$3\mathrm{.}09-3$ Hop$-$by$-$hop congestion control $($c$).$ We learned about end$-$to$-$end and network$-$assisted congestion control. A third type of congestion control $($mostly deployed only in internal data$-$center networks$)$ is hop$-$by$-$hop congestion control. In this approach a downstream router$/$host explicitly informs the previous hop upstream router$/$host about the amount of free buffer space it has available, and the upstream router will only forward a packet when it knows the downstream router has buffer space.

Consider the three$-$hop scenario shown below, where the first and third links have capacity R$,$ and the link between the routers has capacity R$/2.$ With hop$-$by$-$hop congestion control and no congestion loss, might the sender still have to sometimes perform retransmission?