1. Part 4: Peer to Peer - P2P Applications
IT 424
Networks2
Chapter 2: Application Layer
Part 4: Peer to Peer - P2P Applications
Ack.: Slides are adapted from the slides of the book: “Computer Networking” – J. Kurose, K. Ross

2. Database Distribution
File Distribution: Client-Server vs P2P
BitTorrent File Distribution
Database Distribution
Overview
Skype Application

3. Learning Outcomes 1
To differentiate between the client-server and the P2P application architectures 2
To analyze the performance of both architectures for file-sharing applications 3
To explain how p2p architecture works 4
To describe the bitTorrent file distribution application 5
To analyze the distributed hash table scheme for database distribution 6
To discuss the structure and operation of skype application

4. Database Distribution
File Distribution: Client-Server vs P2P
BitTorrent File Distribution
Database Distribution
Overview
Skype Application

5. Recap: Application Architectures
Application Layer Architectures
Recap: Application Architectures
Possible structure of applications:
Client-server
Peer-to-peer (P2P)
Hybrid of client-server and P2P

6. File Distribution: Client-Server vs P2P
us: server upload capacity u2 d2 u1 d1
di: peer i download capacity
file, size F us
server di uN
network (with abundant
bandwidth) ui dN
ui: peer i upload capacity
Question: How much time to distribute file (size F) from one server to N peers?
Peer upload/download capacity is limited resource

7. File Distribution Time: Client-Server
File Distribution: Client-Server vs P2P
File Distribution Time: Client-Server
Server transmission: must sequentially send (upload) N file copies:
time to send one copy: L/R= F/us
time to send N copies: L/R=NF/us F us di
network ui
increases linearly in N
time to distribute F
to N clients using
client-server approach
If N >> then Delayc-s α N

8. File Distribution Time: P2P
File Distribution: Client-Server vs P2P
File Distribution Time: P2P
Server transmission: must upload at least one copy
time to send one copy: F/us
Client:s as aggregate must upload NF bits
Delay = L/R
L α N
However R also depends upon N
P2P is more scalable F us di
network ui
… but so does this, as each
peer brings service capacity
increases linearly in N
time to distribute F
to N clients using
P2P approach
DelayP2P α N / (us + ui N)

9. Client-server vs. P2P: example
File Distribution: Client-Server vs P2P
Client-server vs. P2P: example

10. Database Distribution
File Distribution: Client-Server vs P2P
BitTorrent File Distribution
Database Distribution
Overview
Skype Application

11. P2P File Distribution: BitTorrent
BitTorrent File Distribution
P2P File Distribution: BitTorrent
tracker: tracks peers
participating in torrent
torrent: group of peers exchanging chunks of a file
Alice arrives …
… obtains list
of peers from tracker
… and begins exchanging
file chunks with peers in torrent
File divided into 256kb chunks
Peers in torrent send/receive file chunks

12. P2P File Distribution: BitTtorrent
Bittorrent File Distribution
P2P File Distribution: BitTtorrent
Peer joining torrent:
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”)
While downloading, peer uploads chunks to other peers
Peer may change peers with whom it exchanges chunks
Churn: peers may come and go
Once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent

13. BitTorrent: Requesting, Sending File Chunks
BitTorrent File Distribution – Sending File
BitTorrent: Requesting, Sending File Chunks
Requesting chunks:
At any given time, different peers have different subsets of file chunks
Periodically, Alice asks each peer for list of chunks that they have
Alice requests missing chunks from peers, “rarest first”
Sending chunks: tit-for-tat
Alice sends chunks to those four 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
Every 30 secs: randomly select another peer, starts sending chunks
“Optimistically unchoke” this peer
Newly chosen peer may join top 4

14. BitTorrent: Tit-For-Tat
BitTorrent File Distribution – Sending File
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
higher upload rate: find better trading partners, get file faster !

15. Database Distribution
File Distribution: Client-Server vs P2P
BitTorrent File Distribution
Database Distribution
Overview
Skype Application

16. Distributed Hash Table (DHT)
Database Distribution
Distributed Hash Table (DHT)
A critical component of many P2P applications is an information index or a simple database supporting search and update operation.
Hash table
Distributed Hash Table (DHT) paradigm
Circular DHT and overlay networks
Peer churn

17. Simple Database Key Value John Washington 132-54-3570
Database Distribution
Simple Database
Key
Value
John Washington
Diana Louise Jones
Xiaoming Liu
Rakesh Gopal
Linda Cohen
…….
………
Lisa Kobayashi
Simple database with (key, value) pairs:
key: human name; value: social security #
key: movie title; value: IP address

