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Toxcore Multi-Device Support Design Proposal
https://docs.google.com/document/d/1op6zGR0KYdF7tTWSSX79KQieJu30vLZ6XG327kIBhxQ/
To be a useable communication system, Tox needs to support multiple devices per user. This is the TokTok proposal.
Objective
This proposal is an RFC for a possible implementation of multi-device support for the Toxcore Messenger system. With the goal of replacing Skype we must come close to feature parity, as well as ease of use. Currently Tox clients need to share a ToxID. This ID inhibits connecting with existing friends as it is only well-suited to machine reading.
The primary goal is to make it simple for clients to securely implement multi-device support, without having to know or manage anything about a contact’s devices.
The following implementation requires some additional features to be added to Toxcore. These features have been requested before; however, the goal of this implementation is to add Multi-Device support only. The additional APIs that would be required (see next section) would only need to be minimal implementations to get multi-device working. And while their feature set should be expanded, that’s not the goal of this implementation.
Background
ToxID’s are difficult to share without having an existing textual communication connection (anything you can copy from or paste to). Sharing ToxID’s in other ways is problematic.
When it comes to multi-device support, at the present time we are lagging behind well-known centralized messenger systems. A few examples are Google Hangouts, Facebook Messenger, and Skype. Every one of these syncs messages to every device which connects to their network. If you need an address, number, or entire friend list it’s a simple login away.
Not the goal of this project but an important benefit, Multi-Device support would solve some of the issues sharing a ToxID: it would allow the clients to connect with mobile devices (NFC, QR codes, other) and be available anywhere.
Prerequisites
Data Syncing (Friends and Messages):
Devices may go online and offline. It’s required that an online device be able to keep other devices in the “device group” in sync, Updating both friends and messages.(a)^1[^2]
Contact Syncing
Contacts (called friends in the Toxcore API) are synced across all devices using a transactional method. As a pair the devices decide which will have the primary role, and which will take secondary. hroughout the entire syncing process, the primary device will send it’s data, followed by a “done” packet. The secondary will then send its data, followed by its own “done” packet. The primary device will then send a “commit” packet, and will then commit the data from temporary storage, into active use. Once receiving the “commit” packet, the secondary device will do the same.
Either Client can send an Error packet at either time, which will then Message ordering
Devices may sync out of order, but messages need to be consistently ordered. To simplify this, messages will be separated into “connection sessions”, each getting a pseudorandom tox_session_ID. Messages will then be ordered within that session by a delta_t. Delta_t in this case will be the elapsed seconds from the start of the tox instance’s first current connection to that peer.
Each session will be identified by a pseudo-unique hash from the previous connect that included a confirmed message receipt. This will ensure that both sessions and messages are ordered correctly once each client is completely synced. (Optionally robust verifiability can be gained by a git style hash of the x parent sessions)
Pseudo-unique Message IDs
Update
typedef void tox_friend_message_cb(Tox *tox, uint32_t friend_number, TOX_MESSAGE_TYPE type, const uint8_t *message, size_t length, void *user_data);
with
typedef void tox_friend_message_cb(Tox *tox, uint32_t friend_number, TOX_MESSAGE_TYPE type, const uint8_t *message, size_t length, uint64_t message_id, uint64_t delta_t, void *user_data);
Required for syncing messages between friends * devices. Each message would include a message_id that would increment with each message. and reset with each new tox_instance (starting message ID is currently undecided). Messages id sections would have to be grouped by a client coming online or going offline. These are per friend, per connection. And would be the same when sending the same message to multiple devices of a single friend.
Each of these are pending an implementation spec. But are self-evident in their use and general implementation.
Implementation
Security and safety is the primary concern, however this needs to be tempered with usability. The user needs to be prompted to choose high availability or high security.
