Delay in OpenSSH

Question

I am using openSSH (8.0p1) and using netmiko client to send some data to server. I see that there is a delay in reply from the server. To debug it, I have put logs in openSSH code to see what can cause the delay and found out that most probably the delay is between when shell returns the data and sshd (user privileges' child) is encrypting the data.

Till now I have tracked the code till here

1. Data from client reaches (after getting read event in select) to => server_loop2 -> 
channel_handler
2. Channel_handler calls “channel_post_open” which in turn calls following 4 functions
    a. channel_handle_rfd => Data from shell to sshd
    b. channel_handle_wfd =>Data to shell from sshd
    c. channel_handle_efd
    d. channel_check_window =>

3. server_loop2 -> process_input => This process input from client and store in ssh buffer.
4. -> process_output => send buffered data to SSH client.

I see that following function is encrypting the data “ssh_packet_send2_wrapped” but I am not sure what functions are being called in between when data is returned from shell (channel_handle_rfd) and data is written to client (process_output).

Can someone help me to find in between functions so that I can put logs to check which particular function is causing the delay ?

Answer 1

void channel_output_poll(void) {
    ....
    packet_start(compat20 ? SSH2_MSG_CHANNEL_DATA : SSH_MSG_CHANNEL_DATA);
    packet_put_int(c->remote_id);
    packet_put_string(buffer_ptr(&c->input), len);
    packet_send();
    ....
}

packet_send -> sshpkt_send ->ssh_packet_send2 ->ssh_packet_send2_wrapped