RFC 9034

Internet Engineering Task Force (IETF)                         L. Thomas
Request for Comments: 9034                                         C-DAC
Category: Standards Track                                 S. Anamalamudi
ISSN: 2070-1721                                        SRM University-AP
                                                            S.V.R. Anand
                                                                M. Hegde
                                             Indian Institute of Science
                                                              C. Perkins
                                                             Lupin Lodge
                                                               June 2021

   Packet Delivery Deadline Time in the Routing Header for IPv6 over
          Low-Power Wireless Personal Area Networks (6LoWPANs)


   This document specifies a new type for the 6LoWPAN routing header
   containing the deadline time for data packets, designed for use over
   constrained networks.  The deadline time enables forwarding and
   scheduling decisions for time-critical machine-to-machine (M2M)
   applications running on Internet-enabled devices that operate within
   time-synchronized networks.  This document also specifies a
   representation for the deadline time values in such networks.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  6LoRHE Generic Format
   4.  Deadline-6LoRHE
   5.  Deadline-6LoRHE Format
   6.  Deadline-6LoRHE in Three Network Scenarios
     6.1.  Scenario 1: Endpoints in the Same DODAG (N1)
     6.2.  Scenario 2: Endpoints in Networks with Dissimilar L2
     6.3.  Scenario 3: Packet Transmission across Different DODAGs (N1
           to N2)
   7.  IANA Considerations
   8.  Synchronization Aspects
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  Modular Arithmetic Considerations

   Authors' Addresses

1.  Introduction

   Low-Power and Lossy Networks (LLNs) are likely to be deployed for
   real-time industrial applications requiring end-to-end delay
   guarantees [RFC8578].  A Deterministic Network ("DetNet") typically
   requires some data packets to reach their receivers within strict
   time bounds.  Intermediate nodes use the deadline information to make
   appropriate packet forwarding and scheduling decisions to meet the
   time bounds.

   This document specifies a new type for the Elective 6LoWPAN Routing
   Header (6LoRHE), Deadline-6LoRHE, so that the deadline time (i.e.,
   the time of latest acceptable delivery) of data packets can be
   included within the 6LoRHE.  [RFC8138] specifies the 6LoWPAN Routing
   Header (6LoRH), compression schemes for RPL (Routing Protocol for
   Low-Power and Lossy Networks) source routing [RFC6554], header
   compression of RPL packet information [RFC6553], and IP-in-IP
   encapsulation.  This document also specifies the handling of the
   deadline time when packets traverse time-synchronized networks
   operating in different time zones or distinct reference clocks.
   Time-synchronization techniques are outside the scope of this
   document.  There are a number of standards available for this
   purpose, including IEEE 1588 [IEEE.1588.2008], IEEE 802.1AS
   [IEEE.802.1AS.2011], IEEE 802.15.4-2015 Time-Slotted Channel Hopping
   (TSCH) [IEEE.802.15.4], and more.

   The Deadline-6LoRHE can be used in any time-synchronized 6LoWPAN
   network.  A 6TiSCH (IPv6 over the TSCH mode of IEEE 802.15.4) network
   is used to describe the implementation of the Deadline-6LoRHE, but
   this does not preclude its use in scenarios other than 6TiSCH.  For
   instance, there is a growing interest in using 6LoWPAN over a
   Bluetooth Low Energy (BLE) mesh network [6LO-BLEMESH] in industrial
   IoT (Internet of Things) [IEEE-BLE-MESH].  BLE mesh time
   synchronization is being explored by the Bluetooth community.  There
   are also cases under consideration in Wi-SUN [PHY-SPEC] [Wi-SUN].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses the terminology defined in [RFC6550] and

3.  6LoRHE Generic Format

   Note: this section is not normative and is included for convenience.
   The generic header format of the 6LoRHE is specified in [RFC8138].
   Figure 1 illustrates the 6LoRHE generic format.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-        ...              -+
     |1|0|1| Length  |      Type     |        Options            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-        ...              -+
                                      <---    length         --->

                          Figure 1: 6LoRHE Format

   Length:  Length of the 6LoRHE expressed in bytes, excluding the first
      2 bytes.  This enables a node to skip a 6LoRHE if the Type is not
      recognized or supported.

   Type (variable length):  Type of the 6LoRHE (see Section 7).

4.  Deadline-6LoRHE

   The Deadline-6LoRHE (see Figure 3) is a 6LoRHE [RFC8138] that
   provides the Deadline Time (DT) for an IPv6 datagram in a compressed
   form.  Along with the DT, the header can include the Origination Time
   Delta (OTD) packet, which contains the time when the packet was
   enqueued for transmission (expressed as a value to be subtracted from
   DT); this enables a close estimate of the total delay incurred by a
   packet.  The OTD field is initialized by the sender based on the
   current time at the outgoing network interface through which the
   packet is forwarded.  Since the OTD is a delta, the length of the OTD
   field (i.e., OTL) will require fewer bits than the length of the DT
   field (i.e., DTL).

