Subject: [bitcoin-dev] Compact Block Relay BIP

From: Matt Corallo <[email-masked]>
Date: Mon, 2 May 2016 22:13:22 +0000

Hi all,

The following is a BIP-formatted design spec for compact block relay
designed to limit on wire bytes during block relay. You can find the
latest version of this document at
https://github.com/TheBlueMatt/bips/blob/master/bip-TODO.mediawiki.

There are several TODO items left on the document as indicated.
Additionally, the implementation linked at the bottom of the document
has a few remaining TODO items as well:

 * Only request compact-block-announcement from one or two peers at a
time, as the spec requires.
 * Request new blocks using MSG_CMPCT_BLOCK where appropriate.
 * Fill prefilledtxn with more than just the coinbase, as noted by the
spec, up to 10K in transactions.

Luke (CC'd): Can you assign a BIP number?

Thanks,
Matt

<pre>
  BIP: TODO
  Title: Compact block relay
  Author: Matt Corallo <[email-masked]>
  Status: Draft
  Type: Standards Track
  Created: 2016-04-27
</pre>

==Abstract==

Compact blocks on the wire as a way to save bandwidth for nodes on the
P2P network.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.

==Motivation==

Historically, the Bitcoin P2P protocol has not been very bandwidth
efficient for block relay. Every transaction in a block is included when
relayed, even though a large number of the transactions in a given block
are already available to nodes before the block is relayed. This causes
moderate inbound bandwidth spikes for nodes when receiving blocks, but
can cause very significant outbound bandwidth spikes for some nodes
which receive a block before their peers. When such spikes occur, buffer
bloat can make consumer-grade internet connections temporarily unusable,
and can delay the relay of blocks to remote peers who may choose to wait
instead of redundantly requesting the same block from other, less
congested, peers.

Thus, decreasing the bandwidth used during block relay is very useful
for many individuals running nodes.

While the goal of this work is explicitly not to reduce block transfer
latency, it does, as a side effect reduce block transfer latencies in
some rather significant ways. Additionally, this work forms a foundation
for future work explicitly targeting low-latency block transfer.

==Specification==

===Intended Protocol Flow===
TODO: Diagrams

The protocol is intended to be used in two ways, depending on the peers
and bandwidth available, as discussed [[#Implementation_Details|later]].
The "high-bandwidth" mode, which nodes may only enable for a few of
their peers, is enabled by setting the first boolean to 1 in a
"sendcmpct" message. In this mode, peers send new block announcements
with the short transaction IDs already, possibly even before fully
validating the block. In some cases no further round-trip is needed, and
the receiver can reconstruct the block and process it as usual
immediately. When some transactions were not available from local
sources (ie mempool), a getblocktxn/blocktxn roundtrip is neccessary,
bringing the best-case latency to the same 1.5*RTT minimum time that
nodes take today, though with significantly less bandwidth usage.

The "low-bandwidth" mode is enabled by setting the first boolean to 0 in
a "sendcmpct" message. In this mode, peers send new block announcements
with the usual inv/headers announcements (as per BIP130, and after fully
validating the block). The receiving peer may then request the block
using a MSG_CMPCT_BLOCK getdata reqeuest, which will receive a response
of the header and short transaction IDs. In some cases no further
round-trip is needed, and the receiver can reconstruct the block and
process it as usual, taking the same 1.5*RTT minimum time that nodes
take today, though with significantly less bandwidth usage. When some
transactions were not available from local sources (ie mempool), a
getblocktxn/blocktxn roundtrip is neccessary, bringing the best-case
latency to 2.5*RTT, again with significantly less bandwidth usage than
today. Because TCP often exhibits worse transfer latency for larger data
sizes (as a multiple of RTT), total latency is expected to be reduced
even when full the 2.5*RTT transfer mechanism is used.

===New data structures===
Several new data structures are added to the P2P network to relay
compact blocks: PrefilledTransaction, HeaderAndShortIDs,
BlockTransactionsRequest, and BlockTransactions. Additionally, we
introduce a new variable-length integer encoding for use in these data
structures.

For the purposes of this section, CompactSize refers to the
variable-length integer encoding used across the existing P2P protocol
to encode array lengths, among other things, in 1, 3, 5 or 9 bytes.

====New VarInt====
TODO: I just copied this out of the src...Something that is
wiki-formatted and more descriptive should be used here isntead.

