MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI

Transcription

1 MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI UNIT IV INTEGRATED AND DIFFERENTIATED SERVICES Part A (2 Marks) 1. What are the two types of traffic on internet? Traffic on network or internet is classified into two broad categories: 1. Elastic 2. Inelastic 2. Define Elastic traffic. Elastic traffic is that which can adjust to changes in delay and throughout across an internet and still meet the needs of its applications. 3. Mention some of the applications that are classified as elastic. The applications that operate over i. File transfer (FTP) ii. Electronic mail (SMTP) iii. Remote logon (TELNET) iv. Network management (SNMP) v. Web access (HTTP) 4. Define Inelastic Traffic. Inelastic traffic does not easily adapt to changes in delay and throughput across the internet. 5. What are the requirements for inelastic traffic? i. Throughput ii. Delay iii. Jitter iv. Packet loss

2 6. What are the problems in inelastic traffic? i. Difficult to meet requirements on network with variable queuing delays and congestion ii. Need preferential treatment iii. Applications need to state requirements 1. Ahead of time (preferably) or on the fly 2. Using fields in IP header 3. Resource reservation protocol iv. Must still support elastic traffic 1. Deny service requests that leave too few resources to handle elastic traffic demands 7. What is the need of ISA? The purpose of ISA is to enable the provision of QoS support over IP-based internets. The central design issue of ISA is how to share the available capacity in times of congestion 8. What are the tools used by a router for controlling congestion in IP networks? i. Routes can be selected to minimize the delay. ii. When a buffer overflows, it discards the packets 9. What are the functions used by ISA for controlling congestion and provide QoS transport? i. Admission control ii. Routing algorithm iii. Queuing discipline iv. Discard policy 10. What are the components of ISA? i. Reservation protocol ii. Admission protocol iii. Management agent iv. Routing protocol 11. What is a packet scheduler?

3 Packet scheduler function manages one or more queues for each output port. It determines the order in which queued packets are transmitted and the selection of packets for discard. Decisions are made based on a packet s class, the contents of the traffic control database and the current and past activity on this outgoing port. 12. What are the services provides by ISA? i. Guaranteed ii. Controlled load iii. Best effort 13. What is token bucket traffic specification? Token bucket traffic specification is a way of characterizing traffic. 14. What are the advantages of Token-Bucket scheme? i. Many traffic sources can be defined by token bucket scheme ii. Provides concise description of load imposed by flow iii. Easy to determine resource requirements iv. Provides input parameters to policing function 15. What are the key elements of guaranteed service? i. Assured capacity level or data rate ii. Specific upper bound on queuing delay through network which is added to propagation delay or latency to get total delay iii. No queuing losses i.e. no buffer overflow 17. What are the key elements of controlled load? i. Tightly approximates to best efforts under unloaded conditions ii. No upper bound on queuing delay. 1. High percentage of packets do not experience delay over minimum transit delay iii. Very high percentage of transmitted packets will be delivered iv. Almost no queuing loss 18. What is meant by Fair-queuing? i. In fair queuing each incoming packet is placed in the appropriate queues. ii. Queues are serviced in round robin fashion, taking one packet from each queue.

4 iii. Each busy queue (flow) gets exactly one packet per cycle iv. Short packets are penalized as each queue sends one packet per cycle 19. Write about BRFQ. In BRFQ (Bit Round Fair Queuing) a bit-by-bit round robin discipline is followed. In this we set up multiple queues and transmit one bit from each queue on each round. In this way, longer packets no longer receive an advantage and each busy source receives exactly the same amount of capacity. This bit-by-bit approach is called processor sharing (PS). 20. What is Generalized Processor sharing technique? In GPS each flow is assigned a weight that determines how many bits are transmitted from that queue during each round. If a weight for a given flow is 5, then during each round that the queue is non-empty, 5 bits will be transmitted 21. Mention the RED design goals. i. Congestion avoidance ii. Global synchronization avoidance iii. Avoidance of bias against bursty traffic. iv. Bound on average queue length. 22. What are differentiated services? The differentiated services architecture is designed to provide a simple, easy-to implement, low overhead toll to support a range of network services that are differentiated on the basis of performance. 23.What are the types of Queuing? 1. Input Queuing 2.Output Queuing 3.Shared Queuing 24. What are the three parameters used to describe the switching fabric performance? 1.Switch throughput 2.Average packet delay 3.Packet loss probability 25.Define switch throughput? It is defined as the probability that a packet received on an input link is successfully switched and transmitted by the addressed switch output. 26. What is maximum throughput? the maximum throughput also referred as the switch capacity indicates the load carried by the switch for an offered load _= What is average packet delay? The average number of slots it takes for a packet received at a switch inlet to cross the network and thus to be transmitted downstream by the addressed switch outlet (T= 1).

