diff --git a/lib/arpv4/arpv4.ml b/lib/arpv4/arpv4.ml
index b28f83d64..8493ac1b7 100644
--- a/lib/arpv4/arpv4.ml
+++ b/lib/arpv4/arpv4.ml
@@ -20,16 +20,17 @@ open Lwt.Infix
 let src = Logs.Src.create "arpv4" ~doc:"Mirage ARP module"
 module Log = (val Logs.src_log src : Logs.LOG)
 
-module Make (Ethif : V1_LWT.ETHIF) (Clock : V1.CLOCK) (Time : V1_LWT.TIME) = struct
+module Make (Ethif : V1_LWT.ETHIF) (Clock : V1.MCLOCK) (Time : V1_LWT.TIME) = struct
 
   type result = [ `Ok of Macaddr.t | `Timeout ]
 
   type entry =
     | Pending of result Lwt.t * result Lwt.u
-    | Confirmed of float * Macaddr.t
+    | Confirmed of int64 * Macaddr.t
 
   type t = {
     ethif : Ethif.t;
+    clock : Clock.t;
     cache: (Ipaddr.V4.t, entry) Hashtbl.t;
     mutable bound_ips: Ipaddr.V4.t list;
   }
@@ -48,15 +49,19 @@ module Make (Ethif : V1_LWT.ETHIF) (Clock : V1.CLOCK) (Time : V1_LWT.TIME) = str
   let probe_num = 3 (* how many probes to send before giving up *)
 
   let rec tick t () =
-    let now = Clock.time () in
-    let expired = Hashtbl.fold (fun ip entry expired ->
+    let now = Clock.elapsed_ns t.clock in
+    let remove_expired ip entry =
         match entry with
-        | Pending _ -> expired
-        | Confirmed (t, _) -> if t >= now then ip :: expired else expired) t.cache []
+        | Pending _ -> Some entry
+        | Confirmed (expiry, _) ->
+          if Int64.compare expiry now > -1
+          then Some entry
+          else begin
+            Log.info (fun f -> f "ARP: timeout %a" Ipaddr.V4.pp_hum ip);
+            None
+          end
     in
-    List.iter (fun ip ->
-         Log.info (fun f -> f "ARP: timeout %a" Ipaddr.V4.pp_hum ip); Hashtbl.remove t.cache ip
-      ) expired;
+    Hashtbl.filter_map_inplace remove_expired t.cache;
     Time.sleep_ns arp_timeout >>= tick t
 
   let to_repr t =
@@ -64,7 +69,7 @@ module Make (Ethif : V1_LWT.ETHIF) (Clock : V1.CLOCK) (Time : V1_LWT.TIME) = str
       let key = Ipaddr.V4.to_string ip in
       match entry with
        | Pending _ -> acc ^ "\n" ^ key ^ " -> " ^ "Pending" 
-       | Confirmed (time, mac) -> Printf.sprintf "%s\n%s -> Confirmed (%s) (expires %f)\n%!" 
+       | Confirmed (time, mac) -> Printf.sprintf "%s\n%s -> Confirmed (%s) (expires %Lu)\n%!" 
                                     acc key (Macaddr.to_string mac) time
     in
     Lwt.return (Hashtbl.fold print t.cache "")
@@ -77,8 +82,8 @@ module Make (Ethif : V1_LWT.ETHIF) (Clock : V1.CLOCK) (Time : V1_LWT.TIME) = str
     match Ipaddr.V4.is_multicast ip || (Ipaddr.V4.compare ip Ipaddr.V4.any = 0) with
     | true -> Log.debug (fun f -> f "Ignoring ARP notification request for IP %a" Ipaddr.V4.pp_hum ip)
     | false ->
-      let now = Clock.time () in
-      let expire = now +. Duration.to_f arp_timeout in
+      let now = Clock.elapsed_ns t.clock in
+      let expire = Int64.add now arp_timeout in
       try
         match Hashtbl.find t.cache ip with
         | Pending (_, w) ->
@@ -197,10 +202,10 @@ module Make (Ethif : V1_LWT.ETHIF) (Clock : V1.CLOCK) (Time : V1_LWT.TIME) = str
       Lwt.async (retry 0);
       response
 
-  let connect ethif =
+  let connect ethif clock =
     let cache = Hashtbl.create 7 in
     let bound_ips = [] in
-    let t = { ethif; cache; bound_ips } in
+    let t = { clock; ethif; cache; bound_ips } in
     Lwt.async (tick t);
     Log.info (fun f -> f "Connected arpv4 device on %s" (Macaddr.to_string (
                Ethif.mac t.ethif)));
diff --git a/lib/arpv4/arpv4.mli b/lib/arpv4/arpv4.mli
index 602eac32b..bb87aa6a7 100644
--- a/lib/arpv4/arpv4.mli
+++ b/lib/arpv4/arpv4.mli
@@ -15,11 +15,11 @@
  *
  *)
 
-module Make (Ethif : V1_LWT.ETHIF) (Clock : V1.CLOCK) (Time : V1_LWT.TIME) : sig
+module Make (Ethif : V1_LWT.ETHIF) (Clock : V1.MCLOCK) (Time : V1_LWT.TIME) : sig
   include V1_LWT.ARP
 
   type ethif = Ethif.t
 
   (** [connect] creates a value of type [t]. *)
-  val connect : ethif -> [> `Ok of t | `Error of error ] Lwt.t
+  val connect : ethif -> Clock.t -> [> `Ok of t | `Error of error ] Lwt.t
 end
diff --git a/lib/ipv6/ipv6.ml b/lib/ipv6/ipv6.ml
index fc4ab6b05..578c75fda 100644
--- a/lib/ipv6/ipv6.ml
+++ b/lib/ipv6/ipv6.ml
@@ -21,7 +21,7 @@ module I = Ipaddr
 
 open Lwt.Infix
 
-module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
+module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.MCLOCK) = struct
   type ethif    = E.t
   type 'a io    = 'a Lwt.t
   type buffer   = Cstruct.t
@@ -31,6 +31,7 @@ module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
 
   type t =
     { ethif : E.t;
+      clock : C.t;
       mutable ctx : Ndpv6.context }
 
   type error =
@@ -39,7 +40,7 @@ module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
 
   let start_ticking t =
     let rec loop () =
-      let now = C.time () in
+      let now = C.elapsed_ns t.clock in
       let ctx, bufs = Ndpv6.tick ~now t.ctx in
       t.ctx <- ctx;
       Lwt_list.iter_s (E.writev t.ethif) bufs >>= fun () ->
@@ -51,7 +52,7 @@ module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
     Ndpv6.allocate_frame t.ctx dst proto
 
   let writev t frame bufs =
-    let now = C.time () in
+    let now = C.elapsed_ns t.clock in
     let dst =
       Ndpv6.ipaddr_of_cstruct
         (Ipv6_wire.get_ipv6_dst (Cstruct.shift frame Ethif_wire.sizeof_ethernet))
@@ -64,7 +65,7 @@ module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
     writev t frame [buf]
 
   let input t ~tcp ~udp ~default buf =
-    let now = C.time () in
+    let now = C.elapsed_ns t.clock in
     let _, bufs, actions = Ndpv6.handle ~now t.ctx buf in
     Lwt_list.iter_s (function
         | `Tcp (src, dst, buf) -> tcp ~src ~dst buf
@@ -81,7 +82,7 @@ module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
   let src t ~dst = Ndpv6.select_source t.ctx dst
 
