Will MQTT topic subscribe occupies a new port?

[Koma Yip]

Hi all, I just want to know when subscribing a topic , will a new port is occupied between the client and the broker? Thanks

[Thomas]

Hi Koma,

No, there is only one TCP/IP connection between the server and the client, regardless of the number of subscriptions.

Thomas

[Gambit Communications]

It is actually an implementation issue how many TCP connections are used by
a client. Consider a multi-homed, multi-threaded subscriber multiplexing
subscriptions over more than one TCP connection, for whatever reason (levels
of service, load-balancing over layer 2 interfaces, etc).

[Paul Fremantle]

No, it happens using the existing TCP connection.

Paul

On 7 Jul 2017 3:02 am, "Koma Yip" <kom...@gmail.com> wrote:

Hi all, I just want to know when subscribing a topic , will a new port is occupied between the client and the broker? Thanks

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[Paul Fremantle]

That is not true. The definition of an MQTT client is that it has a connection to the broker. If another connection is made with the same ClientID then the old connection is automatically terminated by the broker. What you are describing is something that would need a new name and could not be described as an "MQTT Client" according to the specification.

Paul

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Paul Fremantle

Doctoral Researcher, University of Portsmouth, School of Computing

Visiting Scientist, Institute of the Architecture of Application Systems, Stuttgart

Visiting Lecturer, Software Engineering Programme, Oxford University

Co-Founder, WSO2
Apache Member and Committer

email: paul.fr...@port.ac.uk, pa...@fremantle.org

twitter: pzfreo / skype: paulfremantle / blog: http://pzf.fremantle.org

[Vardharajulu]

MQTT lies above the transport layer. TCP ports are defined in the transport layer. The port is used for the TCP connection between broker and client. All the other control packets of MQTT (including connect and disconnect packet of MQTT) are messages of a TCP connection. MQTT Connect packet retains TCP connection and Disconnect packet close the TCP connection. MQTT SUBSCRIBE, PUBLISH are just a transport layer message.

Vardharajulu K N,

Technical Architect,

Bevywise Networks, India.

[Gambit Communications]

You are correct using the narrow interpretation of "client". One client session / ClientID, one TCP connection.
The original post did not restrict to one client ID, and I used the application-centric concept of "client"
to mean "client application".
A client application can open multiple client sessions, each one using one and only one transport connection.
Thank you for the clarification.

On Monday, July 17, 2017 at 9:44:15 AM UTC-4, Paul Fremantle wrote:

That is not true. The definition of an MQTT client is that it has a connection to the broker. If another connection is made with the same ClientID then the old connection is automatically terminated by the broker. What you are describing is something that would need a new name and could not be described as an "MQTT Client" according to the specification.

Paul

On 17 July 2017 at 14:19, Gambit Communications <gambit.com...@gmail.com> wrote:

It is actually an implementation issue how many TCP connections are used by
a client. Consider a multi-homed, multi-threaded subscriber multiplexing
subscriptions over more than one TCP connection, for whatever reason (levels
of service, load-balancing over layer 2 interfaces, etc).

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[Paul Fremantle]

Completely agreed! 

Paul

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