6. The RPL Packet Information 6LoRH (RPI-6LoRH) Section 11.2 of the RPL document [RFC6550] specifies the RPL Packet Information (RPI) as a set of fields that are placed by RPL routers in IP packets to identify the RPL Instance, detect anomalies, and trigger corrective actions. In particular, the SenderRank, which is the scalar metric computed by a specialized Objective Function such as described in RFC 6552 [RFC6552], indicates the Rank of the sender and is modified at each hop. The SenderRank field is used to validate that the packet progresses in the expected direction, either upwards or downwards, along the DODAG.
RPL defines the "The Routing Protocol for Low-Power and Lossy Networks (RPL) Option for Carrying RPL Information in Data-Plane Datagrams" [RFC6553] to transport the RPI, which is carried in an IPv6 Hop-by-Hop Options Header [RFC2460], typically consuming 8 bytes per packet. With RFC 6553 [RFC6553], the RPL Option is encoded as 6 octets, which must be placed in a Hop-by-Hop header that consumes two additional octets for a total of 8 octets. To limit the header's range to just 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-byte overhead is detrimental to LLN operation, particularly with regard to bandwidth and battery constraints. These bytes may cause a containing frame to grow above maximum frame size, leading to Layer 2 or 6LoWPAN [RFC4944] fragmentation, which in turn leads to even more energy expenditure and issues discussed in "LLN Fragment Forwarding and Recovery" [FORWARD-FRAG]. 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. For that reason, this specification defines an IP-in-IP-6LoRH header in Section 7, but it must be noted that removal of a 6LoRH header does not require manipulation of the packet in the LOWPAN_IPHC, and thus, if the source address in the LOWPAN_IPHC is the node that inserted the IP-in-IP-6LoRH header, then this situation alone does not mandate an IP-in-IP-6LoRH header. Note: It was found that some implementations omit the RPI for packets going down the RPL graph in Non-Storing mode, even though RPL indicates that the RPI should be placed in the packet. With this specification, the RPI is important to indicate the RPLInstanceID, so the RPI should not be omitted. As a result, a RPL packet may bear only an RPI-6LoRH header and no IP-in-IP-6LoRH header. In that case, the source and destination of the packet are specified by the LOWPAN_IPHC. As with RFC 6553 [RFC6553], 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 header, which is a Critical 6LoWPAN Routing Header that is designed to transport the RPI in 6LoWPAN LLNs. 6.1. Compressing the RPLInstanceID RPL Instances are discussed in Section 5 of the RPL specification [RFC6550]. A number of simple use cases do not require more than one RPL Instance, and in such cases, the RPL Instance is expected to be the Global Instance 0. A global RPLInstanceID is encoded in a RPLInstanceID field as follows: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |0| ID | Global RPLInstanceID in 0..127 +-+-+-+-+-+-+-+-+ Figure 8: RPLInstanceID Field Format for Global Instances For the particular case of the Global Instance 0, the RPLInstanceID field is all zeros. This specification allows the compressor to elide a RPLInstanceID field that is all zeros and defines an I flag that, when set, signals that the field is elided. 6.2. Compressing the SenderRank 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 of the RPL specification [RFC6550] as: DAGRank(rank) = floor(rank/MinHopRankIncrease) If MinHopRankIncrease is set to a multiple of 256, the least significant eight 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 RFC 6550 [RFC6550], by defining DEFAULT_MIN_HOP_RANK_INCREASE as 256, and in RFC 6552 [RFC6552], which defaults MinHopRankIncrease to DEFAULT_MIN_HOP_RANK_INCREASE. This specification allows for the SenderRank to be encoded as either 1 or 2 bytes and defines a K flag that, when set, signals that a single byte is used. 6.3. The Overall RPI-6LoRH Encoding The RPI-6LoRH header provides a compressed form for the RPL RPI. Routers that need to forward a packet with a RPI-6LoRH header are expected to be RPL routers that support this specification.
