The following are the design issues for the layers:

1. Reliability: It is a design issue of making a network that operates correctly even when it is made up of unreliable components.

2. Addressing: There are multiple processes running on one machine. Every layer needs a mechanism to identify senders and receivers.

3. Error Control: It is an important issue because physical communication circuits are not perfect. Many error detecting and error correcting codes are available. Both sending and receiving ends must agree to use any one code.

4. Flow Control: If there is a fast sender at one end sending data to a slow receiver, then there must be flow control mechanism to control the loss of data by slow receivers. There are several mechanisms used for flow control such as increasing buffer size at receivers, slow down the fast sender, and so on. Some process will not be in position to accept arbitrarily long messages. This property leads to mechanisms for disassembling, transmitting and the reassembling messages.

5. Multiplexing and De-multiplexing: If the data has to be transmitted on transmission media separately, it is inconvenient or expensive to setup separate connection for each pair of communicating processes. So, multiplexing is needed in the physical layer at sender end and de-multiplexing is need at the receiver end.

6. Scalability: When network gets large, new problem arises. Thus scalability is important so that network can continue to work well when it gets large.

7. Routing: When there are multiple paths between source and destination, only one route must be chosen. This decision is made on the basis of several routing algorithms, which chooses optimized route to the destination.

8. Confidentiality and Integrity: Network security is the most important factor. Mechanisms that provide confidentiality defend against threats like eavesdropping. Mechanisms for integrity prevent faulty changes to messages.