Computer Networks & internet protocols.pptx

1
COMPUTER NETWORKS AND
INTERNET PROTOCOLS
Application Layer – I
I (DNS)
SOUMYA K GHOSH
COMPUTER SCIENCE AND ENGINEERING
I
I
T KHARAGPUR
SANDIP CHAKRABORTY
I
I
T KHARAGPUR

Application Layer Examples
• Domain Name System (DNS)
• File Transfer Protocol (FTP)
• Hypertext Transfer Protocol (HTTP)
• Simple Mail Transport Protocol (SMTP)
• Simple Network Management Protocol
(SNMP)
• Telnet
• ….

What is DNS?
• The global database system for Internet addressing, mail
and other information.
– Much easier to use and memorize.
• Concept of domains and sub-domains.
– Domain management is distributed.
– DNS servers translate domain names to IP addresses.

Top Level Domains
• com – Commercial
• org – Non-profit
• net – Network service provider
• gov – US govt.
• mil – military
• edu – Education
• au – Australian
• at – Austrian
• ca – Canadian
• dk – Dutch
• fr – French
• de – German
• in – Indian
• it – Italian
• jp – Japanese
• kr – Korean
• nz – New Zealand
• es – Spanish
• tw – Taiwanese
• uk – British or Irish
• us – U.S.

Domain Name Structure
• Domain names are arranged in a hierarchical tree-like
structure.
in
ac
iitkgp
iitb
math
cse
cse
ece
mech
nic Eg:
cse.iitkgp.ac.in

Fully qualified domain names (FQDNs)
• If a domain name ends in a dot it is assumed to be complete.
This is called a fully qualified domain name (FQDN) or an absolute
domain name.
• If a domain name does not end in a dot, it is incomplete and
the DNS resolver may complete this by appending a suffix
to the domain name. The rules for doing this are
implementation-dependent and locally configurable.

Generic TLDs
• The top-level names are called the generic top-level domains
(gTLDs), and can be three characters or more in length.
the
Internet
• These names are registered withand
maintained by Corporation for Assigned Names
and Numbers (ICANN).
http://www.icann.org

Country Domains
• Top-level domains named for the each of the ISO 3166
international 2- character country codes (from ae for the
United Arab Emirates to zw for Zimbabwe). These are called
the country domains or the geographical domains.
• Many countries have their own second-level domains underneath
which parallel the generic top-level domains.

What is a Zone?
• Domains are broken into zones for which individual DNS
servers are responsible.
– A domain represents the entire set of names/machines
that are contained under an organizational
domain name.
– A zone is a domain minus any sub-domains delegated
to other DNS servers.

The Concept
• Each domain name is typically served by 2 or more DNS
servers for redundancy.
– Referred to as primary and secondary.
• Only one DNS server should be configured as primary for a
zone.
– Several secondary DNS servers possible.
– The primary server contains master copy of the data for
a zone.
– Secondary servers get copies of this data
through zone transfers.

Zone Transfer
• A primary server loads all information from the disk file.
• The secondary server loads all information from the primary
server
.
• When the primary downloads information from the
secondary, it is called zone transfer.

Name Resolution Process
• The commonly used server is BIND (Berkeley Internal
NameDomain).
– Runs under UNIX as a process called named.
• When an application needs some information from the
server
, it invokes the DNS name resolver.
– DNS translates a fully qualified domain name
into the corresponding IP address.
– Using the command nslookup.

Name Resolution Process (contd…)
– If the name server does not have the information
locally, it asks its primary server
, and so on.
– For redundancy, each host may also have one or
more secondary name servers which may be
queried when the primary fails.
• “nslookup” command

Hierarchy of Name Servers
root
stanford.edu
com
edu
rediff.com
au in
yahoo.com
gatech.edu

Iterative Name Resolution
• Client sequentially sends queries to DNS servers and receives
response.
– If response is negative, the DNS serverto query next is also
returned.
– Unlike recursive name resolution, where only one response
is finally returned back to the client.

Domain name stub resolver
• Stub resolver, a routine linked with the user program, that forwards the queries
to a name server for processing.
• On most platforms, the stub resolver is implemented by two library routines
(or by some variation of these routines): gethostbyname() and
gethostbyaddr().

