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Understanding SSM

SSM is a datagram delivery model that best supports one-to-many applications, also known as Internet broadcast applications. SSM is a core networking technology for the Cisco implementation of IP multicast solutions targeted for applications such as audio and video broadcasting.

To run SSM with IGMPv3, SSM must be supported in the Cisco IOS router, in the host where the application is running, and in the application itself. IGMP v3lite and URD, two Cisco-developed transition solutions, enable the immediate development and deployment of SSM services, without the need to wait for the availability of full IGMPv3 support in host operating systems and SSM receiver applications. IGMPv3, IGMP v3lite, and URD interoperate with each other so that both IGMP v3lite and URD can easily be used as transitional solutions toward full IGMPv3 support in hosts.

This section covers the following topics:

  • Differences Between SSM and ISM
  • SSM IP Address Range
  • SSM Operations

Differences between SSM and ISM

The Internet Standard Multicast (ISM) service is described in RFC 1112, "Host Extensions for IP Multicasting." This service consists of the delivery of IP datagrams from any source to a group of receivers called the multicast host group. Datagram traffic for the multicast host group consists of datagrams with an arbitrary IP unicast source address S and the multicast group address G as the IP destination address. Systems receive this traffic when they become members of the host group. Membership in a host group simply requires signaling the host group through IGMP Version 1, 2, or 3.

In SSM, delivery of datagrams is based on (S, G) channels. Traffic for one (S, G) channel consists of datagrams with an IP unicast source address S and the multicast group address G as the IP destination address. Systems receive this traffic when they become members of the (S, G) channel rather than the host group. No signaling is required to become a source in either SSM or ISM. However, in SSM, receivers must subscribe or unsubscribe to (S, G) channels to receive or stop receiving traffic from specific sources. In other words, in SSM, receivers can receive traffic only from (S, G) channels to which they are subscribed. In ISM, receivers need not know the IP addresses of sources from which they receive their traffic. The proposed standard approach for channel subscription signaling uses IGMP INCLUDE mode membership reports, which are supported only in IGMPv3.

SSM IP Address Range

SSM can coexist with ISM service by applying the SSM delivery model to a configured subset of the IP multicast group address range. The Internet Assigned Numbers Authority (IANA) has reserved the address range 232.0.0.0 through 232.255.255.255 for SSM applications and protocols. Cisco IOS software allows SSM configuration for an arbitrary subset of the IP multicast address range from 224.0.0.0 through 239.255.255.255. When an SSM range is defined, existing IP multicast receiver applications will not receive any traffic when they try to use addresses in the SSM range unless the application is modified to use explicit (S, G) channel subscription or is SSM-enabled through URD.

SSM Operations

An established network in which IP multicast service is based on PIM-SM can support SSM services. You can also deploy SSM alone in a network without the full range of protocols that are required for interdomain PIM-SM (for example, MSDP, Auto-RP, or bootstrap router [BSR]) if only SSM service is needed. However, multiprotocol BGP might be required (and Cisco recommends its use) to maintain IP multicast connectivity if multiple autonomous systems are deployed in a network.

If SSM is deployed in a network already configured for PIM-SM (Cisco IOS Software Release 12.0 or later is recommended), only the last hop routers must be upgraded to a Cisco IOS Software Release 12.1(5)T or later that supports SSM. Routers that are not directly connected to receivers can run Cisco IOS Software Release 12.0 or later releases. In general, non-last hop routers must run only PIM-SM in the SSM range and might need additional access control configuration to suppress MSDP signalling, registering, or PIM-SM shared tree operations from occurring within the SSM range.

In Cisco IOS Software Release 12.1(3)T and later releases, you enable the SSM operation mode by configuring the SSM range through the ip pim ssm global configuration command. This configuration has the following effects:

  • For groups within the SSM range, (S, G) channel subscriptions are accepted through IGMPv3 INCLUDE mode membership reports, IGMP v3lite, or URD. Each of these methods must be configured on a per-interface basis. IGMP v3lite and URD (S, G) channel subscriptions are ignored for groups outside the SSM range.

  • PIM operations within the SSM range of addresses change to PIM source-specific mode (PIM-SSM). PIM-SSM, the routing protocol that supports the implementation of SSM, is derived from PIM-SM. In PIM-SSM mode, only PIM (S, G) join and prune messages are generated by the router, and no (S, G) rendezvous point tree (RPT) or (*, G) RPT messages are generated. Incoming messages related to RPT operations are ignored or rejected, and incoming PIM register messages are immediately answered with register-stop messages. PIM-SSM is backward-compatible with PIM-SM, unless the router is a last hop router. Routers that are not last hop routers can run PIM-SM for SSM groups (for example, if they do not yet support SSM).

  • No MSDP Source-Active (SA) messages within the SSM range will be accepted, generated, or forwarded.

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