   let set_ip t ip =
-    let now = C.time () in
+    let now = C.elapsed_ns t.clock in
     let ctx, bufs = Ndpv6.add_ip ~now t.ctx ip in
     t.ctx <- ctx;
     Lwt_list.iter_s (E.writev t.ethif) bufs
@@ -90,7 +91,7 @@ module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
     Ndpv6.get_ip t.ctx
 
   let set_ip_gateways t ips =
-    let now = C.time () in
+    let now = C.elapsed_ns t.clock in
     let ctx = Ndpv6.add_routers ~now t.ctx ips in
     t.ctx <- ctx;
     Lwt.return_unit
@@ -102,7 +103,7 @@ module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
     Ndpv6.get_prefix t.ctx
 
   let set_ip_netmask t pfx =
-    let now = C.time () in
+    let now = C.elapsed_ns t.clock in
     let ctx = Ndpv6.add_prefix ~now t.ctx pfx in
     t.ctx <- ctx;
     Lwt.return_unit
@@ -127,11 +128,11 @@ module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) = struct
     | Some x -> f x >>= g
     | None -> g ()
 
-  let connect ?ip ?netmask ?gateways ethif =
+  let connect ?ip ?netmask ?gateways ethif clock =
     Log.info (fun f -> f "IP6: Starting");
-    let now = C.time () in
+    let now = C.elapsed_ns clock in
     let ctx, bufs = Ndpv6.local ~now (E.mac ethif) in
-    let t = {ctx; ethif} in
+    let t = {ctx; clock; ethif} in
     Lwt_list.iter_s (E.writev t.ethif) bufs >>= fun () ->
     (ip, set_ip t) >>=? fun () ->
     (netmask, Lwt_list.iter_s (set_ip_netmask t)) >>=? fun () ->
diff --git a/lib/ipv6/ipv6.mli b/lib/ipv6/ipv6.mli
index 13f7c0003..c69634985 100644
--- a/lib/ipv6/ipv6.mli
+++ b/lib/ipv6/ipv6.mli
@@ -14,11 +14,11 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  *)
 
-module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (C : V1.CLOCK) : sig
+module Make (E : V1_LWT.ETHIF) (T : V1_LWT.TIME) (Clock : V1.MCLOCK) : sig
   include V1_LWT.IPV6 with type ethif = E.t
   val connect :
     ?ip:Ipaddr.V6.t ->
     ?netmask:Ipaddr.V6.Prefix.t list ->
     ?gateways:Ipaddr.V6.t list ->
-    ethif -> [> `Ok of t | `Error of error ] Lwt.t
+    ethif -> Clock.t -> [> `Ok of t | `Error of error ] Lwt.t
 end
diff --git a/lib/ipv6/ndpv6.ml b/lib/ipv6/ndpv6.ml
index 19d1826eb..2da24a68e 100644
--- a/lib/ipv6/ndpv6.ml
+++ b/lib/ipv6/ndpv6.ml
@@ -44,6 +44,7 @@ module Ipaddr = Ipaddr.V6
 type buffer = Cstruct.t
 type ipaddr = Ipaddr.t
 type prefix = Ipaddr.Prefix.t
+type time   = int64
 
 module BoundedMap (K : Map.OrderedType) : sig
   type 'a t
@@ -74,14 +75,14 @@ let solicited_node_prefix =
   Ipaddr.(Prefix.make 104 (of_int16 (0xff02, 0, 0, 0, 0, 1, 0xff00, 0)))
 
 module Defaults = struct
-  let _max_rtr_solicitation_delay = 1.0
+  let _max_rtr_solicitation_delay = Duration.of_sec 1
   let _ptr_solicitation_interval  = 4
   let _max_rtr_solicitations      = 3
   let max_multicast_solicit      = 3
   let max_unicast_solicit        = 3
   let _max_anycast_delay_time     = 1
   let _max_neighbor_advertisement = 3
-  let delay_first_probe_time     = 5.0
+  let delay_first_probe_time     = Duration.of_sec 5
 
   let link_mtu                   = 1500 (* RFC 2464, 2. *)
   let min_link_mtu               = 1280
@@ -90,8 +91,8 @@ module Defaults = struct
 
   let min_random_factor          = 0.5
   let max_random_factor          = 1.5
-  let reachable_time             = 30.0
-  let retrans_timer              = 1.0
+  let reachable_time             = Duration.of_sec 30
+  let retrans_timer              = Duration.of_sec 1
 end
 
 let ipaddr_of_cstruct cs =
@@ -138,20 +139,12 @@ let multicast_mac =
     Cstruct.BE.set_uint32 pbuf 2 n;
     Macaddr.of_bytes_exn (Cstruct.to_string pbuf)
 
-(* let float_of_uint32 n = Uint32.to_float (Uint32.of_int32 n)
-   but we can't use uint on Xen. *)
-let float_of_uint32 n =
-  if n >= 0l then
-    Int32.to_float n
-  else
-    let m = Int32.logand n 0x7fffffffl in
-    Int32.to_float m +. 2. ** 31.
-
-let compute_reachable_time dt =
-  let r =
-    Defaults.(min_random_factor +. Random.float (max_random_factor -. min_random_factor))
+(* vary the reachable time by some random factor between 0.5 and 1.5 *)
+let compute_reachable_time reachable_time =
+  let factor =
+    Defaults.(min_random_factor +. (max_random_factor -. min_random_factor))
   in
-  r *. dt
+  Int64.of_float (factor *. Int64.to_float reachable_time)
 
 let cksum_buf =
   let pbuf = Io_page.to_cstruct (Io_page.get 1) in
@@ -270,15 +263,15 @@ type ns =
 type pfx =
   { pfx_on_link : bool;
     pfx_autonomous : bool;
-    pfx_valid_lifetime : float option;
-    pfx_preferred_lifetime : float option;
+    pfx_valid_lifetime : time option;
+    pfx_preferred_lifetime : time option;
     pfx_prefix : Ipaddr.Prefix.t }
 
 type ra =
   { ra_cur_hop_limit : int;
-    ra_router_lifetime : float;
-    ra_reachable_time : float option;
-    ra_retrans_timer : float option;
+    ra_router_lifetime : time;
+    ra_reachable_time : time option;
+    ra_retrans_timer : time option;
     ra_slla : Macaddr.t option;
     ra_prefix : pfx list }
 
