Protocol Numbers for SCHC

Introduction The Static Context Header Compression (SCHC) Architecture originally envisioned SCHC used at the Network layer to enable IPv6 over selected Low-Power Wide Area Networking (LPWAN) radio technologies, encompassing IP and Transport, by the network provider. Then SCHC would be used by the application; this would include any security envelope. In the evolution of SCHC, compression and fragmentation are available at any layer. After applying SCHC, the protocol information is reduced to a RuleID and the compression residue (if any). We need to identify SCHC to recognise when a protocol header has been compressed by SCHC. The identifier to be used depends on the protocol/layer in the stack where SCHC is applied. It MUST be unambiguous to transform SCHC into an application. And this identifier could be a protocol number and port numbers. This approach brakes down when dealing with Diet ESP . When Next Header is ESP, it is challenging for the ESP process to determine if an incoming ESP payload is regular ESP or a diet ESP payload. Careful allocation of the incoming SPI can mitigate this and have an implicit SCHC header, but it is not sound protocol design. If the Next Header in the IP header were SCHC, not ESP, a clear segregation of incoming traffic is directly supportable. Additionally, SCHC can then be the Next Header within the ESP header with 'regular' SCHC rules for processing this content. This approach will greatly simplify . DTLS 1.3 adds further complications. DTLS 1.3 headers themselves are typically already very compressed and SCHC would not provide much value. But the UDP header in front of DTLS would benefit of a separate compression from the IP Header compression. Where it is possible with ESP's SPI to mitigate inbound packet processing challenges implicit SCHC would generate, DTLS header does not safely even provide this and a SCHC IP number is necessary to separate traffic. New IETF work has started with the SCHC WG that is chartered to:

provide specifications for the application of SCHC over underlying layers, where underlying layers include but are not limited to UDP tunnels, IP, PPP, and Ethernet, as well as the use of SCHC by upper-layer protocols.

To achieve its charter, the SCHC working group needs the allocations that are requested in this document. These issues carry over to IP Header compression if SCHC were available as an Ethertype (for 802 networking) and if SCHC were available as a TCP/UDP port number (for the application layer). At each layer, SCHC solves a problem that protocol designers, using constrained networks, currently have to design around.

Basic use case for SCHC as an Internet Protocol Number A mobile node, or network, may use different links over a period of time. In some cases the node has the multiple interfaces and, in theory, could tune the compression to each interface. In other cases, it is the whole network that is mobile and individual nodes have no "knowledge" of which link with what characteristics is actively handling the traffic. In either case, the node administrator is aware that some links are constrained and use of SCHC compression is highly recommended. One example is an UA that uses different links over the duration of an operation (i.e. flight).

Operation starts using Veriport's WiFi service.
On gaining altitude, UA transitions to a Cellular service.
On gaining more altitude, UA transitions to a constrained 700MHz UHF service.
On approach to destination vertiport, link transition is reversed.

The UA could use SCHC compression only on the UHF link, but this may complicate the implementation. A more complex example is an Unmanned Cargo Aircraft that has multiple avionics systems, all Ethernet connected to an onboard router that has the multiple interfaces. Here the nodes each manage their own secure path to their ground-based server, but have no knowledge of which link is in use to intelligently use compression.

Basic use case for SCHC as an Ethertype In the case of a classical LPWAN link such as LoRa , the use of SCHC to compress the transported protocol, as well as the SCHC session (called instance) to use, are implicit. The MAC-Layer endpoints are preconfigured so there can be only one session, and there can be only SCHC. When extended to Ethernet and more powerful endpoint, this model is way too restrictive, and it is necessary to signal both the use of SCHC and the SCHC session to be used. While the SCHC WG is charterd to produce the latter, the Ethertype defined in this document will be used to signal SCHC as the upper* layer protocol. As an example that will leverage this, Aircraft-to-anything (A2X) and Aircraft-to-Ground protocols are specific cases that can benefit from SCHC as an Ethertype. These can use IEEE 802 wireless technology and lessen spectrum contention in high traffic or long-range situations by minimizing the datagram size via SCHC. In the above uses, SCHC compresses the IPv6 header completely (all 40 bytes), leaving only destination address (32 bytes, source address calcuated from content), or only 8 bytes (needs both addresses) at the cost of the 1-byte SCHC RuleID. The 2-byte payload length may be needed in some cases (as in ). Since the whole point of SCHC is to reduce payload size, SCHC directly over an 802 technology cannot be addressed via the Ethernet Protocol Assignment under the IANA OUI. A distinct Ethertype is needed by SCHC to actually reduce payload overhead.

Basic use case for SCHC as a UDP port There is a need to allow carrying SCHC-compressed data units (i.e., SCHC datagrams [draft-ietf-schc-architecture]) atop UDP. For example, SCHC-based header compression for Constrained Application Protocol (CoAP) has been specified . The document entitled 'Transmission of SCHC-compressed packets over IEEE 802.15.4 networks' aims to exploit the opportunity of carrying SCHC-compressed CoAP messages on top of UDP. To support this functionality, there is a need for UDP port numbers known by both endpoints (sender and receiver) that identifies the presence of a SCHC Stratum atop UDP, i.e., that the UDP payload is a SCHC datagram (in this case, a SCHC-compressed CoAP message). In addition, note that it is possible to use traditional 6LoWPAN header compression to compress IPv6 and UDP headers, but not to compress CoAP headers. Therefore, the only way to support CoAP header compression on devices running 6LoWPAN is by means of SCHC, which again requires to place a SCHC Stratum on top of UDP. SCHC header compression is also being developed for further protocols carried by UDP (e.g., QUIC ). In the future, SCHC may be applied to any protocol at any layer, such as DTLS and TCP.