If a non-RPL router receives a packet with a RPI-6LoRH header, there was a routing or a configuration error (see Section 8). The desired reaction for the non-RPL router is to drop the packet as opposed to skip the header and forward the packet, which could end up forming loops by reinjecting the packet in the wrong RPL Instance. The Dispatch Value Bit Pattern for the SRH-6LoRH header indicates it is Critical. Routers that understand the 6LoRH general format detailed in Section 4 cannot ignore a 6LoRH header of this type and will drop the packet if it is unknown to them. Since the RPI-6LoRH header is a Critical header, the TSE field does not need to be a length expressed in bytes. Here, the field is fully reused for control bits that encode the O, R, and F flags from the RPI, as well as the I and K flags that indicate the compression format. The RPI-6LoRH is Type 5. The RPI-6LoRH header 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 2 bytes. The I and K flags in the RPI-6LoRH header indicate whether the RPLInstanceID is elided and/or the SenderRank is compressed. Depending on these bits, the Length of the RPI-6LoRH may vary as described hereafter. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... -+-+-+ |1|0|0|O|R|F|I|K| 6LoRH Type=5 | Compressed fields | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... -+-+-+ Figure 9: The Generic RPI-6LoRH Format O, R, and F bits: The O, R, and F bits are defined in Section 11.2 of RFC 6550 [RFC6550]. I flag: If it is set, the RPLInstanceID 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 of RFC 6550 [RFC6550]. K flag: If it is set, the SenderRank is compressed into 1 octet, with the least significant octet elided. If it is not set, the SenderRank is fully inlined as 2 octets.
In Figure 10, 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: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|O|R|F|1|1| 6LoRH Type=5 | SenderRank | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I=1, K=1 Figure 10: The Most Compressed RPI-6LoRH In Figure 11, the RPLInstanceID is the Global RPLInstanceID 0, but both bytes of the SenderRank are significant so it cannot be compressed: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|O|R|F|1|0| 6LoRH Type=5 | SenderRank | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I=1, K=0 Figure 11: Eliding the RPLInstanceID In Figure 12, the RPLInstanceID is not the Global RPLInstanceID 0, and the MinHopRankIncrease is a multiple of 256: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|O|R|F|0|1| 6LoRH Type=5 | RPLInstanceID | SenderRank | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I=0, K=1 Figure 12: Compressing SenderRank
In Figure 13, the RPLInstanceID is not the Global RPLInstanceID 0, and both bytes of the SenderRank are significant: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|O|R|F|0|0| 6LoRH Type=5 | RPLInstanceID | Sender-... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...-Rank | +-+-+-+-+-+-+-+-+ I=0, K=0 Figure 13: The Least Compressed Form of RPI-6LoRH 7. The IP-in-IP 6LoRH Header The IP-in-IP 6LoRH (IP-in-IP-6LoRH) header 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 IP-in-IP 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 the last router prior to the Destination to remove a field such as the RPI, but this can be done in the compressed form by removing the RPI-6LoRH, so an IP-in-IP- 6LoRH encapsulation is not required for that sole purpose. The Dispatch Value Bit Pattern for the SRH-6LoRH header indicates it is Elective. This field is not Critical for routing since it does not indicate the destination of the packet, which is either encoded in an SRH-6LoRH header or in the inner IP header. A 6LoRH header of this type can be skipped if not understood (per Section 4), and the 6LoRH header indicates the Length in bytes. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+ |1|0|1| Length | 6LoRH Type 6 | Hop Limit | Encaps. Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+ Figure 14: The IP-in-IP-6LoRH
The Length of an IP-in-IP-6LoRH header 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 RFC 2460 [RFC2460]. If the Length of an IP-in-IP-6LoRH header 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. The most efficient compression of an IP-in-IP encapsulation that can be achieved with this specification is obtained when an endpoint of the packet is the root of the RPL DODAG associated to the RPL Instance that is used to forward the packet, and the root address is known implicitly as opposed to signaled explicitly in the data packets. If the Length of an IP-in-IP-6LoRH header is greater 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 Length of 3 indicates that the Encapsulator Address is compressed to 2 bytes. The reference for the compression is the address of the root of the DODAG. The way the address of the root is determined is discussed in Section 4.3.2. With RPL, the destination address in the IP-in-IP header is implicitly the root in the RPL graph for packets going upwards; in Storing mode, it is the destination address in the LOWPAN_IPHC for packets going downwards. In Non-Storing mode, there is no implicit value for packets going downwards. If the implicit value is correct, the destination IP address of the IP-in-IP encapsulation can be elided. Else, the destination IP address of the IP-in-IP header is transported in an SRH-6LoRH header as the first entry of the first of these headers. If the final destination of the packet is a leaf that does not support this specification, then the chain of 6LoRH headers 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 a 6LoRH header is used to route 6LoWPAN fragments, the destination address is not accessible in the LOWPAN_IPHC on all fragments and can be elided only for the first fragment and for packets going upwards.
