Download - Chapter 2
Chapter 2
The Application Layer
Goals of this Chapter
• To understand common application protocols work– Web (http)– Email (smtp)– FTP– DNS– P2P– DHT (distributed hash table)
• To understand how the design alternatives for application layer networking protocols– A network application runs on many hosts, it is a distributed
application– This chapter discusses several designs of distributed
applications
Road Map
• Application networking basics• Web• Email• FTP• DNS• P2P• DHP
Road Map
• Application networking basics• Web• Email• FTP• DNS• P2P• DHT
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
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
An app-layer networking 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
Public-domain protocols:• defined in RFCs• allows for
interoperability• e.g., HTTP, SMTP
Proprietary protocols:• e.g., Skype
Which application gets a newly arriving packet?
Web server
SSH server
SSH client
Web browser
Skype
IM
Operating System
TransportNetwork
Link LayerLink Layer
Physical Layer
Applications
Dest IP: 74.125.115.99
IP: 74.125.115.99
IP: 128.174.13.63
Transport layer multiplexing: TCP
TCP port 80Web server
app
socket
OperatingSystem
Network
Link layer
OperatingSystem
Web browser
app
TCP port 23421
Network
Link layer
I would like to communicate
with 74.125.115.99
port 80
I would like to accept
communication on port 80
Dest IP: 74.125.115.99Source IP: 128.174.13.63Dest port: 80Source port: 23421
IP: 74.125.115.99
•An application is identified by the hosts IP addresses, transport protocol, and ports•A TCP connection is identified by the pair of IPs, the pair of ports, and the transport protocol
IP: 128.174.13.63
Transport layer multiplexing: TCP
TCP port 80Web server
app
socket
OperatingSystem
Network
Link layer
OperatingSystem
Web browser
app
TCP port 23421
Network
Link layer
I would like to communicate
with 74.125.115.99
port 80
IP: 74.125.115.99
•An application is identified by the hosts IP addresses, transport protocol, and ports•A TCP connection is identified by the pair of IPs, the pair of ports, and the transport protocol
Dest IP: 128.174.13.63Source IP: 74.125.115.99Dest port: 23421Source port:80
socket
IP: 128.174.13.63
Transport layer multiplexing: TCP
TCP port 80Web server
app
socket
OperatingSystem
Network
Link layer
OperatingSystem
Web browser
app
TCP port 23421
Network
Link layer
IP: 74.125.115.99
socket
•An application is identified by the hosts IP addresses, transport protocol, and ports•A TCP connection is identified by the pair of IPs, the pair of ports, and the transport protocol
socketDest IP: 74.125.115.99Source IP: 128.174.13.63Dest port: 80Source port: 23421
IP: 128.174.13.63
Transport layer multiplexing: TCP
TCP port 80Web server
app
socket
OperatingSystem
Network
Link layer
Web browser
app
TCP port 23421
socket
Network
Link layer
OperatingSystem
I would like to send data:
XXSFGFEWRV
Dest IP: 74.125.115.99Source IP: 128.174.13.63Protocol: TCPDest port: 80Source port: 23421Data: XXSFGFEWRV
Dest IP: 74.125.115.99Source IP: 128.174.13.63Protocol: TCPDest port: 80Source port: 23421Data: XXSFGFEWRV
Dest IP: 74.125.115.99Source IP: 128.174.13.63Protocol: TCPDest port: 80Source port: 23421Data: XXSFGFEWRVData: XXSFGFEWRV
Network
TCP port 80
Dest IP: 74.125.115.99Source IP: 128.174.13.63Protocol: TCPDest port: 80Source port: 23421Data: XXSFGFEWRV
IP: 74.125.115.99
socket
•An application is identified by the hosts IP address, transport protocols, and port•A TCP connection is identified by the pair of IPs, the pair of ports, and the transport protocol
IP: 128.174.13.63
Transport layer multiplexing: UDP
IM server
app
socket
OperatingSystem
Network
Link layer
socket
OperatingSystem
IM clientapp
UDP port 23421
Network
Link layer
I would like to send/receive
data over UDP port 23421
I would like to receive any data
on UDP port 1401
IP: 74.125.115.99
UDP port 1401
•An application is identified by the hosts IP address, transport protocols, and port
IP: 128.174.13.63
Transport layer multiplexing: UDP
IMServer
app
socket
OperatingSystem
Network
Link layer
socket
OperatingSystem
IM clientapp
UDP port 23421
Network
Link layer
Send data to 74.125.115.99
port 1401Data: xxadre
Dest IP: 74.125.115.99Source IP: 128.174.13.63Protocol: UDPDest port: 1401Source port: 23421Data: xxadre
IP: 74.125.115.99
•An application is identified by the hosts IP address, transport protocols, and port
UDP port 1401
Data: xxadre
Transport layer multiplexing
TCP• Applications accept new
connections based on the destination port
– An client app that would like to communicate a server app must know
• the IP of the host that is running the server app and
• the port on which the server app is listening
• When a connection is created, a socket is made.
– Data is sent and received over this socket
– This socket is identified by two pairs of IP-port, and the transport layer protocol, i.e., the tuple (IP, port, IP, port, transport layer)
UDP• The application is identified by the
port on which the application is listening.
• The application can use the source IP and port to further multiplex
Project 1 – Send a Message via TCP and UDP
• UDP– Client
• Set up socket
• Send message
• Wait for reply
• If no reply comes, give up
• If reply comes, print it
– Server• Set up socket
• Wait for message
• When message arrives– Print message– Send reply
• TCP– Client
• Set up socket
• Send message
• Wait for reply
• If no reply comes, give up
• If reply comes, print it
– Server• Set up socket
• Wait for connection
• When connect arrives, get socket for connection
• Wait for message over connection socket
• When message arrives– Print message
– Send reply
• Make client program to send message to server and then wait for message from server
• Make server program to wait for message from client and then respond with a message
Steps in Visual Studio
• Open visual studio
• File -> new ->project– Win32 console application
• Select name and directory
– Leave defaults• Console app
• Uncheck empty project
• Checked precompiled header
– Paste code from web page into source code
• Build->build solution– The output window is in the lower
middle frame
– Might need to adjust the frames to see the output window
– There will be many warnings, but, hopefully, no errors
– the bottom of the output gives the directory where the program is located
• Open two command windows• Change to the directory where the
program is located• Run the program in each command
windows– Testudp 1
testudp 0
• Start the server first (the one with 1 as an argument
What transport service does an app need?
