Chakchai So-In, Ph.D.

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Application Layer

Functionality and

Protocols

Khon Kaen University

Chakchai So-In, Ph.D.

Department of Computer Science

Faculty of Science, Khon Kaen University 123 Mitaparb Rd., Naimaung,

Maung, Khon Kaen, 40002 Thailand [email protected]

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Agenda

Define the application layer as the source and destination of

data for communication across networks.

Explain the role of protocols in supporting communication

between server and client processes.

Describe the features, operation, and use of well-known

TCP/IP application layer services (HTTP, DNS, SMTP).

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Explain that applications provide the means for generating and

receiving data that can be transported on the network

Applications

Applications -- The Interface Between

The Interface Between

Human and Data Networks

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Explore the role of applications, services and protocols in

converting communication to data that can be transferred across the data network

Applications

Applications -- The Interface Between

The Interface Between

Human and Data Networks (Cont.)

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Define the separate roles applications, services and protocols

play in transporting data through networks

Applications

Applications -- The Interface Between

The Interface Between

Human and Data Networks (Cont.)

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Application layer is the layer that provides

the interface between the applications we use to communicate and the underlying network over which our messages are transmitted.

Application layer protocols are used to

Applications

Applications -- The Interface Between

The Interface Between

Human and Data Networks (Cont.)

Application layer protocols are used to

exchange data between programs running on the source and destination hosts.

Application layer protocols specify the

format and control information necessary for many of the common Internet

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Example of application layer protocols

Applications

Applications -- The Interface Between

The Interface Between

Human and Data Networks (Cont.)

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The protocols in the TCP/IP suite are generally defined by Requests for

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The Presentation layer has three primary functions:

Coding and conversion of Application layer data to ensure

that data from the source device can be interpreted by the appropriate application on the destination device.

Compression of the data in a manner that can be

decompressed by the destination device.

Encryption of the data for transmission and the decryption

The Presentation Layer

Encryption of the data for transmission and the decryption

of data upon receipt by the destination.

E,g., Quicktime, MPEG,

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The Session Layer

The Session layer creates and maintains dialogs between source

and destination applications.

The Session layer handles the exchange of information to

initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.

E.g. Network File System (NFC), RPC (Remote Procedure

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Applications provide people a way to create messages.

Network-Aware Applications: they implement the application

layer protocols and are able to communicate directly with the lower layers of the protocol stack. E-mail clients and web

browsers.

Application Layer Services: other programs may need the

Applications

Applications -- The Interface Between

The Interface Between

Human and Data Networks (Cont.)

Application Layer Services: other programs may need the

assistance of Application layer services to use network resources, like file transfer or network print spooling.

Application Layer Services establish an interface to the network. Protocols provide the rules and formats that govern how data is

treated. Protocols specify what messages are exchanged between the source and destination hosts, the syntax of the control

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Applications

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User Applications/Services/Application

Layer Protocol [Animation

Layer Protocol [Animation 3

3..1

1..3

3..1

1]]

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The Role of Protocols in Supporting

Communication (Client/Server Models)

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A server is usually a computer that contains information to be

shared with many client systems e.g. web pages, documents, databases, pictures, video, and audio files

Different types of server applications may have different

requirements for client access e.g. some servers may require authentication

The server runs a service, or process, sometimes called a server

Servers

The server runs a service, or process, sometimes called a server

daemon.

Daemons listen for a request from a client

When a daemon "hears" a request from a client, it exchanges

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The Role of Protocols in Supporting

Communication

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Compare and contrast client server networking with

peer-to-peer networks and peer-to-peer-to-peer-to-peer applications

The Role of Protocols in Supporting

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Compare and contrast client server networking with peer-to-peer

networks and peer-to-peer applications (Centralized vs. Distributed)

Difficulty: User Management/Policy Allow: Hybrid (index; resource)

A peer-to-peer application (P2P), unlike a peer-to-peer network,

The Role of Protocols in Supporting

Communication (Cont.)

A peer-to-peer application (P2P), unlike a peer-to-peer network,

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Domain Name System (DNS) - TCP/UDP Port 53 Hypertext Transfer Protocol (HTTP) - TCP Port 80 Simple Mail Transfer Protocol (SMTP) - TCP Port 25 Post Office Protocol (POP) - TCP Port 110

Telnet - TCP Port 23

Dynamic Host Configuration Protocol - UDP Ports 67 and 68

Application Layer Protocols: Examples

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Describe the features of the DNS protocol and how this protocol

supports DNS services [Animation 3.3.1.1]

Features, Operation, and Use of TCP/IP

Application Layer Services

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DNS is a client/server service.

DNS differs from other services - While other services use a

client that is an application (such as web browser, e-mail client), the DNS client runs as a service itself.

The DNS server, sometimes called the DNS resolver, supports

name resolution for our other network applications and other services that need it.

DNS Services and Protocol

services that need it.

DNS Verification

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A DNS server provides the name resolution using the

name daemon, which is often called named.

The DNS server stores different types of resource records

used to resolve names. These records contain the name, address, and type of record.

A - an end device address

NS - an authoritative name server

DNS Services and Protocol (Cont.)

NS - an authoritative name server

CNAME - the canonical name (or Fully Qualified Domain

Name) for an alias; used when multiple services have the single network address but each service has its own entry in DNS

MX - mail exchange record; maps a domain name to a list

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DNS uses a hierarchical system to create a name database to

provide name resolution.

