When to use RFC 6553, 6554 and IPv6-in-IPv6

RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks) is a routing protocol for constrained networks. RFC 6553 defines the "RPL option" (RPI), carried within the IPv6 Hop-by-Hop header to quickly identify inconsistencies (loops) in the routing topology. RFC 6554 defines the "RPL Source Route Header" (RH3), an IPv6 Extension Header to deliver datagrams within a RPL routing domain, particularly in non-storing mode. These various items are referred to as RPL artifacts, and they are seen on all of the data-plane traffic that occurs in RPL routed networks; they do not in general appear on the RPL control plane traffic at all which is mostly hop-by-hop traffic (one exception being DAO messages in non-storing mode). It has become clear from attempts to do multi-vendor interoperability, and from a desire to compress as many of the above artifacts as possible that not all implementors agree when artifacts are necessary, or when they can be safely omitted, or removed. An interim meeting went through the 24 cases defined here to discover if there were any shortcuts, and this document is the result of that discussion. This document should not be defining anything new, but it may clarify what is correct and incorrect behaviour. The related document A Routing Header Dispatch for 6LoWPAN (6LoRH) defines a method to compress RPL Option information and Routing Header type 3 , an efficient IP-in-IP technique, and use cases proposed for the involving 6loRH.

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 RFC 2119. Terminology defined in applies to this document: LBR, LLN, RPL, RPL Domain and ROLL. RPL-node: It is device which implements RPL, thus we can say that the device is RPL-capable or RPL-aware. Please note that the device can be found inside the LLN or outside LLN. In this document a RPL-node which is a leaf is called RPL-aware-leaf. RPL-not-capable: It is device which do not implement RPL, thus we can say that the device is not-RPL-aware. Please note that the device can be found inside the LLN. In this document a not-RPL-node which is a leaf is called not-RPL-aware-leaf.

The term "hop-by-hop IPv6-in-IPv6" header refers to: adding a header that originates from a node to an adjacent node, using the addresses (usually the GUA or ULA, but could use the link-local addresses) of each node. If the packet must traverse multiple hops, then it must be decapsulated at each hop, and then re-encapsulated again in a similar fashion.

A RPL network is composed of a 6LBR (6LoWPAN Border Router), Backbone Router (6BBR), 6LR (6LoWPAN Router) and 6LN (6LoWPAN Node) as leaf logically organized in a DODAG structure (Destination Oriented Directed Acyclic Graph). RPL defines the RPL Control messages (control plane), a new ICMPv6 message with Type 155. DIS (DODAG Information Solicitation), DIO (DODAG Information Object) and DAO (Destination Advertisement Object) messages are all RPL Control messages but with different Code values. A RPL Stack is showed in Figure 1. RPL supports two modes of Downward traffic: in storing mode (RPL-SM), it is fully stateful or an in non-storing (RPL-NSM), it is fully source routed. A RPL Instance is either fully storing or fully non-storing, i.e. a RPL Instance with a combination of storing and non-storing nodes is not supported with the current specifications at the time of writing this document.

In Figure 2 is showed the reference RPL Topology for this document. The numbers in or above the nodes are there so that they may be referenced in subsequent sections. In the figure, a 6LN can be a router or a host. The 6LN leafs marked as (21) is a RPL host that does not have forwarding capability and (25) is a RPL router. The leaf marked 6LN (24) is a device which does not speak RPL at all (not-RPL-aware), but uses Router-Advertisements, 6LowPAN DAR/DAC and efficient-ND only to participate in the network . In the document this leaf (24) is often named IPv6 node. The 6LBR in the figure is the root of the Global DODAG. This document is in part motivated by the work that is ongoing at the 6TiSCH working group. The 6TiSCH architecture draft explains the network architecture of a 6TiSCH network.