8. Management Considerations Though it is possible to decompress a packet at any hop, this specification is optimized to enable that a packet is forwarded in its compressed form all the way, and it makes sense to deploy homogeneous networks where all nodes, or no nodes at all, use the compression technique detailed therein. This specification aims at a simple implementation running in constrained nodes, so it does indeed expect a homogeneous network and, as a consequence, it does not provide a method to determine the level of support by the next hops at forwarding time. Should an extension to this specification provide such a method, forwarding nodes could compress or decompress the RPL artifacts appropriately and enable a backward compatibility between nodes that support this specification and nodes that do not. It results that this specification does not attempt to enable such backwards compatibility. It does not require extraneous code to exchange and handle error messages to automatically correct mismatch situations either. When a packet is expected to carry a 6LoRH header but does not, the node that discovers the issue is expected to send an ICMPv6 error message to the root. It should be sent at an adapted rate-limitation and with a type 4 (indicating a "Parameter Problem") and a code 0 (indicating an "Unrecognized Next Header field encountered"). The relevant portion of the received packet should be embedded and the offset therein where the 6LoRH header was expected should be pointed out. When a packet is received with a 6LoRH header that is not recognized, the node that discovers the issue is expected to send an ICMPv6 error message to the root. It should be sent at an adapted rate-limitation and with a type 4 (indicating a "Parameter Problem") and a code 1 (indicating an "Unrecognized Next Header type encountered"). The relevant portion of the received packet should be embedded and the offset therein where the 6LoRH header was expected should be pointed out. In both cases, the node SHOULD NOT place a 6LoRH header as defined in this specification in the resulting message, and the node should either omit the RPI or place it uncompressed after the IPv6 header. Additionally, in both cases, an alternate management method may be preferred in order to notify the network administrator that there is a configuration error.
Keeping the network homogeneous is either a deployment issue, by deploying only devices with a same capability, or a management issue, by configuring all devices to either use or not use a certain level of this compression technique and its future additions. In particular, the situation where a node receives a message with a Critical 6LoWPAN Routing Header that it does not understand is an administrative error whereby the wrong device is placed in a network, or the device is misconfigured. When a mismatch situation is detected, it is expected that the device raises some management alert indicating the issue, e.g., that it has to drop a packet with a Critical 6LoRH. 9. Security Considerations The security considerations of RFC 4944 [RFC4944], RFC 6282 [RFC6282], and RFC 6553 [RFC6553] apply. Using a compressed format as opposed to the full in-line format is logically equivalent and is believed not to create an opening for a new threat when compared to RFC 6550 [RFC6550], RFC 6553 [RFC6553], and RFC 6554 [RFC6554], noting that, even though intermediate hops are removed from the SRH header as they are consumed, a node may still identify that the rest of the source-routed path includes a loop or not (see the "Security" section of RFC 6554). It must be noted that if the attacker is not part of the loop, then there is always a node at the beginning of the loop that can detect it and remove it. 10. IANA Considerations 10.1. Reserving Space in 6LoWPAN Dispatch Page 1 This specification reserves Dispatch Value Bit Patterns within the 6LoWPAN Dispatch Page 1 as follows: 10 1xxxxx: for Elective 6LoWPAN Routing Headers 10 0xxxxx: for Critical 6LoWPAN Routing Headers Additionally, this document creates two IANA registries: one for the Critical 6LoWPAN Routing Header Type and one for the Elective 6LoWPAN Routing Header Type, each with 256 possible values, from 0 to 255, as described below. Future assignments are made by IANA using the "RFC Required" procedure [RFC5226].
