CS计算机代考程序代写 dns database Java file system cache FTP Advanced Network Technologies

Advanced Network Technologies

Week 2:
Network performance
Network application

School of Computer Science
Dr. Wei Bao | Lecturer

Network Performance:
Throughput

Throughput

› throughput: rate (bits/time unit) at which bits transferred between
sender/receiver

– instantaneous: rate at given point in time

– average: rate over longer period of time

server, with
file of F bits

to send to client

link capacity
Rs bits/sec

link capacity
Rc bits/sec

Throughput (cont’d)

› Rs < Rc What is average end-end throughput? › Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

link on end-end path that constrains end-end throughput
bottleneck link

Rs bits/sec Rc bits/sec

Throughput (cont’d)

Internet Scenario

› per-connection end-end
throughput: min(Rc,Rs,R/10)

10 connections (fairly) share
backbone bottleneck link R bits/sec

Rs

Rs
Rs

Rc

Rc

Rc

R

Bit and byte

› bit: basic unit. “b”

› byte: 8 bits. “B”

› bps: bit per second

› Network/Telecom:
– Kb/Mb/Gb: 103,106,109 bit

– Kbps/Mbps/Gbps: 103,106,109 bit per second

– By default in this course

› File system:
– KB/MB/GB: 210,220,230 byte (1024,10242,10243 byte)

Bit and byte

210 byte

109 bit per second

Network Performance:
Fairness

Network Fairness and Bandwidth Allocation

› Efficiency
› Fairness

› However, they are contradicting!

In reality: two considerations

Network Fairness, Bandwidth allocation

Three flows: A-B, B-C, A-C

A B C

1 Mbps 1 Mbps

Q: How can we allocate the link bandwidths to the three flows?

Network Fairness, Bandwidth allocation

Three flows: A-B, B-C, A-C

A B C

1 Mbps 1 Mbps

0.5 Mbps 0.5 Mbps

0.5 Mbps

Very fair!

However: Network throughput, only 1.5Mbps

Network Fairness, Bandwidth allocation

Three flows: A-B, B-C, A-C

A B C

1 Mbps 1 Mbps

1 Mbps 1 Mbps

0 Mbps

Very unfair!

However: Network throughput, 2Mbps

Fairness

Bottleneck for a flow: The link that limits the data rate of the flow

A B C

1 Mbps 10 Mbps

Bottleneck for A-B, A-C Bottleneck for B-C

Max-min Fairness

› Maximize the minimum
› Try to increase the “poorest” as much as possible

– A richer flow can be sacrificed.
› Try to increase the second “poorest” as much as possible

– A richer flow can be sacrificed.
– A poorer flow cannot be sacrificed.

› Try to increase the third “poorest” as much as possible
› …

Max-min Fairness

› Max-min Fairness criteria: if we want to improve one
flow, we can only achieve this by sacrificing a poorer
or equal flow.

Max-min Fairness

Bottleneck for a flow: The link limits its data rate

A B C

1 Mbps 10 Mbps
0.5 Mbps

0.5 Mbps

9.5 Mbps

Even this is large,
but it does hurt
poorer flows

Bottleneck approach

› 1 Start with all zero flows, potential flow set = {all flows}
› 2 Slowly increase flows in the potential flow set until there is a (new) link

saturated
– “Pouring water in the network”

› 3 Hold fix the flows that are bottlenecked, remove them from the potential
flow set

› 4 If potential flow set is not empty, go to step 2 (still has potential to
increase)

Bottleneck approach

Bottleneck approach

A

B

C

D

Each link between two routes with capacity 1

0

0

0

0

Potential flow set {A, B, C, D}

1/3

1/3
1/3
1/3

Bottleneck!

Bottleneck approach

A

B

C

D

Each link between two routes with capacity 1

1/3

1/3

1/3

1/3

Potential flow set {A}

1/3

1/3
1/3
1/3

Bottleneck!

Bottleneck!

2/3

Bottleneck approach

A

B

C

D

Each link between two routes with capacity 1

2/3

1/3

1/3

1/3

Potential flow set {}

1/3
1/3
1/3

Bottleneck!

Bottleneck!

2/3

Can you solve the following problem?