In data plane context a combination of RFC6553, RFC6554 and IPv6-in-IPv6 encapsulation is going to be analyzed for the following traffic flows. This version of the document assumes the changes in are passed (at the time to write this specification, the draft is on version 05). RPL-aware-leaf to root root to RPL-aware-leaf not-RPL-aware-leaf to root root to not-RPL-aware-leaf RPL-aware-leaf to Internet Internet to RPL-aware-leaf not-RPL-aware-leaf to Internet Internet to not-RPL-aware-leaf RPL-aware-leaf to RPL-aware-leaf (storing and non-storing) RPL-aware-leaf to not-RPL-aware-leaf (non-storing) not-RPL-aware-leaf to RPL-aware-leaf (storing and non-storing) not-RPL-aware-leaf to not-RPL-aware-leaf (non-storing) This document assumes the rule that a Header cannot be inserted or removed on the fly inside an IPv6 packet that is being routed. This is a fundamental precept of the IPv6 architecture as outlined in . Extensions may not be added or removed except by the sender or the receiver. But, options in the Hop-by-Hop option which are marked with option type 01 ( section 4.2 and ) SHOULD be ignored when received by a host or router which does not understand that option. This means that in general, any packet that leaves the RPL domain of an LLN (or leaves the LLN entirely) will NOT be discarded, when it has the RPL Option Header known as the RPI or SRH3 Extension Header (S)RH3. The recent change to the second of these rules it means that the RPI Hop-by-Hop option MAY be left in place even if the end host does not understand it. NOTE: There is some possible security risk when the RPI information is released to the Internet. At this point this is a theoretical situation. It is clear that the RPI option would waste some network bandwidth when it escapes. An intermediate router that needs to add an extension header (SHR3 or RPI Option) must encapsulate the packet in an (additional) outer IP header. The new header can be placed is placed after this new outer IP header. A corollory is that an SHR3 or RPI Option can only be removed by an intermediate router if it is placed in an encapsulating IPv6 Header, which is addressed to the intermediate router. When it does so, the whole encapsulating header must be removed. (A replacement may be added). This sometimes can result in outer IP headers being addressed to the next hop router using link-local addresses. Both RPI and RH3 headers may be modified in very specific ways by routers on the path of the packet without the need to add to remove an encapsulating header. Both headers were designed with this modification in mind, and both the RPL RH and the RPL option are marked mutable but recoverable: so an IPsec AH security header can be applied across these headers, but it can not secure the values which mutate. RPI should be present in every single RPL data packet. There is one exception in non-storing mode: when a packet is going down from the root. In a downward non-storing mode, the entire route is written, so there can be no loops by construction, nor any confusion about which forwarding table to use (as the root has already made all routing decisions). There still may be cases (such as in 6tisch) where the instanceID portion of the RPI header may still be needed to pick an appropriate priority or channel at each hop. In the tables present in this document, the term "RPL aware leaf" is has been shortened to "Raf", and "not-RPL aware leaf" has been shortened to "~Raf" to make the table fit in available space. The earlier examples are more extensive to make sure that the process is clear, while later examples are more consise.

In storing mode (fully stateful), the sender cannot determine whether the destination is RPL-capable and thus would need an IP-in-IP header. The IP-in-IP header needs to be addressed on a hop-by-hop basis so that the last 6LR can remove the RPI header. Additionally, The sender can determine if the destination is inside the LLN by looking if the destination address is matched by the DIO's PIO option. The following table summarizes what headers are needed in the following scenarios, and indicates when the IP-in-IP header must be inserted on a hop-by-hop basis, and when it can target the destination node directly. There are three possible situations: hop-by-hop necessary (indicated by "hop"), or destination address possible (indicated by "dst"). In all cases hop by hop can be used. In cases where no IP-in-IP header is needed, the column is left blank. The leaf can be a router 6LR or a host, both indicated as 6LN. Use Case RPI RH3 IP-in-IP IP-in-IP dst Raf to root Yes No No -- root to Raf Yes No No -- root to ~Raf Yes No No -- ~Raf to root Yes No Yes root Raf to Int Yes No No -- Int to Raf Yes No Yes raf ~Raf to Int Yes No Yes root Int to ~Raf Yes No Yes hop Raf to Raf Yes No No -- Raf to ~Raf Yes No No -- ~Raf to Raf Yes No Yes dst ~Raf to ~Raf Yes No Yes hop

