Flow Label Payload length Next header Hop limit Source IPv6 address (128 bits) Destination IPv6 address (128 bits) Figure 2.1 Comparison of IPv4 and IPv 6packet headers. Regarding the QoS on the network layer, the most important field in IP headers is Type of Service (ToS) in IPv4 as well as DSCP (Differentiated Services Code Point) in IPv6. Both fields have the same length (1 byte, i.e. 1 octet of bits) and can be used for traffic differentiation on the network layer, for marking packets (with this field in the IP headers) which belong to the same aggregate traffic type (also called a traffic class or QoS class), such as voice, TV, various data, VPN (virtual private network), etc. IP packets travel across different IP-based networks which are using different under- lying 2 of transport technologies (this refers to network interfaces defined with layers 1 and the protocol stack). For example, the most used local area network (LAN) for Inter- WiFi net access in home and business environments is Ethernet (IEEE 802.3 standard) and (IEEE 802.11 family of standards) as unified wireless LAN (WLAN). Both main Internet access standards, Ethernet and WiFi, are standardized by the Institute of Elec- trical and Electronics Engineers (IEEE) only on layer 1 and layer 2 (i.e. physical layer and data-link layer, respectively), while on layer 3 it is expected to carry primarily IP pack- ets, that is, to have IP on the protocol layer 3. So, from inception (Ethernet appeared in the 1980s and WiFi at the end of the 1990s) they were built to carry Internet traffic in the local area networks. Of course, one should note that these access networks are typically connected via other broadband access technologies (fixed or mobile) to the 2 Internet QoS core networks of the network providers. However, due to their influence globally on the Internet network infrastructures, the size limit in their layer 2 payloads (that is Medium access control (MAC) frame payload), which is approximately 1500 bytes, in fact defines the maximum size of IP p