 size) in response to congestion in its path. In the current Internet, the source algorithm is carried out by TCP. In case of congestion TCP reduces the send speed. This speed reduction and the subsequent speed recovery has been a subject for much interest during the last few years. Why? Let us first look at Figure 7.11. A reasonable QoS goal is that the inherent bit rate of a particular system should be possible to use for traffic, and thus generate corresponding income. The traffic types using TCP such as e-mail, file transfer or web browsing will be affected by a reduced transfer speed caused by TCP. Real-time applications such as IP telephony do not normally use TCP (but UDP). This traffic cannot slow down or retransmit, and tends to both cause congestion and suffer from loss. TCP behaviour can force the carried traffic to become much lower than system speed would permit. The TCP algorithm uses window size, threshold and acknowledgement as the main parameters. A window is the maximum traffic volume that is not yet acknowl- edged by the receiver. It can be expressed as the maximum number of sent packets that are not acknowledged. In Figure 7.11 the traffic is segmented and the sender is waiting for six segment acknowledgements travelling between receiver and sender. Figure 7.11 also shows the round trip time, RTT (or round trip delay). RTT is equal to the time it takes for a packet to travel to the receiver plus the time it would take to travel in the opposite direction. A sending window is the window that is agreed with the receiver. A threshold is a speed limit set by congestion indications in an iterative way, in order to operate below congestion. It is therefore also called congestion avoidance. Quality of Service Fully-reliable byte stream service Mobile Internet network Mobile Server terminal Segment Segment Segment Segment Segment Segment round window Bandwidth-delay product = Minimum number of outstanding bytes TCP needs to fully utilize its available bandwidth Figure 7.11 Important factors in TCP/IP o