# $Id$
# TM Module README
# Module depends on: none

TM Module enables stateful processing of SIP transactions.
The main use of stateful logic, which is costly in terms of
memory and CPU, is some services inherently need state.
For example, transaction-based accounting (module acc) needs
to process transaction state as opposed to individual messages,
and any kinds of forking must be implemented statefuly.
Other use of stateful processing is it trading CPU caused by 
retransmission processing for memory. That makes however only
sense if CPU consumption per request is huge. For example,
if you want to avoid costly DNS resolution for every retransmission
of a request to an unresolveable destination, use stateful mode.
Then, only the initial message burdens server by DNS queries,
subsequent retranmissions will be dropped and will not result in
more processes blocked by DNS resolution. The price is more 
memory consumption and higher processing latency.

From user's perspective, there are two major functions :
t_relay and t_relay_to. Both setup transaction state,
absorb retransmissions from upstream, generate downstream
retransmissions and correlate replies to requests.
t_relay forwards to current uri (be it original request's
uri or a uri changed by some of uri-modifying functions,
such as sethost). t_relay_to forwards to a specific address.

In general, if TM is used, it copies clones of received SIP
messages in shared memory. That costs the memory and also CPU
time (memcpys, lookups, shmem locks, etc.) Note that non-TM
functions operate over the received message in private memory,
that means that any core operations will have no effect on
statefuly processed messages after creating the transactional
state. For example, calling addRecordRoute *after* t_relay
is pretty useless, as the RR is added to privately held message
whereas its TM clone is being forwarded.

TM is quite big and uneasy to programm -- lot of mutexes, shared
memory access, malloc & free, timers -- you really need to be careful
when you do anything. To simplify TM programming, there is the
instrument of callbacks. The callback mechanisms allow programmers
to register their functions to specific event. See t_hooks.h
for a list of possible events.

Other things programmers may want to know is UAC -- it is 
a very simplictic code which allows you to generate your own
transactions. Particularly useful for things like NOTIFYs or
IM gateways. The UAC takes care of all the transaction
machinery: retransmissions , FR timeouts, forking, etc.
See t_uac prototype in uac.h for more details. Who wants to
see the transaction result may register for a callback.

Exported parameters:

Name:		fr_timer
Type:		int (seconds)
Default:	FR_TIME_OUT=30
Desc:		timer which hits if no final reply for a request or
			ACK for a negative INVITE reply arrives

Name:		fr_inv_timer
Type:		int(seconds)
Default:	INV_FR_TIME_OUT=120
Desc:		timer which hits if no final reply for an INVITE
			arrives after a provisional message was received

Name:		wt_timer
Type:		int (seconds)
Default:	WT_TIME_OUT=5
Desc:		time for which a transaction stays in memory to absorb
			delayed messages after it completed; also, when this
			timer hits, retransmission of local cancels is stopped
			(a puristic but complex behviour would be not to enter
			wait state until local branches are finished by a final
			reply or FR timer -- we simplified)

Name:		delete_timer
Type:		int (seconds)
Default:	DEL_TIME_OUT=2
Desc:		time after which a to-be-deleted transaction currently
			ref-ed by a process will be tried to be deleted again

Name:		retr_timer1p1, 2, 3
Type:		int (seconds)
Default:	RETR_T1=1, 2*RETR_T1, 4*RETR_T1
Desc:		retransmission period

Name:		retr_timer2
Type:		int (seconds)
Default:	RETR_T2=4
Desc:		maximum retransmission period

Name:		noisy_ctimer
Type:		int (boolean)
Default:	0 (FALSE)
Desc:		if set, on FR timer INVITE transactions will be 
			explicitly cancelled if possible, silently dropped
			otherwise; preferably, it is turned off to allow
			very long ringing; this behaviour is overridden if
			a request is forked, or some functionality explicitly
			turned it off for a transaction (like acc does to avoid
			unaccounted transactions due to expired timer)

Exported Functions:

For use in scripts, t_relay_to and t_relay are design. All other
functions are advanced and should be used with care.

Name: 	t_relay_to
Params:	ip address, port number
Desc:	relay a message statefuly to a fixed destination; this along with
		t_relay is the function most users want to use -- all other are
		mostly for programming; programmers interested in writing TM
		logic should review how t_relay is implemented in tm.c and how
		TM callbacks work

Name:	t_relay
Params:	0
Desc:	relay a message statefuly to destination indicated in current URI;
		(if the original URI was rewritten by UsrLoc, RR, strip/prefix,
		etc., the new URI will be taken); returns a negative value on
		failure -- you may still want to send a negative reply upstream
		statelessly not to leave upstream UAC in lurch
Example: if (!t_relay()) { sl_reply_error(); break; };

