Application Layer

Topics to be covered under

this:
r 2.1 Principles of
network applications
r 2.2 Web and HTTP
r 2.3 FTP
r 2.4 Electronic Mail
❖ SMTP, POP3, IMAP
r 2.5 DNS
r 2.6 Security aspects
of Application layer

2
Some network apps
r e-mail
r web
r instant messaging
r remote login
r P2P file sharing
r multi-user network games
r streaming stored video
clips
r voice over IP
r real-time video
conferencing
r grid computing

3
Application architectures
r Client-server
r Peer-to-peer (P2P)
r Hybrid of client-server and P2P

4
Client-server architecture
server:
❖ always-on host
❖ permanent IP address
❖ server farms for
scaling
clients:
❖ communicate with server
client/server
❖ may be intermittently
connected
❖ may have dynamic IP
addresses
❖ do not communicate
directly with each other
5
Pure P2P architecture
r no always-on server
r arbitrary end systems
directly communicate peer-peer
r peers are intermittently
connected and change IP
addresses

Highly scalable but

difficult to manage

6
Hybrid of client-server and P2P
Instant messaging
❖ chatting between two users is P2P
❖ centralized service: client presence
detection/location
• user registers its IP address with central
server when it comes online
• user contacts central server to find IP
addresses of buddies
• Example: spype

7
Processes communicating
Process: program running
within a host.
r within same host, two Client process: process
processes communicate that initiates
using inter-process communication
communication (IPC Server process:
defined by OS). process that waits
r processes in different to be contacted
hosts communicate by
exchanging messages

8
 Sockets
host or host or
r process sends/receives server server
messages to/from its
socket controlled by
app developer
process
r socket analogous to door process
socke socke
❖ sending process shoves t t
message out door TCP with TCP with
buffers, Internet buffers,
❖ sending process relies on
variables variables
transport infrastructure on
other side of door which
brings message to socket controlled
by OS
at receiving process

9
Addressing processes
r to receive messages, process must have identifier
r host device has unique 32-bit IP address

Ques: does IP address of host suffice for identifying

the process?

10
 Answer:
No, many processes can be running on same host

r identifier includes both IP address and port

numbers associated with process on host.
r Example port numbers:
❖ HTTP server: 80
❖ Mail server: 25
r to send HTTP message to gaia.cs.umass.edu web
server:
❖ IP address: 128.119.245.12
❖ Port number: 80

Just as every device on an IP network has a unique IP address, every instance

of every application protocol also has a unique port number that is used to
identify it to the transport protocols on the local system.
11
r Can 2 programs use same port?

2: Application Layer 12
Application Layer Protocols

2: Application Layer 13
2: Application Layer 14
 App-layer protocol defines:

how an application processes running on different

end systems pass messages to each other
r Types of messages exchanged,
❖ e.g., request, response

r Message syntax:
❖ what fields in messages & how fields are defined

r Message semantics
❖ meaning of information in fields

r Rules for when and how processes send & respond to

messages

15
 What transport service does an app need?
Data loss Throughput
r some apps (e.g., audio) can r some apps (e.g.,
tolerate some loss known multimedia) require
as loss tolerant minimum amount of
applications.
throughput to be
r other apps (e.g., file
effective “Bandwidth
transfer, telnet) require
100% reliable data sensitive applications”
transfer r other apps (“elastic apps”)
make use of whatever
Timing throughput they get
r some apps (e.g.,
Security
Internet telephony,
interactive games) r Encryption, data integrity,
require low delay to …
be “effective”
16
 Transport service requirements of common apps

Application Data loss Throughput Time Sensitive

file transfer no loss elastic no

e-mail no loss elastic no
Web documents no loss elastic no
real-time audio/video loss-tolerant audio: 5kbps-1Mbps yes, 100’s msec
video:10kbps-5Mbps
stored audio/video loss-tolerant same as above yes, few secs
interactive games loss-tolerant few kbps up yes, 100’s msec
instant messaging no loss elastic yes and no

17
Internet apps: application, transport protocols

Application Underlying
Application layer protocol transport protocol

e-mail SMTP [RFC 2821] TCP

remote terminal access Telnet [RFC 854] TCP
Web HTTP [RFC 2616] TCP
file transfer FTP [RFC 959] TCP
streaming multimedia HTTP (eg Youtube), TCP or UDP
RTP [RFC 1889]
Internet telephony SIP, RTP, proprietary
(e.g., Skype) typically UDP

