Internet-Draft SCHC compression over IEEE 802.15.4 September 2023
Gomez & Minaburo Expires 2 April 2024 [Page]
Workgroup:
6lo Working Group
Internet-Draft:
draft-ietf-6lo-schc-15dot4-03
Published:
Intended Status:
Standards Track
Expires:
Authors:
C. Gomez
UPC
A. Minaburo
Consultant

Transmission of SCHC-compressed packets over IEEE 802.15.4 networks

Abstract

A framework called Static Context Header Compression and fragmentation (SCHC) has been designed with the primary goal of supporting IPv6 over Low Power Wide Area Network (LPWAN) technologies [RFC8724]. One of the SCHC components is a header compression mechanism. If used properly, SCHC header compression allows a greater compression ratio than that achievable with traditional 6LoWPAN header compression [RFC6282]. For this reason, it may make sense to use SCHC header compression in some 6LoWPAN environments, including IEEE 802.15.4 networks. This document specifies how a SCHC-compressed packet can be carried over IEEE 802.15.4 networks. The document also enables the transmission of SCHC-compressed UDP/CoAP headers over 6LoWPAN-compressed IPv6 packets.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 4 March 2024.

▲

Table of Contents

1. Introduction

RFC 6282 is the main specification for IPv6 over Low power Wireless Personal Area Network (6LoWPAN) IPv6 header compression [RFC6282]. That RFC was designed assuming IEEE 802.15.4 as the layer below the 6LoWPAN adaptation layer, and it has also been reused (with proper adaptations) for IPv6 header compression over many other technologies relatively similar to IEEE 802.15.4 in terms of characteristics such as physical layer bit rate, layer 2 maximum payload size, etc. Examples of such technologies comprise BLE, DECT-ULE, ITU G.9959, MS/TP, NFC, and PLC. RFC 6282 provides additional functionality, such as a mechanism for UDP header compression.

In the best cases, RFC 6282 allows to compress a 40-byte IPv6 header down to a 2-byte compressed header (for link-local interactions) or a 3-byte compressed header (when global IPv6 addresses are used). On the other hand, RFC 6282 typically compresses a UDP header to a size of 2 to 4 bytes. Therefore, in advantageous conditions, a 48-byte uncompressed IPv6/UDP header may be compressed down to a 4- to 6-byte format (when using link-local addresses) or a 5- to 7-byte format (for global interactions) by using RFC 6282.

Recently, a framework called Static Context Header Compression (SCHC) has been designed with the primary goal of supporting IPv6 over Low Power Wide Area Network (LPWAN) technologies [RFC8724]. SCHC comprises header compression and fragmentation functionality tailored to the extraordinary constraints of LPWAN technologies, which are more severe than those exhibited by IEEE 802.15.4 or other relatively similar technologies. SCHC header compression allows a greater compression ratio than that of RFC 6282. If used properly, SCHC allows to compress an IPv6/UDP header down to e.g. a single byte. In addition, SCHC can be used to compress Constrained Application Protocol (CoAP) headers [RFC7252][RFC8824], which further increases the achievable performance improvement of using SCHC header compression, since there is no 6LoWPAN header compression mechanism defined for CoAP. Therefore, it may make sense to use SCHC header compression in some 6LoWPAN environments, including IEEE 802.15.4 networks, considering its greater efficiency.

This document specifies how a SCHC-compressed packet can be carried over IEEE 802.15.4 networks. In order to ease a transition from existing 6LoWPAN/6Lo implementations to support SCHC header compression, the document also enables the transmission of SCHC-compressed UDP/CoAP headers over 6LoWPAN-compressed IPv6 packets. Further transition approaches are also described.

Note that, as per this document, and while SCHC defines fragmentation mechanisms as well, 6LoWPAN/6Lo fragmentation is used when necessary to transport SCHC-compressed packets over IEEE 802.15.4 networks [RFC4944][RFC8930][RFC8931].

This specification updates RFC 8138 and RFC 9008.

2. Terminology

2.1. Requirements language

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 BCP14 [RFC2119], [RFC8174], when, and only when, they appear in all capitals, as shown here.

2.2. Background on previous specifications

The reader is expected to be familiar with the terms and concepts defined in specifications of 6LoWPAN frame formats [RFC4944], RPL [RFC6550] and companion documents [RFC6553][RFC6554][RFC9008], 6LoWPAN Routing Header [RFC8138], SCHC [RFC8724], and SCHC for CoAP [RFC8824].

RFC 8724 defines the Rule concept, whereby a Rule may be used to support header compression or fragmentation functionality. In the present document, Rules are only used for header compression.

RFC 6775 defines the term 6LoWPAN Node (6LN) as the following: "A 6LoWPAN node is any host or router participating in a LoWPAN. This term is used when referring to situations in which either a host or router can play the role described." In this document, as in RFC 9008, 6LN acts as a leaf.

3. Architecture

3.1. Protocol stacks

3.1.1. Main protocol stack

The traditional 6LoWPAN-based protocol stack for constrained devices (Figure 1, left) places the 6LoWPAN adaptation layer between IPv6 and an underlying technology such as IEEE 802.15.4. Suitable upper layer protocols include CoAP [RFC7252] and UDP. (Note that, while CoAP has also been specified over TCP, and TCP may play a significant role in IoT environments [RFC9006], 6LoWPAN header compression has not been defined for TCP, as of the writing.)

6LoWPAN can be envisioned as a set of two main sublayers, where the upper one provides header compression, while the lower one offers fragmentation.

This document defines an alternative approach for packet header compression over IEEE 802.15.4, which leads to a modified protocol stack (Figure 1, right). Fragmentation functionality remains the one defined by 6LoWPAN [RFC4944] and 6Lo [RFC8930][RFC8931].


     +------------+          +------------+
     | CoAP, other|          | CoAP, other|
     +------------+          +------------+
     | UDP, other |          | UDP, other |
     +------------+          +------------+
     |    IPv6    |          |    IPv6    |
     +------------+          +------------+
     | 6LoWPAN HC |          |  SCHC HC   |  <-- NEW
     +------------+          +------------+
     |6LoWPAN Frag|          |6LoWPAN Frag|
     +------------+          +------------+
     |  802.15.4  |          |  802.15.4  |
     +------------+          +------------+
Figure 1: Traditional 6LoWPAN-based protocol stack over IEEE 802.15.4 (left) and alternative protocol stack using SCHC for header compression (right). HC and Frag stand for Header Compression and Fragmentation, respectively.

SCHC header compression may be applied to the headers of different protocols or sets of protocols. Some examples include: i) IPv6 packet headers, ii) joint IPv6 and UDP packet headers, iii) joint IPv6, UDP and CoAP packet headers, etc.

3.1.2. Transition protocol stacks

In order to ease a transition from existing 6LoWPAN implementations to support SCHC header compression, the present document also: i) illustrates two possible protocol stacks, where 6LoWPAN header compression is used to compress IPv6/UDP headers while SCHC compresses CoAP headers (see Section 5.1), and ii) enables the transmission of SCHC-compressed UDP/CoAP headers over 6LoWPAN-compressed IPv6 packets (see Section 5.2). However, note that the greatest header compression performance can be achieved by using SCHC to also compress the UDP header.

