的一个重点在於下层的协定提供服务给其上一层.ppt

2019/6/17,1,Transport Layer,2019/6/17,2,Outline,Functions of Transport Layer Transport Layer Protocols that are in use TCP UDP,2019/6/17,3,Transport layer,2019/6/17,4,Transport layer,Transport layer servicing its upper layer Transport layer relieving its lower layers burden,2019/6/17,5,Layered 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？,2019/6/17,6,Application Layer 有哪些應用？,2019/6/17,7,Facts 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),2019/6/17,8,First problem: 很多應用，同時經由網路與遠端另一主機溝通 ,2019/6/17,9,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.,2019/6/17,10,Transports 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 data,2019/6/17,11,Segmentation,2019/6/17,12,Segmentation,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.,2019/6/17,13,Tracking multiple streams,Various applications running concurrently,2019/6/17,14,Identifying the Applications (with port number),2019/6/17,15,Re-assembling segments (with sequence number),2019/6/17,16,實例：TCP & UDP Headers,What are these fields related to The above mentioned functions?,2019/6/17,17,Second problem: 每種應用的特性不同,2019/6/17,18,Data 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 it to be of use. In other cases, an application can tolerate some loss of data during transmission over the network.,2019/6/17,19,Consequently ,The different Transport layer protocols have different rules allowing devices to handle these diverse data requirements In todays converged networks, applications with very different transport needs may be communicating on the same network.,2019/6/17,20,Example - 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 presented.,2019/6/17,21,Example - 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 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.,2019/6/17,22,Transport layers functions may also need to include ,If required, ensures that all the data is received reliably and in order by the correct application Employs error handling mechanisms,2019/6/17,23,The 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 destination application is known as a conversation Both TCP and UDP support these two functions,2019/6/17,24,Some Transport layer protocol may need more ,Connection-oriented conversations Reliable delivery Ordered data reconstruction Flow control Only TCP support these functions,2019/6/17,25,Thats why there are ,TCP : Transport Control Protocol and UDP : User Datagram Protocol,2019/6/17,26,TCP & UDP Headers - again,What are these two different protocol related to the above mentioned functions?,2019/6/17,27,How can Transport layer ,Establish a connection-oriented connection? Ensure reliable delivery? Maintain same order delivery? Execute flow control? Later when we talk about TCP,2019/6/17,28,When do we need reliability?,It depends on the applications Applications developer choose the appropriate transport layer protocol based on the nature of the application,2019/6/17,29,Different applications require different transport layer support,2019/6/17,30,Applications 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 overhead (as a result of ensuring reliability) is considered to be required for these applications. Use TCP,2019/6/17,31,Applications 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. This may appear as distortion in the image but may not even be noticeable to the user. Use UDP,2019/6/17,32,TCP in details,The key distinction between TCP and UDP is reliability,2019/6/17,33,How does TCP ensure reliability?,Sequence number in each segment Positively acknowledge to the sender upon receiving segments Retransmit segments if no acknowledge is received from the receiver,2019/6/17,34,Sequence number & Ack number,2019/6/17,35,Positive Acknowledgement,2019/6/17,36,TCP Retransmission,2019/6/17,37,Flags,URG - Urgent pointer field ACK - Acknowledgement field PSH - Push function RST - Reset the connection SYN - Synchronize sequence numbers FIN - No more data from sender,2019/6/17,38,TCP connection establishment establishing connection-oriented connection (three-way handshake),2019/6/17,39,Details on 3-way handshake - 1,2019/6/17,40,Details on 3-way handshake - 2,2019/6/17,41,Details on 3-way handshake - 3,2019/6/17,42,Simple Acknowledgement,2019/6/17,43,Whats the problem with Simple Acknowledgement?,Network usage efficiency,2019/6/17,44,Adjust the Window Size (=3),2019/6/17,45,Window 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 receive an acknowledgement before it can send any more messages,2019/6/17,46,Window Size TCPs way,2019/6/17,47,Window Size TCPs way,2019/6/17,48,Sliding 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 just like SLIDE the window to allow more or less air in,2019/6/17,49,TCP connection termination,2019/6/17,50,TCP Termination - FIN,2019/6/17,51,TCP Termination - ACK,2019/6/17,52,TCP Termination - FIN,2019/6/17,53,FIN vs RST,A 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.,2019/6/17,54,Re-assembling segments (with sequence number),2019/6/17,55,Re-assembling segments,During session setup, an initial sequence number (ISN) is set 3-Way Handshake This 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. Miss