Add Reliable Asynchronous Transfer Protocol

This patch adds support for Reliable Asynchronous Transfer Protocol (RATP)
as described in RFC916.

Communication over RS232 is often unreliable as characters are lost or
misinterpreted. This protocol allows for a reliable packet based communication
over serial lines.

The implementation simply follows the state machine described in the RFC
text with one exception. RFC916 uses a plain checksum for the
transferred data. We decided to use CRC16 for greater robustness. Since
this is the only RFC916 implementation we currently know of interoperability
with other implementations should not matter.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Andrey Smirnov <andrew.smirnov@gmail.com>

3 files changed, 1843 insertions, 0 deletions

diff --git a/lib/Kconfig b/lib/Kconfig
index fbf9f0f348..a7e067e571 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -55,6 +55,14 @@ config LIBMTD
 config STMP_DEVICE
 	bool
 
+config RATP
+	select CRC16
+	bool
+	help
+	  Reliable Asynchronous Transfer Protocol (RATP) is a protocol for reliably
+	  transferring packets over serial links described in RFC916. This implementation
+	  is used for controlling barebox over serial ports.
+
 source lib/gui/Kconfig
 
 source lib/fonts/Kconfig
diff --git a/lib/Makefile b/lib/Makefile
index abb34cfbdb..0694721e9e 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -56,3 +56,4 @@ obj-y			+= gcd.o
 obj-y			+= hexdump.o
 obj-$(CONFIG_FONTS)	+= fonts/
 obj-$(CONFIG_BAREBOX_LOGO)     += logo/
+obj-$(CONFIG_RATP)	+= ratp.o
diff --git a/lib/ratp.c b/lib/ratp.c
new file mode 100644
index 0000000000..d596a0e8b2
--- /dev/null
+++ b/lib/ratp.c
@@ -0,0 +1,1834 @@
+/*
+ * barebox RATP implementation.
+ * This is the barebox implementation for the Reliable Asynchronous
+ * Transfer Protocol (RATP) as described in RFC916.
+ *
+ * Copyright (C) 2015 Pengutronix, Sascha Hauer <s.hauer@pengutronix.de>
+ *
+ * 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.
+ */
+
+#define pr_fmt(fmt)  "ratp: " fmt
+
+#include <common.h>
+#include <malloc.h>
+#include <getopt.h>
+#include <ratp.h>
+#include <crc.h>
+#include <clock.h>
+#include <asm/unaligned.h>
+
+/*
+ * RATP packet format:
+ *
+ * Byte No.
+ *
+ *        +-------------------------------+
+ *        |                               |
+ *    1   |          Synch Leader         | Hex 01
+ *        |                               |
+ *        +-------------------------------+
+ *        | S | A | F | R | S | A | E | S |
+ *    2   | Y | C | I | S | N | N | O | O | Control
+ *        | N | K | N | T |   |   | R |   |
+ *        +-------------------------------+
+ *        |                               |
+ *    3   |      Data length (0-255)      |
+ *        |                               |
+ *        +-------------------------------+
+ *        |                               |
+ *    4   |        Header Checksum        |
+ *        |                               |
+ *        +-------------------------------+
+ *
+ */
+
+struct ratp_header {
+	uint8_t	synch;
+	uint8_t	control;
+	uint8_t	data_length;
+	uint8_t	cksum;
+};
+
+#define RATP_CONTROL_SO		(1 << 0)
+#define RATP_CONTROL_EOR	(1 << 1)
+#define RATP_CONTROL_AN		(1 << 2)
+#define RATP_CONTROL_SN		(1 << 3)
+#define RATP_CONTROL_RST	(1 << 4)
+#define RATP_CONTROL_FIN	(1 << 5)
+#define RATP_CONTROL_ACK	(1 << 6)
+#define RATP_CONTROL_SYN	(1 << 7)
+
+enum ratp_state {
+	RATP_STATE_LISTEN,
+	RATP_STATE_SYN_SENT,
+	RATP_STATE_SYN_RECEIVED,
+	RATP_STATE_ESTABLISHED,
+	RATP_STATE_FIN_WAIT,
+	RATP_STATE_LAST_ACK,
+	RATP_STATE_CLOSING,
+	RATP_STATE_TIME_WAIT,
+	RATP_STATE_CLOSED,
+};
+
+struct ratp_message {
+	void *buf;
+	size_t len;
+	struct list_head list;
+	void (*complete)(void *ctx, int status);
+	void *complete_ctx;
+	int eor;
+};
+
+static char *ratp_state_str[] = {
+	[RATP_STATE_LISTEN]       = "LISTEN",
+	[RATP_STATE_SYN_SENT]     = "SYN_SENT",
+	[RATP_STATE_SYN_RECEIVED] = "SYN_RECEIVED",
+	[RATP_STATE_ESTABLISHED]  = "ESTABLISHED",
+	[RATP_STATE_FIN_WAIT]     = "FIN_WAIT",
+	[RATP_STATE_LAST_ACK]     = "LAST_ACK",
+	[RATP_STATE_CLOSING]      = "CLOSING",
+	[RATP_STATE_TIME_WAIT]    = "TIME_WAIT",
+	[RATP_STATE_CLOSED]       = "CLOSED",
+};
+
+struct ratp_internal {
+	struct ratp *ratp;
+
+	enum ratp_state state;
+	int sn_sent;
+	int sn_received;
+	int active;
+
+	void *recvbuf;
+	void *sendbuf;
+	int sendbuf_len;
+
+	struct list_head recvmsg;
+	struct list_head sendmsg;
+
+	struct ratp_message *sendmsg_current;
+
+	uint64_t timewait_timer_start;
+	uint64_t retransmission_timer_start;
+	int max_retransmission;
+	int retransmission_count;
+	int srtt;
+	int rto;
+
+	int status;
+
+	int in_ratp;
+};
+
+static bool ratp_sn(struct ratp_header *hdr)
+{
+	return hdr->control & RATP_CONTROL_SN ? 1 : 0;
+}
+
+static bool ratp_an(struct ratp_header *hdr)
+{
+	return hdr->control & RATP_CONTROL_AN ? 1 : 0;
+}
+
+#define ratp_set_sn(sn) (((sn) % 2) ? RATP_CONTROL_SN : 0)
+#define ratp_set_an(an) (((an) % 2) ? RATP_CONTROL_AN : 0)
+
+static inline int ratp_header_ok(struct ratp_internal *ri, struct ratp_header *h)
+{
+	uint8_t cksum;
+	int ret;
+
+	cksum = h->control;
+	cksum += h->data_length;
+	cksum += h->cksum;
+
+	ret = cksum == 0xff ? 1 : 0;
+
+	if (ret)
+		pr_vdebug("Header ok\n");
+	else
+		pr_vdebug("Header cksum failed: %02x\n", cksum);
+
+	return ret;
+}
+
+static bool ratp_has_data(struct ratp_header *hdr)
+{
+	if (hdr->control & RATP_CONTROL_SO)
+		return 1;
