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Q and A for Ch. 20

Q and A for Ch. 20. Victor Norman CS332, 2014. Last Friday’s scenario.

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Q and A for Ch. 20

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  1. Q and A for Ch. 20 Victor Norman CS332, 2014

  2. Last Friday’s scenario Q: In the scenario with the two rooms and the middle-man translator, you said that it would be a problem to give the translator a message in English for the French guy, because then the translator would have to change your content of “Donut please" to “Beignet s'ilvous plait". But what's the problem with that? They mean the same thing. A: The problem is that the request is at layer 3, which doesn’t change across heterogeneous layer 2 (English vs. French) networks.

  3. Router vs. Modem Q: How does a router differ from a modem? I always thought it was my cable modem which converted cable signals to ethernet. A: It is a difference of functionality. A modem converts an analog signal (from a carrier phone system, DSL, cable) to a signal that can be used in a LAN. A router inspects packets at layer 3 to determine which interface to forward them on.

  4. It is an illusion? Q: I found Comer's assurances on page 339, last paragraph that "although a combination of hardware and software provides the illusion of a uniform network system, no such network exists," to be totally off the wall. That the network exists virtually means it exists, and it is not an illusion. This is why the principal of hardware and software equivalence is so important. A: He is saying that the network appears as in Fig. 20.3 a, but is actually more like 20.3 b. You have no idea how your data is carried… (I have DSL, and my data leaves my house on a (ATM) virtual circuit… I didn’t know that and I don’t really care as long as it work.)

  5. Non IP protocols? Q: Are any other internetworking protocols commonly used aside from IP? A: Only IPv6, as far as I know.

  6. TCP/IP Q: Comer seems to imply that TCP/IP go together as a pair. Is that true, i.e. can TCP use other layer 3 applications, and vice versa? A: Theoretically, TCP can be used over other layer 3 protocols that provide the correct functionality. (Q: what is that functionality?)

  7. Internet = TCP/IP ? Q: Does the Internet have to use TCP/IP? Could we implement a new protocol and use that instead (actually, I suppose the difficulty of rolling out IPv6 indicates no) A: Using a new protocol on the Internet would require quite a few changes: 1. The internet routers would have to speak that new layer 3, and know how to move packets correctly. 2. The clients and servers would have to know how to use the new protocol (or we’d have to find a better way to isolate the location of services at layer 3 and below).

  8. Mr. T Q: So going back to our discussion Friday, Mr. T (the translator) is a router because he connects two different LANs? A: Right. He connects LANs that use different layer 2 protocols (English vs. French). He knows how to move stuff from Room 1 to Room 2, and vice versa.

  9. What layer do routers work at? Q: Are routers used on all layers except layer 5? A: A router is a layer 3 device. It receives de-encapsulated data from layer 2 below, looks at layer 3 data only in the packet, and sends the data on back down to layer 2 to another interface to be sent out. In general, it does not change values in the layer 3 header, but does replace the layer 2 headers.

  10. Router = host? Q: The book separates routers from hosts; is that really accurate? My understanding was that routers were usually just specialized hosts. A: No, a router is a specialized device. It can be small like a home router or big with many 100s of ports. You’ll see a real router on Wednesday. It is always something that you log in to to configure.

  11. IPv6 Q: How common is IPv6? What uses it? A: It is now 3% of traffic that accesses Google around the world. It is being deployed because last year the IANA gave out the last block of IPv4 addresses to Regional Internet Registries. So, some RIRs have blocks to allocate yet, but when they run out, that’s it…

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