Moving from Multipeer Connectivity to Network Framework

App & System Services Networking
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DTS Engineer OP
Apple
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I see a lot of folks spend a lot of time trying to get Multipeer Connectivity to work for them. My experience is that the final result is often unsatisfactory. Instead, my medium-to-long term recommendation is to use Network framework instead. This post explains how you might move from Multipeer Connectivity to Network framework.

If you have questions or comments, put them in a new thread. Place it in the App & System Services > Networking topic area and tag it with Multipeer Connectivity and Network framework.

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

Moving from Multipeer Connectivity to Network Framework

Multipeer Connectivity has a number of drawbacks:

  • It has an opinionated networking model, where every participant in a session is a symmetric peer. Many apps work better with the traditional client/server model.

  • It offers good latency but poor throughput.

  • It doesn’t support flow control, aka back pressure, which severely constrains its utility for general-purpose networking.

  • It includes a number of UI components that are effectively obsolete.

  • It hasn’t evolved in recent years. For example, it relies on NSStream, which has been scheduled for deprecation as far as networking is concerned.

  • It always enables peer-to-peer Wi-Fi, something that’s not required for many apps and can impact the performance of the network (see Enable peer-to-peer Wi-Fi, below, for more about this).

  • Its security model requires the use of PKI — public key infrastructure, that is, digital identities and certificates — which are tricky to deploy in a peer-to-peer environment.

  • It has some gnarly bugs.

IMPORTANT Many folks use Multipeer Connectivity because they think it’s the only way to use peer-to-peer Wi-Fi. That’s not the case. Network framework has opt-in peer-to-peer Wi-Fi support. See Enable peer-to-peer Wi-Fi, below.

If Multipeer Connectivity is not working well for you, consider moving to Network framework. This post explains how to do that in 13 easy steps (-:

  1. Plan for security

  2. Select a network architecture

  3. Create a peer identifier

  4. Choose a protocol to match your send mode

  5. Discover peers

  6. Design for privacy

  7. Configure your connections

  8. Manage a listener

  9. Manage a connection

  10. Send and receive reliable messages

  11. Send and receive best effort messages

  12. Start a stream

  13. Send a resource

Finally, at the end of the post you’ll find two appendices:

  • Final notes contains some general hints and tips.

  • Symbol cross reference maps symbols in the Multipeer Connectivity framework to sections of this post. Consult it if you’re not sure where to start with a specific Multipeer Connectivity construct.

Plan for security

The first thing you need to think about is security. Multipeer Connectivity offers three security models, expressed as choices in the MCEncryptionPreference enum:

  • .none for no security

  • .optional for optional security

  • .required for required security

For required security each peer must have a digital identity.

Optional security is largely pointless. It’s more complex than no security but doesn’t yield any benefits. So, in this post we’ll focus on the no security and required security models.

Your security choice affects the network protocols you can use:

  • QUIC is always secure.

  • WebSocket, TCP, and UDP can be used with and without TLS security.

QUIC security only supports PKI. TLS security supports both TLS-PKI and pre-shared key (PSK). You might find that TLS-PSK is easier to deploy in a peer-to-peer environment.

To configure the security of the QUIC protocol:

func quicParameters() -> NWParameters {
    let quic = NWProtocolQUIC.Options(alpn: ["MyAPLN"])
    let sec = quic.securityProtocolOptions
    … configure `sec` here …
    return NWParameters(quic: quic)
}

To enable TLS over TCP:

func tlsOverTCPParameters() -> NWParameters {
    let tcp = NWProtocolTCP.Options()
    let tls = NWProtocolTLS.Options()
    let sec = tls.securityProtocolOptions
    … configure `sec` here …
    return NWParameters(tls: tls, tcp: tcp)
}

To enable TLS over UDP, also known as DTLS:

func dtlsOverUDPParameters() -> NWParameters {
    let udp = NWProtocolUDP.Options()
    let dtls = NWProtocolTLS.Options()
    let sec = dtls.securityProtocolOptions
    … configure `sec` here …
    return NWParameters(dtls: dtls, udp: udp)
}

