Static Context Header Compression (SCHC) Architecture
draft-ietf-schc-architecture-04
| Document | Type | Active Internet-Draft (schc WG) | |
|---|---|---|---|
| Authors | Alexander Pelov , Pascal Thubert , Ana Minaburo | ||
| Last updated | 2025-02-06 | ||
| Replaces | draft-ietf-lpwan-architecture | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
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| Send notices to | (None) |
draft-ietf-schc-architecture-04
SCHC Working Group A. Pelov
Internet-Draft IMT Atlantique
Intended status: Informational P. Thubert
Expires: 10 August 2025
A. Minaburo
Consultant
6 February 2025
Static Context Header Compression (SCHC) Architecture
draft-ietf-schc-architecture-04
Abstract
This document defines the SCHC architecture.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 10 August 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Building Blocks . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. SCHC Stratum (plural: strata) . . . . . . . . . . . . . . 4
4.2. Discriminator . . . . . . . . . . . . . . . . . . . . . . 5
4.3. SCHC Stratum Header end point . . . . . . . . . . . . . . 6
4.3.1. SCHC Stratum Header . . . . . . . . . . . . . . . . . 7
4.4. SCHC Payload end point . . . . . . . . . . . . . . . . . 8
4.4.1. SCHC Payload . . . . . . . . . . . . . . . . . . . . 9
4.5. SCHC Profiles . . . . . . . . . . . . . . . . . . . . . . 10
4.6. SCHC Operation . . . . . . . . . . . . . . . . . . . . . 10
4.6.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . 11
4.6.2. SoR identification . . . . . . . . . . . . . . . . . 11
4.7. SCHC Management . . . . . . . . . . . . . . . . . . . . . 11
4.7.1. SCHC Instance Manager . . . . . . . . . . . . . . . . 12
4.7.2. SCHC Data Model . . . . . . . . . . . . . . . . . . . 12
5. SCHC Architecture . . . . . . . . . . . . . . . . . . . . . . 14
6. The Static Context Header Compression . . . . . . . . . . . . 17
6.1. SCHC over Network Technologies . . . . . . . . . . . . . 18
6.1.1. SCHC over PPP . . . . . . . . . . . . . . . . . . . . 18
6.1.2. SCHC over Ethernet . . . . . . . . . . . . . . . . . 19
6.1.3. SCHC over IPv6 . . . . . . . . . . . . . . . . . . . 19
6.1.4. SCHC over UDP . . . . . . . . . . . . . . . . . . . . 20
7. SCHC Endpoints for LPWAN Networks . . . . . . . . . . . . . . 21
7.1. SCHC Device Lifecycle . . . . . . . . . . . . . . . . . . 21
7.1.1. Device Development . . . . . . . . . . . . . . . . . 21
7.1.2. Rules Publication . . . . . . . . . . . . . . . . . . 22
7.1.3. SCHC Device Deployment . . . . . . . . . . . . . . . 22
7.1.4. SCHC Device Maintenance . . . . . . . . . . . . . . . 23
7.1.5. SCHC Device Decommissionning . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
9. IANA Consideration . . . . . . . . . . . . . . . . . . . . . 23
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
11.1. Normative References . . . . . . . . . . . . . . . . . . 23
11.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The IETF LPWAN WG defined the necessary operations to enable IPv6
over selected Low-Power Wide Area Networking (LPWAN) radio
technologies. [rfc8376] presents an overview of those technologies.
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The Static Context Header Compression (SCHC) [rfc8724] technology is
the core product of the IETF LPWAN working group and was the basis to
form the SCHC Working Group. [rfc8724] defines a generic framework
for header compression and fragmentation, based on a static context
that is pre-installed on the SCHC endpoints.
This document details the constitutive elements of a SCHC-based
solution, and how the solution can be deployed. It provides a
general architecture for a SCHC deployment, positioning the required
specifications, describing the possible deployment types, and
indicating models whereby the rules can be distributed and installed
to enable reliable and scalable operations.
2. 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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
* C/D. Compression and Decompression.
* Context. All the information related to the Rules for SCHC
Header, Non-Compression, C/D and F/R and CORECONF_Management.
* FID. Field Identifiers, describing the name of the field in a
protocol header.
* F/R. Fragmentation and Reassembly.
* Rule. A description of the header fields to performs compression/
decompression, fragmentation/reassembly, SCHC end points and
CORECONF_Management.
* SCHC Gateway (end point). The SCHC end point located upstream,
e.g., in a Network Core Software.
* SCHC Device (end point). The SCHC end point located downstream,
e.g., in a constrained physical device.
* SCHC end point. An entity (e.g., Device, Application and Network
Gateway) involved in the SCHC process. Each SCHC end point will
have its Set of Rules (SoR), based on the profile, the protocols,
the device, the behaviour and a Set of Variables (SoV).
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* SCHC Instance. The session between SCHC end points in two or more
peer nodes operating SCHC to communicate using a common SoR and a
matching SoV. There are 2 SCHC Instances or more involved per
SCHC stratum, one for the SCHC Stratum Header and one or more for
the SCHC payload, i.e., the SCHC-compressed data.