RFC 8824 defines how SCHC can be used to compress CoAP headers, including Object Security for Constrained RESTful Environments (OSCORE)-protected messages [RFC8613]. On the other hand, it is possible to carry SCHC-compressed CoAP headers over UDP by means of using SCHC UDP ports [I-D.ietf-intarea-schc-protocol-numbers]. Figure 2 (left) shows the resulting protocol stack, where 6LoWPAN header compression is applied to UDP and IPv6. When Datagram Transport Layer Security (DTLS) [RFC9147] is preferred to protect SCHC-compressed CoAP messages, the DTLS layer sits between the SCHC and UDP layers (Figure 2, right).

                                 +------------+
                                 |    CoAP    |
         +------------+          +------------+
         |    CoAP    |          |    SCHC    |
         +------------+          +------------+
         |    SCHC    |          |    DTLS    |
         +------------+          +------------+
         |     UDP    |          |     UDP    |
         +------------+          +------------+
         |    IPv6    |          |    IPv6    |
         +------------+          +------------+
         | 6LoWPAN HC |          | 6LoWPAN HC |
         +------------+          +------------+
         |6LoWPAN Frag|          |6LoWPAN Frag|
         +------------+          +------------+
         |  802.15.4  |          |  802.15.4  |
         +------------+          +------------+
Figure 2: Transition protocol stacks where 6LoWPAN header compression is applied to UDP and IPv6. The leftmost protocol stack supports the use of OSCORE, whereas the rightmost one corresponds to the use of DTLS to protect SCHC-compressed CoAP messages.

Finally, the "transition" protocol stack enabled by this document, which allows the transmission of 6LoWPAN-compressed IPv6 packets containing SCHC-compressed UDP/CoAP data units, is shown in Figure 3 (rightmost).

                                               +------------+
                                               |    CoAP    |
  +------------+   +------------+              +------------+
  | CoAP, other|   | CoAP, other|              |     UDP    |
  +------------+   +------------+              +------------+
  | UDP, other |   | UDP, other |              |   SCHC HC  |  <-- NEW
  +------------+   +------------+              +------------+
  |    IPv6    |   |    IPv6    |              |    IPv6    |
  +------------+   +------------+              +------------+
  | 6LoWPAN HC |   |  SCHC HC   |  <-- NEW     | 6LoWPAN HC |
  +------------+   +------------+              +------------+
  |6LoWPAN Frag|   |6LoWPAN Frag|              |6LoWPAN Frag|
  +------------+   +------------+              +------------+
  |  802.15.4  |   |  802.15.4  |              |  802.15.4  |
  +------------+   +------------+              +------------+
Figure 3: Traditional 6LoWPAN-based protocol stack over IEEE 802.15.4 (left), alternative protocol stack using SCHC for header compression (middle), and transition protocol stack using SCHC for header compression of UDP/CoAP headers (right). HC and Frag stand for Header Compression and Fragmentation, respectively.

3.2. Network topologies

IEEE 802.15.4 supports two main network topologies: the star topology, and the peer-to-peer (i.e., mesh) topology.

SCHC has been designed for LPWAN technologies, which are typically based on a star topology where constrained devices (e.g., sensors) communicate with a less constrained, central network gateway [RFC 8376]. However, as stated in [draft-ietf-schc-architecture], SCHC is generic and it can also be used in networking environments beyond the ones originally considered for SCHC.

SCHC compression is applicable to both star topology and mesh topology IEEE 802.15.4 networks.

3.3. Single-hop communication

In order to support the transmission of SCHC-compressed packets between two endpoints that are single-hop neighbors, both endpoints MUST store the Rules intended for the communication between those two endpoints.

The frame format to be used to carry a SCHC-compressed packet in single-hop communication is described in Section 4.1.

3.4. Multihop communication

6LoWPAN defines two approaches for multihop communication: Route-Over and Mesh-Under [RFC6606]. In Route-Over, routing is performed at the IP layer. In Mesh-Under, routing functionality is located at the adaptation layer, below IP. This section describes how SCHC-compressed packets are transmitted over a multihop IEEE 802.15.4 network, for both Route-Over and Mesh-Under.

3.4.1. Straightforward Route-Over (SRO)

SCHC header compression MAY be used in a Route-Over network in a straightforward approach, whereby all routers (i.e., all 6LRs and 6LBRs) MUST store all the Rules in use by any nodes in the network, whereas a host MUST store the Rules defined for its communication with other endpoints. This approach is called Straightforward Route-Over (SRO). In this case, 6LoWPAN routers are able to decompress (if needed) received packet headers and compress packet headers before being forwarded.

Figure 4 illustrates an example network with the Rules that need to be stored by the nodes in SRO. In this example, RuleID 1 is intended for communication between Host A and Host B, RuleID 2 is intended for communication between Host A and Host C, and RuleID 3 is used for the communication between Host A and an external node called Host E.

                                                 Host E
                                                /
                    (RuleID 1)        +--------+
                    (RuleID 2)    --- |Internet|
                    (RuleID 3)   /    +--------+
                   6LBR ---------
                 /      \
                /        \
              6LR         6LR ------------+                   Pair of endpoints
     (RuleID 1) |         | (RuleID 1)    |          RuleID 1:       A, B
     (RuleID 2) |         | (RuleID 2)    |          RuleID 2:       A, C
     (RuleID 3) |         | (RuleID 3)    |          RuleID 3:       A, E
                |         |               |
             Host A      Host B         Host C
              (RuleID 1)    (RuleID 1)     (RuleID 2)
              (RuleID 2)
              (RuleID 3)
Figure 4: Rules stored by each node in an example network using SRO.

The frame format to be used to carry a SCHC-compressed packet in SRO is described in Section 4.1.

3.4.2. Tunneled, RPL-based Route-Over (TRO)

In a Route-Over network that uses the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) [RFC6550], the RPL non-storing mode [RFC6550, RFC 6554] and [RFC8138] MAY be exploited in order to efficiently transmit SCHC-compressed packets. In this approach, packets sent by a 6LN are tunneled to the root, and packets intended for 6LNs are tunneled from the root (note: a tunnel is not needed when the root itself is the source). Traffic between two 6LNs traverses an Upward tunnel to the root and a Downward tunnel from the root. The present document defines the described approach as Tunneled, RPL-based Route-Over approach (TRO).

In this approach, each 6LoWPAN node (i.e., a host, a 6LR or a 6LBR) MUST store the Rules defined for its communication with other endpoints. A 6LR is thus relieved to store Rules used by pairs of endpoints that do not include the 6LR itself. A 6LBR MUST store all the Rules used by all nodes in the network.

Figure 5 illustrates the Rules that need to be stored by the nodes in TRO, based on the same example network and endpoint pairs shown in Figure 4.

