chapter€2 application€layer - nyu computer science · · 2005-02-16lecture€3:...

Internet and Intranet Protocols andApplications

Lecture 3:Application Layer 2:Email, DNS and P2P

February 1, 2005Arthur Goldberg

Computer Science DepartmentNew York University

[email protected]

Chapter 2Application Layer

Computer Networking:A Top Down

Approach Featuringthe Internet,3rd edition.

Jim Kurose, KeithRoss

AddisonWesley, July2004.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers).They’re in PowerPoint form so you can add, modify, and delete slides(including this one) and slide content to suit your needs. They obviouslyrepresent a lot of work on our part. In return for use, we only ask thefollowing:q If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, we’d like people to use our book!)q If you post any slides in substantially unaltered form on a www site, thatyou note that they are adapted from (or perhaps identical to) our slides, andnote our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 19962004J.F Kurose and K.W. Ross, All Rights Reserved

3

Chapter 2: Application layer• 2.1 Principles of

network applications• 2.2 Web and HTTP• 2.3 FTP• 2.4 Electronic Mail

– SMTP, POP3, IMAP• 2.5 DNS

• 2.6 P2P file sharing• 2.7 Socket

programming withTCP

• 2.8 Socketprogramming withUDP

• 2.9 Building a Webserver

2: Application Layer 4

Electronic Mail

Three major components:user agentsmail serverssimple mail transferprotocol: SMTP

User Agenta.k.a. “mail reader”composing, editing, readingmail messagese.g., Eudora, Outlook, elm,Netscape Messengeroutgoing, incoming messagesstored on server

user mailbox

outgoingmessage queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Click t

o buy NOW!

PDFXCHANGE

www.docutrack.com Clic

k to buy N

OW!PDFXCHANGE

www.docutrack.com

mailto:[email protected]

http://www.docu-track.com/index.php?page=38



Electronic Mail: mail servers

Mail Serversmailbox contains incomingmessages for usermessage queue of outgoing(to be sent) mail messagesSMTP protocol between mailservers to send emailmessages

client: sending mailserver“server”: receiving mailserver

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 clientto server, port 25direct transfer: sending server to receiving serverthree phases of transfer

handshaking (greeting)transfer of messagesclosure

command/response interactioncommands: ASCII textresponse: status code and phrase

messages must be in 7bit ASCII


Scenario: Alice sends message to Bob1) Alice uses UA to compose

message and “to”[email protected]

2) Alice’s UA sends messageto her mail server; messageplaced in message queue

3) Client side of SMTP opensTCP connection with Bob’smail server

4) SMTP client sends Alice’smessage over the TCPconnection

5) Bob’s mail server places themessage in Bob’s mailbox

6) Bob invokes his user agentto 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

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com









Try SMTP interaction for yourself:

telnet servername 25see 220 reply from serverenter HELO, MAIL FROM, RCPT TO, DATA, QUITcommands

above lets you send email without using email client(reader)


SMTP: final words

SMTP uses persistentconnectionsSMTP requires message(header & body) to be in 7bit ASCIISMTP server usesCRLF.CRLF to determineend of message

Comparison with HTTP:HTTP: pullSMTP: push

both have ASCIIcommand/responseinteraction, status codes

HTTP: each objectencapsulated in its ownresponse msgSMTP: multiple objectssent in multipart msg


Mail message format

SMTP: protocol forexchanging email msgs

RFC 822: standard for textmessage format:header lines, e.g.,

To:From:Subject:

different from SMTPcommands!

bodythe “message”, ASCIIcharacters only

header

body

blankline


Message format: multimedia extensions

MIME: multimedia mail extension, RFC 2045, 2056additional lines in msg header declare MIME contenttype

From: [email protected]: [email protected]: Picture of yummy crepe.MIMEVersion: 1.0ContentTransferEncoding: base64ContentType: image/jpeg

base64 encoded data ....................................base64 encoded data

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com






Mail access protocols

SMTP: delivery/storage to receiver’s serverMail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939]• authorization (agent <> server) and download

