Communication and network

1. © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
ITE PC v4.0
Chapter 1 1
Application Layer
Functionality and
Protocols

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Application Layer: OSI and TCP/IP Models
 The communication process between two communicating nodes is
Actually a communication process between two applications on
these devices.
 Application layer protocols are used to exchange data between
programs running on the source and destination hosts.
Service
application
protocol

3. ITE PC v4.0
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© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Application Layer: OSI and TCP/IP
Models

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Client
Server
Model
 Client: the device requesting the information
 Server: the device responding to the request is called a server.
 The client begins the exchange by requesting data from the server.
 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.

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Servers
 The server runs a service, or process, sometimes called a server
daemon.
 Daemons (like other services) typically run in the background and
are not under an end user's direct control.
 Daemons are described as "listening" for a request from a client.
Programmed to respond whenever the server receives a request for the
service provided by the daemon.
 When a daemon "hears" a request from a client:
It exchanges appropriate messages with the client, as required by its
protocol,
Proceeds to send the requested data to the client in the proper format.

6. © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
ITE PC v4.0
Chapter 1 6
Application Layer
Protocols

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HTTP
(WWW)
FTP
SMTP
(email)
Telnet
(file transfer)
(remote login)
DHCP
(IP address
resolution)
DNS
(file sharing)
P2P
(domain name
resolution)
(file sharing)
SMB
We will examine
HTTP in detail.

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DNS

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nslookup
nslookup
 Displays default DNS server for your host
 Can be used to query a domain name and get the IP address

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Name Resolution
Resolver
 DNS client programs used to look up DNS name information.
Name Resolution
 The two types of queries that a DNS resolver (either a DNS client or another DNS server)
can make to a DNS server are the following:
Recursive queries
 Queries performed by Host to Local DNS Server
Iterative queries
 Queries performed Local DNS server to other servers

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DNS Name
Resolution
 User types http://www.example.com
Step 1.
 The DNS resolver on the DNS client sends a recursive query to its
configured Local DNS server.
 Requests IP address for "www.example.com".
 The DNS server for that client is responsible for resolving the name
Cannot refer the DNS client to another DNS server.
1

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Step 2.
 Local DNS Server forwards the query to a Root DNS server.
Step 3.
 Root DNS server
Makes note of .com suffix
Returns a list of IP addresses for TLD (Top Level Domain Servers)
responsible for .com.
DNS Name
Resolution
1
2
2
3

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DNS Name
Resolution
 Root DNS Servers
There are 13 Root DNS servers (labeled A through M)
 TLD Servers
Responsible for domains such as .com, edu, org, .net, .uk, jp, fr
There are redundant servers throughout the world.

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Step 4.
 The local DNS server sends query for www.example.com to one of
the TLD servers.
Step 5.
 TLD Server
Makes note of example.com
Returns IP address for authoritative server example.com (such as
dns.example.com server)
DNS Name Resolution 4
4
5

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Step 6.
 Local DNS server sends query for www.example.com directly to
DNS server for example.com
Step 7.
 example.com DNS server responds with its IP address for
www.example.com
6
6
DNS Name Resolution
7

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Step 8.
 Local DNS server sends the IP address of www.example.com to the
DNS client.
 DNS Caching
 When a DNS server receives a DNS reply (mapping hostname to an
IP address) it can cache the information in its local memory.
8
7
DNS Name Resolution

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DNS Name
Resolution
 ipconfig /displaydns
After a certain amount of time, specified in the Time to Live (TTL)
associated with the DNS resource record, the resolver discards the
record from the cache.
ipconfig /flushdns – Manually deletes entries
 The default TTL for positive responses is 86,400 seconds (1 day).
 The default TTL for negative responses is 300 seconds.

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DHCP – Dynamic Host Configuration Protocol
 IP addresses and other information can be obtained:
Statically
Dynamically (DHCP)

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DHCP
 DHCP Information can
include:
IP address
Subnet mask
Default gateway
Domain name
DNS Server
 DHCP servers can be:
Server on LAN
Router
Server at ISP

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DHCP
 We will discuss DHCP more when
we discuss IPv4.

