CS3001_Computer_Networks_Chapter_2_v8.1.pptx

Chapter 2:
Application
Layer
Lect ure Slides f rom the Boo k authors
http://gaia.cs.umas s.edu/kuro se_ross /
interactive/

Layers
ZESHAN.KHAN@NU.EDU.PK Application Layer: 2-2
APPLICATION PRESENTATION SESSION TRANSPORT NETWORK LINK
PHYSICAL

Chapter 2:
Application
layer
ZESHAN.KHAN@NU.EDU.PK
Application Layer:
2-3
Principles of network applications
Web and HTTP
E-mail, SMTP, IMAP
The Domain Name System DNS
P2P applications
video streaming and content distribution networks
socket programming with UDP and TCP

Application layer: overview
Our goals:
 conceptual and implementation aspects of application-layer protocols
• transport-layer service models
• client-server paradigm
• peer-to-peer paradigm
 learn about protocols by examining popular application-layer protocols and infrastructure
• HTTP
• SMTP, IMAP
• DNS
• video streaming systems, CDNs
 programming network applications
• socket API
Application Layer: 2-
4

Some network apps
 social networking
 Web
 text messaging
 e-mail
 multi-user network games
 streaming stored video
(YouTube, Hulu, Netflix)
 P2P file sharing
 voice over IP (e.g., Skype)
 real-time video conferencing
(e.g., Zoom, meet)
 Internet search
 remote login
 …
5

mobile network
home network
enterprise
network
national or global ISP
local or
regional ISP
datacenter
network
content
provider
network
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
Creating a network app
6
write programs that:
 run on (different) end systems
 communicate over network
 e.g., web server software
communicates with browser software
no need to write software for
network-core devices
 network-core devices do not run user
applications
 applications on end systems allows
for rapid app development,
propagation

mobile network
home network
enterprise
network
local or
regional ISP
datacenter
network
content
provider
network
Client-server paradigm
7
server:
 always-on host
 permanent IP address
 often in data centers, for scaling
clients:
 contact, communicate with server
 may be intermittently connected
 may have dynamic IP addresses
 do not communicate directly with
each other
 examples: HTTP, IMAP, FTP

mobile network
home network
enterprise
network
local or
regional ISP
datacenter
network
content
provider
network
Peer-peer architecture
8
 no always-on server
 arbitrary end systems directly
communicate
 peers request service from other
peers, provide service in return to
other peers
• self scalability – new peers bring new
service capacity, as well as new service
demands
 peers are intermittently connected
and change IP addresses
• complex management
 example: P2P file sharing

Processes communicating
9
process: program running
within a host
within same host, two
processes communicate
using inter-process
communication (defined by
OS)
processes in different hosts
communicate by exchanging
messages
 note: applications with
P2P architectures have
client processes &
server processes
client process: process that
initiates communication
server process: process
that waits to be contacted
clients, servers

Sockets
10
 process sends/receives messages to/from its socket
 socket analogous to door
• sending process shoves message outdoor
• sending process relies on transport infrastructure on other side of door to deliver
message to socket at receiving process
• two sockets involved: one on each side
Internet
controlled
by OS
controlled by
app developer
transport
application
physical
link
network
process
transport
application
physical
link
network
process
socket

Addressing processes
11
 to receive messages, process
must have identifier
 host device has unique 32-bit
IP address
 Q: does IP address of host on
which process runs suffice for
identifying the process?
 A: no, many processes can be
running on same host
 identifier includes both IP address and
port numbers associated with process
on host.
 example port numbers:
• HTTP server: 80
• mail server: 25
 to send HTTP message to
gaia.cs.umass.edu web server:
• IP address: 128.119.245.12
• port number: 80
 more shortly…

An application-layer protocol defines:
12
 types of messages exchanged,
• e.g., request, response
 message syntax:
• what fields in messages & how
fields are delineated
 message semantics
• meaning of information in fields
 rules for when and how processes
send & respond to messages
open protocols:
 defined in RFCs, everyone
has access to protocol
definition
 allows for interoperability
 e.g., HTTP, SMTP
proprietary protocols:
 e.g., Skype, Zoom

What transport service does an app need?
13
data integrity
 some apps (e.g., file transfer,
web transactions) require
100% reliable data transfer
 other apps (e.g., audio) can
tolerate some loss
timing
 some apps (e.g., Internet
telephony, interactive games)
require low delay to be “effective”
throughput
 some apps (e.g., multimedia)
require minimum amount of
throughput to be “effective”
 other apps (“elastic apps”)
make use of whatever
throughput they get
security
 encryption, data integrity,
…

Transport
service
requirements:
common apps
Application Layer:
2-14
application data loss throughput time sensitive?
file transfer/download 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
video:10Kbps-
5Mbps
yes, 10’s msec
streaming audio/video loss-tolerant audio: 5Kbps-
1Mbps
video:10Kbps-
5Mbps
yes, few secs
interactive games loss-tolerant Kbps yes, 10’s msec
text messaging no loss elastic yes and no

Internet transport protocols services
15
TCP service:
 reliable transport between sending and
receiving process
 flow control: sender won’t overwhelm
receiver
 congestion control: throttle sender when
network overloaded
 connection-oriented: setup required
between client and server processes
 does not provide: timing, minimum
throughput guarantee, security
UDP service:
 unreliable data transfer
between sending and receiving
process
 does not provide: reliability,
flow control, congestion
control, timing, throughput
guarantee, security, or
connection setup.
Q: why bother? Why
is there a UDP?

