Protocol Numbers for SCHC HTT Consulting
Oak Park MI 48237 USA rgm@labs.htt-consult.com
Roquefort-les-Pins 06330 France pascal.thubert@gmail.com
UPC
C/Esteve Terradas, 7 Castelldefels 08860 Spain carles.gomez@upc.edu
Consultant
Rue de Rennes Cesson-Sevigne 35510 France anaminaburo@gmail.com
Viagenie
Canada marc.blanchet@viagenie.ca
Internet SCHC RFC Request for Comments I-D Internet-Draft SCHC This document requests an Internet Protocol Number, an Ethertype assignment, a CCSDS (Consultative Committee for Space Data Systems) Encapsulation Number, and well known ports for SCHC. The SCHC architecture, the SCHC instance establishment, and the SCHC compression/decompression processes are simplified when SCHC is easily recognised. Well-known protocol and port numbers are needed. The Internet Protocol Number request is so that SCHC can be used for IP independent of other transports such as UDP and ESP. The Ethertype and the CCSDS Encapsulation Number are to support generic use of native SCHC over any IEEE 802 technology and CCSDS link layer technology, respectively, for IP and non-IP protocols.
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. The identifier has to be unambiguous to ensure correct SCHC processing at the receiver side; it could be a protocol number or port number. This approach breaks 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 compressing the DTLS header. But the UDP header in front of DTLS would benefit with 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 or UDP number is necessary to separate traffic for SCHC processing. 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 IEEE 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.
Terms and Definitions
Requirements Terminology 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.
SCHC Protocol Number Use Cases
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 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 Uncrewed Aircraft (UA) that uses different links over the duration of an operation (i.e., flight).
  • Operation starts using a vertiport'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 Uncrewed 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 chartered 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 calculated from content), or only 8 bytes (needs both addresses) at the cost of a 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 packet size, SCHC directly over an IEEE 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 ) atop UDP. For example, SCHC-based header compression for the Constrained Application Protocol (CoAP ) has been specified and is being updated by . 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 (defined in ) 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.
Basic Use case for SCHC over connection-oriented communication In a connection-oriented communication, two endpoints establish a session to transfer data reliably, with error detection and reordering of received data. During the connection establishment (3-way handshake), both hosts must identify SCHC with the layer-4 port number and exchange and agree on the Set of Rules (SoR). Through the data transfer, the management of the SoR uses the Yang data model as described in the . Both endpoints must make the same changes to keep the integrity of the flow control. The SoR may contain dedicated Rules for Acknowledgements and connection termination. This approach is essential for critical business applications where data loss or corruption could have serious financial or legal consequences.
Basic use case for SCHC over Space Links Space communications is a very bandwidth-constraint environment. Space links are typically point-to-point links. The deployment of IP in deep space is described in the TIPTOP (Taking IP To Other Planets) use case and architecture documents. It specifies the use of SCHC on space link layers as defined by the Consultative Committee for Space Data Systems (CCSDS).
SCHC Port Numbers and protocol number as identifiers In the current SCHC architecture, the SCHC Stratum Header adds signalling information to the SCHC datagram. It may be fully compressed, and it does not add any overhead in that case. The SCHC Stratum Header helps to identify the use of SCHC and selects the correct instance and SoR in the SCHC process. The SCHC Stratum Header format includes an identifier that depends on the compressed stack layer. These identifiers are the protocol number at layer three and port numbers at layer four.
Internet Protocol Number for SCHC The transport of a SCHC datgram as payload of an IP packet SHOULD be indicated in the IPv4 "Protocol" field or the IPv6 "Next Header" field with a value of TBD1 (recommended: 145) as shown below: Internet Protocol Numbers
Decimal Keyword Protocol IPv6 Extension Header Reference
TBD1 (145) SCHC Static Context Header Compression This RFC
The SCHC compressed header with payload is shown below. The size of the SCHC RuleID is variable as described in . An implementation should have a table of source IP address and RuleID size. The addresses should be represented in prefix format to allow for groups of addresses having the same RuleID size.
SCHC Datagram
The RuleID may be statically configured per , or may be negotiated within a protocol as in IKE .
Ethertype for SCHC The use of SCHC as an Ethertype is similar to that as in above. Immediately after the SCHC Ethertype is the RuleID as in . If the rules associated with the RuleID does not provide the datagram length, the datagram length MUST be explicit in the Compression Residue, as the IEEE 802 header may not provide the needed length information to properly process the datagram.
Transport Port Numbers for SCHC SCHC's first function is to compress the header; with this action, the protocol ports are hidden from the application. To identify SCHC in the upper layers, the protocols do not have a next header field. The port numbers are necessary to be aware that the protocol's header has been compressed.
CCSDS Encapsulation Number for SCHC The CCSDS link layers have a common encapsulation named Internet Protocol Extension (IPE) . The codepoints are managed by the Space Assigned Numbers Authority (SANA) under the IPE registry . This registry already specifies the encapsulation of previous IP header compression techniques. This document requests SANA through CCSDS to allocate a codepoint for SCHC in the IPE registry.
IANA Considerations
IANA Internet Protocol Number Registry Update This document requests IANA to make the following change to the "Assigned Internet Protocol Numbers" registry:
Internet Protocol Number:
This document defines the new Internet Protocol Number value TBD1 (suggested: 145) () in the "Assigned Internet Protocol Numbers" registry.
Decimal Keyword Protocol IPv6 Extension Header Reference
TBD1 (145) SCHC Static Context Header Compression This RFC
IANA Ethertype Request IANA is requested using the process in , to request the Ethertype for SCHC.
Security Considerations None additional over already noted in , and .
References Assigned Internet Protocol Numbers IANA IP OVER CCSDS SPACE LINKS, Blue Book 702 Consultative Committee on Space Data Systems(CCSDS) Internet Protocol Extension Header Space Assigned Numbers Authority
Acknowledgments Discussions with Pascal Thubert, lpwan co-chair, helped develop this approach of using SCHC E2E below the current Transport Layers. Adam Wiethuechter and Stuart W. Card of AX Enterprize, LLC provided extensive input and review for use of SCHC for aviation air-to-air and air-to-ground communications. Carles Gomez has been funded in part by MCIU/AEI/10.13039/501100011033/FEDER/UE through project PID2023-146378NB-I00, and by Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya with the grant number 2021 SGR 00330.