Understanding Reticulum

This chapter will briefly describe the overall purpose and operating principles of Reticulum. It should give you an overview of how the stack works, and an understanding of how to develop networked applications using Reticulum.

This chapter is not an exhaustive source of information on Reticulum, at least not yet. Currently, the only complete repository, and final authority on how Reticulum actually functions, is the Python reference implementation and API reference. That being said, this chapter is an essential resource in understanding how Reticulum works from a high-level perspective, along with the general principles of Reticulum, and how to apply them when creating your own networks or software.

After reading this document, you should be well-equipped to understand how a Reticulum network operates, what it can achieve, and how you can use it yourself. If you want to help out with the development, this is also the place to start, since it will provide a pretty clear overview of the sentiments and the philosophy behind Reticulum, what problems it seeks to solve, and how it approaches those solutions.

Motivation

The primary motivation for designing and implementing Reticulum has been the current lack of reliable, functional and secure minimal-infrastructure modes of digital communication. It is my belief that it is highly desirable to create a reliable and efficient way to set up long-range digital communication networks that can securely allow exchange of information between people and machines, with no central point of authority, control, censorship or barrier to entry.

Almost all of the various networking systems in use today share a common limitation: They require large amounts of coordination and centralised trust and power to function. To join such networks, you need approval of gatekeepers in control. This need for coordination and trust inevitably leads to an environment of central control, where it’s very easy for infrastructure operators or governments to control or alter traffic, and censor or persecute unwanted actors. It also makes it completely impossible to freely deploy and use networks at will, like one would use other common tools that enhance individual agency and freedom.

Reticulum aims to require as little coordination and trust as possible. It aims to make secure, anonymous and permissionless networking and information exchange a tool that anyone can just pick up and use.

Since Reticulum is completely medium agnostic, it can be used to build networks on whatever is best suited to the situation, or whatever you have available. In some cases, this might be packet radio links over VHF frequencies, in other cases it might be a 2.4 GHz network using off-the-shelf radios, or it might be using common LoRa development boards.

At the time of release of this document, the fastest and easiest setup for development and testing is using LoRa radio modules with an open source firmware (see the section Reference Setup), connected to any kind of computer or mobile device that Reticulum can run on.

The ultimate aim of Reticulum is to allow anyone to be their own network operator, and to make it cheap and easy to cover vast areas with a myriad of independent, interconnectable and autonomous networks. Reticulum is not one network, it is a tool to build thousands of networks.

Networks without kill-switches, surveillance, censorship and control. Networks that can freely interoperate, associate and disassociate with each other, and require no central oversight. Networks for human beings. Networks for the people.

Goals

To be as widely usable and efficient to deploy as possible, the following goals have been used to guide the design of Reticulum:

  • Fully useable as open source software stack

    Reticulum must be implemented with, and be able to run using only open source software. This is critical to ensuring the availability, security and transparency of the system.

  • Hardware layer agnosticism

    Reticulum must be fully hardware agnostic, and shall be useable over a wide range of physical networking layers, such as data radios, serial lines, modems, handheld transceivers, wired ethernet, wifi, or anything else that can carry a digital data stream. Hardware made for dedicated Reticulum use shall be as cheap as possible and use off-the-shelf components, so it can be easily modified and replicated by anyone interested in doing so.

  • Very low bandwidth requirements

    Reticulum should be able to function reliably over links with a transmission capacity as low as 500 bits per second.

  • Encryption by default

    Reticulum must use strong encryption by default for all communication.

  • Initiator Anonymity

    It must be possible to communicate over a Reticulum network without revealing any identifying information about oneself.

  • Unlicensed use

    Reticulum shall be functional over physical communication mediums that do not require any form of license to use. Reticulum must be designed in a way, so it is usable over ISM radio frequency bands, and can provide functional long distance links in such conditions, for example by connecting a modem to a PMR or CB radio, or by using LoRa or WiFi modules.

