diff --git a/CHANGES b/CHANGES
index 9caa524226..a0dffe6841 100644
--- a/CHANGES
+++ b/CHANGES
@@ -63,6 +63,12 @@ ChanSpy
    experienced when using ChanSpy, but may introduce some delay in the audio
    feed on the listening channel.
 
+Codecs
+------------------
+ * Added format attribute negotiation for the iLBC audio codec. Format attribute
+   negotiation is provided by the res_format_attr_ilbc module. iLBC 20 is the
+   default now. Falls back to iLBC 30, when the remote party requests this.
+
 ConfBridge
 ------------------
  * Added the ability to pass options to MixMonitor when recording is used with
diff --git a/codecs/codec_ilbc.c b/codecs/codec_ilbc.c
index 3e480e8fd1..fc713beb77 100644
--- a/codecs/codec_ilbc.c
+++ b/codecs/codec_ilbc.c
@@ -34,10 +34,13 @@
 
 ASTERISK_REGISTER_FILE()
 
-#include "asterisk/translate.h"
+#include "asterisk/codec.h"             /* for AST_MEDIA_TYPE_AUDIO */
+#include "asterisk/format.h"            /* for ast_format_get_attribute_data */
+#include "asterisk/frame.h"             /* for ast_frame, etc */
+#include "asterisk/linkedlists.h"       /* for AST_LIST_NEXT, etc */
+#include "asterisk/logger.h"            /* for ast_log, ast_debug, etc */
 #include "asterisk/module.h"
-#include "asterisk/utils.h"
-#include "asterisk/linkedlists.h"
+#include "asterisk/translate.h"         /* for ast_trans_pvt, etc */
 
 #ifdef ILBC_WEBRTC
 #include <ilbc.h>
@@ -52,13 +55,10 @@ typedef float         ilbc_block;
 #define BUF_TYPE uc
 #endif
 
-#define USE_ILBC_ENHANCER	0
-#define ILBC_MS 			30
-/* #define ILBC_MS			20 */
+#include "asterisk/ilbc.h"
 
-#define	ILBC_FRAME_LEN	50	/* apparently... */
-#define	ILBC_SAMPLES	240	/* 30ms at 8000 hz */
-#define	BUFFER_SAMPLES	8000
+#define USE_ILBC_ENHANCER 0
+#define BUFFER_SAMPLES    8000
 
 /* Sample frame data */
 #include "asterisk/slin.h"
@@ -69,13 +69,16 @@ struct ilbc_coder_pvt {
 	iLBC_Dec_Inst_t dec;
 	/* Enough to store a full second */
 	int16_t buf[BUFFER_SAMPLES];
+	int16_t inited;
 };
 
 static int lintoilbc_new(struct ast_trans_pvt *pvt)
 {
 	struct ilbc_coder_pvt *tmp = pvt->pvt;
+	struct ilbc_attr *attr = pvt->explicit_dst ? ast_format_get_attribute_data(pvt->explicit_dst) : NULL;
+	const unsigned int mode = attr ? attr->mode : 30;
 
-	initEncode(&tmp->enc, ILBC_MS);
+	initEncode(&tmp->enc, mode);
 
 	return 0;
 }
@@ -84,7 +87,7 @@ static int ilbctolin_new(struct ast_trans_pvt *pvt)
 {
 	struct ilbc_coder_pvt *tmp = pvt->pvt;
 
-	initDecode(&tmp->dec, ILBC_MS, USE_ILBC_ENHANCER);
+	tmp->inited = 0; /* we do not know the iLBC mode, yet */
 
 	return 0;
 }
@@ -93,13 +96,19 @@ static int ilbctolin_new(struct ast_trans_pvt *pvt)
 static int ilbctolin_framein(struct ast_trans_pvt *pvt, struct ast_frame *f)
 {
 	struct ilbc_coder_pvt *tmp = pvt->pvt;
+	struct ilbc_attr *attr = ast_format_get_attribute_data(f->subclass.format);
+	const unsigned int mode = attr ? attr->mode : 30;
+	const unsigned int sample_rate = pvt->t->dst_codec.sample_rate;
+	const unsigned int samples_per_frame = mode * sample_rate / 1000;
+	const unsigned int octets_per_frame = (mode == 20) ? 38 : 50;
+
 	int plc_mode = 1; /* 1 = normal data, 0 = plc */
 	/* Assuming there's space left, decode into the current buffer at
 	   the tail location.  Read in as many frames as there are */
 	int x,i;
 	int datalen = f->datalen;
 	int16_t *dst = pvt->outbuf.i16;
-	ilbc_block tmpf[ILBC_SAMPLES];
+	ilbc_block tmpf[samples_per_frame];
 
