diff --git a/i2c-internal.h b/i2c-internal.h
new file mode 100644
index 0000000..95ab81d
--- /dev/null
+++ b/i2c-internal.h
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later */
+#pragma once
+
+#include <assert.h>
+#include "libmctp.h"
+#include "libmctp-i2c.h"
+
+/* Limited by bytecount field */
+static_assert(I2C_BTU <= 254);
+
+#ifndef MCTP_I2C_NEIGH_COUNT
+#define MCTP_I2C_NEIGH_COUNT 4
+#endif
+
+struct mctp_i2c_hdr {
+	uint8_t dest;
+	uint8_t cmd;
+	uint8_t bytecount;
+	uint8_t source;
+};
+
+struct mctp_i2c_neigh {
+	bool used;
+	/* 7-bit address */
+	uint8_t addr;
+	uint8_t eid;
+	/* from platform_now(), for LRU eviction */
+	uint64_t last_seen_timestamp;
+};
+
+struct mctp_binding_i2c {
+	struct mctp_binding binding;
+
+	struct mctp_i2c_neigh neigh[MCTP_I2C_NEIGH_COUNT];
+
+	uint8_t own_addr;
+
+	uint8_t tx_storage[sizeof(struct mctp_i2c_hdr) +
+			   MCTP_PKTBUF_SIZE(I2C_BTU)] __attribute((aligned(8)));
+	uint8_t rx_storage[sizeof(struct mctp_i2c_hdr) +
+			   MCTP_PKTBUF_SIZE(I2C_BTU)] __attribute((aligned(8)));
+
+	mctp_i2c_tx_fn tx_fn;
+	void *tx_ctx;
+};