An Internet server is running both FTP and HTTP services. How does the server know which of these applications should handle an incoming segment?

Question

BlackTom AI · Accepted Answer

Transport-layer demultiplexing uses the segment’s destination port number to deliver incoming data to the correct application process (e.g., HTTP or FTP).

1) Relevant knowledge points and concepts
- The TCP and UDP headers include both source and destination port numbers.  
- Operating systems demultiplex incoming segments by the destination port number, matching it against sockets bound by server applications.  
- Well-known server ports: HTTP typically listens on TCP port 80 (or 443 for HTTPS); FTP uses ports 21 (control) and 20 (data).  
- IP packet headers identify network-layer addressing (source/destination IP) and protocol (e.g., TCP), but not the specific application protocol.

2) Step-by-step reasoning
- A segment arrives at the host and the NIC/OS processes it up the stack.  
- The IP layer verifies the destination IP and hands the packet to the transport layer according to the IP protocol field (TCP or UDP).  
- The transport layer parses the TCP/UDP header and reads the destination port number.  
- The OS looks up which socket or server process has bound to that destination port (e.g., port 80 → web server, port 21 → FTP server).  
- The transport layer delivers the segment’s payload to that application’s socket.

3) Calculations / intermediate steps
- No numeric calculation is required; the lookup is a direct mapping from destination port → listening socket/process (e.g., dest port = 80 → web server).

4) Why the correct answer is correct
- The destination port is the standard mechanism for identifying which application should receive the incoming segment; it is how the OS demultiplexes transport-layer traffic to the appropriate server process.

5) Why the other options are incorrect
- a. “The packet header identifies it as an HTTP or FTP packet.” Incorrect: the IP header identifies network addresses and the protocol (e.g., TCP) but does not label the application protocol (HTTP/FTP).  
- c. “The source port number is associated with one of these well known server applications.” Incorrect: the source port is usually an ephemeral client port, not the server’s well-known port, so it cannot reliably identify the server application.  
- d. “The data in the segment is specially formatted for either HTTP or FTP.” Incorrect: although payload formats differ, OS demultiplexing does not rely on parsing application data to decide delivery; it uses the destination port.

6) Conclusion
The correct answer is b. The segment destination port number identifies the application that should handle it. This matches the provided answer.

An Internet server is running both FTP and HTTP services. How does the server know which of these applications should handle an incoming segment?单项选择题

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An Internet server is running both FTP and HTTP services. How does the server know which of these applications should handle an incoming segment?

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