Disable Nagle's algorithm for NSOutputStream

Question

I am creating a multiplayer game using MPCF. You control a spacehip on the iPad using the iPhone.

I am experiencing various amount of lag and latency and buffering/pauses at random times and intervals and have now landed on Apples Technical Q&A NW26 paper that talks about disabled the Nagle Algorithm. Im trying it out but my program keeps crashing and I dont understand why. It seem to be that CFWriteStreamCopyProperty always return NULL.

- (void)stream:(NSStream *)stream handleEvent:(NSStreamEvent)eventCode
{
    switch (eventCode) {
        case NSStreamEventOpenCompleted:

            // Trying to get a handle to the native socket
            CFSocketNativeHandle rawsock;

            // This always return NULL
            CFDataRef socketData = CFWriteStreamCopyProperty((__bridge CFWriteStreamRef)(stream), kCFStreamPropertySocketNativeHandle); 

            // And this row always crash (coz of socketData being NULL i guess)
            CFDataGetBytes(socketData, CFRangeMake(0, sizeof(CFSocketNativeHandle)), (UInt8 *)&rawsock); 
            CFRelease(socketData);

            // Code example from Apple that need a handle to the native socket, that I am trying to get above
            int err; 
            static const int kOne = 1;  
            err = setsockopt(rawsock, IPPROTO_TCP, TCP_NODELAY, &kOne, sizeof(kOne)); 
            if (err < 0) {     
                err = errno; 
            }

            break;

        default:
            break;
    }
}

Any help is appreciated.

Answer 1

You assuming that stream is an NSOutputStream, but there could be passed NSInputStream also. So it crashes on that line.

Here is a sample code that can handle both scenarios and fixes a crash:

- (void)disableNaglesAlgorithmForStream:(NSStream *)stream {

    CFDataRef socketData;

    // Get socket data
    if ([stream isKindOfClass:[NSOutputStream class]]) {
        socketData = CFWriteStreamCopyProperty((__bridge CFWriteStreamRef)((NSOutputStream *)stream), kCFStreamPropertySocketNativeHandle);
    } else if ([stream isKindOfClass:[NSInputStream class]]) {
        socketData = CFReadStreamCopyProperty((__bridge CFReadStreamRef)((NSInputStream *)stream), kCFStreamPropertySocketNativeHandle);
    }

    // get a handle to the native socket
    CFSocketNativeHandle rawsock;

    CFDataGetBytes(socketData, CFRangeMake(0, sizeof(CFSocketNativeHandle)), (UInt8 *)&rawsock);
    CFRelease(socketData);

    // Disable Nagle's algorythm

    // Debug info
    BOOL isInput = [stream isKindOfClass:[NSInputStream class]];
    NSString * streamType = isInput ? @"INPUT" : @"OUTPUT";

    int err;
    static const int kOne = 1;
    err = setsockopt(rawsock, IPPROTO_TCP, TCP_NODELAY, &kOne, sizeof(kOne));
    if (err < 0) {
        err = errno;
        NSLog(@"Could Not Disable Nagle for %@ stream", streamType);
    } else {
        NSLog(@"Nagle Is Disabled for %@ stream", streamType);
    }
}

This should be called right in the case NSStreamEventOpenCompleted:

For anyone wondering why it does not compile when copy-pasted:

#import <arpa/inet.h>       // for IPPROTO_TCP
#include <netinet/tcp.h>    // for TCP_NODELAY

However, it does NOT help with periodical latency issues. I'm still searching for a way to prevent that.

I recorded a simple video that demonstrates the issue and I believe you are experiencing the same one: https://www.dropbox.com/s/omdqkbckph4b1y2/Multipeer%20Connectivity.mov?dl=0

EDIT: I was able to find a way to fix a periodical latency issues. It is an MCNearbyServiceAdvertiser that causes the issues. You need to stop advertising a peer in order to get rid of lags: call a stopAdvertisingPeer method on your MCNearbyServiceAdvertiser instance right after the connection succeeded.

MCNearbyServiceAdvertiser *nearbyServiceAdvertiser;

- (void)session:(MCSession *)session peer:(MCPeerID *)peerID didChangeState:(MCSessionState)state {

    switch (state) {

        case MCSessionStateConnected: {
            dispatch_async(dispatch_get_main_queue(), ^{
                [self.nearbyServiceAdvertiser stopAdvertisingPeer];
            });
        ...
    }
    ...
}

BUT the delays will turn off only after 30 seconds. I don't know how to make it disappear right away.