From 80cdf6d0b3545516d4c45dfc995caa8e0ab09eda Mon Sep 17 00:00:00 2001
From: Pietro <pietro.abate@inria.fr>
Date: Wed, 21 Mar 2018 15:38:41 +0100
Subject: [PATCH] Shell: A bit of randomization for gap in block locators.

The random seed is computed from the sender and receiver `peer_id`.
---
 src/lib_base/block_locator.ml            | 149 +++++++++++++++++------
 src/lib_base/block_locator.mli           |  47 +++++--
 src/lib_shell/block_locator_iterator.ml  |  93 --------------
 src/lib_shell/block_locator_iterator.mli |  55 ---------
 src/lib_shell/bootstrap_pipeline.ml      |   8 +-
 src/lib_shell/chain.ml                   |  15 +--
 src/lib_shell/chain.mli                  |   4 +-
 src/lib_shell/chain_validator.ml         |   5 +-
 src/lib_shell/distributed_db.ml          |  32 ++++-
 src/lib_shell/distributed_db.mli         |   6 +-
 src/lib_shell/peer_validator.ml          |  27 ++--
 src/lib_shell/state.ml                   |  47 +++++--
 src/lib_shell/state.mli                  |  12 +-
 src/lib_shell/test/test_locator.ml       |  13 +-
 src/lib_shell/test/test_state.ml         |  37 +-----
 15 files changed, 275 insertions(+), 275 deletions(-)
 delete mode 100644 src/lib_shell/block_locator_iterator.ml
 delete mode 100644 src/lib_shell/block_locator_iterator.mli

diff --git a/src/lib_base/block_locator.ml b/src/lib_base/block_locator.ml
index c1cb85043..6d69ca4a9 100644
--- a/src/lib_base/block_locator.ml
+++ b/src/lib_base/block_locator.ml
@@ -10,8 +10,6 @@
 open Lwt.Infix
 
 type t = raw
-
-(** Non private version of Block_store_locator.t for coercions *)
 and raw = Block_header.t * Block_hash.t list
 
 let raw x = x
@@ -27,7 +25,8 @@ let pp ppf (hd, h_lst) =
     | hd :: tl ->
         let new_d = if r > 1 then d else d * coef in
         let new_r = if r > 1 then r - 1 else repeats in
-        Format.fprintf ppf "%a (%i)\n%a" Block_hash.pp hd acc pp_hash_list (tl , acc - d , new_d , new_r) in
+        Format.fprintf ppf "%a (%i)\n%a"
+          Block_hash.pp hd acc pp_hash_list (tl , acc - d , new_d , new_r) in
   Format.fprintf ppf "%a (head)\n%a"
     Block_hash.pp (Block_header.hash hd)
     pp_hash_list (h_lst , -1, 1, repeats - 1)
@@ -44,50 +43,130 @@ let encoding =
      (req "current_head" (dynamic_size Block_header.encoding))
      (req "history" (dynamic_size (list Block_hash.encoding))))
 
