IEEE 802.2 standard, can be handled the
      same way as Ethernet for the purpose of sending multicast IP
      datagrams.  For a network that supports broadcast but not
      multicast, such as the Experimental Ethernet, all IP host group
      addresses can be mapped to a single local broadcast address (at
      the cost of increased overhead on all local hosts).  For a
      point-to-point networks like the ARPANET or a public data network


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Host Extensions for IP Multicasting


      (X.25), all IP host group addresses might be mapped to the
      well-known local address of an IP multicast agent; an agent on
      such a network would take responsibility for completing multicast
      delivery within the network as well as among networks.

7.  RECEIVING MULTICAST IP DATAGRAMS

   7.1. Extensions to the IP Service Interface

      No change to the IP service interface is required to support the
      reception of multicast IP datagrams.  Incoming multicast IP
      datagrams are delivered to upper-layer protocol modules using the
      same "Receive IP" operation as normal, unicast datagrams.

   7.2. Extensions to the IP Module

      To support the reception of multicast IP datagrams, the IP module
      must be extended to recognize the addresses of IP host groups to
      which the host currently belongs, in addition to the host's
      individual IP address(es).  An incoming datagram destined to one
      of those group addresses is processed exactly the same way as
      datagrams destined to one of the host's individual addresses.
      Incoming datagrams destined to groups to which the host does not
      belong are discarded without generating any error report.

      On hosts attached to more than one network, if a datagram arrives
      via one network interface, destined for a group to which the host
      belongs only on a different interface, the datagram is quietly
      discarded.  (This should occur only as a result of inadequate
      multicast address filtering in the local network module.)

      An incoming datagram is not rejected for having an IP host group
      address in its source address field or anywhere in a source
      routing option.

      An ICMP error message (Destination Unreachable, Time Exceeded,
      Parameter Problem, Source Quench, or Redirect) is never generated
      in response to a datagram destined to an IP host group.

   7.3. Extensions to the Local Network Service Interface

      No change to the local network service interface is required to
      support the reception of multicast IP datagrams.  Incoming local
      network packets, whether multicast or unicast, are delivered to
      the IP module using the same "Receive Local" operation.




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RFC 988                                                        July 1986
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   7.4. Extensions to an Ethernet Local Network Module

      To support the reception of multicast IP datagrams, an Ethernet
      module must be able to receive packets addressed to the Ethernet
      multicast addresses that correspond to the host's IP host group
      addresses.  It is highly desirable to take advantage of any
      address filtering capabilities that the Ethernet hardware
      interface may have, so that the host only receives packets that
      are destined to it.

      Unfortunately, many current Ethernet interfaces have a small limit
      on the number of addresses that the hardware can be configured to
      recognize.  However, an implementation must be capable of
      listening on an arbitrary number of Ethernet multicast addresses,
      which may mean "opening up" the address filter to accept all
      multicast packets during those periods when the number of
      addresses exceeds the limit of the filter.

      For interfaces with inadequate hardware address filtering, it may
      be desirable (for performance reasons) to perform Ethernet address
      filtering within the software of the Ethernet module.  This is not
      mandatory, however, because the IP module performs its own
      filtering based on IP destination addresses.

   7.5. Extensions to Local Network Modules other than Ethernet

      Other multicast networks, such as IEEE 802.2 networks, can be
      handled the same way as Ethernet for the purpose of receiving
      multicast IP datagrams.  For pure broadcast networks, such as the
      Experimental Ethernet, all incoming broadcast packets can be
      accepted and passed to the IP module for IP-level filtering.  On a
      point-to-point network, multicast IP datagrams will arrive as
      local network unicasts, so no change to the local network module
      should be necessary.















