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.
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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.
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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 : provides a remote logon capability, which enables a user at a terminal or personal computer to logon to a remote computer and function as if directly connected to that computer. TELNET is implemented in 2 modules (user telnet which interacts with the terminal I/O module to communicate with a local terminal & server telnet which interacts with an app, acting as a surrogate terminal handler) - Traffic between the 2 modules is carried on a TCP connection.
etc.
Protocol Interfaces
Each layer in the TCP/IP protocol suite interacts with its immediate adjacent layers. This use of each individual layer is not required by the architecture.
Most applications require a reliable end-to-end protocol and thus make use of TCP. Some special-purpose applications do not need the services of TCP. Some of these applications, such as the Simple Network Management Protocol (SNMP), use an alternative end-to-end protocol known as the User Datagram Protocol (UDP); others may make use of IP directly.
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