   The DT field contains the value of the deadline time for the packet
   -- in other words, the time by which the application expects the
   packet to be delivered to the receiver.

      packet_deadline_time = packet_origination_time + max_delay

   In order to support delay-sensitive, deterministic applications, all
   nodes within the network should process the Deadline-6LoRHE.  The DT
   and OTD packets are represented in time units determined by a scaling
   parameter in the Routing Header.  The Network ASN (Absolute Slot
   Number) can be used as a time unit in a time-slotted synchronized
   network (for instance, a 6TiSCH network, where global time is
   maintained in the units of slot lengths of a certain resolution).

   The delay experienced by packets in the network is a useful metric
   for network diagnostics and performance monitoring.  Whenever a
   packet crosses into a network using a different reference clock, the
   DT field is updated to represent the same deadline time, but
   expressed using the reference clock of the interface into the new
   network.  Then the origination time is the same as the current time
   when the packet is transmitted into the new network, minus the delay
   already experienced by the packet, say 'current_dly'.  In this way,
   within the newly entered network, the packet will appear to have
   originated 'current_dly' time units earlier with respect to the
   reference clock of the new network.

      new_network_origin_time = time_now_in_new_network - current_dly

   The following example illustrates these calculations when a packet
   travels between three networks, each in a different time zone (TZ).
   'x' can be 1, 2, or 3.  Suppose that the deadline time as measured in
   TZ1 is 1050, and the origination time is 50.  Suppose that the
   difference between TZ2 and TZ1 is 900, and the difference between TZ2
   and TZ3 is 3600.  In the figure, OT is the origination time as
   measured in the current time zone, and is equal to DT - OTD, that is,
   DT - 1000.  Figure 2 uses the following abbreviations:

   TxA:  Time of arrival of packet in the network 'x'

   TxD:  Departure time of packet from the network 'x'

   dlyx:  Delay experienced by the packet in the previous network(s)

   TZx:  The time zone of network 'x'

               TZ1                      TZ2                    TZ3
     T1A=50|                 |                             |
           |----  dly1=50    |                             |
           |     \           |                             |
           |      \          |                             |
           |       \ T1D=100 |T2A=1000                     |
           |        -------->|-----           dly2=450     |
           |                 |     \                       |
           |                 |      \                      |
           |                 |       \          T2D=1400   | T3A=5000
           |                 |         ------------------->|---------->
           |                 |                             |
           v                 v                             v

      dly0 = 0          dly1 = T1D-OT1      dly2 = T2D-OT2
                             = 100-50            = 1400 - 950
                             = 50                = 450

      OT1 = T1A-dly0     OT2 = T2A-dly1     OT3 = T3A-dly2
          = 50               = 1000-50          = 5000 - 450
                             = 950              = 4550

                   Figure 2: Deadline Time Update Example

   There are multiple ways that a packet can be delayed, including
   queuing delay, Media Access Control (MAC) layer contention delay,
   serialization delay, and propagation delay.  Sometimes there are
   processing delays as well.  For the purpose of determining whether or
   not the deadline has already passed, these various delays are not

5.  Deadline-6LoRHE Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |1|0|1| Length  |  6LoRH Type   |D| TU|  DTL  | OTL | BinaryPt  |
       |      DT (variable length)     | OTD(variable length)(optional)|

                      Figure 3: Deadline-6LoRHE Format

   Length (5 bits):  Length represents the total length of the Deadline-
      6LoRHE Type measured in octets.

   6LoRH Type:  7 (See Section 7.)

   D flag (1 bit):  The 'D' flag, set by the sender, qualifies the
      action to be taken when a 6LoWPAN Router (6LR) detects that the
      deadline time has elapsed.

      If 'D' bit is 1, then the 6LR MUST drop the packet if the deadline
      time is elapsed.

      If 'D' bit is 0, the packet MAY be forwarded on an exception
      basis, if the forwarding node is NOT in a situation of constrained
      resource, and if there are reasons to suspect that downstream
      nodes might find it useful (delay measurements, interpolations,

   TU (2 bits):  Indicates the time units for DT and OTD fields.  The
      encodings for the DT and OTD fields use the same time units and

      00  Time represented in seconds and fractional seconds
      01  Reserved
      10  Network ASN
      11  Reserved

   DTL (4 bits):  Length of the DT field as an unsigned 4-bit integer,
      encoding the length of the field in hex digits, minus one.

