Protagonist International Journal of Management And Technology (PIJMT) Online ISSN Vol 2 No 3 (May-2015) Active Queue Management

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1 Protagonist International Journal of Management And Technology (PIJMT) Online ISSN Vol 2 No 3 (May-2015) Active Queue Management For Transmission Congestion control Manu Yadav M.Tech Student Dr. Pushpender Sarao Professor Dept.of Computer Sci. & Engineering Dept.of Computer Sci. & Engineering Somany Institute of Tech. & Mgt.Rewari Haryana,India Somany Institute of Tech. & Mgt.,Rewari Haryana,India 1

2 Introduction Computer networks is defined as network, which consists of one or more computers or any other devices like routers, switches, hub, server etc. that are linked together to interact to each other and shares data and other resources. The devices on the network are referred to as nodes. These nodes communicate with each other using medium such as twisted pair cable, Ethernet cable, Optical fiber cables and radio waves and they all can be arranged according to various topologies such as bus, ring, tree etc. Computer networks have gone through a sudden growth over the past few years and with that growth have come severe congestion problems. Internet congestion occurs as the demand increases than available resources. The congestion creates many problems like data loss, long delay, waste of resources and much more. It has a huge influence to both wired network and wireless network and causes the problem of packet loss, packet delay and lock out. To control congestion there are many techniques, such as exponential back off, congestion control in TCP, priority schemes and queue management. Exponential back off is used in CSMA/CA which is sensing scheme of The sender senses the channel before transmission. If the channel is busy it wait until idle and sends the data after a random period of time. The random period is calculated by exponential back off. Congestion control in TCP consists of slow start, fast transmission, fast recovery, congestion avoidance [1]. It is a method to controlling the transmission rate of the sender. The TCP flows starts at a very slow rate and increase exponentially to a threshold. Congestion avoidance then happens and congestion window increases by one segment each time for one successful transmission. Congestion control in TCP defines TCP's four intertwined congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. Priority queues marks the packets into different priorities and drops low priority packets when it is needed. It is not a real congestion control method but improves the performance with other methods. 2

3 Queue management is a way to control the queue size of the bottlenecks. It contains passive queue management, which drops packet when the queue is full and active queue management which drops the packets before buffer getting full. Drop Tail and random Early Detection (RED) are algorithms that represent the two ways respectively. RED is more complicated but can avoid congestion and lockout. Background of AQM: The role of Active Queue Management (AQM) in IP networks was to equilibrate the utilization of end-system protocols such as the Transmission Control Protocol (TCP) in congestion control. Ultimately the performance of a communications network will be judged by the Quality of Service (QoS) perceived by users. This end user QoS can be affected by many factors outside of the control of the network operators. Queuing delay or latency is the total time it occupies the sender s packet, once it goes in the network, to be delivered to its destination. Fig 1.1 [2] shows a single sender-receiver connection. A packet contains of data in a sequence of binary digits in a packet-switched network. Transmission Control Protocol (TCP) divides a file into adequate sized packets that are separately numbered. A packet consists of an IP header, destination Internet address, packet number protocol, source address (TCP sender), and data. The TCP conveys the packet in the network where it is then routed out of diverse links and hubs. Every link is connected to a hub server that routes packets through routers and switches that receive a queue size and possibly a different and effective algorithm for handling queue congestion. The hub server s router and switches get input links and route the packets to the suitable output link. When a packet enters a hub server, it will be situated into a routers queue when congestion occurs. If there is no congestion the packet will be sent instantly with almost zero delay. Depending on the length of the queue and where the packet was set in the queue determines the sum of the time it will reside in queue until it is transmitted. Formerly, all the packets have arrived to its destination; the TCP receiver will reassemble the file in the receiver TCP by putting the packets in-order and assembling the data back into a file [3]. 3

4 Current Interpretations of TCP rely on loss as needle of congestion. Clearly, this is undesirable if one wants to run the network at low-levels of loss. On the other side, losses are good needles of congestion and one needs other signals from the network to produce congestion-control diagnosis with very small or no loss. The schematic of a sender, receiver connection is examined in Figure. Explicit Congestion Notification (ECN) has been lately offered by providing early reading of sources about imminent congestion in the network. ECN marking is a phenomenon to provide such data about the network to the users. A schematic of a sender-receiver connection [2] 4

