Consider the problem of creating a simple bridge between two protocols, say XMPP and CoAP introduced in the previous chapters. For a device that communicates through XMPP to be able to interact with a device that uses CoAP, a bridge needs to be inserted in between. This bridge would need to be able to translate requests made using XMPP to a request made using CoAP, then translate the CoAP response back to an XMPP response. In the same way, if a CoAP device wants to request something from an XMPP device, the bridge would need to translate the CoAP request into an XMPP request and then translate the XMPP response back to a CoAP response. It is easy to see we need a pair of translators from one protocol to the other and vice versa.

However, what happens if we want to introduce a third protocol, say MQTT, into the picture? In this case, three pairs of protocol translators would be required: CoAP and XMPP, MQTT and XMPP, and CoAP and MQTT. Another complexity arises...