diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 18ad3ff..7907734 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -136,7 +136,8 @@
 #define DLBSSGP		-21	/*!< Osmocom BSSGP layer */
 #define DLNSDATA	-22	/*!< Osmocom NS layer data pdus */
 #define DLNSSIGNAL	-23	/*!< Osmocom NS layer signal pdus */
-#define OSMO_NUM_DLIB	23	/*!< Number of logging sub-systems in libraries */
+#define DLIUUP		-24	/*!< Osmocom IuUP layer */
+#define OSMO_NUM_DLIB	24	/*!< Number of logging sub-systems in libraries */
 
 /* Colors that can be used in log_info_cat.color */
 #define OSMO_LOGCOLOR_NORMAL NULL
diff --git a/include/osmocom/gsm/iuup.h b/include/osmocom/gsm/iuup.h
new file mode 100644
index 0000000..b1651c5
--- /dev/null
+++ b/include/osmocom/gsm/iuup.h
@@ -0,0 +1,122 @@
+#pragma once
+
+#include <stdint.h>
+
+#include <osmocom/core/prim.h>
+#include <osmocom/gsm/protocol/gsm_25_415.h>
+
+/***********************************************************************
+ * Primitives towards the lower layers (typically RTP transport)
+ ***********************************************************************/
+enum osmo_iuup_tnl_prim_type {
+	OSMO_IUUP_TNL_UNITDATA,
+};
+
+struct osmo_iuup_tnl_prim {
+	struct osmo_prim_hdr oph;
+};
+
+/***********************************************************************
+ * Primitives towards the upper layers at the RNL SAP
+ ***********************************************************************/
+
+/* 3GPP TS 25.415 Section 7.2.1 */
+enum osmo_iuup_rnl_prim_type {
+	OSMO_IUUP_RNL_CONFIG,
+	OSMO_IUUP_RNL_DATA,
+	OSMO_IUUP_RNL_STATUS,
+	OSMO_IUUP_RNL_UNIT_DATA,
+};
+
+/* TS 25.413 9.2.1.3*/
+#define IUUP_MAX_SUBFLOWS 7
+#define IUUP_MAX_RFCIS 64
+
+#define IUUP_TIMER_INIT_T_DEFAULT 1000
+#define IUUP_TIMER_TA_T_DEFAULT 500
+#define IUUP_TIMER_RC_T_DEFAULT 500
+#define IUUP_TIMER_INIT_N_DEFAULT 3
+#define IUUP_TIMER_TA_N_DEFAULT 1
+#define IUUP_TIMER_RC_N_DEFAULT 1
+struct osmo_iuup_rnl_config_timer {
+	uint32_t t_ms;	/* time in ms */
+	uint32_t n_max;	/* max number of repetitions */
+};
+struct osmo_iuup_rnl_config {
+	/* transparent (true) or SMpSDU (false): */
+	bool transparent;
+
+	/* should we actively transmit INIT in SmpSDU mode? */
+	bool active;
+
+	/* Currently Version 0 or 1: */
+	uint8_t data_pdu_type;
+
+	/* Supported mode versions */
+	uint16_t supported_versions_mask;
+	uint8_t num_rfci;
+	uint8_t num_subflows;
+	uint16_t subflow_sizes[IUUP_MAX_RFCIS][IUUP_MAX_SUBFLOWS];
+	bool IPTIs_present;
+	uint8_t IPTIs[IUUP_MAX_RFCIS]; /* values range 0-15, 4 bits */
+
+	/* TODO: Indication of delivery of erroneous SDUs*/
+	struct osmo_iuup_rnl_config_timer t_init;
