标签：网络基础 应用层 传输层 网络应用 会话层 cisco
1. Applications – The Interface Between the Networks
two programs access to the network: applications and services.
Applications are designed to interact with us. Application is software for the user. If the device is a computer, the application is typically initiated by the user. Although there may be many layers of support underneath, application software provides an interface between humans and the hardware. The application will initiate the data transfer process when the user presses the Send button, or a similar action.
Services are background programs that perform a particular function in the data network. Services are invoked by a device connecting to the network or by an application. For example, a network service can provide functions that transmit data or provide conversion of data in a network. In general, services are not directly accessible or seen by the end user. They provide the connection between an application and the network.
2. Application Layer
provides the interface between the applications we use to communicate and the underlying network over which our messages are transmitted.
3. Application layer protocols
used to exchange data between programs running on the source and destination hosts.
3.1 Application Layer Protocol Functions
(1) Define Processes on either end of the communication
(2) Define the types of message
(3) Define the syntax of messages
(4) Define the meaning of any informational fields
(5) Define two messages are sent and the expected response
(6) Define interaction with the next lower layer.
4. Network applications
Make communication in the workplace easier
Affect the amount of work completed on a daily basis
Reduce data communications time and costs
5. The Presentation layer has three primary functions:
Formats, or presents, data from the source device into a compatible form for receipt by the destination device. (Quick Time, MPEG, GIF JPEG, PNG)
Compression of the data in a manner that can be decompressed by the destination device.
Encryption of the data for transmission and the decryption of data upon receipt by the destination.
6. The Session Layer functions
This layer create and maintain dialog between source and destination applications. The Session layer handles the exchange of information to initiate dialog, keep them active, and to restart sessions that are disrupted or idle for a long period of time.
7. TCP/IP Application layer protocols:
DNS: resolve Internet names to IP addresses. TCP/UDP 53
HTTP(Hypertext Transfer Protocol): transfer files that make up the Web pages of the World Wide Web. TCP 80
SMTP(Simple Mail Transfer Protocol): the transfer of mail messages and attachments. TCP 25
POP(Post Office Protocol): used by email Clients to retrieve email from a remote server.
Telnet:a terminal emulation protocol, is used to provide remote access to servers and networking devices. TCP 23
FTP(File Transfer Protocol): interactive file transfer between systems. TCP 21/20
TFTP(Trivial File Transfer Protocol): connectionless active file transfer
DHCP: assign an IP address,subnet mask, default gateway, and DNS server address to a host.
BOOTP(Bootstrap Protocol): a precursor to the DHCP protocol. Obtain IP address
IMAP(Internet Message Access Protocol): another protocol for email retrieval
8. Application Layer Protocols and Services Examples
8.1 DNS Format
DNS is a client/server service; mapping name-to-IP address for internal hosts & forwarding name resolution requests between servers . Computer operating systems also have a utility called nslookup that allows the user to manually query the name servers to resolve a given host name.
A DNS server provides the name resolution using the name daemon, which is often called named
The DNS server stores different types of resource records used to resolve names. These records contain the name, address, and type of record. Some of these record types are:
A - an end device address
NS - an authoritative name server
CNAME - the canonical name(or Fully Qualified Domain Name) for an alias; used when multiple services have the single network address but each service has its own entry in DNS
MX - mail exchange record; maps a domain name to a list of mail exchange servers for that domain
The ipconfig /displaydns command displays all of the cached DNS entries on a Windows XP or 2000 computer system.
The Domain Name System uses a hierarchical system to create a name database to provide name resolution.
8.2 WWW Service and HTTP
HTTP specifies a request/response protocol. When a client, typically a web browser, sends a request message to a server, the HTTP protocol defines the message types the client uses to request the web page and also the message types the server uses to respond. The three common message types are GET, POST, and PUT.
GET is a client request for data. A web browser sends the GET message to request pages from a web server.
POST and PUT are used to send messages that upload data to the web server.
HTTPS(HTTP Secure): can use authentication and encryption to secure data as it travels between the client and server.
8.3 E-mail Services and SMTP/POP Protocols
SMTP is used to send mail from the client to the server but POP3 is used to download mail from the server to the client.
