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Application Layer Overlays
IS250
Spring 2010
John Chuang
Application Layer Overlay
 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)
John Chuang
2
Application Layer Overlay
 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
John Chuang
3
Overlay Networks
 Logical topology
 Self-organized
 Dynamic
 Application specific
Application layer overlay
Network layer
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Early Examples




Domain Name Service (DNS)
6bone: IPv6 over IPv4
Mbone: multicast over unicast IP
X-Bone
John Chuang
http://graphics.stanford.edu/papers/mbone/morepix/world-6bone.jpeg
http://www.mbone.cl.cam.ac.uk/mbone/mbone-small.gif
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Some Overlay Networks
 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)
John Chuang
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Web Caching
 Improves download latency, content availability by
storing local copy of popular web objects
 Web caches are L7 boxes
web
server
client
proxy
cache
John Chuang
network
caches
reverse
proxy
cache
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Content Delivery Networks
 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
John Chuang
CDN
servers
web
server
8
Method 1: DNS Redirect
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
John Chuang
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Method 1: DNS Redirect
Step 2: client contacts replica for object
Local DNS
publisher DNS
publisher
client
Nearest
replica
John Chuang
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Method 2: URL Redirect
Step 1: client queries DNS for IP address of
www.publisher.com
Local DNS
publisher
client
CDN DNS
CDN server
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Method 2: URL Redirect
Step 2: client contacts publisher;
publisher returns HTML with embedded
objects’ URLs pointing to best CDN server
Local DNS
publisher
client
CDN DNS
CDN server
John Chuang
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Method 2: URL Redirect
Step 3: client queries DNS for IP address of
CDN server
Local DNS
publisher
client
CDN DNS
CDN server
John Chuang
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Method 2: URL Redirect
Local DNS
Step 4: client contacts CDN server;
CDN server returns embedded objs
publisher
client
CDN DNS
CDN server
John Chuang
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Some Overlay Networks
 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)
John Chuang
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IP Multicast
 Network routers must implement IP Multicast to
construct delivery tree and forward packets to multicast
group receivers
routers
client
John Chuang
server
16
Application Layer Multicast
 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
John Chuang
server
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Some Overlay Networks
 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)
John Chuang
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IP Source Route
 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
John Chuang
server
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User Directed Routing
 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
John Chuang
server
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Onion Routing
 Application layer overlay for
anonymous routing
- Existence of communication between
Alice and Bob not revealed to any 3rd
party
http://tor.eff.org/overview.html.en
 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
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Some Overlay Networks
 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)
John Chuang
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P2P
 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
John Chuang
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P2P File-Sharing Networks
 1st generation: centralized index
- e.g., Napster
 2nd generation: decentralized indices
- e.g., Gnutella v0.4, Freenet
 3rd generation: hierarchical
- e.g., FastTrack (KaZaA, Grokster, Morpheus),
eDonkey2000, Gnutella v0.6
 4th generation:
- Structured topologies using DHTs, e.g., eMule,
Overnet, BitTorrent
- Parallel downloads, e.g., BitTorrent, Avalanche
- Darknets, e.g., WASTE for small-scale “F2F”
networks
John Chuang
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Napster
 Maintains a centralized index that
maps files to machines
m6
 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)
m5
E
F
E?
E
 Advantage:
E?
m5
- Simplicity: easy to implement
sophisticated search engines
on top of the index system
 Disadvantage:
- Single point of failure
John Chuang
m1
m2
m3
m4
m5
m6
D
A
B
C
D
E
F
m4
C
A
B
m3
m1
m2
Slide adapted from Ion Stoica, Nicolas Christin
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Gnutella (v0.4)
 Flood the request
 How to find a file:
m5
- 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
E
m6
F
E
m4
E?
E?
 Advantages:
- Totally decentralized, highly robust
E?
 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
D
E?
C
A
m1
B
m3
m2
Assume: m1’s neighbors are m2 and m3;
m3’s neighbors are m4 and m5;…
Slide adapted from Ion Stoica, Nicolas Christin
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Hierarchical Networks
 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
F
where to find an item, query is
forwarded to other ultra-peers
 Advantage:
- Reduce the amount of network traffic
compared to “naïve” flooding
 Disadvantage:
- Ultra-peers vulnerable to attacks
- Potential convergence problems when
ultra-peers leave abruptly
 Used in FastTrack (KaZaA, Grokster,
Morpheus), eDonkey2000, Gnutella v0.6
John Chuang
E
F?
m4
F?
D
F?
C
A
m1
B
m3
m2
Assume red nodes are ultra-peers
Slide adapted from Ion Stoica, Nicolas Christin
27
Structured Topologies
 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
John Chuang
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Distributed Hash Table (DHT)
 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.
John Chuang
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DHT Example: Chord
 Associate each node and item to a unique
identifier in a one-dimensional space (0..2m)
 Each node x maintains a finger table
- Fingers are neighbors
- i-th entry in finger table is the first node that succeeds or equals
x + 2i
 An item identified by id is stored on the
successor node of id
 Properties
- Routing table size O(log(N)) , where N is the total number of
nodes
- 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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Chord Example
 Assume m = 3, i.e.,
an identifier space
0..7
 Node n1:(1) joins
Finger Table
i id+2i succ
0 2
1
1 3
1
2 5
1
0
1
7
6
2
5
3
4
John Chuang
Slide adapted from Ion Stoica, Nicolas Christin
31
Chord Example
 Assume m = 3, i.e.,
an identifier space
0..7
 Node n1:(1) joins
 Node n2:(2) joins
Finger Table
i id+2i succ
0 2
2
1 3
1
2 5
1
0
1
7
6
2
Finger Table
5
3
4
John Chuang
i id+2i succ
0 3
1
1 4
1
2 6
1
Slide adapted from Ion Stoica, Nicolas Christin
32
Chord Example
Finger Table
 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+2i succ
0 7
0
1 0
0
2 2
2
i id+2i succ
0 1
1
1 2
2
2 4
6
Finger Table
i id+2i succ
0 2
2
1 3
6
2 5
6
0
1
7
6
2
Finger Table
5
3
4
John Chuang
i id+2i succ
0 3
6
1 4
6
2 6
6
Slide adapted from Ion Stoica, Nicolas Christin
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Insertion
Finger Table
i
i id+2
0 1
1 2
2 4
 Items inserted: f1:(7), f2:(1)
Items
7
succ
1
2
6
0
1
7
Finger Table
i id+2i succ
0 7
0
1 0
0
2 2
2
6
2
Finger Table
5
3
4
John Chuang
Finger Table Items
i id+2i succ 1
0 2
2
1 3
6
2 5
6
i id+2i succ
0 3
6
1 4
6
2 6
6
Slide adapted from Ion Stoica, Nicolas Christin
34
Query
 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
Finger Table
i id+2i succ
0 7
0
1 0
0
2 2
2
Finger Table
i
i id+2
0 1
1 2
2 4
0
Finger Table Items
i id+2i succ 1
0 2
2
1 3
6
2 5
6
1
7
query(7)
6
2
Finger Table
5
3
4
John Chuang
Items
7
succ
1
2
6
i id+2i succ
0 3
6
1 4
6
2 6
6
Slide adapted from Ion Stoica, Nicolas Christin
35
Summary
 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