Variable-length integers: bytes are a MSB base-128 encoding of the number.
The high bit in each byte signifies whether another digit follows. To make
sure the encoding is one-to-one, one is subtracted from all but the last
digit.
Thus, the byte sequence a[] with length len, where all but the last byte
has bit 128 set, encodes the number:

(a[len-1] & 0x7F) + sum(i=1..len-1, 128^i*((a[len-i-1] & 0x7F)+1))

Properties:
* Very small (0-127: 1 byte, 128-16511: 2 bytes, 16512-2113663: 3 bytes)
* Every integer has exactly one encoding
* Encoding does not depend on size of original integer type
* No redundancy: every (infinite) byte sequence corresponds to a list
  of encoded integers.

0:         [0x00]  256:        [0x81 0x00]
1:         [0x01]  16383:      [0xFE 0x7F]
127:       [0x7F]  16384:      [0xFF 0x00]
128:  [0x80 0x00]  16511: [0x80 0xFF 0x7F]
255:  [0x80 0x7F]  65535: [0x82 0xFD 0x7F]
2^32:           [0x8E 0xFE 0xFE 0xFF 0x00]

Several uses of New VarInts below are "differentially encoded". For
these, instead of using raw indexes, the number encoded is the
difference between the current index and the previous index, minus one.
For example, a first index of 0 implies a real index of 0, a second
index of 0 thereafter refers to a real index of 1, etc.

====PrefilledTransaction====
A PrefilledTransaction structure is used in HeaderAndShortIDs to provide
a list of a few transactions explicitly.

{|
|Field Name||Type||Size||Encoding||Purpose
|-
|index||New VarInt||1-3 bytes||[[#New_VarInt|New VarInt]],
differentially encoded since the last PrefilledTransaction in a
list||The index into the block at which this transaction is
|-
|tx||Transaction||variable||As encoded in "tx" messages||The transaction
which is in the block at index index.
|}

====HeaderAndShortIDs====
A HeaderAndShortIDs structure is used to relay a block header, the short
transactions IDs used for matching already-available transactions, and a
select few transactions which we expect a peer may be missing.

{|
|Field Name||Type||Size||Encoding||Purpose
|-
|header||Block header||80 bytes||First 80 bytes of the block as defined
by the encoding used by "block" messages||The header of the block being
provided
|-
|nonce||uint64_t||8 bytes||Little Endian||A nonce for use in short
transaction ID calculations
|-
|shortids_length||CompactSize||1, 3, 5, or 9 bytes||As used elsewhere to
encode array lengths||The number of short transaction IDs in shortids
|-
|shortids||List of uint64_ts||8*shortids_length bytes||Little
Endian||The short transaction IDs calculated from the transactions which
were not provided explicitly in prefilledtxn
|-
|prefilledtxn_length||CompactSize||1, 3, 5, or 9 bytes||As used
elsewhere to encode array lengths||The number of prefilled transactions
in prefilledtxn
|-
|prefilledtxn||List of PrefilledTransactions||variable
size*prefilledtxn_length||As defined by PrefilledTransaction definition,
above||Used to provide the coinbase transaction and a select few which
we expect a peer may be missing
|}

====BlockTransactionsRequest====
A BlockTransactionsRequest structure is used to list transaction indexes
in a block being requested.

{|
|Field Name||Type||Size||Encoding||Purpose
|-
|blockhash||Binary blob||32 bytes||The output from a double-SHA256 of
the block header, as used elsewhere||The blockhash of the block which
the transactions being requested are in
|-
|indexes_length||New VarInt||1-3 bytes||As defined in [[#New_VarInt|New
VarInt]]||The number of transactions being requested
|-
|indexes||List of New VarInts||1-3 bytes*indexes_length||As defined in
[[#New_VarInt|New VarInt]], differentially encoded||The indexes of the
transactions being requested in the block
|}

====BlockTransactions====
A BlockTransactions structure is used to provide some of the
transactions in a block, as requested.

{|
|Field Name||Type||Size||Encoding||Purpose
|-
|blockhash||Binary blob||32 bytes||The output from a double-SHA256 of
the block header, as used elsewhere||The blockhash of the block which
the transactions being provided are in
|-
|transactions_length||New VarInt||1-3 bytes||As defined in
[[#New_VarInt|New VarInt]]||The number of transactions provided
|-
|transactions||List of Transactions||variable||As encoded in "tx"
messages||The transactions provided
|}

====Short transaction IDs====
Short transaction IDs are used to represent a transaction without
sending a full 256-bit hash. They are calculated by:
# single-SHA256 hashing the block header with the nonce appended (in
little-endian)
# XORing each 8-byte chunk of the double-SHA256 transaction hash with
each corresponding 8-byte chunk of the hash from the previous step
# Adding each of the XORed 8-byte chunks together (in little-endian)
iteratively to find the short transaction ID

===New messages===
A new inv type (MSG_CMPCT_BLOCK == 4) and several new protocol messages
are added: sendcmpct, cmpctblock, getblocktxn, and blocktxn.

