• IP-within-IP encapsulation:

fig: IP-within-IP encapsulation:

In IP-within-IP Encapsulation the entire IP datagram becomes the payload in a new IP datagram as shown above. The inner, original IP header is unchanged except to decrement TTL by 1. The outer header is a full IP header. Two fields (indicated as unshaded in the figure) are copied from the inner header: The version number is 4, which is the protocol identifier for IPv4, and the type of service requested for the outer IP datagram is the same as that requested for the inner IP datagram.

In the inner IP header, the source address refers to the host that is sending the original datagram, and the destination address is the home address of the intended recipient. In the outer IP header, the source and destination addresses refer to the entry and exit points of the tunnel. Thus, the source address typically is the IP address of the home agent, and the destination address is the care-of address for the intended destination.

• Minimal Encapsulation:

Fig: Minimal Encapsulation:

Minimal encapsulation results in less overhead and can be used if the mobile node, home agent, and foreign agent all agree to do so. With minimal encapsulation, the new header is inserted between the original IP header and the original IP payload shown in above figure. It includes the following fields:

• Protocol: Copied from the destination address field in the original IP header. This field identifies the protocol type of the original IP payload and thus identifies the type of header than begins the original IP payload.

• S: If 0, the original source address is not present, and the length of this header is 8 octets. If 1, the original source address is present, and the length of this header is 12 octets.

• Header Checksum: Computed over all the fields of this header.

• Original Destination Address: Copied from the destination address field in the original IP header.

• Original Source Address: Copied from the source address field in the original IP header. This field is present only if the S bit is 1. The field is not present if the encapsulator is the source of the datagram (i.e., the datagram originates at the home agent).

In the processing of minimal encapsulation the encapsulator (home agent) prepares the encapsulated datagram with the format as shown in above figure. This datagram is now suitable for tunneling and is delivered across the Internet to the care-of address. At the care-of address, the fields in the minimal forwarding header are restored to the original IP header and the forwarding header is removed from the datagram. The total length field in the IP header is decremented by the size of the minimal forwarding header (8 or 12) and the header checksum field is recomputed.