Reticulum/RNS/Transport.py

import os
import RNS
import time
import math
import struct
import threading
import traceback
from time import sleep
from .vendor import umsgpack as umsgpack

class Transport:
    """
    Through static methods of this class you can interact with the
    Transport system of Reticulum.
    """
    # Constants
    BROADCAST    = 0x00;
    TRANSPORT    = 0x01;
    RELAY        = 0x02;
    TUNNEL       = 0x03;
    types        = [BROADCAST, TRANSPORT, RELAY, TUNNEL]

    REACHABILITY_UNREACHABLE = 0x00
    REACHABILITY_DIRECT      = 0x01
    REACHABILITY_TRANSPORT   = 0x02

    APP_NAME = "rnstransport"

    PATHFINDER_M    = 128       # Max hops
    """
    Maximum amount of hops that Reticulum will transport a packet.
    """
    PATHFINDER_C    = 2.0        # Decay constant
    PATHFINDER_R    = 1          # Retransmit retries
    PATHFINDER_T    = 10         # Retry grace period
    PATHFINDER_RW   = 10         # Random window for announce rebroadcast
    PATHFINDER_E    = 60*60*24*7 # Path expiration in seconds

    # TODO: Calculate an optimal number for this in
    # various situations
    LOCAL_REBROADCASTS_MAX = 2          # How many local rebroadcasts of an announce is allowed

    PATH_REQUEST_GRACE  = 0.35          # Grace time before a path announcement is made, allows directly reachable peers to respond first
    PATH_REQUEST_RW     = 2             # Path request random window

    LINK_TIMEOUT         = RNS.Link.KEEPALIVE * 2
    REVERSE_TIMEOUT      = 30*60        # Reverse table entries are removed after max 30 minutes
    DESTINATION_TIMEOUT  = PATHFINDER_E # Destination table entries are removed if unused for one week
    MAX_RECEIPTS         = 1024         # Maximum number of receipts to keep track of

    interfaces           = []           # All active interfaces
    destinations         = []           # All active destinations
    pending_links        = []           # Links that are being established
    active_links         = []           # Links that are active
    packet_hashlist      = []           # A list of packet hashes for duplicate detection
    receipts             = []           # Receipts of all outgoing packets for proof processing

    # TODO: "destination_table" should really be renamed to "path_table"
    # Notes on memory usage: 1 megabyte of memory can store approximately
    # 55.100 path table entries or approximately 22.300 link table entries.
    announce_table       = {}           # A table for storing announces currently waiting to be retransmitted
    destination_table    = {}           # A lookup table containing the next hop to a given destination
    reverse_table        = {}           # A lookup table for storing packet hashes used to return proofs and replies
    link_table           = {}           # A lookup table containing hops for links
    held_announces       = {}           # A table containing temporarily held announce-table entries
    announce_handlers    = []           # A table storing externally registered announce handlers
    tunnels              = {}           # A table storing tunnels to other transport instances

    # Transport control destinations are used
    # for control purposes like path requests
    control_destinations = []
    control_hashes       = []

    # Interfaces for communicating with
    # local clients connected to a shared
    # Reticulum instance
    local_client_interfaces = []

    jobs_locked = False
    jobs_running = False
    job_interval = 0.250
    receipts_last_checked    = 0.0
    receipts_check_interval  = 1.0
    announces_last_checked   = 0.0
    announces_check_interval = 1.0
    hashlist_maxsize         = 1000000
    tables_last_culled       = 0.0
    tables_cull_interval     = 5.0

    identity = None

    @staticmethod
    def start(reticulum_instance):
        Transport.owner = reticulum_instance

        if Transport.identity == None:
            transport_identity_path = RNS.Reticulum.storagepath+"/transport_identity"
            if os.path.isfile(transport_identity_path):
                Transport.identity = RNS.Identity.from_file(transport_identity_path)                

            if Transport.identity == None:
                RNS.log("No valid Transport Identity in storage, creating...", RNS.LOG_VERBOSE)
                Transport.identity = RNS.Identity()
                Transport.identity.to_file(transport_identity_path)
            else:
                RNS.log("Loaded Transport Identity from storage", RNS.LOG_VERBOSE)

        packet_hashlist_path = RNS.Reticulum.storagepath+"/packet_hashlist"
        if os.path.isfile(packet_hashlist_path):
            try:
                file = open(packet_hashlist_path, "rb")
                Transport.packet_hashlist = umsgpack.unpackb(file.read())
                file.close()
            except Exception as e:
                RNS.log("Could not load packet hashlist from storage, the contained exception was: "+str(e), RNS.LOG_ERROR)

        # Create transport-specific destinations
        Transport.path_request_destination = RNS.Destination(None, RNS.Destination.IN, RNS.Destination.PLAIN, Transport.APP_NAME, "path", "request")
        Transport.path_request_destination.set_packet_callback(Transport.path_request_handler)
        Transport.control_destinations.append(Transport.path_request_destination)
        Transport.control_hashes.append(Transport.path_request_destination.hash)

        Transport.tunnel_synthesize_destination = RNS.Destination(None, RNS.Destination.IN, RNS.Destination.PLAIN, Transport.APP_NAME, "tunnel", "synthesize")
        Transport.tunnel_synthesize_destination.set_packet_callback(Transport.tunnel_synthesize_handler)
        Transport.control_destinations.append(Transport.tunnel_synthesize_handler)
        Transport.control_hashes.append(Transport.tunnel_synthesize_destination.hash)

        thread = threading.Thread(target=Transport.jobloop)
        thread.setDaemon(True)
        thread.start()

        if RNS.Reticulum.transport_enabled():
            destination_table_path = RNS.Reticulum.storagepath+"/destination_table"
            tunnel_table_path = RNS.Reticulum.storagepath+"/tunnels"

            if os.path.isfile(destination_table_path) and not Transport.owner.is_connected_to_shared_instance:
                serialised_destinations = []
                try:
                    file = open(destination_table_path, "rb")
                    serialised_destinations = umsgpack.unpackb(file.read())
                    file.close()

                    for serialised_entry in serialised_destinations:
                        destination_hash = serialised_entry[0]
                        timestamp = serialised_entry[1]
                        received_from = serialised_entry[2]
                        hops = serialised_entry[3]
                        expires = serialised_entry[4]
                        random_blobs = serialised_entry[5]
                        receiving_interface = Transport.find_interface_from_hash(serialised_entry[6])
                        announce_packet = Transport.get_cached_packet(serialised_entry[7])

                        if announce_packet != None and receiving_interface != None:
                            announce_packet.unpack()
                            # We increase the hops, since reading a packet
                            # from cache is equivalent to receiving it again
                            # over an interface. It is cached with it's non-
                            # increased hop-count.
                            announce_packet.hops += 1
                            Transport.destination_table[destination_hash] = [timestamp, received_from, hops, expires, random_blobs, receiving_interface, announce_packet]
                            RNS.log("Loaded path table entry for "+RNS.prettyhexrep(destination_hash)+" from storage", RNS.LOG_DEBUG)
                        else:
                            RNS.log("Could not reconstruct path table entry from storage for "+RNS.prettyhexrep(destination_hash), RNS.LOG_DEBUG)
                            if announce_packet == None:
                                RNS.log("The announce packet could not be loaded from cache", RNS.LOG_DEBUG)
                            if receiving_interface == None:
                                RNS.log("The interface is no longer available", RNS.LOG_DEBUG)

                    if len(Transport.destination_table) == 1:
                        specifier = "entry"
                    else:
                        specifier = "entries"

                    RNS.log("Loaded "+str(len(Transport.destination_table))+" path table "+specifier+" from storage", RNS.LOG_VERBOSE)

                except Exception as e:
                    RNS.log("Could not load destination table from storage, the contained exception was: "+str(e), RNS.LOG_ERROR)

            if os.path.isfile(tunnel_table_path) and not Transport.owner.is_connected_to_shared_instance:
                serialised_tunnels = []
                try:
                    file = open(tunnel_table_path, "rb")
                    serialised_tunnels = umsgpack.unpackb(file.read())
                    file.close()

                    for serialised_tunnel in serialised_tunnels:
                        tunnel_id = serialised_tunnel[0]
                        interface_hash = serialised_tunnel[1]
                        serialised_paths = serialised_tunnel[2]
                        expires = serialised_tunnel[3]

