Computer networks unit iv
9 likes1,785 views
This document provides a summary of the transport layer of computer networks. It discusses key topics like UDP, TCP, connection management, flow control, congestion control, and quality of service. UDP provides a connectionless, unreliable service while TCP provides connection-oriented, reliable byte streaming. TCP uses three-way handshaking for connection establishment and termination. It implements flow control using sliding windows and employs congestion control algorithms like additive increase/multiplicative decrease. The document also covers transport layer concepts like ports, checksums, and retransmission methods in TCP.
Computer networks unit iv
- 1. RMK College of Engineering and Technology CS 6551 Computer Networks Department of Electronics and Communication Engineering
- 2. Unit 4 Transport Layer Prepared by Jai Ganesh S Asst.Professor - ECE
- 3. Syllabus • Overview of transport layer • UDP – User Datagram Protocol • Reliable Byte Stream – TCP • Connection management • Flow control • Retransmission • TCP Congestion Control • Congestion Avoidance • Quality of Service • Application Requirements.
- 5. Overview of Transport Layer • This layer is responsible for process to process delivery of entire message. • The system runs several process. Transport layer is responsible for delivering the message to the correct process. • This can be achieved by a special type of address called as “service point address”
- 6. IP address vs Port Address
- 7. Concept of PORT Addressing
- 8. TCP protocols • There are 2 main protocols used in this layer. • Connection Less – UDP • Connection Oriented - TCP
- 9. User Datagram Protocol • This is an connection less un reliable transport layer protocol. • They have very low error checking features.
- 11. Checksum Calculation • The UDP check sum has 3 different sections. • Pseudo header • UDP header • Data from application Layer
- 12. Functions of UDP • Connection less services • Flow control and error control • Encapsulation and de capsulation • Queuing
- 13. Uses of UDP • They are suitable for simple communications where error control and flow control are little concern • They are suitable for multicasting • They are well suited for updating the router information.
- 14. End of UDP
- 15. Reliable Byte Stream • In contrast to the unreliable transmission protocol we have a reliable byte stream . • Protocol used here is “Transmission Control Protocol” (TCP)
- 16. TCP Services • Process to process communication • Stream delivery service • Full duplex communication • Connection oriented service • Reliable service
- 17. 1. Process to process communication • There are few well known ports for the process identification.
- 18. 2. Stream delivery service • The data are sent as a stream of bytes. • This creates an virtual connection between the source and the destination.
- 19. • Full duplex communication: • TCP allows full duplex communication in which the data flow in both the directions at the same time. • Connection Oriented Services: • Data transmission occurs in 3 stages, • Connection is established between the source & destination • Data are exchanged between the source and destination • Connection is terminated. • Reliable Services: • TCP is an reliable protocol. • It uses an acknowledgement mechanism to check the safe and sound arrival of data
- 20. Features of TCP • Numbering system: • They have 2 types of numbering system • Sequence number - to identify the order of the segment • Acknowledgement number – used to conform the reception of segment and the next segment to be sent. • Flow control: • Controls the amount of data sent by the sender to the receiver. To avoid the loss of packets • Error control: • TCP implements the error control mechanism to ensure the correct delivery of data. • Congestion control: • TCP implements certain strategies to ensure there is no congestion on the network.
- 22. Connection Management in TCP • There are 3 phases in connection management. Connection Establishment Data transfer Connection termination
- 23. 1. Connection Establishment • Connection is established by a 3 way handshaking process. • Consider the scenario shown in the figure, where client need to establish a connection with the server.
- 24. 2. Data transfer • After the connection is established , data can flow in bi direction. • The client and server can send data and ACK • The figure shows the timing diagram for the data transfer phase.
- 25. 3. Connection Termination • Connection termination also happens in 3 way handshaking process. • The client ends the termination by sending FIN signal and the same is acknowledged from the server.
- 27. Flow Control in TCP
- 28. • TCP uses sliding window to handle the flow control • The major difference between the sliding window flow control in data link layer and the Transport layer is as follows • Byte oriented in TCP, Frame oriented in DLL • Window size is variable in TCP, Fixed in DLL • A window can be opened, Closed or Shrink.
