RFC3569: An Overview of Source-Specific Multicast (SSM)¶

July 2003
Category: Informational

1. Introduction¶

This document provides an overview of the Source-Specific Multicast (SSM) service and its deployment using the PIM-SM and IGMP/MLD protocols.
The network layer service provided by SSM is a “channel”, identified by an SSM destination IP address (G) and a source IP address S.
An IPv4 address range has been reserved by IANA for use by the SSM service. An SSM destination address range already exists for IPv6.
A source S transmits IP datagrams to an SSM destination address G.
A receiver can receive these datagrams by subscribing to the channel (Source, Group) or (S,G).
Channel subscription is supported by version 3 of the IGMP protocol for IPv4 and version2 of the MLD protocol for IPv6.
The interdomain tree for forwarding IP multicast datagrams is rooted at the source S, and is constructed using the PIM Sparse Mode [9] protocol.

备注

This document is not intended to be a standard for Source-Specific Multicast (SSM). Instead, its goal is to serve as an introduction to SSM and its benefits for anyone interested in deploying SSM services.

2. Terminology¶

Any-Source Multicast (ASM)¶

普通的多播

Source-Specific Multicast (SSM)¶

An IP datagram is transmitted by a source S to an SSM destination address G, and receivers can receive this datagram by subscribing to channel (S,G). SSM provides host applications with a “channel” abstraction, in which each channel has exactly one source and any number of receivers.
The address range 232/8 has been assigned by IANA for SSM service in IPv4.
For IPv6, the range FF3x::/96 is defined for SSM services.

Source-Filtered Multicast (SFM)¶

This is a variant of the ASM service model, and uses the same address range as ASM (224.0.0.0-239.255.255.255).
It extends the ASM service model as follows. Each “upper layer protocol module” can now request data sent to a host group G by only a specific set of sources, or can request data sent to host group G from all BUT a specific set of sources.
Support for source filtering is provided by version 3 of the Internet Group Management Protocol (or IGMPv3) for IPv4, and version 2 of the Multicast Listener Discovery (or MLDv2) protocol for IPv6.
We shall henceforth refer to these two protocols as “SFM-capable”. Earlier versions of these protocols - IGMPv1/IGMPv2 and MLDv1 - do not provide support for source-filtering, and are referred to as “non-SFM-capable”.
Note that while SFM is a different model than ASM from a receiver standpoint, there is no distinction between the two for a sender.

3. The IGMP/PIM-SM/MSDP/MBGP Protocol Suite for ASM¶

One of the most common multicast protocol suites for supporting ASM consists of IGMP version 2 [2], PIM-SM [8,9], MSDP [13] and MBGP [26].
IGMPv2 is the most commonly used protocol for hosts to specify membership in a multicast group
PIM-SM [8,9] is the most widely used protocol for building multicast tree among all receivers and sources in the same administrative domain
Inter-domain multicast service (i.e., where sources and receivers are located in different domains) requires additional protocols - MSDP [13] and MBGP [26] are the most commonly used ones.

4. Problems with Current Architecture¶

Address Allocation: The current multicast architecture does not provide a deployable solution to prevent address collisions among multiple applications.
Lack of Access control: In the ASM service model, a receiver cannot specify which specific sources it would like to receive when it joins a given group.
Inefficient handling of well-known sources

5. Source Specific Multicast (SSM): Benefits and Requirements¶

The SSM service model alleviates all of the deployment problems described earlier:

- Address Allocation: SSM defines channels on a per-source basis
    i.e., the channel (S1,G) is distinct from the channel (S2,G)

- Access Control: When a receiver subscribes to an (S,G) channel, it receives data sent only by the source S.