   OTL (3 bits):  Length of the OTD field as an unsigned 3-bit integer,
      encoding the length of the field in hex digits.  If OTL == 0, the
      OTD field is not present.  The value of OTL MUST NOT exceed the
      value of DTL plus one.

      For example, DTL = 0b0000 means the DT field in the 6LoRHE is 1
      hex digit (4 bits) long.  OTL = 0b111 means the OTD field is 7 hex
      digits (28 bits) long.

   BinaryPt (6 bits):  If zero, the number of bits of the integer part
      the DT is equal to the number of bits of the fractional part of
      the DT.  If nonzero, the BinaryPt is a (2's complement) signed
      integer determining the position of the binary point within the
      value for the DT.  This allows BinaryPt to be within the range

      *  If BinaryPt value is positive, then the number of bits for the
         integer part of the DT is increased by the value of BinaryPt,
         and the number of bits for the fractional part of the DT is
         correspondingly reduced.  This increases the range of DT.

      *  If BinaryPt value is negative, then the number of bits for the
         integer part of the DT is decreased by the value of BinaryPt,
         and the number of bits for the fractional part of the DT is
         correspondingly increased.  This increases the precision of the
         fractional seconds part of DT.

   DT Value (4..64 bits):  An unsigned integer of DTL+1 hex digits
      giving the DT value.

   OTD Value (4..28 bits):  If present, an unsigned integer of OTL hex
      digits giving the origination time as a negative offset from the
      DT value.

   Whenever a sender initiates the IP datagram, it includes the
   Deadline-6LoRHE along with other 6LoRH information.  For information
   about the time-synchronization requirements between sender and
   receiver, see Section 8.

   For the chosen time unit, a compressed time representation is
   available as follows.  First, the application on the originating node
   determines how many time bits are needed to represent the difference
   between the time at which the packet is launched and the deadline
   time, including the representation of fractional time units.  That
   number of bits (say, N_bits) determines DTL as follows:

      DTL = ((N_bits - 1) / 4)

   The number of bits determined by DTL allows the counting of any
   number of fractional time units in the range of interest determined
   by DT and the OT.  Denote this number of fractional time units to be
   Epoch_Range(DTL) (i.e., Epoch_Range is a function of DTL):

      Epoch_Range(DTL) = 2^(4*(DTL+1))

   Each point of time between OT and DT is represented by a time unit
   and a fractional time unit; in this section, this combined
   representation is called a rational time unit (RTU).  1 RTU measures
   the smallest fractional time that can be represented between two
   points of time in the epoch (i.e., within the range of interest).

   DT - OT cannot exceed 2^(4*(DTL+1)) == 16^(DTL+1).  A low value of
   DTL leads to a small Epoch_Range; if DTL = 0, there will only be 16
   RTUs within the Epoch_Range (i.e., Epoch_Range(DTL) = 16^1) for any
   TU.  The values that can be represented in the current epoch are in
   the range [0, (Epoch_Range(DTL) - 1)].

   Assuming wraparound does not occur, OT is represented by the value
   (OT mod Epoch_Range), and DT is represented by the value (DT mod
   Epoch_Range).  All arithmetic is to be performed modulo
   (Epoch_Range(DTL)), yielding only positive values for DT - OT.

   In order to allow fine-grained control over the setting of the
   deadline time, the fields for DT and OTD use fractional seconds.
   This is done by specifying a binary point, which allocates some of
   the bits for fractional times.  Thus, all such fractions are
   restricted to be negative powers of 2.  Each point of time between OT
   and DT is then represented by a time unit (either seconds or ASNs)
   and a fractional time unit.

   Let OT_abs, DT_abs, and CT_abs denote the true (absolute) values (on
   the synchronized timelines) for OT, DT, and current time.  Let N be
   the number of bits to be used to represent the integer parts of
   OT_abs, DT_abs, and CT_abs:

         N = {4*(DTL+1)/2} + BinaryPt

   The originating node has to pick a segment size (2^N) so that DT_abs
   - OT_abs < 2^N, and so that intermediate network nodes can detect
   whether or not CT_abs > DT_abs.

   Given a value for N, the value for DT is represented in the deadline-
   time format by DT = (DT_abs mod 2^N).  DT is typically represented as
   a positive value (even though negative modular values make sense).
   Also, let OT = OT_abs mod 2^N and CT = CT_abs mod 2^N, where both OT
   and CT are also considered as non-negative values.

   When the packet is launched by the originating node, CT_abs == OT_abs
   and CT == OT.  Given a particular value for N, then in order for
   downstream nodes to detect whether or not the deadline has expired
   (i.e., whether DT_abs > CT_abs), the following is required:

         Assumption 1: DT_abs - OT_abs < 2^N.

