PowerPoint Presentation
Chapter 2
Application Layer
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All material copyright 1996-2020
J.F Kurose and K.W. Ross, All Rights Reserved
Application Layer: 2-1
Computer Networking: A Top-Down Approach
8th edition n
Jim Kurose, Keith Ross
Pearson, 2020
Version History
8.1. Updates throughout, including a lot more animations. These are the ppt files used to make Chapter 2 online lectures in Sept. 2020.
8.0 (April 2020)
All slides reformatted for 16:9 aspect ratio
All slides updated to 8th edition material
Use of Calibri font, rather that Gill Sans MT
Add LOTS more animation throughout
1
Application layer: overview
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: 2-2
2
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: 2-3
3
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)
Internet search
remote login
…
Q: your favorites?
Application Layer: 2-4
4
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
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
Application Layer: 2-5
5
mobile network
home network
enterprise
network
national or global ISP
local or regional ISP
datacenter
network
content
provider
network
Client-server paradigm
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
Application Layer: 2-6
6
mobile network
home network
enterprise
network
national or global ISP
local or regional ISP
datacenter
network
content
provider
network
Peer-peer architecture
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
Application Layer: 2-7
7
Processes communicating
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
Application Layer: 2-8
8
Sockets
process sends/receives messages to/from its socket
socket analogous to door
sending process shoves message out door
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
Application Layer: 2-9
9
Addressing processes
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?
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…
A: no, many processes can be running on same host
Application Layer: 2-10
10
An application-layer protocol defines:
types of messages exchanged,
e.g., request, response
message syntax:
what fields in messages & how fields are delineated
message semantics
meaning of information in fields
rules for when and how processes send & respond to messages
open protocols:
defined in RFCs, everyone has access to protocol definition
allows for interoperability
e.g., HTTP, SMTP
proprietary protocols:
e.g., Skype, Zoom
Application Layer: 2-11
11
What transport service does an app need?
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, …
Application Layer: 2-12
12
Transport service requirements: common apps
application
file transfer/download
e-mail
Web documents
real-time audio/video
streaming audio/video
interactive games
text messaging
data loss
no loss
no loss
no loss
loss-tolerant
loss-tolerant
loss-tolerant
no loss
throughput
elastic
elastic
elastic
audio: 5Kbps-1Mbps
video:10Kbps-5Mbps
same as above
Kbps+
elastic
time sensitive?
no
no
no
yes, 10’s msec
yes, few secs
yes, 10’s msec
yes and no
Application Layer: 2-13
13
Internet transport protocols services
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?
Application Layer: 2-14
14
Internet applications, and transport protocols
application
file transfer/download
e-mail
Web documents
Internet telephony
streaming audio/video
interactive games
application
layer protocol
FTP [RFC 959]
SMTP [RFC 5321]
HTTP 1.1 [RFC 7320]
SIP [RFC 3261], RTP [RFC 3550], or proprietary HTTP [RFC 7320], DASH
WOW, FPS (proprietary)
transport protocol
TCP
TCP
TCP
TCP or UDP
TCP
UDP or TCP
Application Layer: 2-15
15
Securing TCP
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
TSL implemented in application layer
apps use TSL libraries, that use TCP in turn
cleartext sent into “socket” traverse Internet encrypted
more: Chapter 8
Application Layer: 2-16
16
Application layer: overview
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: 2-17
17
Web and HTTP
First, a quick review…
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
Application Layer: 2-18
18
HTTP overview
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
Application Layer: 2-19
19
HTTP overview (continued)
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
Application Layer: 2-20
20
HTTP connections: two types
Non-persistent HTTP
TCP connection opened
at most one object sent over TCP connection
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
Application Layer: 2-21
21
Non-persistent HTTP: example
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
Application Layer: 2-22
22
Non-persistent HTTP: example (cont.)
