diff --git a/src/gsupclient/gsup_req.c b/src/gsupclient/gsup_req.c
new file mode 100644
index 0000000..4a2ff23
--- /dev/null
+++ b/src/gsupclient/gsup_req.c
@@ -0,0 +1,410 @@
+/* Copyright 2019 by sysmocom s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * 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.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <errno.h>
+#include <inttypes.h>
+
+#include <osmocom/core/logging.h>
+#include <osmocom/gsm/gsm23003.h>
+
+#include <osmocom/gsupclient/gsup_req.h>
+
+/*! Create a new osmo_gsup_req record, decode GSUP and add to a provided list of requests.
+ *
+ * Rationales:
+ *
+ * - osmo_gsup_req makes it easy to handle GSUP requests asynchronously. Before this, a GSUP message struct would be
+ *   valid only within a read callback function, and would not survive asynchronous handling, because the struct often
+ *   points directly into the received msgb. An osmo_gsup_req takes ownership of the msgb and ensures that the data
+ *   remains valid, so that it can easily be queued for later handling.
+ * - osmo_gsup_req unifies the composition of response messages to ensure that all IEs that identify it to belong to
+ *   the initial request are preserved / derived, like the source_name, destination_name, session_id, etc (see
+ *   osmo_gsup_make_response() for details).
+ * - Deallocation of an osmo_gsup_req is implicit upon sending a response. The idea is that msgb memory leaks are a
+ *   recurring source of bugs. By enforcing a request-response relation with implicit deallocation, osmo_gsup_req aims
+ *   to help avoid most such memory leaks implicitly.
+ *
+ * The typical GSUP message sequence is:
+ *   -> rx request,
+ *   <- tx response.
+ *
+ * With osmo_gsup_req we can easily expand to:
+ *   -> rx request,
+ *   ... wait asynchronously,
+ *   <- tx response.
+ *
+ * Only few GSUP conversations go beyond a 1:1 request-response match. But some have a session (e.g. USSD) or more
+ * negotiation may happen before the initial request is completed (e.g. Update Location with interleaved Insert
+ * Subscriber Data), so osmo_gsup_req also allows passing non-final responses.
+ * The final_response flag allows for:
+ *    -> rx request,
+ *    ... wait async,
+ *    <- tx intermediate message to same peer (final_response = false, req remains open),
+ *    ... wait async,
+ *    -> rx intermediate response,
+ *    ... wait async,
+ *    <- tx final response (final_response = true, req is deallocated).
+ *
+ * This function takes ownership of the msgb, which will, on success, be owned by the returned osmo_gsup_req instance
+ * until osmo_gsup_req_free(). If a decoding error occurs, send an error response immediately, and return NULL.
+ *
+ * The original CNI entity that sent the message is found in req->source_name. If the message was passed on by an
+ * intermediate CNI peer, then req->via_proxy is set to the immediate peer, and it is the responsibility of the caller
+ * to add req->source_name to the GSUP routes that are serviced by req->via_proxy (usually not relevant for clients with
+ * a single GSUP conn).
+ * Examples:
+ *
+ *   "msc" ---> here
+ *   source_name = "msc"
+ *   via_proxy = <empty>
+ *
+ *   "msc" ---> "proxy-HLR" ---> here (e.g. home HLR)
+ *   source_name = "msc"
+ *   via_proxy = "proxy-HLR"
+ *
+ *   "msc" ---> "proxy-HLR" ---> "home-HLR" ---> here (e.g. EUSE)
+ *   source_name = "msc"
+ *   via_proxy = "home-HLR"
+ *
+ * An osmo_gsup_req must be concluded (and deallocated) by calling one of the osmo_gsup_req_respond* functions.
+ *
+ * Note: osmo_gsup_req API makes use of OTC_SELECT to allocate volatile buffers for logging. Use of
+ * osmo_select_main_ctx() is mandatory when using osmo_gsup_req.
+ *
+ * \param[in] ctx  Talloc context for allocation of the new request.
+ * \param[in] from_peer  The IPA unit name of the immediate GSUP peer from which this msgb was received.