To configure TLS with a local digital identity and custom server trust evaluation:

func configureTLSPKI(sec: sec_protocol_options_t, identity: SecIdentity) {
    let secIdentity = sec_identity_create(identity)!
    sec_protocol_options_set_local_identity(sec, secIdentity)
    if disableServerTrustEvaluation {
        sec_protocol_options_set_verify_block(sec, { metadata, secTrust, completionHandler in
            let trust = sec_trust_copy_ref(secTrust).takeRetainedValue()
            … evaluate `trust` here …
            completionHandler(true)
        }, .main)
    }
}

To configure TLS with a pre-shared key:

func configureTLSPSK(sec: sec_protocol_options_t, identity: Data, key: Data) {
    let identityDD = identity.withUnsafeBytes { DispatchData(bytes: $0) }
    let keyDD = identity.withUnsafeBytes { DispatchData(bytes: $0) }
    sec_protocol_options_add_pre_shared_key(
        sec,
        keyDD as dispatch_data_t,
        identityDD as dispatch_data_t
    )
    sec_protocol_options_append_tls_ciphersuite(
        sec,
        tls_ciphersuite_t(rawValue: TLS_PSK_WITH_AES_128_GCM_SHA256)!
    )
}

Select a network architecture

Multipeer Connectivity uses a star network architecture. All peers are equal, and every peer is effectively connected to every peer. Many apps work better with the client/server model, where one peer acts on the server and all the others are clients. Network framework supports both models.

To implement a client/server network architecture with Network framework:

  1. Designate one peer as the server and all the others as clients.

  2. On the server, use NWListener to listen for incoming connections.

  3. On each client, use NWConnection to made an outgoing connection to the server.

To implement a star network architecture with Network framework:

  1. On each peer, start a listener.

  2. And also start a connection to each of the other peers.

This is likely to generate a lot of redundant connections, as peer A connects to peer B and vice versa. You’ll need to a way to deduplicate those connections, which is the subject of the next section.

IMPORTANT While the star network architecture is more likely to create redundant connections, the client/server network architecture can generate redundant connections as well. The advice in the next section applies to both architectures.

Create a peer identifier

Multipeer Connectivity uses MCPeerID to uniquely identify each peer. There’s nothing particularly magic about MCPeerID; it’s effectively a wrapper around a large random number.

To identify each peer in Network framework, generate your own large random number. One good choice for a peer identifier is a locally generated UUID, created using the system UUID type.

Some Multipeer Connectivity apps persist their local MCPeerID value, taking advantage of its NSSecureCoding support. You can do the same with a UUID, using either its string representation or its Codable support.

IMPORTANT Before you decide to persist a peer identifier, think about the privacy implications. See Design for privacy below.

Avoid having multiple connections between peers; that’s both wasteful and potentially confusing. Use your peer identifier to deduplicate connections.

Deduplicating connections in a client/server network architecture is easy. Have each client check in with the server with its peer identifier. If the server already has a connection for that identifier, it can either close the old connection and keep the new connection, or vice versa.

Deduplicating connections in a star network architecture is a bit trickier. One option is to have each peer send its peer identifier to the other peer and then the peer with the ‘best’ identifier wins. For example, imagine that peer A makes an outgoing connection to peer B while peer B is simultaneously making an outgoing connection to peer A. When it receives the incoming connection, each peer compares its peer identifier to that of the other. If it’s peer identifier is larger, it drops the incoming connection, on the assumption that it’s outgoing connection will ‘win’.

Choose a protocol to match your send mode

Multipeer Connectivity offers two send modes, expressed as choices in the MCSessionSendDataMode enum:

  • .reliable for reliable messages

  • .unreliable for best effort messages

Best effort is useful when sending latency-sensitive data, that is, data where retransmission is pointless because, by the retransmission arrives, the data will no longer be relevant. This is common in audio and video applications.

In Network framework, the send mode is set by the connection’s protocol:

  • A specific QUIC connection is either reliable or best effort.

