Merge the changes from issue #10665 from the team/group/sip_session_timers branch.

This set of changes introduces SIP session timers support (RFC 4028).  In short,
this prevents stuck SIP sessions that were not properly torn down due to network
or endpoint failures during an established SIP session.

To quote some of the documentation supplied with the patch:
"The SIP Session-Timers is an extension of the SIP protocol that allows end-points and proxies to
refresh a session periodically. The sessions are kept alive by sending a RE-INVITE or UPDATE
request at a negotiated interval. If a session refresh fails then all the entities that support Session-
Timers clear their internal session state. In addition, UAs generate a BYE request in order to clear
the state in the proxies and the remote UA (this is done for the benefit of SIP entities in the path
that do not support Session-Timers)."

(closes issue #10665)
Reported by: rjain
Patches:
      chan_sip.c.1.diff uploaded by rjain (license 226)
      chan_sip.c.diff uploaded by rjain (license 226)
      sip.conf.sample.diff uploaded by rjain (license 226)
      proc_422_rsp_comment.diff uploaded by rjain (license 226)
      chan_sip.c.cache.diff uploaded by rjain (license 226)
      chan_sip.memalloc uploaded by rjain (license 226)
      chan_sip.memalloc.bugfix uploaded by rjain (license 226)

      Patches tracked in team/group/sip_session_timers, with some additional fixes
      by russell and oej.

Tested by: jtodd, rjain, loloski


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@98978 65c4cc65-6c06-0410-ace0-fbb531ad65f3

3 files changed, 1067 insertions, 50 deletions

diff --git a/CREDITS b/CREDITS
index 1386fb069..cb59dbe1b 100644
--- a/CREDITS
+++ b/CREDITS
@@ -17,6 +17,9 @@ nic.at - ENUM support in Asterisk
 
 Paul Bagyenda, Digital Solutions - for initial Voicetronix driver development
 
+John Todd, TalkPlus, Inc.  and JR Richardson, Ntegrated Solutions. - for funding
+    the development of SIP Session Timers support.
+
 === WISHLIST CONTRIBUTERS ===
 Jeremy McNamara - SpeeX support
 Nick Seraphin - RDNIS support
diff --git a/channels/chan_sip.c b/channels/chan_sip.c
index b21a82045..c36706763 100644
--- a/channels/chan_sip.c
+++ b/channels/chan_sip.c
@@ -86,6 +86,45 @@
         <depend>res_features</depend>
  ***/
 
+/*!  \page sip_session_timers SIP Session Timers in Asterisk Chan_sip
+
+	The SIP Session-Timers is an extension of the SIP protocol that allows end-points and proxies to
+	refresh a session periodically. The sessions are kept alive by sending a RE-INVITE or UPDATE
+	request at a negotiated interval. If a session refresh fails then all the entities that support Session-
+	Timers clear their internal session state. In addition, UAs generate a BYE request in order to clear
+	the state in the proxies and the remote UA (this is done for the benefit of SIP entities in the path
+	that do not support Session-Timers).
+
+	The Session-Timers can be configured on a system-wide, per-user, or per-peer basis. The peruser/
+	per-peer settings override the global settings. The following new parameters have been
+	added to the sip.conf file.
+		session-timers=["accept", "originate", "refuse"]
+		session-expires=[integer]
+		session-minse=[integer]
+		session-refresher=["uas", "uac"]
+
+	The session-timers parameter in sip.conf defines the mode of operation of SIP session-timers feature in
+	Asterisk. The Asterisk can be configured in one of the following three modes:
+
+	1. Accept :: In the "accept" mode, the Asterisk server honors session-timers requests
+		made by remote end-points. A remote end-point can request Asterisk to engage
+		session-timers by either sending it an INVITE request with a "Supported: timer"
+		header in it or by responding to Asterisk's INVITE with a 200 OK that contains
+		Session-Expires: header in it. In this mode, the Asterisk server does not 
+		request session-timers from remote end-points. This is the default mode.
+	2. Originate :: In the "originate" mode, the Asterisk server requests the remote 
+		end-points to activate session-timers in addition to honoring such requests
+		made by the remote end-pints. In order to get as much protection as possible
+		against hanging SIP channels due to network or end-point failures, Asterisk
+		resends periodic re-INVITEs even if a remote end-point does not support
+		the session-timers feature.
+	3. Refuse :: In the "refuse" mode, Asterisk acts as if it does not support session-
+		timers for inbound or outbound requests. If a remote end-point requests
+		session-timers in a dialog, then Asterisk ignores that request unless it's
+		noted as a requirement (Require: header), in which case the INVITE is 
+		rejected with a 420 Bad Extension response.
+
+*/
 
 #include "asterisk.h"
 
@@ -198,6 +237,9 @@ static int expiry = DEFAULT_EXPIRY;
 
 #define INITIAL_CSEQ                 101              /*!< our initial sip sequence number */
 
+#define DEFAULT_MAX_SE               1800             /*!< Session-Timer Default Session-Expires period (RFC 4028) */
+#define DEFAULT_MIN_SE               90               /*!< Session-Timer Default Min-SE period (RFC 4028) */
+
 /*! \brief Global jitterbuffer configuration - by default, jb is disabled */
 static struct ast_jb_conf default_jbconf =
 {
@@ -347,6 +389,22 @@ enum sipregistrystate {
 		/* fatal - no chance to proceed */
 };
 
+/*! \brief Modes in which Asterisk can be configured to run SIP Session-Timers */
+enum st_mode {
+        SESSION_TIMER_MODE_INVALID = 0, /*!< Invalid value */ 
+        SESSION_TIMER_MODE_ACCEPT,      /*!< Honor inbound Session-Timer requests */
+        SESSION_TIMER_MODE_ORIGINATE,   /*!< Originate outbound and honor inbound requests */
+        SESSION_TIMER_MODE_REFUSE       /*!< Ignore inbound Session-Timers requests */
+};
+
+/*! \brief The entity playing the refresher role for Session-Timers */
+enum st_refresher {
+        SESSION_TIMER_REFRESHER_AUTO,    /*!< Negotiated                      */
+        SESSION_TIMER_REFRESHER_UAC,     /*!< Session is refreshed by the UAC */
+        SESSION_TIMER_REFRESHER_UAS      /*!< Session is refreshed by the UAS */
+};
+
+
 /*! \brief definition of a sip proxy server
  *
  * For outbound proxies, this is allocated in the SIP peer dynamically or
@@ -458,6 +516,8 @@ static const struct  cfsip_methods {
 #define SIP_OPT_NOREFERSUB	(1 << 14)
 #define SIP_OPT_HISTINFO	(1 << 15)
 #define SIP_OPT_RESPRIORITY	(1 << 16)
+#define SIP_OPT_UNKNOWN		(1 << 17)
+
 
 /*! \brief List of well-known SIP options. If we get this in a require,
    we should check the list and answer accordingly. */
@@ -472,8 +532,8 @@ static const struct cfsip_options {
 	{ SIP_OPT_REPLACES,	SUPPORTED,	"replace" },	
 	/* RFC3262: PRACK 100% reliability */
 	{ SIP_OPT_100REL,	NOT_SUPPORTED,	"100rel" },	
-	/* RFC4028: SIP Session Timers */
-	{ SIP_OPT_TIMER,	NOT_SUPPORTED,	"timer" },
+	/* RFC4028: SIP Session-Timers */
+	{ SIP_OPT_TIMER,	SUPPORTED,	"timer" },
 	/* RFC3959: SIP Early session support */
 	{ SIP_OPT_EARLY_SESSION, NOT_SUPPORTED,	"early-session" },
 	/* RFC3911: SIP Join header support */
@@ -512,7 +572,7 @@ static const struct cfsip_options {
 #define ALLOWED_METHODS "INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY"
 
 /*! \brief SIP Extensions we support */
-#define SUPPORTED_EXTENSIONS "replaces" 
+#define SUPPORTED_EXTENSIONS "replaces, timer" 
 
 /*! \brief Standard SIP port from RFC 3261. DO NOT CHANGE THIS */
 #define STANDARD_SIP_PORT	5060
@@ -666,6 +726,12 @@ static int regobjs = 0;                  /*!< Registry objects */
 static struct ast_flags global_flags[2] = {{0}};        /*!< global SIP_ flags */
 static char used_context[AST_MAX_CONTEXT]; /*!< name of automatically created context for unloading */
 
+static enum st_mode global_st_mode;           /*!< Mode of operation for Session-Timers           */
+static enum st_refresher global_st_refresher; /*!< Session-Timer refresher                        */
+static int global_min_se;                     /*!< Lowest threshold for session refresh interval  */
+static int global_max_se;                     /*!< Highest threshold for session refresh interval */
+
+
 AST_MUTEX_DEFINE_STATIC(netlock);
 
 /*! \brief Protect the monitoring thread, so only one process can kill or start it, and not
@@ -1026,6 +1092,37 @@ struct sip_refer {
 	enum referstatus status;			/*!< REFER status */
 };
 
+
+/*! \brief Structure that encapsulates all attributes related to running 
+ *   SIP Session-Timers feature on a per dialog basis.
+ */
+struct sip_st_dlg {
+	int st_active;                          /*!< Session-Timers on/off */ 
+	int st_interval;                        /*!< Session-Timers negotiated session refresh interval */
+	int st_schedid;                         /*!< Session-Timers ast_sched scheduler id */
+	enum st_refresher st_ref;               /*!< Session-Timers session refresher */
+	int st_expirys;                         /*!< Session-Timers number of expirys */
+	int st_active_peer_ua;                  /*!< Session-Timers on/off in peer UA */
+	int st_cached_min_se;                   /*!< Session-Timers cached Min-SE */
+	int st_cached_max_se;                   /*!< Session-Timers cached Session-Expires */
+	enum st_mode st_cached_mode;            /*!< Session-Timers cached M.O. */
+	enum st_refresher st_cached_ref;        /*!< Session-Timers cached refresher */
+};
+
+
+/*! \brief Structure that encapsulates all attributes related to configuration 
+ *   of SIP Session-Timers feature on a per user/peer basis.
+ */
+struct sip_st_cfg {
+	enum st_mode st_mode_oper;      /*!< Mode of operation for Session-Timers           */
+	enum st_refresher st_ref;       /*!< Session-Timer refresher                        */
+	int st_min_se;                  /*!< Lowest threshold for session refresh interval  */
+	int st_max_se;                  /*!< Highest threshold for session refresh interval */
+};
+
+
+
+
 /*! \brief sip_pvt: structures used for each SIP dialog, ie. a call, a registration, a subscribe.
  * Created and initialized by sip_alloc(), the descriptor goes into the list of
  * descriptors (dialoglist).
@@ -1098,6 +1195,7 @@ struct sip_pvt {
 	int timer_t1;				/*!< SIP timer T1, ms rtt */
 	int timer_b;            /*!< SIP timer B, ms */
 	unsigned int sipoptions;		/*!< Supported SIP options on the other end */
+	unsigned int reqsipoptions;		/*!< Required SIP options on the other end */
 	struct ast_codec_pref prefs;		/*!< codec prefs */
 	int capability;				/*!< Special capability (codec) */
 	int jointcapability;			/*!< Supported capability at both ends (codecs) */
@@ -1118,6 +1216,8 @@ struct sip_pvt {
 	char tag[11];				/*!< Our tag for this session */
 	int sessionid;				/*!< SDP Session ID */
 	int sessionversion;			/*!< SDP Session Version */
+	int sessionversion_remote;		/*!< Remote UA's SDP Session Version */
+	int session_modify;			/*!< Session modification request true/false  */
 	struct sockaddr_in sa;			/*!< Our peer */
 	struct sockaddr_in redirip;		/*!< Where our RTP should be going if not to us */
 	struct sockaddr_in vredirip;		/*!< Where our Video RTP should be going if not to us */
@@ -1167,8 +1267,10 @@ struct sip_pvt {
 							before strolling to the Grokyzpå
 							(A bit unsure of this, please correct if
 							you know more) */
+	struct sip_st_dlg *stimer;		/*!< SIP Session-Timers */              
 };
 