                                                 Host E
                                                /
                    (RuleID 1)        +--------+
                    (RuleID 2)    --- |Internet|
                    (RuleID 3)   /    +--------+
                   6LBR ---------
                 /      \
                /        \
              6LR         6LR ------------+                    Pair of endpoints
     (no Rules) |         | (no Rules)    |           RuleID 1:       A, B
                |         |               |           RuleID 2:       A, C
                |         |               |           RuleID 3:       A, E
                |         |               |
             Host A      Host B         Host C
              (RuleID 1)    (RuleID 1)     (RuleID 2)
              (RuleID 2)
              (RuleID 3)
Figure 5: Rules stored by each node in an example network using TRO.

RFC 9008 describes how the communication between a 6LN and another endpoint (another 6LN or the root of the same RPL domain, or an external node, e.g., on the Internet) is performed. For the sake of description clarity, Figure 6 (adapted from Figure 3 in RFC 9008) provides a reference topology including nodes referred to in the remainder of this subsection.

                     +------------+
                     |  INTERNET  |---------+
                     +------------+         |
                                          Z |
                                      +-------+
                                      | 6LBR  |
                          +-----------|(root) |--------+
                          |           +-------+        |
                          |                            |
                          | Y                          |X
                      +---|---+                    +---|---+
                      |  6LR  |                    |  6LR  |
              +-------|       |--+              +--|       |--+
              |       +-------+  |              |  +-------+  |
              | W                |  V           |             |
          +---|---+          +---|---+          |             |
          |  6LR  |          |  6LR  |          |             |
          |       |          |       |          |             |
          +---|---+          +-|---|-+          |             |
              |                |   |            |             |
              |           +----+   |            |             |
           U  |         T |        | S        R |           Q |
        +-----+-+   +-------+  +---|--+     +---|---+     +---|---+
        |  RAL  |   | RUL   |  | RAL  |     |  RAL  |     | RUL   |
        |  6LN  |   |  6LN  |  | 6LN  |     |  6LN  |     |  6LN  |
        +-------+   +-------+  +------+     +-------+     +-------+
Figure 6: Reference topology to support the description of TRO.

In RPL non- storing mode, for Downward traffic, the root adds a source-routing header. The root also performs IPv6-in-IPv6 encapsulation, except when the root itself is the packet source. The IPv6-in-IPv6 encapsulation terminates at the 6LN (if it is a RAL, e.g., U, S or R) or at the last 6LR, e.g., V or X, (if the 6LN is a RUL, e.g., T or Q). For Upward traffic, IPv6-in-IPv6 encapsulation is performed by the first 6LR, e.g. V or X, when the 6LN is a RUL, e.g., T or Q, that sends a packet to an external node or to another 6LN in the same RPL domain, but not to the root. When the 6LN is a RAL (e.g., U, S or R) that sends packets to the same destinations, IPv6-in-IPv6 encapsulation may be performed (by the RAL itself). The destination in the outer header of the IPv6-in-IPv6 encapsulation for Upward traffic is the root.

This document updates RFC 9008 by specifying that, in TRO, when a 6LN transmits an IPv6 packet whose header is compressed by means of SCHC instead of 6LoWPAN header compression (RFC 6282), the SCHC-compressed packet MUST be tunneled by means of IPv6-in-IPv6 encapsulation up to the root. This applies regardless of the inner, SCHC-compressed packet destination.

For Upward traffic, when the 6LN is a RAL (e.g., U, S or R), the 6LN itself performs the IPv6-in-IPv6 encapsulation. However, if the 6LN is a RUL (e.g., T or Q), IPv6- in-IPv6 encapsulation is performed by the first 6LR (e.g., E or C, respectively). In the latter case, in order to enable efficient packet transmission in the first hop from the 6LN, the first 6LR SHOULD be provided with SCHC Rules allowing efficient header compression of packets sent by that 6LN.

For Downward traffic, when the 6LN is a RUL (e.g., G or J), in order to enable efficient packet transmission in the last hop to the 6LN, the last 6LR (e.g., V or X, respectively) SHOULD be provided with SCHC Rules allowing efficient header compression of packets sent to that 6LN.

Not providing such SCHC Rules to the first or last 6LR (for Upward or Downward traffic, respectively) should only happen if it is not practical or possible to do so (e.g., due to lack of available memory at the 6LR).

For the sake of efficiency, RFC 8138 MUST be used to compress IPv6-in-IPv6 headers, the RPL Option (RFC 6553) and the source routing header (RPL Routing Header type 3, RFC 6554).

The frame format to be used to carry a SCHC-compressed packet in TRO is described in Section 4.3.

3.4.3. Pointer-based Route-Over (PRO)

In the previous approach, TRO, intermediate nodes do not have to know the IPv6 destination address of a SCHC-compressed IPv6 packet to be able to forward it. Another approach where intermediate nodes do not have to store the Rules used by the endpoints for packet header compression/decompression, which in addition does not require IPv6- in-IPv6 encapsulation, non-storing mode RPL and RFC 8138 compression, is called Pointer-based Route-Over (PRO).

In PRO, a SCHC pointer is prepended to the SCHC-compressed packet, in order to indicate the location and length of the destination address residue in the SCHC-compressed header. Therefore, a 6LR is able to determine the IPv6 destination address of a SCHC-compressed packet, and route the packet, without the need to store the corresponding Rules. Note that, in PRO, each 6LoWPAN node (i.e., a host, a 6LR, or a 6LBR) MUST store the Rules defined for its communication as an endpoint with other endpoints. A 6LBR MUST store the Rules used by any network node for communication with external nodes.

Figure 7 illustrates the Rules that need to be stored by the nodes in PRO, based on the same example network and endpoint pairs shown in Figure 4 and Figure 5.

                                                  Host E
                                                /
                                      +--------+
                                  --- |Internet|
                    (RuleID 3)   /    +--------+
                   6LBR ---------
                 /      \
                /        \
              6LR         6LR ------------+                     Pair of endpoints
     (no Rules) |         | (no Rules)    |           RuleID 1:       A, B
                |         |               |           RuleID 2:       A, C
                |         |               |           RuleID 3:       A, E
                |         |               |
             Host A      Host B         Host C
              (RuleID 1)    (RuleID 1)     (RuleID 2)
              (RuleID 2)
              (RuleID 3)
Figure 7: Rules stored by each node in an example network using PRO.

PRO is compatible with RPL storing mode, as well as with other routing protocols.

3.4.4. Mesh-Under

When SCHC header compression is used in a Mesh-Under network, Mesh-Under operates as described in RFC 4944. The frame format to be used to carry a SCHC-compressed packet in the Mesh-Under approach is described in Section 4.3.

For header compression in a Mesh-Under network, a network node MUST store the Rules defined for its communication with other endpoints.

In this case, a RuleID MAY be reused across disjoint pairs of endpoints, to identify different Rules used by such disjoint pairs of endpoints, at the expense of increased RuleID management and device configuration complexity.

4. Frame Format

This section defines the frame formats that can be used when a SCHC-compressed packet is carried over IEEE 802.15.4. Such formats are carried as IEEE 802.15.4 frame payload.

