Transcript Requirements for QoS Signaling in Wireless Networks

Application Layer Functionality and
Protocols
Md. Asif Hossain
Typically the applications that we use are intuitive, meaning we can access and use
them without knowing how they work. However, for network professionals, it is
important to know how an application is able to format, transmit and interpret
messages that are sent and received across the network.
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Applications – The Interface between the Networks
The Application layer
Layer seven, is the top layer of both the OSI and TCP/IP
models. It 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 exchange data
between programs running on the source and destination
hosts. There are many Application layer protocols and
new protocols are always being developed.
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Applications – The Interface between the Networks
The Presentation Layer
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 of data upon receipt by the destination.
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Applications – The Interface between the Networks
The Presentation Layer
Some well-known standards for video:
QuickTime and Motion Picture Experts Group (MPEG). QuickTime
is an Apple Computer specification for video and audio, and MPEG
is a standard for video compression and coding.
Among the well-known graphic image formats are: Graphics
Interchange Format (GIF)
Joint Photographic Experts Group (JPEG)
Tagged Image File Format (TIFF).
GIF and JPEG are compression and coding standards for graphic
images, and TIFF is a standard coding format for graphic images
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Applications – The Interface between the Networks
The Session Layer
As the name of the Session layer implies, functions at this layer
create and maintain 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.
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Applications – The Interface between the Networks
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Application Layer Protocol Functions
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Making Provision for Applications and Services
The Client-Server Model
• The device requesting the information is called a client and the
device responding to the request is called a server.
• Client and server processes are considered to be in the Application
layer.
• The client begins the exchange by requesting data from the server,
which 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.
• In addition to the actual data transfer, this exchange may also
require control information, such as user authentication and the
identification of a data file to be transferred.
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Making Provision for Applications and Services
The Client-Server Model
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Making Provision for Applications and Services
Servers
• In a general networking context, any device that responds to requests
from client applications is functioning as a server.
• A server is usually a computer that contains information to be shared
with many client systems.
• For example, web pages, documents, databases, pictures, video, and
audio files can all be stored on a server and delivered to requesting
clients.
• In other cases, such as a network printer, the print server delivers the
client print requests to the specified printer.
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Making Provision for Applications and Services
Servers
•In a client/server network, the server runs a service, or process,
sometimes called a server daemon.
• Like most services, daemons 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,
because they are 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, and
proceeds to send the requested data to the client in the proper format.
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Making Provision for Applications and Services
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Peer-to-Peer Networking and Applications (p2p)
Peer-to-Peer Networks
In a peer-to-peer network, two or more computers are connected via a
network and can share resources (such as printers and files) without having
a dedicated server. Every connected end device (known as a peer) can
function as either a server or a client. One computer might assume the role
of server for one transaction while simultaneously serving as a client for
another. The roles of client and server are set on a per request basis.
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Peer-to-Peer Networking and Applications (p2p)
Peer-to-Peer Applications
A peer-to-peer application (P2P), unlike a peer-to-peer network, allows a device to act
as both a client and a server within the same communication. In this model, every client
is a server and every server a client. Both can initiate a communication and are
considered equal in the communication process. However, peer-to-peer applications
require that each end device provide a user interface and run a background service.
When you launch a specific peer-to-peer application it invokes the required user
interface and background services. After that the devices can communicate directly.
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DNS Services and Protocol
• In data networks, devices are labeled with numeric IP addresses, so that
they can participate in sending and receiving messages over the network.
However, most people have a hard time remembering this numeric address.
• Hence, domain names were created to convert the numeric address into a
simple, recognizable name.
• On the Internet these domain names, such as www.cisco.com , are much
easier for people to remember than 198.132.219.25, which is the actual
numeric address for this server.
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DNS Services and Protocol
• When networks were small, it was a simple task to maintain the
mapping between domain names and the addresses they represented.
However, as networks began to grow and the number of devices
increased, this manual system became unworkable.
• The Domain Name System (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.
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DNS Services and Protocol
DNS server provides the name resolution using the name daemon, which
is often called named, (pronounced name-dee).
The DNS server stores different types of resource records used to resolve
names. These records contain the name, address, and type of record.
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DNS records
DNS: distributed db storing resource records (RR)
RR format: (name,
Type=A
name is hostname
value is IP address


• Type=NS
– name is domain (e.g.
foo.com)
– value is IP address of
authoritative name server
for this domain
value, type, ttl)
Type=CNAME
name is alias name for some
“cannonical” (the real) name
www.ibm.com is really

