1. Decentralized peer discovery performance in swarm-protocols
Raul Jimenez
KTH, Sweden
KTH logo here
tslab.ssvl.kth.se/raul

2. Overview BitTorrent Content Discovery Data Exchange Peer Discovery
Centralized Tracker
Decentralized Tracker (DHT)
Gossiping (PEX)
DHT Issues
Experiments

3. BitTorrent Content discovery Metadata (.torrent file)
Little tree
Content discovery Metadata (.torrent file)
Peer discovery List of peers
Data exchange Content
tree

4. Metadata (.torrent file)
supergadget3000.pdf
blender_foundation_-_elephants_dream.torrent
Product:
SuperGadget 3000
Article #:
1234-ABC98
Specifications:
.....
Updated list of shops:
Name:
info_hash:
c70bcf4657cac1253d12acda95b04d1e4dcd23f4
Hash for every piece:
Tracker (updated list of peers):
Elephants Dream HD
.....
Product Picture here (webhallen) hide product and spec

5. Peer Discovery Centralized Tracker Decentralized PEX (Peer Exchange)
DHT (Distributed Hash Table)
Follow tree

6. Centralized Tracker 1 2 3 4 5 6 7 8 tracker.kth.se 1
Represent this better 8
Add comments

7. Centralized Tracker 1 2 3
tracker.kth.se 4 5 6 1 4 7 8

8. Centralized Tracker 1 2 3
tracker.kth.se 4 5 6 1 4 7 7 8

9. Peer Discovery Centralized Tracker Decentralized PEX (Peer Exchange)
DHT (Distributed Hash Table)
Follow the tree

10. Peer Exchange (PEX) Gossiping protocol
Messages between peers in the same swarm
Need external bootstrap
Example, better description

11. Distributed Hash Table (DHT)
Simple demo
4-bits identifiers (24 = 16 identifiers)
1 node per bucket
10 nodes
Properties
iterative
full tracker

12. DHT: Routing Table Short version for iniciated Routing table Level 0
MyNode 1001

13. DHT: Routing Table 1xxx 0xxx Routing table Level 0 0010 Level 1
MyNode 1001

14. DHT: Routing Table 11xx 00xx Routing table 10xx 01xx Level 0 0010
1111
Level 2
Level 3
MyNode 1001

15. DHT: Routing Table 111x 000x 110x 001x 101x 010x Routing table Level 0
0010
Level 1
1111
100x
011x
Level 2
1011
Level 3
MyNode 1001

16. DHT: Routing Table 111x 000x 110x 001x 101x 010x Routing table Level 0
0010
Level 1
1111
1001
011x
Level 2
1011
1000
Level 3
------
MyNode 1001

18. DHT: Find Peers 1xxx 0xxx Level 0 0010 0101 torrentID Level 1 1111
1011
Level 3
MyNode 1001

19. DHT: Find Peers Level 0 1001 Level 1 0110 Level 2 0000 11xx 00xx
0011
10xx
01xx
0101 torrentID
MyNode 1001

20. DHT: Find Peers 111x 000x 110x 001x 101x 010x 0101 torrentID 100x 011x
Level 0
1101
Level 1
0000
MyNode 1001
Level 2
0100
Level 3

21. DHT: Find Peers 111x 000x 110x 001x Level 0 1100 101x 010x Level 1
0011
Level 2
0110
Level 3
0101
0101 torrentID
100x
011x
MyNode 1001

22. Distributed Hash Table (DHT)
Kademlia
Iterative routing algorithm
160-bit identifiers
8 nodes per bucket
1 million nodes ?

23. DHT Issues
NAT behaviour
BitTorrent & NATs
DHT & NATs

24. Network Address Translation (NAT)
NAT hole open
partially open
NAT hole closed after NAT timeout,
or full table, or ...
Syncro closed with square
Red cross

25. BitTorrent & NATs 1 2 3
tracker.kth.se 4 5 6 1 7 8
Add more NATs

26. BitTorrent & NATs 1 2 3
tracker.kth.se 4 5 6 1 4 7 8

27. BitTorrent & NATs 1 2 3
tracker.kth.se 4 5 6 1 4 7 7 8

28. DHT & NATs
Higher (apparent) churn
Broken routes

29. Higher churn A new node joins Add to the routing table
Dead node (remove) 1 2
A new node joins
...
Show routing table?

30. Broken routes Node 1 finds node 2 N1 adds N2 to routing table
N3 sends message to N1
N2 is the next hop
N2 is unreachable from N3 3
Show routing tables?

31. What to do? Prevent my routing table from pollution
Check for global reachability before adding a node
No pollute others' routing tables
Tell others “don't add me to your routing table”

32. Prevent pollution Check candidate node's reachability
from the DHT (same IP, same port)
opening a new connection (same IP, different port)
from another host (different IP)
Wait for a long time (>NAT timeout)
Zero 0%
Very cheap
Exp data
Expensive
100% ? ?

33. Prevent pollution
Experiment results here

34. No pollute others Flag myself as “unreachable” new protocol message
a flag in messages
select a special port range
don't reply any message

35. Thank you!