Internet DRAFT - draft-ietf-6lo-schc-15dot4
draft-ietf-6lo-schc-15dot4
6lo Working Group C. Gomez
Internet-Draft UPC
Intended status: Standards Track A. Minaburo
Expires: 31 August 2024 Consultant
February 2024
Transmission of SCHC-compressed packets over IEEE 802.15.4 networks
draft-ietf-6lo-schc-15dot4-05
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
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Protocol stacks . . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Main protocol stack . . . . . . . . . . . . . . . . . 5
3.1.2. Transition protocol stacks . . . . . . . . . . . . . 6
3.2. Network topologies . . . . . . . . . . . . . . . . . . . 8
3.3. Single-hop communication . . . . . . . . . . . . . . . . 8
3.4. Multihop communication . . . . . . . . . . . . . . . . . 8
3.4.1. Straightforward Route-Over (SRO) . . . . . . . . . . 9
3.4.2. Tunneled, RPL-based Route-Over (TRO) . . . . . . . . 10
3.4.3. Pointer-based Route-Over (PRO) . . . . . . . . . . . 12
3.4.4. Mesh-Under . . . . . . . . . . . . . . . . . . . . . 14
4. Frame Format . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Single-hop or SRO frame format . . . . . . . . . . . . . 15
4.1.1. SCHC Dispatch . . . . . . . . . . . . . . . . . . . . 15
4.1.2. SCHC-compressed Header . . . . . . . . . . . . . . . 16
4.1.3. Padding . . . . . . . . . . . . . . . . . . . . . . . 16
4.2. TRO frame format . . . . . . . . . . . . . . . . . . . . 16
4.3. PRO frame format . . . . . . . . . . . . . . . . . . . . 18
4.4. Mesh-Under frame format . . . . . . . . . . . . . . . . . 20
4.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 21
5. Enabling the transition protocol stack . . . . . . . . . . . 22
6. SCHC compression for IPv6, UDP, and CoAP headers . . . . . . 23
6.1. SCHC compression for IPv6 and UDP headers . . . . . . . . 23
6.1.1. Compression of IPv6 addresses . . . . . . . . . . . . 24
6.1.2. UDP checksum field . . . . . . . . . . . . . . . . . 25
6.2. SCHC compression for CoAP headers . . . . . . . . . . . . 25
7. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . 25
8. Fragmentation and reassembly . . . . . . . . . . . . . . . . 26
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
12.1. Normative References . . . . . . . . . . . . . . . . . . 27
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12.2. Informative References . . . . . . . . . . . . . . . . . 29
Appendix A. Header compression examples . . . . . . . . . . . . 29
A.1. Single-hop or SRO frame format . . . . . . . . . . . . . 30
A.2. TRO frame format . . . . . . . . . . . . . . . . . . . . 31
A.3. PRO frame format . . . . . . . . . . . . . . . . . . . . 31
A.4. Mesh-Under frame format . . . . . . . . . . . . . . . . . 31
A.5. Enabling the transition protocol stack . . . . . . . . . 31
Appendix B. Analysis of route-over multihop approaches . . . . . 33
B.1. SRO . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
B.2. TRO . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
B.3. PRO . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
B.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
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
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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.
The mechanism to be used to provide the SCHC header compression
context to the nodes in an IEEE 802.15.4 network is out of the scope
of this document.
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.
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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 |
+------------+ +------------+
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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).
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+------------+
| 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 |
+------------+ +------------+ +------------+
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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. The mechanism to be used to provide
the SCHC header compression context to the nodes in an IEEE 802.15.4
network is out of the scope of this document.
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.
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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. In SRO, a RuleID MUST NOT be reused across disjoint
pairs of endpoints.
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.
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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 TRO, 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.
If all 6LNs in the 6LoWPAN network are RALs, 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 complexity. Else, RuleIDs MUST NOT be reused
across disjoint pairs of endpoints.
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)
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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
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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 compression/decompression Rules used by the
endpoints, 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).
