Fabrice Drouin [ARCHIVE] on Nostr: 📅 Original date posted:2018-02-05 📝 Original message: Hi, On 5 February 2018 at ...

📅 Original date posted:2018-02-05
📝 Original message:
Hi,

On 5 February 2018 at 14:02, Christian Decker
<decker.christian at gmail.com> wrote:
> Hi everyone
>
> The feature bit is even, meaning that it is required from the peer,
> since we extend the `init` message itself, and a peer that does not
> support this feature would be unable to parse any future extensions to
> the `init` message. Alternatively we could create a new
> `set_gossip_timestamp` message that is only sent if both endpoints
> support this proposal, but that could result in duplicate messages being
> delivered between the `init` and the `set_gossip_timestamp` message and
> it'd require additional messages.

We chose the other aproach and propose to use an optional feature

> The reason I'm using timestamp and not the blockheight in the short
> channel ID is that we already use the timestamp for pruning. In the
> blockheight based timestamp we might ignore channels that were created,
> then not announced or forgotten, and then later came back and are now
> stable.

Just to be clear, you propose to use the timestamp of the most recent
channel updates to filter
the associated channel announcements ?

> I hope this rather simple proposal is sufficient to fix the short-term
> issues we are facing with the initial sync, while we wait for a real
> sync protocol. It is definitely not meant to allow perfect
> synchronization of the topology between peers, but then again I don't
> believe that is strictly necessary to make the routing successful.
>
> Please let me know what you think, and I'd love to discuss Pierre's
> proposal as well.
>
> Cheers,
> Christian

Our idea is to group channel announcements by "buckets", create a
filter for each bucket, exchange and use them to filter out channel
announcements.

We would add a new `use_channel_announcement_filters` optional feature
bit (7 for example), and a new `channel_announcement_filters` message.

When a node that supports channel announcement filters receives an
`init` message with the `use_channel_announcement_filters` bit set, it
sends back its channel filters.

When a node that supports channel announcement filters receives
a`channel_announcement_filters` message, it uses it to filter channel
announcements (and, implicitly ,channel updates) before sending them.

The filters we have in mind are simple:
- Sort announcements by short channel id
- Compute a marker height, which is `144 * ((now - 7 * 144) / 144)`
(we round to multiples of 144 to make sync easier)
- Group channel announcements that were created before this marker by
groups of 144 blocks
- Group channel announcements that were created after this marker by
groups of 1 block
- For each group, sort and concatenate all channel announcements short
channel ids and hash the result (we could use sha256, or the first 16
bytes of the sha256 hash)

The new `channel_announcement_filters` would then be a list of
(height, hash) pairs ordered by increasing heights.

This implies that implementation can easily sort announcements by
short channel id, which should not be very difficult.
An additional step could be to send all short channel ids for all
groups for which the group hash did not match. Alternatively we could
use smarter filters

The use case we have in mind is mobile nodes, or more generally nodes
which are often offline and need to resync very often.

Cheers,
Fabrice