Hi Tony,
My responses are inline below with [HC].
Best Regards,
Huaimo
practically speaking "backwards compatibility" here is restricted by the fact
that
1) we have in most largest networks de facto mp-tlv deployed and relied on for
operations implemented by all major vendors.
2) we cannot encode mp-tlv deployed in parallel with "something new that we
flip over once e'one has it" since the encoding space is limited and there are
deployments that consume on some node so many fragments re-encoding twice until
cut over would immediately break those networks
[HC]: We have encoded TLVs > 255 in two different ways (or say, deployed in
parallel) already.
RFC 8202 specifies one way for IID-TLV (TLV 7) > 255 on page 4-6 (quoted some
text below for your convenience).
“A single IID-TLV will support advertisement of up to 126 ITIDs. If
multiple IID-TLVs are present in an IIH PDU, the supported set of
ITIDs is the union of all ITIDs present in all IID-TLVs.”
RFC5311 defines another way for extended IS reachability TLV 22 and MT-ISN TLV
222 > 255, in which two new TLVs: IS Neighbor Attribute TLV 23 and MT IS
Neighbor Attribute TLV 223, are defined. Some texts from RFC 5311 are quoted
below for your convenience.
“If the attribute information does not conflict, it MUST be
considered additive.”
“In cases where information about the same neighbor/link/attribute
appears in both TLV 22 and TLV 23 (or TLV 222 and TLV 223 for the
same MTID) then the information in TLV 22 (or TLV 222) MUST be used
and the information in TLV 23 (or TLV 223) MUST be ignored.”
“Utilization of the new TLVs for neighbor attribute information would
provide additional benefits that include:
Easier support for a set of TE information associated with a single
link that exceeds the 255-byte TLV limit by allowing the
interpretation of multiple TLVs to be considered additive rather
than mutually exclusive.”
These two different ways do not affect each other. There is no flip over.
A new way for TLVs > 255 will not affect any existing way. There will not be
any flip over.
Corollary I: a flag day on such networks is NOT possible unless it's seamlessly
flipping to something new while disabling old
[HC]: Corollary 1 seems not valid since the Theorem on which Corollary 1 based
is not valid. The Theorem is 1) and 2) above. The Theorem is not valid (or
true) since 2) is false.
Collorary II: no matter how many times something that does not meet those 2
criteria is suggested is repeated ad nauseam it will not make it practically
relevant or feasible.
[HC]: Corollary 2 seems not valid since the Theorem on which Corollary 2 based
is not valid.
what we need is documentation to the extent possible of existing mp-tlv and
framework for new tlv/sub-tlv/sub.. to describe intended behavior in case of
mp-tlv of those. All in distributed fashion without gating it on a single
include file ;-) gathering documentation about existing is laudable and could
be attempted in an application draft if someone is willing (TonyL's statements
about that are true however). mp-tlv draft should probably add that every new
draft doing new tlv/sub-tlv has to provide a mp-tlv section then (and make sure
that it does not break recursion of all including parent hierarchy in some way
[i.e. it's of no use to say that a sub-tlv has mp-tlv capability if the partent
doesn't since the parent cannot repeat and will never have space hence for more
than one sub-tlv already due to length restrictons)
[HC]: Some issues on the draft are raised in the LSR mailing list.
all that has been repeated enough already and no matter of wishful thinking and
ASCII text will shift this reality on the ground
[HC]: More discussions, more understanding on MP-TLV and Big-TLV.
-- tony
On Tue, Dec 5, 2023 at 8:11 AM Les Ginsberg (ginsberg)
<[email protected]<mailto:[email protected]>> wrote:
Folks –
The term “backwards compatibility” is getting abused here.
What does “backwards compatibility” mean in the context of a routing protocol
like IS-IS?
It means that protocol extensions can be advertised and safely used in the
presence of legacy nodes (which do not understand the new advertisements).
Neither MP nor Big-TLV are backwards compatible.
The authors of MP draft do not claim it is backwards compatible.
