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Protocol: Sessions and Compact Frames

This document summarizes the session handshake and compact header format. For the full wire layout, see WIRE_FORMAT.md.

Session init

Session init packets are regular sdec packets with SESSION_INIT set in the v2 header flags. They carry the schema hash and header mode so both sides can agree on how to decode subsequent snapshots.

Key properties:

  • SESSION_INIT is exclusive (no full/delta flags set).
  • Schema hash must match the local schema.
  • The receiver must accept the session before processing compact deltas.

Compact session header

Compact headers reduce per-packet overhead by using:

  • 1 byte flags (full vs delta).
  • Varint-encoded tick delta (relative to last tick).
  • Varint-encoded baseline delta (relative to tick).
  • Varint-encoded payload length.

This format is defined in wire::session and used by codec::SessionState to enforce ordering and baseline constraints.

Packed sparse indices

Packed sparse updates are used for per-client visibility deltas:

  • Indices are encoded densely to avoid a bitmap per component.
  • Update entries remain strictly ordered.
  • Limits are enforced during encode/decode.

See WIRE_FORMAT.md for the exact sparse layout.