diff --git a/.flake8 b/.flake8
new file mode 100644
index 0000000..2bcd70e
--- /dev/null
+++ b/.flake8
@@ -0,0 +1,2 @@
+[flake8]
+max-line-length = 88
diff --git a/.gitignore b/.gitignore
index 4f595f8..b6db07d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,8 +1,6 @@
-*.pyc
-*.pyo
-*.so
-*.coverage
-
-build/
+__pycache__/
+.mypy_cache/
+.pytest_cache/
+.vscode/
 dist/
-sdist/
+env/
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..2e12ed5
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,12 @@
+# Contributing
+To contribute to this repository, use GitHub pull-requests to contribute code
+to this repository.
+
+A few things to pay attention to:
+
+- Dependencies are managed using [poetry](https://python-poetry.org/).
+- Code is formatted using [Black](https://github.com/psf/black) and imports are
+  sorted using [isort](https://pycqa.github.io/isort/).
+- Documentation conforms to the
+  [Google Python Style Guide](https://google.github.io/styleguide/pyguide.html#doc-function-args).
+- Your branch is linear and logical.
diff --git a/LICENSE b/LICENSE.md
similarity index 96%
rename from LICENSE
rename to LICENSE.md
index 24c7a71..41afdb6 100644
--- a/LICENSE
+++ b/LICENSE.md
@@ -1,4 +1,4 @@
-Copyright (c) 2014-2015 Bas Stottelaar
+Copyright (c) 2014-2022 Bas Stottelaar
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
diff --git a/README.md b/README.md
index d4d4c8b..8593074 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,10 @@ Frame-based streaming protocol for embedded applications.
 [![Build Status](https://travis-ci.org/basilfx/python-tinylink.svg?branch=master)](https://travis-ci.org/basilfx/python-tinylink)
 
 ## Introduction
-This is a general purpose Python module to provide a bi-directional frame-based streaming protocol for low-speed embedded applications, such as serial connected devices. It allowes the receiver to 'jump into' a stream of data frames. Every frame starts with a preamble, so the receiver can synchronize. Any mismatch in checksum will the receiver.
+This is a general purpose Python module to provide a bi-directional frame-based
+streaming protocol for low-speed embedded applications, such as serial
+connected devices. It allowes the receiver to 'jump into' a stream of data
+frames. Every frame starts with a preamble, so the receiver can synchronize.
 
 A payload is optional.
 
@@ -22,28 +25,40 @@ X = Body payload (max. 65536 bytes)
 Y = CRC32 checksum over header + body
 ```
 
-The flags field can have arbitrary values, but the following flags are reserved.
+The flags field can have arbitrary values, but the following flags are
+reserved.
 
 * `0x01 = RESET`
 * `0x02 = ERROR`
 * `0x04 = PRIORITY`
 
-Error correction is not implemented and the bytes are not aligned. The endianness is customizable.
+Error correction is not implemented and the bytes are not aligned. The
+endianness is customizable.
 
 ## State chart diagram
 Below is a simplified statechart diagram of the receiver.
 ![Alt text](docs/statechart.png)
 
 ## Installation
-The latest development version can be installed via `pip install git+https://github.com/basilfx/python-tinylink`.
-
-## Tests
-Tests can be executed with `nosetests`. Check the `tests/` folder for more information.
+The latest development version can be installed via
+`pip install git+https://github.com/basilfx/python-tinylink`.
 
 ## CLI
-A simple serial CLI is included. When installed, run `tinylink /dev/tty.PORT_HERE` to start it. You can use it to send raw bytes via the link and display what comes back.
+A simple serial CLI is included. When installed, run
+`tinylink /dev/tty.PORT_HERE` to start it. You can use it to send raw bytes via
+the link and display what comes back.
+
+The CLI supports so-called modifiers to modify the outgoing data. For example,
+the input `\flags=1 hello world` would send a reset frame with the value
+'hello world'.
 
 PySerial is required to run this CLI.
 
+## Tests
+To run the tests, please clone this repository and run `poetry run pytest`.
+
+## Contributing
+See the [`CONTRIBUTING.md`](CONTRIBUTING.md) file.
+
 ## License
-See the `LICENSE` file (MIT license).
