Modem protocols

On top of the modem layer (see Modem networking) lies application protocols, using some transport, session and presentation layers. This page mostly describes protocols implemented using ComputerCraft modems or equivalent, which are broadcast-only networks with very basic mechanisms, where most protocols are actually made to be between two devices, which leads to transport protocols introducing addressing on top of the modem API.

Hence, two approaches co-exist:

  • The Rednet approach: using port numbers as addresses on a network, with protocols being part of the message as a string; see sProtocol in rednet.send.

  • The “one port, one protocol” approach: using port numbers as protocol identifiers, with messages not compatible with Rednet messages.

The first approach is embedded in the ComputerCraft ROM, therefore is accessible to any computer attached to a modem on any Minecraft server including the mod. It is the officially recommended way to network. The second approach has mainly be used by third-party OSes such as OneOS or OPUS OS, and also has limits which we will describe in this section.

Note however that Rednet is not the only protocol of its kind, and others have been created either for answering Rednet’s identified weaknesses, or simply for funsies.

Rednet

Rednet uses port numbers as adresses, with the exception of two special port numbers:

  • 65533 (known as rednet.CHANNEL_REPEAT): the repeat channel, which allows physical networks to communicate with each other using physical relays.

  • 65535 (known as rednet.CHANNEL_BROADCAST): the broadcast channel, which allows a message to be sent to all computers on all connected physical networks (repeaters share these messages as any other Rednet message).

Any Rednet message is a table composed of the following members:

  • nMessageID: a pseudo-random number in the [1, 2^{32} - 1] range. This member allows a Rednet-enabled device to recognize the message as a Rednet message, and repeaters to mitigate loops by not transmitting the same message in a given timespan (currently 30 seconds).

  • nRecipient: the ID of the computer to which the message is sent.

  • message: the message.

  • sProtocol: the protocol as a string or nil.

As a globally unique number, Rednet uses the computer ID, an integer which is guaranteed to be unique for a given world (NOT server) in the [0, 2^{32} - 1] range. To determine the source of a Rednet message, the “reply channel” is used by filling it with the computer ID.

Warning

The computer ID can be non-unique in case computers with a given ID are duplicated (through cheating or otherwise), although this might cause other problems such as intricated filesystems between the two computers having the same ID.

Plus, the computer ID can be spoofed; the most straightforward way of doing this is by reimplementing rednet over the modem API but getting the ID to use through other means, such as having a idToSpoof constant.

On a Rednet-enabled device, two channels are opened by default for receiving messages: the broadcast channel, and the modem channel with the computer ID as the port number, e.g. channel 14 for a computer which ID is 14, for direct messages or answers; see rednet.open.

Warning

Notice that there is no limit to the ID a computer can be assigned except the Java int type maximum value, which is 2^{32} - 1, where the modem number can only between 0 and 2^{16} - 1. There are two cases that might cause unexpected behaviours:

  • The computer ID exceeds 2^{16} - 1, which is not a valid port number.

  • The computer ID is a reserved Rednet channel number amongst 65533 (known in CraftOS as rednet.CHANNEL_BROADCAST) and 65535 (known in CraftOS as rednet.CHANNEL_REPEAT), or even 65534 (known in CraftOS as rednet.CHANNEL_GPS) for that matter.

The computer identifier should be lesser or equal to 65532 for it to have no unexpected behaviour on a Rednet network, which is NOT GUARANTEED. Note that the computer identifier should be unique to the world it is into, so that means the computer counter (not computer count) should not exceed 65532. This should cause an anomaly on servers using ComputerCraft in an intense manner, although I have not found the case to have happened on forums.

See IDAssigner.getNextID and Java Primitive Types and Values.

When receiving a new message, the Rednet event loop checks if the message is a table and has a nMessageID member set to understand it as a Rednet message, which allows the Rednet and the “port protocols” approaches to coexist on the network; see rednet.run.

Note

This means that when implementing your own protocol not using the Rednet API, you should be aware that Rednet messages might arrive on your port number; you need to either not use tables as messages, or use tables but have a member that allows you to recognize messages in your protocol’s vocabulary, or at least ignore Rednet messages defining the nMessageID field.

The advantages of using protocols on top of Rednet are the following:

  • There is a “hardware” limit to the number of ports you can open on a modem: 128 ports, which even if unlikely is still reachable. Using the Rednet approach, there is no such arbitrary limit, as all string combinations (for the sProtocol field) are allowed.

  • Rednet has a “repeat” system, using the repeat channel; this is especially useful for messages on wireless modems, which can travel so many blocks at once (64 blocks, or 16 during a storm, in the default configuration). With this system, likely to be implemented by other players as well on a server, you can have Rednet repeaters, which by default behave like hubs (they repeat messages on all other modems, but do not optimize traffic by remembering which host is available through which modem), with loop mitigation; see the CraftOS repeat command.