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In PRO, a pointer (called "SCHC Pointer") is prepended to the SCHC-
compressed packet, in order to indicate the location and length of
the Hop Limit and the destination address residues in the SCHC-
compressed header. Therefore, a 6LR is able to determine the IPv6
destination address of a SCHC-compressed packet, decrement its Hop
Limit 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.
In PRO, a RuleID MAY be reused across disjoint pairs of endpoints.
To identify different Rules used by such disjoint pairs of endpoints,
the endpoint nodes store an additional identifier along with each
RuleID and its corresponding Rule. This identifier may be the IPv6
address of the other endpoint or a SCHC Header session ID
[I-D.ietf-schc-architecture].
Figure 7 illustrates the Rules that are stored by the nodes in an
example network based on PRO. Note that, in this example, the
network exploits the fact that PRO allows a given RuleID to be reused
by disjoint pairs of endpoints.
Host E
/
+--------+- Host F
(RuleID 3, AE) --- |Internet|
(RuleID 3, BF) / +--------+
6LBR -----------
/ \
/ \
6LR 6LR ------------+ Pair of endpoints
(no Rules)/| | (no Rules) | RuleID 1: A, B
/ | | | RuleID 2: A, C
/ | | | RuleID 2: D, B
/ | | | RuleID 3: A, E
Host D Host A Host B Host C RuleID 3: B, F
(RuleID 2) (RuleID 1) (RuleID 1) (RuleID 2)
(RuleID 2) (RuleID 2)
(RuleID 3) (RuleID 3)
Figure 7: In this example, RuleID 2 and RuleID 3 are used by two
disjoint pairs of endpoints.
PRO is compatible with RPL storing mode, as well as with other
routing protocols.
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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.
Figure 8 illustrates the Rules that need to be stored by the nodes
when SCHC is used for header compression in a Mesh-under network,
based on the same example network and endpoint pairs shown in
Figure 7. Note that, in this example, the network exploits the fact
that Mesh-under allows a given RuleID to be reused by disjoint pairs
of endpoints, even if the Rules sharing the same RuleID are
different. As in PRO, these Rules are distinguished by an
identifier, which may be the IPv6 address of the other endpoint or a
SCHC Header session ID [I-D.ietf-schc-architecture]. Nodes denoted
"m" in Figure 8 correspond to Mesh-Under forwarders [RFC 6606].
Host E
/
+--------+- Host F
(RuleID 3, AE) --- |Internet|
(RuleID 3, BF) / +--------+
6LBR -----------
/ \
/ \
m m --------------+ Pair of endpoints
(no Rules)/| | (no Rules) | RuleID 1: A, B
/ | | | RuleID 2: A, C
/ | | | RuleID 2: D, B
/ | | | RuleID 3: A, E
Host D Host A Host B Host C RuleID 3: B, F
(RuleID 2) (RuleID 1) (RuleID 1) (RuleID 2)
(RuleID 2) (RuleID 2)
(RuleID 3) (RuleID 3)
Figure 8: Rules stored by each node in an example network using
Mesh-Under. In this example, RuleID 2 and RuleID 3 are used by
disjoint pairs of endpoints.
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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 9 illustrates
the described frame format.