IMAP: Internet Mail Access Protocol [RFC 3501]• more features (more complex)• manipulation of stored messages on server

HTTP: Hotmail, Yahoo! Mail, etc.

useragent

sender’s mailserver

useragent

SMTP SMTP accessprotocol

receiver’s mailserver


POP3 protocol

authorization phaseclient commands:

user: declare username

pass: passwordserver responses

+OK

ERR

transaction phase, client:

list: list message numbers

retr: retrieve message bynumber

dele: deletequit

C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off

S: +OK POP3 server readyC: user bobS: +OKC: pass hungryS: +OK user successfully logged on


POP3 (more) and IMAPMore about POP3

Previous example uses“download and delete”mode.Bob cannot reread email if he changesclient“Downloadandkeep”:copies of messages ondifferent clientsPOP3 is statelessacross sessions

IMAPKeep all messages inone place: the serverAllows user toorganize messages infoldersIMAP keeps user stateacross sessions:

names of folders andmappings betweenmessage IDs and foldername


Chapter 2: Application layer

2.1 Principles ofnetwork applications2.2 Web and HTTP2.3 FTP2.4 Electronic Mail

SMTP, POP3, IMAP2.5 DNS

2.6 P2P file sharing2.7 Socket programmingwith TCP2.8 Socket programmingwith UDP2.9 Building a Webserver

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




DNS: Domain Name System

People: many identifiers:SSN, name, passport #

Internet hosts, routers:IP address (32 bit) used for addressingdatagrams“name”, e.g.,ww.yahoo.com used byhumans

Q: map between IPaddresses and name ?

Domain Name System:distributed databaseimplemented in hierarchy ofmany name serversapplicationlayer protocolhost, routers, name servers tocommunicate to resolve names(address/name translation)

note: core Internetfunction, implemented asapplicationlayer protocolcomplexity at network’s“edge”


DNSWhy not centralize DNS?

single point of failuretraffic volumedistant centralized databasemaintenance

doesn’t scale!

DNS servicesHostname to IP addresstranslationIP address to HostnametranslationHost aliasing

Canonical and alias namesMail server aliasingLoad distribution

Replicated Web servers:set of IP addresses forone canonical nameBut performs poorly –suppose Aol cached one IPaddress


Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS serversyahoo.com

DNS serversamazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approx:Client queries a root server to find com DNSserverClient queries com DNS server to get amazon.comDNS serverClient queries amazon.com DNS server to get IPaddress for www.amazon.com


DNS: Root name serverscontacted by local name server that cannot resolve nameroot name server:

contacts authoritative name server if name mapping not knowngets mappingreturns mapping to local name server

84 root nameservers worldwide

See www.rootservers.orgb USCISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto,CA (and 17 other locations)

i Autonomica, Stockholm (plus 3other locations)

k RIPE London (also Amsterdam,Frankfurt)

m WIDE Tokyo

a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 11 locations)

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com

http://www.amazon.com;

http://www.amazon.com

http://www.root-servers.org




TLD and Authoritative Servers

Toplevel domain (TLD) servers: responsiblefor com, org, net, edu, etc, and all toplevelcountry domains uk, fr, ca, jp

Network solutions maintains servers for com TLDEducause for edu TLD

Authoritative DNS servers: organization’sDNS servers, providing authoritativehostname to IP mappings for organization’sservers (e.g., Web and mail)

Can be maintained by organization or serviceprovider


Local Name Server

Does not strictly belong to hierarchyEach ISP (residential ISP, company,university) has one

Also called “default name server”When a host makes a DNS query, query issent to its local DNS server

