Advanced Networks - Assignment Example

ASSIGNMENT- ADVANCED NETWORKS Q1. a. Reasons for inexhaustive availability of IP addresses: (Ossining, 2006 IPv4 supports a little over 4 billion IP addresses which have proven sufficient till date, however with increased demand on IP addresses, IPv6 offers promising solution for future availability of IP addresses with a higher capacity to withstand load. 2. In a LAN setup, normally private IP addresses are used for all the PCs and these are translated to a single IP/Pool of IPs by either a proxy server or a router. 3. Another reason is provision of NAT which can be explained as follows: One thing which tends to reduce IP addresses is the way in which pieces of that range are distributed. Normally, when pieces of the IP address space are allocated, they are allocated in units which are powers of 2. But not all of those addresses will be used by the entity to which they are allocated, so some are lost that way. (The entities are typically ISPs or corporations.) An example would be, if I need 600 IP addresses, I would probably be allocated 1024 and effectively waste about 400 of them. Things like NAT (network address translation) increase the effective number of available addresses. This is done by using a single IP address to serve multiple computers, and using the PORT number, which is an adjunct to the IP address, to differentiate among computers and among specific connections. Q1. a. (Tanenbaum 2003) Class type field: IP addresses having divided into different categories is recognized as classful addressing. Class type field helps in specifying the exact network configuration with respect to source as well as destination. There are different classes; A, B, C, D and E which specify different number of networks to be interconnected. Disadvantage of IP addresses including fixed size classes: Class type addressing has definite size of chunks which are not always put to use resulting in wastage of those unused chunks. Q1.b. (Tanenbaum 2003) VLAN: It is basically a Virtual LAN. The concept resulting from a way to rewire buildings entirely in software is called as VLAN. It deals with the problem of decoupling logical topology from physical topology. It has widespread use because: 1. It offers more flexibility to reflect organizational structure in an efficient manner. In traditional LANs, organizational changes had to be catered for by system administrators spending a lot of time pulling out plugs and pushing them back in somewhere else. 2. It deals with problem of load distribution on a network. 3. Offers increased security by virtually keeping all the information intact. Q2. a. (Behrouz & Fourouzan 2006) Preamble: This field contains seven byte (56 bits) of alternating 0s and 1s that alter the receiving system to the coming frame, and enable it to synchronize its input timing. The preamble is actually added at the physical layer and is not (formally) part of the frame. Frame Delimiter (FD): The FD field (one byte: 10101011) signals the beginning of the frame. The FD gives the station a last chance for synchronization. The last two bits are 11 to signal that the next field is the destination address. Destination: The Destination Address (DA) field is six bytes (48 bits) and contains physical address of the next station. Source: The Source Address (SA) field is also six byte (48 bits) and contains the physical address of previous station. Length: The length field has one of the two meanings. If the value of the field is less than 1518, it is a length field and defines the length of the data field that follows. If the value oh this field is greater than 1538, it defines the upper layer protocol that uses the services of the Internet. Data and Padding: The data field carries data encapsulated from the upper layer protocol. CRC: The last field of the frame contains the error detection information. Q2. b. (Tanenbaum 2003) Encoding scheme with Cat 5, 100 Base-TX: Instead of usual binary encoding, a scheme called as 4B/5B is used. It is taken from FDDI (Fiber Distributed Data Interface) and is compatible with it. Every group of 5 clock periods, each containing one of two signal values, yields 32 combinations 16 of which transmit four bit groups and remaining are used for control purposes. Q2. c. and Q2. d. Time space diagram: Solution: Whenever two nodes transmit at the same time and eventually happen to collide; they back off for a random time interval and retransmit. We have node A and node B transmitting at T=0; and K=1 for A