DP/IP, in which case retransmissions are not used. However, in the case of IPTV over TCP/IP (which is also possible), that will add delays and bitrate fluctuation due to TCP retransmissions of lost packets, which may result in long playout times at the set-top boxes (e.g. tens of seconds or even minutes). There- fore TCP versions used in legacy operating systems are not feasible for IPTV. However, proprietary transport protocols (with a combination of TCP and UDP features) may be developed for IPTV service delivery by telecom operators when they are at the same time service provider, network provider, and user set-top box provider. Why does packet loss have the greatest influence on IPTV QoE? Well, it leads to video pixelization or blocking, frame freezing, and/or set-top box lockup. However, the impact of packet losses is dependent on the type of video frame that is affected with it. Because IPTV is based on MPEG standards (MPEG-4 or MPEG-2), the worst case is when the I-frames are affected by the packet loss because they serve as the reference for all frames in a GoP and the loss of an I-frame (or part of it) can affect the whole GoP. Similarly, losses that affect P frames influence B frames in the same GoP. Packet loss also can cause longer channel change times because the decoders at the set-top box wait for the next I reference frame before presentation of an image to the viewer. 6.4 QoS for Popular Internet Services The popular Internet services are those which have appeared on the Internet and which are inseparably connected with Internet technologies and IP networks. One of the old- est and still most used in all types of communications is the email, in both personal and business environments. However, the most important native Internet service (or appli- cation) is the Web (i.e. the World Wide Web), based on the HTTP/TCP/IP protocol stack, which contributed to the global growth of the Internet with its appearance in the early 1990s. Email has replaced fax communications (used in the late decades of the t