EE5412 TCPip QA1

EE5412 [TcpIpQA1.doc] [Garcia]
27. What is the difference between a physical address, a network address, and a domain name?

Solution:
The physical address is the unique hardware address that identifies an interface of a machine on a physical network such as a LAN. Physical addresses are used in the data link layer.

A network address is a machine's logical address on a network. The network address is used in the network layer. The network address used on the Internet is the IP address.

Domain names are used as an aid to identify hosts and networks in the Internet, since names are easier to remember than numbers. The DNS system is used to translate between domain names and IP addresses. The domain name for the network address 128.100.132.30 is toronto.edu.
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28. The Domain Name System has a hierarchical structure, for example, comm.toronto.edu. Explain how a DNS query might proceed if the local name server does not have the IP address for a given host.

Solution:
The domain name comm.toronto.edu has three levels of domains, comm.toronto.edu, toronto.edu, and edu. We can envision that name servers are arranged in a tree topology with a "root" server at the top; "edu" and other servers, such as "com", "org" "gov", "ca", at the second level; and below each second-level server, third level servers such as toronto.edu; and so on.

To find an IP address, in principle the host needs to contact the root server, that, if necessary, contacts an appropriate server below it, for example, edu. This second-level server can then, if necessary, contact a server in the level below it, for example, toronto.edu.

This process continues until the name is resolved.
This approach, however, can place large loads on the root server. In practice most queries involve local names, and so queries are first directed to a local name server. If the local name server cannot resolve a name, then the query is directed to another server in the domain system, for example, the parent server in the above tree hierarchy.
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31. Suppose a computer is moved from one department to another.
Does the physical address need to change?
Does the IP address need to change?
Does it make a difference if the computer is a laptop?

Solution:
The physical address does not change. It is globally unique to the computer's NIC card.

The IP address needs to be changed to reflect the new sub-network id and host id.
The situation is the same for laptops.
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38. Explain why it is useful for application layer programs to have a "well-known" TCP port number?
Solution:

The TCP layer entity uses the port number to determine which application program the packets belong to. In the TCP connection setup process it is very convenient to have a unique well-known port number, otherwise some protocol or procedure would be required to find the desired number.

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41. Which of the TCP/IP transport protocol (UDP or TCP) would you select for the following applications: packet voice, file transfer, remote login, multicast communication (i.e., multiple destinations).

Solution:
Packet Voice - This example involves the transfer of a stream of information in real time across the network. At first, it may appear that TCP is suitable because of its connection orientation.

However the acknowledgment and retransmission mechanisms in TCP introduce too much delay in the transfer of packets, and so UDP is the preferred approach to transferring a real-time voice stream across the network.

File Transfer - In general, file transfer requires reliable transfer and so TCP is preferred.

Remote Login - TCP is preferable because it provides for the reliable transfer of the stream of keystrokes that forms the basis for a remote login application.

Multicast Communication - In multicast services, a source sends information to a subset of
destinations attached to the network. It is easy to imagine multicast applications that require reliable transfer of a stream of information to a set of destinations, and multicast applications that require only best effort transfer of individual messages.

Therefore neither TCP nor UDP is preferred. A more pertinent point is that providing reliable multicast stream transfer service is quite difficult to implement, and TCP is not designed for this.
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46. Suppose regularly spaced PING packets are sent to a remote host. What can you conclude from the following results?

Solutions follow questions:

a. No replies arrive back.
Possibilities are: the remote host is down; the remote host or the network is extremely congested; the remote host is set up not to reply.

b. Some replies are lost.
Some packets are discarded due to congestion at the remote-host listening-buffer or congestion at the network routers.

c. All replies arrive but with variable delays.
The packets traverse network routes that have different path length or traffic load.

d. What kind of statistics would be useful to calculate for the round-trip delays?
Time(reply packet arrival) - Time( the echo packet is sent).
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