+ * \param[in] msg  The message buffer containing the received GSUP message, where msgb_l2() shall point to the GSUP
+ *                 message start. The caller no longer owns the msgb when it is passed to this function: on error, the
+ *                 msgb is freed immediately, and on success, the msgb is owned by the returned osmo_gsup_req.
+ * \param[in] send_response_cb  User specific method to send a GSUP response message, invoked upon
+ *				osmo_gsup_req_respond*() functions. Typically this invokes encoding and transmitting the
+ *				GSUP message over a network socket. See for example gsup_server_send_req_response().
+ * \param[inout] cb_data  Context data to be used freely by the caller.
+ * \param[inout] add_to_list  List to which to append this request, or NULL for no list.
+ * \return a newly allocated osmo_gsup_req, or NULL on error. If NULL is returned, an error response has already been
+ *         dispatched to the send_response_cb.
+ */
+struct osmo_gsup_req *osmo_gsup_req_new(void *ctx, const struct osmo_ipa_name *from_peer, struct msgb *msg,
+					osmo_gsup_req_send_response_t send_response_cb, void *cb_data,
+					struct llist_head *add_to_list)
+{
+	static unsigned int next_req_nr = 1;
+	struct osmo_gsup_req *req;
+	int rc;
+
+	if (!msgb_l2(msg) || !msgb_l2len(msg)) {
+		LOGP(DLGSUP, LOGL_ERROR, "Rx GSUP from %s: missing or empty L2 data\n",
+		     osmo_ipa_name_to_str(from_peer));
+		msgb_free(msg);
+		return NULL;
+	}
+
+	req = talloc_zero(ctx, struct osmo_gsup_req);
+	OSMO_ASSERT(req);
+	/* Note: req->gsup is declared const, so that the incoming message cannot be modified by handlers. */
+	req->nr = next_req_nr++;
+	req->msg = msg;
+	req->send_response_cb = send_response_cb;
+	req->cb_data = cb_data;
+	if (from_peer)
+		req->source_name = *from_peer;
+	rc = osmo_gsup_decode(msgb_l2(req->msg), msgb_l2len(req->msg), (struct osmo_gsup_message*)&req->gsup);
+	if (rc < 0) {
+		LOGP(DLGSUP, LOGL_ERROR, "Rx GSUP from %s: cannot decode (rc=%d)\n", osmo_ipa_name_to_str(from_peer), rc);
+		osmo_gsup_req_free(req);
+		return NULL;
+	}
+
+	LOG_GSUP_REQ(req, LOGL_DEBUG, "new request: {%s}\n", osmo_gsup_message_to_str_c(OTC_SELECT, &req->gsup));
+
+	if (req->gsup.source_name_len) {
+		if (osmo_ipa_name_set(&req->source_name, req->gsup.source_name, req->gsup.source_name_len)) {
+			LOGP(DLGSUP, LOGL_ERROR,
+			     "Rx GSUP from %s: failed to decode source_name, message is not routable\n",
+			     osmo_ipa_name_to_str(from_peer));
+			osmo_gsup_req_respond_msgt(req, OSMO_GSUP_MSGT_ROUTING_ERROR, true);
+			return NULL;
+		}
+
+		/* The source of the GSUP message is not the immediate GSUP peer; the peer is our proxy for that source.
+		 */
+		if (osmo_ipa_name_cmp(&req->source_name, from_peer))
+			req->via_proxy = *from_peer;
+	}
+
+	if (!osmo_imsi_str_valid(req->gsup.imsi)) {
+		osmo_gsup_req_respond_err(req, GMM_CAUSE_INV_MAND_INFO, "invalid IMSI: %s",
+					  osmo_quote_str(req->gsup.imsi, -1));
+		return NULL;
+	}
+
+	if (add_to_list)
+		llist_add_tail(&req->entry, add_to_list);
+	return req;
+}
+
+/*! Free an osmo_gsup_req and its msgb -- this is usually implicit in osmo_gsup_req_resond_*(), it should not be
+ * necessary to call this directly. */
+void osmo_gsup_req_free(struct osmo_gsup_req *req)
+{
+	LOG_GSUP_REQ(req, LOGL_DEBUG, "free\n");
+	if (req->msg)
+		msgb_free(req->msg);
+	if (req->entry.prev)
+		llist_del(&req->entry);
+	talloc_free(req);
+}
+
+/*! Send a response to a GSUP request.
