Protocol :

Sending a request :

<request protocol="nebia-1">
    <rid>random id</rid>
    <did>device id</did>
    <sport id="BALL" />
    <sport id="CYCL" />
    <sendername id='uid'>Alice</sendername>
</request>

Sending a reply :

<answer protocol="nebia-1">
    <rid>same id</rid>
    <did>devide id</did>
    <sport id="BALL" />
    <sendername id='uid'>Bob</sendername>
    <presence>1</presence>
</answer>
The presence item can be :
1: Interested

Confirming the event:

<confirmation protocol="nebia-1">
    <rid>same id</rid>
    <sport id="BALL" />
    <username id='uid'>Alice</username>
    <username id='uid'>Bob</username>
    <username id='uid'>Eve</username>
</confirmation>

The protocol sends the name of the different people to be able to remind the
user if some day, the same people are taking part to an activity.