  • Supplied software

    In addition to the core networking stack and API, that allows a developer to build applications with Reticulum, a basic set of Reticulum-based communication tools must be implemented and released along with Reticulum itself. These shall serve both as a functional, basic communication suite, and as an example and learning resource to others wishing to build applications with Reticulum.

  • Ease of use

    The reference implementation of Reticulum is written in Python, to make it easy to use and understand. A programmer with only basic experience should be able to use Reticulum to write networked applications.

  • Low cost

    It shall be as cheap as possible to deploy a communication system based on Reticulum. This should be achieved by using cheap off-the-shelf hardware that potential users might already own. The cost of setting up a functioning node should be less than $100 even if all parts needs to be purchased.

Introduction & Basic Functionality

Reticulum is a networking stack suited for high-latency, low-bandwidth links. Reticulum is at it’s core a message oriented system. It is suited for both local point-to-point or point-to-multipoint scenarios where alle nodes are within range of each other, as well as scenarios where packets need to be transported over multiple hops in a complex network to reach the recipient.

Reticulum does away with the idea of addresses and ports known from IP, TCP and UDP. Instead Reticulum uses the singular concept of destinations. Any application using Reticulum as it’s networking stack will need to create one or more destinations to receive data, and know the destinations it needs to send data to.

All destinations in Reticulum are represented as a 10 byte hash, derived from truncating a full SHA-256 hash of identifying characteristics of the destination. To users, the destination addresses will be displayed as 10 bytes in hexadecimal representation, as in the following example: <80e29bf7cccaf31431b3>.

The truncation size of 10 bytes (80 bits) for destinations has been choosen as a reasonable tradeoff between address space and packet overhead. The address space accomodated by this size can support many billions of simultaneously active devices on the same network, while keeping packet overhead low, which is essential on low-bandwidth networks. In the very unlikely case that this address space nears congestion, a one-line code change can upgrade the Reticulum address space all the way up to 256 bits, ensuring the Reticulum address space could potentially support galactic-scale networks. This is obviusly complete and ridiculous over-allocation, and as such, the current 80 bits should be sufficient, even far into the future.

By default Reticulum encrypts all data using elliptic curve cryptography. Any packet sent to a destination is encrypted with a derived ephemeral key. Reticulum can also set up an encrypted channel to a destination with Forward Secrecy and Initiator Anonymity using a elliptic curve cryptography and ephemeral keys derived from a Diffie Hellman exchange on Curve25519. In Reticulum terminology, this is called a Link. The multi-hop transport, coordination, verification and reliability layers are fully autonomous and also based on elliptic curve cryptography.

Reticulum also offers symmetric key encryption for group-oriented communications, as well as unencrypted packets for broadcast purposes, or situations where you need the communication to be in plain text.

Reticulum can connect to a variety of interfaces such as radio modems, data radios and serial ports, and offers the possibility to easily tunnel Reticulum traffic over IP links such as the Internet or private IP networks.

Destinations

To receive and send data with the Reticulum stack, an application needs to create one or more destinations. Reticulum uses three different basic destination types, and one special:

  • Single

    The single destination type is the most common type in Reticulum, and should be used for most purposes. It is always identified by a unique public key. Any data sent to this destination will be encrypted using ephemeral keys derived from an ECDH key exchange, and will only be readable by the creator of the destination, who holds the corresponding private key.

  • Plain

    A plain destination type is unencrypted, and suited for traffic that should be broadcast to a number of users, or should be readable by anyone. Traffic to a plain destination is not encrypted. Generally, plain destinations can be used for broadcast information intended to be public. Plain destinations are only reachable directly, and packets adressed to plain destinations are never transported over multiple hops in the network. To be transportable over multiple hops in Reticulum, information must be encrypted, since Reticulum uses the per-packet encryption to verify routing paths and keep them alive.

