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The IPv6 Flow Label within a RPL domain

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Author Pascal Thubert
Last updated 2014-04-17
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ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                          April 17, 2014
Expires: October 17, 2014

                The IPv6 Flow Label within a RPL domain


   This document present how the Flow Label can be used inside a RPL
   domain as a replacement to the RPL option and provides rules for the
   root to set and reset the Flow Label when forwarding between the
   inside of RPL domain and the larger Internet, in both direction.
   This new operation saves 44 bits in each frame, and an eventual IP-
   in-IP encapsulation within the RPL domain that is required for all
   packets that reach outside of the RPL domain.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on October 17, 2014.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2

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     1.1.  On LLN flows . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  On Wasted Resources  . . . . . . . . . . . . . . . . . . .  4
     1.3.  On Compatibility With Existing Standards . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Flow Label Format Within the RPL Domain  . . . . . . . . . . .  6
   4.  Root Operation . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Incoming Packets . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Outgoing Packets . . . . . . . . . . . . . . . . . . . . .  7
   5.  RPL node Operation . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     9.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9

1.  Introduction

   The emergence of radio technology enabled a large variety of new
   types of devices to be interconnected, at a very low marginal cost
   compared to wire, at any range from Near Field to interplanetary
   distances, and in circumstances where wiring would be less than
   practical, for instance rotating devices.

   In particular, IEEE802.14.5 [IEEE802154] that is chartered to specify
   PHY and MAC layers for radio Lowpower Lossy Networks (LLNs), defined
   the TimeSlotted Channel Hopping [I-D.ietf-6tisch-tsch] (TSCH) mode of
   operation as part of the IEEE802.15.4e MAC specification in order to
   address Time Sensitive applications.

   The  6TISCH architecture  [I-D.ietf-6tisch-architecture] specifies
   the operation IPv6 over the IEEE802.15.4e TimeSlotted Channel Hopping
   [I-D.ietf-6tisch-tsch] (TSCH) wireless networks attached and
   synchronized by backbone routers.  In that model, route Computation
   may be achieved in a centralized fashion by a Path Computation
   Element (PCE), in a distributed fashion using the Routing Protocol
   for Low Power and Lossy Networks  [RFC6550] (RPL), or in a mixed
   mode.  The Backbone Routers may typically serve as roots for the RPL

   6TiSCH was created to simplify the adoption of IETF technology by
   other Standard Defining Organizations (SDOs), in particular in the
   Industrial Automation space, which already relies on variations of
   IEEE802.15.4e TSCH for Wireless Sensor Networking.  ISA100.11a
   [ISA100.11a] is an example of such industrial WSN standard, using
   IEEE802.15.4e over the classical IEEE802.14.5 PHY. In that case,
   after security is applied, roughly 80 octets are available per frame
   for IP and Payload.  In order to 1) avoid fragmentation and 2)
   conserve energy, the SDO will scrutinize any bit in the frame and
   reject any waste.

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   The challenge to obtain the adoption of IPv6 in the original standard
   was really to save any possible bit in the frames, including the UDP
   checksum which was an interesting discussion on its own.  This work
   was actually one of the roots for the 6LoWPAN Header Compression
   [RFC6282] work, which goes down to the individual bits to save space
   in the frames for actual data, and allowed ISA100.11a to adopt IPv6.

1.1.  On LLN flows

   In industrial applications such as control systems [RFC5673], a
   packet loss is usually acceptable but jitter and latency must be
   strictly controlled as they can play a critical role in the
   interpretation of the measured information.  Sensory systems are
   often distributed, and the control information can in fact be
   originated from multiple sources and aggregated.  As a result, it can
   be a requirement for related measurements from multiple sources to be
   treated as a single flow following a same path over the Internet in
   order to experience similar jitter and latency.  The traditional
   tuple of source, destination and ports might then not be the proper
   indication to isolate a meaningful flow.

