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Transport Layer2024/7/101OutlineFunctions of Transport LayerTransport Layer Protocols that are in useTCPUDP2024/7/102Transport layer2024/7/103Transport layerTransport layer servicing its upper layerTransport layer relieving its lower layers burden2024/7/104Layered protocols 的一個重點在於下層的協定提供服務給其上一層1.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？2024/7/105Application Layer 有哪些應用？2024/7/106Facts 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)2024/7/107First problem:很多應用，同時經由網路與遠端另一主機溝通 2024/7/108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 multiplexed over the media.2024/7/109Transports primary responsibilitiesSegmenting data and managing each pieceIdentifying&Tracking individual communication between applications on the source and destination hostsReassembling the segments(seq.#)into streams of application data2024/7/1010Segmentation2024/7/1011SegmentationDividing 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.2024/7/1012Tracking multiple streamsVarious applications running concurrently2024/7/1013Identifying the Applications(with port number)2024/7/1014Re-assembling segments(with sequence number)2024/7/1015實例：TCP&UDP HeadersWhat are these fields related toThe above mentioned functions?2024/7/1016Second problem:每種應用的特性不同2024/7/1017Data Requirements VaryDifferent 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 it to be of use.In other cases,an application can tolerate some loss of data during transmission over the network.2024/7/1018Consequently The different Transport layer protocols have different rules allowing devices to handle these diverse data requirementsIn todays converged networks,applications with very different transport needs may be communicating on the same network.2024/7/1019Example-1An 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 presented.2024/7/1020Example-2Occasionally missing small parts of a telephone conversation might be considered acceptable.One can either infer the missing audio from the context of the conversationor ask the other person to repeat what they said.This is considered preferable to the delays that would 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.2024/7/1021Transport layers functions may also need to include If required,ensures that all the data is received reliably and in order by the correct applicationEmploys error handling mechanisms2024/7/1022The primary functions all Transport layer protocols should supportSegmentation and ReassemblyConversation MultiplexingAt the Transport layer,each particular set of pieces flowing between a source application and a destination application is known as a conversation Both TCP and UDP support these two functions 2024/7/1023Some Transport layer protocol may need more Connection-oriented conversationsReliable deliveryOrdered data reconstructionFlow control Only TCP support these functions 2024/7/1024Thats why there are TCP:Transport Control ProtocolandUDP:User Datagram Protocol2024/7/1025TCP&UDP Headers-againWhat are these two different protocolrelated to the above mentioned functions?2024/7/1026How can Transport layer Establish a connection-oriented connection?Ensure reliable delivery?Maintain same order delivery?Execute flow control?Later when we talk about TCP2024/7/1027When do we need reliability?It depends on the applicationsApplications developer choose the appropriate transport layer protocol based on the nature of the application2024/7/1028Different applications require different transport layer support2024/7/1029Applications that need reliabilityDatabases,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 overhead(as a result of ensuring reliability)is considered to be required for these applications.Use TCP2024/7/1030Applications that do not need reliabilityAudio&Video streamFor example,if one or two segments of a video stream fail to arrive,it would only create a momentary disruption in the stream.This may appear as distortion in the image but may not even be noticeable to the user.Use UDP2024/7/1031TCP in detailsThe key distinction between TCP and UDP is reliability2024/7/1032How does TCP ensure reliability?Sequence number in each segmentPositively acknowledge to the sender upon receiving segmentsRetransmit segments if no acknowledge is received from the receiver2024/7/1033Sequence number&Ack number2024/7/1034Positive Acknowledgement2024/7/1035TCP Retransmission2024/7/1036FlagsURG-Urgent pointer field ACK-Acknowledgement field PSH-Push functionRST-Reset the connectionSYN-Synchronize sequence numbersFIN-No more data from sender2024/7/1037TCP connection establishmentestablishing connection-oriented connection(three-way handshake)2024/7/1038Details on 3-way handshake-12024/7/1039Details on 3-way handshake-22024/7/1040Details on 3-way handshake-32024/7/1041Simple Acknowledgement2024/7/1042Whats the problem with SimpleAcknowledgement?Network usage efficiency2024/7/1043Adjust the Window Size(=3)2024/7/1044Window Size TCPs wayWindow size determines the amount of data that can be transmitted at one time before receiving an acknowledgementAfter a host transmits the window-sized number of bytes,it must receive an acknowledgement before it can send any more messages2024/7/1045Window Size TCPs way2024/7/1046Window Size TCPs way2024/7/1047Sliding WindowsWindow Size is changingTo achieve FLOW CONTROLSend more packets when remote buffer can accommodate,send less packets when remote buffer is almost fullIt is just like SLIDE the window to allow more or less air in2024/7/1048TCP connection termination2024/7/1049TCP Termination-FIN2024/7/1050TCP Termination-ACK2024/7/1051TCP Termination-FIN2024/7/1052FIN vs RSTA graceful end by sending a FIN packet followed by a received ACK FIN packet.A not-so-graceful end when one part just hangs-up.In this case TCP sends a RST(reset)packet closing the connection.2024/7/1053Re-assembling segments(with sequence number)2024/7/1054Re-assembling segmentsDuring session setup,an initial sequence number(ISN)is set 3-Way HandshakeThis ISN represents the starting value for the bytes for 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 segments can be identified.2024/7/1055How to re-assembly&maintain order?Can UDP do the same?i.e.,maintaining order?2024/7/1056Can UDP do the same?Can UDP do the same?i.e.,maintaining order?2024/7/1057UDP in detailsTrading reliability for lower overhead2024/7/1058UDP:lower overhead transmission2024/7/1059Whats Overhead?(1)2024/7/1060Whats Overhead?(2)Those session establishment,ACK,retransmission,etc,stuff that TCP has to manage 2024/7/10612024/7/10622024/7/1063ReviewHow 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?2024/7/1064Wireshark LabIdentify TCP header fields and operation using a Wireshark FTP session capture.Identify UDP header fields and operation using a Wireshark TFTP session capture.2024/7/1065供娄浪颓蓝辣袄驹靴锯澜互慌仲写绎衰斡染圾明将呆则孰盆瘸砒腥悉漠堑脊髓灰质炎(讲课2019)脊髓灰质炎(讲课2019)
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