Internet
applications,
and transport
protocols
Application Layer:
2-16
Application application
layer protocol
transport
protocol
file
transfer/download
FTP [RFC 959] TCP
e-mail SMTP [RFC 5321] TCP
Web documents HTTP 1.1 [RFC 7320] TCP
Internet telephony SIP [RFC 3261], RTP
[RFC 3550], or
proprietary
TCP or UDP
streaming
audio/video
HTTP [RFC 7320],
DASH
TCP
interactive games WOW, FPS
(proprietary)
TCP or UDP

Securing TCP
17
Vanilla TCP & UDP sockets:
 no encryption
 cleartext passwords sent into socket
traverse Internet in cleartext (!)
Transport Layer Security (TLS)
 provides encrypted TCP connections
 data integrity
 end-point authentication
TLS implemented in
application layer
 apps use TLS libraries, that
use TCP in turn
 cleartext sent into “socket”
traverse Internet encrypted
 more: Chapter 8

Chapter 2:
Application
layer
Application Layer:
2-18
Web and HTTP
E-mail, SMTP, IMAP
P2P applications

Web and HTTP
Transport Layer: 3-
19
 web page consists of objects, each of which can be stored on
different Web servers
 object can be HTML file, JPEG image, Java applet, audio file,…
 web page consists of base HTML-file which includes several
referenced objects, each addressable by a URL, e.g.,
www.someschool.edu/someDept/pic.gif
host name path name

HTTP overview
Transport Layer: 3-
20
HTTP: hypertext transfer protocol
 Web’s application-layer protocol
 client/server model:
• client: browser that requests,
receives, (using HTTP protocol) and
“displays” Web objects
• server: Web server sends (using
HTTP protocol) objects in response
to requests
HTTP request
HTTP response
HTTP request
HTTP response
iPhone running
Safari browser
PC running
Firefox browser
server running
Apache Web
server

HTTP overview (continued)
Transport Layer: 3-
21
HTTP uses TCP:
 client initiates TCP connection
(creates socket) to server, port 80
 server accepts TCP connection
from client
 HTTP messages (application-layer
protocol messages) exchanged
between browser (HTTP client) and
Web server (HTTP server)
 TCP connection closed
HTTP is “stateless”
 server maintains no
information about past client
requests
protocols that maintain “state”
are complex!
 past history (state) must be
maintained
 if server/client crashes, their views
of “state” may be inconsistent, must
be reconciled
aside

HTTP connections: two types
Transport Layer: 3-
22
Non-persistent HTTP
1. TCP connection opened
2. at most one object sent
over TCP connection
3. TCP connection closed
downloading multiple
objects required multiple
connections
Persistent HTTP
TCP connection opened to
a server
multiple objects can be
sent over single TCP
connection between client,
and that server
TCP connection closed

Non-persistent HTTP: example
Transport Layer: 3-
23
User enters URL:
1a. HTTP client initiates TCP
connection to HTTP server
(process) at www.someSchool.edu on
port 80
2. HTTP client sends HTTP
request message (containing
URL) into TCP connection
socket. Message indicates
that client wants object
someDepartment/home.index
1b. HTTP server at host
www.someSchool.edu waiting for TCP
connection at port 80 “accepts”
connection, notifying client
3. HTTP server receives request message,
forms response message containing
requested object, and sends message
into its socket
time
(containing text, references to 10 jpeg images)
www.someSchool.edu/someDepartment/home.index

Non-persistent HTTP: example (cont.)
Transport Layer: 3-
24
User enters URL:
(containing text, references to 10 jpeg images)
www.someSchool.edu/someDepartment/home.index
5. HTTP client receives response
message containing html file,
displays html. Parsing html file,
finds 10 referenced jpeg objects
6. Steps 1-5 repeated for
each of 10 jpeg objects
4. HTTP server closes TCP
connection.
time

Non-persistent HTTP: response time
Transport Layer: 3-
25
RTT (definition): time for a small
packet to travel from client to
server and back
HTTP response time (per object):
 one RTT to initiate TCP connection
 one RTT for HTTP request and first few
bytes of HTTP response to return
 object/file transmission time
time to
transmit
file
initiate TCP
connection
RTT
request file
RTT
file received
time time
Non-persistent HTTP response time = 2RTT+ file transmission time

Persistent HTTP (HTTP 1.1)
Transport Layer: 3-
26
Non-persistent HTTP issues:
 requires 2 RTTs per object
 OS overhead for each TCP
connection
 browsers often open multiple
parallel TCP connections to
fetch referenced objects in
parallel
Persistent HTTP (HTTP1.1):
 server leaves connection open after
sending response
 subsequent HTTP messages between
same client/server sent over open
connection
 client sends requests as soon as it
encounters a referenced object
 as little as one RTT for all the referenced
objects (cutting response time in half)

HTTP request message
Transport Layer: 3-
27
 two types of HTTP messages: request, response
 HTTP request message:
• ASCII (human-readable format)
header
lines
GET /index.html HTTP/1.1rn
Host: www-net.cs.umass.edurn
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X
10.15; rv:80.0) Gecko/20100101 Firefox/80.0 rn
Accept: text/html,application/xhtml+xmlrn
Accept-Language: en-us,en;q=0.5rn
Accept-Encoding: gzip,deflatern
Connection: keep-alivern
rn
carriage return character
line-feed character
request line (GET, POST,
HEAD commands)
carriage return, line feed
at start of line indicates
end of header lines

HTTP request message: general format
Transport Layer: 3-
28
request
line
header
lines
body
method sp sp cr lf
version
URL
cr lf
value
header field name
cr lf
value
header field name
~
~ ~
~
cr lf
entity body
~
~ ~
~

Other HTTP request messages
Transport Layer: 3-
29
POST method:
 web page often includes form
input
 user input sent from client to
server in entity body of HTTP
POST request message
GET method (for sending data to server):
 include user data in URL field of HTTP
GET request message (following a ‘?’):
www.somesite.com/animalsearch?monkeys&banana
HEAD method:
 requests headers (only) that
would be returned if specified URL
were requested with an HTTP
GET method.
PUT method:
 uploads new file (object) to server
 completely replaces file that exists
at specified URL with content in
entity body of POST HTTP request
message

