DCN-Protocol

HyperText Transfer Protocol (HTTP)

HTTP (Hypertext Transfer Protocol) is perhaps the most popular application protocol used in the Internet (or The WEB).

• HTTP is an asymmetric request-response client-server protocol as illustrated.  An HTTP client sends a request message to an HTTP server.  The server, in turn, returns a response message.  In other words, HTTP is a pull protocol, the client pulls information from the server (instead of server pushes information down to the client).

• HTTP is a stateless protocol. In other words, the current request does not know what has been done in the previous requests.
• HTTP permits negotiating of data type and representation, so as to allow systems to be built independently of the data being transferred.
• Quoting from the RFC2616: "The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name servers and distributed object management systems, through extension of its request methods, error codes and headers."

Transmission Control Protocol (TCP)

TCP is connection-oriented in the sense that prior to transmission end points need to establish a connection first. TCP protocol data units are called segments. The sending and receiving TCP entities exchange data in the form of segments, which consist of a fixed 20-byte header followed by a variable size data field.
TCP is responsible for breaking down a stream of bytes into segments and reconnecting them at the other end, retransmitting whatever might be lost and also organizing the segments in the correct order

• Source Port and Destination Port fields together identify the two local end points of the particular connection. A port plus its hosts’ IP address forms a unique end point. Ports are used to communicate with the upper layer and distinguish different application sessions on the host.
• The Sequence Number and Acknowledgment Number fields specify bytes in the byte stream. The sequence number is used for segment differentiation and is useful for reordering or retransmitting lost segments. The Acknowledgment number is set to the next segment expected.
• Data offset or TCP header length indicates how many 4-byte words are contained in the TCP header.
• The Window field indicates how many bytes can be transmitted before an acknowledgment is received.
• The Checksum field is used to provide extra reliability and security to the TCP segment.
• The actual user data are included after the end of the header.

User Datagram Protocol (UDP)

UDP protocol consists of fewer fields compared to TCP. The reason for that is because certain data types do not require reliable delivery and extra overhead. Real-time traffic for example, needs to be transported in an efficient way without error correction and retransmission mechanisms.
UDP is considered to be a connectionless protocol. It leaves reliability to be handled by the application layer.

TCP (Transmission Control Protocol) is a transport-layer protocol, responsible for establish a connection between two machines. TCP consists of 2 protocols: TCP and UDP (User Datagram Package).  TCP is reliable, each packet has a sequence number, and an acknowledgement is expected.  A packet will be re-transmitted if it is not received by the receiver.  Packet delivery is guaranteed in TCP.  UDP does not guarantee packet delivery, and is therefore not reliable.  However, UDP has less network overhead and can be used for applications such as video and audio streaming, where reliability is not critical.

IP

IP (Internet Protocol) is a network-layer protocol, deals with network addressing and routing. In an IP network, each machine is assigned an unique IP address (e.g., 165.1.2.3), and the IP software is responsible for routing a message from the source IP to the destination IP. In IPv4 (IP version 4), the IP address consists of 4 bytes, each ranges from 0 to 255, separated by dots, which is called a quad-dotted form.  This numbering scheme supports up to 4G addresses on the network.  The latest IPv6 (IP version 6) supports more addresses.  Since memorizing number is difficult for most of the people, an english-like domain name, such as www.test101.com is used instead.  The DNS (Domain Name Service) translates the domain name into the IP address (via distributed lookup tables). A special IP address 127.0.0.1 always refers to your own machine.  It's domian name is "localhost" and can be used for local loopback testing.

ARP stands for Address Resolution Protocol and RARP stands for Reverse Address Resolution Protocol. ARP convert IP address to physical address(i.e. ethernet address). RARP convert physical address to the IP address

IMAP versus POP3 comparison chart
	IMAP	POP3
Introduction (from Wikipedia)	Internet message access protocol (IMAP) is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol (POP).	In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection.
Speed	Slow	Fast
Storage of content	Always on server	Downloaded onto local device, unless ‘keep a copy on the server’ selected.
Mail Syncing	Yes	No
Direction	Bi-directional - Whatever changes you make on server or device, the other side shows the changes too.	One-direction - Changes made on device have no effect on server content.
Offline Use (no internet connection)	Mails downloaded can be read or deleted when offline. Changes made on the device are synced with the server when device goes online.	Mails can be read offline once downloaded; device only needs to go online to download email.
Port used by server	Port 143	Port 110
Consolidation of multiple accounts	No	Yes
Keep messages on the server	Required	Possible
Current version	IMAP4rev1	POP3
Year of Creation	Imap 2 was first specified in 1990 (RFC 1176)	First specified in paper 1988 (RFC 1081)
Previous versions	IMAP, IMAP2, IMAP3 (never accepted), IMAP2bis (renamed IMAP4)	POP (POP1), POP2, POP3, POP4 (proposed)

The File Transfer Protocol (FTP) 

is used to transfer files between two computers over a network and Internet.Transferring files from a client computer to a server computer is called "uploading" and transferring from a server to a client is "downloading".

Active and Passive Connection Mode

The FTP server may support Active or Passive connections, or both.  In an Active FTP connection, the client opens a port and listens and the server actively connects to it.  In a Passive FTP connection, the server opens a port and listens (passively) and the client connects to it.  You must grant Auto FTP Manager access to the Internet and to choose the right type of FTP Connection Mode.
Most FTP client programs select passive connection mode by default because server administrators prefer it as a safety measure.  Firewalls generally block connections that are "initiated" from the outside.  Using passive mode, the FTP client (like Auto FTP Manager) is "reaching out" to the server to make the connection.  The firewall will allow these outgoing connections, meaning that no special adjustments to firewall settings are required

 

 stateless protocol  vs statefull protocol

In stateless protocol there is no record of the state is saved at server end.Client send request to the server and server response back according to current state. A stateless server does not keeps state between connections.So,When you send a request to a stateless server, it does not create any objects that track information regarding your requests. ex.UDP , HTTP etc.

On the other hand in statefull protocol there is tight dependency between client and server . If client send a request to the server then it expects some kind of response, if it does not get any responce then resend request.A stateful server keeps state of connections. ex.FTP , Talnet.
Even you can force to make stateless behave like stateful if need by keep data using session , cookies.

connectionless and connection oriented communication

	connection less	connection oriented
connection between source (sender) and destination	no need	must established before data transfer
reliable	less	more
HANDSHAKING process	Not use	use
bandwidth	less	high
guaranteed delivery	no	yes
example	IP ,UDP, ICMP, DNS, TFTP , SNMP	TCP,HTTP,SMTP,FTP,TELNET

* In conection-oriented system a HANDSHAKING process is used i.e the process tsarts from the sender computer by 

sending a request to start the transfer that is to be acknowledged by the destination device before the data can be send.

while in CONNECTIONLESS system no handshaking process is used. if one computer wants to send data,it just 

sends the data packet to the end system.