Postal and layered networks

Networks may (or may not) check for errors at various points. The layer at which the error is detected is responsible for taking corrective action.
  1. What happens if there is a misspelled word in a letter? Where is that error detected?
  2. What happens if the letter does not have a stamp on it? Where is that error detected?
  3. What happens if your friend has moved and left a forwarding address? Where is that error detected?
  4. What happens if your friend has moved and not left a forwarding address? Where is that error detected?
  5. What happens if you accidently write the wrong Zip Code? Where is that error detected?
  6. What happens if you accidently write the wrong street address? Where is that error detected?
  7. In a TCP/IP network, which layer is responsible for checking for spelling errors? What happens when they occur?
  8. In a TCP/IP network, which layer is responsible for checking transmission errors within the local area network? What happens when they occur?
  9. In a TCP/IP network, which layer is responsible for checking for errors when a message reaches its destination? What happens when they occur?

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