OSI layer

Which layer of the OSI/RM stack controls the flow of information between hosts?

At which layer of the OSI/RM do packet filters function?

Which layer of the OSI/RM do proxy servers usually address?

Which layer in the OSI reference model is responsible for determining the availability of the receiving program and checking to see if enough resources exist for that communication?

HTTP, FTP, SMTP reside at which layer of the OSI model?

Which layer of the OSI model does the problem most likely exist?