 	if (!f->data.ptr && datalen) {
 		ast_debug(1, "issue 16070, ILIB ERROR. data = NULL datalen = %d src = %s\n", datalen, f->src ? f->src : "no src set");
@@ -108,27 +117,32 @@ static int ilbctolin_framein(struct ast_trans_pvt *pvt, struct ast_frame *f)
 	}
 
 	if (datalen == 0) { /* native PLC, set fake datalen and clear plc_mode */
-		datalen = ILBC_FRAME_LEN;
-		f->samples = ILBC_SAMPLES;
+		datalen = octets_per_frame;
+		f->samples = samples_per_frame;
 		plc_mode = 0;	/* do native plc */
-		pvt->samples += ILBC_SAMPLES;
+		pvt->samples += samples_per_frame;
 	}
 
-	if (datalen % ILBC_FRAME_LEN) {
-		ast_log(LOG_WARNING, "Huh?  An ilbc frame that isn't a multiple of 50 bytes long from %s (%d)?\n", f->src, datalen);
+	if (datalen % octets_per_frame) {
+		ast_log(LOG_WARNING, "Huh?  An ilbc frame that isn't a multiple of %u bytes long from %s (%d)?\n", octets_per_frame, f->src, datalen);
 		return -1;
 	}
 
-	for (x=0; x < datalen ; x += ILBC_FRAME_LEN) {
-		if (pvt->samples + ILBC_SAMPLES > BUFFER_SAMPLES) {
+	if (!tmp->inited) {
+		initDecode(&tmp->dec, mode, USE_ILBC_ENHANCER);
+		tmp->inited = 1;
+	}
+
+	for (x = 0; x < datalen; x += octets_per_frame) {
+		if (pvt->samples + samples_per_frame > BUFFER_SAMPLES) {
 			ast_log(LOG_WARNING, "Out of buffer space\n");
 			return -1;
 		}
 		iLBC_decode(tmpf, plc_mode ? f->data.ptr + x : NULL, &tmp->dec, plc_mode);
-		for ( i=0; i < ILBC_SAMPLES; i++)
+		for (i = 0; i < samples_per_frame; i++)
 			dst[pvt->samples + i] = tmpf[i];
-		pvt->samples += ILBC_SAMPLES;
-		pvt->datalen += 2*ILBC_SAMPLES;
+		pvt->samples += samples_per_frame;
+		pvt->datalen += samples_per_frame * 2;
 	}
 	return 0;
 }
@@ -155,20 +169,26 @@ static struct ast_frame *lintoilbc_frameout(struct ast_trans_pvt *pvt)
 	struct ast_frame *last = NULL;
 	int samples = 0; /* output samples */
 
-	while (pvt->samples >= ILBC_SAMPLES) {
+	struct ilbc_attr *attr = ast_format_get_attribute_data(pvt->f.subclass.format);
+	const unsigned int mode = attr ? attr->mode : 30;
+	const unsigned int sample_rate = pvt->t->dst_codec.sample_rate;
+	const unsigned int samples_per_frame = mode * sample_rate / 1000;
+	const unsigned int octets_per_frame = (mode == 20) ? 38 : 50;
+
+	while (pvt->samples >= samples_per_frame) {
 		struct ast_frame *current;
-		ilbc_block tmpf[ILBC_SAMPLES];
+		ilbc_block tmpf[samples_per_frame];
 		int i;
 
 		/* Encode a frame of data */
-		for (i = 0 ; i < ILBC_SAMPLES ; i++)
+		for (i = 0; i < samples_per_frame; i++)
 			tmpf[i] = tmp->buf[samples + i];
 		iLBC_encode((ilbc_bytes *) pvt->outbuf.BUF_TYPE, tmpf, &tmp->enc);
 
-		samples += ILBC_SAMPLES;
-		pvt->samples -= ILBC_SAMPLES;
+		samples += samples_per_frame;
+		pvt->samples -= samples_per_frame;
 
-		current = ast_trans_frameout(pvt, ILBC_FRAME_LEN, ILBC_SAMPLES);
+		current = ast_trans_frameout(pvt, octets_per_frame, samples_per_frame);
 		if (!current) {
 			continue;
 		} else if (last) {
@@ -226,7 +246,8 @@ static struct ast_translator lintoilbc = {
 	.frameout = lintoilbc_frameout,
 	.sample = slin8_sample,
 	.desc_size = sizeof(struct ilbc_coder_pvt),
-	.buf_size = (BUFFER_SAMPLES * ILBC_FRAME_LEN + ILBC_SAMPLES - 1) / ILBC_SAMPLES,
+	/* frame len (38 bytes), frame size (160 samples), ceil (+ 160 - 1) */
+	.buf_size = (BUFFER_SAMPLES * 38 + 160 - 1) / 160,
 };
 
 static int unload_module(void)
diff --git a/codecs/ex_ilbc.h b/codecs/ex_ilbc.h
index 3a79b09181..3fe2749dde 100644
--- a/codecs/ex_ilbc.h
+++ b/codecs/ex_ilbc.h
@@ -7,6 +7,9 @@
  *
  */
 
