| Internet-Draft | SCHC compression over IEEE 802.15.4 | July 2023 |
| Gomez & Minaburo | Expires 6 January 2024 | [Page] |
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.¶
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Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
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, an RFC 6282 compressed UDP header has a typical size of 4 bytes. Therefore, in advantageous conditions, a 48-byte uncompressed IPv6/UDP header may be compressed down to a 6-byte format (when using link-local addresses) or a 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 as well [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.¶
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.¶
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.¶
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.¶
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.)¶
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 |
+------------+ +------------+
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.¶
In order to ease a transition from existing 6LoWPAN implementations to support SCHC header compression, the document also enables the transmission of SCHC-compressed UDP/CoAP headers over 6LoWPAN-compressed IPv6 packets. The "transition" protocol stack is shown in Figure 2 (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 |
+------------+ +------------+ +------------+
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.¶
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.¶
SCHC header compression MAY be used in a Route-Over network in a straightforward approach, whereby all network nodes MUST store all the Rules in use by any nodes in the network. In this case, 6LoWPAN routers are able to decompress (if needed) received packet headers and compress packet headers before being forwarded.¶
The frame format to be used to carry a SCHC-compressed packet in the straightforward Route-Over approach is described in Section 4.1.¶
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.¶
In this approach, a network node 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.¶
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. 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) or at the last 6LR (if the 6LN is a RUL). For Upward traffic, IPv6-in-IPv6 encapsulation is performed by the first 6LR when the 6LN is a RUL 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 that sends packets to the same destinations, IPv6-in-IPv6 encapsulation may be performed (by the RAL). 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 the tunneled, RPL-based Route-Over approach, 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, the 6LN itself performs the IPv6-in-IPv6 encapsulation. However, if the 6LN is a RUL, IPv6-in-IPv6 encapsulation is performed by the first 6LR. 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, in order to enable efficient packet transmission in the last hop to the 6LN, the last 6LR 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 the tunneled, RPL-based Route-Over approach is described in Section 4.3.¶
In the previous approach, intermediate nodes do not have to know the IPv6 destination address of a SCHC-compressed IPv6 packet to be able to forward it. An alternative approach where intermediate nodes neither have to store the Rules used by the endpoints for packet header compression/decompression, which also does not require IPv6-in-IPv6 encapsulation, non-storing mode RPL and RFC 8138 compression, is the Pointer-based Route-Over approach.¶
In this approach, a SCHC pointer is added after the SCHC Dispatch, in order to indicate the location and length of the destination address residue in the SCHC-compressed header. Note that, in this approach, a network node MUST store the Rules defined for its communication as an endpoint with other endpoints.¶
The Pointer-based Route-over approach is compatible with RPL storing mode, as well as with other routing protocols.¶
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.¶
The different transmission alternatives enabled by the present document are shown in Figure 3:¶
+-------------+----------------------------------------------------------+ | One hop | Multihop | +-------------+----------+-----------------------------------------------+ | |Mesh-under| Route-Over | | | +----------------+---------------+--------------+ | | | RPL-based, non-storing | RPL storing | | | +----------------+---------------+ or other | | | | Up | Down | routing | +-------------+----------+----------------+---------------+--------------+ |SCHC Dispatch|Mesh Hdrs,|IP-in-IP, 6LoRH,|IP-in-IP,6LoRH,| SCHC Dispatch| | |SCHC Disp | SCHC Dispatch | SCHC Dispatch | (with ptr) | +-------------+----------+----------------+---------------+--------------+ | see 4.1 | see 4.4 | see 4.2 | see 4.2 | see 4.3 | +-------------+----------+----------------+---------------+--------------+
This section defines the frame format to be used when a SCHC-compressed packet is carried over IEEE 802.15.4. Such format is carried as IEEE 802.15.4 frame payload.¶
This subsection defines the frame format for carrying SCHC-compressed packets over IEEE 802.15.4 for single-hop communication or when the straightforward Route-Over approach (see 3.3.1) is used. This format comprises a SCHC Dispatch Type, a SCHC Packet (i.e. a SCHC-compressed packet (RFC 8724), and Padding bits, if any). Figure 4 illustrates the described frame format.¶
<---------- IEEE 802.15.4 frame payload ---------->
<----- SCHC Packet ----->
+---------------+-------------+---------+ - - - - +
| SCHC Dispatch | SCHC Header | Payload | Padding |
+---------------+-------------+---------+ - - - - +