TO-DO: align, if needed, with current SCHC WG discussion regarding SCHC headers.

4.1. Single-hop or SRO frame format

This subsection defines the frame format for carrying SCHC-compressed packets over IEEE 802.15.4 for single-hop communication (see 3.3) or when SRO is used for multihop communication (see 3.4.1). This format comprises a SCHC Dispatch Type, a SCHC Packet (i.e. a SCHC-compressed packet (RFC 8724), and Padding bits, if any). Figure 8 illustrates the described frame format.


        <---------- IEEE 802.15.4 frame payload ---------->

                        <----- SCHC Packet ----->
        +---------------+-------------+---------+ - - - - +
        | SCHC Dispatch | SCHC Header | Payload | Padding |
        +---------------+-------------+---------+ - - - - +
Figure 8: Encapsulated, SCHC-compressed packet, for single-hop or SRO transmission. Padding bits are added if needed.

4.1.1. SCHC Dispatch

Adding SCHC header compression to the panoply of header compression mechanisms used in 6LoWPAN/6Lo environments creates the need to signal when a packet header has been compressed by using SCHC. To this end, the present document specifies the SCHC Dispatch. The SCHC Dispatch indicates that the next field in the frame format is a SCHC-compressed header (SCHC Header in Figure 8, see 4.1.2)).

This document defines the SCHC Dispatch as a 6LoWPAN Dispatch Type for SCHC header compression [RFC4944]. With the aim to minimize overhead, the present document allocates a 1-byte pattern in Page 0 [RFC8025] for the SCHC Dispatch Type:

SCHC Dispatch Type bit pattern: 01000100 (Page 0) (Note: to be confirmed by IANA))

4.1.2. SCHC Header

SCHC Header (Figure 8) corresponds to a packet header that has been compressed by using SCHC. As defined in [RFC8724], the SCHC Header comprises a RuleID, and a compression residue. As per the present specification, a RuleID size between 1 and 16 bits is RECOMMENDED. In order to decide the RuleID size to be used in a network, the trade-off between (compressed) header overhead and the number of Rules needs to be carefully assessed.

4.1.3. Padding

If SCHC header compression leads to a SCHC Packet size of a non-integer number of bytes, padding bits of value equal to zero MUST be appended to the SCHC Packet as appropriate to align to an octet boundary.

4.2. TRO frame format

This subsection defines the frame formats for carrying SCHC-compressed packets over IEEE 802.15.4 in TRO (see 3.3.2). Such formats are based on RFC 8138; however, instead of RFC 6282 header compression, this specification uses SCHC header compression. Accordingly, this specification updates RFC 8138 by stating that a 6LoRH header MUST always be placed before the LOWPAN_IPHC as defined in RFC 6282 [RFC6282] or the SCHC Dispatch, followed by the SCHC-compressed packet, as defined in the present specification.

Since 6LoRH uses Dispatch Types in Page 1, the present specification also defines a SCHC Dispatch Type in Page 1, with the same bit pattern as the one in Page 0: 01000100 (to be confirmed by IANA).

In the TRO frame formats, the SCHC-compressed header is preceded by the SCHC Dispatch (in this case, in Page 1).

The frame format for Downward transmission, except when the SCHC-compressed packet source is a RPL root, is shown in Figure 9:


        <----------------- IEEE 802.15.4 frame payload ---------------------->

                                                          <- SCHC pkt ->
        +-- ... -+-- ... --+- ... -+--- ... --+---- ... -+-----+-------+ - - +
        |11110001|SRH-6LoRH| RPI-  | IP-in-IP | 01000100 |SCHC |payload| pad |
        |Page 1  |         | 6LoRH |  6LoRH   |SCHCDsptch| hdr |       |     |
        +-- ... -+-- ... --+- ... -+--- ... --+---- ... -+-----+-------+ - - +
                                                (Page 1)

                                              <----- This specification ----->
Figure 9: Downward frame format for SCHC-compressed packets in TRO, when the source is not a RPL root.

The frame format for Downward transmission, when the SCHC-compressed packet source is a RPL root, is shown in Figure 10:


         <-------------- IEEE 802.15.4 frame payload -------------->

                                               <- SCHC pkt ->
        +-- ... -+-- ... --+- ... -+---- ... -+-----+-------+ - - +
        |11110001|SRH-6LoRH| RPI-  | 01000100 |SCHC |payload| pad |
        |Page 1  |         | 6LoRH |SCHCDsptch| hdr |       |     |
        +-- ... -+-- ... --+- ... -+---- ... -+-----+-------+ - - +
                                     (Page 1)

                                   <----- This specification ----->
Figure 10: Downward frame format for SCHC-compressed packets in TRO, when the source is a RPL root.

The frame format for Upward transmission is shown in Figure 11 (note that it does not include the source routing header that is present in the Downward frame format):


        <------------- IEEE 802.15.4 frame payload ---------------->

                                                <- SCHC pkt ->
        +-- ... -+- ... -+--- ... --+---- ... -+-----+-------+ - - +
        |11110001| RPI-  | IP-in-IP | 01000100 |SCHC |payload| pad |
        |Page 1  | 6LoRH |  6LoRH   |SCHCDsptch| hdr |       |     |
        +-- ... -+- ... -+--- ... --+---- ... -+-----+-------+ - - +
                                      (Page 1)

                                    <----- This specification ----->
Figure 11: Upward frame format for SCHC-compressed packets in TRO.

4.3. PRO frame format

This subsection describes the frame format for carrying SCHC-compressed packets over IEEE 802.15.4 in PRO (see 3.3.3). Such format is shown in Figure 12:


 <------------------ IEEE 802.15.4 frame payload ------------------->

                                  <----- SCHC Packet ----->
 +-----------------+--------------+-------------+---------+ - - - - +
 | SCHC Ptr Dsptch | SCHC Pointer | SCHC Header | Payload | Padding |
 +-----------------+--------------+-------------+---------+ - - - - +
                        v              <->
                        |               |
                        +---------------+
                     compr. dest. addr. residue
Figure 12: frame format for SCHC-compressed packets in PRO.

The first field in Figure 12 is defined as the SCHC Pointer Dispatch, which signals that the next field is the SCHC Pointer and not a SCHC-compressed header. This document defines the SCHC Pointer Dispatch as a 6LoWPAN Dispatch Type [RFC4944] for SCHC header compression. With the aim to minimize overhead, the present document allocates a 1-byte pattern in the 6LoWPAN Dispatch Type Page 0 [RFC8025] for the SCHC Pointer Dispatch Type:

SCHC Pointer Dispatch Type bit pattern: 01000101 (Page 0) (Note: to be confirmed by IANA))

The SCHC Pointer indicates the position of the first bit of the IPv6 destination address residue in the SCHC Header (note that the latter starts with the RuleID), and the length (in bits) of the IPv6 destination address residue. The SCHC Pointer format is shown in Figure 13:


                0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
               +-+-------------+-+-------------+
               |R|    Bit      |0|   Address   |
               | |   pointer   | |   length    |
               +-+-------------+-+-------------+
Figure 13: SCHC Pointer format.