5 To provide ECN marks, the routers need to mark packets smartly that conveys information of the current state of network to the users. Algorithms which the routers employ to convey such information are called as Active Queue Management (AQM) schemes [AQM techniques]. Active Queue Management (AQM) Generally, AQM schemes controls the congestion by controlling flow. Congestion is measured and control actions are taken. There are two approaches for measuring congestion [4]. Queue based: In queue based AQMs congestion is measured by queue size. And action is taken by maintaining a set of queues by Internet routers, one per interface, that hold packets scheduled to start extinct on that interface. In such queues a packet is set onto the queue if the queue is shorter than its upper limit size, and dropped otherwise. The limitation of this is that a backlog of packets is inherently required by the control mechanism when the congestion is observed in queue is already positive. Flow based: In Flow based AQMs, congestion is observed and action is taken based on the packet arrival rate. For such schemes, backlog, and all its unfavourable implications, is not necessary for the control mechanism. Goals of AQM are summarised as following: 1. Controls average queue size. 2. Absorbs bursts without dropping packets. 3. Prevents bias against bursty connections. 4. Avoids global synchronization of TCP. 5. Reduces the number of timeouts in TCP. 6. Punishes misbehaving flows. There are many AQM schemes that have proposed in the literature. Here are the recently proposed AQM algorithms. Drop tail 5

6 Random early detection (RED) Blue Random Exponential Marking (REM) Blue and Stochastic Fair Blue (SFB) Robust Random Early Detection (RRED) Existing Systems Drop Tail: Tail Drop is the default congestion avoidance mechanism. Tail Drop is when the packets are dropped when they arrive on a congested interface. It also impacts the efficiency of network bandwidth utilization. When the Output Queue is full and packets arrive in on the Input Queue, then the packets which are arriving on the interface will be dropped. It does not matter if it s a voice packet or a data packet, everything will be dropped by default when Tail Drop is in action. This method has served the Internet well for years, but it has three important drawbacks [5]. Lock-Out: In some situations drop tail allows a single connection or a few flows to monopolize queue space, preventing other connections from getting room in the queue. This "lock-out" phenomenon is often the result of synchronization or other timing effects. Full Queues: The drop tail discipline allows queues to maintain a full (or, almost full) status for long periods of time, since tail drop signals congestion (via a packet drop) only when the queue has become full. It is important to reduce the steady state queue size, and this is perhaps queue management's most important goal. Global TCP Synchronization: When TCP Slow Start kicks in, all senders on the network back off and you can see a drop in the bandwidth, then slowly everyone starts sending packets at higher rate as they see no more packet loss, so all senders on the network start sending the packets again at higher rate and you see peaks in the network bandwidth. At this time the interfaces can get congested again and packets can be dropped, which then makes all senders to drop their sending rate and wait for certain time interval where they see no more packet loss, this 6

7 leads TCP Senders to again increase the sending rate. This goes on in cycles and this behaviour means a lot of bandwidth is just getting wasted. If you are monitoring the bandwidth with a graph, you will something like below graph in the utilization charts. Drop Tail mechanism of bandwidth utilization This behaviour is also called as Global TCP Synchronization and it is responsible for a lot of network bandwidth wastage. Nevertheless, drop tail has some weakness, such as the bad fairness, sharing among TCP connections and the throughput and link efficiency suffer severe degradation if congestion is making worse. Source Quench: The ICMP Source Quench is the only congestion control mechanism in the Network layer of the TCP/IP protocol suite. Both routers and hosts participate in the mechanism to control congestion. When a router believes itself to be congested, it sends source quench packets back to the source of the packets causing the congestion [6]. On receipt of these packets, the host will regulate its data rate so as to prevent router congestion. It will then slowly increase its data rate, until source quench packets begin to arrive again. Drawbacks Source Quench's effectiveness as a congestion control mechanism is flawless due to two problems. 7