@@ -299,9 +292,9 @@ type action =
 module AddressList = struct
 
   type state =
-    | TENTATIVE of (float * float option) option * int * float
-    | PREFERRED of (float * float option) option
-    | DEPRECATED of float option
+    | TENTATIVE of (time * time option) option * int * time
+    | PREFERRED of (time * time option) option
+    | DEPRECATED of time option
 
   type t =
     (Ipaddr.t * state) list
@@ -331,19 +324,19 @@ module AddressList = struct
         let timeout = match timeout with
           | None -> None
           | Some (preferred_lifetime, valid_lifetime) ->
-            Some (now +. preferred_lifetime, valid_lifetime)
+            Some (Int64.add now preferred_lifetime, valid_lifetime)
         in
         Log.debug (fun f -> f "SLAAC: %a --> PREFERRED" Ipaddr.pp_hum ip);
         Some (ip, PREFERRED timeout), []
       else
         let dst = Ipaddr.Prefix.network_address solicited_node_prefix ip in
-        Some (ip, TENTATIVE (timeout, n+1, now +. retrans_timer)),
+        Some (ip, TENTATIVE (timeout, n+1, Int64.add now retrans_timer)),
         [SendNS (`Unspecified, dst, ip)]
     | ip, PREFERRED (Some (preferred_timeout, valid_lifetime)) when preferred_timeout <= now ->
       Log.debug (fun f -> f "SLAAC: %a --> DEPRECATED" Ipaddr.pp_hum ip);
       let valid_timeout = match valid_lifetime with
         | None -> None
-        | Some valid_lifetime -> Some (now +. valid_lifetime)
+        | Some valid_lifetime -> Some (Int64.add now valid_lifetime)
       in
       Some (ip, DEPRECATED valid_timeout), []
     | ip, DEPRECATED (Some t) when t <= now ->
@@ -371,7 +364,7 @@ module AddressList = struct
   let add al ~now ~retrans_timer ~lft ip =
     match List.mem_assoc ip al with
     | false ->
-      let al = (ip, TENTATIVE (lft, 0, now +. retrans_timer)) :: al in
+      let al = (ip, TENTATIVE (lft, 0, Int64.add now retrans_timer)) :: al in
       let dst = Ipaddr.Prefix.network_address solicited_node_prefix ip in
       al, [SendNS (`Unspecified, dst, ip)]
     | true ->
@@ -419,7 +412,7 @@ end
 module PrefixList = struct
 
   type t =
-    (Ipaddr.Prefix.t * float option) list
+    (Ipaddr.Prefix.t * time option) list
 
   let link_local =
     [Ipaddr.Prefix.link, None]
@@ -436,7 +429,7 @@ module PrefixList = struct
   let add pl ~now pfx ~vlft =
     let vlft = match vlft with
       | None -> None
-      | Some dt -> Some (now +. dt)
+      | Some dt -> Some (Int64.add now dt)
     in
     match List.mem_assoc pfx pl with
     | false ->
@@ -473,18 +466,18 @@ module PrefixList = struct
     Log.debug (fun f -> f "ND6: Processing PREFIX option in RA");
     if Ipaddr.Prefix.link <> pfx then
       match vlft, List.mem_assoc pfx pl with
-      | Some 0.0, true ->
+      | Some 0L, true ->
         Log.debug (fun f -> f "ND6: Removing PREFIX: pfx=%a" Ipaddr.Prefix.pp_hum pfx);
         List.remove_assoc pfx pl, []
-      | Some 0.0, false ->
+      | Some 0L, false ->
         pl, []
       | Some dt, true ->
-        Log.debug (fun f -> f "ND6: Refreshing PREFIX: pfx=%a lft=%f" Ipaddr.Prefix.pp_hum pfx dt);
+        Log.debug (fun f -> f "ND6: Refreshing PREFIX: pfx=%a lft=%Lu" Ipaddr.Prefix.pp_hum pfx dt);
         let pl = List.remove_assoc pfx pl in
-        (pfx, Some (now +. dt)) :: pl, []
+        (pfx, Some (Int64.add now dt)) :: pl, []
       | Some dt, false ->
-        Log.debug (fun f -> f "ND6: Received new PREFIX: pfx=%a lft=%f" Ipaddr.Prefix.pp_hum pfx dt);
-        (pfx, Some (now +. dt)) :: pl, []
+        Log.debug (fun f -> f "ND6: Received new PREFIX: pfx=%a lft=%Lu" Ipaddr.Prefix.pp_hum pfx dt);
+        (pfx, Some (Int64.add now dt)) :: pl, []
       | None, true ->
         Log.debug (fun f -> f "ND6: Refreshing PREFIX: pfx=%a lft=inf" Ipaddr.Prefix.pp_hum pfx);
         let pl = List.remove_assoc pfx pl in
@@ -494,30 +487,16 @@ module PrefixList = struct
         (pfx, None) :: pl, []
     else
       pl, []
-    (* TODO check for 0 (this is checked in update_prefix currently), infinity *)
-    (* if vlft >= plft && Ipaddr.Prefix.link <> pfx then *)
-    (*   let pl, acts = *)
-    (*     if on_link then *)
-    (*       update pl ~now ~valid:vlft pfx *)
-    (*     else *)
-    (*       pl, [] *)
-    (*   in *)
-    (*   if aut && (vlft :> float) > 0.0 then *)
-    (*     pl, acts, Some (pfx, plft, vlft) *)
-    (*   else *)
-    (*     pl, acts, None *)
-    (* else *)
-    (*   pl, [], None *)
 end
 
 module NeighborCache = struct
 
   type state =
-    | INCOMPLETE of float * int
-    | REACHABLE of float * Macaddr.t
+    | INCOMPLETE of time * int
+    | REACHABLE of time * Macaddr.t
     | STALE of Macaddr.t
-    | DELAY of float * Macaddr.t
-    | PROBE of float * int * Macaddr.t
+    | DELAY of time * Macaddr.t
+    | PROBE of time * int * Macaddr.t
 
