Cisco Systems 78-11741-02 Wireless Office Headset User Manual


 
Configuring Source Specific Multicast
How SSM Differs from Internet Standard Multicast
IPC-460
Cisco IOS IP Configuration Guide
deploy receiver applications that are not yet SSM enabled (through support for IGMPv3). 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.
How SSM Differs from Internet Standard Multicast
The current IP multicast infrastructure in the Internet and many enterprise intranets is based on the
PIM-SM protocol and Multicast Source Discovery Protocol (MSDP). These protocols have proved to be
reliable, extensive, and efficient. However, they are bound to the complexity and functionality
limitations of the Internet Standard Multicast (ISM) service model. For example, with ISM, the network
must maintain knowledge about which hosts in the network are actively sending multicast traffic. With
SSM, this information is provided by receivers through the source addresses relayed to the last hop
routers by IGMPv3, IGMP v3lite, or URD. SSM is an incremental response to the issues associated with
ISM and is intended to coexist in the network with the protocols developed for ISM. In general, SSM
provides a more advantageous IP multicast service for applications that utilize SSM.
ISM service is described in RFC 1112. This service consists of the delivery of IP datagrams from any
source to a group of receivers called the multicast host group. The 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 will receive this traffic by becoming members of the host group.
Membership to a host group simply requires signalling 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 will receive this traffic by becoming members of the (S, G) channel. In both SSM and
ISM, no signalling is required to become a source. However, in SSM, receivers must subscribe or
unsubscribe to (S, G) channels to receive or not receive traffic from specific sources. In other words,
receivers can receive traffic only from (S, G) channels to which they are subscribed, whereas in ISM,
receivers need not know the IP addresses of sources from which they receive their traffic. The proposed
standard approach for channel subscription signalling utilizes IGMP INCLUDE mode membership
reports, which are supported only in IGMP Version 3.
SSM IP Address Range
SSM can coexist with the 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 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.
SSM can also be deployed 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.