Host Extensions for "Any Source" IP Multicasting (ASM)

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3.1. 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 described in section 4 of this memo.¶

3.2. 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 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.¶

3.3. 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 (IGMP) for IP and the equivalent Multicast Listener Discovery Protocol (MLD) for IPv6 and extension of the IP and local network service interfaces within the host.¶

The current protocol versions are [IGMPv3] and [MLDv2] or lightweight versions of either protocol [IGMPv3LITE].¶

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

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 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 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. These packets are not IP Multicast packets but simply invalid packets.¶

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 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 IP and IPv6 (as well as for SSM). Note that these are UDP socket extions (and not IP/IPv6 socket extensions due to the absence of widely available/used IP/IPv6 level socket APIs).¶

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 group.¶

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 IP and/or the MLD protocol for IPv6 (depending on the version of IP to be supported). 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 adequately.¶

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.¶

8.1. Moving RFC1112 and IGMPv1 to historic status

This document moves [RFC1112] to historic status so that the IGMP version 1 protocol as specified in Appendix 1 of [RFC1112] is moved to historic status. This protocol is not included in the text of this document anymore, which hence renders IGMPv1 historic.¶

All other aspects of [RFC1112] beside IGMPv1 are inherited and enhanced by this document and maintain their current Internet Standard designation from [RFC1112] through the normative status of this document.¶

8.2. Backward compatibility with IGMPv1

Newer versions of IGMP or other protocols/mechisms including but not necessary limited to [IGMPv2], [IGMPv3] or [IGMPv3LITE] do or may (such as in [IGMPsnooping]) include backward compatibility with IGMPv1, which requires the [RFC1112] specification of IGMPv1.¶

This document does not ask for any change to any specifications or implementations that includes any form of support for IGMPv1 for backward compatibility reasons as long as it also includes compatibility with a newer version of IGMP starting with [IGMPv2]. Any new or updated specification with such backward compatibility needs to continue to reference the specification of IGMPv1 in [RFC1112]. Any future reference for new or updated work to any other definition from [RFC1112] needs to refer to this document instead.¶

9.1. Normative language

This document introduces the use of normative language through capitalization. [RFC1112] preceeded this method and hence did not have this.¶

TBD: This version is an initial run across the text to find the appropriate places. It may be incomplete.¶

9.2. Superceeding references to IGMPv1

References to IGMPv1 in [RFC1112] are replaced by references to [IGMPv3] in this text.¶

9.3. Introduction of the term Any-Source Multicast (ASM)

This update introduces the term "ASM IP multicast" (ASM) as another term for "Host Extensions for IP multicast". This term was introduced when [SSM] introduced another service model for IP Multicast called "Source Specific Multicast" (SSM), and hence, the service described in [RFC1112] and this update is more precisely called Any Source Multicast (ASM) IP multicast.¶

[RFC1112] defines and uses the term "host group". This term is not applicable to IP/IPv6 multicast group addresses that are not used for ASM but SSM according to [SSM]. New text in this document explains this.¶

No functional changes to the IP Multicast service are incurred by these changes.¶

9.4. Applicability to both IP and IPv6

This update is written to apply to both IP and IPv6 by adding equivalent detail for IPv6 where [RFC1112] only covered IP: addressing and protocols in support of the service - Multicast Listener Discovery [MLDv2] for IPv6 versus IGMP for IP.¶

Note: IPv6 documents such as [RFC1883] and all its updates (e.g.: [RFC8200]) are defining multicasting in the assumption of the service of [RFC1112] for IPv6, but without being able to refer to [RFC1112], as it was only defined for IP. Future documents can refer to this document as the IP Multicast / ASM service for both IP and IPv6.¶

Additional text provides references for IETF UDP socket API specifications that instantiate the abstract APIs defined in this document.¶

No functional changes to the IP Multicast service are incurred by these text changes.¶

9.5. Standard for IP multicasting in controlled networks

This document removes the claim in the abstract of [RFC1112], that these host extensions are "... the recommended standard for IP multicasting in the Internet."¶

The reason for this is that [RFC8815] deprecated the ASM Service across the Internet because there is no Internet Standard solution for protocols to support interdomain ASM except for [RFC3956], which is only applicable to IPv6, and even that solution does not resolve the challenges to source access control in interdomain deployments.¶

