APPLICATION LAYER PROTOCOLS –20 Marks

• The application layer is closest to the end user.

• Network applications enable users to send and receive data with ease.

• The application layer acts as interface between the applications and

the underlying network.
• Application layer protocols help
exchange data between programs
running on the source and
destination hosts.
• The TCP/IP application layer
performs the functions of the
upper three layers of the OSI
model.
• Common application layer protocols
include: HTTP, FTP, TFTP, DNS.
• TCP/IP application protocols specify the format and control
information necessary for common Internet functions.
• Application layer protocols must be implemented in both the source
and destination devices.
• Application layer protocols implemented on the source and
destination host must be compatible to allow communication.
• The device requesting the information is called a client.

• The device responding to the request is called a server.

• Client and server processes are considered to be in the application layer.

• The client initiates the exchange by requesting data from the server.

• The server responds by sending one

or more streams of data to the
client.
• Application layer protocols describe
the format of the requests and
responses between clients and
servers.
• The contents of the data exchange
will depend of the application in use.
• Email is an example of a
Client-Server
interaction.
• In the peer-to-peer (P2P) networking model, the data is
accessed without the use of a dedicated server.
• Two or more computers can be connected to a P2P network to
share resources.
• Every connected end device (a peer) can function as both a server
and a client.
• The roles of client and
server are set on a
per request basis.
• Some P2P applications use a hybrid system.

• In hybrid P2P, resource sharing is decentralized.

• Indexes that point to resource locations are stored in a

centralized directory.
• In a hybrid system, each
peer accesses an index
server to get the
location of a resource
stored on another peer.
• Common P2P networks include: BitTorrent, Bitcoin.

• Many P2P applications allow users to share pieces of many files

with each other at the same time.
• A small torrent file contains information about the location of other
users and tracker computers.
• Trackers are computers keeping
track of the files hosted by
users.
• This technology is called BitTorrent.

• There are many BitTorrent clients,

including BitTorrent, uTorrent,
Frostwire, and qBittorrent.
• A web address or uniform resource locator (URL) is a reference to a web server.
A URL allows a web browser to establish a connection to that web server.
• URLs and Uniform Resource Identifier (URIs) are the names most
people associate with web addresses.
• The URL http://cisco.com/index.html has three basic parts:
o http (the protocol or scheme)
o www.cisco.com (the server name)
o index.html (the specific filename requested) HTTP Protocol Step 1
• Using DNS, the server name portion of
the URL is then translated to the
associated IP address before the server
can be contacted.
 HTTP Protocol Step 2
• The browser sends a GET request
to the server’s IP address and
asks for the index.html file.
• The server sends the requested
file to the client.
• The index.html was specified
in the URL and contains the
HTML code for this web page.
HTTP Protocol Step 3
• The browser processes the HTML
code and formats the page for the
browser window based on the code
in the file.
• HTTP
o Is a request/response protocol.
o Has three common message types: GET, POST, PUT.
o Is not secure. Messages can be intercepted.
• HTTPS uses authentication and
encryption to secure data.
• Email is a store-and-forward method of sending, storing, and
retrieving electronic messages.
• Email messages are stored in databases on mail servers.
• Email clients communicate with mail
servers to send and receive email.
• Mail servers communicate with other
mail servers to transport messages
from one domain to another.
• Email clients do not
communicate directly when
sending email.
• Email relies on three separate protocols
for operation: SMTP (sending),POP (retrieving),
IMAP (retrieving).
• SMTP message formats require a message header and body.
• The body can contain any amount of text.
• The header must have a properly formatted recipient email address and
a sender address.
• An SMTP client sends an email
by connecting to a SMTP server
on port 25.
• The server receives the message and
stores it message in a local mailbox or
relays the message to another mail server.
• Users use email clients to retrieve
messages stored on the server.
• IMAP and POP are two protocols
commonly used by email clients to retrieve
messages.
• Messages are downloaded from the server to the client.

• The server listens on port 110 TCP for client requests.

• Email clients direct their POP requests to mail servers on port TCP 110.