+	if (hdr->data_length)
+		return 1;
+	return 0;
+}
+
+static void ratp_print_header(struct ratp_internal *ri, struct ratp_header *hdr,
+		const char *prefix)
+{
+	uint8_t control = hdr->control;
+
+	pr_debug("%s>%s %s %s %s %s %s %s %s< len: %-3d\n",
+		prefix,
+		control & RATP_CONTROL_SO  ? "so" : "--",
+		control & RATP_CONTROL_EOR ? "eor" : "---",
+		control & RATP_CONTROL_AN ? "an" : "--",
+		control & RATP_CONTROL_SN ? "sn" : "--",
+		control & RATP_CONTROL_RST ? "rst" : "---",
+		control & RATP_CONTROL_FIN ? "fin" : "---",
+		control & RATP_CONTROL_ACK ? "ack" : "---",
+		control & RATP_CONTROL_SYN ? "syn" : "---",
+		hdr->data_length);
+
+#ifdef VERBOSE_DEBUG
+	if (hdr->data_length)
+		memory_display(hdr + 1, 0, hdr->data_length, 1, 0);
+#endif
+}
+
+static void ratp_create_packet(struct ratp_internal *ri, struct ratp_header *hdr,
+		uint8_t control, uint8_t length)
+{
+	hdr->synch = 0x1;
+	hdr->control = control;
+	hdr->data_length = length;
+	hdr->cksum = (control + length) ^ 0xff;
+}
+
+static void ratp_state_change(struct ratp_internal *ri, enum ratp_state state)
+{
+	pr_debug("state %-10s -> %-10s\n", ratp_state_str[ri->state],
+			ratp_state_str[state]);
+
+	ri->state = state;
+}
+
+#define RATP_CONTROL_SO		(1 << 0)
+#define RATP_CONTROL_EOR	(1 << 1)
+#define RATP_CONTROL_AN		(1 << 2)
+#define RATP_CONTROL_SN		(1 << 3)
+#define RATP_CONTROL_RST	(1 << 4)
+#define RATP_CONTROL_FIN	(1 << 5)
+#define RATP_CONTROL_ACK	(1 << 6)
+#define RATP_CONTROL_SYN	(1 << 7)
+
+static int ratp_send_pkt(struct ratp_internal *ri, void *pkt, int length)
+{
+	struct ratp_header *hdr = (void *)pkt;
+
+	ratp_print_header(ri, hdr, "send");
+
+	if (ratp_has_data(hdr) ||
+	    (hdr->control & (RATP_CONTROL_SYN | RATP_CONTROL_RST | RATP_CONTROL_FIN))) {
+		memcpy(ri->sendbuf, pkt, length);
+		ri->sn_sent = ratp_sn(hdr);
+		ri->sendbuf_len = length;
+		ri->retransmission_timer_start = get_time_ns();
+		ri->retransmission_count = 0;
+	}
+
+	return ri->ratp->send(ri->ratp, pkt, length);
+}
+
+static int ratp_send_hdr(struct ratp_internal *ri, uint8_t control)
+{
+	struct ratp_header hdr = {};
+
+	ratp_create_packet(ri, &hdr, control, 0);
+
+	return ratp_send_pkt(ri, &hdr, sizeof(hdr));
+}
+
+static int ratp_recv_char(struct ratp_internal *ri, uint8_t *data, int poll_timeout_ms)
+{
+	uint64_t start;
+	int ret;
+
+	start = get_time_ns();
+
+	while (1) {
+		ret = ri->ratp->recv(ri->ratp, data);
+		if (ret < 0 && ret != -EAGAIN)
+			return ret;
+
+		if (ret == 0)
+			return 0;
+
+		if (is_timeout(start, poll_timeout_ms * MSECOND))
+			return -EAGAIN;
+	}
+}
+
+static int ratp_recv_pkt_header(struct ratp_internal *ri, struct ratp_header *hdr,
+		int poll_timeout_ms)
+{
+	int ret;
+	uint8_t buf;
+
+	do {
+		ret = ratp_recv_char(ri, &buf, 0);
+		if (ret < 0)
+			return ret;
+		hdr->synch = buf;
+	} while (hdr->synch != 1);
+	ret = ratp_recv_char(ri, &buf, poll_timeout_ms);
+	if (ret < 0)
+		return ret;
+
+	hdr->control = buf;
+	ret = ratp_recv_char(ri, &buf, poll_timeout_ms);
+	if (ret < 0)
+		return ret;
+
+	hdr->data_length = buf;
+
+	ret = ratp_recv_char(ri, &buf, poll_timeout_ms);
+	if (ret < 0)
+		return ret;
+
+	hdr->cksum = buf;
+
+	if (!ratp_header_ok(ri, hdr))
+		return -EAGAIN;
+
+	return 0;
+}
+
+static int ratp_recv_pkt_data(struct ratp_internal *ri, void *data, uint8_t len,
+		int poll_timeout_ms)
+{
+	uint16_t crc_expect, crc_read;
+	int ret, i;
+
+	for (i = 0; i < len + 2; i++) {
+		ret = ratp_recv_char(ri, data + i, poll_timeout_ms);
+		if (ret < 0)
+			return ret;
+	}
+
+	crc_expect = cyg_crc16(data, len);
+
+	crc_read = get_unaligned_be16(data + len);
+
+	if (crc_expect != crc_read) {
+		pr_vdebug("Wrong CRC: expected: 0x%04x, got 0x%04x\n",
+				crc_expect, crc_read);
+		return -EBADMSG;
+	} else {
+		pr_vdebug("correct CRC: 0x%04x\n", crc_expect);
+	}
+
+	return 0;
+}
+
+static int ratp_recv_pkt(struct ratp_internal *ri, void *pkt, int poll_timeout_ms)
+{
+	struct ratp_header *hdr = pkt;
+	void *data = pkt + sizeof(struct ratp_header);
+	int ret;
+
+	ret = ratp_recv_pkt_header(ri, hdr, poll_timeout_ms);
+	if (ret < 0)
+		return ret;
+
+	if (hdr->control & (RATP_CONTROL_SO | RATP_CONTROL_RST | RATP_CONTROL_SYN |
+		RATP_CONTROL_FIN))
+		return 0;
+
+	if (hdr->data_length) {
+		ret = ratp_recv_pkt_data(ri, data, hdr->data_length,
+				poll_timeout_ms);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
+static bool ratp_an_expected(struct ratp_internal *ri, struct ratp_header *hdr)
+{
+	return ratp_an(hdr) == (ri->sn_sent + 1) % 2;
+}
+
+static bool ratp_sn_expected(struct ratp_internal *ri, struct ratp_header *hdr)
+{
+	return ratp_sn(hdr) != ri->sn_received;
+}
+
+static int ratp_send_ack(struct ratp_internal *ri, struct ratp_header *hdr)
+{
+	uint8_t control = RATP_CONTROL_ACK;
+	int ret;
+
+	if (hdr->control & RATP_CONTROL_SN)
+		control |= RATP_CONTROL_AN;
+	else
+		control |= 0;
+
+	ret = ratp_send_hdr(ri, control);
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
+static int ratp_send_next_data(struct ratp_internal *ri)
+{
+	uint16_t crc;
+	uint8_t control = RATP_CONTROL_ACK;
+	struct ratp_header *hdr;
+	int pktlen;
+	struct ratp_message *msg;
+	int len;
+
+	if (ri->sendmsg_current) {
+		pr_err("%s: busy\n", __func__);
+		return -EBUSY;
+	}
+
+	if (list_empty(&ri->sendmsg))
+		return 0;
+
+	msg = list_first_entry(&ri->sendmsg, struct ratp_message, list);
+
+	ri->sendmsg_current = msg;
+
+	list_del(&msg->list);
+
+	len = msg->len;
+
+	control = ratp_set_sn(ri->sn_sent + 1) |
