2,Application Layer 1
Chapter 2
Application Layer
Computer Networking,
A Top Down Approach
Featuring the Internet,
2nd edition,
Jim Kurose,Keith Ross
Addison-Wesley,July
2002,
The PowerPoint Slides are based on
the material provided by
J.F Kurose and K.W,Ross.
2,Application Layer 2
Chapter 2,Application Layer
Our goals:
? conceptual,
implementation
aspects of network
application protocols
? transport-layer
service models
? client-server
paradigm
? peer-to-peer
paradigm
? learn about protocols
by examining popular
application-level
protocols
? HTTP
? FTP
? SMTP / POP3 / IMAP
? DNS
? programming network
applications
? socket API
2,Application Layer 3
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 4
Network applications,some jargon
Process,program running
within a host.
? within same host,two
processes communicate
using interprocess
communication (defined
by OS).
? processes running in
different hosts
communicate with an
application-layer
protocol
user agent,interfaces
with user,above” and
network,below”,
? implements user
interface &
application-level
protocol
? Web,browser
? E-mail,mail reader
? streaming audio/video,
media player
2,Application Layer 5
Applications and application-layer protocols
Application,communicating,
distributed processes
? e.g.,e-mail,Web,P2P file
sharing,instant messaging
? running in end systems
(hosts)
? exchange messages to
implement application
Application-layer protocols
? one,piece” of an app
? define messages
exchanged by apps and
actions taken
? use communication services
provided by lower layer
protocols (TCP,UDP)
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
2,Application Layer 6
App-layer protocol defines
? Types of messages
exchanged,eg,request
& response messages
? Syntax of message
types,what fields in
messages & how fields
are delineated
? Semantics of the
fields,ie,meaning of
information in fields
? Rules for when and
how processes send &
respond to messages
Public-domain protocols:
? defined in RFCs
? allows for
interoperability
? eg,HTTP,SMTP
Proprietary protocols:
? eg,KaZaA
2,Application Layer 7
Client-server paradigm
Typical network app has two
pieces,client and server
application
transport
network
data link
physical
application
transport
network
data link
physical
Client:
? initiates contact with server
(“speaks first”)
? typically requests service from
server,
? Web,client implemented in
browser; e-mail,in mail reader
request
reply
Server:
? provides requested service to client
? e.g.,Web server sends requested Web
page,mail server delivers e-mail
2,Application Layer 8
Processes communicating across network
? process sends/receives
messages to/from its
socket
? socket analogous to door
? sending process shoves
message out door
? sending process asssumes
transport infrastructure
on other side of door which
brings message to socket
at receiving process
process
TCP with
buffers,
variables
socket
host or
server
process
TCP with
buffers,
variables
socket
host or
server
Internet
controlled
by OS
controlled by
app developer
? API,(1) choice of transport protocol; (2) ability to fix
a few parameters (lots more on this later)
2,Application Layer 9
Addressing processes:
? For a process to
receive messages,it
must have an identifier
? Every host has a unique
32-bit IP address
? Q,does the IP address
of the host on which
the process runs
suffice for identifying
the process?
? Answer,No,many
processes can be
running on same host
? Identifier includes
both the IP address
and port numbers
associated with the
process on the host.
? Example port numbers:
? HTTP server,80
? Mail server,25
? More on this later
2,Application Layer 10
What transport service does an app 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”
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
2,Application Layer 11
Transport service requirements of common apps
Application
file transfer
e-mail
Web documents
real-time audio/video
stored audio/video
interactive games
instant messaging
Data loss
no loss
no loss
no loss
loss-tolerant
loss-tolerant
loss-tolerant
no loss
Bandwidth
elastic
elastic
elastic
audio,5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
Time Sensitive
no
no
no
yes,100’s msec
yes,few secs
yes,100’s msec
yes and no
2,Application Layer 12
Internet transport protocols services
TCP service:
? connection-oriented,setup
required between client and
server processes
? reliable transport between
sending and receiving process
? flow control,sender won’t
overwhelm receiver
? congestion control,throttle
sender when network
overloaded
? does not providing,timing,
minimum bandwidth
guarantees
UDP service:
? unreliable data transfer
between sending and
receiving process
? does not provide,
connection setup,
reliability,flow control,
congestion control,timing,
or bandwidth guarantee
Q,why bother? Why is
there a UDP?
2,Application Layer 13
Internet apps,application,transport protocols
Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
Internet telephony
Application
layer protocol
SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
proprietary
(e.g,RealNetworks)
proprietary
(e.g.,Dialpad)
Underlying
transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
typically UDP
2,Application Layer 14
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 15
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
2,Application Layer 16
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
2,Application Layer 17
HTTP overview (continued)
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
2,Application Layer 18
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
2,Application Layer 19
Nonpersistent HTTP
Suppose user enters URL
www.someSchool.edu/someDepartment/home.index
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)
2,Application Layer 20
Nonpersistent HTTP (cont.)
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
2,Application Layer 21
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
time to
transmit
file
initiate TCP
connection
RTT
request
file
RTT
file
received
time time
2,Application Layer 22
Persistent HTTP
Nonpersistent HTTP issues:
? requires 2 RTTs per object
? OS must work and allocate
host resources for each TCP
connection
? but 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
are sent over connection
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
? client sends requests as
soon as it encounters a
referenced object
? as little as one RTT for all
the referenced objects
2,Application Layer 23
HTTP request message
? two types of HTTP messages,request,response
? HTTP request message:
? ASCII (human-readable format)
GET /somedir/page.html HTTP/1.1
Host,www.someschool.edu
User-agent,Mozilla/4.0
Connection,close
Accept-language:fr
(extra carriage return,line feed)
request line
(GET,POST,
HEAD commands)
header
lines
Carriage return,
line feed
indicates end
of message
2,Application Layer 24
HTTP request message,general format
2,Application Layer 25
Uploading form input
Post method:
? Web page often
includes form input
? Input is uploaded to
server in entity body
URL method:
? Uses GET method
? Input is uploaded in
URL field of request
line:
www.somesite.com/animalsearch?monkeys&banana
2,Application Layer 26
Method types
HTTP/1.0
? GET
? POST
? HEAD
? asks server to leave
requested object out of
response
HTTP/1.1
? GET,POST,HEAD
? PUT
? uploads file in entity
body to path specified
in URL field
? DELETE
? deletes file specified in
the URL field
2,Application Layer 27
HTTP response message
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,.,
status line
(protocol
status code
status phrase)
header
lines
data,e.g.,
requested
HTML file
2,Application Layer 28
HTTP response status 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
In first line in server->client response message.
A few sample codes:
2,Application Layer 29
Trying out HTTP (client side) for yourself
1,Telnet to your favorite Web server:
Opens TCP connection to port 80
(default HTTP server port) at www.eurecom.fr.
Anything typed in sent
to port 80 at www.eurecom.fr
telnet www.eurecom.fr 80
2,Type in a GET HTTP request:
GET /~ross/index.html HTTP/1.0 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!
2,Application Layer 30
User-server interaction,authorization
Authorization, control access to
server content
? authorization credentials,
typically name,password
? stateless,client must present
authorization in each request
? authorization,header line in
each request
? if no authorization,header,
server refuses access,
sends
WWW authenticate:
header line in response
client server
usual http request msg
401,authorization req.