-(** Computes a locator for block [b] picking 10 times the immediate
-    predecessors of [b], then 10 times one predecessor every 2, then
-    10 times one predecessor every 4, ..., until genesis or it reaches
-    the desired size. *)
-let compute ~predecessor ~genesis b header size =
-  if size < 0 then
-    invalid_arg "Block_locator.compute: negative size"
-  else
-    let repeats = 10 in (* number of repetitions for each power of 2 *)
-    let rec loop acc size step cnt b =
-      if size = 0 then
-        Lwt.return (List.rev acc)
+type seed = {
+  sender_id: P2p_peer.Id.t ;
+  receiver_id: P2p_peer.Id.t ;
+}
+
+(* Random generator for locator steps.
+
+   We draw steps by sequence of 10. The first sequence's steps are of
+   length 1 (consecutive). The second sequence's steps are of a random
+   length between 1 and 2. The third sequence's steps are of a random
+   length between 2 and 4, and so on...
+
+   The sequence is deterministic for a given triple of sender,
+   receiver and block hash. *)
+module Step : sig
+
+  type state
+  val init: seed -> Block_hash.t -> state
+  val next: state -> int * state
+
+end = struct
+
+  type state = int * int * Cstruct.t
+
+  let init seed head =
+    let seed =
+      Nocrypto.Hash.digest `SHA256 @@
+      Cstruct.concat
+        [ Cstruct.of_bigarray @@ P2p_peer.Id.to_bytes seed.sender_id ;
+          Cstruct.of_bigarray @@ P2p_peer.Id.to_bytes seed.receiver_id ;
+          Cstruct.of_bigarray @@ Block_hash.to_bytes head ] in
+    (1, 9, seed)
+
+  let draw seed n =
+    Int32.to_int (MBytes.get_int32 (Cstruct.to_bigarray seed) 0) mod n,
+    Nocrypto.Hash.digest `SHA256 seed
+
+  let next (step, counter, seed) =
+    let random_gap, seed =
+      if step <= 1 then
+        0, seed
       else
-        predecessor b step >>= function
-        | None ->      (* reached genesis before size *)
-            if Block_hash.equal b genesis then
-              Lwt.return (List.rev acc)
-            else
-              Lwt.return (List.rev (genesis :: acc))
-        | Some pred ->
-            if cnt = 1 then
-              loop (pred :: acc) (size - 1) (step * 2) repeats pred
-            else
-              loop (pred :: acc) (size - 1) step (cnt - 1) pred in
+        draw seed (1 + step/2) in
+    let new_state =
+      if counter = 0 then
+        (step * 2, 9, seed)
+      else
+        (step, counter - 1, seed) in
+    step - random_gap, new_state
+
+end
+
+let estimated_length seed (head, hist) =
+  let rec loop acc state = function
+    | [] -> acc
+    | _ :: hist ->
+        let step, state = Step.next state in
+        loop (acc + step) state hist in
+  let state = Step.init seed (Block_header.hash head) in
+  let step, state = Step.next state in
+  loop step state hist
+
+let fold ~f ~init (head, hist) seed =
+  let rec loop state acc = function
+    | [] | [_] -> acc
+    | block :: (pred :: rem as hist) ->
+        let step, state = Step.next state in
+        let acc = f acc ~block ~pred ~step ~strict_step:(rem <> []) in
+        loop state acc hist in
+  let head = Block_header.hash head in
+  let state = Step.init seed head in
+  loop state init (head :: hist)
+
+type step = {
+  block: Block_hash.t ;
+  predecessor: Block_hash.t ;
+  step: int ;
+  strict_step: bool ;
+}
+
+let to_steps seed locator =
+  fold locator seed
+    ~init:[]
+    ~f: begin fun acc ~block ~pred ~step ~strict_step ->
+      { block ; predecessor = pred ; step ; strict_step } :: acc
+    end
+
+let compute ~predecessor ~genesis block_hash header seed ~size =
+  let rec loop acc size state block =
     if size = 0 then
-      Lwt.return (header, [])
+      Lwt.return (List.rev acc)
     else
-      predecessor b 1 >>= function
-      | None -> Lwt.return (header, [])
-      | Some p ->
-          loop [p] (size-1) 1 (repeats-1) p >>= fun hist ->
-          Lwt.return (header, hist)
+      let step, state = Step.next state in
+      predecessor block step >>= function
+      | None ->
+          (* We reached genesis before size *)
+          if Block_hash.equal block genesis then
+            Lwt.return (List.rev acc)
+          else
+            Lwt.return (List.rev (genesis :: acc))
+      | Some pred ->
+          loop (pred :: acc) (size - 1) state pred in
+  if size <= 0 then
+    Lwt.return (header, [])
+  else
+    let state = Step.init seed block_hash in
+    let step, state = Step.next state in
+    predecessor block_hash step >>= function
+    | None -> Lwt.return (header, [])
+    | Some p ->
+        loop [p] (size-1) state p >>= fun hist ->
+        Lwt.return (header, hist)
 
 type validity =
   | Unknown
   | Known_valid
   | Known_invalid
 
-let unknown_prefix cond (head, hist) =
+let unknown_prefix ~is_known (head, hist) =
   let rec loop hist acc =
     match hist with
     | [] -> Lwt.return_none
     | h :: t ->
-        cond h >>= function
+        is_known h >>= function
         | Known_valid ->
             Lwt.return_some (h, (List.rev (h :: acc)))
         | Known_invalid ->
@@ -95,7 +174,7 @@ let unknown_prefix cond (head, hist) =
         | Unknown ->
             loop t (h :: acc)
   in
-  cond (Block_header.hash head) >>= function
+  is_known (Block_header.hash head) >>= function
   | Known_valid ->
       Lwt.return_some (Block_header.hash head, (head, []))
   | Known_invalid ->
diff --git a/src/lib_base/block_locator.mli b/src/lib_base/block_locator.mli
index 3a741b29b..504d122cd 100644
--- a/src/lib_base/block_locator.mli
+++ b/src/lib_base/block_locator.mli
@@ -14,17 +14,43 @@ and raw = Block_header.t * Block_hash.t list
 (** Non private version of Block_store_locator.t for coercions *)
 
 val raw: t -> raw
-
-val pp : Format.formatter -> t -> unit
-
-val pp_short : Format.formatter -> t -> unit
-
+val pp: Format.formatter -> t -> unit
+val pp_short: Format.formatter -> t -> unit
 val encoding: t Data_encoding.t
 
-val compute: predecessor: (Block_hash.t -> int -> Block_hash.t option Lwt.t) ->
-  genesis:Block_hash.t -> Block_hash.t -> Block_header.t -> int -> t Lwt.t
-(** [compute block max_length] compute the sparse block locator for
-    the [block]. The locator contains at most [max_length] elements. *)
+type seed = {
+  sender_id: P2p_peer.Id.t ;
+  receiver_id: P2p_peer.Id.t
+}
+(** Argument to the seed used to randomize the locator. *)
+
+val estimated_length: seed -> t -> int
+(** [estimated_length seed locator] estimate the length of the chain
+    represented by [locator] using [seed]. *)
+
+val compute:
+  predecessor: (Block_hash.t -> int -> Block_hash.t option Lwt.t) ->
+  genesis:Block_hash.t ->
+  Block_hash.t -> Block_header.t -> seed -> size:int -> t Lwt.t
+(** [compute block seed max_length] compute the sparse block locator using
+    [seed] to compute random jumps for the [block]. The locator contains at
+    most [max_length] elements. *)
+
+type step = {
+  block: Block_hash.t ;
+  predecessor: Block_hash.t ;
+  step: int ;
+  strict_step: bool ;
+}
+(** A 'step' in a locator is a couple of consecutive hashes in the
+    locator, and the expected difference of level between the two
+    blocks (or an upper bounds when [strict_step = false]). *)
+
+val to_steps: seed -> t -> step list
+(** Build all the 'steps' composing the locator using a given seed,
+    starting with the oldest one (typically the predecessor of the
+    first step will be `genesis`).
+    All steps contains [strict_step = true], except the oldest one. *)
 
