/**@MODULEPAGE "soa" - SDP Offer/Answer Engine Module

@section soa_meta Module Information

The Sofia SIP @b soa module consists of an asynchronous SDP Offer/Answer engine
library. The interface to library is defined in <soa.h>.

@CONTACT Pekka Pessi <Pekka.Pessi@nokia.com>

@STATUS @SofiaSIP Core library

@LICENSE LGPL

@section soa_oveview Using soa engine

SIP uses SDP and a negotiation procedure known as "SDP
Offer-Answer Model" to establish the multimedia sessions. The
SDP Offer-Answer negotiation is specified in 
<a href="http://ietf.org/rfc/rfc3264.txt">RFC 3264</a>.

The soa engine is implemented in object-oriented manner. The default soa
object just implements the basic SDP negotiation and basic SIP call model. A
more complex soa object implementation can manipulate the call model and
initiate actions on behalf of application.

@section soa_model SDP Offer/Answer Model

The basic capabilities provided by Offer/Answer mechanism include
-# generating SDP offer (section 5)
-# processing SDP offer, generating SDP answer (section 6)
-# processing SDP answer (section 7)
-# modifying session (section 8)
-# indicating capabilities (section 9)

The offerer indicates its capabilities in the offer: 
- the media streams it wants to establish
- transport addresses it uses to receive by media streams 
  (IP addresses, port numbers, transport protocols) 
- the codecs used by particular streams
- codec parameters (for instance, codec profile used by H.263)

The answerer indicates which parts of the offer are acceptable to
it in the answer:
- the media streams it agrees to establish
- transport addresses answerer uses to receive by media streams 
- the codecs and codec parameters used by particular streams

Note that the capabilites indicate what the party generating the
SDP is prepared to receive. They can send anything the other end
accepts.

There may be other things, like encryption keys included in the
session description.

The advanced capabilities are required by more complicated negotiation
involving two or more offer-answer rounds. For instance, an extension known
as session <i>preconditions</i> is defined
<a href="http://ietf.org/rfc/rfc3312.txt">RFC 3312</a>.
Another example of two-phase negotiation is presented in RFC
3264 section 10.2, showing how a single codec can be selected. 

@section soa_motivation SOA Design 

Why to have simple interface? Is it not simple enough to include SDP offer
with your INVITE, and act on SDP answer in 200 OK?

Our design goal is to allow application to follow the simple call
model, regardless of the underlying complications - early
sessions, preconditions, session timers, 3rd party call control. 
In other words, we would like a have a simple "cooked" interface
toward naive applications even if the underlying call follows the
byzantine call model chosen by 3GPP.

@section soa_with_sip Using SDP Offer/Answer with SIP

Using SDP Offer/Answer with SIP is specified in 
<a href="http://ietf.org/rfc/rfc3261.txt">RFC 3261</a>, 
<a href="http://ietf.org/rfc/rfc3262.txt">RFC 3262 (100rel and PRACK)</a>, and
<a href="http://ietf.org/rfc/rfc3211.txt">RFC 3311 (UPDATE)</a>.

There is a @ref soa_sdp_oa_use_cases "separate page listing scenarios".

The rules for sending offers:
- offer may be sent in INVITE
- if there was no offer in INVITE, offer MUST be sent in first
  reliable response to INVITE
- offer may be sent in 100rel (reliable 1XX series response)
- offer may be sent in PRACK
- offer may be sent in UPDATE

PRACK may only be sent when an unacknowledged 100rel (reliable 1XX
series response) is received. UPDATE may be sent during early or
established dialog. 

Only one INVITE request may be pending within a dialog. Only one
non-INVITE request may be pending within a dialog (in one
direction): it is not possible to send UPDATE if no final response
has been received to PRACK.

If there is already an offer/answer exchange in progress, no offer
MUST be sent. Offer/answer exchange is in progress if offer has
been sent but no answer has been received, or if an offer has been
received but no answer has been generated.