HTTP response message
Transport Layer: 3-
30
status line (protocol status code status phrase)
header
lines
data, e.g., requested
HTML file
Date: Tue, 08 Sep 2020 00:53:20 GMT
Server: Apache/2.4.6 (CentOS)
OpenSSL/1.0.2k-fips PHP/7.4.9
mod_perl/2.0.11 Perl/v5.16.3
Last-Modified: Tue, 01 Mar 2016 18:57:50 GMT
ETag: "a5b-52d015789ee9e"
Accept-Ranges: bytes
Content-Length: 2651
Content-Type: text/html; charset=UTF-8
rn
data data data data data ...
HTTP/1.1 200 OK

HTTP response
status codes
Transport Layer: 3-
31
 status code appears
in 1st line in server-
to-client response
message.
 some sample codes:
• request succeeded, requested object later in this message
200 OK
• requested object moved, new location specified later in this
message (in Location: field)
301 Moved Permanently
• request msg not understood by server
400 Bad Request
• requested document not found on this server
404 Not Found
505 HTTP Version Not Supported

Maintaining user/server state: cookies
Transport Layer: 3-
33
Recall: HTTP GET/response
interaction is stateless
 no notion of multi-step exchanges of
HTTP messages to complete a Web
“transaction”
• no need for client/server to track
“state” of multi-step exchange
• all HTTP requests are independent of
each other
• no need for client/server to “recover”
from a partially-completed-but-never-
completely-completed transaction
a stateful protocol: client makes
two changes to X, or none at all
time time
OK
OK
unlock X
OK
update X X’
update X X’’
lock data record X
OK
X
X
X’
X’’
X’’
t’
Q: what happens if network connection or
client crashes at t’ ?

Transport Layer: 3-
34
Web sites and client browser use cookies
to maintain some state between
transactions
four components:
1) cookie header line of HTTP response
message
2) cookie header line in next HTTP request
message
3) cookie file kept on user’s host, managed by
user’s browser
4) back-end database at Web site
Example:
 Susan uses browser on laptop, visits
specific e-commerce site for first time
 when initial HTTP requests arrives at
site, site creates:
• unique ID (aka “cookie”)
• entry in backend database for ID
• subsequent HTTP requests from
Susan to this site will contain cookie
ID value, allowing site to “identify”
Susan

Transport Layer: 3-
35
client
server
usual HTTP response msg
usual HTTP response msg
cookie file
one week later:
usual HTTP request msg
cookie: 1678 cookie-
specific
action
access
ebay 8734
usual HTTP request msg Amazon server
creates ID
1678 for user create
entry
usual HTTP response
set-cookie: 1678
ebay 8734
amazon 1678
usual HTTP request msg
cookie: 1678 cookie-
specific
action
access
ebay 8734
amazon 1678
backend
database
time time

HTTP cookies: comments
Transport Layer: 3-
36
What cookies can be used for:
 authorization
 shopping carts
 recommendations
 user session state (Web e-mail)
cookies and privacy:
 cookies permit sites to
learn a lot about you on
their site.
 third party persistent
cookies (tracking cookies)
allow common identity
(cookie value) to be
tracked across multiple
web sites
aside
Challenge: How to keep state?
 at protocol endpoints: maintain state at
sender/receiver over multiple
transactions
 in messages: cookies in HTTP messages
carry state

Web caches
Transport Layer: 3-
37
 user configures browser to
point to a (local) Web cache
 browser sends all HTTP
requests to cache
• if object in cache: cache
returns object to client
• else cache requests object
from origin server, caches
received object, then
returns object to client
Goal: satisfy client requests without involving origin server
client
Web
cache
client
HTTP request
HTTP response
HTTP request HTTP request
origin
server
HTTP response HTTP response

Web caches (proxy servers)
Transport Layer: 3-
38
 Web cache acts as both
client and server
• server for original
requesting client
• client to origin server
Why Web caching?
 reduce response time for client
request
• cache is closer to client
 reduce traffic on an institution’s
access link
 Internet is dense with caches
• enables “poor” content providers
to more effectively deliver content
 server tells cache about
object’s allowable caching in
response header:

Caching example
Transport Layer: 3-
39
origin
servers
public
Internet
institutional
network
1 Gbps LAN
1.54 Mbps
access link
Scenario:
 access link rate: 1.54 Mbps
 RTT from institutional router to server: 2 sec
 web object size: 100K bits
 average request rate from browsers to origin
servers: 15/sec
 avg data rate to browsers: 1.50 Mbps
Performance Access Link
 A=packet arrival rate (requests/second)=15/sec
 R=transmission rate (bits/second)=1.54M/s
 L=Packet size=100K
 Traffic Intensity=LA/R=100K*15/1.54M=0.97

Caching example
Transport Layer: 3-
40
origin
servers
public
Internet
institutional
network
1 Gbps LAN
1.54 Mbps
access link
Scenario:
servers: 15/sec
Performance Institute LAN
 A=packet arrival rate (requests/second)=15/sec
 R=transmission rate (bits/second)=1G/s
 L=Packet size=100K
 Traffic Intensity=LA/R=100K*15/1G=0.0015

Caching example
Transport Layer: 3-
41
origin
servers
public
Internet
institutional
network
1 Gbps LAN
1.54 Mbps
access link
Performance:
 access link utilization = .97
 LAN utilization: .0015
 end-end delay = Internet delay +
access link delay + LAN delay
= 2 sec + minutes + micro secs
Scenario:
servers: 15/sec
problem: large
queueing delays at
high utilization!

Performance:
 access link utilization = .97
 LAN utilization: .0015
 end-end delay = Internet delay +
access link delay + LAN delay
= 2 sec + minutes + micro secs
Option 1: buy a faster access link
Transport Layer: 3-
42
origin
servers
public
Internet
institutional
network
1 Gbps LAN
1.54 Mbps
access link
Scenario:
servers: 15/sec
154 Mbps
154 Mbps
.0097
msecs
Cost: faster access link (expensive!)

