Application Layer
The
application layer is the OSI layer that is closest to the user.
This layer provides network services to the user's applications. It differs
from the other layers in that it does not provide services to any other OSI
layer, but only to applications outside the OSI reference model. Applications
layer provide a platform to access the data of remote computer.
The application layer protocols that you should know are as follows:
- SNMP (Simple Network Management
Protocol)— Communicates
status and allows control of networked devices.
- TFTP (Trivial File Transfer
Protocol)— Simple,
lightweight file transfer.
- DNS (Domain Naming System)— Translates a website name
(easy for people) to an IP address (easy for computers).
- DHCP (Dynamic Host
Configuration Protocol)—
Assigns IP, mask, and DNS server (plus a bunch of other stuff) to hosts.
- Telnet— Provides a remote terminal
connection to manage devices to which you are not close enough to use a
console cable.
- HTTP (Hypertext Transfer
Protocol)— Browses
web pages.
- FTP (File Transfer Protocol)— Reliably sends/retrieves all
file types.
- SMTP (Simple Mail Transfer
Protocol)— Sends
email.
- POP3 (Post Office Protocol
v.3)— Retrieves
email.
- NTP (Network Time Protocol)— Synchronizes networked device
clocks.
presentation layer
The
presentation layer is responsible for formatting data so that
application-layer protocols (and then the users) can recognize and work with
it. Presentation layer format the file extensions—such as .doc, .jpg, .txt,
.avi, and so on. you realize that each of these file types is formatted for use
by a particular type of application. The presentation layer taking the
application layer data and marking it with the formatting codes so that it can
be viewed reliably when accessed later. If necessary, the presentation layer
might be able to translate between multiple data formats by using a common format.
The Session Layer
The
session layer establishes, manages, and terminates sessions
between two communicating hosts. It provides its services to the presentation
layer. The session layer also synchronizes dialogue between the presentation
layers of the two hosts and manages their data exchange. For example, web
servers have many users, so many communication processes are open at a given
time. Therefore, keeping track of which user communicates on which path is
important.
Transport Layer
The transport layer is possibly the most important layer for exam study
purposes. A lot is going on here, and it is heavily tested.
The transport layer's main jobs
- It sets
up and maintains a session connection between two devices.
- It can
provide for the reliable or unreliable delivery of data across this
connection.
- It
multiplexes connections, allowing multiple applications to simultaneously
send and receive data. When
- Implementing
a reliable connection, sequence numbers and acknowledgments (ACKs) are
used.
- Flow control
(through the use of windowing or acknowledgements)
- Reliable
connections (through the use of sequence numbers and Acknowledgement )
Transport layer use two protocols for sending data TCP and UDP.
TCP
TCP is connection oriented protocols. Connection-oriented transmission is said
to be reliable. Thinks TCP as registry AD facility available in Indian post
office. For this level of service, you have to buy extra ticket and put a bunch
of extra labels on it to track where it is going and where it has been. But,
you get a receipt when it is delivered, you are guaranteed delivery, and you
can keep track of whether your shipment got to its destination. All of this
costs you more—but it is reliable!
UDP
UDP is connection less protocols. Connection-less transmission is said to be
unreliable. Now, don't get too wrapped up in the term "unreliable"
this doesn't mean that the data isn't going to get there; it only means that it
isn't guaranteed to get there. Think of your options when you are sending a
postcard, put it in the mailbox, and chances are good that it will get where
it's supposed to go—but there is no guarantee, and stuff does go missing once
in a while. On the other hand, it's cheap.
Reliability
When reliability is necessary, it should cover these four items:
- recognizing
lost packets and having them re-sent
- recognizing
packets that arrive out of order and reordering them
- detecting
duplicate packets and dropping the extra ones
- Avoiding
congestion
Connection Multiplexing/Application Mapping
Transport layer assigns a unique set of numbers for each connection. These
numbers are called port or socket numbers. TCP, and UDP, provide a multiplexing
function for a device: This allows multiple applications to simultaneously send
and receive data.
Imagine a server that performs a number of functions—for example email, web
pages, FTP, and DNS. The server has a single IP address, but can perform all
these different functions for all the hosts that want to connect to it. The
transport layer (layer 4) uses port numbers to distinguish between different
types of traffic that might be headed for the same IP address.
Port numbers are divided into ranges by the IANA. Following are the
current port ranges:
|
Port number
|
descriptions
|
0–1023
|
Well-Known—For common TCP/IP functions and applications
|
1024–49151
|
Registered—For applications built by companies
|
49152–65535
|
Dynamic/Private—For dynamic connections or unregistered applications
|
Common TCP and UDP Port Numbers
|
TCP
|
UDP
|
|
FTP
|
20, 21
|
DNS
|
53
|
|
Telnet
|
23
|
DHCP
|
67,68
|
|
SMTP
|
25
|
TFTP
|
69
|
|
DNS
|
53
|
NTP
|
123
|
|
HTTP
|
80
|
SNMP
|
161
|
|
POP
|
110
|
|
|
|
NNTP
|
119
|
|
|
|
HTTPS
|
443
|
|
|
Network Layer
The network layer provides a logical topology and layer-3 addresses. Routers
function at the network layer. This layer is responsible for three main
functions:
- Defines
logical addresses used at layer-3
- Finds
paths, based on the network numbers of logical addresses, to reach
destination devices
- Connects
different data link types together, such as Ethernet, FDDI, Serial, and
Token Ring
IP packet
Where the transport layer uses segments to transfer information between
machines, the Internet layer uses datagram's. Datagram is just another word for
packet.
