The following candidates are being presented in the most cases:
- MQTT and its variants like MQTT-S
- REST API
Maybe it's also worth mentioning that there are some other similar protocols - like HTTPU, which is used by UPnP / DLNA for the home networking needs. And it will stay for a long time in the home space and must be considered by IoT ecosystem as an important protocol.
If you will try to Google for the pending discussion groups about the choices for these protocols, then you may find a lot of them. There is also the following publicly available document, which becomes more and more popular in such discussion groups: link to GoogleDocs
And the hot discussion happens not only in the space of development communities and the topic researchers, but in the large enterprise circles as well: MQTT is pushed forward by IBM and CoAP with XMPP are actively promoted by Cisco, smaller companies have their own case studies for MQTT or CoAP deployments etc.
Still this question is open and there is no decision here. Maybe it doesn't make sense to identify such a single standard IoT protocol, but to stick with the suitable choices for the project requirements?
Somehow this story reminds me the old days about "SIP over UDP" vs "SIP over TCP/TLS" discussions. With the wish to find the perfect balance between scalability (UDP wins) and NAT traversal options (TCP wins). There were some other reasons for SIP over TCP in addition to NAT traversal - larger MTU size and larger SIP REGISTER periods for prevening extreme server spamming. But still the main point in this story was not about UDP or TCP, but in the hot discussion itself ;-)
To make the long story shorter - here is my self-made summary for the protocol choices in IoT space: