RE: [mpls] Comments on draft-decraene-mpls-ldp-interarea
"DECRAENE Bruno RD-CORE-ISS" <bruno.decraene@orange-ftgroup.com> Thu, 04 January 2007 10:38 UTC
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Subject: RE: [mpls] Comments on draft-decraene-mpls-ldp-interarea
Date: Thu, 04 Jan 2007 11:38:05 +0100
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Thread-Topic: [mpls] Comments on draft-decraene-mpls-ldp-interarea
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From: DECRAENE Bruno RD-CORE-ISS <bruno.decraene@orange-ftgroup.com>
To: erosen@cisco.com
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Eric, First, thank you for writing down your comments. Let me start by summarizing my understanding of your mail. You believe the draft should work but you have some concerns regarding: - A potential significant impact on the implementation - LDP interactions with the RIB - Specific cases Regarding these points I believe that: - The complexity of the changes on the implementation depends on the implementation itself. For some implementations there may be minor impacts while for others there may be more impacts. - The interactions with the RIB are very similar to the BGP and RIB interactions where BGP need to find and maintain the best match for the BGP next hop. AFAIK, these interactions are not specified in the BGP specification (RFC 4271) and different implementations had different strategies. Additionaly I believe the same kind of interactions are also needed for PIM (RPF check) and mLDP. So is this draft the best place for this discussion? - The specific cases described are indeed valid technical points but most -if not all- are not specific to the draft and also applies to RFC 3036. As next steps, may I suggest that: - LDP coders express their opinion regarding the draft. For example do you think the draft would work? Can be implemented? How hard would it be compared to the implementations of others drafts/RFC? (eg Gracefull Restart, mLDP, ...). - Service Providers express their opinion regarding the draft. For example do you think it would be usefull that LDP be able to set up inter area LSPs while performing IP aggregation on Area Border Router? Would you use the hierarchical solution (using BGP plus LDP to set up inter area LSPs) for all deploiments? Would you use a flat solution (LDP) if available? - You send some feebacks and possibly improve the text I proposed to add in the draft to answer some of your concerns? Following are more detailled answers to the points you raised. Eric Rosen wrote: > In San Diego I expressed some concerns about > draft-decraene-mpls-ldp- interarea, and Loa asked me to write them up > for this list. > > I certainly can't say at this time that the draft just won't work, > but I'm not currently convinced that the complexity is fully > understood, or that the proposed procedures will really achieve all > their goals. > > A summary of my concerns is the following: > > - The draft purports to be making a simple change to the LDP > procedures, but I believe it's actually a fairly involved change > to the implementation. > In particular the interaction between > routing, LDP, and the maintenance of the forwarding tables is made > more complicated, but the details are not fully specified. > > - The draft has LDP put prefixes into the forwarding table that > would not otherwise be there. Currently, LDP doesn't do this, > it just puts in labels for prefixes that have been installed by > routing. This means that LDP would have functionality that was > formerly in the domain of routing. The interactions of LDP with > the various routing algorithms under a variety of circumstances > must therefore be carefully considered. However, the draft has very > little about this. To be precise, we don't necessarily need the LSR to act as LER for this FEC (ie "forward packets that arrive unlabeled, but which are to be labeled before being forwarded"). We need this FEC be available to other protocols; typically MP-BGP to have an LSP toward the BGP next-hop. Nothing requires that LDP puts prefixes in the forwarding table. We need to put these prefixes in the RIB and some implementations have some part of the RIB which is not downloaded in the FIB. To come to your point, agreed LDP now needs to put prefixes in the RIB. But only for prefixes for a which routing protocols have already advertised IP reachability (through a superset) and defined the route to use to reach these prefixes. In others words, the advertisement of IP reachability and the selection of the route to reach these IP prefixes are still required to be done by routing protocols. LDP does not take any routing decision and does not change at all the path that will be followed for a given prefix. Hence it cannot be considered as a routing protocol. Installing entries in the FIB is not a sufficient condition to be a routing protocol. The routing protocol is in charge of determining a route for a given prefix, which LDP does not do. In short, the IGP already advertises IP reachability and the path to use to reach the remote PE. IP is working just fine. What we are lacking for MPLS to also be working are