📝 BitTorrent protocol explained (10 MCQs)

📖 From Data Communication and Networks • 29. Peer to Peer Paradigm • 10 questions available

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What is BitTorrent protocol explained?

BitTorrent is a P2P file-sharing protocol that breaks large files into small pieces and allows users to download different pieces simultaneously from multiple peers, dramatically improving transfer speeds and reducing server load.

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📝 All BitTorrent protocol explained MCQs

Q1. What is the primary function of a tracker in the original BitTorrent protocol?

A.Store the actual file data

B.Maintain a list of peers sharing each torrent ✅

C.Encrypt all traffic between peers

D.Provide DNS resolution for peer addresses

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The tracker does not hold any file chunks; its role is to keep a dynamic directory that maps torrent identifiers to the IP addresses of peers that currently possess those files. When a client contacts the tracker, it receives a list of peers to connect with, making option B the correct choice.

Q2. If a peer stops sending 'have' messages after downloading a piece, what is the most likely immediate effect on other peers' download strategy?

A.They will request that piece more often

B.They will assume the piece is unavailable ✅

C.They will increase their upload bandwidth

D.They will disconnect from the swarm

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Peers rely on 'have' messages to learn which pieces are available elsewhere. When a peer ceases to announce possession of a piece, other peers interpret the lack of notification as the piece being unavailable, prompting them to avoid requesting it. Hence, the most immediate effect is the assumption that the piece cannot be obtained, making option B correct.

Q3. In a trackerless BitTorrent network that uses Kademlia, how does a large XOR distance between a node and a target key affect the node's ability to locate the desired piece?

A.It enables the node to locate the piece more quickly

B.It slows down the search because the node is farther from the target in the key space ✅

C.It has no impact on search speed

D.It prevents the node from storing the piece altogether

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Kademlia organizes nodes in a binary key space where distance is measured by XOR. A larger XOR distance means the node is farther from the target key, requiring more hops through intermediate nodes to reach the responsible peer. Consequently, the search takes longer, so option B accurately describes the effect.

Q4. Suppose a torrent swarm contains 100 peers, each with an upload capacity of 0.5 Mbps, and the original seed has a download capacity of 2 Mbps. If the seed stops uploading, what is the maximum sustainable download rate for any individual peer, assuming perfect tit‑for‑tat fairness?

A.0.5 Mbps ✅

B.0.25 Mbps

C.1 Mbps

D.2 Mbps

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: With the seed no longer contributing upload bandwidth, the total upload capacity in the swarm is 100 × 0.5 Mbps = 50 Mbps. Under perfect tit‑for‑tat, each peer can download at most the average share of this upload capacity, which is 50 Mbps / 100 = 0.5 Mbps. Therefore, the highest sustainable download rate per peer is 0.5 Mbps, making option A correct.

Q5. Which of the following statements best compares centralized (tracker‑based) and decentralized (trackerless) BitTorrent architectures?

A.Centralized systems rely on a single point of failure, while decentralized systems distribute the directory function across peers ✅

B.Decentralized systems require a central server to coordinate piece exchange, unlike centralized systems

C.Both architectures use identical peer discovery mechanisms, differing only in encryption methods

D.Centralized architectures provide better anonymity than decentralized ones

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: In a centralized model, a tracker acts as a single point that maintains the list of peers, creating a potential failure point. Decentralized (trackerless) systems replace this with a distributed hash table where each peer contributes to the directory, eliminating a single point of failure. Thus, option A captures the core distinction.

Q6. How does the choke/unchoke algorithm influence fairness in a BitTorrent swarm?

A.It randomly selects peers to disconnect, ensuring equal bandwidth distribution

B.It prioritizes peers that provide higher upload rates, encouraging reciprocal sharing ✅

C.It forces all peers to upload at the same rate regardless of capacity

D.It disables upload to peers with low download speeds

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The choke/unchoke mechanism evaluates the upload rates received from each neighbor and preferentially unchokes those that contribute the most data. This reciprocal incentive promotes fairness by rewarding peers that share bandwidth, while penalizing free‑riders. Consequently, option B correctly describes the algorithm's impact on fairness.

Q7. What is the primary trade‑off when choosing a larger piece size for a torrent file?

A.Larger pieces reduce protocol overhead but increase latency for rare pieces ✅

B.Larger pieces increase overhead and decrease latency for rare pieces

C.Larger pieces have no effect on overhead or latency

D.Larger pieces improve anonymity but reduce download speed

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Increasing piece size means fewer pieces overall, which lowers the number of protocol messages (reducing overhead). However, if a piece is rare, a larger block takes longer to obtain, raising latency for that scarce piece. This balance of reduced overhead versus higher latency for rare pieces is captured in option A.

Q8. Applying the rarest‑first strategy, what is the expected effect on overall swarm health when a newly joined peer selects the rarest piece it does not have?

A.It accelerates the distribution of common pieces

B.It helps balance piece availability across the swarm ✅

C.It causes the peer to download slower than average

D.It reduces the need for a tracker

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The rarest‑first policy directs peers to request the pieces that have the fewest copies in the swarm. By doing so, it spreads those scarce pieces more quickly, preventing bottlenecks and ensuring a more uniform distribution of all pieces. This improves overall swarm health, making option B the correct answer.

Q9. In a Distributed Hash Table (DHT) used by trackerless BitTorrent, how is churn (peers frequently joining and leaving) typically mitigated?

A.By storing multiple redundant copies of each node’s routing table on a central server

B.By periodically refreshing bucket entries and replicating key‑value pairs to nearby nodes ✅

C.By preventing any node from leaving the network once it joins

D.By using static IP addresses for all peers

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: DHTs handle churn by continuously updating routing buckets; nodes ping neighbors to confirm liveness and replace failed entries. Additionally, key‑value pairs are replicated to nodes that are close in the XOR space, ensuring that the loss of a single node does not erase data. This combination of bucket refresh and replication is described in option B.

Q10. What is the relationship between the size of a BitTorrent swarm and the average download speed experienced by its members?

A.Larger swarms generally provide higher average download speeds due to more available upload capacity ✅

B.Larger swarms always decrease download speed because of increased network congestion

C.Swarm size has no impact on download speed

D.Smaller swarms guarantee faster downloads because they have fewer peers to coordinate with

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: A larger swarm means more peers contributing upload bandwidth, which increases the total pool of data that any individual can download. While extreme congestion can occur, in typical scenarios the added capacity outweighs coordination overhead, resulting in higher average download speeds. Therefore, option A correctly describes the relationship.

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