 type validity =
   | Unknown
@@ -32,7 +58,8 @@ type validity =
   | Known_invalid
 
 val unknown_prefix:
-  (Block_hash.t -> validity Lwt.t) -> t -> (Block_hash.t * t) option Lwt.t
+  is_known:(Block_hash.t -> validity Lwt.t) ->
+  t -> (Block_hash.t * t) option Lwt.t
 (** [unknown_prefix validity locator] keeps only the unknown part of
     the locator up to the first valid block. If there is no known valid
     block or there is a known invalid one, None is returned. *)
diff --git a/src/lib_shell/block_locator_iterator.ml b/src/lib_shell/block_locator_iterator.ml
deleted file mode 100644
index c20c3fb8c..000000000
--- a/src/lib_shell/block_locator_iterator.ml
+++ /dev/null
@@ -1,93 +0,0 @@
-(**************************************************************************)
-(*                                                                        *)
-(*    Copyright (c) 2014 - 2018.                                          *)
-(*    Dynamic Ledger Solutions, Inc. <contact@tezos.com>                  *)
-(*                                                                        *)
-(*    All rights reserved. No warranty, explicit or implicit, provided.   *)
-(*                                                                        *)
-(**************************************************************************)
-
-let estimated_length locator =
-  let (_head, hist) = Block_locator.raw locator in
-  let rec loop acc step cpt = function
-    | [] -> acc
-    | _ :: hist ->
-        if cpt = 0 then
-          loop (acc+step) (step*2) 9 hist
-        else
-          loop (acc+step) step (cpt-1) hist
-  in
-  loop 1 1 9 hist
-
-let fold ~f acc locator =
-  let (head, hist) = Block_locator.raw locator in
-  let rec loop step cpt acc = function
-    | [] | [_] -> acc
-    | block :: (pred :: rem as hist) ->
-        let step, cpt =
-          if cpt = 0 then
-            2 * step, 9
-          else
-            step, cpt - 1 in
-        let acc = f acc ~block ~pred ~step ~strict_step:(rem <> []) in
-        loop step cpt acc hist
-  in
-  loop 1 10 acc (Block_header.hash head :: hist)
-
-type step = {
-  block: Block_hash.t ;
-  predecessor: Block_hash.t ;
-  step: int ;
-  strict_step: bool ;
-}
-
-let to_steps locator =
-  fold
-    ~f:begin fun acc ~block ~pred ~step ~strict_step -> {
-        block ; predecessor = pred ; step ; strict_step ;
-      } :: acc
-    end
-    [] locator
-
-let block_validity chain_state block : Block_locator.validity Lwt.t =
-  State.Block.known chain_state block >>= function
-  | false ->
-      if Block_hash.equal block (State.Chain.faked_genesis_hash chain_state) then
-        Lwt.return Block_locator.Known_valid
-      else
-        Lwt.return Block_locator.Unknown
-  | true ->
-      State.Block.known_invalid chain_state block >>= function
-      | true ->
-          Lwt.return Block_locator.Known_invalid
-      | false ->
-          Lwt.return Block_locator.Known_valid
-
-let known_ancestor chain_state locator =
-  Block_locator.unknown_prefix (block_validity chain_state) locator >>= function
-  | None -> Lwt.return_none
-  | Some (tail, locator) ->
-      if Block_hash.equal tail (State.Chain.faked_genesis_hash chain_state) then
-        State.Block.read_exn
-          chain_state (State.Chain.genesis chain_state).block >>= fun genesis ->
-        Lwt.return_some (genesis, locator)
-      else
-        State.Block.read_exn chain_state tail >>= fun block ->
-        Lwt.return_some (block, locator)
-
-let find_new chain_state locator sz =
-  let rec path sz acc h =
-    if sz <= 0 then Lwt.return (List.rev acc)
-    else
-      State.read_chain_data chain_state begin fun chain_store _data ->
-        Store.Chain_data.In_main_branch.read_opt (chain_store, h)
-      end >>= function
-      | None -> Lwt.return (List.rev acc)
-      | Some s -> path (sz-1) (s :: acc) s in
-  known_ancestor chain_state locator >>= function
-  | None -> Lwt.return_nil
-  | Some (known, _) ->
-      Chain.head chain_state >>= fun head ->
-      Chain_traversal.common_ancestor known head >>= fun ancestor ->
-      path sz [] (State.Block.hash ancestor)
-
diff --git a/src/lib_shell/block_locator_iterator.mli b/src/lib_shell/block_locator_iterator.mli
deleted file mode 100644
index a97cc6e64..000000000
--- a/src/lib_shell/block_locator_iterator.mli
+++ /dev/null
@@ -1,55 +0,0 @@
-(**************************************************************************)
-(*                                                                        *)
-(*    Copyright (c) 2014 - 2018.                                          *)
-(*    Dynamic Ledger Solutions, Inc. <contact@tezos.com>                  *)
-(*                                                                        *)
-(*    All rights reserved. No warranty, explicit or implicit, provided.   *)
-(*                                                                        *)
-(**************************************************************************)
-
-val fold:
-  f:('a ->
-     block:Block_hash.t -> pred:Block_hash.t ->
-     step:int -> strict_step:bool -> 'a) ->
-  'a -> Block_locator.t -> 'a
-(** [map f l] applies [f] to each block of the locator, the last one
-    excepted. The function also receives the expected predecessor
-    [pred] of the [block] after [step] steps, i.e. the next block in
-    the locator. When [strict_step] is [true], then [step] is the
-    exact number of predecessor to be followed before to found
-    [pred]. Otherwise, it is only an upper bound. *)
-
-type step = {
-  block: Block_hash.t ;
-  predecessor: Block_hash.t ;
-  step: int ;
-  strict_step: bool ;
-}
-(** A 'step' in a locator is a couple of consecutives hashes in the
-    locator, and the expected difference of level the two blocks (or
-    an upper bounds when [strict_step = false]). *)
-
-val to_steps: Block_locator.t -> step list
-(** Build all the 'steps' composing the locator, starting with the
-    oldest one (typically the predecessor of the first step will be
-    `genesis`). All steps contains [strict_step = true], except the
-    first one. *)
-
-val estimated_length: Block_locator.t -> int
-(** [estimated_length locator] estimate the length of the chain
-    represented by [locator]. *)
-
-val known_ancestor:
-  State.Chain.t -> Block_locator.t -> (State.Block.t * Block_locator.t) option Lwt.t
-(** [known_ancestor chain_state locator] computes the first block of
-    [locator] that is known to be a valid block. It also computes the
-    'prefix' of [locator] with end at the first valid block.  The
-    function returns [None] when no block in the locator are known or
-    if the first known block is invalid. *)
-
-val find_new:
-  State.Chain.t -> Block_locator.t -> int -> Block_hash.t list Lwt.t
-(** [find_new chain locator max_length] returns the blocks from our
-    current branch that would be unknown to a peer that sends us the
-    [locator]. *)
-
diff --git a/src/lib_shell/bootstrap_pipeline.ml b/src/lib_shell/bootstrap_pipeline.ml
index 107087c39..3c38609a7 100644
--- a/src/lib_shell/bootstrap_pipeline.ml
+++ b/src/lib_shell/bootstrap_pipeline.ml
@@ -31,7 +31,7 @@ type t = {
   mutable errors: Error_monad.error list ;
 }
 