+See the [`LICENSE.md`](LICENSE.md) file (MIT license).
diff --git a/poetry.lock b/poetry.lock
new file mode 100644
index 0000000..2eac2ef
--- /dev/null
+++ b/poetry.lock
@@ -0,0 +1,396 @@
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+tests-no-zope = ["cloudpickle", "coverage[toml] (>=5.0.2)", "hypothesis", "mypy (>=0.900,!=0.940)", "pympler", "pytest (>=4.3.0)", "pytest-mypy-plugins"]
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diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..36e799c
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,53 @@
+[tool.black]
+exclude = '''
+(
+    \.git/
+  | \.vscode/
+  | env/
+)
+'''
+line-length = 88
+target-version = ["py39"]
+
+[tool.isort]
+profile = "black"
+
+[tool.poetry]
+authors = ["Bas Stottelaar <basstottelaar@gmail.com>"]
+classifiers = [
+    "Development Status :: 5 - Production/Stable",
+    "Intended Audience :: Developers",
+    "Topic :: System :: Networking",
+    "Topic :: Software Development :: Embedded Systems",
+]
+description = "Frame-based streaming protocol for embedded applications."
+homepage = "https://github.com/basilfx/python-tinylink"
+include = [
+    "LICENSE.md",
+]
+keywords = ["python", "embedded", "tinylink", "streaming", "serial"]
+license = "MIT"
+name = "tinylink"
+packages = [
+    { include = "tinylink" }
+]
+readme = "README.md"
+repository = "https://github.com/basilfx/python-tinylink"
+version = "2.0.0"
+
+[tool.poetry.dependencies]
+python = "^3.9"
+
+[tool.poetry.group.dev.dependencies]
+black = "^22.10.0"
+isort = "^5.10.1"
+flake8 = "^5.0.4"
+flake8-mypy = "^17.8.0"
+pytest = "^7.2.0"
+
+[tool.poetry.scripts]
+tinylink = 'tinylink.cli:run'
+
+[build-system]
+build-backend = "poetry.core.masonry.api"
+requires = ["poetry-core"]
diff --git a/setup.py b/setup.py
deleted file mode 100644
index e387695..0000000
--- a/setup.py
+++ /dev/null
@@ -1,31 +0,0 @@
-from setuptools import setup
-
-# Setup definitions
-setup(
-    name="python-tinylink",
-    version="2.0.0",
-    description="Frame-based streaming protocol for embedded applications.",
-    author="Bas Stottelaar",
-    author_email="basstottelaar@gmail.com",
-    packages=["tinylink"],
-    license="MIT",
-    keywords="python embedded arm arduino tinylink streaming serial",
-    setup_requires=["nose"],
-    install_requires=["six"],
-    zip_safe=False,
-    entry_points={
-        "console_scripts": [
-            "tinylink = tinylink.cli:run",
-        ]
-    },
-    classifiers=[
-        "Development Status :: 5 - Production/Stable",
-        "Intended Audience :: Developers",
-        "License :: OSI Approved :: MIT License",
-        "Topic :: System :: Networking",
-        "Topic :: Software Development :: Embedded Systems",
-        "Programming Language :: Python :: 2",
-        "Programming Language :: Python :: 2.7",
-        "Programming Language :: Python :: 3.4"
-    ]
-)
diff --git a/tests/test_tinylink.py b/tests/test_tinylink.py
index c929dec..a1ccfc2 100644
--- a/tests/test_tinylink.py
+++ b/tests/test_tinylink.py
@@ -1,6 +1,5 @@
-from six.moves import xrange
-
 import unittest
+
 import tinylink
 
 
@@ -9,25 +8,29 @@ class DummyHandle(object):
     Dummy handler, so the TinyLink class can exchange data with itself.
     """
 
-    def __init__(self):
+    stream: bytes
+    index: int
+    length: int
+
+    def __init__(self) -> None:
         self.stream = bytearray()
         self.index = 0
         self.length = 0
 
-    def write(self, data):
+    def read(self, size: int) -> bytes:
+        data = self.stream[self.index : min(self.length, self.index + size)]
+        self.index += len(data)
+
+        # Return data.
+        return bytes(data)
+
+    def write(self, data: bytes) -> int:
         self.stream.extend(data)
         self.length += len(data)
 
-        # Return number of bytes written
+        # Return number of bytes written.
         return len(data)
 
-    def read(self, count):
-        data = self.stream[self.index:min(self.length, self.index + count)]
-        self.index += len(data)
-
-        # Return data
-        return bytes(data)
-
 
 class TinyLinkTest(unittest.TestCase):
     """
@@ -46,10 +49,14 @@ def test_basic(self):
         size = link.write(message)
 
         self.assertEqual(
-            size, tinylink.LEN_PREAMBLE + tinylink.LEN_HEADER +
-            tinylink.LEN_BODY + len(message))
-
-        # Read `size' bytes to receive the full frame, test it partially
+            size,
+            tinylink.LEN_PREAMBLE
+            + tinylink.LEN_HEADER
+            + tinylink.LEN_BODY
+            + len(message),
+        )
+
+        # Read `size` bytes to receive the full frame, test it partially.
