diff --git a/include/osmocom/gsm/protocol/gsm_23_003.h b/include/osmocom/gsm/protocol/gsm_23_003.h
new file mode 100644
index 0000000..a2109b6
--- /dev/null
+++ b/include/osmocom/gsm/protocol/gsm_23_003.h
@@ -0,0 +1,23 @@
+#pragma once
+
+/* Chapter 2.2 */
+#define GSM23003_IMSI_MAX_DIGITS	15
+/* Chapter 2.4 */
+#define GSM23003_TMSI_NUM_BYTES		4
+/* Chapter 2.5 */
+#define GSM23003_LMSI_NUM_BYTES		4
+/* Chapter 2.6 */
+#define GSM23003_TLLI_NUM_BYTES		4
+/* Chapter 2.7 */
+#define GSM23003_PTMSI_SIG_NUM_BYTES	3
+/* Chapter 2.8 */
+#define GSM23003_MME_CODE_NUM_BYTES	1
+#define GSM23003_MME_GROUP_NUM_BYTES	2
+#define GSM23003_MTMSI_NUM_BYTES	4
+/* Chapter 6.2.1 */
+#define GSM23003_IMEI_TAC_NUM_DIGITS	8
+#define GSM23003_IMEI_SNR_NUM_DIGITS	6
+#define GSM23003_IMEI_NUM_DIGITS	(GSM23003_IMEI_TAC_NUM_DIGITS + \
+					 GSM23003_IMEI_SNR_NUM_DIGITS + 1)
+#define GSM23003_IMEISV_NUM_DIGITS	(GSM23003_IMEI_TAC_NUM_DIGITS + \
+					 GSM23003_IMEI_SNR_NUM_DIGITS + 2)