*Request for Comments (RFC)

Internet Engineering Task Force (IETF) is an organization

18
r Flow control is an
end-to-end (sender &
receiver) mechanism
that controls the
traffic between a
sender and a
receiver. Flow
control occurs in
the data link layer and
the transport layer.
r Congestion
control is used by a
network to control
congestion in
the network.
r Why we use UDP?
r TCP is Connection-oriented and it
take time to establish connection
r For Speed
19
Web and HTTP

20
First some jargon

r Web page also called document consists of objects

r Object can be HTML file, JPEG image, Java
applet, audio file,…
r Web page consists of base HTML-file which
includes several referenced objects.
r Base HTML references the object in the page
with the object’s addressable URL
r Example URL:

www.someschool.edu/someDept/pic.gif

host name path name

21
http://www.someschools.edu:80/calender.cgi
?month=july#week3

2: Application Layer 22
HTTP overview

HTTP: hypertext
transfer protocol HT
TP
req
r Web’s application layer PC running HT ues
TP t
protocol Explorer res
pon
se
r client/server model
❖ client: browser that
st
requests, receives, que
r e
P nse Server
“displays” Web objects H TT e sp o
running
r
❖ server: Web server TP Apache Web
HT
server
sends objects in
response to requests
Mac running
Navigator

2: Application Layer 23
HTTP overview (continued)
Uses TCP: HTTP is “stateless”
r client initiates TCP r server maintains no
connection (creates socket) information about
to server, port 80 past client requests
r server accepts TCP
connection from client aside
Protocols that maintain
r HTTP messages “state” are complex!
(application-layer protocol
r past history (state) must
messages) exchanged
be maintained
between browser (HTTP
r if server/client crashes,
client) and Web server
(HTTP server) their views of “state” may
be inconsistent, must be
r TCP connection closed
reconciled

24
HTTP connections
Non-persistent HTTP Persistent HTTP
r At most one object is r Multiple objects can
sent over a TCP be sent over single
connection. TCP connection
between client and
server.
r Also known as HTTP
Keep-alive or HTTP
Connection reuse.

25
 Nonpersistent HTTP
(contains text,
Suppose user enters URL references to 10
www.someSchool.edu/someDepartment/home.index jpeg images)

1a. HTTP client initiates TCP

connection to HTTP server
(process) at
1b. HTTP server at host
www.someSchool.edu waiting
www.someSchool.edu on port
for TCP connection at port 80.
80
“accepts” connection, notifying
client
2. HTTP client sends HTTP
request message (containing
URL) into TCP connection 3. HTTP server receives request
socket. Message indicates message, forms response
that client wants object message containing requested
someDepartment/home.index object, and sends message
into its socket

time
26
 Nonpersistent HTTP (cont.)

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

27
Non-Persistent HTTP: Response time
Definition of RTT: time for
a small packet to travel
from client to server
and back. initiate TCP
connection
Response time: RTT
r one RTT to initiate TCP request
file
connection time to
RTT
r one RTT for HTTP transmit
file
request and first few file
received
bytes of HTTP response
to return time time
r file transmission time
total = 2RTT+transmit time
28
r Serial fetching because:
❖ Multiple connection (TCP connection)
❖ Multiple tabs
❖ Download objects in parallel
2: Application Layer 29
HTTP 1.0 : default connection is non-persistent
2: Application Layer 30
2: Application Layer 31
2: Application Layer 32
2RTT+.25RTT +(2RTT+.25RTT)=4.5RTT
2: Application Layer 33
2: Application Layer 34
HTTP 1.1 : default connection is persistent
2: Application Layer 35
2: Application Layer 36
In Pipelined
connection 2RTT for
connection
establishment and
then 1RTT
(assuming no
window limit) for all
the objects i.e.
images/text

2: Application Layer 37
Advantages of persistent connections :
1) Lower CPU and memory usage because there are less
number of connections.
2) Allows HTTP pipelining of requests and responses.
3) Reduced network congestion (fewer TCP connections).
4) Reduced latency in subsequent requests (no handshaking).
5) Errors can be reported without the penalty of closing the
TCP connection.
Disadvantages of persistent connections :
Resources may be be kept occupied even when not needed
and may not be available to others.

