ch7(第七章讲义)

1、2022-5-1Computer Networks - 071/60COMPUTER NETWORKSChapter 07The Application Layer2022-5-1Computer Networks - 072/60Chapter 7 The Application Layer Domain Name ServerElectronic MailWorld Wide WebMultimedia 2022-5-1Computer Networks - 073/607.1 DNS-Domain Name System MotivationsWe are sending email t

2、o rather than to syangIn the ARPANET, hosts.txt lists all the hosts and their IP addresses.DNS: a hierarchical, domain-based naming scheme and a distributed database system, for mapping host names and e-mail destinations to IP addresses.2022-5-1Computer Networks - 074/60DNSDomain Name Sys

3、tem DNS is the invention of a hierarchical, domain-based naming scheme and a distributed database system for implementing the scheme. Basic idea: Every host has a worldwide unique name that is bound to an IP address. DNS provides namelookup facilities: when given a hostname, it returns that hosts IP

4、 address. How DNS worksAn application program calls a library procedure, the resolver, passing it the name as a parameter.The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP address to the resolver, which then returns it to the caller.2022-5-1Comput

5、er Networks - 075/60A portion of the Internet domain name space A domain name is a path from a leaf node up to the root. A domain is a sub-tree in the domain name space. Each domain can have a set of resource records which are stored (in a file) at name servers. There are different types of records,

6、 including one for storing the binding between a name and an IP address.2022-5-1Computer Networks - 076/607.1.2 Resource Records Every domain has a set of resource records associated with it. The most common resource records is just its IP address. The primary function of DNS is to map domain names

7、onto resource records. A resource record is a five-tuple: Domain_name Time_to_live Class Type Value2022-5-1Computer Networks - 077/602022-5-1Computer Networks - 078/60 IN SOA . . (674 10800 1800 3600000 86400 ) IN NS . IN NS . IN MX 10 . IN MX 20 . IN MX 30 .; 重要服务器重要服务器ns IN A email IN A

8、 email2 IN A email3 IN A infonetv6 IN AAAA 2001:da8:d800:75:202:38:75:75; Division of Teaching Affairs$ORIGIN . IN A 2TLMIS IN A 6mail IN A 2www IN A 2dtal CNAME .$ORIGIN . IN A IN NS .dnsserver IN A

9、2022-5-1Computer Networks - 079/607.1.3 Name Servers To avoid the problems associated with having only a single source of information, the DNS name space is divided into non-overlapping zones. A zone will have one primary name server, which gets its information from a file on its disk, and one or mo

10、re secondary name servers, which get their information from the primary name server. 2022-5-1Computer Networks - 0710/60Name Servers (2) When a resolver has a query about a domain name, it passes the query to one of the local name servers. If the domain being sought falls under the name server, it r

11、eturns the authoritative resource records. If the domain is remote and no information about the requested domain is available locally, the name server sends a query message to the top-level name server for the domain requested. Ex. A resolver looks up a remote name in eight steps.2

12、022-5-1Computer Networks - 0711/60Observations Note: There can be several servers per zone. Most of the time, secondary masters pull in their information from a primary master. The latter gets its info from domain administrators. A resolver is capable of sending DNS queries to a name server. A resol

13、ver is often just a collection of library routines that can be linked to an application. What DNS really does is map symbolic names for machines onto their IP addresses. It does not help locate people, resources, services, or objects. LDAP (Light-weight Directory Access Protocol) has been defined, w

14、hich is a simplified version of X.500 directory service. It organizes information as a tree and allows searches on different components. It can be regarded as a “white pages” telephone book. 2022-5-1Computer Networks - 0712/607.2 Electronic Mail Email Architecture and Services Basic functionsComposi

15、tion, Transfer, Reporting, Displaying, Disposition2022-5-1Computer Networks - 0713/60Envelopes and messagesEnvelopes and messages. (a) Paper mail. (b) Electronic mail.2022-5-1Computer Networks - 0714/60Reading E-mailAn example display of the contents of a mailbox.2022-5-1Computer Networks - 0715/60M

16、essage Formats RFC 822RFC 822 header fields related to message transport.2022-5-1Computer Networks - 0716/60Message Formats RFC 822 (2)Some fields used in the RFC 822 message header.2022-5-1Computer Networks - 0717/60MIME Multipurpose Internet Mail Extensions The basic idea of MIME is to continue to

17、 use the RFC 822 format, but to add structure to the message body and define encoding rules for non-ASCII messages. MIME messages can be sent using the existing mail programs and protocols. All that has to be changed are the sending and receiving programs. 2022-5-1Computer Networks - 0718/60The MIME

18、 types and subtypes defined in RFC 2045.2022-5-1Computer Networks - 0719/60A multipart message containing enriched and audio alternatives.2022-5-1Computer Networks - 0720/60Message Transfer - SMTP2022-5-1Computer Networks - 0721/60Final Delivery(a) Sending and reading mail when the receiver has a pe

19、rmanent Internet connection and the user agent runs on the same machine as the message transfer agent. (b) Reading e-mail when the receiver has a dial-up connection to an ISP.2022-5-1Computer Networks - 0722/60POP3: Post Office Protocol V3 POP3 protocol goes through three stages in sequence: authori

20、zation, transactions, update. Ex: Using POP3 to fetch three messages.2022-5-1Computer Networks - 0723/60IMAP: Internet Message Access ProtocolA comparison of POP3 and IMAP.2022-5-1Computer Networks - 0724/607.3 The World Wide Web Architectural Overview Static Web Documents Dynamic Web Documents HTTP

21、 The HyperText Transfer Protocol Performance Enhancements The Wireless Web2022-5-1Computer Networks - 0725/607.3.1 Architectural OverviewThe parts of the Web model.2022-5-1Computer Networks - 0726/60The client side and the server side(a) A browser plug-in. (b) A helper application.A multithreaded We

22、b server with a front end and processing modules2022-5-1Computer Networks - 0727/60URLs Uniform Resource Locators What is the page called? Where is the page located? How can the page be accessed? URL is the solution chosen identifies pages in a way that solve all three problems at once. Some common URLs are listed as:2022-5-1Computer Networks - 0728/607.3.2 Static Web Documents HTML HyperText Markup Language(a) The HTML for a sample Web page. (b) The formatted page.(b)2022-5-1Computer Networks - 0729/60H