  Current approaches to the practical implementation of network coding are
batch-based, and often do not use feedback, except possibly to signal
completion of a file download. In this paper, the various benefits of using
feedback in a network coded system are studied. It is shown that network coding
can be performed in a completely online manner, without the need for batches or
generations, and that such online operation does not affect the throughput.
Although these ideas are presented in a single-hop packet erasure broadcast
setting, they naturally extend to more general lossy networks which employ
network coding in the presence of feedback. The impact of feedback on queue
size at the sender and decoding delay at the receivers is studied. Strategies
for adaptive coding based on feedback are presented, with the goal of
minimizing the queue size and delay. The asymptotic behavior of these metrics
is characterized, in the limit of the traffic load approaching capacity.
Different notions of decoding delay are considered, including an
order-sensitive notion which assumes that packets are useful only when
delivered in order. Our work may be viewed as a natural extension of Automatic
Repeat reQuest (ARQ) schemes to coded networks.
