The Stellar Consensus Protocol (SCP)
draft-mazieres-dinrg-scp-04
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| Authors | Nicolas Barry , Giuliano Losa , David Mazieres , Jed McCaleb , Stanislas Polu | ||
| Last updated | 2018-06-29 (Latest revision 2018-06-13) | ||
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draft-mazieres-dinrg-scp-04
Network Working Group N. Barry
Internet-Draft Stellar Development Foundation
Intended status: Experimental G. Losa
Expires: December 31, 2018 UCLA
D. Mazieres
Stanford University
J. McCaleb
Stellar Development Foundation
S. Polu
Stripe Inc.
June 29, 2018
The Stellar Consensus Protocol (SCP)
draft-mazieres-dinrg-scp-04
Abstract
SCP is an open Byzantine agreement protocol resistant to Sybil
attacks. It allows Internet infrastructure stakeholders to reach
agreement on a series of values without unanimous agreement on what
constitutes the set of important stakeholders. A big differentiator
from other Byzantine agreement protocols is that, in SCP, nodes
determine the composition of quorums in a decentralized way: each
node selects sets of nodes it considers large or important enough to
speak for the whole network, and a quorum must contain such a set for
each of its members.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 31, 2018.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Model . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Configuration . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Input and output . . . . . . . . . . . . . . . . . . . . 4
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Federated voting . . . . . . . . . . . . . . . . . . . . 5
3.2. Basic types . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Quorum slices . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Nominate message . . . . . . . . . . . . . . . . . . . . 10
3.5. Ballots . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.6. Prepare message . . . . . . . . . . . . . . . . . . . . . 13
3.7. Commit message . . . . . . . . . . . . . . . . . . . . . 16
3.8. Externalize message . . . . . . . . . . . . . . . . . . . 18
3.9. Summary of phases . . . . . . . . . . . . . . . . . . . . 18
3.10. Message envelopes . . . . . . . . . . . . . . . . . . . . 19
4. Security considerations . . . . . . . . . . . . . . . . . . . 20
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1. Normative References . . . . . . . . . . . . . . . . . . 21
6.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Various aspects of Internet infrastructure depend on irreversible and
transparent updates to data sets such as authenticated mappings [cite
Li-Man-Watson draft]. Examples include public key certificates and
revocations, transparency logs [RFC6962], preload lists for HSTS
[RFC6797] and HPKP [RFC7469], and IP address delegation
[I-D.paillisse-sidrops-blockchain].
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The Stellar Consensus Protocol (SCP) specified in this draft allows
Internet infrastructure stakeholders to collaborate in applying
irreversible transactions to public state. SCP is an open Byzantine
agreement protocol that resists Sybil attacks by allowing individual
parties to specify minimum quorum memberships in terms of specific
trusted peers. Each participant chooses combinations of peers on
which to depend such that these combinations can be trusted in
aggregate. The protocol guarantees safety so long as these
dependency sets transitively overlap and contain sufficiently many
honest nodes correctly obeying the protocol.
Though bad configurations are theoretically possible, several
analogies provide an intuition for why transitive dependencies
overlap in practice. For example, given multiple entirely disjoint
Internet-protocol networks, people would have no trouble agreeing on
the fact that the network containing the world's top web sites is
_the_ Internet. Such a consensus can hold even without unanimous
agreement on what constitute the world's top web sites. Similarly,
if network operators listed all the ASes from whom they would
consider peering or transit worthwhile, the transitive closures of
these sets would contain significant overlap, even without unanimous
agreement on the "tier-1 ISP" designation. Finally, while different
browsers and operating systems have slightly different lists of valid
certificate authorities, there is significant overlap in the sets, so
that a hypothetical system requiring validation from "all CAs" would
be unlikely to diverge.
A more detailed abstract description of SCP and its rationale,
including an English-language proof of safety, is available in [SCP].
In particular, that reference shows that a necessary property for
safety, termed _quorum intersection despite ill-behaved nodes_, is
sufficient to guarantee safety under SCP, making SCP optimally safe
against Byzantine node failure for any given configuration.
This document specifies the end-system logic and wire format of the
messages in SCP.
2. The Model
This section describes the configuration and input/output values of
the consensus protocol.
2.1. Configuration
Each participant or _node_ in the SCP protocol has a digital
signature key and is named by the corresponding public key, which we
term a "NodeID".
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Each node also selects one or more sets of nodes (each of which
includes itself) called _quorum slices_. A quorum slice represents a
large or important enough set of peers that the node selecting the
quorum slice believes the slice collectively speaks for the whole
network.