+
 /*! Max entires in the history list for a sip_pvt */
 #define MAX_HISTORY_ENTRIES 50
 
@@ -1273,6 +1375,7 @@ struct sip_user {
 	struct ast_variable *chanvars;	/*!< Variables to set for channel created by user */
 	int maxcallbitrate;		/*!< Maximum Bitrate for a video call */
 	int autoframing;
+	struct sip_st_cfg stimer;	/*!< SIP Session-Timers */
 };
 
 /*!
@@ -1359,8 +1462,9 @@ struct sip_peer {
 	struct ast_variable *chanvars;	/*!<  Variables to set for channel created by user */
 	struct sip_pvt *mwipvt;		/*!<  Subscription for MWI */
 	int autoframing;
-	int timer_t1;		/*!<  The maximum T1 value for the peer */
-	int timer_b;      /*!<  The maximum timer B (transaction timeouts) */
+	struct sip_st_cfg stimer;	/*!<  SIP Session-Timers */
+	int timer_t1;			/*!<  The maximum T1 value for the peer */
+	int timer_b;			/*!<  The maximum timer B (transaction timeouts) */
 };
 
 
@@ -1545,7 +1649,7 @@ static int transmit_response_using_temp(ast_string_field callid, struct sockaddr
 static int transmit_response(struct sip_pvt *p, const char *msg, const struct sip_request *req);
 static int transmit_response_reliable(struct sip_pvt *p, const char *msg, const struct sip_request *req);
 static int transmit_response_with_date(struct sip_pvt *p, const char *msg, const struct sip_request *req);
-static int transmit_response_with_sdp(struct sip_pvt *p, const char *msg, const struct sip_request *req, enum xmittype reliable);
+static int transmit_response_with_sdp(struct sip_pvt *p, const char *msg, const struct sip_request *req, enum xmittype reliable, int oldsdp);
 static int transmit_response_with_unsupported(struct sip_pvt *p, const char *msg, const struct sip_request *req, const char *unsupported);
 static int transmit_response_with_auth(struct sip_pvt *p, const char *msg, const struct sip_request *req, const char *rand, enum xmittype reliable, const char *header, int stale);
 static int transmit_response_with_allow(struct sip_pvt *p, const char *msg, const struct sip_request *req, enum xmittype reliable);
@@ -1553,7 +1657,7 @@ static void transmit_fake_auth_response(struct sip_pvt *p, struct sip_request *r
 static int transmit_request(struct sip_pvt *p, int sipmethod, int inc, enum xmittype reliable, int newbranch);
 static int transmit_request_with_auth(struct sip_pvt *p, int sipmethod, int seqno, enum xmittype reliable, int newbranch);
 static int transmit_invite(struct sip_pvt *p, int sipmethod, int sdp, int init);
-static int transmit_reinvite_with_sdp(struct sip_pvt *p, int t38version);
+static int transmit_reinvite_with_sdp(struct sip_pvt *p, int t38version, int oldsdp);
 static int transmit_info_with_digit(struct sip_pvt *p, const char digit, unsigned int duration);
 static int transmit_info_with_vidupdate(struct sip_pvt *p);
 static int transmit_message_with_text(struct sip_pvt *p, const char *text);
@@ -1607,7 +1711,7 @@ static void add_codec_to_sdp(const struct sip_pvt *p, int codec, int sample_rate
 static void add_noncodec_to_sdp(const struct sip_pvt *p, int format, int sample_rate,
 				struct ast_str **m_buf, struct ast_str **a_buf,
 				int debug);
-static enum sip_result add_sdp(struct sip_request *resp, struct sip_pvt *p);
+static enum sip_result add_sdp(struct sip_request *resp, struct sip_pvt *p, int oldsdp);
 static void do_setnat(struct sip_pvt *p, int natflags);
 static void stop_media_flows(struct sip_pvt *p);
 
@@ -1844,6 +1948,21 @@ static int transmit_response_with_t38_sdp(struct sip_pvt *p, char *msg, struct s
 static struct ast_udptl *sip_get_udptl_peer(struct ast_channel *chan);
 static int sip_set_udptl_peer(struct ast_channel *chan, struct ast_udptl *udptl);
 
+/*------ Session-Timers functions --------- */
+static void proc_422_rsp(struct sip_pvt *p, struct sip_request *rsp);
+static int  proc_session_timer(const void *vp);
+static void stop_session_timer(struct sip_pvt *p);
+static void start_session_timer(struct sip_pvt *p);
+static void restart_session_timer(struct sip_pvt *p);
+static const char *strefresher2str(enum st_refresher r);
+static int parse_session_expires(const char *p_hdrval, int *const p_interval, enum st_refresher *const p_ref);
+static int parse_minse(const char *p_hdrval, int *const p_interval);
+static int st_get_se(struct sip_pvt *, int max);
+static enum st_refresher st_get_refresher(struct sip_pvt *);
+static enum st_mode st_get_mode(struct sip_pvt *);
+static struct sip_st_dlg* sip_st_alloc(struct sip_pvt *const p);
+
+
 /*! \brief Definition of this channel for PBX channel registration */
 static const struct ast_channel_tech sip_tech = {
 	.type = "SIP",
@@ -2106,6 +2225,14 @@ static unsigned int parse_sip_options(struct sip_pvt *pvt, const char *supported
 				break;
 			}
 		}
+
+		/* This function is used to parse both Suported: and Require: headers.
+		Let the caller of this function know that an unknown option tag was 
+		encountered, so that if the UAC requires it then the request can be 
+		rejected with a 420 response. */
+		if (!found)
+			profile |= SIP_OPT_UNKNOWN;
+
 		if (!found && sipdebug) {
 			if (!strncasecmp(next, "x-", 2))
 				ast_debug(3, "Found private SIP option, not supported: %s\n", next);
@@ -2526,6 +2653,9 @@ static void sip_scheddestroy(struct sip_pvt *p, int ms)
 	if (p->do_history)
 		append_history(p, "SchedDestroy", "%d ms", ms);
 	p->autokillid = ast_sched_add(sched, ms, __sip_autodestruct, dialog_ref(p));
+
+	if (p->stimer && p->stimer->st_active == TRUE && p->stimer->st_schedid > 0)
+		stop_session_timer(p);
 }
 
 /*! \brief Cancel destruction of SIP dialog.
@@ -2867,7 +2997,7 @@ static int sip_sendhtml(struct ast_channel *chan, int subclass, const char *data
 		break;
 	case AST_STATE_UP:
 		if (!p->pendinginvite) {		/* We are up, and have no outstanding invite */
-			transmit_reinvite_with_sdp(p, FALSE);
+			transmit_reinvite_with_sdp(p, FALSE, FALSE);
 		} else if (!ast_test_flag(&p->flags[0], SIP_PENDINGBYE)) {
 			ast_set_flag(&p->flags[0], SIP_NEEDREINVITE);	
 		}	
@@ -3298,6 +3428,7 @@ static struct sip_peer *find_peer(const char *peer, struct sockaddr_in *sin, int
 static void sip_destroy_user(struct sip_user *user)
 {
 	ast_debug(3, "Destroying user object from memory: %s\n", user->name);
+
 	ast_free_ha(user->ha);
 	if (user->chanvars) {
 		ast_variables_destroy(user->chanvars);
@@ -3784,6 +3915,13 @@ static void __sip_destroy(struct sip_pvt *p, int lockowner, int lockdialoglist)
 		p->registry = registry_unref(p->registry);
 	}
 
+	/* Destroy Session-Timers if allocated */
+	if (p->stimer) {
+		if (p->stimer->st_active == TRUE && p->stimer->st_schedid > -1)
+			ast_sched_del(sched, p->stimer->st_schedid);
+		ast_free(p->stimer);
+	}
+
 	/* Unlink us from the owner if we have one */
 	if (p->owner) {
 		if (lockowner)
@@ -4338,7 +4476,7 @@ static int sip_answer(struct ast_channel *ast)
 			ast_debug(2,"T38State change to %d on channel %s\n", p->t38.state, ast->name);
 			res = transmit_response_with_t38_sdp(p, "200 OK", &p->initreq, XMIT_CRITICAL);
 		} else 
-			res = transmit_response_with_sdp(p, "200 OK", &p->initreq, XMIT_CRITICAL);
+			res = transmit_response_with_sdp(p, "200 OK", &p->initreq, XMIT_CRITICAL, FALSE);
 	}
 	sip_pvt_unlock(p);
 	return res;
@@ -4371,7 +4509,7 @@ static int sip_write(struct ast_channel *ast, struct ast_frame *frame)
 				if ((ast->_state != AST_STATE_UP) &&
 				    !ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT) &&
 				    !ast_test_flag(&p->flags[0], SIP_OUTGOING)) {
-					transmit_response_with_sdp(p, "183 Session Progress", &p->initreq, XMIT_UNRELIABLE);
+					transmit_response_with_sdp(p, "183 Session Progress", &p->initreq, XMIT_UNRELIABLE, FALSE);
 					ast_set_flag(&p->flags[0], SIP_PROGRESS_SENT);	
 				}
 				p->lastrtptx = time(NULL);
@@ -4388,7 +4526,7 @@ static int sip_write(struct ast_channel *ast, struct ast_frame *frame)
 				if ((ast->_state != AST_STATE_UP) &&
 				    !ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT) &&
 				    !ast_test_flag(&p->flags[0], SIP_OUTGOING)) {
-					transmit_response_with_sdp(p, "183 Session Progress", &p->initreq, XMIT_UNRELIABLE);
+					transmit_response_with_sdp(p, "183 Session Progress", &p->initreq, XMIT_UNRELIABLE, FALSE);
 					ast_set_flag(&p->flags[0], SIP_PROGRESS_SENT);	
 				}
 				p->lastrtptx = time(NULL);
@@ -4405,7 +4543,7 @@ static int sip_write(struct ast_channel *ast, struct ast_frame *frame)
 				if ((ast->_state != AST_STATE_UP) &&
 				    !ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT) &&
 				    !ast_test_flag(&p->flags[0], SIP_OUTGOING)) {
-					transmit_response_with_sdp(p, "183 Session Progress", &p->initreq, XMIT_UNRELIABLE);
+					transmit_response_with_sdp(p, "183 Session Progress", &p->initreq, XMIT_UNRELIABLE, FALSE);
 					ast_set_flag(&p->flags[0], SIP_PROGRESS_SENT);	
 				}
 				p->lastrtptx = time(NULL);
@@ -4601,7 +4739,7 @@ static int sip_indicate(struct ast_channel *ast, int condition, const void *data
 		    !ast_test_flag(&p->flags[0], SIP_PROGRESS_SENT) &&
 		    !ast_test_flag(&p->flags[0], SIP_OUTGOING)) {
 			p->invitestate = INV_EARLY_MEDIA;
-			transmit_response_with_sdp(p, "183 Session Progress", &p->initreq, XMIT_UNRELIABLE);
+			transmit_response_with_sdp(p, "183 Session Progress", &p->initreq, XMIT_UNRELIABLE, FALSE);
 			ast_set_flag(&p->flags[0], SIP_PROGRESS_SENT);	
 			break;
 		}
@@ -5053,7 +5191,7 @@ static struct ast_frame *sip_read(struct ast_channel *ast)
 			if (!p->pendinginvite) {
 				ast_debug(3, "Sending reinvite on SIP (%s) for T.38 negotiation.\n",ast->name);
 				p->t38.state = T38_LOCAL_REINVITE;
-				transmit_reinvite_with_sdp(p, TRUE);
+				transmit_reinvite_with_sdp(p, TRUE, FALSE);
 				ast_debug(2, "T38 state changed to %d on channel %s\n", p->t38.state, ast->name);
 			}
 		} else if (!ast_test_flag(&p->flags[0], SIP_PENDINGBYE)) {
@@ -5107,6 +5245,26 @@ static void make_our_tag(char *tagbuf, size_t len)
 	snprintf(tagbuf, len, "as%08lx", ast_random());
 }
 
