Subject: question on E-LSPs, draft-ietf-mpls-diff-ext-02

From: Mark Duffy <[email-masked]>
Date: Sun, 14 Nov 1999 23:41:48 -0500

Hi,

This is a question for the authors of draft-ietf-mpls-diff-ext-02.txt,
regarding E-LSPs.

I'm trying to understand the intended operation with more than one E-LSP
for a given FEC.  Section 1.4 states there may be "zero or any number of
E-LSPs".  When there are more than one, are they constrained to each carry
the same set of (8 or fewer) PHBs?

Section 2.3 states that the same PHB<-->EXP mapping must apply to all
E-LSPs on the LSR, which pretty much requires that the answer to the above
question must be yes.  Also, the definition of E-LSPs states that the PHB
is totally encoded in the EXP field, which also implies that all E-LSPs
must have the same PHB<-->EXP mapping.

However, section 2.2.4, second list item, implies that when there *is*
Local Policy and Traffic Conditioning, the outgoing label depends on
something else besides the incoming label, presumably the PHB.  Does this
imply another E-LSP with a different set or PHBs?  Or does this relate to
changing the packet's PHB to one that is carried on an L-LSP?  Also, in
section 1.2, the first two benefits listed for E-LSPs also seem to imply
that more than 8 BAs per FEC may be obtained by using multiple E-LSPs.  Or
again is that referring to using L-LSPs if there are more than 8 BAs?

Thanks,
Mark Duffy

---

From: Francois Le Faucheur <[email-masked]>
Date: Mon, 15 Nov 1999 16:41:21 +0100

Mark,

>the same set of (8 or fewer) PHBs?

First, let me restate that in the "normal/simple" situation, you would
carry the same set of up to 8 BAs over all E-LSPs (ie single E-LSP per
FEC). However, we are simply noting that someone may want to do it
differently in some cases (eg support only one Ordered Aggregate per E-LSP).

I suggest you have a look at section B.5 (under Appendix B) which gives an
example scenario for this case.
(note that there is an unfortunate typo in the 3rd "hyphenated point" so
that it currently says "one L-LSP per <FEC,OA> when it shoudl say "one
E-LSP per <FEC,OA>"; this will be fixed in next version).

The subtlety is that E-LSPs are actually constrained to carry A SUBSET of
the same set of (8 or fewer) PHBs (as opposed to necessarily carrying the
same set).
For instance I can have one E-LSP carrying only AF1 and another E-LSP
carrying AF2.
But clearly, there cannot be more than eight Behavior Aggregates
transported over all E-LSPs for a given FEC.
We will put in some words capturing this clarification.

>something else besides the incoming label, presumably the PHB.  

No.
The "Local Policy and Traffic Conditioning" is not related to E-LSPs and
L-LSPs. It applies to both. It is optional. It is there for "special" cases
such as boundaries between Diff-Serv domains or if, for some reasons, you
want to do Diff-Serv Traffic Conditioning in the middle of the MPLS cloud
(eg rate limit the amount of traffic from a particular Behavior Aggregate,
demote/promote, etc..).

so when there is "Local Policy and Traffic Conditioning", the outgoing PHB
can be modified compared to the incoming PHB according to arbitrary rules .

>Does this
>that more than 8 BAs per FEC may be obtained by using multiple E-LSPs.  

The sentence was trying to say that when there are less than 8 BAs, then
those can be transported over a single LSP (they don't have to but they
can). But I can see that the current wording may suggest that you can put 8
BAs on one E-LSP and another 8 BAs on anther E-LSP. We will try to modify
the wording to clarify that you can only put 8 or less BAs on E-LSPs. 

I hope this clarifies operations.

Thanks for the questions/comments.

Francois

>Or

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Francois Le Faucheur   
Development Engineer, IOS Layer 3 Services 
Cisco Systems 
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