the labels for each remote PE. Isn't LDP the good candidate for this task ? Regarding the addition of a prefix in the RIB by a non routing protocol, is this a real issue? Doesn't RSVP-TE also install the tunnel destination prefix in the RIB? > - The draft focuses on the use of the new procedures in a particular > sort of deployment scenario, and doesn't say much about the effects > on the network if the procedures were to be used in other > deployment scenarios, or if the procedures were misconfigured, etc. The deployement scenario is the set up of inter area LSPs and the draft describe procedure for this. Do you think of others deployement scenario for which this procedure should be useful? Regarding misconfiguration, in the general case, any protocol will not behave as expected if the network operator makes a mistake. If the procedure is misconfigured (activated by error or activated on FECs which are normally advertised in the routing protocols and should behave according to RFC 3036), in case of egress LER failure, the LSP to this LER may not be immediately removed following the IGP convergence because some IP aggregate prefixe may still match the FEC. In this case the LSP will be removed by LDP when the withdraw messages are propagated (cf draft section 6.2). Would this be fine for you if we add such text? > - I think that the major purpose of the proposal is to facilitate the > use of LDP-based node protection for area border routers. > However, I am not convinced that the proposal really achieves this > goal. One goal is to be able to keep IP aggregation between IGP area. I believe we both agree that this is a desirable goal. To achieve this goal, some may favor a hierarchical solution. Others may favor a flat solution. Regarding Orange/France Telecom, at least for one deployement we favored the flat solution when we introduced IGP areas because this is easier: PE routers in edge areas have the same configuration and use the same protocols as existing routers in the backbone areas. (more on this latter) > Let me start by summarizing my understanding of the draft. The > problem > addressed by the draft is the following. There are a number of > situations in which an ingress PE needs to send a packet on an LSP > tunnel whose "LSP egress" is a PE router (the "egress PE"). This > requires the "tunnel label" > to be bound to a /32 prefix for the egress, which in turn requires > that the ingress PE's forwarding table contain a /32 prefix for each > potential egress PE. > > The authors describe a deployment scenario in which the service > provider's backbone is a multi-area IS-IS or OSPF network, containing > tens of thousands of PE routers distributed across the areas. The > usual way of getting all the /32s in each ingress PE's forwarding > table is to have all IGP area border routers leak all the /32s into > each area. Then the IGP ensures that each ingress PE's > forwarding table is populated with all the /32s. > > The problem is that the intra-area IGP protocols don't scale up well > as the number of prefixes increases into the tens of thousands. > Therefore it would be desirable to have some other way of getting > all the /32s into the forwarding tables of the ingress > PEs. > > The proposal is to use LDP, rather than the IGP, to distribute the > /32s and to ensure that the ingress PE's forwarding tables are > properly populated with the /32s. > Note that this does not really reduce the resources in the core > needed to support the LSPs to the PEs, it simply changes the > protocol which is used to allocate some of those resources. you don't change the protocol as you still use LDP. You use one protocol instead of two. Instead of advertising the prefixes twice you advertise them only once. > Resource consumption in the P routers of a given area is still > proportional to the total number of PEs in all the areas. This allow to advertise the /32s only in LDP rather than both in LDP and the IGP. This is an obvious significant gain for the IGP. Agreed there is no gain (but no loss either) for LDP or the forwarding table. Besides, this seems reasonable for me to put the burden on LDP rather than the IGP because IP doesn't need these /32 whereas MPLS does. Additionaly LDP uses incremental messages over TCP so seems better suited than link state IGP. Not to mention that link states IGP are not designed for advertising tens of thousands of prefixes, especially when advertised by a single node (the ABR in our case). For instance with IS-IS there is an hard limitation around 42K IPv4 prefixes (max LSP fragment size = 1496 and max #fragments = 256) (and even less if IPv6 is deployed). In return with LDP there is not such limitation. > It might seem that a better solution would be to use hierarchy, so > that the P routers do not have to maintain an LSP for each egress > PE. For instance, BGP between the border routers could carry the > /32s, without any need for the P routers to maintain the /32s are > all. However, I think that the authors actually want to the P > routers maintain an LSP for each egress PE, because they believe > this to be a necessary condition of applying node protection when > a border router fails. So they frame the problem not as one of > reducing the resource usage in the core, but as one of finding a > better protocol to allocate those resources in the core. The use of hierarchy is a perfectly valid solution. It has pro & con compared to the use of LDP inter area. As you said the advantage is that it does not require P routers to maintain the /32. However, one may weight this advantage in regard to the (usually) small numbers of P routers compare to the high number of PE routers. One disadvantage is that it requires BGP on ABR. Depending on some engineering choice, it may also requires that ABRs be iBGP route reflectors and change the BGP next hop to themselves. Another disadvantage is that the set up of PE to PE LSP requires two protocols (LDP plus BGP) and two labels instead of one. This may add complexity to the monitoring and the troubleshooting. > One could of course put IBGP in all the P routers and use IBGP to > distribute the /32s. However, the authors do not like the > operational implications of putting BGP in the core. I'm not sure > they've considered a restricted use of BGP for distributing only the > egress PE addresses; I believe we did. The use of BGP to carry labelled IPv4 unicast adresses (as per RFC 3107) is only needed on ABR and PE (and probably iBGP route reflector), not on P routers. I would not call this a "restricted use" as is requires modifying BGP configurations (new AFI/SAFI, probably new iBGP peers) on all PE and RR ie 90% of the network equipments in the network. To conclude on the hierarchical solution, do you think we should add a section in the doc to present it and explain why this alternative does not fit all deployments ? Personnaly I'm not sure this would fit well in a solution draft. Such analysis should be part of an applicability statement draft. > often when we talk of a "BGP free core", what > it really important is that the core be free of > external routes, rather than that it be free of BGP protocol. I agree that removing the external routes really matter but if you can also get rid of BGP on P routers that's an additional benefit. Some service providers do have BGP free core and they did it on purpose. > But in any event they propose to use LDP. > > I think the proposed operational model is the following: > > - A PE, say PE1, is configured to bind a label to its own address > A (as a /32 prefix),a and to use LDP distribute this label > binding to its neighbors. > > - If P1 is a neighbor of PE1, and PE1 is P1's next hop for A, > and P1 receives a label binding for A from PE1, then P1 must put > an entry in its FIB for A, with the label distributed by PE1. > > - P1 must then assign a label of its own to A, and use LDP to > advertise the binding A and that label to P1's neighbors. > > - Using LDP ordered mode, this process proceeds out to the edges > of the network. However, it does not go beyond the edges. > > This operational model is not completely clear to me. I surmise > that each > PE needs to be configured to begin this process. I'm not sure to follow you. Especially I fail to see the difference between your above text and the existing LDP ordered distribution mode. This does not necessarily require any configuration, this can be done by default (advertise FEC for which you're egress eg your loopback), this is how many LDP implementations work today. > I'm less clear > about how other nodes know whether or not they are on the "edge" > of the network. They don't need to know that they are on the edge. FEC propagation stops when there is no LDP session with the downstream (Typically on the edge of the AS) or when the downstream has no IP route with the upstream for the FEC. Just like today. The only change beeing the "match algorithm" between FEC and RIB prefixes. > I think the authors have in mind a deployment > scenario in which the "edges" > are inter-AS interfaces and VRF interfaces, but inter-area > interfaces are not considered to be edges. But I'm not sure what the > model is supposed to > be in general. Are VRF interfaces always edges, even if Carrier's > Carrier is being used? In section 5 "Application examples" of the draft, the authors have described a deployment scenario in which the "edges" of the SP network are the PE ("Provider Edge"). Regarding VRF Carrier's Carrier, even if LDP is used to distribute MPLS labels to the VPN customer (rather than BGP with RFC 3107), they advertise customers prefixes, not SP ones. So there is no relation between the LDP in the core and the LDP in the VRF carrier's carrier; Just like there is no relation between the routing protocol in the core (the SP one) and the routing protocol in the VRF (the customer one). > Are inter-AS interfaces always edges, even if a single SP has broken > his network into several ASes. If you refer to BGP confederation, I suppose it depends if you use one IGP or multiple IGP. If have one IGP, you don't have IGP frontier or borders so you don't have issue with LDP mandating a longest match. If you have multiple IGPs, - if ASBRs change the BGP next hop I don't see the need to leak the /32 in the IGP and LDP from one AS to the other. - if