Difficulty: Medium
Correct Answer: It stores per connection state information such as sequence numbers, acknowledgment numbers, window sizes, timers, and buffer pointers for a TCP connection
Explanation:
Introduction / Context:
Transmission Control Protocol is connection oriented and maintains complex state for each active connection. Internally, TCP implementations use a data structure called the Transmission Control Block, abbreviated as TCB, to store this state. Understanding what a TCB holds is important for networking students and is a common topic in computer networks exams. The question asks you to identify the role of the TCB in managing a TCP connection.
Given Data / Assumptions:
Concept / Approach:
A TCP connection goes through several states and uses sequence and acknowledgment numbers to ensure reliable, ordered delivery. It also manages sliding windows, congestion control variables, timers, and pointers to send and receive buffers. All of this information needs to be stored in a structured way. The TCB is that structure: one instance per active TCP connection, containing all state variables necessary to process incoming segments and generate outgoing segments correctly. Therefore, the correct option must mention that the TCB stores per connection state information.
Step-by-Step Solution:
Step 1: Recall from networking textbooks that each TCP connection is uniquely identified by a four tuple and has its own state.
Step 2: The implementation uses a TCB to hold variables such as send sequence number, receive sequence number, acknowledgment number, window size, and congestion control parameters.
Step 3: The TCB also points to send and receive buffers in memory, and tracks timers associated with retransmission and timeouts.
Step 4: Option a describes the TCB exactly as a container for per connection state such as sequence numbers, acknowledgment numbers, window sizes, timers, and buffer pointers.
Step 5: Options b, c, and d describe hardware devices, user interface elements, or encryption algorithms, none of which match the definition of a TCB.
Step 6: Therefore, the correct answer is option a.
Verification / Alternative check:
To verify, imagine a server handling hundreds of simultaneous TCP connections. For each connection, the stack must remember what data has been sent but not yet acknowledged, what data has been received out of order, and what the current window size is. It would be impossible to manage this correctly without storing state for each connection. Operating system networking code typically allocates a TCB structure for each connection where all this information lives. Debugging tools often display parts of the TCB when showing connection state, which aligns with the description in option a.
Why Other Options Are Wrong:
Option b is wrong because a TCB is not a physical signal amplifier; that role belongs to devices like repeaters or hubs at lower layers. Option c is incorrect because TCBs are hidden implementation details, not user interface panels. Option d is wrong because encryption algorithms operate on data; they do not store connection state like sequence numbers and window sizes.
Common Pitfalls:
Learners sometimes confuse the TCB with protocol control blocks of other protocols or think of it as a hardware module. Another pitfall is to focus only on sequence numbers and forget that the TCB also includes timers and buffer pointers critical for retransmission and flow control. Understanding the TCB as a comprehensive container of per connection state helps in grasping how TCP achieves reliability and in reasoning about performance issues such as buffer exhaustion and connection limits.
Final Answer:
It stores per connection state information such as sequence numbers, acknowledgment numbers, window sizes, timers, and buffer pointers for a TCP connection.
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