   Otherwise the ambiguity inherent in the modulus arithmetic yielding
   OT and DT will cause failure: one cannot measure time differences
   greater than 2^N using numbers in a time segment of length less than

   Under Assumption 1, downstream nodes must effectively check whether
   or not their current time is later than the DT -- but the value of
   the DT has to be inferred from the value of DT in the 6LoRHE, which
   is a number less than 2^N.  This inference cannot be expected to
   reliably succeed unless Assumption 1 is valid, which means that the
   originating node has to be careful to pick proper values for DTL and
   for BinaryPt.

   Since OT is not necessarily provided in the 6loRHE, there may be a
   danger of ambiguity.  Surely, when DT = CT, the deadline time is
   expiring and the packet should be dropped.  However, what if an
   intermediate node measures that CT = DT+1?  Was the packet launched a
   short time before arrival at the intermediate node, or has the
   current time wrapped around so that CT_abs - OT_abs > 2^N?

   In order to solve this problem, a safety margin has to be provided,
   in addition to requiring that DT_abs - OT_abs < 2^N.  The value of
   this safety margin is proportional to 2^N and is determined by a new
   parameter, called the "SAFETY_FACTOR".  Then, for safety the
   originating node MUST further ensure that (DT_abs - OT_abs) <

   Each intermediate node that receives the packet with the Deadline-
   6LoRHE must determine whether ((CT - DT) mod 2^N) >
   SAFETY_FACTOR*2^N.  If this test condition is not satisfied, the
   deadline time has expired.  See Appendix A for more explanation about
   the test condition.  All nodes that receive a packet with a Deadline-
   6LoRHE included MUST use the same value for the SAFETY_FACTOR.  The
   SAFETY_FACTOR is to be chosen so that a packet with the Deadline-
   6LoRHE included will be tested against the current time at least once
   during every subinterval of length SAFETY_FACTOR*2^N.  In this way,
   it can be guaranteed that the packet will be tested often enough to
   make sure it can be dropped whenever CT_abs > DT_abs.  The value of
   SAFETY_FACTOR is specified in this document to be 20%.

      Example: Consider a 6TiSCH network with time-slot length of 10 ms.
      Let the time units be ASNs (TU == (binary)0b10).  Let the current
      ASN when the packet is originated be 54400, and the maximum
      allowable delay (max_delay) for the packet delivery be 1 second
      from the packet origination, then:

         deadline_time = packet_origination_time + max_delay

            = 0xD480 + 0x64 (Network ASNs)

            = 0xD4E4 (Network ASNs)

         Then, the Deadline-6LoRHE encoding with nonzero OTL is:

            DTL = 3, OTL = 2, TU = 0b10, BinaryPt = 8, DT = 0xD4E4,
            OTD = 0x64

6.  Deadline-6LoRHE in Three Network Scenarios

   In this section, the Deadline-6LoRHE operation is described for three
   network scenarios.  Figure 4 depicts a constrained time-synchronized
   LLN that has two subnets, N1 and N2, connected through 6LoWPAN Border
   Routers (6LBRs) [RFC8929] with different reference clock times, T1
   and T2.

                          | Time-Synchronized |
                          |      Network      |
                     |                             |
                  +-----+                       +-----+
                  |     | Backbone              |     | Backbone
             o    |     | router                |     | router
                  +-----+                       +-----+
             o                  o               o
                 o    o   o               o  o   o  o   o  o
            o      LLN    o                 o  LLN   o  o
               o   o    o      o             o o o     o  o
         6LoWPAN Network (subnet N1)   6LoWPAN Network (subnet N2)

                 Figure 4: Intra-Network Time Zone Scenario

6.1.  Scenario 1: Endpoints in the Same DODAG (N1)

   In Scenario 1, shown in Figure 5, the Sender 'S' has an IP datagram
   to be routed to a Receiver 'R' within the same Destination-Oriented
   Directed Acyclic Graph (DODAG).  For the route segment from the
   sender to the 6LBR, the sender includes a Deadline-6LoRHE by encoding
   the deadline time contained in the packet.  Subsequently, each 6LR
   will perform hop-by-hop routing to forward the packet towards the
   6LBR.  Once the 6LBR receives the IP datagram, it sends the packet
   downstream towards 'R'.

   In the case of a network running in RPL non-storing mode, the 6LBR
   generates an IPv6-in-IPv6 encapsulated packet when sending the packet
   downwards to the receiver [RFC9008].  The 6LBR copies the Deadline-
   6LoRHE from the sender-originated IP header to the outer IP header.
   The Deadline-6LoRHE contained in the inner IP header is removed.

