SCHC over PPP

Introduction The Point-to-Point Protocol (PPP) 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)" 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" 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" 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 also apply to more traditional links where PPP applies. The "Static Context Header Compression (SCHC) and fragmentation for LPWAN, application to UDP/IPv6" is a new technology that effectively provides an extreme compression performance but requires a matching state to be provisionned on both ends before it can be operated. The "SCHC 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 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 and Operational Technology (OT) control flows over IEEE Std 802.1 Time-Sensitive Networking (TSN) or one of the RAW Technologies.

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 when, and only when, they appear in all capitals, as shown here.

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: 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 :

The IPv6-Compression-Protocol Configuration Option 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 ... +-+-+-+-+ This specification indicates a new IPv6-Compression-Protocol field value for (see ), and enables to transport a Uniform Resource Identifier (URI) 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" encoded in JSON as specified in . 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.

Profiling SCHC for high speed links Appendix D of specifies the profile information that technology specifications such as this must provide. The following section address this requirement.

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 .

Typical Deployment over PPP ]]> 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. 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.

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:
SCHC Compressed Packet
This specification only supports the No-ACK Mode of SCHC fragmentation as specified in section 8.4.1 of . 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 and to 0 otherwise. There is no W field (M=0).
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.

Resulting Packet Format In the case of PPPoE, the sequence of compression and encapsulation is as follows:

Stack Operation (no fragment) + v | | | +--------------------+ | | +-------------------+ | SCHC Fragmentation | | | | SCHC Reassembly | +--------------------+ | | +-------------------+ | | | ^ +<------------------+ | No | | +-- fragmentation -+ v | +--------------------+ +--------------------+ | PPP Session encaps | | PPP Session decaps | +--------------------+ +--------------------+ | ^ | | v | +------------------+ +------------------+ | PPPoE(oE) encaps | | PPPoE(oE) encaps | +------------------+ +------------------+ | ^ | | +-------------------------------------------+ Sender Receiver ]]> In the case of PPPoE, a frame that transports an IPv6 packet compressed with SCHC with no fragmentation shows as follows:

SCHC over PPP over Ethernet Format

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 .

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 : IP Header Compression Configuration Option Suboption Types

Value	Description	Reference
4	Static Context Header Compression (SCHC)	This document

Acknowledgments