How do I implement low-latency streaming between two local iOS devices?

Multipeer Connectivity only works on the local network, so I presume that’s a constraint that you’re prepared to accept. However, there’s one wrinkle here. Multipeer Connectivity supports two local network scenarios [1]:

• Infrastructure Wi-Fi, where both devices are on the same infrastructure Wi-Fi network

• Peer-to-peer, where they are not

Do you care about the latter?

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

[1] There’s nothing special about Multipeer Connectivity here. Many other networking APIs, including Network framework, support peer-to-peer Wi-Fi.

I only care about local peer-to-peer for this specific use-case.

The term “local peer-to-peer” means different things to do different people. Do you expect these devices to be on the same infrastructure Wi-Fi?

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Don’t worry about not understanding this stuff. Peer-to-peer Wi-Fi is a complex topic. Even if you limit yourself to the Apple ecosystem, there are two implementations in play! [1] And there many more outside of the Apple ecosystem.

I do not expect the devices to be connected to the same infrastructure Wi-Fi

OK.

with the session being purely and directly between them

Just for context, that’s not how Apple peer-to-peer Wi-Fi works. Apple peer-to-peer Wi-Fi is not point-to-point. Rather, the peers negotiate and then use a shared Wi-Fi network.

There’s a trade-off in play here. In general, infrastructure Wi-Fi is faster than peer-to-peer Wi-Fi. Moreover, the use of peer-to-peer can degrade the performance of infrastructure. So, if you’re looking for best performance, it’s better to require infrastructure Wi-Fi.

This is one (of the many :-) place where Network framework is better than Multipeer Connectivity because Network framework gives you control over whether to use peer-to-peer on not (via its includesPeerToPeer property). If you using Multipeer Connectivity, you always opt in to peer-to-peer.

Coming back to your original question, you wrote:

I'd like to know whether Multipeer Connectivity (MPC) between two modern iOS devices can support cross-streaming of video content at low latency and relatively high resolution

I don’t think Multipeer Connectivity is an issue here; it adds some overhead but not a substantial amount [2].

The real issue is whether the underlying peer-to-peer network can deal with your load. It’s hard to answers that for two reasons:

• You didn’t post any concrete info about your load.

• Even if you had, network performance tends to vary a lot based on the situation, both with the devices themselves and the local RF environment.

What I recommend is that you set up some performance tests. First look at the video side of this, so you can get a handle on your latency and bandwidth requirements. Then create a tiny test app that transfers data using Network framework, run it in realistic circumstances, and see if it gets anywhere near those requirements.

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—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

[1] You could see evidence for this in older versions of macOS where the Finder > Go > AirDrop window had a “search for older devices” option. Fortunately that older protocol has now fallen by the wayside.

[2] Assuming you restrict yourself to its datagram features. The high-level stream and resource transfer facilities are more complex and thus its hard to evaluate their performance.

Oh, I should’ve posted this link earlier: Wi-Fi Fundamentals. It defines the terms that I use in answers like this.

If I have one device set up a hotspot, and have the other connect to it, would I be able to use the resulting network as though both devices are sharing an infrastructure Wi-Fi?

Hmmm, tricky. Personal Hotspot certainly does create an infrastructure Wi-Fi network. The problem with relying on it is that it doesn’t guarantee connectivity between your code running on a STA and your code running on the AP. I believe that this connectivity works in practice, but if you build a product around it then you’re relying on it working that way forever.

This is an issue with infrastructure Wi-Fi in general. To quote the post I referenced about: Some APs refuse to forward STA-to-STA traffic.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Edit:

@Eskimo

Err come to think about it I should ask for one clarification regarding APs not forwarding STA-to-STA traffic before I burn myself:

In your previous response you were ambiguous about the device brand, probably due to all the Peer-to-Peer Wi-Fi specifications out there.

My use case is only between Apple products. Period.

Does the phenomenon of some APs not forwarding STA-to-STA traffic apply to iPhone Hotspots talking with iPhones or iPads or Macs? E.g. Can an iPhone hosting a Hotspot Connection (being both AP and STA) and the iPhone being hosted (the pure STA) communicate with each other (e.g. cross streaming videos)?

I did a double take and came back to ask because this is critical to me.

I should ask for one clarification regarding APs not forwarding STA-to-STA traffic

The problem here is that Personal Hotspot is a user-facing feature, not a developer API. Last I checked it will forward STA-to-STA traffic [1]. However, that’s not really a part of Personal Hotpot’s core mission, which is to provide Internet connectivity to the STAs. You could imagine a world where Apple changed this behaviour. The vast majority of Personal Hotspot users wouldn’t notice, but your use case would be completely toast.

So, is this behaviour going to change? I can’t predict The Future™ in general, and that’s doubly so for user-level features like Personal Hotspot.

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—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

[1] Including the not-really-a-STA case of an app running on the device running Personal Hotspot.