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Type:
Improvement
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Status:
Closed
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Priority:
Major
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Resolution:
Fixed
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Affects Version/s:
3.1.1
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Proposal:
Hide
WD11 1232 - 1254
Remove existing content in this section and replace with:
When a Client reconnects with CleanSession = 0, both the client and server MUST redeliver any previous
in-flight messages. This means re-sending any unacknowledged PUBLISH packets (where QoS > 0) and PUBREL packets.
This is the only circumstance where a Client or Server is REQUIRED to redeliver messages.
Clients MAY resend SUBSCRIBE and UNSUBSCRIBE packets on reconnect but are not REQUIRED to do this.
While a modern TCP network is unlikely to lose packets, a Client or Server is permitted to attempt redelivery
at other times. However, redelivery is not encouraged unless a network failure has been detected.
Non-normative comment
Historically retransmission of Control Packets was required to overcome data loss on some older TCP networks.
This might remain a concern where MQTT 3.1.1 implementations are to be deployed in such environments.
Show
WD11 1232 - 1254
Remove existing content in this section and replace with:
When a Client reconnects with CleanSession = 0, both the client and server MUST redeliver any previous
in-flight messages. This means re-sending any unacknowledged PUBLISH packets (where QoS > 0) and PUBREL packets.
This is the only circumstance where a Client or Server is REQUIRED to redeliver messages.
Clients MAY resend SUBSCRIBE and UNSUBSCRIBE packets on reconnect but are not REQUIRED to do this.
While a modern TCP network is unlikely to lose packets, a Client or Server is permitted to attempt redelivery
at other times. However, redelivery is not encouraged unless a network failure has been detected.
Non-normative comment
Historically retransmission of Control Packets was required to overcome data loss on some older TCP networks.
This might remain a concern where MQTT 3.1.1 implementations are to be deployed in such environments.
WD08 (section 4.3) states that if no response is received from PUBLISH, PUBREL, SUBSCRIBE, UNSUBSCRIBE within a reasonable period of time then the sender may retry delivery.
This attempts to cater for unreliable TCP/IP networks. Since modern compliant TCP/IP networks should not lose packets, then the sender should not retry unless a network failure is detected.
The spec should be changed to reflect this and add non-normative content to describe the historical problem.
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I've always thought this was more of a problem when a connection is re-established after network failure - were the first thing a client should do is retry delivery of failed stuff. I've never been too clear if that include SUBSCRIBE and UNSUBSCRIBE...