Vic 20 multi thread TCP by Lee Davison

The Vic 20 network code allows a Vic 20 to be connected to an ethernet network and, while it works, it suffers from one drawback, it can only support one TCP connection at any time. This is most noticeable when opening a web page with multiple images on it as once one connection is open all the others wait at least one time out time before trying again. This meant that in spite of the first HTTP GET request being serviced in less than 200ms, the pages on the Vic 20 web server would still take ten seconds or so to complete.

The solution to this is to replace the single thread TCP code with multi thread code that can handle more than one connection at any time. This introduces a new level of complexity into the code and increases RAM use but this is more than made up for by the aparent improvement in performance. For example, a web page with three embedded images is now served in less than 800ms.

The biggest change is to the TCP code. This has been modified to send RST packets, should they be needed, and now behaves more like the specification in RFC 793. It has also been changed to support multiple simutaneous connections and in this case up to eight simultaneous connections are supported, enough to serve two http clients with no noticeable loss in performance. RAM permitting, sixty four connections should be possible with only minimal code changes.

The software has changed in more than just it's ability to handle multiple TCP connections. Changes have also been made in the way the different layers of code interact, giving better definition to which layer each part of the code belongs to. There is also greater division between different protocols at the same level and, while this has increased code size a little, it should make it easier to add or remove code should need arise.

The low level code that handles the network interface card has not changed, neither has the ROM filesystem or the web pages held in it and there have only been cosmetic changes to the display code to show the status of the extra TCP connections. In all there are just over 530 bytes left in the 8KB ROM area that holds the data and code.

You can download the latest source as zipped .asm file and you can download the binary ROM image . The version is 00.69 multi.

There are still a few issues to address with the current TCP implementation. Firstly the sent TCP window size is not calculated but is just blindly set, while this is not a problem with a TCP message that fits in this set size it may cause problems later. Also the incomming window size is ignored and the data just sent in one packet. This again has not been a problem so far but it should be taken into account.

The timeout code could be much improved. At the moment when a TCB times out it is reset to listen and the connection discarded, this could be improved by retransmitting the last data sent by that TCB but it would mean having to hold more information in each TCB. This would include such data as the number of retries and the last acknowledged data pointer as well as information on the size and location of the data being sent. Adding features such as these soon eats up the often scarce resources in an 8 bit system.

Another feature that is lacking is the ability to transmit and receive TCP data that spans more than one packet. While this is a desirable feature it introduces another degree of complexity to the code and the need to keep track of what has been sent or received requires more of the limited system memory.

Last page update: 20th June, 2004.

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