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 4, see 4.2)).¶
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))¶
SCHC Header (Figure 4) 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.¶
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.¶
This subsection defines the frame formats for carrying SCHC-compressed packets over IEEE 802.15.4 in the tunneled, RPL-based Route-Over approach (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 tunneled, RPL-based Route-Over 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 5:¶
<----------------- 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 ----->
The frame format for Downward transmission, when the SCHC-compressed packet source is a RPL root, is shown in Figure 6:¶
<-------------- 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 ----->
The frame format for Upward transmission is shown in Figure 7 (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 ----->
This subsection describes the frame format for carrying SCHC-compressed packets over IEEE 802.15.4 in the Pointer-based Route-Over approach (see 3.3.3). Such format is shown in Figure 8:¶
<------------------ IEEE 802.15.4 frame payload ------------------->
<----- SCHC Packet ----->
+-----------------+--------------+-------------+---------+ - - - - +
| SCHC Ptr Dsptch | SCHC Pointer | SCHC Header | Payload | Padding |
+-----------------+--------------+-------------+---------+ - - - - +
v <->
| |
+---------------+
compr. dest. addr. residue
The first field in Figure 8 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 9:¶
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-------------+-+-------------+
|R| Bit |0| Address |
| | pointer | | length |
+-+-------------+-+-------------+
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).¶
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 10:¶
<-------------------- IEEE 802.15.4 frame payload ---------------------->
<----- SCHC Packet ----->
+-----------+-------------+---------------+-------------+---------+ - - +
| Mesh Type | Mesh Header | SCHC Dispatch | SCHC Header | Payload | pad |
+-----------+-------------+---------------+-------------+---------+ - - +
The frame format for a SCHC-compressed packet to be sent by means of Mesh-Under, which also requires fragmentation, is shown in Figure 11:¶
<-------------------- IEEE 802.15.4 frame payload -------------------->
<---- SCHC Packet --->
+-------+-------+-------+-------+----------+----------+---------+ - - +
| M Typ | M Hdr | F Typ | F Hdr | SCHC Dsp | SCHC Hdr | Payload | Pad |
+-------+-------+-------+-------+----------+----------+---------+ - - +
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 12:¶
<-------------------- IEEE 802.15.4 frame payload -------------------->
<---- SCHC Packet --->
+-------+-------+-------+-------+----------+----------+---------+ - - +
| M Typ | M Hdr | B Dsp | B Hdr | SCHC Dsp | SCHC Hdr | Payload | Pad |
+-------+-------+-------+-------+----------+----------+---------+ - - +
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.¶
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-ip-protocol-number]. 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 13. 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 | | |
+-----------------------+------------------+--------------+- - -+
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 14. 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 | | |
+--------+------------+------------------+---------------+--------------+ - - +
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.¶
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.¶
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.¶
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.¶
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.¶
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.)¶
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.¶
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.¶
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.¶
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, and Kiran Makhijani 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.¶
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 15.¶
+----------------+--+--+--+-------------+------+----------++------+ | 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-* || | +================+==+==+==+=============+======+==========++======+
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¶
TO-DO¶
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¶
TO-DO¶
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 16.¶
+----------------+--+--+--+-------------+------+----------++------+ | 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 || | +================+==+==+==+=============+======+==========++======+
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¶
This section provides an analysis of the features, pros and cons of the route-over multihop approaches defined in this document: i) Straightforward; ii) Tunneled, RPL-based; and iii) Pointer-based.¶
The Straightforward Route-Over approach 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.¶
The Tunneled, RPL-based Route-Over approach 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).¶
The Pointer-based Route-Over approach 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 the Tunneled, RPL-based Route-Over 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.¶
A potential advantage of the Pointer-based Route-Over approach is that, in contrast with the Tunneled, RPL-based Route-Over approach, 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.¶
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.¶