DNS Resource Records
(RR)
• Domain Name System's distributed database is composed of resource records
(RRs), which are divided into classes for different kinds of networks.
• Resource records provide a mapping between domain names and network objects.
• The most common network objects are the addresses of Internet hosts, but the
Domain Name System is designed to accommodate a wide range of different
objects.
• A zone consists of a group of resource records, beginning with a Start of
Authority (SOA) record.
• The SOA record identifies the domain name of the zone.
• There will be a name server (NS) record for the primary name server for this
zone. There might also be NS records for the secondary name servers.
• The NS records are used to identify which of the name servers are
authoritative.

QR: Query/Response
OpCode:0 standard, 1 inverse, 2 server
status AA: Authoritative
TC: Truncated
RD: Recursion Desired
RA: Recursion
Available rCode: Status of
the error
Flag Fields

admin.atc.fhda.edu.
Query Name Format

Example 1
• A resolver sends a query message to a local server to find the IP
address for the host “chal.fhda.edu”. We discuss the query and
response messages separately.

Example 1: The Response Message

Example 2
• An FTP server has received a packet from an FTP client with IP
address
153.2.7.9. The FTP server wants to verify that the FTP client
is anauthorized client.

Example 2: Inverse Query Message

Example 2: Inverse Response Message

1
INTERNET PROTOCOLS
Application Layer – I
I [Cli-Srv, FTP]
SOUMYA K GHOSH
I
I
T KHARAGPUR
SANDIP CHAKRABORTY
I
I
T KHARAGPUR

Client-server Model
• Standard model for developing network
applications.
• Notion of client and server.
– A server is a process that is offering some
service.
– A client is a process that is requesting the
service.
– Server or client may be running on different
machines.
– Server waits for requests from client(s).

CLIENT
CLIENT
SERVER CLIENT
Client-Server Model (contd.)

• Typical scenario:
– The server process starts on some computer system.
• Initializes itself, then goes to sleep waiting for a client request.
– A client process starts, either on the same system or on
some other system.
• Sends a request to the server.

– When the server process has finished providing its service to
the client, the server goes back to sleep, waiting for the next
client request to arrive.
• The process repeats.

• Roles of the client and the server processes are
asymmetric.
• Two types of servers:
– Iterative servers.
– Concurrent servers.

Iterative Servers
• Used when the server process knows in advance how long it
takes to handle each request and it handles each request
itself.
– Single copy of server runs at all times.
– A client may have to wait if the server is busy.

Concurrent Servers
• Used when the amount of work required to handle a request is
unknown; the server starts another process to handle each
request.
– A copy of the server caters to a client’s request in a dedicated
fashion.
– As many copies of server as there are client requests.

Using TCP or UDP
• Before start of communication, a connection has to be
established between the two hosts.
• Five components in a connection:
– Protocol used
– Source IP address
– Source port number
– Destination IP address
– Destination port number

Develop a Network Application
• The best way is to use some standard and well-accepted
protocol.
– At the data link layer level, use Ethernet.
– At the network layer level, use IP.
– At the transport layer level, use TCP.
– At the application layer level, use a standardAPI like the
Berkeley Socket Interface.

What is a Socket?
• The socket is the method for achieving inter-process
communication (IPC).
• It is used to allow one process to speak to another (on
same or different machine).
– Analogy: Like the telephone is used to allow one
person to speak to another.

Socket - Basic Idea
• When two processes located on two machines
communicate, we define association and socket.
– Association: basically a 5-tuple
• Protocol
• Local IP address
• Local port number
• Remote IP address
• Remote port number

Socket - Basic Idea
– Socket: also called half-association (a 3-
tuple)
• Protocol, local IP address, local port number
• Protocol, remote IP address, remote port
number

FTP
• Facilitates transfer of files over
network
• Server/Client model
• FTP often works with
• Transmission Control Protocol
(TCP)
• Telnet Protocol
• Defined as RFC959

Overview of FTP
• FTP uses TCP as a transport protocol to provide reliable end-to-end
connections and implements two types of connections in managing
data transfers.
• The FTP client initiates the first connection, referred to as the control
connection, to well-known port 21 (the client’s port is typically
ephemeral). It is on this port that an FTP server listens for and accepts
new connections.
• The control connection is used for all of the control commands a client
user uses to log on to the server
, manipulate files, and terminate a
session. This is also the connection across which the FTP server will
send messages to the client in response to these control commands.
Ref: IBM Redbools

Overview of FTP (contd.)
• The second connection used by FTP is referred to as the data connection.
• Typically, the data connection is established on server port 20. However
,
depending on how the data connection is established, both the client and
server might use ephemeral ports.
• FTP transfers the data over data connection. FTP only opens a data connection
when a client issues a command requiring a data transfer
, such as a request to
retrieve a file, or to view a list of the files available. It is possible for an entire
FTP session to open and close without a data connection ever having been
opened.
• The data connection is unidirectional. FTP can transfer data only from the
client to the server
, or from the server to the client, but not both.
• The data connection can be initiated from either the client or the server
. Data
connections initiated by the server are active, while those initiated by the client
are passive.

