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Indirect Device Connection and Message Routing
Most of the explanations that I have
provided in the other topics of this section have discussed the mechanisms
by which machines connect to each other over a network directly.
However, one of the most powerful aspects of networking is that it is
possible to create internetworksnetworks of networkswhich
allow devices to be connected indirectly. For example, machine
A may send a message to machine B without really
even knowing where it is on the network at all.
If a message is being sent between
devices that are not on the same network, then it must be passed between
directly-connected networks until it reaches its final destination.
The process of transmitting a message from one network to another is
called forwarding, and the collective process of forwarding from
one device to another is routing. These concepts are fundamental
to all internetworking, including the Internet itself. Every time you
access an Internet resource such as a Web site, you are sending messages
that get routed to that site, and the responses you receive get routed
Note: Even though the technically-correct term for moving a message from one network to an adjacent network is forwarding, over time the term routing has come to often be used both for a single network-to-network transfer as well as the overall process of transmitting a message from one device to another.
In the context of the OSI Reference
Model, routing is an activity that generally takes place at the network
layerlayer 3. Recall that data
encapsulation causes a higher-layer message
to be surrounded by headers and/or footers at the lower layers. When
a message is routed, here's what happens:
- A high-level application on a machine decides
to send a datagram to a distant computer. The datagram is packaged,
and then passed
down vertically through the protocol stack
on the originating machine. Each layer encapsulates the data as described
earlier. The datagram is addressed to the final destination device.
When the message gets to the network layer and below, however, it is
not packaged for local delivery directly to its ultimate destination,
but rather to an intermediate device. This is the device that
is responsible for routing to that destination network. The message
is passed down to the data link layer and then the physical layer for
transmission to that intermediate device.
- The intermediate device (often called a router)
receives the message at the physical layer. It is passed up to the data
link layer, where it is processed, checked for errors and so on, and
the data link layer headers are removed. The resulting packet is passed
up to the network layer. There, the intermediate device determines if
the destination machine is on its local network, or if it needs to be
forwarded to another intermediate device. It then repackages the message
and passes it back down to the data link layer to be sent on
the next leg of its journey.
- After several potential intermediate devices
handle the message, it eventually reaches its destination.
Here, it travels back up the protocol stack until it reaches the same
layer as the one of the application that generated the message on the
Figure 17: Message Routing in the OSI Reference Model
This diagram shows how routing is accomplished conceptually in the OSI model. The intermediate device connects the networks of the message transmitter and recipient. When data is sent, it is passed up to the network layer on the intermediate device, where it is repackaged and sent back down the stack for the next leg of its transmission. Note that the intermediate device actually has two different layer 1 and 2 implementations; one for the interface to each network. Also note that while the layer 3 protocol must be the same across the internetwork, each network can use different technologies at layers 1 and 2.
The key to this description is that
in the intermediate devices, the message travels back up the OSI layers
only to the network layer. It is then repackaged and sent back
along its way. The higher layers are only involved on the source and
destination devices. The protocol used at layer 3 must be common across
the internetwork but each individual network can be different. This
demonstrates some of the power of layering, by enabling even rather
dissimilar physical networks to be connected together. The process is
illustrated in Figure 17.
Key Concept: In the OSI model, the process of routing occurs when data is sent not directly from transmitter to ultimate recipient, but indirectly through the use of an intermediate system. That device, normally called a router, connects to two or more physical networks and thus has multiple interfaces to layer two. When it receives data, the data passes up only to the network layer, where it is repackaged and then sent on the next leg of its journey over the appropriate layer two interface.
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Version 3.0 - Version Date: September 20, 2005
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