* SCHC Instance Manager. Provides the management of SCHC end
points, the SoR of each end point and the dialog between hosts to
keep the SCHC synchronization, and the establishment of SCHC
Instances with peer nodes.
* SoR (Set of rules). Group of Rules used in a SCHC end point. The
set of rules contains Rules for different nature as compression,
no compression, fragmentation, SCHC end points and CORECONF
management.
* SoV (Set of Variables). External information that needs to be
known to identify the correct protocol, the SCHC Instance id, and
the flow when there is one.
* SCHC Stratum. A SCHC Stratum is the SCHC analoguous to a
classical layer in the IP architecture, but its operation may
cover multiple IP layers or only a subset of a layer.
4. Building Blocks
This section specifies the principal blocks defined for building and
using the SCHC architecture in any network topology and protocol.
4.1. SCHC Stratum (plural: strata)
A SCHC Stratum is the SCHC analoguous to a classical layer in the IP
architecture, but its operation may cover multiple IP layers or only
a subset of a layer, e.g., IP only, IP+UDP, CoAP, or OSCORE
[rfc8824]. The term stratum is thus used to avoid confusion with
traditional layers. Also, SCHC Strata are not stacked, though they
can be nested.
The SCHC Stratum data in a datagram is composed of a SCHC Stratum
Header (which may be compressed to the point that it is fully
implicit and thus elided), a SCHC payload (that is used to uncompress
a section of the SCHC datagram), and user payload that is unaffected
by the SCHC Stratum. The SCHC Stratum operation requires at least 2
end points, one for the SCHC Stratum Header and one or more for the
SCHC payload.
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A SCHC compressed packet may contain multiple stratum data, to be
handled by sequential (nested) SCHC Strata, where the inner (nested)
Stratum operates within the payload of the outter (nesting) Stratum.
A SCHC Stratum is instantiated in participating nodes as a pair of
SCHC end points, and matching SCHC end points in communicating nodes
are associated to form a SCHC end point. A SCHC end point may be
Point to point (P2P), or Point to Multipoint. A P2MP SCHC end point
is unidirectional, meaning that all the SCHC datagrams are generated
by the same node. A P2P SCHC end point may be unidirectional or
bidirectional, symmetrical (between peers) or asymetrical (between a
device and an application).
A SCHC end point operates datagram fragmentation and/or data
compression and decompression, and maintains the state and timers
associated with the Stratum operation over the consecutive packets or
fragments.
The SCHC end points that handle the compression for nested Strata
might differ for the same packet, meaning that the payload of a given
Stratum might be compressed/uncompressed by a different entity,
possibly in a different node. It results that the degree of
compression (the number of Strata) for a given packet may vary as the
packet progresses through the layers inside a node and then through
the network.
4.2. Discriminator
The key to determine how to decompress a SCHC Stratum Header in a
stratum is called a Discriminator.
The Discriminator is typically extrinsic to the stratum data.
It may be found in the packet context, e.g., the ID of the interface,
VLAN, SSID, or PPP SCHC Instance on which the packet is received.
It may also be received in the packet, natively or uncompressed from
a nesting stratum, e.g.:
* A source and destination MAC or IP addresses of the packet
carrying SCHC packets
* A source and destination port number of the transport layer
carrying SCHC packets
* A next header field
* An MPLS label
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* A TLS Association
* Any other kind of connection id.
The Discriminator enables to determine the SCHC end point that is
used to decompress the SCHC Stratum Header, called a SCHC Stratum
Header end point.
Once uncompressed, the SCHC Stratum Header enables to determine the
SCHC end point, called a SCHC Payload end point, that is used to
restore the packet data that is compressed in the stratum.
4.3. SCHC Stratum Header end point
The SCHC Stratum Header end point manages the SCHC Stratum Headers
and provides the information and the selection of a SCHC Payload end
point.
The rules for that end point might be such that all the fields in the
SCHC Stratum Header are well-known, in which case the header is fully
elided in the stratum data and recreated from the rules.
The rules might also leverage intrinsic data that is found in-line in
the stratum data, in which case the first bits of the stratum data
are effectively residue to the compression of the SCHC Stratum
Header. Finally, the rules may leverage extrinsic data as the
Discriminator does.
Figure 1 illustrates the case where a given stratum may compress
multiple protocols SCHC Instances, each corresponding to a different
SCHC Payload end point.
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+---------------+---------------+---------------+ S
| SCHC Payload | SCHC Payload | SCHC Payload | C
| end point___ | end point___ | end point___ | H
| [SoR] | [SoR] | [SoR] | C
| [___] | [___] | [___] |
| | | | S
| | | | T
+----inst_id1---+----inst_id2---+----inst_id3---+ R
. SCHC Stratum end point ___ . A
. [SoR] . T
. [___] . U
+...............................................+ M
_____________^
/
/
+-- Discriminator: (SCHC Stratum Header)(SCHC PAYLOAD)
Each SCHC Payload End point uses its own Set of Rules,
but share the same SCHC Stratum Header.