The rules for sending answer:
- when offer is received with INVITE
  - answer MAY be sent with next end-to-end 1XX or 2XX response
  - answer MUST be sent in a reliable response (100rel or 2XX)
- when offer is received in 2XX response
  - answer MUST be sent in ACK
- when offer is received with 100rel response
  - answer MUST be sent with PRACK
- when offer is received with PRACK or UPDATE
  - answer MUST be sent with 2XX response to PRACK or UPDATE

Offer or answer in PRACK MUST be processed even if we have already
sent 2XX to INVITE.

The rules for receiving answer:
- if offer was sent in INVITE, first session description in any
  non-error response to INVITE is treated as the answer
- if offer was sent in 2XX response, session description in
  ACK is answer 
- if offer was sent in 100rel response, session description in
  PRACK is answer 
- if offer was sent in PRACK or UPDATE, session description in 
  2XX response is answer

Rules for situations when endpoint MUST ignore the SDP: 
- If offer was sent in INVITE, only the first session description in
  any non-error (1XX or 2XX) response to INVITE is processed, rest
  are ignored
- If no offer was sent in 2XX response to INVITE, SDP in ACK is ignored
- If no offer was sent in PRACK, SDP in response to it is ignored
- If no offer was sent in UPDATE, SDP in response to it is ignored

The re-INVITEs and UPDATEs are sent for two different purposes:
updating or modifying SIP state, or updating or modifying the
associated session. Session timer extension (not yet an rfc) is an
example of the first. Putting a call on hold, or adding video to
audio-only call is an example of the second. So, upon receiving
re-INVITE, there might be quite different things happening. The
application can just return a 200 OK with previous SDP, sometimes
it must indicate call being on hold and sometimes ask user for
permission (for adding video).

Rules for resolving glare (both endpoints trying to send offer at the same
time):
- if a offer is received while UAS has generated an offer, 
  it must be rejected (with SIP 491 response).

@section soa_use_cases SOA and SDP Offer/Answer Scenarios

Note that due to limitations in space
- soa_set_params() is referred as @c set_params
- soa_set_remote_sdp() is referred as @c set_remote
- soa_generate_offer() followed by soa_get_session_sdp() 
  is referred as @c gen_offer  
- soa_generate_answer() followed by soa_get_session_sdp() 
  is referred as @c gen_answer 
- soa_process_answer() is referred as @c proc_answer 

@subsection soa_uc_basic_out Basic Call Out

This is the "basic" outbound call model.

<pre>
       APPL	       NUA	       SOA		      REMOTE
	|		|		|			|
 0      |		|		|			|
 1	|----INVITE---->|		|			|
 2	|		|--set_params-->|			|
 3      |		|---gen_offer-->|			|
 4	|		|		|			|
 5	|		|-------------------INVITE(sdp offer)-->|
 6      |		|		|			|
 7	|		|		|			|
 8      |		|		|     			|
 9      |		|< - - - - - - - - - - 180 Ringing - - -|
10      |< - - 180 - - -|		|			|
11      |		|		|			|
12      |		|<-------------------200(sdp answer)----|
13	|		|--set_remote-->|			|
14	|		|--proc_answer->|			|
15	|<-----200------|		|			|
16      |		|               | 			|
17      |		|----activate-->|			|
18	|<----active----|		|			|
19	|		|-------------------------ACK---------->|
20	|		|		|			|
21	|		|		|			|
22	|		|		|			|
        |		|		|			|
</pre>

@subsection soa_uc_basic_in Basic Call In

This is the "basic" inbound call model.

