Overview

This article describes how blocks are exchanged between nodes. See Protocol rules for more information on how blocks are validated.

Upon initial connection, if the connection was not inbound[1], or in other words, if the connection was initiated by the local node, the version message is queued for sending immediately. When the remote node receives the version message it replies with its own version message.[2]

When a node receives a "version" message, it may send a "getblocks" request to the remote node if EITHER:

  1. The node has never sent an initial getblocks request to any node yet.
  2. Or, this is the only active node connection. Presumably the node had zero connections prior to this connection, so maybe it was disconnected for a long time. So, it will ask for blocks to catch up.

The getblocks message contains multiple block hashes that the requesting node already possesses, in order to help the remote note find the latest common block between the nodes. The list of hashes starts with the latest block and goes back ten and then doubles in an exponential progression until the genesis block is reached.[3] Since both nodes are hard coded with the genesis block, they are guaranteed to a least start there. If that block does not match for some reason, no blocks are exchanged.

Inventory Messages

Note that the node receiving the getblocks request does not actually send full blocks in response. The node sends an "inv" message containing just the hashes of the series of blocks that fit the request, which verifies that the node does indeed have blocks to send that the remote node does not have (but does not presume the remote node wants the full blocks yet).

When the local node receives the "inv" message, it will request the actual blocks with a "getdata" message. See Below.

But first, here is more detail how the remote node sends the "inv" message in response to the "getblocks" request sent by the local node. The remote node calls pFrom->PushInventory which is a method on the CNode instance that represents the node that requested the blocks (the local node in this walkthrough), and PushInventory adds the block hash to the vInventoryToSend variable of the CNode. The SendMessages function in main.cpp will take the inv items out of vInventoryToSend and add it to a vInv variable which means they are really ready for sending.[4] The reason for the seperate variable is that some inventory items (transactions only right now) may be "trickled" to the remote node, which means they may kept from from being sent right away. When the vInv variable fills up with 1000 entries, a message is queued with those 1000 entries and the loop continues. At the end, any remaining entries are sent in a final "inv" message.

When the local node receives the "inv" message, it will request the actual block with a "getdata" message. To be precise, the node calls pfrom->AskFor to request the block, and that method queues the request for the blocks in mapAskFor, and the multipurpose SendMessage() sends the "getdata" requests in batches of 1000 entries from the map.[5]

The code attempts to limit redundant requests to every 2 minutes for the same block by using a map called mapAlreadyAskedFor to delay the message if necessary.[6]

Block Batching

The responding node to a "getblocks" request attempts to limit the response to the requestor to 500 blocks.[7]

However, in a peculiar twist, if the requestor appears to have diverged from the main branch, the node will send as many blocks as necessary to replace the entire bad chain of the requestor, from the lastest common block between the nodes, up to the last block the requestor has (on their bad main branch). That is in addition to the 500 "catch up" blocks for main branch updates that will also be sent.[8]

Note that in addition to a flat limit on the number of blocks queued for sending, bitcoind also limits the total size of the blocks that are being queued. This is currently limited to half the send buffer size[9], which is 10MB, for a limit of 5MB of queued blocks for send.[10]

Batch Continue Mechanism

When a node is finished sending a batch of block inventory, it records the hash of the last block in the batch.[11] When the node receives a request for that full block, it realizes the remove node is done with the current batch and directly queues a special "inv" message (bypassing the normal SendMessage mechanism) with one block hash entry containing the latest block hash.[12] When the remote node receives that "inv" message, it will see that it does not have that block and it it will ask for that block as described above. However, this time when it receives the block and processes it, it will notice that it does not have the previous block, so it will record the latest block as an "orphan" block, and will request a block update starting with the latest block it has up to the block before the orphan [13], in order to fill in the gap. That goes out as a "getblocks" request and the whole batch process repeats itself.

However, there is a twist. When the next batch finishes, the remote node sending the blocks will send the "inv" with latest block hash as usual, and the local node will notice it already has this block in the orphan block map this time and so it will skip requesting the block and directly ask for a block update.[14] This process will continue until the last block prior to the latest block is received. At the end of processing that block, it will notice there is an orphan that pointed to this block and will process the orphan block, (and any other orphans, recursively) thus completing the entire process.[15]

Stall Recovery

If the batching processes is interrupted for some reason, such as the remote node failing to honor the "Batch Continue Mechanism" or if a disconnection occurs, there is a way for the process to restart. When a new block is solved and advertised around[16], any nodes that are behind will notice the new block in the "inv" and that will trigger it to request a "getblocks" update from the node that sent it the message. That will cause blocks to be sent starting from wherever in the block chain that the node that is behind is currently at.

Long Orphan Chains

In various tests, it has proven relatively common (say more than one in ten) to discover nodes that are significantly behind on the block chain, probably because they are in the process of catching up as well. Since a well connected node will have at least 8 and up to dozens of connections, it is fairly likely that a new node will connect to another node that is also catching up.

Nodes that are catching up will advertise the blocks they are processing, as they accept blocks into their main chain, to every other node.[16] While there is code to prevent advertising old blocks before a certain checkpoint, that code also has a clause that does advertise blocks to remote nodes if the block height is over the remote node's current best height minus 2000 blocks.[17] This appears to allow nodes to "help" other nodes catch up, even if they are both processing old blocks.

