现代计算机网络讲义7(英语)+应用层ppt课件

1、1,The Application Layer,Chapter 7,2,7.1 DNS The Domain Name System,Map ASCII strings onto network addresses The reasons for using DNS The size of the file would become too large. Host name conflicts would occur constantly The load and latency. A hierarchical Domain-based naming scheme A distributed

2、database system The DNS Name Space Resource Records Name Servers,3,7.1.1 The DNS Name Space,A portion of the Internet domain name space,4,7.1.2 Resource Records (1,Every domain can have a set of resource records DNS is to map domain names onto resource records A resource record is a five-tuple: Doma

3、in_name Time_to_live Class Type Value The principal DNS resource records types,5,7.1.2 Resource Records (2,A portion of a possible DNS database for cs.vu.nl,6,7.1.3 Name Servers(1,Part of the DNS name space showing the division into zones. Each zone contains some part of the tree and also contains n

4、ame servers holding the information about that zone,7,7.1.3 Name Servers (2,Recursive query a resolver looks up a remote name in eight steps. Or return the name of the next server to try,8,7.2 Electronic Mail,Architecture and Services The User Agent Message Formats Message Transfer Final Delivery,9,

5、7.2.1 Architecture and Services,Basic functions Composition Transfer Reporting Displaying Disposition,E-mail Messages: a primitive envelope some number of header fields, a blank line, the message body,10,7.2.2 The User Agent,Sending E-mail User must provide the message, the destination address, and

6、possibly some other parameters. Reading E-mail From the users mailbox for incoming e-mail,11,7.2.3 Message Formats-RFC 822(1,RFC 822 header fields related to message transport,12,7.2.3 Message Formats RFC 822 (2,Some fields used in the RFC 822 message header,13,Problems with international languages:

7、 Languages with accents (French, German). Languages in non-Latin alphabets (Hebrew, Russian). Languages without alphabets (Chinese, Japanese). Messages not containing text at all (audio or images). MIME Multipurpose Internet Mail Extensions,7.2.3 Message Formats RFC 822 (3,14,7.2.3 Message Formats R

8、FC 822 (4,RFC 822 headers added by MIME,15,7.2.3 Message Formats RFC 822 (5,The MIME types and subtypes defined in RFC 2045,16,A multipart message containing enriched and audio alternatives,17,7.2.4 Message Transfer (1,SMTPThe Simple Mail Transfer Protocol a simple ASCII protocol Establishing the TC

9、P connection at port 25 Sending machine, operating as the client Receiving machine, operating as the server, talk first. The server starts by sending its identity and telling whether it is prepared to receive mail. If it is not, the client releases the connection and tries again later. If it is, the

10、 client announces whom the e-mail is coming from and whom it is going to. If such a recipient exists at the destination, the server gives the client the go-ahead to send the message. Then the client sends the message and the server acknowledges it,18,7.2.4 Message Transfer (2,Transferring a message

11、from to ,19,Some one is not On-line POP3 (Post Office Protocol Version 3) The user agents to copy e-mail from the transfer agent Three status Authorization. Transactions. Update,7.2.5 Final Delivery (1,20,7.2.5 Final Delivery (2,the user agent and the message transfer agent runs on the same machine.

12、 the receiver has a dial-up connection to an ISP,21,7.2.5 Final Delivery (3,Using POP3 to fetch three messages. POP3 server listens at port 110,22,7.3 The World Wide Web,Architectural Overview Static Web Documents Dynamic Web Documents HTTP The HyperText Transfer Protocol Performance Enhancements Th

13、e Wireless Web,23,7.3.1 Architectural Overview (1,The parts of the Web model,24,The Client Side (1,The browser determines the URL (by seeing what was selected). The browser asks DNS for the IP address. DNS replies with address The browser makes a TCP connection to port 80 It then sends a request ask

14、ing for file The server sends the file The TCP connection is released. The browser displays all the text in the file. The browser fetches and displays all images in this file,25,The Client Side (2,To display the file, the browser has to understand its format. HTML interpreter A browser plug-in, from

15、 a special directory on the disk A helper application, programs independently of the browser,26,The Server Side (1,Accept a TCP connection from a client (a browser). Get the name of the file requested. Get the file (from disk). Return the file to the client. Release the TCP connection,27,The Server

16、Side (2,Get the file Cache, savings in time should worth the expense Multithread Multiple disk,28,The Server Side (3,do more than just return files Resolve the name of the Web page requested. Authenticate the client. Access control on the client. Access control on the Web page. Check the cache. Fetc

17、h the requested page from disk. Determine the MIME type to include in the response. Take care of miscellaneous odds and ends. Return the reply to the client. Make an entry in the server log,29,The Server Side (4,A server farm. Multiple CPU Front end keep track of where it sends each request and send

18、 subsequent requests for the same page to the same node,30,The Server Side (5,a) Normal request-reply message sequence. (b) Sequence when TCP handoff is used,31,7.3.4 HTTPThe HyperText Transfer Protocol,Connection HTTP1.0, one connection only for one request HTTP1.1, persistent connection, for multi

19、ple request The built-in HTTP request methods GET HEAD PUT POST DELETE TRACE CONNECT OPTIONS Message Headers,32,7.3.5 Performance Enhancement,World Wide Wait Caching Server replication Content delivery networks,33,Caching(1,Who should do the catching? Hierarchical caching with three proxies,34,Cachi

