IS250
Spring 2010
John Chuang

 The Internet infrastructure, based on TCP/IP, provides:
Global reachability
Reliable end-to-end transport
 Highly successful in supporting one-to-one (unicast) communication
 But there are some limitations:
Difficult to deploy new network services (e.g., IP multicast, IP anycast, QoS, IPv6)
Lack of support for one-to-many (multicast) or even many-tomany (“peer-to-peer”) communication
End hosts have no control over what goes on in the network
(e.g., no source routing or user-directed routing)
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 One strategy: build an overlay network at the application layer
 End hosts gain control over topology formation, routing, to meet specific application needs
 New applications and services can be deployed without changes to the TCP/IP infrastructure
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 Logical topology
 Self-organized
 Dynamic
 Application specific
Application layer overlay
Network layer
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 Domain Name Service (DNS)
 6bone: IPv6 over IPv4
 Mbone: multicast over unicast IP
 X-Bone
John Chuang http://www.mbone.cl.cam.ac.uk/mbone/mbone-small.gif
http://graphics.stanford.edu/papers/mbone/morepix/world-6bone.jpeg
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 Web Caching and Content Distribution Networks (CDNs)
 Application Layer Multicast (ALM)
 User Directed Routing
Anonymous Routing
Resilient overlay network
 Peer-to-Peer (P2P)
Unstructured P2P: gnutella, FreeNet, kazaa,…
Structured P2P: Distributed Hash Tables (DHTs)
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 Improves download latency, content availability by storing local copy of popular web objects
 Web caches are L7 boxes client
John Chuang proxy cache network caches reverse proxy cache web server
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 Clients are intelligently redirected to nearest CDN server to download publisher content
 IP anycast (if it exists) could accomplish this easily…
 In the absence of IP anycast, companies like Akamai constructs
CDNs as application layer overlay networks client
CDN servers web server
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Step 1: client queries DNS for IP address of www.publisher.com; based on client’s IP address, reconfigured publisher DNS returns IP address of replica closest to client
Local DNS publisher DNS publisher client
Nearest replica
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Step 2: client contacts replica for object
Local DNS publisher DNS publisher client
Nearest replica
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Local DNS
Step 1: client queries DNS for IP address of www.publisher.com
publisher client
CDN server
CDN DNS
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Step 2: client contacts publisher; publisher returns HTML with embedded
Local DNS objects’ URLs pointing to best CDN server publisher client
CDN server
CDN DNS
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Local DNS
Step 3: client queries DNS for IP address of
CDN server publisher client
CDN server
CDN DNS
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Local DNS
Step 4: client contacts CDN server;
CDN server returns embedded objs publisher client
CDN server
CDN DNS
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 Web Caching and Content Distribution Networks (CDNs)
 Application Layer Multicast (ALM)
 User Directed Routing
Anonymous Routing
Resilient overlay network
 Peer-to-Peer (P2P)
Unstructured P2P: gnutella, FreeNet, kazaa,…
Structured P2P: Distributed Hash Tables (DHTs)
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 Network routers must implement IP Multicast to construct delivery tree and forward packets to multicast group receivers routers client server
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 End hosts self-organize to construct multicast delivery tree; messages sent using IP unicast
 Sacrifice some efficiency (latency stretch) for deployability
 Various systems: ESM, Overcast, Promise, Scattercast, SplitStream,
Yoid, … routers client server
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 Web Caching and Content Distribution Networks (CDNs)
 Application Layer Multicast (ALM)
 User Directed Routing
Anonymous Routing
Resilient overlay network
 Peer-to-Peer (P2P)
Unstructured P2P: gnutella, FreeNet, kazaa,…
Structured P2P: Distributed Hash Tables (DHTs)
