SSM mapping supports
in cases where neither URD nor IGMP v3lite is available, or
when supporting SSM on the end system is impossible or unwanted due
to administrative or technical reasons. Using SSM to deliver live streaming
video to legacy STBs that do not support IGMPv3
is a typical application of SSM mapping.
Prior to the introduction of SSM mapping, the following
conditions would have prevented SSM transition in the case of legacy STB
deployments with STB receivers that only support IGMPv1 or IGMPv2:
system on the receivers do not support IGMPv3; thus, IGMPv3 cannot be used to
application running on the receivers cannot be upgraded to support SSM; thus,
IGMPv3 lite cannot be used to support SSM transition.
exacerbate the issue, the application itself cannot be started through a web
browser; thus, URD cannot be used to support SSM transition.
SSM mapping provides an SSM transition solution for hosts and
applications that meet those conditions.
In a typical STB
deployment, each TV channel uses one separate IP multicast group and has one
active server host sending the TV channel. A single server may of course send
multiple TV channels, but each to a different group. In this network
environment, if a router receives an IGMPv1 or IGMPv2 membership report for a
particular group G, the report implicitly addresses the well-known TV server
for the TV channel associated with the multicast group.
introduces a means for the last hop router to discover sources sending to
groups. When SSM mapping is configured, if a router receives an IGMPv1 or
IGMPv2 membership report for a particular group G, the router translates this
report into one or more (S, G) channel memberships for the well-known sources
associated with this group.
As is the case for the other SSM transition solutions (URD and
IGMP v3lite), SSM mapping only needs to be configured on the last hop router
connected to receivers. No support is needed on any other routers in the
network. SSM mapping, in addition, is fully compatible with IGMPv3, IGMP
v3lite, and URD.
When the router
receives an IGMPv1 or IGMPv2 membership report for group G, the router uses SSM
mapping to determine one or more source IP addresses for group G. SSM mapping
then translates the membership report as an IGMPv3 report INCLUDE (G, [S1, G],
[S2, G]...[Sn, G] and continues as if it had received an IGMPv3 report. The
router then sends out PIM joins toward (S1, G) to (Sn, G) and continues to be
joined to these groups as long as it continues to receive the IGMPv1 or IGMPv2
membership reports and as long as
the SSM mapping for the group
remains the same. SSM mapping, thus, enables you to leverage SSM for video
delivery to legacy STBs that do not support IGMPv3 or for applications that do
not take advantage of the IGMPv3 host stack.
SSM mapping enables
the last hop router to determine the source addresses either by a statically
configured table on the router or by consulting a DNS server. When the
statically configured table is changed, or when the DNS mapping changes, the
router will leave the current sources associated with the joined groups.
DNS-Based SSM Mapping
DNS-based SSM mapping
enables you to configure the last hop router to perform a reverse DNS lookup to
determine sources sending to groups (see the figure below). When DNS-based SSM
mapping is configured, the router constructs a domain name that includes the
group address G and performs a reverse lookup into the DNS. The router looks up
IP address resource records (IP A RRs) to be returned for this constructed
domain name and uses the returned IP addresses as the source addresses
associated with this group. SSM mapping supports up to 20 sources for each
group. The router joins all sources configured for a group.
Figure 1. DNS-Based
The SSM mapping
mechanism that enables the last hop router to join multiple sources for a group
can be used to provide source redundancy for a TV broadcast. In this context,
the redundancy is provided by the last hop router using SSM mapping to join two
video sources simultaneously for the same TV channel. However, to prevent the
last hop router from duplicating the video traffic, it is necessary that the
video sources utilize a server-side switchover mechanism where one video source
is active while the other backup video source is passive. The passive source
waits until an active source failure is detected before sending the video
traffic for the TV channel. The server-side switchover mechanism, thus, ensures
that only one of the servers is actively sending the video traffic for the TV
To look up one or
more source addresses for a group G that includes G1, G2, G3, and G4, the
following DNS resource records (RRs) must be configured on the DNS server:
multicast-domain argument is a configurable DNS
prefix. The default DNS prefix is in-addr.arpa. You should only use the default
prefix when your installation is either separate from the internet or if the
group names that you map are global scope group addresses (RFC 2770 type
addresses that you configure for SSM) that you own.
argument configures the length of time for which the router performing SSM
mapping will cache the DNS lookup. This argument is optional and defaults to
the timeout of the zone in which this entry is configured. The timeout
indicates how long the router will keep the current mapping before querying the
DNS server for this group. The timeout is derived from the cache time of the
DNS RR entry and can be configured for each group/source entry on the DNS
server. You can configure this time for larger values if you want to minimize
the number of DNS queries generated by the router. Configure this time for a
low value if you want to be able to quickly update all routers with new source
Refer to your DNS
server documentation for more information about configuring DNS RRs.
DNS-based SSM mapping in the software, you must configure a few global commands
but no per-channel specific configuration is needed. There is no change to the
configuration for SSM mapping if additional channels are added. When DNS-based
SSM mapping is configured, the mappings are handled entirely by one or more DNS
servers. All DNS techniques for configuration and redundancy management can be
applied to the entries needed for DNS-based SSM mapping.