User manual

Introduction

To address described limitation and enable packet interchange and hence network connection between

the host and VAX MP (or between VAX MP instance and other virtual machines running on the local

host) one can optionally configure virtual Ethernet interface on the host, such as TAP and VDE devices

on Linux and OS X or Microsoft Loopback Adaptor on Windows

By default this leads to a configuration (called hereafter dual-homed set-up) where VAX MP instance

uses two virtual DEQNA or DELQA adapters: one attached to host NIC and used for communication with

physically remote hosts, the other attached to virtual Ethernet interface (such as TAP device or MS

Loopback Adapter) and used for communication with host machine or with other virtual machines

running on the local host. Each of DEQNA/DELQA interfaces is assigned its own IP address. Likewise, host

machine is assigned two IP addresses too: one is host’s address on the NIC, the other is host’s address

on virtual Ethernet segment used to connect between the host and VAX MP. See illustration for dual-

homed set-up in the section for Microsoft Windows.

Some host operating systems (including Linux and Windows but not OS X at this point) also offer an

option for bridged set-up. In bridged set-up both VAX MP instance and host machine each use one IP

address, rather than two addresses. In bridged set-up VAX MP utilizes one virtual DEQNA or DELQA

interface attached to host virtual Ethernet interface (such as Microsoft Loopback Adapter on Windows

or TAP device on Linux). Using host OS interface bridging facilities, virtual Ethernet interface is bridged

with host NIC. Bridging works similarly to Ethernet hub and provides full exchange of packets between

two adapters (host NIC and virtual Ethernet) thus obviating the need for two separate communication

channels: (channel 1) between VAX MP instance and local host and (channel 2) between VAX MP

instance and remote hosts, i.e. dual-homed set-up. Bridged set-up is illustrated and explained in the

section for Linux below. Bridged set-up is equally possible for Windows as well, albeit it is of little benefit

under Windows since default (on Windows) single-interface configuration already performs full packet

interchange between VAX MP and the host.

Only Ethernet NICs and virtual Ethernet adapters such as Microsoft Loopback Adapter can be attached

to XQ or XQB for mapping virtual DEQNA or DELQA adapter. Host WiFi interfaces might superficially look

similar to Ethernet interfaces but they are actually not Ethernet and cannot be used for VAX MP or SIMH

Ethernet networking.

Furthermore, some newer NICs can implement TOE (TCP off-load-engine) processing. One variant of TOE

is also known as TCP Chimney Offload. TOE adaptors build IP frames within the card itself rather than

within the host network stack, and therefore IP data sent by host to TOE adaptor is not captured by

pcap and is not propagated to virtual machines such as VAX MP. TOE adaptors therefore cannot be used

for communication between VAX instance and host machine (as long as TOE mode is enabled in

To avoid confusion due to similarity in naming: Microsoft Loopback Adapter is not the same as TCP/IP loopback

adapter with IP address 127.0.0.1. Microsoft Loopback Adapter is a virtual device that on the application’s side

appears as virtual Ethernet controller connected to virtual wire hooked into Windows network kernel. It thus

establishes virtual subnet between the host machine and application or applications running on the host and

making use of Microsoft Loopback Adapter. Microsoft Loopback Adapter is similar to Linux and OS X TAP and VDE

devices.