ASBRs preserve the BGP next-hop, you need to advertise to the other AS IP reachability for the routers next-hop and you typically have the same use as in the inter area case described in the draft: either you advertise all the /32 in the IGP, or you need a hiearchical solution, or you need the LDP inter area draft. If you refer to inter AS VPN model C, this is basically identical to the last case described in the above text. (you can use a hiearchical solution or the ldp-inter-area draft) > Is it true that > inter-area links are never edges, or is this meant to be > configurable? Inter-area links are edge from the point of view of the link state IGP. If you leak all /32, you don't have issue with LDP mandating the exact match. If you don't, you need a solution (again, hierarchy or LDP inter area). This is a matter of choice for the SP. > If the latter, are there deployment requirements for > all area border routers to be configured the same? Currently, IGP policy allows you to have different rules (leaking or aggregation) per ABR/area but usually SPs perform the choice once and apply it globally. The draft does not change this. (I'm not sure I got your point so please elaborate on this if I missed it) > In any event, the authors note that the use of LDP in this > manner is explicitly prohibited by the LDP specification, which > requires that LDP not install a label for any prefix if that exact > prefix has not already been installed in the forwarding table by > routing. That is, LDP cannot bind a label to a prefix unless that > prefix is an exact match to one installed by routing. > > This prohibition didn't get there by accident. It was put there > deliberately to ensure that LDP cannot "second guess" routing by > creating paths which differ from the paths created by routing. Could you provide any relevant reference where the exact reason(s) for this limit is/are detailed ? In the inter area draft, LDP entirely relies on the routing decisions taken by the routing protocol. LDP DOES NOT TAKE ANY ROUTING DECISION. Could you please provide one example where the draft would have LDP create path which would differ from the paths created by routing? > After all, LDP does > not have its own procedures for deciding which path an LSP should > follow, it depends upon routing for this. Absolutely. This is definitely not changed by the draft and LSPs set up by LDP still strictly follow the path chosen by routing protocols. > The authors propose to remove this prohibition, and instead impose > a more sophisticated restriction, which they express as follows: > > "This procedure is similar to the one defined in [LDP] but > performs a longest match when searching the FEC element in the RIB > ... > > With this new longest match label mapping procedure, a LSR > receiving a Label Mapping message from a neighbor LSR for a Prefix > Address FEC Element SHOULD use the label for MPLS forwarding if > its routing table contains an entry that matches the FEC Element > and the advertising LSR is a next hop to reach the FEC. If so, it > SHOULD advertise the FEC Element and a label to its LDP peers. > > By 'matching FEC Element', one should understand an IP longest > match." > > The intention is to maintain the requirement that LDP not alter the > routing, while allowing LDP to install prefixes that are not > installed by routing. > They propose to allow LDP to install a prefix if and only if there > is a corresponding less specific prefix that has been installed by > routing. > Further, the next hop of the LDP-installed prefix is constrained to > be the same as the next hop of the less specific prefix, as > determined by routing. > This should ensure that labeled packets always follow the path that > has been determined by routing to be the best path. I understand from your conclusion that removing the LDP requirement for an *exact* match should be ok from a routing point of view. > It must be understood that this procedure adds considerable > complexity to the maintenance of the forwarding tables. > > In existing LDP deployments, all LDP does with regard to the > forwarding table is to supply the label for a given <prefix, next > hop> pair which routing has installed in the forwarding table. > In the proposal, LDP has a number of additional jobs to do with > regard to the forwarding table. Let's define some terminology: > > - Let RP be a prefix distributed by routing. > - Let NH(RP) be the next hop for RP. > - Let LP(i) be a prefix distributed by neighbor i using LDP. > - Let RP(i) be the set of LDP-distributed prefixes from neighbor i > for which RP is the longest match. > > If NH(RP) changes from i to j, things aren't too hard, as long as > RP(i) is identical to RP(j). Then you just have to change the next > hop and outgoing label for each prefix in RP(i). Agreed. BTW this is basically the current LDP behavior. The difference being that currently, at most one FEC have its next-hop change where as with the ldp-interarea draft, multiple FECs can have their next-hop changed. > It's a bit more complicated if RP(j) is different from RP(i). > Now some prefixes need to get their nh and label changed, but others > need