5 27. What is packet loss probability? Probability that a packet received at a switch input is lost due to buffer overflow (0<p<=1). 28. What are the internal protocols available to enable the downstream transmission of packets? 1. Backpressure 2. Queue loss 29. What is back pressure? Signals are exchanged between switching elements in adjacent stages so that the generic SE can grant a packet transmission to its upstream SE s only within the current idle buffer capacity. 30. What are the types of back pressure? 1. Gobal back pressure 2. Local back pressure 31. Define local back pressure? The number of buffer places that can be filled in the generic SE in stage i at slot t by upstream SE s is simply given by the number of idle positions at the end of slot t Define global back pressure? The number of buffer places that can be filled in the generic SE in stage i at slot t by upstream SE s is simply given by the number of idle positions at the end of slot t-1 increased by the number of packets that are going to be transmitted by the SE in the slot. 33. Define input queuing? Cells addressing different switch outlets are stored at the switch input interface as long as there conflict-free switching through the inter connection network is possible. 34. Define output queuing? Multiple cells addressing the same switch outlet are first switched through the interconnection network and then stored in the switch output while waiting to be transmitted down stream. 35. What is shared queuing? The queuing capability shared by all switch input and output interfaces is available for all cells that cannot be switched immediately to the desired switch outlet. 36. What are the blocks involved in an N M ATM switch? 1. N input port controller 2. Non blocking interconnection network 3. M output port controller

6 37. What are the assumptions made in an input queuing? Bi> 0,Bo=Bs=0 and K=1 38. What are the algorithms involved for an input queuing? 1. Three phase algorithm 2. Ring reservation algorithm 39. What are the phases present in three phase switch? 1. probe phase 2. Acknowledgement phase 3. data phase 40. What is signal latency in a network? The number of bit times it takes for a signal to cross the network is called signal latency. 41. Why combined input and output queuing is necessary? The combined architecture adopt a k-non blocking self routing multistage structure where the shared queue is removed. The virtual queue, input and output queue are mutually independent discrete time systems. In this queuing technique the number of cells entering the virtual queues in a slot approaches infinity and the queue joined by each cell is randomly and independently selected. 42. What is the assumption made in an output queuing? Bo>0, Bi=Bs=0 and Output speed up K>1 43. What is cross bar tree switch? Cross bar tree switch consists of a set of N planes each inter connecting a switch inlet to all the N output concentrators. 44. What is the assumption made in a shared queuing? Bs>0,Bi=Bo=0 and K= What is the need for an delay network in the starlite switch? The recirculation or delay network of size P P acts as a distributed shared buffer and feeds back to the routing network up to P=N Bs packets that could not be switched in the preceding slot. 46. What are the blocks involved in a trap network? 1. Marker 2. Running adder winner 3. Running adder loser

7 4. Concentrator. 47. What is meant by elastic traffic? Give example. Elastic traffic can adjust over wide ranges to changes in delay and throughput across an internet and still meet the needs of its applications. Example: File transfer Web access 48. What is meant by inelastic traffic? Give example. Inelastic traffic cannot adjust to changes in delay and throughput across an internet. Example: Voice chat 49. Define Delay Jitter. Tele conferencing The delay jitter is the maximum variation in delay experienced by packets in a single session. 50. What is meant by best effort service? Flows that are not reserving resources are provided with best effort service. The network will put best effort to deliver the packet but if congestion occurs severely discard the packet. 51. What is meant by guaranteed service? Flows that are reserving resources are provided with guaranteed service. The service provides assured capacity levels. 52. Define global synchronization. Due to packet discard during congestion, many TCP connections entered slow start at the same time. As a result, the network is unnecessarily under utilized for some time. The TCP connections which entered into slow start, will come out of slow start at about time causing congestion again. This phenomenon is called global synchronization. 53. What are the design goals of RED algorithm?