Acts as a proxy, forwards query into hierarchy


requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

78

TLD DNS server

Example

Host at cis.poly.eduwants IP address forgaia.cs.umass.edu

2. Find gaia.cs.umass.edu3. Find gaia.cs.umass.edu4. List of IPs for TLD

servers for edu5. Find gaia.cs.umass.edu6. IP for authoritative

server for umass.edu7. Find gaia.cs.umass.edu8. IP for gaia.cs.umass.edu9. IP for gaia.cs.umass.edu



gaia.cs.umass.edu

root DNS server


1

2

45

6


7

8

TLD DNSserver

3

Recursive queriesrecursive query:

Name server findsanswerputs burden of nameresolution oncontacted nameserverheavy load?

iterated query:contacted serverreplies with name ofserver to contact“I don’t know thisname, but ask thisserver”

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




DNS: caching and updating records

When a name server learns a mapping, it cachesthe mapping

A server discards cached entries after a timeout(typically 2 days)TLD servers typically cached in local name servers

• Thus root name servers queried infrequently

update/notify mechanisms under design by IETFRFC 2136www.ietf.org/html.charters/dnsindcharter.html


DNS recordsDNS: distributed db storing resource records (RR)

Type=NSname is domain (e.g.foo.com)

value is IP address ofauthoritative nameserver for this domain

RR format: (name, value, type, ttl)

Type=Aname is hostname

value is IP address

Type=CNAMEname is alias name for some“cannonical” (the real) name

www.ibm.com is really servereast.backup2.ibm.com

value is cannonical nameType=MX

value is name of mailserverassociated with name


DNS protocol, messagesDNS protocol : query and reply messages, both with

same message format

msg headeridentification: 16 bit #for query, reply to queryuses same #flags:

query or replyrecursion desiredrecursion availablereply is authoritative


DNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com

http://www.ietf.org/html.charters/dnsind-charter.html

http://www.ibm.com




Inserting records into DNS

Example: just created startup “Network Utopia”Register name networkuptopia.com at a registrar(e.g., Network Solutions)

Need to provide registrar with names and IP addresses ofyour authoritative name server (primary and secondary)Registrar inserts two RRs into the com TLD server:

(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)

Put in authoritative server Type A record forwww.networkuptopia.com and Type MX record fornetworkutopia.comHow do people get the IP address of your Web site?



2.1 Principles ofnetwork applications

app architecturesapp requirements

2.2 Web and HTTP2.4 Electronic Mail




P2P file sharing

ExampleAlice runs P2P clientapplication on hernotebook computerIntermittentlyconnects to Internet;gets new IP addressfor each connectionAsks for “Hey Jude”Application displaysother peers that havecopy of Hey Jude.

Alice chooses one ofthe peers, Bob.File is copied fromBob’s PC to Alice’snotebook: HTTPWhile Alice downloads,other users uploadingfrom Alice.Alice’s peer is both aWeb client and atransient Web server.

All peers are servers =highly scalable!


P2P: centralized directory

original “Napster” design1) when peer connects, it

informs central server:IP addresscontent

2) Alice queries for “HeyJude”

3) Alice requests file fromBob

centralizeddirectory server

peers

Alice

Bob

1

1

1

12

3

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com

http://www.networkuptopia.com




P2P: problems with centralized directory

Single point of failurePerformancebottleneckCopyrightinfringement

file transfer isdecentralized, butlocating content ishighly decentralized


Query flooding: Gnutella

fully distributedno central server

public domain protocolmany Gnutella clientsimplementing protocol

overlay network: graphedge between peer Xand Y if there’s a TCPconnectionall active peers andedges is overlay netEdge is not a physicallinkGiven peer willtypically be connectedwith < 10 overlayneighbors


Gnutella: protocol

Query

QueryHit

Query

Query

QueryHit

Query

Query

QueryHit

File transfer:HTTP Query message

sent over existing TCPconnections peers forward

Query message QueryHit

sent overreversepath

Scalability:limited scopeflooding


Gnutella: Peer joining

1. Joining peer X must find some other peer inGnutella network: use list of candidate peers

2. X sequentially attempts to make TCP with peerson list until connection setup with Y

3. X sends Ping message to Y; Y forwards Pingmessage.

4. All peers receiving Ping message respond withPong message

5. X receives many Pong messages. It can thensetup additional TCP connections

Peer leaving: see homework problem!

