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  • ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 4, June 2012 An Analysis on Buffer Sizing in Routers and Investigating its Impact on Network Parameters Neetha K.Nataraj, Dr.S.Karthik reasons for uncertainty in internet is router buffers. Buffers Abstract—The need for buffering is very important for when they overflow cause packet loss and they degrade thepacket switching networks. Buffers in routers absorb the throughput when they underflow. So sizing of router bufferstransient bursts that occur naturally in such networks. It are very much important.reduces the frequency of packet loss and even they can avoid An earlier experimental study by Villamizar and Songunder-utilization when TCP connections back off due to packet recommended that the buffer size should be equal to thedrops. After several years of research with packet switchingnetworks, it is surprising that we still do not know how to bandwidth delay product (BDP) of that link [9].But the BDPdimension the buffer of a router interface. At the same time the rule results in large buffer .The paper [1] shows that the BDPissue of buffer sizing has become important in practice. rule was outdated and the size of buffer can be made smallerRecently there have been many interesting debates regarding i.e. the buffer requirement at the link decrease with the squarethe sizing of router buffers. From all the previous work we can root of the number of flows N as described in section II.say that the buffer size on the routers depends on several Section III explains that the size of the buffers can furthernetwork parameters such as number of flows, output/input reduced, perhaps to 10-20 packets at the cost of small amountcapacity ratios, speed, window size etc i.e. it is difficult to arrive of bandwidth utilization. Traffic and hardware considerationsat a simple formula for sizing router buffer. This article while sizing router buffers are also discussed.describes the impact of these networks parameters on buffer How control theory has been used to size the router buffersizing in routers. is mentioned in next section (IV). Then dependency of buffer Index Terms— Buffer sizing, Core routers, Internet routers, size on window size is explained (V),which also opted forlink utilization, TCP. small buffer size .All the previous discussion were opting for small buffers, but the use of small buffers can lead to excessively high loss rates in congested access links that I. INTRODUCTION carry many flows(VI). Our analysis points out that core to access speed ratio is also a key parameter that determines the Routers knit together the constituent networks of the buffer size. This parameter along with buffer size determinesglobal Internet, creating the illusion of a unified whole. the typical number of flows in the network, i.e. buffer sizeWhile their primary role is to transfer packets from a set of and number of flows are dependent quantities (VII).Lastinput links to a set of output links, they must also deal with section(VIII) describes the impact of buffer size on TCPheterogeneous link technologies, provide scheduling support performance.for differential service, and participate in complex distributedalgorithms to generate globally coherent routing tables.These demands, along with an insatiable need for bandwidth II. SIZING ROUTER BUFFERin the Internet, complicate their design. Routers are found atevery level in the Internet Size of the buffer is determined by the dynamics of the All Internet routers contain buffers which are mainly TCP’s congestion algorithm .The goal is to make sure thatindented for holding the packets during time of congestion. when a link is congested ,it is busy 100% of the time: whichThe buffer in router has several roles. It accommodates is equivalent to making sure its buffer never goes empty. Intransient bursts in traffic, without having to drop packets. It the paper [1] they argue that the rule of thumb [9]which stateskeeps a reserve of packets, so that the link doesn’t go idle. It that each link needs a buffer of size B=RTT×C (where RTT isalso introduces queuing delay and jitter. One of the main the average round –trip time of the flows passing across the link and C is the data rate of the link)is now outdated and Manuscript received May, 2012. incorrect for backbone routers. The rule is found inNeetha K.Nataraj, Final year M.E-CSE, Department of Computer Science architectural guidelines [2], too.Engineering,SNS College of Technology, Sathy Main Road, Using some theory, simulation an experiments on aCoimbatore-641035,India,Telephone:+91-4842464349,Mobile:+91-94004 network of real routers[1] ,they show that a link with N flows14349, requires not more than B=(RTT×C)/Sqrt(N) for long livedDr.S.Karthik, Dean cum Professor and Head, Department of Computer TCP flows.ie flows requires only 10Mbits of buffering whichScience Engineering(PG),SNS College of Technology, Sathy Main Road, can be implemented using fast, on chip SRAM. Actually theCoimbatore-641035,India,Telephone:+91-422-2666264,Mobile:+91-9842720118 paper [1] is not the last word; the goal is to persuade one or 20 All Rights Reserved © 2012 IJARCSEE
  • ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 4, June 2012more network operators to try reduced router buffers in their With small buffers, bursty flows in slow–start willbackbone network. be unfairly penalized because their packets arrive back to The consequences of over buffering are first, it will make back and are more likely to be dropped. Large buffers in turnthe design of high speed router complicated by high power increases RTT and queuing delay but reduces packet dropconsumption and lower density. Second, over buffering probability .But the statement they argued in [10] is thatincreases end-to-end delay when there is congestion. While applications can protect themselves against packet drop bythe paper [1] has evidence that the buffers can be made resending the packets but lost time can never be recapturedsmaller, but haven’t tested the hypothesis in a real operational i.e. packet loss is better signal than delay.network. It will be a risk for the operator to remove 99% oftheir buffers. But that has to be the next step and will see theresults pointing out in that paper as a first step for the IV. CONTROL THEORY FOR BUFFER SIZINGoperator to try it out. Here it is mentioned how control theory can be helpful in Even if operator verifies that small buffers work fine, it is sizing the core router buffer. The stability of the network isdifficult for router manufacturers, because in a competitive predicted using control theory i.e. whether the TCP flows aremarket place, it is not obvious that a router vendor will build desynchronized. If so small buffers are sufficient[1] i.e. smalla router with 1% the buffers of its competitors. But it will be buffers promote desynchronization.difficult to build line cards from commercial memory chips A network will generally be stable for certain buffer sizesfor building router using current rule of thumb. So as a and unstable for others. In the paper [8] it is explaining howconclusion we can say that necessity may force buffers to be to choose buffer sizes to make it stable. Active queuesmaller. management parameters, round trip time, TCP congestion avoidance algorithm all these will affects stability. It is described how the entire network is made less prone to III. BUFFER SIZING IN CORE ROUTERS synchronization by changing TCP’s rules for increasing and The study of buffer size for core routers [10] says that the decreasing window size.size of the buffers can be reduced to 10-20 packets (where The approach consist of writing down a system ofearlier it was reduced from 1000000 packets to 10000 differential equations and analyse to see whether the systemwithout loss in performance), at the cost of small amount of is stable .Stable solution corresponds to desychronization sobandwidth utilization. Theory and simulations illustrate why that we can go with small buffers as argued in [10]. Stabilitybuffers could be much smaller [4][8] without making also depends on buffer size in two ways one is that the orderchanges to TCP. This will encourages network operator to of magnitude of buffer size determines which differentialexperiment with small buffers. The main issues are raised in equation to be written down and other is, the actual value of[10]. the buffer size affects the stability of equation. In small buffer regime, buffers larger than 50 packets forA. Hardware Considerations While Sizing Router Buffer. small window sizes will cause instability. In large buffer The first reason why we go for small buffer is that the regime large buffer sizes are required for flows with largememory in a router line card has to be as fast as the line window sizes. So the recommendation is that buffer’s be norate-much faster than the speed of the memory in desktop larger than 50 packets for best overall scalability. Therecomputers. Second according to Moore’s Law switching would be no instability and network could have small buffersspeed doubles every 18 months ,memory access speed double if the window increase and decrease