Internet DRAFT - draft-thubert-intarea-schc-over-ppp
draft-thubert-intarea-schc-over-ppp
LPWAN P. Thubert, Ed.
Internet-Draft Cisco Systems
Updates: 5172 (if approved) 21 April 2021
Intended status: Standards Track
Expires: 23 October 2021
SCHC over PPP
draft-thubert-intarea-schc-over-ppp-03
Abstract
This document extends RFC 5172 to signal the use of SCHC as the
compression method between a pair of nodes over PPP. Combined with
RFC 2516, this enables the use of SCHC over Ethernet and Wi-Fi.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 23 October 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Extending RFC 5172 . . . . . . . . . . . . . . . . . . . . . 3
4. Profiling SCHC for high speed links . . . . . . . . . . . . . 4
4.1. Mapping the SCHC Architecture . . . . . . . . . . . . . . 4
4.2. SCHC Parameters . . . . . . . . . . . . . . . . . . . . . 5
4.2.1. Resulting Packet Format . . . . . . . . . . . . . . . 6
4.3. Security Considerations . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
7. Normative References . . . . . . . . . . . . . . . . . . . . 9
8. Informative References . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The Point-to-Point Protocol (PPP) [RFC5172] provides a standard
method of encapsulating network-layer protocol information over
serial (point-to-point and bus) links. "A Method for Transmitting
PPP Over Ethernet (PPPoE)" [RFC2516] transports PPP over Ethernet
between a pair of nodes. It is compatible with a translating bridge
to Wi-Fi, and therefore enables PPP over Wi-Fi as well.
PPP also proposes an extensible Link Control Protocol and a family of
Network Control Protocols (NCPs) for establishing and configuring
different network-layer protocols. "IP Version 6 over PPP" [RFC5072]
specifies the IPv6 Control Protocol (IPV6CP), which is an NCP for a
PPP link, and allows for the negotiation of desirable parameters for
an IPv6 interface over PPP. "Negotiation for IPv6 Datagram
Compression Using IPv6 Control Protocol" [RFC5172] defines the IPv6
datagram compression option that can be negotiated by a node on the
link through the IPV6CP.
PPP is not commonly used in Low-Power Wide Area Networks (LPWAN) but
the extreme compression techniques that are defined for use in LPWAN
may be applicabel to more traditional links where PPP applies.
The "Static Context Header Compression (SCHC) and fragmentation for
LPWAN, application to UDP/IPv6" [SCHC] is a new technology that can
provide an extreme compression performance but requires a same state
to be provisionned on both ends before it can be operated.
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The "SCHC Architecture" [I-D.pelov-lpwan-architecture] enables a peer
to peer SCHC operation in addition to the classical device to network
LPWAN paradigm, e.g., over a PPP connection. To enable SCHC over PPP
and therefore Ethernet and Wi-Fi, this specification extends
[RFC5172] to signal SCHC as an additional compression method for use
over PPP.
An example use case for SCHC over PPP over Ethernet (SCHCoPPPoE) is
to apply SCHC to periodic flows and maintain them at a protocol-
independant size and rate. The constant size may be too small for a
particular flow or protocol. The SCHC fragmentation can then be used
to transport a protocol data unit (PDU) as N compressed SCHC
fragments, in which case the effective PDU rate is the TSN frame rate
divided by N.
This can be useful to streamline the frames and simplifies the
scheduling of Deterministic Networking [DetNet] and Operational
Technology (OT) control flows over IEEE Std 802.1 Time-Sensitive
Networking (TSN) [IEEE802.1TSNTG] or one of the RAW Technologies [RAW
Technologies].
2. BCP 14
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. Extending RFC 5172
With this specification, a PPP session defines a vitual link where a
SCHC context is established with a particular set of Rules, which is
indicated at the set up of the PPP session as follows:
[RFC5172] defines an IPV6CP option called the IPv6-Compression-
Protocol Configuration option with a type of 2. The option contains
an IPv6-Compression-Protocol field value that indicates a compression
protocol and an optional data field as shown in Figure 1:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | IPv6-Compression-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
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Figure 1: The IPv6-Compression-Protocol Configuration Option
This specification indicates a new IPv6-Compression-Protocol field
value for [SCHC] (see Section 5), and enables to transport a Uniform
Resource Identifier (URI) [RFC3986] of the set of rules in the
optional data. The default format for the set of rules is YANG using
the "Data Model for SCHC" [SCHC_DATA_MODEL] encoded in JSON as
specified in [RFC7951]. The size of the URL is computed based on the
Length of the option as Length-4. If the encoding is asymetrical,
the initiator of the session is considered downstream, playing the
role of the device in an LPWAN network.