The SCHC Pointer Format comprises three fields, namely: R, Bit pointer and Address length.

The first field, R, is one bit reserved for future use.

The Bit pointer gives the starting position of the IPv6 destination address residue in the SCHC Header (in bits), starting after the SCHC Pointer Dispatch and before the first field of the SCHC Header (i.e., the RuleID). For example, if the IPv6 destination address residue is the only residue in a SCHC-compressed IPv6 packet header (i.e., such residue starts right after the RuleID in the SCHC-compressed header), then the Bit pointer will have a value of RuleID length in bits.

Address length indicates the size of the IPv6 destination address residue (in bits).

4.4. Mesh-Under frame format

This subsection describes the frame formats for carrying SCHC-compressed packets over IEEE 802.15.4 in the Mesh-Under approach (see 3.3.3). Note that the formats are provided in this section for the sake of clarity and completeness, since they are the same as those in RFC 4944, except for the fact that SCHC-compressed packets are carried.

The frame format for a SCHC-compressed packet to be sent by means of Mesh-Under, when fragmentation is not needed, is shown in Figure 14:


    <-------------------- IEEE 802.15.4 frame payload ---------------------->

                                              <----- SCHC Packet ----->
    +-----------+-------------+---------------+-------------+---------+ - - +
    | Mesh Type | Mesh Header | SCHC Dispatch | SCHC Header | Payload | pad |
    +-----------+-------------+---------------+-------------+---------+ - - +
Figure 14: Encapsulated, SCHC-compressed packet, for Mesh-Under transmission (without fragmentation). Padding bits are added if needed.

The frame format for a SCHC-compressed packet to be sent by means of Mesh-Under, which also requires fragmentation, is shown in Figure 15:


   <-------------------- IEEE 802.15.4 frame payload -------------------->

                                              <---- SCHC Packet --->
   +-------+-------+-------+-------+----------+----------+---------+ - - +
   | M Typ | M Hdr | F Typ | F Hdr | SCHC Dsp | SCHC Hdr | Payload | Pad |
   +-------+-------+-------+-------+----------+----------+---------+ - - +
Figure 15: Encapsulated, SCHC-compressed packet, for Mesh-Under transmission (with fragmentation). Padding bits are added if needed.

The frame format for a SCHC-compressed packet to be sent by means of Mesh-Under, which also requires a broadcast header to support mesh broadcast/multicast, is shown in Figure 16:


   <-------------------- IEEE 802.15.4 frame payload -------------------->

                                              <---- SCHC Packet --->
   +-------+-------+-------+-------+----------+----------+---------+ - - +
   | M Typ | M Hdr | B Dsp | B Hdr | SCHC Dsp | SCHC Hdr | Payload | Pad |
   +-------+-------+-------+-------+----------+----------+---------+ - - +
Figure 16: Encapsulated, SCHC-compressed packet, for mesh broadcast/multicast in Mesh-Under transmission (without fragmentation). Padding bits are added if needed. 'B Dsp' and 'B Hdr' stand for 'Broadcast Dispatch' and 'Broadcast Header', respectively.

As in RFC 4944, when more than one LoWPAN header is used in the same packet, they MUST appear in the following order: Mesh Addressing Header, Broadcast Header, Fragmentation Header.

4.5. Summary

The different transmission alternatives enabled by the present document are shown in Figure 17:

   +-------------+----------------------------------------------------------+
   |  Single-hop |                        Multihop                          |
   +-------------+-------------------------------------------+--------------+
   |             |                Route-Over                 |  Mesh-Under  |
   |             +-----------+----------------+--------------+--------------+
   |             |    SRO    |      TRO       |     PRO      |              |
   +-------------+-----------+----------------+--------------+--------------+
   |SCHC Dispatch| SCHC Disp |IP-in-IP, 6LoRH,|SCHC Ptr Disp,| Mesh Headers,|
   |             |           | SCHC Dispatch  | SCHC Pointer | SCHC Dispatch|
   +-------------+-----------+----------------+--------------+--------------+
   |   see 4.1   |  see 4.1  |    see 4.2     |   see 4.3    |    see 4.4   |
   +-------------+-----------+----------------+--------------+--------------+
Figure 17: Summary of alternatives for the transmission of SCHC-compressed packets over IEEE 802.15.4 enabled by the present document, and corresponding artifacts

5. Enabling the transition protocol stack

In order to enable the transition protocol stack, (i.e., supporting SCHC-compressed UDP/CoAP headers over 6LoWPAN-compressed IPv6 packets), the present document exploits the work that is being done by the INTAREA WG, to define a new Internet Protocol Number for SCHC [I-D.ietf-intarea-schc-protocol-numbers]. In this approach, the NH field of the RFC 6282-compressed IPv6 header format is set to 0. The Next Header field of the IPv6 header remains an 8-bit (uncompressed) field carrying the SCHC Internet Protocol Number. The resulting protocol encapsulation and corresponding format for an unfragmented packet, which is carried as IEEE 802.15.4 frame payload, is shown in Figure 18. Padding is added as needed to align the format to an octet boundary.


    <---------------- IEEE 802.15.4 frame payload ------------------>
    +-----------------------+------------------+--------------+ - - +
    | RFC6282-compressed    |                  |              |     |
    |     IPv6 header       | SCHC-compressed  | CoAP Payload | Pad |
    |(NH=0,Next Header=SCHC)| UDP/CoAP headers |              |     |
    +-----------------------+------------------+--------------+- - -+
Figure 18: Protocol data unit encapsulation and format for the transition protocol stack using a SCHC Internet Protocol Number

For networks using the transition protocol stack based on RPL routing, the formats defined in RFC 8138 may also be used for the sake of efficiency, as shown in Figure 19. In this figure, the first field is the Page switch with value 1, followed by RFC 8138-compressed routing artifacts, then followed by the RFC 6282-compressed IPv6 header (which indicates that the next header data unit is a SCHC Packet).


    <------------------------ IEEE 802.15.4 frame payload ------------------------>
    +--------+------------+------------------+---------------+--------------+ - - +
    |11110001|8138-cmprssd|  6282-compressed |               |              |     |
    |(Page 1)|  routing   |   IPv6 header    |SCHC-compressed| CoAP Payload | Pad |
    |        | artifacts  |(NH=0,NxtHdr=SCHC)| UDP/CoAP hdrs |              |     |
    +--------+------------+------------------+---------------+--------------+ - - +
Figure 19: Protocol data unit encapsulation and format for the transition protocol stack using a SCHC Internet Protocol Number and RFC 8138-compressed routing artifacts

6. SCHC compression for IPv6, UDP, and CoAP headers

SCHC header compression may be applied to the headers of different protocols or sets of protocols. Some examples include: i) IPv6 packet headers, ii) joint IPv6 and UDP packet headers, iii) joint IPv6, UDP and CoAP packet headers, etc.