B

CA

link rate: AB=BC=1, CA=2

Can you solve the following problem?

B

CA

link rate: AB=BC=1, CA=2

demand 1,2,3 =1/3 demand 4 =2/3

demand 5=4/3

More comments

A B C

1 Mbps 1 Mbps

0.5 Mbps 0.5 Mbps

0.5 Mbps

You are using two links. How can we get a same share?

More comment: Max-min fairness is too fair!

More comments

A B C

1 Mbps 1 Mbps

2/3 Mbps 2/3 Mbps

1/3 Mbps

Longer routes are penalized

Another form of fairness
proportional fairness

The Application Layer

Some network applications

› e-mail
› web
› text messaging
› remote login
› P2P file sharing
› multi-user network games
› streaming stored video

(YouTube, Netflix)

› voice over IP (e.g., Skype)
› real-time video conferencing
› social networking
› search
› …
› …

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
transport
network
data link
physical

application
transport
network
data link
physical

application
transport
network
data link
physical

Application architectures

Possible structure of applications

› Client-server
› Peer-to-peer (P2P)

Client-server architecture

server:

› always-on
› permanent IP address
› data centers for scaling

clients:

› communicate with server
› may be intermittently connected
› may have dynamic IP addresses
› do not communicate directly with

each other

client/server

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, as well as new service
demands

› peers are intermittently
connected and change IP
addresses
– complex management

application
transport
network
data link
physical

application
transport
network
data link
physical

application
transport
network
data link
physical

Process 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

client process: process that
initiates communication

server process: process that
waits to be contacted

v aside: applications with P2P
architectures have client
processes & server
processes

clients, servers

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

Addressing processes

› 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…

› to receive messages, process
must have identifier

› host device has unique 32-bit
IP address (or 128 in IPv6)

› 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

App-layer protocol defines

› types of messages exchanged,
– e.g., request, response

› message syntax:
– what fields in messages & how

fields are delineated

– e.g. First line: method. Second line: URL
– message semantics
– meaning of information in fields
– e.g. 404 means “not found”

› rules for when and how processes
send & respond to messages

open protocols:

› defined in RFCs
› allows for interoperability
› e.g., HTTP, SMTP
proprietary protocols:

› e.g., Skype

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
v some apps (e.g.,

multimedia) require
minimum amount of
throughput to be
“effective”

v other apps (“elastic apps”)
make use of whatever
throughput they get

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

› does not provide: timing,
minimum throughput
guarantee

› connection-oriented: setup
required between client and
server processes

UDP service:
› unreliable data transfer

between sending and
receiving process

› does not provide: reliability,
flow control, congestion
control, timing, throughput
guarantee, or connection
setup,

application

e-mail
remote terminal access

Web
file transfer

streaming multimedia

Internet telephony

application
layer protocol

SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
HTTP
RTP [RFC 1889]
SIP, RTP, proprietary
(e.g., Skype)

underlying
transport protocol

TCP
TCP
TCP
TCP
TCP or UDP

TCP or UDP

Internet apps: application, transport protocols

Web and HTTP

Web and HTTP

First, a review…
› web page consists of base HTML-file which includes several referenced

objects

– HTML: HyperText Markup Language

› object can be JPEG image, Java applet, audio file,…
› each object is addressable by a URL (Uniform Resource Locator), e.g.,

www.someschool.edu/someDept/pic.gif

host name path name

Web and HTTP

xxxxxxxxx
www.aaa.edu/Obj1.jpg

yyyyyyyyyyyy
www.aaa.edu/Obj2.jpg

zzzzzzzzz

xxxxxxxxx

yyyyyyyyyyyy

zzzzzzzzz

File: usually base-html file
(HyperText Markup Language) Browser shows

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

PC running
Firefox browser

server
running

Apache Web
server

iPhone running
Safari browser

HTTP requestHTTP response

HT
TP

req
ues

t

HT
TP

res
pon

se

HTTP overview (cont’d)

uses TCP:
› client initiates TCP connection

(creates socket) to server, port
80
– How to know IP address?
– DNS (Domain Name System)
› 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!

