r contains the properties and bandwidth requirements of the whole packet stream - this is described as flow switching or IP switching - 'a sequence of packets treated identically by a possibly complex routing function'. This would seem like a good idea, particularly as session persistency increases over time. However, session properties and session characteristics tend to change as a session progresses, SO more flexibility is needed. It is difficult to deliver flexibility and deterministic routing or switch performance. 17.4 Significance of the IPv6 Transition An additional challenge for an IPv4 router is that it never quite knows what type of packet it will have to deal with next. IPv6 tries to simplify things by using a fixed-length rather than variable-length header and by reducing the 14 fields used in IPv4 to 8 fields - the protocol version number, traffic class (similar to type of service in IPv4), a flow label to manage special priority requests, payload length, next header, hop limit, source address and destination address. This is one of the reasons Japanese vendors are keen on mandatory IPv6 in routers - it makes deterministic performance easier to achieve. 17.5 Router Hardware/Software Partitioning Performance can also be made more predictable by adding in a hardware coprocessor (or parallel coprocessors) to the router. Figure 17.1 shows the packet-flow sequence in a packet processor. To minimise delay, the router divides packet processing into a number of tasks. The data comes in from the physical layer and is demultiplexed in accordance with the MAC (medium access control) layer rule set, packets within frames within multiframes. The packet Network Software - The User Experience Lookup IP Co-processor Framer or MAC Queuing Address Traffic Modification Shaping Incoming Classification Traffic Management Multiple Outputs Figure 17.1 Router packet processing. 1 is then sent for classification. At this point a hardware coprocessor may be used to improve look-up performance. Performance is defined by the n