FTP – Basic working
⚫ FTP has to be on both server and client computers to work
⚫ Connection
⚫ Control connection (port 21)
⚫ Used to send and receive FTP commands
⚫ Data connection (port 20)
⚫ Used to upload and download files
⚫ Processes
⚫ Data Transfer Process (DTP)
⚫ Establishing the connection and managing the data channel
⚫ Protocol Interpreter (PI)
⚫ Interprets the protocol
⚫ let DTP be
controlled using
commands received
over the control
channel

(contd.)
⚫ Transferring mode between server and client
⚫ Active mode
⚫ Control connection port: Client: Large port (N>1023);
Server: 21
⚫ Data connection port: Client: N+1; Server: 20
⚫ Passive mode
⚫ Control connection port: Client: Large port (N>1023);
Server: 21
⚫ Data connection port: Client: N+1; Server: large port
(P>1023)
⚫ File Transferring mode
⚫ ASCII mode
⚫ .txt, .html, .asp, .vbs,.js
⚫ Binary
⚫ .doc, .pdf, .mp3/mp4

(contd.)
• The client FTP application is built with a protocol interpreter (PI), a data
transfer process (DTP), and a user interface.
• The server FTP application typically only consists of a PI and DTP
Ref: IBM Redbools

(contd.)
• FTP client’s user interface communicates with the protocol interpreter
(PI), which manages the control connection.
• PI translates any application-specific commands to the RFC architected FTP
commands, and then communicates these control commands to the FTP
server.
• The FTP server’s PI receives these commands, and then initiates the
appropriate processes to service the client’s requests. If the requests require
the transfer of data, data management is performed by the DTPs on both the
client and server applications.
• After the completion of the data transfer
, the data connection is closed, and
control is returned to the PIs of the client and server applications.
• Only one data transfer can occur for each data connection. If multiple data
transfers are required for a single FTP session, one distinct control connection
will be opened for each transfer.
Ref: IBM Redbools

FTP Operation – User’s Perspective
When using FTP
, the user performs some or all of the following
operations:
• Connect to a remote host.
• Navigate and manipulate the directory structure.
• List files available for transfer.
• Define the transfer mode, transfer type, and data structure.
• Transfer data to or from the remote host.
• Disconnect from the remote host.
Ref: IBM Redbools

A Typical FTP scenario
Ref: IBM Redbools

Trivial File Transfer Protocol (TFTP)
• TFTP file transfer is a disk-to-disk data transfer
, and is an simple protocol used to
transfer files. The simplicity of the architecture is deliberate in order to facilitate ease of
implementation.
• This simplistic approach has many benefits over traditional FTP
, including:
– Use by diskless devices to download firmware at boot time
– Use by any automated process for which the assignment of a user ID or password
is not feasible
– Small application size, allowing it to be implemented inexpensively and in
environments where resources are constricted
• TFTP is implemented on top of the User Datagram Protocol.
• The TFTP client initially sends read/write request through well-known port 69. The server
and the client then determine the port that they will use for the rest of the connection.
• TFTP lacks most of the features of FTP
, and instead is limited to only reading a file from a
server or writing a file to a server.
• TFTP has no provisions for user authentication; in that respect, it is an insecure protocol.
Ref: IBM Redbools

FTP – Access Commands
Ref: TCP/IP Protocol Suite

FTP – File Management Commands

FTP – Data Formatting Commands

FTP – File Transfer
Commands

1
INTERNET PROTOCOLS
Application Layer – IV [HTTP
, HTML, TELNET]
SOUMYA K GHOSH
I
I
T KHARAGPUR
SANDIP CHAKRABORTY
I
I
T KHARAGPUR

HyperText Transfer Protocol (HTTP)
• HTTP is the protocol that supports communication
between webbrowsers and web servers.
• A “Web Server” is a HTTP server
• A “Web Browser” is a HTTP client
• Most clients/servers run version 1.1, but 1.0 is also in
use.
– RFC 1945 (HTTP 1.0)
– RFC 2616 (HTTP 1.1)
• HTTP version 1.1 specifies a persistent connection by
default.

