diff --git a/src/libmsc/Makefile.am b/src/libmsc/Makefile.am
index d834896..e6a2dc1 100644
--- a/src/libmsc/Makefile.am
+++ b/src/libmsc/Makefile.am
@@ -64,6 +64,7 @@
 	ran_peer.c \
 	rrlp.c \
 	rtp_stream.c \
+	sdp_msg.c \
 	silent_call.c \
 	sms_queue.c \
 	transaction.c \
diff --git a/src/libmsc/sdp_msg.c b/src/libmsc/sdp_msg.c
new file mode 100644
index 0000000..7880978
--- /dev/null
+++ b/src/libmsc/sdp_msg.c
@@ -0,0 +1,576 @@
+/* Minimalistic SDP parse/compose implementation, focused on GSM audio codecs */
+/*
+ * (C) 2019 by sysmocom - s.m.f.c. GmbH <info@sysmocom.de>
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: AGPL-3.0+
+ *
+ * Author: Neels Hofmeyr
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <string.h>
+#include <errno.h>
+
+#include <osmocom/core/utils.h>
+#include <osmocom/core/logging.h>
+
+#include <osmocom/msc/debug.h>
+#include <osmocom/msc/sdp_msg.h>
+
+/* Compare name, rate and fmtp, returning typical cmp result: 0 on match, and -1 / 1 on mismatch.
+ * Do *not* compare the payload_type number.
+ * The fmtp is only string-compared -- e.g. if AMR parameters appear in a different order, it amounts to a mismatch even
+ * though all parameters are the same. */
+int sdp_audio_codec_cmp(const struct sdp_audio_codec *a, const struct sdp_audio_codec *b)
+{
+	int rc;
+	if (a == b)
+		return 0;
+	if (!a)
+		return -1;
+	if (!b)
+		return 1;
+	rc = strncmp(a->subtype_name, b->subtype_name, sizeof(a->subtype_name));
+	if (rc)
+		return rc;
+
+	if (a->rate < b->rate)
+		return -1;
+	if (a->rate > b->rate)
+		return 1;
+
+	rc = strncmp(a->fmtp, b->fmtp, sizeof(a->fmtp));
+	if (rc)
+		return rc;
+
+	return 0;
+}
+
+/* Given a predefined fixed payload_type number, add an SDP audio codec entry, if not present yet.
+ * The payload_type must exist in sdp_msg_payload_type_names.
+ * Return the audio codec created or already existing for this payload type number.
+ */
+struct sdp_audio_codec *sdp_audio_codec_add(struct sdp_audio_codecs *ac, unsigned int payload_type,
+					    const char *subtype_name, unsigned int rate, const char *fmtp)
+{
+	struct sdp_audio_codec *codec;
+
+	/* Does an entry already exist? */
+	codec = sdp_audio_codec_by_payload_type(ac, payload_type, false);
+	if (codec) {
+		/* Already exists, sanity check */
+		if (!codec->subtype_name[0])
+			OSMO_STRLCPY_ARRAY(codec->subtype_name, subtype_name);
+		else if (strcmp(codec->subtype_name, subtype_name)) {
+			/* There already is an entry with this payload_type number but a mismatching subtype_name. That is
+			 * weird, rather abort. */
+			return NULL;
+		}
+		if (codec->rate != rate
+		    || (fmtp && strcmp(fmtp, codec->fmtp))) {
+			/* Mismatching details. Rather abort */
+			return NULL;
+		}
+		return codec;
+	}
+
+	/* None exists, create codec entry for this payload type number */
+	codec = sdp_audio_codec_by_payload_type(ac, payload_type, true);
+	/* NULL means unable to add an entry */
+	if (!codec)
+		return NULL;
+
+	OSMO_STRLCPY_ARRAY(codec->subtype_name, subtype_name);
+	if (fmtp)
+		OSMO_STRLCPY_ARRAY(codec->fmtp, fmtp);
+	return codec;
+}
+
+struct sdp_audio_codec *sdp_audio_codec_add_copy(struct sdp_audio_codecs *ac, const struct sdp_audio_codec *codec)
+{
+	return sdp_audio_codec_add(ac, codec->payload_type, codec->subtype_name, codec->rate,
+				   codec->fmtp[0] ? codec->fmtp : NULL);
+}
+
