1. © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public 1 Version 4.0 OSI Transport Layer Network Fundamentals – Chapter 4

2. 2 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Objectives  Explain the role of Transport Layer protocols and services in supporting communications across data networks.  Analyze the application and operation of TCP mechanisms that support reliability.  Analyze the application and operation of TCP mechanisms that support reassembly and manage data loss.  Analyze the operation of UDP to support communicate between two processes on end devices.

3. 3 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Transport Layer Role and Services  Transport layer is responsible for overall end-to-end transfer of application data

4. 4 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Transport Layer Role and Services  Transport layer enables applications on devices to communicate  Fig 4.2

5. 5 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Purpose of Transport Layer  Tracking the individual communications between applications on the source and destination hosts  Segmenting data and managing each piece  Reassembling the segments into streams of application data  Identifying the different applications  Performing flow control between end users  Enabling error recovery  Initiating a session

6. 6 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public 1. Tracking Individual Conversation  Any host can have multiple applications communicating across the network  TL maintain the multiple communication streams between these applications.  Ex. Email, IM, Websites and VoIP simultaneously  Fig 4-3

7. 7 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Segmenting Data  AL passes large amounts of data to TL  TL has to break data into smaller pieces =segments – suited transmission  Without segmentations, only one application would be able to receive data  Fig 4.4

8. 8 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Reassembling Segments  Networks prove multiple routes – diff trans times  Data can arrives in the wrong order  Numbering and sequencing – TL ensure segments are reassembled into the proper order  Each segment of data reassembled and directed to appropriate application.

9. 9 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Identifying the Applications  To pass data streams to the proper applications  TL must identify target application  TL assigns an identifier to an application  In TCP/IP, the identifier = port number  Port number used in TL header to indicate which application the data is associated  Each particular set of pieces flowing between a source application and destination application is known as conversation  Dividing data into small parts enables many diff comm to be interleaved/multiplexed on the same network

10. 10 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Flow Control  Network hosts have limited resources – memory, bandwidth  When TL aware, some protocol can request the sending application to reduce the rate of data flow  This is done at TL by regulating the amount of data source transmits as a group.  =FLOW CONTROL  Prevent the loss of segments on net and avoid retransmission

11. 11 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Error Recovery  Possible  Piece of data corrupted or lost while transmitted over the network  TL ensure all pieces reach destination – source device retransmit any data that is lost Initiating a Session  TL can provide connection orientation by creating a session between the app.  These connections prepare the app to communicate with each other before any data transmitted  Data can be closely managed

12. 12 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Transport Layer Role and Services  Supporting Reliable Communication Diff apps have diff requirements for their data Diff transport protocols have been developed to meet these requirements TCP is a TL protocol – ensure reliable delivery In networking, reliability means each piece of data the source sends arrives at the destination Three basic operation at TL to support reliability Tracking transmitted data Acknowledging received data Retransmitting any unacknowledged data More control data (in Layer 4 header) is exchanged to support acknowledgement, tracking and retransmission

13. 13 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Transport Layer Role and Services  Supporting Reliable Communication

14. 14 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP and UDP  Two most common TL protocols of TCP/IP are  Transmission Control Protocol (TCP) and User Datagram Protocol (TCP)

15. 15 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public UDP  Simple and connectionless protocol  Low-overhead data delivery  8 bytes of overhead  UDP segments = datagrams  UDP sends datagrams as ‘best effort’  Applications of UDP DNS Video Streaming VoIP

16. 16 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Transport Layer Role and Services  Identify the basic characteristics of the UDP and TCP protocols

17. 17 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP  Connection-Oriented protocol  Additional overhead to gain functions  Additional functions Same-order delivery Reliable delivery Flow-control Each TCP segments has 20 bytes of overhead Applications Web browsers E-mail File transfer

18. 18 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Transport Layer Role and Services  Identify the basic characteristics of the UDP and TCP protocols

19. 19 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Transport Layer Role and Services

20. 20 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Port Addressing  TCP and UDP based services keep track of various applications.  To differentiate segments and datagrams for each application – TCP and UDP have header fields that uniquely identify these apps.  Header contains source and dest port  Server processes have static port number assigned  Client dynamically choose a port number for each conversation.

