1. Lecture XV: Real P2P Systems
CMPT
Dr. Alexandra Fedorova

2. Outline
Skype
BitTorrent

3. Skype IP telephony system Allows users to make phone calls
to Skype users
to regular phone users
Calls are routed via Skype nodes
Central login server
Super-nodes
Nodes that know about location of other nodes

4. Skype P2P Network
Super-nodes are nodes with powerful CPU and network bandwidth
One has to qualify to become a super-node
Super-nodes know about locations of other nodes
Ordinary nodes contact super-nodes to place calls

5. Skype Components
Skype client – the client program used to make phone calls
Host cache – list of other known Skype users, maintained at each Skype client
Skype login server – a centralized component. Processes account information, authentication

6. Skype Bootstrap Procedure
Skype client depends on host cache to find other users
How does a new Skype client populate its host cache?
There is a list of bootstrap supernodes hard-coded into the Skype client
Skype client contacts them to
Find the login server
Find names of other Skype peers
Skype client continues maintaining and refreshing its host cache throughout its lifetime
It’s a good idea to let Skype running for a while before making a phone call!

7. Skype User Search Procedure
A Skype client making a phone call needs to find other users
It contacts super-nodes from its host cache, asking them to help find the user
Super-nodes return a list of nodes to contact
The client contacts those nodes
If unsuccessful, the client asks for more nodes
Guarantees to find any user that has logged in within the last 72 hours
Not much specific information on Skype protocol is available…

8. Outline
Skype
BitTorrent

10. BitTorrent Overview
Website allowing peers to share music, video and other media files
Central server helps users find initial set of peers that have pieces of the file
Tracker server keeps track of peers possessing content of individual files
Users download the file by participating in exchange:
They exchange pieces that they have
for pieces that they don’t have
Therefore, for the system to work, users must have incentive to give
Users who just get, but do not give are called free riders
Protocol must discourage free riding

11. BitTorrent Download Mechanism
Peers download pieces they need and upload pieces they have
Keep track of download rates from all links
Keep those with highest rate
Keep uploading to those peers
Choke the others (stop upload)
fatter link = higher rate

12. Selective Unchoking
Once in a while selectively unchoke one of the links
Allow some time (30 sec.) to see if the download rate will increase
If the download rate becomes greater than the rate at one of the existing links, choke the slowest of the existing links

13. Game Theory on Exchange
Two traders A and B simultaneously exchange goods
They have to decide whether to participate in the exchange without knowing the other person’s decision
If a trader makes the exchange, he cooperates
If a trader does not make the exchange, he defects
Possible outcomes:
A cooperates, B cooperates – both benefit from receiving the other trader’s good
A cooperates, B defects – B benefits by receiving A’s good, by not giving away his own good
A defects, B defects – none receives the other trader’s good, but each gets to keep his own good

14. Incentives P – payoff P(C) – payoff to cooperating player
P(D) – payoff to defecting player
Cooperate
Defect
P(C) = 3, P(C) = 3
P(C) = 0, P(D) = 5
P(D) = 5, P(C) = 0
P(D) = 1, P(D) = 1

15. Strategies Encouraged By Incentives
Cooperate
Defect
P(C) = 3, P(C) = 3
P(C) = 0, P(D) = 5
P(D) = 5, P(C) = 0
P(D) = 1, P(D) = 1
Best individual strategy is to defect
But if both traders defect they are worse off than if they both co-operated
So the best strategy is to co-operate, but only if the other player co-operates

16. Winning the Exchange Game: TIT-FOR-TAT
Repeat rounds of exchanges with other players
Remember what other players did in previous rounds
Goal: maximize personal payoff
The winning strategy: TIT-FOR-TAT
Cooperate in the first round
In subsequent rounds do that which the other player did
Properties of the winning player:
Nice – he begins by cooperating
Retaliatory – he won’t cooperate with players that do not cooperate with him
Forgiving – will cooperate again if bad players fix their behavior
Clear – clearly communicates his intentions

17. Analogy With BitTorrent
Goods exchange: exchange the piece of file that you have for the piece that you don’t have
Cooperate: upload the piece you have in exchange for downloading the piece you need
Defect: download the piece you need, but don’t upload (free riding)
Payoff:
The piece you download
minus the cost of uploading the file

18. Is BitTorrent Using the Winning Strategy?
Is BitTorrent player NICE?
No, it may choke an existing link upon finding a faster one
Is BitTorrent player RETALIATORY?
No, it uses optimistic unchoking
Is BitTorrent player FORGIVING?
Not necessarily – unchoked links are chosen arbitrarily
Is BitTorrent player CLEAR?
No, unchoked links are chosen arbitrarily

19. Improved Incentive Mechanism
For each link, maintain:
Download amount d
Upload amount u
Upload to the peer as long as:
u – d ≤ f * c
c – the fragment size
f – some constant
If u – d is large, the other peer is not cooperating
So you stop cooperating in return d u

20. Comparison of the Mechanisms
Old mechanism: everyone experiences the same download time, regardless of the upload rate they provide
New mechanism: nodes that upload more enjoy faster completion time

21. Comparison of the Mechanisms (cont.)
FO – free riders + old players
FN – free riders + new players
FNO – free riders + new players + old players

22. Summary Skype BitTorrent IP telephony system
Relies on super-nodes, recorded in host cache
Skype client learns about initial super-nodes from bootstrap super-nodes
Centralized login server
BitTorrent
Media sharing network
File location is found in a centralized fashion (central server + tracker)
File download is P2P
System must encourage co-operation, discourage free riding
The right incentive mechanism is key