通信網(wǎng)絡(luò)技術(shù)課件：Chapter 7 Application Layer1

1、Chapter 7 The Application LayerUpon completion you will be able to:7.1 Domain NameSystem: DNS Understand how the DNS is organized Know the domains in the DNS Know how a name or address is resolved Be familiar with the query and response formats Understand the need for DDNSObjectives 17.1.1 NAME SPAC

2、EThe names assigned to machines must be unique because the addresses are unique. A name space that maps each address to a unique name can be organized in two ways: flat or hierarchical.The topics discussed in this section include:Flat Name Space Hierarchical Name Space 27.1.2 DOMAIN NAME SPACEThe do

3、main name space is hierarchical in design. The names are defined in an inverted-tree structure with the root at the top. The tree can have 128 levels: level 0 (root) to level 127.The topics discussed in this section include:Label Domain Name Domain 3Figure 7.1 Domain name space4Figure 7.2 Domain nam

4、es and labels5Figure 7.3 FQDN and PQDN6Figure 7.4 Domains77.1.3 DISTRIBUTION OF NAME SPACEThe information contained in the domain name space is distributed among many computers called DNS servers. The topics discussed in this section include:Hierarchy of Name Servers Zone Root Server Primary and Sec

5、ondary Servers 8Figure 7.5 Hierarchy of name servers9Figure 7.6 Zones and domains10A primary server loads all information from the disk file; the secondary server loads all information from the primary server. When the secondary downloads information from the primary, it is called zone transfer.Note

6、:117.1.4 DNS IN THE INTERNETThe domain name space (tree) is divided into three different sections: generic domains, country domains, and the inverse domain.The topics discussed in this section include:Generic Domains Country Domains Inverse Domain Registrar 12Figure 7.7 DNS used in the Internet13Fig

7、ure 7.8 Generic domains14Table 7.1 Generic domain labels15Table 7.1 Generic domain labels (Continued)16Figure 7.9 Country domains17Figure 7.10 Inverse domain187.1.5 RESOLUTIONMapping a name to an address or an address to a name is called name-address resolution.The topics discussed in this section i

8、nclude:Resolver Mapping Names to Addresses Mapping Addresses to Names Recursive Resolution Iterative Resolution Caching 19Figure 7.11 Recursive resolution20Figure 7.12 Iterative resolution217.1.6 DNS MESSAGESThe DNS query message consists of a header and question records; the DNS response message co

9、nsists of a header, question records, answer records, authoritative records, and additional records.The topics discussed in this section include:Header 22Figure 7.13 DNS messages23Figure 7.14 Query and response messages24Figure 7.15 Header formatTCP/IP Protocol Suite25Figure 7.16 Flags fieldTCP/IP P

10、rotocol Suite26Table 7.2 Values of rCode277.1.7 TYPES OF RECORDSTwo types of records are used in DNS. The question records are used in the question section of the query and response messages. The resource records are used in the answer, authoritative, and additional information sections of the respo

11、nse message.The topics discussed in this section include:Question Record Resource Record 28Figure 7.17 Question record format29Figure 7.18 Query name format30Table 7.3 Types31Table 7.4 ClassesTCP/IP Protocol Suite32Figure 7.19 Resource record format337.1.8 COMPRESSIONDNS requires that a domain name

12、be replaced by an offset pointer if it is repeated. DNS defines a 2-byte offset pointer that points to a previous occurrence of the domain name or part of it.34Figure 7.20 Format of an offset pointer35A resolver sends a query message to a local server to find the IP address for the host “.”. We disc

13、uss the query and response messages separately.Example 1QR OpCode AA TC RD RA Reserved rCode0 0000 0 0 1 0 000 0000Figure 17.21 shows the query message sent by the resolver. The first 2 bytes show the identifier (1333). It is used as a sequence number and relates a response to a query. Because a res

14、olver may even send many queries to the same server, the identifier helps to sort responses that arrive out of order. The next bytes contain the flags with the value of 0 x0100 in hexadecimal. In binary it is 0000000100000000, but it is more meaningful to divide it into the fields as shown below:36F

15、igure 7.21 Example 1: Query message37Example 1 (Continued)QR OpCode AA TC RD RA Reserved rCode1 0000 0 0 1 1 000 0000The QR bit defines the message as a query. The OpCode is 0000, which defines a standard query. The recursion desired (RD) bit is set. (Refer back to Figure 17.16 for the flags field d

16、escriptions.) The message contains only one question record. The domain name is 4chal4fhda3edu0. The next 2 bytes define the query type as an IP address; the last 2 bytes define the class as the Internet.Figure 17.22 shows the response of the server. The response is similar to the query except that

17、the flags are different and the number of answer records is one. The flags value is 0 x8180 in hexadecimal. In binary it is 1000000110000000, but again we divide it into fields as shown below:38Example 1 (Continued)The QR bit defines the message as a response. The OpCode is 0000, which defines a sta

18、ndard response. The recursion available (RA) and RD bits are set. The message contains one question record and one answer record. The question record is repeated from the query message. The answer record has a value of 0 xC00C (split in two lines), which points to the question record instead of repe

19、ating the domain name. The next field defines the domain type (address). The field after that defines the class (Internet). The field with the value 12,000 is the TTL (12,000 s). The next field is the length of the resource data, which is an IP address (05).39Figure 7.22 Example 1: Response message4

