Hi JL,
I have a question. Suppose, a path computation request has two constraints:
link colors and link bandwidth. Suppose the path computation fails. The
question is how the PCE can figure out what is the reason of this
computation failure? As an implementer of a Path Computation Engine I
always found it a difficult problem. Normally, presented with a path
computation request with link type constraints as in the example I am
describing, a PCE would prune out the arcs form the network graph associated
with links that do not satisfy at least one of the constraints, and after
that run some algorithm. If the algorithm fails the only thing is clear that
there are no links with proper colors that have enough bandwidth, that is,
entire set of constraints cannot be satisfied. But it is impossible to say
which constraint in particular has caused the failure. Even if you start
removing constraints one by one and rerunning the algorithm (which would be
very unpractical to do), you will find out that it does not help. The path
computation of the example may succeed in both these iterations because:
a) there are links with proper colors;
b) there are links with enough bandwidth.
The bottom line is that path computation fails because of the entire set of
constraints rather than because of one or several of them, and it is
unrealistic to think that PCE in its negative reply would mark constraints
which cause the failure.
So, I recommend to have two flags (say, P and I). One of them should be used
to mark mandatory constraints, while another - optional constraints.
Recommend to make an assumption that PCE will run path computation at most
two times:
1) trying to accommodate all constraints;
2) in case 1) fails, removing all optional constraints
PCE should return:
a) successful response without flags when 1) succeeds;
b) successful response with flag I when 2) succeeds;
3) negative response with no flags when 2 fails.
Igor
----- Original Message -----
From: "LE ROUX Jean-Louis RD-CORE-LAN" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>;
<[EMAIL PROTECTED]>
Sent: Friday, July 14, 2006 10:46 AM
Subject: RE: [Pce] P & I flags
Hi Dimitri,
-----Message d'origine-----
De : dimitri papadimitriou [mailto:[EMAIL PROTECTED]
Envoyé : vendredi 14 juillet 2006 15:44
À : [EMAIL PROTECTED]
Objet : [Pce] P & I flags
the comments around the P and I flags are
o) we have two levels of optionality in listing the protocol
object and usage level of the protocol object - however it is
much more appropriate to be sure that mandatory constraints
have been taken rather than having the optional having been
taken into account
As clearly pointed out in the same setion of the draft, if a mandatory
constraint cannot be handled the PCE replies with PCErr message
"If the PCE does not understand an object with the P Flag set or
understands the object but decides to ignore the object, the entire
PCEP message MUST be rejected and the PCE MUST send a PCErr message
with Error-Type="Unknown Object" or "Not supported Object".
If the PCE can handle all mandatory constraints it replies with a PCRep
message (either positive or negative if there is no path), else it sends a
PCErr message.
Hence the PCC knows whether mandatory constraints have been taken into
account.
P flag (Processing-Rule - 1-bit): the P flag allows a PCC
to specify
in a PCReq message sent to a PCE whether the object must be taken
into account by the PCE during path computation or is
just optional.
When the P flag is set, the object MUST be taken into
account by the
PCE. Conversely, when the P flag is cleared, the object
is optional
and the PCE is free to ignore it if not supported.
o) on the I flag issue is identical why include an object
which has not considered during computation ?
I don't catch your point, the PCC may want to know the constraints that were
not taken into account during path computation, this sounds quite important
isn'it?
there are so
many things that the PCE has not taken into account during
computation so why bother ?
Hum I don't follow you here "there are so many things...", What do you mean?
it is imho
more important to include the mandatory object not taken
into account
If a mandatory object is not taken into account, you cannot compute the path
and you send an error message. I agree with you that this would be useful to
include within the PCErr the set of objects not supported, this can be
clarified in next revision.
I flag (Ignore - 1 bit): the I flag is used by a PCE in a PCRep
message to indicate to a PCC whether or not an optional object was
processed. The PCE MAY include the ignored optional object in its
reply and set the I flag to indicate that the optional object was
ignored during path computation. When the I flag is
cleared, the PCE
indicates that the optional object was processed during the path
computation. The setting of the I flag for optional objects is
purely indicative and optional. The I flag MUST be
cleared if the P
flag is set.
o) the result of all these flags is that a PCC can be
wrapping around unknown constraints create processing churn
on the PCE;
OK, if you mean that we should include the unsupported object(s) within the
PCErr, then we are in sync
Thanks for the comment
Regards
JL
If the PCE does not understand an object with the P Flag set or
understands the object but decides to ignore the object,
the entire
PCEP message MUST be rejected and the PCE MUST send a
PCErr message
with Error-Type="Unknown Object" or "Not supported Object".
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