Computer Networks Essay

I. profit and forge protocol architecture Q1. (5 blooms) In the layered protocol architecture the sustain layer functionality includes over-crowding control and flaw retrieval (e.g., retransmission). One suggested that this functionality should be d i strictly at the closedown points (i.e., at the hosts) without aid from the web. Do you agree? Why? Elabo tote up masking the see solelyot- take outs. Answer (5 points) In general, error recovery (e.g., re-transmission) is circumstantial to screening fatalitys. Some applications require 100% packet recovery, even with check outs and jitters (such as TCP-based applications, http, ftp and telnet traffic). Other applications whitethorn be tolerant to loss further less tolerant to retards and jitter, such as voice applications. Re-transmissions and packet recovery add to the jitters and the delays and hence may not be suited for real duration or voice applications. Hence it is not a good idea, in general, to include er ror recovery at the ne twainrk layer (that is not aware of application pick ups) and it is better to implement such functionality at the transport layer end-to-end.In cases of lossy posts in the engagement (such as X.25 in the early communicateing days, or wireless fall ins) it may be preferable to decrease the bit error rates on those links by including error recovery at the end points of those links. In general, most links present have very low BER, and for wireless links the MAC (such as IEEE 802.11) layer forgets Acked delivery. For congestion control, a similar argument may be given. That is, congestion reaction may be application specific and is better implemented end-to-end. Congestion notification, on the other hand, may provide useful nurture to the end points to react appropriately. Since losses in the network may be due to congestion or other actors, a signal from the network to the end point may help complete congestion errors from other errors.Only congestio n errors should spark off back off or rate cut at the end points. So, network attention in congestion notification may help in near scenarios. extra In other scenarios network assistance may prevent synchronising effects of congestion control, e.g., RED, or may prevent/isolate misbehavior, e.g., WFQ.. Q2. (5 points) What utility does a circuit-switched network have over a packetswitched network? How repel aside it establish such profit? Answer A circuit-switched network substructure guarantee a certain amount of end-to-end bandwidth for the duration of a call. Most packet-switched networks today (including the profit) burn downnot fixate any end-to-end guarantees for bandwidth. Circuit-switched networks use admittance control, and reserve a circuit (in TDM it is done in the form of an delegate cartridge holder slot per source that no other source contribute use). The allocated resources are never exceeded.Q3. (10 points) What are the advantages and disadvantages of havi ng a layered protocol architecture for the Internet? (mention at least 3 advantages and 2 disadvantages) Answer Is it sure that the change in any of the layers does not affect the other layers? (support your break up/arguments with manakins) Advantages Allows an explicit structure to identify relationships between various pieces of the complex Internet structure, by providing a reference present for contendion. Provides a modular design that facilitates maintenance, updating/upgrading of protocols and implementations (by various vendors) at the various layers of the stack. Supports a flexible textile for future advances and inventions (such as wandering or sensor networks). Disadvantages overhead of headers, tautology of functions ( slightlytimes not require) such as reliability as the transport layer and the link layer, or routing at the network layer and some link layer protocols (such as ATM)It is true in legion(predicate) cases that the change in one layer does not affe ct the change in the other layers, solely not al rooms. Examples of change that did not affect the other layers change from FDDI to minimum ring, to Ethernet at the MAC layer. Examples of change that affected other layers wireless vs. wire ( transaction of TCP and routing degraded drastically). Introduction of 802.11 for wireless and ad hoc networks (a change in the physical and MAC layers), does affect in a major way routing at the network layer and the transport layers. In that case, many of the protocols needed re-design. Q4. (10 essential points) Design parameters In order to be able to break apart accomplishment of the Internet protocols a researcher needs to model some parameters, such as enactment of bosss in the network, in addition to many other parameters.a. cover 4 different main parameters one would need to model in order to evaluate the performance of Internet protocols. Elaborate on the definition of these parameters and their moral forces b. Discuss 2 more par ameters for mobile wireless networks these two parameters are not needed for the fit out Internet Answer a. Traffic model temporal and spacial (packet reach processes, session/flow comer processes, spatial distribution of traffic (src-dst) pair distribution across the topology), topology/connectivity model, node failure model, membership dynamics (for multicast) spatio-temporal models. Any honest 4 parameters are ok, with 1.5 points per parameterb. For mobile wireless networks there is a need to model mobility (spatio-temporal), and wireless channel dynamics/loss/bandwidth since it changes with time much more drastically than the wired Internet (in which virtually the max bandwidth of a channel/link is static) Any 2 reasonable parameters are ok, with 2 points per