servereast.backup2.ibm.com

value is cannonical name
 Type=MX

value is name of mailserver
associated with name
202:
Applicatio
DNS Services and Protocol
When a client makes a query, the server's "named" process first looks at its
own records to see if it can resolve the name. If it is unable to resolve the
name using its stored records, it contacts other servers in order to resolve
the name.
The request may be passed along to a number of servers, which can take
extra time and consume bandwidth. Once a match is found and returned to
the original requesting server, the server temporarily stores the numbered
address that matches the name in cache.
If that same name is requested again, the first server can return the address
by using the value stored in its name cache. Caching reduces both the DNS
query data network traffic and the workloads of servers higher up the
hierarchy. The DNS Client service on Windows PCs optimizes the
performance of DNS name resolution by storing previously resolved names
in memory, as well.
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DNS Services and Protocol
The Domain Name System uses a hierarchical system to create a name database to
provide name resolution. The hierarchy looks like an inverted tree with the root at the
top and branches below.
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 secondary level
domain servers and so on.
The different top-level domains represent the either the type of organization or the
country or origin. Examples of top-level domains are:
.au - Australia
.bd - Bangladesh
.com - a business or industry
.jp - Japan
.org - a non-profit organization; .edu-Educational
After top-level domains are second-level domain names, and below them are other
lower level domains.
Each domain name is a path down this inverted tree starting from the root.
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DNS Services and Protocol
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DNS: Root name servers
• contacted by local name server that can not resolve name
• root name server:
– contacts authoritative name server if name mapping not known
– gets mapping
– returns mapping to local name server
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also Los Angeles)
d U Maryland College Park, MD
k RIPE London (also Amsterdam,
g US DoD Vienna, VA
Frankfurt)
i Autonomica, Stockholm
(plus 3
h ARL Aberdeen, MD
other
locations)
j Verisign, ( 11 locations)
m WIDE Tokyo
e NASA Mt View, CA
f Internet Software C. Palo Alto,
CA (and 17 other locations)
13 root name servers
worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
Web and HTTP
First some jargon
• 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
• Example URL:
www.someschool.edu/someDept/pic.gif
host name
path name
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HTTP overview
HTTP: hypertext transfer
protocol
• Web’s application layer protocol
• client/server model
– client: browser that requests,
receives, “displays” Web
objects
– server: Web server sends
objects in response to
requests
• HTTP 1.0: RFC 1945
• HTTP 1.1: RFC 2068
PC running
Explorer
Server
running
Apache Web
server
Mac running
Navigator
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HTTP overview (continued)
Uses TCP:
HTTP is “stateless”
• client initiates TCP connection
(creates socket) to server, port
80
• server accepts TCP connection
from client
• HTTP messages (applicationlayer protocol messages)
exchanged between browser
(HTTP client) and Web server
(HTTP server)
• TCP connection closed
• server maintains no
information about past
client requests
aside
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
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HTTP connections
Nonpersistent HTTP
• At most one object is sent
over a TCP connection.
• HTTP/1.0 uses
nonpersistent HTTP
Persistent HTTP
• Multiple objects can be
sent over single TCP
connection between client
and server.
• HTTP/1.1 uses persistent
connections in default
mode
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WWW Service and HTTP
Browsers can interpret and present many data types, such as plain text or Hypertext
Markup Language (HTML, the language in which web pages are constructed).
Other types of data, however, may require another service or program, typically referred
to as plug-ins or add-ons. To help the browser determine what type of file it is
receiving, the server specifies what kind of data the file contains.
To better understand how the web browser and web client interact, we can examine how
a web page is opened in a browser. For this example, we will use the URL:
http://www.cisco.com/web-server.htm.
First, the browser interprets the three parts of the URL:
1. http (the protocol or scheme)
2. www.cisco.com(the server name)
3. web-server.htm (the specific file name requested).
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WWW Service and HTTP
• The browser then checks with a name server to convert www.cisco.com
<http://www.cisco.com > into a numeric address, which it uses to connect to the server.
• Using the HTTP protocol requirements, the browser sends a GET request to the server
and asks for the file web-server.htm.
• The server in turn sends the HTML code for this web page to the browser.
•Finally, the browser deciphers the HTML code and formats the page for the browser
window.
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WWW Service and HTTP
•The Hypertext Transfer Protocol (HTTP), one of the protocols in the TCP/IP
suite, was originally developed to publish and retrieve HTML pages and is now
used for distributed, collaborative information systems.
•HTTP is used across the WWW for data transfer and is one of the most used
application protocols.
• HTTP specifies a request/response protocol. When a client, typically a web
browser, sends a request message to a server, the HTTP protocol defines the
message types the client uses to request the web page and also the message
types the server uses to respond.
• The three common message types are GET, POST, and PUT.
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WWW Service and HTTP
• GET is a client request for data. A web browser sends the GET message to request
pages from a web server. As shown in the figure, once the server receives the GET
request, it responds with a status line, such as HTTP/1.1 200 OK, and a message of its
own, the body of which may be the requested file, an error message, or some other
information.
• POST and PUT are used to send messages that upload data to the web server. For
example, when the user enters data into a form embedded in a web page, POST