  • WebSocket and TCP are reliable.

  • UDP is best effort.

Start with a reliable connection. In many cases you can stop there, because you never need a best effort connection.

If you’re not sure which reliable protocol to use, choose WebSocket. It has key advantages over other protocols:

  • It supports both security models: none and required. Moreover, its required security model supports both TLS-PKI and TLS PSK. In contrast, QUIC only supports the required security model, and within that model it only supports TLS-PKI.

  • It allows you to send messages over the connection. In contrast, TCP works in terms of bytes, meaning that you have to add your own framing.

If you need a best effort connection, get started with a reliable connection and use that connection to set up a parallel best effort connection. For example, you might have an exchange like this:

  1. Peer A uses its reliable WebSocket connection to peer B to send a request for a parallel best effort UDP connection.

  2. Peer B receives that, opens a UDP listener, and sends the UDP listener’s port number back to peer A.

  3. Peer A opens its parallel UDP connection to that port on peer B.

Note For step 3, get peer B’s IP address from the currentPath property of the reliable WebSocket connection.

If you’re not sure which best effort protocol to use, use UDP. While it is possible to use QUIC in datagram mode, it has the same security complexities as QUIC in reliable mode.

Discover peers

Multipeer Connectivity has a types for advertising a peer’s session (MCAdvertiserAssistant) and a type for browsering for peer (MCNearbyServiceBrowser).

In Network framework, configure the listener to advertise its service by setting the service property of NWListener:

let listener: NWListener = …
listener.service = .init(type: "_example._tcp")
listener.serviceRegistrationUpdateHandler = { change in
    switch change {
    case .add(let endpoint):
        … update UI for the added listener endpoint …
        break
    case .remove(let endpoint):
        … update UI for the removed listener endpoint …
        break
    @unknown default:
        break
    }
}
listener.stateUpdateHandler = … handle state changes …
listener.newConnectionHandler = … handle the new connection …
listener.start(queue: .main)

This example also shows how to use the serviceRegistrationUpdateHandler to update your UI to reflect changes in the listener.

Note This example uses a service type of _example._tcp. See About service types, below, for more details on that.

To browse for services, use NWBrowser:

let browser = NWBrowser(for: .bonjour(type: "_example._tcp", domain: nil), using: .tcp)
browser.browseResultsChangedHandler = { latestResults, _ in
    … update UI to show the latest results …
}
browser.stateUpdateHandler = … handle state changes …
browser.start(queue: .main)

This yields NWEndpoint values for each peer that it discovers. To connect to a given peer, create an NWConnection with that endpoint.

About service types

The examples in this post use _example._tcp for the service type. The first part, _example, is directly analogous to the serviceType value you supply when creating MCAdvertiserAssistant and MCNearbyServiceBrowser objects. The second part is either _tcp or _udp depending on the underlying transport protocol. For TCP and WebSocket, use _tcp. For UDP and QUIC, use _udp.

Service types are described in RFC 6335. If you deploy an app that uses a new service type, register taht service type with IANA.

Discovery UI

Multipeer Connectivity also has UI components for advertising (MCNearbyServiceAdvertiser) and browsing (MCBrowserViewController). There’s no direct equivalent to this in Network framework. Instead, use your preferred UI framework to create a UI that best suits your requirements.

Discovery TXT records

The Bonjour service discovery protocol used by Network framework supports TXT records. Using these, a listener can associate metadata with its service and a browser can get that metadata for each discovered service.

To advertise a TXT record with your listener, include it it the service property value:

let listener: NWListener = …
let peerID: UUID = …
var txtRecord = NWTXTRecord()
txtRecord["peerID"] = peerID.uuidString
listener.service = .init(type: "_example._tcp", txtRecord: txtRecord.data)

To browse for services and their associated TXT records, use the .bonjourWithTXTRecord(…) descriptor:

let browser = NWBrowser(for: .bonjourWithTXTRecord(type: "_example._tcp", domain: nil), using: .tcp)
browser.browseResultsChangedHandler = { latestResults, _ in
    for result in latestResults {
        guard
            case .bonjour(let txtRecord) = result.metadata,
            let peerID = txtRecord["peerID"]
        else { continue }
        // … examine `result` and `peerID` …
        _ = peerID
    }
}

This example includes the peer identifier in the TXT record with the goal of reducing the number of duplicate connections, but that’s just one potential use for TXT records.

