diff --git a/include/osmocom/core/tdef.h b/include/osmocom/core/tdef.h
new file mode 100644
index 0000000..92b7159
--- /dev/null
+++ b/include/osmocom/core/tdef.h
@@ -0,0 +1,172 @@
+/*! \file tdef.h
+ * API to define Tnnn timers globally and use for FSM state changes.
+ */
+/*
+ * (C) 2018-2019 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * Author: Neels Hofmeyr <neels@hofmeyr.de>
+ *
+ * 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/>.
+ */
+#pragma once
+
+#include <stdint.h>
+#include <osmocom/core/utils.h>
+
+struct osmo_fsm_inst;
+
+/*! \defgroup Tdef  Tnnn timer configuration
+ * @{
+ * \file tdef.h
+ */
+
+enum osmo_tdef_unit {
+	OSMO_TDEF_S = 0,	/*!< most T are in seconds, keep 0 as default. */
+	OSMO_TDEF_MS,		/*!< milliseconds */
+	OSMO_TDEF_M,		/*!< minutes */
+	OSMO_TDEF_CUSTOM,	/*!< unspecified unit, explained in osmo_tdef.desc. */
+};
+
+extern const struct value_string osmo_tdef_unit_names[];
+/*! \return enum osmo_tdef_unit value as human readable unit letter, or "custom-unit". */
+static inline const char *osmo_tdef_unit_name(enum osmo_tdef_unit val)
+{ return get_value_string(osmo_tdef_unit_names, val); }
+
+/*! Define a GSM timer of the form Tnnn, with unit, default value and doc string.
+ * Typically used as an array with the last entry being left zero-initialized, e.g.:
+ *
+ *         struct osmo_tdef tdefs[] = {
+ *                 { .T=10, .default_val=6, .desc="RR Assignment" },
+ *                 { .T=101, .default_val=10, .desc="inter-BSC Handover MT, HO Request to HO Accept" },
+ *                 { .T=3101, .default_val=3, .desc="RR Immediate Assignment" },
+ *                 {}
+ *         };
+ *
+ * Program initialization should call osmo_tdefs_reset() so that all timers return the default_val, until e.g. the VTY
+ * configuration sets user-defined values (see osmo_tdef_vty_init()).
+ */
+struct osmo_tdef {
+	/*! T1234 number; type corresponds to struct osmo_fsm_inst.T. Negative and zero T numbers are actually possible,
+	 * but be aware that osmo_tdef_fsm_inst_state_chg() interprets T == 0 as "no timer". */
+	const int T;
+	/*! Timeout duration (according to unit), default value; type corresponds to osmo_fsm_inst_state_chg()'s
+	 * timeout_secs argument. Note that osmo_fsm_inst_state_chg() clamps the range. */
+	const unsigned long default_val;
+	const enum osmo_tdef_unit unit;
+	/*! Human readable description. For unit == OSMO_TDEF_CUSTOM, this should include an explanation of the value's
+	 * unit. Best keep this a short one-liner (e.g. for VTY output). */
+	const char *desc;
+	/*! Currently active timeout value, e.g. set by user config. This is the only mutable member: a user may
+	 * configure the timeout value, but neither unit nor any other field. */
+	unsigned long val;
+};
+
+/*! Iterate an array of struct osmo_tdef, the last item should be fully zero, i.e. "{}".
+ * Example:
+ *
+ *     struct osmo_tdef *t;
+ *     osmo_tdef_for_each(t, tdefs) {
+ *             printf("%lu %s %s\n", t->val, osmo_tdef_unit_name(t->unit), t->desc);
+ *     }
+ *
+ * \param[inout] t  A struct osmo_tdef *t used for iteration, will point at the current entry inside the loop scope.
+ * \param[in] tdefs  Array of struct osmo_tdef to iterate, zero-terminated.
+ */
+#define osmo_tdef_for_each(t, tdefs) \
+	for (t = tdefs; t && (t->T || t->default_val || t->desc); t++)
+
+void osmo_tdefs_reset(struct osmo_tdef *tdefs);
+unsigned long osmo_tdef_get(const struct osmo_tdef *tdefs, int T, enum osmo_tdef_unit as_unit,
+			    unsigned long val_if_not_present);
+struct osmo_tdef *osmo_tdef_get_entry(struct osmo_tdef *tdefs, int T);
+
+/*! Using osmo_tdef for osmo_fsm_inst: array entry for a mapping of state numbers to timeout definitions.