whereas K=0 for node B. Usually K=0 suggests a very large random time interval. Hence A will be the first one to transmit the information during second attempt i.e. at T=1. Time taken by A to transmit information to C = (T2-T1) seconds Where T2, T1 = Propagation delays as shown in diagram. A completes transmission; C processes the information and sends an ACK signal back to A (refer time space diagram below). Time taken by C to acknowledge A = (Tp + (T3-T2)) seconds Where, Tp = Processing delay and T3, T2 = Propagation delays as shown in diagram. Fig: Time Space diagram Q3. a. (Tanenbaum 2003) Distance vector routing algorithms operate by having each router maintain a table having best known distance to each destination and which line to use to get there. Tables are updated by exchanging information with neighbors. According to the diagram provided, router E works as follows: To node Cost from E Path 1 (Iteration 1) Remarks E 0 -   B 2 E-B Neighbor D 5 E-D Neighbor C 8 E-C Neighbor A 11 E-C-A C shows lesser cost to A. F 15 E-C-F C is the direct path to F. To node Cost from E Path 2 (Iteration 2) Remarks E 0 -   B 2 E-B Neighbor D 5 E-D Neighbor C 8 E-C Neighbor A 9 E-D-A Although C shows lesser cost to A, entire cost from E to A is lesser through D than C. Hence E updates the table with cost=9 and path=E-D-A. F 19 E-D-A-C-F Although there is an alternate path available, its cost is higher than the cost in first iteration. Hence E updates table with cost=15 and path=E-C-F. Q3. b. (Tanenbaum 2003) In a Link state algorithm, each router does the following: 1. Discover its neighbor and learn their network address. 2. Measure the cost to each of its neighbors. 3. Construct a packet telling all it has first learned. 4. Send this packet to all other routers. 5. Compute shortest path to every other router. Distance vector routing and Link state algorithm ultimately have same results. To node Cost from E Next Router Remarks and Updated Path E 0 - - A 9 D Selected path. A 11 C Higher cost hence rejected. B 2 - Neighbor C 8 - Neighbor, lesser cost. C 12 D, A Higher cost hence rejected. D 5 - Neighbor D 15 C, A Higher cost hence rejected. F 15 C Selected path. F 19 D, A, C Higher cost hence rejected. Q3. c. (Kahate A 2003) The principal of confidentiality specifies that only the sender and the intended recipient(s) should be able access the contents of a message. “Non-Symmetric Key Algorithm” is also called as Public Key Cryptography, in which a key pair is used, one key is used for encryption and the other corresponding key must be used for decryption. No other key can decrypt the message – not even the original key used for encryption. One of the keys is called as public key and the other is the private key. The private key remains as a secret, it must not be disclosed with anybody, and the public key is for general public. Confidentiality can be maintained with non-symmetric key algorithm as follows: 1. When A wants to send a message to B, A encrypts the message using B’s public key. This is possible because A knows B’s public key. 2. A sends this encrypted message to B. 3. B decrypts A’s message using B’s private key. As only B knows her private key, message can be decrypted only by B’s private key and nothing else. No one can manage to intercept the message as the intruder does not know about B’s private key. Similarly, when B wants to send a message to A, B encrypts the message using A’s public key. Therefore, only A can decrypt the message, using her private key. Fig.: Asymmetric Key Cryptography When the contents of a message are changed after the sender sends it, but before it reaches the intended recipient, we say that the integrity of the message is lost. With the help of Digital Signature, data integrity can be maintained in non- symmetric key algorithm. To ensure data integrity, the message being sent is hashed to create a message digest using MD5 or SHA-1. Q4. a. (Walke B 2002), (Tanenbaum A 2003), (Piconet and Scatternet 2007) Piconet and Scatternet in the context of Bluetooth networking:  Piconet: The basic unit of a Bluetooth system is a piconet, which consists of master node and up to seven active slave nodes within a distance of 10 meters. Multiple piconets can exist in the same (large) room and can even be connected via a bridge node. Piconet usually supports 8 stations and the connected ends in piconet behave as master and slave for the time the connection between them is valid. Scatternet: A group of piconets with connections between different piconets