6LoWPAN Routing Header And Paging Dispatches

The design of Low Power and Lossy Networks (LLNs) is generally focused 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 often available from primary batteries that are expected to last for years, or is 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. Controlling the amount of data transmission is one possible venue to save energy. In a number of LLN standards, the frame size is limited to much smaller values than the IPv6 maximum transmission unit (MTU) of 1280 bytes. In particular, an LLN that relies on the classical Physical Layer (PHY) of IEEE 802.14.5 is limited to 127 bytes per frame. The need to compress IPv6 packets over IEEE 802.14.5 led to the 6LoWPAN Header Compression work (6LoWPAN-HC). Innovative Route-over techniques have been and are still being developed for routing inside a LLN. In a general fashion, such techniques require additional information in the packet to provide loop prevention and to indicate information such as flow identification, source routing information, etc. For reasons such as security and the capability to send ICMP errors back to the source, an original packet must not be tampered with, and any information that must be inserted in or removed from an IPv6 packet must be placed in an extra IP-in-IP encapsulation. This is the case when the additional routing information is inserted by a router on the path of a packet, for instance a mesh root, as opposed to the source node. This is also the case when some routing information must be removed from a packet that will flow outside the LLN. As an example, the Routing Protocol for Low Power and Lossy Networks (RPL) is designed to optimize the routing operations in constrained LLNs. As part of this optimization, RPL requires the addition of RPL Packet Information (RPI) in every packet, as defined in Section 11.2 of . The RPL Option for Carrying RPL Information in Data-Plane Datagrams specification indicates how the RPI can be placed in a RPL Option for use in an IPv6 Hop-by-Hop header. This representation demands a total of 8 bytes when in most cases the actual RPI payload requires only 19 bits. Since the Hop-by-Hop header must not flow outside of the RPL domain, it must be removed from packets that leave the domain, and be inserted in packets entering the domain. In both cases, this operation implies an IP-in-IP encapsulation.

Additionally, in the case of the Non-Storing Mode of Operation (MOP), RPL requires a Routing Header type 3 (RH3) as defined in the IPv6 Routing Header for Source Routes with RPL specification, for all packets that are routed down a RPL graph. With Non-Storing RPL, even if the source is a node in the same LLN, the packet must first reach up the graph to the root so that the root can insert the RH3 to go down the graph. In any fashion, whether the packet was originated in a node in the LLN or outside the LLN, and regardless of whether the packet stays within the LLN or not, as long as the source of the packet is not the root itself, the source-routing operation also implies an IP-in-IP encapsulation at the root to insert the RH3. 6TiSCH specifies the operation of IPv6 over the TimeSlotted Channel Hopping (TSCH) mode of operation of IEEE 802.14.5. The architecture requires the use of both RPL and the 6lo adaptation layer framework (, ) over IEEE 802.14.5. Because it inherits the constraints on the frame size from the MAC layer, 6TiSCH cannot afford to spend 8 bytes per packet on the RPI. Hence the requirement for a 6LoWPAN header compression of the RPI. 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. This specification extends the 6lo adaptation layer framework so as to carry routing information for Route-over use cases. The specification includes the formats necessary for RPL and is extensible for additional formats.

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 `Terminology in Low power And Lossy Networks’ and . The terms Route-over and Mesh-under are defined in . Other terms in use in LLNs are found in . The term “byte” is used in its now customary sense as a synonym for “octet”.

This draft adapts 6LoWPAN while maintaining backward compatibility with IPv6 over IEEE 802.15.4 by introducing a concept of context in the 6LoWPAN parser, a context being identified by a Page number, and defines 16 Pages. Pages are delimited in a 6LoWPAN packet by a Paging Dispatch value that indicates the next current Page. The Page number is encoded in a Paging Dispatch with the Value Bit Pattern of 1111xxxx where xxxx is the Page number, 0 to 15, as follows:

Values of the Dispatch byte defined in are considered as belonging to a Page 0 parsing context, which is the default and does not need to be signaled explicitly at the beginning of a 6LoWPAN packet. That way, backward compatibility with existing implementations in ensured. Note: This specification does not use the Escape Dispatch, which extends Page 0 to more values, but rather allocates another Dispatch Bit Pattern (1111xxxx) for a new Paging Dispatch, that is present in all Pages, including Page 0 and Pages defined in future specifications, to indicate the next parsing context represented by its Page number.

This draft defines a new Page 1 Paging Dispatch with a Dispatch Value of 11110001 that indicates a context switch in the 6LoWPAN parser to a Page 1. The Dispatch bits defined in Page 0 by are free to be reused in the next Pages, 1 to 15. On the other hand, the Dispatch bits defined in Page 0 for the Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks are defined with the same values in Page 1 so there is no need to switch context back from Page 1 to Page 0 to address LOWPAN_IPHC and LOWPAN_NHC.