   In a typical LLN application, the bulk of the traffic consists of
   small chunks of data (in the order few bytes to a few tens of bytes)
   at a time.  In the industrial case, a typical frequency is 4Hz but it
   can be a lot slower than that for, say, environmental monitoring.
   The granularity of traffic from a single source is too small to make
   a lot of sense in load balancing application.

   In such cases, related packets from multiple sources should not be
   load-balanced along their path in the Internet; load-balancing can be
   discouraged by tagging those packets with a same Flow Label in the
   IPv6  [RFC2460] header.  This can be achieved if the Flow Label in
   packets outgoing a RPL domain are set by the root of the RPL
   structure as opposed to the actual source.  It derives that the Flow
   Label could be reused inside the RPL domain.

   In a LLN, each transmitted bit represents energy and every saving
   counts dearly.  Considering that the value for which the Flow Label
   is used in the IPv6 Flow Label Specification  [RFC6437] is to serve
   load balancing in the core, it is unlikely that LLN devices will
   consume energy to generate and then transmit a Flow Label to serve
   interests in some other place.  On the other hand, it makes sense to
   recommend the computation of a stateless Flow Label at the root of
   the LLN towards the Internet.

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   Reciprocally, [RFC6437] requires that once set, a non-zero flow label
   value is left unchanged.  The value for that setting is consumed once
   the packet has traversed the core and reaches the LLN.  Then again,
   there is little value but a high cost for the LLN in spending 20 bits
   to transport a Flow Label from the Internet over the constrained
   network to the destination node.  It results that the MUST in
   [RFC6437] should be alleviated for packets coming from the outside on
   the LLN, and that it should be acceptable that the compression over
   the LLN erases the original flow label.  It should also be acceptable
   that the Flow Label field is reused in the LLN as proposed in this

1.2.  On Wasted Resources

   The Routing Protocol for Low Power and Lossy Networks (RPL)
   [RFC6550] specification defines a generic Distance Vector protocol
   that is adapted to a variety of LLNs.  RPL forms Destination Oriented
   Directed Acyclic Graphs (DODAGs) which root often acts as the Border
   Router to connect the RPL domain to the Internet.  The root is
   responsible to select the RPL Instance that is used to forward a
   packet coming from the Internet into the RPL domain.

   A classical RPL implementation will use the RPL Option for Carrying
   RPL Information in Data-Plane Datagrams  [RFC6553] to tag a packet
   with the Instance ID and other information that RPL requires for its
   operation within the RPL domain.  In particular, the Rank, which is
   the scalar metric computed by an specialized Objective Function such
   as [RFC6552], is modified at each hop and allows to validate that the
   packet progresses in the execpted direction each upwards or downwards
   in along the DODAG.

   With [RFC6553] the RPL option is encoded as 6 Octets; it must be
   placed in a Hop-by-Hop header that represents 2 additional octets for
   a total of 8. In order to limit its range to the inside the RPL
   domain, the Hop-by-Hop header must be added to (or removed from)
   packets that cross the border of the RPL domain.  For reasons such as
   the capability to send ICMP errors back to the source, this operation
   involves an extra IP-in-IP encapsulation inside the RPL domain for
   all the packets which path is not contained within the RPL domain.

   The 8-octets overhead is detrimental to the LLN operation, in
   particular with regards to bandwidth and battery constraints.  The
   extra encapsulation may cause a containing frame to grow above
   maximum frame size, leading to Layer 2 or 6LoWPAN [RFC4944]
   fragmentation, which in turn cause even more energy spending and
   issues discussed in the LLN Fragment Forwarding and Recovery [I-D

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               ------+---------                            ^
                     |          Internet                   |
                     |                                     | Native IPv6
                  +-----+                                  |
                  |     | Border Router (RPL Root)    ^    |    ^
                  |     |                             |    |    |
                  +-----+                             |    |    | IPv6 +
                     |                                |    |    | HbH
               o    o   o    o                        |    |    | headers
           o o   o  o   o  o  o o   o                 |    |    |
          o  o o  o o    o   o   o  o  o              |    |    |
          o   o    o  o     o  o    o  o  o           |    |    |
         o  o   o  o   o         o   o o              v    v    v
         o        o  o         o        o o
         o          o             o     o

