November 26, 2002TeleSim Research Group1 Performance and Robustness Testing of Wireless Web Servers Guangwei Bai Kehinde Oladosu Carey Williamson

November 26, 2002TeleSim Research Group2 1. Introduction and Motivation  Observation: the same wireless technology that allows a Web client to be mobile also allows Web servers to be mobile  Idea: portable, short-lived, ad hoc networks  Possible applications: o classroom area networks, seminars o press conferences, media events o sporting events, gaming, exhibitions o conferences and trade shows o disaster recovery sites, field work, etc.

November 26, 2002TeleSim Research Group3 Background: Portable Networks  Assumptions: the characteristics of a portable short-lived network are: o set it up when needed; tear down after o only needed for minutes or hours o when may not be known a priori o where may not be known a priori o no existing infrastructure of any kind o general Internet access not available o general Internet access not required o pre-defined content; target audience o users; mobile; limited bw needed

November 26, 2002TeleSim Research Group4 2. Objectives  to assess feasibility of portable networks  to benchmark the performance capabilities and limitations of an Apache Web server in a wireless ad hoc network  to identify the performance bottlenecks  to understand impacts of different factors o number of clients o Web object size o persistent connections o transmit power (energy consumption) o wireless channel conditions

November 26, 2002TeleSim Research Group5 3. Experimental Setup Compaq Notebooks (1.2GHz Pentium III, 128MB RAM, 512 KB L2 cache, Cisco Aironet 350 network cards) RedHat Linux 7.3, httperf, Apache , SnifferPro 4.6 Network: 11 Mbps IEEE b wireless LAN, ad hoc mode

November 26, 2002TeleSim Research Group6 Experimental Setup (Cont’d) IEEE b: a standard for wireless LANs Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), up to 11 Mbps data rate at physical layer ad hoc mode frames are addressed directly from sender to receiver httperf Web benchmarking software tool developed at HP Labs Web server: Apache (version ) Process-based, flexible, powerful, HTTP/1.1-compliant SnifferPro 4.6 real-time capture, recording all wireless channel activity, enabling protocol analysis at MAC, IP, TCP and HTTP layers

7 4. Experimental Design Factor Levels Number of Clients 1, 2, 4 HTTP Transaction Rate (per-client)10, 20, 30, …, 160 HTTP Transfer Size (KB)1, 2, 4, 8, …, 100 Persistent Connectionsno, yes HTTP Requests per Connection1, 5, 10, 15, …, 60 Transmit Power (mW)1, 5, 20, 30, 50, 100 Client-Server Distance (m)1, 10, 100 Impacts of different factors on wireless Web server performance (one-factor-at-a-time) Experimental Factors and Levels Performance metrics – HTTP transaction rate, throughput, response time, error rate at Application Layer, – TCP connection duration at Network Layer – Transmit queue behaviour at Link Layer,

November 26, 2002TeleSim Research Group8 5. Measurement Results and Analyses - Expt 1: Request Rate - Expt 2: Transfer Size - Expt 3: Number of Clients - Expt 4: Persistent Connections - Expt 5: Transmit Power - Expt 6: Wireless Channel

November 26, 2002TeleSim Research Group9 Experiment 1: Request Rate Purpose : to determine the range of feasible and sustainable loads for the wireless Web server Design : Number of Clients: 1 HTTP transaction rate: 10, 20, …, 160 req/sec HTTP transfer size: 1 KB (fixed) Persistent connections: no Transmit power: 100 mW Client-server distance: 1 meter (on same desk)

November 26, 2002TeleSim Research Group10 Wireless Web Performance at Application Layer Main observation: As the offered load increases: linear increase  instability  lower plateau Peak throughput < 1 Mbps for 1 KB transfers

November 26, 2002TeleSim Research Group11 Transmit Queue Behaviour for Experiment 1 Main observation: Wireless LAN is the bottleneck Packet drops occur from link-layer queue (client side) Even before they get on the wireless LAN!!! Reason: No flow control / backpressure mechanism Note: default queue size is 100 in the Linux kernel

November 26, 2002TeleSim Research Group12 Wireless Web Performance at Application Layer (Cont’d) Main observation: the response time is about 9 ms at low load, increase significantly to over 2 sec at high load (>85 req/sec) failures occur frequently under overload

November 26, 2002TeleSim Research Group13 Measurement at Network Layer Overload: 100 req/sec Queue buildup,Packet drops, Retransmissions,TCP resets Low load: 10 req/sec Stable performance Mean: 9.7ms Medium load: 50 req/sec Greater variation, 2 spikes Mean: 10ms High load: 80 req/sec More variability, some spikes, slight skew

November 26, 2002TeleSim Research Group14 Experiment 2: Transfer Size Purpose : to study impact of HTTP response size Design : Number of Clients: 1 HTTP transaction rate: 10 req/sec (fixed) HTTP transfer size (KB): 1, 2, 4, 8, … Persistent connections: no Transmit power: 100 mW Client-server distance: 1 meter (on same desk)

November 26, 2002TeleSim Research Group15 Measurement at Network Layer General observation: as HTTP transfer size increases, mean TCP connection duration increases, as does the variance of distribution.