WWW authenticate:
usual http request msg
+ Authorization,<cred>
usual http response msg
usual http request msg
+ Authorization,<cred>
usual http response msg time
2,Application Layer 31
Cookies,keeping,state”
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 e-
commerce 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
2,Application Layer 32
Cookies,keeping,state” (cont.)
client server
usual http request msg
usual http response +
Set-cookie,1678
usual http request msg
cookie,1678
usual http response msg
usual http request msg
cookie,1678
usual http response msg
cookie-
specific
action
cookie-
spectific
action
server
creates ID
1678 for user
Cookie file
amazon,1678
ebay,8734
Cookie file
ebay,8734
Cookie file
amazon,1678
ebay,8734
one week later:
2,Application Layer 33
Cookies (continued)
What cookies can bring:
? authorization
? shopping carts
? recommendations
? user session state
(Web e-mail)
Cookies and privacy:
? cookies permit sites to
learn a lot about you
? you may supply name
and e-mail to sites
? search engines use
redirection & cookies
to learn yet more
? advertising companies
obtain info across
sites
aside
2,Application Layer 34
Conditional GET,client-side caching
? Goal,don’t send object if
client has up-to-date cached
version
? client,specify date of
cached copy in HTTP request
If-modified-since,
<date>
? server,response contains no
object if cached copy is up-
to-date,
HTTP/1.0 304 Not
Modified
client server
HTTP request msg
If-modified-since,
<date>
HTTP response
HTTP/1.0
304 Not Modified
object
not
modified
HTTP request msg
If-modified-since,
<date>
HTTP response
HTTP/1.0 200 OK
<data>
object
modified
2,Application Layer 35
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 36
FTP,the file transfer protocol
? 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
file transfer FTP
server
FTP
user
interface
FTP
client
local file
system
remote file
system
user
at host
2,Application Layer 37
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.
FTP
client
FTP
server
TCP control connection
port 21
TCP data connection
port 20
? Server opens a second TCP
data connection to transfer
another file.
? Control connection:,out of
band”
? FTP server maintains,state”,
current directory,earlier
authentication
2,Application Layer 38
FTP commands,responses
Sample commands:
? sent as ASCII text over
control channel
? USER username
? PASS password
? LIST return list of file in
current directory
? RETR filename retrieves
(gets) file
? STOR filename stores
(puts) file onto remote
host
Sample return codes
? status code and phrase (as
in HTTP)
? 331 Username OK,
password required
? 125 data connection
already open;
transfer starting
? 425 Can’t open data
connection
? 452 Error writing
file
2,Application Layer 39
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 40
Electronic Mail
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
user mailbox
outgoing
message queue
mail
server
user
agent
user
agent
user
agentmailserver
user
agent
user
agent
mail
server
user
agent
SMTP
SMTP
SMTP
2,Application Layer 41
Electronic Mail,mail servers
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
user
agent
user
agentmailserver
user
agent
user
agent
mail
server
user
agent
SMTP
SMTP
SMTP
2,Application Layer 42
Electronic Mail,SMTP [RFC 2821]
? uses TCP to reliably transfer email message from client
to server,port 25
? direct transfer,sending server to receiving server
? three phases of transfer
? handshaking (greeting)
? transfer of messages
? closure
? command/response interaction
? commands,ASCII text
? response,status code and phrase
? messages must be in 7-bit ASCII
2,Application Layer 43
Scenario,Alice sends message to Bob
1) Alice uses UA to compose
message and,to”
bob@someschool.edu
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
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
user
agent
mail
server
mail
server useragent
1
2 3 4
5
6
2,Application Layer 44
Sample SMTP interaction
S,220 hamburger.edu
C,HELO crepes.fr
S,250 Hello crepes.fr,pleased to meet you
C,MAIL FROM,<alice@crepes.fr>
S,250 alice@crepes.fr..,Sender ok
C,RCPT TO,<bob@hamburger.edu>
S,250 bob@hamburger.edu,.,Recipient ok
C,DATA
S,354 Enter mail,end with "." on a line by itself
C,Do you like ketchup?
C,How about pickles?
C,,
S,250 Message accepted for delivery
C,QUIT
S,221 hamburger.edu closing connection
2,Application Layer 45
Try SMTP interaction for yourself:
? telnet servername 25
? see 220 reply from server
? enter HELO,MAIL FROM,RCPT TO,DATA,QUIT
commands
above lets you send email without using email client
(reader)
2,Application Layer 46
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
2,Application Layer 47
Mail message format
SMTP,protocol for
exchanging email msgs
RFC 822,standard for text
message format:
? header lines,e.g.,
? To:
? From:
? Subject:
different from SMTP
commands!
? body
? the,message”,ASCII
characters only
header
body
blank
line
2,Application Layer 48
Message format,multimedia extensions
? MIME,multimedia mail extension,RFC 2045,2056
? additional lines in msg header declare MIME content
type
From,alice@crepes.fr
To,bob@hamburger.edu
Subject,Picture of yummy crepe,
MIME-Version,1.0
Content-Transfer-Encoding,base64
Content-Type,image/jpeg
base64 encoded data,...,
........................,
......base64 encoded data
multimedia data
type,subtype,
parameter declaration
method used
to encode data
MIME version
encoded data
2,Application Layer 49
MIME types
Content-Type,type/subtype; parameters
Text
? example subtypes,plain,
html
Image
? example subtypes,jpeg,
gif
Audio
? exampe subtypes,basic
(8-bit mu-law encoded),
32kadpcm (32 kbps
coding)
Video
? example subtypes,mpeg,
quicktime
Application
? other data that must be
processed by reader
before,viewable”
? example subtypes,
msword,octet-stream
2,Application Layer 50
Multipart Type
From,alice@crepes.fr
To,bob@hamburger.edu
Subject,Picture of yummy crepe,
MIME-Version,1.0
Content-Type,multipart/mixed; boundary=StartOfNextPart
--StartOfNextPart
Dear Bob,Please find a picture of a crepe.
--StartOfNextPart
Content-Transfer-Encoding,base64
Content-Type,image/jpeg
base64 encoded data,...,
........................,
......base64 encoded data
--StartOfNextPart
Do you want the reciple?
2,Application Layer 51
Mail access protocols
? 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.
user
agent
sender’s mail
server
user
agent
SMTP SMTP access
protocol
receiver’s mail
server
2,Application Layer 52
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,<message 1 contents>
S,,
C,dele 1
C,retr 2
S,<message 1 contents>
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
2,Application Layer 53
POP3 (more) and IMAP
More about POP3
? Previous example uses
“download and delete”
mode.
? Bob cannot re-read e-
mail if he changes
client
?,Download-and-keep”,
copies of messages on
different clients
? POP3 is stateless
across sessions
IMAP
? Keep all messages in
one place,the server
? Allows user to
organize messages in
folders
? IMAP keeps user state
across sessions:
? names of folders and
mappings between
message IDs and folder
name
2,Application Layer 54
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 55
DNS,Domain Name System
People,many identifiers:
? SSN,name,passport #
Internet hosts,routers:
? IP address (32 bit) -
used for addressing
datagrams
?,name”,e.g.,
gaia.cs.umass.edu - used
by humans
Q,map between IP
addresses and name?
Domain Name System:
? distributed database
implemented in hierarchy of
many name servers
? application-layer protocol
host,routers,name servers to
communicate to resolve names
(address/name translation)
? note,core Internet
function,implemented as
application-layer protocol
? complexity at network’s
“edge”
2,Application Layer 56
DNS name servers
? no server has all name-
to-IP address mappings
local name servers:
? each ISP,company has
local (default) name server
? host DNS query first goes
to local name server
authoritative name server:
? for a host,stores that
host’s IP address,name
? can perform name/address
translation for that host’s
name
Why not centralize DNS?
? single point of failure
? traffic volume
? distant centralized
database
? maintenance
doesn’t scale!