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 IP source route allows end hosts to exercise some degree of route control
 However, many ISPs turned off IP source routing option for security reasons routers client server
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 Some applications would benefit from having some degree of control over route selection
Resiliency: e.g., resilient overlay network (RON), Detour
Anonymity: onion routing, MIX-nets, … routers client server
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 Application layer overlay for anonymous routing
Existence of communication between
Alice and Bob not revealed to any 3rd party
 Alice constructs onion where message is successively encrypted with keys of intermediate routing nodes
 Each intermediate node ‘peels’ one layer of onion and forward to next node
 Example system: Tor
John Chuang http://tor.eff.org/overview.html.en
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 Web Caching and Content Distribution Networks (CDNs)
 Application Layer Multicast (ALM)
 User Directed Routing
Anonymous Routing
Resilient overlay network
 Peer-to-Peer (P2P)
Unstructured P2P: gnutella, FreeNet, kazaa,…
Structured P2P: Distributed Hash Tables (DHTs)
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 Self-organized overlay network to support distributed storage, search and retrieval of content
The killer-app: free music and movies
 Individual peers contribute resources
Content
Network management (e.g., forwarding query messages)
 Desirable properties:
Scalability
Performance (latency, recall)
Robustness
Anonymity, censorship-resistance
 Design challenges:
Dynamic membership
Various forms of attacks
Free-riding behavior
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 1 st generation: centralized index
e.g., Napster
 2 nd generation: decentralized indices
e.g., Gnutella v0.4, Freenet
 3 rd generation: hierarchical
e.g., FastTrack (KaZaA, Grokster, Morpheus), eDonkey2000, Gnutella v0.6
 4 th generation:
Structured topologies using DHTs, e.g., eMule,
Overnet, BitTorrent
Parallel downloads, e.g., BitTorrent, Avalanche
Darknets, e.g., WASTE for small-scale “F2F”
John Chuang networks
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 Maintains a centralized index that maps files to machines
 How to find a file
Query the index system  return a list of peers that store the requested file
Transfer the file directly from peer(s)
 Advantage: m6
F
Simplicity: easy to implement sophisticated search engines on top of the index system
 Disadvantage:
Single point of failure
E?
m1
A
E
E?
m5 m5
E m1 A m2 B m3 C m4 D m5 E m6 F m2
B
John Chuang
Slide adapted from Ion Stoica, Nicolas Christin
25 m4
D m3
C

 Flood the request
 How to find a file:
Send request to all neighbors
Neighbors recursively propagate the request
Eventually a machine that has the file receives the request, and it sends back the answer m6
 Advantages:
Totally decentralized, highly robust
 Disadvantages:
The entire network can be swamped with a request
Can be alleviated using TTLs, but can then fail to locate files (and still high resource usage)
John Chuang
F m1
E
A
E?
m5
E?
E m2
B
E?
E?
Slide adapted from Ion Stoica, Nicolas Christin m4 m3
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C
Assume: m1’s neighbors are m2 and m3; m3’s neighbors are m4 and m5;…
D

 Use two-level hierarchy
F
Some nodes are elected as “super nodes” or “ultra-peers”
Each ultra-peer serves as centralized index for a portion of the network
If an ultra-peer does not know where to find an item, query is forwarded to other ultra-peers
 Advantage:
F
F?
E
F?
m4
Reduce the amount of network traffic compared to “naïve” flooding
F?
 Disadvantage:
Ultra-peers vulnerable to attacks
A
B
C m3
Potential convergence problems when ultra-peers leave abruptly
 Used in FastTrack (KaZaA, Grokster,
Morpheus), eDonkey2000, Gnutella v0.6
John Chuang m1 m2
Assume red nodes are ultra-peers
Slide adapted from Ion Stoica, Nicolas Christin
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D

 Gnutella and KaZaA topologies are unstructured
Neighbor selection largely random
No guarantee that a file can be located, even if it exists in the network
 Distributed hash tables (DHTs) offer to solve this problem by constructing highly structured topologies
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 Applications: distributed search (e.g., p2p, CDNs, cooperative caching), application layer overlays for multicast, anycast, etc.
