Q and A for Ch. 1, 2, 3 CS 332 Spring 2014

Structure of the class Q: Comer describes five aspects of networking around which he has structured his text. How closely will this course’s content match that structure? A: We’ll follow it closely, but we’ll skip most of section 2.

TCP/IP stack vs. OSI stack Q: How do the stacks differ? Why use the TCP/IP stack? A: The Presentation and Session layers are theoretically good, but are not actually implemented in any TCP/IP protocol, AFAIK.

Network topology Q: What is a network topology? A: It is the configuration of a network – which machines are connected to which, etc. It is layer 1.

(De)multiplexing Q: What do multiplexing and demultiplexing mean? A: Multiplexing is the gathering of multiple “things” to share one resource; demultiplexing is the opposite: taking “things” that share a resource and splitting them out, according to some criteria.

Internet Usage

Public vs. Private Network Q: Comer defines private networks as networks which have their use restricted to a specific group of users. He gives ISP-driven networks (which are open to anyone paying a subscription fee) as an example of a public network. Wouldn’t those networks technically also be private networks, with the “specific group” being the subscribers? A: Not according to his definition…The way I see it, there is no need to encrypted data in a private network – it is all your data. But, not so in a public network.

Private network types Q: Comer lists the scale categories that vendors use to describe network sizes (Consumer, SOHO, SMB, Large Enterprise). How have these categories and definitions changed over the history of networking technology? A: I don’t know… I never worked for a company that did private network stuff…

Communication Protocols Q: Is it correct to say that communications protocols govern all five aspects of networking listed by Comer? A: Yes. If you want your computer to communicate with another computer, you have to define a protocol so each knows what the other is saying. This is true at all layers.

Physical Layer Headers Q: The text says that Physical layer headers are not in common use. Under what conditions would such a header be used? A: There must be some physical layers that prepend and append signals to a packet. These would be signals to do synchronization between endpoints, provide reliability, etc. I don’t know much about physical layer stuff…

Building packets in memory Q: In sending a message through the TCP/IP stack, how do the headers get prepended to the message in memory? Is space reserved for the headers before (or after) the message location, or is it some mess of pointers? Does the location of the headers not matter, and one of the layers ensures they get sent across the network in the correct order?

Building packets in memory A: In general, there is a pool of memory structures called “mbufs” that are used for building up packets. It has been a long time since I’ve used them and I don’t remember the details, but they are nasty… Space is not reserved at higher layers for lower layer headers. The location of headers totally matters. Lower layer modules only look at data in their layer, which must be on the front of the packet.

First packet switching technology Q: Was ARPANet the first technology to implement packet switching? A: I don’t know… Lots of places were building their own little networks with their own little protocols…

Networking history Q: How important to the study of current networking theory and practice is an in-depth understanding of previous technical implementations? A: Not that important, to me…

World Wide Web Q: What precisely is meant by the term “World Wide Web”? A: It is all computers that can communicate with each other, using the http protocol. E.g., some refrigerators, pop machines, TVs, etc.

Message lengths Q: Are the limits on message length in a message-based communication paradigm protocol-dependent? A: Yes. Some message lengths are limited by the protocol, but ultimately message lengths are limited by the physical layer, which gets its message lengths (min and max) from physical properties.

Stream-based paradigms Q: Why are stream-based paradigms not favored for multimedia? A: Because in a stream-based paradigm (TCP), data is not delivered to the application (layer 5) if *any* previous data is missing. It must be delivered in order. But, in streaming real-time multiple applications, old data is worthless data. Q: Are stream paradigms packet-based? A: Yes. The stream paradigm is built on top of the packet-based deliver in the TCP/IP Internet.

Server/client identification Q: Does the server need to identify the client in the same way the client identifies the server, with both an IP address and a port number? A: Yes. Both endpoints are identified by the pair.

Data during disconnects Q: What happens when there is a disconnect in the network when streaming a service, does the network continue to send packets after the client is back up, or does the client need to reopen the stream? A: Client and server need to reestablish their connection… We’ll find out all about this later…

Questions about sockets Q: Various questions about socket API definitions and rationale… A: I cannot nor will I attempt to defend decisions made by the authors of the sockets API. It is a mess. Nasty, nasty mess. Ask them your questions, and then beat them with a wet noodle. (There is lots of history and compromise involved with sockets…)

Stream vs. message vs. … Q: Are there other communication paradigms than “stream” and “message”? A: There are some paradigms that are sort of in between. Stream protocols provide reliability, but require setup. Message protocols don’t. Some have tried to make some protocols that provide reliability without setup…

Communication Speeds Q: Have communication speeds increased with Moore's law? Are they expected to continue to increase? A: I don’t know, for sure. But, communication speeds have certainly increased a lot during my lifetime, and are still increasing.