[net2-wg] anycast forwarding requirements

[henri dubois-ferriere]

Here are some link-layer requirements  for anycast forwarding.
I am explicitly *not* making any proposals related to link-layer
interfaces; maybe all of this can be done above an abstracted link
layer and has no effect on it (though i would be surprised ;)).

1) Multiple candidate relays:
It must be possible for the routing protocol to indicate a list of
candidate next hops for a packet (*). This is not the same as having
backups in case a packet doesn't go through.

2) ACKs with payloads
It should be possible to fit 1-2 fields in an ACK frame, maybe stg
like  32 bits total. This does not mean that *all* ACKs  must carry
this overhead. So an open question is how the contents of this field
are communicated down to the link-layer from above, on a per-packet
basis.

On the receive side, this means that an ack is not just a 1 bit
metadata field; the receiver must have some way of obtaining the ACK
payloads, including for ACKs corresponding to a packet sent by another
node.

3) ACK timing and slotted ACKs
It must be possible to specify some timing on ACKs, ie say "i want the
ack to go out in X ms; alternatively to have logical timeslots and say
"i want the ack to go out in the Xth slot".


(*) This is not required by every conceivable anypath/opportunistic
routing protocol. E.g. in some instances, one can simply send to
broadcast address; every receiver with cost lower than the sender is a
candidate relay (packet of course then has a cost field).