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RFC 988                                                        July 1986
Host Extensions for IP Multicasting


8.  MANAGING GROUP MEMBERSHIP

   8.1. Extensions to the IP Service Interface

      To allow upper-layer protocol modules to request that their host
      create, join, or leave a host group, the IP service interface must
      be extended to offer the following three new operations:

         CreateGroup ( private, loopback )
                                  --> outcome, group-address, access-key

      The CreateGroup operation requests the creation of a new,
      transient host group, with this host as its only member.  The
      "private" argument specifies if the group is to be private or
      public.  The "loopback" argument specifies whether or not
      datagrams sent from this host to the group should be delivered
      locally as well as to other member hosts.  The "outcome" result
      indicates whether the request is granted or denied.  If it is
      granted, a new 32-bit IP host group address is returned, along
      with a 64-bit access key which is zero for public groups and
      non-zero for private groups.  The request may be denied due to
      lack of response from a multicast agent, or lack of resources.

         JoinGroup ( group-address, access-key, loopback ) --> outcome

      The JoinGroup operation requests that this host become a member of
      the host group identified by "group-address", with the specified
      access key. The "loopback" argument specifies whether or not
      datagrams sent from this host to the group should be delivered
      locally as well as to other member hosts.  The "outcome" result
      indicates whether the request is granted or denied.  The request
      may be denied due to lack of response from a multicast agent, lack
      of resources, an invalid group address, an incorrect access key,
      or already being a member.

         LeaveGroup ( group-address, access-key ) --> outcome

      The LeaveGroup operation requests that this host give up its
      membership in the host group identified by "group-address", with
      the specified access key.  The "outcome" result indicates whether
      the request is granted or denied.  The request may be denied due
      to an invalid group address, an incorrect access key, or not
      currently being a member.

      Each of these operations may take up to a minute or more to
      complete, depending on the number of IGMP retransmissions



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RFC 988                                                        July 1986
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      performed within the IP module, and time required for a multicast
      agent to generate a reply. However, typical delays should be on
      the order of a few seconds.

      Besides the LeaveGroup operation, a host loses its membership in a
      group whenever the host or its IP module crashes, or, in rare
      circumstances, when a multicast agent revokes its membership.  The
      IP service interface should provide some means of informing an
      upper-layer module when its membership has been revoked.
      Membership may be revoked due to lack of resources, deallocation
      of the group address, or the discovery of another host group using
      the same group address with a different access key.  (See Appendix
      II for discussion of address recycling issues.)

      It is important to observe that IP group membership is per-host,
      rather than per-process.  An IP service interface should not allow
      multiple processes to invoke JoinGroup operations for the same
      group as a way of achieving delivery to more than process.  The IP
      module delivers each incoming datagram, whether multicast or
      unicast, to the single upper-layer protocol module identified by
      the protocol field in the datagram's IP header; it is an
      upper-layer issue whether or not to deliver incoming datagrams to
      more than one process, perhaps using the concept of "process
      groups" or "shared ports".

   8.2. Extensions to the IP Module

      Within the IP module, the membership management operations are
      supported by the Internet Group Management Protocol (IGMP),
      specified in Appendix I. As well as having messages corresponding
      to each of the operations specified above, IGMP also specifies a
      "deadman timer" procedure whereby hosts periodically confirm their
      memberships with the multicast agents.

      The IP module must maintain a data structure listing the IP
      addresses of all host groups to which the host currently belongs,
      along with each group's loopback policy, access key, and timer
      variables.  This data structure is used by the IP multicast
      transmission service to know which outgoing datagrams to loop
      back, and by the reception service to know which incoming
      datagrams to accept.  The purpose of IGMP and the management
      interface operations is to maintain this data structure.

      On hosts attached to more than one network, each membership is
      associated with a particular network interface.  On such a host
      the management interface operations above may each require an
      additional parameter specifying to which interface the create,


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      join, or leave request applies.  The group membership data
      structure must also be extended to associate an interface with
      each membership.  If a host joins the same host group on more than
      one network interface, it can expect to receive multiple copies of
      each datagram sent to that group.

   8.3. Extensions to the Local Network Service Interface

      To allow an IP module to control what packets should be accepted
      by the local network module, it is necessary to extend the local
      network service interface with the following two new operations:

         AcceptAddress ( group-address )

         RejectAddress ( group-address )

      where "group-address" is an IP host group address.  The
      AcceptAddress operation requests the local network module to
      accept and deliver up subsequently arriving packets destined to
      the local network address corresponding to "group-address".  The
      RejectAddress operation requests the local network module to stop
      delivering up packets destined to the local network address
      corresponding to "group-address".