21. 21 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Port Addressing  Identifying Conversations

22. 22 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Port Addressing  The combination of TL port number and NL IP address uniquely identifies a particular process running on specific host device.  This combination = socket  Ex : Web server on host 192.168.1.20, web browser (dynamically assigned port 49152) on host 192.168.100.48  Destined socket = 192.168.1.10:80  Socket for the web page = 192.168.100.48:49152

23. 23 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Port Addressing

24. 24 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Port Addressing  Different types of port numbers Well-known ports (0-1023) Reserved for services and apps FTP 20, SMTP 25, HTTP 80 Registered Ports (1024-49151) Assigned to user processes or application Dynamic or private ports (49152-65535) Netstat command

25. 25 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Connection Establishment and Termination  TCP Three-Way Handshake  Three steps in TCP connection establishment: 1. Initiating client sends a segment containing an initial sequence value. 2. Server responds with segment containing an acknowledgement value of received sequence+1, plus its own synchronizing sequence value 3. Initiating client responds with an acknowledgement received sequence+1  Fig 4.11

26. 26 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Connection Establishment and Termination

27. 27 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Session Termination  Client has no more data to send, it sends a segment with FIN flag set  The server sends an ACK to acknowledge the receipt of the FIN to terminate the session from client to server  The server sends a FIN to the client to terminate server to client session  Client responds with ACK to acknowledge the FIN from server  Fig 4-12

28. 28 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Session Termination

29. 29 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Managing TCP Sessions  Describe how TCP sequence numbers are used to reconstruct the data stream with segments placed in the correct order

30. 30 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Managing TCP Sessions  TCP Acknowledgement with Windowing The sequence number indicates the relative number of bytes that have been transmitted in this session, including the bytes in the current segment. TCP use acknowledgement number in segment sent back to indicate the next byte expects to receive. Fig 4-13 But if A had to wait for ack of the receipt of each 10 bytes – lot of overhead

31. 31 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Managing TCP Sessions  TCP Acknowledgement with Windowing

32. 32 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Managing TCP Sessions  TCP Acknowledgement with Windowing Multiple segments of data can be sent and ack with a single TCP message in opposite direction. Ex start with SEQ = 2000, if 10 segments of 1000bytes were received, an ACK = 12001 would be returned to the source. Amount of data can be transmitted before ACK = window size Window size is a field in TCP header used for management of lost data and flow control

33. 33 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Retransmission  Data loss will occasionally occur  Dest host using TCP only ack data for contiguous sequence bytes  If one or more segments are missing, only segments that complete the stream is ack  Ex. Segments with SEQ=1500 to 3000 and SEQ=3400 to 3500 were received, the ACK will be =3001

34. 34 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Retransmission

35. 35 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Congestion Control  Flow Control & Dynamic Window Sizes

36. 36 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Congestion Control  Flow Control Adjusting the effective rate of data flow. Window size field in TCP header specifies amount of data can be transmitted before an ACK Initial window size determined through 3-way handshake TCP feedback mechanism adjusts the effective rate to the maximum flow network and destination can support without loss Fig 4-14 During the delay in receiving the ACK, sender will not sent additional segments for the session

37. 37 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Congestion Control  Dynamic Window Size When network resources are constrained, TCP can reduce the window size After no data losses or constrained resources, receiver will begin to increase the window size field This dynamic increasing and decreasing of window size is a continuous process in TCP

38. 38 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TCP Congestion Control

39. 39 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public UDP Protocol  Communicating with low overhead

40. 40 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public UDP Protocol  Simple protocol  Basic TL functions  Not connection oriented  Does not provide sophisticated retransmission, sequencing and flow control mechanism  UDP does not provide reliability..so be careful..  but, are not always unreliable just the reliability is not provided

41. 41 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public UDP Datagram Reassembly  Session are not established  When app has data to send, it simply sends the data  When larger amounts of data – split into multiple segments =datagrams  Multiple datagrams – may take different paths and arrives in the wrong order  UDP has no way to reorder the datagrams into their transmission order.  UDP simply reassembles the data in order it was received and forward to app  If seq is important for app, the app will have to identify the proper sequence and how it should be processed

42. 42 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public UDP Datagram Reassembly

43. 43 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public UDP Protocol  Servers use port numbers to identify a specified application layer process and direct segments to the proper service or application

44. 44 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public UDP Protocol  UDP protocol and port numbers are utilized in client- server communication

45. 45 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Summary

46. 46 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public