v past history (state) must be
maintained

v if server/client crashes, their views
of “state” may be inconsistent,
must be reconciled

aside

HTTP connections

non-persistent HTTP

› at most one object sent
over TCP connection

– connection then closed
› downloading multiple
objects required multiple
connections

persistent HTTP

› multiple objects can be
sent over single TCP
connection between
client, server

Non-persistent HTTP
suppose 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 into TCP connection
socket. Message indicates that client
wants page
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 page, and sends message

(contains text,
references to 10

jpeg images)
www.someSchool.edu/someDepartment/home.index

time

5. HTTP client receives response message
containing html file, displays html. Parsing
html file, finds 10 referenced jpeg objects
to download

4. HTTP server closes TCP connection.

Non-persistent HTTP
suppose 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 into TCP connection
socket. Message indicates that client
wants object
someDepartment/object1.jpg

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

(contains text,
references to 10

jpeg images)
www.someSchool.edu/someDepartment/home.index

time

5. HTTP client receives response message
containing object, displays the object.

6. Steps 1-5 repeated for each of 10 jpeg
objects

4. HTTP server closes TCP connection.

HTTP: response time

RTT (definition): time for a small
packet to travel from client to
server and back

HTTP response time:
› one RTT to initiate TCP

connection
› one RTT for HTTP request and

first few bytes of HTTP response
to return
› file transmission time
› non-persistent HTTP response

time =
2RTT+ file transmission time

time to
transmit
file

initiate TCP
connection

RTT
request
file

RTT

file
received

time time

Persistent HTTP
suppose 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 into TCP connection
socket. Message indicates that client
wants page
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 page, and sends message

(contains text,
references to 10

jpeg images)
www.someSchool.edu/someDepartment/home.index

time

5. HTTP client receives response message
containing html file, displays html. Parsing
html file, finds 10 referenced jpeg objects
to download

TCP is still on

Persistent HTTP
suppose user enters URL:

2. HTTP client sends HTTP request
message into TCP connection
socket. Message indicates that client
wants object
someDepartment/object1.jpg

3. HTTP server receives request message,
forms response message containing
requested object, and sends message

(contains text,
references to 10

jpeg images)
www.someSchool.edu/someDepartment/home.index

time

4. HTTP client receives response message
containing object, displays the object.

Repeated for each of 10 jpeg objects 10 rounds later HTTP server closes TCP
connection.

Non-persistent vs. persistent

OK
Request file

time

time

File

Request obj 1

Obj1

initiate TCP
connection

OK

initiate TCP
connection

Non-persistent

OK
Request file

time

time

File

Request obj 1

Obj1

initiate TCP
connection

Persistent

Persistent HTTP

non-persistent HTTP issues:
› requires 2 RTTs + file

transmission time per object

persistent HTTP:
› 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 + file
transmission time for all the
referenced objects

HTTP request message

› two types of HTTP messages: request, response
› HTTP request message:

– ASCII (human-readable format)

request line
(GET, POST,
HEAD commands)

header
lines

carriage return,
line feed at start
of line indicates
end of header lines

GET /index.html HTTP/1.1\r\n
Host: www-net.cs.umass.edu\r\n
User-Agent: Firefox/3.6.10\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
Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n
Keep-Alive: 115\r\n
Connection: keep-alive\r\n
\r\n

carriage return character
line-feed character

request
line

header
lines

body

method sp sp cr lfversionURL

cr lfvalueheader field name

cr lfvalueheader field name

~~ ~~

cr lf

entity body~~ ~~

HTTP request message: general format

Uploading form input

POST method:
› web page often includes form input
› input is uploaded to server in entity body

GET method

Method types

HTTP/1.0:
› GET
› POST
› HEAD

– asks server to leave
requested object out of
response

HTTP/1.1:
› GET, POST, HEAD
› PUT

– uploads file in entity body
to path specified in URL
field

› DELETE
– deletes file specified in the

URL field

status line
(protocol
status code
status phrase)

header
lines

data, e.g.,
requested
HTML file

HTTP/1.1 200 OK\r\n
Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n
Server: Apache/2.0.52 (CentOS)\r\n
Last-Modified: Tue, 30 Oct 2007 17:00:02

GMT\r\n
ETag: “17dc6-a5c-bf716880″\r\n
Accept-Ranges: bytes\r\n
Content-Length: 2652\r\n
Keep-Alive: timeout=10, max=100\r\n
Connection: Keep-Alive\r\n
Content-Type: text/html; charset=ISO-8859-