+		ratp_set_an(ri->sn_received + 1) |
+		RATP_CONTROL_ACK;
+
+	hdr = msg->buf;
+
+	if (msg->eor)
+		control |= RATP_CONTROL_EOR;
+
+	if (len > 1) {
+		void *data = hdr + 1;
+		pktlen = sizeof(*hdr) + len + 2;
+		crc = cyg_crc16(data, len);
+		put_unaligned_be16(crc, data + len);
+	} else {
+		pktlen = sizeof(struct ratp_header);
+		control |= RATP_CONTROL_SO;
+		len = 0;
+	}
+
+	ratp_create_packet(ri, hdr, control, len);
+
+	ri->retransmission_count = 0;
+
+	ratp_send_pkt(ri, msg->buf, pktlen);
+
+	return 0;
+}
+
+static void ratp_start_time_wait_timer(struct ratp_internal *ri)
+{
+	ri->timewait_timer_start = get_time_ns();
+}
+
+static void ratp_msg_done(struct ratp_internal *ri, struct ratp_message *msg, int status)
+{
+	int alpha, beta, rtt;
+
+	if (!status) {
+		rtt = (unsigned long)(get_time_ns() - ri->retransmission_timer_start) / MSECOND;
+
+		alpha = 8;
+		beta = 15;
+
+		ri->srtt = (alpha * ri->srtt + (10 - alpha) * rtt) / 10;
+		ri->rto = max(200, beta * ri->srtt / 10);
+
+		pr_debug("%s: done. SRTT: %dms RTO: %dms status: %d\n",
+			__func__, ri->srtt, ri->rto, ri->status);
+	}
+
+	if (msg->complete)
+		msg->complete(msg->complete_ctx, status);
+
+	free(msg->buf);
+	free(msg);
+}
+
+/*
+ * This procedure details the behavior of the LISTEN state.  First
+ * check the packet for the RST flag.  If it is set then packet is
+ * discarded and ignored, return and continue the processing
+ * associated with this state.
+ *
+ * We assume now that the RST flag was not set.  Check the packet
+ * for the ACK flag.  If it is set we have an illegal condition
+ * since no connection has yet been opened.  Send a RST packet
+ * with the correct response SN value:
+ *
+ * <SN=received AN><CTL=RST>
+ *
+ * Return to the current state without any further processing.
+ *
+ * We assume now that neither the RST nor the ACK flags were set.
+ * Check the packet for a SYN flag.  If it is set then an attempt
+ * is being made to open a connection.  Create a TCB for this
+ * connection.  The sender has placed its MDL in the LENGTH field,
+ * also specified is the sender's initial SN value.  Retrieve and
+ * place them into the TCB.  Note that the presence of the SO flag
+ * is ignored since it has no meaning when either of the SYN, RST,
+ * or FIN flags are set.
+ *
+ * Send a SYN packet which acknowledges the SYN received.  Choose
+ * the initial SN value and the MDL for this end of the
+ * connection:
+ *
+ * <SN=0><AN=received SN+1 modulo 2><CTL=SYN, ACK><LENGTH=MDL>
+ *
+ * and go to the RATP_STATE_SYN_RECEIVED state without any further
+ * processing.
+ *
+ * Any packet not satisfying the above tests is discarded and
+ * ignored.  Return to the current state without any further
+ * processing.
+ */
+static void ratp_behaviour_a(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if (hdr->control & RATP_CONTROL_RST)
+		return;
+
+	if (hdr->control & RATP_CONTROL_ACK) {
+		uint8_t control = RATP_CONTROL_RST;
+
+		if (hdr->control & RATP_CONTROL_AN)
+			control |= RATP_CONTROL_SN;
+
+		ratp_send_hdr(ri, control);
+
+		return;
+	}
+
+	if (hdr->control & RATP_CONTROL_SYN) {
+		struct ratp_header synack = {};
+		uint8_t control = RATP_CONTROL_SYN | RATP_CONTROL_ACK;
+
+		if (!(hdr->control & RATP_CONTROL_SN))
+			control |= RATP_CONTROL_AN;
+
+		ratp_create_packet(ri, &synack, control, 255);
+		ratp_send_pkt(ri, &synack, sizeof(synack));
+
+		ratp_state_change(ri, RATP_STATE_SYN_RECEIVED);
+	}
+}
+
+/*
+ * This procedure represents the behavior of the SYN-SENT state
+ * and is entered when this end of the connection decides to
+ * execute an active OPEN.
+ *
+ * First, check the packet for the ACK flag.  If the ACK flag is
+ * set then check to see if the AN value was as expected.  If it
+ * was continue below.  Otherwise the AN value was unexpected.  If
+ * the RST flag was set then discard the packet and return to the
+ * current state without any further processing, else send a
+ * reset:
+ *
+ * <SN=received AN><CTL=RST>
+ *
+ * Discard the packet and return to the current state without any
+ * further processing.
+ *
+ * At this point either the ACK flag was set and the AN value was
+ * as expected or ACK was not set.  Second, check the RST flag.
+ * If the RST flag is set there are two cases:
+ *
+ * . If the ACK flag is set then discard the packet, flush the
+ * retransmission queue, inform the user "Error: Connection
+ * refused", delete the TCB, and go to the CLOSED state without
+ * any further processing.
+ *
+ * 2. If the ACK flag was not set then discard the packet and
+ * return to this state without any further processing.
+ *
+ * At this point we assume the packet contained an ACK which was
+ * Ok, or there was no ACK, and there was no RST.  Now check the
+ * packet for the SYN flag.  If the ACK flag was set then our SYN
+ * has been acknowledged.  Store MDL received in the TCB.  At this
+ * point we are technically in the ESTABLISHED state.  Send an
+ * acknowledgment packet and any initial data which is queued to
+ * send:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK><DATA>
+ *
+ * Go to the ESTABLISHED state without any further processing.
+ *
+ * If the SYN flag was set but the ACK was not set then the other
+ * end of the connection has executed an active open also.
+ * Acknowledge the SYN, choose your MDL, and send:
+ *
+ * <SN=0><AN=received SN+1 modulo 2><CTL=SYN, ACK><LENGTH=MDL>
+ *
+ * Go to the SYN-RECEIVED state without any further processing.
