Speaking as WG member:
Hi Gunder, Tony, Les,
I'm also not a fan of the Multi-Part TLV Capability flag. While the intent of
the draft is to encourage multi-part TLV advertisement and usage, the addition
of this flag and the requirement for advertisement will most likely have the
opposite effect. Given that IS-IS implementations are already advertising
multi-part TLVs but none are advertising the proposed capability flag,
implementation of the draft as currently written would inhibit Multi-Part TLV
usage and be a step backwards. Of course, we know implementations that are
already advertising these multi-part TLVs will most likely ignore the
recommendation and continue to advertise them even when not all IS-IS routers
within the scope of the Multi-Part TLV advertise the capability.
Rather, I propose that the draft eliminates the capability flag and introduces
a recommended configuration parameter that would allow Multi-Part TLVs to be
suppressed. The recommended default would be FALSE. This would provide an out
if these Multi-Part TLVs did, in fact, have dire consequences.
Thanks,
Acee
On 8/25/22, 6:53 AM, "Lsr on behalf of Van De Velde, Gunter (Nokia -
BE/Antwerp)" <lsr-boun...@ietf.org on behalf of gunter.van_de_ve...@nokia.com>
wrote:
Inline: GV>
From: Tony Li <tony1ath...@gmail.com> On Behalf Of Tony Li
Sent: Wednesday, August 24, 2022 5:26 PM
To: Van De Velde, Gunter (Nokia - BE/Antwerp)
<gunter.van_de_ve...@nokia.com>
Cc: lsr <lsr@ietf.org>
Subject: Re: [Lsr] New Version Notification for
draft-pkaneria-lsr-multi-tlv-01.txt
Hi Gunter,
I am having troubles understanding the value of ‘The Multi-part TLV
Capability’ flag.
What would break if ‘Multi-part TLV Capability’ flag would not exist?
A system that supported MP-TLVs would not be able to determine that there
were other systems in the area that did not support MP-TLVs. The system might
then advertise MP-TLVs and they would be misinterpreted or cause system crashes
in the systems that did not support them.
GV> crashes? I really hope that is not happening.
GV> When a legacy router receives MP-TLVs from another system and legacy
router has no support for handling MP-TLV, then yes, things get misinterpreted.
There is nothing wrong with that, is there? Do you have an example where things
go wrong?
If we want to introduce MP-TLVs, that change would warrant the existence of
the flag.
GV> I am not convinced yet how a MP-TLV catch-all flag would make ISIS
behave better
I dispute that a binary flag warrants the word ‘complexity’.
GV> living without binary flag is simpler and less complex then dealing
with a binary flag. (i.e. what, when, how, why, who sets this flag?)
Note: thoughts about the flag: What if a system by accident sends
flip-flopping (set/unset/set/unset/etc) of this flag?
Then other systems might misinterpret the results and generate inconsistent
TLVs. That would be bad.
GV> correct, no good at all.
What if an advertising system support multi-tlv for TLV ‘A’ but not for TLV
‘B’?
We are not allowing that level of granularity. A system that is going to
support MP-TLVs should take care to operate correctly for ALL TLVs before
advertising that it supports them.
GV> I suspect that 'ALL TLVs' is a reference to ALL TLVs supported by the
local system. This means that e.g. when new TLVs would be supported after a
system upgrade, that the operator has to be aware and correct the flag during
each single upgrade.
GV> Unfortunately I remain to have troubles understanding the value
"Multi-part TLV Capability’ flag brings to an ISIS network.
* Without flag it is indeed uncertain if area wide mp-tlv is supported
(sub-optimal).
* but with catch all MP-TLV flag I am not sure we improve ISIS operation:
** Who guarantees that the flag is set correctly on all systems at all
times
** Maybe all systems falls back to advertise single TLV because another
(legacy?) system advertise a wrong flag (sub-optimal)
** Legacy system with MP-TLV support gets upgraded and now supports
additional TLVs but not with MP-TLV... ?manual intervention? (sub-optimal)
** what, when, how, why, who sets the MP-TLV flag? What with flapping of
MP-TLV flag (undefined)
G/
Tony
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