Static Context Header Compression over Narrowband Internet of Things
Extracted elements (29)
For devices acting as capillary gateways, several SCHC rules are needed to match the diversity of devices and protocols; simpler devices may have predetermined protocols and fixed parameters, motivating a configurable RuleID size.
For NB-IoT, 9 bytes of radio protocol overhead are assumed (5 bytes PDCP for header and integrity protection, 4 bytes RLC and MAC), constraining the minimum effective bits for SCHC header to 12 bits in the smallest Transport Block.
IoT devices use Power Save Mode and Idle Mode DRX (battery life up to 10 years), so the Inactivity Timer and Retransmission Timer for SCHC should be set based on 3GPP radio timer values defined in TS 24.008, ranging from N to 3N hours.
SCHC fragmentation is disabled for Radio Link and DoNAS scenarios because the RLC layer already handles segmentation; the radio protocols' link adaptation (varying TB selection per TTI) would make SCHC tiling overhead equal to or greater than native RLC fragmentation.
The recommended 3GPP MTU is 1358 bytes, smaller than the radio network's 1600-byte limit, to avoid fragmentation in the network backbone due to payload encryption size (which must be a multiple of 16) and additional core transport overhead.
For RLC Unacknowledged Mode (UM), available only on terminals compatible with 3GPP Release 15, reliability mechanisms are disabled and only MAC HARQ provides reliability, limiting DoNAS overhead but capping segmentation support at 1600 bytes.
SCHC can be deployed in the PDCP layer of the 3GPP radio stack without significant changes to 3GPP specifications, because the 3GPP architecture already supports Robust Header Compression (ROHC, RFC 5795) at the same layer.
SCHC fragmentation SHOULD NOT be used in AM or UM RLC modes where the RLC layer handles reliability and segmentation; SCHC fragmentation may be considered only for TM (Transparent Mode) transmissions in the future.
SCHC over NIDD is backward compatible with legacy NB-IoT devices: Non-IP transmission features of the operator network can carry SCHC-compressed packets for devices that do not natively support SCHC, using NGW-PGW or NGW-SCEF as intermediaries.
For NIDD scenarios, No-ACK fragmentation mode MAY be enough in point-to-point connections given that 3GPP uses reliability functions; ACK-on-Error MAY be activated for Non-IP packets on the NGW-MME.
For the Radio Link (Section 5.2.1) and DoNAS (Section 5.2.2) scenarios, SCHC fragmentation is disabled and SHOULD NOT be used; the RLC layer handles segmentation into suitable units.
MAX_ACK_REQ is RECOMMENDED to be 2, but applications MAY change this value based on transmission conditions.
SCHC overhead SHOULD NOT exceed the available number of effective bits of the smallest physical Transport Block (TB) to optimize the transmission; with the minimum TB, the maximum SCHC header size is 12 bits.
The context distribution for SCHC over NIDD MUST be according to the application's capabilities, perhaps utilizing IP data transmissions up to context initialization.
The L2 Word for NB-IoT MUST be considered 8 bits, and padding treatment must use this value accordingly, because NB-IoT and 3GPP wireless access assume a byte-aligned payload.
The maximum RECOMMENDED MTU size for SCHC over NIDD is 1358 bytes, since SCHC packets traverse the whole 3GPP network infrastructure (core and radio), not only the radio.
The use of SCHC fragmentation with ACK-on-Error mode is RECOMMENDED for NIDD to secure additional reliability on packets transmitted with low QoS, where packets may be lost before reaching the 3GPP radio network.
WINDOW_SIZE of (2^N)-1 is RECOMMENDED for SCHC fragmentation parameters in NB-IoT deployments.
SCHC over DoNAS (Data over Non-Access Stratum) places SCHC entities in the NAS protocol layer; on the network side, the NGW-MME in the core network is the terminating node, not the RGW-eNB as in the Radio Link case.
SCHC over Non-IP Data Delivery (NIDD) places SCHC compression entities at the application layer of the Dev-UE and Application Server (or broker at the operator edge); the network treats compressed packets as Non-IP traffic and delivers them directly over L2 without protocol stack intervention.
SCHC over the Radio Link places SCHC header compression entities within the PDCP layer of the Dev-UE and RGW-eNB, analogous to how ROHC (RFC 5795) is deployed in the 3GPP PDCP layer without significant architectural changes.
The NB-IoT architecture defines three SCHC deployment scenarios: (1) SCHC over NIDD as a normative end-to-end application-layer scenario, (2) SCHC over the Radio Link (Dev-UE to RGW-eNB in PDCP), and (3) SCHC over DoNAS/NAS (Dev-UE to NGW-MME).
The Reassembly Check Sequence (RCS) follows the default size defined in RFC 8724 Section 8.2.3, with a length equal to the L2 Word (8 bits for NB-IoT).
The RuleID field size for SCHC over NB-IoT may range from 2 bits (4 rules) to 8 bits (256 rules); the maximum 256-rule limit is considered reasonable even for devices acting as NATs, and the size may be negotiated dynamically before context delivery.
Tiles for SCHC fragmentation over NB-IoT MAY keep a fixed value of 4 or 8 bits to avoid padding, except when the transfer block equals 16 bits where Tiles may be 2 bits; the header size must be a multiple of 4.
This document has no IANA actions; no new registries are created and no existing registries are updated.
This document does not add new security considerations; it follows the security model of RFC 8724 (SCHC) and the 3GPP access security document TS 33.122, which specifies security aspects for the Common API Framework for 3GPP northbound APIs.
For Transfer Blocks larger than 304 bits, a 16-bit Header_size configuration is used with: RuleID 8-10 bits, DTag 1-2 bits, FCN 3 bits, W 2-3 bits.
For Transfer Blocks smaller than or equal to 304 bits, an 8-bit Header_size configuration is used with: RuleID 1-3 bits, DTag 1 bit, FCN 3 bits, W 1 bit.