The authors of Big-TLV claim it is “backwards compatible” – but this is a false
statement. Any attempt to use Big-TLV advertisements in the presence of legacy
nodes will result in inconsistent and potentially dangerous behavior.
Big-TLV authors like to say “you can send Big-TLV but not use it until all
nodes support it” – but this does meet the criteria for backwards
compatibility. If Big-TLV were “backwards compatible” there would be no need
for a capability advertisement to determine when it is safe to use the
advertisements.
Les
From: [email protected]<mailto:[email protected]>
<[email protected]<mailto:[email protected]>>
Sent: Monday, December 4, 2023 10:35 PM
To: Huaimo Chen <[email protected]<mailto:[email protected]>>;
Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>>; Tony
Li <[email protected]<mailto:[email protected]>>; Linda Dunbar
<[email protected]<mailto:[email protected]>>
Cc: Yingzhen Qu <[email protected]<mailto:[email protected]>>;
[email protected]<mailto:[email protected]>;
lsr <[email protected]<mailto:[email protected]>>
Subject: Re: Re: [Lsr] WG Adoption Call - draft-pkaneria-lsr-multi-tlv
(11/17/2023 - 12/09/2023)
Hello All,
It seems Big-TLV is backward compatible. Backward compatible is an important
point that should be considered when we introduce new features in a protocol,
especially the widely used protocols like ISIS, BGP etc.
________________________________
Best Regards,
Zhenqiang Li
China Mobile
[email protected]<mailto:[email protected]>
From: Huaimo Chen<mailto:[email protected]>
Date: 2023-12-04 21:57
To: Les Ginsberg (ginsberg)<mailto:[email protected]>; Tony
Li<mailto:[email protected]>; Linda Dunbar<mailto:[email protected]>
CC: Yingzhen Qu<mailto:[email protected]>;
[email protected]<mailto:[email protected]>;
lsr<mailto:[email protected]>
Subject: Re: [Lsr] WG Adoption Call - draft-pkaneria-lsr-multi-tlv (11/17/2023
- 12/09/2023)
Hi Les,
My responses are inline below with [HC].
Best Regards,
Huaimo
Linda –
When we have polarized positions (for whatever reasons), coming to consensus is
often difficult. Each side tends to dismiss the arguments of the other –
sometimes regardless of merit.
So, maybe the following won’t help – but I am going to give it a try.
Point #1: There are existing TLVs for which MP behavior was explicitly stated
in existing RFCs and there are already many deployments that conform to those
RFCs (see Introduction of MP draft for a list).
If we choose to use a different encoding to support > 255 bytes for other
codepoints, this both complicates implementations and confuses the definition
of new protocol extensions. When defining a new codepoint should I choose MP or
should I choose a different encoding? And what criteria can be used to make
this choice a sensible one?
And since MP is already REQUIRED for those TLVs where it was explicitly
defined, we will always have to support that – at least for some codepoints.
[HC]: When Big-TLV is used for a TLV > 255, it does not affect the existing
MP-TLV that has been used for another TLV > 255. When defining a new codepoint
for a new TLV, it seems better to choose Big-TLV if backward compatibility is
needed for the new TLV since Big-TLV is backward compatible.
I have explained it (i.e. ,“Big-TLV is backward compatible”) in detail. In
addition, some other people indicate it in the LSR mailing list.
Point #2: MP for IS-Neighbor/Prefix Reachability TLVs has already been
implemented by multiple vendors, tested in both partial deployment and full
deployment scenarios. We know that it works and we know what the problems are
in partial deployment.
This cannot be said for new alternatives.
With due respect to Huaimo, he tends to characterize the implementation
problems to be solved for Big-TLV as “easy to resolve” but given the absence of
implementation I think this is an overly optimistic and somewhat naïve POV. (No
offense intended)
[HC]: It seems that the implementation of an idea/solution in a draft is not
required by LSR WG for the draft to be adopted, or even for the draft to become
RFC.