   type info =
     { state : state;
@@ -537,7 +516,7 @@ module NeighborCache = struct
       if tn < Defaults.max_multicast_solicit then begin
         Log.info (fun f -> f "NUD: %a --> INCOMPLETE [Timeout]" Ipaddr.pp_hum ip);
         let dst = Ipaddr.Prefix.network_address solicited_node_prefix ip in
-        IpMap.add ip {nb with state = INCOMPLETE (now +. retrans_timer, tn+1)} nc,
+        IpMap.add ip {nb with state = INCOMPLETE ((Int64.add now retrans_timer), tn+1)} nc,
         [SendNS (`Specified, dst, ip)]
       end else begin
         Log.info (fun f -> f "NUD: %a --> UNREACHABLE [Discarding]" Ipaddr.pp_hum ip);
@@ -549,12 +528,12 @@ module NeighborCache = struct
       IpMap.add ip {nb with state = STALE mac} nc, []
     | DELAY (t, dmac) when t <= now ->
       Log.info (fun f -> f "NUD: %a --> PROBE" Ipaddr.pp_hum ip);
-      IpMap.add ip {nb with state = PROBE (now +. retrans_timer, 0, dmac)} nc,
+      IpMap.add ip {nb with state = PROBE ((Int64.add now retrans_timer), 0, dmac)} nc,
       [SendNS (`Specified, ip, ip)]
     | PROBE (t, tn, dmac) when t <= now ->
       if tn < Defaults.max_unicast_solicit then begin
         Log.info (fun f -> f "NUD: %a --> PROBE [Timeout]" Ipaddr.pp_hum ip);
-        IpMap.add ip {nb with state = PROBE (now +. retrans_timer, tn+1, dmac)} nc,
+        IpMap.add ip {nb with state = PROBE ((Int64.add now retrans_timer), tn+1, dmac)} nc,
         [SendNS (`Specified, ip, ip)]
       end else begin
         Log.info (fun f -> f "NUD: %a --> UNREACHABLE [Discarding]" Ipaddr.pp_hum ip);
@@ -616,7 +595,7 @@ module NeighborCache = struct
         IpMap.add tgt nb nc, [SendQueued (tgt, new_mac)]
       | INCOMPLETE _, Some new_mac, true, _ ->
         Log.info (fun f -> f "NUD: %a --> REACHABLE" Ipaddr.pp_hum tgt);
-        let nb = {nb with state = REACHABLE (now +. reachable_time, new_mac)} in
+        let nb = {nb with state = REACHABLE ((Int64.add now reachable_time), new_mac)} in
         IpMap.add tgt nb nc, [SendQueued (tgt, new_mac)]
       | INCOMPLETE _, None, _, _ ->
         let nc =
@@ -628,11 +607,11 @@ module NeighborCache = struct
         nc, []
       | PROBE (_, _, mac), Some new_mac, true, false when mac = new_mac ->
         Log.info (fun f -> f "NUD: %a --> REACHABLE" Ipaddr.pp_hum tgt);
-        let nb = {nb with state = REACHABLE (now +. reachable_time, new_mac)} in
+        let nb = {nb with state = REACHABLE ((Int64.add now reachable_time), new_mac)} in
         IpMap.add tgt nb nc, []
       | PROBE (_, _, mac), None, true, false ->
         Log.info (fun f -> f "NUD: %a --> REACHABLE" Ipaddr.pp_hum tgt);
-        let nb = {nb with state = REACHABLE (now +. reachable_time, mac)} in
+        let nb = {nb with state = REACHABLE ((Int64.add now reachable_time), mac)} in
         IpMap.add tgt nb nc, []
       | (REACHABLE _ | STALE _ | DELAY _ | PROBE _), None, _, _ ->
         let nc =
@@ -648,7 +627,7 @@ module NeighborCache = struct
         IpMap.add tgt nb nc, []
       | (REACHABLE _ | STALE _ | DELAY _ | PROBE _), Some new_mac, true, true ->
         Log.info (fun f -> f "NUD: %a --> REACHABLE" Ipaddr.pp_hum tgt);
-        let nb = {nb with state = REACHABLE (now +. reachable_time, new_mac)} in
+        let nb = {nb with state = REACHABLE ((Int64.add now reachable_time), new_mac)} in
         IpMap.add tgt nb nc, []
       | (REACHABLE (_, mac) | STALE mac | DELAY (_, mac) | PROBE (_, _, mac)),
         Some new_mac, false, true when mac <> new_mac ->
@@ -675,11 +654,11 @@ module NeighborCache = struct
         nc, Some dmac, []
       | STALE dmac ->
         let dt = Defaults.delay_first_probe_time in
-        let nc = IpMap.add ip {nb with state = DELAY (now +. dt, dmac)} nc in
+        let nc = IpMap.add ip {nb with state = DELAY (Int64.add now dt, dmac)} nc in
         nc, Some dmac, []
     with
     | Not_found ->
-      let nb  = {state = INCOMPLETE (now +. reachable_time, 0); is_router = false} in
+      let nb  = {state = INCOMPLETE (Int64.add now reachable_time, 0); is_router = false} in
       let nc  = IpMap.add ip nb nc in
       let dst = Ipaddr.Prefix.network_address solicited_node_prefix ip in
       nc, None, [SendNS (`Specified, dst, ip)]
@@ -697,7 +676,7 @@ end
 module RouterList = struct
 
   type t =
-    (Ipaddr.t * float) list
+    (Ipaddr.t * time) list
 
   let empty =
     []
@@ -705,10 +684,11 @@ module RouterList = struct
   let to_list rl =
     List.map fst rl
 
-  let add rl ~now ?(lifetime = max_float) ip =
+  let add rl ~now ?(lifetime = Duration.of_year 1) ip =
     (* FIXME *)
     (* yomimono 2016-06-30: fix what? *)
-    (ip, now +. lifetime) :: rl
+    (* yomimono 2016-08-17: maybe fix this default lifetime. *)
+    (ip, Int64.add now lifetime) :: rl
 
   (* FIXME if we are keeping a destination cache, we must remove the stale routers from there as well. *)
   let tick rl ~now =
@@ -718,15 +698,15 @@ module RouterList = struct
     match List.mem_assoc src rl with
     | true ->
       let rl = List.remove_assoc src rl in
-      if lft > 0.0 then begin
-        Log.info (fun f -> f "RA: Refreshing Router: src=%a lft=%f" Ipaddr.pp_hum src lft);
-        (src, now +. lft) :: rl, []
+      if lft > 0L then begin
+        Log.info (fun f -> f "RA: Refreshing Router: src=%a lft=%Lu" Ipaddr.pp_hum src lft);
+        (src, Int64.add now lft) :: rl, []
       end else begin
         Log.info (fun f -> f "RA: Router Expired: src=%a" Ipaddr.pp_hum src);
         rl, []
       end
     | false ->
-      if lft > 0.0 then begin
+      if lft > 0L then begin
 	Log.info (fun f -> f "RA: Adding Router: src=%a" Ipaddr.pp_hum src);
         (add rl ~now ~lifetime:lft src), []
       end else
@@ -735,7 +715,7 @@ module RouterList = struct
   let add rl ~now:_ ip =
     match List.mem_assoc ip rl with
     | true -> rl
-    | false -> (ip, max_float) :: rl
+    | false -> (ip, Duration.of_year 1) :: rl
 
   let select rl reachable ip =
     let rec loop = function
@@ -793,13 +773,13 @@ module Parser = struct
           let n = Ipv6_wire.get_opt_prefix_valid_lifetime opt in
           match n with
           | 0xffffffffl -> None
-          | n -> Some (float_of_uint32 n)
+          | n -> Some (Int64.of_int32 n)
         in
         let pfx_preferred_lifetime =
           let n = Ipv6_wire.get_opt_prefix_preferred_lifetime opt in
           match n with
           | 0xffffffffl -> None
-          | n -> Some (float_of_uint32 n)
+          | n -> Some (Int64.of_int32 n)
         in
         let pfx =
           {pfx_on_link; pfx_autonomous; pfx_valid_lifetime; pfx_preferred_lifetime; pfx_prefix}
@@ -814,20 +794,20 @@ module Parser = struct
   let parse_ra buf =
     let ra_cur_hop_limit = Ipv6_wire.get_ra_cur_hop_limit buf in
     let ra_router_lifetime =
-      float_of_int (Ipv6_wire.get_ra_router_lifetime buf)
+      Int64.of_int (Ipv6_wire.get_ra_router_lifetime buf)
     in
     let ra_reachable_time =
       let n = Ipv6_wire.get_ra_reachable_time buf in
       if n = 0l then None
       else
-        let dt = (float_of_uint32 n) /. 1000.0 in
+        let dt = Int64.of_int32 @@ Int32.div n 1000l in
         Some dt
     in
     let ra_retrans_timer =
       let n = Ipv6_wire.get_ra_retrans_timer buf in
       if n = 0l then None
       else
-        let dt = (float_of_uint32 n) /. 1000.0 in
+        let dt = Int64.of_int32 @@ Int32.div n 1000l in
         Some dt
     in
     let opts = Cstruct.shift buf Ipv6_wire.sizeof_ra in
@@ -1063,9 +1043,9 @@ type context =
     address_list : AddressList.t;
     link_mtu : int;
     cur_hop_limit : int;
-    base_reachable_time : float;
-    reachable_time : float;
-    retrans_timer : float;
+    base_reachable_time : time;
+    reachable_time : time;
+    retrans_timer : time;
     packet_queue : (Macaddr.t -> Cstruct.t list) PacketQueue.t }
 
