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Subject: [OASIS Issue Tracker] (MQTT-253) Subscription identification

    [ https://issues.oasis-open.org/browse/MQTT-253?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=64315#comment-64315 ] 

Ken Borgendale commented on MQTT-253:

I do not understand the paragraph stating that the subscription IDs are not part of the client session state.  If the client wants to make use of them they need to be in the session state, and if the client does not want to use them it is free to not ever specify one.  Using a subscription ID in a durable session and then forgetting about it seems a bit strange.

We should point out the retransmission is not the only case where there is ambiguity when changing the subscription ID.  If a message cannot be delivered immediately and is queued, it might get either the old subID or the new subID depending on where it is queued.  

Given this issue, it makes sense to suggest in non-normative text that the best way to avoid this problem as a client is to continue to use the same subID for the life of a subscription.  

I would also note that this ability to change the subID of an existing subscription is likely to be the most costly part of implementing this feature.  What we say for changing the QoS of a subscription is that we replace the existing subscription with a new subscription without  interrupting the flow of messages and we might want to keep similar text or just put the subID into that paragraph.

As always, if some server wants to not implement this function is can just reject any subscription which uses one. However, I think we should encourage servers to implement this by not providing the CONNACK flag.  

> Subscription identification
> ---------------------------
>                 Key: MQTT-253
>                 URL: https://issues.oasis-open.org/browse/MQTT-253
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: New Feature
>          Components: futures
>    Affects Versions: 5
>            Reporter: Andrew Schofield
>            Assignee: Andrew Banks
>            Priority: Minor
> Subscribe and unsubscribe operations are performed on topic filters in MQTT 3.1.1. Adding subscription identifiers would enable unambiguous identification of subscriptions.

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