hi j-p - see in-line
JP Vasseur <[EMAIL PROTECTED]>
26/06/2006 01:14
To: Dimitri PAPADIMITRIOU/BE/[EMAIL PROTECTED]
cc: "Adrian Farrel" <[EMAIL PROTECTED]>,
[EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED]
Subject: Re: [Pce] Re: Comments on
draft-vasseur-pce-brpc-00.txt
hi dimitiri,
On Jun 25, 2006, at 2:08 PM, [EMAIL PROTECTED] wrote:
> the following comment was also discussed during last meeting
>
> "==
> Section 5.1
> I like the simplicity of section 5.1, but I think you need to point
> out
> that
> if the sequence of domains is known a priori (can you determine it a
> priori?) then the path computed might not be 'optimal' in the usual
> sense
> of
> the word, but that this is OK because the ordered list of domains
> constitutes an additional constraint on the computation so the
> resultant
> path *is* optimal within the constraints applied.
>
> Can't agree more. Good suggestion, thanks.
>
> Updated text: "The PCE-based BRPC procedure applies to the
> computation of
> an optimal constrained inter-domain TE LSP. The sequence of domains
> to be
> traversed can either be determined a priori or during the path
> computation
> procedure. The BRPC procedure guarantees to compute the optimal path
> across a specific set of traversed domains (which constitutes
> an additional constraint). In the case of an arbitrary set of meshed
> domains, the BRPC procedure can be used to compute the optimal path
> across
> each domain set in order to get to optimal constrained path between
> the
> source and the destination of the TE LSP."
>
> => in brief, how to determine the sequence through which path
> computation
> has to be performed because if this sequence was known there is no
> need
> for such procedure (only inter-domain link selection would be
> required) -
Not correct; even if you know the sequence you still need the BRPC
procedure to figure out the set of BN (boundary Nodes) to traverse to
get the shortest inter-domain TE LSP: this is the whole issue with
inter-domain, otherwise per-domain path computation would be sufficient.
[dp] inter-domain link selection (as i mentioned) implies what you mean
with boundary nodes selection - you could rephrase by access/exit point
selection exact same business
> now adrian mentions that pre- determination becomes a new constraint -
> well actually this is not a constraint for the TE LSP itself but a
> restriction on the domain set known to be (a-priori) supportive of the
> method such as to make the method not returning an error message
>
No, this is a constraint: "Give me the shortest path (potentially
made of loose hops (the BNs)) that provides the shortest end to end
path, considering that the additional constraint is to traverse the
following set of domains <D1, D2, ....>.
[dp] incorrect, this is not a "TE LSP" constraint - since so far when
a set of domains do not support/or support a given computation has not
yet become a constraint
> hence sentence "The BRPC procedure guarantees to compute the
> optimal path
> across a specific set of traversed domains (which constitutes
> an additional constraint). In the case of an arbitrary set of meshed
> domains, the BRPC procedure can be used to compute the optimal path
> across
> each domain set in order to get to optimal constrained path between
> the
> source and the destination of the TE LSP."
>
> should as far as i understand the method fulfill the following
>
> a) this method provides a result iff all domains traversed by the
> request
> support the proposed method
BRPC is a multi-PCE based approach where you need at least one PCE
per-domain.
[dp] indeed, but one PCE supporting that method and having FULL
visibility of the domain
> b) this method provides an "optimal" result iff
>
> b1) all possible sequences of domains have been traversed (starting
> from
> the root) and are supportive of that method - as there might be an
> optimal
> path through a domain not supporting this method -
>
heu ... yes indeed. If a domain does not support the method, the
method cannot be used to compute the path.
> b2) there is no discontinuity in the availability of the inter-AS link
> information on each PCE performing a given selection - indeed a domain
> might be supportive of the method without supporting inter-AS link
> flooding
No this is not a requirement, just an optimization. The only
requirement is for the PCE to get the TE-related data about the Inter-
AS links. I'll detail a bit more in the next revision.
[dp] then you can never achieve - using your own words to make it more
straighforward -
"you still need the BRPC procedure to figure out the set of BN (boundary
Nodes) to traverse to get the shortest inter-domain TE LSP: this is the
whole issue with inter-domain, otherwise per-domain path computation would
be sufficient."