20、0An FTP server has received a packet from an FTP client with IP address . The FTP server wants to verify that the FTP client is an authorized client. The FTP server can consult a file containing the list of authorized clients. However, the file consists only of domain names. The FTP server has only

21、the IP address of the requesting client, which was the source IP address in the received IP datagram. The FTP server asks the resolver (DNS client) to send an inverse query to a DNS server to ask for the name of the FTP client. We discuss the query and response messages separately.Example 241Example

22、 2 (Continued)QR OpCode AA TC RD RA Reserved rCode0 0001 0 0 1 0 000 0000Figure 17.23 shows the query message sent from the resolver to the server. The first 2 bytes show the identifier (0 x1200). The flags value is 0 x0900 in hexadecimal. In binary it is 0000100100000000, and we divide it into fiel

23、dsas shown below:The OpCode is 0001, which defines an inverse query. The message contains only one question record. The domain name is 19171231537in-addr4arpa. The next 2 bytes define the query type as PTR, and the last 2 bytes define the class as the Internet.42Figure 7.23 Example 2: Inverse query

24、message43Example 2 (Continued)QR OpCode AA TC RD RA Reserved rCode 1 0001 1 0 1 1 000 0000Figure 17.24 shows the response. The flags value is 0 x8D80 in hexadecimal. In binary it is 1000110110000000, and we divide it into fields as shown below:44Figure 7.24 Example 2: Inverse response message45In UN

25、IX and Windows, the nslookup utility can be used to retrieve address/name mapping. The following shows how we can retrieve an address when the domain name is given.Example 3$ nslookup Name: Address: The nslookup utility can also be used to retrieve the domain name when the address is given as shown

26、below:$ nslookup 53. name = .4617.9 DDNSThe Dynamic Domain Name System (DDNS) updates the DNS master file dynamically. 477.1.10 ENCAPSULATIONDNS uses UDP as the transport protocol when the size of the response message is less than 512 bytes. If the size of the response message is more than 512 bytes

27、, a TCP connection is used. 48DNS can use the services of UDP or TCP using the well-known port 53.Note:49Upon completion you will be able to:7.2 Electronic Mail:SMTP, POP, and IMAP Understand four configurations of email architecture Understand the functions and formats of a user agent Understand MI

28、ME and its capabilities and data types Understand the functions and commands of an MTA Understand the function of POP3 and IMAP4Objectives 507.2.1 ARCHITECTURETo explain the architecture of email, we give four scenarios. We begin with the simplest situation and add complexity as we proceed. The four

29、th scenario is the most common in the exchange of email.The topics discussed in this section include:First Scenario Second Scenario Third Scenario Fourth Scenario 51Figure 7.25 First scenario52When the sender and the receiver of an email are on the same system, we need only two user agents.Note:53Fi

30、gure 7.26 Second scenario54When the sender and the receiver of an email are on different systems, we need two UAs and a pair of MTAs (client and server).Note:55Figure 2.27 Third scenario56When the sender is connected to the mail server via a LAN or a WAN, we need two UAs and two pairs of MTAs (clien

31、t and server).Note:57Figure 7.28 Fourth scenario58Figure 7.29 Push vs. pull59When both sender and receiver are connected to the mail server via a LAN or a WAN, we need two UAs, two pairs of MTAs (client and server), and a pair of MAAs (client and server). This is the most common situation today.Note

32、:607.2.2 USER AGENTThe user agent (UA) provides service to the user to make the process of sending and receiving a message easier.The topics discussed in this section include:Services Provided by a User Agent User Agent Types Sending Mail Receiving Mail Addresses Mailing List MIME 61Figure 7.30 User

33、 agent62Some examples of command-driven user agents are mail, pine, and elmNote:63Some examples of GUI-based user agents are Eudora, Outlook, and Netscape.Note:64Figure 7.31 Format of an email65Figure 7.32 Email address66Figure 7.33 MIME67Figure 7.34 MIME header68Table 7.5 Data types and subtypes in

34、 MIME69Table 7.35 Data types and subtypes in MIME (Continued)70Table 7.36 Content-transfer-encoding71Figure 7.37 Base6472Table 7.6 Base64 encoding tableTCP/IP Protocol Suite73Figure 7.38 Quoted-printable747.2.3 MESSAGE TRANSFER AGENT: SMTPThe actual mail transfer requires message transfer agents (MT

35、As). The protocol that defines the MTA client and server in the Internet is called Simple Mail Transfer Protocol (SMTP).The topics discussed in this section include:Commands and Responses Mail Transfer Phases 75Figure 7.39 SMTP range76Figure 7.40 Commands and responses77Figure 7.41 Command format78T

36、able 7.7 Commands79Table 7.8 Responses80Table 7.8 Responses (Continued)81Figure 7.42 Connection establishment82Figure 7.43 Message transfer83Figure 7.44 Connection termination84Example 4$ telnet 25Trying 00.Connected to (00).Let us see how we can directly use SMTP to send an email and simulate the c

37、ommands and responses we described in this section. We use TELNET to log into port 25 (the well-known port for SMTP). We then use the commands directly to send an email. In this example, forouzanb is sending an email to himself. The first few lines show TELNET trying to connect to the adelphia mail server.After connection, we can type the SMTP commands and then receive the responses as shown