This specification introduces a new 6LoWPAN Routing Header (6LoRH) to carry IPv6 routing information. The 6LoRH may contain source routing information such as a compressed form of RH3, as well as other sorts of routing information such as the RPL Packet Information and IP-in-IP encapsulation. The 6LoRH is expressed in a 6loWPAN packet as a Type-Length-Value (TLV) field, which is extensible for future uses. This specification uses the bit pattern 10xxxxxx in Page 1 for the new 6LoRH Dispatch. The 6LoRH uses on a 1/4th of the Dispatch space in Page 1, and this specification only uses a limited portion of the TLV space in the 6LoRH to encode RPL artifacts as detailed in . It is expected that in the future, other specification with extend the 6LoRH for other features related to packet routing and forwarding in 6LoWPAN networks.

In a zone of a packet where Page 1 is active, which means once a Page 1 Paging Dispatch is parsed, and as long as no other Paging Dispatch is parsed, the parsing of the packet MUST follow this specification if the 6LoRH Bit Pattern [] is found. Mesh Headers represent Layer-2 information and are processed before any Layer-3 information that is encoded in Page 1. If a 6LoWPAN packet requires a Mesh header, the Mesh Header MUST always be placed in the packet before the first Page 1 Paging Dispatch, if any. For the same reason, Fragments Headers as defined in MUST always be placed in the packet before the first Page 1 Paging Dispatch, if any. It must be noted that the NALP Dispatch Bit Pattern as defined in is only defined for the first octet in the packet. Switching back to Page 0 for NALP inside a 6LoWPAN packet appears non-sensical. It results that there is no need so far for restoring the Page 0 parsing context after a context was switched to Page 1, so the value for the Page 0 Paging Dispatch of 11110000 may not actually be seen in packets following the 6LoWPAN specifications that are available at the time of this writing.

With this specification, the 6LoRH [] is only defined in Page 1, so it MUST be placed in the packet in a zone where the Page 1 context is active. One or more 6LoRHs MAY be placed in a 6LoWPAN packet and MUST always be placed before the LOWPAN_IPHC . A 6LoRH being placed in a Page 1 context, it MUST always be placed after any Fragmentation Header and/or Mesh Header , even if a sur-compression mechanism is used that elides the Paging Dispatches.

In its canonical form, the 6LoRH reuses in Page 1 the Dispatch Value Bit Pattern of 10xxxxxx that is defined in Page 0 for the Mesh Header in . The Dispatch Value Bit Pattern is split in two forms of 6LoRH: Elective (6LoRHE) that may skipped if not understood Critical (6LoRHC) that may not be ignored

In its canonical form, the 6LoRHE uses the Dispatch Value Bit Pattern of 101xxxxx. A 6LoRHE may be ignored and skipped in parsing. If it is ignored, the 6LoRHE is forwarded with no change inside the LLN.

]]> Length of the 6LoRHE expressed in bytes, excluding the first 2 bytes. This is done to enable a node to skip a 6LoRH that it does not support and/or cannot parse, for instance if the Type is not known. Type of the 6LoRHE

In its canonical form, the 6LoRHC uses the Dispatch Value Bit Pattern of 100xxxxx. A node which does not support the 6LoRHC Type MUST silently discard the packet. Note: there is no provision for the exchange of error messages; such a situation should be avoided by judicious use of administrative control and/or capability indications.

]]> Type Specific Extension. The meaning depends on the Type, which must be known in all of the nodes. The interpretation of the TSE depends on the Type field that follows. For instance, it may be used to transport control bits, the number of elements in an array, or the length of the remainder of the 6LoRHC expressed in a unit other than bytes. Type of the 6LoRHC

The Routing Header type 3 (RH3) 6LoRH (RH3-6LoRH) is a Critical 6LoWPAN Routing Header that provides a compressed form for the RH3, as defined in for use by RPL routers. Routers that need to forward a packet with a RH3-6LoRH are expected to be RPL routers and expected to support this specification. If a non-RPL router receives a packet with a RPI-6LoRH, this means that there was a routing error and the packet should be dropped so the Type cannot be ignored.

