1. P2P Networking and Content Distribution
March 28, 2013
2: Application Layer

2. Announcements H/W due today (Calendar, Packet pair)
Calendar app 1 week extension is possible (but w/ 10% point deduction)
Meeting w/ project mentors by Monday
Project plan presentation
Introduction/background
Problem definition (or research questions)
Related work (no need to be complete)
Approach (+supporting materials)
Plans (including refining research questions + experimenting about ideas)
2: Application Layer

3. Reviews Network app: client-server, p2p, hybrid Programming: socket
Addressing issues
Transport layer vs. service requirements
TCP vs. UDP (differences)
HTTP: persistent vs. non-persistent
HTTP: cookies
DNS: distributed, hierarchical DB
DNS name hierarchy vs. Internet's topology
DNS resolution: iterative vs. recursive
2: Application Layer

4. Contents P2P architecture and benefits P2P content distribution
Content distribution network (CDN)
2: Application Layer

5. 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
2: Application Layer

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

7. File distribution time: server-client
server sequentially sends N copies:
NF/us time
client i takes F/di time to download F u2 u1 d1 d2 us
Network (with
abundant bandwidth) dN uN
= dcs = max {NF/us, F/min(di) } i
Time to distribute F
to N clients using
client/server approach
increases linearly w.r.t. N (for large N)
2: Application Layer

8. File distribution time: P2P
Server
server must send one copy: F/us time
client i takes F/di time to download
NF bits must be downloaded (aggregate) F u2 u1 d1 d2 us
Network (with
abundant bandwidth) dN uN
fastest possible upload rate: us + Sui
dP2P = max { F/us, F/min(di) , NF/(us + Sui) } i
2: Application Layer

9. Server-client vs. P2P: example
Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
Client server ~ NF/us vs. P2P ~ NF/(us + Sui)
2: Application Layer

10. Contents P2P architecture and benefits P2P content distribution
Content distribution network (CDN)
2: Application Layer

11. P2P content distribution issues
Group management and data search
Reliable and efficient file exchange
Security/privacy/anonymity/trust
Approaches for group management and data search (i.e., who has what?)
Centralized (e.g., BitTorrent tracker)
Unstructured (e.g., Gnutella)
Structured (Distributed Hash Tables [DHT])
2: Application Layer

12. Centralized model (Napster)
Bob
original “Napster” design
1) when peer connects, it informs central server:
IP address
content
2) Alice queries for “Hey Jude”; server notifies that Bob has the file..
3) Alice requests file from Bob
centralized
directory server 1 3
peers 1 1 1 2
Q: “Hey Jude”
A: Bob has it
Alice
2: Application Layer

13. Centralized model
Bob
Alice
Jane
Judy
file transfer is decentralized, but locating content is highly centralized
Upload index to central server when you come online
To search, consult central server
Request doc directly
2: Application Layer

14. Centralized model Benefits: Drawbacks: Low per-node state
Limited bandwidth usage
Short search time
High success rate
Fault tolerant
Drawbacks:
Single point of failure
Limited scale
Possibly unbalanced load
Bob
Alice
Upload index to central server when you come online
To search, consult central server
Request doc directly
Judy
Jane
2: Application Layer

15. File distribution: BitTorrent
P2P file distribution
tracker: tracks peers
participating in torrent
torrent: group of
peers exchanging
chunks of a file
obtain a list
of peers
trading
chunks
peer
2: Application Layer

16. BitTorrent (1) file divided into 256KB chunks. peer joining torrent:
has no chunks, but will accumulate them over time
registers with a tracker to get list of peers, connects to subset of peers (“neighbors”)
while downloading, peer uploads chunks to other peers.
peers may come online and go offline
once peer has entire file, it may (selfishly) leave or (altruistically) remain
2: Application Layer

