Re: Protocol Specification in Lua

[Jerome Vuarand <jerome.vuarand@...>]

2010/10/1 John Passaniti <john@japanisshinto.com>:
> I'm looking for a little design inspiration from others.  I have a
> basic approach (which I'll describe, below), but chances are good that
> someone here has already implemented something like this or can see a
> better solution.
>
> I have a UDP-based communications protocol for controlling a device.
> The protocol is implemented in C and specified in a tedious Microsoft
> Word document.  The base protocol is simple and extensible and isn't
> interesting.  What is interesting is there are a large number of
> binary messages, each with a unique structure.  I'm bothered that when
> I change the C code to add a new message, I also have to update the
> Word document.  The two are separate things.
>
> I'd like to write a tool that could parse these messages and display
> them in human readable form.  And I'd like another tool that does the
> reverse-- specify a message in a human-readable form, and get back out
> a message.  And I'd like to be able to write a Wireshark dissector for
> these messages.  And I'd like... well, more stuff.
>
> The solution would seem to be to express the protocol in some
> abstract, high-level representation (like... a Lua table).  Once in
> this form, I can write a tool that will dump out a description of the
> protocol in HTML.  I could also have it generate optimized C code to
> implement the protocol parser (such as converting to a finite state
> machine).  The table would also be at the heart of the Wireshark
> dissector and the tools to display and create packets.  And so on.
>
> Now whenever I make changes to the protocol, I edit the Lua table that
> describes the messages, and I'm done.  Everything flows from that.
>
> So how about something like this:
>
> {
>    OPT_MUTE = {
>        option = 0x02,
>        read = true,
>        write = true,
>        message = {
>            {
>                name = "type",
>                type = OutputInput,
>                desc = "Channel Type"
>            },
>            {
>                name = "number"
>                type = ChannelNumber,
>                desc = "Channel Number"
>            },
>            {
>                name = "mute"
>                type = OffOn,
>                desc = "Mute Status"
>            }
>        }
>    }
> }
>
> So I have a message named OPT_MUTE that has a binary value of 0x02,
> it's both readable and writable, and has three items in the payload.
> The three items are named type, number, and mute and are of specific
> types (OutputInput, ChannelNumber, and OffOn).  Each of these types is
> itself a table that describes them further:
>
> ChannelNumber = {
>    bytes = 1,
>    low = 0,
>    high = 23,
> }
>
> There are (potentially) a variety of other fields to describe messages
> (which devices implement them, numeric ranges), and the payload
> section can have repetitions of a item or a larger structure, or
> optional data at the end of a message.
>
> What I'm looking for is if anyone has done this kind of thing before
> and what kind of representation you came up with.  The biggest
> stumbling block I have is the representation of repeated and optional
> data in the payload.
>
> Suggestions?

I'm a little late on the topic, but you can have a look at Lunary,
which is a module to convert binary packets to and from Lua, with the
help of a a Lua based packet description. It's available there :

http://lunary.luaforge.net/
http://piratery.net/lunary/ (mirror)

Feel free to ask for help on using it, I don't have many users yet and
the doc may be confusing.