Subject: Re: Label assignment and distribution

From: "Gangadharan Ramakrishnan" <[email-masked]>
Date: Wed, 15 Aug 2001 02:52:12 +0000

Gopal,
See my comments inline....

Cheers,
ganges...

>From: "Gopal@Cisco" <[email-masked]>
>To: Gangadharan Ramakrishnan <[email-masked]>
>CC: [email-masked], [email-masked]
>Subject: Re:
>Date: Tue, 14 Aug 2001 18:18:14 -0700
>
>Ganges,
>prefix-X------Rd--------Ru--
>
>Data packets flow from upstream to downstream towards the destination.
>Ru will use the label distributed by Rd to impose/insert/swap into the
>data packet. Rd will take the forwarding decision based on the label it
>looks in the data packet.
>So it is necessary for Ru to use the label distributed by Rd for proper
>transmission.
>"> Why the bindings are assigned by the downstream LSR"--> i believe you
>meant labels are assigned.....
Ganges::; Yes, i mean that exactly.
>
>" What makes the architecture to expect aggsinement and distribution of
> > label bindings from downstream to upstream not the other way?"
> >
>Label distribution can happen bothways(depending on distribution
>methods: unsolicited or ondemand). However, label assignement depends on 
>the routing table. Ru's routing table point to Rd as the next-hop for

Ganges:::: Well, but the rfc says that,
3.4. Label Assignment and Distribution

   In the MPLS architecture, the decision to bind a particular label L
   to a particular FEC F is made by the LSR which is DOWNSTREAM with
   respect to that binding.  The downstream LSR then informs the
   upstream LSR of the binding.  Thus labels are "downstream-assigned",
   and label bindings are distributed in the "downstream to upstream"
   direction.

In both distribution methods: Unsolicited or OnDemand, is a way how Ru gets 
the label assignment information and it is not specifying who is 
distributing the label assignment. DOWNSTREAM LSR is the one which 
distributes the label assiagnment in both the methods. Please, correct me if 
i'm wrong.

>prefix-X. Hence Ru will use the label distributed by Rd while forwarding
>data to preix X. however Rd's routing entry to prefix-X will not point
>to Ru as the next-hop. Hence Rd will not use the label distributed by Ru
>for prefix X.
>
>rgds,
>gopal
>
>
>Gangadharan Ramakrishnan wrote:
> >
> > Hi,
> >
> > I'm new to this MPLS domain. I have a question on MPLS Arch (rfc3031). I
> > think, may be some one else might have asked this question earlier. 
>Well,
> > the question is,
> >
> > Why the bindings are assigned by the downstream LSR and why not the 
>upstream
> > LSR? What makes the architecture to expect aggsinement and distribution 
>of
> > label bindings from downstream to upstream not the other way?
> >
> > Thanks in advance....
> >
> > Cheers,
> > ganges....
> >
> > _________________________________________________________________
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>--
>Gopal(www-tac/~gnaganab),CCIE#6209,1.408.525.9922,RP-TAC,9:00AM-5:00PM(PST)
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---

From: "Gopal@Cisco" <[email-masked]>
Date: Thu, 16 Aug 2001 13:12:22 -0700

Ganges,
> distributes the label assiagnment in both the methods. Please, correct me if

I am not very knowledgeable in this but this is my understanding. so,
others, pls correct if wrong.

prefix-X----R1--R2-----Rd1-------Ru-----
       \                       /
        \                     /
         --------------Rd2---/

Talking about unsolicited-distribution, Rd1, Rd2, Ru distribute labels
to each other fro the prefix-X. However, they install the right
out-label(remote-binding) in their forwarding table. This is driven by
the control plane(routing table + label binding table). So I wd say, the
decision to use the label is local to the LSR.
now from the rfc, === "In the MPLS architecture, the decision to bind a
particular label L
>    to a particular FEC F is *made* by the LSR which is DOWNSTREAM with
>    respect to that binding."

I personally not comfortable reading "*made* by". May be i
misunderstood.

If i have to assume RFC is *GOD*, then i wd try this:
when we say DOWNSTREAM, it is the next-hop. For Ru, if Rd1 is the
next-hop in his routing table, then Rd1 'is and only is':-) Ru's
DOWNSTREAM router. 
I think this explaination goes with the rfc.

About your comments----"" DOWNSTREAM LSR is the one which > distributes
the label assiagnment in both the methods. ""
Logically, it is imposible for any router to know it is the downstream
of another router. For example, Rd1 and Rd2 can not know if they are
DOWNSTREAM rtrs for Ru. However Ru wd know who the DOWNSTREAM router is
for the prefix-X from his routing table. Hence it is impossible for any
router to *assign* labels for another router but can only
advertise/distribute to other rotuers. (assign ---i understood as
installing in the label forwarding table)
Please remember, I may be wrong! Can some one comment??

Thnx, Gopal