8 It is not clearly stated as to how a router or destination decides when it is congested, who to send a source quench message to, how often to send source quench messages, and when to stop. It is not clearly stated as to how a host should react to a source quench message; by how much it should reduce its output rate, if it should inform upper layers in the network stack, and how to properly increase its output rate in the absence of source quench messages. Even worse, a router or destination may be congested and not send any source quench messages. Sources in receipt of a source quench message cannot determine if a router or destination dropped the datagram that caused the source quench message. The source does not know to what rate it should reduce its transmission to prevent further source quench messages. First In First Out (FIFO): In First In First Out (FIFO) method, during monitoring the average queue size at the gateway, and in an initial stage of congestion at the connections, it is assumed that the required queues at the gateway traffic from a number of connections is multiplexed together with FIFO scheduling [7]. Not only is FIFO scheduling useful for sharing delay among connections and reducing delay for a particular connection during its period of burstiness, but it scales well and is easy to implement efficiently. DEC- Bit: In DEC-bit congestion avoidance scheme, the gateway uses congestion notification bit in packet headers to provide feedback about congestion in the network. When the average queue size at the gateway exceeds one, the gateway sets the ECN bit in the packet header of arriving packets. The source uses window flow control, and updates its window once every two round-trip times [8]. If at least half of the packets in the last window had the ECN- bit set, then the congestion window decreased multiplicatively. Otherwise, the congestion window is increased additively. In contrast to the DEC bit schemes, for the ECN mechanism proposed, a single packet with the ECN bit is to be interpreted by the transport-level source as an indication of congestion. Congestion indication bit Congestion indication bit 8 Ack packet

9 1 0 1 Data packet Data packet Congestion indication bit Source Router Congested router Destination The DEC-bit Scheme Random Drop Router: In the Random Drop (RD) method the intention is to give feedback to users whose traffic congests on the gateway by dropping packets on a statistical basis as shown in figure 2. The key to this policy is the hypothesis that a packet is randomly selected from all incoming traffic belonging to a particular user with a probability proportional to the average rate of transmission of that user [1]. Random Drop Router Scheme Random Early Detection (RED): Random Early Detection (RED) [10] was proposed by Floyd and Jacobson as an efficient congestion avoidance mechanism in the network routers/gateways. It also helps to prevent the global synchronization in the TCP connections sharing a congested router and to decrease the bias against bursty connections. It is assumed to solve the traditional problems of queue management techniques. It was an improvement over the previous techniques such as Random 9

10 Drop and Early Random Drop. RED use probabilistic discard methodology of queue fill before overflow conditions are reached. By detecting incipient congestion early and to convey congestion notification to the end-hosts, allowing them to decrease their transmission rates before queues in the network overflow and packets are dropped. The RED gateway computes the average queue size by using a low pass filter along with an exponential weighted moving average [24]. The average queue size is compared with two thresholds: a minimum and a maximum threshold. When the size of average queue is less than the minimum threshold, no packets are marked. When the size of average queue is greater than the maximum threshold, every arriving packet from gateway is marked. If marked packets are, in fact, dropped or if all source nodes are collaborative, this assures that the average queue size does not significantly exceed the maximum threshold. Future Work Future study includes evaluation of this framework for more AQM algorithms. Although much research effort has been focused on understanding and utilizing RRED algorithm to leverage the current network, some interesting research topics are yet to be investigated in more detail in future. For example, we have seen the throughput of RRED in presence of LDoS which is better than RED. We can improve the RRED algorithm in future so that it increases the throughput. Reference: S Floyd, M Handley, J Padhye, J Widmer, Equation-based congestion control for unicast applications. Proceedings of ACM SIGCOMM Conference, September 2000, S Floyd, V Jacobson, Random early detection gateways for congestion avoidance. IEEE/ACM Transactions on Networking 1(4), (1993). Publisher Full Text OpenURL H Park, EF Burmeister, S Bjorline, JE Bowers, 40-Gb/s optical buffer design and simulation. Proceedings of the 4th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD '04), August 2004,

11 Fisher, US natinal science foundation and the future Internet design. ACM SIGCOMM Computer Communication Review 37(3), (2007). Publisher Full Text OpenURL G Appenzeller, I Keslassy, N McKeowen, Sizing router buffers. Proceedings of the SIGCOM, 2004,, New York, NY, USA (ACM Press), pp S Ryu, C Rump, C Qiao, Advances in active queue management (AQM) based TCP congestion control. Telecommunication Systems 25(3-4), (2004) SS Kunniyur, R Srikant, An adaptive virtual queue (AVQ) algorithm for active queue management. IEEE/ACM Transactions on Networking 12(2), (2004). Publisher Full Text OpenURL. 11

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