+/*! \brief Allocate Session-Timers struct w/in dialog */
+static struct sip_st_dlg* sip_st_alloc(struct sip_pvt *const p)
+{
+	struct sip_st_dlg *stp;
+
+	if (p->stimer) {
+		ast_log(LOG_ERROR, "Session-Timer struct already allocated\n");
+		return p->stimer;
+	}
+
+	if (!(stp = ast_calloc(1, sizeof(struct sip_st_dlg))))
+		return NULL;
+
+	p->stimer = stp;
+
+	stp->st_schedid = -1;           /* Session-Timers ast_sched scheduler id */
+
+	return p->stimer;
+}
+
 /*! \brief Allocate sip_pvt structure, set defaults and link in the container.
  * Returns a reference to the object so whoever uses it later must
  * remember to release the reference.
@@ -5132,6 +5290,9 @@ static struct sip_pvt *sip_alloc(ast_string_field callid, struct sockaddr_in *si
 	p->autokillid = -1;
 	p->subscribed = NONE;
 	p->stateid = -1;
+	p->sessionversion_remote = -1;
+	p->session_modify = TRUE;
+	p->stimer = NULL;
 	p->prefs = default_prefs;		/* Set default codecs for this call */
 
 	if (intended_method != SIP_OPTIONS) {	/* Peerpoke has it's own system */
@@ -5231,6 +5392,7 @@ static struct sip_pvt *sip_alloc(ast_string_field callid, struct sockaddr_in *si
 	}
 	ast_string_field_set(p, context, default_context);
 
+
 	/* Add to active dialog list */
 	dialoglist_lock();
 	p->next = dialoglist;
@@ -5646,6 +5808,9 @@ static int process_sdp(struct sip_pvt *p, struct sip_request *req)
 	const char *m;		/* SDP media offer */
 	const char *c;
 	const char *a;
+	const char *o;		/* Pointer to o= line */
+	char *o_copy;		/* Copy of o= line */
+	char *token;
 	char host[258];
 	int len = -1;
 	int portno = -1;		/*!< RTP Audio port number */
@@ -5687,6 +5852,7 @@ static int process_sdp(struct sip_pvt *p, struct sip_request *req)
 	int last_rtpmap_codec=0;
 
 	char buf[BUFSIZ];
+	int rua_version;
 
 	if (!p->rtp) {
 		ast_log(LOG_ERROR, "Got SDP but have no RTP session allocated.\n");
@@ -5712,6 +5878,50 @@ static int process_sdp(struct sip_pvt *p, struct sip_request *req)
 	/* Update our last rtprx when we receive an SDP, too */
 	p->lastrtprx = p->lastrtptx = time(NULL); /* XXX why both ? */
 
+	/* Store the SDP version number of remote UA. This will allow us to 
+	distinguish between session modifications and session refreshes. If 
+	the remote UA does not send an incremented SDP version number in a 
+	subsequent RE-INVITE then that means its not changing media session. 
+	The RE-INVITE may have been sent to update connected party, remote  
+	target or to refresh the session (Session-Timers).  Asterisk must not 
+	change media session and increment its own version number in answer 
+	SDP in this case. */ 
+	
+	o = get_sdp(req, "o");
+	if (ast_strlen_zero(o)) {
+		ast_log(LOG_WARNING, "SDP sytax error. SDP without an o= line\n");
+		return -1;
+	}
+
+	o_copy = ast_strdupa(o);
+	token = strsep(&o_copy, " ");  /* Skip username   */
+	if (!o_copy) { 
+		ast_log(LOG_WARNING, "SDP sytax error in o= line username\n");
+		return -1;
+	}
+	token = strsep(&o_copy, " ");  /* Skip session-id */
+	if (!o_copy) { 
+		ast_log(LOG_WARNING, "SDP sytax error in o= line session-id\n");
+		return -1;
+	}
+	token = strsep(&o_copy, " ");  /* Version         */
+	if (!o_copy) { 
+		ast_log(LOG_WARNING, "SDP sytax error in o= line\n");
+		return -1;
+	}
+	if (!sscanf(token, "%d", &rua_version)) {
+		ast_log(LOG_WARNING, "SDP sytax error in o= line version\n");
+		return -1;
+	}
+
+	if (p->sessionversion_remote < 0 || p->sessionversion_remote != rua_version) {
+ 		p->sessionversion_remote = rua_version;
+		p->session_modify = TRUE;
+	} else if (p->sessionversion_remote == rua_version) {
+		p->session_modify = FALSE;
+		ast_debug(2, "SDP version number same as previous SDP\n");
+		return 0;
+	} 
 
 	/* Try to find first media stream */
 	m = get_sdp(req, "m");
@@ -6598,6 +6808,16 @@ static int respprep(struct sip_request *resp, struct sip_pvt *p, const char *msg
 		add_header(resp, "User-Agent", global_useragent);
 	add_header(resp, "Allow", ALLOWED_METHODS);
 	add_header(resp, "Supported", SUPPORTED_EXTENSIONS);
+
+	/* Add Session-Timers related headers if the feature is active for this session */
+	if (p->stimer && p->stimer->st_active == TRUE && p->stimer->st_active_peer_ua == TRUE) {
+		char se_hdr[256];
+		snprintf(se_hdr, sizeof(se_hdr), "%d;refresher=%s", p->stimer->st_interval, 
+			strefresher2str(p->stimer->st_ref));
+		add_header(resp, "Require", "timer");
+		add_header(resp, "Session-Expires", se_hdr);
+	}
+
 	if (msg[0] == '2' && (p->method == SIP_SUBSCRIBE || p->method == SIP_REGISTER)) {
 		/* For registration responses, we also need expiry and
 		   contact info */
@@ -6729,6 +6949,17 @@ static int reqprep(struct sip_request *req, struct sip_pvt *p, int sipmethod, in
 		ast_string_field_set(p, url, NULL);
 	}
 
+	/* Add Session-Timers related headers if the feature is active for this session */
+	if (p->stimer && p->stimer->st_active == TRUE && p->stimer->st_active_peer_ua == TRUE) {
+		char se_hdr[256];
+		snprintf(se_hdr, sizeof(se_hdr), "%d;refresher=%s", p->stimer->st_interval, 
+			strefresher2str(p->stimer->st_ref));
+		add_header(req, "Require", "timer");
+		add_header(req, "Session-Expires", se_hdr);
+		snprintf(se_hdr, sizeof(se_hdr), "%d", st_get_se(p, FALSE));
+		add_header(req, "Min-SE", se_hdr);
+	}
+
 	return 0;
 }
 
@@ -6840,6 +7071,23 @@ static int transmit_response_with_unsupported(struct sip_pvt *p, const char *msg
 	return send_response(p, &resp, XMIT_UNRELIABLE, 0);
 }
 
+/*! \brief Transmit 422 response with Min-SE header (Session-Timers)  */
+static int transmit_response_with_minse(struct sip_pvt *p, const char *msg, const struct sip_request *req, int minse_int)
+{
+	struct sip_request resp;
+	char minse_str[20];
+
+	respprep(&resp, p, msg, req);
+	append_date(&resp);
+
+	snprintf(minse_str, sizeof(minse_str), "%d", minse_int);
+	add_header(&resp, "Min-SE", minse_str);
+
+	add_header_contentLength(&resp, 0);
+	return send_response(p, &resp, XMIT_UNRELIABLE, 0);
+}
+
+
 /*! \brief Transmit response, Make sure you get an ACK
 	This is only used for responses to INVITEs, where we need to make sure we get an ACK
 */
@@ -7222,8 +7470,13 @@ static void get_our_media_address(struct sip_pvt *p, int needvideo,
 
 #define SDP_SAMPLE_RATE(x) (x == AST_FORMAT_G722) ? 16000 : 8000
 
-/*! \brief Add Session Description Protocol message */
-static enum sip_result add_sdp(struct sip_request *resp, struct sip_pvt *p)
+/*! \brief Add Session Description Protocol message 
+
+    If oldsdp is TRUE, then the SDP version number is not incremented. This mechanism
+    is used in Session-Timers where RE-INVITEs are used for refreshing SIP sessions 
+    without modifying the media session in any way. 
+*/
+static enum sip_result add_sdp(struct sip_request *resp, struct sip_pvt *p, int oldsdp)
 {
 	int len = 0;
 	int alreadysent = 0;
@@ -7276,8 +7529,10 @@ static enum sip_result add_sdp(struct sip_request *resp, struct sip_pvt *p)
 	if (!p->sessionid) {
 		p->sessionid = (int)ast_random();
 		p->sessionversion = p->sessionid;
-	} else
-		p->sessionversion++;
+	} else {
+		if (oldsdp == FALSE)
+			p->sessionversion++;
+	}
 
 	capability = p->jointcapability;
 
