First, you want to keep a pointer to the timer
self.packetTimer = [NSTimer timerWithTimeInterval:CONNECTION_TIMEOUT target:self selector:@selector(connectionTimeout:) userInfo:nil repeats:NO];
[[NSRunLoop currentRunLoop] addTimer:packetTimer forMode:NSDefaultRunLoopMode];
If somewhere else in your code you want to cancel it, just call:
[self.packetTimer invalidate];
self.packetTimer = nil;