diff --git a/proto.py b/proto.py
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+++ b/proto.py
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+# This file is a part of sedbgmux, an open source DebugMux client.
+# Copyright (c) 2022  Vadim Yanitskiy <axilirator@gmail.com>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# 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 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
+# 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/>.
+
+from construct import *
+
+import crcmod
+
+# DebugMux frame definition
+class DbgMuxFrame:
+	# Kudos to Stefan @Sec Zehl for finding the CRC function parameters
+	fcs_func = crcmod.mkCrcFun(0x11021, rev=True, initCrc=0x0, xorOut=0xffff)
+
+	MsgType = Enum(subcon=Int8ul,
+		Enquiry		= 0x65, # 'e'
+		Ident		= 0x66, # 'f'
+		Ping		= 0x67, # 'g'
+		Pong		= 0x68, # 'h'
+
+		DPAnnounce	= 0x69, # 'i'
+		# TODO:		= 0x6a, # 'j'
+		ConnEstablish	= 0x6b, # 'k'
+		ConnEstablished	= 0x6c, # 'l'
+		ConnTerminate	= 0x6d, # 'm'
+		ConnTerminated	= 0x6e, # 'n'
+		ConnData	= 0x6f, # 'o'
+
+		# TODO:		= 0x70, # 'p'
+		Ack		= 0x71, # 'q'
+	)
+
+	Frame = Struct(
+		'Magic' / Const(b'\x42\x42'),
+		'Length' / Rebuild(Int16ul, lambda ctx: len(ctx.MsgData) + 5),
+		'TxCount' / Int8ul,
+		'RxCount' / Int8ul,
+		'MsgType' / MsgType,
+		'MsgData' / Bytes(lambda ctx: ctx.Length - 5),
+		'FCS' / Int16ul, # fcs_func() on all preceeding fields
+	)
+
+	# MsgType.Ident structure
+	MsgIdent = Struct(
+		'Magic' / Bytes(4), # TODO
+		'Ident' / PascalString(Int8ul, 'ascii'),
+	)
+
+	# MsgType.{Ping,Pong} structure
+	MsgPingPong = PascalString(Int8ul, 'ascii')
+
+	# MsgType.DPAnnounce structure
+	MsgDPAnnounce = Struct(
+		'DPRef' / Int16ul,
+		'Name' / PascalString(Int8ul, 'ascii'),
+	)
+
+	# MsgType.ConnEstablish[ed] structure
+	MsgConnEstablish = Struct('DPRef' / Int16ul)
+	MsgConnEstablished = Struct(
+		'DPRef' / Int16ul,
+		'ConnRef' / Int16ul,
+		'DataBlockLimit' / Int16ul,
+	)
+
+	# MsgType.ConnTerminate[ed] structure
+	MsgConnTerminate = Struct('ConnRef' / Int16ul)
+	MsgConnTerminated = Struct(
+		'DPRef' / Int16ul,
+		'ConnRef' / Int16ul,
+	)
+
+	# MsgType.ConnData structure
+	MsgConnData = Struct(
+		'ConnRef' / Int16ul,
+		'Data' / GreedyBytes,
+	)