Fishery

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Thin-wire Ethernet, or thin net, uses cable type RG-58. With 10Base2, the transceiver functionality is processed in the NIC. BNC T connectors link the cable to the NIC . As with every media type, due to signal degradation, a thin net segment is limited to fewer than 185 meters, with a maximum of 30 stations per segment of 1,024 stations total.

Ethernet comes in various flavors. The actual physical arrangement of nodes in a structure is termed the network topology. Ethernet topology examples include bus, star, and point-to-point.

The primary extended functions of NetBIOS are part of the NetBIOS Extended User Interface, or NetBEUI, technology. Basically, NetBEUI is a derivative of NetBIOS that utilizes NetBIOS addresses and ports for upper-layer communications. NetBEUI is an unreliable protocol, limited in scalability, used in local Windows NT, LAN Manager, and IBM LAN server networks for file and print services. The technology offers a small, efficient, optimized stack. Due to its simplicity, vendors recommend NetBEUI for small departmental-sized networks with fewer than 200 clients.

UDP messages are called user datagrams. These datagrams are encapsulated in IP, including the UDP header and data, as it travels across the Internet. Basically, UDP adds a header to the data that a user sends, and passes it along to IP. The IP layer then adds a header to what it receives from UDP. Finally, the network interface layer inserts the datagram in a frame before sending it from one machine to another. As just mentioned, UDP messages contain smaller headers and consume fewer overheads than TCP. The UDP datagram format is shown in Figure 1.18, and its components are defined in the following list.

Source/Destination Port. A 16-bit UDP port number used for datagram processing. Message Length. Specifies the number of octets in the UDP datagram. Checksum. An optional field to verify datagram delivery. Data. The data handed down to the TCP protocol, including upperlayer headers.

The RARP Daemon (RARPd) is a service that responds to RARP requests. Diskless systems typically use RARP at boot time to discover their 32-bit IP address, given their 48-bit hardware Ethernet address. The booting machine sends its Ethernet address, encapsulated in a frame as a RARP request message. The server running RARPd must have the machine’s name-to-IP-address entry, or it must be available from the Domain Name Server (DNS) with its name-to-Ethernetaddress. With these sources available, the RARPd server maps this Ethernet address with the corresponding IP address.

IP has many weaknesses, one of which is unreliable packet delivery—packets may be dropped due to transmission errors, bad routes, and/or throughput degradation. The Transmission Control Protocol (TCP) helps reconcile these issues by providing reliable, stream-oriented connections. In fact, 23 TCP/IP is predominantly based on TCP functionality, which is based on IP, to make up the TCP/IP suite. These features describe a connection-oriented process of communication establishment.
There are many components that result in TCP’s reliable service delivery. Following are some of the main points:
· Streams. Data is systematized and transferred as a stream of bits, organized into 8-bit octets or bytes. As these bits are received, they are passed on in the same manner.
· Buffer Flow Control. As data is passed in streams, protocol software may divide the stream to fill specific buffer sizes. TCP manages this process, and assures avoidance of a buffer
overflow. During this process, fast-sending stations may be stopped periodically to keep up with slow-receiving stations.
· Virtual Circuits. When one station requests communication with another, both stations inform their application programs, and agree to communicate. If the link or communications between these stations fail, both stations are made aware of the breakdown and inform their respective software applications. In this case, a coordinated retry is attempted.
· Full Duplex Connectivity. Stream transfer occurs in both directions, simultaneously, to reduce overall network traffic.

Another possible diversion is using a technique I’ve discussed before: strong-naming your assembly. Unfortunately, the best you can do with strong-naming your assembly is that you can prevent others from reverse-engineering your code and putting your brand on it. It doesn’t prevent others from at least being able to see the underlying code.

There are a couple of options you may try to prevent a user from reverse-engineering your assembly. In this section, I’ll cover the following techniques:
•    Native image generation
•    Strong-naming the assembly
•    Hiding the implementation via a Web service
•    Obfuscation
Native Image Generation
One technique you may try is to create a native image of your assembly. This is done via the ngen tool that comes with .NET. This tool doesn’t work with CIL source files; rather, it takes a .NET assembly and transforms it into a native assembly. For example, you can create a native image of AI.dll as follows:
ngen AI.dll
The tool creates a native image of the assembly, but it puts the result into the Native Image Cache (NIC). You can see what’s in the cache by using the /show argument of ngen:
ngen /show
Unfortunately, the results aren’t what you might expect. The problem is that you still need the original assembly for the client code to run properly. ngen essentially creates implementations of your methods, but the metadata is contained in the original assembly. If you try to run an assembly that uses AI.dll after you employed ngen on it and deleted (or renamed) the source AI.dll, the client will throw a FileNotFoundException.
The runtime will always load the CIL-based assembly, but it’s smart enough to use the native implementations if they exist in the NIC. Therefore, you can’t distribute a native image without the regular assembly. Someone will always be able to reengineer the code like I did before, and if you rename the resulting assembly and target the client to use this new assembly, you can prevent the runtime from using the native implementations.

The CAS mechanism in .NET allows for tighter control over code usage of resources by the user of a particular machine. For example, let’s say I bought a component object model (COM) server from a component vendor that has one coclass called CCompletelySafe that implements the IItsOK interface. This interface has one method called NothingBadWillHappen(). To get this to run, I add the reference to my VB application, and start coding away, like so:

Dim oOK As IItsOK
Set oOK = CreateObject(”TrustMe.CCompletelySafe”)
oOK.NothingBadWillHappen()

Unfortunately, in no time at all I’ve e-mailed everyone in the company a bunch of links that will probably get me fired. Plus, my files are being deleted at a furious rate, and my computer is also creating a bunch of HTTP traffic. The real problem here is that I can’t determine what that method call will do until I invoke it.