[SOLVED] RFC 1034/1035

Computer Networking and Applications
Domain Name System (DNS) RFC 1034/1035
The Internet iden4fies hosts by their IP address (e.g. 129.127.12.6)
People tend to prefer names (e.g. www.cs.adelaide.edu.au)

Copyright By Assignmentchef assignmentchef

There are lots of Internet hosts. A centralised database wouldnt scale. The DNS is a distributed database.
organisa4ons can change host names and IP addresses within their domain without informing a central authority
an organisa4on will typically have a name server
A DNS server provides name resolu+on = conversion from a domain name to an IP address
a name server is a process listening on UDP/TCP port 53 for requests when detected, the name is resolved, and a reply is sent
Lots of applica4ons are DNS clients, including web browsers.

Computer Networking and Applications
Root name server (13worldwide)
DNS hierarchical database
abletoresolveallqueriesoridentifyanotherintermediatenameserver
Top-level domain (TLD) servers:
responsibleforcom,org,net,edu,etc,andalltop-levelcountrydomainsuk,fr, ca, jp.
NetworkSolutionsmaintainsserversforcomTLD,EducauseforeduTLD
Authoritative DNS servers:
organizationsDNSservers,providingauthoritativehostnametoIPmappings for organizations servers (e.g., Web, mail).
canbemaintainedbyorganizationorserviceprovider
Local name server handles local DNS requests. Must know at least one root
server. Caches resolved addresses.
Root DNS Servers org DNS servers
pbs.org DNS servers
com DNS servers
edu DNS servers
poly.edu umass.edu DNS serversDNS servers
yahoo.com DNS servers
amazon.com DNS servers

Computer Networking and Applications
DNS name resolution example
hostatcis.poly.eduwants IP address for
gaia. cs. umass. edu
iterated query:
vcontacted server replies with name of server to contact
vI dont know this name, but ask this server
root DNS server
authoritative DNS server
dns.cs.umass.edu
gaia.cs.umass.edu
TLD DNS server
local DNS server
dns.poly.edu
requesting host
cis.poly.edu

Computer Networking and Applications
DNS name resolution example
recursive query:
vputs burden of name resolution on
contacted name server vheavy load at upper levels of hierarchy?
root DNS server
TLD DNS server
local DNS server
dns.poly.edu
requesting host
cis.poly.edu
authoritative DNS server
dns.cs.umass.edu
gaia.cs.umass.edu

Computer Networking and Applications
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)
name is hostname
value is IP address
Type=NS
name is domain (e.g.
value is hostname of authorita4ve name server for this domain
Type=CNAME
name is alias name for some
canonical (the real) name
www.ibm.com is really
servereast.backup2.ibm.com value is canonical name
value is name of mailserver associated with name

Computer Networking and Applications
Inserting records into DNS
Example: just created startup Network Utopia
Register name networkuptopia.com at a registrar (e.g., Network Solu4ons)
Need to provide registrar with names and IP addresses of your authorita4ve 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,
Put in authorita4ve server Type A record for www.networkuptopia.com and Type MX record for networkutopia.com
How do people get the IP address of your Web site?

Computer Networking and Applications
Attacking DNS
DDoS attacks
Bombard root servers with traffic
Not successful to date
Traffic Filtering
Local DNS servers cache IPs of TLD servers, allowing root server bypass
Bombard TLD servers
Potentially more dangerous
Redirect a]acks
v Man-in-middle
Intercept queries
v DNS poisoning
Send bogus relies to DNS
server, which caches
Exploit DNS for DDoS
v Send queries with spoofed source address: target IP
v Requires amplifica4on

Computer Networking and Applications
Models of Interaction central storage of informa4on in always on
Client Server server
dis4nc4on between client which receives service and server which provides service
note that it is possible for a host to act as both a client and as a server in different interac4ons.
Web, e-mail, FTP
Peer to Peer
distributed storage of informa4on
no clear dis4nc4on between clients and servers. Hosts share typically equal control of processing and data
Peers dynamically join and leave
Bit Torrent
client/server

Computer Networking and Applications
Pure P2P architecture
noalways-onserver
arbitraryendsystemsdirectly communicate
peersareintermittentlyconnectedand change IP addresses
Advantages
Distributes load of serving files.
Challenges
How to find resources Fairness
examples:
file distribution (BitTorrent), Streaming (KanKan), VoIP (Skype)

Computer Networking and Applications
File distribution: client-server vs P2P Question: how much time to distribute file (size F)
from one server to N peers?
peer upload/download capacity is limited resource
us: server upload capacity
file, size F
di: peer i download capacity
ui: peer i upload capacity
network (with abundant bandwidth)

Computer Networking and Applications
File distribution time: client-server server transmission: must
sequentially send (upload) N file copies:
time to send one copy: F/us
dmin = min client download rate
min client download time: F/dmin time to distribute F
to N clients using client-server approach
time to send N copies: NF/us
client: each client must download
Dc-s > max{NF/us,,F/dmin}
increases linearly in N
Application Layer

Computer Networking and Applications
File distribution time: P2P
servertransmission:mustupload at least one copy
time to send one copy: F/us
client: each client must download
min client download time: F/dmin
vclients: as aggregate must download NF bits max upload rate (limting max download rate) is us + ui
time to distribute F to N clients using
P2P approach
DP2P > max{F/us,,F/dmin,,NF/(us + ui)} increases linearly in N
but so does this, as each peer brings service capacity
Application Layer 2-12

Computer Networking and Applications
3.5 3 2.5 2 1.5 1 0.5 0
Comparing Client-server, P2P architectures
Client-Server
0 5 10 15 20 25 30 35 N
Minimum Distribution Time

Computer Networking and Applications
P2P file distribution: BitTorrent
v file divided into 256Kb chunks
v peers in torrent send/receive file chunks
tracker: tracks peers participating in torrent
Alice arrives
obtains list
of peers from tracker
and begins exchanging file chunks with peers in torrent
torrent: group of peers exchanging chunks of a file

Computer Networking and Applications
P2P file distribution: BitTorrent
peerjoiningtorrent:
has no chunks, but will accumulate them over time from other peers
registers with tracker to get list of peers, connects to subset of peers (neighbors)
vwhile downloading, peer uploads chunks to other peers vpeer may change peers with whom it exchanges chunks vchurn: peers may come and go
once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent

Computer Networking and Applications
BitTorrent: requesting, sending file chunks
requesting chunks: sending chunks: tit-for-tat
at any given time, different vAlice sends chunks to those four
peers have different subsets of file chunks
periodically, Alice asks each peer for list of chunks that they have
Alicerequestsmissingchunks from peers, rarest first
peers currently sending her chunks
at highest rate
other peers are choked by Alice (do not receive chunks from her)
re-evaluate top 4 every10 secs vevery 30 secs: randomly select
another peer, starts sending chunks optimistically unchoke this peer newly chosen peer may join top 4

Computer Networking and Applications
Food for thought
While P2P applica4ons scale well to begin
with, there are some concerns about the effect of mass-usage!
On what assump4ons is a packet-switching network based?
On what assump4ons is BitTorrent based?

CS: assignmentchef QQ: 1823890830 Email: [email protected]