[aprssig] APRS-IS Duplicate Packets

[Matti Aarnio]

On Mon, May 03, 2010 at 03:57:42PM -0500, Pete Loveall AE5PL Lists wrote:
> 
> As indicated by Lynn, the assumption -must- be that trailing whitespace
> is considered part of the packet.  Anyone who has spent any time in network
> design and implementation knows that the transport protocol and transport
> level software -never- alters the transported data.  It is a bad design
> from day one for anyone to consider that it is "ok" to ignore data in
> a packet regardless of how inconsequential the software author may think
> that data is and then modify those packets and their handling based on
> their errant assumption.

Pete, that is wishfull thinking.

The reality is that much used UNMAINTAINED closed source firmwares on
radio layer do keep adding space characters to packets that they relay.

So iGate A hears the packet before such broken digipeater, and relays
it to APRSIS.  Then iGate B hears the same packet after digipeat but
now with extra space (0x20) byte at the tail, and relays it to APRSIS.

This is unfortunate fact in our radio layer.

Neither APRSIS nor iGates are allowed to MODIFY the packets that they pass,
but when comparing against duplicate one must see if a packet is variation
of some other that has gotten extra spaces at the end.

Essentially for duplicate lookup hashing and subsequent comparison one
really needs to ignore all 0x20 bytes at the end of the packet.
Call it "duplicateComparisonLength". Most of the time it is equals to
packet payload length, but not always.

> 73,
> 
> Pete Loveall AE5PL
> pete at ae5pl dot net

73 de Matti, OH2MQK