Could not not install ionic diagnostic plugin. Why?

Dear Friends,
I am using ionic 3 . My issue is , when I try to install
this plugin using command

ionic cordova plugin add cordova.plugins.diagnostic

I got error as

(node:22025) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): CordovaError: Failed to fetch plugin cordova.plugins.diagnostic via registry.
Probably this is either a connection problem, or plugin spec is incorrect.
Check your connection and plugin name/version/URL.
Error: npm: Command failed with exit code 1 Error output:
npm ERR! code EPROTO
npm ERR! errno EPROTO
npm ERR! request to failed, reason: write EPROTO 140536491255616:error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol:../deps/openssl/openssl/ssl/s23_clnt.c:827:
npm ERR!

npm ERR! A complete log of this run can be found in:
npm ERR! /home/user/.npm/_logs/2018-03-14T12_07_38_150Z-debug.log
(node:22025) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.    

my ionic info result is

@ionic/cli-plugin-proxy : 1.5.7
@ionic/cli-utils : 1.19.1
ionic (Ionic CLI) : 3.19.1

global packages:

cordova (Cordova CLI) : 8.0.0

local packages:

@ionic/app-scripts : 3.1.8
Cordova Platforms : android 6.4.0 browser 5.0.3
Ionic Framework : ionic-angular 3.9.2


Android SDK Tools : 26.1.1
Node : v8.9.4
npm : 5.6.0
OS : Linux 4.13

Environment Variables:

ANDROID_HOME : /home/user/Android/Sdk
HTTP_PROXY : not set
http_proxy :
HTTPS_PROXY : not set
https_proxy :
PROXY : not set
proxy : not set


backend : pro 

Please advise How I can install this plugin ??



Dear Friends,

Solved the issue by using command

ionic cordova plugin add cordova.plugins.diagnostic --nofetch

thanks alot @firozerakib