--- a/libsip/Makefile	Fri Sep 23 19:18:07 2022 -0800
+++ b/libsip/Makefile	Sat Sep 24 15:13:53 2022 -0800
@@ -1,7 +1,7 @@
 CC=	gcc
 CFLAGS=	-O2
-OBJS=	get_header.o grok_from.o out_msg.o primary_parse.o req_supp.o sdp_gen.o\
-	sdp_parse.o to_tag.o uas_basic.o uri_utils.o
+OBJS=	get_header.o grok_from.o out_msg.o primary_parse.o req_supp.o \
+	resp_ident.o sdp_gen.o sdp_parse.o to_tag.o uas_basic.o uri_utils.o
 LIB=	libsip.a
 
 all:	${LIB}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libsip/resp_ident.c	Sat Sep 24 15:13:53 2022 -0800
@@ -0,0 +1,52 @@
+/*
+ * In this module we implement an essential step in the process
+ * of handling incoming SIP response messages: extracting Call-ID
+ * and CSeq headers and preparsing the latter, providing key info
+ * for matching these incoming responses with call state and
+ * with outstanding UAC requests.
+ */
+
+#include <ctype.h>
+#include <string.h>
+#include <strings.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include "parse.h"
+#include "resp_ident.h"
+
+extern char *get_single_header();
+
+sip_resp_extract_ident(msg, id)
+	struct sip_pkt_rx *msg;
+	struct sip_resp_ident *id;
+{
+	char *hval, *cp;
+	int dup_flag = 0;
+
+	hval = get_single_header(msg, "Call-ID", "i", &dup_flag);
+	if (!hval || dup_flag) {
+		id->error_field = "Call-ID";
+		return(-1);
+	}
+	id->call_id = hval;
+	hval = get_single_header(msg, "CSeq", (char *) 0, &dup_flag);
+	if (!hval || dup_flag) {
+bad_cseq:	id->error_field = "CSeq";
+		return(-1);
+	}
+	if (!isdigit(*hval))
+		goto bad_cseq;
+	id->cseq_num = strtoul(hval, &cp, 10);
+	if (!isspace(*cp))
+		goto bad_cseq;
+	while (isspace(*cp))
+		cp++;
+	if (!isupper(*cp))
+		goto bad_cseq;
+	id->cseq_method = cp;
+	while (isalnum(*cp))
+		cp++;
+	if (*cp)
+		goto bad_cseq;
+	return(0);
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libsip/resp_ident.h	Sat Sep 24 15:13:53 2022 -0800
@@ -0,0 +1,13 @@
+/*
+ * Here we define struct sip_resp_ident, a structure that captures
+ * Call-ID and CSeq headers from initial parsing of SIP responses,
+ * allowing these responses to be matched with call state and
+ * with UAC requests that elicited them.
+ */
+
+struct sip_resp_ident {
+	char		*call_id;
+	unsigned	cseq_num;
+	char		*cseq_method;
+	char		*error_field;
+};