These advertisements cause the local node to request those blocks from the remote node, which could be blocks well into the future compared to what has been processed locally. Due to the way blocks are requested, the remote node will send a large batch of blocks in response and will continue sending blocks to the local node until it reaches the end. Note that this is likely to occur at the same time the local node is downloading earlier blocks on the main chain from another node. That process may eventually catch up with the orphan chain and produce a very, very long operation to revalidate and connect up all the orphan blocks. Orphan chains over ten thousand blocks long, taking over an hour to process are possible.

Therefore, two nodes talking to each other that are both catching up can lead to suboptimal interactions, especially when one both are far behind and one is far ahead of the other.

Flood Limit Effects

Even with the batching mechanism described above, there are scenarios that occur that result in the remote node overflowing the local receive buffer while blocks are being exchanged.

For example, if a remote node is "catching up", it will advertise each block it processes to the local node in certain circumstances (see above [17]). The local node will request each of those blocks right away. There is no protection against the local node requesting too many of these blocks. The remote node will send all blocks requested. There is no protection against the remote node sending too many blocks before the local node has time to process them, in this circumstance.

The local receive buffer can fill up. When the local node notices a receive buffer is full, it disconnects that node connection.[18] If sets the fDisconnect flag, and once the buffers are empty[19], the socket is closed.

Performance

As of September 1, 2011, on a server class computer circa 2005 running Ubuntu with a Comcast cable internet connection takes over 10 hours to download and process the block chain. While it is debatable what the bottleneck is early in the download process, it is clear from the processing of recent blocks that the network is not the bottleneck for all but the slowest internet connections.

Blocks are taking over a second, on average, to process once downloaded.[20] However, the average size of a block is only around 24 kilobytes in August 2011. It certainly does not take 1 second to download 24K. Also, testing reveals very large queues of blocks being processed per message loop, which is not what you would expect if the thread was pulling them out of the queue as they arrive on the sockets.

There are a number of "false signals" that lead one to believe the problem is with network performance. The first false signal is that, as of August 2011, nearly all of the first 60 or 70% of blocks downloaded are very small. Recent average block sizes are around one hundred times bigger! So, almost all of a sudden, the block rate goes from very fast to very slow. It looks like something went wrong. In reality, if you measure the rate of block processing by kilobyte, the rate remains relatively constant.

Another false signal is related to the fact that message queues are processed to completion, one at a time per node. This can result in big backups of messages from other nodes. So, a long period of increasing blocks may freeze for long periods as other nodes are serviced. Consider that block downloads typically come from just one remote node (at least until a miner or other relaying or downloading node advertises a late block and disrupts the process) and so all the work might be on one node. Things go fast processing the blocks from a node, and then that looks like it stops as "addr" messages are processed from other nodes and other work is done. But it looks like something is wrong.

Also, the orphaning effects described above can lead to excessive block processing with nothing to show for it until the orphan chain is connected. Also, you do ocassionally run into a node that is slow to respond, perhaps because they are also processing blocks or because they have a slow machine or connection.

All of the above contributes to heavy "jitter" in the block download process, and that is a more frustrating user experience than a constant download rate.

Footnotes

  1. See pfrom->fInbound where pfrom is a CNode.

  2. See ProcessMessage() in main.cpp where strCommand == "version".

  3. See CBlockLocator in main.h.

  4. See Message: inventory in SendMessage in main.cpp.

  5. See Message: getdata at the end of SendMessage in main.cpp.

  6. See CNode::AskFor in net.h.

  7. See ProcessMessage() in main.cpp where strCommand =="getblocks".

  8. See
    int nLimit = 500 + locator.GetDistanceBack();
    in ProcessMessage in main.cpp where strCommand =="getblocks".

  9. See
    if (--nLimit <= 0 nBytes >= SendBufferSize()/2)
    in ProcessMessage() in main.cpp where strCommand =="getblocks".

  10. See
    inline unsigned int SendBufferSize() {
    return 1000GetArg("-maxsendbuffer", 101000); }
    in net.h.

  11. See pfrom->hashContinue = pindex->GetBlockHash(); in ProcessMessage() in main.cpp where strCommand =="getblocks".

  12. See: if (inv.hash == pfrom->hashContinue) in ProcessMessage() in main.cpp where strCommand =="getdata".

  13. See:
    // Ask this guy to fill in what we're missing if (pfrom) pfrom->PushGetBlocks(pindexBest, GetOrphanRoot(pblock2)); in ProcessBlock() in main.cpp.

  14. See:
    else if (inv.type == MSG_BLOCK && mapOrphanBlocks.count(inv.hash)) pfrom->PushGetBlocks(pindexBest, GetOrphanRoot(mapOrphanBlocks?inv.hash)); in ProcessMessage() in main.cpp where strCommand =="inv".

  15. See:
    // Recursively process any orphan blocks that depended on this one in ProcessBlock() in main.cpp.

  16. See the last block of code in AcceptBlock in main.cpp.

  17. See:
    if (nBestHeight > (pnode->nStartingHeight != -1 ? pnode->nStartingHeight - 2000 : 134444)) in AcceptBlock() in main.cpp.

  18. See:
    if (nPos > ReceiveBufferSize()) { in ThreadSocketHandler2() in net.cpp.

  19. See:
    if (pnode->fDisconnect (pnode->GetRefCount() <= 0 && pnode->vRecv.empty() && pnode->vSend.empty())) in ThreadSocketHandler2() in net.cpp.

  20. This is from the authors experience and also
    see https://bitcointalk.org/index.php?topic=31376.0.