For my replace-by-fee implementation(1) I used service bit 26 to let
preferential peering work so that replace-by-fee nodes could easily find
each other. Of course, that's a temporary/experimental usage that can be
dropped after wider adoption, so I included the following comment:

    // Reserve 24-31 for temporary experiments
    NODE_REPLACE_BY_FEE = (1 << 26)

Service bits are never a guaranteed thing anyway, so occasional
collisions can and should be tolerated by applications using these
experimental service bits.


Alternately Wladimir J. van der Laan brought up elsewhere(2) the
possibility for a wider notion of an extension namespace. I'm personally
not convinced of the short-term need - we've got 64 service bits yet
NODE_BLOOM is the first fully fleshed out proposal to use one - but it's
worth thinking about for the long term.

1) https://github.com/petertodd/bitcoin/tree/replace-by-fee-v0.9.1
2) https://github.com/bitcoin/bitcoin/pull/4351#issuecomment-46272958

-- 
'peter'[:-1]@petertodd.org
000000000000000058ca7ee3a40438ea5a96e499910638352468c6d69abdb226