HTTP – Overview
• “HTTP is an application-level protocol with the lightness and
speed necessary for distributed, hypermedia information
systems.”
• Transport Independence
– HTTP protocol generally takes place over a TCP
connection,
– However
, the protocol itself is not dependent on a
specific transport layer.

Request - Response
• HTTP has a simple structure:
– client sends a request
– server returns a reply.
• HTTP can support multiple request-reply exchanges over a
single TCP connection.
• The “well known” TCP port for HTTP servers is port 80.
– Other ports also can be used

ARCHITECTURE
WWW is a distributed client/server service, in which a HTTP
client (browser) can access a service from a HTTP server.
• Client (Browser)
• Server
• Uniform Resource Locator
• Cookies

Operation
Ref: Data Communication and Networking, Forouzan

HTTP Client (Browser)

WEB DOCUMENTS
Web documents can be grouped into three broad categories:
• Static
• Dynamic
• Active

Static document

Dynamic document using CGI (Common Gateway
Interface)

Dynamic document using Server-site script

Active document using Java applet

Active document using Client-site script

HTTP: Request and Response messages

HTTP Methods

This example retrieves a document. We use the GET method to retrieve an
image with the path /usr/bin/image1. The request line shows the method
(GET), the URL, and the HTTP version (1.1).
The header has two lines that show that the client can accept images in the
GIF or JPEG format. The request does not have a body. The response message
contains the status line and four lines of header. The header lines define the
date, server, MIME version, and length of the document. The body of the
document follows the header.
Example 1

Clien
t
Request (GET
method)
GET /usr/bin/image1
HTTP/1.1 Accept: image/gif
Accept: image/jpeg
HTTP/1.1 200 OK
Date: Mon, 07-Jan-05 13:15:14 GMT
Server:
Challenger
MIME-version:
1.0
Content-length:
2048
(Body of the
document)
Respons
e
Serve
r

In this example, the client wants to send data to the
server. Use the POST method. The request line shows the
method (POST), URL, and HTTP version (1.1). There
are four lines of headers. The request body contains the
input information. The response message contains the
status line and four lines of headers. The created
document, which is a CGI document, is included as the
body
Example 2

Connecting HTTP server using TELNET

HTTP Proxy Server
HTTP
Server
Browser
HTTP
Proxy

What is HTML?
• Stands for Hyper Text Markup Language
• Computer language used to create web pages
• HTML file = text file containing markup tags
such <p>
• T
ags tell Web browser how to display a page
• Can have either *.htm or *.html file extension

HTML Elements
• T
ags are the elements that create the components of
a page
• T
ags surrounded by angle brackets < >
• Usually come in pairs
– Example: Start tag <p> and end tag </p>
• Stuff between is called “element content”
• T
ags are not case sensitive
– New standard is to use lower case

Y
our created HTML document
<html>
<head>
<title> …document title…
</title>
</head>
<body>
…your page content…
</body>
</html>

Page Components
• <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-
transitional.dtd">
– First line of code
– Declaration of version of HTML
• <html>…</html>
– Container for the
document
• <head>…</head>
– <title> Title of page
</title>
• <body>…</body>
–
Content of page
<html>
<head>
<title> …document
title… </title>
</head>
<body>
…your page
content…
</body>
</html>

Computer Networks & internet protocols.pptx

Basic T
ags
to <h6>…</
h6>
• Headings
– <h1>…
</h1>
– Like in
Word
• Paragraph
– <p>… </p>
– Inserts a line space before and after a
paragraph

Link T
ag
• Link
– Anchor tag <a>…</a>
– 3 kinds
• Link to page in same folder
• Link to page in different folder
• Link to outside webpage on the
Internet.