A _quorum_ is a non-empty set of nodes containing at least one quorum
slice of each of its members. For instance, suppose "v1" has the
single quorum slice "{v1, v2, v3}", while each of "v2", "v3", and
"v4" has the single quorum slice "{v2, v3, v4}". In this case, "{v2,
v3, v4}" is a quorum because it contains a slice for each member. On
the other hand "{v1, v2, v3}" is not a quorum, because it does not
contain a quorum slice for "v2" or "v3". The smallest quorum
including "v1" in this example is the set of all nodes "{v1, v2, v3,
v4}".
Unlike traditional Byzantine agreement protocols, nodes in SCP only
care about quorums to which they belong themselves (and hence that
contain at least one of their quorum slices). Intuitively, this is
what protects nodes from Sybil attacks. In the example above, if
"v3" deviates from the protocol, maliciously inventing 96 Sybils "v5,
v6, ..., v100", the honest nodes' quorums will all still include one
another, ensuring that "v1", "v2", and "v4" continue to agree on
output values.
Every message in the SCP protocol specifies the sender's quorum
slices. Hence, by collecting messages, a node dynamically learns
what constitutes a quorum and can decide when a particular message
has been sent by a quorum to which it belongs. (Again, nodes do not
care about quorums to which they do not belong themselves.)
2.2. Input and output
SCP produces a series of output _values_ for consecutively numbered
_slots_. At the start of a slot, higher-layer software on each node
supplies a candidate input value. Nodes then exchange protocol
messages to agree on one or a combination of nodes' input values as
the slot's output value. After a pause to assemble new input values,
the process repeats for the next slot, with a 5-second interval
between slots.
A value typically encodes a set of actions to apply to a replicated
state machine. During the pause between slots, nodes accumulate the
next set of actions, amortizing the cost of consensus on one slot
over arbitrarily many individual state machine operations.
In practice, only one or a small number of nodes' input values
actually affect the output value for any given slot. As discussed in
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Section 3.4, which nodes' input values to use depends on a
cryptographic hash of the slot number and node public keys. A node's
chances of affecting the output value depend on how often it appears
in other nodes' quorum slices.
From SCP's perspective, values are just opaque byte arrays whose
interpretation is left to higher-layer software. However, SCP
requires a _validity_ function (to check whether a value is valid)
and a _combining function_ that reduces multiple candidate values
into a single _composite_ value. When nodes nominate multiple values
for a slot, SCP nodes invoke this function to converge on a single
composite value. By way of example, in an application where values
consist of sets of transactions, the combining function could take
the union of transaction sets. Alternatively, if values represent a
timestamp and a set of transactions, the combining function might
pair the highest nominated timestamp with the transaction set that
has the highest hash value.
3. Protocol
The protocol consists of exchanging digitally-signed messages bound
to nodes' quorum slices. The format of all messages is specified
using XDR [RFC4506]. In addition to quorum slices, messages
compactly convey votes on sets of conceptual statements. The core
technique of voting with quorum slices is termed _federated voting_.
We describe federated voting next, then detail protocol messages in
the subsections that follow.
The protocol goes through four phases: NOMINATE, PREPARE, COMMIT, and
EXTERNALIZE. The NOMINATE and PREPARE phases run concurrently
(though NOMINATE's messages are sent earlier and it ends before
PREPARE ends). The COMMIT and EXTERNALIZE phrases are exclusive,
with COMMIT occurring immediately after PREPARE and EXTERNALIZE
immediately after COMMIT.
3.1. Federated voting
Federated voting is a process through which nodes _confirm_
statements. Not every attempt at federated voting may succeed--an
attempt to vote on some statement "a" may get stuck, with the result
that nodes can confirm neither "a" nor its negation "!a". However,
when a node succeeds in confirming a statement "a", federated voting
guarantees two things:
1. No two well-behaved nodes will confirm contradictory statements
in any configuration and failure scenario in which any protocol
can guarantee safety for the two nodes (i.e., quorum intersection
for the two nodes holds despite ill-behaved nodes).
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2. If a node that is guaranteed safety by #1 confirms a statement
"a", and that node is a member of one or more quorums consisting
entirely of well-behaved nodes, then eventually every member of
every such quorum will also confirm "a".
Intuitively, these conditions are key to ensuring agreement among
nodes as well as a weak form of liveness (the non-blocking property
[building-blocks]) that is compatible with the FLP impossibility
result [FLP].
