diff --git a/core.c b/core.c
new file mode 100644
index 0000000..575d14f
--- /dev/null
+++ b/core.c
@@ -0,0 +1,197 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+
+#include <assert.h>
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#undef pr_fmt
+#define pr_fmt(fmt) "core: " fmt
+
+#include "libmctp.h"
+#include "libmctp-alloc.h"
+#include "libmctp-log.h"
+
+/* Internal data structures */
+
+struct mctp_bus {
+	mctp_eid_t		eid;
+	struct mctp_binding	*binding;
+
+	/* todo: routing */
+};
+
+struct mctp {
+	/* todo: multiple busses */
+	struct mctp_bus	busses[1];
+
+	struct mctp_pktbuf	txbuf;
+
+	/* Message RX callback */
+	mctp_rx_fn		message_rx;
+	void			*message_rx_data;
+};
+
+#ifndef BUILD_ASSERT
+#define BUILD_ASSERT(x) \
+	do { (void)sizeof(char[0-(!(x))]); } while (0)
+#endif
+
+struct mctp_pktbuf *mctp_pktbuf_alloc(uint8_t len)
+{
+	struct mctp_pktbuf *buf;
+
+	BUILD_ASSERT(MCTP_PKTBUF_SIZE <= 0xff);
+
+	/* todo: pools */
+	buf = __mctp_alloc(sizeof(*buf));
+
+	buf->start = MCTP_PKTBUF_BINDING_PAD;
+	buf->end = buf->start + len;
+	buf->mctp_hdr_off = buf->start;
+
+	return buf;
+}
+
+void mctp_pktbuf_free(struct mctp_pktbuf *pkt)
+{
+	__mctp_free(pkt);
+}
+
+struct mctp_hdr *mctp_pktbuf_hdr(struct mctp_pktbuf *pkt)
+{
+	return (void *)pkt->data + pkt->mctp_hdr_off;
+}
+
+void *mctp_pktbuf_data(struct mctp_pktbuf *pkt)
+{
+	return (void *)pkt->data + pkt->mctp_hdr_off + sizeof(struct mctp_hdr);
+}
+
+uint8_t mctp_pktbuf_size(struct mctp_pktbuf *pkt)
+{
+	return pkt->end - pkt->start;
+}
+
+void *mctp_pktbuf_alloc_start(struct mctp_pktbuf *pkt, uint8_t size)
+{
+	assert(size <= pkt->start);
+	pkt->start -= size;
+	return pkt->data + pkt->start;
+}
+
+void *mctp_pktbuf_alloc_end(struct mctp_pktbuf *pkt, uint8_t size)
+{
+	void *buf;
+
+	assert(size < (MCTP_PKTBUF_SIZE - pkt->end));
+	buf = pkt->data + pkt->end;
+	pkt->end += size;
+	return buf;
+}
+
+int mctp_pktbuf_push(struct mctp_pktbuf *pkt, void *data, uint8_t len)
+{
+	void *p;
+
+	assert(pkt->end + len <= MCTP_PKTBUF_SIZE);
+
+	if (pkt->end + len > MCTP_PKTBUF_SIZE)
+		return -1;
+
+	p = pkt->data + pkt->end;
+
+	pkt->end += len;
+	memcpy(p, data, len);
+
+	return 0;
+}
+
+struct mctp *mctp_init(void)
+{
+	struct mctp *mctp;
+
+	mctp = __mctp_alloc(sizeof(*mctp));
+	memset(mctp, 0, sizeof(*mctp));
+
+	return mctp;
+}
+
+int mctp_set_rx_all(struct mctp *mctp, mctp_rx_fn fn, void *data)
+{
+	mctp->message_rx = fn;
+	mctp->message_rx_data = data;
+	return 0;
+}
+
+static struct mctp_bus *find_bus_for_eid(struct mctp *mctp,
+		mctp_eid_t dest __attribute__((unused)))
+{
+	return &mctp->busses[0];
+}
+
+unsigned long mctp_register_bus(struct mctp *mctp,
+		struct mctp_binding *binding,
+		mctp_eid_t eid)
+{
+	assert(!mctp->busses[0].binding);
+	mctp->busses[0].binding = binding;
+	mctp->busses[0].eid = eid;
+	return 0;
+}
+
+void mctp_bus_rx(struct mctp *mctp, unsigned long bus_id,
+		struct mctp_pktbuf *pkt)
+{
+	struct mctp_bus *bus = &mctp->busses[bus_id];
+	size_t len;
+	void *p;
+
+	len = pkt->end - pkt->mctp_hdr_off - sizeof(struct mctp_hdr);
+	p = pkt->data + pkt->mctp_hdr_off + sizeof(struct mctp_hdr),
+	mctp->message_rx(bus->eid, mctp->message_rx_data, p, len);
+}
+
+static int mctp_packet_tx(struct mctp *mctp __attribute__((unused)),
+		struct mctp_bus *bus,
+		struct mctp_pktbuf *pkt)
+{
+	return bus->binding->tx(bus->binding, pkt);
+}
+
+int mctp_message_tx(struct mctp *mctp, mctp_eid_t eid,
+		void *msg, size_t msg_len)
+{
+	struct mctp_pktbuf *pkt;
+	struct mctp_hdr *hdr;
+	struct mctp_bus *bus;
+	int rc;
+
+	/* todo: multiple-packet messages, sequence numbers */
+	assert(msg_len <= MCTP_MTU);
+
+	bus = find_bus_for_eid(mctp, eid);
+
+	pkt = mctp_pktbuf_alloc(msg_len + sizeof(*hdr));
+	hdr = mctp_pktbuf_hdr(pkt);
+
+	/* todo: tags */
+	hdr->ver = bus->binding->version & 0xf;
+	hdr->dest = eid;
+	hdr->src = bus->eid;
+	hdr->flags_seq_tag = MCTP_HDR_FLAG_SOM |
+		MCTP_HDR_FLAG_EOM |
+		(0 << MCTP_HDR_SEQ_SHIFT) |
+		MCTP_HDR_FLAG_TO |
+		(0 << MCTP_HDR_TAG_SHIFT);
+
+	/* todo: zero copy? */
+	memcpy(mctp_pktbuf_data(pkt), msg, msg_len);
+
+	rc = mctp_packet_tx(mctp, bus, pkt);
+
+	return rc;
+}