Figure 1: SCHC end points for a stratum
4.3.1. SCHC Stratum Header
The SCHC Stratum Header carries information that is required for the
SCHC strata operation. For example, it selects the correct end point
and checks the validity of the datagram. There IS NOT always a
RuleID if there is only one Rule for the SCHC Stratum Header, whose
length is 0. The SCHC Stratum Header format is not fixed, and the
SoR MUST have one or more Rules describing the formats. SCHC Stratum
Header contains different fields. For end point, when the SCHC
Stratum Header may identify the next protocol in the stack, the
format of the SCHC Stratum Header takes the format as Figure 2 shows.
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Non-compressed SCHC Stratum Header Format:
+- - - - - - +- - - - - - -+- - -+
| SCHC Instance ID | Protocol ID | CRC |
+- - - - - - +- - - - - - -+- - -+
SCHC Stratum Header Compressed:
+- - - - -+- - - - - - - - - - +
| Rule ID | Compressed Residue |
+- - - - -+- - - - - - - - - - +
Rule uses to compressed the SCHC Stratum Header:
RuleID
+------------+--+---+--+-----+------+-----------+
| FID |FL|POS|DI| TV | MO | CDA |
+------------+--+---+--+-----+------+-----------+
| SCHC.sesid |10| 1 |Bi|0x00 |MSB(7)| LSB |
| SCHC.proto | 8| 1 |Bi|value|equal | not-sent |
| SCHC.CRC | 8| 1 |Bi| |ignore| value-sent|
+------------+--+---+--+-----+------+-----------+
Figure 2: Example of SCHC Stratum Header Format and the
corresponding Rule
In this example the Rule defines:
* A SCHC InstanceID is 10 bits length and it is used to identify the
SoR used for this end point of SCHC.
* A Protocol ID in 1-byte length giving the value send in the layer
below the SCHC packet to identify the uncompressed protocol stack.
* And A CRC. The CRC field is 8 bits length and covers the SCHC
Stratum Header and the SCHC packet from error. When it is elided
by the compression, the layer-4 checksum MUST be replaced by
another validation sequence.
4.4. SCHC Payload end point
SCHC Payload end point is characterized by a particular SoR common
with the corresponding distant end point. The [rfc8724] defines a
protocol operation between a pair of peers. In a SCHC strata,
several SCHC end points may contain different SoR.
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When the SCHC Device is a highly constrained unit, there is typically
only one end point for that Device, and all the traffic from and to
the device is exchanged with the same Network Gateway. All the
traffic can thus be implicitly associated with the single end point
that the device supports, and the Device does not need to manipulate
the concept. For that reason, SCHC avoids to signal explicitly the
end point identification in its data packets.
The Network Gateway, on the other hand, maintains multiple end
points, one per SCHC Device. The end point is derived from the lower
layer, typically the source of an incoming SCHC packet as a
discriminator in the Figure 1. The end point is used in particular
to select the set of rules that apply to the SCHC Device, and the
current state of their exchange, e.g., timers and previous fragments.
4.4.1. SCHC Payload
When compressed, the SCHC Payload is composed of a RuleID followed by
the content described in the Rule. The content may be a C/D packet,
a F/R packet, a CORECONF_Management or a Non Compressed packet. As
defined in the [rfc8724], the SCHC packet for C/D is composed of the
Compressed Header followed by the payload from the original packet.
Figure 3 shows the compressed header format that is composed of the
RuledID and a Compressed Residue, which is the output of compressing
a packet header with a Rule.
C/D Compressed Packet:
+------------+----------------------+
| RuleID | Compressed Residue |
+------------+----------------------+
F/R Compressed Packet:
+------------+----------------------+-----
| RuleID | Fragmentation Header | Tiles
+------------+----------------------+-----
CORECONF_Management Compressed Packet:
+------------+----------------------+
| RuleID | Compressed Residue |
+------------+----------------------+
Figure 3: SCHC Packet
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4.5. SCHC Profiles
A SCHC profile is the specification to adapt the use of SCHC with the
necessities of the technology to which it is applied. In the case of
star topologies and because LPWAN technologies [rfc8376] have strict
yet distinct constraints, e.g., in terms of maximum frame size,
throughput, and directionality, also a SCHC end point and the
fragmentation model with the parameters' values for its use.
Appendix D. "SCHC Parameters" of [rfc8724] lists the information
that an LPWAN technology-specific document must provide to profile
SCHC fragmentation for that technology.
As an example, [rfc9011] provides the SCHC fragmentation profile for
LoRaWAN networks.
4.6. SCHC Operation
The SCHC operation requires a shared sense of which SCHC Device is
Uplink (Dev to App) and which is Downlink (App to Dev), see
[rfc8376]. In a star deployment, the hub is always considered Uplink
and the spokes are Downlink. The expectation is that the hub and
spoke derive knowledge of their role from the network configuration
and SCHC does not need to signal which is hub thus Uplink vs. which
is spoke thus Downlink. In other words, the link direction is
determined from extrinsic properties, and is not advertised in the
protocol.
Nevertheless, SCHC is very generic and its applicability is not
limited to star-oriented deployments and/or to use cases where
applications are very static and the state provisioned in advance.
In particular, a peer-to-peer (P2P) SCHC end point (see Section 4.4)
may be set up between peers of equivalent capabilities, and the link
direction cannot be inferred, either from the network topology nor
from the device capability.