  • Group

    The group special destination type, that defines a symmetrically encrypted virtual destination. Data sent to this destination will be encrypted with a symmetric key, and will be readable by anyone in possession of the key, but as with the plain destination type, packets to this type of destination are not currently transported over multiple hops, although a planned upgrade to Reticulum will allow globally reachable group destinations.

  • Link

    A link is a special destination type, that serves as an abstract channel to a single destination, directly connected or over multiple hops. The link also offers reliability and more efficient encryption, forward secrecy, initiator anonymity, and as such can be useful even when a node is directly reachable. It also offers a more capable API and allows easily carrying out requests and responses, large data transfers and more.

Destination Naming

Destinations are created and named in an easy to understand dotted notation of aspects, and represented on the network as a hash of this value. The hash is a SHA-256 truncated to 80 bits. The top level aspect should always be a unique identifier for the application using the destination. The next levels of aspects can be defined in any way by the creator of the application.

Aspects can be as long and as plentiful as required, and a resulting long destination name will not impact efficiency, as names are always represented as truncated SHA-256 hashes on the network.

As an example, a destination for a environmental monitoring application could be made up of the application name, a device type and measurement type, like this:

app name  : environmentlogger
aspects   : remotesensor, temperature

full name : environmentlogger.remotesensor.temperature
hash      : fa7ddfab5213f916dea

For the single destination, Reticulum will automatically append the associated public key as a destination aspect before hashing. This is done to ensure only the correct destination is reached, since anyone can listen to any destination name. Appending the public key ensures that a given packet is only directed at the destination that holds the corresponding private key to decrypt the packet.

Take note! There is a very important concept to understand here:

  • Anyone can use the destination name environmentlogger.remotesensor.temperature

  • Each destination that does so will still have a unique destination hash, and thus be uniquely addressable, because their public keys will differ.

In actual use of single destination naming, it is advisable not to use any uniquely identifying features in aspect naming. Aspect names should be general terms describing what kind of destination is represented. The uniquely identifying aspect is always acheived by the appending the public key, which expands the destination into a uniquely identifyable one. Reticulum does this automatically.

Any destination on a Reticulum network can be addressed and reached simply by knowning its destination hash (and public key, but if the public key is not known, it can be requested from the network simply by knowing the destination hash). The use of app names and aspects makes it easy to structure Reticulum programs and makes it possible to filter what information and data your program receives.

To recap, the different destination types should be used in the following situations:

  • Single

    When private communication between two endpoints is needed. Supports multiple hops.

  • Group

    When private communication between two or more endpoints is needed. Supports multiple hops indirectly, but must first be established through a single destination.

  • Plain

    When plain-text communication is desirable, for example when broadcasting information, or for local discovery purposes.

To communicate with a single destination, you need to know it’s public key. Any method for obtaining the public key is valid, but Reticulum includes a simple mechanism for making other nodes aware of your destinations public key, called the announce. It is also possible to request an unknown public key from the network, as all transport instances serve as a distributed ledger of public keys.

Note that public key information can be shared and verified in other ways than using the built-in announce functionality, and that it is therefore not required to use the announce and path request functionality to obtain public keys. It is by far the easiest though, and should definitely be used if there is not a very good reason for doing it differently.

Public Key Announcements

An announce will send a special packet over any relevant interfaces, containing all needed information about the destination hash and public key, and can also contain some additional, application specific data. The entire packet is signed by the sender to ensure authenticity. It is not required to use the announce functionality, but in many cases it will be the simplest way to share public keys on the network. The announce mechanism also serves to establish end-to-end connectivity to the announced destination, as the announce propagates through the network.

As an example, an announce in a simple messenger application might contain the following information:

  • The announcers destination hash

  • The announcers public key

  • Application specific data, in this case the users nickname and availability status

  • A random blob, making each new announce unique

  • An Ed25519 signature of the above information, verifying authenticity

With this information, any Reticulum node that receives it will be able to reconstruct an outgoing destination to securely communicate with that destination. You might have noticed that there is one piece of information lacking to reconstruct full knowledge of the announced destination, and that is the aspect names of the destination. These are intentionally left out to save bandwidth, since they will be implicit in almost all cases. The receiving application will already know them. If a destination name is not entirely implicit, information can be included in the application specific data part that will allow the receiver to infer the naming.