                   ^          | 6LBR  |       |
                   |          |       |       |
                   |          +-------+       |
           Upward  |         /      /| \      | Downward
           routing |      (F)      / |  \     | routing
                   |     /  \    (C) |  (D)   |
                   |    /    \    |  | / |\   |
                   |  (A)    (B)  : (E)  : R  |
                   |  /|\     | \   / \       |
                   | S : :    : :  :  :       v

           Figure 5: Endpoints within the Same DODAG (Subnet N1)

   At the tunnel endpoint of the encapsulation, the Deadline-6LoRHE is
   copied back from the outer header to inner header, and the inner IP
   packet is delivered to 'R'.

6.2.  Scenario 2: Endpoints in Networks with Dissimilar L2 Technologies

   In Scenario 2, shown in Figure 6, the Sender 'S' (belonging to DODAG
   1) has an IP datagram to be routed to a Receiver 'R' over a time-
   synchronized IPv6 network.  For the route segment from 'S' to 6LBR,
   'S' includes a Deadline-6LoRHE.  Subsequently, each 6LR will perform
   hop-by-hop routing to forward the packet towards the 6LBR.  Once the
   deadline time information reaches the 6LBR, the packet will be
   encoded according to the mechanism prescribed in the other time-
   synchronized network depicted as "Time-Synchronized Network" in
   Figure 6.  The specific data encapsulation mechanisms followed in the
   new network are beyond the scope of this document.

                              | Time-          |
                              | Synchronized   |------R
                              | Network        |
                     ^                |
                     |            +---+---+
                     |            | 6LBR  |
            Upward   |            |       |
            routing  |            +------++
                     |        (F)/      /| \
                     |       /  \      / |  \
                     |      /    \   (C) |  (D)
                     |    (A)    (B)  |  | / |\
                     |    /|\     |\  : (E)  : :
                     |   S : :    : :   / \
                                       :   :

       Figure 6: Packet Transmission in Dissimilar L2 Technologies or

   For instance, the IP datagram could be routed to another time-
   synchronized, deterministic network using the mechanism specified in
   In-situ Operations, Administration, and Maintenance (IOAM)
   [IOAM-DATA], and then the deadline time would be updated according to
   the measurement of the current time in the new network.

6.3.  Scenario 3: Packet Transmission across Different DODAGs (N1 to N2)

   Consider the scenario depicted in Figure 7, in which the Sender 'S'
   (belonging to DODAG 1) has an IP datagram to be sent to Receiver 'R'
   belonging to another DODAG (DODAG 2).  The operation of this scenario
   can be decomposed into a combination of Scenarios 1 and 2.  For the
   route segment from 'S' to 6LBR1, 'S' includes the Deadline-6LoRHE.
   Subsequently, each 6LR will perform hop-by-hop operations to forward
   the packet towards 6LBR1.  Once the IP datagram reaches 6LBR1 of
   DODAG1, 6LBR1 applies the same rule as described in Scenario 2 while
   routing the packet to 6LBR2 over a (likely) time-synchronized wired
   backhaul.  The wired side of 6LBR2 can be mapped to the receiver of
   Scenario 2.  Once the packet reaches 6LBR2, it updates the Deadline-
   6LoRHE by adding or subtracting the difference of time of DODAG2 and
   sends the packet downstream towards 'R'.

                    Time-Synchronized Network
                   |                           |
      DODAG1   +---+---+                   +---+---+   DODAG2
               | 6LBR1 |                   | 6LBR2 |
               |       |                   |       |
               +-------+                   +-------+
           (F)/      /| \              (F)/      /| \
          /  \      / |  \            /  \      / |  \
         /    \   (C) |  (D)         /    \   (C) |  (D)
       (A)    (B)  |  | / |\       (A)    (B)  |  |   |\
       /|\     |\  : (E)  : :      /|\     |\  : (E)  : :
      S : :    : :   / \          : : :    : :   / \
                    :   :                       :   R
   Network N1, time zone T1      Network N2, time zone T2

        Figure 7: Packet Transmission in Different DODAGs (N1 to N2)

   Consider an example of a 6TiSCH network in which S in DODAG1
   generates the packet at ASN 20000 to R in DODAG2.  Let the maximum
   allowable delay be 1 second.  The time-slot length in DODAG1 and
   DODAG2 is assumed to be 10 ms.  Once the deadline time is encoded in
   Deadline-6LoRHE, the packet is forwarded to 6LBR1 of DODAG1.  Suppose
   the packet reaches 6LBR1 of DODAG1 at ASN 20030.

      current_time = ASN at 6LBR * slot_length_value

      remaining_time = deadline_time - current_time

         = ((packet_origination_time + max_delay) - current time)

         = (20000 + 100) - 20030

         = 30 (in Network ASNs)

         = 30 * 10^3 milliseconds

   Once the deadline time information reaches 6LBR2, the packet will be
   encoded according to the mechanism prescribed in the other time-
   synchronized network.

