Last Call Review of draft-ietf-trill-mtu-negotiation-05
review-ietf-trill-mtu-negotiation-05-genart-lc-carpenter-2017-06-23-00

Gen-ART Last Call review of draft-ietf-trill-mtu-negotiation-05

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Document: draft-ietf-trill-mtu-negotiation-05.txt
Reviewer: Brian Carpenter
Review Date: 2017-06-24
IETF LC End Date: 2017-06-28
IESG Telechat date: 2017-07-06

Summary: Ready with issues
--------

Minor issues: 
-------------

> 2. Link-Wide TRILL MTU Size
...
>   ...RBridges MUST support the Extended L1 Circuit-Scoped
>   (E-L1CS) flooding scope LSP (FS-LSP). They use that flooding to
>   exchange their maximally supportable value of "Lz".

Where does that value come from? Is it configured, derived from
the interface in some way, or discovered?

> 2.1. Operations
>
>   Lz is reported using a originatingSNPBufferSize TLV that MUST occur
>   in fragment zero of the RBridge's E-L1CS FS-LSP. An
>   originatingSNPBufferSize APPsub-TLV occurring in any other fragment
>   is ignored.

Is that really what you mean? I thought Lz was an optional extra. So I think
you mean:

2.1. Operations

   Lz MAY be reported using a originatingSNPBufferSize TLV that occurs
   in fragment zero of the RBridge's E-L1CS FS-LSP. An
   originatingSNPBufferSize APPsub-TLV occurring in any other fragment
   MUST be ignored.

> 3. Link MTU Size Testing
...
>   Step 0:
...
>      b) Link MTU size is set to 1470, lowerBound is set to 1470,
>         upperBound is set to the link-wide Lz, linkMtuSize is set to
>         [(lowerBound + upperBound)/2] (Operation "[...]" returns the
>         fraction-rounded-up integer.).  

This is confusing. "linkMtuSize" was defined as a local variable.
But what is "Link MTU size"? Is that another local variable?
If so, how is it different from "linkMtuSize"?
It is used again in part 2) of step 2 below.

Also, I assume "Lz" is the value previously agreed among the nodes,
but that should be made clear to the reader.

Nits:
-----

> 1. Introduction
...
>   topology. While in this document, a new RECOMMENDED link-wide minimum
>   inter-RBridge MTU size, Lz, is specified. By calculating a using Lz
>   as specified herein, link-scoped PDUs can be formatted greater than
>   the campus-wide Sz up to the link-wide minimum acceptable inter-
>   RBridge MTU size potentially improving the efficiency of link
>   utilization and speeding link state convergence.

I cannot parse those two sentences. What does the "While" refer to? 
What does "By calculating a using Lz" mean?

> 3. Link MTU Size Testing
...
>      b) Link MTU size is set to 1470, lowerBound is set to 1470,
>         upperBound is set to the link-wide Lz, linkMtuSize is set to
>         [(lowerBound + upperBound)/2] (Operation "[...]" returns the
>         fraction-rounded-up integer.).  

This would be easier to understand:

3. Link MTU Size Testing
...
      b) Link MTU size is set to 1470, lowerBound is set to 1470,
         upperBound is set to the link-wide Lz, linkMtuSize is set to
         [(lowerBound + upperBound)/2], rounded up to the nearest integer.

Repeat this in the following two cases; a normal reader will not
remember the rounding rule:

...
   1) If RB1 fails to receive an MTU-ack from RB2 after k tries: 

         upperBound is set to linkMtuSize and linkMtuSize is set to
         [(lowerBound + upperBound)/2], rounded up to the nearest integer.

   2) If RB1 receives an MTU-ack to a probe of size linkMtuSize from
      RB2:

         link MTU size is set to linkMtuSize, lowerBound is set to
         linkMtuSize and linkMtuSize is set to [(lowerBound +
         upperBound)/2], rounded up to the nearest integer.

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