Application layer characteristics
Each application layer protocol is to solve a certain type of application problem
. The specific content of the application layer: specifies the protocol that the application process follows when communicating
. Many protocols in the application layer are based on the client-server approach . The client is the service requester and the server is the service. provider.
6.1 Domain Name System DNS
DNS is a system for naming machines on the network.
Distributed
domain name and IP address are mapped
to each other . The resolution of names to IP addresses is done by several domain name server programs. The domain name server program runs on a dedicated node, and the machine running the program is called a domain name server .
Domain Name Structure The only hierarchical name in the
hierarchical tree structure is the domain name ...Third-level domain name. Second-level domain name. Top-level domain name Top-level domain name TLD National top-level domain nTLD .cn .us .uk Generic top-level domain gTLD .com .net .org .edu. gov … China divides the second-level domain names into "category domain names" (6) and "administrative region domain names" (34)
Domain name server
The scope of a server is called a zone . All nodes in a zone can be connected.
Set up the corresponding authority domain name server for each district
Tree structure DNS domain name server
Root domain name server: Know the domain names and IP addresses of all top-level domain name servers (13 different IP addresses).
Top-level domain name server: Manage all the second-level domain name
authority registered in the domain name server. Domain name server: responsible for one The domain name server of the zone
Local domain name server: the domain name cannot be resolved, first turn to the root domain name server.
DNS port number: 53
query request message UDP is sent to the local domain name server.
Domain name resolution process
. The query from the host to the local domain name server is generally recursive query. The query from the
local server to the root domain name server is usually iterative query .
Each server maintains a high-speed cache for storing recently used names and a record of where name mapping information was obtained.
6.2 File Transfer Protocol FTP
Used to transfer files between a remote host and a local host or between two remote hosts.
Interactive,
suitable for any computer in a heterogeneous network to transfer files
Basic working principle
Based on TCP , reliable transmission
uses client-server method . There are two FTP server processes: a master process (responsible for receiving new requests) and a slave process (responsible for handling a single process).
The
port number of the
master process 21. The master process and the slave hexadecimal concurrent
simple file transfer protocol TFTP , based on UDP, cannot reliably transmit the
network file system NFS : realize the sharing of files and directories
6.3 Email
Three main components
User agent UA
mail server: send and receive mail, and send
mail protocol and mail read protocol to the sender at the same time
E-mail protocol
E-mail sends the mail to the ISP mail server.
Sender: Simple mail transfer protocol SMPT
reads the mail POP3 and IMPA
general Internet mail expansion
SMTP
specifies how to exchange information between two SMTP processes that communicate with each other. The
client-server method is sent out. The SMTP process on the one side is the client, and the server responsible for receiving the SMTP process ( port number 25 )
POP3
client server
is initiated by the client to read the mail protocol
The mail reading protocol IMAP
can operate the mailbox of the ISP's mail server, just like the local operation ( online protocol ) is
different from POP3. The mail is still stored on the server after IMAP is read. IMAP must be connected to the Internet to read the mail, while POP3 can be viewed locally.
E-mail composition: envelope and content
Format: recipient's mailbox name@the domain name of the host where the mailbox is located
Web-based e-mail
uses HTTP protocol
The transmission between two mail servers uses SMTP
HTTP->SMTP->HTTP
6.4 World Wide Web
Overview of
distributed hypermedia system
in a client-server mode work
Uniform Resource Locator UPL
<protocol>://<host>:<port>/<path>
Protocol: FTP, HTTP, News
Port number: 80
Hypertext Transfer Protocol HTTP
establishes TCP connection
HTTP request message
HTTP response message
releases TCP connection
Documents on the World Wide Web
Static documents: HTML language, XML language, CSS
Dynamic documents: no pre-stored in the server, use CGI to create dynamic documents
Active documents (programs)
Information Retrieval on the World Wide Web
Search Engine: Full-text Search Search, Category Search
Dynamic Host Configuration Protocol DHCP
Plug and play networking.
Internet access needs to be configured: IP address, subnet mask, default router (gateway) IP address, DNS server IP address
Client-server mode
broadcasts a discovery message to the DHCP server when it is started, and the host becomes a DHCP client at this time.
DHCP first searches the database for configuration information, and if it is not allocated, the
server's response message is called an offer message .
Lease period: The IP address assigned by DHCP is temporary, and this limited period of time is the lease period.
The value of the lease period is determined by the server itself.
Customers can also put forward the requirements for the lease period in the messages they send.
Use UDP to transmit messages.
Server port: 67
Client: 68