18. Hash Table Original Key Key Value John Washington 8962458 132-54-3570
Database Distribution - Hash Table
Hash Table
Original Key
Key
Value
John Washington
Diana Louise Jones
Xiaoming Liu
Rakesh Gopal
Linda Cohen
…….
………
Lisa Kobayashi
More convenient to store and search on numerical representation of key
key = hash (original key)

19. Distributed Hash Table (DHT)
Database Distribution - Distributed Hash Table
Distributed Hash Table (DHT)
Distribute (key, value) pairs over millions of peers
Pairs are evenly distributed over peers
Any peer can query database with a key
Database returns value for the key
To resolve query, small number of messages exchanged among peers
Each peer only knows about a small number of other peers
Robust to peers coming and going (churn)

20. Assign Key-Value Pairs To Peers
Database Distribution - Distributed Hash Table
Assign Key-Value Pairs To Peers
Rule: assign key-value pair to the peer that has the closest ID.
Convention: closest is the immediate successor of the key.
E.G., ID space {0,1,2,3,…,63}
Suppose 8 peers: 1,12,13,25,32,40,48,60
If key = 51, then assigned to peer 60
If key = 60, then assigned to peer 60
If key = 61, then assigned to peer 1

21. Database Distribution - Circular DHT
Each peer only aware of immediate successor and predecessor. 1 12 13 25 32 40 48 60
“overlay network”

22. Resolving A Query 1 value 12 60 13 48 25 40 32
Database Distribution - Circular DHT
Resolving A Query
What is the value associated with key 53 ? 1
value 12
O(N) messages
On avgerage to resolve
Query, when there
Are N peers 60 13 48 25 40 32

23. Circular DHT With Shortcuts
Database Distribution - Circular DHT
Circular DHT With Shortcuts 1 12 13 25 32 40 48 60
What is the value for
key 53
value
Each peer keeps track of IP addresses of predecessor, successor, short cuts.
Reduced from 6 to 3 messages.
Possible to design shortcuts with O(log N) neighbors, O(log N) messages in query

24. Peer Churn 1 3 15 4 12 5 10 8 Handling peer churn:
Database Distribution – Peer Churn
Peer Churn 1 3 4 5 8 10 12 15
Handling peer churn:
Peers may come and go (churn)
Each peer knows address of its two successors
Each peer periodically pings its two successors to check aliveness
If immediate successor leaves, choose next successor as new immediate successor
Example: peer 5 abruptly leaves
peer 4 detects peer 5’s departure; makes 8 its immediate successor
4 asks 8 who its immediate successor is; makes 8’s immediate successor its second successor.

25. Database Distribution
File Distribution: Client-Server vs P2P
BitTorrent File Distribution
Database Distribution
Overview
Skype Application

26. Voice-over-IP: Skype Skype clients (SC)
Skype Application
Voice-over-IP: Skype
Skype clients (SC)
Proprietary application-layer protocol
P2p (PC-to-PC, PC-to-phone, phone-to-PC) Voice-over-IP (VoIP) application
Also Instant Messaging
Encrypted msgs
P2P components:
Skype
login server
supernode (SN)
supernode
overlay
network
Clients: skype peers connect directly to each other for voip call
Super nodes (SN): skype peers with special functions
Overlay network: among SNs to locate SCs
Login server

27. Voice-over-IP: Skype Skype client operation:
Skype Application
Voice-over-IP: Skype
Skype client operation:
Skype
login server
1. Joins skype network by contacting SN (IP address cached) using TCP
2. Logs-in (usename, password) to centralized skype login server
3. Call: SC contacts SN will callee ID
SN contacts other SNs (unknown protocol, maybe flooding) to find addr of callee; returns addr to SC
4. Initiate call directly to callee

28. Voice-over-IP: Skype Problem: both Alice, Bob are behind NATs
Skype Application
Voice-over-IP: Skype
Problem: both Alice, Bob are behind NATs
NAT prevents outside peer from initiating connection to insider peer
Inside peer can initiate connection to outside
Relay solution: Alice, Bob maintain open connection
to their SNs
Alice signals her SN to connect to bob
Alice’s SN connects to bob’s SN
Bob’s SN connects to bob over open connection bob initially initiated to his SN

29. Conclusion There are three application architectures:
Client server (e.g. FTP, Web)
Pure HTP (e.g. bitTorrent)
Hybrid architecture (e.g skype)
File distribution time using P2P architecture is less than using client-server architecture
Four paradigm for file distribution:
Hash table
Distributed Hash Table paradigm
Circular DHT and overlay networks
Peer churn

30. References
References
Computer Networking: A Top-Down Approach Featuring the Internet by James Kurose and Keith Ross, Addison Wesley, (chapter 2 )
Peer-to-Peer (P2P) Architecture (RFC5694)