> b3) at each step there is no PCE that has partial visibility of a
> given
> domain
>
This is not specific to BRPC. Even with intra-domain if you want to
compute the path for a TE LSP, you cannot always use the PCE if it
has a partial visibility. Note that this also applies to the case of
co-located PCE (when the path is computed by the head-end).
[dp] hence, to BRPC as well
> b4) during the selection process no network conditions changes
> (that would
> make the returned result void)
>
Again, this applies to other cases as well.
[dp] ditto
thanks.
JP.
>
>
>
>
>
> JP Vasseur <[EMAIL PROTECTED]>
> 23/06/2006 16:11
>
> To: "Adrian Farrel" <[EMAIL PROTECTED]>
> cc: [EMAIL PROTECTED], [EMAIL PROTECTED],
> [EMAIL PROTECTED]
> Subject: [Pce] Re: Comments on
> draft-vasseur-pce-brpc-00.txt
>
>
> Many Thanks Adrian for the comments.
>
> We incorporated the vast majority of them in the new revision (just
> posted).
>
> See below for the comments requesting some clarification.
>
> On Jun 10, 2006, at 8:49 AM, Adrian Farrel wrote:
>
> Hi,
>
> Thanks for keeping these ideas alive.
>
> Here are some picky thoughts that you may want to include in the next
> revision to make it more robust as a document. I don't have any issues
> with
> the basic process.
>
> Thanks,
> Adrian
>
> ==
> Is this I-D likely to be Informational or Standards Track? From
> Appendix
> A,
> this looks like Standards Track. Can you remove Appendix A and put the
> status in the usual place in the header?
> ==
>
> Status has been updated (Informational) but let's discuss this
> later on.
> FYI, it does not appear on the front page when using the xml2rfc
> editor
> (which by the way is excellent), thus the appendix addition.
>
> It would be nice if the Abstract contained a pithy summary of brpc
> and/or
> gave the motivation for the draft. For example, what is the method
> trying
> to
> achieve that isn't achievable by other methods? Or, what is the
> fundamental
> element of brpc that is not in other methods?
>
> Agree. Here is the new Abstract:
> "The ability to compute constrained shortest Traffic Engineering (TE)
> Label Switched Paths (LSPs) in Multiprotocol Label Switching (MPLS)
> and
> Generalized MPLS (GMPLS) networks across multiple domains (where a
> domain
> is referred to as a collection of network elements within a common
> sphere
> of address management or path computational responsibility such as IGP
> areas and Autonomous Systems) has been identified as a key
> requirement .
> This document specifies a procedure relying on the use of multiple
> Path
> Computation Elements (PCEs) in order to compute such inter-domain
> shortest
> constraint paths, using a backward recursive path computation
> technique
> while fully preserving confidentiality requirements across domains. "
>
>
> ==
> I'm assuming that section 1 is for short term information only and
> will be
> deleted (in the next revision?)
> ==
>
> Indeed, just to provide a bit of history for the ones who have been
> following inter-domain path computation for a while.
>
> ==
> Section 4
> - No topology or resource information is distributed between domains
> (as mandated per [RFC4105] and [RFC4216]), which is critical to
> preserve IGP/BGP scalability and confidentiality in the case of TE
> LSPs spanning multiple domains.
> You should clarify what you are saying here, because many would
> argue that
> reachability information (i.e. external routes) constitutes
> topology and
> resource information.
>
>
> OK fine
>
> ==
> Section 4
> - While certain constraints like bandwidth can be used across
> different domains, certain other TE constraints like resource
> affinity, color, metric, etc. as listed in [RFC2702] could be
> translated at domain boundaries. If required, it is assumed
> that, at
> the domain boundary LSRs, there will exist some sort of local
> mapping
> based on offline policy agreement, in order to translate such
> constraints across domain boundaries during the inter-PCE
> communication process.
> Why does the policy agreement need to be offline?
>
>
> It should actually read "pre-determined". Actually the best thing
> to do
> here is to remove "off-line"
>
> ==
> Section 5
> No assumption is made on the actual path computation algorithm in
> use
> by the PCE (it can be any variant of CSPF, algorithm based on
> linear-
> programming to solve multi-constraints optimization problems and so
> on).
> Suggest that you delete the text in parenthesis because it is making
> suggestions that border on assumptions!