2: Application Layer 38
Nonpersistent HTTP issues: Persistent HTTP
r requires 2 RTTs per object r server leaves connection
r OS overhead for each TCP open after sending
connection response
r browsers often open parallel r subsequent HTTP messages
TCP connections to fetch between same
referenced objects client/server sent over
open connection
r client sends requests as
soon as it encounters a
referenced object
r as little as one RTT for all
the referenced objects

* With Pipelining and without

pipelining

39
r Suppose within your Web browser you click on a link to obtain a Web page.
The IP address for the associated URL is not cached in your local host, so a
DNS lookup is necessary to obtain the IP address. Suppose that n DNS servers
are visited before your host receives the IP address from DNS; the successive
visits incur an RTT of RTT1, . . ., RTTn. Further suppose that the Web page
associated with the link contains exactly one object, consisting of a small
amount of HTML text. Let RTT0 denote the RTT between the local host and
the server containing the object. Assuming zero transmission time of the
object, how much time elapses from when the client clicks on the link until the
client receives the object?

2: Application Layer 40
r suppose the HTML file references eight very small objects on the
same server. Neglecting transmission times, how much time elapses
with
r a. Non-persistent HTTP with no parallel TCP connections?
r b. Non-persistent HTTP with the browser configured for 5 parallel
connections?
r c. Persistent HTTP? (with pipelining)
r d. Persistent HTTP? (without pipelining)

2: Application Layer 41
2: Application Layer 42
Question
Suppose the HTML file references three very small objects on
the same server. Neglecting transmission times how much time
elapses with
a) Non persistent HTTP?
b) Persistent HTTP without pipelining?
c) Persistent HTTP with pipelining?

2: Application Layer 43
HTTP request and resposne
message

2: Application Layer 44
 HTTP request message

r two types of HTTP messages: request, response

r HTTP request message:
❖ ASCII (human-readable format)

request line
(GET, POST, GET /somedir/page.html HTTP/1.1
HEAD commands) Host: www.someschool.edu
User-agent: Mozilla/4.0
header Connection: close
lines Accept-language:fr

Carriage return,
(extra carriage return, line feed)
line feed
indicates end
of message
45
HTTP request message: general format

2: Application Layer 46
2: Application Layer 47
HTTP request Method types
HTTP/1.0 HTTP/1.1
1. GET r GET, POST, HEAD
2. POST r PUT
3. HEAD ❖ uploads an object used
in conjunction with web
publishing tools.
❖ uploads an object to a
specific path
(directory)in URL field
on a specific web
server.
r DELETE
❖ deletes file specified in
the URL field
48
 Request Methods
r The three most commonly used request methods are: GET, POST
and HEAD.
r GET: Retrieve a Document
❖ This is the main method used for retrieving html, images, JavaScript, CSS,
etc. Most data that loads in your browser was requested using this method.
r POST: Send Data to the Server
❖ Even though you can send data to the server using GET and the query string,
in many cases POST will be preferable. Sending large amounts of data using
GET is not practical and has limitations.
r HEAD: Retrieve Header Information
❖ HEAD is identical to GET, except the server does not return the content in
the HTTP response. When you send a HEAD request, it means that you are
only interested in the response code and the HTTP headers, not the
document itself.

2: Application Layer 49
 Conditional GET

r Goal: don’t send object if cache server

cache has up-to-date cached HTTP request msg
version If-modified-since:
<date>
object
r cache: specify date of
not
cached copy in HTTP request modified
If-modified-since: HTTP response
HTTP/1.0
<date> 304 Not Modified
r server: response contains no
object if cached copy is
up-to-date: HTTP request msg
If-modified-since:
HTTP/1.0 304 Not <date> object
Modified modified
HTTP response
HTTP/1.0 200 OK
<data>
50
HTTP request Header field lines
These fields are used to pass additional information about the
request or about the client or to add conditions to the request.

Common HTTP request headers:-

1.Host
2. Content Length
3. Content-type
4. Authentication
5. Use Agent
6. Accept language
8. Cookie

51
HTTP response message
status line
(protocol
status code HTTP/1.1 200 OK
status phrase) Connection close
Date: Thu, 06 Aug 1998 12:00:15 GMT
header Server: Apache/1.3.0 (Unix)
lines Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html

data, e.g., data data data data data ...

requested
HTML file

2: Application Layer 52
HTTP response status codes
1xx :- indicates informational message only

2xx :- indicates that all is well with the request

3xx :- indicates some form of redirection i.e.

redirects the client to another URL

4xx :- indicates an error and server is blaming the

browser for doing something wrong

5xx :- indicates an error on the servers part.

