owever, it is clear that IPv6 will replace ver- sion 4 but it may take many more years, perhaps a decade. IPv6 header is shown in Figure 6.42 and it has a fixed length of 40 bytes. The version field in Figure 6.42 has value 9 for IPv6 to tell the network layer how other fields should be interpreted. The priority field contains a higher value for higher priority. Values 0 to 7 are for typical data applications where transmissions can slow down in the case of congestion. For example, the recommended value for FTP is 4 and for Telnet 6 because a few seconds delay of one packet in the FTP stream is not noticeable but in the case of Tel- net it is frustrating. Values 8 through 15 are for real-time traffic, such as audio or video, whose sending rate is constant. The flow label field is zero for low-priority traffic and other values may be used to indicate to which data flow between source and destination hosts this packet belongs. Many flows may be active between a pair of hosts. Then routers may, for example, be set up to handle real-time traffic SO that con- stant bandwidth is reserved for a certain flow. The payload length field tells how many bytes follow the 40-byte header in Figure 6.42. The next header field allows extension to the header following Introduction to Telecommunications Network Engineering 32 bits, 4 bytes 12345678 312345678 4 5 6 7 8 12345678 Version Flow label Payload length Next header Hop limit Source address Destination address Figure 6.42 IPv6 packet header. the fixed header in the figure. They are optional and, for example, the exten- sion header for information exchange between routers for authentication and encryption is specified [3]. If there are no extension headers, the Next header field specifies a higher layer protocol such as TCP or UDP. The hop limit field prevents packets from living forever and it is used the same way as the time-to-live field in IPv4 header. If we compare IPv4 in Figure 6.41 and IPv6 header in Figure 6.42, the IHL field is not needed in IPv6 because the header le