Design for privacy

This section lists some privacy topics to consider as you implement your app. Obviously this isn’t an exhaustive list. For general advice on this topic, see Protecting the User’s Privacy.

There can be no privacy without security. If you didn’t opt in to security with Multipeer Connectivity because you didn’t want to deal with PKI, consider the TLS-PSK options offered by Network framework. For more on this topic, see Plan for security.

When you advertise a service, the default behaviour is to use the user-assigned device name as the service name. To override that, create a service with a custom name:

let listener: NWListener = …
let name: String = …
listener.service = .init(name: name, type: "_example._tcp")

It’s not uncommon for folks to user the peer identifier as the service name.

There are good reasons to persist your peer identifier, but doing so isn’t great for privacy. Persisting the identifier allows for tracking of your service over time and between networks. Consider whether you need a persistent peer identifier at all. If you do, consider whether it makes sense to rotate it over time.

A persistent peer identifier is especially worrying if you use it as your service name or put it in your TXT record.

Configure your connections

Multipeer Connectivity’s symmetric architecture means that it uses a single type, MCSession, to manage the connections to all peers.

In Network framework, that role is fulfilled by two types:

  • NWListener to listen for incoming connections.

  • NWConnection to make outgoing connections.

Both types require you to supply an NWParameters value that specifies the network protocol and options to use. In addition, when creating an NWConnection you pass in an NWEndpoint to tell it the service to connect to. For example, here’s how to configure a very simple listener for TCP:

let parameters = NWParameters.tcp
let listener = try NWListener(using: parameters)
… continue setting up the listener …

And here’s how you might configure an outgoing TCP connection:

let parameters = NWParameters.tcp
let endpoint = NWEndpoint.hostPort(host: "example.com", port: 80)
let connection = NWConnection.init(to: endpoint, using: parameters)
… continue setting up the connection …

NWParameters has properties to control exactly what protocol to use and what options to use with those protocols.

To work with QUIC connections, use code like that shown in the quicParameters() example from the Security section earlier in this post.

To work with TCP connections, use the NWParameters.tcp property as shown above.

To enable TLS on your TCP connections, use code like that shown in the tlsOverTCPParameters() example from the Security section earlier in this post.

To work with WebSocket connections, insert it into the application protocols array:

let parameters = NWParameters.tcp
let ws = NWProtocolWebSocket.Options(.version13)
parameters.defaultProtocolStack.applicationProtocols.insert(ws, at: 0)

To enable TLS on your WebSocket connections, use code like that shown in the tlsOverTCPParameters() example to create your base parameters and then add the WebSocket application protocol to that.

To work with UDP connections, use the NWParameters.udp property:

let parameters = NWParameters.udp

To enable TLS on your UDP connections, use code like that shown in the dtlsOverUDPParameters() example from the Security section earlier in this post.

Enable peer-to-peer Wi-Fi

By default, Network framework doesn’t use peer-to-peer Wi-Fi. To enable that, set the includePeerToPeer property on the parameters used to create your listener and connection objects.

parameters.includePeerToPeer = true

IMPORTANT Enabling peer-to-peer Wi-Fi can impact the performance of the network. Only opt into it if it’s a significant benefit to your app.

If you enable peer-to-peer Wi-Fi, it’s critical to stop network operations as soon as you’re done with them. For example, if you’re browsing for services with peer-to-peer Wi-Fi enabled and the user picks a service, stop the browse operation immediately. Otherwise, the ongoing browse operation might affect the performance of your connection.