         link.read(1)
         link.read(1)
         link.read(1)
@@ -68,14 +75,14 @@ def test_multiple(self):
 
         size = 0
 
-        for i in xrange(5):
+        for i in range(5):
             size += link.write(bytes([97 + i]))
 
         frames = link.read(size)
 
         self.assertEqual(len(frames), 5)
 
-        for i in xrange(5):
+        for i in range(5):
             self.assertEqual(frames[i].data, bytes([97 + i]))
 
     def test_sync(self):
@@ -86,7 +93,7 @@ def test_sync(self):
         handle = DummyHandle()
         link = tinylink.TinyLink(handle)
 
-        garbage = b"Garbage here that doesn't synchronize."
+        garbage = b"Garbage here that does not synchronize."
         message = b"Hi!"
 
         size = handle.write(garbage) + link.write(message)
@@ -103,7 +110,7 @@ def test_sync_small(self):
         handle = DummyHandle()
         link = tinylink.TinyLink(handle, max_length=4)
 
-        garbage = b"Garbage here that doesn't synchronize."
+        garbage = b"Garbage here that does not synchronize."
         message = b"Hi!"
 
         size = handle.write(garbage) + link.write(message)
@@ -153,7 +160,7 @@ def test_damaged_a(self):
         message = b"Hello, this is a test"
 
         size = link.write(message)
-        handle.stream[-tinylink.LEN_CRC:] = [0x00] * tinylink.LEN_CRC
+        handle.stream[-tinylink.LEN_CRC :] = [0x00] * tinylink.LEN_CRC
         frames = link.read(size)
 
         self.assertEqual(len(frames), 1)
@@ -162,7 +169,7 @@ def test_damaged_a(self):
 
     def test_damaged_b(self):
         """
-        Test damaged frame (header) that won't return anything.
+        Test damaged frame (header) that will not return anything.
         """
 
         handle = DummyHandle()
@@ -171,7 +178,7 @@ def test_damaged_b(self):
         message = b"Hello, this is a test"
 
         size = link.write(message)
-        handle.stream[tinylink.LEN_PREAMBLE+tinylink.LEN_HEADER-1] = 0x00
+        handle.stream[tinylink.LEN_PREAMBLE + tinylink.LEN_HEADER - 1] = 0x00
         frames = link.read(size)
 
         self.assertEqual(len(frames), 0)
diff --git a/tinylink/__init__.py b/tinylink/__init__.py
index 591878b..b419def 100644
--- a/tinylink/__init__.py
+++ b/tinylink/__init__.py
@@ -1,232 +1,4 @@
-from tinylink import utils
+from .consts import *  # noqa
+from .link import Frame, TinyLink  # noqa
 
-import struct
-import six
-
-__version__ = "1.1"
-
-__all__ = ["Frame", "TinyLink"]
-
-# This can be anything, and is used to synchronize a frame
-PREAMBLE = 0xAA55AA55
-
-# Endianness
-LITTLE_ENDIAN = "<"
-BIG_ENDIAN = ">"
-
-# Protocol states.
-WAITING_FOR_PREAMBLE = 1
-WAITING_FOR_HEADER = 2
-WAITING_FOR_BODY = 3
-
-# Message flags (reserved).
-FLAG_NONE = 0x00
-FLAG_RESET = 0x01
-FLAG_ERROR = 0x02
-FLAG_PRIORITY = 0x04
-
-# Don't change these values!
-LEN_PREAMBLE = 4
-LEN_FLAGS = 2
-LEN_LENGTH = 2
-LEN_XOR = 1
-LEN_CRC = 4
-LEN_HEADER = LEN_FLAGS + LEN_LENGTH + LEN_XOR
-LEN_BODY = LEN_CRC
-
-
-class Frame(object):
-    """
-    Represents a frame.
-    """
-
-    def __init__(self, data=None, flags=FLAG_NONE, damaged=False):
-        if data is not None:
-            if not type(data) == six.binary_type:
-                raise ValueError("Provided data must be encoded as bytes.")
-        else:
-            data = bytes()
-
-        self.data = data
-        self.flags = flags
-        self.damaged = damaged
-
-    def __repr__(self):
-        return "%s(%s, flags=%d, damaged=%s)" % (
-            self.__class__.__name__, repr(self.data), self.flags, self.damaged)
-
-
-class TinyLink(object):
-    """
-    TinyLink state machine for streaming communication with low-speed embedded
-    applications that only use RX/TX. Every message is encapsulated in a frame.