Data reliability• some apps (e.g., audio) can
tolerate some loss• other apps (e.g., file transfer,
telnet) require 100% reliable data transfer
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 “useful” (i.e., in order for the user to gain utility)
• other apps (“elastic apps”) make use of whatever throughput they get
Security• Encryption, data integrity, …
Transport service requirements of common apps
Application
file transfere-mail
Web documentsreal-time audio/video
stored audio/videointeractive gamesinstant messaging
Data loss
no lossno lossno lossloss-tolerant
loss-tolerantloss-tolerantno loss
Throughput
elasticelasticsome what elasticaudio: 5kbps-1Mbpsvideo:10kbps-5Mbpssame as above few kbps upelastic
Time Sensitive
nononot reallyyes, 100’s msec
yes, few secsyes, 100’s msecyes and no
Internet transport protocols services
TCP service:• connection-oriented: setup
required between client and server processes
• reliable transport between sending and receiving process
• does not provide: timing, minimum throughput guarantees, or even when packets are transmitted
• flow control: sender won’t overwhelm receiver
• congestion control: throttle sender when network overloaded
UDP service:• No connection set-up
needed• unreliable data transfer
between sending and receiving process
• Packets can be sent at any rate/time desired (but this might be cause considerable congestion)
• does not provide: flow control, congestion control, timing, throughput guarantee, or security
Internet apps: application, transport protocols
Application
e-mailremote terminal access
Web file transfer
streaming multimedia
Internet telephony
Applicationlayer protocol
SMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]HTTP (eg Youtube), RTP [RFC 1889]SIP, RTP, proprietary(e.g., Skype)
Underlyingtransport protocol
TCPTCPTCPTCPTCPUDP
typically UDP
Road Map
• Application basics• Web• Email• FTP• DNS• P2P• DHT
Web and HTTP• Web page consists of objects• Object can be HTML file, JPEG image, Java applet, audio file,…• Web page consists of base HTML-file which includes several
referenced objects• The browser first requests the base file• The base file specifies text and URLs of objects• The browser requests these objects, where ever they are (not
always on the same server)• HTTP is used to request the base file and all the other files• Note, that HTTP can be used for other applications besides web• Each object is addressable by a URL• Example URL:
www.someschool.edu/someDept/pic.gif
host name path name
HTTP overview
HTTP: hypertext transfer protocol
• Web’s application layer protocol
• client/server model– client: browser that
requests, receives, “displays” Web objects
– server: Web server sends objects in response to requests
PC runningExplorer
Server running
Apache Webserver
Mac runningNavigator
HTTP request
HTTP request
HTTP response
HTTP response
HTTP overview (continued)
Uses TCP:• client initiates TCP connection
(creates socket) to server, port 80
• server accepts TCP connection from client
• HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)
• TCP connection closed
HTTP is “stateless”• server maintains no
information about past client requests
Protocols that maintain “state” are complex!
• past history (state) must be maintained
• if server/client crashes, their views of “state” may be inconsistent, must be reconciled
aside
HTTP connections
Nonpersistent HTTP• At most one object is
sent over a TCP connection.
Persistent HTTP• Multiple objects can
be sent over single TCP connection between client and server.
Nonpersistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index
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
(contains text, references to 10
jpeg images)
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.
filereceived
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
time time
Non-Persistent HTTP: Response time
Definition of RTT: time for a small packet to travel from client to server and back.
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
total = 2RTT + data transmit time
10 objects require
20RTT+10 data transmit times
Persistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index
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
(contains text, references to 10
jpeg images)
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects, requests these 10 references
4. HTTP server sends each of the 10 objects.
base received
time to transmit file
initiate TCP connection
RTT
requestfile
RTT
(Non) Persistent HTTP: Response time
time to transmit file
RTT
requestfile
RTT
initiate TCP connection
10 objects require 20RTT+10 data transmit times
Non-persistent
base received
time to transmit file
initiate TCP connection
RTT
requestfile
RTT
time to transmit file
RTT
Request files
10 objects require 3RTT+10 data transmit times
Persistent
Object 1 received
Object 1 received
Object 2 received
(non) Persistent HTTP
• Advantages of persistent HTTP are only valid if the objects are the same server.
– Usually some objects are on the server, but many are not
• Instead of using a single persistent HTTP connection, a browser could use many non-persistent connections in parallel
– This is a bit unfair.
• As far as I know, persistent HTTP is usually not supported by the server or browser
base received
time to transmit file
initiate TCP connection
RTT
requestfile
RTT
time to transmit file
RTT
request 5 files
RTT
Initiate 5 TCP connection
Non-persistent
Object 1 receivedObject 2 received
Object 3 receivedObject 4 receivedObject 5 received
HTTP request message
• two types of HTTP messages: request, response• HTTP request message:
– ASCII (human-readable format)
GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr
(extra carriage return, line feed)
request line(GET, POST,
HEAD commands)
header lines
Carriage return, line feed
indicates end of message
HTTP request message: general format
HTTP response message
HTTP/1.1 200 OK Connection closeDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ...
status line(protocol
status codestatus phrase)
header lines
data, e.g., requestedHTML file
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
(Location:)
400 Bad Request– request message not understood by server
404 Not Found– requested document not found on this server
505 HTTP Version Not Supported
In first line in server->client response message.
A few sample codes:
Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
Opens TCP connection to port 80(default HTTP server port) at www.eecis.udel.edu.Anything typed in sent to port 80 at www.eecis.udel.edu
telnet www.eecis.udel.edu 80
2. Type in a GET HTTP request:
GET / HTTP/1.1Host: www.eecis.udel.edu
By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server
3. Look at response message sent by HTTP server!
Wireshark (ethereal)
• Wireshark captures all packets that pass through the hosts interface• To run Wireshark , libpcap (linux) or winpcap (windows) must be installed. It
comes with wireshark package• Then, run wireshark• Select Capture• Find the active interface
– E.g., not generic dialup, nor vnp, nor packet scheduler, but wireless …. With IP address
– Then select prepare– Let’s watch TCP packets on port 80
• Next to capture filter, enter TCP port 80– Select update in realtime and autoscroll– Might need to enable or disable “capture in promiscuous mode”– Press start– Press close
• Load www.eecis.udel.edu page in browser• Press stop in Wireshark • Find http request to 128.4.40.10.