Device IDs
Device IDs need to be unique and never changing. On each connect, each
client should query each friend they connect to, to verify if they
support multi-device AND are in a group. This can be done by asking
for the deviceID at __device0. If there is a device at that
address; then enumerate for each other device. Devices will be found
by the string_to_id service using the protected __device[0-9]+
group. (Toxcore must protect strings that start with two underscores
such as “__[string]”, and not allow a client to use them in the
string_to_id API.)
Starting with __device0, the client will attempt to resolve the
ToxID at each incrementing __device#. Once a __device#
returns empty, the client should stop. Each __device# string
should be queried separately, ideally in a pseudo-random order to
prevent clear text attacks on the encryption. {under discussion} For
each ToxID at each __device# the client must add them to their
list of devices for this friend/user, and probe each device in the
same way. The default should be high security mode, in which if any
list at any device do not match, toxcore should report that friend to
be in an unsecure state, and shouldn’t send any messages to any of the
devices.
The __device# list for every client must match exactly to what every other device reports. And each __device# once used must always resolve to that ID.
To enhance security once a __device# (number) is taken by a
device, that __device# must always resolve to either the same
ToxID, or to all zeroes [0] if the device has been removed from the
Tox “group” by the user, in a case such as losing the private key, or
uninstalling the Tox client. Toxcore will maintain a local list of
each device for each user, changes to a __device# will be
assumed to be an impersonation attack. Removed devices will still keep
their __device#, but that __device# will instead resolve
to zeros. A string of zeros signifies that there was a device, but it
was removed intentionally. A __device# once resolved to zeros,
must never change back to a ToxID.
To remove or add a device, you will tell toxcore to add the device at
ToxID to this group. That toxcore instance will then, try to connect
to that device and verify that it also wants to connect to the
existing device group. Once accepted, the toxcore instance on the
primary device will assign a __device# to the new device, and
sync both it’s ID, to the other devices already in the group, as well
as the existing devices to the new device. Once all the devices in the
group are in sync, any of the existing and online devices can sync the
friend/contact list, and the selected amount of history text history
to the new device.
A max limit of devices will need to be chosen later to limit worst case usage of enumerating devices. (Additional protocol requirements will be detailed later, i.a. security limitations, verifiable, master device, slave device, deauthed devices.)
Message sending
When a client sends a message to a friend/user, Toxcore will only attempt to send the message to the last 3 devices that sent data from the user that are online.(c)[^3] (d)[^4] (e)[^5] E.g., Device_A sends a message, Device_B comes online, Device_C sends a file, Device_D sends a message then goes off line, Device_F sends a picture, Device_H comes online. Toxcore will attempt to send only to devices A, C and F. While H did have the latest data sent/received, it wasn’t data generated by the user, so we assume that the user is most likely to be on one of those 3 devices. Toxcore will only have to verify that one client received the message, and may stop. This will protect the sender from having to maintain a list of who needs what message, and leave the additional data syncing up to the receiving client.
It is considered the job of the receiving client to keep all the other devices in sync with the rest of the group.
Alternatives Considered
- Not supporting multi-device (separate friends in the contact list for each device).
- Not considered, this feature is required to be competitive. Not user friendly, no syncing, too hard to create and maintain friend lists.
-
Sharing the private key of a single ToxID to multiple devices.
- Non starter. Security nightmare.
- Requires protocol rewrite.
Unsolved Issues
How will messages be hashed to generate the seed for the message ID?
Effective and secure history syncing across devices.
How to add a user to your contact list
-
By Primary ToxID(f)[^6](g)[^7] -
By any deviceID -
Something else?
Roadmap
- Unique message IDs
- incrementing numbers
- delta-from last
- Device network
- deviceID vs toxID
- Adding a device
- Removing a device
- Sending a message to more than one device
- Syncing data
- contact list
- messages
- other?
Notes & FAQ:
Conversations that need to be added to the spec:
<nurupo> say Alice has a tox client running on a phone and another tox client on pc. Alice has Bob in contacts. Bob is online and sends Alice a file.
<nurupo> will Alice see the file request on both clients?