                        tunnel_paths = {}
                        for serialised_entry in serialised_paths:
                            destination_hash = serialised_entry[0]
                            timestamp = serialised_entry[1]
                            received_from = serialised_entry[2]
                            hops = serialised_entry[3]
                            expires = serialised_entry[4]
                            random_blobs = serialised_entry[5]
                            receiving_interface = Transport.find_interface_from_hash(serialised_entry[6])
                            announce_packet = Transport.get_cached_packet(serialised_entry[7])

                            if announce_packet != None:
                                announce_packet.unpack()
                                # We increase the hops, since reading a packet
                                # from cache is equivalent to receiving it again
                                # over an interface. It is cached with it's non-
                                # increased hop-count.
                                announce_packet.hops += 1

                                tunnel_path = [timestamp, received_from, hops, expires, random_blobs, receiving_interface, announce_packet]
                                tunnel_paths[destination_hash] = tunnel_path

                        tunnel = [tunnel_id, None, tunnel_paths, expires]
                        Transport.tunnels[tunnel_id] = tunnel

                    if len(Transport.destination_table) == 1:
                        specifier = "entry"
                    else:
                        specifier = "entries"

                    RNS.log("Loaded "+str(len(Transport.tunnels))+" tunnel table "+specifier+" from storage", RNS.LOG_VERBOSE)

                except Exception as e:
                    RNS.log("Could not load tunnel table from storage, the contained exception was: "+str(e), RNS.LOG_ERROR)



            RNS.log("Transport instance "+str(Transport.identity)+" started", RNS.LOG_VERBOSE)

        # Synthesize tunnels for any interfaces wanting it
        for interface in Transport.interfaces:
            interface.tunnel_id = None
            if hasattr(interface, "wants_tunnel") and interface.wants_tunnel:
                Transport.synthesize_tunnel(interface)

    @staticmethod
    def jobloop():
        while (True):
            Transport.jobs()
            sleep(Transport.job_interval)

    @staticmethod
    def jobs():
        outgoing = []
        Transport.jobs_running = True
        try:
            if not Transport.jobs_locked:
                # Process receipts list for timed-out packets
                if time.time() > Transport.receipts_last_checked+Transport.receipts_check_interval:
                    while len(Transport.receipts) > Transport.MAX_RECEIPTS:
                        culled_receipt = Transport.receipts.pop(0)
                        culled_receipt.timeout = -1
                        culled_receipt.check_timeout()

                    for receipt in Transport.receipts:
                        receipt.check_timeout()
                        if receipt.status != RNS.PacketReceipt.SENT:
                            Transport.receipts.remove(receipt)

                    Transport.receipts_last_checked = time.time()

                # Process announces needing retransmission
                if time.time() > Transport.announces_last_checked+Transport.announces_check_interval:
                    for destination_hash in Transport.announce_table:
                        announce_entry = Transport.announce_table[destination_hash]
                        if announce_entry[2] > Transport.PATHFINDER_R:
                            RNS.log("Dropping announce for "+RNS.prettyhexrep(destination_hash)+", retries exceeded", RNS.LOG_DEBUG)
                            Transport.announce_table.pop(destination_hash)
                            break
                        else:
                            if time.time() > announce_entry[1]:
                                announce_entry[1] = time.time() + math.pow(Transport.PATHFINDER_C, announce_entry[4]) + Transport.PATHFINDER_T + Transport.PATHFINDER_RW
                                announce_entry[2] += 1
                                packet = announce_entry[5]
                                block_rebroadcasts = announce_entry[7]
                                attached_interface = announce_entry[8]
                                announce_context = RNS.Packet.NONE
                                if block_rebroadcasts:
                                    announce_context = RNS.Packet.PATH_RESPONSE
                                announce_data = packet.data
                                announce_identity = RNS.Identity.recall(packet.destination_hash)
                                announce_destination = RNS.Destination(announce_identity, RNS.Destination.OUT, RNS.Destination.SINGLE, "unknown", "unknown");
                                announce_destination.hash = packet.destination_hash
                                announce_destination.hexhash = announce_destination.hash.hex()
                                
                                new_packet = RNS.Packet(
                                    announce_destination,
                                    announce_data,
                                    RNS.Packet.ANNOUNCE,
                                    context = announce_context,
                                    header_type = RNS.Packet.HEADER_2,
                                    transport_type = Transport.TRANSPORT,
                                    transport_id = Transport.identity.hash,
                                    attached_interface = attached_interface
                                )

                                new_packet.hops = announce_entry[4]
                                if block_rebroadcasts:
                                    RNS.log("Rebroadcasting announce as path response for "+RNS.prettyhexrep(announce_destination.hash)+" with hop count "+str(new_packet.hops), RNS.LOG_DEBUG)
                                else:
                                    RNS.log("Rebroadcasting announce for "+RNS.prettyhexrep(announce_destination.hash)+" with hop count "+str(new_packet.hops), RNS.LOG_DEBUG)
                                outgoing.append(new_packet)

                                # This handles an edge case where a peer sends a past
                                # request for a destination just after an announce for
                                # said destination has arrived, but before it has been
                                # rebroadcast locally. In such a case the actual announce
                                # is temporarily held, and then reinserted when the path
                                # request has been served to the peer.
                                if destination_hash in Transport.held_announces:
                                    held_entry = Transport.held_announces.pop(destination_hash)
                                    Transport.announce_table[destination_hash] = held_entry
                                    RNS.log("Reinserting held announce into table", RNS.LOG_DEBUG)

                    Transport.announces_last_checked = time.time()


                # Cull the packet hashlist if it has reached max size
                if len(Transport.packet_hashlist) > Transport.hashlist_maxsize:
                    Transport.packet_hashlist = Transport.packet_hashlist[len(Transport.packet_hashlist)-Transport.hashlist_maxsize:len(Transport.packet_hashlist)-1]

                if time.time() > Transport.tables_last_culled + Transport.tables_cull_interval:
                    # Cull the reverse table according to timeout
                    stale_reverse_entries = []
                    for truncated_packet_hash in Transport.reverse_table:
                        reverse_entry = Transport.reverse_table[truncated_packet_hash]
                        if time.time() > reverse_entry[2] + Transport.REVERSE_TIMEOUT:
                            stale_reverse_entries.append(truncated_packet_hash)

                    # Cull the link table according to timeout
                    stale_links = []
                    for link_id in Transport.link_table:
                        link_entry = Transport.link_table[link_id]
                        if time.time() > link_entry[0] + Transport.LINK_TIMEOUT:
                            stale_links.append(link_id)

                    # Cull the path table
                    stale_paths = []
                    for destination_hash in Transport.destination_table:
                        destination_entry = Transport.destination_table[destination_hash]
                        attached_interface = destination_entry[5]

                        if time.time() > destination_entry[0] + Transport.DESTINATION_TIMEOUT:
                            stale_paths.append(destination_hash)
                            RNS.log("Path to "+RNS.prettyhexrep(destination_hash)+" timed out and was removed", RNS.LOG_DEBUG)
                        elif not attached_interface in Transport.interfaces:
                            stale_paths.append(destination_hash)
                            RNS.log("Path to "+RNS.prettyhexrep(destination_hash)+" was removed since the attached interface no longer exists", RNS.LOG_DEBUG)

                    # Cull the tunnel table
                    stale_tunnels = []
                    ti = 0
                    for tunnel_id in Transport.tunnels:
                        tunnel_entry = Transport.tunnels[tunnel_id]

                        expires = tunnel_entry[3]
                        if time.time() > expires:
                            stale_tunnels.append(tunnel_id)
                            RNS.log("Tunnel "+RNS.prettyhexrep(tunnel_id)+" timed out and was removed", RNS.LOG_DEBUG)
                        else:
                            stale_tunnel_paths = []
                            tunnel_paths = tunnel_entry[2]
                            for tunnel_path in tunnel_paths:
                                tunnel_path_entry = tunnel_paths[tunnel_path]