 let next_hop ctx ip =
@@ -1216,7 +1196,7 @@ let handle_ra ~now ctx ~src ~dst ra =
          let vlft = pfx.pfx_valid_lifetime in
          let prefix_list, acts = PrefixList.handle_ra state.prefix_list ~now ~vlft pfx.pfx_prefix in
          match pfx.pfx_autonomous, vlft with
-         | _, Some 0.0 ->
+         | _, Some 0L ->
            {state with prefix_list}, acts
          | true, Some _ ->
            let plft = pfx.pfx_preferred_lifetime in
diff --git a/lib/ipv6/ndpv6.mli b/lib/ipv6/ndpv6.mli
index d0bc0d184..958695509 100644
--- a/lib/ipv6/ndpv6.mli
+++ b/lib/ipv6/ndpv6.mli
@@ -17,6 +17,7 @@
 type buffer = Cstruct.t
 type ipaddr = Ipaddr.V6.t
 type prefix = Ipaddr.V6.Prefix.t
+type time   = int64
 
 val ipaddr_of_cstruct : buffer -> ipaddr
 val ipaddr_to_cstruct_raw : ipaddr -> buffer -> int -> unit
@@ -29,12 +30,12 @@ type event =
 
 type context
 
-val local : now:float -> Macaddr.t -> context * buffer list list
+val local : now:time -> Macaddr.t -> context * buffer list list
 (** [local ~now mac] is a pair [ctx, bufs] where [ctx] is a local IPv6 context
     associated to the hardware address [mac].  [bufs] is a list of ethif packets
     to be sent. *)
 
-val add_ip : now:float -> context -> ipaddr -> context * buffer list list
+val add_ip : now:time -> context -> ipaddr -> context * buffer list list
 (** [add_ip ~now ctx ip] is [ctx', bufs] where [ctx'] is [ctx] updated with a
     new local ip and [bufs] is a list of ethif packets to be sent. *)
 
@@ -49,30 +50,30 @@ val select_source : context -> ipaddr -> ipaddr
 (** [select_source ctx ip] returns the ip that should be put in the source field
     of a packet destined to [ip]. *)
 
-val handle : now:float -> context -> buffer -> context * buffer list list * event list
+val handle : now:time -> context -> buffer -> context * buffer list list * event list
 (** [handle ~now ctx buf] handles an incoming ipv6 packet.  It returns [ctx',
     bufs, evs] where [ctx'] is the updated context, [bufs] is a list of packets to
     be sent and [evs] is a list of packets to be passed to the higher layers (udp,
     tcp, etc) for further processing. *)
 
-val send : now:float -> context -> ipaddr -> buffer -> buffer list -> context * buffer list list
+val send : now:time -> context -> ipaddr -> buffer -> buffer list -> context * buffer list list
 (** [send ~now ctx ip frame bufs] starts route resolution and assembles an ipv6
     packet for sending with header [frame] and body [bufs].  It returns a pair
     [ctx', bufs] where [ctx'] is the updated context and [bufs] is a list of
     packets to be sent. *)
 
-val tick : now:float -> context -> context * buffer list list
+val tick : now:time -> context -> context * buffer list list
 (** [tick ~now ctx] should be called periodically (every 1s is good).  It
     returns [ctx', bufs] where [ctx'] is the updated context and [bufs] is a list of
     packets to be sent. *)
 
-val add_prefix : now:float -> context -> prefix -> context
+val add_prefix : now:time -> context -> prefix -> context
 (** [add_prefix ~now ctx pfx] adds a local prefix to [ctx]. *)
 
 val get_prefix : context -> prefix list
 (** [get_prefix ctx] returns the list of local prefixes known to [ctx]. *)
 
-val add_routers : now:float -> context -> ipaddr list -> context
+val add_routers : now:time -> context -> ipaddr list -> context
 (** [add_routers ~now ctx ips] adds a list of gateways to [ctx] to be used for
     routing. *)
 
diff --git a/lib/tcp/ack.ml b/lib/tcp/ack.ml
index 918ba6cf3..0ba4ab29f 100644
--- a/lib/tcp/ack.ml
+++ b/lib/tcp/ack.ml
@@ -105,7 +105,7 @@ module Delayed (Time:V1_LWT.TIME) : M = struct
     let delayedack = last in
     let r = {send_ack; delayedack; delayed; pushpending} in
     let expire = ontimer r in
-    let period = 0.1 in
+    let period = (Duration.of_f 0.1) in
     let timer = TT.t ~period ~expire in
     {r; timer}
 
diff --git a/lib/tcp/flow.ml b/lib/tcp/flow.ml
index 0bb91b37e..3c42d4ffb 100644
--- a/lib/tcp/flow.ml
+++ b/lib/tcp/flow.ml
@@ -23,7 +23,7 @@ exception Refused
 let src = Logs.Src.create "flow" ~doc:"Mirage TCP Flow module"
 module Log = (val Logs.src_log src : Logs.LOG)
 
-module Make(IP:V1_LWT.IP)(TM:V1_LWT.TIME)(C:V1.CLOCK)(R:V1.RANDOM) = struct
+module Make(IP:V1_LWT.IP)(TM:V1_LWT.TIME)(C:V1.MCLOCK)(R:V1.RANDOM) = struct
 
   module Pcb = Pcb.Make(IP)(TM)(C)(R)
 
@@ -83,7 +83,7 @@ module Make(IP:V1_LWT.IP)(TM:V1_LWT.TIME)(C:V1.CLOCK)(R:V1.RANDOM) = struct
   let writev t views = Pcb.writev t views >|= err_rewrite
   let write_nodelay t view = Pcb.write_nodelay t view >>= err_raise
   let writev_nodelay t views = Pcb.writev_nodelay t views >>= err_raise
-  let connect ipv4 = ok (Pcb.create ipv4)
+  let connect ipv4 clock = ok (Pcb.create ipv4 clock)
   let disconnect _ = Lwt.return_unit
 
   let create_connection tcp (daddr, dport) =
diff --git a/lib/tcp/flow.mli b/lib/tcp/flow.mli
index e8d5ad7bf..8f9ba8876 100644
--- a/lib/tcp/flow.mli
+++ b/lib/tcp/flow.mli
@@ -18,10 +18,10 @@ exception Refused
 (** {b NOTE}: to be removed in favor of a proper result type in
     V1.write_nodelay and V1.writev_nodelay.*)
 
-module Make (IP:V1_LWT.IP)(TM:V1_LWT.TIME)(C:V1.CLOCK)(R:V1.RANDOM) : sig
+module Make (IP:V1_LWT.IP)(TM:V1_LWT.TIME)(C:V1.MCLOCK)(R:V1.RANDOM) : sig
   include V1_LWT.TCP
     with type ip = IP.t
      and type ipaddr = IP.ipaddr
      and type ipinput = src:IP.ipaddr -> dst:IP.ipaddr -> Cstruct.t -> unit Lwt.t
-  val connect : ip -> [> `Ok of t | `Error of error ] Lwt.t
+  val connect : ip -> C.t -> [> `Ok of t | `Error of error ] Lwt.t
 end
diff --git a/lib/tcp/pcb.ml b/lib/tcp/pcb.ml
index f52d842a3..d4149abd2 100644
--- a/lib/tcp/pcb.ml
+++ b/lib/tcp/pcb.ml
@@ -20,7 +20,7 @@ open Lwt.Infix
 let src = Logs.Src.create "pcb" ~doc:"Mirage TCP PCB module"
 module Log = (val Logs.src_log src : Logs.LOG)
 
