thanks, - dimitri.
Igor Bryskin wrote:
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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