8 i. Congestion avoidance ii. Global synchronization avoidance iii. Round on average queue length 54. Define behavior aggregate in per hop behavior. A set of packets with the same Ds code point crossing a link in particular direction is called behaviour aggregate. 55. Define DS code point. A specified value of 6 bit DS code point portion of the 8 bit DS field in the IP header which indicate to which class packet belongs and its drop precedence. 56. What is meant by traffic conditioning agreement? An agreement that specify rules that are to apply for packets selected by the classifier. Control functions performed in TCA are metering, marking, shaping and dropping. 57. Define DS boundary node. A DS node that connects one Ds domain to the node in another domain. 58. Define DS interior node. A node in DS domain, which is not the boundary node is called Ds interior node. 59. Define Ds node. A router that supports DS policies is called as DS node. A host system that use DS for application is called as DS node. 60. What is meant by differentiated service? a. It does not attempt to view the total traffic demand in integrated sense. b. It does not reserve network capacity in advance.

9 c. It provides differential level of QOS to different traffic flows. 61. What is meant by integrated services? The Is provider a. Views the total of current traffic demand. b. Limits the demand with respect to the current capacity handled by the network. c. Reserve resources with in the domain to provide a particular QOS guaranteed. Part B (16 Marks) 1. Integrated Services Architecture (ISA) IPv4 header fields for precedence and type of service usually ignored ATM only network designed to support TCP, UDP and real-time traffic May need new installation Need to support Quality of Service (QoS) within TCP/IP Add functionality to routers Means of requesting QoS Internet Traffic Elastic Can adjust to changes in delay and throughput E.g. common TCP and UDP application insensitive to delay changes FTP User expect delay proportional to file size Sensitive to changes in throughput SNMP delay not a problem, except when caused by congestion Web (HTTP), TELNET sensitive to delay Not per packet delay total elapsed time E.g. web page loading time For small items, delay across internet dominates For large items it is throughput over connection Need some QoS control to match to demand Internet Traffic Inelastic Does not easily adapt to changes in delay and throughput Real time traffic Throughput Minimum may be required Delay E.g. stock trading Jitter - Delay variation More jitter requires a bigger buffer E.g. teleconferencing requires reasonable upper bound

10 Packet loss Inelastic Traffic Problems Difficult to meet requirements on network with variable queuing delays and congestion Need preferential treatment Applications need to state requirements Ahead of time (preferably) or on the fly Using fields in IP header Resource reservation protocol Must still support elastic traffic Deny service requests that leave too few resources to handle elastic traffic demands 2. ISA Approach Provision of QoS over IP Sharing available capacity when congested Router mechanisms Routing Algorithms Select to minimize delay Packet discard Causes TCP sender to back off and reduce load Enahnced by ISA Flow IP packet can be associated with a flow Distinguishable stream of related IP packets From single user activity Requiring same QoS E.g. one transport connection or one video stream Unidirectional Can be more than one recipient Multicast Membership of flow identified by source and destination IP address, port numbers, protocol type IPv6 header flow identifier can be used but isnot necessarily equivalent to ISA flow ISA Functions Admission control For QoS, reservation required for new flow RSVP used Routing algorithm Base decision on QoS parameters Queuing discipline Take account of different flow requirements Discard policy Manage congestion Meet QoS

11 ISA Implementation in Router Background Functions Forwarding functions 3. ISA Components Background Functions Reservation Protocol RSVP Admission control Management agent Can use agent to modify traffic control database and direct admission control Routing protocol ISA Components Forwarding Classifier and route selection Incoming packets mapped to classes Single flow or set of flows with same QoS E.g. all video flows Based on IP header fields Determines next hop Packet scheduler Manages one or more queues for each output Order queued packets sent Based on class, traffic control database, current and past activity on outgoing port Policing

