Updated documentation

Merge branch 'master' of github.com:markqvist/Reticulum
Added output to echo example
2024-11-26 15:30:18 +00:00 · 2023-03-02 12:47:55 +01:00 · 2023-03-02 12:05:06 +01:00 · 2023-03-02 12:04:50 +01:00 · 2023-03-02 11:34:00 +01:00 · 2023-03-02 11:31:31 +01:00
44 changed files with 2157 additions and 31 deletions
--- a/Examples/Channel.py
+++ b/Examples/Channel.py
@ -0,0 +1,395 @@
+##########################################################
+# This RNS example demonstrates how to set up a link to  #
+# a destination, and pass structured messages over it    #
+# using a channel.                                       #
+##########################################################
+
+import os
+import sys
+import time
+import argparse
+from datetime import datetime
+
+import RNS
+from RNS.vendor import umsgpack
+
+# Let's define an app name. We'll use this for all
+# destinations we create. Since this echo example
+# is part of a range of example utilities, we'll put
+# them all within the app namespace "example_utilities"
+APP_NAME = "example_utilities"
+
+##########################################################
+#### Shared Objects ######################################
+##########################################################
+
+# Channel data must be structured in a subclass of
+# MessageBase. This ensures that the channel will be able
+# to serialize and deserialize the object and multiplex it
+# with other objects. Both ends of a link will need the
+# same object definitions to be able to communicate over
+# a channel.
+#
+# Note: The objects we wish to use over the channel must
+# be registered with the channel, and each link has a
+# different channel instance. See the client_connected
+# and link_established functions in this example to see
+# how message types are registered.
+
+# Let's make a simple message class called StringMessage
+# that will convey a string with a timestamp.
+
+class StringMessage(RNS.MessageBase):
+    # The MSGTYPE class variable needs to be assigned a
+    # 2 byte integer value. This identifier allows the
+    # channel to look up your message's constructor when a
+    # message arrives over the channel.
+    #
+    # MSGTYPE must be unique across all message types we
+    # register with the channel. MSGTYPEs >= 0xf000 are
+    # reserved for the system.
+    MSGTYPE = 0x0101
+
+    # The constructor of our object must be callable with
+    # no arguments. We can have parameters, but they must
+    # have a default assignment.
+    #
+    # This is needed so the channel can create an empty
+    # version of our message into which the incoming
+    # message can be unpacked.
+    def __init__(self, data=None):
+        self.data = data
+        self.timestamp = datetime.now()
+
+    # Finally, our message needs to implement functions
+    # the channel can call to pack and unpack our message
+    # to/from the raw packet payload. We'll use the
+    # umsgpack package bundled with RNS. We could also use
+    # the struct package bundled with Python if we wanted
+    # more control over the structure of the packed bytes.
+    #
+    # Also note that packed message objects must fit
+    # entirely in one packet. The number of bytes
+    # available for message payloads can be queried from
+    # the channel using the Channel.MDU property. The
+    # channel MDU is slightly less than the link MDU due
+    # to encoding the message header.
+
+    # The pack function encodes the message contents into
+    # a byte stream.
+    def pack(self) -> bytes:
+        return umsgpack.packb((self.data, self.timestamp))
+
+    # And the unpack function decodes a byte stream into
+    # the message contents.
+    def unpack(self, raw):
+        self.data, self.timestamp = umsgpack.unpackb(raw)
+
+
+##########################################################
+#### Server Part #########################################
+##########################################################
+
+# A reference to the latest client link that connected
+latest_client_link = None
+
+# This initialisation is executed when the users chooses
+# to run as a server
+def server(configpath):
+    # We must first initialise Reticulum
+    reticulum = RNS.Reticulum(configpath)
+    
+    # Randomly create a new identity for our link example
+    server_identity = RNS.Identity()
+
+    # We create a destination that clients can connect to. We
+    # want clients to create links to this destination, so we
+    # need to create a "single" destination type.
+    server_destination = RNS.Destination(
+        server_identity,
+        RNS.Destination.IN,
+        RNS.Destination.SINGLE,
+        APP_NAME,
+        "channelexample"
+    )
+
+    # We configure a function that will get called every time
+    # a new client creates a link to this destination.
+    server_destination.set_link_established_callback(client_connected)
+
+    # Everything's ready!
+    # Let's Wait for client requests or user input
+    server_loop(server_destination)
+
+def server_loop(destination):
+    # Let the user know that everything is ready
+    RNS.log(
+        "Link example "+
+        RNS.prettyhexrep(destination.hash)+
+        " running, waiting for a connection."
+    )
+
+    RNS.log("Hit enter to manually send an announce (Ctrl-C to quit)")
+
+    # We enter a loop that runs until the users exits.
+    # If the user hits enter, we will announce our server
+    # destination on the network, which will let clients
+    # know how to create messages directed towards it.
+    while True:
+        entered = input()
+        destination.announce()
+        RNS.log("Sent announce from "+RNS.prettyhexrep(destination.hash))
+
+# When a client establishes a link to our server
+# destination, this function will be called with
+# a reference to the link.
+def client_connected(link):
+    global latest_client_link
+    latest_client_link = link
+
+    RNS.log("Client connected")
+    link.set_link_closed_callback(client_disconnected)
+
+    # Register message types and add callback to channel
+    channel = link.get_channel()
+    channel.register_message_type(StringMessage)
+    channel.add_message_handler(server_message_received)
+
+def client_disconnected(link):
+    RNS.log("Client disconnected")
+
+def server_message_received(message):
+    """
+    A message handler
+    @param message: An instance of a subclass of MessageBase
+    @return: True if message was handled
+    """
+    global latest_client_link
+    # When a message is received over any active link,
+    # the replies will all be directed to the last client
+    # that connected.
+
+    # In a message handler, any deserializable message
+    # that arrives over the link's channel will be passed
+    # to all message handlers, unless a preceding handler indicates it
+    # has handled the message.
+    #
+    #
+    if isinstance(message, StringMessage):
+        RNS.log("Received data on the link: " + message.data + " (message created at " + str(message.timestamp) + ")")
+
+        reply_message = StringMessage("I received \""+message.data+"\" over the link")
+        latest_client_link.get_channel().send(reply_message)
+
+        # Incoming messages are sent to each message
+        # handler added to the channel, in the order they
+        # were added.
+        # If any message handler returns True, the message
+        # is considered handled and any subsequent
+        # handlers are skipped.
+        return True
+
+
+##########################################################
+#### Client Part #########################################
+##########################################################
+
+# A reference to the server link
+server_link = None
+
+# This initialisation is executed when the users chooses
+# to run as a client
+def client(destination_hexhash, configpath):
+    # We need a binary representation of the destination
+    # hash that was entered on the command line
+    try:
+        dest_len = (RNS.Reticulum.TRUNCATED_HASHLENGTH//8)*2
+        if len(destination_hexhash) != dest_len:
+            raise ValueError(
+                "Destination length is invalid, must be {hex} hexadecimal characters ({byte} bytes).".format(hex=dest_len, byte=dest_len//2)
+            )
+            
+        destination_hash = bytes.fromhex(destination_hexhash)
+    except:
+        RNS.log("Invalid destination entered. Check your input!\n")
+        exit()
+
+    # We must first initialise Reticulum
+    reticulum = RNS.Reticulum(configpath)
+
+    # Check if we know a path to the destination
+    if not RNS.Transport.has_path(destination_hash):
+        RNS.log("Destination is not yet known. Requesting path and waiting for announce to arrive...")
+        RNS.Transport.request_path(destination_hash)
+        while not RNS.Transport.has_path(destination_hash):
+            time.sleep(0.1)
+
+    # Recall the server identity
+    server_identity = RNS.Identity.recall(destination_hash)
+
+    # Inform the user that we'll begin connecting
+    RNS.log("Establishing link with server...")
+
+    # When the server identity is known, we set
+    # up a destination
+    server_destination = RNS.Destination(
+        server_identity,
+        RNS.Destination.OUT,
+        RNS.Destination.SINGLE,
+        APP_NAME,
+        "channelexample"
+    )
+
+    # And create a link
+    link = RNS.Link(server_destination)
+
+    # We set a callback that will get executed
+    # every time a packet is received over the
+    # link
+    link.set_packet_callback(client_message_received)
+
+    # We'll also set up functions to inform the
+    # user when the link is established or closed
+    link.set_link_established_callback(link_established)
+    link.set_link_closed_callback(link_closed)
+
+    # Everything is set up, so let's enter a loop
+    # for the user to interact with the example
+    client_loop()
+
+def client_loop():
+    global server_link
+
+    # Wait for the link to become active
+    while not server_link:
+        time.sleep(0.1)
+
+    should_quit = False
+    while not should_quit:
+        try:
+            print("> ", end=" ")
+            text = input()
+
+            # Check if we should quit the example
+            if text == "quit" or text == "q" or text == "exit":
+                should_quit = True
+                server_link.teardown()
+
+            # If not, send the entered text over the link
+            if text != "":
+                message = StringMessage(text)
+                packed_size = len(message.pack())
+                channel = server_link.get_channel()
+                if channel.is_ready_to_send():
+                    if packed_size <= channel.MDU:
+                        channel.send(message)
+                    else:
+                        RNS.log(
+                            "Cannot send this packet, the data size of "+
+                            str(packed_size)+" bytes exceeds the link packet MDU of "+
+                            str(channel.MDU)+" bytes",
+                            RNS.LOG_ERROR
+                        )
+                else:
+                    RNS.log("Channel is not ready to send, please wait for " +
+                            "pending messages to complete.", RNS.LOG_ERROR)
+
+        except Exception as e:
+            RNS.log("Error while sending data over the link: "+str(e))
+            should_quit = True
+            server_link.teardown()
+
+# This function is called when a link
+# has been established with the server
+def link_established(link):
+    # We store a reference to the link
+    # instance for later use
+    global server_link
+    server_link = link
+
+    # Register messages and add handler to channel
+    channel = link.get_channel()
+    channel.register_message_type(StringMessage)
+    channel.add_message_handler(client_message_received)
+
+    # Inform the user that the server is
+    # connected
+    RNS.log("Link established with server, enter some text to send, or \"quit\" to quit")
+
+# When a link is closed, we'll inform the
+# user, and exit the program
+def link_closed(link):
+    if link.teardown_reason == RNS.Link.TIMEOUT:
+        RNS.log("The link timed out, exiting now")
+    elif link.teardown_reason == RNS.Link.DESTINATION_CLOSED:
+        RNS.log("The link was closed by the server, exiting now")
+    else:
+        RNS.log("Link closed, exiting now")
+    
+    RNS.Reticulum.exit_handler()
+    time.sleep(1.5)
+    os._exit(0)
+
+# When a packet is received over the link, we
+# simply print out the data.
+def client_message_received(message):
+    if isinstance(message, StringMessage):
+        RNS.log("Received data on the link: " + message.data + " (message created at " + str(message.timestamp) + ")")
+        print("> ", end=" ")
+        sys.stdout.flush()
+
+
+##########################################################
+#### Program Startup #####################################
+##########################################################
+
+# This part of the program runs at startup,
+# and parses input of from the user, and then
+# starts up the desired program mode.
+if __name__ == "__main__":
+    try:
+        parser = argparse.ArgumentParser(description="Simple link example")
+
+        parser.add_argument(
+            "-s",
+            "--server",
+            action="store_true",
+            help="wait for incoming link requests from clients"
+        )
+
+        parser.add_argument(
+            "--config",
+            action="store",
+            default=None,
+            help="path to alternative Reticulum config directory",
+            type=str
+        )
+
+        parser.add_argument(
+            "destination",
+            nargs="?",
+            default=None,
+            help="hexadecimal hash of the server destination",
+            type=str
+        )
+
+        args = parser.parse_args()
+
+        if args.config:
+            configarg = args.config
+        else:
+            configarg = None
+
+        if args.server:
+            server(configarg)
+        else:
+            if (args.destination == None):
+                print("")
+                parser.print_help()
+                print("")
+            else:
+                client(args.destination, configarg)
+
+    except KeyboardInterrupt:
+        print("")
+        exit()
--- a/Examples/Echo.py
+++ b/Examples/Echo.py
@ -210,6 +210,7 @@ def client(destination_hexhash, configpath, timeout=None):
            # If we do not know this destination, tell the
            # user to wait for an announce to arrive.
            RNS.log("Destination is not yet known. Requesting path...")
+            RNS.log("Hit enter to manually retry once an announce is received.")
            RNS.Transport.request_path(destination_hash)

 # This function is called when our reply destination
--- a/2
+++ b/2
@ -47,7 +47,7 @@ documentation:
 	make -C docs html

 manual:
-	make -C docs latexpdf
+	make -C docs latexpdf epub

 release: test remove_symlinks build_wheel build_pure_wheel documentation manual create_symlinks

--- a/README.md
+++ b/README.md
@ -71,6 +71,7 @@ To learn more about the direction and future of Reticulum, please see the [Devel
 If you want to quickly get an idea of what Reticulum can do, take a look at the
 following resources.

