Re: [vnrg] Logical vs. virtual

[Joe Touch <touch at isi.edu>]

Hi, Didier,

(again, speaking as a participant only),

IMO,

On 7/15/2010 6:31 AM, Didier Colle wrote:
> Dear all,
>
> Joe raised the issue of logical vs. virtual.
> I believe it is worth spending a separate thread on clarifying this point.
>
> At the bottom of this email is my proposal.
...
> Conclusion: not really clear definition of both terms.

Agreed.

> My proposal:
> * Logical is opposite of physical:

The same could be said of virtual.

> logical always requires mapping to
> physical. Of course, logical can be mapped on some other logical thing
> that itself needs mapping to physical.

I think the same is true here as well.

I tend to think of a logical device as not really adding to the 
capabilities of a physical one, though, whereas a virtual one seems to 
(at least to me).

Maybe these are really just two aspects of a single thing, i.e., I would 
differentiate between:

- a device which does not map 1:1 to a physical entity
(could be a part of one, a group of many, or a group of parts)

- a device which provides supports virtual networking

Since we call the latter "virtual networking" (the name of this RG), I 
would propose that the latter would be the virtual device, and the 
former be the logical one.

This matches the term 'logical' in many other places, e.g., logical 
disks (which can be part of a single disk, or a group [ala RAID], or a 
group of parts [RAID done on disk partitions]).

> More formally, this mapping is a
> general function: 1) nothing should prohibit mapping multiple logical
> names/addresses to a single physical address/thing, 2) not all physical
> entities must have a logical counterpart. Also the source and target of
> the mapping should not be restricted in terms of types: names can be
> mapped onto other names or names can be mapped onto address for examples.

I agree with the above.

> * Virtual is a specialized form of logical, as the virtualization
> technology still needs to map a virtual entity onto a single
> non-virtualized (physical or isolated uniquely identifiable logical)
> entity, while guaranteeing isolation.

I don't think of it as a 'specialized form', but rather the fact that a 
single virtual router CANNOT be mapped onto a single physical device 
with no other logical components.

I.e., a virtual router needs a base router to connect to the links of 
the tunnel. A virtual host needs a base router (as well) for the same 
reason.

> More formally, this mapping is an
> injection of tupples of <entity, virtual instance ID> onto entities of
> the same type (thus not mapping of names into addresses for example:
> virtualization of name space, means mapping <virtual name, instance ID>
> on to unique identifiable <name>, virtualization of virtual memory (I
> would rather call this logical memory) page tables <virtual page number
> (seen by application), instance ID> on to <system wide page number>).

This part is a stretch to me, though.

Joe