12 4. ISA Services Traffic specification (TSpec) defined as service for flow On two levels General categories of service Guaranteed Controlled load Best effort (default) Particular flow within category TSpec is part of contract Token Bucket Many traffic sources can be defined by token bucket scheme Provides concise description of load imposed by flow Easy to determine resource requirements Provides input parameters to policing function Token Bucket Diagram ISA Services Guaranteed Service Assured capacity level or data rate Specific upper bound on queuing delay through network Must be added to propagation delay or latency to get total delay Set high to accommodate rare long queue delays No queuing losses I.e. no buffer overflow E.g. Real time play back of incoming signal can use delay buffer for incoming signal but will not tolerate packet loss ISA Services Controlled Load Tightly approximates to best efforts under unloaded conditions No upper bound on queuing delay High percentage of packets do not experience delay over minimum transit delay

13 Propagation plus router processing with no queuing delay Very high percentage delivered Almost no queuing loss Adaptive real time applications Receiver measures jitter and sets playback point Video can drop a frame or delay output slightly Voice can adjust silence periods 5. Queuing Discipline Traditionally first in first out (FIFO) or first come first served (FCFS) at each router port No special treatment to high priority packets (flows) Small packets held up by large packets ahead of them in queue Larger average delay for smaller packets Flows of larger packets get better service Greedy TCP connection can crowd out altruistic connections If one connection does not back off, others may back off more Fair Queuing (FQ) Multiple queues for each port One for each source or flow Queues services round robin Each busy queue (flow) gets exactly one packet per cycle Load balancing among flows No advantage to being greedy Your queue gets longer, increasing your delay Short packets penalized as each queue sends one packet per cycle FIFO and FQ Processor Sharing Multiple queues as in FQ Send one bit from each queue per round Longer packets no longer get an advantage Can work out virtual (number of cycles) start and finish time for a given packet

14 However, we wish to send packets, not bits Bit-Round Fair Queuing (BRFQ) Compute virtual start and finish time as before When a packet finished, the next packet sent is the one with the earliest virtual finish time Good approximation to performance of PS Throughput and delay converge as time increases Comparison of FIFO, FQ and BRFQ Generalized Processor Sharing (GPS) BRFQ can not provide different capacities to different flows Enhancement called Weighted fair queue (WFQ) From PS, allocate weighting to each flow that determines how many bots are sent during each round If weighted 5, then 5 bits are sent per round Gives means of responding to different service requests Guarantees that delays do not exceed bounds Weighted Fair Queue Emulates bit by bit GPS Same strategy as BRFQ

15 FIFO v WFQ \ Proactive Packet Discard Congestion management by proactive packet discard Before buffer full Used on single FIFO queue or multiple queues for elastic traffic E.g. Random Early Detection (RED) 6. Random Early Detection (RED) Motivation Surges fill buffers and cause discards On TCP this is a signal to enter slow start phase, reducing load Lost packets need to be resent Adds to load and delay

16 Global synchronization Traffic burst fills queues so packets lost Many TCP connections enter slow start Traffic drops so network under utilized Connections leave slow start at same time causing burst Bigger buffers do not help Try to anticipate onset of congestion and tell one connection to slow down RED Design Goals Congestion avoidance Global synchronization avoidance Current systems inform connections to back off implicitly by dropping packets Avoidance of bias to bursty traffic Discard arriving packets will do this Bound on average queue length Hence control on average delay RED Algorithm Overview Calculate average queue size avg if avg < TH min queue packet else if TH min avg Th max calculate probability P a with probability P a discard packet else with probability 1-P a queue packet else if avg TH max discard packet RED Buffer