It is important to note that announces will be forwarded throughout the network according to a certain pattern. This will be detailed in the section The Announce Mechanism in Detail.

In Reticulum, destinations are allowed to move around the network at will. This is very different from protocols such as IP, where an address is always expected to stay within the network segment it was assigned in. This limitation does not exist in Reticulum, and any destination is completely portable over the entire topography of the network, and can even be moved to other Reticulum networks than the one it was created in, and still become reachable. To update it’s reachability, a destination simply needs to send an announce on any networks it is part of. After a short while, it will be globally reachable in the network.

Seeing how single destinations are always tied to a private/public key pair leads us to the next topic.

Identities

In Reticulum, an identity does not necessarily represent a personal identity, but is an abstraction that can represent any kind of verifiable entity. This could very well be a person, but it could also be the control interface of a machine, a program, robot, computer, sensor or something else entirely. In general, any kind of agent that can act, or be acted upon, or store or manipulate information, can be represented as an identity. An identity can be used to create any number of destinations.

A single destination will always have an identity tied to it, but not plain or group destinations. Destinations and identities share a multilateral connection. You can create a destination, and if it is not connected to an identity upon creation, it will just create a new one to use automatically. This may be desirable in some situations, but often you will probably want to create the identity first, and then use it to create new destinations.

As an example, we could use an identity to represent the user of a messaging application. Destinations can then be created by this identity to allow communication to reach the user. In all cases it is of great importance to store the private keys associated with any Reticulum Identity securely and privately, since obtaining access to the identity keys equals obtaining access and controlling reachability to any destinations created by that identity.

Getting Further

The above functions and principles form the core of Reticulum, and would suffice to create functional networked applications in local clusters, for example over radio links where all interested nodes can directly hear each other. But to be truly useful, we need a way to direct traffic over multiple hops in the network.

In the following sections, two concepts that allow this will be introduced, paths and links.

Reticulum Transport

The methods of routing used in traditional networks are fundamentally incompatible with the physical medium types and circumstances that Reticulum was designed to handle. These mechanisms mostly assume trust at the physical layer, and often needs a lot more bandwidth than Reticulum can assume is available. Since Reticulum is designed to survive running over open radio spectrum, no such trust can be assumed, and bandwidth is often very limited.

To overcome such challenges, Reticulum’s Transport system uses asymmetric elliptic curve cryptography to implement the concept of paths that allow discovery of how to get information closer to a certain destination. It is important to note that no single node in a Reticulum network knows the complete path to a destination. Every Transport node participating in a Reticulum network will only know the most direct way to get a packet one hop closer to it’s destination.

Node Types

Currently, Reticulum distinguishes between two types of network nodes. All nodes on a Reticulum network are Reticulum Instances, and some are alo Transport Nodes. If a system running Reticulum is fixed in one place, and is intended to be kept available most of the time, it is a good contender to be a Transport Node.

Any Reticulum Instance can become a Transport Node by enabling it in the configuration. This distinction is made by the user configuring the node, and is used to determine what nodes on the network will help forward traffic, and what nodes rely on other nodes for wider connectivity.

If a node is an Instance it should be given the configuration directive enable_transport = No, which is the default setting.

If it is a Transport Node, it should be given the configuration directive enable_transport = Yes.

