Re: xAP EOM identifier in xAP v1.3

[Kevin Hawkins <yahoogroupskh@xxxxxxxxxxxxxx>]

One of the issues seems to be that there is conflicting views as to
the
length of a a UDP data packet payload.  Some people cite 500 or 512
characters and some 1500.  Regardless in some low memory/performance
devices it is being reported, that even with UDP,  packets are being
received from the buffer either truncated or appended back to back.
The latter I assume is due to the speed of the device in servicing the
buffer.

We have an opportunity to protect against this with v1.3 and the double
'0A' seems the most compatible.   I would be loathe to support anything
that wasn't backward compatible.

K

Patrick Lidstone wrote:
>
>
> I will dig it out - it included an optional checksum I think, and IIRC
> was framed by stx and etx (a kind of pseudo industry standard). I
> certainly used it with the PIC serial stuff and the xAP-serial bridge.
> Re.: long message truncation and concatenation: If we need to support
> messages that are larger than one UDP packet, then there are
> additional complexities and the proposed scheme won't work as intended
> as the ordering of UDP messages is not guaranteed. I'm happy to help
> refine a spec to support these capabilities, but it is moving away
> from the basic ethos of xAP somewhat, as devices would have to be able
> to buffer received messages, and that raises the bar considerably.
> Perhaps there is scope for co-existence of a long and standard message
> protocol though?
>
> Patrick
>
> 2009/9/13 Kevin Hawkins <yahoogroupskh@xxxxxxx
> <mailto:yahoogroupskh@xxxxxxx>>
>
>     ... Oh ... where is that in the spec ?  it might be all we need.
>
>        This is also tied in with some aspects of long message
truncation
>     and concatenation of messages received in UDP receive buffers
>     though...
>
>      K
>
>     Patrick Lidstone wrote:
>     >
>     >
>     > The original xap spec provides extensions for framing a
message over
>     > async serial which also delimit the start of the message -
you don't
>     > need this 'hack' if you follow the original spec for non-UDP
>     transports.
>     >
>     > Patrick
>     >
>     > 2009/9/13 Kevin Hawkins <yahoogroupskh@xxxxxxx
>     <mailto:yahoogroupskh@xxxxxxx>
>     > <mailto:yahoogroupskh@xxxxxxx
>     <mailto:yahoogroupskh@xxxxxxx>>>
>     >
>     >       We have been asked on several occasions how to detect
the
>     end of a
>     >     xAP messager as there is no unique EOM character. 
Typically
>     in any
>     >     reasonable sized packet structured transport eg UDP then
the
>     packet
>     >     provides such an indication but on systems with small
packets or
>     >     non eg
>     >     asynchronous serial this is not useable.
>     >
>     >       In discussing this with the specification team we must
>     consider
>     >     backwards compatability and so we do not wish to alter
the
>     >     specification
>     >     to include a unique EOM character.  What we do propose
>     however is that
>     >     xAP v1.3 will specify that all messages should end with
two
>     >     consecutive
>     >     chr(10)'s immediately after the closing '}'
>     >
>     >     ie  ..... 7D 0A 0A
>     >
>     >      Some apps, even v1.2 ones,  already do this.  We don't
>     believe this
>     >     will cause any backwards compatibility issues and it will
>     always be
>     >     unique within a xAP message.
>     >
>     >      So, in developing xAP v1.3 apps could you therefore
append
>      two 0A's
>     >     at the end of your messages , and of course handle
incoming
>     messages
>     >     containing such.
>     >
>     >       K
>     >
>     >
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