Light ethereum subprotocol
Performance considerations Request latency The only thing a light client always has in its database is the last few thousand block headers. This means that retrieving anything else requires the client to send requests and get answers from light servers. The light client tries to optimize request distribution and collects statistical data of each server's usual response times in order to reduce latency.
Latency is the key performance parameter of a light client. If many requests are made sequentially to perform an operation, it may result in a slow response time for the user. Running API functions in parallel whenever possible can greatly improve performance. Searching for events in a long history of blocks Full clients employ a so-called "MIP mapped" bloom filter to find events quickly in a long list of blocks so that it is reasonably cheap to search for certain events in the entire block history.
Unfortunately, using a MIP-mapped filter is not easy to do with a light client, as searches are only performed in individual headers, which is a lot slower. Searching a few days' worth of block history usually returns after an acceptable amount of time, but at the moment you should not search for anything in the entire history because it will take an extremely long time.
Memory, disk and bandwidth requirements Here is the good news: a light client does not need a big database since it can retrieve anything on demand. With garbage collection enabled which scheduled to be implemented , the database will function more like a cache, and a light client will be able to run with as low as 10Mb of storage space. Note that the current Geth implementation uses around Mb of memory, which can probably be further reduced.
Bandwidth requirements are also lower when the client is not used heavily. Future improvements Reducing overall latency by remote execution Sometimes it is unnecessary to pass data back and forth multiple times between the client and the server in order to evaluate a function. It would be possible to execute functions on the server side, then collect all the Merkle proofs proving every piece of state data the function accessed and return all the proofs at once so that the client can re-run the code and verify the proofs.
Verifying complex calculations indirectly One of the main limitations we are working to improve is the slow search speed of log histories. Many of the limitations mentioned above, including the difficulty of obtaining MIP-mapped bloom filters, follow the same pattern: the server which is a full node can easily calculate a certain piece of information, which can be shared with the light clients.
But the light clients currently have no practical way of checking the validity of that information, since verifying the entire calculation of the results directly would require so much processing power and bandwidth, which would make using a light client pointless. Fortunately there is a safe and trustless solution to the general task of indirectly validating remote calculations based on an input dataset that both parties assume to be available, even if the receiving party does not have the actual data, only its hash.
The network participants are required to evaluate every transaction starting from the genesis block, which requires a large amount of network, computing, and storage resources. This is impractical for many devices with either limited computing resources or intermittent network connectivity.
To overcome this drawback Ethereum defines a light client protocol where the light client fetches the blockchain state from a node operating as a light protocol server. Light clients are unable to maintain blockchain state internally, and as a consequence can only perform partial validation on blocks. Thus they rely on the light server for full block validation and to provide the updated blockchain state. Light clients connect to multiple light servers to mitigate the risk of relying on a single potentially dishonest server.
Ethereum light clients are known to suffer from a probabilistic security model, but they are widely assumed to be secure under normal operating conditions. In fact, the implicit security assumptions of light clients have not been formally characterised in the literature.
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STRATEGY FOREX SIMPLE RENKO
Client Side Flow Control Any node which takes on a server role in the the LES protocol needs to be able to somehow limit the amount of work it does for each client peer during a given time period. They can always just serve requests slowly if they are overloaded, but it is beneficial to give some sort of flow control feedback to the clients.
This way, clients could and would have incentive to behave nicely and not send requests too quickly in the first place and then possibly timeout and resend while the server is still working on them. They could also distribute requests better between multiple servers they are connected to.
And if clients can do this, servers can expect them to do this and throttle or drop them if they break the flow control rules. The Model Let us assume that serving each request has a cost depending on type and parameters for the server.
This cost is determined by the server, but it has an upper limit for any valid request. The server assigns a "buffer" for each client from which the cost of each request is deduced. The buffer has an upper limit the "buffer limit" and a recharge rate cost per second. The server can decide to recharge it more quickly at any time if it has more free resources, but there is a guaranteed minimum recharge rate. If a request is received that would drain the client's buffer below zero, the client has broken the flow control rules and is throttled or disconnected.
