As is apparent from the hourglass model of the Internet architecture, an application is hidden from the details of the lower layers by the abstraction of IP. This means it's not possible to determine directly the types of networks across which an IP packet will have traveled ”it could be anything from a 14.4-kilobit cellular radio connection to a multi-gigabit optical fiber ”or the level of congestion of that network. The only means of discovering the performance of the network are observation and measurement. So what do we need to measure, and how do we measure it? Luckily, the design of the IP layer means that the number of parameters is limited, and that number often can be further constrained by the needs of the application. The most important questions we can ask are these:
The next section provides some sample measurements for the first four listed parameters. The maximum rate is closely tied to the probability that packets are lost in the network, as discussed in Chapter 10, Congestion Control. What affects such measurements? The obvious factor is the location of the measurement stations . Measurements taken between two systems on a LAN will clearly show properties different from those of a transatlantic connection! But geography is not the only factor; the number of links traversed (often referred to as the number of hops ), the number of providers crossed, and the times at which the measurements are taken all are factors. The Internet is a large, complex, and dynamic system, so care must be taken to ensure that any measurements are representative of the part of the network where an application is to be used. We also have to consider what sort of network is being used, what other traffic is present, and how much other traffic is present. To date, the vast majority of network paths are fixed, wired (either copper or optical fiber) connections, and the vast majority of traffic (96% of bytes, 62% of flows, according to a recent estimate 123 ) is TCP based. The implications of these traffic patterns are as follows :
If the composition of the network infrastructure or the traffic changes, other sources of loss may become important. For example, a large increase in the number of wireless users would likely increase the proportion of loss due to packet corruption and interference on the wireless links. In another example, if the proportion of multimedia traffic using transports other than TCP increased, and those transports did not react to loss in the same way as TCP does, the loss patterns would probably change because of variation in the dynamics of congestion control. As we develop new applications that run on IP, we have to be aware of the changes we are bringing to the network, to ensure that we don't cause problems to other users. Chapter 10, Congestion Control, discusses this issue in more detail. |