As a node "v" collects signed copies of a federated voting message
"m" from peers, two thresholds trigger state transitions in "v"
depending on the message. We define these thresholds as follows:
o _quorum threshold_: When every member of a quorum to which "v"
belongs (including "v" itself) has issued message "m"
o _blocking threshold_: When at least one member of each of "v"'s
quorum slices (a set that does not necessarily include "v" itself)
has issued message "m"
Each node "v" can send several types of message with respect to a
statement "a" during federated voting:
o _vote_ "a" states that "a" is a valid statement and constitutes a
promise by "v" not to vote for any contradictory statement, such
as "!a".
o _accept_ "a" says that nodes may or may not come to agree on "a",
but if they don't, then the system has experienced a catastrophic
set of Byzantine failures to the point that no quorum containing
"v" consists entirely of correct nodes. (Nonetheless, accepting
"a" is not sufficient to act on it, as doing so could violate
agreement, which is worse than merely getting stuck from lack of a
correct quorum.)
o _vote-or-accept_ "a" is the disjunction of the above two messages.
A node implicitly sends such a message if it sends either _vote_
"a" or _accept_ "a". Where it is inconvenient and unnecessary to
differentiate between _vote_ and _accept_, a node can explicitly
send a _vote-or-accept_ message.
o _confirm_ "a" indicates that _accept_ "a" has reached quorum
threshold at the sender. This message is interpreted the same as
_accept_ "a", but allows recipients to optimize their quorum
checks by ignoring the sender's quorum slices, as the sender
asserts it has already checked them.
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Figure 1 illustrates the federated voting process. A node "v" votes
for a valid statement "a" that doesn't contradict statements in past
_vote_ or _accept_ messages sent by "v". When the _vote_ message
reaches quorum threshold, the node accepts "a". In fact, "v" accepts
"a" if the _vote-or-accept_ message reaches quorum threshold, as some
nodes may accept "a" without first voting for it. Specifically, a
node that cannot vote for "a" because it has voted for "a"'s negation
"!a" still accepts "a" when the message _accept_ "a" reaches blocking
threshold (meaning assertions about "!a" have no hope of reaching
quorum threshold barring catastrophic Byzantine failure).
If and when the message _accept_ "a" reaches quorum threshold, then
"v" has confirmed "a" and the federated vote has succeeded. In
effect, the _accept_ messages constitute a second vote on the fact
that the initial vote messages succeeded. Once "v" enters the
confirmed state, it may issue a _confirm_ "a" message to help other
nodes confirm "a" more efficiently by pruning their quorum search at
"v".
"vote-or-accept a" "accept a"
reaches reaches
quorum threshold quorum threshold
+-----------------+ +-----------------+
| | | |
| V | V
+-----------+ +-----------+ +-----------+
a is +---->| voted a | |accepted a | |confirmed a|
valid | +-----------+ +-----------+ +-----------+
| | ^
+-----------+ | | "accept a" reaches
|uncommitted|------+-----------------+ blocking threshold
+-----------+ |
| |
| +-----------+
+---->| voted !a |
+-----------+
Figure 1: Federated voting process
3.2. Basic types
SCP employs 32- and 64-bit integers, as defined below.
typedef unsigned int uint32;
typedef int int32;
typedef unsigned hyper uint64;
typedef hyper int64;
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SCP uses the SHA-256 cryptograhpic hash function [RFC6234], and
represents hash values as a simple array of 32 bytes.
typedef opaque Hash[32];
SCP employs the Ed25519 digital signature algorithm [RFC8032]. For
cryptographic agility, however, public keys are represented as a
union type that can later be compatibly extended with other key
types.
typedef opaque uint256[32];
enum PublicKeyType
{
PUBLIC_KEY_TYPE_ED25519 = 0
};
union PublicKey switch (PublicKeyType type)
{
case PUBLIC_KEY_TYPE_ED25519:
uint256 ed25519;
};
// variable size as the size depends on the signature scheme used
typedef opaque Signature<64>;
Nodes are public keys, while values are simply opaque arrays of
bytes.
typedef PublicKey NodeID;
typedef opaque Value<>;
3.3. Quorum slices
Theoretically a quorum slice can be an arbitrary set of nodes.
However, arbitrary predicates on sets cannot be encoded concisely.
Instead we specify quorum slices as any set of k-of-n members, where
each of the n members can either be an individual node ID, or,
recursively, another k-of-n set.