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Questions about Gnutella, 1

What are ‘littleendian’ and ‘bigendian’?Why does the protocol have to specifythem?Unique identifiers: How are uniqueDescriptor IDs and Servent Identifiersgenerated?The spec says (p 3, para 2) “if a serventbecomes out of synch with its input stream,it should drop the connection”. How wouldit know?



In the section ‘Descriptor Routing’ on page 5, thespec says “Pong descriptors may only be sent alongthe same path that carried the incoming Pingdescriptor” and “Push descriptors may only be sentalong the same path that carried the incomingQueryHit descriptor.” How would this beimplemented?In the section ‘Firewalled Servents’ the spec says“A servent can request a file push by routing aPush request back to the servent that sent theQueryHit descriptor describing the target file.”How is this possible? Isn’t the latter serventbehind a firewall?


Exploiting heterogeneity: KaZaA

Each peer is either agroup leader or assignedto a group leader.

TCP connection betweenpeer and its group leader.TCP connections betweensome pairs of groupleaders.

Group leader tracks thecontent in all itschildren.

ordinary peer

groupleader peer

neighoring relationshipsin overlay network


KaZaA: Querying

Each file has a hash and a descriptorClient sends keyword query to its groupleaderGroup leader responds with matches:

For each match: metadata, hash, IP addressIf group leader forwards query to othergroup leaders, they respond with matchesClient then selects files for downloading

HTTP requests using hash as identifier sent topeers holding desired file

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Kazaa tricks

Limitations on simultaneous uploadsRequest queuingIncentive prioritiesParallel downloading

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com



Internet and Intranet Protocols andApplications

Lecture 3:Application Layer 2:Email, DNS and P2P

February 1, 2005Arthur Goldberg

Computer Science DepartmentNew York University

[email protected]

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




2

Chapter 2Application Layer

Computer Networking:A Top Down

Approach Featuringthe Internet,3rd edition.

Jim Kurose, KeithRoss

AddisonWesley, July2004.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers).They’re in PowerPoint form so you can add, modify, and delete slides(including this one) and slide content to suit your needs. They obviouslyrepresent a lot of work on our part. In return for use, we only ask thefollowing:q If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, we’d like people to use our book!)q If you post any slides in substantially unaltered form on a www site, thatyou note that they are adapted from (or perhaps identical to) our slides, andnote our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 19962004J.F Kurose and K.W. Ross, All Rights Reserved

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com



3

Chapter 2: Application layer• 2.1 Principles of

network applications• 2.2 Web and HTTP• 2.3 FTP• 2.4 Electronic Mail

– SMTP, POP3, IMAP• 2.5 DNS

• 2.6 P2P file sharing• 2.7 Socket

programming withTCP

• 2.8 Socketprogramming withUDP

• 2.9 Building a Webserver

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Electronic Mail

Three major components:user agentsmail serverssimple mail transferprotocol: SMTP

User Agenta.k.a. “mail reader”composing, editing, readingmail messagese.g., Eudora, Outlook, elm,Netscape Messengeroutgoing, incoming messagesstored on server

user mailbox

outgoingmessage queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Electronic Mail: mail servers

Mail Serversmailbox contains incomingmessages for usermessage queue of outgoing(to be sent) mail messagesSMTP protocol between mailservers to send emailmessages

client: sending mailserver“server”: receiving mailserver

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Electronic Mail: SMTP [RFC 2821]

uses TCP to reliably transfer email message from clientto server, port 25direct transfer: sending server to receiving serverthree phases of transfer

handshaking (greeting)transfer of messagesclosure

command/response interactioncommands: ASCII textresponse: status code and phrase

messages must be in 7bit ASCII

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Scenario: Alice sends message to Bob1) Alice uses UA to compose

message and “to”[email protected]

