       Features:
       ========

oSIP is a not a complete SIP stack and will never implements
all the specifications. Instead, the goal is to provide
a limited set of features common to any kind of SIP Agents.
Thus oSIP is not oriented towards any particular implementations
and can be used for implementing SIP End-Point, Proxy
or any kind of more specific SIP Agent such as B2BUA.

The minimal common required features for any SIP Agent
provided by osip are describe below.

However, some extra facilities oriented towards SIP End-Points
are provided.

       SIP parser:
       ==========

The initial feature implemented in osip is a SIP parser. There
is not much to say about it: it is capable of parsing and
reformating SIP requests and answers.

The details of the parsing tools available are listed below:

 1  SIP request/answer
 2  SIP uri
 3  specific headers
 4  Via
 5  CSeq
 6  Call-ID
 7  To, From, Route, Record-Route...
 8  Authentication related headers
 9  Content related headers
 10 Accept related headers
 11 ...
 12 Generic header
 13 Attachement parser (should support mime)
 14 SDP parser

       SIP transaction state machines:
       ==============================

The interesting and somewhat complex feature implemented
by osip is the 4 states machines that applied to the different
transactions defined by the SIP rfc.

SIP defines the following 4 state machines, abreviations
used in osip are provided below:

  * ICT : Invite Client Transaction (Section 17.1.1) 
  * NICT: Non Invite Client Transaction (Section 17.1.2) 
  * IST : Invite Server Transaction (Section 17.2.1) 
  * NIST: Non Invite Server Transaction (Section 17.2.2) 

As you can notice if you have read the rfc (do it!), those
4 state machines are provided as drawings within the SIP
rfc3261.txt (section 17.1 and 17.2)

As an exemple of what you'll find in the rfc3261, here is the
drawing that apply to the "Invite Client Transaction" (page 127)


                               |INVITE from TU
             Timer A fires     |INVITE sent
             Reset A,          V                      Timer B fires
             INVITE sent +-----------+                or Transport Err.
               +---------|           |---------------+inform TU
               |         |  Calling  |               |
               +-------->|           |-------------->|
                         +-----------+ 2xx           |
                            |  |       2xx to TU     |
                            |  |1xx                  |
    300-699 +---------------+  |1xx to TU            |
   ACK sent |                  |                     |
resp. to TU |  1xx             V                     |
            |  1xx to TU  -----------+               |
            |  +---------|           |               |
            |  |         |Proceeding |-------------->|
            |  +-------->|           | 2xx           |
            |            +-----------+ 2xx to TU     |
            |       300-699    |                     |
            |       ACK sent,  |                     |
            |       resp. to TU|                     |
            |                  |                     |      NOTE:
            |  300-699         V                     |
            |  ACK sent  +-----------+Transport Err. |  transitions
            |  +---------|           |Inform TU      |  labeled with
            |  |         | Completed |-------------->|  the event
            |  +-------->|           |               |  over the action
            |            +-----------+               |  to take
            |              ^   |                     |
            |              |   | Timer D fires       |
            +--------------+   | -                   |
                               |                     |
                               V                     |
                         +-----------+               |
                         |           |               |
                         | Terminated|<--------------+
                         |           |
                         +-----------+

                 Figure 5: INVITE client transaction


As you can expect, with osip an Invite Client Transaction may be
in the CALLING, PROCEEDING, COMPLETED or TERMINATED state. To
"execute" the state machine, you will build events, provide them
to the correct transaction context and the the state of the
transaction will be updated if the event is allowed in the current
state.

Events are divided in three categories:

* SIP messages
* Timers
* transport errors

       Other features:
       ==============

To ease development of SIP portable application, osip
provide abstractions methods for threads, semaphore,
and mutex.

You still have the choice to compile osip without thread
support which you may be required on some embedded platforms.

Please report any porting issue to the mailing list <osip@atosc.org>
(subscribe first!) or directly to me <jack@atosc.org>.

If you are building a new port, I'll certainly think about merging
it.

       Other Facilities:
       ================

oSIP also give small facilities which are completly optional.

Among them, the dialog facility(osip_dialog.h), mainly to
be used by SIP End-Points, is a stable extra feature. It will
allow you to build a structure described in the rfc3261 (Section
12: Dialogs). Once built, you can reuse this information to match
received requests with a particular SIP call or to build a new
request within a particular SIP call.

*** An old feature has been implemented in osip: a SDP negotiator.
*** I advise you to not use this old facility which is not powerfull
*** often uncompliant and not flexible enough. It's also quite
*** unreadable and poorly written (by me!). I'll delete it anyway in
*** future version of osip. I have warned you...

   ->>> UPDATE 27/11/2005: the SDP negotiator have been removed

oSIP finaly contains a MD5 implementation (osip_md5.h) which
you'll find usefull when implementing SIP Digest authentication.