to either be put into or pulled out of the forwarding table entirely. IMHO, the difference is the same: with the draft multiple FECs may have to be updated whereas currently the "set of LDP-distributed prefixes" is limited to 1 (or 0) element. In RFC 3036 (current LDP) for a given FEC three cases are possible following a routing change - nh and label changed - new FEC put in - FEC pulled out We have the same cases with the draft. The difference is that the set is not limited to 1 FEC and multiple FECs may need to be updated. > It's even more complicated if routing installs a new prefix, RP2, > which is more specific than an already installed prefix RP1, but > less specific than some members of RP1(i). (And possibly more > specific than some other members > of RP1(i)). Now one has to figure out which members of RP1(i) > remain in > RP1(i) and which are now in RP2(i). In fact, one has to figure this > out for each of the neighbors. Indeed the case where the routing install a new prefix more specific than the one used to match some FEC is a new behavior. Indeed for this case the LSR need to detect that the new prefix is a better match. The impact on the implementation is probably very implementation dependent so its hard to discuss it from a general point of view. One implementation strategy is that LDP ask the RIB the next hop to use for its FEC (ie the IP best match) and ask to be notified of a change. In this case, LDP only has to deal with three cases: - next-hop change - next-hop disappeared (there is no route for the FEC) - next-hop appeared (routing advertised a new route which for a FEC which had previously no route) These 3 cases would not be different from the existing cases from RFC 3036 so this would not "add considerable complexity". You would probably argue that the complexity is pushed to the RIB but BGP has exactly the same job to do to resolve its BGP Next Hop so this is definitely doable and has been so for some years. > Certainly this could all be made to work, but it's not exactly the > simple change that the draft suggests it is. The change is minimal from a specification point of view. The impact on the implementation may be significant or not but this is very implementation dependent. > It would be nice to > see section 4 replaced with a much more comprehensive treatment of > how the interaction between routing events and LDP events affects > the forwarding tables. I am not at all sure that I yet > understand all the implications. Ok. What about adding something like: "As per RFC 3036, LDP has already some interactions with the RIB. In particular, it needs to be aware of the following events: - prefix UP when a new IP prefix appears in the RIB - prefix DOWN when an existing prefix disappears - next hop change when an existing prefix have new next hop following a routing change. With the longest match procedure, some LSR reactions following a RIB event are changed: - When a new prefix appears in the RIB, the LSR MUST check if this prefix is not a better match for some existing FECs. (Eg the FEC elements 10.0.0.1/32 and 10.0.0.2/32 used the IP RIB entry 10.0/16 and a new more specific IP RIB entry 10.0.0/24 appears). This may result in changing the LSR used as next hop and hence a change of outgoing label for this FEC. - When a prefix disappears in the RIB, the LSR MUST check all FEC elements which were using this RIB prefix as best match. For each FEC, if another RIB prefix is found as best match, LDP MUST use it which may result in changing LSR used as next hop for this FEC and hence a change of outgoing label for this FEC. Otherwise, the LSR MUST remove the FEC binding and send a label withdraw message." As your detailed comments shows you've started the analysis, your input or comments on this would be welcomed. However the following protocols already have similar interactions with routing event: BGP (from BGP next hop resolution), PIM (for RPF check), mLDP (to determine the 'upstream LSR') and probably others. They don't specify this interaction so I'm not sure to understand why suddenly this becomes needed for the inter area draft. > The draft also needs a lot more consideration of "corner cases". > > For instance, what if BGP and LDP each distribute the same /32 prefix, > while IGP distributes a less specific prefix? > It's not immediately obvious > whether the LDP-distributed prefix is supposed to match the > IGP-distributed less specific prefix, thereby replacing the > BGP-distributed prefix, or whether the LDP-distributed prefix is > supposed to match the BGP-distributed > prefix. This is not really specific to the draft. I don't think that RFC 3036 specifies that the routing protocol advertising the prefix in the RIB should be taken into account. This should probably not be changed. Do you think different implementations made different choice? Ie some implementations consider the whole RIB whereas some other implementations filter out the RIB prefixes learnt through BGP? In that case: - which would be the correct behavior (originally intended by RFC 3036)? - would both implementations be compliant with RFC 3036? - that would create an interoperability issue