ation, something that is typical for OTT voice services. Unlike UDP, which is very light (regarding implementation and resource consumption at the host) and has no particular function except providing port numbers (which is good for some real-time applications), TCP can be considered as one of the most important Internet technology protocols. Why? Because the initial concept of Internet networking for end-to-end QoS support is implemented with TCP at the end hosts. While in tradi- tional telecommunication networks, the PSTNs, the intelligence was located inside the network and hosts were very simple devices (traditional telephones), in the Internet the intelligence for dealing with end-to-end QoS (e.g. congestion control) is implemented at end hosts (which are typically computers such as personal computers, smartphones, or other type of devices with hardware and software on them) via TCP. But TCP has also evolved with the Internet. During the ARPANET era in the 1970s, TCP functionalities were in fact implemented in one protocol together with network functionalities (which are now part of IP), called a network control program (NCP), as a single protocol imple- mented between the applications on the top and the network interface at the bottom of the hosts. However, with the standardization of IP and TCP in 1981, the NCP split into two main protocols: IP as a network protocol and transport protocols (TCP and UDP) over it. This was first implemented in the BSD (Berkeley Software Distribution) Unix os in the 1980s and implemented in all Internet hosts (several hundred of them) on January 1, 1983, known as Internet flag day. Because of the low number of hosts connected to the Internet at the time, it was possible to change the os in all of them in a short period of time. However, is another flag day possible? Well, when the Internet has billions and tens of billions of hosts (including things attached to the Internet, besides humans with their devices), it becomes almost impossible to implement such change in all hosts