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Versions: 00                                                            
PIM WG                                                       S. Deering
Internet-Draft                                                  Retired
Intended status: Standards Track                        T. Eckert (Ed.)
Obsoletes: RFC1112                                        Futurewei USA
Expires: January 13, 2022                                 July 12, 2021

      Host Extensions IP Multicasting - Any Source Multicast (ASM)


   This memo specifies the extensions required of a host implementation
   of the Internet Protocol (IP) to support Any Source Multicast (ASM)
   IP Multicasting or abbreviated IP Multicast.
   Distribution of this memo is unlimited.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."
   This Internet-Draft will expire on January 13, 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


   The host extensions defined in this memo are called
   Any Source Multicast (ASM) IP multicast or abbreviated IP multicast.
   The term Any Source Multicast is used to distinguish these extensions
   from Source Specific Multicast (SSM) IP multicast as defined by [RFC4607].
   The abbreviation IP multicast always refers to this memo's extensions.

   This memo applies to both IPv4 and IPv6. When it uses the term IP it
   implies either or both version of the IP protocol. It uses
   the terms IPv4 and/or IPv6 explicitly when referring to functions
   applicable to only a specific version of the IP protocol.

   This document is a revision of [RFC1112]. See Appendix II. for a detailled
   list of changes from that memo.

   IP multicasting is 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 intact 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.  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.  Those IP
   multicast addresses that are not reserved for permanent groups are
   available for dynamic assignment to transient groups which exist only
   as long as they have members.

   Internetwork forwarding of IP multicast datagrams is handled by
   "multicast routers" which may be co-resident with, or separate from,
   internet gateways.  A host transmits an IP multicast datagram as a
   local network multicast which reaches all immediately-neighboring
   members of the destination host group.  If the datagram has an IP
   time-to-live greater than 1, the multicast router(s) attached to the
   local network take responsibility for forwarding it towards all other
   networks that have members of the destination group.  On those other
   member networks that are reachable within the IP time-to-live, an
   attached multicast router completes delivery by transmitting the
   datagram as a local multicast.

   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 acting as multicast
   routers.  The algorithms and protocols used within and between
   multicast routers are transparent to hosts and will be specified in
   separate documents.  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.


   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 IPv4 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 described in section 4 of this memo.

      Level 1: support for sending but not receiving multicast IP

   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 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

      Level 2: full support for IP multicasting.

   Level 2 allows a host to join and leave host groups, as well as send
   IP datagrams to host groups.  Most IPv6 hosts require Level 2 support
   because IPv6 Neighbor Discovery ([RFC4861], as used on most link types)
   depends on multicast and requires that nodes join Solicited Node multicast addresses.

   Level 2 requires implementation of the Internet Group Management Protocol (IGMPv)
   for IPv4 and the equivalent Multicast Listener Discovery Protocol (MLDv) for IPv6
   and extension of the IP and local network service interfaces within the host.

   The current protocol versions are IGMPv3 [RFC3376] and MLDv2 [RFC3810] or lightweight
   versions of either protocol [RFC5790].

   All of the following sections of this memo are applicable to level 2


   IPv4 Host groups are identified by class D IP addresses, i.e., those with
   "1110" as their high-order four bits.  Class E IP addresses, i.e.,
   those with "1111" as their high-order four bits, are reserved for
   future addressing modes.

   In Internet standard "dotted decimal" notation, host group addresses
   range from to  The address is
   guaranteed not to be assigned to any group, and is assigned
   to the permanent group of all IPv4 hosts (including gateways).  This is
   used to address all IPv4 multicast hosts on the directly connected
   network.  There is no multicast address (or any other IP address) for
   all hosts on the total Internet.  The addresses of other well-known,
   permanent groups are to be published in "Assigned Numbers".

   IPv6 Host groups are identified by IPv6 addresses as defined in [RFC4291] section 2.7
   and updated by [RFC7346], [RFC7371].

   IPv4 and IPv6 addresses as specified in [RFC4607] are not used for
   ASM IP multicast and are not considered IP host groups. They are instead
   only the destination address part G of Source Specific Multicast (SSM)
   IP multicast (S,G) channels.