- Handling of well-known sources: SSM requires only source-based forwarding trees;
    This implies that neither the RP-based shared tree infrastructure of PIM-SM nor the MSDP protocol is required.
    Thus the complexity of the multicast routing infrastructure for SSM is low

6. SSM Framework¶

SSM Framework: elements in end-to-end model:

--------------------------------------------------------------
 IANA assigned 232/8 for IPv4             ADDRESS ALLOCATION
      FF3x::/96 for IPv6
--------------------------------------------------------------
             |
             v
    +--------------+ session directory/web page
    | source,group |                      SESSION DESCRIPTION
--------------------------------------------------------------
           ^ |
     Query | | (S,G)
           | v
  +-----------------+ host
  |   SSM-aware app |                     CHANNEL DISCOVERY
--------------------------------------------------------------
  |   SSM-aware app |                   SSM-AWARE APPLICATION
--------------------------------------------------------------
  |   IGMPv3/MLDv2  |              IGMPv3/MLDv2 HOST REPORTING
  +-----------------+
            |(source specific host report)
--------------------------------------------------------------
            v
  +-----------------+  Querier Router
  |   IGMPv3/MLDv2  |                         QUERIER
--------------------------------------------------------------
    |   PIM-SSM  |                        PIM-SSM ROUTING
    +------------+     Designated Router
            |
            | (S,G) Join only
            v
      +-----------+  Backbone Router
      |  PIM-SSM  |
      +-----------+
            |
            | (S,G) Join only
            V

6.1. Address Allocation¶

For IPv4, the address range of 232/8 has been assigned by IANA for SSM.
In case of IPv6, [23] has defined an extension to the addressing architecture to allow for unicast prefix-based multicast addresses. See RFC 3306 for details.

6.2. Session Description and Channel Discovery¶

An SSM receiver application must know both the SSM destination address G and the source address S before subscribing to a channel.

6.3. SSM-Aware Applications¶

There are two main issues in making multicast applications “SSM-aware”:

-  An application that wants to receive an SSM session must first discover the channel address in use.
-  A receiving application must be able to specify
    both a source address and a destination address to the network layer protocol module on the end-host.

6.4. IGMPv3/MLDv2 Host Reporting and Querier¶

The ability to specify an SSM channel address c is provided by IGMP version 3 [3] and MLD version 2 [20].

6.5. PIM-SSM Routing¶

Provides guidelines for how a PIM-SM implementation should handle source-specific host reports as required by SSM.

7. Interoperability with Existing Multicast Service Models¶

备注

旨在解决在现有的IP多播模型与SSM之间的互操作性问题。

应用¶

备注

SSM协议可用于各种应用场景，如点对点视频会议、流媒体传输、网络游戏等

SSM示例¶

场景示例:

A streaming video service that wants to multicast a live sports event to a select group of viewers.
The service knows the IP address of the source and the IP addresses of the viewers who have subscribed to this particular event.

multicast address生成示例:
ff3e::1:2:3:4 (source IP address) + ff31::1234 (group address)
=>
ff3e::1:2:3:4:ff31:1234 (SSM multicast address)

The streaming service would then send the multicast stream to this SSM address.
The subscribers would receive the stream by subscribing to this specific SSM address, rather than a broader multicast address.

场景示例2:

A company with multiple offices located around the world.
The company wants to use multicast to distribute internal training videos to employees in different offices.
However, the company is concerned about the security of the multicast transmissions
    and wants to ensure that the videos can only be accessed by employees in specific offices.

The company would assign a unique SSM multicast group address for each office that needs to receive the training videos.
For example:
    Office A: ff3e::1:2:3:4:ff31:1234
    Office B: ff3e::1:2:3:4:ff31:5678
    Office C: ff3e::1:2:3:4:ff31:9abc

The training videos would be streamed to these SSM addresses,
    and only employees in the specific offices with access to the SSM group address
    would be able to receive the multicast transmission.
This helps to ensure the security of the transmission and prevent unauthorized access.

场景示例3:

A research institute that wants to use multicast to distribute scientific data to researchers around the world.
The data is highly specialized and only a specific group of researchers need to access it.
The data is time-sensitive and needs to be transmitted quickly to ensure that researchers can analyze it in a timely manner.

地址转换示例:

源地址: 2001:db8::1
多播地址: ff31::8000:1234
=>
源定多播: ff3e::8000:1234:800:1234

地址转换示例:

源地址: 2001:db8:1234:5678:9abc:def0:1234:5678
多播地址: ff31::8000:1234
=>
源定多播: ff33::1234:5678

地址转换示例2:

192.0.2.1
239.255.0.1
=>
ff3e::c000:201

地址转换示例3:

2001:db8:85a3::8a2e:370:7334
ff3a::c
=>
ff3f::8a2e:370:7334

参考¶

https://www.rfc-editor.org/rfc/rfc3569