diff --git a/include/osmocom/msc/codec_filter.h b/include/osmocom/msc/codec_filter.h
new file mode 100644
index 0000000..16315c7
--- /dev/null
+++ b/include/osmocom/msc/codec_filter.h
@@ -0,0 +1,66 @@
+/* Filter/overlay codec selections for a voice call, across MS, RAN and CN limitations */
+/*
+ * (C) 2019-2022 by sysmocom - s.m.f.c. GmbH <info@sysmocom.de>
+ * All Rights Reserved
+ *
+ * Author: Neels Hofmeyr
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 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 General Public License for more details.
+ */
+#pragma once
+
+#include <osmocom/gsm/gsm_utils.h>
+#include <osmocom/gsm/mncc.h>
+#include <osmocom/mgcp_client/mgcp_client.h>
+
+#include <osmocom/msc/sdp_msg.h>
+
+struct gsm0808_speech_codec_list;
+
+/* Combine various codec selections to obtain a resulting set of codecs allowed by all of them.
+ * Members reflect the different entities/stages that select codecs in a voice call.
+ * Call codec_filter_run() and obtain the resulting set of codecs in codec_filter.result. */
+struct codec_filter {
+	/* The fixed set of codecs available on the RAN type, per definition. */
+	struct sdp_audio_codecs ran;
+	/* The codecs advertised by the MS Bearer Capabilities */
+	struct sdp_audio_codecs ms;
+	/* If known, the set of codecs the current RAN cell allows / has available.
+	 * This may not be available if the BSC does not issue this information early enough.
+	 * Should be ignored if empty. */
+	struct sdp_audio_codecs bss;
+
+	/* SDP as last received from the remote call leg. */
+	struct sdp_msg remote;
+
+	/* After a channel was assigned, this reflects the chosen codec. */
+	struct sdp_audio_codec assignment;
+
+	/* Resulting choice of supported codecs, usually the intersection of the above,
+	 * and the local RTP address to be sent to the remote call leg.
+	 * The RTP address:port in result.rtp is not modified by codec_filter_run() -- set it once. */
+	struct sdp_msg result;
+};
+
+void codec_filter_init(struct codec_filter *codec_filter);
+void codec_filter_set_ran(struct codec_filter *codec_filter, enum osmo_rat_type ran_type);
+void codec_filter_set_ms_from_bc(struct codec_filter *codec_filter, const struct gsm_mncc_bearer_cap *ms_bearer_cap);
+void codec_filter_set_bss(struct codec_filter *codec_filter,
+			  const struct gsm0808_speech_codec_list *codec_list_bss_supported);
+int codec_filter_set_remote(struct codec_filter *codec_filter, const char *remote_sdp);
+void codec_filter_set_local_rtp(struct codec_filter *codec_filter, const struct osmo_sockaddr_str *rtp);
+int codec_filter_run(struct codec_filter *codec_filter);
+
+int codec_filter_to_str_buf(char *buf, size_t buflen, const struct codec_filter *codec_filter);
+char *codec_filter_to_str_c(void *ctx, const struct codec_filter *codec_filter);
+const char *codec_filter_to_str(const struct codec_filter *codec_filter);