In that case, by convention, the device that initiates the connection
that sustains the SCHC end point is considered as being Downlink,
i.e. it plays the role of the Dev in [rfc8724].
This convention can be reversed, e.g., by configuration, but for
proper SCHC operation, it is required that the method used ensures
that both ends are aware of their role, and then again this
determination is based on extrinsic properties.
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4.6.1. SCHC Rules
SCHC Rules are a description of the header protocols fields, into a
list of Field Descriptors. The [rfc8724] gives the format of the
Rule description for C/D, F/R and non-compression. In the same
manner the SCHC Stratum Header and SCHc CORECONF_Management will use
the [rfc8724] field descriptors to compress the format information.
Each type of Rule is identified with a RuleID. There are different
types of Rules: C/D, F/R, SCHC Stratum Header, CORECONF_Management
and No Compression. Notice that each Rule type used an independent
range of RuleID to identify its rules.
A Rule does not describe how the compressor parses a packet header.
Rules only describe the behavior for each header field.
SCHC Action. ToDo
4.6.2. SoR identification
ToDo
4.7. SCHC Management
RFC9363 writes that only the management can be done by the two
entities of the end point, and other SoR cannot be manipulated.
Management rules are explicitly define in the SoR, see Figure 4.
They are compression Rules for CORECONF messages to get or modify the
SoR of the end point. The management can be limited with the
[I-D.ietf-schc-access-control] access definition.
+-----------------+ +-----------------+
| ^ | | ^ |
| C/D ! M ___ | | ! M ___ |
| +-->[SoR] | ... | +-->[SoR] |
| ! [___] | | ! [___] |
| ! | | ! |
| F/R | | F/R |
+------ins_id1----+-----ins_idi-----+------ins_idn----+
. C/D ! ___ .
. +--------------------->[SoR] .
. F/R M [___] .
+.................. Discriminator ....................+
Figure 4: Inband Management
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4.7.1. SCHC Instance Manager
The SCHC Instance Manager provides the management of SCHC end points,
the SoR of each end point and the dialog between hosts to keep the
SCHC synchronization, and the establishment of SCHC Instances with
peer nodes. Changes that involve the SoR must be transactional, in a
way that ensures that the compression and decompression of a packet
is done with the same SoR on every end points.
The management of the SCHC end points includes the capability for the
end-points to modify the common SoR, by:
* modifyng rules values (such as TV, MO or CDA) in existing rules,
* adding or
* removing rules.
The rule management uses the CORECONF interface based on CoAP. The
management traffic is carried as SCHC compressed packets tagged to
some specific rule IDs.
4.7.2. SCHC Data Model
A SCHC end point, summarized in the Figure 5, implies C/D and/or F/R
and CORECONF_Management and SCHC end points Rules present in both end
and that both ends are provisioned with the same SoR.
----- -----
[ SoR ] [ SoR ]
----- -----
. .
. .
. .
+- M ---+ +- M ---+
<===| R & D |<=== <===| C & F |<===
===>| C & F |===> ===>| R & D |===>
+-------+ +-------+
Figure 5: Summarized SCHC elements
A common rule representation that expresses the SCHC rules in an
interoperable fashion is needed to be able to provision end-points
from different vendors to that effect, [rfc9363] defines a rule
representation using the YANG [rfc7950] formalism.
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[rfc9363] defines a YANG data model to represent the rules. This
enables the use of several protocols for rule management, such as
NETCONF[RFC6241], RESTCONF[RFC8040], and
CORECONF[I-D.ietf-core-comi]. NETCONF uses SSH, RESTCONF uses HTTPS,
and CORECONF uses CoAP(s) as their respective transport layer
protocols. The data is represented in XML under NETCONF, in
JSON[RFC8259] under RESTCONF and in CBOR[RFC8949] under CORECONF.
create
----- read +=======+
[ SoR ]<------->|Rule |<-----+ NETCONF,
----- update |Manager| | RESTCONF or
delete +=======+ | CORECONF
+--------------------------+ request
|
v M
+---+---+
<===| R & D |<===
===>| C & F |===>
+-------+
Figure 6: Summerized SCHC elements
The Rule Manager (RM) is in charge of handling data derived from the
YANG Data Model and apply changes to the context and SoR of each SCHC
end point Figure 6.
The RM is an Application using the Internet to exchange information,
therefore:
* for the network-level SCHC, the communication does not require
routing. Each of the end-points having an RM and both RMs can be
viewed on the same link, therefore wellknown Link Local addresses
can be used to identify the Device and the core RM. L2 security
MAY be deemed as sufficient, if it provides the necessary level of
protection.
* for application-level SCHC, routing is involved and global IP
addresses SHOULD be used. End-to-end encryption is RECOMMENDED.
Management messages can also be carried in the negotiation protocol,
for instace, the [I-D.ietf-schc-over-ppp] proposes a solution. The
RM traffic may be itself compressed by SCHC: if CORECONF protocol is
used, [rfc8824] can be applied.
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5. SCHC Architecture
As described in [rfc8824], SCHC combining several SCHC end points.