2) Alice’s UA sends messageto her mail server; messageplaced in message queue

3) Client side of SMTP opensTCP connection with Bob’smail server

4) SMTP client sends Alice’smessage over the TCPconnection

5) Bob’s mail server places themessage in Bob’s mailbox

6) Bob invokes his user agentto read message

useragent

mailserver

mailserver user

agent

1

2 3 4 56

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com





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

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com








Try SMTP interaction for yourself:

telnet servername 25see 220 reply from serverenter HELO, MAIL FROM, RCPT TO, DATA, QUITcommands

above lets you send email without using email client(reader)

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




SMTP: final words

SMTP uses persistentconnectionsSMTP requires message(header & body) to be in 7bit ASCIISMTP server usesCRLF.CRLF to determineend of message

Comparison with HTTP:HTTP: pullSMTP: push

both have ASCIIcommand/responseinteraction, status codes

HTTP: each objectencapsulated in its ownresponse msgSMTP: multiple objectssent in multipart msg

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Mail message format

SMTP: protocol forexchanging email msgs

RFC 822: standard for textmessage format:header lines, e.g.,

To:From:Subject:

different from SMTPcommands!

bodythe “message”, ASCIIcharacters only

header

body

blankline

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Message format: multimedia extensions

MIME: multimedia mail extension, RFC 2045, 2056additional lines in msg header declare MIME contenttype

From: [email protected]: [email protected]: Picture of yummy crepe.MIMEVersion: 1.0ContentTransferEncoding: base64ContentType: image/jpeg

base64 encoded data ....................................base64 encoded data

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com






Mail access protocols

SMTP: delivery/storage to receiver’s serverMail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939]• authorization (agent <> server) and download

IMAP: Internet Mail Access Protocol [RFC 3501]• more features (more complex)• manipulation of stored messages on server

HTTP: Hotmail, Yahoo! Mail, etc.

useragent

sender’s mailserver

useragent

SMTP SMTP accessprotocol

receiver’s mailserver

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




POP3 protocol

authorization phaseclient commands:

user: declare username

pass: passwordserver responses

+OK

ERR

transaction phase, client:

list: list message numbers

retr: retrieve message bynumber

dele: deletequit

C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off

S: +OK POP3 server readyC: user bobS: +OKC: pass hungryS: +OK user successfully logged on

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




POP3 (more) and IMAPMore about POP3

Previous example uses“download and delete”mode.Bob cannot reread email if he changesclient“Downloadandkeep”:copies of messages ondifferent clientsPOP3 is statelessacross sessions

IMAPKeep all messages inone place: the serverAllows user toorganize messages infoldersIMAP keeps user stateacross sessions:

names of folders andmappings betweenmessage IDs and foldername

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com





2.1 Principles ofnetwork applications2.2 Web and HTTP2.3 FTP2.4 Electronic Mail



Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




DNS: Domain Name System

People: many identifiers:SSN, name, passport #

Internet hosts, routers:IP address (32 bit) used for addressingdatagrams“name”, e.g.,ww.yahoo.com used byhumans

Q: map between IPaddresses and name ?

Domain Name System:distributed databaseimplemented in hierarchy ofmany name serversapplicationlayer protocolhost, routers, name servers tocommunicate to resolve names(address/name translation)

note: core Internetfunction, implemented asapplicationlayer protocolcomplexity at network’s“edge”

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




DNSWhy not centralize DNS?

single point of failuretraffic volumedistant centralized databasemaintenance

doesn’t scale!

