<pre>
  BIP: 61
  Layer: Peer Services
  Title: Reject P2P message
  Authors: Gavin Andresen <gavinandresen@gmail.com>
  Comments-Summary: Controversial; some recommendation, and some discouragement
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0061
  Status: Deployed
  Type: Specification
  Assigned: 2014-06-18
</pre>

==Abstract==

This BIP describes a new message type for the Bitcoin peer-to-peer network.

==Motivation==

Giving peers feedback about why their blocks or transactions are rejected, or
why they are being banned for not following the protocol helps
interoperability between different implementations.

It also gives SPV (simplified payment verification) clients a hint that something
may be wrong when their transactions are rejected due to insufficient priority
or fees.

==Specification==

Data types in this specification are as described at https://en.bitcoin.it/wiki/Protocol_specification

===reject===

One new message type "reject" is introduced. It is sent directly to a peer in response to a "version", "tx" or "block" message.

For example, the message flow between two peers for a relayed transaction that is rejected for some reason would be:

 --> inv
 <-- getdata
 --> tx
 <-- reject

All implementations of the P2P protocol version 70,002 and later should support the reject message.

====common payload====


Every reject message begins with the following fields. Some messages append extra, message-specific data.

{|
| Field Size || Name || Data type || Comments
|-
| variable || response-to-msg || var_str || Message that triggered the reject
|-
| 1 || reject-code || uint8_t || 0x01 through 0x4f (see below)
|-
| variable || reason || var_string || Human-readable message for debugging
|}

The human-readable string is intended only for debugging purposes; in particular, different implementations may
use different strings. The string should not be shown to users or used for anything besides diagnosing
interoperability problems.

The following reject code categories are used; in the descriptions below, "server" is the peer generating
the reject message, "client" is the peer that will receive the message.

{|
| Range || Category
|-
| 0x01-0x0f || Protocol syntax errors
|-
| 0x10-0x1f || Protocol semantic errors
|-
| 0x40-0x4f || Server policy rule
|}

==== rejection codes common to all message types ====

{|
| Code || Description
|-
| 0x01 || Message could not be decoded
|}

==== reject version codes ====

Codes generated during the initial connection process in response to a "version" message:

{|
| Code || Description
|-
| 0x11 || Client is an obsolete, unsupported version
|-
| 0x12 || Duplicate version message received
|}

==== reject tx payload, codes ====

Transaction rejection messages extend the basic message with the transaction id hash:

{|
| Field Size || Name || Data type || Comments
|-
| 32 || hash || char[32] || transaction that is rejected
|}

The following codes are used:

{|
| Code || Description
|-
| 0x10 || Transaction is invalid for some reason (invalid signature, output value greater than input, etc.)
|-
| 0x12 || An input is already spent
|-
| 0x40 || Not mined/relayed because it is "non-standard" (type or version unknown by the server)
|-
| 0x41 || One or more output amounts are below the 'dust' threshold
|-
| 0x42 || Transaction does not have enough fee/priority to be relayed or mined
|}

==== payload, reject block ====

Block rejection messages extend the basic message with the block header hash:

{|
| Field Size || Name || Data type || Comments
|-
| 32 || hash || char[32] || block (hash of block header) that is rejected
|}

Rejection codes:

{|
| code || description
|-
| 0x10 || Block is invalid for some reason (invalid proof-of-work, invalid signature, etc)
|-
| 0x11 || Block's version is no longer supported
|-
| 0x43 || Inconsistent with a compiled-in checkpoint
|}

Note: blocks that are not part of the server's idea of the current best chain, but are otherwise valid, should not trigger reject messages.

== Compatibility ==

The reject message is backwards-compatible; older peers that do not recognize the reject message will ignore it.

== Implementation notes ==

Implementers must consider what happens if an attacker either sends them
reject messages for valid transactions/blocks or sends them random
reject messages, and should beware of possible denial-of-service attacks.
For example, notifying the user of every reject message received
would make it trivial for an attacker to mount an annoy-the-user attack.
Even merely writing every reject message to a debugging log could make
an implementation vulnerable to a fill-up-the-users-disk attack.
