I considered that as well, but it seemed to me that other software on the
network (say, different wallet implementations) might prefer the option of
being able to bump up their protocol version in the future to pick up some
other change, without having to also opt-in to receiving
headers-announcements for blocks.

In particular, inv-based block announcements aren't going away (even in my
implementation of headers announcements, there are some edge cases where
the code would need to fall back to an inv announcement), so forcing all
software on the network to upgrade to supporting headers announcements,
whether now or in the future, seems too drastic -- I could imagine some
software not being very concerned about optimizing block relay in this
particular way.

On Thu, Sep 24, 2015 at 2:41 PM, Peter Todd <pete@petertodd.org> wrote:

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>
>
> On 24 September 2015 14:37:40 GMT-04:00, Suhas Daftuar via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
> >On Thu, Sep 24, 2015 at 2:17 PM, Tier Nolan via bitcoin-dev <
> >bitcoin-dev@lists.linuxfoundation.org> wrote:
> >
> >>
> >> Is there actually a requirement for the new message?  New nodes could
> >just
> >> unilaterally switch to sending headers and current nodes would be
> >> compatible.
> >>
> >
> >I don't believe that unilaterally switching to headers announcements
> >would
> >work for all network participants -- both for users running older
> >Bitcoin
> >Core versions (anything before 0.10, which I believe all ignore headers
> >messages) and for non-Bitcoin Core software that participates on the
> >network (which may ignore headers messages too, I'm not sure what all
> >is
> >out there).
>
> You can enable the behaviour based on advertised p2p network version.
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