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Difficulty: HardTFTP and FTP Operations for System Images and Configuration Management

During a network maintenance window, a network engineer attempts to transfer a software image to a Cisco IOS XE switch located across an enterprise WAN link with mild latency and occasional packet loss. When using Trivial File Transfer Protocol (TFTP), the transfer repeatedly stalls and experiences severe throughput degradation whenever a single packet is dropped. However, when switching to File Transfer Protocol (FTP) over the same link, the transfer completes smoothly despite identical network conditions. Which underlying transport protocol characteristic explains why FTP recovers from packet loss far more efficiently than TFTP during file transfers?

  1. FTP operates over TCP using dynamic sliding window mechanisms and selective acknowledgments, whereas TFTP operates over UDP using a lock-step stop-and-wait acknowledgment mechanism where each individual block must be acknowledged before the next block is transmitted.Answer
  2. B
    FTP utilizes TCP port 21 to negotiate a stateless UDP data stream, whereas TFTP relies exclusively on unacknowledged UDP broadcasts across port 69.
  3. C
    FTP incorporates native application-layer forward error correction (FEC) to rebuild missing segments without retransmission, whereas TFTP lacks error detection headers entirely.
  4. D
    FTP utilizes TCP windowing to dynamically compress payload data when packet drops occur, whereas TFTP on UDP forces the switch to restart the entire file transfer from the first block upon a timeout.

Answer

FTP operates over TCP using sliding window mechanisms, whereas TFTP operates over UDP using a lock-step stop-and-wait acknowledgment mechanism where each block must be acknowledged prior to sending the next.
The correct answer highlights the foundational difference in flow control and reliability mechanics. FTP utilizes TCP, which features sliding window mechanisms that allow continuous stream transmission and selective retransmission of missing segments. TFTP uses UDP and requires an explicit ACK for every single data block before transmitting the next block (stop-and-wait), creating massive throughput bottlenecks when packets are dropped over WAN links.

Step-by-Step Solution

1
Analyze transport protocols utilized by TFTP and FTP.
TFTP runs over UDP on port 69, while FTP runs over TCP on ports 20 and 21.
Understanding the transport layer foundation determines reliability mechanics.
2
Examine TFTP application-layer behavior during file transfer.
TFTP uses a lock-step (stop-and-wait) protocol design where block NN must be explicitly acknowledged by the receiver before block N+1N+1 can be sent.
Because UDP does not provide native sequence numbers or windowing, TFTP enforces retransmission at the application layer per block.
3
Examine FTP transport-layer behavior during file transfer.
FTP relies on TCP's connection-oriented sliding window flow control, sequence numbers, and cumulative/selective ACKs.
TCP allows multiple segments to be transmitted concurrently without waiting for individual acknowledgments, drastically diminishing loss recovery overhead over WAN links.

Key Concept

TFTP (UDP 69, lock-step stop-and-wait per block) vs FTP (TCP 20/21, sliding windows, reliable streaming)
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Topics

TFTP and FTP Operations for System Images and Configuration Management

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