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
Application Layer: 2-23
23
Non-persistent HTTP: response time
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
obect/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
Application Layer: 2-24
24
Persistent HTTP (HTTP 1.1)
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)
Application Layer: 2-25
25
HTTP request message
two types of HTTP messages: request, response
HTTP request message:
ASCII (human-readable format)
header
lines
GET /index.html HTTP/1.1\r\n
Host: www-net.cs.umass.edu\r\n
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:80.0) Gecko/20100101 Firefox/80.0 \r\n
Accept: text/html,application/xhtml+xml\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Connection: keep-alive\r\n
\r\n
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
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/
Application Layer: 2-26
26
HTTP request message: general format
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
~
~
~
~
Application Layer: 2-27
27
Other HTTP request messages
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
Application Layer: 2-28
28
HTTP response message
status line (protocol
status code status phrase)
header
lines
data, e.g., requested
HTML file
HTTP/1.1 200 OK
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
\r\n
data data data data data …
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/
Application Layer: 2-29
29
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 (in Location: field)
400 Bad Request
request msg not understood by server
404 Not Found
requested document not found on this server
505 HTTP Version Not Supported
status code appears in 1st line in server-to-client response message.
some sample codes:
Application Layer: 2-30
30
Trying out HTTP (client side) for yourself
1. netcat to your favorite Web server:
opens TCP connection to port 80 (default HTTP server port) at gaia.cs.umass. edu.
anything typed in will be sent to port 80 at gaia.cs.umass.edu
% nc -c -v gaia.cs.umass.edu 80
3. look at response message sent by HTTP server!
(or use Wireshark to look at captured HTTP request/response)
2. type in a GET HTTP request:
GET /kurose_ross/interactive/index.php HTTP/1.1
Host: gaia.cs.umass.edu
by typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server
Application Layer: 2-31
31
Maintaining user/server state: cookies
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’ ?
Application Layer: 2-32
t’
32
Maintaining user/server state: cookies
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
Application Layer: 2-33
t’
33
Maintaining user/server state: cookies
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
Application Layer: 2-34
t’
34
HTTP cookies: comments
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 inHTTP messages carry state
Application Layer: 2-35
t’
35
Web caches
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
Application Layer: 2-36
t’
36
Web caches (aka proxy servers)
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:
Application Layer: 2-37
re: poor content providers – like in real-estate, location is everything. servers want to be close to clients.
37
Caching example
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 + usecs
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
problem: large queueing delays at high utilization!
Application Layer: 2-38
explain why minutes (recall earlier delay versus arrival rate curve from Chapter 1)
38
Performance:
access link utilization = .97
LAN utilization: .0015
end-end delay = Internet delay +
access link delay + LAN delay
= 2 sec + minutes + usecs
Option 1: buy a faster access link
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
154 Mbps
154 Mbps
.0097
msecs
Cost: faster access link (expensive!)
Application Layer: 2-39
t’
39
Performance:
LAN utilization: .?
access link utilization = ?
average end-end delay = ?
Option 2: install a web cache
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
How to compute link
utilization, delay?
Cost: web cache (cheap!)
local web cache
Application Layer: 2-40
t’
40
Calculating access link utilization, end-end delay with cache:
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!)
Application Layer: 2-41
t’
41
Conditional GET
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:
server: response contains no object if cached copy is up-to-date:
HTTP/1.0 304 Not Modified
HTTP request msg
If-modified-since:
HTTP response
HTTP/1.0
304 Not Modified
object
not
modified
before
HTTP request msg
If-modified-since:
HTTP response
HTTP/1.0 200 OK
object
modified
after
client
server
Application Layer: 2-42
t’
42
HTTP/2
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
Application Layer: 2-43
t’
43
HTTP/2
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
Application Layer: 2-44
t’
44
HTTP/2: mitigating HOL blocking
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
Application Layer: 2-45
t’
45
HTTP/2: mitigating HOL blocking
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
Application Layer: 2-46
shortest job first: decreased average delay!
46
HTTP/2 to HTTP/3
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
Application Layer: 2-47
t’
47
Application layer: overview
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: 2-48
48
E-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., 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
Application Layer: 2-49
49
E-mail: mail servers
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
Application Layer: 2-50
50
SMTP RFC (5321)
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
Application Layer: 2-51
51
Scenario: Alice sends e-mail to Bob
1) Alice uses UA to compose e-mail message “to”
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
Application Layer: 2-52
52
Sample SMTP interaction
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM: < >
S: 250 … Sender ok
C: RCPT TO: < >
S: 250 … 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: 2-53
53
SMTP: observations
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
Application Layer: 2-54
54
Mail message format
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
Application Layer: 2-55
55
Retrieving email: mail access protocols
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
Application Layer: 2-56
56
Application Layer: Overview
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: 2-57
57
DNS: Domain Name System
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”
Application Layer: 2-58
58
DNS: services, structure
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
Application Layer: 2-59
59
Thinking about the DNS
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
Application Layer: 2-60
60
DNS: a distributed, hierarchical database
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
…
…
…
…
Application Layer: 2-61
61
DNS: root name servers
official, contact-of-last-resort by name servers that can not resolve name
Application Layer: 2-62
62
DNS: root name servers
official, contact-of-last-resort by name servers that can not 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)
Application Layer: 2-63
63
Top-Level Domain, and authoritative servers
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
Application Layer: 2-64
64
Local DNS name servers
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
Application Layer: 2-65
65
DNS name resolution: iterated query
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
Application Layer: 2-66
66
DNS name resolution: recursive query
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
Application Layer: 2-67
67
Caching DNS Information
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!
