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16
Chapter 1
Internetworking
end-to-end data transport services and can establish a logical connection
between the sending host and destination host on an internetwork.
Some of you are probably familiar with TCP and UDP already. (But if
you're not, no worries--I'll tell you all about them in Chapter 3.) If so, you
know that both work at the Transport layer, and that TCP is a reliable ser-
vice and UDP is not. This means that application developers have more
options because they have a choice between the two protocols when working
with TCP/IP protocols.
The Transport layer is responsible for providing mechanisms for multi-
plexing upper-layer applications, establishing sessions, and tearing down
virtual circuits. It also hides details of any network-dependent information
from the higher layers by providing transparent data transfer.
The term "reliable networking" can be used at the Transport layer.
Flow Control
Data integrity is ensured at the Transport layer by maintaining flow control
and by allowing users to request reliable data transport between systems.
Flow control prevents a sending host on one side of the connection from
overflowing the buffers in the receiving host--an event that can result in lost
data. Reliable data transport employs a connection-oriented communications
session between systems, and the protocols involved ensure that the follow-
ing will be achieved:
The segments delivered are acknowledged back to the sender upon
their reception.
Any segments not acknowledged are retransmitted.
Segments are sequenced back into their proper order upon arrival at
their destination.
A manageable data flow is maintained in order to avoid congestion,
overloading, and data loss.
Connection-Oriented Communication
In reliable transport operation, one device first establishes a connection-
oriented session with its peer system. This is called a call setup, or a three-way
handshake (Cisco's term). Data is then transferred, and when finished, a call
termination takes place to tear down the virtual circuit.
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