+- You can use the [rnsh](https://github.com/acehoss/rnsh) program to establish remote shell sessions over Reticulum.
 - For an off-grid, encrypted and resilient mesh communications platform, see [Nomad Network](https://github.com/markqvist/NomadNet)
 - The Android, Linux and macOS app [Sideband](https://github.com/markqvist/Sideband) has a graphical interface and focuses on ease of use.
 - [LXMF](https://github.com/markqvist/lxmf) is a distributed, delay and disruption tolerant message transfer protocol built on Reticulum
--- a/RNS/Channel.py
+++ b/RNS/Channel.py
@ -0,0 +1,521 @@
+# MIT License
+#
+# Copyright (c) 2016-2023 Mark Qvist / unsigned.io and contributors.
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+from __future__ import annotations
+import collections
+import enum
+import threading
+import time
+from types import TracebackType
+from typing import Type, Callable, TypeVar, Generic, NewType
+import abc
+import contextlib
+import struct
+import RNS
+from abc import ABC, abstractmethod
+TPacket = TypeVar("TPacket")
+
+
+class ChannelOutletBase(ABC, Generic[TPacket]):
+    """
+    An abstract transport layer interface used by Channel.
+
+    DEPRECATED: This was created for testing; eventually
+    Channel will use Link or a LinkBase interface
+    directly.
+    """
+    @abstractmethod
+    def send(self, raw: bytes) -> TPacket:
+        raise NotImplemented()
+
+    @abstractmethod
+    def resend(self, packet: TPacket) -> TPacket:
+        raise NotImplemented()
+
+    @property
+    @abstractmethod
+    def mdu(self):
+        raise NotImplemented()
+
+    @property
+    @abstractmethod
+    def rtt(self):
+        raise NotImplemented()
+
+    @property
+    @abstractmethod
+    def is_usable(self):
+        raise NotImplemented()
+
+    @abstractmethod
+    def get_packet_state(self, packet: TPacket) -> MessageState:
+        raise NotImplemented()
+
+    @abstractmethod
+    def timed_out(self):
+        raise NotImplemented()
+
+    @abstractmethod
+    def __str__(self):
+        raise NotImplemented()
+
+    @abstractmethod
+    def set_packet_timeout_callback(self, packet: TPacket, callback: Callable[[TPacket], None] | None,
+                                    timeout: float | None = None):
+        raise NotImplemented()
+
+    @abstractmethod
+    def set_packet_delivered_callback(self, packet: TPacket, callback: Callable[[TPacket], None] | None):
+        raise NotImplemented()
+
+    @abstractmethod
+    def get_packet_id(self, packet: TPacket) -> any:
+        raise NotImplemented()
+
+
+class CEType(enum.IntEnum):
+    """
+    ChannelException type codes
+    """
+    ME_NO_MSG_TYPE      = 0
+    ME_INVALID_MSG_TYPE = 1
+    ME_NOT_REGISTERED   = 2
+    ME_LINK_NOT_READY   = 3
+    ME_ALREADY_SENT     = 4
+    ME_TOO_BIG          = 5
+
+
+class ChannelException(Exception):
+    """
+    An exception thrown by Channel, with a type code.
+    """
+    def __init__(self, ce_type: CEType, *args):
+        super().__init__(args)
+        self.type = ce_type
+
+
+class MessageState(enum.IntEnum):
+    """
+    Set of possible states for a Message
+    """
+    MSGSTATE_NEW       = 0
+    MSGSTATE_SENT      = 1
+    MSGSTATE_DELIVERED = 2
+    MSGSTATE_FAILED    = 3
+
+
+class MessageBase(abc.ABC):
+    """
+    Base type for any messages sent or received on a Channel.
+    Subclasses must define the two abstract methods as well as
+    the ``MSGTYPE`` class variable.
+    """
+    # MSGTYPE must be unique within all classes sent over a
+    # channel. Additionally, MSGTYPE > 0xf000 are reserved.
+    MSGTYPE = None
+    """
+    Defines a unique identifier for a message class.
+    
+    * Must be unique within all classes registered with a ``Channel``
+    * Must be less than ``0xf000``. Values greater than or equal to ``0xf000`` are reserved.
+    """
+
+    @abstractmethod
+    def pack(self) -> bytes:
+        """
+        Create and return the binary representation of the message
+
+        :return: binary representation of message
+        """
+        raise NotImplemented()
+
+    @abstractmethod
+    def unpack(self, raw: bytes):
+        """
+        Populate message from binary representation
+
+        :param raw: binary representation
+        """
+        raise NotImplemented()
+
+
+MessageCallbackType = NewType("MessageCallbackType", Callable[[MessageBase], bool])
+
+
+class Envelope:
+    """
+    Internal wrapper used to transport messages over a channel and
+    track its state within the channel framework.
+    """
+    def unpack(self, message_factories: dict[int, Type]) -> MessageBase:
+        msgtype, self.sequence, length = struct.unpack(">HHH", self.raw[:6])
+        raw = self.raw[6:]
+        ctor = message_factories.get(msgtype, None)
+        if ctor is None:
+            raise ChannelException(CEType.ME_NOT_REGISTERED, f"Unable to find constructor for Channel MSGTYPE {hex(msgtype)}")
+        message = ctor()
+        message.unpack(raw)
+        return message
+
+    def pack(self) -> bytes:
+        if self.message.__class__.MSGTYPE is None:
+            raise ChannelException(CEType.ME_NO_MSG_TYPE, f"{self.message.__class__} lacks MSGTYPE")
+        data = self.message.pack()
+        self.raw = struct.pack(">HHH", self.message.MSGTYPE, self.sequence, len(data)) + data
+        return self.raw
+
+    def __init__(self, outlet: ChannelOutletBase, message: MessageBase = None, raw: bytes = None, sequence: int = None):
+        self.ts = time.time()
+        self.id = id(self)
+        self.message = message
+        self.raw = raw
+        self.packet: TPacket = None
+        self.sequence = sequence
+        self.outlet = outlet
+        self.tries = 0
+        self.tracked = False
+
+
+class Channel(contextlib.AbstractContextManager):
+    """
+    Provides reliable delivery of messages over
+    a link.
+
+    ``Channel`` differs from ``Request`` and
+    ``Resource`` in some important ways:
+
+     **Continuous**
+        Messages can be sent or received as long as
+        the ``Link`` is open.
+     **Bi-directional**
+        Messages can be sent in either direction on
+        the ``Link``; neither end is the client or
+        server.
+     **Size-constrained**
+        Messages must be encoded into a single packet.
+
+    ``Channel`` is similar to ``Packet``, except that it
+    provides reliable delivery (automatic retries) as well
+    as a structure for exchanging several types of
+    messages over the ``Link``.
+
+    ``Channel`` is not instantiated directly, but rather
+    obtained from a ``Link`` with ``get_channel()``.
+    """
+    def __init__(self, outlet: ChannelOutletBase):
+        """
+
+        @param outlet:
+        """
+        self._outlet = outlet
+        self._lock = threading.RLock()
+        self._tx_ring: collections.deque[Envelope] = collections.deque()
+        self._rx_ring: collections.deque[Envelope] = collections.deque()
+        self._message_callbacks: [MessageCallbackType] = []
+        self._next_sequence = 0
+        self._message_factories: dict[int, Type[MessageBase]] = {}
+        self._max_tries = 5
+
+    def __enter__(self) -> Channel:
+        return self
+
+    def __exit__(self, __exc_type: Type[BaseException] | None, __exc_value: BaseException | None,
+                 __traceback: TracebackType | None) -> bool | None:
+        self._shutdown()
+        return False
+
+    def register_message_type(self, message_class: Type[MessageBase]):
+        """
+        Register a message class for reception over a ``Channel``.
+
+        Message classes must extend ``MessageBase``.
+
+        :param message_class: Class to register
+        """
+        self._register_message_type(message_class, is_system_type=False)
+
+    def _register_message_type(self, message_class: Type[MessageBase], *, is_system_type: bool = False):
+        with self._lock:
+            if not issubclass(message_class, MessageBase):
+                raise ChannelException(CEType.ME_INVALID_MSG_TYPE,
+                                       f"{message_class} is not a subclass of {MessageBase}.")
+            if message_class.MSGTYPE is None:
+                raise ChannelException(CEType.ME_INVALID_MSG_TYPE,
+                                       f"{message_class} has invalid MSGTYPE class attribute.")
+            if message_class.MSGTYPE >= 0xf000 and not is_system_type:
+                raise ChannelException(CEType.ME_INVALID_MSG_TYPE,
+                                       f"{message_class} has system-reserved message type.")
+            try:
+                message_class()
+            except Exception as ex:
+                raise ChannelException(CEType.ME_INVALID_MSG_TYPE,
+                                       f"{message_class} raised an exception when constructed with no arguments: {ex}")
+
+            self._message_factories[message_class.MSGTYPE] = message_class
+
+    def add_message_handler(self, callback: MessageCallbackType):
+        """
+        Add a handler for incoming messages. A handler
+        has the following signature:
+
+        ``(message: MessageBase) -> bool``
+
+        Handlers are processed in the order they are
+        added. If any handler returns True, processing
+        of the message stops; handlers after the
+        returning handler will not be called.
+
+        :param callback: Function to call
+        """
+        with self._lock:
+            if callback not in self._message_callbacks:
+                self._message_callbacks.append(callback)
+
+    def remove_message_handler(self, callback: MessageCallbackType):
+        """
+        Remove a handler added with ``add_message_handler``.
+
+        :param callback: handler to remove
+        """
+        with self._lock:
+            self._message_callbacks.remove(callback)
+
+    def _shutdown(self):
+        with self._lock:
+            self._message_callbacks.clear()
+            self._clear_rings()
+
+    def _clear_rings(self):
+        with self._lock:
+            for envelope in self._tx_ring:
+                if envelope.packet is not None:
+                    self._outlet.set_packet_timeout_callback(envelope.packet, None)
+                    self._outlet.set_packet_delivered_callback(envelope.packet, None)
+            self._tx_ring.clear()
+            self._rx_ring.clear()
+
+    def _emplace_envelope(self, envelope: Envelope, ring: collections.deque[Envelope]) -> bool:
+        with self._lock:
+            i = 0
+            for existing in ring:
+                if existing.sequence > envelope.sequence \
+                   and not existing.sequence // 2 > envelope.sequence:  # account for overflow
+                    ring.insert(i, envelope)
+                    return True
+                if existing.sequence == envelope.sequence:
+                    RNS.log(f"Envelope: Emplacement of duplicate envelope sequence.", RNS.LOG_EXTREME)
+                    return False
+                i += 1
+            envelope.tracked = True
+            ring.append(envelope)
+            return True
+
+    def _prune_rx_ring(self):
+        with self._lock:
+            # Implementation for fixed window = 1
+            stale = list(sorted(self._rx_ring, key=lambda env: env.sequence, reverse=True))[1:]
+            for env in stale:
+                env.tracked = False
+                self._rx_ring.remove(env)
+
+    def _run_callbacks(self, message: MessageBase):
+        with self._lock:
+            cbs = self._message_callbacks.copy()
+
+        for cb in cbs:
+            try:
+                if cb(message):
+                    return
+            except Exception as ex:
+                RNS.log(f"Channel: Error running message callback: {ex}", RNS.LOG_ERROR)
+
+    def _receive(self, raw: bytes):
+        try:
+            envelope = Envelope(outlet=self._outlet, raw=raw)
+            with self._lock:
+                message = envelope.unpack(self._message_factories)
+                is_new = self._emplace_envelope(envelope, self._rx_ring)
+                self._prune_rx_ring()
+            if not is_new:
+                RNS.log("Channel: Duplicate message received", RNS.LOG_DEBUG)
+                return
+            RNS.log(f"Message received: {message}", RNS.LOG_DEBUG)
+            threading.Thread(target=self._run_callbacks, name="Message Callback", args=[message], daemon=True).start()
+        except Exception as ex:
+            RNS.log(f"Channel: Error receiving data: {ex}")
+
+    def is_ready_to_send(self) -> bool:
+        """
+        Check if ``Channel`` is ready to send.
+
+        :return: True if ready
+        """
+        if not self._outlet.is_usable:
+            RNS.log("Channel: Link is not usable.", RNS.LOG_EXTREME)
+            return False
+
+        with self._lock:
+            for envelope in self._tx_ring:
+                if envelope.outlet == self._outlet and (not envelope.packet
+                                                        or self._outlet.get_packet_state(envelope.packet) == MessageState.MSGSTATE_SENT):
+                    RNS.log("Channel: Link has a pending message.", RNS.LOG_EXTREME)
+                    return False
+        return True
+
+    def _packet_tx_op(self, packet: TPacket, op: Callable[[TPacket], bool]):
+        with self._lock:
+            envelope = next(filter(lambda e: self._outlet.get_packet_id(e.packet) == self._outlet.get_packet_id(packet),
+                                   self._tx_ring), None)
+            if envelope and op(envelope):
+                envelope.tracked = False
+                if envelope in self._tx_ring:
+                    self._tx_ring.remove(envelope)
+                else:
+                    RNS.log("Channel: Envelope not found in TX ring", RNS.LOG_DEBUG)
+        if not envelope:
+            RNS.log("Channel: Spurious message received.", RNS.LOG_EXTREME)
+
+    def _packet_delivered(self, packet: TPacket):
+        self._packet_tx_op(packet, lambda env: True)
+
+    def _packet_timeout(self, packet: TPacket):
+        def retry_envelope(envelope: Envelope) -> bool:
+            if envelope.tries >= self._max_tries:
+                RNS.log("Channel: Retry count exceeded, tearing down Link.", RNS.LOG_ERROR)
+                self._shutdown()  # start on separate thread?
+                self._outlet.timed_out()
+                return True
+            envelope.tries += 1
+            self._outlet.resend(envelope.packet)
+            return False
+
+        self._packet_tx_op(packet, retry_envelope)
+
+    def send(self, message: MessageBase) -> Envelope:
+        """
+        Send a message. If a message send is attempted and
+        ``Channel`` is not ready, an exception is thrown.
+
+        :param message: an instance of a ``MessageBase`` subclass
+        """
+        envelope: Envelope | None = None
+        with self._lock:
+            if not self.is_ready_to_send():
+                raise ChannelException(CEType.ME_LINK_NOT_READY, f"Link is not ready")
+            envelope = Envelope(self._outlet, message=message, sequence=self._next_sequence)
+            self._next_sequence = (self._next_sequence + 1) % 0x10000
+            self._emplace_envelope(envelope, self._tx_ring)
+        if envelope is None:
+            raise BlockingIOError()
+
+        envelope.pack()
+        if len(envelope.raw) > self._outlet.mdu:
+            raise ChannelException(CEType.ME_TOO_BIG, f"Packed message too big for packet: {len(envelope.raw)} > {self._outlet.mdu}")
+        envelope.packet = self._outlet.send(envelope.raw)
+        envelope.tries += 1
+        self._outlet.set_packet_delivered_callback(envelope.packet, self._packet_delivered)
+        self._outlet.set_packet_timeout_callback(envelope.packet, self._packet_timeout)
+        return envelope
+
+    @property
+    def MDU(self):
+        """
+        Maximum Data Unit: the number of bytes available
+        for a message to consume in a single send. This
+        value is adjusted from the ``Link`` MDU to accommodate
+        message header information.
+
+        :return: number of bytes available
+        """
+        return self._outlet.mdu - 6  # sizeof(msgtype) + sizeof(length) + sizeof(sequence)
+
+
+class LinkChannelOutlet(ChannelOutletBase):
+    """
+    An implementation of ChannelOutletBase for RNS.Link.
+    Allows Channel to send packets over an RNS Link with
+    Packets.
+
+    :param link: RNS Link to wrap
+    """
+    def __init__(self, link: RNS.Link):
+        self.link = link
+
+    def send(self, raw: bytes) -> RNS.Packet:
+        packet = RNS.Packet(self.link, raw, context=RNS.Packet.CHANNEL)
+        packet.send()
+        return packet
+
+    def resend(self, packet: RNS.Packet) -> RNS.Packet:
+        if not packet.resend():
+            RNS.log("Failed to resend packet", RNS.LOG_ERROR)
+        return packet
+
+    @property
+    def mdu(self):
+        return self.link.MDU
+
+    @property
+    def rtt(self):
+        return self.link.rtt
+
+    @property
+    def is_usable(self):
+        return True  # had issues looking at Link.status
+
+    def get_packet_state(self, packet: TPacket) -> MessageState:
+        status = packet.receipt.get_status()
+        if status == RNS.PacketReceipt.SENT:
+            return MessageState.MSGSTATE_SENT
+        if status == RNS.PacketReceipt.DELIVERED:
+            return MessageState.MSGSTATE_DELIVERED
+        if status == RNS.PacketReceipt.FAILED:
+            return MessageState.MSGSTATE_FAILED
+        else:
+            raise Exception(f"Unexpected receipt state: {status}")
+
+    def timed_out(self):
+        self.link.teardown()
+
+    def __str__(self):
+        return f"{self.__class__.__name__}({self.link})"
+
+    def set_packet_timeout_callback(self, packet: RNS.Packet, callback: Callable[[RNS.Packet], None] | None,
+                                    timeout: float | None = None):
+        if timeout:
+            packet.receipt.set_timeout(timeout)
+
+        def inner(receipt: RNS.PacketReceipt):
+            callback(packet)
+
+        if packet and packet.receipt:
+            packet.receipt.set_timeout_callback(inner if callback else None)
+
+    def set_packet_delivered_callback(self, packet: RNS.Packet, callback: Callable[[RNS.Packet], None] | None):
+        def inner(receipt: RNS.PacketReceipt):
+            callback(packet)
+
+        if packet and packet.receipt:
+            packet.receipt.set_delivery_callback(inner if callback else None)
+
+    def get_packet_id(self, packet: RNS.Packet) -> any:
+        return packet.get_hash()
--- a/RNS/Cryptography/HKDF.py
+++ b/RNS/Cryptography/HKDF.py
@ -39,9 +39,6 @@ def hkdf(length=None, derive_from=None, salt=None, context=None):
    if salt == None or len(salt) == 0:
        salt = bytes([0] * hash_len)

-    if salt == None:
-        salt = b""
-
    if context == None:
        context = b""

--- a/RNS/Interfaces/AutoInterface.py
+++ b/RNS/Interfaces/AutoInterface.py
@ -23,6 +23,7 @@
 from .Interface import Interface
 import socketserver
 import threading
+import re
 import socket
 import struct
 import time
@ -53,6 +54,13 @@ class AutoInterface(Interface):
            return AutoInterfaceHandler(callback, *args, **keys)
        return create_handler

+    def descope_linklocal(self, link_local_addr):
+        # Drop scope specifier expressd as %ifname (macOS)
+        link_local_addr = link_local_addr.split("%")[0]
+        # Drop embedded scope specifier (NetBSD, OpenBSD)
+        link_local_addr = re.sub(r"fe80:[0-9a-f]*::","fe80::", link_local_addr)
+        return link_local_addr
+
    def __init__(self, owner, name, group_id=None, discovery_scope=None, discovery_port=None, data_port=None, allowed_interfaces=None, ignored_interfaces=None, configured_bitrate=None):
        import importlib
        if importlib.util.find_spec('netifaces') != None:
@ -158,9 +166,9 @@ class AutoInterface(Interface):
                        for address in addresses[self.netifaces.AF_INET6]:
                            if "addr" in address:
                                if address["addr"].startswith("fe80:"):
-                                    link_local_addr = address["addr"]
-                                    self.link_local_addresses.append(link_local_addr.split("%")[0])
-                                    self.adopted_interfaces[ifname] = link_local_addr.split("%")[0]
+                                    link_local_addr = self.descope_linklocal(address["addr"])
+                                    self.link_local_addresses.append(link_local_addr)
+                                    self.adopted_interfaces[ifname] = link_local_addr
                                    self.multicast_echoes[ifname] = time.time()
                                    RNS.log(str(self)+" Selecting link-local address "+str(link_local_addr)+" for interface "+str(ifname), RNS.LOG_EXTREME)