In storing mode, RFC 6553 (RPI) is used to send RPL Information instanceID and rank information. As stated in Section 16.2 of a RPL-aware-leaf node does not generally issue DIO messages; a leaf node accepts DIO messages from upstream. (When the inconsistency in routing occurs, a leaf node will generate a DIO with an infinite rank, to fix it). It may issue DAO and DIS messages though it generally ignores DAO and DIS messages. In this case the flow comprises: RPL-aware-leaf (6LN) --> 6LR1,... --> 6LRN --> root (6LBR) As it was mentioned In this document 6LRs, 6LBR are always full-fledge RPL routers. The 6LN inserts the RPI header, and sends the packet to 6LR which decrements the rank in RPI and sends the packet up. When the packet arrives at 6LBR, the RPI is removed and the packet is processed. No IP-in-IP header is required. The RPI header can be removed by the 6LBR because the packet is addressed to the 6LBR. The 6LN must know that it is communicating with the 6LBR to make use of this scenario. The 6LN can know the address of the 6LBR because it knows the address of the root via the DODAGID in the DIO messages. Header 6LN 6LR 6LBR Inserted headers RPI -- -- Removed headers -- -- RPI Re-added headers -- -- -- Modified headers -- RPI -- Untouched headers -- -- --

In this case the flow comprises: root (6LBR) --> 6LR1,... --> 6LRN --> RPL-aware-leaf (6LN) In this case the 6LBR inserts RPI header and sends the packet down, the 6LR is going to increment the rank in RPI (examines instanceID for multiple tables), the packet is processed in 6LN and RPI removed. No IP-in-IP header is required. Header 6LBR 6LR 6LN Inserted headers RPI -- -- Removed headers -- -- RPI Re-added headers -- -- -- Modified headers -- RPI -- Untouched headers -- -- --

In this case the flow comprises: root (6LBR) --> 6LR1,... --> 6LRN --> not-RPL-aware-leaf (IPv6) As the RPI extension can be ignored by the not-RPL-aware leaf, this situation is identical to the previous scenario. Header 6LBR 6LR(1..N) 6LN Inserted headers RPI -- -- Removed headers -- -- -- Re-added headers -- -- -- Modified headers -- RPI -- Untouched headers -- -- RPI (Ignored)

In this case the flow comprises: not-RPL-aware-leaf (IPv6) --> 6LR1,... --> 6LRN --> root (6LBR) When the packet arrives from IPv6 node to 6LR, the 6LR1 will insert an RPI header, encapsuladed in a IPv6-in-IPv6 header. The IPv6-in-IPv6 header can be addressed to the next hop, or to the root. The root removes the header and processes the packet. Header IPv6 6LR1 6LRN 6LBR Inserted headers -- IP-in-IP(RPI) -- -- Removed headers -- -- -- IP-in-IP(RPI) Re-added headers -- -- -- -- Modified headers -- -- IP-in-IP(RPI) -- Untouched headers -- -- -- --

RPL information from RFC 6553 MAY go out to Internet as it will be ignored by nodes which have not been configured to be RPI aware. In this case the flow comprises: RPL-aware-leaf (6LN) --> 6LR1,... --> 6LRN --> root (6LBR) --> Internet No IP-in-IP header is required. Header 6LN 6LR(1..N) 6LBR Internet Inserted headers RPI -- -- -- Removed headers -- -- -- -- Re-added headers -- -- -- -- Modified headers -- RPI -- -- Untouched headers -- -- -- RPI (Ignored)

In this case the flow comprises: Internet --> root (6LBR) --> 6LR1,... --> 6LRN --> RPL-aware-leaf (6LN) When the packet arrives from Internet to 6LBR the RPI header is added in a outer IPv6-in-IPv6 header and sent to 6LR, which modifies the rank in the RPI. When the packet arrives at 6LN the RPI header is removed and the packet processed. Header Internet 6LBR 6LR(1...N) 6LN Inserted headers -- IP-in-IP(RPI) -- -- Removed headers -- -- -- IP-in-IP(RPI) Re-added headers -- -- -- -- Modified headers -- -- IP-in-IP(RPI) -- Untouched headers -- -- -- --