parameter II. Statistical multiplexing and queuing theory Note You may want to make use of the following equations M/D/1 queuing delay Ts is service time & is link utilizationM/D/1 average queue length or caramel brown line of work M/M/1 queuing delay , caramel lineQ5. (8 points) Consider two queuing strategys, serving packets with lengths that have exponential distribution, and the packet arrival process is Poisson. The first queuing system (system I) has a undivided queue and a single legion, and hence the packet arrival rate is X, and the emcee stimulate is Y. The second queuing system (system II) has two queues and two servers, and hence the packet arrival rate is X/2, and the server speed is Y/2. Derive a relation between the delays in each of these systems. What conclusion asshole you make? Answer (8 points) We use the M/M/1 queue (because the research states Poisson arrival and exponentially distributed service time). For the first system (I) Tq=Ts/(1-)=1/M(1-/M)=1/Y(1-X/Y), For the second system (II) Tq=2/Y(1-X/Y)=2Tq (of system I) That is, using 1 queuing system performs better than using 2 queues with fractional of the arrival rate and half of the output link susceptibi lity.Q6. (5 points) In an Internet experiment it was noted that the queuing performance of the switches/routers was worse than foretelled. One designer suggested increasing the buffer size in the routers drastically to withstand any executable burst of data. Argue for or against this suggestion, and justify your position. A6. Increasing the buffer size allows switches to store more packets (which may reduce loss). However, it does not alleviate the congestion. If this was the only cure proposed, then we expect the queues to configuration up, increasing the buffer line, and increasing the delays. If the build up persists (due to inadequacy of congestion control for example) the queues shall incur losses and extended delays. Delays may tinge re-transmission timers to expire (for reliable protocols, such as TCP) deuce-aceing to re-transmissions. Also, the TTL value in the header of each packet is reduced based on time (and hop count). So, many of the TTLs may expire leading to t he discard of packets. So, in general, only increasing thebuffer sizes does not help improve the queuing performance. Q7. (7 points) mark the network design trade-off introduced by using statistical multiplexing and destine and describe a metric that captures this trade-off. A7. (7 points 3.5 for the link between stat muxing and congestion and 3.5 for the trade off metric (network power) and its description). Statistical multiplexing allows the network to admit flows with aggregate readiness exceeding the network capacity (even if momentarily). This leads to the need for buffering and the store and forward model. Subsequently, queuing delays and build up may be understandd as the load on the network is increased. Two major design goals of the network is to provide uttermost throughput (or goodput) with least (or min) delay.However, these two goals are conflicting. In order to increase the throughput, the congestion increases and so does the delay. In order to reduce the queuing delays then we need to reduce the load on the network and hence the goodput of the flows would decrease. This is the throughput-delay trade off in network design. One metric that captures both measures is the network power=Tput/Delay, as the Tput increases, so does the network power, and when the delay decreases the network power increases. Q8. (8 points) Flows in the Internet vary widely in their characteristics. Someone suggested that in order to be fair to the various heterogeneous flows then we need the different flows to experience the same delay at the different queues. Argue for or against this suggestion. A8. (8 points 4 points for the constant ratio and the link to the fluid flow model, 4 points for the unfairnes/ avariciousness description)In order to provide the same delay for the various flows we need to maintain the rate/capacity ratio constant (this is based on the fluid flow model we introduced in class). Hence, if the different flows arrive at various rates, then th e capacity tryst should reflect such variation. The allocation leading to same delays would favor (i.e., allocate more capacity to) flows with risque rates at the expense of flows with low rates. This strategy encourages greed in the network and cannot achieve fairness, where the existence of uplifted rate (large) flows in the network would adversely affect low rate (small) flows in the network by increasing the overall delay experienced by all the flows. Q9. (12 total points)Consider a network that uses statistical multiplexing. The network has N number of ON/OFF sources, each sending at a rate of R packets per second when ON. All the sources are multiplexed through a single output link. The capacity of the output link is M. A. (3 points) What is the condition on N, R and M in order to stabilize this network? When the number of sources to be supported is increased from R to 10R, there were two suggestions to interchange the networkSuggestion I is to replicate the above syste m 10 times. That is, create 10 links, each with capacity of M handling R sources. Suggestion II is to replace the link with another link of capacity 10 M B. (9 points) Which suggestion do you support and why? Argue gift expressions for the delay/buffer performance of each system. cut into both the advantages and disadvantages of each case Answer = A. (3 points) The conditions for a stable network are N.R. < M, N.R > M, where is the fraction of the time the sources are ON (on average) If N.R. > M, then this leads to constant build up of the queue with no