Application Layer: 2-68
68
DNS records
DNS: distributed database storing resource records (RR)
type=NS
name is domain (e.g., foo.com)
value is hostname 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 “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
Application Layer: 2-69
69
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
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
Application Layer: 2-70
70
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
Application Layer: 2-71
71
Getting your info into the DNS
example: new startup “Network Utopia”
register name networkuptopia.com at DNS registrar (e.g., Network Solutions)
provide names, IP addresses of authoritative name server (primary and secondary)
registrar inserts NS, A RRs into .com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A)
create authoritative server locally with IP address 212.212.212.1
type A record for www.networkuptopia.com
type MX record for networkutopia.com
Application Layer: 2-72
72
DNS security
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
Application Layer: 2-73
73
Application Layer: Overview
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: 2-74
74
mobile network
home network
enterprise
network
national or global ISP
local or regional ISP
datacenter
network
content
provider
network
Peer-to-peer (P2P) architecture
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)
Application Layer: 2-75
75
Introduction: 1-76
File distribution: client-server vs P2P
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
76
Introduction: 1-77
File distribution time: client-server
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}
77
File distribution time: P2P
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 …
Application Layer: 2-78
78
Client-server vs. P2P: example
client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
Application Layer: 2-79
79
P2P file distribution: BitTorrent
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
Application Layer: 2-80
80
P2P file distribution: BitTorrent
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
Application Layer: 2-81
81
BitTorrent: requesting, sending file chunks
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
Application Layer: 2-82
82
BitTorrent: tit-for-tat
(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 !
Application Layer: 2-83
83
Application layer: overview
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: 2-84
84
Video Streaming and CDNs: context
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
Application Layer: 2-85
85
Multimedia: video
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
Application Layer: 2-86
86
Multimedia: video
……………………..
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:
MPEG 1 (CD-ROM) 1.5 Mbps
MPEG2 (DVD) 3-6 Mbps
MPEG4 (often used in Internet, 64Kbps – 12 Mbps)
Application Layer: 2-87
87
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
Streaming stored video
simple scenario:
video server
(stored video)
client
Internet
Application Layer: 2-88
88
Streaming stored video
video
recorded (e.g., 30 frames/sec)
2. video
sent
Cumulative 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)
Application Layer: 2-89
89
Streaming stored video: challenges
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
Application Layer: 2-90
90
Streaming stored video: playout buffering
constant bit
rate video
transmission
Cumulative 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
Application Layer: 2-91
91
Streaming multimedia: DASH
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
Application Layer: 2-92
92
…
…
…
Streaming multimedia: DASH
“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
?
Application Layer: 2-93
93
Content distribution networks (CDNs)
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
Application Layer: 2-94
94
Content distribution networks (CDNs)
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
Application Layer: 2-95
95
…
…
…
…
…
…
subscriber requests content, service provider returns manifest
Content distribution networks (CDNs)
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
Application Layer: 2-96
96
Akamai: 100,000+ servers in 1000+ clusters in 1000+ networks in 70+ countries serving trillions of requests a day.
How many people use Netflix?
…
…
…
…
…
…
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?