– Right click and select follow TCP stream
User-server state: cookies
Many major Web sites use cookies
Four components:1) cookie header line of HTTP
response message2) cookie header line in HTTP
request message3) cookie file kept on user’s
host, managed by user’s browser
4) back-end database at Web site
Example:• Susan always access
Internet always from PC• visits specific e-commerce
site for first time• when initial HTTP
requests arrives at site, site creates: – unique ID– entry in backend
database for ID
Cookies: keeping “state” (cont.)client
server
usual http response msg
usual http response msg
cookie file
one week later:
usual http request msgcookie: 1678 cookie-
specificaction
access
ebay 8734usual http request msg Amazon server
creates ID1678 for user create
entry
usual http response Set-cookie: 1678
ebay 8734amazon 1678
usual http request msgcookie: 1678 cookie-
spectificaction
accessebay 8734amazon 1678
backenddatabase
Cookies (continued)
What cookies can bring:• authorization• shopping carts• recommendations• user session state (Web
e-mail)
Cookies and privacy: cookies permit sites to learn a
lot about you you may supply name and e-
mail to sites
aside
How to keep “state”: protocol endpoints: maintain state at
sender/receiver over multiple transactions cookies: http messages carry state
Web Serving Systems• LAMP Stack – very popular
– Linux (OS)– Apache (web sever)
• Receives http request and generates http response
• The generation of response can involve many steps
• Other servers are also popular: – nginx – open source, reverse proxy, load balancer, popularity: apache, microsoft, nginx, google– lighttpd – open source, small and fast. Good for high load. E.g., youtube, meeboo. Can handle
1000 hits per sec
– MySQL or MariaDB (mySQL is oracle. MariaDB is a branch of mySQL, but not under oracle’s control)
• Open source database• Very popular• PostgreSQL is also very popular• New, noSQL (not only SQL), databases are also playing a role
– Casandra– mongoDB
– Php• Php scripts make the html that is delivered by the web server to the client
• Other application– Perl– Python– Java (with a tomcat server)
• Facebook converts php to C and save 30% in speed (which means 30% in servers, which is a huge amount of money)
Simple LAMP Topology
Faster Topology
Apache+PHP+APCApache+PHP+APC
MySQLMySQL
SquidSquid
memcachedmemcached
Web cache: holds recently requested pages and generate response if the desired page is in cache. Otherwise, the request is forward to the web server
Cache for sql queries. A giant hash table. Give it a string (key) and if the response is in cache, it gets it. Otherwise, the request is sent to database and the result is also saved the response in cacheThe key can be anything. The programmer decides.
APC is a cache for compiled php code
MySQL Replication
MySQL MySQL mastermaster slaveslave slaveslave
•All writes are to the master •Reads are from the master or the slaves•There is a slight delay from when the master is updated and the update is reflected by all slaves
Where’d my edit go???
Data Sharding MySQL
MySQL group s1MySQL group s1 English-language Wikipedia
Next 19 biggest wikisMySQL group s2MySQL group s2
MySQL group s3MySQL group s3 Next 764 wikis
Load BalancerLoad Balancer
Apache Apache Apache
tomcat tomcat tomcat tomcat tomcat tomcat
•1/3 of HTTP request to each server•Might keep request with the same cookie to the same server (sticky sessions)•Might decrypt SSL
Also load balances
Load balancers also check if machines are healthy and will stop sending requests if they seem unhealthy
Content distribution networksEven with a very fast server architecture, RTT is still large to some users
Locations of Amazon’s “cloudFront” servers
CDNs•allow you to put parts of your web page (e.g., logo, javascripts, audio, video, multicast live video) on servers that are close to the client•Act as web proxy. See next slides•If the content is always unique and changing (facebook), then the design of the CDN is more complicated
CDN example: AWS CloudFront
• cloudFront machines are scattered around the world and are close to most people– Close in terms of RTT
• Documents and media can be cached in a cloud– If the document is not in the CloudFront machine, the request is forwarded to the permanent
storage/original server and given to the user and saved in cache
• E.g., – Create a distribution for the image in cloudfront, and get a new url, e.g., http://mydomain.cloudfront.net– If image is at http://mydomain.com/images/pic1.jpg– Your web pages should refer to http://mydomain.cloudfront.net/images/pic1.jpg not to
http://mydomain.com/images/pic1.jpg
• A request to mydomain.cloudfront.com should go to the nearest cloudfront server farm.
– How?
• If the cache document changes, then different cloudfront machines will have different versions. Eventually they will be updated, usually within a few minutes, depending on the timeout value set
Web caches (proxy server)
• user sets browser: Web accesses via cache
• browser sends all HTTP requests to cache– object in cache: cache
returns object – else cache requests
object from origin server, then returns object to client
Goal: reduce network utilization by satisfying client request without involving original server
client
Proxyserver
client
HTTP request
HTTP response
HTTP request HTTP request
origin server
origin server
HTTP response HTTP response
More about Web caching
• cache acts as both client and server
• typically cache is installed by ISP (university, company, residential ISP, e.g., satellite-based ISP) or as part of a CDN
Why Web caching?• reduce response time for
client request• reduce traffic on an
institution’s access link.• Internet dense with
caches: enables “poor” content providers to effectively deliver content (similar objective as P2P file sharing)
Caching example
Assumptions• average object size = 100,000 bits• avg. request rate from institution’s
browsers to origin servers = 15/sec
• delay from institutional router to any origin server and back to router = 2 sec
Consequences• utilization on LAN = 15%• utilization on access link = 100%• total delay = Internet delay + access
delay + LAN delay
= 2 sec + minutes + milliseconds
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
Caching example (cont)
possible solution• increase bandwidth of access
link to, say, 10 Mbps
consequence• utilization on LAN = 15%
• utilization on access link = 15%
• Total delay = Internet delay + access delay + LAN delay
= 2 sec + msecs + msecs
• often a costly upgrade
originservers
public Internet
institutionalnetwork 10 Mbps LAN
10 Mbps access link
institutionalcache
Caching example (cont)
possible solution: install cache
• suppose hit rate is 0.4
consequence• 40% requests will be satisfied
almost immediately• 60% requests satisfied by origin
server• utilization of access link reduced
to 60%, resulting in negligible delays (say 10 msec)
• total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
Conditional GET
• Goal: don’t send object if cache has up-to-date cached version
• cache: specify date of cached copy in HTTP requestIf-modified-since:
<date>
• server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not
Modified
cache server
HTTP request msgIf-modified-since:
<date>
HTTP responseHTTP/1.0
304 Not Modified
object not
modified
HTTP request msgIf-modified-since:
<date>
HTTP responseHTTP/1.0 200 OK
<data>
object modified
Road Map
• Application basics• Web• FTP• Email• DNS• P2P• DHT
FTP: the file transfer protocol
• transfer file to/from remote host• client/server model
– client: side that initiates transfer (either to/from remote)– server: remote host
• ftp: RFC 959• ftp server: listens on port 21
file transfer FTPserver
FTPuser
interface
FTPclient
local filesystem
remote filesystem
user at host
FTP is weird: separate control and data connections• FTP client contacts FTP server at port 21,
TCP is transport protocol• client authorized over control connection
– This is done in “clear text” (i.e., unencrypted)– So if some one if sniffing packets, your
password might be learned.– Sniffing packets is difficult on ethernet,
encrypted wifi, and DSL, but is possible on cable modems, and unencrypted wifi
• client browses remote directory by sending commands over control connection.
