How can I pause reading packets from NEUDPSession

Question

Created Mar ’21

Replies 3

Boosts 0

Participants 2

Hi There,

NWUDPSession has a setReadHandler:maxDatagrams: method to register a callback for reading packets. I am wondering how can I pause this handler not to be called for a short period(pause it for a while)?

I can think of few options, but all of them seem to be bad.

Cancel the session when paused, and create a new one when resumed.

Code Block -(void)stopReadingPackets {
  [self.session cancel];
}
-(void)resume {
  self.session = ...
  [self.session
      setReadHandler:^(NSArray<NSData*>* datagrams, NSError* error) {
       // Handle packets here.
      }
        maxDatagrams:1];
}

2. Block the handler to be executed. For example:

Code Block [self.session
      setReadHandler:^(NSArray<NSData*>* datagrams, NSError* error) {
        while (self.paused) {}
        // Handle datagrams here.
      }
        maxDatagrams:1];

Thanks.

Boost

Answer 1

DTS Engineer OP

Apple

Mar ’21

Yeah, that’s tricky. Do you need to use NWUDPSession here? I generally encourage folks, even folks working on Network Extension providers, to switch to using NWConnection for UDP. NWConnection has a different model for reading, one that supports flow control.

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

0

Answer 2

yijiexu OP

Mar ’21

Hi Quinn,

Thanks for the response. I think we can use NWConnection over NWUDPSession as all we need is to connect to our VPN server to send/receive encrypted packets and perform handshakes.

But I am not sure how I can pause reading packets using NWConnection either, one guess would be:

Code Block func startReadingPackets() {
    connection.receiveMessage { [weak self] data, context, success, error in
        /* Handle packets or error here. */
        while (self?.paused) {}
        self?.startReadingPackets()
    }
}

0

Answer 3

DTS Engineer OP

Apple

Mar ’21

Code Block  while (self?.paused) {}

Yikes! don’t do that. There’s two problems here:

You’re polling, which is never a good idea.
You’re blocking the packet receive thread, which is also a bad idea.

Rather, to exert read-side flow control you should simply not call startReadingPackets(…) in your completion handler. Then, when you want to release that flow control, call startReadingPackets(…) from the code that’s doing that release.

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

0