Support forking SIP calls

When a SIP call is forked, the client will send an INVITE with the
same Call ID and same From tag as an earlier INVITE, but with a
different Branch ID in the topmost Via header.

There were two issues with sofia-sip which prevented call forking from
working; first, its logic for merging SIP requests (as per RFC3261
section 8.2.2.2) did not respect the Branch ID. It would merge requests
with a different Branch ID, which is contrary to RFC3261 section 17.2.3.

Second, sofia-sip has some logic for checking whether incoming requests
are part of an established SIP dialog or not. The matching criteria
were such that incoming INVITE requests forking a call would be
treated as part of the already established dialog, and then an error
would be returned to the client, essentially telling the client that
this new INVITE is invalid because the call is already established.

Therefore, add some extra matching conditions which ensure that an
INVITE forking a call will not be treated as part of the previously
established call leg. The matching conditions are as specific as
possible, to minimize the chances of unintentionally affecting how
other types of SIP messages are handled.

Implementing these new matching conditions can only be done by
recording the Branch ID for established calls, so we can check whether
another INVITE which comes later has the same Branch ID or a different
one. This requires adding a new member to nta_leg_s.

Co-Authored-By: João Arruda <[email protected]>

Author: alexdowad

libsofia-sip-ua/nta/nta.c

@@ -399,6 +399,7 @@ struct nta_leg_s
 
   url_t const      *leg_url;	/**< Match incoming requests. */
   char const       *leg_method;	/**< Match incoming requests. */
+  char const       *leg_branch_id; /**< Match incoming INVITE to identify forked calls */
 
   uint32_t	    leg_seq;    /**< Sequence number for next transaction */
   uint32_t	    leg_rseq;   /**< Remote sequence number */
@@ -685,7 +686,8 @@ static nta_leg_t *leg_find(nta_agent_t const *sa,
 			   url_t const *request_uri,
 			   sip_call_id_t const *i,
 			   char const *from_tag,
-			   char const *to_tag);
+			   char const *to_tag,
+			   char const *branch_id);
 static nta_leg_t *dst_find(nta_agent_t const *sa, url_t const *u0,
 			   char const *method);
 static void leg_recv(nta_leg_t *, msg_t *, sip_t *, tport_t *);
@@ -3134,7 +3136,8 @@ void agent_recv_request(nta_agent_t *agent,
 		      method_name, url,
 		      sip->sip_call_id,
 		      sip->sip_from->a_tag,
-		      sip->sip_to->a_tag))) {
+		      sip->sip_to->a_tag,
+		      sip->sip_via ? sip->sip_via->v_branch : NULL))) {
     /* Try existing dialog */
     SU_DEBUG_5(("nta: %s (%u) %s\n",
 		method_name, cseq, "going to existing leg"));
@@ -4316,6 +4319,7 @@ nta_leg_t *nta_leg_tcreate(nta_agent_t *agent,
   su_home_t *home;
   url_t *url;
   char const *what = NULL;
+  const sip_via_t *via = NULL;
 
   if (agent == NULL)
     return su_seterrno(EINVAL), NULL;
@@ -4333,6 +4337,7 @@ nta_leg_t *nta_leg_tcreate(nta_agent_t *agent,
 	  SIPTAG_TO_REF(to),
 	  SIPTAG_TO_STR_REF(to_str),
 	  SIPTAG_ROUTE_REF(route),
+	  SIPTAG_VIA_REF(via),
 	  NTATAG_TARGET_REF(contact),
 	  NTATAG_REMOTE_CSEQ_REF(rseq),
 	  SIPTAG_CSEQ_REF(cs),
@@ -4353,6 +4358,13 @@ nta_leg_t *nta_leg_tcreate(nta_agent_t *agent,
     return NULL;
   home = leg->leg_home;
 
+  if (via) {
+    SU_DEBUG_9(("nta_leg_tcreate(): setting branch ID to: %s\n", via->v_branch));
+    leg->leg_branch_id = su_strdup(home, via->v_branch);
+  } else {
+    leg->leg_branch_id = NULL;
+  }
+
   leg->leg_agent = agent;
   nta_leg_bind(leg, callback, magic);
 