November 26, 2002TeleSim Research Group16 Light load: 8 KB Duration: 24 msec Throughput: 2.8 Mbps Overload: 64 KB Duration: >100 msec Throughput: 4.1 Mbps Medium load: 32 KB Duration: 67 msec Throughput: 3.9 Mbps Measurement at Network Layer

November 26, 2002TeleSim Research Group17 Experiment 3: Number of Clients Purpose : to study impact of high load generated by multiple clients Design : Number of Clients: 2, 3, 4 HTTP transaction rate: 10, 20, …, 160 req/sec HTTP transfer size: 1 KB (fixed) Persistent connections: no Transmit power: 100 mW Client-server distance: 1 meter (on same desk)

18 Wireless Web Performance at Application Layer (4 Clients)

November 26, 2002TeleSim Research Group19 Main observation: 4 clients share network and server resources equally 30% higher aggregate throughput (110 conns/sec) bottleneck is now at server network card (drops!!) Wireless Web Performance at Application Layer (4 Clients)

November 26, 2002TeleSim Research Group20 Wireless Web Performance at Application Layer (2 or 3 Clients)

November 26, 2002TeleSim Research Group21 Wireless Web Performance at Application Layer (2 or 3 Clients) Main observation: unfairness problem at high loads: one client obtained a higher proportion of the throughput at expense of another (don’t know why?)

November 26, 2002TeleSim Research Group22 Experiment 4: Persistent Connections Persistent Connections: Multiple HTTP transactions can be sent on the same TCP connection. amortize overhead of TCP connection processing reduce memory consumption for TCP state Purpose of this experiment: to study impact of persistent connection on wireless Web performance Design: Number of Clients: 1 and 2 HTTP transaction rate: 10 req/sec (fixed) HTTP transfer size: 1 KB (fixed) Persistent connections: yes Transmit power: 100 mW Client-server distance: 1 meter (on same desk)

November 26, 2002TeleSim Research Group23 Achieved Throughput for Experiment with Persistent Connections Main observation: Peak throughput: 3.22 Mbps, 3.5x improvement over non-persistent connections (0.9 Mbps), two clients share the server and network resources equally

November 26, 2002TeleSim Research Group24 Experiment 5: Transmit Power Energy consumption- an important issue for mobile Clients and Server. Purpose : to see what transmit power is required for acceptable performance in classroom setting Design : Number of Clients: 1 HTTP transaction rate: 10 req/sec (fixed) HTTP transfer size: 1 KB (fixed) Persistent connections: no Transmit power: 1, 5, 20, 100 mW Client-server distance: 10 meter (same floor)

November 26, 2002TeleSim Research Group25 Measurement at Network Layer General observation: If transmit power<10 mW: MAC-layer retransmits rightward skew unacceptable perf. If transmit power  20 mW: acceptable performance

November 26, 2002TeleSim Research Group26 Experiment 6: Wireless Channel Characteristics Wireless Internet is characterized by limited bandwidth, high error rates, and interference. Purpose : to study the impact of the wireless channel characteristics on wireless Web performance Design : Number of Clients: 1 HTTP transaction rate: 10 req/sec (fixed) HTTP transfer size: 1 KB (fixed) Persistent connection: no Transmit power: 100 mW Client-server distance: 1m, 10m, 100m

November 26, 2002TeleSim Research Group27 Low load: 10 req/sec Significant skew to the tail of the distribution, Some periodicity (why?) Medium load: 50 req/sec Significant skew to the tail of the distribution Measurement at Network Layer (100m scenario)

November 26, 2002TeleSim Research Group28 6. Summary and Conclusions  What we did: wireless Web server, portable nw Application-layer measurements (httperf) Network-layer measurements (Wireless Sniffer)  Our results show: Server capability: 100 conn/sec for non-persistent HTTP with throughputs up to 4 Mbps (adequate?) Bottleneck: at wireless network interface Some “network thrashing” for large HTTP transfers when the network utilization is high (aborts, resets) Effect of wireless channel on performance at TCP and HTTP-level (MAC-layer retransmits) Power consumption issue for mobile client and server

November 26, 2002TeleSim Research Group29 7. Future Work  Explaining the anomalies (fairness, periodicity)  Better system instrumentation (Linux)  More realistic Web workloads  Larger WLAN testing (classroom scenario)  Repeat experiments with IEEE a (55 Mbps)  Kenny’s M.Sc. Thesis...  Another paper?