Summary

  • RFC 5270 defines four cross-layer primitives: detection, an imminent link decision, an IP-to-radio switch command and link readiness after IEEE 802.16e network entry. They are not interchangeable success signals.
  • After LINK_UP, the IP layer still has to identify the network, choose predictive, reactive or recovery processing, deal with NCoA confirmation and DAD, release buffered packets and observe the application. A green link is an input to completion, not its receipt.

Four timestamps survived until the dashboard

The first timestamp said a new base station had been detected. The second said the link layer had selected a target and intended to move. The third recorded a command from the IP layer telling the radio when to switch. The fourth said network entry on the new link was complete.

An operations model can preserve those facts, or it can replace them with one Boolean. The Boolean feels simpler. It is also causally weak. Detection may lead only to discovery traffic. A decision may be reversed by radio conditions. A command expresses intent from another layer. Link readiness says the local radio state machine can transmit; it says nothing yet about the correctness of the new IP location.

RFC 5270 is useful here not because it supplies a current deployment recipe. It is an Informational mapping from IEEE 802.16e hard handover into the Fast Mobile IPv6 design of RFC 5268. RFC 5568 later obsoleted RFC 5268. The value of the older document is architectural: its four local primitives expose exactly where one layer's observation stops.

Detection opens a question

An 802.16e base station periodically broadcasts `MOB_NBR-ADV), including neighboring BSIDs and channel information. Scanning contributes fresher link-quality evidence. Optional association can reveal ranging and service availability. None of that names the next IP router by itself.

When the link layer sees a new base station, NEW_LINK_DETECTED rises to the IP layer. The response is investigation: the mobile node can send RtSolPr and receive PrRtAdv through the previous access router, resolving the radio candidate toward access-router information.

This event therefore means “a candidate deserves discovery work,” not “a handover has begun.” That distinction also contains a security lesson. RFC 5270 notes that broadcast neighbor advertisements may be unauthenticated. Forged BSIDs or a flood of different advertisements can cause many detection events and useless RtSolPr traffic. Scanning is recommended to test the advertised neighbor information. The local primitive is not dangerous because it crosses a network; its triggering premise can be false.

An imminent decision is not an outcome

The link layer can receive a response to a mobile-initiated request or a base-station-initiated handover request. It then raises LINK_HANDOVER_IMPEND with the chosen target. This is stronger than detection. A link-layer decision exists and execution is near.

The IP layer still has work. It checks whether the target belongs to the same subnet. If so, FMIPv6 is unnecessary. If the subnet differs, the node formulates a prospective new care-of address and sends FBU. The previous and new routers may exchange HI and HAck, prepare a tunnel and return FBack.

The adjective “prospective” matters. The decision refers to a predicted target, not a later observation of the place actually reached. RFC 4907's warning about unreliable link indications remains relevant: filtered signal strength should be joined with other loss evidence. A handover decision is a control input with provenance, not a future fact.

The command travels downward

LINK_SWITCH is the one primitive in the set that runs from IP to link. Its direction is part of its meaning. An implementation can keep the previous radio link long enough to receive FBack; after the acknowledgement arrives, the IP layer can tell the link layer to switch. The mechanism tries to preserve predictive operation.

If a system records only “handover event,” it loses whether radio evidence informed IP or IP ordered radio. That destroys accountability. The producer of a signal determines what it could observe and what authority it exercised. A command can be authenticated, accepted and logged without proving that the commanded transition finished.

Timing changes the branch. Once MOB_HO-IND is sent, the serving base station releases connections and resources. If FBack has not arrived, reactive processing becomes likely. The absence of an acknowledgement before disconnect is not an empty field; it is evidence explaining why later work moved to the new link.

LINK_UP starts the last IP decision

After switching, the mobile node synchronizes with the target base station and performs 802.16e network entry. Ranging, capability negotiation, key management, registration and service-flow creation may occur, with some steps omitted when context was transferred. When the link is ready for data transmission, the link layer raises LINK_UP.

That green ring has a precise scope. It shows that the local link-layer state machine completed network entry. The IP layer must now ask where it actually arrived.

If the target is in the same subnet, there is no FMIPv6 completion. If it is the predicted network and FBack arrived on the old link, the node follows the predictive path and sends UNA. If it reached the predicted network without FBack, it follows the reactive path and sends both UNA and FBU. If it reached an unpredicted network, it must update the earlier binding so packets are redirected to the confirmed location.

One LINK_UP can therefore precede several mutually exclusive IP outcomes. A status model that declares success at that point cannot distinguish ordinary predictive completion, delayed reactive repair and a wrong-target recovery.

The address remains provisional after the radio is ready

The node uses the formulated NCoA as the source of UNA and then starts Duplicate Address Detection. The NAR may supply a different NCoA in an RA carrying a NAACK option. Link readiness and address validity are visibly separate in the protocol order.

After receiving UNA, the NAR updates neighbor state and can forward buffered packets. That action proves an attempt to deliver at a protocol boundary. It does not prove that the mobile node received every packet, that transport state survived, or that an application completed useful work.

This is the evidence hierarchy the single green light conceals: authenticated radio entry, IP-network identity, address authority, neighbor state, buffer release, bidirectional packets, transport recovery and application acknowledgement. Each later claim needs a later observer.

Preserve a cross-layer receipt

A defensible handover record should retain:

  • candidate BSID, advertisement provenance, scan result and radio measurements;
  • NEW_LINK_DETECTED time and the resulting RtSolPr/PrRtAdv mapping;
  • target, policy inputs and LINK_HANDOVER_IMPEND generation;
  • FBU, HI/HAck and FBack state, including whether FBack arrived before disconnect;
  • the producer, consumer and execution result of LINK_SWITCH;
  • MOB_HO-IND, old-link release and new 802.16e entry generation;
  • LINK_UP with authenticated entry and service-flow evidence;
  • observed versus predicted network, selected completion branch and any recovery FBU;
  • formulated and accepted NCoA, NAACK replacement and DAD result;
  • UNA, neighbor-cache change, buffered-packet release and first bidirectional packet; and
  • transport and application receipts when continuity is the stated outcome.

The record should not force these facts into one status column. Their order, direction and missing edges are the explanation.

Sources