A message can travel five ways. CHANNEL tries them in order and takes the first one that works: Bluetooth, then direct Wi-Fi, then the local network, then straight over the internet, then our mailbox. Each has a letter, and the letter on a message tells you which one carried it. Direct Wi-Fi and the local network send only to their phone, once it has proved on that connection that it is theirs; when no such phone is there, the message goes the next way. With the internet available, CHANNEL sends by Bluetooth only to a nearby phone it knows is theirs. Without it, and with Relay for Others set to Anyone, every phone in Bluetooth range is offered the message, since one may carry it on; in any other position only their own phone is used, and in Contacts Only a contact's phone nearby may carry it on. The message shows no letter until their phone confirms how it arrived.
| Mark | Path | What it needs |
|---|---|---|
| B | Bluetooth | Both phones nearby |
| W | Direct Wi-Fi | Both phones nearby, no network at all, and both the same kind of phone |
| L | Local Network | Both phones on the same Wi-Fi |
| 6 | Direct over the internet | Both phones have an IPv6 internet address |
| I | Internet mailbox | Our server, from anywhere |
W works between two iPhones or between two Android phones, not between one of each. The two systems use different radios for it — Apple's own, and the Wi-Fi Alliance's — and they do not talk to each other. Nothing is lost: an iPhone and an Android in the same room use B, or L if they share Wi-Fi.
The first three never leave the room. 6 crosses the internet without touching our server: when both phones have an IPv6 address, which most mobile networks and many home networks give, CHANNEL connects the two phones straight to each other. The first message to someone goes by I while that connection is made, and the ones after it go by 6 while it stays open. The networks in between can see that your two addresses are talking to each other, which the mailbox does not show them; the Internet switch under Settings › Reach turns 6 off too. Only I touches our server, and you can switch it off for a profile under Settings › Reach.
Each profile uses only the routes switched on for it under Settings › Reach, for sending and for listening alike. A family profile can use the internet while another profile on the same phone keeps to B and L, and nothing about one profile's traffic travels by a route the other has switched off.
Settings › Reach opens on This profile: a switch changed there changes only the profile you are using. All profiles at the top changes every profile on this phone, until you leave the page.
Every profile on the phone listens on the routes it has switched on, all the time and not only while it is on screen: a message for a profile you are not using arrives by B, W or L when that profile has the route on, and a transfer under way goes on when you switch profiles. The phone has one presence on each of those routes for all its profiles, so a phone nearby sees one device and never one per profile.
Between two people, the stricter of the two settings wins. Your card tells the other person's phone which routes you take, and their phone sends to you only by a route both of you allow, in the order above. A phone that has not yet heard which routes the other person takes tries them in that order; a route the other phone is not listening on delivers nothing, so the message goes by the next one.
Five letters sit under a contact's name. They show which paths are open right now. A path you have switched off in Settings is not shown at all.
They are always in the same order, B W L 6 I, and that is the order CHANNEL would try them in. So the leftmost lit letter is the path a message would take if you sent one now. The same order is used in the Reach summary in Settings, and on the letter a message carries once it has gone. Tap the letters in a conversation, or on a contact's page, and a small popup names each letter shown and says whether that path is available now; tap anywhere else and it closes.
The letters do not all mean the same kind of thing.
On a device of a shared profile, a contact the device reaches only through the phone shows the letters the phone holds for that contact, behind a small phone symbol, and a screen reader says Ways to reach this contact through your <device>. The phone passes along what it sees, and a device shows nothing rather than letters that are out of date. A contact the device can reach itself keeps its own letters. A message a contact sends is marked with the letter of the copy that reached this device.
L is on when CHANNEL first runs, and it is the one path that says something about you to people who are not your contacts.
To find another phone on the same Wi-Fi, CHANNEL announces itself on that network. The announcement is a random label that changes every fifteen minutes and whenever you move to another network, under a service name that does not mention CHANNEL or messaging, with a random host name that changes with it. It carries no device name, no account and no contact.