17 RED Algorithm Detail

18 7. Differentiated Services (DS) ISA and RSVP complex to deploy May not scale well for large volumes of traffic Amount of control signals Maintenance of state information at routers DS architecture designed to provide simple, easy to implement, low overhead tool Support range of network services Differentiated on basis of performance Characteristics of DS Use IPv4 header Type of Service or IPv6 Traffic Class field No change to IP Service level agreement (SLA) established between provider (internet domain) and customer prior to use of DS DS mechanisms not needed in applications Build in aggregation All traffic with same DS field treated same E.g. multiple voice connections DS implemented in individual routers by queuing and forwarding based on DS field State information on flows not saved by routers Services Provided within DS domain Contiguous portion of Internet over which consistent set of DS policies administered Typically under control of one administrative entity Defined in SLA Customer may be user organization or other DS domain Packet class marked in DS field Service provider configures forwarding policies routers Ongoing measure of performance provided for each class DS domain expected to provide agreed service internally If destination in another domain, DS domain attempts to forward packets through other domains Appropriate service level requested from each domain SLA Parameters Detailed service performance parameters Throughput, drop probability, latency Constraints on ingress and egress points Indicate scope of service Traffic profiles to be adhered to Token bucket Disposition of traffic in excess of profile

19 Example Services Qualitative A: Low latency B: Low loss Quantitative C: 90% in-profile traffic delivered with no more than 50ms latency D: 95% in-profile traffic delivered Mixed E: Twice bandwidth of F F: Traffic with drop precedence X has higher delivery probability than that with drop precedence Y DS Field Detail Leftmost 6 bits are DS codepoint 64 different classes available 3 pools xxxxx0 : reserved for standards : default packet class xxx000 : reserved for backwards compatibility with IPv4 TOS xxxx11 : reserved for experimental or local use xxxx01 : reserved for experimental or local use but may be allocated for future standards if needed Rightmost 2 bits unused Configuration Diagram Configuration Interior Routers Domain consists of set of contiguous routers Interpretation of DS codepoints within domain is consistent

20 Interior nodes (routers) have simple mechanisms to handle packets based on codepoints Queuing gives preferential treatment depending on codepoint Per Hop behaviour (PHB) Must be available to all routers Typically the only part implemented in interior routers Packet dropping rule dictated which to drop when buffer saturated Configuration Boundary Routers Include PHB rules Also traffic conditioning to provide desired service Classifier Separate packets into classes Meter Measure traffic for conformance to profile Marker Policing by remarking codepoints if required Shaper Dropper DS Traffic Conditioner Per Hop Behaviour Expedited forwarding Premium service Low loss, delay, jitter; assured bandwidth end-to-end service through domains Looks like point to point or leased line Difficult to achieve Configure nodes so traffic aggregate has well defined minimum departure rate EF PHB Condition aggregate so arrival rate at any node is always less that minimum departure rate Boundary conditioners Per Hop Behaviour Explicit Allocation Superior to best efforts Does not require reservation of resources Does not require detailed discrimination among flows Users offered choice of number of classes Monitored at boundary node In or out depending on matching profile or not

21 Inside network all traffic treated as single pool of packets, distinguished only as in or out Drop out packets before in packets if necessary Different levels of service because different number of in packets for each user PHB - Assured Forwarding Four classes defined Select one or more to meet requirements Within class, packets marked by customer or provider with one of three drop precedence values Used to determine importance when dropping packets as result of congestion Codepoints for AF PHB 8.Explain the Integrated and differentiated services approach in detail. Modern Internet applications demand services not provided by a best-effort service model Two complementary, yet fundamentally different, traffic management frameworks have evolved: Integrated Services (IS, ISA, IntServ): reserve resources per session and limit total demand to the capacity that can be handled by the network Differentiated Services (DS, DiffServ): classify traffic into a number of traffic groups and handle traffic based on its group Traffic control mechanisms: queuing discipline, packet discard policy Services are specified within a given domain Internet Traffic Elastic Traffic traffic that can adapt, over a wide range, to delay and throughput changes