• Data is transferred over different connections. Two approaches
– Active– Passive
FTPclient
FTPserver
TCP control connection
port 21
TCP data connectionport 20
• Active– The client opens a TCP socket with
on some port (port number >1024)– The client sends the server the port– The server connects to the client’s
port where the servers source port is 20
• Active mode is a problem for firewalls
– If my desktop is not a server, it should not receive any requests for connections.
– But FTP servers will make such a requests
FTP Passive mode
• When a file is to be transferred, the server opens a port (number>1024 and not 20)
• The server sends this port number information over the command connection
• The client connects to the servers over this port.
FTPclient
FTPserver
TCP control connection
port 21
TCP data connectionhigh port
• Drawback of passive– Some enterprises (companies) like to
control which applications are used• E.g., web browsing is ok, but skype is
not
– One way to do this is to block out going connections based on the port.
– However, this will cause FTP to fail, unless the device that blocks connections is smart
Road Map
• Application basics• Web• FTP• Email• DNS• P2P• DHT
Email Protocol Design• Basic assumption: weak user agents and strong mail servers
– The user wants to send the mail and leave– The user wants to get the mail– The user may come and go whenever (e.g., roaming laptop)– It should be possible to send mail between users even if neither user is online at the same time.– We conclude that there must be a middle man/mail server.
• Servers are not that strong: The protocol must be as robust as possible to servers being offline – No single server – why
• Single point of failure• The server would have to be too big (congestion)
– We conclude that there should be many mail servers
• Two types of hosts– Users– Mail servers
• Each user has a mail box in its mail server– Users retrieve mail from their mail server at their convenience
• Users give mail to their mail servers to deliver the mail• Mail servers communicate with
– The users that have mail boxes in the server– Other mail servers
useragent
mailserver
mailserver user
agent
Email Protocol Design• Two types of hosts
– Users– Mail servers
• Each user has a mail box in its mail server– Users retrieve mail from their mail server at there convenience
• Users give mail to their mail servers to deliver the mail• Mail servers communicate with
– The users that have mail boxes in the server– Other mail servers
useragent
mailserver
mailserver user
agent
User composes mail and sends it to its mail server (or a mail server that will send mail for it)
Mail server finds the destination mail server and attempts to send the mail
Destination user requests emails from mailbox
Destination server gives mails to user
Email Protocol Design• Two types of hosts
– Users– Mail servers
• Each user has a mail box in its mail server– Users retrieve mail from their mail server at there convenience
• Users give mail to their mail servers to deliver the mail• Mail servers communicate with
– The users that have mail boxes in the server– Other mail servers
useragent
mailserver
mailserver user
agent
User composes mail and sends it to its mail server (or a mail server that will send mail for it)
Mail server finds the destination mail server and attempts to send the mail
Destination user requests emails from mailbox
Destination server gives mails to user
SMTP SMTP POP3IMAP…
Electronic Mail: Details
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., Eudora, Outlook, elm, Mozilla Thunderbird
• Put outgoing on server (with SMTP)
• Get incoming messages from server
user mailbox
outgoing message queue
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
Electronic Mail: mail servers
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
• Reliable: several attempts and provide notification if delivery fails
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
Electronic Mail: SMTP [RFC 2821]
• uses TCP to reliably transfer email message from client to server, port 25
• direct transfer: sending server to receiving server• Emails are pushed to servers (but users pull messages from
servers)• three phases of transfer
– handshaking (greeting)– transfer of messages– closure
• command/response interaction– commands: ASCII text– response: status code and phrase
• messages must be in 7-bit ASCII– Makes it difficult to send attachments
Scenario: Alice sends message to Bob
1) Alice uses UA to compose message and “to” [email protected]
2) Alice’s UA sends message to her mail server; message placed in message queue
3) Client side of SMTP opens TCP connection with Bob’s mail server
4) SMTP client sends Alice’s message over the TCP connection
5) Bob’s mail server places the message in Bob’s mailbox
6) Bob invokes his user agent to read message
useragent
mailserver
mailserver user
agent
1
2 3 4 56
Sample SMTP interaction
S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... 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
Client connects to server
Try SMTP interaction for yourself:
• telnet mail.eecis.udel.edu 25• see 220 reply from server
• enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands
above lets you send email without using email client (reader)
SMTP: final words
• 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: pull• SMTP: push
• both have ASCII command/response interaction, status codes
• HTTP: each object encapsulated in its own response msg
• SMTP: multiple objects sent in multipart msg
Mail access
• POP3 and IMAP are two protocols for access mail on a mail server
• Web-based mail works differently, the web mail server and the mail server can be integrated, so that there is no user agent.
Mail access protocols
• SMTP: delivery/storage to receiver’s server• Mail access protocol: retrieval from server
– POP: Post Office Protocol [RFC 1939]• authorization (agent <-->server) and download
– IMAP: Internet Mail Access Protocol [RFC 1730]• more features (more complex)• manipulation of stored msgs on server
– HTTP: gmail, Hotmail, Yahoo! Mail, etc.
useragent
sender’s mail server
useragent
SMTP SMTP accessprotocol
receiver’s mail server
Road Map
• Application basics• Web• FTP• Email• DNS• P2P• DHT
DNS – domain name system
• Change names, like www.yahoo.com into IP address.• Services provided by DNS
– Name to address translation– Host aliasing
• A host relay1.west-coast.yahoo.com could have two aliases, yahoo.com and www.yahoo.com.
• In this case, the canonical hostname is relay1.west-coast.yahoo.com. • DNS can provide canonical host names
– Mail server aliasing• When a mail server wants to send a mail to [email protected], it does not send it to
www.udel.edu, but to mail.udel.edu. Or maybe udmail.udel.edu. DNS can translate udel.edu to mail.udel.edu
– (Cheap) Load distribution • Cnn.com has several servers.• DNS will respond with all address, • but it will reorder the addresses every time.• If the client uses the first address listed, then each client will use different servers.