1\r\n
\r\n
data data data data data …

HTTP response message

HTTP response status codes

200 OK
– request succeeded, requested object later in this msg
301 Moved Permanently

– requested object moved, new location specified later in this msg
(Location:)

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:

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

usual http request msg Amazon server
creates ID

1678 for user create
entry

usual http response
set-cookie: 1678amazon 1678

usual http request msg
cookie: 1678 cookie-

specific
action

access
amazon 1678

backend
database

Cookies: keeping “state” (cont’d)

User-server state: cookies

many Web sites use cookies
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

Cookies (cont’d)

what cookies can be used for:
› authorization
› shopping carts
› recommendations
› user session state (Web e-mail)

how to keep “state”:
• protocol endpoints: maintain state at sender/receiver over

multiple transactions
• cookies: http messages carry state

Web caches (proxy server)

› user sets browser: Web
accesses via cache

› browser sends all HTTP
requests to cache

› if object in cache:
– then cache returns object
– else cache requests object

from origin server, then
returns object to client

goal: satisfy client request without involving origin server

client

proxy
server

client

HT
TP

req
ues

t

HT
TP

res
pon

se

HTTP request HTT
P re

que
st

origin
server

origin
server

HTTP response HTT
P re

spo
nse

More about Web caching

› Q: Does the cache act as a client or a server?

More about Web caching

› R: cache acts as both
client and server
– server for original requesting

client

– client to origin server

› typically cache is
installed by ISP
(university, company,
residential ISP)

why Web caching?

› reduce response time for
client request

› reduce traffic on an
institution’s access link

origin
servers

public
Internet

institutional
network

1 Gbps LAN

1.54 Mbps
access link

assumptions:
v avg object size: 100K bits
v avg request rate from browsers to origin

servers:15/sec (1.5 Mbps service )
v RTT from institutional router to any

origin server: 2 sec
v access link rate: 1.54 Mbps

consequences:
v LAN utilization: 0.15%
v LANU = avg req rate * size / link

bandwidth
v access link utilization = 99%
v ALU = avg req rate * size / link

bankwidth
v total delay = 2 sec + minutes + usecs

Q: what happens with fatter access link?

problem!

Caching example

assumptions:
v avg object size: 100K bits
v avg request rate from browsers to origin

servers:15/sec
v RTT from institutional router to any

origin server: 2 sec
v access link rate: 1.54 Mbps

consequences:
v LAN utilization: 0.15%
v access link utilization = 99%
v total delay = 2 sec + minutes + usecs

origin
servers

1.54 Mbps
access link

154 Mbps

154 Mbps

msecs

Cost: increased access link speed (not cheap!)

0.99%

public
Internet

institutional
network

1 Gbps LAN

Caching example: fatter access link

institutional
network

1 Gbps LAN

origin
servers

1.54 Mbps
access link

local web
cache

assumptions:
v avg object size: 100K bits
v avg request rate from browsers to origin

servers:15/sec
v RTT from institutional router to any

origin server: 2 sec
v access link rate: 1.54 Mbps

consequences:
v LAN utilization: 0.15%
v access link utilization = 0%
v total delay = usecs

Cost: web cache (cheap!)

public
Internet

Caching example: install local cache

Calculating access link utilization, delay with
cache:

› suppose cache hit rate is 0.4
– 40% requests satisfied at cache,
– 60% requests satisfied at origin

› access link utilization:
– 60% of requests use access link
› average total delay

= 0.6 * (delay from origin servers) +0.4 * (delay
when satisfied at cache)
Link utilization is around 60%, queueing delay is
small enough
= 0.6 (~2.x second) + 0.4 (~usecs)

less than with 154 Mbps link (and cheaper
too!)

origin
servers

1.54 Mbps
access link

public
Internet

institutional
network

1 Gbps LAN

local web
cache

Caching example: install local cache

Conditional GET

› Goal: don’t send object if client
has up-to-date cached version

– no object transmission delay

– lower link utilization

› 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

HTTP request msg
If-modified-since:

HTTP response
HTTP/1.0 200 OK

object
modified

after

client server