2: Application Layer1 DNS: Domain Name System People have many identifiers: SSN, name, passport number Internet hosts, routers have identifiers, too: IP.

2: Application Layer 1

DNS: Domain Name SystemPeople have many identifiers:

SSN, name, passport number

Internet hosts, routers have identifiers, too:IP address (32 bit) - used for addressing datagrams“name”, e.g., www.yahoo.com - used by humans

Domain Name System:Distributed database Distributed database implemented in hierarchy of many name serversname servers

Application-layer protocolApplication-layer protocol host, routers, name servers to communicate to resolveresolve names (address/name translation)DNS is core Internet function, implemented as application-layer protocolComplexity at network’s “edge”

2: Application Layer 2

DNS DNS services

Hostname to IP address translation and vice versaHost aliasing – canonical and alias namesMail server aliasingLoad distribution – e.g. replicated Web servers: set of IP addresses for one canonical name

Why not centralize DNS?Single point of failureTraffic volumeDistant centralized databaseMaintenance

2: Application Layer 3

Root DNS Servers

.com DNS servers .org DNS servers .edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com A straightforward strategy:

Client queries a root server to find .com DNS serverClient queries .com DNS server to get amazon.com DNS serverClient queries amazon.com DNS server to get IP address for www.amazon.com

2: Application Layer 4

DNS: Root name servers

Contacted by local name server that can not resolve nameRoot name server:

Returns mapping of the domain server to local name server

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 17 other locations)

i Autonomica, Stockholm (plus 3 other 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)

2: Application Layer 5

TLD and Authoritative Servers

Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.

Network solutions maintains servers for .com TLDEducause for .edu TLD

Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail).

Can be maintained by organization or service provider

2: Application Layer 6

Local Name Server

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

Also called “default name server”

When a host makes a DNS query, query is sent to its local DNS server

Acts as a cacheIf no record for this query, forwards query into hierarchyCaches replies

2: Application Layer 7

requesting hosttitan.cis.udel.edu

lever.cs.ucla.edu131.179.192.136

root DNS server

local DNS serverdns.udel.edu

1

23

4

5

6

authoritative DNS serverdns.ucla.edu

78

TLD DNS server.edu

Example

Host titan.cis.udel.edu wants IP address for lever.cs.ucla.edu

2: Application Layer 8

requesting hosttitan.cis.udel.edu

lever.cs.ucla.edu131.179.192.136

root DNS server

local DNS serverdns.udel.edu

1

2

45

6

authoritative DNS serverdns.ucla.edu

7

8

TLD DNS server.edu

3

Recursive queriesrecursive query:

puts burden of name resolution on contacted name serverheavy load?

iterated query:contacted server replies with name of server to contact“I don’t know this name, but ask this server”

2: Application Layer 9

DNS: caching and updating records

Once (any) name server learns mapping, it cachescaches this mapping

Cache entries timeout (disappear) after some timeTLD servers typically cached in local name servers Thus root name servers are not often visited

Update/notify mechanisms under design by IETF

RFC 2136http://www.ietf.org/html.charters/dnsind-charter.html

2: Application Layer 10

DNS resource records (RR)

Type=NSname is domain (e.g. foo.com)value is hostname of authoritative name server for this domain

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

Type=Aname is hostnamevalue is IP address

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

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

value is canonical name

Type=MXvalue is name of mailserver associated with name

And more!And more!

2: Application Layer 11

DNS resource records (RR)

mycompany.com IN SOA nameserv.mycompany.comroot.mymachine.mycompany.com (1999112701 ; Serial number

10800 ; Refresh in seconds3600 ; Retry in seconds 604800 ; Expire86400) ; Minimum TTL

mycompany.com IN NS nameserv.mycompany.com.

mycompany.com IN MX 10 mailserv.mycompany.com

nameserv.mycompany.com

IN A 10.1.0.100

mailserv.mycompany.com

IN A 10.1.0.4

www.mycompany.com IN CNAME webhost1.mycompany.com

webhost1.mycompany.com

IN A 10.1.0.13

2: Application Layer 12

DNS protocol, messagesDNS protocol has queryquery and replyreply messages, both with same message format

16 bit number for query, reply to query uses same number

flags: query or reply recursion desired recursion available reply is authoritative

2: Application Layer 13

DNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used

2: Application Layer 14

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

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

(networkutopia.com NS dns1.networkutopia.com)(networkutopia.com NS dns2.networkutopia.com)(dns1.networkutopia.com A 212.212.212.1)(dns2.networkutopia.com A 212.212.212.3)

At dns1.networkutopia.com put in more information for various hostsHow do people get the IP address of your Web site?