Example of Anchor T
ag
<a href="http://www.iitkgp.ac.in">Go to the IITKgp home page</a>
address text in page
– Two components
• Address
• Text or description – this is what you see on
the page

Image Source T
ag
• Empty tag – no closing tag
• Components of Img tag
<img src="url“ alt = “description of
image” />
– url = points to location of the image
file
– alt = describes image for screen
readers

File location
• Same folder: “samplePic.gif”
– Document-relative link
– Look for image in same folder
• Different folder named images:
“/images/samplePic.gif”

Division T
ag
• <div>…</div>
– Division or section of document
– Use to group elements to apply
formatting or style
– Example:
<div style="color: #1F00FF">
<h1> Title of section</h1>
<p> Computer
Networks</p>
</div>
<html>
<head>
<title> Sample - Page</title>
</head>
<body>
Hello from NPTEL!
<div style="color: #1F00FF">
<h1> Title of section</h1>
<p> Computer Networks
</p>
</div>
</body>
</html>

Typical HTML document
<html>
<head>
<title> …document title…
</title>
</head>
<body>
…your page content…
</body>
</html>

End
Product
• <html>
• <head>
• <title>Author’s Page</title>
• </head>
• <body>
• <div>
• <a href="index.html>Home</a><br />
• <a href="courses.html">Courses</a><br />
• <a href="personal.html">Personal</a><br />
• </div>
• <p>Hello my name is XYZ and I am writing about myself.
Contact info:
<a href="http://www.hawaii.edu/slis/webteam">Web
Team</a>
<div>
<img src="palmtree.jpg"alt=”a picture of a palm
tree”/>
</div>
•
•
•
•
•
• </
div>
• </
body>
• </
html>

Next Mission
• Choose colors for your
page
– Text color
– Link color
– Background color
• Choose font size
– Type of font
– Font size

TELNET vs.
telnet
• TELNET is a protocol that provides “a general, bi-
directional, eight-bit byte oriented communications
facility”.
• telnet is a program that supports the TELNET protocol
over TCP
.
• Many application protocols are built upon the TELNET
protocol.

The TELNET
Protocol
 Reference: RFC 854
• TCP connection (Popular port: 23)
• Data and control over the same connection.
• Network Virtual Terminal
– intermediate representation of a generic terminal.
– provides a standard language for communication of
terminal control functions.

Network Virtual Terminal (NVT)
NVT
TCP
Server
Process
NVT
TCP

Negotiated Options
• All NVTs support a minimal set of capabilities.
– Some terminals have more capabilities than the minimal set.
• The set of options is not part of the TELNET protocol,
– so that new terminal features can be incorporated without
changing the TELNET protocol.
• Two endpoints negotiate a set of mutually acceptable options
– Line mode vs. character mode
– echo modes
– character set (EBCDIC vs. ASCII)

Control Functions
• TELNET includes support for a series of
control functions commonly supported by
servers.
• This provides a uniform mechanism for
communication of (the supported) control
functions.

Control Functions
• Interrupt Process (IP)
– suspend/abort process.
• Abort Output (AO)
– send no more output to user’s
terminal.
• Are Y
ou There (AYT)
– check to see if system is still
running.
• Erase Character (EC)
– delete last character sent
• Erase Line (EL)
– delete all input in current line.

Command Structure
• All TELNET commands and data flow through the
same TCPconnection.
• Commands start with a special character called the
Interpret as Command escape character
– The IAC code is 255.
– If a 255 is sent as data - it must be followed by
another 255.
• If IAC is found and the next byte is IAC
– a single byte is presented to application/terminal
• If IAC is followed by any other code
– the TELNET layer interprets this as a command.

TELNET Operations
• You can use the telnet program to play with the TELNET
protocol.
• telnet is a generic TCP client.
– Sends whatever you type to the TCP socket.
– Prints whatever comes back through the TCP socket
– Useful for testing TCP servers (ASCII based protocols).
• Many Unix systems have these servers running (by default):
– echo port 7
discard
– daytime port 13
chargen
port 9
port
19

telnet hostname port
> telnet skg.cse.edu 7
Trying 234.197.140.246...
Connected to skg.cse.edu
(234.197.140.246).
Escape character is '^]'.
Hi SKG
Hi SKG
Testing telnet
Testing telnet
^]
telnet> quit
Connection closed.