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


   The multicast extensions to a host IP implementation are specified in
   terms of the layered model illustrated below.  In this model, ICMP/ICMPv6
   and (for level 2 hosts) IGMP/MLD 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 -----------------------
         |                            |              |              |
         |                            | IPv4:        | IPv6:        |
         |                            | ICMP+ICMP    | ICMPv6+MLD   |
         |    IP [IPv4 and/or IPv6]   |______________|______________|
         |           Module(s)                                      |
         |                                                          |

      ---------------- Local Network Service Interface -----------------
         |                            |                             |
         |           Local            | IP-to-local address mapping |
         |          Network           |         (e.g., ARP/ND)      |
         |          Modules           |_____________________________|
         |      (e.g., Ethernet)                                    |
         |                                                          |

   To provide level 1 multicasting, a host IP implementation must
   support the transmission of multicast IP datagrams.  To provide level
   2 multicasting, a host must also support the reception of multicast
   IP datagrams.  Each of these two new 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.


6.1. Extensions to the IP Service Interface

   Multicast IP datagrams are sent using the same "Send IP" operation
   used to send unicast IP datagrams; an upper-layer protocol module
   merely specifies an IP host group address, rather than an individual
   IP address, as the destination.  However, a number of extensions may
   be necessary or desirable.

   First, the service interface should provide a way for the upper-layer
   protocol to specify the IP time-to-live of an outgoing multicast
   datagram, if such a capability does not already exist.  If the
   upper-layer protocol chooses not to specify a time-to-live, it should
   default to 1 for all multicast IP datagrams, so that an explicit
   choice is required to multicast beyond a single network.

   Second, for hosts that may be attached to more than one network, the
   service interface should provide a way for the upper-layer protocol
   to identify which network interface is be used for the multicast
   transmission.  Only one interface is used for the initial
   transmission; multicast routers are responsible for forwarding to any
   other networks, if necessary.  If the upper-layer protocol chooses
   not to identify an outgoing interface, a default interface should be
   used, preferably under the control of system management.

   Third (level 2 implementations only), for the case in which the host
   is itself a member of a group to which a datagram is being sent, the
   service interface should provide a way for the upper-layer protocol
   to inhibit local delivery of the datagram; by default, a copy of the
   datagram is looped back.  This is a performance optimization for
   upper-layer protocols that restrict the membership of a group to one
   process per host (such as a routing protocol), or that handle
   loopback of group communication at a higher layer (such as a
   multicast transport protocol).

   IPv6 socket extensions supporting these functions are defined in [RFC3493], section 5.2.

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

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

   To allow multicast transmissions, the routing logic must be changed

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

   If the sending host is itself a member of the destination group on
   the outgoing interface, a copy of the outgoing datagram must be
   looped-back for local delivery, unless inhibited by the sender.
   (Level 2 implementations only.)

   The IP source address of the outgoing datagram must be one of the
   individual addresses corresponding to the outgoing interface.

   A host group address must never be placed in the source address field
   or anywhere in a source route or record route option of an outgoing
   IP datagram.

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 IPv4 host group address is mapped to an Ethernet multicast address
   by placing the low-order 23-bits of the IP address into the low-order
   23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
   Because there are 28 significant bits in an IP host group address,
   more than one host group address may map to the same Ethernet
   multicast address.

   Mapping of IPv6 host group addresses to Ethernet is defined in
   [RFC2464] and [RFC6085].

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, may 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 may be mapped to a
   single local broadcast address (at the cost of increased overhead on
   all local hosts).  For a point-to-point link joining two hosts (or a
   host and a multicast router), multicasts should be transmitted
   exactly like unicasts.  For a store-and-forward network like the
   ARPANET or a public X.25 network, all IP host group addresses might
   be mapped to the well-known local address of an IP multicast router;
   a router on such a network would take responsibility for completing
   multicast delivery within the network as well as among networks.


7.1. Extensions to the IP Service Interface

   Incoming multicast IP datagrams are received by upper-layer protocol
   modules using the same "Receive IP" operation as normal, unicast
   datagrams.  Selection of a destination upper-layer protocol is based
   on the protocol field in the IP header, regardless of the destination
   IP address.  However, before any datagrams destined to a particular
   group can be received, an upper-layer protocol must ask the IP module
   to join that group.  Thus, the IP service interface must be extended
   to provide two new operations:

                 JoinHostGroup  ( group-address, interface )

                 LeaveHostGroup ( group-address, interface )

   The JoinHostGroup operation requests that this host become a member
   of the host group identified by "group-address" on the given network
   interface.  The LeaveGroup operation requests that this host give up
   its membership in the host group identified by "group-address" on the
   given network interface.  The interface argument may be omitted on
   hosts that support only one interface.  For hosts that may be
   attached to more than one network, the upper-layer protocol may
   choose to leave the interface unspecified, in which case the request
   will apply to the default interface for sending multicast datagrams
   (see section 6.1).