In this case the flow comprises: not-RPL-aware-leaf (IPv6) --> 6LR1,... --> 6LRN --> root (6LBR) --> Internet The 6LR1 node will add an IP-in-IP(RPI) header addressed either to the root, or hop-by-hop such that the root can remove the RPI header before passing upwards. The originating node will ideally leave the IPv6 flow label as zero so that it can be better compressed through the LLN, and the 6LBR will set the flow label to a non-zero value when sending to the Internet. Header IPv6 6LR1 6LBN 6LBR Internet Inserted headers -- IP-in-IP(RPI) -- -- -- Removed headers -- -- -- IP-in-IP(RPI) -- Re-added headers -- -- -- -- -- Modified headers -- -- IP-in-IP(RPI) -- -- Untouched headers -- -- -- -- --

In this case the flow comprises: Internet --> root (6LBR) --> 6LR1,... --> 6LRN --> not-RPL-aware-leaf (IPv6) The 6LBR will have to add an RPI header within an IP-in-IP header. The IP-in-IP can be addressed to the not-RPL-aware-leaf, leaving the RPI inside. The 6LBR MAY set the flow label on the inner IP-in-IP header to zero in order to aid in compression, as the packet will not emerge again from the LLN. Header Internet 6LBR 6LR(1...N) IPv6 Inserted headers -- IP-in-IP(RPI) -- -- Removed headers -- -- IP-in-IP(RPI) -- Re-added headers -- -- -- -- Modified headers -- -- IP-in-IP(RPI) -- Untouched headers -- -- -- RPI (Ignored)

In RPL allows a simple one-hop optimization for both storing and non-storing networks. A node may send a packet destined to a one-hop neighbor directly to that node. Section 9 in . In this case the flow comprises: 6LN --> 6LR1 --> common parent (6LRx) --> 6LRN --> 6LN This case is assumed in the same RPL Domain. In the common parent, the direction of RPI is changed (from increasing to decreasing the rank). While the 6LR nodes will update the RPI, no node needs to add or remove the RPI, so no IP-in-IP headers are necessary. This may be done regardless of where the destination is, as the included RPI will be ignored by the receiver. Header 6LN src 6LR1 6LRx (common parent) 6LRN 6LN dst Inserted headers RPI -- -- -- -- Removed headers -- -- -- -- RPI Re-added headers -- -- -- -- -- Modified headers -- RPI (decreasing rank) RPI (increasing rank) -- -- Untouched headers -- -- -- -- --

In this case the flow comprises: 6LN --> 6LR1 --> common parent (6LRx) --> 6LRN --> not-RPL-aware 6LN (IPv6) This situation is identical to the previous situation Header 6LN src 6LR1 6LRx (common parent) 6LRN IPv6 Inserted headers RPI -- -- -- -- Removed headers -- -- -- -- RPI Re-added headers -- -- -- -- -- Modified headers -- RPI (decreasing rank) RPI (increasing rank) -- -- Untouched headers -- -- -- -- RPI(Ignored)

In this case the flow comprises: not-RPL-aware 6LN (IPv6) --> 6LR1 --> common parent (6LRx) --> 6LRN --> 6LN The 6LR1 receives the packet from the the IPv6 node and inserts and the RPI header encapsulated in IPv6-in-IPv6 header. The IP-in-IP header is addressed to the destination 6LN. Header IPv6 6LR1 common parent (6LRx) 6LRn 6LN Inserted headers -- IP-in-IP(RPI) -- -- -- Removed headers -- -- -- -- IP-in-IP(RPI) Re-added headers -- -- -- -- -- Modified headers -- -- IP-in-IP(RPI) IP-in-IP(RPI) -- Untouched headers -- -- -- -- --