Simple Mail Transfer Protocol (SMTP)

SMTP
• Protocol originated in 1982 (RFC821, Jon Postel)
• Standard message format (RFC822,2822, D. Crocker)
• Goal: To transfer mail reliably and efficiently

SMTP
• SMTP clients and servers have two
main components
– User Agents – Prepares the
message, encloses it in an envelope.
(ex. Thunderbird, Eudora)
– Mail Transfer Agent – Transfers the
mail across the internet (ex.
Sendmail, Exim)
– Analogous to the postal system in
many ways

SMTP
• SMTP also allows the use of
Relays allowing other MTAs to
relay the mail
• Mail Gateways are used to
relay mail prepared by a
protocol other than SMTP and
convert it to SMTP

Typical form of an Email
• Mail is a text file
• Envelope –
– sender address
– receiver address
– other information
• Message –
– Mail Header – defines the sender,
the receiver, the subject of the
message, and other information
– Mail Body – Contains the actual
information in the message

SMTP Keywords
Keyword Arguments
HELO Sender’s Host Domain Name
MAIL FROM: Email Address of sender
RCPT TO: Email of Intended recipient
DATA Body of the message
QUIT

Keyword Arguments
RSET
VRFY Name to be verified
NOOP
TURN
EXPN Mailing list to expand
HELP Command Name
SMTP Keywords

Status
Codes
• The Server responds with a 3 digit code that may be
followed by text info
– 2## - Success
– 3## - Command can be accepted with more
information
– 4## - Command was rejected, but error
condition is temporary
– 5## - Command rejected, Bad User!

Connection
Establishment
TCP Connection Establishment

Connection Termination
TCP Connection Termination

Solution: SMTP extensions
• MIME – Multipurpose Internet Mail Extensions
– Transforms non-ASCII data to NVT (Network Virtual Terminal) ASCII data
• Text
• Application
• Image
• Audio
• Video
RFC 1425, 1426, 1521

MIME Headers
• Located between the Email Header and Body
– MIME-Version: 1.1
– Content-Type: type/subtype
– Content-Transfer-Encoding: encoding type
– Content-Id: message id
– Content-Description: textual explanation of non-
textual contents

MIME Headers
(cont’d)
• Content-Type – Type of data used in the Body
– Text: plain, unformatted text; HTML
– Multipart: Body contains different data types
– Message: Body contains a whole, part, or pointer to a message
– Image: Message contains a static image (JPEG, GIF)
– Video: Message contains an animated image (MPEG)
– Audio: Message contains a basic sound sample (8kHz)
– Application: Message is of data type not previously defined
• Content-Transfer-Encoding – How to encode the message
– 7 bit – no encoding needed
– 8 bit – Non-ASCII, short lines
– Binary – Non-ASCII, unlimited length lines
– Base64 – 6 bit blocks encoded into 8-bit ASCII
– Quoted-printable – send non-ASCII characters as 3 ASCII characters, =##, ## is the hex representation
of the byte

MT
As and Mail Access
Protocols
• The MT
A delivers email to the user’s mailbox
• Can be complex with numerous delivery methods, routers, and ACLs
• Exim, Postfix, Sendmail
• The Mail Access Protocols are used by the users to retrieve the email
from the mailbox
– POP3
– IMAP4

POP vs.
IMAP All Messages
POP3:
IMAP
:
Johnr
Internet
Friends
…
Intern
.
et

Post Office Protocol
v3
• Simple
• Allows the user to obtain a list of their Emails
• Users can retrieve their emails
• Users can either delete or keep the email on their
system
• Minimizes server resources

Internet Mail Access Protocol (IMAP)
v4
• Has more features than POP3
• User can check the email header before downloading
• Emails can be accessed from any location
• Can search the email for a specific string of characters
before downloading
• User can download parts of an email
• User can create, delete, or rename mailboxes on a
server

Simple Network Management Protocol
(SNMP)

Network Management
The development of SNMP was to be kept simple, facilitating rapid deployment of
the protocol throughout the Internet community. After the immediate management
needs were met, albeit temporarily, by SNMP, thorough research and development
could be performed on CMIS/CMIP. Ultimately, this protocol would then be
deployed as a permanent solution, replacing SNMP.

Simple Network Management Protocol (SNMP)
Fundamental objective of Simple Network Management Protocol (SNMP) is to
manage all aspects of a network, as well as applications related to that network.
 Monitor: SNMP implementations allow network administrators to monitor their
networks in order to--among other things--ensure the health of the network,
forecast usage and capacity, and in problem determination.
 Manage: SNMP provides the capability for network administrators to affect
aspects with the network. Values which regulate network operation can be
altered, allowing administrators to quickly respond to network problems,
dynamically implement new network changes, and to perform real-time testing
on how changes may affect their network.

