Hi,

I would like to discuss the following bitcoin protocol improvement proposal:

         Adding request/reply id in all messages (in the message header,
based on what was done for the "checksum" field)

The original reason is that I found it very hard to implement robust
blockchain downloading as we are missing context information:
The download protocol relies on the other peer sending one (or more) "inv"
reponse(s) to "getblocks" and sending the "hashContinue".
But if the other peer doesn't send a response to getblock, send a partial
response to getblocks, mixes it with some unrelated blocks inventories
or doesn't send the "hashContinue" it is very hard to detect and handle
(e.g. ban the peer and switch to another).  This could cause some DoS
attacks by misbehaving peers.
The problems are that 1/ we don't know how many "inv" messages to wait for
after "getblocks" and 2/ we don't know how to distinguish between result of
"getblocks" and spontaneous "inv" notifications.
The same is true for  "addr" messages (it is both a notification and reply)
but this is less of a problem as a lack of response to getaddr
doesn't constitute a DoS.

The idea would be to add a new "requestid" field in messages and define the
following:
      - verack sends back the requestid given in version.
      - inv sends back the requestid given in getblocks or a special value
in case of a notification.
      - addr sends back the requestid given in getaddr or a special value
in case of a notification.
      - tx sends back the requestid  given in getdata.
      - block sends back the requestid given in getdata
      - headers sends back the requestid given in getheaders.
      - reply sends back the requestid given in checkorder/submitorder.
      - pong sends back the requestid given in ping.

With the following rules:
       - getblocks always results in a single"inv" reply (0-500 elements
allowed).
       - getdata still result in multiple 'tx' or 'block' 's (as currently).

This is inspired by the common request/reply pattern (the id is sometimes
called "correlation id").
This would permit better asynchronous messaging (multiple request of the
same type in parallel. This could be useful for
getblocks/getdata/getheaders and future messages).

Historic note: It seems similar to the existing mechanism of
"CRequestTracker, hashReply & PushRequest" in the current bitcoin source
code except requestId  would only be a 32bits instead of the 256bits
of hashReply .

I have implemented a prototype here: https://github.com/sirk390/bitcoin/
This could allow to remove "hashContinue" in a second step like this:
https://github.com/sirk390/bitcoin/tree/better_block_download

I'm discussing it now as I read the PONG BIP and saw some similar nonce
field added.
I was not sure if this nonce field would still be required if we had
request/reply ids .

What do you think?

Best Regards,
Christian Bodt, France
Note: I am working on a python implementation of bitcoin.
See https://github.com/sirk390/coinpy