Best way to parse SSDP Discovery in Objective C

Question

I am working on a very simple application to discover a device using SSDP and I am trying to find the easiest way to parse the response from this command. I am trying to avoid having to do a bunch of NSString or regular expressions manipulations.

I have tried the following two approaches:

Approach 1: Using GCDAsyncUdpSocket, I am able to successfully send the discovery command and get the following response:

HTTP/1.1 200 OK
Cache-Control: max-age=300
ST: roku:ecp
USN: uuid:roku:ecp:1234567890
Ext:
Server: Roku UPnP/1.0 MiniUPnPd/1.4
Location: http://192.168.XX.XX:8060/

This looks like a regular HTTP response, but using GCDAsyncUdpSocket, I am getting the response as an NSData object, which I can easily convert to an NSString. However, what would be ideal is to somehow cast this to an NSHTTPURLResponse and then use its methods to get the field values. Any idea if this can be done?

Approach 2: I have tried using a regular NSURLRequest to try to send this command and then I would be able to get an NSHTTPURLResponse back. However, I keep on getting an error because the SSDP discovery command requires me to send this request to port 1900. I use the following code to send the "HTTP" request, I know it is not strictly HTTP, but I thought it may be an easier way to send this UDP command as the requirements look very similar to HTTP.

NSURL *url = [NSURL URLWithString:@"http://239.255.255.250:1900"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url
                            cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0];


[request setHTTPMethod:@"M-SEARCH *"];
[request setValue:@"239.255.255.250:1900" forHTTPHeaderField:@"Host"];
[request setValue:@"\"ssdp:discover\"" forHTTPHeaderField:@"Man"];
[request setValue:@"roku:ecp" forHTTPHeaderField:@"ST"];

NSURLConnection *connection = [[NSURLConnection alloc]initWithRequest:request delegate:self startImmediately:YES];
    if (connection)
    {
        NSLog(@"Connection success!");
    }
    else
    {
        NSLog(@"Connection failed!");   
    }

When I do this, the connection is successful, but I get the following error in the didFailWithError delegate for NSURLConnection:

failed Error Domain=NSPOSIXErrorDomain Code=47 "The operation couldn’t be completed. Address family not supported by protocol family" UserInfo=0x8875940 {NSErrorFailingURLKey=http://239.255.255.250:1900/, NSErrorFailingURLStringKey=http://239.255.255.250:1900/}

This error only happens if I use port 1900, if I leave this out or use another more HTTP friendly port such as 8080, then this works, but obviously the device I am trying to discover does not respond correctly unless it gets the request in port 1900.

Thanks for any help you can provide.

Pavel Zdenek Pavel Zdenek · Accepted Answer · 2012-11-21T09:33:43

Port 1900 is fixed for SSDP. All UPnP devices are fixed to listen on this port and the connecting nodes expect that. You can't change it. It's a part of the UPnP design goal to "work out of the box". Furthermore, my Cocoa expertise is very limited, but i think that NSHTTPURLResponse won't make things simpler for you. [NSHTTPURLResponse allHeaderFields] returns NSDictionary, which means that you don't know anymore what was the original order of the header fields. And you need to know what was coming after Ext: which is a meta-header.

I suggest either parsing the response yourself, which shouldn't be much more complicated than one cycle, separating the response by lines and then splitting by :. Or instead of trying to roll your own SSDP handshake, use ready made Cocoish library like Upnpx, Cyberlink or Platinum. It might feel like inappropriately heavy artillery just for the discovery phase, but i wonder what else you would do with the device after that, other than actually trying to invoke some actions on the device.

Best way to parse SSDP Discovery in Objective C

1 Answers