In this case the flow comprises: not-RPL-aware 6LN (IPv6 src)--> 6LR1 --> 6LR2 --> root (6LBR) --> 6LRn --> not-RPL-aware 6LN (IPv6 dst) This flow is identical to The 6LR receives the packet from the the IPv6 node and inserts the RPI header (RPIa) encapsulated in IPv6-in-IPv6 header. The IPv6-in-IPv6 header is addressed to the 6LBR. The 6LBR remove the IPv6-in-IPv6 header and insert another one (RPIb) with destination to 6LRn node. Header IPv6 src 6LR1 6LR2 6LBR 6LRn IPv6 dst Inserted headers -- IP-in-IP(RPIa) -- IP-in-IP(RPIb) -- -- Removed headers -- -- -- -- -- -- Re-added headers -- -- -- -- IP-in-IP(RPIb) -- Modified headers -- -- IP-in-IP(RPIa) -- IP-in-IP(RPIb) -- Untouched headers -- -- -- -- -- --

Use Case RPI RH3 IP-in-IP IP-in-IP dst Raf to root Yes No No -- root to Raf Opt Yes No -- root to ~Raf No Yes Yes 6LR ~Raf to root Yes No Yes root Raf to Int Yes No Yes root Int to Raf Opt Yes Yes dst ~Raf to Int Yes No Yes root Int to ~Raf Opt Yes Yes 6LR Raf to Raf Yes Yes Yes root/dst Raf to ~Raf Yes Yes Yes root/6LR ~Raf to Raf Yes Yes Yes root/6LN ~Raf to ~Raf Yes Yes Yes root/6LR

In non-storing mode the leaf node uses default routing to send traffic to the root. The RPI header must be included to avoid/detect loops. RPL-aware-leaf (6LN) --> 6LR --> root (6LBR) This situation is the same case as storing mode. Header 6LN 6LR 6LBR Inserted headers RPI -- -- Removed headers -- -- RPI Re-added headers -- -- -- Modified headers -- RPI -- Untouched headers -- -- --

In this case the flow comprises: root (6LBR)--> 6LR --> RPL-aware-leaf (6LN) The 6LBR will insert an RH3, and may optionally insert an RPI header. No IP-in-IP header is necessary as the traffic originates with an RPL aware node, the 6LBR. The destination is known to RPL-aware because, the root knows the whole topology in non-storing mode. Header 6LBR 6LR 6LN Inserted headers (opt: RPI), RH3 -- -- Removed headers -- -- RH3,RPI Re-added headers -- -- -- Modified headers -- RH3 -- Untouched headers -- -- --

In this case the flow comprises: root (6LBR)--> 6LR1...-->6LRn --> not-RPL-aware-leaf (IPv6) In 6LBR the RH3 is added, modified in each intermediate 6LR (6LR1 and so on) and it is fully consumed in the last 6LR (6LRn), but left there. If RPI is left present, the IPv6 node which does not understand it will ignore it (following 2460bis), thus encapsulation is not necesary. Due the complete knowledge of the topology at the root, the 6LBR is able to address the IP-in-IP header to the last 6LR. Header 6LBR 6LR1 6LRn IPv6 Inserted headers (opt: RPI), RH3 -- -- -- Removed headers -- RH3 -- -- Re-added headers -- -- -- -- Modified headers -- (opt: RPI), RH3 (opt: RPI), RH3 -- Untouched headers -- -- -- RPI

In this case the flow comprises: IPv6-node --> 6LR1 ...--> 6LRn --> root (6LBR) In this case the RPI is added by the first 6LR (6LR1), encapsulated in an IP-in-IP header, and is modified in the followings 6LRs. The RPI and entire packet is consumed by the root. Header IPv6 6LR1 6LR2 6LBR Inserted headers -- IP-in-IP(RPI) -- -- Removed headers -- -- -- IP-in-IP(RPI) Re-added headers -- -- -- -- Modified headers -- IP-in-IP(RPI) IP-in-IP(RPI) -- Untouched headers -- -- -- --

In this case the flow comprises: RPL-aware-leaf (6LN) --> 6LR1 ...--> 6LRn --> root (6LBR) --> Internet This case is identical to storing-mode case. The IPv6 flow label should be set to zero to aid in compression, and the 6LBR will set it to a non-zero value when sending towards the Internet. Header 6LN 6LR(1..N) 6LBR Internet Inserted headers RPI -- -- -- Removed headers -- -- -- -- Re-added headers -- -- -- -- Modified headers -- RPI -- -- Untouched headers -- -- -- RPI (Ignored)

