CLASS 30.pdf

COMPUTER COMMUNICATION NETWORKS (UE22EC351A) Department of Electronics and Communication Engineering 1 COMPUTER COMMUNICATION NETWORKS Unit 2 – Transmission Control & IP – Class 30 - Principles of reliable data transfer: Stop and Wait...

COMPUTER COMMUNICATION NETWORKS (UE22EC351A) Department of Electronics and Communication Engineering 1 COMPUTER COMMUNICATION NETWORKS Unit 2 – Transmission Control & IP – Class 30 - Principles of reliable data transfer: Stop and Wait protocols Department of Electronics and Communication Engineering 2 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer ▪ Text Book Reference – Section 3.4.1 – pages – 230 to 241 3 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer Introduction Packet loss is said to occur due to: ▪ Corrupt packet discarded at the receiver ▪ Packet was discarded at a router due to lack of buffer space ▪ Packets experience long queuing delays in a router ▪ Reliable data transfer (RDT) ▪ It is a fool-proof mechanism for overcoming packet loss ▪ It requires a connection oriented approach (i.e., sender and receiver must agree to some parameters for monitoring packets using some handshaking) ▪ Throughput and delay may be compromised ▪ By default, UDP does not guarantee reliable data transfer ▪ TCP offers reliable rata transfer (the only QoS guaranteed) 4 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer System model ▪ Consider two hosts A (Sender) and B (Receiver) which wish to communicate over a network reliably ▪ Reliable data transfer between hosts A and B is achieved when they agree to monitor the packets exchanged and notify one another if packet losses are detected ▪ This is accomplished by some handshaking between A and B before data packets are exchanged ▪ We build the principles of reliable data transfer systematically ▪ We start with an ideal case and incrementally add complexity to the transport layer protocol ▪ Hereafter, the transport layer protocol is referred to as RDT(Reliable Data Transfer) protocol 5 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer Overview of the stages of development of the RDT protocol Stop and wait RDT protocols: ▪ Host A sends one packet at a time ▪ Host A waits for an acknowledgement from the host B to transmit the next packet ▪ We will see four versions of the stop and wait RDT protocols with each version addressing one limitation of its predecessor ▪ Pipelining RDT protocols: ▪ Host A sends multiple packets to host B at a time ▪ Host A waits for the acknowledgements from host B within a fixed time interval (referred to as a Timeout) ▪ Upon learning successfully delivery of the packets in previous round, the next batch of packets are transmitted 6 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer ▪ Pipelining RDT protocols (contd.): ▪ We will see two versions of the pipelining RDT protocols (GBN (Go-Back N and SR(Selective Repeat) ). ▪ Compared to the stop and wait RDT protocols, these pipelining RDT protocols provide better link utilization ▪ Transmission control protocol (TCP): ▪ It is a hybrid of the above pipelining RDT protocols but with sophistication of its own. ▪ TCP is robust compared to the above pipelining RDT protocols. ▪ TCP makes host A adaptive to network congestion and packet overflow problems that may occur at host B. 7 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer Stop and wait RDT protocol – Version 1 8  COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer Stop and wait RDT protocol – Version 1 (RDT1.0): ▪ No bit errors and no packet delays ▪ What should be the role of the RDT protocol in this context? ▪ An application in host A generates a message. ▪ Host A segments the message into several packets. ▪ Transport layer in host A inserts the source and destination port numbers (i.e., encapsulation) and passes it to network layer. ▪ The network layer protocol delivers the datagram to host B. 9  COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer Stop and wait RDT protocol – Version 2 (RDT 2.0): ▪ Bit errors in packet transmissions from A to B but no packet delays. ▪ How should RDT1.0 be modified to handle the above issue? ▪ Host A introduces checksum (i.e., error detection code) into the packet before passing it to the network layer. ▪ Host B verifies if packet is corrupt or not. ▪ If packet is corrupt, then host B sends an NAK packet; otherwise host B sends an ACK packet. ▪ If host A receives NAK packet, it retransmits the old packet. ▪ If host A receives ACK packet, it retransmits next packet with new checksum. 10 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer Stop and wait RDT protocol – Version 3 (RDT2.2): ▪ Bit errors in two way packet transmissions but no packet delays. ▪ Now, it is difficult to distinguish between old packets and new packets exchanged between A and B. ▪ Two types of packets are used by host B (ACK and NAK) which is unnecessary given that they may get corrupted as well ▪ How should RDT2.0 be modified to handle the above issues? ▪ Hosts introduce sequence numbers to identify packets (0 and 1 used alternatively) besides using the checksum to detect errors. ▪ Host B just uses ACK packets with sequence number (0 or 1) ▪ Example: Host A sent packet 0, then it deems the transmission successful only when an ACK packet with sequence number 0. Otherwise, Host A retransmits the packet 0. 11 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer Stop and wait RDT protocol – Version 4 (RDT3.0): ▪ Bit errors and packet delays occur in two way packet transmissions. ▪ Now, host A may end up waiting endlessly hoping the packet is stuck in some intermediate router’s queue ▪ How should RDT2.2 be modified to handle the above issues? ▪ Host A starts a timer as soon as it transmitted a packet using a sequence number and checksum. ▪ Host B replies just as in RDT2.2 depending on whether the received packet is corrupt, new or old. In case ACK packet is lost/corrupt/old, host A retransmits after timer expires. ▪ Otherwise, host A sends the next packet and starts timer 12 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer 13 COMPUTER COMMUNICATION NETWORKS Unit 2 – Class 30 - Principles of reliable data transfer 14 THANK YOU Department of Electronics and Communication Engineering 15