2,Application Layer 57
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
b USC-ISI Marina del Rey,CA
l ICANN Marina del Rey,CA
e NASA Mt View,CA
f Internet Software C,Palo Alto,
CA
i NORDUnet Stockholm
k RIPE London
m WIDE Tokyo
a NSI Herndon,VA
c PSInet Herndon,VA
d U Maryland College Park,MD
g DISA Vienna,VA
h ARL Aberdeen,MD
j NSI (TBD) Herndon,VA
13 root name
servers worldwide
2,Application Layer 58
Simple DNS example
host surf.eurecom.fr
wants IP address of
gaia.cs.umass.edu
1,contacts its local DNS
server,dns.eurecom.fr
2,dns.eurecom.fr contacts
root name server,if
necessary
3,root name server contacts
authoritative name server,
dns.umass.edu,if
necessary requesting hostsurf.eurecom.fr gaia.cs.umass.edu
root name server
authorititive name server
dns.umass.edu
local name server
dns.eurecom.fr
1
2
3
4
5
6
2,Application Layer 59
DNS example
Root name server:
? may not know
authoritative name
server
? may know
intermediate name
server,who to
contact to find
authoritative name
server
requesting host
surf.eurecom.fr
gaia.cs.umass.edu
root name server
local name server
dns.eurecom.fr
1
2
3
4 5
6
authoritative name server
dns.cs.umass.edu
intermediate name server
dns.umass.edu
7
8
2,Application Layer 60
DNS,iterated queries
recursive query:
? puts burden of name
resolution on
contacted name
server
? heavy load?
iterated query:
? contacted server
replies with name of
server to contact
?,I don’t know this
name,but ask this
server”
requesting host
surf.eurecom.fr
gaia.cs.umass.edu
root name server
local name server
dns.eurecom.fr
1
2
3
4
5 6
authoritative name server
dns.cs.umass.edu
intermediate name server
dns.umass.edu
7
8
iterated query
2,Application Layer 61
DNS,caching and updating records
? once (any) name server learns mapping,it caches
mapping
? cache entries timeout (disappear) after some
time
? update/notify mechanisms under design by IETF
? RFC 2136
? http://www.ietf.org/html.charters/dnsind-charter.html
2,Application Layer 62
DNS records
DNS,distributed db storing resource records (RR)
? Type=NS
? name is domain (e.g,
foo.com)
? value is IP address of
authoritative name
server for this domain
RR format,(name,value,type,ttl)
? Type=A
? name is hostname
? value is IP address
? 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
2,Application Layer 63
DNS protocol,messages
DNS protocol, query and reply messages,both with
same message format
msg header
? identification,16 bit #
for query,reply to query
uses same #
? flags:
? query or reply
? recursion desired
? recursion available
? reply is authoritative
2,Application Layer 64
DNS protocol,messages
Name,type fields
for a query
RRs in reponse
to query
records for
authoritative servers
additional,helpful”
info that may be used
2,Application Layer 65
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 66
Socket programming
Socket API
? introduced in BSD4.1 UNIX,
1981
? explicitly created,used,
released by apps
? client/server paradigm
? two types of transport
service via socket API,
? unreliable datagram
? reliable,byte stream-
oriented
a host-local,
application-created,
OS-controlled interface
(a,door”) into which
application process can
both send and
receive messages to/from
another application
process
socket
Goal,learn how to build client/server application that
communicate using sockets
2,Application Layer 67
Socket-programming using TCP
Socket,a door between application process and end-
end-transport protocol (UCP or TCP)
TCP service,reliable transfer of bytes from one
process to another
process
TCP with
buffers,
variables
socket
controlled by
application
developer
controlled by
operating
system
host or
server
process
TCP with
buffers,
variables
socket
controlled by
application
developer
controlled by
operating
system
host or
server
internet
2,Application Layer 68
Socket programming with TCP
Client must contact server
? server process must first
be running
? server must have created
socket (door) that
welcomes client’s contact
Client contacts server by:
? creating client-local TCP
socket
? specifying IP address,port
number of server process
? When client creates
socket,client TCP
establishes connection to
server TCP
? When contacted by client,
server TCP creates new
socket for server process to
communicate with client
? allows server to talk with
multiple clients
? source port numbers
used to distinguish
clients (more in Chap 3)
TCP provides reliable,in-order
transfer of bytes (“pipe”)
between client and server
application viewpoint
2,Application Layer 69
Stream jargon
? A stream is a sequence of
characters that flow into
or out of a process.
? An input stream is
attached to some input
source for the process,eg,
keyboard or socket.
? An output stream is
attached to an output
source,eg,monitor or
socket.
2,Application Layer 70
Socket programming with TCP
Example client-server app:
1) client reads line from
standard input (inFromUser
stream),sends to server via
socket (outToServer
stream)
2) server reads line from socket
3) server converts line to
uppercase,sends back to
client
4) client reads,prints modified
line from socket
(inFromServer stream)
o
u
tT
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S
e
r
v
e
r
to n e tw o r k f r o m n e tw o r k
i
n
F
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o
m
S
e
r
v
e
r
i
n
F
r
o
m
U
s
e
r
k e y b o a r d m o n i to r
P ro c e s s
c l ie n tSo c k e t
in p u t
s tr e a m
in p u t
s tr e a m
o u tp u t
s tr e a m
T C P
s o c k e t
Client
process
client TCP
socket
2,Application Layer 71
Client/server socket interaction,TCP
wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
create socket,
port=x,for
incoming request:
welcomeSocket =
ServerSocket()
create socket,
connect to hostid,port=x
clientSocket =
Socket()
close
connectionSocket
read reply from
clientSocket
close
clientSocket
Server (running on hostid) Client
send request using
clientSocketread request from
connectionSocket
write reply to
connectionSocket
TCP
connection setup
2,Application Layer 72
Example,Java client (TCP)
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception
{
String sentence;
String modifiedSentence;
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname",6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
Create
input stream
Create
client socket,
connect to server
Create
output stream
attached to socket
2,Application Layer 73
Example,Java client (TCP),cont.
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER," + modifiedSentence);
clientSocket.close();
}
}
Create
input stream
attached to socket
Send line
to server
Read line
from server
2,Application Layer 74
Example,Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception
{
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient =
new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));
Create
welcoming socket
at port 6789
Wait,on welcoming
socket for contact
by client
Create input
stream,attached
to socket
2,Application Layer 75
Example,Java server (TCP),cont
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence);
}
}
}
Read in line
from socket
Create output
stream,attached
to socket
Write out line
to socket
End of while loop,
loop back and wait for
another client connection
2,Application Layer 76
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 77
Socket programming with UDP
UDP,no,connection” between
client and server
? no handshaking
? sender explicitly attaches
IP address and port of
destination to each packet
? server must extract IP
address,port of sender
from received packet
UDP,transmitted data may be
received out of order,or
lost
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
2,Application Layer 78
Client/server socket interaction,UDP
close
clientSocket
Server (running on hostid)
read reply from
clientSocket
create socket,
clientSocket =
DatagramSocket()
Client
Create,address (hostid,port=x,
send datagram request
using clientSocket
create socket,
port=x,for
incoming request:
serverSocket =
DatagramSocket()
read request from
serverSocket
write reply to
serverSocket
specifying client
host address,
port number
2,Application Layer 79
Example,Java client (UDP)
s
e
n
d
P
a
c
k
e
t
to n e tw o r k f r o m n e tw o r k
r
e
c
e
i
v
e
P
a
c
k
e
t
in
F
r
o
m
U
s
e
r
k e y b o a r d m o n i to r
P ro c e s s
c l ie n tSo c k e t
UDP
p a c k e t
in p u t
s tr e a m
UDP
p a c k e t
UDP
s o c k e t
Output,sends
packet (TCP sent
“byte stream”)
Input,receives
packet (TCP
received,byte
stream”)
Client
process
client UDP
socket
2,Application Layer 80
Example,Java client (UDP)
import java.io.*;
import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception
{
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
Create
input stream
Create
client socket
Translate
hostname to IP
address using DNS
2,Application Layer 81
Example,Java client (UDP),cont.
