The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ...

Chapter 39Web cache as an applicationAll applications described above are “virtual” applications, in the sense that they do not actually transfer their own datain the simulator; all that matter is the size and the time when data are transferred. Sometimes we may want applicationsto transfer their own data in simulations. One such example is web caching, where we want HTTP servers to send HTTPheaders to caches and clients. These headers contain page modification time information and other caching directives, whichare important for some cache consistency algorithms.In the following, we first describe general issues regarding transmitting application-level data in ns, then we discuss specialissues, as well as APIs, related to transmitting application data using TCP as transport. We will then proceed to discuss theinternal design of HTTP client, server, and proxy cache.39.1 Using application-level data in nsIn order to transmit application-level data in ns, we provide a uniform structure to pass data among applications, and topass data from applications to transport agents (Figure 39.1). It has three major components: a representation of a uniformapplication-level data unit (ADU), a common interface to pass data between applications, and two mechanisms to pass databetween applications and transport agents.39.1.1 ADUThe functionality of an ADU is similar to that of a Packet. It needs to pack user data into an array, which is then included inthe user data area of an nspacket by an Agent (this is not supported by current Agents. User must derive new agents to acceptuser data from applications, or use an wrapper like TcpApp. We’ll discuss this later).Compared with Packet, ADU provides this functionality in a different way. In Packet, a common area is allocated for allpacket headers; an offset is used to access different headers in this area. In ADU this is not applicable, because some ADUallocates their space dynamically according the the availability of user data. For example, if we want to deliver an OTclscript between applications, the size of the script is undetermined beforehand. Therefore, we choose a less efficient but moreflexible method. Each ADU defines its own data members, and provides methods to serialize them (i.e., pack data into anarray and extract them from an array). For example, in the abstract base class of all ADU, AppData, we have:class AppData {341