2: Application Layer 15

Try out DNS!nslookup www.microsoft.com

Server: 127.0.0.1Address: 127.0.0.1#53

Non-authoritative answer:www.microsoft.com canonical name = toggle.www.ms.akadns.nettoggle.www.ms.akadns.net canonical name = g.www.ms.akadns.netg.www.ms.akadns.net canonical name = lb1.www.ms.akadns.netName: lb1.www.ms.akadns.netAddress: 207.46.199.30Name: lb1.www.ms.akadns.netAddress: 207.46.225.60Name: lb1.www.ms.akadns.netAddress: 207.46.18.30Name: lb1.www.ms.akadns.netAddress: 207.46.19.30

2: Application Layer 16

Try out DNS!nslookup www.yahoo.com

Server: 127.0.0.1Address: 127.0.0.1#53

Non-authoritative answer:www.yahoo.com canonical name = www.yahoo.akadns.net.Name: www.yahoo.akadns.netAddress: 68.142.226.56Name: www.yahoo.akadns.netAddress: 68.142.226.33Name: www.yahoo.akadns.netAddress: 68.142.226.35Name: www.yahoo.akadns.netAddress: 68.142.226.37Name: www.yahoo.akadns.netAddress: 68.142.226.38

2: Application Layer 17

Try out DNS!nslookup –debug www.yahoo.com

QUESTIONS: www.yahoo.com, type = A, class = IN ANSWERS: -> www.yahoo.com canonical name = www.yahoo.akadns.net. -> www.yahoo.akadns.net internet address = 68.142.226.34 -> www.yahoo.akadns.net internet address = 68.142.226.36 -> www.yahoo.akadns.net internet address = 68.142.226.43 -> www.yahoo.akadns.net internet address = 68.142.226.45 -> www.yahoo.akadns.net internet address = 68.142.226.47 -> www.yahoo.akadns.net internet address = 68.142.226.52

-> www.yahoo.akadns.net internet address = 68.142.226.53 -> www.yahoo.akadns.net internet address = 68.142.226.56 AUTHORITY RECORDS: -> akadns.net nameserver = use1.akadns.net. -> akadns.net nameserver = use9.akadns.net. -> akadns.net nameserver = usw5.akadns.net. -> akadns.net nameserver = asia5.akadns.net. -> akadns.net nameserver = asia9.akadns.net. -> akadns.net nameserver = eur4.akadns.net. -> akadns.net nameserver = eur5.akadns.net. -> akadns.net nameserver = usc4.akadns.net.

2: Application Layer 18

P2P file sharingAlice runs P2P client application such as Kazaaon her notebook computerIntermittently connects to Internet; gets new IP address for each connectionAsks for “Hey Jude”Application displays peers that have a copy Alice chooses one of the peers, Bob.File is copied from Bob’s PC to Alice’s notebook: HTTPWhile Alice downloads, other users uploading other songs from Alice.Alice’s application is both a Web client and a transient Web server.All peers are servers = highly scalable!

2: Application Layer 19

Centralized directory

Original “Napster” design1) when peer connects, it informs

central server: IP address content

2) Alice queries server for “Hey Jude”3) Server tells Alice which users

(including Bob) have the file4) Alice requests file from Bob

centralizeddirectory server

peers

Alice

Bob

1

1

1

12/3

4

2: Application Layer 20

Problems with centralized directory

Single point of failurePerformance bottleneckCopyright infringement

file transfer is decentralized, but locating content is highly centralized

2: Application Layer 21

Query flooding: Gnutella

Fully distributedNo central server

Public domain protocolMany Gnutella clients implementing protocolOverlay network is a graph: edge between peer X and Y if there’s a TCP connection

Edge is not a physical link

All active peers and edges form overlay netGiven peer will typically be connected with < 10 overlay neighbors

2: Application Layer 22

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 over reversepath

Scalability:limited scopeflooding

2: Application Layer 23

Gnutella: Peer joining

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

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

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

4. All peers receiving Ping message respond with Pong message

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

2: Application Layer 24

Exploiting heterogeneity: KaZaA

Each peer is either a group leader or assigned to a group leader.

TCP connection between peer and its group leader.TCP connections between some pairs of group leaders.

Group leader tracks the content in all its children.

ordinary peer

group-leader peer

neighoring relationshipsin overlay network

2: Application Layer 25

KaZaA: Querying

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

For each match: metadata, hash, IP address

If group leader forwards query to other group leaders, they respond with matchesClient then selects files for downloading

HTTP requests using hash as identifier sent to peers holding desired file

2: Application Layer 26

KaZaA tricks

Limitations on simultaneous uploadsRequest queuingIncentive prioritiesParallel downloading

For more info:J. Liang, R. Kumar, K. Ross, “Understanding KaZaA,”(available via cis.poly.edu/~ross)