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// supports things like: A,B,C,(D,E,F),(G,H,(I,J,K,L))
// only allows 2 levels of nesting
struct SCPSlices
{
uint32 threshold; // the k in k-of-n
PublicKey validators<>;
SCPSlices1 innerSets<>;
};
struct SCPSlices1
{
uint32 threshold; // the k in k-of-n
PublicKey validators<>;
SCPSlices2 innerSets<>;
};
struct SCPSlices2
{
uint32 threshold; // the k in k-of-n
PublicKey validators<>;
};
Let "k" be the value of "threshold" and "n" the sum of the sizes of
the "validators" and "innerSets" vectors in a message sent by some
node "v". A message "m" sent by "v" reaches quorum threshold at "v"
when three things hold:
1. "v" itself has issued (digitally signed) the message,
2. The number of nodes in "validators" who have signed "m" plus the
number "innerSets" that (recursively) meet this condition is at
least "k", and
3. These three conditions apply (recursively) at some combination of
nodes sufficient for condition #2.
A message reaches blocking threshold at "v" when the number of
"validators" making the statement plus (recursively) the number
"innerSets" reaching blocking threshold exceeds "n-k". (Blocking
threshold depends only on the local node's quorum slices and hence
does not require a recursive check on other nodes like step #3
above.)
As described in Section 3.10, every protocol message is paired with a
cryptographic hash of the sender's "SCPSlices" and digitally signed.
Inner protocol messages described in the next few sections should be
understood to be received alongside such a quorum slice specification
and digital signature.
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3.4. Nominate message
For each slot, the SCP protocol begins in a NOMINATE phase, whose
goal is to devise one or more candidate output values for the
consensus protocol. In this phase, nodes send nomination messages
comprising a monotonically growing set of values:
struct SCPNominate
{
Value voted<>; // X
Value accepted<>; // Y
};
The "voted" and "accepted" sets are disjoint; any value that is
eligible for both sets is placed only in the "accepted" set.
"voted" consists of candidate values that the sender has voted to
nominate. Each node progresses through a series of nomination
_rounds_ in which it may increase the set of values in its own
"voted" field by adding the contents of the "voted" and "accepted"
fields of "SCPNominate" messages received from a growing set of
peers. In round "n" of slot "i", each node determines an additional
peer whose nominated values it should incorporate in its own
"SCPNominate" message as follows:
o Let "Gi(m) = SHA-256(i || m)", where "||" denotes the
concatenation of serialized XDR values. Treat the output of "Gi"
as a 256-bit binary number in big-endian format.
o For each peer "v", define "weight(v)" as the fraction of quorum
slices containing "v".
o Define the set of nodes "neighbors(n)" as the set of nodes v for
which "Gi(1 || n || v) < 2^{256} * weight(v)", where "1" and "n"
are both 32-bit XDR "int" values.
o Define "priority(n, v)" as "Gi(2 || n || v)", where "2" and "n"
are both 32-bit XDR "int" values.
For each round "n" until nomination has finished (see below), a node
starts _echoing_ the available peer "v" with the highest value of
"priority(n, v)" from among the nodes in "neighbors(n)". To echo
"v", the node merges any valid values from "v"'s "voted" and
"accepted" sets into its own "voted" set.
XXX - expand "voted" with only the 10 values with lowest Gi hash in
any given round to avoid blowing out the message size?
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Note that when echoing nominations, nodes must exclude and neither
vote for nor accept values rejected by the higher-layer application's
validity function. This validity function must not depend on state
that can permanently differ across nodes. By way of example, it is
okay to reject values that are syntactically ill-formed, that are
semantically incompatible with the previous slot's value, that
contain invalid digital signatures, that contain timestamps in the
future, or that specify upgrades to unknown versions of the protocol.
By contrast, the application cannot reject values that are
incompatible with the results of a DNS query or some dynamically
retrieved TLS certificate, as different nodes could see different
results when doing such queries.
Nodes must not send an "SCPNominate" message until at least one of
the "voted" or "accepted" fields is non-empty. When these fields are
both empty, a node that has the highest priority among its neighbors
in the current round (and hence should be echoing its own votes) adds
the higher-layer software's input value to its "voted" field. Nodes
that do not have the highest priority wait to hear "SCPNominate"
messages from the nodes whose nominations they are echoing.
If a particular valid value "x" reaches quorum threshold in the
messages sent by peers (meaning that every node in a quorum contains
"x" either in the "voted" or the "accepted" field), then the node at
which this happens moves "x" from its "voted" field to its "accepted"
field and broadcasts a new "SCPNominate" message. Similarly, if "x"
reaches blocking threshold in a node's peers' "accepted" field
(meaning every one of a node's quorum slices contains at least one
node with "x" in its "accepted" field), then the node adds "x" to its
own "accepted" field (removing it from "voted" if applicable). These
two cases correspond to the two conditions for entering the
"accepted" state in Figure 1.
