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