@ -278,7 +286,7 @@ class AutoInterface(Interface):
                        for address in addresses[self.netifaces.AF_INET6]:
                            if "addr" in address:
                                if address["addr"].startswith("fe80:"):
-                                    link_local_addr = address["addr"].split("%")[0]
+                                    link_local_addr = self.descope_linklocal(address["addr"])
                                    if link_local_addr != self.adopted_interfaces[ifname]:
                                        old_link_local_address = self.adopted_interfaces[ifname]
                                        RNS.log("Replacing link-local address "+str(old_link_local_address)+" for "+str(ifname)+" with "+str(link_local_addr), RNS.LOG_DEBUG)
--- a/RNS/Link.py
+++ b/RNS/Link.py
@ -22,6 +22,7 @@

 from RNS.Cryptography import X25519PrivateKey, X25519PublicKey, Ed25519PrivateKey, Ed25519PublicKey
 from RNS.Cryptography import Fernet
+from RNS.Channel import Channel, LinkChannelOutlet

 from time import sleep
 from .vendor import umsgpack as umsgpack
@ -163,6 +164,7 @@ class Link:
        self.destination = destination
        self.attached_interface = None
        self.__remote_identity = None
+        self._channel = None
        if self.destination == None:
            self.initiator = False
            self.prv     = X25519PrivateKey.generate()
@ -462,6 +464,8 @@ class Link:
            resource.cancel()
        for resource in self.outgoing_resources:
            resource.cancel()
+        if self._channel:
+            self._channel._shutdown()
            
        self.prv = None
        self.pub = None
@ -642,6 +646,16 @@ class Link:
                if pending_request.request_id == resource.request_id:
                    pending_request.request_timed_out(None)

+    def get_channel(self):
+        """
+        Get the ``Channel`` for this link.
+
+        :return: ``Channel`` object
+        """
+        if self._channel is None:
+            self._channel = Channel(LinkChannelOutlet(self))
+        return self._channel
+
    def receive(self, packet):
        self.watchdog_lock = True
        if not self.status == Link.CLOSED and not (self.initiator and packet.context == RNS.Packet.KEEPALIVE and packet.data == bytes([0xFF])):
@ -788,6 +802,14 @@ class Link:
                        for resource in self.incoming_resources:
                            resource.receive_part(packet)

+                    elif packet.context == RNS.Packet.CHANNEL:
+                        if not self._channel:
+                            RNS.log(f"Channel data received without open channel", RNS.LOG_DEBUG)
+                        else:
+                            plaintext = self.decrypt(packet.data)
+                            self._channel._receive(plaintext)
+                            packet.prove()
+
                elif packet.packet_type == RNS.Packet.PROOF:
                    if packet.context == RNS.Packet.RESOURCE_PRF:
                        resource_hash = packet.data[0:RNS.Identity.HASHLENGTH//8]
--- a/RNS/Packet.py
+++ b/RNS/Packet.py
@ -75,6 +75,7 @@ class Packet:
    PATH_RESPONSE  = 0x0B   # Packet is a response to a path request
    COMMAND        = 0x0C   # Packet is a command
    COMMAND_STATUS = 0x0D   # Packet is a status of an executed command
+    CHANNEL        = 0x0E   # Packet contains link channel data
    KEEPALIVE      = 0xFA   # Packet is a keepalive packet
    LINKIDENTIFY   = 0xFB   # Packet is a link peer identification proof
    LINKCLOSE      = 0xFC   # Packet is a link close message
--- a/RNS/Utilities/rnodeconf.py
+++ b/RNS/Utilities/rnodeconf.py
@ -831,6 +831,9 @@ class RNode():
                            print("     WARNING! This device is NOT verifiable and should NOT be trusted.")
                            print("     Someone could have added privacy-breaking or malicious code to it.")
                            print("     ")
+                            print("     Please verify the signing key is present on this machine.")
+                            print("     Autogenerated keys will not match another machine's signature.")
+                            print("     ")
                            print("     Proceed at your own risk and responsibility! If you created this")
                            print("     device yourself, please read the documentation on how to sign your")
                            print("     device to avoid this warning.")
@ -948,6 +951,7 @@ def ensure_firmware_file(fw_filename):
                        RNS.log("Failed to retrive latest version information for your board.")
                        RNS.log("Check your internet connection and try again.")
                        RNS.log("If you don't have Internet access currently, use the --fw-version option to manually specify a version.")
+                        RNS.log("You can also use --extract to copy the firmware from a known-good RNode of the same model.")
                        exit()

                    import shutil
@ -972,6 +976,7 @@ def ensure_firmware_file(fw_filename):
                    os.makedirs(UPD_DIR+"/"+selected_version)

                if not os.path.isfile(UPD_DIR+"/"+selected_version+"/"+fw_filename):
+                    RNS.log("Firmware "+UPD_DIR+"/"+selected_version+"/"+fw_filename+" not found.")
                    RNS.log("Downloading missing firmware file: "+fw_filename+" for version "+selected_version)
                    urlretrieve(update_target_url, UPD_DIR+"/"+selected_version+"/"+fw_filename)
                    RNS.log("Firmware file downloaded")
@ -986,6 +991,7 @@ def ensure_firmware_file(fw_filename):
                            selected_hash = release_info.split()[1]
                        except Exception as e:
                            RNS.log("Could not read locally cached release information.")
+                            RNS.log("Ensure "+UPD_DIR+"/"+selected_version+"/"+fw_filename+".version exists and has the correct format and hash.")
                            RNS.log("You can clear the cache with the --clear-cache option and try again.")

                        if selected_hash == None:
@ -1132,6 +1138,11 @@ def main():

        if args.fw_version != None:
            selected_version = args.fw_version
+            try: 
+                check_float = float(selected_version)
+            except ValueError:
+                RNS.log("Selected version \""+selected_version+"\" does not appear to be a number.")
+                exit()

        if args.force_update:
            force_update = True
--- a/RNS/init.py
+++ b/RNS/init.py
@ -32,6 +32,7 @@ from ._version import __version__
 from .Reticulum import Reticulum
 from .Identity import Identity
 from .Link import Link, RequestReceipt
+from .Channel import MessageBase
 from .Transport import Transport
 from .Destination import Destination
 from .Packet import Packet
--- a/Roadmap.md
+++ b/Roadmap.md
@ -21,6 +21,7 @@ For each release cycle of Reticulum, improvements and additions from the five [P
  - [x] Add bluetooth pairing code output to rnodeconf
  - [x] Add `rnid` utility with encryption, signing and Identity funcionality
  - [x] JSON output mode for rnstatus
+  - [ ] Add `Buffer` class to the API
  - [ ] Add automatic retries to all use cases of the `Request` API
  - [ ] Updating the documentation to reflect recent changes and improvements
 - Targets for related applications
--- a/docs/Makefile
+++ b/docs/Makefile
@ -30,3 +30,8 @@ help:
 		cp -r build/latex/reticulumnetworkstack.pdf ./Reticulum\ Manual.pdf; \
 		echo "PDF Manual Generated"; \
 	fi
+
+	@if [ $@ = "epub" ]; then \
+		cp -r build/epub/ReticulumNetworkStack.epub ./Reticulum\ Manual.epub; \
+		echo "EPUB Manual Generated"; \
+	fi
--- a/docs/Reticulum
+++ b/docs/Reticulum
--- a/docs/Reticulum
+++ b/docs/Reticulum
--- a/docs/manual/.buildinfo
+++ b/docs/manual/.buildinfo
@ -1,4 +1,4 @@
 # Sphinx build info version 1
 # This file hashes the configuration used when building these files. When it is not found, a full rebuild will be done.
-config: 9738e36d7743271d1e6118c152b32349
+config: f6ba2c305fffe594c412b86b0152889d
 tags: 645f666f9bcd5a90fca523b33c5a78b7
--- a/docs/manual/_sources/examples.rst.txt
+++ b/docs/manual/_sources/examples.rst.txt
@ -92,6 +92,18 @@ The *Request* example explores sendig requests and receiving responses.

 This example can also be found at `<https://github.com/markqvist/Reticulum/blob/master/Examples/Request.py>`_.

+.. _example-channel:
+
+Channel
+=======
+
+The *Channel* example explores using a ``Channel`` to send structured
+data between peers of a ``Link``.
+
+.. literalinclude:: ../../Examples/Channel.py
+
+This example can also be found at `<https://github.com/markqvist/Reticulum/blob/master/Examples/Channel.py>`_.
+
 .. _example-filetransfer:

 Filetransfer
--- a/docs/manual/_sources/gettingstartedfast.rst.txt
+++ b/docs/manual/_sources/gettingstartedfast.rst.txt
@ -42,6 +42,13 @@ transceivers or infrastructure just to try it out. Launching the programs on sep
 devices connected to the same WiFi network is enough to get started, and physical
 radio interfaces can then be added later.

+Remote Shell
+^^^^^^^^^^^^
+
+The `rnsh <https://github.com/acehoss/rnsh>`_ program lets you establish fully interactive
+remote shell sessions over Reticulum. It also allows you to pipe any program to or from a
+remote system, and is similar to how ``ssh`` works.
+
 Nomad Network
 ^^^^^^^^^^^^^

--- a/docs/manual/_sources/index.rst.txt
+++ b/docs/manual/_sources/index.rst.txt
@ -5,6 +5,10 @@ This manual aims to provide you with all the information you need to
 understand Reticulum, build networks or develop programs using it, or
 to participate in the development of Reticulum itself.

+.. only:: html
+
+This manual is also available in `PDF <https://github.com/markqvist/Reticulum/releases/latest/download/Reticulum.Manual.pdf>`_ and `EPUB <https://github.com/markqvist/Reticulum/releases/latest/download/Reticulum.Manual.epub>`_ formats.
+
 .. only:: html

   Table Of Contents
--- a/docs/manual/_sources/reference.rst.txt
+++ b/docs/manual/_sources/reference.rst.txt
@ -121,6 +121,34 @@ This chapter lists and explains all classes exposed by the Reticulum Network Sta
 .. autoclass:: RNS.Resource(data, link, advertise=True, auto_compress=True, callback=None, progress_callback=None, timeout=None)
   :members:

+.. _api-channel:
+
+.. only:: html
+
+   |start-h3| Channel |end-h3|
+
+.. only:: latex
+
+   Channel
+   -------
+
+.. autoclass:: RNS.Channel.Channel()
+   :members:
+
+.. _api-messsagebase:
+
+.. only:: html
+
+   |start-h3| MessageBase |end-h3|
+
+.. only:: latex
+
+   MessageBase
+   -----------
+
+.. autoclass:: RNS.MessageBase()
+   :members:
+
 .. _api-transport:

 .. only:: html
--- a/docs/manual/examples.html
+++ b/docs/manual/examples.html
@ -861,6 +861,7 @@ the Packet interface.</p>
            <span class="c1"># If we do not know this destination, tell the</span>
            <span class="c1"># user to wait for an announce to arrive.</span>
            <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Destination is not yet known. Requesting path...&quot;</span><span class="p">)</span>
+            <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Hit enter to manually retry once an announce is received.&quot;</span><span class="p">)</span>
            <span class="n">RNS</span><span class="o">.</span><span class="n">Transport</span><span class="o">.</span><span class="n">request_path</span><span class="p">(</span><span class="n">destination_hash</span><span class="p">)</span>

