Re: Lifetime of topic alias map

[Takatoshi Kondo <redboltz@gmail.com>]

Hi,

> When resending the publish packet that has an empty topic message and
> Topic Alias property, the empty topic name must be replaced with the
> topic name that is corresponding to the Topic Alias at the first
> sending.

Typo. "an empty topic message" should be "an empty topic name".


On Tue, Jan 19, 2021 at 8:31 PM Takatoshi Kondo <redboltz@gmail.com> wrote:
>
> Hi,
>
> I noticed that if topic alias map is a part of the session state, then
> implementing broker cluster would become very difficult.
> So as the spec said, the topic alias map should be cleared when the
> connection is disconnected.
>
> I think that the spec should define clearer the behavior retrying a
> publish message that has empty topic name and Topic Alias Property
>
> At the following chapter,
>
> 4.4 Message delivery retry
> https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901238
>
> I propose adding the clause is something like as follows:
>
> When resending the publish packet that has an empty topic message and
> Topic Alias property, the empty topic name must be replaced with the
> topic name that is corresponding to the Topic Alias at the first
> sending.
>
> NOTE: That means changing meaning from **use topic alias** to
> **register topic alias**.
>
> MQTT client needs to implement the replacement logic. It can avoid
> infinity protocol error and reconnecting loop.
>
> ---
> Thanks,
> Takatoshi
>
> On Wed, Sep 2, 2020 at 10:09 AM Takatoshi Kondo <redboltz@gmail.com> wrote:
> >
> > On Fri, Aug 14, 2020 at 1:53 PM Takatoshi Kondo <redboltz@gmail.com> wrote:
> > >
> > > Hi,
> > >
> > > I have a question about the lifetime of TopicAlias map.
> > >
> > > According to https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Topic_Alias
> > >
> > > > Topic Alias mappings exist only within a Network Connection and last only for the lifetime of that Network Connection.
> > >
> > > It seems that the lifetime of the map is over when the network
> > > connection is closed.
> > >
> > > Consider the following scenario.
> > >
> > > 1. Client connects to the broker with CleanStart: false
> > > SessionExpiryInterval=0xfffffffff(infinity)
> > > 2. Client publishes topic1 with TopicAlias: 1, QoS: 1, PacketId: 1
> > >    Broker inserts topic1 - ToppicAlias 1 entry to the map.
> > > 3. Client publishes empty topic  with TopicAlias: 1, QoS: 1, PacketId: 2
> > >    Broker finds topic1 from the map using TopicAlias 1 as the key.
> > > 4. Client receives PUBACK PacketId: 1.
> > > 5. Disconnect between the client and the broker. Broker clears the map here.
> > > 6. Client connects to the broker with CleanStart: false
> > > SessionExpiryInterval=0xfffffffff(infinity)
> > > 7. Client publishes empty topic  with TopicAlias: 1, QoS: 1, PacketId:
> > > 2 (PUBACK PacketId: 2 is not received so resend step 3 message)
> > > 8. Broker can't find the topic corresponding to the TopicAlias: 1 in
> > > the map because the map has been cleared at the step 5.
> > >    What should happen here? Should the broker disconnect the client
> > > due to no matching TopicAlias with empty TopicName ?
> >
> > According to the spec,  DISCONNECT with Reason Code of 0x82 (Protocol
> > Error) should happen.
> >
> > See
> >
> > ---
> > https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901120
> >
> > 3)    If the receiver does not already have a mapping for this Topic Alias
> > a)    If the packet has a zero length Topic Name field it is a
> > Protocol Error and the receiver uses DISCONNECT with Reason Code of
> > 0x82 (Protocol Error) as described in section 4.13.
> > ---
> >
> > That indicates that re-send and disconnect happen again and again if
> > Topic Alias map lifetime lasts only for the lifetime of that Network
> > Connection.
> > If Topic Alias map lifetime lasts the same as session lifetime, the
> > re-send won't cause DISCONNECT.
> >
> > ---
> > Thanks,
> > Takatoshi
> >
> > >
> > > If the client continues to re-connect with the same parameter, the
> > > publish packet at the step 3 is never processed by the broker.
> > > The client needs to set CleanStart: true to erase re-send publish
> > > message. But it avoids message deliverty guarantee.
> > >
> > > I think that if the lifetime of the map is the same as session
> > > lifetime, it is easy to use.
> > > At the step 5, broker dossn't clear the map. The lifetime of the map
> > > is extended until the session lifetime is over.
> > >
> > > However, session is defined as follows:
> > >
> > > https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc473619931
> > >
> > > > A stateful interaction between a Client and a Server. Some Sessions last only as long as the Network Connection, others can span multiple consecutive Network Connections between a Client and a Server.
> > >
> > > It seems ttat Network Connection and session are different concept.
> > >
> > > I think that TopicAlias mapping should have the same lifetime as the
> > > session lifetime.
> > >
> > > Any ideas?
> > >
> > > ----
> > > Thanks,
> > > Takatoshi