<pre>
       APPL	       NUA	       SOA		      REMOTE
	|		|		|			|
 0      |		|		|			|
 1	|		|<------------------INVITE(sdp offer)---|
 2      |		|		|			|
 3	|		|--set_remote-->|			|
 4      |		|		|			|
 5	|<---INVITE-----|		|			|
 6      |		|		|			|
 7      |		|		|			|
 8	|- - -180- - - >|		|			|
 9	|		|- - - - - - - - - - 180 Ringing - - - >|
10      |		|		|			|
11      |		|		|			|
12	|-----200------>|		|			|
13	|		|--set_params-->|			|
14      |		|		|			|
15	|		|--gen_answer-->|			|
16	|		|               |			|
17	|<----active----|		|			|
18	|		|----activate-->|			|
19	|		|--------------------200 (sdp answer)-->|
20	|		|		|			|
21	|		|		|	 		|
22	|		|<------------------------ACK-----------|
23	|<-----ACK------|		|			|
24	|		|		|			|
	|		|		|			|
</pre>

@subsection soa_uc_basic_3p 3rd Party Call In 

The 3rd-party call model just reverses the O/A roles of callee and caller.

<pre>
 t     APPL	       NUA	       SOA		      REMOTE
	|		|		|			|
 0      |		|		|			|
 1	|		|<----------------------INVITE----------|
 2	|    		|		|			|
 3      |<---INVITE-----|		|			|
 4	|		|		|			|
 5	|		|		|			|
 6      |    		|		|			|
 7	|----200 OK---->|   		|			|
 8      |		|--set_params-->|			|
 9      |		| 		|			|
10      |		|--gen_offer--->|			|
11	|		|		|			|
12      |		|		|			|
13      |		|----------------------200 (off) ------>|
14	|		|		|			|
15	|		|		|	 		|
16	|     		|<---------------------ACK (ans)--------|
17      |		|--set_remote-->|			|
18	|		|--proc_answer->|			|
19	|<----ACK-------|		|			|
20	|<----active----|		|			|
21	|		|----activate-->|			|
22      |		|		|			|
        |		|		|			|
</pre>

@subsection soa_uc_early_out Callout with Early Media

It is possible to establish media session before call is completed. In this
case, the 180 Ringing contains the SDP answer. A copy of SDP answer is
included in the 200 OK response, too, because the 180 Ringing is not
acknowledged and it may be lost.

This is preferred to the basic call model above, as the endpoints has more
time to establish the media session.

<pre>
 t     APPL	       NUA	       SOA		      REMOTE
	|		|		|			|
 0      |		|		|			|
 1	|----INVITE---->|		|			|
 2      |		|--set_params-->|			|
 3	|		|               |			|
 4      |		|--gen_offer--->|			|
 5	|		|               |			|
 6      |		|		|			|
 7      |		|-------------------INVITE(sdp offer)-->|
 8	|		|		|			|
 9	|		|		|			|
10      |		|		|			|
11      |		|<-------------------180(sdp answer)----|
12      |		|--set_remote-->|			|
13	|<-----180------|--proc_answer->|	    		|
14	|   		|		|			|
15      |		|		|			|
16	|		|		|			|
17	|		|<-----------------200(copy of answer)--|
18	|	(copy is ignored)	|			|
19	|		|		|			|
20	|<-----200------|		|			|
21	|		|----activate-->|			|
22	|<----active----|		|			|
23	|		|-------------------------ACK---------->|
24	|		|		|			|
	|		|		|			|
</pre>

@subsection soa_uc_early_in Call In Establishing Early Media

The mirror of the previous scenario:

<pre>
 t     APPL	       NUA	       SOA		      REMOTE
	|		|		|			|
 0      |		|		|			|
 1	|		|<------------------INVITE(sdp offer)---|
 2      |		|--set_remote-->|			|
 3	|<---INVITE-----|		|			|
 4      |		|		|			|
 5      |   		|		|			|
 6	|---180 Ring--->|   		|			|
 7      |		|--set_params-->|			|
 8      |		|--gen_offer--->| 			|
 9      |		|	     (Note 1)			|
10      |		|		|			|
11      |		|-------------------180 (sdp answer)--->|
12	|		|		|			|
13	|		|		|			|
14	|		|		|			|
15	|----200 OK---->|		|			|
16	|		|--set_params-->|			|
17	|		|		|			|
18	|		|		|			|
19	|		|-----------------200 (copy of answer)->|
20	|		|----activate-->|			|
21	|<----active----|		|			|
22	|		|<------------------------ACK-----------|
23	|<-----ACK------|		|			|
24      |		|		|			|
	|		|		|			|
</pre>
<b>Note 1:</b> the user expectation (set by ordinary telephone) here is
that callee sends a ringing tone towards caller and discards any media sent
by caller until the call is accepted (200 OK is sent towards caller).

