Benefits Of Source Specific Multicast - Cisco Catalyst 3650 Configuration Manual

Configuring SSM

Benefits of Source Specific Multicast

IP Multicast Address Management Not Required
In the ISM service, applications must acquire a unique IP multicast group address because traffic distribution
is based only on the IP multicast group address used. If two applications with different sources and receivers
use the same IP multicast group address, then receivers of both applications will receive traffic from the
senders of both applications. Even though the receivers, if programmed appropriately, can filter out the
unwanted traffic, this situation would cause generally unacceptable levels of unwanted traffic.
Allocating a unique IP multicast group address for an application is still a problem. Most short-lived applications
use mechanisms like Session Description Protocol (SDP) and Session Announcement Protocol (SAP) to get
a random address, a solution that does not work well with a rising number of applications in the Internet. The
best current solution for long-lived applications is described in RFC 2770, but this solution suffers from the
restriction that each autonomous system is limited to only 255 usable IP multicast addresses.
In SSM, traffic from each source is forwarded between routers in the network independent of traffic from
other sources. Thus different sources can reuse multicast group addresses in the SSM range.
Denial of Service Attacks from Unwanted Sources Inhibited
In SSM, multicast traffic from each individual source will be transported across the network only if it was
requested (through IGMPv3, IGMP v3lite, or URD memberships) from a receiver. In contrast, ISM forwards
traffic from any active source sending to a multicast group to all receivers requesting that multicast group. In
Internet broadcast applications, this ISM behavior is highly undesirable because it allows unwanted sources
to easily disturb the actual Internet broadcast source by simply sending traffic to the same multicast group.
This situation depletes bandwidth at the receiver side with unwanted traffic and thus disrupts the undisturbed
reception of the Internet broadcast. In SSM, this type of denial of service (DoS) attack cannot be made by
simply sending traffic to a multicast group.
Easy to Install and Manage
SSM is easy to install and provision in a network because it does not require the network to maintain which
active sources are sending to multicast groups. This requirement exists in ISM (with IGMPv1, IGMPv2, or
IGMPv3).
The current standard solutions for ISM service are PIM-SM and MSDP. Rendezvous point (RP) management
in PIM-SM (including the necessity for Auto-RP or BSR) and MSDP is required only for the network to learn
about active sources. This management is not necessary in SSM, which makes SSM easier than ISM to install
and manage, and therefore easier than ISM to operationally scale in deployment. Another factor that contributes
to the ease of installation of SSM is the fact that it can leverage preexisting PIM-SM networks and requires
only the upgrade of last hop routers to support IGMPv3, IGMP v3lite, or URD.
Ideal for Internet Broadcast Applications
The three benefits previously described make SSM ideal for Internet broadcast-style applications for the
following reasons:
• The ability to provide Internet broadcast services through SSM without the need for unique IP multicast
OL-29890-01
addresses allows content providers to easily offer their service (IP multicast address allocation has been
a serious problem for content providers in the past).
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
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