PESIT Bangalore South Campus Hosur road, 1km before Electronic City, Bengaluru -100 Department of Computer Science and Engineering

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1 INTERNAL ASSESSMENT TEST 2 Date : Marks : 0 Subject & Code :Computer Networks 2(10CS64) Class : VI CSE B Name of faculty : Surbhi Agrawal Time : 8:30 to 10:00a.m Note: wer ALL Questions 1 a B A university has 10 LANs with 100 hosts in each LAN. Design an appropriate subnet addressing scheme if the university has one class B address A class B address has 14bit for netid & 16 for hostid. Since we have 100 hosts in each LAN 7 bits are sufficient to identify the host. Subnet can use 16 7=9 bits. With 9 bits, 2^9=12 LANs can be covered easily, where we have requirement only for 10 LANs. Subnet mask here will be as 7 bits are used for identifying host so 2^7 give a maximum of Net id Subnetid hostid With an example, explain classless interdomain routing. mask. An arbitrary prefix length to indicate the n/w number known as CIDR was used. A prefix of of length 22 is written as /22. /22 indicates that n/w mask is 22 bits. Packets are routed according to the prefix of the IP address.the entries in a CIDR routing table has 32 bit IP address & 32 bit mask.it enables a technique called Supernetting, to allow a single routing entry to cover a block of classful addresses. Example Instead of having 4 entries , , , ,CIDR allows a single routing entry /22 which includes all IP addresses from to

2 Class C Address Class C Address Class C Address Class C Address Mask Supernet Address A What do you mean by tunneling? Briefly explain the changes from IPv4 to Ipv6. In order to do the migration from IPv4 to IPv6, tunneling is used. Source Tunnel head-end Tunnel tail-end Destination Tunnel (a) IPv6 header IPv6 network IPv4 network IPv4 header IPv6 network In order to do the migration from IPv4 to IPv6, tunneling is used. IPv6 islands can tunnel across IPv4 networks Encapsulate user packet insider IPv4 packet.tunnel endpoint at source host, intermediate router, or destination host.tunneling can be recursive.

3 Changes from IPv4 to IPv6 Longer address field:128 bits can support up to 3.4 x 1038 hosts Simplified header format:simpler format to speed up processing of each header.all fields are of fixed size IPv4 vs IPv6 fields: Same: Version Dropped: Header length, ID/flags/frag offset, header checksum Replaced: Datagram length by Payload length Protocol type by Next header TTL by Hop limit B. Explain fragmentation & reassembly in IP networks. The packets have to be sent from different network interface. Each interface allows a maximum size (or bytes) of packets to be sent through known as its MTU or maximum transmission unit. If the packet s size is more than the MTU of the network interface then the packet must be fragmented. Each packet has following 3 fields in the header for fragmentation & reassembly ( reassembly is done at the destination end). Identification identifies a particular packet Flags = (unused, don t fragment/df, more fragment/mf) Fragment offset identifies the location of a fragment within a packet Example A packet is to be forwarded to a network with MTU of 76 bytes. The packet has an IP header of 20 bytes and a data part of 1484 bytes. and of each fragment.maximum data length per fragment = = 6 bytes.we set maximum data length to 2 bytes to get multiple of 8. So, 3 fragments of length 72,72 & 400 will be sent over the network with same id say x for all these packets. Fragment offset will be 0,69,138 respectively for 1 st,2 nd & 3 rd fragment. Draw fig as well.

4 3 A What do you mean by remote login and also explain secure shell(ssh) protocol. Users want to run application program at a remote site and create results that can be transferred to their local site.for example, employee working at home can log in to her work server to access application programs for doing a project.this can be done by client/server application program.two protocols TELNET SSH(secure shell protocol) SSH Based on UNIX programs. Uses TCP for communications.. SSH provides a secure communication by encrypting and authenticating messages. SSH provides several additional data transfers over the same connection by multiplexing multiple channels that are used for remote login.ssh security is implemented by using public key encryption.data being transmitted remains confidential.ssh implements an authentication.ssh starts a shell for the session.ssh can handle several data transfers simultaneously in a same session.establish a secure tunnel to perform port forwarding 4

5 B b)define: network management and explain SNMP and SNMP messages. Network management is done to monitor, manage, and control a network. To monitor, test, and analyze the hardware, software, and human elements of a network and then to configure and control those elementsthe network administrator can periodically send management packets to communicate with a certain network entity. 6 SNMP Designed to monitor the performance of network protocols and devices. It uses a UDP datagram and Uses client/server configurations. Managed devices, such as routers and hosts, are objects, and each object has a formal ASN.1 definition.some examples of managed objects include routers, concentrators, hosts, servers and applications like Oracle, Microsoft SMS, Lotus Notes, and MS Mail. The managed object does not have to be a piece of hardware, but should rather be depicted as a function provided on the network. For each object, MIB accommodates a database of information that describes its characteristics. The task of SNMP is to transport MIB information among managing centers and agents executing on behalf of managing centers.when an interface or device goes down, the protocol can also inform the managing center. SNMPv2 has more messaging options, resulting in more effective network management. SNMPv3 has more security options. SNMP Messages 1. GetRequest is used to obtain a MIB object value. 2. GetNextRequest is used to obtain the next value of a MIB object. 3. GetBulkRequest gets multiple values, equivalent to multiple GetRequests but without using multiple overheads. 4. InformRequest is a manager to manager message that two communicating management centers are remote to each other.. SetRequest is used by a managing center to initiate the value of a MIB object. 6. Response is a reply message to a request type PDU.