Name:	t_on_negative
Params:	reply_route
Desc:	sets reply routing block, to which control is passed after
		a transaction completed with a negative result but before
		sending a final reply; In the refered block, you can
		either start a new branch (good for services such as
		forward_on_no_reply) or send a final reply on your own
		(good for example for message silo, which received 
		a negative reply from upstream and wants to tell
		upstream "202 I will take care of it"); Note that the
		set of command which are useable within reply_routes is
		strictly limited to rewriting URI, initiating new branches,
		logging, and sending 'unsafe' replies (t_reply_unsafe). Any 
		other commands may result in unpredictable behaviour and 
		possible server failure.
		Note that whenever reply_route is entered, uri is reset to
		value which it had on relaying. If it temporarily changed
		during a reply_route processing, subsequent reply_route
		will ignore the changed value and use again the original
Example: route { t_on_negative("1"); t_relay(); } reply_route[1] {
			revert_uri(); setuser("voicemail"); append_branch(); }

		see test/onr.cfg for a more complex example of combination
		of serial with parallel forking

Name:	append_branch (actually part of core now)
Params:	uri
Desc:	adds a new destination to destination set; if used,
		a subsequent call to t_relay (or t_forward_nonack, 
		on which t_relay is based) than introduces
	    a new branch and forks a transaction; append_branch may
		also be called from reply processing -- this may 
	    be particularly useful for services such as

Name:	append_branch
Params:	0
Desc:	similarly to t_fork_to, it extends destination set
		by a new entry; the difference is that current uri
		is taken as new entry;
Example: set_user("john"); t_fork(); set_user("alice");
		 t_fork(); t_relay();

-----   ----  ---- medium-advanced commands here --- on -----

Name:	t_newtran
Params: 0
Desc:	creates a new transaction, returns a negative value on 
	    error; this is the only way a script can add a new transaction 
	    in an atomic way; typically, it is used to deploy a UAS
Example: see test/uas.cfg: 
	    if (t_newtran()) { log("UAS logic"); t_reply("999","hello"); }
		else sl_reply_error();

Name:	t_reply
Params:	code, reason phrase
Desc: 	sends a stateful reply after a transaction has been
		established; see t_newtran for usage; note: never use
	    t_reply from within reply_route ... always use t_reply_unsafe

----- only --- advanced --- commands --- from --- here --- on -----

Name:	t_lookup_request
Params:	0
Desc:	checks if a transaction exists; returns a positive value
		if so, negative otherwise; most likely you will not want
	    to use it, as a typicall application of a looku-up is to
	    introduce a new transaction if none was found; however
	    this is safely (atomically) done using t_newtran

Name:	t_retransmit_reply
Params:	0
Desc:	retransmits a reply sent previously by UAS transaction

Name:	t_release
Params:	0
Desc:	remove transaction from memory (it will be first put on
		a wait timer to absorb delayed messages)

Name:	t_forward_nonack
Params:	ip, port
Desc:	mainly for internal -- forward a non-ACK request statefuly

Name:	register_tmcb
Params:	callback type, callback function
Desc:	for programmatic use only -- register a function to be called
		back on an event; see t_hooks.h for more details

Name:	load_tm
Params:	*import_structure
Desc:	for programmatic use only -- import exported TM functions;
		see the acc module for an example of use

Name:	t_reply_unsafe
Params:	code, reason phrase
Desc: 	sends a stateful reply after a transaction has been
		established; it can only be used from reply processing;
	    using it from regular processing will introduce erroneous
	    conditions; using t_reply from reply_processing will
	    introduce a deadlock

External Usage of TM
There are applications which would like to generate SIP
transactions without too big onvolvement in SIP stack, transaction
management, etc. An example of such an application
is sending instant messages from a website. To address needs
of such apps, SER accepts requests for new transactions via
fifo pipes too. If you want to enable this feature, statrt
FIFO server by configuration option
Then, an application can easily launch a new transaction by
writing a transaction request to this named pipe. The request
must follow very simple format, whic is
 :t_uac:[<file_name>]\n<method>\n<dst uri>\n<CR_separated_headers>\n<body>\n\n\n
(Filename is to where a report will be dumped. ser assumes /tmp
as file's directory.)

A convenience library fifo_uac.c implements this simple functionality.
Note the the request write must be atomic, otherwise the request
might get intermixes with writes from other writers.
You can easily use it via Unix command-line tools, see the following
[jiri@bat jiri]$ cat > /tmp/fifo
p_header: p_value

body body body
yet body
end of body

or use an example file and call cat test/transaction.fifo > /tmp/fifo

Known Issues
- need to revisit profiling again
- review whether there is not potential for to-tag
  rewriting and ACK matching
- we don't have authentication merging on forking
- branch tid is not used yet
- local ACK/CANCELs copy'n'pastes Route and ignores deleted
- 6xx should be delayed 
- possibly, performance could be improved by not parsing non-INVITEs,
  as they do not be replied with 100, and do not result in ACK/CANCELs,
  and other things which take parsing. However, we need to rethink
  whether we don't need parsed headers later for something else.
  Remember, when we now conserver a request in sh_mem, we can't apply
  any pkg_mem operations to it any more. (that might be redesigned too)
- t_replicate should be done more cleanly -- Vias, Routes, etc. should
  be removed from a message prior to replicating it
- SNMP support
- lookup fails to recognize subsequent requests which have additional 
  leading spaces in header field values
- make UAC session-aware (as opposed to just transaction aware) -- needed
  for keeing SUB-NOT dialog state, etc. Currently, there are only
  place-holders for in in TM.
- places labeled with "HACK" strongly deserve beautification