<---------- IEEE 802.15.4 frame payload ---------->
<------ SCHC Packet ----->
+---------------+--------------+---------+ - - - - +
| SCHC Dispatch | Cmprd Header | Payload | Padding |
+---------------+--------------+---------+ - - - - +
Figure 9: 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 9, 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))
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4.1.2. SCHC-compressed Header
The SCHC-compressed Header ("Cmprd Header" in Figure 9) corresponds
to a packet header that has been compressed by using SCHC. As
defined in [RFC8724], a SCHC-compressed 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 10:
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<----------------- IEEE 802.15.4 frame payload ---------------------->
<- SCHC pkt ->
+-- ... -+-- ... --+- ... -+--- ... --+---- ... -+-----+-------+ - - +
|11110001|SRH-6LoRH| RPI- | IP-in-IP | 01000100 |Cmprd|payload| pad |
|Page 1 | | 6LoRH | 6LoRH |SCHCDsptch| Hdr | | |
+-- ... -+-- ... --+- ... -+--- ... --+---- ... -+-----+-------+ - - +
(Page 1)
<----- This specification ----->
Figure 10: 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 11:
<-------------- IEEE 802.15.4 frame payload -------------->
<- SCHC pkt ->
+-- ... -+-- ... --+- ... -+---- ... -+-----+-------+ - - +
|11110001|SRH-6LoRH| RPI- | 01000100 |Cmprd|payload| pad |
|Page 1 | | 6LoRH |SCHCDsptch| Hdr | | |
+-- ... -+-- ... --+- ... -+---- ... -+-----+-------+ - - +
(Page 1)
<----- This specification ----->
Figure 11: 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 12 (note
that it does not include the source routing header that is present in
the Downward frame format):
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<------------- IEEE 802.15.4 frame payload ---------------->
<- SCHC pkt ->
+-- ... -+- ... -+--- ... --+---- ... -+-----+-------+ - - +
|11110001| RPI- | IP-in-IP | 01000100 |Cmprd|payload| pad |
|Page 1 | 6LoRH | 6LoRH |SCHCDsptch| Hdr | | |
+-- ... -+- ... -+--- ... --+---- ... -+-----+-------+ - - +
(Page 1)
<----- This specification ----->
Figure 12: 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 13:
<--------- IEEE 802.15.4 frame payload ---------->
<----- SCHC Packet ----->
+--------------+-------------+---------+ - - - - +
| PRO Header | Cmprd Header| Payload | Padding |
+--------------+-------------+---------+ - - - - +
v <->
| |
+---------------+
SCHC Pointer
Figure 13: frame format for SCHC-compressed packets in PRO.
The PRO Header format is shown in Figure 14:
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3
+---------------+-+ - - - +-------------+-+-------------+
| SCHC |C| | |H| |
| Pointer |I| DCI | Bit Pointer |L| Address |
| Dispatch |D| | |M| Length |
+---------------+-+ - - - +-------------+-+-------------+
Figure 14: PRO Header format.
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The first field in Figure 14 is defined as the SCHC Pointer Dispatch,
which signals the start of a PRO Header format. This document
defines the SCHC Pointer Dispatch as a 6LoWPAN Dispatch Type
[RFC4944] for SCHC header compression.
With the aim to minimize header 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 next field in the PRO Header is the Context IDentifier (CID)
flag, which is set to 1 to signal that the Destination Context
Identifier (DCI) field (see PRO_header_format) is present in the
frame. When CID is set to 0, the DCI field is not present.
The DCI field is optional. When present, it has a size of 4 bits.
Similarly to RFC 6282, this field identifies the prefix of the IPv6
destination address. How such prefix context is distributed and
maintained is out of the scope of the present document.
The Bit pointer gives the starting position of the Hop Limit followed
by the IPv6 destination address in the SCHC residue of the SCHC-
compressed IPv6 header (in bits), starting after the Address Length
field and before the first field of the SCHC-compressed IPv6 header
(i.e., the RuleID). For example, if the Hop Limit and the IPv6
destination address residue are the only residues 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.
The Hop Limit (HLM) flag is 1 bit that indicates the length of the
Hop Limit field residue in the SCHC-compressed IPv6 header. When HLM
equals 0, the Hop Limit compression residue has a size of 4 bits. In
this case, the 4 most significant bits of the uncompressed Hop Limit
field are equal to 0. Therefore, Hop Limit compression applies only
to Hop Limit values between 15 and 0. When HLM is set to 1, the Hop
Limit compression residue has a size of 8 bits (i.e., it is
uncompressed).
Address Length indicates the size of the IPv6 destination address
residue (in bits). It can be up to 128 bits to allow representing
the complete destination address, if needed.
PRO requires a special SCHC Rule design where the FIDs of the IPv6
Destination and Source addresses are swapped (see 6.1.1).
