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This 4 page Class Notes was uploaded by Nishtha on Wednesday May 18, 2016. The Class Notes belongs to INFO 330 at Drexel University taught by Dikran Kassabian in Spring 2016. Since its upload, it has received 20 views. For similar materials see Networking in INFO at Drexel University.
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Date Created: 05/18/16
1 INFO 330 – HW2 – Application Layer, Transport Layer Name: Nishtha H. Dalal This assignment is to be done individually. Please use complete sentences for your responses. You will need to use the textbook as a reference guide to answer many of the questions, and for some questions you may want to have your laptop connected to the Internet to research or capture results. 1. (10 points) A student types in www.ed.ac.uk in their web browser to learn about the University of Edinburgh. The IP address of the web server is initially unknown. Name all application and transport layer protocols that are required in this web session in addition to HTTP? DNS UDP, TCP, if secure, then TLS/SSL 2. (8 points) Use the SMTP RFC 5321 posted in LEARN to assist you with this question. a. What are the differences between MAIL FROM: in SMTP and From: in an actual mail message itself? MAIL FROM: is the message from the SMTP client that identifies the sender of the mail message to the SMTP server. The From: is just a line in the body text, not an SMTP message. b. What command in SMTP is used to see if an email address is valid? HELO 3. (8 points) Concerning electronic mail. a. List (2) mail access protocols described in the text. IMAP and POP b. What mail access protocol might be better suited for a smartphone? Explain, and include one specific reason for the protocol you chose. IMAP because it supports push (the email gets to you when it’s sent, as opposed to waiting for the user to open the mail application and download the email). IMAP is also better because if the user downloads the email on their device using POP(3), and then also on their PC, they would have two duplicated repositories of their email. 4. (8 points) TCP can be enhanced with SSL to provide processtoprocess encryption. Between what two layers does SSL operate? If an application developer needs TCP to be enhanced with SSL, what does the developer need to do? SSL operates at the application layer. It takes data that is not encrypted from the application layer, encrypts it and then passes it to the TCP socket. If the application developer wants TCP to be enhanced with SSL, they have to include the SSL code in the application 5. (8 points) Suppose you are asked to write (2) new applications. Application 1 must transfer sensitive information using encryption. It is very important that the information is transmitted reliably and completely over the network. Application 2 will be designed to send out small bits of information, 1 2 INFO 330 – HW2 – Application Layer, Transport Layer such as alerts and notifications, from one server to many other servers as quickly as possible. This second application will continue to send these alerts more than once each, and the receiving servers will collect all the alerts in a local database as time passes. What transport protocol will you use for: a. Application 1? Explain your choice. TCP because it guarantees delivery of packets in order, if any packet is not acknowledged, the sender will resend it b. Application 2? Explain your choice. UDP because it’s stateless, and is able to answer small queries from a large number of clients 6. (9 points) For this question, refer to the DNS section of the text book. a. Explain the role of Root DNS servers (23 sentences) Root name server is a name server for the root zone of it’s Domain Name System(DNS). It directly responds to requests for records in the root zone and answers other requests by returning a list of the authoritative nameservers for the appropriate toplevel domain (TLD) b. Explain the role of TLD DNS servers (23 sentences) and name 3 different TLDs TLD servers are responsible for toplevel domains such as .com, .org, .net, .edu, and .gov Verisign’s Global Registry Service maintains the TLD servers for the .com TLD, and Educause maintains the TLD for the .edu TLD c. Explain the role of Authoritative DNS servers (23 sentences) Every organization with publicly accessible hosts (like web and mail servers) on the internet must provide publicly accessible DNS records that map the names to the DNS records. It provides original and definitive answers to DNS queries. It only returns answers to queries about domain names that are installed in its configuration system. 7. (8 points) Describe the following DNS resource record types (one sentence each). a. A record If the type is A, then the Name is a hostname and Value is the IP address for the hostname. Thus, a Type A record provides the standard hostname to IP address mapping b. AAAA record The AAAA record is also known as a quadA record, which specifies an IPv6 address for the hostname c. TXT record The TXT record is used to provide associations of some arbitrary and unformatted text with a host or other name, such as human readable information about a server, network, data center, and other accounting information 2 3 INFO 330 – HW2 – Application Layer, Transport Layer d. CNAME record If the type is CNAME then the value is a canonical hostname for the alias hostname. This record can provide querying hosts the canonical name for a hostname 8. (8 points) In the DNS, there are different types of servers and different types of queries. a. Explain what recursive queries are (23 sentences). A recursive DNS query happens when the DNS server you asked for an address, doesn't know the answer itself, so it has to check with another server. The DNS server is configured to pass unresolved name queries to another DNS server, in the case of a DNS server configured to use a forwarder. b. Explain what iterative queries are (23 sentences). An iterative name query when a DNS client allows the DNS server to return the best answer it can give based on its cache or zone data. If the queried DNS server does not have a match for the queried name, the best possible information it can return is a referral (which points to a DNS server authoritative for a lower level of the domain). The DNS client can then query the DNS server for which it obtained a referral. It continues this process until it locates a DNS server that is authoritative for the queried name, or until an error or timeout condition is met. 9. (8 points) Describe how DNS caching can reduce delays in DNS name to IP address lookups. Every time a user visits a website, there is a DNS query to translate the hostname to an IP address, by caching this result, the query resolves much faster than it would have. Caching involves storing key value pairs, where the key is the hostname, and the resulting value is the IP address. Because of caching, the local DNS server can immidiately return the IP address without having to query any other DNS servers. A local DNS servers can calso cache the IP addresses of TLD servers, which allows the local DNS servers to bypass the root DNS servers in a query chain, which further reduces delays. 10. (8 points) Describe how Web caching can reduce the delay in receiving a requested object. Will Web caching reduce the delay for all objects requested by a user or for only some of the objects? Web caching brings the requested content closer to the user, and easily accessible, by keeping it on the LAN which the user is connected to. Web caching can reduce delay for all objects, even ones that weren’t previously cached since caching reduces the other traffic that would have been on the link. 11. (8 points) Use the HTTP RFC (2616) posted in LEARN for this question. a. What mechanism is used to signal that a persistent connection is being closed? Idle time is used to signal that a persistent connection is being closed b. Can the client, or server, or both use that signal? Yes, both the client and the server can signal the close of the connection if they aren’t transferring any kind of information. 3 4 INFO 330 – HW2 – Application Layer, Transport Layer 12. (9 points) Research the SNMP protocol. a. What does the acronym SNMP stand for? Simple Network Management Protocol b. What is this protocol used for? Internetstandard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behavior c. What layer in the model does this protocol operate? Operates in the application layer protocol 4
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