BUIP051: add CompactBlocks support
Proposer: freetrader
Submitted: 2017-04-13
Status: draft

Summary
=======

In the interest of supporting better fast block propagation between 
BU and other implementation which only support CompactBlocks (CB)[1] 
but not XthinBlocks (XTB), it is proposed to add BIP152 support to 
the BU client.


Motivation
==========

Currently, bridging between XTB and CB peers is possible by 
connecting via a BitcoinXT node. There are few of these (46 
according to nodecounter.com). Supporting CB in BU would strengthen 
the connections between the BU peers and the rest of the network, 
without having to rely on intermediates.

Also, few other (non-Core) implementation currently support XTB. It 
is more likely that some support CB. Being able to exchange big 
blocks with them might be necessary in the near/medium term, and 
since some of them are quite different from Satoshi clients, 
providing reference XTB implementation for them will not be so easy 
for BU developers. It makes more sense to support their existing 
fast propagation protocol.


Implementation considerations
=============================

The implementation should, for safety reasons, prefer to use the 
more tested XTB with peers that support this service, and fall back 
to using CompactBlocks with those that do not support XTB.

User preference switches are recommended to

1. allow disabling CB support in the client entirely (such a kill 
   switch should be supplied anyway for security purposes, should a 
   flaw be found in the CB implementation, similar to `-use-thinblocks` 
   for XTB)
2. allow the user to prefer CB to XTB with peers that support both


Notes:
======

[1] This has already been implemented in Bitcoin XT, so hopefully 
    together with the BIP152 specification it should be quite easy to 
    port this feature to BU.

[2] This porting work could be encouraged by making this a funded 
    BUIP. The author has no advice for the effort and funding level 
    required, but is open to suggestions, esp. from the XT developers 
    who have already implemented this.


Additional information
======================

[1] https://github.com/bitcoin/bips/blob/master/bip-0152.mediawiki