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A distributed Search Service for Peer-to-Peer File Sharing in Mobile Applications
From U. of Dortmund, Germany
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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
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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
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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
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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
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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
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PDI: Example
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Experiment Parameters:
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Experiment results:
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Tarzan: A Peer-to-Peer Anonymizing Network Layer
from NYU & MIT
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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.
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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
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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)
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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.
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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.
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Tarzan: Architecture Overview
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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
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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)}
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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
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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)
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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
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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
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Tarzan: peer selection
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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
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Tarzan: Mimic Topology
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Experiment result:
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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)