Early Review of draft-litkowski-rtgwg-spf-uloop-pb-statement-02
review-litkowski-rtgwg-spf-uloop-pb-statement-02-rtgdir-early-shand-2015-05-13-00

  
  
    I have been assigned as Routing Directorate QA reviewer for this
    document. 




The following web page contains a briefing on
      the QA process.




​

https://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDirDocQa










The document provides a useful summary of
        the issues leading to micro-loop formation, especially in mixed
        vendor networks and provides examples of the possibilities to
        adversely affect the the micro-loop behaviour by inconsistent
        choices of parameters and algorithms among the routers in the
        network.







I confess that I find the timing diagrams quite
      hard to follow. One of the penalties of being constrained by the
      need to use ASCII art. But I wonder if something could be done to
      make them a little easier to follow without losing the essential
      information?




There are some very simple mitigation
        techniques, such as delaying the locally triggered  SPF/FIB
        installation more than a remotely triggered one, which it may be
        helpful to mention. 







The document goes on to propose future
        work to standardise some behaviours.







Clearly this work is at an early stage and
        the trade-offs between standardisation and allowing vendors
        freedom to innovate for the benefit of their customers must be
        carefully considered.







The document seems like a good starting
        point for this work.  










In reading the document I spotted a few items
      which it would be as well to address.





      General:





      1. micro-loop or microloop. The terminology is used
      inconsistently. RFC 5715 uses micro-loop


      2. There are numerous instances of awkward usage of english. It
      would be helpful to address these at some stage.





      Nits:





      3. "   We will call SPF delay, the delay timer that exists in most


         implementations that makes codes to wait before running SPF


         computation after a SPF trigger is received."





      The phrase "makes codes to wait" is somewhat contrived. How about
      "that specifies the required delay"?





      4. "   Routers have more and more powerful controlplane and
      dataplane that


         reduce the Control plane to Forwarding plane overhead during
      the


         convergence process.  Even if FIB update is still reasonably
      the


         highest contributor in the convergence time for large network,
      its


         duration is reducing more and more and may become comparable to


         protocol timers.  This is particular true in small and medium


         networks."





      I don't understand what is meant by "may become comparable to
      protocol timers"? Are you suggesting that the FIB update latency
      WAS greater than the protocol timers, but has now been reduced to
      a comparable value? 





      The reference to small and medium networks is also confusing,
      since in my experience it is actually the small and medium
      networks which are subject to the LARGEST FIB update times as a
      result of the deployment of under powered hardware. 








      5. "   In multi vendor networks, using different implementations
      of a link


         state protocol may favor micro-loops creation during
      convergence time


         due to deprecancies of timers."





      deprecancies? Do you mean discrepancies?





      6. "4.2 Exponential Backoff"





      "   o  First delay : amount of time to wait before running SPF. 
      This


            delay is used on when SPF is in fast mode."





      I assume "is used only when SPF" is what you mean.





      and similarly in the next bullet











      Mike