-    A frame has a header checksum and a frame checksum, to detect errors as
-    fast as possible (this can happen when you jump right into a stream of
-    packets, without being synchronized).
-
-    A typical frame has 13 bytes overhead, and can have a data payload up to
-    65536 bytes.
-
-    It does not provide error correction and the bytes are not aligned.
-    """
-
-    def __init__(self, handle, endianness=LITTLE_ENDIAN,
-                 max_length=2**(LEN_LENGTH * 8), ignore_damaged=False):
-        """
-        Construct a new TinyLink state machine. A state machine takes a handle,
-        which provides a `read' and `write' method.
-
-        The endianness is either LITTLE_ENDIAN or BIG_ENDIAN. While big endian
-        is common for networking, little endian is directly compatible with ARM
-        microcontrollers, so the microcontrollers don't have to change the
-        endianness.
-
-        Both microcontroller and this instance should agree upon the value of
-        `max_length'. In case a message is received that exceeds this value, it
-        will be silently ignored.
-
-        By default, if a fully received frame is damaged, it will be returned
-        as a `DamagedFrame' instance, unless `ignored_damaged' is True.
-        """
-
-        self.handle = handle
-        self.endianness = endianness
-        self.max_length = max_length
-        self.ignore_damaged = ignore_damaged
-
-        # Set initial state
-        self.state = WAITING_FOR_PREAMBLE
-
-        # Pre-allocate buffer that fits header + body. The premable will be
-        # cleared when it is detected, so it doesn't need space.
-        self.stream = bytearray(max_length + LEN_HEADER + LEN_BODY)
-        self.index = 0
-
-        # Python 2 does not allow unpack from bytearray, but Python 3.
-        if six.PY3:
-            self.buffer = self.stream
-        else:
-            self.buffer = buffer(self.stream)
-
-    def write_frame(self, frame):
-        """
-        Write a frame via the handle.
-        """
-
-        result = bytearray()
-        length = len(frame.data or [])
-
-        # Check length of message
-        if length > self.max_length:
-            raise ValueError(
-                "Message length %d exceeds max length %d" % (
-                    length, self.max_length))
-
-        # Pack header
-        checksum_header = utils.checksum_header(frame.flags, length)
-        result += struct.pack(
-            self.endianness + "IHHB", PREAMBLE, frame.flags, length,
-            checksum_header)
-
-        # Pack data
-        if frame.data is not None:
-            checksum_frame = utils.checksum_frame(frame.data, checksum_header)
-            result += struct.pack(
-                self.endianness + str(length) + "sI", frame.data,
-                checksum_frame)
-
-        # Write to file
-        return self.handle.write(result)
-
-    def write(self, data, flags=FLAG_NONE):
-        """
-        Shorthand for `write_frame(Frame(data, flags=flags))'.
-        """
-
-        return self.write_frame(Frame(data, flags=flags))
-
-    def read(self, limit=1):
-        """
-        Read up to `limit' bytes from the handle and process it. Returns a list
-        of received frames, if any.
-        """
-
-        # List of frames received
-        frames = []
-
-        # Bytes are added one at a time
-        while limit:
-            char = self.handle.read(1)
-
-            if not char:
-                return []
-
-            # Append to stream
-            self.stream[self.index] = ord(char)
-            self.index += 1
-
-            # Decide what to do
-            if self.state == WAITING_FOR_PREAMBLE:
-                if self.index >= LEN_PREAMBLE:
-                    start, = struct.unpack_from(
-                        self.endianness + "I", self.buffer, self.index - 4)
-
-                    if start == PREAMBLE:
-                        # Advance to next state
-                        self.index = 0
-                        self.state = WAITING_FOR_HEADER
-                    elif self.index == self.max_length + LEN_HEADER + LEN_BODY:
-                        # Preamble not found and stream is full. Copy last four
-                        # bytes, because the next byte may form the preamble
-                        # together with the last three bytes.
-                        self.stream[0:4] = self.stream[-4:]
-                        self.index = 4
-
-            elif self.state == WAITING_FOR_HEADER:
-                if self.index == LEN_HEADER:
-                    flags, length, checksum = struct.unpack_from(
-                        self.endianness + "HHB", self.buffer)
-
-                    # Verify checksum
-                    if checksum == utils.checksum_header(flags, length) and \
-                            length <= self.max_length:
-
-                        if length > 0:
-                            self.state = WAITING_FOR_BODY
-                        else:
-                            # Frame without body.
