程序代写代做代考 case study Java FTP database Chapter 1. Introduction to Data Communications
Chapter 1. Introduction to Data Communications
Networks, Security, and Privacy
158.235
A/Prof. Julian Jang-Jaccard
Massey University
Application
Layer
Reading: Chapter 2 in the prescribed textbook
Outline
• Application Architecture
• Application Layer Services
– The Web: HTTP
– Email: SMTP, POP
– Other Application-Layer Protocols
Application Layer
• Layer 5 in the Internet
model
• The software that
enables users to
interact with the
network and
accomplish tasks
Application
Transport
Network
Data Link
Physical
Internet Model
Applications
(e.g., email, web..)
What is a network application?
Is a program that:
• run on (different) end
systems
• communicate over network
no need to write software
for network-core devices
• network-core devices do
not run user applications
• applications on end
systems allows for rapid
app development,
propagation
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
Application Architecture
• The way the functions of the application layer are
spread out across the client and the server
• Four components of applications:
1. Data Storage
2. Data Access Logic
3. Application Logic
4. Presentation Logic
Application Architectures
• Who is doing what between the clients
and servers?
– Host-based Architectures
• Server performs almost all functions
– Client-based architectures
• Client performs most functions
– Client-server architectures
• Functions shared between client and
server (including Cloud Computing)
– Peer to peer architectures
• Computers are both clients and servers
Host-Based Architecture
• Common in the 1960s with mainframes and
terminals
• Server contains all components (“server-based”)
CLIENT SERVER
Presentation Logic
Application Logic
Data Access Logic
Data Storage
Host-Based Architecture
• Advantages
– Very simple
– Single point of control
• Disadvantages
– Host (server) can become a bottleneck
– Upgrades typically expensive (‘lumpy
architecture’)
Client-Based Architecture
• Most common in the 1980s with popularity of PC
• Client contains presentation, application, and
data access logic while server stores the data
CLIENT SERVER
Data Storage
Presentation Logic
Application Logic
Data Access Logic
Client-Based Architecture
• Advantages
– Hardware and applications less expensive
– Simple architecture
• Disadvantages
– Data must travel back and forth between
server and client
Client-Server Architecture
• Most common architecture today
• Thin clients are easier to manage, thick clients have
more functionality
CLIENT SERVER
Data Access Logic
Data Storage
Presentation Logic
Application Logic
CLIENT SERVER
Application Logic
Data Access Logic
Data Storage
Presentation Logic
Thick-Client Architecture Thin-Client Architecture
Client-Server Architecture
• Advantages
– More efficient because of distributed
processing
– Allows hardware/software from different
vendors to be used together
– Less bandwidth required
• Disadvantages
– May be challenges in configuring
hardware/software from different vendors to
work together
– In many cases, middleware is required
Client-Server Architecture
Middleware is software acts as an intermediary by
“sitting between” client and server applications
1.Provides a standard way of translating between
software from different vendors
2.Manages message transfers: Insulates network
changes from the clients (e.g., adding a new server)
Client
Applications
Middleware
(ODBC etc.,)
Server
Applications
Client-Server Architecture
• Example of architecture
SERVER
Application Logic
Data Access
Logic
Data Storage
CLIENT
Presentation
Logic
two-tier
Client-Server Architecture
• Example of architecture
Application Logic
CLIENT
Presentation
Logic
Data Access
Logic
Data Storage
Application
SERVER
Database
SERVER
three-tier
Client-Server Architecture
• Example of architecture
Application Logic
CLIENT
Presentation
Logic
Data Access
Logic
Data Storage
Database
SERVER
Application Logic
n-tier
Application
SERVER
Web
SERVER
Tiered Client-Server Architecture
• Advantages
– Better load balancing: More evenly distributed
processing
– More scalable: Only servers experiencing high demand
need be upgraded
• Disadvantages
– Heavily loaded network: More distributed processing
necessitates more data exchanges
– Difficult to program and test due to increased
complexity
Cloud Computing
• Cloud Computing is the general term for
enabling access to computing services
over the network (most commonly the
Internet)
• Models of cloud computing define who
manages each application function and
associated hardware/software
Case Study: Building a Business
OLD WAY
• Significant Upfront
Investment
• Results in months
• Large total
investment
CLOUD WAY
• No Upfront
investment
• Results in days
• Trivial total
investment
Cloud Computing: Key benefits
• Huge Resources
– Available for everyone with a small fee
– Leasing model compared to buying model
• No Commitment
– No over provisioning (waste of capital)
– No under provisioning (waste of users)
• Pay by use
• Pay only for actual resources consumed
Software as a Service (SaaS)
Use Applications
Applications
Services
Platforms
Server OS
Network
Physical environment
(Electricity, Air Con, Spaces, etc.)