20、ng (2,How long should pages be cached ? the cacheability of a page may vary wildly over time Base the holding time on the Last-Modified time If-Modified-Since proactive caching,35,Server Replication-Mirroring,Static mirrored sites Automatically clone itself to handle a massive increase in traffic Dy

21、namic replicas on a per-page,36,Content Delivery Networks(1,Page preprocess Original Web page Same page after transformation,37,Steps in looking up a URL when a CDN is used,Content Delivery Networks(2,38,7.3.6 The Wireless Web,WAP The Wireless Application Protocol Use the existing digital wireless i

22、nfrastructure. Users can literally call up a WAP gateway over the wireless link and send page requests to it. The gateway then checks its cache for the page requested. If present, sends it; if absent, it fetches it over the wired Internet. Problem: Accessing the Internet on a tiny screen Paying by t

23、he minute,39,WAP1.0 The Wireless Application Protocol (1,The WAP protocol stack,40,WAP 1.0 (2,WAE layer uses a WML (Wireless Markup Language), an application of XML. The WAP architecture,41,WAP2.0Second-Generation Wireless Web (1,New features of WAP 2.0. Push model as well as pull model. Support for

24、 integrating telephony into apps. Multimedia messaging. Voice and data are starting to merge. Inclusion of 264 pictograms. Interface to a storage device. Support for plug-ins in the browser,42,WAP2.0Second-Generation Wireless Web (2,Two protocol stacks XHTML Basic, for small wireless devices.,43,7.4

25、 Multimedia,Introduction to Audio Audio Compression Streaming Audio Internet Radio Voice over IP Introduction to Video* Video Compression* Video on Demand* The MBone The Multicast Backbone,44,7.4.1 Introduction to Audio,A sine wave. Sampling the sine wave. Quantizing the samples to 4 bits,45,7.4.2 A

26、udio Compression,a) The threshold of audibility as a function of frequency. (b) The masking effect,46,7.4.3 Streaming Audio(1,A straightforward way to implement clickable music on a Web page. Delay,47,7.4.3 Streaming Audio (2,Media Player Manage the user interface. Handle transmission errors. Decomp

27、ress the music. Eliminate jitter. Buffers input from the media server Plays from the buffer,48,7.4.3 Streaming Audio (3,Pull server as long as there is room in the buffer, the media player keeps sending requests to the server. Unnecessary data requests. The server knows it has sent the whole file, s

28、o why have the player keep asking? Push server, the media player sends a PLAY request, the server keeps pushing data. The media server runs at normal playback speed perfectly, no control messages are required The server runs faster. Low-water mark & High-water mark Buffer is filled to the high-water

29、 mark. the media player tells it to pause When hits the low-water mark, the media player tells to start again,49,7.4.3 Streaming Audio (4,RTSP(Real Time Streaming Protocol ), to operate a push server Commands from the player to the server. RTP for the data stream,50,The Real-Time Transport Protocol(

30、RTP) To multiplex several real-time data streams onto a single stream of UDP packets The position of RTP in the protocol stack. Packet nesting,7.4.3 Streaming Audio (5,51,7.4.3 Streaming Audio (6,RTP head,52,RTP Head P bit, the packet has been padded to a multiple of 4 bytes. X bit, an extension hea

31、der is present. CC field, tells how many contributing sources are present. M bit, an application-specific marker bit. Payload type field, tells which encoding algorithm has been used Sequence number, incremented on each RTP packet . Retransmission is not a practical option RTP has no flow control, n

32、o error control, no acknowledgements, and no mechanism to request retransmissions. Timestamp, to note when the first sample in the packet was made, allows the destination play each sample at right time, independently of when the packet arrived, allows multiple streams to be synchronized with each ot

33、her. Synchronization source identifier tells which stream the packet belongs to, used to multiplex and demultiplex multiple data streams onto a single stream of UDP packets. Contributing source identifiers, used when mixers are present in the studio,7.4.3 Streaming Audio (7,53,RTCP (Realtime Transpo

34、rt Control Protocol). To provide feedback on delay, jitter, bandwidth, congestion, and other network properties to the sources. Interstream synchronization. Different streams may use different clocks, with different granularities and different drift rates. Naming the various sources, to display on t

35、he receivers screen,7.4.3 Streaming Audio (8,54,Live radio is always broadcast at exactly the rate it is generated and played back Internet radio should use multicasting with the RTP/RTSP protocols. In practice, the sound is sent over the TCP connection from Internet radio station,7.4.4 Internet Rad

36、io (2-1,55,7.4.4 Internet Radio (2-2,A student radio station,56,7.4.5 Voice over IP(1,The H323 architectural model for Internet telephony. Gateway Gatekeeper, controls the end points under its zone,57,7.4.5 Voice over IP (2,The H323 protocol stack. G.711,Encoding & Decoding H.245,negotiates other as

37、pects of the connection RTCP, control the RTP channels. Q.931, establishing and releasing connections of the standard telephony H.225, RAS (Registration/Admission/Status ), talking to the gatekeeper RTP, for the actual data transmission,58,7.4.5 Voice over IP (3,Connect between PC and phone,59,7.4.5 Voice over IP (4,Logical channels between t