Performance:
 LAN utilization: .?
 access link utilization = ?
 average end-end delay = ?
Option 2: install a web cache
Transport Layer: 3-
43
origin
servers
public
Internet
institutional
network
1 Gbps LAN
1.54 Mbps
access link
Scenario:
servers: 15/sec
How to compute link
utilization, delay?
Cost: web cache (cheap!)
local web cache

Calculating access link utilization, end-end delay with cache:
Transport Layer: 3-
44
origin
servers
public
Internet
institutional
network
1 Gbps LAN
1.54 Mbps
access link
local web cache
suppose cache hit rate is 0.4:
 40% requests served by cache, with low (msec)
delay
 60% requests satisfied at origin
• rate to browsers over access link
= 0.6 * 1.50 Mbps = .9 Mbps
• access link utilization = 0.9/1.54 = .58 means
low (msec) queueing delay at access link
 average end-end delay:
= 0.6 * (delay from origin servers)
+ 0.4 * (delay when satisfied at cache)
= 0.6 (2.01) + 0.4 (~msecs) = ~ 1.2 secs
lower average end-end delay than with 154 Mbps link (and cheaper too!)

Conditional GET
Transport Layer: 3-
45
Goal: don’t send object if cache has
up-to-date cached version
• no object transmission delay (or use
of network resources)
 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
HTTP request msg
HTTP response
HTTP/1.0
304 Not Modified
object
not
modified
before
<date>
HTTP request msg
HTTP response
HTTP/1.0 200 OK
<data>
object
modified
after
<date>
client server

HTTP/2
Transport Layer: 3-46
Key goal: decreased delay in multi-object HTTP requests
HTTP1.1: introduced multiple, pipelined GETs over single
TCP connection
• server responds in-order (FCFS: first-come-first-served scheduling) to
GET requests
• with FCFS, small object may have to wait for transmission (head-of-
line (HOL) blocking) behind large object(s)
• loss recovery (retransmitting lost TCP segments) stalls object
transmission

HTTP/2
Transport Layer: 3-
47
HTTP/2: [RFC 7540, 2015] increased flexibility at server in sending
objects to client:
 methods, status codes, most header fields unchanged from HTTP 1.1
 transmission order of requested objects based on client-specified
object priority (not necessarily FCFS)
 push unrequested objects to client
 divide objects into frames, schedule frames to mitigate HOL blocking
Key goal: decreased delay in multi-object HTTP requests

HTTP/2: mitigating HOL blocking
Transport Layer: 3-
48
HTTP 1.1: client requests 1 large object (e.g., video file) and 3 smaller
objects
client
server
GET O1
GET O2
GET O3
GET O4
O1
O2
O3
O4
object data requested
O1
O2
O3
O4
objects delivered in order requested: O2, O3, O4 wait behind O1

HTTP/2: mitigating HOL blocking
Transport Layer: 3-
49
HTTP/2: objects divided into frames, frame transmission interleaved
client
server
GET O1
GET O2
GET O3
GET O4
O2
O4
object data requested
O1
O2
O3
O4
O2, O3, O4 delivered quickly, O1 slightly delayed
O3
O1

HTTP/2 to HTTP/3
Transport Layer: 3-
50
HTTP/2 over single TCP connection means:
 recovery from packet loss still stalls all object transmissions
• as in HTTP 1.1, browsers have incentive to open multiple parallel
TCP connections to reduce stalling, increase overall throughput
 no security over vanilla TCP connection
 HTTP/3: adds security, per object error- and congestion-
control (more pipelining) over UDP
• more on HTTP/3 in transport layer

Chapter 2:
Application
layer
Application Layer:
2-51
Web and HTTP
E-mail, SMTP, IMAP
P2P applications

E-mail
52
Three major components:
user agents
mail servers
simple mail transfer protocol: SMTP
User Agent
“mail reader”
composing, editing, reading mail messages
e.g., Outlook, iPhone mail client
outgoing, incoming messages stored on
server user mailbox
outgoing
message queue
mail
server
mail
server
mail
server
SMTP
SMTP
SMTP
user
agent
user
agent
user
agent
user
agent
user
agent
user
agent

E-mail: mail servers
53
user mailbox
outgoing
message queue
mail
server
mail
server
mail
server
SMTP
SMTP
SMTP
user
agent
user
agent
user
agent
user
agent
user
agent
user
agent
mail servers:
 mailbox contains incoming
messages for user
 message queue of outgoing (to be
sent) mail messages
SMTP protocol between mail
servers to send email messages
 client: sending mail server
 “server”: receiving mail server

SMTP RFC (5321)
54
 uses TCP to reliably transfer email message
from client (mail server initiating
connection) to server, port 25
 direct transfer: sending server (acting like client)
to receiving server
 three phases of transfer
• SMTP handshaking (greeting)
• SMTP transfer of messages
• SMTP closure
 command/response interaction (like HTTP)
• commands: ASCII text
• response: status code and phrase
initiate TCP
connection
RTT
time
220
250 Hello
HELO
SMTP
handshaking
TCP connection
initiated
“client”
SMTP server
“server”
SMTP server
SMTP
transfers

Scenario: Alice sends e-mail to Bob
55
1) Alice uses UA to compose e-mail
message “to” bob@someschool.edu
4) SMTP client sends Alice’s message
over the TCP connection
user
agent
mail
server
mail
server
1
2 3 4
5
6
Alice’s mail server Bob’s mail server
user
agent
2) Alice’s UA sends message to her
mail server using SMTP; message
placed in message queue
3) client side of SMTP at mail server
opens TCP connection with Bob’s mail
server
5) Bob’s mail server places
the message in Bob’s
mailbox
6) Bob invokes his user
agent to read message

SMTP: observations
56
 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: client pull
 SMTP: client push
 both have ASCII command/response
interaction, status codes
 HTTP: each object encapsulated in its
own response message
 SMTP: multiple objects sent in
multipart message

