ICS 5: IBC packet semantics & handling

[cwgoes]

Primarily ported from the original IBC spec.

Will cover:

• IBC packet semantics
• Required data structures
• Packet wire encoding

[cwgoes]

To-do in spec:

• Reference primitives defined in ICS 3 / ICS 4
• Formalize packet handling algorithm
• Enumerate security properties (particularly replay prevention)
• Define packet data structure
• Define options for receipts, timeouts, alterations to handlers

[cwgoes]

Datagram formats that need to be defined in this spec (that I can think of so far):

• PACKETFORWARD - forward packet
• PACKETREVERSE - reverse packet (receipt)
• PACKETTIMEOUT - timeout packet (if it needs to be a packet at all)

[mossid]

My thought was directly inspecting the incoming sequence on the other chain to detect timeout, so in that case, PACKETTIMEOUT will not be needed.

[cwgoes]

I think that depends on which ordering requirements are selected by the channel - sometimes one might want timeouts without ordering - but when possible I agree.

[cwgoes]

Possibly this may not actually require a separate ICS, I think ICS 4 will determine most of this at the channel level.

[cwgoes]

Closing, this should just be merged into ICS 4 (#1).