@@ -4821,12 +4833,12 @@ nta_leg_by_replaces(nta_agent_t *sa, sip_replaces_t const *rp)
 
     id->i_hash = msg_hash_string(id->i_id = rp->rp_call_id);
 
-    leg = leg_find(sa, NULL, NULL, id, from_tag, to_tag);
+    leg = leg_find(sa, NULL, NULL, id, from_tag, to_tag, NULL);
 
     if (leg == NULL && strcmp(from_tag, "0") == 0)
-      leg = leg_find(sa, NULL, NULL, id, NULL, to_tag);
+      leg = leg_find(sa, NULL, NULL, id, NULL, to_tag, NULL);
     if (leg == NULL && strcmp(to_tag, "0") == 0)
-      leg = leg_find(sa, NULL, NULL, id, from_tag, NULL);
+      leg = leg_find(sa, NULL, NULL, id, from_tag, NULL, NULL);
   }
 
   return leg;
@@ -5054,7 +5066,8 @@ nta_leg_t *nta_leg_by_dialog(nta_agent_t const *agent,
 		 NULL, url,
 		 call_id,
 		 remote_tag,
-		 local_tag);
+		 local_tag,
+		 NULL);
 
   if (to_be_freed) su_free(NULL, to_be_freed);
 
@@ -5073,7 +5086,8 @@ nta_leg_t *leg_find(nta_agent_t const *sa,
 		    url_t const *request_uri,
 		    sip_call_id_t const *i,
 		    char const *from_tag,
-		    char const *to_tag)
+		    char const *to_tag,
+		    char const *branch_id)
 {
   hash_value_t hash = i->i_hash;
   leg_htable_t const *lht = sa->sa_dialogs;
@@ -5123,6 +5137,13 @@ nta_leg_t *leg_find(nta_agent_t const *sa,
     if (leg_method && method_name && !su_casematch(method_name, leg_method))
       continue;
 
+    /* Do not match if incoming INVITE To header has no tag AND the topmost Via
+     * Header branch ID in the incoming is different from the existing leg
+     * (this means it's a call being forked)
+     */
+    if (!to_tag && su_casematch(method_name, "INVITE") && branch_id && leg->leg_branch_id && !su_casematch(branch_id, leg->leg_branch_id))
+      continue;
+
     /* Perfect match if both local and To have tag
      * or local does not have tag.
      */
@@ -6238,10 +6259,16 @@ static nta_incoming_t *incoming_find(nta_agent_t const *agent,
 
     /* RFC3261 - section 8.2.2.2 Merged Requests */
     if (return_merge) {
-      if (irq->irq_cseq->cs_method == cseq->cs_method &&
-	  strcmp(irq->irq_cseq->cs_method_name,
-		 cseq->cs_method_name) == 0)
-	*return_merge = irq, return_merge = NULL;
+      /* RFC3261 - section 17.2.3 Matching Requests to Server Transactions */
+      if (irq->irq_via->v_branch &&
+	  su_casematch(irq->irq_via->v_branch, v->v_branch) &&
+	  su_casematch(irq->irq_via->v_host, v->v_host) &&
+	  su_strmatch(irq->irq_via->v_port, v->v_port)) {
+        if (irq->irq_cseq->cs_method == cseq->cs_method &&
+	    strcmp(irq->irq_cseq->cs_method_name,
+		   cseq->cs_method_name) == 0)
+	  *return_merge = irq, return_merge = NULL;
+      }
     }
   }
 

libsofia-sip-ua/nua/nua_stack.c

@@ -1084,6 +1084,7 @@ nua_handle_t *nua_stack_incoming_handle(nua_t *nua,
 				 SIPTAG_CALL_ID(sip->sip_call_id),
 				 SIPTAG_FROM(sip->sip_to),
 				 SIPTAG_TO(sip->sip_from),
+				 SIPTAG_VIA(sip->sip_via),
 				 NTATAG_REMOTE_CSEQ(sip->sip_cseq->cs_seq),
 				 TAG_END());