DNS servicesHostname to IP addresstranslationIP address to HostnametranslationHost aliasing

Canonical and alias namesMail server aliasingLoad distribution

Replicated Web servers:set of IP addresses forone canonical nameBut performs poorly –suppose Aol cached one IPaddress

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS serversyahoo.com

DNS serversamazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approx:Client queries a root server to find com DNSserverClient queries com DNS server to get amazon.comDNS serverClient queries amazon.com DNS server to get IPaddress for www.amazon.com

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com

http://www.amazon.com;

http://www.amazon.com




DNS: Root name serverscontacted by local name server that cannot resolve nameroot name server:

contacts authoritative name server if name mapping not knowngets mappingreturns mapping to local name server

84 root nameservers worldwide

See www.rootservers.orgb USCISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto,CA (and 17 other locations)

i Autonomica, Stockholm (plus 3other locations)

k RIPE London (also Amsterdam,Frankfurt)

m WIDE Tokyo

a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 11 locations)

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com

http://www.root-servers.org




TLD and Authoritative Servers

Toplevel domain (TLD) servers: responsiblefor com, org, net, edu, etc, and all toplevelcountry domains uk, fr, ca, jp

Network solutions maintains servers for com TLDEducause for edu TLD

Authoritative DNS servers: organization’sDNS servers, providing authoritativehostname to IP mappings for organization’sservers (e.g., Web and mail)

Can be maintained by organization or serviceprovider

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Local Name Server

Does not strictly belong to hierarchyEach ISP (residential ISP, company,university) has one

Also called “default name server”When a host makes a DNS query, query issent to its local DNS server

Acts as a proxy, forwards query into hierarchy

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com





gaia.cs.umass.edu

root DNS server


1

23

4

5

6


78

TLD DNS server

Example

Host at cis.poly.eduwants IP address forgaia.cs.umass.edu

2. Find gaia.cs.umass.edu3. Find gaia.cs.umass.edu4. List of IPs for TLD

servers for edu5. Find gaia.cs.umass.edu6. IP for authoritative

server for umass.edu7. Find gaia.cs.umass.edu8. IP for gaia.cs.umass.edu9. IP for gaia.cs.umass.edu

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com





gaia.cs.umass.edu

root DNS server


1

2

45

6


7

8

TLD DNSserver

3

Recursive queriesrecursive query:

Name server findsanswerputs burden of nameresolution oncontacted nameserverheavy load?

iterated query:contacted serverreplies with name ofserver to contact“I don’t know thisname, but ask thisserver”

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




DNS: caching and updating records

When a name server learns a mapping, it cachesthe mapping

A server discards cached entries after a timeout(typically 2 days)TLD servers typically cached in local name servers

• Thus root name servers queried infrequently

update/notify mechanisms under design by IETFRFC 2136www.ietf.org/html.charters/dnsindcharter.html

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com

http://www.ietf.org/html.charters/dnsind-charter.html




DNS recordsDNS: distributed db storing resource records (RR)

Type=NSname is domain (e.g.foo.com)

value is IP address ofauthoritative nameserver for this domain

RR format: (name, value, type, ttl)

Type=Aname is hostname

value is IP address

Type=CNAMEname is alias name for some“cannonical” (the real) name

www.ibm.com is really servereast.backup2.ibm.com

value is cannonical nameType=MX

value is name of mailserverassociated with name

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com

http://www.ibm.com




DNS protocol, messagesDNS protocol : query and reply messages, both with

same message format

msg headeridentification: 16 bit #for query, reply to queryuses same #flags:

query or replyrecursion desiredrecursion availablereply is authoritative

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




DNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Inserting records into DNS

Example: just created startup “Network Utopia”Register name networkuptopia.com at a registrar(e.g., Network Solutions)

Need to provide registrar with names and IP addresses ofyour authoritative name server (primary and secondary)Registrar inserts two RRs into the com TLD server:

(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)

Put in authoritative server Type A record forwww.networkuptopia.com and Type MX record fornetworkutopia.comHow do people get the IP address of your Web site?

