# Chapter 7 - 7.3 - Communication Protocols - Practice Problems - Page 362: 1

A has resent $\mathrm{M}(3),$ presumably because $\mathrm{B}^{\prime} \mathrm{s\ ACK}(3)$ message never reached A. When $\mathrm{B}$ receives the second copy of $\mathrm{M}(3),$ it should again send the $\mathrm{ACK}(3)$ to $\mathrm{A}$ but discard the message because it is a duplicate.

#### Work Step by Step

A has resent $\mathrm{M}(3),$ presumably because $\mathrm{B}^{\prime} \mathrm{s\ ACK}(3)$ message never reached A. When $\mathrm{B}$ receives the second copy of $\mathrm{M}(3),$ it should again send the $\mathrm{ACK}(3)$ to $\mathrm{A}$ but discard the message because it is a duplicate.

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