A 6loRH for BitStrings

The type of information that needs to be present in a packet inside the LLN but not outside of the LLN varies with the routing operation, but there is overall a need for an extensible compression technique that would simplify the IP-in-IP encapsulation, when needed, and optimally compress existing routing artifacts found in LLNs. The 6LoWPAN Routing Header (6LoRH) specification is such a technique, that extends the 6lo adaptation layer framework , so as to carry routing information for Route-over use cases. The original specification includes the formats necessary for RPL such as the Source Route Header (SRH) and is intended to be extended for additional routing artifacts. The Bit Index Explicit Replication (BIER), as introduced in the BIER Architecture , can be used as an alternate artifact to route multicast as well as unicast traffic. The Traffic Engineering for Bit Index Explicit Replication (BIER-TE) adds support for traffic engineering by explicit hop-by-hop forwarding and loose hop forwarding of packets along a unicast route. This specification provides additional formats for the 6LoRH compression to carry BitStrings such as used for Bit Index Explicit Replication and its Traffic Engineering variant (BIER and BIER-TE, respectively).

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in . The Terminology used in this document is consistent with and incorporates that described in Terms Used in Routing for Low-Power and Lossy Networks (LLNs).. Other terms in use in LLNs are found in Terminology for Constrained-Node Networks. The term “byte” is used in its now customary sense as a synonym for “octet”. "RPL", "RPL Packet Information" (RPI) and "RPL Instance" are defined in the RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks specification. The terms Bit-Forwarding Egress Routers (BFR), BFR-id and BitString are defined in . A BitString indicates a continuous sequence of bits indexed by an offset in the sequence. The leftmost bit is bit 0 and corresponds to the value 0x80 of the leftmost octet in the BitString.

BIER and other bit-indexed methods that would leverage BitStrings will generally require additional information in the packet to complement the BitString. For instance, BIER has the concept of a BFR-id and an Entropy value in the BIER header. Since those additional fields depend on the bit-indexed method, they are expected to be transported separately from the BitString. This specification concentrates on the BitString alone. The DetNet Data Plane Protocol and Solution Alternatives document details how BIER-TE can be leveraged to activate the Deterministic Networking Replication and Elimination functions in a manner that is abstract to the data plane forwarding information. An adjacency, which is represented by a bit in the BIER header, can be mapped in the data plane to an Ethernet hop, a Label Switched Path, or it may correspond to a loose or a strict IPv6 Source Routed Path. In the context of LLNs, the 6TiSCH Architecture introduces the concept of a Track that is a directional traffic-engineered path between a source and a destination. A Track is indicated in a packet by a Source or Destination IPv6 Address and a RPL Local Instance. The RPL Instance is carried in an IPv6 packet as part of the RPL Packet Information (RPI), and a bit in the RPI indicates whether the Instance is Local to the Source or the Destination Address. The RPI can be compressed as a RPI 6LoRH header (RPI-6LoRH) as described in . The 6TiSCH requirements for DetNet indicate that a 6TiSCH Track may leverage replication and elimination as defined in DetNet. This specification enables this behavior as follows: if a BIER-6LoRH is positioned right after a RPI-6LoRH, then the BitString in the BIER-6LoRH applies to the context of the Track indicated by the source or destination address of the packet and the local Instance ID associated to the source or destination of the packet.

The BIER 6LoRH (BIER-6LoRH) is a Critical 6LoWPAN Routing Header that provides a variable-size container for a BitString such as, a but not limited to, a BIER BitString. The capability to parse the BIER BitString is necessary to forward the packet so the Type cannot be ignored.

This specification provides a 5-bit Control field that can be used to encode information that is specific to the BitString. The type and size of the BitString are encoded in the 6LoRHType.

In the bit-by-bit case, each bit is mapped in an unequivocal fashion with a single addressable resource in the network. This may rapidly lead to large BitStrings, and BIER allows to divide a network into groups that partition the network so that a given BitString is locally significant to one group only. This specification uses the 5-bits Control field to encode the group. When groups are used, it may be that a packet is sent to different groups at the same time. In that case, multiple BIER-6LoRH headers can be prepended to a same packet, each one for a different group. As the packet flows along the multicast distribution tree, a BIER-6LoRH header that has no more destination in a given branch may be removed to make the packet shorter.

A Bloom Filter can be seen as a compression technique for the BitString. A Bloom Filter may generate false positives, which, in the case of BIER, result in undue forwarding of a packet down a path where no listener exists. As an example, the Constrained-Cast specification employs Bloom Filters as a compact representation of a match or non-match for elements in a set that may be larger than the number of bits in the BitString. In the case of a Bloom Filter, a number of Hash functions must be run to obtain a multi-bit signature of an encoded element. This specification uses the 5-bits Control field to signal an Identifier of the set of Hash functions being used to generate a certain BitString, so as to enable the migration from a set of Hash functions to the next.

The Type of a BIER-6LoRH header indicates the size of words used to build the BitString and whether the BitString is operated as an uncompressed bit-by-bit mapping, or as a Bloom filter.

In order to address a potentially large number of devices, the BitString may grow very large. Yet, the maximum frame size for a given MAC layer may limit the number of bits that can be dedicated to routing. With this specification, a number of BIER-6LoRH headers of a same type (bit-by-bit or Bloom filter) may be placed contiguously in the packet. This results in a larger BitString that is the concatenation of the BitStrings in the individual headers in the order they are appearing in the packet.

A research implementation was developed at Cisco's Paris Innovation Lab (PIRL) by Zacharie Brodard. The implementation is based on openWSN (https://openwsn.atlassian.net/wiki/pages/viewpage.action?pageId=688187), and was tested on OpenMote hardware (http://www.openmote.com/). The implementation covers 6LoRH Types 15 to 18 for 6TiSCH Tracks, and does not attempt to support Bloom Filters.

The security considerations of apply.

This document extends the IANA registry created by for the 6LoWPAN Routing Header Type, and adds the following values: 15..24 : BIER-6LoRH [RFCthis]