Experimental Experiences with Emulating Multihop Path using Dummynet

Transcription

1 Experimental Experiences with Emulating Multihop Path using Dummynet Thuy T.T. Nguyen Introduction Motivations How to emulate multihop path using Dummynet Test setup & Results Analysis Conclusions 13/04/2005 Page 2 Swinburne University of Technology 1

2 Motivations Investigate the network conditions (e.g. congestions, bottleneck bandwidth, multihop path, cross traffic) on flow traffic statistical characteristics The need of simulating multihop path with different hop configurations, testing with live traffic Emulating multihop path using Dummynet FreeBSD kernel-resident tool for traffic shaping with bandwidth, delay, loss configurations? 13/04/2005 Page 3 How to emulate multihop with dummynet Source N Destination Source host Traffic coming in Bridge FreeBSD - Dummynet Pipe 1 Pipe 2 Pipe 3 Pipe N Traffic going out Destination host Configuration: Chage sysctl variable net.inet.ip.fw.one_pass: /04/2005 Page 4 Swinburne University of Technology 2

3 Test Setup and Results Host 1 FreeBSD Mbps Bridge FreeBSD Mbps Host 2 FreeBSD Emulate 1, 2,, 10 hops between host1 and host2 All pipes have same configurations ( and delay) Ping traffic with different Ping packet sizes Clock precision at all machines = 1000 Path between host1 and host2 is full-duplex (different pipe for incoming and outgoing traffic) 13/04/2005 Page 5 Test Setup and Results Host 1 FreeBSD Host 2 FreeBSD Pipe i Pipe i+1 Hop i configurations: ipfw pipe {i} config {1, 2}Mbps delay {0 10}ms ipfw pipe {i + 1} config {1, 2}Mbps delay {0 10}ms ipfw add {i} pipe {i} icmp from to any ipfw add {i + 1} pipe { i + 1} icmp from any to /04/2005 Page 6 Swinburne University of Technology 3

4 Test Setup and Results Host 1 FreeBSD Mbps Bridge FreeBSD Mbps Host 2 FreeBSD Ping from Host 1 to Host 2 Ping Interval 1sec Ping Packet Size 64, 128, 256 bytes Configured at bridge: BW 1, 2, 4 Mbps, Delay 0, 10ms 13/04/2005 Page 7 Test Setup and Results BW 1Mbps, Delay 0ms BW 2Mbps, Delay 0ms BW 4Mbps, Delay 0ms BW 1Mbps, Delay 10ms BW 2Mbps, Delay 10ms BW 4Mbps, Delay 10ms /04/2005 Page 8 Swinburne University of Technology 4

5 Test Setup and Results RTT BW 1Mbps, Delay 0ms RTT BW 2Mbps, Delay 0ms RTT BW 4Mbps, Delay 0ms RTT BW 1Mbps, Delay 10ms RTT BW 2Mbps, Delay 10ms RTT BW 4Mbps, Delay 10ms Tp Ping(64, 128, 256, BW 1Mbps) = 0.512, 1.02, 2.05 () Tp Ping(64, 128, 256, BW 2Mbps) = 0.256, 0.512, 1.02 () 13/04/2005 Page 9 Analysis Dummynet internal timing and scheduling implementation for one hop: Ready_event() Transmit_event() Flow Queue Delay Line Packet In Packet Out t 0 t 1 t 2 t 0 : Time packet arrives and is placed into queue t 1 : Time packet is pulled out of the queue by the scheduler to put into delay line = Time packet entering the delay line t 2 : Time packet is transmitted out of pipe 13/04/2005 Page 10 Swinburne University of Technology 5

6 Analysis Pipe configurations: = bandwidth (Bits/sec) p_delay = delay () p_len = packet length in bytes Let t last_sched = time the first packet arrives, or the previous scheduler event fired (simulator virtual time, unit clock ticks) t0 = the packet current (arrival) time (simulator virtual time, unit clock ticks) 13/04/2005 Page 11 Analysis Number of bytes allowed to be extracted at t 1 is calculated based on the cumulative bandwidth from the last scheduled event. Dummynet calculation [3]: q _ numbytes = ( t t ) 0 last _ sched Then, the number of clock ticks that the packet needs to wait before being put into the delay line is: q _ numticks = p _ len 8 q _ numbytes 1+ (1) 13/04/2005 Page 12 Swinburne University of Technology 6

7 Analysis The number of clock ticks that the packet needs to wait in the delay line is: dl _ numticks = p_ delay 1000 So the total clock ticks from packet coming-in to coming-out is: p_ len 8 q _ numbytes 1 + p _ delay pipe _ numticks = /04/2005 Page 13 Analysis Ambiguity about question (1) Possible explanation: = bits/sec BW = ( bytes / tick) 8 No of bytes allowed to transmit at t0: ( t t ) BW 0 last _ sched The rest of the packet need to be sent after: pkt _ len ( t0 tlast _ sched ) BW ticks BW ( ) 8 H Z 0 last _ sched pkt _ len ( t t ) = 8 H Z pkt _ len 8 = ( t0 tlast _ sched ) pkt _ len 8 ( t0 tlast _ sched ) = pkt _ len 8 q _ numbytes = ( ticks ) 13/04/2005 Page 14 Swinburne University of Technology 7