-let fetch_step pipeline (step : Block_locator_iterator.step)  =
+let fetch_step pipeline (step : Block_locator.step)  =
   lwt_log_info "fetching step %a -> %a (%d%s) from peer %a."
     Block_hash.pp_short step.block
     Block_hash.pp_short step.predecessor
@@ -78,7 +78,11 @@ let fetch_step pipeline (step : Block_locator_iterator.step)  =
 
 let headers_fetch_worker_loop pipeline =
   begin
-    let steps = Block_locator_iterator.to_steps pipeline.locator in
+    let sender_id = Distributed_db.my_peer_id pipeline.chain_db in
+    (* sender and receiver are inverted here because they are from
+       the point of view of the node sending the locator *)
+    let seed = {Block_locator.sender_id=pipeline.peer_id; receiver_id=sender_id } in
+    let steps = Block_locator.to_steps seed pipeline.locator in
     iter_s (fetch_step pipeline) steps >>=? fun () ->
     return ()
   end >>= function
diff --git a/src/lib_shell/chain.ml b/src/lib_shell/chain.ml
index cfe2addf4..9cc05acb7 100644
--- a/src/lib_shell/chain.ml
+++ b/src/lib_shell/chain.ml
@@ -39,7 +39,6 @@ type data = State.chain_data = {
   current_mempool: Mempool.t ;
   live_blocks: Block_hash.Set.t ;
   live_operations: Operation_hash.Set.t ;
-  locator: Block_locator.t Lwt.t lazy_t ;
 }
 
 let data chain_state =
@@ -47,12 +46,11 @@ let data chain_state =
     Lwt.return data
   end
 
-let locator chain_state =
-  data chain_state >>= begin fun data ->
-    Lazy.force data.locator
-  end
+let locator chain_state seed =
+  data chain_state >>= fun data ->
+  State.compute_locator chain_state data.current_head seed
 
-let locked_set_head chain_state chain_store data block =
+let locked_set_head chain_store data block =
   let rec pop_blocks ancestor block =
     let hash = State.Block.hash block in
     if Block_hash.equal hash ancestor then
@@ -89,12 +87,11 @@ let locked_set_head chain_state chain_store data block =
                current_mempool = Mempool.empty ;
                live_blocks ;
                live_operations ;
-               locator = lazy (State.compute_locator chain_state block) ;
              }
 
 let set_head chain_state block =
   State.update_chain_data chain_state begin fun chain_store data ->
-    locked_set_head chain_state chain_store data block >>= fun new_chain_data ->
+    locked_set_head chain_store data block >>= fun new_chain_data ->
     Lwt.return (Some new_chain_data,
                 data.current_head)
   end
@@ -104,7 +101,7 @@ let test_and_set_head chain_state ~old block =
     if not (State.Block.equal data.current_head old) then
       Lwt.return (None, false)
     else
-      locked_set_head chain_state chain_store data block >>= fun new_chain_data ->
+      locked_set_head chain_store data block >>= fun new_chain_data ->
       Lwt.return (Some new_chain_data, true)
   end
 
diff --git a/src/lib_shell/chain.mli b/src/lib_shell/chain.mli
index 8da902444..481168c8e 100644
--- a/src/lib_shell/chain.mli
+++ b/src/lib_shell/chain.mli
@@ -15,7 +15,7 @@ val genesis: State.Chain.t -> State.Block.t Lwt.t
 
