A distributed Search Service for PeertoPeer File Sharing

A distributed Search Service for Peer-to-Peer File Sharing in Mobile Applications From U. of Dortmund, Germany

Motivation: Mobile devices become more powerful (computation, resources) They form spontaneous self-organizing communication structure: Mobile Ad-Hoc Network. (all of them are peers) People shares files among those mobile devices to satisfy more requirements. Challenge: n Efficiently locating the sharing files

Passive Distributed Indexing PDI: provide a general-purpose file search service. Each mobile device maintains: n Repository: a set of sharing files, PDI provides local search services w Doc ID: IP/MAC + local path n Index cache: a set of (keyword, doc ID) w Used to answer query for non-local doc

PDI: Query model: n n A query string contains several keywords “AND” operation on all keywords Broadcast: query/response messages (nature of wireless network) Forward: for a predefined number of hops (using broadcast) By experiments: 2 hops are enough

PDI : Cache: all received query results in the local cache index (all nodes which receives a message) Local cache indexing replacement: n n Least-recently-used algorithm Timeout Exploit locality and erase hotspots

PDI: Messages QUE: n n Query string, SRC, SEQ, TTL Each node stores the highest SEQ for each SRC and prevents retransmission. REP: n n contains local search results (set of Doc Ids) Selectively forwarding: only forward the doc Ids which are not in the local cache index

PDI: Example

Experiment Parameters:

Experiment results:

Tarzan: A Peer-to-Peer Anonymizing Network Layer from NYU & MIT

Motivation People want anonymity for all kinds of reasons There are some entities which are interested in exposing the host’s identity The goal of Internet anonymization: n A host can communicate with an arbitrary server in such a manner that nobody can determine the host’s identity.

Previous work: Proxy: n Trust the proxy, can be blocked by servers, DOS A set of mix relays: n n Onion routing, Zero-knowledge’s freedom Relay may be corrupted, timing analysis, some other same problems with proxy The above two: n n Ignore the attack by observing all network traffic There are some other solutions, but still not good

Tarzan: P 2 P Technique armed Extend mix-net design to a peer-to-peer environment communicate over sequences of mix relays chosen from a pool of volunteer nodes, without centralized component. All peers are potential originators and relays Nobody can tell who is the first hop in a mix path (except the originator itself)

Tarzan: resistant to adversary nodes A new concept: domain Used to remove potential adversarial bias Based on the observation: n An adversary may run hundreds of virtual machines, yet is unlikely to control hundreds of different IP subnets.

Tarzan: more… Cover traffic for packet routing Packets can be routed only between mimics Applications (with Tarzan support) can talk to Applications (without Tarzan support) through special IP tunnels Tarzan is transparent to Applications. Tarzan don’t provide authentication and congestion control functionalities.

Tarzan: Architecture Overview

Tarzan: Packet relay Two types of messages: data & control A flow tag uniquely identifies each link of each tunnel. (used forwarding) Symmetric encryption hides data, MAC protects integrity. Separate keys are used in each direction of each relay

Tarzan: Packet relay (cont. ) Clear IP packet’s src filed, encrypt and encapsulate in a UDP packet T = (h 1, h 2, …, hl, hpnat) For each relay: ekhi, ikhi c(i) = ENC(ekhi, {B(I+1)}) a(i) = MAC(ikhi, {seq, c(i)}) B(i) = {seq, c(I), a(i)}

Tarzan: packet relay (cont. ) The initiator does all the encryption Each relay just decrypts the block, retags it, encapsulates in a new UDP packet and forwards it. On the reverse path, the relays encrypt the packet and the initiator decrypts the final packets

Tarzan: Tunnel Setup Initiator is responsible for that work Includes: n n Generate/distribute symmetric keys Iteratively setup the tunnel one by one an establish request (forward session key are encrypted by the public key of node hi ) Using the existing tunnel to setup next step, so the relays on current tunnel don’t know it’s a data message or control message (for setting up another relay)

Tarzan: IP packet forwarding The last node on the tunnel (PNAT) will send the packet to the server with its own address. Upon receiving the replay, it will send it back along the tunnel. Tunnel failure & reconstruction: n n Periodically ping message Start reconstruction from the failed relay

Tarzan: Peer discovery Gossip Algorithm Each node has a public key n two handshake authentication From weakly connected to fully connected Three different/related operations: n n n Initialization: send entire neighbor set (for fast propagation) Redirection: redirect new nodes to random neighbors (for shed load) Maintenance: an incremental update

Tarzan: peer selection

Tarzan: cover traffic Mimics: node pairs n n Calculated, not randomly selected !! Mimic relationship is symmetric !! Tunnels must be built through mimics Cover traffic is transferred between mimics (adjust according to all incoming traffic and outgoing traffic) So nobody can observe the real user data

Tarzan: Mimic Topology

Experiment result:

Tarzan: conclusion Resistant to attack (a lot of security analysis) Achieve anonymity for end users Overhead: n n n Each node needs to keep some info for all other nodes in the network Packet transfer latency Considerable computation workload (especially on the initiator of the traffic)