@@ -7539,7 +7794,7 @@ static void copy_request(struct sip_request *dst, const struct sip_request *src)
 /*! \brief Used for 200 OK and 183 early media 
 	\return Will return XMIT_ERROR for network errors.
 */
-static int transmit_response_with_sdp(struct sip_pvt *p, const char *msg, const struct sip_request *req, enum xmittype reliable)
+static int transmit_response_with_sdp(struct sip_pvt *p, const char *msg, const struct sip_request *req, enum xmittype reliable, int oldsdp)
 {
 	struct sip_request resp;
 	int seqno;
@@ -7554,7 +7809,7 @@ static int transmit_response_with_sdp(struct sip_pvt *p, const char *msg, const
 			ast_rtp_codec_setpref(p->rtp, &p->prefs);
 		}
 		try_suggested_sip_codec(p);	
-		add_sdp(&resp, p);
+		add_sdp(&resp, p, oldsdp);
 	} else 
 		ast_log(LOG_ERROR, "Can't add SDP to response, since we have no RTP session allocated. Call-ID %s\n", p->callid);
 	if (reliable && !p->pendinginvite)
@@ -7611,27 +7866,38 @@ static int determine_firstline_parts(struct sip_request *req)
 	
 	If t38version is TRUE, we send T38 SDP for re-invite from audio/video to
 	T38 UDPTL transmission on the channel
+
+    If oldsdp is TRUE then the SDP version number is not incremented. This
+    is needed for Session-Timers so we can send a re-invite to refresh the
+    SIP session without modifying the media session. 
 */
-static int transmit_reinvite_with_sdp(struct sip_pvt *p, int t38version)
+static int transmit_reinvite_with_sdp(struct sip_pvt *p, int t38version, int oldsdp)
 {
 	struct sip_request req;
 
 	reqprep(&req, p, ast_test_flag(&p->flags[0], SIP_REINVITE_UPDATE) ?  SIP_UPDATE : SIP_INVITE, 0, 1);
-	
+
 	add_header(&req, "Allow", ALLOWED_METHODS);
 	add_header(&req, "Supported", SUPPORTED_EXTENSIONS);
-	if (sipdebug)
-		add_header(&req, "X-asterisk-Info", "SIP re-invite (External RTP bridge)");
+	if (sipdebug) {
+		if (oldsdp == TRUE)
+			add_header(&req, "X-asterisk-Info", "SIP re-invite (Session-Timers)");
+		else
+			add_header(&req, "X-asterisk-Info", "SIP re-invite (External RTP bridge)");
+	}
+
 	if (p->do_history)
 		append_history(p, "ReInv", "Re-invite sent");
 	if (t38version)
 		add_t38_sdp(&req, p);
 	else
-		add_sdp(&req, p);
+		add_sdp(&req, p, oldsdp);
+
 	/* Use this as the basis */
 	initialize_initreq(p, &req);
 	p->lastinvite = p->ocseq;
-	ast_set_flag(&p->flags[0], SIP_OUTGOING);		/* Change direction of this dialog */
+	ast_set_flag(&p->flags[0], SIP_OUTGOING);       /* Change direction of this dialog */
+
 	return send_request(p, &req, XMIT_CRITICAL, p->ocseq);
 }
 
@@ -7942,6 +8208,20 @@ static int transmit_invite(struct sip_pvt *p, int sipmethod, int sdp, int init)
 		add_header(&req, "Require", "replaces");
 	}
 
+	/* Add Session-Timers related headers */
+	if (st_get_mode(p) == SESSION_TIMER_MODE_ORIGINATE) {
+		char i2astr[10];
+
+		if (!p->stimer->st_interval)
+			p->stimer->st_interval = st_get_se(p, TRUE);
+
+		p->stimer->st_active = TRUE;
+		
+		snprintf(i2astr, sizeof(i2astr), "%d", p->stimer->st_interval);
+		add_header(&req, "Session-Expires", i2astr);
+		add_header(&req, "Min-SE", i2astr);
+	}
+
 	add_header(&req, "Allow", ALLOWED_METHODS);
 	add_header(&req, "Supported", SUPPORTED_EXTENSIONS);
 	if (p->options && p->options->addsipheaders && p->owner) {
@@ -7990,7 +8270,7 @@ static int transmit_invite(struct sip_pvt *p, int sipmethod, int sdp, int init)
 			ast_debug(1, "T38 is in state %d on channel %s\n", p->t38.state, p->owner ? p->owner->name : "<none>");
 			add_t38_sdp(&req, p);
 		} else if (p->rtp) 
-			add_sdp(&req, p);
+			add_sdp(&req, p, FALSE);
 	} else {
 		add_header_contentLength(&req, 0);
 	}
@@ -10829,6 +11109,45 @@ static const char *nat2str(int nat)
 /*! \brief  Report Peer status in character string
  *  \return 0 if peer is unreachable, 1 if peer is online, -1 if unmonitored
  */
+
+
+/* Session-Timer Modes */
+static struct _map_x_s stmodes[] = {
+        { SESSION_TIMER_MODE_ACCEPT,    "Accept"},
+        { SESSION_TIMER_MODE_ORIGINATE, "Originate"},
+        { SESSION_TIMER_MODE_REFUSE,    "Refuse"},
+        { -1,                           NULL},
+};
+
+static const char *stmode2str(enum st_mode m)
+{
+	return map_x_s(stmodes, m, "Unknown");
+}
+
+static enum st_mode str2stmode(const char *s)
+{
+	return map_s_x(stmodes, s, -1);
+}
+
+/* Session-Timer Refreshers */
+static struct _map_x_s strefreshers[] = {
+        { SESSION_TIMER_REFRESHER_AUTO,     "auto"},
+        { SESSION_TIMER_REFRESHER_UAC,      "uac"},
+        { SESSION_TIMER_REFRESHER_UAS,      "uas"},
+        { -1,                               NULL},
+};
+
+static const char *strefresher2str(enum st_refresher r)
+{
+	return map_x_s(strefreshers, r, "Unknown");
+}
+
+static enum st_refresher str2strefresher(const char *s)
+{
+	return map_s_x(strefreshers, s, -1);
+}
+
+
 static int peer_status(struct sip_peer *peer, char *status, int statuslen)
 {
 	int res = 0;
@@ -11510,8 +11829,6 @@ static int manager_sip_show_peer(struct mansession *s, const struct message *m)
 	return 0;
 }
 
-
-
 /*! \brief Show one peer in detail */
 static char *sip_show_peer(struct ast_cli_entry *e, int cmd, struct ast_cli_args *a)
 {
@@ -11667,7 +11984,7 @@ static char *_sip_show_peer(int type, int fd, struct mansession *s, const struct
 		print_codec_to_cli(fd, &peer->prefs);
 		ast_cli(fd, ")\n");
 
-		ast_cli(fd, "  Auto-Framing:  %s \n", cli_yesno(peer->autoframing));
+		ast_cli(fd, "  Auto-Framing :  %s \n", cli_yesno(peer->autoframing));
 		ast_cli(fd, "  100 on REG   : %s\n", ast_test_flag(&peer->flags[1], SIP_PAGE2_REGISTERTRYING) ? "Yes" : "No");
 		ast_cli(fd, "  Status       : ");
 		peer_status(peer, status, sizeof(status));
@@ -11680,6 +11997,11 @@ static char *_sip_show_peer(int type, int fd, struct mansession *s, const struct
 			for (v = peer->chanvars ; v ; v = v->next)
  				ast_cli(fd, "                 %s = %s\n", v->name, v->value);
 		}
+
+		ast_cli(fd, "  Sess-Timers  : %s\n", stmode2str(peer->stimer.st_mode_oper));
+		ast_cli(fd, "  Sess-Refresh : %s\n", strefresher2str(peer->stimer.st_ref));
+		ast_cli(fd, "  Sess-Expires : %d secs\n", peer->stimer.st_max_se);
+		ast_cli(fd, "  Min-Sess     : %d secs\n", peer->stimer.st_min_se);
 		ast_cli(fd,"\n");
 		unref_peer(peer);
 	} else  if (peer && type == 1) { /* manager listing */
@@ -11722,6 +12044,10 @@ static char *_sip_show_peer(int type, int fd, struct mansession *s, const struct
 		astman_append(s, "SIP-UserPhone: %s\r\n", (ast_test_flag(&peer->flags[0], SIP_USEREQPHONE)?"Y":"N"));
 		astman_append(s, "SIP-VideoSupport: %s\r\n", (ast_test_flag(&peer->flags[1], SIP_PAGE2_VIDEOSUPPORT)?"Y":"N"));
 		astman_append(s, "SIP-TextSupport: %s\r\n", (ast_test_flag(&peer->flags[1], SIP_PAGE2_TEXTSUPPORT)?"Y":"N"));
+		astman_append(s, "SIP-Sess-Timers: %s\r\n", stmode2str(peer->stimer.st_mode_oper));
+		astman_append(s, "SIP-Sess-Refresh: %s\r\n", strefresher2str(peer->stimer.st_ref));
+		astman_append(s, "SIP-Sess-Expires: %d\r\n", peer->stimer.st_max_se);
+		astman_append(s, "SIP-Sess-Min: %d\r\n", peer->stimer.st_min_se);
 
