[Nsi-wg] [Nml-wg] Conversation about ITU concepts with Ciena folks

[John Vollbrecht]

comment below
On Sep 7, 2009, at 5:45 AM, Guy Roberts wrote:

> Hi Jeroen,
>
> Thanks for pointing this out - I have misunderstood the definition/ 
> use of transitional links.  But now I really have a problem seeing  
> the point of NML using transitional links.  Re-reading I wonder if  
> this is perhaps a concept only required for George's path  
> computation on a 'flattened multi-layer topology' idea?  This is  
> what he says about this:
>
> "2.4	Transitional Links
>
> The discussion above shows that use of the G.805 link as the  
> topological entity between nodes in a routing graph is too  
> restrictive. We need another term to describe the node – node  
> relationship in the network graph. In graph theory the terms node  
> and edge or vertex and arc are used, and there is value in adopting  
> vertex and arc (or edge) to avoid confusion. Nodes frequently  
> designate equipment.
>
> Inspection of the figure suggests that the links that are important  
> when considering pruning are those links whose CI changes along the  
> link (L1, L2 and L3). L1 and L3 convert CI from the client to the  
> server (and back again) while L2 converts server CI to client at  
> each end. If we include those links, we have a connected graph at  
> the client CI. If we remove those links then we have the server  
> topology left over. It seems that we are perhaps better off  
> retaining the term link, because it has been use so long, and  
> introducing the term transitional link for those links which change  
> CI along the link. This should give us some strong hints when  
> looking for appropriate link semantics."
>
>
> If path computation is the only proposed use for transitional links  
> then I am not sure that we need to use the concept in NML, or are we  
> proposing another use that I have missed?
>

I think that transitional link is a way of flattening topology where  
it is possible to do adaptations between layers.  It requires that a  
device be represented at both layers and have a link between layers  
with a specific adaptation.  When doing pathfinding using such links  
one must be sure adaptations and deadaptations match such that the  
client info is passed through.

I think this concept is highly useful for describing a "routing area"  
over which pathfinding can be done.  We should consider this relative  
to other methods of pathfinding which seem more difficult to use in  
practice.

John

> Guy
>
>
>
> -----Original Message-----
> From: Jeroen van der Ham [mailto:vdham at uva.nl]
> Sent: 04 September 2009 19:13
> To: Guy Roberts
> Cc: John Vollbrecht; NSI WG; Network Markup Language Working Group
> Subject: Re: [Nml-wg] [Nsi-wg] Conversation about ITU concepts with  
> Ciena folks
>
> Guy Roberts wrote:
>> The other document of note is the transitional link document.  I
>> think we need to be careful about adopting this concept since as far
>> as I can see it has been created by ITU-T for a very specific
>> purpose.  They use it for transit between sub-layers as opposed to
>> adaptation between layers.  In their example a transitional link is
>> used for all-optical conversion of wavelengths, where wavelengths are
>> not real layers as there is no termination and adaptation function
>> when converting between wavelengths.
>
> That's not completely true. As far as I see it there are two kinds of
> Transitional Links. The example you are referring to are sub-layer
> transitional links, where the connection on a single layer is not
> terminated, and readapted, but only the "Characteristic Information"  
> is
> transformed, in this case the wavelength.
>
> There is also the Layer Transitional Link. This link does go from a
> client layer to a server layer, without specifying all the details. It
> seems to be an abstraction of a normal link with an adaptation.
>
> I am not sure whether the distinction between sub-layer and layers is
> actually relevant to us. So far I'm inclined to think that it is not
> relevant.
>
> Jeroen.