A node stops adding any new values to its "voted" set as soon as any
value "x" reaches quorum threshold in the "accepted" fields of
received "SCPNominate" messages. Following the terminology of
Section 3.1, this condition corresponds to when the node confirms "x"
as nominated. Note, however, that the node continues adding new
values to "accepted" as appropriate. Doing so may lead to more
values becoming confirmed nominated even after the "voted" set is
closed to new values.
A node always begins nomination in round "1". Round "n" lasts for
"1+n" seconds, after which, if no value has been confirmed nominated,
the node proceeds to round "n+1". A node continues to echo votes
from the highest priority neighbor in prior rounds as well as the
current round. In particular, until any value is confirmed
nominated, a node continues expanding its "voted" field with values
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nominated by highest priority neighbors from prior rounds even when
the values appeared after the end of those prior rounds.
As defined in the next two sections, the NOMINATE phase ends when a
node has confirmed "prepare(b)" for some any ballot "b", as this is
the point at which the nomination outcome no longer influences the
protocol. Until this point, a node must continue to transmit
"SCPNominate" messages as well as to expand its "accepted" set (even
if "voted" is closed because some value has been confirmed
nominated).
3.5. Ballots
Once there is a candidate on which to try to reach consensus, a node
moves through three phases of balloting: PREPARE, COMMIT, and
EXTERNALIZE. Balloting employs federated voting to chose between
_commit_ and _abort_ statements for ballots. A ballot is a pair
consisting of a counter and candidate value:
// Structure representing ballot <n, x>
struct SCPBallot
{
uint32 counter; // n
Value value; // x
};
We use the notation "<n, x>" to represent a ballot with "counter ==
n" and "value == x".
Ballots are totally ordered with "counter" more significant than
"value". Hence, we write "b1 < b2" to mean that either "(b1.counter
< b2.counter)" or "(b1.counter == b2.counter && b1.value <
b2.value)". Values are compared lexicographically as a strings of
unsigned octets.
The protocol moves through federated voting on successively higher
ballots until nodes confirm "commit(b)" for some ballot "b", at which
point consensus terminates and outputs "b.value" for the slot. To
ensure that only one value can be chosen for a slot and that the
protocol cannot get stuck if individual ballots get stuck, there are
two restrictions on voting:
1. A node cannot vote for both "commit(b)" and "abort(b)" on the
same ballot (the two outcomes are contradictory), and
2. A node may not vote for or accept "commit(b)" for any ballot "b"
unless it has confirmed "abort" for every lesser ballot with a
different value.
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The second condition requires voting to abort large numbers of
ballots before voting to commit a ballot "b". We call this
_preparing_ ballot "b", and introduce the following notation for the
associated set of abort statements.
o "prepare(b)" encodes an "abort" statement for every ballot less
than "b" containing a value other than "b.value", i.e.,
"prepare(b) = { abort(b1) | b1 < b AND b1.value != b.value }".
o "vote prepare(b)" stands for a set of _vote_ messages for every
"abort" statement in "prepare(b)".
o Similarly, "accept prepare(b)", "vote-or-accept prepare(b)", and
"confirm prepare(b)" encode sets of _accept_, _vote-or-accept_,
and _confirm_ messages for every "abort" statement in
"prepare(b)".
Using this terminology, a node must confirm "prepare(b)" before
issuing a _vote_ or _accept_ message for the statement "commit(b)".
3.6. Prepare message
The first phase of balloting is the PREPARE phase. During this
phase, as soon as a node has a valid candidate value (see the rules
for "ballot.value" below), it begins sending the following message:
struct SCPPrepare
{
SCPBallot ballot; // b
SCPBallot *prepared; // p
SCPBallot *preparedPrime; // p'
uint32 hCounter; // h.counter or 0 if h == NULL
uint32 cCounter; // c.counter or 0 if !c || !hCounter
};
This message compactly conveys the following (conceptual) federated
voting messages:
o "vote-or-accept prepare(ballot)"
o If "prepared != NULL": "accept prepare(prepared)"
o If "preparedPrime != NULL": "accept prepare(preparedPrime)"
o If "hCounter != 0": "confirm prepare(<hCounter, ballot.value>)"
o If "cCounter != 0": "vote commit(<n, ballot.value>)" for every
"cCounter <= n <= hCounter"
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Note that to be valid, an "SCPPrepare" message must satisfy the
following conditions:
o If "prepared != NULL", then "prepared <= ballot",
o If "preparedPrime != NULL", then "prepared != NULL" and
"preparedPrime < prepared", and
o "cCounter <= hCounter <= ballot.counter".