7.  IANA Considerations

   This document defines a new Elective 6LoWPAN Routing Header Type, and
   IANA has assigned the value 7 from the 6LoWPAN Dispatch Page 1 number
   space for this purpose.

                  | Value | Description     | Reference |
                  | 7     | Deadline-6LoRHE | RFC 9034  |

                      Table 1: Entry in the "Elective
                        6LoWPAN Routing Header Type"

8.  Synchronization Aspects

   The document supports time representation of the deadline and
   origination times carried in the packets traversing networks of
   different time zones having different time-synchronization
   mechanisms.  For instance, in a 6TiSCH network where the time is
   maintained as ASN time slots, the time synchronization is achieved
   through beaconing among the nodes as described in [RFC7554].  There
   could be 6lo networks that employ NTP where the nodes are
   synchronized with an external reference clock from an NTP server.
   The specification of the time-synchronization method that needs to be
   followed by a network is beyond the scope of the document.

   The number of hex digits chosen to represent DT, and the portion of
   that field allocated to represent the integer number of seconds,
   determines the meaning of t_0, i.e., the meaning of DT == 0 in the
   chosen representation.  If DTL == 0, then there are only 4 bits that
   can be used to count the time units, so that DT == 0 can never be
   more than 16 time units (or fractional time units) in the past.  This
   then requires that the time synchronization between sender and
   receiver has to be tighter than 16 units.  If the binary point were
   moved so that all the bits were used for fractional time units (e.g.,
   fractional seconds or fractional ASNs), the time-synchronization
   requirement would be correspondingly tighter.

   A 4-bit field for DT allows up to 16 hex digits, which is 64 bits.
   That is enough to represent the NTP 64-bit timestamp format
   [RFC5905], which is more than enough for the purposes of establishing
   deadline times.  Unless the binary point is moved, this is enough to
   represent time since year 1900.

   For example, suppose that DTL = 0b0000 and the DT bits are split
   evenly; then we can count up to 3.75 seconds by quarter-seconds.

   If DTL = 3 and the DT bits are again split evenly, then we can count
   up to 256 seconds (in steps of 1/256 of a second).

   In all cases, t_0 is defined as specified in Section 5.

      t_0 = [current_time - (current_time mod (2^(4*(DTL+1))))]

   regardless of the choice of TU.

   For TU = 0b00, the time units are seconds.  With DTL == 15, and
   BinaryPt == 0, the epoch is (by default) January 1, 1900, at 00:00
   UTC.  The resolution is then 2^-32 seconds, which is the maximum
   possible.  This time format wraps around every 2^32 seconds, which is
   roughly 136 years.

   For TU = 0b10, the time units are ASNs.  The start time is relative,
   and updated by a mechanism that is out of scope for this document.
   With 10 ms slots, DTL = 15, and BinaryPt == 0, it would take over a
   year for the ASN to wrap around.  Typically, the number of hex digits
   allocated for TU = 0b10 would be less than 15.

9.  Security Considerations

   The security considerations of [RFC4944] (Section 13), [RFC6282]
   (Section 6), and [RFC6553] (Section 5) apply.  Using a compressed
   format as opposed to the full inline format is logically equivalent
   and does not create an opening for a new threat when compared to
   [RFC6550], [RFC6553], and [RFC6554].

   The protocol elements specified in this document are designed to work
   in controlled operational environments (e.g., industrial process
   control and automation).  In order to avoid misuse of the deadline
   information that could potentially result in a Denial of Service
   (DoS) attack, proper functioning of this deadline time mechanism
   requires the provisioning and management of network resources for
   supporting traffic flows with deadlines, performance monitoring, and
   admission control policy enforcement.  The network provisioning can
   be done either centrally or in a distributed fashion.  For example,
   tracks in a 6TiSCH network could be established by a centralized Path
   Computation Element (PCE), as described in the 6TiSCH architecture

   The security considerations of DetNet architecture [RFC8655]
   (Section 5) mostly apply to this document as well, as follows.  To
   secure the request and control of resources allocated for tracks,
   authentication and authorization can be used for each device and
   network controller devices.  In the case of distributed control
   protocols, security is expected to be provided by the security
   properties of the protocols in use.

   The identification of deadline-bearing flows on a per-flow basis may
   provide attackers with additional information about the data flows
   compared to networks that do not include per-flow identification.
   The security implications of disclosing that additional information
   deserve consideration when implementing this deadline specification.

   Because of the requirement of precise time synchronization, the
   accuracy, availability, and integrity of time synchronization is of
   critical importance.  Extensive discussion of this topic can be found
   in [RFC7384].