10.2. New Critical 6LoWPAN Routing Header Type Registry This document creates an IANA registry titled "Critical 6LoWPAN Routing Header Type" and assigns the following values: 0-4: SRH-6LoRH [RFC8138] 5: RPI-6LoRH [RFC8138] 10.3. New Elective 6LoWPAN Routing Header Type Registry This document creates an IANA registry titled "Elective 6LoWPAN Routing Header Type" and assigns the following value: 6: IP-in-IP-6LoRH [RFC8138] 11. References 11.1. Normative References [IEEE.802.15.4] IEEE, "IEEE Standard for Low-Rate Wireless Networks", IEEE 802.15.4-2015, DOI 10.1109/IEEESTD.2016.7460875, <http://ieeexplore.ieee.org/document/7460875/>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, <http://www.rfc-editor.org/info/rfc2460>. [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, DOI 10.17487/RFC4443, March 2006, <http://www.rfc-editor.org/info/rfc4443>. [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, <http://www.rfc-editor.org/info/rfc4944>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, September 2011, <http://www.rfc-editor.org/info/rfc6282>. [RFC6550] Winter, T., Ed., Thubert, P., Ed., 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, DOI 10.17487/RFC6550, March 2012, <http://www.rfc-editor.org/info/rfc6550>. [RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing Protocol for Low-Power and Lossy Networks (RPL)", RFC 6552, DOI 10.17487/RFC6552, March 2012, <http://www.rfc-editor.org/info/rfc6552>. [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, DOI 10.17487/RFC6553, March 2012, <http://www.rfc-editor.org/info/rfc6553>. [RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6 Routing Header for Source Routes with the Routing Protocol for Low-Power and Lossy Networks (RPL)", RFC 6554, DOI 10.17487/RFC6554, March 2012, <http://www.rfc-editor.org/info/rfc6554>. [RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Paging Dispatch", RFC 8025, DOI 10.17487/RFC8025, November 2016, <http://www.rfc-editor.org/info/rfc8025>. 11.2. Informative References [FORWARD-FRAG] Thubert, P., Ed. and J. Hui, "LLN Fragment Forwarding and Recovery", Work in Progress, draft-thubert-6lo-forwarding- fragments-05, April 2017.
[IPv6-ARCH] Thubert, P., Ed., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Work in Progress, draft-ietf-6tisch-architecture-11, January 2017. [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. Bormann, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 6775, DOI 10.17487/RFC6775, November 2012, <http://www.rfc-editor.org/info/rfc6775>. [RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January 2014, <http://www.rfc-editor.org/info/rfc7102>. [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014, <http://www.rfc-editor.org/info/rfc7228>. [RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the Internet of Things (IoT): Problem Statement", RFC 7554, DOI 10.17487/RFC7554, May 2015, <http://www.rfc-editor.org/info/rfc7554>. [RPL-INFO] Robles, M., Richardson, M., and P. Thubert, "When to use RFC 6553, 6554 and IPv6-in-IPv6", Work in Progress, draft-ietf-roll-useofrplinfo-14, April 2017.
Appendix A. Examples A.1. Examples Compressing the RPI The example in Figure 15 illustrates the 6LoRH compression of a classical packet in Storing mode in all directions, as well as in Non-Storing mode for a packet going up the DODAG following the default route to the root. In this particular example, a fragmentation process takes place per RFC 4944 [RFC4944], and the fragment headers must be placed in Page 0 before switching to Page 1: +- ... -+- ... -+-+ ... -+- ... +-+-+ ... -+-+-+-+-+-+-+-+-+-+... |Frag type|Frag hdr |11110001| RPI- |IP-in-IP| LOWPAN_IPHC | ... |RFC 4944 |RFC 4944 | Page 1 | 6LoRH | 6LoRH | | +- ... -+- ... -+-+ ... -+- ... +-+-+ ... -+-+-+-+-+-+-+-+-+-+... <- RFC 6282 -> No RPL artifact +- ... -+- ... -+-+ ... -+-+ ... -+- ... +-+-+-+-+-+-+-+-+-+-+... |Frag type|Frag hdr | |RFC 4944 |RFC 4944 | Payload (cont) +- ... -+- ... -+-+ ... -+-+ ... -+- ... +-+-+-+-+-+-+-+-+-+-+... +- ... -+- ... -+-+ ... -+-+ ... -+- ... +-+-+-+-+-+-+-+-+-+-+... |Frag type|Frag hdr | |RFC 4944 |RFC 4944 | Payload (cont) +- ... -+- ... -+-+ ... -+-+ ... -+- ... +-+-+-+-+-+-+-+-+-+-+... Figure 15: Example Compressed Packet with RPI In Storing mode, if the packet stays within the RPL domain, then it is possible to save the IP-in-IP encapsulation, in which case, only the RPI is compressed with a 6LoRH, as illustrated in Figure 16 in the case of a non-fragmented ICMP packet: +- ... -+-+- ... -+-+-+-+ ... -+-+-+-+ ... -+-+-+-+-+-+-+-+-+-+-+... |11110001| RPI-6LoRH | NH = 0 | NH = 58 | ICMP message ... |Page 1 | Type 5 | 6LOWPAN_IPHC | (ICMP) | (no compression) +- ... -+-+- ... -+-+-+-+ ... -+-+-+-+ ... -+-+-+-+-+-+-+-+-+-+-+... <- RFC 6282 -> No RPL artifact Figure 16: Example ICMP Packet with RPI in Storing Mode