rules were different.only every 10 years. So memory requirements will Scalable TCP [5] was designed along these lines.increasingly become a limited aspect of router design. Thirdall optical routers can buffer 100 packets .If this is sufficient V. ROUTERS HAVING VERY SMALL BUFFER SIZEfor getting good performance then all-optical packetswitched core network is feasible. The size of the router buffer can make comparatively even smaller than all the previous discussions (.i.e. a little as a fewB. Traffic Considerations While Sizing Router Buffer. dozen packets), if we are willing to sacrifice a small amount The naïve view is that the larger the buffer, the higher the of link capacity. The buffer size they specifying is O(logutilization. But the problem with this is that: First, utilization Wmax) where W is the window size[4].This buffer size will beis not necessarily the right metric because it is a useful metric sufficient to produce high throughput provided the TCPwhen capacity is expensive and the network operator wants sources are not over bursty.to be sure that all the capacity is able to be used but today core In order to reduce the buffer size to dozens of packets itrouters are run significantly below 100% utilization and its need some minor modification in TCP.They start with twoneed is not nearly strong. Other metrics like latency and jitter assumptions ,First have to modify the way in which themust be considered and it will definitely be improved with packets are transmitted by the TCP senders(assume the TCPsmall buffers. Second, the paper argues that if TCP flows are sender sends according to poison process).Second thesynchronized then we need large buffer to get high utilization network is over provisioned (i.e. the network will not beand if they are desynchronized then we can go for small congested even if each flow is sending at itsbuffer by statistical multiplexing. maximum).Under these assumptions the above mentioned buffer size is sufficient .Later these assumptions are relaxed. 21 All Rights Reserved © 2012 IJARCSEE
  • ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 4, June 2012 Here they have completely removed the dependency of the results are as follows. In access limited network the corebuffer size on band width delay product [8]. It appears from router speed is several orders of magnitude larger than accessthe simulation thought it have not prove that how much link router speed. First they have derived the buffer sizecapacity is lost. But it can be compensated by making the requirement for a network with fixed number of flows. Thenrouter running faster than the link rate. using AFCT as metric they consider the departure and arrival of the files and show that the core router is not congested due to access speed limitations, i.e. the number of packet drops in core router is less than the number of packet loss in access VI. SOME OPEN ISSUE’S IN BUFFER SIZING OF ROUTERS router. All the previous discussions suggest that the router buffer Theoretical analysis indicates that core router is chosencan be made very small, much less that link’s bandwidth independently of its capacity .To verify this claim ,thedelay product. But through simulations[3] it is shown that the performance of the system was studied at differentuse of such small buffer’s can lead to excessively high loss core-routers speeds without changing the traffic parametersrates in congested link that carry many flows. Small buffers or the access-speed limitations .The external load was chosencan lead to lower throughput even if the link is fully utilized to be 0.8.The core-router buffer size was varied from 20 tofor most large TCP flows .Through simulations[3] it is clear 1000 packets .The results of the simulations are presented inthat the Stanford model mentioned in [1] lead to high loss rate Fig.1 From the figure ,it is clear that core-router buffer sizeup to 5%-15% in congested access links that carry many has no significant affect on AFCT of the flows (because theflows. The key issues in buffer sizing like whether the link is core router is not congested), thus small buffers(i.e. O(1)saturable or not ,the exact type of the traffic(whether Buffering) are sufficient.persistent ,open loop ,closed loop) and the way to estimate For unlimited access networks they study a scenario inthe number of long TCP flows in a link are discussed .These which the access routers do not limit the transmission rate ofissues are ignored in all the previous discussions. the TCP but they change the access speed to 30 Mb/s. In this Simulations were run for each of the above