Know Your Networks

he importance of enabling your software to identify the current network connections and change behavior has never been greater, as more people use laptops, notebooks, and Tablet PCs. With the growth of connectivity and online technology come applications that require network connectivity for their supported features, ranging from regular scans for software updates to online data synchronization. Applications that call through to Web services and utilize Internet technologies are becoming commonplace.

As these applications run on Mobile PCs, it is important that they can cope with joining to and disconnecting from networks.

There are three states with which an application needs to cope:

  1. There is no network connection.
  2. There is a single network connection.
  3. There are multiple network connections.

A well-behaved software application needs to be able to transition smoothly among these three states. These transitions occur in four possible scenarios:

  1. A network connection becomes available when there was not one previously.
  2. A new network connection becomes available when one (or more) was already available.
  3. A network connection becomes unavailable (drops out), leaving no connections.
  4. A network connection becomes unavailable, leaving one (or more) connections.

In an ideal world, end users should not have to interact when network transitions occur, but it’s good practice to provide some non-obtrusive feedback to the user to identify the current state.

The importance of enabling your software to identify the current network connections and change behavior has never been greater.

Current Networks
A good first step is to work out what current network connections are available to the application. The NLA (Network Location Awareness) API provides three methods to help identify the current connections:

  • WSALookupServiceBegin?Initiates iterating through the currently connected networks
  • WSALookupServiceNext?Accesses the next available network (or lets the user know that there are no more network connections)
  • WSALookupServiceEnd?Finishes iterating through the networks.

As these methods are part of the Windows Sockets API, you need to call the WSAStartup method to initialize the process for using the Windows Sockets Library. You must call the WSACleanup method when you have finished using the library.

The WSALookupServiceBegin and WSALookupServiceNext methods both use a structure called a WSAQUERYSET. It is used in the WSALookupServiceBegin method to constrain or filter the results returned. The details for each network connection are then returned in a WSAQUERYSET structure from the WSALookupServiceNext method.

Doing this in managed code (C# in this example) requires that you use the Platform Invoke (P/Invoke) to import the Win32 API functions. Listing 1 shows the code needed to define the structures required in C#. Once defined, the methods can be called to build a list of currently connected networks.

The WSALookupServiceBegin method creates a dataset of the current connections and the WSALookupServiceNext iterates through each network in that dataset. Finally, calling WSALookupServiceEnd indicates that you have finished with that dataset and that you no longer need it maintained.

The WSALookupServiceBegin method creates a dataset of the current connections.

Listing 2 shows a method that returns an ArrayList of the name of each network connection. Of course, you will probably want more information than just the name of the network. You may notice that in this code listing, a large block of memory is allocated to marshal the network information structure. Later, you’ll see how to extract more information about each network and how to allocate the correct amount of memory for the returned information.

Knowing the Network Changes
Now that you have an idea of how to get a list of the current network connections, it would be great if your application knew when the connections changed. A dirty solution would be to poll and keep requesting the current changes. This is not a good idea. Most of the devices that use this code are Mobile PCs, and power consumption is an issue for Mobile PCs (see my other article in this issue on Power Management), so polling is not a good solution.

Instead, what you need is some form of notification. The NLA API provides a function called WSANSPIoctl that provides what you need. The WSANSPIoctl function can be used to block the current thread until a change in the currently connected networks has transpired. The function requires that you first call WSALookupServiceBegin to build a list of the current connections. You can then call WSANSPIoctl, which blocks the current thread until there is a change in that list. The code in Listing 3 demonstrates this in the WaitForNetworkChanges method. As this method is going to block until there is a change, run this method in a background worker thread.

Digging Deeper into the Network Information
The last field in the WSAQUERYSET structure is a pointer to some binary data. This binary data contains more information about the network connection and is not of a fixed size. It can contain a variable amount of information depending on what features of the network are currently supported. This is why, in the first example, I simply allocated a large block of memory. This is obviously not the best thing to do here.