Mail message format
57
SMTP: protocol for exchanging e-mail messages, defined in RFC 5321
(like RFC 7231 defines HTTP)
RFC 2822 defines syntax for e-mail message itself (like HTML defines
syntax for web documents)
header
body
blank
line
 header lines, e.g.,
• To:
• From:
• Subject:
these lines, within the body of the email
message area different from SMTP MAIL FROM:,
RCPT TO: commands!
 Body: the “message” , ASCII characters only

Retrieving email: mail access protocols
58
sender’s e-mail
server
SMTP SMTP
receiver’s e-mail
server
e-mail access
protocol
(e.g., IMAP,
HTTP)
user
agent
user
agent
 SMTP: delivery/storage of e-mail messages to receiver’s server
 mail access protocol: retrieval from server
• IMAP: Internet Mail Access Protocol [RFC 3501]: messages stored on server, IMAP
provides retrieval, deletion, folders of stored messages on server
 HTTP: gmail, Hotmail, Yahoo!Mail, etc. provides web-based interface on
top of STMP (to send), IMAP (or POP) to retrieve e-mail messages

SMTP vs IMAP
SMTP (SIMPLE MAIL TRANSFER
PROTOCOL)
It is primarily used for sending emails from a
client (such as an email application or server)
to an email server or between email servers.
It handles the process of sending outgoing
emails, including routing emails to the
appropriate recipient's email server.
It operates on port 25 (although encrypted
versions like SMTPS or SMTP over TLS may use
different ports) and is responsible for
transferring the email message from the
sender's email client to the recipient's email
server.
IMAP (INTERNET MESSAGE ACCESS
PROTOCOL)
It is used for accessing emails stored on an email server
from a client device (such as an email application
or webmail client).
It allows users to view, manage, and organize their email
messages stored on the server without downloading
them to the client device.
It supports features like folder synchronization, allowing
changes made on one device to be reflected on
other devices accessing the same email account.
It operates on port 143 (or 993 for IMAP over TLS/SSL) and
provides a way for email clients to communicate with the
email server to retrieve and manage emails.
59

Chapter 2:
Application
layer
Application Layer:
2-60
Web and HTTP
E-mail, SMTP, IMAP
P2P applications

DNS: Domain Name System
61
people: many identifiers:
• SSN, name, passport #
Internet hosts, routers:
• IP address (32 bit) - used for
addressing datagrams
• “name”, e.g., cs.umass.edu -
used by humans
Q: how to map between IP
address and name, and vice
versa ?
Domain Name System (DNS):
 distributed database implemented in
hierarchy of many name servers
 application-layer protocol: hosts, DNS
servers communicate to resolve
names (address/name translation)
• note: core Internet function,
implemented as application-layer
protocol
• complexity at network’s “edge”

DNS: services, structure
62
Q: Why not centralize DNS?
 single point of failure
 traffic volume
 distant centralized database
 maintenance
DNS services:
hostname-to-IP-address translation
host aliasing
• canonical, alias names
 mail server aliasing
 load distribution
• replicated Web servers: many IP
addresses correspond to one
name
A: doesn‘t scale!
 Comcast DNS servers alone:
600B DNS queries/day
 Akamai DNS servers alone:
2.2T DNS queries/day

Thinking about the DNS
63
humongous distributed database:
 ~ billion records, each simple
handles many trillions of queries/day:
 many more reads than writes
 performance matters: almost every
Internet transaction interacts with
DNS - msecs count!
organizationally, physically decentralized:
 millions of different organizations
responsible for their records
“bulletproof”: reliability, security

DNS: a distributed, hierarchical database
64
Client wants IP address for www.amazon.com; 1st
approximation:
client queries root server to find .com DNS server
client queries .com DNS server to get amazon.com DNS server
client queries amazon.com DNS server to get IP address for www.amazon.com
.com DNS servers .org DNS servers .edu DNS servers
… …
Top Level Domain
Root DNS Servers Root
nyu.edu
DNS servers
umass.edu
DNS servers
yahoo.com
DNS servers
amazon.com
DNS servers
pbs.org
DNS servers Authoritative
…
… … …

DNS: root name servers
65
 official, contact-of-last-resort by
name servers that can't resolve
name

DNS: root name servers
66
 official, contact-of-last-resort by
name servers that can't resolve
name
 incredibly important Internet
function
• Internet couldn’t function without it!
• DNSSEC – provides security
(authentication, message integrity)
 ICANN (Internet Corporation for
Assigned Names and Numbers)
manages root DNS domain
13 logical root name “servers”
worldwide each “server” replicated
many times (~200 servers in US)

Top-Level Domain, and authoritative servers
67
Top-Level Domain (TLD) servers:
 responsible for .com, .org, .net, .edu, .aero, .jobs, .museums, and all top-level
country domains, e.g.: .cn, .uk, .fr, .ca, .jp
 Network Solutions: authoritative registry for .com, .net TLD
 Educause: .edu TLD
authoritative DNS servers:
 organization’s own DNS server(s), providing authoritative hostname to IP
mappings for organization’s named hosts
 can be maintained by organization or service provider

Local DNS name servers
68
 when host makes DNS query, it is sent to its local DNS server
• Local DNS server returns reply, answering:
• from its local cache of recent name-to-address translation pairs (possibly out
of date!)
• forwarding request into DNS hierarchy for resolution
• each ISP has local DNS name server; to find yours:
• MacOS: % scutil --dns
• Windows: >ipconfig /all
 local DNS server doesn’t strictly belong to hierarchy

DNS name resolution: iterated query
69
Example: host at engineering.nyu.edu
wants IP address for gaia.cs.umass.edu
Iterated query:
 contacted server replies
with name of server to
contact
 “I don’t know this name,
but ask this server”
requesting host at
engineering.nyu.edu
gaia.cs.umass.edu
root DNS server
local DNS server
dns.nyu.edu
1
2
3
4
5
6
authoritative DNS server
dns.cs.umass.edu
7
8
TLD DNS server

DNS name resolution: recursive query
70
requesting host at
engineering.nyu.edu
gaia.cs.umass.edu
root DNS server
local DNS server
dns.nyu.edu
1
2 3
4
5
6
authoritative DNS server
dns.cs.umass.edu
7
8
TLD DNS server
Recursive query:
 puts burden of name
resolution on
contacted name
server
 heavy load at upper
levels of hierarchy?
Example: host at engineering.nyu.edu
wants IP address for gaia.cs.umass.edu

Caching DNS Information
71
 once (any) name server learns mapping, it caches mapping,
and immediately returns a cached mapping in response to a
query
• caching improves response time
• cache entries timeout (disappear) after some time (TTL)
• TLD servers typically cached in local name servers
 cached entries may be out-of-date
• if named host changes IP address, may not be known Internet-
wide until all TTLs expire!
• best-effort name-to-address translation!