      Any local network module is free to ignore RejectAddress requests,
      and may deliver up packets destined to more addresses than just
      those specified in AcceptAddress requests, if it is unable to
      filter incoming packets adequately.

   8.4. Extensions to an Ethernet Local Network Module

      An Ethernet module responds to AcceptAddress operations by adding
      the corresponding Ethernet multicast address to its acceptance
      filter for incoming packets.  A RejectAddress operation causes the
      corresponding Ethernet address to be dropped from the filter.  For
      Ethernet interfaces with a limit on the number of addresses that
      can be added to the filter, the Ethernet software module must
      detect when that threshold is exceeded and open up the filter to
      accept all multicast packets.  It should also detect when the
      number of addresses drops below the threshold and revert to
      individual address filtering.

   8.5. Extensions to Local Network Modules other than Ethernet

      Other multicast networks, such as IEEE 802.2 networks, can be
      handled the same way as Ethernet for the purpose of controlling
      address filtering.  For a pure broadcast network or a


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      point-to-point network, the AcceptAddress and RejectAddress
      operations may have no effect; all incoming packets could be
      passed to the IP module for IP-level filtering.














































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APPENDIX I.  INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)

   The Internet Group Management Protocol (IGMP) is used between IP
   hosts and their immediate neighbor multicast agents to support the
   creation of transient groups, the addition and deletion of members of
   a group, and the periodic confirmation of group membership.  IGMP is
   an asymmetric protocol and is specified here from the point of view
   of a host, rather than a multicast agent.

   Like ICMP, IGMP is a integral part of IP.  It is required to be
   implemented in full by all hosts conforming to level 2 of the IP
   multicasting specification.  IGMP messages are encapsulated in IP
   datagrams, with an IP protocol number of 2.  All IGMP 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            | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                          Identifier                           | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                         Group Address                         | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |                                                               | 
   +                         Access Key                            + 
   |                                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 

   Type

      There are eight types of IGMP message:

         1 = Create Group Request
         2 = Create Group Reply

         3 = Join Group Request
         4 = Join Group Reply

         5 = Leave Group Request
         6 = Leave Group Reply

         7 = Confirm Group Request
         8 = Confirm Group Reply





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   Code

      In a Create Group Request message, the code field indicates if the
      new host group is to be public or private:

         0 = public
         1 = private

      In all other Request messages, the code field contains zero.

      In a Reply message, the Code field specifies the outcome of the
      request:

         0       = request granted
         1       = request denied,  no resources
         2       = request denied,  invalid code
         3       = request denied,  invalid group address
         4       = request denied,  invalid access key
         5 - 255 = request pending, retry in this many seconds

   Checksum

      The checksum is the 16-bit one's complement of the one's
      complement sum of the IGMP message starting with the IGMP Type.
      For computing the checksum, the checksum field should be zero.

   Identifier

      In a Confirm Group Request message, the identifier field contains
      zero.

      In all other Request messages, the identifier field contains a
      value to distinguish the request from other requests by the same
      host.

      In a Reply message, the identifier field contains the same value
      as in the corresponding Request message.












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   Group Address

      In a Create Group Request message, the group address field
      contains zero.

      In all other Request messages, the group address field contains a
      host group address.

      In a Create Group Reply message, the group address field contains
      either a newly allocated host group address (if the request is
      granted) or zero (if denied).

      In all other Reply messages, the group address field contains the
      same host group address as in the corresponding Request message.

   Access Key

      In a Create Group Request message, the access key field contains
      zero.

      In all other Request messages, the access key field contains the
      access key assigned to the host group identified in the Group
      Address field (zero for public groups).

      In a Create Group Reply message, the access key field contains
      either a non-zero 64-bit number (if the request for a private
      group is granted) or zero.

      In all other Reply messages, the access key field contains the
      same access key as in the corresponding Request.



















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   Protocol Rules

      Request messages are sent only by hosts.  Reply messages are sent
      only by multicast agents.  If a host receives an IGMP message of a
      type other than the four Reply types specified above, the message
      is discarded.