The Announce Mechanism in Detail

When an announce for a destination is transmitted by from a Reticulum instance, it will be forwarded by any transport node receiving it, but according to some specific rules:

  • If this exact announce has already been received before, ignore it.
  • If not, record into a table which Transport Node the announce was received from, and how many times in total it has been retransmitted to get here.
  • If the announce has been retransmitted m+1 times, it will not be forwarded any more. By default, m is set to 128.
  • After a randomised delay, the announce will be retransmitted on all interfaces that have bandwidth available for processing announces. By default, the maximum bandwidth allocation for processing announces is set at 2%, but can be configured on a per-interface basis.
  • If any given interface does not have enough bandwidth available for retransmitting the announce, the announce will be assigned a priority inversely proportional to it’s hop count, and be inserted into a queue managed by the interface.
  • When the interface has bandwidth available for processing an announce, it will prioritise announces for destinations that are closest in terms of hops, thus prioritising reachability and connectivity of local nodes, even on slow networks that connect to wider and faster networks.
  • After the announce has been re-transmitted, and if no other nodes are heard retransmitting the announce with a greater hop count than when it left this node, transmitting it will be retried r times. By default, r is set to 1.
  • If a newer announce from the same destination arrives, while an identical one is already waiting to be transmitted, the newest announce is discarded. If the newest announce contains different application specific data, it will replace the old announce.

Once an announce has reached a node in the network, any other node in direct contact with that node will be able to reach the destination the announce originated from, simply by sending a packet addressed to that destination. Any node with knowledge of the announce will be able to direct the packet towards the destination by looking up the next node with the shortest amount of hops to the destination.

According to these rules, an announce will propagate throughout the network in a predictable way, and make the announced destination reachable in a short amount of time. Fast networks that have the capacity to process many announces can reach full convergence very quickly, even when constantly adding new destinations. Slower segments of such networks might take a bit longer to gain full knowledge about the wide and fast networks they are connected to, but can still do so over time, while prioritising full and quickly converging end-to-end connectivity for their local, slower segments.

In general, even extremely complex networks, that utilize the maximum 128 hops will converge to full end-to-end connectivity in about one minute, given there is enough bandwidth available to process the required amount of announces.

Reaching the Destination

In networks with changing topology and trustless connectivity, nodes need a way to establish verified connectivity with each other. Since the network is assumed to be trustless, Reticulum must provide a way to guarantee that the peer you are communicating with is actually who you expect. Reticulum offers two ways to do this.

For exchanges of small amounts of information, Reticulum offers the Packet API, which works exactly like you would expect - on a per packet level. The following process is employed when sending a packet:

  • A packet is always created with an associated destination and some payload data. When the packet is sent to a single destination type, Reticulum will automatically create an ephemeral encryption key, perform an ECDH key exchange with the destinations public key, and encrypt the information.
  • It is important to note that this key exchange does not require any network traffic. The sender already knows the public key of the destination from an earlier received announce, and can thus perform the ECDH key exchange locally, before sending the packet.
  • The public part of the newly generated ephemeral key-pair is included with the encrypted token, and sent along with the encrypted payload data in the packet.
  • When the destination receives the packet, it can itself perform an ECDH key exchange and decrypt the packet.
  • A new ephemeral key is used for every packet sent in this way, and forward secrecy is guaranteed on a per packet level.
  • Once the packet has been received and decrypted by the addressed destination, that destination can opt to prove its receipt of the packet. It does this by calculating the SHA-256 hash of the received packet, and signing this hash with it’s Ed25519 signing key. Transport nodes in the network can then direct this proof back to the packets origin, where the signature can be verified against the destinations known public signing key.
  • In case the packet is addressed to a group destination type, the packet will be encrypted with the pre-shared AES-128 key associated with the destination. In case the packet is addressed to a plain destination type, the payload data will not be encrypted. Neither of these two destination types offer forward secrecy. In general, it is recommended to always use the single destination type, unless it is strictly necessary to use one of the others.

