Bitcoin Book of BIPs

BIP 144

Abstract

This BIP defines new messages and serialization formats for propagation of transactions and blocks committing to segregated witness structures.

Motivation

In addition to defining witness structures and requiring commitments in future blocks (BIP141 - Consensus segwit BIP), new mechanisms must be defined to allow peers to advertise support for segregated witness and to relay the witness structures and request them from other peers without breaking compatibility with older nodes.

Specification

Serialization

A new serialization format for tx messages is added to the peer-to-peer protocol.

The serialization has the following structure:

Field Size Name Type Description
4 version int32_t Transaction data format version
1 marker char Must be zero
1 flag char Must be nonzero
1+ txin_count var_int Number of transaction inputs
41+ txins txin[] A list of one or more transaction inputs
1+ txout_count var_int Number of transaction outputs
9+ txouts txouts[] A list of one or more transaction outputs
1+ script_witnesses script_witnesses[] The witness structure as a serialized byte array
4 lock_time uint32_t The block number or timestamp until which the transaction is locked

Parsers supporting this BIP will be able to distinguish between the old serialization format (without the witness) and this one. The marker byte is set to zero so that this structure will never parse as a valid transaction in a parser that does not support this BIP. If parsing were to succeed, such a transaction would contain no inputs and a single output.

If the witness is empty, the old serialization format must be used.

Currently, the only witness objects type supported are script witnesses which consist of a stack of byte arrays. It is encoded as a var_int item count followed by each item encoded as a var_int length followed by a string of bytes. Each txin has its own script witness. The number of script witnesses is not explicitly encoded as it is implied by txin_count. Empty script witnesses are encoded as a zero byte. The order of the script witnesses follows the same order as the associated txins.

Handshake

A node will signal that it can provide witnesses using the following service bit

   NODE_WITNESS = (1 << 3)
   

Hashes

Transaction hashes used in the transaction merkle tree and txin outpoints are always computed using the old non-witness serialization.

Support for a new hash including the witness data is added that is computed from the new witness serialization. (Note that transactions with an empty witness always use the old serialization, and therefore, they have witness hash equal to normal hash.)

<img src=bip-0144/witnesstx.png></img>

Relay

New inv types MSG_WITNESS_TX (0x40000001, or (1<<30)+MSG_TX) and MSG_WITNESS_BLOCK (0x40000002, or (1<<30)+MSG_BLOCK) are added, only for use in getdata. Inventory messages themselves still use just MSG_TX and MSG_BLOCK, similar to MSG_FILTERED_BLOCK. A further inv type MSG_FILTERED_WITNESS_BLOCK (0x40000003, or (1<<30)+MSG_FILTERED_BLOCK) is reserved for future use.

MSG_WITNESS_TX getdata requests should use the non-witness serialized hash. The peer shall respond with a tx message, and if the witness structure is nonempty, the witness serialization shall be used.

MSG_WITNESS_BLOCK requests will return a block message with transactions that have a witness using witness serialization.

Credits

Special thanks to Gregory Maxwell for originating many of the ideas in this BIP and Luke-Jr for figuring out how to deploy this as a soft fork.

Reference Implementation

https://github.com/bitcoin/bitcoin/pull/8149

This document is placed in the public domain.

  BIP: 152
  Layer: Peer Services
  Title: Compact Block Relay
  Authors: Matt Corallo <bip152@bluematt.me>
  Comments-Summary: Unanimously Recommended for implementation
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0152
  Status: Deployed
  Type: Specification
  Assigned: 2016-04-27
  License: PD