lecture24 - Networking Computer Networking is becoming a...

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Networking Computer Networking is becoming a bigger and bigger issue every day. It’s a versitile and inexpen- sive way to share resources and trade data. This section addresses the basic OS issues involved in commu- nicating between computers. Network vs. System Domain host router host router host router host network host That’s the same diagram from our discussion of the I/O system, only relabelled to represent a com- puter network. Some of the issues are remarkably similar. The system still has to address: Asynchrony: Events on different hosts are not synchronized. Data corruption and reordering: Reordering is similar to the problem of multiplexing responses, and is handled the same way. Data in the packets can be used to order data. Because there are more sources of error in the network, the OS has to address errors directly. Buffering: Each host is responsible for queueing data until the interested process retrieves it, similar to the way disk blocks are queued. There are some significant differences between a network and a hardware system, though: Autonomy: Individual pieces of hardware in a system are all controlled by the same entity, the owner of the machine or the CPU. In a network, each host may be an autonomous (or self-con- trolling) entity, with goals that may be in direct opposition of other hosts and no central author- ity to which to appeal to resolve conflicts. If this wasn’t enough to worry about in the abstract, there is the problem of two communicating entities sitting in different human domains. The legal requirements on the hardware or even the data content are often at issue. Latency: The latency between a CPU and a disk is a few tens of milliseconds at worst, and this is perceived as a glacial pace. The round trip time to a geosynchronous satellite and back is a quarter second. There are documented reports of packets taking minutes to get from host to host in the Internet. The latencies are often considerably higher in a network. But sometimes they are lower. Hosts on an uncongested LAN sometimes use a distributed file system to reduce disk latency. The range of latencies with which systems have to contend in networks is the issue. Connectivity Richness: In a physical box, there are only so many elements that can sit physi- cally on one bus, so the CPU need only concern itself with a few entities. There are millions of computers connected to the Internet. Systems have to exhibit vastly different scaling properties in networks.
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-2- Basic Concepts Because there are so many more elements connected sparsely, issues of naming, addressing, and rout- ing become paramount. It’s important to grasp the distinction between a name, an address, and a route. Names are a convenient way for humans (or programs) to refer to an entity. My name is Ted; my computer’s name is vermouth.isi.edu . In both cases this is just a convenient string of char- acters that refers to a physical entity.
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