The source begins by collecting a batch of packets all destined
to the same host. The source chooses a unique batch
ID and selects a forwarder list (Section 3.4). The source
prepends an ExOR header to each packet of the batch, containing
the batch ID and forwarder list. The batch map in
each header indicates that the only source has received each
packet. The source indicates how many packets it will send
in both the BatchSz and FragSz fields. Finally the source
broadcasts each packet in the batch.