Anyone else on the network who looks can see that something on your phone is announcing, and your phone's network address, as they can for every device on that network. They cannot see from the announcement that it is CHANNEL or whose phone it is.
Nothing is said about who you talk to, and no message is readable.
If that matters where you are, switch Local Network off in Settings › Reach. The other paths are unaffected, and messages still reach the same people. It is one tap, and you can put it back when you leave.
L needs both phones on the same Wi-Fi. That network does not have to be a router in a building. One phone turning on its personal hotspot makes a Wi-Fi network, and the other phone joining it puts both on the same one. CHANNEL needs no setting for this and is told nothing about how the network came to exist: it sees a shared network and uses L.
It is worth knowing because of the difference in speed. A photo or a video that takes many minutes over Bluetooth crosses a local network in a small fraction of that, and the local network usually reaches further than Bluetooth does.
Whoever turns on the hotspot decides whether it has a password. Two things they should know: the phone sharing it can see that traffic passes between the two devices, though not what is in it, and a hotspot with no password can be joined by anyone in range.
A message that goes by radio is not also posted to our server. If any radio can see the contact, the message is offered to those radios alone, and the internet never sees it.
The internet carries a message in two cases:
You set that time on the contact's page: At Once, After a Minute, After 2 Minutes, After 5 Minutes, or Never. A new contact starts at At Once. Never means that person is reached by radio or not at all.
Read receipts, cards and settings updates follow the same rule, with no timer. They go by radio when one can see the contact, and by the internet otherwise.
The radios are tried first either way, and they keep being tried while the timer runs. What the timer decides is only whether the internet may carry the message as well. So a contact set to a wait, whose radio can be seen but will not carry anything, waits that long before the message leaves by another road.
With no internet at all, a message to a contact still reaches them by Bluetooth while their phone is in range: CHANNEL remembers for five minutes which nearby phone showed it was theirs, so a message does not wait to ask again.
The lights are checked every two seconds, so a phone coming into range shows within a few seconds. A Bluetooth send that is not going to work gives up after about six seconds and the message goes by the Internet instead. How long each path is given before the next one is tried is learned from what that path has actually cost on your device, under Settings › Performance, and only ever gets shorter than the shipped setting.
A contact shows Waiting when nothing can carry a message to them yet: they are not nearby and there is no mailbox for them. CHANNEL never says "no profile" or "not on CHANNEL", because it does not know that, and neither should anyone watching.
When every path is switched off, CHANNEL says so as the last one goes off, and a message waits on your device. A line above the message box, Open Reach, opens the Reach page, and the message goes the moment you switch a path on.
On Android, Settings › Reach shows one line under the Bluetooth switch. It tells you whether this device is announcing itself, which is how a phone that has never met yours finds it.
The iPhone does not show this line.
Message another way, on a contact's own page, hands your words to another app: Messages, WhatsApp, Signal or Telegram. It is a choice you make for one message rather than a path that comes and goes, so it never appears among the letters under a contact's name and it never wins or loses a race against a radio. A text message in a thread wears an S of its own.
A text is not sealed by CHANNEL: the phone network can read it. See Messages.
The Internet switch in Settings › Reach belongs to one profile. With it off, that profile asks the server for nothing: it sends nothing there, checks no mailbox, registers for no wake-up notifications, renews no handle and fetches no call servers. Messages, receipts, cards and calls go on over Bluetooth, Wi-Fi Direct and the local network, and a nearby phone with Internet can carry a message on for you. A call between two phones on the same network connects without the server. Whatever needs the server waits, and goes as soon as the switch is on again.
A group that lives on the server shows Internet is off for this profile at the top of its conversation, and its messages wait.
A handle needs the server to stay yours. It is renewed while the Internet is on for the profile. With the switch off, the profile shows Your handle @name lapses on a date unless Internet is on for this profile from a week before that date, and again on the last day. Turning the switch on renews the handle at once.