For exchanges of larger amounts of data, or when longer sessions of bidirectional communication is desired, Reticulum offers the Link API. To establish a link, the following process is employed:

  • First, the node that wishes to establish a link will send out a special packet, that traverses the network and locates the desired destination. Along the way, the Transport Nodes that forward the packet will take note of this link request.
  • Second, if the destination accepts the link request , it will send back a packet that proves the authenticity of it’s identity (and the receipt of the link request) to the initiating node. All nodes that initially forwarded the packet will also be able to verify this proof, and thus accept the validity of the link throughout the network.
  • When the validity of the link has been accepted by forwarding nodes, these nodes will remember the link , and it can subsequently be used by referring to a hash representing it.
  • As a part of the link request, an Elliptic Curve Diffie-Hellman key exchange takes place, that sets up an efficiently encrypted tunnel between the two nodes. As such, this mode of communication is preferred, even for situations when nodes can directly communicate, when the amount of data to be exchanged numbers in the tens of packets, or whenever the use of the more advanced API functions is desired.
  • When a link has been set up, it automatically provides message receipt functionality, through the same proof mechanism discussed before, so the sending node can obtain verified confirmation that the information reached the intended recipient.
  • Once the link has been set up, the initiator can remain anonymous, or choose to authenticate towards the destination using a Reticulum Identity. This authentication is happening inside the encrypted link, and is only revealed to the verified destination, and no intermediaries.

In a moment, we will discuss the details of how this methodology is implemented, but let’s first recap what purposes this methodology serves. We first ensure that the node answering our request is actually the one we want to communicate with, and not a malicious actor pretending to be so. At the same time we establish an efficient encrypted channel. The setup of this is relatively cheap in terms of bandwidth, so it can be used just for a short exchange, and then recreated as needed, which will also rotate encryption keys. The link can also be kept alive for longer periods of time, if this is more suitable to the application. The procedure also inserts the link id , a hash calculated from the link request packet, into the memory of forwarding nodes, which means that the communicating nodes can thereafter reach each other simply by referring to this link id.

The combined bandwidth cost of setting up a link is 3 packets totalling 237 bytes (more info in the Binary Packet Format section). The amount of bandwidth used on keeping a link open is practically negligible, at 0.62 bits per second. Even on a slow 1200 bits per second packet radio channel, 100 concurrent links will still leave 95% channel capacity for actual data.

Resources

For exchanging small amounts of data over a Reticulum network, the Packet interface is sufficient, but for exchanging data that would require many packets, an efficient way to coordinate the transfer is needed.

This is the purpose of the Reticulum Resource. A Resource can automatically handle the reliable transfer of an arbitrary amount of data over an established Link. Resources can auto-compress data, will handle breaking the data into individual packets, sequencing the transfer, integrity verification and reassembling the data on the other end.

Resources are programmatically very simple to use, and only requires a few lines of codes to reliably transfer any amount of data. They can be used to transfer data stored in memory, or stream data directly from files.

Reference Setup

This section will detail a recommended Reference Setup for Reticulum. It is important to note that Reticulum is designed to be usable on more or less any computing device, and over more or less any medium that allows you to send and receive data, which satisfies some very low minimum requirements.

The communication channel must support at least half-duplex operation, and provide an average throughput of around 500 bits per second, and supports a physical layer MTU of 500 bytes. The Reticulum stack should be able to run on more or less any hardware that can provide a Python 3.x runtime environment.

That being said, this reference setup has been outlined to provide a common platform for anyone who wants to help in the development of Reticulum, and for everyone who wants to know a recommended setup to get started experimenting. A reference system consists of three parts:

  • An Interface Device

    Which provides access to the physical medium whereupon the communication takes place, for example a radio with an integrated modem. A setup with a separate modem connected to a radio would also be an interface device.

  • A Host Device

    Some sort of computing device that can run the necessary software, communicate with the interface device, and provide user interaction.

  • A Software Stack

    The software implementing the Reticulum protocol and applications using it.

