1. The layers must have a mechanism for connection establishment. Network usually has more than one computer some of them have multiple processes a way is needed for a process on one machine to specify where it wants to establish a connection. Having multiple destinations they should be a form of addressing in order to specify a destination.
2. Mechanisms for disconnecting the connections when is not required.
3. Data Transfer Rules
- Simplex Communication: With this, data transfer occurs in only one direction that is either from source to destination or destination to source machines.
- Half-duplex Communication: The data transfer occurs in either directions but not simultaneously.
- Full-duplex Communication: With this, the data transfer occurs in either direction simultaneously.
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The protocol must also determine the number of logical channels per connection along with their individual priorities.
4. Error Control Mechanism
It is one of the important issues since physical communication circuits are not perfect. Many error-correcting and detecting codes are known but both ends of the connection must agree on which one is being used. Also the receiver must have some means of telling the sender which messages has been correctly received and which has not.
5. Message Ordering
Not all communication channels preserve the ordering of messages sent on them. To deal with a possible loss of sequencing the protocol must make explicit provision for the receiver to allow the pieces to be put back together properly. The best solution is to number the pieces.
6. A issue that occurs at every level is how to keep a fast sender from swamping a slow receiver with data. All the proposed solutions have some kind of feedback mechanisms wherein the receiver informs its current situation to the sender.
7. Must solved problem at several levels is the inability of all processes to accept arbitrarily long messages. This property leads to mechanisms for disassembling transmitting and then reassembling messages.
8. When it is inconvenient to set up a separate connection for each pair of communicating processes the underlying layer may decide to use the same connection for multiple unrelated conversations. As long as this multiplexing and de-multiplexing is done transparently it can be used by any layer.
9. When there are multiple paths between source and destination a route must be chosen. Sometimes this decision must be split over two or more layers.
Also Read: Network Structures