BA-09 · LESSON 02

Transaction and block relay

KEY DEFINITIONS

Terms used in this lesson

Identifier: Identifier: a value used to distinguish one request, object, or participant from another.
Coinbase transaction: The special first transaction in a Bitcoin block, which claims permitted subsidy and fees and has no ordinary prior-output input.

Compact blocks, headers-first synchronization, feefilters, and relay policies reduce bandwidth and latency.

45 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · transaction relayA transaction propagates through independent local decisionsSelect any state to inspect its exact role.

RELATIONSHIP MAPInputs, rule, and outputs

01
WALLETBroadcast tx A
The wallet submits serialized transaction bytes to one or more peers.
02
NODE 1Policy accepts
The node checks parsing, consensus preconditions, conflicts, and its local mempool policy.
03
INVAnnounce txid
Accepted candidates are advertised so peers can request missing bytes.
04
NODE 2Requests and verifies
Every peer repeats its own checks. It does not inherit Node 1's decision.
05
BOUNDARYRelay is not confirmation
A miner may include a consensus-valid transaction even if common relay policy rejected it.

Rule shown: Relay policy controls mempool and forwarding; consensus rules control whether a transaction can be included in a valid block.

STAGE 1 OF 5WALLET: Broadcast tx A

The wallet submits serialized transaction bytes to one or more peers.

Governing relationship: Relay policy controls mempool and forwarding; consensus rules control whether a transaction can be included in a valid block. Complete path: 1. WALLET, Broadcast tx A. The wallet submits serialized transaction bytes to one or more peers. 2. NODE 1, Policy accepts. The node checks parsing, consensus preconditions, conflicts, and its local mempool policy. 3. INV, Announce txid. Accepted candidates are advertised so peers can request missing bytes. 4. NODE 2, Requests and verifies. Every peer repeats its own checks. It does not inherit Node 1's decision. 5. BOUNDARY, Relay is not confirmation. A miner may include a consensus-valid transaction even if common relay policy rejected it.

1. State transitionWhat exists, what changes, and what results

BEFORE

Block contains coinbase plus A, B, C, D. Receiver already has A, C, D in its mempool.

MECHANISM

A peer sends inv or another protocol-specific announcement for a transaction or block it believes the receiver may not know.

AFTER

Compact relay saves bandwidth by predicting shared mempool contents, not by trusting the sender's block.

Text equivalent: Starting from Block contains coinbase plus A, B, C, D. Receiver already has A, C, D in its mempool. The mechanism begins when A peer sends inv or another protocol-specific announcement for a transaction or block it believes the receiver may not know. The resulting lesson is Compact relay saves bandwidth by predicting shared mempool contents, not by trusting the sender's block.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Announce an object

A peer sends inv or another protocol-specific announcement for a transaction or block it believes the receiver may not know.

Complete path: 1. Announce an object: A peer sends inv or another protocol-specific announcement for a transaction or block it believes the receiver may not know. 2. Request missing data: The receiver checks local state and requests only needed objects, subject to relay policy, rate limits, and peer behavior. 3. Validate before forwarding: A node parses and checks transactions against mempool policy or headers and blocks against consensus before trusted-state changes. 4. Use compact reconstruction: A compact block sends a header, short IDs, and selected transactions. The receiver maps short IDs to its mempool and requests any missing or ambiguous transactions. 5. Handle divergence: If reconstruction fails because mempools differ, the node requests missing transactions or the full block without weakening block validation.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

Transaction relay separately applies consensus, local mempool policy, peer fee filter, inventory request, and full-object delivery; compact blocks reconstruct committed order before validation.

CHANGED CONDITION

Policy can stop one relay path without changing consensus, while missing or ambiguous short IDs require explicit data and an invalid reconstructed transaction rejects the block.

Important distinction: Compact blocks let nodes validate only the header and skip transaction checks. They optimize transport. The receiver reconstructs the complete transaction list and performs normal block validation.

Text equivalent: The accepted path is enforced by Transaction relay separately applies consensus, local mempool policy, peer fee filter, inventory request, and full-object delivery; compact blocks reconstruct committed order before validation. If the required condition changes, Policy can stop one relay path without changing consensus, while missing or ambiguous short IDs require explicit data and an invalid reconstructed transaction rejects the block.

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Learn the idea

Bitcoin relays object announcements rather than pushing every full object immediately. Inventory (inv) messages announce transaction or block identifiers, getdata messages request selected objects, and headers messages announce chain linkage. A fee filter is a peer's announced minimum fee rate for transaction announcements. Compact-block relay reconstructs a block from short transaction identifiers plus transactions already in the receiver's mempool, reducing latency and bandwidth.

GUIDED EXPLANATION1/5 ideas inspected

Announce an object

A peer sends inv or another protocol-specific announcement for a transaction or block it believes the receiver may not know.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Compact blocks let nodes validate only the header and skip transaction checks.”

They optimize transport. The receiver reconstructs the complete transaction list and performs normal block validation.

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