• The POP client and server exchange commands and responses

until the connection is closed or aborted.
• POP allows for email messages
to be downloaded to the client’s
device (computer or phone)
and removed from the server.
• There is no centralized location
where email messages are
kept.
• A downloaded message resides on
the device that triggered the download.
• IMAP is another protocol used to retrieve email messages.
• Allows for messages to be displayed to the user rather than downloaded.
• The original messages reside on the server until manually deleted by the user.
• Users view copies of the messages in their email client software.
• Users can create a folder hierarchy on
the server to organize and store mail.
• That file structure is displayed on
the email client.
• When a user decides to delete a
message, the server synchronizes that
action and deletes the message from the
server.
• While IP addresses are crucial for network communication, they are
not easy to memorize.
• Domain names are created to make server addresses more
user- friendly.
• Domain names such as http://www.cisco.com are user-
friendly addresses associated with the IP address of a specific
server.
• However, computers still need the actual numeric address before
they can communicate.
• The DNS protocol allows for the
dynamic translation of a domain
name into the correct IP
address.
• The DNS protocol communications
using a single format called a
message.
• DNS supports different types of records. Some of these record types are:
o A - An end device IPv4 address
o NS - An authoritative name server
o AAAA - An end device IPv6 address (pronounced quad-A)
o MX - A mail exchange record

• DNS servers will first look at its own records to resolve the name. If the server is
unable to resolve the name using its locally stored records, it relays the query to
other servers.
• The response is then forwarded to
the requesting client.
• The DNS Client service on
Windows PCs also stores
previously resolved names in
memory.
• ipconfig /displaydns displays all of
the cached DNS entries on Windows.
• The DNS protocol uses a hierarchical system, with the root at the top
and branches below. The naming structure is broken down into
small, manageable zones.
• Each DNS server is only responsible for managing name-to-IP
mappings for that small portion of the DNS structure.
• Requests for zones not stored in a specific DNS server are forwarded
to other servers for translation.
• Top-level domains represent either the type
of domain or the country of origin.
Examples of top-level domains are:
o .com - a business or industry
o .org - a non-profit organization
o .au - Australia
o .co - Colombia
• Allows the user to manually place DNS queries.

• It can also be used to troubleshoot name resolution issues.

• Has many options available for extensive testing and verification of

the DNS process.
• Computers need network addresses to communicate over a network.

• Additional crucial information includes gateway address, subnet

mask, and DNS server.
• Manually configuring end devices is not scalable. DHCP allows
for automated distribution of network information.
• DHCP-distributed addresses are
leased for a set period of time.
• Addresses are returned to the
pool for reuse when no longer in
use.
• DHCP supports IPv4 and
DHCPv6 supports IPv6.
• A DHCP client goes through the following basic steps to request an IP:
o The client broadcasts a DHCPDISCOVER.
o A DHCP server replies with a DHCPOFFER message
o The client sends a DHCPREQUEST message to the server it wants to use (in case
of multiple offers).

• A client may also choose to request an address that it had

previously been allocated by the server.
• The server returns a DHCPACK
message to confirm the lease
has been finalized.
• The server would respond with a DHCPNAK if the offer is no longer
valid
• Leases must be renewed before its expiration through
another DHCPREQUEST.
• DHCPv6 has a similar set of messages:
o SOLICIT
o ADVERTISE
o INFORMATION REQUES T
o REPLY
• FTP was developed to allow the transfer of files over the network.

• An FTP client is an application that runs on a client computer used

to push and pull data from an FTP server.
• FTP requires two connections between the client and the server:
one connection for commands and replies and another connection
for the actual file transfer.
• The client initiates and establishes the
first connection to the server for
control traffic on TCP port 21.
• The client then establishes the second
connection to the server for the actual
data transfer on TCP port 20.
• The client can download (pull) data from
the server or upload (push) data to the server.
• SMB is a client/server file sharing protocol.

• All SMB messages share a common format.

• SMB file-sharing and print services have become the mainstay

of Windows networking.
• Microsoft products now support
TCP/IP protocols to directly
support SMB resource sharing.
• After the connection is established,
the user of the client can access the resources
on the server as if the resource is local to the
client host.

• The Mac, LINUX, and UNIX operating

systems have their own implementation of
 • Remote Login

Telnet
• Telnet is a network protocol that allows a user to communicate with a remote device. It
is a virtual terminal protocol used mostly by network administrators to remotely access
and manage devices. Administrator can access the device by telnetting to the IP
address or hostname of a remote device.

• To use telnet, you must have a software (Telnet client) installed. On a remote device, a
Telnet server must be installed and running. Telnet uses the TCP port 23 by default.

• One of the greatest disadvantages of this protocol is that all data, including usernames
and passwords, is sent in clear text, which is a potential security risk. This is the main
reason why Telnet is rarely used today and is being replaced by a much secure protocol
called SSH
 SSH

SSH (Secure Shell)

SSH is a network protocol used to remotely access and manage a device.

The key difference between Telnet and SSH is that SSH uses encryption, which means that
all data transmitted over a network is secure from eavesdropping.

SSH uses the public key encryption for such purposes.

Like Telnet, a user accessing a remote device must have an SSH client installed. On a
remote device, an SSH server must be installed and running. SSH uses the TCP port 22 by
default.

Remote Login
hjhjyou.