 <span class="c1"># This function is called when our reply destination</span>
@ -1898,6 +1899,409 @@ the link has been established.</p>
 </div>
 <p>This example can also be found at <a class="reference external" href="https://github.com/markqvist/Reticulum/blob/master/Examples/Request.py">https://github.com/markqvist/Reticulum/blob/master/Examples/Request.py</a>.</p>
 </section>
+<section id="channel">
+<span id="example-channel"></span><h2>Channel<a class="headerlink" href="#channel" title="Permalink to this heading">#</a></h2>
+<p>The <em>Channel</em> example explores using a <code class="docutils literal notranslate"><span class="pre">Channel</span></code> to send structured
+data between peers of a <code class="docutils literal notranslate"><span class="pre">Link</span></code>.</p>
+<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1">##########################################################</span>
+<span class="c1"># This RNS example demonstrates how to set up a link to  #</span>
+<span class="c1"># a destination, and pass structured messages over it    #</span>
+<span class="c1"># using a channel.                                       #</span>
+<span class="c1">##########################################################</span>
+
+<span class="kn">import</span> <span class="nn">os</span>
+<span class="kn">import</span> <span class="nn">sys</span>
+<span class="kn">import</span> <span class="nn">time</span>
+<span class="kn">import</span> <span class="nn">argparse</span>
+<span class="kn">from</span> <span class="nn">datetime</span> <span class="kn">import</span> <span class="n">datetime</span>
+
+<span class="kn">import</span> <span class="nn">RNS</span>
+<span class="kn">from</span> <span class="nn">RNS.vendor</span> <span class="kn">import</span> <span class="n">umsgpack</span>
+
+<span class="c1"># Let&#39;s define an app name. We&#39;ll use this for all</span>
+<span class="c1"># destinations we create. Since this echo example</span>
+<span class="c1"># is part of a range of example utilities, we&#39;ll put</span>
+<span class="c1"># them all within the app namespace &quot;example_utilities&quot;</span>
+<span class="n">APP_NAME</span> <span class="o">=</span> <span class="s2">&quot;example_utilities&quot;</span>
+
+<span class="c1">##########################################################</span>
+<span class="c1">#### Shared Objects ######################################</span>
+<span class="c1">##########################################################</span>
+
+<span class="c1"># Channel data must be structured in a subclass of</span>
+<span class="c1"># MessageBase. This ensures that the channel will be able</span>
+<span class="c1"># to serialize and deserialize the object and multiplex it</span>
+<span class="c1"># with other objects. Both ends of a link will need the</span>
+<span class="c1"># same object definitions to be able to communicate over</span>
+<span class="c1"># a channel.</span>
+<span class="c1">#</span>
+<span class="c1"># Note: The objects we wish to use over the channel must</span>
+<span class="c1"># be registered with the channel, and each link has a</span>
+<span class="c1"># different channel instance. See the client_connected</span>
+<span class="c1"># and link_established functions in this example to see</span>
+<span class="c1"># how message types are registered.</span>
+
+<span class="c1"># Let&#39;s make a simple message class called StringMessage</span>
+<span class="c1"># that will convey a string with a timestamp.</span>
+
+<span class="k">class</span> <span class="nc">StringMessage</span><span class="p">(</span><span class="n">RNS</span><span class="o">.</span><span class="n">MessageBase</span><span class="p">):</span>
+    <span class="c1"># The MSGTYPE class variable needs to be assigned a</span>
+    <span class="c1"># 2 byte integer value. This identifier allows the</span>
+    <span class="c1"># channel to look up your message&#39;s constructor when a</span>
+    <span class="c1"># message arrives over the channel.</span>
+    <span class="c1">#</span>
+    <span class="c1"># MSGTYPE must be unique across all message types we</span>
+    <span class="c1"># register with the channel. MSGTYPEs &gt;= 0xf000 are</span>
+    <span class="c1"># reserved for the system.</span>
+    <span class="n">MSGTYPE</span> <span class="o">=</span> <span class="mh">0x0101</span>
+
+    <span class="c1"># The constructor of our object must be callable with</span>
+    <span class="c1"># no arguments. We can have parameters, but they must</span>
+    <span class="c1"># have a default assignment.</span>
+    <span class="c1">#</span>
+    <span class="c1"># This is needed so the channel can create an empty</span>
+    <span class="c1"># version of our message into which the incoming</span>
+    <span class="c1"># message can be unpacked.</span>
+    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
+        <span class="bp">self</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="n">data</span>
+        <span class="bp">self</span><span class="o">.</span><span class="n">timestamp</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">()</span>
+
+    <span class="c1"># Finally, our message needs to implement functions</span>
+    <span class="c1"># the channel can call to pack and unpack our message</span>
+    <span class="c1"># to/from the raw packet payload. We&#39;ll use the</span>
+    <span class="c1"># umsgpack package bundled with RNS. We could also use</span>
+    <span class="c1"># the struct package bundled with Python if we wanted</span>
+    <span class="c1"># more control over the structure of the packed bytes.</span>
+    <span class="c1">#</span>
+    <span class="c1"># Also note that packed message objects must fit</span>
+    <span class="c1"># entirely in one packet. The number of bytes</span>
+    <span class="c1"># available for message payloads can be queried from</span>
+    <span class="c1"># the channel using the Channel.MDU property. The</span>
+    <span class="c1"># channel MDU is slightly less than the link MDU due</span>
+    <span class="c1"># to encoding the message header.</span>
+
+    <span class="c1"># The pack function encodes the message contents into</span>
+    <span class="c1"># a byte stream.</span>
+    <span class="k">def</span> <span class="nf">pack</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">bytes</span><span class="p">:</span>
+        <span class="k">return</span> <span class="n">umsgpack</span><span class="o">.</span><span class="n">packb</span><span class="p">((</span><span class="bp">self</span><span class="o">.</span><span class="n">data</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">timestamp</span><span class="p">))</span>
+
+    <span class="c1"># And the unpack function decodes a byte stream into</span>
+    <span class="c1"># the message contents.</span>
+    <span class="k">def</span> <span class="nf">unpack</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">raw</span><span class="p">):</span>
+        <span class="bp">self</span><span class="o">.</span><span class="n">data</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">timestamp</span> <span class="o">=</span> <span class="n">umsgpack</span><span class="o">.</span><span class="n">unpackb</span><span class="p">(</span><span class="n">raw</span><span class="p">)</span>
+
+
+<span class="c1">##########################################################</span>
+<span class="c1">#### Server Part #########################################</span>
+<span class="c1">##########################################################</span>
+
+<span class="c1"># A reference to the latest client link that connected</span>
+<span class="n">latest_client_link</span> <span class="o">=</span> <span class="kc">None</span>
+
+<span class="c1"># This initialisation is executed when the users chooses</span>
+<span class="c1"># to run as a server</span>
+<span class="k">def</span> <span class="nf">server</span><span class="p">(</span><span class="n">configpath</span><span class="p">):</span>
+    <span class="c1"># We must first initialise Reticulum</span>
+    <span class="n">reticulum</span> <span class="o">=</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Reticulum</span><span class="p">(</span><span class="n">configpath</span><span class="p">)</span>
+    
+    <span class="c1"># Randomly create a new identity for our link example</span>
+    <span class="n">server_identity</span> <span class="o">=</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Identity</span><span class="p">()</span>
+
+    <span class="c1"># We create a destination that clients can connect to. We</span>
+    <span class="c1"># want clients to create links to this destination, so we</span>
+    <span class="c1"># need to create a &quot;single&quot; destination type.</span>
+    <span class="n">server_destination</span> <span class="o">=</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Destination</span><span class="p">(</span>
+        <span class="n">server_identity</span><span class="p">,</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">Destination</span><span class="o">.</span><span class="n">IN</span><span class="p">,</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">Destination</span><span class="o">.</span><span class="n">SINGLE</span><span class="p">,</span>
+        <span class="n">APP_NAME</span><span class="p">,</span>
+        <span class="s2">&quot;channelexample&quot;</span>
+    <span class="p">)</span>
+
+    <span class="c1"># We configure a function that will get called every time</span>
+    <span class="c1"># a new client creates a link to this destination.</span>
+    <span class="n">server_destination</span><span class="o">.</span><span class="n">set_link_established_callback</span><span class="p">(</span><span class="n">client_connected</span><span class="p">)</span>
+
+    <span class="c1"># Everything&#39;s ready!</span>
+    <span class="c1"># Let&#39;s Wait for client requests or user input</span>
+    <span class="n">server_loop</span><span class="p">(</span><span class="n">server_destination</span><span class="p">)</span>
+
+<span class="k">def</span> <span class="nf">server_loop</span><span class="p">(</span><span class="n">destination</span><span class="p">):</span>
+    <span class="c1"># Let the user know that everything is ready</span>
+    <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span>
+        <span class="s2">&quot;Link example &quot;</span><span class="o">+</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">prettyhexrep</span><span class="p">(</span><span class="n">destination</span><span class="o">.</span><span class="n">hash</span><span class="p">)</span><span class="o">+</span>
+        <span class="s2">&quot; running, waiting for a connection.&quot;</span>
+    <span class="p">)</span>
+
+    <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Hit enter to manually send an announce (Ctrl-C to quit)&quot;</span><span class="p">)</span>
+
+    <span class="c1"># We enter a loop that runs until the users exits.</span>
+    <span class="c1"># If the user hits enter, we will announce our server</span>
+    <span class="c1"># destination on the network, which will let clients</span>
+    <span class="c1"># know how to create messages directed towards it.</span>
+    <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
+        <span class="n">entered</span> <span class="o">=</span> <span class="nb">input</span><span class="p">()</span>
+        <span class="n">destination</span><span class="o">.</span><span class="n">announce</span><span class="p">()</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Sent announce from &quot;</span><span class="o">+</span><span class="n">RNS</span><span class="o">.</span><span class="n">prettyhexrep</span><span class="p">(</span><span class="n">destination</span><span class="o">.</span><span class="n">hash</span><span class="p">))</span>
+
+<span class="c1"># When a client establishes a link to our server</span>
+<span class="c1"># destination, this function will be called with</span>
+<span class="c1"># a reference to the link.</span>
+<span class="k">def</span> <span class="nf">client_connected</span><span class="p">(</span><span class="n">link</span><span class="p">):</span>
+    <span class="k">global</span> <span class="n">latest_client_link</span>
+    <span class="n">latest_client_link</span> <span class="o">=</span> <span class="n">link</span>
+
+    <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Client connected&quot;</span><span class="p">)</span>
+    <span class="n">link</span><span class="o">.</span><span class="n">set_link_closed_callback</span><span class="p">(</span><span class="n">client_disconnected</span><span class="p">)</span>
+
+    <span class="c1"># Register message types and add callback to channel</span>
+    <span class="n">channel</span> <span class="o">=</span> <span class="n">link</span><span class="o">.</span><span class="n">get_channel</span><span class="p">()</span>
+    <span class="n">channel</span><span class="o">.</span><span class="n">register_message_type</span><span class="p">(</span><span class="n">StringMessage</span><span class="p">)</span>
+    <span class="n">channel</span><span class="o">.</span><span class="n">add_message_handler</span><span class="p">(</span><span class="n">server_message_received</span><span class="p">)</span>
+
+<span class="k">def</span> <span class="nf">client_disconnected</span><span class="p">(</span><span class="n">link</span><span class="p">):</span>
+    <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Client disconnected&quot;</span><span class="p">)</span>
+
+<span class="k">def</span> <span class="nf">server_message_received</span><span class="p">(</span><span class="n">message</span><span class="p">):</span>
+    <span class="sd">&quot;&quot;&quot;</span>
+<span class="sd">    A message handler</span>
+<span class="sd">    @param message: An instance of a subclass of MessageBase</span>
+<span class="sd">    @return: True if message was handled</span>
+<span class="sd">    &quot;&quot;&quot;</span>
+    <span class="k">global</span> <span class="n">latest_client_link</span>
+    <span class="c1"># When a message is received over any active link,</span>
+    <span class="c1"># the replies will all be directed to the last client</span>
+    <span class="c1"># that connected.</span>
+
+    <span class="c1"># In a message handler, any deserializable message</span>
+    <span class="c1"># that arrives over the link&#39;s channel will be passed</span>
+    <span class="c1"># to all message handlers, unless a preceding handler indicates it</span>
+    <span class="c1"># has handled the message.</span>
+    <span class="c1">#</span>
+    <span class="c1">#</span>
+    <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">message</span><span class="p">,</span> <span class="n">StringMessage</span><span class="p">):</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Received data on the link: &quot;</span> <span class="o">+</span> <span class="n">message</span><span class="o">.</span><span class="n">data</span> <span class="o">+</span> <span class="s2">&quot; (message created at &quot;</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">message</span><span class="o">.</span><span class="n">timestamp</span><span class="p">)</span> <span class="o">+</span> <span class="s2">&quot;)&quot;</span><span class="p">)</span>
+
+        <span class="n">reply_message</span> <span class="o">=</span> <span class="n">StringMessage</span><span class="p">(</span><span class="s2">&quot;I received </span><span class="se">\&quot;</span><span class="s2">&quot;</span><span class="o">+</span><span class="n">message</span><span class="o">.</span><span class="n">data</span><span class="o">+</span><span class="s2">&quot;</span><span class="se">\&quot;</span><span class="s2"> over the link&quot;</span><span class="p">)</span>
+        <span class="n">latest_client_link</span><span class="o">.</span><span class="n">get_channel</span><span class="p">()</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">reply_message</span><span class="p">)</span>
+
+        <span class="c1"># Incoming messages are sent to each message</span>
+        <span class="c1"># handler added to the channel, in the order they</span>
+        <span class="c1"># were added.</span>
+        <span class="c1"># If any message handler returns True, the message</span>
+        <span class="c1"># is considered handled and any subsequent</span>
+        <span class="c1"># handlers are skipped.</span>
+        <span class="k">return</span> <span class="kc">True</span>
+
+
+<span class="c1">##########################################################</span>
+<span class="c1">#### Client Part #########################################</span>
+<span class="c1">##########################################################</span>
+
+<span class="c1"># A reference to the server link</span>
+<span class="n">server_link</span> <span class="o">=</span> <span class="kc">None</span>
+
+<span class="c1"># This initialisation is executed when the users chooses</span>
+<span class="c1"># to run as a client</span>
+<span class="k">def</span> <span class="nf">client</span><span class="p">(</span><span class="n">destination_hexhash</span><span class="p">,</span> <span class="n">configpath</span><span class="p">):</span>
+    <span class="c1"># We need a binary representation of the destination</span>
+    <span class="c1"># hash that was entered on the command line</span>
+    <span class="k">try</span><span class="p">:</span>
+        <span class="n">dest_len</span> <span class="o">=</span> <span class="p">(</span><span class="n">RNS</span><span class="o">.</span><span class="n">Reticulum</span><span class="o">.</span><span class="n">TRUNCATED_HASHLENGTH</span><span class="o">//</span><span class="mi">8</span><span class="p">)</span><span class="o">*</span><span class="mi">2</span>
+        <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">destination_hexhash</span><span class="p">)</span> <span class="o">!=</span> <span class="n">dest_len</span><span class="p">:</span>
+            <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span>
+                <span class="s2">&quot;Destination length is invalid, must be </span><span class="si">{hex}</span><span class="s2"> hexadecimal characters (</span><span class="si">{byte}</span><span class="s2"> bytes).&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="nb">hex</span><span class="o">=</span><span class="n">dest_len</span><span class="p">,</span> <span class="n">byte</span><span class="o">=</span><span class="n">dest_len</span><span class="o">//</span><span class="mi">2</span><span class="p">)</span>
+            <span class="p">)</span>
+            
+        <span class="n">destination_hash</span> <span class="o">=</span> <span class="nb">bytes</span><span class="o">.</span><span class="n">fromhex</span><span class="p">(</span><span class="n">destination_hexhash</span><span class="p">)</span>
+    <span class="k">except</span><span class="p">:</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Invalid destination entered. Check your input!</span><span class="se">\n</span><span class="s2">&quot;</span><span class="p">)</span>
+        <span class="n">exit</span><span class="p">()</span>
+
+    <span class="c1"># We must first initialise Reticulum</span>
+    <span class="n">reticulum</span> <span class="o">=</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Reticulum</span><span class="p">(</span><span class="n">configpath</span><span class="p">)</span>
+
+    <span class="c1"># Check if we know a path to the destination</span>
+    <span class="k">if</span> <span class="ow">not</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Transport</span><span class="o">.</span><span class="n">has_path</span><span class="p">(</span><span class="n">destination_hash</span><span class="p">):</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Destination is not yet known. Requesting path and waiting for announce to arrive...&quot;</span><span class="p">)</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">Transport</span><span class="o">.</span><span class="n">request_path</span><span class="p">(</span><span class="n">destination_hash</span><span class="p">)</span>
+        <span class="k">while</span> <span class="ow">not</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Transport</span><span class="o">.</span><span class="n">has_path</span><span class="p">(</span><span class="n">destination_hash</span><span class="p">):</span>
+            <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.1</span><span class="p">)</span>
+
+    <span class="c1"># Recall the server identity</span>
+    <span class="n">server_identity</span> <span class="o">=</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Identity</span><span class="o">.</span><span class="n">recall</span><span class="p">(</span><span class="n">destination_hash</span><span class="p">)</span>
+
+    <span class="c1"># Inform the user that we&#39;ll begin connecting</span>
+    <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Establishing link with server...&quot;</span><span class="p">)</span>
+
+    <span class="c1"># When the server identity is known, we set</span>
+    <span class="c1"># up a destination</span>
+    <span class="n">server_destination</span> <span class="o">=</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Destination</span><span class="p">(</span>
+        <span class="n">server_identity</span><span class="p">,</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">Destination</span><span class="o">.</span><span class="n">OUT</span><span class="p">,</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">Destination</span><span class="o">.</span><span class="n">SINGLE</span><span class="p">,</span>
+        <span class="n">APP_NAME</span><span class="p">,</span>
+        <span class="s2">&quot;channelexample&quot;</span>
+    <span class="p">)</span>
+
+    <span class="c1"># And create a link</span>
+    <span class="n">link</span> <span class="o">=</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Link</span><span class="p">(</span><span class="n">server_destination</span><span class="p">)</span>
+
+    <span class="c1"># We set a callback that will get executed</span>
+    <span class="c1"># every time a packet is received over the</span>
+    <span class="c1"># link</span>
+    <span class="n">link</span><span class="o">.</span><span class="n">set_packet_callback</span><span class="p">(</span><span class="n">client_message_received</span><span class="p">)</span>
+
+    <span class="c1"># We&#39;ll also set up functions to inform the</span>
+    <span class="c1"># user when the link is established or closed</span>
+    <span class="n">link</span><span class="o">.</span><span class="n">set_link_established_callback</span><span class="p">(</span><span class="n">link_established</span><span class="p">)</span>
+    <span class="n">link</span><span class="o">.</span><span class="n">set_link_closed_callback</span><span class="p">(</span><span class="n">link_closed</span><span class="p">)</span>
+
+    <span class="c1"># Everything is set up, so let&#39;s enter a loop</span>
+    <span class="c1"># for the user to interact with the example</span>
+    <span class="n">client_loop</span><span class="p">()</span>
+
+<span class="k">def</span> <span class="nf">client_loop</span><span class="p">():</span>
+    <span class="k">global</span> <span class="n">server_link</span>
+
+    <span class="c1"># Wait for the link to become active</span>
+    <span class="k">while</span> <span class="ow">not</span> <span class="n">server_link</span><span class="p">:</span>
+        <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.1</span><span class="p">)</span>
+
+    <span class="n">should_quit</span> <span class="o">=</span> <span class="kc">False</span>
+    <span class="k">while</span> <span class="ow">not</span> <span class="n">should_quit</span><span class="p">:</span>
+        <span class="k">try</span><span class="p">:</span>
+            <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&gt; &quot;</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s2">&quot; &quot;</span><span class="p">)</span>
+            <span class="n">text</span> <span class="o">=</span> <span class="nb">input</span><span class="p">()</span>
+
+            <span class="c1"># Check if we should quit the example</span>
+            <span class="k">if</span> <span class="n">text</span> <span class="o">==</span> <span class="s2">&quot;quit&quot;</span> <span class="ow">or</span> <span class="n">text</span> <span class="o">==</span> <span class="s2">&quot;q&quot;</span> <span class="ow">or</span> <span class="n">text</span> <span class="o">==</span> <span class="s2">&quot;exit&quot;</span><span class="p">:</span>
+                <span class="n">should_quit</span> <span class="o">=</span> <span class="kc">True</span>
+                <span class="n">server_link</span><span class="o">.</span><span class="n">teardown</span><span class="p">()</span>
+
+            <span class="c1"># If not, send the entered text over the link</span>
+            <span class="k">if</span> <span class="n">text</span> <span class="o">!=</span> <span class="s2">&quot;&quot;</span><span class="p">:</span>
+                <span class="n">message</span> <span class="o">=</span> <span class="n">StringMessage</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
+                <span class="n">packed_size</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">message</span><span class="o">.</span><span class="n">pack</span><span class="p">())</span>
+                <span class="n">channel</span> <span class="o">=</span> <span class="n">server_link</span><span class="o">.</span><span class="n">get_channel</span><span class="p">()</span>
+                <span class="k">if</span> <span class="n">channel</span><span class="o">.</span><span class="n">is_ready_to_send</span><span class="p">():</span>
+                    <span class="k">if</span> <span class="n">packed_size</span> <span class="o">&lt;=</span> <span class="n">channel</span><span class="o">.</span><span class="n">MDU</span><span class="p">:</span>
+                        <span class="n">channel</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">message</span><span class="p">)</span>
+                    <span class="k">else</span><span class="p">:</span>
+                        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span>
+                            <span class="s2">&quot;Cannot send this packet, the data size of &quot;</span><span class="o">+</span>
+                            <span class="nb">str</span><span class="p">(</span><span class="n">packed_size</span><span class="p">)</span><span class="o">+</span><span class="s2">&quot; bytes exceeds the link packet MDU of &quot;</span><span class="o">+</span>
+                            <span class="nb">str</span><span class="p">(</span><span class="n">channel</span><span class="o">.</span><span class="n">MDU</span><span class="p">)</span><span class="o">+</span><span class="s2">&quot; bytes&quot;</span><span class="p">,</span>
+                            <span class="n">RNS</span><span class="o">.</span><span class="n">LOG_ERROR</span>
+                        <span class="p">)</span>
+                <span class="k">else</span><span class="p">:</span>
+                    <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Channel is not ready to send, please wait for &quot;</span> <span class="o">+</span>
+                            <span class="s2">&quot;pending messages to complete.&quot;</span><span class="p">,</span> <span class="n">RNS</span><span class="o">.</span><span class="n">LOG_ERROR</span><span class="p">)</span>
+
+        <span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
+            <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Error while sending data over the link: &quot;</span><span class="o">+</span><span class="nb">str</span><span class="p">(</span><span class="n">e</span><span class="p">))</span>
+            <span class="n">should_quit</span> <span class="o">=</span> <span class="kc">True</span>
+            <span class="n">server_link</span><span class="o">.</span><span class="n">teardown</span><span class="p">()</span>
+
+<span class="c1"># This function is called when a link</span>
+<span class="c1"># has been established with the server</span>
+<span class="k">def</span> <span class="nf">link_established</span><span class="p">(</span><span class="n">link</span><span class="p">):</span>
+    <span class="c1"># We store a reference to the link</span>
+    <span class="c1"># instance for later use</span>
+    <span class="k">global</span> <span class="n">server_link</span>
+    <span class="n">server_link</span> <span class="o">=</span> <span class="n">link</span>
+
+    <span class="c1"># Register messages and add handler to channel</span>
+    <span class="n">channel</span> <span class="o">=</span> <span class="n">link</span><span class="o">.</span><span class="n">get_channel</span><span class="p">()</span>
+    <span class="n">channel</span><span class="o">.</span><span class="n">register_message_type</span><span class="p">(</span><span class="n">StringMessage</span><span class="p">)</span>
+    <span class="n">channel</span><span class="o">.</span><span class="n">add_message_handler</span><span class="p">(</span><span class="n">client_message_received</span><span class="p">)</span>
+
+    <span class="c1"># Inform the user that the server is</span>
+    <span class="c1"># connected</span>
+    <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Link established with server, enter some text to send, or </span><span class="se">\&quot;</span><span class="s2">quit</span><span class="se">\&quot;</span><span class="s2"> to quit&quot;</span><span class="p">)</span>
+
+<span class="c1"># When a link is closed, we&#39;ll inform the</span>
+<span class="c1"># user, and exit the program</span>
+<span class="k">def</span> <span class="nf">link_closed</span><span class="p">(</span><span class="n">link</span><span class="p">):</span>
+    <span class="k">if</span> <span class="n">link</span><span class="o">.</span><span class="n">teardown_reason</span> <span class="o">==</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Link</span><span class="o">.</span><span class="n">TIMEOUT</span><span class="p">:</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;The link timed out, exiting now&quot;</span><span class="p">)</span>
+    <span class="k">elif</span> <span class="n">link</span><span class="o">.</span><span class="n">teardown_reason</span> <span class="o">==</span> <span class="n">RNS</span><span class="o">.</span><span class="n">Link</span><span class="o">.</span><span class="n">DESTINATION_CLOSED</span><span class="p">:</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;The link was closed by the server, exiting now&quot;</span><span class="p">)</span>
+    <span class="k">else</span><span class="p">:</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Link closed, exiting now&quot;</span><span class="p">)</span>
+    
+    <span class="n">RNS</span><span class="o">.</span><span class="n">Reticulum</span><span class="o">.</span><span class="n">exit_handler</span><span class="p">()</span>
+    <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">1.5</span><span class="p">)</span>
+    <span class="n">os</span><span class="o">.</span><span class="n">_exit</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
+
+<span class="c1"># When a packet is received over the link, we</span>
+<span class="c1"># simply print out the data.</span>
+<span class="k">def</span> <span class="nf">client_message_received</span><span class="p">(</span><span class="n">message</span><span class="p">):</span>
+    <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">message</span><span class="p">,</span> <span class="n">StringMessage</span><span class="p">):</span>
+        <span class="n">RNS</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;Received data on the link: &quot;</span> <span class="o">+</span> <span class="n">message</span><span class="o">.</span><span class="n">data</span> <span class="o">+</span> <span class="s2">&quot; (message created at &quot;</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">message</span><span class="o">.</span><span class="n">timestamp</span><span class="p">)</span> <span class="o">+</span> <span class="s2">&quot;)&quot;</span><span class="p">)</span>
+        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&gt; &quot;</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s2">&quot; &quot;</span><span class="p">)</span>
+        <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">flush</span><span class="p">()</span>
+
+
+<span class="c1">##########################################################</span>
+<span class="c1">#### Program Startup #####################################</span>
+<span class="c1">##########################################################</span>
+
+<span class="c1"># This part of the program runs at startup,</span>
+<span class="c1"># and parses input of from the user, and then</span>
+<span class="c1"># starts up the desired program mode.</span>
+<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s2">&quot;__main__&quot;</span><span class="p">:</span>
+    <span class="k">try</span><span class="p">:</span>
+        <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">description</span><span class="o">=</span><span class="s2">&quot;Simple link example&quot;</span><span class="p">)</span>
+
+        <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span>
+            <span class="s2">&quot;-s&quot;</span><span class="p">,</span>
+            <span class="s2">&quot;--server&quot;</span><span class="p">,</span>
+            <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store_true&quot;</span><span class="p">,</span>
+            <span class="n">help</span><span class="o">=</span><span class="s2">&quot;wait for incoming link requests from clients&quot;</span>
+        <span class="p">)</span>
+
+        <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span>
+            <span class="s2">&quot;--config&quot;</span><span class="p">,</span>
+            <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store&quot;</span><span class="p">,</span>
+            <span class="n">default</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span>
+            <span class="n">help</span><span class="o">=</span><span class="s2">&quot;path to alternative Reticulum config directory&quot;</span><span class="p">,</span>
+            <span class="nb">type</span><span class="o">=</span><span class="nb">str</span>
+        <span class="p">)</span>
+
+        <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span>
+            <span class="s2">&quot;destination&quot;</span><span class="p">,</span>
+            <span class="n">nargs</span><span class="o">=</span><span class="s2">&quot;?&quot;</span><span class="p">,</span>
+            <span class="n">default</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span>
+            <span class="n">help</span><span class="o">=</span><span class="s2">&quot;hexadecimal hash of the server destination&quot;</span><span class="p">,</span>
+            <span class="nb">type</span><span class="o">=</span><span class="nb">str</span>
+        <span class="p">)</span>
+
+        <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>
+
+        <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">config</span><span class="p">:</span>
+            <span class="n">configarg</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">config</span>
+        <span class="k">else</span><span class="p">:</span>
+            <span class="n">configarg</span> <span class="o">=</span> <span class="kc">None</span>
+
+        <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">server</span><span class="p">:</span>
+            <span class="n">server</span><span class="p">(</span><span class="n">configarg</span><span class="p">)</span>
+        <span class="k">else</span><span class="p">:</span>
+            <span class="k">if</span> <span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">destination</span> <span class="o">==</span> <span class="kc">None</span><span class="p">):</span>
+                <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&quot;</span><span class="p">)</span>
+                <span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
+                <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&quot;</span><span class="p">)</span>
+            <span class="k">else</span><span class="p">:</span>
+                <span class="n">client</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">destination</span><span class="p">,</span> <span class="n">configarg</span><span class="p">)</span>
+
+    <span class="k">except</span> <span class="ne">KeyboardInterrupt</span><span class="p">:</span>
+        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&quot;</span><span class="p">)</span>
+        <span class="n">exit</span><span class="p">()</span>
+</pre></div>
+</div>
+<p>This example can also be found at <a class="reference external" href="https://github.com/markqvist/Reticulum/blob/master/Examples/Channel.py">https://github.com/markqvist/Reticulum/blob/master/Examples/Channel.py</a>.</p>
+</section>
 <section id="filetransfer">
 <span id="example-filetransfer"></span><h2>Filetransfer<a class="headerlink" href="#filetransfer" title="Permalink to this heading">#</a></h2>
 <p>The <em>Filetransfer</em> example implements a basic file-server program that
@ -2543,7 +2947,7 @@ interface to efficiently pass files of any size over a Reticulum <a class="refer
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
@ -2578,6 +2982,7 @@ interface to efficiently pass files of any size over a Reticulum <a class="refer
 <li><a class="reference internal" href="#link">Link</a></li>
 <li><a class="reference internal" href="#example-identify">Identification</a></li>
 <li><a class="reference internal" href="#requests-responses">Requests &amp; Responses</a></li>
+<li><a class="reference internal" href="#channel">Channel</a></li>
 <li><a class="reference internal" href="#filetransfer">Filetransfer</a></li>
 </ul>
 </li>
--- a/docs/manual/forhumans.html
+++ b/docs/manual/forhumans.html
@ -236,7 +236,7 @@
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/genindex.html
+++ b/docs/manual/genindex.html
@ -222,13 +222,15 @@
          
