rnet hosts), then the connection (a call, a session) is established between two open sockets at the two end hosts. If one of the sockets closes, it means that the given connection is also con- sidered as closed. So, to maintain a given connection means to keep open the sockets between the two ends. That refers to both fixed and mobile Internet access. However, it is more complex to keep the socket open during a connection for a moving host which typically changes a point of attachment to the network (while it is moving), and hence it may change the IP network to which it is attached, which means that the IP address must change in such a case. But if any of the parameters (IP address, port number, transport protocol type) which define a given socket change, it means that a new socket should be opened and the previous one closed. That results in closing (i.e. dropping) the connection and opening a new one (that is, a new connection with a new socket). How- ever, in such a case there is no mobility because mobility means maintaining the same connection of mobile hosts regardless of their movement. Due to such mobility prob- lems in IP network environments, there are certain solutions on the network layer for Internet hosts, which include standardized protocols such as Mobile IP (for IPv4 mobile networks) [7] and Mobile IPv6 (for IPv6 mobile networks) [8]. But in mobile networks 2.1 Overview of Internet Technology Protocols (until and including 4G) the mobility is typically handled on the data-link layer (below the IP layer), which hides the mobility from the network layer up and hence eliminates potential problems that may arise from the IP addressing principles (which were initially created for fixed hosts only). After defining the sockets that bind the IP (on the network layer) and the transport layer protocols, let's see which are the main transport protocols and their main princi- ples influencing QoS. The main transport protocols are: UDP, standardized with RFC 798 in 1980 [3]; TCP, initially standardized in 19