Note

As repeaters behave as hubs and not switches, this means they don’t try to optimize communications by trying to determine on which interface a Rednet-enabled device is, and always repeat on all interfaces. While this means more messages than necessary are exchanged, it also means that several computers can use the same computer ID for Rednet, and that no packet is lost because of bad routing, which has the nice security implication that a device cannot block messages while trying to impersonate a device or censor a device without the repeaters being destroyed.

Todo

It is suggested that the repeat port only could as well be good enough, or at least multiplexing individual channels with a mod by 60000 (e.g. devices 5, 60005 and 120005 share the same port and check if the nRecipient is set correctly to their address; an idea I find really clever by Lupus590).

However, this field is not checked in classic Rednet that the nRecipient corresponds to the computer ID, making this approach non-retrocompatible, which is a real shame…

Wojbie suggested that this number is very high anyway, and that servers having as many computer IDs as that should use other protocols than classic Rednet. As of today, I haven’t decided on what to recommend in this documentation, or how to describe the case.

However, Wojbie suggested an evolution of Rednet where messages would all be sent and received on CHANNEL_REPEAT and Rednet messages could have a nReplyChannel field as the modem equivalent, but which could go far higher than the channel number; and of course, when receiving a modem message, the nRecipient field is checked, on the contrary to classic Rednet. See their suggestion as a valid implementation.

Some known protocols running over Rednet are the following:

sProtocol

Name

"dns"

Rednet lookup protocol; see Rednet lookup protocol.

"tror"

Terminal Redirection over Rednet (TRoR); see Rednet TRoR protocol.

In the next chapters, we’ll go more in depth on protocols that use Rednet.

Rednet derivatives

Todo

Find information about the criticism about Rednet or why the newbies don’t want to use Rednet.

Todo

Find which alternatives to talk about; there are probably some interesting and uninteresting ones. Some alternatives I found:

  • brednet: “better rednet” by LDDestroier, didn’t figure out the differenciating features with basic Rednet yet.

  • CryptoNet: adds encryption and PKI-based trusting.

Plus alternative systems brought by systems for their built-in protocols:

  • OPUS OS: encryption and peer-to-peer-based trusting.

  • OneOS: ?

Port protocols

Some protocols do not use Rednet, but the modem channels directly in a UDP or EtherTypes fashion. Messages in such a protocol are either not tables or tables not defining the nMessageID for distinguishing messages between Rednet and non-Rednet, although it is to be noted not all implementations ignore Rednet messages on the given channels.

Some known port protocols are the following:

Channel

Name

Protocol

19

opus-trust

OPUS OS trust protocol

22

opus-stelnet

OPUS OS telnet protocol

23

opus-telnet

OPUS OS telnet protocol

139

opus-samba

OPUS OS samba protocol

161

opus-snmp

OPUS OS SNMP protocol

180

ldd-drawtoy

drawtoy protocol

188

opus-proxy

OPUS OS proxy protocol

701

ldd-tron

TRON protocol

999

opus-discovery

OPUS OS discovery protocol

1005

ldd-wireless-rs

Wireless redstone

1294

ldd-ldris

LDRis protocol

3773

opus-neural-canvas

4200

oneos-ping

OneOS Ping Protocol

4201

oneos-ping-reply

OneOS Ping Protocol

4202

oneos-turtle-remote

OneOS Turtle Protocol

4203

oneos-turtle-remote-reply

OneOS Turtle Protocol

4204

oneos-transmit-discovery

OneOS Transmit Protocol

4205

oneos-transmit-discovery-reply

OneOS Transmit Protocol

4206

oneos-transmit-request

OneOS Transmit Protocol

4207

oneos-transmit-request-reply

OneOS Transmit Protocol

4208

oneos-transmit-send

OneOS Transmit Protocol

4210

oneos-door-lock-ping

Ultimate Door Lock Protocol

4211

oneos-door-lock-request

Ultimate Door Lock Protocol

4212

oneos-door-lock-request-reply

Ultimate Door Lock Protocol

4242

milo-remote

5222

opus-pickup-remote

5900

opus-vnc

OPUS OS VNC protocol

5901

opus-svnc

OPUS OS VNC protocol

5902

opus-mirror

8366

ldd-progdor2

Progdor2 protocol

11000

ldd-enchat3

Enchat3 protocol

60000

opus-message

65530

ldd-brednet

65534

gps

GPS protocol

Todo

Describe LDDestroier’s brednet protocol, and see if it doesn’t actually fit on the current page.

Remarks:

  • OneOS has an explicit comment about using channels between 4200 and 4300. It also has an Wireless.Channels.Ignored set to 4299, which is not used within the code.

  • OPUS OS uses ports in the [16384, 32767] range for connections.

Some references for this work are:

In the following chapters, we’ll go more in depth on port protocols.