1. The FTP client sends data to the FTP server to describe its operational process. The communication between the two machines is illustrated below:
The detailed explanation is as follows:
1.1. Assume that the initial settings are as follows: The client's FTP port number is 32768, and the server's FTP port number is 21.
1.2. The communication process between two computers on different network segments through the TCP/IP protocol is described as follows: the protocol operates horizontally while the service functions vertically.
The physical layer deals with the transmission of electrical signals and the transparent transfer of bit streams. The link layer transmits data in frames, ensuring error-free transmission between adjacent nodes. The network layer provides communication across different hosts in a packet-switched network, using packets for data transmission. The transport layer manages communication between processes on the host, using segments as the unit of transmission. While the network layer handles point-to-point communication (where "point" refers to a host or router), the transport layer ensures end-to-end communication (with "end" referring to the source and destination host).
1.3. Data Packet Encapsulation Process
Different protocol layers use different names for data packets: the transport layer calls it a segment, the network layer a datagram, and the link layer a frame. After encapsulation into frames, the data is sent over the transmission medium. Upon reaching the destination host, each layer strips off the corresponding headers, and the application layer finally receives the data for processing. When two computers are on different network segments, the data must pass through one or more routers.
1.4. Working Process
(1) On PC1, the original data is encapsulated into a frame and sent through a physical link to port 1 of Switch 1. The resulting frame is as follows:
Note: How does the sender know if the destination is on the same network segment? Each IP address has a network prefix. The sender extracts the network prefix from the destination IP and compares it with its own. If they match, the data can be sent directly. In this case, PC1 and PC2 are on different networks.
(2) After receiving the data, Switch1 checks the destination MAC address and forwards the frame to Router’s S0 port based on its MAC address table.
(3) After receiving the data, the router checks the IP datagram, re-encapsulates the data, and sends it out through the S1 port after consulting the routing table. The new data frame is shown below:
Note: The destination and source IP addresses remain unchanged, but the MAC address is updated to the next hop’s MAC address.
(4) Switch2 receives the data from the router and forwards it to PC2 based on its MAC address table.
(5) Upon receiving the data, PC2 verifies it, then decapsulates the TCP segment, identifies the destination port as 21, and delivers the data to the corresponding FTP application process.
Second, what issues may occur during data transmission, and how can they be resolved?
1. At the data link layer, data may be corrupted or lost, and there may be differences in transmission speeds at both ends. Here’s how these problems are addressed:
(1) Error Control: There are two main methods—ARQ (Automatic Repeat Request) and FEC (Forward Error Correction). ARQ involves requesting retransmission when an error is detected, while FEC allows the receiver to correct errors without needing retransmission. Common error-correcting codes include parity check, CRC, and Hamming code.
(2) Loss Solution: If no ACK is received within a timeout period, the sender retransmits the data frame.
(3) Flow Control: One method is to stop sending after each frame until an acknowledgment is received. Another method uses a sliding window to manage the number of frames that can be sent at once.
2. If an IP datagram is 5000 bytes long and the MTU is set to 1500 bytes, fragmentation and reassembly are required. This process ensures that large datagrams can be transmitted across networks with smaller MTUs.
3. During the application process, errors and loss may occur, and caches at both ends may differ. To control traffic, the transport layer, especially TCP, provides reliable delivery and flow control mechanisms. Port numbers are used to identify specific application processes.
TCP uses logical communication between application processes and supports full-duplex, reliable data transfer. It includes sequence numbers and acknowledgments to ensure data is correctly ordered and delivered. Flow control is implemented via sliding windows, and congestion control is managed using techniques like slow start and congestion avoidance.
Port numbers, which are 16-bit values, serve as access points for the transport layer to deliver data to the correct application process. A socket, combining an IP address and port number, uniquely identifies a connection endpoint.
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