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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Datagram Delivery and Routing

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IP Datagram Delivery and Routing
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IP Routing Concepts and the Process of Next-Hop Routing
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IP Datagram Direct Delivery and Indirect Delivery (Routing)
(Page 1 of 3)

The overall job of the Internet Protocol is to transmit messages from higher layer protocols over an internetwork of devices. These messages must be packaged and addressed, and if necessary fragmented, and then they must be delivered. The process of delivery can be either simple or complex, depending on the proximity of the source and destination devices.

Datagram Delivery Types

Conceptually, we can divide all IP datagram deliveries into two general types, shown graphically in Figure 91:


Figure 91: Direct and Indirect (Routed) Delivery of IP Datagrams

This diagram shows three examples of IP datagram delivery. The first transmission (highlighted in green) shows a direct delivery between two devices on the local network. The second (purple) shows indirect delivery within the local network, between a client and server separated by a router. The third shows a more distant indirect delivery, between a client on the local network and a server across the Internet.

 


  • Direct Datagram Deliveries: When datagrams are sent between two devices on the same physical network, it is possible for datagrams to be delivered directly from the source to the destination. Imagine that you want to deliver a letter to a neighbor on your street. You probably wouldn't bother mailing it through the post office; you'd just put the neighbor’s name on the envelope and stick it right into his or her mailbox.

  • Indirect Datagram Deliveries: When two devices are not on the same physical network, the delivery of datagrams from one to the other is indirect. Since the source device can't see the destination on its local network, it must send the datagram through one or more intermediate devices to deliver it. Indirect delivery is analogous to mailing a letter to a friend in a different city. You don't deliver it yourself—you put it into the postal system. The letter journeys through postal system, possibly taking several intermediate steps, and ends up in your friend's neighborhood, where a postal carrier puts it into his or her mailbox.

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IP Datagram Delivery and Routing
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IP Routing Concepts and the Process of Next-Hop Routing
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Version 3.0 - Version Date: September 20, 2005

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