====sendcmpct====
# The sendcmpct message is defined as a message containing a 1-byte
integer followed by a 8-byte integer where pchCommand == "sendcmpct".
# The first integer SHALL be interpreted as a boolean (and MUST have a
value of either 1 or 0)
# The second integer SHALL be interpreted as a little-endian version
number. Nodes sending a sendcmpct message MUST currently set this value
to 1.
# Upon receipt of a "sendcmpct" message with the first and second
integers set to 1, the node SHOULD announce new blocks by sending a
cmpctblock message.
# Upon receipt of a "sendcmpct" message with the first integer set to 0,
the node SHOULD NOT announce new blocks by sending a cmpctblock message,
but SHOULD announce new blocks by sending invs or headers, as defined by
BIP130.
# Upon receipt of a "sendcmpct" message with the second integer set to
something other than 1, nodes SHOULD treat the peer as if they had not
received the message (as it indicates the peer will provide an
unexpected encoding in cmpctblock, and/or other, messages)
# Nodes SHOULD check for a protocol version of >= 70014 before sending
sendcmpct messages.
# Nodes MUST NOT send a request for a MSG_CMPCT_BLOCK object to a peer
before having received a sendcmpct message from that peer.

====MSG_CMPCT_BLOCK====
# getdata messages may now contain requests for MSG_CMPCT_BLOCK objects.
# Upon receipt of a getdata containing a request for a MSG_CMPCT_BLOCK
object with the hash of a block which was recently announced and after
having sent the requesting peer a sendcmpct message, nodes MUST respond
with a cmpctblock message containing appropriate data representing the
block being requested.
# MSG_CMPCT_BLOCK inv objects MUST NOT appear anywhere except for in
getdata messages.

====cmpctblock====
# The cmpctblock message is defined as as a message containing a
serialized HeaderAndShortIDs message and pchCommand == "cmpctblock".
# Upon receipt of a cmpctblock message after sending a sendcmpct
message, nodes SHOULD calculate the short transaction ID for each
unconfirmed transaction they have available (ie in their mempool) and
compare each to each short transaction ID in the cmpctblock message.
# After finding already-available transactions, nodes which do not have
all transactions available to reconstruct the full block SHOULD request
the missing transactions using a getblocktxn message.
# A node MUST NOT send a cmpctblock message unless they are able to
respond to a getblocktxn message which requests every transaction in the
block.
# A node MUST NOT send a cmpctblock message without having validated
that the header properly commits to each transaction in the block, and
properly builds on top of the existing chain with a valid proof-of-work.
A node MAY send a cmpctblock before validating that each transaction in
the block validly spends existing UTXO set entries.

====getblocktxn====
# The getblocktxn message is defined as as a message containing a
serialized BlockTransactionsRequest message and pchCommand == "getblocktxn".
# Upon receipt of a properly-formatted getblocktxnmessage, nodes which
recently provided the sender of such a message a cmpctblock for the
block hash identified in this message MUST respond with an appropriate
blocktxn message. Such a blocktxn message MUST contain exactly and only
each transaction which is present in the appropriate block at the index
specified in the getblocktxn indexes list, in the order requested.

====blocktxn====
# The blocktxn message is defined as as a message containing a
serialized BlockTransactions message and pchCommand == "blocktxn".
# Upon receipt of a properly-formatted requested blocktxn message, nodes
SHOULD attempt to reconstruct the full block by:
## Taking the prefilledtxn transactions from the original cmpctblock and
placing them in the marked positions.
## For each short transaction ID from the original cmpctblock, in order,
find the corresponding transaction either from the blocktxn message or
from other sources and place it in the first available position in the
block.
# Once the block has been reconstructed, it shall be processed as
normal, keeping in mind that short transaction IDs are expected to
occasionally collide, and that nodes MUST NOT be penalized for such
collisions, wherever they appear.

===Implementation Notes===
# For nodes which have sufficient inbound bandwidth, sending a sendcmpct
message with the first integer set to 1 to up to three peers is
RECOMMENDED. If possible, it is RECOMMENDED that those peers be selected
based on their past performance in providing blocks quickly. This will
allow them to receive some blocks in only 0.5*RTT between them and the
sending peer. It will also reduce their block transfer latency in other
cases due to the smaller amount of data transmitted. Nodes MUST NOT send
such sendcmpct messages to all peers, as it encourages wasting outbound
bandwidth across the network.

# All nodes SHOULD send a sendcmpct message to all appropriate peers.
This will reduce their outbound bandwidth usage by allowing their peers
to request compact blocks instead of full blocks.