DatagramPacket sendPacket =
new DatagramPacket(sendData,sendData.length,IPAddress,9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData,receiveData.length);
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
Create datagram
with data-to-send,
length,IP addr,port
Send datagram
to server
Read datagram
from server
2,Application Layer 82
Example,Java server (UDP)
import java.io.*;
import java.net.*;
class UDPServer {
public static void main(String args[]) throws Exception
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
DatagramPacket receivePacket =
new DatagramPacket(receiveData,receiveData.length);
serverSocket.receive(receivePacket);
Create
datagram socket
at port 9876
Create space for
received datagram
Receive
datagram
2,Application Layer 83
Example,Java server (UDP),cont
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData,sendData.length,IPAddress,
port);
serverSocket.send(sendPacket);
}
}
}
Get IP addr
port #,of
sender
Write out
datagram
to socket
End of while loop,
loop back and wait for
another datagram
Create datagram
to send to client
2,Application Layer 84
Building a simple Web server
? handles one HTTP
request
? accepts the request
? parses header
? obtains requested file
from server’s file
system
? creates HTTP response
message:
? header lines + file
? sends response to client
? after creating server,
you can request file
using a browser (eg IE
explorer)
? see text for details
2,Application Layer 85
Socket programming,references
C-language tutorial (audio/slides):
?,Unix Network Programming” (J,Kurose),
http://manic.cs.umass.edu/~amldemo/courseware/intro.
Java-tutorials:
?,All About Sockets” (Sun tutorial),
http://www.javaworld.com/javaworld/jw-12-1996/jw-12-
sockets.html
?,Socket Programming in Java,a tutorial,”
http://www.javaworld.com/javaworld/jw-12-1996/jw-12-
sockets.html
2,Application Layer 86
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 87
Web caches (proxy 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
Goal,satisfy client request without involving origin server
client
Proxy
server
client
origin
server
origin
server
2,Application Layer 88
More about Web caching
? Cache acts as both client
and server
? Cache can do up-to-date
check using If-modified-
since HTTP header
? Issue,should cache take
risk and deliver cached
object without checking?
? Heuristics are used.
? 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
2,Application Layer 89
Caching example (1)
Assumptions
? average object size = 100,000
bits
? avg,request rate from
institution’s browser to origin
serves = 15/sec
? delay from institutional router
to any origin server and back
to router = 2 sec
Consequences
? utilization on LAN = 15%
? utilization on access link = 100%
? total delay = Internet delay +
access delay + LAN delay
= 2 sec + minutes + milliseconds
origin
servers
public
Internet
institutional
network 10 Mbps LAN
1.5 Mbps
access link
institutional
cache
2,Application Layer 90
Caching example (2)
Possible solution
? increase bandwidth of access
link to,say,10 Mbps
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
origin
servers
public
Internet
institutional
network 10 Mbps LAN
10 Mbps
access link
institutional
cache
2,Application Layer 91
Caching example (3)
Install cache
? suppose hit rate is,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 delay = Internet delay +
access delay + LAN delay
=,6*2 sec +,6*.01 secs +
milliseconds < 1.3 secs
origin
servers
public
Internet
institutional
network 10 Mbps LAN
1.5 Mbps
access link
institutional
cache
2,Application Layer 92
Content distribution networks (CDNs)
? The content providers are
the CDN customers.
Content replication
? CDN company installs
hundreds of CDN servers
throughout Internet
? in lower-tier ISPs,close
to users
? CDN replicates its customers’
content in CDN servers,
When provider updates
content,CDN updates
servers
origin server
in North America
CDN distribution node
CDN server
in S,America CDN server
in Europe
CDN server
in Asia
2,Application Layer 93
CDN example
origin server
? www.foo.com
? distributes HTML
? Replaces:
http://www.foo.com/sports.ruth.gif
with
http://www.cdn.com/www.foo.com/sports/ruth.gif
HTTP request for
www.foo.com/sports/sports.html
DNS query for www.cdn.com
HTTP request for
www.cdn.com/www.foo.com/sports/ruth.gif
1
2
3
Origin server
CDNs authoritative
DNS server
Nearby
CDN server
CDN company
? cdn.com
? distributes gif files
? uses its authoritative
DNS server to route
redirect requests
2,Application Layer 94
More about CDNs
routing requests
? CDN creates a,map”,
indicating distances
from leaf ISPs and
CDN nodes
? when query arrives at
authoritative DNS
server:
? server determines ISP
from which query
originates
? uses,map” to determine
best CDN server
not just Web pages
? streaming stored
audio/video
? streaming real-time
audio/video
? CDN nodes create
application-layer
overlay network
2,Application Layer 95
P2P file sharing
Example
? Alice runs P2P client
application on her
notebook computer
? Intermittently
connects to Internet;
gets new IP address
for each connection
? Asks for,Hey Jude”
? Application displays
other peers that have
copy of Hey Jude.
? Alice chooses one of
the peers,Bob.
? File is copied from
Bob’s PC to Alice’s
notebook,HTTP
? While Alice downloads,
other users uploading
from Alice.
? Alice’s peer is both a
Web client and a
transient Web server.
All peers are servers =
highly scalable!
2,Application Layer 96
P2P,centralized directory
original,Napster” design
1) when peer connects,it
informs central server:
? IP address
? content
2) Alice queries for,Hey
Jude”
3) Alice requests file from
Bob
centralized
directory server
peers
Alice
Bob
1
1
1
12
3
2,Application Layer 97
P2P,problems with centralized directory
? Single point of failure
? Performance
bottleneck
? Copyright
infringement
file transfer is
decentralized,but
locating content is
highly decentralized
2,Application Layer 98
P2P,decentralized directory
? Each peer is either a
group leader or
assigned to a group
leader.
? Group leader tracks
the content in all its
children.
? Peer queries group
leader; group leader
may query other group
leaders.
o r d i n a r y p e e r
g r o u p - l e a d e r p e e r
n e i g h o r i n g r e l a ti o n sh i p s
i n o ve r l a y n e tw o r k
2,Application Layer 99
More about decentralized directory
overlay network
? peers are nodes
? edges between peers
and their group leaders
? edges between some
pairs of group leaders
? virtual neighbors
bootstrap node
? connecting peer is
either assigned to a
group leader or
designated as leader
advantages of approach
? no centralized directory
server
? location service
distributed over peers
? more difficult to shut
down
disadvantages of approach
? bootstrap node needed
? group leaders can get
overloaded
2,Application Layer 100
P2P,Query flooding
? Gnutella
? no hierarchy
? use bootstrap node to
learn about others
? join message
? Send query to neighbors
? Neighbors forward query
? If queried peer has
object,it sends message
back to querying peer
join
2,Application Layer 101
P2P,more on query flooding
Pros
? peers have similar
responsibilities,no
group leaders
? highly decentralized
? no peer maintains
directory info
Cons
? excessive query
traffic
? query radius,may not
have content when
present
? bootstrap node
? maintenance of overlay
network
2,Application Layer 102
Chapter 2,Summary
? application service
requirements:
? reliability,bandwidth,
delay
? client-server paradigm
? Internet transport
service model
? connection-oriented,
reliable,TCP
? unreliable,datagrams,
UDP
Our study of network apps now complete!
? specific protocols:
? HTTP
? FTP
? SMTP,POP,IMAP
? DNS
? socket programming
? content distribution
? caches,CDNs
? P2P
2,Application Layer 103
Chapter 2,Summary
? typical request/reply
message exchange:
? client requests info or
service
? server responds with
data,status code
? message formats:
? headers,fields giving
info about data
? data,info being
communicated
Most importantly,learned about protocols
? control vs,data msgs
? in-band,out-of-band
? centralized vs,decentralized
? stateless vs,stateful
? reliable vs,unreliable msg
transfer
?,complexity at network
edge”
? security,authentication
Chapter 2
Application Layer
Computer Networking,
A Top Down Approach
Featuring the Internet,
2nd edition,
Jim Kurose,Keith Ross
Addison-Wesley,July
2002,
The PowerPoint Slides are based on
the material provided by
J.F Kurose and K.W,Ross.