change of recovering from congestion (and draining the queue), which would lead to unstable system. B. (9 points) Write down the equations, M/D/1 queuing delay Ts is service time & is link utilizationM/D/1 average queue length or buffer occupancy M/M/1 queuing delay , buffer occupancyThe buffer occupancy depends on only. If is the same (i.e., the load on the queue server is the same) then the buffer occupancy is th e same, = . Ts = . N. R / M Increasing the bandwidth of the link to 10M means that we can get the same average buffer occupancy in the two systems. In system I we would need 10 times the buffer size as in system II, so system II is advantageous in that sense. (more sharing and statistical multiplexing) In addition, the queuing delay will be decreased drastically (by a factor of 10) where Tq=Ts. f() (6 points for the above argument) (3 points) On the other hand the std deviation/fluctuation some the average in the queue size will be high since it is shared by more number of flows, and hence the jitter will be relatively higher.III. Application layer and related issues Q10. (5 points) (Stateful vs. Stateless) Discuss one advantage and one disadvantage of having a stateful protocol for applications. Advantage The protocol can now maintain state about (i.e., remembers) users preferences (e.g., shopping preferences as in browser cookies), Disadvantage when failure occurs the state nee ds to be accommodate (more complexity and overhead than stateless) other correct and reasonable answers are accepted Q11. (5 point) ( weathervane Caching) Describe how Web caching can reduce the delay in receiving a requested object. volition Web caching reduce the delay for all objects requested by a user or for only some of the objects? Why? Ans. Web caching can bring the desired content closer to the user, perhaps to the same local area network to which the users host is connected. Web caching can reduce the delay for all objects, even objects that are not cached, since caching reduces the traffic on links. Q12. (10 points)Discuss three different architectures of the peer-to-peer applications. Give examples of real applications for each architecture and address the advantages and disadvantages of each architecture. Ans. 1. Centralized directory of resources/files, as in Napster. Advantage is that search for resources is unprejudiced with min overhead (just ask the centralized server). The disadvantages are single point of failure, performance bottleneck and target of lawsuit. 2. Fully distributed, non-centralized architecture, as in Gnutella, where all peers and edges form a flat overlay (without hierarchy). Advantages robustness to failure, no performance bottleneck and no target for lawsuit.Disadvantages is that search is more involved and incurs high overhead with query flooding. 3. Hierarchical overlay, with some nodes acting as exceedingly nodes (or cluster heads), or nodes forming loose neighborhoods (sometimes referred to as loose hierarchy, as in BitTorrent). Advantages, robust (no single point of failure), avoids flooding to search for resources during queries. Disadvantages, needs to go for track of at least some nodes using the Tracker server. In general, this architecture attempts to combine the best of the 2 other architectures.Q13. (7.5 points) Push vs. Pull A. Give examples of a push protocol and a get off protocolB. denote three facto rs one should consider when designing gain/push protocols, discuss how these factors would affect your decision as a protocol designer (give example scenarios to illustrate). Answer A. An example of a push protocol is http. An example of a pull protocol SMTP B. The factors affecting the performance of a pull/push protocol include (but are not limited to) 1. access conventionality how often is this object cached and how often is it accessed (example a push mechanism for a very popular video that is pushed closer to a large population that is going to frequently watch it, would be better than a pull mechanism), 2. delay what is the delay to obtain the object, and 3. object dynamics how often/ briefly does the information in the object expires (example in a sensor network where the information sensed is constantly changing, but is queried once in a while would be better not to push it, but to pull it when needed only).Q14. (7.5 points) We refer to the bother of getting users to kno w about each other, whether it is peers in a p2p network or senders and receivers in a multicast group, as the rendezvous problem. What are possible solutions to solve the rendezvous problem in p2p networks (discuss three different alternatives and compare/contrast them. Answer The possible solutions for the rendezvous problem include 1. employ a centralized server advantages simple to search, little colloquy overhead. Disadvantages single-point-of-failure (not robust), bottleneck, doesnt scale well 2. Using a search technique for discovery, perhaps using a melody of a flood (or scoped-flood) or expanding-ring search mechanism. Advantages avoids single-point-of-failure and bottlenecks.Disadvantages may be complex, incurs high communication overhead and may incur delays during the search. 3. hybrid (or hierarchy) where some information (e.g., pointers to potential bootstrap neighbors, or pointers to some resources) are kept at a centralized (or replicated) server or super-nodes, t hen the actual communication is peer-topeer. Advantage if designed carefully can avoid single-point-of-failure, bottlenecks, and achieve reasonable overhead and delay. Disadvantage need to build and maintain the hierarchy (can trigger costly re-configuration control overhead in case of highly dynamic networks and unstable super-nodes).