                                if time.time() > tunnel_path_entry[0] + Transport.DESTINATION_TIMEOUT:
                                    stale_tunnel_paths.append(tunnel_path)
                                    RNS.log("Tunnel path to "+RNS.prettyhexrep(tunnel_path)+" timed out and was removed", RNS.LOG_DEBUG)

                            for tunnel_path in stale_tunnel_paths:
                                tunnel_paths.pop(tunnel_path)
                                ti += 1


                    if ti > 0:
                        if ti == 1:
                            RNS.log("Removed "+str(ti)+" tunnel path", RNS.LOG_DEBUG)
                        else:
                            RNS.log("Removed "+str(ti)+" tunnel paths", RNS.LOG_DEBUG)


                    
                    i = 0
                    for truncated_packet_hash in stale_reverse_entries:
                        Transport.reverse_table.pop(truncated_packet_hash)
                        i += 1

                    if i > 0:
                        if i == 1:
                            RNS.log("Dropped "+str(i)+" reverse table entry", RNS.LOG_DEBUG)
                        else:
                            RNS.log("Dropped "+str(i)+" reverse table entries", RNS.LOG_DEBUG)

                    

                    i = 0
                    for link_id in stale_links:
                        Transport.link_table.pop(link_id)
                        i += 1

                    if i > 0:
                        if i == 1:
                            RNS.log("Dropped "+str(i)+" link", RNS.LOG_DEBUG)
                        else:
                            RNS.log("Dropped "+str(i)+" links", RNS.LOG_DEBUG)

                    i = 0
                    for destination_hash in stale_paths:
                        Transport.destination_table.pop(destination_hash)
                        i += 1

                    if i > 0:
                        if i == 1:
                            RNS.log("Removed "+str(i)+" path", RNS.LOG_DEBUG)
                        else:
                            RNS.log("Removed "+str(i)+" paths", RNS.LOG_DEBUG)

                    i = 0
                    for tunnel_id in stale_tunnels:
                        Transport.tunnels.pop(tunnel_id)
                        i += 1

                    if i > 0:
                        if i == 1:
                            RNS.log("Removed "+str(i)+" tunnel", RNS.LOG_DEBUG)
                        else:
                            RNS.log("Removed "+str(i)+" tunnels", RNS.LOG_DEBUG)

                    Transport.tables_last_culled = time.time()

        except Exception as e:
            RNS.log("An exception occurred while running Transport jobs.", RNS.LOG_ERROR)
            RNS.log("The contained exception was: "+str(e), RNS.LOG_ERROR)
            traceback.print_exc()

        Transport.jobs_running = False

        for packet in outgoing:
            packet.send()

    @staticmethod
    def outbound(packet):
        while (Transport.jobs_running):
            sleep(0.01)

        Transport.jobs_locked = True
        # TODO: This updateHash call might be redundant
        packet.update_hash()
        sent = False

        # Check if we have a known path for the destination in the path table
        if packet.packet_type != RNS.Packet.ANNOUNCE and packet.destination_hash in Transport.destination_table:
            outbound_interface = Transport.destination_table[packet.destination_hash][5]

            # If there's more than one hop to the destination, and we know
            # a path, we insert the packet into transport by adding the next
            # transport nodes address to the header, and modifying the flags.
            # This rule applies both for "normal" transport, and when connected
            # to a local shared Reticulum instance.
            if Transport.destination_table[packet.destination_hash][2] > 1:
                if packet.header_type == RNS.Packet.HEADER_1:
                    # Insert packet into transport
                    new_flags = (RNS.Packet.HEADER_2) << 6 | (Transport.TRANSPORT) << 4 | (packet.flags & 0b00001111)
                    new_raw = struct.pack("!B", new_flags)
                    new_raw += packet.raw[1:2]
                    new_raw += Transport.destination_table[packet.destination_hash][1]
                    new_raw += packet.raw[2:]
                    outbound_interface.processOutgoing(new_raw)
                    Transport.destination_table[packet.destination_hash][0] = time.time()
                    sent = True

            # In the special case where we are connected to a local shared
            # Reticulum instance, and the destination is one hop away, we
            # also add transport headers to inject the packet into transport
            # via the shared instance. Normally a packet for a destination
            # one hop away would just be broadcast directly, but since we
            # are "behind" a shared instance, we need to get that instance
            # to transport it onto the network.
            elif Transport.destination_table[packet.destination_hash][2] == 1 and Transport.owner.is_connected_to_shared_instance:
                if packet.header_type == RNS.Packet.HEADER_1:
                    # Insert packet into transport
                    new_flags = (RNS.Packet.HEADER_2) << 6 | (Transport.TRANSPORT) << 4 | (packet.flags & 0b00001111)
                    new_raw = struct.pack("!B", new_flags)
                    new_raw += packet.raw[1:2]
                    new_raw += Transport.destination_table[packet.destination_hash][1]
                    new_raw += packet.raw[2:]
                    outbound_interface.processOutgoing(new_raw)
                    Transport.destination_table[packet.destination_hash][0] = time.time()
                    sent = True

            # If none of the above applies, we know the destination is
            # directly reachable, and also on which interface, so we
            # simply transmit the packet directly on that one.
            else:
                outbound_interface.processOutgoing(packet.raw)
                sent = True

        # If we don't have a known path for the destination, we'll
        # broadcast the packet on all outgoing interfaces, or the
        # just the relevant interface if the packet has an attached
        # interface, or belongs to a link.
        else:
            stored_hash = False
            for interface in Transport.interfaces:
                if interface.OUT:
                    should_transmit = True
                    if packet.destination.type == RNS.Destination.LINK:
                        if packet.destination.status == RNS.Link.CLOSED:
                            should_transmit = False
                        if interface != packet.destination.attached_interface:
                            should_transmit = False
                    if packet.attached_interface != None and interface != packet.attached_interface:
                        should_transmit = False
                            
                    if should_transmit:
                        if not stored_hash:
                            Transport.packet_hashlist.append(packet.packet_hash)
                            stored_hash = True

                        interface.processOutgoing(packet.raw)
                        sent = True

        if sent:
            packet.sent = True
            packet.sent_at = time.time()

            # Don't generate receipt if it has been explicitly disabled
            if (packet.create_receipt == True and
                # Only generate receipts for DATA packets
                packet.packet_type == RNS.Packet.DATA and
                # Don't generate receipts for PLAIN destinations
                packet.destination.type != RNS.Destination.PLAIN and
                # Don't generate receipts for link-related packets
                not (packet.context >= RNS.Packet.KEEPALIVE and packet.context <= RNS.Packet.LRPROOF) and
                # Don't generate receipts for resource packets
                not (packet.context >= RNS.Packet.RESOURCE and packet.context <= RNS.Packet.RESOURCE_RCL)):

                packet.receipt = RNS.PacketReceipt(packet)
                Transport.receipts.append(packet.receipt)
            
            Transport.cache(packet)

        Transport.jobs_locked = False
        return sent

    @staticmethod
    def packet_filter(packet):
        # TODO: Think long and hard about this.
        # Is it even strictly necessary with the current
        # transport rules?
        if packet.context == RNS.Packet.KEEPALIVE:
            return True
        if packet.context == RNS.Packet.RESOURCE_REQ:
            return True
        if packet.context == RNS.Packet.RESOURCE_PRF:
            return True
        if packet.context == RNS.Packet.RESOURCE:
            return True
        if packet.context == RNS.Packet.CACHE_REQUEST:
            return True
        if packet.destination_type == RNS.Destination.PLAIN:
            return True

        if not packet.packet_hash in Transport.packet_hashlist:
            return True
        else:
            if packet.packet_type == RNS.Packet.ANNOUNCE:
                return True

        RNS.log("Filtered packet with hash "+RNS.prettyhexrep(packet.packet_hash), RNS.LOG_DEBUG)
        return False

    @staticmethod
    def inbound(raw, interface=None):
        while (Transport.jobs_running):
            sleep(0.01)
            