6 7. Trap notifies a managing center that an unexpected event has occurred. 4 A Compare secret key and public key cryptography systems, with neat block diagrams. Secret key encryption both sender and receiver conventionally use the same key for an encryption process. Symmetric encryption, or single key encryption protocols. An encryption algorithm, a key, a decryption algorithm. The encrypted message is called ciphertext. Two protocols: Data Encryption Standard (DES). Advanced Encryption Standard (AES). A shared secret key between a transmitter and a receiver. The encryption algorithm produces a different key at any time for a specific transmission. The algorithm need not be kept secret;only the key has to be secret. Public key encryption A sender and a receiver each use a different key. The public key system is more powerful than the secret key system and provides better security and message privacy. Drawback of public key encryption

7 Speed. More complex computationally and may not be practical in many cases. A sender/receiver pair use different keys. Known as asymmetric, or twokey encryption.public key algorithm is based on mathematical functions rather than on substitution or permutation.the security of any encryption scheme depends on the length of the key and the computational work involved in breaking an encrypted message. Two protocols are Rivert, Shamir, and Adleman (RSA) protocol. Diffie Hellman key exchange protocol. Give fig for any 1 of the above protocols. B Apply RSA algorithm for the following: i) Encrypt the plain text m= 13 for a=, b = 11, x = 7 ii) Find the value of y and decrypt the ciphertext. i) N=ab= C= 13^7mod=7 ii) Xymod(a 1)(b 1)=1 X7ymod40=1 Y=23 So, decrypting, 7^23mod =13=m Give short notes on a Diffie Hellman Algorithm Generate keys instead of distributing keys.diffie-hillman exchange - to create a shared key.intruders would not be able to access the transmitted communication between the two users or discover the shared secret code. This protocol is normally used for virtual private networks (VPNs). The essence of this protocol for two users, 1 and 2, is as follows. User1 and user2 have agreed on the use of a large prime number a, about 1000 bits long, and generator number g < a user 1 selects a random integer number x 1, creates y 1 {1, 2,..., a - 1} such that User 2 performs the same function and creates y 2 :

8 User 1 then sends y 1 to user 2. User 2 then sends y 2 to user 1. Now, user 1 forms its key, k 1, using the information its partner sent as User 2 forms its key, k2, using the information its partner sent as It can easily be proved that the two Keys k 1 and k 2 are equal. b. UDP datagram header Best effort datagram service. Multiplexing enables sharing of IP datagram servicesimple transmitter & receiverconnectionless: no handshaking & no connection state Low header overhead No flow control, no error control, no congestion control UDP datagrams can be lost or out-of-orderapplications Source Port Destination Port UDP Length Data UDP Checksum Source port identifies the application in the source host to receive replies. Destination port allows UDP module to demultiplex datagrams to the correct application in a given host. UDP length indicates the number of bytes in the UDP datagram ( including Header & data). UDP checksum is to detect errors in the datagram & its use is optional. Source port identifies the application in the source host to receive replies. Destination port allows UDP module to demultiplex datagrams to the correct application in a given host. UDP length indicates the number of bytes in the UDP datagram ( including Header & data). UDP checksum is to detect errors in the datagram & its use is optional.

9 c. Authentication used to verify identity.message authentication verifies the authenticity of both the message content and the message sender. Message content is authenticated through implementation of a hash function and encryption of the resulting message digest. The sender's authenticity can be implemented by use of a digital signature.a common technique for authenticating a message is to implement a hash function, which is used to produce a "fingerprint" of a message. The hash value is added at the end of message before transmission. The receiver recomputes the hash value from the received message and compares it to the received hash value. If the two hash values are the same, the message was not altered during transmission. Once a hash function is applied on a message, m, the result is known as a message digest, or h(m). Given a message digest h(m), it is computationally infeasible to find m. It is computationally infeasible to find two different messages m 1 and m 2 such that h(m 1 ) = h(m 2 ). * * * * * * * *

EITF25 Internet Techniques and Applications L7: Internet. Stefan Höst

EITF25 Internet Techniques and Applications L7: Internet Stefan Höst What is Internet? Internet consists of a number of networks that exchange data according to traffic agreements. All networks in Internet