+ * For a usage example, see osmo_tdef_get_state_timeout() and test_tdef_state_timeout() in tdef_test.c. */
+struct osmo_tdef_state_timeout {
+	/*! Timer number to match struct osmo_tdef.T, and to pass to osmo_fsm_inst_state_chg(). */
+	int T;
+	/*! If true, call osmo_fsm_inst_state_chg_keep_timer().
+	 * If T == 0, keep previous T number, otherwise also set fi->T. */
+	bool keep_timer;
+};
+
+const struct osmo_tdef_state_timeout *osmo_tdef_get_state_timeout(uint32_t state,
+								  const struct osmo_tdef_state_timeout *timeouts_array);
+
+/*! Call osmo_fsm_inst_state_chg() or osmo_fsm_inst_state_chg_keep_timer(), depending on the timeouts_array, tdefs and
+ * default_timeout.
+ *
+ * A T timer configured in sub-second precision is rounded up to the next full second. A timer in unit =
+ * OSMO_TDEF_CUSTOM is applied as if the unit is in seconds (i.e. this macro does not make sense for custom units!).
+ *
+ * See osmo_tdef_get_state_timeout() and osmo_tdef_get().
+ *
+ * If no T timer is defined for the given state (T == 0), invoke the state change without a timeout.
+ *
+ * Should a T number be defined in timeouts_array that is not defined in tdefs, use default_timeout (in seconds). If
+ * default_timeout is negative, a missing T definition in tdefs instead causes a program abort.
+ *
+ * This is best used by wrapping this function call in a macro suitable for a specific FSM implementation, which can
+ * become as short as: my_fsm_state_chg(fi, NEXT_STATE):
+ *
+ *     #define my_fsm_state_chg(fi, NEXT_STATE) \
+ *             osmo_tdef_fsm_inst_state_chg(fi, NEXT_STATE, my_fsm_timeouts, global_T_defs, 5)
+ *
+ *     my_fsm_state_chg(fi, MY_FSM_STATE_1);
+ *     // -> No timeout configured, will enter state without timeout.
+ *
+ *     my_fsm_state_chg(fi, MY_FSM_STATE_3);
+ *     // T423 configured for this state, will look up T423 in tdefs, or use 5 seconds if unset.
+ *
+ *     my_fsm_state_chg(fi, MY_FSM_STATE_8);
+ *     // keep_timer == true for this state, will invoke osmo_fsm_inst_state_chg_keep_timer().
+ *
+ * \param[inout] fi  osmo_fsm_inst to transition to another state.
+ * \param[in] state  State number to transition to.
+ * \param[in] timeouts_array  Array of struct osmo_tdef_state_timeout[32] to look up state in.
+ * \param[in] tdefs  Array of struct osmo_tdef (last entry zero initialized) to look up T in.
+ * \param[in] default_timeout  If a T is set in timeouts_array, but no timeout value is configured for T, then use this
+ *                             default timeout value as fallback, or pass -1 to abort the program.
+ * \return Return value from osmo_fsm_inst_state_chg() or osmo_fsm_inst_state_chg_keep_timer().
+ */
+#define osmo_tdef_fsm_inst_state_chg(fi, state, timeouts_array, tdefs, default_timeout) \
+	_osmo_tdef_fsm_inst_state_chg(fi, state, timeouts_array, tdefs, default_timeout, \
+				      __FILE__, __LINE__)
+int _osmo_tdef_fsm_inst_state_chg(struct osmo_fsm_inst *fi, uint32_t state,
+				  const struct osmo_tdef_state_timeout *timeouts_array,
+				  const struct osmo_tdef *tdefs, unsigned long default_timeout,
+				  const char *file, int line);
+
+/*! Manage timer definitions in named groups.
+ * This should be defined as an array with the final element kept fully zero-initialized,
+ * to be compatible with osmo_tdef_vty* API. There must not be any tdefs == NULL entries except on the final
+ * zero-initialized entry. */
+struct osmo_tdef_group {
+	const char *name;
+	const char *desc;
+	struct osmo_tdef *tdefs;
+};
+
+/*! Iterate an array of struct osmo_tdef_group, the last item should be fully zero, i.e. "{}".