-                            frames.append(Frame(flags=flags))
-
-                            self.index = 0
-                            self.state = WAITING_FOR_PREAMBLE
-                    else:
-                        # Reset to start state
-                        self.index = 0
-                        self.state = WAITING_FOR_PREAMBLE
-
-            elif self.state == WAITING_FOR_BODY:
-                # Unpack header
-                flags, length, checksum_a = struct.unpack_from(
-                    self.endianness + "HHB", self.buffer)
-
-                if self.index == LEN_HEADER + length + LEN_CRC:
-                    # Unpack body
-                    result, checksum_b = struct.unpack_from(
-                        self.endianness + str(length) + "sI",
-                        self.buffer, LEN_HEADER)
-
-                    # Verify checksum
-                    if checksum_b == utils.checksum_frame(result, checksum_a):
-                        frames.append(Frame(result, flags=flags))
-                    elif not self.ignore_damaged:
-                        frames.append(Frame(result, flags=flags, damaged=True))
-
-                    # Reset to start state
-                    self.index = 0
-                    self.state = WAITING_FOR_PREAMBLE
-
-            # Decrement number of bytes to read
-            limit -= 1
-
-        # Done
-        return frames
+__version__ = "2.0.0"
diff --git a/tinylink/cli.py b/tinylink/cli.py
index ff71ee8..1f74bb6 100644
--- a/tinylink/cli.py
+++ b/tinylink/cli.py
@@ -1,14 +1,13 @@
-from six.moves import xrange
-from six import StringIO
-
+import argparse
 import csv
-import six
-import sys
-import time
 import select
 import struct
+import sys
+import time
+from io import StringIO
+from typing import Optional
+
 import tinylink
-import argparse
 
 try:
     import serial
@@ -16,36 +15,32 @@
     serial = None
 
 
-def run():
-    """
-    Entry point for console script.
-    """
-
-    sys.exit(main())
-
-
-def parse_arguments():
+def parse_arguments(argv: list[str]) -> argparse.Namespace:
     """
     Create and parse command line arguments.
     """
 
-    parser = argparse.ArgumentParser()
+    parser = argparse.ArgumentParser(prog=argv[0])
 
-    # Add option
+    # Add options.
     parser.add_argument("port", type=str, help="serial port")
+    parser.add_argument("baudrate", type=int, default=9600, help="serial baudrate")
     parser.add_argument(
-        "baudrate", type=int, default=9600, help="serial baudrate")
-    parser.add_argument(
-        "--length", type=int, default=2**16, help="maximum length of frame")
+        "--length", type=int, default=2**16, help="maximum length of frame"
+    )
     parser.add_argument(
-        "--endianness", type=str, default="little", choices=["big", "little"],
-        help="maximum length of frame")
+        "--endianness",
+        type=str,
+        default="little",
+        choices=["big", "little"],
+        help="maximum length of frame",
+    )
 
-    # Parse command line
-    return parser.parse_args(), parser
+    # Parse command line.
+    return parser.parse_args(argv[1:])
 
 
-def dump(prefix, data):
+def dump(prefix: str, data: bytes) -> str:
     """
     Dump data as two hex columns.
     """
@@ -53,15 +48,15 @@ def dump(prefix, data):
     result = []
     length = len(data)
 
-    for i in xrange(0, length, 16):
+    for i in range(0, length, 16):
         hexstr = ""
         bytestr = b""
 
-        for j in xrange(0, 16):
+        for j in range(0, 16):
             if i + j < length:
-                b = six.indexbytes(data, i + j)
+                b = data[i + j]
                 hexstr += "%02x " % b
-                bytestr += six.int2byte(b) if 0x20 <= b < 0x7F else b"."
+                bytestr += bytes((b,)) if 0x20 <= b < 0x7F else b"."
             else:
                 hexstr += "   "
 
@@ -70,18 +65,18 @@ def dump(prefix, data):
 
         result.append(prefix + " " + hexstr + bytestr.decode("ascii"))
 
-    # Return concatenated string
+    # Return concatenated string.
     return "\n".join(result)
 
 
-def process_link(link):
+def process_link(link: tinylink.TinyLink) -> None:
     """
     Process incoming link data.
     """
 
     frames = link.read()
 
-    # Print received frames
+    # Print received frames.
     for frame in frames:
         sys.stdout.write("### Type = %s\n" % frame.__class__.__name__)
         sys.stdout.write("### Flags = 0x%04x\n" % frame.flags)
@@ -90,7 +85,7 @@ def process_link(link):
         sys.stdout.write(dump("<<<", frame.data) + "\n\n")
 
 
-def process_stdin(link):
+def process_stdin(link: tinylink.TinyLink) -> Optional[bool]:
     """
     Process stdin commands.