The format in Figure 16 is logically equivalent to the uncompressed format illustrated in Figure 17: +-+-+-+- ... -+-+-+-+ ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+... | IPv6 Header | Hop-by-Hop | RPI in | ICMP message ... | NH = 58 | Header | RPL Option | +-+-+-+- ... -+-+-+-+ ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+... Figure 17: Uncompressed ICMP Packet with RPI For a UDP packet, the transport header can be compressed with 6LoWPAN HC [RFC6282] as illustrated in Figure 18: +-+ ... -+-+-...-+-+- ... -+-+-+-+ ... -+-+-+ ... -+-+-+-+-+... |11110001| RPI- | NH=1 |11110CPP| Compressed | UDP |Page 1 | 6LoRH | LOWPAN_IPHC | UDP | UDP header | Payload +-+ ... -+-+-...-+-+- ... -+-+-+-+ ... -+-+-+ ... -+-+-+-+-+... <- RFC 6282 -> No RPL artifact Figure 18: Uncompressed ICMP Packet with RPI If the packet is received from the Internet in Storing mode, then the root is supposed to encapsulate the packet to insert the RPI. The resulting format would be as represented in Figure 19: +-+ ... -+-+-...-+-+-- ... -+-+-+-+- ... -+-+ ... -+-+-+ ... -+-+-+... |11110001| RPI- | IP-in-IP | NH=1 |11110CPP| Compressed | UDP |Page 1 | 6LoRH | 6LoRH | LOWPAN_IPHC | UDP | UDP header | Payld +-+ ... -+-+-...-+-+-- ... -+-+-+-+- ... -+-+ ... -+-+-+ ... -+-+-+... <- RFC 6282 -> No RPL artifact Figure 19: RPI Inserted by the Root in Storing Mode A.2. Example of a Downward Packet in Non-Storing Mode The example illustrated in Figure 20 is a classical packet in Non- Storing mode for a packet going down the DODAG following a source- routed path from the root. Say that we have four forwarding hops to reach a destination. In the uncompressed form, when the root generates the packet, the last 3 hops are encoded in a Routing Header Type 3 (SRH) and the first hop is the destination of the packet. The intermediate hops perform a swap; the hop count indicates the current active hop as defined in RFC 2460 [RFC2460] and RFC 6554 [RFC6554].
When compressed with this specification, the 4 hops are encoded in SRH-6LoRH when the root generates the packet, and the final destination is left in the LOWPAN_IPHC. There is no swap; the forwarding node that corresponds to the first entry effectively consumes it when forwarding, which means that the size of the encoded packet decreases and that the hop information is lost. If the last hop in an SRH-6LoRH is not the final destination, then it removes the SRH-6LoRH before forwarding. In the particular example illustrated in Figure 20, all addresses in the DODAG are assigned from the same /112 prefix and the last 2 octets encoding an identifier such as an IEEE 802.15.4 short address. In that case, all addresses can be compressed to 2 octets, using the root address as reference. There will be one SRH_6LoRH header with, in this example, three compressed addresses: +-+ ... -+-+ ... +-+- ... -+-+- ... +-+-+-+ ... +-+-+ ... -+ ... +-... |11110001|SRH-6LoRH| RPI- | IP-in-IP | NH=1 |11110CPP| UDP | UDP |Page 1 |Type1 S=2| 6LoRH | 6LoRH |LOWPAN_IPHC| UDP | hdr |Payld +-+ ... -+-+ ... +-+- ... -+-+-- ... -+-+-+ ... +-+-+ ... -+ ... +-... <-8bytes-> <- RFC 6282 -> No RPL artifact Figure 20: Example Compressed Packet with SRH One may note that the RPI is provided. This is because the address of the root that is the source of the IP-in-IP header is elided and inferred from the RPLInstanceID in the RPI. Once found from a local context, that address is used as a Compression Reference to expand addresses in the SRH-6LoRH. With the RPL specifications available at the time of writing, the root is the only node that may incorporate an SRH in an IP packet. When the root forwards a packet that it did not generate, it has to encapsulate the packet with IP-in-IP. But, if the root generates the packet towards a node in its DODAG, then it should avoid the extra IP-in-IP as illustrated in Figure 21: +- ... -+-+-+ ... +-+-+-+ ... -+-+-+-+-+-+-+-++-+- ... -+-+-+-+-+... |11110001| SRH-6LoRH | NH=1 | 11110CPP | Compressed | UDP |Page 1 | Type1 S=3 | LOWPAN_IPHC| LOWPAN-NHC| UDP header | Payload +- ... -+-+-+ ... +-+-+-+ ... -+-+-+-+-+-+-+-++-+- ... -+-+-+-+-+... <- RFC 6282 -> Figure 21: Compressed SRH 4*2bytes Entries Sourced by Root
