的一个重点在於下层的协定提供服务给其上一层课件.ppt

1、Transport Layer2022/9/27Outline Functions of Transport Layer Transport Layer Protocols that are in use TCP UDP2022/9/27Transport layer2022/9/27Transport layer Transport layer servicing its upper layer Transport layer relieving its lower layers burden2022/9/27Layered protocols 的一個重點在於下層的協定提供服務給其上一層1.

2、那 思考一下，Application Layer 有 哪些應用？2.你覺得 Transport 需哪些功能以提供這 些應用所需？2022/9/27Application Layer 有哪些應用？2022/9/27Facts learned from previous slide 有很多應用，同時經由網路與遠端另一主機溝通 Can you name a few of these applications?MSN,IE,etc 每種應用的特性不同 Can you name a few of these characteristics?Email vs Internet phone(Skype)20

3、22/9/27First problem:很多應用，同時經由網路與遠端另一主機溝通 2022/9/27很多應用，同時經由網路與遠端另一主機溝通 Can we transport a full chunk of large application data at one time?If not,what would you do?accepts data from different conversations and segments/passes it down to the lower layers as manageable pieces that can be eventually m

4、ultiplexed over the media.2022/9/27Transports primary responsibilities Segmenting data and managing each piece Identifying&Tracking individual communication between applications on the source and destination hosts Reassembling the segments(seq.#)into streams of application data2022/9/27Segmentation2

5、022/9/27Segmentation Dividing data into small parts,and sending these parts from the source to the destination,enables many different communications to be interleaved(multiplexed)on the same network.So that you could receive e-mails,chat on instant messenger,view web pages or video at the same time.

6、2022/9/27Tracking multiple streamsVarious applications running concurrently2022/9/27Identifying the Applications(with port number)2022/9/27Re-assembling segments(with sequence number)2022/9/27實例：TCP&UDP HeadersWhat are these fields related toThe above mentioned functions?2022/9/27Second problem:每種應用

7、的特性不同2022/9/27Data Requirements Vary Different applications have different requirements,there are multiple Transport layer protocols.For some applications,segments must arrive in a very specific sequence in order to be processed successfully.In some cases,all of the data must be received for any of

8、it to be of use.In other cases,an application can tolerate some loss of data during transmission over the network.2022/9/27Consequently The different Transport layer protocols have different rules allowing devices to handle these diverse data requirements In todays converged networks,applications wi

9、th very different transport needs may be communicating on the same network.2022/9/27Example-1 An e-mail or web page need to be completely received and presented for the information to be considered useful.Slight delays are considered acceptable to ensure that the complete information is received and

10、 presented.2022/9/27Example-2 Occasionally missing small parts of a telephone conversation might be considered acceptable.One can either infer the missing audio from the context of the conversation or ask the other person to repeat what they said.This is considered preferable to the delays that woul

11、d result from asking the network to manage and resend missing segments.In this example,the user-not the network-manages the resending or replacement of missing information.2022/9/27Transport layers functions may also need to include If required,ensures that all the data is received reliably and in o

12、rder by the correct application Employs error handling mechanisms2022/9/27The primary functions all Transport layer protocols should support Segmentation and Reassembly Conversation Multiplexing At the Transport layer,each particular set of pieces flowing between a source application and a destinati

13、on application is known as a conversation Both TCP and UDP support these two functions 2022/9/27Some Transport layer protocol may need more Connection-oriented conversations Reliable delivery Ordered data reconstruction Flow control Only TCP support these functions 2022/9/27Thats why there are TCP:T

14、ransport Control Protocoland UDP:User Datagram Protocol2022/9/27TCP&UDP Headers-againWhat are these two different protocolrelated to the above mentioned functions?2022/9/27How can Transport layer Establish a connection-oriented connection?Ensure reliable delivery?Maintain same order delivery?Execute

15、 flow control?Later when we talk about TCP2022/9/27When do we need reliability?It depends on the applications Applications developer choose the appropriate transport layer protocol based on the nature of the application2022/9/27Different applications require different transport layer support2022/9/2

16、7Applications that need reliability Databases,web pages,and e-mail,require all of the sent data arrive at the destination in its original condition,in order for the data to be useful.Any missing data could cause a corrupt communication that is either incomplete or unreadable.The additional network o

17、verhead(as a result of ensuring reliability)is considered to be required for these applications.Use TCP2022/9/27Applications that do not need reliability Audio&Video stream For example,if one or two segments of a video stream fail to arrive,it would only create a momentary disruption in the stream.T

18、his may appear as distortion in the image but may not even be noticeable to the user.Use UDP2022/9/27TCP in detailsThe key distinction between TCP and UDP is reliability2022/9/27How does TCP ensure reliability?Sequence number in each segment Positively acknowledge to the sender upon receiving segmen

19、ts Retransmit segments if no acknowledge is received from the receiver2022/9/27Sequence number&Ack number2022/9/27Positive Acknowledgement2022/9/27TCP Retransmission2022/9/27FlagsURG-Urgent pointer field ACK-Acknowledgement field PSH-Push functionRST-Reset the connectionSYN-Synchronize sequence numb

20、ersFIN-No more data from sender2022/9/27TCP connection establishmentestablishing connection-oriented connection(three-way handshake)2022/9/27Details on 3-way handshake-12022/9/27Details on 3-way handshake-22022/9/27Details on 3-way handshake-32022/9/27Simple Acknowledgement2022/9/27Whats the problem

21、 with SimpleAcknowledgement?Network usage efficiency2022/9/27Adjust the Window Size(=3)2022/9/27Window Size TCPs way Window size determines the amount of data that can be transmitted at one time before receiving an acknowledgement After a host transmits the window-sized number of bytes,it must recei

22、ve an acknowledgement before it can send any more messages2022/9/27Window Size TCPs way2022/9/27Window Size TCPs way2022/9/27Sliding Windows Window Size is changing To achieve FLOW CONTROL Send more packets when remote buffer can accommodate,send less packets when remote buffer is almost full It is

23、just like SLIDE the window to allow more or less air in2022/9/27TCP connection termination2022/9/27TCP Termination-FIN2022/9/27TCP Termination-ACK2022/9/27TCP Termination-FIN2022/9/27FIN vs RST A graceful end by sending a FIN packet followed by a received ACK FIN packet.A not-so-graceful end when on

24、e part just hangs-up.In this case TCP sends a RST(reset)packet closing the connection.2022/9/27Re-assembling segments(with sequence number)2022/9/27Re-assembling segments During session setup,an initial sequence number(ISN)is set 3-Way Handshake This ISN represents the starting value for the bytes f

25、or this session that will be transmitted to the receiving application.As data is transmitted during the session,the sequence number is incremented by the number of bytes that have been transmitted.This tracking of data byte enables each segment to be uniquely identified and acknowledged.Missing segm

26、ents can be identified.2022/9/27How to re-assembly&maintain order?Can UDP do the same?i.e.,maintaining order?2022/9/27Can UDP do the same?Can UDP do the same?i.e.,maintaining order?2022/9/27UDP in detailsTrading reliability for lower overhead2022/9/27UDP:lower overhead transmission2022/9/27Whats Ove

27、rhead?(1)2022/9/27Whats Overhead?(2)Those session establishment,ACK,retransmission,etc,stuff that TCP has to manage 2022/9/272022/9/272022/9/27Review How does TCP establish a connection-oriented session?How does TCP support reliable delivery?How does TCP support same order delivery?How does TCP support flow control?2022/9/27Wireshark Lab Identify TCP header fields and operation using a Wireshark FTP session capture.Identify UDP header fields and operation using a Wireshark TFTP session capture.2022/9/27