Point #3: As documented in the IANA section of the MP draft, the problem
extends to sub-TLVs of top level TLVs as well. It is then not as simple as
reserving one encap TLV to handle the top-level TLV case. We also have to have
a solution when a sub-TLV requires more than 255 bytes. MP solves this without
additional changes. Big-TLV has yet to discuss this.
[HC]: It seems that MP-TLV has to discuss this.
Assume: a TLV (e.g., TLV 1) > 255 and its sub-TLV > 255, there are MP-TLVs
(e.g., MP-TLV 1 and MP-TLV 2) for the TLV and MP-TLVs (e.g., MP-TLV 3 and
MP-TLV 4) for the sub-TLV. All these MP-TLVs are TLVs. If there is another TLV
(e.g., TLV 2) > 255 and its sub-TLV > 255 and all these sub-TLVs (i.e., the
sub-TLV of TLV 1 and the sub-TLV of TLV 2) have the same type and there are
MP-TLVs (e.g., MP-TLV 5, MP-TLV 6) for TLV 2 and MP-TLVs (e.g., MP-TLV 7,
MP-TLV 8) for the sub-TLV (of TLV 2) > 255, how do you handle this case? How
do you map MP-TLVs (e.g., MP-TLV 3, 4, 7, 8) for the same type of the sub-TLVs
to their TLVs (e.g., TLV 1, 2)?
If Big-TLV actually solved the partial deployment case, we would have a
motivation to look at it more seriously. But it does not. It has the same issue
with partial deployment that MP does. So for me, there is no value add to
Big-TLV – and it does require additional implementation work – not all of which
has even been defined yet.
[HC]: Big-TLV actually solved the partial deployment case. I have explained
this in detail. In addition, some other people indicate that Big-TLV is
backward compatible in the LSR mailing list.
It isn’t better – it is just different – and comes with additional
implementation costs.
[HC]: Big-TLV is better as I have explained in detail.
Les
From: Tony Li <[email protected]<mailto:[email protected]>> On Behalf
Of Tony Li
Sent: Friday, December 1, 2023 2:44 PM
To: Linda Dunbar <[email protected]<mailto:[email protected]>>
Cc: Yingzhen Qu <[email protected]<mailto:[email protected]>>;
[email protected]<mailto:[email protected]>;
lsr <[email protected]<mailto:[email protected]>>
Subject: Re: [Lsr] WG Adoption Call - draft-pkaneria-lsr-multi-tlv (11/17/2023
- 12/09/2023)
Hi Linda,
* Suppose the information to be carried by the Extended IS Reachability
(type 22) (in Example 4.1) is larger than 255. Does it mean the recipient will
receive 2 TLVs (both with the Type 22) in one LSA? For legacy routers, the
second TLV (Type =22) might overwrite the first TLV.
Yes, a legacy implementation may well have bugs. The proposal is to fix that:
expect MP-TLVs.
[Linda] Are you saying only the legacy implementation with bugs will be
confused with two TLVs with the same Type in in one LSA?
No. All implementations have bugs. This is reality.
Implementations that do not understand MP-TLV may be confused. Correct
implementations of MP-TLV support will not be confused.
* Isn’t it more straightforward to have a new TYPE value for carrying the
extra information beyond the 255 bytes? So, the legacy routers can ignore the
TLVs with the unrecognized types.
You could do that, but code points are not free. We certainly cannot afford
another code point for each existing code point. Using just one code point is
less than helpful: it forces us to aggregate information that has no business
being aggregated. Ignoring information is a non-starter because it makes
partial deployments fatal: some of the domain operates with some information
and some of the domain operates with different information.
[Linda] Why not consider having just one additional TYPE code with sub-types to
indicate which original TLVs the value should be appended to?
We have considered it. See all of Les’ emails for why it’s a bad idea.
If it helps simplify this debate: we know that you work for Futurewei/Huawei
and that the discussion has polarized into your Big-TLV faction vs. everyone
else. Repetition of previously made points add zero value to the discussion.
Tony
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