     """
@@ -135,13 +130,13 @@ def process_stdin(link):
                 except:
                     try:
                         # Assume it is an int.
-                        value = struct.pack(
-                            link.endianness + pack, int(item, 0))
+                        value = struct.pack(link.endianness + pack, int(item, 0))
                     except ValueError:
                         # Assume it is a byte string.
-                        item = item.encode("ascii")
+                        item_bytes = item.encode("ascii")
                         value = struct.pack(
-                            link.endianness + str(len(item)) + "s", item)
+                            link.endianness + str(len(item_bytes)) + "s", item_bytes
+                        )
 
                 # Concat to frame.
                 frame.data = (frame.data or bytes()) + value
@@ -149,7 +144,7 @@ def process_stdin(link):
         sys.stdout.write("Parse exception: %s\n" % e)
 
     # Output the data.
-    for i in xrange(repeat):
+    for i in range(repeat):
         sys.stdout.write("### Flags = 0x%04x\n" % frame.flags)
 
         if frame.data:
@@ -164,7 +159,15 @@ def process_stdin(link):
             return
 
 
-def main():
+def run() -> None:
+    """
+    Entry point for console script.
+    """
+
+    sys.exit(main(sys.argv))
+
+
+def main(argv: list[str]) -> int:
     """
     Main entry point.
     """
@@ -172,49 +175,50 @@ def main():
     if serial is None:
         sys.stdout.write(
             "TinyLink CLI uses PySerial, but it is not installed. Please "
-            "install this first.\n")
+            "install this first.\n"
+        )
         return 1
 
-    # Parse arguments
-    arguments, parser = parse_arguments()
+    # Parse arguments.
+    arguments = parse_arguments(argv)
 
     if arguments.endianness == "little":
         endianness = tinylink.LITTLE_ENDIAN
     else:
         endianness = tinylink.BIG_ENDIAN
 
-    # Open  serial port and create link
+    # Open serial port and create link.
     handle = serial.Serial(arguments.port, baudrate=arguments.baudrate)
-    link = tinylink.TinyLink(
-        handle, max_length=arguments.length, endianness=endianness)
+    link = tinylink.TinyLink(handle, max_length=arguments.length, endianness=endianness)
 
-    # Loop until finished
+    # Loop until finished.
     try:
-        # Input indicator
+        # Input indicator.
         sys.stdout.write("--> ")
         sys.stdout.flush()
 
         while True:
             readables, _, _ = select.select([handle, sys.stdin], [], [])
 
-            # Read from serial port
+            # Read from serial port.
             if handle in readables:
                 process_link(link)
 
-            # Read from stdin
+            # Read from stdin.
             if sys.stdin in readables:
                 if process_stdin(link) is False:
                     break
 
-                # Input indicator
+                # Input indicator.
                 sys.stdout.write("--> ")
                 sys.stdout.flush()
     except KeyboardInterrupt:
         handle.close()
 
-    # Done
+    # Done.
     return 0
 
-# E.g. `python tinylink_cli.py /dev/tty.usbmodem1337 --baudrate 9600'
+
+# E.g. `python cli.py /dev/tty.usbmodem1337 --baudrate 9600`.
 if __name__ == "__main__":
     run()
diff --git a/tinylink/consts.py b/tinylink/consts.py
new file mode 100644
index 0000000..e83cf85
--- /dev/null
+++ b/tinylink/consts.py
@@ -0,0 +1,26 @@
+# This can be anything, and is used to synchronize a frame.
+PREAMBLE = 0xAA55AA55
+
+# Endianness.
+LITTLE_ENDIAN = "<"
+BIG_ENDIAN = ">"
+
+# Protocol states.
+WAITING_FOR_PREAMBLE = 1
+WAITING_FOR_HEADER = 2
+WAITING_FOR_BODY = 3
+
+# Message flags (reserved).
+FLAG_NONE = 0x00
+FLAG_RESET = 0x01
+FLAG_ERROR = 0x02
+FLAG_PRIORITY = 0x04
+
+# Do not change these values!
+LEN_PREAMBLE = 4
+LEN_FLAGS = 2
+LEN_LENGTH = 2
+LEN_XOR = 1
+LEN_CRC = 4
+LEN_HEADER = LEN_FLAGS + LEN_LENGTH + LEN_XOR
+LEN_BODY = LEN_CRC
diff --git a/tinylink/link.py b/tinylink/link.py
new file mode 100644
index 0000000..f028c8b
--- /dev/null
+++ b/tinylink/link.py
@@ -0,0 +1,249 @@
+import struct
+from typing import Protocol
+
+from . import consts, utils
+
+
+class Handle(Protocol):
+    """
+    Protocol for a handler.