53
HTTP response status codes
In first line in server->client response message.
A few sample codes:
200 OK
❖ request succeeded, response is included in the content
200 No response
❖ request succeeded, but no response is provided.
301 Moved Permanently
❖ Indicates that URL of the requested resource has changed.
302 Found
❖ It functions like 301 response except that the move is temporary
303 see other
❖ It indicates that the resource has temporarily moved and it is
obtained from new URL via GET request only.

54
400 Bad Request
❖ request message not understood by server due to bad
syntax
401 Unauthorized
❖ Indicates that the requested resource is in a protected
medium.
403 Forbidden
❖ Indicates that client is not allowed to access the
requested resource for some reasons, other than valid
HTTP login.
404 Not Found
❖ requested document not found on this server
408 Request time out

55
500 Internal server error
❖ Indicates that something happened on the server that
caused the transaction to fail.
503 Service unavailable
❖ Indicates that the server is unable to respond to the
request due to a high volume of traffic
505 HTTP Version Not Supported

56
HTTP response server Headers

1. Content length
2. Content type
3. Date
4. Last Modified
5. Location
6. Server
7. Set cookie

57
In this example, the idle timeout is
set to ten seconds and it can accept
up to 100 HTTP requests before
the HTTP Connection is forcibly
closed.

2: Application Layer 58
2: Application Layer 59
State True or False?

60
Ques:

2: Application Layer 61
User-server state: cookies
Example:
Many major Web sites
use cookies r Susan always access
Four components: Internet always from PC
1) cookie header line of r visits specific
HTTP response message e-commerce site for
2) cookie header line in first time
HTTP request message
3) cookie file kept on r when initial HTTP
user’s host, managed by requests arrives at site,
user’s browser
site creates:
4) back-end database at
Web site ❖ unique ID
❖ entry in backend
database for ID
62
 Cookies: keeping “state” (cont.)
client server
ebay 8734
usual http request msg
Amazon server
cookie file usual http response creates ID
Set-cookie: 1678 1678 for user create
ebay 8734 entry
amazon 1678
usual http request msg
cookie: 1678 cookie- access
specific
one week later: usual http response msg action backend
database
access
ebay 8734 usual http request msg
amazon 1678 cookie: 1678 cookie-
spectific
usual http response msg action

63
Cookies (continued)
aside
What cookies can bring: Cookies and privacy:
r authorization r cookies permit sites to
r shopping carts learn a lot about you
r recommendations r you may supply name
and e-mail to sites
r user session state
(Web e-mail)
How to keep “state”:
r protocol endpoints: maintain state
at sender/receiver over multiple
transactions
r cookies: http messages carry state

64
 Web caches (proxy server)
Goal: satisfy client request without involving origin server

r user sets browser: origin

server
Web accesses via
cache HT Proxy
TP e st
r browser sends all req server req
u
clientHTTP ues
t HT
T P
p o nse
HTTP requests to res
pon T P res
se HT
cache ue
s t
req
❖ object in cache: cache T P o nse
HT p
returns object r es
T TP
H
❖ else cache requests
object from origin client
origin
server, then returns
server
object to client
65
More about Web caching
r cache acts as both Why Web caching?
client and server r reduce response time
r typically cache is for client request
installed by ISP r reduce traffic on an
(university, company, institution’s access
residential ISP) link.
r Internet dense with
caches: enables “poor”
content providers to
effectively deliver
content (but so does
P2P file sharing)
66
Consider the networks shown in the figure below. Assume
computers in the institution send out 14 requests per second. Each
object average size is 100,000 bits. Also assume the internet side
delay of a request is 2 seconds. Using M/M/1 queue to model the
access delay in the 1.5Mbps access link. The formula for the
average response time is E[T]=1/(-), where  is the arrival
rate of objects to the access link and  is the service rate of the
access link.

2: Application Layer 67
Caching example
origin
Assumptions servers
r average object size = 100,000
public
bits Internet
r avg. request rate from
institution’s browsers to origin
servers = 15/sec
1.5 Mbps
r delay from institutional router access link
to any origin server and back
institutional
to router = 2 sec network
10 Mbps LAN
Consequences
r utilization on LAN = 15%
r utilization on access link = 100%
r total delay = Internet delay + institutional
access delay + LAN delay cache
= 2 sec + minutes + milliseconds
68
Caching example (cont)
origin
possible solution servers
r increase bandwidth of access
public
link to, say, 10 Mbps Internet
consequence
r utilization on LAN = 15%
r utilization on access link = 15%
10 Mbps
r Total delay = Internet delay + access link
access delay + LAN delay
institutional
= 2 sec + msecs + msecs network
10 Mbps LAN
r often a costly upgrade