• Provides all application components and
associated hardware/software
Platform as a Service (PaaS)
Use Applications
Applications
Platforms
Services
Server OS
Network
Physical environment
(Electricity, Air Con, Spaces, etc.)
• Provides computing platforms (OS,
database, webserver etc.,)
Infrastructure as a Service (IaaS)
Use Applications
Applications
Platforms
Server OS
Network
Services Physical environment
(Electricity, Air Con, Spaces, etc.)
• All hardware is outsourced
Cloud Computing Delivery
Cloud Computing
Traditional Thin-Client
Client-Server
Infrastructure as a
Service (IaaS)
Platform as a Service
(Paas)
Software as a Service
(SaaS)
Internal Outsourced Internal Outsourced Internal Outsourced Internal Outsourced
Application
Logic
X X X X
Data Storage X X X X
Data Access
Logic
X X X X
Operating
System
X X X X
Virtualization
Software
X X X X
Server
Hardware
X X X X
Storage
Hardware
X X X X
Network
Hardware
X X X X
Peer to Peer Architecture
• An older architecture that became popular again
with Napster, BitTorrent, etc., in early 2000s.
• All devices can serve as a client and a server
CLIENT/SERVER
Presentation Logic
Application Logic
Data Access Logic
Data Storage
CLIENT/SERVER
Presentation Logic
Application Logic
Data Access Logic
Data Storage
Peer to Peer Architecture
• Advantages:
• Data can be stored anywhere on the network
• Very resilient to failure
• Distributes bandwidth requirements
• Disadvantages:
• Finding the stored data is hard (no centralised
control)
• Security (everything is everywhere)
Criteria for Choosing Architecture
• Development Costs
– Tools, Software packages etc.,
– Cost of servers, clients, and circuits (infra,
platform etc.,)
• Scalability
– Ability to increase (or decrease) in computing
capacity as network demand changes
– Easier in client-server architectures
• Reliable
– Ability to recover from failures
Outline
• Application Architecture
• Application Layer Services
–The Web: HTTP
–Email: SMTP, POP
–Other Application-Layer Protocols
Web and HTTP
First, a review…
• web page consists of objects
• object can be HTML file, JPEG image, Java
applet, audio file,…
• web page consists of base HTML-file
which includes several referenced objects
• each object is addressable by a URL, e.g.,
www.someschool.edu/someDept/pic.gif
host name path name
HTTP Overview
HTTP: hypertext
transfer protocol
• Web’s application
layer protocol
• client/server model
– client: browser that
requests, receives,
(using HTTP
protocol) and
“displays” Web
objects
– server: Web server
sends (using HTTP
protocol) objects in
response to requests
PC running
Firefox browser
server
running
Apache Web
server
iphone running
Safari browser
HTTP Overview
uses TCP:
• client initiates TCP
connection (creates socket)
to server, port 80
• server accepts TCP
connection from client
• HTTP messages
(application-layer protocol
messages) exchanged
between browser (HTTP
client) and Web server
(HTTP server)
• TCP connection closed
HTTP is “stateless”
• server maintains no
information about
past client requests
protocols that maintain
“state” are complex!
past history (state) must be
maintained
if server/client crashes, their
views of “state” may be
inconsistent, must be
reconciled
aside
HTTP Connections
non-persistent HTTP
• at most one object
sent over TCP
connection
– connection then
closed
• downloading
multiple objects
required multiple
connections
persistent HTTP
• multiple objects
can be sent over
single TCP
connection
between client,
server
Non-persistent HTTP
suppose user enters URL:
1a. HTTP client initiates TCP
connection to HTTP server
(process) at
www.someSchool.edu on
port 80
2. HTTP client sends HTTP request
message (containing URL) into
TCP connection socket.