Each Rule defines the set of protocols whose headers are compressed. For example, in a given deployment, RuleIDs 1 to 3 may be defined for IPv6 header compression only, RuleIDs 4 to 7 may be used for IPv6/UDP header compression, and RuleIDs 8 to 15 may be used for IPv6/UDP/CoAP header compression.

This section describes how IPv6, UDP, and CoAP header fields are compressed.

6.1. SCHC compression for IPv6 and UDP headers

IPv6 and UDP header fields MUST be compressed as per Section 10 of RFC 8724.

IPv6 addresses are split into two 64-bit-long fields; one for the prefix and one for the Interface Identifier (IID).

To allow for a single Rule being used for both directions, RFC 8724 identifies IPv6 addresses and UDP ports by their role (Dev or App) and not by their position in the header (source or destination). This optimization can be used as is in some IEEE 802.15.4 networks (e.g., an IEEE 802.15.4 star topology where the peripheral devices (Devs) send/receive packets to/from a network-side entity (App)).

However, in some types of 6LoWPAN environments (e.g., when a sender and its destination are both peer nodes in a mesh topology network), additional functionality is needed to allow use of the Dev and App roles for C/D. In this case, each SCHC C/D entity needs to know its role (Dev or App) in addition to the Rule(s), and corresponding RuleIDs, for each endpoint it communicates with before such communication occurs [I-D.ietf-schc-architecture]. In such cases, the terms Uplink and Downlink that have been defined in RFC 8724 need to be understood in the context of each specific pair of endpoints.

6.1.1. Compression of IPv6 addresses

Compression of IPv6 source and destination prefixes MUST be performed as per Section 10.7.1 of RFC 8724. Additional guidance is given in the present section.

Compression of IPv6 source and destination IIDs MUST be performed as per Section 10.7.2 of RFC 8724. One particular consideration when SCHC C/D is used in IEEE 802.15.4 networks is that, in contrast with some LPWAN technologies, IEEE 802.15.4 data frame headers include both source and destination fields. If the Dev or App IID are based on an L2 address, in some cases the IID can be reconstructed with information coming from the L2 header. Therefore, in those cases, DevIID and AppIID CDAs can be used.

6.1.2. UDP checksum field

RFC 8724 states that "a SCHC compressor MAY elide the UDP checksum when another layer guarantees at least equal integrity protection for the UDP payload and the pseudo-header".

IEEE 802.15.4 frames carry a 16-bit Frame Check Sequence (FCS), which is computed by means of a 16-bit ITU-T CRC algorithm. Considering the FCS size, the greater error detection capabilities of CRC compared with checksum, and the fact that the IEEE 802.15.4 FCS will be checked at each hop in an IEEE 802.15.4 multihop network, the UDP checksum MUST be elided when using SCHC to compress UDP headers.

6.2. SCHC compression for CoAP headers

CoAP header fields MUST be compressed as per Sections 4 to 6 of RFC 8824. Additional guidance is given in this section.

For CoAP header compression/decompression, the SCHC Rules description uses direction information in order to reduce the number of Rules needed to compress headers.

As stated in 5.1, in some types of 6LoWPAN environments (e.g., when a sender and its destination are both peer nodes in a mesh topology network), each SCHC C/D entity needs to know its role (Dev or App), in addition to the Rule(s), and corresponding RuleIDs, for each endpoint it communicates with before such communication occurs [I-D.ietf-schc-architecture]. Therefore, in such cases, direction information will be specific to each pair of endpoints.

7. Neighbor Discovery

A number of optimizations have been developed in order to efficiently support IPv6 Neighbor Discovery (ND) in 6LoWPAN environments (6LoWPAN ND) [RFC 6775][RFC 8505]. SCHC can also be used to compress 6LoWPAN ND packets. At the time of this writing, compression of ICMPv6 or ICMPv6-based protocols has not been specified. Therefore, currently, only the IPv6 header of a packet carrying a 6LoWPAN ND message can be compressed. Nevertheless, future specifications may define how ICMPv6 and 6LoWPAN ND messages can be compressed. (Note: the charter of the new IETF SCHC WG includes the development of "ICMPv6-based protocols" over SCHC as a potential work item.)

8. Fragmentation and reassembly

After applying SCHC header compression to a packet intended for transmission, if the size of the resulting SCHC Packet (Section 4) exceeds the IEEE 802.15.4 frame payload space available, such SCHC Packet MUST be fragmented, carried and reassembled by means of the fragmentation and reassembly functionality defined by 6LoWPAN [RFC4944] or 6Lo [RFC8930][RFC8931].

In a Route-Over multihop network, the 6LoWPAN fragment forwarding technique called Virtual Reassembly Buffer (VRB) [RFC8930] SHOULD be used. However, VRB might not be the best approach for a particular network, e.g., if at least one of the caveats described in Section 6 of RFC 8930 is unacceptable or cannot be addressed.

9. IANA Considerations

This document requests the allocation of the 6LoWPAN Dispatch Type Field bit pattern 01000100 (in Pages 0 and 1) as SCHC Dispatch Type.

This document also requests the allocation of the 6LoWPAN Dispatch Type Field bit pattern 01000101 (in Page 0) as SCHC Pointer Dispatch Type.

10. Security Considerations

This document does not define SCHC header compression functionality beyond the one defined in RFC 8724. Therefore, the security considerations in section 12.1 of RFC 8724 and in section 9 of RFC 8824 apply.

As a safety measure, a SCHC decompressor implementing the present specification MUST NOT reconstruct a packet larger than 1500 bytes [RFC8724].

IEEE 802.15.4 networks support link-layer security mechanisms such as encryption and authentication. As in RFC 8824, the use of a cryptographic integrity-protection mechanism to protect the SCHC headers is REQUIRED.

11. Acknowledgments

Ana Minaburo and Laurent Toutain suggested for the first time the use of SCHC in environments where 6LoWPAN has traditionally been used. Flavien Moullec is a contributor to this document. Laurent Toutain, Pascal Thubert, Dominique Barthel, Guangpeng Li, Carsten Bormann, Nathan Lecorchet, Stuart Cheshire, Kiran Makhijani, and Georgios Z. Papadopoulos made comments that helped shape this document.

Carles Gomez has been funded in part by the Spanish Government through project PID2019-106808RA-I00, and by Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya 2017 through grant SGR 376 and 2021 throught grant SGR 00330.