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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 15:
<-------------------- IEEE 802.15.4 frame payload ---------------------->
<----- SCHC Packet ----->
+-----------+-------------+---------------+-------------+---------+ - - +
| Mesh Type | Mesh Header | SCHC Dispatch | Cmprd Header| Payload | pad |
+-----------+-------------+---------------+-------------+---------+ - - +
Figure 15: 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 16:
<-------------------- IEEE 802.15.4 frame payload -------------------->
<---- SCHC Packet --->
+-------+-------+-------+-------+----------+-----------+---------+ - - +
| M Typ | M Hdr | F Typ | F Hdr | SCHC Dsp | Cmprd Hdr | Payload | Pad |
+-------+-------+-------+-------+----------+-----------+---------+ - - +
Figure 16: 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 17:
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<-------------------- IEEE 802.15.4 frame payload -------------------->
<---- SCHC Packet --->
+-------+-------+-------+-------+----------+-----------+---------+ - - +
| M Typ | M Hdr | B Dsp | B Hdr | SCHC Dsp | Cmprd Hdr | Payload | Pad |
+-------+-------+-------+-------+----------+-----------+---------+ - - +
Figure 17: 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 18:
+-------------+----------------------------------------------------------+
| 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 18: Summary of alternatives for the transmission of SCHC-
compressed packets over IEEE 802.15.4 enabled by the present
document, and corresponding artifacts
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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 19. 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 19: 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 20. 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 20: Protocol data unit encapsulation and format for the
transition protocol stack using a SCHC Internet Protocol Number
and RFC 8138-compressed routing artifacts
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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-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.
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RFC 8724 (Section 7.1) states that "In a Rule, the Field Descriptors
are listed in the order in which the fields appear in the packet
header". The present specification updates RFC 8724 to state that,
in order to allow IPv6 header compression in PRO, the Field
Descriptors of the IPv6 destination address (i.e., IPv6 DevPrefix and
IPv6 DevIID) MUST appear before the Field Descriptors of the IPv6
source address (i.e., IPv6 AppPrefix and IPv6 AppIID), while the rest
of fields appear in the same order as in the IPv6 packet header.
In PRO, in order to support IPv6 header compression, one Rule MUST be
defined for each direction between the two involved C/D endpoints.
In such a Rule, the IPv6 DevPrefix and IPv6 DevIID FIDs MUST refer to
the destination address (i.e., the destination endpoint takes the
"Dev" role) of the SCHC-compressed IPv6 header. This allows a 6LR to
read the compression residue of the Hop Limit and IPv6 destination
address fields of the SCHC-compressed header by means of the Bit
Pointer.
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.
RFC 8724 states that "If the Rule is intended to compress packets
with different prefix values, match-mapping SHOULD be used"
(Section 10.7.1 of RFC 8724) and "If several IIDs are possible, then
the TV contains the list of possible IIDs, the MO is set to "match-
mapping" and the CDA is set to "mapping-sent"" (Section 10.7.2 of RFC
8724). However, in PRO, a source node MUST NOT use the match-mapping
operator or the "mapping-sent" CDA to compress the IPv6 destination
address prefix or the IPv6 destination IID, because 6LRs do not store
SCHC context, and therefore do not have the match-mapping index
meaning information.
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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.)
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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.
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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 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-
01, 12 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
schc-protocol-numbers-01>.
[I-D.ietf-schc-architecture]
Pelov, A., Thubert, P., and A. Minaburo, "Static Context
Header Compression (SCHC) Architecture", Work in Progress,
Internet-Draft, draft-ietf-schc-architecture-01, 6 October
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
schc-architecture-01>.
[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>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
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[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>.
[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>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<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, March 2021,
<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
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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 21.
+----------------+--+--+--+-------------+------+----------++------+
| 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 21: 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
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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
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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 22.
+----------------+--+--+--+-------------+------+----------++------+
| 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 22: 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
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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.
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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.
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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
Email: carles.gomez@upc.edu
Ana Minaburo
Consultant
Rue de Rennes
35510 Cesson-Sevigne
France
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Email: anaminaburo@gmail.com
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