Message indicates that client
wants object
someDepartment/home.index
1b. HTTP server at host
www.someSchool.edu waiting
for TCP connection at port 80.
“accepts” connection, notifying
client
3. HTTP server receives request
message, forms response
message containing requested
object, and sends message into
its socket
time
(contains text,
references to 10
jpeg images) www.someSchool.edu/someDepartment/home.index
Non-persistent HTTP
5. HTTP client receives response
message containing html file,
displays html. Parsing html
file, finds 10 referenced jpeg
objects
6. Steps 1-5 repeated for each of
10 jpeg objects
4. HTTP server closes TCP
connection.
time
Non-persistent HTTP: response
RTT (definition): time for a small
packet to travel from client to
server and back
HTTP response time:
• one RTT to initiate TCP
connection
• one RTT for HTTP request
and first few bytes of HTTP
response to return
• file transmission time
• non-persistent HTTP
response time =
2RTT+ file transmission
time
time to
transmit
file
initiate TCP
connection
RTT
request
file
RTT
file
received
time time
Persistent HTTP
non-persistent HTTP
issues:
• requires 2 RTTs per object
• OS overhead for each TCP
connection
• browsers often open
parallel TCP connections to
fetch referenced objects
persistent HTTP:
• server leaves connection
open after sending
response
• subsequent HTTP
messages between same
client/server sent over
open connection
• client sends requests as
soon as it encounters a
referenced object
• as little as one RTT for all
the referenced objects
HTTP Request Message
Request line
(command, URL, HTTP version number)
Request header
(information on the browser,
date, and the referring page )
Request body
(information sent to the server,
such as from a form, mainly with
POST command)
required
optional
optional
GET
POST
…
HTTP request message
• two types of HTTP messages: request, response
• HTTP request message:
– ASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
header
lines
carriage return,
line feed at start
of line indicates
end of header lines
GET /index.html HTTP/1.1
Host: www-net.cs.umass.edu
User-Agent: Firefox/3.6.10
Accept: text/html,application/xhtml+xml
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7
Keep-Alive: 115
Connection: keep-alive
carriage return character
line-feed character
HTTP request message: format
request
line
header
lines
body
method sp sp cr lf version URL
cr lf value header field name
cr lf value header field name
~ ~ ~ ~
cr lf
entity body ~ ~ ~ ~
HTTP Response Message
Response status
(http version number, status code, reason)
Response header
(information on the server, date,
URL of the page retrieved, format used )
Response body
(requested web page)
optional
optional
required
HTTP Response Message
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
HTML file
HTTP/1.1 200 OK
Date: Sun, 26 Sep 2010 20:09:20 GMT
Server: Apache/2.0.52 (CentOS)
Last-Modified: Tue, 30 Oct 2007 17:00:02
GMT
ETag: “17dc6-a5c-bf716880″
Accept-Ranges: bytes
Content-Length: 2652
Keep-Alive: timeout=10, max=100
Connection: Keep-Alive
Content-Type: text/html; charset=ISO-8859-
1
data data data data data …
HTTP Response: status codes
200 OK
– request succeeded, requested object later in this msg
301 Moved Permanently
– requested object moved, new location specified later in
this msg (Location:)
400 Bad Request
– request msg not understood by server
404 Not Found
– requested document not found on this server
505 HTTP Version Not Supported
status code appears in 1st line in server-to-
client response message.
