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Glossary
• Layer 3, the Network layer, defines the how to determine the best path for information traversing
the network. Packets and logical
IP
Addresses operate on the network layer.
• Layer 4, the Transport layer, defines connection oriented protocols such as
TCP
and UDP.
• Layer 5, the Session layer, defines protocols for initiating, maintaining, and ending communication
and transactions across the network. Some common examples of protocols that operate on this
layer are network file system (NFS) and structured query language (SQL). Also part of this layer
are communication flows like single mode (device sends information bulk), half-duplex mode
(devices take turns transmitting information in bulk), and full-duplex mode (interactive, where
devices transmit and receive simultaneously).
• Layer 6, the Presentation layer, defines how information is presented to the application. It includes
meta-information about how to encrypt/decrypt and compress/decompress the data. JPEG and
TIFF file formats are examples of protocols at this layer.
• Layer 7, the Application layer, includes protocols like hypertext transfer protocol (HTTP), simple
mail transfer protocol (SMTP), and file transfer protocol (FTP).
Packet
Data and media are transmitted among nodes on a network in the form of packets. Data and
multimedia content is divided up and packaged into packets. A packet includes a small chunk
of the content to be sent along with its destination address and sender address. Packets are
pushed out onto the network and inspected by each node. The node to which it is addressed is
the ultimate recipient.
Packet Loss
Packet Loss describes the percentage of packets transmitted over the network that did not reach
their intended destination. A 0 percent package loss indicates no packets were lost in transmission.
QoS
features are designed to minimize packet loss.
PHY
The Physical Layer (PHY) is the lowest layer in the network layer model (see OSI). The Physical
Layer conveys the bit stream - electrical impulse, light or radio signal — through the network at
the electrical and mechanical level. It provides the hardware means of sending and receiving
data on a medium, including defining cables, NICs, and physical aspects.
Ethernet
and the
802.11
family are protocols with physical layer components.
PID
The Process Identifier (PID) is an integer used by Linux to uniquely identify a process. A PID is
returned by the fork() system call. It can be used by wait() or kill() to perform actions
on the given process.
Port Forwarding
Port Forwarding creates a 'tunnel' through a firewall, allowing users on the Internet access to a
service running on one of the computers on your LAN, for example, a Web server, an FTP or
SSH server, or other services. From the outside user's point of view, it looks like the service is
running on the firewall.
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