institutional
cache

69
Caching example (cont)
origin
possible solution: install servers
cache public
r suppose hit rate is 0.4 Internet
consequence
r 40% requests will be
satisfied almost immediately
r 60% requests satisfied by 1.5 Mbps
origin server access link
r utilization of access link institutional
reduced to 60%, resulting in network
10 Mbps LAN
negligible delays (say 10
msec)
r total avg delay = Internet
delay + access delay + LAN
delay = .6*(2.01) secs + institutional
.4*milliseconds < 1.4 secs cache

70
r Does a Cookie Slow Down a Computer?

2: Application Layer 71
FTP

72
 FTP: the file transfer protocol

FTP file transfer

FTP FTP
user client server
interface
user
at host remote file
local file system
system

r transfer file to/from remote host

r client/server model
❖ client: side that initiates transfer (either to/from remote)
❖ server: remote host
r ftp server: port 21

73
2: Application Layer 74
2: Application Layer 75
2: Application Layer 76
FTP: separate control, data connections
TCP control connection
r FTP client contacts FTP server port 21
at port 21, TCP is transport
protocol TCP data connection
r client authorized over control FTP port 20 FTP
connection client server
r client browses remote
r server opens another TCP
directory by sending commands
data connection to transfer
over control connection.
another file.(data connections
r when server receives file are non persistent)
transfer command, server
r control connection: “out of
opens 2nd TCP connection (for
band”
file) to client
r FTP server maintains “state”:
r after transferring one file,
current directory, earlier
server closes data connection.
authentication
77
r In active mode, the client establishes the
command channel (from client port X to server
port 21) but the server establishes the data
channel (from server port 20 to client port Y,
where Y has been supplied by the client).
r In passive mode, the client establishes both
channels. In that case, the server tells the
client which port should be used for the data
channel.

2: Application Layer 78
FTP commands, responses

Sample commands: Sample return codes

r sent as ASCII text over r status code and phrase (as
control channel in HTTP)
r USER username r 331 Username OK,
r PASS password password required
r LIST return list of file in r 125 data connection
current directory already open;
transfer starting
r RETR filename retrieves r 425 Can’t open data
(gets) file connection
r STOR filename stores r 452 Error writing
(puts) file onto remote file
host

79
80
2: Application Layer 81
2: Application Layer 82
r Use UDP (connectionless) protocol
2: Application Layer 83
1. FTP server listens for connection on port number
a) 20
b) 21
c) 22
d) 23
2. In FTP protocol, client contacts server using ____ as the
transport protocol.
a) transmission control protocol
b) user datagram protocol
c) datagram congestion control protocol
d) stream control transmission protocol
3. In which mode FTP, the client initiates both the control and data
connections.
a) active mode
b) passive mode
c) both (a) and (b)
d) none of the mentioned

84
 Electronic Mail
SMTP, POP3, IMAP

85
2: Application Layer 86
Electronic Mail outgoing
message queue
user mailbox
user
Three major components: agent
r user agents mail
user
r mail servers server
agent
r simple mail transfer SMTP mail
protocol: SMTP server user

User Agent SMTP agent

r a.k.a. “mail reader” SMTP

r composing, editing, reading mail user
server agent
mail messages
r e.g., Eudora, Outlook, elm,
user
Mozilla Thunderbird agent
r outgoing, incoming messages user
stored on server agent

87
 Electronic Mail: mail servers
user
Mail Servers agent
r mailbox contains incoming mail
user
messages for user server
agent
r message queue of outgoing
(to be sent) mail messages
SMTP mail
server user
r SMTP protocol between mail
servers to send email SMTP agent

messages
SMTP
❖ client: sending mail mail user
server server agent

❖ “server”: receiving mail

user
server agent
user
***Multipurpose Internet Mail Extension (MIME) agent
for audio, video, images, application programs
88
Electronic Mail: SMTP

r uses TCP to reliably transfer email message from client

to server, port 25
r direct transfer: sending server to receiving server (No
intermediate server)
r three phases of transfer
❖ handshaking (greeting) ***SMTP is a pure text based protocol.

