# Routing

Understand SWQoS leader delivery, block-engine delivery and transaction privacy.

Source: https://docs.zerocopy.sh/sending/routing

This page describes routing at hosted SWQoS edges. Routes can change, as described in [Terms 5.1](https://zerocopy.sh/terms#sending-1). A response does not identify which path was used.

## Leader delivery [#leaders]

SWQoS forwards each accepted transaction to upcoming leaders over stake-weighted QUIC. The signed transaction bytes are unchanged.

## Block-engine delivery [#engines]

SWQoS may also submit the transaction to the leader's block engine, Jito, Harmonic or Jito BAM, which can place it in a bundle with a ZeroCopy transaction. See [Terms 5.1](https://zerocopy.sh/terms#sending-1).

Submission does not mean an engine will accept the bundle or that any transaction will land. Confirm the signature through your [Solana RPC provider](/sending/confirmation#confirmation).

## Explicit bundles [#bundles]

`sendBundle` goes to block engines only; it has no direct leader route. It accepts one to four customer transactions in execution order. ZeroCopy may add its own transaction to the bundle. See [Send a bundle](/sending/bundles) for tips, control accounts and bundle limits.

If no eligible engine is available, the request returns [`not_ready`](/errors#not_ready) with `waiting_for: block_engine`. Do not submit members separately if they must execute together.

## Preflight and resubmission [#preflight]

Preflight is off by default. When requested, simulation runs on ZeroCopy's RPC infrastructure provider before dispatch. See [preflight options](/sending/transactions#preflight) and [Terms 5.2](https://zerocopy.sh/terms#sending-2).

SWQoS does not resend accepted transactions or bundles. `maxRetries` is ignored. A recently repeated single-transaction signature may be accepted without being sent again. Do not rely on this as global idempotency; explicit bundle resubmissions are not suppressed. Your application owns [confirmation and later resubmission](/sending/confirmation).

## Transaction privacy [#privacy]

SWQoS has no mempool. Delivery is limited to validators scheduled for the next blocks and the block engines described above. Requested preflight also sends the transaction to the RPC infrastructure provider. See [Terms 5.5](https://zerocopy.sh/terms#sending-5).