– Check out Namespace design in http://blogs.technet.com/b/exchange/archive/2014/04/21/the-preferred-architecture.aspx
» The idea is that MS Exchange can be set up to use multiple servers, so a single email is held on multiple servers. Then the question is, how to decide which server to use, i.e.., load balancing. What does this document say?
• Content distribution networks (CDN) are better ways of load balancing
DNS - structure
• Centralized DNS?– Pros – somewhat easy to maintain (there is only one
system). But it must always be online– Cons
• Single point of failure (the system crashes -> no web)• Congestion• Server would be far from some hosts (delay)• Database would be too big• The register bohacek-pc1.pc.udel.edu would require
interacting with the big server
• Instead, a distributed hierarchical database is used.
Domain Hierarchy
edu com gov mil org net uk in
UDel upenn yahoo cisco whitehouse nasa navy arpa acm
eecis art
bohacek_pc1 bohacek_pc10
Administrative Zones in the Domain Hierarchy
edu comgov mil org net uk in
UD upenn yahoo ciscowhitehouse nasa navy arpa acm
eecis art
bohacek_pc1 bohacek_pc10
root
It is possible that .edu and .gov are administered togetherNote that UD administers art but not eecisSome times a single service provider will administer the domains for a large number of .coms
Root servers
• Each layer in the hierarchy knows about the domain names below it• The highest level is the root.
– There are 13 root “servers”
– Each of these servers is actually several servers, and some of the machines that comprise a server are distributed geographically.
13 root name servers worldwide
b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA
e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)
i Autonomica, Stockholm (plus 28 other locations)
k RIPE London (also 16 other locations)
m WIDE Tokyo (also Seoul, Paris, SF)
a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)
Overview
• Top-level domain (TLD) servers– There are around 200 top-level domains– These include com, edu, mil, info, in, uk, cn, – Currently,
• network solutions maintains the TLD servers for com
• Educause maintains the TLD servers for edu
– The root servers know the addresses and names of all top level servers
• Organizations have a hierarchy of DNS servers
DNS queries
• Suppose a host needs the IP address of bohacek-pc1.eecis.udel.edu• If this IP address is not in cache, the host asks its local DNS server.• If the DNS server does not have it in cache, it checks if is had the IP address of the
DNS server of eecis.udel.edu in cache• If not, it checks if IP address of the dns server of udel.edu in cache• If not, it check if it has the IP address of the top-level domain server of edu in cache• It not, it asks the root server for the IP address of the edu TLD server
– The DNS server always has the IP address of the root servers• The local DNS server asks the edu TLD server for address of bohack-
pc1.eecis.udel.edu. • The TLD server does not know that IP address, but instead gives the IP address of
the dns server for UD• The local DNS server asks the UD dns server for the address of bohack-
pc1.eecis.udel.edu.• The UD dns server does not know the address, but instead returns the address of the
eecis dns server.• The local DNS server asks the eecis dns server for the address of bohacek-
pc1.eecis.udel.edu• Eecis dns server replies with the address.• This address is returned to the host that orginally asked the question.
DNS Queries
Browser wants to show www. eecis.udel.edu
Browser needs the IP address of www. eecis.udel.edu
Host asks local DNS server for IP address of www. eecis.udel.edu
• Local DNS server checks if it has the IP address of www.eecis.udel.edu in cache.
• If not, it checks if is had the IP address of the DNS server of eecis.udel.edu in cache
• If not, it checks if IP address of the dns server of udel.edu in cache
• If not, it check if it has the IP address of the top-level domain server of edu in cache
• .if not, …..
What is the IP address of www.eecis.udel.edu?
Root server (IP address are always known)
Root server does not know. Instead, it responds with dns server that might, specifically, the TLD server for .edu
What is the ip address of www.eecis.udel.edu?
TLD server for .edu
TLD server does not know. Instead replies with the name and IP address of the UD DNS server
What is the ip address of www.eecis.udel.edu?
UD dns server does not know. Instead it replies with the name and IP address of the eecis dns server.
What is the ip address of www.eecis.udel.edu?
It is 128.4.1.2
It is 128.4.1.2
Browser wants to show
www.eecis.udel.edu
DNS Queries
Browser needs the IP address of
www.eecis.udel.edu Host asks local DNS server for IP
address of www.eecis.udel.ed
u
1. Local DNS server checks if it has the IP address of www.eecis.udel.edu in cache.
2. If not, it checks if is had the IP address of the DNS server of eecis.udel.edu in cache
3. If not, it checks if it has the IP address of the DNS server of udel.edu in cache
4. If not, it checks if it has the IP address of the top-level domain server of edu in cache
5. .if not, …..
What is the IP address of www.eecis.udel.edu?
Root server (IP addresses are always known)
Root server does not know. Instead, it responds with name and address of a
server that might, specifically, the TLD server
for .eduWhat is the IP address of
www.eecis.udel.edu?TLD server for .edu
TLD server does not know. Instead replies with the name and IP address of
the UDel DNS server
What is the ip address of www.eecis.udel.edu?
UDel DNS server does not know. Instead it replies with the name and IP address of the eecis dns server.
What is the IP address of www.eecis.udel.edu?
It is 128.4.1.2
It is 128.4.1.2
UD DNS server
eecis DNS server
Browser wants to show
www.eecis.udel.edu
DNS Queries
Browser needs the IP address of
www.eecis.udel.edu Host asks local DNS server for IP
address of www.eecis.udel.ed
u
1. Local DNS server checks if it has the IP address of www.eecis.udel.edu in cache.
2. If yes, then return it
It is 128.4.1.2
Browser wants to show
www.eecis.udel.edu
DNS Queries
Browser needs the IP address of
www.eecis.udel.edu Host asks local DNS server for IP
address of www.eecis.udel.ed
u
1. Local DNS server checks if it has the IP address of www.eecis.udel.edu in cache.
2. If not, it checks if is had the IP address of the DNS server of eecis.udel.edu in cache
3. If yes, query it…
What is the IP address of www.eecis.udel.edu?
It is 128.4.1.2
It is 128.4.1.2
eecis DNS server
Browser wants to show
www.eecis.udel.edu
DNS Queries
Browser needs the IP address of
www.eecis.udel.edu Host asks local DNS server for IP
address of www.eecis.udel.ed
u
1. Local DNS server checks if it has the IP address of www.eecis.udel.edu in cache.
2. If not, it checks if is had the IP address of the DNS server of eecis.udel.edu in cache
3. If not, it checks if it has the IP address of the DNS server of udel.edu in cache
4. If not, it checks if it has the IP address of the top-level domain server of edu in cache
5. .if so, then query it…
What is the IP address of www.eecis.udel.edu?