❖ transfer of messages
❖ closure ***Multipurpose Internet Mail
Extension (MIME) for audio, video,
r command/response interaction images, application programs
❖ commands: ASCII text
❖ response: status code and phrase

r messages must be in 7-bit ASCII

89
Scenario: Alice sends message to Bob
1) Alice uses UA to compose 4) SMTP client sends Alice’s
message and “to” message over the TCP
bob@someschool.edu connection
2) Alice’s UA sends message 5) Bob’s mail server places the
to her mail server; message message in Bob’s mailbox
placed in message queue 6) Bob invokes his user agent
3) Client side of SMTP opens to read message
TCP connection with Bob’s
mail server

1 mail
mail
server user
user server
2 agent
agent 3 6
4 5

90
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

91
 SMTP: final words
r SMTP uses persistent Comparison with HTTP:
connections
r HTTP: pull
r SMTP requires message
r SMTP: push
(header & body) to be in
7-bit ASCII r both have ASCII
r SMTP server uses command/response
CRLF.CRLF to determine interaction, status codes
end of message
r HTTP: each object
encapsulated in its own
response msg
***HTTP encapsulates each object r SMTP: multiple objects
in its own HTTP response message. sent in multipart msg
Internet mail places all of the
message’s objects into one message
92
Mail message format

SMTP: protocol for

exchanging email msgs header
blank
RFC 822: standard for text
line
message format:
r header lines, e.g.,
❖ To:
body
❖ From:
❖ Subject:
different from SMTP
commands!
r body
❖ the “message”, ASCII
characters only

93
 Message format: multimedia extensions
r MIME: multimedia mail extension, RFC 2045, 2056
r additional lines in msg header declare MIME content
type

From: alice@crepes.fr
MIME version To: bob@hamburger.edu
Subject: Picture of yummy crepe.
method used MIME-Version: 1.0
to encode data Content-Transfer-Encoding: base64
Content-Type: image/jpeg
multimedia data
type, subtype, base64 encoded data .....
parameter declaration .........................
......base64 encoded data
encoded data

94
Mail access protocols
SMTP SMTP access user
user
agent protocol agent

sender’s mail receiver’s mail

server server

r SMTP: delivery/storage to receiver’s server

r 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: gmail, Hotmail, Yahoo! Mail, etc. (Web mail)

95
POP3 protocol[110 port] S: +OK POP3 server ready
C: user bob
authorization phase S: +OK
C: pass hungry
r client commands: S: +OK user successfully logged on
❖ user: declare username
C: list
❖ pass: password S: 1 498
r server responses S: 2 912
❖ +OK S: .
C: retr 1
❖ -ERR
S: <message 1 contents>
transaction phase, client: S: .
C: dele 1
r list: list message
C: retr 2
numbers S: <message 1 contents>
r retr: retrieve message by S: .
number C: dele 2
r dele: delete C: quit
S: +OK POP3 server signing
r quit off
96
2: Application Layer 97
POP3 (more) and IMAP
More about POP3 IMAP
Two Modes of POP3 r Keep all messages in
1. “download and delete” one place: the server
mode. r Allows user to
2. “Download-and-keep”: organize messages in
copies of messages on folders
different clients r IMAP keeps user state
across sessions:
POP3 is stateless across ❖ names of folders and
mappings between
sessions message IDs and folder
name

98
2: Application Layer 99
WEB-BASED MAIL

2: Application Layer 100

DNS

101
DNS: Domain Name System

People: many identifiers: Domain Name System:

❖ SSN, name, passport # r distributed database
Internet hosts, routers: implemented in hierarchy of
many name servers
❖ IP address (32 bit) -
r application-layer protocol
used for addressing
datagrams host, routers, name servers to
communicate to resolve names
❖ “name”, e.g.,
(address/name translation)
ww.yahoo.com - used by
❖ note: core Internet
humans
function, implemented as
Q: map between IP application-layer protocol
addresses and name ? ❖ complexity at network’s
“edge”

102
 DNS services

r hostname to IP address translation

r host aliasing
❖ Canonical, alias names

r mail server aliasing

r load distribution
❖ replicated Web servers: set of IP addresses for one
canonical name

103
Why not centralize DNS?

r single point of failure

r traffic volume
r distant centralized database
r maintenance

doesn’t scale!