-module Make(Ip:V1_LWT.IP)(Time:V1_LWT.TIME)(Clock:V1.CLOCK)(Random:V1.RANDOM) =
+module Make(Ip:V1_LWT.IP)(Time:V1_LWT.TIME)(Clock:V1.MCLOCK)(Random:V1.RANDOM) =
 struct
 
   module RXS = Segment.Rx(Time)
@@ -47,6 +47,7 @@ struct
 
   type t = {
     ip : Ip.t;
+    clock : Clock.t;
     mutable localport : int;
     channels: (WIRE.id, connection) Hashtbl.t;
     (* server connections the process of connecting - SYN-ACK sent
@@ -284,7 +285,7 @@ struct
     let on_close () = clearpcb t id tx_isn in
     let state = State.t ~on_close in
     let txq, _tx_t =
-      TXS.create ~xmit:(Tx.xmit_pcb t.ip id) ~wnd ~state ~rx_ack ~tx_ack ~tx_wnd_update
+      TXS.create ~clock:t.clock ~xmit:(Tx.xmit_pcb t.ip id) ~wnd ~state ~rx_ack ~tx_ack ~tx_wnd_update
     in
     (* The user application transmit buffer *)
     let utx = UTX.create ~wnd ~txq ~max_size:16384l in
@@ -600,12 +601,12 @@ struct
     th
 
   (* Construct the main TCP thread *)
-  let create ip =
+  let create ip clock =
     Random.self_init ();
     let localport = 10000 + (Random.int 10000) in
     let listens = Hashtbl.create 1 in
     let connects = Hashtbl.create 1 in
     let channels = Hashtbl.create 7 in
-    { ip; localport; channels; listens; connects }
+    { clock; ip; localport; channels; listens; connects }
 
 end
diff --git a/lib/tcp/pcb.mli b/lib/tcp/pcb.mli
index 3a9065c77..7fd02f2dc 100644
--- a/lib/tcp/pcb.mli
+++ b/lib/tcp/pcb.mli
@@ -14,7 +14,7 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  *)
 
-module Make(Ip:V1_LWT.IP)(Time:V1_LWT.TIME)(Clock:V1.CLOCK)(Random:V1.RANDOM) : sig
+module Make(Ip:V1_LWT.IP)(Time:V1_LWT.TIME)(Clock:V1.MCLOCK)(Random:V1.RANDOM) : sig
 
   (** Overall state of the TCP stack *)
   type t
@@ -57,5 +57,5 @@ module Make(Ip:V1_LWT.IP)(Time:V1_LWT.TIME)(Clock:V1.CLOCK)(Random:V1.RANDOM) :
   val write_nodelay: pcb -> Cstruct.t -> (unit, string) Result.result Lwt.t
   val writev_nodelay: pcb -> Cstruct.t list -> (unit, string) Result.result Lwt.t
 
-  val create: Ip.t -> t
+  val create: Ip.t -> Clock.t -> t
 end
diff --git a/lib/tcp/segment.ml b/lib/tcp/segment.ml
index 99803f221..07b575f47 100644
--- a/lib/tcp/segment.ml
+++ b/lib/tcp/segment.ml
@@ -230,7 +230,7 @@ type tx_flags = (* At most one of Syn/Fin/Rst/Psh allowed *)
   | Rst
   | Psh
 
-module Tx (Time:V1_LWT.TIME) (Clock:V1.CLOCK) = struct
+module Tx (Time:V1_LWT.TIME) (Clock:V1.MCLOCK) = struct
 
   module StateTick = State.Make(Time)
   module TT = Tcptimer.Make(Time)
@@ -263,6 +263,7 @@ module Tx (Time:V1_LWT.TIME) (Clock:V1.CLOCK) = struct
                                       with this queue *)
     tx_wnd_update: int Lwt_mvar.t; (* Received updates to the transmit window *)
     rexmit_timer: Tcptimer.t;      (* Retransmission timer for this connection *)
+    clock: Clock.t;                (* whom to ask for the time *)
     mutable dup_acks: int;         (* dup ack count for re-xmits *)
   }
 
@@ -282,7 +283,7 @@ module Tx (Time:V1_LWT.TIME) (Clock:V1.CLOCK) = struct
           match rexmit_seg.seq = seq with
           | false ->
             Log.debug (fun fmt ->
-                fmt "PUSHING TIMER - new time=%f, new seq=%a"
+                fmt "PUSHING TIMER - new time=%Lu, new seq=%a"
                   (Window.rto wnd) Sequence.pp rexmit_seg.seq);
             let ret =
               Tcptimer.ContinueSetPeriod (Window.rto wnd, rexmit_seg.seq)
@@ -304,7 +305,7 @@ module Tx (Time:V1_LWT.TIME) (Clock:V1.CLOCK) = struct
                 (fun () -> xmit ~flags ~wnd ~options ~seq rexmit_seg.data);
               Window.backoff_rto wnd;
               Log.debug (fun fmt ->
-                  fmt "PUSHING TIMER - new time = %f, new seq = %a"
+                  fmt "PUSHING TIMER - new time = %Lu, new seq = %a"
                     (Window.rto wnd) Sequence.pp rexmit_seg.seq);
               let ret =
                 Tcptimer.ContinueSetPeriod (Window.rto wnd, rexmit_seg.seq)
@@ -341,7 +342,7 @@ module Tx (Time:V1_LWT.TIME) (Clock:V1.CLOCK) = struct
     let rec tx_ack_t () =
       let serviceack dupack ack_len seq win =
         let partleft = clearsegs q ack_len q.segs in
-        TX.tx_ack q.wnd (Sequence.sub seq partleft) win;
+        TX.tx_ack q.clock q.wnd (Sequence.sub seq partleft) win;
         match dupack || Window.fast_rec q.wnd with
         | true ->
           q.dup_acks <- q.dup_acks + 1;
@@ -392,14 +393,14 @@ module Tx (Time:V1_LWT.TIME) (Clock:V1.CLOCK) = struct
     in
     tx_ack_t ()
 
-  let create ~xmit ~wnd ~state ~rx_ack ~tx_ack ~tx_wnd_update =
+  let create ~clock ~xmit ~wnd ~state ~rx_ack ~tx_ack ~tx_wnd_update =
     let segs = Lwt_sequence.create () in
     let dup_acks = 0 in
     let expire = ontimer xmit state segs wnd in
     let period = Window.rto wnd in
     let rexmit_timer = TT.t ~period ~expire in
     let q =
-      { xmit; wnd; state; rx_ack; segs; tx_wnd_update; rexmit_timer; dup_acks }
+      { clock; xmit; wnd; state; rx_ack; segs; tx_wnd_update; rexmit_timer; dup_acks }
     in
     let t = rto_t q tx_ack in
     q, t
@@ -418,7 +419,7 @@ module Tx (Time:V1_LWT.TIME) (Clock:V1.CLOCK) = struct
     let seq = Window.tx_nxt wnd in
     let seg = { data; flags; seq } in
     let seq_len = len seg in
-    TX.tx_advance q.wnd seq_len;
+    TX.tx_advance q.clock q.wnd seq_len;
     (* Queue up segment just sent for retransmission if needed *)
     let q_rexmit () =
       match Sequence.(gt seq_len zero) with
diff --git a/lib/tcp/segment.mli b/lib/tcp/segment.mli
index 18f4b6af5..e5675e569 100644
--- a/lib/tcp/segment.mli
+++ b/lib/tcp/segment.mli
@@ -55,7 +55,7 @@ type tx_flags = No_flags | Syn | Fin | Rst | Psh
 (** Either Syn/Fin/Rst allowed, but not combinations *)
 
