ificant disruption of the bitrate of the given connection. However, multi- ple TCP losses within the CW may lead to expiration of TCP timeouts (e.g. maximum time for waiting for acknowledgements), which may result in transition to slow start. Fast retransmit and Timeout fast recovery Packet loss Packet loss Exponential bitrate Linear growth bitrate growth Congestion Congestion Congestion start avoidance avoidance start avoidance Figure 2.3 TCP congestion control. 2 Internet QoS Different TCP versions may implement the above basic congestion control mechanisms partially, modify them in certain aspects (e.g. regarding the phases and their trigger), or combine TCP and UDP characteristics to achieve reliable and low- delay transfer (e.g. for services such as video streaming or IPTV). TCP payload contains data from application protocols that use TCP (e.g. the Web, i.e. Hypertext Transfer Protocol (HTTP)), email protocols, File Transfer Protocol (FTP)). TCP is a client-server protocol which establishes a connection between two end hosts prior to the data transfer. For connection establishment it uses a three-way handshake. "Fake" TCP connection establishment requests are the basis for denial of service (DoS) attacks, which affect the availability of the application or service provided via TCP. 2.1.3 Dynamic Host Configuration Protocol - DHCP The Dynamic Host Configuration Protocol (DHCP) is a standardized protocol for dynamic provision of network configuration parameters to Internet hosts in a given IP network. DHCP exists in every network, including telecom internal networks, and enterprise or home networks. DHCP has prolonged the life of IP version 4 (the initial IP) [12], providing the possibility to serve more users with a limited pool of public IP addresses. However, DHCP has fundamental importance for the global Internet and for the telecommunication world based on the Internet technologies for providing a plug-and-play approach for access to the Internet. How? Well, besides the dynamically allocated IP address, 