Performance Evaluation of TCP over WLAN with the Snoop Performance Enhancing Proxy

Transcription

1 Performance Evaluation of TCP over WLAN with the Snoop Performance Enhancing Proxy Case study Chi-ho Ng, Jack Chow, and Ljiljana Trajković Simon Fraser University 1

2 Roadmap Introducing the problem and the project scope TCP retransmission and window size Wireless LAN using TCP Snoop protocol OPNET implementation Conclusions 2

3 Problem formulation TCP is a reliable protocol with packet retransmission and congestion control (transmission window adjustment) Loss packets are seen as network congestion: packets are retransmitted using a smaller window size This scheme works well in wired networks In wireless networks, with the high bit error rate, TCP reduces window size excessively and under-utilizes the available bandwidth 3

4 Project goals Investigate the TCP congestion control policy and its performance in wireless LAN networks Find possible enhancing algorithms Investigate the Snoop protocol Implement Snoop protocol in OPNET Compare TCP performance with and without Snoop 4

5 TCP retransmission policy Sender Receiver First transmission Duplicate acknowledgements Retransmission ack1 ack1 ack1 ack For every packet received, the recipient returns an Ack Recipient sends duplicate Acks if a packet is lost Sender re-transmits lost packets 5

6 TCP transmission window Window size W Slope = 1/(round-trip time) W/2 TCP increases the window exponentially to determine the available bandwidth When the source fails to receive an acknowledgement, TCP reduces the window size by half. The source increases its window size by one unit every average round trip time 6 Time

7 Wireless LAN using TCP Host server Router Base station Mobile terminal 1. Host server establishes a TCP connection with mobile terminal and starts to send data 3. Missing acknowledgements trigger congestion control at host server 2. High bit error rate in the wireless channel 7

8 Adding the Snoop agent Host server Router Base station with Snoop agent Mobile terminal 1. Host server establishs a TCP connection with mobile terminal and starts to send data 3. Snoop agent re-transmits lost packets locally 2. High bit error rate in the wireless channel 8

9 Snoop protocol Snoop copies packets to its cache It starts a retransmission timer It retransmits the packets if a duplicate Ack is received or if the timer expires When an Ack is received, Snoop deletes the cache entry 9

10 OPNET implementation 10

11 Two additional protocol layers 11

12 PEG process model 12

13 Packet error generator: PEG Generates packet loss (uniformly distributed) Packets dropped by PEG at 30% packet error rate Total packets dropped Time (sec) Total sent Total dropped 13

14 Snoop agent process model 14

15 Snoop cache typedef struct { unsigned int src_ip; unsigned int dest_ip; int src_port; int dest_port; unsigned int seq_num; unsigned int ack_num; unsigned int rcv_win; int urgent_pointer; int data_len; int urg; int ack; int push; int rst; int syn; int fin; } TcpInfo; 15

16 Cache functions Function sncacheinit sncachedpkt sncacheretrieve sncachedestroy Description Initializes the cache records Copies a packet into the cache Retrieves a packet from the cache Deletes a packet from the cache 16

17 Snoop connection table typedef struct { /* Source IP */ unsigned int src_ip; /* Destination IP */ unsigned int dest_ip; /* Source Port */ int src_port; /* Destination Port */ int dest_port; /* Last sent seq number */ unsigned int last_seq_num; /* Last received seq number */ unsigned int last_ack_num; /* Determines if we have received repeat Acks */ int repeat_ack; /* Determines if we have received FIN packet */ int fin_flag; /* Seq num of the fin packet */ unsigned fin_seq_num; /* Timeout event handle */ Evhandle timeout_evt; } struct_sntable; 17

18 Snoop_Data() Packet arrives New packet? no Packet seq num > last ack num? yes 1. Forward packet 2. Reset lcoal retransmission timer Sender retranmission no yes Discard packet In sequence? no Forward packet yes Congestion loss 1. Cache packet 2. Forward packet Common case 18

19 Snoop_Ack() Ack arrives New ack? no yes Duplicate Ack? yes no 1. Free cache record 2. Forward Ack to sender Common case Discard packet False Ack First DupAck? no Discard packet yes Retransmit lost packets Repeated DupAck Packet loss 19

20 TCP parameters TCP Parameters Values Maximum Segment Size (bytes) 2264 Receiver Buffer Size (bytes) 8760 Receiver Buffer Usage Threshold 0.0 Delayed Ack Mechansim Segment/Clock Based Maximum Ack Delay 0.2 Fast Transmit Enabled Fast Recovery Enabled Selective Ack (SACK) Disabled Nagle SWS Algorithm Disabled Karn s Algorithm Enabled Retransmission Threshold Attempt Based Initial RTO 1.0 Minimum RTO 0.5 Maximum RTO 64 RTT Gain Deviation Gain 0.25 RTT Deviation Coefficient 4.0 Timer Granularity 0.5 Persist Timeout

21 Scenario 1: Single mobile upload Mobile 1 uploads a 100,000-byte file to the server 21

22 Scenario 1: parameters Parameters Setup Snoop Model Attribute in Mobile 1 Snoop Enabled Snoop Retransmission Timeout PEG Model Attribute in Mobile 1 Send_Data_Drop_Rate Recv_Data_Drop_Rate Snoop Model Attribute in Server Snoop Enabled Values 1 (Enabled) 1 sec Varied Varied 0 (Disabled) PEG Model Attribute in Server Send_Data_Drop_Rate 0 Recv_Data_Drop_Rate 0 22

23 Upload response time ~ 68 times improvements at 30% error rate Scenario 1: Upload reponse time Time (sec) Packet error rate (%) Snoop enabled Snoop disabled 23

24 Packet caching Transmission window: four packets Number of packets cached at the end: zero Number of cached packets Number of cached packets Time (sec) 24

25 Congestion window size Varies with time TCP congestion window size at 20% packet error rate Congestion window size (bits) Snoop enabled Time (sec) Snoop disabled 25

26 Sequence numbers Each data byte is represented by a sequence number Sequence numbers at 20% packet error rate Transmitted sequence number Snoop enabled Time (sec) Snoop disabled 26

27 Retransmission timeout Persist retransmission timeout period = 1 sec Retransmission timeout at 20% packet error rate Retransmission timeout interval (sec) Time (sec) Snoop enabled Snoop disabled 27

28 Scenario 2: Multiple mobile downloads Snoop enabled at the server 28

29 Scenario 2: parameters Parameters Setup Mobile 1 Snoop Enabled Values 0 (Disabled) PEG Model Attribute in Mobile 1 Send_Data_Drop_Rate 0 Recv_Data_Drop_Rate 0 Mobile 2 Snoop Enabled 0 (Disabled) PEG Model Attribute in Mobile 2 Send_Data_Drop_Rate 0 Recv_Data_Drop_Rate 0 Server Snoop Enabled 1 (Enabled) Snoop Retransmission Timeout (sec) 1 PEG Model Attribute in Server Send_Data_Drop_Rate Recv_Data_Drop_Rate Varied Varied 29

30 Download response time Snoop can handle multiple connections Average download response time Time (sec) Packet error rate (%) Snoop enabled Snoop disabled 30

31 Scenario 3: Multiple mobile uploads Snoop disabled on Mobile 1 and enabled on Mobile 2 31

32 Scenario 3: parameters Parameters Setup Mobile 1 Snoop Enabled Values 0 (Disabled) PEG Model Attribute in Mobile 1 Send_Data_Drop_Rate 0 Recv_Data_Drop_Rate 0 Mobile 2 Snoop Enabled 1 (Enabled) Snoop Retransmission Timeout (sec) 1.0 PEG Model Attribute in Mobile 2 Send_Data_Drop_Rate 0 Recv_Data_Drop_Rate 0 Server Snoop Enabled PEG Model Attribute in Server Send_Data_Drop_Rate Recv_Data_Drop_Rate 0 (Disabled) Varied Varied 32

33 Upload response time Mobile with Snoop has shorter upload time Upload response time Upload respnose time (sec) Packet error rate (%) Snoop enabled Snoop disabled 33

34 Conclusions We implemented Snoop and packet error generator OPNET models They were implemented between the ARP and IP OPNET nodes. Implementation required examination of the ARP and IP node source code and their separation Future work: Vary the retransmission timer based on calculated round trip delays in wireless links 34

35 References [1] IEEE Workgroup: [2] Performance Enchancing Proxy (PEP) Request for Comments (RFC): [3] Improving TCP/IP Performance over Wireless Networks: [4] W. Stevens, TCP/IP Illustrated, Volume 1. Reading, MA: Addison Wesley, Professional Computing Series, [5] A. S. Tanenbaum, Computer Networks, Third edition. Englewood Cliffs, NJ: Prentice-Hall, [6] Wireless LAN Model Description, OPNET Manual. 35