# $Id$
 # 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