--- res/res_format_attr_h264.c	(Asterisk 13.38.1)
+++ res/res_format_attr_h264.c	(working copy)
@@ -39,2 +39,4 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revisi
 #include "asterisk/format.h"
+#include "asterisk/strings.h"           /* for ast_str_append, etc */
+#include "asterisk/utils.h"             /* for MAX, ast_calloc, ast_free, etc */
 
@@ -52,2 +54,6 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revisi
 
+#define H264_DEFAULT_PROFILE_IDC 66  /* 42 */
+#define H264_DEFAULT_PROFILE_IOP 160 /* a0 */
+#define H264_MIN_LEVEL 10            /* 0a */
+
 struct h264_attr {
@@ -133,3 +139,2 @@ static struct ast_format *h264_getjoint(
 	DETERMINE_JOINT(attr, attr1, attr2, PROFILE_IOP);
-	DETERMINE_JOINT(attr, attr1, attr2, LEVEL);
 	DETERMINE_JOINT(attr, attr1, attr2, MAX_MBPS);
@@ -152,2 +157,31 @@ static struct ast_format *h264_getjoint(
 
+	/* https://mailarchive.ietf.org/arch/msg/avt/o-q0iDkaZXZRg1IQUeD67pd4kAo/ */
+	if (!attr->LEVEL_ASYMMETRY_ALLOWED) {
+		if (attr1 && attr2) {
+			/* Level 1b is 0x09 sometimes, see RFC 6184 section 8.2.2:
+			 * Level 1b is one level higher than Level 1.0 */
+			if (9 == attr1->LEVEL ||  9 == attr2->LEVEL) {
+				if (!attr1->LEVEL || 10 == attr1->LEVEL ||
+				    !attr2->LEVEL || 10 == attr2->LEVEL) {
+					attr->LEVEL = 10; /* minimum is Level 1.0 */
+				} else {
+					attr->LEVEL =  9; /* minimum is Level 1b */
+				}
+			/* otherwise minimum level is 1.0; this is forced here again
+			 * because cached formats do not have that as default but 0 */
+			} else {
+				attr->LEVEL = MIN(MAX(attr1->LEVEL,H264_MIN_LEVEL),
+					          MAX(attr2->LEVEL,H264_MIN_LEVEL));
+			}
+			attr1->LEVEL = attr->LEVEL;
+			attr2->LEVEL = attr->LEVEL;
+		} else if (attr1) {
+			attr->LEVEL = attr1->LEVEL;
+		} else if (attr2) {
+			attr->LEVEL = attr2->LEVEL;
+		} else {
+			attr->LEVEL = H264_MIN_LEVEL;
+		}
+	}
+
 	if (attr1 && !ast_strlen_zero(attr1->SPS)) {
@@ -182,3 +216,6 @@ static struct ast_format *h264_parse_sdp
 	attr->PACKETIZATION_MODE = H264_ATTR_KEY_UNSET;
-	attr->LEVEL_ASYMMETRY_ALLOWED = H264_ATTR_KEY_UNSET;
+
+	attr->PROFILE_IDC = H264_DEFAULT_PROFILE_IDC;
+	attr->PROFILE_IOP = H264_DEFAULT_PROFILE_IOP;
+	attr->LEVEL = H264_MIN_LEVEL;
 
@@ -284,5 +321,9 @@ static void h264_generate_sdp_fmtp(const
 	APPEND_IF_NOT_H264_UNSET(attr->PACKETIZATION_MODE, str, "packetization-mode");
-	APPEND_IF_NOT_H264_UNSET(attr->LEVEL_ASYMMETRY_ALLOWED, str, "level-asymmetry-allowed");
 
-	if (attr->PROFILE_IDC && attr->LEVEL) {
+	APPEND_IF_NONZERO(attr->LEVEL_ASYMMETRY_ALLOWED, str, "level-asymmetry-allowed");
+
+	/* cached formats have not the defaults but 0 */
+	if ((attr->PROFILE_IDC && H264_DEFAULT_PROFILE_IDC != attr->PROFILE_IDC) ||
+	    (attr->PROFILE_IOP && H264_DEFAULT_PROFILE_IOP != attr->PROFILE_IOP) ||
+	    (attr->LEVEL && H264_MIN_LEVEL != attr->LEVEL)) { /* could be 0x09 for Level 1b */
 		if (added) {