DNS records
72
DNS: distributed database storing resource records (RR)
type=NS
 name is domain
 value is hostname of authoritative
name server for this domain
 (foo.com, dns.foo.com, NS)
RR format: (name, value, type, ttl)
type=A
 name is hostname
 value is IP address
 (relay1.bar.foo.com, 145.37.93.126, A)
type=CNAME
 name is alias name for some “canonical”
(the real) name
 www.ibm.com is really servereast.backup2.ibm.com
 value is canonical name
type=MX
 value is name of SMTP mail
server associated with name

identification flags
# questions
questions (variable # of questions)
# additional RRs
# authority RRs
# answer RRs
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
2 bytes 2 bytes
DNS protocol messages
73
DNS query and reply messages, both have same format:
message header:
 identification: 16 bit # for query,
reply to query uses same #
 flags:
• query or reply
• recursion desired
• recursion available
• reply is authoritative

identification flags
# questions
questions (variable # of questions)
# additional RRs
# authority RRs
# answer RRs
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
2 bytes 2 bytes
DNS query and reply messages, both have same format:
name, type fields for a query
RRs in response to query
records for authoritative servers
additional “ helpful” info that may
be used
DNS protocol messages
74

Getting your info into the DNS
75
example: new startup “FAST”
 register name fast.com at DNS registrar (e.g., FAST.com)
• provide names, IP addresses of authoritative name server (primary and
secondary)
• registrar inserts NS, A RRs into .com TLD server:
(fast.com, dns1.fast.com, NS)
(dns1.fast.com, 212.212.212.1, A)
 create authoritative server locally with IP address 212.212.212.1
• type A record for www.fast.com
• type MX record for fast.com

DNS security
76
DDoS attacks
 bombard root servers with
traffic
• not successful to date
• traffic filtering
• local DNS servers cache IPs of TLD
servers, allowing root server
bypass
 bombard TLD servers
• potentially more dangerous
Spoofing attacks
 intercept DNS queries,
returning bogus replies
 DNS cache poisoning
 RFC 4033: DNSSEC
authentication services

Chapter 2:
Application
layer
Application Layer:
2-77
Web and HTTP
E-mail, SMTP, IMAP
P2P applications

mobile network
home network
enterprise
network
local or
regional ISP
datacenter
network
content
provider
network
Peer-to-peer (P2P) architecture
78
 no always-on server
 arbitrary end systems directly
communicate
 peers request service from other
peers, provide service in return to
other peers
• self scalability – new peers bring new
service capacity, and new service demands
 peers are intermittently connected
and change IP addresses
• complex management
 examples: P2P file sharing (BitTorrent),
streaming (KanKan), VoIP (Skype)

File distribution: client-server vs P2P
79
Q: how much time to distribute file (size F) from one server to N
peers?
• peer upload/download capacity is limited resource
us
uN
dN
server
network (with abundant
bandwidth)
file, size F
us: server upload
capacity
ui: peer i upload
capacity
di: peer i download
capacity
u2 d2
u1 d1
di
ui

File distribution time: client-server
80
 server transmission: must sequentially
send (upload) N file copies:
• time to send one copy: F/us
• time to send N copies: NF/us
 client: each client must download
file copy
• dmin = min client download rate
• min client download time: F/dmin
us
network
di
ui
F
increases linearly in N
time to distribute F
to N clients using
client-server approach
Dc-s > max{NF/us,,F/dmin}

File distribution time: P2P
81
 server transmission: must upload at
least one copy:
• time to send one copy: F/us
 client: each client must download
file copy
• min client download time: F/dmin
us
network
di
ui
F
 clients: as aggregate must download NF bits
• max upload rate (limiting max download rate) is us + Sui
time to distribute F
to N clients using
P2P approach
DP2P > max{F/us,,F/dmin,,NF/(us + Sui)}
… but so does this, as each peer brings service capacity
increases linearly in N …

Client-server vs. P2P: example
82
client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25 30 35
N
Minimum
Distribution
Time
P2P
Client-Server

P2P file distribution: BitTorrent
83
 file divided into 256Kb chunks
 peers in torrent send/receive file chunks
tracker: tracks peers
participating in torrent
torrent: group of peers
exchanging chunks of a file
Alice arrives …
… obtains list
of peers from tracker
… and begins exchanging
file chunks with peers in torrent

P2P file distribution: BitTorrent
84
 peer joining torrent:
• has no chunks, but will accumulate them
over time from other peers
• registers with tracker to get list of peers,
connects to subset of peers (“neighbors”)
 while downloading, peer uploads chunks to other peers
 peer may change peers with whom it exchanges chunks
 churn: peers may come and go
 once peer has entire file, it may (selfishly) leave or (altruistically) remain
in torrent

BitTorrent: requesting, sending file chunks
85
Requesting chunks:
 at any given time, different
peers have different subsets
of file chunks
 periodically, Alice asks each
peer for list of chunks that
they have
 Alice requests missing
chunks from peers, rarest
first
Sending chunks: tit-for-tat
 Alice sends chunks to those four peers
currently sending her chunks at highest
rate
• other peers are choked by Alice (do not
receive chunks from her)
• re-evaluate top 4 every10 secs
 every 30 secs: randomly select another
peer, starts sending chunks
• “optimistically unchoke” this peer
• newly chosen peer may join top 4

BitTorrent: tit-for-tat
86
(1) Alice “optimistically unchokes” Bob
(2) Alice becomes one of Bob’s top-four providers; Bob reciprocates
(3) Bob becomes one of Alice’s top-four providers
higher upload rate: find better trading
partners, get file faster !

