Subject: [mpls] context labels

From: Loa Andersson <[email-masked]>
Date: Wed, 12 Jul 2006 20:19:11 +0200

Eric,

a question on the context label, where would it go?

+------------------------+------+--------+-------+
|    ip payload          |  ip  |context |   ua  |
|                        |header| label  | label |
+------------------------+------+--------+-------+

or

+------------------------+------+--------+-------+
|    ip payload          |  ip  |   ua   |context|
|                        |header| label  | label |
+------------------------+------+--------+-------+

Regardless where it goes the semantics is similar,
the context label gives you a context in which you may
handle the push/swap/pop the ua label.

This strikes me as being different from the normal
behavior of an LSR. Normally when a label is propped
you can forget it, because the next label in the stack is
unique. This is not true in this case.

When taking the decision on the ua label this is not
necessarily true. Do you intend do this in some type
of VRF? In any case wouldn't this affect hw?

/Loa

-- 
Loa Andersson

Principal Networking Architect
Acreo AB                           phone:  +46 8 632 77 14
Isafjordsgatan 22                  mobile: +46 739 81 21 64
Kista, Sweden                      email:  [email-masked]
                                           [email-masked]

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---

From: Eric Rosen <[email-masked]>
Date: Tue, 18 Jul 2006 12:15:57 -0400

> a question on the context label, where would it go? 

It has to be  above (i.e., closer to the top of the  stack) the UA label for
which it provides  the context.  On an ethernet,  if the ethertype indicates
"UA label",  the context label  would be  expected to be  at the top  of the
stack,   with   the  UA   label   beneath  it.    On   a   MP2MP  LSP,   the
downstream-assigned label assigned by mLDP  would be at the top, and beneath
it would be the context label and then the UA label.

> Normally when a label is propped you can forget it, because the next label
> in the stack is unique

Well, not necessarily.   If it is known, for example, that  when a packet is
received  from interface  X its  top  label is  looked up  in an  interface-
specific label space  determined by X, then the label  is not "unique", it's
only unique relative to X.  We've  always allowed the scope of uniqueness to
be determined by the incoming interface. 

If we allow the  "incoming interface" to be the context in  which a label is
interpreted, why  not generalize  and allow an  LSP egress to  use "incoming
LSP" as the context?  

When using UA  labels over a P2MP LSP,  we don't use PHP on that  LSP, so at
an LSP egress of the P2MP  LSP, the top label identifies the LSP (implicitly
identifying the  transmitter), and  hence provides the  context in  which to
interpret the next label, which is the UA label. 

It's just  a short step  to the generalized  notion of context label  that I
proposed in Montreal.

> This strikes me as being different from the normal behavior of an LSR

Think of it as a way of providing a 40 or 60 bit label space ;-)

This  is  a  change  to  the  forwarding path  and  existing  LSRs  may  not
necessarily be able to handle it.