 (** Pre-transmission queue *)
-module Tx (Time:V1_LWT.TIME)(Clock:V1.CLOCK) : sig
+module Tx (Time:V1_LWT.TIME)(Clock:V1.MCLOCK) : sig
 
   type xmit = flags:tx_flags -> wnd:Window.t -> options:Options.t list ->
     seq:Sequence.t -> Cstruct.t -> unit Lwt.t
@@ -64,7 +64,7 @@ module Tx (Time:V1_LWT.TIME)(Clock:V1.CLOCK) : sig
   (** Queue of pre-transmission segments *)
 
   val create:
-    xmit:xmit -> wnd:Window.t -> state:State.t ->
+    clock:Clock.t -> xmit:xmit -> wnd:Window.t -> state:State.t ->
     rx_ack:Sequence.t Lwt_mvar.t ->
     tx_ack:(Sequence.t * int) Lwt_mvar.t ->
     tx_wnd_update:int Lwt_mvar.t -> t * unit Lwt.t
diff --git a/lib/tcp/tcptimer.ml b/lib/tcp/tcptimer.ml
index b77480860..255d2c5bc 100644
--- a/lib/tcp/tcptimer.ml
+++ b/lib/tcp/tcptimer.ml
@@ -19,14 +19,16 @@ open Lwt.Infix
 let src = Logs.Src.create "tcptimer" ~doc:"Mirage TCP Tcptimer module"
 module Log = (val Logs.src_log src : Logs.LOG)
 
+type time = int64
+
 type tr =
   | Stoptimer
   | Continue of Sequence.t
-  | ContinueSetPeriod of (float * Sequence.t)
+  | ContinueSetPeriod of (time * Sequence.t)
 
 type t = {
   expire: (Sequence.t -> tr Lwt.t);
-  mutable period: float;
+  mutable period: time;
   mutable running: bool;
 }
 
@@ -39,7 +41,7 @@ module Make(Time:V1_LWT.TIME) = struct
     Log.debug (fun f -> f "timerloop");
     Stats.incr_timer ();
     let rec aux t s =
-      Time.sleep_ns (Duration.of_f t.period) >>= fun () ->
+      Time.sleep_ns t.period >>= fun () ->
       t.expire s >>= function
       | Stoptimer ->
         Stats.decr_timer ();
diff --git a/lib/tcp/tcptimer.mli b/lib/tcp/tcptimer.mli
index ffa903c86..b2de5351f 100644
--- a/lib/tcp/tcptimer.mli
+++ b/lib/tcp/tcptimer.mli
@@ -16,13 +16,15 @@
 
 type t
 
+type time = int64
+
 type tr =
   | Stoptimer
   | Continue of Sequence.t
-  | ContinueSetPeriod of (float * Sequence.t)
+  | ContinueSetPeriod of (time * Sequence.t)
 
 module Make(T:V1_LWT.TIME) : sig
-  val t : period: float -> expire: (Sequence.t -> tr Lwt.t) -> t
+  val t : period: time -> expire: (Sequence.t -> tr Lwt.t) -> t
 
-  val start : t -> ?p:float -> Sequence.t -> unit Lwt.t
+  val start : t -> ?p:time -> Sequence.t -> unit Lwt.t
 end
diff --git a/lib/tcp/user_buffer.ml b/lib/tcp/user_buffer.ml
index 5fb832a2b..117f2f3c8 100644
--- a/lib/tcp/user_buffer.ml
+++ b/lib/tcp/user_buffer.ml
@@ -109,7 +109,7 @@ end
    to decide how to throttle or breakup its data production with this
    information.
 *)
-module Tx(Time:V1_LWT.TIME)(Clock:V1.CLOCK) = struct
+module Tx(Time:V1_LWT.TIME)(Clock:V1.MCLOCK) = struct
 
   module TXS = Segment.Tx(Time)(Clock)
 
diff --git a/lib/tcp/user_buffer.mli b/lib/tcp/user_buffer.mli
index 58e9f1314..a6be15678 100644
--- a/lib/tcp/user_buffer.mli
+++ b/lib/tcp/user_buffer.mli
@@ -26,7 +26,7 @@ module Rx : sig
   val monitor: t -> int32 Lwt_mvar.t -> unit
 end
 
-module Tx(Time:V1_LWT.TIME)(Clock:V1.CLOCK) : sig
+module Tx(Time:V1_LWT.TIME)(Clock:V1.MCLOCK) : sig
 
   type t
 
diff --git a/lib/tcp/window.ml b/lib/tcp/window.ml
index 77f66b012..f6b76b31f 100644
--- a/lib/tcp/window.ml
+++ b/lib/tcp/window.ml
@@ -17,6 +17,8 @@
 let src = Logs.Src.create "window" ~doc:"Mirage TCP Window module"
 module Log = (val Logs.src_log src : Logs.LOG)
 
+type time = int64
+
 type t = {
   tx_mss: int;
   tx_isn: Sequence.t;
@@ -47,10 +49,10 @@ type t = {
   mutable rtt_timer_on: bool;
   mutable rtt_timer_reset: bool;
   mutable rtt_timer_seq: Sequence.t;
-  mutable rtt_timer_starttime: float;
-  mutable srtt: float;
-  mutable rttvar: float;
-  mutable rto: float;
+  mutable rtt_timer_starttime: time;
+  mutable srtt: time;
+  mutable rttvar: time;
+  mutable rto: int64;
   mutable backoff_count: int;
 }
 
@@ -96,10 +98,10 @@ let t ~rx_wnd_scale ~tx_wnd_scale ~rx_wnd ~tx_wnd ~rx_isn ~tx_mss ~tx_isn =
   let rtt_timer_on = false in
   let rtt_timer_reset = true in
   let rtt_timer_seq = tx_nxt in
-  let rtt_timer_starttime = 0.0 in
-  let srtt = 1.0 in
-  let rttvar = 0.0 in
-  let rto = 3.0 in
+  let rtt_timer_starttime = 0L in
+  let srtt = 1L in
+  let rttvar = 0L in
+  let rto = 3L in
   let backoff_count = 0 in
   { tx_isn; rx_isn; max_rx_wnd; max_tx_wnd;
     ack_serviced; ack_seq; ack_win;
@@ -158,18 +160,18 @@ let set_tx_wnd t sz =
 let tx_mss t =
   t.tx_mss
 
-module Make(Clock:V1.CLOCK) = struct
+module Make(Clock:V1.MCLOCK) = struct
   (* Advance transmitted packet sequence number *)
-  let tx_advance t b =
+  let tx_advance clock t b =
     if not t.rtt_timer_on && not t.fast_recovery then begin
       t.rtt_timer_on <- true;
       t.rtt_timer_seq <- t.tx_nxt;
-      t.rtt_timer_starttime <- Clock.time ();
+      t.rtt_timer_starttime <- Clock.elapsed_ns clock;
     end;
     t.tx_nxt <- Sequence.add t.tx_nxt b
 