 <section class="genindex-section">
  <h1 id="index">Index</h1>
-  <div class="genindex-jumpbox"><a href="#A"><strong>A</strong></a> | <a href="#C"><strong>C</strong></a> | <a href="#D"><strong>D</strong></a> | <a href="#E"><strong>E</strong></a> | <a href="#F"><strong>F</strong></a> | <a href="#G"><strong>G</strong></a> | <a href="#H"><strong>H</strong></a> | <a href="#I"><strong>I</strong></a> | <a href="#K"><strong>K</strong></a> | <a href="#L"><strong>L</strong></a> | <a href="#M"><strong>M</strong></a> | <a href="#N"><strong>N</strong></a> | <a href="#P"><strong>P</strong></a> | <a href="#R"><strong>R</strong></a> | <a href="#S"><strong>S</strong></a> | <a href="#T"><strong>T</strong></a> | <a href="#V"><strong>V</strong></a></div>
+  <div class="genindex-jumpbox"><a href="#A"><strong>A</strong></a> | <a href="#C"><strong>C</strong></a> | <a href="#D"><strong>D</strong></a> | <a href="#E"><strong>E</strong></a> | <a href="#F"><strong>F</strong></a> | <a href="#G"><strong>G</strong></a> | <a href="#H"><strong>H</strong></a> | <a href="#I"><strong>I</strong></a> | <a href="#K"><strong>K</strong></a> | <a href="#L"><strong>L</strong></a> | <a href="#M"><strong>M</strong></a> | <a href="#N"><strong>N</strong></a> | <a href="#P"><strong>P</strong></a> | <a href="#R"><strong>R</strong></a> | <a href="#S"><strong>S</strong></a> | <a href="#T"><strong>T</strong></a> | <a href="#U"><strong>U</strong></a> | <a href="#V"><strong>V</strong></a></div>
 </section>
 <section id="A" class="genindex-section">
  <h2>A</h2>
  <table style="width: 100%" class="indextable genindextable"><tr>
    <td style="width: 33%; vertical-align: top;"><ul>
        <li><a href="reference.html#RNS.Destination.accepts_links">accepts_links() (RNS.Destination method)</a>
+</li>
+        <li><a href="reference.html#RNS.Channel.Channel.add_message_handler">add_message_handler() (RNS.Channel.Channel method)</a>
 </li>
        <li><a href="reference.html#RNS.Resource.advertise">advertise() (RNS.Resource method)</a>
 </li>
@ -249,6 +251,8 @@
  <table style="width: 100%" class="indextable genindextable"><tr>
    <td style="width: 33%; vertical-align: top;"><ul>
        <li><a href="reference.html#RNS.Resource.cancel">cancel() (RNS.Resource method)</a>
+</li>
+        <li><a href="reference.html#RNS.Channel.Channel">Channel (class in RNS.Channel)</a>
 </li>
        <li><a href="reference.html#RNS.Destination.clear_default_app_data">clear_default_app_data() (RNS.Destination method)</a>
 </li>
@ -330,6 +334,8 @@
  <h2>G</h2>
  <table style="width: 100%" class="indextable genindextable"><tr>
    <td style="width: 33%; vertical-align: top;"><ul>
+        <li><a href="reference.html#RNS.Link.get_channel">get_channel() (RNS.Link method)</a>
+</li>
        <li><a href="reference.html#RNS.Resource.get_data_size">get_data_size() (RNS.Resource method)</a>
 </li>
        <li><a href="reference.html#RNS.Link.get_establishment_rate">get_establishment_rate() (RNS.Link method)</a>
@ -411,6 +417,8 @@
        <li><a href="reference.html#RNS.Link.inactive_for">inactive_for() (RNS.Link method)</a>
 </li>
        <li><a href="reference.html#RNS.Resource.is_compressed">is_compressed() (RNS.Resource method)</a>
+</li>
+        <li><a href="reference.html#RNS.Channel.Channel.is_ready_to_send">is_ready_to_send() (RNS.Channel.Channel method)</a>
 </li>
    </ul></td>
  </tr></table>
@ -456,6 +464,14 @@
  <h2>M</h2>
  <table style="width: 100%" class="indextable genindextable"><tr>
    <td style="width: 33%; vertical-align: top;"><ul>
+        <li><a href="reference.html#RNS.Channel.Channel.MDU">MDU (RNS.Channel.Channel property)</a>
+</li>
+        <li><a href="reference.html#RNS.MessageBase">MessageBase (class in RNS)</a>
+</li>
+    </ul></td>
+    <td style="width: 33%; vertical-align: top;"><ul>
+        <li><a href="reference.html#RNS.MessageBase.MSGTYPE">MSGTYPE (RNS.MessageBase attribute)</a>
+</li>
        <li><a href="reference.html#RNS.Reticulum.MTU">MTU (RNS.Reticulum attribute)</a>
 </li>
    </ul></td>
@ -484,12 +500,14 @@
  <h2>P</h2>
  <table style="width: 100%" class="indextable genindextable"><tr>
    <td style="width: 33%; vertical-align: top;"><ul>
-        <li><a href="reference.html#RNS.Packet">Packet (class in RNS)</a>
+        <li><a href="reference.html#RNS.MessageBase.pack">pack() (RNS.MessageBase method)</a>
 </li>
-        <li><a href="reference.html#RNS.PacketReceipt">PacketReceipt (class in RNS)</a>
+        <li><a href="reference.html#RNS.Packet">Packet (class in RNS)</a>
 </li>
    </ul></td>
    <td style="width: 33%; vertical-align: top;"><ul>
+        <li><a href="reference.html#RNS.PacketReceipt">PacketReceipt (class in RNS)</a>
+</li>
        <li><a href="reference.html#RNS.Transport.PATHFINDER_M">PATHFINDER_M (RNS.Transport attribute)</a>
 </li>
        <li><a href="reference.html#RNS.Packet.PLAIN_MDU">PLAIN_MDU (RNS.Packet attribute)</a>
@ -507,13 +525,17 @@
        <li><a href="reference.html#RNS.Identity.recall_app_data">recall_app_data() (RNS.Identity static method)</a>
 </li>
        <li><a href="reference.html#RNS.Transport.register_announce_handler">register_announce_handler() (RNS.Transport static method)</a>
+</li>
+        <li><a href="reference.html#RNS.Channel.Channel.register_message_type">register_message_type() (RNS.Channel.Channel method)</a>
 </li>
        <li><a href="reference.html#RNS.Destination.register_request_handler">register_request_handler() (RNS.Destination method)</a>
 </li>
-        <li><a href="reference.html#RNS.Link.request">request() (RNS.Link method)</a>
+        <li><a href="reference.html#RNS.Channel.Channel.remove_message_handler">remove_message_handler() (RNS.Channel.Channel method)</a>
 </li>
    </ul></td>
    <td style="width: 33%; vertical-align: top;"><ul>
+        <li><a href="reference.html#RNS.Link.request">request() (RNS.Link method)</a>
+</li>
        <li><a href="reference.html#RNS.Transport.request_path">request_path() (RNS.Transport static method)</a>
 </li>
        <li><a href="reference.html#RNS.RequestReceipt">RequestReceipt (class in RNS)</a>
@ -532,8 +554,12 @@
  <h2>S</h2>
  <table style="width: 100%" class="indextable genindextable"><tr>
    <td style="width: 33%; vertical-align: top;"><ul>
-        <li><a href="reference.html#RNS.Packet.send">send() (RNS.Packet method)</a>
+        <li><a href="reference.html#RNS.Channel.Channel.send">send() (RNS.Channel.Channel method)</a>
+
+        <ul>
+          <li><a href="reference.html#RNS.Packet.send">(RNS.Packet method)</a>
 </li>
+        </ul></li>
        <li><a href="reference.html#RNS.Destination.set_default_app_data">set_default_app_data() (RNS.Destination method)</a>
 </li>
        <li><a href="reference.html#RNS.PacketReceipt.set_delivery_callback">set_delivery_callback() (RNS.PacketReceipt method)</a>
@ -606,6 +632,16 @@
  </tr></table>
 </section>

