Structure Overlay Networks and Chord Presentation by Todd Gardner Figures from: Ion Stoica, Robert Morris, David Liben- Nowell, David R. Karger, M. Frans Kaashoek, Frank Dabek, Hari Balakrishnan, Chord: A Scalable Peer-to- peer Lookup Protocol for Internet Applications. To Appear in IEEE/ACM Transactions on Networking

Peer-to-peer networks, History The first large peer-to-peer network was Napster The first large peer-to-peer network was Napster As we went over in class, Napster relied on a central server to store all routing information, so it was only sort of peer-to-peer As we went over in class, Napster relied on a central server to store all routing information, so it was only sort of peer-to-peer

Peer-to-peer networks, History The later peer-to-peer networks, like Gnutella, are fully distributed, but don’t have the guarantees Napster had for finding files The later peer-to-peer networks, like Gnutella, are fully distributed, but don’t have the guarantees Napster had for finding files If a file lay beyond your number of hops on the network, you might not find it If a file lay beyond your number of hops on the network, you might not find it

Structure Overlay Networks Introducing Chord (and others) Introducing Chord (and others) The idea is to build a peer to peer network, but to offer guarantees about finding data if it is on the network The idea is to build a peer to peer network, but to offer guarantees about finding data if it is on the network This is accomplished by enforcing a topology on the network This is accomplished by enforcing a topology on the network

What do I mean by enforcing a topology? This is where it gets complicated This is where it gets complicated Nodes in a Gnutella-like network join willy-nilly, using a reference to a node on the network, and having a potentially inefficient topology for searching: Nodes in a Gnutella-like network join willy-nilly, using a reference to a node on the network, and having a potentially inefficient topology for searching:

Attempting to correct the problem A simple solution would just be to publish well known nodes A simple solution would just be to publish well known nodes This is just recreating Napster This is just recreating Napster

Chord’s Approach Whenever a node joins a Chord-type network, it joins a “ring” Whenever a node joins a Chord-type network, it joins a “ring” All members of the network are on the same ring All members of the network are on the same ring Your position in the ring is determined by a Key (in most applications, this key will be your IP address) Your position in the ring is determined by a Key (in most applications, this key will be your IP address)

An example ring

Knowledge of the ring from the Node’s perspective The node should know about the next node in the ring – the node’s successor The node should know about the next node in the ring – the node’s successor The node should have a table of log n (n is the number of total keys) other nodes around the ring, with exponentially increasing keys – the node’s finger table The node should have a table of log n (n is the number of total keys) other nodes around the ring, with exponentially increasing keys – the node’s finger table

Other information The node stores the previous node in the ring, it’s predecessor The node stores the previous node in the ring, it’s predecessor The node stores values for the distributed hash table The node stores values for the distributed hash table

Storing/Retrieving Data Data is inserted into the ring with a Key, which is hashed to be put onto a node in the table Data is inserted into the ring with a Key, which is hashed to be put onto a node in the table When one wants to get data, one takes the key, applies the hash function to find the Key of the node it’s on, and uses it’s knowledge of the table When one wants to get data, one takes the key, applies the hash function to find the Key of the node it’s on, and uses it’s knowledge of the table

Best shown by example

So, what does this get us? If the node’s successors are correct, it is guaranteed to find the correct node If the node’s successors are correct, it is guaranteed to find the correct node If the node’s finger table is correct, it can do it in logarithmic time. If the node’s finger table is correct, it can do it in logarithmic time. The protocol includes functions to update these constantly The protocol includes functions to update these constantly

Example: DNS Russ Cox, Athicha Muthitacharoen, Robert T. Morris, Serving DNS using a Peer-to-Peer Lookup Service, In the proceedings of the First International Workshop on Peer-to-Peer Systems (IPTPS '02), March, 2002; Cambridge, MA. Russ Cox, Athicha Muthitacharoen, Robert T. Morris, Serving DNS using a Peer-to-Peer Lookup Service, In the proceedings of the First International Workshop on Peer-to-Peer Systems (IPTPS '02), March, 2002; Cambridge, MA.