In a campus network, limiting high-bandwidth
traffic to specific LANs or regions within the network is essential
to containing or eliminating unnecessary consumption of resources.
One method of containing multicast traffic is to impose constraints
on the forwarding of that traffic by using the Time-To-Live (TTL)
field in the IP packet.
As in unicast routing, the multicast TTL field
controls the lifetime of the packet. The function of TTL is to
prevent packets from being looped forever because of routing errors.
However, the TTL field in multicasting also carries the concept of a
"threshold." (see Figure
)
Multicast-enabled routers have a TTL threshold
assigned to each interface. Those packets with a TTL greater than
the interface threshold are forwarded, and those packets with a TTL
equal to or less than the interface threshold are discarded. The
packet TTL is compared with the interface threshold first. The
router then decrements the packet TTL upon sending the packet out
the interface. A list of TTL thresholds is shown in the Figure
table.
For example, a multicast packet with a TTL of
less than 16 is restricted to the same department, or site, and
should not be forwarded across an interface to other sites in the
same region. Defining the scope of a "site" or
"region" is the responsibility of the network
administrator.
A multicast router will forward a multicast
packet across an interface only if the TTL field in the IP header is
greater than the TTL threshold assigned to the interface. If the TTL
field in the IP header of the packet is equal to or less than the
TTL threshold assigned to the interface, the packet is discarded. If
the interface has no assigned TTL threshold, the packet is
forwarded.
In Figure
,
the interfaces on the router have been configured with the following
TTL thresholds:
- E1: TTL Threshold = 16
- E2: TTL Threshold = 0 (none)
- E3: TTL Threshold = 64
An incoming multicast packet for Group XYZ is
received on Interface E0 with a TTL of 24. The outgoing interface
list for Group XYZ contains Interfaces E1, E0, and S2. The TTL
threshold check is performed on each outgoing interface, as follows:
- E1: TTL (24) > TTL Threshold (16).
FORWARD
- E2: TTL (24) > TTL Threshold (0).
FORWARD
- E3: TTL (24) < TTL Threshold (64).DROP
The TTL is then decremented to 23 by normal
router IP packet processing.