In result, ASM is today "only" a recommended solution for controlled networks including controlled federated networks for applications for which SSM is not usable.¶

However, these limitations to the applicability of ASM to no impact the applicability of most of the host stack described in this document for other forms of IP Multicast, specifically "Source Specific Multicast", [SSM], which inherits all aspects of ASM specified in this document, especially the sending (Section 6, Section 6.2) of IP Multicast packets as well as the mapping to ethernet (Section 6.4). It only amends the joining of IP Multicast traffic on IP Multicast receivers with additional procedures fitting into the host stack described in this document.¶

A.1. 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 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.¶

A.2. 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 [RFC1045]) 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.¶

B.1. Goals of this document

The goal of this document is to allow for IETF to declare [RFC1112] historic and inherit the full INTERNET STANDARD status of [RFC1112] with this document immediately - without going through the otherwise necessary long process.¶

The reason why [RFC1112] needs to be declared historic is so that the IGMP version 1 protocol specified in it can be declared obsolete. This update removes IGMPv1 text.¶

The reason why this document is still needed (as an Internet Standard), is because the IP Multicast service specified in [RFC1112] has since its inception been the Internet Standard for the IP Multicasting service.¶

To allow for this document to gets immediately the intended Internet Standard status, it introduces no functional changes and it deliverately avoids also any unnecessary textual changes. This includes the deliberade non-upgrade of the [RFC1112] language to use [RFC2119] terminology. While the use of that language might be preferred for new work/text, the success of IP Multicasting as defined in [RFC2119] seems to indicate that the existing text was more than sufficient.¶

B.2. Internet Standard status

Note that the removal of the IGMPv1 protocol may raise the question whether the document in its current form still contains specifications sufficient for Internet Standard as opposed to Informational.¶

The core aspects that impacts interoperability (and hence qualifies the document for Internet Standard) is the format of IP packets when IP Multicast service is used, e.g.: IP Multicast addressing and binding to Multicast Ethernet MAC addresses. There is no other RFC that introduces these specifications for IP, because there was never another update to [RFC791] to do this. [SSM], another standards track document bilding on [RFC1112], defining the SSM service / host stack.¶

This update also includes the necessary text for IPv6. Note that for IPv6 the ethernet MAC address mapping of IPv6 multicast packets was later (after [RFC1112]) specified in [RFC1972] and its updates, but scattering the aspects of IPv6 multicast across (currently) [RFC2464], {RFC4291}} and [RFC8200] makes it arguably more difficult for implementers to understand the technology than this document that coalesces all these services aspects - from ethernet bonding to application interfac.¶

Beyond those packet format/ethernet aspects, historically, the Multicast service (API) related text in [RFC1112] would not have been considered to be an Internet Standards scope definition because this classification was not extended to (abstract) APIs, even though they do of course define an interoperability interface between e.g.: operating system providing the API and applications using it.¶

Recently, the IETF has changed its stance on this issue though and is working on [I-D.ietf-taps-interface] with the intent for it to become Internet Standard. With this in mind, all that text of [RFC1112] can also be considered appropriate for Internet Standard.¶

B.3. Authors email

This document does include Steve Deering as the original author of the base rfc [RFC1112] in the same way as [RFC8200] does for [RFC2460] ([RFC8507] is also similar). The mayority of text originates from his original RFC, hence he is the primary co-author. He is not actively involved in editing this -bis document but is in support of the work. Changes are edited by the co-author(s).¶

Being retired and not actively involved, he does not want for his personal email address to be included in the work. Any notifications where all authors need to provide feedback will be forwarded through the co-author.¶

Whereas at the time of [RFC8200] it was possible for a co-author to not have to include an email address, policies on Datatracker have since changed and therefore this draft include a placeholder email address for Steve solely to allow uploading to Datatracker.¶

B.4. Changelog

This document is hosted at https://github.com/toerless/rfc1112bis. Please submit issues with this text as issues to that github and report them on pim@ietf.org.¶

B.5. Open Issues

Need to revisit in more details the logic of upding RFC791. RFC1112 did not claim to be such an update even though it does effectively update RFC791 because it exempts IP Multicast packets from RFC791 processing. And also introduces invalid packets (source address IP Multicast which are neither unicast nor multicast).¶

Likewise the same would apply to RFC8200 which does not specify these details either.¶