Suppose the network layer provides the following service:

a. the network layer at the source host accepts from the transport layer a segment of maximum size 1,000 bytes.
b. this segment includes data, port numbers, and also addresses (the transport layer in this example somehow also deals with addresses as well).
c. the network layer then guarantees delivery of the segment to the transport layer at the destination host.
d. suppose many network application processes can be running at the destination host.