+#include "asterisk/format_cache.h"      /* for ast_format_ilbc */
+#include "asterisk/frame.h"             /* for ast_frame, etc */
+
 static uint8_t ex_ilbc[] = {
 	0xff, 0xa0, 0xff, 0xfa, 0x0f, 0x60, 0x12, 0x11, 0xa2, 0x47, 
 	0x22, 0x8c, 0x00, 0x00, 0x01, 0x02, 0x80, 0x43, 0xa0, 0x40, 
@@ -20,8 +23,8 @@ static struct ast_frame *ilbc_sample(void)
 	static struct ast_frame f = {
 		.frametype = AST_FRAME_VOICE,
 		.datalen = sizeof(ex_ilbc),
-		/* All frames are 30 ms long */
-		.samples = ILBC_SAMPLES,
+		/* example frames are default long (30 ms) */
+		.samples = 240,
 		.mallocd = 0,
 		.offset = 0,
 		.src = __PRETTY_FUNCTION__,
diff --git a/include/asterisk/ilbc.h b/include/asterisk/ilbc.h
new file mode 100644
index 0000000000..2534367cda
--- /dev/null
+++ b/include/asterisk/ilbc.h
@@ -0,0 +1,8 @@
+#ifndef _AST_FORMAT_ILBC_H_
+#define _AST_FORMAT_ILBC_H_
+
+struct ilbc_attr {
+	unsigned int mode;
+};
+
+#endif /* _AST_FORMAT_ILBC_H */
\ No newline at end of file
diff --git a/main/codec_builtin.c b/main/codec_builtin.c
index 50fbf555c0..15147980db 100644
--- a/main/codec_builtin.c
+++ b/main/codec_builtin.c
@@ -31,6 +31,7 @@
 
 ASTERISK_REGISTER_FILE()
 
+#include "asterisk/ilbc.h"
 #include "asterisk/logger.h"
 #include "asterisk/astobj2.h"
 #include "asterisk/codec.h"
@@ -588,7 +589,12 @@ static struct ast_codec speex32 = {
 
 static int ilbc_samples(struct ast_frame *frame)
 {
-	return 240 * (frame->datalen / 50);
+	struct ilbc_attr *attr = ast_format_get_attribute_data(frame->subclass.format);
+	const unsigned int mode = attr ? attr->mode : 30;
+	const unsigned int samples_per_frame = mode * ast_format_get_sample_rate(frame->subclass.format) / 1000;
+	const unsigned int octets_per_frame = (mode == 20) ? 38 : 50;
+
+	return samples_per_frame * frame->datalen / octets_per_frame;
 }
 
 static struct ast_codec ilbc = {
@@ -596,12 +602,12 @@ static struct ast_codec ilbc = {
 	.description = "iLBC",
 	.type = AST_MEDIA_TYPE_AUDIO,
 	.sample_rate = 8000,
-	.minimum_ms = 30,
+	.minimum_ms = 20,
 	.maximum_ms = 300,
-	.default_ms = 30,
-	.minimum_bytes = 50,
+	.default_ms = 20,
+	.minimum_bytes = 38,
 	.samples_count = ilbc_samples,
-	.smooth = 1,
+	.smooth = 0,
 };
 