+ *
+ * Any packet not satisfying the above tests is discarded and
+ * ignored.  Return to the current state without any further
+ * processing.
+ */
+static void ratp_behaviour_b(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if ((hdr->control & RATP_CONTROL_ACK) && !ratp_an_expected(ri, hdr)) {
+		if (!(hdr->control & RATP_CONTROL_RST)) {
+			uint8_t control = RATP_CONTROL_RST;
+
+			control = RATP_CONTROL_RST |
+				ratp_set_sn(ratp_an(hdr));
+
+			ratp_send_hdr(ri, control);
+		}
+		return;
+	}
+
+	if (hdr->control & RATP_CONTROL_RST) {
+		if (hdr->control & RATP_CONTROL_ACK) {
+			ri->status = -ECONNREFUSED;
+
+			pr_debug("Connection refused\n");
+
+			ratp_state_change(ri, RATP_STATE_CLOSED);
+
+		}
+		return;
+	}
+
+	if (hdr->control & RATP_CONTROL_SYN) {
+		uint8_t control;
+
+		if (hdr->control & RATP_CONTROL_ACK) {
+			control = ratp_set_sn(ratp_an(hdr)) |
+				ratp_set_an(!ratp_sn(hdr)) |
+				RATP_CONTROL_ACK;
+		} else {
+			control = ratp_set_an(!ratp_sn(hdr)) |
+				RATP_CONTROL_SYN |
+				RATP_CONTROL_ACK;
+
+		}
+
+		ri->sn_received = ratp_sn(hdr);
+
+		ratp_send_hdr(ri, control);
+		ratp_state_change(ri, RATP_STATE_ESTABLISHED);
+	}
+}
+
+/*
+ * Examine the received SN field value.  If the SN value was
+ * expected then return and continue the processing associated
+ * with this state.
+ *
+ * We now assume the SN value was not what was expected.
+ *
+ * If either RST or FIN were set discard the packet and return to
+ * the current state without any further processing.
+ *
+ * If neither RST nor FIN flags were set it is assumed that this
+ * packet is a duplicate of one already received.  Send an ACK
+ * back:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
+ *
+ * Discard the duplicate packet and return to the current state
+ * without any further processing.
+ */
+static int ratp_behaviour_c1(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	int ret;
+
+	pr_debug("%s\n", __func__);
+
+	if (ratp_sn_expected(ri, hdr)) {
+		pr_vdebug("%s: sn is expected\n", __func__);
+		return 0;
+	}
+
+	if (!(hdr->control & RATP_CONTROL_RST) &&
+			!(hdr->control & RATP_CONTROL_FIN)) {
+		ret = ratp_send_ack(ri, hdr);
+		if (ret)
+			return ret;
+	}
+
+	return 1;
+
+}
+
+/*
+ * Examine the received SN field value.  If the SN value was
+ * expected then return and continue the processing associated
+ * with this state.
+ *
+ * We now assume the SN value was not what was expected.
+ *
+ * If either RST or FIN were set discard the packet and return to
+ * the current state without any further processing.
+ *
+ * If SYN was set we assume that the other end crashed and has
+ * attempted to open a new connection.  We respond by sending a
+ * legal reset:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=RST, ACK>
+ *
+ * This will cause the other end, currently in the SYN-SENT state,
+ * to close.  Flush the retransmission queue, inform the user
+ * "Error: Connection reset", discard the packet, delete the TCB,
+ * and go to the CLOSED state without any further processing.
+ *
+ * If neither RST, FIN, nor SYN flags were set it is assumed that
+ * this packet is a duplicate of one already received.  Send an
+ * ACK back:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
+ *
+ * Discard the duplicate packet and return to the current state
+ * without any further processing.
+ */
+static int ratp_behaviour_c2(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	int ret;
+
+	pr_debug("%s\n", __func__);
+
+	if (!ratp_has_data(hdr))
+		return 0;
+
+	if (ratp_sn_expected(ri, hdr))
+		return 0;
+
+	if ((hdr->control & RATP_CONTROL_RST) ||
+			(hdr->control & RATP_CONTROL_FIN))
+		return 1;
+
+	if (hdr->control & RATP_CONTROL_SYN) {
+		ri->status = -ECONNRESET;
+		pr_debug("Error: Connection reset\n");
+		ratp_state_change(ri, RATP_STATE_CLOSED);
+		return 1;
+	}
+
+	if (!ratp_has_data(hdr))
+		return 1;
+
+	pr_debug("Sending ack for duplicate message\n");
+	ret = ratp_send_ack(ri, hdr);
+	if (ret)
+		return ret;
+
+	return 1;
+}
+
+/*
+ * The packet is examined for a RST flag.  If RST is not set then
+ * return and continue the processing associated with this state.
+ *
+ * RST is now assumed to have been set.  If the connection was
+ * originally initiated from the LISTEN state (it was passively
+ * opened) then flush the retransmission queue, discard the
+ * packet, and go to the LISTEN state without any further
+ * processing.
+ *
+ * If instead the connection was initiated actively (came from the
+ * SYN-SENT state) then flush the retransmission queue, inform the
+ * user "Error: Connection refused", discard the packet, delete
+ * the TCB, and go to the CLOSED state without any further
+ * processing.
+ */
+static int ratp_behaviour_d1(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_RST))
+		return 0;
+
+	if (!(ri->active)) {
+		ratp_state_change(ri, RATP_STATE_LISTEN);
+		return 1;
+	}
+
+	ri->status = -ECONNREFUSED;
+
+	pr_debug("Error: connection refused\n");
+
+	ratp_state_change(ri, RATP_STATE_CLOSED);
+
+	return 1;
+}
+
+/*
+ * The packet is examined for a RST flag.  If RST is not set then
+ * return and continue the processing associated with this state.
+ *
+ * RST is now assumed to have been set.  Any data remaining to be
+ * sent is flushed.  The retransmission queue is flushed, the user
+ * is informed "Error: Connection reset.", discard the packet,
+ * delete the TCB, and go to the CLOSED state without any further
+ * processing.
+ */
+static int ratp_behaviour_d2(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_RST))
+		return 0;
+
+	ri->status = -ECONNRESET;
+
+	pr_debug("connection reset\n");
+
+	return 0;
+}
+
+/*
+ * The packet is examined for a RST flag.  If RST is not set then
+ * return and continue the processing associated with this state.
+ *
+ * RST is now assumed to have been set.  Discard the packet,
+ * delete the TCB, and go to the CLOSED state without any further
+ * processing.
+ */
+static int ratp_behaviour_d3(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_RST))
+		return 0;
+
+	ratp_state_change(ri, RATP_STATE_CLOSED);
+
+	return 1;
+}
+
+/*
+ * Check the presence of the SYN flag.  If the SYN flag is not set
+ * then return and continue the processing associated with this
+ * state.
+ *
+ * We now assume that the SYN flag was set.  The presence of a SYN
+ * here is an error.  Flush the retransmission queue, send a legal
+ * RST packet.