2,Application Layer 2
Chapter 2,Application Layer
Our goals:
? conceptual,
implementation
aspects of network
application protocols
? transport-layer
service models
? client-server
paradigm
? peer-to-peer
paradigm
? learn about protocols
by examining popular
application-level
protocols
? HTTP
? FTP
? SMTP / POP3 / IMAP
? DNS
? programming network
applications
? socket API
2,Application Layer 3
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 4
Network applications,some jargon
Process,program running
within a host.
? within same host,two
processes communicate
using interprocess
communication (defined
by OS).
? processes running in
different hosts
communicate with an
application-layer
protocol
user agent,interfaces
with user,above” and
network,below”,
? implements user
interface &
application-level
protocol
? Web,browser
? E-mail,mail reader
? streaming audio/video,
media player
2,Application Layer 5
Applications and application-layer protocols
Application,communicating,
distributed processes
? e.g.,e-mail,Web,P2P file
sharing,instant messaging
? running in end systems
(hosts)
? exchange messages to
implement application
Application-layer protocols
? one,piece” of an app
? define messages
exchanged by apps and
actions taken
? use communication services
provided by lower layer
protocols (TCP,UDP)
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
2,Application Layer 6
App-layer protocol defines
? Types of messages
exchanged,eg,request
& response messages
? Syntax of message
types,what fields in
messages & how fields
are delineated
? Semantics of the
fields,ie,meaning of
information in fields
? Rules for when and
how processes send &
respond to messages
Public-domain protocols:
? defined in RFCs
? allows for
interoperability
? eg,HTTP,SMTP
Proprietary protocols:
? eg,KaZaA
2,Application Layer 7
Client-server paradigm
Typical network app has two
pieces,client and server
application
transport
network
data link
physical
application
transport
network
data link
physical
Client:
? initiates contact with server
(“speaks first”)
? typically requests service from
server,
? Web,client implemented in
browser; e-mail,in mail reader
request
reply
Server:
? provides requested service to client
? e.g.,Web server sends requested Web
page,mail server delivers e-mail
2,Application Layer 8
Processes communicating across network
? process sends/receives
messages to/from its
socket
? socket analogous to door
? sending process shoves
message out door
? sending process asssumes
transport infrastructure
on other side of door which
brings message to socket
at receiving process
process
TCP with
buffers,
variables
socket
host or
server
process
TCP with
buffers,
variables
socket
host or
server
Internet
controlled
by OS
controlled by
app developer
? API,(1) choice of transport protocol; (2) ability to fix
a few parameters (lots more on this later)
2,Application Layer 9
Addressing processes:
? For a process to
receive messages,it
must have an identifier
? Every host has a unique
32-bit IP address
? Q,does the IP address
of the host on which
the process runs
suffice for identifying
the process?
? Answer,No,many
processes can be
running on same host
? Identifier includes
both the IP address
and port numbers
associated with the
process on the host.
? Example port numbers:
? HTTP server,80
? Mail server,25
2,Application Layer 10
What transport service does an app 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”
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
2,Application Layer 11
Transport service requirements of common apps
Application
file transfer
Web documents
real-time audio/video
stored audio/video
interactive games
instant messaging
Data loss
no loss
no loss
no loss
loss-tolerant
loss-tolerant
loss-tolerant
no loss
Bandwidth
elastic
elastic
elastic
audio,5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
Time Sensitive
no
no
no
yes,100’s msec
yes,few secs
yes,100’s msec
yes and no
2,Application Layer 12
Internet transport protocols services
TCP service:
? connection-oriented,setup
required between client and
server processes
? reliable transport between
sending and receiving process
? flow control,sender won’t
overwhelm receiver
? congestion control,throttle
sender when network
overloaded
? does not providing,timing,
minimum bandwidth
guarantees
UDP service:
? unreliable data transfer
between sending and
receiving process
? does not provide,
connection setup,
reliability,flow control,
congestion control,timing,
or bandwidth guarantee
Q,why bother? Why is
there a UDP?
2,Application Layer 13
Internet apps,application,transport protocols
Application
remote terminal access
Web
file transfer
streaming multimedia
Internet telephony
Application
layer protocol
SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
proprietary
(e.g,RealNetworks)
proprietary
(e.g.,Dialpad)
Underlying
transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
typically UDP
2,Application Layer 14
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 15
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
2,Application Layer 16
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
2,Application Layer 17
HTTP overview (continued)
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
2,Application Layer 18
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
2,Application Layer 19
Nonpersistent HTTP
Suppose user enters URL
www.someSchool.edu/someDepartment/home.index
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)
2,Application Layer 20
Nonpersistent HTTP (cont.)
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
2,Application Layer 21
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
time to
transmit
file
initiate TCP
connection
RTT
request
file
RTT
file
received
time time
2,Application Layer 22
Persistent HTTP
Nonpersistent HTTP issues:
? requires 2 RTTs per object
? OS must work and allocate
host resources for each TCP
connection
? but 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
are sent over connection
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
? client sends requests as
soon as it encounters a
referenced object
? as little as one RTT for all
the referenced objects
2,Application Layer 23
HTTP request message
? two types of HTTP messages,request,response
? HTTP request message:
? ASCII (human-readable format)
GET /somedir/page.html HTTP/1.1
Host,www.someschool.edu
User-agent,Mozilla/4.0
Connection,close
Accept-language:fr
(extra carriage return,line feed)
request line
(GET,POST,
HEAD commands)
header
lines
Carriage return,
line feed
indicates end
of message
2,Application Layer 24
HTTP request message,general format
2,Application Layer 25
Uploading form input
Post method:
? Web page often
includes form input
? Input is uploaded to
server in entity body
URL method:
? Uses GET method
? Input is uploaded in
URL field of request
line:
www.somesite.com/animalsearch?monkeys&banana
2,Application Layer 26
Method types
HTTP/1.0
? GET
? POST
? HEAD
? asks server to leave
requested object out of
response
HTTP/1.1
? GET,POST,HEAD
? PUT
? uploads file in entity
body to path specified
in URL field
? DELETE
? deletes file specified in
the URL field
2,Application Layer 27
HTTP response message
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,.,
status line
(protocol
status code
status phrase)
header
lines
data,e.g.,
requested
HTML file
2,Application Layer 28
HTTP response status 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
In first line in server->client response message.
A few sample codes:
2,Application Layer 29
Trying out HTTP (client side) for yourself
1,Telnet to your favorite Web server:
Opens TCP connection to port 80
(default HTTP server port) at www.eurecom.fr.
Anything typed in sent
to port 80 at www.eurecom.fr
telnet www.eurecom.fr 80
2,Type in a GET HTTP request:
GET /~ross/index.html HTTP/1.0 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!
2,Application Layer 30
User-server interaction,authorization
Authorization, control access to
server content
? authorization credentials,
typically name,password
? stateless,client must present
authorization in each request
? authorization,header line in
each request
? if no authorization,header,
server refuses access,
sends
WWW authenticate:
header line in response
client server
usual http request msg
401,authorization req.
