Packet Delivery Deadline Time in the Routing Header for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)
Extracted elements (25)
DT and OT are represented modulo 2^N (derived from DTL and BinaryPt) rather than as absolute timestamps, enabling compact encoding suitable for constrained 6LoWPAN headers. The originating node must pick N large enough that DT_abs - OT_abs < 2^N, or the modular representation becomes ambiguous.
Network ASN (Absolute Slot Number) is offered as an alternative time unit (TU=10) to fractional seconds (TU=00) because in 6TiSCH networks global time is natively maintained in slot-length units. Using ASNs avoids conversion overhead and keeps the DT representation aligned with the network's inherent scheduling granularity.
OTD is encoded as a delta (offset subtracted from DT) rather than an absolute origination timestamp, because the difference DT - OT is always smaller than DT itself; this reduces the OTD field to fewer bits than would be needed for an absolute value, conserving header space in constrained networks.
SAFETY_FACTOR is fixed at 20% to guarantee that every packet is tested against the current time at least once during every subinterval of length SAFETY_FACTOR*2^N, ensuring timely detection that CT_abs > DT_abs. Without this margin, modular-arithmetic ambiguity (where 'slightly past deadline' looks identical to 'almost a full epoch before deadline') would prevent reliable drop decisions.
The maximum DT field length of 64 bits (DTL=15) is sufficient to represent the full NTP 64-bit timestamp format, giving a maximum epoch of roughly 136 years at 2^-32 second resolution when TU=seconds, which is more than adequate for deadline-time purposes in LLN deployments.
When a packet crosses into a network with a different reference clock, the DT field is updated to re-express the same absolute deadline in the new network's time units, and the OTD is reset to reflect delay already accumulated. This approach preserves end-to-end semantics without requiring a common global epoch across heterogeneous networks.
In RPL non-storing mode, when the 6LBR generates an IPv6-in-IPv6 encapsulated packet for downstream delivery, it copies the Deadline-6LoRHE from the inner IP header to the outer IP header and removes it from the inner header. At the tunnel endpoint, the header is copied back from outer to inner before delivery to the receiver.
The Deadline-6LoRHE is designed for any time-synchronized 6LoWPAN network and is not limited to 6TiSCH; the document notes applicability to IPv6 over Bluetooth Low Energy mesh and Wi-SUN networks as well, provided those networks achieve adequate time synchronization.
When a packet transits from one DODAG to another with a different time zone (reference clock), the 6LBR at the boundary updates the DT field to express the deadline in the new network's time reference and recalculates OTD to account for delay already experienced, using: new_network_origin_time = time_now_in_new_network - current_dly.
All nodes that receive a packet with a Deadline-6LoRHE included MUST use the same value for the SAFETY_FACTOR (specified as 20%) to ensure consistent deadline-expiry detection across the network.
If the D bit is 1, a 6LoWPAN Router (6LR) MUST drop the packet when it detects that the deadline time has elapsed.
The originating node MUST ensure that (DT_abs - OT_abs) < 2^N * (1 - SAFETY_FACTOR), where N is the number of integer-part bits and SAFETY_FACTOR = 20%, so that intermediate nodes can reliably detect deadline expiration using modular arithmetic.
The value of OTL MUST NOT exceed the value of DTL plus one, ensuring the origination time offset field never requires more bits than the deadline time field can support.
BinaryPt (6 bits) is a 2's complement signed integer in the range [-32, 31] that positions the binary point within the DT value. A positive BinaryPt increases the integer-part bits (extending range); a negative BinaryPt increases the fractional-part bits (improving sub-second precision). If zero, bits are split evenly between integer and fractional parts.
Deadline-6LoRHE is a new Elective 6LoWPAN Routing Header (6LoRHE) type that carries the Deadline Time (DT) for an IPv6 datagram in compressed form, optionally accompanied by an Origination Time Delta (OTD) field. It enables intermediate nodes to make forwarding and scheduling decisions to meet end-to-end delay bounds in time-synchronized LLNs.
DTL (4 bits) encodes the length of the DT field as the number of hex digits minus one (i.e., DT field length = DTL+1 hex digits = 4*(DTL+1) bits), allowing DT to range from 4 to 64 bits. OTL (3 bits) encodes the length of the OTD field in hex digits; OTL=0 means OTD is absent.
The D flag (1 bit) set by the sender specifies the forwarding action when a 6LoWPAN Router detects that the deadline time has elapsed: D=1 mandates packet drop; D=0 permits exception-based forwarding if the node is not resource-constrained and downstream nodes may benefit (e.g., for delay measurements or interpolation).
The Origination Time Delta (OTD) field, when present, contains the time at which the packet was enqueued for transmission expressed as a positive offset to be subtracted from DT (i.e., OT = DT - OTD). Because it is a delta, OTL requires fewer bits than DTL, saving header space.
The TU field (2 bits) specifies the time units used for both DT and OTD fields: 00 = seconds and fractional seconds, 10 = Network ASN (Absolute Slot Number); values 01 and 11 are reserved.
IANA has assigned value 7 in the 'Elective 6LoWPAN Routing Header Type' registry (6LoWPAN Dispatch Page 1 number space) for the Deadline-6LoRHE type defined in this document.
Because the protocol requires precise time synchronization, the accuracy, availability, and integrity of the time synchronization mechanism are of critical importance; RFC 7384 provides extensive security requirements for time protocols in packet-switched networks.
Misuse of deadline information could enable Denial of Service (DoS) attacks against time-critical flows. Proper functioning requires provisioning and management of network resources for deadline-bearing flows, performance monitoring, and admission control policy enforcement, applied either centrally (e.g., via a PCE) or in a distributed fashion.
Per-flow identification of deadline-bearing flows may provide attackers with additional information about data flows compared to networks without such identification; the security implications of this disclosure should be considered when implementing the deadline mechanism.
Each intermediate node receiving a Deadline-6LoRHE evaluates the condition ((CT - DT) mod 2^N) > SAFETY_FACTOR * 2^N; if the condition is NOT satisfied, the deadline has expired and the packet is subject to the D-flag action (drop or conditional forward). The node transitions from 'in-deadline' to 'expired' when this test fails.
The Deadline-6LoRHE format begins with the standard 6LoRHE 2-byte header (3-bit prefix 101, 5-bit Length, 8-bit Type=7), followed by a control byte containing D (1 bit), TU (2 bits), DTL (4 bits), OTL (3 bits), and BinaryPt (6 bits), then a variable-length DT field of (DTL+1) hex digits (4–64 bits) and an optional variable-length OTD field of OTL hex digits (4–28 bits).