+ *
+ * If the ACK flag was set then send:
+ *
+ * <SN=received AN><CTL=RST>
+ *
+ * If the ACK flag was not set then send:
+ *
+ * <SN=0><CTL=RST>
+ *
+ * The user should receive the message "Error: Connection reset.",
+ * then delete the TCB and go to the CLOSED state without any
+ * further processing.
+ */
+static int ratp_behaviour_e(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	uint8_t control;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_SYN))
+		return 0;
+
+	ri->status = -ECONNRESET;
+
+	control = RATP_CONTROL_RST;
+
+	if (hdr->control & RATP_CONTROL_ACK)
+		control |= ratp_set_sn(ratp_an(hdr));
+
+	ratp_send_hdr(ri, control);
+
+	pr_debug("connection reset\n");
+
+	ratp_state_change(ri, RATP_STATE_CLOSED);
+
+	return 1;
+}
+
+/*
+ * Check the presence of the ACK flag.  If ACK is not set then
+ * discard the packet and return without any further processing.
+ *
+ * We now assume that the ACK flag was set.  If the AN field value
+ * was as expected then return and continue the processing
+ * associated with this state.
+ *
+ * We now assume that the ACK flag was set and that the AN field
+ * value was unexpected.  If the connection was originally
+ * initiated from the LISTEN state (it was passively opened) then
+ * flush the retransmission queue, discard the packet, and send a
+ * legal RST packet:
+ *
+ * <SN=received AN><CTL=RST>
+ *
+ * Then delete the TCB and go to the LISTEN state without any
+ * further processing.
+ *
+ * Otherwise the connection was initiated actively (came from the
+ * SYN-SENT state) then inform the user "Error: Connection
+ * refused", flush the retransmission queue, discard the packet,
+ * and send a legal RST packet:
+ *
+ * <SN=received AN><CTL=RST>
+ *
+ * Then delete the TCB and go to the CLOSED state without any
+ * further processing.
+ */
+static int ratp_behaviour_f1(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	uint8_t control;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_ACK))
+		return 1;
+
+	if (ratp_an_expected(ri, hdr))
+		return 0;
+
+	control = RATP_CONTROL_RST | ratp_set_sn(ratp_an(hdr));
+	ratp_send_hdr(ri, control);
+
+	if (ri->active) {
+		ratp_state_change(ri, RATP_STATE_CLOSED);
+		ri->status = -ECONNREFUSED;
+
+		pr_debug("connection refused\n");
+	} else {
+		ratp_state_change(ri, RATP_STATE_LISTEN);
+	}
+
+	return 1;
+}
+
+/*
+ * Check the presence of the ACK flag.  If ACK is not set then
+ * discard the packet and return without any further processing.
+ *
+ * We now assume that the ACK flag was set.  If the AN field value
+ * was as expected then flush the retransmission queue and inform
+ * the user with an "Ok" if a buffer has been entirely
+ * acknowledged.  Another packet containing data may now be sent.
+ * Return and continue the processing associated with this state.
+ *
+ * We now assume that the ACK flag was set and that the AN field
+ * value was unexpected.  This is assumed to indicate a duplicate
+ * acknowledgment.  It is ignored, return and continue the
+ * processing associated with this state.
+ */
+static int ratp_behaviour_f2(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_ACK))
+		return 1;
+
+	if (ratp_an_expected(ri, hdr)) {
+		pr_debug("Data succesfully sent\n");
+		if (ri->sendmsg_current)
+			ratp_msg_done(ri, ri->sendmsg_current, 0);
+		ri->sendmsg_current = NULL;
+		return 0;
+	} else {
+		pr_vdebug("%s: an not expected\n", __func__);
+	}
+
+	return 0;
+}
+
+/*
+ * Check the presence of the ACK flag.  If ACK is not set then
+ * discard the packet and return without any further processing.
+ *
+ * We now assume that the ACK flag was set.  If the AN field value
+ * was as expected then continue the processing associated with
+ * this state.
+ *
+ * We now assume that the ACK flag was set and that the AN field
+ * value was unexpected.  This is ignored, return and continue
+ * with the processing associated with this state.
+ */
+static int ratp_behaviour_f3(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_ACK))
+		return 1;
+
+	return 0;
+}
+
+/*
+ * This procedure represents the behavior of the CLOSED state of a
+ * connection.  All incoming packets are discarded.  If the packet
+ * had the RST flag set take no action.  Otherwise it is necessary
+ * to build a RST packet.  Since this end is closed the other end
+ * of the connection has incorrect data about the state of the
+ * connection and should be so informed.
+ *
+ * If the ACK flag was set then send:
+ *
+ * <SN=received AN><CTL=RST>
+ *
+ * If the ACK flag was not set then send:
+ *
+ * <SN=0><AN=received SN+1 modulo 2><CTL=RST, ACK>
+ *
+ * After sending the reset packet return to the current state
+ * without any further processing.
+ */
+static int ratp_behaviour_g(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	uint8_t control;
+
+	pr_debug("%s\n", __func__);
+
+	control = RATP_CONTROL_RST;
+
+	if (hdr->control & RATP_CONTROL_ACK)
+		control |= ratp_set_sn(ratp_an(hdr));
+	else
+		control = ratp_set_an(ratp_sn(hdr) + 1) | RATP_CONTROL_ACK;
+
+	ratp_send_hdr(ri, control);
+
+	return 0;
+}
+
+/*
+ * Our SYN has been acknowledged.  At this point we are
+ * technically in the ESTABLISHED state.  Send any initial data
+ * which is queued to send:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK><DATA>
+ *
+ * Go to the ESTABLISHED state and execute procedure I1 to process
+ * any data which might be in this packet.
+ *
+ * Any packet not satisfying the above tests is discarded and
+ * ignored.  Return to the current state without any further
+ * processing.
+ */
+static int ratp_behaviour_h1(struct ratp_internal *ri, void *pkt)
+{
+	pr_debug("%s\n", __func__);
+
+	ratp_state_change(ri, RATP_STATE_ESTABLISHED);
+
+	return 0;
+}
+
+/*
+ * Check the presence of the FIN flag.  If FIN is not set then
+ * continue the processing associated with this state.
+ *
+ * We now assume that the FIN flag was set.  This means the other
+ * end has decided to close the connection.  Flush the
+ * retransmission queue.  If any data remains to be sent then
+ * inform the user "Warning: Data left unsent."  The user must
+ * also be informed "Connection closing."  An acknowledgment for
+ * the FIN must be sent which also indicates this end is closing:
+ *
+ * <SN=received AN><AN=received SN + 1 modulo 2><CTL=FIN, ACK>
+ *
+ * Go to the LAST-ACK state without any further processing.