Based on the federated vote messages received, each node keeps track
of what ballots have been accepted and confirmed prepared. It uses
these ballots to set the following fields of its own "SCPPrepare"
messages as follows.
ballot
The current ballot that a node is attempting to prepare and
commit. The rules for setting each field are detailed below.
Note that the "value" is updated when and only when "counter"
changes.
ballot.counter
The counter is set according to the following rules:
* Upon entering the PREPARE phase, the "counter" field is
initialized to 1.
* When a node sees messages from a quorum to which it belongs
such that each message's "ballot.counter" is greater than or
equal to the local "ballot.counter", the node arms a timer to
fire in a number of seconds equal to its "ballot.counter + 1"
(so the timeout lengthens linearly as the counter increases).
Note that for the purposes of determining whether a quorum has
a particular "ballot.counter", a node considers "ballot" fields
in "SCPPrepare" and "SCPCommit" messages. It also considers
"SCPExternalize" messages to convey an implicit
"ballot.counter" of "infinity".
* If the timer fires, a node increments the ballot counter by 1.
* If nodes forming a blocking threshold all have "ballot.counter"
values greater than the local "ballot.counter", then the local
node immediately cancels any pending timer, increases
"ballot.counter" to the lowest value such that this is no
longer the case, and if appropriate according to the rules
above arms a new timer. Note that the blocking threshold may
include ballots from "SCPCommit" messages as well as
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"SCPExternalize" messages, which implicitly have an infinite
ballot counter.
* *Exception*: To avoid exhausting "ballot.counter", its value
must always be less then 1,000 plus the number of seconds a
node has been running SCP on the current slot. Should any of
the above rules require increasing the counter beyond this
value, a node either increases "ballot.counter" to the maximum
permissible value, or, if it is already at this maximum, waits
up to one second before increasing the value.
ballot.value
Each time the ballot counter is changed, the value is also
recomputed as follows:
* If any ballot has been confirmed prepared, then "ballot.value"
is taken to to be "h.value" for the highest confirmed prepared
ballot "h". (Note that once this is the case, the node can
stop sending "SCPNominate" messages, as "h.value" supersedes
any output of the nomination protocol.)
* Otherwise (if no such "h" exists), if one or more values are
confirmed nominated, then "ballot.value" is taken as the output
of the deterministic combining function applied to all
confirmed nominated values. Note that because the NOMINATE and
PREPARE phases run concurrently, the set of confirmed nominated
values may continue to grow during balloting, changing
"ballot.value" even if no ballots are confirmed prepared.
* Otherwise, if no ballot is confirmed prepared and no value is
confirmed nominated, but the node has accepted a ballot
prepared (because "prepare(b)" meets blocking threshold for
some ballot "b"), then "ballot.value" is taken as the value of
the highest such accepted prepared ballot.
* Otherwise, if no value is confirmed nominated and no value is
accepted prepared, then a node cannot yet send an "SCPPrepare"
message and must continue sending only "SCPNominate" messages.
prepared
The highest accepted prepared ballot not exceeding the "ballot"
field, or NULL if no ballot has been accepted prepared. Recall
that ballots with equal counters are totally ordered by the value.
Hence, if "ballot = <n, x>" and the highest prepared ballot is
"<n, y>" where "x < y", then the "prepared" field in sent messages
must be set to "<n-1, y>" instead of "<n, y>", as the latter would
exceed "ballot". In the event that "n = 1", the prepared field
may be set to "<0, y>", meaning 0 is a valid "prepared.counter"
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even though it is not a valid "ballot.counter". It is possible to
confirm "prepare(<0, y>)", in which case the next "ballot.value"
is set to "y". However, it is not possible to vote to commit a
ballot with counter 0.
preparedPrime
The highest accepted prepared ballot such that "preparedPrime <
prepared" and "preparedPrime.value != prepared.value", or NULL if
there is no such ballot. Note that together, "prepared" and
"preparedPrime" concisely encode all "abort" statements (below
"ballot") that the sender has accepted.
hCounter
If "h" is the highest confirmed prepared ballot and "h.value ==
ballot.value", then this field is set to "h.counter". Otherwise,
if no ballot is confirmed prepared or if "h.value !=
ballot.value", then this field is 0. Note that by the rules
above, if "h" exists, then "ballot.value" will be set to "h.value"
the next time "ballot" is updated.
cCounter
The value "cCounter" is maintained based on an internally-
maintained _commit ballot_ "c", initially "NULL". "cCounter" is 0
while "c == NULL" or "hCounter == 0", and is "c.counter"
otherwise. "c" is updated as follows:
* If either "(prepared > c && prepared.value != c.value)" or
"(preparedPrime > c && preparedPrime.value != c.value)", then
reset "c = NULL".