+ * \param[inout] g  A struct osmo_tdef_group *g used for iteration, will point at the current entry inside the loop scope.
+ * \param[in] tdefs  Array of struct osmo_tdef_group to iterate, zero-terminated.
+ */
+#define osmo_tdef_groups_for_each(g, tdef_groups) \
+	for (g = tdef_groups; g && g->tdefs; g++)
+
+/*! @} */
diff --git a/include/osmocom/vty/tdef_vty.h b/include/osmocom/vty/tdef_vty.h
new file mode 100644
index 0000000..f55239a
--- /dev/null
+++ b/include/osmocom/vty/tdef_vty.h
@@ -0,0 +1,67 @@
+/*! \file tdef_vty.h
+ * API to configure osmo_tdef Tnnn timers from VTY configuration.
+ */
+/* (C) 2018-2019 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * Author: Neels Hofmeyr <neels@hofmeyr.de>
+ *
+ * All Rights Reserved
+ *
+ * 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/>.
+ */
+#pragma once
+
+#include <stdint.h>
+
+struct vty;
+
+/*! \defgroup Tdef_VTY  Tnnn timer VTY configuration
+ * @{
+ * \file tdef_vty.h
+ */
+
+struct osmo_tdef;
+struct osmo_tdef_group;
+
+#define OSMO_TDEF_VTY_ARG_T "TNNNN"
+#define OSMO_TDEF_VTY_DOC_T "T-number, optionally preceded by 't' or 'T'.\n"
+#define OSMO_TDEF_VTY_ARG_T_OPTIONAL "[" OSMO_TDEF_VTY_ARG_T "]"
+
+#define OSMO_TDEF_VTY_ARG_VAL "(<0-2147483647>|default)"
+#define OSMO_TDEF_VTY_DOC_VAL "New timer value\n" "Set to default timer value\n"
+#define OSMO_TDEF_VTY_ARG_VAL_OPTIONAL "[" OSMO_TDEF_VTY_ARG_VAL "]"
+
+#define OSMO_TDEF_VTY_ARG_SET	OSMO_TDEF_VTY_ARG_T " " OSMO_TDEF_VTY_ARG_VAL
+#define OSMO_TDEF_VTY_DOC_SET	OSMO_TDEF_VTY_DOC_T OSMO_TDEF_VTY_DOC_VAL
+#define OSMO_TDEF_VTY_ARG_SET_OPTIONAL	OSMO_TDEF_VTY_ARG_T_OPTIONAL " " OSMO_TDEF_VTY_ARG_VAL_OPTIONAL
+
+int osmo_tdef_vty_set_cmd(struct vty *vty, struct osmo_tdef *tdefs, const char **args);
+int osmo_tdef_vty_show_cmd(struct vty *vty, struct osmo_tdef *tdefs, const char *T_arg,
+			   const char *prefix_fmt, ...);
+void osmo_tdef_vty_write(struct vty *vty, struct osmo_tdef *tdefs,
+			 const char *prefix_fmt, ...);
+
+void osmo_tdef_vty_out_one(struct vty *vty, struct osmo_tdef *t, const char *prefix_fmt, ...);
+void osmo_tdef_vty_out_all(struct vty *vty, struct osmo_tdef *tdefs, const char *prefix_fmt, ...);
+
+void osmo_tdef_vty_out_one_va(struct vty *vty, struct osmo_tdef *t, const char *prefix_fmt, va_list va);
+void osmo_tdef_vty_out_all_va(struct vty *vty, struct osmo_tdef *tdefs, const char *prefix_fmt, va_list va);
+
+struct osmo_tdef *osmo_tdef_vty_parse_T_arg(struct vty *vty, struct osmo_tdef *tdefs, const char *osmo_tdef_str);
+unsigned long osmo_tdef_vty_parse_val_arg(const char *val_arg, unsigned long default_val);
+
+void osmo_tdef_vty_groups_init(enum node_type parent_node, struct osmo_tdef_group *groups);
+void osmo_tdef_vty_groups_write(struct vty *vty, const char *indent);
+
+/*! @} */