+ */
+static int ratp_behaviour_h2(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	uint8_t control;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_FIN))
+		return 0;
+
+	ri->status = -ENETDOWN;
+
+	control = ratp_set_sn(ratp_an(hdr)) |
+		ratp_set_an(ratp_sn(hdr) + 1) |
+		RATP_CONTROL_FIN |
+		RATP_CONTROL_ACK;
+
+	ratp_send_hdr(ri, control);
+
+	ratp_state_change(ri, RATP_STATE_LAST_ACK);
+
+	return 1;
+}
+
+/*
+ * This state represents the final behavior of the FIN-WAIT state.
+ *
+ * If the packet did not contain a FIN we assume this packet is a
+ * duplicate and that the other end of the connection has not seen
+ * the FIN packet we sent earlier.  Rely upon retransmission of
+ * our earlier FIN packet to inform the other end of our desire to
+ * close.  Discard the packet and return without any further
+ * processing.
+ *
+ * At this point we have a packet which should contain a FIN.  By
+ * the rules of this protocol an ACK of a FIN requires a FIN, ACK
+ * in response and no data.  If the packet contains data we have
+ * detected an illegal condition.  Send a reset:
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=RST, ACK>
+ *
+ * Discard the packet, flush the retransmission queue, inform the
+ * ser "Error: Connection reset.", delete the TCB, and go to the
+ * CLOSED state without any further processing.
+ *
+ * We now assume that the FIN flag was set and no data was
+ * contained in the packet.  If the AN field value was expected
+ * then this packet acknowledges a previously sent FIN packet.
+ * The other end of the connection is then also assumed to be
+ * closing and expects an acknowledgment.  Send an acknowledgment
+ * of the FIN:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
+ *
+ * Start the 2*SRTT timer associated with the TIME-WAIT state,
+ * discard the packet, and go to the TIME-WAIT state without any
+ * further processing.
+ *
+ * Otherwise the AN field value was unexpected.  This indicates a
+ * simultaneous closing by both sides of the connection.  Send an
+ * acknowledgment of the FIN:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
+ *
+ * Discard the packet, and go to the CLOSING state without any
+ * further processing.
+ */
+static int ratp_behaviour_h3(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	uint8_t control;
+	int expected;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_FIN))
+		return 1;
+
+	if (ratp_has_data(hdr)) {
+		control = ratp_set_sn(ratp_an(hdr)) |
+			ratp_set_an(ratp_sn(hdr) + 1) |
+			RATP_CONTROL_RST |
+			RATP_CONTROL_ACK;
+		ratp_send_hdr(ri, control);
+		ri->status = -ECONNRESET;
+		pr_debug("Error: Connection reset\n");
+		ratp_state_change(ri, RATP_STATE_CLOSED);
+		return 1;
+	}
+
+	control = ratp_set_sn(ratp_an(hdr)) |
+		ratp_set_an(ratp_sn(hdr) + 1) |
+		RATP_CONTROL_ACK;
+
+	expected = ratp_an_expected(ri, hdr);
+
+	ratp_send_hdr(ri, control);
+
+	if (expected) {
+		ratp_state_change(ri, RATP_STATE_TIME_WAIT);
+		ratp_start_time_wait_timer(ri);
+	} else {
+		ratp_state_change(ri, RATP_STATE_CLOSING);
+	}
+
+	return 1;
+}
+
+/*
+ * This state represents the final behavior of the LAST-ACK state.
+ *
+ * If the AN field value is expected then this ACK is in response
+ * to the FIN, ACK packet recently sent.  This is the final
+ * acknowledging message indicating both side's agreement to close
+ * the connection.  Discard the packet, flush all queues, delete
+ * the TCB, and go to the CLOSED state without any further
+ * processing.
+ *
+ * Otherwise the AN field value was unexpected.  Discard the
+ * packet and remain in the current state without any further
+ * processing.
+ */
+static int ratp_behaviour_h4(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if (ratp_an_expected(ri, hdr))
+		ratp_state_change(ri, RATP_STATE_CLOSED);
+
+	return 1;
+}
+
+/*
+ * This state represents the final behavior of the CLOSING state.
+ *
+ * If the AN field value was expected then this packet
+ * acknowledges the FIN packet recently sent.  This is the final
+ * acknowledging message indicating both side's agreement to close
+ * the connection.  Start the 2*SRTT timer associated with the
+ * TIME-WAIT state, discard the packet, and go to the TIME-WAIT
+ * state without any further processing.
+ *
+ * Otherwise the AN field value was unexpected.  Discard the
+ * packet and remain in the current state without any further
+ * processing.
+ */
+static int ratp_behaviour_h5(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+
+	pr_debug("%s\n", __func__);
+
+	if (ratp_an_expected(ri, hdr)) {
+		ratp_state_change(ri, RATP_STATE_TIME_WAIT);
+		ratp_start_time_wait_timer(ri);
+	}
+
+	return 0;
+}
+
+/*
+ * This state represents the behavior of the TIME-WAIT state.
+ * Check the presence of the ACK flag.  If ACK is not set then
+ * discard the packet and return without any further processing.
+ *
+ * Check the presence of the FIN flag.  If FIN is not set then
+ * discard the packet and return without any further processing.
+ *
+ * We now assume that the FIN flag was set.  This situation
+ * indicates that the last acknowledgment of the FIN packet sent
+ * by the other end of the connection did not arrive.  Resend the
+ * acknowledgment:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
+ *
+ * Restart the 2*SRTT timer, discard the packet, and remain in the
+ * current state without any further processing.
+ */
+static int ratp_behaviour_h6(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	uint8_t control;
+
+	pr_debug("%s\n", __func__);
+
+	if (!(hdr->control & RATP_CONTROL_ACK))
+		return 1;
+
+	if (!(hdr->control & RATP_CONTROL_FIN))
+		return 1;
+
+	control = ratp_set_sn(ratp_an(hdr) + 1) | RATP_CONTROL_ACK;
+
+	ratp_send_hdr(ri, control);
+
+	ratp_start_time_wait_timer(ri);
+
+	return 0;
+}
+
+static int msg_recv(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	struct ratp_message *msg;
+
+	pr_debug("%s: Put msg in receive queue\n", __func__);
+
+	msg = xzalloc(sizeof(*msg));
+	if (hdr->data_length) {
+		msg->len = hdr->data_length;
+		msg->buf = xzalloc(msg->len);
+		memcpy(msg->buf, pkt + sizeof(struct ratp_header), msg->len);
+	} else {
+		msg->len = 1;
+		msg->buf = xzalloc(1);
+		*(uint8_t *)msg->buf = hdr->data_length;
+	}
+
+	if (hdr->control & RATP_CONTROL_EOR)
+		msg->eor = 1;
+
+	list_add_tail(&msg->list, &ri->recvmsg);
+
+	return 0;
+}
+
+/*
+ * This represents that stage of processing in the ESTABLISHED
+ * state in which all the flag bits have been processed and only
+ * data may remain.  The packet is examined to see if it contains
+ * data.  If not the packet is now discarded, return to the
+ * current state without any further processing.