The e-mail client processes:
Mail User Agent (MUA) : an application that is used to compose email messages. These messages are then sent to MTA which sends it to the MDA for delivery to the destination.
if the mail is addressed to a user whose mailbox is on the local server, the mail is passed to the MDA. If the mail is for a user not on the local server, the MTA routes the email to the MTA on the appropriate server.
The Mail Transfer Agent (MTA) process is used by an email server to forward email messages destined to other email servers. The MDA process is used by the email server to deliver an email message to the mailbox that is hosted on the server. The MUA is an email client that allows a user to compose or read email. The POP3 is a protocol that is used by an email client to retrieve messages from the email server.
Some of the commands specified in the SMTP protocol are:
HELO - identifies the SMTP client process to the SMTP server process
EHLO - Is a newer version of HELO, which includes services extensions
MAIL FROM - Identifies the sender
RCPT TO - Identifies the recipient
DATA - Identifies the body of the message
IMAP and POP are protocols that are used to retrieve email messages. The advantage of using IMAP instead of POP is that when the user connects to an IMAP-capable server, copies of the messages are downloaded to the client application. IMAP then stores the email messages on the server until the user manually deletes those messages.
The destination email server may be offline or busy when email messages are sent. In either of these cases, SMTP spools messages to be sent at a later time. Periodically, the server checks the queue for messages and attempts to send them again. The message is returned to the sender as undeliverable if it is still not delivered after a predetermined expiration time.
8.4 FTP
The client establishes the first connection to the server on TCP port 21. This connection is used forcontrol traffic, consisting of client commands and server replies.
The client establishes the second connection to the server over TCP port 20. This connection is for theactual file transfer and is created every time there is a file transferred.
An FTP server runs an FTP daemon, which is a program that provides FTP services. End users who request services must run an FTP client program.
8.5 DHCP(Dynamic Host Configuration Protocol)
DHCP service enables devices on a network to obtain IP addresses and other information from a DHCP server. This service automates the assignment of IP addresses, subnet masks, gateway and other IP networking parameters.
DHCP is used for general purpose hosts such as end user devices, and fixed addresses are used for network devices such as gateways, switches, servers and printers.
DHCP Discover: is used to any available DHCP server on a network.
DHCP Offer: is used to acknowledge that the lease is successful.
DHCP Request: is used to identify the explicit server and lease offer to accept.
DHCP Acknowledge: is used to suggest a lease to a client.
When a host configured to use DHCP powers up on a network it sends a DHCPDISCOVER message. FF-FF-FF-FF-FF-FF is the L2 broadcast address. A DHCP server replies with a unicast DHCPOFFER message back to the host.
8.6 File Sharing Services and SMB Protocol
SMB (Server Message Block) is a client/server file sharing protocol. request-response protocol
SMB messages can:
Start, authenticate, and terminate sessions
Control file and printer access
Allow an application to send or receive messages to or from another device
IBM developed SMB to describe the structure of shared network resources. Unlike the file sharing supported by FTP, clients establish a long term connection to servers. SAMBA was originally developed by Andrew Tridgell to provide file and print services for various Microsoft Windows operating systems, as well as for UNIX and UNIX-like systems (Linux, Solaris, AIX, Mac OS X 10.2 and above, etc.) When downloading large numbers of files from the same server it be more efficient to use SMB to transfer files instead of FTP.
8.7 Telnet Services and Protocol
Telnet is a client/server protocol and it specifies how a VTY session is established and terminated.
Each Telnet command consists of at least two bytes. The first byte is a special character called the Interpret as Command (IAC) character.
Some sample Telnet protocol commands include:
Are You There (AYT) - Lets the user request that something appear on the terminal screen to indicate that the VTY session is active.
Erase Line (EL) - Deletes all text from the current line.
Interrupt Process (IP) - Suspends, interrupts, aborts, or terminates the process to which the Virtual Terminal is connected. For example, if a user started a program on the Telnet server via the VTY, he or she could send an IP command to stop the program.
As a best practice, network professionals should always use SSH in place of Telnet, whenever possible.
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标签：网络基础 应用层 传输层 网络应用 会话层 cisco
原文地址：http://elewei.blog.51cto.com/7519205/1669375
