300-415 Question #99: Real Exam Question with Answer & Explanation

The correct answer is D: SACK. SACK (Selective Acknowledgment) is the TCP Optimization feature that prevents unnecessary retransmissions by allowing the receiver to acknowledge non-contiguous received segments, so only truly missing segments are retransmitted.

WAN Edge Router Deployment

Question

Which TCP Optimization feature is used by WAN Edge to prevent unnecessary retransmissions and large initial TCP window sizes to maximize throughput and achieve a better quality?

Options

ASEQ
BSYN
CRTT
DSACK

Explanation

SACK (Selective Acknowledgment) is the TCP Optimization feature that prevents unnecessary retransmissions by allowing the receiver to acknowledge non-contiguous received segments, so only truly missing segments are retransmitted.

Common mistakes.

A. SEQ refers to TCP sequence numbers used for ordering segments - it is a fundamental TCP field, not a specific optimization feature for preventing retransmissions.
B. SYN is the TCP flag used during the three-way handshake to initiate connections - while TCP Optimization intercepts SYN packets to proxy connections, SYN itself is not the feature that prevents retransmissions.
C. RTT (Round-Trip Time) is a measurement metric used by TCP to calculate retransmission timers, but it is not an optimization feature that actively prevents unnecessary retransmissions.

Concept tested. Cisco SD-WAN TCP Optimization SACK retransmission prevention

Reference. https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/qos/ios-xe-17/qos-book-xe/m-appqoe-tcp-opt.html

Topics

#TCP Optimization#SACK#WAN Edge#Throughput

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