TLD server for .edu
TLD server does not know. Instead replies with the name and IP address of
the UD DNS server
What is the ip address of www.eecis.udel.edu?
UD DNS server does not know. Instead it replies with the name and IP address of the eecis dns server.
What is the IP address of www.eecis.udel.edu?
It is 128.4.1.2
It is 128.4.1.2
UD DNS server
eecis DNS server
International domains (ccTLD)
• www.videos.cnn.com.cn• A root server will provide the name and address of the .cn dns
server.– However, this dns server could be able to answer com.cn or cnn.com.cn
• It is possible that .cn (china) has its own TLD and .com.cn is a subdomain
• However, usually, com.cn is the TLD– But it does not have to be this way
• We usually call cnn.com.cn the second level domain, even though it is strictly a third level domain
• For performance (i.e., reducing the number of DNS servers with which the local DNS must communicate), it is best if the dns server for .cn can answer cnn.com.cn
• International top-level domains can be identified by two letters, e.g., .cn, and also in language-native script such as arabic or chinese characters
Attack on DNS
• Hackers have tried to bring down DNS by performing a DoS on the root servers– DoS – denial of service. Sends more
packets or requests for service than the server can accommodate. Resulting in poor service for normal users.
• This failed because– There are many very strong root servers and have
firewalls/filters• The attacks used ICMP ping packets• DNS requests would have been more effective
– It is rare that a root server is needed• Usually only the TLD server is needed• Or only a domain server.
DNS Message Details
• DNS Record– (Name, Value, Type, Class, TTL)– If Type = A
• Name is the host name• Value is the IP address of the host
– If Type = NS• Name is a domain name• Value is the name of the DNS server for the domain• E.g., (udel.edu, dns.udel.edu, NS, …, …)
– Type = MX• Name is the domain name• Value is the name of the mail server for the domain• E.g., (udel.edu, mail.udel.edu, MX, …, …)
– Type = CName• Name is a host name• Value is the canonical name of the host• E.g., (www.yahoo.com, relay-east.yahoo.com, CName, …, …)
– TTL is the time to live, so DNS caches can be timed out– Class is no longer used, it is set as IN
DNS query
• (Name, Type, Class)
• (UDel.edu, MX, IN)– Please provide the name of the UD’s mail
server
• (mail.UDel.edu, A, IN)– Please provide the IP address for mail.udel.edu
DNS message format
DNS protocol : query and reply messages, both with same message format
msg header• identification: 16 bit #
for query, reply to query uses same #
• flags:– query or reply– recursion desired – recursion available– reply is authoritative
DNS message format
Name, type fields for a query
RRs in responseto query
records forauthoritative servers
additional “helpful”info that may be used
Browser wants to show
www.eecis.udel.edu
Browser needs the IP address of
www.eecis.udel.edu
DNS Queries
1. Local DNS server checks if it has the IP address of www.eecis.udel.edu in cache.
2. If not, it checks if is had the IP address of the DNS server of eecis.udel.edu in cache
3. If not, it checks if it has the IP address of the DNS server of udel.edu in cache
4. If not, it checks if it has the IP address of the top-level domain server of edu in cache
5. .if not, …..
Root server (IP addresses are always known)
TLD server for .edu
UD DNS server
eecis DNS server
1 00 0
(www.eecis.udel.edu, A,IN)
1 00 0
(www.eecis.udel.edu, A,IN)
0 00 4
(edu, edu-serverA.net, NS, IN)
(edu-serverA.net, 124.5.1.1, A, IN)
(edu, edu-serverB.net, NS, IN)
(edu-serverB.net, 124.5.1.2, A, IN)
1 00 0
(www.eecis.udel.edu, A,IN)
0 00 4
(udel.edu, dns2.udel.edu, NS, IN)
(udel.edu, dns2.udel.edu, 128.178.2.2, A, IN)
(udel.edu, dns1.udel.edu, NS, IN)
(dns1.udel.edu, 128.173.2.1, A, IN)
1 00 0
(www.eecis.udel.edu, A,IN)
0 00 4
(eecis.udel.edu, dns1.eecis.udel.edu, NS, IN)
(dns1.eecis.udel.edu, 128.4.1.10, A, IN)
(eecis.udel.edu, dns2.udel.edu, NS, IN)
(dns2.udel.edu, 128.4.1.11, A, IN)
1 00 0
(www.eecis.udel.edu, A,IN)
0 10 0
(www.eecis.udel.edu, 128.4.1.1, A, IN)
0 10 0
(www.eecis.udel.edu, 128.4.1.1, A, IN)
DNS Header and Flags
• The DNS header has a query ID– The query has this ID and the server copies this ID into the
response
• Flag indicating query or answer• Flag indicating whether the server is the authoritative
server for the answer (as oppose to a cached answer)• A recursive desired flag indicating that the host/server
would like the server to perform the recursive DNS lookup
• A recursive available flag indicating whether the server is available to to the recursive lookup
Exploring DNS with dig
• Dig is installed on stimpy and perhaps other eecis linux machines• A windows version of dig is also available• E.g.,
– >> dig udel.edu
– Returns information about the dns entry for udel
• E.g., – >>dig @dns.eecis.udel.edu udel.edu
– Returns information about the dns entry for udel that is stored in dns.eecis.udel.edu
• E.g.,– >> dig edu
– Returns a list of TLD edu servers
• E.g.,– >> dig
– Returns list of root servers
• E.g.,– >> dig udel.edu MX
– Returns the mail server for udel.edu
DNS
• Which transport protocol should DNS use?
• Why?
Captive Portal Problem• Use Udel Wifi with a new machine.• Instead of google.com, I get some UD web page that requires log in• I log in, and it says that I should reboot.• I don’t reboot, but when I go to google.com, I still get UD login page.• However, if I go to some other page (e..g, NYTimes.com, it works fine)• What is going on? Why would reboot fix it? What else would fix the
problem
• Answer: when a new machine connects to UD wifi, all dns request return the IP of UD login page. After logged in, DNS works correctly. However, my machine’s DNS cache will still point to UD login page, until I reboot.
• DNS cache– Local DNS server cache– DNS cache on machine (this machine)
• Ipconfig /displaydns
DNS Attack
clientLocal DNS
server attackerBoA DNS
server
Request IP of www.BoA.com
Request IP of www.BoA.com
IP of www.BoA.com
1.1.1.1IP of www.BoA.com
1.1.1.1
TCP+HTTP connection to 1.1.1.1
HTTP:: data: “welcome to BoA, please enter your acct# and password
•Client request IP address of BoA from local DNS server•Local DNS server request address from BoA DNS server•Before BoA’s server can respond, the attacker sends a response•The DNS server sends this response to the client,•The client then accesses the incorrect web page, which appears exactly the same as BoA’s web page
Note, this attack has nothing to do with the security of your machine.
