How the Receiving Host Handles the Packet

When the packet arrives on the receiving host, it travels through the TCP/IP protocol stack in the reverse order from that which it took on the sender. Figure 2-1 illustrates this path. Moreover, each protocol on the receiving host strips off header information attached to the packet by its peer on the sending host. Here is what happens.

1. Physical Network Layer receives the packet in its frame form. It computes the CRC of the packet, then sends the frame to the data link layer.

2. Data-Link Layer verifies that the CRC for the frame is correct and strips off the frame header and CRC. Finally, the data link protocol sends the frame to the Internet layer.

3. Internet Layer reads information in the header to identify the transmission and determine if it is a fragment. If the transmission was fragmented, IP reassembles the fragments into the original datagram. It then strips off the IP header and passes the datagram on to transport layer protocols.

4. Transport Layer (TCP and UDP) reads the header to determine which application layer protocol must receive the data. Then TCP or UDP strips off its related header and sends the message or stream up to the receiving application.

5. Application Layer receives the message and performs the operation requested by the sending host.