mentioned setting, access-speed limitations do not limit TCPtraffic models(persistent traffic, open loop traffic ,closed throughput. Furthermore, due to the window flow control mechanism, any TCP connection cannot transmit more thanloop traffic)[3].The observation is that the loss rate is 12% for 64 packets within a RTT. To avoid access routers imposingopen loop,7-8% for persistent flows and closed loop traffic restrictions on TCP, the access-router buffer size was set towith small buffers. While with large buffers the loss rate drop 64 packets. Similar to the previous one, the core-router bufferto 1% for open loop traffic, about 2% for persistent flows and size was varied from 20 to 1000 packets. The load on the4% for closed loop traffic. Thus small buffers result in lower system was varied from 0.5 to 0.8.The simulation results andthroughput and higher loss rate for open traffic model. The the theoretical predictions are presented in Fig.2tradeoffs between loss rate and queuing delay are still an As seen from these results, small buffers in such networksopen issue yet to consider and that is discussed in the coming degrade the performance significantly. For example, whensection. the system load is 0.8, the AFCT can be decreased nearly 85% by increasing the buffer size from 20 to 1000 packets. Similarly, as seen in the simulations, the average throughput VII. IMPACT OF ARRIVAL AND DEPARTURES OF FILE ON increases by about 400% with the increase in the buffer size. BUFFER SIZING IN CORE ROUTERS So in unlimited access networks it is found impossible to All the previous discussion we were not considering the reduce the buffer size without degrading the performance.speed of the routers. But in paper [6] they consider the speed The analysis indicates that AFCT increases by nearly anof router.ie in that they re-examines the buffer size of the order of magnitude when small buffers are used.routers when flow arrives and depart. The discussion in thepaper will come to the conclusion that if the core-to-access–speed ratio is large then O(1) buffer size is requiredotherwise we have to go for large buffer. And one importantthink that they pointing out is that the depending upon thecore-to-access –speed ratio, the buffer size itself may affectthe number of flows in the system, so the two quantities i.e.buffer size and number of flows can’t be treated usindependent quantities. This was not pointed out in any of theprevious discussions. All the above results were under the assumption that thereare N long lived flow in the network. But in reality the Fig.1 Impact of O (1) buffering access limited networksnumber of flows can be varying .So the paper [6] focus on Scenario C=100Mbps,Ca=30Mbps, Avg RTT=50ms“Can the buffer size of the router be significantly reducedwith varying number of flows”. The performance metric theyare considering is AFCT (Average Flow completion Time). In order to find the amount of buffering required theyconsidering two types of network one access limited networkand other network with unlimited access for simulation and 22 All Rights Reserved © 2012 IJARCSEE
  • ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 4, June 2012 especially if we want to increase the throughput of TCP.Otherwise if Γ>1 the loss rate drops with buffer size so the optimal buffer size is small (close to zero). All these are proved through simulations. The difference between small and large flows are discussed and they identified two models for throughput of TCP flows depending on whether a flow lives in mostly in slow-stat(S-model) or in congestion avoidance (L-model).Using these two models along with loss rate and queuing delay they derived the optimal buffer size .For more realistic queuing models they conducted a simulation study [7] which examine the average queuing delay d(B) and loss probability p(B) as a function of buffer size B under heavy load conditions. Then they used the models for d (B) and p (B) to derive the optimal buffer size. IX. CONCLUSION Fig. 2. Impact of core-router buffer size in networks with fast access routers (top) simulations and (bottom) theory. Our study concludes by pointing out that it may not be possible to derive a single universal formula to dimension So the conclusion is that when core routers are severely buffers at any routers interface in a network. Such a formulacongested it is not possible to use small buffers in fact we may not exist in practice. The optimal buffer size at the routerrequired O(C×RTT) buffers to maintain good performance. depends on several parameters. The network administratorThe paper [6] studies the impact of buffer size on short flows. should take into account several factors such as flow sizeThe model for short flows is developed in two steps 1) Study distribution, nature of TCP traffic, out- put/input capacity ratios, speed, window size etc for sizing router buffers.the impact of buffer size on packet loss probability. 