- 29. • Opening a window: • Moving the right wall to the right. This allows new bits to be added for sending. • Closing a window: • Moving left wall to the right. Represents that the bytes are acknowledged. • Shrinking a window: • Moving right wall to the left. Revoking the eligibility of bytes for sending. Problem occurs when the bytes are already sent.
- 30. • Size of the window is determined by 2 parameters. • Receiver window (rwnd) – No of bytes that the receiver can accept. • Congestion window (cwnd) – No of bytes that the network can carry without any congestion. • The window size should be a function of rwnd and cwnd
- 31. Transmission Process • There are 3 mechanisms in transmission process. Deciding the “Maximum Segment Size” (MSS) Flushing the bytes into the buffer. Firing the Timer for retransmission purpose.
- 32. Silly Window Syndrome • Generally receiver decides the window size. • If the available data and receiver window size is same as MSS, no problem will occur. • In case the transmitter accumulates the data bytes and waiting for the window size from the receiver. • Now the receiver sends the window size as MSS/2 (Half the Size of the MSS). • Now the problem occurs at the transmitter that it has to send MSS/2 bytes of data….. Or to wait for some more period to get the full MSS window size. So that the complete data is transmitted. • This confusion leads to a problem called as “Silly Window Syndrome”. Solution for this problem is : “Nagle’s Algorithm”
- 33. • If sender waits too long --> bad for interactive connections. • If it does not wait long enough --> silly window syndrome. • How to solve ?
- 34. Nagle’sAlgorithm • If both available data and Window ≥ MSS, send full segment. • Else, if there is un ACK ed data in flight, buffer new data until ACK returns. • Else, send new data now.
- 36. Retransmission
- 37. • Reliable data transmission is achieved by retransmitting the segments. • The key factor in implementing the retransmission is deciding the Time Out Period • The default method for setting up the Time out period is double the period of Round Trip Time between any 2 nodes on the network
- 38. Problem with RTT • Unfortunately the RTT between any 2 nodes on the network gets varied because of several factors
- 39. Solution • To eliminate this problem, TCP uses an “Adaptive Retransmission Technique”. • Where the Time out Period is adjusted according to the situations. • Several algorithms were proposed on calculating the time out period • Original Algorithm • Karn / Parteidge Algorithm • Jacobson / Karels Algorithm
- 40. 1. Original Algorithm • This is an simple algorithm, in which the sample time is computed for every segment sent. • This sample time is related with the previously computed values and the final time out period is computed as shown below
- 41. Drawbacks of Original Algorithm • RTT cannot be simply calculated only based on the received ACK. • We cannot identify the received ACK is for the original Transmission or the Retransmitted Segment • Consider the given scenarios. • Case 1: ACK Sent by the Receiver is lost – segment retransmitted – received ACK is assumed for the original transmission – Sample RTT is too long. • Case 2: ACK is delayed by the Received – Segment Retransmitted – received ACK is assumed for the retransmitted segment – Sample RTT is too short.
- 42. 2. Karn / Parteidge Algorithm • This algorithm overcomes the drawback of the original algorithm. • This algorithm proposes that the RTT is computed only for the segments that are sent for the first time. (Not for the Retransmitted Segments) • The Final time out is computed based only on the sample RTT’s of first time sent segments
- 43. 3. Jacobson / Karels Algorithm • This algorithm serves better than the predecessor method. • This algorithm considers the variance of sample RTT’s in to account. • Variance -- ?
- 44. What’s Mean
- 45. What’s Variance
- 49. Congestion Control • It is very important factor to control the congestion on the network. • Congestion occurs when the network is overloaded. • Thus to control the congestion the flow of packets should be controlled according to the network capacity. • There are several techniques available. Out of which few techniques are discussed here. • Adaptive Increase and Multiplicative Decrease. • Slow Start • Fast Retransmit and Fast Recovery
- 50. Adaptive Increase and Multiplicative Decrease. (AIDM) • TCP maintains the congestion window size which decides the amount of data to be transmitted. • The size of this window can be increased or decreased according to the occurrence of congestion on the network.