 		/* - is enumerated */
 		astman_append(s, "SIP-DTMFmode: %s\r\n", dtmfmode2str(ast_test_flag(&peer->flags[0], SIP_DTMF)));
@@ -11816,6 +12142,11 @@ static char *sip_show_user(struct ast_cli_entry *e, int cmd, struct ast_cli_args
 		print_group(a->fd, user->pickupgroup, 0);
 		ast_cli(a->fd, "  Callerid     : %s\n", ast_callerid_merge(cbuf, sizeof(cbuf), user->cid_name, user->cid_num, "<unspecified>"));
 		ast_cli(a->fd, "  ACL          : %s\n", cli_yesno(user->ha != NULL));
+ 		ast_cli(a->fd, "  Sess-Timers  : %s\n", stmode2str(user->stimer.st_mode_oper));
+ 		ast_cli(a->fd, "  Sess-Refresh : %s\n", strefresher2str(user->stimer.st_ref));
+ 		ast_cli(a->fd, "  Sess-Expires : %d secs\n", user->stimer.st_max_se);
+ 		ast_cli(a->fd, "  Sess-Min-SE  : %d secs\n", user->stimer.st_min_se);
+
 		ast_cli(a->fd, "  Codec Order  : (");
 		print_codec_to_cli(a->fd, &user->prefs);
 		ast_cli(a->fd, ")\n");
@@ -11826,7 +12157,9 @@ static char *sip_show_user(struct ast_cli_entry *e, int cmd, struct ast_cli_args
 			for (v = user->chanvars ; v ; v = v->next)
  				ast_cli(a->fd, "                 %s = %s\n", v->name, v->value);
 		}
+
 		ast_cli(a->fd,"\n");
+
 		unref_user(user);
 	} else {
 		ast_cli(a->fd,"User %s not found.\n", a->argv[3]);
@@ -12058,6 +12391,10 @@ static char *sip_show_settings(struct ast_cli_entry *e, int cmd, struct ast_cli_
 	ast_cli(a->fd, "  Auto-Framing:           %s\n", cli_yesno(global_autoframing));
 	ast_cli(a->fd, "  Outb. proxy:            %s %s\n", ast_strlen_zero(global_outboundproxy.name) ? "<not set>" : global_outboundproxy.name,
 							global_outboundproxy.force ? "(forced)" : "");
+	ast_cli(a->fd, "  Session Timers:         %s\n", stmode2str(global_st_mode));
+	ast_cli(a->fd, "  Session Refresher:      %s\n", strefresher2str (global_st_refresher));
+	ast_cli(a->fd, "  Session Expires:        %d secs\n", global_max_se);
+	ast_cli(a->fd, "  Session Min-SE:         %d secs\n", global_min_se);
  	ast_cli(a->fd, "  Timer T1:               %d\n", global_t1);
 	ast_cli(a->fd, "  Timer T1 minimum:       %d\n", global_t1min);
  	ast_cli(a->fd, "  Timer B:                %d\n", global_timer_b);
@@ -12438,9 +12775,29 @@ static char *sip_show_channel(struct ast_cli_entry *e, int cmd, struct ast_cli_a
 					if (cur->sipoptions & sip_options[x].id)
 						ast_cli(a->fd, "%s ", sip_options[x].text);
 				}
+				ast_cli(a->fd, "\n");
 			} else
 				ast_cli(a->fd, "(none)\n");
+
+			if (!cur->stimer)
+ 				ast_cli(a->fd, "  Session-Timer:          Uninitiallized\n");
+			else {
+ 				ast_cli(a->fd, "  Session-Timer:          %s\n", cur->stimer->st_active ? "Active" : "Inactive");
+ 				if (cur->stimer->st_active == TRUE) {
+ 					ast_cli(a->fd, "  S-Timer Interval:       %d\n", cur->stimer->st_interval);
+ 					ast_cli(a->fd, "  S-Timer Refresher:      %s\n", strefresher2str(cur->stimer->st_ref));
+ 					ast_cli(a->fd, "  S-Timer Expirys:        %d\n", cur->stimer->st_expirys);
+ 					ast_cli(a->fd, "  S-Timer Sched Id:       %d\n", cur->stimer->st_schedid);
+ 					ast_cli(a->fd, "  S-Timer Peer Sts:       %s\n", cur->stimer->st_active_peer_ua ? "Active" : "Inactive");
+ 					ast_cli(a->fd, "  S-Timer Cached Min-SE:  %d\n", cur->stimer->st_cached_min_se);
+ 					ast_cli(a->fd, "  S-Timer Cached SE:      %d\n", cur->stimer->st_cached_max_se);
+ 					ast_cli(a->fd, "  S-Timer Cached Ref:     %s\n", strefresher2str(cur->stimer->st_cached_ref));
+ 					ast_cli(a->fd, "  S-Timer Cached Mode:    %s\n", stmode2str(cur->stimer->st_cached_mode));
+ 				}
+			}
+
 			ast_cli(a->fd, "\n\n");
+
 			found++;
 		}
 	}
@@ -13457,7 +13814,7 @@ static void check_pendings(struct sip_pvt *p)
 		} else {
 			ast_debug(2, "Sending pending reinvite on '%s'\n", p->callid);
 			/* Didn't get to reinvite yet, so do it now */
-			transmit_reinvite_with_sdp(p, FALSE);
+			transmit_reinvite_with_sdp(p, FALSE, FALSE);
 			ast_clear_flag(&p->flags[0], SIP_NEEDREINVITE);	
 		}
 	}
@@ -13485,6 +13842,8 @@ static void handle_response_invite(struct sip_pvt *p, int resp, char *rest, stru
 	int xmitres = 0;
 	int reinvite = (p->owner && p->owner->_state == AST_STATE_UP);
 	struct ast_channel *bridgepeer = NULL;
+	char *p_hdrval;
+	int rtn;
 	
 	if (reinvite)
 		ast_debug(4, "SIP response %d to RE-invite on %s call %s\n", resp, outgoing ? "outgoing" : "incoming", p->callid);
@@ -13657,6 +14016,39 @@ static void handle_response_invite(struct sip_pvt *p, int resp, char *rest, stru
 			if (!req->ignore)
 				ast_set_flag(&p->flags[0], SIP_PENDINGBYE);	
 		}
+
+		/* Check for Session-Timers related headers */
+		if (st_get_mode(p) != SESSION_TIMER_MODE_REFUSE && p->outgoing_call == TRUE && !reinvite) {
+			p_hdrval = (char*)get_header(req, "Session-Expires");
+        		if (!ast_strlen_zero(p_hdrval)) {
+				/* UAS supports Session-Timers */
+				enum st_refresher tmp_st_ref = SESSION_TIMER_REFRESHER_AUTO;
+				int tmp_st_interval = 0;
+				rtn = parse_session_expires(p_hdrval, &tmp_st_interval, &tmp_st_ref);
+				if (rtn != 0) {
+					ast_set_flag(&p->flags[0], SIP_PENDINGBYE);	
+				}
+				if (tmp_st_ref == SESSION_TIMER_REFRESHER_UAC || 
+					tmp_st_ref == SESSION_TIMER_REFRESHER_UAS) {
+					p->stimer->st_ref = tmp_st_ref;
+				} 
+				if (tmp_st_interval) {
+					p->stimer->st_interval = tmp_st_interval;
+				}
+				p->stimer->st_active = TRUE;
+				p->stimer->st_active_peer_ua = TRUE;
+				start_session_timer(p);
+			} else {
+				/* UAS doesn't support Session-Timers */
+				if (st_get_mode(p) == SESSION_TIMER_MODE_ORIGINATE) {
+					p->stimer->st_ref = SESSION_TIMER_REFRESHER_UAC;
+					p->stimer->st_active_peer_ua = FALSE;
+					start_session_timer(p);
+				}
+			}
+		}
+
+
 		/* If I understand this right, the branch is different for a non-200 ACK only */
 		p->invitestate = INV_TERMINATED;
 		xmitres = transmit_request(p, SIP_ACK, seqno, XMIT_UNRELIABLE, TRUE);
@@ -13711,6 +14103,13 @@ static void handle_response_invite(struct sip_pvt *p, int resp, char *rest, stru
 			ast_queue_control(p->owner, AST_CONTROL_CONGESTION);
 		sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
 		break;
+
+	case 422: /* Session-Timers: Session interval too small */
+		xmitres = transmit_request(p, SIP_ACK, seqno, XMIT_UNRELIABLE, FALSE);
+		ast_string_field_set(p, theirtag, NULL);
+		proc_422_rsp(p, req);
+		break;
+
 	case 487: /* Cancelled transaction */
 		/* We have sent CANCEL on an outbound INVITE 
 			This transaction is already scheduled to be killed by sip_hangup().
@@ -14214,6 +14613,13 @@ static void handle_response(struct sip_pvt *p, int resp, char *rest, struct sip_
 				p->needdestroy = 1;
 			}
 			break;
+
+		case 422: /* Session-Timers: Session Interval Too Small */
+			if (sipmethod == SIP_INVITE) {
+				handle_response_invite(p, resp, rest, req, seqno);
+			}
+			break;
+
 		case 481: /* Call leg does not exist */
 			if (sipmethod == SIP_INVITE) {
 				handle_response_invite(p, resp, rest, req, seqno);
@@ -14936,7 +15342,7 @@ static int handle_invite_replaces(struct sip_pvt *p, struct sip_request *req, in
 		/* We should answer something here. If we are here, the
 			call we are replacing exists, so an accepted 
 			can't harm */
-		transmit_response_with_sdp(p, "200 OK", req, XMIT_RELIABLE);
+		transmit_response_with_sdp(p, "200 OK", req, XMIT_RELIABLE, FALSE);
 		/* Do something more clever here */
 		ast_channel_unlock(c);
 		sip_pvt_unlock(p->refer->refer_call);
@@ -14970,7 +15376,7 @@ static int handle_invite_replaces(struct sip_pvt *p, struct sip_request *req, in
 	   Targetcall is not touched by the masq */
 
 	/* Answer the incoming call and set channel to UP state */
-	transmit_response_with_sdp(p, "200 OK", req, XMIT_RELIABLE);
+	transmit_response_with_sdp(p, "200 OK", req, XMIT_RELIABLE, FALSE);
 		
 	ast_setstate(c, AST_STATE_UP);
 	
@@ -15069,6 +15475,17 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 	unsigned int required_profile = 0;
 	struct ast_channel *c = NULL;		/* New channel */
 	int reinvite = 0;
+	int rtn;
+
+	const char *p_uac_se_hdr;       /* UAC's Session-Expires header string                      */
+	const char *p_uac_min_se;       /* UAC's requested Min-SE interval (char string)            */
+	int uac_max_se = -1;            /* UAC's Session-Expires in integer format                  */
+	int uac_min_se = -1;            /* UAC's Min-SE in integer format                           */
+	int st_active = FALSE;          /* Session-Timer on/off boolean                             */
+	int st_interval = 0;            /* Session-Timer negotiated refresh interval                */
+	enum st_refresher st_ref;       /* Session-Timer session refresher                          */
+	int dlg_min_se = -1;
+	st_ref = SESSION_TIMER_REFRESHER_AUTO;
 
 	/* Find out what they support */
 	if (!p->sipoptions) {
@@ -15081,8 +15498,8 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 	required = get_header(req, "Require");
 	if (!ast_strlen_zero(required)) {
 		required_profile = parse_sip_options(NULL, required);
-		if (required_profile && required_profile != SIP_OPT_REPLACES) {
-			/* At this point we only support REPLACES */
+		if (required_profile && required_profile != SIP_OPT_REPLACES && required_profile != SIP_OPT_TIMER) {
+			/* At this point we only support REPLACES and Session-Timer */
 			transmit_response_with_unsupported(p, "420 Bad extension (unsupported)", req, required);
 			ast_log(LOG_WARNING,"Received SIP INVITE with unsupported required extension: %s\n", required);
 			p->invitestate = INV_COMPLETED;
@@ -15092,6 +15509,11 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 		}
 	}
 
+	/* The option tags may be present in Supported: or Require: headers.
+	Include the Require: option tags for further processing as well */
+	p->sipoptions |= required_profile;
+	p->reqsipoptions = required_profile;
+
 	/* Check if this is a loop */
 	if (ast_test_flag(&p->flags[0], SIP_OUTGOING) && p->owner && (p->owner->_state != AST_STATE_UP)) {
 		/* This is a call to ourself.  Send ourselves an error code and stop
@@ -15218,7 +15640,6 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 		}
 	}
 
-
 	/* Check if this is an INVITE that sets up a new dialog or
 	   a re-invite in an existing dialog */
 
@@ -15343,6 +15764,7 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 			sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
 			return 0;
 		} else {
+
 			/* If no extension was specified, use the s one */
 			/* Basically for calling to IP/Host name only */
 			if (ast_strlen_zero(p->exten))
@@ -15369,10 +15791,141 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 			else
 				ast_debug(2, "Got a SIP re-transmit of INVITE for call %s\n", p->callid);
 		}
+
 		reinvite = 1;
 		c = p->owner;
 	}
 
