Understanding the network.pdf - Back to Home

AppleTalk Echo Protocol (AEP)
Name Binding Protocol (NBP)
• IPX—Service Advertisement Protocol (SAP)
Regardless of the mode, it is the responsibility of Layer 4 to optimize the use of the
network's resources.
The lower layers (3, 2, and 1) of the OSI model are where the "network" actually
starts. Under OSI, the network is slightly more abstract than under the TCP/IP
model because it was created from the perspective of telecommunications providers.
Although this perspective is not apparent when you look at the standards
themselves, it becomes clearer when you try to implement WAN technology.
Layer 3: Network
The network layer is where the actual delivery of the transport data units takes
place. The network layer provides the delivery and addressing services needed to
move the transport data units from host to host. This is accomplished by sequencing
the data into data packets and adding a delivery information header with the source
and destination addresses and any additional sequencing information. This
packeted data is known as a datagram.
Layer 3 is also responsible for delivery of the datagrams, requiring some kind of
routing service. Under the Internet-RM, this is handled at the Internet layer. Under
OSI, the activity of datagram routing is seen as two processes: routing and
forwarding. Routing is the process of seeking and finding network information.
Forwarding is the actual process of using the routing information to move data from
host to host. The OSI model sees routing as a three-tiered hierarchy:
• Tier 1 is route discovery.
• Tier 2 is intradomain routing.
• Tier 3 is interdomain routing.
In an OSI environment, there are two types of network layer transport services:
• Connectionless service handled by connectionless network (CLNP)
• Connection-oriented service handled by CONS X-25 (connection-oriented
network service over X.25)
With connection-oriented transport, confirmation of delivery is required after each
datagram is delivered. With connectionless transport, datagrams are sent by the
best-known path; as the packet is moved from intermediate router to intermediate