[Bitcoin-development] BIP 35: add mempool message

[Jeff Garzik <jgarzik@exmulti.com>]

Consensus was we should do a BIP for all P2P changes, so here it is...
 feedback requested.

https://en.bitcoin.it/wiki/BIP_0035

Abstract
-------------------------------------------
Make a network node's transaction memory pool accessible via a new
"mempool" message.  Extend the existing "getdata" message behavior to permit
accessing the transaction memory pool.


Motivation
-------------------------------------------
Several use cases make it desireable to expore a network node's transaction
memory pool:
* SPV clients, wishing to obtain zero-confirmation transactions sent or
  received.
* Miners, downloading existing network transactions after a restart.
* Remote network diagnostics.


Specification
-------------------------------------------
1) Upon receipt of a "mempool" message, the node will respond
   with an "inv" message containing MSG_TX hashes of all the
   transactions in the node's transaction memory pool.

   An "inv" message is always returned, even if empty.

2) The typical node behavior in response to an "inv" is "getdata".

   However, the reference Satoshi implementation ignores requests
   for transaction hashes outside that which is recently relayed.

   To support "mempool", an implementation must extend its "getdata"
   message support to querying the memory pool.

3) Feature discovery is enabled by checking two "version" message attributes:

   a) Protocol version >= 60002
   b) NODE_NETWORK bit set in nServices


Backwards compatibility
-------------------------------------------
Older clients remain 100% compatible and interoperable after this change.


Implementation
-------------------------------------------
See https://github.com/bitcoin/bitcoin/pull/1641

-- 
Jeff Garzik
exMULTI, Inc.
jgarzik@exmulti.com