22 typically TCP/UDP QoS perceived based on application Inelastic Traffic IntServ Approach traffic does not adapt well requires guarantees on: throughput, delay, jitter, packet loss e.g. traffic generated by real-time applications Two key features form core of architecture Resource reservation routers must maintain state of available resource reserved for each session Call/session setup each router on the session s path must verify availability of required resources for a session and admit sessions only if requirements can be met Call Admission process (more later) Traffic characterization (Tspec) Desired QoS caharterizatio (Rspec) Reservation signaling (RSVP, RFC 2210) Per-element call admission per Tspec and Rspec IntServ Implementation Associate each packet with a flow a distinguishable stream of related IP packets that result from a single user activity and demand the same QoS (per RFC 1633) unidirectional, can have multiple recipients typically identified by: source & destination IP addresses, port numbers and protocol type Provide for enhanced router functions to manage flows: Admission control based on requested QoS and availability of required network resources Routing protocol based on QoS (like OSPF/MOSPF) Queuing/scheduling disciplines based on QoS

23 Packet discard policy based on QoS ISA: 3 Categories of Service Guaranteed Service assured capacity (data rate) specified upper bound on queuing delay through the network no queuing loss (i.e., no buffer overflow) Controlled Load roughly equivalent to best-effort under no-load conditions (dprop + dtrans) no specified upper bound on queuing delay, but will approximate minimum expected transit delay almost no queuing loss Best Effort 9.Describe the types of Queuing Disciplines. Queuing Disciplines Single FIFO queues have numerous drawbacks relative to QoS demands no special treatment based on priority larger packets get better service connections can get an unfair share of resources IntServ allows for multiple queues one per flow separate discipline per flow fair queuing policy Queuing Disciplines (Scheduling)

24 FIFO (First-Come-First-Served) Flows with busy (greedy) sources crowd out others Flows with shorter packets are penalized Round Robin (Fair Queuing)

25 Flows with shorter packets are penalized Processor Sharing Approach Processor Sharing (PS) ideal, but not a practical policy transmit only one bit per round per queue with N queues, each queue receives exactly 1/N of the available capacity consider each queue independently to calculate virtual start and finish times for each transmission EXAMPLE QUEUE QUEUE QUEUE Packet 1 Packet 2 Packet 1 Packet 2 Packet 1 Real arrival time, i Transmission time, Pi Virtual start time, Si Virtual finish time, Fi Bit-Round Fair Queuing Bit-Round Fair Queuing (BRFQ) emulates PS round-robin approach for packets and multiple synchronous queues uses packet length and flow identification (queue) to schedule packets calculate Si and Fi as though PS were running when a packet finishes transmission, send next packet based on smallest

26 value of Fi over all queues algorithm is fair on the basis of amount of data transmitted instead of number of packets Queuing Discipline Priority Queuing Queuing Discipline Weighted Fair Queuing

27 Scheduling vs. Queue Management Closely related, but different performance issues Scheduling: managing allocation of bandwidth between flows by determining which packet to send next (queuing discipline) Queue Management: managing the length of packet queues by proactively dropping packets when necessary (packet discard policy) 10. Explain Random Early Detection in detail. Random Early Detection (RED) Random early detection (RED), also known as random early discard or random early drop is an active queue management algorithm. It is also a congestion avoidance algorithm. [1] In the traditional tail drop algorithm, a router or other network component buffers as many packets as it can, and simply drops the ones it cannot buffer. If buffers are constantly full, the network is congested. Tail drop distributes buffer space unfairly among traffic flows. Tail drop can also lead to TCP global synchronization as all TCP connections "hold back" simultaneously, and then step forward simultaneously. Networks become under-utilized and flooded by turns. RED addresses these issues. RED monitors the average queue size and drops (or marks when used in conjunction with ECN) packets based on statistical probabilities. If the buffer is almost empty, all incoming packets are accepted. As the queue grows, the probability for dropping an incoming packet grows too. When the buffer is full, the probability has reached 1 and all incoming packets are dropped. RED is more fair than tail drop, in the sense that it does not possess a bias against bursty traffic that uses only a small portion of the bandwidth. The more a host transmits, the more likely it is that its packets are dropped as the probability of a host's packet being dropped is proportional to the amount of data it has in a queue. Early detection helps avoid TCP global synchronization. Problems with Classic RED According to Van Jacobson, "there are not one, but two bugs in classic RED." [2] Improvements to the algorithm were developed, and a draft paper [3] was prepared, but the paper was never published, and the improvements were not widely disseminated or implemented. Pure RED does not accommodate quality of service (QoS) differentiation. weighted RED

