Network takes care of routing and also does checksums.
However, this compression only works well if the network does not reorder or drop packets. If the network reorders VJ compressed packets, then the receiver has no option but to throw them all away, waiting for the sender to time out, and send an uncompressed packet. This allows for shorter sequence numbers, and also makes a selective ack mechanism very easy to implement. Between two hosts, TCPL can only support a single bi-directional stream of data. If more than one connection is necessary, a seperate protocol is needed that somehow divides the stream into multiple streams, each for a particular connection.