   Considering that, in the classical IEEE802.14.5 PHY that is used by
   ISA100.11a, roughly 80 octets are available per frame after security
   is applied, and  , any additional transmitted octet weights in the
   energy consumption and drains the batteriesBut [RFC6282] does not
   provide an efficient compression for the RPL option so the cost in
   current implementations can not be alleviated in any fashion.  So
   even for packets that are confined within the RPL domain and do not
   need the 6in6 encapsulation, the use of the flow label instead of the
   RPL option would be a valuable saving.

1.3.  On Compatibility With Existing Standards

   All the packets from all the nodes in a same DODAG that are leaving a
   RPL domain towards the Internet will transit via a same RPL root.
   The RPL root segregates the Internet and the RPL domain, which
   enables the capability to reuse the Flow Label within the RPL domain.

   On the other hand, the operation of writing/rewriting the IPv6 Flow
   Label at the root of a RPL domain may seem in contradiction with the
   IPv6 Flow Label Specification  [RFC6437], in that it is neither the
   source nor the first hop router that sets the final Flow Label for
   use outside the RPL domain.

   Additionally, using the Flow Label to transport the information that
   is classically present in the RPL option implies that the Flow Label
   is modified at each hop inside the RPL domain, which again
   contradicts [RFC6437], which explicitly requires that the flow label
   cannot be modified once set.

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   But if we consider the whole RPL domain as a large virtual host from
   the standpoint of the rest of the Internet, the interests that lead
   to [RFC6437], and in particular load balancing in the core of the
   Internet, are probably better served if the root guarantees that the
   Flow Label is set in a compliant fashion than if we rely on each
   individual sensor that may not use it at all, or use it slightly
   differently such as done in ISA100.11a.

   Additionally, LLN flows can be compound flows aggregating information
   from multiple sources.  The root is an ideal place to rewrite the
   Flow Label to a same value for a same flow across multiple sources,
   ensuring compliance with the rules defined by [RFC6437] for use
   outside of the RPL domain and in particular in the core of the

   It can be noted that [RFC6282] provides an efficient header
   compression for packets that do have the Flow Label set in the IPv6
   header.  It results overhead for transporting the RPL information can
   be down from 64 to 20 bits, alleviating at the same time the need for
   IP-in-IP encapsulation.  This optimization cannot be ignored, and is
   required for the adoption of the 6TiSCH architecture by external
   standard bodies.

   This document specifies how the Flow Label can be reused within the
   RPL domain as a replacement to the RPL option.  The use of the Flow
   Label within a RPL domain is an instance of the stateful scenarios as
   discussed in [RFC6437]where the states include the rank of a node and
   the RPLInstanceID that identifies the routing topology.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The Terminology used in this document is consistent with and
   incorporates that described in `Terminology in Low power And Lossy
   Networks' [RFC7102] and [RFC6550].

3.  Flow Label Format Within the RPL Domain

   [RFC6550] section 11.2 specifies the fields that are to be placed
   into the packets for the purpose of Instance Identification, as well
   as Loop Avoidance and Detection.  Those fields include an 'O', and
   'R' and an 'F' bits, the 8-bit RPLInstanceID, and the 16-bit
   SenderRank.  SenderRank is the result of the DAGRank operation on the
   rank of the sender, where the DAGRank operation is defined in section
   3.5.1 as:

      DAGRank(rank) = floor(rank/MinHopRankIncrease)

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   If MinHopRankIncrease is set to a multiple of 256, it appears that
   the most significant 8 bits of the SenderRank will be all zeroes and
   could be ommitted.  In that case, the Flow Label MAY be used as a
   replacement to the [RFC6553] RPL option.  To achive this, the
   SenderRank is expressed with 8 least significant bits, and the
   information carried within the Flow Label in a packet is constructed

           0                   1                   2
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
                  | |O|R|F|  SenderRank   | RPLInstanceID |

   The first (leftmost) bit of the Flow Label is reserved and should be
   set to zero.