includes the data in the message sent to the server.
• PUT uploads resources or content to the web server.
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HTTP response message
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
HTML file
HTTP/1.1 200 OK
Connection close
Date: Thu, 06 Aug 1998 12:00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html
data data data data data ...
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HTTP response status codes
In first line in server->client response message.
A few sample codes:
200 OK
– request succeeded, requested object later in this message
301 Moved Permanently
– requested object moved, new location specified later in this
message (Location:)
400 Bad Request
– request message not understood by server
404 Not Found
– requested document not found on this server
505 HTTP Version Not Supported
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WWW Service and HTTP
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Response time modeling
Definition of RTT: time to
send a small packet to
travel from client to server
and back.
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
total = 2RTT+transmit time
initiate TCP
connection
RTT
request
file
time to
transmit
file
RTT
file
received
time
time
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User-server state: cookies
Many major Web sites use
cookies
Four components:
1) cookie header line in the
HTTP response message
2) cookie header line in HTTP
request message
3) cookie file kept on user’s
host and managed by user’s
browser
4) back-end database at Web
site
Example:
– Susan access Internet
always from same PC
– She visits a specific ecommerce site for first time
– When initial HTTP requests
arrives at site, site creates a
unique ID and creates an
entry in backend database
for ID
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Web caches (proxy server)
Goal: satisfy client request without involving origin server
• user sets browser: Web
accesses via cache
• browser sends all HTTP
requests to cache
– object in cache: cache
returns object
– else cache requests object
from origin server, then
returns object to client
origin
server
Proxy
server
client
client
origin
server
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More about Web caching
• Cache acts as both client and
server
• Typically cache is installed by
ISP (university, company,
residential ISP)
Why Web caching?
• Reduce response time for client
request.
• Reduce traffic on an institution’s
access link.
• Internet dense with caches
enables “poor” content providers
to effectively deliver content (but
so does P2P file sharing)
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WWW Service and HTTP
• Although it is remarkably flexible, HTTP is not a secure protocol. The
POST messages upload information to the server in plain text that can be
intercepted and read. Similarly, the server responses, typically HTML
pages, are also unencrypted.
• For secure communication across the Internet, the Secure HTTP
(HTTPS) protocol is used for accessing or posting web server
information.
•HTTPS can use authentication and encryption to secure data as it travels
between the client and server.
• e.g. www.mail.yahoo.com
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FTP: the file transfer protocol
FTP
user
interface
user
at host
FTP
client
local file
system
file transfer
FTP
server
remote file
system
• transfer file to/from remote host
• client/server model
– client: side that initiates transfer (either to/from remote)
– server: remote host
• ftp: RFC 959
• ftp server: port 21
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FTP: separate control, data connections
• FTP client contacts FTP server
at port 21, specifying TCP as
transport protocol
• Client obtains authorization over
control connection
• Client browses remote directory
by sending commands over
control connection.
• When server receives a
command for a file transfer, the
server opens a TCP data
connection to client
• After transferring one file, server
closes connection.
TCP control connection
port 21
FTP
client
TCP data connection
port 20
FTP
server
 Server opens a second TCP data
connection to transfer another file.
 Control connection: “out of band”
 FTP server maintains “state”:
current directory, earlier
authentication
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outgoing
message queue
Electronic Mail
user mailbox
user
agent
Three major components:
• user agents
• mail servers
• simple mail transfer protocol:
SMTP
User Agent
• a.k.a. “mail reader”
• composing, editing, reading
mail messages
• e.g., Eudora, Outlook, elm,
Netscape Messenger
• outgoing, incoming messages
stored on server
mail
server
user
agent
SMTP
SMTP
SMTP
mail
server
mail
server
user
agent
user
agent
user
agent
user
agent
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Electronic Mail: mail servers
user
agent
Mail Servers
• mailbox contains incoming
messages for user
• message queue of outgoing (to
be sent) mail messages
• SMTP protocol between mail
servers to send email
messages
– client: sending mail server
– “server”: receiving mail
server
mail
server
user
agent
SMTP
SMTP
SMTP
mail
server
mail
server
user
agent
user
agent
user
agent
user
agent
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Scenario: Alice sends message to
Bob
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
1) Alice uses UA to compose
message and “to”
[email protected]
2) Alice’s UA sends message to
her mail server; message
placed in message queue
3) Client side of SMTP opens
TCP connection with Bob’s
mail server
1
user
agent
2
mail
server
3
mail
server
4
6
user
agent
5
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SMTP: final words
• SMTP uses persistent
connections
• SMTP requires message
(header & body) to be in 7-bit
ASCII
• SMTP server uses CRLF.CRLF
to determine end of message
Comparison with HTTP:
• HTTP: pull
• SMTP: push
• both have ASCII
command/response interaction,
status codes
• HTTP: each object
encapsulated in its own
response msg
• SMTP: multiple objects sent in
multipart msg
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Mail access protocols
SMTP
SMTP
user
agent
sender’s mail
server
access
protocol
user
agent
receiver’s mail
server
• SMTP: delivery/storage to receiver’s server
• Mail access protocol: retrieval from server
– POP: Post Office Protocol [RFC 1939]
• authorization (agent <-->server) and download
– IMAP: Internet Mail Access Protocol [RFC 1730]
• more features (more complex)
• manipulation of stored msgs on server
– HTTP: Hotmail , Yahoo! Mail, etc.
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