+ *
+ * Ensure that the response message contains all GSUP IEs that identify it as a response for the request req, by calling
+ * osmo_gsup_make_response().
+ *
+ * The final complete response message is passed to req->send_response_cb() to take care of the transmission.
+ *
+ * \param req  Request as previously initialized by osmo_gsup_req_new().
+ * \param response  Buffer to compose the response, possibly with some pre-configured IEs.
+ *                  Any missing IEs are added via osmo_gsup_make_response().
+ *                  Must not be NULL. Does not need to remain valid memory beyond the function call,
+ *                  i.e. this can just be a local variable in the calling function.
+ * \param error  True when the response message indicates an error response (error message type).
+ * \param final_response  True when the request is concluded by this response, which deallocates the req.
+ *                        False when the request should remain open after this response.
+ *                        For most plain request->response GSUP messages, this should be True.
+ * \param file  Source file for logging as in __FILE__, added by osmo_gsup_req_respond() macro.
+ * \param line  Source line for logging as in __LINE__, added by osmo_gsup_req_respond() macro.
+ */
+int _osmo_gsup_req_respond(struct osmo_gsup_req *req, struct osmo_gsup_message *response,
+			   bool error, bool final_response, const char *file, int line)
+{
+	int rc;
+
+	rc = osmo_gsup_make_response(response, &req->gsup, error, final_response);
+	if (rc) {
+		LOG_GSUP_REQ_SRC(req, LOGL_ERROR, file, line, "Invalid response (rc=%d): {%s}\n",
+				 rc, osmo_gsup_message_to_str_c(OTC_SELECT, response));
+		rc = -EINVAL;
+		goto exit_cleanup;
+	}
+
+	if (!req->send_response_cb) {
+		LOG_GSUP_REQ_SRC(req, LOGL_ERROR, file, line, "No send_response_cb set, cannot send: {%s}\n",
+				 osmo_gsup_message_to_str_c(OTC_SELECT, response));
+		rc = -EINVAL;
+		goto exit_cleanup;
+	}
+
+	LOG_GSUP_REQ_SRC(req, LOGL_DEBUG, file, line, "Tx response: {%s}\n",
+			 osmo_gsup_message_to_str_c(OTC_SELECT, response));
+	req->send_response_cb(req, response);
+
+exit_cleanup:
+	if (final_response)
+		osmo_gsup_req_free(req);
+	return rc;
+}
+
+/*! Shorthand for _osmo_gsup_req_respond() with no additional IEs and a fixed message type.
+ * Set the message type in a local osmo_gsup_message and feed it to _osmo_gsup_req_respond().
+ * That will ensure to add all IEs that identify it as a response to req.
+ *
+ * \param req  Request as previously initialized by osmo_gsup_req_new().
+ * \param message_type  The GSUP message type discriminator to respond with.
+ * \param final_response  True when the request is concluded by this response, which deallocates the req.
+ *                        False when the request should remain open after this response.
+ *                        For most plain request->response GSUP messages, this should be True.
+ * \param file  Source file for logging as in __FILE__, added by osmo_gsup_req_respond_msgt() macro.
+ * \param line  Source line for logging as in __LINE__, added by osmo_gsup_req_respond_msgt() macro.
+ */
+int _osmo_gsup_req_respond_msgt(struct osmo_gsup_req *req, enum osmo_gsup_message_type message_type,
+				bool final_response, const char *file, int line)
+{
+	struct osmo_gsup_message response = {
+		.message_type = message_type,
+	};
+	return _osmo_gsup_req_respond(req, &response, OSMO_GSUP_IS_MSGT_ERROR(message_type), final_response,
+				      file, line);
+}
+
+/*! Shorthand for _osmo_gsup_req_respond() with an error cause IEs and using the req's matched error message type.
+ * Set the error cause in a local osmo_gsup_message and feed it to _osmo_gsup_req_respond().
+ * That will ensure to add all IEs that identify it as a response to req.