+	/* Session-Timers */
+	if (p->sipoptions == SIP_OPT_TIMER) {
+		/* The UAC has requested session-timers for this session. Negotiate
+		the session refresh interval and who will be the refresher */
+		ast_debug(2, "Incoming INVITE with 'timer' option enabled\n");
+
+		/* Allocate Session-Timers struct w/in the dialog */
+		if (!p->stimer)
+			sip_st_alloc(p);
+
+		/* Parse the Session-Expires header */
+		p_uac_se_hdr = get_header(req, "Session-Expires");
+		if (!ast_strlen_zero(p_uac_se_hdr)) {
+			rtn = parse_session_expires(p_uac_se_hdr, &uac_max_se, &st_ref);
+			if (rtn != 0) {
+				transmit_response(p, "400 Session-Expires Invalid Syntax", req);
+				p->invitestate = INV_COMPLETED;
+				if (!p->lastinvite) {
+					sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
+				}
+				return -1;
+			}
+		}
+
+		/* Parse the Min-SE header */
+		p_uac_min_se = get_header(req, "Min-SE");
+		if (!ast_strlen_zero(p_uac_min_se)) {
+			rtn = parse_minse(p_uac_min_se, &uac_min_se); 
+			if (rtn != 0) {
+        			transmit_response(p, "400 Min-SE Invalid Syntax", req);
+       	   			p->invitestate = INV_COMPLETED;
+       	   			if (!p->lastinvite) {
+					sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
+				}
+				return -1;
+			}
+		}
+
+		dlg_min_se = st_get_se(p, FALSE);
+		switch (st_get_mode(p)) {
+		case SESSION_TIMER_MODE_ACCEPT:
+		case SESSION_TIMER_MODE_ORIGINATE:
+			if (uac_max_se > 0 && uac_max_se < dlg_min_se) {
+				transmit_response_with_minse(p, "422 Session Interval Too Small", req, dlg_min_se);
+				p->invitestate = INV_COMPLETED;
+				if (!p->lastinvite) {
+					sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
+				}
+				return -1;
+			}
+
+			p->stimer->st_active_peer_ua = TRUE;
+			st_active = TRUE;
+			if (st_ref == SESSION_TIMER_REFRESHER_AUTO) {
+				st_ref = st_get_refresher(p);
+			}
+
+			if (uac_max_se > 0) {
+				int dlg_max_se = st_get_se(p, TRUE);
+				if (dlg_max_se >= uac_min_se) {
+					st_interval = (uac_max_se < dlg_max_se) ? uac_max_se : dlg_max_se;
+				} else {
+					st_interval = uac_max_se;
+				}
+			} else {
+				st_interval = uac_min_se;
+			}
+			break;
+
+		case SESSION_TIMER_MODE_REFUSE:
+			if (p->reqsipoptions == SIP_OPT_TIMER) {
+				transmit_response_with_unsupported(p, "420 Option Disabled", req, required);
+				ast_log(LOG_WARNING,"Received SIP INVITE with supported but disabled option: %s\n", required);
+				p->invitestate = INV_COMPLETED;
+				if (!p->lastinvite) {
+					sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
+				}
+				return -1;
+			}
+			break;
+
+		default:
+			ast_log(LOG_ERROR, "Internal Error %d at %s:%d\n", st_get_mode(p), __FILE__, __LINE__);
+			break;
+		}
+	} else {
+		/* The UAC did not request session-timers.  Asterisk (UAS), will now decide
+		(based on session-timer-mode in sip.conf) whether to run session-timers for
+		this session or not. */
+		switch (st_get_mode(p)) {
+		case SESSION_TIMER_MODE_ORIGINATE:
+			st_active = TRUE;
+			st_interval = st_get_se(p, TRUE);
+			st_ref = SESSION_TIMER_REFRESHER_UAS;
+			p->stimer->st_active_peer_ua = FALSE;
+			break;
+
+		default:
+			break;
+		}
+	}
+
+	if (reinvite == 0) {
+		/* Session-Timers: Start session refresh timer based on negotiation/config */
+		if (st_active == TRUE) {
+			p->stimer->st_active   = TRUE;
+			p->stimer->st_interval = st_interval;
+			p->stimer->st_ref      = st_ref;
+			start_session_timer(p);
+		}
+	} else {
+		if (p->stimer->st_active == TRUE) {
+			/* Session-Timers:  A re-invite request sent within a dialog will serve as 
+			a refresh request, no matter whether the re-invite was sent for refreshing 
+			the session or modifying it.*/
+			ast_debug (2, "Restarting session-timers on a refresh - %s\n", p->callid);
+
+			/* The UAC may be adjusting the session-timers mid-session */
+			if (st_interval > 0) {
+				p->stimer->st_interval = st_interval;
+				p->stimer->st_ref      = st_ref;
+			}
+
+			restart_session_timer(p);
+			if (p->stimer->st_expirys > 0) {
+				p->stimer->st_expirys--;
+			}
+		}
+	}
+
 	if (!req->ignore && p)
 		p->lastinvite = seqno;
 
@@ -15537,9 +16090,10 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 					}
 				} 
 				/* Respond to normal re-invite */
-				if (sendok)
+				if (sendok) {
 					/* If this is not a re-invite or something to ignore - it's critical */
-					transmit_response_with_sdp(p, "200 OK", req, (reinvite || req->ignore) ?  XMIT_UNRELIABLE : XMIT_CRITICAL);
+					transmit_response_with_sdp(p, "200 OK", req, (reinvite || req->ignore) ?  XMIT_UNRELIABLE : XMIT_CRITICAL, p->session_modify == TRUE ? FALSE:TRUE); 
+				}
 			}
 			p->invitestate = INV_TERMINATED;
 			break;
@@ -16188,6 +16742,7 @@ static int handle_request_bye(struct sip_pvt *p, struct sip_request *req)
 	}
 
 	stop_media_flows(p); /* Immediately stop RTP, VRTP and UDPTL as applicable */
+    stop_session_timer(p); /* Stop Session-Timer */
 
 	if (!ast_strlen_zero(get_header(req, "Also"))) {
 		ast_log(LOG_NOTICE, "Client '%s' using deprecated BYE/Also transfer method.  Ask vendor to support REFER instead\n",
@@ -17159,6 +17714,331 @@ static int restart_monitor(void)
 	return 0;
 }
 