@subsection soa_uc_100rel_out Call Out with PRACK

Here is second alternative establishing media session before call is
completed. In this case, the 180 Ringing contains the SDP answer. The 180
Ringing is now sent reliably. In other words, it is acknowledged by a PRACK
request.

<pre>
 t     APPL	       NUA	       SOA		      REMOTE
	|		|		|			|
 0      |		|		|			|
 1	|----INVITE---->|		|			|
 2      |		|--set_params-->|			|
 3	|		|               |			|
 4      |		|--gen_offer--->|			|
 5	|		|		|			|
 6      |		|		|			|
 7      |		|-------------------INVITE(sdp offer)-->|
 8	|		|		|			|
 9	|		|		|			|
10      |		|<-------------------183(sdp answer)----|
11      |		|--set_remote-->|			|
12      |		|--proc_answer->|			|
13	|<-----183------|		|	    		|
14	|   		|		|			|
15      |		|-----------------------PRACK---------->|
16	|		|<--------------------200/PRACK---------|
17	|<--200/PRACK---|		|			|
18	|		|		|			|
19	|		|		|			|
20	|		|<--------------------180 Ringing-------|
21	|<-----180------|		|			|
22	|		|-----------------------PRACK---------->|
23	|		|<--------------------200/PRACK---------|
24	|<--200/PRACK---|		|			|
25	|		|		|			|
26	|		|		|			|
27	|		|<----------------------200 OK----------|
28      |<--200/INVITE--|		|			|
29	|		|----activate-->|			|
30	|<----active----|		|			|
31	|		|-----------------------ACK------------>|
32	|		|		|			|
	|		|		|			|
</pre>

@subsection soa_uc_100rel_in Call In with PRACK

The mirror of the previous scenario:

<pre>
 t     APPL	       NUA	       SOA		      REMOTE
	|		|		|			|
 0      |		|		|			|
 1	|		|<------------------INVITE(sdp offer)---|
 2      |		|--set_remote-->|			|
 3	|<---INVITE-----|		|			|
 4      |		|		|			|
 5      |   		|		|			|
 6	|-183 Progress->|   		|			|
 7      |		|--set_params-->|			|
 8      |		|--gen_answer-->|			|
 9	|		|		|			|
10      |		|-------------------183 (sdp answer)--->|
11      |		|		|			|
12      |		|<----------------------PRACK-----------|
13      |<----PRACK-----|		|			|
14	|		|---------------------200/PRACK-------->|
15	|		|		|			|
16      |--180 Ringing->|   		|			|
17      |		|---------------------180 Ringing------>|
18	|		|		|			|
19      |		|<----------------------PRACK-----------|
20      |<----PRACK-----|		|			|
21	|		|---------------------200/PRACK-------->|
22      |		|		|			|
23      |		|		|			|
24      |		|		|			|
25      |----200 OK---->|		|			|
26      |		|----activate-->|			|
27      |<----active----|		|			|
28      |		|---------------------200/INVITE------->|
29      |		|		|			|
30      |		|<-----------------------ACK------------|
31      |<-----ACK------|		|			|
32      |		|		|			|
        |		|		|			|
</pre>

<b>Note 1:</b> the user expectation (set by ordinary telephone) here is that
callee sends a ringing tone towards caller and discards any media sent by
callee until the call is accepted at t=26 (200 OK is sent towards caller).

The application starts to alert user at t=13 when it knows that the
media session has been successfully established.

*/

/**
@page soa_sdp_oa_use_cases Use Cases for SIP and SDP Offer/Answer

This page contains a list of use cases or call scenarios for SIP and SDP
Offer/Answer. 

