[bess] Shepherd's review of draft-ietf-bess-mvpn-expl-track

<stephane.litkowski@orange.com> Wed, 03 January 2018 13:10 UTC

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From: stephane.litkowski@orange.com
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Subject: [bess] Shepherd's review of draft-ietf-bess-mvpn-expl-track
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As shepherd of this document, please find below some comments that I have:

Overall comments:

-          Please add a section that contains all the abbreviations expansions: that may help non expert people to follow the acronyms without looking for the first reference in the text.

-          I usually like figures. For the intro, it may be wonderful to build a figure that reminds the existing S-PMSI/Leaf A-D procedure. So without reading the text, we can remember how it works.

-          The interAS case may also be better with a Figure and an example (or couples of).


"By originating one of these BGP routes, an ingress node advertises that
   it is transmitting a particular multicast flow."
[SLI] Is "is transmitting" correct ? Can't we have situations where an S-PMSI route is/was advertised but no traffic is flowing (no yet started or switched, or stopped).


   suppose that the ingress node wants explicit tracking for each

   individual flow that it transmits (following the procedures of

   [RFC6625] on that P-tunnel."

[SLI] Missing ")"

"This allows the

   ingress node to determine the set of egress nodes that are receiving

   flows from the ingress node."

[SLI] I think the Leaf A-D tells that there is a receiver interested by the flow, but does not tell that it actually receives it.

"   Howver, this procedure requires several clarifications:"

[SLI] There is a typo s/Howver/However/

"The procedures of [RFC6625] do not clearly state how to handle an

      S-PMSI A-D route if its NLRI contains wild cards, but its PTA

      specifies "no tunnel info"."

[SLI] I quickly ran over RFC6625, it does not mention anything on explicit tracking or Leaf A-D routes. So we assume that RFC6513/6514 only applies here.

"   *  The explicit tracking procedures do not allow an ingress node

         to "see" past the boundaries of the segmentation domain.

         This particular problem is not further addressed in this

         revision of this document.


[SLI] Do you plan to address it ? Or do we now consider it as out of scope ?

Section 2:

"Prior specifications define one flag in the PTA, the "Leaf Info

   Required" (LIR) flag, that is used for explicit tracking."

[SLI] Please point to the right reference

"If the LIR-pF flag is set in a given PTA, the LIR flag of that PTA

   SHOULD also be set."

[SLI] Why not using a MUST ?

"one forces a

   a response to be sent an egress node that does not support LIR-pF"

[SLI] Is there a missing word like 'sent by an egress node" ?

Section 3:

"The definition of "match for reception" in [RFC6625] is hereby

   modified as follows:"

[SLI] Please point to the section that you are updating.

In addition, section 3.2 of RFC6625 contains multiple if then else conditions for each cases (C-S,C-G) and (C-*,C-G). Please give some precision in where do you want to insert your new statement in this processing sequence. I guess it is at the beginning.

"When finding the "match for reception" for a given (C-S,C-G) or

      (C-*,C-G), ignore any S-PMSI A-D route that has no PTA, or whose

      PTA specifying "no tunnel information"."
[SLI] I would be in favor to introduce a normative statement here.

"We also introduce a new notion: the "match for tracking".  This

   differs from the "match for reception" as follows:"

[SLI] It would be better to give a definition of the "match for tracking" before giving the rules. Here you're explaining only the rules, not the difference in the meaning. Wouldn't it be easier to tell that the implementation MUST consider only the S-PMSI A-D routes that have a LIR flag and/or LR-pF flag set and then run the same rules as the RFC6625. Why do you want to have S-PMSI routes with PTA and LIR unset in your route list for "match for tracking" ? You need to take care here on your text proposal as one of your previous statement updates the RFC6625 and the match reception procedure, so the rules to be applied are the original one from RFC6625 and not the updated one.

"   Also note that if a match for tracking does not have the LIR flag or

   the LIR-pF flag set, no explicit tracking information will be

   generated.  See Section 5."

[SLI] Again I do not see the value added of keeping such route as a match for tracking as there is no tracking requested.

Section 4:

"Such a route could be an

       I-PMSI A-D route, a (C-*,C-G1) S-PMSI A-D route, a (C-S1,C-*)

       S-PMSI A-D route, or a (C-*,C-*) S-PMSI A-D route. "

[SLI] Is per flow explicit tracking also required for I-PMSI ? I do not see it listed in the goals of the doc ? The goal was to address wildcards S-PMSI AD routes.

"Further, if the ingress node originates a wildcard S-PMSI A-D
       route carrying a PTA specifying the tunnel to be used for
       carrying (C-S1,C-G1) traffic, and if that PTA has the LIR-pF bit
       set, then explicit tracking for (C-S1,C-G1) is requested by that
       S-PMSI A-D route.  Thus the ingress node SHOULD NOT originate a
       (C-S1,C-G1) S-PMSI A-D route whose PTA specifies "no tunnel
       info"; such a route would not provide any additional

[SLI] I do not fully understand this text in the context of your procedure 1. which deals with an origination of an (C-S1,C-G1) S-PMSI A-D route (no wildcard). As I understand the procedure for the wildcard is defined in 2.

"2. The following procedure can be used if (and only if) it is known
       that the egress nodes support the optional LIR-pF flag."

I have some issue with this sentence. It does not seem to be normative as there is no normative statement. However I understand it as a critical thing ("and only if" !) so it may require at least a SHOULD.

Then the issue I see is that this knowledge does not come from the protocol, so it sounds important to me to highlight the impact of a mistake.

BUT, reading the section 5. I understand that it is not as critical as it seems as the receiver will ignore simply the LIR-pF flag and apply the standard procedure (LIR set).

Please clarify the criticity to have or not this knowledge.

"       To terminate explicit tracking that has been initiated by an
       S-PMSI A-D route whose PTA specifies a tunnel, the ingress node
       re-originates the route without the LIR flag set"

[SLI] Do we also need to clear the LIR-pf ? It would make sense to do so.

"If the match for tracking has LIR set and if either (a) the
       egress node does not support LIR-pF, or (b) LIR-pF is not set,
       then the egress node must respond to the match for tracking,
       following procedures specified in other documents for the case
       where LIR is set."

[SLI] Please state the relevant document references here.

Section 5.2

"Note that, per RFC4364, every RD begins with a two-octet type field
   that is either 0, 1, or 2.  By adding 16 to the second octet of the
   RD, we force the type field to be 16, 17, or 18. "

[SLI] It works but we may need to ensure that the types > 16 cannot be used anymore by new applications. Moreover if new RD types are created, new siblings will have to be created.
Wouldn't it be easier to set the MSB of the RD type to 1 ? so we only lock half of the type space.
Or what could be the impact of using the RD of the local VRF of the egress node ?

Section 5.3

"In the case where the egress
   node is not a PE, but rather an ABR or ASBR, it will not know whether
   it needs to receive a given flow unless it receives a Leaf A-D route
   whose NLRI specifies that flow and whose IP-address-specific RT
   specifies an address of the egress node."

[SLI] The sentence works but when reading it I needed multiple reread to understand that the "IP-address-specific RT
   specifies an address of the egress node." was referring to the ASBR/ABR and not the receiver PE.
Do you mind using "this egress node" or "this ABR/ASBR" ?

Section 8.
[SLI] Do we have counter-measures against such "attack" ? Ingress PE dropping ?

Section 9.2
I think RFC7524 needs to be referenced as normative giving that its knowledge is required to understand section 5.3. I think that section 5.3 is also updating/complementing the procedures in RFC7524 with the "no-tunnel info" case.


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Stephane Litkowski
Network Architect
Orange Expert Future Networks
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