+struct sdp_audio_codec *sdp_audio_codec_by_payload_type(struct sdp_audio_codecs *ac, unsigned int payload_type,
+							bool create)
+{
+	struct sdp_audio_codec *codec;
+	foreach_sdp_audio_codec(codec, ac) {
+		if (codec->payload_type == payload_type)
+			return codec;
+	}
+
+	if (!create)
+		return NULL;
+
+	/* Not found; codec points after the last entry now. */
+	if ((codec - ac->codec) >= ARRAY_SIZE(ac->codec))
+		return NULL;
+
+	*codec = (struct sdp_audio_codec){
+		.payload_type = payload_type,
+		.rate = 8000,
+	};
+
+	ac->count = (codec - ac->codec) + 1;
+	return codec;
+}
+
+/* Return a given sdp_msg's codec entry that matches the subtype_name, rate and fmtp of the given codec, or NULL if no
+ * match is found. Comparison is made by sdp_audio_codec_cmp(). */
+struct sdp_audio_codec *sdp_audio_codec_by_descr(struct sdp_audio_codecs *ac, const struct sdp_audio_codec *codec)
+{
+	struct sdp_audio_codec *i;
+	foreach_sdp_audio_codec(i, ac) {
+		if (!sdp_audio_codec_cmp(i, codec))
+			return i;
+	}
+	return NULL;
+}
+
+/* Remove the codec entry pointed at by 'codec'. 'codec' must point at an entry of 'sdp' (to use an external codec
+ * instance, use sdp_audio_codec_by_descr()).
+ * Return 0 on success, -ENOENT if codec does not point at the sdp->codec array. */
+int sdp_audio_codec_remove(struct sdp_audio_codecs *ac, const struct sdp_audio_codec *codec)
+{
+	struct sdp_audio_codec *i;
+	if ((codec < ac->codec)
+	    || ((codec - ac->codec) >= OSMO_MIN(ac->count, ARRAY_SIZE(ac->codec))))
+		return -ENOENT;
+
+	/* Move all following entries one up */
+	ac->count--;
+	foreach_sdp_audio_codec(i, ac) {
+		if (i < codec)
+			continue;
+		*i = *(i+1);
+	}
+	return 0;
+}
+
+/* Convert struct sdp_msg to the actual SDP protocol representation */
+int sdp_msg_to_str(char *dst, size_t dst_size, const struct sdp_msg *sdp)
+{
+	const struct sdp_audio_codec *codec;
+	struct osmo_strbuf sb = { .buf = dst, .len = dst_size };
+	const char *ip = sdp->rtp.ip[0] ? sdp->rtp.ip : "0.0.0.0";
+
+	OSMO_STRBUF_PRINTF(sb,
+			   "v=0\r\n"
+			   "o=OsmoMSC 0 0 IN IP4 %s\r\n"
+			   "s=GSM Call\r\n"
+			   "c=IN IP4 %s\r\n"
+			   "t=0 0\r\n"
+			   "m=audio %d RTP/AVP",
+			   ip, ip,
+			   sdp->rtp.port);
+
+	/* Append all payload type numbers to 'm=audio <port> RTP/AVP 3 4 112' line */
+	foreach_sdp_audio_codec(codec, &sdp->audio_codecs)
+		OSMO_STRBUF_PRINTF(sb, " %d", codec->payload_type);
+	OSMO_STRBUF_PRINTF(sb, "\r\n");
+
+	/* Add details for all codecs */
+	foreach_sdp_audio_codec(codec, &sdp->audio_codecs) {
+		if (codec->subtype_name[0]) {
+			OSMO_STRBUF_PRINTF(sb, "a=rtpmap:%d %s/%d\r\n", codec->payload_type, codec->subtype_name,
+					   codec->rate > 0? codec->rate : 8000);
+		}
+
+		if (codec->fmtp[0])
+			OSMO_STRBUF_PRINTF(sb, "a=fmtp:%d %s\r\n", codec->payload_type, codec->fmtp);
+	}
+
+	OSMO_STRBUF_PRINTF(sb, "a=ptime:%d\r\n", sdp->ptime > 0? sdp->ptime : 20);
+
+	return sb.chars_needed;
+}
+
+/* Return the first line ending (or the end of the string) at or after the given string position. */
+const char *sdp_msg_line_end(const char *src)
+{
+	const char *line_end = strchr(src, '\r');
+	if (!line_end)
+		line_end = strchr(src, '\n');
+	if (!line_end)
+		line_end = src + strlen(src);
+	return line_end;
+}
+
+/* parse a line like 'a=rtpmap:0 PCMU/8000', 'a=fmtp:112 octet-align=1; mode-set=4', 'a=ptime:20'.
