These are needed to support vxlan encapsulation. In a vxlan packet,
the only place we can straightforwardly hold the source and
destination peer identifiers used by the weave router is in the
24-bit VNI (vxlan network identifier) field. This gives us 12 bits to
indicate a peer. The resulting limit of 4096 peers on a weave network
does not seem like a problem in the near future. But it does mean
that decentralized allocation makes collisions fairly likely even in
weave networks of sizes that are common today. Fortunately, the
problem is much more tractable than IPAM because we can detect when
collisions occur and take action to resolve them.
Introduce an intermediary - GossipSender - between GossipChannel and
Connection. This accumulates gossip data (now represented by the new
GossipData interface) from the former until it can be passed onto the
latter, allowing the former to proceed when the latter may be blocked
on i/o.
This
1) prevents deadlocks that arise from cycles in the communication
topology
2) improves performance by allowing GossipChannel and its calling code
to proceed when connection i/o is blocked
3) improves performance by accumulating GossipData - the accumulated
data often is considerably more compact than the sum of all the
accumulated bits, and it is transmitted in a single communication
event.
In order to support accumulation, the GossipData interface has a Merge
method. Furthermore, encoding is deferred until the data can actually
be sent, since accumulation would be hard/impossible to encoded
data. This required changes on some interfaces, most notably Gossiper.
The implementation of GossipData for topology gossip is
TopologyGossipData. It carries a reference to Peers and a set of
PeerNames, indexing into Peers and referencing the Peer entries which
have changed. Previously updates were represented as a list of Peer
entries. The indirection via PeerNames allows Encode to omit entries
which have been removed. The change in representation required changes
to the signature of some methods on Peers.
Note that all the above only applies to the "periodic gossip" portion
of the Gossiper API, which topology gossip in fact uses for *all*
gossip. GossipUnicast and GossipBroadcast are unchanged, but could
conceivably receive the same treatment in the future.
Fixes#445.
David Wragg