The value of this parameter is a table assigning cost values to every on-demand retrieval message in the LES protocol. If BV goes negative, drops the peer, otherwise starts serving the request. The reply message contains a BV value that is the previously calculated BV plus the amount recharged during the time spent serving. Now it is possible and recommended to send a StopMsg instead and then a ResumeMsg when the buffer has been at least partially recharged.
This allows clients to treat the buffer feedback as an optional performance optimization hint instead of a mandatory mechanism and allows simple implementations that do not care about the buffer at all. Request ID Every on-demand request message contains a reqID field, which is simply returned by the server in the corresponding reply message. This helps matching replies for requests on the client side so that each reply doesn't need to be matched against each pending request. This message should be sent just after the connection is established and prior to any other LES messages.
The following keys are required value types are noted after the key string : "protocolVersion" P: is 1 for protocol version one. Integer, as found in block header. Allowed integer values are: none 0 : no Announce messages are sent, i. If the node serves chain data then "serveChainSince" should always be present while "serveRecentChain" is optional.
If the node serves state data then "serveStateSince" should always be present while "serveRecentState" is optional. This allows announcing additional capabilities while staying compatible with past protocol versions. A restrictive change of server capabilities for example, an increase of "serveStateSince" due to state pruning should be announced at least 10 seconds prior to actually restricting those capabilities in order to avoid asynchronous problems.
Changes to unknown keys should be ignored. Changes to known keys that make no sense lead to disconnection. Announcing a head with a lower or equal TD than previously announced or a head that the sending node later refuses to honor with a proceeding GetBlockHeaders message with number and TD also matching is considered bad form, and may lead to disconnection or reduce the reputation of the sending node.
The field reorgDepth contains the number of blocks to be rolled back from the last head announced by the same node in order to find the last common ancestor of the last and current heaviest chain. Adding this field helps the client to minimize the number of requests and the amount of bandwidth required to fetch new headers.
Reply must contain a number of block headers, of rising number when reverse is 0, falling when 1, skip blocks apart, beginning at block block denoted by either number or hash in the canonical chain, and with at most maxHeaders items. The items in the list following the message ID are block headers in the format described in the main Ethereum specification, previously asked for in a GetBlockHeaders message.
The list may be empty if none of the requested block headers were available on the server side. Specify the set of blocks that we're interested in with the hashes. The items in the list following the message ID are some of the blocks, minus the header, in the format described in the main Ethereum specification, previously asked for in a GetBlockBodies message.
If fromLevel is greater than zero, the given number of trie nodes closest to the root can be omitted from the proof. This set will be called a proof set. Compared to Proofs , this message contains a single list of nodes satisfying all requested proofs. The list shouldn't contain duplicate nodes. It retrieves Merkle proofs from different types of "helper tries" which are generated for every fixed-length section of the canonical chain.
Reply must contain a number of block headers, of rising number when reverse is 0, falling when 1, skip blocks apart, beginning at block block denoted by either number or hash in the canonical chain, and with at most maxHeaders items. The items in the list following the message ID are block headers in the format described in the main Ethereum specification, previously asked for in a GetBlockHeaders message. This may validly contain no block headers if no block headers were able to be returned for the GetBlockHeaders query.
Specify the set of blocks that we're interested in with the hashes. The items in the list following the message ID are some of the blocks, minus the header, in the format described in the main Ethereum specification, previously asked for in a GetBlockBodies message.
If fromLevel is greater than zero, the given number of trie nodes closest to the root can be omitted from the proof. Each block delta contains a list of altered trie nodes in no particular order and contract code within the block. Each transaction proof consists of a set of required trie nodes from various tries, in no particular order and code. This can be done impromptu or as part of a request.
The first block by hash which queries may request state data for. The last block by hash which queries may request state data for. Indicates whether this peer can serve block bodies. Indicates whether this peer can serve receipts.
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