The [rfc8724] states that a SCHC end point needs the rules to process
C/D and F/R before the SCHC Instance starts and that the SoR of the
end point control layer cannot be modified. However, the rules may
be updated in certain end points to improve the performance of C/D,
F/R, or CORECONF_Management. The [I-D.ietf-schc-access-control]
defines the possible modifications and who can modify, update, create
and delete Rules or part of them in the end points' SoR.
As represented in Figure 7, the compression of the IP and UDP headers
may be operated by a network SCHC end point whereas the end-to-end
compression of the application payload happens between the Device and
the application. The compression of the application payload may be
split in two end points to deal with the encrypted portion of the
application PDU. Fragmentation applies before LPWAN transmission
layer.
(Device) (NGW) (App)
+--------+ +--------+
A S | CoAP | | CoAP |
p C | inner | | inner |
p H +--------+ +--------+
. C | SCHC | | SCHC |
| inner | cryptographical boundary | inner |
-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-.._.-._.-._.-._.-._
A S | CoAP | | CoAP |
p C | outer | | outer |
p H +--------+ +--------+
. C | SCHC | | SCHC |
| outer | layer / functional boundary | outer |
-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.--._.-._.-._.-._
N . UDP . . UDP .
e .......... .................. ..........
t . IPv6 . . IPv6 . . IPv6 .
w S .......... .................. ..........
o C .SCHC/L3 . . SCHC/L3. . . .
r H .......... .......... . . .
k C . LPWAN . . LPWAN . . . .
.......... .................. ..........
((((LPWAN)))) ------ Internet ------
Figure 7: Different SCHC end points in a global system
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This document defines a generic architecture for SCHC that can be
used at any of these levels. The goal of the architectural document
is to orchestrate the different protocols and data model defined by
the LPWAN and SCHC working groups to design an operational and
interoperable framework for allowing IP application over constrained
networks.
The Figure 8 shows the protocol stack and the corresponding SCHC
stratas enabling the compression of the different protocol headers.
The SCHC Stratum Header eases the introduction of intermediary host
in the end-to-end communication transparently. All the SCHC Stratum
Headers are compressed and in some cases are elided, for example for
LPWAN networks. The layers using encryption does not have a SCHC
Stratum Header in the middle because they are the same entity.
Figure 9 shows an example of an IP/UDP/CoAP in an LPWAN network.
DEV NGW APP
{[(Encrypted Application Layer)]} . . . . . . . . {[(EAL)]}
(Application Layer Protocol) . . . . . . . . . . .({[ALP]})
(SCHC) . . . . . . . . . . . . . . . . . . . . . ({[SCHC]})
{[(Encrypted Security Layer)]} . . . . . . . . . .{[(ESL)]}
{(Security Layer Protocol)}. . . . . . . . . . . . .{(SLP)}
{(SCHC)} . . . . . . . . . . . . . . . . . . . . . {(SCHC)}
(Transport Layer Protocol). . . (TLP) TLP . . . . . .TLP
{(SCHC)} . . . . . . . . . . {(SCHC)}
(Internet Layer Protocol) . . . (IP) IP . . . . . . IP
(SCHC). . . . . . . . . . . . .(SCHC)
Network Layer Protocol . . . . . . . . . . . . . . . NLP
Where: {} Optional; [] Encrypted; () Compressed.
Figure 8: SCHC Architecture
In Figure 8, each line represents a layer or a stratum, parentheses
surround a compressed header, and if it is optional, it has curly
brackets. All the SCHC strata are compressed. Square brackets
represent the encrypted data; if the encryption is optional, curly
brackets precede the square brackets.
Figure 9 represents the stack of SCHC end points that operate over 3
strata, one for OSCORE, one for CoAP, and one for IP and UDP.
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+--------------------------OSCORE-------------------------+
| +-----------------+ +-----------------+ |
| | ^ | | ^ | |
| | C/D ! M ___ | | ! M ___ | |
S | | +-->[SoR] | ... | +-->[SoR] | |
C | | ! [___] | | ! [___] | |
H | | ! | | ! | |
C | | F/R | | F/R | |
| +------ins_id1----+-----ins_idi-----+------ins_idn----+ |
| | C/D ! (OSCORE) ___ | |
| | +--------------------->[SoR] | |
| | F/R M [___] | |
+------- Discriminator: IP:A->B/UDP, prot = OSCORE--------+
IP/UDP,port=CoAP CoAP ( ) (OSCORE)
^ _____^ ^
| / |
| (SCHC Stratum Header)( SCHC-compressed data)
|
+-------- | ---------------CoAP---------------------------+
| +-----------------+ +-----------------+ |
| | ^ | | ^ | |
| | C/D ! M ___ | | ! M ___ | |
S | | +-->[SoR] | ... | +-->[SoR] | |
C | | ! [___] | | ! [___] | |
H | | ! | | ! | |
C | | F/R | | F/R | |
| +------ins_id1----+-----ins_idi-----+------ins_idn----+ |
| | C/D ! (CoAP) ___ | |
| | +--------------------->[SoR] | |
| | F/R M [___] | |
+------- Discriminator: IP:A->B/UDP port=SCHC -----------+
IP/UDP ( ) (CoAP) PAYLOAD2
^ ^ ^_____________
| | \
| +-(SCHC Stratum Header)( SCHC-compressed data)
|
+-------- | --------------IP/UDP--------------------------+
| +------ | --------+ +-----------------+ |
| | | | | ^ | |
| | C/D ! M ___ | | ! M ___ | |
S | | +-->[SoR] | ... | +-->[SoR] | |
C | | ! [___] | | ! [___] | |
H | | ! | | ! | |
C | | F/R | | F/R | |
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| +------ins_id1----+-----ins_idi-----+------ins_idn----+ |
| | C/D ! (IP/UDP) | |
| | +--------------------->[SoR] | |
| | F/R M [___] | |
+-+-----------Discriminator: interface ID -------+-+
N ______________^
E /
T | ( ) (IP/UDP) PAYLOAD1
W | ^ ^_______
| | \
+-(SCHC Stratum Header)( SCHC-compressed data)
Figure 9: SCHC Strata Example
6. The Static Context Header Compression
SCHC [rfc8724] specifies an extreme compression capability based on a
description that must match on the compressor and decompressor side.
