Bitswap is the module that is responsible for requesting blocks over the
network from other ipfs peers.
##Main Operations
Bitswap has three main operations:
###GetBlocks
`GetBlocks` is a bitswap method used to request multiple blocks that are likely to all be provided by the same peer (part of a single file, for example).
###GetBlock
`GetBlock` is a special case of `GetBlocks` that just requests a single block.
###HasBlock
`HasBlock` registers a local block with bitswap. Bitswap will then send that block to any connected peers who want it (strategy allowing), and announce to the DHT that the block is being provided.
##Internal Details
All `GetBlock` requests are relayed into a single for-select loop via channels. Calls to `GetBlocks` will have `FindProviders` called for only the first key in the set initially, This is an optimization attempting to cut down on the number of RPCs required. After a timeout (specified by the strategies `GetRebroadcastDelay`) Bitswap will iterate through all keys still in the local wantlist, perform a find providers call for each, and sent the wantlist out to those providers. This is the fallback behaviour for cases where our initial assumption about one peer potentially having multiple blocks in a set does not hold true.
When receiving messages, Bitswaps `ReceiveMessage` method is called. A bitswap message may contain the wantlist of the peer who sent the message, and an array of blocks that were on our local wantlist. Any blocks we receive in a bitswap message will be passed to `HasBlock`, and the other peers wantlist gets updated in the strategy by `bs.strategy.MessageReceived`.