<grayhatter> the file request will go to all online devices
<nurupo> ok, i'm not finished
<grayhatter> then the file will get canceled / "marked as accpeted on a different device"
<nurupo> so, Alice accepts and completes the file transfer on her phone
(h)[^8] (i)[^9] (j)[^10]
<nurupo> i guess you answered what i wanted to know
<nurupo> what if race condition happens?
<grayhatter> but I hadn't considered that detail... It'll break the existing file transfer API...
<nurupo> Alice accepts files at the same time both on pc and phone?
<grayhatter> nurupo, can't happen
<nurupo> why it can't happen?
<grayhatter> the accept will processed by toxcore on the "sending" device in a single thread, it'll accept for the first device that accepts the transfer, then cancel all other divices
<grayhatter> if two devices accept at the same time, toxcore will choose the device in the lowest number slot only because the loop will start at 0
<nurupo> what if Alice accepts a file on another device before it gets the cancel message?
<grayhatter> it'll show inprogress on that device, with 0 transfer, then toxcore on the sending device will still send the cancel to that device
<nurupo> Bob will get two accepts from two devices of Alice?
<grayhatter> yes, but it will ignore the 2nd
<nurupo> how it knows which to ignore?
<grayhatter> (I'm saying all of this from what my plans are, I haven't even started on filetransfers yet)
<grayhatter> psudo code ->
<nurupo> Alice might have accepted the file transfer on the phone first, but because of network latency, Bob received the acceptance from pc first and only then from the phone
<grayhatter> OH
<grayhatter> bob will in that case cancel the phone then
(k)[^11]
<nurupo> fair enough
<grayhatter> no reason to make it more complicated then it needs to be
Feature Requests:
(Requested publicly, no suggestions have been reviewed or accepted yet.)
-
Per device id priority like with XMPP https://wiki.xmpp.org/web/Jabber_Resources
- Per status+deviceid priority, XMPP does that as well
-
Ability to address device id explicitly
Footnotes
-
(a)[^1] Does this design document define a way in which friends (and perhaps groups) are to be synced? And how would friend/group syncing work across multi-devices that don't have a common time base?
-
(b)[^2] I haven't fully decided on the final spec for this question.
Currently my plan is to retain information about changes as a numerical delta, and use the largest delta. But as I haven't started on this section, really I have no idea.
-
(c)[^3] What about in the case of an A/V call? Does that get broadcast to call connected clients or only the last 3? A tox client running on a mobile device that had not sent a user-generated message missing a call would be bad.
-
(d)[^4] I'm likely to drop that idea in total.
Currently toxcore sends everything to all devices.
-
(e)[^5] With other services such as Discord, when a call comes in depending on what is on, I could have two PCs, a laptop and cellphone all ring, but whatever device I answer on obviously gets the call.
-
(f)[^6] how is primary toxID different from any other ID? which device is the primary one? as far as I understood they are all equal.
-
(g)[^7] I'm not sure there's going to be anything like a primary ID in the final source. When discussing it, the idea of a master device came up as a security tool. But the longer I consider it, the less I think it'll help at all
-
(h)[^8] Does it make sense to think about sending the file to both clients, if the user accepts on multiple devices? Especially for bigger files or multiple recipients a P2P protocol (e.g. torrent) may be useful.
-
(i)[^9] Yeah, I've given that a little thought. The current idea floating around has been to implement something torrent like inside group chats. Maybe we could use that as well? As far as multiple devices, what I think is best is transfer the file to the first to accept, then let the other devices sync the file themselves. That way the sender doesn't have to spend the extra data for the receivers convenience.
-
(j)[^10] I like that idea. Especially if one thinks about those users on a phone with limited traffic. You wouldn't want to use all your available data just so others can sync. You could add a button on top of lets say a picture saying "sync with my other devices" to start the transfer of that picture to all your other devices. But just sending it once should be a lot easier. I'd consider media sync between my devices as a nice-to-have.
-
(k)[^11] Could it be a good idea to allow sync incomplete downloaded files from device A to device B and then start downloading from device B
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