Java Reference
In-Depth Information
In IP multicasting, the TTL limits the multicast geographically. For example, a TTL
value of 16 limits the packet to the local area, generally one organization or perhaps an
organization and its immediate upstream and downstream neighbors. A TTL of 127,
however, sends the packet around the world. Intermediate values are also possible.
However, there is no precise way to map TTLs to geographical distance. Generally, the
farther away a site is, the more routers a packet has to pass through before reaching it.
Packets with small TTL values won't travel as far as packets with large TTL values.
Table 13-3
provides some rough estimates relating TTL values to geographical reach.
Packets addressed to a multicast group from 224.0.0.0 to 224.0.0.255 are never forwar‐
ded beyond the local subnet, regardless of the TTL values used.
Table 13-3. Estimated TTL values for datagrams originating in the continental United
States
Destinations
TTL value
The local host
0
The local subnet
1
The local campus—that is, the same side of the nearest Internet router—but on possibly different LANs
16
High-bandwidth sites in the same country, generally those fairly close to the backbone
32
All sites in the same country
48
All sites on the same continent
64
High-bandwidth sites worldwide
128
All sites worldwide
255
Once the data has been stuffed into one or more datagrams, the sending host launches
the datagrams onto the Internet. This is just like sending regular (unicast) UDP data.
The sending host begins by transmitting a multicast datagram to the local network. This
packet immediately reaches all members of the multicast group in the same subnet. If
the Time-To-Live field of the packet is greater than 1, multicast routers on the local
network forward the packet to other networks that have members of the destination
group. When the packet arrives at one of the final destinations, the multicast router on
the foreign network transmits the packet to each host it serves that is a member of the
multicast group. If necessary, the multicast router also retransmits the packet to the next
routers in the paths between the current router and all its eventual destinations.
When data arrives at a host in a multicast group, the host receives it as it receives any
other UDP datagram—even though the packet's destination address doesn't match the
receiving host. The host recognizes that the datagram is intended for it because it belongs
to the multicast group to which the datagram is addressed, much as most of us accept
mail addressed to “Occupant,” even though none of us are named Mr. or Ms. Occupant.
The receiving host must be listening on the proper port, ready to process the datagram
when it arrives.