+	struct osmo_iuup_rnl_config_timer t_ta;
+	struct osmo_iuup_rnl_config_timer t_rc;
+};
+
+struct osmo_iuup_rnl_data {
+	uint8_t rfci;
+	uint8_t frame_nr;
+	uint8_t fqc;
+};
+
+struct osmo_iuup_rnl_status {
+	enum iuup_procedure procedure;
+	union {
+		struct {
+			enum iuup_error_cause cause;
+			enum iuup_error_distance distance;
+		} error_event;
+		struct {
+			uint16_t supported_versions_mask;
+			uint8_t num_subflows;
+			uint8_t num_rfci;
+			uint16_t subflow_sizes[IUUP_MAX_RFCIS][IUUP_MAX_SUBFLOWS];
+			bool IPTIs_present;
+			uint8_t IPTIs[IUUP_MAX_RFCIS]; /* values range 0-15, 4 bits */
+		} initialization;
+		struct {
+		} rate_control;
+		struct {
+		} time_alignment;
+	} u;
+};
+
+/* SAP on the upper side of IuUP, towards the user */
+struct osmo_iuup_rnl_prim {
+	struct osmo_prim_hdr oph;
+	union {
+		struct osmo_iuup_rnl_config config;
+		struct osmo_iuup_rnl_data data;
+		struct osmo_iuup_rnl_status status;
+		//struct osmo_iuup_rnl_unitdata unitdata;
+	} u;
+};
+
+struct osmo_iuup_instance;
+struct osmo_iuup_instance *osmo_iuup_instance_alloc(void *ctx, const char *id);
+void osmo_iuup_instance_free(struct osmo_iuup_instance *iui);
+
+void osmo_iuup_instance_set_user_prim_cb(struct osmo_iuup_instance *iui, osmo_prim_cb func, void *priv);
+void osmo_iuup_instance_set_transport_prim_cb(struct osmo_iuup_instance *iui, osmo_prim_cb func, void *priv);
+int osmo_iuup_tnl_prim_up(struct osmo_iuup_instance *iui, struct osmo_iuup_tnl_prim *itp);
+int osmo_iuup_rnl_prim_down(struct osmo_iuup_instance *inst, struct osmo_iuup_rnl_prim *irp);
+
+
+int osmo_iuup_compute_header_crc(const uint8_t *iuup_pdu, unsigned int pdu_len);
+int osmo_iuup_compute_payload_crc(const uint8_t *iuup_pdu, unsigned int pdu_len);
+
+struct osmo_iuup_rnl_prim *osmo_iuup_rnl_prim_alloc(void *ctx, unsigned int primitive, unsigned int operation, unsigned int size);
+struct osmo_iuup_tnl_prim *osmo_iuup_tnl_prim_alloc(void *ctx, unsigned int primitive, unsigned int operation, unsigned int size);
diff --git a/include/osmocom/gsm/prim.h b/include/osmocom/gsm/prim.h
index 045e353..73cd7f7 100644
--- a/include/osmocom/gsm/prim.h
+++ b/include/osmocom/gsm/prim.h
@@ -18,4 +18,7 @@
 	SAP_NS,
 
 	SAP_BSSGP_RIM,
+
+	SAP_IUUP_TNL,
+	SAP_IUUP_RNL,
 };
diff --git a/include/osmocom/gsm/protocol/gsm_25_415.h b/include/osmocom/gsm/protocol/gsm_25_415.h
new file mode 100644
index 0000000..5d90fd3
--- /dev/null
+++ b/include/osmocom/gsm/protocol/gsm_25_415.h
@@ -0,0 +1,222 @@
+#pragma once
+/* Iu User Plane (IuUP) Definitions as per 3GPP TS 25.415 */
+/* (C) 2017 by Harald Welte <laforge@gnumonks.org>
+ * All Rights Reserved.