DNS Attack: really?client
Local DNSserver attacker
BoA DNSserver
Request IP of www.BoA.com
Request IP of www.BoA.com
IP of www.BoA.com
1.1.1.1IP of www.BoA.com
1.1.1.1TCP+HTTP connection to 1.1.1.1
HTTP:: data: “welcome to BoA, please enter your acct# and password
•How does the attacker know the exact time to send the fake DNS reply?•It does not, instead the attacker continuously sends replies•But what information is contained in the DNS response that the attacker must guess correctly?
•Source IP address of BoA DNS server•There are not that many
•The local DNS’s server’s source port•The destination port is always 53, but sometimes the source port is also 53, or changes in a predictable way, e.g., increments
•The DNS query Id•This is typically incremented
The attacker, which is a DNS server as well, makes DNS request that cause the DNS server to request to itself. In this way, the attacker can determine various numbers
DNS Attack: really?client
Local DNSserver attacker
BoA DNSserver
Request IP of www.BoA.com
Request IP of www.BoA.com
IP of www.BoA.com
1.1.1.1IP of www.BoA.com
1.1.1.1TCP+HTTP connection to 1.1.1.1
HTTP:: data: “welcome to BoA, please enter your acct# and password
•How does the attacker know the exact time to send the fake DNS reply?•It does not, instead the attacker continuously sends replies•But what information is contained in the DNS response that the attacker must guess correctly?
•Source IP address of BoA DNS server•There are not that many
•The local DNS’s server’s source port•The destination port is always 53, but sometimes the source port is also 53, or changes in a predictable way, e.g., increments
•The DNS query Id•This is typically incremented
By adding randomness to the source port and query Id, the DNS attack becomes very difficult.Still, it only needs to succeed once to gain information.Perhaps it will take months or years, but it might work
DNS poisoning
• A DNS response might include additional responses, e.g., for BoA, but with the incorrect IP.
• This additional response is cached• E.g., send email stating that pictures of naked people can be
found at www.evil.com• But dns.evil.com includes additional records• Solution: ignore this type of additional records
• DNSSec solves the known security problems with DNS
Good DNS “attack”
• OpenDNS keeps track of bad web sites and will return a different IP address when the web address for these bad web sites is requested
Road Map
• Application basics• Web• FTP• Email• DNS• P2P• DHT
Peer-to-peer file sharing
• About P2P– 30% or more of the bytes transferred on the Internet are from
P2P users– Skype is a very successful P2P VoIP app
• Written in 3-4 months
• Topics covered– Scalability– P2P querying– BitTorrent
Pure P2P architecture• Review: What is the difference
between peer-to-peer and client/server?
– Each hosts acts as both a server and a client.
• no always-on server• arbitrary end systems directly
communicate• peers are intermittently
connected and may change IP addresses
• Pure P2P has significant drawbacks.
• P2P-like systems with some central servers are more common.
• But in all cases, the file transfer is between peers, not from servers.
peer-peer
File Distribution: Server-Client vs P2P
Question : How much time to distribute file from one server to N peers?
us
u2d1 d2
u1
uN
dN
Server
Network (with abundant bandwidth)
File, size F
us: server upload bandwidth
ui: peer i upload bandwidth
di: peer i download bandwidth
File distribution time: server-client
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
F
• Time for the server to send a copy to a single client– F/us
• Time for the server send N copies:– NF/us time
• client i takes F/di time to download
increases linearly in N(for large N)
= dcs = max { NF/us, F/min(di) }i
Time to distribute F to N clients using
client/server approach
File distribution time: P2P
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
F• server must send one copy:
– F/us time
• client i download time – F/di
• Total data to be downloaded– NF
• fastest possible transfer rate: us + ui
dP2P = max { F/us, F/min(di) , NF/(us + ui) }i
Can you make a schedule for the download the take this amount?
P2P schedule
• Important: we model data flow as a continuous stream of data, not packets or even bytes
• Assume each host has the same upload data rate, uc
• Server delivers a chunk of size F/N to each host– The server sends chunks to each of the N host
simultaneously
– Duration = F/us
• As data arrives from the server, each host delivers the chunk to all N-1 hosts– Client’s total upload rate is uc
– So each client gets data at rate uc/(N-1)
– Duration to send its chunk (of size F/N) to all of the other hosts
• = (F/N) / (uc/(N-1)) = F/ uc N/(N-1)
• Duration = max(F/us , F/uc N/(N-1))
Server
client client client
us/Nus/N us/N
us
uc/(N-1)
uc/(N-1)uc/(N-1)
• Scheme– Server send chunk of size zi to host i
– Host i send this chunk to all other N-1 hosts• The download to the host runs simultaneous with the upload to the other hosts
• Objective: each host completes its upload at the same time– This a difficult since each host has a different upload data rate
• Let T be the time for each host to send the chunk to all N-1 other hosts. And assume that T is shorter than the time it takes for the server to deliver the file to all hosts (the server is the bottleneck)
• Then T = (N-1) zi / ui
• Solve for zi we get, zi = ui T/(N-1)
• F = Sum zi
• F = sum ui T/(N-1) = T/(N-1) sum ui
• T/(N-1) = F/sum(ui)
• zi =F ui /sum(ui)
• Duration from server = F/ us
• Check our assumption, is F/ us > T ?
– Is F/ us > F (N-1)/sum(ui)
– Is us < sum(ui)/(N-1)
– Is N us – us < sum(ui)
– Is N us < us + sum(ui)
– Our assumption is true if us < (us + sum(ui))/N, or us < sum(ui))/(N-1)
• Suppose that us > sum(ui))/(N-1), then the server could send more data than the previous scheme
• Scheme– The server gives host i chunk of size zi.
– Host i sends this chunk to all other hosts
– Server also sends chunk of size zs to all hosts
• Objective, find zi and zs so that each host and the server finish at the same time
• Let T be the duration of download.
• T = (N-1) zi / ui
– Or zi = T/(N-1) ui
• T = (F-zs)/ us + N zs / us
– Or zs = (T-F/ us)/(N-1)* us
• Must have F = sum(zi) + zs
• Which implies, F = sum(T/(N-1) ui) + (T-F/ us)/(N-1) us
• Which implies F = T/(N-1)(us + sum(ui)) -F/(N-1)
• NF = T (us + sum(ui))
• T = NF/ (us + sum(ui))
• Check that zs >=0
– Is (T-F/ us)/(N-1)* us >0
– Is (T-F/ us)>0
– Is NF/ (us + sum(ui)) > F/ us
– Is (us + sum(ui)) /N < us, yes
File distribution time: P2P
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
F• server must send one copy:
– F/us time
• client i download time – F/di
• Total data to be downloaded– NF
• fastest possible transfer rate: us + ui
dP2P = max { F/us, F/min(di) , NF/(us + ui) }i
Can you make a schedule for the download the take this amount?
