Some simple notes on SIP

- SIPGrammar parses and Request/Response
- SIPRequest is base to requests
- SIPResponse is the response class

- SIPDialog holds state of a dialogue
    - to/from/to tag/from tag...
    - call id...

- SIPTransport is base class for the transport
    - SIPUdpTransport sends and listens on UDP

- SIPUserAgent
    - Holds a SIPTransport
    - Generates various ids and fills requests
    - Will dispatch response to the transaction
    - Will dispatch SIPRequests to a dialog.

- SIPTransactionBase is the base for transactions
    - Transactions work on a Dialog (e.g. for INVITE on a non
      confirmed dialog), the callbacks will pass the dialog of
      of the response, e.g. for INVITE a confirmed dialog with
      a to tag.
    - SIPInviteTransaction is to create a session
    - SIPByeTransaction is to release a session/dialog

- SIPCall handles high level call handling
    - Cancel/Hangup in form of terminate
    - Refusing to deal with multiple dialogs


Timeouts/Retransmission:
    - This is handled by the SIPTransaction base using
      the generic timer framework.



Limitations:
    - Multiple Via's are not supported, no routing information
      is gathered, e.g. one can not have multiple transports