        Transport.jobs_locked = True
        
        packet = RNS.Packet(None, raw)
        packet.unpack()
        packet.receiving_interface = interface
        packet.hops += 1

        if len(Transport.local_client_interfaces) > 0:

            if Transport.is_local_client_interface(interface):
                packet.hops -= 1
        elif Transport.interface_to_shared_instance(interface):
            packet.hops -= 1


        if Transport.packet_filter(packet):
            Transport.packet_hashlist.append(packet.packet_hash)
            Transport.cache(packet)
            
            # Check special conditions for local clients connected
            # through a shared Reticulum instance
            from_local_client         = (packet.receiving_interface in Transport.local_client_interfaces)
            for_local_client          = (packet.packet_type != RNS.Packet.ANNOUNCE) and (packet.destination_hash in Transport.destination_table and Transport.destination_table[packet.destination_hash][2] == 0)
            for_local_client_link     = (packet.packet_type != RNS.Packet.ANNOUNCE) and (packet.destination_hash in Transport.link_table and Transport.link_table[packet.destination_hash][4] in Transport.local_client_interfaces)
            for_local_client_link    |= (packet.packet_type != RNS.Packet.ANNOUNCE) and (packet.destination_hash in Transport.link_table and Transport.link_table[packet.destination_hash][2] in Transport.local_client_interfaces)
            proof_for_local_client    = (packet.destination_hash in Transport.reverse_table) and (Transport.reverse_table[packet.destination_hash][0] in Transport.local_client_interfaces)

            # Plain broadcast packets from local clients are sent
            # directly on all attached interfaces, since they are
            # never injected into transport.
            if not packet.destination_hash in Transport.control_hashes:
                if packet.destination_type == RNS.Destination.PLAIN and packet.transport_type == Transport.BROADCAST:
                    # Send to all interfaces except the originator
                    if from_local_client:
                        for interface in Transport.interfaces:
                            if interface != packet.receiving_interface:
                                interface.processOutgoing(packet.raw)
                    # If the packet was not from a local client, send
                    # it directly to all local clients
                    else:
                        for interface in Transport.local_client_interfaces:
                            interface.processOutgoing(packet.raw)


            # General transport handling. Takes care of directing
            # packets according to transport tables and recording
            # entries in reverse and link tables.
            if RNS.Reticulum.transport_enabled() or from_local_client or for_local_client or for_local_client_link:

                # If there is no transport id, but the packet is
                # for a local client, we generate the transport
                # id (it was stripped on the previous hop, since
                # we "spoof" the hop count for clients behind a
                # shared instance, so they look directly reach-
                # able), and reinsert, so the normal transport
                # implementation can handle the packet.
                if packet.transport_id == None and for_local_client:
                    packet.transport_id = Transport.identity.hash

                # If this is a cache request, and we can fullfill
                # it, do so and stop processing. Otherwise resume
                # normal processing.
                if packet.context == RNS.Packet.CACHE_REQUEST:
                    if Transport.cache_request_packet(packet):
                        return

                # If the packet is in transport, check whether we
                # are the designated next hop, and process it
                # accordingly if we are.
                if packet.transport_id != None and packet.packet_type != RNS.Packet.ANNOUNCE:
                    if packet.transport_id == Transport.identity.hash:
                        if packet.destination_hash in Transport.destination_table:
                            next_hop = Transport.destination_table[packet.destination_hash][1]
                            remaining_hops = Transport.destination_table[packet.destination_hash][2]
                            
                            if remaining_hops > 1:
                                # Just increase hop count and transmit
                                new_raw  = packet.raw[0:1]
                                new_raw += struct.pack("!B", packet.hops)
                                new_raw += next_hop
                                new_raw += packet.raw[12:]
                            elif remaining_hops == 1:
                                # Strip transport headers and transmit
                                new_flags = (RNS.Packet.HEADER_1) << 6 | (Transport.BROADCAST) << 4 | (packet.flags & 0b00001111)
                                new_raw = struct.pack("!B", new_flags)
                                new_raw += struct.pack("!B", packet.hops)
                                new_raw += packet.raw[12:]
                            elif remaining_hops == 0:
                                # Just increase hop count and transmit
                                new_raw  = packet.raw[0:1]
                                new_raw += struct.pack("!B", packet.hops)
                                new_raw += packet.raw[2:]

                            outbound_interface = Transport.destination_table[packet.destination_hash][5]
                            outbound_interface.processOutgoing(new_raw)
                            Transport.destination_table[packet.destination_hash][0] = time.time()

                            if packet.packet_type == RNS.Packet.LINKREQUEST:
                                # Entry format is
                                link_entry = [  time.time(),                    # 0: Timestamp,
                                                next_hop,                       # 1: Next-hop transport ID
                                                outbound_interface,             # 2: Next-hop interface
                                                remaining_hops,                 # 3: Remaining hops
                                                packet.receiving_interface,     # 4: Received on interface
                                                packet.hops,                    # 5: Taken hops
                                                packet.destination_hash,        # 6: Original destination hash
                                                False]                          # 7: Validated

                                Transport.link_table[packet.getTruncatedHash()] = link_entry

                            else:
                                # Entry format is
                                reverse_entry = [   packet.receiving_interface, # 0: Received on interface
                                                    outbound_interface,         # 1: Outbound interface
                                                    time.time()]                # 2: Timestamp

                                Transport.reverse_table[packet.getTruncatedHash()] = reverse_entry

                        else:
                            # TODO: There should probably be some kind of REJECT
                            # mechanism here, to signal to the source that their
                            # expected path failed.
                            RNS.log("Got packet in transport, but no known path to final destination "+RNS.prettyhexrep(packet.destination_hash)+". Dropping packet.", RNS.LOG_DEBUG)

                # Link transport handling. Directs packets according
                # to entries in the link tables
                if packet.packet_type != RNS.Packet.ANNOUNCE and packet.packet_type != RNS.Packet.LINKREQUEST and packet.context != RNS.Packet.LRPROOF:
                    if packet.destination_hash in Transport.link_table:
                        link_entry = Transport.link_table[packet.destination_hash]
                        # If receiving and outbound interface is
                        # the same for this link, direction doesn't
                        # matter, and we simply send the packet on.
                        outbound_interface = None
                        if link_entry[2] == link_entry[4]:
                            # But check that taken hops matches one
                            # of the expectede values.
                            if packet.hops == link_entry[3] or packet.hops == link_entry[5]:
                                outbound_interface = link_entry[2]
                        else:
                            # If interfaces differ, we transmit on
                            # the opposite interface of what the
                            # packet was received on.
                            if packet.receiving_interface == link_entry[2]:
                                # Also check that expected hop count matches
                                if packet.hops == link_entry[3]:
                                    outbound_interface = link_entry[4]
                            elif packet.receiving_interface == link_entry[4]:
                                # Also check that expected hop count matches
                                if packet.hops == link_entry[5]:
                                    outbound_interface = link_entry[2]
                            
                        if outbound_interface != None:
                            new_raw = packet.raw[0:1]
                            new_raw += struct.pack("!B", packet.hops)
                            new_raw += packet.raw[2:]
                            outbound_interface.processOutgoing(new_raw)
                            Transport.link_table[packet.destination_hash][0] = time.time()
                        else:
                            pass


            # Announce handling. Handles logic related to incoming
            # announces, queueing rebroadcasts of these, and removal
            # of queued announce rebroadcasts once handed to the next node.
            if packet.packet_type == RNS.Packet.ANNOUNCE:
                local_destination = next((d for d in Transport.destinations if d.hash == packet.destination_hash), None)
                if local_destination == None and RNS.Identity.validate_announce(packet): 
                    if packet.transport_id != None:
                        received_from = packet.transport_id
                        
                        # Check if this is a next retransmission from
                        # another node. If it is, we're removing the
                        # announce in question from our pending table
                        if RNS.Reticulum.transport_enabled() and packet.destination_hash in Transport.announce_table:
                            announce_entry = Transport.announce_table[packet.destination_hash]
                            
                            if packet.hops-1 == announce_entry[4]:
                                RNS.log("Heard a local rebroadcast of announce for "+RNS.prettyhexrep(packet.destination_hash), RNS.LOG_DEBUG)
                                announce_entry[6] += 1
                                if announce_entry[6] >= Transport.LOCAL_REBROADCASTS_MAX:
                                    RNS.log("Max local rebroadcasts of announce for "+RNS.prettyhexrep(packet.destination_hash)+" reached, dropping announce from our table", RNS.LOG_DEBUG)
                                    Transport.announce_table.pop(packet.destination_hash)

