6lo Working Group C. Gomez
Internet-Draft UPC
Intended status: Standards Track A. Minaburo
Expires: 11 September 2023 F. Moullec
Acklio
March 2023
Transmission of SCHC-compressed packets over IEEE 802.15.4 networks
draft-ietf-6lo-schc-15dot4-01
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
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This Internet-Draft will expire on 2 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements language . . . . . . . . . . . . . . . . . . 4
2.2. Background on previous specifications . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Protocol stack . . . . . . . . . . . . . . . . . . . . . 4
3.2. Network topologies . . . . . . . . . . . . . . . . . . . 6
3.3. Multihop communication . . . . . . . . . . . . . . . . . 6
3.3.1. Straightforward Route-Over approach . . . . . . . . . 7
3.3.2. Tunneled, RPL-based Route-Over approach . . . . . . . 7
3.3.3. Pointer-based Route-over approach . . . . . . . . . . 8
3.3.4. Mesh-Under approach . . . . . . . . . . . . . . . . . 9
3.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Frame Format . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Single-hop or straightforward Route-Over frame format . . 10
4.1.1. SCHC Dispatch . . . . . . . . . . . . . . . . . . . . 10
4.1.2. SCHC Header . . . . . . . . . . . . . . . . . . . . . 10
4.1.3. Padding . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Tunneled, RPL-based Route-Over frame format . . . . . . . 11
4.3. Pointer-based, Route-Over frame format . . . . . . . . . 12
4.4. Mesh-Under frame format . . . . . . . . . . . . . . . . . 14
5. Enabling the transition protocol stack . . . . . . . . . . . 15
6. SCHC compression for IPv6, UDP, and CoAP headers . . . . . . 16
6.1. SCHC compression for IPv6 and UDP headers . . . . . . . . 16
6.1.1. Compression of IPv6 addresses . . . . . . . . . . . . 16
6.1.2. UDP checksum field . . . . . . . . . . . . . . . . . 17
6.2. SCHC compression for CoAP headers . . . . . . . . . . . . 17
7. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . 17
8. Fragmentation and reassembly . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Header compression examples . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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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, 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.
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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.
3. Architecture
3.1. 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.)
6LoWPAN can be envisioned as a set of two main sublayers, where the
upper one provides header compression, while the lower one offers
fragmentation.
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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.
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).
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+------------+
| 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 2: 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-lpwan-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. 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.
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3.3.1. Straightforward Route-Over approach
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.
3.3.2. Tunneled, RPL-based Route-Over approach
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.
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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.
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.
3.3.3. Pointer-based Route-over approach
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
header.
TO-DO: clarify assumption regarding the IPv6 destination prefix.
The Pointer-based Route-over approach is compatible with RPL storing
mode, as well as with other routing protocols.
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3.3.4. Mesh-Under approach
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.
3.4. Summary
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 (or other|
| | +----------------+--------------+ routing) |
| | | Up | Down | storing |
+------------+----------+----------------+--------------+--------------+
|SCHC Disp |Mesh Hdrs,|IP-in-IP, 6LoRH,| 6LoRH, | SCHC Dispatch|
| |SCHC Disp |SCHC Dispatch | SCHC Dsptch | (with ptr) |
+------------+----------+----------------+--------------+--------------+
| see 4.1 | see 4.4 | see 4.2 | see 4.2 | see 4.3 |
+------------+----------+----------------+--------------+--------------+
Figure 3: Summary of transmission alternatives enabled by the
present document
4. Frame Format
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.
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4.1. Single-hop or straightforward Route-Over frame format
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 |
+---------------+-------------+---------+ - - - - +
Figure 4: Encapsulated, SCHC-compressed packet, for single-hop or
straightforward Route-Over 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 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))
4.1.2. SCHC Header
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.
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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. Tunneled, RPL-based Route-Over frame format
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 ----->
Figure 5: Downward frame format for SCHC-compressed packets in
the tunneled, RPL-based Route-Over approach, when the source is
not a RPL root.
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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 ----->
Figure 6: Downward frame format for SCHC-compressed packets in
the tunneled, RPL-based Route-Over approach, when the source is a
RPL root.
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 ----->
Figure 7: Upward frame format for SCHC-compressed packets in the
tunneled, RPL- based Route-Over approach.
4.3. Pointer-based, Route-Over frame format
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:
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<---------------- IEEE 802.15.4 frame payload ------------------->
<----- SCHC Packet ----->
+---------------+--------------+-------------+---------+ - - - - +
| SCHC Dispatch | SCHC Pointer | SCHC Header | Payload | Padding |
+---------------+--------------+-------------+---------+ - - - - +
v <->
| |
+---------------+
compr. resd. addr.
