3. Explain how IEEE 802.1Q virtual LANs are organized. What modification is made

Question

3. Explain how IEEE 802.1Q virtual LANs are organized. What modification is made in the frame format?

5. Briefly explain what is the purpose of the Routing Information Protocol (RIP) and how it works.

7. Briefly explain what is the purpose of the Open Shortest Path First (OSPF) and how it works.

8. Is the UDP protocol connection-oriented or connectionless? For what kind of application is it preferred over the TCP protocol?

10. Suppose that a client is sending data to a server using the TCP protocol but the application running on the server side stops reading the received data for 10 seconds. Does the client notice that the server is no longer accepting data? If yes, how? What is a possible course of action for the client?

11. What fields are there in a UDP segment header?

12. Enumerate at least 5 fields in the TCP segment header.

13. Suppose a TCP segment has 1400 data bytes and no header options (at neither the TCP nor IP levels). What is the expected payload size in the corresponding Ethernet frame?

15. What is the domain name system (DNS) for? Briefly explain how it works.

16. List some application-layer protocols and for each state purpose and on what transport-layer protocol it is based.

17. What is the purpose of the Diffie-Helmann protocol? Mention one application that uses this protocol.

18. What kind of addresses are stored in the IEEE 802.3 (Ethernet) header? How many bytes for each address? Can these addresses be seen from outside of a LAN?

19. Draw block diagrams for persistent, non-persistent and p-persistent CSMA.

20. Briefly explain what is a DHCP server

22. Briefly explain what is the purpose the TCP slow-start method.

23. What is the domain name system (DNS) for? Briefly explain how it works.

24. What is the difference between active and passive FTP modes?

25. What transport protocol is used for HTTP requests? Briefly describe the format of a typical request/answer.

26. Explain how a denial of service attack works.

27. Explain what is the purpose of secret-key encryption techniques such as DES or AES and how are they used.

28. Given a private key (y, n) = (7, 143) and a ciphertext c = 9, explain how the ciphertext can be decoded.

29. Is there any way to find the original prime factors (a and b) given a public key (x, n)? Why this is not feasible in practice?

30. Given a public key for the RSA algorithm (x, n) = (5, 4891), a private key (y, n) = (1901, 4891) and a message m = 4, calculate the ciphertext and explain how can it be decoded (no need to actually decode).

Explanation / Answer

5. Briefly explain what is the purpose of the Routing Information Protocol (RIP) and how it works.

Answer:

Routing Information Protocol (RIP) is a dynamic protocol used to find the best route or path from end-to-end (source to destination) over a network by using a routing metric/hop count algorithm. This algorithm is used to determine the shortest path from the source to destination, which allows the data to be delivered at high speed in the shortest time.

RIP plays an important role providing the shortest and best path for data to take from node to node. The hop is the step towards the next existing device, which could be a router, computer or other device. Once the length of the hop is determined, the information is stored in a routing table for future use. RIP is being used in both local and wide area networks and is generally considered to be easily configured and implemented.

7. Briefly explain what is the purpose of the Open Shortest Path First (OSPF) and how it works.

Answer:

Open Shortest Path First (OSPF) is a link state routing protocol (LSRP) that uses the Shortest Path First (SPF) network communication algorithm (Dijkstra's algorithm) to calculate the shortest connection path between known devices.

The OSPF routing protocol has largely replaced the older Routing Information Protocol (RIP) in corporate networks. Using OSPF, a router that learns of a change to a routing table (when it is reconfigured by network staff, for example) or detects a change in the network immediately multicasts the information to all other OSPF hosts in the network so they will all have the same routing table information. Unlike RIP, which requires routers to send the entire routing table to neighbors every 30 seconds, OSPF sends only the part that has changed and only when a change has taken place. When routes change -- sometimes due to equipment failure -- the time it takes OSPF routers to find a new path between endpoints with no loops (which is called "open") and that minimizes the length of the path is called the convergence time.

Rather than simply counting the number of router hops between hosts on a network, as RIP does, OSPF bases its path choices on "link states" that take into account additional network information, including IT-assigned cost metrics that give some paths higher assigned costs. For example, a satellite link may be assigned higher cost than a wireless WAN link, which in turn may be assigned higher cost than a metro Ethernet link.

OSPF Version 2, as defined by IEEE RFC 2328 for IPv4, is broadly implemented in enterprise routers. IPv6 revisions to this standard are captured in the newer OSPF Version 3 (as defined in IEEE RFC 5340).

Although it is intended to replace RIP, OSPF has RIP support built in both for router-to-host communication and for compatibility with older networks using RIP as their primary protocol.

OSPF bis an Interior Gateway Protocol (IGP) that routes Internet Protocol (IP) packets within a single routing network domain only. OSPF finds the best network layout (topology) by calculating shortest device connection paths using the Shortest Path First (SPF) algorithm.

For example, a person in city A wants to travel to city M and is given two options:

The shortest route is always the one with least amount of distance covered in total. Thus, the ABCM route is the better option (10+5+10=25), even though the person has to travel to two cities as the associated total cost to travel to the destination is less than the second option with a single city (20+10=30). OSPF performs a similar algorithm by first calculating the shortest path between the source and destination based on link bandwidth cost and then allows the network to send and receive IP packets via the shortest route.