+ *
+ * Responding with an error always implies a final response: req is implicitly deallocated.
+ *
+ * \param req  Request as previously initialized by osmo_gsup_req_new().
+ * \param cause  The error cause to include in a OSMO_GSUP_CAUSE_IE.
+ * \param file  Source file for logging as in __FILE__, added by osmo_gsup_req_respond_err() macro.
+ * \param line  Source line for logging as in __LINE__, added by osmo_gsup_req_respond_err() macro.
+ */
+void _osmo_gsup_req_respond_err(struct osmo_gsup_req *req, enum gsm48_gmm_cause cause,
+				const char *file, int line)
+{
+	struct osmo_gsup_message response = {
+		.cause = cause,
+	};
+
+	/* No need to answer if we couldn't parse an ERROR message type, only REQUESTs need an error reply. */
+	if (!OSMO_GSUP_IS_MSGT_REQUEST(req->gsup.message_type)) {
+		osmo_gsup_req_free(req);
+		return;
+	}
+
+	osmo_gsup_req_respond(req, &response, true, true);
+}
+
+/*! This function is implicitly called by the osmo_gsup_req API, if at all possible rather use osmo_gsup_req_respond().
+ * This function is non-static mostly to allow unit testing.
+ *
+ * Set fields, if still unset, that need to be copied from a received message over to its response message, to ensure
+ * the response can be routed back to the requesting peer even via GSUP proxies.
+ *
+ * Note: after calling this function, fields in the reply may reference the same memory as rx and are not deep-copied,
+ * as is the usual way we are handling decoded GSUP messages.
+ *
+ * These fields are set in the reply message, iff they are still unset:
+ * - Set reply->message_type to the rx's matching RESULT code (or ERROR code if error == true).
+ * - IMSI,
+ * - Set reply->destination_name to rx->source_name (for proxy routing),
+ * - sm_rp_mr (for SMS),
+ * - session_id (for SS/USSD),
+ * - if rx->session_state is not NONE, set tx->session_state depending on the final_response argument:
+ *   If false, set to OSMO_GSUP_SESSION_STATE_CONTINUE, else OSMO_GSUP_SESSION_STATE_END.
+ *
+ * If values in reply are already set, they will not be overwritten. The return code is an optional way of finding out
+ * whether all values that were already set in 'reply' are indeed matching the 'rx' values that would have been set.
+ *
+ * \param[in] rx  Received GSUP message that is being replied to.
+ * \param[inout] reply  The message that should be the response to rx, either empty or with some values already set up.
+ * \return 0 if the resulting message is a valid response for rx, nonzero otherwise. A nonzero rc has no effect on the
+ *         values set in the reply message: all unset fields are first updated, and then the rc is determined.
+ *         The rc is intended to merely warn if the reply message already contained data that is incompatible with rx,
+ *         e.g. a mismatching IMSI.
+ */
+int osmo_gsup_make_response(struct osmo_gsup_message *reply,
+			    const struct osmo_gsup_message *rx, bool error, bool final_response)
+{
+	int rc = 0;
+
+	if (!reply->message_type) {
+		if (error)
+			reply->message_type = OSMO_GSUP_TO_MSGT_ERROR(rx->message_type);
+		else
+			reply->message_type = OSMO_GSUP_TO_MSGT_RESULT(rx->message_type);
+	}
+
+	if (*reply->imsi == '\0')
+		OSMO_STRLCPY_ARRAY(reply->imsi, rx->imsi);
+
+	if (reply->message_class == OSMO_GSUP_MESSAGE_CLASS_UNSET)
+		reply->message_class = rx->message_class;
+
+	if (!reply->destination_name || !reply->destination_name_len) {
+		reply->destination_name = rx->source_name;
+		reply->destination_name_len = rx->source_name_len;
+	}
+
+	/* RP-Message-Reference is mandatory for SM Service */
+	if (!reply->sm_rp_mr)
+		reply->sm_rp_mr = rx->sm_rp_mr;
+
+	/* For SS/USSD, it's important to keep both session state and ID IEs */
+	if (!reply->session_id)
+		reply->session_id = rx->session_id;
+	if (rx->session_state != OSMO_GSUP_SESSION_STATE_NONE
+	    && reply->session_state == OSMO_GSUP_SESSION_STATE_NONE) {
+		if (final_response || rx->session_state == OSMO_GSUP_SESSION_STATE_END)
+			reply->session_state = OSMO_GSUP_SESSION_STATE_END;
+		else
+			reply->session_state = OSMO_GSUP_SESSION_STATE_CONTINUE;
+	}
+
+	if (strcmp(reply->imsi, rx->imsi))
+		rc |= 1 << 0;
+	if (reply->message_class != rx->message_class)
+		rc |= 1 << 1;
+	if (rx->sm_rp_mr && (!reply->sm_rp_mr || *rx->sm_rp_mr != *reply->sm_rp_mr))
+		rc |= 1 << 2;
+	if (reply->session_id != rx->session_id)
+		rc |= 1 << 3;
+	return rc;
+}
+
+/*! Print the most important value of a GSUP message to a string buffer in human readable form.