+ *
+ * We assume the packet contained data, that either the SO flag
+ * was set or LENGTH is positive.  That data is placed into the
+ * user's receive buffers.  As these become full the user should
+ * be informed "Receive buffer full."  An acknowledgment is sent:
+ *
+ * <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
+ *
+ * If data is queued to send then it is most efficient to
+ * 'piggyback' this acknowledgment on that data packet.
+ *
+ * The packet is now discarded, return to the ESTABLISHED state
+ * without any further processing.
+ */
+static int ratp_behaviour_i1(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	uint8_t control = 0;
+
+	if (!hdr->data_length && !(hdr->control & RATP_CONTROL_SO))
+		return 1;
+
+	pr_vdebug("%s **received** %d\n", __func__, hdr->data_length);
+
+	ri->sn_received = ratp_sn(hdr);
+
+	msg_recv(ri, pkt);
+
+	if (list_empty(&ri->sendmsg) || ri->sendmsg_current) {
+		control = ratp_set_sn(!ri->sn_sent) |
+			ratp_set_an(ri->sn_received + 1) |
+			RATP_CONTROL_ACK;
+
+		ratp_send_hdr(ri, control);
+	} else {
+		ratp_send_next_data(ri);
+	}
+
+	return 0;
+}
+
+/*
+ * State machine as desribed in RFC916
+ *
+ * STATE                BEHAVIOR
+ * =============+========================
+ * LISTEN       |  A
+ * -------------+------------------------
+ * SYN-SENT     |  B
+ * -------------+------------------------
+ * SYN-RECEIVED |  C1  D1  E  F1  H1
+ * -------------+------------------------
+ * ESTABLISHED  |  C2  D2  E  F2  H2  I1
+ * -------------+------------------------
+ * FIN-WAIT     |  C2  D2  E  F3  H3
+ * -------------+------------------------
+ * LAST-ACK     |  C2  D3  E  F3  H4
+ * -------------+------------------------
+ * CLOSING      |  C2  D3  E  F3  H5
+ * -------------+------------------------
+ * TIME-WAIT    |  D3  E  F3 H6
+ * -------------+------------------------
+ * CLOSED       |  G
+ * -------------+------------------------
+ */
+
+static int ratp_state_machine(struct ratp_internal *ri, void *pkt)
+{
+	struct ratp_header *hdr = pkt;
+	int ret;
+
+	ratp_print_header(ri, hdr, "                      recv");
+	pr_debug(" state %s\n", ratp_state_str[ri->state]);
+
+	switch (ri->state) {
+	case RATP_STATE_LISTEN:
+		ratp_behaviour_a(ri, pkt);
+		break;
+	case RATP_STATE_SYN_SENT:
+		ratp_behaviour_b(ri, pkt);
+		break;
+	case RATP_STATE_SYN_RECEIVED:
+		ret = ratp_behaviour_c1(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_d1(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_e(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_f1(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_h1(ri, pkt);
+		if (ret)
+			return ret;
+		break;
+	case RATP_STATE_ESTABLISHED:
+		ret = ratp_behaviour_c2(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_d2(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_e(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_f2(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_h2(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_i1(ri, pkt);
+		if (ret)
+			return ret;
+		break;
+	case RATP_STATE_FIN_WAIT:
+		ret = ratp_behaviour_c2(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_d2(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_e(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_f3(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_h3(ri, pkt);
+		if (ret)
+			return ret;
+		break;
+	case RATP_STATE_LAST_ACK:
+		ret = ratp_behaviour_c2(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_d3(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_e(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_f3(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_h4(ri, pkt);
+		if (ret)
+			return ret;
+		break;
+	case RATP_STATE_CLOSING:
+		ret = ratp_behaviour_c2(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_d3(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_e(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_f3(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_h5(ri, pkt);
+		if (ret)
+			return ret;
+		break;
+	case RATP_STATE_TIME_WAIT:
+		ret = ratp_behaviour_d3(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_e(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_f3(ri, pkt);
+		if (ret)
+			return ret;
+		ret = ratp_behaviour_h6(ri, pkt);
+		if (ret)
+			return ret;
+		break;
+	case RATP_STATE_CLOSED:
+		ratp_behaviour_g(ri, pkt);
+		break;
+	};
+
+	return 0;
+}
+
+/**
+ * ratp_closed() - Check if a connection is closed
+ *
+ * Return: true if a connection is closed, false otherwise
+ */
+bool ratp_closed(struct ratp *ratp)
+{
+	struct ratp_internal *ri = ratp->internal;
+
+	if (!ri)
+		return true;
+
+	return ri->state == RATP_STATE_CLOSED;
+}
+
+/**
+ * ratp_busy() - Check if we are inside the RATP code
+ *
+ * Needed for RATP debugging. The RATP console uses this to determine
+ * if it is called from inside the RATP code.
+ *
+ * Return: true if we are inside the RATP code, false otherwise
+ */
+bool ratp_busy(struct ratp *ratp)
+{
+	struct ratp_internal *ri = ratp->internal;
+
+	if (!ri)
+		return false;
+
+	return ri->in_ratp != 0;
+}
+
+/**
+ * ratp_poll() - Execute RATP state machine
+ * @ratp: The RATP link
+ *
+ * This function should be executed periodically to keep the RATP state
+ * machine going.
+ *
+ * Return: 0 if successful, a negative error code otherwise.
+ */
+int ratp_poll(struct ratp *ratp)
+{
+	struct ratp_internal *ri = ratp->internal;
+	int ret;
+
+	if (!ri)
+		return -ENETDOWN;
+
+	ri->in_ratp++;
+
+	ret = ratp_recv_pkt(ri, ri->recvbuf, 100);
+	if (ret == 0) {
+
+		if (ri->state == RATP_STATE_TIME_WAIT &&
+		    is_timeout(ri->timewait_timer_start, ri->srtt * 2 * MSECOND)) {
+			pr_debug("2*SRTT timer timed out\n");
+			ret = -ECONNRESET;
+			goto out;
+		}
+
+		ret = ratp_state_machine(ri, ri->recvbuf);
+		if (ret < 0)
+			goto out;
+
+		if (ri->status < 0) {
+			ret = ri->status;
+			goto out;
+		}
+	}
+
+	if (ri->sendmsg_current && is_timeout(ri->retransmission_timer_start,
+	    ri->rto * MSECOND)) {
+
+		ri->retransmission_count++;
+		if (ri->retransmission_count == ri->max_retransmission) {
+			ri->status = ret = -ETIMEDOUT;
+			ri->state = RATP_STATE_CLOSED;
+			goto out;
+		}
+
+		pr_debug("%s: retransmit\n", __func__);
+
+		ratp_print_header(ri, ri->sendbuf, "resend");
+
+		ri->retransmission_timer_start = get_time_ns();
+
+		ret = ri->ratp->send(ratp, ri->sendbuf, ri->sendbuf_len);
+		if (ret)
+			goto out;
+	}
+
+	if (!ri->sendmsg_current && !list_empty(&ri->sendmsg))
+		ratp_send_next_data(ri);
+
+	ret = 0;
+out:
+	ri->in_ratp--;
+
+	return ret;
+}
+
+/**
+ * ratp_establish(): Establish a RATP link
+ * @ratp: The RATP link
+ * @active: if true actively create a connection
+ * @timeout_ms: Timeout in ms to wait until a connection is established. If
+ *              0 wait forever.