* If "c == NULL" and "hCounter == ballot.counter" (meaning
"ballot" is confirmed prepared), then set "c" to "ballot".
A node leaves the PREPARE phase and proceeds to the COMMIT phase when
there is some ballot "b" for which the node confirms "prepare(b)" and
accepts "commit(b)". (If nodes never changed quorum slice mid-
protocol, it would suffice to accept "commit(b)". Also waiting to
confirm "prepare(b)" makes it easier to recover from liveness
failures by removing Byzantine faulty nodes from quorum slices.)
3.7. Commit message
In the COMMIT phase, a node has accepted "commit(b)" for some ballot
"b", and must confirm that statement to act on the value in
"b.counter". A node sends the following message in this phase:
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struct SCPCommit
{
SCPBallot ballot; // b
uint32 preparedCounter; // prepared.counter
uint32 hCounter; // h.counter
uint32 cCounter; // c.counter
};
The message conveys the following federated vote messages, where
"infinity" is 2^{32} (a value greater than any ballot counter
representable in serialized form):
o "accept commit(<n, ballot.value>)" for every "cCounter <= n <=
hCounter"
o "vote-or-accept prepare(<infinity, ballot.value>)"
o "accept prepare(<preparedCounter, ballot.value>)"
o "confirm prepare(<hCounter, ballot.value>)"
o "vote commit(<n, ballot.value>)" for every "n >= cCounter"
A node computes the fields in the "SCPCommit" messages it sends as
follows:
ballot
This field is maintained identically to how it is maintained in
the PREPARE phase, though "ballot.value" can no longer change,
only "ballot.counter". Note that the value "ballot.counter" does
not figure in any of the federated voting messages. The purpose
of continuing to update and send this field is to assist other
nodes still in the PREPARE phase in synchronizing their counters.
preparedCounter
This field is the counter of the highest accepted prepared
ballot--maintained identically to the "prepared" field in the
PREPARE phase. Since the "value" field will always be the same as
"ballot", only the counter is sent in the COMMIT phase.
cCounter
The counter of the lowest ballot "c" for which the node has
accepted "commit(c)". (No value is included in messages since
"c.value == ballot.value".)
hCounter
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The counter of the highest ballot "h" for which the node has
accepted "commit(h)". (No value is included in messages since
"h.value == ballot.value".)
As soon as a node confirms "commit(b)" for any ballot "b", it moves
to the EXTERNALIZE phase.
3.8. Externalize message
A node enters the EXTERNALIZE phase when it confirms "commit(b)" for
any ballot "b". As soon as this happens, SCP outputs "b.value" as
the value of the current slot. In order to help other nodes achieve
consensus on the slot more quickly, a node reaching this phase also
sends the following message:
struct SCPExternalize
{
SCPBallot commit; // c
uint32 hCounter; // h.counter
};
An "SCPExternalize" message conveys the following federated voting
messages:
o "accept commit(<n, commit.value>)" for every "n >= commit.counter"
o "confirm commit(<n, commit.value>)" for every "commit.counter <= n
<= hCounter"
o "confirm prepare(<infinity, commit.value>)"
The fields are set as follows:
commit
The lowest confirmed committed ballot.
hCounter
The counter of the highest confirmed committed ballot.
3.9. Summary of phases
Table 1 summarizes the phases of SCP for each slot. The NOMINATE and
PREPARE phases begin concurrently. However, a node initially does
not send "SCPPrepare" messages but only listens for ballot messages
in case "accept prepare(b)" reaches blocking threshold for some
ballot "b". The COMMIT and EXTERNALIZE phases then run in turn after
PREPARE ends. A node may externalize (act upon) a value as soon as
it enters the EXTERNALIZE phase.
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The point of "SCPExternalize" messages is to help straggling nodes
catch up more quickly. As such, the EXTERNALIZE phase never ends.
Rather, a node should archive an "SCPExternalize" message for as long
as it retains slot state.