+ * \param[out] buf  The buffer to write to.
+ * \param[out] buflen  sizeof(buf).
+ * \param[in] msg  GSUP message to print.
+ */
+size_t osmo_gsup_message_to_str_buf(char *buf, size_t buflen, const struct osmo_gsup_message *msg)
+{
+	struct osmo_strbuf sb = { .buf = buf, .len = buflen };
+	if (!msg) {
+		OSMO_STRBUF_PRINTF(sb, "NULL");
+		return sb.chars_needed;
+	}
+
+	if (msg->message_class)
+		OSMO_STRBUF_PRINTF(sb, "%s ", osmo_gsup_message_class_name(msg->message_class));
+
+	OSMO_STRBUF_PRINTF(sb, "%s:", osmo_gsup_message_type_name(msg->message_type));
+
+	OSMO_STRBUF_PRINTF(sb, " imsi=");
+	OSMO_STRBUF_APPEND(sb, osmo_quote_cstr_buf, msg->imsi, strnlen(msg->imsi, sizeof(msg->imsi)));
+
+	if (msg->cause)
+		OSMO_STRBUF_PRINTF(sb, " cause=%s", get_value_string(gsm48_gmm_cause_names, msg->cause));
+
+	switch (msg->cn_domain) {
+	case OSMO_GSUP_CN_DOMAIN_CS:
+		OSMO_STRBUF_PRINTF(sb, " cn_domain=CS");
+		break;
+	case OSMO_GSUP_CN_DOMAIN_PS:
+		OSMO_STRBUF_PRINTF(sb, " cn_domain=PS");
+		break;
+	default:
+		if (msg->cn_domain)
+			OSMO_STRBUF_PRINTF(sb, " cn_domain=?(%d)", msg->cn_domain);
+		break;
+	}
+
+	if (msg->source_name_len) {
+		OSMO_STRBUF_PRINTF(sb, " source_name=");
+		OSMO_STRBUF_APPEND(sb, osmo_quote_cstr_buf, (char*)msg->source_name, msg->source_name_len);
+	}
+
+	if (msg->destination_name_len) {
+		OSMO_STRBUF_PRINTF(sb, " destination_name=");
+		OSMO_STRBUF_APPEND(sb, osmo_quote_cstr_buf, (char*)msg->destination_name, msg->destination_name_len);
+	}
+
+	if (msg->session_id)
+		OSMO_STRBUF_PRINTF(sb, " session_id=%" PRIu32, msg->session_id);
+	if (msg->session_state)
+		OSMO_STRBUF_PRINTF(sb, " session_state=%s", osmo_gsup_session_state_name(msg->session_state));
+
+	if (msg->sm_rp_mr)
+		OSMO_STRBUF_PRINTF(sb, " sm_rp_mr=%" PRIu8, *msg->sm_rp_mr);
+
+	return sb.chars_needed;
+}
+
+/*! Same as  osmo_gsup_message_to_str_buf() but returns a talloc allocated string. */
+char *osmo_gsup_message_to_str_c(void *ctx, const struct osmo_gsup_message *msg)
+{
+	OSMO_NAME_C_IMPL(ctx, 64, "ERROR", osmo_gsup_message_to_str_buf, msg)
+}