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ */
+
+#include <stdint.h>
+#include <osmocom/core/endian.h>
+
+/* 3GPP TS 25.415 Section 6.6.2.1 */
+struct iuup_pdutype0_hdr {
+#if OSMO_IS_LITTLE_ENDIAN
+	/* control part */
+	uint8_t frame_nr:4,
+		pdu_type:4;
+	uint8_t rfci:6,
+		fqc:2;
+	/* checksum part */
+	uint8_t payload_crc_hi:2, header_crc:6;
+	uint8_t payload_crc_lo;
+
+	/* payload part */
+	uint8_t payload[0];
+#elif OSMO_IS_BIG_ENDIAN
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+	uint8_t pdu_type:4, frame_nr:4;
+	uint8_t fqc:2, rfci:6;
+	uint8_t header_crc:6, payload_crc_hi:2;
+	uint8_t payload_crc_lo;
+	uint8_t payload[0];
+#endif
+} __attribute__((packed));
+
+/* 3GPP TS 25.415 Section 6.6.2.2 */
+struct iuup_pdutype1_hdr {
+#if OSMO_IS_LITTLE_ENDIAN
+	/* control part */
+	uint8_t frame_nr:4,
+		pdu_type:4;
+	uint8_t rfci:6,
+		fqc:2;
+	/* checksum part */
+	uint8_t spare:2,
+		header_crc:6;
+	/* payload part */
+	uint8_t payload[0];
+#elif OSMO_IS_BIG_ENDIAN
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+	uint8_t pdu_type:4, frame_nr:4;
+	uint8_t fqc:2, rfci:6;
+	uint8_t header_crc:6, spare:2;
+	uint8_t payload[0];
+#endif
+} __attribute__((packed));
+
+/* 3GPP TS 25.415 Section 6.6.2.3 */
+struct iuup_pdutype14_hdr {
+#if OSMO_IS_LITTLE_ENDIAN
+	/* control part */
+	uint8_t frame_nr:2,
+		ack_nack:2,
+		pdu_type:4;
+	uint8_t proc_ind:4,
+		mode_version:4;
+	/* checksum part */
+	uint8_t payload_crc_hi:2, header_crc:6;
+	uint8_t payload_crc_lo;
+	/* payload part */
+	uint8_t payload[0];
+#elif OSMO_IS_BIG_ENDIAN
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+	uint8_t pdu_type:4, ack_nack:2, frame_nr:2;
+	uint8_t mode_version:4, proc_ind:4;
+	uint8_t header_crc:6, payload_crc_hi:2;
+	uint8_t payload_crc_lo;
+	uint8_t payload[0];
+#endif
+} __attribute__((packed));
+
+/* 3GPP TS 25.415 Section 6.6.2.3.4.1 */
+struct iuup_ctrl_init_rfci_hdr {
+#if OSMO_IS_LITTLE_ENDIAN
+	uint8_t rfci:6,
+		li:1,
+		lri:1;
+	uint8_t subflow_length[0]; /* 1 or 2 bytes depending on li */
+#elif OSMO_IS_BIG_ENDIAN
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+	uint8_t lri:1, li:1, rfci:6;
+	uint8_t subflow_length[0];
+#endif
+} __attribute__((packed));
+struct iuup_ctrl_init_hdr {
+#if OSMO_IS_LITTLE_ENDIAN
+	uint8_t chain_ind:1,
+		num_subflows_per_rfci:3,
+		ti:1,
+		spare:3;
+	uint8_t rfci_data[0]; /* struct iuup_ctrl_init_rfci_hdr* */
+#elif OSMO_IS_BIG_ENDIAN
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+	uint8_t spare:3, ti:1, num_subflows_per_rfci:3, chain_ind:1;
+	uint8_t rfci_data[0]; /* struct iuup_ctrl_init_rfci_hdr* */
+;
+#endif
+} __attribute__((packed));
+struct iuup_ctrl_init_tail {
+#if OSMO_IS_LITTLE_ENDIAN
+	uint16_t versions_supported;
+	uint8_t spare:4,
+		data_pdu_type:4;
+	uint8_t spare_extension[0];
+#elif OSMO_IS_BIG_ENDIAN
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+	uint16_t versions_supported;