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com

http://www.networkuptopia.com





2.1 Principles ofnetwork applications

app architecturesapp requirements

2.2 Web and HTTP2.4 Electronic Mail



Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




P2P file sharing

ExampleAlice runs P2P clientapplication on hernotebook computerIntermittentlyconnects to Internet;gets new IP addressfor each connectionAsks for “Hey Jude”Application displaysother peers that havecopy of Hey Jude.

Alice chooses one ofthe peers, Bob.File is copied fromBob’s PC to Alice’snotebook: HTTPWhile Alice downloads,other users uploadingfrom Alice.Alice’s peer is both aWeb client and atransient Web server.

All peers are servers =highly scalable!

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




P2P: centralized directory

original “Napster” design1) when peer connects, it

informs central server:IP addresscontent

2) Alice queries for “HeyJude”

3) Alice requests file fromBob

centralizeddirectory server

peers

Alice

Bob

1

1

1

12

3

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




P2P: problems with centralized directory

Single point of failurePerformancebottleneckCopyrightinfringement

file transfer isdecentralized, butlocating content ishighly decentralized

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Query flooding: Gnutella

fully distributedno central server

public domain protocolmany Gnutella clientsimplementing protocol

overlay network: graphedge between peer Xand Y if there’s a TCPconnectionall active peers andedges is overlay netEdge is not a physicallinkGiven peer willtypically be connectedwith < 10 overlayneighbors

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Gnutella: protocol

Query

QueryHit

Query

Query

QueryHit

Query

Query

QueryHit

File transfer:HTTP Query message

sent over existing TCPconnections peers forward

Query message QueryHit

sent overreversepath

Scalability:limited scopeflooding

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Gnutella: Peer joining

1. Joining peer X must find some other peer inGnutella network: use list of candidate peers

2. X sequentially attempts to make TCP with peerson list until connection setup with Y

3. X sends Ping message to Y; Y forwards Pingmessage.

4. All peers receiving Ping message respond withPong message

5. X receives many Pong messages. It can thensetup additional TCP connections

Peer leaving: see homework problem!

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com





What are ‘littleendian’ and ‘bigendian’?Why does the protocol have to specifythem?Unique identifiers: How are uniqueDescriptor IDs and Servent Identifiersgenerated?The spec says (p 3, para 2) “if a serventbecomes out of synch with its input stream,it should drop the connection”. How wouldit know?

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com





In the section ‘Descriptor Routing’ on page 5, thespec says “Pong descriptors may only be sent alongthe same path that carried the incoming Pingdescriptor” and “Push descriptors may only be sentalong the same path that carried the incomingQueryHit descriptor.” How would this beimplemented?In the section ‘Firewalled Servents’ the spec says“A servent can request a file push by routing aPush request back to the servent that sent theQueryHit descriptor describing the target file.”How is this possible? Isn’t the latter serventbehind a firewall?

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Exploiting heterogeneity: KaZaA

Each peer is either agroup leader or assignedto a group leader.

TCP connection betweenpeer and its group leader.TCP connections betweensome pairs of groupleaders.

Group leader tracks thecontent in all itschildren.

ordinary peer

groupleader peer

neighoring relationshipsin overlay network

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




KaZaA: Querying

Each file has a hash and a descriptorClient sends keyword query to its groupleaderGroup leader responds with matches:

For each match: metadata, hash, IP addressIf group leader forwards query to othergroup leaders, they respond with matchesClient then selects files for downloading

HTTP requests using hash as identifier sent topeers holding desired file

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com




Kazaa tricks

Limitations on simultaneous uploadsRequest queuingIncentive prioritiesParallel downloading

Click t

o buy NOW!

PDFXCHANGE


k to buy N

OW!PDFXCHANGE

www.docutrack.com



chapter€2 application€layer - nyu computer science · · 2005-02-16lecture€3:...

Documents