Every writer packs Open because that is what RNS 1.4.2 does at construction. IFAC is an interface-boundary transform, where Transport.transmit masks the raw packet and flips this flag per interface key, and Transport.inbound unmasks or drops.
The trailing context byte: a sub-type tag the engine routes on. Exhaustive over the values RNS defines, plus Unknown(u8) so an unrecognised byte round-trips unchanged (RNS preserves unknown context bytes). Decoding only ever yields Unknown for bytes outside the named set, so a parsed value is always canonical.
RNS 1.4.2 Reticulum.MTU: the maximum byte size of one packet on the broadcast plane (announces, path requests, un-linked Singles), which peers must agree on. A wire-protocol invariant, permanently 500 even on fat pipes; the per-link MTU a LINKREQUEST negotiates is separate.
RNS 1.4.2 Reticulum.HEADER_MAXSIZE. The type-2 (transport-routed) header: flags, hops, transport id, destination, context. Outbound payload budgets reserve this even when emitting type-1, because a relay re-emits the packet with the transport id added.
RNS 1.4.2 Reticulum.IFAC_MIN_SIZE: the smallest per-interface access-code overhead a packet may gain; reserved in every payload budget like the transport header.