AP CSP Day 48: Packet Switching | Cycle 2
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Packet headers contain routing information including source address, destination address, and sequence number, allowing routers to forward packets correctly and enabling the destination to reassemble them in order. Network congestion occurs when too many packets compete for the same link, increasing latency and potentially causing packet loss. AP CSP Cycle 2 packet switching questions may ask students to analyze a scenario where packets arrive out of order or are lost, and determine how the system detects and recovers from the error. Understanding that retransmission is handled by TCP but not UDP is a frequently tested protocol distinction.
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Packet Headers and Reassembly
What Packet Headers Contain
Every packet header includes a source IP address, destination IP address, sequence number for reassembly, and error-checking data. The sequence number allows the receiving system to reorder packets that arrive out of sequence.
TCP vs. UDP Handling of Loss
TCP detects lost packets through acknowledgment timeouts and requests retransmission automatically. UDP does not detect or correct packet loss. Applications using UDP (video streaming, gaming) are designed to tolerate some data loss rather than wait for retransmission.
Practice Question
A 1 MB file is divided into 100 packets and sent over the Internet. During transmission, 3 packets are lost. Which of the following best describes what happens next?
TCP uses sequence numbers and acknowledgments to detect missing packets. When 3 packets are not acknowledged, only those 3 are retransmitted. The destination holds the 97 received packets and reassembles the complete file once the 3 retransmissions arrive.
A) Resending the entire file would be wasteful. TCP retransmits only the specific missing packets. C) TCP ensures reliable delivery — missing packets are retransmitted, not permanently lost. B) Lost packet data cannot be reconstructed from other packets; it must be retransmitted from the source.
Students think either the whole file needs resending or that missing data is lost forever. TCP's selective retransmission is a key feature of reliable data delivery.
TCP provides reliable delivery through acknowledgments and retransmission. Missing packets trigger retransmission of only the missing packets, not the entire message.
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