The values for the RH3-6LoRH Type are an enumeration, 0 to 4. The form of compression is indicated by the Type as follows:

In the case of a RH3-6LoRH, the TSE field is used as a Size, which encodes the number of hops minus 1; so a Size of 0 means one hop, and the maximum that can be encoded is 32 hops. (If more than 32 hops need to be expressed, a sequence of RH3-6LoRH can be employed.) The next Hop is indicated in the first entry of the first RH3-6LoRH. Upon reception, the entry is checked whether it refers to the processing router itself. If it so, the entry is removed from the RH3-6LoRH and the Size is decremented. If the Size is now zero, the whole RH3-6LoRH is removed. If there is no more RH3-6LoRH, the processing node is the last router on the way, which may or may not be collocated with the final destination. The last hop in the last RH3-6LoRH is the last router prior to the destination in the LLN. So even when there is a RH3-6LoRH in the frame, the address of the final destination is in the LoWPAN_IPHC . If some bits of the first address in the RH3-6LoRH can be derived from the final destination is in the LoWPAN_IPHC, then that address may be compressed, otherwise is is expressed in full. Next addresses only need to express the delta from the previous address. All addresses in a RH3-6LoRH are compressed in a same fashion, down to the same number of bytes per address. In order to get different forms of compression, multiple consecutive RH3-6LoRH must be used.

, Section 11.2, specifies the RPL Packet Information (RPI) as a set of fields that are to be added to the IP packets for the purpose of Instance Identification, as well as Loop Avoidance and Detection. In particular, the SenderRank, which is the scalar metric computed by an specialized Objective Function such as , indicates the Rank of the sender and is modified at each hop. The SenderRank allows to validate that the packet progresses in the expected direction, either upwards or downwards, along the DODAG. RPL defines the RPL Option for Carrying RPL Information in Data-Plane Datagrams to transport the RPI, which is carried in an IPv6 Hop-by-Hop Options Header, typically consuming eight bytes per packet. With , the RPL option is encoded as six Octets; it must be placed in a Hop-by-Hop header that consumes two additional octets for a total of eight. 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. The 8-bytes overhead is detrimental to the LLN operation, in particular with regards to bandwidth and battery constraints. These bytes may cause a containing frame to grow above maximum frame size, leading to Layer 2 or 6LoWPAN fragmentation, which in turn cause even more energy spending and issues discussed in the LLN Fragment Forwarding and Recovery. An additional overhead comes from the need, in certain cases, to add an IP-in-IP encapsulation to carry the Hop-by-Hop header. This is needed when the router that inserts the Hop-by-Hop header is not the source of the packet, so that an error can be returned to the router. This is also the case when a packet originated by a RPL node must be stripped from the Hop-by-Hop header to be routed outside the RPL domain. This specification defines an IPinIP-6LoRH in for that purpose, but it must be noted that stripping a 6LoRH does not require a manipulation of the packet in the LOWPAN_IPHC, and thus, if the source address in the LOWPAN_IPHC is the node that inserted the IPinIP-6LoRH then this alone does not mandate an IPinIP-6LoRH. As a result, a RPL packet may bear only a RPI-6LoRH and no IPinIP-6LoRH. In that case, the source and destination of the packet are located in the LOWPAN_IPHC. As with , the fields in the RPI include an ‘O’, an ‘R’, and an ‘F’ bit, an 8-bit RPLInstanceID (with some internal structure), and a 16-bit SenderRank. The remainder of this section defines the RPI-6LoRH, a Critical 6LoWPAN Routing Header that is designed to transport the RPI in 6LoWPAN LLNs.

RPL Instances are discussed in , Section 5. A number of simple use cases will not require more than one instance, and in such a case, the instance is expected to be the global Instance 0. A global RPLInstanceID is encoded in a RPLInstanceID field as follows:

For the particular case of the global Instance 0, the RPLInstanceID field is all zeros. This specification allows to elide a RPLInstanceID field that is all zeros, and defines a I flag that, when set, signals that the field is elided.

The SenderRank is the result of the DAGRank operation on the rank of the sender; here 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, the least significant 8 bits of the SenderRank will be all zeroes; by eliding those, the SenderRank can be compressed into a single byte. This idea is used in by defining DEFAULT_MIN_HOP_RANK_INCREASE as 256 and in that defaults MinHopRankIncrease to DEFAULT_MIN_HOP_RANK_INCREASE. This specification allows to encode the SenderRank as either one or two bytes, and defines a K flag that, when set, signals that a single byte is used.

The RPI-6LoRH provides a compressed form for the RPL RPI. Routers that need to forward a packet with a RPI-6LoRH are expected to be RPL routers and expected to support this specification. If a non-RPL router receives a packet with a RPI-6LoRH, this means that there was a routing error and the packet should be dropped so the Type cannot be ignored. Since the I flag is not set, the TSE field does not need to be a length expressed in bytes. The field is fully reused for control bits so as to encode the O, R and F flags from the RPI, and the I and K flags that indicate the compression that is taking place. The Type for the RPI-6LoRH is 5. The RPI-6LoRH is immediately followed by the RPLInstanceID field, unless that field is fully elided, and then the SenderRank, which is either compressed into one byte or fully in-lined as the whole 2 bytes. The I and K flags in the RPI-6LoRH indicate whether the RPLInstanceID is elided and/or the SenderRank is compressed and depending on these bits, the Length of the RPI-6LoRH may vary as described hereafter.