The reference setup can be considered a relatively stable platform to develop on, and also to start building networks or applications on. While details of the implementation might change at the current stage of development, it is the goal to maintain hardware compatibility for as long as entirely possible, and the current reference setup has been determined to provide a functional platform for many years into the future. The current Reference System Setup is as follows:

  • Interface Device

    A data radio consisting of a LoRa radio module, and a microcontroller with open source firmware, that can connect to host devices via USB. It operates in either the 430, 868 or 900 MHz frequency bands. More details can be found on the RNode Page.

  • Host Device

    Any computer device running Linux and Python. A Raspberry Pi with a Debian based OS is recommended.

  • Software Stack

    The most recently released Python Implementation of Reticulum, running on a Debian based operating system.

To avoid confusion, it is very important to note, that the reference interface device does not use the LoRaWAN standard, but uses a custom MAC layer on top of the plain LoRa modulation! As such, you will need a plain LoRa radio module connected to an controller with the correct firmware. Full details on how to get or make such a device is available on the RNode Page.

With the current reference setup, it should be possible to get on a Reticulum network for around 100$ even if you have none of the hardware already, and need to purchase everything.

This reference setup is of course just a recommendation for getting started easily, and you should tailor it to your own specific needs, or whatever hardware you have available.

Protocol Specifics

This chapter will detail protocol specific information that is essential to the implementation of Reticulum, but non critical in understanding how the protocol works on a general level. It should be treated more as a reference than as essential reading.

Packet Prioritisation

Currently, Reticulum is completely priority-agnostic regarding general traffic. All traffic is handled on a first-come, first-serve basis. Announce re-transmission are handled according to the re-transmission times and priorities described earlier in this chapter.

Interface Access Codes

Reticulum can create named virtual networks, and networks that are only accessible by knowing a preshared passphrase. The configuration of this is detailed in the Common Interface Options section. To implement these feature, Reticulum uses the concept of Interface Access Codes, that are calculated and verified per packet.

An interface with a named virtual network or passphrase authentication enabled will derive a shared Ed25519 signing identity, and for every outbound packet generate a signature of the entire packet. This signature is then inserted into the packet as an Interface Access Code before transmission. Depending on the speed and capabilities of the interface, the IFAC can be the full 512-bit Ed25519 signature, or a truncated version. Configured IFAC length can be inspected for all interfaces with the rnstatus utility.

Upon receipt, the interface will check that the signature matches the expected value, and drop the packet if it does not. This ensures that only packets sent with the correct naming and/or passphrase parameters are allowed to pass onto the network.

Wire Format

== Reticulum Wire Format ======

A Reticulum packet is composed of the following fields:

[HEADER 2 bytes] [ADDRESSES 10/20 bytes] [CONTEXT 1 byte] [DATA 0-477 bytes]

* The HEADER field is 2 bytes long.
  * Byte 1: [IFAC Flag], [Header Type], [Propagation Type], [Destination Type] and [Packet Type]
  * Byte 2: Number of hops

* Interface Access Code field if the IFAC flag was set.
  * The length of the Interface Access Code can vary from
    1 to 64 bytes according to physical interface
    capabilities and configuration.

* The ADDRESSES field contains either 1 or 2 addresses.
  * Each address is 10 bytes long.
  * The Header Type flag in the HEADER field determines
    whether the ADDRESSES field contains 1 or 2 addresses.
  * Addresses are Reticulum hashes truncated to 10 bytes.

* The CONTEXT field is 1 byte.
  * It is used by Reticulum to determine packet context.

* The DATA field is between 0 and 477 bytes.
  * It contains the packets data payload.

