Closes#697.
per https://github.com/ZcashFoundation/zebra/issues/697#issuecomment-662742971
The response to a getblocks message is an inv message with the hashes of the
following blocks. However, inv messages are also sent unsolicited to gossip new
blocks across the network. Normally, this wouldn't be a problem, because for
every other request we filter only for the messages that are relevant to us.
But because the response to a getblocks message is an inv, the network layer
doesn't (and can't) distinguish between the response inv and the unsolicited
inv.
But there is a mitigation we can do. In our sync algorithm we have two phases:
(1) "ObtainTips" to get a set of tips to chase down, (2) repeatedly call
"ExtendTips" to extend those as far as possible. The unsolicited inv messages
have length 1, but when extending tips we expect to get more than one hash. So
we could reject responses in ExtendTips that have length 1 in order to ignore
these messages. This way we automatically ignore gossip messages during initial
block sync (while we're extending a tip) but we don't ignore length-1 responses
while trying to obtain tips (while querying the network for new tips).
Closes#617.
Closes#698.
The remaining work on the syncer is alluded to in a new comment:
1. Correctly constructing a block locator object
2. Detecting when we've stopped making progress syncing and restarting obtain_tips.
* fix: Resist CheckpointVerifier memory DoS attacks
Allow a maximum of 2 queued blocks at each height, as a tradeoff between
efficient bad block rejection, and memory usage.
Closes#628.
* fix: Make max queued blocks at height equal to fanout
* fix: Just allocate all the capacity upfront
* fix: Use with_capacity(1) and reserve_exact(1)
* also download tips and filter tips
* dispatch all block downloads together
* tweek to match henry's changes
* switch to more intuitive match
Co-authored-by: Jane Lusby <jane@zfnd.org>
Henry de Valence
teor
Jane Lusby