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Introduction to Networks Session 3
Transcript of Introduction to Networks Session 3
But HTTP is not secured..
Solution is SHTTP
How does DNS work?
Application Layer Functionality & Protocols
Peer 2 Peer Network?
DNS Service and Protocol
WWW and HTTP Services
Which product do you
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. PUT uploads resources or content to the web server.
DNS is a client/server service is different from the other client/server services that we are examining. While other services use a client that is an application (such as web browser, e-mail client), the DNS client runs as a service itself. The DNS client, sometimes called the DNS resolver. 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.
The e-mail server operates two separate processes:
Mail Transfer Agent (MTA)
Mail Delivery Agent (MDA)
The Mail Transfer Agent (MTA) process is used to forward e-mail. As shown in the figure, the MTA receives messages from the MUA or from another MTA on another e-mail server. Based on the message header, it determines how a message has to be forwarded to reach its 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 e-mail to the MTA on the appropriate server.
A DNS server provides the name resolution using the name daemon. 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
How DNS Works?
Everbody likes to laugh...
What is OSI / TCP IP Model?
Can you remember the 7 layers of OSI Model?
The Presentation layer has three primary functions:
Coding and conversion of Application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device.
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.
Widely used TCP IP protocols are:
Domain Name Service Protocol (DNS) is used to resolve Internet names to IP addresses.
Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the Web pages of the World Wide Web.
Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and attachments.
Telnet, a terminal emulation protocol, is used to provide remote access to servers and networking devices.
File Transfer Protocol (FTP) is used for interactive file transfer between systems.
Protocols establish consistent rules for exchanging data between applications and services loaded on the participating devices. Protocols specify how data inside the messages is structured and the types of messages that are sent between source and destination.
Many different types of applications communicate across data networks. Therefore, Application layer services must implement multiple protocols to provide the desired range of communication experiences. Each protocol has a specific purpose and contains the characteristics required to meet that purpose.
What is a Server?
In a general networking context, any device that responds to requests from client applications is functioning as a server. A server is usually a computer that contains information to be shared with many client systems. For example, web pages, documents, databases, pictures, video, and audio files can all be stored on a server and delivered to requesting clients. In other cases, such as a network printer, the print server delivers the client print requests to the specified printer.
In a client/server network, the server runs a service, or process, sometimes called a server daemon. Like most services, daemons typically run in the background and are not under an end user's direct control. Daemons are described as "listening" for a request from a client, because they are programmed to respond whenever the server receives a request for the service provided by the daemon. When a daemon "hears" a request from a client, it exchanges appropriate messages with the client, as required by its protocol, and proceeds to send the requested data to the client in the proper format.
How to protect the organisation from these threats?
EMail Services & SMTP Protocol
Two example Application layer protocols are Post Office Protocol (POP) and Simple Mail Transfer Protocol (SMTP). When people compose e-mail messages, they typically use an application called a Mail User Agent (MUA), or e-mail client. The MUA allows messages to be sent and places received messages into the client's mailbox, both of which are distinct processes.
In order to receive e-mail messages from an e-mail server, the e-mail client can use POP. Sending e-mail from either a client or a server uses message formats and command strings defined by the SMTP protocol. Usually an e-mail client provides the functionality of both protocols within one application.
The Application layer, Layer seven, is the top layer of both the OSI and TCP/IP models. It is the layer that provides the interface between the applications we use to communicate and the underlying network over which our messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts. There are many Application layer protocols and new protocols are always being developed.
The Presentation Layer
What session layer will do?
Port numbers identify applications and Application layer services that are the source and destination of data. Server programs generally use predefined port numbers that are commonly known by clients.
Domain Name System (DNS) - TCP/UDP Port 53
Hypertext Transfer Protocol (HTTP) - TCP Port 80
Simple Mail Transfer Protocol (SMTP) - TCP Port 25
Post Office Protocol (POP) - TCP Port 110
Telnet - TCP Port 23
Dynamic Host Configuration Protocol - UDP Ports 67 and 68
File Transfer Protocol (FTP) - TCP Ports 20 and 21
Virtual Communication Ports
Try the command ipconfig /displaydns which will show you the cached DNS
The Domain Name System uses a hierarchical system to create a name database to provide name resolution. The hierarchy looks like an inverted tree with the root at the top and branches below.
At the top of the hierarchy, the root servers maintain records about how to reach the top-level domain servers, which in turn have records that point to the secondary level domain servers and so on.
Top level domain
au - Australia
.co - Colombia
.com - a business or industry
.jp - Japan
.org - a non-profit organization
When a web address (or URL) is typed into a web browser, the web browser establishes a connection to the web service running on the server using the HTTP protocol. URLs (or Uniform Resource Locator) and URIs (Uniform Resource Identifier) are the names most people associate with web addresses. To better understand how the web browser and web client interact, we can examine how a web page is opened in a browser. For this example, we will use the URL: http://www.cisco.com/web-server.htm. First, the browser interprets the three parts of the URL:
1. http (the protocol or scheme)
2. www.cisco.com (the server name)
3. web-server.htm (the specific file name requested).
The browser then checks with a name server to convert www.cisco.com into a numeric address, which it uses to connect to the server. Using the HTTP protocol requirements, the browser sends a GET request to the server and asks for the file web-server.htm. The server in turn sends the HTML code for this web page to the browser. Finally, the browser deciphers the HTML code and formats the page for the browser window.
The File Transfer Protocol (FTP) is another commonly used Application layer protocol. FTP was developed to allow for file transfers between a client and a server. An FTP client is an application that runs on a computer that is used to push and pull files from a server running the FTP daemon (FTPd).
To successfully transfer files, FTP requires two connections between the client and the server: one for commands and replies, the other for the actual file transfer.
The client establishes the first connection to the server on TCP port 21. This connection is used for control traffic, consisting of client commands and server replies. The file transfer can happen in either direction. The client can download (pull) a file from the server or, the client can upload (push) a file to the server.
Dynamic Host Configuration Protocol
The 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 allows a host to obtain an IP address dynamically when it connects to the network. The DHCP server is contacted and an address requested. The DHCP server chooses an address from a configured range of addresses called a pool and assigns ("leases") it to the host for a set period.
How DHCP Works?
With P2P applications based on the Gnutella protocol, people can make files on their hard disks available to others for downloading. Gnutella-compatible client software allows users to connect to Gnutella services over the Internet and to locate and access resources shared by other Gnutella peers.
Many client applications are available for accessing the Gnutella network, including: BearShare, Gnucleus, LimeWire, Morpheus, WinMX and XoloX (see a screen capture of LimeWire in the figure). While the Gnutella Developer Forum maintains the basic protocol, application vendors often develop extensions to make the protocol work better on their applications.
Lets prepare a cable for network connection
Recap of today's session: Different types of protocols which work in Application Layer