--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/sip-in/invite_dup.c	Sat Oct 08 19:31:05 2022 -0800
@@ -0,0 +1,81 @@
+/*
+ * Here we process SIP INVITE retransmissions and/or re-INVITEs
+ * for existing calls.
+ */
+
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <sys/time.h>
+#include <netinet/in.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <strings.h>
+#include <syslog.h>
+#include "../libsip/parse.h"
+#include "../libsip/uas_basic.h"
+#include "../libsip/sdp.h"
+#include "../libsip/out_msg.h"
+#include "call.h"
+
+void
+invite_existing_call(req, ess, sin, call)
+	struct sip_pkt_rx *req;
+	struct uas_parse_hdrs *ess;
+	struct sockaddr_in *sin;
+	struct call *call;
+{
+	struct sip_msg_out resp;
+	int rc;
+
+	if (ess->cseq_num != call->invite_cseq) {
+		start_response_out_msg(&resp, "501 Re-INVITE not supported");
+		rc = add_resp_basic_headers(&resp, ess, req->req_method);
+		if (rc < 0) {
+			syslog(LOG_ERR,
+		"sending 501 Re-INVITE error: response length exceeded");
+			return;
+		}
+		out_msg_finish(&resp);
+		sip_tx_packet(&resp, sin);
+		return;
+	}
+	/* it's a retransmission, not a re-INVITE */
+	switch (call->sip_state) {
+	case SIP_STATE_INVITE_PROC:
+		start_response_out_msg(&resp, "100 Proceeding");
+		fill_invite_resp_from_call(&resp, call);
+		out_msg_finish(&resp);
+		sip_tx_packet(&resp, sin);
+		return;
+	case SIP_STATE_RINGING:
+		start_response_out_msg(&resp, "180 Ringing");
+		fill_invite_resp_from_call(&resp, call);
+		out_msg_finish(&resp);
+		sip_tx_packet(&resp, sin);
+		return;
+	case SIP_STATE_RINGING_REL:
+		start_response_out_msg(&resp, "180 Ringing");
+		fill_invite_resp_from_call(&resp, call);
+		out_msg_add_header(&resp, "Require", "100rel");
+		out_msg_add_header(&resp, "RSeq", "1");
+		out_msg_finish(&resp);
+		sip_tx_packet(&resp, sin);
+		return;
+	case SIP_STATE_INVITE_200:
+	case SIP_STATE_CONNECTED:
+		fill_invite_200_resp(&resp, call);
+		sip_tx_packet(&resp, sin);
+		return;
+	case SIP_STATE_INVITE_ERR:
+		start_response_out_msg(&resp, call->invite_fail);
+		fill_invite_resp_from_call(&resp, call);
+		out_msg_finish(&resp);
+		sip_tx_packet(&resp, sin);
+		return;
+	default:
+		/* silently discard */
+		return;
+	}
+}