+<section id="U" class="genindex-section">
+  <h2>U</h2>
+  <table style="width: 100%" class="indextable genindextable"><tr>
+    <td style="width: 33%; vertical-align: top;"><ul>
+        <li><a href="reference.html#RNS.MessageBase.unpack">unpack() (RNS.MessageBase method)</a>
+</li>
+    </ul></td>
+  </tr></table>
+</section>
+
 <section id="V" class="genindex-section">
  <h2>V</h2>
  <table style="width: 100%" class="indextable genindextable"><tr>
@ -628,7 +664,7 @@
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/gettingstartedfast.html
+++ b/docs/manual/gettingstartedfast.html
@ -253,6 +253,12 @@ links, such as local WiFi, wired Ethernet, the Internet, or any combination.</p>
 transceivers or infrastructure just to try it out. Launching the programs on separate
 devices connected to the same WiFi network is enough to get started, and physical
 radio interfaces can then be added later.</p>
+<section id="remote-shell">
+<h3>Remote Shell<a class="headerlink" href="#remote-shell" title="Permalink to this heading">#</a></h3>
+<p>The <a class="reference external" href="https://github.com/acehoss/rnsh">rnsh</a> program lets you establish fully interactive
+remote shell sessions over Reticulum. It also allows you to pipe any program to or from a
+remote system, and is similar to how <code class="docutils literal notranslate"><span class="pre">ssh</span></code> works.</p>
+</section>
 <section id="nomad-network">
 <h3>Nomad Network<a class="headerlink" href="#nomad-network" title="Permalink to this heading">#</a></h3>
 <p>The terminal-based program <a class="reference external" href="https://github.com/markqvist/nomadnet">Nomad Network</a>
@ -604,7 +610,7 @@ section of this manual.</p>
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
@ -634,6 +640,7 @@ section of this manual.</p>
 <li><a class="reference internal" href="#">Getting Started Fast</a><ul>
 <li><a class="reference internal" href="#standalone-reticulum-installation">Standalone Reticulum Installation</a></li>
 <li><a class="reference internal" href="#try-using-a-reticulum-based-program">Try Using a Reticulum-based Program</a><ul>
+<li><a class="reference internal" href="#remote-shell">Remote Shell</a></li>
 <li><a class="reference internal" href="#nomad-network">Nomad Network</a></li>
 <li><a class="reference internal" href="#sideband">Sideband</a></li>
 </ul>
--- a/docs/manual/hardware.html
+++ b/docs/manual/hardware.html
@ -442,7 +442,7 @@ connectivity for client devices.</p>
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/index.html
+++ b/docs/manual/index.html
@ -226,6 +226,7 @@
 <p>This manual aims to provide you with all the information you need to
 understand Reticulum, build networks or develop programs using it, or
 to participate in the development of Reticulum itself.</p>
+<p>This manual is also available in <a class="reference external" href="https://github.com/markqvist/Reticulum/releases/latest/download/Reticulum.Manual.pdf">PDF</a> and <a class="reference external" href="https://github.com/markqvist/Reticulum/releases/latest/download/Reticulum.Manual.epub">EPUB</a> formats.</p>
 <section id="table-of-contents">
 <h2>Table Of Contents<a class="headerlink" href="#table-of-contents" title="Permalink to this heading">#</a></h2>
 </section>
@ -242,6 +243,7 @@ to participate in the development of Reticulum itself.</p>
 <li class="toctree-l1"><a class="reference internal" href="gettingstartedfast.html">Getting Started Fast</a><ul>
 <li class="toctree-l2"><a class="reference internal" href="gettingstartedfast.html#standalone-reticulum-installation">Standalone Reticulum Installation</a></li>
 <li class="toctree-l2"><a class="reference internal" href="gettingstartedfast.html#try-using-a-reticulum-based-program">Try Using a Reticulum-based Program</a><ul>
+<li class="toctree-l3"><a class="reference internal" href="gettingstartedfast.html#remote-shell">Remote Shell</a></li>
 <li class="toctree-l3"><a class="reference internal" href="gettingstartedfast.html#nomad-network">Nomad Network</a></li>
 <li class="toctree-l3"><a class="reference internal" href="gettingstartedfast.html#sideband">Sideband</a></li>
 </ul>
@ -353,6 +355,7 @@ to participate in the development of Reticulum itself.</p>
 <li class="toctree-l2"><a class="reference internal" href="examples.html#link">Link</a></li>
 <li class="toctree-l2"><a class="reference internal" href="examples.html#example-identify">Identification</a></li>
 <li class="toctree-l2"><a class="reference internal" href="examples.html#requests-responses">Requests &amp; Responses</a></li>
+<li class="toctree-l2"><a class="reference internal" href="examples.html#channel">Channel</a></li>
 <li class="toctree-l2"><a class="reference internal" href="examples.html#filetransfer">Filetransfer</a></li>
 </ul>
 </li>
@ -375,6 +378,8 @@ to participate in the development of Reticulum itself.</p>
 <li class="toctree-l2"><a class="reference internal" href="reference.html#RNS.Link"><code class="docutils literal notranslate"><span class="pre">Link</span></code></a></li>
 <li class="toctree-l2"><a class="reference internal" href="reference.html#RNS.RequestReceipt"><code class="docutils literal notranslate"><span class="pre">RequestReceipt</span></code></a></li>
 <li class="toctree-l2"><a class="reference internal" href="reference.html#RNS.Resource"><code class="docutils literal notranslate"><span class="pre">Resource</span></code></a></li>
+<li class="toctree-l2"><a class="reference internal" href="reference.html#RNS.Channel.Channel"><code class="docutils literal notranslate"><span class="pre">Channel</span></code></a></li>
+<li class="toctree-l2"><a class="reference internal" href="reference.html#RNS.MessageBase"><code class="docutils literal notranslate"><span class="pre">MessageBase</span></code></a></li>
 <li class="toctree-l2"><a class="reference internal" href="reference.html#RNS.Transport"><code class="docutils literal notranslate"><span class="pre">Transport</span></code></a></li>
 </ul>
 </li>
@ -408,7 +413,7 @@ to participate in the development of Reticulum itself.</p>
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/interfaces.html
+++ b/docs/manual/interfaces.html
@ -954,7 +954,7 @@ that a large span of network types can seamlessly <em>co-exist</em> and intercon
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/networks.html
+++ b/docs/manual/networks.html
@ -421,7 +421,7 @@ connected outliers are now an integral part of the network.</p>
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/objects.inv
+++ b/docs/manual/objects.inv
--- a/docs/manual/reference.html
+++ b/docs/manual/reference.html
@ -1114,6 +1114,17 @@ thus preserved. This method can be used for authentication.</p>
 be used if a new link to the same destination is established.</p>
 </dd></dl>

+<dl class="py method">
+<dt class="sig sig-object py" id="RNS.Link.get_channel">
+<span class="sig-name descname"><span class="pre">get_channel</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#RNS.Link.get_channel" title="Permalink to this definition">#</a></dt>
+<dd><p>Get the <code class="docutils literal notranslate"><span class="pre">Channel</span></code> for this link.</p>
+<dl class="field-list simple">
+<dt class="field-odd">Returns</dt>
+<dd class="field-odd"><p><code class="docutils literal notranslate"><span class="pre">Channel</span></code> object</p>
+</dd>
+</dl>
+</dd></dl>
+
 <dl class="py method">
 <dt class="sig sig-object py" id="RNS.Link.set_link_closed_callback">
 <span class="sig-name descname"><span class="pre">set_link_closed_callback</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">callback</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#RNS.Link.set_link_closed_callback" title="Permalink to this definition">#</a></dt>
@ -1367,6 +1378,153 @@ the resource advertisement it will begin transferring.</p>

