diff --git a/src/ipv6/ipv6.ml b/src/ipv6/ipv6.ml
index a57d51a95..800e864c5 100644
--- a/src/ipv6/ipv6.ml
+++ b/src/ipv6/ipv6.ml
@@ -90,7 +90,8 @@ module Make (E : Mirage_protocols.ETHERNET)
 
   let input t ~tcp ~udp ~default buf =
     let now = C.elapsed_ns () in
-    let _, outs, actions = Ndpv6.handle ~now ~random:R.generate t.ctx buf in
+    let ctx, outs, actions = Ndpv6.handle ~now ~random:R.generate t.ctx buf in
+    t.ctx <- ctx;
     Lwt_list.iter_s (function
         | `Tcp (src, dst, buf) -> tcp ~src ~dst buf
         | `Udp (src, dst, buf) -> udp ~src ~dst buf
diff --git a/src/ipv6/ndpv6.ml b/src/ipv6/ndpv6.ml
index b5cfecee6..666b41713 100644
--- a/src/ipv6/ndpv6.ml
+++ b/src/ipv6/ndpv6.ml
@@ -158,13 +158,13 @@ module Allocate = struct
     let size' = size + Ipv6_wire.sizeof_ipv6 in
     let fill ipbuf =
       Ipv6_wire.set_ipv6_version_flow ipbuf 0x60000000l; (* IPv6 *)
+      Ipv6_wire.set_ipv6_len ipbuf size;
       ipaddr_to_cstruct_raw src (Ipv6_wire.get_ipv6_src ipbuf) 0;
       ipaddr_to_cstruct_raw dst (Ipv6_wire.get_ipv6_dst ipbuf) 0;
       Ipv6_wire.set_ipv6_hlim ipbuf hlim;
       Ipv6_wire.set_ipv6_nhdr ipbuf (Ipv6_wire.protocol_to_int proto);
       let hdr, payload = Cstruct.split ipbuf Ipv6_wire.sizeof_ipv6 in
       let len' = fillf hdr payload in
-      assert (len' <= size') ;
       len' + Ipv6_wire.sizeof_ipv6
     in
     (size', fill)
@@ -233,8 +233,8 @@ module Allocate = struct
       Ipv6_wire.set_pingv6_id icmpbuf id;
       Ipv6_wire.set_pingv6_seq icmpbuf seq;
       Ipv6_wire.set_pingv6_csum icmpbuf 0;
-      Ipv6_wire.set_pingv6_csum icmpbuf @@ checksum hdr (icmpbuf :: data :: []);
       Cstruct.blit data 0 icmpbuf Ipv6_wire.sizeof_pingv6 (Cstruct.len data);
+      Ipv6_wire.set_pingv6_csum icmpbuf @@ checksum hdr [ icmpbuf ];
       size
     in
     hdr ~src ~dst ~hlim ~proto:`ICMP ~size fillf
diff --git a/test/test.ml b/test/test.ml
index ede9b9b8b..64e7f4e3e 100644
--- a/test/test.ml
+++ b/test/test.ml
@@ -35,6 +35,7 @@ let run test () =
   Lwt_main.run (test ())
 
 let () =
+  Printexc.record_backtrace true;
   (* someone has to call Mirage_random_test.initialize () *)
   Mirage_random_test.initialize ();
   (* enable logging to stdout for all modules *)
diff --git a/test/test_iperf.ml b/test/test_iperf.ml
index 6fa29472e..ad49b9774 100644
--- a/test/test_iperf.ml
+++ b/test/test_iperf.ml
@@ -43,9 +43,9 @@ module Test_iperf (B : Vnetif_backends.Backend) = struct
     client : V.Stackv4.t;
   }
 
-  let default_network ?(backend = B.create ()) () =
-    V.create_stack ~cidr:client_cidr ~gateway backend >>= fun client ->
-    V.create_stack ~cidr:server_cidr ~gateway backend >>= fun server ->
+  let default_network ?mtu ?(backend = B.create ()) () =
+    V.create_stack ?mtu ~cidr:client_cidr ~gateway backend >>= fun client ->
+    V.create_stack ?mtu ~cidr:server_cidr ~gateway backend >>= fun server ->
       Lwt.return {backend; server; client}
 
   let msg =
@@ -196,8 +196,11 @@ let test_tcp_iperf_two_stacks_basic amt timeout () =
     (Test.tcp_iperf ~server ~client amt timeout)
 
 let test_tcp_iperf_two_stacks_mtu amt timeout () =
-  let module Test = Test_iperf (Vnetif_backends.Mtu_enforced) in
-  Test.default_network () >>= fun { backend; Test.client; Test.server } ->
+  let mtu = 1500 in
+  let module Test = Test_iperf (Vnetif_backends.Frame_size_enforced) in
+  let backend = Vnetif_backends.Frame_size_enforced.create () in
+  Vnetif_backends.Frame_size_enforced.set_max_ip_mtu backend mtu;
+  Test.default_network ?mtu:(Some mtu) ?backend:(Some backend) () >>= fun { backend; Test.client; Test.server } ->
   Test.V.record_pcap backend
     (Printf.sprintf "tcp_iperf_two_stacks_mtu_%d.pcap" amt)
     (Test.tcp_iperf ~server ~client amt timeout)
diff --git a/test/test_ipv6.ml b/test/test_ipv6.ml
index 57211068b..bdcd862f1 100644
--- a/test/test_ipv6.ml
+++ b/test/test_ipv6.ml
@@ -49,22 +49,26 @@ let listen ?(tcp = noop) ?(udp = noop) ?(default = noop) stack =
 
 let udp_message = Cstruct.of_string "hello on UDP over IPv6"
 