Looking in the MSWSock.h file, you can discover that the NLA_BLOB structure is represented as a structure with nested structures and a union of other structures.

To make your life easier, I have represented the NLA_BLOB structure as a number of C# classes and structures in Listing 4. You can see that in order to represent the union, I have used the LayoutKind.Explicit and the FieldOffset attributes to indicate to the CLR exactly how I expect the memory to be laid out.

To extract the data from the pointer field in the WSAQUERYSET, it will be necessary to pin the memory so that it can be mapped between the managed and unmanaged code. In C#, this means that you need to mark your code as unsafe. This needs to be done around the code blocks and for the whole project from the build configuration in the project properties.

To retrieve the network information, you need to start by marshalling the last field (a pointer) in the WSAQUERYFIELD to a BLOB object, as defined in Listing 4.

   protected BLOB GetBlob(WSAQUERYSET qsResult)   {      BLOB blob = new BLOB();      Marshal.PtrToStructure(qsResult.lpBlob, blob);      return blob;   }

You can then use the pInfo field in the BLOB class to marshal the data into an instance of an NLA_Info class and extract the data, which is shown in the two methods in Listing 5.

You can use the WSANSPIoctl function to block the current thread until a change in the currently connected networks has transpired.

Remember that the NLA_Info object contains a nextOffset field within the NLA_Header structure. This nextOffset field indicates the byte offset to the next NLA_Info object in memory. In order to read out all of the network information for a specific connection, you will need to repeat the process for each object, as shown here.

   BLOB blob = GetBlob(qsResult);   byte* pInfo = (byte*)blob.pInfo;   NLA_Info info;   do   {      info = SetNLAInfo(new IntPtr(pInfo));      pInfo += info.header.nextOffset;   } while (0 != info.header.nextOffset);

The first method you examined (Listing 2), allocated a large block of memory to read in the network information. This is obviously not a good practice. You can refine that now by making two calls to the WSALookupServiceNext function. In the first call, pass in an empty buffer and a null pointer to the WSAQUERYSET parameter. Doing this causes the WSALookupServiceNext function to return the size of the buffer required for the network information. You can then use this to allocate the correct amount of memory and begin extracting the details of the network connection, which is done in Listing 6.

If you have put all of this together, you should now be retrieving some information about the network connections currently available to your application. There is one more step you can take that provides you with a fuller set of information. The first call to WSALookupServiceBegin uses the second parameter (dwControlFlags) to determine what information to return. The code thus far has used the LUP_RETURN_ALL flag that returns all the known information about the currently connected networks. You can combine this with the LUP_DEEP flag to force the network connections to be queried further. Be aware that you should only use this flag when calling WSALookupServiceBegin and not on subsequent calls to WSALookupServiceNext. Using the LUP_DEEP flag creates network traffic and forces the current set of networks to be rebuilt?and doing this on a call to WSALookupServiceNext causes a problem as the set of networks you are iterating through will have changed. The code snippet below demonstrates using the flags for the WSALookupServiceBegin and then changing the flags back before iterating through the collection of networks.

   dwControlFlags = 0x0FF1;//LUP_RETURN_ALL|LUP_DEEP   int nResult = WSALookupServiceBegin(      qsRestrictions, dwControlFlags, ref valHandle);   if (0 !=nResult)   {       CheckResult(nResult);   }   dwControlFlags = 0x0FF0;    while (0 == nResult)   { ...

In this article, you have seen how to import the unmanaged NLA functions from the Windows Sockets Library. You have also explored how to use the various unmanaged structures that are required to use those functions and how to become aware of changes to the network connections. Finally, you should now be able to retrieve extra information about the connected networks by marshalling the unmanaged structures into managed classes and structures.

Together, these capabilities let you build applications that are aware of the current connections and the features of those connections. This should enable you to create more valuable software that provides a better user experience.

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