Manage a listener

In Network framework, use NWListener to listen for incoming connections:

let parameters: NWParameters = .tcp
… configure parameters …
let listener = try NWListener(using: parameters)
listener.service = … service details …
listener.serviceRegistrationUpdateHandler = … handle service registration changes …
listener.stateUpdateHandler = { newState in
    … handle state changes …
}
listener.newConnectionHandler = { newConnection in
    … handle the new connection …
}
listener.start(queue: .main)

For details on how to set up parameters, see Configure your connections. For details on how to set up up service and serviceRegistrationUpdateHandler, see Discover peers.

Network framework calls your state update handler when the listener changes state:

let listener: NWListener = …
listener.stateUpdateHandler = { newState in
    switch newState {
    case .setup:
        // The listener has not yet started.
        …
    case .waiting(let error):
        // The listener tried to start and failed.  It might recover in the
        // future.
        …
    case .ready:
        // The listener is running.
        …
    case .failed(let error):
        // The listener tried to start and failed irrecoverably.
        …
    case .cancelled:
        // The listener was cancelled by you.
        …
    @unknown default:
        break
    }
}

Network framework calls your new connection handler when a client connects to it:

var connections: [NWConnection] = []

let listener: NWListener = listener
listener.newConnectionHandler = { newConnection in
    … configure the new connection …
    newConnection.start(queue: .main)
    connections.append(newConnection)
}

IMPORTANT Don’t forget to call start(queue:) on your connections.

In Multipeer Connectivity, the session (MCSession) keeps track of all the peers you’re communicating with. With Network framework, that responsibility falls on you. This example uses a simple connections array for that purpose. In your app you may or may not need a more complex data structure. For example:

  • In the client/server network architecture, the client only needs to manage the connections to a single peer, the server.

  • On the other hand, the server must managed the connections to all client peers.

  • In the star network architecture, every peer must maintain a listener and connections to each of the other peers.

Understand UDP flows

Network framework handles UDP using the same NWListener and NWConnection types as it uses for TCP. However, the underlying UDP protocol is not implemented in terms of listeners and connections. To resolve this, Network framework works in terms of UDP flows. A UDP flow is defined as a bidirectional sequence of UDP datagrams with the same 4 tuple (local IP address, local port, remote IP address, and remote port). In Network framework:

  • Each NWConnection object manages a single UDP flow.

  • If an NWListener receives a UDP datagram whose 4 tuple doesn’t match any known NWConnection, it creates a new NWConnection.

Manage a connection

In Network framework, use NWConnection to start an outgoing connection:

var connections: [NWConnection] = []

let parameters: NWParameters = …
let endpoint: NWEndpoint = …
let connection = NWConnection(to: endpoint, using: parameters)
connection.stateUpdateHandler = … handle state changes …
connection.viabilityUpdateHandler = … handle viability changes …
connection.pathUpdateHandler = … handle path changes …
connection.betterPathUpdateHandler = … handle better path notifications …
connection.start(queue: .main)

connections.append(connection)

As in the listener case, you’re responsible for keeping track of this connection.

Each connection supports four different handlers. Of these, the state and viability update handlers are the most important. For information about the path update and better path handlers, see the NWConnection documentation.

Network framework calls your state update handler when the connection changes state:

let connection: NWConnection = …
connection.stateUpdateHandler = { newState in
    switch newState {
    case .setup:
        // The connection has not yet started.
        …
    case .preparing:
        // The connection is starting.
        …
    case .waiting(let error):
        // The connection tried to start and failed.  It might recover in the
        // future.
        …
    case .ready:
        // The connection is running.
        …
    case .failed(let error):
        // The connection tried to start and failed irrecoverably.
        …
    case .cancelled:
        // The connection was cancelled by you.
        …
    @unknown default:
        break
    }
}

If you a connection is in the .waiting(_:) state and you want to force an immediate retry, call the restart() method.

Network framework calls your viability update handler when its viability changes:

let connection: NWConnection = …
connection.viabilityUpdateHandler = { isViable in
    … react to viability changes …
}

A connection becomes inviable when a network resource that it depends on is unavailable. A good example of this is the network interface that the connection is running over. If you have a connection running over Wi-Fi, and the user turns off Wi-Fi or moves out of range of their Wi-Fi network, any connection running over Wi-Fi becomes inviable.