 (** The current head of the chain. *)
 val head: State.Chain.t -> State.Block.t Lwt.t
-val locator: State.Chain.t -> Block_locator.t Lwt.t
+val locator: State.Chain.t -> Block_locator.seed -> Block_locator.t Lwt.t
 
 (** All the available chain data. *)
 type data = {
@@ -23,13 +23,11 @@ type data = {
   current_mempool: Mempool.t ;
   live_blocks: Block_hash.Set.t ;
   live_operations: Operation_hash.Set.t ;
-  locator: Block_locator.t Lwt.t lazy_t ;
 }
 
 (** Reading atomically all the chain data. *)
 val data: State.Chain.t -> data Lwt.t
 
-
 (** The current head and all the known (valid) alternate heads. *)
 val known_heads: State.Chain.t -> State.Block.t list Lwt.t
 
diff --git a/src/lib_shell/chain_validator.ml b/src/lib_shell/chain_validator.ml
index c834fa8b2..a3a10dc48 100644
--- a/src/lib_shell/chain_validator.ml
+++ b/src/lib_shell/chain_validator.ml
@@ -216,10 +216,7 @@ let broadcast_head w ~previous block =
         nv.parameters.chain_db block ;
       Lwt.return_unit
     end else begin
-      let chain_state = Distributed_db.chain_state nv.parameters.chain_db in
-      Chain.locator chain_state >>= fun locator ->
-      Distributed_db.Advertise.current_branch
-        nv.parameters.chain_db locator
+      Distributed_db.Advertise.current_branch nv.parameters.chain_db
     end
   end
 
diff --git a/src/lib_shell/distributed_db.ml b/src/lib_shell/distributed_db.ml
index 28768da15..ec0a696f4 100644
--- a/src/lib_shell/distributed_db.ml
+++ b/src/lib_shell/distributed_db.ml
@@ -340,6 +340,8 @@ let state { disk } = disk
 let chain_state { chain_state } = chain_state
 let db { global_db } = global_db
 
+let my_peer_id chain_db = P2p.peer_id chain_db.global_db.p2p
+
 let read_block_header { disk } h =
   State.read_block disk h >>= function
   | Some b ->
@@ -466,7 +468,10 @@ module P2p_reader = struct
           ignore
           @@ P2p.try_send global_db.p2p state.conn
           @@ Get_current_branch chain_id ;
-        Chain.locator chain_db.chain_state >>= fun locator ->
+        let seed = {
+          Block_locator.receiver_id=state.gid;
+          sender_id=(my_peer_id chain_db) } in
+        (Chain.locator chain_db.chain_state seed) >>= fun locator ->
         ignore
         @@ P2p.try_send global_db.p2p state.conn
         @@ Current_branch (chain_id, locator) ;
@@ -681,6 +686,7 @@ let raw_try_send p2p peer_id msg =
   | None -> ()
   | Some conn -> ignore (P2p.try_send p2p conn msg : bool)
 
+
 let create disk p2p =
   let global_request =
     { data = () ;
@@ -924,10 +930,28 @@ module Advertise = struct
     send chain_db ?peer @@
     Current_head (chain_id, State.Block.header head, mempool)
 
-  let current_branch chain_db ?peer locator =
+  let current_branch ?peer chain_db =
     let chain_id = State.Chain.id chain_db.chain_state in
-    send chain_db ?peer @@ Current_branch (chain_id, locator) ;
-    Lwt.return_unit
+    let chain_state = chain_state chain_db in
+    let sender_id = my_peer_id chain_db in
+    match peer with
+    | Some receiver_id ->
+        let seed = {
+          Block_locator.receiver_id=receiver_id; sender_id } in
+        (Chain.locator chain_state seed) >>= fun locator ->
+        let msg = Message.Current_branch (chain_id, locator) in
+        try_send chain_db receiver_id msg;
+        Lwt.return_unit
+    | None ->
+        Lwt_list.iter_p
+          (fun (receiver_id,state) ->
+             let seed = {
+               Block_locator.receiver_id=receiver_id; sender_id } in
+             (Chain.locator chain_state seed) >>= fun locator ->
+             let msg = Message.Current_branch (chain_id, locator) in
+             ignore (P2p.try_send chain_db.global_db.p2p state.conn msg);
+             Lwt.return_unit
+          ) (P2p_peer.Table.fold (fun k v acc -> (k,v)::acc) chain_db.active_connections [])
 
 end
 
diff --git a/src/lib_shell/distributed_db.mli b/src/lib_shell/distributed_db.mli
index 904a93b51..8f6ad59f9 100644
--- a/src/lib_shell/distributed_db.mli
+++ b/src/lib_shell/distributed_db.mli
@@ -58,6 +58,9 @@ val disconnect: chain_db -> P2p_peer.Id.t -> unit Lwt.t
 val chain_state: chain_db -> State.Chain.t
 val db: chain_db -> db
 
