Having trouble working out this SqlException:

A transport-level error has occurred when sending the request to the server. (provider: Session Provider, error: 19 - Physical connection is not usable)

Any ideas what this is about? Couldn't get any hits on google or msdn on this one. The connection is MARS and Asynchronous in my own SQL pool. But don't get all excited about that - that is old code that has been stable for a long time!

Exception details pasted below:

  System.Data.SqlClient.SqlException occurred
  Message=A transport-level error has occurred when sending the request to the server. (provider: Session Provider, error: 19 - Physical connection is not usable)
  Source=.Net SqlClient Data Provider
  ErrorCode=-2146232060
  Class=20
  LineNumber=0
  Number=-1
  Server=SERVER
  State=0
  StackTrace:
       at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection)
       at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection)
       at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning()
       at System.Data.SqlClient.TdsParserStateObject.SNIWriteAsync(SNIHandle handle, SNIPacket packet, DbAsyncResult asyncResult)
       at System.Data.SqlClient.TdsParserStateObject.WriteSni()
       at System.Data.SqlClient.TdsParserStateObject.WritePacket(Byte flushMode)
       at System.Data.SqlClient.TdsParserStateObject.ExecuteFlush()
       at System.Data.SqlClient.TdsParser.TdsExecuteRPC(_SqlRPC[] rpcArray, Int32 timeout, Boolean inSchema, SqlNotificationRequest notificationRequest, TdsParserStateObject stateObj, Boolean isCommandProc)
       at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)
       at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
       at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe)
       at System.Data.SqlClient.SqlCommand.BeginExecuteNonQuery(AsyncCallback callback, Object stateObject)
       at CrawlAbout.Library.SQL.SQLAdapter.<ExecuteNonQueryTask>d__12.MoveNext() in D:\Work\Projects\CrawlAbout.com 2.0\CrawlAbout.Library.SQL\SQLAdapter.cs:line 149
  InnerException: