CSE 489/589 Programming Assignment 2

Transcription

1 CSE 489/589 Programming Assignment 2

2 Objective In a given simulator, implement three reliable data transport protocols: Alternating-Bit (ABT), Go-Back-N (GBN), and Selective- Repeat (SR). 2

3 Outline Part 0 An Overview to Reliable Transport Protocols Part 1 How to implement your protocols Programming environment Code Structure The Routines You Will Write The Alternating-Bit Version Part 2 Analysis and Report 3

4 Part 0: An Overview to Reliable Transport Protocols 4

5 The Alternating-Bit (rdt 3.0) Assumption: underlying channel can lose and corrupt packets (data or ACKs) checksum, seq. #, ACKs, retransmissions will be of help. Page 216,217 Approach: sender waits reasonable amount of time for ACK retransmits if no ACK received in this time if pkt (or ACK) just delayed (not lost): retransmission will be duplicate, but use of seq. # s already handles this receiver must specify seq # of pkt being ACKed requires countdown timer 5

6 Go-Back-N Sender window of up to N, consecutive unack ed pkts allowed timer for oldest unacked pkt Page 223 timeout(n): retransmit pkt n and all higher seq # pkts in window may receive duplicate ACKs (see receiver) 6

7 Selective-Repeat Receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer Sender only resends pkts for which ACK not received sender timer for each unacked pkt Sender window N consecutive seq # s again limits seq #s of sent, unack ed pkts Page 228 7

8 Part 1: How to implement your protocols 8

9 Programming Environment In a simulator close to an actual UNIX environment Simulator Sender - A Receiver - B Simulator 9

10 Simulator vs. Real Environment 10

11 Real Environment (PA1) Sending/receiving process run on different hosts A and B Hosts A and B communicate over the network Set of routers/links with packet loss/corruption characteristics User has no control over them Application implemented in user space, transport/network layer implemented in the kernel App/transport talk to each other through sockets Each host has its own local clock/time 11

12 Simulator (PA2) Simulated hosts A and B run on single real host Hosts A and B communicate over simulated network User specifies packet loss/corruption All layers implemented in user space Both simulated hosts have the same time measured in time units get_sim_time() Simulated time different from real time! 12

13 Design Goals To insure the data generated from sender A s application layer is delivered in-order and correctly to the receiver B s application layer Simulator Data packets will get lost or corrupted at lower layers! 13

14 Code Structure A Simulator for Transport and lower layers 14

15 The Routines You Will Write A_output(message); A_input(packet); A_timerinterrupt(); A_init(); B_input(packet); B_init(); 15

16 The Alternating-Bit Version Take a journey when there s no packet loss/ corruption No timer in this journey 16

17 The Alternating-Bit Version data received at B s app layer Simulator tolayer5() Simulator data generated at A s app layer A_output() tolayer3() Simulator ACK received by A A_input() Simulator B_input() tolayer3() 17

18 Messages and packets structure 18

19 Functions A_output() Called whenever the upper layer at sender A has a message to send Function parameters - message 19

20 Functions A_output() Will call tolayer3() inside A_output(message) { tolayer3(0, packet); } 20

21 The Alternating-Bit Version data received at B s app layer Simulator tolayer5() Simulator data generated at A s app layer A_output() tolayer3() Simulator ACK received by A A_input() Simulator B_input() tolayer3() 21

22 Functions tolayer3() Function parameters AorB, packet AorB int AorB AorB = 0 [when being called by A] AorB = 1 [when being called by B] packet struct pkt packet 22

23 Functions struct msg message struct pkt packet The only two types of data can be recognized by the simulator. Both data packets and ack packets are of struct pkt type. Both data packets and ack packets need checksum Don t change the definitions of struct msg or struct pkt 23

24 Functions Checksum Any content - payload, seqnum, acknum or checksum can be corrupted by the simulator s lower layers. It should include the data, seqnum and acknum fields. 24

25 Question If the data generating speed at A s app layer is higher than the speed at which lower layers send packets, what should we do? Buffer! 25

26 The Alternating-Bit Version data received at B s app layer Simulator tolayer5() Simulator data generated at A s app layer A_output() tolayer3() Simulator ACK received by A A_input() Simulator B_input() tolayer3() 26

27 Functions B_input() Function parameters - packet Validate the checksum first May call tolayer3() and tolayer5() inside B_input(packet) { // validate the checksum; // if the checksum is correct // send ack to A tolayer3(1, ack); // deliver the data to upper layer tolayer5(1, payload); } 27

28 Functions tolayer5() Function parameters AorB, datasent AorB int AorB AorB = 0 // called by A AorB = 1 // called by B datasent // different from tolayer3() here char datasent[20] // payload 28

29 The Alternating-Bit Version data received at B s app layer Simulator tolayer5() Simulator data generated at A s app layer A_output() tolayer3() Simulator ACK received by A A_input() Simulator B_input() tolayer3() 29

30 Functions A_input() Function parameters - packet Validate the ack s checksum No need to deliver the ack to upper layer 30

31 The Alternating-Bit Version The journey is complete! 31

32 Packet corruption and Loss If packet corruption and loss happen at the lower layers, what should we do? NAK is not allowed to be used in this assignment Retransmit with timer! 32

33 Timer starttimer(calling_entity, increment) Starts a timer. Function parameters AorB, increment AorB int AorB AorB = 0 // called by A AorB = 1 // called by B Increment float increment Only ONE timer provided by the simulator!

34 Timer stoptimer(calling_entity) Its usage is similar to starttimer() Stops the timer A_timerinterrupt() No function parameter Will be called when A s timer expires

35 Questions What value should we use in the timer? Hint: A packet sent into the network takes an average of 5 time units to arrive at the other side when there are no other packets in the medium. Where should we start and stop the timer? This will be different in the 3 protocols

36 About the Simulator When will the simulation end? The simulation is controlled by the number of messages generated by A s app layer the number of packets you type in before the simulation start The simulator will stop as soon as that number of messages have been passed down from A s app layer.

37 About the Simulator What is the average time between messages from sender s layer 5? Controls the source data rate Non zero, positive value Smaller the value, faster the packets will be arriving to the sender s transport layer. Be careful about the relationship between the buffer size and this value Sender A shouldn t drop any message at the transport layer Best is to use a buffer size of 1000

38 Analysis: Result Application_A #Packets sent from the Application Layer of Sender A Transport_A #Packets sent from the Transport Layer of Sender A Transport_B #Packets received at the Transport Layer of Receiver B Application_B #Packets received at the Application Layer at Receiver B Throughput Application_B/Simulation_time

39 Analysis: Experiments PA2 Analysis part carries significant points. PA2 handout lists out 2 experiments (see section 6). We of course encourage you to do more! You NEED to use the run_experiments script, provided with the template. Do NOT run the binaries manually for analysis. The script will give you 10 results (different seeds). You may take average for reporting.

40 Questions?