+ *
+ * This function establishes a link with the remote end. It expects the
+ * send and receive functions to be set, all other struct ratp_internal members can
+ * be left uninitialized.
+ *
+ * Return: 0 if successful, a negative error code otherwise.
+ */
+int ratp_establish(struct ratp *ratp, bool active, int timeout_ms)
+{
+	struct ratp_internal *ri;
+	int ret;
+	uint64_t start;
+
+	ri = xzalloc(sizeof(*ri));
+	ri->ratp = ratp;
+	ratp->internal = ri;
+
+	ri->recvbuf = xmalloc(512);
+	ri->sendbuf = xmalloc(512);
+	INIT_LIST_HEAD(&ri->recvmsg);
+	INIT_LIST_HEAD(&ri->sendmsg);
+	ri->max_retransmission = 100;
+	ri->srtt = 100;
+	ri->rto = 100;
+	ri->active = active;
+
+	ri->in_ratp++;
+
+	if (ri->active) {
+		ratp_send_hdr(ri, RATP_CONTROL_SYN);
+
+		ratp_state_change(ri, RATP_STATE_SYN_SENT);
+	}
+
+	start = get_time_ns();
+
+	while (1) {
+		ret = ratp_poll(ri->ratp);
+		if (ret < 0)
+			goto out;
+
+		if (ri->state == RATP_STATE_ESTABLISHED) {
+			ret = 0;
+			goto out;
+		}
+
+		if (timeout_ms && is_timeout(start, MSECOND * timeout_ms)) {
+			ret = -ETIMEDOUT;
+			goto out;
+		}
+	}
+
+out:
+	if (ret) {
+		free(ri->recvbuf);
+		free(ri->sendbuf);
+		free(ri);
+		ratp->internal = NULL;
+	}
+
+	ri->in_ratp--;
+
+	return ret;
+}
+
+void ratp_close(struct ratp *ratp)
+{
+	struct ratp_internal *ri = ratp->internal;
+	struct ratp_message *msg, *tmp;
+	struct ratp_header fin = {};
+
+	if (!ri)
+		return;
+
+	if (ri->state == RATP_STATE_ESTABLISHED) {
+		uint64_t start;
+		u8 control;
+
+		pr_debug("Closing...\n");
+
+		ratp_state_change(ri, RATP_STATE_FIN_WAIT);
+
+		control = ratp_set_sn(!ri->sn_sent) |
+			ratp_set_an(ri->sn_received + 1) |
+			RATP_CONTROL_FIN | RATP_CONTROL_ACK;
+
+		ratp_create_packet(ri, &fin, control, 0);
+
+		ratp_send_pkt(ri, &fin, sizeof(fin));
+
+		start = get_time_ns();
+
+		while (!is_timeout(start, ri->srtt * MSECOND * 2))
+			ratp_poll(ratp);
+	}
+
+	list_for_each_entry_safe(msg, tmp, &ri->sendmsg, list)
+		ratp_msg_done(ri, msg, -ECONNRESET);
+
+	free(ri);
+	ratp->internal = NULL;
+
+	pr_info("Closed\n");
+}
+
+/**
+ * ratp_send_complete(): Send data over a RATP link
+ * @ratp: The RATP link
+ * @data: The data buffer
+ * @len: The length of the message to send
+ * @complete: The completion callback for the message
+ * @complete_ctx: context pointer for the completion callback
+ *
+ * Queue a RATP message for transmission. This only queues the message,
+ * ratp_poll has to be called to actually transfer the message.
+ * @complete will be called upon completion of the message.
+ *
+ * Return: 0 if successful, a negative error code otherwise.
+ */
+int ratp_send_complete(struct ratp *ratp, const void *data, size_t len,
+		   void (*complete)(void *ctx, int status), void *complete_ctx)
+{
+	struct ratp_internal *ri = ratp->internal;
+	struct ratp_message *msg;
+
+	if (!ri || ri->state != RATP_STATE_ESTABLISHED)
+		return -ENETDOWN;
+
+	if (!len)
+		return -EINVAL;
+
+	ri->in_ratp++;
+
+	while (len) {
+		int now = min((int)len, 255);
+
+		msg = xzalloc(sizeof(*msg));
+		msg->buf = xzalloc(sizeof(struct ratp_header) + now + 2);
+		msg->len = now;
+		memcpy(msg->buf + sizeof(struct ratp_header), data, now);
+
+		list_add_tail(&msg->list, &ri->sendmsg);
+
+		len -= now;
+	}
+
+	msg->eor = 1;
+	msg->complete = complete;
+	msg->complete_ctx = complete_ctx;
+
+	ri->in_ratp--;
+
+	return 0;
+}
+
+/**
+ * ratp_send(): Send data over a RATP link
+ * @ratp: The RATP link
+ * @data: The data buffer
+ * @len: The length of the message to send
+ *
+ * Queue a RATP message for transmission. This only queues the message,
+ * ratp_poll has to be called to actually transfer the message.
+ *
+ * Return: 0 if successful, a negative error code otherwise.
+ */
+int ratp_send(struct ratp *ratp, const void *data, size_t len)
+{
+	return ratp_send_complete(ratp, data, len, NULL, NULL);
+}
+
+/**
+ * ratp_recv() - Receive data from a RATP link
+ * @ratp: The RATP link
+ * @data: Pointer to data
+ * @len: The length of the data in bytes
+ *
+ * If a message is available it fills @data with a pointer to the data.
+ * This function does not wait for new messages. If no data is available
+ * -EAGAIN is returned. If data is received @data has to be freed by the
+ * caller.
+ *
+ * Return: 0 if successful, a negative error code otherwise.
+ */
+int ratp_recv(struct ratp *ratp, void **data, size_t *len)
+{
+	struct ratp_internal *ri = ratp->internal;
+	struct ratp_message *msg, *tmp;
+	void *pos;
+	int num = 0;
+
+	*len = 0;
+
+	if (!ri || ri->state != RATP_STATE_ESTABLISHED)
+		return -ENETDOWN;
+
+	if (list_empty(&ri->recvmsg))
+		return -EAGAIN;
+
+	list_for_each_entry(msg, &ri->recvmsg, list) {
+		*len += msg->len;
+		num++;
+		if (msg->eor)
+			goto eor;
+	}
+
+	return -EAGAIN;
+
+eor:
+	*data = malloc(*len);
+	if (!*data)
+		return -ENOMEM;
+
+	pos = *data;
+
+	list_for_each_entry_safe(msg, tmp, &ri->recvmsg, list) {
+		memcpy(pos, msg->buf, msg->len);
+		pos += msg->len;
+
+		list_del(&msg->list);
+
+		free(msg->buf);
+		free(msg);
+	}
+
+	return 0;
+}
\ No newline at end of file