This description comprises a set of Compression/Decompression (C/D)
rules.
The SCHC Parser analyzes incoming packets and creates a list of
fields that it matches against the compression rules. The rule that
matches is used to compress the packet, and the rule identifier
(RuleID) is transmitted together with the compression residue to the
decompressor. Based on the RuleID and the residue, the decompressor
can rebuild the original packet and forward it in its uncompressed
form over the Internet. When no Rule matches the header, the No
Compression Rule is used. When several Rules match the header the
implementation must choose one. How it is done or based on which
parameters is out of the scope of this document. SCHC compresses
datagrams and there is no notion of flows.
[rfc8724] also provides a Fragmentation/Reassembly (F/R) capability
to cope with the maximum and/or variable frame size of a Link, which
is extremely constrained in the case of an LPWAN network.
If a SCHC-compressed packet is too large to be sent in a single Link-
Layer PDU, the SCHC fragmentation can be applied on the compressed
packet. The process of SCHC fragmentation is similar to that of
compression; the fragmentation rules that are programmed for this
Device are checked to find the most appropriate one, regarding the
SCHC packet size, the link error rate, and the reliability level
required by the application.
The ruleID allows to determine if it is a compression or
fragmentation rule or any other type of Rule.
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6.1. SCHC over Network Technologies
SCHC can be used in multiple environments and multiple protocols. It
was designed by default to work on native MAC frames with LPWAN
technologies such as LoRaWAN[rfc9011], IEEE std 802.15.4
[I-D.ietf-6lo-schc-15dot4], and SigFox[rfc9442].
To operate SCHC over Ethernet, IPv6, and UDP, the definition of,
respectively, an Ethertype, an IP Protocol Number, and a UDP Port
Number are necessary, more in
[I-D.ietf-intarea-schc-protocol-numbers]. In either case, there's a
need for a SCHC Stratum Header that is sufficient to identify the
SCHC peers (endpoints) and their role (device vs. app), as well as
the SCHC Instance between those peers that the packet pertains to.
In either of the above cases, the expectation is that the SCHC
Stratum Header is transferred in a compressed form. This implies
that the rules to uncompress the header are well known and separate
from the rules that are used to uncompress the SCHC payload. The
expectation is that for each stratum, the format of the SCHC Stratum
Header and the compression rules are well known, with enough
information to identify the SCHC Instance at that stratum, but there
is no expectation that they are the same across strata.
6.1.1. SCHC over PPP
The LPWAN architecture (Figure 14) generalizes the model to any kind
of peers. In the case of more capable devices, a SCHC Device may
maintain more than one end point with the same peer, or a set of
different peers. Since SCHC does not signal the end point in its
packets, the information must be derived from a lower layer point to
point information. For end point, the SCHC end point control can be
associated one-to-one with a tunnel, a TLS SCHC Instance, or a TCP or
a PPP connection.
For end point, [I-D.ietf-schc-over-ppp] describes a type of
deployment where the C/D and/or F/R operations are performed between
peers of equal capabilities over a PPP [rfc2516] connection. SCHC
over PPP illustrates that with SCHC, the protocols that are
compressed can be discovered dynamically and the rules can be fetched
on-demand using CORECONF messages Rules, ensuring that the peers use
the exact same set of rules.
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+----------+ Wi-Fi / +----------+ ....
| IP | Ethernet | IP | .. )
| Host +-----/------+ Router +----------( Internet )
| SCHC C/D | Serial | SCHC C/D | ( )
+----------+ +----------+ ...
<-- SCHC -->
over PPP
Figure 10: PPP-based SCHC Deployment
In that case, the SCHC end point is derived from the PPP connection.
This means that there can be only one end point per PPP connection,
and that all the flow and only the flow of that end point is
exchanged within the PPP connection. As discussed in Section 7, the
Uplink direction is from the node that initiated the PPP connection
to the node that accepted it.
