【導讀】V. CSI5321. Dec122020. Submittedby. ArunChokkalingam&FirasathRiyaz. 1Torrent(MetaInfo)File. bencoded.Bencoding. StringsNormal. Strings[series. ofcontinuous. characters]. <string. length>:<string. data>. 7:work. IntegersNormal. integers. i<integer>ei3e. represents3.ListsTheyarelists. oftypes. [strings,I. integers,lists,dictionaries].l<bencoded. type>e. l8:advanced7:worke. ["advanced","work"]. mappingof. keystovalues. d<bencoded. string><bencoded. element>e. Note:Thekeys. arebencoded. strings. StructureofTorrent(MetaInfo)File. sub-directorystructure].KeyDescription. 20-byteSHA1hashvalues,oneper. piece.(rawbinaryencoded). 1-Jan-197000:00:00UTC). Comment(optional)Freeformtextments.(string). create.directorystructure].KeyDescription. keys. stringelements:"dir1","dir2",and. "".20-byteSHA1hashvalues,oneper. piece.(rawbinaryencoded)

　　

【正文】 n ? index: integer specifying the zerobased piece index ? begin: integer specifying the zerobased byte offset within the piece ? Length: integer specifying the requested length. This value must not exceed 2^17 bytes, typical values are 2^15 bytes. The observant reader will note that a block is typically smaller than a piece (which is monly = 2^18 bytes). A client should close the connection if it receives a request for more than 2^17 bytes. Java example: (13)。 (6)。 (piece_index)。 (begin)。 (length)。 Piece: len=0009+Xid=7indexbeginblock The piece message is variable length, where X is the length of the block. The payload contains the following information ? index: integer specifying the zerobased piece index ? begin: integer specifying the zerobased byte offset within the piece ? Block: block of data, which is a subset of the piece specified by index. Java example: (9 + 256)。 (7)。 (piece_index)。 (begin)。 (block)。 block is a byte array of size 256 for this example Cancel: len=0013id=8indexbeginlength The cancel message is fixed length, and is used to cancel block requests. The payload is identical to that of the request message. It is typically used during End Game (see the Algorithms section below). Java example: (13)。 (8)。 (piece_index)。 (begin)。 (length)。 Algorithms: Super Seeding: (This was not part of the original specification) The superseed feature in and on is a new seeding algorithm designed to help a torrent initiator with limited bandwidth pump up a large torrent, reducing the amount of data it needs to upload in order to spawn new seeds in the torrent. When a seeding client enters superseed mode, it will not act as a standard seed, but masquerades as a normal client with no data. As clients connect, it will then inform them that it received a piece a piece that was never sent, or if all pieces were already sent, is very rare. This will induce the client to attempt to download only that piece. When the client has finished downloading the piece, the seed will not inform it of any other pieces until it has seen the piece it had sent previously present on at least one other client. Until then, the client will not have access to any of the other pieces of the seed, and therefore will not waste the seed39。s bandwidth. This method has resulted in much higher seeding efficiencies, by both inducing peers into taking only the rarest data, reducing the amount of redundant data sent, and limiting the amount of data sent to peers which do not contribute to the swarm. Prior to this, a seed might have to upload 150% to 200% of the total size of a torrent before other clients became seeds. However, a large torrent seeded with a single client running in superseed mode was able to do so after only uploading 105% of the data. This is 150200% more efficient than when using a standard seed. Superseed mode is NOT remended for general use. While it does assist in the wider distribution of rare data, because it limits the selection of pieces a client can downlad, it also limits the ability of those clients to download data for pieces they have already partially retrieved. Therefore, superseed mode is only remended for initial seeding servers. Why not rename it to . Initial Seeding Mode or Releaser Mode then? Piece downloading strategy: Clients may choose to download pieces in random order. A better strategy is to download pieces in rarest first order. The client can determine this by keeping the initial bitfield from each peer, and updating it with every have message. Then, the client can download the pieces that appear least frequently in these peer bitfields. End Game: When a download is almost plete, there39。s a tendency for the last few blocks to trickle in slowly. To speed this up, the client sends requests for all of its missing blocks to all of its peers. To keep this from being horribly inefficient, the client also sends a cancel to everyone else every time a block arrives. There is no documented thresholds, remended percentages, or block counts that could be used as a guide or Remended Best Practice here. Choking and Optimistic Unchoking: Choking is done for several reasons. TCP congestion control behaves very poorly when sending over many connections at once. Also, choking lets each peer use a titfortatish algorithm to ensure that they get a consistent download rate. The choking algorithm described below is the currently deployed one. It is very important that all new algorithms work well both in a work consisting entirely of themselves and in a work consisting mostly of this one. There are several criteria a good choking algorithm should meet. It should cap the number of simultaneous uploads for good TCP performance. It should avoid choking and unchoking quickly, known as 39。fibrillation39。. It should reciprocate to peers who let it download. Finally, it should try out unused connections once in a while to find out if they might be better than the currently used ones, known as optimistic unchoking. The currently deployed choking algorithm avoids fibrillation by only changing choked peers once every ten seconds. Reciprocation and number of uploads capping is managed by unchoking the four peers which have the best upload rate and are interested. This maximizes the client39。s download rate. These four peers are referred to as downloaders, because they are interested in downloading from the client. Peers which have a better upload rate (as pared to the downloaders) but aren39。t interested get unchoked. If they bee interested, the downloader with the worst upload rate gets choked. If a client has a plete file, it uses its upload rate rather than its download rate to decide which peers to unchoke. For optimisti