+-------------+---------------------------------+-------------------+
| Phase | Begin | End |
+-------------+---------------------------------+-------------------+
| NOMINATE | previous slot externalized and | some ballot is |
| | 5 seconds have elapsed since | confirmed |
| | NOMINATE ended for that slot | prepared |
| | | |
| PREPARE | begin with NOMINATE, but send | accept |
| | "SCPPrepare" only once some | "commit(b)" for |
| | value confirmed nominated or | some ballot "b" |
| | accept "prepare(b)" for some | |
| | ballot b | |
| | | |
| COMMIT | accept "commit(b)" for some | confirm |
| | ballot "b" | "commit(b)" for |
| | | some ballot "b" |
| | | |
| EXTERNALIZE | confirm "commit(b)" for some | slot state |
| | ballot "b" | garbage-collected |
+-------------+---------------------------------+-------------------+
Table 1: Phases of SCP for a slot
3.10. Message envelopes
In order to provide full context for each signed message, all signed
messages are part of an "SCPStatement" union type that includes the
"slotIndex" naming the slot to which the message applies, as well as
the "type" of the message. A signed message and its signature are
packed together in an "SCPEnvelope" structure.
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enum SCPStatementType
{
SCP_ST_PREPARE = 0,
SCP_ST_COMMIT = 1,
SCP_ST_EXTERNALIZE = 2,
SCP_ST_NOMINATE = 3
};
struct SCPStatement
{
NodeID nodeID; // v (node signing message)
uint64 slotIndex; // i
Hash quorumSetHash; // hash of serialized SCPSlices
union switch (SCPStatementType type)
{
case SCP_ST_PREPARE:
SCPPrepare prepare;
case SCP_ST_COMMIT:
SCPCommit commit;
case SCP_ST_EXTERNALIZE:
SCPExternalize externalize;
case SCP_ST_NOMINATE:
SCPNominate nominate;
}
pledges;
};
struct SCPEnvelope
{
SCPStatement statement;
Signature signature;
};
4. Security considerations
If nodes do not pick quorum slices well, the protocol will not be
safe.
5. Acknowledgments
The Stellar development foundation supported development of the
protocol and produced the first production deployment of SCP. The
IRTF DIN group including Dirk Kutscher, Sydney Li, Colin Man, Piers
Powlesland, Melinda Shore, and Jean-Luc Watson helped with the
framing and motivation for this specification. We also thank Bob
Glickstein for finding bugs in drafts of this document and offering
many useful suggestions.
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6. References
6.1. Normative References
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <https://www.rfc-editor.org/info/rfc4506>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
6.2. Informative References
[building-blocks]
Song, Y., van Renesse, R., Schneider, F., and D. Dolev,
"The Building Blocks of Consensus", 9th International
Conference on Distributed Computing and Networking pp.
54-72, 2008.
[FLP] Fischer, M., Lynch, N., and M. Lynch, "Impossibility of
Distributed Consensus with One Faulty Process", Journal of
the ACM 32(2):374-382, 1985.
[I-D.paillisse-sidrops-blockchain]
Paillisse, J., Rodriguez-Natal, A., Ermagan, V., Maino,
F., Vegoda, L., and A. Cabellos-Aparicio, "An analysis of
the applicability of blockchain to secure IP addresses
allocation, delegation and bindings.", draft-paillisse-
sidrops-blockchain-02 (work in progress), June 2018.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
<https://www.rfc-editor.org/info/rfc6797>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
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[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <https://www.rfc-editor.org/info/rfc7469>.
[SCP] Mazieres, D., "The Stellar Consensus Protocol: A Federated
Model for Internet-level Consensus", Stellar Development
Foundation whitepaper , 2015,
<https://www.stellar.org/papers/
stellar-consensus-protocol.pdf>.
Authors' Addresses
Nicolas Barry
Stellar Development Foundation
170 Capp St., Suite A
San Francisco, CA 94110
US
Email: nicolas@stellar.org
Giuliano Losa
UCLA
3753 Keystone Avenue #10
Los Angeles, CA 90034
US
Email: giuliano@cs.ucla.edu
David Mazieres
Stanford University
353 Serra Mall, Room 290
Stanford, CA 94305
US
Email: dm@uun.org
Jed McCaleb
Stellar Development Foundation
170 Capp St., Suite A
San Francisco, CA 94110
US
Email: jed@stellar.org
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Stanislas Polu
Stripe Inc.
185 Berry Street, Suite 550
San Francisco, CA 94107
US
Email: stan@stripe.com
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