      A Request message is sent with its IP destination field containing
      the well-known address of the Multicast Agent Group.  The IP
      time-to-live field is initialized by the sender to 1, in order to
      limit the scope of the request to immediate neighbor multicast
      agents only.  The IP source address field contains the individual
      IP address of the sending host.

      A Reply message is sent only in response to a Request message.
      Its IP destination address field contains the individual address
      of the host that sent the corresponding Request.  (A Confirm Group
      Reply may also be sent to the host group address specified in its
      corresponding Confirm Group Request.)  The IP source address field
      contains the individual IP address of the replying multicast
      agent.

      When a host sends a new Create Group, Join Group, or Leave Group
      Request message, it supplies an arbitrary identifier that it has
      not used within the last T0 seconds.  (It is usually sufficient
      just to increment the identifier for each new request.)  The host
      initializes a timer to T1 seconds and a retransmission counter to
      zero.  If a Reply message with a matching identifier is not
      received before the timer expires, it is reset to T1 seconds and
      the retransmission counter is incremented.  If the counter is less
      than N1, the host retransmits the Request message with the same
      identifier.  If the counter equals N1, the host gives up; if the
      request was to create or join a group, it is deemed to have
      failed; if the request was to leave a group, it is deemed to have
      succeeded.

      If a "request pending" code is received in a matching reply to a
      Create Group, Join Group, or Leave Group Request, the timer is
      reset to the number of seconds specified by the code and the
      retransmission counter is reset to zero.  The new timer value
      applies to one timeout interval only -- if the timer expires, it
      is reset to T1 seconds, the counter is incremented, and the
      request is retransmitted.

      The first matching Reply to a Create Group, Join Group, or Leave
      Group Request containing a "request granted" or "request denied"
      code determines the outcome of the request.  Any subsequent or


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      non-matching Replies are discarded by the host.  However, if a
      host receives an affirmative Create Group Reply or Join Group
      Reply that neither matches an outstanding Request nor contains the
      address of a group to which the host belongs, the host should
      immediately send a Leave Group Request for the unexpected group
      address.

      A "request granted" reply to a Create Group Request implies that,
      as well as the group being created, the requesting host is granted
      membership in the group, i.e. there is no need to send a separate
      Join Group Request.

      Confirm Group Request messages must be sent periodically by hosts
      to inform the neighboring multicast agent(s) of the hosts'
      continuing membership in the specified groups.  If an agent does
      not receive a Confirm Group Request message for a particular group
      within an agent-defined interval, it stops delivering datagrams
      destined to that group.

      For each group to which it belongs, a host maintains a
      confirmation timer and a variable t.  The variable t is
      initialized to T2 seconds. Whenever the host's request to create
      or join a group is granted, and whenever the host either sends a
      Confirm Group Request or receives a Confirm Group Reply with a
      "request granted" code for the group, the host sets the group's
      timer to a random number uniformly distributed between t and t +
      T3 seconds.  If the host receives a a Confirm Group Reply with a
      "request pending" code, t is changed to the value of the code and
      the timer is reset to a random number between the new t and t +
      T3.  The variable t retains its value until another "request
      pending" code is received.  Whenever the timer expires, the host
      sends a Confirm Group Request.

      Even if a host fails to receive Confirm Group Replies to its
      Requests, it continues to consider itself a member of the group,
      because it may still be able to receive multicast datagrams from
      other hosts on the same local network.  Only if a host receives a
      "request denied" code in a Confirm Group Reply does it stop
      sending Confirm Group Requests and consider its membership to be
      revoked.

      Multicast agents respond to Confirm Group Request messages by
      sending Confirm Group Reply messages either to the individual
      sender of the Request or to the host group address specified in
      the Request.  By sending back a Confirm Group Reply to all
      neighboring members of a group, a multicast agent is able to reset
      every member's timer with a single packet.  The randomization of


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RFC 988                                                        July 1986
Host Extensions for IP Multicasting


      the timers is intended to cause only the one member whose timer
      expires first to send a Confirm Group Request, stimulating a Reply
      to reset all the timers.  The use of the "request pending" codes
      allows the multicast agent to control the rate at which it
      receives Confirm Group Requests.

