Incentive for propagating transactions?

[clinton]

From what I understand, if I want a transaction to finalised, I send to my peers I'm connected to, who send it to peers they're connected to, until eventually one of those people solve a block.

Whatever person solves the block which includes my transaction, gets, along with the 50 bitcoins for solving the block, any transaction fees offered in any transactions they've included in the block.

These transaction fees give nodes incentive to include transactions in blocks.

However, is there any incentive for nodes to propagate transactions. Say node A a transaction to node B. Is there any incentive for node B to send the transaction to node C, or are nodes just altruistic when it comes to propagating transactions?

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Also, I was just wondering, what's the current average transaction fees included in solved blocks? Is there a graph of this over time? I'm interested at the moment whether transaction fees or the 50 bitcoins for solving the block is greater.

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[hashcoin]

There is no incentive, you are correct that propagation occurs only because "the default client is generous".  Greedy nodes could choose to keep transactions to themselves.

If that began occurring regularly, it could be defeated by having the client connect to more peers after a bit and asking them if they've seen the transaction (I don't think this is implemented, but would be a simple extension "do you have transaction 0xabcd in your memory pool"?).  If they haven't, some vertex cut of peers separating you from them is withholding.  You would now choose new peers.

In an utterly greedy world where everyone withholds, the big mining pool operators would publish an address where you could submit your transactions.  You would then have to submit your TX to each of them.

As for your second question, the 50btc is MUCH greater.  You can see recent blocks at
http://blockexplorer.com/

The last few blocks have total fees under 0.001 BTC.

[Maged]

In an utterly greedy world where everyone withholds, the big mining pool operators would publish an address where you could submit your transactions.  You would then have to submit your TX to each of them.

As an extension to this, you would then likely see multiple relay networks pop up that charge some sort of fee to users, either per-transaction or monthly. Once they start becoming popular, they will also start charging miners for access to the relayed transactions. If that happens, the future is good for people who relay transactions.