is called a scatternet. In other words, an interconnected collection of piconets is called scatternet. Fig. piconet and scatternet Q4. b. (Walke B 2002) Active mode station: A packet from master containing specific address of intended slave is sent through the information channel. Upon receiving this address, the addressed slave enters into a state called as active mode. Sniff mode station: In this mode, the slave no longer listens to the ACL slots but only to some slots called sniff slots. Consequently the master only transmits the slave in these slots. Hold mode station: The ACL connection of a slave can be transferred to hold mode which means that the connection is no longer supported. This mode creates capacity for scanning, paging and enquiry. Before a slave comes into hold mode, it comes on an agreement with the master for how long will it stay there. Parked mode station: If a slave does not want to participate in piconet but wants to remain synchronized, it jumps to a low power state with little activity called as parked mode. Q4. c. (Tanenbaum 2003) Difference between peer to peer and hub configurations for wireless LAN networks: Peer to Peer systems: 1. Are totally distributed. 2. All nodes are symmetric and there is no central control or hierarchy. 3. Huge central database. Hub configuration system: 1. Wireless hub configurations imply a central node with many input and output lines. 2. Information arriving at any of the lines is sent on all others which is sometimes undesirable. Q4. d. (Tanenbaum 2003) PCF protocol: In 802.11, most radios are half duplex, meaning they cannot transmit and listen for noise bursts at the same time on a single frequency. As a result of this, 802.11 do not use CSMA/CD as Ethernet does. To deal with this problem, we have two modes of operation: DCF (Distributed Co-ordination Function) and PCF (Point Co-ordination Function). In PCF mode, base station polls the other stations asking them if they have any frames to send. Since transmission order is completely controlled by base station in PCF mode, no collisions ever occur. The standard prescribes the mechanism for polling, but not for the polling frequency, polling order, or even whether all stations need to get equal service. Q5. a. (Kahate A 2003) a) Non-symmetric key cryptography solves the problem of key agreement and key exchange, which occurs in symmetric key cryptography. Non-symmetric key public key encryption works as follows: 1. When A wants to send a message to B, A encrypts the message using B’s public key. This is possible because A knows B’s public key. 2. A sends this encrypted message to B. 3. B decrypts A’s message using B’s private key. As only B knows her private key, message can be decrypted only by B’s private key and nothing else. Advantages: Characteristics Symmetric key cryptography Non-symmetric key cryptography 1. Key used for encryption/decryption Same key is used for encryption and decryption One key is used for encryption and another, different key is used for decryption 2. Key agreement/ exchange A big problem No problem at all 3. Number of keys required as compared to number of participants in message exchange Equals about the square of the number of participants, so scalability is an issue Same as the number of participants, so scales up quite well Disadvantages: Characteristics Symmetric key cryptography Non-symmetric key cryptography 1. Speed of encryption/ decryption Very fast Slower 2. Size of resulting encryption text Usually same as or less than original clear text size More than the original clear text size Q5. b. (Tanenbaum 2003) Ethernet LANs are based on use of switches rather than hubs: All lines coming into a hub need to operate at same speed. Hub is used for connection establishment at physical layer. In case of a hub, frames arriving on any of the lines are sent out on all others; thus the entire hub forming a single collision domain. Switches are similar to bridges in that both route on frame addresses and operate at data link layer. Switch is most often used to connect individual computers and hence a switch actively forwards a frame to desired machine without loss of data. Each port is its own collision domain; switches never lose frames to collisions. Advantages of cut through switches: If frames come in faster than they can be retransmitted, the switch may run out of its buffer space and have to start discarding frames. To alleviate this problem, modern switches, referred to as cut through switches, start forwarding frames