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Telnet
 Allows a user to remotely access another device (host, router,
switch).
 A connection using Telnet is called a Virtual Terminal (VTY) session,
or connection.
 Telnet uses software to create a virtual device that provides the same
features of a terminal session with access to the server command
line interface (CLI).
 Telnet clients (Teraterm ,Hyperterm)
Server
Telnet
Telnet

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Telnet
 Telnet supports user authentication, but does not encrypt data.
 All data exchanged during a Telnet sessions is transported as
plain text.
 Secure Shell (SSH) protocol offers an alternate and secure
method for server access.
Stronger authentication
Encrypts data

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HTTP (HyperText Transfer Protocol)
HTTP
Server
HTTP
Client
Web page (html document)
Web page consists of
Objects
HTML file ,JPEG
image ,GIF
image ,JAVA applet,
Audio file
Browser – The user
agent for the Web.
Displays requested
Web page and
provides navigational
and configuration
features.

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Web
Caching
1. Client/browser sends HTTP Request to Web cache (Proxy server).
2. Web cache checks to see if it has a local copy of the object.
2a. Local copy: Web cache sends object to client’s browser.
2b. No Local copy: Web cache sends HTTP request to origin server.
3. Origin server sends object to Web cache.
4. Web cache stores a local copy of the object.
5. Web cache forwards copy of the object to the client browser.
Note: TCP connections are also created between Client and Web Cache; Web cache and Origin
server (later).
Web
Cache or
Proxy
Server
Client
Client
Origin
Server
Origin
Server
HTTP
Request
HTTP
Request
HTTP
Request
HTTP
Request
HTTP Response
HTTP Response
HTTP Response
HTTP Response

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FTP (File Transfer Protocol)
 FTP was developed to allow for file transfers between a client and a server.
 Used to push and pull files from a server running the FTP daemon (FTPd).
 Uses get and put commands
FTP
Client
FTP
Server

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 Internet mail involves:
User agents
Allows users to read, reply, compose, forward, save, etc., mail messages
GUI user agents: Outlook, Eudora, Messenger
Mail servers
Stores user mail boxes, communicates with local user agents and other
mail servers.
SMTP
Principle application layer protocol for Internet mail
Sent over TCP
Mail access protocols: POP3, IMAP, HTTP (Web-based email)
SMTP
SMTP
POP3
IMAP
User agent User agent
Mail server Mail server
SMTP – Simple Mail Transfer Protocol

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SMTP
Mail software, processes used: MTA and MDA
 MUA (Mail User Agent) – Email client software.
 MTA (Mail Transfer Agent) – Software that governs transfer of email between
mail servers.
Includes UNIX sendmail, Microsoft Exchange Server, Postfix, and Exim
 MDA (Mail Delivery Agent) – Software that governs transfer of email from mail
servers to clients.
On Unix systems, procmail and maildrop are the most popular MDAs.
MTA
 receives email from the
client's MUA
 passes email to the MDA for
final delivery
 uses SMTP to route email
between servers

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P2P File Sharing
 P2P (Peer-to-Peer) file sharing accounts for more traffic on the
Internet than any other application (2004).
 Peers (hosts) act as both clients and servers.
 No centralized file server.
 HTTP GET and responses are commonly used.

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1. Peer A starts P2P application
2. Informs centralized directory server of its:
IP address
Names of objects making available for sharing (MP3, videos, etc.)
3. Directory server collects information from each peer that becomes active.
Dynamic database
Maps IP addresses with object names
4. Peer A queries directory server for IP addresses of other peers for specific content
Directory Server returns IP addresses for those peers (Peer B)
5. Peer A establishes TCP connection and downloads file (i.e. HTTP GET) from other peer, Peer B.
6. Directory server removes Peer from database when Peer closes application or disconnects from
Internet (periodic messages – pings – from server).
1 – Inform and Update
Centralized
Directory
Server
P2P – Centralized Directory
1 – Inform and Update
1 – Inform and Update
1 – Inform and Update
Peer
Peer B
Peer
Peer A
2 – Query for
content
3 – File Transfer

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Peer A searches for a file
1. Peer A sends query to all neighboring peers.
2. If neighboring peer does not have file, forwards query to all its neighboring peers
3. If any peer has the file it returns a query hit message.
4. Peer A selects a peer, Peer C, to retrieve file (HTTP GET)
5. A direct TCP connection is made with selected peer, Peer C.
6. HTTP response is used to send file.
Peer A
Peer C
Peer B
Peer D
P2P – Query
Flooding
Peer E
Peer F
Query
Query
Query
Query
Query
Query hit
Query hit
Query hit
File transfer

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 Kazaa combines ideas from Napster and Gnutella
 2004 – Contributed to more traffic on Internet than any other application
 2007 – Bittorrent became the leading application
 Proprietary technology
P2P - Combination
Group
Leader
Group
Leader
Group
Leader
Query
Query
Query
Query Reply

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Summary
applications
DNS DHCP TELNET
HTTP
SMTP
P2P FILE SHARING