Question
Suppose the network layer provides the following service. The network layer in the source host accepts a segment of maximum size 1,200 bytes and a des- tination host address from the transport layer. The network layer then guaran- tees to deliver the segment to the transport layer at the destination host. Suppose many network application processes can be running at the destination host.
a. Design the simplest possible transport-layer protocol that will get applica- tion data to the desired process at the destination host. Assume the operat- ing system in the destination host has assigned a 4-byte port number to each running application process.
b. Modify this protocol so that it provides a “return address” to the destina- tion process.
c. In your protocols, does the transport layer “have to do anything” in the core of the computer network?
Answer
a)
The Simple Transport Protocol takes data not exceeding 1196 bytes at the sender side.
- It accepts four byte of destination port number and host address.
- The Simple Transport Protocol gives the destination host address and the resulting segment to the network layer.
- The network layer sends the segment to Simple Transport Protocol at the destination host.
- The Simple Transport Protocol observes the port number.
- Abstracts the data from the segment in the Simple Transport Protocol.
- Finally, send the data to the process recognized by the port number.
b)
Consider the two header fields in the segment:
- Source port field
- Destination port field
The Simple Transport Protocol creates application data, source and destination port numbers in the segment. It sends the destination host address to the network layer. Then, The Simple Transport Protocol is receiving host address and provides the process the source port number and the application data.
c)
No, the transport layer does not have to do anything in the core.
The reason is that, the transport layer "lives" in the end systems.