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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Layer 3
Commonly referred to as the routing layer, this layer provides logical partitioning of networks
(subnetting), scalability, security, and Quality of Service (QoS).
The backbone of the Internet is built using Layer 3 functions. IP is the premier Layer 3 protocol.
IP is itself, only one protocol in the IP protocol suite. More extensive capabilities are found in the other
protocols of the IP suite. For example; the Domain Name System (DNS) associates IP addresses with
text names, the Dynamic Host Configuration Protocol (DHCP) eases the administration of IP addresses,
and routing protocols such as the Routing Information Protocol (RIP), the Open Shortest Path First
(OSPF), and the Border Gateway Protocol (BGP) enable Layer 3 devices to direct data traffic to the
intended destination. IP security allows for authentication and encryption. IP not only allows for user-
to-user communication, but also for transmission from point-to-multipoint (known as IP multicasting).
Layer 4
This layer, known as the transport layer, establishes the communication path between user
applications and the network infrastructure and defines the method of communicating. TCP and UDP
are well-known protocols in the transport layer. TCP is a “connection-oriented” protocol, and requires
the establishment of parameters for transmission prior to the exchange of data. Web technology is
based on TCP. UDP is “connectionless” and requires no connection setup. This is important for
multicast traffic, which cannot tolerate the overhead and latency of TCP. TCP and UDP also differ in the
amount of error recovery provided and whether or not it is visible to the user application. Both TCP and
UDP are layered on IP, which has minimal error recovery and detection. TCP forces retransmission of
data that was lost by the lower layers, UDP does not.
Layer 7
This layer, known as the application layer, provides access to either the end user application software
such as a database. Users communicate with the application, which in turn delivers data to the
transport layer. Applications do not usually communicate directly with lower layers. They are written to
use a specific communication library, like the popular WinSock library.
Software developers must decide what type of transport mechanism is necessary. For example, Web
access requires reliable, error-free access and would demand TCP. Multimedia, on the other hand,
requires low overhead and latency and commonly uses UDP.
TCP/IP
The TCP/IP protocol suite is a set of protocols that allow computers to share resources across a
network. TCP and IP are only two of the Internet suite of protocols, but they are the best known and it
has become common to refer the entire family of Internet protocols as TCP/IP.
TCP/IP is a layered set of protocols. An example, such as sending e-mail, can illustrate this. There is
first a protocol for sending and receiving e-mail. This protocol defines a set of commands to identify the
sender, the recipient, and the content of the e-mail. The e-mail protocol will not handle the actual
communication between the two computers as this is done by TCP/IP. TCP/IP handles the actual
sending and receiving of the packets that make up the e-mail exchange.
TCP makes sure the e-mail commands and messages are received by the appropriate computers. It
keeps track of what is sent and what is received, and retransmits any packets that are lost or dropped.
TCP also handles the division of large messages into several Ethernet packets, and makes sure these
packets are received and reassembled in the correct order.
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