Re: [trill] Tsvart telechat review of draft-ietf-trill-mtu-negotiation-06

[Warren Kumari <[e-mail]>]

I wanted to quickly thank Magnus for this review - it is well thought
out, and clear.

I usually focus on the OpsDir reviews, but wanted to explicitly call
this out as a good one.

W

On Wed, Jul 5, 2017 at 9:02 AM, Magnus Westerlund
<[e-mail]> wrote:
> Reviewer: Magnus Westerlund
> Review result: Not Ready
>
> This TSV-ART review is influenced by that I did the review of
> draft-ietf-trill-over-ip.
>
> 1. So draft-ietf-trill-over-ip-10 has MTU discovery needs for
> determining if the UDP encapsulation will work or not. It references  in
> Section 8.4 the old RFC, i.e. RFC 6325, which is updated by
> draft-ietf-trill-mtu-negotiation.
>
>     TRILL IS-IS MTU PDUs, as specified in Section 5 of [RFC6325] and in
>     [RFC7177], can be used to obtain added assurance of the MTU of a
>     link.
>
> However, this is not quite true, as if the IP path MTU is below 1470
> bytes, which is not unheard of, the algorithm in the MTU negotiation
> draft can't determine it. It will only report the IP path as having an
> MTU to small when the 1470 bytes probe fail.
>
> So, if the trill-over-ip authors want to use this as a mechanism, then the MTU
> negotiation draft needs to be expanded to have more flexible lower
> boundaries. However, that appear to affect MTU negotiation quite
> significant as it needs to separate algorithm for finding MTU, from the
> different usage of the algorithm with different starting points. Where
> the normal will have a lower bound of 1470, and be more tightly coupled
> to Sz when finding Lz. While the Trill-over-IP has a different usage.
>
> I think the trill WG needs to decide on how to slice this. If the
> MTU-negotiation only targets the explicit targets in the current draft and goes
> forward now. Or if they want to meet trill-over-ip's goals which will require
> restructuring.
>
> 2. Another issue, is that I think the algorithm is a bit short on
> transmission scheduling recommendations:
>
>     1) If RB1 successfully receives the MTU-ack from RB2 to the probe of
>        the value of link-wide Lz within k tries (where k is a
>        configurable parameter whose default is 3), link MTU size is set
>        to the size of link-wide Lz and stop.
>
> If I do this test with all three packets back to back at line rate, I could
> potentially get all probes lost in the same burst loss in router queue or
> switch fabric. What I think is needed here is a specification on how these
> probes are transmitted. Spaced in a particular way, or at least minimal
> distance, and are the additional probes only sent after the previous has been
> judged to have been lost, which makes it interact with the next issue.
>
> 3. This is also unclear on what the criteria is for determining that something
> is lost:
>
>       a) If RB1 fails to receive an MTU-ack from RB2 after k tries, RB1
>           sets the "failed minimum MTU test" flag for RB2 in RB1's Hello
>           and stop.
>
> I fail to see any specification for the criteria when an MTU-ack should be
> considered to have failed to reach the probing entity. So this appear to
> require a timeout, and thus a timeout interval. Is the RTT known so that one
> can define something as lost after N*RTT? Are there possible delays in sending
> the MTU-ack that are considered okay that can affect this?
>
> 4. Section 3, the algorithm in Step 1 is unable to reach the first termination
> condition (3) "If lowerBound >= upperBound" in some cases.
>
>   Step 1: RB1 tries to send an MTU-probe padded to the size x.
>
>    1) If RB1 fails to receive an MTU-ack from RB2 after k tries:
>
>          upperBound is set to x and x is set to [(lowerBound +
>          upperBound)/2], rounded up to the nearest integer.
>
>    2) If RB1 receives an MTU-ack to a probe of size x from RB2:
>
>          link MTU size is set to x, lowerBound is set to x and x is set
>          to [(lowerBound + upperBound)/2], rounded up to the nearest
>          integer.
>
>    3) If lowerBound >= upperBound or Step 1 has been repeated n times
>       (where n is a configurable parameter whose default value is 5),
>       stop.
>
>    4) Repeat Step 1.
>
> I run this on the input data: Lower bound = 1470, Upper bound = 9216 and with
> an MTU of 7935 and gets the following sequence:
>
> Lower   Upper   X
> 1470    9216    5343
> 5343    9216    7280
> 7280    9216    8248
> 7280    8248    7764
> 7764    8248    8006
> 7764    8006    7885
> 7885    8006    7946
> 7885    7946    7916
> 7916    7946    7931
> 7931    7946    7939
> 7931    7939    7935
> 7935    7939    7937
> 7935    7937    7936
> 7935    7936    7936
> 7935    7936    7936
>
> Thus, the termination condition needs to change. The second I notice is that
> having a limitation on number of steps as 5, results in quite a large gap
> between upper and lower bound in which the MTU exists in.
>
> 5. I frankly gets confused by the application of the binary search. First it
> will in many case not be run to termination where the actual MTU is determined.
> Then the result of the upper and lower bound are just used to confirm the Sz
> value. There are no discussion about using the MTU search to determine a new
> possible value for Sz. The text is not even explicit that lower bound is the
> highest known to work Transmission unit size at the time of testing. I think
> section 3, should conclude in determine some TU value, and if that is Sz or
> something other appears quite relevant for what to do in the later sections.
>
>



-- 
I don't think the execution is relevant when it was obviously a bad
idea in the first place.
This is like putting rabid weasels in your pants, and later expressing
regret at having chosen those particular rabid weasels and that pair
of pants.
   ---maf