Amy Chen @amy Moulindra Muchumari @mm1729 Dylan Herman @oneTimePad

IMPLEMENTATION DETAILS

Dependency Diagram:

Client --GET list of peers-> Tracker---Contact Tracker--> Tracker server | --peers list---> PeerDownloader---get bittorrent packets--> Message | --send/get pieces--> PieceManager --write data-> FileWriter --write to disk-> I/O disk

///////////////////////////////// // Download & Upload Bitfields // /////////////////////////////////

Per peer, PeerDownload contacts all peers.

Handshakes Peer: client tells peer its client ID & torrent hash Handshake from Peer: peer sends its ID and torrent hash SUCCESS: if handshake is successful, EXCHANGE BITFIELDS

Exchange Bitfields: A bitfield represents the pieces that a peer has. The peer and client exchange bitfields. On the client, we have 2 byte slices. One slice represents the bits we have received (aka currently own). The other slice represents pieces we are missing (aka pieces we need to request from the peer).

our pieces      [0 0 0 0 0 0]
missing pieces  [1 1 1 1 1 1]

When we receive a piece, we flip the bit for the ones we receive. To calculate the missing: !(our pieces) & (peer pieces) = (missing pieces)

Because our bitfields are represented in byte slices, the following calculation is needed:

PIECE INDEX = (byte index * 8) + bit index

////////////////////////////// // DOWNLOAD & UPLOAD PIECES // //////////////////////////////

If PieceManager determines we need a piece, it returns TRUE from compareBitField. Otherwise we move on to the next peer.

We send the peer an INTERESTED message to notify we would like to download a piece(s).

We wait for an UNCHOKE message from the peer, which indicates that the peer is ready to send pieces. After receiving the unchoke message, our connection has state UNCHOKED and INTERESTED. At this point, we are able to send request messages and receive piece messages as response.

Next, we ask PieceManager for the piece index to be requested. Then, we send the REQUEST message to the peer. After sending the request, we wait for a PIECE message for the REQUEST sent. We currently only send one REQUEST at a time and wait until the corresponding PIECE message is received to the send the next REQUEST. If the peer receives a CHOKE message at any time in this loop, we wait until we receive an UNCHOKE message to resume sending REQUESTS.

We added a time calculation of the total time it takes to download all the pieces.

////////////////// // PIECEMANAGER // //////////////////

PieceManager keeps track of what pieces need to be downloaded. Currently, it works with one peer at a time. It stores the peer bitfield and the client bitfield to check what pieces it is missing. PieceManager stores the requests (10 by default) in a queue to quickly send the requests

///////// // RTT // /////////

findRTT - Finds the round trip time for a given peer. This calculates the time it takes to ping a peer 10 times. Then divide this number by 10 to find the average.

findMinRTT - Finds the minimum round trip time of the 3 peers that are contacted.