How the Receiving Host Handles the Packet

When the packet arrives on the receiving host, the packet travels through the TCP/IP protocol stack in the reverse order from which it was sent. Figure 1–1 illustrates this path. Moreover, each protocol on the receiving host strips off header information that is attached to the packet by its peer on the sending host. The following process occurs:

1. The physical network layer receives the packet in its frame form. The physical network layer computes the CRC of the packet, then sends the frame to the data link layer.

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

3. The Internet layer reads information in the header to identify the transmission. Then, the Internet layer determines if the packet is a fragment. If the transmission is fragmented, IP reassembles the fragments into the original datagram. IP then strips off the IP header and passes the datagram on to transport layer protocols.

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

5. The application layer receives the message. The application layer then performs the operation that the sending host requested.