4. Profiling SCHC for high speed links
Appendix D of [SCHC] specifies the profile information that
technology specifications such as this must provide. The following
section address this requirement.
4.1. Mapping the SCHC Architecture
This specification leverages SCHC between an end point that is an IP
Host and possibly a serial DTE (Data Terminal Equipment), and another
that is an IP Node (either another IP Host or a Router) and possibly
a serial DCE (Data Control Equipment), or a more modern physical or
emulated endpoint, e.g., Ethernet devices that echange IP packets
over PPPoE.
Both endpoints MUST support the function of SCHC Compressor/
Decompressor (C/D) as shown in Figure 2.
+----------+ Wi-Fi / +----------+ ....
| IP | Ethernet | IP | .. )
| Host +-----/------+ Router +----------( Internet )
| SCHC C/D | Serial | SCHC C/D | ( )
+----------+ +----------+ ...
<-- SCHC -->
over PPP
Figure 2: Typical Deployment
The SCHC Fragmenter/Reassembler (F/R) is generally not needed,
because the maximum transmission unit (MTU) is expected to be large
enough and SCHC only reduces the frame size vs. native IP. But it
may be used to obtain a small protocol-independant frame size for the
compressed packets, possibly way smaller than MTU.
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A context may be generated for a particular upper layer application,
such as a control loop using an industrial automation protocol, to
protect the particular flow with a DetNet service. The context can
be asymetric, e.g., when connecting a primary and a secondary
endpoints, a client and a server, or a programmable logic controller
with a sensor or an actuator.
4.2. SCHC Parameters
Compared to typical LPWANs, most serial links and emulations such as
PPPoE are very fast and most of the constraints can be alleviated.
For this reason, the SCHC profile for PPP is defined as follows:
RuleID numbering scheme: The RuleID for a compression rule is
expressed as 2 bytes. The first (leftmost) 2 bits of that RuleId
MUST be set to 0 This leaves 14 bits to index the rule. A SCHC
compressed packet is always in the form:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+----------...--------+~~~~~~~~~~
|0 0 RuleID | Compression Residue | Payload
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+----------...--------+~~~~~~~~~~
|------- Compressed Header (byte aligned) ------------|
Figure 3: SCHC Compressed Packet
This specification only supports the No-ACK Mode of SCHC
fragmentation as specified in section 8.4.1 of [SCHC]. The SCHC
Fragment Header is 2 bytes long.
The RuleID for a fragmentation rule is expressed as 4 bits. The
bits MUST all set to 1 for a fragmentation rule in No-ACK Mode.
The DTag field is 11 bits long (T=11) and the FCN field is one bit
(N=1), which is set to 1 on the last fragment as illustrated in
Appendix B of [SCHC] and to 0 otherwise. There is no W field
(M=0).
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--------...-------+~~~~~~~~~~~
|1 1 1 1| DTag |F| Fragment Payload | padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--------...-------+~~~~~~~~~~~
|--------- T ---------|N|
|---- SCHC Fragment Header ----|
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Figure 4: SCHC Fragment
The No-ACK mode has been designed under the assumption that data
unit out-of-sequence delivery does not occur between the entity
performing fragmentation and the entity performing reassembly and
a DetNet PREOF function might be needed to reorder the fragments.
Maximum packet size: MAX_PACKET_SIZE is aligned to the PPP Link MTU.
Padding: The Compression Residue MUST be aligned to the L2 word.
For Ethernet, the L2 word is one byte, so padding is needed up to
the next byte boundary. If a compression rule produces a residue
that is not byte aligned, then it is implicitly terminated with a
statement that indicates padding till the next byte boundary. The
padding bit is 0.
