Transcript notes - Academic Server| Cleveland State University

EEC-484/584
Computer Networks
Lecture 4
Wenbing Zhao
[email protected]
(Part of the slides are based on Drs. Kurose &
Ross’s slides for their Computer Networking book)
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• Next Monday, Lab1-HTTP
– Homework: Lab0-GettingStarted
• If you have access to a computer: Install wireshark, carry out
all exercises, no need to submit report for lab0
• If you don’t have access to a computer, at least read the
instructions!
• Lab report requirement:
– Typed hardcopy, must include questions/tasks, your
answers, and snapshots to backup your answers
• Today’s topics
– Principles of networked applications
– Web and HTTP
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Application Layer Protocols
• Principles of networked applications
– Client server model
– Sockets
– Addressing
– Protocol
– What do we need from transport layer?
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Creating a Network Application
• Write programs that
– run on different end
systems and
– communicate over a
network
application
transport
network
data link
physical
• No need to write code
for devices in subnet
– Subnet devices do not run
user application code
– application on end systems
allows for rapid app
development, propagation
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application
transport
network
data link
physical
EEC-484/584: Computer Networks
application
transport
network
data link
physical
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Inter-Process Communications
• Process: program
• Client process:
running within a host
process that initiates
communication
• Processes in different
hosts communicate
• Server process:
by exchanging
process that waits to
messages
be contacted
More accurately, client and server should be regarded
as the roles played by a process. A process can be
both a client and a server
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Sockets
• Process sends/receives
messages to/from its
socket
• For each point-to-point
connection, there are two
sockets, one on each side
• API (Application
Programming Interface):
(1) choice of transport
protocol; (2) ability to fix a
few parameters
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host or
server
host or
server
Controlled by
app developer
process
process
socket
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
Controlled
by OS
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Addressing
• To receive messages, a process must have an
identifier
• Each host device has a unique 32-bit IP address
• Question: Does the IP address of the host on
which the process runs suffice for identifying the
process?
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Addressing
• Identifier includes both IP address and port
numbers (16-bit) associated with process on
host
• Example port numbers:
– HTTP server: 80
– SSH server: 22
• To send HTTP request to
academic.csuohio.edu Web server:
– IP address: 137.148.49.46
– Port number: 80
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Application Layer Protocol Defines
• Types of messages exchanged
– e.g., request, response
• Message syntax
– what fields in messages & how
fields are delineated
• Message semantics
– meaning of information in fields
• Rules for when and how
processes send & respond to
messages
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Public-domain protocols:
• defined in RFCs
• allows for
interoperability
• e.g., HTTP, SMTP
Proprietary protocols:
• e.g., KaZaA
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Wenbing Zhao
What Transport Service Does an
Application Need?
Data loss
• some apps (e.g., audio) can
tolerate some loss
• other apps (e.g., file
transfer, telnet) require
100% reliable data transfer
Timing
• some apps (e.g.,
Internet telephony,
interactive games)
require low delay to be
“effective”
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Bandwidth
• some apps (e.g.,
multimedia) require
minimum amount of
bandwidth to be
“effective”
• other apps (“elastic
apps”) make use of
whatever bandwidth
they get
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The World Wide Web
• Creation of Tim Berners-Lee, in 1989 CERN
nuclear physics research
– Mosaic – first graphical interface, creation of Marc
Andersson (and others), precursor to Netscape
• Uses a client-server architecture
– Web server
– Web browser
• Runs on HTTP over TCP
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Web and HTTP
• Web page consists of objects
• Object can be HTML file, JPEG image, Java
applet, audio file,…
• A Web page consists of a base HTML-file which
includes several referenced objects
• Each object is addressable by a URL
• The idea of having one page point to another is
called hypertext
– Invented by Vannevar Bush, a MIT EE professor, in
1945
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URL – Uniform Resource Locater
• Example URL:
http://www.someschool.edu/someDept/pic.gif
protocol name
host name
path name
• URL encodes three types of information
– What is the page called – local path name uniquely
indicating the specific page
– Where is the page located – Host name of the
server on which the page is located
– How can the page be accessed – protocol, e.g.,
http, ftp
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Some Common URLs
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HTTP Overview
HTTP: HyperText
Transfer Protocol
• Web’s application layer
protocol
• client/server model
• 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
• 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
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HTTP Overview
• 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
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HTTP Connections
Nonpersistent HTTP
• At most one object is
sent over a TCP
connection
• HTTP/1.0 uses
nonpersistent HTTP
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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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Nonpersistent HTTP
Suppose user enters URL
(contains text,
references to 10
jpeg images)
http://www.someSchool.edu/someDept/home.index
1a. HTTP client initiates TCP
connection to HTTP server at
1b. HTTP server at host
www.someSchool.edu on port
www.someSchool.edu
80
waiting for TCP connection
at port 80. “accepts”
connection, notifying client
2. HTTP client sends HTTP
request message (containing
3. HTTP server receives
URL) into TCP connection
request message, forms
socket. Message indicates
response message
that client wants object
containing requested object,
someDept/home.index
and sends message into its
socket
time
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Nonpersistent HTTP
4. HTTP server closes TCP
connection.