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <https://www.rfc-editor.org/info/rfc7384>.

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015,

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <https://www.rfc-editor.org/info/rfc8138>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,

   [RFC9030]  Thubert, P., Ed., "An Architecture for IPv6 over the Time-
              Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)",
              RFC 9030, DOI 10.17487/RFC9030, May 2021,

10.2.  Informative References

              Gomez, C., Darroudi, S. M., Savolainen, T., and M. Spoerk,
              "IPv6 Mesh over BLUETOOTH(R) Low Energy using IPSP", Work
              in Progress, Internet-Draft, draft-ietf-6lo-blemesh-10, 22
              April 2021,

              Leonardi, L., Patti, G., and L. Lo Bello, "Multi-Hop Real-
              Time Communications Over Bluetooth Low Energy Industrial
              Wireless Mesh Networks", IEEE Access, Vol 6, pp.
              26505-26519, DOI 10.1109/ACCESS.2018.2834479, May 2018,

              IEEE, "IEEE Standard for a Precision Clock Synchronization
              Protocol for Networked Measurement and Control Systems",
              DOI 10.1109/IEEESTD.2008.4579760, July 2008,

              IEEE, "IEEE Standard for Low-Rate Wireless Networks", IEEE
              Standard 802.15.4-2015, DOI 10.1109/IEEESTD.2016.7460875,
              April 2016,

              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks - Timing and Synchronization for Time-Sensitive
              Applications in Bridged Local Area Networks", IEEE Std
              802.1AS-2011, DOI 10.1109/IEEESTD.2011.5741898, March
              2011, <https://doi.org/10.1109/IEEESTD.2011.5741898>.

              Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi,
              Ed., "Data Fields for In-situ OAM", Work in Progress,
              Internet-Draft, draft-ietf-ippm-ioam-data-12, 21 February
              2021, <https://tools.ietf.org/html/draft-ietf-ippm-ioam-

   [PHY-SPEC] Wi-SUN Alliance, "Wi-SUN PHY Specification V1.0", March
              2016, <http://wi-sun.org>.

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,

   [RFC8929]  Thubert, P., Ed., Perkins, C.E., and E. Levy-Abegnoli,
              "IPv6 Backbone Router", RFC 8929, DOI 10.17487/RFC8929,
              November 2020, <https://www.rfc-editor.org/info/rfc8929>.

   [RFC9008]  Robles, M.I., Richardson, M., and P. Thubert, "Using RPI
              Option Type, Routing Header for Source Routes, and IPv6-
              in-IPv6 Encapsulation in the RPL Data Plane", RFC 9008,
              DOI 10.17487/RFC9008, April 2021,

   [Wi-SUN]   Harada, H., Mizutani, K., Fujiwara, J., Mochizuki, K.,
              Obata, K., and R. Okumura, "IEEE 802.15.4g Based Wi-SUN
              Communication Systems", IEICE Transactions on
              Communications, Volume E100.B, Issue 7, pp. 1032-1043,
              DOI 10.1587/transcom.2016SCI0002, January 2017,

Appendix A.  Modular Arithmetic Considerations

   Graphically, one might visualize the timeline as follows:

              OT_abs        CT_abs        DT_abs

                 Figure 8: Absolute Timeline Representation

   In Figure 8, the value of CT_abs is envisioned as traveling to the
   right as time progresses, getting farther away from OT_abs and
   getting closer to DT_abs.  The timeline is considered to be
   subdivided into time subintervals [i,j] starting and ending at
   absolute times equal to k*(2^N), for integer values of k.  Let I_k =
   k*(2^N) and I_(k+1) = (k+1)*2^N.  Intervals starting at I_k and
   I_(k+1) may occur at various placements in the above timeline.  Even
   though OT_abs is _always_ less than DT_abs, it could be that DT < OT
   because of the way that DT and OT are represented within the range
   [0, 2^N) and similarly for CT_abs and CT compared to OT and DT.

   Representing the above situation in time segments of length 2^N (and
   values OT, CT, DT) results in several cases where the deadline time
   has not elapsed:

   1) OT < CT < DT  (e.g., I_k < OT_abs < CT_abs < DT_abs < I_(k+1) )

   2) DT < OT < CT  (e.g., I_k < OT_abs < CT_abs < I_(k+1) < DT_abs )

   3) CT < DT < OT  (e.g., I_k < OT_abs < I_(k+1) < CT_abs < DT_abs )

   In the following cases, the deadline time has elapsed and the packet
   should be dropped.

   4) DT < CT < OT

   5) OT < DT < CT

   6) CT < OT < DT

   Again in Figure 8, consider CT_abs as time moving away from OT_abs
   and towards DT_abs.  For times CT_abs before the expiration of the
   deadline time, we also have CT_abs - OT_abs == CT - OT mod 2^N and
   similarly for DT_abs - CT_abs.

