* Things to do *

- How should we handle the case where MTU > MRU?
  Should we reduce the IP MTU to the link MRU (so TCP MSS is correct)?

- Provide ways to:
  + set the IP address, possibly based on the peer's identity
  + have external checking of PAP user/password, CHAP response
  + supply the PAP user/password, CHAP secret
  + decide which tty to use (locking, DTR issues)

- Integrate callback stuff?

- Implement link quality monitoring

- Implement other network control protocols