When the switch comes on, the profile renews what is due, registers for wake-up notifications, checks for mail and sends what waited, all at once. Performance shows how many requests the switch held, as Internet off for this profile.
When two phones meet, each can carry sealed mail the other cannot send itself, and post it to the recipient's mailbox once it reaches the internet. The carrying phone cannot read what it carries. Settings › Relay › Relay for Others controls whether a profile does so, and each profile has its own: turning it on for the profile you are using leaves every other profile on the phone unchanged. A phone carrying your message posts it within an hour of when it was sealed; after that it stops trying, and your own phone sends the message as soon as it has a way.
It has three positions. Off, where nothing is carried in either direction. Contacts Only, where this profile carries for your contacts and only a contact carries yours: a message with no other way out is handed to a contact's phone nearby once that phone has proved who it is. And Anyone, where it carries for any phone nearby and any phone nearby may carry yours. Over the local network only a contact's phone takes part; Bluetooth reaches other phones as well, in the Anyone position.
The reason the setting works in both directions at once is that it is one bargain, not two. A phone you will not carry for is a phone that never offers to carry for you, and there is no position that takes without giving. Your own messages are carried only while Relay for Others is on for your profile.
What another phone learns by carrying for you: that some sealed message passed through it. Not what it says, not who wrote it, and not who it is for.
What a carried message shows. A message that has gone no further than the phones around you says so under the conversation and in Message Info. Carried by a nearby device means a nearby phone confirmed that it took the message; it is delivered when that phone reaches the internet. Waiting: no route means no phone has confirmed taking it, and the line says why: it was handed to nearby phones and none confirmed carrying it, Relay for Others is off so none of them may carry it on, or nothing nearby can carry it. A waiting message also goes by the internet as soon as the internet reaches them. When everything is already on the server, the line reads Waiting for their phone to collect it from the server, and the message is delivered when their phone fetches it. A phone keeps what it carries for others when it is restarted, until it hands it on or a day has passed. Once a carried message reaches them, it shows Delivered as usual, when the receipt reaches you by the internet or through a nearby phone that fetches your mail.
A nearby device carries a message of up to 16 MB. A larger one reads Too large to be carried by a nearby device; it is sent when this device reaches the Internet.
Fetching for a phone with no internet. With the setting on, a phone that has the internet also fetches mail for a nearby phone that has none, over Bluetooth or the local network, and yours can have its mail fetched the same way, including when the internet is switched off for your profile under Settings › Reach. The request is sealed so that the phone fetching it cannot read it, and so is the answer. What that phone learns is when you checked for mail and roughly how much was waiting, never who sent it or what it says. It costs the fetching phone its data and battery. A profile with the internet switched off fetches nothing for anyone.
Settings › Reach › Let Nearby Devices Carry My Messages to the Internet is on for each profile unless you turn it off. When a nearby device carries your message, our server sees that device's connection, never yours, and nobody can read the message. The carrier and our server can still tell that a message passed through near that device at that time; if that alone could put you at risk, turn this off.
Off, nothing this profile sends is handed to a nearby device. It goes by a direct route (Bluetooth, Wi-Fi Direct or the local network) or waits, and the line under the conversation reads Waiting for a direct route; nearby devices are not allowed to carry this profile's messages.
Faster Bluetooth carries messages and pictures between two phones over a Bluetooth channel made for larger transfers. It is on unless you turn it off in Settings › Previews, and it is used when both phones have it; otherwise, or if the channel will not open, Bluetooth carries everything the usual way. What it carries is sealed exactly as before. The choice is stored on this device only, and a change applies from the next time CHANNEL starts.
Keep Bluetooth Ready, also under Previews, keeps the Bluetooth link to a contact you are writing to open while they are nearby, so messages to them go at once. It uses more battery.
English is the reference text; any translation is a convenience.
CHANNEL