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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 neighbors 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 yourselfyou 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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Version 3.0 - Version Date: September 20, 2005
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