eMule behind the scenes

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eMule behind the scenes
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Overview

• Extends the eDonkey protocol
• File sharing network
• Several hundreds of eMule servers
• Millions of eMule clients
• Each server is an independent centralized
  indexing service and does not store any files

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eMule Client

• Preconfigured with a list of servers and a list
  of shared files
• Uses a single TCP connection for logging into the
  network
• Uses several TCP connections to upload/ download
  files to/from other clients

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Client to Server Connection

• Upon startup the client connects using TCP a
  single eMule server
• Receives a client ID that is valid only through
  the connection lifetime
• Sends its list of shared files
• Sends its download list
• Receives a download sources lists
• The connection is kept open during all the
  clients session
• After establishment
• Further search queries can be made and replied
  with sources list
• A filename query is answered with a set of
  matching hashes

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Client to Client Connection

• An eMule client connects another client (source)
  to download a file
• Files are fragmented and clients may download
  fragments of the same file form several sources
  concurrently
• When two clients connect the uploading client
  serves the downloading client or adds it to its
  downloading queue
• When a downloading client reaches the head of an
  uploading client the uploading client initiates a
  connection in order to send him the needed
  fragments

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High Level Network Diagram
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Data Transfer

• Compressed to save bandwidth
• Erasure coded to support fast recovery
• When a complete chunk of 9500 kilobytes is
  downloaded and verified this data is also shared
  by the downloader

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Client ID

• A client can receive a low-id or a high-id
• A low-id is assigned if the server fails to
  initiate a TCP connection to the client
• Low-id restricts the clients use of the network
  and might result with the server rejecting the
  client
• High-id is based on the client IP address and
  will remain constant as long as the IP address
  does not change

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Call back Mechanism

• Allows high-id clients to utilize low-id clients
  in case they are connected to the same server
• The server sends the low-id client the high-id
  clients IP address
• The low-id client then initiates a TCP connection
  to the high-id client and serve it

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Credit System

• A local tit-for-tat mechanism to encourage
  uploads
• A downloading client may identify itself upon
  clients connection establishment through a simple
  RSA challenge response
• Clients records the upload/download ratio for
  other clients and promote friendly clients in
  their queues