>
> Well I'd suggest to keep it as is since the point is to
> differentiate the
> path computation procedure from the algorithm to compute each intra-
> domain
> path segment. Furthermore, we'd like to highlight the fact that the
> BRPC
> procedure does not mandate for each PCE to use CSPF, could be other
> algorithm.
>
> ==
> Section 5.1 and throughout
> "BRPC path computation" seems to be tautologous
>
> It is actually a double tautology ;-) (fixed).
>
> ==
> Section 5.1
> I like the simplicity of section 5.1, but I think you need to point
> out
> that
> if the sequence of domains is known a priori (can you determine it a
> priori?) then the path computed might not be 'optimal' in the usual
> sense
> of
> the word, but that this is OK because the ordered list of domains
> constitutes an additional constraint on the computation so the
> resultant
> path *is* optimal within the constraints applied.
>
> Can't agree more. Good suggestion, thanks.
>
> Updated text: "The PCE-based BRPC procedure applies to the
> computation of
> an optimal constrained inter-domain TE LSP. The sequence of domains
> to be
> traversed can either be determined a priori or during the path
> computation
> procedure. The BRPC procedure guarantees to compute the optimal path
> across a specific set of traversed domains (which constitutes
> an additional constraint). In the case of an arbitrary set of meshed
> domains, the BRPC procedure can be used to compute the optimal path
> across
> each domain set in order to get to optimal constrained path between
> the
> source and the destination of the TE LSP."
>
> ==
> Section 5.2
> The introduction of the term Virtual Shortest Path Tree is hard to
> parse
> and
> to my mind the definition is arse about tit. I think you could
> usefully
> add
> some textual definition of what the VSPT is trying to achieve, before
> presenting it in pictorial or abstract form.
> Even more important (I think) is the fact that the tree as presented
> "represents the shortest path between the destination and BR-en
> (j,i)..."
> Frankly, we are building directional paths in exactly the oposite
> direction!
>
> This is an abstract form of representation but see below,
>
> So the VSPT is still a tree, but it is an MP2P tree, not a P2MP
> tree as
> described.
> I suspect that this is just a textual descriptive issue rather than
> anything
> broken in the process.
>
>
> I clarified the text to avoid any ambiguity (VSPT defined as a MP2P
> tree).
>
> ==
> Section 5.2
> I suggest that you pull the note (Note: in term of computation...) out
> into
> a separate section. It doesn't belong in the middle of section 5.2.
> You will probably want to re-write the text as well since the
> unidirectional
> limit is not necessary. Also to explain 'flood' - flood to where,
> and how,
> and by whom? And you will want to add to pieces of information:
> - the link to the I-D that defines how to do this
> - a description of what happens if this process is not used.
>
> This only works for unidirectional LSPs: indeed, the exit boundary
> node
> can flood TE-related data in its LSA/LSP for the link
> exit-BN(i,k)->entry-BN(i+1,k') even in the absence of any IGP
> running on
> the link but how would a node in domain (i) get the TE-related data
> for
> the link entry(i+1,k')->exit-BN(i,k) ? I can't thus the restriction on
> unidirectional link. Are we in sync ?
>
> ==
> Section 5.2
> The paragraph that begins "BRPC guarantees to find the optimal..."
> is a
> nascent Applicability Statement. I suggest moving this to another
> Section
> and significantly beefing it up. You need to describe more fully:
> - how ECMP works (the PCC has to request the return of more than
> one path)
> - how diversity works (i.e. section 9 fits in here)
> - how some freedom of selection of domains can be offered
> - how to mix the method with other mechanisms
>
> This was (and is) planned for future revision. Note that ECMP and
> diversity are discussed already but we're planning the flesh out these
> sections in further revision.
>
> ==
> Section 7
> If the BRPC procedure cannot be completed because a PCE along the
> domain path does not support the procedure, a PCErr message is
> returned by the upstream PCE with a Error-Type "BRPC procedure
> completion failure". The PCErr message MUST be relayed to the
> requesting PCC.
> Not clear to me whether PCE(n) is supposed to know that PCE(n+1)
> does not
> support brpc, presumably through capabilities
> configuration/advertisement/negotiation, or whether PCE(n+1) is
> supposed
> to
> respond to the path computation request saying that it does not
> support
> bit-V of the RP object. If the latter, presumably the PCErr comes from
> PCE(n+1) and not from PCE(n) as documented.