+	uint8_t data_pdu_type:4, spare:4;
+	uint8_t spare_extension[0];
+#endif
+} __attribute__((packed));
+
+/* 3GPP TS 25.415 Section 6.6.2.3.4.4 */
+struct iuup_ctrl_error_event {
+#if OSMO_IS_LITTLE_ENDIAN
+	struct iuup_pdutype14_hdr hdr;
+	uint8_t error_cause:6,
+		error_distance:2;
+	uint8_t spare_extension[0];
+#elif OSMO_IS_BIG_ENDIAN
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+	struct iuup_pdutype14_hdr hdr;
+	uint8_t error_distance:2, error_cause:6;
+	uint8_t spare_extension[0];
+#endif
+} __attribute__((packed));
+
+/* 3GPP TS 25.415 Section 6.6.2.3.2 */
+struct iuup_ctrl_ack {
+	struct iuup_pdutype14_hdr hdr;
+	uint8_t spare_extension[0];
+} __attribute__((packed));
+
+/* 3GPP TS 25.415 Section 6.6.2.3.3 */
+struct iuup_ctrl_nack {
+#if OSMO_IS_LITTLE_ENDIAN
+	struct iuup_pdutype14_hdr hdr;
+	uint8_t spare:2,
+		error_cause:6;
+	uint8_t spare_extension[0];
+#elif OSMO_IS_BIG_ENDIAN
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+	struct iuup_pdutype14_hdr hdr;
+	uint8_t error_cause:6, spare:2;
+	uint8_t spare_extension[0];
+#endif
+} __attribute__((packed));
+
+/* 3GPP TS 25.415 Section 6.6.2 + 6.6.3.1 */
+enum iuup_pdu_type {
+	IUUP_PDU_T_DATA_CRC	= 0,
+	IUUP_PDU_T_DATA_NOCRC	= 1,
+	IUUP_PDU_T_CONTROL	= 14,
+};
+
+/* 3GPP TS 25.415 Section 6.6.3.2 */
+enum iuup_ack_nack {
+	IUUP_AN_PROCEDURE	= 0,
+	IUUP_AN_ACK		= 1,
+	IUUP_AN_NACK		= 2,
+};
+
+/* 3GPP TS 25.415 Section 6.6.3.5 */
+enum iuup_fqc {
+	IUUP_FQC_FRAME_GOOD	= 0,
+	IUUP_FQC_FRAME_BAD	= 1,
+	IUUP_FQC_FRAME_BAD_RADIO = 2,
+};
+
+/* 3GPP TS 25.415 Section 6.6.3.7 */
+enum iuup_procedure {
+	IUUP_PROC_INIT		= 0,
+	IUUP_PROC_RATE_CTRL	= 1,
+	IUUP_PROC_TIME_ALIGN	= 2,
+	IUUP_PROC_ERR_EVENT	= 3,
+};
+
+
+/* 3GPP TS 25.415 Section 6.6.3.15 */
+enum iuup_error_distance {
+	IUUP_ERR_DIST_LOCAL		= 0,
+	IUUP_ERR_DIST_FIRST_FWD		= 1,
+	IUUP_ERR_DIST_SECOND_FWD	= 2,
+	IUUP_ERR_DIST_RESERVED		= 3,
+};
+
+
+/* 3GPP TS 25.415 Section 6.6.3.16 */
+enum iuup_error_cause {
+	IUUP_ERR_CAUSE_CRC_ERR_HDR	= 0,
+	IUUP_ERR_CAUSE_CRC_ERR_DATA	= 1,
+	IUUP_ERR_CAUSE_UNEXPECTED_FN	= 2,
+	IUUP_ERR_CAUSE_FRAME_LOSS	= 3,
+	IUUP_ERR_CAUSE_UNKNOWN_PDUTYPE	= 4,
+	IUUP_ERR_CAUSE_UNKNOWN_PROC	= 5,
+	IUUP_ERR_CAUSE_UNKNNOWN_RES_VAL	= 6,
+	IUUP_ERR_CAUSE_UNKNNOWN_FIELD	= 7,
+	IUUP_ERR_CAUSE_FRAME_TOO_SHORT	= 8,
+	IUUP_ERR_CAUSE_MISSING_FIELDS	= 9,
+	IUUP_ERR_CAUSE_UNEXPECTED_PDU_T	= 16,
+	IUUP_ERR_CAUSE_UNEXPECTED_PROC	= 18,
+	IUUP_ERR_CAUSE_UNEXPECTED_RFCI	= 19,
+	IUUP_ERR_CAUSE_UNEXPECTED_VALUE	= 20,
+	IUUP_ERR_CAUSE_INIT_FAILURE	= 42,
+	IUUP_ERR_CAUSE_INIT_FAILURE_NET_TMR = 43,
+	IUUP_ERR_CAUSE_INIT_FAILURE_REP_NACK = 44,
+	IUUP_ERR_CAUSE_RATE_CTRL_FAILURE = 45,
+	IUUP_ERR_CAUSE_ERR_EVENT_FAIL	= 46,
+	IUUP_ERR_CAUSE_TIME_ALIGN_NOTSUPP = 47,
+	IUUP_ERR_CAUSE_REQ_TIME_ALIGN_NOTPOSS = 48,
+	IUUP_ERR_CAUSE_MODE_VERSION_NOT_SUPPORTED = 49,
+};