12. References

12.1. Normative References

[I-D.ietf-intarea-schc-protocol-numbers]
Moskowitz, R., Card, S. W., Wiethuechter, A., and P. Thubert, "Protocol Numbers for SCHC", Work in Progress, Internet-Draft, draft-ietf-intarea-schc-protocol-numbers-00, , <https://datatracker.ietf.org/doc/html/draft-ietf-intarea-schc-protocol-numbers-00>.
[I-D.ietf-schc-architecture]
Pelov, A., Thubert, P., and A. Minaburo, "LPWAN Static Context Header Compression (SCHC) Architecture", Work in Progress, Internet-Draft, draft-ietf-schc-architecture-00, , <https://datatracker.ietf.org/doc/html/draft-ietf-schc-architecture-00>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[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, , <https://www.rfc-editor.org/info/rfc4944>.
[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, , <https://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, , <https://www.rfc-editor.org/info/rfc6550>.
[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, , <https://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, , <https://www.rfc-editor.org/info/rfc6554>.
[RFC6606]
Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem Statement and Requirements for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Routing", RFC 6606, DOI 10.17487/RFC6606, , <https://www.rfc-editor.org/info/rfc6606>.
[RFC7252]
Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <https://www.rfc-editor.org/info/rfc7252>.
[RFC8025]
Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Paging Dispatch", RFC 8025, DOI 10.17487/RFC8025, , <https://www.rfc-editor.org/info/rfc8025>.
[RFC8065]
Thaler, D., "Privacy Considerations for IPv6 Adaptation-Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065, , <https://www.rfc-editor.org/info/rfc8065>.
[RFC8138]
Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie, "IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138, , <https://www.rfc-editor.org/info/rfc8138>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8613]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, , <https://www.rfc-editor.org/info/rfc8613>.
[RFC8724]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. Zuniga, "SCHC: Generic Framework for Static Context Header Compression and Fragmentation", RFC 8724, DOI 10.17487/RFC8724, , <https://www.rfc-editor.org/info/rfc8724>.
[RFC8824]
Minaburo, A., Toutain, L., and R. Andreasen, "Static Context Header Compression (SCHC) for the Constrained Application Protocol (CoAP)", RFC 8824, DOI 10.17487/RFC8824, , <https://www.rfc-editor.org/info/rfc8824>.
[RFC8930]
Watteyne, T., Ed., Thubert, P., Ed., and C. Bormann, "On Forwarding 6LoWPAN Fragments over a Multi-Hop IPv6 Network", RFC 8930, DOI 10.17487/RFC8930, , <https://www.rfc-editor.org/info/rfc8930>.
[RFC8931]
Thubert, P., Ed., "IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Selective Fragment Recovery", RFC 8931, DOI 10.17487/RFC8931, , <https://www.rfc-editor.org/info/rfc8931>.
[RFC9008]
Robles, M.I., Richardson, M., and P. Thubert, "Using RPI Option Type, Routing Header for Source Routes, and IPv6-in-IPv6 Encapsulation in the RPL Data Plane", RFC 9008, DOI 10.17487/RFC9008, , <https://www.rfc-editor.org/info/rfc9008>.
[RFC9147]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, , <https://www.rfc-editor.org/info/rfc9147>.

12.2. Informative References

[RFC9006]
Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage Guidance in the Internet of Things (IoT)", RFC 9006, DOI 10.17487/RFC9006, , <https://www.rfc-editor.org/info/rfc9006>.

Appendix A. Header compression examples

Uplink packet

Source address: fd00::202:2:2:2 with port 8765
Destination address: 2001::1 with port 5678
Payload: "Hello 1" 68 65 6C 6C 6F 20 31

Uncompressed IPv6/UDP packet:
60 00 00 00 00 17 00 40    FD 00 00 00 00 00 00 00
02 02 00 02 00 02 00 02    20 01 00 00 00 00 00 00
00 00 00 00 00 00 00 01    22 3D 16 2E 00 0F 33 68
68 65 6C 6C 6F 20 31

IPv6/UDP header length: 48 bytes
Total length:           55 bytes

In this example, for SCHC compression of IPv6/UDP headers, RuleID 0x20 is used. The Rule corresponding to RuleID 0x20 is shown in Figure 20.

 +----------------+--+--+--+-------------+------+----------++------+
 |       FID      |FL|FP|DI|      TV     |  MO  |    CDA   || Sent |
 |                |  |  |  |             |      |          ||[bits]|
 +----------------+--+--+--+-------------+------+----------++------+
 |IPv6 Version    |4 |1 |Bi|6            |ignore| not-sent ||      |
 |IPv6 Diffserv   |8 |1 |Bi|0            |equal | not-sent ||      |
 |IPv6 Flow Label |20|1 |Bi|0            |equal | not-sent ||      |
 |IPv6 Length     |16|1 |Bi|             |ignore|compute-* ||      |
 |IPv6 Next Header|8 |1 |Bi|17           |equal | not-sent ||      |
 |IPv6 Hop Limit  |8 |1 |Bi|64           |ignore| not-sent ||      |
 |IPv6 DevPrefix  |64|1 |Bi|FD00::/64    |equal | not-sent ||      |
 |IPv6 DevIID     |64|1 |Bi|             |ignore|value-sent|| 64   |
 |IPv6 AppPrefix  |64|1 |Bi|2001::/64    |equal | not-sent ||      |
 |IPv6 AppIID     |64|1 |Bi|::1          |equal | not-sent ||      |
 +================+==+==+==+=============+======+==========++======+
 |UDP DevPort     |16|1 |Bi|8765         |equal | not-sent ||      |
 |UDP AppPort     |16|1 |Bi|5678         |equal | not-sent ||      |
 |UDP Length      |16|1 |Bi|             |ignore|compute-* ||      |
 |UDP checksum    |16|1 |Bi|             |ignore|compute-* ||      |
 +================+==+==+==+=============+======+==========++======+
Figure 20: Illustration of an example Rule with RuleID 0x20

A.1. Single-hop or SRO frame format 

SCHC-compressed packet:
44 20 02 02 00 02 00 02
00 02 68 65 6C 6C 6F 20
31

Header length: 10 bytes
SCHC Dispatch: 44 (01000100)
SCHC RuleID: 0x20 (1 byte)
SCHC residue: 02 02 00 02 00 02 00 02
Payload: 68 65 6C 6C 6F 20 31
Total length: 17 bytes

A.2. TRO frame format

TO-DO

A.3. PRO frame format 

SCHC-compressed packet:
45 88 40 20 02 02 00 02
00 02 00 02 68 65 6C 6C
6F 20 31

Header length: 12 bytes
SCHC Pointer Dispatch: 45 (01000101)
SCHC Pointer: 88 40
SCHC Pointer P: 1
SCHC Pointer Bit Pointer: 8
SCHC Address length: 64 bits
SCHC RuleID: 0x20 (1 byte)
SCHC residue: 02 02 00 02 00 02 00 02
Payload: 68 65 6C 6C 6F 20 31
Total length: 19 bytes

A.4. Mesh-Under frame format

TO-DO

A.5. Enabling the transition protocol stack

Uplink packet

Source address: fe80::201:1:1:1 with port 46487
Destination address: fe80::1 with port 5683
Payload (Temperature value): DA 8C E8 75 15 66 3B 00 1B 37
SCHC protocol number: 145 (0x91)

Uncompressed IPv6/UDP/CoAP packet:
60 0D 4E 65 00 25 11 40    FE 80 00 00 00 00 00 00
02 01 00 01 00 01 00 01    FE 80 00 00 00 00 00 00
00 00 00 00 00 00 00 01    B5 97 16 33 00 25 00 38
50 02 B6 F7 BA 74 65 6D    70 65 72 61 74 75 72 D1
EA 00 FF DA 8C E8 75 15    66 3B 00 1B 37

IPv6/UDP/CoAP header length: 67 bytes
Total length: 77 bytes

In this example, for SCHC compression of UDP/CoAP headers, RuleID 0x22 is used. The Rule corresponding to RuleID 0x22 is shown in Figure 21.