There are a few call scenarios that we expect to see when dealing with more
telephone-like side of SIP: 
- calling to existing PSTN networks (early session)
- doing resource reservations
- calling to 3G IMS
- call hunting
- having external party setting up your call, etc.

@section soa_use_case_1 Case #1: Basic Call

This is the basic SIP call model with the most simple offer-answer exchange.

<pre>
	A		        B
	|			|
	|----INVITE (offer)---->|
	|			|
	|			|
	|< - - 180 Ringing - - -|
	|			|
	|			|
	|<----200 (answer)------|
	|----------ACK--------->|
	|			|
	|			|
</pre>

@section soa_use_case_2 Case #2: Early Media

Another case, slightly more complex. The SDP answer is sent with
180 Ringing in order to establish an "early session". B might
not be a SIP phone, but a gateway to PSTN, for instance. Using this
"early session" B can play tones like "burr-burr" or "the
subscriber you tried to reach is not in the coverage...":

<pre>
	A		        B
	|			|
	|----INVITE (offer)---->|
	|			|
	|			|
	|<----180 (answer)------|
	|			|
	|			|
	|<----200 (answer')-----|
	|----------ACK--------->|
	|			|
	|			|
</pre>

After receiving answer in 180 Ringing, A simply ignores SDP in
subsequent responses.

Nothing special here, right? But SIP is not so simple,
unfortunately. There are hairy cases because of "early sessions",
"forking", "preconditions" and other reasons.

Now lets go through some hairy cases.

@section soa_use_case_3 Case #3: Early Dialog, Early Media

This case is a call using 100rel, early dialog and early media. It
is used when the session should be established before call alerts.

<pre>
	A		        B
	|			|
	|----INVITE (offer)---->|
	|			|
	|<----183 (answer)------|
	|--------PRACK--------->|
	|<-----200/PRACK------->|
	|			|
	|<---------180----------|
	|--------PRACK--------->|
	|<-----200/PRACK------->|
	|			|
	|			|
	|			|
	|<---------200----------|
	|----------ACK--------->|
	|			|
	|			|
</pre>

@section soa_use_case_4 Case #4: UPDATE with Offer

This is a call model with two rounds of offer/answer and 100rel. 
It can be used, for instance, when the endpoints have to make sure
that there are enough network capacity for the call to succeed. 
They can establish a new radio bearer, for instance, before
progressing with the call. The initial offer would contain SDP
attribute "inactive", the second "sendrev":

<pre>
	A		        B
	|			|
	|----INVITE (offer)---->|
	|			| 
	|			| 
	|<----183 (answer)------| 
	|--------PRACK--------->|
	|<-----200/PRACK--------|
	|			|
	|			|
	|----UPDATE (offer2)--->|
	|<-200/UPDATE (answer2)-|
	|			|
	|<---------180----------|
	|--------PRACK--------->|
	|<-----200/PRACK------->|
	|			|
	|			|
	|<---------200----------|
	|----------ACK--------->|
	|			|
	|			|
</pre>


@section soa_use_case_5 Case #5: 2nd Offer in PRACK

Alternative 1 to above, two rounds of offer/answer and 100rel, no
UPDATE. It can used, for instance, when caller wants to make sure
there is only one audio or video codec that is used during the
call. The initial offer would contain SDP attribute "inactive";
the second "sendrev":

<pre>
	A		        B
	|			|
	|----INVITE (offer)---->|
	|			|
	|<----183 (answer)------|
	|-----PRACK(offer2)---->|
	|<--200/PRACK(answer2)--|
	|			|
	|			|
	|<---------180----------|
	|--------PRACK--------->|
	|<-----200/PRACK------->|
	|			|
	|			|
	|<---------200----------|
	|----------ACK--------->|
	|			|
	|			|
</pre>


@section soa_use_case_6 Case #6: Callee Making 2nd Offer 

Alternative 2 to above: two rounds of offer/answer and 100rel, but
now it is B who wants to do two rounds and initiates second
Offer-Answer exchange:

<pre>
	A		        B
	|			|
	|----INVITE (offer)---->|
	|			|
	|<----183 (answer)------|
	|--------PRACK--------->|
	|<-----200/PRACK------->|
	|			|
	|			|
	|<---UPDATE (offer2)----|
	|-200/UPDATE (answer2)->|
	|			|
	|<---------180----------|
	|--------PRACK--------->|
	|<-----200/PRACK------->|
	|			|
	|			|
	|<---------200----------|
	|----------ACK--------->|
	|			|
	|			|
</pre>


@section soa_use_case_7 Case #7: 3GPP Call Model

Here is a third alternative, know as "3GPP call model", where
there is not 2 but 3 offer-answer rounds, allowing A and B to do
precise resource reservations after they have agreed on media,
codecs and transport addresses used during the call:

<pre>
        	A		        B
        	|			|
        	|----INVITE (offer)---->|
        	|			| 
        	|			| 
        	|<----183 (answer)------| 
        	|-----PRACK(offer2)---->|
        	|<--200/PRACK(answer2)--|
        	|			|
        << resource reservations are done now >>
        	|			|
        	|----UPDATE (offer3)--->|
        	|<-200/UPDATE (answer3)-|
        	|			|
        	|<---------180----------|
        	|--------PRACK--------->|
        	|<-----200/PRACK------->|
        	|			|
        	|			|
        	|<---------200----------|
        	|----------ACK--------->|
        	|			|
        	|			|
</pre>


@section soa_use_case_8 Case #8: Forking

Now, another complication. Here B has two terminals, let say,
fixed (B1) and mobile (B2) phone, both alert when B receives a
call using procedure known as forking:

<pre>
	A		   B's proxy		B1		B2
	|			|		|		|
	|----INVITE (offer)---->|		|		|
	|			|-INVITE (off)->|		|
	|			|-----------INVITE (off)------->|
	|			|		|		|
	|			|<--180 (ans1)--|		|
	|<------180 (ans1)------|		|		|
	|			|		|		|
	|			|<----------180 (ans2)----------|
	|<------180 (ans2)------|		|		|
	|			|		|		|
	|			|		|		|
	|			|<----------200 (ans2')---------|
	|<------200 (ans2')-----|		|		|
	|			|----CANCEL---->|		|
	|			|<--200/CANCEL--|		|
	|			|<-----487------|		|
	|			|		|		|
	|----------ACK----------------------------------------->|
	|			|		|		|
	|			|		|		|
</pre>

Here we have two calls initially established, but the call to B1
along with "early session" is should be dropped when B2 picks up
the call (and 200 OK is returned).

@section soa_use_case_9 Case #9: 3rd Party Call Control

Now something different: 3rd party call model, where "C"
establishes the call:

<pre>
	A		        C		        B
	|			|			|
	|<-------INVITE---------|			|
	|			|			|
	|			|			|
	|------200 (offer)----->|  	  		|
	|			|----INVITE (offer)---->|
	|			|			|
	|			|			|
	|			|<-----200 (answer)-----|
	|<-----ACK (answer)-----|			|
	|			|     			|
	|			|----------ACK--------->|
	|			|			|
</pre>

@section soa_use_case_10 Case #10: Upgrade Session with Re-INVITE

The session is upgraded: a new media is added to the session. 

<pre>
	A		        B
	|			|
	|----INVITE (offer1)--->|
	|			|
	|			|
	|< - - 180 Ringing - - -|
	|			|
	|			|
	|<----200 (answer2)-----|
	|----------ACK--------->|
	|			|
	|			|
	|			|
	|			|
	|----INVITE (offer2)--->|
	|			|
	|<----200 (answer2)-----|
	|----------ACK--------->|
	|			|
	|			|
</pre>

@section soa_use_case_10 Case #10: Upgrade Session with Re-INVITE

The session upgraded is rejected.

<pre>
	A		        B
	|			|
	|----INVITE (offer1)--->|
	|			|
	|			|
	|< - - 180 Ringing - - -|
	|			|
	|			|
	|<----200 (answer2)-----|
	|----------ACK--------->|
	|			|
	|			|
	|			|
	|			|
	|----INVITE (offer2)--->|
	|			|
	|<---------488----------|
	|----------ACK--------->|
	|			|
	|			|
</pre>


*/

/** @typedef struct soa_session soa_session_t;

@brief "soa" session object. 