The inviable state is not necessarily permanent. To continue the above example, the user might re-enable Wi-Fi or move back into range of their Wi-Fi network. If the connection becomes viable again, Network framework calls your viability update handler with a true value.

It’s a good idea to debounce the viability handler. If the connection becomes inviable, don’t close it down immediately. Rather, wait for a short while to see if it becomes viable again.

If a connection has been inviable for a while, you get to choose as to how to respond. For example, you might close the connection down or inform the user.

To close a connection, call the cancel() method. This gracefully disconnects the underlying network connection. To close a connection immediately, call the forceCancel() method. This is not something you should do as a matter of course, but it does make sense in exceptional circumstances. For example, if you’ve determined that the remote peer has gone deaf, it makes sense to cancel it in this way.

Send and receive reliable messages

In Multipeer Connectivity, a single session supports both reliable and best effort send modes. In Network framework, a connection is either reliable or best effort, depending on the underlying network protocol.

The exact mechanism for sending a message depends on the underlying network protocol. A good protocol for reliable messages is WebSocket. To send a message on a WebSocket connection:

let connection: NWConnection = …
let message: Data = …
let metadata = NWProtocolWebSocket.Metadata(opcode: .binary)
let context = NWConnection.ContentContext(identifier: "send", metadata: [metadata])
connection.send(content: message, contentContext: context, completion: .contentProcessed({ error in
    // … check `error` …
    _ = error
}))

In WebSocket, the content identifier is ignored. Using an arbitrary fixed value, like the send in this example, is just fine.

Multipeer Connectivity allows you to send a message to multiple peers in a single send call. In Network framework each send call targets a specific connection. To send a message to multiple peers, make a send call on the connection associated with each peer.

If your app needs to transfer arbitrary amounts of data on a connection, it must implement flow control. See Start a stream, below.

To receive messages on a WebSocket connection:

func startWebSocketReceive(on connection: NWConnection) {
    connection.receiveMessage { message, _, _, error in
        if let error {
            … handle the error …
            return
        }
        if let message {
            … handle the incoming message …
        }
        startWebSocketReceive(on: connection)
    }
}

IMPORTANT WebSocket preserves message boundaries, which is one of the reasons why it’s ideal for your reliable messaging connections. If you use a streaming protocol, like TCP or QUIC streams, you must do your own framing. A good way to do that is with NWProtocolFramer.

If you need the metadata associated with the message, get it from the context parameter:

connection.receiveMessage { message, context, _, error in
    …
    if let message,
       let metadata = context?.protocolMetadata(definition: NWProtocolWebSocket.definition) as? NWProtocolWebSocket.Metadata
    {
        … handle the incoming message and its metadata …
    }
    …
}

Send and receive best effort messages

In Multipeer Connectivity, a single session supports both reliable and best effort send modes. In Network framework, a connection is either reliable or best effort, depending on the underlying network protocol.

The exact mechanism for sending a message depends on the underlying network protocol. A good protocol for best effort messages is UDP. To send a message on a UDP connection:

let connection: NWConnection = …
let message: Data = …
connection.send(content: message, completion: .idempotent)

IMPORTANT UDP datagrams have a theoretical maximum size of just under 64 KiB. However, sending a large datagram results in IP fragmentation, which is very inefficient. For this reason, Network framework prevents you from sending UDP datagrams that will be fragmented. To find the maximum supported datagram size for a connection, gets its maximumDatagramSize property.

To receive messages on a UDP connection:

func startUDPReceive(on connection: NWConnection) {
    connection.receiveMessage { message, _, _, error in
        if let error {
            … handle the error …
            return
        }
        if let message {
            … handle the incoming message …
        }
        startUDPReceive(on: connection)
    }
}

This is exactly the same code as you’d use for WebSocket.

