Mirage test that sends one byte to TCP endpoint (10.0.0.1/7001), with tracing on

config.ml

open Mirage

let main = foreign "Unikernel.Main" (stackv4 @-> job)

let stack console =
  match get_mode () with
  | `Xen -> direct_stackv4_with_default_ipv4 console tap0
  | `Unix -> assert false

let tracing = mprof_trace ~size:1000000 ()

let () =
  register "bench" ~tracing [
    main $ stack default_console;
  ]

unikernel.ml

open Lwt

let laptop_ip = Ipaddr.V4.of_string_exn "10.0.0.1"

let () =
  let open Tcp in
  [Pcb.info; Pcb.debug; State.debug] |> List.iter (fun log ->
    Log.enable log;
    Log.set_stats log false
  )

let () =
  Lwt.async_exception_hook := fun ex -> Printf.eprintf "Uncaught exception: %s\n%!" (Printexc.to_string ex)

module Main (S: V1_LWT.STACKV4) = struct
  let buffer = Io_page.get 1 |> Io_page.to_cstruct

  let start s =
    let t = S.tcpv4 s in

    match_lwt S.TCPV4.create_connection t (laptop_ip, 7001) with
    | `Error _err -> failwith "TCP error 7001"
    | `Ok flow ->
    
    let payload = Cstruct.sub buffer 0 1 in
    Cstruct.set_char payload 0 '!';

    match_lwt S.TCPV4.write flow payload with
    | `Error _ | `Eof -> assert false
    | `Ok () ->

    S.TCPV4.close flow >>= fun () ->
    OS.Time.sleep 10.0
end

To test, run netcat on 10.0.01 to accept the data (or change laptop_ip):

nc -l -p 7001

Then run the unikernel. With mirage/mirage-tcpip#156, the output is:

Xen Minimal OS!
[...]
Manager: connect
Manager: configuring
Manager: Interface to 10.0.0.2 nm 255.255.255.0 gw [10.0.0.1]

 sg:true gso_tcpv4:true rx_copy:true rx_flip:false smart_poll:false
ARP: sending gratuitous from 10.0.0.2
Manager: configuration done
ARP: transmitting probe -> 10.0.0.1
ARP: retrying 10.0.0.1 (n=1)
ARP: transmitting probe -> 10.0.0.1
ARP: updating 10.0.0.1 -> fe:ff:ff:ff:ff:ff
Tcp.PCB: process-synack: [channels=0 listens=0 connects=1]
Tcp.PCB: new-client-connection: [channels=0 listens=0 connects=0]
Tcp.State: Closed  - Send_syn(466439307) -> Syn_sent(466439307)
Tcp.State: Syn_sent(466439307)  - Recv_synack(466439308) -> Established
Tcp.PCB: TX.close
Tcp.State: Established  - Send_fin(466439309) -> Fin_wait_1(466439309)
Tcp.State: Fin_wait_1(466439309)  - Recv_ack(466439309) -> Fin_wait_1(466439309)
Tcp.State: finwait2timer 10.00
Tcp.State: Fin_wait_1(466439309)  - Recv_ack(466439310) -> Fin_wait_2(0)
Tcp.State: timewait 2.00
Tcp.State: Fin_wait_2(0)  - Recv_fin -> Time_wait
Tcp.State: Time_wait  - Recv_rst -> Time_wait
Tcp.State: Time_wait  - Recv_fin -> Time_wait
Tcp.State: Time_wait  - Recv_rst -> Time_wait
Tcp.State: Time_wait  - Recv_fin -> Time_wait
Tcp.State: Time_wait  - Recv_rst -> Time_wait
Tcp.State: Time_wait  - Recv_fin -> Time_wait
...

Perfect, thanks; I was just recreating something from the snippet you put in the issue but I'll grab this and give it a try instead.