In this case the flow comprises: Internet --> root (6LBR) --> 6LR1...--> 6LRn --> RPL-aware-leaf (6LN) The 6LBR must add an RH3 header. As the 6LBR will know the path and address of the target node, it can address the IP-in-IP header to that node. The 6LBR will zero the flow label upon entry in order to aid compression. The RPI may be added or not, it is optional. Header Internet 6LBR 6LR 6LN Inserted headers -- IP-in-IP(RH3,opt:RPI) -- -- Removed headers -- -- -- IP-in-IP(RH3,opt:RPI) Re-added headers -- -- -- -- Modified headers -- -- IP-in-IP(RH3,opt:RPI) -- Untouched headers -- -- -- --

In this case the flow comprises: not-RPL-aware-leaf (IPv6) --> 6LR1..--> 6LRn --> root (6LBR) --> Internet In this case the flow label is recommended to be zero in the IPv6 node. As RPL headers are added in the IPv6 node, the first 6LN will add an RPI header inside a new IP-in-IP header. The IP-in-IP header will be addressed to the root. This case is identical to the storing-mode case (Section 5.7). Header IPv6 6LR1 6LRn 6LBR Internet Inserted headers -- IP-in-IP(RPI) -- -- -- Removed headers -- -- -- IP-in-IP(RPI) -- Re-added headers -- -- -- -- -- Modified headers -- -- IP-in-IP(RPI) -- -- Untouched headers -- -- -- -- --

In this case the flow comprises: Internet --> root (6LBR) --> 6LR1...--> 6LRn --> not-RPL-aware-leaf (IPv6) The 6LBR must add an RH3 header inside an IP-in-IP header. The 6LBR will know the path, and will recognize that the final node is not an RPL capable node as it will have received the connectivity DAO from the nearest 6LR. The 6LBR can therefore make the IP-in-IP header destination be the last 6LR. The 6LBR will set to zero the flow label upon entry in order to aid compression. Header Internet 6LBR 6LR1 6LRn IPv6 Inserted headers -- IP-in-IP(RH3,opt:RPI) -- -- -- Removed headers -- -- -- IP-in-IP(RH3, RPI) -- Re-added headers -- -- -- -- -- Modified headers -- -- IP-in-IP(RH3, RPI) IP-in-IP(RH3, RPI) -- Untouched headers -- -- -- -- RPI

In this case the flow comprises: 6LN --> 6LR1 --> root (6LBR) --> 6LRN --> 6LN This case involves only nodes in same RPL Domain. The originating node will add an RPI header to the original packet, and send the packet upwards. The originating node SHOULD put the RPI into an IP-in-IP header addressed to the root, so that the 6LBR can remove that header. If it does not, then additional resources are wasted on the way down to carry the useless RPI option. The 6LBR will need to insert an RH3 header, which requires that it add an IP-in-IP header. It SHOULD be able to remove the RPI, as it was contained in an IP-in-IP header addressed to it. Otherwise, there MAY be an RPI header buried inside the inner IP header, which should get ignored. Networks that use the RPL P2P extension are essentially non-storing DODAGs and fall into this scenario or scenario , with the originating node acting as 6LBR. Header 6LN src 6LR1 6LBR 6LRN 6LN dst Inserted headers IP-in-IP(RPI1) -- IP-in-IP(RH3 to 6LN, opt RPI2) -- -- Removed headers -- -- IP-in-IP(RPI1) -- IP-in-IP(RH3, opt RPI2) Re-added headers -- -- -- -- -- Modified headers -- -- -- -- -- Untouched headers -- -- -- -- --

In this case the flow comprises: 6LN --> 6LR1 --> root (6LBR) --> 6LRn --> not-RPL-aware (IPv6) As in the previous case, the 6LN will insert an RPI (RPI1) header which MUST be in an IP-in-IP header addressed to the root so that the 6LBR can remove this RPI. The 6LBR will then insert an RH3 inside a new IP-in-IP header addressed to the 6LN destination node. The RPI is optional from 6LBR to 6LRn (RPI2). Header 6LN 6LR1 6LBR 6LRn IPv6 Inserted headers IP-in-IP(RPI1) -- IP-in-IP(RH3, opt RPI2) -- -- Removed headers -- -- IP-in-IP(RPI1) IP-in-IP(RH3, opt RPI2) -- Re-added headers -- -- -- -- -- Modified headers -- IP-in-IP(RPI1) -- IP-in-IP(RH3, opt RPI2) -- Untouched headers -- -- -- -- opt RPI2