28 (WRED) and RED with In and Out (RIO) [4] considerations. provide early detection with QoS Other variants In Weighted RED you can have different probabilities for different priorities (IP precedence, DSCP) and/or queues. [5] The Adaptive / Active RED (ARED) algorithm [6] infers whether to make RED more or less aggressive based on the observation of the average queue length. If the average queue length oscillates around min threshold then early detection is too aggressive. On the other hand if the average queue length oscillates around max threshold then early detection is being too conservative. The algorithm changes the probability according to how aggressive it senses it has been discarding traffic. See Srikant [7] for an in-depth account on these techniques and their analysis. [ RRED: Robust RED Main article: Robust random early detection The existing Random Early Detection (RED) algorithm and its variants are found vulnerable to emerging attacks, especially the Low-rate Denial-of-Service (LDoS) attacks. Experiments have confirmed that the existing RED-like algorithms are notably vulnerable under LDoS attacks due to the oscillating TCP queue size caused by the attacks. Recent Publications in low-rate Denial-of-Service (DoS) attacks A Robust RED (RRED) algorithm was proposed to improve the TCP throughput against LDoS attacks. The basic idea behind the RRED is to detect and filter out attack packets before a normal RED algorithm is applied to incoming flows. RRED algorithm can significantly improve the performance of TCP under Low-rate Denial-of-Service attacks. Queuing discipline with proactive packet discard anticipate congestion and take early avoidance action improved performance for elastic traffic by not penalizing bursty traffic avoids global synchronization phenomenon at congestion onset control average queue length (buffer size) within deterministic bounds therefore, control average queuing delay.

29 11. Explain RED Buffer Management and Generalized RED Algorithm. RED Buffer Management Discard probability is calculated for each packet arrival at the output queue based on: the current weighted average queue size, and the number of packets sent since the previous packet discard Generalized RED Algorithm calculate the average queue size, avg if avg < THmin queue the packet else if THmin avg < THmax calculate probability Pa with probability Pa discard the packet else with probability 1 Pa queue the packet else if avg THmax discard the packet RED Algorithm avg lags considerably behind changes in actual queue size (weight, wq, is small typ ) avg (1 wq)avg + wqq

30 prevents reaction to short bursts count, number of packets passed without discard, increases incrementally while Thmin < avg < Thmax probability of discard, Pa, increases as count increases helps ensure fairness across multiple flows.

31 12.Write short notes on Weighted fair queuing. Weighted fair queuing Weighted fair queuing (WFQ) is a data packet scheduling technique allowing different scheduling priorities to statistically multiplexed data flows. WFQ is a generalization of fair queuing (FQ). Both in WFQ and FQ, each data flow has a separate FIFO queue. In FQ, with a link data rate of R, at any given time the N active data flows (the ones with non-empty queues) are serviced simultaneously, each at an average data rate of R / N. Since each data flow has its own queue, an ill-behaved flow (who has sent larger packets or more packets per second than the others since it became active) will only punish itself and not other sessions. Contrary to FQ, WFQ allows different sessions to have different service shares. If N data flows currently are active, with weights w 1,w 2...w N, data flow number i will achieve an average data rate of It can be proven [1] that when using a network with WFQ switches and a data flow that is leaky bucket constrained, an end-to-end delay bound can be guaranteed. By regulating the WFQ weights dynamically, WFQ can be utilized for controlling the quality of service, for example to achieve guaranteed data rate. Proportional fairness can be achieved by setting the weights to w i = 1 / c i, where c i is the cost per data bit of data flow i. For example in CDMA spread spectrum cellular networks, the cost may be the required energy (the interference level), and in dynamic channel allocation systems, the cost may be the number of nearby base station sites that can not use the same frequency channel, in view to avoid co-channel interference

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