17. BitTorrent (2) 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
newly chosen peer may join top 4
“optimistically unchoke”
Pulling Chunks
at any given time, different peers have different subsets of file chunks
periodically, a peer (Alice) asks each neighbor for a list of chunks that it has.
Alice sends requests for her missing chunks
rarest first
2: Application Layer

18. 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
With higher upload rate, can find better trading
partners & get file faster!
2: Application Layer

19. P2P Case study: Skype inherently P2P: pairs of users communicate.
Skype clients (SC)
inherently P2P: pairs of users communicate.
proprietary application-layer protocol (inferred via reverse engineering)
hierarchical overlay with super nodes (SNs)
Index maps usernames to IP addresses; distributed over SNs
Skype
login server
Supernode
(SN)
2: Application Layer

20. Peers as relays Problem when both Alice and Bob are behind “NATs”.
NAT prevents an outside peer from initiating a call to insider peer
Solution:
Using Alice’s and Bob’s SNs, Relay is chosen
Each peer initiates session with relay.
Peers can now communicate through NATs via relay
2: Application Layer

21. Contents P2P architecture and benefits P2P content distribution
Content distribution network (CDN)
2: Application Layer

22. Why Content Networks?
More hops between client and Web server  more congestion!
Same data flowing repeatedly over links between clients and Web server C1 C4 C2 C3 S
- IP router
2: Application Layer
Slides from

23. Why Content Networks?
Origin server is bottleneck as number of users grows
Flash Crowds (for instance, Sept. 11)
The Content Distribution Problem: Arrange a rendezvous between a content source at the origin server ( and a content sink (us, as users)
2: Application Layer
Slides from

24. Example: Web Server Farm
Simple solution to the content distribution problem: deploy a large group of servers
L4-L7 Switch
Request from
grad.umd.edu
ren.cis.udel.edu
(Copy 1)
(Copy 3)
(Copy 2)
Arbitrate client requests to servers using an “intelligent” L4-L7 switch
Pretty widely used today
2: Application Layer

25. Example: Caching Proxy
Majorly motivated by ISP business interests – reduction in bandwidth consumption of ISP from the Internet
Reduced network traffic
Reduced user perceived latency
ISP
Other
traffic
Client
ren.cis.udel.edu
Intercepters
Internet
TCP port 80
traffic
Client
merlot.cis.udel.edu
Proxy
2: Application Layer

26. But on Sept. 11, 2001 Web Server www.cnn.com 1000,000 other hosts ISP
User
mslab.kaist.ac.kr
1000,000
other hosts
New Content
WTC News!
old
content
request
- Caching Proxy
ISP
Congestion /
Bottleneck
2: Application Layer

27. Problems with discussed approaches: Server farms and Caching proxies
Server farms do nothing about problems due to network congestion
Caching proxies serve only their clients, not all users on the Internet
Content providers (say, Web servers) cannot rely on existence and correct implementation of caching proxies
Accounting issues with caching proxies.
For instance, needs to know the number of hits to the webpage for advertisements displayed on the webpage
2: Application Layer

28. Again on Sept. 11, 2001 with CDN Web Server www.cnn.com WA CA MI IL MA
User
mslab.kaist.ac.kr
New Content
WTC News! FL IL DE NY MA MI CA WA
1000,000
other users
request
new
content
Distribution
Infrastructure
- Surrogate
2: Application Layer

29. Web replication - CDNs
Overlay network to distribute content from origin servers to users
Avoids large amount of same data repeatedly traversing potentially congested links on the Internet
Reduces Web server load
Reduces user perceived latency
Tries to route around congested networks
2: Application Layer

30. CDN vs. Caching Proxies
Caches are used by ISPs to reduce bandwidth consumption, CDNs are used by content providers to improve quality of service to end users
Caches are reactive, CDNs are proactive
Caching proxies cater to their users (web clients) and not to content providers (web servers), CDNs cater to the content providers (web servers) and clients
CDNs give control over the content to the content providers, caching proxies do not
2: Application Layer

