Post on 27-Dec-2015
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Lecture05: Application layer
Principles of network applications
DNS
P2P and DHT
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Some network apps
e-mail web instant messaging remote login P2P file sharing multi-user network
games streaming stored
video clips
voice over IP real-time video
conferencing VoD (Youtube) Facebook Twitter Cloud ….
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Application architectures
Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P
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Client-server architectureserver:
always-on host permanent IP address server farms for
scalingclients:
communicate with server (speak first)
may have dynamic IP addresses
do not communicate directly with each other
client/server
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Pure P2P architecture
no always-on server arbitrary end systems
directly communicate peers are
intermittently connected and change IP addresses
Highly scalable but difficult to manage
peer-peer
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Hybrid of client-server and P2PSkype
voice-over-IP P2P application centralized server: finding address of
remote party: client-client connection: direct (not through
server) Instant messaging
chatting between two users is (can be) P2P centralized service: client presence
detection/location• user registers its IP address with central
server when it comes online• user contacts central server to find IP
addresses of buddies
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What transport service does an app need?
Data loss 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 “effective”
other apps (“elastic apps”) make use of whatever throughput they get
Security Encryption, data
integrity, …
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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
TCPTCPTCPTCPTCP or UDP
typically UDP
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Lecture05: Application layer
Principles of network applications
DNS
DHT
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DNS: Domain Name SystemPeople: many
identifiers: SSN, name, passport
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Internet hosts, routers: IP address (32 bit)
- used for addressing datagrams
“name”, e.g., www.yahoo.com - used by humans
Required: map between name and IP addresses
Domain Name System: distributed database
implemented as a hierarchy of many name servers
application-layer protocol used to resolve names (address/name translation) note: core Internet
function, implemented as application-layer protocol
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DNS
Why not centralize DNS? single point of failure traffic volume distant centralized
database maintenance
doesn’t scale!
DNS services Hostname to IP
address translation Mail server aliasing Load distribution
Replicated Web servers: set of IP addresses for one canonical name
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Root DNS Servers
com DNS servers org DNS servers edu DNS servers
poly.eduDNS servers
umass.edu DNS servers
yahoo.comDNS servers
amazon.comDNS servers
pbs.orgDNS servers
Distributed, Hierarchical Database
Client wants IP address for www.amazon.com; 1st approx:
Client queries a root server to find com DNS server Client queries com DNS server to get amazon.com
DNS server Client queries amazon.com DNS server to get IP
address for www.amazon.com
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DNS: Root name servers
13 root name servers worldwideb USC-ISI Marina del Rey, CA
l 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)
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Local Name Server
does not strictly belong to hierarchy each ISP (residential ISP, company,
university) has one. also called “default name server”
when host makes DNS query, query is sent to its local DNS server acts as proxy, forwards query into hierarchy
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requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
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23
4
5
6
authoritative DNS serverdns.cs.umass.edu
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TLD DNS server
DNS name resolution example
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
iterated query: contacted server
replies with name of server to contact
“I don’t know this name, but ask this server”
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requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
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2
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6
authoritative DNS serverdns.cs.umass.edu
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TLD DNS server
3recursive query: puts burden of
name resolution on contacted name server
DNS name resolution example
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DNS: caching and updating records once (any) name server learns mapping, it
caches mapping cache entries timeout (disappear) after
some time typically cached in local name servers
• Thus root name servers not often visited
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Lecture05: Application layer
Principles of network applications
DNS
P2P and DHT
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Pure P2P architecture
no always-on server arbitrary end systems
directly communicate peers are intermittently
connected and change IP addresses
Three topics: File distribution Searching for information Case Study: Skype
peer-peer
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File Distribution: Server-Client vs P2PQuestion : How much time to distribute file
from one server to N peers?
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
File, size F
us: server upload capacity (bps)
ui: peer i upload capacity (bps)
di: peer i download capacity (bps)
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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
Client-server vs. P2P: example
Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
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File distribution: BitTorrent
tracker: tracks peers participating in torrent
torrent: group of peers exchanging chunks of a file
obtain listof peers
trading chunks
peer
P2P file distribution
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BitTorrent (1)
file divided into 256KB chunks. peer joining torrent:
registers with tracker to get list of peers, connects to subset of peers (“neighbors”)
has no chunks, but will accumulate them over time
while downloading, peer uploads chunks to other peers.
peers may come and go once a peer has entire file, it may (selfishly) leave
or (altruistically) remain
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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
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 “optimistically
unchoke”
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Finding a better trading partner(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!
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Distributed Hash Table (DHT)
DHT = distributed P2P database Database has (key, value) pairs;
key: ss number; value: human name key: content type; value: IP address
Peers query database with key database returns values that match the key
Peers can also insert (key, value) pairs into database
Finding “needles” requires that the P2P system be structured
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DHT Identifiers
Assign integer identifier to each peer in range [0,2n-1]. Each identifier can be represented by n bits.
Require each key to be an integer in same range.
To get integer keys, hash original key. e.g., key = h(“Led Zeppelin IV”) This is why database is called a distributed “hash”
table
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How to assign keys to peers?
Central issue: Assigning (key, value) pairs to peers.
Rule: assign to the peer that has the ID closest to key.
Convention in lecture: closest is the immediate successor of the key (or equal to)
Example: 4 bits; peers: 1,3,4,5,8,10,12,14; key = 13, then successor peer = 14 key = 15, then successor peer = 1
Consistent Hashing For n = 6, # of identifiers is 64. The following DHT ring has 10 nodes
and stores 5 keys. The successor of key 10 is node 14.
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1
3
4
5
810
12
15
Circular DHT (1)
Each peer only aware of immediate successor and predecessor.
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Circle DHT (2)
0001
0011
0100
0101
10001010
1100
1111
Who’s resp
for key 1110 ?I am
O(N) messageson avg to resolvequery, when thereare N peers
1110
1110
1110
1110
1110
1110
Define closestas closestsuccessor
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Circular DHT with Shortcuts
Each peer keeps track of IP addresses of predecessor, successor, and short cuts.
Reduced from 6 to 3 messages. Can design shortcuts such that O(log N) neighbors per
peer, O(log N) messages per query
0001
0011
0100
0101
10001010
1100
1111
Who’s resp for key 1110?
Scalable Key Location – Finger Tables
0
4
26
5
1
3
7
124
130
finger tablestart succ.
keys1
235
330
finger tablestart succ.
keys2
457
000
finger tablestart succ.
keys6
0+20
0+21
0+22
For.
1+20
1+21
1+22
For.
3+20
3+21
3+22
For.
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Peer Churn
•To handle peer churn, require each peer to know the IP address of its two successors •Each peer periodically pings its two successors to see if they are still alive •Limited solution for single join or single failure
Consistent Hashing – Node Join
0
4
26
5
1
3
7
keys1
keys2
keys
keys
7
5
Consistent Hashing – Node Dep.
0
4
26
5
1
3
7
keys1
keys2
keys
keys6
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Problem of DHT
No good solution to maintain both scalable and consistent finger table under Churn.
Solution of BitTorrent Maintain trackers (servers) as DHT, which
are more reliable Users queries trackers to get the locations
of the file File sharing are not structured.
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DNS vs. DHT
DNS
provides a host name to IP address mapping
relies on a set of special root servers
names reflect administrative boundaries
is specialized to finding named hosts or services
DHT
can provide same service: Name = key, value = IP
requires no special servers
imposes no naming structure
can also be used to find data objects that are not tied to certain machines
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End of Lecture05