   It is permissible to join the same group on more than one interface,
   in which case duplicate multicast datagrams may be received.  It is
   also permissible for more than one upper-layer protocol to request
   membership in the same group.

   Both operations should return immediately (i.e., they are non-
   blocking operations), indicating success or failure.  Either
   operation may fail due to an invalid group address or interface
   identifier.  JoinHostGroup may fail due to lack of local resources.
   LeaveHostGroup may fail because the host does not belong to the given
   group on the given interface.  LeaveHostGroup may succeed, but the
   membership persist, if more than one upper-layer protocol has
   requested membership in the same group.

   IPv6 socket extensions supporting these functions are defined in [RFC3493], section 5.2.
   [RFC3678] specifies these functions for IPv4 and IPv6 (as well as for SSM).

7.2. Extensions to the IP Module

   To support the reception of multicast IP datagrams, the IP module
   must be extended to maintain a list of host group memberships
   associated with each network interface.  An incoming datagram
   destined to one of those groups 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 or log
   entry.  On hosts with more than one network interface, if a datagram
   arrives via one interface, destined for a group to which the host
   belongs only on a different interface, the datagram is quietly
   discarded.  (These cases should occur only as a result of inadequate
   multicast address filtering in a local network module.)

   An incoming datagram is not rejected for having an IP time-to-live of
   1 (i.e., the time-to-live should not automatically be decremented on
   arriving datagrams that are not being forwarded).  An incoming
   datagram with an IP host group address in its source address field is
   quietly discarded.  An ICMP/ICMPv6 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

   The list of host group memberships is updated in response to
   JoinHostGroup and LeaveHostGroup requests from upper-layer protocols.
   Each membership should have an associated reference count or similar
   mechanism to handle multiple requests to join and leave the same
   group.  On the first request to join and the last request to leave a
   group on a given interface, the local network module for that
   interface is notified, so that it may update its multicast reception
   filter (see section 7.3).

   The IP module must also be extended to implement the IGMP protocol for
   IPv4 and the MLD protocol for IPv6.  IGMP/MLD are used to keep neighboring multicast
   routers informed of the host group memberships present on a
   particular local network.

7.3. Extensions to the Local Network Service Interface

   Incoming local network multicast packets are delivered to the IP
   module using the same "Receive Local" operation as local network
   unicast packets.  To allow the IP module to tell the local network
   module which multicast packets to accept, the local network service
   interface is extended to provide two new operations:

                      JoinLocalGroup  ( group-address )

                      LeaveLocalGroup ( group-address )

   where "group-address" is an IP host group address.  The
   JoinLocalGroup operation requests the local network module to accept
   and deliver up subsequently arriving packets destined to the given IP
   host group address.  The LeaveLocalGroup operation requests the local
   network module to stop delivering up packets destined to the given IP
   host group address.  The local network module is expected to map the
   IP host group addresses to local network addresses as required to
   update its multicast reception filter.  Any local network module is
   free to ignore LeaveLocalGroup requests, and may deliver up packets
   destined to more addresses than just those specified in
   JoinLocalGroup requests, if it is unable to filter incoming packets

   The local network module must not deliver up any multicast packets
   that were transmitted from that module; loopback of multicasts is
   handled at the IP layer or higher.

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 receives only those 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.  Nevertheless, 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 point-to-point or
   store-and-forward networks, multicast IP datagrams will arrive as
   local network unicasts, so no change to the local network module
   should be necessary.


   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

   It is important to understand that an IP host group address is NOT
   bound to a set of IP unicast addresses.  The multicast routers do not
   need to maintain a list of individual members of each host group.
   For example, a multicast router 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.

Allocation of Transient Host Group Addresses

   This memo does not specify how transient group address are allocated.
   It is anticipated that different portions of the IP transient host
   group address space will be allocated using different techniques.
   For example, there may be a number of servers that can be contacted
   to acquire a new transient group address.  Some higher-level
   protocols (such as VMTP, specified in RFC-1045) may generate higher-
   level transient "process group" or "entity group" addresses which are
   then algorithmically mapped to a subset of the IP transient host
   group addresses, similarly to the way that IP host group addresses
   are mapped to Ethernet multicast addresses.  A portion of the IP
   group address space may be set aside for random allocation by
   applications that can tolerate occasional collisions with other
   multicast users, perhaps generating new addresses until a suitably
   "quiet" one is found.