+
+/*! \brief Session-Timers: Restart session timer */
+static void restart_session_timer(struct sip_pvt *p)
+{
+	if (!p->stimer) {
+		ast_log(LOG_WARNING, "Null stimer in restart_session_timer - %s\n", p->callid);
+		return;
+	}
+
+	if (p->stimer->st_active == TRUE) {
+		if (ast_sched_del(sched, p->stimer->st_schedid) != 0) {
+			ast_log(LOG_WARNING, "ast_sched_del failed: %d - %s\n", p->stimer->st_schedid, p->callid);
+		}
+
+		ast_debug(2, "Session timer stopped: %d - %s\n", p->stimer->st_schedid, p->callid);
+		start_session_timer(p);
+	}
+}
+
+
+/*! \brief Session-Timers: Stop session timer */
+static void stop_session_timer(struct sip_pvt *p)
+{
+	if (!p->stimer) {
+		ast_log(LOG_WARNING, "Null stimer in stop_session_timer - %s\n", p->callid);
+		return;
+	}
+
+	if (p->stimer->st_active == TRUE) {
+		p->stimer->st_active = FALSE;
+		ast_sched_del(sched, p->stimer->st_schedid);
+		ast_debug(2, "Session timer stopped: %d - %s\n", p->stimer->st_schedid, p->callid);
+	}
+}
+
+
+/*! \brief Session-Timers: Start session timer */
+static void start_session_timer(struct sip_pvt *p)
+{
+	if (!p->stimer) {
+		ast_log(LOG_WARNING, "Null stimer in start_session_timer - %s\n", p->callid);
+		return;
+	}
+
+	p->stimer->st_schedid  = ast_sched_add(sched, p->stimer->st_interval * 1000 / 2, proc_session_timer, p);
+	if (p->stimer->st_schedid < 0) {
+		ast_log(LOG_ERROR, "ast_sched_add failed.\n");
+	}
+	ast_debug(2, "Session timer started: %d - %s\n", p->stimer->st_schedid, p->callid);
+}
+
+
+/*! \brief Session-Timers: Process session refresh timeout event */
+static int proc_session_timer(const void *vp)
+{
+	struct sip_pvt *p = (struct sip_pvt *) vp;
+	int sendreinv = FALSE;
+
+	if (!p->stimer) {
+		ast_log(LOG_WARNING, "Null stimer in proc_session_timer - %s\n", p->callid);
+		return 0;
+	}
+
+	ast_debug(2, "Session timer expired: %d - %s\n", p->stimer->st_schedid, p->callid);
+
+	if (!p->owner) {
+		if (p->stimer->st_active == TRUE) {
+			stop_session_timer(p);
+		}
+		return 0;
+	}
+
+	if ((p->stimer->st_active != TRUE) || (p->owner->_state != AST_STATE_UP)) {
+		return 0;
+	}
+
+	switch (p->stimer->st_ref) {
+	case SESSION_TIMER_REFRESHER_UAC:
+		if (p->outgoing_call == TRUE) {
+	  		sendreinv = TRUE;
+		}
+		break;
+	case SESSION_TIMER_REFRESHER_UAS:
+		if (p->outgoing_call != TRUE) {
+  			sendreinv = TRUE;
+		}
+		break;
+	default:
+		ast_log(LOG_ERROR, "Unknown session refresher %d\n", p->stimer->st_ref);
+		return -1;
+	}
+
+	if (sendreinv == TRUE) {
+		transmit_reinvite_with_sdp(p, FALSE, TRUE);
+	} else {
+		p->stimer->st_expirys++;
+		if (p->stimer->st_expirys >= 2) {
+			ast_log(LOG_WARNING, "Session-Timer expired - %s\n", p->callid);
+			stop_session_timer(p);
+
+			while (p->owner && ast_channel_trylock(p->owner)) {
+				sip_pvt_unlock(p);
+				usleep(1);
+				sip_pvt_lock(p);
+          		}
+
+           		ast_softhangup_nolock(p->owner, AST_SOFTHANGUP_DEV);
+           		ast_channel_unlock(p->owner);
+		}
+	}
+	return 1;
+}
+
+
+/* Session-Timers: Function for parsing Min-SE header */
+int parse_minse (const char *p_hdrval, int *const p_interval)
+{
+	if (ast_strlen_zero(p_hdrval)) {
+		ast_log(LOG_WARNING, "Null Min-SE header\n");
+		return -1;
+	}
+
+	*p_interval = 0;
+	p_hdrval = ast_skip_blanks(p_hdrval);
+	if (!sscanf(p_hdrval, "%d", p_interval)) {
+		ast_log(LOG_WARNING, "Parsing of Min-SE header failed %s\n", p_hdrval);
+		return -1;
+	}
+
+	ast_debug(2, "Received Min-SE: %d\n", *p_interval);
+	return 0;
+}
+
+
+/* Session-Timers: Function for parsing Session-Expires header */
+int parse_session_expires(const char *p_hdrval, int *const p_interval, enum st_refresher *const p_ref)
+{
+	char *p_token;
+	int  ref_idx;
+	char *p_se_hdr;
+
+	if (ast_strlen_zero(p_hdrval)) {
+		ast_log(LOG_WARNING, "Null Session-Expires header\n");
+		return -1;
+	}
+
+	*p_ref = SESSION_TIMER_REFRESHER_AUTO;
+	*p_interval = 0;
+
+	p_se_hdr = ast_strdupa(p_hdrval);
+	p_se_hdr = ast_skip_blanks(p_se_hdr);
+
+	while ((p_token = strsep(&p_se_hdr, ";"))) {
+		p_token = ast_skip_blanks(p_token);
+		if (!sscanf(p_token, "%d", p_interval)) {
+			ast_log(LOG_WARNING, "Parsing of Session-Expires failed\n");
+			return -1;
+		}
+
+		ast_debug(2, "Session-Expires: %d\n", *p_interval);
+
+		if (!p_se_hdr)
+			continue;
+		
+		ref_idx = strlen("refresher=");
+		if (!strncasecmp(p_se_hdr, "refresher=", ref_idx)) {
+			p_se_hdr += ref_idx;
+			p_se_hdr = ast_skip_blanks(p_se_hdr);
+
+			if (!strncasecmp(p_se_hdr, "uac", strlen("uac"))) {
+				*p_ref = SESSION_TIMER_REFRESHER_UAC;
+				ast_debug(2, "Refresher: UAC\n");
+			} else if (!strncasecmp(p_se_hdr, "uas", strlen("uas"))) {
+				*p_ref = SESSION_TIMER_REFRESHER_UAS;
+				ast_debug(2, "Refresher: UAS\n");
+			} else {
+				ast_log(LOG_WARNING, "Invalid refresher value %s\n", p_se_hdr);
+				return -1;
+			}
+			break;
+		}
+	}
+	return 0;
+}
+
+
+/*! \brief Handle 422 response to INVITE with session-timer requested
+
+   Session-Timers:   An INVITE originated by Asterisk that asks for session-timers support
+   from the UAS can result into a 422 response. This is how a UAS or an intermediary proxy 
+   server tells Asterisk that the session refresh interval offered by Asterisk is too low 
+   for them.  The proc_422_rsp() function handles a 422 response.  It extracts the Min-SE 
+   header that comes back in 422 and sends a new INVITE accordingly. */
+static void proc_422_rsp(struct sip_pvt *p, struct sip_request *rsp)
+{
+	int rtn;
+	const char *p_hdrval;
+	int minse;
+
+	p_hdrval = get_header(rsp, "Min-SE");
+	if (ast_strlen_zero(p_hdrval)) {
+		ast_log(LOG_WARNING, "422 response without a Min-SE header %s\n", p_hdrval);
+		return;
+	}
+	rtn = parse_minse(p_hdrval, &minse);
+	if (rtn != 0) {
+		ast_log(LOG_WARNING, "Parsing of Min-SE header failed %s\n", p_hdrval);
+		return;
+	}
+	p->stimer->st_interval = minse;
+	transmit_invite(p, SIP_INVITE, 1, 2); 
+}
+
+
+/*! \brief Get Max or Min SE (session timer expiry)
+	\param max if true, get max se, otherwise min se
+*/
+int st_get_se(struct sip_pvt *p, int max)
+{
+	if (max == TRUE) {
+		if (p->stimer->st_cached_max_se) {
+			return p->stimer->st_cached_max_se;
+		} else {
+			if (p->username) {
+				struct sip_user *up = find_user(p->username, 1);
+				if (up) {
+					p->stimer->st_cached_max_se = up->stimer.st_max_se;
+					return (p->stimer->st_cached_max_se);
+				}
+			} 
+			if (p->peername) {
+				struct sip_peer *pp = find_peer(p->peername, NULL, 1);
+				if (pp) {
+					p->stimer->st_cached_max_se = pp->stimer.st_max_se;
+					return (p->stimer->st_cached_max_se);
+				}
+			}
+		}
+		p->stimer->st_cached_max_se = global_max_se;
+		return (p->stimer->st_cached_max_se);
+	} else {
+		if (p->stimer->st_cached_min_se) {
+			return p->stimer->st_cached_min_se;
+		} else {
+			if (p->username) {
+				struct sip_user *up = find_user(p->username, 1);
+				if (up) {
+					p->stimer->st_cached_min_se = up->stimer.st_min_se;
+					return (p->stimer->st_cached_min_se);
+				}
+			} 
+			if (p->peername) {
+				struct sip_peer *pp = find_peer(p->peername, NULL, 1);
+				if (pp) {
+					p->stimer->st_cached_min_se = pp->stimer.st_min_se;
+					return (p->stimer->st_cached_min_se);
+				}
+			}
+		}
+		p->stimer->st_cached_min_se = global_min_se;
+		return (p->stimer->st_cached_min_se);
+	}
+}
+
+
+/*! \brief Get the entity (UAC or UAS) that's acting as the session-timer refresher 
+	\param sip_pvt pointer to the SIP dialog 
+*/
+enum st_refresher st_get_refresher(struct sip_pvt *p)
+{
+	if (p->stimer->st_cached_ref != SESSION_TIMER_REFRESHER_AUTO) 
+		return p->stimer->st_cached_ref;
+
+	if (p->username) {
+		struct sip_user *up = find_user(p->username, 1);
+		if (up) {
+			p->stimer->st_cached_ref = up->stimer.st_ref;
+			return up->stimer.st_ref;
+		}
+	} 
+
+	if (p->peername) {
+		struct sip_peer *pp = find_peer(p->peername, NULL, 1);
+		if (pp) {
+			p->stimer->st_cached_ref = pp->stimer.st_ref;
+			return pp->stimer.st_ref;
+		}
+	}
+	
+	p->stimer->st_cached_ref = global_st_refresher;
+	return global_st_refresher;
+}
+
+
+/*! \brief Get the session-timer mode 
+	\param sip_pvt pointer to the SIP dialog 
+*/
+enum st_mode st_get_mode(struct sip_pvt *p)
+{
+	if (!p->stimer) 
+		sip_st_alloc(p);
+
+	if (p->stimer->st_cached_mode != SESSION_TIMER_MODE_INVALID) 
+		return p->stimer->st_cached_mode;
+
+	if (p->username) {
+		struct sip_user *up = find_user(p->username, 1);
+		if (up) {
+			p->stimer->st_cached_mode = up->stimer.st_mode_oper;
+			return up->stimer.st_mode_oper;
+		}
+	} 
+	if (p->peername) {
+		struct sip_peer *pp = find_peer(p->peername, NULL, 1);
+		if (pp) {
+			p->stimer->st_cached_mode = pp->stimer.st_mode_oper;
+			return pp->stimer.st_mode_oper;
+		}
+	}
+
+	p->stimer->st_cached_mode = global_st_mode;
+	return global_st_mode;
+}
+
+
 /*! \brief React to lack of answer to Qualify poke */
 static int sip_poke_noanswer(const void *data)
 {
@@ -17191,7 +18071,7 @@ static int sip_poke_peer(struct sip_peer *peer)
 
 	if (!peer->maxms || !peer->addr.sin_addr.s_addr) {
 		/* IF we have no IP, or this isn't to be monitored, return
-		  imeediately after clearing things out */
+		  immediately after clearing things out */
 		if (peer->pokeexpire > -1)
 			ast_sched_del(sched, peer->pokeexpire);
 		peer->lastms = 0;
@@ -17798,6 +18678,11 @@ static struct sip_user *build_user(const char *name, struct ast_variable *v, int
 	if (global_callcounter)
 		user->call_limit=999;
 	user->prefs = default_prefs;
+	user->stimer.st_mode_oper = global_st_mode;	/* Session-Timers */
+	user->stimer.st_ref = global_st_refresher;
+	user->stimer.st_min_se = global_min_se;
+	user->stimer.st_max_se = global_max_se;
+
 	/* set default context */
 	strcpy(user->context, default_context);
 	strcpy(user->language, default_language);
@@ -17875,7 +18760,38 @@ static struct sip_user *build_user(const char *name, struct ast_variable *v, int
 			user->maxcallbitrate = atoi(v->value);
 			if (user->maxcallbitrate < 0)
 				user->maxcallbitrate = default_maxcallbitrate;
+		} else if (!strcasecmp(v->name, "session-timers")) {
+			int i = (int) str2stmode(v->value); 
+			if (i < 0) {
+				ast_log(LOG_WARNING, "Invalid session-timers '%s' at line %d of %s\n", v->value, v->lineno, config);
+				user->stimer.st_mode_oper = global_st_mode;
+			} else {
+				user->stimer.st_mode_oper = i;
+			}
+		} else if (!strcasecmp(v->name, "session-expires")) {
+			if (sscanf(v->value, "%d", &user->stimer.st_max_se) != 1) {
+				ast_log(LOG_WARNING, "Invalid session-expires '%s' at line %d of %s\n", v->value, v->lineno, config);
+				user->stimer.st_max_se = global_max_se;
+			} 
+		} else if (!strcasecmp(v->name, "session-minse")) {
+			if (sscanf(v->value, "%d", &user->stimer.st_min_se) != 1) {
+				ast_log(LOG_WARNING, "Invalid session-minse '%s' at line %d of %s\n", v->value, v->lineno, config);
+				user->stimer.st_min_se = global_min_se;
+			} 
+			if (user->stimer.st_min_se < 90) {
+				ast_log(LOG_WARNING, "session-minse '%s' at line %d of %s is not allowed to be < 90 secs\n", v->value, v->lineno, config);
+				user->stimer.st_min_se = global_min_se;
+			} 
+		} else if (!strcasecmp(v->name, "session-refresher")) {
+			int i = (int) str2strefresher(v->value); 
+			if (i < 0) {
+				ast_log(LOG_WARNING, "Invalid session-refresher '%s' at line %d of %s\n", v->value, v->lineno, config);
+				user->stimer.st_ref = global_st_refresher;
+			} else {
+				user->stimer.st_ref = i;
+			}
 		}
+
 		/* We can't just report unknown options here because this may be a
 		 * type=friend entry.  All user options are valid for a peer, but not
 		 * the other way around.  */
@@ -17930,6 +18846,11 @@ static void set_peer_defaults(struct sip_peer *peer)
 	peer->pickupgroup = 0;
 	peer->maxms = default_qualify;
 	peer->prefs = default_prefs;
+	peer->stimer.st_mode_oper = global_st_mode;	/* Session-Timers */
+	peer->stimer.st_ref = global_st_refresher;
+	peer->stimer.st_min_se = global_min_se;
+	peer->stimer.st_max_se = global_max_se;
+
 	peer->timer_t1 = global_t1;
 	peer->timer_b = global_timer_b;
 	clear_peer_mailboxes(peer);
@@ -18240,6 +19161,36 @@ static struct sip_peer *build_peer(const char *name, struct ast_variable *v, str
 			peer->maxcallbitrate = atoi(v->value);
 			if (peer->maxcallbitrate < 0)
 				peer->maxcallbitrate = default_maxcallbitrate;
+		} else if (!strcasecmp(v->name, "session-timers")) {
+			int i = (int) str2stmode(v->value); 
+			if (i < 0) {
+				ast_log(LOG_WARNING, "Invalid session-timers '%s' at line %d of %s\n", v->value, v->lineno, config);
+				peer->stimer.st_mode_oper = global_st_mode;
+			} else {
+				peer->stimer.st_mode_oper = i;
+			}
+		} else if (!strcasecmp(v->name, "session-expires")) {
+			if (sscanf(v->value, "%d", &peer->stimer.st_max_se) != 1) {
+				ast_log(LOG_WARNING, "Invalid session-expires '%s' at line %d of %s\n", v->value, v->lineno, config);
+				peer->stimer.st_max_se = global_max_se;
+			} 
+		} else if (!strcasecmp(v->name, "session-minse")) {
+			if (sscanf(v->value, "%d", &peer->stimer.st_min_se) != 1) {
+				ast_log(LOG_WARNING, "Invalid session-minse '%s' at line %d of %s\n", v->value, v->lineno, config);
+				peer->stimer.st_min_se = global_min_se;
+			} 
+			if (peer->stimer.st_min_se < 90) {
+				ast_log(LOG_WARNING, "session-minse '%s' at line %d of %s is not allowed to be < 90 secs\n", v->value, v->lineno, config);
+				peer->stimer.st_min_se = global_min_se;
+			} 
+		} else if (!strcasecmp(v->name, "session-refresher")) {
+			int i = (int) str2strefresher(v->value); 
+			if (i < 0) {
+				ast_log(LOG_WARNING, "Invalid session-refresher '%s' at line %d of %s\n", v->value, v->lineno, config);
+				peer->stimer.st_ref = global_st_refresher;
+			} else {
+				peer->stimer.st_ref = i;
+			}
 		}
 	}
 	if (!sip_cfg.ignore_regexpire && peer->host_dynamic && realtime) {
@@ -18423,6 +19374,11 @@ static int reload_config(enum channelreloadreason reason)
 	ast_set_flag(&global_flags[1], SIP_PAGE2_ALLOWOVERLAP);		/* Default for peers, users: TRUE */
 	sip_cfg.peer_rtupdate = TRUE;
 