+    """
+
+    def read(self, size: int) -> bytes:
+        """ "
+        Read up to  `size` bytes.
+        """
+
+    def write(self, data: bytes) -> int:
+        """
+        Write data and return the number of bytes written.
+        """
+
+
+class Frame:
+    """
+    Represents a frame.
+    """
+
+    data: bytes
+    flags: int
+    damaged: int
+
+    def __init__(
+        self, data: bytes = None, flags: int = consts.FLAG_NONE, damaged: bool = False
+    ) -> None:
+        if data is not None:
+            if type(data) is not bytes:
+                raise ValueError("Provided data must be encoded as bytes.")
+        else:
+            data = bytes()
+
+        self.data = data
+        self.flags = flags
+        self.damaged = damaged
+
+    def __repr__(self) -> str:
+        return "%s(%s, flags=%d, damaged=%s)" % (
+            self.__class__.__name__,
+            repr(self.data),
+            self.flags,
+            self.damaged,
+        )
+
+
+class TinyLink:
+    """
+    TinyLink state machine for streaming communication with low-speed embedded
+    applications that only use RX/TX. Every message is encapsulated in a frame.
+    A frame has a header checksum and a frame checksum, to detect errors as
+    fast as possible (this can happen when you jump right into a stream of
+    packets, without being synchronized).
+
+    A typical frame has 13 bytes overhead, and can have a data payload up to
+    65536 bytes.
+
+    It does not provide error correction and the bytes are not aligned.
+    """
+
+    handle: Handle
+    endianness: str
+    max_length: int
+    ignore_damaged: bool
+
+    def __init__(
+        self,
+        handle: Handle,
+        endianness: str = consts.LITTLE_ENDIAN,
+        max_length: int = 2 ** (consts.LEN_LENGTH * 8),
+        ignore_damaged: bool = False,
+    ) -> None:
+        """
+        Construct a new TinyLink state machine. A state machine takes a handle,
+        which provides a `read` and `write` method.
+
+        The endianness is either `consts.LITTLE_ENDIAN` or `consts.BIG_ENDIAN`.
+        While big endian is common for networking, little endian is directly
+        compatible with ARM microcontrollers, so the microcontrollers do not
+        have to perform conversion of endianness.
+
+        Both microcontroller and this instance should agree upon the value of
+        `max_lengthz. In case a message is received that exceeds this value, it
+        will be silently ignored.
+
+        By default, if a fully received frame is damaged, it will be returned
+        as an instance of `DamagedFrame` instance, unless `ignored_damaged` is
+        set to `True`.
+        """
+
+        self.handle = handle
+        self.endianness = endianness
+        self.max_length = max_length
+        self.ignore_damaged = ignore_damaged
+
+        # Set initial state
+        self.state = consts.WAITING_FOR_PREAMBLE
+
+        # Pre-allocate buffer that fits header + body. The premable will be
+        # cleared when it is detected, so it does not need space.
+        self.stream = bytearray(max_length + consts.LEN_HEADER + consts.LEN_BODY)
+        self.index = 0
+
+        # Python 2 does not allow unpack from bytearray, but Python 3.
+        self.buffer = self.stream
+
+    def write_frame(self, frame: Frame) -> int:
+        """
+        Write a frame via the handle.
+        """
+
+        result = bytearray()
+        length = len(frame.data or [])
+
+        # Check length of message.
+        if length > self.max_length:
+            raise ValueError(
+                "Message length %d exceeds max length %d" % (length, self.max_length)
+            )
+
+        # Pack header.
+        checksum_header = utils.checksum_header(frame.flags, length)
+        result += struct.pack(
+            self.endianness + "IHHB",
+            consts.PREAMBLE,
+            frame.flags,
+            length,
+            checksum_header,
+        )
+
+        # Pack data.
+        if frame.data is not None:
+            checksum_frame = utils.checksum_frame(frame.data, checksum_header)
+            result += struct.pack(
+                self.endianness + str(length) + "sI", frame.data, checksum_frame
+            )
+
+        # Write to file.
+        return self.handle.write(result)
+
+    def write(self, data: bytes, flags: int = consts.FLAG_NONE) -> int:
+        """
+        Shorthand for `write_frame(Frame(data, flags=flags))`.
