This is a description of onion layer in Tox.

## Paths

Onion routing can be described using this chart:

To send a message via an onion path, first we construct the path by choosing three random nodes. Then, we put the message in several layers:

1. OnionRequest2 contains the address of the message receiver and the message encrypted with the third's node key
2. OnionRequest1 contains the address of the second node and OnionRequest2 encrypted with the second's node key, and
3. OnionRequest0 contains the address of the first node and encrypted OnionRequest0

A node that belongs to the path receives a message, decrypts the payload, attaches OnionReturn and sends the result to the next node in the path.

An OnionReturn of the n'th node is the pair of IP_Port and OnionReturn of the previous node (if it exists), encrypted with the n'th node's key. It allows the receiver to send a response to the sender using the same onion path.

## Client announce

An iteration of self announce:

1. Clean up the announce_list from timeouted nodes
2. For node n in announce_list:
• If the node is out of ping credit, skip it
• Check whether the node is not announced, announced or stable
• If it's time to send an announce request, send it to the node
• Update time and increment ping_debt of the node
3. If announce_list is not full, choose whether we should send to a path node
• If so, send the announce request to a random path node (ping_id = 0)

A node is considered timeouted if the last ping was more than NODE_TIMEOUT seconds ago and the node is out of ping credit.

A node is out of ping credit if ping_debt of the node is more than MAX_NODE_PINGS.

A node is announced if is_stored is not 0 and client's self_paths contains a path with the path_num of the node. An announced node is called stable if the node by itself is stable and the corresponding path is stable.

A node is stable by itself if it was added more than TIME_TO_STABLE seconds ago, its ping_debt is zero, the node was pinged less than NODE_TIMEOUT seconds ago.

A path is called stable if it was created more than TIME_TO_STABLE seconds ago, the usage_debt of the path is zero and the path was not used for more than PATH_TIMEOUT.

It is time to send an announce request if:

1. A node was pinged more than interval seconds ago, where interval is one of ANNOUNCE_INTERVAL_NOT_ANNOUNCED, ANNOUNCE_INTERVAL_ANNOUNCED, ANNOUNCE_INTERVAL_STABLE, depending on the node type
2. Or the last announce was more than NODE_PING_INTERVAL seconds ago and random(MAX_ONION_CLIENTS_ANNOUNCE - i) == 0, where i is the index of the node in the announce_list

The process of sending a self announce request to a node is the following:

1. First, get a random path with a given path_num.
2. Store a sendback in announce_ping_array, getting the sendback_id
3. Construct an announce request payload with
• search_id = real_pk
• data_pk = temp_pk
• ping_id = ping_id
• sendback_data = sendback_id
• Where real_pk is the real public key, temp_pk is the temporary public key of the onion client.
4. The payload is encrypted using {dest_pk, real_sk}, where dest_pk is the public key of the destination, real_sk is the real secret key
5. Construct the request with the payload and {pk = real_pk}
6. Send the request via the onion path

Getting a random path:

1. If path_num is not None, set path_index = path_num % NUMBER_ONION_PATHS. Otherwise, set path_index = random(0..NUMBER_ONION_PATHS)
2. If path_list[path_index] is timed out or doesn't exist:
1. Get ONION_PATH_LENGTH random path nodes
2. Check whether the last node belongs to some path
3. If yes, then use that path
4. If not:
1. Create a new onion path
2. Set times for the path
3. Set path_num = r(random_u32(), NUMBER_ONION_PATHS) + path_index, where $$r(a, n) = a - (a \bmod n)$$ – a “modulo rounding” function
3. Otherwise, just use the existing path
4. If the path is not out of usage credit, update last_path_used
5. Increment the usage debt of the path
6. Return the path

An announce response is handled in the following way:

1. Get the sendback from announce_ping_array using the sendback_data
2. Decrypt the payload using {sb.node.pk, real_sk}, where sb.node.pk is the public key stored in the sendback, real_sk is the real secret key
3. Set path timeouts using {sb.friend_num, sb.path_num}
4. Add the announce node to announce_list
5. Add the node to path_nodes
6. Get nodes from the payload and ping them (if any)

The process of pinging a node is the following:

1. Clean up announce_list from timeouted nodes
2. A node in the payload is pinged if:
1. It is closer to us than nodes in the announce_list or the list is not full
2. And it doesn't belong to the list
3. And it is good to ping
3. Pinging is sending an announce request with ping_id = 0 via a random path

A node is good to ping if this is not the last pinged node unless the node was pinged more than MIN_NODE_PING_TIME ago.

A sequence chart for the beginning of self announce process (messages are sent via onion):

## Friend search and DHTPK announce

The overall process has two steps:

• First, we use announce requests to find announce nodes that stores paths to our friend
• This process is similar to the announce process
• Then, we send (via onion) DataRequest to found announce nodes

This is the chart of the second step:

Specifically:

1. We start with constructing DhtPkData:
• no_replay = now where now is the current time
• dht_pk = dht_pk where dht_pk is our dht public key
• nodes = closest where closest is a list of closest to us dht nodes
2. Serialize it into bytes and send as an onion data or via dht

The process of sending onion data is the following:

1. The client_list of a friend is cleaned up of timeouted nodes
2. Good nodes are with is_stored != 0
3. It should be more than (num_nodes - 1) / 4 + 1 good nodes to continue where num_nodes is the number of nodes in client_list
4. Generate a random nonce
5. The data is encrypted using friend's real public key, our real secret key and the nonce
6. Construct OnionData:
• real_pk is our real public key
• dht_pk_data is the encrypted data
7. For each good node:
1. Get a random friend path
2. Construct a DataRequest:
• dest_pk = friend.real_pk
• nonce = nonce. The same nonce that we used before
• temp_pk = random_pk. We generate a random keypair
• The payload is the onion data encryped with the node's data public key and the random secret key, using the same nonce
3. Send the request via the onion path

## Notes on data structures

Both path_nodes and announce_list are arrays of limited size. But the way they are updated are different.

When path_nodes is full, adding a new element replaces an old one in a circular manner: first adding an element replaces path_nodes[0], then path_nodes[1], and so on til we get to the end of the array. After that, we begin again with path_nodes[0].

announce_list is different. It is sorted by distance to real our public key. When an element is added, it is checked against the farest node. If the element is closer to us than the node, the node is removed and the element is inserted. Otherwise, the element is simply discarded.

Some packet data structures:

struct DhtPkData {
no_replay: u64,
dht_pk: PublicKey,
nodes: Vec<PackedNode>,
}

layout DhtPkData {
u8 = ONION_DATA_DHTPK,
u64,
[u8; PUBLIC_KEY_SIZE],
[[u8; PACKED_NODE_SIZE]; 0..MAX_SENT_NODES]
}


## Toxcore notes

In the C implementation, ping_debt and usage_debt are called unsuccessful_pings and last_path_used_times correspondingly. New names are chosen to represent the meaning of these variables more clearly.