- 51. Operation of AIDM • AIDM performs slow increase in the congestion window size when the congestion is less. • AIDM performs sudden drop in the congestion window size when there is a congestion on the network
- 52. 2. Slow Start Method • The draw back with the previous method is that, the system take too long time to reach the optimum flow level. • This methods initially sets the window size to 1 and when the ACK is received, then it doubles the window size. • This method is well suited in 2 conditions • Initial stage of the transmission when the system do not know anything about the capacity of the network. • When the line is down for a while and again back on to service.
- 53. Fast retransmit and Fast Recovery • Fast Retransmit: • The convention system waits for the time out period to retransmit the lost frame. • Instead to speed up the process Fast retransmit is used. • In this method the receivers send the previous copy of ACK to the sender to intimate the loss of packets. • When the receiver keeps on receiving the Duplicate ACK then the frame is Retransmitted. Packet 1 Packet 2 Packet 3 Packet 4 Packet 5 Packet 6 Retransmit packet 3 ACK 1 ACK 2 ACK 2 ACK 2 ACK 6 ACK 2 Sender Receiver
- 54. • Fast Recovery: • This methods modifies the operation of slow start method. • This methods reduces the size of the congestion window to half, Instead of starting form the beginning. • This method helps in fast recovery of the system when there is any occurrence of congestion in the network.
- 57. Introduction • The previously discussed control mechanisms measures the amount of congestion occurred in the network and take steps to control. • But it is always better to avoid congestion than controlling.
- 58. Congestion avoidance Mechanisms • There are several congestion avoidance mechanisms proposed. Few of them are discussed here. • Destination experiencing Congestion Bit (DEC Bit) • Random Early Detection (RED). • Source Based Congestion Avoidance.
- 59. DEC Bit • This system introduces an congestion bit to the packets transmitted by the sender. • This bit can either set or reset according to the congestion level. • This bit is set when the queue length if more than 50%. • When the receiver detects the congestion bit is set then the window size is reduced by 0.875 times the original value. • When the queue length is less than 50% then the congestion window size is incremented by 1 byte.
- 60. Random Early Detection • This bit avoids congestion by dropping the packets. • This method measures the queue length and fixes minimum threshold and maximum threshold values.
- 61. • When the queue length is below the minimum threshold value then no packets are dropped. • When the queue length lies between the minimum and maximum threshold values then the packets are dropped according to the predefined configuration. • When the queue length is above the maximum threshold level then all the packets are dropped.
- 64. Quality of service (QoS)
- 65. QoS - Introduction • Qos deals with the regular flow of data throughout the network. • There are several mechanisms incorporated to improve the QoS. • Scheduling • Traffic shaping • Admission control • Resource reservation.
- 66. 1. Scheduling • Scheduling is nothing but prioritizing the various flow of packets. • There are 3 methods. • First In First Out Queuing (FIFO) • Priority Queuing • Weighted Priority Queuing
- 67. FIFO Queuing
- 68. Priority Queuing
- 70. 2. Traffic Shaping • This method regulates the flow of data from various rates of flow of packets. • This can be implemented by 2 techniques. • Leaky Bucket Mechanism • Token Bucket Mechanism
- 73. Token Bucket Mechanism • Packets will be sent based on the token system • Each packet consumes one token. • The ideal time of the node are effectively utilised in this mechanism
- 75. 3. Admission Control • Packets will be accepted or rejected based on the pre defined configuration. • If the packet passes the test then the packet is accepted. Else rejected.
- 76. 4. Resource Reservation • Network resources like bandwidth, queue memory can be reserved before hand for priority applications .
- 77. End of QoS
- 79. Application Requirements • It is first important to classify the types of application for choosing the requirements. • Real Time Applications • Non – Real Time Applicaations
- 80. Real Time Applications • The best examples of real time applications are Audio transmission. • In this applications the minimum delay are most important requirement than any other parameters.
- 81. Non Real time applications • Such kind of applications do not require minimum delay as the important requirement. • The parameters like security, correct delivery can be important requirements based on the applications • Email, FTP and HTTP are few examples of Non Real time Applications.
- 82. Approaches for Deciding the QoS • There are 2 main approaches in providing the QoS. • Fine Grained Approach • Provides QoS for individual Applications • Coarse Grained Approach • Provides QoS for large class of data or Aggregate Traffic.