MQTT SN Subcommittee

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       
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
 



Great catch Simon! I agree with your approach as consistent with MQTT5. I am anyway scared about the fact that the Protocol Version 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.


 


ZjQcmQRYFpfptBannerEnd




Great catch Simon!
I agree with your approach as consistent with MQTT5.
I am anyway scared about the fact that the Protocol Version in CONNECT is represented over 1 byte and in PUBLISH-1 is represented over 2 bits.
 
I propose the following 2 options to have a future-proof solution:
 
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 Protocol Version
 
Option 2 (Packet type refactoring)

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


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


 



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



ERRATUM: That should be bits 7 &  6 for the flags and 5 - 0 for packet type

 

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

Good morning folks,

 


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 decided to 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 have a mechanism in the PUBLISH packet to note protocol version WITHOUT
adding any further bytes.


 


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 warrant this change - and give us further flexibility for saving space elsewhere.


 


I still havn't gained access to the ticketing system, but will press OASIS next week and raise a ticket when I can. I'm happy to create a WD version (25) with just this change applied to all the message types this week for review.


 


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


 


Best,


Si


 






 







Unless otherwise stated above: IBM United Kingdom Limited Registered in England and Wales with number 741598 Registered office: PO Box 41, North Harbour, Portsmouth, Hants. PO6 3AU

I fear we may have our hand forced actually; the ENCAPSULATED message type sits at 0xFE right at the end of the reserved range in 1.2 which precludes the use of bits 0-1 for any nefarious purposes 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 received by 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. S 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 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 Great catch Simon! I agree with your approach as consistent with MQTT5. I am anyway scared about the fact that the Protocol Version 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. ZjQcmQRYFpfptBannerEnd Great catch Simon! I agree with your approach as consistent with MQTT5. I am anyway scared about the fact that the Protocol Version in CONNECT is represented over 1 byte and in PUBLISH-1 is represented over 2 bits. I propose the following 2 options to have a future-proof solution: 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 Protocol Version Option 2 (Packet type refactoring) PUBLISH 0x0C (MQTT-SN v2) PUBLISHv1_2 0x1E (MQTT-SN v1.2) Bye Davide 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 **** This is an EXTERNAL email. It was sent from outside of u-blox. **** ERRATUM: That should be bits 7 & 6 for the flags and 5 - 0 for packet type On Sun, 8 Jan 2023 at 10:13, Simon Johnson < thesii@gmail.com > wrote: Good morning folks, 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 decided to 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 have a mechanism in the PUBLISH packet to note protocol version WITHOUT adding any further bytes. 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 warrant this change - and give us further flexibility for saving space elsewhere. I still havn't gained access to the ticketing system, but will press OASIS next week and raise a ticket when I can. I'm happy to create a WD version (25) with just this change applied to all the message types this week for review. Further, Davide, I have made the changes to the doc RE: network connection and will submit WD 24 this coming week. Best, Si Unless otherwise stated above: IBM United Kingdom Limited Registered in England and Wales with number 741598 Registered office: PO Box 41, North Harbour, Portsmouth, Hants. PO6 3AU