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

Magnus Westerlund <magnus.westerlund@ericsson.com> Wed, 05 July 2017 13:02 UTC

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Subject: [trill] Tsvart telechat review of draft-ietf-trill-mtu-negotiation-06
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Reviewer: Magnus Westerlund
Review result: Not Ready

This TSV-ART review is influenced by that I did the review of

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

    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

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

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

   3) If lowerBound >= upperBound or Step 1 has been repeated n times
      (where n is a configurable parameter whose default value is 5),

   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.