8 Analysis So total number of ticks that packet needs to wait in the flow queue is : pkt _ len 8 q _ numbytes + 1( ticks) To avoid waiting for an extra tick in the case when bytes to send bytes allowed to be sent, adding a rounding factor. Therefore: q _ numticks = p_ len 8 q_ numbytes+ 1 Total number of ticks that packet needs to wait in the pipe: p_ len 8 q_ numbytes + 1 p_ delay pipe _ numticks = /04/2005 Page 15 Analysis Packet In t 0 t 1 t2 i t p i +1 Clock Ticks One-way delay for 1 hop (OWD): pipe _ numticks OWD = floor( t p + ) t p(sec) One-way delay for N hop (OWDN): OWDN = ( floor( t p + pipe numticks ) t p(sec)) + ( N 1) pipe numticks (sec) 13/04/2005 Page 16 Swinburne University of Technology 8

9 Analysis Question: What factors needed to take into account while experimenting with dummynet? 13/04/2005 Page 17 Analysis - More empirical results RTT for Ping PktSize 64 Bytes, BW 1Mbps, 1000 samples ecdf(rtt01[, 1]) rtt01[, 1] Fn(x) Index x 13/04/2005 Page 18 Swinburne University of Technology 9

10 Analysis More empirical results tp factor : Taking the microseconds part from pkt timestamp Q-Q plot Uniform Dist. - Tp forward Tp forward at Bridge, =1000 x.data0 tp1 x.runif0 Index BW1Mbps, Delay = 0ms, 1 hop 13/04/2005 Page 19 Analysis More empirical results Q-Q plot Uniform Dist. - Tp backward Tp backward at Bridge, =1000 x.data tp2 x.runif Index BW1Mbps, Delay = 0ms, 1 hop 13/04/2005 Page 20 Swinburne University of Technology 10

11 Analysis More empirical results a = transmission time of pkt at 100Mbps( s) t = packet interarrival time (ping interval) Ks and Kb = clock skewing factor at source and bridge Since the test environment is near-perfect synchronous condition, at Bridge, S, and D are equal = 1000, and t = sec Source Bridge t01 t11 = t01 + a tp1 t02 t12 = t01 + t + a + Ks + Kb tp2 = tp1+ Kb + Ks t03 t13 = t01+2 t + a + 2(Ks+ Kb) tp3 = tp1+ 2(Ks + Kb) And so on.tpn = tp1 + (N-1)(Ks+Kb) that s why the curve in previous Tp graphs 13/04/2005 Page 21 Analysis Tp Forward at Bridge, = 589 Tp Backward at Bridge, = Index at Source = at Destination = 1000 tp factor : assume tp[1] = 0, i 2: t[ i] = mod(( ti [ ] ti [ 1]) + t[ i 1]*1000) /( / 589)( ) p p Index *note: tp shown in the graph is calculated as relative to tp[1] 13/04/2005 Page 22 Swinburne University of Technology 11

12 Analysis ecdf(rtt0) rtt0 Fn(x) Index 12 x 13/04/2005 Page 23 Analysis Question: What factors needed to take into account while experimenting with dummynet? Answer: Dummynet s results might be affected by the real system, in which the components are usually asynchronous, e.g: Clock granularity differences between machines Clock skewness within a single machine and between different machines Kernel Load Interfering traffic 13/04/2005 Page 24 Swinburne University of Technology 12

13 Some implications Configure Delay with BW simultaneously RTT for Delay 5ms - 1 hop RTT for Delay 5ms - 5 hop BW100Mbps BW-Unlimited BW100Mbps BW-Unlimited index index p _ len 8 numbits 1 + BW p _ delay pipe _ numticks = + BW /04/2005 Page 25 Some Implications Configure Delay only (without BW) The higher the delay, the more precise the dummynet [1] p_ delay OWD = floor( + t p) t 1000 p 1 t p Error = 100% p _ delay when =1000 Real Delay is not influenced by the packet rate [1] Only true if the clock granularity of the sending source is the processing time of the packet & the clock granularity at the bridge Otherwise, would be affected by packet rate and packet sizes caused by queuing delay at the delay line 13/04/2005 Page 26 Swinburne University of Technology 13

14 Conclusions Dummynet can be used for multihop path emulation, however, need to be aware of the effects of testbed factors, e.g. clock granularity, skewness of machine s clock, synchronisation of simulation time and system time, load at server, source and bridge, precision of interface s timestamp More factors to investigate: cross traffic, pkt loss 13/04/2005 Page 27 References [1] W.A. Vanhonacker, " Evaluation of the FreeBSD dummynet network performance simulation tool on a Pentium 4-based Ethernet Bridge", CAIA Technical Report A, December 2003 [2] L. Rizzo, Dummynet: a simple approach to the evaluation of network protocols, ACM SIGCOMM Computer communications Review, Volume 27, Issue 1, January 1997 [3] Dummynet source code FreeBSD /04/2005 Page 28 Swinburne University of Technology 14

15 Acknowledgement Great thanks to Grenville and Lawrence! 13/04/2005 Page 29 THANK YOU! 13/04/2005 Page 30 Swinburne University of Technology 15