6.1.2. SCHC over Ethernet
Before the SCHC compression takes place, the SCHC Stratum Header
showed in the Figure 11, is virtually inserted before the real
protocol header and data that are compressed in the SCHC Instance,
e.g. a IPv6 in this figure.
|---- SCHC PACKET ----|
+------------------+------------------+---------+-----------+
| IEEE 802 Header | SCHC Stratum Hdr | Rule ID | Compressed|
| Ethertype = SCHC | Ethertype = IPv6 | | Residue |
+------------------+------------------+---------+-----------+
<-
SCHC overhead
->
Figure 11: SCHC over Ethernet
6.1.3. SCHC over IPv6
In the case of IPv6, the expectation is that the Upper Layer Protocol
(ULP) checksum can be elided in the SCHC compression of the ULP,
because the SCHC Stratum Header may have its own checksum that
protects both the SCHC Stratum Header and the whole ULP, header and
payload.
The SCHC Stratum Header between IPv6 and the ULP is not needed
because of the Next Header field on the IPv6 header format.
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|---- SCHC Packet -----|
+-------------+-------------+---------+------------+
| IPv6 Header | SCHC Str Hdr| Rule ID | Compressed |
| NH=SCHC | NH = ULP | | Residue |
+-------------+-------------+---------+------------+
<-
SCHC overhead
->
Figure 12: SCHC over IPv6
In the air, both the SCHC Stratum Header and the ULP are compressed.
The SCHC Instance endpoints are typically identified by the source
and destination IP addresses. If the roles are well-known, then the
endpoint information can be elided and deduced from the IP header.
If there is only one SCHC Instance, it can be elided as well,
otherwise a rule and residue are needed to extract the SCHC Instance
ID.
6.1.4. SCHC over UDP
When SCHC operates over the Internet, middleboxes may block packets
with a next header that is SCHC. To avoid that issue, it would be
desirable to prepend a UDP header before the SCHC Stratum Header as
shown in figure Figure 13.
|---- SCHC Packet -----|
+-------------+-------------+-------------+---------+------------+
| IPv6 Header | UDP Header | SCHC Str Hdr| Rule ID | Compressed |
| NH=UDP | Port = SCHC | NH = ULP | | Residue |
+-------------+-------------+-------------+---------+------------+
<-
SCHC overhead
->
~
Figure 13: SCHC over UDP
In that case, the destination port can indicate SCHC as in an header
chain, and the source port can indicate the SCHC Instance in which
case it can be elided in the compressed form of the SCHC Stratum
Header. The UDP checksum protects both the SCHC Stratum Header and
the whole ULP, so the SCHC and the ULP checksums can both be elided.
In other words, in the SCHC over UDP case, the SCHC Stratum Header
can be fully elided, but the packet must carry the overhead of a full
UDP header.
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7. SCHC Endpoints for LPWAN Networks
Section 3 of [rfc8724] depicts a typical network architecture for an
LPWAN network, simplified from that shown in [rfc8376] and reproduced
in Figure 14.
() () () |
() () () () / \ +---------+
() () () () () () / \======| ^ | +-----------+
() () () | | <--|--> | |Application|
() () () () / \==========| v |=============| Server |
() () () / \ +---------+ +-----------+
Dev RGWs NGW App
Figure 14: Typical LPWAN Network Architecture
Typically, an LPWAN network topology is star-oriented, which means
that all packets between the same source-destination pair follow the
same path from/to a central point. In that model, highly constrained
Devices (Dev) exchange information with LPWAN Application Servers
(App) through a central Network Gateway (NGW), which can be powered
and is typically a lot less constrained than the Devices. Because
Devices embed built-in applications, the traffic flows to be
compressed are known in advance and the location of the C/D and F/R
functions (e.g., at the Dev and NGW), and the associated rules, can
be pre provisioned in the system before use.
7.1. SCHC Device Lifecycle
In the context of LPWANs, the expectation is that SCHC rules are
associated with a physical device that is deployed in a network.
This section describes the actions taken to enable an automatic
commissioning of the device in the network.
7.1.1. Device Development
The expectation for the development cycle is that message formats are
documented as a data model that is used to generate rules. Several
models are possible:
1. In the application model, an interface definition language and
binary communication protocol such as Apache Thrift is used, and
the parser code includes the SCHC operation. This model imposes
that both ends are compiled with the generated structures and
linked with generated code that represents the rule operation.
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2. In the device model, the rules are generated separately. Only
the device-side code is linked with generated code. The Rules
are published separately to be used by a generic SCHC engine that
operates in a middle box such as a SCHC gateway.
3. In the protocol model, both endpoint generate a packet format
that is imposed by a protocol. In that case, the protocol itself
is the source to generate the Rules. Both ends of the SCHC
compression are operated in middle boxes, and special attention
must be taken to ensure that they operate on the compatible SoR,
basically the same major version of the same SoR.
Depending on the deployment, the tools that generate the Rules should
provide knobs to optimize the SoR, e.g., more rules vs. larger
residue.
7.1.2. Rules Publication
In the device model and in the protocol model, at least one of the
endpoints must obtain the SoR dynamically. The expectation is that
the SoR are published to a reachable repository and versionned
(minor, major). Each SoR should have its own Uniform Resource Names
(URN) [RFC8141] and a version.
The SoR should be authenticated to ensure that it is genuine, or
obtained from a trusted app store. A corrupted SoR may be used for
multiple forms of attacks, more in Section 8.
