Subject: Opsdir last call review of draft-ietf-teas-sr-rsvp-coexistence-rec-02

From: Al Morton <[email-masked]>
Date: Thu, 12 Apr 2018 17:12:48 -0700

Reviewer: Al Morton
Review result: Has Nits

    Recommendations for RSVP-TE and Segment Routing LSP co-existence
             draft-ietf-teas-sr-rsvp-coexistence-rec-02.txt

Overall:
This memo considers coexistence of SR LSPs and RSVP-TE LSPs,
or migration to SR, and lists 5 methods as solutions to the
problems posed.  Although only one of the solutions (3.5) is
developed in detail, the text never makes a clear recommendation
among the 5 solutions. At least two seem viable (3.1 and 3.5).
Operators would probably appreciate a clear recommendation from 
the authors, if possible.

Edits/nits follow, see [acm]:

1.  Introduction

   The problem space can be generalized as a dark bandwidth problem to
   cases where any other service exists in the network that runs in
   parallel across common links and whose bandwidth is not reflected in
   the available and reserved values in the TED.  The general problem is
   management of dark bandwidth pools and can be generalized to cases
   where any other service exists in the network that runs in parallel
   across common links and whose bandwidth is not reflected in the
   available and reserved values in the TED. 
[acm]
You've written two sentences above that essentially say the same thing.
Although it appears in the Abstract, TED should be spelled-out in the 
body text.

   In most practical
   instances given the static nature of the traffic demands, limiting
   the available reservable bandwidth available to RSVP-TE has been an
   acceptable solution.  However, in the case of SR traffic, there is
   assumed to be very dynamic traffic demands and there is considerable
   risk associated with stranding capacity or overbooking service
   traffic resulting in traffic drops.

   The high level requirements or assumptions to consider are:

   1.  Placement of SR LSPs in the same domain as RSVP-TE LSPs MUST NOT
       introduce inaccuracies in the TED used by distributed or
       centralized path computation engines.

   2.  Engines that compute RSVP-TE paths MAY have no knowledge of the
       existence of the SR paths in the same domain.
[acm] suggest 
s/2.../Knowledge of the
       existence of the SR paths in the same domain is OPTIONAL for
	   engines that compute RSVP-TE paths./
or, s/MAY/may/ since this is only an assumption?

   3.  Engines that compute RSVP-TE paths SHOULD NOT require a software
       upgrade or change to their path computation logic.

   4.  Protocol extensions SHOULD be avoided or be minimal as in many
       cases this co-existence of RSVP-TE and SR MAY be needed only
       during a transition phase.

   5.  Placement of SR LSPs in the same domain as RSVP-TE LSPs that are
       computed in a distributed fashion MUST NOT require migration to a
       central controller architecture for the RSVP-TE LSPs.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Solution options

3.1.  Static partitioning of bandwidth

   In this model, the static reservable bandwidth of an interface can be
   statically partitioned between SR and RSVP-TE and each can operate
   within that bandwidth allocation and SHOULD NOT preempt each other.

   While it is possible to configure RSVP-TE to only reserve up to a
   certain maximum link bandwidth and manage the remaining link
   bandwidth for other services, this is a deployment where SR and RSVP-
   TE are separated in the same network (ships in the night) and can
   lead to suboptimal link bandwidth utilization not allowing each to
   consume more, if required and constraining the respective
   deployments.

   The downside of this approach is the inability to use the reservable
   bandwidth effectively and inability to use bandwidth left unused by
   the other protocol.
[acm] 
... so this option satisfies all requirements and assumptions?

3.2.  Centralized management of available capacity

   In this model, a central controller performs path placement for both
   RSVP-TE and SR LSPs.  The controller manages and updates its own view
   of the in-use and the available capacity.  As the controller is a
   single common entity managing the network it can have a unified and
   consistent view of the available capacity at all times.
[acm]
Comment and a question:
This also makes the central controller a single point of failure.
What is the Recovery & Restoration Strategy? (care to cite a reference?)

