Re: [mqtt-comment] MQTT 5.0 WD13 - Message retaining

["Ken Borgendale" <kwb@us.ibm.com>]

The initial purpose of the retain as published flag is to allow server to server bridging where it is desirable for the original value of the retain bit to be preserved rather than the default behavior which is to set the retain bit to whether the message is sent from a retained source. This is stated in a non-normative comment. However, this function is useful for some other purposes, and so it is documented as how it works and not its intended use.

The statement in section 3.8.3.1 could perhaps be better worded to clarify that it applies to the forwarded PUBLISH packet. If the Retain As Published is 0, any forwarded message has its RETAIN flag set to 0. Regardless of the setting of Retain As Published, any message coming from a retained source has the RETAIN flag set to 1. Thus from the point of view of the client receiving the publish, if Retain As Published is 0, the RETAIN flag indicates whether or not the message came from a retained source.


Ken Borgendale -- kwb@us.ibm.com


Boris Verkhovykh ---05/30/2017 12:00:43 PM---Hello! I think I have found a discrepancy in the message retaining description:

From: Boris Verkhovykh <b_a_v@mail.ru>
To: mqtt-comment <mqtt-comment@lists.oasis-open.org>
Date: 05/30/2017 12:00 PM
Subject: [mqtt-comment] MQTT 5.0 WD13 - Message retaining
Sent by: <mqtt-comment@lists.oasis-open.org>

---

Hello!

I think I have found a discrepancy in the message retaining description:

1734 3.3.1.3 RETAIN
1735 Position: byte 1, bit 0.
...
1771 The setting of the RETAIN flag in an Application Message forwarded by the Server from an established
1772 connection is controlled by the Retain As Published subscription option. Refer to section 3.8.3.1 for a
1773 definition of the Subscription Options.
1774
1775   If the value of Retain As Published subscription option is set to 0, the Server MUST set the RETAIN
1776 flag to 0 when forwarding an Application Message regardless of how the RETAIN flag was set in the
1777 received PUBLISH packet [MQTT-3.3.1-12].
1778   If the value of Retain As Published subscription option is set to 1, the Server MUST set the RETAIN
1779 flag equal to the RETAIN flag in the received PUBLISH packet [MQTT-3.3.1-13].

And it appears that [MQTT-3.3.1-12] (RaP bit == 0) contradicts the last sentence from the following chapter:

2332 3.8.3.1 Subscription Options
...
2341 Bit 3 of the Subscription Options represents the Retain As Published option. If 1, Application Messages
2342 forwarded using this subscription keep the RETAIN flag they were published with. If 0, the RETAIN flag in
2343 the PUBLISH packet indicates whether it came from a retained source or is a new publication.

Or, is it different functionality for bridging? Then it should be articulated more the clearly.

Best regards,

Boris Verkhovykh