48 Networking Explained, Second Edition
Additionally, each layer consists of two parts: a service definition, which defines the type
of service a layer provides, and a protocol specification that details the rules governing the
implementation of a particular service. Lower layers provide services to upper layers.
42. What kinds of services do layers provide to each other?
There are two different types of services: connection-oriented and connectionless.
Some layers have an additional type of service called multiplexing, but this does not neces-
sarily transcend all layers of the architecture. Services are available at service access
points (SAPs), with each SAP having a corresponding address. (Note: In UNIX, a SAP is
called a socket, and a SAP address is a socket number.)
43. Define connection-oriented service.
Connection-oriented implies that prior to the transfer of data a physical (and virtual)
link is established between the sending and receiving nodes. This link remains in effect for
the duration of the session. After the session is completed, the link is removed. Character-
istics of a connection-oriented service include: wasted bandwidth, because the link must
remain established even during idle periods of a transmission; a high potential for a hung
network, since there is always a possibility that a link will not be terminated; and (on the
bright side) guaranteed sequential arrival of packets at the destination node.
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2
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6
5
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Data Message
Data Message
FIGURE 2.13 How layers work. Each layer “envelops” the data with its protocol. Each layer has a
corresponding layer on the remote (destination) node, which is called a peer.

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