Den 2017-07-08 kl. 07:57, skrev Zhangmingui (Martin):
Hi Magnus,
Thanks for your careful review. Please see the reponses inline below.
________________________________________
From: Magnus Westerlund [magnus(_dot_)westerlund(_at_)ericsson(_dot_)com]
Sent: Wednesday, July 05, 2017 21:02
To: tsv-art(_at_)ietf(_dot_)org
Cc: draft-ietf-trill-mtu-negotiation(_dot_)all(_at_)ietf(_dot_)org;
ietf(_at_)ietf(_dot_)org; trill(_at_)ietf(_dot_)org
Subject: Tsvart telechat review of draft-ietf-trill-mtu-negotiation-06
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.
[Mingui] I copied the relevant text from RFC 6325.
"The desired minimum acceptable inter-RBridge link MTU for the
campus, that is, originatingLSPBufferSize. This is a 16-bit
unsigned integer number of octets that defaults to 1470 bytes,
which is the minimum valid value. Any lower value being
advertised by an RBridge is ignored."
So the minimum value of Sz would be 1470. IOW, IP path with MTU below 1470 will
not be qualified as an adjaceny of the TRILL network topology.
So this issue (1) is mostly bringing up what I saw as an discrepancy
between trill-over-ip and this document. I think it is quite reasonable
to push the needed extensions for more generalized IP path MTU discovery
in the Trill context onto the trill-over-ip document. However, I want to
note that from my perspective if one think one can run Trill-over-IP
then one better have a mechanism that can do fragmentation and
reassembly to handle IP MTUs that will not let trill packets of 1470
bytes through as that is quite common, in fact IPv6/UDP/Trill over
regular ethernet with 1500 bytes IP MTU is sufficient to fail that
criteria.
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.
[Mingui] This seems an implementation space. However, the document may offer
recommendations. The being recommended minimum interval between two successive
probes would affect the boot up speed of a TRILL campus. One RTT is a
reasonable value.
Okay, for this there might be some implementation variations. However,
it can clearly affect the performance of the implementation so some
recommendation are likely good. I also think a spacing of one per RTT is
quite reasonable. However, that leads to the question, how does the
sender know what RTT there is? Is there something in the TRILL protocol
that will determine the RTT and have a current value? Sorry, I don't
have time to digest the whole TRILL suit of specifications.
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?
[Mingui] Yes, this makes sense. An MTU-ack should be considered to have failed
two RTT after the probe is sent out.
So two questions on this. First, can a receiver know which MTU probe it
gets response to, i.e. are there some token or sequence number being
acked? Second, 2*RTT appear to be to short to make that conclusion in.
The reasons I say so is that there will be networks where the jitter of
the path is larger than the RTT sample, thus leading to
misinterpretation. Thus, I would recommend a bit more robustness to
misinterpretation and likely some minimal value that avoids low latency
paths wrongly classify the path. Note, I think it is needed to define
what the criteria is for when an MTU is considered lost, just because it
appears that it affects both startup and robustness of the probing. It
also becomes a question of what tools in the probes that are used for
this, and if you actually need some additional ones.
4. Section 3, the algorithm in Step 1 is unable to reach the first termination
condition (3) "If lowerBound >= upperBound" in some cases.
[Mingui] This algorithm has been updated through a few rounds of revisions. Let
me insert a few minor updates to the cited text as below.
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.
[Mingui] s/uppperBound is set to x/uppperBound is set to x-1/
[Mingui] s/rounded up to the nearest integer./rounded down 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.
[Mingui] s/rounded up to the nearest integer./rounded down to the nearest
integer./
[Mingui] Append one condistion to this step 2): If lowerBound equals
upperBound-1 then x is set to upperBound.
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.
[Mingui] After the update of the text, the sequence would become:
Lower Upper X
1470 9216 5343
5343 9216 7279
7279 9216 8247
7279 8246 7762
7762 8246 8004
7762 8003 7882
7882 8003 7942
7882 7941 7911
7911 7941 7926
7926 7941 7933
7933 7941 7937
7933 7936 7934
7934 7936 7935
7935 7936 7935
Shouldn't the above line result in that X becomes 7936, as the probe
before it succeeds, and then the new additional rule in step (2) then X
becomes upper bound.
7935 7936 7936
7935 7935 7935
The second I notice is that having a limitation on number of steps as 5,
[Mingui] Since the testing might be too resource consuming, implementors suggested
this limitation. Afterall, the purpose of testing a Lz value is to improve the
efficiency (if Lz > Sz) rather than reach the optimal efficiency.
Ok.
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.
[Mingui] Because the MTU search will NOT be used to determine a new possible
value for Sz. It is only applicable to Lz.
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.
[Mingui] As specified in Section 3, “link MTU size” is already set to the lower bound. This tested
“link MTU size” is the “TU” value. This value is potentially larger than Sz as explained in
the introduction and Section 2.
Ok, I think I am getting what the different values are for.
Cheers
Magnus Westerlund
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