cs.Receiver has a receive method taking a DataInput:

void receive(Address sender, DataInput in) throws Exception;

This is always called by a method which knows the length of the data to be read, because - when sending data - the length (an int is sent first and - when receiving - the length is also read first. However, it is not passed to the above method.

This is a problem for applications which need data framing; ie., applications which need to know how much data they need to read, e.g. to add that data to a byte array.
Such applications would need to send the length of their data before sending the data, but this is superfluous, and adding length would provide the length that is read anyway to them