+ * The src should point at the character after 'a=', e.g. at the start of 'rtpmap', 'fmtp', 'ptime'
+ */
+int sdp_parse_attrib(struct sdp_msg *sdp, const char *src)
+{
+	unsigned int payload_type;
+	struct sdp_audio_codec *codec;
+#define A_RTPMAP "rtpmap:"
+#define A_FMTP "fmtp:"
+#define A_PTIME "ptime:"
+#define A_RTCP "rtcp:"
+#define A_SENDRECV "sendrecv"
+#define A_SENDONLY "sendonly"
+#define A_RECVONLY "recvonly"
+
+	if (osmo_str_startswith(src, A_RTPMAP)) {
+		char *audio_name;
+		unsigned int channels = 1;
+		if (sscanf(src, A_RTPMAP "%u", &payload_type) != 1)
+			return -EINVAL;
+
+		audio_name = strchr(src, ' ');
+		if (!audio_name || audio_name >= sdp_msg_line_end(src))
+			return -EINVAL;
+
+		codec = sdp_audio_codec_by_payload_type(&sdp->audio_codecs, payload_type, true);
+		if (!codec)
+			return -ENOSPC;
+
+		if (sscanf(audio_name, " %31[^/]/%u/%u", codec->subtype_name, &codec->rate, &channels) < 1)
+			return -EINVAL;
+
+		if (channels != 1)
+			return -ENOTSUP;
+	}
+
+	else if (osmo_str_startswith(src, A_FMTP)) {
+		char *fmtp_str;
+		const char *line_end = sdp_msg_line_end(src);
+		if (sscanf(src, A_FMTP "%u", &payload_type) != 1)
+			return -EINVAL;
+
+		fmtp_str = strchr(src, ' ');
+		if (!fmtp_str)
+			return -EINVAL;
+		fmtp_str++;
+		if (fmtp_str >= line_end)
+			return -EINVAL;
+
+		codec = sdp_audio_codec_by_payload_type(&sdp->audio_codecs, payload_type, true);
+		if (!codec)
+			return -ENOSPC;
+
+		/* (+1 because osmo_strlcpy() interprets it as size including the '\0') */
+		osmo_strlcpy(codec->fmtp, fmtp_str, line_end - fmtp_str + 1);
+	}
+
+	else if (osmo_str_startswith(src, A_PTIME)) {
+		if (sscanf(src, A_PTIME "%u", &sdp->ptime) != 1)
+			return -EINVAL;
+
+	}
+
+	else if (osmo_str_startswith(src, A_RTCP)) {
+		/* TODO? */
+	}
+
+	else if (osmo_str_startswith(src, A_SENDRECV)) {
+		/* TODO? */
+	}
+
+	else if (osmo_str_startswith(src, A_SENDONLY)) {
+		/* TODO? */
+	}
+
+	else if (osmo_str_startswith(src, A_RECVONLY)) {
+		/* TODO? */
+	}
+
+	return 0;
+}
+
+const struct value_string sdp_msg_payload_type_names[] = {
+	{ 0, "PCMU" },
+	{ 3, "GSM" },
+	{ 8, "PCMA" },
+	{ 18, "G729" },
+	{ 110, "GSM-EFR" },
+	{ 111, "GSM-HR-08" },
+	{ 112, "AMR" },
+	{ 113, "AMR-WB" },
+	{}
+};
+
+/* Return payload type number matching given string ("AMR", "GSM", ...) or negative if not found. */
+int sdp_subtype_name_to_payload_type(const char *subtype_name)
+{
+	return get_string_value(sdp_msg_payload_type_names, subtype_name);
+}
+
+/* Parse a line like 'm=audio 16398 RTP/AVP 0 3 8 96 112', starting after the '=' */
+static int sdp_parse_media_description(struct sdp_msg *sdp, const char *src)
+{
+	unsigned int port;
+	int i;
+	const char *payload_type_str;
+	const char *line_end = sdp_msg_line_end(src);