+	global_st_mode = SESSION_TIMER_MODE_ACCEPT;    /* Session-Timers */
+	global_st_refresher = SESSION_TIMER_REFRESHER_UAS;
+	global_min_se  = DEFAULT_MIN_SE;
+	global_max_se  = DEFAULT_MAX_SE;
+
 	/* Initialize some reasonable defaults at SIP reload (used both for channel and as default for peers and users */
 	ast_copy_string(default_context, DEFAULT_CONTEXT, sizeof(default_context));
 	default_subscribecontext[0] = '\0';
@@ -18733,6 +19689,36 @@ static int reload_config(enum channelreloadreason reason)
 				default_maxcallbitrate = DEFAULT_MAX_CALL_BITRATE;
 		} else if (!strcasecmp(v->name, "matchexterniplocally")) {
 			global_matchexterniplocally = ast_true(v->value);
+		} else if (!strcasecmp(v->name, "session-timers")) {
+			int i = (int) str2stmode(v->value); 
+			if (i < 0) {
+				ast_log(LOG_WARNING, "Invalid session-timers '%s' at line %d of %s\n", v->value, v->lineno, config);
+				global_st_mode = SESSION_TIMER_MODE_ACCEPT;
+			} else {
+				global_st_mode = i;
+			}
+		} else if (!strcasecmp(v->name, "session-expires")) {
+			if (sscanf(v->value, "%d", &global_max_se) != 1) {
+				ast_log(LOG_WARNING, "Invalid session-expires '%s' at line %d of %s\n", v->value, v->lineno, config);
+				global_max_se = DEFAULT_MAX_SE;
+			} 
+		} else if (!strcasecmp(v->name, "session-minse")) {
+			if (sscanf(v->value, "%d", &global_min_se) != 1) {
+				ast_log(LOG_WARNING, "Invalid session-minse '%s' at line %d of %s\n", v->value, v->lineno, config);
+				global_min_se = DEFAULT_MIN_SE;
+			} 
+			if (global_min_se < 90) {
+				ast_log(LOG_WARNING, "session-minse '%s' at line %d of %s is not allowed to be < 90 secs\n", v->value, v->lineno, config);
+				global_min_se = DEFAULT_MIN_SE;
+			} 
+		} else if (!strcasecmp(v->name, "session-refresher")) {
+			int i = (int) str2strefresher(v->value); 
+			if (i < 0) {
+				ast_log(LOG_WARNING, "Invalid session-refresher '%s' at line %d of %s\n", v->value, v->lineno, config);
+				global_st_refresher = SESSION_TIMER_REFRESHER_UAS;
+			} else {
+				global_st_refresher = i;
+			}
 		}
 	}
 
@@ -18962,7 +19948,7 @@ static int sip_set_udptl_peer(struct ast_channel *chan, struct ast_udptl *udptl)
 	if (!ast_test_flag(&p->flags[0], SIP_GOTREFER)) {
 		if (!p->pendinginvite) {
 			ast_debug(3, "Sending reinvite on SIP '%s' - It's UDPTL soon redirected to IP %s:%d\n", p->callid, ast_inet_ntoa(udptl ? p->udptlredirip.sin_addr : p->ourip.sin_addr), udptl ? ntohs(p->udptlredirip.sin_port) : 0);
-			transmit_reinvite_with_sdp(p, TRUE);
+			transmit_reinvite_with_sdp(p, TRUE, FALSE);
 		} else if (!ast_test_flag(&p->flags[0], SIP_PENDINGBYE)) {
 			ast_debug(3, "Deferring reinvite on SIP '%s' - It's UDPTL will be redirected to IP %s:%d\n", p->callid, ast_inet_ntoa(udptl ? p->udptlredirip.sin_addr : p->ourip.sin_addr), udptl ? ntohs(p->udptlredirip.sin_port) : 0);
 			ast_set_flag(&p->flags[0], SIP_NEEDREINVITE);
@@ -19017,7 +20003,7 @@ static int sip_handle_t38_reinvite(struct ast_channel *chan, struct sip_pvt *pvt
 					ast_debug(3, "Sending reinvite on SIP '%s' - It's UDPTL soon redirected to IP %s:%d\n", p->callid, ast_inet_ntoa(p->udptlredirip.sin_addr), ntohs(p->udptlredirip.sin_port));
 				else
 					ast_debug(3, "Sending reinvite on SIP '%s' - It's UDPTL soon redirected to us (IP %s)\n", p->callid, ast_inet_ntoa(p->ourip.sin_addr));
-				transmit_reinvite_with_sdp(p, TRUE);
+				transmit_reinvite_with_sdp(p, TRUE, FALSE);
 			} else if (!ast_test_flag(&p->flags[0], SIP_PENDINGBYE)) {
 				if (flag)
 					ast_debug(3, "Deferring reinvite on SIP '%s' - It's UDPTL will be redirected to IP %s:%d\n", p->callid, ast_inet_ntoa(p->udptlredirip.sin_addr), ntohs(p->udptlredirip.sin_port));
@@ -19197,7 +20183,7 @@ static int sip_set_rtp_peer(struct ast_channel *chan, struct ast_rtp *rtp, struc
 			ast_debug(1, "Early remote bridge setting SIP '%s' - Sending media to %s\n", p->callid, ast_inet_ntoa(rtp ? p->redirip.sin_addr : p->ourip.sin_addr));
 		} else if (!p->pendinginvite) {		/* We are up, and have no outstanding invite */
 			ast_debug(3, "Sending reinvite on SIP '%s' - It's audio soon redirected to IP %s\n", p->callid, ast_inet_ntoa(rtp ? p->redirip.sin_addr : p->ourip.sin_addr));
-			transmit_reinvite_with_sdp(p, FALSE);
+			transmit_reinvite_with_sdp(p, FALSE, FALSE);
 		} else if (!ast_test_flag(&p->flags[0], SIP_PENDINGBYE)) {
 			ast_debug(3, "Deferring reinvite on SIP '%s' - It's audio will be redirected to IP %s\n", p->callid, ast_inet_ntoa(rtp ? p->redirip.sin_addr : p->ourip.sin_addr));
 			/* We have a pending Invite. Send re-invite when we're done with the invite */
diff --git a/configs/sip.conf.sample b/configs/sip.conf.sample
index 0a0cebcbf..acca09c6c 100644
--- a/configs/sip.conf.sample
+++ b/configs/sip.conf.sample
@@ -236,6 +236,28 @@ srvlookup=yes			; Enable DNS SRV lookups on outbound calls
 				; when we're on hold (must be > rtptimeout)
 ;rtpkeepalive=<secs>		; Send keepalives in the RTP stream to keep NAT open
 				; (default is off - zero)
+
+;--------------------------- SIP Session-Timers (RFC 4028)------------------------------------
+; SIP Session-Timers provide an end-to-end keep-alive mechanism for active SIP sessions.
+; This mechanism can detect and reclaim SIP channels that do not terminate through normal
+; signaling procedures. Session-Timers can be configured globally or at a user/peer level.
+; The operation of Session-Timers is driven by the following configuration parameters:
+;
+; * session-timers    - Session-Timers feature operates in the following three modes:
+;                            originate : Request and run session-timers always
+;                            accept    : Run session-timers only when requested by other UA
+;                            refuse    : Do not run session timers in any case
+;                       The default mode of operation is 'accept'.
+; * session-expires   - Maximum session refresh interval in seconds. Defaults to 1800 secs.
+; * session-minse     - Minimum session refresh interval in seconds. Defualts to 90 secs.
+; * session-refresher - The session refresher (uac|uas). Defaults to 'uas'.
+;
+;session-timers=originate
+;session-expires=600
+;session-minse=90
+;session-refresher=uas
+
+
 ;--------------------------- SIP DEBUGGING ---------------------------------------------------
 ;sipdebug = yes			; Turn on SIP debugging by default, from
 				; the moment the channel loads this configuration
@@ -616,13 +638,14 @@ srvlookup=yes			; Enable DNS SRV lookups on outbound calls
 ; videosupport		      videosupport
 ; maxcallbitrate	      maxcallbitrate
 ; rfc2833compensate           mailbox
-;			      busylevel
-;                             template
-;                             fromdomain
-;                             regexten
-;                             fromuser
-;                             host
-;                             port
+; session-timers             busylevel
+; session-expires            
+; session-minse              template
+; session-refresher          fromdomain
+;                            regexten
+;                            fromuser
+;                            host
+;                            port
 ;                             qualify
 ;                             defaultip
 ;                             defaultuser
@@ -633,10 +656,15 @@ srvlookup=yes			; Enable DNS SRV lookups on outbound calls
 ;                             rfc2833compensate
 ;                             callbackextension
 ;                             registertrying
+;                             session-timers
+;                             session-expires
+;                             session-minse
+;                             session-refresher
 ;                             timert1
 ;                             timerb
 ;                             qualifyfreq
 
+
 ;[sip_proxy]
 ; For incoming calls only. Example: FWD (Free World Dialup)
 ; We match on IP address of the proxy for incoming calls