Introduction
This Article will provide a basic overview of some common
networking
concepts - basic terminology, common protocols, and
the
responsibilities and characteristics of the different
layers of networking.
Networking Glossary
Before we begin discussing networking with any depth, we must
define
some common terms that you will see throughout this guide,
and in other
guides and documentation regarding networking.
These terms will be expanded upon in the appropriate sections
that follow:
·
Connection: In
networking, a connection refers to pieces of related information that
are transfered through a network. This geneally infers that a
connection is built before the data transfer (by following the procedures
laid out in a protocol) and then is deconstructed at the at the end of the
data transfer.
·
Packet:
A packet is, generally speaking, the most basic unit that is
transfered over a network. When communicating over a network, packets are
the envelopes that carry your data (in pieces) from one end point to the other.
Packets have a header portion that contains information about the packet
including the source and destination, timestamps, network hops, etc. The main
portion of a packet contains the actual data being transfered. It is sometimes
called the body or the payload.
·
Network
Interface: A
network interface can refer to any kind of software interface to networking
hardware. For instance, if you have two network cards in your computer, you can
control and configure each network interface associated with them individually.
A network interface may be associated with a physical device, or it may be a
representation of a virtual interface. The "loopback" device, which
is a virtual interface to the local machine, is an example of this.
·
LAN:
LAN stands for "local area network". It refers to a
network or a portion of a network that is not publicly accessible to the
greater internet. A home or office network is an example of a LAN.
·
WAN:
WAN stands for "wide area network". It means a network
that is much more extensive than a LAN. While WAN is the relevant term to use
to describe large, dispersed networks in general, it is usually meant to mean
the internet, as a whole.
If an interface is said to be connected to the
WAN, it is generally assumed that it is reachable through the internet.
·
Protocol:
A protocol is a set of rules and standards that basically define
a language that devices can use to communicate. There are a great number of
protocols in use extensively in networking, and they are often implemented in
different layers.Some low level protocols are TCP, UDP, IP, and ICMP. Some
familiar examples of application layer protocols, built on these lower
protocols, are HTTP (for accessing web content), SSH, TLS/SSL, and FTP.
·
Port: A
port is an address on a single machine that can be tied to a specific piece of
software. It is not a physical interface or location, but it allows your server
to be able to communicate using more than one application.
·
Firewall: A
firewall is a program that decides whether traffic coming into a server or
going out should be allowed. A firewall usually works by creating rules for
which type of traffic is acceptable on which ports. Generally, firewalls block
ports that are not used by a specific application on a server.
·
NAT: NAT
stands for network address translation. It is a way to translate requests that
are incoming into a routing server to the relevant devices or servers that it
knows about in the LAN. This is usually implemented in physical LANs as a way
to route requests through one IP address to the necessary backend servers.
·
VPN: VPN
stands for virtual private network. It is a means of connecting separate LANs
through the internet, while maintaining privacy. This is used as a means of
connecting remote systems as if they were on a local network, often for
security reasons.
Network Layers
While networking is often discussed in terms of topology in a
horizontal way, between hosts, its implementation is layered in a vertical
fashion throughout a computer or network.
What this means is that there are multiple technologies and
protocols that are built on top of each other in order for communication to
function more easily. Each successive, higher layer abstracts the raw data a
little bit more, and makes it simpler to use for applications and users.
It also allows you to leverage lower layers in new ways without
having to invest the time and energy to develop the protocols and applications
that handle those types of traffic.
The language that we use to talk about each of the layering
scheme varies significantly depending on which model you use. Regardless of the
model used to discuss the layers, the path of data is the same.
As data is sent out of one machine, it begins at the top of the
stack and filters downwards. At the lowest level, actual transmission to
another machine takes place. At this point, the data travels back up through
the layers of the other computer.
Each layer has the ability to add its own "wrapper"
around the data that it receives from the adjacent layer, which will help the
layers that come after decide what to do with the data when it is passed off.
OSI Model
Historically, one method of talking about the different layers
of network communication is the OSI model. OSI stands for Open Systems Interconnect.
This model defines seven separate layers. The layers in this
model are:
·
Application: The
application layer is the layer that the users and user-applications most often
interact with. Network communication is discussed in terms of availability of
resources, partners to communicate with, and data synchronization.
·
Presentation: The
presentation layer is responsible for mapping resources and creating context.
It is used to translate lower level networking data into data that applications
expect to see.
·
Session: The
session layer is a connection handler. It creates, maintains, and destroys
connections between nodes in a persistent way.
·
Transport: The
transport layer is responsible for handing the layers above it a reliable
connection. In this context, reliable refers to the ability to verify that a
piece of data was received intact at the other end of the connection.
This layer can resend information that has been dropped or
corrupted and can acknowledge the receipt of data to remote computers.
·
Network: The
network layer is used to route data between different nodes on the network. It
uses addresses to be able to tell which computer to send information to. This
layer can also break apart larger messages into smaller chunks to be
reassembled on the opposite end.
·
Data
Link: This layer is implemented as a method of
establishing and maintaining reliable links between different nodes or devices
on a network using existing physical connections.
·
Physical: The
physical layer is responsible for handling the actual physical devices that are
used to make a connection. This layer involves the bare software that manages
physical connections as well as the hardware itself (like Ethernet).
As you can see, there are many different layers that can be
discussed based on their proximity to bare hardware and the functionality that
they provide.
TCP/IP Model
The TCP/IP model, more commonly known as the Internet protocol
suite, is another layering model that is simpler and has been widely adopted.