• SNMP implements a manager/agent/subagent model, which conforms very closely to the
client/server model.
• RFC 1157 defines the components and interactions involved in an SNMP community,
which include:
 A Management Information Base
 An SNMP agent
 An manager
 SNMP subagents
S N M P a ge nt
S N M P
m a n a g e r
S N M P a ge nt
S N M P a ge nt
S N M P
protocol
m e s s a g e s

• SNMP agent is software that runs on a piece of network equipment (host,
router, printer, or others) and that maintains information about its configuration
and current state in a database
• Information in the database is described by Management Information
Bases (MIBs)
• An SNMP manager is an application program that contacts an SNMP agent
to query or modify the database at the agent.
• SNMP protocol is the application layer protocol used by SNMP agents
and managers to send and receive data.

SNMP - Interactions
S N M P
M a n a g e r
P ro c e s s
Replies
Queries
Traps
S N M P
U D P
IP IP
U D P
S N M P
S N M P A g e n t
P ro c e s s
Access
object
s
Dat
a
I P N e t w o r k
M a n a g e m e n t
Stat io n
S N M P m e s s a g e s
Trap
s
M a n a g e d S y s t e m
M I B

Management Information Bases (MIBs)
• A MIB specifies the managed objects
• MIB is a text file that describes managed objects using the syntax of ASN.1
(Abstract Syntax Notation 1)
• ASN.1 is a formal language for describing data and its properties
• In Linux, MIB files are in the directory /usr/share/snmp/mibs
–Multiple MIB files
–MIB-II (defined in RFC 1213) defines the managed objects of TCP/IP networks

Managed Objects
• Each managed object is assigned an object identifier (OID)
• OID is specified in a MIB file.
• An OID can be represented as a sequence of integers separated by
decimal points or by a text string:
• When an SNMP manager requests an object, it sends the OID to the
SNMP agent.

SNMP Protocol
• SNMP manager and an SNMP agent communicate using the SNMP
protocol
–Generally: Manager sends queries and agent responds
–Exception: Traps are initiated by agent.
g e t-
r e q u e s t
g e t- n ex t -
r e q u e s t
se t - re q u e s t
trap
P o r t 1 6 1
P o r t 1 6 1
P o r t 1 6 1
P o r t 1 6 2
S N M P a g e n t
S N M P m a n a g e r
g e t- r e s p o n s e

SNMP Protocol
• Get-request. Requests the values of one or more objects
• Get-next-request. Requests the value of the next object, according to
a lexicographical ordering of OIDs.
• Set-request. A request to modify the value of one or more objects
• Get-response. Sent by SNMP agent in response to a get-request, get-next-
request, or set-request message.
• Trap. An SNMP trap is a notification sent by an SNMP agent to an SNMP
manager, which is triggered by certain events at the agent.

SNMP Versions
• Three versions are in use today:
–SNMPv1 (1990)
–SNMPv2c (1996)
•Adds “GetBulk” function and some new types
•Adds RMON (remote monitoring) capability
–SNMPv3 (2002)
•SNMPv3 started from SNMPv1 (and not SNMPv2c)
•Addresses security
• All versions are active
• Many SNMP agents and managers support all three versions of the
protocol.

Format of SNMP Packets
• SNMPv1 Get/Set messages:
Version Community SNMP PDU
PDU Type Request ID
Error Status Error Index
Object 1, Value 1
Object 2, Value 2
...
Cleartext string that is
used as a password
PDU type, e.g.:
32: SNMPv1 Get
64: SNMPv2 Get
Unique ID to match
requests with replies
Sequence of name-value
pairs

SNMP Security
• SNMPv1 uses plain text community strings for authentication as plain
text without encryption
• SNMPv2 was supposed to fix security problems, but effort de-railed
(The “c” in SNMPv2c stands for “community”).
• SNMPv3 has numerous security features:
– Ensure that a packet has not been tampered with (integrity),
– Ensures that a message is from a valid source (authentication)
– Ensures that a message cannot be read by unauthorized (privacy).

References
1. “TCP/IP Tutorial and Technical Overview”, An IBM Redbooks publication, 2006.
2. Data Communications & Networking, Behrouz A. Forouzan
3. TCP/IP Protocol Suite, B. Forouzan
4. SMTP, TCP/IP and Upper Layer Protocols, Ezra Kissel, M. Lacroce and J.
Watson
5. Internet resources

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