In P2PD all data messages are sent and received via pipes. Pipes are simply the name given to the object providing a common list of functions for transmission and message processing. A pipe supports UDP or TCP and IPv4 or IPv6.

async def pipe_open(proto, route, dest=None, sock=None, msg_cb=None, conf=NET_CONF):
    proto = TCP or UDP.
    route = Route object that's been bound with await route.bind().
    dest  = If it's a client pipe a destionation should be included.
                await Address('host/ip', port, route).res()
            A server includes no destination.
    sock  = Used to wrap a pre-existing socket in a pipe. If the protocol is
            TCP and dest is included the socket is assumed to be connected.
    cb    = A message handler registered with servers before they're started
            so that messages aren't received before a handler is setup.
    conf  = A dictionary describing many different configuration options for
            changing various properties of the pipe.

    More details on msg_cb format and conf format later.

    Returns: a pipe object.

Whether a pipe is for a client or server, UDP or TCP, IPv4 or IPv6, every pipe works the same. Pipes have been designed to process messages as they arrive. They pass these messages to any registered handlers (to process in real-time) or message queues (to be processed later).

TCP echo server example

Starts a simple TCP server that writes back received data down the client pipes for the sender. If this example works you should see nothing. Notice that msg handlers include a field for the senders addressing information and a pipe that can be used to interact with that client.

from p2pd import *

async def msg_cb(msg, client_tup, pipe):
    await pipe.send(msg, client_tup)

async def example():
    # Start default interface and get the first route.
    # No AF for route use i.supported()[0] 
    i = await Interface().start()
    route = await i.route().bind() # Port 0 = any unused port.
    # Start the server and use msg_cb to process messages.
    server = await pipe_open(TCP, route, msg_cb=msg_cb)
    # Connect to the server.
    # Use the IP of the route and unused port for the destination.
    dest = await Address(*server.sock.getsockname()[0:2], route).res()
    client = await pipe_open(TCP, route, dest)
    # Send data to the server and check receipt.
    msg = b"test msg."
    await client.send(msg)
    out = await client.recv()
    assert(msg == out)
    # Close both.
    await client.close()
    await server.close()

# From inside the async REPL.
if __name__ == '__main__':

UDP await example

In Python if you want to do asynchronous programming you’re likely going to be writing different code for TCP and UDP. This is because TCP is ‘stream-based’ and UDP is ‘packet-based.’ TCP streams are reliable and ordered. UDP communication is not. So in Python for TCP connections you will be dealing with ‘streams’ while for UDP you will use protocol classes.

Only stream readers are ‘asynchronous’ e.g. you can await ‘draining’ a writer or await a reader - while there is no such equivalent for UDP. It’s all very inconvenient. Wouldn’t it be great if you could use asynchronous awaits for UDP and TCP? Further: wouldn’t it be great if you modelled interactions in such a way that the same code would work for both?

Here’s an example of how simple P2PD makes this. Here I’m using await for UDP which is based on message queues. Since there is no delivery guarantees for UDP it’s possible this example throws a timeout error for you. Real-world code that deals with TCP usually has retransmissions built-in after a set duration. But no such logic here has been included. Note that the await for the recv is fully asynchronous. The event loop is free to run other tasks until a match occurs.

import random
import binascii
from p2pd import *

async def example():
    # Open default interface.
    # Get a route for the first AF supported.
    i = await Interface().start()
    route = await i.route().bind()
    # Open a UDP pipe to stunprotocol.org.
    # Subscribe to all messages.
    pipe = await pipe_open(
        await Address("stunserver.stunprotocol.org", 3478, route).res()
    # Build a STUN request and send it.
    msg_id = ''.join([str(random.randrange(10, 99)) for _ in range(16)])
    req_hex = "00010000" + msg_id
    req_buf = binascii.unhexlify(req_hex)
    await pipe.send(req_buf)
    # Get the response.
    resp = await pipe.recv()
    await pipe.close()

if __name__ == '__main__':

Pipe methods

Pipes are an instance of the BaseProto class that provides many useful methods and properties for working with connections (TCP or UDP.) Assume all of these methods are of the form ‘pipe.method_name()’ and that they ‘belong’ to a BaseProto class instance.

def subscribe(self, sub=SUB_ALL, handler=None)

Install a new message queue indexed by the regex pair sub = [msg_regex, client_tup_regex]. Doing this enables the use of publish-subscribe e.g. push / pull style awaits for a message. By default a pipe will subscribe to all messages (SUB_ALL) if a pipe has a destination given.

