Application Layer
Functionality and Protocols
These presentations are the result of a collaboration among
the instructors at St. Clair College in Windsor, Ontario.o IBM Lotus Notes
o Novell Groupwise
o Microsoft Exchange
o Web based e-mail:
o Hotmail
o Gmail

E-Mail Protocols
Post Office Protocol (POP3):
o Uses TCP port 110
o Download-and-delete mode:
o Retrieves messages from the server
o Stores the message locally
o Deletes the message from the server
o Download-and-keep mode:
o Does not delete messages on server when retrieved.Internet Message Access Protocol (IMAP):
o E-mail is not downloaded, but retained on server
o Any received email is associated with the user's INBOX
o Users can create and manage remote folders
o Users can retrieve portions of the email:
o Message header: Subject line and Sender
o Web Based E-mail:
o Introduced with Hotmail in mid-1990's.o A - an end device address
o NS - an authoritative name server
o CNAME - the Fully Qualified Domain Name
o MX - mail exchange record to identify mail servers

WWW Service and
Hypertext Transfer Protocol
HTTP
Web browsers are the client applications used to interpret the
HTTP application protocol received from a web server.o The resolver is responsible for issuing requests and
processing responses from the DNS server

Domain Name System (DNS)
All DNS servers store
different types of
resource records to
resolve names.Application Layer Functionality and Protocols
Applications: The Interface
Between the Networks

Application Layer - OSI and TCP/IP Models
The Application layer is the top layer of both the OSI and
TCP/IP models.o The Telnet daemon resumes listening and repeats the
process for each unique connection

Peer-to-Peer (P2P) Networking
Two or more computers are connected via a network and can
share resources (such as printers and files) without having a
dedicated server.Functionality of the TCP/IP Application Layer protocols fit
roughly into the top three layers of the OSI Model.o Most TCP/IP application layer protocols were developed
before PCs, GUIs and multimedia objects.o Three methods:
o Client/Server Model
o Application Layer Services and Protocols
o Peer-to-Peer (P2P) Networking and Applications

The biggest advantage of the client/server model
is the centralization of resources.o Sends the requested data in the proper format

Application Layer Services and Protocols
Typically, a server will
have multiple clients
requesting services
at the same time.o In other words, in this model, every client is a server and every
server a clien


Application Layer Protocols
and Services Examples
Commonly used protocols:
o The OSI Model Transport Layer uses an addressing
scheme called a port number.E-Mail Server Processes: MTA and MDA
Other alternatives with their own internal e-mail format and
proprietary protocol.Dynamic Host Configuration Protocol
DHCP
IP addresses and other configuration information can be
obtained dynamically.o Linux / Unix have a similar protocol - SAMBA
SMB messages can:
o Start, authenticate, and terminate sessions.o They implement the application layer protocols and are
able to communicate directly with the lower layers of the
protocol stack.o Email Clients
o Web Browsers

Application Layer Services:
o Other programs may need the assistance of Application
Layer services to use network resources such as:
o File transfer
o Network print spooling
o These services are the programs that interface with the
network and prepare the data for transfer.o Are described as "listening" for a request
from a client. o Programmed to respond whenever the server receives a
request for the service provided by the daemon. o When a daemon "hears" a request from a client:
o It exchanges appropriate messages with the client.Domain Name System (DNS)
The DNS protocol defines an automated service that
matches resource names with the required numeric network
address.o Examples:
o Apache, Microsoft Internet Information Server (MIIS)

HTTP is a request/response type of protocol.o Simple Mail Transfer Protocol (SMTP)
o Post Office Protocol (POP and POP3)
o Internet Message Access Protocol (IMAP)

E-Mail Server Processes: MTA and MDA
Mail Transfer Agent (MTA):
o Used to forward e-mail.IP address
o Subnet Mask
o Default Gateway
o Domain Name
o DNS Server
o Others....Application Layer - OSI and TCP/IP Models
Two important concepts:
o Application Layer:
o The first step for getting data on to the network.Application Layer Software
Within the Application
layer, there are two forms
of software programs or
processes that provide
access to the network:
o Applications
o Services

Network-Aware Applications:
o The software programs used by people to communicate.o If anyone finds any errors or omissions, please let me know
at:
o tdame@stclaircollege.ca.o Provides the interface between the applications we use to
communicate and the underlying network.Making Provisions for
Applications and Services
Introduction
When accessing
information on a device,
the data may not be
physically stored on
that device.All Policies and User Names and
Passwords must be maintained on each peer device.Peer-to-Peer (P2P) Applications
A peer-to-peer application, unlike a peer-to-peer network,
allows a device to act as both a client and a server within the
same communication.o Server programs generally use predefined port numbers
that are commonly known by clients.o DHCP servers can be on a LAN, on a router or at an ISP.o IBM in the late 1980s
o Describes the structure of shared network resources
o Directories, files, printers, and serial ports.o For example:
o When displaying a web page:
o The Application Layer uses the HTTP Protocol.o They implement very little of the Presentation and
Session layer functionality.o The Telnet daemon
listens for connection
requests that are received on port 23.o End devices (peers) can function as either a server or client
depending upon the required service.One big disadvantage of P2P networking is that it
decentralizes the services on a multiuser network.However, instead of the client
being a browser or email client application, the DNS client
(Resolver) runs as a service itself.o When a client requests a web page, HTTP defines the
types of messages exchanged.e.g. GET, PUT, POST

E-Mail Services and SMTP/POP Protocols
Revolutionized how people communicate.o They can be accessed remotely by sites on a WAN.