+	if (sscanf(src, "audio %u RTP/AVP", &port) < 1)
+		return -ENOTSUP;
+
+	if (port < 0 || port > 0xffff)
+		return -EINVAL;
+
+	sdp->rtp.port = port;
+
+	/* skip "audio 12345 RTP/AVP ", i.e. 3 spaces on */
+	payload_type_str = src;
+	for (i = 0; i < 3; i++) {
+		payload_type_str = strchr(payload_type_str, ' ');
+		if (!payload_type_str)
+			return -EINVAL;
+		while (*payload_type_str == ' ')
+			payload_type_str++;
+		if (payload_type_str >= line_end)
+			return -EINVAL;
+	}
+
+	/* Parse listing of payload type numbers after "RTP/AVP" */
+	while (payload_type_str < line_end) {
+		unsigned int payload_type;
+		struct sdp_audio_codec *codec;
+		const char *subtype_name;
+		if (sscanf(payload_type_str, "%u", &payload_type) < 1)
+			return -EINVAL;
+
+		codec = sdp_audio_codec_by_payload_type(&sdp->audio_codecs, payload_type, true);
+		if (!codec)
+			return -ENOSPC;
+
+		/* Fill in subtype name for fixed payload types */
+		subtype_name = get_value_string_or_null(sdp_msg_payload_type_names, codec->payload_type);
+		if (subtype_name)
+			OSMO_STRLCPY_ARRAY(codec->subtype_name, subtype_name);
+
+		payload_type_str = strchr(payload_type_str, ' ');
+		if (!payload_type_str)
+			payload_type_str = line_end;
+		while (*payload_type_str == ' ')
+			payload_type_str++;
+	}
+
+	return 0;
+}
+
+/* parse a line like 'c=IN IP4 192.168.11.151' starting after the '=' */
+static int sdp_parse_connection_info(struct sdp_msg *sdp, const char *src)
+{
+	char ipv[10];
+	char addr_str[INET6_ADDRSTRLEN];
+	if (sscanf(src, "IN %s %s", ipv, addr_str) < 2)
+		return -EINVAL;
+
+	/* supporting only IPv4 */
+	if (strcmp(ipv, "IP4"))
+		return -ENOTSUP;
+
+	osmo_sockaddr_str_from_str(&sdp->rtp, addr_str, sdp->rtp.port);
+	return 0;
+}
+
+/* Parse SDP string into struct sdp_msg. Return 0 on success, negative on error. */
+int sdp_msg_from_str(struct sdp_msg *sdp, const char *src)
+{
+	const char *pos;
+	*sdp = (struct sdp_msg){};
+
+	for (pos = src; pos && *pos; pos++) {
+		char attrib;
+		int rc = 0;
+
+		if (*pos == '\r' || *pos == '\n')
+			continue;
+
+		/* Expecting only lines starting with 'X='. Not being too strict about it is probably alright. */
+		if (pos[1] != '=')
+			goto next_line;
+
+		attrib = *pos;
+		pos += 2;
+		switch (attrib) {
+			/* a=... */
+			case 'a':
+				rc = sdp_parse_attrib(sdp, pos);
+				break;
+			case 'm':
+				rc = sdp_parse_media_description(sdp, pos);
+				break;
+			case 'c':
+				rc = sdp_parse_connection_info(sdp, pos);
+				break;
+			default:
+				/* ignore any other parameters */
+				break;
+		}
+
+		if (rc) {
+			size_t line_len;
+			const char *line_end = sdp_msg_line_end(pos);
+			pos -= 2;
+			line_len = line_end - pos;
+			switch (rc) {
+			case -EINVAL:
+				LOGP(DMNCC, LOGL_ERROR,
+				     "Failed to parse SDP: invalid line: %s\n", osmo_quote_str(pos, line_len));
+				break;
+			case -ENOSPC:
+				LOGP(DMNCC, LOGL_ERROR,
+				     "Failed to parse SDP: no more space for: %s\n", osmo_quote_str(pos, line_len));
+				break;
+			case -ENOTSUP:
+				LOGP(DMNCC, LOGL_ERROR,
+				     "Failed to parse SDP: not supported: %s\n", osmo_quote_str(pos, line_len));
+				break;
+			default:
+				LOGP(DMNCC, LOGL_ERROR,
+				     "Failed to parse SDP: %s\n", osmo_quote_str(pos, line_len));
+				break;
+			}
+			return rc;
+		}
+next_line:
+		pos = strstr(pos, "\r\n");
+		if (!pos)
+			break;
+	}
+
+	return 0;
+}
+
+/* Leave only those codecs in 'ac_dest' that are also present in 'ac_other'.
+ * The matching is made by sdp_audio_codec_cmp(), i.e. payload_type numbers are not compared and fmtp parameters are
+ * compared 1:1 as plain strings.
+ * If translate_payload_type_numbers has an effect if ac_dest and ac_other have mismatching payload_type numbers for the
+ * same SDP codec descriptions. If translate_payload_type_numbers is true, take the payload_type numbers from ac_other.
+ * If false, keep payload_type numbers in ac_dest unchanged. */
+void sdp_audio_codecs_intersection(struct sdp_audio_codecs *ac_dest, const struct sdp_audio_codecs *ac_other,
+				   bool translate_payload_type_numbers)
+{
+	int i;
+	for (i = 0; i < ac_dest->count; i++) {
+		struct sdp_audio_codec *codec = &ac_dest->codec[i];
+		struct sdp_audio_codec *other;
+		OSMO_ASSERT(i < ARRAY_SIZE(ac_dest->codec));
+
+		other = sdp_audio_codec_by_descr((struct sdp_audio_codecs*)ac_other, codec);
+
+		if (!other) {
+			OSMO_ASSERT(sdp_audio_codec_remove(ac_dest, codec) == 0);
+			i--;
+			continue;
+		}
+
+		/* Doing payload_type number translation of part of the intersection because it makes the algorithm
+		 * simpler: we already know ac_dest is a subset of ac_other, and there is no need to resolve payload
+		 * type number conflicts. */
+		if (translate_payload_type_numbers)
+			codec->payload_type = other->payload_type;
+	}
+}
+
+/* Make sure the given codec is listed as the first codec. 'codec' must be an actual codec entry of the given audio
+ * codecs list. */
+void sdp_audio_codecs_select(struct sdp_audio_codecs *ac, struct sdp_audio_codec *codec)
+{
+	struct sdp_audio_codec tmp;
+	struct sdp_audio_codec *pos;
+	OSMO_ASSERT((codec >= ac->codec)
+		    && ((codec - ac->codec) < OSMO_MIN(ac->count, ARRAY_SIZE(ac->codec))));
+
+	/* Already the first? */
+	if (codec == ac->codec)
+		return;
+
+	tmp = *codec;
+	for (pos = codec - 1; pos >= ac->codec; pos--)
+		pos[1] = pos[0];
+
+	ac->codec[0] = tmp;
+	return;
+}
+
+/* Short single-line representation of an SDP audio codec, convenient for logging.