Start a stream

In Multipeer Connectivity, you can ask the session to start a stream to a specific peer. There are two ways to achieve this in Network framework:

  • If you’re using QUIC for your reliable connection, start a new QUIC stream over that connection. This is one place that QUIC shines. You can run an arbitrary number of QUIC connections over a single QUIC connection group, and QUIC manages flow control (see below) for each connection and for the group as a whole.

  • If you’re using some other protocol for your reliable connection, like WebSocket, you must start a new connection. You might use TCP for this new connection, but it’s not unreasonable to use WebSocket or QUIC.

If you need to open a new connection for your stream, you can manage that process over your reliable connection. Choose a protocol to match your send mode explains the general approach for this, although in that case it’s opening a parallel best effort UDP connection rather than a parallel stream connection.

The main reason to start a new stream is that you want to send a lot of data to the remote peer. In that case you need to worry about flow control. Flow control applies to both the send and receive side.

IMPORTANT Failing to implement flow control can result in unbounded memory growth in your app. This is particularly bad on iOS, where jetsam will terminate your app if it uses too much memory.

On the send side, implement flow control by waiting for the connection to call your completion handler before generating and sending more data. For example, on a TCP connection or QUIC stream you might have code like this:

func sendNextChunk(on connection: NWConnection) {
    let chunk: Data = … read next chunk from disk …
    connection.send(content: chunk, completion: .contentProcessed({ error in
        if let error {
            … handle error …
            return
        }
        sendNextChunk(on: connection)
    }))
}

This acts like an asynchronous loop. The first send call completes immediately because the connection just copies the data to its send buffer. In response, your app generates more data. This continues until the connection’s send buffer fills up, at which point it defers calling your completion handler. Eventually, the connection moves enough data across the network to free up space in its send buffer, and calls your completion handler. Your app generates another chunk of data

For best performance, use a chunk size of at least 64 KiB. If you’re expecting to run on a fast device with a fast network, a chunk size of 1 MiB is reasonable.

Receive-side flow control is a natural extension of the standard receive pattern. For example, on a TCP connection or QUIC stream you might have code like this:

func receiveNextChunk(on connection: NWConnection) {
    let chunkSize = 64 * 1024
    connection.receive(minimumIncompleteLength: chunkSize, maximumLength: chunkSize) { chunk, _, isComplete, error in
        if let chunk {
            … write chunk to disk …
        }
        if isComplete {
            … close the file …
            return
        }
        if let error {
            … handle the error …
            return
        }
        receiveNextChunk(on: connection)
    }
}

IMPORTANT The above is cast in terms of writing the chunk to disk. That’s important, because it prevents unbounded memory growth. If, for example, you accumulated the chunks into an in-memory buffer, that buffer could grow without bound, which risks jetsam terminating your app.

The above assumes that you can read and write chunks of data synchronously and promptly, for example, reading and writing a file on a local disk. That’s not always the case. For example, you might be writing data to an accessory over a slow interface, like Bluetooth LE. In such cases you need to read and write each chunk asynchronously.

This results in a structure where you read from an asynchronous input and write to an asynchronous output. For an example of how you might approach this, albeit in a very different context, see Handling Flow Copying.

Send a resource

In Multipeer Connectivity, you can ask the session to send a complete resource, identified by either a file or HTTP URL, to a specific peer. Network framework has no equivalent support for this, but you can implement it on top of a stream:

  • To send, open a stream and then read chunks of data using URLSession and send them over that stream.

  • To receive, open a stream and then receive chunks of data from that stream and write those chunks to disk.

In this situation it’s critical to implement flow control, as described in the previous section.

Final notes

This section collects together some general hints and tips.

Concurrency

In Multipeer Connectivity, each MCSession has its own internal queue and calls delegate callbacks on that queue. In Network framework, you get to control the queue used by each object for its callbacks. A good pattern is to have a single serial queue for all networking, including your listener and all connections.

In a simple app it’s reasonable to use the main queue for networking. If you do this, be careful not to do CPU intensive work in your networking callbacks. For example, if you receive a message that holds JPEG data, don’t decode that data on the main queue.