It defines the four separate layers, some of which overlap with the OSI model:
·
Application: In
this model, the application layer is responsible for creating and transmitting
user data between applications. The applications can be on remote systems, and
should appear to operate as if locally to the end user.
The communication is said to take place between peers.
·
Transport: The
transport layer is responsible for communication between processes. This level
of networking utilizes ports to address different services. It can build up
unreliable or reliable connections depending on the type of protocol used.
·
Internet: The
internet layer is used to transport data from node to node in a network. This
layer is aware of the endpoints of the connections, but does not worry about
the actual connection needed to get from one place to another. IP addresses are
defined in this layer as a way of reaching remote systems in an addressable
manner.
·
Link: The
link layer implements the actual topology of the local network that allows the
internet layer to present an addressable interface. It establishes connections
between neighboring nodes to send data.
As you can see, the TCP/IP model, is a bit more abstract and
fluid. This made it easier to implement and allowed it to become the dominant
way that networking layers are categorized.
Interfaces
Interfaces are networking communication points for your
computer. Each interface is associated with a physical or virtual networking
device.
Protocols
Networking works by piggybacking a number of different protocols
on top of each other. In this way, one piece of data can be transmitted using
multiple protocols encapsulated within one another.
We will talk about some of the more common protocols that you
may come across and attempt to explain the difference, as well as give context
as to what part of the process they are involved with.
We will start with protocols implemented on the lower networking
layers and work our way up to protocols with higher abstraction.
Media Access Control
Media access control is a communications protocol that is used
to distinguish specific devices. Each device is supposed to get a unique MAC
address during the manufacturing process that differentiates it from every
other device on the internet.
Addressing hardware by the MAC address allows you to reference a
device by a unique value even when the software on top may change the name for
that specific device during operation.
Media access control is one of the only protocols from the link
layer that you are likely to interact with on a regular basis.
IP
The IP protocol is one of the fundamental protocols that allow
the internet to work. IP addresses are unique on each network and they allow
machines to address each other across a network. It is implemented on the
internet layer in the IP/TCP model.
Networks can be linked together, but traffic must be routed when
crossing network boundaries. This protocol assumes an unreliable network and
multiple paths to the same destination that it can dynamically change between. There are a number of different implementations of the protocol.
The most common implementation today is IPv4, although IPv6 is growing in
popularity as an alternative due to the scarcity of IPv4 addresses available
and improvements in the protocols capabilities.
ICMP
ICMP stands for internet control message protocol. It is used to
send messages between devices to indicate the availability or error conditions.
These packets are used in a variety of network diagnostic tools, such as ping
and traceroute.
Usually ICMP packets are transmitted when a packet of a
different kind meets some kind of a problem. Basically, they are used as a
feedback mechanism for network communications.
TCP
TCP stands for transmission control protocol. It is implemented
in the transport layer of the IP/TCP model and is used to establish reliable
connections.
TCP is one of the protocols that encapsulates data into packets.
It then transfers these to the remote end of the connection using the methods
available on the lower layers. On the other end, it can check for errors,
request certain pieces to be resent, and reassemble the information into one
logical piece to send to the application layer. The protocol builds up a connection prior to data transfer using
a system called a three-way handshake. This is a way for the two ends of the
communication to acknowledge the request and agree upon a method of ensuring
data reliability.
After the data has been sent, the connection is torn down using
a similar four-way handshake. TCP is the protocol of choice for many of the most popular uses
for the internet, including WWW, FTP, SSH, and email. It is safe to say that
the internet we know today would not be here without TCP.
UDP
UDP stands for user datagram protocol. It is a popular companion
protocol to TCP and is also implemented in the transport layer.
The fundamental difference between UDP and TCP is that UDP
offers unreliable data transfer. It does not verify that data has been received
on the other end of the connection. This might sound like a bad thing, and for
many purposes, it is. However, it is also extremely important for some
functions.
Because it is not required to wait for confirmation that the
data was received and forced to resend data, UDP is much faster than TCP. It
does not establish a connection with the remote host, it simply fires off the
data to that host and doesn't care if it is accepted or not.
Because it is a simple transaction, it is useful for simple
communications like querying for network resources. It also doesn't maintain a
state, which makes it great for transmitting data from one machine to many
real-time clients. This makes it ideal for VOIP, games, and other applications
that cannot afford delays.
HTTP
HTTP stands for hypertext transfer protocol. It is a protocol
defined in the application layer that forms the basis for communication on the
web.
HTTP defines a number of functions that tell the remote system
what you are requesting. For instance, GET, POST, and DELETE all interact with
the requested data in a different way.
FTP
FTP stands for file transfer protocol. It is also in the
application layer and provides a way of transferring complete files from one
host to another.
It is inherently insecure, so it is not recommended for any
externally facing network unless it is implemented as a public, download-only
resource.
DNS
DNS stands for domain name system. It is an application layer
protocol used to provide a human-friendly naming mechanism for internet
resources. It is what ties a domain name to an IP address and allows you to
access sites by name in your browser.
SSH
SSH stands for secure shell. It is an encrypted protocol
implemented in the application layer that can be used to communicate with a
remote server in a secure way. Many additional technologies are built around
this protocol because of its end-to-end encryption and ubiquity.
There are many other protocols that we haven't covered that are
equally important. However, this should give you a good overview of some of the
fundamental technologies that make the internet and networking possible.
Conclusion
At this point, you should be familiar with some basic networking
terminology and be able to understand how different components are able to
communicate with each other. This should assist you in understanding other
articles and the documentation of your system.