WWW authenticate:
usual http request msg
+ Authorization,<cred>
usual http response msg
usual http request msg
+ Authorization,<cred>
usual http response msg time
2,Application Layer 31
Cookies,keeping,state”
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 e-
commerce 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
2,Application Layer 32
Cookies,keeping,state” (cont.)
client server
usual http request msg
usual http response +
Set-cookie,1678
usual http request msg
cookie,1678
usual http response msg
usual http request msg
cookie,1678
usual http response msg
cookie-
specific
action
cookie-
spectific
action
server
creates ID
1678 for user
Cookie file
amazon,1678
ebay,8734
Cookie file
ebay,8734
Cookie file
amazon,1678
ebay,8734
one week later:
2,Application Layer 33
Cookies (continued)
What cookies can bring:
? authorization
? shopping carts
? recommendations
? user session state
(Web e-mail)
Cookies and privacy:
? cookies permit sites to
learn a lot about you
? you may supply name
and e-mail to sites
? search engines use
redirection & cookies
to learn yet more
? advertising companies
obtain info across
sites
aside
2,Application Layer 34
Conditional GET,client-side caching
? Goal,don’t send object if
client has up-to-date cached
version
? client,specify date of
cached copy in HTTP request
If-modified-since,
<date>
? server,response contains no
object if cached copy is up-
to-date,
HTTP/1.0 304 Not
Modified
client server
HTTP request msg
If-modified-since,
<date>
HTTP response
HTTP/1.0
304 Not Modified
object
not
modified
HTTP request msg
If-modified-since,
<date>
HTTP response
HTTP/1.0 200 OK
<data>
object
modified
2,Application Layer 35
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 36
FTP,the file transfer protocol
? 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
file transfer FTP
server
FTP
user
interface
FTP
client
local file
system
remote file
system
user
at host
2,Application Layer 37
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.
FTP
client
FTP
server
TCP control connection
port 21
TCP data connection
port 20
? Server opens a second TCP
data connection to transfer
another file.
? Control connection:,out of
band”
? FTP server maintains,state”,
current directory,earlier
authentication
2,Application Layer 38
FTP commands,responses
Sample commands:
? sent as ASCII text over
control channel
? USER username
? PASS password
? LIST return list of file in
current directory
? RETR filename retrieves
(gets) file
? STOR filename stores
(puts) file onto remote
host
Sample return codes
? status code and phrase (as
in HTTP)
? 331 Username OK,
password required
? 125 data connection
already open;
transfer starting
? 425 Can’t open data
connection
? 452 Error writing
file
2,Application Layer 39
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 40
Electronic Mail
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
user mailbox
outgoing
message queue
server
user
agent
user
agent
user
agentmailserver
user
agent
user
agent
server
user
agent
SMTP
SMTP
SMTP
2,Application Layer 41
Electronic Mail,mail servers
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
server
user
agent
user
agent
user
agentmailserver
user
agent
user
agent
server
user
agent
SMTP
SMTP
SMTP
2,Application Layer 42
Electronic Mail,SMTP [RFC 2821]
? uses TCP to reliably transfer email message from client
to server,port 25
? direct transfer,sending server to receiving server
? three phases of transfer
? handshaking (greeting)
? transfer of messages
? closure
? command/response interaction
? commands,ASCII text
? response,status code and phrase
? messages must be in 7-bit ASCII
2,Application Layer 43
Scenario,Alice sends message to Bob
1) Alice uses UA to compose
message and,to”
bob@someschool.edu
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
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
user
agent
server
server useragent
1
2 3 4
5
6
2,Application Layer 44
Sample SMTP interaction
S,220 hamburger.edu
C,HELO crepes.fr
S,250 Hello crepes.fr,pleased to meet you
C,MAIL FROM,<alice@crepes.fr>
S,250 alice@crepes.fr..,Sender ok
C,RCPT TO,<bob@hamburger.edu>
S,250 bob@hamburger.edu,.,Recipient ok
C,DATA
S,354 Enter mail,end with "." on a line by itself
C,Do you like ketchup?
C,How about pickles?
C,,
S,250 Message accepted for delivery
C,QUIT
S,221 hamburger.edu closing connection
2,Application Layer 45
Try SMTP interaction for yourself:
? telnet servername 25
? see 220 reply from server
? enter HELO,MAIL FROM,RCPT TO,DATA,QUIT
commands
above lets you send email without using email client
(reader)
2,Application Layer 46
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
2,Application Layer 47
Mail message format
SMTP,protocol for
exchanging email msgs
RFC 822,standard for text
message format:
? header lines,e.g.,
? To:
? From:
? Subject:
different from SMTP
commands!
? body
? the,message”,ASCII
characters only
header
body
blank
line
2,Application Layer 48
Message format,multimedia extensions
? MIME,multimedia mail extension,RFC 2045,2056
? additional lines in msg header declare MIME content
type
From,alice@crepes.fr
To,bob@hamburger.edu
Subject,Picture of yummy crepe,
MIME-Version,1.0
Content-Transfer-Encoding,base64
Content-Type,image/jpeg
base64 encoded data,...,
........................,
......base64 encoded data
multimedia data
type,subtype,
parameter declaration
method used
to encode data
MIME version
encoded data
2,Application Layer 49
MIME types
Content-Type,type/subtype; parameters
Text
? example subtypes,plain,
html
Image
? example subtypes,jpeg,
gif
Audio
? exampe subtypes,basic
(8-bit mu-law encoded),
32kadpcm (32 kbps
coding)
Video
? example subtypes,mpeg,
quicktime
Application
? other data that must be
processed by reader
before,viewable”
? example subtypes,
msword,octet-stream
2,Application Layer 50
Multipart Type
From,alice@crepes.fr
To,bob@hamburger.edu
Subject,Picture of yummy crepe,
MIME-Version,1.0
Content-Type,multipart/mixed; boundary=StartOfNextPart
--StartOfNextPart
Dear Bob,Please find a picture of a crepe.
--StartOfNextPart
Content-Transfer-Encoding,base64
Content-Type,image/jpeg
base64 encoded data,...,
........................,
......base64 encoded data
--StartOfNextPart
Do you want the reciple?
2,Application Layer 51
Mail access protocols
? 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.
user
agent
sender’s mail
server
user
agent
SMTP SMTP access
protocol
receiver’s mail
server
2,Application Layer 52
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,<message 1 contents>
S,,
C,dele 1
C,retr 2
S,<message 1 contents>
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
2,Application Layer 53
POP3 (more) and IMAP
More about POP3
? Previous example uses
“download and delete”
mode.
? Bob cannot re-read e-
mail if he changes
client
?,Download-and-keep”,
copies of messages on
different clients
? POP3 is stateless
across sessions
IMAP
? Keep all messages in
one place,the server
? Allows user to
organize messages in
folders
? IMAP keeps user state
across sessions:
? names of folders and
mappings between
message IDs and folder
name
2,Application Layer 54
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 55
DNS,Domain Name System
People,many identifiers:
? SSN,name,passport #
Internet hosts,routers:
? IP address (32 bit) -
used for addressing
datagrams
?,name”,e.g.,
gaia.cs.umass.edu - used
by humans
Q,map between IP
addresses and name?
Domain Name System:
? distributed database
implemented in hierarchy of
many name servers
? application-layer protocol
host,routers,name servers to
communicate to resolve names
(address/name translation)
? note,core Internet
function,implemented as
application-layer protocol
? complexity at network’s
“edge”
2,Application Layer 56
DNS name servers
? no server has all name-
to-IP address mappings
local name servers:
? each ISP,company has
local (default) name server
? host DNS query first goes
to local name server
authoritative name server:
? for a host,stores that
host’s IP address,name
? can perform name/address
translation for that host’s
name
Why not centralize DNS?
? single point of failure
? traffic volume
? distant centralized
database
? maintenance
doesn’t scale!