Overriding protocol defaults

Many network protocols, most notably TCP and QUIC, are intended to be deployed at vast scale across the wider Internet. For that reason they use default options that aren’t optimised for local networking. Consider changing these defaults in your app.

TCP has the concept of a send timeout. If you send data on a TCP connection and TCP is unable to successfully transfer it to the remote peer within the send timeout, TCP will fail the connection.

The default send timeout is infinite. TCP just keeps trying. To change this, set the connectionDropTime property.

TCP has the concept of keepalives. If a connection is idle, TCP will send traffic on the connection for two reasons:

  • If the connection is running through a NAT, the keepalives prevent the NAT mapping from timing out.

  • If the remote peer is inaccessible, the keepalives fail, which in turn causes the connection to fail. This prevents idle but dead connections from lingering indefinitely.

TCP keepalives default to disabled. To enable and configure them, set the enableKeepalive property. To configure their behaviour, set the keepaliveIdle, keepaliveCount, and keepaliveInterval properties.

Symbol cross reference

If you’re not sure where to start with a specific Multipeer Connectivity construct, find it in the tables below and follow the link to the relevant section.

[Sorry for the poor formatting here. DevForums doesn’t support tables properly, so I’ve included the tables as preformatted text.]

| For symbol                          | See                         |
| ----------------------------------- | --------------------------- |
| `MCAdvertiserAssistant`             | *Discover peers*            |
| `MCAdvertiserAssistantDelegate`     | *Discover peers*            |
| `MCBrowserViewController`           | *Discover peers*            |
| `MCBrowserViewControllerDelegate`   | *Discover peers*            |
| `MCNearbyServiceAdvertiser`         | *Discover peers*            |
| `MCNearbyServiceAdvertiserDelegate` | *Discover peers*            |
| `MCNearbyServiceBrowser`            | *Discover peers*            |
| `MCNearbyServiceBrowserDelegate`    | *Discover peers*            |
| `MCPeerID`                          | *Create a peer identifier*  |
| `MCSession`                         | See below.                  |
| `MCSessionDelegate`                 | See below.                  |

Within MCSession:

| For symbol                                                | See                                  |
| --------------------------------------------------------- | ------------------------------------ |
| `cancelConnectPeer(_:)`                                   | *Manage a connection*                |
| `connectedPeers`                                          | *Manage a listener*                  |
| `connectPeer(_:withNearbyConnectionData:)`                | *Manage a connection*                |
| `disconnect()`                                            | *Manage a connection*                |
| `encryptionPreference`                                    | *Plan for security*                  |
| `myPeerID`                                                | *Create a peer identifier*           |
| `nearbyConnectionData(forPeer:withCompletionHandler:)`    | *Discover peers*                     |
| `securityIdentity`                                        | *Plan for security*                  |
| `send(_:toPeers:with:)`                                   | *Send and receive reliable messages* |
| `sendResource(at:withName:toPeer:withCompletionHandler:)` | *Send a resource*                    |
| `startStream(withName:toPeer:)`                           | *Start a stream*                     |

Within MCSessionDelegate:

| For symbol                                                             | See                                  |
| ---------------------------------------------------------------------- | ------------------------------------ |
| `session(_:didFinishReceivingResourceWithName:fromPeer:at:withError:)` | *Send a resource*                    |
| `session(_:didReceive:fromPeer:)`                                      | *Send and receive reliable messages* |
| `session(_:didReceive:withName:fromPeer:)`                             | *Start a stream*                     |
| `session(_:didReceiveCertificate:fromPeer:certificateHandler:)`        | *Plan for security*                  |
| `session(_:didStartReceivingResourceWithName:fromPeer:with:)`          | *Send a resource*                    |
| `session(_:peer:didChange:)`                                           | *Manage a connection*                |

Revision History

2025-03-11 Expanded the Enable peer-to-peer Wi-Fi section to stress the importance of stopping network operations once you’re done with them. Added a link to that section from the list of Multipeer Connectivity drawbacks.

2025-03-07 First posted.

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