   As time proceeds, DT_abs - CT_abs gets smaller.  When the deadline
   time expires, DT_abs - CT_abs begins to grow negative.  A proper
   selection for SAFETY_FACTOR allows it to go _slightly negative_ but
   for an intermediate point to _detect_ that it has gone negative.
   Note that in modular arithmetic, "slightly negative" means _exactly_
   the same as "almost as large as the modulus (i.e., 2^N)".  Now
   consider the test condition ((CT - DT) mod 2^N) > SAFETY_FACTOR*2^N.

   (DT_abs - OT_abs) < 2^N*(1-SAFETY_FACTOR) satisfies the test
   condition when CT_abs == OT_abs (i.e., when the packet is launched).
   In modular arithmetic, 2^N*(1-SAFETY_FACTOR) == 2^N -
   2^N*SAFETY_FACTOR == -2^N*(SAFETY_FACTOR).  Then DT_abs - OT_abs <
   -2^N*(1-SAFETY_FACTOR).  Inverting the inequality, OT_abs - DT_abs >
   2^N*(1-SAFETY_FACTOR), and thus at launch CT_abs - DT_abs >

   As CT_abs grows larger, CT_abs - DT_abs gets LARGER in (non-negative)
   modular arithmetic until the time at which CT_ABS == DT_ABS, and
   suddenly CT_ABS - DT_abs becomes zero.  Also suddenly, the test
   condition is no longer fulfilled.

   As CT_abs grows still larger, CT_abs > DT_abs, and we need to detect
   this condition as soon as possible.  Requiring the SAFETY_FACTOR
   enables this detection until CT_abs exceeds DT_abs by an amount equal

   A note about "inverting the inequality".  Observe that a < b implies
   that -a > -b on the real number line.  Also, (a - b) == -(b - a).
   These facts hold also for modular arithmetic.

   During the times prior to the expiration of the deadline, for Safe =
   2^N*SAFETY_FACTOR we have:

         (DT_abs - 2^N) < OT_abs < CT_abs < DT_abs < DT_abs+Safe

   Naturally, DT_abs - 2^N == DT_abs mod 2^N == DT.

   Again considering Figure 8, it is easy to see that {CT_abs - (DT_abs
   - 2^N)} gets larger and larger until the time at which CT_abs =
   DT_abs, which is the first time at which CT - DT == 0 mod 2^N.  As
   CT_abs increases past the deadline time, 0 < CT_abs - DT_abs < Safe.
   In this range, any intermediate node can detect that the deadline has
   expired.  As CT_abs increases past DT_abs+Safe, it is no longer
   possible for an intermediate node to determine with certainty whether
   or not the deadline time has expired.  These statements also apply
   when reduced to modular arithmetic in the modulus 2^N.

   In particular, the test condition no longer allows detection of
   deadline expiration when the current time becomes later than
   (DT_abs+Safe).  In order to maintain correctness even for packets
   that are forwarded after expiration (i.e., the 'D' flag), N has to be
   chosen to be so large that the test condition will not fail -- i.e.,
   that in all scenarios of interest, the packet will be dropped before
   the current time becomes equal to DT_abs+2^N*SAFETY_FACTOR.


   The authors thank Pascal Thubert for suggesting the idea and
   encouraging the work.  Thanks to Shwetha Bhandari's suggestions,
   which were instrumental in extending the timing information to
   heterogeneous networks.  The authors acknowledge the 6TiSCH WG
   members for their inputs on the mailing list.  Special thanks to
   Jerry Daniel, Dan Frost (Routing Directorate), Charlie Kaufman
   (Security Directorate), Seema Kumar, Tal Mizrahi, Avinash Mohan,
   Shalu Rajendran, Anita Varghese, and Dale Worley (General Area Review
   Team (Gen-ART) review) for their support and valuable feedback.

Authors' Addresses

   Lijo Thomas
   Centre for Development of Advanced Computing
   Trivandrum 695033

   Email: lijo@cdac.in

   Satish Anamalamudi
   SRM University-AP
   Amaravati Campus
   Amaravati, Andhra Pradesh 522 502

   Email: satishnaidu80@gmail.com

   S.V.R. Anand
   Indian Institute of Science
   Bangalore 560012

   Email: anandsvr@iisc.ac.in

   Malati Hegde
   Indian Institute of Science
   Bangalore 560012

   Email: malati@iisc.ac.in

   Charles E. Perkins
   Lupin Lodge
   20600 Aldercroft Heights Rd.
   Los Gatos, CA 95033
   United States of America

   Email: charliep@computer.org