The O, R, and F bits as defined in , Section 11.2. If it is set, the Instance ID is elided and the RPLInstanceID is the Global RPLInstanceID 0. If it is not set, the octet immediately following the type field contains the RPLInstanceID as specified in section 5.1. If it is set, the SenderRank is be compressed into one octet, and the lowest significant octet is elided. If it is not set, the SenderRank, is fully inlined as 2 octets. In , the RPLInstanceID is the Global RPLInstanceID 0, and the MinHopRankIncrease is a multiple of 256 so the least significant byte is all zeros and can be elided:

In , the RPLInstanceID is the Global RPLInstanceID 0, but both bytes of the SenderRank are significant so it can not be compressed:

In , the RPLInstanceID is not the Global RPLInstanceID 0, and the MinHopRankIncrease is a multiple of 256:

In , the RPLInstanceID is not the Global RPLInstanceID 0, and both bytes of the SenderRank are significant:

A typical packet in RPL non-storing mode going down the RPL graph requires an IPinIP encapsulating the RH3, whereas the RPI is usually omitted, unless it is important to indicate the RPLInstanceID. To match this structure, an optimized IPinIP 6LoRH is defined in . And the types include the setting of I and K as follows:

The IP-in-IP 6LoRH (IPinIP-6LoRH) is an Elective 6LoWPAN Routing Header that provides a compressed form for the encapsulating IPv6 Header in the case of an IP-in-IP encapsulation. An IPinIP encapsulation is used to insert a field such as a Routing Header or an RPI at a router that is not the source of the packet. In order to send an error back regarding the inserted field, the address of the router that performs the insertion must be provided. The encapsulation can also enable a router down the path removing a field such as the RPI, but this can be done in the compressed form by removing the RPI-6LoRH, so an IPinIP-6LoRH encapsulation is not required for that sole purpose. This field is not critical for routing so the Type can be ignored, and the TSE field contains the Length in bytes.

The Length of an IPinIP-6LoRH is expressed in bytes and MUST be at least 1, to indicate a Hop Limit (HL), that is decremented at each hop. When the HL reaches 0, the packet is dropped per If the Length of an IPinIP-6LoRH is exactly 1, then the Encapsulator Address is elided, which means that the Encapsulator is a well-known router, for instance the root in a RPL graph. If the Length of an IPinIP-6LoRH is strictly more than 1, then an Encapsulator Address is placed in a compressed form after the Hop Limit field. The value of the Length indicates which compression is performed on the Encapsulator Address. For instance, a Size of 3 indicates that the Encapsulator Address is compressed to 2 bytes. When it cannot be elided, the destination IP address of the IP-in-IP header is transported in a RH3-6LoRH as the first address of the list. With RPL, the destination address in the IP-in-IP header is implicitly the root in the RPL graph for packets going upwards, and the destination address in the IPHC for packets going downwards. If the implicit value is correct, the destination IP address of the IP-in-IP encapsulation can be elided. If the final destination of the packet is a leaf that does not support this specification, then the chain of 6LoRH must be stripped by the RPL/6LR router to which the leaf is attached. In that example, the destination IP address of the IP-in-IP header cannot be elided. In the special case where the 6LoRH is used to route 6LoWPAN fragments, the destination address is not accessible in the IPHC on all fragments and can be elided only for the first fragment and for packets going upwards.

The security considerations of , , and apply. Using a compressed format as opposed to the full in-line format is logically equivalent and does not create an opening for a new threat when compared to , and .

This document creates a IANA registry for the 6LoWPAN Routing Header Type, and assigns the following values: 0..4 : RH3-6LoRH [RFCthis] 5 : RPI-6LoRH [RFCthis] 6 : IPinIP-6LoRH [RFCthis]

The authors wish to thank Martin Turon, James Woodyatt, Samita Chakrabarti, Jonathan Hui, Gabriel Montenegro and Ralph Droms for constructive reviews to the design in the 6lo Working Group. The overall discussion involved participants to the 6MAN, 6TiSCH and ROLL WGs, thank you all. Special thanks to the chairs of the ROLL WG, Michael Richardson and Ines Robles, and Brian Haberman, Internet Area A-D, and Adrian Farrel, Routing Area A-D, for driving this complex effort across Working Groups and Areas.