Note: The RPI is not represented, though RPL [RFC6550] generally expects it. In this particular case, since the Compression Reference for the SRH-6LoRH is the source address in the LOWPAN_IPHC, and the routing is strict along the source route path, the RPI does not appear to be absolutely necessary. In Figure 21, all the nodes along the source route path share the same /112 prefix. This is typical of IPv6 addresses derived from an IEEE802.15.4 short address, as long as all the nodes share the same PAN-ID. In that case, a Type 1 SRH-6LoRH header can be used for encoding. The IPv6 address of the root is taken as reference, and only the last 2 octets of the address of the intermediate hops are encoded. The Size of 3 indicates 4 hops, resulting in an SRH-6LoRH of 10 bytes. A.3. Example of SRH-6LoRH Life Cycle This section illustrates the operation specified in Section 5.6 of forwarding a packet with a compressed SRH along an A->B->C->D source route path. The operation of popping addresses is exemplified at each hop. Packet as received by node A ---------------------------- Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA AAAA Type 1 SRH-6LoRH Size = 0 BBBB Type 2 SRH-6LoRH Size = 1 CCCC CCCC DDDD DDDD Step 1: Popping BBBB, the first entry of the next SRH-6LoRH Step 2: If larger value (2 vs. 1), the SRH-6LoRH is removed Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA AAAA Type 2 SRH-6LoRH Size = 1 CCCC CCCC DDDD DDDD Step 3: Recursion ended; coalescing BBBB with the first entry Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA BBBB Step 4: Routing based on next segment endpoint to B Figure 22: Processing at Node A
Packet as received by node B ---------------------------- Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA BBBB Type 2 SRH-6LoRH Size = 1 CCCC CCCC DDDD DDDD Step 1: Popping CCCC CCCC, the first entry of the next SRH-6LoRH Step 2: Removing the first entry and decrementing the Size (by 1) Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA BBBB Type 2 SRH-6LoRH Size = 0 DDDD DDDD Step 3: Recursion ended; coalescing CCCC CCCC with the first entry Type 3 SRH-6LoRH Size = 0 AAAA AAAA CCCC CCCC Step 4: Routing based on next segment endpoint to C Figure 23: Processing at Node B Packet as received by node C ---------------------------- Type 3 SRH-6LoRH Size = 0 AAAA AAAA CCCC CCCC Type 2 SRH-6LoRH Size = 0 DDDD DDDD Step 1: Popping DDDD DDDD, the first entry of the next SRH-6LoRH Step 2: The SRH-6LoRH is removed Type 3 SRH-6LoRH Size = 0 AAAA AAAA CCCC CCCC Step 3: Recursion ended; coalescing DDDD DDDDD with the first entry Type 3 SRH-6LoRH Size = 0 AAAA AAAA DDDD DDDD Step 4: Routing based on next segment endpoint to D Figure 24: Processing at Node C Packet as received by node D ---------------------------- Type 3 SRH-6LoRH Size = 0 AAAA AAAA DDDD DDDD Step 1: The SRH-6LoRH is removed Step 2: No more header; routing based on inner IP header Figure 25: Processing at Node D
Acknowledgements The authors wish to thank Tom Phinney, Thomas Watteyne, Tengfei Chang, 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 Michael Richardson and Ines Robles (the Chairs of the ROLL WG), Brian Haberman (the Internet Area AD), and Alvaro Retana and Adrian Farrel (Routing Area ADs) for driving this complex effort across working groups and areas.
Authors' Addresses Pascal Thubert (editor) Cisco Systems Building D - Regus 45 Allee des Ormes BP1200 MOUGINS - Sophia Antipolis 06254 France Phone: +33 4 97 23 26 34 Email: firstname.lastname@example.org Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63921 Email: email@example.com Laurent Toutain IMT Atlantique 2 rue de la Chataigneraie CS 17607 Cesson-Sevigne Cedex 35576 France Email: Laurent.Toutain@IMT-Atlantique.fr Robert Cragie ARM Ltd. 110 Fulbourn Road Cambridge CB1 9NJ United Kingdom Email: firstname.lastname@example.org