Space Link Extension (SLE)
The CCSDS Space Link Extension protocol (911.1-B-4, 911.2-B-3) provides a standardized interface between a mission control center and ground station antennas. Where the other protocol layers handle the space link itself, SLE handles the ground segment: how mission software connects to a remote antenna to send commands or receive telemetry.
SLE runs over TCP using ISP1 (Internet SLE Protocol 1) framing. Each message is a length-prefixed BER-encoded PDU. The protocol follows a client-provider model where the mission control center (client) binds to a service instance on the ground station (provider).
Services
Two services are supported:
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RAF (Return All Frames) — the provider delivers downlink telemetry frames to the client. The client can filter by frame quality (good, erred, or all). Frames arrive in transfer buffers containing one or more annotated TM frames with earth receive times and data link continuity indicators.
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CLTU (Forward CLTU) — the client sends Command Link Transmission Units to the provider for uplink radiation. Each CLTU is assigned a sequence ID and the provider reports whether the unit was radiated, expired, or interrupted.
Session lifecycle
A session proceeds through three phases:
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Bind — the client authenticates with optional SHA-1 credentials (time + nonce + password hash) and identifies the service instance by its OID path (e.g.
sagr=1.spack=1.raf=onlt1). The provider responds with a bind result (success, access denied, unsupported service type, or version mismatch). -
Start/Transfer — the client starts the service. For RAF, this includes optional start/stop time bounds and a frame quality filter. For CLTU, this includes the first CLTU sequence ID. Data then flows: transfer buffers for RAF, transfer data invocations and returns for CLTU.
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Stop/Unbind — the client stops the service and unbinds from the provider.
ISP1 framing
ISP1 provides a simple transport layer over TCP. Each PDU is prefixed with a 4-byte big-endian length field:
| Field | Size | Description |
|---|---|---|
| Length | 4 bytes | Payload size (BE u32) |
| PDU | N bytes | BER-encoded SLE PDU |
Authentication
Bind invocations carry optional credentials: an 8-byte CDS timestamp, a 4-byte random nonce, and a 20-byte SHA-1 hash computed over the concatenation of time, nonce, and a shared password. This prevents replay attacks and authenticates the caller without transmitting the password.
BER encoding
All SLE PDUs use ASN.1 Basic Encoding Rules. Each operation is wrapped in a context-specific tagged CHOICE that identifies the operation type (bind, start, transfer data, etc.). Fields within each PDU are standard BER types: INTEGER, OCTET STRING, ENUMERATED, SEQUENCE, and NULL.