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Subject: [OASIS Issue Tracker] (MQTT-236) Consolidate Acks, enable negative acknowledgements.


    [ https://issues.oasis-open.org/browse/MQTT-236?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=61742#comment-61742 ] 

Ed Briggs commented on MQTT-236:
--------------------------------

On January 28, 2016 at the MQTT-TC face-to-face meeting in Hursley, a majority of the attendees voted for the following in connection with this JIRA (MQTT-236). The vote was taken by show of hands.

1. ACK and NAK consolidation is rejected.  The existing message types and encoding scheme will be preserved, and a return status code field will be added. A majority felt this would reduce the number of changes to the protocol.

2. QoS 0 will NOT use NAKS.  A majority felt that a) simple QoS 0 client applications expect a very simple 'fire-and-forget' programmatic interface and receiving asynchronous notification of status would cause difficulties for these applications, and b) that in the absence of a packet identifier in QoS 0, it would not always be possible to correlate commands and responses.

3.  The DISCONNECT packet will be modified to carry a 'reason' field to convey the reason for disconnection. 

4.   Servers will be permitted to send DISCONNECT commands to signal errors such as session eviction or errors which can not be recovered by retry or re-connection.  Clients are already permitted to send DISCONNECT commands in MQTT 3.1.1.


> Consolidate Acks, enable negative acknowledgements.
> ---------------------------------------------------
>
>                 Key: MQTT-236
>                 URL: https://issues.oasis-open.org/browse/MQTT-236
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: New Feature
>          Components: futures
>    Affects Versions: 3.1.1
>            Reporter: Andrew Banks
>            Priority: Critical
>
> Consolidate Acks, enable negative acknowledgements.
> ---------------------------------------------------
> Connack, SubAck, UnsubAck, PubAck, PubRec, PubRel, PubComp, PingResp
> are all flavours of an Ack. Currently there are no negative Acks.
> I suggest consolidating all of the acks into a single packet type eg the current Connack
> and using bits 3-0 of the fixed header to indicate the ack'd packet type. 
> Connect Ack     : 0010 0001 RL, SP, RC   
> Publish Ack     : 0010 0011 RL, ID, RC  (Ack Qos=0,1)
> Publish Ack     : 0010 0100 RL, ID, RC  (Ack Qos=2)
> Publish Ack     : 0010 0101 RL, ID      (Rel Qos=2)
> Publish Ack     : 0010 0110 RL, ID, RC  (Comp Qos=2)
> Subscribe Ack   : 0010 1000 RL, ID, RC(,RC)(,RC)... (Per topic)
> UnSubscribe Ack : 0010 1010 RL, ID, RC(,RC)(,RC)... (Per topic)
> Ping Ack        : 0010 1011 RL
> RL=RamainingLength, SP=SessionPresent, RC=ResponseCode ID=PacketIdentifier
> This would free up the remaining  SubAck, UnsubAck, PubAck, PubRec, PubRel, PubComp, 
> PingResp packet types for other uses.
> Connect Ack response codes
> --------------------------
> 0	0x00 Connection Accepted.
> 1-127   Reserved for future use.
> 128	0x80 Connection Refused, Malformed Packet, eg user name indicated but not present. Followed by network disconnection.
> 129	0x81 Connection Refused, reason unspecified. Followed by network disconnection.
> 130	0x82 Connection Refused, the Client is not authorized to connect. Followed by network disconnection.
> 131     0x83 Connection Refused, implementation specific, eg. server capacity reached.
>                                  Followed by network disconnection.
> 132	0x84 Connection Refused, unacceptable protocol version. The Server does not support
>                                  the level of the MQTT protocol requested by the Client.
>                                  Followed by network disconnection.
> 133	0x85 Connection Refused, identifier rejected. The Client identifier is correct UTF-8
>                                  but not allowed by the Server. Followed by network disconnection.
> 134	0x86 Connection Refused, Server unavailable. The Network Connection has been made but the 
>                                  MQTT service is unavailable. Followed by network disconnection.