+(** Return the peer id of the node *)
+val my_peer_id: chain_db -> P2p_peer.Id.t
+
 (** {1 Sending messages} *)
 
 module Request : sig
@@ -85,8 +88,7 @@ module Advertise : sig
   (** Notify a given peer, or all known active peers for the
       chain, of a new head and its sparse history. *)
   val current_branch:
-    chain_db -> ?peer:P2p_peer.Id.t ->
-    Block_locator.t -> unit Lwt.t
+    ?peer:P2p_peer.Id.t -> chain_db -> unit Lwt.t
 
 end
 
diff --git a/src/lib_shell/peer_validator.ml b/src/lib_shell/peer_validator.ml
index be58d99f6..98dd4742f 100644
--- a/src/lib_shell/peer_validator.ml
+++ b/src/lib_shell/peer_validator.ml
@@ -26,13 +26,15 @@ module Request = struct
 
   type _ t =
     | New_head : Block_hash.t * Block_header.t -> unit t
-    | New_branch : Block_hash.t * Block_locator.t -> unit t
+    | New_branch : Block_hash.t * Block_locator.t * Block_locator.seed -> unit t
 
   let view (type a) (req : a t) : view = match req with
     | New_head (hash, _) ->
         New_head hash
-    | New_branch (hash, locator) ->
-        New_branch (hash, Block_locator_iterator.estimated_length locator)
+    | New_branch (hash, locator, seed) ->
+        (* the seed is associated to each locator
+           w.r.t. the peer_id of the sender *)
+        New_branch (hash, Block_locator.estimated_length seed locator)
 end
 
 type limits = {
@@ -89,7 +91,11 @@ let set_bootstrapped pv =
 
 let bootstrap_new_branch w _ancestor _head unknown_prefix =
   let pv = Worker.state w in
-  let len = Block_locator_iterator.estimated_length unknown_prefix in
+  let sender_id = Distributed_db.my_peer_id pv.parameters.chain_db in
+  (* sender and receiver are inverted here because they are from
+     the point of view of the node sending the locator *)
+  let seed = {Block_locator.sender_id=pv.peer_id; receiver_id = sender_id } in
+  let len = Block_locator.estimated_length seed unknown_prefix in
   debug w
     "validating new branch from peer %a (approx. %d blocks)"
     P2p_peer.Id.pp_short pv.peer_id len ;
@@ -201,7 +207,7 @@ let may_validate_new_branch w distant_hash locator =
   let distant_header, _ = (locator : Block_locator.t :> Block_header.t * _) in
   only_if_fitness_increases w distant_header @@ fun () ->
   let chain_state = Distributed_db.chain_state pv.parameters.chain_db in
-  Block_locator_iterator.known_ancestor chain_state locator >>= function
+  State.Block.known_ancestor chain_state locator >>= function
   | None ->
       debug w
         "ignoring branch %a without common ancestor from peer: %a."
@@ -228,7 +234,7 @@ let on_request (type a) w (req : a Request.t) : a tzresult Lwt.t =
         Block_hash.pp_short hash
         P2p_peer.Id.pp_short pv.peer_id ;
       may_validate_new_head w hash header
-  | Request.New_branch (hash, locator) ->
+  | Request.New_branch (hash, locator, _seed) ->
       (* TODO penalize empty locator... ?? *)
       debug w "processing new branch %a from peer %a."
         Block_hash.pp_short hash
@@ -300,7 +306,7 @@ let table =
   let merge w (Worker.Any_request neu) old =
     let pv = Worker.state w in
     match neu with
-    | Request.New_branch (_, locator) ->
+    | Request.New_branch (_, locator, _) ->
         let header, _ = (locator : Block_locator.t :> _ * _) in
         pv.last_advertised_head <- header ;
         Some (Worker.Any_request neu)
@@ -346,7 +352,12 @@ let create
 let notify_branch w locator =
   let header, _ = (locator : Block_locator.t :> _ * _) in
   let hash = Block_header.hash header in
-  Worker.drop_request w (New_branch (hash, locator))
+  let pv = Worker.state w in
+  let sender_id = Distributed_db.my_peer_id pv.parameters.chain_db in
+  (* sender and receiver are inverted here because they are from
+     the point of view of the node sending the locator *)
+  let seed = {Block_locator.sender_id=pv.peer_id; receiver_id=sender_id } in
+  Worker.drop_request w (New_branch (hash, locator, seed))
 
 let notify_head w header =
   let hash = Block_header.hash header in
diff --git a/src/lib_shell/state.ml b/src/lib_shell/state.ml
index 561824419..01d1ec508 100644
--- a/src/lib_shell/state.ml
+++ b/src/lib_shell/state.ml
@@ -60,7 +60,6 @@ and chain_data = {
   current_mempool: Mempool.t ;
   live_blocks: Block_hash.Set.t ;
   live_operations: Operation_hash.Set.t ;
-  locator: Block_locator.t Lwt.t lazy_t ;
 }
 
