Using P2C

How to receive the post-pack stream and act on it.

Payment paths (use one or both):

• PSA — selling / reselling pre-conf. You pay an epoch fee based on txn Updates.
• MCA — backrun. Share 65% of backrun profit; pre-conf is free for backrun-only (no PSA). If you also sell, fund a PSA too.

The validator connects out to your gRPC server and pushes transactions at the point of no return. Your reply bundle still needs a tip for scheduling priority — see Minimum Tip Requirements.

This page covers Relayer gRPC services, packet layout, reply bundles, and validator Admin RPC for endpoints.

Audience: Searchers and TIN partners consuming the stream; validator operators managing endpoints.

Related: Post-pack overview · PSA · MCA · Setup guide · Tips

1. Setup: Required gRPC services

Run a gRPC server and share that URL with the Rakurai team — see Setup guide. Keep a funded PSA if you resell, and/or an active MCA if you backrun — otherwise the matching stream stops.

Sample URL to share:

https://sample-server.com:20000

1.1. Services your server must expose

Protos: auth.proto, block_engine.proto.

1.2. Relayer streams — which RPC to implement

Pick your use case first. That decides which streams you need.

All streams send the same PacketBatchUpdate message type. Tell them apart by which gRPC method delivered the message — there is no source field on the packet. Details: §2.2 · Setup guide §8.1.

enable_tpu_p2c_update is set per P2C URL (on-chain / Admin). When it is false, the validator does not open the TPU stream even if your server implements it.

1.3. Reusing your block-engine URL

If one host handles both bundles (you → validator) and P2C (validator → you), register that single URL for both paths. The server must expose the Relayer role and the Validator role — otherwise bundles may work while P2C never starts.

Full details: Setup guide — same URL for bundles and P2C. Or register a separate P2C URL that implements Relayer + GetBlockEngineEndpoints (and leaves bundle subscribe RPCs unimplemented).

Once your endpoint is on-chain, opted-in Rakurai validators push transactions to you. Settle with PSA for resell, MCA for backrun, or both — see Post-pack — access and payments.

2. Transaction / packet structure

packet.proto

For each transaction, the validator sends one PacketBatchUpdate with msg = batches on either the scheduler or TPU Relayer stream (same shape; which stream you accepted is the source):

PacketBatchUpdate
  └── batches: ExpiringPacketBatch
        ├── header.ts
        ├── batch: PacketBatch
        │     └── packets[]: Packet
        │           ├── data    ← raw Solana wire transaction bytes
        │           └── meta: Meta
        │                 ├── size
        │                 ├── addr
        │                 ├── port
        │                 ├── flags: PacketFlags
        │                 └── sender_stake
        └── expiry_ms = <working-bank slot as u32>   ← field name is historical; value is a slot, not a timeout in ms

2.1. Per-slot counts (StartP2cUpdateCountStream)

When enabled, the validator pushes one P2cUpdateCount once per slot (and flushes on disconnect). Each message is a count of transactions successfully sent through the P2C update streams for that slot:

2.2. meta fields

Meta and PacketFlags are the packet gRPC types in packet.proto. Decode the transaction from data; treat meta as P2C fills it, not as a TPU/relayer packet.

PacketFlags (present in the proto, not set on P2C):

Do not use sender_stake, port, or flags to decide whether to backrun. When you reply, put the original Packet unchanged — data and meta — as the first packet(s) of the bundle.

Decode in Rust:

use solana_transaction::versioned::VersionedTransaction;

let txn: VersionedTransaction = bincode::deserialize(&packet.data)?;

packet.data is the raw Solana wire transaction. Decode it, inspect accounts and instructions, then decide whether to backrun.

3. Send a bundle

Receive the post-pack confirmation Packet, then send back a bundle (SendBundle through block-engine).

When building the bundle, include:

1. The original post-pack confirmation packet(s) unchanged (the same Packet you received)
2. Any additional transactions (e.g. backrun / arbitrage)
3. A transaction or instruction with a tip to one of Rakurai’s tip accounts

A Bundle is a header plus a list of Packets — the same packet shape as the stream:

Bundle
  ├── header
  └── packets[]: Packet     ← original post-pack packet(s) first, then your txs

4. Commands

Searcher / TIN partner (on-chain money paths):

Validator operators use Admin RPC below to inspect which post-pack endpoints are active and to blocklist services. Registering a new endpoint (adding your gRPC URL on-chain) is not done here — share your endpoint with the Rakurai team via Setup guide.

Admin IPC is request/response: keep the socket open briefly so socat can read the reply before stdin closes.

4.1. getPostPackConfirmationConfig

Returns the live status maintained by the scheduler (admin + on-chain merge, blocklist, and what is actually connected).

(echo '{"jsonrpc":"2.0","id":1,"method":"getPostPackConfirmationConfig","params":[]}'; sleep 1) \
  | socat - UNIX-CONNECT:admin.rpc | jq

Example response:

{
  "admin_entries": [
    {"url":"http://127.0.0.1:20000","uuid":"PostPackConfig2","enable_tpu_p2c_update":true},
    {"url":"http://127.0.0.1:10000","uuid":"PostPackConfig1","enable_tpu_p2c_update":false}
  ],
  "onchain_entries": [],
  "blocklisted_uuids": ["PostPackConfig1"],
  "blocklisted_entries": [
    {"url":"http://127.0.0.1:10000","uuid":"PostPackConfig1","enable_tpu_p2c_update":false}
  ],
  "active_entries": [
    {"url":"http://127.0.0.1:20000","uuid":"PostPackConfig2","enable_tpu_p2c_update":true}
  ]
}

4.2. setPostPackConfirmationUuidBlocklist

Blocklists post-pack confirmation endpoints by UUID. Each call replaces the full blocklist. Pass an empty array to clear.

Blocklisted UUIDs are removed from active_entries on the next scheduler config sync. If a blocklisted endpoint already has an open gRPC connection, it is torn down immediately on sync; other endpoints stay connected.

Example — block one endpoint by UUID:

(echo '{"jsonrpc":"2.0","id":1,"method":"setPostPackConfirmationUuidBlocklist","params":[["PostPackConfig1"]]}'; sleep 1) \
  | socat - UNIX-CONNECT:admin.rpc

Example — clear blocklist (reconnect blocklisted endpoints on next sync):

(echo '{"jsonrpc":"2.0","id":1,"method":"setPostPackConfirmationUuidBlocklist","params":[[]]}'; sleep 1) \
  | socat - UNIX-CONNECT:admin.rpc

Related

• Tips — tip for reply bundles and virtual priority
• PSA · MCA
• Setup guide — Validator gRPC and discovery