# Nodes with limited inbound bandwidth SHOULD request blocks using
MSG_CMPCT_BLOCK/getblocktxn requests, when possible. While this
increases worst-case message round-trips, it is expected to reduce
overall transfer latency as TCP is more likely to exhibit poor
throughput on low-bandwidth nodes.

# Nodes sending cmpctblock messages SHOULD make an attempt to not place
too many transactions into prefilledtxn (ie should limit prefilledtxn to
only around 10KB of transactions). When in doubt, nodes SHOULD only
include the coinbase transaction in prefilledtxn.

# Nodes MAY pick one nonce per block they wish to send, and only build a
cmpctblock message once for all peers which they wish to send a given
block to. Nodes SHOULD NOT use the same nonce across multiple different
blocks.

# Nodes MAY impose additional requirements on when they announce new
blocks by sending cmpctblock messages. For example, nodes with limited
outbound bandwidth MAY choose to announce new blocks using inv/header
messages (as per BIP130) to conserve outbound bandwidth.

# Note that the MSG_CMPCT_BLOCK section does not require that nodes
respond to MSG_CMPCT_BLOCK getdata requests for blocks which they did
not recently announce. This allows nodes to calculate cmpctblock
messages at announce-time instead of at request-time. Thus, nodes MUST
NOT request blocks using MSG_CMPCT_BLOCK getdatas unless it is in
response to an inv/headers block announcement (as per BIP130), and MUST
NOT request blocks using MSG_CMPCT_BLOCK getdatas in response to headers
messages which were, themselves, responses to getheaders requests.

# While the current version sends transactions with the same encodings
as is used in tx messages and elsewhere in the protocol, the version
field in sendcmpct is intended to allow this to change in the future.
For this reason, it is recommended that the code used to decode
PrefilledTransaction and BlockTransactions messages be prepared to take
a different transaction encoding, if and when the version field in
sendcmpct changes in a future BIP.

==Justification==

====Protocol design====
There have been many proposals to save wire bytes when relaying blocks.
Many of them have a two-fold goal of reducing block relay time and thus
rely on the use of significant processing power in order to avoid
introducing additional worst-case RTTs. Because this work is not focused
primarily on reducing block relay time, its design is much simpler (ie
does not rely on set reconciliation protocols). Still, in testing at the
time of writing, nodes are able to relay blocks without the extra
getblocktxn/blocktxn RTT around 90% of the time. With a smart
compact-block-announcement policy, it is thus expected that this work
might allow blocks to be relayed between nodes in 0.5*RTT instead of
1.5*RTT at least 75% of the time.

====Use of New VarInts====
Bitcoin has long had a variable-length integer implementation (referred
to as CompactSize in this document), making a second a strange protocol
quirk. However, in this protocol most of our variable-length integers
are between 0 and 2000. For both encodings, small numbers (<100) are
encoded as 1-byte. For numbers over 250, the CompactSize encoding begins
to use 3 bytes instead of 1, whereas the New VarInt encoding uses 2.
Because the primary motivation for this work is to save bytes during
block relay, the extra byte of saving per transaction-difference is
considered worth the extra design complexity.

====Short transaction ID calculation====
The short transaction ID calculation is designed to take absolutely
minimal processing time during block compaction to avoid introducing
serious DoS vulnerabilities such as those introduced by the
bloom-filtering in BIP 37. As such, it is possible for a node to
construct one compact-block representation of a block for relay to
multiple peers. Additionally, only one cryptographic hash (2 SHA rounds)
is used when calculating the short transaction IDs for an entire block.

The XOR-and-add method is used for calculating short transaction IDs
primarily because it is fast and is reasonably able to limit the ability
of an attacker who does not know the block hash or nonce to cause
collisions in short transaction IDs. If an attacker were able to cause
such collisions, filling mempools (and, thus, blocks) with them would
cause poor network propagation of new (or non-attacker, in the case of a
miner) blocks.

The 8-byte nonce in short transaction ID calculation is used to
introduce additional entropy on a per-node level. While the use of 8
bytes is sufficient for an attacker to maliciously cause short
transaction ID collisions in their own block relay, this would have less
of an effect than if such an attacker were relaying headers/invs and not
responding to requests for the full block.

==Backward compatibility==

Older clients remain fully compatible and interoperable after this change.

==Implementation==

https://github.com/TheBlueMatt/bitcoin/tree/udp

==Acknowledgements==

Thanks to Gregory Maxwell for the initial suggestion as well as a lot of
back-and-forth design and significant testing.

==Copyright==

This document is placed in the public domain.