 and block = {
@@ -139,7 +138,7 @@ let store_predecessors (store: Store.Block.store) (b: Block_hash.t) : unit Lwt.t
    and in O(|chain|) after that.
    @raise Invalid_argument "State.predecessors: negative distance"
 *)
-let predecessor_n (store: Store.Block.store) (b: Block_hash.t) (distance: int)
+let predecessor_n (store: Store.Block.store) (block_hash: Block_hash.t) (distance: int)
   : Block_hash.t option Lwt.t =
   (* helper functions *)
   (* computes power of 2 w/o floats *)
@@ -167,9 +166,9 @@ let predecessor_n (store: Store.Block.store) (b: Block_hash.t) (distance: int)
   if distance <= 0 then
     invalid_arg ("State.predecessor: distance <= 0"^(string_of_int distance))
   else
-    let rec loop b distance =
+    let rec loop block_hash distance =
       if distance = 1
-      then Store.Block.Predecessors.read_opt (store, b) 0
+      then Store.Block.Predecessors.read_opt (store, block_hash) 0
       else
         let (power,rest) = closest_power_two_and_rest distance in
         let (power,rest) =
@@ -179,24 +178,24 @@ let predecessor_n (store: Store.Block.store) (b: Block_hash.t) (distance: int)
             let rest = distance - (power_of_2 power) in
             (power,rest)
         in
-        Store.Block.Predecessors.read_opt (store, b) power >>= function
+        Store.Block.Predecessors.read_opt (store, block_hash) power >>= function
         | None -> Lwt.return_none (* reached genesis *)
         | Some pred ->
             if rest = 0
             then Lwt.return_some pred (* landed on the requested predecessor *)
             else loop pred rest       (* need to jump further back *)
     in
-    loop b distance
+    loop block_hash distance
 
-let compute_locator_from_hash (chain : chain_state) ?(size = 200) head_hash =
+let compute_locator_from_hash (chain : chain_state) ?(size = 200) head_hash seed =
   Shared.use chain.block_store begin fun block_store ->
     Store.Block.Contents.read_exn (block_store, head_hash) >>= fun { header } ->
     Block_locator.compute ~predecessor:(predecessor_n block_store)
-      ~genesis:chain.genesis.block head_hash header size
+      ~genesis:chain.genesis.block head_hash header seed ~size
   end
 
-let compute_locator chain ?size head =
-  compute_locator_from_hash chain ?size head.hash
+let compute_locator chain ?size head seed =
+  compute_locator_from_hash chain ?size head.hash seed
 
 type t = global_state
 
@@ -259,7 +258,6 @@ module Chain = struct
         current_mempool = Mempool.empty ;
         live_blocks = Block_hash.Set.singleton genesis.block ;
         live_operations = Operation_hash.Set.empty ;
-        locator = lazy (compute_locator_from_hash chain_state current_head) ;
       } ;
       chain_data_store ;
     }
@@ -645,6 +643,33 @@ module Block = struct
     context block >>= fun context ->
     Context.get_test_chain context
 
+  let block_validity chain_state block : Block_locator.validity Lwt.t =
+    known chain_state block >>= function
+    | false ->
+        if Block_hash.equal block (Chain.faked_genesis_hash chain_state) then
+          Lwt.return Block_locator.Known_valid
+        else
+          Lwt.return Block_locator.Unknown
+    | true ->
+        known_invalid chain_state block >>= function
+        | true ->
+            Lwt.return Block_locator.Known_invalid
+        | false ->
+            Lwt.return Block_locator.Known_valid
+
+  let known_ancestor chain_state locator =
+    Block_locator.unknown_prefix
+      ~is_known:(block_validity chain_state) locator >>= function
+    | None -> Lwt.return_none
+    | Some (tail, locator) ->
+        if Block_hash.equal tail (Chain.faked_genesis_hash chain_state) then
+          read_exn
+            chain_state (Chain.genesis chain_state).block >>= fun genesis ->
+          Lwt.return_some (genesis, locator)
+        else
+          read_exn chain_state tail >>= fun block ->
+          Lwt.return_some (block, locator)
+
 end
 
 let read_block { global_data } ?pred hash =
diff --git a/src/lib_shell/state.mli b/src/lib_shell/state.mli
index c502c6aa3..18fc33a07 100644
--- a/src/lib_shell/state.mli
+++ b/src/lib_shell/state.mli
@@ -148,6 +148,14 @@ module Block : sig
 
   val watcher: Chain.t -> block Lwt_stream.t * Lwt_watcher.stopper
 
+  val known_ancestor:
+    Chain.t -> Block_locator.t -> (block * Block_locator.t) option Lwt.t
+    (** [known_ancestor chain_state locator] computes the first block of
+        [locator] that is known to be a valid block. It also computes the
+        'prefix' of [locator] with end at the first valid block.  The
+        function returns [None] when no block in the locator are known or
+        if the first known block is invalid. *)
+
 end
 
 val read_block:
@@ -156,7 +164,7 @@ val read_block:
 val read_block_exn:
   global_state -> ?pred:int -> Block_hash.t -> Block.t Lwt.t
 
-val compute_locator: Chain.t -> ?size:int -> Block.t -> Block_locator.t Lwt.t
+val compute_locator: Chain.t -> ?size:int -> Block.t -> Block_locator.seed -> Block_locator.t Lwt.t
 
 val fork_testchain:
   Block.t -> Protocol_hash.t -> Time.t -> Chain.t tzresult Lwt.t
@@ -166,7 +174,6 @@ type chain_data = {
   current_mempool: Mempool.t ;
   live_blocks: Block_hash.Set.t ;
   live_operations: Operation_hash.Set.t ;
-  locator: Block_locator.t Lwt.t lazy_t ;
 }
 
 val read_chain_data:
@@ -217,4 +224,3 @@ module Current_mempool : sig
       not the provided one. *)
 
 end
-
diff --git a/src/lib_shell/test/test_locator.ml b/src/lib_shell/test/test_locator.ml
index 310b45771..3d4cd3ede 100644
--- a/src/lib_shell/test/test_locator.ml
+++ b/src/lib_shell/test/test_locator.ml
@@ -226,6 +226,10 @@ let test_pred (base_dir:string) : unit tzresult Lwt.t =
   let repeats = 100 in
   return (repeat (fun () -> test_once (1 + Random.int range)) repeats)
 
