APPLICATION LAYER

An interface between the networks is called application.

This section introduces two important concepts:
• Application Layer: The application layer of the OSI model provides the first step
of getting data onto the network.

Left Column:

Application Software: Applications are the software

programs used by people to communicate over the net-
work. Examples of application software, includes HTTP,
FTP, e-mail, and others, used to explain the differences
between these two concepts.

In the OSI model, information is passed from one layer

to the next, starting at the application layer on the transmit-
ting host and proceeding down the hierarchy to the physical
layer, then passing over the communications channel to the
destination host, where the information proceeds back up
the hierarchy, ending at the application layer.

The following six steps explain the procedure:

1. People create the communication.

2. The application layer prepares human communication
for transmission over the data network.
3. Software and hardware converts communication to
digital format.
4. Application layer services initiate the data transfer.
5. Each layer plays its role. The OSI layers encapsulate
data down the stack. Encapsulated data travels across
the media to the destination. OSI layers at the destina-
tion unencapsulate the data.
6. The application layer receives data from the network
and prepares it for human use.

The application layer, layer 7, is the top layer of both the OSI
and TCP/IP models. Layer 7 provides the interface between the
application you use to communicate and the underlying net-
work over which your messages are transmitted. Application
layer protocols are used to exchange data between programs,
running on the source and destination hosts.

TCP/IP Application Layer Protocol

The most widely known TCP/IP application layer protocols
are those that provide the exchange of user information.
These protocols specify the format and control information
necessary for many of the common internet communication
functions. Among these, TCP/IP protocols are the following.

* Domain name system (DNS) is used to resolve internet

names to IP addresses.
* Hypertext transfer protocol (HTTP) is used to transfer
files that make up the web pages of the world wide web.
* Simple mail transfer protocol (SMTP) is used for the
transfer of mail messages and attachments.
* Telnet, a terminal emulation protocol, is used to provide
remote access to servers and networking devices.
* File transfer protocol (FTP) is used for interactive file
transfers between systems.

Application Layer Services

Programs such as file transfer or network print spool-
ing, might need the assistance of application layer ser-
vices to use network resources. Although transparent to

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the user, these services have interface with the networ

and prepares the data for transfer. Different types of da
whether it is text, graphics or video require different ne
work services to ensure that it is properly prepared fo
processing by the functions occurring at the lower lay
ers of OSI model. Application layer services establish a
interface to the network and protocols provide the rule
and formats that govern how data is treated, a single exe
cutable program can use all three components. For exam
ple, while discussing “Telnet”, you could be referring t
the Telnet application, the Telnet service, or the Telne
protocol.

Application Layer Protocol Functions

Both the source and destination devices use application
layer protocols during a communication session. For the
communications to be successful, the application layer pro-
tocols implemented on the source and destination host must
match.

Protocols perform the following tasks

• Establish consistent rules for exchanging data between
applications and services loaded on the participating
devices.
• Specifies how data inside the messages is structured and
the types of messages that are sent between source and
destination. These messages can be requests for services,
acknowledgements, data messages, status messages, or
error messages.
• Defines message dialogues, ensuring that a message
being sent is met by the expected response and that the
correct services are invoked when data transfer occurs.

Applications and services can also use multiple proto-

cols in the course of a single conversation. One protocol
might specify how to establish the network connection
and another might describe the process for the data
transfer when the message is passed to the next lower
layer.
A single application can employ many different sup-
porting application layer services. Thus, what appears
to the user as one request for a web page might, in fact,
amount to dozens of individual requests. For each reques
multiple processes can be executed. For example, the FTP
requires a client to initiate a control process and a data
stream process to a server. Additionally, servers typically
have multiple clients requesting information at the same
time, as shown in the figure below. For example, a Telnet
server can have many clients requesting connections to d
These individual client requests must be handled simul-
taneously and separately for the network to succeed. The
application layer processes and services rely on support
from lower layer functions to successfully manage the mul-
tiple conversations.

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Figure 6 Multiple client's service Requests

[Diagram description: Telnet server application connected to multiple clients via
Application layer services.]

APPLICATION LAYER PROTOCOLS

The transport Layer uses an addressing scheme called a port number. Port numbers
identify application layer services that are source and destination of data. Server
programs generally use predefined port numbers that are commonly known by clients.

Some of these services are:

- Domain Name System (DNS): TCP/UDP Port 53
- HTTP: TCP Port 80
- Simple Mail Transfer Protocol (SMTP): TCP Port 25
- Post office Protocol (POP): UDP Port 110
- Telnet: TCP Port 23
- DHCP: UDP Port 67
- FTP: TCP Ports 20 and 21

Internet Control Message Protocol (ICMP)

- Used by hosts and gateways to send notification of datagram problems back to the
sender.
- Used for error reporting and query messages.
- Helpful in network debugging.
- Uses the services of TCP and UDP with the port number 7 as the ping command which
is used for testing; this testing is done from a source which starts at the
application layer and reaches network through transport layer.
- ICMP is encapsulated into an IP datagram and then transmitted into the network;
if the protocol filed in the IP datagram is 1 then the IP datagram is said to be
carrying ICMP message.

Types of messages
Error reporting
- Destination Unreachable: The packet is discarded due to the host not present in
the network or the host is not responding to the request.
- Source Quench: The packet is discarded due to the congestion in the network.
- Parameter Problem: The packet is discarded due to the processing problem
observing a change in the header format of the I/P datagram.
- Time Exceeded: The packet is discarded because the TTL value is decremented to
zero(0).