7.1.3. SCHC Device Deployment
The device and the network should mutually authenticate themselves.
The autonomic approach [RFC8993] provides a model to achieve this at
scale with zero touch, in networks where enough bandwidth and compute
are available. In highly constrained networks, one touch is usually
necessary to program keys in the devices.
The initial handshake between the SCHC endpoints should comprise a
capability exchange whereby URN and the version of the SoR are
obtained or compared. SCHC may not be used if both ends can not
agree on an URN and a major version.
Manufacturer Usage Descriptions (MUD) [RFC8520] may be used for that
purpose in the device model.
Upon the handshake, both ends can agree on a SoR, their role when the
rules are asymmetrical, and fetch the SoR if necessary. Optionally,
a node that fetched a SoR may inform the other end that it is reacy
from transmission.
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7.1.4. SCHC Device Maintenance
URN update without device update (bug fix) FUOTA => new URN =>
reprovisioning
7.1.5. SCHC Device Decommissionning
Signal from device/vendor/network admin
8. Security Considerations
SCHC is sensitive to the rules that could be abused to form arbitrary
long messages or as a form of attack against the C/D and/or F/R
functions, say to generate a buffer overflow and either modify the
Device or crash it. It is thus critical to ensure that the rules are
distributed in a fashion that is protected against tempering, e.g.,
encrypted and signed.
9. IANA Consideration
This document has no request to IANA
10. Acknowledgements
The authors would like to thank (in alphabetic order): Laurent
Toutain
11. References
11.1. Normative References
[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/rfc/rfc2119>.
[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<https://www.rfc-editor.org/rfc/rfc8141>.
[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/rfc/rfc8174>.
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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/rfc/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/rfc/rfc8824>.
11.2. Informative References
[I-D.ietf-6lo-schc-15dot4]
Gomez, C. and A. Minaburo, "Transmission of SCHC-
compressed packets over IEEE 802.15.4 networks", Work in
Progress, Internet-Draft, draft-ietf-6lo-schc-15dot4-07, 2
October 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-6lo-schc-15dot4-07>.
[I-D.ietf-core-comi]
Veillette, M., Van der Stok, P., Pelov, A., Bierman, A.,
and C. Bormann, "CoAP Management Interface (CORECONF)",
Work in Progress, Internet-Draft, draft-ietf-core-comi-19,
3 November 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-comi-19>.
[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-
02, 8 April 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-intarea-schc-protocol-numbers-02>.
[I-D.ietf-schc-access-control]
Minaburo, A., Toutain, L., and I. Martinez, "SCHC Access
Control", Work in Progress, Internet-Draft, draft-ietf-
schc-access-control-00, 13 December 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-schc-
access-control-00>.
[I-D.ietf-schc-over-ppp]
Thubert, P., "SCHC over PPP", Work in Progress, Internet-
Draft, draft-ietf-schc-over-ppp-00, 25 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-schc-
over-ppp-00>.
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[rfc2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,
and R. Wheeler, "A Method for Transmitting PPP Over
Ethernet (PPPoE)", RFC 2516, DOI 10.17487/RFC2516,
February 1999, <https://www.rfc-editor.org/rfc/rfc2516>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/rfc/rfc6241>.
[rfc7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/rfc/rfc7950>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/rfc/rfc8040>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/rfc/rfc8259>.
[rfc8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/rfc/rfc8376>.
[RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520,
DOI 10.17487/RFC8520, March 2019,
<https://www.rfc-editor.org/rfc/rfc8520>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
[RFC8993] Behringer, M., Ed., Carpenter, B., Eckert, T., Ciavaglia,
L., and J. Nobre, "A Reference Model for Autonomic
Networking", RFC 8993, DOI 10.17487/RFC8993, May 2021,
<https://www.rfc-editor.org/rfc/rfc8993>.
[rfc9011] Gimenez, O., Ed. and I. Petrov, Ed., "Static Context
Header Compression and Fragmentation (SCHC) over LoRaWAN",
RFC 9011, DOI 10.17487/RFC9011, April 2021,
<https://www.rfc-editor.org/rfc/rfc9011>.
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[rfc9363] Minaburo, A. and L. Toutain, "A YANG Data Model for Static
Context Header Compression (SCHC)", RFC 9363,
DOI 10.17487/RFC9363, March 2023,
<https://www.rfc-editor.org/rfc/rfc9363>.
[rfc9442] Zúñiga, J., Gomez, C., Aguilar, S., Toutain, L., Céspedes,
S., Wistuba, D., and J. Boite, "Static Context Header
Compression (SCHC) over Sigfox Low-Power Wide Area Network
(LPWAN)", RFC 9442, DOI 10.17487/RFC9442, July 2023,
<https://www.rfc-editor.org/rfc/rfc9442>.
Authors' Addresses
Alexander Pelov
IMT Atlantique
rue de la Chataigneraie
35576 Cesson-Sevigne Cedex
France
Email: alexander.pelov@imt-atlantique.fr
Pascal Thubert
06330 Roquefort les Pins
France
Email: pascal.thubert@gmail.com
Ana Minaburo
Consultant
35510 Cesson-Sevigne Cedex
France
Email: anaminaburo@gmail.com
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