8)Answer:

The device sending a message simply sends it addressed to the intended recipient. If there are problems with the transmission, it may be necessary to resend the data several times. The Internet Protocol (IP) and User Datagram Protocol (UDP) areconnectionless protocols.

11. What fields are there in a UDP segment header?

Answer:

Length*

Source Port*

Checksum

15. What is the domain name system (DNS) for? Briefly explain how it works.

Answer:

domain name system (DNS):

Domain name system (DNS) is a hierarchical naming system built on a distributed database. This system transforms domain names to IP addresses and makes it possible to assign domain names to groups of Internet resources and users, regardless of the entities' physical location.

Web browsing and most other internet activity rely on DNS to quickly provide the information necessary to connect users to remote hosts. DNS mapping is distributed throughout the internet in a hierarchy of authority. Access providers and enterprises, as well as governments, universities and other organizations, typically have their own assigned ranges of IP addresses and an assigned domain name; they also typically run DNS servers to manage the mapping of those names to those addresses. Most URLs are built around the domain name of the web server that takes client requests.

DNS servers answer questions from both inside and outside their own domains. When a server receives a request from outside the domain for information about a name or address inside the domain, it provides the authoritative answer. When a server receives a request from inside its own domain for information about a name or address outside that domain, it passes the request out to another server -- usually one managed by its internet service provider. If that server does not know the answer or the authoritative source for the answer, it will reach out to the DNS servers for the top-level domain -- e.g., for all of .com or .edu. Then, it will pass the request down to the authoritative server for the specific domain -- e.g., techtarget.com or stkate.edu; the answer flows back along the same path.

To promote efficiency, servers can cache the answers they receive for a set amount of time. This allows them to respond more quickly the next time a request for the same lookup comes in. For example, if everyone in an office needs to access the same training video on a particular website on the same day, the local DNS server will ordinarily only have to resolve the name once, and then it can serve all the other requests out of its cache. The length of time the record is held -- the time to live -- is configurable; longer values decrease the load on servers, shorter values ensure the most accurate responses.

17)Answer:

The purpose of the Diffie-Hellman protocol is to enable two users to exchange a secret key securely that can then be used for subsequent encryption of messages. ... Finally an improved key exchange schema based on hash function is given, which improves the security and practicality of Diffie-Hellman protocol.

20. Briefly explain what is a DHCP server

Answer:

DHCP (Dynamic Host Configuration Protocol) is a network management protocol used to dynamically assign an Internet Protocol (IP) address to any device, or node, on a network so they can communicate using IP. DHCP automates and centrally manages these configurations rather than requiring network administrators to manually assign IP addresses to all network devices. DHCP can be implemented on small local networks as well as large enterprise networks.

DHCP will assign new IP addresses in each location when devices are moved from place to place, which means network administrators do not have to manually initially configure each device with a valid IP address or reconfigure the device with a new IP address if it moves to a new location on the network. Versions of DHCP are available for use in Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6).

How DHCP works

DHCP runs at the application layer of the Transmission Control Protocol/IP (TCP/IP) protocol stack to dynamically assign IP addresses to DHCP clients and to allocate TCP/IP configuration information to DHCP clients. This includes subnet maskinformation, default gateway IP addresses and domain name system (DNS) addresses.

DHCP is a client-server protocol in which servers manage a pool of unique IP addresses, as well as information about client configuration parameters, and assign addresses out of those address pools. DHCP-enabled clients send a request to the DHCP server whenever they connect to a network.

Clients configured with DHCP broadcast a request to the DHCP server and request network configuration information for the local network to which they're attached. A client typically broadcasts a query for this information immediately after booting up. The DHCP server responds to the client request by providing IP configuration information previously specified by a network administrator. This includes a specific IP address as well as for the time period, also called a lease, for which the allocation is valid. When refreshing an assignment, a DHCP client requests the same parameters, but the DHCP server may assign a new IP address based on policies set by administrators

A DHCP server manages a record of all the IP addresses it allocates to network nodes. If a node is relocated in the network, the server identifies it using its Media Access Control (MAC) address, which prevents accidentally configuring multiple devices with the same IP address.

DHCP is not a routable protocol, nor is it a secure one. DHCP is limited to a specific local area network (LAN), which means a single DHCP server per LAN is adequate, or two servers for use in case of a failover. Larger networks may have a wide area network (WAN) containing multiple individual locations. Depending on the connections between these points and the number of clients in each location, multiple DHCP servers can be set up to handle the distribution of addresses. If network administrators want a DHCP server to provide addressing to multiple subnets on a given network, they must configure DHCP relay services located on interconnecting routers that DHCP requests have to cross. These agents relay messages between DHCP clients and servers located on different subnets.

DHCP lacks any built-in mechanism that would allow clients and servers to authenticate each other. Both are vulnerable to deception (one computer pretending to be another) and to attack, where rogue clients can exhaust a DHCP server's IP address pool.

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