 static struct ast_codec g722 = {
diff --git a/res/res_format_attr_ilbc.c b/res/res_format_attr_ilbc.c
new file mode 100644
index 0000000000..0fac55cb0c
--- /dev/null
+++ b/res/res_format_attr_ilbc.c
@@ -0,0 +1,172 @@
+/*
+ * Asterisk -- An open source telephony toolkit.
+ *
+ * Copyright (C) 2016, Alexander Traud
+ *
+ * Alexander Traud <pabstraud@compuserve.com>
+ *
+ * See http://www.asterisk.org for more information about
+ * the Asterisk project. Please do not directly contact
+ * any of the maintainers of this project for assistance;
+ * the project provides a web site, mailing lists and IRC
+ * channels for your use.
+ *
+ * This program is free software, distributed under the terms of
+ * the GNU General Public License Version 2. See the LICENSE file
+ * at the top of the source tree.
+ */
+
+/*!
+ * \file
+ * \brief iLBC format attribute interface
+ *
+ * \author Alexander Traud <pabstraud@compuserve.com>
+ *
+ * \note http://tools.ietf.org/html/rfc3952
+ */
+
+/*** MODULEINFO
+	<support_level>core</support_level>
+ ***/
+
+#include "asterisk.h"
+
+#include "asterisk/module.h"
+#include "asterisk/format.h"
+#include "asterisk/strings.h"           /* for ast_str_append */
+#include "asterisk/utils.h"             /* for ast_calloc, ast_free */
+
+#include "asterisk/ilbc.h"
+
+static struct ilbc_attr default_ilbc_attr = {
+	.mode = 20,
+};
+
+static void ilbc_destroy(struct ast_format *format)
+{
+	struct ilbc_attr *attr = ast_format_get_attribute_data(format);
+
+	ast_free(attr);
+}
+
+static int ilbc_clone(const struct ast_format *src, struct ast_format *dst)
+{
+	struct ilbc_attr *original = ast_format_get_attribute_data(src);
+	struct ilbc_attr *attr = ast_malloc(sizeof(*attr));
+
+	if (!attr) {
+		return -1;
+	}
+
+	if (original) {
+		*attr = *original;
+	} else {
+		*attr = default_ilbc_attr;
+	}
+
+	ast_format_set_attribute_data(dst, attr);
+
+	return 0;
+}
+
+static struct ast_format *ilbc_parse_sdp_fmtp(const struct ast_format *format, const char *attributes)
+{
+	struct ast_format *cloned;
+	struct ilbc_attr *attr;
+	const char *kvp;
+	unsigned int val;
+
+	cloned = ast_format_clone(format);
+	if (!cloned) {
+		return NULL;
+	}
+	attr = ast_format_get_attribute_data(cloned);
+
+	if ((kvp = strstr(attributes, "mode")) && sscanf(kvp, "mode=%30u", &val) == 1) {
+		attr->mode = val;
+	} else {
+		attr->mode = 30; /* optional attribute; 30 is default value */
+	}
+
+	return cloned;
+}
+
+static void ilbc_generate_sdp_fmtp(const struct ast_format *format, unsigned int payload, struct ast_str **str)
+{
+	struct ilbc_attr *attr = ast_format_get_attribute_data(format);
+
+	if (!attr) {
+		attr = &default_ilbc_attr;
+	}
+
+	/* When the VoIP/SIP client Zoiper calls Asterisk and its
+	 * iLBC 20 is disabled but iLBC 30 enabled, Zoiper still
+	 * falls back to iLBC 20, when there is no mode=30 in the
+	 * answer. Consequently, Zoiper defaults to iLBC 20. To
+	 * make that client happy, Asterisk sends mode always.
+	 * tested in June 2016, Zoiper Premium 1.13.2 for iPhone
+	 */
+	/* if (attr->mode != 30) */ {
+		ast_str_append(str, 0, "a=fmtp:%u mode=%u\r\n", payload, attr->mode);
+	}
+}
+
+static struct ast_format *ilbc_getjoint(const struct ast_format *format1, const struct ast_format *format2)
+{
+	struct ast_format *jointformat;
+	struct ilbc_attr *attr1 = ast_format_get_attribute_data(format1);
+	struct ilbc_attr *attr2 = ast_format_get_attribute_data(format2);
+	struct ilbc_attr *attr_res;
+
+	if (!attr1) {
+		attr1 = &default_ilbc_attr;
+	}
+
+	if (!attr2) {
+		attr2 = &default_ilbc_attr;
+	}
+
+	jointformat = ast_format_clone(format1);
+	if (!jointformat) {
+		return NULL;
+	}
+	attr_res = ast_format_get_attribute_data(jointformat);
+
+	if (attr1->mode != attr2->mode) {
+		attr_res->mode = 30;
+	}
+
+	return jointformat;
+}
+
+static struct ast_format_interface ilbc_interface = {
+	.format_destroy = ilbc_destroy,
+	.format_clone = ilbc_clone,
+	.format_cmp = NULL,
+	.format_get_joint = ilbc_getjoint,
+	.format_attribute_set = NULL,
+	.format_parse_sdp_fmtp = ilbc_parse_sdp_fmtp,
+	.format_generate_sdp_fmtp = ilbc_generate_sdp_fmtp,
+};
+
+static int load_module(void)
+{
+	if (ast_format_interface_register("ilbc", &ilbc_interface)) {
+		return AST_MODULE_LOAD_DECLINE;
+	}
+
+	return AST_MODULE_LOAD_SUCCESS;
+}
+
+static int unload_module(void)
+{
+	return 0;
+}
+
+AST_MODULE_INFO(ASTERISK_GPL_KEY, AST_MODFLAG_LOAD_ORDER,
+	"iLBC Format Attribute Module",
+	.support_level = AST_MODULE_SUPPORT_CORE,
+	.load = load_module,
+	.unload = unload_module,
+	.load_pri = AST_MODPRI_CHANNEL_DEPEND,
+);