Chapter 2:
Application
layer
Application Layer:
2-87
Web and HTTP
E-mail, SMTP, IMAP
P2P applications

Video Streaming and CDNs: context
88
 stream video traffic: major
consumer of Internet bandwidth
• Netflix, YouTube, Amazon Prime: 80% of
residential ISP traffic (2020)
 challenge: scale - how to reach
~1B users?
 challenge: heterogeneity
 different users have different capabilities (e.g., wired
versus mobile; bandwidth rich versus bandwidth poor)
 solution: distributed, application-level infrastructure

Multimedia: video
89
 video: sequence of images displayed
at constant rate
• e.g., 24 images/sec
 digital image: array of pixels
• each pixel represented by bits
 coding: use redundancy within and
between images to decrease # bits
used to encode image
• spatial (within image)
• temporal (from one image to next)
……………………..
spatial coding example: instead
of sending N values of same
color (all purple), send only two
values: color value (purple) and
number of repeated values (N)
……………….…….
frame i
frame i+1
temporal coding example:
instead of sending
complete frame at i+1,
send only differences from
frame i

Multimedia: video
90
……………………..
spatial coding example: instead
of sending N values of same
color (all purple), send only two
values: color value (purple) and
number of repeated values (N)
……………….…….
frame i
frame i+1
temporal coding example:
instead of sending
complete frame at i+1,
send only differences from
frame i
 CBR: (constant bit rate): video
encoding rate fixed
 VBR: (variable bit rate): video
encoding rate changes as
amount of spatial, temporal
coding changes
 examples:
• MPEG1 (CD-ROM) 1.5 Mbps
• MPEG2 (DVD) 3-6 Mbps
• MPEG4 (often used in
Internet, 64Kbps – 12 Mbps)

Streaming stored video
Main challenges:
server-to-client bandwidth will vary over time, with changing network congestion levels (in house,
access network, network core, video server)
packet loss, delay due to congestion will delay playout, or result in poor video quality
91
simple scenario:
video server
(stored video)
client
Internet

Streaming stored video
92
1. video
recorded
(e.g., 30
frames/sec)
2. video
sent
Cumulati
ve
data
streaming: at this time, client playing out
early part of video, while server still sending
later part of video
time
3. video received, played out at client
(30 frames/sec)
network delay
(fixed in this
example)

Streaming stored video: challenges
93
 continuous playout constraint: during client
video playout, playout timing must match
original timing
• … but network delays are variable (jitter), so will
need client-side buffer to match continuous playout
constraint
 other challenges:
• client interactivity: pause, fast-forward, rewind, jump through video
• video packets may be lost, retransmitted

Streaming stored video: playout buffering
94
constant bit
rate video
transmission
Cumula
tive
data
time
variable
network
delay
client video
reception
constant bit
rate video
playout at client
client playout
delay
buffered
video
client-side buffering and playout delay: compensate for
network-added delay, delay jitter

Streaming multimedia: DASH
95
server:
 divides video file into multiple chunks
 each chunk encoded at multiple different rates
 different rate encodings stored in different files
 files replicated in various CDN nodes
 manifest file: provides URLs for different chunks
client
?
client:
 periodically estimates server-to-client bandwidth
 consulting manifest, requests one chunk at a time
• chooses maximum coding rate sustainable given current bandwidth
• can choose different coding rates at different points in time (depending on
available bandwidth at time), and from different servers
...
...
...
Dynamic, Adaptive
Streaming over HTTP

...
...
...
Streaming multimedia: DASH
96
“intelligence” at client: client
determines
• when to request chunk (so that buffer
starvation, or overflow does not occur)
• what encoding rate to request (higher
quality when more bandwidth available)
• where to request chunk (can request
from URL server that is “close” to
client or has high available
bandwidth)
Streaming video = encoding + DASH + playout buffering
client
?

Content distribution networks (CDNs)
97
challenge: how to stream content (selected from millions of
videos) to hundreds of thousands of simultaneous users?
 option 1: single, large “mega-
server”
• single point of failure
• point of network congestion
• long (and possibly congested) path to
distant clients
….quite simply: this solution doesn’t scale

98
challenge: how to stream content (selected from millions of
videos) to hundreds of thousands of simultaneous users?
• enter deep: push CDN servers deep into many access networks
• close to users
• Akamai: 240,000 servers deployed
in > 120 countries (2015)
 option 2: store/serve multiple copies of videos at multiple
geographically distributed sites (CDN)
• bring home: smaller number (10’s) of
larger clusters in POPs near access nets
• used by Limelight

…
…
…
…
…
…
subscriber requests content, service provider returns manifest
99
 CDN: stores copies of content (e.g. MADMEN) at CDN nodes
where’s Madmen?
manifest file
• using manifest, client retrieves content at highest supportable rate
• may choose different rate or copy if network path congested

…
…
…
…
…
…
Internet host-host communication as a service
OTT challenges: coping with a congested Internet from the “edge”
 what content to place in which CDN node?
 from which CDN node to retrieve content? At which rate?
100
OTT: “over the top”

Chapter 2:
Application
layer
Application Layer:
2-101
Web and HTTP
E-mail, SMTP, IMAP
P2P applications

Socket programming
102
goal: learn how to build client/server applications that
communicate using sockets
socket: door between application process and end-end-transport
protocol
Internet
controlled
by OS
controlled by
app developer
transport
application
physical
link
network
process
transport
application
physical
link
network
process
socket

Socket programming
Two socket types for two transport services:
 UDP: unreliable datagram
 TCP: reliable, byte stream-oriented
103
Application Example:
1. client reads a line of characters (data) from its keyboard and sends
data to server
2. server receives the data and converts characters to uppercase
3. server sends modified data to client
4. client receives modified data and displays line on its screen