                            if packet.hops-1 == announce_entry[4]+1 and announce_entry[2] > 0:
                                now = time.time()
                                if now < announce_entry[1]:
                                    RNS.log("Rebroadcasted announce for "+RNS.prettyhexrep(packet.destination_hash)+" has been passed on to next node, no further tries needed", RNS.LOG_DEBUG)
                                    Transport.announce_table.pop(packet.destination_hash)

                    else:
                        received_from = packet.destination_hash

                    # Check if this announce should be inserted into
                    # announce and destination tables
                    should_add = False

                    # First, check that the announce is not for a destination
                    # local to this system, and that hops are less than the max
                    if (not any(packet.destination_hash == d.hash for d in Transport.destinations) and packet.hops < Transport.PATHFINDER_M+1):
                        random_blob = packet.data[RNS.Identity.KEYSIZE//8:RNS.Identity.KEYSIZE//8+RNS.Reticulum.TRUNCATED_HASHLENGTH//8]
                        announce_emitted = int.from_bytes(random_blob[5:10], "big")
                        random_blobs = []
                        if packet.destination_hash in Transport.destination_table:
                            random_blobs = Transport.destination_table[packet.destination_hash][4]

                            # If we already have a path to the announced
                            # destination, but the hop count is equal or
                            # less, we'll update our tables.
                            if packet.hops <= Transport.destination_table[packet.destination_hash][2]:
                                # Make sure we haven't heard the random
                                # blob before, so announces can't be
                                # replayed to forge paths.
                                # TODO: Check whether this approach works
                                # under all circumstances
                                if not random_blob in random_blobs:
                                    should_add = True
                                else:
                                    should_add = False
                            else:
                                # If an announce arrives with a larger hop
                                # count than we already have in the table,
                                # ignore it, unless the path is expired, or
                                # the emission timestamp is more recent.
                                now = time.time()
                                path_expires = Transport.destination_table[packet.destination_hash][3]
                                
                                path_announce_emitted = 0
                                for path_random_blob in random_blobs:
                                    path_announce_emitted = max(path_announce_emitted, int.from_bytes(path_random_blob[5:10], "big"))
                                    if path_announce_emitted >= announce_emitted:
                                        break

                                if (now >= path_expires):
                                    # We also check that the announce hash is
                                    # different from ones we've already heard,
                                    # to avoid loops in the network
                                    if not random_blob in random_blobs:
                                        # TODO: Check that this ^ approach actually
                                        # works under all circumstances
                                        RNS.log("Replacing destination table entry for "+str(RNS.prettyhexrep(packet.destination_hash))+" with new announce due to expired path", RNS.LOG_DEBUG)
                                        should_add = True
                                    else:
                                        should_add = False
                                else:
                                    if (announce_emitted > path_announce_emitted):
                                        if not random_blob in random_blobs:
                                            RNS.log("Replacing destination table entry for "+str(RNS.prettyhexrep(packet.destination_hash))+" with new announce, since it was more recently emitted", RNS.LOG_DEBUG)
                                            should_add = True
                                        else:
                                            should_add = False

                        else:
                            # If this destination is unknown in our table
                            # we should add it
                            should_add = True

                        if should_add:
                            now                = time.time()
                            retries            = 0
                            expires            = now + Transport.PATHFINDER_E
                            announce_hops      = packet.hops
                            local_rebroadcasts = 0
                            block_rebroadcasts = False
                            attached_interface = None
                            retransmit_timeout = now + math.pow(Transport.PATHFINDER_C, packet.hops) + (RNS.rand() * Transport.PATHFINDER_RW)
                            
                            random_blobs.append(random_blob)

                            if (RNS.Reticulum.transport_enabled() or Transport.from_local_client(packet)) and packet.context != RNS.Packet.PATH_RESPONSE:
                                # If the announce is from a local client,
                                # we announce it immediately, but only one
                                # time.

                                if Transport.from_local_client(packet):
                                    retransmit_timeout = now
                                    retries = Transport.PATHFINDER_R

                                Transport.announce_table[packet.destination_hash] = [
                                    now,
                                    retransmit_timeout,
                                    retries,
                                    received_from,
                                    announce_hops,
                                    packet,
                                    local_rebroadcasts,
                                    block_rebroadcasts,
                                    attached_interface
                                ]

                            # If we have any local clients connected, we re-
                            # transmit the announce to them immediately
                            if (len(Transport.local_client_interfaces)):
                                announce_identity = RNS.Identity.recall(packet.destination_hash)
                                announce_destination = RNS.Destination(announce_identity, RNS.Destination.OUT, RNS.Destination.SINGLE, "unknown", "unknown");
                                announce_destination.hash = packet.destination_hash
                                announce_destination.hexhash = announce_destination.hash.hex()
                                announce_context = RNS.Packet.NONE
                                announce_data = packet.data

                                if Transport.from_local_client(packet) and packet.context == RNS.Packet.PATH_RESPONSE:
                                    for local_interface in Transport.local_client_interfaces:
                                        if packet.receiving_interface != local_interface:
                                            new_announce = RNS.Packet(
                                                announce_destination,
                                                announce_data,
                                                RNS.Packet.ANNOUNCE,
                                                context = announce_context,
                                                header_type = RNS.Packet.HEADER_2,
                                                transport_type = Transport.TRANSPORT,
                                                transport_id = Transport.identity.hash,
                                                attached_interface = local_interface
                                            )
                                            
                                            new_announce.hops = packet.hops
                                            new_announce.send()

                                else:
                                    for local_interface in Transport.local_client_interfaces:
                                        if packet.receiving_interface != local_interface:
                                            new_announce = RNS.Packet(
                                                announce_destination,
                                                announce_data,
                                                RNS.Packet.ANNOUNCE,
                                                context = announce_context,
                                                header_type = RNS.Packet.HEADER_2,
                                                transport_type = Transport.TRANSPORT,
                                                transport_id = Transport.identity.hash,
                                                attached_interface = local_interface
                                            )

                                            new_announce.hops = packet.hops
                                            new_announce.send()

                            destination_table_entry = [now, received_from, announce_hops, expires, random_blobs, packet.receiving_interface, packet]
                            Transport.destination_table[packet.destination_hash] = destination_table_entry
                            RNS.log("Path to "+RNS.prettyhexrep(packet.destination_hash)+" is now "+str(announce_hops)+" hops away via "+RNS.prettyhexrep(received_from)+" on "+str(packet.receiving_interface), RNS.LOG_VERBOSE)

                            # If the receiving interface is a tunnel, we add the
                            # announce to the tunnels table
                            if hasattr(packet.receiving_interface, "tunnel_id") and packet.receiving_interface.tunnel_id != None:
                                tunnel_entry = Transport.tunnels[packet.receiving_interface.tunnel_id]
                                paths = tunnel_entry[2]
                                paths[packet.destination_hash] = destination_table_entry
                                expires = time.time() + Transport.DESTINATION_TIMEOUT
                                tunnel_entry[3] = expires
                                RNS.log("Path to "+RNS.prettyhexrep(packet.destination_hash)+" associated with tunnel "+RNS.prettyhexrep(packet.receiving_interface.tunnel_id), RNS.LOG_VERBOSE)

                            # Call externally registered callbacks from apps
                            # wanting to know when an announce arrives
                            if packet.context != RNS.Packet.PATH_RESPONSE:
                                for handler in Transport.announce_handlers:
                                    try:
                                        # Check that the announced destination matches
                                        # the handlers aspect filter
                                        execute_callback = False
                                        if handler.aspect_filter == None:
                                            # If the handlers aspect filter is set to
                                            # None, we execute the callback in all cases
                                            execute_callback = True
                                        else:
                                            announce_identity = RNS.Identity.recall(packet.destination_hash)
                                            handler_expected_hash = RNS.Destination.hash_from_name_and_identity(handler.aspect_filter, announce_identity)
                                            if packet.destination_hash == handler_expected_hash:
                                                execute_callback = True
                                        if execute_callback:
                                            handler.received_announce(
                                                destination_hash=packet.destination_hash,
                                                announced_identity=announce_identity,
                                                app_data=RNS.Identity.recall_app_data(packet.destination_hash)
                                            )
                                    except Exception as e:
                                        RNS.log("Error while processing external announce callback.", RNS.LOG_ERROR)
                                        RNS.log("The contained exception was: "+str(e), RNS.LOG_ERROR)