2,Application Layer 57
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
b USC-ISI Marina del Rey,CA
l ICANN Marina del Rey,CA
e NASA Mt View,CA
f Internet Software C,Palo Alto,
CA
i NORDUnet Stockholm
k RIPE London
m WIDE Tokyo
a NSI Herndon,VA
c PSInet Herndon,VA
d U Maryland College Park,MD
g DISA Vienna,VA
h ARL Aberdeen,MD
j NSI (TBD) Herndon,VA
13 root name
servers worldwide
2,Application Layer 58
Simple DNS example
host surf.eurecom.fr
wants IP address of
gaia.cs.umass.edu
1,contacts its local DNS
server,dns.eurecom.fr
2,dns.eurecom.fr contacts
root name server,if
necessary
3,root name server contacts
authoritative name server,
dns.umass.edu,if
necessary requesting hostsurf.eurecom.fr gaia.cs.umass.edu
root name server
authorititive name server
dns.umass.edu
local name server
dns.eurecom.fr
1
2
3
4
5
6
2,Application Layer 59
DNS example
Root name server:
? may not know
authoritative name
server
? may know
intermediate name
server,who to
contact to find
authoritative name
server
requesting host
surf.eurecom.fr
gaia.cs.umass.edu
root name server
local name server
dns.eurecom.fr
1
2
3
4 5
6
authoritative name server
dns.cs.umass.edu
intermediate name server
dns.umass.edu
7
8
2,Application Layer 60
DNS,iterated queries
recursive query:
? puts burden of name
resolution on
contacted name
server
? heavy load?
iterated query:
? contacted server
replies with name of
server to contact
?,I don’t know this
name,but ask this
server”
requesting host
surf.eurecom.fr
gaia.cs.umass.edu
root name server
local name server
dns.eurecom.fr
1
2
3
4
5 6
authoritative name server
dns.cs.umass.edu
intermediate name server
dns.umass.edu
7
8
iterated query
2,Application Layer 61
DNS,caching and updating records
? once (any) name server learns mapping,it caches
mapping
? cache entries timeout (disappear) after some
time
? update/notify mechanisms under design by IETF
? RFC 2136
? http://www.ietf.org/html.charters/dnsind-charter.html
2,Application Layer 62
DNS records
DNS,distributed db storing resource records (RR)
? Type=NS
? name is domain (e.g,
foo.com)
? value is IP address of
authoritative name
server for this domain
RR format,(name,value,type,ttl)
? Type=A
? name is hostname
? value is IP address
? 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
2,Application Layer 63
DNS protocol,messages
DNS protocol, query and reply messages,both with
same message format
msg header
? identification,16 bit #
for query,reply to query
uses same #
? flags:
? query or reply
? recursion desired
? recursion available
? reply is authoritative
2,Application Layer 64
DNS protocol,messages
Name,type fields
for a query
RRs in reponse
to query
records for
authoritative servers
additional,helpful”
info that may be used
2,Application Layer 65
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 66
Socket programming
Socket API
? introduced in BSD4.1 UNIX,
1981
? explicitly created,used,
released by apps
? client/server paradigm
? two types of transport
service via socket API,
? unreliable datagram
? reliable,byte stream-
oriented
a host-local,
application-created,
OS-controlled interface
(a,door”) into which
application process can
both send and
receive messages to/from
another application
process
socket
Goal,learn how to build client/server application that
communicate using sockets
2,Application Layer 67
Socket-programming using TCP
Socket,a door between application process and end-
end-transport protocol (UCP or TCP)
TCP service,reliable transfer of bytes from one
process to another
process
TCP with
buffers,
variables
socket
controlled by
application
developer
controlled by
operating
system
host or
server
process
TCP with
buffers,
variables
socket
controlled by
application
developer
controlled by
operating
system
host or
server
internet
2,Application Layer 68
Socket programming with TCP
Client must contact server
? server process must first
be running
? server must have created
socket (door) that
welcomes client’s contact
Client contacts server by:
? creating client-local TCP
socket
? specifying IP address,port
number of server process
? When client creates
socket,client TCP
establishes connection to
server TCP
? When contacted by client,
server TCP creates new
socket for server process to
communicate with client
? allows server to talk with
multiple clients
? source port numbers
used to distinguish
clients (more in Chap 3)
TCP provides reliable,in-order
transfer of bytes (“pipe”)
between client and server
application viewpoint
2,Application Layer 69
Stream jargon
? A stream is a sequence of
characters that flow into
or out of a process.
? An input stream is
attached to some input
source for the process,eg,
keyboard or socket.
? An output stream is
attached to an output
source,eg,monitor or
socket.
2,Application Layer 70
Socket programming with TCP
Example client-server app:
1) client reads line from
standard input (inFromUser
stream),sends to server via
socket (outToServer
stream)
2) server reads line from socket
3) server converts line to
uppercase,sends back to
client
4) client reads,prints modified
line from socket
(inFromServer stream)
o
u
tT
o
S
e
r
v
e
r
to n e tw o r k f r o m n e tw o r k
i
n
F
r
o
m
S
e
r
v
e
r
i
n
F
r
o
m
U
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e
r
k e y b o a r d m o n i to r
P ro c e s s
c l ie n tSo c k e t
in p u t
s tr e a m
in p u t
s tr e a m
o u tp u t
s tr e a m
T C P
s o c k e t
Client
process
client TCP
socket
2,Application Layer 71
Client/server socket interaction,TCP
wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
create socket,
port=x,for
incoming request:
welcomeSocket =
ServerSocket()
create socket,
connect to hostid,port=x
clientSocket =
Socket()
close
connectionSocket
read reply from
clientSocket
close
clientSocket
Server (running on hostid) Client
send request using
clientSocketread request from
connectionSocket
write reply to
connectionSocket
TCP
connection setup
2,Application Layer 72
Example,Java client (TCP)
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception
{
String sentence;
String modifiedSentence;
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname",6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
Create
input stream
Create
client socket,
connect to server
Create
output stream
attached to socket
2,Application Layer 73
Example,Java client (TCP),cont.
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER," + modifiedSentence);
clientSocket.close();
}
}
Create
input stream
attached to socket
Send line
to server
Read line
from server
2,Application Layer 74
Example,Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception
{
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient =
new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));
Create
welcoming socket
at port 6789
Wait,on welcoming
socket for contact
by client
Create input
stream,attached
to socket
2,Application Layer 75
Example,Java server (TCP),cont
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence);
}
}
}
Read in line
from socket
Create output
stream,attached
to socket
Write out line
to socket
End of while loop,
loop back and wait for
another client connection
2,Application Layer 76
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 77
Socket programming with UDP
UDP,no,connection” between
client and server
? no handshaking
? sender explicitly attaches
IP address and port of
destination to each packet
? server must extract IP
address,port of sender
from received packet
UDP,transmitted data may be
received out of order,or
lost
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
2,Application Layer 78
Client/server socket interaction,UDP
close
clientSocket
Server (running on hostid)
read reply from
clientSocket
create socket,
clientSocket =
DatagramSocket()
Client
Create,address (hostid,port=x,
send datagram request
using clientSocket
create socket,
port=x,for
incoming request:
serverSocket =
DatagramSocket()
read request from
serverSocket
write reply to
serverSocket
specifying client
host address,
port number
2,Application Layer 79
Example,Java client (UDP)
s
e
n
d
P
a
c
k
e
t
to n e tw o r k f r o m n e tw o r k
r
e
c
e
i
v
e
P
a
c
k
e
t
in
F
r
o
m
U
s
e
r
k e y b o a r d m o n i to r
P ro c e s s
c l ie n tSo c k e t
UDP
p a c k e t
in p u t
s tr e a m
UDP
p a c k e t
UDP
s o c k e t
Output,sends
packet (TCP sent
“byte stream”)
Input,receives
packet (TCP
received,byte
stream”)
Client
process
client UDP
socket
2,Application Layer 80
Example,Java client (UDP)
import java.io.*;
import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception
{
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
Create
input stream
Create
client socket
Translate
hostname to IP
address using DNS
2,Application Layer 81
Example,Java client (UDP),cont.