+        """
+
+        return self.write_frame(Frame(data, flags=flags))
+
+    def read(self, limit: int = 1) -> list[Frame]:
+        """
+        Read up to `limit` bytes from the handle and process it. Returns a list
+        of received frames, if any.
+        """
+
+        # List of frames received.
+        frames = []
+
+        # Bytes are added one at a time.
+        while limit:
+            char = self.handle.read(1)
+
+            if not char:
+                return []
+
+            # Append to stream.
+            self.stream[self.index] = ord(char)
+            self.index += 1
+
+            # Decide what to do.
+            if self.state == consts.WAITING_FOR_PREAMBLE:
+                if self.index >= consts.LEN_PREAMBLE:
+                    (start,) = struct.unpack_from(
+                        self.endianness + "I", self.buffer, self.index - 4
+                    )
+
+                    if start == consts.PREAMBLE:
+                        # Advance to next state.
+                        self.index = 0
+                        self.state = consts.WAITING_FOR_HEADER
+                    elif (
+                        self.index
+                        == self.max_length + consts.LEN_HEADER + consts.LEN_BODY
+                    ):
+                        # Preamble not found and stream is full. Copy last four
+                        # bytes, because the next byte may form the preamble
+                        # together with the last three bytes.
+                        self.stream[0:4] = self.stream[-4:]
+                        self.index = 4
+
+            elif self.state == consts.WAITING_FOR_HEADER:
+                if self.index == consts.LEN_HEADER:
+                    flags, length, checksum = struct.unpack_from(
+                        self.endianness + "HHB", self.buffer
+                    )
+
+                    # Verify checksum.
+                    if (
+                        checksum == utils.checksum_header(flags, length)
+                        and length <= self.max_length
+                    ):
+
+                        if length > 0:
+                            self.state = consts.WAITING_FOR_BODY
+                        else:
+                            # Frame without body.
+                            frames.append(Frame(flags=flags))
+
+                            self.index = 0
+                            self.state = consts.WAITING_FOR_PREAMBLE
+                    else:
+                        # Reset to start state.
+                        self.index = 0
+                        self.state = consts.WAITING_FOR_PREAMBLE
+
+            elif self.state == consts.WAITING_FOR_BODY:
+                # Unpack header.
+                flags, length, checksum_a = struct.unpack_from(
+                    self.endianness + "HHB", self.buffer
+                )
+
+                if self.index == consts.LEN_HEADER + length + consts.LEN_CRC:
+                    # Unpack body.
+                    result, checksum_b = struct.unpack_from(
+                        self.endianness + str(length) + "sI",
+                        self.buffer,
+                        consts.LEN_HEADER,
+                    )
+
+                    # Verify checksum.
+                    if checksum_b == utils.checksum_frame(result, checksum_a):
+                        frames.append(Frame(result, flags=flags))
+                    elif not self.ignore_damaged:
+                        frames.append(Frame(result, flags=flags, damaged=True))
+
+                    # Reset to start state.
+                    self.index = 0
+                    self.state = consts.WAITING_FOR_PREAMBLE
+
+            # Decrement number of bytes to read.
+            limit -= 1
+
+        # Done.
+        return frames
diff --git a/tinylink/utils.py b/tinylink/utils.py
index 3fbdde1..69c0773 100644
--- a/tinylink/utils.py
+++ b/tinylink/utils.py
@@ -1,5 +1,3 @@
-from six.moves import xrange
-
 CRC32_POLYNOMIAL = 0xEDB88320
 CRC32_INITIAL = 0x00000000
 
@@ -13,9 +11,9 @@ def crc32(buf):
 
     def crc32_value(c):
         ulTemp1 = (result >> 8) & 0x00FFFFFF
-        ulCRC = (result ^ c) & 0xff
+        ulCRC = (result ^ c) & 0xFF
 
-        for i in xrange(8):
+        for i in range(8):
             if ulCRC & 0x01:
                 ulCRC = (ulCRC >> 1) ^ CRC32_POLYNOMIAL
             else:
@@ -23,7 +21,7 @@ def crc32_value(c):
 
         return ulTemp1 ^ ulCRC
 
-    # Execute above function for each byte.
+    # Execute function for each byte.
     for b in buf:
         result = crc32_value(b)
 
@@ -48,5 +46,4 @@ def checksum_frame(data, checksum_header):
     Calculate checksum of both the checksum header and the data.
     """
 
-    return crc32(
-        memoryview(data).tobytes() + bytearray([checksum_header])) & 0xFFFFFFFF
+    return crc32(memoryview(data).tobytes() + bytearray([checksum_header])) & 0xFFFFFFFF