re serving the clients for a given (same) application or service. The servers can be centralized into a sin- gle data center or distributed among several geographically dispersed data centers (e.g. content distribution network (CDN) [17]). For example, global OTT service providers have many data centers, which are situated as close as possible to the end-users in a given region because besides balancing of the servers' load, another important param- eter for client-server communication is the delay in receiving a response from a server for a given client request (also referred to as RTT). Overall, client-server architectures are Internet native and fundamental; however, they are not symmetric regarding client and server functionalities and their roles. Both main types of transport protocols (TCP, UDP) as well as others (SCTP, DCCP) are also based on the client-server principles. The transport protocol on the side of the client application acts as a client (e.g. TCP client, UDP client) and the transport protocol on the side of the server acts as a server (e.g. TCP server, UDP server). Each protocol communication is peer-to-peer (P2P), which means that a given protocol (e.g. application, transport protocol) must communicate with exactly the same protocol on the other side of the connection (i.e. the same application protocol and the same transport protocol, respectively). 2 Internet QoS 2.2.2 Peer-to-Peer Internet Networking P2P networking is based on network architectures in which each host (computer, smartphone, etc.) or network node (e.g. router) has similar capabilities and functional- ities as other peers. P2P applications for residential users appeared by the end of 1990s. One of the first globally known P2P architectures (besides those used in research) was Napster, which was created and used for sharing of music files (mp3) around the year 2000. It was followed by other file-sharing systems (e.g. Gnutella, Kazaa, eDonkey, BitTorrent). All conversational communications over IP networks, such as VoIP or video