What does the W3C `traceparent` HTTP header carry, field by field, and what should a receiving service do with its sampled flag and with the accompanying `tracestate` header?

## The header, field by field ``` traceparent: 00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01 | | | | | | | +-- trace-flags (1 byte hex) | | +------------------- parent-id / span-id (8 bytes hex) | +---------------------------------------------------- trace-id (16 bytes hex) +------------------------------------------------------- version (1 byte hex) ``` **version** — `00` is the only defined version of W3C Trace Context Level 1. The specification defines forward compatibility: an implementation seeing a higher version should still attempt to parse the first three fields it understands rather than discard the header, so future extensions do not break existing hops. Version `ff` is invalid. **trace-id** — 16 bytes, 32 lowercase hex characters, globally unique for the whole distributed operation and constant across every hop. All zeroes is invalid, which is why an implementation that has no trace must omit the header rather than send zeros. **parent-id** (also called span-id) — 8 bytes, 16 hex characters: the id of the span *in the sender* that made this call. The receiver uses it as the parent of the spans it creates, which is how the tree is stitched. All zeroes is invalid. **trace-flags** — one byte of flags. Only bit 0 is defined: `01` means the caller recorded/sampled this trace, `00` means it did not. It is explicitly *not* a command and not a guarantee that data was exported; it is a hint about the caller's sampling decision. The header is case-insensitive by HTTP rules and must appear at most once; a duplicated or malformed header is treated as absent, in which case the receiver starts a new trace as a root. ## What the receiver does The SDK's propagator *extracts* a remote span context: `{trace_id, span_id, flags, trace_state, remote=true}`. That context becomes the current context, so when instrumentation starts the server span it inherits the trace id and sets its parent to the received span id. Nothing else about the caller crosses — no span name, no attributes, no timing. Correlation happens in the backend, which assembles a trace by grouping spans that share a trace id and linking by parent id. The sampled bit interacts with sampling. The default OpenTelemetry sampler is parent-based: if a remote parent exists, follow its decision; only when there is no parent does the configured root sampler (for example a ratio sampler) decide. This is what makes head-based sampling produce *whole* traces — the decision is taken once at the edge and every downstream service obeys it. Configuring an independent ratio sampler at every service instead produces partial traces, because each service tosses its own coin. ## `tracestate` `tracestate: vendorA=abc123,vendorB=xyz` is a comma-separated ordered list of up to 32 key/value entries, each key namespaced to a vendor or tenant. It exists because different tracing systems need to carry a little of their own state alongside the standard ids without colliding. Two rules matter operationally: your system may add or update *its own* key and should move it to the leftmost position (most recently mutated first), and you must forward all other entries unchanged. Dropping unknown entries silently breaks correlation for whichever system owns them — a common bug in hand-written gateway code that rebuilds headers from an allowlist. Entries have size limits (the list should be kept under 512 bytes; implementations may drop the rightmost, least recently used entries when trimming), so `tracestate` is not a general-purpose data channel. Application data belongs in Baggage, not here. ## Common operational notes - If `traceparent` is absent, the service becomes a trace root and generates a new trace id. Traces starting at the wrong hop usually means the header was stripped by a proxy, gateway or a strict forwarded-header allowlist, not that instrumentation is missing. - Accepting `traceparent` from the public internet lets outsiders choose your trace ids and set the sampled bit, which can be used to force expensive sampling or to pollute traces. Many teams strip and re-root at the edge, accepting inbound context only from partners they trust. - The header is transport-specific by design. The same span context travels over gRPC metadata, message-broker record headers or any other carrier through the same propagator API; only the carrier changes. ## In an interview Recite the four fields with their sizes, say that the third field is the *caller's* span id and becomes the parent, explain that the flag is a sampling hint honoured by the parent-based sampler, and note that `tracestate` must be forwarded intact except for your own entry.