# Match any message containing meow.
# Allow only hosts from the subnet.
# Put them into the same queue.
sub = [b"meow", b"192[.]168[.][0-9]+[.][0-9]+:[0-9]+"]

# Wait for a message that fits into the sub queue.
await pipe.recv(sub, timeout=4)

def unsubscribe(self, sub)

Delete the queue and its resources marked by sub (if it exists.) No longer copy messages that fit this subscription into this queue.

async def recv(self, sub=SUB_ALL, timeout=2, full=False)

Given a queue identified by the subscription ‘sub’ – wait for a message that suites it. Waiting is done asynchronously and other tasks may be done by the event loop until a message arrives. Timeout specifies the total duration to attempt to wait. After the duration an exception will be thrown. Set this to 0 to disable timeouts (not recommended.)

# Wait for any message from a loopback client.
out = await pipe.recv([b"[\s\S]+", "[0-9]+"])

By default this function only returns the message received on the pipe. Some pipes receive messages from multiple destinations (like UDP.) To also show the sender set the full flag to True. The return value will end up being [msg_bytes, client_tup].

async def send(self, data, dest_tup=None)

Wait for data to be transmitted down the pipe (non-blocking.) For TCP / UDP connections (with a fixed destination) the dest_tup does not need to be set. But it’s a good practice to include it in servers because the same socket in UDP servers is used to receive messages from multiple clients and the pipe by itself won’t be able to disambiguate what the destination should be. This is also one reasons why msg_cbs include a client_tup for a message sender.

def add_msg_cb(self, msg_cb)

When a pipe receives a message it will also forward it to any installed message handlers. The format for a message handler is:

async def msg_cb(msg, client_tup, pipe)

The msg_cb also doesn’t have to be an async callback but keep in mind if it’s given as a regular function you will have to use asyncio.create_task to schedule any callbacks and you won’t be able to await them. Since the whole library uses async await it’s best just to use an async msg_cb.

Using message handlers like this is very useful because you can install them for either a server pipe or a client pipe and it will automatically be called when there’s a new message. No need to run your own loop and call awaits on some object. The event loop handles it.

def del_msg_cb(self, msg_cb)

Removes a function reference designated by msg_cb from the pipe’s msg_cbs.

def add_end_cb(self, end_cb)

When a connection is closed manually or forcefully the end_cb handlers are called. These are useful for cleanup. The format is:

async def end_cb(msg, client_tup, pipe)

Where message is set to None.

def del_end_cb(self, end_cb)

Removes a function reference designated by end_cb from the pipe’s end_cb handlers.

def add_pipe(self, pipe)

Pipes can be made to route messages to other pipes. You can connect two pipes together by adding each pipe to each other.

  1. Messages received at pipe_a will be sent down pipe_b.

  2. Messages received at pipe_b will be sent down pipe_a.

This doesn’t cause looping as the messages get sent to the destination rather than the pipe itself. Linking pipes together is the trick used in the P2PD REST API for ‘converting’ an active HTTP connection into a two-way relay to an active P2P connection in only two lines of code.

def del_pipe(self, pipe)

Unlink ‘pipe’ from self.

async def close(self)

Closes all resources associated with a pipe. If it’s a server it will stop serving any clients and all client connections will be closed. All sockets will be closed forcefully. Server’s that immediately reuse the same port may experience errors where they fail to receive designated packets. There may be a solution to this by setting SO_LINGER to enabled and using a zero timeout. But using this option on client TCP sockets on Windows prevents the hole punching algorithm from working so this needs to only be considered for server sockets.

Additional pipe options

A default dictionary of configuration options is passed to each pipe. The options look like this:

    # Protocol family used for the socket.socket function.
    "sock_proto": 0,

    # N seconds before a registering recv timeout.
    "recv_timeout": 2,

    # Only applies to TCP.
    "con_timeout": 2,

    # No of messages to receive per subscription.
    "max_qsize": 0,

    # Require unique messages or not.
    "enable_msg_ids": 0,

    # Number of message IDs to keep around.
    "max_msg_ids": 1000,

    # Reuse address tuple for bind() socket call.
    "reuse_addr": False,

    # Setup socket as a broadcast socket.
    "broadcast": False,

    # Buf size for asyncio.StreamReader.
    "reader_limit": 2 ** 16,

    # Return the sock instead of the base proto.
    "sock_only": False,

    # Disable closing sock on error.
    "no_close": False,

    # Whether to set SO_LINGER. None = off.
    # Non-none = linger value.
    "linger": None,

    # Ref to an event loop.
    "loop": None

# Here's where to use these options.
pipe = pipe_open(TCP, route, dest, conf=NET_CONF)