-let check_for_one_udp_packet netif on_received_one ~src ~dst buf =
-  Alcotest.(check ip) "sender address" (Ipaddr.V6.of_string_exn "fc00::23") src;
-  Alcotest.(check ip) "receiver address" (Ipaddr.V6.of_string_exn "fc00::45") dst;
+let check_for_one_udp_packet on_received_one ~src ~dst buf =
   (match Udp_packet.Unmarshal.of_cstruct buf with
   | Ok (_, payload) ->
+    Alcotest.(check ip) "sender address" (Ipaddr.V6.of_string_exn "fc00::23") src;
+    Alcotest.(check ip) "receiver address" (Ipaddr.V6.of_string_exn "fc00::45") dst;
     Alcotest.(check cstruct) "payload is correct" udp_message payload
   | Error m -> Alcotest.fail m);
   (try Lwt.wakeup_later on_received_one () with _ -> () (* the first succeeds, the rest raise *));
-  (*after receiving 1 packet, disconnect stack so test can continue*)
-  V.disconnect netif
+  Lwt.return_unit
 
 let send_forever sender receiver_address udp_message =
   let rec loop () =
-    Printf.fprintf stderr "Udp.write\n%!";
-    Udp.write sender.udp ~dst:receiver_address ~dst_port:1234 udp_message
-    >|= Rresult.R.get_ok >>= fun () ->
+    (* Check that we have an IP before sending *)
+    if List.length (Ipv6.get_ip sender.ip) >= 1 then
+    begin
+            Udp.write sender.udp ~dst:receiver_address ~dst_port:1234 udp_message
+            >|= Rresult.R.get_ok
+    end else
+            Lwt.return_unit
+    >>= fun () ->
     Time.sleep_ns (Duration.of_ms 50) >>= fun () ->
     loop () in
   loop ()
@@ -77,7 +81,7 @@ let pass_udp_traffic () =
   get_stack backend receiver_address >>= fun receiver ->
   let received_one, on_received_one = Lwt.task () in
   Lwt.pick [
-    listen receiver ~udp:(check_for_one_udp_packet receiver.netif on_received_one);
+    listen receiver ~udp:(check_for_one_udp_packet on_received_one);
     listen sender;
     send_forever sender receiver_address udp_message;
     received_one; (* stop on the first packet *)
diff --git a/test/test_mtus.ml b/test/test_mtus.ml
index 18f8211ee..0d46e5817 100644
--- a/test/test_mtus.ml
+++ b/test/test_mtus.ml
@@ -5,7 +5,7 @@ let client_cidr = Ipaddr.V4.Prefix.of_string_exn "192.168.1.10/24"
 
 let server_port = 7
 
-module Backend = Vnetif_backends.Mtu_enforced
+module Backend = Vnetif_backends.Frame_size_enforced
 module Stack = Vnetif_common.VNETIF_STACK(Backend)
 
 let default_mtu = 1500
@@ -36,7 +36,7 @@ let get_stacks ?client_mtu ?server_mtu backend =
   Stack.create_stack ~cidr:client_cidr ~mtu:client_mtu backend >>= fun client ->
   Stack.create_stack ~cidr:server_cidr ~mtu:server_mtu backend >>= fun server ->
   let max_mtu = max client_mtu server_mtu in
-  Backend.set_mtu max_mtu;
+  Backend.set_max_ip_mtu backend max_mtu;
   Lwt.return (server, client)
 
 let start_server ~f server =
diff --git a/test/vnetif_backends.ml b/test/vnetif_backends.ml
index 0a7262d53..73a07ff54 100644
--- a/test/vnetif_backends.ml
+++ b/test/vnetif_backends.ml
@@ -21,23 +21,49 @@ module type Backend = sig
   val create : unit -> t
 end
 
-(** This backend enforces an MTU. *)
-module Mtu_enforced = struct
+(** This backend enforces an Ethernet frame size. *)
+module Frame_size_enforced = struct
   module X = Basic_backend.Make
-  include X
+  type t = {
+    xt : X.t;
+    mutable frame_size : int;
+  }
 
-  let mtu = ref 1500
+  type macaddr = X.macaddr
+  type 'a io = 'a X.io
+  type buffer = X.buffer
+  type id = X.id
+
+  let register t =
+    X.register t.xt
+
+  let unregister t id =
+    X.unregister t.xt id
+
+  let mac t id =
+    X.mac t.xt id
+
+  let set_listen_fn t id buf =
+    X.set_listen_fn t.xt id buf
+
+  let unregister_and_flush t id =
+    X.unregister_and_flush t.xt id
 
   let write t id ~size fill =
-    if size > !mtu then
+    if size > t.frame_size then
       Lwt.return (Error `Invalid_length)
     else
-      X.write t id ~size fill
+      X.write t.xt id ~size fill
+
+  let set_frame_size t m = t.frame_size <- m
+  let set_max_ip_mtu t m = t.frame_size <- m + Ethernet_wire.sizeof_ethernet
 
-  let set_mtu m = mtu := m
+  let create ~frame_size () =
+    let xt = X.create ~use_async_readers:true ~yield:(fun() -> Lwt_main.yield () ) () in
+    { xt ; frame_size }
 
   let create () =
-    X.create ~use_async_readers:true ~yield:(fun() -> Lwt_main.yield () ) ()
+    create ~frame_size:(1500 + Ethernet_wire.sizeof_ethernet) ()
 
 end