application layer overlays

Application Layer Overlays

IS250Spring 2010

John Chuang

John Chuang 2

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-to-many (“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)

Application Layer Overlay

John Chuang 3

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

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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

http://www.mbone.cl.cam.ac.uk/mbone/mbone-small.gifhttp://graphics.stanford.edu/papers/mbone/morepix/world-6bone.jpeg

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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)

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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 serverclient

proxycache

networkcaches

reverse proxycache

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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

web serverclient

CDN servers

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publisher DNS

publisher

client

Nearestreplica

Method 1: DNS Redirect

Local DNS

Step 1: client queries DNS for IP address of www.publisher.com; based on client’s IP address, reconfigured publisher DNSreturns IP address of replica closest to client

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publisher DNS

publisher

client

Nearestreplica

Method 1: DNS Redirect

Local DNS

Step 2: client contacts replica for object

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Local DNS

publisher

client

CDN server

Method 2: URL Redirect

CDN DNS

Step 1: client queries DNS for IP address of www.publisher.com

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Local DNS

publisher

client

CDN server


CDN DNS

Step 2: client contacts publisher;publisher returns HTML with embedded objects’ URLs pointing to best CDN server

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Local DNS

publisher

client

CDN server


CDN DNS

Step 3: client queries DNS for IP address of CDN server

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Local DNS

publisher

client

CDN server


CDN DNS

Step 4: client contacts CDN server;CDN server returns embedded objs

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IP Multicast Network routers must implement IP Multicast to construct delivery tree and forward packets to multicast group receivers

serverclient

routers

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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, …

serverclient

routers

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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

serverclient

routers

default route

IP sourc

e

route

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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, …

serverclient

routers

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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

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

http://tor.eff.org/overview.html.en

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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

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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

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Napster 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: - Simplicity: easy to implement sophisticated search engines on top of the index system

Disadvantage:- Single point of failure

AB

C

D

E

F

m1m2

m3

m4

m5

m6

m1 Am2 Bm3 Cm4 Dm5 Em6 F

E?

m5

E?E

Slide adapted from Ion Stoica, Nicolas Christin

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Gnutella (v0.4) 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

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)

AB

C

D

E

F

m1m2

m3

m4

m5

m6

E?

E?

E?E?

E

Assume: m1’s neighbors are m2 and m3; m3’s neighbors are m4 and m5;…


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Hierarchical Networks

Use two-level hierarchy- 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:- 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

A B

C

D

EF

m1 m2

m3

m4

F?

F?

F?F

Assume red nodes are ultra-peers


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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

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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.

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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


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Chord Example Assume m = 3, i.e., an identifier space 0..7

Node n1:(1) joins

01

2

34

5

6

7

i id+2i succ0 2 11 3 12 5 1

Finger Table


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Chord Example

01

2

34

5

6

7

i id+2i succ0 2 21 3 12 5 1

Finger Table

i id+2i succ0 3 11 4 12 6 1

Finger Table

Assume m = 3, i.e., an identifier space 0..7

Node n1:(1) joins Node n2:(2) joins


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Chord Example

01

2

34

5

6

7

i id+2i succ0 2 21 3 62 5 6

Finger Table

i id+2i succ0 3 61 4 62 6 6

Finger Table

i id+2i succ0 1 11 2 22 4 6

Finger Table

i id+2i succ0 7 01 0 02 2 2

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


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Insertion

Items inserted: f1:(7), f2:(1)

01

2

34

5

6

7 i id+2i succ0 2 21 3 62 5 6

Finger Table

i id+2i succ0 3 61 4 62 6 6

Finger Table

i id+2i succ0 1 11 2 22 4 6

Finger Table

7

Items 1

Items

i id+2i succ0 7 01 0 02 2 2

Finger Table


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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

01

2

34

5

6

7 i id+2i succ0 2 21 3 62 5 6

Finger Table

i id+2i succ0 3 61 4 62 6 6

Finger Table

i id+2i succ0 1 11 2 22 4 6

Finger Table

7

Items 1

Items

i id+2i succ0 7 01 0 02 2 2

Finger Table

query(7)


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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)- …

application layer overlays

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