 +----------------+--+--+--+-------------+------+----------++------+
 |       FID      |FL|FP|DI|      TV     |  MO  |    CDA   || Sent |
 |                |  |  |  |             |      |          ||[HEX] |
 +----------------+--+--+--+-------------+------+----------++------+
 |UDP DevPort     |16|1 |Bi|             |ignore|value-sent||B5 97 |
 |UDP AppPort     |16|1 |Bi|5683         |equal | not-sent ||      |
 |UDP Length      |16|1 |Bi|             |ignore|compute-* ||      |
 |UDP checksum    |16|1 |Bi|             |ignore|compute-* ||      |
 +================+==+==+==+=============+======+==========++======+
 |CoAP Version    |16|1 |Bi|1            |equal | not sent ||      |
 |CoAP Type       |16|1 |Up|01           |equal | not sent ||      |
 |CoAP TKL        |32|1 |Bi|0x00         |equal | not sent ||      |
 |CoAP Code       |8 |1 |Up|0.02         |equal | not-sent ||      |
 |CoAP MID        |16|1 |Bi|             |ignore|value-sent||B6 F7 |
 |CoAP OptUri-Path|10|1 |Up|/temperature |equal | not-sent ||      |
 |CoAP Opt No-Resp|1 |1 |Up|00           |equal | not-sent ||      |
 |CoAP Opt EndOpt |8 |1 |Up|0xFF         |equal | not-sent ||      |
 +================+==+==+==+=============+======+==========++======+
Figure 21: Illustration of an example Rule with RuleID 0x22 
IPv6 packet (with uncompressed header) carrying the SCHC-compressed UDP/CoAP headers:
60 0D 4E 65 00 25 91 40    FE 80 00 00 00 00 00 00
02 01 00 01 00 01 00 01    FE 80 00 00 00 00 00 00
00 00 00 00 00 00 00 01    22 B5 97 B6 F7 DA 8C E8
75 15 66 3B 00 1B 37

Compressed packet (IPv6 using 6LoWPAN + UDP/CoAP using SCHC):
6A 11 0D 4E 65 91 02 01    00 01 00 01 00 01 00 00
00 00 00 00 00 01 22 B5    97 B6 F7 DA 8C E8 75 15
66 3B 00 1B 37

Header length: 27 bytes
IPHC: 6A 11
  Dispatch: 011
  TF: 01
  NH: 0
  HLIM: 10
  CID: 0
  SAC: 0
  SAM: 01
  M: 0
  DAC: 0
  DAM: 01

  Traffic Class: 0D4E65
  Next Header: 91
  Src. Address: 201:1:1:1
  Dst. Address: ::1

Next Header: 91 (SCHC)
SCHC RuleID: 0x22
SCHC Residue:
  UDP Dev Port: B5 97 (46487)
  CoAP MID: B6 F7 (46839)

Total length: 37 bytes

Appendix B. Analysis of route-over multihop approaches

This section provides an analysis of the features, pros and cons of the route-over multihop approaches defined in this document: i) SRO, ii) TRO, and iii) PRO.

TO-DO: align with latest descriptions of SRO, TRO and PRO.

B.1. SRO

SRO incurs the lowest header overhead among the considered Route-Over approaches, as it only requires the SCHC Dispatch (1 byte). However, it is the most demanding approach in terms of memory usage, since all network nodes (including intermediate nodes) need to store all the Rules in use in the network. Therefore, it will be suitable for rather small networks and/or where nodes have sufficient memory. Also, SCHC context should be ideally and actually be as static as possible, in order to avoid frequent network- wide stored SCHC context updates.

B.2. TRO

TRO incurs a header overhead that includes a fixed part (a Page Switch plus the SCHC Dispatch, of 1 byte each), plus a variable part that comprises RFC 8138-compressed routing artifacts.

Regarding the latter, in a Downward transmission, it would include the SRH-6LoRH (of variable size, of 4 bytes in the best case, or e.g., 8 bytes as in Fig. 20 of RFC 8138), the RPI-6LoRH (3 bytes in the best case) and the IP-in-IP header (not present if the source is the Root, at least 3 bytes otherwise). In the cases considered, and when the Root is not the packet source, the total header overhead of this approach would be of at least 12-16 bytes.

For upward transmission, the variable part of the header overhead for this approach would include only the RPI-6LoRH (at least, 3 bytes) and the IP-in-IP header (at least, 3 bytes). Therefore, in the cases considered, the total header overhead of this approach would be of at least 8 bytes.

An advantage of this approach is that a node only has to store the Rules for the communications it is involved in as an endpoint, which minimizes memory requirements and the impact of potential SCHC context updates. For example, pure intermediate nodes do not have to store SCHC context.

Note that this approach requires the network to use RPL, non-storing mode. Furthermore, the paths for communication between two nodes in the same network or with external nodes will need to traverse the Root. For communication with external nodes, traversing the Root will be needed anyway, therefore this feature does not pose any issue. However, this constraint will preclude the usage of optimal routes (when they do not include the Root node).

B.3. PRO

PRO incurs a header overhead that includes a 2-byte fixed part (the SCHC Pointer Dispatach plus the SCHC Pointer itself) and a variable part (i.e., the destination address compression residue). The size of the latter will depend on and will need to be planned for the intended use case of the network:

A.- In special cases (e.g., if there is only one possible destination that is known beforehand), there will not be a destination address residue.

B.- If interactions are always intranetwork (i.e., the prefix is known by intermediate nodes), and there can be several possible destinations in the network, the destination address residue will be up to 8 bytes (it could be less depending on how the addresses in that network are built, for example, it could be just 2 bytes).

C.- If interactions can occur with various external networks (i.e., the destination prefix is not known beforehand), the destination address residue will have to be the whole address (16 bytes), since an intermediate node does not know which is the destination prefix.

An advantage of this approach, as in TRO, is that a node only has to store the Rules for the communications it is involved in as an endpoint, which minimizes memory requirements and the impact of potential SCHC context updates. For example, pure intermediate nodes do not have to store SCHC context.

A potential advantage of PRO is that, in contrast with TRO, paths for intranetwork communication are not necessarily constrained to traversing a root node. Therefore, for intranetwork communication, the chances of using optimal paths are greater. Another feature is that the routing solution to be used is not tied to RPL non-storing mode.

B.4. Summary

Assessing the suitability of the different approaches requires considering the following dimensions: network size, node memory capabilities, header overhead, routing constraints / path optimality, intra- or inter-network communication.

TO-DO: to be completed.

Authors' Addresses

Carles Gomez
UPC
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Ana Minaburo
Consultant
Rue de Rennes
35510 Cesson-Sevigne
France