Re: Suggested Label Doubt

[Yangguang Xu <xuyg@lucent.com>]

Eric,

Please see my response in line.


> > In my understanding, the purpose of "suggested label" is to speed up circuit
> > connection procedure by letting the upstream node to specify a label rather than
> > following the conventional MPLS signaling procedure that receiving the label in
> > the response.
> >
> > My questions are:
> >
> > (1) From a protocol design perspective, what's the benefit to suggest something
> > that could be declined?
> 
> The benefit is in optimizing the likely.  As you point out below, it is likely
> that the suggestion will be accepted, therefore - for the optimization reason
> that you mention above (connection speedup) - it makes sense to allow the
> 'suggestion'.
> 

The key is that the likely is not likely in optical network. 

Basically, "suggested label" adds the local negotiation capability.
Therotically, it's a good idea. But it's not necessary in the optical network.

> >
> >
> > (2) Label is of local significance. In optical network, labels are physically
> > bound, not logically bound. This is different from conventional IP network. This
> > is also why there is no difference of which node (upstream or downstream) to
> > choose the label in optical network. (Normally upstream and downstream node
> > share almost the same knowledge of links that connect them.)
> 
> This is a presumption, not necessarily guaranteed.
> 

Would you please give me an example that what I said is not true?

Comparing to conventional IP MPLS, many "logical" value become "physical". This
has many implications to GMPLS.

> >
> >
> > (3) In optical transport network, a upstream node may decline a label from a
> > downstream node as a downstream node may decline a "suggested label" from a
> > upstream node. The rejections are of the same reasons and frequency.
> 
> True.
> 
> >
> >
> >     Why do we need to make the suggested label's operation so special? and not
> > necessary.
> 
> As you have just described, the semantics - as they are now specified
> - make use of suggested label (from upstream) and distributed label
> (from downstream) equally in the form of a suggestion.  But, look at how
> this true: if the upstream peer does not like a label distributed to it, it is
> required to request a new one (while holding onto the current one) and
> then release the one it does not want to use.  The analog behavior for
> a more forceful 'suggested' label woud be for the downstream to issue
> a new label and then withdraw the requested label.  This behavior is
> either explicitly disallowed or very much not supported in signaling.
> 
> Therefore, I would tend to look at the current approach as making the
> suggested label NOT special.
> 

I don't disagree with your point.

Indeed, you make me believe that "suggested label" may be a good idea for
conventional MPLS. Although it may add some complications.

However, for optical network, I just don't see any application that can benefit
from the current semantics of the "suggested label". Can you give me an example?

> >
> >
> > So I propose to change the semantic of the suggested label as such: (1) The
> > suggested label object/TLV is optional (2) However, if it is presented, it is
> > treated as the final decision. That is if the downstream node couldn't accept
> > the "suggested label", a connection rejection is responded.
> >
> > I am looking forward to hearing your opinions.
> 
> I disagree.  For one thing, given the current signaling specifications, what
> is likely to prevent the upstream from suggesting the exact same label
> again?
> 

Then, what is likely to prevent the downstream node from distributing the exact
same label again?

Again, because of the change of many concepts from "logical" to "physical", the
role of upstream and downstream nodes are not different anymore in optical
network. In this case, it's not a "suggested" label anymore. It's just a
"distributed" label from upstream.

Thanks,

Yangguang