Other Kinds of Firewalls Packet lters are the crudest kind of rewall: they operate at the network level, and generally look only at TCP, UDP, and IP headers. One can also build rewalls that restrict trafc according to the contents of the data elds; these are known as application-layer rewalls, or application rewalls for short. Application rewalls have some security advantages, because they can enforce more restrictive security policies and because they can transform data on the y. We may come back to the topic of application rewalls later in the course. We've only scratched the surface of the topic in this lecture. For more information on rewalls, the authoritative reference is Cheswick, Bellovin, and Rubin: Firewalls and Internet Security: Repelling the Wily Hacker. Packet ltering software is available for many operating systems: e.g., Linux has iptables, OpenBSD/FreeBSD has PF, and Windows XP has its own rewall. 5 Pr
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The Motivation for Firewalls Suppose you are given a machine, and asked to harden it against external attack. How do you do it? One starting point is to look at the network services that this machine is providing to the outside world. If any of its network services are buggy or have security holes, a hacker may be able to penetrate your machine by interacting with that application. As we know, bugs are inevitable, and bugs in security-critical applications often lead to security holes. Thus, the more network services your machine runs, the great
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Registers versus Cache In order to devise a coordinated scheme for management of registers and cache, it is first necessary to develop a better understanding of the differences and similarities between these two types of buffer memory. 2.1. Registers 2.1.1. Concepts of Registers Registers, or a "register file”, constitute a relatively small, fast, local memory residing in an address -space distinguished memory. The structure of a is given in Figure 1. from that of main register memory cell name: Figure 1. Structure of Register Memory Cell Since registers are the absolute top of the memory hierarchy (typically with cache just below), register access time is the fastest of all memory systems in a computer and there are typically fewer memory cells in a register file than there are cells in any other level of the memory hierarchy. Each register is usually one word wide, with a total of perhaps 16
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Abstract In current computer memory system hierarchy, registers and cache are both used to bridge the reference delay gap between the fast processor(s) and the slow main memory. While registers are managed by the compiler using program flow analysis, cache is mainly controlled by hardware without any program understanding. Due to the lack of coordination in managing these two memory structures, significant loss of system performance results because: 0 Cache space is wasted to hold inaccessible copies of values in registers. 0 Inaccessible copies of values replace those accessible ones from cache. 0 Despite the fact that register allocation has long recognized the benefits of live range analysis, current cache management has completely ignored live range information. In this paper, we propose an unified management of registers and cache using liveness and cache bypass. By using a single model to manage
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