Subject: label stack depth

From: "Robert Streijl" <[email-masked]>
Date: Thu, 21 Jun 2001 10:59:00 -0400

Hi,

Is there any maximum defined for the depth of a label stack? What is
common, two, three labels, more?

I known that the bottom will indicate the last label. Theoratically it
seems that there can be an undefinite amount. Practically it will probably
introduce some management complications. I don't recall having seen any
limitations in the drafts and RFCs.

I guess that before transmitted onto the layer 2 transport  technology
(e.g. ATM, Ethernet, etc.) it can be fragmented to a size supported by that
transport technology so that should not matter.

Any insights are welcomed.

Thanks.
Robert

---

From: Eric Gray <[email-masked]>
Date: Thu, 21 Jun 2001 13:04:27 -0400

Robert,

    Please see below.

You wrote:

> Hi,
>
> Is there any maximum defined for the depth of a label stack?

    In effect, a maximum label stack depth is defined by what any
specific application considers to be a minimum usable payload
and the maximum transmission unit size for the portion of the
network that carries packets for this application and is currently
under examination.

>
> What is common, two, three labels, more?

Past performance is no guarantee of future results.  Most common
at this point is probably still zero.  Once this is no longer true, the
number may be very much higher than 2 or 3.

>
>
> I known that the bottom will indicate the last label. (Theoretically) it
> seems that there can be an (indefinite) amount.

> Practically it will probably introduce some management complications.
> I don't recall having seen any limitations in the drafts and RFCs.

What's practical depends in part on the real diameter of the network
using MPLS.  To get any real utility out of label switching, at least one
LSR on any given LSP needs to perform a simple label switch operation
based on the top level label.  On this basis, you would have to have at
least 3 LSRs in a row (discounting PHP or egress pop at a second LSR)
before it makes sense to have a stack depth of even 1 label.

Beyond this, however, it depends a lot on complexity of the routing (or
traffic engineering) topology and the extent to which implementations
seeks to take advantage of work previously done in aggregating and
deaggregating traffic.  For example, you could build a case for a scenario
in which a label stack depth of 2 labels is justified for traffic spanning
only 4 LSRs, 3 labels for 5 LSRs, and so on.

Management complications are limited to the scope of the label stack
depth that is managed.  If a provider offers a service for transparently
transporting opaque (or - to some extent - quasi opaque) data between
two points, they need only to manage the labels (if any) that they use to
support this service (not any that they may transport using this service).

> Any insights are welcomed.

    Hope this helps.

--
Eric Gray