            # Handling for linkrequests to local destinations
            elif packet.packet_type == RNS.Packet.LINKREQUEST:
                for destination in Transport.destinations:
                    if destination.hash == packet.destination_hash and destination.type == packet.destination_type:
                        packet.destination = destination
                        destination.receive(packet)
            
            # Handling for local data packets
            elif packet.packet_type == RNS.Packet.DATA:
                if packet.destination_type == RNS.Destination.LINK:
                    for link in Transport.active_links:
                        if link.link_id == packet.destination_hash:
                            packet.link = link
                            link.receive(packet)
                else:
                    for destination in Transport.destinations:
                        if destination.hash == packet.destination_hash and destination.type == packet.destination_type:
                            packet.destination = destination
                            destination.receive(packet)

                            if destination.proof_strategy == RNS.Destination.PROVE_ALL:
                                packet.prove()

                            elif destination.proof_strategy == RNS.Destination.PROVE_APP:
                                if destination.callbacks.proof_requested:
                                    if destination.callbacks.proof_requested(packet):
                                        packet.prove()

            # Handling for proofs and link-request proofs
            elif packet.packet_type == RNS.Packet.PROOF:
                if packet.context == RNS.Packet.LRPROOF:
                    # This is a link request proof, check if it
                    # needs to be transported
                    if (RNS.Reticulum.transport_enabled() or for_local_client_link or from_local_client) and packet.destination_hash in Transport.link_table:
                        link_entry = Transport.link_table[packet.destination_hash]
                        if packet.receiving_interface == link_entry[2]:
                            # TODO: Should we validate the LR proof at each transport
                            # step before transporting it?
                            RNS.log("Link request proof received on correct interface, transporting it via "+str(link_entry[4]), RNS.LOG_DEBUG)
                            new_raw = packet.raw[0:1]
                            new_raw += struct.pack("!B", packet.hops)
                            new_raw += packet.raw[2:]
                            Transport.link_table[packet.destination_hash][7] = True
                            link_entry[4].processOutgoing(new_raw)
                        else:
                            RNS.log("Link request proof received on wrong interface, not transporting it.", RNS.LOG_DEBUG)
                    else:
                        # Check if we can deliver it to a local
                        # pending link
                        for link in Transport.pending_links:
                            if link.link_id == packet.destination_hash:
                                link.validate_proof(packet)

                elif packet.context == RNS.Packet.RESOURCE_PRF:
                    for link in Transport.active_links:
                        if link.link_id == packet.destination_hash:
                            link.receive(packet)
                else:
                    if packet.destination_type == RNS.Destination.LINK:
                        for link in Transport.active_links:
                            if link.link_id == packet.destination_hash:
                                packet.link = link
                                
                    if len(packet.data) == RNS.PacketReceipt.EXPL_LENGTH:
                        proof_hash = packet.data[:RNS.Identity.HASHLENGTH//8]
                    else:
                        proof_hash = None

                    # Check if this proof neds to be transported
                    if (RNS.Reticulum.transport_enabled() or from_local_client or proof_for_local_client) and packet.destination_hash in Transport.reverse_table:
                        reverse_entry = Transport.reverse_table.pop(packet.destination_hash)
                        if packet.receiving_interface == reverse_entry[1]:
                            RNS.log("Proof received on correct interface, transporting it via "+str(reverse_entry[0]), RNS.LOG_DEBUG)
                            new_raw = packet.raw[0:1]
                            new_raw += struct.pack("!B", packet.hops)
                            new_raw += packet.raw[2:]
                            reverse_entry[0].processOutgoing(new_raw)
                        else:
                            RNS.log("Proof received on wrong interface, not transporting it.", RNS.LOG_DEBUG)

                    for receipt in Transport.receipts:
                        receipt_validated = False
                        if proof_hash != None:
                            # Only test validation if hash matches
                            if receipt.hash == proof_hash:
                                receipt_validated = receipt.validate_proof_packet(packet)
                        else:
                            # TODO: This looks like it should actually
                            # be rewritten when implicit proofs are added.

                            # In case of an implicit proof, we have
                            # to check every single outstanding receipt
                            receipt_validated = receipt.validate_proof_packet(packet)

                        if receipt_validated:
                            if receipt in Transport.receipts:
                                Transport.receipts.remove(receipt)

        Transport.jobs_locked = False

    @staticmethod
    def synthesize_tunnel(interface):
        interface_hash = interface.get_hash()
        public_key     = RNS.Transport.identity.get_public_key()
        random_hash    = RNS.Identity.get_random_hash()
        
        tunnel_id_data = public_key+interface_hash
        tunnel_id      = RNS.Identity.full_hash(tunnel_id_data)

        signed_data    = tunnel_id_data+random_hash
        signature      = Transport.identity.sign(signed_data)
        
        data           = signed_data+signature

        tnl_snth_dst   = RNS.Destination(None, RNS.Destination.OUT, RNS.Destination.PLAIN, Transport.APP_NAME, "tunnel", "synthesize")

        packet = RNS.Packet(tnl_snth_dst, data, packet_type = RNS.Packet.DATA, transport_type = RNS.Transport.BROADCAST, header_type = RNS.Packet.HEADER_1, attached_interface = interface)
        packet.send()

        interface.wants_tunnel = False

    @staticmethod
    def tunnel_synthesize_handler(data, packet):
        try:
            expected_length = RNS.Identity.KEYSIZE//8+RNS.Identity.HASHLENGTH//8+RNS.Reticulum.TRUNCATED_HASHLENGTH//8+RNS.Identity.SIGLENGTH//8
            if len(data) == expected_length:
                public_key     = data[:RNS.Identity.KEYSIZE//8]
                interface_hash = data[RNS.Identity.KEYSIZE//8:RNS.Identity.KEYSIZE//8+RNS.Identity.HASHLENGTH//8]
                tunnel_id_data = public_key+interface_hash
                tunnel_id      = RNS.Identity.full_hash(tunnel_id_data)
                random_hash    = data[RNS.Identity.KEYSIZE//8+RNS.Identity.HASHLENGTH//8:RNS.Identity.KEYSIZE//8+RNS.Identity.HASHLENGTH//8+RNS.Reticulum.TRUNCATED_HASHLENGTH//8]
                
                signature      = data[RNS.Identity.KEYSIZE//8+RNS.Identity.HASHLENGTH//8+RNS.Reticulum.TRUNCATED_HASHLENGTH//8:expected_length]
                signed_data    = tunnel_id_data+random_hash

                remote_transport_identity = RNS.Identity(create_keys=False)
                remote_transport_identity.load_public_key(public_key)

                if remote_transport_identity.validate(signature, signed_data):
                    Transport.handle_tunnel(tunnel_id, packet.receiving_interface)

        except Exception as e:
            RNS.log("An error occurred while validating tunnel establishment packet.", RNS.LOG_DEBUG)
            RNS.log("The contained exception was: "+str(e), RNS.LOG_DEBUG)