5. HTTP client receives response
message containing html file,
displays html. Parsing html
file, finds 10 referenced jpeg
objects
time
6. Steps 1-5 repeated for each
of 10 jpeg objects
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Non-Persistent HTTP:
Response Time
Definition of RTT: time
to send a small packet
to travel from client to
server and back
(Round Trip Time)
initiate TCP
connection
RTT
request
file
RTT
file
received
time
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time to
transmit
file
time
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Non-Persistent HTTP:
Response Time
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
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Non-Persistent HTTP Issues
• Requires 2 RTTs per object
• OS overhead for each TCP connection
To reduce response time, browsers
often open parallel TCP connections to
fetch referenced objects
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Persistent HTTP
• Server leaves connection open after
sending response
• Subsequent HTTP messages between
same client/server sent over open
connection
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Persistent HTTP
Persistent without pipelining:
• Client issues new request only when previous response
has been received
• One RTT for each referenced object
Persistent with pipelining:
• Default in HTTP/1.1
• Multiple requests are sent over the same connection
concurrently. That is, after the first request, the second
request is sent before the reply for the first request is
received
• As little as one RTT for all the referenced objects
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HTTP Request Message
• Two types of HTTP messages: request, response
• HTTP request message:
– ASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.1
Host: www.someschool.edu
User-agent: Mozilla/4.0
header Connection: close
lines Accept-language:fr
Carriage return,
line feed
indicates end
of message
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(extra carriage return, line feed)
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HTTP Request Message:
General Format
HTTP header is pure ASCII based. It is very different from
lower layer protocols such as TCP, which is binary based
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Method Types
HTTP/1.0
• GET
• POST
• HEAD
HTTP/1.1
• GET, POST, HEAD
• PUT
– Asks server to include
only the header part in
response
– Uploads file in entity
body to path specified
in URL field
• DELETE
– Deletes file specified
in the URL field
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HTTP Response Message
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
HTML file
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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
Status code is in first line of the response message:
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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Trying out HTTP
1. Telnet to your favorite Web server:
telnet cis.poly.edu 80
Opens TCP connection to port 80
(default HTTP server port) at cis.poly.edu.
Anything typed in sent
to port 80 at cis.poly.edu
2. Type in a GET HTTP request:
GET /~ross/ HTTP/1.1
Host: cis.poly.edu
By typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to HTTP server
3. Look at response message sent by HTTP server!
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Wenbing Zhao
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Web Caching
Goal: satisfy client request without involving origin server
• user sets browser:
Web accesses via
proxy server
• browser sends all
HTTP requests to
proxy server
origin
server
client
– object in cache: returns
cached object
– else cache requests
object from origin
server, then returns
object to client
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client
EEC-484/584: Computer Networks
Proxy
server
origin
server
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More about Web Caching
• Proxy server acts as both
client and server
• Typically proxy server is
installed by ISP
(university, company,
residential ISP)
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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
EEC-484/584: Computer Networks
Wenbing Zhao
Conditional GET: HTTP Build-in
Support for Caching
Proxy
• Goal: don’t send object if server
HTTP request msg
cache is up-to-date
If-modified-since:
• Proxy server: specify date of
<date>
cached copy in HTTP
HTTP response
request
HTTP/1.0
If-modified-since: <date>
• Origin server: response
contains no object if cached
copy is up-to-date:
HTTP/1.0 304 Not Modified
Origin
Server
object
not
modified
304 Not Modified
HTTP request msg
If-modified-since:
<date>
HTTP response
object
modified
HTTP/1.0 200 OK
<data>
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Wenbing Zhao
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Non-Caching Example
origin
servers
Assumptions
• Average object size =
100,000 bits
• Avg. request rate from
institution’s browsers to
origin servers = 15/sec
• Delay from institutional
router to any origin server
and back to router = 2 sec
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public
Internet
1.5 Mbps
access link
institutional
network
EEC-484/584: Computer Networks
10 Mbps LAN
Wenbing Zhao
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Non-Caching Example
origin
servers
Consequences
• Utilization on LAN = 15%
• Utilization on access link =
100%
• Total delay = Internet
delay + access delay +
LAN delay
= 2 sec + minutes +
milliseconds
Spring 2009
public
Internet
1.5 Mbps
access link
institutional
network
EEC-484/584: Computer Networks
10 Mbps LAN
Wenbing Zhao
37
Non-Caching Example
origin
servers
Possible solution
• Increase bandwidth of
access link to, say, 10
Mbps
public
Internet
Consequences
• Utilization on LAN = 15%
• Utilization on access link = 15%
• Total delay = Internet delay +
access delay + LAN delay
= 2 sec + msecs + msecs
• Often a costly upgrade
Spring 2009
10 Mbps
access link
institutional
network
EEC-484/584: Computer Networks
10 Mbps LAN
Wenbing Zhao
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Caching Example
origin
servers
Install proxy server
• Suppose hit rate is 0.4
Consequence
• 40% requests will be satisfied almost
immediately
• 60% requests satisfied by origin
server
• Utilization of access link
reduced to 60%, resulting in
negligible delays (say 10
msec)
• Total avg delay = Internet
delay + access delay + LAN
delay = .6*(2.01) secs +
.4*milliseconds < 1.4 secs
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public
Internet
institutional
network
EEC-484/584: Computer Networks
1.5 Mbps
access link
10 Mbps LAN
Institutional
Proxy server
Wenbing Zhao