The IPv6 Flow Label within a RPL domain
draft-thubert-6man-flow-label-for-rpl-04

Abstract

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.

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This Internet-Draft will expire on February 5, 2015.

Copyright Notice

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

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Table of Contents

• 1. Introduction
• 1.1. On Wasted Energy
• 1.2. LLN flows
• 1.3. On Compatibility With Existing Standards
• 2. Terminology
• 3. Applicability
• 4. Flow Label Format Within the RPL Domain
• 5. Root Operation
• 5.1. Incoming Packets
• 5.2. Outgoing Packets
• 6. RPL node Operation
• 7. Security Considerations
• 8. IANA Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Author's Address

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 TSCH wireless networks attached and synchronized by backbone routers.

With 6TiSCH, the 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.

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.

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 Wasted Energy

The design of Lowpower Lossy Networks is generally focussed on saving energy, which is the most constrained resource of all. The other constraints, such as the memory capacity and the duty cycling of the LLN devices, derive from that primary concern. Energy is typically available from batteries that are expected to last for years, or scavenged from the environment in very limited quantities. Any protocol that is intended for use in LLNs must be designed with the primary concern of saving energy as a strict requirement.

The Routing Protocol for Low Power and Lossy Networks (RPL) [RFC6550] specification defines a generic Distance Vector protocol that is indeed designed for very low energy consumption and 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 and set the related RPL information in the packets.

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 expected 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.

           ------+---------                            ^
                 |          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
 
                       LLN 

Figure 1: IP-in-IP Encapsulation within the LLN

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.thubert-6lo-forwarding-fragments].

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 bit weights in the energy consumption and drains the batteries.

Regrettably, [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 IP-in-IP encapsulation, the use of the flow label instead of the RPL option would be a valuable saving.

1.2. LLN flows

In Industrial Automation and Control Systems (IACS) [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. In such cases, related packets from multiple sources should not be load-balanced along their path in the Internet.

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. 4Hz is a typical loop frequency in Process Control, though it can be a lot slower than that in, say, environmental monitoring. The granularity of traffic from a single source is too small to make a lot of sense in load balancing application.

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 so as to experience similar jitter and latency. The traditional tuple of source, destination and ports might then not be the proper indication to isolate a consistent flow. On the other hand, the flow integrity can be preserved in a simple manner if the setting of the Flow Label in the IPv6 header of packets outgoing a RPL domain, is centralized to the root of the RPL DODAG structure, as opposed to distributed across the actual sources.

Considering that the goal for setting the Flow Label as prescribed in the IPv6 Flow Label Specification [RFC6437] is to improve load balancing in the core of the Internet, it is unlikely that LLN devices will consume energy to generate and then transmit a Flow Label to serve outside interests and the Flow Label is generally left to zero so as to be elided in the 6LoWPAN [RFC6282] compression. So in a general manner the interests of the core are better served if the RPL roots systematically rewrite the flow label rather than if they never do.

For packets coming into the RPL domain from the Internet, the value for setting the Flow Label as prescribed in [RFC6437] 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 a destination node that has no use of it.

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 resetting or reusing the IPv6 Flow Label at the root of a RPL domain is a deviation from 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 is a limited deviation from [RFC6437], which explicitly requires that the flow label cannot be modified once set.

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 Internet.

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 that the 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 can make the difference for the adoption of RPL and 6TiSCH 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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. Applicability

This specification applies to a RPL [RFC6282] domain that forms a stub LLN and is connected to the Internet by and only by its RPL root(s), which act(s) as Border Router(s) for the LLN. With RPL, a root is the bottleneck for all the traffic between the Internet and the Destination-Oriented Directed Acyclic Graph (DODAG) that it serves.

In that context, the specification entitles a RPL root to rewrite the IPv6 [RFC2460] Flow Label of all packets entering or leaving the RPL domain in both directions, from and towards the Internet, regardless of its original setting. This may seem contradictory with the IPv6 Flow Label Specification [RFC6437] which stipulates that once it is set, the Flow Label is left unchanged; but the RFC also indicates a violation to the rule can be accepted for compelling reasons, and that security is a case justifying such a violation. This specification suggests that energy-saving is another compelling reason for a violation to the aforementioned rule.

For the compelling reason of saving energy, this specification allows that regardless of its original setting, a root of a RPL domain MAY reset the Flow Label of IPv6 packets entering the RPL domain to zero for an optimal Header Compression by 6LoWPAN [RFC6282]. The specification also allows that the root and LLN routers MAY reuse the Flow Label inside the LLN for LLN purposes, such as to carry the RPL Information as detailed hereafter.

This specification also allows that regardless of its original setting, a root of a RPL domain MAY set the Flow Label of IPv6 packets that exit the RPL domain in a manner that SHOULD conform to the prescriptions in [RFC6437]. Therefore a source in the RPL domain MUST NOT assume that its setting of the Flow Label be preserved end-to-end. From there, the capability by RPL routers inside the LLN to alter a non-zero Flow Label between the source and the root is another minor deviation to [RFC6437] that is also acceptable since it is transparent to the core of the Internet.

4. 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)

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 omitted. In that case, the Flow Label MAY be used as a replacement to the [RFC6553] RPL option. To achieve this, the SenderRank is expressed with 8 least significant bits, and the information carried within the Flow Label in a packet is constructed follows:

        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 |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+