+let seed =
+  let receiver_id = P2p_peer.Id.of_string_exn (String.make P2p_peer.Id.size 'r') in
+  let sender_id = P2p_peer.Id.of_string_exn (String.make P2p_peer.Id.size 's') in
+  {Block_locator.receiver_id=receiver_id ; sender_id }
 
 (* compute locator using the linear predecessor *)
 let compute_linear_locator (chain:State.Chain.t) ~size block =
@@ -233,16 +237,16 @@ let compute_linear_locator (chain:State.Chain.t) ~size block =
   let block_hash = State.Block.hash block in
   let header = State.Block.header block in
   Block_locator.compute ~predecessor:(linear_predecessor_n chain)
-    ~genesis:genesis.block block_hash header size
+    ~genesis:genesis.block block_hash header ~size seed
 
 
-(* given the size of a chain, returns the size required for a locator 
+(* given the size of a chain, returns the size required for a locator
    to reach genesis *)
 let compute_size_locator size_chain =
   let repeats = 10. in
   int_of_float ((log ((float size_chain) /. repeats)) /. (log 2.) -. 1.) * 10
 
-(* given the size of a locator, returns the size of the chain that it 
+(* given the size of a locator, returns the size of the chain that it
    can cover back to genesis *)
 let compute_size_chain size_locator =
   let repeats = 10. in
@@ -284,7 +288,7 @@ let test_locator base_dir =
   let check_locator size : unit tzresult Lwt.t =
     State.Block.read chain head >>=? fun block ->
     time ~runs:runs (fun () ->
-        State.compute_locator chain ~size:size block) |>
+        State.compute_locator chain ~size:size block seed) |>
     fun (l_exp,t_exp) ->
     time ~runs:runs (fun () ->
         compute_linear_locator chain ~size:size block) |>
@@ -312,7 +316,6 @@ let test_locator base_dir =
   in
   loop 1
 
-
 let wrap n f =
   Alcotest_lwt.test_case n `Quick begin fun _ () ->
     Lwt_utils_unix.with_tempdir "tezos_test_" begin fun dir ->
diff --git a/src/lib_shell/test/test_state.ml b/src/lib_shell/test/test_state.ml
index 112fbeddb..4acce23dd 100644
--- a/src/lib_shell/test/test_state.ml
+++ b/src/lib_shell/test/test_state.ml
@@ -234,12 +234,17 @@ let test_ancestor s =
 
 (****************************************************************************)
 
+let seed =
+  let receiver_id = P2p_peer.Id.of_string_exn (String.make P2p_peer.Id.size 'r') in
+  let sender_id = P2p_peer.Id.of_string_exn (String.make P2p_peer.Id.size 's') in
+  {Block_locator.receiver_id=receiver_id ; sender_id }
+
 (** Chain_traversal.block_locator *)
 
 let test_locator s =
   let check_locator length h1 expected =
     State.compute_locator s.chain
-      ~size:length (vblock s h1) >>= fun l ->
+      ~size:length (vblock s h1) seed >>= fun l ->
     let _, l = (l : Block_locator.t :> _ * _) in
     if List.length l <> List.length expected then
       Assert.fail_msg
@@ -378,35 +383,6 @@ let test_new_blocks s =
   return ()
 
 
-(****************************************************************************)
-
-(** Block_locator.find_new *)
-
-let test_find_new s =
-  let test s h expected =
-    State.compute_locator s.chain ~size:50 (vblock s h) >>= fun loc ->
-    Block_locator_iterator.find_new
-      s.chain loc (List.length expected) >>= fun blocks ->
-    if List.length blocks <> List.length expected then
-      Assert.fail_msg
-        "Invalid find new length %s (found: %d, expected: %d)"
-        h (List.length blocks) (List.length expected) ;
-    List.iter2
-      (fun h1 h2 ->
-         if not (Block_hash.equal h1 (State.Block.hash @@ vblock s h2)) then
-           Assert.fail_msg "Invalid find new %s.%d (expected: %s)" h (List.length expected) h2)
-      blocks expected ;
-    Lwt.return_unit
-  in
-  Chain.set_head s.chain (vblock s "A8") >>= fun _ ->
-  test s "A6" [] >>= fun () ->
-  test s "A6" ["A7";"A8"] >>= fun () ->
-  test s "A6" ["A7"] >>= fun () ->
-  test s "B4" ["A4"] >>= fun () ->
-  test s "B7" ["A4";"A5";"A6";"A7"] >>= fun () ->
-  return ()
-
-
 (****************************************************************************)
 
 
@@ -420,7 +396,6 @@ let tests : (string * (state -> unit tzresult Lwt.t)) list = [
   "head", test_head ;
   "mem", test_mem ;
   "new_blocks", test_new_blocks ;
-  "find_new", test_find_new ;
 ]
 
 let wrap (n, f) =