some sample codes:
• Mail User Agent (MUA): Mail Client
– Formal name for mail client software
– e.g., Outlook, Apple Mail, Thunderbird
• Mail Transfer Agent (MTA): Mail Server
– Formal name for mail server software
– e.g., Sendmail, Postfix,
• Simple Mail Transfer Protocol (SMTP)
– Protocol used to send a message to a MTA
– Originally only handled text files
• Internet Message Access Protocol (IMAP) or Post
Office Protocol (POP)
– Protocols used by a MUA to retrieve messages from an MTA
Email in Action
user
agent
1) Alice uses MUA to compose
message “to”
bob@someschool.edu
2) Alice’s MUA sends message
to her mail server; message
placed in message queue
3) client side of SMTP opens
TCP connection with Bob’s
mail server
4) SMTP client sends Alice’s
message over the TCP
connection
5) Bob’s mail server places the
message in Bob’s mailbox
6) Bob invokes his user agent to
read message
server
server
1
2 3
4
5
6
Alice’s mail server Bob’s mail server
user
agent
Mail Access Protocols
• SMTP: delivery/storage to receiver’s server
• Mail access protocol: retrieval from server
– POP: Post Office Protocol [RFC 1939]:
authorization, download
– IMAP: Internet Mail Access Protocol [RFC 1730]:
more features, including manipulation of stored
msgs on server
– HTTP: gmail, Hotmail, Yahoo! Mail, etc.
sender’s mail
server
SMTP SMTP
mail access
protocol
receiver’s mail
server
(e.g., POP,
IMAP)
user
agent
user
agent
POP3 vs IMAP
POP3
• previous example uses
POP3 “download and
delete” mode
– Bob cannot re-read e-
mail if he changes client
• POP3 “download-and-
keep”: copies of
messages on different
clients
• POP3 is stateless
across sessions
IMAP
• keeps all messages in
one place: at server
• allows user to organize
messages in folders
• keeps user state across
sessions:
– names of folders and
mappings between
message IDs and folder
name
Email Message Format
• SMTP Message format
– RFC 822: standard for
text message format
• Header lines
– Contains information
about the message
(e.g., to, from, subject)
• Body Section
– Contains the ‘content
of the message’
– Begins with the ‘DATA’
keyword
– Only uses ASCII
characters
header
body
blank
line
POP3 protocol
authorization phase
• client commands:
– user: declare username
– pass: password
• server responses
– +OK
– -ERR
transaction phase,
client:
• list: list message
numbers
• retr: retrieve message by
number
• dele: delete
• quit
C: list
S: 1 498
S: 2 912
S: .
C: retr 1
S:
S: .
C: dele 1
C: retr 2
S:
S: .
C: dele 2
C: quit
S: +OK POP3 server signing off
S: +OK POP3 server ready
C: user bob
S: +OK
C: pass hungry
S: +OK user successfully logged on
MIME
• Multipurpose Internet Mail Extension
– A graphic-capable mail transfer agent protocol (to send
graphical information in addition to text)
• SMTP was designed years ago for text transfer only
– MIME software is included as part of an e-mail client
– Superimposes a format for the body text, so a graphic
can be represented using text, and then sent via SMTP
(as a special attachment)
– Receiver’s e-mail client then translates the MIME
attachment from text back into graphical format
Telnet/SSH
• Allows one computer to log into another
computer
– Remote login enabling full control of the host
• Requires account name and password
– Anonymous sites similar to FTP approach
• Most popular Telnet software is PuTTY
– Open source
– Uses SSH encryption for security
• Remote Desktop (windows)
– Most advanced, connecting Window-based
machines, provide full access to Window
interface
Instant Messaging (IM)
• One of the fastest
growing Internet
applications
• Allows users to
exchange real-time
typed messages or
chat with friends
Videoconferencing
• Provides real-time transmission of video
and audio signals
• Combined video/audio signals sent via
WAN (Wide Area Network)
• Desktop videoconferencing is fast
growing (Skype, FaceTime etc.,)
• Require a lot of network capacity thus use
data compression
• Most often compatibility is an issue
Thank You