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25 30 35
N
Min
imu
m D
istr
ibut
ion
Tim
e P2P
Client-Server
Server-client vs. P2P: exampleClient upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
Conclusion: P2P systems are scalable. But the load is distributed to all users, so P2P users have more load than clients in the client-server model.
Project 2
Peer-to-peer querying• While the file is transferred from the peer, how to find the file• Options
– Centralize directory• Napster• Single point of failure• Congestion
– Server would be the performance bottleneck
• Target for the RIAA• Always up (as oppose to user machine that goes up and down)• Easy to find• Easy protocol
– Query flooding• Gnutella• Hosts find other host and forms a network of neighbors (overlay network) • Search for a file (covered on the next slide)• How to set up the network – bootstrap?
– Have a central list of peers– Have distributed lists of peers– Search out a peer by scanning (Project 2)
• Flood the network to answer query
Flooding Search
Osearcher
Flooding Search
Osearcher
Searcher: Send a message to all neighbors that searcher is looking for file xyz
Flooding Search
Osearcher
Receive the search message and respond if they have the file, otherwise, …
Flooding Search
Osearcher
Every host that received message: Send a message to all neighbors that searcher is looking for file xyz
Flooding Search
Osearcher
Receive the search message and respond if they have the file, otherwise, …
Flooding Search
Osearcher
Every host that received message: Send a message to all neighbors that searcher is looking for file xyz
Flooding Search
Osearcher
Every host that received message: Send a message to all its neighbors that searcher is looking for file xyz
Flooding Search
Osearcher
Received message for the first time
Received message again,do not retransmit message
Querying Flooding State Diagram
User Request for File
wait
Generate Id
Send request message(with message Id)
Originator of search
Querying Flooding State Diagram
wait
Request arrives
Get message Id
Have seen request before
Check for file in directory
Send response to peer that requested file
File is in local dir
Send request to all neighbors
Listening peer
Querying FloodNodes that don’t have the file
Nodes that do have the file
Don’t flood the entire network when searching.
Expanding Ring Search
Osearcher
destination
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=1
Time To Live
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=2
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=2
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=2
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=3
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=3
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=3
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=3
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=4
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=4
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=4
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=4
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=4
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait TO seconds
1. If answer arrives, then exit3.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
Expanding Ring Search
Osearcher
destination
TTL=4
Originator:0. set K=11.Generate ID and message with TTL=K2.Wait K*TO seconds
1. If answer arrives, then exit3.If K>max_K, exit4.K=K+1, go to 1
Peer: 1.Wait for message2.If message Id is old, go to 13.Check is query can be answer,
1. If so, send answer to originator and go to 1
4.Message TTL—5.If TTL>0, send message to all neighbors, go to 1
(hierarchical peer-to-peer network)• KaZaA
• Not all peers are equal – super peers (?)– Super peers (group leaders) have higher bit-rate connections, are more stable,
etc.
• Peers connect to group leaders
• The group leaders keep a list of files shared by all their children.
• group leaders connect to a small number of other group leaders
• A child host will ask its group leader for a file, if the group leader does not know where it is, it will flood the network of group leaders. The response from other group leaders follows a reverse path to the asking group leader (so other leader can cache the response)
• A file is identified with a ID (e.g., MD5) that can take a string (e.g., file) and come to a unique ID. A small change in the file causes a large change in the ID. It is not possible to construct two files that have the same ID. The ID is a finger print.
• Since files are ID-ed, multiple copies of the same file can be found and these copies can be downloaded from multiple hosts in parallel.
BitTorrent
• Centralized P2P– A centralized server, or tracker, tracks the clients
involved in the P2P transfer– This is similar to Napster– Companies that host these site get sued and are
attacked by DDoS
• Components of BitTorrent System– Torrent Files– Trackers– Seeders– Peers
• Trackerless is also possible
Torrent File
• Required to download• Can be found on web sites or sent by email• Contains information about the file and the tracker
– Announce: the URL of the tracker– Creation date– Info
• Length of file• Name of file• Length of each piece (except for the last)• Pieces – the 20B SHA-1 value of each piece
• If the download contains multiple files, then a single torrent file will contain information about all files.
Tracker
• Make a HTTP Get request to the tracker specifying the SHA-1 hash of the file to be downloaded– The request also includes the number of bytes
downloaded and the number uploaded– If the client does not upload enough, the tracker might
not provide a reply
• The reply contains– The time when the tracker information should be
refreshed (usually 30 minutes)– A list of the peers that have the file
• IP address and port (usually 6881)• Peer ID
File distribution with BitTorrent
tracker: tracks peers participating in torrent
obtain listof peers
trading chunks
peer
BitTorrent (1)
• file divided into 256KB chunks.• peer joining torrent:
– has no chunks, but will accumulate them over time– registers with tracker to get list of peers, connects to subset of
peers (“neighbors”)• while downloading, peer uploads chunks to other peers. • peers may come and go• once peer has entire file, it may (selfishly) leave or (altruistically)
remain
BitTorrent (2)
Pulling Chunks• at any given time, different
peers have different subsets of file chunks
• periodically, a peer (Alice) asks each neighbor for list of chunks that they have.
• Alice sends requests for her missing chunks– rarest first– So rarest chunks are
spread, and chunks are uniformly common
Sending Chunks: tit-for-tat• Alice sends chunks to four
neighbors currently sending her chunks at the highest rate – re-evaluate top 4 every 10
secs• every 30 secs: randomly select
another peer, starts sending chunks– newly chosen peer may join
top 4– “optimistically unchoke”
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
With higher upload rate, can find better trading partners & get file faster!
BitTorrent Pros/Cons
• Centralized server• Slow to get the transfer started
– Web transfers start much faster and will achieve a sustained rate
• Peers must upload– Some peers might not be in position to upload (e.g.,
mobile phone)• Chunks can be corrupted
– HBO distributed fake chunks– Since the SHA-1 hash does not match what is given
in the Torrent File, the chunk is dropped after it is downloaded
• This wastes bandwidth and can increase download time
Road Map
• Application basics• Web• FTP• Email• DNS• P2P• DHT