Socket programming with UDP
UDP: no “connection” between client and server:
no handshaking before sending data
sender explicitly attaches IP destination address and port # to each packet
receiver extracts sender IP address and port# from received packet
104
UDP: transmitted data may be lost or received out-of-order
Application viewpoint:
 UDP provides unreliable transfer of groups of bytes (“datagrams”)
between client and server processes

Client/server socket interaction: UDP
105
close
clientSocket
read datagram from
clientSocket
create socket:
clientSocket =
socket(AF_INET,SOCK_DGRAM)
Create datagram with serverIP address
And port=x; send datagram via
clientSocket
create socket, port= x:
serverSocket =
socket(AF_INET,SOCK_DGRAM)
read datagram from
serverSocket
write reply to
serverSocket
specifying
client address,
port number
server (running on serverIP) client

Example app: UDP client
106
from socket import *
serverName = ‘hostname’
serverPort = 12000
clientSocket = socket(AF_INET, SOCK_DGRAM)
message = raw_input(’Input lowercase sentence:’)
clientSocket.sendto(message.encode(), (serverName,
serverPort))
modifiedMessage, serverAddress =
clientSocket.recvfrom(2048)
print modifiedMessage.decode()
clientSocket.close()
Python UDPClient
include Python’s socket library
create UDP socket for server
get user keyboard input
attach server name, port to message; send into socket
print out received string and close socket
read reply characters from socket into string

Example app: UDP server
107
Python UDPServer
serverPort = 12000
serverSocket = socket(AF_INET, SOCK_DGRAM)
serverSocket.bind(('', serverPort))
print (“The server is ready to receive”)
while True:
message, clientAddress = serverSocket.recvfrom(2048)
modifiedMessage = message.decode().upper()
serverSocket.sendto(modifiedMessage.encode(),
clientAddress)
create UDP socket
bind socket to local port number 12000
loop forever
Read from UDP socket into message, getting
client’s address (client IP and port)
send upper case string back to this client

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 TCP socket, specifying IP
address, port number of server
process
when client creates socket: client TCP
establishes connection to server TCP
108
 when contacted by client, server
TCP creates new socket for server
process to communicate with that
particular client
• allows server to talk with multiple
clients
• source port numbers used to
distinguish clients (more in Chap 3)
TCP provides reliable, in-order
byte-stream transfer (“pipe”)
between client and server
processes
Application viewpoint

Client/server socket interaction: TCP
109
server (running on hostid) client
wait for incoming
connection request
connectionSocket =
serverSocket.accept()
create socket,
port=x, for incoming
request:
serverSocket = socket()
create socket,
connect to hostid, port=x
clientSocket = socket()
send request using
clientSocket
read request from
connectionSocket
write reply to
connectionSocket
TCP
connection setup
close
connectionSocket
read reply from
clientSocket
close
clientSocket

Example app: TCP client
110
serverName = ’servername’
serverPort = 12000
clientSocket = socket(AF_INET, SOCK_STREAM)
clientSocket.connect((serverName,serverPort))
sentence = raw_input(‘Input lowercase sentence:’)
clientSocket.send(sentence.encode())
modifiedSentence = clientSocket.recv(1024)
print (‘From Server:’, modifiedSentence.decode())
clientSocket.close()
Python TCPClient
create TCP socket for server,
remote port 12000
No need to attach server name, port

Example app: TCP server
111
serverPort = 12000
serverSocket = socket(AF_INET,SOCK_STREAM)
serverSocket.bind((‘’,serverPort))
serverSocket.listen(1)
print ‘The server is ready to receive’
while True:
connectionSocket, addr = serverSocket.accept()
sentence = connectionSocket.recv(1024).decode()
capitalizedSentence = sentence.upper()
connectionSocket.send(capitalizedSentence.
encode())
connectionSocket.close()
Python TCPServer
create TCP welcoming socket
server begins listening for
incoming TCP requests
loop forever
server waits on accept() for incoming
requests, new socket created on return
read bytes from socket (but
not address as in UDP)
close connection to this client (but not
welcoming socket)

Chapter 2: Summary
 application architectures
• client-server
• P2P
 application service requirements:
• reliability, bandwidth, delay
 Internet transport service model
• connection-oriented, reliable: TCP
• unreliable, datagrams: UDP
112
our study of network application layer is now complete!
 specific protocols:
• HTTP
• SMTP, IMAP
• DNS
• P2P: BitTorrent
 video streaming, CDNs
 socket programming:
TCP, UDP sockets

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
Application Layer:
2-113

CDN content access: a closer look
114
netcinema.com
KingCDN.com
1
1. Bob gets URL for video
http://netcinema.com/6Y7B23V
from netcinema.com web page
2
2. resolve http://netcinema.com/6Y7B23V
via Bob’s local DNS
netcinema’s
authoratative DNS
3
3. netcinema’s DNS returns CNAME for
http://KingCDN.com/NetC6y&B23V 4
5
6. request video from
KINGCDN server,
streamed via HTTP
KingCDN
authoritative DNS
Bob’s
local DNS
server
Bob (client) requests video http://netcinema.com/6Y7B23V
 video stored in CDN at http://KingCDN.com/NetC6y&B23V

Case study: Netflix
115
1
Bob manages
Netflix account
Netflix registration,
accounting servers
Amazon cloud
CDN
server
2
Bob browses
Netflix video
Manifest file,
requested
returned for
specific video
DASH server
selected, contacted,
streaming begins
upload copies of
multiple versions of
video to CDN servers
CDN
server
CDN
server
3
4

CS3001_Computer_Networks_Chapter_2_v8.1.pptx

More Related Content

Similar to CS3001_Computer_Networks_Chapter_2_v8.1.pptx (20)

More from MujtabaVlogs (19)

Recently uploaded (20)

CS3001_Computer_Networks_Chapter_2_v8.1.pptx

Editor's Notes