for the ldp-interarea draft because different implementation could select different prefixes in the RIB and that could lead to permanent routing loops. So we would need to mandate only one behavior. We will select it based on your and the MPLS WG feedback. > It's also not clear just what you'd have to do if the > LDP-distributed prefix matched a BGP-distributed prefix, because the > notions of "next hop" are different. Not sure I fully got the point but it seems to me that the question is not really specific to the draft and is also valid for RFC 3036 : LDP: FEC 10.0.0.1/32 BGP NLRI 10.0.0.1/32 NH ABR IGP: ABR -> interface I > To add a further > complication, suppose that the BGP-distributed prefix is really a > labeled-IPv4 prefix. What is the relation between the > BGP-distributed label and the LDP-distributed label? > Are they both used, or not? Questions like these need to be > addressed, and > the implications of answering them one way or another need to be > considered. This is definitely not specific to the draft. The same question is already valid with the current LDP specification (RFC 3036). Additionaly we have the same question with IPv4 unicast prefixes. Usually implementations I'm aware of use some kind of administrative distance / preference ranking protocols priorities. > Here's another case worth considering. Consider an intermediate P > router, P1, with neighbors P2 and P3. Suppose: > > - IGP distributes a route to prefix X. > - Using LDP, P2 distributes a label for prefix Y, where X is the > "longest match" for Y. > - P3 does not use LDP to distribute a label for Y. > - At time t0, the next hop at P1 for X is P2. > - At time t1, the next hop at P1 for X is P3. > > I think this means that at time t1, any packets which P1 receives > that have been labeled for Y will be dropped by P1. Why? The more > specific prefix Y will have to be removed from the forwarding table > at t1, as the conditions for having it in the forwarding table no > longer seem to hold, given the P3 has not distributed a label for > the longer prefix. Thus the labels that correspond to Y will also > be removed, and packets which have already been labeled for Y will > find that there is no outgoing label corresponding to the incoming > label. This means they must be dropped. This is not specific to the draft. We have exactly the same issue with RFC 3036 (replacing "Y" by "X" and "longest match" by "exact match"). > One could dismiss this as a transient, but if the whole reason for > doing any of this is to make node protection work, i.e., to avoid the > bad effects of routing transients, then one presumably is not > willing to just dismiss transient effects. > > This scenario could occur, e.g., if a given egress PE can be reached > via two different ABRs, where one of them is implementing the draft > but the other is not (or where one is configured to use the draft > procedures but the other is not). One could dismiss this as an > operator error, This is not specific to the draft. This scenario can already occur with the current RFC 3036 if the operator forgot to configure LDP on the backup P/path. > but it can also happen if a new ABR comes up and > the distribution of routing info across the > network proceeds faster than the distribution of LDP info. This is not specific to the draft. It also already occurs with RFC 3036 if a new P comes up and the distribution of routing info across the network proceeds faster than the distribution of LDP info. (BTW it also occurs with iBGP (IGP convergence precede iBGP sessions going up and running. Hence customer traffic is dropped) > Note that on a > single link, we can implement procedures which keep the link from > coming up to routing until LDP has distributed labels over the link, > in order to keep things like this from happening. This is not specific to the draft. It's already implemented on some boxes to fixe the two above cases. > That's fine as > long as LDP operations are per-link. When we start to provide LDP > operations that have edge-to-edge significance, such as using LDP to > build a path for X across the network, we don't have any similar way > to synchronize the LDP and IGP info. I'm not sure to follow your point. Could you please be more specific? > There may be other corner cases as well. > > If a decision is made to pursue this draft, I think it needs to > have a fairly comprehensive treatment of (a) all the possible > interactions between and routing, and (b) the behavior of the scheme > when deployed in other than > the intended manner. > Also, if the main purpose of the draft is to facilitate an > LDP-based node protection scheme for border routers, it should provide > a better assurance that it doesn't leave a number of transient > conditions unprotected. > Without this additional work, I think it is difficult to tell > whether this is ultimately a direction worth pursuing. _______________________________________________ mpls mailing list mpls@lists.ietf.org https://www1.ietf.org/mailman/listinfo/mpls
- [mpls] Comments on draft-decraene-mpls-ldp-intera… Eric Rosen
- RE: [mpls] Comments on draft-decraene-mpls-ldp-in… DECRAENE Bruno RD-CORE-ISS