 Similar to traditional hash table data structure, except data is stored in distributed peer nodes
Each node is analogous to a bucket in a hash table
Put(), Get() interface like a regular hash table:
-
put( id , item ); item = get( id );
 Designed to scale to large numbers of nodes and to handle continual node arrivals, departures, or failures.
 Various DHT designs:
CAN, Chord, Kademlia, Pastry, Tapestry, Viceroy, etc.
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 Associate each node and item to a unique identifier in a one
 Each node x
-
-
Fingers are neighbors i -th entry in finger table is the first node that succeeds or equals x + 2 i successor node of dimensional space (0..2
maintains a finger table
 An item identified by id id is stored on the m )
 Properties
Routing table size O(log( nodes
N )) , where N is the total number of
Guarantees that a file (if it exists) is found in O(log( N )) steps
John Chuang
Slide adapted from Ion Stoica, Nicolas Christin
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 Assume m = 3, i.e., an identifier space
0..7
 Node n1:(1) joins
7
6
0
1
2
Finger Table i id +2 i succ
0 2 1
1 3 1
2 5 1
John Chuang
5 3
4
Slide adapted from Ion Stoica, Nicolas Christin
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 Assume m = 3, i.e., an identifier space
0..7
 Node n1:(1) joins
 Node n2:(2) joins
7
John Chuang
0
1
Finger Table i id +2 i succ
0 2 2
1 3 1
2 5 1
6 2
5
4
3
Finger Table i id +2 i succ
0 3 1
1 4 1
2 6 1
Slide adapted from Ion Stoica, Nicolas Christin
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 Assume m = 3, i.e., an identifier space
0..7
 Node n1:(1) joins
 Node n2:(2) joins
 Nodes n3:(0), n4:(6) join
Finger Table i id +2 i succ
0 7 0
1 0 0
2 2 2
6
7
John Chuang
0
Finger Table i id +2 i succ
0 1 1
1 2 2
2 4 6
1
2
Finger Table i id +2 i succ
0 2 2
1 3 6
2 5 6
5
4
3
Finger Table i id +2 i succ
0 3 6
1 4 6
2 6 6
Slide adapted from Ion Stoica, Nicolas Christin
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 Items inserted: f1:(7), f2:(1)
6
7
0
Finger Table i id +2 i succ
0 1 1
1 2 2
2 4 6
Items
7
1
Finger Table i id +2 i succ
0 2 2
1 3 6
2 5 6
Items
1
2 Finger Table i id +2 i succ
0 7 0
1 0 0
2 2 2
John Chuang
5
4
3
Finger Table i id +2 i succ
0 3 6
1 4 6
2 6 6
Slide adapted from Ion Stoica, Nicolas Christin
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 Upon receiving a query for item id , a node
Checks if item is cached locally
If not, forwards the query to the largest node in its successor table that does not exceed id
7
6 Finger Table i id +2 i succ
0 7 0
1 0 0
2 2 2
5
John Chuang
0
Finger Table i id +2 i succ
0 1 1
1 2 2
2 4 6
Items
7 query(7)
1
Finger Table i id +2 i succ
0 2 2
1 3 6
2 5 6
Items
1
2
4
3
Finger Table i id +2 i succ
0 3 6
1 4 6
2 6 6
Slide adapted from Ion Stoica, Nicolas Christin
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 Difficult to deploy new network services at network layer
 Response: build overlay network at the application layer
End hosts gain control over topology formation, routing, to meet specific application needs
New applications and services can be deployed without changes to the
TCP/IP infrastructure
 Many flavors of application layer overlay networks:
Web Caching and Content Distribution Networks (CDNs)
Application Layer Multicast (ALM)
Anonymous Routing (Tor)
Resilient overlay network (RON)
P2P file-sharing networks
Distributed Hash Tables (DHTs)
…
John Chuang 36