4.2.1. Resulting Packet Format
In the case of PPPoE, the sequence of compression and encapsulation
is as follows:
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A packet (e.g., an IPv6 packet)
| ^ (padding bits
v | dropped)
+------------------+ +--------------------+
| SCHC Compression | | SCHC Decompression |
+------------------+ +--------------------+
| ^
+-- No -+ |
| fragmentation | +------------------>+
v | | |
+--------------------+ | | +-------------------+
| SCHC Fragmentation | | | | SCHC Reassembly |
+--------------------+ | | +-------------------+
| | | ^
+<------------------+ | No |
| +-- fragmentation -+
v |
+--------------------+ +--------------------+
| PPP Session encaps | | PPP Session decaps |
+--------------------+ +--------------------+
| ^
| |
v |
+------------------+ +------------------+
| PPPoE(oE) encaps | | PPPoE(oE) encaps |
+------------------+ +------------------+
| ^
| |
+-------------------------------------------+
Sender Receiver
Figure 5: Stack Operation (no fragment)
In the case of PPPoE, a frame that transports an IPv6 packet
compressed with SCHC with no fragmentation shows as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Source MAC Address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Destination MAC Address +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethernet Frame Type(0x8864) | Ver=1 | Type=1| Code=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPP Protocol (IPv6) = 0x0057 |0|0| SCHC RuleID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
... Compression ...
| Residue +-+-+-+
| | Pad |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Uncompressed |
... Original ...
| Payload |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: SCHC over PPP over Ethernet Format
4.3. Security Considerations
This draft enables to use the SCHC compression and fragmentation over
PPP and therefore Ethernet and Wi-Fi with PPPoE. It inherits the
possible threats against SCHC listed in the "Security considerations"
section of [SCHC].
5. IANA Considerations
This document requests the allocation of a new value in the registry
"IPv6-Compression-Protocol Types" for "SCHC". A suggested value is
proposed in Table 1:
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+=======+==========================================+===============+
| Value | Description | Reference |
+=======+==========================================+===============+
| 4 | Static Context Header Compression (SCHC) | This document |
+-------+------------------------------------------+---------------+
Table 1: IP Header Compression Configuration Option Suboption Types
6. Acknowledgments
7. 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/info/rfc2119>.
[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/info/rfc2516>.
[RFC5072] Varada, S., Ed., Haskins, D., and E. Allen, "IP Version 6
over PPP", RFC 5072, DOI 10.17487/RFC5072, September 2007,
<https://www.rfc-editor.org/info/rfc5072>.
[RFC5172] Varada, S., Ed., "Negotiation for IPv6 Datagram
Compression Using IPv6 Control Protocol", RFC 5172,
DOI 10.17487/RFC5172, March 2008,
<https://www.rfc-editor.org/info/rfc5172>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[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>.
[SCHC] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zúñiga, "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>.
8. Informative References
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[I-D.pelov-lpwan-architecture]
Pelov, A., Thubert, P., and A. Minaburo, "Static Context
Header Compression (SCHC) Architecture", Work in Progress,
Internet-Draft, draft-pelov-lpwan-architecture-00, 19
January 2021, <https://tools.ietf.org/html/draft-pelov-
lpwan-architecture-00>.
[SCHC_DATA_MODEL]
Minaburo, A. and L. Toutain, "Data Model for Static
Context Header Compression (SCHC)", Work in Progress,
Internet-Draft, draft-ietf-lpwan-schc-yang-data-model-03,
10 July 2020, <https://tools.ietf.org/html/draft-ietf-
lpwan-schc-yang-data-model-03>.
[RAW Technologies]
Thubert, P., Cavalcanti, D., Vilajosana, X., Schmitt, C.,
and J. Farkas, "Reliable and Available Wireless
Technologies", Work in Progress, Internet-Draft, draft-
thubert-raw-technologies-05, 18 May 2020,
<https://tools.ietf.org/html/draft-thubert-raw-
technologies-05>.
[RFC7951] Lhotka, L., "JSON Encoding of Data Modeled with YANG",
RFC 7951, DOI 10.17487/RFC7951, August 2016,
<https://www.rfc-editor.org/info/rfc7951>.
[DetNet] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
[IEEE802.1TSNTG]
IEEE, "Time-Sensitive Networking (TSN) Task Group",
<https://1.ieee802.org/tsn/>.
Author's Address
Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
06254 Mougins - Sophia Antipolis
France
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
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