Re: Packet Type Field Change

I fear we may have our hand forced actually; the ENCAPSULATED message type sits at 0xFE right at the end of the reservedÂrange in 1.2 which precludes the use of bits 0-1 for any nefariousÂpurposes such as this so the available solutions to this as I see it are as follows:

Create a NEW packet type for version 2 which is a new PUBLISH_MINUS_1 specific packet format such that version 1.2 PUBLISH -1's receivedÂby a gateway will decode successfully in the old format and can be differentiated
Modify the meaning of flags (byte 3) field bit 3 (presently reserved in the current PUBLISH spec) where 0 means "old style PUBLISH" and 1 means "new style -1 PUBLISH" - this mean old style packets will still decode ok since they are forced to send 0 here, and there is no extra byte required - this is important since many old client impls still exist and we will want to be able to accept valid 1.2 traffic on new gateways
Use one of those reserved bits above to mean "protocol" version byte follows, which when set to 1 will decode the a protocol byte inserted at Byte 4
Ignore and accept the broken change
Other suggestions....

Now we have the packet versions broken out for QoS -1 & 0 in the document anyway, this seems less impactful - but as Andy said, we need to ensure we're good going forward.

On Mon, 9 Jan 2023 at 09:26, Andy Stanford-Clark <ANDYSC@uk.ibm.com> wrote:

Well spotted, Si ð

Isn't this just kicking the can down the road, Davide?
Perhaps we should bite the bullet now and take the extra byte for the protocol version?

Our future selves will one day thank us ð

I really like the "drive-by publish" of QoS -1, but given that it is completely "self contained" it should not surprise us that it's a bit bigger than a normal "in session" publish.

I admit to being torn on this one, as "spare bits" always make me unhappy!

Andy

Andy Stanford-Clark

Innovation Leader - IBM SPEED team

Distinguished Engineer, Master Inventor, IBM Quantum Ambassador,
Member IBM Academy of Technology, Fellow BCS
Visiting Professor at Newcastle, Southampton and UEA.

Tel: +44 (0)7801 787096 Â ÂÂÂ twitter: @andysc

From: mqtt-sn@lists.oasis-open.org <mqtt-sn@lists.oasis-open.org> on behalf of Davide Lenzarini <davide.lenzarini@u-blox.com>
Sent: 09 January 2023 08:52
To: Simon Johnson <thesii@gmail.com>
Cc: Ian Craggs <icraggs@gmail.com>; tara.walker@microsoft.com <tara.walker@microsoft.com>; mqtt-sn@lists.oasis-open.org <mqtt-sn@lists.oasis-open.org>
Subject: [EXTERNAL] [mqtt-sn] RE: Packet Type Field Change
Â

Great catch Simon! I agree with your approach as consistent with MQTT5. I am anyway scared about the fact that the âProtocol Versionâ in CONNECT is represented over 1 byte and in PUBLISH-1 is represented over 2 bits. I propose the following

ZjQcmQRYFpfptBannerStart

This Message Is From an External Sender

This message came from outside your organization.

Â

ZjQcmQRYFpfptBannerEnd

Great catch Simon!

I agree with your approach as consistent with MQTT5.

I am anyway scared about the fact that the âProtocol Versionâ in CONNECT is represented over 1 byte and in PUBLISH-1 is represented over 2 bits.

Â

I propose the following 2 options to have a future-proof solution:

Â

Option 1 (Packet type 2 bits flag extension):

00 means MQTT-SN v1.2
01 means MQTT-SN v2
10 is reserved
11 means there is a following dedicated byte providing the âProtocol Versionâ

Â

Option 2 (Packet type refactoring)

PUBLISH 0x0C (MQTT-SN v2)
PUBLISHv1_2 0x1E (MQTT-SN v1.2)

Â

Bye

Davide

Â

From: Simon Johnson <thesii@gmail.com>
Sent: Sunday, January 8, 2023 11:31 AM
To: mqtt-sn@lists.oasis-open.org
Cc: Ian Craggs <icraggs@gmail.com>; tara.walker@microsoft.com; Davide Lenzarini <davide.lenzarini@u-blox.com>
Subject: Re: Packet Type Field Change

Â

**** This is an EXTERNAL email. It was sent from outside of u-blox. ****

ERRATUM: That should be bits 7 & Â6 for the flags and 5 - 0 for packet type

Â

On Sun, 8 Jan 2023 at 10:13, Simon Johnson <thesii@gmail.com> wrote:

Good morning folks,

Â

I have been working with PUBLISH -1 in v2. I have found a flaw with the new packet type layouts. Presently you can PUBLISH -1 without a session, and it is the SESSION that determines the protocol version you have decidedÂto use. Without a session, the GATEWAY has no mechanism to determine the version of the PUBLISH packet to use to decode it. So to maintain the ability for a GW to support both protocol versions, we must haveÂa mechanism in the PUBLISH packet to note protocol version WITHOUT adding any further bytes.

Â

I was looking, the packet type field seems to be a great chance to squeeze some more space out of - in a similar fashion to tt5, so we coolocate a generic flags field in bits 1 & 0, using 6 bits of space (7-2) for packet type (64 available types - we currently use ~30) so plenty of growing room - then the 2 new flags can be used for flags on the message without needing a new flags field. In the case of PUBLISH this can be protocol version, but also it could be used for a few other types specifying and using only 1 or 2 flags saving space elsewhere. I feel this is a fundamental enough issue to warrantÂthis change - and give us further flexibility for saving space elsewhere.

Â

I still havn'tÂgained access to the ticketing system, but will press OASIS next week and raise a ticket whenÂI can. I'm happy to create a WD version (25) with just this change applied to all the messageÂtypes this week for review.

Â

Further, Davide, I have made the changes to the doc RE: network connection and will submit WD 24 this coming week.

Â

Best,

Si

Â

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