as soon as the destination header field has come in, but before the rest of the frame has arrived. These switches do not use store-and-forward switching. Q5. c. (Tanenbaum 2003) MPLS: It stands for MultiProtocol Label Switching. MPLS capable routers are employed for different purposes. Since IP packets were not designed for virtual circuits, there is no field available for virtual circuit numbers within IP header. The MPLS header in front of IP header solves this problem. MPLS network element can forward both IP packets and ATM cells, depending on what shows up. With traditional virtual circuit routing, it is not possible to group several distinct paths with different end points onto the same virtual circuit identifier because there would be no way to distinguish them at final destination. With MPLS, the packets still contain their final destination address, in addition to the label, so that at the end of the labeled route the label header can be removed and forwarding can continue the usual way using network layer administration address. Q6. a. Solution: (1000m/0.65c) + 0.6µs + 0.6 µs + 0.2µs + 0.2µs + (1500m/0.77c) + (200/0.65c) = 14.246µs Where, 1000m = point to point long cable 1500m = coaxial cable 200m = drop cable Round trip delay = 14.246 + 14.246 + (base value = 10+10) = 48.492µs which is closer to official one of 52.1µs Assuming 48 bits of jam signal the minimum packet size would be around 586bytes with an assumption that complete jam signal is received for detection. As this is very large packet size which can result in wastage of bandwidth, hence officially it is set to 512 bits. Q6. b. (Downey A) To find: effect on min packet size when delay time is constant and transmission rate rises to 100 Mbps. Solution: Suppose the round trip time propagation delay for an Ethernet is 46.4 us. This yields a minimum packet size of 512 bits (464 bits corresponding to propagation delay + 48 bits of jam signal). Now if the signaling rate rises to 100 Mbps, then 46.4 us * 100 Mbps = 4640 bits 4640 bits + 48 bits = 4688 bits Drawback: It imposes an overhead on small packets and wastes bandwidth. Q6. d. (Downey A) Effect on packet size at 1Gbps, cable length: If we try to run a network faster than its maximum baud rate, its limiting factor is the ability to distinguish between two signals with a very low error rate. If we push a network above its limit, the error rates climb quickly. It is normal to reduce the cable length for better data handling, efficient bandwidth utilization and fully utilized network with minimum transfer time. Q7. a. Given data: Router B: Router D: Destination Delay A 13 B 15 C 2 D 0 E 9 F 5 Solution: Ideally router C has its own table consisting of distances to different routers from itself. With the help of that table it calculates the distances to B and D. Assuming that C initially has its own developed routing table, distances to B and D shall be 6 and 2 respectively. With the knowledge of two routing tables; that for B and D, new routing table for C is as follows. It shows outgoing lines and expected delay values. Router C: Q7. b. Solution: With the help of information provided we can have costs for different links as follows: 6 5 2 5 1 4 Link state and distance vector routing algorithms have same final results. As described in answer 3.b., link state algorithm makes routers follow same steps. The routing table for C will be as follows: Q7. c. (Behrouz & Fourouzan 2006) Advantages of IPv6 over IPv4: 1. Larger address space. An IPv6 address is 128 bits long. Compared with the 32-bit address of IPv4, this is a huge (2^96) increase in the address space. 2. Better header format. IPv6 uses a new header format, in which options are separated from the base header and inserted, when needed, between the base header and the upper-layer data. This simplifies and speeds up the routing process because most of the options do not need to be checked by router. 3. Allowance for extension. IPv6 is designed to allow the extension of the protocol it required bye new technologies or application. 4. Support for resource allocation. In IPv6, the type-of-service field has been removed, but a mechanism (called flow label) has been added to enable the source to request special handling of the packet such as real-time audio and video. 