104
Distributed, Hierarchical Database
Root DNS Servers

com DNS servers org DNS servers edu DNS servers

pbs.org poly.edu umass.edu

yahoo.com amazon.com
DNS servers DNS serversDNS servers
DNS servers DNS servers

Client wants IP for www.amazon.com; 1st approx:

r client queries a root server to find com DNS server
r client queries com DNS server to get amazon.com
DNS server
r client queries amazon.com DNS server to get IP
address for www.amazon.com
105
 1. Root name servers
r contacted by local name server that can not resolve name
r root name server:
❖ contacts authoritative name server if name mapping not known
❖ gets mapping
❖ returns mapping to local name server

a Verisign, Dulles, VA
c Cogent, Herndon, VA (also LA)
d U Maryland College Park, MD k RIPE London (also 16 other locations)
g US DoD Vienna, VA
h ARL Aberdeen, MD i Autonomica, Stockholm (plus
j Verisign, ( 21 locations) 28 other locations)
e NASA Mt View, CA m WIDE Tokyo (also Seoul,
f Internet Software C. Palo Alto, Paris, SF)
CA (and 36 other locations)

13 root name
servers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA

106
2. Top-level domain (TLD) servers:
❖ responsible for com, org, net, edu, etc, and all
top-level country domains uk, fr, ca, jp.
❖ Network Solutions maintains servers for com TLD
❖ Generic domains, country domains and inverse
domains

3. Authoritative DNS servers:

❖ organization’s DNS servers, providing
authoritative hostname to IP mappings for
organization’s servers (e.g., Web, mail).
❖ can be maintained by organization or service
provider

107
4. Local Name Server

r does not strictly belong to hierarchy

r each ISP (residential ISP, company,
university) has one.
❖ also called “default name server”
r when host makes DNS query, query is sent
to its local DNS server
❖ acts as proxy, forwards query into hierarchy

108
109
Domain Name Syntax
❑ URL
❑ Host name
❑ domain name: Any name registered in the DNS is a domain name.
❑ A fully qualified domain name (FQDN) is a domain name that is
completely specified in the hierarchy of the DNS, having no omitted parts.
❑ partially-qualified domain name (PQDN) only specifies a portion of a
domain name. It is a relative name that has meaning only within a particular
context; the partial name must be interpreted within that context to fully
identify the node.
❑ Top level Domain
❑ Second level Domain
❑ Labels: A domain name consists of one or more parts, technically
called labels, that are conventionally concatenated, and delimited by dots
❑ Each label may contain up to 63 characters.
❑ Processing is done for sequence of domain labels from right to left,
going top to bottom within the tree.

110
r Tree: The DNS hierarchy can be visualized as a tree where each node in
the tree corresponds to a domain and the tree corresponds to the hosts
being named.
r Zones: partition of the hierarchy into sub trees called zones.
r Zone files: server makes a database called zone file and keeps all the
information for every node under that domain.
r Primary server
r Secondary server
r Zone transfer
❑ DNS uses TCP for Zone Transfer over Port: 53
❑ DNS uses UDP for DNS queries over Port:53
❑ Mapping names to addresses
❑ Mapping adresses to names

111
DNS name root DNS server

resolution example
2
3
r Host at cis.poly.edu TLD DNS server
wants IP address for 4
gaia.cs.umass.edu 5

iterated query: local DNS server

dns.poly.edu
r contacted server 7 6
replies with name of 1 8
server to contact
r “I don’t know this authoritative DNS server
dns.cs.umass.edu
name, but ask this requesting host
server” cis.poly.edu

gaia.cs.umass.edu

112
DNS name
root DNS server
resolution example
recursive query: 2 3
r puts burden of name 6
7
resolution on
TLD DNS server
contacted name
server
r heavy load? local DNS server
dns.poly.edu 5 4

1 8

authoritative DNS server

dns.cs.umass.edu
requesting host
cis.poly.edu

gaia.cs.umass.edu

113
DNS: caching and updating records
r once (any) name server learns mapping, it caches
mapping
❖ cache entries timeout (disappear) after some
time
❖ TLD servers typically cached in local name
servers
• Thus root name servers not often visited

114
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)

r Type=A r Type=CNAME
❖ name is hostname ❖ name is alias name for some
❖ value is IP address “canonical” (the real) name
www.ibm.com is really
r Type=NS
servereast.backup2.ibm.com
❖ name is domain (e.g.
❖ value is canonical name
foo.com)
❖ value is hostname of
r Type=MX
authoritative name server
❖ value is name of mailserver
for this domain
associated with name

115
DNS protocol, messages
DNS protocol : query and reply messages, both with
same message format

message header
r identification: 16 bit #
for query, reply to query
uses same #
r flags:
❖ 1 bit query or reply
❖ 1 bit recursion desired
❖ 1 bit recursion available
❖ 1 bit reply is
authoritative