>
> Good catch, typo: it should read "a PCErr message is returned to the
> upstream".
> The plan is to define a capability bit (IGP) in a further revision.
>
> ==
> Section 8
> Metric normalisation is a great potential solution, but how does it
> fit
> with
> AS topology confidentiality? What is the solution to providing
> discretion
> about what TE connectivity is available within an AS, and operating
> brpc?
>
> Metric normalization is handled locally by the PCE and does not
> impact the
> BRPC procedure per-say. As far as confidentiality is concerned,
> this only
> requires for the SPs to exchange metric mapping information (nothing
> related to the network topology or resources); for example
> SP 1 Links Cost = F (link_bandwidth) with F (OC192)=1
> SP 2 Links Cost = F (link_bandwidth) with F (OC192)=10
>
> A simple metric normalization can then be performed on the PCE
> thanks to
> simple cost mapping tables.
>
> Section 10
> Some of this text has been seen before in this I-D
> Why do you single out LSP stitching as possibly breaking end-to-end
> optimality. There is functionally no difference between stitching and
> nesting in this respect.
> But further, your statement about stitching is pejorative! If you
> say that
> "In the case where a domain has more than one BR-en or more than one
> BR-ex,
> optimality after some network change within the domain can only be
> guaranteed by re-executing the full brpc computation" then I would
> agree
> with you. But as phrased, it implies that making a per-domain
> re-optimisation could make the path *less* optimal. This (of
> course) is
> not
> the case - per-domain optimisation will always improve or leave the
> same
> end-to-end optimality (in the case where new resources are made
> available
> this is obvious; in the case of a network failure, *any* path is
> better
> than
> no path!)
> And, lastly, the text about re-optimisation belongs in the next
> section,
> not
> in this section.
>
>
> Yes I wanted to leave that text to keep track of who would read the
> ID,
> expecting reaction ;-)) Per-domain reoptimization would always
> improve the
> current path *but* in case of stitched LSPs, TE LSP segments are
> computed
> by the stitching point; thus in case of failure, they could end up
> following a non optimal path whereas in the case of contiguous LSP,
> the
> BRPC procedure would be called again thus leading to the optimal path
> again. The point that I was trying to make is "if you have a
> stitched LSP
> and rely on BRPC when first signaled in an attempt to get an
> optimal path,
> you need to consider the interaction between the local reopt mode
> and BRPC
> if the goal is to preserve an optimal path at any given time". Anyway,
> we're on the same page and I agree and the text should be reworded
> according to your proposal to avoid any misunderstanding.
>
> ==
> Section 13
> I would like you to discuss the issues of confidentiality of topology
> information at greater length. It seems that you would possibly use
> this
> technique in conjunction with path cookies, which is fine. But, are
> there
> two things you cannot escape from?
> - indicating which BR-en provides best connectivity
> - indicating the metric for multiple BR-ens
> Maybe there are some things that can be done here based on policy,
> including
> not reporting all of the BR-ens, ranking rather than giving absolute
> metrics, etc. But these would compromise optimality.
>
> Indeed.
>
> Maybe the PCE(n+1) should carefully monitor and limit
> (statistically and
> by
> policy) brpc requests so that excessive information is not divulged?
>
> Since one of objectives for using the BRPC procedure is to compute an
> optimal path, we need to know at least the BN (Boundary Nodes) and the
> cost.
>
> ==
> I wouldn't mind seeing a Management Considerations section.
> Do we need to be able to examine the pruned pieces of VSPT at transit
> PCEs?
> What are the implications of reporting multiple VSPT branches in
> PCEP to
> the
> MIB modules?
> Is brpc support something that needs to be configured: per PCE?; per
> PCE-neighbor?
> What are the implications on existing protocols (IGPs? PCEP?) of using
> brpc?
>
> Absolutely ! I put a placeholder for a further revision.
>
> Fully supportive of adding such section.
>
> Many Thanks for your comments !
>
> JP.
>
> etc.
> ==
>
>
>
>
>
> I think Jean-Louis' email address needs updating
>
>
> ==
> The document would benefit from a little care in the formatting,
> and you
> could usefully re-read it for missing words, etc.
> ==
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