31. CDN Architecture Origin Server CDN Client 2: Application Layer Request
Routing
Infrastructure
Distribution
& Accounting
Infrastructure
Surrogate
2: Application Layer

32. CDN Components Distribution Infrastructure:
Moving or replicating content from content source (origin server, content provider) to surrogates
Request Routing Infrastructure:
Steering or directing content request from a client to a suitable surrogate
Content Delivery Infrastructure:
Delivering content to clients from surrogates
Accounting Infrastructure:
Logging and reporting of distribution and delivery activities
2: Application Layer

33. Server Interaction with CDN
Origin
Server
Distribution
Infrastructure 1
Origin server pushes new content to CDN OR
CDN pulls content from origin server
Accounting
Infrastructure 2
2. Origin server requests logs and other accounting info from CDN OR
CDN provides logs and other accounting info to origin server
2: Application Layer

34. Client Interaction with CDN
Surrogate
(NY)
(CA)
CDN
california.cnn.akamai.com
newyorkcnn.akamai.com 1
1. Hi! I need 2
Go to surrogate
newyork.cnn.akamai.com
Request
Routing
Infrastructure 3
3. Hi! I need content /sept11 Q:
How did the CDN choose the New York surrogate over the California surrogate ?
2: Application Layer

35. Request Routing Techniques
Request routing techniques use a set of metrics to direct users to “best” surrogate
Proprietary, but underlying techniques known:
DNS based request routing
Content modification (URL rewriting)
Anycast based (how common is anycast?)
URL based request routing
Transport layer request routing
Combination of multiple mechanisms
2: Application Layer

36. DNS based Request-Routing
Common due to the ubiquity of DNS as a directory service
Specialized DNS server inserted in a DNS resolution process
DNS server is capable of returning a different set of A, NS or CNAME records based on policies/metrics
2: Application Layer

37. DNS based Request-Routing
Surrogate
Akamai
CDN
test.nyu.edu
newyork.cnn.akamai.com
california.cnn.akamai.com
Q: How does the Akamai DNS know which surrogate is closest ?
Akamai DNS
DNS response: A
DNS query:
Session
local DNS server (dns.nyu.edu)
1) DNS query:
DNS response: A
2: Application Layer

38. DNS based Request-Routing
Akamai DNS
Surrogate
Akamai
CDN
test.nyu.edu
Measurement results
Measure to
Client DNS
DNS query
Measurements
local DNS server
(dns.nyu.edu)
DNS query
2: Application Layer

39. DNS based Request-Routing
Client DNS
Surrogate
Akamai DNS
Akamai
CDN
Client
Requesting DNS
Surrogate
Requesting DNS
Available Bandwidth = 10 kbps
RTT = 10 ms
Available Bandwidth = 5 kbps
RTT = 100 ms A
TTL = 10s
2: Application Layer

40. DNS based Request Routing: Discussion
Originator Problem: Client may be far removed from client DNS
Client DNS Masking Problem: Virtually all DNS servers, except for root DNS servers honor requests for recursion
Q: Which DNS server resolves a request for test.nyu.edu?
Q: Which DNS server performs the last recursion of the DNS request?
Hidden Load Factor: A DNS resolution may result in drastically different load on the selected surrogate – issue in load balancing requests, and predicting load on surrogates
2: Application Layer

41. CDN Strategies
Pushing content closer to the users: hop count reduction (overall network traffic reduction)
CDN Strategies:
Limelight  placing CDN servers near a small # of ISP core nets
Akamai  placing CDN servers deep into a large # of ISP networks’ sites
Nano Data Center (NaDa)  home gateways (STBs/modems) as CDN servers (peer-to-peer delivery among NaDa servers)
Edge Router
Core Router
ONT
OLT
DSLAM
Modem
Access
Metro/Edge Network
Core Network
NaDa
Digital Media Delivery Platform

42. Summary P2P architecture and its benefits P2P content distribution
BitTorrent, Skype
Content distribution network (CDN)
DNS-based request routing
2: Application Layer