In this case the flow comprises: not-RPL-aware 6LN (IPv6) --> 6LR1 --> root (6LBR) --> 6LRn --> 6LN This scenario is mostly identical to the previous one. The RPI is added by the first 6LR (6LR1) inside an IP-in-IP header addressed to the root. The 6LBR will remove this RPI, and add it's own IP-in-IP header containing an RH3 header and optional RPI (RPI2). Header IPv6 6LR1 6LBR 6LRn 6LN Inserted headers -- IP-in-IP(RPI1) IP-in-IP(RH3, opt RPI2) -- -- Removed headers -- -- IP-in-IP(RPI1) -- IP-in-IP(RH3, opt RPI2) Re-added headers -- -- -- -- -- Modified headers -- -- -- IP-in-IP(RH3, opt RPI2) -- Untouched headers -- -- -- -- --

In this case the flow comprises: not-RPL-aware 6LN (IPv6 src)--> 6LR1 --> root (6LBR) --> 6LRn --> not-RPL-aware (IPv6 dst) This scenario is the combination of the previous two cases. Header IPv6 src 6LR1 6LBR 6LRn IPv6 dst Inserted headers -- IP-in-IP(RPI1) IP-in-IP(RH3) -- -- Removed headers -- -- IP-in-IP(RPI1) IP-in-IP(RH3, opt RPI2) -- Re-added headers -- -- -- -- -- Modified headers -- -- -- -- -- Untouched headers -- -- -- -- --

shows that this hop-by-hop IP-in-IP header can be compressed down to {TBD} bytes. There are potential significant advantages to having a single code path that always processes IP-in-IP headers with no options. Thanks to the relaxation of the RFC2406 rule about discarding unknown Hop-by-Hop options, there is no longer any uncertainty about when to use an IPIP header in the storing mode case. The RPI header SHOULD always be added when 6LRs originate packets (without IPIP headers), and IPIP headers should always be added (addressed to the root when on the way up, to the end-host when on the way down) when a 6LR finds it needs to insert an RPI header. (XXX - this is a problem for storing mode optimization) In order to support the above two cases with full generality, the different situations (always do IP-in-IP vs never use IP-in-IP) should be signaled in the RPL protocol itself.

In the non-storing case, dealing with non-RPL aware leaf nodes is much easier as the 6LBR (DODAG root) has complete knowledge about the connectivity of all DODAG nodes, and all traffic flows through the root node. The 6LBR can recognize non-RPL aware leaf nodes because it will receive a DAO about that node from the 6LN immediately above that node. This means that the non-storing mode case can avoid ever using hop-by-hop IP-in-IP headers. shows how the destination=root, and destination=6LN IP-in-IP header can be compressed down to {TBD} bytes. Unlike in the storing mode case, there is no need for all nodes to know about the existence of non-RPL aware nodes. Only the 6LBR needs to change when there are non-RPL aware nodes. Further, in the non-storing case, the 6LBR is informed by the DAOs when there are non-RPL aware nodes.

The proposes a compression method for RPI, RH3 and IPv6-in-IPv6. In Storing Mode, for the examples of Flow from RPL-aware-leaf to non-RPL-aware-leaf and non-RPL-aware-leaf to non-RPL-aware-leaf comprise an IP-in-IP and RPI compression headers. The type of this case is critical since IP-in-IP is encapsulating a RPI header.

There are no IANA considerations related to this document.

The security considerations covering of and apply when the packets get into RPL Domain.

This work is partially funded by the FP7 Marie Curie Initial Training Network (ITN) METRICS project (grant agreement No. 607728). The authors would like to acknowledge the review, feedback, and comments of Robert Cragie, Simon Duquennoy, Cenk Gündogan, Peter van der Stok, Xavier Vilajosana and Thomas Watteyne.