> 135	0x87 Connection Refused, bad user name or password. The data in the user name or 
>                                  password is malformed. Followed by network disconnection.
> 136-255	Reserved for future use.
> Publish Ack response codes, Ack Qos=0,1
> ---------------------------------------
> A packet identifier of zero indicates an ack for a Qos=0 message, 
> rc=0,1 is not allowed for Qos=0 messages, instead no Ack should be sent.
> 0	0x00 Message Accepted, Publication of Qos=1 message proceeds, not allowed for Qos=0.
> 1	0x01 Message Accepted, No matching subscribers. Not allowed for Qos=0.
> 2-127   Reserved for future use.
> 128	0x80 Message Refused, Malformed Packet, eg invalid topic name. Followed by network disconnection.
> 129	0x81 Message Refused, reason unspecified.
> 130	0x82 Message Refused, publication not authorized.
> 131     0x83 Message Refused, implementation specific, eg. client quota reached.
> 132-255	Reserved for future use.
> Publish Ack response codes, Ack Qos=2
> -------------------------------------
> 0	0x00 Message Accepted, Publication of Qos=2 message proceeds.
> 1	0x01 Message Accepted, No matching subscribers. Should be used in preference to 0x00 
>                                If the server cannot determine whether there are any subscribers, eg because it is 
>                                in a cluster, then use response code 0x00.
>                                No PubRel,PubComp expected.
> 2-127	Reserved for future use.
> 128	0x80 Message Refused, Malformed Packet, eg packet identifier=0. Followed by network disconnection.
> 129	0x81 Message Refused, reason unspecified.  No PubRel,PubComp expected.
> 130	0x82 Message Refused, publication not authorized.  No PubRel,PubComp expected.
> 131     0x83 Message Refused, implementation specific, eg. client quota reached.  No PubRel,PubComp expected.
> 132-255	Reserved for future use.
> Publish Ack response codes, Rel Qos=2
> -------------------------------------
> There are no Publish Ack Rel Qos=2 response codes, should we add a 0x00 byte as a placeholder?
> Publish Ack response codes, Comp Qos=2
> --------------------------------------
> 0	0x00 Message Released, Publication of Qos=2 message completed.
> 1	0x01 Message Released, No matching subscribers.  Should be used in preference to 0x00 
>                                If the server cannot determine whether there are any subscribers, eg because it is 
>                                in a cluster, then use response code 0x00.
> 2	0x02 Message Released, No message existed eg. expired.
> 3-127	Reserved for future use.
> 128	0x80 Message Release Refused, Malformed Packet, eg packet identifier=0. Followed by network disconnection.
> 129	0x81 Message Release Refused, reason unspecified.
> 130	0x82 Message Release Completed without publication, publication not authorized.
> 131     0x83 Message Release Refused, implementation specific, eg. client quota reached.
> 132-255	Reserved for future use.
> Subscribe Ack response codes
> ----------------------------
> 0	0x00 Subscribe Success - Maximum QoS 0.
> 1	0x01 Subscribe Success - Maximum QoS 1.
> 2	0x02 Subscribe Success - Maximum QoS 2.
> 3-127	Reserved for future use.
> 128	0x80 Subscribe Refused, Malformed Packet, eg packet identifier=0. Followed by network disconnection.
> 129	0x81 Subscribe Refused, reason unspecified.
> 130	0x82 Subscribe Refused, not authorized.
> 131     0x83 Subscribe Refused, implementation specific, eg. client quota reached.
> 132-255	Reserved for future use.
> UnSubscribe Ack response codes
> ------------------------------
> 0	0x00 UnSubscribe Success.
> 1	0x01 UnSubscribe Success, No subscription existed.
> 2-127	Reserved for future use.
> 128	0x80 UnSubscribe Refused, Malformed Packet, eg. packet identifier=0. Followed by network disconnection.
> 129	0x81 UnSubscribe Refused, reason unspecified.
> 130	0x82 UnSubscribe Refused, not authorized.
> 131     0x83 UnSubscribe Refused, implementation specific, eg. client quota reached.
> 122-255	Reserved for future use.
> Ping Resp response codes
> -------------------------
> There are no Ping Resp response codes, should we add a 0x00 byte as a placeholder?



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