 </dd></dl>

+<p id="api-channel"><h3> Channel </h3></p>
+<dl class="py class">
+<dt class="sig sig-object py" id="RNS.Channel.Channel">
+<em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">RNS.Channel.</span></span><span class="sig-name descname"><span class="pre">Channel</span></span><a class="headerlink" href="#RNS.Channel.Channel" title="Permalink to this definition">#</a></dt>
+<dd><p>Provides reliable delivery of messages over
+a link.</p>
+<p><code class="docutils literal notranslate"><span class="pre">Channel</span></code> differs from <code class="docutils literal notranslate"><span class="pre">Request</span></code> and
+<code class="docutils literal notranslate"><span class="pre">Resource</span></code> in some important ways:</p>
+<blockquote>
+<div><dl class="simple">
+<dt><strong>Continuous</strong></dt><dd><p>Messages can be sent or received as long as
+the <code class="docutils literal notranslate"><span class="pre">Link</span></code> is open.</p>
+</dd>
+<dt><strong>Bi-directional</strong></dt><dd><p>Messages can be sent in either direction on
+the <code class="docutils literal notranslate"><span class="pre">Link</span></code>; neither end is the client or
+server.</p>
+</dd>
+<dt><strong>Size-constrained</strong></dt><dd><p>Messages must be encoded into a single packet.</p>
+</dd>
+</dl>
+</div></blockquote>
+<p><code class="docutils literal notranslate"><span class="pre">Channel</span></code> is similar to <code class="docutils literal notranslate"><span class="pre">Packet</span></code>, except that it
+provides reliable delivery (automatic retries) as well
+as a structure for exchanging several types of
+messages over the <code class="docutils literal notranslate"><span class="pre">Link</span></code>.</p>
+<p><code class="docutils literal notranslate"><span class="pre">Channel</span></code> is not instantiated directly, but rather
+obtained from a <code class="docutils literal notranslate"><span class="pre">Link</span></code> with <code class="docutils literal notranslate"><span class="pre">get_channel()</span></code>.</p>
+<dl class="py method">
+<dt class="sig sig-object py" id="RNS.Channel.Channel.register_message_type">
+<span class="sig-name descname"><span class="pre">register_message_type</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">message_class</span></span><span class="p"><span class="pre">:</span></span><span class="w"> </span><span class="n"><span class="pre">Type</span><span class="p"><span class="pre">[</span></span><a class="reference internal" href="#RNS.MessageBase" title="RNS.Channel.MessageBase"><span class="pre">MessageBase</span></a><span class="p"><span class="pre">]</span></span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#RNS.Channel.Channel.register_message_type" title="Permalink to this definition">#</a></dt>
+<dd><p>Register a message class for reception over a <code class="docutils literal notranslate"><span class="pre">Channel</span></code>.</p>
+<p>Message classes must extend <code class="docutils literal notranslate"><span class="pre">MessageBase</span></code>.</p>
+<dl class="field-list simple">
+<dt class="field-odd">Parameters</dt>
+<dd class="field-odd"><p><strong>message_class</strong> – Class to register</p>
+</dd>
+</dl>
+</dd></dl>
+
+<dl class="py method">
+<dt class="sig sig-object py" id="RNS.Channel.Channel.add_message_handler">
+<span class="sig-name descname"><span class="pre">add_message_handler</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">callback</span></span><span class="p"><span class="pre">:</span></span><span class="w"> </span><span class="n"><span class="pre">MessageCallbackType</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#RNS.Channel.Channel.add_message_handler" title="Permalink to this definition">#</a></dt>
+<dd><p>Add a handler for incoming messages. A handler
+has the following signature:</p>
+<p><code class="docutils literal notranslate"><span class="pre">(message:</span> <span class="pre">MessageBase)</span> <span class="pre">-&gt;</span> <span class="pre">bool</span></code></p>
+<p>Handlers are processed in the order they are
+added. If any handler returns True, processing
+of the message stops; handlers after the
+returning handler will not be called.</p>
+<dl class="field-list simple">
+<dt class="field-odd">Parameters</dt>
+<dd class="field-odd"><p><strong>callback</strong> – Function to call</p>
+</dd>
+</dl>
+</dd></dl>
+
+<dl class="py method">
+<dt class="sig sig-object py" id="RNS.Channel.Channel.remove_message_handler">
+<span class="sig-name descname"><span class="pre">remove_message_handler</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">callback</span></span><span class="p"><span class="pre">:</span></span><span class="w"> </span><span class="n"><span class="pre">MessageCallbackType</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#RNS.Channel.Channel.remove_message_handler" title="Permalink to this definition">#</a></dt>
+<dd><p>Remove a handler added with <code class="docutils literal notranslate"><span class="pre">add_message_handler</span></code>.</p>
+<dl class="field-list simple">
+<dt class="field-odd">Parameters</dt>
+<dd class="field-odd"><p><strong>callback</strong> – handler to remove</p>
+</dd>
+</dl>
+</dd></dl>
+
+<dl class="py method">
+<dt class="sig sig-object py" id="RNS.Channel.Channel.is_ready_to_send">
+<span class="sig-name descname"><span class="pre">is_ready_to_send</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span> <span class="sig-return"><span class="sig-return-icon">&#x2192;</span> <span class="sig-return-typehint"><span class="pre">bool</span></span></span><a class="headerlink" href="#RNS.Channel.Channel.is_ready_to_send" title="Permalink to this definition">#</a></dt>
+<dd><p>Check if <code class="docutils literal notranslate"><span class="pre">Channel</span></code> is ready to send.</p>
+<dl class="field-list simple">
+<dt class="field-odd">Returns</dt>
+<dd class="field-odd"><p>True if ready</p>
+</dd>
+</dl>
+</dd></dl>
+
+<dl class="py method">
+<dt class="sig sig-object py" id="RNS.Channel.Channel.send">
+<span class="sig-name descname"><span class="pre">send</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">message</span></span><span class="p"><span class="pre">:</span></span><span class="w"> </span><span class="n"><a class="reference internal" href="#RNS.MessageBase" title="RNS.Channel.MessageBase"><span class="pre">MessageBase</span></a></span></em><span class="sig-paren">)</span> <span class="sig-return"><span class="sig-return-icon">&#x2192;</span> <span class="sig-return-typehint"><span class="pre">Envelope</span></span></span><a class="headerlink" href="#RNS.Channel.Channel.send" title="Permalink to this definition">#</a></dt>
+<dd><p>Send a message. If a message send is attempted and
+<code class="docutils literal notranslate"><span class="pre">Channel</span></code> is not ready, an exception is thrown.</p>
+<dl class="field-list simple">
+<dt class="field-odd">Parameters</dt>
+<dd class="field-odd"><p><strong>message</strong> – an instance of a <code class="docutils literal notranslate"><span class="pre">MessageBase</span></code> subclass</p>
+</dd>
+</dl>
+</dd></dl>
+
+<dl class="py property">
+<dt class="sig sig-object py" id="RNS.Channel.Channel.MDU">
+<em class="property"><span class="pre">property</span><span class="w"> </span></em><span class="sig-name descname"><span class="pre">MDU</span></span><a class="headerlink" href="#RNS.Channel.Channel.MDU" title="Permalink to this definition">#</a></dt>
+<dd><p>Maximum Data Unit: the number of bytes available
+for a message to consume in a single send. This
+value is adjusted from the <code class="docutils literal notranslate"><span class="pre">Link</span></code> MDU to accommodate
+message header information.</p>
+<dl class="field-list simple">
+<dt class="field-odd">Returns</dt>
+<dd class="field-odd"><p>number of bytes available</p>
+</dd>
+</dl>
+</dd></dl>
+
+</dd></dl>
+
+<p id="api-messsagebase"><h3> MessageBase </h3></p>
+<dl class="py class">
+<dt class="sig sig-object py" id="RNS.MessageBase">
+<em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">RNS.</span></span><span class="sig-name descname"><span class="pre">MessageBase</span></span><a class="headerlink" href="#RNS.MessageBase" title="Permalink to this definition">#</a></dt>
+<dd><p>Base type for any messages sent or received on a Channel.
+Subclasses must define the two abstract methods as well as
+the <code class="docutils literal notranslate"><span class="pre">MSGTYPE</span></code> class variable.</p>
+<dl class="py attribute">
+<dt class="sig sig-object py" id="RNS.MessageBase.MSGTYPE">
+<span class="sig-name descname"><span class="pre">MSGTYPE</span></span><em class="property"><span class="w"> </span><span class="p"><span class="pre">=</span></span><span class="w"> </span><span class="pre">None</span></em><a class="headerlink" href="#RNS.MessageBase.MSGTYPE" title="Permalink to this definition">#</a></dt>
+<dd><p>Defines a unique identifier for a message class.</p>
+<ul class="simple">
+<li><p>Must be unique within all classes registered with a <code class="docutils literal notranslate"><span class="pre">Channel</span></code></p></li>
+<li><p>Must be less than <code class="docutils literal notranslate"><span class="pre">0xf000</span></code>. Values greater than or equal to <code class="docutils literal notranslate"><span class="pre">0xf000</span></code> are reserved.</p></li>
+</ul>
+</dd></dl>
+
+<dl class="py method">
+<dt class="sig sig-object py" id="RNS.MessageBase.pack">
+<em class="property"><span class="pre">abstract</span><span class="w"> </span></em><span class="sig-name descname"><span class="pre">pack</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span> <span class="sig-return"><span class="sig-return-icon">&#x2192;</span> <span class="sig-return-typehint"><span class="pre">bytes</span></span></span><a class="headerlink" href="#RNS.MessageBase.pack" title="Permalink to this definition">#</a></dt>
+<dd><p>Create and return the binary representation of the message</p>
+<dl class="field-list simple">
+<dt class="field-odd">Returns</dt>
+<dd class="field-odd"><p>binary representation of message</p>
+</dd>
+</dl>
+</dd></dl>
+
+<dl class="py method">
+<dt class="sig sig-object py" id="RNS.MessageBase.unpack">
+<em class="property"><span class="pre">abstract</span><span class="w"> </span></em><span class="sig-name descname"><span class="pre">unpack</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">raw</span></span><span class="p"><span class="pre">:</span></span><span class="w"> </span><span class="n"><span class="pre">bytes</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#RNS.MessageBase.unpack" title="Permalink to this definition">#</a></dt>
+<dd><p>Populate message from binary representation</p>
+<dl class="field-list simple">
+<dt class="field-odd">Parameters</dt>
+<dd class="field-odd"><p><strong>raw</strong> – binary representation</p>
+</dd>
+</dl>
+</dd></dl>
+
+</dd></dl>
+
 <p id="api-transport"><h3> Transport </h3></p>
 <dl class="py class">
 <dt class="sig sig-object py" id="RNS.Transport">
@ -1494,7 +1652,7 @@ will announce it.</p>
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
@ -1604,6 +1762,7 @@ will announce it.</p>
 <li><a class="reference internal" href="#RNS.Link.inactive_for"><code class="docutils literal notranslate"><span class="pre">inactive_for()</span></code></a></li>
 <li><a class="reference internal" href="#RNS.Link.get_remote_identity"><code class="docutils literal notranslate"><span class="pre">get_remote_identity()</span></code></a></li>
 <li><a class="reference internal" href="#RNS.Link.teardown"><code class="docutils literal notranslate"><span class="pre">teardown()</span></code></a></li>
+<li><a class="reference internal" href="#RNS.Link.get_channel"><code class="docutils literal notranslate"><span class="pre">get_channel()</span></code></a></li>
 <li><a class="reference internal" href="#RNS.Link.set_link_closed_callback"><code class="docutils literal notranslate"><span class="pre">set_link_closed_callback()</span></code></a></li>
 <li><a class="reference internal" href="#RNS.Link.set_packet_callback"><code class="docutils literal notranslate"><span class="pre">set_packet_callback()</span></code></a></li>
 <li><a class="reference internal" href="#RNS.Link.set_resource_callback"><code class="docutils literal notranslate"><span class="pre">set_resource_callback()</span></code></a></li>
@ -1633,6 +1792,21 @@ will announce it.</p>
 <li><a class="reference internal" href="#RNS.Resource.is_compressed"><code class="docutils literal notranslate"><span class="pre">is_compressed()</span></code></a></li>
 </ul>
 </li>
+<li><a class="reference internal" href="#RNS.Channel.Channel"><code class="docutils literal notranslate"><span class="pre">Channel</span></code></a><ul>
+<li><a class="reference internal" href="#RNS.Channel.Channel.register_message_type"><code class="docutils literal notranslate"><span class="pre">register_message_type()</span></code></a></li>
+<li><a class="reference internal" href="#RNS.Channel.Channel.add_message_handler"><code class="docutils literal notranslate"><span class="pre">add_message_handler()</span></code></a></li>
+<li><a class="reference internal" href="#RNS.Channel.Channel.remove_message_handler"><code class="docutils literal notranslate"><span class="pre">remove_message_handler()</span></code></a></li>
+<li><a class="reference internal" href="#RNS.Channel.Channel.is_ready_to_send"><code class="docutils literal notranslate"><span class="pre">is_ready_to_send()</span></code></a></li>
+<li><a class="reference internal" href="#RNS.Channel.Channel.send"><code class="docutils literal notranslate"><span class="pre">send()</span></code></a></li>
+<li><a class="reference internal" href="#RNS.Channel.Channel.MDU"><code class="docutils literal notranslate"><span class="pre">MDU</span></code></a></li>
+</ul>
+</li>
+<li><a class="reference internal" href="#RNS.MessageBase"><code class="docutils literal notranslate"><span class="pre">MessageBase</span></code></a><ul>
+<li><a class="reference internal" href="#RNS.MessageBase.MSGTYPE"><code class="docutils literal notranslate"><span class="pre">MSGTYPE</span></code></a></li>
+<li><a class="reference internal" href="#RNS.MessageBase.pack"><code class="docutils literal notranslate"><span class="pre">pack()</span></code></a></li>
+<li><a class="reference internal" href="#RNS.MessageBase.unpack"><code class="docutils literal notranslate"><span class="pre">unpack()</span></code></a></li>
+</ul>
+</li>
 <li><a class="reference internal" href="#RNS.Transport"><code class="docutils literal notranslate"><span class="pre">Transport</span></code></a><ul>
 <li><a class="reference internal" href="#RNS.Transport.PATHFINDER_M"><code class="docutils literal notranslate"><span class="pre">PATHFINDER_M</span></code></a></li>
 <li><a class="reference internal" href="#RNS.Transport.register_announce_handler"><code class="docutils literal notranslate"><span class="pre">register_announce_handler()</span></code></a></li>
--- a/docs/manual/search.html
+++ b/docs/manual/search.html
@ -241,7 +241,7 @@
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/searchindex.js
+++ b/docs/manual/searchindex.js
--- a/docs/manual/support.html
+++ b/docs/manual/support.html
@ -288,7 +288,7 @@ report issues, suggest functionality and contribute code to Reticulum.</p>
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/understanding.html
+++ b/docs/manual/understanding.html
@ -1126,7 +1126,7 @@ those risks are acceptable to you.</p>
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/using.html
+++ b/docs/manual/using.html
@ -757,7 +757,7 @@ WantedBy=multi-user.target
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/manual/whatis.html
+++ b/docs/manual/whatis.html
@ -390,7 +390,7 @@ want to help out with this, or can help sponsor an audit, please do get in touch
        <div class="bottom-of-page">
          <div class="left-details">
            <div class="copyright">
-                Copyright &#169; 2022, Mark Qvist
+                Copyright &#169; 2023, Mark Qvist
            </div>
            Generated with <a href="https://www.sphinx-doc.org/">Sphinx</a> and 
            <a href="https://github.com/pradyunsg/furo">Furo</a>
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@ -10,7 +10,7 @@ sys.path.insert(0, os.path.abspath('../..'))

 # -- Project information -----------------------------------------------------
 project = 'Reticulum Network Stack'
-copyright = '2022, Mark Qvist'
+copyright = '2023, Mark Qvist'
 author = 'Mark Qvist'

 # The full version, including alpha/beta/rc tags
--- a/docs/source/examples.rst
+++ b/docs/source/examples.rst
@ -92,6 +92,18 @@ The *Request* example explores sendig requests and receiving responses.

 This example can also be found at `<https://github.com/markqvist/Reticulum/blob/master/Examples/Request.py>`_.

+.. _example-channel:
+
+Channel
+=======
+
+The *Channel* example explores using a ``Channel`` to send structured
+data between peers of a ``Link``.
+
+.. literalinclude:: ../../Examples/Channel.py
+
+This example can also be found at `<https://github.com/markqvist/Reticulum/blob/master/Examples/Channel.py>`_.
+
 .. _example-filetransfer:

 Filetransfer
--- a/docs/source/gettingstartedfast.rst
+++ b/docs/source/gettingstartedfast.rst
@ -42,6 +42,13 @@ transceivers or infrastructure just to try it out. Launching the programs on sep
 devices connected to the same WiFi network is enough to get started, and physical
 radio interfaces can then be added later.

+Remote Shell
+^^^^^^^^^^^^
+
+The `rnsh <https://github.com/acehoss/rnsh>`_ program lets you establish fully interactive
+remote shell sessions over Reticulum. It also allows you to pipe any program to or from a
+remote system, and is similar to how ``ssh`` works.
+
 Nomad Network
 ^^^^^^^^^^^^^

--- a/docs/source/index.rst
+++ b/docs/source/index.rst
@ -5,6 +5,10 @@ This manual aims to provide you with all the information you need to
 understand Reticulum, build networks or develop programs using it, or
 to participate in the development of Reticulum itself.

+.. only:: html
+
+This manual is also available in `PDF <https://github.com/markqvist/Reticulum/releases/latest/download/Reticulum.Manual.pdf>`_ and `EPUB <https://github.com/markqvist/Reticulum/releases/latest/download/Reticulum.Manual.epub>`_ formats.
+
 .. only:: html

   Table Of Contents
--- a/docs/source/reference.rst
+++ b/docs/source/reference.rst
@ -121,6 +121,34 @@ This chapter lists and explains all classes exposed by the Reticulum Network Sta
 .. autoclass:: RNS.Resource(data, link, advertise=True, auto_compress=True, callback=None, progress_callback=None, timeout=None)
   :members:

+.. _api-channel:
+
+.. only:: html
+
+   |start-h3| Channel |end-h3|
+
+.. only:: latex
+
+   Channel
+   -------
+
+.. autoclass:: RNS.Channel.Channel()
+   :members:
+
+.. _api-messsagebase:
+
+.. only:: html
+
+   |start-h3| MessageBase |end-h3|
+
+.. only:: latex
+
+   MessageBase
+   -----------
+
+.. autoclass:: RNS.MessageBase()
+   :members:
+
 .. _api-transport:

 .. only:: html
--- a/tests/all.py
+++ b/tests/all.py
@ -4,6 +4,7 @@ from .hashes import TestSHA256
 from .hashes import TestSHA512
 from .identity import TestIdentity
 from .link import TestLink
+from .channel import TestChannel

