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: Efficiently locating the sharing files

Passive Distributed Indexing

PDI: provide a general-purpose file search service.Each mobile device maintains: Repository: a set of sharing files, PDI

provides local search services Doc ID: IP/MAC + local path

Index cache: a set of (keyword, doc ID) Used to answer query for non-local doc

PDI: QueryQuery model: 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 : CacheCache: all received query results in the local cache index (all nodes which receives a message)Local cache indexing replacement: Least-recently-used algorithm Timeout

Exploit locality and erase hotspots

PDI: Messages

QUE: Query string, SRC, SEQ, TTL Each node stores the highest SEQ for each

SRC and prevents retransmission.

REP: 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

MotivationPeople want anonymity for all kinds of reasonsThere are some entities which are interested in exposing the host’s identityThe goal of Internet anonymization: A host can communicate with an arbitrary

server in such a manner that nobody can determine the host’s identity.

Previous work:Proxy: Trust the proxy, can be blocked by servers, DOS

A set of mix relays: Onion routing, Zero-knowledge’s freedom Relay may be corrupted, timing analysis, some

other same problems with proxy

The above two: Ignore the attack by observing all network traffic There are some other solutions, but still not good

Tarzan: P2P Technique armed

Extend mix-net design to a peer-to-peer environmentcommunicate over sequences of mix relays chosen from a pool of volunteer nodes, without centralized component.All peers are potential originators and relaysNobody can tell who is the first hop in a mix path (except the originator itself)

Tarzan: resistant to adversary nodes

A new concept: domainUsed to remove potential adversarial biasBased on the observation: An adversary may run hundreds of

virtual machines, yet is unlikely to control hundreds of different IP subnets.

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

Tarzan: Architecture Overview

Tarzan: Packet relayTwo types of messages: data & controlA flow tag uniquely identifies each link of each tunnel. (used for 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 packetT = (h1, h2,…, hl, hpnat)For each relay: ekhi, ikhic(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 encryptionEach 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 SetupInitiator is responsible for that workIncludes: 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: Periodically ping message Start reconstruction from the failed

relay

Tarzan: Peer discoveryGossip AlgorithmEach node has a public key two handshake authentication

From weakly connected to fully connectedThree different/related operations: 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 trafficMimics: node pairs Calculated, not randomly selected !! Mimic relationship is symmetric !!

Tunnels must be built through mimicsCover 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 usersOverhead: 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)