   Protocol Timing Constants

      The following timing constants are specified for IGMP.  They are
      subject to change as a result of operational experience.

      T0 = 300 seconds  minimum recycle time for identifiers

      T1 = 2 seconds    retrans. interval for Create/Join/Leave Requests

      N1 = 5 tries      retrans. limit for Create/Join/Leave Requests

      T2 = 15 seconds   initial value for Confirm Request variable t

      T3 = 15 seconds   random range for Confirm Request variable t





























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RFC 988                                                        July 1986
Host Extensions for IP Multicasting


APPENDIX II.  HOST GROUP ADDRESS ISSUES

   This appendix is not part of the IP multicasting specification, but
   provides background discussion of several issues related to IP host
   group addresses.

   Group Address Binding

      The binding of IP host group addresses to physical hosts may be
      considered a generalization of the binding of IP unicast
      addresses.  An IP unicast address is statically bound to a single
      local network interface on a single IP network.  An IP host group
      address is dynamically bound to a set of local network interfaces
      on a set of IP networks.

      It is important to understand that an IP host group address is NOT
      bound to a set of IP unicast addresses.  The multicast agents do
      not need to maintain a list of individual members of each host
      group.  For example, a multicast agent attached to an Ethernet
      need associate only a single Ethernet multicast address with each
      host group having local members, rather than a list of the
      members' individual IP or Ethernet addresses.

   Group Addresses as Logical Addresses

      Host group addresses have been defined specifically for use in the
      destination address field of multicast IP datagrams.  However, the
      fact that group addresses are location-independent (they are not
      statically bound to a single network interface) suggests possible
      uses as more general "logical addresses", both in the source as
      well as the destination address field of datagrams.  For example,
      a mobile IP host might have a host group address as its only
      identity, used as the source of datagrams it sends.  Whenever the
      mobile host moved from one network to another, it would join its
      own group on the new network and depart from the group on the old
      network.  Other hosts communicating with the mobile one would deal
      only with the group address and would be unaware of, and
      unaffected by, the changing network location of the mobile host.

      Host group addresses cannot, however, be used to solve all
      problems of internetwork logical addressing, such as delivery to
      the "nearest" or the "least loaded" network interface of a
      multi-homed host. Furthermore, there are hazards in using group
      addresses in the source address field of datagrams when the group
      actually contains more than one host.  For instance, the IP
      datagram reassembly algorithm relies on every host using a
      different source address.  Also, errors in a datagram sent with a


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RFC 988                                                        July 1986
Host Extensions for IP Multicasting


      group source address may result in error reports being returned to
      all members of the group, not just the sender.  In view of these
      hazards, this memo specifies the use of host group addresses only
      as destinations of datagrams, either in the destination address
      field or as the last element of a source routing option.  However,
      it is recommended that datagrams with a group source address be
      accepted without complaint, thereby allowing other implementations
      to experiment with logical addressing applications of host group
      addresses.

   Recycling of Transient Host Group Addresses

      Since host group addresses are of fixed, relatively small size,
      transient group addresses must be recycled to satisfy continuing
      requests for creation of new groups.  The multicast agents make an
      effort to ensure that a group has no members anywhere in the
      internet before allocating its address to a new group.  However,
      under certain conditions of internetwork partitioning and
      membership migration, it is impossible to guarantee unique
      allocation of an address without seriously compromising the
      availability and robustness of host groups. Furthermore, hosts
      that are unaware that a particular group has ceased to exist may
      send datagrams to it long after its address has been assigned to a
      new group.  Therefore, hosts should be prepared for the
      possibility of misdelivery of multicast IP datagrams to unintended
      hosts, even in private groups.  Such misdelivery can only be
      detected at a level above IP, using higher-level identifiers or
      authentication tokens.  (The access key of a private group might
      be used in some applications as such an identifier.)  Of course,
      there are other threats to privacy of communication in the
      internet, besides group address collision, such as untrustworthy
      gateways or unsecured networks. End-to-end encryption is an
      effective defense against such threats.
















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