 if __name__ == '__main__':
    unittest.main(verbosity=2)
--- a/tests/channel.py
+++ b/tests/channel.py
@ -0,0 +1,375 @@
+from __future__ import annotations
+import threading
+import RNS
+from RNS.Channel import MessageState, ChannelOutletBase, Channel, MessageBase
+from RNS.vendor import umsgpack
+from typing import Callable
+import contextlib
+import typing
+import types
+import time
+import uuid
+import unittest
+
+
+class Packet:
+    timeout = 1.0
+
+    def __init__(self, raw: bytes):
+        self.state = MessageState.MSGSTATE_NEW
+        self.raw = raw
+        self.packet_id = uuid.uuid4()
+        self.tries = 0
+        self.timeout_id = None
+        self.lock = threading.RLock()
+        self.instances = 0
+        self.timeout_callback: Callable[[Packet], None] | None = None
+        self.delivered_callback: Callable[[Packet], None] | None = None
+
+    def set_timeout(self, callback: Callable[[Packet], None] | None, timeout: float):
+        with self.lock:
+            if timeout is not None:
+                self.timeout = timeout
+            self.timeout_callback = callback
+
+
+    def send(self):
+        self.tries += 1
+        self.state = MessageState.MSGSTATE_SENT
+
+        def elapsed(timeout: float, timeout_id: uuid.uuid4):
+            with self.lock:
+                self.instances += 1
+            try:
+                time.sleep(timeout)
+                with self.lock:
+                    if self.timeout_id == timeout_id:
+                        self.timeout_id = None
+                        self.state = MessageState.MSGSTATE_FAILED
+                        if self.timeout_callback:
+                            self.timeout_callback(self)
+            finally:
+                with self.lock:
+                    self.instances -= 1
+
+        self.timeout_id = uuid.uuid4()
+        threading.Thread(target=elapsed, name="Packet Timeout", args=[self.timeout, self.timeout_id],
+                         daemon=True).start()
+
+    def clear_timeout(self):
+        self.timeout_id = None
+
+    def set_delivered_callback(self, callback: Callable[[Packet], None]):
+        self.delivered_callback = callback
+        
+    def delivered(self):
+        with self.lock:
+            self.state = MessageState.MSGSTATE_DELIVERED
+            self.timeout_id = None
+        if self.delivered_callback:
+            self.delivered_callback(self)
+
+
+class ChannelOutletTest(ChannelOutletBase):
+    def get_packet_state(self, packet: Packet) -> MessageState:
+        return packet.state
+
+    def set_packet_timeout_callback(self, packet: Packet, callback: Callable[[Packet], None] | None,
+                                    timeout: float | None = None):
+        packet.set_timeout(callback, timeout)
+
+    def set_packet_delivered_callback(self, packet: Packet, callback: Callable[[Packet], None] | None):
+        packet.set_delivered_callback(callback)
+
+    def get_packet_id(self, packet: Packet) -> any:
+        return packet.packet_id
+
+    def __init__(self, mdu: int, rtt: float):
+        self.link_id = uuid.uuid4()
+        self.timeout_callbacks = 0
+        self._mdu = mdu
+        self._rtt = rtt
+        self._usable = True
+        self.packets = []
+        self.packet_callback: Callable[[ChannelOutletBase, bytes], None] | None = None
+
+    def send(self, raw: bytes) -> Packet:
+        packet = Packet(raw)
+        packet.send()
+        self.packets.append(packet)
+        return packet
+
+    def resend(self, packet: Packet) -> Packet:
+        packet.send()
+        return packet
+
+    @property
+    def mdu(self):
+        return self._mdu
+
+    @property
+    def rtt(self):
+        return self._rtt
+
+    @property
+    def is_usable(self):
+        return self._usable
+
+    def timed_out(self):
+        self.timeout_callbacks += 1
+
+    def __str__(self):
+        return str(self.link_id)
+
+
+class MessageTest(MessageBase):
+    MSGTYPE = 0xabcd
+
+    def __init__(self):
+        self.id = str(uuid.uuid4())
+        self.data = "test"
+        self.not_serialized = str(uuid.uuid4())
+
+    def pack(self) -> bytes:
+        return umsgpack.packb((self.id, self.data))
+
+    def unpack(self, raw):
+        self.id, self.data = umsgpack.unpackb(raw)
+
+
+class SystemMessage(MessageBase):
+    MSGTYPE = 0xf000
+
+    def pack(self) -> bytes:
+        return bytes()
+
+    def unpack(self, raw):
+        pass
+
+
+class ProtocolHarness(contextlib.AbstractContextManager):
+    def __init__(self, rtt: float):
+        self.outlet = ChannelOutletTest(mdu=500, rtt=rtt)
+        self.channel = Channel(self.outlet)
+
+    def cleanup(self):
+        self.channel._shutdown()
+
+    def __exit__(self, __exc_type: typing.Type[BaseException], __exc_value: BaseException,
+                 __traceback: types.TracebackType) -> bool:
+        # self._log.debug(f"__exit__({__exc_type}, {__exc_value}, {__traceback})")
+        self.cleanup()
+        return False
+
+
+class TestChannel(unittest.TestCase):
+    def setUp(self) -> None:
+        print("")
+        self.rtt = 0.001
+        self.retry_interval = self.rtt * 150
+        Packet.timeout = self.retry_interval
+        self.h = ProtocolHarness(self.rtt)
+
+    def tearDown(self) -> None:
+        self.h.cleanup()
+
+    def test_send_one_retry(self):
+        print("Channel test one retry")
+        message = MessageTest()
+
+        self.assertEqual(0, len(self.h.outlet.packets))
+
+        envelope = self.h.channel.send(message)
+
+        self.assertIsNotNone(envelope)
+        self.assertIsNotNone(envelope.raw)
+        self.assertEqual(1, len(self.h.outlet.packets))
+        self.assertIsNotNone(envelope.packet)
+        self.assertTrue(envelope in self.h.channel._tx_ring)
+        self.assertTrue(envelope.tracked)
+
+        packet = self.h.outlet.packets[0]
+
+        self.assertEqual(envelope.packet, packet)
+        self.assertEqual(1, envelope.tries)
+        self.assertEqual(1, packet.tries)
+        self.assertEqual(1, packet.instances)
+        self.assertEqual(MessageState.MSGSTATE_SENT, packet.state)
+        self.assertEqual(envelope.raw, packet.raw)
+
+        time.sleep(self.retry_interval * 1.5)
+
+        self.assertEqual(1, len(self.h.outlet.packets))
+        self.assertEqual(2, envelope.tries)
+        self.assertEqual(2, packet.tries)
+        self.assertEqual(1, packet.instances)
+
+        time.sleep(self.retry_interval)
+
+        self.assertEqual(1, len(self.h.outlet.packets))
+        self.assertEqual(self.h.outlet.packets[0], packet)
+        self.assertEqual(3, envelope.tries)
+        self.assertEqual(3, packet.tries)
+        self.assertEqual(1, packet.instances)
+        self.assertEqual(MessageState.MSGSTATE_SENT, packet.state)
+
+        packet.delivered()
+
+        self.assertEqual(MessageState.MSGSTATE_DELIVERED, packet.state)
+
+        time.sleep(self.retry_interval)
+
+        self.assertEqual(1, len(self.h.outlet.packets))
+        self.assertEqual(3, envelope.tries)
+        self.assertEqual(3, packet.tries)
+        self.assertEqual(0, packet.instances)
+        self.assertFalse(envelope.tracked)
+
+    def test_send_timeout(self):
+        print("Channel test retry count exceeded")
+        message = MessageTest()
+
+        self.assertEqual(0, len(self.h.outlet.packets))
+
+        envelope = self.h.channel.send(message)
+
+        self.assertIsNotNone(envelope)
+        self.assertIsNotNone(envelope.raw)
+        self.assertEqual(1, len(self.h.outlet.packets))
+        self.assertIsNotNone(envelope.packet)
+        self.assertTrue(envelope in self.h.channel._tx_ring)
+        self.assertTrue(envelope.tracked)
+
+        packet = self.h.outlet.packets[0]
+
+        self.assertEqual(envelope.packet, packet)
+        self.assertEqual(1, envelope.tries)
+        self.assertEqual(1, packet.tries)
+        self.assertEqual(1, packet.instances)
+        self.assertEqual(MessageState.MSGSTATE_SENT, packet.state)
+        self.assertEqual(envelope.raw, packet.raw)
+
+        time.sleep(self.retry_interval * 7.5)
+
+        self.assertEqual(1, len(self.h.outlet.packets))
+        self.assertEqual(5, envelope.tries)
+        self.assertEqual(5, packet.tries)
+        self.assertEqual(0, packet.instances)
+        self.assertEqual(MessageState.MSGSTATE_FAILED, packet.state)
+        self.assertFalse(envelope.tracked)
+
+    def test_multiple_handler(self):
+        print("Channel test multiple handler short circuit")
+
+        handler1_called = 0
+        handler1_return = True
+        handler2_called = 0
+
+        def handler1(msg: MessageBase):
+            nonlocal handler1_called, handler1_return
+            self.assertIsInstance(msg, MessageTest)
+            handler1_called += 1
+            return handler1_return
+
+        def handler2(msg: MessageBase):
+            nonlocal handler2_called
+            self.assertIsInstance(msg, MessageTest)
+            handler2_called += 1
+
+        message = MessageTest()
+        self.h.channel.register_message_type(MessageTest)
+        self.h.channel.add_message_handler(handler1)
+        self.h.channel.add_message_handler(handler2)
+        envelope = RNS.Channel.Envelope(self.h.outlet, message, sequence=0)
+        raw = envelope.pack()
+        self.h.channel._receive(raw)
+
+        self.assertEqual(1, handler1_called)
+        self.assertEqual(0, handler2_called)
+
+        handler1_return = False
+        envelope = RNS.Channel.Envelope(self.h.outlet, message, sequence=1)
+        raw = envelope.pack()
+        self.h.channel._receive(raw)
+
+        self.assertEqual(2, handler1_called)
+        self.assertEqual(1, handler2_called)
+
+    def test_system_message_check(self):
+        print("Channel test register system message")
+        with self.assertRaises(RNS.Channel.ChannelException):
+            self.h.channel.register_message_type(SystemMessage)
+        self.h.channel._register_message_type(SystemMessage, is_system_type=True)
+
+
+    def eat_own_dog_food(self, message: MessageBase, checker: typing.Callable[[MessageBase], None]):
+        decoded: [MessageBase] = []
+
+        def handle_message(message: MessageBase):
+            decoded.append(message)
+
+        self.h.channel.register_message_type(message.__class__)
+        self.h.channel.add_message_handler(handle_message)
+        self.assertEqual(len(self.h.outlet.packets), 0)
+
+        envelope = self.h.channel.send(message)
+        time.sleep(self.retry_interval * 0.5)
+
+        self.assertIsNotNone(envelope)
+        self.assertIsNotNone(envelope.raw)
+        self.assertEqual(1, len(self.h.outlet.packets))
+        self.assertIsNotNone(envelope.packet)
+        self.assertTrue(envelope in self.h.channel._tx_ring)
+        self.assertTrue(envelope.tracked)
+
+        packet = self.h.outlet.packets[0]
+
+        self.assertEqual(envelope.packet, packet)
+        self.assertEqual(1, envelope.tries)
+        self.assertEqual(1, packet.tries)
+        self.assertEqual(1, packet.instances)
+        self.assertEqual(MessageState.MSGSTATE_SENT, packet.state)
+        self.assertEqual(envelope.raw, packet.raw)
+
+        packet.delivered()
+
+        self.assertEqual(MessageState.MSGSTATE_DELIVERED, packet.state)
+
+        time.sleep(self.retry_interval * 2)
+
+        self.assertEqual(1, len(self.h.outlet.packets))
+        self.assertEqual(1, envelope.tries)
+        self.assertEqual(1, packet.tries)
+        self.assertEqual(0, packet.instances)
+        self.assertFalse(envelope.tracked)
+
+        self.assertEqual(len(self.h.outlet.packets), 1)
+        self.assertEqual(MessageState.MSGSTATE_DELIVERED, packet.state)
+        self.assertFalse(envelope.tracked)
+        self.assertEqual(0, len(decoded))
+
+        self.h.channel._receive(packet.raw)
+
+        self.assertEqual(1, len(decoded))
+
+        rx_message = decoded[0]
+
+        self.assertIsNotNone(rx_message)
+        self.assertIsInstance(rx_message, message.__class__)
+        checker(rx_message)
+
+    def test_send_receive_message_test(self):
+        print("Channel test send and receive message")
+        message = MessageTest()
+
+        def check(rx_message: MessageBase):
+            self.assertIsInstance(rx_message, message.__class__)
+            self.assertEqual(message.id, rx_message.id)
+            self.assertEqual(message.data, rx_message.data)
+            self.assertNotEqual(message.not_serialized, rx_message.not_serialized)
+
+        self.eat_own_dog_food(message, check)
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)
--- a/tests/link.py
+++ b/tests/link.py
@ -6,6 +6,8 @@ import threading
 import time
 import RNS
 import os
+from tests.channel import MessageTest
+from RNS.Channel import MessageBase

 APP_NAME = "rns_unit_tests"

@ -346,6 +348,54 @@ class TestLink(unittest.TestCase):
        time.sleep(0.5)
        self.assertEqual(l1.status, RNS.Link.CLOSED)

+    def test_10_channel_round_trip(self):
+        global c_rns
+        init_rns(self)
+        print("")
+        print("Channel round trip test")
+
+        # TODO: Load this from public bytes only
+        id1 = RNS.Identity.from_bytes(bytes.fromhex(fixed_keys[0][0]))
+        self.assertEqual(id1.hash, bytes.fromhex(fixed_keys[0][1]))
+
+        dest = RNS.Destination(id1, RNS.Destination.OUT, RNS.Destination.SINGLE, APP_NAME, "link", "establish")
+
+        self.assertEqual(dest.hash, bytes.fromhex("fb48da0e82e6e01ba0c014513f74540d"))
+
+        l1 = RNS.Link(dest)
+        time.sleep(1)
+        self.assertEqual(l1.status, RNS.Link.ACTIVE)
+
+        received = []
+
+        def handle_message(message: MessageBase):
+            received.append(message)
+
+        test_message = MessageTest()
+        test_message.data = "Hello"
+
+        channel = l1.get_channel()
+        channel.register_message_type(MessageTest)
+        channel.add_message_handler(handle_message)
+        channel.send(test_message)
+
+        time.sleep(0.5)
+
+        self.assertEqual(1, len(received))
+
+        rx_message = received[0]
+
+        self.assertIsInstance(rx_message, MessageTest)
+        self.assertEqual("Hello back", rx_message.data)
+        self.assertEqual(test_message.id, rx_message.id)
+        self.assertNotEqual(test_message.not_serialized, rx_message.not_serialized)
+        self.assertEqual(1, len(l1._channel._rx_ring))
+
+        l1.teardown()
+        time.sleep(0.5)
+        self.assertEqual(l1.status, RNS.Link.CLOSED)
+        self.assertEqual(0, len(l1._channel._rx_ring))
+

    def size_str(self, num, suffix='B'):
        units = ['','K','M','G','T','P','E','Z']
@ -404,6 +454,13 @@ def targets(yp=False):
        link.set_resource_strategy(RNS.Link.ACCEPT_ALL)
        link.set_resource_started_callback(resource_started)
        link.set_resource_concluded_callback(resource_concluded)
+        channel = link.get_channel()
+
+        def handle_message(message):
+            message.data = message.data + " back"
+            channel.send(message)
+        channel.register_message_type(MessageTest)
+        channel.add_message_handler(handle_message)

    m_rns = RNS.Reticulum("./tests/rnsconfig")
    id1 = RNS.Identity.from_bytes(bytes.fromhex(fixed_keys[0][0]))
Author	SHA1	Message	Date
Mark Qvist	58004d7c05	Updated documentation	2023-03-02 12:47:55 +01:00
Mark Qvist	aa0d9c5c13	Merge branch 'master' of github.com:markqvist/Reticulum	2023-03-02 12:05:06 +01:00
Mark Qvist	9e46950e28	Added output to echo example	2023-03-02 12:04:50 +01:00
markqvist	a6551fc019	Merge pull request #246 from gdt/fix-transmit-hash AutoInterface: Drop embedded scope identifier on fe80::	2023-03-02 11:34:00 +01:00
markqvist	a06ae40797	Merge pull request #236 from faragher/master Additional error messages for offline flashing.	2023-03-02 11:31:31 +01:00
markqvist	1db08438df	Merge pull request #248 from Erethon/hkdf-remove-dead-code hkdf: Remove duplicate check if the salt is None	2023-03-02 11:29:18 +01:00
markqvist	89aa51ab61	Merge pull request #245 from acehoss/feature/channel Channel: reliable delivery over Link	2023-03-02 11:27:15 +01:00
Dionysis Grigoropoulos	ddb7a92c15	hkdf: Remove duplicate check if the salt is None The second if isn't needed since we initialize the salt with zeroes earlier. If instead we meant to pass an empty bytes class to the HMAC implementation, the end result would be the same, since it's gonna get padded with zeroes in the HMAC code.	2023-03-01 16:22:51 +02:00
Greg Troxel	e273900e87	AutoInterface: Drop embedded scope identifier on fe80:: The code previously dropped scope identifiers expressed as a trailing "%ifname", which happens on macOS. On NetBSD and OpenBSD (and likely FreeBSD, not tested), the scope identifier is embedded. Drop that form of identifier as well, because we keep address and ifname separate, and because the scope identifier must not be part of computing the hash of the address. Resolves #240, failure to peer on NetBSD and OpenBSD.	2023-02-28 10:19:46 -05:00
Aaron Heise	d2d121d49f	Fix broken Channel test	2023-02-28 08:38:36 -06:00
Aaron Heise	9963cf37b8	Fix exceptions on Channel shutdown	2023-02-28 08:38:23 -06:00
Aaron Heise	72300cc821	Revert "Only send proof if link is still active"	2023-02-28 08:24:13 -06:00
Aaron Heise	8168d9bb92	Only send proof if link is still active	2023-02-28 08:13:07 -06:00
Aaron Heise	8f0151fed6	Tidy up PR	2023-02-27 21:33:50 -06:00
Aaron Heise	d3c4928eda	Tidy up PR	2023-02-27 21:31:41 -06:00
Aaron Heise	68f95cd80b	Tidy up PR	2023-02-27 21:30:13 -06:00
Aaron Heise	42935c8238	Make the PR have zero deletions	2023-02-27 21:15:25 -06:00
Aaron Heise	118acf77b8	Fix up documentation even more	2023-02-27 21:10:28 -06:00
Aaron Heise	661964277f	Fix up documentation for building	2023-02-27 19:05:25 -06:00
Aaron Heise	464dc23ff0	Add some internal documenation	2023-02-27 17:36:04 -06:00
Aaron Heise	44dc2d06c6	Add channel tests to all test suite Also print name in each test	2023-02-26 11:47:46 -06:00
Aaron Heise	c00b592ed9	System-reserved channel message types - a message handler can return logical True to prevent subsequent message handlers from running - Message types >= 0xff00 are reserved for system/framework messages	2023-02-26 11:39:49 -06:00
Aaron Heise	e005826151	Allow channel message handlers to short circuit - a message handler can return logical True to prevent subsequent message handlers from running	2023-02-26 11:23:38 -06:00
Aaron Heise	a61b15cf6a	Added channel example	2023-02-26 07:26:12 -06:00
Aaron Heise	fe3a3e22f7	Expose Channel on Link Separates channel interface from link Also added: allow multiple message handlers	2023-02-26 07:25:49 -06:00
Aaron Heise	68cb4a6740	Initial work on Channel	2023-02-25 18:23:25 -06:00
Mark Qvist	9f06bed34c	Updated readme and roadmap	2023-02-23 17:27:05 +01:00
Mark Qvist	3b1936ef48	Added EPUB output to documentation build	2023-02-23 17:25:38 +01:00
Michael Faragher	5b3d26a90a	Additional error messages for offline flashing.	2023-02-22 12:49:24 -06:00