The @soa session object is responsible for
@ref soa_with_sip "SDP offer/answer negotiation"
defined in @RFC3264. The session object is used to
store the SDP template from user, remote SDP received from the network and
the negotiation result, local SDP.

@par Functions used to create, copy and destroy "soa" session objects:
@code
soa_session_t *soa_create(char const *name, su_root_t *, soa_magic_t *);

soa_session_t *soa_clone(soa_session_t *, su_root_t *, soa_magic_t *);

void soa_destroy(soa_session_t *);
@endcode

@par Functions used to set and get parameters for "soa" session objects:
@code
int soa_set_params(soa_session_t *ss,
                   tag_type_t tag, tag_value_t value, ...);
int soa_get_params(soa_session_t const *ss, 
                   tag_type_t tag, tag_value_t value, ...);

tagi_t *soa_get_paramlist(soa_session_t const *ss,
                          tag_type_t tag, tag_value_t value, ...);
@endcode

@par Functions used to obtain status information from "soa" session objects:
@code
int soa_error_as_sip_response(soa_session_t *soa, 
                              char const **return_phrase);

char const *soa_error_as_sip_reason(soa_session_t *soa);

int soa_get_warning(soa_session_t *ss, char const **return_phrase);
@endcode


@par Functions used to store and retrieve SDP descriptions: 
@code
int soa_set_capability_sdp(soa_session_t *ss, 
                           struct sdp_session_s const *sdp,
                           char const *str, issize_t len);

int soa_get_capability_sdp(soa_session_t const *ss,
                           struct sdp_session_s const **return_sdp,
                           char const **return_sdp_str,
                           isize_t *return_len);

int soa_set_remote_sdp(soa_session_t *ss, 
                       struct sdp_session_s const *sdp,
                       char const *str, issize_t len);

int soa_get_remote_sdp(soa_session_t const *ss,
                       struct sdp_session_s const **return_sdp,
                       char const **return_sdp_str,
                       isize_t *return_len);

int soa_clear_remote_sdp(soa_session_t *ss);

int soa_get_remote_version(soa_session_t const *ss);

int soa_set_user_sdp(soa_session_t *ss, 
                     struct sdp_session_s const *sdp,
                     char const *str, issize_t len);

int soa_get_user_sdp(soa_session_t const *ss,
                     struct sdp_session_s const **return_sdp,
                     char const **return_sdp_str,
                     isize_t *return_len);

int soa_get_user_version(soa_session_t const *ss);

int soa_get_local_sdp(soa_session_t const *ss,
                      struct sdp_session_s const **return_sdp,
                      char const **return_sdp_str,
                      isize_t *return_len);
@endcode


@par Functions for executing Offer/Answer negotiation steps:
@code
int soa_init_offer_answer(soa_session_t *ss);
int soa_generate_offer(soa_session_t *, int always, soa_callback_f *);
int soa_generate_answer(soa_session_t *, soa_callback_f *);
int soa_process_answer(soa_session_t *, soa_callback_f *);
int soa_process_reject(soa_session_t *, soa_callback_f *);
int soa_is_complete(soa_session_t const *ss);
@endcode

@par Functions for signaling events of session signaling:
@code
int soa_activate(soa_session_t *, char const *option);
int soa_deactivate(soa_session_t *, char const *option);
void soa_terminate(soa_session_t *, char const *option);
@endcode
These functions are used to activate actions taken by @soa, for instance,
a COMEDIA connection is established with soa_activate().

@par Functions for Checking Activated Media:
@code
int soa_is_audio_active(soa_session_t const *ss);
int soa_is_video_active(soa_session_t const *ss);

int soa_is_remote_audio_active(soa_session_t const *ss);
int soa_is_remote_video_active(soa_session_t const *ss);
@endcode

*/