5. Support for more security. The encryption and authentication options in IPv6 provide confidentiality and integrity of the packet. Q8. a. (Schiller 2003) Mobile Network Architecture: Base transceiver station (BTS): A BTS comprises all radio equipment i.e. antennas, signal processing, amplifiers necessary for radio transmission. Base station controller (BSC): It manages BTS; it reserves radio frequencies, handles the handover from one BTS to another within BSS and performs paging of the MS Base Station subsystem (BSS): mobile network contains many BSS, each controlled by BSC. Mobile station (MS): MS comprises all user and equipment and software needed for communication. An MS consists of user independent hard- and software and of the Subscriber identity module which stores all user specific data. Mobile services switching center (MSC): MSCs are high performance digital ISDN switches. They setup the connection to other MSC and to BSC via interface and form fixed backbone network. Home location register (HLR): HLR is the most imp database as it stores all user relevant information Visitor location register (VLR): VLR associated to each MSC is a dynamic database which stores all important information needed for MS users currently in Location Area that is associated to MSC. Q8. b. (Mcgill) Effect of mobility on end-to-end delays of datagram between the source and destination: Because datagram must be first forwarded to the home agent, and from there to the mobile, the delays will generally be longer than via direct routing. Note that it is possible, however, that the direct delay from the correspondent to the mobile (i.e., if the datagram is not routed through the home agent) could actually be smaller than the sum of the delay from the correspondent to the home agent and from there to the mobile. It would depend on the delays on these various path segments. Note that indirect routing also adds a home agent processing (e.g., encapsulation) delay. Q8. c. (Schiller 2003) Two mobile nodes in a foreign network having a foreign agent can use same care of address. Care of address (COA) defines the current location of the mobile node from IP point of view. All IP packets sent to the MN are delivered to the COA, not directly to the IP address of Mobile Node (MN). Packet delivery towards the MN is done using a tunnel; The COA marks the tunnel end point i.e. the address where packet exits the tunnel. The COA located at Foreign Agent (FA) is the IP address of the FA. The FA is the tunnel end point and forwards packets to the MN. Many MN using FA can share this COA as common COA Q8. d. (Schiller 2003) Sequence of messages: The correspondent node (CN) request current location from Home Agent (HA). If allowed by the Mobile Node (MN), the Home Agent (HA) returns the Care of address (COA) of MN via an update message. Now CN can send its data directly to the Foreign Agent FAold. FAold forwards the packet to MN. Now MN might change its location and register with new foreign agent, FAnew. FAnew informs FAold about the new registration of MN. Passing this information is achieved via an update message, which is acknowledged by FAold. Without the information provided by new FA the, old FA would not know anything about the new location of MN. CN still tunnels its packets to FAold. Now FAold notices packets with destination MN, but also knows that it is not the current FA of the MN. FAold might forward these packets to new COA of MN which is FAnew. Forwarding packets is optimization of basic mobile IP providing smooth handover, without this optimization, all packets in transit would be lost while the MN moves from one FA to another. HA sends an update to inform the CN about new location, which is acknowledged. Now CN can send its packet directly to FAnew. References Ossining 2006, Answers from yahoo: Blog, Available from: [16 April, 2009]. Tanenbaum, AS 2003, Computer Networks, Pearson Education, Delhi, pp.285, 296-299, 311, 327-330, 357-360, 415, 437 Behrouz A & Fourouzan 2006, TCP/IP Protocol Suite, Tata McGraw Hill, New Delhi, pp.46, 690 Kahate A 2003, Cryptography and Network Security, Tata McGraw Hill, New Delhi, pp.7, 154, 155, 160-161 Walke B 2002, Mobile radio networks: networking, protocols, and traffic performance, John Wiley and Sons, Available from: [16 April 2009] pp.906, 909 Survival Guide- Encryption and Authentication, ZYTRAX, Inc, Hampstead, QC Canada, Available from: [16 April 2009] Schiller 2003, Mobile Communication, Pearson Education, New Delhi, pp. 120-124, 238, 340-341 Mcgill, Canada, Available from: [16 April 2009] Piconet and Scatternet, 2007, Available from: Google images: [16 April 2009] Downey A, cs398 Lecture Notes, Olin College of Engineering, Available from: [21 April, 2009] Read More