Boost async_read不会给我一个“帧结束”标志

Question

I'm still working on some kind of client for communication with an IP Camera. Now I have the following issue:

I send a request to the camera ( a RTSP DESCRIBE in particular ). Now I get it's answer which looks like this:

RTSP/1.0 200 OK
CSeq: 2
Date: Thu, Jan 01 1970 00:31:41 GMT
Content-Base: rtsp://192.168.0.42/mpeg4?mode=Live&stream=-1&buffer=0&seek=0&fps=100&    metainfo=/
Content-Type: application/sdp
Content-Length: 517

This is the header of the answer, followed by a so called Session Description which has the size shown in the field Content-Length . Actually I don't care much for the Session Description , I'm just interested in the Content-Base field. But still, since there is some communication following on the same socket, I need to get rid of all the data.

For receiving'm using the async_read calls from boost::asio . My code looks ( simplified ) like this:

CommandReadBuffer::CallbackFromAsyncWrite()
{
boost::asio::async_read_until(*m_Socket, m_ReceiveBuffer,"\r\n\r\n",
            boost::bind(&CommandReadBuffer::handle_rtsp_describe, this->shared_from_this(),
                    boost::asio::placeholders::error,boost::asio::placeholders::bytes_transferred));
}

This one reads at least the header ( shown above ) since its terminated by a blank line. As usual for async_write it just reads some more of the data, but nevermind. Now to the next callback function:

void CommandReadBuffer::handle_rtsp_describe(const boost::system::error_code& err,size_t bytesTransferred)
{
std::istream response_stream(&m_ReceiveBuffer);
std::string header;
// Just dump the data on the console 
while (std::getline(response_stream, header))
{
   // Normally I would search here for the desired content-base field
   std::cout << header << "\n";
}
 boost::asio::async_read(*m_Socket, m_ReceiveBuffer, boost::asio::transfer_at_least(1),
         boost::bind(&CommandReadBuffer::handle_rtsp_setup, this->shared_from_this(),
                            boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
}

Now this works fine as well, if I print out the number of received bytes it's always 215.

Now we go on to the critical callback:

void CommandReadBuffer::handle_rtsp_setup(const boost::system::error_code& err, size_t bytesTransferred)
{
std::cout << "Error: " << err.message() << "\n";
    if (!err)
    {
      // Write all of the data that has been read so far.
      std::cout << &m_ReceiveBuffer;

    // Continue reading remaining data until EOF.
    m_DeadlineTimer->async_wait(boost::bind(&CommandReadBuffer::handleTimeout, this->shared_from_this(),boost::asio::placeholders::error));
      boost::asio::async_read(*m_Socket, m_ReceiveBuffer, boost::asio::transfer_at_least(1),
             boost::bind(&CommandReadBuffer::handle_rtsp_setup, this->shared_from_this(),
                                boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
}
else if (err != boost::asio::error::eof)
{
  std::cout << "Error: " << err.message() << "\n";
}
else
{
    std::cout << "End of Frame " << err.message() << "\n";
}
}

This part reads 220 Bytes. If I look at console Output from this call and compare it with the actualy payload of the frame ( as seen in Wireshark ) I can see that all data has been received. Now I would actually assume that async_read would set me the eof error. But instead the return code of error is success and so it calls async_read again. This time there is no data to be received and it never calls the callback function ( since there will be no more incoming data ).

Now I actually don't know how I could determine that all data has been sent. Actually I would expect the error flag to be set.

Now this is very similar to the implementation of the Boost Example for an Async HTTP client. Also it is done the same way in the Example Boost Async HTTP Client . I implemented this in another call and there it actually works.

Now in my opinion it should make no difference for the async_read call wether it is HTTP or RTSP - end of frame is end of frame, if there is no more data to read.

I'm also aware that according to the boost documentation I am using

void async_read(
    AsyncReadStream & s,
    basic_streambuf< Allocator > & b,
    CompletionCondition completion_condition,
    ReadHandler handler);

which means the function will continue until

 The supplied buffer is full (that is, it has reached maximum size). The completion_condition function object returns 0. 

So if there is no more data to read, it just continues. But I also tried the overloaded function without the CompletionCondition parameter, which should return when an error occurs ( EOF !!! ) - But this just won't callback either...

Any suggestions? I just don't get what I'm doing wrong...

Answer 1

I have written an RTSP client and server library using boost asio and can offer the following advice: The RTSP message syntax is generic: there is no need for different DESCRIBE and SETUP handlers. In general

• write an RTSP request
• to read the response do a boost::asio::async_read_until("\\r\\n\\r\\n")
• then check for the Content-Length header
• if content_length > 0 do a boost::asio::transfer_at_least(content_length)

Further, why are you expecting an EOF? The connection is still open: the server is waiting for either another SETUP or a PLAY request and typically won't close the connection until the RTSP TCP connection has been timed out, which has a default value of 60 seconds according to RFC2326 .

If in your application, you have completed interaction with the RTSP server, close the connection after you have read the response.