At the top of the hierarchy, the root servers maintain records

about how to reach the top-level domain servers, which in turn have records that point to the second-level domain

servers and so on.

The resource records list domain names that the server can

DNS Services and Protocol

The resource records list domain names that the server can

resolve and alternative servers that can also process requests.

If a given server has resource records that correspond to its

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Describe the features of the HTTP protocol and how this protocol

supports the delivery of web pages to the client

Features, Operation, and Use of TCP/IP

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Describe the features of the HTTP protocol and how this protocol

supports the delivery of web pages to the client

Features, Operation, and Use of TCP/IP

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HTTP is used across the WWW for data transfer and is one of

the most used application protocols.

HTTP specifies a request/response protocol. Three common message types

GET is a client request for data.

POST and PUT are used to send messages that upload

WWW Service and HTTP

POST and PUT are used to send messages that upload data to the web server.

POST includes the data in the message sent to the

server.

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The MUA allows messages to be sent and places received

messages into the client's mailbox.

In order to receive mail messages from an mail server, the

e-mail client can use POP.

Sending e-mail from either a client or

a server uses message formats and

E

E--Mail Services and POP/SMTP

Mail Services and POP/SMTP

Protocol

a server uses message formats and

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Describe the features of the POP and SMTP protocols, and how

these protocols support e-mail services

Mail Transfer Agent (MTA) - forwards e-mail to MDA

Mail Delivery Agent (MDA) - receives all the inbound mail

from the MTA and places it into the appropriate users' mailboxes

Mail User Agent (MUA)

E

E--Mail Services

Mail Services

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Some of the commands specified in the SMTP protocol are:

HELO - identifies the SMTP client process to the SMTP

server process

EHLO - Is a newer version of HELO, which includes services

extensions

MAIL FROM - Identifies the sender

E

E--Mail Services and POP/SMTP

Mail Services and POP/SMTP

Protocol (Cont.)

MAIL FROM - Identifies the sender RCPT TO - Identifies the recipient

DATA - Identifies the body of the

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Describe the features of the FTP protocol and

the role it plays in supporting file sharing (FTPd)

TCP Port 21 (Control) and 20 (Data)

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DHCP Services

Describe the features of the DHCP

protocol and the role it plays in allocating IP addresses to network nodes (IP, Netmask, Gateway, etc.)

Come and Go (Dynamic

Allocation)

IP Pool and Lease IP Pool and Lease Security = Mixed

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Describe the features of the SMB protocol and the role it plays

in supporting file sharing in Microsoft-based networks

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File Sharing Service/ SMB Protocol

(Cont.)

The Server Message Block (SMB) (IBM/Microsoft) describes

the structure of shared network resources, such as directories, files, printers, and serial ports. (SAMBA = Linux/Unix)

Unlike the file sharing supported by FTP, clients establish a

long term connection to servers. (Map Network Drive)

Once the connection is established, the user of the client can Once the connection is established, the user of the client can

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File Sharing Service and SMB Protocol

The SMB protocol describes file system access and how clients

can make requests for files

All SMB messages share a common format. This format uses a

fixed-sized header followed by a variable-sized parameter and data component.

Start, authenticate, and terminate sessions

Control file and printer access

Allow an application to send or receive messages to or from

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P

P2

2P Services and Gnutella Protocol

P Services and Gnutella Protocol

Similar to FTP/SMB, P2P = File Sharing (Internet) based on

Gnutella protocol, e.g., BearShare, Gnucleus, LimeWire, Morpheus, WinMX and XoloX

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Connected Gnutella service, the client applications will search

for other Gnutella nodes to connect to.

These nodes handle queries for resource locations and replies

to those requests.

Govern control messages, which help the service discover other

nodes. Actual file transfers = HTTP 5 Gnutella Messages

P

P2

2P Services/ Gnutella Protocol (Cont.)

P Services/ Gnutella Protocol (Cont.)

5 Gnutella Messages

ping - for device discovery pong - as a reply to a ping query - for file location

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Before = Text-based system (display

terminal) physical

Telnet = virtual terminal (software) based

on TCP/IP to create session (VTY – Virtual Terminal); command line interface (CLI)

Telnet Client E.g., telnet (cmd), hyper

terminal, TeraTerm

Telnet Services and Protocol

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Telnet is a client/server protocol; how a VTY session is

established and terminated; provides the syntax and commands used to initiate the Telnet session

Command = at least two bytes.

The first byte is a special character called the Interpret as

Command (IAC) character.

As its name implies, the IAC defines the next byte as a

Telnet Services and Protocol (Cont.)

As its name implies, the IAC defines the next byte as a

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Telnet Services and Protocol (Cont.)

Some sample Telnet protocol commands include:

Are You There (AYT) - Lets the user request that

something appear on the terminal screen to indicate that the VTY session is active.

Erase Line (EL) - Deletes all text from the current line. Interrupt Process (IP) - Suspends, interrupts, aborts, or

terminates the process to which the Virtual Terminal is terminates the process to which the Virtual Terminal is connected.

For example, if a user started a program on the Telnet

server via the VTY, he or she could send an IP command to stop the program.

Drawback: Not secure (Plaintext) only authentication

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Summary

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Thank you and Questions?

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