    @staticmethod
    def handle_tunnel(tunnel_id, interface):
        expires = time.time() + Transport.DESTINATION_TIMEOUT
        if not tunnel_id in Transport.tunnels:
            RNS.log("Tunnel endpoint "+RNS.prettyhexrep(tunnel_id)+" established.", RNS.LOG_DEBUG)
            paths = {}
            tunnel_entry = [tunnel_id, interface, paths, expires]
            interface.tunnel_id = tunnel_id
            Transport.tunnels[tunnel_id] = tunnel_entry
        else:
            RNS.log("Tunnel endpoint "+RNS.prettyhexrep(tunnel_id)+" reappeared. Restoring paths...", RNS.LOG_DEBUG)
            tunnel_entry = Transport.tunnels[tunnel_id]
            tunnel_entry[1] = interface
            tunnel_entry[3] = expires
            interface.tunnel_id = tunnel_id
            paths = tunnel_entry[2]

            deprecated_paths = []
            for destination_hash, path_entry in paths.items():
                received_from = path_entry[1]
                announce_hops = path_entry[2]
                expires = path_entry[3]
                random_blobs = path_entry[4]
                receiving_interface = interface
                packet = path_entry[6]
                new_entry = [time.time(), received_from, announce_hops, expires, random_blobs, receiving_interface, packet]

                should_add = False
                if destination_hash in Transport.destination_table:
                    old_entry = Transport.destination_table[destination_hash]
                    old_hops = old_entry[2]
                    old_expires = old_entry[3]
                    if announce_hops <= old_hops or time.time() > old_expires:
                        should_add = True
                    else:
                        RNS.log("Did not restore path to "+RNS.prettyhexrep(packet.destination_hash)+" because a newer path with fewer hops exist", RNS.LOG_DEBUG)
                else:
                    if time.time() < expires:
                        should_add = True
                    else:
                        RNS.log("Did not restore path to "+RNS.prettyhexrep(packet.destination_hash)+" because it has expired", RNS.LOG_DEBUG)

                if should_add:
                    Transport.destination_table[destination_hash] = new_entry
                    RNS.log("Restored path to "+RNS.prettyhexrep(packet.destination_hash)+" is now "+str(announce_hops)+" hops away via "+RNS.prettyhexrep(received_from)+" on "+str(receiving_interface), RNS.LOG_DEBUG)
                else:
                    deprecated_paths.append(destination_hash)

            for deprecated_path in deprecated_paths:
                RNS.log("Removing path to "+RNS.prettyhexrep(deprecated_path)+" from tunnel "+RNS.prettyhexrep(tunnel_id), RNS.LOG_DEBUG)
                paths.pop(deprecated_path)


                


    @staticmethod
    def register_destination(destination):
        destination.MTU = RNS.Reticulum.MTU
        if destination.direction == RNS.Destination.IN:
            for registered_destination in Transport.destinations:
                if destination.hash == registered_destination.hash:
                    raise KeyError("Attempt to register an already registered destination.")
            
            Transport.destinations.append(destination)

            if Transport.owner.is_connected_to_shared_instance:
                if destination.type == RNS.Destination.SINGLE:
                    destination.announce(path_response=True)

    @staticmethod
    def deregister_destination(destination):
        if destination in Transport.destinations:
            Transport.destinations.remove(destination)

    @staticmethod
    def register_link(link):
        RNS.log("Registering link "+str(link), RNS.LOG_DEBUG)
        if link.initiator:
            Transport.pending_links.append(link)
        else:
            Transport.active_links.append(link)

    @staticmethod
    def activate_link(link):
        RNS.log("Activating link "+str(link), RNS.LOG_DEBUG)
        if link in Transport.pending_links:
            Transport.pending_links.remove(link)
            Transport.active_links.append(link)
            link.status = RNS.Link.ACTIVE
        else:
            RNS.log("Attempted to activate a link that was not in the pending table", RNS.LOG_ERROR)

    @staticmethod
    def register_announce_handler(handler):
        """
        Registers an announce handler.

        :param handler: Must be an object with an *aspect_filter* attribute and a *received_announce(destination_hash, announced_identity, app_data)* callable. See the :ref:`Announce Example<example-announce>` for more info.
        """
        if hasattr(handler, "received_announce") and callable(handler.received_announce):
            if hasattr(handler, "aspect_filter"):
                Transport.announce_handlers.append(handler)

    @staticmethod
    def deregister_announce_handler(handler):
        """
        Deregisters an announce handler.

        :param handler: The announce handler to be deregistered.
        """
        while handler in Transport.announce_handlers:
            Transport.announce_handlers.remove(handler)

    @staticmethod
    def find_interface_from_hash(interface_hash):
        for interface in Transport.interfaces:
            if interface.get_hash() == interface_hash:
                return interface

        return None

    @staticmethod
    def should_cache(packet):
        if packet.context == RNS.Packet.RESOURCE_PRF:
            return True

        return False

    # When caching packets to storage, they are written
    # exactly as they arrived over their interface. This
    # means that they have not had their hop count
    # increased yet! Take note of this when reading from
    # the packet cache.
    @staticmethod
    def cache(packet, force_cache=False):
        if RNS.Transport.should_cache(packet) or force_cache:
            try:
                packet_hash = RNS.hexrep(packet.get_hash(), delimit=False)
                interface_reference = None
                if packet.receiving_interface != None:
                    interface_reference = str(packet.receiving_interface)

                file = open(RNS.Reticulum.cachepath+"/"+packet_hash, "wb")  
                file.write(umsgpack.packb([packet.raw, interface_reference]))
                file.close()

            except Exception as e:
                RNS.log("Error writing packet to cache", RNS.LOG_ERROR)
                RNS.log("The contained exception was: "+str(e))

    @staticmethod
    def get_cached_packet(packet_hash):
        try:
            packet_hash = RNS.hexrep(packet_hash, delimit=False)
            path = RNS.Reticulum.cachepath+"/"+packet_hash

            if os.path.isfile(path):
                file = open(path, "rb")
                cached_data = umsgpack.unpackb(file.read())
                file.close()

                packet = RNS.Packet(None, cached_data[0])
                interface_reference = cached_data[1]

                for interface in Transport.interfaces:
                    if str(interface) == interface_reference:
                        packet.receiving_interface = interface

                return packet
            else:
                return None
        except Exception as e:
            RNS.log("Exception occurred while getting cached packet.", RNS.LOG_ERROR)
            RNS.log("The contained exception was: "+str(e), RNS.LOG_ERROR)

    @staticmethod
    def cache_request_packet(packet):
        if len(packet.data) == RNS.Identity.HASHLENGTH/8:
            packet = Transport.get_cached_packet(packet.data)

            if packet != None:
                # If the packet was retrieved from the local
                # cache, replay it to the Transport instance,
                # so that it can be directed towards it original
                # destination.
                Transport.inbound(packet.raw, packet.receiving_interface)
                return True
            else:
                return False
        else:
            return False

    @staticmethod
    def cache_request(packet_hash, destination):
        cached_packet = Transport.get_cached_packet(packet_hash)
        if cached_packet:
            # The packet was found in the local cache,
            # replay it to the Transport instance.
            Transport.inbound(packet.raw, packet.receiving_interface)
        else:
            # The packet is not in the local cache,
            # query the network.
            RNS.Packet(destination, packet_hash, context = RNS.Packet.CACHE_REQUEST).send()

    @staticmethod
    def has_path(destination_hash):
        """
        :param destination_hash: A destination hash as *bytes*.
        :returns: *True* if a path to the destination is known, otherwise *False*.
        """
        if destination_hash in Transport.destination_table:
            return True
        else:
            return False

    @staticmethod
    def hops_to(destination_hash):
        """
        :param destination_hash: A destination hash as *bytes*.
        :returns: The number of hops to the specified destination, or ``RNS.Transport.PATHFINDER_M`` if the number of hops is unknown.
        """
        if destination_hash in Transport.destination_table:
            return Transport.destination_table[destination_hash][2]
        else:
            return Transport.PATHFINDER_M

    @staticmethod
    def next_hop(destination_hash):
        """
        :param destination_hash: A destination hash as *bytes*.
        :returns: The destination hash as *bytes* for the next hop to the specified destination, or *None* if the next hop is unknown.
        """
        if destination_hash in Transport.destination_table:
            return Transport.destination_table[destination_hash][1]
        else:
            return None

    @staticmethod
    def next_hop_interface(destination_hash):
        """
        :param destination_hash: A destination hash as *bytes*.
        :returns: The interface for the next hop to the specified destination, or *None* if the interface is unknown.
        """
        if destination_hash in Transport.destination_table:
            return Transport.destination_table[destination_hash][5]
        else:
            return None

    @staticmethod
    def request_path(destination_hash):
        """
        Requests a path to the destination from the network. If
        another reachable peer on the network knows a path, it
        will announce it.

