Understanding transport header in Reliable data transfer

I am trying to implement a reliable Data transfer (RDT 3.0) protocol
http://www2.ic.uff.br/~michael/kr1999/3-transport/3_040-principles_rdt.htm
Reliable Data Transfer (nyu.edu) 


I have the above text using which I am implementing it. I need some help from experts in order to
understand it. I am mainly confused at the line which says -

The message must be sent in multiple
segments to the recipient based on the segment’s size. For this project, you must use a segment size of 10 bytes (transport header & payload together make up exactly 10 bytes). This means that if the complete message to be delivered is 50 bytes long, then you will have more than 5 segments that need to be sent to the recipient. In tests, the grader will have messages that will be longer than 10 bytes

Then there is this network layer part which seems to be separate from the segment.

What I am trying to implement is basically a Sender.java, Network.java, Reciever.java
The sender sends the packet to the receiver through the network in segments and the Reciever accumulates those segments and forms the complete message.