2) Studythe impact of loss probability on AFCT. REFERENCES All the flows less than 90Kb are considered as short flowsand simulations prove that they required a small buffer size [1] G. Appenzeller, I. Keslassy, and N. McKeown, “Sizing router.For example even with flow size of 127 kb the core router buffers,” in Proc. ACM SIGCOMM, 2004, pp. 281–292. [2] R. Bush and D. Meyer. RFC 3439: Some internet architecturalrequires no more than 2.4 MB of buffering to maintain a loss guidelines and philosophy, December 2003.rate of 0.01%.And loss rate of 0.01% do not cause significant [3] A. Dhamdhere and C. Dovrolis,”Open issues in router buffer sizing”degradation in AFCT. Comput.Commun.Rev, vol.36, no.1, pp.87-92, Jan 2006. [4] M. Enachescu, Y. Ganjali, A. Geol, T. Roughgarden, and N. McKeown “Part III: Routers with very small buffers,” Comput.Commun.Rev,vol 35,no.3,pp.7-7,Jul.2005.VIII. THE ROLE OF OUTPUT/INPUT CAPACITY RATIO IN SIZING [5] T. Kelly.On engineering a stable and scalable TCP variant Technical ROUTER BUFFER FOR TCP TRAFFIC Report CUED/F-INFENG/TR.435.Cambridge University Engineering Department.2002. Apart from all the above discussions which were focusing [6] A. lakshmikantha, C. Beck, and R. Srikanth, “Impact of File Arrivalson only persistent flows [7] considers the non persistent and Departures on Buffer Sizing in Core Routers”. April 2011. [7] R .S .Prasad, C .Dovrolis and M. Thottan, “Router Buffer Sizing forflows and the impact on it on buffer sizing. The two main TCP Traffic and the Role of the Output/Input Capacity Ratio”.things discussed in the paper [7] are First, it considers non [8] G. Raina, D. Towsley, and D. Wischik,.”Part II: Control theory forpersistent flows with heavy tailed file distribution which was buffer sizing.” Comput.Commun. Rev, vol.35.no.3.pp.79-82, Julnot considered in [6].Second, the main consideration is given 2005. [9] C. Villamiazar and C. Song,”High Performance TCP into TCP performance and its impact on buffer sizing rather ANSNET,”Comput. Commun. Rev. Vol 24, no.5, pp 45 -60, 1994.than focusing on link metrics. Through simulations done in [10] D.Wischik and N.mcKeown,”Part I: Buffer sizes for core routers”.[7] they came to the conclusion that buffer size depends on Comput.Commun. Rev.vol.35, no.3, pp.75-78, Jul 2005.output/input capacity ratio.i.e. If the ratio greater than one theloss rate drops exponentially with the buffer size and if theratio is less than one then significant buffering is needed. The paper [7] also focus on structural characteristics of thelink that was ignored in most of the papers except [4][6].The Neetha K.Nataraj is presently Pursuing MEcharacteristics is the output/input capacities. Let Cout be the in Computer Science & Engineering from SNS College of Technology, Anna University-output capacity and Cin be the input capacity. Then it turns Coimbatore, Tamilnadu, India. She received theout that the ratio Γ=Cout/Cin largely determines the relation BTech degree from TocH Institute of Science &between loss probability and buffer size and consequently, Technology, Cochin University, Kerala. Herthe relation between TCP throughput and buffer size .If Γ<1 interested area is networking.the loss rate can be approximated by a power law of thebuffer size then we need a significant amount of buffering 23 All Rights Reserved © 2012 IJARCSEE
  • ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 4, June 2012 Professor Dr.S.Karthik is presently Professor & Dean in the Department of Computer Science & Engineering, SNS College of Technology, affiliated to Anna University- Coimbatore, Tamilnadu, India.. He received the M.E degree from the Anna University Chennai and PhD degree from Ann University of Technology, Coimbatore. His research interests include network security, web services and wireless systems. In particular ,he is currently working in a research groupdeveloping new security architectures and active defense systems againstDDoS attacks. Dr.S.Karthik published more than 35 papers in refereedinternational journals and 25 papers in conferences and has been involvedmany international conferences as Technical Chair and tutorial presenter. He isan active member of IEEE, ISTE, IAENG, IACSIT and Indian ComputerSociety. 24 All Rights Reserved © 2012 IJARCSEE