4.  Root Operation

   [RFC6437] section 3 intentionally does not consider flow label values
   in which any of the bits have semantic significance.  However, the
   present specification assigns semantics to various bits in the flow
   label, destroying  within the edge network that is the RPL domaina
   property of belonging to a      statistically uniform distribution
   that is desirable in the rest of the Internet.  This property MUST be
   restored by the root for outgoing packets.

   It can be noted that the rationale for the statistically uniform
   distribution does not necessarily bring a lot of value within the RPL
   domain.  In a specific use case where it would, that value must be
   compared with that of the battery savings in order to decide which
   technique the deployment will use to transport the RPL information.

4.1.  Incoming Packets

   When routing a packet towards the RPL domain, the root applies a
   policy to determine whether the Flow Label is to be used to carry the
   RPL information.  If so, the root MUST reset the Flow Label and then
   it MUST set all the fields in the Flow Label as prescribed by
   [RFC6553] using the format specified in Figure 2. In particular, the
   root selects the Instance that will be used to forward the packet
   within the RPL domain.

4.2.  Outgoing Packets

   When routing a packet outside the RPL domain, the root applies a
   policy to determine whether the Flow Label was used to carry the RPL
   information.  If so, the root MUST reset the Flow Label.  The root
   SHOULD recompute a Flow Label following the rules prescribed by
   [RFC6553].  In particular, the root MAY ignore the source address but
   it SHOULD use the RPLInstanceID for the computation.

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5.  RPL node Operation

   Depending on the policy in place, the source of a packet will decide
   whether to use this specification to transport the RPL information in
   the IPv6 packets.  If it does, the source in the LLN SHOULD set the
   Flow Label to zero and MUST NOT expect that the flow label will be
   conserved end-to-end".

6.  Security Considerations

   The process of using the Flow Label as opposed to the RPL option does
   not appear to create any opening for new threat compared to

7.  IANA Considerations

   No IANA action is required for this specification.

8.  Acknowledgements

   The author wishes to thank Brian Carpenter for his in-depth review
   and constructive approach to the problem and its resolution.

9.  References

9.1.  Normative References

              IEEE standard for Information Technology, "IEEE std.
              802.15.4, Part.  15.4: Wireless Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks", June 2011.

              ISA, "ISA100, Wireless Systems for Automation", May 2008,

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

   [RFC2460]  Deering, S.E. and R.M. Hinden, "Internet Protocol, Version
              6 (IPv6) Specification", RFC 2460, December 1998.

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

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437, November 2011.

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   [RFC6550]  Winter, T., Thubert, P., 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, March 2012.

   [RFC6552]  Thubert, P., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)", RFC
              6552, 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, March

9.2.  Informative References

              Thubert, P., Watteyne, T. and R. Assimiti, "An
              Architecture for IPv6 over the TSCH mode of IEEE
              802.15.4e", Internet-Draft draft-ietf-6tisch-
              architecture-01, February 2014.

              Watteyne, T., Palattella, M. and L. Grieco, "Using
              IEEE802.15.4e TSCH in an LLN context: Overview, Problem
              Statement and Goals", Internet-Draft draft-ietf-6tisch-
              tsch-00, November 2013.

              Thubert, P. and J. Hui, "LLN Fragment Forwarding and
              Recovery", Internet-Draft draft-thubert-6lo-forwarding-
              fragments-01, February 2014.

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

   [RFC5673]  Pister, K., Thubert, P., Dwars, S. and T. Phinney,
              "Industrial Routing Requirements in Low-Power and Lossy
              Networks", RFC 5673, October 2009.

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, January 2014.

Author's Address

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   Pascal Thubert, editor
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3
   Biot - Sophia Antipolis, 06410
   Phone: +33 4 97 23 26 34

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