RE: [xAP] Protocol revisions?

["Mark Harrison" <Mark.Harrison@xxxxxxx>]

1: Config discovery.

Well, it makes certain inut plugins harder to develop.

I think that we should specify the functionality, and require input plugin
developers to declare whether or not they support it, rather than being
mandatory.

(As an example - it means that the outlook plug _can't_ be written in VBA -
it would have to move out to a full VB app in order to support a port
listener as well as a broadcaster.)

2: Looks fine to me. With only 4 xAP plugins out there then it's probably
the right time to change. I'll modify the Outlook one this evening.

3: What you're proposing looks sensible - I'll modify the Outlook one
tonight.

4: I think I follow. OK. (I won't be using the in-message STX stuff, but
I'll certainly include the <STX> / <ETX> wrappers.

M.

-----Original Message-----
From: PatrickLidstone [mailto:patrickl@xxxxxxx]
Sent: 15 August 2002 12:24
To: ukha_d@xxxxxxx
Subject: [ukha_d] [xAP] Protocol revisions?


--- In ukha_d@y..., "Mark Harrison" <Mark.Harrison@e...>
wrote:
> Agreed - we need to start maintaining a registry of the custom-
extensions...
> 
> M.
> 

Further proposed change to the xAP protocol:

1. Config Discovery.

Convert config discovery to a heartbeart message. Heartbeat contains 
flag indicating request for config info. This allows the status of 
all devices on the network to be monitored from any point.

2. Name-space.

Allow support for hierarchial, nested and bridged messages by 
grouping name-value pairs:

e.g.

xAP-source=Outlook<0x0A>
xAP-instance=Mark<0x0a>
unread=19<0x0a>
last subject=101 useful things to do with string<0x0a>


becomes

xAP-header {<0x0A>
  hop=1<0x0a>
  source=Outlook<0x0A>
  instance=Mark<0x0a>
}

Outlook { <0x0A>
unread=19<0x0a>
last subject=101 useful things to do with string<0x0a>
} <0x0A>

[xAP-header.hop increments for each re-broadcast, allowing for auto-
loop recognition in bridged networks]

The introduction of {} actually reduces the message size, especially 
in the xAP header.


3. Framing for Serial protocols

Serial protocols generally benefit from framing, and from the 
identification of the start and end of a valid message.

I propose wrapping a xAP message in <STX> and <ETX> - even over
TCP, 
to support dumb bridging. STX and ETX will generally only occur in 
the body of messages where that message contains a binary blob, which 
is probably to be discouraged, but is in some cases unavoidable.

STX and ETX will be escaped in-body by (a) preceding with the <ESC>
char and (b) setting the top bit of the escaped character.
The <ESC> code itself is escaped in the same way.

4. Plain text descriptor
A dumb device would not be able to compose a useful display text from 
the message below. I propose that information ASCII text is included 
in the body of a message aimed at dumb display. As a rule of thumb, 
this message should never be parsed (the constituent values should be 
available explicitly in the body of the message, and the format of 
this text may change at the programmers discretion), should be human 
readable and concise. Think of it as an event-log message and you 
won't go far wrong.


Summary:

<STX>

xAP-header {<0x0A>
  hop=1<0x0a>
  source=Outlook<0x0A>
  instance=Mark<0x0a>
}

Outlook { <0x0A>
unread=19<0x0a>
last subject=101 useful things to do with string<0x0a>
demo escape stx=this is an embedded stx char<ESC><0x82>
<0x0a>
} <0x0A>

<ETX>




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