116
DNS protocol, messages

Name, type fields

for a query

RRs in response
to query

records for
authoritative servers

additional “helpful”
info that may be used

117
Inserting records into DNS
r example: new startup “Network Utopia”
r register name networkuptopia.com at DNS registrar
(e.g., Network Solutions)
❖ provide names, IP addresses of authoritative name server
(primary and secondary)
❖ registrar inserts two RRs into com TLD server:

(networkutopia.com, dns1.networkutopia.com, NS)

(dns1.networkutopia.com, 212.212.212.1, A)

r create authoritative server Type A record for

www.networkuptopia.com; Type MX record for
networkutopia.com
r How do people get IP address of your Web site?

118
Questions..

2: Application Layer 119

1. In a _______ name space, a name is assigned to an address.A
name in this space is a sequence of characters without
structure.
A) Flat B) hierarchical C) organized D) none of the above

2.In a _________name space, each name is made of several

parts.
A)Flat B) hierarchical C)organized D)none of the above

3.In the DNS, the names are defined in ___________

structure.
A)a linear list B)an inverted-tree C)a graph D)none of the above

4. Each node in the tree has a _______, which is a string with

a maximum of ___ characters.
A)label; 127 B)name; 255 C)label; 63 D)none of the above

120
5. The root of the DNS tree is _______.
A)a string of characters
B)a string of 63 characters
C)an empty string
D)none of the above

6. A full domain name is a sequence of labels separated by ________.

A)Semicolons B) dots C) colons D) none of the above

7. If a label is terminated by a null string, it is called a __________.

A)PQDN B)FQDN C)SQDN D)none of the above

8. If a label is not terminated by a null string, it is called a

__________.
A)PQDN
B)FQDN
C)SQDN
D)none of the above 121
9. In __________ resolution, the resolver expects the server
To supply the final answer.
A)Iterative B)recursive C)straight D)none of the above

10. 15. A DNS client is called

A) DNS updater
B) DNS resolver
C) DNS handler
D) none of the mentioned

11. A _______ is a server whose zone consists of the whole

tree.
A)domain server B)zone server C)root server D)none of the
Above

122
12. The ____________ domain section uses two-character
country abbreviations.
A)Generic B)country C)inverse D)none of the above

13. In the domain name chal.atc.fhda.edu, _______ is the least specific label.
A)Chal
B)atc
C)edu
D)none of the above

14. The entire hostname has a maximum of

A) 255 characters
b) 127 characters
C) 63 characters
D) 31 characters

123
Question:

2: Application Layer 124

Question: What would be the type for the resource
record (RR) that contains the hostname of the mail server?

Question: Suppose within your Web browser you click on a link

to obtain a Web page. The Web page you are getting is at
server S0. Its size is very small, but it contains four objects
Stored at servers S1, S2, S3 and S4. Each object has size of
1000bits. Assume that each TCP connection established from
your host has throughput of 10000bits/sec. Let RTTi denote
the RTT between the local host and the server Si. How much
time elapses from when the client clicks on the link until the
client receives all the objects?
Note: In this question you should assume (i) that the IP
addresses for all URLs are cached in your local host so no DNS
lookup is required and (ii) your browser is NOT using persistent
connections and also is Not using parallel connections.

125
Question : Is the user-agent (e.g., mail-reader) of the receiver
Of an email message uses SMTP to download the message from
The receiver’s mailbox. If yes/no then how it is done.

Question.: In SMTP, show the connection establishment phase

from aaa@xxx.com to bbb@yyy.com.

Question: In SMTP, show the message transfer phase from

aaa@xxx.com to bbb@yyy.com.

The message is "Good morning my friend.“

Question: In SMTP, show the connection termination phase

from aaa@xxx.com to bbb@yyy.com.

126
Question: Suppose within your web browser you click on a link
to obtain a Web page. The IP address for the associated URL is
not cached in your local host, so a DNS lookup is necessary to
obtain the IP address. Suppose that n DNS servers are visited
before your host received the IP address from DNS; the
successive visits incur an RTT of RTT1, RTT2, …, RTTn. Further
Suppose that the Web page associated with the link contains
exactly one object, consisting of a small amount of HTML text.
Let RTT0 denote the RTT between the local host and the
Server containing the object. Assuming zero transmission time
of the object, how much time elapses between when the client
clicks on the link until it receives the object?

127