IFAC Flag
-----------------
open             0  Packet for publically accessible interface
authenticated    1  Interface authentication is included in packet


Header Types
-----------------
type 1           0  Two byte header, one 10 byte address field
type 2           1  Two byte header, two 10 byte address fields


Propagation Types
-----------------
broadcast       00
transport       01
reserved        10
reserved        11


Destination Types
-----------------
single          00
group           01
plain           10
link            11


Packet Types
-----------------
data            00
announce        01
link request    10
proof           11


+- Packet Example -+

   HEADER FIELD           DESTINATION FIELDS            CONTEXT FIELD  DATA FIELD
 _______|_______   ________________|________________   ________|______   __|_
|               | |                                 | |               | |    |
01010000 00000100 [HASH1, 10 bytes] [HASH2, 10 bytes] [CONTEXT, 1 byte] [DATA]
|| | | |    |
|| | | |    +-- Hops             = 4
|| | | +------- Packet Type      = DATA
|| | +--------- Destination Type = SINGLE
|| +----------- Propagation Type = TRANSPORT
|+------------- Header Type      = HEADER_2 (two byte header, two address fields)
+-------------- Access Codes     = DISABLED


+- Packet Example -+

   HEADER FIELD   DESTINATION FIELD   CONTEXT FIELD  DATA FIELD
 _______|_______   _______|_______   ________|______   __|_
|               | |               | |               | |    |
00000000 00000111 [HASH1, 10 bytes] [CONTEXT, 1 byte] [DATA]
|| | | |    |
|| | | |    +-- Hops             = 0
|| | | +------- Packet Type      = DATA
|| | +--------- Destination Type = SINGLE
|| +----------- Propagation Type = BROADCAST
|+------------- Header Type      = HEADER_1 (two byte header, one address field)
+-------------- Access Codes     = DISABLED


+- Packet Example -+

   HEADER FIELD     IFAC FIELD    DESTINATION FIELD   CONTEXT FIELD  DATA FIELD
 _______|_______   ______|______   _______|_______   ________|______   __|_
|               | |             | |               | |               | |    |
10000000 00000111 [IFAC, N bytes] [HASH1, 10 bytes] [CONTEXT, 1 byte] [DATA]
|| | | |    |
|| | | |    +-- Hops             = 0
|| | | +------- Packet Type      = DATA
|| | +--------- Destination Type = SINGLE
|| +----------- Propagation Type = BROADCAST
|+------------- Header Type      = HEADER_1 (two byte header, one address field)
+-------------- Access Codes     = ENABLED


Size examples of different packet types
---------------------------------------

The following table lists example sizes of various
packet types. The size listed are the complete on-
wire size counting all fields including headers,
but excluding any interface access codes.

- Path Request    :    33  bytes
- Announce        :    151 bytes
- Link Request    :    77  bytes
- Link Proof      :    77  bytes
- Link RTT packet :    83  bytes
- Link keepalive  :    14  bytes

Announce Propagation Rules

The following table illustrates the rules for automatically propagating announces from one interface type to another, for all possible combinations. See the Interface Modes section for a conceptual overview of the different interface modes, and how they are configured.

Full ────── ✓ ──┐              ┌── ✓ ── Full
AP ──────── ✓ ──┼───> Full >───┼── ✕ ── AP
Boundary ── ✓ ──┤              ├── ✓ ── Boundary
Roaming ─── ✓ ──┘              └── ✓ ── Roaming

Full ────── ✕ ──┐              ┌── ✓ ── Full
AP ──────── ✕ ──┼────> AP >────┼── ✕ ── AP
Boundary ── ✕ ──┤              ├── ✓ ── Boundary
Roaming ─── ✕ ──┘              └── ✓ ── Roaming

Full ────── ✓ ──┐              ┌── ✓ ── Full
AP ──────── ✓ ──┼─> Roaming >──┼── ✕ ── AP
Boundary ── ✕ ──┤              ├── ✕ ── Boundary
Roaming ─── ✕ ──┘              └── ✕ ── Roaming

Full ────── ✓ ──┐              ┌── ✓ ── Full
AP ──────── ✓ ──┼─> Boundary >─┼── ✕ ── AP
Boundary ── ✓ ──┤              ├── ✓ ── Boundary
Roaming ─── ✕ ──┘              └── ✕ ── Roaming