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

The question asks: how does the Internet server know which application (FTP or HTTP) should handle an incoming segment?
Option a: 'The source port number is associated with one of these well known server applications.' While the source port can be used to identify the client side, it is not what identifies the destination service on the server. The server uses the destination port to determine which local service to invoke, not the source port.
Option b: 'The packet header identifies it as an HTTP or FTP packet.' In TCP/IP, there isn’t a field that labels the payload as HTTP or FTP at the IP or TCP header level. Higher-level protocols (HTTP/FTP) sit atop TCP, and the appropriate service is selected by the destination port number, not by an explicit protocol label in the header.
Option c: 'The data in the segment is specially formatted for either HTTP or FTP.' The presence of data formatting alone does not tell the kernel which process should handle the segment. The operating system relies on the port-based demultiplexing mechanism, not on detecting the format of the payload to decide the destination application.
Option d: 'The segment destination port number identifies the application that should handle it.' This is the correct mechanism: servers listen on well-known ports (e.g., 80 for HTTP, 21 for FTP), and when a segment arrives, the system uses the destination port to route the segment to the appropriate process or socket. This is how demultiplexing is implemented in TCP/IP sockets and is fundamental to how multiple services coexist on a single host.
In summary, the demultiplexing decision is driven by the destination port in the segment header, which maps to the corresponding server application. The other options misattribute the mechanism or rely on assumptions that do not reflect how TCP/IP networking routes segment data to the correct service.

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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