The IP protocol is mainly responsible for these functions:
- Connectionless
data delivery: best effort delivery with no data recovery capabilities
- Hierarchical
logical addressing to provide for highly scalable internetworks
IP
addresses are broken into two components:
- Network
component Defines on what segment, in the network, a device is located
- Host
component defines the specific device on a particular network segment
Two types of packets are used at the Network layer: data and route updates.
Data
packets
Used to transport user data through the internetwork. Protocols used to support
data traffic are called routed protocols; examples of routed protocols are IP
and IPv6.
Route
update packets
Used to update neighboring routers about the networks connected to all routers
within the internetwork. Protocols that send route update packets are called
routing protocols; examples of some common ones are RIP, RIPv2, EIGRP, and
OSPF. Route update packets are used to help build and maintain routing tables
on each router.
IP Classes
- Class A
addresses range from 1-126: 00000001-01111111.
- Class B
addresses range from 128-191: 10000000-10111111.
- Class C
addresses range from 192-223: 11000000-11011111.
- Class D
addresses range from 224-239: 11100000-11101111.
- Class E
addresses range from 240-254:
- 0 is
reserved and represents all IP addresses;
- 127 is a
reserved address and is used for testing, like a loop back on an
interface:
- 255 is a
reserved address and is used for broadcasting purposes.
Public addresses are Class A, B, and C addresses that can be used to access
devices in other public networks, such as the Internet. Public IP address assign
authority The Internet Assigned Numbers Authority (IANA) is ultimately
responsible for handing out and managing public addresses. Normally you get
public addresses directly from your ISP, which, in turn, requests them from one
of five upstream address registries:
- American
Registry for Internet Numbers (ARIN)
- Reseaux IP
Europeans Network Coordination Center (RIPE NCC)
- Asia
Pacific Registry for Internet Numbers (APNIC)
- Latin
American and Caribbean Internet Address Registry (LACNIC)
- African
Network Information Centre (AfriNIC)
Private IP and ISP
Private ip address can be used to configure private network. You can use
private ip to build your network without paying a single rupees. But one
biggest problem with private ip is that with private you can not access the
internet. This is the point where ISP comes from. ISP purchase a bulk of public
ip address and provide them on rent. Whatever you pay to ISP for accessing
internet is actually the charge of using public ip address.
Private
ip address:- Not route able in public network
- Class A:
10.0.0.0-10.255.255.255 (1 Class A network)
- Class B:
172.16.0.0-172.31.255.255 (16 Class B networks)
- Class C:
192.168.0.0-192.168.255.255 (256 Class C networks)
|
Protocol
|
Description
|
|
IP
|
IP of TCP/IP, featuring routable 32-bit addressing.
|
|
IPX
|
The equivalent of IP in Novell Netware.
|
|
ICMP
|
Internet Connection Management Protocol. Incorporates Ping and Traceroute,
which are layer 3 link-testing utilities.
|
|
OSPF, IGRP, EIGRP, RIP, ISIS
|
Dynamic routing protocols that learn about remote networks and the best
paths to them from other routers running the same protocol.
|
|
ARP, RARP
|
Address Resolution Protocol (and Reverse ARP). ARP learns what MAC address
is associated with a given IP address. Reverse ARP learns an IP address given
a MAC address.
|
Data link layer
Main functions of data link layer is
- Defining
the Media Access Control (MAC) or hardware addresses
- Defining
the physical or hardware topology for connections
- Defining
how the network layer protocol is encapsulated in the data link layer
frame
- Providing
both connectionless and connection-oriented services
- Defines
hardware (MAC) addresses as well as the communication process that occurs
within a media.
- The first
six hexadecimal digits of a MAC address form the OUI.
- MAC
addresses only need to be unique in a broadcast domain,
- You can
have the same MAC address in different broadcast domains (virtual LANs).
There are two specifications of Ethernet frame Ethernet II and
802
802.2 use a SAP or SNAP field to differentiate between encapsulatedlayer-3
payloads.
With a SNAP frame, the SAP fields are set to 0xAA and the type field is used
to indicate the layer-3 protocol. One of the issues of the original SAP field
in the 802.2 SAP frame is that even though it is eight bits (one byte) in
length, only the first six bits are used for identifying upper-layer protocols,
which allows up to 64 protocols.
802.2 SNAP frame support of up to 65,536 protocols
Ethernet II's Version of Ethernet
- Ethernet
II does not have any sub layers, while IEEE 802.2/3 has two: LLC and MAC.
- Ethernet
II has a type field instead of a length field (used in 802.3). IEEE 802.2
defines the type for IEEE Ethernet
Physical Layer
The Physical layer communicates directly with the various types of actual communication
media. Different kinds of media represent these bit values in different ways.
Some use audio tones, while others utilize state transitions—changes in voltage
from high to low and low to high. Specific protocols are needed for each type
of media to explain the proper bit patterns to be used, how data is encoded
into media signals, and the various qualities of the physical media’s
attachment interface.
