Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 278
DVMRP Tunneling

RFC 2003 describes IP encapsulation or tunneling. It is not necessary and can be the case many times, to have all routers running a multicast protocol. The question is, how do you get multicast datagrams through the Internet? Most routers on the Internet are not running a multicast protocol. The answer is tunneling. With tunneling, you build “islands” of multicast autonomous networks and they communicate with one another over the Internet by tunneling the multicast datagram over the Internet.

DVMRP supports the ability to tunnel a multicast datagram through nonmulticast routers. The multicast datagram is encapsulated in a unicast IP packet and addressed to the routers that do support native multicast routing. In other words, we wrap the multicast packet in an IP header and tell it which path to take to a destination multicast router.

To encapsulate an IP datagram using IP-in-IP encapsulation, an IP header is inserted before the existing IP datagram header. The source and destination address of the outer IP header are described in the input and output of the tunnel or the tunnel endpoints. The original IP header contains the IP source and destination address of the originator and final destination of the datagram.

DVMRP Tunneling