Re: [mpls] Comments on draft-ietf-mpls-spring-entropy-label

[" 徐小虎(义先) " <[e-mail]>]

Hi Stephane,

Thanks for your reply. Please see my response inline with [Xiaohu]
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From:stephane.litkowski <[e-mail]>
Send Time:2018年7月6日(星期五) 06:19
To:徐小虎(义先) <[e-mail]>; [e-mail] <[e-mail]>
Cc:[e-mail] <[e-mail]>; [e-mail] <[e-mail]>
Subject:RE: [mpls] Comments on draft-ietf-mpls-spring-entropy-label


Hi,

Thanks for your comment.
Pls find some inline replies

Brgds,

Stephane

From: mpls [mailto:[e-mail]] On Behalf Of ???(??)
Sent: Thursday, July 05, 2018 05:34
To: [e-mail]
Cc: [e-mail]; [e-mail]
Subject: [mpls] Comments on draft-ietf-mpls-spring-entropy-label
Hi all,

I have the following comments and hope it' s not too late.

1. In fact, RFC6790 doesn't require intermediate routers to have the capability of performing EL-based load-balancing mechanism. Instead, it just provides an entropy in the MPLS packet which may be available for intermediate routers to perform load-balancing.  In contrast, the recommended approach as defined in draft-ietf-mpls-spring-entropy-label requires the ingress of a given SR-TE path to take into account the ERLD capability of all intermediate routers on that path. However, in the loose explicit route case, those intermediate routers that the explicit path traverses may change over time due to IGP convergence or there may exist multiple ECMPs from one segment towards the next segment. That would make the ELI/EI imposition decision much complex. I personally believe that the principle used in RFC6790 would make the implementation and deployment much easier and therefore should be kept.

[SLI] Using SRTE and label stacking is not different from nested LSP. Each tail-end of a segment should be ELC to ensure that it can pop the ELI/EL if the ingress decides to push it. The current text does not mandate anything regarding the analysis of transit nodes. It says without using normative language that the implementation may try to find the minimum ERLD along the path. But behaving as RFC6790 is for sure simpler and is compliant.

[Xiaohu] If so, I wonder whether it should be Informational rather than Standard Track?


2. It said in section 4 that "
   The Entropy Readable Label Depth (ERLD) is defined as the number of
   labels a router can both:
 
   a.  Read in an MPLS packet received on its incoming interface(s)
       (starting from the top of the stack).
 
   b.  Use in its load-balancing function.
:

However, it said later that:

 To advertise an ERLD value, a SPRING router:
 
   o  MUST be entropy label capable and, as a consequence, MUST apply
      the dataplane procedures defined in [RFC6790].
 
   o  MUST be able to read an ELI/EL which is located within its ERLD
      value.
 
   o  MUST take into account this EL in its load-balancing function.

Why should intermediate routers be required to meet the first requirement (e.g. the ELC as defined in RFC6790 ) if they would never be used as an LSP egress?
[SLI] If they are pure transit for a node SID, there is no need for them to advertise the ERLD.

[Xiaohu] Yes there is no need for them to advertise the ELC. However, there is a need for them to advertise the capability of reading the maximum label stack depth and performing EL-based load-balancing, if I understood it correctly. IMHO, it seems better that the ELC and the ERLD are defined as two independent capabilities, in other words, the ERLD semantics should not include the ELC semantics (i.e., the first requirement should be removed). 

3. Section 5 introduces the MSD concept. I wonder whether this concept is aligned with the MSD concept as defined in the PCEP-SR draft or the MSD concept as defined in the IGP-MSD draft. In PCEP-SR draft, it said "
The "Maximum SID Depth" (1
   octet) field (MSD) specifies the maximum number of SIDs (MPLS label
   stack depth in the context of this document) that a PCC is capable of
   imposing on a packet.
 
In the IGP-MSD draft, it said "
MSD of type 1 (IANA Registry), called Base MSD is used to signal the
   total number of SIDs a node is capable of imposing, to be used by a
   path computation element/controller.  "
 
If I understand it correctly, the MSD in this draft==the MSD in PCEP-SR draft==the Base MSD (i.e., the MSD of type 1), No?

[SLI] Today, the two IGP drafts does not seem to agree on the definition
ISIS says:” Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
   labels a node is capable of imposing, including all
   service/transport/special labels.”
OSPF says:” MSD of type 1 (IANA Registry) is used to signal the number of SIDs a
   node is capable of imposing, to be used by a path computation
   element/controller and is only relevant to the part of the stack
   created as the result of the computation.”

MSD is just MSD is defines a maximum number of labels to be pushed. This is the definition we use and it is compliant with the one used in PCEP-SR:
“The "Maximum SID Depth" (1
   octet) field (MSD) specifies the maximum number of SIDs (MPLS label
   stack depth in context of this document) that a PCC is capable of
   imposing on a packet.”

As we also say: “This includes any kind of labels (service, entropy, transport...).”, we are compliant with the BMI-MSD defined in IS-IS.

[Xiaohu] Oh, Should the semantics of MSD be unified among the relevant drafts ASAP? Otherwise, it would introduce unnecessary confusion to implementors and operators.

Best regards,
Xiaohu



Best regards,
Xiaohu
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