   In general, a host cannot assume that datagrams sent to any host
   group address will reach only the intended hosts, or that datagrams
   received as a member of a transient host group are intended for the
   recipient.  Misdelivery must be detected at a level above IP, using
   higher-level identifiers or authentication tokens.  Information
   transmitted to a host group address should be encrypted or governed
   by administrative routing controls if the sender is concerned about
   unwanted listeners.

APPENDIX II. Changes from RFC1112

This document updates RFC1112 with the following changes:

o It removes the claim that these host extensions are "... the recommended standard for IP multicasting in the Internet." (Status of the memo section). This is reflecting the experience of the past 30 years, in which Interdomain and even more so across the Internet has produced no full IETF standard solution for IPv4 and that SSM IP multicast is now considered the preferred standard for Interdomain/Internet deployments. See [draft-ietf-mboned-deprecate-interdomain-asm].

o It is written to apply to both IPv4 and IPv6 by adding equivalent detail for IPv4 where RFC1112 only covered IPv4: addressing and protocol (MLD vs. IGMP).

o It introduces the term "ASM IP multicast" as another term for "Host Extensions for IP multicast". This became necessary because RFC4607 introduced another service model for IP Multicast called "Source Specific Multicast" (SSM), and since then, the original service of RFC1112 is more precisely called Any Source Multicast (ASM) IP multicast.

o It removes the original appendix I of RFC1112 that defined the IGMP version 1 protocol and text in the main document referring to details of IGMPv1 because that protocol is intended to be made obsolete as it is superceeded by IGMPv3/IGMPv3.

APPENDIX III. Discussion and Explanations

[RFC-editor: Please remove this section]


This document is intended to be an update to RFC1112 for the following reasons:

RFC1112 is at the time of this writing the only FULL INTERNET STANDARD describing the ASM IP Multicast Service Model for IPv4. There is no eqivalent document for IPv6. RFC6434 section 5.10 is a stand-in, but it only describes MLD protocol version aspects but not the overall aspects of the host extensions and specifically the ASM IP multicast service. Instead it just refers to the term ASM and RFC1112.

RFC1112 includes the specification of IGMPv1. PIM WG would like to make IGMPv1 historic, but not the ASM host extensions / service. Instead, it ideally wants a FULL INTERNET STANDARD normative reference for ASM that applies to IPv4 and IPv6. This RFC1112bis is the attempt to do this.

Open Issues:

Has any document after RFC1112 (re-)defined the mapping of IPv4 multicast
group addresses to ethernet multicast MAC ? If so, then we should include
a reference to it and update the appropriate text in this rfc1112bis.

This document uses/defines the term "host group", which really is only a term
relevant in this traditional ASM service model, but not in SSM. Therefore new
text stating that IP multicast group addresses from RFC4607 are not included
in this ASM definition. This is hopefully aligned with the text in RFC4607.

Is it appropriate to have included text to refer to socket API for ASM
eg: rfc3678. These socket APIs are primarily about UDP sockets, and only
rarely for IP level. This document only specifies an IP Service Interface.
Note that RFC4607 (SSM) also refers to the socket interface extensions for
SSM under a section called "Extensions to the IP Module Interface".

Discuss: Why would this document still need to be a standards track document ?
IETF typically does not assign standard track to pure API/service-interface
document. With IGMPv1 removed from the document, what does it still make it
standards track, aka: define in a normative fashion interoperability
impacting behacior of nodes ?

Answer1: Definition of the IPv4 ASM address space, Definition of
the IP Multicast group to ethernet MAC address mapping for IPv4. The document
now also contains references to these standards aspects for IPv6, but those
are references to prior standards track documents.

Answer2: This document is similar in scope to RFC4607 (SSM) which is standards
track. The newly defined interop impacting behavior on the wire are also
limited: address ranges for IPv4/IPv6 SSM, and it is referring to RFC1112.

Discuss: Which documents should we claim this document is updating ? Hopefully
none other than rfc1112 - rfc1112 itself is referenced by > 60 RFCs.

Author's Addresses

   Stephen E. Deering
   Vancouver, British Columbia
   Email: bob.hinden@gmail.com (email secretary)

   Toerless Eckert (editor)
   Futurewei Technologies Inc. USA
   2220 Central Expy
   Santa Clara, CA 95050
   United States of America
   Email: tte@cs.fau.de