        :param destination_hash: A destination hash as *bytes*.
        """
        path_request_data = destination_hash + RNS.Identity.get_random_hash()
        path_request_dst = RNS.Destination(None, RNS.Destination.OUT, RNS.Destination.PLAIN, Transport.APP_NAME, "path", "request")
        packet = RNS.Packet(path_request_dst, path_request_data, packet_type = RNS.Packet.DATA, transport_type = RNS.Transport.BROADCAST, header_type = RNS.Packet.HEADER_1)
        packet.send()

    @staticmethod
    def request_path_on_interface(destination_hash, interface):
        path_request_data = destination_hash + RNS.Identity.get_random_hash()
        path_request_dst = RNS.Destination(None, RNS.Destination.OUT, RNS.Destination.PLAIN, Transport.APP_NAME, "path", "request")
        packet = RNS.Packet(path_request_dst, path_request_data, packet_type = RNS.Packet.DATA, transport_type = RNS.Transport.BROADCAST, header_type = RNS.Packet.HEADER_1, attached_interface = interface)
        packet.send()

    @staticmethod
    def path_request_handler(data, packet):
        if len(data) >= RNS.Identity.TRUNCATED_HASHLENGTH//8:
            Transport.path_request(
                data[:RNS.Identity.TRUNCATED_HASHLENGTH//8],
                Transport.from_local_client(packet),
                packet.receiving_interface
            )

    @staticmethod
    def path_request(destination_hash, is_from_local_client, attached_interface):
        RNS.log("Path request for "+RNS.prettyhexrep(destination_hash), RNS.LOG_DEBUG)
        
        local_destination = next((d for d in Transport.destinations if d.hash == destination_hash), None)
        if local_destination != None:
            RNS.log("Destination is local to this system, announcing", RNS.LOG_DEBUG)
            local_destination.announce(path_response=True)

        elif (RNS.Reticulum.transport_enabled() or is_from_local_client or len(Transport.local_client_interfaces) > 0) and destination_hash in Transport.destination_table:
            RNS.log("Path found, inserting announce for transmission", RNS.LOG_DEBUG)
            packet = Transport.destination_table[destination_hash][6]
            received_from = Transport.destination_table[destination_hash][5]

            now = time.time()
            retries = Transport.PATHFINDER_R
            local_rebroadcasts = 0
            block_rebroadcasts = True
            announce_hops      = packet.hops

            if is_from_local_client:
                retransmit_timeout = now
            else:
                # TODO: Look at this timing
                retransmit_timeout = now + Transport.PATH_REQUEST_GRACE # + (RNS.rand() * Transport.PATHFINDER_RW)

            # This handles an edge case where a peer sends a past
            # request for a destination just after an announce for
            # said destination has arrived, but before it has been
            # rebroadcast locally. In such a case the actual announce
            # is temporarily held, and then reinserted when the path
            # request has been served to the peer.
            if packet.destination_hash in Transport.announce_table:
                held_entry = Transport.announce_table[packet.destination_hash]
                Transport.held_announces[packet.destination_hash] = held_entry
            
            Transport.announce_table[packet.destination_hash] = [now, retransmit_timeout, retries, received_from, announce_hops, packet, local_rebroadcasts, block_rebroadcasts, attached_interface]

        elif is_from_local_client:
            # Forward path request on all interfaces
            # except the local client
            for interface in Transport.interfaces:
                if not interface == attached_interface:
                    Transport.request_path_on_interface(destination_hash, interface)

        elif not is_from_local_client and len(Transport.local_client_interfaces) > 0:
            # Forward the path request on all local
            # client interfaces
            for interface in Transport.local_client_interfaces:
                Transport.request_path_on_interface(destination_hash, interface)

        else:
            RNS.log("No known path to requested destination, ignoring request", RNS.LOG_DEBUG)

    @staticmethod
    def from_local_client(packet):
        if hasattr(packet.receiving_interface, "parent_interface"):
            return Transport.is_local_client_interface(packet.receiving_interface)
        else:
            return False

    @staticmethod
    def is_local_client_interface(interface):
        if hasattr(interface, "parent_interface"):
            if hasattr(interface.parent_interface, "is_local_shared_instance"):
                return True
            else:
                return False
        else:
            return False

    @staticmethod
    def interface_to_shared_instance(interface):
        if hasattr(interface, "is_connected_to_shared_instance"):
            return True
        else:
            return False

    @staticmethod
    def detach_interfaces():
        for interface in Transport.interfaces:
            interface.detach()

        for interface in Transport.local_client_interfaces:
            interface.detach()


    @staticmethod
    def exit_handler():
        try:
            if not RNS.Reticulum.transport_enabled():
                Transport.packet_hashlist = []
            else:
                RNS.log("Saving packet hashlist to storage...", RNS.LOG_VERBOSE)

            packet_hashlist_path = RNS.Reticulum.storagepath+"/packet_hashlist"
            file = open(packet_hashlist_path, "wb")
            file.write(umsgpack.packb(Transport.packet_hashlist))
            file.close()

        except Exception as e:
            RNS.log("Could not save packet hashlist to storage, the contained exception was: "+str(e), RNS.LOG_ERROR)

        if not Transport.owner.is_connected_to_shared_instance:
            RNS.log("Saving path table to storage...", RNS.LOG_VERBOSE)
            try:
                serialised_destinations = []
                for destination_hash in Transport.destination_table:
                    # Get the destination entry from the destination table
                    de = Transport.destination_table[destination_hash]
                    interface_hash = de[5].get_hash()

                    # Only store destination table entry if the associated
                    # interface is still active
                    interface = Transport.find_interface_from_hash(interface_hash)
                    if interface != None:
                        # Get the destination entry from the destination table
                        de = Transport.destination_table[destination_hash]
                        timestamp = de[0]
                        received_from = de[1]
                        hops = de[2]
                        expires = de[3]
                        random_blobs = de[4]
                        packet_hash = de[6].get_hash()

                        serialised_entry = [
                            destination_hash,
                            timestamp,
                            received_from,
                            hops,
                            expires,
                            random_blobs,
                            interface_hash,
                            packet_hash
                        ]

                        serialised_destinations.append(serialised_entry)

                        Transport.cache(de[6], force_cache=True)

                destination_table_path = RNS.Reticulum.storagepath+"/destination_table"
                file = open(destination_table_path, "wb")
                file.write(umsgpack.packb(serialised_destinations))
                file.close()
                RNS.log("Done saving "+str(len(serialised_destinations))+" path table entries to storage", RNS.LOG_VERBOSE)
            except Exception as e:
                RNS.log("Could not save path table to storage, the contained exception was: "+str(e), RNS.LOG_ERROR)

            RNS.log("Saving tunnel table to storage...", RNS.LOG_VERBOSE)
            try:
                serialised_tunnels = []
                for tunnel_id in Transport.tunnels:
                    te = Transport.tunnels[tunnel_id]
                    interface = te[1]
                    tunnel_paths = te[2]
                    expires = te[3]

                    if interface != None:
                        interface_hash = interface.get_hash()
                    else:
                        interface_hash = None

                    serialised_paths = []
                    for destination_hash in tunnel_paths:
                        de = tunnel_paths[destination_hash]

                        timestamp = de[0]
                        received_from = de[1]
                        hops = de[2]
                        expires = de[3]
                        random_blobs = de[4]
                        packet_hash = de[6].get_hash()

                        serialised_entry = [
                            destination_hash,
                            timestamp,
                            received_from,
                            hops,
                            expires,
                            random_blobs,
                            interface_hash,
                            packet_hash
                        ]

                        serialised_paths.append(serialised_entry)

                        Transport.cache(de[6], force_cache=True)


                    serialised_tunnel = [tunnel_id, interface_hash, serialised_paths, expires]
                    serialised_tunnels.append(serialised_tunnel)

                tunnels_path = RNS.Reticulum.storagepath+"/tunnels"
                file = open(tunnels_path, "wb")
                file.write(umsgpack.packb(serialised_tunnels))
                file.close()
                RNS.log("Done saving "+str(len(serialised_tunnels))+" tunnel table entries to storage", RNS.LOG_VERBOSE)
            except Exception as e:
                RNS.log("Could not save tunnel table to storage, the contained exception was: "+str(e), RNS.LOG_ERROR)