Figure 8: frame format for SCHC-compressed packets in the
Pointer-based Route- Over approach.
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 0 1 2 3 4 5 6
+-+-----------+-+-----------+
|P| Bit |0| Address |
| | pointer | | length |
+-+-----------+-+-----------+
Figure 9: SCHC Pointer format.
The SCHC Pointer Format comprises three fields, namely: P, Bit
pointer and Address length.
The first field, P, is an indication of whether the Bit pointer and
Address length fields are present (P=1) or not (P=0).
If P is set to 1, the Bit pointer gives the starting position of the
IPv6 destination address residue in the SCHC Header (in bits), and
Address length indicates the size of the IPv6 destination address
residue (in bits).
If P is set to 0, neither the Bit Pointer nor the Address length
fields are present (note: the "0" that precedes the Address length is
not present either).
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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 10:
<-------------------- IEEE 802.15.4 frame payload ---------------------->
<----- SCHC Packet ----->
+-----------+-------------+---------------+-------------+---------+ - - +
| Mesh Type | Mesh Header | SCHC Dispatch | SCHC Header | Payload | pad |
+-----------+-------------+---------------+-------------+---------+ - - +
Figure 10: 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 11:
<-------------------- IEEE 802.15.4 frame payload -------------------->
<---- SCHC Packet --->
+-------+-------+-------+-------+----------+----------+---------+ - - +
| M Typ | M Hdr | F Typ | F Hdr | SCHC Dsp | SCHC Hdr | Payload | Pad |
+-------+-------+-------+-------+----------+----------+---------+ - - +
Figure 11: 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 12:
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<-------------------- IEEE 802.15.4 frame payload -------------------->
<---- SCHC Packet --->
+-------+-------+-------+-------+----------+----------+---------+ - - +
| M Typ | M Hdr | B Dsp | B Hdr | SCHC Dsp | SCHC Hdr | Payload | Pad |
+-------+-------+-------+-------+----------+----------+---------+ - - +
Figure 12: 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.
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-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, 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 | | |
+-----------------------+------------------+--------------+- - -+
Figure 13: Protocol data unit encapsulation and format for the
transition protocol stack using a SCHC Internet Protocol Number
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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-lpwan-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.
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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 IPv6/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-lpwan-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
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ICMPv6 and 6LoWPAN ND messages can be compressed. (Note: at the time
of this writing, the LPWAN WG is discussing a new charter, which
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 Dispatch Type Field bit
pattern 01000100 (in Pages 0 and 1) as SCHC 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.
Laurent Toutain, Pascal Thubert, Dominique Barthel, Guangpeng Li,
Carsten Bormann, and Nathan Lecorchet made comments that helped shape
this document.
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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.
12. References
12.1. Normative References
[I-D.ietf-intarea-schc-ip-protocol-number]
Moskowitz, R., Card, S. W., and A. Wiethuechter, "Internet
Protocol Number for SCHC", Work in Progress, Internet-
Draft, draft-ietf-intarea-schc-ip-protocol-number-00, 6
October 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-intarea-schc-ip-protocol-number-00>.
[I-D.ietf-lpwan-architecture]
Pelov, A., Thubert, P., and A. Minaburo, "LPWAN Static
Context Header Compression (SCHC) Architecture", Work in
Progress, Internet-Draft, draft-ietf-lpwan-architecture-
02, 30 June 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-lpwan-architecture-02>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<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, September 2007,
<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, September 2011,
<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, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
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[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams", RFC 6553,
DOI 10.17487/RFC6553, March 2012,
<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, March 2012,
<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, May 2012,
<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, June 2014,
<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, November 2016,
<https://www.rfc-editor.org/info/rfc8025>.
[RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation-
Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
February 2017, <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,
April 2017, <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,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
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[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, June 2021,
<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, November 2020,
<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, November 2020,
<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, April 2021,
<https://www.rfc-editor.org/info/rfc9008>.
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, March 2021,
<https://www.rfc-editor.org/info/rfc9006>.
Appendix A. Header compression examples
TO-DO: provide examples for IPv6-only, IPv6/UDP and IPv6/UDP/CoAP.
Authors' Addresses
Carles Gomez
UPC
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: carles.gomez@upc.edu
Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
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Email: ana@ackl.io
Flavien Moullec
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: flavien@ackl.io
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