Fill in the blank with the appropriate term. The _____________is an application layer protocol that is used between workstations and routers for transporting SNA/NetBIOS traffic over TCP sessions…

Question

Options

• ASequence Number
• BHeader Length
• CAcknowledgment Number
• DSource Port Address

• A
  6% (1)
• B
  6% (1)
• D
  89% (16)

Explanation

DCAP (Data Link Switching Client Access Protocol) is identified within TCP sessions via the Source Port Address (D) because DCAP uses a well-known TCP port (port 1996) to transport SNA/NetBIOS traffic between workstations and routers - the port number is what tells the network stack which application layer protocol is in use.

Why the distractors are wrong:

• A (Sequence Number) is a TCP field used to order packets for reassembly, not to identify application protocols.
• B (Header Length) indicates the size of the TCP header in 32-bit words - purely structural metadata, unrelated to protocol identification.
• C (Acknowledgment Number) confirms receipt of data by indicating the next expected byte - a flow-control mechanism, not a protocol identifier.

Memory tip: Think "Port = Protocol Passport" - just like a passport identifies who you are at a border, the Source Port Address identifies which application protocol (like DCAP on port 1996) is presenting itself at the TCP layer. When you see a question about identifying or distinguishing application-layer protocols within TCP sessions, the port number (Source or Destination Port) is almost always the right answer.

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