+ * Like "AMR/8000:octet-align=1#122" */
+int sdp_audio_codec_name_buf(char *buf, size_t buflen, const struct sdp_audio_codec *codec)
+{
+	struct osmo_strbuf sb = { .buf = buf, .len = buflen };
+	OSMO_STRBUF_PRINTF(sb, "%s", codec->subtype_name);
+	if (codec->fmtp[0])
+		OSMO_STRBUF_PRINTF(sb, ":%s", codec->fmtp);
+	return sb.chars_needed;
+}
+
+char *sdp_audio_codec_name_c(void *ctx, const struct sdp_audio_codec *codec)
+{
+	OSMO_NAME_C_IMPL(ctx, 32, "sdp_audio_codec_name_c-ERROR", sdp_audio_codec_name_buf, codec)
+}
+
+const char *sdp_audio_codec_name(const struct sdp_audio_codec *codec)
+{
+	return sdp_audio_codec_name_c(OTC_SELECT, codec);
+}
+
+/* Short single-line representation of a list of SDP audio codecs, convenient for logging */
+int sdp_audio_codecs_name_buf(char *buf, size_t buflen, const struct sdp_audio_codecs *ac)
+{
+	struct osmo_strbuf sb = { .buf = buf, .len = buflen };
+	const struct sdp_audio_codec *codec;
+	if (!ac->count)
+		OSMO_STRBUF_PRINTF(sb, "(no-codecs)");
+	foreach_sdp_audio_codec(codec, ac) {
+		bool first = (codec == ac->codec);
+		if (!first)
+			OSMO_STRBUF_PRINTF(sb, ",");
+		OSMO_STRBUF_APPEND(sb, sdp_audio_codec_name_buf, codec);
+	}
+	return sb.chars_needed;
+}
+
+char *sdp_audio_codecs_name_c(void *ctx, const struct sdp_audio_codecs *ac)
+{
+	OSMO_NAME_C_IMPL(ctx, 128, "sdp_audio_codecs_name_c-ERROR", sdp_audio_codecs_name_buf, ac)
+}
+
+const char *sdp_audio_codecs_name(const struct sdp_audio_codecs *ac)
+{
+	return sdp_audio_codecs_name_c(OTC_SELECT, ac);
+}
+
+/* Short single-line representation of an SDP message, convenient for logging */
+int sdp_msg_name_buf(char *buf, size_t buflen, const struct sdp_msg *sdp)
+{
+	struct osmo_strbuf sb = { .buf = buf, .len = buflen };
+	if (!sdp) {
+		OSMO_STRBUF_PRINTF(sb, "NULL");
+		return sb.chars_needed;
+	}
+
+	OSMO_STRBUF_PRINTF(sb, OSMO_SOCKADDR_STR_FMT, OSMO_SOCKADDR_STR_FMT_ARGS(&sdp->rtp));
+	OSMO_STRBUF_PRINTF(sb, "{");
+	OSMO_STRBUF_APPEND(sb, sdp_audio_codecs_name_buf, &sdp->audio_codecs);
+	OSMO_STRBUF_PRINTF(sb, "}");
+	return sb.chars_needed;
+}
+
+char *sdp_msg_name_c(void *ctx, const struct sdp_msg *sdp)
+{
+	OSMO_NAME_C_IMPL(ctx, 128, "sdp_msg_name_c-ERROR", sdp_msg_name_buf, sdp)
+}
+
+const char *sdp_msg_name(const struct sdp_msg *sdp)
+{
+	return sdp_msg_name_c(OTC_SELECT, sdp);
+}