DatagramPacket sendPacket =
new DatagramPacket(sendData,sendData.length,IPAddress,9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData,receiveData.length);
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
Create datagram
with data-to-send,
length,IP addr,port
Send datagram
to server
Read datagram
from server
2,Application Layer 82
Example,Java server (UDP)
import java.io.*;
import java.net.*;
class UDPServer {
public static void main(String args[]) throws Exception
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
DatagramPacket receivePacket =
new DatagramPacket(receiveData,receiveData.length);
serverSocket.receive(receivePacket);
Create
datagram socket
at port 9876
Create space for
received datagram
Receive
datagram
2,Application Layer 83
Example,Java server (UDP),cont
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData,sendData.length,IPAddress,
port);
serverSocket.send(sendPacket);
}
}
}
Get IP addr
port #,of
sender
Write out
datagram
to socket
End of while loop,
loop back and wait for
another datagram
Create datagram
to send to client
2,Application Layer 84
Building a simple Web server
? handles one HTTP
request
? accepts the request
? parses header
? obtains requested file
from server’s file
system
? creates HTTP response
message:
? header lines + file
? sends response to client
? after creating server,
you can request file
using a browser (eg IE
explorer)
? see text for details
2,Application Layer 85
Socket programming,references
C-language tutorial (audio/slides):
?,Unix Network Programming” (J,Kurose),
http://manic.cs.umass.edu/~amldemo/courseware/intro.
Java-tutorials:
?,All About Sockets” (Sun tutorial),
http://www.javaworld.com/javaworld/jw-12-1996/jw-12-
sockets.html
?,Socket Programming in Java,a tutorial,”
http://www.javaworld.com/javaworld/jw-12-1996/jw-12-
sockets.html
2,Application Layer 86
Chapter 2 outline
? 2.1 Principles of app
layer protocols
? clients and servers
? app requirements
? 2.2 Web and HTTP
? 2.3 FTP
? 2.4 Electronic Mail
? SMTP,POP3,IMAP
? 2.5 DNS
? 2.6 Socket programming
with TCP
? 2.7 Socket programming
with UDP
? 2.8 Building a Web
server
? 2.9 Content distribution
? Network Web caching
? Content distribution
networks
? P2P file sharing
2,Application Layer 87
Web caches (proxy 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
Goal,satisfy client request without involving origin server
client
Proxy
server
client
origin
server
origin
server
2,Application Layer 88
More about Web caching
? Cache acts as both client
and server
? Cache can do up-to-date
check using If-modified-
since HTTP header
? Issue,should cache take
risk and deliver cached
object without checking?
? Heuristics are used.
? 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
2,Application Layer 89
Caching example (1)
Assumptions
? average object size = 100,000
bits
? avg,request rate from
institution’s browser to origin
serves = 15/sec
? delay from institutional router
to any origin server and back
to router = 2 sec
Consequences
? utilization on LAN = 15%
? utilization on access link = 100%
? total delay = Internet delay +
access delay + LAN delay
= 2 sec + minutes + milliseconds
origin
servers
public
Internet
institutional
network 10 Mbps LAN
1.5 Mbps
access link
institutional
cache
2,Application Layer 90
Caching example (2)
Possible solution
? increase bandwidth of access
link to,say,10 Mbps
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
origin
servers
public
Internet
institutional
network 10 Mbps LAN
10 Mbps
access link
institutional
cache
2,Application Layer 91
Caching example (3)
Install cache
? suppose hit rate is,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 delay = Internet delay +
access delay + LAN delay
=,6*2 sec +,6*.01 secs +
milliseconds < 1.3 secs
origin
servers
public
Internet
institutional
network 10 Mbps LAN
1.5 Mbps
access link
institutional
cache
2,Application Layer 92
Content distribution networks (CDNs)
? The content providers are
the CDN customers.
Content replication
? CDN company installs
hundreds of CDN servers
throughout Internet
? in lower-tier ISPs,close
to users
? CDN replicates its customers’
content in CDN servers,
When provider updates
content,CDN updates
servers
origin server
in North America
CDN distribution node
CDN server
in S,America CDN server
in Europe
CDN server
in Asia
2,Application Layer 93
CDN example
origin server
? www.foo.com
? distributes HTML
? Replaces:
http://www.foo.com/sports.ruth.gif
with
http://www.cdn.com/www.foo.com/sports/ruth.gif
HTTP request for
www.foo.com/sports/sports.html
DNS query for www.cdn.com
HTTP request for
www.cdn.com/www.foo.com/sports/ruth.gif
1
2
3
Origin server
CDNs authoritative
DNS server
Nearby
CDN server
CDN company
? cdn.com
? distributes gif files
? uses its authoritative
DNS server to route
redirect requests
2,Application Layer 94
More about CDNs
routing requests
? CDN creates a,map”,
indicating distances
from leaf ISPs and
CDN nodes
? when query arrives at
authoritative DNS
server:
? server determines ISP
from which query
originates
? uses,map” to determine
best CDN server
not just Web pages
? streaming stored
audio/video
? streaming real-time
audio/video
? CDN nodes create
application-layer
overlay network
2,Application Layer 95
P2P file sharing
Example
? Alice runs P2P client
application on her
notebook computer
? Intermittently
connects to Internet;
gets new IP address
for each connection
? Asks for,Hey Jude”
? Application displays
other peers that have
copy of Hey Jude.
? Alice chooses one of
the peers,Bob.
? File is copied from
Bob’s PC to Alice’s
notebook,HTTP
? While Alice downloads,
other users uploading
from Alice.
? Alice’s peer is both a
Web client and a
transient Web server.
All peers are servers =
highly scalable!
2,Application Layer 96
P2P,centralized directory
original,Napster” design
1) when peer connects,it
informs central server:
? IP address
? content
2) Alice queries for,Hey
Jude”
3) Alice requests file from
Bob
centralized
directory server
peers
Alice
Bob
1
1
1
12
3
2,Application Layer 97
P2P,problems with centralized directory
? Single point of failure
? Performance
bottleneck
? Copyright
infringement
file transfer is
decentralized,but
locating content is
highly decentralized
2,Application Layer 98
P2P,decentralized directory
? Each peer is either a
group leader or
assigned to a group
leader.
? Group leader tracks
the content in all its
children.
? Peer queries group
leader; group leader
may query other group
leaders.
o r d i n a r y p e e r
g r o u p - l e a d e r p e e r
n e i g h o r i n g r e l a ti o n sh i p s
i n o ve r l a y n e tw o r k
2,Application Layer 99
More about decentralized directory
overlay network
? peers are nodes
? edges between peers
and their group leaders
? edges between some
pairs of group leaders
? virtual neighbors
bootstrap node
? connecting peer is
either assigned to a
group leader or
designated as leader
advantages of approach
? no centralized directory
server
? location service
distributed over peers
? more difficult to shut
down
disadvantages of approach
? bootstrap node needed
? group leaders can get
overloaded
2,Application Layer 100
P2P,Query flooding
? Gnutella
? no hierarchy
? use bootstrap node to
learn about others
? join message
? Send query to neighbors
? Neighbors forward query
? If queried peer has
object,it sends message
back to querying peer
join
2,Application Layer 101
P2P,more on query flooding
Pros
? peers have similar
responsibilities,no
group leaders
? highly decentralized
? no peer maintains
directory info
Cons
? excessive query
traffic
? query radius,may not
have content when
present
? bootstrap node
? maintenance of overlay
network
2,Application Layer 102
Chapter 2,Summary
? application service
requirements:
? reliability,bandwidth,
delay
? client-server paradigm
? Internet transport
service model
? connection-oriented,
reliable,TCP
? unreliable,datagrams,
UDP
Our study of network apps now complete!
? specific protocols:
? HTTP
? FTP
? SMTP,POP,IMAP
? DNS
? socket programming
? content distribution
? caches,CDNs
? P2P
2,Application Layer 103
Chapter 2,Summary
? typical request/reply
message exchange:
? client requests info or
service
? server responds with
data,status code
? message formats:
? headers,fields giving
info about data
? data,info being
communicated
Most importantly,learned about protocols
? control vs,data msgs
? in-band,out-of-band
? centralized vs,decentralized
? stateless vs,stateful
? reliable vs,unreliable msg
transfer
?,complexity at network
edge”
? security,authentication