   A practical drawback of this model is that it requires the
   introduction of a central controller managing the RSVP-TE LSPs as a
   prerequisite to the deployment of any SR LSPs.  Therefore, this
   approach is not practical for networks where distributed TE with
   RSVP-TE LSPs is already deployed, as it requires a redesign of the
   network and is not backwards compatible.  This does not satisfy
   requirement 5.

   Note that it is not enough for the controller to just maintain the
   unified view of the available capacity, it must also perform the path
   computation for the RSVP-TE LSPs, as the reservations for the SR LSPs
   are not reflected in the TED.  This does not fit with assumption 2
   mentioned earlier.
[acm] 
... So, this option fails to satisfy key requirements and assumptions.
(suggest to state that)

...

3.5.  TED consistency by reflecting SR traffic

...

   The following methodology can be used at every TE node for this
   solution:
[acm]
sugest to identify this list as Parameters for the methodology.
s/solution:/solution, using the following parameters:/

   o  T: Traffic statistics collection time interval

   o  N: Traffic averaging calculation (adjustment) interval such that N
      = k * T, where k is a constant integer multiplier greater or equal
      to 1.  Its purpose is to provide a smoothing function to the
      statistics collection.
[acm]
k should be a separate list item, preceding N.

---

From: Harish Sitaraman <[email-masked]>
Date: Mon, 16 Apr 2018 18:57:16 +0000

Hi Al,

Thanks for the review comments. Comments are inline [HS]. Let me know if you've further questions. 

----
On 4/12/18, 5:12 PM, "Al Morton" <[email-masked]> wrote:

    Reviewer: Al Morton
    Review result: Has Nits

    Overall:

[HS] The intent behind the recommendations is to not take a position on which solution is preferable. All solutions are valid but some do not satisfy all the requirements. However, if a solution is acceptable (based on their deployment of SR and RSVP and knowing which requirements are not satisfied) for an operator then such a solution can be chosen.

    1.  Introduction

[HS] Thanks for catching this. We can remove the 2nd sentence and TED can be again expanded in the Introduction.
    
       In most practical

    

[HS] We can s/MAY/may/ instead. The existing deployed path computation engines for RSVP-TE in the network may not know about the SR paths as they could be computed by another entity. As an assumption, this requirement is basically stating that solutions to co-exist RSVP-TE and SR should not assume or mandate that existing RSVP-TE path compute engines will know about the SR paths.

    

    
    3.  Solution options

    

[HS] Correct. The document points out specific requirements that are not satisfied under such solutions.

[HS] It is not necessarily a single point of failure as it depends on the deployment model of the controller in the network. The wording “single common entity” generically refers to a centralized controller managing path computation for all RSVP-TE and SR paths in the network. Recovery and restoration of the controller seems orthogonal to how the operator chooses to deploy 3.2.

[HS] Correct. The above text states that 5 and 2 are not satisfied.
    
    ...

    ...

[HS] Ok. New text: “The following methodology can be used at every TE node for this solution, using the following parameters:”

[HS] Ok. The new text is

  o k: The number of traffic statistics samples that can provide a smoothing function 
     to the statistics collection. The value of k is a constant integer multiplier greater or equal
     to 1.  
  o  N: Traffic averaging calculation (adjustment) interval such that N = k * T.

--
Harish

---

From: Harish Sitaraman <[email-masked]>
Date: Thu, 26 Apr 2018 13:43:44 +0000

Hi Al,

We've posted a -03 version of the draft. Let us know if it helps with your comments based on the clarifications below.

Harish

On 4/16/18, 11:57 AM, "Harish Sitaraman" <[email-masked]> wrote:

    Hi Al,

    ----
    
        Reviewer: Al Morton
        Review result: Has Nits

        Overall:

        1.  Introduction

        
           In most practical

        

        

        
        
        3.  Solution options

        

        

        

        
        ...

        ...

        

    [HS] Ok. The new text is

    --
    Harish