OTT: “over the top”
Content distribution networks (CDNs)
Application Layer: 2-97
97
peak load: 7million viewers, 2 Tbytes via
Application Layer: Overview
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: 2-98
98
Socket programming
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
Application Layer: 2-99
99
Socket programming
Two socket types for two transport services:
UDP: unreliable datagram
TCP: reliable, byte stream-oriented
Application Example:
client reads a line of characters (data) from its keyboard and sends data to server
server receives the data and converts characters to uppercase
server sends modified data to client
client receives modified data and displays line on its screen
Application Layer: 2-100
100
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
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
Application Layer: 2-101
101
Client/server socket interaction: UDP
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
Application Layer: 2-102
102
Example app: UDP client
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
Application Layer: 2-103
103
Example app: UDP server
Python UDPServer
from socket import *
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
Application Layer: 2-104
104
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
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
Application Layer: 2-105
105
Client/server socket interaction: TCP
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
Application Layer: 2-106
106
Example app: TCP client
from socket import *
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
Application Layer: 2-107
107
Example app: TCP server
from socket import *
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)
Application Layer: 2-108
108
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
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
Application Layer: 2-109
109
Chapter 2: Summary
Most importantly: learned about protocols!
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(payload) being communicated
important themes:
centralized vs. decentralized
stateless vs. stateful
scalability
reliable vs. unreliable message transfer
“complexity at network edge”
Application Layer: 2-110
110
Application Layer: 2-111
Additional Chapter 2 slides
Version History
7.01 (January 2020)
All slides reformatted for 16:9 aspect ratio
Use of Calibri font, rather that Gill Sans MT
Updated slide content for 2020 link technologies and applications
Add more animation throughout
New Master slide
111
Sample SMTP interaction
Application Layer: 2-112
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM: < >
S: 250 … Sender ok
C: RCPT TO: < >
S: 250 … 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
Retired this slide, since telnet isn’t really used anymore, and few non https: mail servers around anymore….
112
CDN content access: a closer look
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
Application Layer: 2-113
This slide retired, since there’s the Netflix example which is visually better
113
Case study: Netflix
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
Application Layer: 2-114
This slide retired, since there’s the new Netflix example which is visually better
114
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1.5
2
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3
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05101520253035
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P2P
Client-Server
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26 26
27 27
28 28
29 29
30 30
31 31
32 32
P2P
Client-Server
N
Minimum Distribution Time
0.1
0.1
0.1666666667
0.2
0.2307692308
0.3
0.2857142857
0.4
0.3333333333
0.5
0.375
0.6
0.4117647059
0.7
0.4444444444
0.8
0.4736842105
0.9
0.5
1
0.5238095238
1.1
0.5454545455
1.2
0.5652173913
1.3
0.5833333333
1.4
0.6
1.5
0.6153846154
1.6
0.6296296296
1.7
0.6428571429
1.8
0.6551724138
1.9
0.6666666667
2
0.6774193548
2.1
0.6875
2.2
0.696969697
2.3
0.7058823529
2.4
0.7142857143
2.5
0.7222222222
2.6
0.7297297297
2.7
0.7368421053
2.8
0.7435897436
2.9
0.75
3
0.756097561
3.1
0.7619047619
3.2
Sheet1
L P2P Client-Server ratio factor
1 0.1 0.1 1 10
2 0.1666666667 0.2
3 0.2307692308 0.3
4 0.2857142857 0.4
5 0.3333333333 0.5
6 0.375 0.6
7 0.4117647059 0.7
8 0.4444444444 0.8
9 0.4736842105 0.9
10 0.5 1
11 0.5238095238 1.1
12 0.5454545455 1.2
13 0.5652173913 1.3
14 0.5833333333 1.4
15 0.6 1.5
16 0.6153846154 1.6
17 0.6296296296 1.7
18 0.6428571429 1.8
19 0.6551724138 1.9
20 0.6666666667 2
21 0.6774193548 2.1
22 0.6875 2.2
23 0.696969697 2.3
24 0.7058823529 2.4
25 0.7142857143 2.5
26 0.7222222222 2.6
27 0.7297297297 2.7
28 0.7368421053 2.8
29 0.7435897436 2.9
30 0.75 3
31 0.756097561 3.1
32 0.7619047619 3.2
33 0.7674418605 3.3
34 0.7727272727 3.4
35 0.7777777778 3.5
36 0.7826086957 3.6
37 0.7872340426 3.7
38 0.7916666667 3.8
39 0.7959183673 3.9
40 0.8 4
41 0.8039215686 4.1
42 0.8076923077 4.2
43 0.8113207547 4.3
44 0.8148148148 4.4
45 0.8181818182 4.5
46 0.8214285714 4.6
47 0.8245614035 4.7
48 0.8275862069 4.8
49 0.8305084746 4.9
Sheet1
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
P2P
Client-Server
N
Minimum Distribution Time
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Sheet2
Sheet3