To make clear that the total communications facility may consist of multiple networks, the constituent networks are usually referred to as subnetworks. This protocol enables the host to send data across the subnetwork to another host or, if the target host is on another subnetwork, to a router that will forward the data. IP is implemented in all of the end systems and the routers. It acts as a relay to move a block of data from one host, through one or more routers, to another host.
TCP keeps track of the blocks of data to assure that all are delivered reliably to the appropriate application.
Suppose that a process, associated with port 3 at host A, wishes to send a message to another process, associated with port 2 at host B. The process at A hands the message down to TCP with instructions to send it to host B, port 2.TCP hands the message down to IP with instructions to send it to host B. Note that IP need not be told the identity of the destination port. All it needs to know is that the data are intended for host B. Next, IP hands the message down to the network access layer (e.g., Ethernet logic) with instructions to send it to router J (the first hop on the way to B).
The network access layer appends its own header, creating a packet, or frame.The packet is transmitted across the subnetwork to router J. The packet header contains the information that the subnetwork needs to transfer the data across the subnet.
At router J, the packet header is stripped off and the IP header examined. On the basis of the destination address information in the IP header, the IP module in the router directs the datagram out across subnetwork 2 to B.
TCP and UDP
TCP provides a reliable connection for the transfer of data between applications : A logical connection refers to a given pair of port values. In addition to TCP, there is one other transport-level protocol that is in common use as part of the TCP/IP protocol suite: the User Datagram Protocol (UDP). UDP does not guarantee delivery, preservation of sequence, or protection against duplication.
UDP enables a procedure to send messages to other procedures with a minimum of protocol mechanism : for example, SNMP (Simple Network Management Protocol).
IP and IPv6
For decades, the keystone of the TCP/IP protocol architecture has been IP.
The header, together with the segment from the transport layer, forms an IP-level PDU referred to as an IP datagram or an IP packet.The header includes 32-bit source and destination addresses.
In 1995, the Internet Engineering Task Force (IETF), which develops protocol standards for the Internet, issued a specification for a next-generation IP, known then as IPng. This specification was turned into a standard in 1996 known as IPv6.
DS : Differentiated services field
ECN : Explicit congestion notification field
TCP/IP Applications
A number of applications have been standardized to operate on top of TCP :
SMTP (Simple Mail Transfer Protocol) : basic electronic mail transport facility, mechanism for transferring messages among separate hosts. Once a message is created, SMTP accepts the message and makes use of TCP to send it to an SMTP module on another host.
FTP (File Transfer Protocol) : used to send files from one system to another under user command. FTP sets up a TCP connection to the target system for the exchange of control messages. This connection allows user ID and password to be transmitted and allows the user to specify the file and file actions desired.
TELNET
etc.