Computer Diagrams

Introduction to Computer Networks?

Consider the Go-Back-N protocol, and suppose that base of the senders window is X. Is it possible for the sender to receive an ACK for a packet that has a smaller sequence number than X. If so, sketch out a timeline-diagram showing sender and receiver messages sent and received that shows how this is possible. If not, explain why this can never happen.

Public Comments

1. I'd remove the "?" mark in your subject line, it sounds like your ARE in Introduction to Computer Networks.
2. Yes, because the Go-Back-N protocol uses what is called cumulative acknowledgment. Here are two ways of explaining this (same issue but worded slighly different): 1. Say an ACK for the first packet sent by the sender is not received as the timer goes down. Because of this, the packet will be sent again. However, in the time that the first packet ACK is received, the sender will already have emptied it's buffer and then filled them with new packets (for another window). While this occurring, an ACK for a second frame be received. In this case, the ACK received would fall outside of the current window. 2. Say you have a window of size 2. The sender transmits two packets (SYN1 and SYN2). However, the sender receives ACK2 and then receives ACK1. Once the sender has received ACK2, it can safely assume that both ACK1 and ACK2 were properly received. As such, the sender empties packet1 and packet 2 from its window and loads packet3 and packet4. When ACK1 is received, the sequence number (1) falls outside of the current window (3 to 4).