   (* An ACK was received - use it to adjust cwnd *)
-  let tx_ack t r win =
+  let tx_ack clock t r win =
     set_tx_wnd t win;
     if t.fast_recovery then begin
       if Sequence.gt r t.snd_una then
@@ -187,16 +189,20 @@ module Make(Clock:V1.CLOCK) = struct
         t.snd_una <- r;
         if t.rtt_timer_on && Sequence.gt r t.rtt_timer_seq then begin
           t.rtt_timer_on <- false;
-          let rtt_m = Clock.time () -. t.rtt_timer_starttime in
+          let rtt_m = Int64.sub (Clock.elapsed_ns clock) t.rtt_timer_starttime in
           if t.rtt_timer_reset then begin
             t.rtt_timer_reset <- false;
-            t.rttvar <- (0.5 *. rtt_m);
+            t.rttvar <- Int64.div rtt_m 2L;
             t.srtt <- rtt_m;
           end else begin
-            t.rttvar <- (((1.0 -. beta) *. t.rttvar) +. (beta *. (abs_float (t.srtt -. rtt_m))));
-            t.srtt <- (((1.0 -. alpha) *. t.srtt) +. (alpha *. rtt_m));
+            let adjusted_rttvar = (1.0 -. beta) *. (Int64.to_float t.rttvar) in
+            let rttvar_addition = beta *. Int64.(sub t.srtt rtt_m |> abs |> to_float) in
+            let adjusted_srtt = (1.0 -. alpha) *. (Int64.to_float t.srtt) in
+            let srtt_addition = alpha *. (Int64.to_float rtt_m) in
+            t.rttvar <- Int64.of_float (adjusted_rttvar +. rttvar_addition);
+            t.srtt <- Int64.of_float (adjusted_srtt +. srtt_addition);
           end;
-          t.rto <- (max 1.0 (t.srtt +. (4.0 *. t.rttvar)));
+          t.rto <- (max 1L (Int64.add t.srtt (Int64.mul 4L t.rttvar)));
         end;
       end;
       let cwnd_incr = match t.cwnd < t.ssthresh with
@@ -244,7 +250,7 @@ let alert_fast_rexmit t _ =
 let rto t =
   match t.backoff_count with
   | 0 -> t.rto
-  | _ -> t.rto *. (2. ** (float_of_int t.backoff_count))
+  | _ -> Int64.(mul t.rto (shift_left 2L t.backoff_count))
 
 let backoff_rto t =
   t.backoff_count <- t.backoff_count + 1;
diff --git a/lib/tcp/window.mli b/lib/tcp/window.mli
index cab17f8f6..2449606cd 100644
--- a/lib/tcp/window.mli
+++ b/lib/tcp/window.mli
@@ -28,9 +28,9 @@ val rx_advance_inseq : t -> Sequence.t -> unit
 val rx_nxt : t -> Sequence.t
 val rx_nxt_inseq : t -> Sequence.t
 
-module Make(C:V1.CLOCK) : sig
-  val tx_advance : t -> Sequence.t -> unit
-  val tx_ack: t -> Sequence.t -> int -> unit
+module Make(C:V1.MCLOCK) : sig
+  val tx_advance : C.t -> t -> Sequence.t -> unit
+  val tx_ack: C.t -> t -> Sequence.t -> int -> unit
 end
 
 val tx_nxt : t -> Sequence.t
@@ -63,7 +63,7 @@ val max_tx_wnd : t -> int32
 
 val alert_fast_rexmit : t -> Sequence.t -> unit
 
-val rto : t -> float
+val rto : t -> int64
 val backoff_rto : t -> unit
 val max_rexmits_done : t -> bool
 
diff --git a/lib_test/static_arp.ml b/lib_test/static_arp.ml
index 2be99d26e..bbb4044e1 100644
--- a/lib_test/static_arp.ml
+++ b/lib_test/static_arp.ml
@@ -1,6 +1,6 @@
 open Lwt.Infix
 
-module Make(E : V1_LWT.ETHIF)(Clock : V1.CLOCK) (Time : V1_LWT.TIME) = struct
+module Make(E : V1_LWT.ETHIF)(Clock : V1.MCLOCK) (Time : V1_LWT.TIME) = struct
   module A = Arpv4.Make(E)(Clock)(Time)
   (* generally repurpose A, but substitute input and query, and add functions
      for adding/deleting entries *)
@@ -34,7 +34,7 @@ module Make(E : V1_LWT.ETHIF)(Clock : V1.CLOCK) (Time : V1_LWT.TIME) = struct
   let pp fmt repr =
     Format.fprintf fmt "%s" repr
   
-  let connect e = A.connect e >>= function
+  let connect e clock = A.connect e clock >>= function
     | `Ok base -> Lwt.return (`Ok { base; table = (Hashtbl.create 7) })
     | `Error e -> Lwt.return (`Error e)
   
diff --git a/lib_test/test_arp.ml b/lib_test/test_arp.ml
index c9d08a426..7f55bcc68 100644
--- a/lib_test/test_arp.ml
+++ b/lib_test/test_arp.ml
@@ -4,42 +4,15 @@ let time_reduction_factor = 60
 
 module Time = Vnetif_common.Time
 module Fast_clock = struct
+  let last_read = ref 0
 
-  let last_read = ref (Clock.time ())
-
-  (* from mirage/types/V1.mli module type CLOCK *)
-  type tm =
-    { tm_sec: int;               (** Seconds 0..60 *)
-      tm_min: int;               (** Minutes 0..59 *)
-      tm_hour: int;              (** Hours 0..23 *)
-      tm_mday: int;              (** Day of month 1..31 *)
-      tm_mon: int;               (** Month of year 0..11 *)
-      tm_year: int;              (** Year - 1900 *)
-      tm_wday: int;              (** Day of week (Sunday is 0) *)
-      tm_yday: int;              (** Day of year 0..365 *)
-      tm_isdst: bool;            (** Daylight time savings in effect *)
-    }
-
-  let gmtime time = 
-    let tm = Clock.gmtime time in
-    { 
-      tm_sec = tm.Clock.tm_sec;
-      tm_min = tm.Clock.tm_min;
-      tm_hour = tm.Clock.tm_hour;
-      tm_mday = tm.Clock.tm_mday;
-      tm_mon = tm.Clock.tm_mon;
-      tm_year = tm.Clock.tm_year;
-      tm_wday = tm.Clock.tm_wday;
-      tm_yday = tm.Clock.tm_yday;
-      tm_isdst = tm.Clock.tm_isdst;
-    }
-
-  let time () = 
-    let this_time = Clock.time () in
-    let clock_diff = ((this_time -. !last_read) *. (float_of_int time_reduction_factor)) in
-    last_read := this_time;
-    this_time +. clock_diff
+  let advance_clock ns =
+    last_read := !last_read + ns
 
+  let elapsed_ns _ = 
+    !last_read
+
+  let period_ns _ = None
 end
 module Fast_time = struct
   type 'a io = 'a Lwt.t
@@ -349,6 +322,9 @@ let entries_expire () =
   let test =
     Time.sleep_ns (Duration.of_ms 100) >>= fun () ->
     set_and_check ~listener:listen.arp ~claimant:speak first_ip >>= fun () ->
+    (* our custom clock requires some manual time-travel *)
+    Fast_clock.advance_clock (Duration.of_sec 90);
+    (* sleep for 1s to make sure we hit `tick` *)
     Time.sleep_ns (Duration.of_sec 1) >>= fun () ->
     (* asking now should generate a query *)
     not_in_cache ~listen:speak.netif expected_arp_query listen.arp first_ip;