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Network Working Group                                           N. Barry
Internet-Draft                            Stellar Development Foundation
Intended status: Experimental                                    G. Losa
Expires: December 15, 2018                                          UCLA
                                                             D. Mazieres
                                                     Stanford University
                                                              J. McCaleb
                                          Stellar Development Foundation
                                                                 S. Polu
                                                             Stripe Inc.
                                                           June 13, 2018

                  The Stellar Consensus Protocol (SCP)


   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 15, 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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   the Trust Legal Provisions and are provided without warranty as
   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.  Nomination  . . . . . . . . . . . . . . . . . . . . . . .  10
     3.5.  Ballots . . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.6.  Prepare messages  . . . . . . . . . . . . . . . . . . . .  13
     3.7.  Commit messages . . . . . . . . . . . . . . . . . . . . .  16
     3.8.  Externalize messages  . . . . . . . . . . . . . . . . . .  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

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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

   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,

   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.  SCP's goal is to ensure that non-
   faulty nodes agree on one or a combination of nodes' input values for
   the slot's output.  5 seconds after completing one slot, the protocol
   runs again for the next slot.

   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, thus amortizing the cost of consensus 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
   Section 3.4, which nodes' input values to use depends on a

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   cryptographic hash of the slot number, output history, 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: NOMINATION, PREPARE, COMMIT,
   and EXTERNALIZE.  The NOMINATION and PREPARE phases run concurrently
   (though NOMINATIONS'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

                      "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

     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

                         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 SCPQuorumSet
              uint32 threshold;            // the k in k-of-n
              PublicKey validators<>;
              SCPQuorumSet1 innerSets<>;
          struct SCPQuorumSet1
              uint32 threshold;            // the k in k-of-n
              PublicKey validators<>;
              SCPQuorumSet2 innerSets<>;
          struct SCPQuorumSet2
              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

   As described in Section 3.10, every protocol message is paired with a
   cryptographic hash of the sender's "SCPQuorumSet" 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.  Nomination

   For each slot, the SCP protocol begins in a NOMINATION 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 SCPNomination
                           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 "SCPNomination" 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
   "SCPNomination" 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 "SCPNomination" 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 "SCPNomination"
   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 "SCPNomination" 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 "SCPNomination" 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 NOMINATION 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
   "SCPNomination" messages as well as to expand its "accepted" set
   (even if "voted" is closed because some value has been confirmed

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

   Using this terminology, a node must confirm "prepare(b)" before
   issuing a _vote_ or _accept_ message for the statement "commit b".

3.6.  Prepare messages

   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 == NULL

   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.

      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"

      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.

      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 "SCPNomination" 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 NOMINATION
         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

      *  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 "SCPNomination"

      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

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      exceed "ballot".  In the event that "n = 1", the prepared field
      may be set to "<0, y>", meaning 0 is a valid "prepared.counter"
      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.

      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.

      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.

      The value "cCounter" is maintained based on an internally-
      maintained _commit ballot_ "c", initially "NULL".  "cCounter" is 0
      while "c" is "NULL" 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 messages

   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 <=

   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

      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.

      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.

      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".)


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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 messages

   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

   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:

      The lowest confirmed committed ballot.

      The counter of the highest confirmed committed ballot.

3.9.  Summary of phases

   Table 1 summarizes the phases of SCP for each slot.  The NOMINATION
   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                |
   |  NOMINATION | previous slot externalized and | some value is      |
   |             | 5 seconds have elapsed since   | confirmed          |
   |             | NOMINATION ended for that slot | nominated or some  |
   |             |                                | ballot is          |
   |             |                                | confirmed prepared |
   |             |                                |                    |
   |     PREPARE | begin with NOMINATION, but     | accept "commit b"  |
   |             | send "SCPPrepare" only once    | for some ballot    |
   |             | some value confirmed nominated | "b"                |
   |             | or accept "prepare(b)" for     |                    |
   |             | some ballot b                  |                    |
   |             |                                |                    |
   |      COMMIT | accept "commit b" for some     | confirm "commit b" |
   |             | ballot "b"                     | for some ballot    |
   |             |                                | "b"                |
   |             |                                |                    |
   | EXTERNALIZE | confirm "commit b" for some    | never              |
   |             | ballot "b"                     |                    |

                                  Table 1

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 SCPQuorumSet

            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:
                SCPNomination nominate;

        struct SCPEnvelope
            SCPStatement statement;
            Signature signature;

4.  Security considerations

   If nodes do not pick quorum slices well, the protocol will not be

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, 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,

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,

6.2.  Informative References

              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.

              Paillisse, J., Rodriguez-Natal, A., Ermagan, V., Maino,
              F., and A. Cabellos-Aparicio, "An analysis of the
              applicability of blockchain to secure IP addresses
              allocation, delegation and bindings.", draft-paillisse-
              sidrops-blockchain-01 (work in progress), October 2017.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,

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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,

Authors' Addresses

   Nicolas Barry
   Stellar Development Foundation
   170 Capp St., Suite A
   San Francisco, CA  94110

   Email: nicolas@stellar.org

   Giuliano Losa
   3753 Keystone Avenue #10
   Los Angeles, CA  90034

   Email: giuliano@cs.ucla.edu

   David Mazieres
   Stanford University
   353 Serra Mall, Room 290
   Stanford, CA  94305

   Email: dm@uun.org

   Jed McCaleb
   Stellar Development Foundation
   170 Capp St., Suite A
   San Francisco, CA  94110

   Email: jed@stellar.org

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   Stanislas Polu
   Stripe Inc.
   185 Berry Street, Suite 550
   San Francisco, CA  94107

   Email: stan@stripe.com

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