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-   -   PID vs. PDL (http://www.rdmprotocol.org/forums/showthread.php?t=1140)

erwin May 1st, 2012 07:36 AM

PID vs. PDL
 
1 Attachment(s)
Hey all,

while writing the Wireshark dissector I ran into a RDM packet that I don't know how to handle. It has a PID of 0x0401 (LAMP_HOURS) and a PDL of 2. But the spec (just bought the new one to verify) says that LAMP_HOURS should be a 32bit value. So, or the PID is wrong, or the PDL is wrong.

How should one deal with situations where PID and PDL disagree ?

- Erwin

erwin May 1st, 2012 07:38 AM

Hmm the image was resize to unreadable size, but it just shows how Wireshark dissected the wrong packet.

prwatE120 May 1st, 2012 08:13 AM

Erwin

not sure which version you have, but in

ANSI E1.20 - 2010
Entertainment Technology
RDM
Remote Device Management
Over DMX512 Networks
Copyright 2011 PLASA NA. All rights reserved.
CP/2009-1017r2
Approved as an American National Standard by the ANSI Board of Standards Review on 4 January 2011.

section 10.8.2 Lamp Hours PID is described with a PDL of 0x04 as you would expect.

It is also correct in a final draft which I, as a task group member, have.

What page are you referring to ?

Peter Willis

erwin May 1st, 2012 08:31 AM

Hey Peter,

Yes the spec says LAMP_HOURS is 32bit, and that's the way I implemented it in Wireshark. But I have a capture file from someone that has a LAMP_HOURS PID with a PDL of 2.

The question should probably more be like; when PID and PDL disagree, should I assume spec is always right and mark the packet as broken?

prwatE120 May 1st, 2012 08:46 AM

In essence YES - even if the spec is "broken". It is the spec, warts and all, and is the only common reference we have!

In the example you cite, the capture file is evidence of a non-compliant device.

Peter

ericthegeek May 1st, 2012 12:10 PM

Quote:

Originally Posted by erwin (Post 2396)
The question should probably more be like; when PID and PDL disagree, should I assume spec is always right and mark the packet as broken?

YES!!! Mark the packet as broken. If the received PDL doesn't match the expected value for that PID, then the packet is corrupt and should be flagged as such.

This is a common problem, I've seen many devices that send incorrect PDLs, especially on ACK SET_RESPONSE packets. Such behavior can cause interoperability problems.


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