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homework II

1.Define protocol.Define protocol data unit(PDU).Which layers have PDUs? Which layers have end-to-end communication?
Protocol is a standard that governs interactions between peer process at the same layer but on different machines.
Protocal data units(PDUs) is massages that governed by protocols .
Application, transport, internet and data link layers have PDUs.
Data Link layer have end-to-end communication.

2.Why do we need nested PDU transmission(why can't the two application layer programs merely exchange PDUs without nesting)?
Because PDU at each layer can reach its peer process on aniother machince. After all,the application layer processes are on different machines. They aren't communicate directly.PDUs are nested, one inside another .So we look at nested PDU on router and on the distination host.

3.In nested PDU transmission,what three thing normally can be in PDU's data filed? What is exception to this rule?
In nested PDU transmission, HTTP-Request-PDU, TCP-PDU and IP packet can be in PDU's data filed.
Physical layer is expection to this rule.

4.Why are there multiple HTPP request-response cycles to download a single webpage?
Because this process of multiple cycles is why webpages appear the way they do on your screen.First ,text appears.Then ,individual images and other elements appear,one at a time.

5.What is an octet?
Octet is a string of eight bits (byte),to help you get used to this common piece of network terminology.

6.What is reliability?Why is it good? Why is it not used at all layer?
-Reliability is TCP.
-Because it does error detection and correction.
-Because checking for errors at all layer is not necessary and would add too must to costs.

7.What is connection-oriented sevice? Explain the benefits of sequence numbers.
Connect-oriented service is that each HTTP request and response is a seperate interaction.

8.What is flow control?
Flow Control is the ability to ask the other side to pause or slow down.

9.What is framgmentation? Why is it necessary? What program does reassembly?
Framgmentation is divides information to be sent in to several PDUs. At the receiving end, the information from several PDUs must be defragmented before being passed to the next-higher-layer process.

10.What are the two parts in two-part internet addresses? What are the three parts in three-part internet addresses?
Internet address have two part , a network part and a local part
Internet address have three part , network part ,subnet part , host part

11.With two-part internet addressing, a router connects two or more networks. To what do routers connect three-part network addressing?
-Two part addressing routers use the network part to decide whether they can deliver an IP packet directly to the destination host or must pass it on to another router on another network.
-Three part addressing , routers use both the network and subnet parts to decide whether ther can deliver an IP packet directly to the destination host or must pass it on to another router on different subnet.

12.What does a router look at in two-part addressing to decide whether it can deliver the IP packet to its destination host or must pass it on to another router? What does a router look at in three-part addresses?
- Two- part addressing is used,A router has to look first at only the network part of an internet address .
- Three-part addresing is used,A router now has to look first atb only the combined network part and the subnet part of the internet address .

13.Why do we need the Domain Name System (DNS)? What information does the clint PC send in a DNS-Request-PDU? What new information does the DNS host send back in its DNS-Response-PDU?
- Because DNS host contains a table listing host names and the internet address associated with each host name.
- The internet layer on the user PC,with out the user's knowledge ,sends a DNS-Request-PDU to the DNS host.This PDU contains the host name you tpyed in the URL.
- The DNS host then sends the internet address of the web server back to your interner layer process,through a DNS-Response-PDU.

14.Distinguish between top-level domains, second-level domains,and host names.
Top level domain are type of organization owning the domain name,and there are national top-level domains.
Second- level domain are beneath the top level domain,define individual organizations .
Host name are domain name of individual host computer ,can know about internet address of official addresses on the internet.

15.You wish to get internet addresses and host names for the computers in your firm. Describe what you would do to accomplish this.
It should be able to give you the internet address of all local hosts.There are also a small number of root hosts that have comprehensive kwonledge of the internet.

16.Why do we use autoconfiguation instead of assigning permanent internet addresses to each client PC? What new information is returned in an Autoconfiguration-Response-PDU?
- Because client PCs,organization normally use autoconfiguration hosts to assign temporary internet addresses to user PCs on and as-needed basis.
- The autoconfiguration host has a pool of internet addresses to assign.It selects and unused internet address form the pool and send this internet address back to the user PC in and Autoconfiguration-Response-PDU.