col stack. However, with the availability of broadband access to the Internet, many video sharing sites pro- vide content by using the HTTP/TCP/IP protocol stack, with longer delays for playback point due to retransmission of all lost segments by TCP. Also, proprietary transport pro- tocol implementations, based on certain characteristic of UDP (e.g. datagram transfer) and TCP (e.g. acknowledgments and retransmissions of lost IP packets), are being used for video traffic (e.g. for IPTV). However, video streaming (as unidirectional video, from video servers toward the video player of the end-user) is also considered as a real-time service, although it has less strict end-to-end delay requirements when compared with voice (e.g. it can tolerate delays up to several seconds). 2.3.3 Non-Real-Time Traffic Characterization NRT traffic has no strict requirements on delay. Such traffic may include applications which are interactive (e.g. Web, FTP) or message based (e.g. email). Most of this NRT traffic is carried by using TCP/IP protocol stack, SO it has lossless transmission end-to-end, which is provided by TCP on the transport layer at the end hosts (e.g. Web browser and Web server as two end sides for Web communication). NRT is considered to be one of the main reasons for Internet congestion at bottlenecks in the networks for 2.3 Internet Traffic Characterization Bottleneck capacity for the TCP flow (end-to-end point of view) Actual TCP flow throughput (it is always below the bottleneck end- to-end capacity for the given flow) Figure 2.7 TCP behavior at congestion. a given connection end-to-end (a bottleneck may appear anywhere in access, core, or transit networks). Congestion typically appears on network interfaces when all packets that should be sent through them cannot be sent in the required time interval, which results in long buffer queues and finally in packet losses (packets are dropped when the buffers are filled over a certain threshold). What are the reasons for Internet congestion? Well, most Internet traff