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- Redirection: Here the packet is not discarded but redirected to a network as the
host doesn’t belong to this network.
Query message
Router solicitation and router advertisement request and reply: Router solicitation
is a request generated by the source requesting the router’s presence in the
network.
The response is a router advertisement generated by the router broadcasting its
network id and its presence in the network.

Address mask request and reply: If by any means the node is unable to identify the
network bits in its I/P address then this request is used by the source to a router
requesting for the network id; the reply is also unicast in this scenario.

Time stamp echo request and reply: This is used to calculate the round trip time of
a packet for network diagnose or debugging.

Echo request and reply: This is used to see the presence of a host or a router in
the network. For example PING.

SMTP
- SMTP stands for simple mail transfer protocol.
- It uses the services of TCP on port number 25.
- It is a push protocol. Even when the destination is not interested to receive the
message this push approach of the SMTP makes the receiver receive the message.
- Components of SMTP:
1. User Agent (UA):
(i) It provides Graphical User Interface access to the user.
Example: Netscape navigation, Mozilla Firefox.
It also provides command-driven access in early days.
(ii) It handles the inbox transactions:
(a) Composing messages: Helps the user compose the e-mail message to be
sent out.
(b) Reading messages: Helps to read incoming messages by checking the
mail in the incoming mail box.
(c) Replying to messages: Sends the message to the sender or recipients
of the copy.
(d) Forwarding messages: Sends the message to a third party.
(e) Handling mailboxes: Two mailboxes, an inbox and an outbox are created
by the user agent. The inbox keeps all the received e-mails until they are deleted
by the user. The outbox keeps all the sent e-mails until the user deletes them.

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FTP(TCP) – requires username and password.

TFTP(UDP) – requires no username and password.

Types of files supported by FTP:

i. ASCII: By default FTP follows ASCII mode for file transfer. It is composed of 7-
bit + 1 parity bit.
ii. EBCDIC: If any node supports EBCDIC then this type of technique is used for
file transfer. BCDIC supports 8 bits data format and is used in IBM. There is no
error control i.e., there is no parity bit.
iii. Image file: If the file to be sent is very large then continuous streams of 0s
and 1s are sent to the transport layer. This is image file. Here FTP does not care
of code, it is done by the lower layers.

Transmission mode of FTP:

FTP can transfer a file across the data connection by using one of the following
three transmission modes:
i. Stream mode: This is the default mode. Data are delivered from FTP to TCP as a
continuous stream of bytes.
ii. Block mode: Data is delivered from FTP to TCP in blocks. Each block is preceded
by a 3-byte header. The first byte is called the block descriptor, the next two
bytes define the size of the block in bytes.
iii. Compressed mode: If the file is big then the data is compressed. The
compression method which is mostly used is run-length encoding. Consecutive
appearances of a data unit are replaced by one occurrence and the number of
repetitions. In a binary file, null characters are compressed.

DNS
- Stands for Domain Name System.
- The DNS is a client/server application that identifies each host on the Internet
with a unique user-friendly name i.e., it is used to map an Uniform Resource
Locator (URL) to an IP address.
- DNS can use the services of UDP or TCP using the well-known port 53.
- If the size of the response message is more than 512 bytes, it uses the TCP
connection.
- When the size of the response message is less than 512 bytes, UDP connection is
used. Even though the size of message is not known then also UDP can be used. The
UDP server will truncate the message if the message size is more than 512 bytes.
- DNS organizes the namespace in a hierarchical structure to decentralize the
responsibilities involved in naming.
- DNS can be pictured as an inverted hierarchical tree structure with one root node
at the top and a maximum of 128 levels. Each node in the tree has a domain name.
For example, on the Internet, the domain names, such as http://www.cisco.com, are
much easier for people to

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remember than 198.132.219.25. Also if cisco decides to change the numeric address,
it is transparent to the user, because the domain name will remain
http://www.cisco.com. The new address will simply linked to the existing domain
name and connectivity is maintained as shown in the figure.

[Figure 7 DNS addresses]

[Diagram: www.cisco.com → DNS server → Network → Client]
[Table: Name | Address → www.cisco.com | 198.133.219.25]
[Text boxes: "The DNS server matches the domain name with the numeric address" and
"The devices use numbers"]

When networks were small, it was a simple task to maintain the mapping between
domain names and the addresses they represent. However, as networks began to grow
and the number of devices increased, this manual system became unworkable. DNS was
created for domain name to address resolution for these networks. DNS uses a
distributed set of servers to resolve the names associated with these numbered
addresses.

The DNS protocol defines an automated service that matches resource names with the
required numeric network address. It includes the format for queries, responses,
and data formats. DNS protocol communications use a single format called a message.
This message format is used for all types of client queries and server responses,
error messages, and the transfer of resource record information between servers.
DNS is a client/server service, however, it differs from the other client/server
services. Where as other services use a client that is an application (Web browser,
e-mail, client, and so on) the DNS client runs as a service itself. The DNS client,
sometimes called the DNS resolver, supports name resolution for the other network
applications and other services that need it.

When configuring a network device, you generally provide one or more DNS server
addresses that the DNS client can use for name resolution. Usually the Internet
Service Provider (ISP) gives you the address to use for the DNS servers. When a
user’s application requests to connect to a remote device by name, the requesting
DNS client queries one of these DNS servers to resolve the name to a numeric
address.

- The domain name space consists of a tree of domain names. Each node or leaf in
the tree has zero or more resource records, which holds information associated with
the domain name. The tree sub-divides into zones beginning at the root zone. A DNS
zone consists of a collection of connected nodes authoritatively served by an
authoritative name server.