[OASIS Issue Tracker] (MQTT-385) Simple clients skipping properties

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    [ https://issues.oasis-open.org/browse/MQTT-385?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=64892#comment-64892 ] 

Ken Borgendale commented on MQTT-385:
-------------------------------------

I do not see that there are any properties which must be universally used.  The most common thing for simple clients is that they are not trying for global interoperability but in fact work in a specific environment which after initial testing will continue to work in the same way.  While this may not be the most aesthetically pleasing design, it is a common one for small devices.  Such a client "knows" what the setting for various "max" properties are for a given server and just lives within them.  If we consider a device which simply publishes small QoS=0 messages to the server, the goal is to make MQTTv5.0 not any harder to code than MQTTv3.1.1.  

For a client using a full function client library such as Paho the equivalent is to have the client library parse the various max properties and have the client application ignore them.  The only difference is that any problems will most likely be caught at the client library level rather than by the server. 

The paragraph as it stands intends on conveying the following: "if a client uses any property values in a packet it must validate the properties.  If it uses none of them it is not required to validate any properties.".  We can change the words used to say this if it does not change the meaning.  

> Simple clients skipping properties
> ----------------------------------
>
>                 Key: MQTT-385
>                 URL: https://issues.oasis-open.org/browse/MQTT-385
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Bug
>          Components: edits
>    Affects Versions: 5, wd10
>            Reporter: Brian Raymor
>            Assignee: Ken Borgendale
>            Priority: Minor
>             Fix For: 5
>
>
> What is the rationale for this exemption -and- how is it interoperable?
> 757 A Client MAY process the Packet without checking the Properties by skipping over the number of bytes
> 758 specified by Property Length. A Client which does not check the Properties MUST NOT make use of any
> 759 Property it has not checked. A Server MUST check all Properties in a Control Packet.
> 760
> 761 Non normative comment
> 762 This exemption for Clients is designed to allow for simple Clients that do not require the
> 763 information sent in Properties. A full function Client should check and use the information in the
> 764 Properties.



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